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Fabrication and Applications of Self-Assembled Nanopillars

Hai-Feng Ji 1*

, Morasae Samadi

1,2, Hao Gu

1, Veronica Tomchak

1, Zhen Qiao

1

1Department of Chemistry, Drexel University, Philadelphia, PA 19104.

2Institute for Nanoscience and Nanotechnology (INST), Sharif University of Technology, Tehran, Iran. P.O. Box

14588-8969.

* CORRESPONDING AUTHOR, Email: [email protected], Phone: 01-215-895-2562, Fax: 01-215-895-1265

doi:10.5618/chem.20131 || Received: 2012-02-16, Accepted: 2012-03-20, Available online: 2013-03-23

Abstract

In this mini-review, we summarize fabrication

methods, formation mechanisms, factors controlling

the characteristics, and applications of self-

assembled nanopillars. Nanopillars prepared both in

the gas phase and in solutions are discussed.

Keywords: Self-assembly; nanopillar.

1. Introduction.

At the nanoscale, materials possess characteristics that

are dramatically different from their corresponding

macroscale ones. This is one of the main driving forces

for the recent explosive increase in research on

nanomaterials such as nanowires, nanoparticles, and

quantum dots. However, a key to realizing their

potential applications for devices is the ability to

assemble them into desirable patterned nanostructures.

The two major design strategies in coupling

functionalities with materials on the micro- or nanoscale

are via multilayering or by forming vertical

heteroepitaxial nanocomposites. In recent years,

fabrication and applications of nanopillars have attracted

more and more attention. There are two principle

approaches to nanopillar fabrication: top-down

approaches such as lithography and etching, and

bottom-up growth techniques. Bottom-up fabrication

methods of nanopillars include chemical vapor

deposition (CVD),1 physical vapor deposition (PVD),

2

template synthesis,3,4

and self-assembled methods. In

this mini-review, we intend to summarize fabrication

methods, formation mechanisms, factors controlling

characteristics, and applications of nanopillars based on

the self-assembly approach, with more attention being

focused on the factors that affect the growth of the

nanopillars. It is noteworthy that although, to a certain

extent, CVD and PVD belong to the self-assembled

class of approaches and have been widely used to make

nanopillars, such as nanopillars of ZnO, CdS, etc., we

will only briefly summarize the reports which

mentioned the concept of self-assembly. The purpose of

this review is to provide new researchers in this area

with a quick overview on the body of work containing

self-assembled nanopillars. For reviews on nanopillars

from CVD or PVD, readers are suggested to read other

recent reviews.5,6,7,8

2. Fabrication methods and formation mechanisms.

The self-assembly approach offers a simple and efficient

method for growth aligned nanostructures. This

includes:

a. self-assembled inorganic nanopillars in the

gas phase by deposition of aerosol, and

pulsed laser deposition (PLD).

b. self-assembled inorganic nanopillars in

solutions by hydrothermal + polymer

assisted template.

c. Self-assembled organic compounds in

solution on surfaces through liquid

evaporation.

2a. Self-assembled inorganic nanopillars in the gas

phase by deposition of aerosol or pulsed laser

deposition (PLD). In a film-on-substrate geometry,

epitaxial composite films can be created in two forms,

horizontal and vertical. Nanocomposite films with a

vertical architecture, i.e. the nanopillar geometry, offer

numerous advantages over the conventional horizontal

multilayers, such as a larger interfacial area and intrinsic

heteroepitaxy in three dimensions.9,10

Lebedev et al discovered11

that (La0.67Ca0.33MnO3)1-x

:(MgO)x (LCMO) composite films on a (100) MgO

substrate have microstructure and magneto-transport

properties depending on the MgO concentration. The

LCMO films were prepared by metal organic aerosol

deposition on a (100) MgO substrate for different

concentrations of the (MgO) phase (0<x<0.8) (Figure 1

mailto:[email protected]



left). At x≈0.3 a percolation threshold in conductivity is

reached, at which point an infinite insulating MgO

cluster forms around the La0.67Ca0.33MnO3 grains. This

yields a drastic increase in the electrical resistance for

films with x>0.3. The films exhibit a remarkable

structure consisting of domains of LCMO material

surrounded by an epitaxially intergrown MgO thin layer.

This MgO layer creates a wall around the LCMO

domains and, as a result, a uniform 3D stress is built up.

