AVS2012 Session SS-TuP: Surface Science Poster Session

Femtosecond two-photon photoemission spectroscopy is used to investigate and compare the interfacial electronic structures of thiophenol and p-fluorothiophenol films on Cu(111) as a function of molecular coverage. A new state is found to emerge as the coverage is increased; simultaneously, the Cu(111) Shockley surface state disappears for both molecular species. This similarity in behavior is shown to originate from spatial lateral confinement of the surface electron. In addition, the change in the workfunction vs. coverage shows that the two thiophenols exhibit almost identical behavior until an inflection point at ~1/3 ML coverage but then subsequently diverge. This divergent behavior is attributed to the changing orientation of the phenyl group with coverage. At a full monolayer, the net change in the workfunction for the two molecules have opposite signs, which can be explained using a quantitative model based on a surface and molecular dipole moments.

Rare-earth doped yttrium aluminum garnet (YAG) and lanthanum zirconate (LZO) are luminescent ceramics that have been used in TV’s, LED’s, metal oxide transistors, and as laser sources. These materials are thermally and chemically stable. Recent work at Vanderbilt by the Walker and Rogers research groups has shown that the emitted spectrum of proton irradiated LZO particles differs from that of non-irradiated particles, suggesting these materials may be used as passive radiation exposure indicators.

We will discuss the combustion synthesis and characterization of YAG and LZO particles. Combustion synthesis involves heating a mixture of metal nitrates and a fuel until the mixture ignites. If the proper conditions are used, the energy released by the combustion is sufficient to form polycrystalline material. The type and amount of fuel used in the synthesis affect the amount of gaseous by-products produced and flame temperature achieved during a reaction, both of affect the crystallite size formed. The organic fuels included in this study are urea and glycine with adiabatic flame temperatures 1780ºC and 1210ºC, respectively. Urea’s higher flame temperature makes this fuel attractive for combustion syntheses. However, urea shows signs of degradation beginning around 120 °C, well below its ignition temperature. Glycine does not appear to degrade until approximately 230ºC much closer to its ignition temperature. The trade-off between degradation and adiabatic flame temperature suggests the temperature ramp rate used will significantly affect the performance of combustion syntheses carried out with these fuels.

We will present results of detailed thermogravimetric analysis and differential scanning calorimetry (TGA/DSC) experiments used to study the affects of heating rate on the combustion process and on the characteristics of the material formed. TGA/DSC-determined heats of combustion and heat capacities of the reactants and products will also be presented.

Since the photoinduced decomposition of water on TiO₂electrodes were discovered, various characteristics based on photocatalyst have attracted extensive interest. TiO₂ is anticipated as one of materials which are alternative for an existing solar cell based on silicon. TiO₂ shows relatively high reactivity and chemical stability under UV light whose energy exceeds a band gap of 3.2 eV in the anatase crystalline phase. The sun can provide an abundant source of photons, however, UV light accounts for the only small fraction (5 %) of the sun’s energy compared to the visible region (45 %). Many techniques have been examined to achieve this purpose.

In this study, the glass (Corning#1737) was used as the substrate. After the TiO₂ layer was prepared by reactive magnetron sputtering using a Ti target in an Ar/O₂ gas mixture, the Ni layer was deposited by using DC sputtering. Finally the TiO₂ layer was coated on the Ni layer. The TiO₂/Ni/TiO₂ multi-layer films were constituted with the first TiO₂ layer of 100 - 200 nm, second Ni layer of 25 nm and the third TiO₂ surface layer of 0 - 100 nm. Composition and microstructure of these films were investigated by X-ray photoelectron spectroscopy and X-ray diffraction, respectively. Chromatic change of a methylene blue solution was applied to photocatalytic evaluation. Light irradiation to TiO₂ films in a methylene blue solution was carried out using a commercial sterilizing lamp as ultraviolet light and an artificial sun lamp as visible light. Transmittance of a methylene blue solution was measured by a spectrophotometer. In this experiment the relationship between the photocatalytic effect and the magnetic field was investigated. Magnets with magnetic field intensity of 0.15 - 0.23 T were placed on the outside of a methylene blue cell. We anticipated that the magnetic field affected separation of an electron and a hole.

