Phillip T. Sprunger

The Role of Interstitial Sites in the Ti3d Defect State in the Band Gap of Titania

Titanium dioxide (TiO2) has a number of uses in catalysis, photochemistry, and sensing that are linked to the reducibility of the oxide. Usually, bridging oxygen (Obr) vacancies are assumed to cause the Ti3d defect state in the band gap of rutile TiO2(110). From high-resolution scanning tunneling microscopy and photoelectron spectroscopy measurements, we propose that Ti interstitials in the near-surface region may be largely responsible for the defect state in the band gap. We argue that these donor-specific sites play a key role in and may dictate the ensuing surface chemistry, such as providing the electronic charge required for O2 adsorption and dissociation. Specifically, we identified a second O2 dissociation channel that occurs within the Ti troughs in addition to the O2 dissociation channel in Obr vacancies. Comprehensive density functional theory calculations support these experimental observations.

Electrochemical Reduction of CO2 to CH3OH at Copper Oxide Surfaces

The direct reduction of CO2 to CH3OH is known to occur at several types of electrocatalysts including oxidized Cu electrodes. In this work, we examine the yield behavior of an electrodeposited cuprous oxide thin film and explore relationships between surface chemistry and reaction behavior relative to air-oxidized and anodized Cu electrodes. CH3OH yields (43 μmol cm-2 h-1) and Faradaic efficiencies (38%) observed at cuprous oxide electrodes were remarkably higher than air-oxidized or anodized Cu electrodes suggesting Cu(I) species may play a critical role in selectivity to CH3OH. Experimental results also show CH3OH yields are dynamic and the copper oxides are reduced to metallic Cu in a simultaneous process. Yield behavior is discussed in comparison with photoelectrochemical and hydrogenation reactions where the improved stability of Cu(I) species may allow continuous CH3OH generation.

Photoemission study of pristine and photodegraded poly(methyl methacrylate)

Degradation of poly(methyl methacrylate) (PMMA) thin films by vacuum ultraviolet (VUV) monochromatic synchrotron radiation was investigated by ultraviolet photoelectron spectroscopy. The photodegradation reaction was analyzed, for the first time, by different spectrometry techniques and ab initio molecular orbital calculations. It is concluded that the main degradation mechanism in PMMA by VUV photons is ascribed to the disappearance of ester groups and formation of double bonds in the polymer chain. The final product of the degradation seems to possess a relatively rich conjugation of unsaturated bonds. The rate constant of the degradation by VUV photons is evaluated to be 2.4×10−17 photons−1 cm2.

RADIATION DAMAGE OF POLY(METHYLMETHACRYLATE) THIN FILMS ANALYZED BY UPS

Abstract Radiation damage of poly(methylmethacrylate) (PMMA) caused by vacuum ultraviolet monochromatic synchrotron radiation was studied by ultraviolet photoelectron spectroscopy. An independent atomic center approximation combined with molecular orbital calculation was employed to study the radiation damage of PMMA quantitatively for the first time. The results of the analysis indicate that main photodegradation is ascribed to abstraction of ester groups and formation of double bonds in the polymer chain.

Direct extraction of the Eliashberg function for electron-phonon coupling: A case study of Be(1010)

We propose a systematic procedure to directly extract the Eliashberg function for electron-phonon coupling from high-resolution angle-resolved photoemission measurement. The procedure is successfully applied to the Be(10(-)10) surface, providing new insights into electron-phonon coupling at this surface. The method is shown to be robust against imperfections in experimental data and suitable for wider applications.

Electronic structure and molecular orientation of well-ordered polyethylene oligomer (n-C44H90) on Cu(100) and Au(111) surfaces studied by UV photoemission and low energy electron diffraction

Abstract The electronic structure and molecular orientation of tetratetracontane (n-C44H90) films on Cu(100) and Au(111) surfaces were investigated by angle-resolved UV photoemission spectroscopy (ARUPS) and low energy electron diffraction (LEED). The observed ARUPS spectra showed the drastic take-off angle dependence due to intramolecular band dispersion. A 2×1-like LEED pattern was observed for both substrates. From these results and theoretical simulation of ARUPS spectra based on independent-atomic center (IAC) approximation, we found that the C–C–C plane of the adsorbed TTC molecule is parallel to the substrate surface and its molecular axis is along a [110] direction for both substrates. We also measured the work function change by adsorption of TTC. The observed values were c.a. −0.3eV and −0.7eV for Cu(100) and Au(111) systems, respectively. Such decrease of the work function indicates the existence of a dipole layer at the interfaces in contrast to the traditional picture of energy level alignment at organic/metal interface assuming a common vacuum level at the interface. The dipole formation in such physisorbed systems can be explained by the polarization of the TTC molecule due to an image force.

Thin crystalline functional group copolymer poly(vinylidene fluoride–trifluoroethylene) film patterning using synchrotron radiation

The photodegradation mechanism due to synchrotron radiation exposure of crystalline poly[vinylidene fluoride–trifluoroetylene, P(VDF–TrFE)] copolymer thin films has been studied with ultraviolet photoemission spectroscopy (UPS) and mass spectroscopy. Upon increasing exposure to x-ray white light (hν⩽1000 eV), UPS measurements reveal that substantial chemical modifications occur in P(VDF–TrFE) 5 monolayer films, including the emergence of new valence band features near the Fermi level, indicating a semimetallic photodegradeted product. The photodetached fragments of the copolymer consist mainly of H2, HF, CHF, CH2. This x-ray exposure study demonstrates that P(VDF–TrFE) films, possessing unique technologically important properties, can be directly patterned by x-ray lithographic processes.

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 morphology and electronic structure of Ni/Ag(100)

The growth morphology and electronic structure of Ni on Ag(100) has been studied with scanning tunneling microscopy (STM) and synchrotron based angle resolved photoemission spectroscopy. At deposition temperatures at or below 300 K, STM reveals Ni cluster growth on the surface along with some subsurface growth. Upon annealing to 420 K, virtually all Ni segregates into the subsurface region forming embedded nanoclusters. The electronic structure of Ni d bands in the unannealed surface shows dispersion only perpendicular to the surface whereas the annealed surface has Ni d bands that exhibit a three-dimensional-like structure. This is a result of the increased Ni d–Ag sp hybridization bonding and increased coordination of the embedded Ni nanoclusters.

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.