Abstract
The electronic band structure of MoS2, MoSe2, WS2, and WSe2, crystals has been studied at various hydrostatic pressures experimentally by photoreflectance (PR) spectroscopy and theoretically within the density functional theory (DFT). In the PR spectra direct optical transitions (A and B) have been clearly observed and pressure coefficients have been determined for these transitions to be: αA = 2.0 ± 0.1 and αB = 3.6 ± 0.1 meV/kbar for MoS2, αA = 2.3 ± 0.1 and αB = 4.0 ± 0.1 meV/kbar for MoSe2, αA = 2.6 ± 0.1 and αB = 4.1 ± 0.1 meV/kbar for WS2, αA = 3.4 ± 0.1 and αB = 5.0 ± 0.5 meV/kbar for WSe2. It has been found that these coefficients are in an excellent agreement with theoretical predictions. In addition, a comparative study of different computational DFT approaches has been performed and analyzed. For indirect gap the pressure coefficient have been determined theoretically to be −7.9, −5.51, −6.11, and −3.79, meV/kbar for MoS2, MoSe2, WS2, and WSe2, respectively. The negative values of this coefficients imply a narrowing of the fundamental band gap with the increase in hydrostatic pressure and a semiconductor to metal transition for MoS2, MoSe2, WS2, and WSe2, crystals at around 140, 180, 190, and 240 kbar, respectively.
Highlights
The electronic band structure of MoS2, MoSe2, WS2, and WSe2, crystals has been studied at various hydrostatic pressures experimentally by photoreflectance (PR) spectroscopy and theoretically within the density functional theory (DFT)
Bulk transition metal dichalcogenides (TMDs) have been investigated in the past[6,7], but since the very first paper on MoS2 layers[1], it is well established that the electronic band structure of TMDs strongly varies with the number of layers and exhibits indirect-to-direct band gap transition with the size reduction to a single layer
Numerous theoretical studies have discussed the influence of hydrostatic pressure and strain on band structure of few-layers TMDs and notable physical effects such as strain-induced direct-to-indirect band gap transition and semiconductor to metal transition have been predicted[10,11,12,13,14,15,16,17,18,19,20,21,22]
Summary
The electronic band structure of MoS2, MoSe2, WS2, and WSe2, crystals has been studied at various hydrostatic pressures experimentally by photoreflectance (PR) spectroscopy and theoretically within the density functional theory (DFT). Numerous theoretical studies have discussed the influence of hydrostatic pressure and strain on band structure of few-layers TMDs and notable physical effects such as strain-induced direct-to-indirect band gap transition and semiconductor to metal transition have been predicted[10,11,12,13,14,15,16,17,18,19,20,21,22]. In this paper to investigate the electric band structure of MoS2, MoSe2, WS2, and WSe2 under hydrostatic pressure we applied photoreflectance (PR) spectroscopy This technique due to its absorption-like character is an excellent tool to investigate the direct optical transitions between both ground and excited states[28,29,30]. The hydrostatic pressure for which the semiconductor to metal transition is expected in MoS2, MoSe2, WS2, and WSe2 is estimated
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