They concluded that the structural phase transition is

coupled with the percolation threshold. The reason

seems to be a drastic increase of the 3D stress

contribution when all the LCMO domains are

surrounded by epitaxially grown MgO layers. The

formation mechanics were proposed as shown in Figure

1 right.

Figure 1. Left: Cross-section electron microscopy image of the interface (La0.67Ca0.33MnO3)1-x-(MgO)x /MgO(100)

for x=0.5. Right: Schematic representation of the growth mechanism of the composite film at different stages: (a)

Nucleation of the LCMO film on the MgO substrate. (b) Enlargement of one LCMO island on the MgO substrate.

Close to the interface the LCMO region is under a two-dimensional tensile stress; the upper part of the LCMO island

is under a two-dimensional compressive stress. The shape of LCMO becomes pyramidal. (c) Nucleation of the

(LCMO)1-x:(MgO)x composite film on the MgO substrate. (d) The columnar structure of the LCMO film on the

MgO substrate. (e) Structure of the (LCMO)1-x :(MgO)x film for x=0.33. Intergrowth of the MgO islands in between

LCMO domains occurs. (f) Structure of the (LCMO)1-x :(MgO)x film for x=0.5. Every LCMO grain is surrounded by

MgO through the complete film thickness. Reprinted with permission from the American Physics Society.

In a later work, Moshnyaga et al. confirmed the

formation of vertical nanopillar films with a

composition of (La0.7Ca0.3MnO3)1-x:(MgO)x.12

The

structural and magneto-transport properties of the

La0.7Ca0.3MnO3 nanoclusters were tuned through the

tensile stress originating from the MgO second phase.

In later reports, the pulsed laser deposition (PLD)

method becomes the most popular method in this area

and it has been successfully used in the fabrication of

ceramic materials with self-assembled nanopillars

embedded in films. PLD is a thin film deposition

technique where a high power pulsed laser beam is

focused inside a vacuum chamber to strike a target of

the material that is to be deposited. The material is

vaporized from the target in a plasma plume which

deposits it as a thin film on a substrate. Ceramic pulsed-

laser deposition target compositions were fabricated by

milling appropriate starting chemicals, such as La2O3,

MnCO3, SrCO3, ZnO, Bi2O3, Fe2O3 and Sm2O3,

BiFeO3:Sm2O3, BiFeO3:NiFe2O4, BaTiO3:CoFe2O4, and



La0.5Sr0.5CoO3:Ce0.9Gd0.1O1.95 systems with vertical

strain tuning, vertical interfacial coupling, and, thus,

unique functionalities.13

To name a few examples, Li et

al.14

prepared self-assembled PbTiO3-CoFe2O4 epitaxial

nanostructures on SrTiO3 single crystals from a

composite 0.67PbTiO3-0.33CoFe2O4 target by using a

PLD approach. The nanostructures on both [001] and

[110] orientations of SrTiO3 consisted of vertical rod- or

platelet-like columns of CoFe2O4 dispersed in a PbTiO3

matrix. In their later work,15

they also demonstrated

multiferroic xCoFe2O4–(1–x) PbTiO3 films on (111)

SrTiO3 substrates.

It has been generally accepted that the formation of

vertical nanostructures in films is related to the

minimization of the relative interfacial and elastic strain

energies between two materials and between each

material and the substrate. Both the morphology and the

scale of self-assembled nanostructures are controlled by

varying the lattice misfit with a substrate.16

By selecting

two appropriate materials and their phases, it is possible

to grow vertically aligned nanopillars in an epitaxial

thin-film form. The properties of such nanostructures

are dependent on the nanostructure morphologies

including domain patterns and shapes as well as

structures and properties of the interfaces. The two

requirements for self-assembled two-phase nano-

composites are 1) the intrinsic similarity in crystal

chemistry between two materials that leads to crystal

lattice parameters that are reasonably commensurate; 2)

the compounds have little solid solubility into each

other. The two phases in the nanostructure can be

epitaxial with respect to each other as well as with

respect to a common substrate.13

In the above mentioned

papers, Li et al14

demonstrate that varying the stress

conditions via differently oriented single-crystal

substrates yields different phase morphologies in the

epitaxial two-phase films. For the [001] orientation the

platelet habits were parallel to the [110] planes, whereas

for the [110] orientation the platelets were parallel to the

[111] planes (Figure 2). They also demonstrated that the

morphologies of self-assembled multiferroic

nanostructures can be varied by modifying the epitaxial

stress state in the film. Their results confirm that the in-

plane elastic anisotropy of the film, as determined by the

substrate orientation, has a profound effect on the

morphologies of the resulting two phase nanostructures,

and that the major morphological differences can be

explained using a thermodynamic theory.

