AVS2000 Session SS-TuP: Poster Session
Tuesday, October 3, 2000 5:30 PM in Room Exhibit Hall C & D
Tuesday Afternoon
Time Period TuP Sessions | Topic SS Sessions | Time Periods | Topics | AVS2000 Schedule
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SS-TuP-1 Oriented Growth of Rod-like Second Phase Precipitate on the (1014) Calcite Surface
A.S. Lea, D.R. Baer, T.T. Hurt, J.E. Amonette (Pacific Northwest National Laboratory) An apparent epitaxial growth from solution of a Mn-rich phase on calcite (CaCO3) has been observed by atomic force microscopy (AFM). Calcite, which is the most widely distributed carbonate mineral, interacts with and incorporates contaminants from the environment and serves as a major reservoir in the global carbon cycle. We have used an AFM equipped with a fluid cell to examine the influence of various solution contaminant ions (Mn, Sr, PO4) on the rates of dissolution of pure calcite. Here, we report on the influence of a soluble Mn2+ impurity on the dissolution behavior of calcite. At Mn2+ concentrations well below saturation for bulk MnCO3, the Mn2+ inhibited dissolution uniformly. However, at Mn2+ concentrations near the MnCO3 saturation level, we observed the formation and oriented growth of rod-like precipitates on the surface. These rods grew along the [221] direction of the surface until they reached a step edge where they could grow no further. While these rods could grow many microns in length, their width was limited to 120 to 180 nm and their thickness was measured consistently at 25 Å, which is indicative of a critical thickness of growth. XPS analysis of the rod covered surfaces showed the presence of Mn with a +2 or +3 valence. EPR analysis of the bulk sample after rod formation shows an intense broad signal that is indicative of dipole-dipole alignment between neighboring Mn(II) ions and distinct from the sextuplet (commonly seen for isolated Mn(II) ions in bulk calcite) observed in the untreated sample. Together these analyses suggest formation of an epitaxial Mn(II) phase on the surface of calcite. |
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SS-TuP-2 Photocatalytic Degradation of Self-Assembled Monolayers Anchored at the Vicinity of Titanium Dioxide Domains, as a Probe for Surface Diffusion
Y. Paz, H. Haick (Technion-IIT, Israel) The presented work demonstrates the ability of self-assembled monolayers to serve as a tool for gaining information on surface diffusion of oxidizing species formed photocatalitically on titanium dioxide. In order to study the photodegradation of molecules located at the vicinity of a titanium dioxide photocatalyst, well-defined structures comprised of alternating micro-stripes of TiO2 and silicon were prepared on silicon wafers. Onto these structures, a cross-linked self-assembled monolayer (SAM) of Octadecyltrichlorosilane (OTS) was chemisorbed. The kinetics of the photodegradation of the anchored SAM on the hybrid structure was then measured in-situ by FTIR under controlled humidity. It was found that the photogenerated oxidizing species, formed on the titanium dioxide well-defined micro-domains, are capable of inducing, within minutes, the mineralization of the aliphatic chains anchored to the inert silicon domains, even when these chains were located as far as 20 microns away from the titanium dioxide micro-zones. In contrast, no remote mineralization could be observed when the same experiments were performed with alkanethiols on gold, in Au/TiO2 hybrid systems. A mechanism, explaining these results and their dependency upon various parameters (diffusion length, temperature, humidity) is presented. The implications of these observations on the design and modeling of porous photocatalysts having "dark" pores are discussed as well. |
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SS-TuP-3 Reactions of HCl and geminal-dichloroethylene on Pd(111) Studied by TDS, LITD, and STM
D.E. Hunka (University of California, Davis); D.C. Herman (University of North Carolina, Chapel Hill); L.I. Lopez (University of California, Berkeley); K.D. Lormand, D. Futaba, S. Chiang, D.P. Land (University of California, Davis) Small organic halides are common groundwater pollutants and remediation of these contaminants has become a popular field of study. Among these, chloroethylenes are the most abundant. Catalytic degradation on transition metal surfaces offers a promising method for the alleviation of this ubiquitous problem. The reactions of HCl and geminal-dichloroethylene on Pd(111) have been investigated using thermal desorption spectroscopy (TDS), laser-induced thermal desorption coupled with FT-mass spectrometry (LITD-FTMS), variable temperature scanning tunneling microscopy, as well as Auger electron spectroscopy and low energy electron diffraction. Although HCl is seen to desorb from the surface in four peaks during TDS, only three distinct surface species exist: two of which are dissociative in nature and one molecular surface species. HCl and H2 are the exclusive desorption products from the decomposition of gem-DCE, and HCl is found to desorb in two desorption peaks at exposures over 0.33 L. These peaks occur at temperatures well above those expected for HCl on clean Pd(111) and are ascribed to a stepwise decomposition of gem-DCE. The two surface intermediates in this stepwise decomposition have been identified using FT-RAIRS. The first intermediate, which is attributed to a chlorothylidyne species, forms by 150 K and is not seen to decompose further until ~400 K. Further decomposition of this intermediate produces a chlorovinylidene species which does not decompose further until ~600 K. Variable temperature scanning tunneling microscopy has been used to observe the reactions of both HCl and gem-DCE on Pd, as well, with the results supporting the above assignments. |
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SS-TuP-4 Lubrication Mechanisms of Tricresylphosphate, TCP, on Cr and Fe Surfaces at Elevated Temperatures: An Atomic-Scale View
M. Abdelmaksoud, B. Borovsky, J. Krim (North Carolina State University) TCP is a high-temperature vapor-phase lubricant which is known for its demonstrated anti-wear properties for macroscopic systems. Although this lubricant has been the subject of much research for over 40 years, the atomic-scale details of its lubrication mechanisms are far from being satisfactorily understood. In this study, we evaporated Fe and Cr substrates in UHV conditions onto Quartz Crystal Microbalances, (QCM), and then monitored the uptake rates of TCP on these substrates at different temperatures. Using an in-situ Auger spectrometer, the samples' surface chemistries were investigated in parallel with uptake rates at the various temperatures. Graphitic carbon was found to be the dominant surface constituent. To understand the role of oxygen in the lubrication process, some samples have been exposed to oxygen before TCP deposition. Comparing deposition rates of pure and oxide surfaces and the corresponding Auger scans allowed determination of whether or not oxygen has an effective part in the lubrication of these two metals with TCP. Our results suggest that oxygen is not a factor in controlling TCP interaction with Fe. Overall, we observed TCP to diffuse more readily into Fe than Cr, supporting recent conclusion by N. Forster.1 on the diffusion mechanisms underlying the lubricants effectiveness. In addition, we observed the TCP to greatly inhibit the uptake of oxygen from the gas phase. |
