Fullerene Molecules as Building Blocks for Nanomaterials 1308 views

The study of surfaces at the atomic and molecular scale has improved our understanding of many physical and chemical processes such as oxidation, carburization and has contributed critically to advancements in nanoscience and technology. In this thesis, fullerene (C60) molecules were used as a critical building block for modification, enhancement, and material synthesis for two distinct material systems. The first material system included in this study was graphene on copper substrates. Intercalation of C60 molecules at the interface between copper and graphene by annealing was used to modulate the electronic properties of graphene. The intercalation was confirmed by comparing topography and electronic properties of the graphene-C60-copper system with graphene wrinkles and moiré patterns. The intercalated molecules create a local strain/deformation on the graphene layer, and its magnitude is controlled by the intermolecular distance. This study provided a pathway to control local strain, which can influence the electronic properties of graphene. Tungsten carbide thin films were the second material system studied in this thesis. These thin films were synthesized by co-deposition of C60 molecules as carbon precursor and tungsten atoms on a MgO(100) substrate, which was held at 1073 K. Prior to thin film synthesis, a number of experiments designed to study the interaction between C60 and W during annealing at moderate temperature revealed remarkable results about this interaction. Scanning tunneling microscopy/spectroscopy (STM/STS) measurements demonstrated that the surface band gap of the C60 molecular layer can be adjusted from a wide band gap (>2.5 eV) to a metallic molecular surface during annealing from 300 to 800 K. Density functional theory calculations were used to develop possible reaction pathways and explain the experimentally observed electronic structure modulation. Tungsten carbide shows a unique combination of properties and exhibits catalytic activity, which makes this material an attractive substitute for expensive noble metal catalysts. However, the use of tungsten carbide thin films is limited by surface oxidation, which is accompanied by a depression of activity. In this thesis, non-stoichiometric carbon rich tungsten carbide thin films were prepared to study the possibility of carburization of the oxidized regions via embedded free carbon in the thin films. Thin films with different C:W ratios were synthesized to illustrate the effect of free carbon on surface morphology and susceptibility to oxidation. Several oxidation/ carburization cycles were performed on carbon rich tungsten carbide films and the results indicated that the annealing of these films results in diffusion of free carbon from lower layers and re-carburization of the oxidized area. Therefore, this is a viable method to significantly increase the lifetime of the catalytic surface of tungsten carbide. For this study, synchrotron-based ambient pressure x-ray photoelectron spectroscopy (AP-XPS) and STM/STS measurements, were performed.

PHD (Doctor of Philosophy)

Scanning tunneling micorscopy; Superlattice; Intercalation; Graphene; Density functional theory; Scanning tunneling spectroscopy; Molecule-metal interface; Tungsten Carbide; Electronic structure

English

2017-03-18