II–IV semiconductor nanocrystal synthesis to the synthesis of semiconductor
nanorods [329, 330a], a unique version of nanowires with relatively shorter aspect
ratio. Xia and co-workers used a polyol method to generate the Ag nanowires by
reducing silver nitrate with ethylene glycol in the presence of polyvinyl pyrrolidone
(PVP) [330b–d]. The key to the formation of a 1D nanostructure is the use of PVP
as a polymeric capping reagent and the introduction of a seeding step. Silver nitrate reduces in the presence of the seed (Pt, or Ag nanoparticles) to form silver
nanoparticles with bimodal size distribution produced via homogeneous and heterogeneous nucleation processes. Ag nanorods grow at the expense of the small Ag
nanoparticles, as directed by the capping reagent (PVP). In the presence of PVP,
most silver particles can be confined and directed to grow into nanowires of uniform diameter. These nanowires have FCC structure with mean diameter @40 nm.
Solvothermal Synthesis Solvothermal methodology has been extensively examined
as a possible solution route to semiconductor nanowires and nanorods. In this
process, a solvent is mixed with certain metal precursors and possibly crystal
growth regulating or templating agents such as amines. This solution mixture was
then placed in an autoclave kept at relatively high temperature and pressure to
carry out the crystal growth and assembly process. The methodology seems to be
quite versatile and has been demonstrated to be able to produce many different
crystalline semiconductor nanorods and nanowires [331, 332]. Xia et al. [307]
demonstrated a solution-phase approach by refluxing selenious acid and excess
hydrazine for the synthesis of uniform nanowires of selenium with lateral dimensions controllable in the range 10–30 nm and with lengths of up to hundreds of
micrometers.
8.4.2.4 Growth Control
A significant challenge for the chemical synthesis is how to rationally control the
nanostructure assembly so that the size, dimensionality, interfaces and ultimately
the 2-D and 3-D superstructures can be tailormade to a desired functionality. Many
physical and thermodynamic properties are diameter dependent. Yang et al. have
used monodispersed Au clusters with sizes 15.3, 20, 25, 29, 52 nm and obtained
uniform nanowires with sizes from 23 to 57 nm respectively.
Controlling the growth orientation is important for the applications of nanowires. By applying the conventional epitaxial crystal growth technique to the VLS
process, a new vapor–liquid–solid epitaxy (VLSE) technique, has been developed
for the controlled synthesis of nanowire arrays. Nanowires generally have preferred
growth directions. For example, Si nanowires prefer to grow along the h111i direction. Hence if (111) Si wafer is used as substrate, Si nanowires will grow epitaxially and vertically on the substrate [333, 334]. Similarly ZnO nanowires prefer
to grow along the h001i direction [335].
It is clear from the VLS nanowire growth mechanism that the positions of the
nanowires can be controlled by the initial positions of the Au clusters or Au thin
films. By creating desired patterns of Au using the lithographic technique it is
possible to grow ZnO nanowires of the same designed pattern since they grow
8.4 Nanowires 273
nanorods [329, 330a], a unique version of nanowires with relatively shorter aspect
ratio. Xia and co-workers used a polyol method to generate the Ag nanowires by
reducing silver nitrate with ethylene glycol in the presence of polyvinyl pyrrolidone
(PVP) [330b–d]. The key to the formation of a 1D nanostructure is the use of PVP
as a polymeric capping reagent and the introduction of a seeding step. Silver nitrate reduces in the presence of the seed (Pt, or Ag nanoparticles) to form silver
nanoparticles with bimodal size distribution produced via homogeneous and heterogeneous nucleation processes. Ag nanorods grow at the expense of the small Ag
nanoparticles, as directed by the capping reagent (PVP). In the presence of PVP,
most silver particles can be confined and directed to grow into nanowires of uniform diameter. These nanowires have FCC structure with mean diameter @40 nm.
Solvothermal Synthesis Solvothermal methodology has been extensively examined
as a possible solution route to semiconductor nanowires and nanorods. In this
process, a solvent is mixed with certain metal precursors and possibly crystal
growth regulating or templating agents such as amines. This solution mixture was
then placed in an autoclave kept at relatively high temperature and pressure to
carry out the crystal growth and assembly process. The methodology seems to be
quite versatile and has been demonstrated to be able to produce many different
crystalline semiconductor nanorods and nanowires [331, 332]. Xia et al. [307]
demonstrated a solution-phase approach by refluxing selenious acid and excess
hydrazine for the synthesis of uniform nanowires of selenium with lateral dimensions controllable in the range 10–30 nm and with lengths of up to hundreds of
micrometers.
8.4.2.4 Growth Control
A significant challenge for the chemical synthesis is how to rationally control the
nanostructure assembly so that the size, dimensionality, interfaces and ultimately
the 2-D and 3-D superstructures can be tailormade to a desired functionality. Many
physical and thermodynamic properties are diameter dependent. Yang et al. have
used monodispersed Au clusters with sizes 15.3, 20, 25, 29, 52 nm and obtained
uniform nanowires with sizes from 23 to 57 nm respectively.
Controlling the growth orientation is important for the applications of nanowires. By applying the conventional epitaxial crystal growth technique to the VLS
process, a new vapor–liquid–solid epitaxy (VLSE) technique, has been developed
for the controlled synthesis of nanowire arrays. Nanowires generally have preferred
growth directions. For example, Si nanowires prefer to grow along the h111i direction. Hence if (111) Si wafer is used as substrate, Si nanowires will grow epitaxially and vertically on the substrate [333, 334]. Similarly ZnO nanowires prefer
to grow along the h001i direction [335].
It is clear from the VLS nanowire growth mechanism that the positions of the
nanowires can be controlled by the initial positions of the Au clusters or Au thin
films. By creating desired patterns of Au using the lithographic technique it is
possible to grow ZnO nanowires of the same designed pattern since they grow
8.4 Nanowires 273