No description available.

No description available.

The investigation and control of molecular properties is currently a dynamic research field. Here I present molecular level studies of porphyrin molecules adsorbed on metal surfaces via Low Temperature Scanning Tunneling Microscopy/Spectroscopy (STM/STS), supported by complementary X-ray absorption experiments. Intermolecular and molecule-surface interactions of tetrapyrdil porphyrin (TPyP) on Ag(111) and Cu(111) were investigated. TPyP self-assembles on Ag(111) over a wide sample temperature range into large, highly-ordered 2D chiral domains. By contrast, adsorption of TPyP on the more reactive Cu(111) leads to temperature dependent assemblies, governed decisively by the strong substrate influence. The increased metal-surface interactions on Cu(111) are accompanied by a conformational distortion of the porphyrin macrocycle. The TPyP’s pyridil groups were coordinated with single iron molecules, forming metal-organic complexes. Furthermore, the porphyrin’s macrocycle was metalated by exposing a layer of well-ordered TPyP to an iron atom beam, demonstrating a novel approach towards the fabrication of metallo-tetraaryl porphyrins performed in two dimensions under ultrahigh vacuum conditions. This method was similarly used to form lanthanideporphyrinates by coordinating tetraphenyl porphyrin (TPP) macrocycles with cerium. The influence of the metal center on the porphyrins’ electronic structure was investigated via STS for TPP, TPyP,Fe−TPyP, Fe−TPP, Ce−TPP, and Co−TPP, whereby the inhomogenous electron density distribution associated with individual frontier orbitals were imaged via dI/dV mapping. The symmetry and form of the molecular orbitals could be directly correlated to the saddle-shaped conformational adaptation for the case of Co −TPP.

The investigation and control of molecular properties is currently a dynamic research field. Here I present molecular level studies of porphyrin molecules adsorbed on metal surfaces via Low Temperature Scanning Tunneling Microscopy/Spectroscopy (STM/STS), supported by complementary X-ray absorption experiments. Intermolecular and molecule-surface interactions of tetrapyrdil porphyrin (TPyP) on Ag(111) and Cu(111) were investigated. TPyP self-assembles on Ag(111) over a wide sample temperature range into large, highly-ordered 2D chiral domains. By contrast, adsorption of TPyP on the more reactive Cu(111) leads to temperature dependent assemblies, governed decisively by the strong substrate influence. The increased metal-surface interactions on Cu(111) are accompanied by a conformational distortion of the porphyrin macrocycle. The TPyP’s pyridil groups were coordinated with single iron molecules, forming metal-organic complexes. Furthermore, the porphyrin’s macrocycle was metalated by exposing a layer of well-ordered TPyP to an iron atom beam, demonstrating a novel approach towards the fabrication of metallo-tetraaryl porphyrins performed in two dimensions under ultrahigh vacuum conditions. This method was similarly used to form lanthanideporphyrinates by coordinating tetraphenyl porphyrin (TPP) macrocycles with cerium. The influence of the metal center on the porphyrins’ electronic structure was investigated via STS for TPP, TPyP,Fe−TPyP, Fe−TPP, Ce−TPP, and Co−TPP, whereby the inhomogenous electron density distribution associated with individual frontier orbitals were imaged via dI/dV mapping. The symmetry and form of the molecular orbitals could be directly correlated to the saddle-shaped conformational adaptation for the case of Co −TPP.

Graphene, a honeycomb lattice of sp2-bonded carbon atoms, has received considerable attention in the scientific community due to its unique electronic properties. Distinct symmetries of the graphene wave functions lead to unusual quantum properties, such as a unique half-integer quantum Hall effect. As an added consequence of these symmetries, back-scattering in graphene is strongly prohibited leading to long coherence lengths of carriers. These charge carriers at low energy exhibit linear energy-momentum dispersion, much like neutrinos. Thus, carriers in graphene can be described as massless Dirac fermions. Graphene grown epitaxially on semiconducting substrates offers the possibility of large-scale production and deterministic patterning of graphene for nanoelectronics. In this work, epitaxial graphene is created on SiC(0001) by annealing in vacuum. Sequential scanning tunneling microscopy (STM) and spectroscopy (STS) are performed in ultrahigh vacuum at a temperature of 4.2 K and 300 K. These atomic-scale studies address the growth, interfacial properties, stacking order, and quasiparticle coherence in epitaxial graphene. STM topographic images show the atomic structure of successive graphene layers on the SiC substrate, as well as the character of defects and adatoms within and below the graphene plane. STS differential conductance (dI/dV) maps provide spatially and energy resolved snapshots of the local density of states. Such maps clearly show that scattering from atomic defects in graphene gives rise to energy-dependent standing wave patterns. We derive the carrier energy dispersion of epitaxial graphene from these data sets by quantifying the dominant wave vectors of the standing waves for each tunneling bias.

