High pressure anomalies in exfoliated MoSe₂: resonance Raman and x-ray diffraction studies

Abstract

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. Slope change in the linear behavior of reduced pressure (H) with respect to Eulerian strain ( f E ) is observed at about 13 GPa in hexagonal phase and at about 17 GPa for the triclinic phase. High pressure Raman measurements using two different pressure transmitting media (PTM) show three linear pressure regions, separated by pressure values around which anomalies in the structure are observed. A broad minimum in the FWHM values of E g 2 1 mode at about 10-12 GPa indicate to an electron-phonon coupling. 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.

Pressure induced lattice expansion and phonon softening in layered ReS2

“…Similar bulk modulus values are also observed in other transition metal dichalcogenides in their triclinic phase. 25,26 The fitting of the Eulerian strain vs normalized pressure in the study by Wang et al taking 8.9 GPa unit-cell volume as reference volume produced a bulk modulus of 90:1 + 2:2 GPa and a pressure derivative of 5:1 + 0:3, and the value of bulk modulus in their study is found to be very close to the present study. 18 We have shown the EOS fitting in Fig.…”

“…Pressure induced strain has a very important role in changing the structural and the electronic properties of materials. 25,26,[42][43][44] To see the response of strain in our sample, we have estimated Eulerian strain (f E ) and corresponding normalized pressure (H) using the following equations: 42,45…”

“…So, pressure induced strain has an important impact in the stabilization of the structure of ReS 2 similar to other exfoliated TMDs. 25,26 More theoretical and experimental investigation are required to understand the exact effect of pressure induced strain on these systems.…”

“…7,20 The effect of high pressures in the hexagonal phase of TMDs has been extensively studied. 3,[21][22][23][24][25][26] An electronic transition from a semiconducting to a metallic phase is reported during isostructural transition (2H c À 2H a ) at high pressures for powder samples. 3,[21][22][23][24] In contrast, in exfoliated samples, the same electronic transition is reported during strain induced structural transition from the hexagonal to the triclinic phase at high pressure.…”

“…3,[21][22][23][24] In contrast, in exfoliated samples, the same electronic transition is reported during strain induced structural transition from the hexagonal to the triclinic phase at high pressure. 25,26 Due to very weak van der Waals interlayer interaction in distorted 1T, ReS 2 application of high pressure can reduce the interlayer distances rapidly and, hence, can change material properties easily. 7,[17][18][19][20]27,28 A few high pressure XRD studies on powder samples show structural transition from the distorted 1T to 1T 0 phase in the pressure range of 7.7-11 GPa.…”