synthesis mechanism of nanowires and nanotubes

8/14/2019 synthesis mechanism of nanowires and nanotubes

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SYNTHESIS MECHANISM OF NANOWIRES

AND NANOTUBES

RAJASEKARAKUMARVADAPOO

by

UPR Physics Dept., Rio piedras Campus, San juan, PR-00931, USA.

More info: http://nanophysics.wordpress.com/


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Differences between growth methods &growth mechanism

Growth methods

Provide a kinetic & thermodynamic rationaleGrowth mechanism

How one dimensional growth occurs

Be predictable & applicable to a widevariety of systems

PVD, CVD, Solution techniques, ..


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General synthetic strategies for 1D growth

Dictation by the anisotropic

crystallographic structures of a solid.

The introduction of a liquid/ solidinterface to reduce the symmetry of the

seed.

direction through the use of a

template. kinetic control provided by a

capping reagent.

self assembly of 0D nanostructures.

size reduction of a 1D

microstructure.Fig.1. General strategies for 1D growth

Mat Law et al., Annu. Rev. Mater. Res. 2004, 34:83-122


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VLS ( Vapor- Liquid- Solid) Mechanism

-Self Catalytic VLS

VS ( Vapor- Solid) Mechanism

Solution-Phase selective Capping Mechanism

Shown to produce high

quality materials

III-V Compounds

( GaN, GaAs, GaP, InAs)

VLS

Pure & Doped Inorganic Materials

II-VI Compounds

( ZnS, ZnSe, CdS, CdSe)

Oxides, Carbides & Nitrides

(InSnO, ZnO, MgO, SiO2,

CdO, SiC, B4C, Si3N4 )


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VLS- mechanism

Fig.2. a) Schematic of VLS process

b)Gold- Ge binary phase diagram

Key Steps

Establishment of

symmetry breaking Solid-

Liquid interface -important Stoichiometry, Lattice

symmetry- less relevant

Wu y, Yang P.,Am. Chem. Soc., 2001, 123:3165-66


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Fig.3. a) Au nanoclusters in solid state at 500C

(b) alloying initiates at 800C, at this stage Au existsmostly in solid state (c) liquid Au/Ge alloy;

(d) the nucleation of Ge nanocrystal on the alloy surface;

(e) Ge nanocrystal elongates with further Ge

condensation & (f) eventually forms a wire.

Advantages:

high purity

High uniformity in diameter

Usual dia.: 10 nm

length : > 1m

Challenges:

Only for Eutectic compound

Contaminant of the Seed

metal.

Wu y, Yang P.,Am. Chem. Soc., 2001, 123:3165-66


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Fig. 4. (a) Field-effect scanning electron

microscope (FESEM) image of the GaN

nanowires grown on a gold-coated c-

plane sapphire substrate. Inset shows a

nanowire with its triangular cross section.

(b) TEM image of a GaN nanowire with a

gold metal alloy droplet on its tip. Insets

are electron diffraction patterns taken

along the [001] zone axis. The lower inset

is the same electron diffraction pattern

but purposely defocused to reveal the

wire growth direction.

(c) Lattice-resolved TEM image of the

nanowire.

Kuykendall T et al., Nano Lett., 2003, 1063-66


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Self- Catalytic VLS

Temp: > 8500C & Press: 10-7 torr.

GaN (s) Ga (l) + 0.5 N (g) + 0.25 N2(g).

&

GaN (s) GaN (g) or [GaN]x (g).

Fig. 5. A series of video frames grabbed from

observations of GaN decomposition at ~1050C,

showing the real-time GaN nanowire growth

process. The number on the bottom left corner ofeach frame is the time (second:millisecond).

Challenges:

Precise control of nanowire length &

diameter- yet to demonstrate.

Versatility of this approach yet to

demonstrate.

Stach EA et al., Nano Lett., 2003, 3: 867-69


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VLS- vapor phase methods

Laser Ablation MBEMOCVD

Less High HighMobility of

carriers

Integration to

the thin film

technology

Less High High

Flexibility Normal GreatGreatest

VLS- vapor phase Methods

Nanowire Growth

Mat Law et al., Annu. Rev. Mater. Res. 2004, 34:83-122

Fig.6. Simple vapor growth


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VS - Mechanism

F+P= C+2

F No. of degrees of freedom

P No. of Phases

C No. of Chemical constituents

Phase Rule

Different defect induced growth mechanism

Thermodynamic &

kinetic considerations-1D growth

An anisotropic growth mechanism(Binding of gas phase reactants along specific XtalFacets (TKP), min. surface energy (TP))

Franks screw dislocation mechanism

(Specific defects- high sticking coefficient of gasPhase species)

Self catalytic VLS

oxide assisted growth mechanism success

structurally uniform

complex morphology

Lacks in compelling thermodynamic &

kinetic justification for 1D growth.Mat Law et al., Annu. Rev. Mater. Res. 2004, 34:83-122

Fig. 7. phase diagram


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Growth in solution

poly vinyl pyrrolidone (PVP)- used PVP- bind the {100} facets of silver {111} facets to allow growth

controllable dia.: 30- 60 nm length: ~50 m

Hydrolysis of Zn salt in the presence of amines

controllable dia.: 30-100 nm. Length: 2-10 m.

Sun Y et al., Chem. Mater., 2002, 14:4736-45,Greene LE et al., Angew. Chem. Int. Ed., 2003, 42:3031-34

Fig.8. solution growth mechanism

Fig. 9. ZnO nanowires using solution growth


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HetrostructuresLongitudinal

coaxial

Si/ SiGe

GaN/ AlGaN

Wu Y et al., Nano Lett., 2002, 2: 83-86Gudiksen MS et al., nature, 2002, 415:617-20Bjoerk MT et al., Nano lett., 2002,2:87-89

Fig. 11. Si/ SiGe hetrostructure using PLD


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Conclusion Synthetic capabilities continue to expand quickly

Several outstanding scientific challenges need to be addressed

integration & interfacing problems

precise control over- size uniformity, dimensionality, growthdirection, dopant distribution within the semiconductornanostructures

accurate appropriate theoretical simulations


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Back- Ups

Cooling

Water

Cooling

Water

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Source Materials

Carrying

GasPump

Substrate

synthesis mechanism of nanowires and nanotubes

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