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.

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The transition metal dichalcogenide (TMD) ReS is a promising material for optoelectronic devices because of its remarkable quantum yield. Pressure can effectively tune the optoelectronic properties of TMDs through control of the atomic displacement. Here, we systematically investigated the lattice and electronic structural evolutions of compressed multilayer ReS. Both Raman spectra and first-principles calculations suggest the occurrence of an intralayer phase transition followed by an interlayer transition. A transition from one indirect to another indirect bandgap at 2.7 GPa was revealed by both high-pressure photoluminescence (PL) measurements and first-principles calculations, this behavior was elucidated by considering the fundamental relationship between lattice variation and electronic evolution. Moreover, by comparing the high-pressure behavior of MoS and ReS, we demonstrated interlayer coupling plays a critical role in determining the lattice and electronic structures in compressed TMDs. Our findings suggest the potential application of ReS in fabricating various stacking devices with tailored properties.

Interlayer coupling plays critical roles in determining the lattice vibrations of two-dimensional transition-metal dichalcogenides. When compressed, the effects of interlayer coupling remain ambiguous. Pressure-dependent vibrational properties of trilayer and quadlayer MoS2 up to 12.7 GPa were investigated through in situ high pressure Raman spectroscopy measurement. The Raman spectrum reveals different responses to pressure in trilayer and quadlayer MoS2 due to their thickness-dependent interlayer coupling interaction. Combining this data with the first-principles calculations, we demonstrate that the quadlayer MoS2 transforms into an AB′ stacking configuration above 8.6 GPa, where all Mo atoms sit exactly over the Mo atoms in their neighboring layer and all S atoms sit over the centers of the hexagons, while the trilayer MoS2 possesses a distorted and wrinkled 2H structure within our studied pressure range. Our study demonstrates that high pressure Raman spectroscopy measurement is an effective method to explore the structural transformation of ultrathin MoS2 at extreme conditions as well as to explore their complicated interlayer coupling interaction. It should also be of great benefit for the development of nanotechnology, especially for the design and fabrication of different stacking nanometer devices with tailored properties for specific applications.

Cadmium ions have variable degrees of toxicity on the environment and organisms. An electrochemical sensor based on self-doped polyaniline (SPAN) modified Metal-organic Framework

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