AVS1997 Session SS-MoP: Surface Science Poster Session I
Monday, October 20, 1997 5:30 PM in Room Exhibit Hall 1
Monday Afternoon
Time Period MoP Sessions | Topic SS Sessions | Time Periods | Topics | AVS1997 Schedule
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SS-MoP-1 Epitaxial Growth of Ge on Si(001) with Atomic Hydrogen Surfactant
S.J. Kahng, J.Y. Park, Y. Kuk (Seoul National University, Korea) Surfactant mediated growth of Ge on Si has drawn much attention in strained layer heteroepitaxial growth. Group V elements have been used to suppress 3 dimensional islands, but they work as dopants at the same time. A newly designed, dynamically controlled flow of atomic hydrogen was used as a surfactant. Both the microscopic origin and the feasibility of hydrogen surfactant mediated growth of Ge on Si(001)-(2X1) surface were studied by the UHV scanning tunneling microscopy. Isotropic island growth, and limited surface diffusion of Ge adatom under atomic hydrogen exposure were observed. These could be explained by the favorable exchange between the preadsorbed hydrogen atom and the newly deposited Ge atom, which is consistent with the recent first principle calculation. By controlling the atomic hydrogen flux, it was possible to control the transition from hut cluster formation to the layer-by-layer growth mode. |
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SS-MoP-2 Surface Monomers and Dinitrosyls in the Reaction of NO on Clean and Oxygen-Covered Mo(110).
K.T. Queeney, C.M. Friend (Harvard University) The formation of dinitrosyl species (two NO molecules bound to one metal atom) on various MoO3-containing catalysts for selective catalytic reduction of NO is well known and has been implicated in the improved reactivity of these catalysts upon addition of molybdena. We have identified a surface dinitrosyl species as an intermediate to N2O formation from NO adsorbed on Mo(110). This identification is made via a combination of vibrational spectroscopies (high resolution electron energy loss and infrared reflectance absorbance spectroscopies) and temperature programmed reaction of isotopically mixed overlayers. The dinitrosyl is characterized by a νs(NO) peak at 1820 cm-1 in the infrared spectrum of a saturated NO overlayer; the analogous spectrum of a mixed overlayer of approximately equal amounts of 15NO and 14NO exhibits the vibrational signature of a combination of the two isotopically "pure" dinitrosyls in coexistence with the isotopically mixed species. Adsorption and reactivity of NO on a variety of oxygen overlayers on Mo(110) are characterized in order to understand the role of surface oxygen in formation of various types of adsorbed NO, including the dinitrosyl species. In general, surface oxygen inhibits NO dissociation, which accounts for ~73% of the reaction of a saturated NO overlayer on clean Mo(110). In addition, the presence of oxygen in specific sites on Mo(110) is found to alter dramatically the type and distribution of adsorbed NO moieties, as evidenced by complex vibrational spectra of these surfaces. Vibrational spectroscopy and temperature programmed reaction of isotopically mixed overlayers--both sequentially and simultaneously dosed, as well as some prepared via heating followed by repopulation--are used to correlate reactivity with NO structure. |
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SS-MoP-3 The Role of Carbon Monoxide in the Mechanism and Kinetics of Methanol Electrooxidation on Platinum Surfaces
T.D. Jarvi, S. Sriramulu, E.M. Stuve (University of Washington) Electrochemical oxidation of methanol gains technological significance from the desire to develop a direct methanol fuel cell. Unfortunately, methanol electrooxidation suffers from self-poisoning with adsorbed carbon monoxide, and the role of CO in the reaction mechanism (as necessary intermediate or undesired side product) is not yet clear. To study the influence of adsorbed CO on reaction kinetics, methanol was reacted at constant electrode potential on Pt(100) with CO coverages ranging from 0.0 to 0.52 ML. The results show dramatic inhibition of the reaction by CO; only 0.04 ML decreases the reaction rate by 26% relative to the rate on the clean surface. The coverage dependent results may be explained through defect control of the surface reaction or through an ensemble effect. To examine the role of CO as a possible reaction intermediate, constant potential reaction experiments were followed immediately by stripping voltammetry of the CO adlayer. A simple analysis allowed for calculation of the CO and CO2 yields from the electrochemical measurements. On both the (100) and (111) surfaces the yield analysis suggests that CO2 production occurs where CO does not oxidize in methanol-free electrolyte. Direct oxidation of methanol to CO2 or a reduction of the overpotential for CO oxidation, brought about by solution phase methanol, may explain the results. |
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SS-MoP-4 Phosphorus Effects on the Reactivity of the Si(100) Surface
M.L. Jacobson, M.C. Chiu, J.E. Crowell (University of California, San Diego) As the dimensions of integrated circuits approach the nanometer scale, the industry demands manufacturing processes that can better control the dopant profile than simple ion implantation. A promising approach is the codeposition of silicon and phosphorus (or boron) by chemical vapor deposition (CVD). Adatoms such as phosphorus are known to significantly modify surface properties, and thus change the reactivity. This can have an effect on both the growth rate and film quality. Phosphorous sits on epitaxial silicon sites and forms both P-P dimers and P-Si heterodimers. Surface phosphorous contains a lone pair of electrons rather than a dangling bond and is thus passive to adsorbing gas molecules, and acts as a site blocker to species diffusing on the surface. This study uses Temperature Programmed Desorption (TPD) and Auger Electron Spectroscopy (AES) to determine the effect of pre-adsorbed phosphorus on the reactivity of the surface. Disilane (Si2H6) reacts with the Si(100) surface at room temperature to form a mixture of SiHx species. At higher temperatures, recombination reactions occur which produce silane (SiH4) or H2 that subsequently desorb. The reactivity of the surface is measured by the relative amounts of silane and hydrogen produced. The amount of silane produced per available Si site increases while the activation energy for SiH4 desorption decreases with phosphorus coverage. An increase in desorption energy was seen for hydrogen desorption from both disilane decomposition and atomic hydrogen exposure on the Si(100) surface. These results will be discussed within the framework of electronic and morphological modifications of the surface caused by phosphorus incorporation. M.L. Jacobson, 1996/7 Stacey Memorial Fellow |
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SS-MoP-5 Surface and Intergallery Catalytic Properties of Cu(II)-Exchanged Hectorite: A Scanning Force Microscopy Study
T.L. Porter, M.E. Hagerman, M.E. Eastman, J.L. Attuso, B.P. Reynolds (Northern Arizona University) Hectorite is a naturally occurring clay mineral that displays unique catalytic properties with regard to organic molecules. Hectorite readily intercalates organic molecules of many types. The layered silicate structure of hectorite contains gallery regions separating the individual layers, within which are exchangeable metal cations. These gallery cations initialize oxidation of organic monomers within the confines of the gallery regions, resulting in in-situ polymerization reactions. The polymers thus formed may exhibit unique mechanical, thermal and electrical properties owing to their nearly two-dimensional confinement. In addition, surface polymerization of organics may also occur, with initial nucleation occurring at surface steps or "micro-pores". We study the surface and intergallery polymerization of a variety of organic molecules using the techniques of scanning force microscopy, electron paramagnetic resonance spectroscopy, and X-ray diffraction. The dipole moment of the organic monomer is seen to play a predominant role in the intercalation and subsequent polymerization reaction. Novel reactions such as the polymerization of amino acids into peptides and polypeptides are seen for the first time. |
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SS-MoP-6 Reaction of Gaseous H Atoms with Physisorbed Trimethylboron: Dimerization to Tetramethyldiboron.
