chapter 4 one-dimensional nanostructures: nanowires and...

Chapter 4 One-dimensional Nanostructures: Nanowires and Nanorods

Introduction4.2 Spontaneous Growth

4.2.1 Evaporation-condensation growth4.2.2 Vapor-liquid-solid growth

4.3 Template-based Synthesis4.3.1 Electrochemical deposition4.3.2 Electrophoretic deposition4.3.3 Template filling4.3.4 Converting through chemical reactions

4.4 Electrospinning4.5 Lithography

Six Different Strategies for achieving 1D growth

(a) dictation by the anisotropic crystallographic

structure of a solid.(b) confinement by a liquid droplet as in the

VLS process. (c) direction through the use of a template.(d) kinetic control provided by a capping reagent.(e) self assembly of 0D nanostructures.(f) size reduction of 1D microstructure.

4.2 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 (Vapor-liquid-solid)

Chemical reaction (unstable-stable, chemical potential; high-low)

Release of stress (ordered-disordered)

For the 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.

4.2.1 Evaporation-condensation growth4.2.1.1 Fundamentals of evaporation-condensation growth

Evaporation-condensation = Vapor-solid growth = vapor phase growth

1D structure growth requires “Anisotropic Growth”→wire/rod growth

Ex. Different facets (selection of growth facet with fast growth rate)

Presents of imperfections screw dislocations

Preperantial accumulation of


Process 1: Fast, not a rate limiting process.

Process 2: Can be rate limiting if the supersaturation or concentration of growth species is low.

Process 4: Will be the rate limiting process when a sufficient supersaturation or a high concentration of growth species is present.

Process 3, 5 and 6: Not rate limiting processes in general.


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).

When step 2 is rate limiting, the growth rate is determined by condensation rate,

Accommodation coefficient s is the fraction of impinging growth species that becomes accommodated on the growing surface, and is a surface specific property.

A surface with a high s will have a high growth rate as compared with low s surfaces.

A significant difference in accommodation coefficients in different facets would result in anisotropic growth.

a: accommodation coefficients=(P-P0)/P0, supersaturationT: temperaturem: atomic weight of the growth speciesk: Boltzmann constant

mkT

PJ o

πασ2

= (atoms/cm2 sec),


When the concentration of the growth

species is very low, the adsorption is

more likely a rate-limiting step.

The growth rate increases with the

increase in the concentration of growth

species.

Further increase in the concentration of

growth species would result in a change

from an adsorption limited to surface

growth limited process.

When the surface growth becomes a

rate-limiting step, the growth rate

becomes independent of the

concentration of the growth species.


Residence time (τs) for a growth species on the growing surface

Diffusion coefficient of a growth species on the growing surface

Mean diffusion distance of a growth species on the growing surface

⎟⎠⎞

⎜⎝⎛=

kT

Edess exp

1

ντ

⎟⎠⎞

⎜⎝⎛ −=

kT

EaD s

s exp2

10ν

⎟⎠⎞

⎜⎝⎛ −

==kT

EEaDX sdes

ss exp2 0τ

Accommodation coefficientIf the mean diffusion distance is far longer than the distance between two growth sites such as kinks ledges, all adsorbed growth species will be incorporated in to the crystal structure and the accommodation coefficient would be unity. If the mean diffusion distance is far shorter than the distance between two growth sites, all adatoms will escape back to the vapor and the accommodation coefficient will be zero. The accommodation coefficient is dependent on desorptionenergy, activation energy of surface diffusion, and the density of growth sites.


KSV theory: Classic step-growth theory for a flat surface, developed by Kossel, Stranski and Volmer, based on reorganization that a crystal surface is not smooth on the atomic scale.For a simple cubic crystal, each atom as a cube has a 6 coordination number (6 chemical bonds).

An atom adsorbed (adatom) on a terrace would form 1 chemical bond between the atom and the surface.If adatom diffuses to a ledge site, it would form 2 chemical bonds. If an atom were incorporated to a ledge-kink site, 3 chemical bonds would be form.An atom incorporated into a kink site would form 4 chemical bonds.


