Orhan Kizilkaya

The local structure of transition metal doped semiconducting boron carbides

Transition metal doped boron carbides produced by plasma enhanced chemical vapour deposition of orthocarborane (closo-1,2-C2B10H12) and 3d metal metallocenes were investigated by performing K-edge extended x-ray absorption fine structure and x-ray absorption near edge structure measurements. The 3d transition metal atom occupies one of the icosahedral boron or carbon atomic sites within the icosahedral cage. Good agreement was obtained between experiment and models for Mn, Fe and Co doping, based on the model structures of two adjoined vertex sharing carborane cages, each containing a transition metal. The local spin configurations of all the 3d transition metal doped boron carbides, Ti through Cu, are compared using cluster and/or icosahedral chain calculations, where the latter have periodic boundary conditions.

Ni doping of semiconducting boron carbide

The wide band gap, temperature stability, high resistivity, and robustness of semiconducting boron carbide make it an attractive material for device applications. Undoped boron carbide is p type; Ni acts as a n-type dopant. Here we present the results of controlled doping of boron carbide with Ni on thin film samples grown using plasma enhanced chemical vapor deposition. The change in the dopant concentration within the thin film as a function of the dopant flow rate in the precursor gas mixture was confirmed by x-ray photoelectron spectroscopy measurements; with increasing dopant concentration, current-voltage (I-V) curves clearly establish the trend from p-type to n-type boron carbide.

Performance of the infrared microspectroscopy beamline at CAMD

The first infrared (IR) beamline at the Center for Advanced Microstructures and Devices (CAMD) at Louisiana State University has been successfully constructed and commissioned. The beamline features a simple optical design with a minimal number of optical components. A pair of mirrors, planar and toroidal, is utilized for extracting synchrotron radiation (50 and 15 mrad, in horizontal and vertical directions, respectively) from the bending magnet port to a diamond window located outside of the shielding wall. Synchrotron radiation is then collimated by an off-axis parabolic mirror and fed into a Thermo Nicolet Continuum microscope through a Thermo Nicolet Nexus 670 FT-IR spectrometer. The microscope’s performances with synchrotron-radiation and conventional-thermal sources were compared in the mid-IR spectral range (11700–400cm−1). Effective beam spot size at sample position of the microscope was measured to be 35×12μm2 (FWHM). It was also determined that synchrotron radiation has substantial advantages o...

Formation of aluminum oxide thin films on FeAl(110) studied by STM

The surface morphology and atomic structure of clean and oxidized FeAl(1 1 0) surfaces have been investigated with scanning tunneling microscopy (STM). An incommensurate reconstructed structure, having FeAl2 stoichiometry confined to the outmost layer, is observed on the clean surface due to preferential Al segregation upon annealing to 1125 K. When the reconstructed clean surface is exposed to oxygen at elevated temperatures, an ordered ultra-thin aluminum oxide film is formed. Based on STM data, a structural model of the oxide film is proposed, which exhibits a quasi-hexagonal oxygen layer and accommodates an even mix of octahedral and tetrahedral occupancy of Al ions arranged in an alternating zigzag–stripe structure. STM imaging with tunnel voltages in the range of the bulkband gap implies that the thin film oxide electronic structure differs substantially from the bulkoxide, and indicates a local density of states around the oxide constituents within the bulkband gap. 2003 Elsevier Science B.V. All rights reserved.

Surface reconstruction of FeAl(110) studied by scanning tunnelling microscopy and angle-resolved photoemission spectroscopy

The surface geometric and electronic structure of the FeAl(110) intermetallic alloy has been investigated by scanning tunnelling microscopy and angle-resolved photoemission spectroscopy (ARPES). Preferential sputtering results in depletion of Al in the surface region and subsequent annealing promotes surface segregation of Al and gives rise to new reconstructed phases. A bulk terminated surface structure is obtained after annealing the surface to 400 °C. However, an incommensurate phase develops above 800 °C with a stoichiometry consistent with an FeAl2 structure in the topmost layer. The ARPES measurements confirm the Al segregation with increased density of states (DOS) near the Fermi level. The increased DOS is believed to be due to hybridization between the Fe d and Al sp states. The increased intensity of the Al 2p core level for the incommensurate phase also confirms the higher Al surface concentration for this phase.

