AVS1998 Session SS-MoP: Surface Science Division Poster Session
Monday, November 2, 1998 5:30 PM in Room Hall A
Monday Afternoon
Time Period MoP Sessions | Topic SS Sessions | Time Periods | Topics | AVS1998 Schedule
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SS-MoP-2 Adsorption Dynamics of Monoenergetic Oxygen on Al(111)
A.J. Komrowski, Y. Liu, A.C. Kummel (University of California, San Diego) The interaction of oxygen with aluminum has become a prominent system in the study of metal oxidation. An intriguing scanning tunneling microscope (STM) study observed that dissociated O atoms on Al(111) at low coverages are separated by >80 Å when the surface is dosed with thermal O2 molecules1. More recently, a detailed energy-resolved sticking probability experiment using supersonic molecular beams reported a strong O2 translational energy dependence of the sticking coefficient on Al(111), demonstrating an activated process with no indication of precursor-mediated adsorption2. Further, the sticking measurements suggested that high incident energy O2 may access regions of the multipotential energy surface unavailable to thermal O2 and therefore chemisorb by a different mechanism. We will present results of the adsorption of monoenergetic O2 molecules on the Al(111) surface over a range of incident energies using supersonic molecular beam techniques and STM. We will show how the chemisorption site distribution changes with incident translational energy.
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SS-MoP-3 Gas Phase Oxidation of Uranium-Niobium Alloys by O2 and H2O
W.L. Manner, M.T. Paffett, R.J. Hanrahan (Los Alamos National Laboratory) Secondary concentrations of certain transition metals (e.g. niobium, titanium, and chromium) alloyed with uranium are known to improve numerous physical and mechanical properties. One such property is the enhancement of corrosion resistance exhibited by the uranium alloy with six weight percent niobium (denoted as U6Nb) relative to the unalloyed uranium. Despite a tremendous knowledge base concerning the bulk metallurgical properties of this material, very little is known concerning the surface chemistry of U6Nb toward corrosion by O2 or H2O. Specifically, we seek to understand the role of niobium toward oxidation resistance of this alloy. We have initiated a series of studies using surface-sensitive techniques that include X-ray photoelectron spectroscopy (XPS), thermal desorption-mass spectroscopy (TDMS), and secondary-ion mass spectroscopy (SIMS) in order to better understand the chemistry between this alloy and O2 or H2O. XPS studies of the oxidation of clean U6Nb by O2 at 300 K produces a thin oxide overlayer of stoichiometric UO2.0 intermixed with Nb2O5. While the same stoichiometry is exhibited for uranium when the oxide is prepared at 500 K with O2, niobium is much less oxidized showing a mixture of NbO and Nb. Depth profiling studies reveal that oxidation by O2 is much greater than that exhibited by H2O. Only the first layer or two is oxidized using H2O as an oxidant at 300 K (the oxidation by O2 is approximately an order of magnitude higher). Formation of a critical density of Nb2O5 is suggested to be responsible for the enhanced corrosion resistance by preventing diffusion of O- (O2-) or OH- into the oxide/metal interface region. |
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SS-MoP-4 Vibrational Spectra and Structure of Hydrogen Adsorbed on Ni(111)
M. Gostein, Q.Y. Yang, S.T. Ceyer (Massachusetts Institute of Technology) The structure and adsorption site of hydrogen on Ni(111) were investigated using high-resolution electron energy loss spectroscopy. At 0.5 ML coverage where the adsorbed hydrogen forms a (2x2)2H unit cell, the vibrational spectra show a pair of fundamentals at 733 and 791 cm-1 and a pair at 1077 and 1109 cm-1, as well as three overtones at 1260, 1396, and 2180 cm-1. The pair of fundamentals at the lower frequency and the two lower frequency overtones are assigned to H-Ni modes parallel to the surface, while the higher frequency pair and highest frequency overtone are assigned to modes perpendicular to the surface. These assignments are based on the expected anharmonicity of the modes and on the angular distributions of the loss features. The features in each fundamental pair, which are closely-spaced in frequency, are resolved by exploiting their different electron impact energy dependence. Preliminary results indicate that the fundamentals are split into pairs because of different potential energy surfaces at the hcp versus fcc three-fold hollow binding sites of hydrogen in a structure previously proposed from the observation of a (2x2)2H unit cell. |
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SS-MoP-5 C-C Bond Breaking in Cyclopropane on the Ni(411) Surface
