Pressure-induced formation of highly controlled branched silicon nanowires suitable for broadband absorption

We report the realization of branched silicon nanowires (SiNWs) by means of controlling the pressure during vapor–liquid–solid growth. Gold nanoparticles migrated from the main catalyst droplet act as seeds for the secondary growth of SiNWs across the trunk sidewall. To have gold nanoparticles migration as well as fewer kinks in the trunks, it is crucial to gradually reduce the pressure, which results in fully covered branched SiNWs. Transmission electron microscopy confirmed the crystallinity of the Si nano-branches by SAED analysis. From optical spectroscopy, it is observed that the light absorption is around 98 % over a wide range of wavelengths from 300 to 600 nm and drops to 90 % for higher wavelengths. It is shown that high absorption is due to the excitation of Mie resonances and waveguide modes in Si nanostructures. In order to confirm the presence of these optical modes, Raman spectroscopy has been used. Raman spectrum of branched SiNWs indicates higher enhancement factor compared to plain SiNWs.