The XRD patterns of TiO₂ prepared by reactive magnetron sputtering showed anatase (101) and rutile (110) of TiO₂. The anatase content for the crystal structure of the TiO₂ was 82 %. When the magnetic field was applied in the parallel direction to the substrate surface, the TiO₂/Ni/TiO₂ thin film with the thinner TiO₂ surface layer, i.e. the third layer of 50 nm, showed the higher photocatalytic property. It was clear that the magnetic field affected a photocatalytic property.

Characteristics based on photo-inducement of TiO₂ have attracted various interests in many fields. One of those characteristics is a photocatalytic effect. The photocatalytic effect shows antifouling or antimicrobial activity and also has the ability to decompose environmental pollutants. The most important characteristic as a photocatalyst of TiO₂ is well known that photo-excited state is very stable and does not cause self-decomposition. Therefore, the electrolysis of water is performed under ultraviolet irradiation to TiO₂. However the light reaction region of TiO₂ is limited at ultraviolet region corresponded with only about 3 % of sunlight.

In this study, to improve the photo-function property of TiO2 the double layer films were fabricated by the constitution of the TiO₂ layer with 3.0 - 3.2 eV and the Cu₂O layer with 2.2 eV in a band gap energy. Each constitution of the film with TiO₂/Cu₂O and Cu₂O/TiO₂ was also investigated about optical permeability. Furthermore, to prevent diffusion of Cu from the Cu₂O layer to the TiO₂ layer, the TiN layer was inserted between the TiO₂ layer and the Cu₂O layer. TiN has a high melting point, stability and the suitable electric property as a barrier layer. Those TiO₂/Cu₂O and TiN were fabricated by reactive magnetron sputtering. Composition and microstructure of these films were investigated by X-ray photoelectron spectroscopy and X-ray diffraction, respectively. Chromatic change of a methylene blue solution was applied for the photocatalytic evaluation. Light irradiation to the sample in a methylene blue solution was carried out using a commercial sterilizing lamp as ultraviolet light and an artificial sun lamp as visible light. Transmittance of a methylene blue solution was measured by a spectrophotometer after irradiation for 6 hours by each lamp.

The XRD pattern of the TiO₂/Cu₂O thin film showed the strong peak of the anatase-rutile TiO from an upper layer and the weak peak of Cu₂O from lower layer. The suitable photocatalytic effect was obtained by the constitution of TiO2 with 300 nm and Cu₂O with 200 nm in thickness, when the photocatalytic effect showed about 31 % under an artificial sun lamp and about 90 % under a sterilization lamp. In the case of the TiO₂/ TiN /Cu₂O film it was estimated that the diffusion of Cu was prevented by inserting TiN from the XRD pattern, however, the high photocatalytic effect was not obtained. It was considered that the photocatalytic effect depended on thickness of the TiN layer.

Recently, TiO₂ is considered as one of attractive materials. Since the photoinduced decomposition of water on the T iO₂electrode was discovered, the photocatalysis based on semiconductor property has attracted extensive interest. TiO₂ is an n-type semiconductor with a band gap energy of 3.0 - 3.2 eV and is well known as a versatile material. From the view of solar cells TiO₂ is applied in development of dye-sensitized solar cells (DSSCs) or quasi-one-dimensional TiO₂ nanotube structure. On the other hand Cu₂O is a p-type semiconductor with a direct band gap of 2.0 eV and is a promising material on solar cell applications because of its nontoxicity, low cost and high absorption coefficient in the visible region. In this study, the photovoltaic property of p-Cu₂O/n-TiO₂ solar cells which was prepared by magnetron sputtering was investigated. Furthermore Cu buffer layer between TiO₂ and Cu₂O was used to obtain high efficiency.

