Optical properties of a hexagonal C/BN framework with sp and sp hybridized bonds.

We investigated the optical properties and roles of sp- and sp-hybridized bonds of a hexagonal C/BN family using first-principles calculations. The calculated phonon dispersions confirm the dynamic stability of Hex-(BN)C and Hex-C(BN). The complex dielectric function evolves from the infrared to the ultraviolet region and has a significant anisotropy for different polarizations. The reflectivity and refractive index spectra show that the sp-hybridized C atoms are more sensitive to the light from infrared to visible region than B-N pairs while the C atoms and B-N pairs have a similar sensitivity to high frequencies. The sharp peaks of the energy-loss spectrum are all concentrated in the 23-30 eV energy region, which can be used to identify these hexagonal structures. The calculated band structures show Hex-C and Hex-(BN)C are metals, but Hex-C(BN) and Hex-(BN) are semiconductors with indirect band gaps of 3.47 and 3.25 eV, respectively. The electronic states near the Fermi level primarily originate from sp-hybridized atoms. In addition, sp-hybridized bonds are the main elements affecting the optical and electronic structure of C/BN materials with sp- and sp-hybridizations. We expect that the results presented will help understand the optical properties of C/BN materials containing sp- and sp-hybridized C atoms and B-N pairs.