Perceptible exciton-to-trion conversion and signature of defect mediated vibronic modes and spin relaxation in nanoscale WS₂ exposed to γ-rays

Abstract

In this work, we report manifested optical, optoelectronic and spin–spin relaxation features of a few layered tungsten disulphide (WS2) nanosheets subjected to energetic γ-photons (∼1.3 MeV) emitted from a Co60 source. Upon intense irradiation (dose = 96 kGy), a slight departure from the pure hexagonal phase was realized with the introduction of the trigonal phase at large. Moreover, in the Raman spectra, as a consequence of the radiation-induced effect, an apparent improvement of the E-to-A mode intensity and a reduction in phonon lifetimes have been realized, with the latter being dependent on the linewidths. The emergence of the new peak (D) maxima observable at ∼406 cm−1 in the Raman spectra and ∼680 nm in the photoluminescence (PL) spectra can be attributed to the introduction of defect centres owing to realization of sulphur vacancies (V S) in the irradiated nanoscale WS2. Additionally, neutral exciton to charged exciton (trion) conversion is anticipated in the overall PL characteristics. The PL decay dynamics, while following bi-exponential trends, have revealed ample improvement in both the fast parameter (0.39 ± 0.01 ns to 1.88 ± 0.03 ns) and the slow parameter (2.36 ± 0.03 ns to 12.1 ± 0.4 ns) after γ-impact. We attribute this to the finite band gap expansion and the incorporation of new localized states within the gap, respectively. A declining exciton annihilation rate is also witnessed. The isotropic nature of the electron paramagnetic resonance spectra as a consequence of γ-exposure would essentially characterize a uniform distribution of the paramagnetic species in the system, while predicting a three-fold improvement of relative spin density at 96 kGy. Exploring defect dynamics and spin dynamics in 2D nanoscale systems does not only strengthen fundamental insight but can also offer ample scope for designing suitable components in the areas of miniaturized optoelectronic and spintronic devices.

Identifying Defect-Induced Trion in Monolayer WS2 via Carrier Screening Engineering

“…In our investigation, DT A exhibited the red-shift peak energy behavior similar to the trion case. Moreover, increasing excitation intensity beyond the neutral-to-trion conversion exhibits diminishing neutral exciton and increasing trion intensities (at a high screening region) as observed previously. ,, A similar behavior was observed in DT A , as shown in Figure d,e. This observation enabled us to conclusively determine the DT A nature as the defect-induced trion state (distinct from the neutral state).…”

“…Consequently, the A 0 energy keeps increasing by the band-filling effect with a negligible carrier screening effect. Similar phenomena were also observed in trions (Figure a–c) with a more prominent carrier screening effect due to conversion from neutron excitons to trions at high excitation intensity (Figure d–f). , The slope of the T A peak was much steeper in region II than region I owing to high carrier screening, as observed before. , Furthermore, T A peak intensity became higher than A 0 in region II due to the neutral exciton-to-trion conversion (schematically shown in Figure d inset). , Moreover, the trion peak energies gradually decreased with a slope (in region I) of approximately 6.1 × 10 –3 in Re-WS 2 /SiO 2 , much higher than the WS 2 /SiO 2 sample (2.2 × 10 –3 shown in Figure a,b). This observation suggested an enhancement of the carrier screening effect due to Re-doping in the Re-WS 2 /SiO 2 sample.…”

“…Similar phenomena were also observed in trions (Figure 4a−c) with a more prominent carrier screening effect due to conversion from neutron excitons to trions at high excitation intensity (Figure 4d−f). 34,35 The slope of the T A peak was much steeper in region II than region I owing to high carrier screening, as observed before. 10,29 Furthermore, T A peak intensity became higher than A 0 in region II due to the neutral exciton-to-trion conversion (schematically shown in Figure 4d inset).…”

“…10,29 Furthermore, T A peak intensity became higher than A 0 in region II due to the neutral exciton-to-trion conversion (schematically shown in Figure 4d inset). 34,35 Moreover, the trion peak energies gradually decreased with a slope (in region I) of approximately 6.1 × 10 −3 in Re-WS 2 / SiO 2 , much higher than the WS 2 /SiO 2 sample (2.2 × 10 −3 shown in Figure 4a,b). This observation suggested an enhancement of the carrier screening effect due to Re-doping in the Re-WS 2 /SiO 2 sample.…”

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

“…For instance, in a study reported by Sarmah et al, a γ irradiation dose of 96 kGy on exfoliated nanoscale WS 2 transitioned its structural symmetry from the 2H to 1T phase and also induced n-type doping. 14 In one study reported by Elafandi et al, the radiation hardness of monolayer WS 2 and WSe 2 was investigated under a γ irradiation dose up to 560 kGy and they are presented as potential materials for space and nuclear science applications. 15 Felix et al reported changes in the magnetic phase from diamagnetic to ferromagnetic and enhancement in the p-type doping in monolayer WS 2 after 400 Gy irradiation.…”