Prof.P. Ravindran, - Universitetet i oslofolk.uio.no/ravi/cutn/nmnt/14.1D Nanostructures -...

P.Ravindran, Nanomaterials and Nanotechnology, Spring 2016 1D Nanostructure – Nanowires/rods etc.

.

http://folk.uio.no/ravi/cutn/NMNT

Prof.P. Ravindran, Department of Physics, Central University of Tamil

Nadu, India

&Center for Materials Science and Nanotechnology,

University of Oslo, Norway

1D Nanostructure – Nanowires/rods etc.

1


.

1-D Nanostructures

Whiskers

Fibers or fibrils

Nanowires

Nanorods

Whiskers and nanorods are generally considered to be

shorter than fibers and nanowires

2


.

Synthesis and formation 1-D nanostructured materials: Techniques

1. Spontaneous growth:

a. Evaporation (or dissolution)-condensation

b. Vapour (or solution)-liquid-solid (VLS or SLS) growth

c. Stress-induced recrystallization

2. Template-based synthesis:

(a) electroplating and electrophoretic deposition

(b) colloid dispersion, melt, or solution filling

(c) conversion with chemical reaction

3. Electrospinning

4. Lithography

3


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Bottom-up approaches

1. Spontaneous growth

2. Template-based synthesis

3. Electrospinning

Top-down approaches

4. Lithography

Spontaneous growth commonly results in the formation single crystal nanowires and nanorods along a preferential crystal growth direction depending on the crystal structures and surface properties of the nanowire materials.

Template-based synthesis mostly produces polycrystalline or even amorphous products

4


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Spontaneous growth

Spontaneous growth is a process driven by the reduction of Gibbs free energy or chemical potential.

the reduction of Gibbs free energy is commonly realized by phase transformation or chemical reaction or the release of stress

For formation of nanowires or nanorods, anisotropic growth is required, i.e. the crystal grows along a certain orientation faster than other directions

Uniformly sized nanowires, i.e. the same diameter along the longitudinal direction of a given nanowire, can be obtained when crystal growth proceeds along one direction, whereas no growth along other directions

Morphology of final product affected by defects and impurities

5


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Evaporation-condensation growth &

Dissolution-condensation growth

Evaporation-condensation process aka vapour-solid process

Driving force for the synthesis of nanorods and nanowires by spontaneous growth: a decrease in Gibbs free energy, arising from either recrystallization or a decrease in super saturation

Nanowires and nanorods grown by evaporation-condensation methods are commonly single crystals with fewer imperfections

The formation of nanowires, nanorods or nanotubules is due to anisotropic growth

6


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Examples of mechanisms resulting

in anisotropic growth

Different facets in a crystal have different growth rate. e.g.

The growth of {111} facets is smaller than that of {110} in Si

with a diamond structure.

Presence of imperfections in specific crystal directions such

as screw dislocation

Preferential accumulation of or poisoning by impurities on

specific facets.

7


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Vapour-Liquid-Solid (VLS) or Solution-

Liquid-Solid (SLS) Growth

In the VLS growth an impurity/catalyst (2nd phase material) isintroduced to direct and confine the crystal growth on to a specificorientation and within a confined area. A catalyst forms a liquid dropletby itself or by alloying with growth material during the growth, whichacts a trap of growth species. Enriched growth species in the catalystdroplets subsequently precipitates at the growth surface resulting in theone-dimensional growth.

VLS theory proposed by Wagner (over 40 years ago)

8


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Requirements for the VLS growth

The catalyst or impurity must form a liquid solution with the crystalline material to be grown at the deposition temperature.

The distribution coefficient of the catalyst or impurity must be less than unity at the deposition temperature.

The equilibrium vapor pressure of the catalyst or impurity over the liquid droplet must be very small.

The catalyst or impurity must be inert chemically.

The interfacial energy plays a very important role. The wetting characteristics influence the diameter of the grown nanowire. For a given volume of liquid droplet, a small wetting angle results in a large growth area, leading to a large diameter of nanowires.

For a compound nanowire growth, one of the constituents can serve as the catalyst.

For controlled unidirectional growth, the solid-liquid interface must be well defined crystallographically. One of the simplest methods is to choose a single crystal substrate with desired crystal orientation.

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Six steps in crystal growth

For most crystal growth, rate-limiting step is either adsorption-

desorption of growth species on the growth surface (step 2) or

surface growth (step 4)

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Nanowires

Nanowires are especially attractive for nanoscience studies as well

as for nanotechnology applications.

