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Modifications of electronic structure around defects in 2D materials by first-principles simulations and STM experiments (MODEST) 

DAAD - PPP Programme for Project-Related Personal Exchange with Finland 

Two-dimensional (2D) materials have recently attracted wide interest in the materials research community both for their interesting physical properties as well as for their potential in flexible optoelectronics, sensing, and catalysis applications. All of these devices require careful control of electrons (and holes) within the device structure. Due to the extreme thinness of the 2D sheets, there is an unavoidable interaction with the environment that modifies the electrostatics in a manner quite distinct from the 3D counterpart. It is manifested in the form of charge transfer and modification of the screening. While these effects can be beneficial or completely destroy the device, surprisingly little is known about these issues. 

In this project we investigate theoretically and experimentally the modification of electronic structure around defects, edges, and grain boundaries in 2D materials. We use STM and STS to obtain direct information on the spatial changes in the density of states, which can be used to infer the electrostatics acting in the system. 

A first milestone of the project was the investigation of charge transfer and band bending next to mirror twin boundaries in MoS2 monolayers on graphene. Strong upward bending of valence and conduction bands toward the line defects is found for the mirror twin boundary and island edges by STS. Quantized energy levels in the valence band are observed wherever upward band bending takes place. We also provide a first measurement of the theoretically predicted quantized polarization charge on a domain boundary. These results provide insight into how mirror twin boundaries impair charge transport of electrons and holes [more]. 

The project involves the experimental group at Universität zu Köln and the computational group of Hannu-Pekka Komsa at Aalto University Oulu, Finland [website]. Joint workshops for the experimental group on density functional theory calculations in Aalto and for the theory group on scanning tunnelling microscopy and spectroscopy in Köln were educational highlights of this project.

Top: STM topography of a mirror twin boundary (MTB) and its atomic structure. Middle: Constant current spectroscopy along the white arrow of the STM topography, displaying the valance band quantization next to the MTB. Right: 1D model for the quantized energy levels of holes at the valence band edge next to the MTB. Bottom: Side-view model of density functional theory calculations of MoS2 with a MTB on graphene with charge transfer indicated (blue: depletion; red accumulation) [more].