Tailoring Excitonic and Optoelectronic Properties of Transition Metal Dichalcogenide Bilayers
Julian F. R. V. Silveira, Rafael Besse, Alexandre C. Dias, Naidel A. M. S. Caturello, Juarez L. F. Da Silva
Abstract
Transition-metal dichalcogenides have drawn attention as possible frameworks for a wide range of applications. Recently, the introduction and rise to prominence of Janus monolayers and vertical heterostructures have allowed further tuning of the properties of these systems. To comprehensively explore these aspects, including excitonic and optoelectronic characterization, we present a theoretical study of the MX2/MX2, MXY/MX2, MX2/MXY, and MXY/MXY bilayer systems, where M = Mo, W; X = S, Se; and Y = S, Se. We established the interlayer binding mechanism as a combination of van der Waals and weak hybridization of out-of-plane states, which competes with electrostatic repulsion. Band gap deviations from Anderson’s rule were observed and traced to the hybridization of out-of-plane electronic states and dipole-derived effects. These aspects were shown to be mostly indifferent to the presence of Janus monolayers, while the absolute positions of band edges are very sensitive to the orientation of the Janus monolayers. Exciton binding energies were expressive but smaller than monolayer binding energies, and many systems displayed small yet optically active interlayer transitions. The power conversion efficiency of the bilayer systems ranged from 0.5% up to 3.0%, which may be small but is significant considering the nanometer-scale thickness of these materials. Furthermore, our findings on how the properties of the bilayers affecting the power conversion efficiency contribute to a deeper understanding of the limitations of ultrathin systems in solar energy harvesting technology.