functionalization of metal oxide nanostructures via self ... · measured using dynamic light...

Functionalization of Metal Oxide Nanostructures via Self-Assembly.

Implications and Applications.

Funktionalisierung von Metalloxid-Nanostrukturen mit

selbstorganisierenden Monolagen. Implikationen und Anwendungen.

Der Technischen Fakultät

der Friedrich-Alexander-Universität

Erlangen-Nürnberg

zur

Erlangung des Doktorgrades Dr.-Ing.

vorgelegt von

Luis Francisco Portilla Berlanga

aus Torreon (Coahuila, Mexico)

Als Dissertation genehmigt

von der Technischen Fakultät

der Friedrich-Alexander-Universität Erlangen-Nürnberg

Tag der mündlichen Prüfung: 19.01.2017

Vorsitzende des Promotionsorgans : Prof. Dr.-Ing. Reinhard Lerch

Gutachter: Prof. Dr. rer. nat. Marcus Halik

Prof. Dr. rer. nat. Carola Kryschi

Abstract

Self-assembled monolayers (SAMs) were employed for the surface modification of a variety of metal

oxide nanostructures (NSs). This resulted in an remarkable assortment of inorganic-organic (core-

shell) hybrid materials. These materials were fabricated, characterized and employed in different

concepts. SAMs allowed to incorporate the endless realm of organic chemistry into the less vast

territory of inorganic materials. An in depth, step by step analysis of the aspects involved in the

process, of going from a pristine inorganic material to an inorganic-organic (core-shell) hybrid

material, is described. In addition, the general theoretical background, which is required to fully grasp

the processes and characterization is herein described. The final outcome was, well characterized

core-shell building blocks with a solid groundwork regarding their new properties. The latter was of

paramount importance, as this hybrid NSs were meant to be further employed in other complex

processes.

The pristine NSs cores and core-shell NSs were characterized with a variety of techniques to ensure

that a complete and well controlled surface modification has taken place. Chemical information about

the composition of the organic shell of the core-shell NSs was obtained via Fourier transform infra red

(FTIR) spectroscopy. The amount of organic vs. inorganic material (SAM grafting density) was

characterized with thermogravimetric analysis (TGA). The size and zeta potential of NSs were

measured using dynamic light scattering techniques (DLS). Surface energies and wettability of the

NSs surface were characterized with goniometry via static contact angle (SCA) measurements. A

comparison of the robustness between different types of SAM molecules, phosphonic acids,

carboxylic acids and catechols was performed as well.

The new properties of the inorganic materials which arose from the effects of surface modification,

were carefully tailored and exploited for several applications. Due to the flexibility allowed by

incorporating organic chemistry, the resulting applications often had a multidisciplinary character.

These applications are also described within this work. Some of them are still work in progress and are

therefore described with less detail than others. Example applications include: Solution processing,

polymer wrapping, thin films, coatings, self-organization of NSs, polymer composites, nanooncology

as well as waste water treatment.

Kurzfassung

Selbstorganisierte Monolagen (SAMs) wurden für die Oberflächenmodifizierung einer Vielzahl von

Metalloxid-Nanostrukturen (NSs) verwendet. Daraus ging eine bemerkenswerte Anzahl an

anorganisch-organischen (Kern-Hülle) Hybridmaterialien hervor. Diese Materialien wurden

hergestellt, charakterisiert und in unterschiedlichen Konzepten angewendet. Die Verwendung

organischer SAMs ermöglicht die Vielfalt der organischen Chemie mit hoch funktionalen

anorganischen Materialien zu verbinden. Eine schrittweise Analyse der in den Prozess eingebunden

Parameter wird beschrieben, wie aus einem ursprünglichen anorganischen Material ein

anorganisch-organisches (Kern-Hülle) Hybridmaterial wird. Darüber hinaus ist der allgemeine

theoretische Hintergrund beschrieben, der erforderlich ist, um die Prozesse und die Charakterisierung

zu erfassen. Das Endergebnis waren vollständig charakterisierte Kern-Hülle NSs mit einem

Schwerpunkt in Bezug auf ihre einzigartigen Eigenschaften. Dies war von größter Wichtigkeit, da

diese Hybrid-NSs weiter in anderen komplexeren Anwendungen eingesetzt wurden.

Die unfunktionalisierten NSs Kerne und Kern-Hülle NSs wurden mit einer Vielzahl von Techniken

charakterisiert, um sicherzustellen, dass eine vollständige und gut gesteuerte Oberflächenmodifikation

stattgefunden hat. Informationen über die chemische Zusammensetzung der organischen Hülle des

Kern-Hülle NSs wurde mittels Fourier-Transformations-Infrarot (FTIR) Spektroskopie erhalten. Das

Verhältnis von organischem zu anorganischem Material (SAM Ankerdichte) wurde durch

thermogravimetrische Analyse (TGA) bestimmt. Die Größe und das Zeta-Potential von NSs wurden

mittels dynamischer Lichtstreuung (DLS) gemessen. Oberflächenenergien und Benetzbarkeit der NSs

Oberfläche wurden mit Goniometrie durch die Messung der statischen Kontaktwinkel (SCA)

charakterisiert. Ebenfalls wurde ein Vergleich der Robustheit zwischen verschiedenen Arten von

SAM-Molekülen, Phosphonsäuren, Carbonsäuren und Katechinen durchgeführt.

Die neuen Eigenschaften der anorganischen Materialien konnten, ermöglicht durch

Oberflächenmodifizierung, für verschiedenste Anwendungen präzise angepasst und genutzt werden.

Die daraus resultierenden Anwendungen besitzen oft einen multidisziplinären Charakter, der durch

den flexiblen Einsatz organischer Moleküle ermöglicht wird. Diese Anwendungen werden auch im

Rahmen der vorliegenden Arbeit beschrieben, wobei einige noch weiterentwickelt und daher weniger

detailliert behandelt werden. Als beispielhafte Anwendungen werden Lösungsprozessierung, dünne

Schichten, Polymerkomposite, Beschichtungen, Nanoonkologie sowie Abwasserbehandlung

beschrieben.

i

Table of contents

Abstract .................................................................................................................................................. iii

Kurzfassung ............................................................................................................................................. v

1. Introduction and motivation ................................................................................................................ 1

2. Theoretical background ....................................................................................................................... 3

2.1. Nanostructured materials and nanostructures ............................................................................... 3

2.2. Surface area of nanostructures ...................................................................................................... 5

2.3. Self-assembled monolayers .......................................................................................................... 6

2.3.1. Anchor groups ....................................................................................................................... 9

2.3.2. Backbone ............................................................................................................................. 10

2.3.3. Head group .......................................................................................................................... 11

2.3.4. Studied anchor groups ......................................................................................................... 11

2.4. Stability of 0D and 1D NSs in solution ...................................................................................... 15

2.4.1. Electrostatic stabilization ..................................................................................................... 16

2.4.2. Steric stabilization ............................................................................................................... 20

2.4.3. Electrosteric ......................................................................................................................... 21

2.4.4. Impact of size and geometry of nanostructures ................................................................... 21

2.5. Thin film transistors ................................................................................................................... 23

2.6. Chapter summary........................................................................................................................ 24

3. Functionalization and characterization of NSs .................................................................................. 25

3.1. Functionalization of 2D materials .............................................................................................. 25

3.2. Functionalization of 0D materials .............................................................................................. 26

3.2.1. Chemical characterization (FTIR-ATR) .............................................................................. 27

3.2.2. Saturation threshold (TGA) ................................................................................................. 33

3.2.3. Mixed monolayers (SCA, FTIR & Zeta potential) .............................................................. 39

3.3. Anchor group stability ................................................................................................................ 43

3.3.1. Desorption ........................................................................................................................... 43

3.3.2. Exchange on 2D NSs ........................................................................................................... 46

3.4. Chapter summary........................................................................................................................ 51

ii Table of contents

4. Applications ....................................................................................................................................... 53

4.1. Solution processing .................................................................................................................... 53

4.1.1. Green processing ................................................................................................................. 53

4.1.2. Any medium processing ...................................................................................................... 55

4.1.3. Shell by shell (double shell) ................................................................................................ 56

4.1.4. Polymer wrapping ............................................................................................................... 59

4.2. Thin films. From 0D to 2D ......................................................................................................... 64

4.2.1. Flexible dielectrics ............................................................................................................... 64

4.2.2. Coatings ............................................................................................................................... 68

4.3. Self-assembled thin films ........................................................................................................... 70

4.3.1. Regio-selective deposition of nanoparticles ........................................................................ 70

4.3.2. Block co-polymer phase matching ...................................................................................... 74

4.4. Polymer composites .................................................................................................................... 76

4.5. Nanooncology............................................................................................................................. 78

4.6. Magnetic water cleaning ............................................................................................................. 83

4.7. Chapter summary........................................................................................................................ 85

5. Conclusion and outlook ..................................................................................................................... 87

6. Characterization methods and materials ............................................................................................ 89

6.1. SCA ............................................................................................................................................ 89

6.2. DLS ............................................................................................................................................ 89

6.3. FTIR-ATR .................................................................................................................................. 89

6.4. TGA ............................................................................................................................................ 90

6.5. Electrical ..................................................................................................................................... 90

6.6. Spray coating .............................................................................................................................. 90

6.7. Materials ..................................................................................................................................... 90

6.8. Functionalization procedures ...................................................................................................... 91

6.8.3. AlOx (Sigma A.), ITO (Sigma A.) and TiO2 (Nanograde 30 nm) ....................................... 91

6.8.4. CeO2 .................................................................................................................................... 91

6.8.5. Fe3O4 (Plasmachem ~10 nm) ............................................................................................... 92

6.8.6. TiO2 (Plasmachem 8 nm) ..................................................................................................... 93

Table of contents iii

6.8.7. Fe3O4 and CoFe3O4 (Prof. Kryschi) ..................................................................................... 94

7. Appendix ........................................................................................................................................... 95

8. List of Figures ................................................................................................................................. 101

9. List of tables .................................................................................................................................... 107

10. Abbreviations ................................................................................................................................ 109

11. Bibliography .................................................................................................................................. 111

12. Acknowledgements ....................................................................................................................... 121

13. Curriculum vitae ............................................................................................................................ 123

1

1. Introduction and motivation

The advent of tool making marked a point in history which started a continuous technological

evolution. Ever since, this understanding has gradually developed and has given shape to our

contemporary world and society. In present day technology, manipulation of materials at the nano

scale is employed for the creation of new materials and device fabrication. This happens both at

industrial and research scales. As the nanorevolution develops, so do our demands for improved

devices and technologies. Towards the fulfillment of these requirements, nanoscience provides “plenty

of room” to elegantly innovate and expand current designs [1].

The working principle of all modern or primitive devices is based on one simple concept. Essentially,

it consists in generating a contrast between two or more materials arranged in a particular

configuration. This allows for the differing properties of materials to be beneficially exploited for a

specific purpose. In retrospect, the fabrication of a device lacking this contrast would be impractical,

or simply impossible to conceive. It can therefore be stated, that material contrast is at the core of any

instrument or device that has ever been fabricated. The impact of the previous statement is even more

accurate when dwelling with device fabrication at the nanoscale, where properties of materials may

differ from those of the bulk. At such scale, material properties can be dependent on their dimensions,

geometry, or neighboring materials. This results in additional degrees of freedom when it comes to

device fabrication [2], [3]. It is by exploiting these concepts, that this thesis explores the fundamental

and technical implications of creating a material contrast via the tailoring of the surface of 0D

nanostructures (NSs) with self-assembled monolayers (SAMs). The NSs surface properties are

carefully tuned in order to be employed as building blocks for their integration into nanostructured

materials (NSMs) and consequentially device fabrication.

The research and application fields of NSMs are as broad as the NSMs themselves [2]. The most

notable area involving NSMs is the field of semiconductors, being this area the most fruitful of the

efforts. However, they very often encompass an interdisciplinary mixture of every natural science. For

instance: energy generation and storage [4], [5] enhanced mechanical properties by creating

nanocomposite materials [6], nanomedicine (in areas such as targeted drug delivery and nanooncology

[7]–[9]), environmental [5], even intricate self-assembly [10] and bottom up fabrication schemes [11],

just to name a few.

2 Introduction and motivation

Just as NSMs, SAMs encompass a wide area of research and applications themselves [12]–[15]. The

field of SAMs and the field of NSMs are closely related, mostly because SAMs offer a versatile

bottom-up approach for surface tuning and control (tailoring) of the NSs. Without SAMs, a total and

direct surface control of the NSs would often remain out of reach. These SAMs which assemble

around the surface of the NSs, create a material contrast between the inorganic core and the organic

SAM (core-shell). Naturally, this contrast can be exploited for a specific function. It becomes evident

now, that the combination of NSMs and SAMs adds additional degrees of freedom to the already

diverse system of NSMs. This in turn, provides a way for new solution processing techniques which

involve the self-assembly of "smart" NSs for the formation of NSMs. In addition, this thesis deals with

the necessary steps required to provide well characterized and controllable core-shell 0D materials.

Lastly, their applications are also discussed. As the prospective applications of the core-shell NSs are

described, the multidisciplinary makeup and diverse potential of NSMs and SAMs becomes more and

more palpable. A concise description of these applications has been partly published in peer reviewed

journals. These applications include: Coatings [11], [16], flexible electronics [17], self-assembly

concepts of NSMs [11], [18], as well as some more elementary studies [19]. Other applications which

may or may not be under preparation for publication, are also described. However, before discussing

these applications in detail, there are a few fundamental concepts that must be elaborated in order to

properly discuss these ideas.

3

2. Theoretical background

In this chapter, we will look into the concept of nanostructures (NSs) and nanostructured materials

(NSMs) and their classification. This is followed by the theoretical background of self-assembled

monolayers (SAMs), which is concisely described using 2D NSs as an archetypal model. Lastly, the

comparatively more intricate consequences of SAMs grafted onto 0D NSs are more exhaustively

analyzed.

2.1. Nanostructured materials and nanostructures

Primarily, NSMs should be differentiated from NSs. NSMs consist of an arrangement of smaller

building blocks. These building blocks (of which NSM are constituted) consist of NSs that can be

classified into four classes in function of their dimensionality: 0D, 1D, 2D and 3D (Figure 2.1). As

NSs, we distinguish structures in which at least one of its dimensions is of a critical magnitude. This

critical magnitude gives rise to size dependent material properties. This critical dimension is

frequently in the submicron and nano regimes. However, the exact magnitude of these critical

dimensions is difficult to demarcate as they are dependent on various physical phenomena. Therefore,

they are typically approached on a case to case basis.

Figure 2.1.: Representation of nanostructures. 0D (red sphere), 1D (green cylinder), 2D (blue rectangular

parallelepiped) and in 3D (cylinder 3D matrix).

4 Theoretical background

However, a peculiarity needs to be noted in this classification scheme. The discrepancy between a 3D

NS and a NSM at this point seems to be vague. Yet, a clear difference between them exists. 3D NSs

refer to a 3D matrix comprised of a single material. For example, a phase separated block-copolymer

matrix or a porous and hollow structure [20]. Whereas NSMs are comprised of an array of 0D, 1D, 2D

or 3D NSs. However, in reality (since the only 3D NS is a matrix) the terms 3D NSs and NSM are

used ambiguously in literature [3], [4], [21]. Going into specifics, a systematic classification of NSs

has already been proposed by V. Pokropivny in 2006 [21]. It has been adapted in Figure 2.2.

Figure 2.2.: NSs classified on basis of their dimensionality as suggested by V. Pokropivny in 2007 [21].

Reprinted from [21] with permission from Elsevier.

Theoretical background 5

2.2. Surface area of nanostructures

0D and 1D NSs are well known for their high surface area to volume ratio when compared to their

bulk equivalents. By using a cube, a classical example of this concept is portrayed in Figure 2.3. In

this example, as we progress from Figure 2.3a to d, the initial cube is split into eight smaller cubes

followed by the splitting of the smaller cubes into eight cubes again and so forth. What we observe is,

that by splitting the cube into smaller components we have exponentially increased the total amount of

exposed surface, while the initial total volume remains unchanged. Pushing the analogy of Figure 2.3

further into nanoscale proportions, the vast amount of surface area available becomes palpable

compared to that of the bulk material. In layman terms this translates to, that a few milligram (mg) of

nanoparticles could in fact have more real state than your current shared flat apartment. In terms of

SAM formation it means, that a very high quantity of molecules are going to be required if the intent is

to fully cover the surface of the nanoparticles (or your entire flat) with SAM molecules. This in great

contrast to 2D NSs, were as far as they are concerned, the area involved is simply very much related to

the substrate dimensions. How to categorize and measure this surface area is now explained below.

Figure 2.3.: Surface area to volume ratio. The increment of surface area vs. volume is graphically represented

from a) lowest ratio, to d) highest ratio.

The standard property of solids by which the surface area of a solid (or powder) is defined, is called

Specific Surface Area (SSA). Commonly employed units of SSA are usually specified in m2/kg or

m2/g. What the SSA property achieves, is to simply correlate a specific amount mass to a specific

amount of area. There are two main methods from which the SSA of a nanomaterial can be obtained.

By measuring the amount of adsorbate gas to the nanomaterial powder based on the Brunauer–

Emmett–Teller isotherm (BET) [22]. Or, geometrically estimated from a particle size distribution of

the nanomaterial as it was performed within this work.

a) d)c)b)

S. Area = 6 u2

Volume = 1 u3

SA/V ratio = 6:1

S. Area = 12 u2

Volume = 1 u3

SA/V ratio = 12:1

S. Area = 24 u2

Volume = 1 u3

SA/V ratio = 24:1

S. Area = 384 u2

Volume = 1 u3

SA/V ratio = 384:1

Increase of surface area to volume ratio (SA:V)


When there is access to the mean particle size or a particle size distribution, the SSA of a nanopowder

can be approximated by simple geometric calculations. This method however, is most accurate when

the morphological discrepancies between individual particles are minimal. That is, all particles tend to

have the same shape (spheroidal, cuboid, rods, etc.). For the sake of argument, let's assume the case in

which all of the particles are spheres with exactly the same size. In this case we can relate the volume

of a sphere, density of the material and the mass of material to a finite amount of particles resulting in

Equation 2.1, were m = mass, d = density and r = mean radius of particles.

𝐴𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 𝑝𝑎𝑟𝑡𝑖𝑐𝑙𝑒𝑠 = 3𝑚

4𝑑𝜋𝑟3 2.1

Once the amount of particles is known, simply multiplying the area of a spherical particle by the

amount of particles results in the total available area (Equation 2.2). To get some perspective into this,

using the proposed scenario, if we calculate the total area of 100 mg of alumina (d = 3.95 g/cm3)

particles with a diameter of 50 nm the total area results in 3.03 m2. However, doing the same

calculation but with 3 nm particles results in more striking results with an area of 50.63 m2.

𝑇𝑜𝑡𝑎𝑙 𝐴𝑟𝑒𝑎 = 𝐴𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 𝑝𝑎𝑟𝑡𝑖𝑐𝑙𝑒𝑠 × 4𝜋𝑟2 2.2

Finally, since the SSA unit is in function of mass, diving the total area in m2 by the mass gives the

final result (Equation 2.3).

𝑆𝑆𝐴 = 𝑇𝑜𝑡𝑎𝑙 𝐴𝑟𝑒𝑎

𝑚 2.3

Naturally, this method is not restricted to a single particle size. It is also applicable if a nanoparticle

size distribution is available. This is achieved by calculating the total area of the different nanoparticle

diameters from the distribution and doing a sum of the calculated areas.

In conclusion, the SSA is a critical parameter to know before functionalizing 0D or 1D NSs as it will

define a theoretical starting point for the amount of required SAM molecules. A major disadvantage of

estimating the SSA via geometric calculations is the possibility of a big margin of error, in particular

for porous samples or with a rich morphological diversity. Therefore, complementing the SSA

calculations with either BET or electron microscopy measurements is highly advisable whenever

possible.

2.3. Self-assembled monolayers

Concisely, a self-assembled monolayer consists of a spontaneous, highly ordered, self-terminating

reaction on a surface. These reacting species usually consist of chain like organic molecular reagents,

[12], [14], [23]. The deposition of SAMs typically requires the solvation of the molecules in a liquid or


gas medium which allows for the free movement of the SAM molecules [24]. This allows them to self-

organize on a surface that is also present within the solvation medium of the SAM molecules. Figure

2.4a shows a graphic representation of the self-assembly process on a 2D NS. It starts with a vacant

surface which is subsequently exposed to the SAM molecules. The self-assembly process begins when

the free SAM molecules start to move around the vicinity of the NSs surface. At this point they may

become physisorbed to the surface. This physisorption, is usually followed by chemisorption onto the

surface, however, it is not a requirement for SAM formation. As more molecules bind to the surface,

the SAM starts to take shape, this binding of molecules continues until the surface is covered by only

one layer of the SAM molecules. This is possible because the surface has a limited amount of reactive

sites and the SAM molecules cannot bind between them. In parallel, the intermolecular forces of the

SAM molecules can also contribute to the organization and package density of the SAM. By

organization of a SAM, we can understand at least two things: SAM tilt angle (Figure 2.4b) and the

formation of crystalline domains (Figure 2.4c). The way these parameters are affected depends mainly

on the chemical structure of the molecules and the deposition conditions. Eventually, when no more

anchoring sites are available at the surface, the SAM molecules are packed as tight as possible and the

SAM formation is finished.

Analogously depicted in Figure 2.5, the process of SAM formation on 0D NSs follows exactly the

process described above. Conversely, as similar as the processes might be, the consequences of SAMs

grafted onto 0D NSs diverge in many degrees from those grafted onto 2D NSs. Naturally, it goes

without saying, that both systems have complications of their own, which are more thoroughly

described later in this work.

The structure of self-assembling molecules can be broken down into 3 principal components: The

anchor group, the backbone and the head group. Each component plays a crucial role in SAM

formation and each one is described in detail throughout this section. Figure 2.6 shows a prototypical

graphical representation of the geometrical hierarchy of a SAM grafted onto a 0D NS. Figure 2.6 also

describes some commonly employed SAM molecule configurations. At the center of Figure 2.6, there

is the NS core, which in our case consists of an inorganic oxide material. The core provides part of the

functionality of such building blocks. Core functionalities are diverse, some commonly employed core

properties are magnetic, conductive, dielectric, transparent, opaque, used as carriers, etc. As we

proceed outwards (in red), we encounter the anchor group which interacts and attaches the molecule to

the NS surface. Afterwards in blue, there is the backbone or spacer of the SAM molecule. As its name

implies, this component mainly acts as a support for the SAM molecule. However, being the largest

component of the molecule, it often provides functionality in an indirect manner rather than on

purpose. Finally, there is the head group (in green), which usually defines the main functionality of the

SAM molecule. However, the roles of the SAM components are more complex than just described, for

this reason they are explained in more detail below.


Figure 2.4.: a) Schematic process of the formation of a C18-PA SAM on a 2D NS. b) Side view depiction of

SAM tilt angle. c) Top view illustration of the crystalline and amorphous domains that may be present after the

SAM formation.

Figure 2.5.: Schematic process of SAM formation of onto a 0D NS.

θSAM tilt angle

a)

amorphous

crystalline

crystalline

c)

b)

2

D

S

AM

f o

rm

at i

on

0D SAM formation


Figure 2.6.: Anatomy of a core-shell 0D building block.

2.3.1. Anchor groups

The anchor group is the most crucial component of any SAM molecule, it is responsible for the

binding of the SAM molecule to the surface. The nature of the bond between the surface and the

anchor group is of paramount importance and must be chosen adequately depending on the SAM

molecule structure, NS surface properties and the nature of desired application. In order for self-

assembly to properly occur, the interaction between the anchor group and the NS surface must be the

governing attraction force. This being particularly significant whenever the backbone or the head

group of the SAM molecule consist of any number of electrostatic or polar species that could in

principle, also interact with the NS surface. Therefore, anchor groups capable of covalent bonding are

preferred, especially when a permanent modification of the surface is required. Weaker interactions

(electrostatic, hydrogen bonding, orthogonal or Van der Waals) are ideally only sought after when a

Anchor group (stability of SAM)•Phosphonic acid

•Carboxylic acid

•Catechol

Backbone (solubility and geometry)•Orthogonal

•Polar

•Ionic•Etc.

Head group (functionality or solubility)•Inert

•Reactive

•Polar•Ionic

•Semiconducting•Etc.

Nanoparticle core (functionality)•Alumina

•Titania

•Iron oxide•Etc.


less permanent or reversible alteration of the surface is required [24], [25]. These weaker interactions

can also be employed as permanent modifications, however, they may only be so under more strictly

controlled circumstances [18].

2.3.2. Backbone

It is very often the case for SAM molecules, that the backbone is the biggest constituent of all

components. Usually, the role of the backbone typically comes down to purely geometrical aspects.

Intuitively, the backbone acts as a spacer between the anchor group and the head group. In addition,

this spacing allows for the head group to be more freely exposed by being further away from the

surface. This is predominantly true for surfaces with a pronounced curvature as it is later showcased in

Figure 2.17b and d.

Nevertheless, the backbone can also have strong influence on the ordering of the SAM as well as the

tilt angle. Generally speaking, the stronger the backbone to backbone interactions the more ordered the

final structure of the SAM will be. Take for example Figure 2.7. When using these molecules to build

a SAM, the bigger the carbon chain backbone is, the stronger the van der Waals interactions between

them will be. This results in better ordered SAMs [26]. However, it must be noted that there must be a

threshold to this approach, in which increasing the chain length won't necessarily incur in higher

ordering, instead the contrary can happen. It is also possible, that the backbone interactions play a

negative role on the final ordering of the SAM if they are of a repulsive or other non favorable nature.

Figure 2.7.: Phosphonic acid SAM molecules of increasing length. As the backbone (black) length increases, so

do the van der Waals interactions between them and therefore also the final ordering of the SAM [26].

Less ordered SAM More ordered SAM


On a more chemical side of things, the backbone being the biggest component, has a strong influence

on its solubility. By solubility we refer to the facility of the SAM molecule to be solvated in the

medium of deposition, as well as the dispersibility it may provide by means of steric or electrostatic

stabilization when grafted onto a 0D or 1D NSs. These effects are thoroughly described in Section 2.4

As a final point, SAM Backbones are usually of a non-conductive nature. Therefore, very short

backbones are usually sought after when electrical conductivity through the SAM is required [27].

However, for the most part the backbone usually plays a highly insulating role in most of applications

[17], [28]. More exotic roles also exist, where the backbone is employed as a 2D organic-inorganic

hybrid dielectric in self-assembled field-effect transistors (SAMFETs) [29], [30] or even as charge

storage layers in thin film transistors memories [31].

2.3.3. Head group

The head group being the outermost component (and therefore the most exposed) of the SAM, has the

strongest impact in the overall functionality of the SAM. Common effects of head groups are the

modification of wettability characteristics of the SAM or adjustment of dipole moment of the SAM

molecule [32]. This properties are particularly important when the SAM is grafted onto 0D NSs, as

these properties can play a dominating role in the stability of the 0D NSs in solution (Section 2.4). It is

also the most diverse component in terms of chemical structures available. Head groups often vary

from inert to reactive and even polymerizable groups, as well as polar, electrostatic, semiconducting,

etc. (Figure 2.6). Fundamentally, whatever is considered as organic chemistry, can potentially be

employed as a head group. Due to these reasons, it is perhaps the most interesting part of the SAM

components. To put it another way, the head group is usually the only reason we want to employ a

SAM in the first place.

Spatial factors tend to be an issue with head groups, especially when the bulkiness of the head group is

of a much greater magnitude than the anchor group. Having a SAM with a bulky head group may lead

to a poorly packed and disordered SAM. A possibility to overcome this problem is to do a co-

deposition of mixed SAM. In such an approach, a SAM with a smaller head group fills the gaps left

between molecules while consequentially acting as a support for the bulkier head groups [33], [34].

Similarly, a SAM exchange scheme where a bulkier SAM replaces a less bulky and weakly bound

SAM is also a successful strategy [35].

