Controlling the interaction of light and matter at atomic length scales has recently become possible by exploiting the unique properties of van der Waal heterostructures. In particular, structures with a Type II band alignment have many potential applications and can provide detailed insight into the carrier-photon conversion processes. To model and understand these processes, it is crucial to obtain the energies of the interlayer exciton states existing at the interface. However, the widely used density functional theory (DFT) is known to fail in predicting band gaps and band alignment at the interfaces and does not consider the excitonic effects. This makes the development of theoretical methods for predicting band lineup and interlayer excitons in van der Waals heterostructures an outstanding problem for 2D Materials science.
A team of researchers led by professor Kristian S. Thygesen at Technical University of Denmark reported a general first principles method that can be used to compute the electronic quasi-particle band structure and the excitonic binding energies of incommensurate van der Waals heterostructures. Their work focused on showing how to overcome the limitations encountered, by applying the recently developed quantum electrostatic heterostructure (QEH) model to compute interlayer exciton binding energies and electronic bands of complex van der Waals heterostructures. The general method was illustrated for the specific case of MoS2-WSe2 bilayers with intercalated spacer layers of the insulating hexagonal boron-nitride (hBN). Their work is now published in Nano Letters.
First, the band structures of different configurations of twisted MoS2-WSe2 bilayers were calculated at the density functional theory level and it was demonstrated that the effect orbital hybridization between the two layers was negligible. By performing highly accurate GW many-body calculations, the band structures of the isolated MoS2 and WSe2 monolayers were obtained and aligned relative to a common vacuum level. To include the effect of interlayer screening on the band energies, the authors employed the QEH model to compute the change in the screened Coulomb interaction and electron self-energy relative to the isolated monolayers. With these ingredients, the researchers could obtain the band energies of the heterostructures at the GW level and overcome the problems of unreliable DFT energies and the lattice mismatch between the different 2D crystals of the heterostructures.
In a second step, the dielectric function of the heterostructures obtained with the QEH model, and the calculated effective band masses, were used as input to a 2D Mott-Wannier model to obtain the intra- and interlayer excitons of MoS2-hBN-WSe2 heterostructures. Despite the spatial separation of the electron and hole, the binding energy of the interlayer excitons was found to be surprisingly large (0.3 eV for the MoS2-WSe2 bilayer). The resulting energies of the lowest bound excitons were in excellent agreement with the experimental photoluminescence spectra.
From the study conducted it is evident that the new and general first principles method is highly promising for computing the electronic quasi-particle band structure and the excitonic binding energies of realistic incommensurate van der Waals heterostructures. A comparison to the density functional theory computations demonstrates the key role of self-energy and electron-hole interaction effects.
Simone Latini1,2, Kirsten T. Winther1, Thomas Olsen1, and Kristian S. Thygesen1,2. Interlayer Excitons and Band Alignment in Molybdenum disulfide/ hexagonal boron nitride /Tungsten diselenide van der Waals Heterostructures. Nano Letters volume 17 (2017) pages 938-945.Show Affiliations
- Center for Atomic-Scale Materials Design (CAMD), Department of Physics, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark
- Center for Nanostructured Graphene (CNG), Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark
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