Transition Metal Atoms Doped Zinc Oxide Nanotubes First-principles Study

Recently theoretical works verified that the single-walled ZnONTs can exist stably and be regarded as a cylinder rolled up from ZnO sheet.3d TM doped ZnONTs is a scientifically interesting and technologically important area of research with many current and future applications in nanoelectronic,optoelectronic and photonic devices.In this paper, we have performed the first-principles calculations onto the structural, electronic and magnetic properties of seven3d TM(TM=V,Cr,Mn,Fe,Co,Ni and Cu) atom substituting cation Zn in both zigzag (10,0) and armchair (6,6) zinc oxide nanotubes (ZnONTs). We presented a detailed first-principles analysis of seven3d TM (V, Cr, Mn, Fe, Co, Ni and Cu) doped zigzag (10,0) and armchair (6,6) ZnONTs using the projector-augmented-wave (PAW) potential approach to the density-functional theory (DFT) within the generalized-gradient approximation (GGA) implemented in Vienna ab-initio simulation package (VASP).The following results are obtained:(1) The structures of the pristine (10,0) and (6,6) ZnONTs are optimized. The results show that both zigzag (10,0) and armchair (6,6) ZnONTs are all semiconducting. The3d TM atom substituting a cation Zn induces a structural distortion around3d TM impurity with respect to the pristine ZnONTs. The bond lengths are shorter than the for Zn-O bonds in two pristine ZnONTs is because the ionic radii of these3d TM2+ions are all shorter than substituted Zn2+ion, except for Mn2+ion may be resulted from the repulsion interaction between Mn2+ion in the highest spin state and its nearest-neighbor O-2ion being ferromagnetic coupling with Mn2+ion. The various bond angles are observed but the average values of them are all slightly smaller than120°due to curvature effects of the tubes.(2)3d TM-doped (10,0) and (6,6) ZnONTs implying the magnetic moments of these TM-doped ZnONTs originate from the doped3d TM impurities. The magnetic moment values and their variation trend versus the atomic number are nearly identical for3d TM-doped (10,0) and (6,6) ZnONTs, implying they are independent on the tube chirality. With increasing the atomic number, the magnetic moments of the3d TM-doped (10,0) and (6,6) ZnONTs increase firstly and reach their maximum for Mn-,and then decrease simultaneously. The calculated values of the magnetic moments are smaller than the predicted values of Hund's rule due to strong hybridization between 2p orbitals of the nearest-neighbor O atoms of ZnONTs and3d orbitals of the TM impurities.(3) Substituting an uncompletely filled3d-orbital TM ion for a completely filled3d-orbital Zn ion of ZnONTs will change the number of up-spin and/or down-spin states in the valence band of ZnONTs. The five up-spin (down-spin) states are not completely filled and there exist unoccupied (hole) up-spin (down-spin) states at the top of valence band. Due to hybridization between TM ion and O ions, these hole up-spin (down-spin) states of TM ion are partly occupied by up-spin (down-spin) electrons of O and consequently up-spin (down-spin) states of O become less occupied than down-spin (up-spin) states. This gives rise to a negative (positive) magnetic moment for the nearest-neighbor O ions of TM ions.(4) At the Fermi level, a finite PDOS in up-spin (down-spin) channel and a zero PDOS in down-spin (up-spin) channel show the half-metallic behaviors, These partially filled TM3d states and a few O2p states contribute to the conductivity at the Fermi level only in one spin channel. The V-and Cr-doped (10,0) and (6,6) ZnONTs are magnetic metals. The Mn-, Fe-, Co-, Cu-doped (10,0) and (6,6) ZnONTs are all half-metals and magnetic. The Ni-doped (10,0) and (6,6) ZnONTs are magnetic semiconductors, implying they are depend strongly on the TM impurities dependent, independent on the tube chirality(5)The charge carriers and local spins arise from the same TM electron states which are respectively incorporated in the crystal-field-split orbitals eg and t2g states but mutually coupled by the strong intra-atomic exchange interaction (Hund's-rule coupling) or hybridized with2p states of the nearest-neighbor O atoms of ZnONTs.