Role of carbon impurities at grain boundaries and Stone-Wales defects in hBN: a potential new class of single photon emitters

Hexagonal boron nitride (hBN) has emerged as a viable host for point defects with bright, room temperature single photon emission. The large-scale synthesis of hBN often introduces extended defects in addition to the presence of point defects. Here, we computationally investigate carbon impurities combined with topological defects and grain boundaries accommodating non-hexagonal rings in hBN. We show that the relative formation energy for carbon containing Stone-Wales defects (SW-C) is significantly reduced. Under uniaxial strain, the SW-C defects become energetically favorable in certain cases. The inclusion of carbon impurities at grain boundaries can resolve unfavorable B-B and N-N bonds and result in energetically preferable atomic structures. In addition, we find that majority of the SW-C defects as well as the carbon impurities at grain boundaries can give rise to optical transitions in the visible spectral range. Our work sheds light on a new class of defects that may appear at highly strained areas and grain boundaries in hBN, where bright single photon emitters have already been observed.