Papers

AVS2001 Session SS+SC-WeM: Adsorption on Semiconductor and Metal Oxide Surfaces

Wednesday, October 31, 2001 8:20 AM in Room 122

Wednesday Morning

Start	Invited?	Item
8:20 AM		SS+SC-WeM-1 Empirical Density Functionals and the Adsorption of Organic Molecules on Si(100) M.A. Phillips, N.A. Besley, P.M.W. Gill, P. Moriarty, P.H. Beton (The University of Nottingham, UK) Density Functional Theory (DFT) has been used in the study of the adsorption of organic molecules such as ethene and ethyne for a number of years. Typically, for semiconductor surfaces, DFT methods are used in conjunction with a cluster model of the substrate.¹ However, the computational expense of DFT calculations implies an upper limit on the size of the substrate model, and thus on the size of adsorbate molecule. As such, the study of larger adsorbates requires some reduction in computational expense, often leading to the use of parameterised semi-empirical methods. Unfortunately, such methods do not give good agreement with experiment for many semiconducting elements: another approach is required for these materials. The Empirical Density Functional, EDF1, has been shown to give very good agreement with values of atomisation energy, ionisation potential and proton affinity for the majority of species included in the G2 experimental data set.² Furthermore, this functional does not require the evaluation of 'exact exchange' contributions, and is therefore significantly less computationally expensive than 'hybrid' functionals. However, the success of EDF1 in the determination of any other physio-chemical properties, in particular adsorption geometry, adsorption energy and vibrational structure, has never been tested. Here, we present the results from a density functional study of the adsorption of organic molecules on the Si(100) surface using EDF1, and discuss their validity through comparison to experimental measurements and results from equivalent calculations using the well known functional, B3LYP. We also compare the computational expense when using EDF1 in these calculations to that when B3LYP is used. ¹ R. Konecný and D. J. Doren, Surf. Sci. 417 (1998), 169-188. ² R. D. Adamson, P. M. W. Gill and J. A. Pople, Chem. Phys. Lett. 284 (1998), 6-11.
8:40 AM		SS+SC-WeM-2 Scanning Tunneling Microscope Investigation of Adsorption of Titanium Atoms on Si(111)-7x7 Surface H.F. Hsu, M.C. Lu (National Tsing Hua University, Taiwan, R.O.C.); H.-L. Hsiao (Tunghai University, Taiwan, R.O.C.); L.J. Chen (National Tsing Hua University, Taiwan, R.O.C.) C54-TiSi₂ has been the primary silicide for contact application in ULSI devices. The initial stages of interfacial reactions of Ti thin films on silicon are of both scientific and the technological interests. The strong reactivity of Ti with Si was manifested by the interatomic mixing at room temperature. The atomic scale studies of initial Ti-Si reactions at very low coverage have been scarce. In particular, it is not clear whether Ti atoms react with the Si substrate from the very beginning of deposition or the formation of a silicide-like compound starts at some critical coverage. In the present study, an atomic-scale characterization of room temperature titanium adsorption on the Si(111)-7x7 surface has been investigated. Preferential adsorption of Ti atoms on the Si center adatom sites at very low coverage was found. At higher coverage, a peculiar contrast feature is observed. The observed feature is attributed to the charge redistribution caused by the adsorption of more than one Ti atoms on this subunit cell. At very low Ti coverage (~0.007 ML), some of the Si adatom sites appear to be brighter in both filled-state and empty-state STM images. The contract behavior is likely to be due to the adsorption of Ti atoms on the Si adatom sites. It was found that Ti atoms adsorb preferentially on top of the Si center adatoms and, to a less extent, on top of the Si corner adatoms. Upon further deposition of Ti, two center adatoms and corresponding corner adatom become darker and the center of these three adatoms become brighter. The observed change in the apparent height is caused by the redistribution of charge near the Si adatom sites. The result indicates that more than one Ti atoms were adsorbed on a triangular subunit cell.