A. Horn, J. Biener, J. Küppers (Universität Bayreuth, Germany) The interaction of gaseous H atoms with trimethylboron B(CH3)3 (TMB) adsorbed at 80 K on monolayer graphite covered Pt(100)/C surfaces was studied with TDS and HREELS methods. H atoms were produced in a thermal (2000 K) source. TMB monolayers were prepared by exposing 2 L TMB to Pt/C surfaces at 80 K and subsequently removing the multilayer species by flashing to 100 K. The multilayer species desorb at 97 K whereas TMB from the monolayer desorb at 140 K. TMB desorption from the Pt/C surface is exclusively molecular due to the efficient shielding of Pt by the graphite monolayer. HREEL spectra of adsorbed TMB exhibit strong CH3 rocking and deformation vibration losses at 920 and 1430 cm-1 in addition to a C-H stretch loss at 2910 cm-1. After subjecting TMB monolayers to fluences of ca 15*1015 H atoms/cm2 a amu 69 desorption peak develops at 188 K and increases with increasing H fluence. As TMB has a mass of 56 amu, this amu 69 peak indicates that a reaction has been induced by H impact. The reaction product is more strongly bound on Pt/C than TMB. In accordance with this, HREEL spectra recorded after reaction exhibit a new loss at 1590 cm- 1. Analysis of scan TD spectra reveal that the reaction product is tetramethyldiboron (CH3)2BH2B(CH3)2 (TMDB). The loss at 1590 cm-1 is due to excitation of a B-H-B vibrational mode. For the reaction pathway from TMB to TMDB two possibilities exist. Either it starts with a methyl group abstraction by H via methane formation leaving a dimethylboron radical as the product. In a second step this radical is hydrogenated by impact of a further H to (CH3)2BH. Alternatively, the latter molecule is formed direct by a H/methyl substitution via an intermediate with fourfold coordination. Upon dimerization of the molecule TMDB forms. Interaction of D atoms with adsorbed TMB reveals an analogous product with B-D-B bonding in the dimer instead of B-H-B bonding. In addition, through H abstraction from the methyl groups followed by D addition, deuterated TMDB species are generated. |
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SS-MoP-7 Reactions of Gaseous H Atoms with Coadsorbed D/methyliodine on Ni(100) Surfaces: Coverage Dependence.
Th. Kammler, J. Küppers (Universität Bayreuth, Germany) The adsorption of methyliodine, CH3I and CD3I, on H (D) precovered Ni(100) surfaces was studied with TDS and AES. The reactions induced by impact of gaseous H atoms at mixed D/CH3I adlayers were investigated as a function of the iodine coverage. Adsorption of CH3I (CD3I) on Ni(100) surfaces precovered with a monolayer of hydrogen (or deuterium) is purely molecular. Desorption of molecular iodine in the monolayer regime occurrs at 153 K, multilayer iodine desorbs between 125 K and 135 K. Only a small fraction, less than 1%, of the iodine dissociates and gives rise to methane recombination products like CH3D or CD3H. It is assumed that this fraction originates from methyl groups produced by direct iodine/Ni(100) interaction as even at a saturation exposure with molecular D2 no complete D monolayer can be achieved. Upon directing thermal H atoms (2000 K) at CH3I covered D/Ni(100) surfaces at 120 K, HD, D2, CH3D, and CH4 were observed as gaseous products. As at 120 K surface temperature neither desorption of nor Langmuir-Hinshelwood reactions between these products is expected, the observed products must stem from direct, Eley-Rideal (ER) and hot atom (HA) type reactions with or induced by the impinging H atoms. The HD and CH4 products are in line with ER or HA type abstractions of adsorbed D and methyl from the iodine. D2 and CH3D products must stem from HA processes mediated by adsorbed D atoms via the following process: Hgas -> H*, H* + Dad -> Had + D*, in which the stared species are hot atoms. Hot D atoms then either can recombine with Dad to produce D2, or react with iodine to form methane. These reaction schemes apply irrespective of iodine coverage and isotope combinations, i.e. with H or D monolayers and coadsorbed CH3I or CD3I. |
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SS-MoP-8 Thermal Decomposition of Trimethylamine on Pt(111):Spectroscopic Identification of Surface Intermediates.