Figure 2. TEM images of the PbTiO3-CoFe2O4 nanostructures grown on (001) SrTiO3 (a) cross section, (b) plane

view of the structures on (110) SrTiO3. (c) cross section, (d) plane view the structures on (110) SrTiO3. The image

shown in (b) is a bright field image recorded near the [001] orientation parallel to the surface normal. In all the

images, the CoFe2O4 phase appears as bright regions. Reprinted with permission from the American Physics

Society.

2b. self-assembled inorganic nanopillars in solutions

by hydrothermal + polymer assisted template. Deng

et al.17

developed self-assembled (CoFe2O4)0.3–

(BaTiO3)0.7 (CFO–BTO) feather-like nanopillars (Figure

3) by using a cost-effective, combined hydrothermal

reaction and polymer-assisted deposition method. The

feather-like nanostructures have single-crystal CFO

nanopillars embedded in the BTO matrix. The CFO–

BTO nanostructures exhibit good ferromagnetic and

ferroelectric properties. The growth process of the

CFO–BTO nanostructures has been studied and the

growth mechanism is proposed, which may be applied

to other perovskite-spinel nanostructures.



Figure 3. (a) Typical XRD pattern of the as-prepared CFO–BTO samples, (b) TEM image of the CFO–BTO

nanostructures with the EDX spectrum in the upper right inset, (c) TEM image and (d) High resolution TEM image

of an individual CFO–BTO nanostructure, (e) High resolution TEM image of a CFO nanopillar embedded in the

BTO matrix. Reprinted with permission from the Elsevier B. V.

2c. Self-assembled organic compounds in solution on

surfaces through liquid evaporation.

Ji et al. have recently developed a novel, surface-

assisted self-assembly approach to harvest vertically

aligned single-crystalline organic nanopillar

semiconductors from a solution of organic chemicals. 18

In these experiments, when cyanuric acid (CA) and

melamine (M) are mixed at lower concentrations on a

gold surface, an array of nearly 100% hexagonal

nanopillars of the CA·M complex was obtained

(scanning electron microscopy (SEM) images in Figure

4). The nanopillar is composed of a cyclic CA3M3

rosette assembly19

(Figure 4 left). CA3M3 rosette

molecular assemblies are hierarchically stacked or

coordinated into insoluble nano- or micro-structures in

3D space due to hydrogen bonding, π-π stacking,

hydrophobic interactions, and van der waals

interactions. They showed that the CA·M complexes

nucleate as a thin, hexagonal plate on the gold surface at

the onset stage, and grow wider and taller after more

solvent evaporates. The unique aspect of this result is

that this assembly is the first of such well-defined self-

assembled nanopillars made of small organic molecules.

It is expected that arrays of pillars can be developed

from several other complexes. The nanopillars prepared

by this method can be made in large quantities at a low

cost due to the facile self-assembling method.

Figure 4. Molecular structures (left) and SEM images of 1:1 CA·M hexagonal nanopillars on a gold surface.

Reprinted with permission from the American Chemical Society.



3. Factors affecting the characteristics of the self-

assembled nanopillars

The property of surfaces is the main factor that controls

the formation and characteristics of nanopillars. For the

nanopillars from the PSD method, it has been believed

that the in-plane compressive strain caused by the misfit

between evaporated materials and the substrate restricts

the lateral size of the nanopillars and drives them to

grow along the c-axis (the direction perpendicular to the

substrate surface) forming a nanopillar structure.20

The

evaporated compounds and substrate should have a

similar structure and chemistry at their interface to

facilitate the nucleation.20

So it is possible to control

relative lattice orientation of the constituent materials

and the subsequent vertical phase boundaries in a

systematic way to achieve desired and tunable physical

property of the nanostructures, i.e. the physical property

of the nanostructures depends on the substrate-film

lattice misfit and thus can be adjusted by the choice of

the substrate. This leads to the possibility of designing

new types of the structures with predictable physical

properties.

