Biomedical Engineering Reference
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metal, followed by the nucleation and growth of single-crystalline rods and then wires.
Since the diameter of the NWs is determined by the diameter of the catalyst particles, this
method provides an efficient means to obtain uniformly sized NWs. This mechanism has
been widely accepted and applied to the growth of various NWs. Indeed, the synthesis of
NWs with controlled composition, size, purity, and crystallinity required an increased
understanding of the nucleation and growth processes at the nanoscale. In situ high-
temperature TEM provided real-time observations of Ge NW growth that confirmed the
VLS growth mechanism [53]. Experimental observations suggest that there are three
growth stages: metal alloying, crystal nucleation, and axial growth (Figure 12.10).
12.3.2.1.2 Vapor-Solid Growth
The vapor-solid grow method, also called evaporation-condensation growth, is also a
popular and simple method to grow oxide NWs. In this process, the vapor species is first
generated by evaporation, chemical reduction, and gaseous reaction. The vapor is subse-
quently transported and condensed onto a substrate. Using this method, various oxide
NWs of Zn, Sn, In, Cd, Mg, Ga, and Si have been synthesized. For example, Zhu and
(A)
(B)
20 nm
(C)
(D)
(E)
(F)
FIGURE 12.10
The birth of a Ge nanowire on a Au nanocluster, as observed using in situ TEM. It is clearly seen that the Au nan-
ocluster started to melt after the formation of Ge-Au alloy, and this was followed by an increase in the liquid
droplet size during the Ge vapor condensation process. When the droplet was supersaturated with the Ge com-
ponent, a Ge nanowire grew out of this droplet of Au-Ge alloy and became longer as time elapsed. (From Wu,
Y., Yang, P. (2001). Direct Observation of Vapor-Liquid-Solid Nanowire Growth. J. Am. Chem. Soc. , 123,
3165-3166.)
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