MATERIALS SCIENCE AND CHEMISTRY

First-principles atomistic computer simulations which make it possible to simulate various materials without any input parameters from the experiment (except for the chemical elements the material consists of) are powerful tools in the modern materials science. Although they require supercomputers, they not only reproduce the structure and properties of the known materials, but also make it possible to predict new ones and describe the behavior of the system under various conditions, e.g., electron irradiation.  In this project, irradiation effects in two-dimensional (2D) inorganic materials were studied with the main focus on transition metal dichalcogenides. The intercalation of Li atoms into bilayer graphene was also addressed.

Understanding the interaction of energetic electrons with matter is the key to tailoring materials structure and properties by, e.g., controllable introduction of defects and to the assessment of irradiation damage in the materials in radiation-harsh environments such as cosmic space, or in a laboratory – the column of a transmission electron microscope (TEM). This is particularly relevant to two-dimensional (2D) materials, which have recently been in the focus of research due to their unique properties and potential applications. However, a growing body of experimental facts indicate that many concepts of energetic particle-solid interaction are not applicable to these systems, as the conventional approaches based on averaging over many scattering events do not work due to their very geometry, e.g., in graphene--a membrane just one atom thick--or require substantial modifications. Among other issues which still are not understood, the results will explain in particular the development of irradiation damage in inorganic 2D materials under electron irradiation. Also, complete understanding of the behavior of composite materials, e.g. bilayer graphene with intercalated Li atoms, under electron beam is still a challenge.

In this research project, irradiation effects in 2D inorganic materials were studied using first-principles methods with the main focus on transition metal dichalcogenides. Novel computational atomistic techniques based on the density-functional theory (DFT), time-dependent DFT and the non-adiabatic Ehrenfest dynamics were used, which enable studying radiation effects from first-principles with account for electronic excitations. The intercalation of Li atoms into bilayer graphene was also addressed. Close collaboration with several leading experimental groups made an immediate verification and implementation of the theoretical concepts in state-of-the-art experiments possible opening thus new avenues for irradiation-mediated post-synthesis tailoring of the properties of 2D materials.

None of these insights could be achieved at the atomistic levels without supercomputers, as such calculations without any empirical parameters are computationally very expensive, and require massive parallel architecture. To this end, the access to the High-Performance Computing Center Stuttgart’s (HLRS) Hazel Hen supercomputer was crucial for the success of this project.

Specifically, the effects of the electron beam on the structure and electronic properties of 2D transition metal dichalcogenides (TMDs) TaS₂ and TaSe₂ was investigated using first-principles computer simulations and TEM experiments [1,3]. The formation of defects, such as vacancies, was correlated with the changes in the TEM images of the so-called charge-density-wave patterns in these systems. The first-principles calculations provided insights into the energetics of the transformations as well as electronic structure of 2D MoTe2 under electron beam irradiation [5], and pointed out that some of the experimentally observed defects have localized magnetic moments. The results indicated that various nano-scale structures, including metallic quantum dots consisting of T’-phase islands and one-dimensional metallic systems such as vacancy lines and mirror twin boundaries embedded into a semiconducting host material can be realized in single-layer MoTe2, and defect-associated magnetism can also be added, paving new paths towards control of optical and electronic properties of two-dimensional materials. Overall, the results made it possible to better understand the effects of imperfections and defects on the electronic properties of TMDs, which are promising materials for applications in electronics and catalysis. 2D materials with interstitial-type defects were further investigated [4].

By combining TEM experiments and first-principles simulations within the framework of the density-functional theory, reversible superdense ordering of lithium between two graphene sheets was studied [2]. This research project was interesting in the context of energy storage, better understanding of the operation of Li-ion batteries and potentially their optimization and characteristic improvements. It was shown that not only single, but also multi-layer Li structure can be formed (at least in the bi-layer graphene), contrary to all previous works, albeit carried out for bulk graphite. The findings thus pointed to the possible existence of distinct storage arrangements of ions in 2D layered materials as compared to their bulk parent compounds. Identification of the atomic structure of Li between graphene sheets was possible due to extensive first-principles calculations. The simulations also provided insights into the dynamics of the process, that is diffusion of Li atoms, and addressed the role of the electron beam in the diffusion. They also made it possible to understand the role of electron-irradiation-induced defects in the nucleation of the Li phases.

With respect to the benefits of the society from the simulations, the results provided theoretical support to the ongoing fundamental experimental studies on 2D materials in the context of their future applications in electronics and energy solutions, and in particular explained the development of irradiation damage in inorganic 2D materials under electron irradiation.