γ-Ray-Induced Surface-Charge Redistribution and Change of the Surface Morphology in Monolayer WS₂

This work reports the effect of γ radiation on the surface morphology and surface-charge redistribution in a monolayer WS 2 film by comparing the film before and after irradiation (1, 50, 100, 200, and 400 kGy dosage). The surface morphology was monitored through optical microscopy and atomic force microscopy. Raman and photoluminescence spectroscopy were used to study the effect on phonon modes and excitonic properties. The results indicated p-type doping and increased trion-to-exciton transitions. Because of the high energy and lower atomic mass of sulfur atoms, γ irradiation induces sulfur vacancies, which creates dangling bonds at vacant sites. The adsorption of oxygen at these reactive sites results in a charge-transfer mechanism, in which electrons get transferred from the WS 2 film to the adsorbed oxygen, which forms oxides and induces p-type doping. An increase in the work function of the film from 4.50 eV for a pristine film to 4.82 eV for an irradiated film (at 200 kGy) was calculated from Kelvin probe force microscopy, which indicates shifting of the Fermi level toward the valence band (VB) maxima. Further, VB spectra deduced from X-ray photoelectron spectroscopy showed a red shift of 0.17 eV after irradiation and confirms p-type doping.