Chapter 6: Photoluminescence Modulation of Monolayer WS 2 by ZnO encapsulation and
6.4. Conclusion
We have grown high-quality 1L-WS2 films on sapphire, SiO2, and quartz substrates by a CVD method. The as-grown 1L-WS2 film shows a sharp PL emission at room temperature dominated by the neutral excitons. However, the low-temperature PL spectra are broader showing additional peaks originating from biexcitons. Interestingly, in the case of the transferred 1L-WS2
film, there is a significant contribution from biexcitonic emission, even at room temperature. We further construct a quantum well structure using ZnO, which is a higher band gap semiconductor.
Thus, we devise an effective way to manipulate the exciton and other quasiparticle densities in the system. The origin of the excitonic states and the overall spectral features of both systems are explored by careful examination of the power-dependent and temperature-dependent PL spectra.
At low powers, the influx of charge carriers from ZnO to WS2 aids in pronounced PL enhancement.
Biexcitons are less likely to be formed at low excitation powers, through the collision of excitons, and also trion formation rate is comparatively low. However, at higher excitation powers, the possibility of trion formation dominates in ZnO/WS2/ZnO, which hinders the enhancement of PL intensity. Additionally, the presence of trap states plays a vital role in the PL emission properties of the system. Higher the trap centers, the lesser the probability of biexciton formation. The preferential formation of trions or biexcitons in the QW system can either limit or facilitate PL
successfully modulate the optical properties of monolayer WS2. We believe this study would be highly relevant in enabling a deeper qualitative understanding of encapsulation-induced changes in the intrinsic properties of 1L-TMDs in quantum well-like systems.
155 | P L M o d u l a t i o n o f 1 L - W S2 b y Z n O e n c a p s u l a t i o n a n d Q u a n t u m w e l l e f f e c t
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