Cu₂O/Cu/TiO₂ thin films with p-n heterojunction were fabricated by reactive magnetron sputtering. Firstly, glasses (Corning#1737) and ITO-film coated glasses as a substrate were ultrasonically cleaned by an acetone rinse. The TiO₂ thin film was deposited on glass substrates using pure metallic titanium (99.99%) as a sputtering target material in an oxygen gas atmosphere. The flow rates of a sputtering argon gas and an oxygen gas were 20 sccm and 1.5 sccm, respectively. Secondly, the Cu thin film was deposited on the TiO₂ thin film using pure metallic copper (99.99%) as a sputtering target material. Thirdly, the Cu₂O thin film was deposited on the Cu/TiO₂ thin film. The flow rates of a sputtering argon gas and an oxygen gas were 15 sccm and 10 sccm, respectively. Each thickness of the TiO₂ and Cu₂O layer was about 100 nm and 200 nm. Composition and microstructure of these films were investigated by the X-ray photoelectron spectroscopy and the X-ray diffraction. Transmittance of the TiO₂ and Cu₂O thin film was measured by a spectrophotometer. The photovoltaic property was evaluated by measuring the current–voltage curve.

The Cu₂O/Cu/TiO₂ thin films with p-n heterojunction were successfully fabricated by reactive magnetron sputtering. The XRD diffraction pattern of TiO₂ layers deposited at an oxygen flow rate of 1.5 sccm showed a mixture structure. The open voltage of Cu₂O/Cu/TiO₂ thin films showed a higher value under artificial sun light than Cu₂O/TiO₂ thin films. It was confirmed that the buffer layer of Cu improved the photovoltaic property of Cu₂O/TiO₂ thin films.

Ceria has attracted great attentions in catalysis due to its unique redox properties and oxygen storage capacity. To improve the properties of ceria, suitable metal dopants such as Ti and Mn can be incorporated into it and form dopant-ceria mixed oxides. The additional metal component can modify the physical/chemical properties of ceria. To understand the intriguing chemistry at the metal dopant-ceria interfaces, we examined their structures at the nano-scale using X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM) techniques. Dopants including Ti and Mn were vapor-deposited onto well-ordered reducible CeO₂(111) thin films grown on Ru(0001) under ultrahigh vacuum conditions. Both Mn and Ti are oxidized at the cost of ceria reduction upon deposition at 300 K. Ti is in the +4 formal oxidation state. However, +2 state of Mn is the predominate species on ceria. STM studies show the formation of two-dimensional structures on ceria which can further develop into chain structures or triangular domains upon heating. Our studies suggest a strong interaction between metal dopants and ceria. The nature of dopant-ceria interfaces greatly depends on the dopant type. The research is sponsored by the School of Energy Resources at University of Wyoming.

A novel approach for determining the photo catalytically active surface region is presented. In this method the photo activity of well-defined epitaxial anatase and rutile TiO₂ thin films is measured by methyl orange decomposition. The photo activity as a function of film thickness then enables to extract the active surface region, which is closely related to the bulk charge carrier diffusion length. Using this methodology we are able to compare differences in the photoactive region of anatase and rutile polymorphs of TiO₂ as well as to investigate the effect of dopants (nitrogen and tungsten) on the depth of the active surface layer. The latter highlights the trade-off between enhanced (visible light) absorption and reduction of active volume of the photocatalyst. These studies are the first that quantifies the differences in rutile and anatase and the influence of dopants on charge carrier diffusion and thus photocatalytic activity.

Fundamental charge transfer processes and chemical reaction mechanisms at gas-solid interfaces require better understanding for catalysis, advanced sensing, and energy conversion and storage applications. Here, we report on distinct regimes of hydrogen oxidation reaction on catalytic Pt/TiO2/Ti porous nanostructures with a potential barrier, identified via analysis of long term chemicurrent kinetics recorded in the course of the surface reactions. Three regimes in total have been observed, where the reaction turnover rate, thermal effect and chemicurrent production efficiency vary by orders of the magnitude. One the regimes is characterized with a nearly negligible thermal effect, but a surprisingly high yield of 0.1-0.4 electrons per water molecule produced, being highly encouraging for novel energy conversion applications of the present system. Correlations between occurrence of the distinct regimes and conditions in the gas phase are also established.

