Synthesis of copper silicide nanocrystallites embedded in siliconnanowires for enhanced transport properties

Abstract

Here we report the in situ doping of Si nanowires with Cu, which results in nanowires containing nanocrystalline inclusions of Cu₃Si and significantly enhanced electrical conductivity. These nanowires are of interest for use in secondary Li batteries as well as nanowire arrays that can be directly sensitized for photovoltaic applications. This synthesis route is based on controlling the vapour-phase flux of precursor materials into the catalyst tip whereby the flux of the Cu is much less than that of Si. A compositional study utilizing SEM–EDS, XRD, and TEM–EDS techniques of vapour–liquid–solid (VLS) grown Si nanowires in the presence of Cu vapour confirms that the bulk nanowire matrix is Si doped with crystalline Cu₃Si and low concentrations of Cu. The electronic transport measurements conducted on single nanowires indicate that the electronic resistivity of the doped nanowires is several orders of magnitude lower than undoped Si, thereby making them more conductive. Based on the data collected from the nanowire growth in conjunction with the in situ VLS doping mechanism, the doping density can be controlled by varying the gas-phase concentration of the dopant or the thermodynamic conditions of the nanowire growth. Both approaches will result in a change in the relative fluxes from the gas phase into the VLS catalyst as well as the kinetics for Cu₃Si formation. This is advantageous because dopant density can be used to tune both the electronic and the optical properties of the nanowires.