Page 145 - Science at the nanoscale

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June 9, 2009
6.3. Quantum Dots, Quantum Wires
nanostructures. In the furnace, parameters such as pressure, gas
flow rate and temperature gradient are varied to optimise the
growth. In addition, the substrates are sometimes decorated with
catalytic nanoparticles to promote the growth of 1D nanowires.
A wide variety of nanowire products have been achieved by
various research groups. Depending on the type of nanowires
fabricated, different mechanisms for the growth of the nanos-
tructure have been proposed. These include the vapour-solid
(VS) growth and vapour-liquid-solid (VLS) growth mechanisms.
Direct vapour-solid (VS) growth is one of the simplest routes to
fabricate nanowires. In this technique, the vapour is generated by
evaporating source materials or decomposing precursor compo-
nents, which then condense on target substrates at lower temp-
eratures.
VLS growth has achieved great success in the fabrication of
nanostructures with high crystal quality and in large quantities.
This method makes use of metallic catalysts, which form eutectic
liquids with the source materials at the appropriate temperatures.
The precursor vapour dissolves into liquid drops that eventually
become saturated. Solid nuclei precipitate after saturation and
subsequently continue to grow into nanostructures. Thus VLS
growth is often characterised by the presence of catalyst particles
located at the tips of nanostructures. VLS growth has been used to
synthesise a rich variety of inorganic nanostructures including ele-
mental semiconductors, III-V semiconductors, and II-VI semicon-
ductors. Nanostructures of metal oxides such as ZnO, MgO, TiO 2 ,
SnO 2 etc. have also been synthesised by the VLS method. One of
the main advantages of VLS growth is that the diameter and posi-
tion of 1D nanostructures can be controlled by the size and posi-
tion of the catalysts. Thus, the 1D nanostructure products can be 135 ch06
highly uniform in diameter and readily patterned. In addition, the
nanostructures fabricated can be well aligned. Figure 6.17 shows
a SEM image of aligned array of ZnO nanowires synthesised on Si
substrates.
Nanoscale metal oxide materials with fascinating morphologies
can also be simply synthesised by heating pure metallic foils or
plates on a hotplate in ambient or appropriate atmospheres. Such
7
a method was described as early as the 1950s by Pfefferkron and
7 G. Pfefferkorn, Umschau Wiss u. Tech. 21, 654 (1954).