Interlayer and intralayer excitons in MoS2/WS2 and MoSe2/WSe2 heterobilayer

Abstract

Accurately described excitonic properties of transition metal dichalcogenide heterobilayers (HBLs) are crucial to comprehend the optical response and the charge carrier dynamics of them. Excitons in multilayer systems possess an inter-or intralayer character whose spectral positions depend on their binding energy and the band alignment of the constituent single layers. In this paper, we report the electronic structure and the absorption spectra of MoS 2 /WS 2 and MoSe 2 /WSe 2 HBLs from first-principles calculations. We explore the spectral positions, binding energies, and the origins of inter-and intralayer excitons and compare our results with experimental observations. The absorption spectra of the systems are obtained by solving the Bethe-Salpeter equation on top of a G 0 W 0 calculation, which corrects the independent-particle eigenvalues obtained from density-functional theory. Our calculations reveal that the lowest energy exciton in both HBLs possess an interlayer character which is decisive regarding their possible device applications. Due to the spatially separated nature of the charge carriers, the binding energy of interlayer excitons might be expected to be considerably smaller than that of intralayer ones. However, according to our calculations, the binding energy of lowest energy interlayer excitons is only ∼20% lower due to the weaker screening of the Coulomb interaction between layers of the HBLs. Therefore, it can be deduced that the spectral positions of the interlayer excitons with respect to intralayer ones are mostly determined by the band offset of the constituent single layers. By comparing oscillator strengths and thermal occupation factors, we show that in luminescence at low temperature, the interlayer exciton peak becomes dominant, while in absorption it is almost invisible.

Shedding light on moiré excitons: A first-principles perspective

“…The other two independent parameters  = 0.06 and  = 0.03 are determined by reproducing the fundamental gap of MoS 2 /WS 2 heterojunction obtained from the one-shot GW calculations. The exciton binding energy, which is the difference between the fundamental gap and the optical gap E b = E g − E opt is 0.50 eV, in good agreement with previous experimental and theoretical results (68)(69)(70). Thanks to the large unit cells (1626 atoms) in our LR-TDDFT calculations of the moiré excitons, only the  point is sampled in the Brillouin zone.…”

Interlayer dielectric function of a type-II van der Waals semiconductor: The HfS2/PtS2 heterobilayer

“…In bilayers both intra-and inter-layer excitons can have binding energies that are some fraction of an eV. However, the latter energy is generally significantly smaller than the former [36][37][38]. Thus, the actual relative positions of the excitations can be different from those calculated here, with the PtS 2 intra-layer excitations being at a lower energy than the inter-layer excitations.…”

Ab initio and semiempirical modeling of excitons and trions in monolayer TiS3

“…3 up to the third excited state, along with the symmetry of the corresponding wave functions of each state. High binding energies, of the order of hundreds of meV, are observed, which is a hallmark of 2D materials [54][55][56][57][58][59]. Besides, similar to, e.g., black phosphorus, which also exhibits strongly anisotropic bands, excitonic p states in monolayer TiS 3 are found to be nondegenerate, a feature that can be experimentally probed by two-photon absorption [60], while the degeneracy between s and p states is lifted by the non-Coulombic form of the electron-hole potential [44].…”