two-dimensional semiconductors for future optical...

i

Two-dimensional semiconductors for future

optical applications

A thesis submitted in fulfilment of the requirements for the degree of Doctor of

Philosophy

Benjamin John Carey

BEng (Electrical and Electronic Engineering)

School of Engineering

College of Science, Engineering and Health

RMIT University

August 2017

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Abstract of Thesis

Field of two dimensional (2D) materials has experienced rapid development and

received considerable attention in recent years. Although various methods of

producing 2D materials have been presented with ample success across numerous

families of materials and for many applications, there are still several factors

limiting their performance in various fields. The field of 2D materials is mostly

focused on stratified crystals, out of which ultra-thin (a single or few monolayers)

planes can be readily extracted. There are however still many unexplored

materials which would be suitable candidates in their 2D morphologies for future

applications. However, due to their naturally occurring non-stratified crystals,

their synthesis and implementation in 2D form have posed significant challenges

to date.

Access to colloidal suspensions of 2D sheets provides a pragmatic avenue for

numerous applications. The processes for obtaining colloidal suspensions can be

scaled up to the industrial scales with relative ease. These processes can be as

simple as applying ultrasound energy into a suitable solvent containing bulk

layered materials. However, there are still several factors which affect the quality

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of the sheets within these suspensions and hence their suitability for use in various

applications. This issue is highlighted by the fact that several solvents which assist

high yield production can also inhibit potential applications due to surface bound

solvent residues and breakdown products which can be difficult to remove.

Thus, the first objective of this thesis was to investigate the solvent effects within

the ultrasound exfoliation process for obtaining colloidal suspensions of 2D

materials. For this tungsten disulphide (WS2) was used as it is a well-known

material with a high potential for future implementation across numerous fields.

This objective is to attempt alternative strategies for WS2 exfoliation which

provide high yield, while also providing less detrimental effects on the desired

properties. In this PhD research, comparative evaluation of organic solvents used

in grinding assisted ultrasonic exfoliation revealed the importance of the pre-

exfoliation grinding step and that careful consideration of the solvent(s) used in

both grinding and ultrasonication is necessary. It was shown that such alterations

vastly affect the quality of the resultant suspension. Through these considerations

it was found possible to provide comparatively high yields of unobstructed 2D WS2

Although the outcomes of the previous objective provide insight into the solvent

effects on production and application of colloidal suspensions of 2D

semiconductors, the exfoliation method is limited to materials which exist

naturally with a layered (stratified) crystal system. It is not possible to use this

exfoliation process non-stratified materials to produce 2D sheets. Thus, the

following objective was to develop the processes which may be utilized in the

synthesis of 2D semiconductors from non-stratified materials.

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In this PhD work, in order to achieve the synthesis of 2D semiconductors for non-

stratified materials, the self-limiting oxide layer which exists at the interface of

many liquid-metals was utilized. These surface skins are in essence a naturally

occurring 2D material and therefore can offer utilization potential for creating

large area atomically thin coatings. By manipulating the interfacial layer of a

gallium based liquid metal alloy, this PhD research investigates the possibility of

forming suspensions of ultrathin non-stratified semiconductors. The

investigations shown, demonstrated that with relevant chemistry the interfacial

layer could be utilized. A eutectic alloy of gallium, indium, tin and zinc was

repeatedly reacted with a solution of sodium polysulphide to form 2D

semiconducting zinc sulphide (ZnS).

This concept was extended further to the direct deposition of ultrathin metal

oxides. It was shown within this thesis that liquid gallium could be efficiently used

to deposit a 2D layer of gallium oxide. The oxide was then chemically treated in

vapour phase reactions to form 2D semiconducting gallium sulphide (GaS). Using

this concept GaS based optical and electronic devices were developed on the

wafer-scale demonstrating the potential for this concept.

In conclusion, this Ph.D. research resulted in several new advances and novel

synthesis paths in the field of 2D materials and the methods and outcomes

presented herein will serve as launching points for many future investigations into

the synthesis and application of 2D materials.

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Declaration

I certify that except where due acknowledgement has been made, the work is that

of the author alone; the work has not been submitted previously, in whole or in

part, to qualify for any other academic award; the content of the thesis is the result

of work which has been carried out since the official commencement date of the

approved research program; any editorial work, paid or unpaid, carried out by a

third party is acknowledged; and, ethics procedures and guidelines have been

followed.

I acknowledge the support I have received for my research through the provision

of an Australian Government Research Training Program Scholarship.

Benjamin John Carey

25th of August 2017

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Acknowledgements

I would like to express my gratitude toward all the people whose help, guidance

and support have made this Ph.D. thesis Possible

Firstly, I would like to thank Professor Kourosh Kalantar-zadeh providing me with

the opportunity to undertake this work and for offering the guidance and

motivation to follow it to completion.

Secondly, I would like to thank my secondary supervisors Dr. Torben Daeneke and

Dr. Jianzhen Ou. Thank you for all the guidance, support, and education you have

given me. Without you, this thesis would not have been possible.

Thirdly, I would also like to extend my appreciation to the CSIRO specifically my

past CSIRO supervisor Professor Serge Zhuiykov and my new CSIRO supervisor Dr.

Anthony Chesman who have always offered support and assistance in any way

possible.

Many thanks to the numerous research facilities and technical staff at RMIT

University including the Micro-Nano Research Facility (MNRF), the former

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Microelectronics and Materials Technology Centre (MMTC), the RMIT microscopy

and Microanalysis Facilities (RMMF) and the Centre of Excellence for Nanoscale

BioPhotonics (CNBP).

I would like to thank the technical staff, researchers and students of which there

are too many to name, both from within RMIT and from other institutions. Thank

you for all the help and support that you have offered. A special mention is due

to professor Salvy Russo for assistance with density function theory computation.

I would like to thank my peers and friends, who have helped keep morale thought

the years, Mr. Christopher Harrison, Ms Emily Nguyen, Ms. Rhiannon Clarke, Mr

Naresh (Shaun) Pillai, Mr. Paul Atkin, Mr. Nam Ha, Mr Steffen Schoenhardt and Mr

Peter Thurgood for all the support, heated discussions and distractions throughout

the last years. You have made this Ph.D. tolerable.

Lastly, I would like to thank my family for all the love and support, especially my

parents who have always offered all the support and assistance they can. And to

my fiancé who has never given up on me.

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Table of Contents

Abstract of Thesis .................................................................................................... ii

Declaration ............................................................................................................... v

Acknowledgements ................................................................................................ vi

Table of Contents .................................................................................................. viii

List of Figures .........................................................................................................xiii

List of Tables ........................................................................................................ xxiv

Abbreviations ........................................................................................................ xxv

Chapter 1 ................................................................................................................. 1

Motivations and Objectives ..................................................................................... 1

1.1 Motivations ....................................................................................................... 1

1.2 Organisation of thesis ...................................................................................... 4

1.3 References ......................................................................................................... 5

Chapter 2 ................................................................................................................. 7

Literature Review ..................................................................................................... 7

2.1 Two dimensional (2D) materials .................................................................... 7

2.1.1 Tungsten disulphide ......................................................................... 10

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2.1.2 Gallium sulphide ............................................................................... 11

2.2 Liquid exfoliation of stratified crystal .......................................................... 12

2.3 Potential of liquid metal as a reaction medium for synthesising non-

stratified 2D materials ......................................................................................... 15

2.3.1 High yield synthesis of colloidal materials .................................... 17

2.3.2 Large scale printing .......................................................................... 18

2.4 This Ph.D. work ............................................................................................... 19

2.4.2 Chapter 4 ........................................................................................... 20

2.4.3 Chapter 5 ........................................................................................... 20

2.4.4 Chapter 6 ........................................................................................... 20

2.5 References ....................................................................................................... 21

Chapter 3 ............................................................................................................... 30

Two Solvent Grinding Sonication Method for the Synthesis of Two-dimensional

Tungsten Disulphide Flakes ................................................................................... 30

3.1 Introduction .................................................................................................... 30

3.2 Experimental Section ..................................................................................... 32

3.2.1 Materials ............................................................................................ 32

3.2.2 Synthesis of Two-dimensional WS2 flakes ..................................... 32

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3.3.3 Measurement and Characterisation .............................................. 32

3.3 Results and Discussion .................................................................................. 34

3.4 Conclusions ..................................................................................................... 52

3.5 References ....................................................................................................... 54

Chapter 4 ............................................................................................................... 56

A Liquid Metal Flow-Reaction for the Synthesis of Ultra-Thin Zinc Sulphide ........ 56

4.1 Introduction .................................................................................................... 56

4.2 Experimental Section ..................................................................................... 58

4.2.1 Materials ............................................................................................ 58

4.2. Preparation of Galinstan .................................................................... 58

4.3.3 Preparation of Zn-Galinstan Alloy: ................................................. 59

4.3.4 Preparation of Polysulphide Solution ............................................ 59

4.3.5 Synthesis of ZnS ................................................................................ 59

4.3.6 Characterisation ................................................................................ 60


4.4 Conclusions ..................................................................................................... 78

4.5 References ....................................................................................................... 79

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Chapter 5 ............................................................................................................... 81

Wafer Scale Two Dimensional Semiconductors from Printed Oxide Skin of Liquid

Metals .................................................................................................................... 81

5.1 Introduction .................................................................................................... 81

5.2 Experimental section ..................................................................................... 84

5.2.1Materials ............................................................................................. 84

5.2.2 Preparation of materials .................................................................. 84

5.2.3 Synthesis of 2D GaS .......................................................................... 84

5.2.4 Fabrication of structures ................................................................. 85

5.2.5 Fabrication of transistors ................................................................. 86

5.2.6 Characterisation: .............................................................................. 86

5.2.7 Computational analysis of band structure .................................... 88


5.4 Conclusions ................................................................................................... 111

5.5 References ..................................................................................................... 113

Chapter 6 ............................................................................................................. 116

Conclusions and Future Work.............................................................................. 116

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6.1 Conclusions ................................................................................................... 116

6.1.1 Stage 1 .............................................................................................. 117

6.1.2 Stage 2 .............................................................................................. 117

6.1.3 Stage 3 .............................................................................................. 118

6.2 Future Work .................................................................................................. 119

6.3 Journal Publications ..................................................................................... 120

6.3.1 First Author Publications ............................................................... 120

6.3.2 Co-author Publications .................................................................. 120

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List of Figures

Figure 2.1 Comparison of the band gap values for different 2D semiconductor

material families studied so far. The crystal structure is also

displayed to highlight the similarities and differences between the

different families. The horizontal degraded bars indicate the range

of band-gap values that can be spanned by changing the number

of layers, straining or alloying.

8

Figure 2.2 Stick and ball representation of GaS crystal structure. Displaying

the top view (top) and side view (bottom). The blue balls represent

Gallium atoms and yellow Sulphur

11

Figure 2.3 Sonication-assisted exfoliation. The layered crystal is

sonicated in a solvent, resulting in exfoliation and nanosheet

formation. In “good” solvents—those with appropriate

surface energy—the exfoliated nanosheets are stabilized

against reaggregation. Otherwise, for “bad” solvents

reaggregation and sedimentation will occur. This mechanism

also describes the dispersion of graphene oxide in polar

solvents, such as water. NB, solvent molecules are not shown

in this figure.

13

Figure 3.1 (a) Image of the 2D WS2 flakes suspensions prepared with NMP (i),

ACN (ii), ethanol/water (iii) and the two-solvent method (iv). (b)

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UV-Vis absorption spectra of the 2D WS2 suspensions with the A,

B and C excitons marked. Note both the NMP and the two solvent

(ACN-ETH/Water) solutions were diluted 1 part in 4 from the

optical image for the UV-Vis measurement. (b) & (c) Raman

spectra of 2D WS2 flakes. Spectra have been normalized to the

𝐴1𝑔(𝛤) phonon peak. (c) The position of the 2𝐿𝐴(𝑀) and 𝐴1𝑔(𝛤)

phonons (from bulk WS2) have been marked and spectra have

been vertically offset for clarity.

Figure 3.2 (a) Photograph and (b) UV-Vis absorption spectra of WS2

suspension after the process involved grinding in ethanol/water

(50/50) and sonication in ACN.

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Figure 3.3 Sample AFM images of the exfoliated WS2 flakes deposited on Si

substrates.

40

Figure 3.4 (a-d) Statistical distribution of WS2 flake thickness analysed

from AFM imagining. (e-h) statistical distribution of lateral

dimensions analysed from TEM imagining. The blue overlay is

an approximation of flake size based on DLS Approximation

of 2D flake lateral dimensions as demonstrated by Lotya et al.

13 (i-l) TEM images of the WS2 flakes synthesized with

different solvent methods.

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Figure 3.5 Conductive Atomic Force Micrograph of WS2 flakes exfoliated via

the two-solvent method on Au substrate representing both (a)

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height and (c) tip current of the same area. B and D are the profiles

the white dashed line in A and B respectively.

Figure 3.6 (a) XRD of bulk WS2 powder and WS2 powder after grinding

(pre-exfoliation). The results have been normalized to the

002 peak, offset vertically and the 002 peak has been clipped

for clarity. (b) Paracrystallinity factor (g) of the vertical crystal

planes for bulk WS2 powder (black) and WS2 powder after

grinding in NMP (green), ACN (purple) and ethanol/water

(50/50, red)

44

Figure 3.7 High resolution transmission electron micrograph (HRTEM) of the

exfoliated WS2. Insert is a selected area electron diffraction (SAED)

image of the same section.

49

Figure 3.8 Thermal gravimetric analysis (TGA) of the dried 2D WS2 flakes

prepared with both only NMP and ACN-ethanol/water two

solvent methods tested in nitrogen gas. The boiling points of

both NMP and water are marked (202°C and 100°C at

standard pressure)

50

Figure 4.1 Images are taken from a video recording of the reaction with

graphical representation of the surface of the liquid metal.

62

Figure 4.2 Depiction of the synthetic method. (a) Alloying of liquid

Galinstan and Zn powder. (b) Flow-reaction between Zn-

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Galinstan alloy and polysulphide (i) Dispersion of ZnS and by-

products. (ii) ZnS forming on the interface between Na2Sx

solution and the alloy. (c) Washing process displaying (i)

solution before washing and iodine treatment, (ii) solution

after centrifugation (iii) resuspended ZnS in DMF.

Figure 4.3 Morphology of the 2D ZnS. (a) TEM image of the ZnS flakes.

(b) HRTEM image of the ZnS which lattice dimensions marked,

with fast Fourier transform (FFT) pattern (inset). (c) Statistical

distribution of the lateral dimension of the ZnS flakes. (d)

AFM micrograph of a flake with height profile (insert) along

the blue line.

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Figure 4.4 TEM images of 2D ZnS sheets produced via the reaction of

Na2Sx with the Zn alloy.

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Figure 4.5 TEM images (a-c) and HRTEM (d, FFT insert) of ZnS produced

with a modified reaction chamber where the Na2Sx is not

pumped directly into the Galinstan from underneath, but

instead from the side.

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Figure 4.6 TEM image (a) of ZnS produced via a modified reaction

chamber where a 0.1 mol L-1M Na2S solution is also

substituting the polysulphide solution and is not pumped

directly into Galinstan from underneath, but instead from the

side. (b & c) An AFM image and profile, which demonstrate

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that the thickness of the flakes is not altered by a change in

the reaction protocol.

Figure 4.7 XPS spectra of the (a) S 2p and (b) Zn 2p regions of the ZnS

flakes. Peak fitting is demonstrated for all the relevant shells.

71

Figure 4.8 Figure 4.8. Schematic representation of the simplified flow

dynamics within the liquid metal reactor. F, V, A, T, D, t, and

N represent the volume of the flow rate, the volume of

bubbles, the surface area of bubbles (i.e. interfacial area), the

thickness of ZnS form, density of ZnS, time of reaction, and

the total number of bubbles during the reaction, respectively.

72

Figure 4.9 TGA of the centrifuged ZnS solution demonstrating weight

reduction relative to (a) temperature and (b) time. The red

dashed line indicates the solid weight of the dried ZnS which

was utilized for the yield calculations. (a, inset) The change in

weight as the N2 flow is replaced with an air flow at 600°C.

74

Figure 4.10. Spectroscopic characterisation of the ZnS suspensions. (a) PL

emission spectrum excited at 275 nm with simplified

electronic band structure (inset). The red lines indicate the

conduction and valence band and the black line is the fermi

level. (b) UV-Vis absorption spectrum with Tauc plot (inset).

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Figure 4.11 Valence band analysis from XPS with line fitted on the valance

band edge for clear determination of energy difference

between the valence band maxima and the Fermi level.

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Figure 5.1. GaS representation and printing process of the 2D layers: stick and

ball representation of GaS crystal: (a) side view of bilayer GaS,

showing a unit cell c = 15.492Å made of two GaS layers. (b) Top

view of the GaS crystal. The GaS crystal lattice is composed of Ga-

Ga and Ga-S covalent bonds, which extend in two dimensions,

forming a stratified crystal made of planes that are held together

by van der Waals attractions. The Ga atoms are green and the S

atoms are blue. (c and d) Functionalisation of the substrate with

FDTES changes the contact angle between a Ga drop and the

substrate by 12.9°. The SiO2 substrate becomes more hydrophobic

and thus resists wetting by liquid Ga. (e to h) Schematics of the

synthesis process for patterning 2D GaS via printing the skin oxide

of liquid Ga. (e) Lithography process for establishing the negative

pattern of the photoresist. (f) Covering the exposed area of the

substrate with vaporised FDTES. (g) Placing Ga liquid metal and

removing it with soft PDMS that leaves a cracked layer of Ga oxide.

(h) Two concurrent steps of chemical vapour treatment: firstly,

GaCl3 layer is formed via exposure to HCl vapour and secondly,

sulphurisation by exposure to S vapour forming GaS.

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Figure 5.2 (a) AFM image of cracked layer of gallium oxide and corresponding

profile (b).

91

Figure 5.3 Analysis of 2D In2S3 synthesised via printing of liquid indium. (a)

Raman spectrum, (b) Indium 3d region of X-ray photoelectron

spectrum (XPS) and (c) AFM image including profile (d) along the

blue line shown in (c).