A.J. Guikema, J.L. Gland (University of Michigan) Thermal C-C bond breaking in cyclopropane has been observed on the stepped Ni(411) surface indicating that (111) step sites on a (100) terrace can break C-C bonds both in the presence and absence of coadsorbed hydrogen. Propane resulting from C-C bond activation in the presence of hydrogen is observed near 220 K. With increasing coverages of coadsorbed hydrogen and cyclopropane the yield of propane at 220 K increases. Above half saturation coverages of hydrogen and cyclopropane the yield of propane decreases with increasing coadsorbed coverages. The maximum propane yield corresponds to approximately 25% of cyclopropane saturation coverage. Experiments with coadsorbed deuterium clearly indicate that coadsorbed deuterium participates in propane formation. These observations support a Langmuir-Hinshelwood mechanism for propane formation associated with active sites in the step or near-step region. In the absence of coadsorbed hydrogen, cyclopropane dehydrogenation on the step sites dominates. A small amount of methane and ethane are observed around 120 K indicating multiple bond breaking during disproportionation. No deuterium incorporation is observed for methane and ethane in the presence of coadsorbed deuterium indicating that intermolecular hydrogen transfer dominates during disproportionation. These results indicate that the most reactive disproportionation sites responsible for multiple bond breaking react first and result in formation of about 5% of a monolayer of methane and ethane below 150 K. |
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SS-MoP-6 LEED and STM Measurement of NO/Pt(111) at Low Temperature
M. Matsumoto (Institute of Molecular Science, Japan); T. Yamada (University of Tsukuba, and CREST, Japan); N. Tatsumi, T. Itoyama (University of Tokyo, Japan); K. Miyake, K. Hata, H. Shigekawa (University of Tsukuba, and CREST, Japan); K. Fukutani, T. Okano (University of Tokyo, Japan) The chemisorption of nitric oxide on Pt(111) at low temperature has been studied by electron energy loss spectroscopy (EELS), infrared absorption spectroscopy (IRAS) and low energy electron diffraction(LEED). The vibrational spectroscopy showed that N-O stretching frequency is 1490 cm-1 at low coverage(<0.5L ) and 1710 cm-1 at high coverage(>0.5L).1 At high coverage, 2X2 LEED pattern was observed and its structure was attributed to the fcc hollow site by LEED dynamic theory.2 But there is no reasonable account for the difference of the N-O stretching frequency between low and high coverage regions and the structure is controversial yet. We measured the LEED I-V curves and STM images of NO/Pt(111) surface at several temperature and coverage conditions . At 175K, diffuse 2X2 LEED pattern could be seen even at low dosage(0.05L) and the I-V curve was the same as that at high dosage(1L). This indicates that (2X2)-NO islands grow with coverage increase but the local structure does not change at this temperature region.
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SS-MoP-7 Chemical Contrast by Funtionalized Tips and Movement of Highly Excited "Hot" Adsorbates: LT-STM Experiments with CO on Cu(111)
L. Bartels (Paul-Drude-Institut, Germany); G. Meyer, K.-H. Rieder (Freie Universität Berlin, Germany) Chemical Contrast between species looking otherwise very similar in STM images (like CO and oxygen on Cu(111), both of which image as circular indentations) can be achieved by controlled in-situ functionalization of the STM tip apex with a single molecule: Electrons tunneling from a STM-tip to a CO molecule on Cu(111) at a sample bias exceeding 2.4V1 can lead to the excitation of the addressed molecule resulting in its hopping either to the tip apex or to a nearby site on the substrate with approx. 1:3 probability. Thus, tips bearing a CO molecule on their apex can be produced. As putting down the CO molecule from the tip apex is easily accomplished using inverted bias, this technique can be used for a whole range of new manipulation (transfer) experiments. Additionally, imaging with a CO tip the apparent shape of CO molecules is inverted to a protrusion, while the shape of oxygen (and several other molecules) remains unchanged, thus allowing for easy discrimination between them2. It could be shown that the transfer of the CO molecule is achieved by single electron attachment to the CO 2π* level, leading after electronic deexcitation to highly vibrational excited CO molecules. Thus, events, in which the CO molecule does not jump onto the STM tip but ends up on the substrate surface, can be used to study the diffusion of highly excited ("hot") adsorbates. We found, that their diffusion is limited in 2/3 of the cases to one adsite distance. In case of longer diffusion distance, scattered movement dominates. A Monte-Carlo simulation of the measured distribution of diffusion paths, however, allows to estimate that in 1/3 of the cases an adsite is transgressed, it is transgresses without scattering the movement of the "hot" molecule.3
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SS-MoP-8 Phenyl Containing Radicals on Cu[111]
G.S. McCarty (Pennsylvania State University); M.K. Kamna (Intel); P.S. Weiss (Pennsylvania State University) Surface features such as step edges, impurities, and adsorbates cause modification of the local electronic properties of a surface. We are taking advantage of this phenomenon to produce active sites for surface reactions. We explore the atomic scale electronic effects on reactions of phenyl containing species to understand the catalytic properties of copper. Copper catalyzes reactions of phenyl containing species through the Ullmann coupling reaction. A low temperature UHV STM was used to study Cu[111] dosed with iodobenzene, di-iodobenzene, and biphenyl. The dissociation of iodobenzene into iodine and phenyl was observed. The reaction can be driven to produce biphenyl. We observed the intermediate complex pairs responsible for the exquisite specificity of this reaction. |