Because of their unique density of electronic states, nanowires in

the limit of small diameters are expected to exhibit significantly

different optical, electrical, and magnetic properties from their

bulk 3-D crystalline counterparts.

11


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Characterization and Physical Properties

of Nanowires

Fig. SEM image of GaN

nanowires in a mat arrangement

synthesized by laser-assisted

catalytic growth. The nanowires

have diameters and lengths on the

order of 10 nm and 10μm,

respectively

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Nanowires

The sizes of nanowires are typically large enough (> 1 nm in thequantum confined direction) to have local crystal structures closelyrelated to their parent materials.

Furthermore, nanowires have been shown to provide a promisingframework for applying the “bottom-up” approach for the design ofnanostructures.

Driven by:

the smaller and smaller length scales now being used in thesemiconductor, opto-electronics, and magnetics industries.

13


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Self-Assembled Nanopores in Alumina

for growing nanowires/nanotubes

Nanowire array

SEM image of the surface of an

anodic alumina template with self-

assembled nanopore structure.

Nanowire array

Template Dissolution

Free-standing wires

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Materials of Templates ( pores, self-assembled)

Al2O3 (anodic alumina )

nano-channel glass

ion track-etched polymers

and mica films

Nanowires -Template-Assisted

Synthesis

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Materials of Nanowires (self-assembled)

Metal: Ag, Au, Cu, Fe, Ge, Bi, In, Sn, and Al

Semiconductors: Se, Te, GaSb, and Bi2Te3

Filling methods

– Pressure

– Vapor

– Electrochemical

Nanowires -Template-Assisted

Synthesis

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Nanowires-Template-Assisted Synthesis

Fig. (a) SEM images of the top

surfaces of porous anodic alumina

templates anodized with an average pore

diameter of 44 nm.

(b) SEM image of the particle track-

etched polycarbonate membrane聚碳酸酯薄膜 , with a pore diameter of 1μm.

Porous anodic alumina templates

17


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Nanowires-Electrochemical Deposition

Fig. (a) SEM image of a Bi2Te3 nanowire array in

cross section showing a relatively high pore filling

factor. (b) SEM image of a Bi2Te3 nanowire array

composite along the wire axis.

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Nanowires-VLS Method for Nanowire Synthesis

Some of the recent successful syntheses of semiconductor

nanowires are based on the so-called vapor-liquid-solid (VLS)

mechanism of anisotropic crystal growth.

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Nanowires

Fig. 4.2a–c XRD patterns of

bismuth/anodic alumina

nano-composites with average

bismuth wire diameters of

(a) 40 nm,

(b) 52 nm, and

(c) 95 nm

20


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Four special forms of Si

nanowires.

(a) A springshaped

Si nanowire;

(b) fishboneshaped

(indicated by a

solid arrow) and

frog-egg-shaped

(indicated by a

hollow arrow)

Si nanowires;

(c) pearl-shaped nanowires；(d) shows poly-sites for the nucleation of

silicon

nanowires (indicated by arrows)

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Nanowires

(a) TEM image of a single Co(10 nm)/Cu(10 nm) multilayered nanowire.

(b) A selected region of the sample at high magnification.

22


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STM height images, obtained in the constant current mode, of MoSe

chains deposited on an Au(111) substrate. (a) A single chain image, and

(b) a MoSe wire bundle. (c) and (d) Images of MoSe wire fragments

containing 5 and 3 unit cells. The scale bars are all 1 nm

23


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Nanowires

Fig. 4.6 (a) TEM images of Si nanowires produced after laser ablating a Si0.9Fe0.1

target. The dark spheres with a slightly larger diameter than the wires are

solidified catalyst clusters.

(b) Diffraction contrast TEM image of a Si nanowire. The crystalline Si core

appears darker than the amorphous oxide surface layer. The inset shows the

convergent beam electron diffraction pattern recorded perpendicular to the wire

axis, confirming the nanowire crystallinity.

(c) STEM image of Si/Si1−xGex superlattice nanowires in the bright field mode.

The scale bar is 500 nm.

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a–d SEM images of (a) 6-fold (b) 4-fold and (c) 2-fold symmetry

nanobrushes made of an In2O3 core and ZnO nanowire brushes, and

of (d) ZnO nanonails

A different category of nontrivial nanowires is that of nanowires

having a nonlinear structure, resulting from multiple one-dimensional

growth steps. Members of this category are tetrapods. In this process, a

tetrahedral quantum-dot core is first grown, and then the conditions are

modified to induce a one-dimensional growth of a nanowire from each

one of the facets of the tetrahedron. A similar process produced high-

symmetry In2O3/ZnO hierarchical nanostructures.