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Figure 5.4 Material characterisations of printed GaS: (a) X-ray Diffraction

(XRD) of printed GaS film (after multiple printing steps) with XRD

of the printed oxide skin and SiO2/Si substrate for comparison and

GaS planes indicated (traces have been normalised and offset for

clarity). (b) Raman spectrum of the 2D GaS with GaS vibrational

peak shifts indicated. (c & d) XPS of the 2D GaS for the regions of

interest (c) Ga 3d and (d) S 2p.

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Figure 5.5 (a-c) XPS study of GaS film displaying (a) survey spectrum, (b) Cl 2p

and (c) O 1s regions of the spectrum. (d-f) EDX of printed 2D GaOx

film (d), the film after treatment with HCl (e) and after

sulphurisation (f).

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Figure 5.6 XPS patterns of the printed metal oxide film before (a-e) and after

(f-j) treatment with HCl vapour. (a & f) Survey, (b & g) Ga 3d, (c &

h) Ga 3s, (d & i) Cl 2p and (e & j) O 1s regions.

97

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Figure 5.7 Morphology and characteristics of the printed 2D films: (a) AFM

image with (b) height profile) of the printed 2D GaS layer

confirming bilayer deposition. The profile is offset to the

substrate’s surface. (c&d) Confocal PL map shows a patterned area

made of 2D GaS (green) on a SiO2/Si substrate (black)

demonstrating a near-uniform 2D layer is obtained in a large area

along with the PL spectra from areas noted on map (c) patterns 1

and 2). Due to the excitation wavelength limitation of confocal PL,

the deep trap emissions are used for assessing the uniformity of

the PL pattern. (e) PL emission spectra demonstrating

contributions of the interband transitions (red) and deep trap

recombinations (black). The deep trap emission spectrum is scaled

by a factor of 10 for clarity. (f) HRTEM of a flake mechanically

scratched from the printed GaS film.

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Figure 5.8. AFM image (a) of 2D GaS film and statistical distribution of the

measured pixel heights (b) of the substrate (blue) and film (red)

within the areas in the blue and red squares, respectively. Scale

bar in (a) is 5 µm.

100

Figure 5.9 Statistical Raman analysis of the 2D GaS films. Raman line scan

analysis. (a) Optical image, the line scan was performed on the

black line indicated. (b) Contour plot with phonon modes

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indicated. (c) Relative intensity of the E12gphonon peak with

standard error indicated.

Figure 5.10 Raman spectra (a) of 2D GaS spaced at approximately 1 mm

intervals. (b & c) Analysis of crystal grains of the printed 2D GaS

film featuring (b) a HRTEM image of a grain boundary and (c) a

map of grain boundaries determined via polarised confocal PL as

has been previously demonstrated by Pan et al. Different shades

of grey indicate areas that luminesce at different polarisation

angles which are associated with crystal domains and their lattice

orientation within the 2D GaS layer.

102

Figure 5.11 (a) HRTEM of a GaS flake scratched from the printed film.

Structural defects are visible in the crystal, which are mainly

ascribed to lattice dislocations (example in the red square). Inset:

magnified image highlighting the dislocation of the crystal lattice.

(b) SCLC measurement of 2D GaS. Ohmic and trap filled limited TFL

regions are marked with their respective colours.

104

Figure 5.12 Optical and electronic properties of 2D GaS. Hybrid density

functional theory (DFT) computation of the band structures in

bilayer (a) and bulk (b) GaS. (c) Tauc plot used for determining the

electronic band transition with simplified electronic band diagram

(inset). Red lines are the valence and conduction band edges, black

is the Fermi level and blue is the location of the deep trap band.

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(d) PESA demonstrating the valence band maximum (VBM) energy

is −5.15 eV. (e) XPS valance analysis revealing an energy difference

of 1.8 eV between the VBM and Fermi level. (f) PL decay with fitted

exponential (red) and mean excitonic decay (blue) time indicated.

Figure 5.13 Hybrid DFT computation of the band structures in (a) tri-layer,

and (b) four-layer GaS.

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Figure 5.14 Schematic representation for device fabrication. (a) Clean

SiO2/Si substrate. (b) Tungsten (W) film e-beam deposited onto

the substrate. (c) Photoresist is deposited and then exposed

through a photolithography mask. (d) Remaining photoresist

after image reversal and developing. (e) Immersion of the of the

unprotected W in the etchant. (f) Sample after etching and lift-

off. (g) Deposition of liquid Ga. (h) Formation of 2D GaOx

between electrodes after Ga is removed. (i) Exposure to HCl

vapour. (j) GaCl3 film between electrodes. (k) Exposure to S

vapour. (l) Final device. Extra photolithography steps can also

be included as presented in Figure 5.1.

107

Figure 5.15 Characterisation of 2D GaS FETs. (a) Schematic representation

of back gated GaS FET with tungsten/WS2 electrodes featuring

band energy diagram. (b) Raman spectrum of the W electrodes

after sulphurisation. Phonon modes of WS2 are indicated

demonstrating the growth of WS2 on the skin of the electrodes.

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(c) optical images of the fabricated devices and electrode gap

(inset). The scale bars are 500 µm and 20 µm respectively. (d)

I/V characteristics of the FET at different gate voltages. (e) I/V

phototransistor characteristics with no gate bias.

Figure 5.16. Statistical analysis of photodetector devices. (a) Responsivity of

each individual photodetector device and (b) distribution of

responsivities. (c) On/Off ratio of each device and (d)

distribution of ratios.

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List of Tables

Table 3.1. Summary of Raman shift peak intensities and locations for

the 2𝐿𝐴(𝑀), 𝐸2𝑔1 (𝛤) and 𝐴1𝑔(𝛤) phonons (intensities are

normalized to the A1g(𝛤) peak).

38

Table 3.2 Summary of vertical plane XRD results for the comparison of the

crystallographic effects of grinding WS2 with different solvents.

46

Table 4.1 Thermodynamics of the compounds relevant to the

interfacial reaction.

63

Table 4.2 Quantum yield estimations based on comparative analysis

with quinine sulphate.

76

Table 5.1 Comparison of established methods for deposition of 2D materials. 112

xxv

Abbreviations

2D Two-Dimensional

ACN Acetonitrile

AFM Atomic Force Microscopy

Au Gold

Bi Bismuth

CBM Conduction Band Minimum

Cl Chlorine

CVD Chemical Vapour Deposition

DFT Density Functional Theory

DI De-Ionized

DLS Dynamic Light Scattering

DMF N,N-dimethylformamide

EDX Energy Dispersive X-ray Spectroscopy

FDTES 1h,1h,2h,2h-perfluorodecyltriethoxysilane

FET Field Effect Transistor

FFT Fast Fourier Transform

Ga Gallium

GaCl3 Gallium Chloride

GaOx Gallium Oxide

GaS Gallium Sulphide

HBr Hydrobromic acid

HCl Hydrochloric acid

HRTEM High Resolution Transmission Electron Microscopy

I Iodine

In Indium

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In2S3 Indium Sulphide

IPA Isopropyl Alcohol

MoS2 Molybdenum Disulphide

N2 Nitrogen

Na Sodium

NMP N-methyl-2- Pyrrolidine

O Oxygen

PESA Photoelectron Spectroscopy in Air

PF Paracrystallinity Factor

PL Photoluminescence

QD Quantum Dots

S Sulphur

SCLC Space Charge Limited Current

Se Selenium

Si Silicon

SiO2 Silicon Dioxide

Sn Tin

STEM Scanning Transmission Electron Microscopy

Te Telurium

TEM Transmission Electron Microscopy

TFL Trap Filled Limited

TGA Thermo Gravimetric Analysis

UV Ultra Violet

W Tungsten

WS2 Tungsten Disulphide

XPS X-ray Photoelectron Spectroscopy

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XRD X-ray Diffraction

Zn Zinc

ZnO Zinc Oxide

ZnS Zinc Sulphide

1

Chapter 1

Motivations and Objectives

1.1 Motivations

Ever since the isolation of the atomically thin form of carbon known as

graphene in 20041 the properties and applications of two-dimensional (2D)

materials have been the focus of consistent and ongoing studies. 2D

materials which consist of atomically thin sheets have received much

attention as they present many promising avenues for future technological

development due to their diverse and remarkable characteristics.2-5 These,

materials can be useful for many future applications in industries such as

electronics, optics, and chemical engineering.

However, there are still many barriers to the adoption of using 2D

semiconductors in the electronics and optics industries stemming from the

challenges involved in large scale production of these materials. In

particular 2D semiconductors could potentially revolutionise these

2

industries due to their inherent material properties4. Many methods to

produce 2D materials have been demonstrated to date, both top-down

(exfoliation of sheets from a layered bulk crystal) and bottom up

(preferential growth of atomically thin layers) however, there are many

more challenges which must be overcome before large scale

industrialisation can be realised.

This thesis will discuss various synthesis approaches, initially investigating

the process and problems of the exfoliation of metal-chalcogenides from

layered crystals to produce 2D semiconducting materials. There are still

many knowledge gaps in the literature which hamper the potential for the

development of commercial electronic and optical devices based up liquid

exfoliated 2D semiconductors.

❖ The first inquisition of this Ph.D. work is to improve on such liquid

exfoliation. In hope of finding methodology capable of efficient and

effective production as well as an investigation into the fundamental

factors affecting this process.

The question of ‘what if the desired 2D material does not exist in a layered

crystal system?’ unavoidably arises. If there is no stratified form of the

desired material then exfoliation techniques are consequently not possible.

Therefore, a new medium is required to create these ultrathin layers of the

material desired. This medium could either exist as atomically thin

precursors which can be chemically altered or as a reaction medium which

facilitates the 2D formation.

3

The requirement of a new medium leads to the consideration of utilising naturally

occurring interfacial compounds such as the oxide layer formed upon the surface of

certain metals. Many metals such as aluminium, gallium, indium and tin will form an

atomically thin and self-limiting layer of oxide when exposed to air.6,7 These

atomically thin layers are in essence naturally occurring 2D materials. Furthermore,

unlike chemical vapour deposition (CVD) the compounds grown on the surface of

these metals hypothetically do not require nucleation points.8,9 As a result, if the

surface skin of these materials can be utilised it could produce large surfaces that are

atomically thin and with the least number of boundaries that can be hypothetically

obtained

Extending upon this concept, If the metal precursor is liquid then this ultrathin layer

forms at the interface between the metal and the environment and in cases such as

gallium and gallium based alloys also have very low binding energy to the bulk metal7

resulting in an atomically thin layer which can be separated from the bulk. This leads

to the consideration of low melting point metal such as gallium, indium and gallium

based alloys as prime candidates for investigation as a new medium allowing access

to various 2D metal compounds.

❖ The later sections of this Ph.D. work will be a quest to explore this idea for

potentially using liquid metal as a reaction medium and precursor for the

synthesis of 2D semiconductors for future optical devices.

4

1.2 Organisation of thesis

The primary objectives of this Ph.D. research are to investigate and

demonstrate new understandings of the synthesis of 2D semiconductors

and to demonstrate potential for future optical technological progress.

Chapter 2 of this thesis presents a review of current relevant literature and

research on the field of 2D materials synthesis and metal-chalcogenides.

This chapter will also serve to provide background knowledge and provide

further rationale to the reader on the relevant 2D materials discussed in

later chapters and on the metal/metal alloys used therein.

Chapter 3 of this thesis investigates grinding assisted liquid phase

exfoliation of tungsten disulphide (WS2) and provides an understanding of

this process specifically the role the solvent plays during exfoliation. This

chapter will also discuss a potential solution to many of the issues involved

in liquid phase exfoliation by presenting a ‘two-solvent grinding sonication

method’.

Chapter 4 of this thesis presents the concept and investigation of utilising a

liquid metal alloy as a reaction medium for the production of 2D zinc

sulphide (ZnS). This chapter will demonstrate the viability of using a liquid

metal alloy (in this case an alloy of zinc, gallium, indium and tin) to create

2D sheets of a material which does not have a layered structure and may

offer access to a simple, scalable solution for synthesis with the devised

‘flow reactor’ system.

5

Chapter 5 of this thesis investigates another potential step toward

producing high quality, large area 2D sheets in a scalable method via the

printed oxide skin of liquid gallium. This chapter presents an investigation

of producing true wafer scale 2D semiconductors and investigates their

potential as basic optical and electronic devices, including transistors and

photodetectors.

Chapter 6 of this thesis will contain a discussion of the conclusions of the research

performed in chapters 3, 4, and 5. This chapter will also discuss the future

endeavours to be undertaken based on the work present in this thesis.

1.3 References

1 Novoselov, K. S. et al. Electric field effect in atomically thin carbon films. Science 306, 666-669 (2004).

2 Xu, M., Liang, T., Shi, M. & Chen, H. Graphene-like two-dimensional materials. Chem. Rev. 113, 3766-3798 (2013).

3 Novoselov, K. S. et al. Two-dimensional gas of massless Dirac fermions in graphene. Nature 438, 197-200 (2005).

4 Wang, Q. H., Kalantar-Zadeh, K., Kis, A., Coleman, J. N. & Strano, M. S. Electronics and optoelectronics of two-dimensional transition metal dichalcogenides. Nat. Nanotechnol. 7, 699-712 (2012).

5 Miro, P., Audiffred, M. & Heine, T. An atlas of two-dimensional materials. Chem. Soc. Rev. 43, 6537-6554 (2014).

6 Downs, A. J. Chemistry of aluminium, gallium, indium and thallium. (Springer Science & Business Media, 1993).

6

7 Dickey, M. D. Emerging Applications of Liquid Metals Featuring Surface Oxides. ACS App. Mater. Interfaces 6, 18369-18379 (2014).

8 Tang, S. et al. Nucleation and growth of single crystal graphene on hexagonal boron nitride. Carbon 50, 329-331 (2012).

9 Ling, X. et al. Role of the seeding promoter in MoS2 growth by chemical

vapor deposition. Nano letters 14, 464-472 (2014).

7

Chapter 2

Literature Review

2.1 Two dimensional (2D) materials

2D materials which comprise of atomically thin sheets of material present

many promising avenues for future technologies due to their diverse and

remarkable characteristics1-5. These materials offer a wide range of

exploitable bandgaps for optical applications6 ranging from 2D conductors

(e.g. graphene2,7), semiconductors3,6 to wide bandgap insulators.8

8

Figure 2.1. Comparison of the band gap values for different 2D

semiconductor material families studied so far. The crystal structure is also

displayed to highlight the similarities and differences between the different

families. The horizontal degraded bars indicate the range of band-gap

values that can be spanned by changing the number of layers, straining or

alloying. [Reproduced with permission: Castellanos-Gomez, Nature

Photonics 10, 202–204 (2016)6]

2D semiconductors exist across many different families of crystal systems

including monoelemental pnictogens such as 2D black phosphorous known

as phosphorene.9 and recently 2D forms of arsenic, antimony, and

bismuth10 however, these forms still require much more fundamental

studies. There are also many forms of bi-elemental metal compounds of

9

various bandgaps (displayed in Figure 2.1.) which have received significant

attention. Of these metallic compounds chalcogenides present many great

candidates. 2D metal chalcogenides include transition metal

dichalcogenides (TMDs) of the form MX2 (where M = Mo, W, Re etc, and X

= S, Se, Te)5 and post-transition metal chalcogenides (PTMCs) which come

in many forms as follows:

❖ MX (where M = Ga, In, Sn and X = S, Se)11-13

❖ MX2 (where M = Sn and X = S, Se) 14

❖ M2X3 (where M = Ga, In, Sn, Bi and X = S, Se)15-17

TMDs and PTMC have recently attracted significant attention for many

applications including electronic and optical devices such as transistors11,18

and photodetectors,19-21 sensors for gas14,22 and biosensors,23 catalysis,24,25

and energy storage.26,27

2D materials can be produced using several different methodologies such

as exfoliation techniques of bulk stratified crystals (compromising of

covalently bonded sheets held together via van der Walls forces) either

through mechanical plying of the layers7,28 or via agitation in liquid phase

through ion intercalation,29,30 laser ablation,31,32 or ultrasound.33,34 As well

as grown directly onto substrates via methods such as chemical vapour

deposition,35-37 atomic layer deposition,38 or thermolysis.39

❖ Exfoliation from bulk crystals

➢ Mechanical exfoliation

➢ Liquid-phase exfoliation (LPE)

10

▪ Ionic intercalation

▪ Laser ablation

▪ Ultrasonic exfoliation

❖ Growth of ultrathin layers

➢ Chemical vapour deposition (CVD)

▪ Metal-organic CVD

➢ Atomic layer deposition

➢ Thermolysis

2.1.1 Tungsten disulphide

One particularly promising 2D TMD is tungsten disulphide (WS2).5,40 In

comparison to many the other 2D TMDs, 2D WS2 offers better chemical

stabilities, while featuring certain favourable electronic characteristics.41,42

There are now well-established methods for obtaining durable p and n type

semiconducting 2D WS2 that makes it amongst the most suitable 2D TMDs

for developing future integrated circuits, optical components, catalysts as

well as analytical and biological systems.3,43 Producing high quality, large

surface area and relatively pure 2D WS2 flakes utilising a scalable method is

still a challenge and further research is necessary to investigate the

suitability of liquid phase exfoliation techniques for this purpose.

11

2.1.2 Gallium sulphide

A representative example of the MX form of PTMCs is gallium (II) sulphide (GaS),

which has a hexagonal crystal structure with a unit cell of a = b = 3.587 Å, c = 15.492

Å (c is equal to two fundamental layers of GaS as shown in Figure 2.2). 2D GaS has

been recently explored for applications in transistors11, energy storage44,

optoelectronics20, gas sensing45, and nonlinear optics46.

Figure 2.2. Stick and ball representation of GaS crystal structure. Displaying the top

view (top) and side view (bottom). The blue balls represent Gallium atoms and yellow

Sulphur

The electronic band structure of 2D GaS is of particular interest.4,47,48 Especially,

bilayer GaS, which has a thickness equivalent to c of the unit cell, is an indirect

bandgap semiconductor with a conduction band minimum (CBM) at the M point in

bouillon space and an associated bandgap of ~3.1 eV and a direct transition at the

point with only a ~210 meV wider bandgap.48 Due to this modest energy difference

between CBMs at the M and points, free carriers can be exchanged between

12

valleys via room temperature thermal excitation48. Therefore, significant radiative

exciton decay occurs, which produces photoluminescence (PL)45.

