An anomalous interlayer exciton in MoS2

Abstract

The few layer transition metal dichalcogenides are two dimensional materials that have an intrinsic gap of the order of ≈2 eV. The reduced screening in two dimensions implies a rich excitonic physics and, as a consequence, many potential applications in the field of opto-electronics. Here we report that a layer perpendicular electric field, by which the gap size in these materials can be efficiently controlled, generates an anomalous inter-layer exciton whose binding energy is independent of the gap size. We show this originates from the rich gap control and screening physics of TMDCs in a bilayer geometry: gating the bilayer acts on one hand to increase intra-layer screening by reducing the gap and, on the other hand, to decrease the inter-layer screening by field induced charge depletion. This constancy of binding energy is both a striking exception to the universal reduction in binding energy with gap size that all materials are believed to follow, as well as evidence of a degree of control over inter-layer excitons not found in their well studied intra-layer counterparts.

Optical control of polarization in ferroelectric heterostructures

“…However, at a sufficiently strong field, the energy splitting of VBM and CBM, and thus the reduction of the band gap, get large enough, such that an inter-layer exciton becomes the lowest optical excitation. Earlier, a similar conclusion was reached for bilayer MoS 2 37 . This scenario is quantitatively verified by our calculations of the optical absorption spectra with and without the electric field (Fig.…”

Utilizing Interlayer Excitons in Bilayer WS2 for Increased Photovoltaic Response in Ultrathin Graphene Vertical Cross-Bar Photodetecting Tunneling Transistors

“…25 The interlayer excitons formed between two layers of WS2 also have a low binding energy, with a similar value of binding energy to that of intralayer excitons when the applied electric field is at a low value. 48 The lifetime of the interlayer exciton in the MoSe2/WSe2 heterostructure is as long as 1.8ns, because of the low exciton energy and its spatially indirect nature, even when the lifetime of the intralayer exciton in monolayer MoSe2 and WSe2 is short. 49 So in our bilayer WS2 devices, the out-of-plane interlayer exciton is also assumed to have a long lifetime on the order of ns, which is much longer than the lifetime of excitons in monolayer Device Fabrication: For making graphene electrodes, one layer of positive photoresist was spincoated on top of graphene before the spin-coating of a layer of negative photoresist to ensure the clean surface of graphene.…”

Adiabatic generalized gradient approximation kernel in time-dependent density functional theory

“…It is well known that the q-dependent behavior of the XC kernel is of vital importance for predicting the optical response of materials. For example, the XC kernel must go as 1/q 2 in order to capture excitonic effects 7,22,25,[46][47][48] in the long-wavelength limit (q → 0). However, the first A comparison is made with reported theoretical work 26,28,29,38,39 which used the ALDA kernel only and also the experimental result (green squares) taken from Mook et al 32,40 for Nickel, Balashov et al 41 for Cobalt, and Lynn 42 for Iron.…”