2.3.4. Studied anchor groups

Commonly employed SAM anchor groups for the modification of metal oxides include phosphonic

acids (PAs), carboxylic acids (CAs) and catechols [12]. A study performed during this work revealed

that; under ambient conditions (room temperature, neutral pH), phosphonic acids have a far stronger

binding affinity to titanium oxide NSs in comparison to carboxylic acids and catechol groups [19]. The


same trend is generally true for a variety of other metal oxides we have employed [36]. However,

mixed results were often obtained for the CAs and catechols anchor groups. This results are

thoroughly described in Section 3.3.

Since, it is critical for all of the applications discussed in this work to have a robust and reliable

modification of the NSs surface. In this work, the majority of the experiments involve SAM molecules

with phosphonic acids acting as anchor groups. Nonetheless, a few experiments involved carboxylic

acids and catechols anchor groups and therefore they are described as well.

2.3.4.1. Phosphonic acids

The first studies of PAs binding to alumina surfaces date back to the mid-late 1980's [37], [38]. Since

then, the covalent binding mechanism of the PAs to a metal oxide surface has remained partly

uncontested and is generally accepted as displayed in Figure 2.8. In short, the P-OH groups of the PA

molecule react with the OH groups of the metal oxide surface. This results in an acid-base

condensation reaction which has as a by-product H2O [39]–[43]. However, the final faith of the double

bonded oxygen (P=O) is not so clear. Partly because of this, the final binding configuration of the PA

to the metal oxide surface is somewhat of a gray zone. This is mainly due to all the possible binding

configurations that arise from having three active motifs (Figure 2.9).

The generally accepted trend is that, either a monodentate or bidentate non-chelating configurations

are the dominant configurations (Figure 2.9a, c, i and j). The previous statement can be made from

several theoretical and experimental studies of PAs grafted onto several metal oxides surfaces [39]–

[43]. Moreover, in this thesis, similar observations were obtained via FTIR-ATR of functionalized NP

powders with PA molecules (Section 3.2.1). Ultimately, it might as well be that all possible binding

configurations co-exist, and that their state merely depends on the accessibility the PA group has to the

surface anchoring hydroxyl sites. That being said, other factors such as the metal oxide employed,

crystallinity (or lack of) and exposed plane should also be considered, since all of them can have an

impact on the amount of hydroxyl groups available. Yet, hydroxyl independent reaction pathways of

PA molecules have been reported on SiO2 wafer substrates (T-BAG method) [44], however, this

requires harsher reaction conditions (140 °C).

Figure 2.8.: Phosphonic acid binding mechanism to a metal oxide surface [39]–[43].


Figure 2.9.: Various phosphonic acid binding modes. M = metal. a) and b) monodentate, c) and d) bridging

bidentate, e) bridging tridentate, f) and g) chelating bidentate, h) chelating tridentate, i) j) k) l) other viable

hydrogen bonding modes. Reprinted from [41] with permission from the American Chemical Society.

The strong binding affinity of PAs to metal oxides makes them excellent candidates for surface

modification. This affinity translates into simpler chemistry, monolayer robustness and overall lower

material requirements in order to achieve full surface coverage of the NSs [19], [36]. Lastly, due to

their ease of synthesis and storage [41], [43] (long term ambient, no self-polymerization), they are

widely commercially available in a variety of different configurations (on a par with thiols and silanes,

minus the stench and difficult handling conditions respectively).

2.3.4.2. Carboxylic acids

Just as PAs, the first studies of carboxylic acid SAMs on aluminum oxide substrates were reported in

the mid-late 1980's [14], [45]. As is often the case with other anchor groups, the coordination of the

carboxylic acid with the metal oxide surface is controversial and highly situational. Figure 2.10

displays various carboxylic acid binding modes. In an infrared spectroscopy study by K. Dobson et al.

[46], it is stated that coordination of CA group to the metal oxide surface mostly happens as a

deprotonated carboxylate (Figure 2.10d, e, f). The study included a variety of metal oxide surfaces

such as TiO2, ZrO2, Al2O3, and Ta2O5. The substrates were functionalized in DI water under ambient

conditions with several CA molecules containing only one carboxylic acid group. It was found that a

strong binding was only present for the ZrO2 substrates, while very weak or not existent for the other

substrates. They concluded that the higher affinity of the CA to ZrO2 was attributed to the higher IEP

) ))) )

) )) )

) ))


(at pH 9) of the ZrO2 substrates. In contrast, TiO2 substrates exhibited their IEP at pH 5. Meaning the

ZrO2 had a surface positive charge at neutral pH, allowing for more favorable reaction conditions.

Figure 2.10.: Various carboxylic acid binding modes) M = metal. a) electrostatic attraction, b) H-bonds to

bridging oxygen, c) H-bonds to carboxylic oxygen, d) monodentate metal-ester, e) bidentate bridging, f)

bidentate chelating. Adapted from [12] with permission from Wiley.

In summary, in order for the carboxylic acid to form a strong bond with the metal oxide surface, it

usually requires non-ambient conditions (pH, temperature) according to other studies [47]–[49].

However, it is also purely situational. Take for example the experiments described in Section 0, where

CA SAMs formed a stable bond on indium tin oxide (ITO) nanoparticles and zinc oxide nanorods.

Whereas, they did not do so on ITO and zinc oxide 2D substrates even when comparable reaction

conditions were employed.

The ample natural occurrence of carboxylic acids (e.g., fatty acids) is perhaps one of the most

attractive characteristics of this anchor group. This natural occurrence results in a more "eco-friendly"

accepted approach under the argument that similar molecules already exist in nature [49].

Furthermore, simpler synthetic routes when compared to phosphonic acids or silanes is also a

desirable trait [12]. However, due to their weaker binding strength in ambient conditions [19], [36],

[46] carboxylic acids have not yet found their way into mainstream commercial availability in terms of

SAM molecules. Such status is currently held only by phosphonic acids and silanes for metal oxides,

as well as thiols for metals.

2.3.4.3. Catechols

The catechol is a relatively new anchor group, it was introduced in 2007 by a bio-mimetics study

based on the remarkable adhesion of mussels to organic and inorganic surfaces [50]. Since then the

catechol has been employed in numerous surface modification schemes [51], [52]. Despite abundant

reports of catechol anchor groups being used for non-reversible modification of metal oxide surfaces

via SAMs. In this thesis, we could not obtain credible evidence (Section 3.3) that supports the

formation of a covalent bond between the catechol and metal oxide surface under the employed

conditions (ambient, neutral pH) [19].

a) d)c)b) e) f)


The suggested binding configurations obtained from literature [12], [53] of the catechol group to a

TiO2 surface are shown in Figure 2.11. Important to note the EWG group, which stands for "electron

withdrawing group". The EWG plays two roles. It prevents the oxidation of the catechol group which

would otherwise render it useless as acknowledged by M. Rodenstein et al. [54]. Furthermore,

according to B. Malisova et al. [55], the EWG plays a critical role on the reactivity of the catechol

since it can directly impact the acidity (pKa) of the catechol group. It was concluded by B. Malisova et

al. that the ideal pH conditions for catechol reactivity occur when the metal oxide surface is as close to

the isoelectric point (IEP) as possible, as well as to the IEP of the catechol, which varies depending on

its pKa. Therefore, the EWG can be used to tune the catechol anchor group disassociation constant in

order to be employed with a specific metal oxide in function of its IEP.

Figure 2.11.: Miscellaneous catechol binding modes on titanium oxide surface. EWG = electron withdrawing

group. a) H-bonds, b) monodentate with H-bond, c) bidentate chelating, d) monodentate with bridging H-bond,

e) bidentate bridging. Adapted from [12] with permission from Wiley.

As is the case with CAs, the binding of the catechol to the metal oxide surface should be approached

on a case by case basis. Often, the reported reaction conditions for the catechols vary greatly and

results are often controversial. From a purely practical point of view, there is no particular good reason

to employ a catechol anchor group whenever carboxylic acid anchor group is also available. However,

the catechol is still a young and perhaps misunderstood anchor group. More research is required to

fully understand its employment as a SAM molecule anchor group.

2.4. Stability of 0D and 1D NSs in solution

Stability is a pivotal property of 0D and 1D NSs in solution. The primordial reason for this is, that

having an unstable dispersion renders it completely useless in terms of solution processability.

Therefore, having a stable dispersion is an especially sought-after attribute. By stability, we refer to

the ability of the NSs to remain dispersed in solution over a period of time without precipitating to the

bottom of the flask. Curiously, the expression "stable dispersion" is frequently used in vague terms and

a) d)c)b) e)


the stability of the dispersion can often vary from years to a mere few hours. Figure 2.12 shows

images of nanoparticle dispersions with varying degrees of stability.

Figure 2.12.: Photographs of TiO2 and Fe3O4 nanoparticle dispersions with varying degrees of stability. a)

Unstable TiO2 dispersion during flocculation. b) Non-transparent stable TiO2 dispersion. c) Transparent stable

TiO2 dispersion. d) Unstable, already flocculated Fe3O4 dispersion. e) Transparent stable Fe3O4 dispersion.

NSs dispersed in solution are under constant movement (Brownian motion) which makes them bump

into each other all the time. When such collision occurs, the NSs have a propensity to attract each

other (agglomerate) in a process known as Ostwald ripening. Essentially, such process occurs simply

because bigger particles (agglomerates) are more thermodynamically stable due to their lesser surface

area [56]. As more and more NSs collide with each other, eventually their size increases reducing their

solubility which causes flocculation to occur rather rapidly. However, this effect can be avoided if

supplementary forces are present that directly counteract the attractions of the NSs or even more,

avoiding the collisions altogether. If such forces are implemented successfully, they may delay or even

completely prevent the flocculation process. To this end, there are two main mechanisms to avoid NSs

agglomeration in solution known as electrostatic and steric stabilization.

2.4.1. Electrostatic stabilization

Electrostatic stabilization of 0D and 1D NSs is in concept very simple. It consists in charging the

surface of the nanoparticles, therefore, forming an electrostatic barrier as shown in Figure 2.13a. These

particles having equal charge, when being in proximity of each other they repel themselves due to

a) c)b)

d) e)


coulombic interactions as shown in Figure 2.13b. The greater the absolute magnitude of the surface

charge, the greater the repulsion, which results in better dispersion stability. The exact magnitude of

this surface charge is hard to measure. Therefore, it is measured by a property of colloidal dispersions

(and surfaces also), known as zeta (ζ) potential. The zeta potential is directly related to the charge of

the surface. However it does not measure the exact charge present at the surface. Examples of

dispersions stabilized via electrostatic interactions can be found in Section 4.5.

Figure 2.13.: Schematic representation of electrostatic stabilization of nanoparticles. a) A nanoparticle with

charged species at its surface. b) Nanoparticles having an equal charge repel each other avoiding agglomeration.

2.4.1.1. Zeta potential

The zeta (ζ) potential is a property of any surface in solution. However, under the context of this work,

it is purely related to the surface charge of 0D or 1D NSs dispersed in solution. The zeta potential is

measured in millivolts (mV). It is generally accepted that dispersions having a zeta potential with an

absolute magnitude higher than 20 mV already form moderately stable dispersions. Whereas,

outstanding stability is usually found for dispersions with potentials higher than 40 mV. The latter is

only true under the concept of an electrostatic stabilization scheme, as highly stable dispersions with a

zeta potential of 0 mV can be obtained via a steric stabilization system.

To better understand what the zeta potential is, Figure 2.14 depicts the electric double layer (EDL)

model. The EDL is the classical model by which the zeta potential is conveniently explained. The

model starts with a charged nanoparticle surface and then it proceeds outwards from the nanoparticle

surface in arbitrary units. Immediately after the charged surface we find the Stern layer. This shell is

comprised of any polar species that have opposing charge from the surface. As we continue outwards,

the influence of the surface charge diminishes gradually forming an ever weaker bound shell of polar

Electrostatic barrier

Charged species

on surface

Electrostatic

nanoparticle repulsion

a) b)


species. At which point, even polar species of opposite magnitude may exist but in lesser number.

Finally, we reach the slipping plane which is defined as the point until the whole system acts "as one",

meaning without any influence from the bulk phase (solvent). It is only until this point that there is

clear distinction between the phases. It is here, that it is possible to measure a nanoparticle "charge" or

more fittingly the zeta potential. Therefore, prior to the slipping plane there is no notable distinction of

the systems. As a result, it is difficult to measure the real surface charge. It now becomes clear (in

concept) that zeta potential is not completely synonymous to surface charge, instead just related.

Figure 2.14.: Schematic representation of the electric double layer (EDL) on a nanoparticle. Red and blue

spheres represent charged species of opposite magnitude.

Lastly, a brief mention of the most relevant mechanisms by which a zeta potential can arise [57] was

compiled.

1. Ionization of surface groups.

2. Difference of electron affinity between the dispersion phase and the nanoparticle surface.

3. Physical entrapment of non-mobile charge.

4. Preference of a phase for ions of a specific charge.

Distance from surface0

Surface potential

ζ(mV)

Stern potential

ζ potential

Charged surface

Stern layer

Slipping plane


Mechanism 1 is one of the most relevant mechanisms when dealing with metal oxides and it is

described in more detail in the immediate section below. As to mechanism 2, we can find some

evidence suggesting that this effect is the cause of the zeta potential of the nanoparticles described in

Section 3.2.3.3. We can also find mechanism 3 very clearly in action in Section 4.5. Whereas, no

situation in this work involved mechanism 4. It is possible however, that these effects may combine to

either enhance or hinder the overall zeta potential. Therefore, a discrete distinction between the

mechanisms that may be at play in a particular situation is often difficult to pin point.

2.4.1.2. Isoelectric point and ionization of surface groups

In the absence of any adsorpted species at the surface of a metal oxide nanopowder, the hydroxyl

groups provide a system that is often employed for electrostatic stabilization of 0D and 1D pristine

metal oxides.

The isoelectric point (IEP) of a metal oxide is defined as the point of neutral surface charge. In

aqueous solutions, hydroxyl terminated surfaces, such as metal oxides, can exhibit a surface charge in

function (mainly) of the pH of the solution (Figure 2.15). The charge arises from the protonation

(OH2) or deprotonation (O-) of the hydroxyl groups of the metal oxide surface. Therefore, having a pH

level below the IEP of the metal oxide results in the protonation of the surface hydroxyl groups.

Whereas, a pH above the IEP results in the deprotonation of the hydroxyl groups. This results in a

positively or negatively charged surface. Moreover, a protonated or deprotonated surface can play a

crucial role in reactivity of weaker anchor groups such as carboxylic acids and catechols as already

explained in Section 2.3.4.

Figure 2.15.: Schematic representation of the isoelectric point (IEP) in function of pH.

The IEP between metal oxides varies greatly, since it is heavily influenced by other factors such as:

Surface impurities, crystallinity and even physical factors like temperature [58], [59]. Because of this,

pH

+

-

Isoelectric

point

ζ(mV)


values for specific materials in the literature often differ from each other [60]. It is not uncommon to

obtain the IEP of a metal oxide powder by measuring the zeta potential of aqueous nanoparticle

dispersions of varying pH. The pH at which the zeta potential = 0 mV (dispersion will most likely

precipitate) is then considered to be IEP.

2.4.2. Steric stabilization

Steric stabilization, as its name implies, refers to a stabilization mechanism that relies on spatial

hindrance. The stabilization is provided by a shell of less dense material surrounding the far denser 0D

or 1D NSs core. This steric barrier (Figure 2.16) prevents collisions between the dispersed NSs cores,

effectively preventing any attraction between them. In addition, the shell providing the spatial

hindrance at the same time plays a chemical stabilization role via solvation effects [61]. This effect of

increased solvation of the NSs can effectively thwart nanoparticle core interactions. In other words, it

can provide an intermediate interface, by which the normally insoluble core would now be "more in

phase" with the dispersion medium as it would be in its pristine form. The nature of this steric barrier

preventing nanoparticle collision is usually an organic layer that surrounds the nanostructures

dispersed in solution. To this end, the organic molecules may form monolayers or multilayers around

the nanostructure depending on their nature.

Figure 2.16.: Schematic representation of steric stabilization of nanoparticles. Physically, the molecules grafted

onto the nanoparticle avoid direct nanoparticle collision and nanoparticle core interaction. Chemically, the

molecules provide solvation in the dispersion media effectively thwarting nanoparticle core interactions.

It is fairly common that oligomers or polymers are employed for this purpose. Like for example, a

myriad of polyethylene glycol (PEG) derivatives [62]. It goes without saying, that SAMs are the

perfect toolkit for this purpose as they allow for a precise monolayer deposition onto the NSs surface.

Steric barrier

Grafted molecules

on surface


An example application of such is presented in this work (Section 4.1.1), in which a phosphonic acid

molecule with a small ethylene glycol chain was employed as a steric stabilization agent [17]. A

similar example can also be found at Section 4.5 as well.

2.4.3. Electrosteric

A combination of both electrostatic and steric stabilization schemes is also possible. This occurs when

the "polar nature" of the layer surrounding the nanoparticle is of a non-negligible magnitude. While at

the same time this layer also provides a non-negligible level of steric stabilization. Examples of

electrosteric stabilization usually involve the usage of polyelectrolytes wrapped around the surface of

the NSs providing both steric and coulombic repulsion forces [63]. Not surprisingly, electrosteric

stabilization with SAM molecules is also possible. Electrosteric stabilization with SAMs can be

achieved in two main ways, the SAM molecules have an ionic moiety of some sort at the backbone or

head group, or by dipole moment alignment of the molecules on the surface [64]. Both effects were

observed in several occasions during the course of this work, and they are catalogued in Section

3.2.3.3 and 4.5. Lastly, under specific circumstances we hypothesized, that the geometry of the

dispersed NSs can have an influence in the stabilization mechanism of the NSs dispersions. This is

explained in more detail in the following section.

2.4.4. Impact of size and geometry of nanostructures

The size and geometry of 0D and 1D NSs play a critical role on the structural properties of the SAM.

These structural differences may also affect the dispersibility of the NSs. In order to understand this,

let us take a 5 nm and 50 nm spherical nanoparticles with a grafted SAM as the prototypical examples

of 0D NSs. Figure 2.17 represents cross-sectional drawings of the functionalized spherical

nanoparticles. In these drawings, the SAM molecules were equally distributed perpendicularly among

the spherical surface. The molecules were placed at a grafting density of 6 molecules per nm2 and

consist of octadecylphosphonic acid (C18-PA) (Appendix Figure 7.1). At first glance, they do not

appear to be much different other than the core size (Figure 2.17a, c), however, when having a closer

look, things are bit different (Figure 2.17b, d). The curvature of a sphere is defined by the sphere's

radius, therefore, as the nanoparticle radius becomes smaller, the surface curvature of the nanoparticle

increases (Figure 2.17b). This is important because as the curvature increases so does the free volume

between each SAM molecule [65]. This extra space negatively affects the weaker intermolecular

interactions (van der Waals) of the SAM molecules that can only exist when the molecules are in close

proximity from each other. Therefore, it becomes increasingly difficult for the SAM molecules to form

highly ordered, densely packed monolayers. Furthermore, smaller NSs posses a higher surface area,

which increases the free energy of the system and usually leads to a higher degree (faster) of

agglomeration [66], [67].


Figure 2.17.: Nanoparticle surface curvatures. a) and c) are nanoparticle illustrations (up to scale) of

functionalized 5 and 50 nm spherical particles. b) and d) represent a zoomed-in up to scale nanoparticle surface

illustration. It becomes apparent in illustrations b) and d) how can surface curvature play a critical role on SAM

crystallinity and SAM dipole moment alignment. Also of importance to note, the free space available between

the molecules.

In summary, having a high surface curvature leads to the following situations which can strongly

impact the solubility of the NSs.

1. Having the SAM molecule backbone and head group more exposed allows for better solvent

accessibility.

2. Having more space available between molecules can facilitate surface access and therefore

resulting in a higher grafting density (not SAM packing density), especially when compared to

a flat surface.

3. The cooperative effect of dipole moment alignment of the SAM molecules may be completely

mitigated due to reduced molecule alignment [64].

If there mechanisms are true, a steric stabilization scheme is favored for NSs with a high surface

curvature due to the improved solvation provided by the more readily exposed SAM. On the other

a) b)

c) d)

5 nm

50 nm


hand, when employing NSs with a smaller surface curvature, SAM exposure would be limited since

the SAM forms a more ordered and denser monolayer. In which case, a steric stabilization scheme is

hindered to a certain degree due to the more limited exposure of the organic shell. Furthermore, in NSs

with low surface curvature (bigger particles), the effect molecule alignment may have to be factored

into the proposed stabilization mechanism. Particularly, for molecules with a strong dipole moment.

This is because of the potentially attainable higher order and higher packing density of the SAM. We

hypothesized, that under such circumstances the combination of both stabilization mechanisms come

into play. Therefore, an electrosteric stabilization seems to be the most viable scenario. In which, the

SAM molecules provide certain degree of steric stabilization, while the dipole moment alignment of

the SAM polarizes the surface resulting in an electrostatic stabilization at the same time. However, this

remains as a hypothesis as the effect of the ligand dipole moment was not fully investigated. Still, the

exact implications of SAM dipole moments are a debated topic [64], [68], [69]. Section 3.2.3.3

provides some experimental insight regarding this topic from a nanoparticle perspective.

2.5. Thin film transistors

In short, a transistor is an electronic switch which is operated by modulating a voltage. This is the case

of field effect transistors (FETs), or by modulating a current in the case of bipolar junction transistors

(BJTs). In the matter at hand, which is thin film transistors (TFTs), they are operated by voltage

modulation which classifies them as a type of FET. Structurally, a key difference between FETs and

TFTs is that all the components of a TFT are built upon an insulating surface which exclusively acts as

carrier substrate. Whereas, in traditional FETs, the substrate (typically a silicon wafer) acts as an

active component. TFTs as well as FETs can be built in a variety of different configurations and

complications, each one with both their pros and cons. Plenty of other technicalities between them

exist, however, describing them would be beyond the scope of this thesis [70]. Therefore, it is

restricted purely to the configurations employed during this work.

Figure 2.18 shows a schematic representation of a bottom gate staggered n-type TFT. The TFT

consists of 3 electrodes namely source, drain and gate. When the transistor is in its "off" state, the

passing of electric current from drain to source (IDS) is impeded due to the low conductivity of the

channel. However, when the transistor is in its "on" state, the conductivity of the channel is increased

several orders of magnitude allowing a higher IDS to pass. The amount of current passing through the

channel is modulated by the modulation of the VGS voltage. The proportion of VGS vs. IDS begins with

a linear behavior known as the "linear regime" and it eventually reaches a non-linear regime called the

"saturation regime". These regimes are described by Equation 2.4 for the linear regime and 2.5 for the

saturation regime, where µ and C are the carrier mobility within the regime and capacitance of the

dielectric respectively. A parameter of particular importance is the carrier mobility (µ) in the


saturation regime (Equation 2.6), since this characteristic is used in literature as the benchmark by

which transistor performance is categorized.

Figure 2.18.: Schematic representation of a bottom gate TFT

Linear regime 𝐼𝐷 = 𝑊𝜇𝐶

𝐿[(𝑉𝐺𝑆 − 𝑉𝑡ℎ)𝑉𝐷𝑆 −

𝑉𝐷𝑆2

2] 2.4

Saturation regime 𝐼𝐷 = 𝑊𝜇𝐶

2𝐿(𝑉𝐺𝑆 − 𝑉𝑡ℎ)2 2.5

Saturation mobility 𝜇𝑠𝑎𝑡 = 2𝐿𝑚2

𝑊𝐶∙ 2.6

2.6. Chapter summary

The essential theoretical aspects required for the discussion of the experimental aspects of this thesis

have been covered. A clear distinction between nanostructures (NSs) and nanostructured materials

(NSMs) was explained as well as the system employed for their classification. The relation of surface

area to volume of NSs was summarized as it is one of the most important characteristics of NSs when

dealing with functionalization. The concept of self-assembled monolayers (SAMs), their composition

and working principle were discussed as well. This was followed by a description of the impact of

grafting a SAM onto the surface of NSs in terms of solution dispersibility. The role that the geometry

of 0D and 1D NSs might play in NSs dispersibility was also taken into account. The mechanisms

behind stable and unstable NSs dispersions were also described. Lastly, the general working principle

of thin film transistors (TFTs) and characterization equations employed for their benchmarking were

described.

Carrier substrateGate Dielectric

Drain Source

Channel

VDS

+-

+-

VGS

ID

LW

25

3. Functionalization and characterization of NSs

This chapter focuses on the key factors involved during the modification of the NSs surface, such as:

starting material, complete surface coverage, impact of the particle dimensions, deposition of mixed

ligand monolayers and anchor group stability. Special attention is given to the high surface to volume

ratio of 0D and 1D NSs. Since it plays a critical role during functionalization and is easy to

underestimate. But first, as a basis for discussion, the relatively simpler functionalization of 2D NSs is

briefly described.

3.1. Functionalization of 2D materials

Deposition of SAMs from solution onto 2D substrates is a fairly straightforward procedure (Figure

3.1). First, the metal oxide surface is exposed to an oxygen plasma treatment (5 min, 0.2 mbar,

200 W). This helps to activate the OH groups at the oxide surface as well as cleaning other organic

species that may be present. The plasma treated substrate is then immersed into the desired SAM

solution (IPA, 0.2 mM) for 24 hr to ensure a tightly bound SAM is formed [36].

Figure 3.1.: Steps for SAM deposition on 2D substrates. a) Plasma treatment of the surface. b) immersion of the

substrate into the SAM solution. c) Schematic of the finalized SAM.

a)

AlOx, ITO, ZnO or TiO2 2D substrate

O2 plasma

24 hrs

Immerse 2D substrate in SAM solution for 24 hours

Rinsed (IPA) and dried (N2, hotplate @60 C) substrate with final SAM

b) c)

Remove from

solution

26 Functionalization and characterization of NSs

Finally, the substrate is taken out of the solution and is vigorously rinsed with pure solvent (the same

solvent used for deposition) to remove any excess unbound molecules. After rinsing, the substrate is

blown dry with nitrogen and heated to 60 °C for a few minutes to remove any remaining solvent at

which point the process is finished. As an added step, the quality of the deposited SAM is verified via

a few quick SCA measurements.

3.2. Functionalization of 0D materials

The deposition of a SAM onto a dispersion of 0D or 1D NSs is also a fairly simple procedure once

suitable conditions have been identified (Figure 3.2). However, in order to properly determine these

conditions an experimental approach is required. This section describes in detail the step by step

characterization from beginning to end as performed during this work.

Ideally, the functionalization process begins with a highly stable dispersion of material (Figure 3.2a).

A highly stable dispersion assures that the surface of the NSs is exposed to the dispersion medium

because little or no agglomeration is present. This guarantees the SAM molecules access to NSs

surface. Nevertheless, whenever a dispersion with poor stability must be functionalized, the process of

sonication during the functionalization step (Figure 3.2b) can help to compensate for the poor stability

of the dispersion. In some cases, the functionalization improves the dispersibility of the NSs, in this

case the functionalization process itself will gradually reduce the agglomeration. This has as a

consequence, the full exposure of the NSs surface. It must be noted that for the purpose of

functionalization and under the same conditions, the sonication process proved to be more valuable

than magnetic stirring. Particularly, for magnetic nanoparticles were magnetic stirring is not feasible.

The detailed differences between stirring and sonication are not discussed. It suffices to say that

stirring often resulted in poorer SAM surface coverage. After the sonication process is finished, any

excess unreacted SAM molecules must be washed away (Figure 3.2c). This is achieved by

centrifugation of the NSs followed by removal of the supernatant. Afterwards, the addition of a

washing solvent to the centrifuged NSs is performed. A washing solvent is a solvent in which the

SAM molecules are highly soluble and cannot be isolated by centrifugation. This process is repeated at

least twice to guarantee full removal of the excess molecules. When centrifugation is not possible (e.g.

highly stable smaller particles), the use of an anti-solvent (technically an anti-dispersant) in excess can

be used to encourage the precipitation of the particles. Alternatively, evaporation of the solvent under

vacuum and heat in a rotary evaporator (rotavap) can also be used for exchanging the functionalization

solvent to a washing solvent. After the washing procedure is finished, the nanoparticles are dispersed

in the solvent of choice or kept as a powder for storage or characterization.