Significant changes to the band structure of GaS occur when the number of layers is

increased. Both direct and indirect bandgaps narrow simultaneously, due to the

strong interlayer interactions along the c axis. For bulk GaS, the bandgap is decreased

by 0.4-0.6 eV in comparison to that of bilayer11,47,49.

2.2 Liquid exfoliation of stratified crystal

Liquid phase exfoliation can be used as a high yield, scalable method for

producing 2D TMDs, however, the relevant outcomes from this process are

highly sensitive to the procedures’ parameters.31-35,50,51 A vast host of liquid

phase exfoliation techniques have been introduced. Such as Intercalation,

29,50 laser ablation,31,32 and ultrasound exfoliation.33,34 With grinding

assisted ultrasonic exfoliation being one of the most attractive methods due

to its high yield while avoiding the use of hazardous reagents such as

butyllithium or hydrazine.29,30

13

Figure 2.3. Sonication-assisted exfoliation. The layered crystal is sonicated

in a solvent, resulting in exfoliation and nanosheet formation. In “good”

solvents—those with appropriate surface energy—the exfoliated

nanosheets are stabilized against reaggregation. Otherwise, for “bad”

solvents reaggregation and sedimentation will occur. This mechanism also

describes the dispersion of graphene oxide in polar solvents, such as water.

NB, solvent molecules are not shown in this figure. [Reproduced with

permission: Nicolosi, et al. Science 340, 1226419 (2013). 50]

Many studies have investigated the importance of solvents50,52-54 and

surfactants50,55,56 during exfoliation via sonication. During ultrasonic

exfoliation, the solvent selection plays an important part since physical

properties such as boiling point,53 surface tension and energy,34,42 along

with solubility parameters54 affect the process and hence the resulting

morphology and stability of the 2D flakes as well as the electronic and

optical properties of the suspended flakes.52,57 Of these many studies have

recommended N-methyl-2-pyrrolidone (NMP), for its high production yield.

34,51,54,57,58 However, residual NMP solvent residues can often be found on

the resulting 2D materials due to its relatively high boiling point (202°C) and

surface tension (40.79 mJ m2 at 20°C),59 even when post processing

14

methods aiming at the solvent removal are used, this surface bound solvent

residue often retards the desired qualities of the material due to the

additional expression of NMP’s.54 Furthermore, in the presence of oxygen

NMP forms many breakdown products even at moderate temperatures (as

low as 320k60).

The most common of these products60,61

• N-methyl succinimide

• formyl-pyrrolidone

• N-hydroxymethyl pyrrolidone

• 5-hydroxy-N-methyl pyrrolidone

• 2-pyrrolidone

• Methylamine

• Formamide

• Acetamide

• N-methyl formamide

• N-ethyl acetamide

• Dimethyl acetamide

Residues and degradation products from NMP and other

solvents/surfactants are detrimental to many applications of 2D

semiconductors such as catalysis and sensing which require a pristine

surface.24,25,58 Furthermore the electronic properties of the resulting flakes

are altered by organic residues.62,63

15

There are still many challenges and unknowns when utilising sonication

exfoliation methodologies and further work is necessary in order to

understand the process and improve the quality of the resulting 2D

semiconducting flakes.

2.3 Potential of liquid metal as a reaction medium for

synthesising non-stratified 2D materials

The element gallium (Ga) was first predicted by Mendeleev and isolated by

de Boisbaudran in 1875.64 Ga is a metal with a melting temperature just

below 30°C. When exposed to an ambient environment containing oxygen,

it establishes an atomically thin, self-limiting oxide on its surface65,66 which

has been shown to comprise of only one-unit cell.67

Reports from as early as the 1960’s show that transition and post-transition

metals such as zinc,68 antimony,69 bismuth,69 and palladium70 have been

alloyed with gallium and used in many applications such as:

• medicine71-73

• dentistry70,74

• soldering75

• soft electronics76-78

• antennas79,80

• sensors81

• coolants82

16

• microfluidics83

Many alloys of gallium have even lower melting temperatures than that of

gallium itself. The most recognized of these room temperature liquid metal

alloys include:

❖ Eutectic gallium-indium (Egain; 85:15 wt% Ga:In) melting point

~15°C.84,85

❖ Eutectic gallium-indium-tin (Galinstan®; 68:22:10 wt% Ga:In:Sn) melting

point ~−19°C.86,87

Gallium and its alloys are gaining popularity for many applications

previously held by mercury due to their low toxicity and their low vapor

pressure. 65,88 Ga also has a low bulk viscosity (approximately twice that of

water).89 Furthermore, with such low melting temperatures and boiling

points of >1300°C these alloys have tremendous dynamic range.79

One of the main considerations regarding Ga based alloys has been the

heterogeneity, which depends on the thermodynamics of the elements

within the alloys.90 Other important factors are the properties that emerge

as a result of the interfacial characteristics of the self-limiting surface

compounds. Such properties depend on whether the alloyed metal takes

part in the formation of the surface layer or whether gallium remains as the

dominant compound forming metal at the interface.79,90,91

17

2.3.1 High yield synthesis of colloidal materials

Hypothetically if the self-limiting interfacial layer may also be able to host

compounds other than oxides, for instance, chalcogenide compounds,

provided that the environment consists of chalcogen rich reagents, and if

the atomically thin interfacial layer could be controlled to be predominantly

composed of the alloyed additive rather than gallium, liquid metal based

reactions could provide access to 2D crystals of a hypothetically immense

number of metal compounds.

If gallium alloys can also be utilized as a liquid reaction medium by delivering

co-alloyed metals as the predominant interfacial component, this synthetic

approach could lead to an entirely new avenue for production of 2D metal

compounds. It has been previously shown that the surface formation is

dependent upon the enthalpy and Gibbs free energy of the potential

products.66,92

Also based on the previous hypothesises, by replacing the ambient oxygen

with chalcogen rich reagents, potentially 2D metal chalcogen can be

obtained. To expand this concept further, the approach would be especially

attractive for the synthesis of 2D compounds that do not occur as a

stratified crystal naturally and are thus inaccessible via traditional

exfoliation techniques.34,50,53,93

Hypothetically if a metal such as Zn was alloyed into Galinstan, the surface

formation should comprise of zinc compounds based on the

18

thermodynamic considerations.94 A chemical flow-reaction causing

repeated exposure could utilise this for a high yield synthesis of Zn based

ultrathin materials.

Sodium sulphide (Na2S) and sodium polysulphide (Na2Sx) are soluble and

reactive compounds, which have been previously used in the synthesis of

colloidal zinc sulphide (ZnS) from zinc metal and zinc oxide (ZnO).95,96 ZnS is

a well-known luminescent material that has been used for many optical

applications.97-99 ZnS is frequently found to be synthesized as quantum dots

or nanoparticles,95,100 however, multiple attempts at making ultrathin ZnS

sheets such as through vapor phase101 and thermo-chemical102 methods

have been met with limited success or been synthetically challenging.93

2.3.2 Large scale printing

The initial step in fabricating devices based on 2D materials is the formation of the

2D sheet on a chosen substrate. Many methods have been proposed for the

synthesis of 2D materials including (as discussed in section 2.1) techniques for

directly growing 2D layers on substrates. However the production of large-scale, high

quality and homogeneous 2D sheets has proven to be a major challenge with many

reports of direct growth limited to expensive and impractical substrates such as

sapphire.37,103 So far only one report has addressed the wafer-scale homogeneity for

the deposition of MoS2 using a metal–organic chemical vapour based technique.104

However, the temperatures used are above 550C, which is incompatible with many

electronic industry processes, and more negatively, the deposition process takes up

to 24 hours, which significantly adds to the cost and practicality.

19

The surface oxide of Ga and Ga alloys has been shown to adhere strongly to (wet)

many materials particularly oxides (e.g. SiO2)66. Ga and Ga alloys have previously

been used for printing of conducting electrical tracks.105-107 However, after the

placement of the liquid metal with its self-oxide on the substrate, the metal-oxide

experiences well dispersed van der Waals forces66 while the bulk liquid metal

contains relatively low binding energy to its surface oxide skin and thus can be

removed readily. Furthermore, via functionalisation with fluorocarbons it has been

shown possible to control the deposition of suspended 2D flakes108 and using the

similar functionalisation techniques it is possible to inhibit the wetting effect of Ga.109

If it is possible to deposit only the oxide skin adhered to the substrate than an

ultrathin layer of oxide could be produced which may provide a suitable precursor

for the development of Ga based 2D semiconductors.

2.4 This Ph.D. work

Considering the discussions presented in chapter 1 and the aforementioned

studies in this chapter, the author has identified several major research gaps

in the literature which could offer vast potential for the future in the fields

of 2D materials. In light of this, the succeeding chapter will deal with the

following prospects.

20

2.4.1 Chapter 3

This chapter will deal with the issues regarding solvent effects on grinding

assisted LPE of stratified crystals. In particular, it will explore the effects of

solvent on the exfoliation of WS2 and attempt to overcome the drawbacks

of high yield solvent NMP by exploring combinations of alternate solvents.

2.4.2 Chapter 4

This chapter will deal with the potential to manipulate the interfacial

compounds in Ga based liquid metal alloys. Such that it will explore the

possibility for the use of selectively alloying precursor metals into a Ga alloy

as a reactor media in order to produce 2D sheets of non-stratified materials.

The primary goal will be the production of ultrathin ZnS sheets by

continuously reacting Na2Sx with an alloy of Galinstan and zinc.

2.4.3 Chapter 5

This chapter will deal primarily with wafer scale production of 2D

semiconducting films and devices based upon such semiconductors via the

deposition of the oxide skin of liquid gallium. This chapter will aim to use

deposited oxide layer as a precursor for sulphurisation to produce devices

based upon 2D GaS.

2.4.4 Chapter 6

This chapter will present an overview of the conclusions and research outcomes

present in chapters 3, 4, and 5.

21

Furthermore, this work will form a starting platform for future works based upon

many more candidate compounds synthesised in similar manners to this. For

instance, many of the metals which could be considered potential candidates for

these methods have previously had their oxide forms shown to produce other

chalcogenides (e.g. GaSe, In2Se3, SnS2, SnSe2, etc.), oxides (Ga2O3, In2O3, SnO2 etc.)

and pnictogens (e.g. GaN, GaP, InP, etc.).

2.5 References

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6 Castellanos-Gomez, A. Why all the fuss about 2D semiconductors? Nat. Photon. 10, 202-204 (2016).

7 Novoselov, K. S. et al. Electric field effect in atomically thin carbon films. Science 306, 666-669 (2004).

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9 Yang, J. et al. Optical tuning of exciton and trion emissions in monolayer phosphorene. Light Sci. Appl. 4, e312 (2015).

22

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23

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33 Stengl, V., Henych, J., Slusna, M. & Ecorchard, P. Ultrasound exfoliation of inorganic analogues of graphene. Nanoscale Res. Lett. 9, 167, doi:10.1186/1556-276x-9-167 (2014).

34 Coleman, J. N. et al. Two-Dimensional Nanosheets Produced by Liquid Exfoliation of Layered Materials. Science 331, 568-571 (2011).

35 Das, S., Kim, M., Lee, J.-w. & Choi, W. Synthesis, Properties, and Applications of 2-D Materials: A Comprehensive Review. Crit. Rev. Solid State Mater. Sci. 39, 231-252, doi:10.1080/10408436.2013.836075 (2014).

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24

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40 Chen, Y. et al. Tunable Band Gap Photoluminescence from Atomically Thin Transition-Metal Dichalcogenide Alloys. Acs Nano 7, 4610-4616, doi:10.1021/nn401420h (2013).

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50 Nicolosi, V., Chhowalla, M., Kanatzidis, M. G., Strano, M. S. & Coleman, J. N. Liquid exfoliation of layered materials. Science 340, 1226419 (2013).

25

51 Coleman, J. N. Liquid Exfoliation of Defect-Free Graphene. Acc. Chem. Res. 46, 14-22 (2013).

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54 Halim, U. et al. A Rational Design of Cosolvent Exfoliation of Layered Materials by Directly Probing Liquid–Solid Interaction. Nat. Commun. 4, 2213 (2013).

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57 O'Neill, A., Khan, U. & Coleman, J. N. Preparation of High Concentration Dispersions of Exfoliated MoS2 with Increased Flake Size. Chem. Mater. 24, 2414-2421 (2012).

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30

Chapter 3

Two Solvent Grinding Sonication Method for the

Synthesis of Two-dimensional Tungsten

Disulphide Flakes

3.1 Introduction

As discussed in chapter 2 and inspired the unique of two-dimensional (2D)

transition metal dichalcogenides (TMDs) this chapter focusses on some of

the boundaries which need to be crossed for large scale production of 2D

TMDCs.

In this chapter, the effects of solvent selection were experimentally

assessed for grinding assisted ultrasound liquid phase exfoliation by

evaluating a methodology that comprises of using different solvents for the

grinding and sonication processes. A popular high yield solvent, N-methyl-

2-pyrrolidone (NMP), which has been recommended in several studies was

31

employed as a high yield reference. 1-3 Even though high yields are obtained

when using NMP, solvent residues were found to be difficult to remove due

to its relatively high boiling point (202°C) and surface tension (40.79 mJ m2

at 20°C). 4 Residual NMP can often be found on the resulting 2D materials,

even when post processing methods aiming at the solvent removal are

used.5 The inherent instability of NMP is further complicating its removal

since NMP is known to polymerize at elevated temperatures which occur

during sonication and solvent evaporation.6 Solvent residues and

degradation products are detrimental to many applications of 2D TMDs

such as catalysis and sensing which require a pristine surface. Furthermore,

the electronic properties of the resulting flakes are likely to be altered by

organic residues. In order to find alternative solutions, able to overcome the

limitations of exfoliation in NMP, acetonitrile (ACN) and a 50/50

ethanol/water mixture were initially investigated as solvents. These good

process solvents are chosen as they offer moderate boiling points and their

residues can be readily removed.

The outcomes of this investigation have been peer reviewed and published

in Chemical Communications.7

32

3.2 Experimental Section

3.2.1 Materials

Tungsten disulphide (WS2) 2 µm powder (CAS Number 12138-09-9) was

purchased from Sigma-Aldrich®. N-methyl-1-pyrrolidine (NMP), acetonitrile (ACN)

and absolute ethanol were purchased from commercial chemical suppliers and

were not purified or treated in any way and the water used was Milli-Q filtered

water (>18 MΩ).

3.2.2 Synthesis of Two-dimensional WS2 flakes

For each of the four batches 1 g of WS2 was ground with a mortar and pestle for

30 minutes with 1 ml of solvent added periodically for a total of 5 ml/g. in the case

of the single solvent method additional solvent was added to form 15 ml of slurry.

For the two-solvent method, the grinding solvent was left to dry for 1 hour at

room temperature before the second solvent was added to form 15 ml of slurry.

The solutions were then probe sonicated at 100 W for 90 minutes in an ice bath

to prevent excessive heating. The solutions were then centrifuged at 2500 RPM

for 45 minutes and the supernatant was collected.

3.3.3 Measurement and Characterisation

Transmission electron microscope (TEM) imaging was performed with a JEOL

1010 TEM operating at 100 kV, and JEOL 2100F scanning transmission electron

microscope (STEM) operating at 80 kV both fitted with SC600 CCD Cameras (Gatan

Orius), 10-100 µl dripped onto a 300-mesh copper holey carbon grid (ProSciTech,

33

Australia).Atomic force microscopy (AFM) imaging was performed using a Phillips

D3100 scanning probe microscope (AFM/SPM) in tapping mode, 5 µL of solution

was drop-casted onto a standard silicon substrate. Conductive AFM images were

obtained using a Bruker MultiMode with an installed Peak Force TUNA module

(MM8-PFTUNA for MultiMode8 AFM system).

UV-Vis absorption was measured using a Cary 60 spectrometer (Algilent

Technologies) with baseline correction in a standard UV glass cuvette. Samples

were diluted 1:4 in the case of NMP and the two-solvent method exfoliated flakes.

Raman spectroscopy was carried out using a Renishaw inVia confocal microscope

system. Specimens were illuminated with an argon laser (514 nm wavelength)

through a 50× objective, laser was approximately 7 mW and the spot size in the

range of 1 μm. 20 µL of solution was drop-casted onto a gold coated (200 nm)

silicon substrate.

X-ray diffraction (XRD) patterns were obtained using RMIT Bruker D4 Endeavour

wide-angle diffractometric with a (Cu K-alpha) 0.15418 nm X-ray source. Dynamic-

Light scattering (DLS) particle sizing was completed using an ALV Fast DLS particle

sizing spectrometer. Thermo-gravimetric analysis (TGA) was carried out using a

Perkin Elmer, Pyris1 TGA from 50°C-800°C under nitrogen gas flow and then from

800-850°C in air. 5 mg of dried WS2 flakes were used, the NMP exfoliated flakes

were dried in a vacuum oven (~500 mmHg) at 70°C for 100 hours. In the case of

the two solvent exfoliated flakes the sample was dried at 70°C for 5 hours under

a stream of nitrogen. Prior to the actual TGA measurement the sample was kept

34

at 70°C for 30 minutes while recording the weight loss. No weight loss was

observed indicating that a stable state had been reached and the sample was dry.

3.3 Results and Discussion

The liquid exfoliation process using NMP, ACN and ethanol/water solvents,

involved the WS2 bulk powder separately ground in each solvent and then

sonicated in the same solvent as grinding. Figure 3.1a shows an image of

the resulting 2D WS2 suspensions. While the NMP solution resulted in high

yield (Figure 3.1a i), as expected, the observed yields for the ACN and

ethanol/water solutions were low, evident through the low coloration of

the suspensions (Figure 3.1a ii and iii).

A fourth suspension was investigated with a modified process step to

possibly increase the yield. In this case, the grinding and sonication solvents

were chosen to be different. This process is referred to as the ‘two solvent

method’ which has previously been shown effective for obtaining 2D

molybdenum disulphide.8 ACN was used during the grinding step and the

50/50 ethanol/water mixture during the sonication step (after the grinding

solvent was evaporated). In this case, a dramatic increase of the yield was

evident due to the more intense coloration of the fourth suspension (Figure

3.1a iv). The following characterisations will demonstrate why the choice of

ACN as the grinding solvent and the ethanol/water mix as the sonication

solvent provided this improvement.