Zheng et al,21

reported that the morphology of self-

assembled perovskite-spinel nanostructures can be

controlled simply by selecting single-crystal substrates

with different orientations; a (001) substrate results in

rectangular-shaped CoFe2O4 nanopillars in a BiFeO3

matrix; in contrast, a (111) substrate leads to triangular-

shaped BiFeO3 nanopillars in a CoFe2O4 matrix,

irrespective of the volume fraction of the two phases

(Figure 5). This reversal can be understood as a primary

consequence of different nucleation modes due to the

large differences in surface energy anisotropy.

Figure 5. Morphologies of the BiFeO3-CoFe2O4 nanostructures grown on a (001)-oriented (left) and a (111)-

oriented (right) SrTiO3 substrate. (a) Z-contrast image from a TEM sample. (b) A TEM image of a single CoFe2O4

pillar embedded in a BiFeO3 matrix. (c) A high-resolution TEM image from the interface region marked by the

rectangle in b. (d) Structural model of the interface between CoFe2O4 and BiFeO3 showing that the interfaces are

110planes (left) or 112planes (right), both along 110directions. (e) Cross-sectional TEM image of a single

CoFe2O4 pillar. (f) SEM image of the CoFe2O4 pillars. (g) A schematic of a CoFe2O4 pillar. (h) A schematic of a

CoFe2O4 pillar showing three crystal facets. (left) or A schematic of a BiFeO3 pillar showing (100), (010), and (100)

facets (right). Reprinted with permission from the American Chemical Society.

Control of crystallographic orientations and physical

properties of self-assembled nanostructures via rational

selections of substrates has also been demonstrated by

Liao et al. 22

They showed that in the perovskitespinel

BiFeO3/CoFe2O4 model system the crystal orientation of

self-assembled CoFe2O4 nanopillars can be tuned among

(001), (011), and (111), while that of the BiFeO3 matrix

is fixed in (001). Moreover, the resultant CoFe2O4

nanopillars appear in various shapes: pyramid, roof, and

triangular platform, respectively (Figure 6). The tunable

nanostructures through this approach enable the control

of material functionality such as the magnetic anisotropy

of CoFe2O4.

The substrates play a major role in the growth of the

CA·M nanopillars.18

The density of the CA·M

hexagonal nanopillars was approximately 30 /m2 on a

glass or silicon surface, which was much greater than

the density of 0.1 / m2 found on the gold surface. The

CA·M nanopillars on a glass or silicon surface were

shorter and smaller in diameter than those made on the

gold surface.



Figure 6. AFM topography of BiFeO3-CoFe2O4 films. (a-c) 2D (3 μm × 3 μm) and 3D (500 nm × 500 nm) images

of the films grown on different substrates. Reprinted with permission from the American Chemical Society.

Other than substrate surfaces, factors to control the

structure and subsequent properties of nanopillars

include the following:

a) Horizontal (lateral) strain within layered

heterostructures is fundamental to many

semiconductor device technologies. It was recently

shown that the vertical strain also controls the

characteristics of the nanopillar films. Macmanus-

Driscoll et al.23

selected second phases with

different crystal structures and they demonstrated

that careful selection of materials can yield self-

assembled nanostructures in which the vertical

lattice mismatch between nanocolumns dominates

the overall strain state of the film and can be used to

control it. They showed spontaneously ordered

structures were produced in predicted compositions

from La0.70Sr0.3MnO3/ZnO (LSMO/ZnO) on a

single-crystal SrTiO3 (STO) and the BiFeO3/Sm2O3

(BFO/SmO) on STO. Vertical strain was proven to

dominate the strain state in films above 20 nm

thickness and strain manipulation was demonstrated

by selection of phases with the appropriate elastic

moduli. Yang et al24

demonstrated the vertical

interface effect on lattice parameters, dielectric

properties, and leakage current of BFO:SmO

nanocomposite films via comparing the physical

properties of these nanocomposites with pure BFO

and SmO thin films. They also confirmed that the

vertical strain is the dominant factor determining

the out-of-plane lattice parameters and strain state

of the BFO and SmO phases. The nanocomposite

shows a lower than expected dielectric loss, due to

the presence of a larger vertical interfacial area in

the nanocomposite, causing a reduction of the

oxygen depletion in the BFO phase relative to a

pure BFO film. In other reports, Varanasi25

and Zhu

showed26

that in YBa2Cu3O7-x (YBCO)/BaSnO3

(BSO) nanocomposite thin films, the BSO grows as

self-assembled vertically aligned nanopillars

uniformly distributed in the superconducting YBCO

matrix and the BSO lattice is strongly tuned by

YBCO rather than the substrate.

b) Temperature can affect the size of the nanopillars.

