The top-down approach limits the dimensions of devices to what is technically achievable using

controlled selective growth, including the formation of a QD at the apex of a pyramid. 
(c) illustrates the principles of SK growth, where a thin strained film spontaneously
converts into nanoscale islands, the so called self-assembly of QDs.

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was limited to the formation of micron-scale structures that
were described then as whiskers. To explain the way in which
a Si whisker formed from a melt containing Au and Si,
Wagner described the phase diagram (Fig. 6, top right) for the
mixture and the possible ways of controlling the transition
between the liquid alloy and solid Si. The transition between
these phases, the liquidus line, can be reached and the melt
kept supersaturated so that Si solidifies out of the melt. With
a seed present in the form of a nucleated Si crystal in contact
with the melt, the excess forms a continuous extension of
the whisker, effectively lifting the metallic particle as the
nanowire grows. In an analogous fashion, a compound III-V
semiconductor like GaAs can catalytically form at the
interface between an alloyed molten particle containing Au,
Ga, and As, as illustrated in the schematic in Fig. 6. This
process resembles the way in which III-V semiconductors like
GaAs and GaP are formed out of a melt during the classical
growth mode liquid phase epitaxy. 
It was primarily Hiruma 
et al. working at Hitachi during
the early to mid 1990s who developed the technique to allow
the growth of wires with dimensions on the nanoscale
11,12
.
Hiruma focused on the epitaxial nucleation of III-V nanowires
on a III-V substrate, using ultra-small Au particles formed
from a thin evaporated film of Au, which broke up during
annealing and reshaped into nanoscale droplets. These Au
nanoparticles then alloy with Ga taken from the GaAs
substrate on which the Au particles rest. Using either
molecular beam epitaxy or metal-organic vapor phase
epitaxy, Hiruma showed that by adding precursor molecules
for Ga and As he could control the transformation of
mixtures of Au, Ga, and As in the melt into a stoichiometric
single-crystalline GaAs layer at the interface between the 
Au-alloy melt and the single-crystal on top of which the
metal was resting (Fig. 6). Hiruma also demonstrated the
potential of III-V nanowires, with their good optical
properties and ability to be doped to form 
pn-junctions, by
producing nanowire light-emitting diodes (LEDs)
12
. He even
demonstrated the principles of formation of heterostructure
interfaces between GaAs and InAs
13
. His research was halted
before he was able to demonstrate such heterostructure
functionality in nanowires. This possibility has, as I will show,
led to significant progress in our recent work.
This method of growing Si and III-V semiconductor
nanowires has been adopted by Lieber 
et al. at Harvard
REVIEW
FEATURE
October 2003
2 6
Fig. 6 Formation of size-controlled nanowires using aerosol Au particles as seeds for vapor-liquid-solid (VLS) growth. A schematic on the left shows how the wafer is seeded by Au
nanoparticles, which alloy with the constituents (like Ga and As for GaAs) and grow the size-controlled nanowires. In the top-right corner is shown the phase diagram for the mixture of Au
and Si, which governs the transformation from the supersaturated molten alloy into the wire
10
. On the bottom right is a scanning electron micrograph of GaAs nanowires grown by this
method. (© Wiley 1970.)

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Fig. 7 Size-, shape-, and position-controlled growth of GaAs nanowires