Karina Schulte

Oxygen Intercalation under Graphene on Ir(111): Energetics, Kinetics, and the Role of Graphene Edges.

Using X-ray photoemission spectroscopy (XPS) and scanning tunneling microscopy (STM) we resolve the temperature-, time-, and flake size-dependent intercalation phases of oxygen underneath graphene on Ir(111) formed upon exposure to molecular oxygen. Through the applied pressure of molecular oxygen the atomic oxygen created on the bare Ir terraces is driven underneath graphene flakes. The importance of substrate steps and of the unbinding of graphene flake edges from the substrate for the intercalation is identified. With the use of CO titration to selectively remove oxygen from the bare Ir terraces the energetics of intercalation is uncovered. Cluster decoration techniques are used as an efficient tool to visualize intercalation processes in real space.

Tuning the spin state of iron phthalocyanine by ligand adsorption

The future use of single-molecule magnets in applications will require the ability to control and manipulate the spin state and magnetization of the magnets by external means. There are different approaches to this control, one being the modification of the magnets by adsorption of small ligand molecules. In this paper we use iron phthalocyanine supported by an Au(111) surface as a model compound and demonstrate, using x-ray photoelectron spectroscopy and density functional theory, that the spin state of the molecule can be tuned to different values (S ∼ 0, [Formula: see text], 1) by adsorption of ammonia, pyridine, carbon monoxide or nitric oxide on the iron ion. The interaction also leads to electronic decoupling of the iron phthalocyanine from the Au(111) support.

Ordering of the Nanoscale Step Morphology As a Mechanism for Droplet Self-Propulsion

We establish a new mechanism for self-propelled motion of droplets, in which ordering of the nanoscale step morphology by sublimation beneath the droplets themselves acts to drive them perpendicular and up the surface steps. The mechanism is demonstrated and explored for Ga droplets on GaP(111)B, using several experimental techniques allowing studies of the structure and dynamics from micrometers to the atomic scale. We argue that the simple assumptions underlying the propulsion mechanism make it relevant for a wide variety of materials systems.

X-ray absorption and photoemission spectroscopy of zinc protoporphyrin adsorbed on rutile TiO2(110) prepared by in situ electrospray deposition.

Zinc-protoporphyrin, adsorbed on the rutile TiO(2)(110) surface, has been studied using photoemission spectroscopy and near-edge absorption fine structure spectroscopy to deduce the nature of the molecule-surface bonding and the chemical environment of the central metal atom. To overcome the difficulties associated with sublimation of the porphyrin molecules, samples were prepared in situ using ultrahigh vacuum electrospray deposition, a technique which facilitates the deposition of nonvolatile and fragile molecules. Monolayers of Zn protoporphyrin are found to bond to the surface via the oxygen atoms of the deprotonated carboxyl groups. The molecules initially lie largely parallel to the surface, reorienting to an upright geometry as the coverage is increased up to a monolayer. For those molecules directly chemisorbed to the surface, the interaction is sufficiently strong to pull the central metal atom out of the molecule.

Electrospray deposition of carbon nanotubes in vacuum

Here we report on a novel and effective technique for the deposition of carbon nanotubes onto surfaces in vacuum directly from a liquid suspension. The technique, based on in-vacuum electrospray ionization, has the potential to bridge the gap between high resolution techniques requiring ultra-high vacuum conditions, and non-volatile molecules and nanostructures such as carbon nanotubes. Atomic force microscopy of double-walled nanotubes deposited onto silicon surfaces in vacuum show individual nanotubes and low density bundles.

Ligand Identification in Titanium Complexes Using X-ray Valence-to-Core Emission Spectroscopy

The identification of ligands in metalloorganic complexes is crucial for understanding many important biological and chemical systems. Nonresonant Kβ valence-to-core X-ray emission spectroscopy (XES) has been demonstrated as a ligand identification technique which is complementary to other spectroscopies, such as X-ray absorption. In this study we show the Kβ valence-to-core XES alongside the Ti K-edge X-ray absorption near edge structure spectra for a series of chemically relevant low-symmetry Ti organometallic complexes. The spectra are modeled using density functional theory calculations. XES spectra are analyzed in terms of the molecular orbitals probed, in order to understand the effects of bond length, bond nature, orbital hybridization, and molecular symmetry on the observed spectral features.

