Estudo teórico de defeitos em nanotubos de BN

Abstract

Boron nitride (BN) is a compound formed by covalent bonds between boron and nitrogen atoms. In the crystalline phase it can be found in different structures, such cubic (c-BN), simgle hexagonal (h-BN), wurtzite (w-BN) and rhombohedral (r- BN). Similarly to graphite h-BN can form two dimensional structures, which can be cut to form tubes. Otherwise, c-BN has hardness similar to diamond, which is retained up to 2000 °C while diamond turns into to graphite at about 900 °C. These properties make BN a promissor material for nanoelectronics in a hot environment. Here, we investigate structural and electronic properties of BN nanotubes. Our study is directed not only to BN nanotubes in the pristine form (no defects), but also when topological defects (vacancies) and dopant impurities (Carbon) are present. The first principles calculations are based on the density functional theory with the generalized gradient approximation for the exchange-correlation term. The calculations were performed using the SIESTA computer code using gaussian functions to expand the Khon-Sham orbitals. We observe that C impurities have low formation energies when compared to vacancies have high formation energies. All the pristine BN nanotubes studied are non magnetic semiconductor with a band gap energy around 3.5 eV, which is almost independent of the tube chirality and diameter. Carbon impurities introduce localized electronic levels into the band gap while vacancies give magnetic moments to the BN nanotubes. Calculations for complex defects (carbon impurities and vacancies) show that the formation of these complex defects have lower formation energies when compared to the sum of the formation energies for isolated defects, indicating that these defects have higher probability of occurring. Double vacancies have formation energies close to those for single vacancies and in the equilibrium geometry, occurs a reconstruction where a pentagon-octagon-pentagon (5−8−5) structure is present. In the optimal geometry dangling bonds are not present and the magnetic moment is zero, but non-spin-polarized electronic levels are present in the band gap.