Temperature-driven crossover of excitonic radiative channels and valley polarization in monolayer WS₂

Valley polarization in monolayer WS₂ is governed by a competition between intervalley depolarization processes and the redistribution of carriers among multiple radiative channels that evolve with temperature. Achieving device-relevant valleytronic functionality therefore requires a channel-resolved and quantitative understanding of this competition under realistic substrate conditions. Here, we perform helicity-resolved photoluminescence spectroscopy on unencapsulated exfoliated monolayer WS₂ on SiO₂/Si, a technologically relevant baseline platform for scalable devices, over a wide temperature range from 80 to 360 K, using both co- and cross-circular polarization detection. By combining the spectral degree of valley polarization P(E), with a global Voigt deconvolution analysis, the low-temperature spectra are resolved into a defect-localized emission band L₂, a charged biexciton XX⁻, a trion X⁻, and a neutral exciton X⁰. We identify a pronounced radiative-channel turnover at 140–160 K, where the L₂ and XX⁻ emissions quench to below detectability, while X⁻ and X⁰ remain as the dominant radiative channels and persist up to 360 K. In the high-temperature regime (160–360 K), both X⁻ and X⁰ exhibit monotonic redshifts and linewidth broadening consistent with exciton–phonon renormalization, establishing a quantitative temperature benchmark for unencapsulated WS₂ on SiO₂, in contrast to idealized encapsulated systems. Component-resolved intensity analysis reveals a redistribution of population into X⁻ and X⁰ channels following the turnover, whereas the corresponding valley polarizations, P_X⁻(T) and P_X⁰(T), decrease systematically with temperature and exhibit a clear inflection near the turnover. This behavior is consistent with a rate-competition picture in which phonon-assisted intervalley scattering progressively shortens the depolarization time relative to radiative recombination. Together, these results provide a channel-resolved and device-compatible framework for quantifying and engineering valley polarization in monolayer WS₂ at technologically relevant operating temperatures.