Catalyst solubility and self-doping in ZnS nanostructures

Abstract

We demonstrate that the variation in the solubility of the catalyst during nanostructure growth via vapor-liquid-solid technique is dependent on the catalyst dimensions. This property can be exploited in a simple way for controlled doping of the nanostructures. Specifically, we investigate the role of Au droplet size on its solubility in ZnS nanostructures. The size-dependent variations in the surface tension of the catalyst droplet can change its solubility in the nanostructures. The solubility of the catalyst (Au) has been observed to decrease drastically with the increase in its droplet radius. X ray diffraction and scanning electron microscopy were used to determine the phase, shape, size, and density of nanostructures. XPS results revealed that concentration of Au in the ZnS nanostructures was high for a small Au droplet and dropped considerably with increase in the droplet size. The experimental findings were in good agreement with the theoretical model, based on the thermodynamical equations for the doping in nanowires. The critical size of the catalyst for the surface and core doping is determined for the zinc blende and Wurtzite phases of ZnS. In the end, we show that, by using a suitable catalyst with high miscibility, nanostructures can be doped during growth, thus eliminating a step in the device fabrication.