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SS-MoP-9 Adsorbate Azimuthal Orientation from Reflectance Anisotropy Spectroscopy
B.G. Frederick (University of Maine); J.R. Power, R.J. Cole, C.C. Perry, Q. Chen, S. Haq, T. Bertrams (University of Liverpool, United Kingdom); N.V. Richardson (University of St. Andrews, Scotland, U.K.); P. Weightman (University of Liverpool, United Kingdom) We have determined the azimuthal orientation of an adsorbate on a metal surface from an intramolecular-transition-derived feature in reflectance anisotropy spectroscopy (RAS). Adsorption of 9-anthracene carboxylic acid onto p(2x1)O/Cu(110) led to an ordered structure with a strong (2%), derivative-like feature at 4.5 eV. Fresnel theory predicts the measured intensity, functional behavior, and sense of the RAS signal for the molecule aligned along [110]. IR measurements confirm that the molecular plane is perpendicular to the surface and STM measurements support the azimuthal orientation. |
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SS-MoP-10 Nickelocene Adsorption on Single Crystal Surfaces
D.L. Pugmire, M.A. Langell (University of Nebraska, Lincoln) The adsorption and decomposition of nickelocene on Ag(100), Ni(100), and NiO(100) has been studied by high resolution electron energy loss spectroscopy (HREELS), x-ray photoelectron spectroscopy (XPS), and temperature programmed desorption (TPD). Nickelocene adsorbs molecularly on less reactive surfaces such as Ag(100) at 140K, and desorbs molecularly at approximately 212K. In contrast to this, more reactive surfaces such as Ni(100) dissociatively adsorb nickelocene at 140K and desorb a variety of metallocene fragments. The orientation of molecularly adsorbed nickelocene as a function of surface coverage and the identification of decomposition products will be presented. |
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SS-MoP-11 Analysis of Desorption Behavior of Sulfur from Pd by Temperature Programmed XPS
K. Dohmae (TOYOTA Central R&D Labs, Inc., Japan) Desorption of sulfur in oxygen gas from polycrystalline palladium plate was investigated by temperature programmed X-ray photoelectron spectroscopy (TP-XPS). The temperature programmed experiments in a range from room temperature to 773K were performed in an ambience of O2 gas at 1x10-6 Pa under a base pressure of 2x10-8 Pa. As the initial coverage of sulfur on Pd increases in the range below 0.5 ML, the temperature that the quantity of sulfur on Pd decrease to half of its initial value becomes higher. Over 0.5 ML of the initial coverage of sulfur, the coverage of sulfur on Pd remained more than 0.5 ML at 773K, though the adsorbed sulfur decreases slightly as the temperature went up. The results were compared with calculations of Langmuir-Hinshelwood model with an assumption that the adsorbed sulfur react with the adsorbed oxygen on Pd and desorbe as sulfur dioxide molecule from Pd. The desorption behavior of sulfur in the experiments was well explained by the calculation model. The adsorption and desorption behavior of oxygen did not much agree with the calculations. Though the calculations suggest that the quantity of adsorbed oxygen on Pd decreased when the sulfur desorbe from Pd, the quantity of adsorbed oxygen did not decrease at the time in the experiments. Furthermore, total quantity of adsorbed sulfur and oxygen on Pd became over 1 ML at higher initial coverage of sulfur than 0.5 ML. They suggest that adsorbed sulfur on Pd affects the ability of adsorption of oxygen onto Pd. |
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SS-MoP-12 Atom Probe Analysis of Dissociation of CO and N2 Gas on a W(110) Oriented Tip
T. Shimizu, A. Ohi, H. Tokumoto (JRCAT, Japan) An atom probe ( AP ) is an attractive mass spectrometer of single-ion sensitivity. But the application of the AP technique to gas-surface phenomena is less advanced than the corresponding study of metallurgical processes. Because the high electric field for desorption induces the dissociation of gases, which hinders us from investigating the gas-surface catalytic phenomena themselves. We measured the dissociative and non-dissociative ions and estimated the temperature dependence of the dissociation probability. Here we introduced 10-7 Torr x 10 s of CO and N2 gas and investigated the dissociation on a W(110) oriented tip by using voltage-pulsed AP without a probe hole. The temperature of introduced gas was kept at RT and the tip temperature was changed from 50 K to RT. The count of gas ions increased with lowering temperature. Further AP at 50 K could detect two different states by the desorption field strength. A state desorbing at low field strength is related to the physisorption at low temperature, which was not observed at higher temperature. A state desorbing at high field strength is related to the trapping state following the dissociation. The dissociation probability decreased with lowering temperature. But below 100 K, the dissociation probability became almost constant ( 41% ( CO ), 45% ( N2) ), which is caused by the field induced dissociation of chemisorbed gas. We postulated the ratio of field induced dissociation is almost constant up to RT and estimated the energy barrier from the trapped to the dissociation state as 30 meV (CO) and 22 meV ( N2 ). |
