Effects of lattice imperfections on the optical and electronic properties of two-dimensional materials

After the first exfoliation of graphene in 2004, the field of two-dimensional (2D) materials has received a lot of research attention due to their unique properties. 2D materials are just one layer thick and they exhibit novel features compare to their bulk counterpart and they have become a platform for discovering exotic phenomena and developing innovative applications.
At the same time, the last few years have seen a renewed interest in studying defects in materials. This happened with a paradigm shift: from an unintended and ubiquitous presence in materials to a source for developing new applications. Therefore, defects are no longer considered to limit the material’s performance, but more as a tool to design specific target properties.
In this thesis, from the title Effects of lattice imperfections on the optical and electronic properties of two-dimensional materials, the methods to investigate the effects of defects in 2D materials from first principles were developed with the
electronic structure code GPAW.
The electronic properties were characterized by describing the symmetry of new defect states that appear in the material’s band gap. A symmetry analysis of such states was performed for thousands of defects states, for vacancy and antisite defects in 2D materials. Then, it was also considered the impact of the defects on the optical properties. This was done by studying the optical transitions between defect states and looking at the interaction of the defects with vibrations, an important aspect for application of defects. The focus was also on the nonradiative recombination of photo-excited charge carriers to the defect states, which is the main loss mechanism in opto-electronic devices. The modern state of the art methods to describe such transitions from first principles were implemented with GPAW and within the ASR framework.
A part of the work was also spent in predicting new 2D materials with a high throughput approach. This method was applied to a new class of 2D material, the Janus monolayers. These materials exhibit an intrinsic finite dipole moment that comes from the difference in electronegativity in the chemical elements on the two sides of the layer. A set of new possible monolayers of this class was proposed. Finally, the structural instabilities of 2D materials were also considered. A simple method was used and validated to assess the stability of a monolayer without more expensive calculations, that are not suitable for high throughput computational studies. The effect of the lattice distortion on the electronic properties was studied for a set of distorted monolayers.