The investigation and control of molecular properties is currently a dynamic research field. Here I present molecular level studies of porphyrin molecules adsorbed on metal surfaces via Low Temperature Scanning Tunneling Microscopy/Spectroscopy (STM/STS), supported by complementary X-ray absorption experiments. Intermolecular and molecule-surface interactions of tetrapyrdil porphyrin (TPyP) on Ag(111) and Cu(111) were investigated. TPyP self-assembles on Ag(111) over a wide sample temperature range into large, highly-ordered 2D chiral domains. By contrast, adsorption of TPyP on the more reactive Cu(111) leads to temperature dependent assemblies, governed decisively by the strong substrate influence. The increased metal-surface interactions on Cu(111) are accompanied by a conformational distortion of the porphyrin macrocycle. The TPyP’s pyridil groups were coordinated with single iron molecules, forming metal-organic complexes. Furthermore, the porphyrin’s macrocycle was metalated by exposing a layer of well-ordered TPyP to an iron atom beam, demonstrating a novel approach towards the fabrication of metallo-tetraaryl porphyrins performed in two dimensions under ultrahigh vacuum conditions. This method was similarly used to form lanthanideporphyrinates by coordinating tetraphenyl porphyrin (TPP) macrocycles with cerium. The influence of the metal center on the porphyrins’ electronic structure was investigated via STS for TPP, TPyP,Fe−TPyP, Fe−TPP, Ce−TPP, and Co−TPP, whereby the inhomogenous electron density distribution associated with individual frontier orbitals were imaged via dI/dV mapping. The symmetry and form of the molecular orbitals could be directly correlated to the saddle-shaped conformational adaptation for the case of Co −TPP. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

No description available.

No description available.

No description available.

The oxidation of Pd single crystals: Pd(111), Pd(100) and Pd(110) was studied using Temperature Programmed Desorption (TPD), X-ray Photoelectron Spectroscopy (XPS), Auger Electron Spectroscopy (AES), Low Electron Energy Diffraction (LEED) and Scanning Tunneling Microscopy (STM) as they were subjected to O2 in the pressure range between 1 and 150 Torr at temperatures 600-900 K. The oxygen species formed during oxidation, the oxygen uptake dependence on the sample history, the Pd single crystal surface morphology transformations, and the catalytic methane combustion over Pd single crystals were investigated in detail. The Pd single crystal oxidation proceeded through a three-step mechanism. Namely, (1) oxygen dissociatively adsorbed on Pd surface, forming chemisorbed oxygen and then surface oxide; (2) atomic oxygen diffused through a thin surface oxide layer into Pd metal, forming near surface and bulk oxygen; (3) bulk PdO formed when a critical oxygen concentration was reached in the near surface region. The diffusion of oxygen through thin surface oxide layer into Pd metal decreased in the order: Pd(110)>Pd(100)>Pd(111). The oxygen diffusion coefficient was estimated to be around 10-16 cm2 s-1 at 600 K, with an activation energy of 80 kJ mol-1. Once bulk PdO was formed, the diffusion of oxygen through the bulk oxide layer was the rate-determining step for the palladium oxidation. The diffusion coefficient was equal to 10-18 cm2 s-1 at 600 K and the activation energy was approximately 120 kJ mol-1. The oxygen diffusion through thin surface oxide layer and bulk PdO followed the Mott-Cabrera parabolic diffusion law. The oxygen uptake on Pd single crystals depended on the sample history. The uptake amount increased with the population of the bulk oxygen species, which was achieved by high oxygen exposure at elevated temperatures, for example in 1 Torr O2 at above 820 K. Ar+ sputtering or annealing in vacuum at 1300 K depleted the bulk oxygen. The Pd single crystal surface morphology was determined by the oxidation conditions: O2 pressure, treatment temperature and exposure time. When bulk PdO was formed, the single crystal surface was covered with semi-spherical agglomerates 2-4 nm in size, which tended to aggregate to form a“cauliflower-like" superstructure. The single crystal surface area during oxidation, determined by integrating the STM image, experienced three major expansions in consistent with a three-step oxidation mechanism. The surface area on the oxidized single crystals increased in the order: Pd(110)