Estudo teórico das propriedades de nitreto de boro hexagonal e grafeno

Abstract

In this work, we made use of first principles calculations based on the density functional theory formalism to describe structural and electronic properties of carbon and boron nitride nanomaterials. The thesis is composed of three studies, two of them directly involving collaboration with experimental groups, and a third one, motivated by experimental results recently described in the literature, but which was conducted only with a theoretical approach. The first work deals with graphene and hexagonal boron nitride (hBN). Specifically, we investigated structural, energetic and electronic properties involved in the formation of graphene bilayers and multilayers and mixed graphene/hBN bilayers characterized by the relative stacking angles. We studied the role of point defects present in the hBN in the energetic stability of the heterostructures, discussing also possible modulations of the electronic structure. Concerning systems composed only of graphene, we studied structural and electronic properties of bilayers and multilayers which were epitaxially grown by sublimation of a silicon carbide substrate. It was possible to determine the structural properties of these systems, such as supercell lattice parameters and distances between the layers, as well as formation energies for several angles. We related these structural and energetic aspects with experiments performed by our collaborators, discussing the possibility of domain formation with distinct orientations between the layers. Finally, we studied aspects related to the electronic structure of such twisted bilayer graphene. In the third work, we investigated the hBN anomalous dielectric response when characterized by Electric Force Microscopy (EFM). To do that, we made use of molecular dynamics simulations with forces originated from first principles calculations. We analyzed the behavior of the dipole moment of a water layer confined between the hBN monolayer and the SiO2 substrate, and we showed that the molecule interactions with the substrate are responsible to make the interfacial dipole moment direction (due to the water) be independent on the external applied field. In this way, we showed how a resulting internal field was able to mask the dielectric response of the material.