空位及氮掺杂二维ZnO单层材料性质:第一性原理计算与分子轨道分析

Properties of vacancies and N-doping in monolayer g-ZnO:First-principles calculation and molecular orbital theory analysis

采用基于密度泛函理论的第一性原理计算方法,系统地研究了带缺陷的二维类石墨烯结构的ZnO(graphenelike-ZnO,g-ZnO)的几何结构、电子结构、磁性性质和吸收光谱性质.研究的缺陷类型包括锌原子空位(VZn_g-ZnO)、氧原子空位(VO_g-ZnO)、氮原子取代氧原子(NO_g-ZnO)和表面吸附氮原子(N@g-ZnO).研究发现:NO_g-ZnO体系和N@g-ZnO体系形变较小,而空位体系会引入较大的形变;g-ZnO本身无磁矩,引入Zn空位后,VZn_g-ZnO体系总磁矩为2.00μB;VO_g-ZnO体系无磁矩,但N掺杂后的NO_g-ZnO体系和氮吸附的N@g-ZnO体系的总磁矩分别为1.00μB和3.00μB.利用掺杂体系的局域对称性和分子轨道理论分析了杂质能级和磁矩的产生原因,并且通过分析光吸收曲线得知,引入空位缺陷或者N原子掺杂,可以有效增强g-ZnO单层材料的光吸收性能.研究结果对系统地理解g-ZnO及其缺陷模型的性质有重要意义,可以为发展基于g-ZnO的纳米电子器件和光催化应用提供理论参考.

The geometric structure,electronic structure,magnetic properties and absorption spectrum of graphene-like ZnO(g-ZnO)monolayer supercell with defects are systemically studied by the first-principles calculation based on density functional theory in this work.The defect supercell model includes zinc atom vacancy(VZn_g-ZnO),oxygen atom vacancy(VO_g-ZnO),nitrogen atom substituted for oxygen atom(NO_g-ZnO)and nitrogen adsorbed on the g-ZnO monolayer(N@g-ZnO).The results indicate that the geometric deformation induced by N-doping in NO_g-ZnO and N@g-ZnO structure is negligible,while that of supercell with vacancy is relatively large.The O atoms neighboring a Zn vacancy center in VZn_g-ZnO model move away from each other as a result of symmetry breaking.As a contrast,three N atoms around VO center move into VZn_g-ZnO supercell.The pristine g-ZnO is non-magnetic.But the magnetic moment of VZn_g-ZnO is 2.00μB in total as a result of symmetry breaking.The partial magnetic moment mainly results from the p-orbitals of the three neighboring O atoms.VO_g-ZnO has no magnetic moment,but possesses the electronic structure with identical spin-up and spin-down.The total magnetic moment of the N-doped NO_g-ZnO is 1.00μB,and the total magnetic moment of N@g-ZnO is 3.00μB.Their local magnetic moments are mainly contributed by the p-orbitals of N atom.The density of states and the spin density are given to analyze the magnetic properties.Based on the supercell local symmetry and molecular orbital theory,the origin of magnetic moment is well explained.The magnetic VZn_gZnO,NO_g-ZnO and N@g-ZnO supercell are found to have a D3 h,D3 h and C3 v local symmetry,respectively,which well explains that their total magnetic moments are 2.00μB,1.00μB and 3.00μB,respectively.The optical absorption characteristics are also discussed.An enhancement of light absorption can be observed for the defective supercells,due to the introduction of defect states into the band gap.The optical transition between gap state and valance band leads to the below band gap absorption.These results are of insightful guidance for understanding properties of graphene-like ZnO monolayer as well as g-ZnO with vacancy and N dopant,and can be theoretically adopted for investigating the nano-electronic devices and catalytic applications based on gZnO monolayer.