Functionalization and characterization of NSs 27

Figure 3.2.: Steps for SAM deposition on 0D and 1D NSs. a) Pristine nanostructures dispersed in a liquid

medium. b) The SAM molecule is added and with the aid of sonication it forms a SAM around the

nanostructures surface. c) The final functionalized NSs dispersion (after washing) with no unbound SAM

molecules present in solution.

3.2.1. Chemical characterization (FTIR-ATR)

Since the sonication process allows for the use of both stable and unstable dispersions. The first real

concern is to evaluate the "pristine" NSs surface. The main worry to assess is the presence of any other

sort of physisorbed or chemisorbed species to the NSs surface. Some species can hinder or even

negate the adsorption of the SAM molecules to the NSs surface. Specially, since plasma cleaning

cannot be performed on 0D and 1D NSs dispersions. Therefore, a truly pristine NSs dispersion with no

adsorpted species on their surface is highly desirable for functionalization purposes. Alternatively,

NSs with weakly bound species that can be removed by the SAM molecules are also suitable for

functionalization. Therefore, a pre-functionalization chemical characterization of the NSs is required.

This is followed by a post-functionalization characterization by which it can probed whether the SAM

molecules are present on the NSs surface or not. To this end, FTIR-ATR is an excellent technique

which allows to obtain chemical information regarding the makeup of the NSs.

In order to conduct the pre or post functionalization characterization of the NSs via FTIR-ATR, the

NSs need to be isolated as a dry powder. Exclusion of the solvent is paramount, as it would interfere

a)

Dispersed pristine metal oxide 0D or 1D NSs

30 min sonication

Sonicate dispersion for 30 min at room temperature

Functionalized NSs with no free excess SAM molecules

b) c)

Add SAM molecule in

required

concentration

Centrifuge NSs, remove supernatant

and redisperse NSs

in fresh solvent (x2)


with the FTIR measurements. This is achieved by centrifugation of the NSs followed by removal of

the supernatant and drying overnight. The drying process was performed in a dry air oven at a

temperature close to the boiling point of the solvent. Alternatively, if the solvent has a high vapor

pressure, simply leaving the wet centrifuged nanopowder exposed to the negative pressure of a

chemical hood is sufficient. Another effective drying route is drying the nanopowder under vacuum

and moderate heat with a rotary evaporator.

Figure 3.3.: FTIR-ATR spectra of several commercial TiO2 nanoparticles. a) Featureless spectrum of pure TiO2

particles. b) Spectrum of HNO3 stabilized TiO2 particles with signals from the HNO3 species present c) Spectrum

of allegedly pure TiO2 particles containing signals that are unaccounted for.

Once a dry powder is obtained, the measurement can be performed. Figure 3.3 shows the FTIR-ATR

spectra of several commercial TiO2 nanoparticles isolated and dried as received. The ideal case is

shown in Figure 3.3a, which is the featureless spectrum of pure TiO2 nanoparticles. In this case, we

can be sure that the particles are indeed pure and suitable for functionalization. Likewise, similar flat

spectra is also obtained for other pure metal oxide nanopowders. In contrast, Figure 3.3b shows a

feature rich spectrum of TiO2 nanoparticles. This particles contain an arbitrary amount of nitric acid

(HNO3) on their surface for stabilization purposes (according to manufacturer). Lastly in Figure 3.3c,

the spectrum of allegedly pure TiO2 nanoparticles is portrayed. However, as can be observed from the

spectrum, the particles contain some unknown adsorpted species. To conclude, it can be safely

assumed that the particles from Figure 3.3a can be used for functionalization. Whereas, the particles

from Figure 3.3b and Figure 3.3c can conceivably be employed. But only, if the SAM molecule is able

to replace the present species on the nanoparticles surface. To this end, all the particles mentioned

2000 1800 1600 1400 1200 1000 800

Wavenumber (1/cm)

c)

b)

a)

3800 3600 3400 3200 3000 2800 2600

Wavenumber (1/cm)

c)

b)

T (

%)

a)

d)


were functionalized with C16-PA (Figure 3.3d) independently of their purity assessment. After

functionalization, the particles were isolated and dried for measurement.

Figure 3.4.: Color coded FTIR-ATR spectra of several commercial TiO2 nanoparticles after being functionalized

with C16-PA. a) Spectrum of pure TiO2 particles plus the signals from C16-PA. b) Spectrum of HNO3 stabilized

TiO2 particles with the signals from C16-PA. Note that the HNO3 peaks are no longer present. c) Spectrum of

allegedly pure TiO2 particles functionalized with C16-PA still containing signals that are unaccounted for, plus

the overlapping signals of the C16-PA. d) Chemical structure of C16-PA.

As expected for the pure TiO2 particles, the functionalization was successful. The presence of the

bound C16-PA is evident as depicted by the bands present in the post functionalization FTIR spectrum

in Figure 3.4a. The blue circles represent the bands related to the methylene groups of the alkyl chain,

while the red circle pinpoints the band associated with the methyl group at the end of the chain. The

green circle represents the bound phosphonic acid to the TiO2 nanoparticles surface. The FTIR-ATR

spectrum of the pure C16-PA SAM molecule can be found at the appendix in Figure 7.2 and Figure 7.3

for a more in depth comparison. The next example is the HNO3 stabilized nanoparticles (Figure 3.4b),

in here we can observe that the typical bands of the C16-PA are also present suggesting a successful

functionalization. But more importantly, the bands belonging to the HNO3 are barely noticeable after

functionalization. This means that the C16-PA was able to replace the nitric acid in its majority, which

makes these "impure" particles also a good choice for functionalization with phosphonic acids.

Finally, Figure 3.4c shows a partially successful example, in which both the unknown bands of the

particles and those of the C16-PA are now overlapped. In this case, the C16-PA was not able to remove

the unknown species from the particle surface. Instead, both the C16-PA and the unknown species were

present at the surface. Therefore, these particles were never used in any further experiments. Since it is

3800 3600 3400 3200 3000 2800 2600

Wavenumber (1/cm)

c)

b)

T (

%)

a)

2000 1800 1600 1400 1200 1000 800

Wavenumber (1/cm)

c)

b)

a)

d)

v


of paramount importance to have full knowledge and control of the nanoparticles surface after

functionalization.

Finally, we take a look at Figure 3.5, it shows a few other metal oxide NSs characterized in the very

same way as explained in this chapter. A clear trend can now be identified between the pure metal

oxides and C16-PA functionalized ones. All the functionalized powders exhibit a broad signal from

1200 to 900 cm-1, which is attributed to the overlap of the P-O and P=O vibrations of the phosphonic

acid group bonded to the metal oxide surface [71]–[73]. The overlap comprises signals of bonded and

non-bonded P-O and P=O groups, owing to the likelihood of mono-dentate, bi-dentate and tri-dentate

binding modes co-existing on the particles surface [12], [42]. The two peaks around 2920 and

2850 cm-1 correspond to the methylene groups vibrations of the C16-PA aliphatic carbon chain,

followed by a small shoulder appearing at 2970 cm-1 attributed to methyl groups at the tail of the alkyl

chain. A less intense peak in the 1470 cm-1 region is recognized as methylene groups scissoring

vibrations.

Figure 3.5.: Exemplary FTIR-ATR spectra (color coded) of various pure and C16-PA functionalized metal oxide

NSs. The general trend of a featureless spectrum for pure metal oxides can be observed. An unmistakable trend

is also identifiable after functionalization with C16-PA in all metal oxides. The molecular structure of C16-PA is

shown atop.

T (

%)

C16

-PA Ceria

Pristine Ceria

T (

%)

C16

-PA Iron oxide

Pristine Iron oxide

3800 3600 3400 3200 3000 2800 2600

T (

%)

Wavenumber (1/cm)

C16

-PA ZnO Nanorods

Pristine ZnO Nanorods

1800 1600 1400 1200 1000 800

Wavenumber (1/cm)

Fe3O4

CeO2

ZnO


A short hypothesis regarding the binding configuration of the phosphonic acid

As mentioned before, the origin of the broad band from 1200 to 900 cm-1 was attributed to the bound

phosphonic acids to the metal oxide surface. It was mentioned as well, that it is thought to be an

overlap of all the diverse binding configurations. To illustrate this point further, Figure 3.19a shows

the spectrum of Fe3O4 nanoparticles functionalized with C16-PA. The characteristic broad band

belonging to the bound phosphonic acid can also be observed. In Figure 3.19b the spectrum of the

pristine C16-PA is shown, in here the bands corresponding to the free phosphonic acid are clearly

defined (1200 to 900 cm-1). The P-O and P=O bands are clearly defined, because these chemical

groups can only exist in their free forms within a limited set of configurations. However, a bound

phosphonic acid to a metal oxide surface can exist in a variety of different configurations (Section

2.3.4.1). The latter results in a broad band rather than in discrete set of states. This broad band could

originate from the overlap of all phosphonic acid configurations as it is hypothetically depicted in

Figure 3.19c.

Figure 3.6.: a) FTIR-ATR spectrum of Fe3O4 nanoparticles functionalized with C16-PA. b) Spectrum of pristine

C16-PA. c) Hypothetical schematic depicting the diverse vibrations of the phosphonic acid bound to a metal

oxide surface.

Etching of metal oxides

As it will be explained throughout this chapter, the phosphonic acid is the most promising of the

anchor groups due its strong binding to the surface of metal oxides (Section 3.3). However, this strong

binding comes with a price, as it often results in the etching of the metal oxide NSs. In general, it is

difficult to define the chemical stability of metal oxides. Some metal oxides are more prone to etching

than others under a particular set of intricate conditions [74]. However, one particular condition that

concerns the functionalization of metal oxides with phosphonic acid SAMs, is the formation of metal

salts by the employment of n-alkyl phosphonic acids. The high complexing capability of phosphonic

acids with metal ions, which results in the formation metal salts is well documented [75], [76].

Therefore, metal oxides which contain metal ions in their composition are particularly sensitive. To

2000 1800 1600 1400 1200 1000 800 600

Fe3O

4 C

16-PA

C16

-PA

a)

b)

Wavenumber (1/cm)

c)


illustrate this phenomena, a case where Fe3O4 nanoparticles were partially etched with phosphonic

acids is showcased.

FTIR-ATR is valuable tool to detect the etching of metal oxides after being exposed to a phosphonic

acid SAM. This is demonstrated in Figure 3.7, which portrays the FTIR-ATR spectra of Fe3O4

functionalized with increasing concentrations of C16-PA. The first spectrum (Figure 3.7) shows the

typical featureless signal of pristine Fe3O4 nanoparticles. As we proceed downwards in the spectra, the

amount of C16-PA was doubled every time. At the concentrations of 10, 20 and 40 mM, the familiar

smooth valley of the bound phosphonic acid is found between the wavelengths of 1200-900 cm-1.

Figure 3.7.: Fe3O4 nanoparticles functionalized with increasing concentration of C16-PA. As the concentration

increases, the valley corresponding to the anchored phosphonic acid (red square) changes.

However, as the concentration increases the smoothness of the valley changes into a feature rich

valley. This change, from a broad band to a set of discrete bands was hypothesized to be caused by the

etching of the nanoparticles. The reasoning behind this is, that the metal salt has a defined chemical

configuration. This results in the appearance of well defined P-O and P=O vibrations in the FTIR-ATR

spectrum, as seen in the 80 and 160 mM concentrations. In contrast, the binding configuration of the

phosphonic acid to a metal oxide surface is very diverse. Therefore, resulting in a single broader band

3800 3600 3400 3200 3000 2800 2600

Pristine

10 mM

T (

%)

20 mM

40 mM

80 mM

160 mM

Wavenumber (1/cm)

2000 1800 1600 1400 1200 1000 800 600

Wavenumber (1/cm)


as explained before. Experimental proof that band overlapping can have the effect described on

FTIR-ATR spectra can be found in Section 3.2.3.2.

Previous work, done in partial collaboration with Johannes Hirschmann [77], identified the same trend

in FTIR-ATR spectra. In an attempt to functionalize ZnO NSs, he exposed ZnO nanoparticles and

nanorods to a phosphonic acid SAM. In parallel, he purposely synthesized the equivalent Zn

phosphonate salt. When the spectrum of the Zn salt was compared to that of the presumably

phosphonic acid functionalized ZnO. He realized that the spectra were exactly identical. Therefore, the

only viable conclusion was the etching of the ZnO NSs into Zn salts.

In conclusion for this section, the use of FTIR-ATR to assess the purity of metal oxide materials has

been demonstrated. It is highly advisable that before any attempts to perform functionalization, an

assessment of the purity of the starting material is made. FTIR-ATR is also a valuable tool for post

functionalization evaluation, as it can provide chemical information about the surface of the NSs. Two

hypothesis were briefly declared. One regarding the binding configuration of the phosphonic acid as

well as one on the subject of the etching of metal oxides with phosphonic acids. However, FTIR-ATR

analysis provides a purely qualitative characterization in terms of the nature of material present.

Making a quantitative characterization via FTIR-ATR is difficult. However this can be easily

measured via thermogravimetric analysis (TGA), which is now explained.

3.2.2. Saturation threshold (TGA)

As we have already discussed in section 2.2, the surface area of 0D and 1D NSs is far greater than that

of 2D NSs. Therefore, the amount of molecules required for a full surface coverage of 0D and 1D NSs

is much larger. The amount of molecules can be estimated by taking the SSA of the NSs into account.

However, it is regularly the case that the amount of molecules required for full coverage is the

equivalent of several monolayers. This occurs because the volume of the solvent employed can also

play a competing role for the NSs surface. Therefore, rather than an amount of molecules, it comes

down to the concentration of the SAM molecule vs. the surface area available [19]. Therefore, simply

estimating the amount of molecules required is not sufficient to guarantee full surface coverage.

Instead, an experimental approach using thermogravimetric analysis (TGA) can be used to determine

the conditions at which the surface of the NSs is fully saturated. In these experiments, the SAM

molecule C16-PA was employed again (Figure 3.8a). C16-PA is a very good molecule for determination

of surface coverage for a few reasons. It contains no bulky backbone or head groups, with the anchor

group being the bulkiest. Therefore, a spatial limitation of the grafting density will be limited by the

anchor group itself and not by any other groups. Secondly, C16-PA contains no other polar groups

which may strongly interact with the NSs surface or have a strong intermolecular interactions. As

these interactions can interfere or modify in differing forms the self-assembly process [78], [79]. In

addition, the alkyl chain backbone provides enough rigidity to keep the molecule from bending and


interfering with the SAM process. Another important quality is that C16-PA has enough molecular

weight to be easily detected in TGA measurements. Furthermore, C16-PA is readily and economically

available by the hundreds of grams.

Figure 3.8.: a) Chemical structure of C16-PA. b) TGA under N2 of AlOx NPs functionalized with different

concentrations of C16-PA. Adapted from [16] with permission from the American Chemical Society.

Figure 3.8b shows TGA measurements of a fixed amount of AlOx nanoparticles functionalized with

increasing amounts of C16-PA. The higher the concentration, the more molecules that attach to the

surface. It can be observed from Figure 3.8b how the increasing concentration of C16-PA results in a

greater and greater mass loss. This mass loss originates from the decomposition of the grafted C16-PA

due to the gradual increase of temperature. In contrast, there is no dramatic mass loss for the pristine

nanoparticles (black line). Considerable increments of mass loss are consistent until the 10 mM

concentration is used, at which point the higher concentrations of 20 and 40 mM did not result in a

significantly greater mass drop. It is at these concentrations (10, 20 and 40 mM) that we can declare

that the particles surface is fully saturated and no more molecules can attach to the surface.

3.2.2.1. Grafting density

The grafting density of the molecules can be calculated from Equation 3.1. In which wt represents the

mass loss measured by TGA, N is Avogadro's constant, MW is the molecular weight of the SAM

molecule employed and SSA is the specific surface area of the NSs. This is a particular useful tool,

since it allows to verify the success of the functionalization procedure [16]. For example, Table 3.1

0 100 200 300 400 500 600 70086

88

90

92

94

96

98

100

We

igh

t (%

)

Temperature (°C)

Pristine

2.5 mM

5 mM

10 mM

20 mM

40 mM

a)

b)


summarizes the calculated grafting densities from the TGA measurement of Figure 3.8. The resulting

grafting densities are in good agreement with literature [80], but most importantly they are of a

reasonable magnitude (3-7 molecules per nm2). When the maximum grafting density value is

reasonable, unwanted side effects of functionalization such as NSs etching or multilayer coating can

be discarded. However, if the calculated grafting densities are out of range, the functionalization

process needs to be re-evaluated under more favorable conditions.

𝑔𝑟𝑎𝑓𝑡𝑖𝑛𝑔 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 = (𝑤𝑡

100 − 𝑤𝑡) (

𝑁

𝑀𝑊 ∙ 𝑆𝑆𝐴) 3.1

Table 3.1.: Calculated grafting densities of AlOx NPs functionalized with C16-PA, in accordance with the mass

loss from 400 to 650 °C of Figure 3.8 The SSA was calculated from a DLS distribution to be 28.05 m2/g.

concentration employed. (mM) mass loss (wt) grafting density

(molecules per nm2)

2.5 5.27 3.9

5 7.03 5.3

10 7.65 5.8

20 8.01 6.1

40 8.25 6.3

Consideration of anchor group in the calculation

It is a fairly common dilemma whether or not to employ the full molecular weight (MW) of the SAM

molecule to calculate grafting density with Equation 3.1. The problem in question being, whether or

not the totality of the SAM molecule, including the anchor group is removed from the NSs surface

during the TGA measurement. This is particularly significant for smaller SAM molecules in which the

anchor group can represent more than 50% of the total MW. Therefore, it can contribute heavily to the

error of the calculation. In the previous example (Figure 3.8) C16-PA was employed given that it is a

fairly "heavy" molecule. Yet, around 25% of the total MW corresponds to the phosphonic acid anchor

group. To complicate matters, the PA reaction pathways and binding modes from Figure 2.8 and

Figure 2.9 should also be taken into consideration. Since, some of the oxygen atoms might no longer

be present after the PA binds with the metal oxide surface. Yet, other oxygen moieties might just be

hydrogen bonded. As different anchor groups have different binding modes and affinities it gets even


more complex. For the sake of simplicity and consistency, in the entirety of this work the full

molecular weight of the SAM molecule was always employed for the calculation of grafting density.

Figure 3.9.: TGA under N2 from 25 to 1100 °C of AlOx NPs functionalized with C16-PA. The inset shows FTIR-

ATR spectra of the nanoparticle powder at different stages of the TGA measurement. It can be observed that

even after exposing the NP powder to 1100 °C for 2 hours the PA band is still present on the nanopowder. The

nanopowder had a black color up to a 1000 °C, past that temperature the powder was white in appearance.

Figure 3.9 provides excellent qualitative data regarding this topic. For this experiment several TGA

measurements were made with different final temperatures (90, 600, 800, 1000 and 1100 °C) while all

other conditions remained the same (N2, 10 °K/min). At the end of each TGA measurement, a

FTIR-ATR analysis of the remaining powder was performed (Figure 3.9 inset). The sample measured

until 90 °C was merely used as a control and shows the typical bands of the C16-PA as previously

discussed in Section 3.2.1. On the other hand, the FTIR-ATR spectra of the samples measured until

600, 800, 1000 and 1100 °C show no bands related to the methyl and methylene groups of the C16-PA.

Yet, the bands pertaining to the anchored PA group clearly remain even after exposing the

functionalized NP powder to 1100 °C for 2 hours. The latter, gives a strong hint regarding the

remainder of PA groups on the nanoparticle surface after TGA. When comparing the spectra of the

600 and 1100 °C, there seems to be a small decrease in the intensity of the PA band. However, a

quantitative determination based on FTIR alone is not possible. As a final remark, the analyzed

0 25 50 75 100 125 150 175 200 225

0

100

200

300

400

500

600

700

800

900

1000

1100

1200

Te

mp

era

ture

(°C

)

Time (min)

86

88

90

92

94

96

98

100

We

igh

t (%

)

3200 2800 2400 2000 1600 1200 800

Wavenumber (1/cm)

1100 °C

1000 °C

T (

%) 800 °C

600 °C

90 °C

White powder

Black powder


nanopowders had a dark black color up to a 1000 °C, past that temperature the powder was completely

white in appearance. Furthermore, there is a small but steep mass loss around 1050 °C. The origin of

this step was not identified. However, it could be speculated that this mass loss can be related to the

dissociation of the phosphorous-carbon (P-C) bond or some other sort of carbon remnants on the

nanopowder. This latter could explain the color change from black to white, as there would be no

more carbon present.

Grafting density paradox

In the very same way Equation 3.1 can be used to estimate the grafting density, logically, it can also be

used to calculate the SSA. If instead of having a fixed value for the SSA, we have a fixed value for the

grafting density, the SSA of the particles can be calculated from the measured amount of adsorbed

molecules. Fixed values for grafting densities are available from theoretical [42] and experimental

literature for all kinds of anchor groups [19], [80], [81]. Even one of the most reliable methods to

measure the SSA of a nanomaterial, the Brunauer–Emmett–Teller isotherm (BET) follows the same

approach. BET involves measuring the amount of adsorbed gas (usually N2) to the nanopowder

surface. Once it is measured, the SSA is calculated by using a fixed grafting density for the gas.

Naturally, if the theoretical grafting density was to differ from the actual real grafting density the

calculated SSA would be erroneous.

To conclude, the methodology presented here to calculate the grafting density of a SAM molecule

might be prone to some discrepancies. It is difficult to obtain a "beyond reasonable doubt" grafting

density by these methods, since they all involve indirect and complex measurements (BET, DLS,

TGA). Yet, as long as the measurement procedure remains the same for all samples a relative

evaluation between them can be made. Other techniques that allow for precise measurement of

grafting densities of the same systems have been developed. An example of such technique is X-ray

reflectivity measurements (XRR) [81]. However, such technique was not applied during this work.

Re-functionalization of previously functionalized powder

SAM patterning of 2D NSs with 2 or more SAMs, typically involves the full coverage of the substrate

with the first SAM. This is then followed by the masking of the SAM via a patterning technique. The

exposed SAM is then removed with oxygen plasma, while the masked SAM remains unaffected. Next,

the second SAM is deposited on the exposed surface were the removed SAM used to rest. Likewise to

the 0D NSs, the question remains whether the phosphonic acid anchor group remains on the surface of

the 2D NSs after SAM removal. As well as, the impact that this phosphonic acid groups have on the

deposition of the second SAM atop this area. To get some insight into the latter question, AlOx

nanoparticles functionalized with C16-PA were subjected to TGA until 1100°C. The particles were

then subjected to the same functionalization procedure as before with C16-PA and the FTIR-ATR

spectrum was measured at every step.


Figure 3.10.: FTIR-ATR spectra (color coded) of pristine and several C16-PA functionalized AlOx nanoparticles.

a) Spectrum of pristine AlOx nanoparticles. b) C16-PA functionalized AlOx nanoparticles. c) C16-PA

functionalized AlOx nanoparticles after exposure to TGA until 1100 °C. e) The exposed AlOx nanoparticles to

TGA until 1100 °C but re-functionalized with C16-PA. e) The C16-PA re-functionalized AlOx nanoparticles after

being measured again by TGA until 1100 °C.

Figure 3.10a shows the typical featureless spectrum of pristine alumina particles. On Figure 3.10b the

spectrum of nanoparticles functionalized C16-PA can be found. After being exposed to the TGA

measurements the spectrum shows no sign of the alkyl chain. However, the phosphonic acid band is

still present as expected (Figure 3.10c). Looking at the spectrum from Figure 3.10d, it is clear that the

re-functionalization of the particles with C16-PA was successful. However, in this spectrum the

phosphonic acid band which occurs between 1000-1200 cm-1 seems to have a slightly modified shape.

In addition, there is the appearance of a previously unseen band around 1390 cm-1 (purple). The last

mentioned observations, could be an indication of the existence of a different binding configuration of

the phosphonic acid to the surface. In contrast, to the observed binding configurations of the

phosphonic acid to a pristine alumna surface. Finally, the particles were exposed to the TGA analysis

for the last time (Figure 3.10e). As expected, the bands related to the alkyl chain are no longer present,

while the phosphonic acid band remains. Interestingly, the unknown band is no longer present after the

TGA measurement.

3800 3600 3400 3200 3000 2800 2600

Wavenumber (1/cm)

Re-func. TGA AlOx with C

16-PA after TGA @1100 °C

Re-func. TGA AlOx with C

16-PA

T (

%)

Func. AlOx with C

16-PA after TGA @1100 °C

Func. AlOx with C

16-PA

Pristine AlOx

2000 1800 1600 1400 1200 1000 800

Wavenumber (1/cm)

a)

b)

c)

d)

e)


It is clear that particles can be re-functionalized as it is evident from the FTIR-ATR spectra above.

Furthermore, there were no major discrepancies in the calculated grafting densities of both C16-PA

functionalized particles. Therefore, the deposition of a phosphonic acid SAM atop a surface with

remaining phosphonic acid groups is possible in a nearly identical fashion as the first one.

3.2.3. Mixed monolayers (SCA, FTIR & Zeta potential)

SAM layer formation is not limited to one type of molecule at a time, mixed SAM formation is also a

viable and flexible alternative. Stoichiometric mixtures of self-assembled monolayers on 2D AlOx

surfaces have been previously confirmed [82], [83]. In here, a facile solution-based procedure for

tailoring the surface properties of aluminum oxide nanoparticles by the formation of core-shell

systems with mixed self-assembled monolayers will be described [16]. By employing chained

molecules with a phosphonic acid anchor group and either hydrophobic or hydrophilic chains

(Figure 3.11a), the surface properties of the nanoparticles can be changed dramatically. A mixed

monolayer consisting of phosphonic acid molecules with a hydrophobic tail (F17C10-PA) and a

hydrophilic tail (H(OC2H4)3-PA) was grafted onto the alumina nanoparticles by employing the

previously described procedure (Section 3.2.2). However, in this case the surface properties of the

nanoparticles were smoothly tuned by the formation of a mixed ligand monolayer. These layers were

possible by using the corresponding stoichiometric mixtures of the SAM molecules during

functionalization. To this end, the ratio of the hydrophobic:hydrophilic SAM molecules employed was

varied proportionally in the ratios of 0:1, 1:3, 1:1, 3:1 and 1:0. The latter resulted in nanoparticles with

differing surface properties, whose characterization is shown ahead.

3.2.3.1. Surface energy

Using the aforementioned functionalized nanoparticle dispersions in 2-propanol, nanoparticle films

were spray coated onto a Si/SiO2 wafer for static contact angle measurements and surface energy

calculations (Figure 3.11b). The film morphology and coverage were measured by AFM (Appendix

Figure 7.4). The surface roughness of a 10x10 µm area was measured to be approximately 100 nm

RMS which correlates agreeably to the measured DLS particle size distribution.

Spray coated films of the 1:0 and 3:1 particles exhibited superhydrophobic properties with presumably

DI water contact angles higher than 160°. The DI water contact angle of such films cannot be

measured properly, since the adhesion of the water droplet to the dispensing needle is higher than that

to the film (Appendix Figure 7.5). As expected, the nanoparticle film hydrophobicity decreased for the

films with a lesser degree of F17C10-PA in respect to the H(OC2H4)3-PA, and vice versa in terms of

hydrophilicity. However, superhydrophilic properties only manifested at the nanoparticle film in

which F17C10-PA was completely absent. The same tendency in wetting was observed when the films

were probed using diiodomethane and formamide. Using the contact angle measurements of the

probing liquids, the surface energy of the films (not the nanoparticle surface) was calculated. As


expected, films consisting of particles functionalized only with F17C10-PA presented an extremely low

total surface energy of 0.45 mN/m. Whereas, films with H(OC2H4)3-PA NPs exhibited a much higher

surface energy of 67 mN/m.

Figure 3.11.: a) Molecular structure of F17C10-PA and H(OC2H4)3-PA. b) SCA measurements with different

liquids and the calculated surface energy of the nanoparticle spray coated films. Reprinted from [16] with

permission from the American Chemical Society.

