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

Abstract

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.

Modification of WS2 thin film properties using high dose gamma irradiation

“…As a result, one of the sulfur in WS 2 having atomic mass number 32 is replaced by oxygen having atomic mass number 16, which results in the reduction of force constant and effective mass, in turn reducing vibrational frequency. The increasing radiation dose continues to removal of more sulfur atoms and simultaneously occupied by oxygen atoms, further enhancing the peak shift [20]. Hence, the sulfur vacancies can be seen to be dominant in our material and are the reason for red shift of Raman peaks.…”

“…The full survey scan in figure 6 shows the reduction of W and S elements, with a gradual increase in C and O in the gamma irradiated samples. The intensity of peaks corresponding to W and S at 32.13 eV and 34.39 eV, and at 161.62 eV and 162.80 eV, respectively, is observed to decrease by more than 50% in the 100 kGy irradiated sample compared to that of the pristine [20,34]. While the peaks at 285 eV, 289 eV and 530 eV attributed to C, C=O and O respectively are observed to get intense with increasing gamma irradiation [35].…”

“…It is also observed that the value of in-plane strain is higher as compared to the out-of-plane strain. As our WS 2 samples are few layered, the van der Waal interactions are considerable and causing the gradual stiffening of out-of-plane resulting in lesser effect of gamma irradiation than that on the in-plane vibration [20,31,32]. Therefore, the structural characterization results obtained from XRD and Raman spectroscopy suggest evolution of sulfur vacancy, oxygen occupancy and reduction in crystallinity of the samples with increasing gamma irradiation.…”

“…The shift of vibrational frequency (w) may be observed due to the change in force constant (k) and/or effective mass (m e ), caused by radiation induced sulfur vacancies [20,31,32]. The creation of sulfur vacancy (V S ) by gamma irradiation reduces k, resulting in the red shift of the characteristic peaks.…”

“…In one of the studies carried out on single layer WS 2 , Aggarwal et al have grown monolayer WS 2 using CVD and further irradiated the samples with gamma radiation. The irradiation triggers sulfur vacancy and induces p-type doping as seen from the shifts in Raman and x-ray photoelectron spectroscopy (XPS) peak positions [20]. In another work, Felix et al reported gamma irradiated WS 2 samples grown using scalable van der Waals epitaxial method.…”