Sensitivity of Excitonic Transitions to Temperature in Monolayers of TMD Alloys

Two-dimensional transition metal dichalcogenides (TMDs) offer tunable optical and electronic properties, making them highly promising for next-generation optoelectronic devices. One effective approach to engineering these properties is through alloying, which enables continuous control over the bandgap energy and excitonic transitions. In this study, we perform temperature-dependent transmission spectroscopy on monolayers of Mo1–xWxS2 and Mo(S1–xSex)2 alloys, transferred onto the core of an optical fiber and measured within a cryostat over a temperature range of 20–320 K. The use of an all-fiber configuration allowed us to probe interband transitions A, B, and C with high stability and precision. We observe a systematic redshift of excitonic transitions with increasing Se content and a blueshift when Mo is replaced with W. These spectral shifts correlate with alloy composition and enable the tuning of bandgap energies between ∼1.6 and ∼2.0 eV at room temperature. Furthermore, we analyze the temperature sensitivity of the excitonic transitions, revealing that Se incorporation enhances thermal response in a nonmonotonic manner, while W substitution results in a more monotonic and stronger temperature dependence. The splitting between A and B transitions, associated with spin–orbit coupling, also varies with composition. Our findings underscore the potential of compositional engineering in 2D TMD alloys to achieve both spectral and thermal control of optical properties, relevant for the design of robust and tunable optoelectronic systems.