Electron transport through low-dimensional carbon-based systems

Abstract

Carbon-based materials in all their facets yield an extremely large area of electronic applications. Its quasi-unlimited availability, wide ranging tuneability and scalability make them a potent rival to the currently dominant silicon technology providing the possibility to overcome its limitations. This thesis addresses fundamental understanding of electronic transport processes in three selected systems. Graphene is well know for its exceptional electronic character and chemical stability. Here an approach on utilizing graphene as a sensor to molecular specimen of high sensitivity is presented. By adsorbing atomic hydrogen, the inert character of graphene is broken and even smallest amounts of hydrogenation is found to increase the resistance by orders of magnitude. The framework of Anderson localization describes the electron transport in hydrogenated graphene, showing a clear correlation between the localization lengths, the mean free path and the coverage. Upon hydrogenation the band transport of graphene is substituted by variable range hopping. Activation energies for this activated transport were found to range from 26 meV to 48 meV. Those values do not refer to the band gap observed in hydrogenated graphene around the Dirac point and exclude an ordinary band insulator. Furthermore, PbPc molecules were adsorbed to pristine and hydrogenated graphene. While the molecules show no influence on the electronic properties of pristine samples, they significantly interact with the chemisorbed H atoms. Density functional theory shows a barrier-less reaction between the H atoms and the PbPc molecules with an energy gain up to 2.8 eV. This annihilation process is very efficient and restores the transport properties of pristine graphene. Contrary to the two-dimensional graphene carbon can also be arranged in one-dimensional fibers and tubes. Extended compound materials of these fibers are heavily discussed in energy storage technology. The electronic transport properties are investigated by means of four-point probe microscopy. A major influence on the conductivity of this materials is the degree of ordering inside the structures. Pristine and templated carbon nanofibers were analyzed and a strong relation of their internal structure and the resistance is found. Improved crystal quality lowers the resistivity of the single fibers as well as pf the compound material. The compounds were investigated by probe distances in the range of the building blocks as well as at significantly larger scales. The latter allows the system to be treated as two-dimensional and isotropic. The measurements combined with simulations yield a description of the conduction tensor as a combination of band conduction inside the microscopic building blocks and hopping conduction across the fibers. As an ultimate miniaturization of conductive carbon materials, spin selective chiral molecules are heavily investigated. Here measurements of single polyalanine molecules by means of mechanically controllable breakjunctions are presented. Transport dominated by tunneling mechanisms has been found by investigating molecules of different length. For lengths between 2.6nm and 5.4 nm, conductance values ranging from 0.007G0 to 0.01G0 were determined, with a reduction factor of β = 3.5nm−1. Symmetric IV characteristics indicate a interdigitation process of the molecules in between the gold contacts. A ratcheting model is proposed to explain the transport properties in regard to the geometrical configuration of the molecules.