For example, as the substrate temperature increases

from 750 to 950°C, the average diameter of the

CoFe2O4 pillars increases from 9 to 70 nm.13

c) By using a (La0.7Sr0.3MnO3)0.5:(ZnO)0.5

(LSMO:ZnO) self-assembled vertically aligned

nanocomposite thin film on SrTiO3 (STO) (001)

substrates as an example, Chen et al observed27

that

the morphology of nanostructures and their physical

properties can be systematically tuned by changing

the deposition frequency, which is associated with

the vertical phase boundaries. The results

demonstrate that manipulating the structure of the

vertical phase interfaces in these unique films with

vertical architecture is a viable route to tune the

physical properties in nanocomposite thin films.

d) The functional controllability of heterostructures

can be controlled by other external parameters, for

instance, Liu et al28

demonstrated that light (or

photons) can be used as an external control

parameter in a self-assembled nanostructure system

with a large photostrictive SrRuO3 (SRO) as the

matrix material to couple with the large

magetostrictive CoFe2O4 nanopillars.

The

combination of photostrictive SrRuO3 and

magnetostrictive CoFe2O4 in the intimately

assembled nanostructures leads to a light-induced,

ultrafast change in magnetization of the CoFe2O4

nanopillars (Figure 7).



Figure 7. (a) AFM topology and (b) MFM image at the same area of a nanostructured CFO-SRO sample that has

been magnetized by applying a large out-of-plane magnetic field before being illuminated. (c) MFM image at the

same area after being illuminated. We randomly selected 100 nanopillars to conveniently observe the change in

magnetic direction. The red circles in (b) are the nanopillars in (a) with downward magnetization, and half these

pillars flipped upward in (c) are presented in yellow circles, suggesting a “liberation” in magnetization during

illumination. (d-f) Schematic illustration of the process of magnetic domain flipping during the illumination by

ultrafast Ti:sapphire laser pulses. (d) Application of a large out-of-plane magnetic field (7T) to magnetize all

nanopillars downward. (e) Introduction of light on the nanostructured thin film to expand the lattice of SRO and

release the vertical compressive strain of CFO. (f) Removal of light, resulting in the magnetization of the CFO

nanopillars becoming either upward or downward for the energetically preferred state. Reprinted with permission

from the American Chemical Society.

4. Applications of the self-assembled nanopillars.

Self-assembled vertical nanostructures can be used to

design new functionalities. Some applications of these

nanopillars are briefly summarized as follows:

4a. Magnetoelectric. The nanopillar-embedded films

hold potential as an excellent magnetoelectric (ME)

materials. ME materials are materials that exhibit an

induction of magnetization by an electric field or an

induction of electric polarization by a magnetic field.

They offer increased functionality and entirely new

applications for electronic devices. The creation of

nanostructures with uniform size and spacing in

epitaxial heterostructures through self-assembly offers

great potential to tailor the physical properties and

explore the fabrication of new functional devices.

Strong ME responses have been realized in artificial

two-phase vertically aligned nanocomposite films with

well-ordered vertical phase boundaries. Laminated

ceramic composites, fabricated from piezoelectric and

magnetostrictive components, have been found to

exhibit strain-mediated ME responses an order-of-

magnitude larger than those observed in single-phase

materials.

Zheng et al.9,13

prepared self-assembled epitaxial

CoFe2O4-BaTiO3 ferroelectromagnetic nanostructures

on single-crystal SrTiO3 (001) substrates (Figure 8). The

CoFe2O4-BaTiO3 nanocomposites were formed from a

0.65BaTiO3-0.35CoFe2O4 or 0.62BaTiO3–0.38CoFe2O4

target by PLD and magnetic spinel CoFe2O4 pillars were

epitaxially embedded into a ferroelectric BaTiO3 matrix.

