Venturing Further into the Field of 2D Materials and their Laminated Parent Phases

Abstract: The field of 2D materials is a relatively young and rapidly growing area within materials science, which is concerned with atomically thin states of matter. Because of their intrinsic 2D morphology, 2D materials have exceptionally high surface to weight or surface to volume ratio. This renders them excellent candidates for surface-sensitive applications such as catalysis and energy storage, which can aid us in the transition to a more sustainable society. 2D materials are also interesting because they show properties intrinsically different from those of their 3D counterparts, expanding the attainable property space within materials science. A 2D material can be synthesised by either a bottom-up or top-down approach. The focus here is on the latter, where the 2D material is derived by either mechanical exfoliation or selective etching of a 3D nanolaminated parent phase. A 3D laminate can typically be assigned to one of two types, depending on the type of interlayer bonding: van der Waals (vdW) or chemical bonding. In a vdW bonded phase, the constituent layers are kept together into their 3D form by rather weak vdW forces, while in the latter type, the layers are bound more strongly by chemical interactions (i.e., covalent, ionic and metallic bonds). The first 2D materials were derived from vdW-phases, which can be exfoliated by mechanical methods. In a chemically bound laminated phase, the inter layer bonding is stronger, and more complex methods are required for exfoliation of these phases into 2D. This thesis concerns the computational study and development of novel 2D materials through exploration of 3D nanolaminated structures, assessment of their phase stability, and potential for conversion into 2D. The 2D derivatives are in turn studied through prediction of dynamical stability, termination configuration, and evaluation of electronic properties. Paper III and IV each addresses a family of van der Waals structures. The family of 3D materials studied in Paper III was chosen because it was recently demonstrated as possible to use for derivation of so called 2D MX-enes, while the 2D form of NbOCl2, from the family studied in Paper IV, has been shown to exhibit exciting optical properties. Both projects focus on identification of parent 3D materials, their exfoliation from 3D to 2D, and the electronic properties of the studied phases. In each project, a range of different chemical compositions is considered, chosen based on the experimentally known members of the respective families. A 3D structural ground state is predicted for each composition and prototype, and the dynamical stability with respect to lattice vibrations is established for each identified structure. To assure the experimental relevance of each considered 3D phase, the thermodynamical stability of each structure is assessed via the formation enthalpy with respect to competing phases, identifying seven stable structures in Paper III, and 17 in Paper IV, all of which are also found dynamically stable. Evaluation of the exfoliation energy for all these phases indicates that 3D to 2D conversion is possible. The electronic band structure and density of states were evaluated both for the 2D materials –being the primary focus in both projects – and their 3D parent phases. Al-though the bandgap for semiconducting phases is generally increased upon exfoliation, the electronic properties are mostly retained when exfoliating the vdW phases studied in this thesis. Paper I, II and V address chemically bonded 3D phases and their 2D derivatives. In these 3D phases, auxiliary atoms are interleaved between the 2D units, which needs to be selectively etched to form the corresponding 2D material. Additionally, new terminating species – so called terminations –may attach to the surfaces of the 2D units exposed during etching. Paper I presents an analysis of bonding characteristics in a group of nanolaminated 3D chemically bonded borides: Mo2SiB2, Ti4MoSiB2, and Ti5SiB2, out of which only the two former are observed experimentally. We identify a peak of antibonding states at the Fermi level for Ti5SiB2 as a reason why full elemental substitution of Mo is not achieved experimentally. Papers II and V instead focus on 2D materials derived from chemical 3D parent phases. They go beyond the 2D transition metal carbides and nitrides (MXenes), which until recently were the only 2D materials synthesised through selective etching. Paper II is a study of possible termination configurations on the first 2D boride Mo4/3B2−xTz – boridene – which is identified as being a conductor or small bandgap semiconductor, depending on the terminating species and specific configuration. In Paper V, a computational methodology for simulation of the selective etching process is employed to predict the possibility of etching Y from YM2X2, where the transition metal M and metalloid or nonmetal X are chosen to cover a large compositional space. This results in the prediction of 15 stable 2D structures, out of which nine are not previously investigated. All 2D structures are found to be either metallic or semimetallic. In this thesis, several different computational tools are used to predict and study laminated 3D phases and their corresponding 2D derivatives, assessing their properties considering both purely hypothetical and experimentally realised structures. Experimental relevance is central to all calculations, either by complementing already established experimental results, or by rigorous assessment of thermodynamical and dynamical stability to estimate the potential for experimental synthesis. The thesis expands our knowledge of 3D laminated phases and their 2D derivatives, and identifies several new phases which are likely possible to synthesise.