Tuning of Optical Behavior in Monolayer and Bilayer Molybdenum Disulfide Using Hydrostatic Pressure

Abstract

Researchers have shown great interest in two-dimensional crystals recently, because of their thickness-dependent electronic and optical properties. We have investigated the Raman and photoluminescence spectra of free-standing monolayer and bilayer MoS2, as a function of pressure. As the enforcement of layer interaction, an electronic and a crystal phase transition were revealed at ∼6 GPa and ∼16 GPa, respectively, in bilayer MoS2, while no phase transition in the monolayer is observed. The electronic phase transition at ∼6 GPa is supposed to be a direct interband changing to an indirect Λ-K interband transition, and the new structure shown at ∼16 GPa is not metallized and supposed to be a transformation from stacking faults due to layer sliding like 2Hc to 2Ha. The different pressure-induced features of monolayer MoS2, compared with bilayer MoS2, can help to get a better understanding about the importance of interlayer interaction on modifying the optical properties of MoS2 and other fundamental understanding of 2D materials.

Tuning of Interlayer Interaction in MoS2–WS2 van der Waals Heterostructures Using Hydrostatic Pressure

“…This observation may suggest that out-of-plane compression is more favorable than in-plane compression under pressure and that interlayer coupling is more pressure-sensitive than intralayer coupling. These results are consistent with previous research findings on MoS 2 and WS 2 monolayers. ,,, Moreover, Raman peaks of MoS 2 and WS 2 show a similar blue-shift rate for A 1 ′ and E′ modes, respectively, indicating that MoS 2 and WS 2 have similar pressure dependence behaviors, in agreement with the PL evolution observed earlier.…”

“…As shown in Figure a,b, a blue shift is observed based on PL peak positions, as well as a decrease in the intensity with increasing pressure for WS 2 and MoS 2 monolayers. It is observed that the band gap increases with pressure in a manner similar to the results of previous studies. ,,, The blue-shift rates are about 2.2 and about 2.0 meV GPa –1 for the WS 2 and MoS 2 monolayers, respectively. In WS 2 and MoS 2 monolayers, the valence and conduction bands mainly come from the W/Mo-d x 2 – y 2 , d z 2 and S-p x ,p y orbitals, respectively.…”

“…Monolayer samples of 1H-MoS 2 and 1H-WS 2 were prepared by mechanical exfoliation technique from MoS 2 single crystals (99.995%; purchased from Shanghai Onway Technology Co. Ltd) and WS 2 single crystals (99.995%; purchased from HQ Graphene). The number of layers of the MoS 2 and WS 2 was determined by micro-PL and micro-Raman measurement analyses. , A WS 2 monolayer was carefully transferred from poly­(dimethylsiloxane) (PDMS) to the culet of DAC. Subsequently, the MoS 2 monolayer was transferred to the top of WS 2 films by the same method.…”

“…Specifically, the twist angle and lattice shift between HS layers provide an effective method to alter interlayer interactions, which can be done by varying the interlayer distance . For this reason, this method can be used to modulate and affect electronic properties of materials, leading to the induction of new quantum phenomena, including superconductivity, magnetism, and insulating states. ,− Practically, the application of hydrostatic pressure by a diamond anvil cell (DAC) has been proven to be another tool to control interlayer interactions and thus achieve electronic transitions in 2D TMDs and HSs. ,− Indeed, hydrostatic pressure can considerably reduce distances between atoms and modify the nature of their interactions. Furthermore, interlayer interactions are much weaker than intralayer covalent bonds, which makes it possible to modulate them much more efficiently than intralayer interactions by compression.…”

Pressure Tuning Resonance Raman Scattering in Monolayer, Trilayer, and Many-Layer Molybdenum Disulfide

“…35 For these particular bands, the Raman frequencies red-shift with increasing excitation energy. Furthermore, it has been observed that pressure induces a blue shift of the direct energy band gap in 1L-MoS 2 25,26,28,37,38 (see the discussion below). Since the main contribution to the scattering process of the LA(K) + TA(K) and 2LA bands is mediated by electronic transitions between states around the highsymmetry K point of the BZ, the pressure-induced increase in the band gap (keeping the laser excitation energy constant) has an analogous effect to the dispersive behavior of these bands, decreasing the excitation energy at a constant pressure.…”