Theory of nitrogen doping of carbon nanoribbons: edge effects.

Nitrogen doping of a carbon nanoribbon is profoundly affected by its one-dimensional character, symmetry, and interaction with edge states. Using state-of-the-art ab initio calculations, including hybrid exact-exchange density functional theory, we find that, for N-doped zigzag ribbons, the electronic properties are strongly dependent upon sublattice effects due to the non-equivalence of the two sublattices. For armchair ribbons, N-doping effects are different depending upon the ribbon family: for families 2 and 0, the N-induced levels are in the conduction band, while for family 1 the N levels are in the gap. In zigzag nanoribbons, nitrogen close to the edge is a deep center, while in armchair nanoribbons its behavior is close to an effective-mass-like donor with the ionization energy dependent on the value of the band gap. In chiral nanoribbons, we find strong dependence of the impurity level and formation energy upon the edge position of the dopant, while such site-specificity is not manifested in the magnitude of the magnetization.

Theory of the sp-d coupling of transition metal impurities with free carriers in ZnO.

The [Formula: see text] exchange coupling between the spins of band carriers and of transition metal (TM) dopants ranging from Ti to Cu in ZnO is studied within the density functional theory. The [Formula: see text] corrections are included to reproduce the experimental ZnO band gap and the dopant levels. The p-d coupling reveals unexpectedly complex features. In particular, (i) the p-d coupling constants [Formula: see text] vary about 10 times when going from V to Ni, (ii) not only the value but also the sign of [Formula: see text] depends on the charge state of the dopant, (iii) the p-d coupling with the heavy holes and the light holes is not the same; in the case of Fe, Co and Ni, [Formula: see text]s for the two subbands can differ twice, and for Cu the opposite sign of the coupling is found for light and heavy holes. The main features of the p-d coupling are determined by the p-d hybridization between the d(TM) and p(O) orbitals. In contrast, the s-d coupling constant [Formula: see text] is almost the same for all TM ions, and does not depend on the charge state of the dopant. The TM-induced spin polarization of the p(O) orbitals contributes to the s-d coupling, enhancing [Formula: see text].