25


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Fig. a–c SEM images of ZnO nanowire arrays grown on a sapphire

substrate, where (a) shows patterned growth, (b) shows a higher

resolution image of the parallel alignment of the nanowires, and (c)

shows the faceted side-walls and the hexagonal cross section of the

nanowires.

26


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Nanowires -Characterization and Physical

Properties

Because of their unique density of electronic states, nanowires in thelimit of small diameters are expected to exhibit significantly differentoptical, electrical, and magnetic properties from their bulk 3-Dcrystalline counterparts.

Increased surface area, very high density of electronic states enhancedexciton binding energy, diameter-dependent bandgap, and increasedsurface scattering for electrons are just some of the ways in whichnanowires differ from their corresponding bulk materials.

27


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Nanowires Applications

“Bottom-up” to form nanowire diodes

Schottky diodes can be formed by contacting a GaNnanowire with Al electrodes.

p-n junction diodes can be formed at the crossing of twonanowires, such as the crossing of n and p-type InP nanowiresdoped by Te and Zn, or Si nanowires doped by phosphorus(n-type) and boron (p-type).

28


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Nanowire logic gates:

(a) Schematic of logic OR gate

constructed from a 2(p-Si)

by 1(n-GaN) crossed

nanowire junction. The

inset shows the SEM image

(bar: 1μm)

(b) The output voltage of the

circuit in (a) versus the

four possible logic address

level inputs ( logic 0 input

is 0V and logic 1 is 5V).

(c) Schematic of logic AND gate constructed from a 1(p-Si) by 3(n- GaN) crossed

nanowire junction. The inset shows the SEM image (bar: 1μm) of an assembled

AND gate and the symbolic electronic circuit. (d) The output voltage of the

circuit in (c) versus the four possible logic address level inputs

29


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Nanowires

In addition to the crossing of two distinctive nanowires, heterogeneousjunctions have also been constructed inside a singlewire, either along thewire axis in the form of a nanowire superlattice or perpendicular to thewire axis by forming a core-shell structure of silicon and germanium.

These various nanowire junctions not only possess similar currentrectifying properties as expected for bulk semiconductor devices, butthey also exhibit electro-luminescence (EL) as of a crossed junction of nand p-type InP nanowires that may be interesting for optoelectronicapplications.

30


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Fig. Optoelectrical characterization of a crossed nanowire junction formed

between 65-nm n-type and 68-nm p-type InP nanowires. (a)

Electroluminescence (EL) image of the light emitted from a forward-biased

nanowire p-n junction at 2.5V. Inset, photoluminescence (PL) image of the

junction. (b) EL intensity as a function of operation voltage. Inset, the SEM

image and the I–V characteristics of the junction.

Light emitting diodes (LEDs) achieved in junctions between

a p-type and an n-type nanowire

31


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In addition to the two-terminal nanowire devices, such as the p-njunctions described above, it is found that the conductance of asemiconductor nanowire can be significantly modified by applyingvoltage at a third gate terminal, implying the utilization of nanowiresas a field-effect transistor (FET).

This gate terminal can either be the substrate, a separate metal contactlocated close to the nanowire, or another nanowire with a thick oxidecoating in the crossed nanowire junction configuration.

Nanowires as a field-effect transistor (FET)

32


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Fig. Gate-dependent I–V

characteristics of a crossed

nanowire field-effect transistor

(FET). The n-GaN nanowire is used

as the nano-gate, with the gate

voltage indicated (0, 1, 2, and 3 V).

The inset shows the current vs.

Vgate for a nanowire gate (lower

curve) and for a global back-gate

(top curve) when the bias voltage is

set at 1V

Nanowires as a field-effect transistor (FET)

33


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Optical Properties of Nanowires

Light emission from quantum wire p-n junctions is especially

interesting for laser applications, because :

Quantum wires can form lasers with lower excitation thresholds

compared to their bulk counterparts, and also exhibit a decreased

temperature sensitivity in their performance.

Furthermore, the emission wavelength can be tuned for a given material

composition by only altering the geometry of the wire.

34


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Fig. . (a) Topographical and (b) photoluminescence (PL)

near- field scanning optical microscopy (NSOM)

images of a single ZnO nanowire waveguide.

Characterization of Nanowires

35


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Nanowires

Fig. 4.42 A schematic of lasing in ZnO nanowires and the PL spectra of

ZnO nanowires at two excitation intensities. One PL spectra is taken

below the lasing threshold, and the other above it.

