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Ab initio and transport models for graphene nanoribbons under mechanical stress and defects

Thomas Lehmann

Diploma Thesis, TU Dresden, October 2013

The scientific community keeps praising the tremendous potential of carbon nanostructures for their application in electronic nanodevices for several years now. Alongside the need for efficient production processes suitable for mass production, their electronic properties should be thoroughly investigated and well understood. Now, this Diploma thesis addresses the electronic transport properties of graphene nanoribbons and carbon nanotubes and in particular the influence of structural defects and mechanical deformation, using a numerical approach based on the Landauer transport formalism and the efficient recursion method for Green functions. A semi-empirical tight-binding model has been adapted for non-ideal systems by employing distance dependent hopping matrix elements. This combination of ab initio and semi-empirical methods enables the calculation and investigation of electron transport characteristics in the quantum coherent regime. The present study emphasizes on long, high aspect ratio graphene nanoribbons with several vacancy types using various concentrations of defects and further applying uniform planar tension to the non-ideal nanoribbons. Since transport characteristics of graphene nanoribbons are found to be very sensitive to edge termination and aspect ratio, and it has been shown that energy gaps can emerge under critical strain, the interplay of both effects needs to be studied. The results demonstrate that band gap engineering using mechanical strain is still practical for non-ideal armchair ribbons with low disorder, as the oscillatory behaviour of the gap is preserved. Additionally, in carbon nanotube systems the effect of Anderson disorder on the electronic properties is addressed and a modular approach for setting up large defected system geometries is presented.

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