Thickness-dependent phase transition and optical behavior of MoS₂ films under high pressure

We report the Raman and photoluminescence spectroscopic analysis of layered MoS₂ under hydrostatic pressure up to ~ 30 GPa. Unlike previous studies, throughout this work, a special treatment is applied to submerge monolayer, bilayer, multilayer (~ 200 layers), and bulk MoS₂ samples directly into silicone oil without a supporting substrate in a diamond anvil cell, thereby eliminating possible interference from substrate–film contact. A thickness-dependent trend is observed for the 2H_c-to-2H_a phase transition: The transition pressure increases from 19.0 to 25.6 GPa as the system thickness is reduced from bulk to multilayer MoS₂; a further decrease in thickness to the bilayer structure increases the transition pressure to ~ 36 GPa, as predicted theoretically. This exceeds our measured pressure range, indicating the weakening of interlayer repulsive interactions as the MoS₂ film thickness is reduced. Our experiment also reveals a monotonic trend of Raman peak shifting vs. film thickness under applied pressure, suggesting that the Raman vibration modes are more responsive to external pressure in thinner films. The photoluminescence emission peak of the monolayer MoS₂ exhibits a blue shift under applied pressure at the rate of 23.8 meV·GPa^-1. These results show that the structural and optical properties of MoS₂ can be effectively modified by applying hydrostatic pressure.