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SS-TuP-5 Density-Functional Study of NO Dissociation on the Al(111) Surface
B. Razaznejad, B.I. Lundqvist (Chalmers University of Technology, Sweden) A first-principles density-functional study utilizing the generalized-gradient approximation (GGA) for exchange-correlation effects is made for the energetics and electron structure of nitrid monoxide (NO) on the (111) surface of aluminum. The calculated adiabatic potential energy surfaces show that (i) when the molecule is kept perpendicular to the surface, with the oxygen atom down, there exist an activation barrier of 0.6 eV in the exit channel, while (ii) there exist a moleculer chemisorption state, identified as a "NO2-" electron configuration, precursor to dissociation, when the molecule is kept parallel or perpendicular to the surface, with the nitrogen atom down. The complexity of the adiabatic potential energy surface indicates an interesting dynamics for NO dissociation on the (111) surface of aluminum. |
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SS-TuP-6 Electrochemical Characterization and Surface Analysis of Bulk Amorphous Alloys in Aqueous Solutions at Different pH
D.L. Cocke (Lamar University); D.G. Naugle (Texas A&M University); H. McWhinney (Prairie View A&M University); R. Schennach (Lamar University) Bulk amorphous alloys are a new class of materials with a variety of characteristics that make them useful for applications in aqueous environments. While some bulk amorphous metals show increased corrosion resistance, there is still a lack of fundamental electrochemical studies of these materials. Three different compositions of BeCuNiTiZr bulk amorphous alloys have been studied at pH 5, 7 and 8 using Cyclic Voltammetry (CV), X-ray Photoelectron Spectroscopy (XPS) and electron microscopy. While XPS is used to determine the composition of the resulting oxide films, the CV curves are compared to the corresponding binary alloys, to pinpoint differences in the corrosion resistance of the amorphous multi-component alloy. The effect of the amorphizity, multi-component structure and the presence of elements with widely varying interfacial reactivities will be discussed. |
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SS-TuP-8 Characterization of High Surface Area Platinum Black Electrodes for Electrochemical Sensing Applications
P. Neuzil (Institute of Microelectronics, Singapore); B. Ilic, D. Czaplewski (Cornell University); T. Stanczyk (Molex Incorporated); J. Blough, G.J. Maclay (University of Illinois at Chicago) The investigation of adsorbed high surface area noble metal nanoparticulates is important in medicine, nanotechnology, electrochemistry, microelectromechanical systems (MEMS) and sensors, biology, and in industry. In particular, high surface area noble metal electrodes have made a considerable impact in the field of microfabricated solid state, chemical and biological sensors. In our work, we investigate the properties of electrochemically deposited platinum black by atomic force and scanning electron microscopy. Initially the platinum electrodes were fabricated on various substrates utilizing developed silicon surface micromachining techniques. Platinum black was then electrochemically deposited on top of the microfabricated platinum electrodes. The platenized electrodes were subsequently used in an electrochemical cell for carbon monoxide detection (CO). The primary benefit of platinized electrodes is the increase in the surface area of the catalyst, which in turn causes an enhancement in the CO sensitivity by a few orders of magnitude. Deposition time and thermal conditioning were found to influence the quality and morphology of the platinum black layer. Methods of fractal analysis were employed to the acquired tapping mode atomic force micrographs in order to demonstrate the degree of roughness of the platinization. Morphological inclusions were readily observed in films deposited for duration of less than 60 seconds, at a bias of 1.5 V against a platinum counter electrode. Shorting of the microfabricated electrodes due to lateral outgrowth of high surface area platinum black was observed for various electrode geometries when current densities on the order of 100 mA cm-2 were employed. We further show from out CO sensor performance that reproducibility of highly adherent platinized electrodes is achieved. |
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SS-TuP-9 A Novel Technique for Producing Regular Nano-Channel Arrays
S.A. Barnett, S. Sambasivan (Applied Thin Films, Inc.) In this paper, we describe a simple method for producing large area nano-channel arrays with channel widths down to 1 nm. The synthetic method involves deposition of multilayers by magnetron sputtering where the width of each layer can be accurately controlled from 1-100nm. Subsequent to the deposition, one of the layers is preferentially etched to leave behind trenches of the other layer left intact. With appropriate choice of chemistry and etching technique,we have demonstrated the feasibility of this technique for a multilayer system. The residual layer and the intervening channels are quite planar for optimized film growth conditions, and thicknesses can be very accurately controlled. A number of applications can be envisioned for these channel arrays. For example, they should make good media for nano-stamping with the choice of a very hard material as one of the component layers. This should allow for imprinting of a wide range of materials. Initial results on nano-stamping will be presented. |
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SS-TuP-10 In situ Investigation of Stage Formation of Eutectic Alloys during Annealing Amorphous and Crystalline Silicon with Metals
B.B. Bokhonov, M.A. Korchagin (Institute of Solid State Chemistry SB RAS, Russia) The in situ electron microscopic investigation of the formation of eutectic alloys in the system: amorphous film Si or crystalline thin foil (100)Si - particle Me (Me = Au, Ag, Cu, Al), showed the formation of eutectics is preceded by metal diffusion into amorphous silicon with the formation of metastable amorphous metal silicide. Supersaturation and decomposition of metastable metal silicide leads to the evolution of polycrystalline silicon. Morphological characteristics of liquid eutectic formation in system: crystalline silicon foil - metal particle are similar to well known morphological characteristics of the formation of etch pits in crystals. For the systems: crystalline silicon foil - metal particle (gold, silver and copper), oriented formation of the liquid evtectic are observed. The crystallization of liquid eutectic alloys leads to topotaxial evolution of metal silicide islands or compounds at the interface - eutectic alloy/silicon crystal. |