35

Figure 3.1. (a) Image of the 2D WS2 flakes suspensions prepared with NMP

(i), ACN (ii), ethanol/water (iii) and the two-solvent method (iv). (b) UV-Vis

absorption spectra of the 2D WS2 suspensions with the A, B and C excitons

marked. Note both the NMP and the two solvent (ACN-ETH/Water)

solutions were diluted 1 part in 4 from the optical image for the UV-Vis

measurement. (b) & (c) Raman spectra of 2D WS2 flakes. Spectra have been

normalized to the 𝐴1𝑔(𝛤) phonon peak. (c) The position of the 2𝐿𝐴(𝑀) and

𝐴1𝑔(𝛤) phonons (from bulk WS2) have been marked and spectra have been

vertically offset for clarity.

The UV-vis spectra of the suspensions can be observed in Figure 3.1b. From this

the concentrations of the solutions were calculated using the UV-Vis absorption

36

data and the methodology outlined by O’Neill et al.3, whereas the resonant

absorbance per unit length [(𝐴/𝑙)𝑟] is directly related to the concentration

independent of scattering effects:

(𝐴/𝑙)𝑟 = 𝛼𝑟𝐶

where 𝛼𝑟 is the resonant absorption coefficient. 3 The value of 𝛼𝑟 for WS2 was

obtained from Coleman et al.’s work.2 This method yields concentrations of

0.14 mg/ml, 1.46 µg/ml, 0.82 µg/ml and 0.1 mg/ml for NMP, ACN, ethanol/water

and the two-solvent method respectively. Indicating the superiority of the two-

solvent method over both ACN and ethanol/water.

The peaks of 635, 530 and 475 nm (A, B and C respectively) are clearly seen for

the two-solvent processed suspension which are known to be due to the excitonic

absorption of 2D WS2 suspensions, emphasising the existence of a direct bandgap

associated with a small number of layers. 9,10 Excitonic peak A is also observed for

flakes exfoliated in NMP. The excitonic peaks B and C are however not observed

in this case. The disappearance and reduction of the intensity of the excitonic

peaks have been suggested to be associated with the decrease in the lateral

dimensions of the flakes when NMP is used.11 In this case, however, it seems that

the interaction between NMP and/or the NMP decomposition products and the

WS2 surface are likely to alter the electronic structure of the semiconducting

flakes leading to more intense absorption of high energy photons obscuring

excitonic effects.

37

The phonon peaks 2𝐿𝐴(𝑀), 𝐸2𝑔1 (𝛤) and 𝐴1𝑔(𝛤) in the Raman spectrum of

the non-exfoliated (bulk) powder are observed at 350.6, 356 and 420.4 cm1

shift to higher wave numbers (~353, ~358 and ~425 cm1, respectively) after

exfoliation (Figure 3.1b). The shift of the Raman peaks and the increase of

the 𝐸2𝑔1 (𝛤) phonon (Table 3.1) is characteristic for the formation of thin WS2

flakes.12

Raman spectra for NMP exfoliated flakes (Figure 3.1d) showed strong

fluorescence induced by the excitation laser evident in the intense

background signal and the deviation from a flat baseline. This behaviour is

not observed in the two-solvent method exfoliated flakes, highlighting

differences in the electronic structure of the resulting material even after

the solvent drying stage. The phonon peaks are quenched for NMP

exfoliated flakes and a characteristic NMP Raman signal is observed

featuring peaks at 1450 and 1600 cm1 highlighting that a significant

amount of the NMP solvent residues and decomposition products remained

on the 2D WS2 surface, despite of drying the flakes at 70°C for 4 hours. These

residues are likely to interfere with 2D WS2 applications that require pristine

surfaces such as catalysis, gas/bio sensing and certain electronic devices.

The NMP effect also reduces the accuracy of the number of layer

assessment using Raman spectroscopy. However, samples prepared

following the two-solvent exfoliation method showed no signs of residues

in the Raman spectrum confirming the two-solvent approach as a superior

exfoliation technique.

38

Table 3.1. Summary of Raman shift peak intensities and locations for the 2𝐿𝐴(𝑀),

𝐸2𝑔1 (𝛤) and 𝐴1𝑔(𝛤) phonons (intensities are normalized to the A1g(𝛤) peak).

2𝐿𝐴(𝑀) 𝐸2𝑔1 (𝛤) 𝐴1𝑔(𝛤)

Intensity Peak shift

(cm−1) Intensity Peak shift

(cm−1) Intensity Peak shift

(cm−1)

Bulk 0.52 350.6 0.22 356.0 1 420.4

NMP 0.42 352.5 0.28 357.7 1 422.5

ACN 0.41 354.2 0.36 358.2 1 425.5

Ethanol/water 0.41 352.8 0.48 359.8 1 428.0

ACN-ethanol/water 0.48 353.0 0.51 358.9 1 425.2

The information of phonon peak intensities and positions are recorded in Table

3.1. Here it is clear that the 2D and quasi-2D WS2 flakes produce a comparative

increase in the 𝐸2𝑔1 (𝛤) phonon peak which is a phonon in the horizontal plane as

opposed to the 2𝐿𝐴(𝑀) and 𝐴1𝑔(𝛤) phonon peaks that are vertical plane

phonons and thus relay upon interlayer interactions.12

The reverse methodology where the implementation of the two-solvent solution

was reversed (grinding in ethanol/water, sonication in ACN) was attempted as a

control test. The procedure was accomplished using an almost identical method

to the ACN grinding, ethanol/water sonication solution with the exception of an

increase in post-grinding drying time (which was necessary due to the higher

boiling point of water). In the case of the reverse method the WS2 ethanol/water

slurry was left to dry overnight (at room temperature).

39

Figure 3.2. (a) Photograph and (b) UV-Vis absorption spectra of WS2 suspension

after the process involved grinding in ethanol/water (50/50) and sonication in

ACN.

The reverse two solvent method was however unable to effectively increase the

yield. As can be seen in Figure 3.2, the reverse two-solvent method produced low

optical contrast and very little UV-Vis absorbance characteristics analogous to the

results produced by both ACN and ethanol/water single solvent methods. In fact,

the concentration of the reverse method was calculated (see above for method)

to be reduced to 0.77 µg/ml, less than both of the former methods.

40

Figure 3.3. Sample tapping mode AFM images of the exfoliated WS2 flakes

deposited on Si substrates.

Typical AFM images of the WS2 solutions are displayed in Figure 3.3. The statistical

analysis presented in Figure 3.3(a-d) were assessed from AFM images such as (and

including) these by assessing the height of approximately 100 flakes. Similarly,

both the AFM and TEM images such as (and including) those presented in Figure

3.3(i-l) were used for statistical analysis of the lateral dimensions.

41

Figure 3.4. (a-d) Statistical distribution of WS2 flake thickness analysed from

AFM imagining. (e-h) statistical distribution of lateral dimensions analysed

from TEM imaging. The blue overlay is an approximation of flake size based

on DLS Approximation of 2D flake lateral dimensions as demonstrated by

Lotya et al. 13 (i-l) TEM images of the WS2 flakes synthesized with different

solvent methods.

The thickness distribution and polydispersity of the exfoliated WS2 flakes

was assessed by AFM, TEM and dynamic light scattering (DLS) techniques.

42

The physical dimensions are summarized in Figure 3.4. Here the effect that

solvent selection has upon the physical characteristics of the produced

flakes becomes fairly evident. NMP produces many thin flakes (≤4 layers)

with relatively large surface areas (median = 98 nm) while ethanol/water

produced flakes of similar lateral polydispersion (median = 87 nm), however

with a tendency towards thicker, less two-dimensional flakes (≥5 layers).

ACN produced a large fraction of single and two-layer flakes with a large

horizontal polydispersity and overall reduced the lateral sizing (median = 51

nm). The two-solvent method produced flakes with size characteristics of

90 nm median lateral dimension with many ≤4-layer flakes, suggesting a

combination of the flake properties between the ethanol/water and ACN

systems which was overall effectively closer to that of the archetypal NMP

based exfoliation.

43

Figure 3.5. Conductive Atomic Force Micrograph of WS2 flakes exfoliated via the

two-solvent method on Au substrate representing both (a) height and (c) tip

current of the same area. B and D are the profiles the white dashed line in A and

B respectively.

It has been shown that liquid exfoliated transition metal dichalcogonides (TMDs)

can contain defects in the crystal structure, resulting in metallic edges.11

Conductive AFM (Figure 3.5) has been used for elucidating this effect. With the

profiles in Figure 3.5 it is apparent that the edges of most of the WS2 flakes are

conductive.

44

Figure 3.6. (a) XRD of bulk WS2 powder and WS2 powder after grinding (pre-

exfoliation). The results have been normalized to the 002 peak, offset

vertically and the 002 peak has been clipped for clarity. (b) Paracrystallinity

factor (g) of the vertical crystal planes for bulk WS2 powder (black) and WS2

powder after grinding in NMP (green), ACN (purple) and ethanol/water

(50/50, red)

To elucidate the origin of the yield variation and the yield increase following

the implementation of the two-solvent method, X-ray diffraction (XRD)

patterns of the ground (non-exfoliated) and dried powders were taken prior

to sonication. Figure 3.6a) shows several solvent dependent variations in

the XRD pattern indicating changes in crystallinity after grinding the bulk

45

WS2. The 002, 004, 006 and 008 peaks (at 14.5°, 29°, 44°, and 60°,

respectively) are of particular interest since these peaks are arising from the

symmetry of the vertical crystal planes (layer to layer crystallinity). The

paracrystallinity factor (PF) allows to assess the crystallinity of a sample

(Figure 3.6b) via the determination of the distortions in short term order of

the crystal lattice. The PF was calculated using the cold-work distortion

method, 14 which has been previously used for similar materials.15 The

paracrystallinity of the WS2 was calculated as

𝑔 = √⟨𝑑2⟩ − ⟨𝑑⟩2

⟨𝑑⟩2

where g is the paracrystallinity factor, and d is the planar spacing.15

46

Table 3.2. Summary of vertical plane XRD results for the comparison of the

crystallographic effects of grinding WS2 with different solvents.

Paracrystallinity (%)

Crystal plane Bulk NMP ACN

Ethanol/

water

002 0.793 0.730 0.979 0.847

004 1.244 1.128 1.347 1.075

006 1.266 1.259 1.264 1.265

008 0.754 0.545 0.766 0.642

Normalized Peak Intensity


Ethanol/

water

002 1 1 1 1

004 0.082 0.102 0.072 0.083

006 0.083 0.102 0.065 0.075

008 0.023 0.004 0.022 0.009

Normalized Integral Intensity

Crystal Plane Bulk NMP ACN

Ethanol/

water

002 1 1 1 1

004 0.117 0.139 0.102 0.104

006 0.143 0.150 0.110 0.112

008 0.115 0.022 0.080 0.041

Full-width half-maximum (FWHM) (2θ °)


Ethanol/

water

002 0.085 0.105 0.130 0.105

004 0.125 0.140 0.165 0.140

006 0.095 0.180 0.219 0.185

008 0.569 1.517 0.888 0.818

47

Increased PF values indicate lower short-term order. A breakdown of

vertical order as indicated by the PF is just seen for the ACN ground samples.

This reduction in the short-term order of the 002 plane can also be indicated

by the broadening of the 002 peak (the calculated FWHM is presented in

Table 3.2). The FWHM of the 002 peak is most effectively increased when

grinding with ACN. NMP and ethanol/water did however also broaden the

002 peak indicating some reduction in the vertical order. The XRD patterns

are providing an explanation for the increased exfoliation yield when using

the two-solvent method. The vertical order is most effectively broken down

by grinding in ACN. Zhou et al. investigated the suitability of different

ethanol/water mixtures for the sonication step during the WS2 exfoliation

and found a correlation between the exfoliation yield and the Hansen

solubility parameters (HSP) of the solvent.16 The HSP distances 𝑅𝑎 were

calculated using:

𝑅𝑎 = [4(𝛿𝐷,𝑠𝑜𝑙𝑣𝑒𝑛𝑡 − 𝛿𝐷,𝑠𝑜𝑙𝑢𝑡𝑒)2

+ (𝛿𝑃,𝑠𝑜𝑙𝑣𝑒𝑛𝑡 − 𝛿𝑃,𝑠𝑜𝑙𝑢𝑡𝑒)2

+ (𝛿𝐻,𝑠𝑜𝑙𝑣𝑒𝑛𝑡 − 𝛿𝐻,𝑠𝑜𝑙𝑢𝑡𝑒)2

]0.5

where 𝛿𝐷, 𝛿𝑃, and 𝛿𝐻 are the dispersive, polar, and hydrogen-bonding solubility

parameters of the solvent and solute, respectively. In the case of mixed solvent,

the HSP values can be calculated as a linear function of composition thus for the

50/50 ethanol/water mixtures the each effective HSP (D, P and H) value is simply

the mean of ethanol and water.16The HSP values for WS2 and the solvents were

48

obtained from Coleman et al. and Hansen Solubility Parameters, A User's

Handbook, 2nd Ed respectively.2,17

A HSP-distance of 13.1 is calculated for the ethanol/water mixture while

ACN leads to a calculated HSP-distance of 19.2. A lower HSP-distance leads

to a higher expected solubility. The observed difference in HSP-distance

between the solvent systems was comparatively large. These results

demonstrate that the required solvent parameters during grinding and

sonication may be very different and potentially unrelated.

49

Figure 3.7. High resolution transmission electron micrograph (HRTEM) of the

exfoliated WS2. Insert is a selected area electron diffraction (SAED) image of the

same section.

Figure 3.7 displays a HRTEM image of the 2D WS2 here both the 1.0.0 and 1.0.1

crystal planes are visible in two distinct crystal domains. These lattice spacing

correlate with the XRD pattern in Figure 3.6 and Table 3.2 at 32.7° and 33.5°.

50

Figure 3.8. Thermal gravimetric analysis (TGA) of the dried 2D WS2 flakes

prepared with both only NMP and ACN-ethanol/water two solvent methods

tested in nitrogen gas. The boiling points of both NMP and water are

marked (202°C and 100°C at standard pressure)

TGA (Figure 3.8) was employed to assess the extent of NMP (for the single

solvent process using NMP) as well as ACN and ethanol (for the two-solvent

method) residues on the dried 2D WS2 flakes. It can be seen that due to the

large surface area of the 2D WS2 flakes over 60% w/w of the product

constitutes surface bound solvent molecules and solvent decomposition

products, even after drying in vacuum at elevated temperatures (100 hr at

70°C). High temperatures of over 200°C were necessary to start the NMP

desorption process. Flakes prepared following the two-solvent method

show weight loss at significantly lower temperatures indicating that

51

predominantly water and potentially ethanol residue were present on the

WS2 surface which were removed with comparative ease. This exceedingly

high absorbed solvent to solid ratio could potentially open up many

doorways to applications such as dehumidification, solvent absorption and

gas/bio sensing. This property of 2D WS2 can be rationalized considering the

large aspect ratio of the flakes and their surface polarity.

52

3.4 Conclusions

A two-solvent exfoliation method was developed and investigated for the

synthesis of 2D WS2 flakes at high yields. For comparison, the resulted 2D

flake suspension was assessed in reference to those obtained using the

single solvent method. It was demonstrated that the choice of a separate

grinding solvent prior to the sonication in liquid phase exfoliation played a

major role in the determination of the process that has been largely

overlooked to date. The two-solvent method based on grinding with ACN

and sonication in ethanol/water mix resulted in much higher yield in

comparison to single solution grinding sonication method using ACN and

ethanol/water solutions alone.

AFM, TEM and DLS characterisations revealed that the lateral size and

thickness of the exfoliated flakes depend highly on the chosen solvent

during both the grinding and sonication steps. They showed that the two-

solvent method generated flakes comparable to those of NMP solution in

terms of lateral size and thickness. Raman and UV-Vis spectroscopy

provided strong evidence that NMP residues remained on the surface of the

exfoliated flakes, interfering with many of the desired properties of 2D WS2.

However, such adverse effects of the residues seem to be reduced using the

two-solvent method.

XRD was performed on the WS2 powder after the grinding step in order to

understand the origin of the yield variation. The XRD results confirmed that

53

ACN does reduce interlayer crystallinity more efficiently than the other

solvents.

A >60% w/w solvent content was later confirmed with TGA for both NMP

and two solvent exfoliated flakes. The NMP contamination required

however a higher temperature for its removal.

This work helps to increase the understanding of the importance of the

grinding step during the grinding assisted ultrasound exfoliation of 2D

materials which has been poorly understood to date. It also suggests that

choice of two separate solvents for grinding and sonication processes are

highly beneficial. Future work should carefully evaluate, if surface bound

molecules on the 2D WS2 surface can interfere with or even be utilized for

the intended applications. Additionally, further research of different two

solvent method combinations may lead to increased exfoliation yields and

larger lateral dimensions of the flakes with clean surfaces.

54

3.5 References

1 Coleman, J. N. Liquid Exfoliation of Defect-Free Graphene. Acc. Chem. Res. 46, 14-22 (2013).


3 O'Neill, A., Khan, U. & Coleman, J. N. Preparation of High Concentration Dispersions of Exfoliated MoS2 with Increased Flake Size. Chem. Mater. 24, 2414-2421 (2012).

4 Lide, D. R. Physical Constants of Organic Compounds. Vol. 69 (CRC Press, 2005).

5 Halim, U. et al. A Rational Design of Cosolvent Exfoliation of Layered Materials by Directly Probing Liquid–Solid Interaction. Nat. Commun. 4, 2213 (2013).

6 Berrueco, C. et al. Sample Contamination with NMP-oxidation Products and Byproduct-free NMP Removal from Sample Solutions. Energy Fuels 23, 3008-3015 (2009).

7 Carey, B. J. et al. Two solvent grinding sonication method for the synthesis of two-dimensional tungsten disulphide flakes. Chemical Communications 51, 3770-3773 (2015).

8 Nguyen, E. P. et al. Investigation of two-solvent grinding assisted liquid phase exfoliation of layered MoS2. Chem. Mater. 27, 53-59 (2015).

9 Wang, Q. H., Kalantar-Zadeh, K., Kis, A., Coleman, J. N. & Strano, M. S. Electronics and optoelectronics of two-dimensional transition metal dichalcogenides. Nat Nano 7, 699-712 (2012).


11 Backes, C. et al. Edge and confinement effects allow in situ measurement of size and thickness of liquid-exfoliated nanosheets. Nat. Commun. 5, 4576 (2014).

