Browsing by Author "Ketolainen, Tomi"

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  • Computational studies of disordered graphene and graphene nanoribbons
    (2012) Ketolainen, Tomi
     School of Science | Master's thesis
    Graphene is one of the most studied carbon nanomaterials today. Carbon atoms in grapheme form a hexagonal lattice that consists of only one atom layer. This structure leads to interesting electronic properties and makes it possible to examine physical phenomena not usually seen in other materials. A special feature of graphene is the linear band structure near the corners (Dirac points) of the first Brillouin zone. Moreover, graphene has no band gap and the density of states at the Fermi level vanishes. Graphene also possesses very good mechanical and optical properties, which is important from the point of view of applications. Real graphene sheets may have some disorder that results, e.g., from the fabrication method. Small point defects, such as vacancies and impurities, have been observed to induce non-zero localized states in graphene, which causes a sharp peak in the density of states near the Fermi level. In addition, the vacancies and impurities influence the electronic and transport properties of graphene nanoribbons. In this work, the local density of states is calculated in the vicinity of point defects in grapheme and zigzag graphene nanoribbons. The computations are based on a tight-binding model and the number of atoms in the systems is 4000-16800. In particular, the coupling of a single vacancy to the edge state of a zigzag nanoribbon is studied. Furthermore, the local density of states is computed in the middle of an impurity cluster placed on a graphene sheet and on a zigzag nanoribbon. Vacancies and impurities induce similar localized states near the Fermi level and the states enhance in the cluster configuration. The local density of states at the Fermi level has a peak that can split if the defect lies close to the ribbon edge. Armchair- and zigzag-shaped impurity rows on a zigzag nanoribbon are also considered. Only the zigzagshaped impurity row is found to induce a peak at the Fermi level.
  • Electrical conductivity of functionalised carbon nanotube networks
    (2019) Ketolainen, Tomi
     School of Science | Doctoral dissertation (article-based)
    The fabrication of novel electronic devices requires new kinds of materials. The use of carbon nanotubes (CNTs) in various applications has already been demonstrated and therefore the CNTs are also important carbon materials in addition to graphene and fullerenes. Because the electronic properties of individual CNTs depend on their atomic structures, the individual CNTs are not possibly the best choice for building new electronics. Instead, the new devices could be made using thin films or networks of CNTs. The CNT thin films are transparent, flexible, and conduct electricity. Hence, the CNT thin films are expected to be utilised in a remarkable amount of applications including transistors, touch screens, and solar cells. However, a significant challenge related to the CNT thin films is making a film with both high conductivity and transparency simultaneously. Several methods to improve the conductivity of CNT networks have been studied experimentally. The goal of this thesis is to investigate a few methods to increase the conductivity of CNT networks by using density functional theory combined with the standard Green's function electron transport calculations. In particular, the conductance of junctions of CNTs is examined since the CNT junctions mainly determine the conductivity of the whole network. Two different approaches to improve the electrical conductivity of CNT networks are studied. The conductivity can be enhanced by depositing group 6 transition metal (TM) atoms on the CNT networks because the TM atoms are able to link the CNTs. The four-terminal electron transport calculations show that Cr, Mo, and W linker atoms enhance the conductances of the CNT junctions in a similar way. The increase in the conductance is related to the strong hybridisation between the carbon and TM atom orbitals. The second approach is based on functionalising the CNTs with molecules. The interaction of AuCl4 molecules with CNTs leads to a p-type doping effect. In addition, the doping of CNTs with nitric acid is studied and the NO3 molecules also cause a p-type doping effect in CNTs. Interestingly, the doping effect is larger in semiconducting CNTs than in metallic ones. Moreover, water molecules near the NO3 molecules enhance the doping effect. The electron transport through the CNT junctions can be increased by doping the CNTs with AuCl4 or NO3 molecules and no linker molecule is needed if the concentration of the molecules on the CNTs is high enough. A central result is the pinning of the Fermi level to the van Hove singularities and flat molecular states. The results of our work also improve the understanding of previous experimental studies.
  • Galliumarsenidin saturoituvan absorption mittaus
    (2011) Ketolainen, Tomi
     Perustieteiden korkeakoulu | Bachelor's thesis