ZnO 벌크와 표면에서 공극에 의한 강자성 유도:밀도범함수론 계산

Abstract

It has been reported that room-temperature ferromagnetism was represented in various inorganic nanoparticles. It was suggested that vacancies at the surface are origin of the unexpected ferromagnetism since ferromagnetism vanished as the size of nanoparticles increased. Since ZnO is one of the most attractive materials for its various novel physical properties, it was theoretically represented that the effects of vacancies in a ZnO nanocrystal using the first-principles calculations. Here, the nanoparticle was assumed to shape as a faceted particle with surfaces having low surface formation energies. Thus, it can be simplified that vacancies at a nanoparticle to a problem of vacancies at the bulk or surfaces. In order to describe the position of defect levels within the band gap, on-site Coulombic repulsion was rendered on strongly localized electrons at Zn 3d with DFT+U method, where U equals to 6.0 eV. It was found that 2 μB, 1 μB, and 2 μB of magnetization were induced per a Zn vacancy at the bulk, m-plane (NP1), and a-plane (NP2) surface, respectively, while no ferromagnetism was induced by an O vacancy in any case. It was due to a triply degenerate tg states formed by a Zn vacancy in a molecular orbital theory. It was interesting to find that a Zn vacancy at the NP1 surface had a local minimum state which is localized at the Zn vacancy site, while the ground state has a relatively extended nature to the surface chain direction. It was found that Zn vacancies at the NP1 surface coupled with each other in an anti-ferromagnetic configuration, and the range was short. Only in the case of vacancies positioned in the adjacent chain represented ferromagnetic ordering with 70 meV of an exchange energy.