12 Berkdemir, A. et al. Identification of individual and few layers of WS2 using Raman Spectroscopy. Sci. Rep. 3, 1755 (2013).

13 Lotya, M., Rakovich, A., Donegan, J. F. & Coleman, J. N. Measuring the lateral size of liquid-exfoliated nanosheets with dynamic light scattering. Nanotechnology 24, 265703 (2013).

55

14 Warren, B. E. & Averbach, B. L. The effect of cold-work distortion on X-ray patterns. J. App. Phys. 21, 595-599 (1950).

15 Palit, D., Srivastava, S. K., Chakravorti, M. C. & Samantaray, B. K. Study of layer disorder and microstructural parameters of molybdenum-tungsten mixed sulpho-selenide Mo0.5W0.5SxSe2-x (0 ≤ x ≤ 2) by X-ray line profile analysis. J. Mater. Sci. Lett. 15, 1636-1637 (1996).

16 Zhou, K.-G., Mao, N.-N., Wang, H.-X., Peng, Y. & Zhang, H.-L. A Mixed-Solvent Strategy for Efficient Exfoliation of Inorganic Graphene Analogues. Ange. Chem. Int. Ed. 50, 10839-10842 (2011).

17 Hansen, C. M. Hansen solubility parameters: a user's handbook. (CRC press, 2012).

56

Chapter 4

A Liquid Metal Flow-Reaction for the Synthesis of

Ultra-Thin Zinc Sulphide

4.1 Introduction

As discussed in chapter 2 the exfoliation method as presented in chapter 3

is limited to materials with stratified crystal structures. This chapter

focusses on the potential to utilize the naturally occurring surface layer of

gallium (Ga) alloys as a medium for synthesising non-stratified two-

dimensional (2D) zinc sulphide (ZnS).

This chapter introduces a galinstan based flow-reactor for the production of

(2D) ZnS, a well-known luminescent material that has been traditionally

used for many optical applications.1,2 ZnS is frequently found to be

synthesized as quantum dots or nanoparticles,3,4 however, multiple

attempts at making ultrathin ZnS sheets such as through vapor phase5 and

57

thermo-chemical6 methods have been met with limited success or been

synthetically challenging.7 In this work, thin ZnS sheets were produced by

continuously reacting sodium polysulphide (Na2Sx, where 2 ≤ x ≤ 5) with an

alloy of galinstan containing 11.7 wt% zinc, which is calculated to be a

eutectic mixture.8 Sodium sulphide (Na2S) and Na2Sx are reactive

compounds, which have been previously used in the synthesis of ZnS from

zinc metal and zinc oxide (ZnO).3,9

The Outcomes of this investigation are undergoing peer review at this time

by Chemistry, A European Journal

58

4.2 Experimental Section

4.2.1 Materials

Gallium (Ga, 99.99%), indium (In, 99.99%) and tin (Sn, 99.9%) were purchased

from Roto Metals. Sulphur (S, 99.98%), zinc (Zn, 99.9%), sodium sulphide (Na2S,

>97%), iodine (I, 99.99%) and N,N-dimethylformamide (DMF, 99.8%) were

purchased from Sigma-Aldrich. Methanol (99.9%) was purchased from VWR

chemicals. All chemicals were used without further processing. Macroscopic

metal precursors were chosen to avoid the formation of large amounts of surface

oxide, which can occur with micron-sized powders during storage.

4.2. Preparation of Galinstan

Ga, In and Sn were placed in a single glass beaker at a Ga:In:Sn ratio of

68.5:22.5:10 wt%. Tin and indium were purchased in the form of chips and ingots

which could be cut to size in order to achieve an accurate mass fraction. A large

ingot of gallium was melted in a hot water bath (~80°C) and the liquid metal was

transferred and weighed using plastic pipettes. Care was taken to achieve

accurate mass fractions. The system containing all three metals was then heated

to 300°C, which is well above the melting point of tin. Heat was applied until all

metals were molten, with the mixture then stirred thoroughly with a glass rod

before the alloy was cooled to room temperature. The liquid metal was separated

from oxides that formed during the melting process by decanting the liquid metal

using plastic pipettes, and only collecting liquid metal from the centre of the melt.

59

The alloy, which remained liquid at room temperature, is called Galinstan

throughout the manuscript

4.3.3 Preparation of Zn-Galinstan Alloy:

In order to alloy Zn into the mixture, 30 g of Galinstan and 4 g (11.7 wt%) of Zn

powder were mixed together inside a N2 glovebox (O2 content ~ 10-100 ppm) via

light grinding with a mortar and pestle for 10 minutes. After grinding, the alloy

was transferred into a plastic vial using plastic pipettes. The Zn-Galinstan alloy was

then stored in the same glovebox.

4.3.4 Preparation of Polysulphide Solution

20 ml of methanol was degassed with N2 and then transferred into a glovebox.

Na2S (~155 mg, 2 mMol) and sulphur (~65 mg, 2 mMol) were measured by weight

and then added to the solvent in a single glass vial. The solution was stirred at

room temperature for 1 hour before filtration with a 0.22 µm syringe driven filter.

4.3.5 Synthesis of ZnS

The 2D ZnS was synthesized in a flow reaction involving pumping the polysulphide

solution (0.1 mol L-1 Na2Sx in CH3OH) through the liquid metal as shown in Figure

4.2b. A centrifuge tube was modified in order to function as a reaction chamber.

A small hole was drilled into the bottom of the tube and a rubber tube was pushed

into this hole, which was then sealed with parafilm. 5 ml of polysulphide solution

and approximately 3.5 ml of the alloy were added into the centrifuge tube. The

60

polysulphide solution was then cyclically and continuously pumped through the

liquid metal at a rate of 5 ml/min for 2 hours.

After the reaction was completed the methanol phase was removed carefully with

a pipette (the liquid metal remained at the bottom of the tube) and centrifuged

at 15 kRPM for 30 min, the supernatant was then removed and the sediment re-

dispersed in DI water that was centrifuged again. This washing process was

repeated twice. The sediment was then resuspended in a 0.1 mol L-1 iodine

solution in DMF. The sample was then washed thrice more with pure DMF and

finally resuspended in DMF prior to use.

4.3.6 Characterisation

X-ray photoelectron spectroscopy (XPS) was performed using a Thermo Scientific

K-alpha system. The X-ray source was a monochromated Al Kα source and the

measurement was performed on 50 µl of the suspension drop cast onto a Si wafer.

Photoluminescence (PL) spectra were collected using a PerkinElmer LS50 on a

diluted sample (100:1). UV-Vis absorption measurements were performed on a

PerkinElmer Lambda 1050 fitted with an integrating sphere. TEM images were

taken using both a JEOL 1010 TEM and a JEOL 2100F STEM with 100 kV and 200 kV

accelerating voltages, respectively. AFM images were collected using a Bruker

Dimension Icon AFM.

TGA was performed on a PerkinElmer TGA Pyris 7 in a N2 environment (flow rate

20 ml/min). Initially the temperature was increased to 80°C at a rate of 10 °C/min

and held for 30 minutes, before increasing the temperature again to 200°C and

61

holding for a further 10 minutes, with the sample then heated to 750°C at 20

°C/min and the N2 flow replaced with air at 600 °C. To isolate a sample for TGA a

suspension of the material was centrifuged for 1 hour at 15 kRPM. Care was taken

to ensure that the entire sample was transferred into the TGA pan.


The flow-reaction was performed as outlined in Figure 4.2 and in the

Experimental section. The methanolic Na2Sx solution was then continuously

circulated through the Zn-Galinstan alloy from the base of the reaction

vessel with a peristaltic pump, with discrete droplets of the Na2Sx solution

rising through the denser liquid metal (Figure 4.1). The polysulphide reacts

with the metal species at the liquid/liquid interface, leading to sulphide

formation.

62

Figure 4.1. Images are taken from a video recording of the reaction with

graphical representation of the surface of the liquid metal.

Thermodynamic considerations lead to the conclusion that the reaction

between the polysulphide ions and zinc results in the greatest reduction in

Gibbs free energy (see Table 4.1), with the reaction proceeding according

to the following equation:

yZn + Na2Sx → yZnS + Na2Sx−y

63

Table 4.1. Thermodynamics of the compounds relevant to the interfacial

reaction.

Material Enthalpy of formation

(kJ mol−1)

ΔfH°

Gibbs energy

(kJ mol−1)

ΔfG°

Standard entropy

(J mol−1 K−1)

S°

Reference

GaS −144.4 −159.6 21 10

InS −138 −131.8 67 11

SnS −100 −98.3 77 11

ZnS −192.6 −201.3 57.7 11

S2− 33.1 85.8 −14.6 11

Sx2− a 13 – 6.6 77.6 – 66 −22 – 100 12

a x≥2 and x≤5

The possible formation of other products, such as GaS, InS and SnS feature

significantly smaller reductions of Gibbs free energy and are thus not

expected to preferentially form. Degassed methanol was used to avoid the

possibility of forming zinc oxide/hydroxide.

64

Figure 4.2. Depiction of the synthetic method. (a) Alloying of liquid

Galinstan and Zn powder. (b) Flow-reaction between Zn-Galinstan alloy and

polysulphide (i) Dispersion of ZnS and by-products. (ii) ZnS forming on the

interface between Na2Sx solution and the alloy. (c) Washing process

displaying (i) solution before washing and iodine treatment, (ii) solution

after centrifugation (iii) resuspended ZnS in DMF.

Once the reaction was completed, the suspension was decanted from the

liquid metal. A reaction workup procedure, which consists of a multi-stage

65

centrifuge based washing procedure outlined in Figure 4.2c and the

Methods section, was devised to remove remaining polysulphide (Na2Sx), as

well as small metallic droplets of the alloy that were suspended due to the

physical agitation during the synthesis.13 De-ionized (DI) water assisted in

the removal of the polysulphide by transforming Na2Sx into sodium sulphate

(NaSO4), which is highly soluble in H2O.14 Thus, after repeated washings with

water, residual Na2Sx and NaSO4 were completely removed.

After the first purification stage, the suspension was predominantly

composed of ZnS, with some residual metallic precursor particles

remaining. These metallic contaminants were removed using a solution of

N,N-dimethylformamide (DMF) containing iodine, which effectively

transforms any residual unreacted metals such as Ga, In, Sn or Zn into easily

dissolved GaI3/InI3/SnI4/ZnI2. This reaction was followed by repeated

washing in clean DMF which was sufficient to remove any iodine and metal

iodides (Figure 4.2c, ii). The final product of ZnS flakes was resuspended in

DMF and stored prior to further characterisation (Figure 4.2c, iii).

66

Figure 4.3. Morphology of the 2D ZnS. (a) TEM image of the ZnS flakes. (b)

HRTEM image of the ZnS which lattice dimensions marked, with fast Fourier

transform (FFT) pattern (inset). (c) Statistical distribution of the lateral

dimension of the ZnS flakes. (d) AFM micrograph of a flake with height

profile (insert) along the blue line.

Transmission electron microscopy (TEM) revealed that large, thin sheets

with lateral dimensions measuring several microns (predominantly 1–3 µm)

67

were obtained (Figure 4.3a,c). These results, as well as those shown in

Figure 4.4, are a good representation of the distribution of flake sizes.

High-resolution TEM imaging (HRTEM) revealed that the nanosheets are

composed of many small nanocrystals, with the FFT of the area indicating

the sheets had a polycrystalline structure (Figure 4.3b). The structure

possibly formed due to the large number of nucleation sites that developed

at the interface between the polysulphide solution and the liquid metal.

During the synthesis, the interfacial layer is expected to protrude as the

bubble expands, allowing additional nucleation sites to appear, ultimately

leading to the observed polycrystallinity in a single nanosheet. The lattice

spacings of 3.3 Å and 3.1 Å correspond to the 100 and 002 planes,

respectively, of hexagonal ZnS, which is also known as wurtzite ZnS. The

synthesis of wurtzite ZnS typically requires elevated temperatures,15

however, the process described in this work takes place at room

temperature, which can be seen as a considerable advantage over other

synthetic strategies.

Atomic force microscopy (AFM, Figure 4.3d) revealed that the synthesized

2D ZnS nanosheets have a thickness in the order of a few nm, which

corresponds to several unit cells of ZnS (measured in the c direction). The

observation of wrinkles in the TEM also suggests atomically-thin nature of

the sheets. Furthermore, it can be seen in these images that some residual

metal appears on the sheets, sometimes in the form of small balls on the

flakes’ surface

68

Figure 4.4. TEM images of 2D ZnS sheets produced via the reaction of Na2Sx

with the Zn alloy.

Interestingly, the configuration of the flow reactor has a significant impact

on the efficacy and outcome of the reaction and the morphology of the

product. When the polysulphide solution was introduced to the liquid alloy

from the side, bubbling was not observed. This is indicative of the formation

of semi-permanent microchannels within the liquid metal, through which

the methanolic solution may flow, reducing the reaction interface. This

could be attributed to different fluid mechanics that applied to the process,

such as the change in pressure when injecting the solution from the side,

due to a lower height of the liquid metal column. It was also observed that

this alternative approach significantly reduced the lateral size and alters the

shape of the ZnS forming nanoparticles, nanoplatelets and nanoflakes as

opposed to sheets (see Figure 4.5). This indicates that a prolonged

residence time of polysulphide in the alloy may lead to laterally larger sheet

sizes. Furthermore, the crystallinity of the sample also appears to be

improved, which were attribute to the longer reaction times that may apply

within the microchannels.

69

Figure 4.5. TEM images (a-c) and HRTEM (d, FFT insert) of ZnS produced

with a modified reaction chamber where the Na2Sx is not pumped directly

into the Galinstan from underneath, but instead from the side.

Substituting Na2Sx for Na2S gave smaller lateral dimensions, however the

thickness of the flakes remained unaffected by this change (Figure 4.6).

These changes, indicating that the observed thickness corresponds to the

sulphide that grew in a self-limiting interfacial surface reaction.

70

Figure 4.6. TEM image (a) of ZnS produced via a modified reaction chamber

where a 0.1 mol L-1M Na2S solution is also substituting the polysulphide

solution and is not pumped directly into Galinstan from underneath, but

instead from the side. (b & c) An AFM image and profile, which demonstrate

that the thickness of the flakes is not altered by a change in the reaction

protocol.

71

Figure 4.7. XPS spectra of the (a) S 2p and (b) Zn 2p regions of the ZnS flakes.

Peak fitting is demonstrated for all the relevant shells.

Elemental analysis of the product utilising X-ray photoelectron

spectroscopy (XPS) revealed that the chemical composition of the sample is

stoichiometric ZnS. The S 2p region of the XPS spectra features a doublet

which in agreement with S2− (Figure 4.7a). The Zn 2p region displays a

doublet consistent with Zn2+ (Figure 4.7b).15,16 A small peak at 160.7 eV was

observed, which corresponds to the Ga 3s binding energy, indicating the

presence of small amounts of residual gallium.

72

Figure 4.8. Schematic representation of the simplified flow dynamics within

the liquid metal reactor. F, V, A, T, D, t, and N represent the volume of the

flow rate, the volume of bubbles, the surface area of bubbles (i.e. interfacial

area), the thickness of ZnS form, density of ZnS, time of reaction, and the

total number of bubbles during the reaction, respectively.

The flow dynamics of the reactions are depicted in Figure 4.8. Knowing that

the flow of the solution is 5 ml/min (F) and counting the average bubble

rate, the average bubble size was determined to be 125 μl (V). Assuming

the bubbles maintain a constant volume and a spherical shape, the

interfacial area was calculated to be 1.21×10−4 m2 (A) based on simple

geometry. Furthermore, assuming the ZnS layer formed at the interface has

a constant 6 nm thickness (T) The potential volume of ZnS formed at the

interface of the bubble can be calculated. Knowing the density of ZnS (D)

73

each bubble could potentially produce 2.97 μg of ZnS or 17.8 mg (M) over

the course of the 2-hour reaction (t).

Thermogravimetric analysis (TGA) was utilized to determine the solid

content of the final suspension, which permitted an estimation of a reaction

yield of 2.14 mg (Figure 4.9). The theoretical maximum reaction yield of

17.5 mg was obtained using the measured thickness of the ZnS nanosheets

and the total surface area of the Na2Sx droplets passing through the metal

alloy. For this purpose, the average bubble volume was estimate by

correlating the number of observed bubbles with the known flow rate and

assuming the bubbles are approximately spherical within the liquid metal,

and that the entire interface is covered with ZnS (Figure 4.8). As a result, an

overall reaction yield of 12% is reported for the flow reaction. Increasing the

reaction time, by prolonging the time the injected polysulphide solution

resides within the liquid metal, should result in even higher overall reaction

yields. This may be achieved by simply increasing the height of the liquid

metal column.

74

Figure 4.9. TGA of the centrifuged ZnS solution demonstrating weight

reduction relative to (a) temperature and (b) time. The red dashed line

indicates the solid weight of the dried ZnS which was utilized for the yield

calculations. (a, inset) The change in weight as the N2 flow is replaced with

an air flow at 600°C.

Furthermore, an improved workup procedure may limit the amount of

product lost during repeated centrifugation steps. TGA allowed the

estimation of the total amount of residual metal to be less than 5 wt% from

the increase in weight at this temperature range attributed to the oxidation

of the sample (see Figure 4.9). Based on the expected formation of

Zn3O(SO4) due to the oxidation of ZnS and the expected formation of Ga2O3,

In2O3, SnO2 and ZnO the mass fractions of the present metal were estimated

based upon literature values.11,17

75

Figure 4.10. Spectroscopic characterisation of the ZnS suspensions. (a) PL

emission spectrum excited at 275 nm with simplified electronic band

structure (inset). The red lines indicate the conduction and valence band

and the black line is the fermi level. (b) UV-Vis absorption spectrum with

Tauc plot (inset).

The optical properties of a ZnS nanosheet suspension are shown in

Figure 4.10. Photoluminescence (PL) spectroscopy revealed a dominant

emission at approximately 3.15 eV with a quantum yield of 1.41% (see Table

4.2 & Figure 4.10a).

76

Table 4.2. Quantum yield estimations based on comparative analysis with

quinine sulphate.

