An important milestone in the initial phase of LIMA was the launching of the Computational 2D Materials Database (C2DB) – a fully open and searchable online database containing structures and a variety of electronic, optical and magnetic properties for monolayer 2D materials (
https://cmr.fysik.dtu.dk/c2db/c2db.html(öffnet in neuem Fenster)). The first version of C2DB contained about 4000 materials. During the LIMA project, the C2DB has formed the basis for high-throughput screening studies where a number of novel 2D materials were identified and studied. In particular, we have discovered 2D ferromagnetic and ferroelectric semiconductors as well as topological insulators. The results of these screening studies have been published in Physical Review M, npj Computational Materials, and 2D Materials. A new type of 2D materials with broken mirror symmetries – the so-called Janus monolayers – have been investigated thoroughly, and their potential as building blocks for advanced opto-electronic devices has been demonstrated and disseminated in several papers. During the course of the project period the C2DB has been significantly expanded both in terms of new materials and new properties. Using a combination of systematic atom substitution and a deep generative machine learning model, we have identified around 10k new 2D crystals. The basic properties of the 4500 most stable of these crystals have been characterized using first principles calculations and have been made available in the C2DB.
Two comprehensive papers on the C2DB were published in 2D Materials and are among the highest cited papers of the journal.
We have studied a range of properties of excitons (bound electron-hole pairs) in 2D semiconductors; mainly monolayer and bilayer transition metal dichalcogenides (TMDs). In particular, we have performed the first predictions of interlayer trions in 2D heterobilayers. These theoretical predictions were subsequently observed experimentally and good agreement was found with our calculated binding energies. We have calculated the full q-dispersion of the lowest lying excitons in monolayer and homobilayer TMDs and also the properties of the dark excitons in these materials. We introduced and studied the important concept of mixed interlayer excitons in bilayer TMDs. These excitons have a mixed intralayer and interlayer character. In a subsequent collaboration with Prof. Saverio Russo at Exeter University we demonstrated electrical tuning of mixed interlayer excitons in bilayer MoS2. This work was published in Nature Nanotechnology.
A large number of 2D point defects for quantum technology applications (single-photon light sources and qbits) been explored. In particular, the photo-physics of native and carbon-containing defects in hexagonal boron-nitride (hBN) has been studied in detail in collaboration with experimental groups at the Technical University of Denmark. The results of these studies have been published in Nanoscale and 2D Materials. Using high-throughput computations we have systematically calculated the properties of around 1800 native point defects in 80 different 2D materials. The results have been compiled and curated and made available to the scientific community via the QPOD database, which is freely available online. A paper on QPOD was published in npj Computational Materials. Like the BiDB bilayer database, the QPOD database is integrated with the C2DB.
To discover point defects with a triplet ground state and narrow emission lineshapes, we conducted a high-throughput computational screening of 5000 point defects in ten 2D host materials. The defects comprised both native and extrinsic single and double defects. We were able to identify a short list of 20 point defects with very promising properties making them interesting candidates for quantum applications. The results were published in ACS Nano and will be made available via QPOD.