3.2.3.2. FTIR-ATR

The FTIR-ATR analysis of the nanoparticle dry powders is shown in Figure 3.12b. The spectrum from

the particles functionalized with the H(OC2H4)3-PA ligand (0:1) displays a broad valley from 1000 to

1200 cm-1 corresponding to the P-O and P=O vibrations, which is comparable to the case of the

C16-PA. However, in this case the smoothness of the valley is compromised by the overlap of the

C-O-C stretching vibrations which are present in the same region [84]. Similarly for the particles

functionalized with F17C10-PA (1:0) an overlap between the bound phosphonic acid and the C-F2 and

0

20

40

60

80

100

120

140

160

180

0:1 1:3 1:1 3:1 1:0

0.0

0.5

1.0

1.5

10

20

30

40

50

60

70

SE

(m

N/m

)

Polar

Dispersive

SC

A (

°)

F17

C10

-PA : H(OC2H

4)3-PA (Ratio)

DI Water

Formamide

Diiodomethane

not measurable

Tunability

a)

b)


C-F3 vibrations occurs. The carbon fluorine vibrations are represented by the strong peaks at 1150 and

1200 cm-1 and by the smaller adjacent peak at 1250 cm-1 [85], [86]. Further evidence of the presence of

a mixed monolayer is shown in FTIR spectra of particles functionalized with the 1:3, 1:1 and 3:1 ratios

(Figure 3.12b). In particular for the 1:3 spectrum, a clear superposition of the 1:0 and 0:1 signals can

be observed. Equally, for all the spectra, the two small peaks at 2850 and 2920 cm-1 relate to the C-H2

group vibrations. Finally, a trend in intensity is also observable for the methylene groups when

comparing the signals of the H(OC2H4)3-PA (0:1), which possesses six methylene groups, against the

F17C10-PA (1:0) with only two methylene groups.

Figure 3.12.: a) Color coded molecular structure of F17C10-PA and H(OC2H4)3-PA. b) FTIR-ATR spectra of

particles functionalized with different ratios of F17C10-PA and H(OC2H4)3-PA. c) Zeta potential measurements of

the nanoparticle dispersions in 2-Propanol. Adapted from [16] with permission from the American Chemical

Society.

3.2.3.3. Zeta potential

The nanoparticles functionalized with H(OC2H4)3-PA exhibited a positive zeta potential in 2-

propanol of approximately +50 mV (Figure 3.13a). Having such a high zeta potential renders the

nanoparticles highly dispersible in solution. Likewise, particles functionalized with F17C10-PA show a

negative zeta potential in 2-propanol of -50 mV, making them equally dispersible. However, when a

1:1 mixed ligand monolayer is grafted onto the nanoparticles, the measured zeta potential is nearly

zero, which in turn causes the dispersibility of the particles to be very poor (Figure 3.13b). Such effect

was hypothesized to be caused by the formation of a randomly ordered mixed monolayer of both

ligands, effectively cancelling out the electrostatic potential (or stabilization mechanism) induced by

2000 1800 1600 1400 1200 1000 800

Wavenumber (1/cm)

3800 3600 3400 3200 3000 2800 2600

Wavenumber (1/cm)

1:0

3:1

T (

%)

1:1

1:3

0:1b)

a)


either ligand. Under such conditions, agglomeration of the particles occurred due to the increment of

particle to particle interactions. Such effects are of concern regarding the further processability of the

nanoparticle dispersion in terms of agglomeration and sedimentation. Therefore, they should be

considered when solution processing of the nanoparticles is required.

Figure 3.13.: a) Zeta potential measurements of the nanoparticle dispersions in 2-Propanol. b) Photograph of the

functionalized nanoparticle dispersions 20 min after re-dispersion by sonication. Adapted from [16] with

permission from the American Chemical Society.

Furthermore, it was originally hypothesized that if the 0:1 and 1:0 dispersions were mixed, the

dispersions would also precipitate. This was based on their high zeta potentials of opposing charge.

The nanoparticles having an opposing charge would agglomerate and eventually precipitate

completely. However, this was not the case, when a mixture from the 0:1 and 1:0 nanoparticle

dispersions was made, the dispersion remained unaffected as a stable dispersion. This means, that in

regards to the stability of the dispersions, other forces must be at play other than the zeta potential. In

this particular case, we can hypothesize that the orthogonality of the nanoparticles surface did not

allowed for agglomeration to occur, even if their measured zeta potentials were of opposing

magnitudes. Therefore, a combination of the steric and electrostatic stabilization schemes must work

in parallel, but in an independent form. In this sense, a steric stabilization occurs when particles of

differing shells meet, while an electrostatic stabilization prevents the agglomeration of particles of the

same shell. The latter hypothesis, serves as a strong indication that the self-assembly of the mixed

monolayers is mostly random. Thus, it is not dominated by the orthogonal nature of the backbone and

a)

0:1 1:3 1:1 3:1 1:0

-60

-40

-20

0

20

40

60

Ze

ta p

ote

ntia

l (m

V)

F17

C10

-PA : H(OC2H

4)

3-PA (Ratio)

Low

er s

tability

b)

0:1 1:3 1:1 3:1 1:0


head group of the SAMs. If it were predominantly driven by orthogonality, then the 1:1 nanoparticles

would not agglomerate. Since the formed monolayers at the nanoparticle surface would be

predominantly hydrophobic or hydrophilic, which would have as consequence a more stable

dispersion. However, an unstable dispersion was obtained for the 1:1 nanoparticles. Instead, the

process must be random because of the strong interaction of the phosphonic acid with the nanoparticle

surface.

Different SAM molecules have different effects on the stability of the nanoparticles, it is predictable

that different ratios of other SAM molecules can have completely different effects. These beneficial or

detrimental effects on dispersibility are not in any way limited to electrostatic or steric stabilization as

it is thought to be the case for the latter example. Rather, an intricate relation between the shape and

size of the NSs with the highly diverse chemical nature of SAM molecules exists. However, the proper

study of such relationship is beyond the scope of this dataset.

3.3. Anchor group stability

As mentioned above (Section 2.3.1), the anchor group plays a pivotal role on the stability of the SAM.

In the following series of experiments, the attaching affinity of the phosphonic acid, carboxylic acid

and catechol anchor groups is demonstrated on a variety of 0D and 2D metal oxide NSs.

3.3.1. Desorption

Desorption experiments evaluate the stability of a freshly formed SAM while being exposed to the

mildest conditions possible. The desorption experiments as carried out in this work consist in

depositing a SAM on a 0D, 1D or 2D NSs. Afterwards, the successful formation of the SAM is

controlled via SCA measurements. Finally, the freshly functionalized NSs are exposed to "pure

solvent" conditions, without any kind of SAM molecules present. The premise of these experiments

relies on the fact that, whenever the SAM is strongly bound to the surface of the NSs, the exposure of

the formed SAM to the pure solvent should be negligible. On the other hand, when the formed SAM

has a weak interaction with the NSs surface, the impact of solvent exposure is considerable. It is

simple experiments like the ones that will be described in this section, which ultimately lead to the

selection of the materials for a successful functionalization scheme. Therefore, such experiments are of

importance.

3.3.1.1. Desorption on 2D Ns

Figure 3.14 portrays the results obtained for desorption experiments of different materials with

different molecules. The materials compared were AlOx, ZnO and ITO vs. carboxylic acid, catechol

and phosphonic acid anchor groups. The molecules employed in this experiment have one main thing

in common, they form a hydrophobic SAM. This facilitates the monitoring of the SAM formation via


SCA measurements since the substrates in their starting form are highly hydrophilic. The starting

SAMs were deposited over the course of 24 hours on all the substrates as previously described

(Section 3.1), however, this time the substrates were then re-immersed into pure solvent (2-propanol)

for differing amounts of time. After immersion, the substrates were dried with a N2 blow gun and

heated at 60 °C for a few min as per the standard procedure. Then the SCA measurements were

conducted.

Figure 3.14.: SCA measurements of functionalized AlOx, ZnO and ITO 2D substrates. The substrates were

functionalized and afterwards immersed in pure 2-propanol for different amount of time. Contact angles were

measured again after the immersion. a) SCA of substrates functionalized with C17-CA. b) SCA of substrates

functionalized with F21-CAT. c) SCA of substrates functionalized with C16-PA.

The first analyzed anchor group is the carboxylic acid in Figure 3.14a. The starting time "0" relates to

the starting DI water contact angle after deposition of the SAM. The starting contact angle of the

C17-CA SAM is hydrophobic for all substrates (above 90°). The C17-CA functionalized samples were

exposed to the pure solvent for 10, 60 and 1440 minutes (24 hours). The results are quite revealing.

Even after a 10 minute immersion, we can already observe a noticeable drop in the contact angle for

0 200 400 600 800 1000 1200 140020

30

40

50

60

70

80

90

100

110

120

AlOx

ZnO

ITO

DI

Wate

r S

CA

(°)

Time (min)

0 200 400 600 800 1000 1200 140020

30

40

50

60

70

80

90

100

110

120

AlOx

ZnO

ITO

DI

Wate

r S

CA

(°)

Time (min)

0 200 400 600 800 1000 1200 140020

30

40

50

60

70

80

90

100

110

120

DI

Wate

r S

CA

(°)

Time (min)

AlOx

ZnO

ITO

a)b)

c)


ZnO and ITO, this drop on the contact angle is even more dramatic in the case of the AlOx substrate.

The DI water contact angle gradually keeps dropping as evidenced by the 24 hours samples. What we

can conclude from Figure 3.14a, is that the C17-CA forms an ordered SAM on the substrates as

evidenced by the high starting contact angles. However, this SAM is so weakly bound to the surface

that it can be removed by the deposition solvent itself (2-propanol) in all 3 substrates. Particularly for

AlOx, where the contact angle drop is higher. It could be argued that, the carboxylic acid forms the

weakest bond to this surface. In Figure 3.14b, the F21-CAT molecule shows a similar but less dramatic

trend. In this case, however, there were no measurements of 10 and 60 minutes since the contact angle

drop of the 24 hours samples was of such lesser magnitude. What we observe with the F21-CAT is a

far slower desorption in all 3 substrates, compared to the C17-CA. Finally, Figure 3.14c shows the

measurements for the C16-PA molecule. For this molecule, even after a 24 hour immersion there seems

to be no suggestion of any desorption occurring, since the contact angle remained the same for all the

substrates.

After these observations, we can conclude that under ambient conditions C16-PA is the only molecule

that forms a robust SAM compared to the other molecules. Followed by the F21-CAT, with the C17-

CA having the weakest bond of them all. This information is particularly important, especially when

the substrates are going to be exposed to other solution processes, which can undesirably remove the

deposited SAM. Therefore, a stable SAM is highly advantageous attribute whenever a permanent

functionalization is desired. On the other hand, other instances when SAM removal or exchange is a

desired scheme, desorption might prove to be viable strategy [35].

3.3.1.2. Desorption on 0D and 1D NSs

0D and 1D NSs often remain in solution for far longer periods than 2D NSs before their actual usage

in processing. As a matter of fact, storage of functionalized 0D and 1D NSs in solution is a common

practice. Therefore, it is of essential importance to study the effects of SAM desorption on solution

dispersed NSs. For this purpose, AlOx and ITO nanoparticles as well as ZnO nanorods were

functionalized with carboxylic and phosphonic acid SAMs namely C17-CA and C16-PA. Due to the

higher amounts of SAM material required to functionalize 0D and 1D NSs, the employment of the

F21-CAT SAM molecule was not possible as it was limited in availability. However, an in-house study

involving all 3 anchor groups on 0D and 2D TiO2 NSs has already been published [19] with similar

results.

The functionalization procedure with the SAM molecules took place as previously described in

Section 3.2. With the exception that this time, the particles were washed between 1 to 3 times in

2-propanol and later spray coated onto a flat substrate as described in Section 3.2.3.1 and 6.6. After

deposition of the spray coated films, SCA measurements were conducted on the films (Figure 3.15).

As expected (Figure 3.15a), the AlOx nanoparticles functionalized with C17-CA have a strong decrease


in the contact angle as they get washed. This originates from the desorption of the C17-CA from the

nanoparticle surface, which in turn renders the surface more and more hydrophilic after each washing

step. This is in good agreement with the previous 2D desorption studies of the same molecule.

However, for the ITO nanoparticles and ZnO nanorods, there is no decrease in the contact angle after

washing. This is in disagreement with the previous 2D desorption studies, where desorption occurs for

all employed materials. This leads to the conclusion that the binding of the C17-CA is situational and

depends not only on the anchor group and material but primarily on the NS surface characteristics.

This situation was previously described in Section 2.3.4.2. Therefore, the surface of the ITO NP and

ZnO nanorods differs enough from that of the 2D employed substrates that the C17-CA can form a

strong bond at the surface. Essentially, a defined set of rules about the relationship of a material vs.

anchor group cannot be defined without considering other properties.

Figure 3.15.: SCA measurements of spray coated films of functionalized 0D AlOx and ITO and 1D ZnO NSs.

The NSs were washed 1 to 3 times before spray coating. a) Contact angles of spray coated NSs functionalized

with C17-CA. b) Contact angles of spray coated NSs functionalized with C16-PA.

In Figure 3.15b, the same experiment was performed but this time with C16-PA. The results are pretty

straight forward and convincing, as there is no measurable desorption in any of the materials

employed. The higher contact angles of the ZnO rods are related to the higher surface roughness of the

film, since they had bigger dimensions than that of the AlOx and ITO NP [87].

3.3.2. Exchange on 2D NSs

SAM exchange, consists in replacing a previously deposited SAM with another one. This usually

involves replacing a SAM which has a weak attachment to the surface, with a SAM which has a

stronger anchor group. Some SAMs are easily replaced by others, while other SAMs are by all

practical purposes unexchangeable. The studied anchor groups in this section are again, carboxylic and

phosphonic acids as well as catechols on 3 different kinds of 2D NSs AlOx, ITO and ZnO. The

1 2 370

80

90

100

110

120

130

140

150

160

AlO

ZnO

ITO

DI W

ate

r S

CA

(°)

Times washed (counts)

1 2 370

80

90

100

110

120

130

140

150

160

DI W

ate

r S

CA

(°)

Times washed (counts)

AlO

ZnO

ITO

a)b)


exchange experiments were performed by depositing a SAM as described in Section 3.1, followed by

the immersion of the substrate under same SAM deposition conditions employed for the first SAM. By

exchanging hydrophobic with hydrophilic SAMs and vice versa, a contrast in DI water contact angles

can be created and the exchange of the SAM can be monitored rather simply. First example of an

exchange reaction is shown in Figure 3.16. This exchange reaction consists in replacing a carboxylic

acid SAM with another carboxylic acid SAM.

Figure 3.16.: SCA measurements of SAM exchange between two carboxylic acid SAM molecules on AlOx and

ITO 2D substrates. a) Exchange of a C17-CA SAM with a C6-CA SAM. b) Exchange of a C6-CA SAM with a

C17-CA SAM.

Figure 3.16a shows the SCA measurements of the exchange of a hydrophobic SAM molecule C17-CA

by a smaller and less hydrophobic SAM molecule C6-CA. The contact angle of the substrates with the

C17-CA SAM starts at 94 and 102 degrees for AlOx and ITO respectively. After immersion in the

C6-CA solution for 10 minutes, an equilibrium state seems to have already occurred. Samples

0 200 400 600 800 1000 1200 140020

30

40

50

60

70

80

90

100

110

120

DI

Wa

ter

SC

A (

°)

Time (min)

AlOx

ITO

a)

0 200 400 600 800 1000 1200 140020

30

40

50

60

70

80

90

100

110

120

DI

Wa

ter

SC

A (

°)

Time (min)

AlOx

ITO

b)

Exchange

Exchange


immersed for 60 minutes and 24 hours showed no further considerable decrease in the contact angle.

The same occurs vice versa for both the AlOx and ITO substrates when the C6-CA is exchanged by

C17-CA (Figure 3.16b). After exchanging the SAM for 10 minutes, an equilibrium point has been

reached with no strong increase in the contact angle past that point. This fast exchange behavior is

expected, due to the strong desorption of the carboxylic acid molecules that was previously observed.

Furthermore, the availability of the new SAM in respect to the of the old one speeds the process even

more.

When the same exchange experiments are performed with phosphonic acid SAM molecules, no

change in contact angle is observed. In Figure 3.17, the measured contact angles of the exchange

experiments are displayed. Initially, the molecule C11OH-PA forms hydrophilic SAMs with contact

angles around 50 to 60 degrees. After immersion in a C16-PA solution for 24 hours there was no

significant change in the contact angle that indicated any SAM exchange occurring. The same occurs

(not shown) when the inverse exchange reaction is attempted from C16-PA to C11OH-PA. The surface

in this case remains hydrophobic with no considerable change in the contact angle. It can be concluded

then, that within the timeline and concentrations employed, phosphonic acid SAMs cannot be

significantly replaced by another phosphonic acid SAM. At this stage, it becomes ever more clear that

the phosphonic acid anchor group forms a stronger attachment to the surface than carboxylic acids.

But before drawing any conclusions, there is a last remaining group to evaluate, the catechol.

Figure 3.17.: SCA measurements of SAM exchange between two phosphonic acid SAM molecules on AlOx,

ITO and ZnO 2D substrates. Attempt to exchange a C11OH-PA SAM with a C16-PA SAM.

The catechol exchange experiments are not as clear as the carboxylic vs. phosphonic acid exchange

experiments. Yet, the general trend of the phosphonic acids as the superior choice as anchor group is

still present. Just as in the desorption experiments, the catechol anchor group lies somewhere in

between the carboxylic acid and phosphonic acid. In Figure 3.18a, the DI water contact angles of the

0 200 400 600 800 1000 1200 140020

30

40

50

60

70

80

90

100

110

120

AlOx

ZnO

ITO

DI

Wa

ter

SC

A (

°)

Time (min)

Exchange


exchange between C11OH-PA and F21-CAT are shown. With an increase of only around 4 degrees

after 24 hours, it can be concluded that there is no significant exchange of the C11OH-PA with F21-

CAT. On the other hand, the opposite exchange experiment of F21-CAT with C11OH-PA shows a more

favorable exchange rate (Figure 3.18b) of around 15 degrees. Therefore, it can be concluded that the

F21-CAT can be exchanged by the C11OH-PA, albeit slower than a carboxylic acid SAM. However,

other catechols do no form such stable SAM as the F21-CAT.

Figure 3.18.: SCA measurements of SAM exchange between a phosphonic acid and catechol molecules on AlOx,

ITO and ZnO 2D substrates. a) Exchange of a C11OH-PA SAM with a F21-CAT SAM. b) Exchange of a F21-

CAT SAM with a C11OH-PA SAM.

In Figure 3.19, the results from the exchange between a C17-CA SAM and hexanoate-CAT SAM on

ZnO are displayed. In this case, both SAMs can be replaced almost completely by the other SAM.

This indicates that the binding of both SAMs, is of a weak nature. The latter is consistent with the

behavior of the C17-CA, however, not with the previous results of the F21-CAT SAM. The F21-CAT

Exchange

Exchange

0 200 400 600 800 1000 1200 140020

30

40

50

60

70

80

90

100

110

120

DI

Wate

r S

CA

(°)

Time (min)

AlOx

ZnO

ITO

0 200 400 600 800 1000 1200 140020

30

40

50

60

70

80

90

100

110

120

DI

Wate

r S

CA

(°)

Time (min)

AlOx

ZnO

ITO

a)

b)


SAM formed a more stable SAM when compared to C17-CA. Yet, in the case of the hexanoate-CAT,

the formed SAM appears to have the same robustness as a carboxylic acid SAM. This discrepancy,

originates from the impact EWG groups can have on the catechol anchor group, as explained in

Section 2.3.4.3.

Figure 3.19.: SCA measurements of SAM exchange between a carboxylic acid and catechol molecules on ZnO.

a) Exchange of a C17-CA SAM with a hexanoate-CAT SAM. b) Exchange of a hexanoate-CAT SAM with a

C17-CA SAM.

Having a strong anchor group is essential for functionalization. In this section, we have identified

phosphonic acids as the most effective anchor group. The phosphonic acid was the only stable anchor

group under the mild conditions employed. It was followed in robustness, by the catechol and lastly

carboxylic acids. However, due to the more complex chemistry of catechols, some had more affinity to

the metal oxide surface than others.

Exchange

Exchange

0 200 400 600 800 1000 1200 140020

30

40

50

60

70

80

90

100

110

120

DI

Wa

ter

SC

A (

°)

Time (min)

ZnO

0 200 400 600 800 1000 1200 140020

30

40

50

60

70

80

90

100

110

120

DI

Wate

r S

CA

(°)

Time (min)

ZnO

a)

b)



To summarize, this chapter has described the step by step functionalization approach that was

performed during this work. The described procedure and characterization are of paramount

importance in order to identify a successful functionalization scheme. This successful functionalization

scheme consists in obtaining a consistent and reliable process by selecting the appropriate NSs core,

SAM molecules and functionalization conditions. This is particularly important since the finalized

functionalized NSs are further processed for a variety of applications. These applications, are heavily

based upon the NSs functionalization concept. Therefore, a certain robustness of the functionalization

must be ensured to avoid further difficulties along the application process chain. For this matter, this

section has described a universally relevant pathway for reliable NSs functionalization.

First and foremost the characterization of the starting NSs core material was performed with

FTIR-ATR. It is advantageous that the starting material surface is free of organic or inorganic

components that may interfere with the self-assembled monolayer process. However, in some cases

even if the NSs had unknown species at their surface they were removed after the functionalization

procedure. The etching of metal oxides with phosphonic acids was also briefly studied via FTIR-ATR.

After determining that the starting material is suitable for functionalization, the determination of the

conditions required for full coverage of the NSs with a SAM molecule was evaluated by TGA studies.

Moreover, the functionalization allowed for the coverage of the nanoparticle surface with a mixed

monolayer, only by proportionally varying the phosphonic acid ligands concentrations. The

nanoparticle surface was successfully modified to exhibit hydrophobic or hydrophilic properties by

employing different ligand mixtures. Furthermore, the wettability of nanoparticle films can be tuned to

any degree by employing the mixed monolayer approach. The dispersibility of the particles in solution

was demonstrated to be governed by the zeta potential of the particles. This was found to be affected

by the proportions and nature of the ligands employed. Lastly, simple SAM desorption and exchange

experiments on the NSs provided valuable information of the adsorption affinity of specific anchor

groups (catechol, carboxylic and phosphonic acid) to specific materials. The desorption and exchange

experiments, proved that the phosphonic acid is the superior anchor group when compared to

carboxylic acids and catechols.

53

4. Applications

In this section, applications and proof of concept ideas that emanated from this work are showcased.

This thesis has so far covered, how to tune the surface of commercially available materials by

relatively simple procedures. This simplicity is one of the most attractive properties for applications,

specially as we move from simpler to more complex schemes. Some of these ideas are still work in

progress and collaborative work, therefore the detail of the displayed information varies from project

to project. In some applications, a small story concerning the starting point of the project is told in

order to give some insight as to why the application was developed.

4.1. Solution processing

Processing of 0D and 1D NSs from solution allows for wet chemical methods of a normally solid

material. It is somewhat accurate to say that the handling of 0D and 1D NSs is dominated by solution

processing. Therefore, the dispersibility of the NSs in solution is of uttermost relevance and it is the

first application discussed in this section. Several NSs dispersion schemes are described and are

applicable to differing situations that developed during the course of this work.

4.1.1. Green processing

We were asked if we could develop highly stable nanoparticle dispersions in water and lower alcohols

for the fabrication of dielectric mirrors for solar applications via solution processing. We eventually

developed a solution, however, the collaborators lost interest in the project. So here we had, a variety

of water and alcohol dispersible metal oxide nanoparticles.

Ideally, solution processing of NSs is performed by employing a non-toxic dispersion medium such as

water or lower alcohols. Solution processing, particularly from water at neutral pH is a highly sought

after attribute which has an impact in many areas of material science [88]–[90]. In here we

demonstrate an eco-friendly solution processing concept. It is based on the chemical modification of

metal oxide nanoparticles (e.g., TiO2, Fe3O4, AlOx, ITO, and CeO2). It allows for excellent

dispersibility in DI water, as well as in methanol, ethanol and 2-propanol [17]. To achieve this

dispersibility, the nanoparticles were functionalized with the SAM molecule CH3(OC2H4)3-PA (Figure

4.1a). The molecule is comprised of a phosphonic acid anchor group and a hydrophilic tail. This SAM

molecule forms a self-assembled monolayer (SAM) around the nanoparticles surface. The phosphonic

acid moiety acts as an anchor while leaving the hydrophilic tail exposed which is responsible for the

54 Applications

dispersibility enhancement of the nanoparticles. Figure 4.1 showcases TiO2 and Fe3O4 nanoparticles as

the prototypical example of this approach.

Figure 4.1.: a) Molecular structure of CH3(OC2H4)3-PA employed for nanoparticle functionalization; b)

Photograph of dispersed TiO2 nanoparticles in different media before and c) after functionalization. d) Graph of

DLS measurements of TiO2 and e) Fe3O4 nanoparticles before and after functionalization. Reprinted from [17]

with permission from Wiley.

A qualitative indication of improved dispersibility of the functionalized TiO2 nanoparticles with

diameter of ca. 6 nm is showcased in Figure 4.1b and c. From the observed transmittance in the

photographs, it can be concluded that the pristine TiO2 particles in DI water exhibit good dispersibility.

Opaque dispersions of the pristine TiO2 particles were obtained in alcohols, indicating poor

dispersibility in those media. On the other hand, core-shell TiO2 particles functionalized with

CH3(OC2H4)3-PA formed a transparent colloidal solution (Figure 4.1c) when dispersed in any of the

0 5 10 15 20 25 30

0

5

10

15

20

25

30

Nu

mb

er

(%)

Size (nm)

DI water (pristine)

DI water

Methanol

Ethanol

2-Propanol

0 20 40 60 80 100

0

5

10

15

20

25

30

Size (nm)

DI water (pristine)

DI water

Methanol

Ethanol

2-Propanol

a)

b) c)

TiO2 Fe3O4

d) e)

DI Water MeOH EtOH IPADI Water MeOH EtOH IPA

Applications 55

four solvents employed. To better discern the degree of improved dispersibility, Figure 4.1d and 4.1e

display the particle size distribution obtained by DLS of the TiO2 and Fe3O4 nanoparticles before and

after functionalization with CH3(OC2H4)3-PA. Due to particle agglomeration, the size measured for the

pristine particles in DI water showed a misleading distribution, which interprets into bigger particles.

After functionalization, the nanoparticle size distributions portrayed not only smaller diameters close

to the real size of core but also an almost equivalent size distribution in all solvents. Furthermore, the

functionalized NPs can be stored as a powder and redispersed in DI water or alcohols and

subsequently used to create defined dispersions of various concentrations. The latter approach was

employed in water processed flexible nanoparticle dielectrics described in Section 4.2.1. The FTIR-

ATR spectra of the functionalized and pristine nanoparticles can be found in the appendix Figure 7.7.

In conclusion, by using a simple functionalization procedure, diverse metal oxide nanoparticles were

rendered highly dispersible in DI water and lower alcohols. This allows for solution processing of the

materials under a variety of solvents. Furthermore, the same principles could be extended to other

metal oxide materials, extending the potential of the concept.

4.1.2. Any medium processing

At the very end of the course of this work, I came to the realization that NSs can be well dispersed in

any medium as long as you find a proper SAM molecule that fits it. Here is a brief example.