Ledge, ledge-kink, kink sites are all considered as growth sites; incorporation of atoms into these sites is irreversible and results in growth. The growth is due to advancement of the steps (or ledges) and the growth rate will be dependent on the step density. A misoriented surface (vicinal surface) would result in an increased density of steps and consequently lead to a high growth rate.An increased step density would favor the irreversible incorporation of adatomsby reducing the surface diffusion distance between the impinging site and the growth site before adatoms escape back to the vapor phase.

Limitation of KSV theory is how to regenerate growth site if all available growth sites are consumed.


Propose screw dislocation as a continuous source to generate growth sites so that stepped growth would continue. The crystal growth proceeds in a spiral growth.The presence of screw dislocation ensure the continuing advancement of the growth surface and also enhance growth increased density of screw dislocations parallel to the growth direction. It is also 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.

BCF theory: Developed by Burton, Cabrera and Frank in 1951.



All crystal facets can be categorized in to three groups based on the number of broken periodic bond chains (simply understood as it means the number of broken bonds per atom on a given facet) on a given facets.

Flat surface (F-face): {100} surfaces in a simple cubic

Have 1 PBCStepped surface (S-face):{110} surfaces in a simple cubic

Have 2 PBCs. Kinked surface (K-face):{111} surfaces in a simple cubic

Have 3 PBCs.

PBC theory: Periodic Bond Chain theory, developed by Hartmann and Perdok in 1955.

4.2.1 Evaporation-condensation growth4.2.1.2 Evaporation-condensation growth

ZnO nanobeltsCharacteristics of the ZnO nanobelts

Typical thickness: 10 -30 nm.Width to thickness ratio: ~5 to 10.Two growth directions were observed:[0001] and [0110].

No screw dislocation was found through out theentire length of nanobelt, except a single stacking fault parallel to the growth axisin the nanobelts grown along [0110].The surface of nanobelts are clean,atomically sharp and free of any sheathed amorphous phase.



(a) and (b): CuO nanowires synthesizedby heating a copper wire (0.1 mm diameter)in air to a temperature of 500OC for 4h.Each CuO nanowires was bicrystal.[Nano Letters, vol 2, pp. 1333, 2002]

4.2.1 Evaporation-condensation growth4.2.1.3 Dissolution-condensation growth

4.2.2 Vapor-liquid-solid growth4.2.2.1 Fundamental aspects of VLS and SLS growth

Liquid solution (catalyst, impurity)

Distribution coefficient must be less than unity

Evaporation pressure of catalyst must be very small

Catalyst must be chemically inert

Interfacial energy plays an important role

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

For controlled unidirectional growth solid-liquid interface must be well defined crystallographically


Growth proceduresThe growth species is evaporated first and then diffuses and dissolves into a liquid droplet.The surface of the liquid has a large accommodation coefficient, and is therefore a preferred site for deposition.Saturated growth species in the liquid droplet will diffuse to and precipitate at the interface between the substrate and the liquid.The precipitation will first follow nucleation and the crystal growth.Continued precipitation or growth will separate the substrate and the liquid droplet, resulting in a growth of nanowires.

4.2.2.2 VLS growth of various nanowires

4.2.2.2 VLS growth of various nanowires

4.2.2.3 Control of the size of nanowires

4.2.2.4 Precursors and catalysts

)(32)(2

)(4)()(2

)(2)(4)(

22

32

2

2

gxg

glg

ggs

NOOGaOxGaN

COZnCZnO

GeIGeGeI

GeIGeIGe

+→⎟⎠⎞

⎜⎝⎛ ++

+↔+

+→

→+

4.2.2.5 SLS growth

4.2.3 Stress induced recrystallization

Strain/Stress

4.3 Template-based synthesis4.3.1 Electrochemical deposition

)ln(0 ii

g aFn

TREE +=

Nerst Equation



4.3.2 Electrophoretic deposition


( )

kT

zne

aa

Q

r

ii

r

0

22

14

εεκ

κπεξ

∑=

+=


4.3.2 Electrophoretic deposition4.3.3.1 Colloidal dispersion filling


4.3.3.2 Melt and solution filling

4.3.3.3 Chemical vapor deposition

4.3.3.4 Deposition by centrifugation

4.3.4 Converting through chemical reactions

4.4 Electrospinning

4.5 Lithography

4.6 Summary

chapter 4 one-dimensional nanostructures: nanowires and...

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