Dopagem substitucional em nanofitas de grafeno com borda zigue-zague: estabilidade, estrutura eletrônica e resposta óptica preliminar via DFT

Abstract

The absence of an intrinsic band gap in graphene restricts its application in logic nanoelectronic devices, motivating the investigation of nanostructures such as zigzag graphene nanoribbons (ZGNRs). Although lateral quantum confinement promotes the opening of a band gap in ZGNRs associated with inter-edge antiferromagnetic ordering, the high reactivity of their pristine edges requires passivation and modulation, such as substitutional doping. This thesis systematically investigates the thermodynamic stability, electronic structure, and optical response of pristine ZGNRs and those edge-doped with nitrogen or boron, as a function of the ribbon width (n = 2 to 16). Density functional theory was employed, using the PBE functional for geometric optimization and the hybrid HSE06 functional to mitigate the self-interaction error and obtain better quantitative descriptions of the electronic states and the band gap. The optical properties were studied within the framework of the independent particle approximation. The results reveal that nitrogen doping induces a slight lattice contraction with a low-energy cost, preserving the global stability of the system in a manner analogous to pristine ribbons. In contrast, boron doping presents a higher energy cost, especially in the strong confinement regime. The electronic structure analysis demonstrates that all nanoribbons operate as narrow-gap semiconductors, whose magnitude decays asymptotically with widening. It was verified that nitrogen doping preserves the geometric tunability of the band gap and induces a spin splitting in the edge states on the order of 0.2 eV. In boron-doped ribbons, a strong spatial localization of the edge state was observed, associated almost exclusively with the px orbital of the dopant, conferring stability to the magnitude of the band gap against variations in width. In the optical domain, a redshift of the absorption threshold was identified with the widening of the nanoribbon; additionally, boron doping promotes a significant redistribution of the oscillator strength, intensifying absorption in the ultraviolet range (4.0 to 6.0 eV). It was also demonstrated that optical edge effects lose relevance for widths n ≥ 14, where transitions from the graphene-like central portion begin to dominate. It is concluded that N-doped ZGNRs emerge as candidates for spin logic architectures and tunable optoelectronics, while B-doped ones are promising for application in spin filters, consolidating a relevant predictive map for the design of carbon-based nanodevices.