Pressure-induced K–Λ crossing in monolayer WSe2

Abstract

The energy band structures and related room temperature exciton transitions of monolayer and bilayer tungsten diselenide (WSe₂) are investigated using photoluminescence (PL) spectra under hydrostatic pressure up to 5.42 GPa. For monolayer WSe₂, it is found that the conduction band Λ valley is 70 ± 30 meV higher than the K valley at zero pressure, and the K–Λ valley crossover happens at a pressure of approximately 2.25 GPa. The PL peak of exciton related to the direct K–K interband transition in monolayer and bilayer WSe₂ shows a pressure-induced blue-shift at the rates of 31.5 ± 0.6 and 27 ± 1 meV GPa⁻¹, respectively. The indirect Λ–K interband transition for monolayer and bilayer WSe₂ exhibits a distinctly different pressure response. The pressure coefficient is as small as −3 ± 6 meV GPa⁻¹ for monolayer, but a much larger value of −22 ± 1 meV GPa⁻¹ for bilayer WSe₂, indicating that the interlayer coupling has a strong effect on the electronic states at the Λ valley.