Abstract
Abstract Phthalocyanines are frequently used as probing molecules in the field of single-molecule surface-enhanced Raman spectroscopy (SERS) and tip-enhanced Raman spectroscopy (TERS). In this work, we systematically compare the SERS and TERS spectra from a thin cobalt phthalocyanine (CoPc) film that is deposited on a Au film. 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 (636 and 532 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 636 nm excitation through a combination of resonant excitation, high EM enhancement, and chemical enhancement via charge transfer from the metal to the molecule. At 532 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. The insight achieved in this work will help to understand the optical contrast in sub- or single-molecule TERS imaging and apply SERS or TERS in the field of photocatalysis.
Highlights
Surface-enhanced Raman spectroscopy (SERS) has witnessed tremendous development since its discovery in the 1970s [1, 2]
We use a parabolic mirror (PM)-assisted confocal optical microscope, which can be applied for performing tip-enhanced Raman spectroscopy by combining with shear-force scanning probe microscopy [24,25,26]
PM is free from chromatic aberration, and it can be adapted to work with different laser wavelengths
Summary
Surface-enhanced Raman spectroscopy (SERS) has witnessed tremendous development since its discovery in the 1970s [1, 2]. 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. This mechanism is proposed as the main process to explain the Raman enhancement observed in tip-enhanced Raman spectroscopy (TERS). In TERS, the substrate on which the probing molecules are deposited does not necessarily provide a high EM field; This work is licensed under the Creative Commons
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