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SS-TuP-11 The Study of ITO Targets Produced at Various Temperatures
J.H. Choi, H. Choi, J.S. Lee, J.Y. Won, J.C. Lee, C.B. Lim (Samsung Advanced Institute of Technology, Korea) ITO targets were produced by annealing of mixed powder of 90 wt% In2O3 and 10 wt% SnO2 at various temperatures. The products were investigated by using TEM/EELS, XRD, XPS, and AES. The XRD results show that the phases of In2O3 and SnO2 remain unchanged until 1300°C annealing. Whereas only In2O3 phase appears over 1400°C annealing. Further experiments with TEM/EELS demonstrate that in fact the In2O3 over 1400°C annealing is Sn-doped In2O3 which is dominant phase and there is another phase which is Sn-rich In2O3 . As the annealing temperature increases, the grain size increases, but the phases still remain. And many small spots exist inside Sn-doped In2O3 grains. It is In2O3 which is not Sn-doped. It is thought that the content of SnO2 is more than proper to achieve Sn-doped ITO target completely. |
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SS-TuP-12 Structure Analysis of Ta Films Deposited on Si(100), SiO2, and Si3N4 Surface by Tapping Mode Atomic Force Microscopy
F. Wu (Medtronic, Inc.) Ta films deposited on Si(100), SiO2, and Si3N4 surface during growth at different pressure and temperature were investigated using tapping mode Atomic Force Microscopy. The surface morphology of the films was found to depend on the subtrate surface, sputtering pressure and substrate temperature. Film stress and roughness of Ta/different substrate vs different deposition conditions were measured. The surface roughness of the films decreased with decreasing sputtering pressure and with increasing substrate temperature. |
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SS-TuP-13 The Orientational Effects under Ion Grazing Incidence on the GaP(100) Surface
U.O. Kutliev (Urganch State University, Uzbekistan); A.A. Dzhurakhalov (Institute of Electronics, Uzbekistan); B.C. Kalandarov (Urganch State University, Uzbekistan) Research of laws of grazing scattering of ions begins with study of the form of trajectories of scattering particles. The forms these trajectories are rather difficult, as they are determined large by number correlated of collisions. This circumstance not allows to receive the analytical decision of the given task. By unique effective way of account of the form of trajectories grazing scattering is the modeling on the computer. In the present work the scattering processes on the GaP(100) surface under grazing ion bombardment have been investigated by computer simulation. The trajectories of incident positive ions Ne were simulated in the binary collision approximation. Energy and angular distributions of Ne+ ions scattered from the GaP (100) surface in the <110>, <100> directions and target orientations were calculated. The computational results show that in the energy spectrum the characteristic peaks corresponding to scattering of ions by the surface atomic chains and semichannels are observed. The situation and intensity of these peaks depend on the forms of the surface semichannels. This technique can be useful for investigations of semiconductor structures. |
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SS-TuP-14 Tribocharging in Electrostatic Beneficiation of Coal: Effects of Surface Composition on Work Function as Measured by XPS and UPS in Air
S. Trigwell, M.K. Mazumder (University of Arkansas at Little Rock); R. Pellissier (RKI Instruments) The cleaning of coal by electrostatic beneficiation is based on tribocharging characteristics of pulverized (smaller than 120 µmm) coal particles. The variation in the charge distribution of electrostatically separated coal particles, as measured by an E-SPART analyzer, indicates that coal exhibits bi-polar charging. This appears to be in conflict with expectations in that the organic coal particles should charge positively, and the mineral particles, present as impurities such as pyrite, charge negatively, as predicted by the relative work functions of the particles (coal and mineral) and the metal surface (copper or stainless steel) used for tribocharging. UPS measurements in air on specimens of three different coal species showed the work function to be higher than predicted, approximately 5.4 eV. Determination of the coal surface composition by XPS showed the coal and the pyrite to be oxidized to different degrees. Studies by UPS and XPS, on copper, stainless steel, aluminium, and other commonly used tribocharging materials such as glass and PTFE, as well as pure pyrite, showed that the work function varied considerably as a function of surface oxidation. Therefore the reason for the bi-polar charging of the coal particles may be due a too small work function difference. The choice of a material for impaction triboelectric charging for coal or mineral separation should therefore depend upon the oxidized work function difference. The correlation between the charge distribution and work function is discussed. |
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SS-TuP-15 The Secondary Electron Emission Characteristics for Sol-Gel Based SiO2 Thin Films
T. Jeong, J. Lee, S. Yu, S. Jin, J. Heo, W. Yi (Samsung Advanced Institute of Technology, Korea); D. Jeon (Myongji University, Korea); J.M. Kim (Samsung Advanced Institute of Technology, Korea) We have fabricated SiO2 thin films by sol-gel method with six different tetraethyl orthosilicate (TEOS) molarities, which were 0.116 M, 0.058 M, 0.029 M, 0.015 M, and 0.007 M. Each solution of the different TEOS molarity was coated on the Si substrate by a spin coater. The SiO2 layer was formed on the substrate by thermal heating at 450°C for three hours. From Nanospec/AFT Model 200 the thickness of the SiO2 film decreased as the concentration of TEOS decreased; the thicknesses of SiO2 were estimated to be 69 nm, 36 nm, 16 nm, 11 nm, 9 nm, and 7 nm for 0.232, 0.116, 0.058, 0.029, 0.015, and 0.007 molarity of TEOS solution, respectively. Finally, we measured secondary electron emission (SEE) yields for these SiO2 thin layers by bombarding electrons in the vacuum chamber which was maintained at around 110-7 torr. It was observed that the 9 nm thick SiO2 film exhibits the highest SEE yield (about 4) among six samples. This observation agrees well with our previous results obtained from thermal SiO2 films. The higher SEE yields for sol-gel based SiO2 films than thermal SiO2 films suggest that the sol-gel method is useful for secondary electron emission layer formation. In conclusion, the solution based SiO2 layer coating method is promising, especially for secondary electron emission layer for the electron multiplying devices, due to high secondary electron emission yield and its easy application to a porous surface such as alumina. |
