Surface-enhanced Raman scattering spectra of adsorbates on Cu₂O nanospheres: Charge-transfer and electromagnetic enhancement

Surface-enhanced Raman scattering (SERS) spectra of 4-mercaptobenzoic acid (4-MBA) have been investigated on the surface of Cu₂O nanospheres. The SERS signals were believed to originate from the static chemical enhancement, resonant chemical enhancement and electromagnetic enhancement. The coupling between the adsorbates and the semiconductor, evidenced by the shift in absorption spectrum of modified Cu₂O and the enhancement of non-totally symmetric modes of the 4-MBA and 4-mercaptopyridine (4-MPY) molecules, were invoked to explain the experimental results. Furthermore, simulations were employed to investigate the nature of the enhancement mechanisms operative between the molecules and the semiconductor. Density functional theory (DFT) calculations suggested a charge transfer (CT) transition process between the molecules and the Cu₂O nanospheres. Three-dimensional finite-difference time domain (3D-FDTD) simulations were conducted to map out the electromagnetic field around the Cu₂O nanospheres. The experimental and simulation results have revealed the promise of the Cu₂O nanospheres as a good SERS substrate and the prospect of using the SERS substrate as a valuable tool for in situ investigation and assay of the adsorption behavior on semiconductor surfaces.