Computational discovery and characterization of novel 2D materials: A 2D Materials Encyclopedia

Sten Haastrup

Research output: Book/ReportPh.D. thesis

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Abstract

In this thesis we have systematically investigated the ground state and response properties of around 3000 two-dimensional materials using density functional theory (DFT) and many body perturbation theory methods. A computational workflow scheme has been developed which allows us to accurately calculate the structural, thermodynamic, elastic, electronic, magnetic, and optical properties of any two-dimensional material.
Today, around 50 compounds have been synthesised in monolayer form and many more layered materials are known. Using combinatorial lattice decoration, we generate new, hypothetical, structures from the existing ones and systematically calculate their properties. To ensure the physical reasonableness of the hypothetical structures, we carefully investigate their global (thermodynamic) and local (mechanical) stability. This analysis reveals hundreds of novel two-dimensional materials with high stability which it should be possible to synthesise. One of the most exciting features of two-dimensional materials is their ability to form heterostructures; patterned stacks of different two-dimensional materials, with precisely tunable properties. This means that the discovery of a new stable two-dimensional material also represents the discovery of a new building block in this stacking framework.
The systematic calculation of properties for all materials also allows us to investigate the performance of simpler models, and lets us understand in more detail where they break down. An example of this is the behaviour of bound electron-hole pairs: excitons. They are frequently modelled using a hydrogen-like equation, but comparison with the BSE binding energy reveals several regimes where the model performs poorly.
Access to the complete structured database enables us to study structure-property and property-property relations in a data-driven manner. Using this paradigm, we can accurately predict the heat of formation of the two-dimensional materials based on knowledge of the chemical composition and the abstract crystal structure. Further investigations in this direction are likely to yield promising descriptors for more complex properties.
Recognising the unavoidable trade-off between computational depth and computational breadth, we also identify some tens of materials with novel magnetic, plasmonic or transport properties; which would be worth studying in greater detail, using more advanced methods.
Finally, at the complete opposite end of the depth vs. breadth spectrum from the systematic study of 3000 materials, we investigate the dissociation of excitons in a single two-dimensional material, MoS2, upon application of an electric field. From this we conclude that it is possible to generate free electrons and holes using realistic field strengths.
Original languageEnglish
PublisherDepartment of Physics, Technical University of Denmark
Number of pages130
Publication statusPublished - 2019

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