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SS-TuP-16 Absolute Determination of the Stoichiometry of Ultrathin Oxide Films as a Function of Thickness: Antimony Oxide on Gold
K. Stefanov, A.J. Slavin (Trent University, Canada) As the thickness of oxides in microelectronic devices decreases, it has become essential to know how oxide stoichiometry evolves with film thickness. This work uses a high-stability quartz crystal microbalance1 which can provide an absolute determination of stoichiometry as it evolves one molecular layer at a time. In the case of antimony films on a gold substrate, the first two monolayers of Sb oxidize as SbAuO, whereas subsequent layers oxidize as Sb2O3. |
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SS-TuP-17 Dodecanethiol on Cu(110) Studied by Low-temperature STM
A. Kühnle, T.R. Linderoth, E. Laegsgaard, I. Stensgaard, F. Besenbacher (University of Aarhus, Denmark) Monolayers of alkane thiols, primarily on Au(111), have been studied extensively as model systems for self-assembly. A number of UHV-STM investigations have been carried out at room temperature adressing the structure of different phases formed from low coverage where the molecules are lying down to higher coverage where they are standing-up.1 Very little is known, however, about the diffusion dynamics of the individual molecules or the quantitative details of the initial nucleation and growth of molecular aggregates on the surface. We have recently initiated a programme to investigate such questions using a home-built UHV-STM operating at variable temperatures down to 25 K. For dodecanethiol [CH3(CH2)11-SH] deposited onto Cu(110) at a sample temperature of 110 K we find the formation of an ordered overlayer structure with molecules lying with their backbone parallel to the surface. There is thus considerable molecular mobility on the surface already at this temperature. We are currently extending these experiments to lower temperatures where the diffusion of individual molecules can be followed by time-lapse STM. |
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SS-TuP-18 Temperature-induced Morphology Changes for 1D Pt Islands on Pt(110)-(1x2)
T.R. Linderoth, S. Horch, L. Petersen, E. Laegsgaard, I. Stensgaard, F. Besenbacher (University of Aarhus, Denmark) The Pt/Pt(110)-(1x2) system has proven a fascinating one-dimensional model system for the study of adatom dynamics by time-resolved STM. At temperatures between 290-380 K, isolated ad-atoms and islands are restricted to the missing-row troughs where atoms show a quasi-1D diffusion behaviour1 while islands migrate along the troughs by a so-called "leap-frog" mechanism.2 Here we wish to present the results of quench-and-look experiments initiated to investigate ripening in this seemingly 1D system. When a surface with 1D Pt islands situated in the missing-row troughs is annealed at 369-395 K, we find: (a) an increase in the mean island size (coarsening) and (b) a surprising island restructuring, where adatoms move from the troughs up on top of the islands. Quantitative analysis of these findings and comparison to the known rates for island diffusion and other relevant kinetic processes interestingly reveal that the Pt/Pt(110)-(1x2) system can NOT be treated as purely 1D at the higher adatom coverages of 27±4 % used in the present experiments. Dynamic STM data, acquired at these higher adatom coverages, provides direct evidence for novel atomic-scale mechanisms leading to inter-trough mass transport. |
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SS-TuP-19 Friction on Diamond-Like Thin Films : Analysis by XPS
J.M. Campbell, L.-S. Johansson (Helsinki University of Technology, Finland); J. Koskinen (VTT Manufacturing Technology, Finland) Under humid conditions, an anomalous drop occurred in the coefficient of friction for a stainless steel ball bearing impinging on a Diamond-Like Coated (DLC) metal ring. XPS analysis of the wear area revealed minute but significant changes from the native film surface. Metal particles were present on the wear track; however, no chemical change in the DLC film was apparent. Assessment of the inelastic background shows that the metal resides on top of the DLC film. It is believed that the metal particles adsorb water from the humid atmosphere to form a lubricating layer. |
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SS-TuP-20 Scanning Tunneling Microscopy of Single-Crystal Si3N4 Layer Grown on Si(111) by Nitridation
C.-L. Wu, H. Ahn, Y.-C. Chou, S. Gwo (National Tsing-Hua University, Taiwan) It has been known that ordered thin Si3N4 layer can be formed on Si(111) by exposing the Si surface to various nitrogen-containing gases (NH3, NO, N, etc.) at high substrate temperatures. A variety of surface orderings ("8x8", "8/3x8/3", "4x4", and quadruplet) have been reported by previous studies using different surface-sensitive techniques, such as low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM), etc. However, until now there is no proposed structure model can satisfactorily incorporate all the observed features. In this work, by using high-resolution STM images at dual bias polarities in combination with Kikuchi electron holography (KEH), transmission electron microscopy (TEM), and ab initio total-energy calculations, we have determined an atomic model for this surface, which is capable of explaining the previously observed surface orderings. |
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SS-TuP-21 Electron- and Photon-stimulated Desorption of Alkali Atoms from Silicon Dioxide and Ice Surfaces
B.V. Yakshinskiy, T.E. Madey (Rutgers University) To investigate mechanisms for the origin of K and Na in the tenuous atmospheres of the Moon, Mercury and Europa, we are studying the electron- and photon-stimulated desorption (ESD and PSD) of alkali atoms from model surfaces: amorphous SiO2 and water ice films (both crystalline and amorphous) grown on a metal substrate. The measurement scheme for ESD and PSD of alkali atoms includes a highly sensitive detector based on surface ionization, and a time-of-flight technique. For PSD measurements, a mercury arc light source (filtered and chopped) is used. In the present work, we focus on ESD and PSD of K atoms, and compare with our previous studies of Na desorption from SiO2.1 We find that bombardment of the alkali covered surfaces by ultraviolet photons or by low energy electrons (E>4 eV) causes desorption of "hot" K and Na atoms. The velocity distributions (VD) of K and Na desorbing from the silica surface are peaked at 650 and 1000 m/s, respectively, whereas the corresponding VD for desorption from the ice surfaces have maxima at 500 and 800 m/s. These values are consistent with the "hot" components of the lunar, Mercurian and Europian atmospheres. The mechanism of desorption is identified as an electronically excited charge-transfer from the silica or the ice substrate to neutralize alkali ion, followed by desorption of the alkali atom. We conclude that PSD by UV solar irradiation is a dominant source process for K and Na in tenuous planetary atmospheres. |
