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 (SeMo_<i>x</i>W_1-<i>x</i>S), synthesized via a plasma-assisted epitaxial replacement process. This method enables the reliable transformation of Mo_<i>x</i>W_1-<i>x</i>Se₂ 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.