Minimizing Extrinsic Effects in High-Pressure Raman of Monolayer WSe₂ through Substrate and Pressure-Transmitting...

We present a comprehensive study of the Raman spectra of monolayer WSe₂ under high pressure up to 40 GPa, focusing on the influence of both the substrate and the pressure-transmitting medium (PTM). By using diamond as the substrate, which minimizes strain transfer compared to conventional Si/SiO₂, we isolate the intrinsic vibrational response of monolayer WSe₂ from substrate-induced effects. Our results show that the enhancement of LA-related second-order modes occurs only at pressures >15 GPa, much higher than in Si/SiO₂-supported samples, reflecting the weaker strain coupling to diamond. However, once the PTM loses hydrostaticity, non-uniform strain rapidly intensifies these modes. The role of strain-induced disorder is further evidenced by the appearance of wrinkles after decompression, which lead to local symmetry breaking and activate the normally forbidden B_2g mode. Density functional theory (DFT) calculations indicate that the K-Λ valley crossover in the conduction band occurs at higher pressures than previously reported, but still much lower than those required to tune the second-order modes, thus suggesting that their intensification is predominantly driven by strain-induced disorder rather than electronic transitions. These findings provide practical guidelines for minimizing substrate- and PTM-related artifacts, thereby enabling more accurate and reproducible high-pressure Raman characterization of two-dimensional materials.