The adsorption of water is studied on Sb(111) single crystals by using temperature programmed desorption (TPD) and molecular beam scattering. The surface of Sb(111) is characterized by auger electron spectroscopy (AES), low energy electron diffraction (LEED), and x-ray photoelectron spectroscopy (XPS). Interestingly, water TPD shows only a single peak at ~155 K while recording the parent mass of water obeying zeroth-order kinetics. The fact that the water monolayer peak is missing, suggests that the Sb(111) surface is hydrophobic. In addition, the results show that the antimony surface is inert towards the adsorption of small molecules such as CO, CO₂, and NO. Moreover, the co-adsorption of n-butane and water shows site blocking effects for n-butane adsorption only at very large pre-exposures of water, indicating the formation of a porous water film, as expected for a hydrophobic surface.

The oxides of the rare earth metals Tb, Pr and Ce are desirable for several catalytic applications due to their ability to store and release oxygen atoms. In contrast to ceria, terbia surfaces have not been widely investigated, and may exhibit interesting structural behavior since several bulk Tb oxide phases are known to exist. In this study, we investigated the growth of terbium oxide (TbO_x) thin films on Cu(111) in ultrahigh vacuum, using scanning tunneling microscopy (STM) and low energy electron diffraction (LEED) to characterize the surface structures. We used a stepwise procedure to prepare the TbO_x films, with each step involving Tb deposition onto Cu(111) held at 300 K in an O₂ background to produce an ~1 monolayer (ML) film, followed by annealing in O₂ at 750 K. The TbO_­x films grow epitaxially on the Cu(111) substrate to generate TbO_x(111) with unit cell dimensions of about (1.4 x 1.4) relative to the Cu(111) lattice. STM images show that the TbO_x films (~2 - 5 ML) are comprised of large, flat terraces and reveal an atomic-structure consisting of hexagonal, close-packed arrangements of atoms that are separated by rows of oxygen vacancies at a spacing of about 2.2 nm. Estimates of the vacancy concentrations suggest that the oxide stoichiometry corresponds to an O:Tb ratio between 1.67 and 1.71, which agrees well with the composition of the stable iota phase of bulk terbia. The capability of preparing TbO_1.7(111) films with well-defined arrangements of vacancies may provide new opportunities for preparing model catalyst surfaces and studying the interactions of molecular reactants with ensembles of vacancies. Such studies are currently in progress.

magnitude of the chemical reaction - induced thermal currents in metal nanofilm – semiconductor Schottky diodes. As well, we present a comparison of our calculations with the experimental values of the chemicurrents reported earlier.

Bimetallic nanoparticles have become an important area of study because of their unique catalytic properties. We describe the synthesis and characterization of bimetallic copper-zinc and copper-palladium nanoparticles. The Cu-Zn and Cu-Pd bimetallics are synthesized using inverse micelle encapsulation in PS-P2VP diblock copolymers. The size and morphology of these nanoparticles supported on SiO₂/Si(111) and TiO₂(110) are characterized using atomic force microscopy (AFM), as well as scanning tunneling microscopy (STM) under ultra high vacuum. Characterization of electronic and chemical properties is carried out using x-ray photoelectron spectroscopy (XPS). Applications of these nanoparticles supported on γ-Al₂O₃ for the catalytic synthesis of methanol will be shown.

In this work we have investigated the reaction mechanism and active sites for CO oxidation over the Au/TiO₂ model surface and Au single crystal surfaces, along with the role of moisture CO.

