Growth behaviors of ultrathin ZnSe nanowires by Au-catalyzed molecular-beam epitaxy

Ultrathin ZnSe nanowires grown by Au-catalyzed molecular-beam epitaxy show an interesting growth behavior of diameter-dependence of growth rates. The smaller the nanowire diameter, the faster is its growth rate. This growth behavior is totally different from that of the nanowires with diameters greater than 60 nm and can not be interpreted by the classical theories of the vapor-liquid-solid mechanism. For the Au-catalyzed nanowire growth at low temperatures, we found that the surface and interface incorporation and diffusion of the source atoms at the nanowire tips controlled the growth of ultrathin ZnSe nanowires. The unique configuration of the metal catalytic growth of nanowires (also known as the vapor-liquid-solid (VLS) growth) makes it very promising for applications in nanotechnology. The most significant work on the mechanism of the unidirectional growth of semiconductor whiskers through the VLS mechanism was published by Wagner and Ellis. Classically, the unidirectional growth of Si whiskers, for example, can be simply interpreted based on the difference of the sticking coefficients of the impinging vapor source atoms on the liquid (the catalytic droplet) and on the solid surfaces (the whisker and substrate). An ideal liquid surface captures all impinging Si source atoms, while a solid surface of Si rejects almost all source atoms if the temperature is sufficiently high. Due to the presence of metal catalysts, the geometry and atomic structure of the interface between the metal catalyst and the whisker have been found to be very critical to the whisker growth, particularly the growth velocity, growth direction or crystal orientation. In the classical VLS model, it is believed that the metal catalyst is in molten state which absorbs the source materials to form a supersaturated liquid droplet. The precipitation of the source atoms occurs at the dropletwhisker interface, and the precipitation rate is mainly determined by the supersaturation of the droplet. Givargizov et al., determined the whisker growth rate as a function of the driving force of supersaturation (Δμ/kT) and first empirically described the growth rate by