γ-Ray irradiation-induced unprecedent optical, frictional and electrostatic performances on CVD-prepared monolayer WSe(2)

Distinguishing from traditional working environments, we propose the harsh gamma radiation method to study the stability and reliability of the emerging two-dimensional (2D) quantum material tungsten diselenide (WSe(2)). Transmission electron microscopy studies showed clear chemical modulation with an atomically sharp interface, indicating that the selenium vacancy content increased with the irradiation dose. The WSe(2) crystal could be transitioned into an n-doped semiconductor due to the anion vacancies created by radiation. Changes in the lattice vibrational modes induced by the passivation of oxygen was captured via Raman spectroscopy. The frequency shifts in both in-plane and out-of-plane modes are dependent linearly on the selenium vacancy content. The friction of WSe(2) increases with the irradiation dose. Electrostatic properties were investigated by measuring the surface potential via Kelvin probe force microscopy. Due to different environments, molecular collisions lead to an increase in the concentration of vacancy defects, which made our results different from those previously reported. The first principles calculation suggests that an increase in the selenium vacancy population is generally accompanied by a transition from a direct gap material to an indirect one. This opens up a new venue to engineer the optical, frictional and electronic properties of transition metal dichalcogenides using irradiation.