Éxcitons intra- e inter-camada em heteroestruturas de van der Waals controlados por campo elétrico perpendicular

Abstract

A van der Waals solid is characterized by the vertical stacking of atomic layers that interact with each other through van der Waals forces. It is already possible to isolate layers and to stack them with other layered materials in order to form a so-called van der Waals heterostructure. Excitons are quasiparticles formed by a bound electron-hole (e - h) pair that appear in semiconductors when an electron in the valence band is excited to the conduction band. Interlayer excitons can be created in van der Waals heterostructures when the e - h pair is located in different layers in real space. Accessing and controlling interlayer excitons and their interplay with conventional intralayer excitons is crucial to develop novel technologies which require long lifetimes and high energy excitons such as photodetectors and solar cells. In this work, we consider transition metal dichalcogenides heterobilayers and heterotrilayers in different configurations. We investigate the behavior of the exciton binding energy as well as the exciton probability distribution as a function of an external electric field applied perpendicularly to the plane of the stacked layers. First, we find an analytical expression for the dielectric functions in bilayers and trilayers systems, encapsulated or not by other materials. Due to the dielectric screening, the interaction potential between the electron-hole pair is no longer the Coulomb potential. In order to find this potential, we solve the Poisson equation in a medium with N interfaces, which represent the interfaces between different layers. Then, we investigate MoS2, MoSe2, WS2 and WSe2 heterobilayers and heterotrilayers in different symmetric and asymmetric (where the substrate and the layer above the heterostructure are composed of different materials) configurations with respect to the central layer plane. We use a hybrid model involving the effective mass approach and the tight-binding model to construct the exciton Hamiltonian for each system. Finally, we study the behavior of the exciton wave functions, focusing mainly on their degree of electron-hole overlap.