The films are epitaxial in-plane as well as out-of-plane

with self-assembled hexagonal arrays of CoFe2O4

nanopillars with average spacing of 20 to 30

nanometers. SrRuO3 was chosen as the lattice-matched

bottom electrode to enable heteroepitaxy as well as to

facilitate electric measurements. This nanocomposite

exhibited strong magnetoelectric coupling due to the

strong elastic interactions between the two phases.

Temperature dependent magnetic measurements

illustrate the coupling between ferroelectric and

magnetic order parameters, which is manifested as a

change in magnetization at the ferroelectric Curie

temperature.



Figure 8. Left: Schematic illustration of a self-assembled nanostructured thin film formed on the substrate. Middle:

AFM topography image of the film showing a quasi-hexagonal arrangement of the CoFe2O4 nanopillars. Right:

TEM planar view image showing the CoFe2O4 nanostructures in the BaTiO3 matrix. Reprinted with permission from

the AAAS.

Zavaliche et al.29

reported the epitaxial nanostructure

of ~ 200 nm thick films comprised of ferrimagnetic

CoFe2O4 pillars embedded in a BiFeO3 matrix, with a

relative volume ratio of 35/65. The films were grown on

single crystalline (001) SrTiO3 substrates. They

presented direct evidence for room-temperature

magnetization reversal induced by an electric field in

epitaxial ferroelectric BiFeO3-ferrimagnetic CoFe2O4

columnar nanostructures. Similarly, the observed effect

is due to the strong elastic coupling between the two

ferroic constituents as the result of the three-dimensional

heteroepitaxy.

4b. Ferroelectric. These nanopillar-embedded films

are also used as ferroelectric materials, which are used

in applications ranging from energy harvesting to high-

power electronic transducers. Strain control of oxide

films can achieve dramatically enhanced ferroelectricity

properties, and huge changes in the ferroelectric Curie

temperature Tc.

In BiFeO3:Sm2O3,23

high compressive vertical strain

levels of ~1.5% have been demonstrated, with a strong

improvement in dielectric properties and reduced

leakage.24

Harrington et al30

reported practical

ferroelectric materials based on micrometer-thick films

of BaTiO3, which were formed by self-assembled

vertical columns of Sm2O3 throughout the film thickness

(Figure 9). Sm2O3 (SmO) is the ideal strain controlling

second phase because it substitutes only minimally into

the BTO, it has a large elastic modulus (125 GPa,

compared to 67 GPa for BTO) and is insulating. The

Curie temperature of the film is at least 330 oC, and the

tetragonal-to-cubic structural transition temperature to

beyond 800 oC. Such enhancements in thick films are a

consequence of uniform, vertical strain coupling

between stiff nanocolumns of SmO and a surrounding

BTO nanomaterial and have opened a new approach to

using BTO in high-temperature ferroelectric

applications. This method provides strain control in

much thicker films via the growth of vertical

nanocomposites.31,32,33

Figure 9. Self-assembled vertical nanostructure. a–c,

TEM images of a 600-nm-thick BaTiO3:Sm2O3 film on

SrTiO3: cross-section (a); plan view (b). Sm2O3 shows a

darker contrast compared with BaTiO3. Yellow arrow

represents c-axis direction. d, Crystallographic model of

interface matching. Reprinted with permission from the

Nature Publishing Group.

4c. Vortex pinning and high conductivity. The

introduction of nanopillars in membranes is a promising

approach for optimizing high-temperature

superconductors (HTSs) by increasing their critical

current density Jc and reducing their anisotropy. These

goals require the introduction of nanostructures able to

strongly pin magnetic flux vortices so as to oppose

vortex motion induced by the Lorentz force.

Wee et al reported34

formation of columnar defects

comprised of self-assembled Ba2YNbO6 (BYNO)

nanopillars in YBa2Cu3O7-δ (YBCO) films via

simultaneous laser ablation of a YBCO target and a Nb



metal foil. Compared to pure YBCO, YBCO+BYNO

films exhibited no Tc reduction as well as superior Jc

performance with higher self- and in-field Jc by a factor

of 1.5–7 and also exhibited a strong Jc peak for H ‖ c

indicative of strong flux-pinning. Wee et al. also

reported35

rare-earth barium tantalates, Ba2RETaO6

(BRETO, RE=rare earth elements) as promising pinning

additives for superior flux pinning in

YBa2Cu3O7−δ(YBCO) films. BRETO compounds have

excellent chemical inertness and large lattice mismatch

with YBCO.