W. Erley (Forschungszentrum Juelich, IGV, Germany); J.C. Hemminger (University of California, Irvine) The adsorption and decomposition of trimethylamine on a Pt(111) surface have been investigated, using high resolution electron energy loss spectroscopy (HREELS), Fourier transform infrared reflection-absorption spectroscopy (FT- IRAS) and temperature-programmed desorption (TPD). Trimethylamine adsorbs molecularly at 85 K through the nitrogen lone pair on the Pt(111) surface. Upon heating, four steps of decomposition are observed, starting with sequential CN bond cleavage. Three different intermediate surface species can be identified by vibrational spectroscopy. Heating to 280 K generates a dimethylamino species, (CH3)2N. Further heating to 400 K causes the formation of a methylnitrene species, CH3N. This species dehydrogenates to a methylene amido species, CH2N around 450 K. Finally, complete dehydrogenation takes place at temperatures above 500 K. |
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SS-MoP-9 State-resolved Studies of NO Scattering from Ordered Molecular Adlayers on Pt(111)
M.K. Ainsworth, M.R.S. McCoustra, M.A. Chesters (University of Nottingham, United Kingdom) There have been numerous reports of rotational state distributions resulting from the scattering of NO from the clean Pt(111) surface. These studies reflect the presence of the deep attractive potential well in the NO/Pt(111) potential energy surface. We will report the first scattering measurements from well ordered molecular adlayers on this surface. Saturated monolayers of carbon monoxide (CO) and ethylidyne (CCH3) were prepared on the Pt(111) crystal plane and characterised using Reflection Absorption Infrared Spectroscopy (RAIRS). State-resolved scattering from these substrates was investigated using 1+1 Resonance Enhanced Multiphoton Ionisation (REMPI) on the NO A 2Σ+ -X 2Π (0,0) transition. Measurements of both the rotational and state-resolved translational distributions will be reported. |
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SS-MoP-10 Interaction of H(D) Atoms with Octadecylsiloxane Self-Assembled Monolayers on the Si(100) Surface
M. Sander, G.J. Kluth, M.M. Sung, R. Maboudian (University of California, Berkeley) The interaction of H(D) atoms with alkylsiloxane self-assembled monolayers deposited on oxidized Si(100) has been examined under ultrahigh vacuum conditions. High resolution electron energy loss spectroscopy (HREELS) is used to follow the H/D exchange reaction. For low exposures, the exchange reaction occurs relatively rapidly, as indicated by the rate of increase in intensity of the C-D stretch. As exposure increases, the exchange reaction slows due to steric effects. Following exposure to atomic hydrogen the intensity of C-H modes decreases and the intensity of Si-O modes increases, suggesting that other reaction pathways exist. Breakage of C-C bonds may occur as a result of energy dissipation from the highly exothermic addition reaction between a hydrogen atom and an alkyl radical, resulting in etching of the monolayer. Cross linking of chains may also occur through the reaction of adjacent alkyl radicals with each other. The importance of these reaction pathways is examined using HREELS and atomic force microscopy. |
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SS-MoP-11 Self-Assembled Monolayers of n-Dodecanethiol on Electrochemically Modified Polycrystalline Titanium Surfaces
Z. Mekhalif, J. Delhalle, J.J. Pireaux (Facultés Universitaires Notre-Dame de la Paix, Belgium) It is known that the native oxide layer on a Titanium surface hinders the adsorption and subsequent stability of thiol self-assembled layers (see ref 1 for a study on Ni surfaces). In this work, the influence of electrochemical pretreatment on the quality of the organic layer has been studied by X-Ray Photoelectron Spectroscopy (XPS) and electrochemistry. Pure n-dodecanethiol or a solution of the mercaptan in 10-3 M absolute ethanol have been used. On as-received oxidized Ti surfaces, a low quality organic layer is deposited, with about 80% of oxidized sulfur. Sulfonate percentage drops to 25 % after electrochemical reduction of Ti (-1500 mV vs SCE, 15 minutes), when three parameters - reduction potential, dilution, immersion time- have been optimized. XPS was also used to monitor the stability of the SAM layers and their potential to block the metal oxidation in air, during two weeks; that experiment allowed to further assess the better quality of the organic layers when deposited on modified Ti surfaces. Similar studies were also performed for a bifunctional molecule, namely the thiophene-CH2-SH. Work performed under RW contract #3119. (1) Z. Mekhalif, J. Riga, J.J. Pireaux, and J. Delhalle. Langmuir 1997,13,2285 |
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SS-MoP-12 Comparison of the Chemical Reactivity of the (0001) Surfaces of Hafnium and Hafnium Diboride
M. Belyansky, M Trenary (University of Illinois, Chicago); T. Tanaka (National Institute for Research in Inorganic Materials, Japan) The surface chemistry of several gases on Hf(0001), HfB2(0001), and on an epitaxial thin film of HfB2 grown on Hf(0001), have been studied with x-ray photoelectron spectroscopy (XPS), reflection adsorption infrared spectroscopy (RAIRS) and low energy electron diffraction (LEED). The topmost layers of the (0001) surfaces of both Hf and HfB2 consist of hexagonal arrays of hafnium atoms with lattice constants that differ by only 1.7%. The good lattice matching enabled heteroepitaxial growth of hafnium diboride on the Hf(0001) surface to be achieved through thermal decomposition of diborane, B2H6(g). In all respects, the epitaxial HfB2 films displayed the same properties as the HfB2(0001) surface. Despite the fact that the (0001) surfaces of hafnium and hafnium diboride have virtually identical structures, they display distinctly different surface chemistry. Hafnium is highly reactive towards carbon monoxide and oxygen. Carbon monoxide was found to decompose on Hf(0001) at 100 K, while some molecular adsorption was detected on hafnium diboride. Diffusion of oxygen and carbon into the bulk dominates the oxidation of the Hf(0001) surface at high temperatures. In contrast, diffusion of carbon and oxygen into the bulk of hafnium diboride occurs to only a limited extent, if at all. The adsorption and decomposition of diborane on hafnium at 100 K gives rise to an infrared band at 1574 cm-1, which is believed to be due to a metalloborane-like surface species. |
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SS-MoP-13 TEM Studies of Self-Assembled Monolayers on GaAs
M. Yamanaka, H. Ohno, J. Takagi, K. Fujioka, S. Mitarai (Sharp Corporation, Japan); H. Tokumoto (JRCAT - National Institute for Advanced Interdisciplinary Research, Japan) Preparation of highly oriented organic molecules is of great importance for many functional revealations. Moleucular self assembly is considered as one of the basic technology in order to construct organic thin films with well ordered molecular architecture. This method is characterized by using the properties of organic moleculues which spontaneously chemisorb onto a substrate with immersion of an appropriate substrate into a solution contained with an active surfactant molecules. Self-assembled monolayer(SAM) films have been extensively studied for a possibility of future device technologies such as molecular transistors, biosensors and resist materials. Previously we have invesitigated the formation of SAM films on cleaved compound semiconductor surfaces with AFM, XPS and FT-IR. However, there was still little fundamental information concerned with SAM structure in a molecular scale dimension. The observation using the transmission electron microscope (TEM ) technique provides us to elucidate their atomic scale structure at the SAMs/GaAs interface. Samples for the TEM observation were prepared by cleaving GaAs sleeves with a thickness of about 50 micron-meters and a size of about 1mm square in 1mM 16-mercaptohexadecanoic acid [HS(CH2)15COOH] in ethanol solution. After 5hr immersion, the samples were taken out from the solution, and rinsed with absolute ethanol. The SAM samples were then placed on a copper microgrid for TEM observation. SAM on super fine grains were also observed, which had been prepared by the pulverization of a GaAs fragment in a glass mortar together with the SAM solution. In this paper, we will mainly discuss the molecular orientation in the SAM films from high-resolution TEM images and electron diffraction patterns. |
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SS-MoP-14 Metal Overlayers on Organic Functional Groups of Self-Assembled Monolayers. X-ray Photoelectron Spectroscopy of the Ni/COOH Interface.
