Tailoring the electronic structure properties of carbon based materials

Abstract

Graphene, a two-dimensional layer of carbon atoms arranged in a honeycomb lattice is the building block of graphite, carbon nanotubes and fullerenes. Prototypes of graphene electronic devices have been already made in various laboratories worldwide, outperforming the conventional semiconductor transistor technology. However, at this point, the lack of a true energy gap in graphene makes the integration of such electronic devices in commercial products (e.g. computers) impossible. It is well established that structural defects or adsorbents lead to a local change in the density of states of materials. This work describes and quantifies the role of chemisorbed atoms on the local density of states carbon based materials, such as graphite and carbon nanotubes as a function of material dimensionality. The thesis first presents a general introduction, followed by two chapters related to more specific theoretical and experimental knowledge on the subjects discussed in the thesis. Highly Oriented Pyrolytic Graphite (HOPG) has been extensively studied over the last decades but the role of structural defects and/or chemisorbed species on its electron local density of states (LDOS) has not been well understood. Chapter four is focused on characterizing the LDOS of HOPG after chemisorption of hydrogen and deuterium atoms using a scanning tunneling microscope. The electronic structure study was performed in the temperature regime of 4 K to 120 K and as a function of hydrogen coverage on HOPG. The results show that the LDOS of HOPG is reduced around the formed H-islands. If the hydrogen coverage is relatively large and the sample temperature is low (4 K), an energy gap is observed. However, for low hydrogen concentrations and hence smaller H-islands, the LDOS at the Fermi-level has an unexpected Fano-lineshape, indicating that localized electron states of HOPG character are coupled to continuum states such as phonons. At low temperatures, the LDOS at the Fermi-level is reduced over a larger spatial region around the H-islands compared to the high H-coverage case. Since their discovery, carbon nanotubes have attracted the interest of both fundamental and applied research due to some interesting physical properties like: the elastic modulus, the tensile strength, the high thermal conductivity and the ballistic conduction regime. Single walled carbon nanotubes (SWCNT) can be semiconducting or metallic depending on their chirality which is re ected in the DOS. In the quest for understanding the infl uence of chemisorbed species on the LDOS of carbon based materials, chapter five summarizes the results obtained on hydrogen decorated metallic SWCNT. It has been observed that upon hydrogenation, metallic SWNCTs can become locally semiconducting. Furthermore, for a short section (delimited by hydrogen patches) of a metallic SWCNT quantum confinement of electrons has been measured. The results obtained for both carbon-based materials upon hydrogenation are compared in the last part of this chapter. Chapter six of the thesis is aimed at examining the possibility of measuring persistent currents on an ensemble of gold nano-rings prepared by a colloidal lithography method. This method allows the preparation of metallic nanostructures bringing the system more towards the ballistic conduction regime, which facilitates the comparison between theory and experiment. Although persistent current measurements were not performed, the magnetic response of the various prepared samples is intriguing. The origin of largely varying magnetic responses is not fully understood yet but the observations hint to peculiar magnetic behavior of thin metallic film deposited on insulating substrates.