Atomic-Scale Doping of Rhenium on Ti₃C₂ MXene for Noble-Metal-Free Electromagnetic Enhancement and Ultrafast Charge Transfer Resonance

From semiconductors to two-dimensional materials, noble-metal-free surface-enhanced Raman scattering (SERS) materials have been explored frequently. However, induced electromagnetic (EM) enhancement on substrates lacking plasmonic resonance in the visible light region and unveiling the underlying chemical mechanism (CM) still remains challenging. Herein, by controlled doping of atomic-scale, Rhenium (Re), in the intrinsic metallic vacancies of Ti₃C₂ MXene, the electronic structure and visible light absorption of the SERS substrate were exceptionally modulated and tuned. The incorporated Re atoms stimulated Ti₃C₂ MXene to generate an enhanced electric field due to the photo-induced collective resonance of its abundant free valence electrons while enhancing the photo-induced charge transfer (PICT) between molecules and substrates. Transient spectroscopy elucidated an ultrafast PICT pathway between MXene and molecules within 0.5 ps. Moreover, Re atoms occupy intrinsic metallic vacancies in Ti₃C₂ MXene to form a stable lattice, which enables the SERS substrate to withstand harsh environments. Based on the above unique atomic-scale-induced enhancement (AIE) effect, Raman signal augmentation with an analytical enhancement factor (AEF) = 1.83 × 10⁷ could be achieved even when Re doping was only 1% by weight. This work will help to understand the SERS enhancement mechanism by MXenes and will set a new milestone for tuning the SERS performance using atomic-level engineering.