Density Functional-based Tight Binding for novel Nanomaterials : Development of Parameters for Boron and Application of DFTB to Adsorbates on Graphene

Abstract

Nanomaterials with specially tuned properties are an active field of research for many application purposes. These materials are of interest due to their large surface area in catalysis or because of the combination of their dimensions and electrical properties in the field of electronics. A tool to determine these properties and the stability of new nanomaterials is the computational modeling at the atomic scale. With the increase of computational power and the development of approximate methods it is possible to handle systems composed of several hundred atoms with high accuracy. One of the methods used is the Density Functional-basd Tight-Binding (DFTB), which combines computational speed of the semi-empirical methods with the accuracy of more sophisticated methods like Density Functional Theory (DFT). Therefore the method relies on pre-determined parameters, which are needed for every combination of chemical elements present in the problem at hand. The scope of this thesis is two-folded. One point is to provide an extension to an existing parameter set in order to enlarge the application range of said set. Therefore new parameters for the element boron and its combination with hydrogen, carbon and nitrogen have been developed. The performance of the parameters is evaluated by comparison to other computational methods like full DFT for molecular and periodic systems. Computed properties like bond lengths, bond angles, and vibrational frequencies are close to DFT predictions. Hence, the proposed parameterization provides a transferable and balanced description of both finite and periodic systems. The second point is the application of existing parameters to one of the most interesting new materials, graphene, in order to answer the question whether the sublattice symmetry of that system can be broken if atoms adsorb on one of its surfaces. For this purpose different adsorption patterns, adatom concentrations and types, and various degrees of electron doping have been considered. The results show that symmetry breaking should be possible if a specific ratio of adatoms to charge doping is exceeded.