Often, the employment of a "not so green" dispersion media (even not encouraged) for solution

processing it is in many applications a requirement. This could be merely situational or when no other

alternative is available. So, just as NSs can be made highly dispersible in water and alcohols, they can

also be so in any particular medium. As proof of this statement, Figure 4.2 showcases a photograph of

Fe3O4 nanoparticles functionalized to match a specific phase of orthogonal dispersion media (heptane,

water and n-perfluoroheptane). The nanoparticles dispersed in n-heptane were functionalized with

C16-PA, while the particles dispersed in DI water, were functionalized as previously described with

CH3(OC2H4)3-PA. Lastly, in order to make the particles highly dispersible in n-perfluoroheptane the

particles were functionalized with F17C10-PA. Basically, the molecular structure of the SAM molecules

employed was matched as close as possible to the dispersion media. Essentially, the popular aphorism

"like dissolves like" seems to be also applicable to nanoparticle dispersions. However, the reasons

behind their dispersibility might be far more intricate (Section 2.4). The dispersed particles were not

characterized thoroughly with DLS and Zeta potential. Instead, by looking at the picture it is clear that

by choosing the appropriate SAM molecule, NSs can be tuned for dispersibility in any particular

media. This opens up the possibility for solution processing of NSs under an array of different

scenarios. Such as, the deposition of materials from orthogonal media, which is a valuable technique

when depositing one material atop another. In general, nanoparticle dispersibility can be achieved in

any medium, as long as the right SAM is chosen and the nanoparticle surface coverage is well

controlled.

56 Applications

Figure 4.2.: Photograph of Fe3O4 nanoparticles functionalized with different molecules (top) and dispersed in

different orthogonal media (left). a) Particles dispersed in n-heptane, b) particles dispersed in DI water, c)

particles dispersed in n-perfluoroheptane. By tuning the surface of the particles, control on their dispersibility in

different media can be achieved.

4.1.3. Shell by shell (double shell)

I was a bit bored a Friday afternoon so I decided to go play around in the lab, I met Lukas Zeininger

at the hallway and told him if he wanted to a see a "cool trick". The "trick" consisted in adding an

amphiphilic molecule to a poorly dispersed hydrophobic nanoparticle dispersion, upon addition, it

would turn into a perfect nanoparticle dispersion. He flipped!, he knew some colleagues of him had

been synthesizing all kinds of complex amphiphilic molecules that we could try. We tried a few them,

some of them worked better than others but overall there was a general tendency, so we were onto

something. It eventually got late and I went home, however, Lukas (the nerd) kept at it over the

weekend. On Monday we discussed his results and we somewhat came up with the shell by shell

stabilization scheme described herein.

The shell by shell stabilization scheme offers a dynamic approach for dispersion of NSs in solution. In

this approach, hydrophobic forces are exploited towards the formation of a second layer at the surface

of an already functionalized NSs. To a certain extent, van der Waals forces may also play a minor and

indirect role. However, the hydrophobic force was identified as the major driving force. This non-

covalent mechanism for grafting a second layer, is based in the same working principle as a lipid

bilayer would form micelles or liposomes. The developed bilayer stabilization concept is

schematically depicted in Figure 4.3. It starts by fabricating an hydrophobic shell around the

nanoparticles, in this case they were TiO2 nanoparticles fully functionalized with C16-PA (Figure 4.3a).

a) b) c)

Applications 57

These particles are then forcefully dispersed (very poorly) in DI water. However, upon the addition of

an amphiphilic specie to the dispersion, a bilayer as depicted (prototypically) in Figure 4.3b is

potentially formed in a micellar arrangement. To this end, a variety of amphiphilic molecules were

synthesized or commercially obtained (Figure 4.3c). However, to illustrate the proof of concept in this

section we will focus mainly in the employment of molecule 7. This molecule consists of a twelve

carbon long alkyl chain followed by a perylene motif as the hydrophobic component of the

amphiphile. The hydrophilic component of the amphiphile consists of a dendritic structure composed

of nine pyridinium moieties (Figure 4.4a). As for the other molecules a more thorough description of

the results of each amphiphilic molecule can be found online [18].

Figure 4.3.: Shell by shell stabilization concept. a) Nanoparticle is rendered hydrophobic by functionalization

with C16-PA. b) The hydrophobic particle is rendered hydrophilic due to the amphiphilic molecules forming a

bilayer. c) Example of the amphiphilic molecules employed for this study. Adapted from [18] with permission

from Wiley.

58 Applications

The stages of the shell by shell process with the employment of the amphiphilic molecule 7, are

clarified with a simple phase transfer experiment showcased with photographs in Figure 4.4b, c, and d.

First, the pristine unfunctionalized hydrophilic TiO2 nanoparticles can be seen dispersed in the water

phase with a clear toluene phase above them in Figure 4.4b. Then, after the particles have been

rendered hydrophobic after functionalization with C16-PA (as previously described in Section 3.2).

Naturally, the hydrophobic particles are now dispersible in the toluene phase instead of the water

phase (Figure 4.4c). Finally, upon addition of the amphiphilic molecule 7 the dispersibility of the

nanoparticles is reversed once more to the water phase due to the formation of a double layer (Figure

4.4d). Additional support for the double layer formation comes from the measured zeta potential at

this stage, which was highly positive around +40 mV. this suggests an electrostatic stabilization

mechanism provided by the pyridinium motifs engulfing the particles. Also at this point, the particle

dispersion has now a strong red color, due to the perylene motif on the amphiphilic molecule.

Figure 4.4.: Photograph of TiO2 nanoparticles dispersed in DI-water or toluene. a) Hydrophobic and hydrophilic

components of molecule 7. The red coloring of the TiO2 nanoparticles is due to the perylene motif. b) Pristine

hydrophilic particles dispersed in DI-water. c) C16-PA functionalized hydrophobic nanoparticles dispersed in

toluene. d) Upon addition of the amphiphile molecule 7 the nanoparticles are now dispersible in the water phase

due to the formation of a bilayer. Adapted from [18] with permission from Wiley.

The red color arises from both bound and unbound specimens of the molecule 7 to the nanoparticles,

as the molecule by itself is highly water soluble. Fascinatingly, we noticed that when the nanoparticle

Toluene

DI water

b) c) d)

Hydrophobic

Hydrophilic

a)

Applications 59

dispersion was centrifuged at this stage, a clear supernatant and a pink colored particle powder was

obtained. Thus, the nanoparticles can sequestrate and isolate in solution what would normally be a

highly soluble amphiphilic molecule such as 7. This very same concept, eventually led to the

development of nanoparticles that were employed as scavengers as briefly described in Section 4.6.

In summary, an approach for generating a controlled second layer on the surface of functionalized

nanoparticles has been described. The non-covalent grafting nature of the second layer allows for its

later removal if desired. This is possible as the approach described herein relies mainly on

hydrophobic forces, which means that the dispersion medium is limited to water, were water would

provide the primary force holding the bilayer together [91]. Thus, transferring the double shell

nanoparticles from their orthogonal system to a non-orthogonal system (e.g. lower alcohols), should in

principle, collapse (at least partially) the second layer into the solution as individual components.

Whether this reversibility is advantageous or not is merely situational and application dependant.

Lastly, the shell by shell concept is not necessarily limited to hydrophobic interactions, as similar

interactions between oleophobic or fluorophobic forces are not unheard of [92], [93]. However, the

employment of such driving forcers remains to be investigated.

4.1.4. Polymer wrapping

This stabilization scheme was developed when Tobias Rejek required glycol covered nanoparticles

that were highly dispersible in toluene, a somewhat contradictory proposition. The particles were

needed so that they could be combined in solution with block copolymers, the block copolymers

employed were highly soluble in toluene but not in polar solvents. We had a few bumps along the way,

but eventually managed to come up with a working solution, which is described below.

Wrapping nanoparticles with polymers is a fairly common strategy for nanoparticle stabilization [94]–

[96]. In this particular case, we specifically resorted to this stabilization mechanism when the need

arose to have hydrophilic iron oxide particles dispersed in toluene. At the same time, the solution

should also contain a dissolved block copolymer. The need for these highly stable hydrophilic

particles in the conditions described, originated from another project related to nanoparticle self-

organization. The driving force for this organization is driven by chemically matching a specific phase

of a phase separated block copolymer film. This application is briefly described elsewhere

(Section 4.3.2).

The best results under the proposed circumstances were obtained when the particles were

functionalized with the molecule CH3(OC2H4)4C4H8-PA displayed in Figure 4.5a. The polymers

employed consisted of a selected variation of block copolymers and polymers of differing molecular

weights, as displayed in Figure 4.5b. To better portrait the proposed approach, a suggested model of a

nanoparticle wrapped by the block copolymer is depicted in Figure 4.5c. In such model we can

60 Applications

observe the nanoparticle core surrounded by an hydrophilic SAM composed of CH3(OC2H4)4C4H8-PA

(blue). Atop this layer, a far thicker polymer layer lies upon (red and dark blue). The polymer layer is

arranged in such a way, that the hydrophilic phase (dark blue) of the polymer remains towards the

center of the particle whereas the hydrophobic phase (red) tends to reside around the particle

periphery. If this were to happen as described in the model, the hydrophilic particles could be made

dispersible in toluene via the steric stabilization provided by the polystyrene phase surrounding the

nanoparticles.

Figure 4.5.: Polymer wrapping of nanoparticles. a) Phosphonic acid molecule with hydrophilic tail employed to

functionalize the surface of the particles. b) PS-b-PEO block copolymer molecular structure and weights.

c) Schematic representation of a nanoparticle wrapped in an orthogonal block copolymer.

a) b)

c)

Block copolymer

wrapping nanoparticle

Nanoparticle

hydrophilic tails

Nanoparticle core

PS (MW) PEO (MW)

1 55000 10200

2 42000 11500

3 16400 72000

4 280000 0

Applications 61

Originally, we anticipated that the polymer wrapping would occur simply by dissolving the polymer in

toluene and afterwards adding the hydrophilic nanoparticles into the solution. The hydrophilic

particles have an extremely poor dispersibility in toluene. Even so, we expected the polymer to pick

them up, wrap them and gradually turn them into an stable dispersion. However, this was not the case

since the nanoparticles remained heavily agglomerated, even after long stirring or sonication times. At

this point, we hypothesized that in order for the particles to be wrapped by the polymer, they first

needed to be individualized (non-aggregated). Undoubtedly, the hydrophilic particles could be well

dispersed in water, unfortunately the polymers could not. Therefore, we decided to try an approach

which involved both the water and the toluene phase (Figure 4.6a). On one hand, the water phase

contained highly stable (individualized) hydrophilic particles. On the other hand, the toluene phase

contained the dissolved polymer. The two phase solution-dispersion was then vigorously shaken and

treated with ultra sonication until it was transformed into an almost homogenous emulsion-like

product (Figure 4.6b). Finally, in the last step of the process, the solvents (water and toluene) were

evaporated by vacuum at 50 °C in a rotary evaporator. What remained was a brown colored "plastic

spider web" like material. Once all the solvent was removed, pure toluene was added to the dried

product. This time however, it formed a perfectly stable whisky colored transparent

solution-dispersion. The solution-dispersion was now perfectly suited for solution processing.

Figure 4.6.: Photograph of the solution before and after shaking vigorously. Photograph b) shows the solution

only after shaking, not after sonication.

DLS measurements of the dispersions before and after polymer wrapping were conducted and are

displayed in Figure 4.7. The black distribution represents the size measured for the

CH3(OC2H4)4C4H8 PA functionalized particles, while the others represent the size measured after the

polymer wrapping procedure and were dispersed in toluene. It was a good indication, that after the

polymer wrapping, there was a slight increase in the size of all the polymer wrapped nanoparticles.

Before shaking After shaking

a) b)

Dissolved polymer

in toluene

Hydrophilic iron oxide

NPs in DI-water

62 Applications

Since it was expected, that the polymer wrapping would increase the size of the nanoparticles.

However, DLS is a complex and indirect measurement technique which can be affected by several

factors [97]. Therefore, an increase of a few nanometers in the size distributions (while very

consistent) should be taken with skepticism. In search of further evidence to support the polymer

wrapping scheme, the zeta potential of the dispersions was measured and it is displayed in Table 4.1.

The particles covered by CH3(OC2H4)4C4H8-PA were in theory stabilized via steric stabilization.

However, the particles exhibited a clear zeta potential of +30 mV. This suggested an electrosteric

stabilization mechanism for this particle dispersion. Interestingly, after the polymer wrapping

procedure the measured zeta potentials in toluene were all near zero, yet they formed perfectly

transparent dispersions. This lack of zeta potential and perfect stability, was unmistakably due to a

steric stabilization which was provided by the wrapping of the polymer to the nanoparticles. Still, at

this point we could further argue that the solvent itself can have an impact on the measured zeta

potential. In particular, between zeta potential measurements carried out in a polar solvent vs. toluene.

For this purpose, the zeta potentials and DLS size distributions of the CH3(OC2H4)4C4H8-PA covered

particles were all measured under chloroform. A solvent which is more analogous to toluene. It must

be noted, that this measurement was only possible due to the serendipitous discovery that the

CH3(OC2H4)4C4H8-PA covered particles were perfectly dispersible in chloroform. As a matter of fact,

if in the previously described polymer wrapping procedure the DI-water is replaced by chloroform, it

also results in toluene dispersible nanoparticles. With the only difference being that there is no phase

separation between toluene and chloroform.

So far these experiments have made for a nice story. However, there is still one small troublesome

detail left. Surprisingly, when the same procedure is performed by replacing the block copolymer with

a long PS polymer 4, similar results were obtained. As a matter of fact, this polymer was only

employed to act as a negative control vs. the block copolymers. Unexpectedly, it also made the

nanoparticles perfectly dispersible in toluene even when the polymer had no hydrophilic components.

Undoubtedly, the polymer must somehow trap or wrap around the nanoparticles making them

dispersible. Ultimately, we concluded that the length of the PS polymer (MW = 280k) was too long and

therefore wrapped the nanoparticles independently of phase matching or not. This in light of, that

using shorter PS polymers (MW = 35k) for dispersing the particles, yielded good negative controls

(unstable dispersions), as we originally expected.

In conclusion, a procedure for creating stable dispersions of hydrophilic nanoparticles in toluene via

polymer wrapping was described. A key factor for the success of the procedure is to first individualize

the particles in solution so that the polymer can wrap around them. The individualization was possible

as the particles formed stable dispersions in DI water and chloroform. Unexpectedly, in order for

polymer wrapping to occur around the nanoparticles a phase matching block copolymer was not

required, it sufficed that the polymer was long enough to capture and wrap around the nanoparticles.

Applications 63

Figure 4.7.: DLS size distribution of CH3(OC2H4)4C4H8-PA nanoparticles before (black) and after polymer

wrapping.

Table 4.1.: Zeta potential of Fe3O4 nanoparticles. The hydrophilic CH3(OC2H4)4C4H8-PA nanoparticles have a

strong zeta potential due to the electrosteric stabilization caused by the functionalization. After the polymer

wrapping procedure the nanoparticles are still stable, yet their zeta potential is near zero. This effect is caused by

the steric stabilization provided by the polymer wrapping of the nanoparticles.

Material Zeta potential (mV)

CH3(OC2H4)4C4H8-PA Fe3O4 +30

PS55k-PEO10.2k Fe3O4 +1.87

PS42k-PEO11.5k Fe3O4 +2.64

PS16k-PEO72k Fe3O4 +0.9

PS280k Fe3O4 -4.49

Yet, there are still a few open questions regarding this polymer wrapping procedure. For example,

when employing similar hydrophilic SAM molecules and following the same protocol only partially

stable dispersions were obtained. Therefore, further investigations regarding the exact wrapping

mechanisms are required. For the moment however, a simple procedure for polymer wrapping of

nanoparticles was described. Furthermore, the same polymer wrapping procedure has been validated

for TiO2 nanoparticles with a core size of 6 nm as well.

0 10 20 30 400

5

10

15

20

25

30

35

40

Nu

mb

er

(%)

Size (nm)

CH3(OC

2H

4)

4C

4H

8-PA Fe

3O

4

PS55k

-PEO10.2k

Fe3O

4

PS42k

-PEO11.5k

Fe3O

4

PS16.4k

-PEO72k

Fe3O

4

PS280

Fe3O

4

64 Applications

4.2. Thin films. From 0D to 2D

Thin films are corner stone of nanotechnology, their use and application encompass every field of

nanotechnology. Often, nanoparticle thin films can be easily fabricated from nanoparticle dispersions

via solution processing. It is therefore natural, that having gained such control over nanoparticle

dispersibility as previously described, to employ conventional thin film fabrication techniques such as

spin coating, spray coating, doctor blading, etc. for the fabrication of nanoparticle thin films. In here,

example applications of nanoparticle thin films fabricated using the previously described dispersion

methods are presented.

4.2.1. Flexible dielectrics

The water and alcohol NP dispersions described in Section 4.1.1 were used to fabricate the dielectric

layer for organic thin-film transistors (OTFTs). The dielectric layers consisted of several metal oxide

materials namely AlOx, ITO, CeO2, TiO2 and Fe3O4. The films were fabricated by simple spin coating

from water or 2-propanol in ambient air, followed by annealing at a maximum temperature of 100 °C

for ten minutes to remove the solvent. Interestingly, after annealing, the films showed no signs of

degradation (as confirmed by AFM) even after being exposed to a vigorous rinse of DI water or

2-propanol. This potentially allows for spin coating of different layers of different materials atop of

each other. Furthermore, it was observed that the film thickness could also be tuned by changing the

NP concentration of the spin coating solutions, with higher concentrations resulting in thicker films

under the same process conditions. Yet, other factors influencing the NP film thickness include

inherent solvent properties, such as vapor pressure or viscosity. Therefore, spin coating of the NP

films from either DI water or alcohols enables an extra degree of freedom for thickness tunability.

For characterization the core-shell particles were spin coated from aqueous dispersions (0.6 wt-%)

onto a silicon oxide wafer, except that the AlOx particles were spin coated from 2-propanol (0.6 wt-%)

to avoid etching of the particles in DI water. The AlOx particles when dispersed in DI water tend to

agglomerate relatively sooner and permanently, this is not the case when dispersed in 2-propanol.

Cross-section images of the spin coated films obtained via scanning electron microscopy (SEM) are

shown in Figure 4.8a. The films showed long range and consistent thickness as a function of the

particle size. The film surface roughness also relates to the mean nanoparticle diameter of the NPs

employed (Table 4.2). The larger the NPs, the larger the surface roughness measured. The NP films

were also spin coated onto a silicon oxide wafer that was pre-patterned with capacitor and transistor

gate electrodes made of Al with 30 nm thickness. The film quality on top the aluminum electrodes was

characterized with atomic force microscopy (AFM), it showed that Al gate electrodes were completely

covered by the nanoparticle films (Figure 4.8b).

Applications 65

Figure 4.8.: a) SEM cross-sections of the spin coated dielectric layers. b) AFM images of the surface

morphology of the spin coated films. Reprinted from [17] with permission from Wiley.

Table 4.2.: Summary of film properties and electrical characteristics of the films and devices. Reprinted from

[17] with permission from Wiley.

Metal

oxide

Particle

mean

size [nm]

Thickness

[nm]

RMS

surface

roughness

[nm]

Capacitance

[µF/cm2]

µ sat

[cm2/Vs]

@ Vds =

–3 V

Voltage

Threshold

[V]

Ion/Ioff Id/Ig

TiO2 6 8 2.3 1.08 0.14 2.78 25 × 103 12

Fe3O4 10 6 3.3 1.01 0.14 2.80 21 × 103 12

AlOx 67 100 22.7 0.39 0.66 3.12 122 ×

103 76

ITO 57 50 18.6 1.12 0.27 2.63 3.8 × 103 17

CeO2 55 40 18.0 0.92 0.24 2.60 5.0 × 103 27

Cerium Oxide

Indium-Tin Oxide

Aluminum Oxide2 µm

2 µm

2 µm

2 µm

2 µm

Titanium Oxide

Iron Oxide

a) b)

66 Applications

To characterize the dielectric layers, 50 x 50 µm capacitors were fabricated (Figure 4.9a). The

breakdown characteristics of the NP-dielectric layers (Figure 4.9b) exhibited reliable values of ±5.5 V.

The low leakage current at voltages below breakdown indicate excellent, and almost uniform,

insulating behavior with slightly improved behavior for the AlOx layer, due to an increased thickness

(Table 4.2) which arises from the use of 2-propanol for spin coating. The corresponding capacitor

devices without NP layers (native AlOx) exhibited a higher current density of almost two orders of

magnitude. It is also noted that ITO and TiO2 core-shell systems provided uniform insulating

properties, indicating that the shell layer significantly limits the transport as obtained previously for

core-shell NPs of ZnO [28]. The corresponding capacitance values (measured at 10 kHz) are shown in

Table 4.2. In contrast to bulk oxide dielectric layers, the NP-based layers differ in their composition.

Considering the core-shell architecture, which provides different -values for core and shell materials,

and the particulate structure leading to a certain porosity of the layer, the measured capacitances of the

NP layers are larger than those formed from bulk oxide [11], [98], [99].

Figure 4.9.: a) Schematic layout of the fabricated capacitor devices. b) Current density of different NP dielectric

materials of 50x50 µm capacitor devices vs. applied voltage. Reprinted from [17] with permission from Wiley.

In order to demonstrate the use of the NP layers as gate dielectrics, they were integrated into thin-film

transistor (TFT) devices. Figure 4.10a illustrates the bottom-gate, top-contact architecture of the

fabricated devices, which were formed using 2-tridecyl[1]benzothieno[3,2-b][1]benzothiophene

(C13-BTBT) as the organic semiconductor [99]. The transfer curves of the devices are shown in Figure

4.10b, and their characteristics summarized in Table 4.2. Devices with AlOx nanoparticles as dielectric

showed the best performance, with a saturation mobility of 0.66 cm2V–1s–1, followed by ITO and CeO2.

The lower performance of the ITO and CeO2 devices was attributed mainly to the semiconducting

nature of these NPs, and, thus, to relatively low ID/IG ratios. The use of ITO and CeO2 as a dielectric

was still possible. However, due to the organic insulating shell layer encasing the NPs, which retards

Bottom (Al) Substrate

AlOx

Top (Au)

NP dielectric

-8 -6 -4 -2 0 2 4 6 810

-13

10-11

10-9

10-7

10-5

10-3

I (A

)

Bias Voltage (V)

AlOx

ITO

CeO2

Fe3O

4

TiO2

a) b)

Applications 67

transport between the particles [27], [28]. TiO2 and Fe3O4 devices showed the lowest performance in

terms mobility, which can be attributed to the poor wettability of the evaporated C13-BTBT

semiconductor films onto the TiO2 and Fe3O4 films. It seems likely that the variation in wettability is

linked to the nanoparticle core size, i.e., SAMs grafted onto smaller particles possess additional free

space in-between the molecules tails, whereas the ligand to ligand distance is constrained due to a less

pronounced surface curvature in bigger particles [65]. This in turn, impacts the free surface energy of

the nanoparticle film. Consequently, different semiconductor wetting and device performance

behavior was observed, and a direct overall comparison of devices formed from AlOx, ITO, and CeO2

NPs is thus quite difficult [100]. Finally, it is noted that after four months of storage under ambient

conditions, the devices performance remained virtually unaffected in terms of charge carrier mobility,

VTH and ON/OFF ratio.

Figure 4.10.: a) Molecular structure of the semiconductor molecule C13-BTBT and schematic layout of the

fabricated OTFTs devices. b) Transfer curves of the OTFTs with different dielectric materials. c) concave

bending of devices during characterization. d) Transfer characteristics of the devices under different bending

modes. Reprinted from [17] with permission from Wiley.

SubstrateGate (Al)

AlOx

NP dielectric

Drain (Au)Source (Au)

a)

-5 -4 -3 -2 -1 010

-14

10-13

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

I g (

A),

Id (

A)

AlOx

ITO

CeO2

TiO2

Fe3O

4

-5 -4 -3 -2 -1 010

-14

10-13

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

I g (

A),

Id (

A)

Vgs

(V)

concave bending

no bending

convex bending

b)

d)c)

68 Applications

To further establish the universality of the developed deposition concept in terms of eco-friendly

production methods on bendable substrates, devices were fabricated onto flexible polyethylene

naphthalene (PEN) substrates using AlOx nanoparticles as a prototypical dielectric. To evaluate the

mechanical robustness, the flexible devices were characterized under different bending conditions as

depicted in Figure 4.10c. The bending radius was around 5 mm both in the concave and convex

bending modes. Figure 4.10d compares the measured transfer curves of a device under different the

bending conditions. The differences between ID and IG are negligible, falling within the same order of

magnitude regardless of the bending mode. Due the higher surface roughness of the PEN substrates,

the highest mobility measured in the flexible devices was slightly reduced to 0.34 cm2V–1s–1, an effect

what is commonly observed [30], [101]. Experience has been obtained in terms of foldability or cyclic

bending of the devices. Thus, the feasibility of spin coated nanoparticle dielectrics onto flexible

substrates is clearly demonstrated,

In conclusion, the flexibility of the procedure in terms of nanoparticle thin film formation has been

demonstrated. Unfortunately in terms of transistor fabrication, the performance of the devices was

limited. Mainly because of the roughness of the NP films and/or the wetting of the semiconductor.

Both are factors which can severely affect the performance of TFTs.

4.2.2. Coatings

I realized I can functionalize any nanoparticle core with any ligand and simply spray coat it onto any

surface. It would then create some kind of functional surface depending on the core and exposed head

groups. Here is a brief example which involves a superhydrophobic coating, and potentially highly

oleophobic as well.

Once nanoparticles have been modified, they can easily be spray coated onto any surface forming a

nanoparticle coating. There is potential in these kind coatings when you consider that the exterior of

the nanoparticle core can consist of any organic functional group or even any particular combination

of them. As previously described in Section 3.2.3 the nanoparticle surface can be finely tuned with a

variety of SAM molecules. Furthermore, by using different nanoparticle cores or even a mixture of

cores, they can act as light filters, color pigments or even create a magnetically responsive coating.

To demonstrate this concept, superhydrophobic films were fabricated on a glass substrate as well as

cardboard and they are shown in Figure 4.11. These samples were prepared by spray coating of

F17C10-PA functionalized AlOx nanoparticles onto a glass slide and a piece of cardboard in the same

manner as described in Section 3.2.3. The average size of the particles (~50 nm), in conjunction with

their "Teflon-like" functionalization, worked in unison to create a superhydrophobic surface wherever

they were sprayed. The spray-coated layer created a randomly nanostructured surface with substantial

roughness, which is required for creating an superhydrophobic surface [87]. Moreover, the sprayed

Applications 69

nanoparticles have already been rendered hydrophobic, and thus the film required no further treatment.

So by providing both requirements of low surface energy and high surface roughness, a

superhydrophobic surface was achieved with a single step process of spray coating. Furthermore, the

coating was essentially transparent in up-close circumstances (Figure 4.11a). However, in reality it

was only semitransparent as evidenced by Figure 4.11b. Additionally in Figure 4.11c several water

based liquids were placed upon the coated glass. The same situation can be observed in Figure 4.11d

except instead of glass, the coating was sprayed over a piece of cardboard. A video demonstrating the

concept can be found online (Appendix Figure 7.6). Interestingly, placing the droplets in an orderly

fashion over the coating was a dauntingly impossible task. To facilitate the matter, a small scratch on

the film was purposely made wherever a droplet was placed.

Figure 4.11.: Photographs of spray coated hydrophobic coatings on glass and cardboard. a) Coated and uncoated

glass slide for up close transparency comparison. Droplets of different water based liquids dispensed on the top

of coated (c) glass slide and (d) cardboard.

Unfortunately, the spray coated nanoparticle films could be removed easily from the glass substrates

by weak mechanical means, such as finger tips or a tissue paper. This is expected, as the particles are

coated with a "Teflon-like" material, which does not allow for any adhesion to the glass. As for the

cardboard coating, a different situation occurred. In this case, the hydrophobic coating showed much

robust mechanical stability. This is because of the rougher and fibrous nature of the cardboard surface

a) b)

c) d)

Coated Uncoated

Beer Milk

Iron oxide NPAu NP Cerium oxideNP

UV Ink (fluorescein)

70 Applications

which traps the nanoparticles. As a matter of fact, we believe that this roughness acts in a similar way

as the strategy that is employed to adhere a Teflon coating onto a metal surface (cooking pans) [102].

