Novel carbon-hydrogen nanostructures

by Stojkovic, Dragan.

This work concerns design and prediction of properties of carbon and hydrogen materials on the basis of computer simulations. After an overview of the theory behind computational methods used in actual calculations, pure carbon systems are discussed, starting with the traditional carbon materials graphite and diamond and continuing with the novel structures such as nanotubes and nanoporous carbon. For nanoporous carbon we show how non-hexagonal rings introduce electronic aromatic radicals corresponding to additional bands which narrow the band gap. The main part of the thesis contains investigation of interaction between carbon tubular systems and hydrogen. First, physisorption and chemisorption of hydrogen on tubes are discussed. We found that more curved regions of carbon surface have stronger Van der Waals interaction with hydrogen, leading to an increase in physisorption. In chemisorption, if hydrogen is allowed to access the both sides of the carbon surface, the hydrogen cluster becomes favorable after some initial size, due to a collective stabilization of successive adjacent chemisorbed hydrogen atoms mediated by cooperative alternate distortions in the underlying carbon sheet. Afterwards, we show how chemisorption can be used to change mechanical and electronic properties of nanotubes. We present our two original carbonhydrogen tubular structures, the sp3-only tubes and the eye tubes. Small sp3 tubes are very strong mechanically and insulators electronically, while eye tubes are related to the graphitic ribbons, with unique electronic band structures and a magnetic ground state. The zig-zag eye tubes have band gap sensitive to the applied electric field, while the armchair eye tubes have two very close long flat bands around the Fermi level potentially interesting for terahertz applications. In addition, we propose how those structures could in principle be realized in experiment. iii