Induced ferromagnetism in bilayer hexagonal Boron Nitride (h-BN) on vacancy defects at B and N sites

We have investigated the electronic and optical properties of bilayer AA′ stacked hexagonal Boron Nitride (h-BN) with B and N vacancy defects by using density functional theory (DFT). The two single layers of h-BN are stacked in layers to form the h-BN bilayer. The inter-layer interaction between the two layers of h-BN bilayer is governed by the introduction of van der Waals potential (vdW). The calculated energy band gap for the pristine h-BN bilayer is found to be ~4.56 eV. The density of states (DOS) and electronic band structure showed that both Boron and Nitrogen vacancies in bilayer h-BN results in magnetic ground state. Considering the presence of 1,3,4-Boron vacancies, half-metallic character is observed. However, the 2 Boron vacancy resulted in metallic character. The bilayer with 1,2,3,4-Nitrogen vacancies preserved the semiconducting band gaps of different width in both the spin channels which are significantly less than the pristine band gap. Also, B and N vacancy induces ferromagnetism in the h-BN bilayer. The maximum total magnetic moment for the Boron vacant system is 6.583μ_B in case of 4-Boron vacancy defects. In case of Nitrogen vacancy system it is found to be 3.926 μ_B for 4-Nitrogen vacancy defects. The optical response of the system is presented in terms of the absorption coefficient, refractive index and dielectric constant for pristine as well as for the defective configurations. Negative value of dielectric constant for Boron vacant system has been observed in the energy range 0.9–1.4 eV and for Nitrogen vacant system the energy range 0.5–0.8 eV. These results open an opportunity for it's utilization in the negative index optical materials. The current study shows that B and N vacancies in bilayer h-BN could have potential applications in nano-structure based electronics, optoelectronics and spintronic devices.