Interlayer Excitons and Band Alignment in Complex van der Waals Heterostructures

Significance Statement

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.

Interlayer-Excitons-Band-Alignment-Complex-van-der-Waals-Heterostructures-Advances-in-Engineering

About The Author

Kirsten T. Winther got her Bachelor degree in Nanoscience from Aarhus University in 2009, and her Master degree in Physics and Nanotechnology from the Technical University of Denmark (DTU) in 2011. She earned a PhD degree in Physics from DTU in 2015, and continued there as a post doc until 2016. She is currently on maternity leave and will join as a post doc at Stanford University later in 2017.

About The Author

Simone Latini obtained his Bachelor and Master degree in Materials Science from the University of Rome Tor Vergata in 2011 and 2013 respectively. He then moved to the Technical University of Denmark (DTU) where he earned his PhD diploma in 2016 and was employed as a Post-Doc until the end of that year. He is currently working as Post-Doc at the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg.

About The Author

Prof. Kristian S. Thygesen earned his Master and PhD degrees in Physics from the Technical University of Denmark (DTU) in 2002 and 2005, respectively. After a post doctoral position at Freie University Berlin he returned to DTU where he became Associate Professor in 2010 and leader of the Molecular Electronics group at the Lundbeck Foundation’s Center for Atomic-scale Materials Design (CAMD).

He was Director of “NanoDTU” from 2009-2010 and has been Spokesperson for Psi-k working group on Quantum Transport in Nanostructures since 2009. In 2013 he became Professor at the Department of Physics at DTU and in 2015 he became leader of the Section for Computational Atomic-scale Materials Design.

About The Author

Thomas Olsen got his Masters degree in from University of Copenhagen in 2006 and his PhD degree in physics from the Technical University of Denmark (DTU) in 2010. Subsequently, he was employed in post doctoral positions at DTU in 2011-2013 and at the University of the Basque country, San Sebastian in 2014. In 2015 he was employed as assistant professor at the department of physics, DTU. He was awarded the “Research Talent” grant from the Danish Council for Independent Research in 2015.

Reference

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
  1. Center for Atomic-Scale Materials Design (CAMD), Department of Physics, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark
  2. Center for Nanostructured Graphene (CNG), Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark

 

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