Pressure induced metallization with absence of structural transition in layered MoSe2

Abstract: Layered transition-metal dichalcogenides have emerged as exciting material systems with atomically thin geometries and unique electronic properties. Pressure is a powerful tool for continuously tuning their crystal and electronic structures away from the pristine states. Here, we systematically investigated the pressurized behavior of MoSe2 up to ~ 60 GPa using multiple experimental techniques and ab -initio calculations. MoSe2 evolves from an anisotropic two-dimensional layered network to a three-dimensional structure without a structural transition, which is a complete contrast to MoS2. The role of the chalcogenide anions in stabilizing different layered patterns is underscored by our layer sliding calculations. MoSe2 possesses highly tunable transport properties under pressure, determined by the gradual narrowing of its band-gap followed by metallization. The continuous tuning of its electronic structure and band-gap in the range of visible light to infrared suggest possible energy-variable optoelectronics applications in pressurized transition-metal dichalcogenides.

Pressure-Induced Metallization in Layered MoSe₂: An Analysis of Electronic and Structural Characteristics

The study conducted by Zhao et al. provides an extensive examination of the pressure-induced electronic and structural properties of molybdenum diselenide (MoSe₂), specifically focusing on its transition from a semiconducting to a metallic state without undergoing a structural phase transition. This understanding is critical given the unique quasi-two-dimensional nature of transition-metal dichalcogenides (TMDs), making them promising candidates for optoelectronic applications. The investigation utilized multiple experimental techniques, including X-ray diffraction (XRD), Raman spectroscopy, infrared (IR) spectroscopy, and temperature-dependent electrical resistivity measurements, complemented by ab initio calculations, to explore the response of MoSe₂ up to approximately 60 GPa of pressure.

The fundamental finding of this work is the absence of a first-order structural transition in MoSe₂ under high pressure, a sharp contrast with its sulfur counterpart, MoS₂, which experiences a well-documented structural shift near metallization. Despite the isotopic and chemical similarities between MoSe₂ and MoS₂, the disparity in their high-pressure behaviors underscores the influence of chalcogenide anions and the weaker van der Waals interactions due to the more delocalized 4p orbitals of selenium compared to sulfur's 3p orbitals.

Structural and Electronic Properties Under Pressure

XRD and Raman spectroscopy results verified that MoSe₂ maintains its 2H_c crystal structure throughout the pressure range studied, without exhibiting the anticipated 2H_c to 2H_a transition observed in MoS₂. The ab initio calculations further confirmed the stability of MoSe₂'s crystal structure under these conditions. This stability is explained through layer sliding simulations which indicate substantial energy barriers that prevent the structural transition in MoSe₂, unlike in MoS₂.

IR and electrical resistivity data corroborated the semiconducting to metallic transition, revealing a pronounced tuning of the band-gap as pressure increases. The significant narrowing of the band-gap commenced around 20.2 GPa, culminating in metallization beyond 40.7 GPa. Unlike many conventional semiconductors subjected to pressure, the absence of a discontinuous structural change during this transition in MoSe₂ suggests potential advantages for applications requiring stable, tunable electronic characteristics—particularly energy-variable opto-electronics and photovoltaics.

Theoretical Implications and Future Directions

Theoretical ab initio calculations provided an insightful understanding of the electronic structure evolution in MoSe₂. Calculations aligned with experimental findings show a systematic decrease in the indirect band-gap with pressure, corresponding to increased overlap of the Mo dxz and dyz orbitals with Se p orbitals. This interaction results in band dispersions critical for defining MoSe₂'s transport properties upon metallization.

MoSe₂'s behavior under pressure positions it as a candidate for probing advanced physical phenomena such as charge density waves or superconductivity under extreme conditions. Furthermore, the retention of the indirect electronic characteristic under high pressure presents intriguing possibilities for unconventional electronic phases and applications in advanced electronic devices.

The study's insights into TMD behavior under extreme pressure conditions offer important implications for the theoretical framework guiding the design and application of novel materials in next-generation electronic technologies. Future research could explore the potential for MoSe₂ in excitonic insulators and valleytronics, especially under symmetries' alteration by non-uniaxial pressures, enhancing its multifunctionality in electronic applications.

In conclusion, the work by Zhao et al. significantly adds to the comprehension of TMDs under pressure, elucidating pathways for their utilization and manipulation in technologically relevant domains. The combination of robust experimental work and theoretical modeling underscores the transformative capabilities of MoSe₂ as a tunable, stable material in advanced material science research and application.