First-Principles Studies Of Doping Effect On Zinc Oxide And Structural Defects In Graphene

Zinc oxide (ZnO) is a promising material for its wide application in solid-state devices. With the pressure raised from an ambient condition, ZnO transforms from fourfold wurtzite (B4) to sixfold coordinated rocksalt (B1) structure. Doping is an efficient approach to improve the structures and properties of materials. Here we use density-functional theory (DFT) to study doped ZnO and find that the transition pressure from B4phase to B1phase of ZnO always decreases with different types of transition metal (V, Cr, Mn, Fe, Co, or Ni) doped, but the phase transition path is not affected by doping. This is consistent with the available experimental results for Mn-doped ZnO and Co-doped ZnO. Doping in ZnO causes the lattice distortion, which leads to the decrease of the bulk modulus and accelerates the phase transition. By analyzing the density of states (DOS) and partial DOS, we fine that doped ZnO is magnetic and the magnetism is mainly induced by the3d state of the transition metal. Mn-doped ZnO shows the strongest magnetic moment due to its half filled d orbital. For V-doped ZnO and Cr-doped ZnO, the magnetism is enhanced by phase transition from B4to B1. But for Mn-doped ZnO, Fe-doped ZnO, Co-doped ZnO, and Ni-doped ZnO, B1phase shows weaker magnetic moment than B4phase. These results can be explained by the amount of charge transferred from the doped atom to O atom. Our results provide a theoretical basis for the doping approach to change the structures and properties of ZnO.Graphene is a promising material due to its outstanding properties. Point defects can tailor or improve the relative properties of pristine graphene and these defects can be created artificially. Under irradiation or heat treatment, defects may diffuse and aggregate together. To acquire the promising graphene-based materials with interesting properties, we need to pay attention to the dynamical behaviors of the defects. We perform density functional theory (DFT) to illustrate the migration and coalescence processes of the point defects which are commonly researched in mono-layer graphene. Two nearing single vacancies can form a divacancy and the energy barrier is calculated to be1.17eV. In addition, the divacancy defect [V2(5-8-5) defect] may also rearrange itself and form a more stable structure [V2(555-777) defect] and another defect structure V2(5555-6-7777) defect can also be achieved with the further structural rearrangement. The energy barrier of the structural rearrangement is much higher than the migration and coalescence processes of single vacancy. One configuration of the divacancy defect which contain two pentagons and one octagon [V2(5-8-5) defect] can be healed by the neighboring adatom defect and form a single vacancy defect. However, another configuration of the divacancy which contain three pentagons and three heptagon [V2(555-777) defect] don’t be healed by the adatom defect.