reduce the symmetry of a seed, (iii) the use of templates (with 1D morphologies) to
direct the formation of nanowires, (iv) the use of supersaturation control to modify
the growth habit of a seed; (v) the use of capping agents to kinetically control the
growth rates of various facets of a seed and (vi) self-assembly of 0D nanostructures.
They can be usefully categorized into: (1) nanowire growth in the gas phase; (2)
solution-based approaches to nanowires.
8.4.2.1 Vapor Phase Growth of Nanowires
Vapor phase growth is commonly used to produce nanowires. Starting with the
simple evaporation technique in an appropriate atmosphere to produce elemental
or oxide nanowires, vapor–liquid–solid, vapor–solid and other processes are made
use of.
Vapor–Liquid–Solid Growth The growth of a nanowire via a gas phase reaction
involving a vapor–liquid–solid (VLS) process has been extensively studied. Wagner,
during his studies on the growth of large single-crystalline whiskers, proposed in
the 1960s, a mechanism for the growth via a gas phase reaction involving the socalled vapor–liquid–solid (VLS) process [278]. According to this mechanism, the
anisotropic crystal growth is promoted by the presence of a liquid alloy–solid interface. His mechanism was widely accepted and applied to understanding the
growth of the nanowires of Si, Ge and others. The growth of Ge nanowire using
Au clusters as solvent at high temperature can be explained based on the GeaAu
binary phase diagram as shown in Figure 8.28(b). The Ge and Au will form a
liquid alloy when the temperature is higher than the eutectic point (363
C) as
shown in Figure 8.28(a-I). The liquid surface has a large accommodation coefficient and is therefore a preferred deposition site for incoming Ge vapor. After the
liquid alloy becomes supersaturated with Ge, Ge nanowire growth occurs by precipitation at the solid–liquid interface (Figure 8.28(a-II, a-III)). A real time observation of Ge nanowire growth conducted in an in situ high temperature transition
electron microscope shows a sequence of TEM images which directly mirrors the
proposed VLS mechanism [279]. This VLS method has been exploited in the past
several decades to produce 1–100 mm diameter 1D structures (whiskers). By controlling the nucleation and growth, it is possible to produce semiconductor nanowhiskers (e.g., InAs, GaAs) using organometallic vapor phase epitaxy. There are
reports on the VLS growth of elemental semiconductors (e.g., Si and Ge), III–V
semiconductors (e.g., GaAs, InP, InAs), II–VI semiconductors (e.g., ZnS, CdS,
CdSe), oxides (e.g., ZnO, SiO 2 ) [271–273, 277, 279–292]. Lieber and coworkers
have developed and optimized a laser ablation based VLS process to produce
semiconductor nanowires with many different compositions [273, 277]. TEM
studies showed the product obtained after the VLS growth is primarily wire-like
structures with remarkably uniform diameters of the order of 10 nm with lengths
>1 mm. By knowing the equilibrium phase diagram one can predict the catalyst
materials and growth conditions for the VLS approach. By following the VLS
approach, Lee and coworkers [293a] have synthesized highly pure, ultra-long and
uniform-sized semiconductor nanowires in bulk quantities by employing laser
8 Nanotubes and Nanowires
256
direct the formation of nanowires, (iv) the use of supersaturation control to modify
the growth habit of a seed; (v) the use of capping agents to kinetically control the
growth rates of various facets of a seed and (vi) self-assembly of 0D nanostructures.
They can be usefully categorized into: (1) nanowire growth in the gas phase; (2)
solution-based approaches to nanowires.
8.4.2.1 Vapor Phase Growth of Nanowires
Vapor phase growth is commonly used to produce nanowires. Starting with the
simple evaporation technique in an appropriate atmosphere to produce elemental
or oxide nanowires, vapor–liquid–solid, vapor–solid and other processes are made
use of.
Vapor–Liquid–Solid Growth The growth of a nanowire via a gas phase reaction
involving a vapor–liquid–solid (VLS) process has been extensively studied. Wagner,
during his studies on the growth of large single-crystalline whiskers, proposed in
the 1960s, a mechanism for the growth via a gas phase reaction involving the socalled vapor–liquid–solid (VLS) process [278]. According to this mechanism, the
anisotropic crystal growth is promoted by the presence of a liquid alloy–solid interface. His mechanism was widely accepted and applied to understanding the
growth of the nanowires of Si, Ge and others. The growth of Ge nanowire using
Au clusters as solvent at high temperature can be explained based on the GeaAu
binary phase diagram as shown in Figure 8.28(b). The Ge and Au will form a
liquid alloy when the temperature is higher than the eutectic point (363
C) as
shown in Figure 8.28(a-I). The liquid surface has a large accommodation coefficient and is therefore a preferred deposition site for incoming Ge vapor. After the
liquid alloy becomes supersaturated with Ge, Ge nanowire growth occurs by precipitation at the solid–liquid interface (Figure 8.28(a-II, a-III)). A real time observation of Ge nanowire growth conducted in an in situ high temperature transition
electron microscope shows a sequence of TEM images which directly mirrors the
proposed VLS mechanism [279]. This VLS method has been exploited in the past
several decades to produce 1–100 mm diameter 1D structures (whiskers). By controlling the nucleation and growth, it is possible to produce semiconductor nanowhiskers (e.g., InAs, GaAs) using organometallic vapor phase epitaxy. There are
reports on the VLS growth of elemental semiconductors (e.g., Si and Ge), III–V
semiconductors (e.g., GaAs, InP, InAs), II–VI semiconductors (e.g., ZnS, CdS,
CdSe), oxides (e.g., ZnO, SiO 2 ) [271–273, 277, 279–292]. Lieber and coworkers
have developed and optimized a laser ablation based VLS process to produce
semiconductor nanowires with many different compositions [273, 277]. TEM
studies showed the product obtained after the VLS growth is primarily wire-like
structures with remarkably uniform diameters of the order of 10 nm with lengths
>1 mm. By knowing the equilibrium phase diagram one can predict the catalyst
materials and growth conditions for the VLS approach. By following the VLS
approach, Lee and coworkers [293a] have synthesized highly pure, ultra-long and
uniform-sized semiconductor nanowires in bulk quantities by employing laser
8 Nanotubes and Nanowires
256