9:00 AM		SS+SC-WeM-3 High-Resolution Core-Level Study of the Initial Stage of Oxygen Adsorption on a Si(111)-(7x7) Surface K. Sakamoto, H.M. Zhang, R.I.G. Uhrberg (Linköping University, Sweden) As motivated by the technological importance of thin silicon oxide films in semiconductor devices, the initial stage of oxygen adsorption on silicon surfaces has been a topic of experimental and theoretical investigations. Among the various reports on oxygen adsorption, Si 2p core-level studies have played crucial roles in elucidating the structures of the SiO₂/Si interfaces. However, very few Si 2p studies are available for the initial stage of oxygen adsorption on a Si(111)-(7x7) surface. In this paper, we present a high-resolution Si 2p core-level photoemission study of submonolayer oxygen adsorption on the (7x7) surface. The photoemission measurements were performed at beamline 311 at the MAX-II synchrotron radiation facility in Lund, Sweden. The clean (7x7) surface was exposed to 0.1-20 L of oxygen at 120 K, which correspond to coverages below 1 ML. Significant intensity of the Si 2p component due to the Si²⁺ species is observed already at 0.3 L, and the component due to the Si³⁺ species is clearly observed at a dosage higher than 1.0 L. These results indicate that more than two oxygen atoms adsorb to one Si atom even at a very low coverage. After annealing the oxygen adsorbed sample at 600 K, the Si 2p component due to the Si²⁺ species shows a shift of 0.4 eV to the higher binding energy side. This result suggests different atomic configurations for the Si²⁺ species at 120 K and after annealing the sample at 600 K. We will also present detailed studies of the dosage- and temperature-dependent intensities of the Si 2p components due to the suboxide species.
9:20 AM		SS+SC-WeM-4 First-Principles Theory of Finite-Temperature Adsorbate Ordering in Ba/Si(111)-"3x2" S.C. Erwin, C.S. Hellberg (Naval Research Laboratory) Alkali metal adsorbates induce a 3x1 reconstruction of Si(111) widely believed to be a "honeycomb-chain channel" structure. This model is based on a true Si=Si double bond in the surface layer, which at 1/3 ML coverage leads to the elimination of all dangling bonds and thus keeps the fundamental gap free of surface states. Alkaline-earth adsorbates also appear to induce 3x1 (as well as higher-order) reconstructions, yet despite the extra electron the resulting surfaces remain fully gapped---an apparent contradiction to the one-electron band picture. Lee et al. recently suggested that for alkaline-earth adsorbates the coverage is in fact 1/6 ML, and showed that a 3x2 honeycomb-chain channel model is indeed fully gapped.footnte 1 We use density-functional methods first to confirm that the model of Lee et al. is indeed energetically preferred at low coverages. Second, we predict that structurally related 5x2 and 2x1 reconstructions will appear---if they are not preempted---at higher coverages. Finally, we propose an explanation for why the 1/6 ML phase appears 3x2 in STM but 3x1 in LEED: namely, that at moderate temperatures the adsorbates will exhibit only short-range order, due the near energetic degeneracy of H3 and T4 adsorption sites. To demonstrate this, we extract from our density-functional calculations the adsorbate-substrate and adsorbate-adsorbate interactions, and based on these carry out classical Monte Carlo simulations to explore the detailed temperature dependence of adsorbate ordering. footnte 1G. Lee, D. Shin, H. Kim, J. Koo, and S. Hong, Bull. Amer. Phys. Soc. 46 (2001).