36


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Nanowire photo detectors

ZnO nanowires were found to display a strong

photocurrent response to UV light irradiation.

The conductivity of the nanowire increased by four orders

of magnitude compared to the dark state.

The response of the nanowire was reversible and selective

to photon energies above the band gap, suggesting that

ZnO nanowires could be a good candidate for

optoelectronic switches.

37


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Application of Nanowires

Fig. (a) An optical image of many short Au- Ag-Au-Au bar-coded

wires and (b) an FE-SEM image of an Au/Ag barcoded wire with

multiple strips of varying length. The insert in (a) shows a histogram

of the particle lengths for 106 particles in this image.

38


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Chemical and Biochemical Sensing Devices

Fig. (a) Streptavidin molecules bind to a silicon nanowire

functionalized with biotin. The binding of streptavidin to biotin

causes the nanowire to change its resistance. (b) The conductance of

a biotin-modified silicon nanowire exposed to streptavidin in a

buffer solution (regions 1 and 3) and with the introduction of a

solution of antibiotin monoclonal antibody .

Nanowire sensors will

potentially be smaller, more

sensitive, demand less power,

and react faster than their

macroscopic counterparts.

39


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BCF Theory – presence of screw dislocation ensures

continuous growth and enhances the growth rate

40


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Mechanism leading to formation of nanowire

The growth rate of a facet increases with an increased

density of screw dislocations parallel to the growth

direction. It is known that different facets can have a

significantly different ability to accommodate dislocations.

The presence of dislocations on a certain facet can result in

anisotropic growth, leading to the formation of nanowire

or nanorods.

41


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PBC Theory(100), F-face, unstable

(110), S-face

(111), K-face

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Mechanism leading to Anisotropic Growth

Therefore, α = 1 for {111} and {110}, α< 1 for {100}. Thisleads to that growth rate for {111}, {110} is greater thanthat for {100}. For both {111} and {110}, the growthprocess is always adsorption limited. Facets with highgrowth rate (high surface energy) disappears while facetswith low growth rate (low surface energy) survives. Thisleads to anisotropic growth and results in nanowires. Inaddition, defects-induced growth and impurity-inhibitedgrowth are the possible mechanisms for growth along axisof nanowires. A low supersaturation is required foranisotropic growth. A higher supersaturation supportsbulk crystal growth or homogeneous nucleation leading toformation of polycrystalline or powder.

43


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Growth of Single Crystal Nanobelts of

Semiconducting or metal oxides

Evaporating the metal oxides (ZnO, SnO2, In2O3, CdO) athigh temperatures under a vacuum of 300 torr andcondensing on an alumina substrate, placed inside thesame alumina tube furnace, at relatively low temperature.

Or heating the metal oxide or metal nanoparticles atT=780 - 820oC in air, Nanorods can be obtained dependingupon annealing T and time. Nanowires such as ZnO,Ga2O3, MgO, CuO or Si3N4 and SiC can be made by thismethod.

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By controlling growth kinetics, a

consequence of minimizing the total

energy attributed by spontaneous

polarization and elasticity, left-

handed helical nanostructures and

nano-rings can be formed.

48


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Dissolution and Condensation Growth

The growth species first dissolve into a solvent or a

solution, and then diffuse through the solvent and

deposit onto the surface resulting growth of

nanowires.

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Growth of Se Nanowires

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Growth of SeTe Nanowires52


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Growth of Ag Nanowire Using Pt

Nanoparticles as Growth Seeds

Precursor: AgNO3

Reduction agent: ethylene glycol

Surfactant: polyvinyl pyrrolidone (PVP)

The surfactant absorbed on some growth surfaces and

blocks the growth, resulting in the formation of uniform

crystalline silver nanowires.

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55


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Disadvantages of Evaporation – Condensation

Deposition

Nanowire grown by EC most likely have facetedmorphology and are generally short in length withrelatively small aspect ratios, particular when grown inliquid medium. However, anisotropic growth induced byaxial imperfections, such as screw dislocation, microtwinsand stacking faults, or by impurity poisoning, can result inthe growth of nanowires with large aspect ratios.

56


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Vapor (or solution)-Liquid-solid (VLS) Growth

It is noted that the surface of liquid has a large accommodation coefficient,

and is therefore a preferred site for deposition.

57


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Wagn

er sum

marized

the req

uirem

ents

for V

LS

gro

wth

over

30yea

rs ago.