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SS-TuP-22 Using Surface-modified Embedded-atom-potentials to Simulate the Adsorption and Diffusion of Gold and Lead Atoms on the Au(111) Surface
M.C. Robinson (Trent University, Canada); K. De'Bell (University of New Brunswick, Canada); A.J. Slavin (Trent University, Canada) The usual embedded-atom method (EAM) typically underestimates surface energies and surface diffusion barriers on (111) surfaces. However, the EAM potentials can be modified1 to provide a fit to the surface energy without modifying the zero-temperature bulk properties. These modified potentials provide much more reasonable values for other surface properties. For example, they predict a surface-layer compression in agreement with that observed in the Au(111) "herringbone" reconstruction. Molecular-dynamics simulations indicate that the preferred mode of surface diffusion is incorporation of an adatom into the surface at one location with the ejection of another atom to the surface elsewhere. In the case of Pb deposited on the reconstructed Au(111) surface, they also predict an instability in the surface energy for a coverage of about 0.05 monolayers. Comparisons are made with scanning tunneling microscopy data for Pb on the Au(111) surface. Research supported by NSERC Canada. |
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SS-TuP-23 The Adsorption and Trimerization of Acetylene to Benzene on Cu(110) Monitored by X-ray Spectroscopies
H. Öström, K. Weiss, L. Triguero, A. Nilsson (Uppsala University, Sweden) We have studied the adsorption of acetylene on Cu(110) and its trimerization reaction to benzene by high resolution X-ray photoelectron spectroscopy (XPS), near edge X-ray absorption fine structure (NEXAFS) spectroscopy and temperature programmed desorption (TPD). At liquid nitrogen temperature the XP spectra show two different C 1s peaks which correspond to two nonequivalent adsorbed acetylene species. By heating the sample one species transforms into the other. This species disappears above room temperature due to the trimerization of acetylene to benzene. The reaction has been followed by time resolved XP spectroscopy. We have been able to monitor the reaction in real time, finding that benzene leaves the surface as soon as it is formed. The adsorption geometry of the different adsorbate species has been determined by polarization dependent NEXAFS spectra and the experimental results are completed by ab-initio cluster model calculations performed in the framework of density functional theory (DFT). |
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SS-TuP-24 Study of High and Low Work Function Surfaces for HyperThermal Surface Ionization using an Absolute Kelvin Probe
I.D. Baikie, U. Petermann, B. Lägel, K. Dirscherl (Robert Gordon University, UK) We have performed a study of high (> 6eV) and low (<3 eV) work function surfaces in order to identify suitable target materials as an ion source for a new type of mass spectrometer technique based upon Hyperthermal Surface Ionization (HSI). In this application a molecular beam of neutral gas molecules is ionized by supersonic collision on a target surface. High work function surfaces produce positive ions (pHSI), low work function surface negative ions (nHSI). Analytical merits of HSI include very high sensitivity, atmospheric pressure inlet and informative mass spectra. As this technique does not use electron impact filaments the amount of cracking products is substantially reduced. The Ultra-High Vacuum Scanning Kelvin probe (SKP) is a technique producing relative work function topographies between a scanning reference tip and sample in a truly non-invasive fashion with high accuracy (1-2 meV). We demonstrate a novel extension of this technique, using photoelectric (PE) determination, which produces absolute work function data even if the tip work function is not known. Using this hybrid probe, together with SEM and AES, we have followed a) work function topographies of clean surfaces in UHV, b) changes in work function with oxidation that are related to surface cleaning processes, c) the temperature dependent (300 -1000 K) oxidation kinetics of polycrystalline metal surfaces (Re, Pt, Mo, W, Pd) for pHSI and d) electron beam evaporation of Ca, Gd and LaB6 for nHSI. We will report the optimum parameters for target stability and performance under both pHSI and nHSI operating conditions. We will also illustrate informative mass spectra produced by Time-of-Flight HSI. |
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SS-TuP-25 A Double Quartz Crystal Microbalance Sensor for Monitoring High Pressure Heterogeneous Catalytic Reactions on Real Catalysts
I. Zori@aa c@ (Chalmers University of Technology and Göteborgs University, Sweden); P. Borchardt, C. Keller, B. Kasemo (Chalmers University of Technology, Sweden) The quartz crystal microbalance (QCM) is a piezoelectric oscillator with a high sensitivity for micro weighing (<0.01ML of H2) and a fast response. In this work we report a novel sensor consisting of two QCMs mounted on a single, 5 MHz, AT cut quartz crystal. The catalyst consists of metallic, highly dispersed, nanosized Pt particles on an Al2O3 washcoat deposited on one QCM by standard wet deposition methods. The other QCM, coated with washcoat only, was used as a reference allowing for subtraction of the frequency shift due to temperature changes, or in a constant temperature experiment, the separation of gases adsorbed on the noble metal of the catalyst from those adsorbed on the alumina support. The catalytic reaction chosen for testing of the sensor system was the high pressure H2+O2 reaction, catalyzed by nanosized Pt particles on alumina support. The resonant frequencies of the two QCMs, proportional to the total coverage of the reactants, were monitored. The time dependence of the frequency difference (proportional to the total coverage) reflects reaction kinetics on the Pt particles. These measurements are complemented by mass spectrometric measurements of the gas phase products, yielding wealthy information about reaction kinetics on a real catalyst at high pressure. In addition we also demonstrate the ability of the sensor for detection of kinetic phase transitions in the above reaction. The sensitivity of the sensor is estimated to about 30Hz/1015 Pt atoms. The noise level is < 1Hz thus enabling us to measure coverage changes corresponding to 2-3% ML. |
