Ni clusters embedded in multivacancy graphene substrates

Abstract

In the present study, we performed density functional theory (DFT) calculations in order to study the structure and stability of small Ni clusters embedded in graphene multivacancy systems. The Bader charges, band structure, total density of states, partial density of states (PDOS), and spin density magnetization were obtained to understand the effects of the cluster size on the electronic and magnetic properties, and thus to determine their potential applications in spintronic devices. Ni cluster adsorption, modified graphene multivacancy substrate behavior, and conversion from a conductor to a semiconductor could be achieved when the cluster size was changed. A linear dispersion relationship around the reciprocal point K was conserved for most of the systems with the inclusion of a band gap between the Dirac cones, which is important for obtaining semiconductors with massless fermions. Comparisons with Ni clusters adsorbed in pristine graphene showed that combining vacancy defects with Ni allows higher band gaps to be obtained. We also analyzed the interactions between Ni clusters and vacancy defects based on the PDOS results. Ni clusters with different sizes could generate ferromagnetic and ferrimagnetic couplings with graphene that exhibited three, four, and six (D2h and D6h symmetry) vacancies. The generation of ferromagnetism when Ni3 and Ni4 absorbed in graphene (with four and six vacancies, respectively) demonstrates both ferromagnetic and semiconductor behaviors can be obtained, thereby making them good candidates for use as dilute magnetic semiconductors. Ni clusters did not magnetize or exhibit slight magnetization on pristine graphene, and the ferromagnetic coupling was only achieved after the introduction of vacancy defects.