In short, the technique consists on increasing the roughness of the metal surface before applying the

Teflon coating. Superhydrophobic coatings are currently a highly researched field as it can have many

potential applications. The applications are very diverse, a notorious example is the packaging

industry [103], automotive, aerospace and naval industry [104], where anti-icing [105], anti-fouling

[106], self-cleaning [106] are of particular interest. Urban applications as well, were an hydrophobic

coating was applied to street walls to dissuade people from urinating on them. All of these concepts

and many more can be easily tackled with the approach presented above. However, further research is

still required in terms of coating durability. One possible solution to improve this would be the

addition of reactive or polymerizable groups to the nanoparticle shell for increasing the robustness of

the film.

4.3. Self-assembled thin films

This section presents fabrication of thin films as well, however, these thin films involve a special

degree of freedom. These films incorporate the concept of self-assembly at a nanostructure level rather

than just at the molecular level as it is the case for SAMs. Via surface functionalization of 2D and 0D

NSs with SAMs, the creation of "smart materials" that self-assemble into a thin film in an orderly

fashion was achieved.

4.3.1. Regio-selective deposition of nanoparticles

A 2D SAM patterning process using photolithography had already been developed for other projects.

It occurred to us that this same SAM patterning could be used create a pattern and selectively deposit

nanoparticles via a covalent bond to a reactive surface. My undertaking in this project, consisted in

the nanoparticles functionalization while Sebastian Etschel took charge of the far harder task of

reacting them selectively.

The fabrication of regio-selective nanoparticle thin films was achieved by employing reactive

nanoparticles and a 2D reactive surface. To this purpose we employed the Huisgen 1,3-dipolar

cycloaddition which was introduced in 2001 by Sharpless and Méldal [107], [108], which is also

classified as the prototypical click reaction. In summary, the reaction results in a covalent bond

between an azide and alkyne group via the formation of a triazole. It has been demonstrated that the

click reaction can be used for the immobilization of functional materials on self-assembled monolayer

(SAM) substrates [109], [110]. A click reaction is ideal for this purpose as it provides in principle,

high yields and almost no byproducts making it ideal for nanoparticle thin film formation.

Applications 71

The regio-selective deposition of nanoparticles was possible by employing pre-patterned substrates

with an inert surface in conjunction with a reactive surface. The contrast of these surfaces provided the

driving forces for the self-organization. Using SAMs and photolithography, a chemically patterned

substrate was fabricated as portrayed in Figure 4.12a [11]. This chemically patterned substrate

consisted of SAMs with reactive head groups (red) vs. SAMs with inert groups (purple). The patterned

substrate was then immersed in a dispersion of reactive nanoparticles, consisting of the corresponding

building block showcased in Figure 4.12b and Figure 4.12c. The nanoparticles were functionalized by

following the protocol described previously in Section 3.2. Measured FTIR spectra of the AlOx

nanoparticles is shown in the appendix (Figure 7.8). The FTIR spectra of the particles clearly showed

the reactive groups were still present after functionalization.

Figure 4.12.: Schematic representation of the building blocks involved in thin film self-assembly. a) Chemically

patterned 2D substrate. Red represents the reactive sites provided by the Azide-PA SAM molecule. Purple

represents the inert sections of the substrate. the inertness is provided by the F15C18-PA SAM molecule. b)

Reactive Alkyne-PA functionalized nanoparticle. c) Reactive Azide-PA functionalized particle.

A critical parameter during the deposition steps was the stability of the nanoparticle dispersion. A

nanoparticle dispersion with poor stability tended to agglomerate and resulted in poor film formation

and selectivity. On the contrary, a highly stable dispersion yielded denser film formation and better

region selectivity as the nanoparticles can be washed away by the solvent. This situation was of no

b) c)

a)

72 Applications

surprise, it is simply analogous to attempting to conduct a reaction were one of the reactants is not

soluble. Whenever poor dispersibility was obtained after functionalization with the Azide-PA or the

Alkyne-PA, a mixed SAM approach can help make the approach feasible. In a mixed SAM situation, a

specific SAM provides dispersibility while the other SAM provides reactivity. The compromise for the

better dispersibility would be the lesser amount of available reactive groups at the nanoparticles.

Fortunately for this example (AlOx and ITO NPs), after functionalization of the nanoparticles with

either the Azide-PA or the Alkyne-PA a stable dispersion was obtained. Therefore, there was no need

for a mixed SAM approach. Nonetheless, there is work in progress by Sebastian Etschel were the

mixed SAM approach was successfully implemented for regio selective deposition with TiO2

nanorods and other NSs.

Figure 4.13a shows a schematic representation as well as reaction conditions for the deposition of the

first layer. The first deposited layer, consisted of AlOx nanoparticles with a median size of 50 nm

functionalized with Alkyne-PA. Once the first layer was covalently bound, the immobilized

nanoparticles still contained un-reacted chemical groups. Therefore, the deposition of a second layer

atop the first one was possible. Thus, using the complimentary Azide-PA functionalized ITO

nanoparticles a second layer was deposited (Figure 4.13b). Similarly, the ITO nanoparticles also had a

median size of around 50 nm. Furthermore, a third layer of Alkyne-PA functionalized CeO2

nanoparticles was deposited [11], however, it is only portrayed in the cross sectional measurements.

Theoretically, this process could be repeated many times as long as the complimentary reactive NSs

are employed. Strictly speaking however, the regio-selective deposition is limited to a few layers. As

with every new layer deposited, the coverage of the film gradually degrades due to lesser and lesser

availability of reactive groups. Yet, further tuning the reaction conditions can help overcome

difficulties and improve the overall film formation process. However, this was not thoroughly studied

in this work.

Figure 4.13c shows AFM images of the first and second layers as well as the measured cross-section

height of the three consecutively deposited films. The first AlOx nanoparticle layer (depicted in red)

had an average thickness of 60 nm which was in good agreement with the nanoparticle core size. This

was a good indication, in terms of monolayer formation of the deposited nanoparticles. After

deposition of the second layer, an increase of film thickness by an average of 47 nm was measured.

This was also in good agreement with the ITO nanoparticle core size and thus also serves as a good

indication of a monolayer coverage. Lastly, a layer of CeO2 nanoparticles with core size of 55 nm was

deposited atop the ITO layer. Unfortunately, the CeO2 nanoparticle dispersion did not showed as good

dispersibility after functionalization as the AlOx and the ITO nanoparticle dispersions. The

dispersibility however, was enough for processing. After deposition of the CeO2 nanoparticles, an

average increase of thickness around 150 nm was measured. This was not in agreement with the CeO2

Applications 73

nanoparticle core size and was attributed to multilayer formation due the inferior dispersibility of the

CeO2 dispersion.

Finally, in Figure 4.14 several SEM images of the selectively fabricated nanoparticle thin films are

showcased. It becomes clear from the images that the nanoparticles were present only at the reactive

surface with striking fidelity. The latter considering the arrangement of the nanoparticles was driven

purely by self-assembly.

In summary, the presented technique offers plenty of versatility via a dynamic layer by layer

nanoparticle thin film formation. The approach can be extended to variety of metal oxide materials

which would allow for great variability of thin film stacking combinations. More importantly, the NP

films conformed to the chemically pattern on the 2D substrate. In general, the technique leaves open

the possibility for the fabrication of other "smart" materials which can potentially self organize by

using covalent interactions.

Figure 4.13.: a) Schematic representation of the first deposited layer and reaction conditions. b) Schematic

representation of the second deposited layer and reaction conditions. c) AFM scans of first and second

selectively deposited thin films. AFM cross-sectional height measurement of the first and second layers.

a)

b)

1st layer AlOx

1st layer AlOx

2nd layer ITO

1st layer

2nd layer

0 2 4 6 8 10 12 14 16 18 200

50

100

150

200

250

300

350

400

He

igh

t [n

m]

Width [m]

AlOX

ITO

CeO2

c)

74 Applications

Figure 4.14.: SEM images of selectively deposited nanoparticles.

4.3.2. Block co-polymer phase matching

I worked with Johannes Kirschner and Tobias Rejek on this project by providing them with

hydrophilic or hydrophobic metal oxide nanoparticles. Here, I briefly write about their work

regarding the employment of the functionalized nanoparticles.

A block copolymer which exhibits nanophase separation was employed as a 3D matrix in an endeavor

to achieve the self-organization of nanoparticles within the matrix. The copolymers in this case,

consist of polystyrene as an hydrophobic block and polyethylene oxide as an hydrophilic (Figure

4.15a). The phase separation of the block copolymers occurs due to the orthogonality of the copolymer

blocks. First, in order to verify that the phase separation of the copolymer occurs. The block

copolymer was dissolved in toluene and spun coated atop a SiO2 wafer. Figure 4.15b shows an AFM

image of a spun coated block copolymer film which exhibited phase separation. The brighter areas in

the image correspond to the polystyrene phase while the darker ones to the polyethylene oxide phase.

The average lateral distance within phases was 20 nm.

To achieve the self-assembly of the nanoparticles into the specific phase, the block copolymer was

solubilized in toluene. However, before spin coating the copolymer, the hydrophobic or hydrophilic

metal oxide nanoparticles were dispersed in conjunction with the copolymer in the same solution. To

this end, metal oxide nanoparticles with a size between 4-10 nm were functionalized to exhibit either

an hydrophobic or an hydrophilic surface. Naturally, the overall size of the nanoparticles must be

smaller than the measured lateral distance between the phases of the film (20 nm). The hydrophobic

nanoparticles consisted of nanoparticles with a shell of an alkyl chained phosphonic acid SAM. This

Applications 75

alkyl chained SAM rendered the particles highly dispersible in toluene. The particle and copolymer

were then just mixed to desired concentrations resulting in a stable solution-dispersion in toluene. The

hydrophilic particles on the other hand, consisted of more elaborated dispersions which were

thoroughly described already in Section 4.1.4.

Figure 4.15.: a) Molecular structure of block copolymers. The hydrophobic phase is composed of polystyrene

(red) while the hydrophilic phase is composed of polyethylene oxide (blue). b) AFM image of a phase separated

spin coated block copolymer thin film. c) SEM image of a phase separated spin coated block copolymer thin film

with embedded Fe3O4 hydrophilic nanoparticles in the corresponding phase.

Figure 4.15c shows a SEM image of a spun coated copolymer film with hydrophilic nanoparticles.

The film still exhibits phase separation, however, it shows a swollen polyethylene oxide phase. A

possible reason for the swelling could be the that the hydrophilic nanoparticles are within the phase.

Therefore, causing a thickening of the hydrophilic copolymer phase. However, the exact reason for

this swelling is not clear. Understanding how the nanoparticles affect the copolymer phase separation

in terms of concentration and whether the nanoparticle size or even organic shell can also have an

Hydrophobic

particle

Hydrophilic

particle

a) b)

Hydrophilic particles

in the PEO phase

c)

76 Applications

impact still remains a challenge [96]. Yet unmistakably, whichever nanoparticles are exposed in

Figure 4.15c lie within the polyethylene oxide phase. The latter is a pretty good indication towards the

overall validity of the approach. Furthermore, a similar situation occurs for hydrophobic particles.

In conclusion, a system with the potential to fabricate via self-assembly a 3D matrix of metal oxide

materials was briefly described. In principle, the functionality of the thin-film can be changed simply

by changing the core of the metal oxide material, while the location of the nanoparticles can be

changed by the organic shell. Still some fine tuning remains to be done regarding the impact of the

nanoparticles on the overall phase separation and the ideal nanoparticle vs. copolymer concentrations.

4.4. Polymer composites

We had developed particles with a polymerizable head group. Naturally, the idea came to use them

during an in situ polymerization in order to improve the mechanical properties of the polymer. This

task was performed by Simon Scheiner, while I simply found a way to properly functionalize the

nanoparticles. A brief description of the idea is provided below.

Nanoparticles were functionalized with a SAM having a polymerizable head group. The SAM

molecule employed for this purpose is shown in Figure 4.16a. The MMA-PA SAM contains the

equivalent of a methyl methacrylate (MMA) molecule as a head group. This head group can

polymerize with other free MMA molecules (Figure 4.16b). Therefore, MMA was employed as

monomer for the polymerization reaction. The polymerization of MMA results in the formation of

Poly(methyl methacrylate) (PMMA) (Figure 4.16c). The FTIR spectrum of the functionalized particles

shows clear evidence of the polymerizable head group being present at the AlOx nanoparticles surface

(Figure 4.16d). After functionalization with MMA-PA the AlOx particles formed a moderately stable

dispersion under methanol and toluene. Unfortunately, this degree of stability was not good enough to

achieve an even distribution of the nanoparticles along the volume of the solution. It is important for

this approach, that a stable dispersion is obtained under the polymerization conditions. A stable

dispersion is required in order to produce a polymer composite with an even distribution of

nanoparticles along its volume. Serendipitously, in a "like dissolves like" situation, the particles were

highly dispersible in the liquid monomer (MMA), this served as an excellent dispersion medium for

the nanoparticles.

For testing, cylindrically shaped specimens were fabricated. These probes consisted of pure PMMA

(Figure 4.16e), a PMMA composite with unfunctionalized AlOx nanoparticles (Figure 4.16f) and a

PMMA composite with functionalized AlOx MMA-PA nanoparticles (Figure 4.16g). The pure PMMA

probes were transparent, while the probes that contained particles were milky white. The

polymerization of MMA was carried via radical polymerization. The radical starter was added to the

MMA contained in a tube with or without nanoparticles and it was cured in ambient air at 50 °C

Applications 77

overnight. Afterwards, tensile tests were performed on the probes (Figure 4.16h). Consistently, the

probes comprised of unfunctionalized particles and PMMA exhibited the worst mechanical properties.

Followed by the pure PMMA probes with better results. The probes with the MMA-PA functionalized

particles were consistently the most mechanically robust. The latter was attributed to the covalent

bond between the particles and the polymer matrix. Furthermore, the nanoparticle core also acts a 3D

branching point for the polymer matrix which increases the degree of cross linking of the polymer as

graphically depicted in Figure 4.16i.

Figure 4.16.: a) Molecular structure of MMA-PA. b) Molecular structure of MMA. c) Molecular structure of

PMMA. d) Color coded FTIR ATR spectrum of AlOx nanoparticles functionalized with MMA-PA.

e) Cylindrical PMMA probe. f) Cylindrical PMMA probe with unfunctionalized AlOx nanoparticles.

g) Cylindrical PMMA probe with MMA-PA functionalized AlOx nanoparticles. h) Strain test curves of PMMA

and PMMA composites. i) Artistical rendition of nanoparticles covalently attached to a polymer matrix.

This increased degree of cross linking, improved the mechanical robustness compared to the pure and

linear (uncrosslinked) PMMA. Finally, a qualitative proof of the increased degree of cross linking was

3800 3600 3400 3200 3000 2800 2600

Wavenumber (1/cm)

MMA-PA

T (

%)

2000 1800 1600 1400 1200 1000 800

Wavenumber (1/cm)

a) b)

d)

e)

f)

g)

h)

c)

0 1 2 3 40

200

400

600

800

1000

PMMA Pure

PMMA/AlOx

PMMA/AlOx-MMA-PA

Str

ess [N

/mm

²]

Strain [%]

i)

78 Applications

observed. The composite was exposed to a flame and non-melting behavior was observed. Instead, the

composite burned, turning into a black material with bubbles. The latter was attributed, to the high

degree of cross linking modified the thermo plastic behavior as compared to the pure PMMA.

Polymers with nanoparticles as a filler have been researched extensively [96], [111]–[113]. However,

there are fewer reports about the nanoparticle filler being covalently bound as part of the nanoparticle

matrix. Usually, the polymer-filler interactions are of a weaker nature [112], [113]. In here, a

nanoparticle filler has been employed successfully to improve mechanical properties of the polymer.

Evidence suggesting the covalent attachment of the nanoparticles was also briefly described. However,

this project is still work in progress and more research is still required in order to identify the proper

polymerization conditions and ideal nanoparticle polymer ratio.

4.5. Nanooncology

Water dispersible metal oxide nanoparticles under neutral pH conditions, are highly desirable

conditions for bio-applications. Towards this purpose, I was given a set of magnetic nanoparticles

which I should try to make dispersible in water. After some back and forth we obtained very good

results in terms of nanoparticle stability. This is a brief summary of the results obtained so far from

this collaboration with Melek Kizaloglu and Stefanie Klein from Prof. Kryschi group.

Fe3O4 and CoFe3O4 nanoparticles were functionalized with a variety of phosphonic acid molecules

known to us to improve water dispersibility under neutral pH conditions. The nanoparticles were

synthesized by Melek Kizaloglu from Prof. Kryschi group. The synthesis of the particles was

performed with an adaptation of the methods described in literature [114], [115]. After synthesis, the

nanoparticles were washed thoroughly with pure DI water for removal of any excess reagents. Use of

pure DI water is of paramount importance, given that when we employed a phosphate-buffered saline

(PBS) buffer solution for washing, the nanoparticle surface appeared to get "functionalized". We were

able to confirm this by doing an FTIR analysis of washed particles with PBS, the particles showed the

distinct broad valley of the P-O and P=O vibrations. The use of a PBS buffer solution was therefore

discontinued for any part of the process.

The molecules in employed for functionalization are shown in Figure 4.17a. Of particular attention is

the molecule Imidazolium-PA (1) which makes its debut in this section, but more on this particular

molecule is described further below. The other molecules (2 and 3) have already been described in

Sections 3.2.3 and 4.1.1 for similar purposes and conditions. The obtained nanoparticles were

characterized in the same way as all the functionalized NSs in this work. The general functionalization

procedure can be reviewed in Section 3.2 and in detail in Section 6.8 as well as supporting FTIR-ATR

spectra of the nanoparticles before and after functionalization (appendix Figure 7.9).

Applications 79

First proof of improved nanoparticle dispersibility is Figure 4.17b, which shows DLS measurements in

DI water of pristine and functionalized Fe3O4 nanoparticles. The pristine nanoparticles showed a

median size around 70 nm. As expected, after functionalization the nanoparticle dispersions stability

was greatly improved. Naturally, a smaller median size of 30 nm was measured for the functionalized

nanoparticles with all 1, 2 and 3 SAM molecules due to the decreased level of agglomeration after

functionalization. A similar situation was found to be the case for the DLS measurements of the

pristine and functionalized CoFe3O4 nanoparticles. The stability of the nanoparticles can also be

confirmed visually, photographs of the Fe3O4 and CoFe3O4 dispersions can be seen in Figure 4.17c

and Figure 4.17d. It can be observed from the photographs how the functionalized nanoparticles form

perfectly stable transparent dispersions. Whereas the pristine particles precipitate or form a turbid

dispersion. Both photographs were taken 12 hours after redispersion by sonication. It becomes clear

from the photographs that all the SAM molecules improved the dispersibility of the nanoparticles

under neutral pH conditions. Furthermore, the particles remained stable for several months, especially

in the particular case of the Imidazolium-PA were the particles have remained stable over a year.

Having access to stable dispersions of the synthesized particles is already an interesting result, as the

use of the pristine unstable dispersions is very problematic or even unusable.

The zeta potential of the particles was measured to obtain some insight into the stabilization

mechanism of the particles. The measured potentials are resumed in Table 4.3. The pristine

nanoparticles had a weak positive zeta potential which rendered them mildly stable. Not surprisingly,

the nanoparticles functionalized with Imidazolium-PA had positive zeta potentials over 60 mV which

explains their remarkable stability. We can be sure then, that the Imidazolium-PA successfully

generates a strong positive charge at the nanoparticles surface, as well as that the particles are well

dispersed via a strong electrostatic stabilization. Furthermore, the Imidazolium-PA particles have

remained stable over several months, with no apparent signs of agglomeration. Positively charged

nanoparticles have been demonstrated to possess a better cellular intake time and again [116], [117],

[118] as compared to neutral or negatively charged particles. This effect is particularly interesting for

this application as not only provides a good stabilization mechanism but also could potentially

increase cellular uptake. As to the particles functionalized with the molecules 2 and 3, the measured

zeta potential was weaker but positive around 15-30 mV suggesting an electrosteric stabilization

scheme. Since the zeta potential remained positive nonetheless, these particles were also interesting in

terms of cellular uptake.

80 Applications

Figure 4.17.: a) Molecules used for functionalization of nanoparticles. b) DLS distribution of Fe3O4 pristine and

functionalized nanoparticles. c) Photograph of Fe3O4 pristine and functionalized NP dispersions 12 hours after

being redispersed on DI water via sonication. d) Photograph of CoFe3O4 pristine and functionalized NP

dispersions 12 hours after being redispersed on DI water via sonication.

Table 4.3.: Zeta potentials of the Fe-Fe and the Fe-Co nanoparticle dispersions before and after functionalization

with different molecules which portrayed in Figure 4.17.

Metal oxide Pristine 1 2 3

Fe3O4 +14 mV +68 mV +30 mV +25 mV

CoFe3O4 +20 mV +60 mV +12 mV +15 mV

Biocompatibility of the pristine and functionalized particles was tested on human endothelial cancer

cells as well as healthy cells by Stefanie Klein from Prof. Kryschi group. The results of the

biocompatibility tests of CoFe3O4 nanoparticles in function of dosage are shown in Figure 4.18. The

pristine CoFe3O4 nanoparticles were generally toxic towards both cancer and healthy cells. This effect

was observed when a dosage of 10 ug/ml was administered to the cells, which yielded a survival rate

of approximately 30%. However, when the CoFe3O4 particles were functionalized with either of the

ligands, there was only a small decrease in cell viability observed. This is very interesting as it allows

0 20 40 60 80 100 120 1400

5

10

15

20

25

30

Num

ber

(%)

Size (nm)

Pristine

Imidazolium-PA

H(OC2H

4)3-PA

CH3(OC

2H

4)3-PA

a)

c)

b)

d)

1

2

3

1 2 3 1 2 3Pristine Pristine

Applications 81

the CoFe3O4 particles to be present in the cell media without any toxic impact. Thus we hypothesized,

that the organic shell surrounding the nanoparticles acts as an isolation barrier, which isolates the toxic

particle core by presenting a benign organic shell at each of the nanoparticles surface. This effect in

reduced toxicity itself is very interesting, as it can potentially be extended to other toxic metal oxide

materials that need to be incorporated into a cell as well. The biocompatibility tests of the Fe3O4

particles were also successful, though less interesting, as there was no dramatic difference between

pristine and functionalized particles due to the decreased toxicity of the pristine Fe3O4 nanoparticle

core. These results were placed in the appendix Figure 7.10. Having established that the interactions

between the functionalized particles and cells are of a non-toxic nature both in healthy and in

cancerous cells, further studies were performed regarding their potential as nanooncology agents.

Figure 4.18.: Biocompatibility of pristine and functionalized CoFe3O4 nanoparticles in function of dosage. a)

Biocompatibility results on breast cancer cells. b) Biocompatibility results on healthy cells.

The iron and cobalt +2 ions on the Fe3O4 and CoFe3O4 are toxic to cells due the generation of oxygen

radicals via the Fenton reaction [119], [120]. This reaction is depicted in Figure 4.19a. Therefore, in

order to monitor oxygen radical formation, a labeling dye (dichlorofluorescein) was applied to the

cells. Under normal circumstances dichlorofluorescein is non-fluorescent, however, if the dye reacts

with an oxygen radical the dye exhibits a strong fluorescence at around 525 nm when excited at around

1 5 100

50

100

Concentration [g/mL]

Cell v

iab

ilit

y

(% o

f co

ntr

ol)

Uncoated Co-ferrite NP

Imidazol-Co-ferrite NP

Hydroxy-Co-ferrite NP

Methoxy-Co-Ferrite NP

1 5 100

50

100

150


Cell v

iab

ilit

y

(% o

f co

ntr

ol)

Uncoated Co-ferrite NP

Imidazol-Co-ferrite NP

Hydroxy-Co-ferrite NP

Methoxy-Co-Ferrite NP

Breast cancer cells

Healthy cellsb)

a)

Pristine CoFe3O4

1 CoFe3O4

2 CoFe3O4

3 CoFe3O4

Pristine CoFe3O4

1 CoFe3O4

2 CoFe3O4

3 CoFe3O4

82 Applications

488 nm. Consequently, this dye is often used in oxygen radical detection and quantification via

fluorescence microscopy [121]. It is important to note that a certain amount of oxygen radical

generation occurs inside the both cancerous and healthy cells under normal circumstances. Therefore,

the amount of fluorescence of the cells must be measured with and without nanoparticles. To this end,

Figure 4.19b and Figure 4.19c have been normalized in the vertical axis as the percentage of

fluorescence increment compared to the amount of fluorescence exhibited by cells that were not

exposed to any CoFe3O4 nanoparticles. Moving on, Figure 4.19b and Figure 4.19c are proof that the

particles are present inside the cells, as an increase in oxygen radicals can only be due to the presence

of the nanoparticles within the cells membrane. In the same figures a comparison between non-

irradiated and radiated samples is shown. Irradiation of the cells was performed with x-rays and

consisted of one dosage of 1 Gy, which corresponds to dosage a human patient would get. After

irradiation the nanoparticles surface is exposed and the generation of oxygen radicals is dramatically

increased in the case of cancer cells (Figure 4.19b). However, in the case of healthy cells there is no

impact even after irradiation (Figure 4.19c). The dramatic increase of radicals in the cancer cells arises

from their higher metabolic rate (as compared to healthy cells), which speeds up the occurrence of the

Fenton reaction eventually resulting in their demise within a shorter timeframe.

Figure 4.19.: a) Fenton reaction of iron and cobalt. b) Comparison of oxygen radical generation before and after

radiation in cancer cells exposed to pristine and functionalized CoFe3O4 nanoparticles. c) Comparison of oxygen

radical generation before and after radiation in healthy cells exposed to pristine and functionalized CoFe3O4

nanoparticles.

In summary, we have presented a non-toxic pathway for the introduction of normally toxic

nanoparticle species into the cytoplasm of cells. The toxicity of this nanoparticles can later be

reactivated by irradiation of the cells. After which, the impact of this toxicity is higher for cancerous

Unco

ate

Co-f

errite

NP

Imid

azol-C

o-fer

rite

NP

Hyd

roxy

-Co-f

errit

e NP

Met

hoxy-C

o-fer

rite

NP

0

100

200

300

Incre

ase o

f fl

uo

rescen

ce

(% o

f co

ntr

ol)

non-irradiated

irradiated

Unco

ated

Co-f

errit

e NP

Imid

azol-C

o-fer

rite

NP

Hyd

roxy

-Co-f

errit

e NP

Met

hoxy C

o-fer

rite

NP

0

50

100

150

200

250

Incre

ase o

f fl

uo

rescen

ce

(% o

f co

ntr

ol)

non-irradiated

irradiated

Unco

ate

Co-f

errite

NP

Imid

azol-C

o-fer

rite

NP

Hyd

roxy

-Co-f

errit

e NP

Met

hoxy-C

o-fer

rite

NP

0

100

200

300

Incre

ase o

f fl

uo

rescen

ce

(% o

f co

ntr

ol)

non-irradiated

irradiated

Unco

ate

Co-f

errite

NP

Imid

azol-C

o-fer

rite

NP

Hyd

roxy

-Co-f

errit

e NP

Met

hoxy-C

o-fer

rite

NP

0

100

200

300

Incre

ase o

f fl

uo

rescen

ce

(% o

f co

ntr

ol)

non-irradiated

irradiated

Unco

ated

Co-f

errit

e NP

Imid

azol-C

o-fer

rite

NP

Hyd

roxy

-Co-f

errit

e NP

Met

hoxy C

o-fer

rite

NP

0

50

100

150

200

250

Incre

ase o

f fl

uo

rescen

ce

(% o

f co

ntr

ol)

non-irradiated

irradiated

Unco

ated

Co-f

errit

e NP

Imid

azol-C

o-fer

rite

NP

Hyd

roxy

-Co-f

errit

e NP

Met

hoxy C

o-fer

rite

NP

0

50

100

150

200

250

Incre

ase o

f fl

uo

rescen

ce

(% o

f co

ntr

ol)

non-irradiated

irradiated

Breast cancer cells Healthy cellsc)b)

Pristine 1 2 3

Co2+ + H2O2 Co3+ + OH– + OH•

Pristine 1 2 3

a) Fe2+ + H2O2 Fe3+ + OH– + OH•

Applications 83

cells as it is to healthy cells. This contrast in toxicity is possible as the mechanism exploits the higher

metabolism of the cancerous cells which results higher cell mortality within a certain timeframe as

compared to healthy cells. Therefore, providing preferential targeting of cancer cells. In addition to

that, the same pathway also provides outstanding nanoparticle stabilization under water at neutral pH,

as well as providing a positive charge at the nanoparticle surface which is highly beneficial for

nanoparticle intake into cells. In addition to all this, this highly positively charged particles are also

being studied for siRNA delivery into cells with already some initial promising results. We have also

indentified that this approach might have further potential in developing new MRI contrast agents.

