Single reactor deposition of silicon/tungsten oxide core-shell heterostructure nanowires with controllable structure and optical properties

We report the controllable growth of silicon/tungsten oxide (Si/WO3) core-shell heterostructure nanowires via a two-step route using a home-built plasma-assisted hot-wire chemical vapour deposition reactor. Uniform coating of WO3 shell indicates a clear preference for growth on the single crystalline Si nanowires. Structure and crystallinity of the WO3 shell are strongly dependent on the filament temperature (T-f). X-ray diffraction patterns and micro-Raman spectra suggested that there was a structure evolution from amorphous into crystallite WO3 monoclinic structure when Tf was increased to 1300 degrees C and above. The WO3 shell exhibits stoichiometric tungsten trioxide structure as identified by micro-Raman and X-ray photoemission spectra analyses, which showed only W6+-O vibration modes for the former analysis and W6+ energy band for the latter. Microstructure, crystal lattice, interface and growth orientation of the core-shell nanowires were recognized using a high resolution transmission electron microscopy. Our results showed that the core-shell nanowires had preserved the optical transmittance of Si core at a longer wavelength, while showing an additional transmission band edge at a shorter wavelength due to tungsten oxide coating. Their optical absorption increased to 80% and above in visible region, owing to the one-dimensional Si NWs backbone. Optical band gap of the core-shell nanowires showed a variation from 2.4 to 1.8 eV with T-f. This superior visible light absorption core-shell nanowires architecture subsequently enhanced the photocurrent density of the crystalline WO3 nanostructures.