Sample Solvent Refractive index

Absorbance at 325 nm

Integrated emission

Quantum yield

Quinine sulphate

0.1 mol L-1 H2SO4

1.33 0.0469 3.822 × 105 54% (reference 18)

ZnS DMF 1.43 0.0267 5.511 × ×103 1.41% (this work)

These findings are in excellent agreement with previous reports on ZnS.1,19

The observed emission peak also correlates very well with the Tauc plot

derived from UV-Vis absorption spectroscopy (Figure 4.10b), allowing us to

infer that the 3.15 eV emission is due to the band edge of the material. A

simplified band structure of the synthesized ZnS nanosheets based on the

values obtained from optical characterisations and XPS valence band

analysis (Figure 4.11) is displayed as an inset in Figure 4.10a.

77

Figure 4.11. Valence band analysis from XPS with line fitted on the valance

band edge for clear determination of energy difference between the

valence band maxima and the Fermi level.

These observations indicate that the 2D ZnS nanosheets feature p-type

semiconductivity. Interestingly, the determined band gap is slightly lower

than what has been previously reported for h-ZnS (3.4–3.7 eV).2,3,5,15,16 This

may be rationalized with the presence of intrinsic defects within the

wurtzite crystal structure, which are known to be introduced when the

synthesis is conducted at low temperatures.20 These defects have been

reported to reduce the electronic bandgap of ZnS to approximately 3.25

eV.20

78

4.4 Conclusions

In conclusion, this chapter demonstrated a gallium-based room

temperature liquid alloy acting as a reaction medium, which introduces a

new pathway for synthesising 2D ZnS. The reaction was implemented in a

flow reactor and relied on the reaction occurring at the liquid/liquid

interface, resulting in 2D nanosheet formation. The findings are particularly

important since ZnS is a naturally non-stratified crystal system, for which

conventional 2D synthesis techniques, such as mechanical exfoliation,

cannot be applied. The report introduces ambient-temperature liquid

metals as suitable reaction media that can fulfil the function that is

traditionally reserved for covalent systems (solvents) and ionic liquids.

Overall, the liquid metal reaction route is a scalable process, providing

access to 2D compounds within a relatively short reaction time. The process

is likely applicable to other metals, which can be alloyed into the liquid

metal, likely giving access to many other 2D compounds that might

otherwise be inaccessible due to their lack of a stratified crystal structure.

Similarly, precursor solutions, apart from polysulphides, may be

investigated, providing access to 2D morphologies of non-sulphide metal

compounds.

79

4.5 References

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2 Denzler, D., Olschewski, M. & Sattler, K. Luminescence studies of localized gap states in colloidal ZnS nanocrystals. J. App. Phys. 84, 2841-2845 (1998).

3 She, Y.-Y., Juan, Y. & Qiu, K.-Q. Synthesis of ZnS nanoparticles by solid-liquid chemical reaction with ZnO and Na2S under ultrasonic. Trans. Nonferrous Met. Soc. China 20, s211-s215 (2010).

4 Xu, J., Ji, W., Lin, J., Tang, S. & Du, Y. Preparation of ZnS nanoparticles by ultrasonic radiation method. App. Phys. A: Mater. Sci. Process. 66, 639-641 (1998).

5 Yu, S.-H., Yang, J., Qian, Y.-T. & Yoshimura, M. Optical properties of ZnS nanosheets, ZnO dendrites, and their lamellar precursor ZnS·(NH2CH2CH2NH2) 0.5. Chem. Phys. Lett. 361, 362-366 (2002).

6 Zhao, Z., Geng, F., Cong, H., Bai, J. & Cheng, H.-M. A simple solution route to controlled synthesis of ZnS submicrospheres, nanosheets and nanorods. Nanotechnology 17, 4731 (2006).

7 Sun, Y. et al. Fabrication of flexible and freestanding zinc chalcogenide single layers. Nat. Commun. 3, 1057 (2012).

8 Brunet, L., Caillard, J. & André, P. Thermodynamic Calculation of n-Component Eutectic Mixtures. Int. J. Mod. Phys. C 15, 675-687 (2004).

9 Scheel, H. Crystallization of sulfides from alkali polysulfide fluxes. J. Cryst. Growth 24, 669-673 (1974).

10 Nishinaga, T. & Lieth, R. Thermodynamical calculations on the chemical transport of GaS in a closed tube system. J. Cryst. Growth 20, 109-115 (1973).


12 Kamyshny, A., Gun, J., Rizkov, D., Voitsekovski, T. & Lev, O. Equilibrium distribution of polysulfide ions in aqueous solutions at different temperatures by rapid single phase derivatization. Enviro. Sci. Tech. 41, 2395-2400 (2007).

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13 Yamaguchi, A., Mashima, Y. & Iyoda, T. Reversible Size Control of Liquid-Metal Nanoparticles under Ultrasonication. Ange. Chem. Int. Ed. 54, 12809-12813, doi:10.1002/anie.201506469 (2015).

14 Okorafor, O. C. Solubility and Density Isotherms for the Sodium Sulfate−Water−Methanol System. J. Chem. Eng. Data 44, 488-490, doi:10.1021/je980243v (1999).

15 Chen, X. et al. Kinetically controlled synthesis of wurtzite ZnS nanorods through mild thermolysis of a covalent organic− inorganic network. Inorg. Chem. 42, 3100-3106 (2003).

16 Clark, R. M. et al. Two-step synthesis of luminescent MoS2–ZnS hybrid quantum dots. Nanoscale 7, 16763-16772 (2015).

17 Behl, M. Desulfurization by reactive adsorption on oxide/metal composites, University of Illinois at Urbana-Champaign, (2014).

18 Melhuish, W. Quantum efficiencies of fluorescence of organic substances: effect of solvent and concentration of the fluorescent solute1. J. Phys. Chem. 65, 229-235 (1961).

19 Kitai, A. Luminescent materials and applications. Vol. 25 (John Wiley & Sons, 2008).

20 Patel, S. P., Pivin, J., Chandra, R., Kanjilal, D. & Kumar, L. Intrinsic defects and structural phase of ZnS nanocrystalline thin films: effects of substrate temperature. J. Mater. Sci. Mater. Electron. 27, 5640-5645 (2016).

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Chapter 5

Wafer Scale Two Dimensional Semiconductors from

Printed Oxide Skin of Liquid Metals

5.1 Introduction

As discussed in chapter 2 and expanding upon the concepts investigated in

chapter 4 this chapter discusses the potential to use the naturally occurring

oxide surface of gallium (Ga) as a precursor directly deposited onto an oxide

substrate as a precursor for wafer scale production of gallium sulphide

(GaS).

In this chapter, a new method is developed for the deposition and patterning of

wafer scale 2D post-transition metal chalcogenide (PTMC) compounds. The 2D

gallium sulphide deposition process described here utilises a novel approach of

placing a bulk of gallium (Ga) liquid metal directly onto a silicon dioxide (SiO2)

coated substrate that leaves a layer of 2D oxide of gallium on the wettable areas

(Figure 5.1c,d). There are many properties of Ga that make it a compelling

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material for this process: liquid Ga has a low bulk viscosity (approximately twice

that of water)1 that flows with ease, and unlike mercury, Ga has low toxicity and

essentially no vapour pressure at room temperature2. Ga and Ga alloys have

previously been used for printing of conducting electrical tracks3-5 however, here

instead of using the bulk metal the oxide skin which is left on the substrate is

utilized. Like many post-transition metals, Ga rapidly (within milliseconds) forms a

thin oxide layer on its surface when exposed to oxygen6. This gallium oxide layer

is initially one-unit cell thick7, which under ambient atmospheric conditions grows

very slowly with time8. The atomically thin film is very robust and can

mechanically stabilise the liquid metal against deformations. The surface oxide

has been shown to adhere strongly to (wet) many materials and particularly to

oxides (e.g. SiO2)9. Certain surface modifications of oxide substrates, such as the

functionalisation with fluorocarbon chains, can inhibit this wetting effect10.

The technique presented here is based on depositing and transforming the native

interfacial metal oxide layer of metal precursors in liquid form. Group III and IV

metals such as Ga, In and Sn have low melting points. It is known that in an oxygen

containing atmosphere, these metals quickly form an atomically-thin protective

and highly wettable oxide layer in a self-limiting reaction6. The presence of the

oxide layer increases the adhesion of the post-transition liquid metal on oxygen

terminated substrates via weak but surface dispersed Van der Waals forces9,11.

After the placement of the liquid metal with its self-limiting skin onto the

substrate, the metal can be removed readily, due to the relatively low binding

energy to its surface oxide skin, which leaves the atomically thin oxide layer

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behind on the substrate. Harnessing these phenomena, this work devised a model

process that uses low melting point Ga (29.7C) to deposit wafer scale, printable

2D GaS (and also extend it to 2D indium sulphide). Here, the oxide skin of Ga is

placed onto a patterned substrate that are then sulphurised via a relatively low

temperature two-step process, producing large area patterned 2D GaS.

Controlling the surface chemistry of the substrate allows for selective patterning

of the deposited material.

The outcomes of this investigation have been peer reviewed and published in

Nature Communications.

84

5.2 Experimental section

5.2.1Materials

Gallium (Ga) (99.99%), Sulphur (S) (99.98%), Hydrochloric acid (HCl) (36.5-38%),

Hydrobromic acid (HBr) (48%) and 1h-1h-2h-2h-perflourodecyltriethoxysilane

(FDTES) (97%) were purchased from Sigma-Aldrich®. AZ-1512 photoresist, AZ-

400k developer, acetone and isopropyl-alcohol (IPA) were purchased from VWR®

chemicals. Indium (In) (99.99%) was purchased from RotoMetals®. PDMS and

cross-linking agent were purchased from the Dow Corning Corporation. All

chemicals were used without further processing unless otherwise stated.

5.2.2 Preparation of materials

Ga was placed in a 1% v/v solution of HCl to dissolve the surface oxide. PDMS was

cured with a 10:1 ratio (elastomer:crosslinker) and allowed set for a minimum of

48 hours at room temperature under vacuum and then plasma treated under O2

flow. Wafers of 200 nm silicon dioxide (SiO2) on Si (SiO2/Si) were washed with

acetone, IPA and distilled water in sequence, baked at 125°C and cleaned with an

O2 RF plasma cleaner prior to use.

5.2.3 Synthesis of 2D GaS

The Ga in HCl was heated to 60°C and placed onto a SiO2/Si substrate also heated

to 60°C as described in Figure 5.1. The liquid gallium was then spread across the

substrate followed by its removal via wiping using soft PDMS. The gallium oxide

remains on the silicon oxide surface during the wiping process due to the very

85

strong van der Waals adhesion between gallium oxides and the substrate.

However, the liquid gallium has a much weaker adherence to the deposited

gallium oxide and thus can be easily removed by wiping the excess liquid metal,

leaving a thin layer of cracked gallium oxide behind. The printed wafer was then

cooled to room temperature. The substrate was then held directly over warm

(45°C), fuming HCl (37%) for 5 min to achieve chlorination. The chlorinated sample

was then sulphurised via a chemical vapour method in a tube furnace under N2

flow (~100 ccm). For sulphurisation, S powder was placed in a tungsten boat half

way down the furnace and heated to 400°C for 90 min, while the GaCl3 samples

were placed face down on a tungsten boat downstream heated to approximately

300°C. 2D In2S3 was synthesised in the same manner; however, a few minor

changes were applied. Indium metal was heated to 170°C for melting the metal

and during the deposition of the oxide. The oxide was then treated with

hydrobromic acid (HBr) fumes as opposed to HCl. This acid was chosen due to the

more favourable melting temperature of indium bromide which is lower than that

of indium chloride. The HBr treated sample was then sulphurised at 400°C.

5.2.4 Fabrication of structures

AZ 1512 was spin-coated onto a cleaned SiO2/Si wafer at a speed of 6000 rpm to

achieve a thickness of approximately 1 µm. The wafer was then soft baked at 95°C

for 60 s before UV exposure (350-450 nm) through a patterned photomask. The

exposed wafer was then developed in a 1:4 solution of AZ 400K and H2O for 45 s

then rinsed thoroughly with H2O. The wafer with patterned photoresist was then

suspended face down on the roof of a Pyrex® Petri dish containing 200 µl of

86

FDTES. The Petri dish was placed on a 120°C hotplate for 2 hours to functionalise

the exposed SiO2 with a monolayer of fluorocarbon. The roof of the Petri dish was

then flipped and the hotplate heated to 180°C for 30 min to evaporate any excess

FDTES. The wafer was then washed with acetone, IPA and water to remove the

photoresist. The patterned substrate was then printed with Ga as outlined in

section 5.3.3.

5.2.5 Fabrication of transistors

20 nm of tungsten (W) was electron beam (e-beam) deposited onto a SiO2/Si

wafer. AZ 5214E was then spin-coated onto the W coated wafer at a speed of

4000 rpm. The wafer was then soft baked at 110°C for 60 s before UV exposure

through a patterned photomask. This step was followed by reverse baking of the

wafer at 120°C for 120 s followed by flood exposure to UV. The exposed wafer

was then developed in a 1:4 solution of AZ 400K and H2O for 30 s then rinsed

thoroughly with H2O. The substrate was then placed in 50 ml of a 30% solution of

H2O2 and ~100mg of NaOH for 10 s to remove all unprotected W. 2D GaS was then

printed as outlined in section 5.3.3.

5.2.6 Characterisation:

X-ray photoelectron spectroscopy (XPS) was performed using a Themo Scientific

K-alpha system. The X-ray source was a monochromated Al Kα source. EDX

spectra were collected using a FEI Nova nano-SEM fitted with AZTEC X-ray

detector. Spectra were collected with a 3 KeV accelerating voltage chosen in order

to provide sufficient range of measurement whilst attempting to limit substrate

87

contributions. XRD was performed using a Bruker D4 Endeavour featuring a CuKα

X-ray source ( = 1.54 Å). Raman measurements were conducted utilising two

systems a WITec alpha 300R and a Horiba Scientific LabRAM HR evolution Raman

spectrometer both using a 100× objective, a 532 nm laser with a spot diameter of

∼500 nm was used.

AFM images were collected using a Bruker Dimension Icon AFM with scanasyst®

software in peak-force tapping mode. Confocal PL maps were gathered with a

Nikon N-STORM in confocal mode with excitations at 488 and 640 nm.

PL and UV-Vis spectroscopy were conducted on GaS printed on quartz substrates

as opposed to Si/SiO2, the method was otherwise unchanged. PL spectra were

collected using a PerkinElmer LS50 and were averaged over 10 scans. UV-Vis

measurements were performed on a PerkinElmer Lambda 1050 fitted with an

integrating sphere. Additional PL measurements including decay-time

measurements were conducted using on a custom build instrument using a

Fianium, WhiteLase SC400-8 supercontinuum optical source delivering

approximately 200 W average power to the sample with a 10 nm optical

bandwidth and a 100× air objective. A waveplate polariser was incorporated into

the excitation path to obtain polarised PL maps. Polarised maps were collected

successively in increments of 5°.

TEM images were taken using a JEOL 2100F STEM with a 200 kV accelerating

voltage. Preparation of TEM samples involved exfoliating flakes from the printed

GaS film by gently scraping with a razor blade. Photoelectron spectroscopy in-air

(PESA) was conducted using a Riken Keiki AC-2, photoelectron spectrometer. FET

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and photodetector characterisations were performed using a Keithely SCS 4200

semiconductor Characterisation system, Keithely 2001 multimeter, and a ABET

technologies 150 W Xenon light source. Space charge limited currents (SCLC) were

measured using a Keithly 2600 source meter.

5.2.7 Computational analysis of band structure

Spin-dependent hybrid density functional theory (Hybrid DFT) calculations were

performed using Gaussian basis set ab initio package CRYSTAL14.12 The B3LYP

hybrid exchange-correlation functional was used13, augmented with an empirical

London-type correction to the energy to include dispersion contributions to the

total energy. The correction term was the one proposed by Grimme14 and

successfully used with B3LYP to calculate cohesive energies in dispersion bonded

molecular crystals.15 For all atoms a triple zeta valance (TZV) basis set, with

polarisation functions, was used to model the electrons.16,17 A 9×9×9 Monkhorst-

Pack k-point mesh18 was used to calculate band structure and total electronic

density of states for GaS bulk, while a 9×9×1 k-point mesh was used for the mono-

layer, bi-layer, tri-layer and four-layer slabs. Each structure was each

independently geometry optimized prior to calculation of the electronic

properties.

89


90

Figure 5.1. GaS representation and printing process of the 2D layers: stick and ball

representation of GaS crystal: (a) side view of bilayer GaS, showing a unit cell c =

15.492Å made of two GaS layers. (b) Top view of the GaS crystal. The GaS crystal

lattice is composed of Ga-Ga and Ga-S covalent bonds, which extend in two

dimensions, forming a stratified crystal made of planes that are held together by

van der Waals attractions. The Ga atoms are green and the S atoms are blue. (c

and d) Functionalisation of the substrate with FDTES changes the contact angle

between a Ga drop and the substrate by 12.9°. The SiO2 substrate becomes more

hydrophobic and thus resists wetting by liquid Ga. (e to h) Schematics of the

synthesis process for patterning 2D GaS via printing the skin oxide of liquid Ga. (e)

Lithography process for establishing the negative pattern of the photoresist. (f)

Covering the exposed area of the substrate with vaporised FDTES. (g) Placing Ga

liquid metal and removing it with soft PDMS that leaves a cracked layer of Ga

oxide. (h) Two concurrent steps of chemical vapour treatment: firstly, GaCl3 layer

is formed via exposure to HCl vapour and secondly, sulphurisation by exposure to

S vapour forming GaS.

In this work, liquid Ga is placed on a SiO2/Si substrate (as shown in Figure 5.1e-h

and described in the Experimental section), leaving an atomically thin-cracked

layer of gallium oxide behind, upon removal of the liquid metal (Figure 5.2). As

the liquid gallium/gallium oxide is printed, a scattered oxide layer appears with a

strong adherence on the substrate. This cracked thin film of flat gallium oxide

nano-flakes allows for the easy removal of the liquid metal by scraping with a slab

91

of PDMS. This process is implementable even as the liquid metal is printed fresh

because of the rapid formation of the oxide layer.

Selective area deposition of gallium oxide is achieved by selective treatment of

the substrate using 1h-1h-2h-2h-perflourodecyltriethoxysilane (FDTES), as shown

in Figure 5.1 e&f, resulting in non-wettable regions on the surface. The treated

areas of the substrate prohibit the adhesion of the 2D gallium oxide layer (Figure

5.1g), effectively facilitating selected area deposition. The printed pattern is found

to be visible by eye, due to interference effects (Figure 5.1g inset).