Electronic Signatures of a Model Pollutant–Particle System: Chemisorbed Phenol on TiO2(110)

Environmentally persistent free radicals (EPFRs) are a class of composite organic/metal oxide pollutants that have recently been discovered to form from a wide variety of substituted benzenes chemisorbed to commonly encountered oxides. Although a qualitative understanding of EPFR formation on particulate metal oxides has been achieved, a detailed understanding of the charge transfer mechanism that must accompany the creation of an unpaired radical electron is lacking. In this study, we perform photoelectron spectroscopy and electron energy loss spectroscopy on a well-defined model system-phenol chemisorbed on TiO2(110) to directly observe changes in the electronic structure of the oxide and chemisorbed phenol as a function of adsorption temperature. We show strong evidence that, upon exposure at high temperature, empty states in the TiO2 are filled and the phenol HOMO is depopulated, as has been proposed in a conceptual model of EPFR formation. This experimental evidence of charge transfer provides a deeper understanding of the EPFR formation mechanism to guide future experimental and computational studies as well as potential environmental remediation strategies.

Design and synthesis of model and practical palladium catalysts using atomic layer deposition

We investigated the “one-batch” synthesis of model and practical palladium catalysts using atomic layer deposition (ALD). Two types of model catalysts and one type of powder-based nanocatalyst were synthesized simultaneously by ALD under viscous flow conditions. In addition, Pd/TiO2(110) model catalysts were prepared by the identical ALD process but under ultrahigh vacuum conditions. Because of the self-limiting surface reaction that defines ALD, the local structure of all the Pd catalysts were essentially the same as confirmed by a suite of microscopic and spectroscopic characterization techniques. A comprehensive understanding of the Pd-based catalysts was achieved by applying both surface science probes and advanced synchrotron techniques, and by strategically selecting the catalyst substrate best suited for each characterization. Both X-ray absorption spectroscopy and X-ray resonant photoelectron spectroscopy suggested that Pd only weakly interacted with the TiO2 support. The one-batch synthesis approach facilitated by ALD can potentially bridge the “synthesis gap” between model catalysts and practical catalysts.

Facile one-pot synthesis of 3D graphite–SiO2 composite foam for negative resistance devices

In this study, we successfully prepare a porous graphite–SiO2 composite foam via a facile glucose-foaming-based method. The foam was characterized by XRD, XPS, SEM, Raman, and soft X-ray absorption spectroscopy of the carbon and oxygen K-edges and by silicon K-edge X-ray absorption near edge structure (XANES) spectroscopy. The electrical properties of the sample were evaluated using current–voltage measurements. The synthesized graphite–SiO2 composite foam showed negative resistance effect, i.e. an increase in the applied voltage led to a decrease in current. This negative resistance effect can be explained by two possible mechanisms: (1) the existence of defect states and (2) a change in the charge shifting amount.

High-throughput Toroidal Grating Beamline for Photoelectron Spectroscopy at CAMD

A 5 meter toroidal grating (5m-TGM) beamline has been commissioned to deliver 28 mrad of bending magnet radiation to an ultrahigh vacuum endstation chamber to facilitate angle resolved photoelectron spectroscopy. The 5m-TGM beamline is equipped with Au-coated gratings with 300, 600 and 1200 lines/mm providing monochromatized synchrotron radiation in the energy ranges 25-70 eV, 50–120 eV and 100–240 eV, respectively. The beamline delivers excellent flux (~1014-1017 photons/sec/100mA) and a combined energy resolution of 189 meV for the beamline (at 1.0 mm slit opening) and HA-50 hemispherical analyzer was obtained at the Fermi level of polycrystalline gold crystal. Our preliminary photoelectron spectroscopy results of phenol adsorption on TiO2 (110) surface reveals the metal ion (Ti) oxidation.

The electronic structure of ultrathin aluminum oxide film grown on FeAl(110) : A photoemission spectroscopy

The electronic structure of the ultrathin aluminum oxide grown on the FeAl(110) surface has been investigated with angle-resolved photoemission spectroscopy. Previous scanning tunneling microscopy studies have revealed that exposing the clean FeAl(110) surface to 1000l of oxygen at 850°C forms a homogeneous hexagonal oxide film with a thickness of approximately 10A. Core level photoemission spectra of FeAl constituents indicate that Al is the only metal species present in the oxide film. The measured band dispersion of the oxide thin film indicates a two dimensional electronic structure parallel to the plane of the thin film due to the limited thickness of the oxide thin films. The appearance of a peak in the anticipated band gap of the bulk oxide film suggests a unique electronic structure of the two dimensional oxide film. This latter observation is correlated with previous scanning tunneling microscopy results to elucidate the structure of the ultrathin alumina film grown on FeAl(110).