G.C. Herdt (Fusion Semiconductor Systems); A.W. Czanderna (National Renewable Energy Laboratory) As a continuation of prior studies of metal/self-assembled monolayer (SAM) interfaces, the interaction of vacuum deposited Ni with the COOH organic functional groups (OFGs) of mercaptoundecanoic acid [MUA, HS(CH2)10COOH] self-assembled monolayers formed on an Au substrate has been studied using in-situ X-ray Photoelectron Spectroscopy (XPS). XPS spectra were taken after incrementally vacuum depositing from 0.05 to 1.0 nm of Ni onto SAMs of MUA. The XPS C 1s, O 1s, Au 4f7/2 (from the substrate) and Ni 2p3/2 peaks all compare favorably with a layer-by-layer growth as a function of Ni coverage. A steady decrease and complete disappearance of the hydroxyl component of O 1s peak at 532.8 eV provides evidence for the formation of a Ni-O bond at the Ni/COOH interface. Further evidence for interfacial compound formation is provided by a binding energy shift in the high binding energy component of the Ni 2p3/2 peak from 854.8 to 854.4 eV for Ni coverages below ca. 0.2 nm. These results are consistent with the donation of electrons from the deposited Ni to the COOH oxygens at low Ni coverages. The lack of the characteristic satellite feature in the Ni 2p peak rules out the possibility that a full electron charge is donated to each COOH OFG as a bidentate. However, the data are consistent with a two step reaction mechanism in which Ni initially reactions weakly with the oxygens in COOH below one monolayer coverage and then forms a complex above this coverage. |
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SS-MoP-15 Heterogeneous Atmospheric Photochemistry
V.H. Grassian (University of Iowa) The past ten years have witnessed an explosion of interest in heterogeneous atmospheric chemistry. It has become increasingly clear that the surface and near surface regions of all kinds of atmospherically abundant particles- for instance, ice, sulfuric acid, and salt- catalyze and promote reactions that perturb the composition of the troposphere and stratosphere. Given the demonstrated importance of heterogeneous thermal chemistry in the atmosphere, it is surprising how little attention has been directed toward the role of heterogeneous photochemistry. In this talk, different examples of potentially important heterogeneous photoreactions will be discussed. These include photoreactions that occur in and on ice as well as photoreactions on metal oxide particles. |
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SS-MoP-16 Initial Oxidation of Ultrathin Indium Deposits on Au(111) and Polycrystilline Indium.
M.C. Robinson, A.J. Slavin (Trent University, Canada) Previous studies have shown that alloying occurs when indium deposits equivalent to a few monolayers are evaporated onto Au(111). This paper compares the initial oxidation of the ultrathin alloy to that of polycrystalline bulk indium. Auger electron spectroscopy, electron energy loss spectroscopy, low energy electron diffraction and work-function change measurements were conducted under UHV conditions at room temperature. Oxidation of the polycrystalline sample follows first-order Langmuir kinetics, and saturates by 1x103 L (1L = 10-6 Torr s). The stoichiometry of the compound is In2O3, the same as that obtained by oxidizing the alloy. The oxygen uptake of the alloy saturates with only two layers of In-Au alloy present. The results are compared to another study of oxidation of bulk indium, and similar studies of ultrathin Sn and Sb films on the Au(111) surface. |
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SS-MoP-17 Kinetic Analysis of Methanol Synthesis on a Zn-promoted Cu(111) Surface
H. Nishimura, T. Yatsu (University of Tsukuba, Japan); T. Fujitani (National Institute for Resources and Environment, Japan); T. Uchijima, J. Nakamura (University of Tsukuba, Japan) We have reported that a Zn-deposited Cu(111) surface is a good model of Cu/ZnO catalysts for methanol synthesis by the hydrogenation of CO2. In this study, we have analyzed the kinetics of methanol synthesis on the Zn/Cu(111) surface by measuring the rates of elementary steps such as formation, decomposition and hydrogenation of formate species. The experiments were performed in a XPS apparatus equipped with a high-pressure flow reactor (up to 30 atm). The formate synthesis was carried out using a gas mixture of CO2/H2=1 at a total pressure of 1 atm, a total feed-rate of 80 cc/min and a reaction temperature of 323 to 353 K. The formate decomposition was carried out at a constant temperature (380 to 407 K). The initial rate of the formate synthesis at 353 K over Zn-deposited Cu(111) was measured to be 7.6x10-4 formate molecules per site per sec and was the same as that over clean Cu(111), indicating that there was no promotional effect of Zn in the formate synthesis. It was thus considered that the promotional effect of Zn observed in the methanol synthesis was ascribed to the promotion in another elementary step such as the hydrogenation of formate species. The rate constants and the activation energies for the formation and decomposition of formate species were obtained for clean Cu(111). The kinetic data indicated that the Zn deposited on Cu(111) promoted the hydrogenation of formate species. It should be noted that Cu atoms are indispensable for the formation of formate species. That is, Zn sites and Cu atoms cooperatively work to catalyze methanol synthesis. We thus established the kinetics of methanol synthesis on the bifunctional catalytic surface. |
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SS-MoP-18 STM Studies of Formate Species on Cu(111) and Zn/Cu(111) Synthesized from CO2 and H2 at Atmospheric Pressure
T. Fujitani (National Institute for Resources and Environment, Japan); Y. Choi, Y. Kushida, M. Sano, T. Uchijima, J. Nakamura (University of Tsukuba, Japan) We have previously shown that the activity for methanol synthesis by the hydrogenation of CO2 over Cu(111) is significantly promoted by the deposition of Zn, and that the role of Zn is suggested to stabilize formate intermediates by binding to each other. Here, we report the structure and the formation process of formate species synthesized by the hydrogenation of CO2 over clean Cu(111) and Zn-deposited Cu(111) surfaces at atmospheric pressure by STM. The STM measurements were carried out using a UHV-STM apparatus including LEED-AES and a reaction cell in which formate synthesis was performed using a mixture of H2/CO2=1 at reaction temperatures 323 ~ 353 K and 1 atm. STM images of individual formate species synthesized on Cu(111) were obtained at Vs=-0.01 ~ -0.1 V and It=0.8 ~ 1.5 nA. The ordered structure of formate species changed in the order of p(2x4), c(2x8), (7x7), p(2x3), (5x5) and c(2x4) with increasing formate coverage, indicating that various ordered structures appeared corresponded to a small change in the formate coverage. The coverage of formate species in c(2x4) structure corresponded to ΘHCOO=0.25, which was in good agreement with the saturation coverage measured by XPS at the same reaction condition; thus, particles seen in the STM images indicate each formate species. Furthermore, at a low formate coverage (ΘHCOO=0.02), formate chains were observed, which tended to be arranged with their molecular planes parallel to each other. This is probably due to an attractive interaction between the molecular planes of formate species. The structure and the formation process of formate species synthesized by the hydrogenation of CO2 on Zn/Cu(111) have also been investigated. |