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SS-MoP-13 The Kinetics of Methanol Synthesis by Hydrogenation of CO2 Over a Zn-deposited Cu(111) Surface
I. Nakamura, H Nakano (University of Tsukuba, Japan); T. Fujitani (National Institute for Resources and Environment, Japan); T. Uchijima, J. Nakamura (University of Tsukuba, Japan) We have found that Zn deposited on a Cu(111) surface promotes methanol synthesis by the hydrogenation of CO2. In this study, we examined the kinetics of methanol synthesis over the Zn-deposited Cu(111) surface by measuring the rates of elementary steps such as formation, decomposition and hydrogenation of formate species to clarify the promotional effect of Zn. The experiments were carried out in an infrared reflection absorption spectroscopy (IRAS) apparatus with a closed-reactor. The formate synthesis was performed using a gas mixture of CO2/H2=1 at a total pressure of 760 Torr and reaction temperatures of 323-353 K. The formate decomposition was carried out at a constant temperature of 373-403 K under vacuum. The initial formation rate of formate species on a clean Cu(111) surface was obtained by the initial slope of the formate coverage estimated from the peak intensity of the symmetric OCO stretching bands of formate species versus exposure to the CO2/H2 gas mixture. For example, at 353 K the initial formation rate was estimated to be 8.2 x 10-4 formate molecules/site/sec, which agreed with that measured by the previous XPS experiment under the same reaction conditions. From the arrhenius plot of the initial formation rates, the activation energy for the formate synthesis on the clean Cu(111) surface was found to be 56.7 kJ/mol. The rates of formate decomposition was found to be first-order for the formate coverage and the activation energy was 100.7 kJ/mol. Furthermore, the equilibrium coverage of formate species was calculated by the kinetic data of formation and decomposition, which was in good agreement with experimentally measured coverage. |
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SS-MoP-14 Characterization of CO Oxidation over Au/TiO2(110) by Scanning Tunneling Microscopy
X. Lai, M. Valden, D.W. Goodman (Texas A&M University) Au clusters of 1-6 nm in diamteter were vapor deposited onto a TiO2(110)-(1x1) single crystal under ultrahigh vacuum (UHV) condition, monitored by Auger Electron Spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). Fresh Au clusters on TiO2(110) were transferred in situ into a high pressure cell, then exposed in separate experiments to 10 torr CO:O2 (2:1), CO and O2. STM observed the morphology change of Au clusters induced by reaction mixtures at 300 K, indicating the chemisorption of O2 on Au clusters and TiO2 substrate even at room temperature. The maximum activity for the oxidation of CO on Au/TiO2 is obtained with Au clusters less than 3.5 nm in diameter and 1.0 nm in height (~300 atoms/cluster) exhibiting an electronic structure characterized by a band gap in the range of 0.2 V - 0.6 V as measured by STM and scanning tunneling spectroscopy (STM/STS), which suggests that CO oxidation over Au/TiO2 is structure sensitive. |
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SS-MoP-15 Selective Chemical Reaction of HBO2 Molecules on the Si(111)-7x 7 Surface Studied by Scanning Tunneling Microscopy
K. Miyake, K. Hata (University of Tsukuba, and CREST, Japan); R. Morita, M. Yamashita (Hokkaido University, and CREST, Japan); H. Shigekawa (University of Tsukuba, and CREST, Japan) The formation process of boron (B) induced √3x√3 structure by HBO2 irradiation was studied by scanning tunneling microscopy (STM). In the chemical reaction of HBO2 molecules with Si(111)-7x7 surface, change in the charge density on the adatoms of the 7x7 units during the process play an exceedingly important role, which was analyzed by the bias dependence of STM images, i.e., the adatoms with less charge density become darker in the filled state STM image.1,2 The molecules preferentially reacted with the center adatoms in the unfaulted half units first. This result indicates that HBO2 molecules tend to react with the adatoms in less charge density. The center adatoms surrounding the reacted center adatoms became darker compared to those in the normal 7x7 units in the filled state STM images, which indicates some charge redistribution occurred by the reacted center adatoms. Such charge redistribution is supposed to increase the chemical reactivity of the modified center adatoms because HBO2 molecules prefer to react with adatoms in less charge density. In fact, center adatoms in the faulted half units, which were adjacent to the firstly reacted center adatoms, reacted subsequently with the HBO2 molecules. According to this process, chain structures were formed by the reacted center adatoms. With further irradiation, B atoms were found to form an 1-dimensional network with the same √3x√3 phase, resulting in the formation of an ordered √3x√3 surface. Small off-phase √3x√3 domains remained only in the areas occupied by Si atoms. The remaining Si adatoms were replaced by B atoms with further deposition of HBO2 molecules, and a completely ordered √3x√3 surface was formed.
