The Properties Of Defective Two-dimensional H-BN Sheet:a First-principles Study

Since the synthesis of single-layer hexagonal boron nitride (h-BN) honeycomb structure, much attention has been focused on this low dimension nanoscale material. This is not only due to their fascinating properties, such as high chemical stabilities, excellent mechanical properties, high melting points and large band gap, but also due to the geometric similarity of planar, two-dimensional (2D) h-BN to graphene. A pristine2D h-BN sheet is a nonmagnetic nanomaterial, and the finding of wide bandgap is a substantial obstacle for its applications in electronics. As we all know, the existence of different kinds of defects such as impurities, substitutional atoms and vacancies is an effective way to manipulate the electronic and magnetic properties of a material.In this work, we performed first-principles pseudopotential calculations within density-functional theory to investigate the geometric, electronic and magnetic properties of vacancies in a single h-BN sheet. By applying a full spin-polarized description to the system, we show that the boron vacancy (VB) or the nitride vacancy (VN) can induce spontaneous magnetization, while the boron-nitride vacancy (VB+N) is spin unpolarized. More interestingly, the magnetic moment of N atoms near the VB vacancy are0.700,0.701and-0.538μB, respectively. The electronic structure calculation result show that there are no energy states at the Fermi level for VB in single h-BN sheet, revealing semiconductor behavior.Moreover, we also studied the structural, electronic and magnetic properties of2D h-BN sheets with3d transition-metal (Mn, Fe, Co, Ni) atom adsorption on or embedment in three types of vacancies:VB, VN, VB+N. Our calculation result indicates that most adsorbed configurations, except Mn atom on VB+N system, are more stable than the embedded situations in h-BN sheets and exhibit interesting phenomena. For instance, the band gaps and magnetic moments of h-BN sheets can be tuned depending on the doped dopant species and vacancy types. In particular, adsorption such as Mn and Ni on VB lead to half-metallic BN sheets where the spin-polarization reaches100%at the Fermi level, which can be used as potential spin-filtering material in spintronics. A Fe adsorbed VB system becomes a narrow-gap semiconductor with a magnetic moment of5.0μB.Surprisingly, the configurations of Co adsorbed on VB and VN are not magnetic, while the Co adsorbed on VB+N system has a high magnetic moment. From the investigation of the charge density redistribution, we found the magnetic moment of single TM atom reduced mainly due to the charge transfer from4s into3d states of TM atoms as well as the charge transfer from3d majority spin into3d minority spin states. Finally, the minimum energy path (MEP) for impurity-atom migration from their initial metastable sites to the last steady sites in defective h-BN sheets has been further calculated by using Transition State Theory.