First-principles Study On The Interface Properties Of Cubic Boron Nitride/Hexagonal Boron Nitride

As one of the superhard materials, cubic boron nitride (cBN) is widely used in photoelectric device and high-speed cutting field. However, synthesizing cBN single crystal with large particles and high quality is restricted as the synthesis mechanism is ambiguous. cBN is transformed from hexagonal boron nitride (hBN), and cBN/hBN interface is formed in the process of transformation. In this paper, interface poperties of cBN/hBN(hexagonal boron nitride), including structures, work of adhesion and interface electron structures, are investigated by first-principles method based on density functional theory, which provide theoretical basis for the synthesis mechanism of cBN single crystal. Our paper are mainly focus on the following several aspects.Firstly, the crystal structures of cBN and hBN are imported from materials studio and inorganic crystal structure database. Considering terminated atoms, slab thickness and cycle period, we cleave 6 different terminated cBN(111) slab with 12 atom layer, 2 different terminated cBN(110) slab with 6 atom layer,4 different terminated cBN(111) slab with 8 atom layer and hBN(0001) slab. After redefine lattice and lattice matching, we construct 6 different type of cBN(111)/hBN(0001) interfaces,2 different type of cBN(110)/hBN(0001) interfaces and 4 different type of cBN(100)/hBN(0001) interfaces.And then we calculate the work of adhesion (Wad) upon geometry optimization and cell optimization. Besides, a comparative analysis is conducted to reveal how the interface structures change during the optimization. The results indicate that all the cell volume of the interface structures reduce after optimization except structure a [hBN(0001)/cBN(111)-4B-ter]and structure e [hBN(0001)/cBN(111)-1B-nearOB-ter] of cBN(111)/hBN(0001) interface. And the result of work of adhesion show that cBN(110)/hBN(0001) interface is unstable as the value of Wad is negative, while cBN(111)/hBN(0001) interface is much more stable, and the nitrogen atom terminated structure is much more stsble than the boron atom terminated structure.Finally, in chapter four, we study electron structures of the interface. The result of band structure indicate that for cBN(111)/hBN(0001) interface, structure a and e are insulator, structure c is direct-gap semiconductor, structure b、d and f are metallic because their energy gap value is 0. There is a one-to-one correspondence between band structure and density of state (DOS). What is more, the DOS pattern show that among all the three energy level ranges, the lowest energy range is mainly derived from the s orbit of nitrogen atom, the middle energy range is primarily ascribed in p orbit of nitrogen atom and the highest energy range is from the p orbit of boron atom. It can be seen from the electron density and electron difference density that there is obvious charge transfer and new chemistry bond at the interface of cBN(100)/hBN(0001) structure. The result of population ndicate that the new bond forming at cBN(100)/hBN(0001) interface is strong covalent bond because the value of Mulliken population is 1.In summary, for cBN/hBN structure, cBN(100)/hBN(0001) interface is the most stable and new strong covalent bond is found at the interface. And cBN(110)/ hBN(0001) interface is the most unstable.