We examined the effect of moisture on the CO₂ formation rate at the reaction temperature of 300 and 400 K. The CO₂ formation rate at 300 K was increased significantly with increasing H₂O partial pressure up to 0.1 Torr, and then gradually decreased with H₂O pressure. In contrast, no promotional effect of H₂O was observed at the reaction temperature of 400 K. The moisture has an essential role to promote the CO oxidation reaction over Au/TiO₂ catalyst at low temperature, whereas the CO oxidation reaction proceeded without moisture with high reaction temperature. This important observation indicates that the CO oxidation mechanism over Au/TiO₂ is different between 300 and 400 K, considering that the activation process of oxygen molecules strongly depended on a reaction temperature. That is, molecular oxygen has been activated directly over the Au surface at the high temperature while the moisture takes part in the activation of the oxygen molecule at low reaction temperature.

Next, we examined the turnover frequencies (TOFs) for CO₂ formation at the two reaction temperatures as a function of mean gold particle diameter. To determine whether the active sites for CO oxidation were exposed gold atoms on the gold particles or perimeter sites at the interface between the gold particles and the TiO₂ support, we calculated the TOFs in two ways: (i) by normalizing the total number of exposed Au atoms at the gold particles (TOF–S) and (ii) by normalizing the the total number of gold atoms at the perimeter interfaces (TOF–P). The results clearly showed that the relationship between TOF and mean gold particle diameter depended strongly on reaction temperature. At 300 K, TOF–S decreased with increasing mean gold particle diameter, whereas TOF–P remained nearly constant regardless of particle diameter, suggesting that the active sites for CO oxidation were the gold atoms located at the periphery of the gold particles attached to TiO₂. In contrast, TOF–S at 400 K remained nearly constant regardless of the mean gold particle diameter, indicating that the active sites for CO oxidation were newly created on the gold metal surface at the high temperature. Thus, we can conclude that both the reaction mechanisms and the active sites differed between the low temperature region and the high temperature region.

Electronic excitation at the interface between an organic molecular film and a substrate is of general importance for the area of organic electronics and light conversion processes. We demonstrate rubrene/HOPG as a model system for an organic film/substrate interface. Many efforts have been devoted to improve the performance of a rubrene thin film transistor after high carrier mobility was achieved for rubrene single crystals. The reasons for the poor efficiency of the thin films are attributed to the molecular geometry on the surface. To understand the mechanisms of charge transportation for organic molecular devices, it is the primary step to unravel the molecular electronic structures of both occupied and unoccupied states at interfaces between the film and the substrate.

We have performed two-photon photoemission (2PPE) spectroscopy for rubrene films formed on HOPG substrate. It is revealed a prominently enhanced unoccupied molecular peak, which is resonantly excited from the highest occupied molecular orbital (HOMO). Interestingly, the enhancement of the peak becomes less significant at the coverage higher than 1 monolayer, where the image potential state (IPS) peak on the substrate disappears. The resonance enhancement is moderate with s-polarization, by which the transition to IPS is completely suppressed. We ascribe that the excitation of the the level is mediated by the IPS on HOPG. Though the IPS wave function extends outside the molecules, it interacts with the unoccupied molecular orbital at the edges of molecular islands, causing the strong resonance enhancement of the unoccupied molecular level.

By clarifying the mechanism, the excitation process is expected to be useful to highly enhance the efficiency of organic molecular devices and light conversion processes. The energy of IPS is generally governed by the work function. It may be possible to tune the IPS level nearly resonant to an unoccupied level of organic films. This provides a way to tailor the electronic excitation efficiency.

DLC (Diamond-like carbon) classified in new materials is amorphous carbon including hydrogen and has the similar property to diamond. DLC films were formed by the ion beam evaporation method in the early 1970's and after that have been manufactured by various methods. Because the representative property of DLC shows the high hardness and low friction coefficient, DLC is applied in various fields such as motor parts or tools. The ion beam assisted deposition method has many parameters on the film formation condition in comparison with other dry process methods. Therefore this method was anticipated in production of new characteristics such as a high adhesion film.

