High pressure anomalies in exfoliated $MoSe_2$: Resonance Raman and X-ray diffraction studies

Detailed high pressure Resonance Raman ($RR$) Spectroscopy and X-ray diffraction ($XRD$) studies are carried out on 3-4 layered $MoSe_2$ obtained by liquid exfoliation. Analysis of ambient $XRD$ pattern and $RR$ spectra indicate the presence of a triclinic phase along with its parent hexagonal phase. Pressure evolution of prominent Raman modes and their full width at half maximum ($FWHM$) show slope changes at about 13 GPa and 33 GPa, respectively. Slope change in the linear behavior of reduced pressure ($H$) with respect to Eulerian strain ($f_E$) is observed at about 13 GPa. A minimum in the $FWHM$ values of $E_{2g}^1$ and $A_{2u}^2$ modes at the same pressure indicate to an electronic topological transition ($ETT$). Above 33 GPa the sample completely gets converted to the triclinic structure, which indicates the importance of strain in structural as well as electronic properties of two dimensional materials.