Linear Tunability of the Band Gap and Two-Dimensional (2D) to Three-Dimensional (3D) Isostructural Transition in WSe₂ under High Pressure

Transition metal dichalcogenides (TMDs) have recently gained tremendous interest for use in electronic and optoelectronic applications. Unfortunately, the electronic structure or band gap of most TMDs shows noncontinuously tunable characteristics, which limits their application to energy-variable optoelectronics. Thus, layered materials with better tunability in their electronic structures and band gaps are desired. Herein, we experimentally demonstrated that layered WSe₂ possessed highly tunable transport properties under various pressures, with a linearly decreasing band gap that culminates in metallization. Pressure tuned the band gap of WSe₂ linearly, at a rate of 25 meV/GPa. The high tunability of WSe₂ was attributed to the larger electron orbitals of W²⁺ and Se^2– in WSe₂ compared to the Mo²⁺ and S^2– in MoS₂. WSe₂ underwent an isostructural phase transition from a 2D layered structure to a 3D structure at approximately 51.7 GPa, where a conversion from van der Waals (vdW) to covalent-like bonding was observed in the valence electron localization function (ELF). Our results present an important advance toward controlling the band structure of layered materials and suggest significant implications for energy-variable optoelectronic devices via pressure engineering