Unfolding the Electronic Structure of Two-Dimensional Heterostructures

Intense research activity on two-dimensional (2D) heterostructures in recent years is due to their unique properties and potential for applications. Detailed theoretical understanding of these properties is of fundamental interest and facilitates novel materials design. We present results based on density functional theory (DFT) for the atomic and electronic structure of 2D nanostructures with defects and twisted van der Waals heterostructures (vdWHs) consisting of combinations of transition metal dichalcogenide (TMD) monolayers (MLs) and graphene (Gr). Due to the large simulation cells required and strong deviations from periodicity, DFT calculations are challenging and results need careful interpretation. We present methods we develop for the construction of optimized simulation cells and for unfolding the corresponding electronic band structure. The effective band structure (EBS) produced allows for a clear and direct comparison between electronic properties of defected 2D nanostructures and heterostructures with their pristine and constituent ML counterparts. Applying our methodology to Gr/TMD and TMD/TMD vdWHs, several experimental observations are explained and predictions are made [1]. Our methodology also proves very useful in investigating defects and adsorption on 2D MLs. In conjunction with experiments, DFT calculations show how n-doped WSe₂ ML becomes a p-doped semiconductor via photochlorination [2]. Energetics and EBSs show that chlorine fills chalcogen vacancies, neutralizing defect states close to the conduction band minimum and creating defect states close to valence band maximum. Current efforts in combining DFT, atomistic simulations, and experimental results with machine learning models for predicting 2D heterostructure properties will be discussed.

Partial support by S.A.R.F. of the University of Crete, KA11569. Computational time granted by GRNET S.A. in the national HPC facility ARIS, ‘CompNanoMat’ Production Project Access.

1. G. Vailakis, G. Kopidakis, Unfolding the band structure of van der Waals heterostructures, Phys. Rev. Mater. 7, 024004 (2023).
2. E. Katsipoulaki et al., Electron density control in WSe2 monolayers via photochlorination, 2D Mater. 10, 045008 (2023).