10:00 AM		SS+SC-WeM-6 Adsorption Induced Deflection and Frequency Changes in a Silicon Nitride Cantilever due to Ca²⁺ Ions S. Cherian, A. Mehta, T.G. Thundat (Oak Ridge National Laboratory) Cantilever based micromechanical sensors exploit changes in surface stress due to interactions between the analyte species and cantilever surface. Charged groups on the cantilever surface play a significant role in binding induced deflection of the micro-cantilever. The deflection and frequency response of triangular silicon nitride cantilevers when exposed to calcium chloride solution was investigated in a flow system. The silicon nitride cantilever used was 200µm long and 20µm wide. Calcium chloride solutions of increasing concentrations were injected sequentially into the flow cell and the cantilever response measured. The fundamental resonance frequency of the cantilever shifted to lower values with increasing solution concentrations. The deflections due to interaction with the CaCl₂ were towards the gold side. The concentration versus deflection curve followed a Langmuir adsorption isotherm. The cantilever response is attributed to chemisorption of calcium ions onto the silicon nitride side. To verify this a calcium binding protein, calmodulin, was used. A cantilever that was exposed to CaCl₂ solution was subsequently exposed to calmodulin. Calmodulin binding to Ca ²⁺ ions on the cantilever surface resulted in a deflection. This deflection was significantly different from that observed when calmodulin was exposed onto a fresh cantilever. These observations were further confirmed by fluorescent measurements using a fluorescently tagged calmodulin. These results demonstrate that consideration of ionic interactions of charged species in the medium with cantilever surfaces is critical in interpreting deflection data of cantilever based sensors. This also suggests the importance of passivating one of the surfaces in order to make the deflection of the functionalized cantilever specific to the species of interest.
10:20 AM		SS+SC-WeM-7 Adsorption of NO and NO₂ on Barium Oxide: Surface Nitrition and Nitration P.J. Schmitz, R.J. Baird (Ford Motor Company); M. Miletic, J.L. Gland (University of Michigan) Alkaline earth oxides surfaces are known to trap NOx species by chemisorption even under excess oxygen conditions typical of lean burn engines exhaust. These oxides, particularly barium oxide, have been proposed as active components in cyclic automotive NOx abatement strategies because of their ability to store and release NOx under rich/lean exhaust cycling. However, molecular understanding of the adsorption and reactions of NO, and NO2 on alkaline earth oxide surfaces remains incomplete. A series of XPS and temperature programmed studies of NO and NO2 adsorption and reaction on model barium oxide thin films prepared in-situ are reported here. These experiments indicate that charge transfer and reactive chemisorption play an important role even during low temperature adsorption on barium oxide surfaces. Nitrite and nitrate species form preferentially on reactive adsorption sites and can be reactively desorbed in the 400 to 600 K temperature range. The coverage of the precursor molecular species plays an unexpected role both in reactive adsorption and desorption on barium oxide surfaces. These results are discussed in terms of recent DFT calculations which highlight the importance of reactive configurations, charge transfer, and surface oxidation/reduction processes.
10:40 AM		SS+SC-WeM-8 Electronegative Adsorbates on TiO₂: Reducing Effects of S and Cl E.L.D. Hebenstreit, W. Hebenstreit (Tulane University); H. Geisler (Xavier University of Louisiana); C.A. Ventrice, Jr. (University of New Orleans); D.A. Hite, P.T. Sprunger (Louisiana State University); U. Diebold (Tulane University) TiO₂(110) is a well-studied model catalyst with an abundance of technical applications. Sulfur and chlorine are common impurities in many catalytic systems which poison catalytic reactions. The adsorption of molecular S and Cl on TiO₂(110)(1 x 1) has been studied with scanning tunneling microscopy (STM), x-ray and ultraviolet photoelectron spectroscopy (XPS, UPS), and low energy electron diffraction (LEED). At room temperature both adsorbates bind dissociatively to 5-fold coordinated Ti atoms and oxygen vacancies. At elevated temperatures (120°C - 440°C), S and Cl replace surface oxygen atoms. S forms different types of superstructures in dependence on coverage and adsorption temperature. No long-range ordering was found in the a dsorbed layer for Cl. Both adsorbates reduce the surface but S leads to a stronger oxygen depletion than Cl. In photoemission experiments, adsorption of either S or Cl at elevated temperatures cause additional emission at the high binding energy side of t he valence band and increases emission from the defect state. Adsorption of S leads to band gap states which fill the band gap completely. Evidence was found that the reduction state of TiO₂ crystals strongly affects the surface coverage of S and Cl at elevated temperatures. The rate of the site exchange of the adsobates between a weakly bound precursor state on Ti and the replacement of oxygen is kinetically limited by the arrival of diffusing bulk defects at the surface.