4.6. Magnetic water cleaning

The surface of magnetic particles (metal oxide based) can be fine tuned to possess a variety of

characteristics as it has been demonstrated along the course of this thesis. Understandably, having a

magnetic core allows for magnetic extraction of nanoparticles from solution. Based on this, we have

devised to employ such particles for decontamination of water sources acting as a sort of magnetic

washing agents (Figure 4.20a). In such concept, as the nanoparticles are dispersed in the water, their

finely tuned shell will selectively attract a specific contaminant or a variety of contaminants. Once the

particles are loaded with a pollutant, they are removed via magnetic means removing the contaminants

along with them. Even more, once extracted, the particles could potentially be washed and reutilized.

The washing and reutilization is dependent on the nature of the attraction force between the

nanoparticle shell and the extracted pollutants. Weak interactions between the shell and the pollutant

will render the particles washable and reusable, whereas, strong interactions would not as removal of

the pollutant from the shell would be difficult. Two potential applications have been currently

identified. Lipophilic nanoparticles for removal of oil from water (Figure 4.20b) and ion caging

nanoparticles for removal of metal ions from water (Figure 4.20c).

Figure 4.21 showcases the first promising results with magnetic nanoparticles for oil removal. These

measurements were performed by Marco Sarcletti and Tobias Luchs. The experiment, consisted in

introducing a certain amount of the lipophilic nanoparticles into a water-hydrocarbon mixture.

Namely, n-heptane, isooctane and n-cyclohexane. Upon addition of the nanoparticles, the mixture was

vigorously shaken. Afterwards, the nanoparticles were removed with a magnet and redispersed in pure

toluene. After redispersion in toluene, the particles were magnetically removed again. The remaining

toluene was analyzed via gas chromatography and mass spectroscopy (GC-MS). This allowed to

determine the exact amount of the magnetically extracted hydrocarbons from the water mixture. The

obtained extraction rates varied from hydrocarbon to hydrocarbon, heptane being the most efficiently

extracted hydrocarbon, with extraction rates of four times the mass of the nanoparticles.

84 Applications

Furthermore, the simplicity of functionalization process with phosphonic acids, potentially enables the

mass production of the core shell magnetic particles. In addition, the high affinity of the phosphonic

acid to the metal oxides provides an effective, efficient and robust modification of the nanoparticles.

The latter is especially true, when compared to other popular SAM anchor groups such as carboxylic

acids and catechols.

Figure 4.20.: Pollutant extraction scheme from a liquid via functionalized magnetic nanoparticles.

Figure 4.21.: Extracted hydrocarbon weight vs. nanoparticle mass.

Polluted water Mixing of

cleaning agent

Gathering of pollutants

via magnetic field

Removal of pollutants

Ion removal agentOil removal agent

Magnetic core with

lipophilic shell

Adsorbed

hydrocarbon shell

a)

b) c)

Magnetic core with

ion caging shell

Washing of cleaning agent and reuse

Trapped ion

Magnet

0

5

10

15

20

25

30

35

40

45

50

Weig

ht (m

g)

particles weight

hydrocarbon weight

Applications 85


By employing a variety of simple schemes, the potential applications of organic-inorganic hybrid NSs

was demonstrated. A description of the applications and projects that have been developed over the

course of this work were described. Being of primordial importance to solution processing, the

applications regarding nanoparticle dispersibility were discussed first. Essentially, this consisted on

adequately matching the organic shell of the NSs to the dispersion media. Examples regarding the

dispersion of NSs in water, lower alcohols as well as non polar media was addressed. More complex

dispersion schemes which involved a two level organic layer (double shell) which allowed for

dispersion of NSs in orthogonal media were also described. This double shell systems consisted of

small molecules double shells as well as polymeric double shells. Real examples of steric, electrostatic

and electrosteric stabilization mechanisms were involved along this chapter.

A de facto application of solution processing is thin-films. An example concerning the fabrication of

dielectric thin films from NSs dispersions via spin coating was described. Thicker films of NSs were

also fabricated via spray coating on a variety of surfaces. These semi transparent films rendered the

surfaces extremely hydrophobic, this being is an interesting effect in terms of coating applications.

More sophisticated solution based thin film fabrication techniques were also detailed, in such, the NSs

themselves self-assembled into specific configurations. One technique exploited covalent reactions

between nanoparticles and a 2D patterned surface, whereas another technique relied on orthogonal

properties as the main driving force.

Other interesting hybrid materials were also described, such as polymer composites in which the NSs

themselves formed part of the polymer matrix. The latter was possible by grafting SAMs with

polymerizable head groups on the NSs surface. The polymerizable nanoparticles act as a cross linker

by providing a 3D branching point for polymer chain formation. This potentially results in stronger

polymer materials due to the increased degree of cross linking.

Furthermore, the toxicity of cobalt-iron oxide nanoparticles was mitigated by covering the

nanoparticles with a bio-compatible organic shell. The latter allowed for their employment in a

nanooncology application. Furthermore, this same particles are also being studied for siRNA delivery

into cells, with already some initial promising results. This also opened the way for their potential

employment as a MRI contrast agent as well, however to this date this has not been investigated.

Finally, a proof of concept application were magnetic nanoparticles can be employed as magnetic

cleaning agents for contaminants present in water was also described.

87

5. Conclusion and outlook

This thesis serves as a comprehensible guideline for solution processing and surface functionalization

of metal oxide nanostructures with organic SAM molecules. In addition, it also serves to demonstrate

the vast assortment of applications enabled by the orderly and controlled incorporation of organic

chemistry into inorganic components. In the first part, the theoretical background regarding

nanostructures, nanostructured materials, self-assembled monolayers and the solution processing of

nanostructures were addressed. This serves as a basis for the clear comprehension of this work.

The second part describes a step by step experimental and characterization procedure. The first step

consisted in analyzing the starting metal oxide material. The dry nanopowder materials were assessed

via a non-destructive technique (FTIR-ATR). This allowed to detect the existence of organic

components and other impurities that may have been present in the starting materials. Naturally, after

functionalization, a FTIR-ATR characterization was performed as well. This allowed to easily take a

glance into the organic composition of the functionalized 0D and 1D NSs. In some instances, the

impurities were removed after functionalization, leading to the correct functionalization of the surface.

However, in other situations the impurities were not removed, this resulted in poorly controlled NSs

functionalization. Consistently, in all cases in which the material was pre-characterized as "pure" the

functionalization was successful. Therefore, FTIR-ATR was demonstrated to be a valuable tool to

recognize commercial 0D and 1D metal oxide NSs with good functionalization potential. It also

proved valuable for the identification of specific chemical groups after functionalization of 0D and 1D

NSs. Furthermore, it allowed to easily detect potential anomalies after functionalization, such as NSs

etching or multilayer formation.

TGA was successfully employed for the quantification of the adsorbed SAM molecules at the surface

of the 0D and 1D NSs. This allowed for the determination of reaction conditions which resulted in a

fully saturated surface. In combination with a size distribution obtained by DLS, the grafting density

of the SAM molecules was calculated. The grafting density was in good agreement with literature and

expected values. When obtaining reasonable values, it was possible to fully discard functionalization

side effects, such as etching or multilayer formation. On the other hand, if the calculated grafting

density values were inconsistent, the process/material was reevaluated/discarded.

Formation of mixed SAMs on 0D NSs involving a stoichiometric variation of the SAM molecules was

demonstrated. The SAM molecules employed were of a hydrophobic and hydrophilic character. This

88 Conclusion and outlook

allowed to create measurable contrast in terms of surface energy. The formation of mixed SAMs was

confirmed by the formation of nanoparticle films onto a flat surface via spray coating. SCA

measurements with different liquids allowed for the calculation of the surface energy of the films. The

surface energy of the films varied from 0.45 to 67 mN/m. The films behavior gradually varied from

superhydrophobic to superhydrophilic. FTIR-ATR spectra of the nanopowders confirmed the presence

of the mixed SAMs at the surface. Interestingly, the mixed monolayers had a negative impact on the

dispersibility of the nanoparticles in solution. The fully hydrophobic nanoparticles had a zeta potential

of -50 mV which rendered them highly dispersible in 2-propanol. Whereas, the fully hydrophilic

particles exhibited a zeta potential of +50 mV making them highly dispersible in 2-propanol as well.

This high zeta potentials are believed to originate from the dipole moments of the SAM molecules.

However, mixed monolayers exhibited zeta potentials of decreasing magnitudes, reaching values near

0 mV when a 1:1 ratio of the SAM molecules was employed. The latter rendered the particles

completely unstable in 2-propanol. This effect on the zeta potential was attributed to the difference in

dipole moments of the molecules, effectively cancelling each other when a 1:1 proportion was used.

Therefore, hindering the particles solution processability when employing specific SAM ratios.

The functionalization of diverse metal oxide NSs was demonstrated with carboxylic, phosphonic acids

and catechols as anchor groups of the SAM molecules. Consistently, the phosphonic acid molecules

were the most effective, efficient and robust when compared to the other anchor groups. Carboxylic

acids and catechols were also effective, but not as efficient or robust as phosphonic acids. Therefore,

the majority of this work is based on phosphonic acid molecules.

The final chapter focuses on the applications derived from the functionalization of the metal oxide

NSs. The applications encompassed different fields, e.g. ecological, electronics, biology, polymers,

coatings as well as bottom up fabrication approaches. All of which, were made possible by the

incorporation of phosphonic acid SAMs onto 0D and 1D NSs. In addition, the potential of having

access to precise fabrication and tuning of core-shell hybrid materials via self-assembly, was

demonstrated. Ultimately, the simplicity of the process necessary for the modification of the NSs,

makes it feasible for this work to materialize into the real world, rather than remain strictly as a

scientific essay.

89

6. Characterization and experimental procedures

6.1. SCA

Static contact angle (SCA). SCA measurements were investigated by the sessile drop method

utilizing DI water, formamide and diiodomethane (1.0 μL) as probe liquids (Dataphysics OCA, Data

Physics Instruments GmbH, Germany).

6.2. DLS

Dynamic light scattering. Nanoparticle size distribution was obtained by measuring a 0.2 wt. %

stable dispersion of nanoparticles in an appropriate solvent (Zetasizer Nano, Malvern, U. K.). Ideally,

before measurement particles were passed through a 0.8 µm membrane filter. However, when a filter

was not employed most of the times the measurements were the same with or without filtering.

Zeta potential. Nanoparticles dispersed in 2-Propanol or DI water (0.2 wt. %) were placed inside a

folded capillary cell (DTS1070, Malvern, U. K.) for carrying out zeta potential measurements

(Zetasizer Nano, Malvern, U. K.). Zeta potential values were determined by measuring the

electrophoretic mobility of the nanoparticles by employing laser Doppler anemometry technique.

In both DLS and zeta potential, depending on the material and solvent the concentration sometimes

needed to be slightly adjusted. The concentration adjustment was made until a signal of 100-300

kilocounts per second (Kcps) was obtained by the Zetasizer Nano. From experience, this is the "sweet

spot" between too much or to less concentrated.

6.3. FTIR-ATR

Fourier transform infrared spectroscopy attenuated total reflection (FTIR ATR). FTIR ATR

measurements of the nanoparticle dry powder were obtained (IR Prestige-21, Shimadzu, Japan)

utilizing an ATR setup with a Diamond/ZnSe crystal plate (MIRacle ATR, Pike Technologies,

U.S.A.). Transmission spectra were collected at a resolution of 8 cm-1 (64 scans) by clamping dry

nanoparticle powder to the ATR crystal plate. The nanopowder is required to be solvent free, it was

dried by heat close to the boiling point of the solvent or by a combination of vacuum and heat.

90 Characterization and experimental procedures

6.4. TGA

Thermogravimetric analysis (TGA). TGA of the nanoparticle dry powders were carried out under a

N2 atmosphere at a heating rate of 10 °C/min (Q500, TA Instruments, U.S.A.) or (TG 209 F1 Libra,

Netzsch, Germany)

6.5. Electrical

Capacitors and TFTs were fabricated on silicon wafers with 100 nm thermally grown oxide and

flexible TFTs onto polyethylene naphthalene films (PEN, DuPont Teijin Films, U.K.) with a thickness

of 125 µm. Metal electrodes and organic semiconductor were thermally evaporated under vacuum

utilizing a shadow mask for patterning. After evaporation of the aluminum capacitor or gate

electrodes, the substrate was treated with a 3 min oxygen plasma treatment at 200 W at pressure of 0.2

mbar (Pico, Diener electronic GmbH, Germany). The dielectric layers were spin coated (500 rpm pre-

spin and 2000 rpm final speed) from (0.6 wt-%) solutions in water and from 2-propanol in the case of

AlOx to avoid etching of the particles. All electrical characterizations were performed in ambient air

(B1500A, Agilent, U.S.A.).

6.6. Spray coating

For the spray coating of nanoparticle films. Nanoparticle films were manually spray coated onto

Si/SiO2 wafers with a 100 nm thermal oxide layer. Before deposition of the films, the wafers were

cleaned with at least a 3 min oxygen plasma treatment at 200 W at pressure of 0.2 mbar (Pico, Diener

electronic GmbH, Germany). Substrate was heated up to temperature close to the boiling point of the

solvent employed during the spray coating process. The spray coating process was usually performed

from 0.2 wt % solutions of NP. However, the concentration of the NP solutions was found to be a

rather arbitrary parameter. Therefore, the formation of the films was carried out until the film was

optically evident. Afterwards, the film morphology and coverage was studied by AFM to ensure

complete coverage of the original substrate surface.

6.7. Materials

The AlOx, ITO and CeO2 nanoparticles employed were purchased from Sigma Aldrich

(702129, 70460 and 643009, U.S.A.).

The TiO2 and Fe3O4 nanoparticles employed were purchased from PlasmaChem GmbH (PL-

TiO-NO and PL-A-Fe3O4, Germany).

The 30 nm TiO2 nanoparticles were specially ordered from Nanograde AG (Switzerland).

The ZnO nano rods were purchased from Sigma Aldrich (U.S.A.) and employed as received.

Characterization and experimental procedures 91

The Fe3O4 and CoFe3O4 nanoparticles employed for nanooncology, were synthesized by Prof.

Kryschi group.

Phosphonic acid molecules were purchased from SiKEMIA (France), Sigma Aldrich (U.S.A.),

PCI Synthesis (U.S.A.) and employed as received.

Carboxylic acid molecules were purchased from Sigma Aldrich (U.S.A.) and employed as

received.

Catechol molecules were purchased from Sigma Aldrich (U.S.A.) or synthesized by Prof.

Hirsch group. The molecules were employed as received.

Polymers were purchased from Sigma Aldrich (U.S.A.) or Polymer Source (Canada). The

polymers were employed as received.

Any other molecule not available commercially displayed during this work was synthesized

by Prof. Hirsch group.

6.8. Functionalization procedures

6.8.3. AlOx (Sigma A.), ITO (Sigma A.) and TiO2 (Nanograde 30 nm)

The nanoparticles originally come dispersed in a concentrated dispersion in 2-propanol. The

dispersions were diluted with 2-propanol into 0.2 wt. % dispersions. 10 ml (0.2 wt %) of nanoparticles

dispersed in 2-propanol were used for functionalization. Afterwards, 3 ml of a 10 mM solution of the

desired functional molecule dissolved in methanol, ethanol or 2-propanol (depending on molecule

solubility) was added. The dispersion was then sonicated for 30 min, yielding a slightly opaque, stable

or unstable dispersion of functionalized particles.

In order to remove the unreacted excess ligands, the particles were then centrifuged at (10K-14K

RPM, 10-20 min) and a clear supernatant was removed, the particles were then redispersed in a

washing solvent resulting again in a slightly opaque, stable or unstable dispersion. The washing

procedure was repeated at least two more times. After washing the particles, they were dried overnight

at room temperature under the negative pressure of a chemical hood. Alternatively, they were

immediately redispersed in the solvent of choice.

6.8.4. CeO2

The nanoparticles originally come dispersed in a concentrated dispersion in water. The dispersions

were diluted with DI-water into 0.2 wt. % dispersions. 10 ml (0.2 wt %) of nanoparticles dispersed in

2-propanol were used for functionalization. Afterwards, 3 ml of a 10 mM solution of the desired

functional molecule dissolved in methanol, ethanol or 2-propanol (depending on molecule solubility)

was added. The dispersion was then sonicated for 30 min, yielding a slightly opaque, stable or unstable

dispersion of functionalized particles.


In order to remove the unreacted excess ligands, the particles were then centrifuged at (10K-14K

RPM, 10-20 min) and a clear supernatant was removed, the particles were then redispersed in washing

solvent resulting again in a slightly opaque, stable or unstable dispersion. The washing procedure was

repeated at least two more times. After washing the particles, they were dried overnight at room

temperature under the negative pressure of a chemical hood. Alternatively, they were immediately

redispersed in the solvent of choice.

6.8.5. Fe3O4 (Plasmachem ~10 nm)

The Fe3O4 nanoparticles originally come dispersed as a 3 wt. % dispersion in water. The dispersion

was diluted with DI water in order to obtain a 0.3 wt. % dispersion. 5 ml of the diluted dispersion were

employed for functionalization. The diluted nanoparticles form a transparent whisky colored

dispersion. At this point, 3 ml of a 40 mM solution of the desired functional molecule dissolved in

methanol, ethanol or 2-propanol (depending on molecule solubility) was added. Afterwards, the

solution was sonicated for 30 min. After sonication, the impact of the functional molecules on the

dispersibility of the nanoparticles becomes visually evident, the effect varies depending on the nature

of the ligand employed. Excess unreacted molecules are still present at this point. The washing

procedure to remove excess ligands varies depending on whether the dispersed nanoparticles are

highly stable or not at this point. To remove the excess molecules from poorly stable dispersions, the

dispersion is centrifuged (10K-14K RPM, 10-20 min) until the nanoparticle powder is isolated at the

bottom of the container. The supernatant is removed and fresh solvent (methanol, ethanol or 2-

propanol) is added to the nanoparticles. The nanoparticles are then redispersed via sonication and

centrifuged again to add fresh solvent. This procedure was repeated at least 2 times for all

experiments.

To remove the excess molecules from highly stable dispersions were centrifugation does not isolate

the nanoparticles. The dispersions were dried using a rotavapor (@20 mbar 50 °C) to remove the DI

water employed during functionalization and then redispersed in 5ml of ethanol. After redispersion in

ethanol, the nanoparticles may or may not form a transparent whisky colored stable dispersion

depending on the ligand employed. If the nanoparticles formed again a highly stable dispersion,

heptane was added to the solutions until a slightly turbid solution was formed, at this point the

particles can be centrifuged and the supernatant was removed. Then the nanoparticle powder was

redispersed in fresh solvent. This procedure was repeated at least 2 times for all experiments.

After washing the particles, they were dried overnight at room temperature under the negative pressure

of a chemical hood. Alternatively, they were immediately redispersed in the solvent of choice.

Heptane (or an anti-solvent) is required to reduce the solubility of the particles to allow for

centrifugation of the particles. Otherwise the particles are so stable that a conventional centrifuge

Characterization and experimental procedures 93

cannot cause the particles to precipitate. Before the addition of heptane removal of all DI water is

required since heptane and DI water are not miscible. DI water was used as the starting solvent for

functionalization since the pristine nanoparticles showed a considerably higher degree of

dispersibility in water than they did in alcohols.

6.8.6. TiO2 (Plasmachem 8 nm)

15 mg of the nanoparticles were immersed into 10 ml of DI water. The dispersions were sonicated

until the nanoparticle powder appeared to be thoroughly dispersed with no visual indication of major

agglomerates being present. The nanoparticles form a transparent almost colorless dispersion. At this

point, 3 ml of a 40 mM solution of the desired functional molecule dissolved in methanol, ethanol or 2-

propanol (depending on molecule solubility) was added. Afterwards, the solution was sonicated for 30

min. After sonication, the impact of the functional molecules on the dispersibility of the nanoparticles

becomes visually evident, the effect varies depending on the nature of the ligand employed. Excess

unreacted molecules are still present at this point. The washing procedure to remove excess ligands

varies depending on whether the dispersed nanoparticles are highly stable or not at this point. To

remove the excess molecules from poorly stable dispersions, the dispersion is centrifuged (10K-14K

RPM, 10-20 min) until the nanoparticle powder is isolated at the bottom of the container. The

supernatant is removed and fresh solvent (methanol, ethanol or 2-propanol) is added to the

nanoparticles. The nanoparticles are then redispersed via sonication and centrifuged again to add fresh

solvent. This procedure was repeated at least 2 times for all experiments.

To remove the excess molecules from highly stable dispersions were centrifugation does not isolate

the nanoparticles. The dispersions were dried using a rotavapor (@20 mbar 50 °C) to remove the DI

water employed during functionalization and then redispersed in 5 ml of ethanol. After redispersion in

ethanol, the nanoparticles may or may not form a transparent stable dispersion depending on the ligand

employed. If the nanoparticles formed again a highly stable dispersion, heptane was added to the

solutions until a slightly turbid solution was formed, at this point the particles can be centrifuged and

the supernatant was removed. Then the nanoparticle powder was redispersed in fresh solvent. This

procedure was repeated at least two times for all experiments.


of a chemical hood. Alternatively, they were immediately redispersed in the solvent of choice.








6.8.7. Fe3O4 and CoFe3O4 (Prof. Kryschi)

Both Fe3O4 and CoFe3O4 nanoparticles were obtained as a dry powder and functionalized using the

same procedure. It was found that crushing the nanoparticles prior to dispersion in water greatly

speeds up the process. Therefore, using a mortar and pestle the nanoparticles were crushed until a

small powder was obtained. Afterwards, 15 mg of the nanoparticles were immersed into 10 ml of DI

water. The dispersions were sonicated until the nanoparticle powder appeared to be thoroughly

dispersed with no visual indication of major agglomerates being present. At this point, 3 ml of a 40

mM solution of the desired functional molecule dissolved in MeOH for Imidazolium-PA or in IPA for

glycol-PAs was added. Afterwards, the solution was sonicated for 30 min. After sonication, the impact

of the functional molecules on the dispersibility of the nanoparticles becomes visually evident, the

dispersions are now fully transparent whisky colored solutions. Any remaining non-dispersed major

agglomerates were removed at this point by extracting the well dispersed nanoparticles with a pipette

into a new flask. However, excess unreacted molecules are still present at this point. To remove the

excess molecules the dispersions were dried using a rotavapor (@20 mbar 50 °C) and then redispersed

in 5 ml of ethanol forming again a stable whisky like colored dispersion. Heptane was added to the

solutions until a slightly turbid solution was formed, at this point the particles where centrifuged and

the supernatant was removed. Then the nanoparticle powder was redispersed in ethanol again.


of a chemical hood. Finally, they were redispersed as a dry powder in DI water in the desired

concentrations.







95

7. Appendix

Figure 7.1.: a) Octadecyl phosphonic acid (C18-PA). b) Van der Waals model of C18-PA. c) Solvent accessible

area of C18-PA as calculated by Chemdraw. d) Hybrid model employed in the 3D models thorough this work.

C18-PA

solvent accessible area

C18-PA

van der Waals radius

C18-PA

a) b)

C18-PA

hybrid

c) d)

96 Appendix

Figure 7.2.: FTIR-ATR spectra of several phosphonic acid molecules.

Figure 7.3.: Continuation of Figure 7.2. FTIR-ATR spectra of several phosphonic acid molecules.

4000 3800 3600 3400 3200 3000 2800 2600 2400 2200 2000 1800 1600 1400 1200 1000 800 600 400

Wavenumber (1/cm)

T (

%)

4000 3800 3600 3400 3200 3000 2800 2600 2400 2200 2000 1800 1600 1400 1200 1000 800 600 400

Wavenumber (1/cm)

T (

%)

Appendix 97

Figure 7.4.: (Top) 10x10 µm AFM image of a spray coated nanoparticle film. (Bottom) 3D render of the same

AFM image. The surface roughness of the measured film was 100 nm (RMS).

https://www.youtube.com/watch?v=wotvGJFQFLw

Figure 7.5.: Hyperlink to video of a SCA measurement of a super hydrophobic film.

https://www.youtube.com/watch?v=N_OKvDxOWtQ

Figure 7.6.: Hyperlink to video of a super hydrophobic coating on a piece of cardboard.

https://www.youtube.com/watch?v=wotvGJFQFLw

https://www.youtube.com/watch?v=N_OKvDxOWtQ

98 Appendix

Figure 7.7.: FTIR-ATR spectra of pristine and CH3(OC2H4)3-PA functionalized nanoparticles. The FTIR-ATR

spectrum of the ITO nanoparticles was not possible to obtain due to the lack of transparency of ITO in the

infrared region.

3800 3600 3400 3200 3000 2800 2600

T (

%)

Wavenumber (1/cm)

Fe3O

4 (CH

3(OC

2H

4)3-PA

Fe3O

4 Pristine

2000 1800 1600 1400 1200 1000 800

T (

%)

Wavenumber (1/cm)

Fe3O

4 (CH

3(OC

2H

4)3-PA

Fe3O

4 Pristine

3800 3600 3400 3200 3000 2800 2600

T (

%)

Wavenumber (1/cm)

TiO2 (CH

3(OC

2H

4)3-PA

TiO2 Pristine

2000 1800 1600 1400 1200 1000 800

T (

%)

Wavenumber (1/cm)

TiO2 (CH

3(OC

2H

4)3-PA

TiO2 Pristine

3800 3600 3400 3200 3000 2800 2600

T (

%)

Wavenumber (1/cm)

CeO2 (CH

3(OC

2H

4)3-PA

CeO2 Pristine

2000 1800 1600 1400 1200 1000 800

T (

%)

Wavenumber (1/cm)

CeO2 (CH

3(OC

2H

4)3-PA

CeO2 Pristine

3800 3600 3400 3200 3000 2800 2600

AlOx (CH

3(OC

2H

4)3-PA

T (

%)

Wavenumber (1/cm)

AlOx Pristine

2000 1800 1600 1400 1200 1000 800

AlOx (CH

3(OC

2H

4)3-PA

T (

%)

Wavenumber (1/cm)

AlOx Pristine

Appendix 99

Figure 7.8.: FTIR-ATR spectra (color coded) of Azide-PA and Alkyne-PA AlOx reactive nanoparticles.

Figure 7.9.: FTIR-ATR spectra (color coded) of pristine and functionalized Fe3O4 nanoparticles. a) Spectra of

Fe-Fe nanoparticles. b) Spectra of Fe-Co nanoparticles.

3800 3600 3400 3200 3000 2800 2600

T (

%)

Wavenumber (1/cm)

Alkyne-PA AlOx

Azide-PA AlO

x

2200 2000 1800 1600 1400 1200 1000 800

Wavenumber (1/cm)

4000 3800 3600 3400 3200 3000 2800 2600

Wavenumber (1/cm)

T (

%)

Fe-Co CH3(OC

2H

4)

3-PA

Fe-Co H(OC2H

4)

3-PA

Fe-Co Imidazolium-PA

Fe-Co Pristine

2000 1800 1600 1400 1200 1000 800

Wavenumber (1/cm)

Fe-Co CH3(OC

2H

4)

3-PA

Fe-Co H(OC2H

4)

3-PA

Fe-Co Imidazolium-PA

Fe-Co Pristine

2000 1800 1600 1400 1200 1000 800

Wavenumber (1/cm)

Fe-Fe CH3(OC

2H

4)

3-PA

Fe-Fe H(OC2H

4)

3-PA

Fe-Fe Imidazolium-PA

Fe-Fe Pristine

4000 3800 3600 3400 3200 3000 2800 2600

Wavenumber (1/cm)

Fe-Fe CH3(OC

2H

4)

3-PA

Fe-Fe H(OC2H

4)

3-PA

T (

%)

Fe-Fe Imidazolium-PA

Fe-Fe Pristinea)

b)

100 Appendix

Figure 7.10.: Biocompatibility of pristine and functionalized Fe3O4 nanoparticles in function of dosage. a)

Biocompatibility results on breast cancer cells. b) Biocompatibility results on healthy cells.

