Non-resonant Raman spectroscopy of individual ZnO nanowires via Au nanorod surface plasmons

We present a non-resonant Raman spectroscopy study of individual ZnO nanowires mediated by Au nanorod surface plasmons. In this approach, selective excitation of the plasmonic oscillations with radiation energy below the semiconductor bandgap was used to probe surface optical modes of individual ZnO nanowires without simultaneous excitation of bulk phonons modes or band-edge photoluminescence. The development of a reproducible method for decoration of nanowires with colloidal Au nanorods allowed performing an extensive statistical analysis addressing the variability and reproducibility of the Raman features found in the hybrid nanostructures. An estimated field enhancement factor of 10³ was calculated, which greatly exceeded previously reported values, and resulted in the detection of a surface optical mode not observable in bare ZnO nanowires under comparable experimental conditions. The role played by Au nanorods in the observed enhancement was investigated both theoretically and experimentally. Specifically, evidence of the superior capabilities in enhancing Raman signals of nanorod longitudinal surface plasmons compared to nanorod transversal surface plasmons is provided. Finite-difference-time domain (FDTD) simulations were used to support the experimental findings and corroborate the use of plasmonic resonances for spectroscopic investigation of individual semiconductor nanostructures.