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SS-TuP-26 Effect of Step Edges upon CO Dissociation over Ni Surfaces
H. Nakano (University of Tsukuba, Japan); T. Fujitani (National Institute for Resources and Environment, Japan); J. Nakamura (University of Tsukuba, Japan) We have studied the formation process of surface carbide on Ni(111) and Ni(977) by Boudouard reaction (2CO --> CO2, g + Ca) using UHV-STM in order to clarify the dissociation site of CO. A reconstructed structure consisting of square units due to carbide was observed at the step-edges on Ni(111) when the carbide coverage was about θ c = 0.01. Islands of the reconstructed structure were formed on the upper terrace in the vicinity of specific step-edges. The same square units were then observed on the terrace at a carbide coverage of 0.13. This structure consisted of three domains in which one direction of the square unit corresponded to that of the Ni(111) substrate's hexagonal unit. Thus, the surface reconstruction caused by carbide formation was found to occur from step-edges. The formation rate of carbon on Ni(111) depended on annealing temperature after Ar ion sputtering. Increasing anneal temperature from 850 to 1100 K resulted in a decrease in formation rate of carbon by about 50 %. STM results showed that the number of defects on the Ni(111) surface decreased upon the annealing at 1100 K. These results indicate that Boudouard reaction or the CO dissociation occurs at defect sites. The formation rate of carbon on a stepped Ni(977) surface was promoted remarkably compared with Ni(111). The apparent activation energies of the carbon formation on Ni(977) and Ni(111) were measured to be - 36.4 and + 7.2 kJ/mol, respectively. It is considered that the formation of carbon consists of the CO dissociation at step sites and subsequent migration onto (111) terraces. |
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SS-TuP-27 Ion Desorption from Molybdenum Oxide by Specific Core-to-Valence Resonant Excitations at the Mo L2,3, Mo M2,3 and O K-edges
G. Wu, Y. Baba, I. Shimoyama, T. Sekiguchi (Japan Atomic Energy Research Institute, Japan) Irradiation of X-rays on solid surface induces various chemical changes such as decomposition and desorption. Due to the localized nature of inner-shell electrons, the primary excitation localizes around specific element or specific chemical bond. The question is whether or not the primarily localized excitation leads to the specific chemical reaction. The examples for specific chemical-bond scission and ion desorption induced by core-level excitation have been reported for adsorbed molecules on solid surfaces. Here we present the results for the ion desorption from bulk material induced by a specific core-to-valence resonant excitation. Molybdenum trioxide was chosen as a sample, because this material is known to be sensitive to irradiation of X-ray or electrons. In the X-ray absorption near-edge structure (XANES) spectra, the resonance peaks corresponding to the excitations from Mo2p to 4d have two components due to the ligand-field splitting of 4d state, which are assigned as transitions from Mo 2p to triply degenerated t2g-type orbits and doubly degenerated eg-type orbits. When we compare the desorption-yield curves with XANES spectra taken by total electron yield, the O+ ion yield at the resonance from Mo 2p3/2 into the eg-type orbits was enhanced compared to that into the t2g-type orbits. Similar enhancements were also observed for the Mo M3 and O K-edge excitations. Considering the antibonding character of the eg-type orbits, the enhancement of desorption is explained by the localization of excited electrons in the eg-type orbits which leads to the specific Mo-O bond breaking and O+ desorption. The detailed mechanism will be presented on the basis of the results for the Auger decay spectra. |
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SS-TuP-28 Surfactant Behaviors of Bi and As in the Growth of Ge/Si(001)-2x1 Systems Studied with Photoemission Spectroscopy
P. De Padova (Consiglio Nazionale delle Ricerche, Italy); R. Larciprete (ENEA, Italy); C. Quaresima, A. Reginelli, S. Priori, E. Paparazzo, L. Moretto, P. Perfetti (Consiglio Nazionale delle Ricerche, Italy) Heteroepitaxial Ge/Si(001)-2x1 systems show promising applications in the fabrication of infrared detectors. A layer-by-layer growth is essential for smooth films to be obtained, and this is greatly facilitated by addition of surfactant elements, such as As, Bi and Sb. Among these, Bi is a good candidate, as it little affects the Ge chemistry and has a low desorption temperature, whereas As has been generally regarded as being the ideal surfactant element. We used synchrotron radiation (SR) photoemission and XPS techniques to comparatively study the roles that Bi and As play in the growth of thin and thick Ge films on Si substrates. Angle-resolved SR core level spectra allowed us to identify two distinct growth modes in Ge/Bi/Si(001) systems as a function of Ge coverage. Sub-monolayer Ge coverages involve Ge- and Bi-atoms exchanging sites with each other, whereas greater Ge coverages involve the Ge-atoms lying both underneath Bi-atoms, as well as on top of the Si substrate. XPS depth-profiling conducted into a 15-monolayer Ge-film reveals that the Bi-atoms remain confined to the true surface. SR spectra of the Ge/As/Si(001) system show that the As-atoms lie anchored to the Si substrate, even for great Ge coverages, which is at striking variance with the expected layer-by-layer growth. Ge growth, carried out via codeposition of As and Ge on Si substrates, gives spectral evidences suggestive of the As atoms diffusing downward the Ge film, with no Ge-As site exchange taking place. We discuss our results in light of theoretical and experimental reports available in the literature, and propose an interpretation to account for the different surfactant behaviors of Bi and As. |
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SS-TuP-29 Electron Transfer in Organic Monolayers
K. Raiber, B. Zeysing, A. Terfort (University of Hamburg, Germany) The electron transfer through organic monolayers is a field of research which is receiving an increased interest since these monolayers on conductive materials are not only used as barrier and protection materials1,2 but also as conductors. These new applications for the monolayers require a detailed understanding of the processes during the electron transfer through the monolayer. There are three different possible conduction mechanisms through monolayers discussed in literature:3 the through bond conduction, a through-bond tunneling and a through-space tunneling. To discriminate between the conduction and the tunneling mechanisms we investigated the electron transfer properties of unsubstituted aliphatic and aromatic molecules by cyclovoltammetry. To avoid problems with the match of Fermi-niveaus we attached an electrochemically active group to an aliphatic and an aromatic thiol, respectively, of approximately the same length and investigated the electron transfer within SAMs formed by these molecules on gold surfaces. The exact knowledge of the place of electron generation, which is determined by the immobilisation of the electrochemical active group on a gold electrode, allowed a better determination of the conduction mechanism. |
