Dark-to-Bright Exciton Transition in 2D Janus Excitonic Semiconductors via Metal Cation Alloying

Monolayer Janus transition metal dichalcogenides (TMDs) are intrinsically polarized two-dimensional (2D) semiconductors with broken mirror symmetry, offering additional degrees of freedom for exciton and spin–orbit engineering. However, controlled access to tunable excitonic ground states has remained largely inaccessible. Here, we report a composition-dependent transition from bright to dark excitonic behavior in alloyed Janus TMDs (SeMoxW1–xS), synthesized via a plasma-assisted epitaxial replacement process. This method enables the reliable transformation of MoxW1–xSe2 into structurally ordered Janus alloys across a wide compositional range. Atomic-resolution imaging and optical spectroscopy reveal that the excitonic character switches abruptly from dark to bright exciton complexes at a critical Mo concentration (∼25%), confirmed by first-principles calculations. This crossover arises from the interplay between spin–orbit coupling and band-edge alignment in the alloyed Janus lattice. Our findings demonstrate a route for engineering dark and bright excitonic ground states in Janus 2D materials and establish a broadly tunable platform for investigating spin–valley physics in 2D Janus TMDs.