Experimentally establishing the optical dielectric function of 2D Janus TMDs

Janus transition metal dichalcogenide (TMD) monolayers exhibit unique optoelectronic properties, arising from their broken mirror symmetry and out-of-plane dipole moments, which distinguish them from their conventional counterparts. While extensive theoretical studies have provided valuable insights, experimental probing of the optical dielectric function of Janus TMDs remains elusive. In this work, we experimentally determine the complex optical dielectric function of single-layer excitonic Janus SeMoS and SeWS monolayers using normal-incidence reflectance spectroscopy combined with Kramers–Kronig constrained analysis, spanning photon energies from 1.5 to 3.0 eV. Our results reveal the fundamental presence of prominent excitonic resonances, band nesting features, and notable spin–orbit coupling effects. Furthermore, by investigating partially converted Janus TMD samples, we demonstrate the tunability of the dielectric function and associated optical properties. These findings offer a comprehensive experimental foundation for understanding the optical responses of Janus TMDs.