58


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VLS Growth Process

59


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60


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61


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The growth rate for VLS is much faster

The liquid surface can be considered as a rough surface.Rough surface is composed of only ledge, ledge-kink, or kinksites. That is, every site over the entire surface is to trap theimpinging growth species. The accommodation coefficient isunity. It is reported that the growth rate of silicon nanowireusing a liquid Pt-Si alloy is about 60 times higher than directlyon the silicon substrate at 900oC.

The liquid acts as a sink for the growth species in the vaporphase, it also act as a catalyst for the heterogeneous reactionor deposition.

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67


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Compound Semiconductor Nanowires

Nanowires of binary group III-V materials (GaAs, GaP,

InAs, and InP), ternary III-V materials (GaAs/P,

InAs/P), binary II-VI compounds (ZnS, ZnSe, CdS, and

CdSe), and binary IV-IV SiGe alloys have been made in

bulk quantities as high purity (>90%) single crystals.

68


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69


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Table 1. Summary of single crystal nanowires synthesized. The growth temperatures

correspond to ranges explored in these studies. The minimum and average nanowire

diameters were determined from TEM and FESEM

images. Structures were determined using electron diffraction and lattice resolved TEM

imaging: ZB, zinc blende; W, wurtzite; and D, diamond structure types. Compositions

were determined from EDX measurements made on individual nanowires. All of the

nanowires were synthesized using Au as the catalyst, except GaAs, for which Ag and Cu

were also used. The GaAs nanowires obtained with Ag and Cu catalysts have the same

size distribution, structure, and composition as those obtained with the Au catalyst

70


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Choice of Catalyst

The catalysts for VLS growth can be chosen in the absenceof detailed phase diagrams by identifying metals in whichthe nanowire component elements are soluble in the liquidphase but that do not form solid compounds more stablethan the desired nanowire phase; i.e., the ideal metalcatalyst should be physically active but chemically stable.From this perspective the noble metal Au should representa good starting point for many materials. This noble metalalso has been used in the past for the VLS growth of surfacesupported nanowires by metal-organic chemical vapordeposition (MOCVD).

71


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.

73


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In general, the nanowires grown by VLS have a cylindrical morphology, i.e. without facets on

the side surface and having a uniform diameter. This is attributed to the growth at a

temperature greater than the roughening temperature (surface undergoing a transition from

faceted (smooth) to rough surface).

74


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75


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76


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VLS Growth of Nanowires

Precursors: Compound gas (e.g. SiCl4), evaporation of

solids, Laser ablation of solid targets.

77


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78


.

79


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Methods for Growth of CNTs

Furnace at 1200 C

Ar gas

Graphite

targetLase

r

Water-cooled

copper

collector

Nanotube

growing along tip

of collector

Laser Ablation Process

Arc-Discharge System

Water

Water

inin out

out

graphite, cathodegraphite anode

Water in

Hepump

Water out

Power Supply

mass flow controllerauto pressure controller

Formation of

nanotubes

Note: The target may be

made by pressing Si

powder mixed with 0.5%

iron.

80


.

81


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Advantages of Oxide Assisted Growth

(OAG) Si Nanowires

pp. 635-640.

82


.

83


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SLS Growth of InP Nanowires84


.

Gro

wth

of S

ilicon

Na

no

wires b

y S

LS

meth

od

85


.

86


.

87


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Template-Based Synthesis-Electrochemical

Deposition

88


.

89


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90


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91


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92


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Electroless Electrolysis93


.

Usin

g p

oly

carb

on

ate

mem

bra

ne a

s te

mp

late

–

Ele

ctro

ch

em

ical D

ep

ositio

n (fo

r co

nd

uctin

g

po

lym

er) o

r ele

ctro

less

ele

ctro

lysis

(for

po

lym

er)

94


.

95


.

Electrophoretic Deposition

Stabilization of colloids is generally achieved by

electrostatic double layer mechanism.

96


.

97


.

Us

ing

po

lyc

arb

on

ate

me

mb

ran

ce

as

tem

pla

te

98


.

99


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Template Filling10

0


.

Template Filling

10

1


.

Incomplete Filling of the Template10

2


.

Template Filling Assisted with Centrifugation

Force

Centrifugation Force which must be greater than the

repulsion force between particles

10

3


.

Converting Through Chemical Reactions

10

4


.

Electro

spin

ing

–u

sing

electric

forces (o

verco

me su

rface ten

sion

)

to p

ord

uce

poly

mer fib

ers

10

5

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Prof.P. Ravindran, - Universitetet i oslofolk.uio.no/ravi/cutn/nmnt/14.1D Nanostructures -...

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