Exciton Dynamics and Anisotropy in 2D Silver Phenylchalcogenolates

Silver phenylselenolate (AgSePh) and silver phenyltelluroate (AgTePh) are novel two-dimensional (2D) van der Waals semiconductors. However, despite having a similar crystal structure and composition, AgSePh and AgTePh exhibit strikingly different excitonic properties. While the three distinct excitonic absorption resonances of AgSePh are close in energy, AgTePh exhibits a large energetic separation between the two lowest excitonic absorption resonances. More strikingly, whereas AgSePh exhibits narrow, fast luminescence with a minimal Stokes shift, AgTePh exhibits comparatively slow, significantly red-shifted and broadened luminescence. In this presentation, we will present the synthesis, structural and optical properties of single crystals and thin films of AgSePh and AgTePh. Using time-resolved and temperature-dependent optical spectroscopy, combined with sub-gap photoexcitation studies, we will show that exciton dynamics in AgTePh films are dominated by intrinsic exciton self-trapping behavior, whereas dynamics in AgTePh films are dominated by the interaction of free-excitons with extrinsic defect states. Furthermore, using polarization-dependent optical spectroscopic studies on single crystals of AgSePh and AgTePh, we will show the strikingly different exciton anisotropy in these materials. Whereas the lowest and the second lowest free-excitons in AgSePh are polarized along [010] and [100] direction, respectively, the two lowest free-excitons and self-trapped excitons in AgTePh are all polarized along [010] direction. Finally, we will discuss GW plus Bethe-Salpeter equation calculations to understand anisotropic excitons in these materials. Overall, this work highlights the properties of a novel class of 2D excitonic semiconductors and lays the foundation for understanding exciton dynamics and anisotropy in these materials.