Computational Prediction of Two-Dimensional Metals and Metallic Oxides: Density Functional Theory Studies on Structural, Thermal, and Electronic Properties

Abstract

Two-dimensional (2D) atomic layer crystals with exotic electronic, optical, and mechanical properties have attracted tremendous attention in the last two decades due to their potential applications in electronic, energy storage, and conversion technologies. Among these 2D materials, metallic crystals are relatively unexplored, although 2D allotropes of gallium (gallenene) have been synthesized on various substrates. Motivated by these experimental findings, this thesis systematically investigates group 13 metals (Al, Ga, and In) for their novel 2D allotropes using first-principles density functional theory (DFT) calculations and an unbiased structural search based on a particle swarm optimization algorithm. The electronic structure, bonding characteristics, and electronic and phonon properties of the predicted 2D allotropes of these metals are calculated, including the expected effects of strain induced by substrates on the dynamical stability. The theoretical results predict that most group 13 elements have one or more stable 2D allotropes, with the preferred allotrope depending on the cell shape relaxation and strain, indicating that the substrate will determine the overall allotrope preferred. The second part of this thesis explores all possible 2D allotropes of an emerging wide-bandgap transparent conductive oxide, β-Ga₂O₃, for their structural and thermal stability using DFT calculations. Although β-Ga₂O₃ is not a van derWaals material, the results predict that one or two of its allotropes are stable. In addition, accurate band structures of these 2D semiconducting oxides are calculated using both the generalized gradient approximation (GGA) and LDA-1/2 method. Remarkably, monolayer Ga₂O₃(100) appears to be the most stable and has a large indirect band gap of 4 eV. The subsequent investigation examines the surface properties and electronic structures of low-index surfaces, i.e., (010), (100), and (001), of β-Ga₂O₃ using DFT calculations, as these are readily available for comparison with the experimental findings. The stability of various surface planes and terminations is assessed via the calculation of their respective surface energies. From the considered surfaces, the most stable (100)-B surface, is consistent with the observed tendency of β-Ga₂O₃ single crystals to cleave on the (100) plane. The band structures of all surfaces indicate the presence of surface states on the top of valence band except for the most stable surface (100)-B having the highest bandgap of 4.10 eV. These theoretical findings demonstrate a new avenue for the discovery of thermodynamically stable 2D metallic layers and β-Ga₂O₃–based nanodevices with enhanced electronic properties.