The contributions from electromagnetic EM , resonance, and charge-transfer enhancements are discussed. Radially and azimuthally polarized vector beams are used to investigate the influences of molecular orientation and the localized surface plasmon resonance SPR.
Furthermore, two different excitation wavelengths and nm are used to study the resonant excitation effect as well as the involvement of the charge-transfer processes between CoPc and the Au substrate.
It is shown that the Raman peaks of CoPc are mostly enhanced by nm excitation through a combination of resonant excitation, high EM enhancement, and chemical enhancement via charge transfer from the metal to the molecule. At nm excitation, however, the SERS and TERS spectra are dominated by photoluminescence, which originates from a photo-induced charge-transfer process from the optically excited molecule to the metal.
The contributions of the different enhancement mechanisms explain the optical contrasts seen in the TERS images of Au nanodisks covered by the CoPc film. Surface-enhanced Raman spectroscopy SERS has witnessed tremendous development since its discovery in the s [ 1 ], [ 2 ]. It has been widely applied to the fields of sensing [ 3 ], [ 4 ], spectroelectrochemistry [ 5 ], [ 6 ], single-molecule spectroscopy [ 7 ], and many others.
The physical mechanisms of SERS have been intensively discussed and extended, especially when new types of SERS substrates have been proposed [ 8 ], [ 9 ]. Electromagnetic EM enhancement is believed to contribute dominantly to SERS on metallic substrates, by providing greatly enhanced EM fields both in the excitation and scattering steps.