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SS-TuP-30 Reactions of Hydrazoic Acid (N3H) on Water and Gold Surfaces Characterized Using RAIRS and XPS
S.R. Carlo, J. Torres, D.H. Fairbrother (Johns Hopkins University) The chemistry of hydrazoic acid (N3H) on gold and amorphous ice surfaces was studied as a function of N3H exposure, temperature, and X-ray irradiation using reflection absorption infrared spectroscopy (RAIRS) and XPS. N3H adsorbed on ice at 100K initially led to azide production (N3-), and subsequent N3H deposition. On Au, N3H adsorbed molecularly at 100K. On either surface annealing caused increased order in the film and loss of N3H correlated with N3- production. Three new IR bands were observed in the NH stretching region at T>180 K. The adsorption characteristics and thermal chemistry of N3H on ice suggest a chemisorbed state. These NH and N3- bands were stable to T>220K. Experiments were performed using N3D to elucidate the nature of these species. On both surfaces X-ray irradiation induced significant molecular desorption, increased azide concentration and production of the same three bands in the NH stretching region as observed when heating. |
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SS-TuP-31 Adsorption and Desorption Processes of Alkanethiol Self-Assembled Monolayers on Au(111)
J. Noh (Frontier Research System, RIKEN, Japan); T. Araki (Saitama University, Japan); K. Nakajima, M. Hara (Frontier Research System, RIKEN, Japan) Adsorption and desorption processes of alkanethiol self-assembled monolayers (SAMs) on Au(111) have been examined by scanning tunneling microscopy (STM) and thermal desorption spectroscopy (TDS). New intermediate phases that are expected by a recent diffraction study from coverage-dependant SAMs prepared by a wet process were observed in a molecular level, revealing phase transitions from the striped phase to the upright phase. In addition, surface structure and molecular species desorbing from such SAM samples as a function of surface temperature were monitored by TDS and STM. TDS spectra for dimer molecules as well as alkyl groups formed after cleavage of C-S bond clearly show second-order desorption behavior implying the associative desorption of alkanethiolate adsorbed on Au(111) surface. STM images clearly show structural transitions due to desorption of molecules at elevated surface temperature. Moreover desorption process and stability of pre-covered alkanethiol SAMs in pure solvents at room temperature were investigated. In this study, it is revealed that desorption of alkanethiol SAMs under solvent and UHV conditions initiate at domain boundaries and near depressions that have weaker lateral interactions compared to close-packed domains, followed by desorption of molecules in close-packed domains. From these results, we discuss the adsorption states in alkanethiol SAMs as well as the precise model of the adsorption and desorption processes. |
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SS-TuP-32 Hydroxymethylcyclopropane on Oxygen-covered Mo(110): A Radical Clock on a Metal Surface?
I. Kretzschmar, J.A. Levinson, C.M. Friend (Harvard University) Radical clocks are a well-established means for the determination of radical lifetimes in liquid and gas-phase organic chemistry.1 However, no equivalent tools yet exist for the evaluation of radical lifetimes on surfaces, although radicals are often proposed as the most important intermediates in surface reactions. In this study, the reaction of hydroxymethylcyclopropane has been investigated on clean and oxygen-covered Mo(110) surfaces. Since methylcyclopropane is the expected product upon C-O bond scission in hydroxymethylcyclopropane and represents the smallest possible model system for a radical clock, this molecule seems to be a promising system for gauging radical lifetimes in the vicinity of the surface. Changes in binding and structure of the adsorbate are monitored using a combination of temperature programmed reaction spectrometry and reflectance-infrared absorbance spectroscopy. These studies reveal that the surface species formed upon adsorption of hydroxymethylcyclopropane onto oxygen-covered Mo(110) is stable up to 400 K. Above 450 K, 1-butene, 1,3-butadiene, and ethene are produced. In addition, two new vibrational peaks develop at 1245 cm-1 and 1645 cm-1 in the infrared spectra obtained after heating to 450 K. Both peaks are attributed to ring-opened surface intermediates: the 1645 cm-1 peak to alkoxide species and the 1245 cm-1 peak to the formation of a metal-bound alkyl species after ring opening. The observation of ring-opened products points to the fact that ring opening is faster than hydrogen abstraction from the surface. The experiments are discussed in the general framework of alkyl oxidation processes. |
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SS-TuP-33 LEED and STM Investigations of the Heteroepitaxy of Perylenetetracarboxylic-dianhydride (PTCDA) on Hexa-peri-benzocoronene (HBC) on Reconstructed Au(111) Surfaces
T. Fritz, F. Sellam, T. Schmitz-Huebsch, M. Toerker, S. Mannsfeld, H. Proehl, K. Leo (TU Dresden, Institut fuer Angewandte Photophysik, Germany) The fabrication of multilayered films and superlattices consisting of different molecules has recently attracted considerable interest because of the prospective applications of those structures as active optoelectronic components in modern integrated devices. While the epitaxy of inorganic semiconductor heterojunctions is already well established, only little is known about the growth of organic heterostructures. We present the result of the combination of two OMBE (organic molecular beam epitaxy) grown materials of planar molecules, namely perylene-tetracarboxylic-dianhydride (PTCDA) and hexa-peri-benzocoronene (HBC). As substrate the Au(111) surface was chosen. Two complementary surface analysis methods which are low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) have been applied to characterize the growth of the ultrathin organic molecular heterojunction and to determine the epitaxial relation between the two lattices. In a first step, we investigated the growth of HBC on reconstructed Au(111) surfaces. Large domains which are defect free on the molecular scale are observed by STM. The LEED investigations reveal that HBC grows on reconstructed Au(111) in a commensurate hexagonal structure. The HBC lattice is rotated by 30° with respect to the Au[110] direction and therefore it appears only in a single domain orientation. In a second step, a monolayer of PTCDA was deposited on HBC and structurally characterized. The LEED and STM measurements show the well-known herringbone alignment of the PTCDA molecules, similar to that in the (102) bulk plane with two molecules in a rectangular unit cell, indicating the p2gg space symmetry group. Besides these findings, the pronounced two-dimensional Moiré pattern in the STM images was used to determine the relative orientation of the PTCDA unit cell with respect to the underlying HBC lattice, revealing a parallel orientation between the short PTCDA lattice vector and the HBC[10] direction. |