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SS-MoP-19 Evidence for Special Formate Species Adsorbed on Cu-Zn Active Sites for Methanol Synthesis
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 evaporatively deposited on a Cu(111) surface promotes methanol synthesis by the hydrogenation of CO2. It was suggested that the role of the Zn deposited on Cu(111) was to create active sites stabilizing formate species as an intermediate by binding to each other. Here, we report evidence for the presence of formate species on the active sites by in-situ infrared reflection absorption spectroscopy (IRAS). Formate synthesis by the hydrogenation of CO2 over clean Cu(111) and Zn-deposited Cu(111) surfaces was performed using an in-situ IRAS apparatus with a closed-reactor at 343-353 K and 760 Torr (CO2/H2=1). During the reaction, the peak attributed to the symmetric OCO stretching vibration of formate species on copper was observed at ~1330cm-1 on both surfaces. On the other hand, the peaks at 1350 and 1585 cm-1 were observed only on the Zn-deposited Cu(111) surface (ΘZn =0.15). The peak at 1585 cm-1 agreed with the frequency of the asymmetric OCO stretching vibration of formate species, indicating tilted bidentate formate species adsorbed on two different atoms such as a Cu-Zn site. We concluded that this was formate species formed on the active sites for methanol synthesis by the hydrogenation of CO2. Furthermore, it was observed that the formate species present on the Cu-Zn active site was hydrogenated to methoxy species. |
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SS-MoP-20 Interaction of C6 Cyclic Hydrocarbons with a Si(100)-2x1 Surface: Adsorption and Hydrogenation by Atomic Hydrogen.
A.V. Teplyakov, M.J. Kong, J. Jagmohan, J.G. Lyubovitsky, S.F. Bent (New York University) The localized electronic structure of a Si(100)-2x1 surface makes it highly reactive toward unsaturated hydrocarbons. The study presented here was aimed at the differences in bonding to this surface among unsaturated cyclic hydrocarbons. Cyclic hydrocarbons were the compounds of choice because of their constrained configuration, allowing us to examine not only the role of conjugation but also the role of geometrical factors on the chemisorption process. Interaction of cyclohexene, 1,3-cyclohexadiene, 1,4-cyclohexadiene, and benzene with a Si(100)-2x1 surface has been studied by thermal desorption and Multiple Internal Reflection Infrared (MIR-IR) spectroscopy. The IR spectra of the hydrocarbons chemisorbed at room temperature, together with experiments involving hydrogenation by atomic hydrogen, allow for the identification of the reaction products. Possible mechanisms for the interaction of these cyclic hydrocarbons with a Si(100)-2x1 surface are suggested and the dependence of the chemical properties of this surface on the surface preparation procedure are discussed. Thermal reactivity studies show that each of these compounds undergoes dehydrogenation to some extent: dienes predominantly form benzene while benzene undergoes decomposition in competition with molecular desorption. |
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SS-MoP-21 Chemistry of Iodobenzene on Metal Surfaces Studied by LITD-FTMS: Coupling vs. Decomposition
D.M. Jaramillo, D.P. Land (University of California, Davis) The formation of biaryls from aryl iodides is catalyzed by metal in the Ullmann coupling reaction. This synthesis is important for the production of both symmetric and asymmetric biaryls. The chemistry of iodobenzene, the simplest aryl iodide, has been studied by laser-induced thermal desorption with Fourier transform mass spectrometry (LITD-FTMS). We will compare iodobenzene decomposition and phenyl coupling on palladium with the results on copper. |
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SS-MoP-22 Photochemical Processing of Water/Methane Ices
T.R. Dillingham, D.M. Cornelison, S.C. Tegler, B.L. Lutz (Northern Arizona University) The investigation of ices and the photochemical processes that can occur in these ices have important applications in atmospheric physics and planetary astronomy. In this study, results are presented concerning water/methane ices formed at liquid nitrogen temperature. The ices are characterized using x-ray photoelectron spectroscopy (XPS) and the chemical changes are monitored while the ices are continuosly exposed to x-rays for periods of up to 6 hours. Significant changes are observed in the XPS core-level lineshapes associated with these ices. A quadrapole mass spectrometer was also used to monitor the species evolving from the ice surface during the photoprocessing. The XPS and mass spectrometer results are correlated and the possible mechanisms associated with the photochemical processing of the ices are presented. Supported by the National Science Foundation under Grant No. DMR-9217526, the NAU Organized Research Program, and the NASA Origins of Solar Systems Program. |
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SS-MoP-23 The Effects of Oxygen and Sulfur Adsorbates on the Selectivity for Alcohol Reaction on Mo(110) and Co-covered Mo(110)
D.A. Chen, C.M. Friend (Harvard University) Controlling the reactivity and selectivity of commercial catalysts presents one of the most challenging problems in heterogeneous catalysis. The chemistry of oxygen-containing molecules such as alcohols on Mo(110) and Co-covered Mo(110) is of particular interest, since Mo-based catalysts with Co added as a promoter are employed in commercial deoxygenation reactions. In this work, we demonstrate that the selectivity for alcohol reactions on Mo(110) and Co thin films supported on Mo(110) can be modified by the addition of electronegative adsorbates such as oxygen or sulfur. For ethanol reaction on both oxygen-covered and sulfur-covered Mo(110), the selectivity for ethylene production over nonselective decomposition is increased compared to that on the clean surface. However, sulfur decreases the total amount of ethanol reactivity via site blocking, whereas reactivity remains high on the oxygen overlayers for oxygen coverages ≥0.75 ML. This sustained reactivity on the oxygen overlayers is attributed to the ability of oxygen to leave the surface as H2O and to migrate subsurface, thus freeing surface sites for further reaction. In methanol reaction on oxygen-covered Co thin films, oxygen also passivates the Co surface toward nonselective C-H bond breaking; therefore, selective C-H bond breaking to produce formaldehyde is favored over decomposition to CO and H2, which is the main reaction pathway on the pure Co films. Furthermore, in the reactions of ethanol on Co films deposited on top of oxygen- or sulfur-covered Mo(110), the presence of oxygen or sulfur underneath the Co layer promotes selective C-H bond scission, producing acetaldehyde, at the expense of nonselective decomposition. This inhibition of nonselective C-H bond breaking on Mo(110) as well as the Co thin films is attributed to an electronic surface modification. |
