Single-molecule enhanced Raman spectroscopy in nanocavities: mechanisms and recent advances

Surface-enhanced Raman spectroscopy (SERS) refers to the phenomenon where the Raman signals of molecules are significantly enhanced when they are adsorbed onto or located near the surface of substrates with specific nanostructures. By integrating with scanning probe microscopy techniques, SERS effectively overcomes the low sensitivity of conventional Raman spectroscopy and has been widely applied in surface science, biological sciences, and other fields. This review provides a comprehensive overview of recent theoretical advances in understanding the substrate-induced enhancement mechanisms of molecular Raman signals, as well as progress in the design and optimization of surface plasmonic configurations. The primary objective is to explore effective strategies for achieving high-resolution Raman signal enhancement. We elaborate in detail on the individual mechanisms of electromagnetic and chemical enhancement, as well as their synergistic interplay. Several key factors influencing surface plasmon-enhanced effects are systematically discussed, including charge transfer, external electric fields, and the role of different substrate materials in enhancing single-molecule Raman responses. In addition, we briefly highlight recent developments in first-principles studies using dispersion-corrected density functional theory (DFT), which incorporate long-range corrections to describe weak interactions between molecules and substrates within van der Waals radii. Finally, we offer perspectives on the future theoretical and experimental directions of surface plasmon and tip-enhanced techniques in the single-molecule regime. We also discuss the potential of integrating single-molecule junction sensors with machine learning and DFT for deeper spectroscopic insights, aiming to further advance the field of single-molecule spectroscopy.