Zigzag Graphene Nanoribbons Band Structure And Absorption Spectra

Since single-layer graphite (graphene) was successfully manufactured in 2004, graphene has excited scientists' wide attention and interest for its unique mechanical, electronic and optical properties. Today, with the rapid development of science and technology, graphene has become a new kind of carbon nano materials, from which graphene nanoribbons (GNRs) are produced. GNRs have a bright future in application. GNRs are materials with only one dimension. The geometry mainly decides electronic structure, so it is of great importance to study the geometry of GNRs. According to its' different edges, ZGNRs are mainly clas-sified to zigzag graphene nanoribbons (ZGNRs) with zigzag edges and armchair graphene nanoribbos (AGNRs) with armchair edges.Based on the tight-binding model, in ZGNRs the influence of boundary structure on the band structure, specially the electrons of the valence and conductor bands near the Fermi level, are studied in detailed. We construct the band structure and distribution of electrons on different atoms in. a unit cell on the valence band near the Fermi level of ZGNRs with seven reasonable boundary structures. We find that NN-ZGNRs with no dangling atoms at both two edges, DN-ZGNRs with dangling atoms only at one edge, SPP-ZGNRs and ASPP-ZGNRs with pentagons at both two edges are metallic. DD-ZGNRs with dangling atoms at both two edges, PN-ZGNRs with the defective structure of pentagons at one edge and no dangling atoms at the other edge, PD-ZGNRs with pentagons at one edge and dangling atoms at the other edge are semiconducting, and the energy gap is inversely proportional to the width of nanoribbons. But for DD-ZGNRs and PD-ZGNRs, the energy gap quickly reduces to zero with the increasing of width; for PN-ZGNRs, the energy gap decreases in index to a limited number of 0.154eV. It's found that different boundary structures have different effect on the distribution of electrons on the valence band near the Fermi level. And the probability of electrons on the valence band near the Fermi level on the atoms of two edges of nanoribbons is relatively large. Furthermore, the optical properties of ZGNRs are studied. We calculate the absorption optical spectrum with the gradient approximation and demonstrate the main electronic excitations and changes of distribution of electrons before and after excitations. We find that the optical spectrum has connection with both velocity matrix element and joint density of states (JDOS).