Zhang et al showed20

the structure and chemistry of

the self-assembled oxide nanopillars that form in

superconducting Co-doped BaFe2As2 thin film grown by

PLD. The oxide nanopillars consist of a BaFeO2 phase,

form epitaxially on the SrTiO3 template, and grow

coherently with the BaFe2As2 film. The high density of

self-assembled non-superconducting oxide nanopillars

provides very effective flux pinning centers in the Ba-

122. The nanopillars provide exceptionally strong vortex

pinning and high critical current density due to the very

close correlation of pillar and vortex core diameters

(Figure 10). The use of self-assembled nanopillars as

correlated defects for strong vortex pinning provides an

efficient method for increasing Jc without any apparent

film thickness dependence or need for post-processing.

Several other pinning materials including Y2BaCuO5,

BaZrO3, BaIrO3, Nd2O3, YSZ, Y2O3, and BaSnO3 have

been investigated to improve the flux pinning properties

of YBCO films. For example, Kang et al36

demonstrated

short segments of a superconducting wire in

substantially thicker films (3-micrometers) that meets or

exceeds performance requirements for many large-scale

applications of high-temperature superconducting

materials, especially those requiring a high supercurrent

and/or a high engineering critical current density in

applied magnetic fields. Enhancements of the critical

current were achieved in the thick films via

incorporation of a periodic array of extended nanopillars

extending through the entire thickness of the film. These

nanopillars are highly effective in pinning the

superconducting vortices or flux lines, thereby resulting

in the substantially enhanced performance of this wire.

Figure 10. A high angle annular dark field (Z-contrast)

image showing the atomic structure of an epitaxial

nanopillar in the Ba-122 film. Reprinted with

permission from the American Chemical Society.

Figure 11. (left) (a) Lattice structure of LAO (100). (b) High quality LAO wafer. (c) Schematic diagram of two-

phase growth mode. A 3D c-plane GaN nanopillar and a 2D M-plane GaN film are simultaneously grown on a -

LiAlO2 substate, forming a 120o angle between them. Reprinted with permission from the Japan Society of Applied

Physics.



4d. FET spintronics. GaN-based quantum wells (QWs)

are a promising candidate for optoelectronic devices,

such as spin-polarized field effect transistors (spin-

polarized FETs).37

When light-polarized M-plane GaN

(Figure 11) and spin-polarized C-plane GaN are

fabricated on the same substrate (e.g., -LiAlO2), two-

phase GaN provides an opportunity to combine

optoelectronics and spintronics to an optically controlled

spin-polarized FET by the coupling of polarized light

and the polarized spin of 2DEG.38

Hsieh et al showed

that the formation of self-assembled c-plane GaN

nanopillars is through nucleation on hexagonal anionic

bases of LiAlO2. This heterostructure might be used in

optically controlled spintronic devices.

5. Conclusion and Further perspectives.

The self-assembled nanopillars hold great promise for

novel device applications and functional property

control that goes substantially beyond what is possible

in established systems. The nanostructures

spontaneously form over large areas which has

important ramifications for many areas of functional

device materials. Compared with the lateral interface,

the effect of vertical interfaces on the physical

properties of metal oxide films is profound.

A functional material that spontaneously forms

nanopillars embedded in a matrix of another material

during the thin-film growth offers attractive new

possibilities for device applications. In addition to the

individual functionalities of each constituent phase,

materials can display coupling between the order

parameters. Investigations which are ongoing in this

field include the creation of ordered structures (and their

formation on a large scale) and different combinations

of materials and control of strain coupling between the

phases. More applications of the these structures are

expected—for instance, recently, a large photon-induced

strain of ∼0.5% to 1.5% was demonstrated in

SRO/SrTiO3 39

and SRO/Pb(Zr,Ti)O3 superlattices;40

these can be used as photosensitive materials.

Furthermore, similar two phase systems have not been

achieved in organic based materials since self-

assembled nanopillars are still in their infancy.

Realization of organic based two-phase, vertical

nanocomposites may lead to new forms of ordered

nanostructures for multifunctional applications and will

open up a new level of control in films so that electric or

optical properties of materials can be tuned by the

appropriate choice of materials. This would also enable

more straightforward basic research studies of physical

property measurements in strained systems to be

undertaken.

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