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SS-MoP-16 Electron-Stimulated Desorption of Na from SiO2 Films
B.V. Yakshinskiy, T.E. Madey (Rutgers University) As part of a program to probe the mechanisms by which Na atoms are produced in tenuous planetary atmospheres (e.g., Mercury, The Moon, etc.) we have studied the electron-stimulated desorption (ESD) of sodium ions and neutral atoms from ultrathin amorphous stoichiometric silica films (~100Å thick) grown on a Re(0001) surface. The desorbing neutral Na flux is detected by using a novel surface ionization detector, which also permits measurements of energy distributions of neutral atoms by means of a time-of-flight method. Sodium ions, desorbing from the surface under electron bombardment, are analyzed by a guadrupole mass spectrometer. Approximately the same appearance threshold (~25eV) is found for both Na+ and Na0 desorption, corresponding to O2s core level excitation. A Coulomb pair O+- Na+ forms as a result of intraatomic Auger decay in the oxygen atom, leading to Na+ desorption. The sodium neutral atom desorbs due to Pauli repulsion between Na and the neutralized oxygen ion. The total ESD cross section for Na is ~3x10-19cm2 at an electron energy of 300eV. The yield of Na atoms grows linearly with increasing sodium concentration in the monolayer, whereas the yield of Na+ ions passes through a maximum at ~0.5ML. Velocity and energy distributions of desorbing sodium neutrals demonstrate evidence for the ESD of "hot" atoms, with most probable kinetic energies of ~0.5eV. The results are compared with recent photon stimulated data from oxides, and with observations of Na in planetary atmospheres. |
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SS-MoP-17 Reaction of Atomic Hydrogen and Activated Methane with Si(111) as Precursor for Diamond Nucleation Studied with High Temperature STM
F. Schaefer, H. Bethge, A. Bianco, U.K. Koehler (Ruhr-Universitaet Bochum, Germany) An STM has been designed especially for studies under the extreme conditions of hot-filament-CVD diamond growth. As a first step the etch-attack of atomic hydrogen and the reaction of activated methane with Si(111) has been investigated by in-situ, time resolved STM in the temperature range between 500°C and 800°C. Gas activation was done by the iongetter pump of the UHV system or by hot filaments. The etch-attack of atomic hydrogen on Si(111) starts at step edges and preferential etching along crystallographic directions is found. The etch-rate has been investigated in dependence on hydrogen flux and substrate temperature. Etching is found to be an activated process with an acitvation energy (E=0.8eV) close to the diffusion energy of Si-atoms on Si(111). Activated methane does not react with Si(111) at temperatures below 500°C. At higher temperatures a layer-by-layer consumption of substrate material and the formation of disordered clusters up to 60 Å height can be seen. Two different contributions to the consumption of Si are discussed: formation of SiC and etching due to the hydrogen supplied by the activated methane. The ratio of consumed substrate material to Si incorporated in the clusters is about 1:1, indicating a consumption of Si due to the formation of amorphous SiC clusters. A quantitative analysis of the nucleation behavior has been carried out. Although the clusters are amorphous their density evolution with growth-rate and temperature can be described according to classical rate equation theories. Different activation energies for nucleation for the two activation methods are found and can be assigned to different species responsible for the growth. As a further step to real hot-filament-CVD diamond growth conditions the reaction of activated methane diluted with hydrogen (1%CH4/H2) has been investigated for pressures up to 1 mbar. |
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SS-MoP-18 Molecular State Identification of the Adsorbate Present on the Si(111)-(7x7) Surface Reacted with Oxygen. A Cs+ Reactive Scattering Study
K.-Y. Kim, T.-H. Shin, H. Kang (Pohang University of Science and Technology, South Korea) While the interaction of molecular oxygen with a Si(111)-(7x7) surface has been one of the most actively studied surface reactions, the nature of the surface adsorbate, or the precursor state produced during initial oxidation of the Si surface, is still an open question. Interpretation of the experimental results on this system varies from molecular oxygen, either in the peroxide or superoxide form, to atomic oxygen bonded to various sites of the surface. To this surface we have applied a recently developed technique of Cs+ reactive scattering.1,2 From this investigation we have found that oxygen exists on the surface only in the oxide form, and that the molecular oxygen bonded to a surface does not exist. The present finding allows explanation for many of the controversial issues regarding the initial oxidation mechanism of the Si surface.
