VLS process explained

The Vapour Liquid Solid VLS process is used for the growth of Si nanowire by using Au clusters as the solvent at high temperature. Based on Si_Au binary phase diagram, Si (from the decomposition of SiH4 , for example) and Au will form a liquid alloy when the temperature is higher than the eutectic point. The liquid surface has a
large accommodation coefficient and is therefore a preferred deposition site for incoming Si vapor. After the liquid alloy becomes supersaturated with Si, Si nanowire growth occurs by precipitation at the solid } liquid interface.

Recently, real-time observation of Ge nanowire growth was conducted in a high-temperature in situ transmission electron microscope (TEM). The experiment result clearly shows three growth stages: formation of Au_Ge alloy, nucleation of Ge nanocrystal and elongation of Ge nanowire. This experiment unambiguously demonstrates the validity of the VLS mechanism for nanowire growth. The establishment of VLS mechanism at the nanometer scale is very important for the rational control of inorganic nanowires, since it provides the necessary underpinning
for the prediction of metal solvents and preparation conditions.

Based on our mechanism study of the nanowire growth, it is conceivable that one can achieve controlled growth of nanowires at different levels. First of all, one can, in principle, synthesize nanowires of different compositions by choosing suitable solvents and growth temperatures. A good solvent should be able to form a liquid alloy with the desired nanowire material, ideally they should be able to form a eutectic alloy. Meantime, the growth temperature should be set between the eutectic point and the melting point of the nanowire material.



Both physical methods (laser ablation, arc discharge, thermal evaporation) and chemical methods (chemical vapour transport and chemical vapour deposition) can be used to generate the vapor species required during the nanowire growth.

Wu, Y. et al., 2002 ‘Inorganic Semiconductor Nanowires: Rational Growth, Assembly, and Novel Properties’, Chemistry – A European Journal, vol. 8, issue 6, pp.1260-1268.