Breast cancer cells

Healthy cellsb)

a)

Pristine Fe3O4

1 Fe3O4

2 Fe3O4

3 Fe3O4

Pristine Fe3O4

1 Fe3O4

2 Fe3O4

3 Fe3O4

1 5 100

50

100

150


Cell v

iab

ilit

y

(% o

f co

ntr

ol)

Uncoated ferrite NP

Imidazol-ferrite NP

Hydroxy-ferrite NP

Methoxy-ferrite NP

1 5 100

50

100


Cell v

iab

ilit

y

(% o

f co

ntr

ol)

Uncoated ferrite NP

Imidazol-ferrite NP

Hydroxy-ferrite NP

Methoxy-ferrite NP

101

8. List of Figures

Figure 2.1.: Representation of nanostructures. 0D (red sphere), 1D (green cylinder), 2D (blue

rectangular parallelepiped) and in 3D (cylinder 3D matrix). ............................................. 3

Figure 2.2.: NSs classified on basis of their dimensionality as suggested by V. Pokropivny in 2007

[21]. Reprinted from [21] with permission from Elsevier.................................................. 4

Figure 2.3.: Surface area to volume ratio. The increment of surface area vs. volume is graphically

represented from a) lowest ratio, to d) highest ratio........................................................... 5

Figure 2.4.: a) Schematic process of the formation of a C18-PA SAM on a 2D NS. b) Side view

depiction of SAM tilt angle. c) Top view illustration of the crystalline and amorphous

domains that may be present after the SAM formation. ..................................................... 8

Figure 2.5.: Schematic process of SAM formation of onto a 0D NS. ................................................... 8

Figure 2.6.: Anatomy of a core-shell 0D building block. ...................................................................... 9

Figure 2.7.: Phosphonic acid SAM molecules of increasing length. As the backbone (black) length

increases, so do the van der Waals interactions between them and therefore also the final

ordering of the SAM [26]. ................................................................................................ 10

Figure 2.8.: Phosphonic acid binding mechanism to a metal oxide surface [39]–[43]. ....................... 12

Figure 2.9.: Various phosphonic acid binding modes. M = metal. a) and b) monodentate, c) and d)

bridging bidentate, e) bridging tridentate, f) and g) chelating bidentate, h) chelating

tridentate, i) j) k) l) other viable hydrogen bonding modes. Reprinted from [41] with

permission from the American Chemical Society. ........................................................... 13

Figure 2.10.: Various carboxylic acid binding modes) M = metal. a) electrostatic attraction, b) H-

bonds to bridging oxygen, c) H-bonds to carboxylic oxygen, d) monodentate metal-ester,

e) bidentate bridging, f) bidentate chelating. Adapted from [12] with permission from

Wiley. ............................................................................................................................... 14

Figure 2.11.: Miscellaneous catechol binding modes on titanium oxide surface. EWG = electron

withdrawing group. a) H-bonds, b) monodentate with H-bond, c) bidentate chelating, d)

monodentate with bridging H-bond, e) bidentate bridging. Adapted from [12] with

permission from Wiley. .................................................................................................... 15

Figure 2.12.: Photographs of TiO2 and Fe3O4 nanoparticle dispersions with varying degrees of

stability. a) Unstable TiO2 dispersion during flocculation. b) Non-transparent stable TiO2

dispersion. c) Transparent stable TiO2 dispersion. d) Unstable, already flocculated Fe3O4

dispersion. e) Transparent stable Fe3O4 dispersion. ......................................................... 16

102 List of Figures

Figure 2.13.: Schematic representation of electrostatic stabilization of nanoparticles. a) A

nanoparticle with charged species at its surface. b) Nanoparticles having an equal charge

repel each other avoiding agglomeration. ........................................................................ 17

Figure 2.14.: Schematic representation of the electric double layer (EDL) on a nanoparticle. Red and

blue spheres represent charged species of opposite magnitude. ...................................... 18

Figure 2.15.: Schematic representation of the isoelectric point (IEP) in function of pH. .................... 19

Figure 2.16.: Schematic representation of steric stabilization of nanoparticles. Physically, the

molecules grafted onto the nanoparticle avoid direct nanoparticle collision and

nanoparticle core interaction. Chemically, the molecules provide solvation in the

dispersion media effectively thwarting nanoparticle core interactions. ........................... 20

Figure 2.17.: Nanoparticle surface curvatures. a) and c) are nanoparticle illustrations (up to scale) of

functionalized 5 and 50 nm spherical particles. b) and d) represent a zoomed-in up to

scale nanoparticle surface illustration. It becomes apparent in illustrations b) and d) how

can surface curvature play a critical role on SAM crystallinity and SAM dipole moment

alignment. Also of importance to note, the free space available between the molecules. 22

Figure 2.18.: Schematic representation of a bottom gate TFT ............................................................ 24

Figure 3.1.: Steps for SAM deposition on 2D substrates. a) Plasma treatment of the surface. b)

immersion of the substrate into the SAM solution. c) Schematic of the finalized SAM. 25

Figure 3.2.: Steps for SAM deposition on 0D and 1D NSs. a) Pristine nanostructures dispersed in a

liquid medium. b) The SAM molecule is added and with the aid of sonication it forms a

SAM around the nanostructures surface. c) The final functionalized NSs dispersion (after

washing) with no unbound SAM molecules present in solution. ..................................... 27

Figure 3.3.: FTIR-ATR spectra of several commercial TiO2 nanoparticles. a) Featureless spectrum of

pure TiO2 particles. b) Spectrum of HNO3 stabilized TiO2 particles with signals from the

HNO3 species present c) Spectrum of allegedly pure TiO2 particles containing signals that

are unaccounted for. ......................................................................................................... 28

Figure 3.4.: Color coded FTIR-ATR spectra of several commercial TiO2 nanoparticles after being

functionalized with C16-PA. a) Spectrum of pure TiO2 particles plus the signals from C16-

PA. b) Spectrum of HNO3 stabilized TiO2 particles with the signals from C16-PA. Note

that the HNO3 peaks are no longer present. c) Spectrum of allegedly pure TiO2 particles

functionalized with C16-PA still containing signals that are unaccounted for, plus the

overlapping signals of the C16-PA. d) Chemical structure of C16-PA. ............................. 29

Figure 3.5.: Exemplary FTIR-ATR spectra (color coded) of various pure and C16-PA functionalized

metal oxide NSs. The general trend of a featureless spectrum for pure metal oxides can

be observed. An unmistakable trend is also identifiable after functionalization with C16-

PA in all metal oxides. The molecular structure of C16-PA is shown atop. ..................... 30

List of Figures 103

Figure 3.6.: a) FTIR-ATR spectrum of Fe3O4 nanoparticles functionalized with C16-PA. b) Spectrum

of pristine C16-PA. c) Hypothetical schematic depicting the diverse vibrations of the

phosphonic acid bound to a metal oxide surface. ............................................................. 31

Figure 3.7.: Fe3O4 nanoparticles functionalized with increasing concentration of C16-PA. As the

concentration increases, the valley corresponding to the anchored phosphonic acid (red

square) changes. ............................................................................................................... 32

Figure 3.8.: a) Chemical structure of C16-PA. b) TGA under N2 of AlOx NPs functionalized with

different concentrations of C16-PA. Adapted from [16] with permission from the

American Chemical Society. ............................................................................................ 34

Figure 3.9.: TGA under N2 from 25 to 1100 °C of AlOx NPs functionalized with C16-PA. The inset

shows FTIR-ATR spectra of the nanoparticle powder at different stages of the TGA

measurement. It can be observed that even after exposing the NP powder to 1100 °C for

2 hours the PA band is still present on the nanopowder. The nanopowder had a black

color up to a 1000 °C, past that temperature the powder was white in appearance. ........ 36

Figure 3.10.: FTIR-ATR spectra (color coded) of pristine and several C16-PA functionalized AlOx

nanoparticles. a) Spectrum of pristine AlOx nanoparticles. b) C16-PA functionalized AlOx

nanoparticles. c) C16-PA functionalized AlOx nanoparticles after exposure to TGA until

1100 °C. e) The exposed AlOx nanoparticles to TGA until 1100 °C but re-functionalized

with C16-PA. e) The C16-PA re-functionalized AlOx nanoparticles after being measured

again by TGA until 1100 °C. ........................................................................................... 38

Figure 3.11.: a) Molecular structure of F17C10-PA and H(OC2H4)3-PA. b) SCA measurements with

different liquids and the calculated surface energy of the nanoparticle spray coated films.

Reprinted from [16] with permission from the American Chemical Society. ................. 40

Figure 3.12.: a) Color coded molecular structure of F17C10-PA and H(OC2H4)3-PA. b) FTIR-ATR

spectra of particles functionalized with different ratios of F17C10-PA and H(OC2H4)3-PA.

c) Zeta potential measurements of the nanoparticle dispersions in 2-Propanol. Adapted

from [16] with permission from the American Chemical Society. .................................. 41

Figure 3.13.: a) Zeta potential measurements of the nanoparticle dispersions in 2-Propanol. b)

Photograph of the functionalized nanoparticle dispersions 20 min after re-dispersion by

sonication. Adapted from [16] with permission from the American Chemical Society. . 42

Figure 3.14.: SCA measurements of functionalized AlOx, ZnO and ITO 2D substrates. The substrates

were functionalized and afterwards immersed in pure 2-propanol for different amount of

time. Contact angles were measured again after the immersion. a) SCA of substrates

functionalized with C17-CA. b) SCA of substrates functionalized with F21-CAT. c) SCA

of substrates functionalized with C16-PA. ........................................................................ 44

Figure 3.15.: SCA measurements of spray coated films of functionalized 0D AlOx and ITO and 1D

ZnO NSs. The NSs were washed 1 to 3 times before spray coating. a) Contact angles of

104 List of Figures

spray coated NSs functionalized with C17-CA. b) Contact angles of spray coated NSs

functionalized with C16-PA. ............................................................................................. 46

Figure 3.16.: SCA measurements of SAM exchange between two carboxylic acid SAM molecules on

AlOx and ITO 2D substrates. a) Exchange of a C17-CA SAM with a C6-CA SAM. b)

Exchange of a C6-CA SAM with a C17-CA SAM. ........................................................... 47

Figure 3.17.: SCA measurements of SAM exchange between two phosphonic acid SAM molecules

on AlOx, ITO and ZnO 2D substrates. Attempt to exchange a C11OH-PA SAM with a

C16-PA SAM. ................................................................................................................... 48

Figure 3.18.: SCA measurements of SAM exchange between a phosphonic acid and catechol

molecules on AlOx, ITO and ZnO 2D substrates. a) Exchange of a C11OH-PA SAM with

a F21-CAT SAM. b) Exchange of a F21-CAT SAM with a C11OH-PA SAM................... 49

Figure 3.19.: SCA measurements of SAM exchange between a carboxylic acid and catechol

molecules on ZnO. a) Exchange of a C17-CA SAM with a hexanoate-CAT SAM. b)

Exchange of a hexanoate-CAT SAM with a C17-CA SAM. ............................................ 50

Figure 4.1.: a) Molecular structure of CH3(OC2H4)3-PA employed for nanoparticle functionalization;

b) Photograph of dispersed TiO2 nanoparticles in different media before and c) after

functionalization. d) Graph of DLS measurements of TiO2 and e) Fe3O4 nanoparticles

before and after functionalization. Reprinted from [17] with permission from Wiley. ... 54

Figure 4.2.: Photograph of Fe3O4 nanoparticles functionalized with different molecules (top) and

dispersed in different orthogonal media (left). a) Particles dispersed in n-heptane, b)

particles dispersed in DI water, c) particles dispersed in n-perfluoroheptane. By tuning

the surface of the particles, control on their dispersibility in different media can be

achieved. .......................................................................................................................... 56

Figure 4.3.: Shell by shell stabilization concept. a) Nanoparticle is rendered hydrophobic by

functionalization with C16-PA. b) The hydrophobic particle is rendered hydrophilic due to

the amphiphilic molecules forming a bilayer. c) Example of the amphiphilic molecules

employed for this study. Adapted from [18] with permission from Wiley. ..................... 57

Figure 4.4.: Photograph of TiO2 nanoparticles dispersed in DI-water or toluene. a) Hydrophobic and

hydrophilic components of molecule 7. The red coloring of the TiO2 nanoparticles is due

to the perylene motif. b) Pristine hydrophilic particles dispersed in DI-water. c) C16-PA

functionalized hydrophobic nanoparticles dispersed in toluene. d) Upon addition of the

amphiphile molecule 7 the nanoparticles are now dispersible in the water phase due to

the formation of a bilayer. Adapted from [18] with permission from Wiley. .................. 58

Figure 4.5.: Polymer wrapping of nanoparticles. a) Phosphonic acid molecule with hydrophilic tail

employed to functionalize the surface of the particles. b) PS-b-PEO block copolymer

molecular structure and weights. c) Schematic representation of a nanoparticle wrapped

in an orthogonal block copolymer. ................................................................................... 60

List of Figures 105

Figure 4.6.: Photograph of the solution before and after shaking vigorously. Photograph b) shows the

solution only after shaking, not after sonication. ............................................................. 61

Figure 4.7.: DLS size distribution of CH3(OC2H4)4C4H8-PA nanoparticles before (black) and after

polymer wrapping. ........................................................................................................... 63

Figure 4.8.: a) SEM cross-sections of the spin coated dielectric layers. b) AFM images of the surface

morphology of the spin coated films. Reprinted from [17] with permission from Wiley.65

Figure 4.9.: a) Schematic layout of the fabricated capacitor devices. b) Current density of different

NP dielectric materials of 50x50 µm capacitor devices vs. applied voltage. Reprinted

from [17] with permission from Wiley. ........................................................................... 66

Figure 4.10.: a) Molecular structure of the semiconductor molecule C13-BTBT and schematic layout

of the fabricated OTFTs devices. b) Transfer curves of the OTFTs with different

dielectric materials. c) concave bending of devices during characterization. d) Transfer

characteristics of the devices under different bending modes. Reprinted from [17] with

permission from Wiley. .................................................................................................... 67

Figure 4.11.: Photographs of spray coated hydrophobic coatings on glass and cardboard. a) Coated

and uncoated glass slide for up close transparency comparison. Droplets of different

water based liquids dispensed on the top of coated (c) glass slide and (d) cardboard. .... 69

Figure 4.12.: Schematic representation of the building blocks involved in thin film self-assembly. a)

Chemically patterned 2D substrate. Red represents the reactive sites provided by the

Azide-PA SAM molecule. Purple represents the inert sections of the substrate. the

inertness is provided by the F15C18-PA SAM molecule. b) Reactive Alkyne-PA

functionalized nanoparticle. c) Reactive Azide-PA functionalized particle. ................... 71

Figure 4.13.: a) Schematic representation of the first deposited layer and reaction conditions. b)

Schematic representation of the second deposited layer and reaction conditions. c) AFM

scans of first and second selectively deposited thin films. AFM cross-sectional height

measurement of the first and second layers. .................................................................... 73

Figure 4.14.: SEM images of selectively deposited nanoparticles. ..................................................... 74

Figure 4.15.: a) Molecular structure of block copolymers. The hydrophobic phase is composed of

polystyrene (red) while the hydrophilic phase is composed of polyethylene oxide (blue).

b) AFM image of a phase separated spin coated block copolymer thin film. c) SEM

image of a phase separated spin coated block copolymer thin film with embedded Fe3O4

hydrophilic nanoparticles in the corresponding phase. .................................................... 75

Figure 4.16.: a) Molecular structure of MMA-PA. b) Molecular structure of MMA. c) Molecular

structure of PMMA. d) Color coded FTIR ATR spectrum of AlOx nanoparticles

functionalized with MMA-PA. e) Cylindrical PMMA probe. f) Cylindrical PMMA probe

with unfunctionalized AlOx nanoparticles. g) Cylindrical PMMA probe with MMA-PA

functionalized AlOx nanoparticles. h) Strain test curves of PMMA and PMMA

106 List of Figures

composites. i) Artistical rendition of nanoparticles covalently attached to a polymer

matrix. .............................................................................................................................. 77

Figure 4.17.: a) Molecules used for functionalization of nanoparticles. b) DLS distribution of Fe3O4

pristine and functionalized nanoparticles. c) Photograph of Fe3O4 pristine and

functionalized NP dispersions 12 hours after being redispersed on DI water via

sonication. d) Photograph of CoFe3O4 pristine and functionalized NP dispersions 12

hours after being redispersed on DI water via sonication. ............................................... 80

Figure 4.18.: Biocompatibility of pristine and functionalized CoFe3O4 nanoparticles in function of

dosage. a) Biocompatibility results on breast cancer cells. b) Biocompatibility results on

healthy cells. ..................................................................................................................... 81

Figure 4.19.: a) Fenton reaction of iron and cobalt. b) Comparison of oxygen radical generation

before and after radiation in cancer cells exposed to pristine and functionalized CoFe3O4

nanoparticles. c) Comparison of oxygen radical generation before and after radiation in

healthy cells exposed to pristine and functionalized CoFe3O4 nanoparticles. .................. 82

Figure 4.20.: Pollutant extraction scheme from a liquid via functionalized magnetic nanoparticles. . 84

Figure 4.21.: Extracted hydrocarbon weight vs. nanoparticle mass. ................................................... 84

Figure 7.1.: a) Octadecyl phosphonic acid (C18-PA). b) Van der Waals model of C18-PA. c) Solvent

accessible area of C18-PA as calculated by Chemdraw. d) Hybrid model employed in the

3D models thorough this work. ........................................................................................ 95

Figure 7.2.: FTIR-ATR spectra of several phosphonic acid molecules. .............................................. 96

Figure 7.3.: Continuation of Figure 7.2. FTIR-ATR spectra of several phosphonic acid molecules. . 96

Figure 7.4.: (Top) 10x10 µm AFM image of a spray coated nanoparticle film. (Bottom) 3D render of

the same AFM image. The surface roughness of the measured film was 100 nm (RMS).

.......................................................................................................................................... 97

Figure 7.5.: Hyperlink to video of a SCA measurement of a super hydrophobic film. ....................... 97

Figure 7.6.: Hyperlink to video of a super hydrophobic coating on a piece of cardboard. .................. 97

Figure 7.7.: FTIR-ATR spectra of pristine and CH3(OC2H4)3-PA functionalized nanoparticles. The

FTIR-ATR spectrum of the ITO nanoparticles was not possible to obtain due to the lack

of transparency of ITO in the infrared region. ................................................................. 98

Figure 7.8.: FTIR-ATR spectra (color coded) of Azide-PA and Alkyne-PA AlOx reactive

nanoparticles. ................................................................................................................... 99

Figure 7.9.: FTIR-ATR spectra (color coded) of pristine and functionalized Fe3O4 nanoparticles. a)

Spectra of Fe-Fe nanoparticles. b) Spectra of Fe-Co nanoparticles. ................................ 99

Figure 7.10.: Biocompatibility of pristine and functionalized Fe3O4 nanoparticles in function of

dosage. a) Biocompatibility results on breast cancer cells. b) Biocompatibility results on

healthy cells. ................................................................................................................... 100

List of tables 107

9. List of tables

Table 3.1.: Calculated grafting densities of AlOx NPs functionalized with C16-PA, in accordance with

the mass loss from 400 to 650 °C of Figure 3.8 The SSA was calculated from a DLS

distribution to be 28.05 m2/g. ........................................................................................... 35

Table 4.1.: Zeta potential of Fe3O4 nanoparticles. The hydrophilic CH3(OC2H4)4C4H8-PA

nanoparticles have a strong zeta potential due to the electrosteric stabilization caused by

the functionalization. After the polymer wrapping procedure the nanoparticles are still

stable, yet their zeta potential is near zero. This effect is caused by the steric stabilization

provided by the polymer wrapping of the nanoparticles. ................................................. 63

Table 4.2.: Summary of film properties and electrical characteristics of the films and devices.

Reprinted from [17] with permission from Wiley............................................................ 65

Table 4.3.: Zeta potentials of the Fe-Fe and the Fe-Co nanoparticle dispersions before and after

functionalization with different molecules which portrayed in Figure 4.17. ................... 80

109

10. Abbreviations

AFM atomic force microscopy

BET Brunauer–Emmett–Teller

CA carboxylic acid

CAT catechol

DI water de-ionized water

DLS dynamic light scattering

EDL electric double layer

FTIR-ATR Fourier transform infrared spectroscopy - attenuated total reflectance

GC-MS gas chromatography and mass spectroscopy

MMA methyl methacrylate

MW molecular weight

NP nanoparticle

NS nanostructure

NSM nanostructured material

OTFT organic thin film transistor

PA phosphonic acid

PBS Phosphate-buffered saline

PEN polyethylene naphthalene

PMMA poly(methyl methacrylate)

RMS root mean square

rotavap rotary evaporator

SAM self-assembled monolayer

SAMFET self-assembled monolayer field effect transistor

SCA static contact angle

SSA specific surface area

TFT thin film transistor

TGA thermogravimetric analysis

XRR x-ray reflectivity

111

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121

12. Acknowledgements

First I would like to extend my gratitude towards Prof. Marcus Halik for giving me the opportunity to

be part of his research group. To all the members of the group as well (colleagues and friends) Atefeh

Yousefi Amin, Michael Salinas, Johannes Hirschmann, Sebastien Pequeur, Saeideh Grünler, Zhenxing

Wang, Thomas Schmaltz, Artoem Khassanov, Sebastian Etschel, Johannes Kirschner, Simon

Scheiner, Bastian Gothe, Tobias Rejek, Hyoungwon Park and Judith Wittmann. I would also like to

extend my gratitude to Lukas Zeininger from Prof. Hirsch's group for all the fruitful work we did

together. It was very enjoyable to sit down over coffee or beer and having useful, useless or plain

crazy discussions with you all. Many good and also bad ideas came out of these conversations, yet

they were all very constructive for the basis of my Ph. D. work. I would like to specially thank you all

for having the patience to answer and correct all of my misguided ideas and irrational thinking

regarding chemistry. I hope I was able to contribute to your work as well.

I would like to thank as well Melek Kizaloglu and Stefanie Klein from Prof. Kryschi's group for all the

interesting research regarding iron oxide nanoparticles for bio-applications. It was a very nice project

and I was very happy to make this collaboration.

I also would like to thank all the people from the LSP (Lehrstuhl für Polymerwerkstoffe), in particular

Jenifer Reiser for some TGA measurements and Alfred Frey for helping me fix the evaporator (ELKE)

power supply which was actually not broken, I was just measuring DC instead of AC voltage.

Our office neighbors "the Guldis" in particular Ruben Casillas, thanks for all those free beers at the

office and being cool about me not bringing anything, even though my house was less than a block

away.

To Liping Sun as well, thank you for proof reading my thesis and for your invaluable company during

this time. To all my friends from Erlangen, in particular my flat mates Girish, Rye and Valeria it was

always great fun going out or just staying at home with you guys. Last but not least, I would like thank

all of my family, while all of you are very far away, I knew I could always count with your support

and love which made this work easier to realize.

123

13. Curriculum vitae

Name: Luis Francisco Portilla Berlanga

Contact: [email protected]

Date of birth: March 22 1984

Nationalities: Mexican, Spanish

Place of birth: Torreon, Mexico

Academic and Professional

10/2012 – 12/2015 Friedrich-Alexander Universität Erlangen-Nürnberg (FAU)

Ph. D. in Materials Science

Supervisor: Prof. Dr. Marcus Halik

10/2010 – 02/2012 Universitat de Barcelona (UB)

M. Sc. in Nanoscience and Nanotechnology

Supervisor: Prof. Dr. Anna Vila

07/2007-07/2010 Materials and Technologies (MATECH)

Automation Engineer

10/2002 – 06/2007 Instituto Tecnológico y de Estudios Superiores de Monterrey (ITESM)

B. Sc. in Electronics Engineering

Publications

[1] A. Vilà, A. Gomez, L. Portilla, and J. R. Morante, “Influence of In and Ga additives onto SnO2

inkjet-printed semiconductor,” Thin Solid Films, vol. 553, pp. 118–122, Feb. 2014.

[2] L. Portilla and M. Halik, “Smoothly tunable surface properties of aluminum oxide core-shell

nanoparticles by a mixed-ligand approach.,” ACS Appl. Mater. Interfaces, vol. 6, no. 8, pp.

5977–82, Apr. 2014.

[3] S. H. Etschel, L. Portilla, J. Kirschner, M. Drost, F. Tu, H. Marbach, R. R. Tykwinski, and M.

Halik, “Region-selective Deposition of Core-Shell Nanoparticles for 3 D Hierarchical

Assemblies by the Huisgen 1,3-Dipolar Cycloaddition,” Angew. Chemie Int. Ed., vol.

201501957, no. 32, p. n/a–n/a, Aug. 2015.

[4] L. Portilla, S. H. Etschel, R. R. Tykwinski, and M. Halik, “Green Processing of Metal Oxide

Core-Shell Nanoparticles as Low-Temperature Dielectrics in Organic Thin-Film Transistors,”

Adv. Mater., vol. 27, no. 39, pp. 5950–5954, Oct. 2015.

mailto:[email protected]

124 Curriculum vitae

[5] L. Zeininger, S. Petzi, J. Schönamsgruber, L. Portilla, M. Halik, and A. Hirsch, “Very Facile

Polarity Umpolung and Noncovalent Functionalization of Inorganic Nanoparticles: A Tool Kit

for Supramolecular Materials Chemistry,” Chem. - A Eur. J., vol. 21, no. 40, pp. 14030–14035,

Sep. 2015.

[6] H. Dietrich, S. Scheiner, L. Portilla, D. Zahn, and M. Halik, “Improving the Performance of

Organic Thin-Film Transistors by Ion Doping of Ethylene-Glycol-Based Self-Assembled

Monolayer Hybrid Dielectrics,” Adv. Mater., vol. 27, no. 48, pp. 8023–8027, Dec. 2015.

[7] L. Zeininger, L. Portilla, M. Halik, and A. Hirsch, “Quantitative Determination and

Comparison of the Surface Binding of Phosphonic Acid, Carboxylic Acid, and Catechol

Ligands on TiO 2 Nanoparticles,” Chem. - A Eur. J., Jul. 2016.

[8] J. Kirschner, L. Portilla, J. Will, M. Berlinghof, H. Steinrück, T. Unruh, M. Halik "Organic

Thin Film Memory Devices Based On Tio2-Loaded Ps-Peo Blockcopolymer Dielectrics,"

manuscript in preparation.

[9] S. Klein, M. Kızaloğlu, L. Portilla, L. Distel, M. Halik, C. Kryschi, "Non-toxic water soluble

cobalt ferrite nanoparticles for low dose radiation therapy," manuscript in preparation.

Patents

1. "Kern-Hülle-Partikel," PCT/EP2016/066051, Jul. 2016.

Conference Contributions

1. Technical session at the MRS 2015 Spring Meeting, with the subject: "Fine Tuning of Mixed

Self-Assembled Monolayers Grafted onto 0D and 2D Metal Oxides Nanostructures".

2. Poster session participation at the Electronic Processes in Organic Materials Gordon

Conference 2014, with the subject: “Fine Tuning of Mixed Self-Assembled Monolayers Grafted

onto 0D and 2D Metal Oxides Nanostructures”.

3. Conference paper at the MRS 2013 Spring Meeting, with the title: "Metal Oxides as functional

semiconductors. An Inkjet Approach".

4. Poster session participation at the ITC 2012 8th International Thin-Film Transistor Conference,

with the subject: “SnO2-based TFTs by inkjet”.

functionalization of metal oxide nanostructures via self ... · measured using dynamic light...

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