Figure 5.2. (a) AFM image of cracked layer of gallium oxide and corresponding

profile (b).

A hydrochloric acid (HCl) vapour treatment (Figure 5.1h) is used as an

intermediary step between patterned oxide deposition and sulphurisation of the

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oxide. The HCl vapour transforms the thin gallium oxide layer into primarily

GaCl39,19, which aids subsequent sulphurisation via two means. Firstly, gallium

oxide is chemically inert and its direct sulphurisation requires high temperatures

(>900C) together with exposure to toxic H2S gas6. The reduced process

temperature is highly advantageous since it allows the inclusion of a broad range

of substrates, including certain polymers and glasses. Secondly, the temperature

used herein during sulphurisation is still higher than the melting point of the

halide, thus encouraging efficient recrystallisation and continuity in the resultant

2D layer. Eventually, sulphurisation of GaCl3 (Figure 5.1h) is conducted at low

temperature (~300°C) that is compatible with existing electronic device

processing standards. This process can be extended to forming other 2D PTMCs.

An example for depositing atomically thin In2S3 is also presented in Figure 5.3,

here it can be seen that the In2S3 film is smooth and continuous in a relatively

large area range. The thickness of the 2D In2S3 is on average 2.6 nm, which is about

three times the fundamental plane thickness (0.9 nm) of this material.

93

Figure 5.3: Analysis of 2D In2S3 synthesised via printing of liquid indium. (a) Raman

spectrum, (b) Indium 3d region of X-ray photoelectron spectrum (XPS) and (c)

AFM image including profile (d) along the blue line shown in (c).

94

Figure 5.4. Material characterisations of printed GaS: (a) X-ray Diffraction (XRD)

of printed GaS film (after multiple printing steps) with XRD of the printed oxide

skin and SiO2/Si substrate for comparison and GaS planes indicated (traces have

been normalised and offset for clarity). (b) Raman spectrum of the 2D GaS with

GaS vibrational peak shifts indicated. (c & d) XPS of the 2D GaS for the regions of

interest (c) Ga 3d and (d) S 2p.

X-ray diffraction (XRD) is utilised to assess the obtained phase and phase purity of

the sulphurised few-layer films (several layers of GaS are sequentially printed to

increase the XRD signal intensity). The XRD patterns shown in Figure 5.4 present

GaS peaks and the lack of peaks from other stoichiometries (e.g. Ga2S3)

demonstrate that the synthesised film has a stratified structure and the desired

95

monosulphide phase20-23. Furthermore, The GaS peak corresponding to the 002

plane (11.6°) reveals a layer spacing of 7.62 Å, which is in good agreement with

previous studies20,23 Raman spectroscopy (Figure 5.4b) conducted on a single

printed film reveals the characteristic features of a few layer thick GaS, including

the A11g (187.5 cm-1), A1

2g (374.8 cm-1) and E12g (303.75 cm-1) vibrational peaks,

matching those reported previously20,24. The observed relative intensity of the

phonon peaks between the out-of-plane (A11g, A1

2g) and in-plane (E12g) vibrations

of 0.25 and 0.56 for A11g/E1

2g and A12g/E1

2g respectively, are uniquely associated

with bilayer GaS23,24. X-ray photoelectron spectroscopy (XPS) further confirms the

stoichiometry of the printed layer. The main Ga 3d peaks (Figure 5.4c) are

associated with stoichiometric GaS (Ga2+), with minor contributions from Ga at

other oxidation states such as Ga0 and Ga3+. A small peak from elemental Ga is

also observed in the Ga 3d region, which has been associated with the slow

decomposition of GaS in moist air.20 The S 2p and Ga 3s peaks (Figure 5.4d) as well

as analysis of the Cl 2p and O 1s ranges (Figure 5.5) further confirm the

stoichiometry of the 2D material.

The O 1s range (Figure 5.5c) of the XPS spectrum demonstrates a significant

contribution from SiO2 (532.65 eV) while there are other minor contributions from

other oxides including gallium-oxides20. The Cl 2p range (Figure 5.5b) of the XPS

spectra also depicts some residual Cl. The intensity of this peak however (~1000

CPS above baseline ~5800 CPS) is almost negligible when compared to that of Ga

or S peaks which are significantly more intense (~16000 and ~8000 CPS above

96

baseline respectively). XPS analysis of the printed 2D GaOx and GaCl3 are shown in

Figure 5.6.

Figure 5.5. (a-c) XPS study of GaS film displaying (a) survey spectrum, (b) Cl 2p and

(c) O 1s regions of the spectrum. (d-f) EDX of printed 2D GaOx film (d), the film

after treatment with HCl (e) and after sulphurisation (f).

97

Figure 5.6. XPS patterns of the printed metal oxide film before (a-e) and after (f-j)

treatment with HCl vapour. (a & f) Survey, (b & g) Ga 3d, (c & h) Ga 3s, (d & i) Cl

2p and (e & j) O 1s regions.

98

Figure 5.7. Morphology and characteristics of the printed 2D films: (a) AFM

image with (b) height profile) of the printed 2D GaS layer confirming bilayer

deposition. The profile is offset to the substrate’s surface. (c&d) Confocal PL map

shows a patterned area made of 2D GaS (green) on a SiO2/Si substrate (black)

demonstrating a near-uniform 2D layer is obtained in a large area along with the

PL spectra from areas noted on map (c) patterns 1 and 2). Due to the excitation

wavelength limitation of confocal PL, the deep trap emissions are used for

assessing the uniformity of the PL pattern. (e) PL emission spectra demonstrating

contributions of the interband transitions (red) and deep trap recombinations

(black). The deep trap emission spectrum is scaled by a factor of 10 for clarity. (f)

HRTEM of a flake mechanically scratched from the printed GaS film.

99

Atomic force microscopy (AFM, Figure 5.7a-b) reveals that the films have a

thickness of approximately 1.5 nm, corresponding to two layers of GaS, which is

in good agreement with the Raman spectrum. High resolution transmission

electron microscopy (HRTEM) (Figure 3f) shows a lattice spacing of 3.1 Å which

correlates to the 101 plane of GaS21.

Confocal PL microscopy (Figure 5.7d) verifies that the GaS has been successfully

patterned. This PL is only seen after sulphurisation (Figure 5.7c-e). The PL has two

major components centred at 393 and 641 nm. The peak at 393 nm is due to the

direct bandgap recombination at the Γ point. The energy difference associated

with this peak is in excellent agreement with our density functional theory (see

section 5.3.7 for computation information) band structure calculations of bilayer

GaS (Figure 5.12a). Additionally, defects in GaS crystals are known to produce

deep trap recombination states25, which are associated with the 641 nm peak.

One of the main outcomes of this work is the deposition of patterned 2D GaS

displaying near-uniform PL on a large scale with consistent PL intensity (Figure

5.7c). The dimensions of the patterned area shown in Figure 5.7d exceed 1 mm,

proving that 2D GaS covering areas of mm2 can be readily achieved.

100

Figure 5.8. AFM image (a) of 2D GaS film and statistical distribution of the

measured pixel heights (b) of the substrate (blue) and film (red) within the areas

in the blue and red squares, respectively. Scale bar in (a) is 5 µm.

The surface of the 2D film is also shown to be atomically smooth (Figure 5.8). The

statistical distribution the 2D GaS shows that the film is highly continuous and the

height distribution of the substrate and the film are similar for values below than

the mean height. However, above the mean, the distribution is extended further

to the higher values for the substrate measurement leading to the skewed

appearance. This is due to the hydrophobic nature of the substrate which leads to

the deposition of some particles on its surface. In comparison, the surface of 2D

GaS is much cleaner than the substrate, indicating controlled film growth. The

standard deviation for the 2D GaS film is 0.137 nm which is associated with a

101

continuous deposited film and the absence of nano or sub-micron size cracks as

well as the absence of regions of overlapping growth that could occur at domain

boundaries. Furthermore, there is no contribution from the substrate level (0 nm)

in the red pattern attesting the absence of cracks and holes even further.

Figure 5.9. Statistical Raman analysis of the 2D GaS films. Raman line scan analysis.

(a) Optical image, the line scan was performed on the black line indicated. (b)

Contour plot with phonon modes indicated. (c) Relative intensity of the

E12gphonon peak with standard error indicated.

A statistical analysis using Raman spectroscopy investigating the uniformity of the

bilayer GaS is also in Figure 5.9. The measurements and analysis show a good

102

uniformity for the deposited bilayer GaS. The relative intensity of the E12g peak is

shown to have a standard error of only 2.5% (Figure 5.9c) across an area spanning

100 µm.

Raman spectra of 2D GaS (Figure 5.10a) spaced at much wider area than

presented in Figure 5.9. The relative intensity of the E12g peak only slightly varies

slightly when macroscopic distances of millimetres are investigated,

demonstrating the strong consistency of the bilayer GaS across a large surface

area. It can be seen from (Figure 5.10b) that the boundaries are laterally

connected with no overlap, however there are significant lattice defects around

the boundaries. Furthermore, from (Figure 5.10c) the domain sizes are seen to

range from ~1 µm2 to 10’s of µm2.

Figure 5.10. (a) Raman spectra of 2D GaS spaced at approximately 1 mm intervals.

(b & c) Analysis of crystal grains of the printed 2D GaS film featuring (b) a HRTEM

image of a grain boundary and (c) a map of grain boundaries determined via

polarised confocal PL as has been previously demonstrated by Pan et al26.

Different shades of grey indicate areas that luminesce at different polarisation

angles which are associated with crystal domains and their lattice orientation

within the 2D GaS layer.

103

Additional HRTEM image are presented in Figure 5.11a as part of an investigation

of the concentration and nature of defects. The dislocation points shown in

Figure 5.11a introduce strain which cause bent lattice planes around the

dislocation points.27 From analysing the dislocations seen in many HRTEM images

the defect density was calculated to be ~6.25×1011 cm-2. Using the threshold

voltage of the trap filled limited (TFL) region of the space charge limited current

(SCLC, Figure 5.11b) and the method as outlined by Shi et al28, a trap density of

~2.85×1011 cm-2 was determined which is in agreement with the determined

defect density from HRTEM.

104

Figure 5.11. (a) HRTEM of a GaS flake scratched from the printed film. Structural

defects are visible in the crystal, which are mainly ascribed to lattice dislocations

(example in the red square). Inset: magnified image highlighting the dislocation of

the crystal lattice. (b) SCLC measurement of 2D GaS. Ohmic and trap filled limited

TFL regions are marked with their respective colours.

105

Figure 5.12. Optical and electronic properties of 2D GaS. Hybrid DFT computation

of the band structures in bilayer (a) and bulk (b) GaS. (c) Tauc plot used for

determining the electronic band transition with simplified electronic band

diagram (inset). Red lines are the valence and conduction band edges, black is the

Fermi level and blue is the location of the deep trap band. (d) PESA demonstrating

the valence band maximum (VBM) energy is −5.15 eV. (e) XPS valence analysis

revealing an energy difference of 1.8 eV between the VBM and Fermi level. (f) PL

decay with fitted exponential (red) and mean excitonic decay (blue) time

indicated.

DFT calculations show indirect and direct recombination paths of 3.08 and

3.29 eV, respectively (Figure 5.12a). The bandgap of the 2D GaS film is determined

as 3.02 eV, using UV-Vis absorption spectroscopy (Figure 5.12c). The measured

bandgap is in good agreement with the indirect transition of bilayer GaS further

106

evidencing the near-uniform nature of the films. Significant PL is also experienced

(Figure 5.7e). The measured PL values (from deep trap emissions and direct

recombinations) are within suitable wavelength for establishing optical devices in

the middle and high end of the visible range. To obtain a more comprehensive

picture of the semiconducting properties for the printed 2D GaS, the band

structure of the material is analysed utilising the PL measurements (Figure 5.7e)

in combination with photo-electron spectroscopy in-air (PESA) and XPS valence

spectra analysis (Figures 5.12d-e). PESA determines a valence band maximum

energy of 5.15 eV and the XPS valance spectrum determined the Fermi level to

reside at 3.35 eV, leading to the energy diagram shown in Figure 5.12c inset. This

shows that the material is n-doped. It is also possible to obtain the p-doped areas

of this 2D GaS by selectively exposing it to gases such as NO229, in order to obtain

the complimentary semiconductor for creating electronics. For comparison, the

DFT calculations for three and four-layer GaS are presented in Figure 5.13.

Figure 5.13. Hybrid DFT computation of the band structures in (a) tri-layer, and

(b) four-layer GaS.

107

The PL decay of deep trap emissions (Figure 5.12f) within 2D GaS show a mean

excitonic recombination time of 315 ps, which is faster than many other 2D

semiconductors including 2D MoS2 (~800 ps)30,31.

Figure 5.14. Schematic representation for device fabrication. (a) Clean SiO2/Si

substrate. (b) Tungsten (W) film e-beam deposited onto the substrate. (c)

Photoresist is deposited and then exposed through a photolithography mask. (d)

Remaining photoresist after image reversal and developing. (e) Immersion of the

of the unprotected W in the etchant. (f) Sample after etching and lift-off. (g)

Deposition of liquid Ga. (h) Formation of 2D GaOx between electrodes after Ga is

removed. (i) Exposure to HCl vapour. (j) GaCl3 film between electrodes. (k)

Exposure to S vapour. (l) Final device. Extra photolithography steps can also be

included as presented in Figure 5.1.

108

Figure 5.15. Characterisation of 2D GaS FETs. (a) Schematic representation of back

gated GaS FET with tungsten/WS2 electrodes featuring band energy diagram. (b)

Raman spectrum of the W electrodes after sulphurisation. Phonon modes of WS2

are indicated demonstrating the growth of WS2 on the skin of the electrodes. (c)

optical images of the fabricated devices and electrode gap (inset). The scale bars

are 500 µm and 20 µm respectively. (d) I/V characteristics of the FET at different

gate voltages. (e) I/V phototransistor characteristics with no gate bias.

To investigate the potential for electronic and optoelectronic devices, field effect

transistors (FETs) are developed via the deposition of tungsten (W) electrodes

109

prior to deposition of Ga (Figure 5.15a,c). This fabrication process of the devices

is outlined in the experimental section and shown in Figure 5.14. W is chosen as

an electrode material for several reasons. Firstly, Ga does not amalgamate with

W as it does with many other metals32. Secondly, W readily forms WS2 under the

same conditions which is use for sulphurising the gallium oxide layer

(Figure 5.15b). WS2 is known to act as a hole injector which can be used to

overcome the Schottky barrier33. A maximum electron mobility of ~0.2 cm2 V-1 s-1

is obtained, which is better than previously reported mobilities for GaS22 and

higher than that of many other printable semiconductors34. Five FETs at different

locations were tested and the mobility measurements have been consistent. The

on/off ratios of the FETs are above ~150 (Figure 5.15d). Furthermore, a median

optical on/off ratios of ~170 in phototransistors is achieved (Figure 5.15e) and a

median responsivity of ~6.4 A W-1 under a solar simulator, which is comparable

with previous reports23. A statistical analysis of the performance of many

phototransistors on fabricated using the here reported wafer-scale process was

conducted. 66 devices were fabricated across two ~1×4 cm substrates (30 and 36

devices, respectively). Of these devices 59 were functional (27 and 32 on each

substrate, respectively) resulting in an average success rate of 89.4% for the

device fabrication. This distribution of responsivity and optical on/off ratio are

shown in Figure 5.16.

110

Figure 5.16. Statistical analysis of photodetector devices. (a) Responsivity of each

individual photodetector device and (b) distribution of responsivities. (c) On/Off

ratio of each device and (d) distribution of ratios.

111

5.4 Conclusions

This chapter describes a printing method that allows the selective patterning of

atomically thin semiconducting layers of 2D PTMCs. The process relies on the

efficient transformation of an ultra-thin oxide layer on the surface of liquid

elemental gallium onto an oxide-coated substrate. Low temperature

sulphurisation leads to the formation of highly uniformly distributed

semiconducting GaS bilayers of ~1.5 nm thickness. There are several advantages

of this method including the relative simplicity and low temperature, cost

effectiveness and scalability of the process (a comparative assessment is

presented in Table 5.1). The method is offers etchless patterning and deposition

of 2D semiconductors on a wafer scale. Furthermore, the development of

electronic and optical devices based on 2D GaS were demonstrated, with the

performance better than or comparable to previous reports on exfoliated GaS

layers. This process for printing 2D semiconductors, which can be extended to the

deposition of other 2D PTMCs and will provide a new paradigm for future

electronics, optics, optoelectronics, sensors and other devices.

112

Table 5.1. Comparison of established methods for deposition of 2D materials.

Temperature (°C)

Time (hours)

Materials Lateral dimensions

Comments References

Atomic layer deposition (ALD)

30035 ~235 MoS235 µm’s35 Requires

annealing at 800°C35

35 Tan et al 2014

Chemical vapour deposition (CVD)

80036, 85037

~236, ~137

WS236,

MoS237

µm’s36,37 36 Elias et al 2013 37Jeon et al 2015

Metalorganic CVD

55038 ~2638 MoS238 µm’s-

mm’s38 38 Kang et al

2015

Thermolysis 100039 1.539 MoS239 mm’s39 39 Liu et al

2012

Mechanical Exfoliation

GaS22, MoS2

40,

WSe240

µm’s-10s µm22,40

Difficult to create reproducibility

22 Late et al 2012 40 Li et al 2015

Liquid phase epitaxy (LPE)

~920 GaS20, MoS2

41, WS2

41, h-BN41

nm’s- 100s nm20, 10s nm-µm’s41

20 Harvey et al 2015. 41 Coleman et al 2011

This work 300 ~2 GaS, In2S3 mm’s

This work can be possibly expanded to create other types of 2D metal compounds.

Gallium can alloy a large number of different metals. As such, the self-limiting

oxide layer can be tuned depending on the type and concentration of the alloyed

metal, allowing the possibility of printing a large variety of pure and mixed oxides.

The 2D oxides can be deposited and patterned on wafer scale and

chalcogenetated. This means that gallium (or many other low melting liquid

metals) can be potentially used as a reaction media to establish desired 2D metal

based compounds which should be further investigated.

113

5.5 References

1 Spells, K. The determination of the viscosity of liquid gallium over an extended nrange of temperature. Proc. Phys. Soc. London 48, 299 (1936).


