Exploring point defects in hexagonal boron-nitrogen monolayers

A comprehensive theoretical study of selected point defects for a monolayer of hexagonal boron nitride (h-BN) is presented. Two-dimensional structures were simulated through large h-BN molecular clusters and used to examine various defects, like: atom vacancies, atom substitutions, or distortions of the hexagonal lattice. Since carbon contaminations are very common in the h-BN technology, a particular attention has been paid to carbon impurities. The calculations of IR spectra for the doped molecular clusters reveal the presence of additional frequencies, which in many cases correspond to defect-bound modes. In particular, when two carbon atoms are close to each other, a localized stretching C- C mode of a high intensity has been found, with a frequency value of about 100-200 cm(-1) higher than for collective BN stretch frequencies. Absorption UV-Vis spectra obtained from time-dependent density functional theory show that the inclusion of impurities results in an emergence of several low-energy electronic excitations, from which some are localized on a defect, while other are delocalized. Energies of these excitations are strongly dependent on the defect type, and they range from about 0.7 to 6.1 eV for the lowest excitations. Based on UV-Vis spectra we propose several candidates which could be responsible for the experimental 4 eV color band. These defects are built from two or four adjacent carbon atoms and have the lowest excitation ranging from 3.9 to 4.8 eV, which is strongly localized on the defect.