Daily life in modern society is highly dependent on many different materials and techniques for manipulating them, and the technological forefront is constantly pushed further by new discoveries. Hence, materials science is a very important field of research. The field of 2D materials is a rather young subfield within materials science, sprung from the realisation of the first 2D material graphene. 2D materials have, due to their 2D morphology, a very high surface-to-weight ratio, which makes them clearly attractive for applications where the material surface is an important characteristic, such as for energy storage and catalysis.

The family of 2D materials called MXenes contrast to other 2D materials through the methods used to synthesise them. Traditionally, 2D materials are mechanically exfoliated from a 3D bulk structure in which the 2D sheets are only kept together by weak van der Waals forces, while MXenes are instead chemically exfoliated by selectively etching the A element from a member of the MAX phase family. A MAX phase is a hexagonal nanolaminated crystal structure on the formula M_n+1AX_n, with n = 1 – 4, where the M indicates one or several transition metals, A stands for an "A element", commonly a metalloid, and X stands for C or N. After etching away the A element from the MAX phase the M_n+1X_n-layers are left, making up the MXene. MXenes thus show an unusual structural and chemical diversity, and the composition spectra is even further expanded by atoms and small molecules, called surface terminations, attaching to the MXene surface upon etching. These terminations in turn also influence the properties of the MXene. Hence, the MXene family shows great potential for property tailoring towards many different applications.

Besides MAX phases there are many other nanolaminated materials which can not be mechanically exfoliated like graphene, and the natural question arises: can other nanolaminated materials be etched into completely new 2D materials? This thesis is concerned with the so called MAB phases – a family of laminated materials similar to MAX phases, but with B instead of C or N – and their 2D derivatives from a computational perspective. More specifically, paper I concerns the quaternary out-of-plane-ordered MAB (o-MAB) phase Ti₄MoSiB₂ – which has been etched into a 2D titanium oxide – and its related ternary counterparts Mo₅SiB₂ and Ti₅SiB₂. In paper II the properties and possible termination configurations of a 2D MXene-analogue named boridene is studied.

Both projects concern novel materials that have recently been experimentally realised, and the main aim of the first principles calculations presented here has been to complement and explain the experimental results. In paper I bonding characteristics of Ti₄MoSiB₂, Mo₅SiB₂ and Ti₅SiB₂ are studied, with the goal of better understanding why the two former are experimentally realisable while the latter has never been reported. In Ti₄MoSiB₂ Ti and Mo populate two symmetrically inequivalent lattice sites, and the bond between these two sites was found to display a large peak of bonding states just below the Fermi level. This peak is fully populated in Ti₄MoSiB₂ and Mo₅SiB₂, but only partially populated in Ti₅SiB₂, which was identified to be the key difference causing Ti₅SiB₂ to be unstable.

Paper II instead focuses on the 2D material boridene, derived from a 3D MAB phase with in-plane ordering (i-MAB). The i-MAB phase is similar in structure to i-MAX phases, and the boridene show similar structure and properties as the corresponding i-MXene etched from i-MAX, including a high activity for the hydrogen evolution reaction (HER). The boridene surface was experimentally found to be terminated by O, F and OH species, and the first principles investigations were aimed at screening the possible termination compositions using dynamical stability analysis, and how the electronic properties of boridene are influenced by the terminations. It was found that the terminations are critical to the dynamical stability of boridene, while the specific composition is less important. For termination with only a single species, the material was predicted to be a small bandgap semiconductor with varying bandgap for different species, while for termination with mixed species, the material was found to be metallic.

Hence, this thesis has slightly expanded the theoretical knowledge of MAB phases and their first 2D derivative, boridene, by detailed first principles characterisation. Hopefully, these studies can contribute in further development of the considered and related materials, and bring meaningful insight into the behaviour and properties of MAB phases and their 2D derivatives.

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 Mo_4/3B_2−xT_z – 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.