3 Zheng, Y., He, Z., Gao, Y. & Liu, J. Direct desktop printed-circuits-on-paper flexible electronics. Sci. Rep. 3, 1786 (2013).

4 Boley, J. W., White, E. L. & Kramer, R. K. Mechanically sintered gallium–indium nanoparticles. Adv. Mater. 27, 2355-2360 (2015).

5 Wang, Q., Yu, Y., Yang, J. & Liu, J. Fast Fabrication of Flexible Functional Circuits Based on Liquid Metal Dual‐Trans Printing. Adv. Mater. 27, 7109-7116 (2015).


7 Regan, M. et al. X-ray study of the oxidation of liquid-gallium surfaces. Phys. Rev. B 55, 10786 (1997).

8 Plech, A., Klemradt, U., Metzger, H. & Peisl, J. In situ x-ray reflectivity study of the oxidation kinetics of liquid gallium and the liquid alloy. J. Phys. Condens. Matter. 10, 971 (1998).

9 Doudrick, K. et al. Different shades of oxide: From nanoscale wetting mechanisms to contact printing of gallium-based liquid metals. Langmuir 30, 6867-6877 (2014).

10 Yoon, Y., Kim, D. & Lee, J.-B. Hierarchical micro/nano structures for super-hydrophobic surfaces and super-lyophobic surface against liquid metal. Micro Nano Syst. Lett. 2, 1-18 (2014).


12 Dovesi, R. et al. CRYSTAL14: A program for the ab initio investigation of crystalline solids. Int. J. Quantum Chem. 114, 1287-1317 (2014).

13 Becke, A. D. Density‐functional thermochemistry. III. The role of exact exchange. J. Chem. Phys. 98, 5648-5652 (1993).

14 Grimme, S. Semiempirical GGA‐type density functional constructed with a long‐range dispersion correction. J. Comput. Chem. 27, 1787-1799 (2006).

114

15 Civalleri, B., Zicovich-Wilson, C. M., Valenzano, L. & Ugliengo, P. B3LYP augmented with an empirical dispersion term (B3LYP-D*) as applied to molecular crystals. Cryst. Eng. Comm. 10, 405-410 (2008).

16 Peintinger, M. F., Oliveira, D. V. & Bredow, T. Consistent Gaussian basis sets of triple‐zeta valence with polarization quality for solid‐state calculations. J. Comput. Chem. 34, 451-459 (2013).


18 Monkhorst, H. J. & Pack, J. D. Special points for Brillouin-zone integrations. Phys. Rev. B 13, 5188 (1976).

19 Kim, D. et al. Recovery of nonwetting characteristics by surface modification of gallium-based liquid metal droplets using hydrochloric acid vapor. ACS Appl. Mater. Interfaces 5, 179-185 (2012).

20 Harvey, A. et al. Preparation of Gallium Sulfide Nanosheets by Liquid Exfoliation and Their Application As Hydrogen Evolution Catalysts. Chem. Mater. 27, 3483-3493 (2015).

21 Shen, G., Chen, D., Chen, P.-C. & Zhou, C. Vapor-Solid Growth of One-Dimensional Layer-Structured Gallium Sulfide Nanostructures. Acs Nano 3, 1115-1120 (2009).

22 Late, D. J. et al. GaS and GaSe ultrathin layer transistors. Adv. Mater. 24, 3549-3554 (2012).

23 Hu, P. et al. Highly Responsive Ultrathin GaS Nanosheet Photodetectors on Rigid and Flexible Substrates. Nano Lett. 13, 1649-1654 (2013).

24 Jung, C. S. et al. Red-to-ultraviolet emission tuning of two-dimensional gallium sulfide/selenide. ACS Nano 9, 9585–9593 (2015).

25 Aydinli, A., Gasanly, N. & Gökşen, K. Donor–acceptor pair recombination in gallium sulfide. J. Appl. Phys. 88, 7144-7149 (2000).

26 Yang, S. et al. High performance few-layer GaS photodetector and its unique photo-response in different gas environments. Nanoscale 6, 2582-2587 (2014).

27 Shi, H. et al. Exciton dynamics in monolayer and few-layer MoS2 2D crystals. ACS Nano 7, 1072–1080 (2014).

28 Yuan, L. & Huang, L. Exciton dynamics and annihilation in WS2 2D semiconductors. Nanoscale 7, 7402-7408 (2015).

115


30 Jin, Z., Li, X., Mullen, J. T. & Kim, K. W. Intrinsic transport properties of electrons and holes in monolayer transition-metal dichalcogenides. Phys. Rev. B 90, 045422 (2014).

31 Kwak, D., Lim, J. A., Kang, B., Lee, W. H. & Cho, K. Self‐Organization of Inkjet‐Printed Organic Semiconductor Films Prepared in Inkjet‐Etched Microwells. Adv. Funct. Mater. 23, 5224-5231 (2013).

32 Tan, L. K. et al. Atomic layer deposition of a MoS2 film. Nanoscale 6, 10584-10588 (2014).

33 Elias, A. L. et al. Controlled synthesis and transfer of large-area WS2 sheets: from single layer to few layers. Acs Nano 7, 5235-5242 (2013).

34 Jeon, J. et al. Layer-controlled CVD growth of large-area two-dimensional MoS2 films. Nanoscale 7, 1688-1695 (2015).

35 Kang, K. et al. High-mobility three-atom-thick semiconducting films with wafer-scale homogeneity. Nature 520, 656-660 (2015).

36 Liu, K.-K. et al. Growth of large-area and highly crystalline MoS2 thin layers on insulating substrates. Nano letters 12, 1538-1544 (2012).

37 Li, H., Wu, J., Yin, Z. & Zhang, H. Preparation and applications of mechanically exfoliated single-layer and multilayer MoS2 and WSe2 nanosheets. Acc. Chem. Res. 47, 1067-1075 (2014).


39 Mayrhofer, P. H., Mitterer, C., Hultman, L. & Clemens, H. Microstructural design of hard coatings. Prog. Mater. Sci. 51, 1032-1114 (2006).

40 Shi, D. et al. Low trap-state density and long carrier diffusion in organolead trihalide perovskite single crystals. Science 347, 519-522 (2015).

41 Pan, Z. et al. Polarization-resolved spectroscopy imaging of grain boundaries and optical excitations in crystalline organic thin films. Nature Comm. 6, 8201 (2015).

116

Chapter 6

Conclusions and Future Work

6.1 Conclusions

The work presented in this PhD thesis was intended to investigate the

research gaps presented in chapter 2 and overcome the present challenges

outlined therein. As outlined in chapter 1 this research was undertaken to

explore potential pathways for the future development and

industrialisation of two-dimensional (2D) semiconductors.

The research outlined within this thesis focused on two main objectives; the

first was to investigate solvents’ effects on the exfoliation of layered

materials by ultrasonication and the changes the solvent imbue upon

colloidal suspensions of 2D semiconductors, and the second was to explore

the concept of novel, pragmatic synthesis of 2D semiconductors via the

utilisation of the surface chemistry of certain metals (e.g. gallium). This

117

research was organised and pursued in three stages presented in chapters

3, 4 and 5, respectively.

6.1.1 Stage 1

The work presented in this thesis (chapter 3) investigates colloidal suspensions

of 2D semiconductors, the effects that solvents have during synthesis and the

impacts solvents can have upon the material properties. Therein the major

conclusions arrive from the comparison of a new method combining two well

known yet low yield solvents with the high yield, well known solvent n-methyl-

2-pyridine. Using Raman and UV-Vis absorption analysis it becomes apparent

that residues from solvents can adversely affect the desired characteristics.

This work aims to highlight the importance solvent particularly during

ultrasound exfoliation of 2D materials which has been poorly understood

prior. This stage demonstrates the importance of solvent in colloidal systems

of 2D semiconductors and needs that should be considered

6.1.2 Stage 2

While the previous stage deals with exfoliation of stratified crystals for

producing high yield suspensions of 2D semiconductors, this stage focuses

on producing non-stratified systems.

This stage demonstrates how a Ga based alloy can be used as a reaction

medium for synthesising 2D ZnS. The reaction relies on the chemistry

occurring at the liquid/liquid interface between the reagent and alloy which

118

results in the formation of 2D nanosheets. ZnS is not a stratified material

and hence, conventional 2D synthesis techniques, such as exfoliation

cannot be applied. Thus, this stage introduces room-temperature liquid

metals as suitable reaction media for synthesising ultrathin sheets. The

process can be explored for other metals, and likely granting access to many

other 2D compounds that are currently inaccessible due to a lack of

stratified crystal systems.

6.1.3 Stage 3

This final stage investigates the transformation of the ultra-thin oxide layer

on the surface of liquid elemental gallium onto an oxide-coated substrate

and subsequent sulphurisation for the wafer scale formation of

semiconducting 2D gallium sulphide (GaS). This method is found to be highly

versatile allowing for patterning via pre-functionalisation of the substrate

and device fabrication via pre-deposition of electrodes. Several advantages

of this method such as the relative simplicity and low temperature, cost

effectiveness and scalability of the process provide much potential for

future development and industrialisation. The method is compatible with

many current industrial processes. This method can potentially be extended

to the deposition of other 2D post-transition metal chalcogenides for future

development.

119

6.2 Future Work

The research presented in this Ph.D. thesis open opportunities for future

investigation further extending the scope of this work.

The work presented in chapter 4 in which a Ga based alloy was used as a medium

for synthesising zinc sulphide, has a lot of potential for expansion into many

further materials by manipulating the interfacial chemistry. Hypothetically the

methodology outlined in this chapter could be expanded to produce 2D sheets of

many other compounds either by changing the composition of the metal alloy or

by selecting different reagents. It may be possible to produce 2D sheets based on

alternative metals alloyed into Ga alloys or potentially even tin (Sn) and bismuth

(Bi) based alloys and utilising the correct reagents may be possible to create

suspensions with many different combinations.

The work presented in chapter 5 where Ga and indium (In) were used to produce

large areas of atomically thin gallium and indium sulphides (GaS and In2S3

respectively), presents many potential avenues for further work. The process

outlined in chapter 5 could potentially be expanded to many other low melting-

point metals such as Sn and Bi. Furthermore, investigation into low melting-point

metal alloys such as Ga and Bi based alloys which could hypothetically enable a

vast number of atomically thin metal compounds, although this would require

extensive investigation into the chemical makeup of the atomically thin oxide

deposited. There are also possibilities to expand upon the chemical reactants that

120

the oxide layer is exposed to resulting in different atomically thin metal

compounds

6.3 Journal Publications

This Ph.D. research resulting in the follow articles published in peer

reviewed journals.

6.3.1 First Author Publications (presented in this thesis).

Wafer-scale two-dimensional semiconductors from printed oxide skin of

liquid metals.

Carey, B. J.; Ou, J. Z.; Clark, R. M.; Berean, K. J.; Zavabeti, A.; Chesman, A. S.;

Russo, S. P.; Lau, D. W.; Xu, Z.-Q.; Bao, Q.; Kahevei, O.; Gibson B. C.; Dickey,

M. D.; Kaner, R. B. Daeneke, T.; Kalantar-zadeh, K., Nat. Commun. 2017, 8,

14482.

Two solvent grinding sonication method for the synthesis of two-

dimensional tungsten disulphide flakes.

Carey, B. J.; Daeneke, T.; Nguyen, E. P.; Wang, Y.; Ou, J. Z.; Zhuiykov, S.;

Kalantar-zadeh, K., Chem. Commun. 2015, 51 (18), 3770-3773.

A Liquid Metal Flow-Reaction for the Synthesis of Ultra-Thin Zinc Sulfide.

Carey B. J.; Zavabeti, A.; Mohiuddin, M.; Chesman, A. S. R.; Kalantar-zadeh K.;

Daeneke, T., Chem. Eur. J. 2017, (Under Review)

6.3.2 Co-author Publications (not presented in this thesis).

Exfoliation solvent dependent plasmon resonances in two-dimensional sub-

stoichiometric molybdenum oxide nanoflakes.

Alsaif, M. M.; Field, M. R.; Daeneke, T.; Chrimes, A. F.; Zhang, W.; Carey, B. J.;

Berean, K. J.; Walia, S.; van Embden, J.; Zhang, B., ACS App Mater Interfaces

2016, 8 (5), 3482-3493.

121

2D WS 2/carbon dot hybrids with enhanced photocatalytic activity.

Atkin, P.; Daeneke, T.; Wang, Y.; Carey, B. J.; Berean, K.; Clark, R.; Ou, J.;

Trinchi, A.; Cole, I.; Kalantar-Zadeh, K., Journal of Materials Chemistry A 2016,

4 (35), 13563-13571.

2D MoS2 PDMS Nanocomposites for NO2 Separation.

Berean, K. J.; Ou, J. Z.; Daeneke, T.; Carey, B. J.; Nguyen, E. P.; Wang, Y.; Russo,

S. P.; Kaner, R. B.; Kalantar‐zadeh, K., Small 2015, 11 (38), 5035-5040.

Patterned films from exfoliated two-dimensional transition metal

dichalcogenides assembled at a liquid–liquid interface.

Clark, R.; Berean, K.; Carey, B. J.; Pillai, N.; Daeneke, T.; Cole, I.; Latham, K.;

Kalantar-zadeh, K., J. Mater. Chem. C 2017, 5 (28), 6937-6944.

Two-step synthesis of luminescent MoS2–ZnS hybrid quantum dots.

Clark, R. M.; Carey, B. J.; Daeneke, T.; Atkin, P.; Bhaskaran, M.; Latham, K.;

Cole, I. S.; Kalantar-Zadeh, K., Nanoscale 2015, 7 (40), 16763-16772.

Exfoliation of quasi-stratified Bi2S3 crystals into micron-scale ultrathin

corrugated nanosheets.

Clark, R. M.; Kotsakidis, J. C.; Weber, B.; Berean, K. J.; Carey, B. J.; Field, M. R.;

Khan, H.; Ou, J. Z.; Ahmed, T.; Harrison, C. J., Chem. Mater. 2016.

Light driven growth of silver nanoplatelets on 2D MoS2 nanosheet

templates.

Daeneke, T.; Carey, B. J.; Chrimes, A.; Ou, J. Z.; Lau, D.; Gibson, B.; Bhaskaran,

M.; Kalantar-Zadeh, K., Journal of Materials Chemistry C 2015, 3 (18), 4771-

4778.

Reductive exfoliation of substoichiometric MoS2 bilayers using hydrazine

salts.

Daeneke, T.; Clark, R. M.; Carey, B. J.; Ou, J. Z.; Weber, B.; Fuhrer, M. S.;

Bhaskaran, M.; Kalantar-zadeh, K., Nanoscale 2016, 8 (33), 15252-15261.

Investigation of two-solvent grinding assisted liquid phase exfoliation of

layered MoS2.

Nguyen, E. P.; Carey, B. J.; Daeneke, T.; Ou, J. Z.; Latham, K.; Zhuiykov, S.;

Kalantar-Zadeh, K., Chem. Mater. 2015, 27 (1), 53-59.

122

Excitation dependent bidirectional electron transfer in phthalocyanine-

functionalised MoS2 nanosheets.

Nguyen, E. P.; Carey, B. J.; Harrison, C. J.; Atkin, P.; Berean, K. J.; Della Gaspera,

E.; Ou, J. Z.; Kaner, R. B.; Kalantar-Zadeh, K.; Daeneke, T., Nanoscale 2016, 8

(36), 16276-16283.

Electronic tuning of 2D MoS2 through surface functionalization.

Nguyen, E. P.; Carey, B. J.; Ou, J. Z.; van Embden, J.; Gaspera, E. D.; Chrimes,

A. F.; Spencer, M. J.; Zhuiykov, S.; Kalantar‐zadeh, K.; Daeneke, T., Advanced

Materials 2015, 27 (40), 6225-6229.

Physisorption-based charge transfer in two-dimensional SnS2 for selective

and reversible NO2 gas sensing.

Ou, J. Z.; Ge, W.; Carey, B. J.; Daeneke, T.; Rotbart, A.; Shan, W.; Wang, Y.; Fu,

Z.; Chrimes, A. F.; Wlodarski, W., ACS Nano 2015, 9 (10), 10313-10323.

Acoustically-driven trion and exciton modulation in piezoelectric two-

dimensional MoS2.

Rezk, A. R.; Carey, B. J.; Chrimes, A. F.; Lau, D. W.; Gibson, B. C.; Zheng, C.;

Fuhrer, M. S.; Yeo, L. Y.; Kalantar-zadeh, K., Nano letters 2016, 16 (2), 849-855.

Intercalated 2D MoS2 Utilizing a Simulated Sun Assisted Process: Reducing

the HER Overpotential.

Wang, Y.; Carey, B. J.; Zhang, W.; Chrimes, A. F.; Chen, L.; Kalantar-zadeh, K.;

Ou, J. Z.; Daeneke, T., The Journal of Physical Chemistry C 2016, 120 (4), 2447-

2455.

Enhanced quantum efficiency from a mosaic of two dimensional MoS2

formed onto aminosilane functionalised substrates.

Wang, Y.; Della Gaspera, E.; Carey, B. J.; Atkin, P.; Berean, K. J.; Clark, R. M.;

Cole, I. S.; Xu, Z.-Q.; Zhang, Y.; Bao, Q., Nanoscale 2016, 8 (24), 12258-12266.

Plasmon resonances of highly doped two-dimensional MoS2.

Wang, Y.; Ou, J. Z.; Chrimes, A. F.; Carey, B. J.; Daeneke, T.; Alsaif, M. M.;

Mortazavi, M.; Zhuiykov, S.; Medhekar, N.; Bhaskaran, M., Nano letters 2015,

15 (2), 883-890.

Liquid metal/metal oxide frameworks with incorporated Ga2O3 for

photocatalysis.

Zhang, W.; Naidu, B. S.; Ou, J. Z.; O’Mullane, A. P.; Chrimes, A. F.; Carey, B. J.;

Wang, Y.; Tang, S.-Y.; Sivan, V.; Mitchell, A., ACS applied materials & interfaces

2015, 7 (3), 1943-1948.

123

Proton intercalated two-dimensional WO 3 nano-flakes with enhanced

charge-carrier mobility at room temperature.

Zhuiykov, S.; Kats, E.; Carey, B. J.; Balendhran, S., Nanoscale 2014, 6 (24),

15029-15036.

two-dimensional semiconductors for future optical...

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