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SS-MoP-24 Interaction of Atomic Hydrogen and Oxygen with Potassium on Mg(100)
H.H. Huang, X. Jiang, M. Xin, Z. Zou, G.Q. Xu (National University of Singapore); J.F. Deng (Fudan University, P.R. China) The growth of potassium thin film and its interaction with hydrogen atoms and oxygen on MgO(100) have been studied by thermal desorption spectroscopy (TDS) and X-ray photoelectron spectroscopy (XPS). The desorption energy of adsorbed K-atoms was found to decrease from 82.3 to 78.5 kJ mol-1 with increasing potassium coverage in the range of θK ≤ 1 ML, which is attributed to the repulsive dipole interaction between adsorbed K-atoms. At θK < 1 ML, H-atoms are adsorbed and bound to the K-adatoms on Mg(100), forming the H-K-O species. On multilayer K-covered surfaces, two hydrogen desorption peaks, α1 and α2, were observed due to the thermal decomposition of two possible bulk hydrides formed beneath the surface. The ß peak in hydrogen desorption at a lower temperature is probably related to the surface-adsorbed hydrogen species. In addition, hydrogen diffusion into the bulk during H-atoms adsorption or thermal annealing has been observed. The coadsorption of oxygen with potassium on Mg(100) results in several oxygen species on the surface. An atomic oxygen species is initially present at lower θK as shown by the evidence of O1s BE at 528.5 eV, together with the consistent but not coincident desorption of potassium and oxygen at 610 and 525 K, respectively. The oxygen desorption at a low temperature of 425 K coupled with the O1s BE at 532.1 eV indicates the associative molecular oxygen adsorption on oxygen vacancies in the substrate. The non-stochiometric 3D K-O clusters form on the surfaces for θK ≥ 1 ML. The desorption of K2O2 followed by K2O at ~ 600 K was observed. |
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SS-MoP-25 Photochemistry of OCS and H2S on GaAs(100)
H.H. Huang, Z. Zou, X. Jiang, W.Y. Chan, G.Q. Xu (National University of Singapore) The photochemistry of both OCS and H2S on GaAs(100) has been investigated using TDS and PIDS. It was found that OCS adsorbs molecularly on GaAs(100) and thermally desorbs at 160K. Adsorbed OCS molecules either dissociate into gaseous CO, leaving S adsorbed on the surface, or desorb as molecular OCS under irradiation of λ = 300~700nm. The photodissociation cross section is ~ 5.0x10-19 cm2 at 300nm, which is about two order of magnitude higher than that for gaseous OCS at 250nm and 10 times greater even compared with that for OCS on Ag(111) at 254nm. The cross section is strongly dependent of the coverage of OCS on GaAs(100), changing from 2.0x10-18cm2 for a 0.03L exposure to 3.6x10-19 cm2 for a 3.3L OCS at 320nm. In addition, a 2.4eV red shift of threshold photon energy for photodissociation of OCS on GaAs(100) was observed in our experiment relative to gaseous OCS. H2S adsorbed on GaAs(100) dissociates into H(ad) and HS(ad) under photon irradiation in a range of λ < 550nm, resulting in an enhanced H2S thermal desorption peak at 340K due to the recombination of H(ad) and HS(ad), as well as a large H2 desorption yield. The cross section shows a declining feature with increasing wavelength and demonstrates a typical value of ~10-20 cm2 in the range between 250 and 450nm. The ratio between the remaining H2S coverage after UV irradiation and its initial value decreases exponentially with irradiation time and light intensity. This result shows that the photo-induced dissociation of H2S(ad) is a single photon process. H2 ejecting from the surface during irradiation was also detected, which can be attributed to the direct reaction between the 'hot' H-atom formed from H2S(ad) photodissociation and the neighbouring adsorbed H2S molecules or adsorbed H-atoms. |
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SS-MoP-26 CO Desorption in a Laser Assisted Reaction of O2 with a Graphite
I. Kamioka (National Research Institute for Metals and University of Tsukuba, Japan); M. Kitajima (National Research Institute for Metals, Japan); T. Kawabe (University of Tsukuba, Japan); K.G. Nakamura (National Research Institute for Metals, Japan) Translational distribution of reaction product CO desorbed from a graphite surface on irradiation with fundamental output of a Nd:YAG laser and exposure of O2 molecule has been investigated. The reaction product CO was detected by time-of-flight (TOF) mass spectroscopy. The used samples were pyrolytic graphite. O2 gas was introduced into the chamber through a variable leak valve from a nozzle with a diameter of 1 mm at normal incidence to the surface. The surface was irradiated with the fundamental output of a Nd:YAG laser (1064 nm) for heating. Photo-ionization was performed with an ArF excimer laser (193 nm) and ionization spot was 6 cm from the surface. The ions were measured with a time-of-flight mass spectrometer. Only CO was observed as the reaction product. C, C2, and C3 were also observed as vaporized species from a graphite surface. All the desorbed species were neutral, and no ion was detected. The TOF signals of CO was well approximated by thermal Maxwell-Boltzmann distributions. All the TOF signals consist of fast and slow components. The mean velocities of CO were 1.2 x 103 and 5.9 x 102 m/s for fast and slow components, respectively. The translational temperature of CO for fast component was estimated to be ~ 2570 K, which was lower than those of C2 (~ 3100 K) and C3 (~ 3460 K), but was higher than that of C (~1500 K). The translational temperature of CO for slow component was estimated to be ~ 580 K, which was lower than those of C2 (~ 730 K) and C3 (~ 790 K), but was higher than that of C (~470 K). The translational temperatures of the vaporized carbon clusters increase as the number of carbon atoms increases, and the translational temperature of CO is lower than those of the carbon clusters. The calculation of the surface temperature of the graphite will be discussed. |
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SS-MoP-27 Adsorption of Acetic Acid on NiO(100)/Mo(100): TPD and XPS Studies
W.S. Oh, C. Xu, D.W. Goodman (Texas A&M University) The adsorption and reaction of acetic acid (CH3COOH) on NiO thin films epitaxially grown on Mo(100) have been studied as a function of exposure using temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). Acetic acid dissociates to form an acetate species upon adsorption at 100 K. Two reaction pathways (ketonization and dehydration) have been identified from the decomposition of adsorbed acetate upon heating. The branching ratios of these two reaction pathways have been found to be strongly dependent upon the coverage of acetic acid. Electron induced decomposition of reaction products or intermediates of acetic acid on the NiO thin films has been observed leading to a carbon-covered NiO thin films. |
