Pressure confinement effect in MoS₂monolayers

Abstract

With ever increasing interest in layered materials, molybdenum disulfide has been widely investigated due to its unique optoelectronic properties. Pressure is an effective technique to tune the lattice and electronic structure of materials such that high pressure studies can disclose new structural and optical phenomena. In this study, taking MoS2 as an example, we investigate the pressure confinement effect on monolayer MoS2 by in situ high pressure Raman and photoluminescence (PL) measurements. Our results reveal a structural deformation of monolayer MoS2 starting from 0.84 GPa, which is evidenced by the splitting of E(1)2g and A1g modes. A further compression leads to a transition from the 1H-MoS2 phase to a novel structure evidenced by the appearance of two new peaks located at 200 and 240 cm(-1). This is a distinct feature of monolayer MoS2 compared with bulk MoS2. The new structure is supposed to have a distorted unit with the S atoms slided within a single layer like that of metastable 1T'-MoS2. However, unlike the non-photoluminescent 1T'-MoS2 structure, our monolayer shows a remarkable PL peak and a pressure-induced blue shift up to 13.1 GPa. This pressure-dependent behavior might enable the development of novel devices with multiple phenomena involving the strong coupling of the mechanical, electrical and optical properties of layered nanomaterials.

E’’ Raman Mode in Thermal Strain-Fractured CVD-MoS2

“…Table 1 summarizes the positions of Raman modes for 1H-(normal), OF-, SF-and 1T/1T'-MoS 2 . In general, the emergence of a new Raman mode indicates a structural transformation [28] The E" mode can only be observed for SF-MoS 2 , indicating that SF-MoS 2 might have a new polymorph in compared with normal and OF-MoS 2 . Furthermore, the intensity of E" mode is decreased with aging shown in Figure S3b in SI, indicating the thermal stress is released.…”

“…Pressure-induced intralayer hybridization between Mo d orbitals and S p orbitals may lead to a crossover from direct to indirect transition and to PL quenching [29,30]. Defects and metallic nature also lead to PL quenching [24,28]. With the increasing concentration of S vacancies, the exciton absorption intensity decreases.…”

“…With the increasing of electronic doping level, the A'1 mode will have a red shift, accompanied by an increase in the line width of its Raman mode [37]. Upon compression, both E' and A'1 modes of single layered MoS2 show an apparent blue shift and broadening due to the increasing of the interaction between atoms [28]. Theoretically, with the increasing of sulfur vacancies concentration (Vs) in 1H-MoS2, the wavenumber difference between E' and A'1 modes increase obviously.…”

“…Table 1 summarizes the positions of Raman modes for 1H-(normal), OF-, SF-and 1T/1T'-MoS2. In general, the emergence of a new Raman mode indicates a structural transformation [28] The E'' mode can In comparison with normal CVD-MoS 2 [33], E' mode has a red shift of~7.8 cm −1 while A' 1 mode has a blue shift of~2.8 cm −1 . The wavenumber difference between E' and A' 1 modes of SF-MoS 2 is 30.9 cm −1 , which is much larger than previous reported of~20.3 cm −1 for single layered CVD-MoS 2 and~24.8 cm −1 for bulk MoS 2 [32,34].…”

“…Both peaks blue shift by 150 meV, indicating that the bandgap is enlarged by 150 meV. The d orbitals of Mo atoms, which contributes to the direct K-K interband transition between the conduction band minimum (CBM) and valence band maximum (VBM) at the K point in the Brillouin zone, may moves away from Fermi level and lead to an increase in the band gap with the compressive stress increases [28]. Thus, a compressive stress may exist in SF-MoS 2 , which may result in the base plane shrink following fracturing.…”