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SS-MoP-19 Reactivity of Hydrogen with Fluorinated LaNi5
A.R. Layson, C.J. Jenks (Iowa State University) Much interest has been shown in hydrogen storage alloys as possible replacements for cadmium-based rechargeable batteries. Recent work under atmospheric conditions has shown that fluorination of the hydrogen storage material LaNi5 substantially improves its ability to adsorb and subsequently desorb hydrogen. We report on studies aimed at understanding the effects of fluorination of LaNi5. These are conducted in an ultrahigh vacuum environment using a single crystal of LaNi5. Despite LaNi5 being notoriously difficult to clean we have developed a method of cleaning which leaves less than 2% impurities at the surface. We follow surface compositional changes with Auger electron spectroscopy and hydrogen release with thermal desorption spectroscopy; these studies are the first such studies on LaNi5. We compare results for D2 adsorption/desorption on the clean surface to that of an oxidized surface and a surface that has been fluorinated after oxidation. We find that fluorination of the oxidized surface results in a low temperature channel for D2 evolution not present on the surface that has only been oxidized. In addition the fluorinated surface enhances the amount of D2 dissociation possible. |
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SS-MoP-20 The Study on Activated Chemisorption on Surface of La Thin Film
S.M. Shao, W. Qian, J.P. Song, S.L. Li, Z.Q. Zhuo, G.K. Xi (Nankai University, China) The surface characteristics of evaporated rare-earth metal La thin film were studied by ex-situ and in-situ AES and molecular beam techniques. In case 1, the La thin film was prepared in the vacuum system, then taken out of the system exposing under atmospheric condition and finally put into the UHV chamber to be studied. In case 2, the La thin film was prepared and in-situ studied in the UHV chamber. The surface composition by ex-situ AES studies corresponds to mainly La(OH)3 which can be attributed to a surface modification by O2 and water vapor from the ambient atmosphere. The in-situ AES studies reveals the surface composition of La. The activated chemisorption of CH4 on La thin film was studied in both cases using molecular beam technique.1,2 After surface cleaning treatment of samples, chemisorption probability was measured as a function of translational kinetic energy. The initial sticking probability was found to depend strongly on the incident kinetic energy E,scaling En=Ecos2θi. The initial sticking probability increases linearly with the translational kinetic energy of the incident molecular beam, but the threshold value and the activated energy were not changed when the surface temperature was increased from 500 K to 700 K.}
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SS-MoP-21 Electron Induced Reactions of Ammonia and Nitric Oxide Adsorbed on Si(100)
C. Bater, J.H. Craig, J.H. Campbell (University of Texas, El Paso) Electron beam enhanced nitridation of Si(100) by ammonia and nitric oxide was studied using XPS, TPD, ESD and HREELS. At low coverages, both ammonia and nitric oxide dissociatively adsorbed on Si(100) at 120 K while molecularly adsorbed ammonia and nitric oxide were detected by TPD and HREELS at higher exposures. Ammonia condenses on the surface at 110 K. During electron beam irradiation, adsorbed hydrogen atoms were effectively removed from the surface so that the site poisoning effect of adsorbed hydrogen was reduced. The electron stimulated dissociation of the adsorbed NHx species enhanced the nitridation process. Enhanced nitride formation occurred when weakly physisorbed and condensed ammonia were present on Si(100) during electron beam irradiation at 110 K. H+ ESD KEDs were used to deduce the surface reaction occurring during electron beam irradiation, and HREELS following electron beam irradiation was used to confirm and enhance the conclusions drawn from the ESD study. The formation of nitride and oxynitride was observed using the peak shifts of the N 1s and Si 2p X-ray photoelectron peaks. From the O+ ESD KED signals, we distinguished O+ originating from NO(a) and O(a), which we used to monitor the surface reaction occurring during electron beam irradiation. Changes in surface chemistry due to different levels of surface nitridation were also studied extensively. From the H+ and O+ ESD decay curves, the ESD cross sections for different surface conditions were obtained. |
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SS-MoP-22 The Interactions of XeF2 and F2 with Si(100)
M.R. Tate, M.F. Bertino, S.C. Eckman, J.R. Holt, S.T. Ceyer (Massachusetts Institute of Technology) The interactions of molecular fluorine (F2) and xenon difluoride (XeF2) with Si(100) are model semiconductor etching systems. Despite the similar energetics and chemical nature of these two etchant molecules, XeF2 is a much better etchant of Si than F2. This comparative study probes the dynamics of the interactions of these two gas-surface systems and seeks to understand the molecular origins of this disparity in reactivity. Using gas-surface scattering techniques, three reaction channels are identified for both etchants including a novel gas-surface mechanism, atom abstraction, in which the surface abstracts a fluorine atom from the incident molecule and ejects the remaining particle into the gas phase. The fluorinated silicon surfaces resulting from exposure to F2 and XeF2 are nearly indistinguishable until the silicon dangling bonds are saturated - that is, only a small fraction of Si-Si lattice bonds are broken until the fluorination of the dangling bonds is complete. Beyond one monolayer of fluorine, however, only XeF2 is able to attack the Si-Si lattice bonds, and SiF4, an etch product, is observed to desorb. A detailed analysis of the energetics of the scattered products yields insight into the disparate reactivities of F2 and XeF2 with Si. |
