Theoretical Study Of Electronic Structure And Optical Properties Of B_nC_n(N=1-13) Clusters

At present, the research on the modification of graphene with boron has become a hot research topic. As researches of the structures of boron carbide clusters are continually reported, scientists are gradually focus on the physical or chemical properties of them. The main goal of this work is a theoretical description of the electronic structure and optical properties of BnCn(n=1-13) clusters (include anion and cation).To gain the electronic characteristic of these clusters, first principles calculation based on density functional theory (DFT) is employed. Time-dependent DFT is also applied for studying the optical excitation in all clusters.Our results show that the binding energy of BnCn (n=1-13) clusters (include anionic and cationic ones increases with growth of cluster size, which suggests the growth of structural stability. All the binding energies of BnCn (n=3,5,7,9,11,13) clusters are lower than that of BnCn-(n=3,5,7,9,11,13) clusters, but higher than BnCn+(n=3,5,7,9,11,13) clusters. The HOMO-LUMO gaps of the clusters display odd-even oscillations as a function of cluster size n, and the even numbered clusters have a larger gap. By analyzing densities of states, it is clear that the highest occupied orbitals and lowest unoccupied orbitals are mainly constructed by p orbitals of boron and carbon atoms.As for the optical properties, we focus on the optical absorption spectrum of neutral BnCn(n=4,6,8,10,12) clusters, anion BnCn-(n=3,5,7,9,11,13) clusters and cation BnCn+(n=3,5,7,9,11,13) clusters. We found that the absorption peaks become abundant with the increase of the number of atoms in the cluster. This can be explained by the increase of the molecular orbitals of the cluster, resulting the electron transition occur more frequently. The value of the first excitation energy of the optical absorption spectra of the three kinds of clusters is significantly related to the HOMO-LUMO energy gap of the corresponding clusters. The clusters with larger gaps always possess higher first optical excitations. Furthermore, the places of absorption peaks are related to the densities of states. The position of the absorption peak of the cluster has a great relationship with the density of states, and the intensity of the absorption peak corresponds to the larger density of states.