Fine Structure Splitting in van der Waals Layered Transition Metal Dichalcogenide Single Crystal

Alloying of van der Waals layered transition metal dichalcogenides (TMDCs) MoS(Se)₂ and WS(Se)₂ provides materials containing versatile and tunable properties with great potential in optoelectronics. Alloying by substituting metal atoms causes an insignificant effect on the lattice strain but significantly affects the optical and electrical properties. Here, using ground-state optical reflectance and femtosecond transient differential reflectance (DR) spectroscopy measurement, we demonstrate fine structure splitting in MoWSe₂ single crystal in addition to MoWS₂. Our studies reveal the vital role of the size of anion atoms, which results in the additional features in the DR spectra. Moreover, density functional theory and Bethe-Salpeter equation calculations support our experimental findings. Our systematic studies offer critical insights into the unexplored domain of alloy TMDCs for improving optoelectronic devices.