A Computational Venture into the Realm of Laminated Borides and their 2D Derivatives

Abstract: 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 Mn+1AXn, 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 Mn+1Xn-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.