Electromagnetic and Chemical Enhancements of Surface-Enhanced Raman Scattering Spectra from Cu₂O Hexagonal Nanoplates

Surface-enhanced Raman scattering (SERS) has been extensively studied, but quantifiable results on electromagnetic enhancement (EM) are primarily focused on noble metals; as for metal oxides, limited studies exist, and a common impression regarding Raman scatterings is not able to elaborate mechanisms involving EM and multiplicative chemical enhancements. Leveraging on the shape-controlled synthesis of Cu₂O, the hexagonal nanoplates exhibit a remarkable SERS activity and reproducible measurements, enabling a systematic investigation of the SERS mechanism of Cu₂O. Here the shape-dependent SERS activity is elaborated by applying finite-difference time-domain simulation; the dominant contributions to SERS activity ascribed to chemical-bonding and charge-transfer (CT) enhancements are investigated using ab initio calculation. As the example, it is shown that the chemisorption of 4-aminothiophenol (PATP) on Cu₂O nanoplates leads to the shift of highest occupied molecular orbital and lowest unoccupied molecular orbital levels, promoting efficient CT processes. The vibronic coupling of the conduction and valence bands of Cu₂O with the molecular orbital of PATP further enhances the Raman scattering. This study on the combined EM and CT channels on Cu₂O nanoplates is promising to identify a new group of SERS-active substrates and advance the fundamental understanding of SERS mechanism, which also opens up a prospect of biocompatible and in situ diagnosis.