Density Functional Theory Study of Two-Dimensional Boron Nitride Films

Since graphene was isolated in 2004, the number of two-dimensional (2D) materials and their scientific relevance have grown exponentially. Besides graphene, one of the most important and technolocially promizing 2D materials that has emerged in recent years is hexagonal boron nitride, in its monolayer or multilayer form. In my thesis work, I used density functional theory (DFT) calculations to investigate the properties of boron nitride films. In particular, I first studied the properties (i.e. formation energy, defect states, and structure) of point charged defects in monolayer and bilayer hexagonal boron nitride, and subsequently, I focused on the linear and nonlinear mechanical properties of boron nitride films with sp2 and sp3 bonding structures. Experimental detection and characterization of defects in 2D materials are challenging tasks. My research work has been focused on points defects in monolayer (1L-) and bilayer hexagonal boron nitride (2L-hBN). I carried out technical developments and DFT calculations, and I have investigated the structural, formation energy, and electronic properties of both neutral and charged nitrogen and boron vacancy defects, as well as carbon substitutions for nitrogen and boron sites in 1L-hBN and 2L-hBN. These studies have shown that, due to an electrostatic polarization effect, the formation energies of charged defects in 2L-hBN are, in all cases, about 0.5 eV lower than in monolayer 1L-hBN. Moreover, I found that, under the assumption that vacancies and carbon substitutions defects are all present in a 2D h-BN film, there is at least one point defect species that is in a charged state, regardless the value of the Fermi energy. Carbon and boron nitride form a variety of solid allotropes, including layered materials such as graphite and h-BN, and sp3-bonded materials such as diamond and cubic BN. Similarly to the case of graphene, which can be considered a single layer of graphite, in recent years experimental investigations have shown that a diamond or sp3-rich phase can exist in the ultrathin down to the monolayer form. In particular, these experimental studies agree that under pressure and/or upon surface passivation (with hydrogen or hydroxyl groups), a few-layer graphene can transform into a sp3-rich C film, exhibiting a stiffness comparable to that one of diamond. As for BN, although high-purity polycrystalline h-BN is known to form a sp3-bonded phase upon compression, to the best of our knowledge only one very recent experimental study has shown that upon compression, nanosheets of h-BN transform into films rich in sp3 bonds. In this work, the sp3-bonded BN film was proposed to have a surface in contact with the substrate, and hydroxyl groups terminating the surface exposed to air. The aforementioned experimental works underline the importance of studying the mechanical properties of BN ultrathin membranes having both a sp2 and sp3 bonding structure. In my thesis work, I have used a DFT approach to calculate linear and nonlinear elastic constants of BN membranes. These elastic constants were used to estimate the ideal breaking strength under biaxial strain, and this theoretical parameter was then used to quantify the effects of film thickness, surface passivants, and structure on the mechanical strength of the membranes. In my work, I found that compression of a few-layer h-BN film can lead to the formation of various plausible conformations of an ultrathin BN film with a sp3 bonding structure. Most of these sp3-bonded 2D films are energetically stable upon passivation of at least one surface. Nonetheless, I found that three-layer BN films can form stable ultra- thin films with a sp3 bonding structure and clean surfaces. Although the BN sp3-bonded membranes exhibit longitudinal mechanical properties comparable to those of the layered BN film, the benefits of a sp3-bonded membrane include the enhanced mechanical strength in the transverse direction of the film, and potentially, the possibility to form conformations of the films with anisotropic and tunable mechanical and electrical properties