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SS-MoP-28 Kinetics and Mechanisms of Halocarbon Decomposition at Iron and Palladium Surfaces Studied by LITD-FTMS and RAIRS
J.S. Loring, D.P. Land (University of California, Davis) An understanding of halocarbon decomposition processes at metal surfaces is essential for the enhancement of both the remediation of waste halocarbon contaminants and the improvement of lubricant tribological properties. Two metals, iron and, more recently palladium, are receiving considerable attention due to the high rates at which many halocarbons decompose at their surfaces. We have used laser-induced thermal desorption Fourier transform mass spectrometry, complimented by reflection absorption infrared spectroscopy, to investigate the reduction of some chlorinated methanes at the surfaces of iron and palladium single crystals and foils as a function of temperature and reaction time. We will present the results of these studies, including the mechanisms we have deduced and the rates, activation energies and preexponential factors we have calculated for the reactions of these small chlorocarbons on iron and palladium. |
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SS-MoP-29 Surface Chemistry of Alkynes on Palladium (III): Methylacetylene and Analogs
D.C. Herman, D.P. Land (University of California, Davis) The relationships between transition metals and adsorbed species have played an important role in many chemical processes. The catalytic ability of the metal to activate processes by modifying adsorbates has played a significant role in petroleum refining, tribology, boundary-layer lubricants, and environmental and medical science. However, little is known about the mechanisms and kinetics involved. Such an understanding is crucial for modeling systems which can be applied efficiently in the field. This ongoing project investigates the interactions between a palladium (III) crystal surface and methylacetylene, as well as other acetylene derivatives, by employing Laser-Induced Thermal Desorption coupled with Fourier Transform Mass Spectroscopy supported by other surface analysis techniques including Auger Electron Spectroscopy, Low Energy Electron Diffraction, and Thermal Desoption Spectroscopy. These studies will investigate adsorption/desorption mechanisms, cyclization and other surface reactions, and the kinetics of these processes. |
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SS-MoP-30 Polymerization Kinetics by State-Selective Laser Photochemistry
R.M. Rao (Brookhaven National Laboratory); J. Dvorak (University of Pennsylvania); R.J. Beuhler, M. White (Brookhaven National Laboratory) State-selective REMPI methods have been used to study the dynamics of photo-induced fragmentation and polymerization of formaldehyde (CH2O) physisorbed on a Ag(111) substrate. Previous studies of the formaldehyde/Ag(111) system by Dai and coworkers have shown that polymerization can be induced by UV radiation (≤ 355 nm) and the rate can be conveniently controlled by varying the surface temperature. Polymerization is initiated by radical intermediates which result from UV photodissociation of the formaldehyde monomer, e.g. CH2O + hν -> HCO· + H. Chain formation is exothermic, but the reaction exhibits a small reaction barrier which has been difficult to measure in condensed phases. Using coherent VUV radiation for single-photon and (1+1') REMPI detection of desorbed CH2O(g) and CO(g), we have investigated the competing fragmentation and polymerization pathways following irradiation at 266 nm. Production of CO(g) results from secondary photodissociation of the formyl radical intermediates, i.e., HCO· + hν -> CO(g) + H. Two distinct CO desorption channels are observed corresponding to translationally and rotationally "hot" fragments and a near thermal channel which appears to be associated with the surface reactions of the radical intermediate. By measuring the competing CO(g) production and the direct monomer (CH2O(g)) desorption rates over a wide range (20-100 K) of Ag surface temperatures, we have been able to extract the activation energy (0.68 ± 0.1 kcal/mole) for polymerization. This small activation energy represents the diffusion barrier for adsorbed formaldehyde monomers moving on the Ag surface. |
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SS-MoP-31 The Decomposition of Fe(CO)5 on Pd(111)
M.N. Rocklein, D.P. Land (University of California, Davis) In conjunction with Fourier transform mass spectrometry (FTMS), both temperature programmed desorption (TPD) and laser induced thermal desorption (LITD) have been used to monitor the surface chemistry of Fe(CO)5 on Pd(111). TPD indicates that some decomposition is occurring since significant amounts of CO leave the surface after molecular desorption of Fe(CO)5. Because electron ionization caused massive fragmentation for this molecule, the ability of LITD to detect small changes in surface concentration is enhanced by using a soft-ionization technique, specifically, proton transfer chemical ionization. LITD should indicate the degree of dissociative adsorption at low temperatures. The LITD data may also suggest that the TPD Fe(CO)5 peak is a recombinative process and the partial decarbonylation is reversible. |
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SS-MoP-32 Surface Stoichiometry and Adsorbate Coverage during GaAs MOCVD
J.R. Creighton, H.K. Moffat, K.C. Baucom (Sandia National Laboratories) As a complement to our conventional surface science studies of GaAs, we have recently implemented in-situ diagnostics to directly examine the state of GaAs surfaces during MOCVD. The most successful technique to date has been reflectance-difference spectroscopy (RDS), as pioneered by Aspnes. Under many MOCVD conditions (typically above 900 K), RDS indicates that the GaAs(100) surface is super arsenic-rich, meaning that it is covered with more than one ML of As. However, there are many other deposition conditions which yield RDS lineshapes that were not observed in the original MBE "benchmarking" experiments, so the state of the surface could not be immediately ascertained. We have therefore measured the RDS of several new surface reconstructions, including some with adsorbates, in an attempt to find a match with the unknowns seen during MOCVD. One important surface state often observed at intermediate temperatures was found to be a metastable version of the super As-rich surface. At lower temperatures (700-800 K) and/or higher TMGa partial pressures the surface is also As-rich, but it is now decorated with a considerable coverage of methyl groups. The stability of this surface state is in good agreement with predictions based on our surface science derived kinetics. We will also summarize our recent attempts to directly monitor surface methyl groups using FTIR. |
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SS-MoP-33 Dehydrogenation of Cyclohexene to Benzene on Pd(111)
