Mechanism of Surface-Enhanced Raman Scattering on Multilayer Ti$_{\mathbf{3}}$\textbf{C}$_{\mathbf{2}}$\textbf{T}$_{\mathbf{X}}$\textbf{ Nanosheets}

MXenes have attracted great attention~as~flat substrates~for surface-enhanced Raman scattering (SERS) applications. However, the underlying SERS mechanism has not been a focus of any investigation. Herein, we report the first systematic experimental study on the SERS activity of Ti$_{3}$C$_{2}$T$_{X\, }$nanosheets with thickness ranging from 5 to 120 nm, using methylene blue (MB) as a probe molecule. We found that SERS intensity increases with the MXene nanosheet thickness. The thickness-dependence of the Raman enhancement can be accounted for by the adsorption and intercalation of MB molecules into the interlayer spacing of Ti$_{3}$C$_{2}$T$_{X}$. Furthermore, by combining experimental observations and numerical calculation, we confirm that the charge transfer mechanism is dominantly responsible for Raman enhancement on Ti$_{3}$C$_{2}$T$_{X}$. Additionally, we report an observation of resonance coupling of charge transfer and molecular transition as a contributing factor to the higher EF obtained with a 633 nm laser excitation. Taken together, these findings have significant implications for cost and performance optimization in designing MXene-based SERS substrates for next-generation chemical and biological sensing platforms.