Hierarchical ZnO/Si nanowire arrays as an effective substrate for surface-enhanced Raman scattering application

Hierarchical arrays of ZnO/Si nanowires were prepared through very convenient two-step solution method on silicon substrates. The tree-like nanowire structures showed high crystal quality, where well aligned Si nanowires were grown vertical to the substrate surface, whereas uniform assembly of branched ZnO nanowires was grown normal to the Si nanowires (backbones). Raman analysis indicated that among all the specimens having identical chemical compositions and dimensions but different forms (thin films or nanowires), Ag-decorated hierarchical arrays of ZnO/Si nanowires possessed the highest intensity peak in surface enhanced Raman scattering spectra. Such arrays demonstrated exceptional ability of detecting Rhodamine 6 G, limiting to a value as low as 1×10−8 mol/L and an enhancement factor up to 1.5 × 104. Both, experimental designing and band structure analysis indicated that electromagnetic effect of the localized surface plasmon resonance of Ag nanoparticles caused eight times enhancement, whereas chemical effect of hierarchical arrays of ZnO/Si nanowires played a more critical role in the high Raman sensitivity. Our results would be helpful to better understand the enhancement mechanisms and meanwhile, could be extended for further potential applications of hierarchical semiconducting nanowires.