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SS-MoP-23 Atomic Hydrogen Reactions with Si(100) Surfaces
S.K. Jo, J.H. Kang (Kyung Won University, South Korea); B. Gong, D.E. Brown, J.M. White, J.G. Ekerdt (University of Texas, Austin) Chemical reactions of hydrogen atoms with Si(100)2x1 have been investigated over a wide range of substrate temperatures (Ts = 110 - 635 K) by using temperature-programmed desorption (TPD) and low-energy electron diffraction (LEED) techniques. Thermal-energy hydrogen atoms generated from a hot tungsten filament were found to react with Si(100) surfaces over the entire temperature regime, resulting in silicon etching, amorphization, and hydrogen penetration into the crystalline substrate. Extensive silicon etching occurred at all substrate temperatures investigated. A large hydrogen uptake of more than 4 monolayers (1 ML = 6.8 x 1014 H atoms/cm2) and destruction of 2x1 LEED patterns upon H exposure at Ts = 415 K and 635 K indicate that the etching continues at substrate temperatures where tri- and di-hydride species are not stable. Moreover, in addition to the H2 desorption peak at Ts = 780 K, a new desorption peak at Ts = 850 K grew in with increasing hydrogen exposure for Ts ≥ 415 K. Deuterium exchange experiments suggest that the 850-K peak is due to H2 evolution from the crystalline silicon bulk. At Ts ≤ 300 K, amorphization as well as etching of Si(100) occurred readily. The observed opposing Ts effects on the rates of hydrogenation [Si(s) + xH(g) -> SiHx(a)] and etching [SiHx(a) + (4-x)H(g) -> SiH4(g)] reactions have been elucidated. Terrace etching and step etching have been invoked to explain the observed etching at low (≤ 415 K) and high (≥ 635 K) substrate temperatures, respectively. Implications for damage-free, dry etching of crystalline silicon surfaces by thermal-energy hydrogen atom beams are discussed. |
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SS-MoP-24 Hydrogen Diffusion and Desorption from the Si(100)-2x1 Surface
E.J. Buehler, J.J. Boland (University of North Carolina, Chapel Hill) The mechanisms of hydrogen diffusion on the Si(100) surface and desorption from the 2x1 monohydride surface are being studied using high temperature scanning tunneling microscopy. Several desorption mechanisms have been proposed in the literature to explain the observed first-order kinetics, large energy barrier to adsorption, and similarity between hydrogen molecules desorbing from the decomposition of the mono- and dihydride phases. The possible role of surface defects has also been discussed. Using high temperature imaging conditions at which the tip does not induce desorption, we have observed pairs of vacant dangling bond (DB) sites on the surface following desorption. The spatial distribution of DB’s on the surface shows no correlation with the locations of steps and defects. A particular surface feature has been identified, however, which may be a stable intermediate formed during the hydrogen desorption process. The structure and stability of this intermediate are discussed, as is the possible role of this feature in hydrogen desorption. |
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SS-MoP-25 Adsorption and Reaction of H2O on GaAs(100)
X.M. Wei, Q.P. Liu, Y.T. Wong, H.H. Huang, G.Q. Xu (National University of Singapore) The adsorption and reaction of H2O on Ga-rich GaAs(100) surface have been investigated using TDS and HREELS. During the H2O dosage at 100 K, we believe the adsorption proceeds as follows: initially, molecular H2O is adsorbed, probably onto Ga. For a higher exposure at about 0.28 L, a 2-D H2O monolayer is built. A subsequent dose increase leads to phase transition from a 2-D monolayer to a 3-D H2O multilayer. During heating to 500 K, the desorption follows the reverse scheme. The H2O multilayer is first desorbed at about 170 K, as detected by TDS, which makes the surface resemble that prepared by exposure at 0.28 L. On heating to a higher temperature, the remaining H2O molecules are desorbed by 250 K. On the other hand, the first monolayer H2O dissociates to give hydroxyls and hydrides. Hydroxyls are bound to Ga sites while hydrides tend to occupy As sites. More hydrides are formed from the further dissociation of Ga-bound hydroxyl. These hydrides adsorb onto Ga sites which are left vacant after the recombinative desorption of Ga-bound hydroxyl. At ~500 K, H2 gas is also liberated. |
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SS-MoP-26 Sites for Arsine Adsorption on GaAs(001)-(4x2)
Q. Fu, L. Li, M.J. Begarney, B.-K. Han, R.F. Hicks (University of California, Los Angeles) Arsine adsorption on GaAs(001)-c(8x2) at 298K-693K has been studied by internal-reflectance infrared spectroscopy and x-ray photoelectron spectroscopy. At 298K, AsH3 dissociatively adsorbs on terminal Ga sites on step edges and transfers hydrogen to terminal As sites. No gallium hydride was observed during the dosing of AsH3 at room temperature. c(8x2) reconstruction was well maintained when GaAs(001) surface was under extended exposure of AsH3 between 298K and 573K. Upon dosing AsH3 with a dosage of 9600L at 650K, surface reconstruction was converted from c(8x2) to a mixture of c(6x4) and (4x2) domains as illustrated by LEED. XPS also revealed a small increase of As/Ga area ratio after dosing at 650K. Further adsorption of arsine at 650K transformed surface to As rich (2x4) reconstruction. We found that it is necessary to have enough adsorption sites( terminal As) to accomodate hydrogen from arsine in order to incorporate As efficiently during epitaxy growth. |
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SS-MoP-27 A New GaCl Molecular Beam Cell for Surface Dynamic Studies