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SS-TuP-34 DeNOx Reactions on MgO(100): Photoemission and Density Functional Studies
T. Jirsak, J.Z. Larese (Brookhaven National Laboratory); A. Maiti (Molecular Simulations Inc.); J.A. Rodriguez (Brookhaven National Laboratory) In environmental catalysis the destruction or removal of nitrogen oxides (DeNOx process) is receiving a lot of attention. Nitrogen oxides (NO and NO2) are formed in automotive engines and industrial combustion systems. Metal oxides can be useful in the control of environmental pollution. It has been found that MgO has the ability of trapping the NO2 formed in automotive engines during the burning of fuels under oxygen-rich conditions. Synchrotron-based photoemission and first-principles density-functional calculations (DF-GGA) have been used to study the chemistry of NO and NO2 on MgO(100). NO is weakly adsorbed on the oxide surface. On flat MgO(100), DF results predict an NO adsorption energy of 6 kcal/mol, which is close to the value of 5 kcal/mol observed experimentally. At steps of the surface, NO interacts with tetra- and penta-coordinated Mg atoms and the adsorption energy can increase to 9 kcal/mol. On MgO1-x, O vacancies and defect sites enhance the bonding energy of NO, and lead to the production of N2O at 100 K plus the deposition of N above 200 K. NO2 is very reactive on terraces and steps of MgO(100). Adsorbed NO3 and NO2 are detected after exposing the oxide to NO2 at 150 K. Strong bonding interactions between Mg sites and NO3 favor disproportionation of adsorbed NO2. The large differences in the reactivity of NO and NO2 reflect the fact that nitrogen dioxide is a much better electron acceptor. The role of metal promoters (K,Cs,Cr,Ni) on DeNOx operations on MgO will be discussed. |
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SS-TuP-35 Oxygen Loss and Recovering Induced by Ultra High Vacuum and Oxygen Annealing on WO3 Thin Film Surfaces
M. Pssacantando, L. Lozzi, C. Cantalini, S. Santucci (University of L'Aquila, Italy) Thin films of WO3 with thickness in the range of 150 nm have been deposited onto silicon substrates by thermal evaporation. The films as deposited and annealed at 300 and 500°C in oxygen for 24 hours were submitted to UHV annealing. XPS measurements (W4f and valence band) show for the as deposited sample annealed at 200°C in steps of 50°C a marked increase of the metallic features of the surface attributable to oxygen loss. This film extracted from the UHV ambient and annealed in oxygen at 200° shows XPS features meaningful of a renewed oxygen incorporation which is again reduced by the UHV annealing demonstrating an evident reversibility towards the oxygen loss and recovery for this kind of sample. Also the WO3 film treated by oxygen annealing at 300° shows an increase of the metallic character of the film surface due to oxygen loss as a consequence of the UHV annealing up to 300°C but the sample extracted from the UHV ambient and submitted to a second annealing in oxygen at 300°C for 24 h recovers oxygen remaining substantially stable and scarcely influenced by successive UHV annealing. Finally the surface of the sample initially annealed in oxygen at 500°C appears at the XPS measurements evidently stable after the UHV annealing up to 500°C. These results have been confirmed by Scanning Tunneling Microscopy measurements which evidenced for the as deposited sample a narrowing of the band gap when the annealing temperature in UHV overcomes 300°C. Gas sensing towards NO2 of samples with UHV annealing modified surfaces have been also investigated giving encouraging results respect to the increase of the sensitivity of the sensor. |
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SS-TuP-36 Spectral Changes in Far and Near Field Optical Microscopy of Surface Clusters
M. Xiao (CCMC-UNAM) We theoretically study the spectra in far and near field optical microscopy of a group of nanoscopic scatterers such metallic and semiconductor quantum dots two-dimensionally distributed on a plane surface. Our emphasis is on the spectral changes in the far and near field spectra as compared with spectra of individual scatterers. We show that extra spectral changes can be observed when the coupling between scatterers becomes strong. If the coupling is mainly due to the evanescent field, the produced spectral changes are localized in the near field zone. The extra spectral changes include both red and blue shift, as well as deformation. |
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SS-TuP-37 1-to-9 keV Electron-Induced Surface Chemistry in Ultra High Vacuum Systems
Q. Ma, R.A. Rosenberg (Argonne National Laboratory) Electrons in accelerators are ubiquitous. They range in energies from MeV down to a few eV (secondary electrons). Collisions of these electrons with surfaces of accelerator components may produce reactions that will modify the properties of the surface/vacuum interface. In order to gain insight into these reactions, we have studied the interaction of 1-to-9 keV electrons with the Ar-ion sputtered surfaces of Al, TiN, and Si, under ultra high vacuum conditions using scanning Auger electron spectroscopy. Electron-induced surface chemistry takes place on all these surfaces. For both Al and Si, layer-by-layer growth of oxides was produced by electron beam bombardment. In the case of Al, a clear threshold behavior of the oxidation rate is observed for excitation energies near the Al K-core hole (1560 eV), which suggests participation of secondary electrons in the surface reaction. In the case of TiN films, surface carbonation occurred with 9 keV electron bombardment while no reaction was observed with 5 keV el ectrons. No enhancement of the surface oxidation rate was observed. It will also be shown that the deposited carbonaceous layer is graphitic in nature, resulting in a decrease of the secondary electron yield and thus an increase of the observed sample current. This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences under Contract No. W-31-109-ENG-38. |