Excitons in hexagonal nanonetwork materials
  Optical excitations in hexagonal nanonetwork materials, for example, Boron-Nitride (BN) sheets and nanotubes, are investigated theoretically. A permanent electric dipole moment, whose direction is from the B site to the N site, is considered along the BN bond. When the exciton hopping integral is restricted to the nearest neighbors, the flat band of the exciton appears at the lowest energy. The symmetry of this exciton band is optically forbidden, indicating that the excitons relaxed to this band will show quite long lifetime which will cause luminescence properties. 
Kikuo Harigaya