Alloying as a new route to generating interlayer excitons

Abstract

Heterobilayers formed by stacking two-dimensional atomic crystals are particularly promising for low-dimensional semiconductor optics, as they host interlayer excitons, bound states of electrons and holes residing in different layers. They inherit the valley-contrasting physics of the individual monolayers, leading to a range of unique properties that distinguish them from other solid-state nanostructures. Here, we propose a novel route for the generation of interlayer excitons based on the synthesis of a transition metal dichalcogenide bilayer alloy material, WS$_{2x}$Se$_{2(1-x)}$. Using piezoelectric force microscopy, we demonstrate the existence of an internal electric field oriented in the out-of-plane direction. Interlayer excitons have so far been mostly observed in heterostructures with a type-II band alignment. In the presence of an internal electric field, a similar alignment occurs in the alloy bilayer resulting in an efficient generation of interlayer excitons. Photoluminescence spectroscopy measurements involving circularly polarised light come up with key observations like a negative degree of circular polarization of the interlayer excitons which increases as a function of temperature. A simple theoretical model provides a physical understanding of the major experimentally observed features. With experimentally fitted parameter values, the dominant contribution to the degree of circular polarization is shown to arise from spin polarization and not from valley polarization, a consequence of the spin-valley-layer coupling characteristic of a TMDC bilayer. The room-temperature interlayer excitonic transition in bilayer TMDCs has key implications for fundamental physics, including Bose-Einstein condensation and high-temperature superfluidity, while enabling advanced valleytronic and quantum information functionalities.