Density Functional Theorey Investigations Of Carbon-based Nanostrucutres Involving Weak Interaction

Carbon based nanostructures, including carbon nanotubes (CNTs), fullerene, and graphene, have attracted extensive research interests, due to their special structures and novel electronic, electronical, optical, and magnetic properties. Noncovalent weak interactions are ubiquitous in the carbon based nanostructures. Information on the weak interactions is essential for understanding the structures and properties of carbon based nanostructures, and improving design strategies for the relative nanodevices. The intermolecular interactions are also of utmost importance and research interest in many areas of chemistry, biology and materials science. The experimental measurements of weakly-binding systems require a number of assumptions, and the diverse experimental studies yielded apparent contradictory results. In this regard, theoretical calculations are anticipated to provide more detailed information on the origin, strength and orientational dependence of intermolecular interaction and its influence on the various properties of the systems. The work detailed in this thesis involves the application of density functional theory (DFT) and DFT-based tight binding (DFTB) calculations in the investigations of theπ-πinteractions and hydrogen bonds in various carbon based nanostructures.Firstly, theπ-πinteraction in the various configurations of benzene dimers was studied by using a DFT method, augmented with an empirical R-6 term for the description of long-range dispersive interaction (DFT-D). It shows that, compared with the data from previous ab initio calculations, our method provide sufficiently accurate binding energies and equilibrium intermolecular geometries, when using PBE functional and moderate polarized triple-ζquality basis sets. This consistency shows considerable promise that our method can be utilized to evaluate theπ-πinteractions involving larger biological, organic, and even infinite systems.Secondly, the interlayerπ-πinteraction between graphene sheet models was investigated by using the above DFT-D method. For the stacked graphene sheet model dimers and multilayers, it is found that their binding energies and HOMO-LUMO energy gaps depended strongly on their sizes, while the orders and numbers of stacking display minor influence. The significantly broad distribution of energy gap ranging from 1.0 eV to 2.5 eV, due mainly to the size-dependence effect, indicates that the well-designed graphene sheets or nanomaterials composed ofπ-πinteracting polycyclic aromatic hydrocarbon fragments will show luminescence at different energy locations.Thirdly, it has been reported that water molecules can form cylindrical crystalline structures, referred as ice nanotubes (INTs), by hydrogen bonding under confinement within CNTs. In this study, the geometry structures and vibrational infrared (IR) spectra of various INTs were systematically studied, using the dispersion corrected DFTB method (DFTB-D). Compared to the conventional hexagonal ice Ih phase, the vibrational bands of INTs show red- or blue-shift in their IR spectra, because the hydrogen bond networks of the INTs are weaker than that of ice-Ih phase.Fourthly, the bulk modulus of solid face-centered-cubic (fcc) C60 was calculated with DFTB-D. The predicted modulus of 9.1 GPa is in good agreement with the experimental measurements. It is found that, under the compression, the structures of C60 molecules are considerably changed, due to the variations in the type of hybridized carbon atoms, which also induces the reduction of HOMO–LUMO energy gap of the system.