Electronic structures and conductance of carbon nanotubes

Electronic structures of finite length single-walled carbon nanotubes (SWNT) with up to more than 500 carbon atoms are calculated using density functional theory (DFT). The HOMO-LUMO gap results from the fixed geometry and optimized geometry structures can be categorized into three families. One of the families, 3Q, shows a nearly constant HOMO-LUMO gap versus SWNT length. The simulation results from analytical Coulson model confirmed that one family of the SWNT electronic structures has a constant HOMO-LUMO gap before reaching a "critical point" where it begins decreasing. The electronic Structures of fixed geometry and optimized geometry SWNT are also compared.; The SWNT-molecules-SWNT junction calculations inspired by recent experiments are modeled with tight-binding theory and the non-equilibrium Green's function formalism. Three situations: polyene, polyacene and two parallel polyenes are studied in the model. The conductance through the junction is calculated in idealized situation that the resonance peaks are on/off the Fermi energy. The maximum conductance for one molecule reconnected junctions is 1 G 0. The interference between two reconnection contact points is also studied. For the case two or more molecules in reconnection of the junctions, the conductance varies from 0 to 2 G0.