High-temperature observation of intralayer, interlayer, and Rydberg excitons in bulk van der Waals alloy single crystals

Transition metal dichalcogenides exhibit remarkable optical properties due to the diverse number of strongly bound excitons, which can be fine-tuned by alloying. Despite a flurry of research activity in characterizing these excitons, a comprehensive and profound understanding of their behavior with temperature is lacking. Here, we report the rich spectrum of excitonic features within bulk van der Waals alloy ${\mathrm{Mo}}_{0.5}{\mathrm{W}}_{0.5}{\mathrm{S}}_{2}$ and ${\mathrm{Mo}}_{0.5}{\mathrm{W}}_{0.5}{\mathrm{Se}}_{2}$ single crystals through temperature-dependent reflectance spectroscopy and first-principles calculations. We observed Rydberg excitons and interlayer excitons in both the single crystals. Notably, we provide the first experimental evidence of highly energetic ${\mathrm{A}}^{\ensuremath{'}}$ and ${\mathrm{B}}^{\ensuremath{'}}$ excitons in ${\mathrm{Mo}}_{0.5}{\mathrm{W}}_{0.5}{\mathrm{S}}_{2}$ at room temperature. The strong carrier-phonon scattering significantly broadens the ${\mathrm{A}}^{\ensuremath{'}}, {\mathrm{B}}^{\ensuremath{'}}$, and interlayer excitons at room temperature in bulk ${\mathrm{Mo}}_{0.5}{\mathrm{W}}_{0.5}{\mathrm{S}}_{2}$ single crystal compared to its selenide. Our findings, supported by density functional theory and Bethe-Salpeter equation calculations, signify the crucial role of carrier-phonon interactions. These results open pathways for next-generation optoelectronic devices and quantum technologies operating at high temperature.