From the result of our research, DLC thin films prepared by the ion beam assist method using an N₂ gas showed the excellent low friction coefficient in the atmosphere. In this study, the behavior on friction in a vacuum of those DLC films was investigated. The DLC films were formed using N₂⁺ ion beam assisted deposition in a toluene (C₇H₈) atmosphere. The formation conditions of DLC films were changed with an ion beam current density and an accelerating voltage. Stainless steels (304SS) were used for the sample substrate. The mechanical properties of hardness and friction coefficient were measured using the dynamic micro knoop hardness tester and the ball-on-disk tribotester respectively. Atomic concentration and structure of the films were investigated by X-ray photoelectron spectroscopy and Raman spectroscopy. The friction coefficient in a vacuum was measured under 5 x10^-4 Pa. The conditions of the ball-on-disk test were 0.98 N in a weight and 135 rpm in a sliding speed.

The suitable friction property of DLC films was obtained by the condition with an accelerating voltage of 5 kV at a current density of 10 μA/cm². The minimum friction coefficient in a vacuum was 0.016 for an SUJ2 ball of the counter material, however, the DLC thin film started to cause partial peeling-off in a sliding distance of 52 m. It is anticipated that the property in a vacuum of these DLC films is applied in space technology.

Bismuth (Bi) is a nearly ideal surfactant on GaAs owing to its large size relative to Ga and As, smoothing the surface morphology of the GaAs(001) surface. Surface modification in this manner is potentially useful in high-quality device growth, but corresponding atomic structure and its effects on subsequent device growth are unknown. Reflection high-energy electron diffraction indicates that the Bi surfactant induces a (1×3) surface reconstruction. Experimental scanning tunneling microscopy clearly shows a nm-length-scale step structure with a high density of alternating up/down steps and reconstruction rows of ×3 periodicity.From these observations, we propose a mechanism for the Bi surfactant surface smoothing. The alternating step heights on the nm length scale are likely a result of a Bi-induced surface energy change. However, compositional disorder obscures the atomic structure within the rows. Thus, the atomic structure of the (1×3) reconstruction cannot be revealed through experiment, but must be determined from simulation.

Ab-initio­ density functional theory and cluster expansion calculations were carried out to determine the relative stability of reconstruction in the Bi/GaAs(001) system. Differences in stability originate from two sources: structure, determined by surface bonding topology, and configuration, arising from the arrangement of Ga, As, and Bi species over the surface dimer sites. For the Bi/GaAs system, the (2×1), α2(2 ×4), β2(2 ×4), (4 ×3), and c(4 ×4) reconstructions and their species configurations were considered. Calculations show the observed (1×3) reconstruction explained by the (4×3) structure first proposed for AlSb and GaSb, which has a large number of stable configurations at 0K. At typical growth temperatures, the calculated Monte Carlo entropy approaches that of ideal mixing, indicating thermal excitations of these configurations produce the experimentally observed disorder.

PMMA is an important thermoplastic material which has been utilized in a variety of engineering areas ranging from aeronautical applications to electronics industries, due to its attractive physical and optical properties. The physical and chemical interactions have been observed at the interfaces between metals and PMMA, which can play a crucial role in the device performance. As a result, significant attention has been paid to the interfaces in order to promote devices performance and stability.

The forward scattering of heavy ionic projectiles from oriented crystal surfaces gives evidence of collective effects that are far more pronounced than those seen in scattering of light ionic projectiles. For example, molecules containing heavy atoms such as XeF₂ are known to initiate novel surface chemistry effects. To study these kinematic trends, MD and trajectory simulations were performed for hyperthermal energy ions. By sequentially increasing the mass of the projectile across the periodic table, we have observed energy and angle resolved trends that show a clear deviation from standard scattering events, which are typically interpreted as sequential binary collisions. The simulation results were compared to experimental data for alkali ions on metal surfaces, specifically for the cases of K⁺ and Cs⁺ scattered from Cu(001).