D.E. Hunka, D.P. Land (University of California, Davis) Dehydrogenation of cyclic hydrocarbons to aromatic species has historically been of interest to the petrochemical industry. Temperature programmed desorption (TPD) and laser induced thermal desorption (LITD) coupled with Fourier transform mass spectrometry (FTMS) have been employed to investigate the dehydrogenation of cyclohexene to benzene on a single crystal of Pd(111). While TPD shows the desorption peak of benzene centered at 530 K, recent results using LITD-FTMS have shown that benzene forms on the surface at 150 K with no stable intermediate. These results indicate a difference in the mechanism of this dehydrogenation from those found on Pt(111). Studies of this reaction as a function of temperature and coverage dependence are used to elucidate the mechanism and surface structure as it relates to coverage. |
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SS-MoP-34 Surface Derivatization of Self-Assembled Monolayers Studied with Multi-technique Surface Characterization
S. Pan, B.D. Ratner (University of Washington) Self-assembled monolayers (SAMs) with different terminal groups have potential applications in bioengineering, such as biosensing, biomimetic, and novel biomaterials due to their highly organized structure along with high stability. The study of the accessibility and reactivity of the outermost functional groups is critical in developing specific surface properties for new artificial biomaterials. Trifluoroacetic anhydride (TFAA) was chosen to surface-derivatize a self-assembled monolayer of 16-mercapato-1-hexandecanol (MHD) on a poly-crystalline gold surface. Given the order and organization of SAM surfaces, the accessibility and reactivity of the outermost functional groups, reaction kinetics and stoichiometry are studied by complementary surface analytical techniques (external reflection-absorption FT-IR, ESCA and TOF-SIMS). In the IR spectra, the reacted trifluoromethyl ester group has several characteristic C-F stretching bands in the 1100-1300 cm -1 region. The static contact angle (H2O) data show a significant decrease of wettability after derivatization. We expect that the study of surface derivatization will lead to a straightforward and reproducible method to modify the physical and chemical surface properties at the molecular level and provide the possibilities of using SAMs as starting material to develop new, well-characterized, and reproducible surfaces for biomolecule and cell interaction studies. |
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SS-MoP-35 Surface Analysis Studies of Self-Assembled (3-mercaptopropyl)trimethoxysilane on Gold
C.R. Cabrera, J. Garcia-Orozco (University of Puerto Rico) The self-assembly technique has enabled the formation of highly ordered films at a molecular level, allowing the controlled modification of surfaces. (3-mercaptopropyl) trimethoxysilane (MPS) is an end-group-functionalized short chain mercaptan that has been used as an organic adhesive for metal evaporated films1. It has been used in the preparation of cluster modified gold surfaces2, in molecular recognition3, as an interfacial adhesive4, and in the elimination of interferants in glucose biosensors5. Several structures for self-assembled MPS have been proposed. Therefore, a detailed surface analysis study of the structure of self-assembled MPS is needed. In this work, surface analysis techniques such as quartz crystal microbalance, ion scattering spectroscopy, X-ray photoelectron spectroscopy, and ellipsometry were used to study the composition and structure of self- assembled MPS onto gold evaporated surfaces from an ethanol solution. Ellipsometry results indicated a film thickness of 14.6 Å. This is in accordance with a film thickness of just one molecular layer, tilted 39.6° from the surface normal, which suggests loose packing of the film. X-ray photoelectron spectroscopy studies indicated that the -SiOCH3 moieties of the MPS film remain intact at the air-film interface; therefore, an unpolymerized film is obtained. High-resolution XPS studies showed the presence of S-Au bonds, indicating that the MPS film adsorbs to the gold through the sulfur functionality. However, angle-dependent XPS studies indicated a small contribution from -SH moieties at the air-film interface. Therefore, the adsorption of a monolayer of MPS at the gold surface is proposed, together with the adsorption of a second MPS molecule at some regions of the monolayer.
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SS-MoP-36 A Temperature Dependent Photoemission Study of the Electronic Structure of Ni(100).
S. Tkatchenko, A.P.J. Stampfl, R.C.G. Leckey (La Trobe University, Australia); Y.Q. Cai, A.M. Bradshaw (Fritz-Haber-Institut der Max-Planck-Gesellschaft, Germany) The changes appearing in the valence band photoemission spectra of Ni as its temperature is raised through the magnetic phase transition point has been under debate for a number of years 1. One problem is to clearly show, using photoemission spectroscopy, that the band structure of Ni obeys one of the models describing the magnetic state above and below the magnetic phase transition temperature. This is difficult to achieve because, the magnetic exchange splitting is small, between 0.1 to 0.3 eV and in general several bands close in energy contribute to the spectra. Constant initial state (CIS) spectroscopy may provide a partial solution since small splittings in binding energy can be magnified to several electron volts in the CIS spectra by using sufficiently large excitation energies. An analysis of the CIS spectra taken in the valence band region is given whereby the spin up and down peaks are identified and their temperature dependencies discussed.
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SS-MoP-37 The Electronic Band Structure of ZnSe(100).
J. Xue, A.P.J. Stampfl (La Trobe University, Australia); D. Wolfframm, D.A. Evans (North East Wales Institute, United Kingdom); J.D. Riley, R.C.G. Leckey (Latrobe University, Australia); A. Ziegler, R. Graupner, B. Mattern, M. Hollering, L. Ley (Universität Erlangen-Nürnberg, Germany) There are two reconstructions known to exist for the clean and ordered surface of MBE grown face-centred-cubic ZnSe(100): the Se-rich (2X1) and the Zn-rich c(2X2) reconstructions. Valence band photoemission spectra were acquired along principle directions of each surface Brillouin zone for a range of surface sensitive photon energies between 25 and 50 eV. Our analysis showed that there may be several surface resonance states lying in the first 3 eV from the top of the valence band maximum which extend over most of the length of the Brillouin zone for both reconstructions. The gap surface state previously predicted for the Se-dimer terminated (2X1) surface was barely resolvable and was found to lie near the vicinity of the valence band maximum at the centre of the Brillouin zone. Our results are discussed in terms of the currently proposed geometrical structures for each reconstruction and published theoretical results. |