11:00 AM		SS+SC-WeM-9 Electronic and Chemical Properties of Ce_0.8Zr_0.2O₂(111) Surfaces: Photoemission, XANES, Density Functional, and NO₂ Adsorption Studies G. Liu, J.A. Rodriguez, J. Hrbek, J. Dvorak (Brookhaven National Laboratory); C.H.F. Peden (Pacific Northwest National Laboratory) Synchrotron-based photoemission,conventional XPS, XANES, and first-principles density functional (DF) calculations were used to study the electronic properties of a Ce_0.8Zr_0.2O₂ mixed-metal oxide. The results of DF calculations show that the band gap in bulk Ce_0.8Zr_0.2O₂ is ~ 0.6 eV smaller than in bulk CeO₂, with the Zr atoms in the mixed-metal oxide showing smaller positive charges than the cations in ZrO₂ or CeO₂. When present in a lattice of CeO₂, the Zr atoms are forced to adopt larger metal-O distances than in ZrO₂, leading to a reduction in the oxidation state of this element. Due to non-equivalent Zr-O distances, at least three different types of oxygen atoms are found in the Ce_0.8Zr_0.2O₂ system. O K-edge XANES spectra for a series of Ce_1-xZr_xO₂ (x=0, 0.1, 0.2, 0.3, 1) compounds show a distinctive line shape for the mixed-metal oxides that can not be attributed to a sum of CeO₂ and ZrO₂ features, supporting the idea that the O atoms in Ce_1-xZr_xO₂ are in a special chemical environment. XPS Ce 3d spectra show the presence of Ce³⁺ cations which may be related to the relative stability of oxygen vacancy defects upon incorporation of zirconia into ceria. The interaction of NO₂ gas with Ce_0.8Zr_0.2O_2-x(111), CeO_2-x(111), and Zr(Y)O_2-x(111) reduced surfaces was examined. Ne⁺ ion sputtering was used to generate substantial concentrations of Ce³⁺, Zr²⁺ and Zr0 centers on the oxide surfaces. On CeO_2-x(111), NO₃, NO₂ and N were seen upon adsorption of NO₂. In contrast, only NO₂ and N were detected after adsorption of NO₂ on Ce_0.8Zr_0.2O_2-x(111) and Zr(Y)O_2-x(111). Adsorption of NO₂ induced an increase in the oxidation state of the metal cations (Ce³⁺ to Ce⁴⁺; Zr0 to Zr²⁺).
11:20 AM		SS+SC-WeM-10 Probing Chemical and Topological Heterogeneity of Carbonaceous Surfaces via Temperature Programmed Desorption of Simple Molecules from Model Carbonaceous Surfaces J. Kwon, R. Vidic, E. Borguet (University of Pittsburgh) Carbonaceous surfaces find applications in fields ranging from tribology to environmental remediation to catalysis. These surfaces are generally characterized by varying extent of chemical and topological heterogeneity that affects key elementary processe s such as adsorption and desorption. Temperature programmed desorption of model adsorbents (propane and acetone), representative of polar and non-polar organic compounds, was used to investigate the role of surface chemical and topological heterogeneity on the physical and chemical properties of model carbonaceous surfaces (air cleaved and plasma oxidized highly oriented pyrolytic graphite - HOPG). We observed that hydrogen and oxygen containing functional groups, which tend to block available adsorption sites, exist on air cleaved and plasma oxidized HOPG. Thermal treatment leads to removal of these groups and to over an order of magnitude increase in adsorption capacity. Thermal treatment (> 900 oC) of carbonaceous surfaces appears essential for maximum accessibility to adsorption sites. The surface defects induced by plasma oxidation yield greater surface area available for adsorption and higher energy sites. This novel approach promises a better understanding of chemically and topologically heterogeneous nanoporous carbons used in practical applications.