Structural anomalies in exfoliated WS2: High pressure investigations on monolayer and nanocrystalline tungsten disulfide

Abstract

A detailed high pressure X-ray diffraction and Raman spectroscopy study is carried out on monolayer WS2 and nanocrystalline WS2. The monolayer sample is obtained by liquid exfoliation. Photoluminescence and Raman measurements show it to consist of a monolayer. Careful analysis of ambient and high pressure data indicates the emergence of a triclinic phase at about 5.8 GPa in patches embedded in the parent hexagonal phase. This raises a question mark over the structural purity of the exfoliated monolayer materials beyond certain stress conditions. Raman mode values and their full width at half maximum of the monolayer sample show anomalous changes at about 27 GPa, the pressure where the sample completely gets converted to the triclinic structure indicating the importance of strain in structural as well as electronic properties of two dimensional materials.

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

“….04(7) Å 3 and Z=4 and it is consistent with the literature [16,34]. In figure 3(a) we have shown Le-Bail refinement of the triclinic phase along with the Rietveld fit of the hexagonal phase.…”

“…Similar features are also found in 1-5 layers of MoSe 2 using excitation wavelength 465 nm by Soubelet et al [31] but remain unresolved. Additional Raman modes observed in earlier Raman studies on exfoliated WS 2 and MoS 2 are assigned to different crystal structures embedded in parent hexagonal phase [16,21]. Therefore it is important to discuss XRD measurements before proceeding to further analysis of high pressure measurements.…”

“…Few layers of MoSe 2 sample are prepared by liquid exfoliation [16,22] of crystalline MoSe 2 (from Sigma Aldrich, powder, −325 mesh, 99.9% trace metals basis). After exfoliation samples of approximate size 10-15 μm are chosen for loading in the DAC.…”

“…Pressure can be an excellent tool to study the structural and electronic phase transitions in layered TMD's to understand their behaviour under strained conditions. There are several reports suggesting metallization of TMD's under pressure [7][8][9][10][11][12][13][14][15][16]. Some of the above reports suggest possible structural transition along with metallization with pressure.…”

“…Though the above studies on layered MoSe 2 show metallization of the sample without any structural transition under stress, a complete experimental characterization of the exfoliated MoSe 2 sample at high pressure is lacking. Exfoliated MoSe 2 layers can develop inherent surface strain similar to MoS 2 and WS 2 [16,21] and therefore a careful detailed high pressure study is needed to address the effect of strain in the structural and electronic behavior of exfoliated MoSe 2 .…”