CO-Induced Smoluchowski Ripening of Pt Cluster Arrays on the Graphene/Ir(111) Moire

Regular Pt cluster arrays grown on the moiré template formed by graphene on Ir(111) were tested for their stability with respect to CO gas exposure. Cluster stability and adsorption-induced processes were analyzed as a function of cluster size, with in situ scanning tunneling microscopy and X-ray photoelectron spectroscopy. Small clusters containing fewer than 10 atoms were unstable upon CO adsorption. They sintered through Smoluchowski ripening-cluster diffusion and coalescence-rather than the frequently reported Ostwald ripening mediated by metal-adsorbate complexes. Larger clusters remained immobile upon CO adsorption but became more three-dimensional. Careful analysis of the experimental data complemented by ab initio density functional theory calculations provides insight into the origin of the CO-induced Pt cluster ripening and shape transformations.

Bulk and surface characterization of In2O3(001) single crystals

A comprehensive bulk and surface investigation of high-quality In2O3(001) single crystals is reported. The transparent-yellow, cube-shaped single crystals were grown using the flux method. Inductively coupled plasma mass spectrometry (ICP-MS) reveals small residues of Pb, Mg, and Pt in the crystals. Four-point-probe measurements show a resistivity of 2.0 +/- 0.5 x 10(5) Omega cm, which translates into a carrier concentration of approximate to 10(12) cm(-3). The results from x-ray diffraction (XRD) measurements revise the lattice constant to 10.1150(5) angstrom from the previously accepted value of 10.117 angstrom. Scanning tunneling microscopy (STM) images of a reduced (sputtered/annealed) and oxidized (exposure to atomic oxygen at 300 degrees C) surface show a step height of 5 angstrom, which indicates a preference for one type of surface termination. The surfaces stay flat without any evidence for macroscopic faceting under any of these preparation conditions. A combination of low-energy ion scattering (LEIS) and atomically resolved STM indicates an indium-terminated surface with small islands of 2.5 angstrom height under reducing conditions, with a surface structure corresponding to a strongly distorted indium lattice. Scanning tunneling spectroscopy (STS) reveals a pronounced surface state at the Fermi level (E-F). Photoelectron spectroscopy (PES) shows additional, deep-lying band gap states, which can be removed by exposure of the surface to atomic oxygen. Oxidation also results in a shoulder at the O 1s core level at a higher binding energy, possibly indicative of a surface peroxide species. A downward band bending of 0.4 eV is observed for the reduced surface, while the band bending of the oxidized surface is of the order of 0.1 eV or less.

Ammonia adsorption on iron phthalocyanine on Au(111): influence on adsorbate-substrate coupling and molecular spin.

The adsorption of ammonia on Au(111)-supported monolayers of iron phthalocyanine has been investigated by x-ray photoelectron spectroscopy, x-ray absorption spectroscopy, and density functional theory calculations. The ammonia-induced changes of the x-ray photoemission lines show that a dative bond is formed between ammonia and the iron center of the phthalocyanine molecules, and that the local spin on the iron atom is quenched. This is confirmed by density functional theory, which also shows that the bond between the iron center of the metalorganic complex and the Au(111) substrate is weakened upon adsorption of ammonia. The experimental results further show that additional adsorption sites exist for ammonia on the iron phthalocyanine monolayer.

Charge transfer between the Au(111) surface and adsorbed C-60: Resonant photoemission and new core-hole decay channels

The interaction of C(60) with the Au(111) surface has been investigated using synchrotron radiation-based electron spectroscopy. Resonant photoelectron spectroscopy and autoionization spectroscopy have been used to probe the coupling between the molecule and the substrate. Three distinct high energy spectator Auger features were observed that are only evident for a monolayer of C(60) chemisorbed to the Au(111) surface and not a multilayer or the clean surface itself. Combined with C 1s x-ray absorption and valence band spectra, the data suggest a decay process not previously reported for this system. This is a spectator decay channel involving electrons transferred from the gold substrate to the adsorbed molecule, either in the ground state or during the timescale of the core-hole lifetime. Both possibilities are considered in the interpretation of the results, which support, on balance, a ground state charge transfer.