This work reports the effect of γ radiation on
the surface
morphology and surface-charge redistribution in a monolayer WS2 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 WS2 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.