Theoretical Investigation On The Nonlinear Optical Responses Of Graphene Based Carbon Nanostructures

Due to the extraordinary thermal, mechanical, optical, and electrical properties, graphene-based carbon nanomaterials have been the exciting objects for experimental and theoretical studies since the first fabrication of graphene. Because of the important role that nonlinear optical(NLO) materials have played in the field of photoelectronics and photonics, the studies of NLO molecules with various properties have always been the key basis for designing and synthesizing new NLO materials. In view of the success in getting graphene nanoribbon with controllable length and width by means of longitudinal unzipping of carbon nanotubes(CNTs), as well as the important role that organic ?-conjugation systems have played in NLO domain, the electronic properties and second order NLO responses of the intermediate structures in the rolling processes to form CNTs from finite size graphene(F-graphene) sheet have been investigated using density functional theory(DFT).On one hand, from a same piece of F-graphene with chemical formula C72H24, metallic(3, 3) and(6, 0) CNT clips can be obtained based on different rolling manners. On the other hand, from the same piece of F-graphene with chemical formula C128H32, metallic(4, 4) and semi-conducting(8, 0) CNT clips can be obtained based on different rolling manners. The results show that the nature of the electronic state appears different variation pattern with different rolling manners, and is significantly affected by the localized edge states of the zigzag edges. Passivation of the intermediate structures with oxygen not only enhances the NLO properties but changes the electronic states of the intermediate structures with passivation at the open zigzag edges. Moreover, the relationships between the change of the frontier molecular orbitals and the properties of the formed CNTs, the change of the energy gap in different rolling processes, and the dependence of the electronic ground states on the size of the F-graphene have also been explored.Considering the decisive influence that the zigzag edge has on the electronic states of the intermediate structures, the zigzag edge of zigzag graphene nanoribbon(ZGNR) has been doped with B and N atoms to get a comprehensive understanding of the localized edge states at the zigzag edges. It is found that the doping of the zigzag edge of the ZGNR with B or N disturbs its electron distributions and further modulates its geometric and electronic structures. In addition, the studies on the NLO responses of the systems find out that the NLO response of the ZGNR was enhanced by doping only one zigzag edge with B or N, but the combination of the B doped zigzag edge and N doped zigzag edge was disadvantageous to improve the NLO responses of the systems.The intermediate structures during the experimental procedure to get GNR through longitudinal unzipping of CNT have been investigated using quantum chemistry methods to understand the correlation between the structures and properties from a microscopic view. Based on the systematic DFT calculations, it has been found that the decisive factor for the electronic states of the intermediate structures is also of great importance NLO properties, and those findings provide a theoretical basis for the future design and syntheses of new graphene-based carbon nanomaterials.