In the past decade the interest in molecular electronic devices has escalated. The synthesis of molecular crystals has improved, providing single crystals or thin films with mobility comparable with or even higher than amorphous silicon. Their mechanical flexibility admits new types of applications and usage of electronic devices. Some of these organic crystals also display magnetic effects. Furthermore, the fullerene and carbon nanotube allotropes of carbon are prominent candidates for various types of applications. The carbon nanotubes, in particular, are suitable for molecular wire applications with their robust, hollow and almost one-dimensional structure and diverse band structure. In this thesis, we have theoretically investigated carbon based materials, such as carbon nanotubes, pentacene and spiro-biphenalenyl neutral radical molecular crystals. The work mainly deals with the electron structure and the transport properties thereof. The first studies concerns effects and defects in devices of finite carbon nanotubes. The transport properties, that is, conductance, are calculated with the Landauer approach. The device setup contains two metallic leads attached to the carbon nanotubes. Structural defects as vacancies and bending are considered for single-walled carbon nanotubes. For the multi-walled carbon nanotubes the focus is on inter-shell interaction and telescopic junctions. The current voltage characteristics of these systems show clear marks of quantum dot behaviour. The influence of defects as vacancies and geometrical deformations are significant for infinite systems, but in these devices they play a minor role. The rest of the studies concern molecular crystals, treated with density-functional theory (DFT). Inspired by the enhance of the electrical conductivity obtained experimentally by doping similar materials with alkali metals, calculations were performed on bundles of single-walled carbon nanotubes and pentacene crystals doped with potassium. The most prominent effect of the potassium intercalation is the shift of Fermi level in the nanotube bands. A sign of charge transfer of the valence electrons of the potassium atoms. Semi-conducting bundles become metallic and metallic bundles gain density of states at the Fermi level. In the semi-conducting pristine pentacene crystals structural transitions occur upon doping. The herringbone arrangement of the pristine pentacene molecules relaxes to a more π-stacked structure causing more dispersive bands. The charge transfer shifts the Fermi level into the lowest unoccupied molecular orbital band and turns the crystal metallic. Finally, we have studied molecular crystals of spiro-biphenalenyl neutral radicals. According to experimental studies, some of these materials show simultaneous electrical, optical and magnetical bistability. The electronic properties of these crystals are investigated by means of DFT with a focus on the possible intermolecular interactions of radical spins.

The solid state of free radicals of bis(9-cyclohexylimino-1-oxyphenalenyl) boron is one of few neutral molecular systems which exhibit high conductivity at room temperature. The system is crystalline but highly anistotropic. We have performed band structure calculations based on the density functional theory on this system. It is evident from these studies that the system is quasi-one-dimensional with strong intermolecular interactions along one crystal direction. The bandwidth is 0.424 eV along this direction as compared to 0.055 eV perpendicular to it. Despite the quasi-one-dimensional character of the system, no signature of a Peierls distortion towards a dimerized state is observed. We argue that this is due to the fact that the Fermi energy lies slightly below the middle of the conduction band. In this case the electronic driving force for the distortion to occur is weakened and overcome by the restoring force of the lattice. Also the optical properties of the system have been investigated and the origin of the low lying excitations in the system has been clarified.

We calculate the ground state geometrical structure of potassium-intercalated pentacene lattices using molecular mechanics and the density-functional theory. Both methods result in a structural phase transition in going from the pristine form to the intercalated state with one potassium ion per pentacene molecule. The phase transition is characterized by a sliding of adjacent pentacene molecules relative to each other. The electronic properties of this phase is studied with the density-functional theory. As a result of the geometrical changes, the - overlap in the direction perpendicular to the molecular planes of the layered pristine pentacene structure increases substantially and many of the electronic bands show strong dispersion in this direction. The Fermi energy of the doped phase appears in the middle of the conduction band where the density of states is maximum. The bandwidth of the conduction band is 0.7  eV.

We present results from density-functional theory calculations on the geometrical and electronic structure of potassium-intercalated (4,4) armchair and (7,0) zigzag single-walled carbon nanotubes. Intercalation of potassium results in notable changes in the geometrical structure, in particular in the zigzag system in which the carbon–carbon bond lengths in the unit cell vary between 1.40 Å and 1.45 Å. The most prominent effect of K intercalation on the electronic band structure is a shift of the Fermi energy which occurs as a result of charge transfer from potassium to the carbon nanotube. In the case of the potassium-intercalated (7,0) nanotube the band structure and the position of the Fermi energy indicate a very good metallic conductor. The (4,4) nanotube has the potential to become superconducting due to the very high density of states at the Fermi energy which is obtained at high intercalation densities.

The effects on charge transport of inter chain interactions in conjugated polymers and inter shell interactions in multi-wall carbon nanotubes is studied theoretically. In the case of conjugated polymers the charge carriers are polarons, which in regions of well ordered chains extend over several such chains. When an external electric field is applied, the polarons drift rapidly through these regions. The disordered regions, however, act as barriers for charge transport and a fairly strong field is needed for the polaron to hop across these regions. Although the inter shell orbital overlap in multi-wall carbon nanotubes is larger than between polymer chains, the effective interaction is weak and the current through a multi-wall carbon nanotube is dominated by the transport along a single shell.

A computational study of the electron transport properties of multiwall carbon nanotubes (MWNT’s) is presented. The study is focused on the role of intershell interactions and the resistance associated with transport between neighboring shells. The tight-binding approximation is used to describe finite double-wall nanotubes [(10,10)@(5,5)] connected to two semi-infinite metallic leads. The current-voltage characteristics are calculated from the multichannel Landauer formula. When contact is made to the outer shell the calculations show that the current is distributed over both coaxial NT’s but the overall resistance is independent if the inner tube is present or not. A device in which the leads are contacted to different coaxial shells of the MWNT (telescopic junction) is also investigated. The resistance of this system depends crucially on the extent of delocalization of the wave functions over several NT’s.

Electron transport through nanotubes is studied theoretically using the Landauer formalism. The studies are carried out for finite metallic nanotubes that bridge two contacts pads. The current is observed to increase stepwise with the applied voltage. Each step corresponds to resonance tunneling including one single-particle eigenstate of the nanotube. Moderate bending of the nanotube results in a shift of the single-particle levels but the overall current remains essentially unaffected. For large bending, however, the π electron system becomes more disturbed, which introduces backscattering and a marked decrease in the conductivity along the tube. A single carbon vacancy in the nanotube is shown to have very small effect on the conductivity in the center of the metallic band whereas, by increasing the defect concentration the conductivity decreases in the same way as for the strongly bent tubes.