Nanotechnology-Enabled Sensors

148 Chapter 4: Nano Fabrication and Patterning Techniques
growth. The theory of VLS was first described by Wagner et al in the
60s. 35 They claimed that formation of one dimensional structures can not
be simply defined using the evaporation-condensation equations which
were presented earlier. In this method, a liquid-like sphere is always present
at either the tip or at the starting point (stem) of the one-dimensional
structures.
Fig. 4.8 Vapour-liquid-solid (VLS) growth: (left) catalyst droplet and nucleation
(right) growth of one-dimensional structures.
In this deposition process, the catalyst forms a liquid droplet when
heated, with the distribution coefficient of the catalyst being less than one.
The vapour pressure of the growth species should be small and the catalyst
should be an inert material so that it does not to react with the crystals. The
wettability of the surface by the catalyst is also very important. If the wetting
contact angle is large then the catalyst droplet will be small (Fig. 4.9).
However, if the surface is highly wettable by the catalyst then, due to the
spread of the liquid, the base diameter will become large. This will result
in larger base size one-dimensional structures.
Fig. 4.9 The wettability of the substrate: (a) a large contact angle on a less wettable
surface (b) the spread of the same droplet volume on a more wettable surface.