M. Ohashi, M. Ozeki (JRCAT-ATP, Japan) An understanding of the surface reaction dynamics between source molecules and solid surface is required for the development of an advanced technological base for III-V semiconductor growth. Gallium chloride (GaCl) is one of the most important sources for III-V epitaxial growth. GaCl has an advantage of controlled chemical reaction, because it easily dissociates with H2 on GaAs surface in spite of its strong bonding. However it is necessary to produce GaCl in-situ in the production cell, because GaCl preferably exists at higher temperature above 870 K. In order to study the surface reaction dynamics between GaCl molecule and GaAs surface, high purity GaCl molecular beam is necessary. We developed a new GaCl molecular beam cell, where high purity GaCl molecular beam was produced by direct reaction between Ga metal and Cl2 gas. We optimized the gas flow rate of Cl2 gas for the production of GaCl molecular beam and the cell temperature from 920 to 1230 K. The byproduct of GaCl3, which is stable and form large particles at low temperature, was observed under excess supply of Cl2 gas. Under the optimized condition, which is the gas flow rate of Cl2 at .25 ccm, the only GaCl was produced and no Cl2 was observed. The Flux density at the sample surface, which is 40 cm away from the nozzle of this cell, was estimated 1.1x1013 molecules cm-2 s-1. The newly developed GaCl molecular beam cell would be useful for the study of the surface reaction dynamics between GaCl source and GaAs surface. This work is supported by New Energy and Industrial Technology Development Organization (NEDO). |
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SS-MoP-29 Electronic Promotion of Graphite Oxidation in the Presence of Cs Trapped between the Basal Planes
J.R. Hahn, K.-Y. Kim, H. Kang (Pohang University of Science and Technology, South Korea) Oxidation of graphite is investigated by using scanning tunneling microscopy (STM) in the presence of Cs atom trapped between the carbon basal planes. Low energy (< 150 eV) bombardment of Cs+ ions onto a graphite surface produces Cs interstitial defects (Cs-ID), where a Cs atom is trapped between the first and the second graphite layers. Oxygen adsorption onto the defected surface and the subsequent heating at 560 °C inside vacuum leads to formation of the pits with a depth of monolayer and a diameter of several nm. The pit formation starts from the Cs-ID's, indicating its promotion effect on the reaction with oxygen. The average diameter of the pit increases with the amount of supplied oxygen molecule. The experimental results suggest that a Cs-ID donates electron density to the neighboring carbon atoms, making the upper-layer carbons to be reactive with the oxygen molecule. The electron transfer from Cs-ID is the prevailing mechanism for the promoted oxidation, which is different from the direct mechanism proceeding via complex formation between the adsorbed Cs and the oxygen molecule, proposed from the previous studies of the oxidation of graphite covered with Cs. |
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SS-MoP-30 The Penetration Range of Low Energy (50-500 eV) Ar+ and Kr+ Ions Impinging onto a Graphite Surface Studied by the Oxidative Etching Method and STM
J.R. Hahn, H. Kang (Pohang University of Science and Technology, South Korea) Penetration of low energy (50-500 eV) Ar+ or Kr+ ions into a graphite surface results in the formation of surface vacancy defect (VD), formed by knock-out of carbon atoms, and interstitial defect (ID) where the incident atom is trapped between the carbon layers1, 2, 3. Thermal oxidation of the defected graphite surface etches away the carbon atoms surrounding a defect, leading to the formation of a pit of a large diameter and nearly circular shape. The etching process, when occurs from a defect of multi-layer depth, removes the carbons at and above the defect-containing layers simultaneously. Such phenomena enable us to locate the spatial position of the ion-generated defects by measuring the STM topography of the etched pits. The yield for production of multi-layer defect increases with ion collision energy. The depth distribution of the defect obtained by this method agrees well the result of theoretical trajectory calculation1. Lateral displacement of the incident ion inside the basal planes, upon penetration into the first carbon layer, is measured by STM from the distance between the VD and the ID formed in a pair. The average distance of the lateral travel varies with the incidence angle and the mass of ion.
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SS-MoP-31 Scanning Tunneling Microscopy Studies of the Growth of Copper, Silver and Gold Overlayers on TiO2(110) in Ultrahigh Vacuum
D.A. Chen, K.F. McCarty, R.Q. Hwang (Sandia National Laboratories) The growth of metal films on oxide surfaces has become an area of increasing technological importance for applications involving electronic devices, sensors, catalysis and ceramic joining. We are investigating the growth of copper, silver and gold overlayers on a TiO2(110) surface in ultrahigh vacuum in order to develop a fundamental understanding of metal-oxide interactions. After annealing at 1000 K in vacuum, the titania substrate is reduced and becomes sufficiently conductive for scanning tunneling microscopy experiments. The resulting (1x1) TiO2 surface is characterized by low energy electron diffraction, Auger electron spectroscopy as well as STM. Previous work by Diebold and Madey et al. have shown that many transition metals, including copper, grow by three-dimensional island formation on titania. Scanning tunneling microscopy is used to study the nucleation, growth and size distributions of the metal islands as a function of annealing temperature and coverage. Furthermore, by comparing the characteristics of copper, silver and gold overlayers deposited on titania, we will investigate how varying the interaction strength between oxygen at the surface and the admetal affects film growth and morphology. This work was performed under the U.S. Department of Energy contract DE-AC04-94AL85000 and supported in part by the USDOE-OBESDivision of Materials Sciences. |