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

Abstract

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 T_f was increased to 1300 °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.