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.
|Publication status||Published - 2019|
|MoE publication type||G5 Doctoral dissertation (article)|
- density functional theory, electronic transport, carbon nanotubes