We have studied the deposition of aluminum films by triisobutylaluminum (TIBA) pyrolysis on clean and oxidized Si(lll) (7×7), Si(100) (2×1), and an evaporated aluminum film on silicon. On all of the silicon surfaces, HBA has a very low reaction probability (<0.01). On aluminum, the reaction probability is much higher, and the rate of steady state aluminum deposition is limited by β-hydride elimination from surface-bound isobutyl species to give isobutene and hydrogen. In the steady state deposition of aluminum on top of silicon, gas phase silanes such as isobutyl- and diisobutylsilane are also evolved. This clearly indicates that hydrogen and alkyl ligand transfer from aluminum to silicon (which has diffused up through the growing aluminum film) can occur. We have also found that alkyl species generated on aluminum surfaces by. the adsorption of alkyl iodides have surface chemistries analogous to the aluminum alkyl ligands.

Results in the literature indicate that Cl2 etches GaAs at room temperature but HCl etches GaAs at a measurable rate only at temperatures above ∼670 K. In this work, molecular beam scattering and surface analysis techniques have been applied to address the fundamental kinetic differences between these two systems. The results indicate that the onset of GaAs etching by Cl2 is determined by the kinetics of Ga-removal as GaCl3 while etching by HCl is limited by As evaporation as As2. The results also suggest that HCl selectively etches gallium from GaAs at temperatures between 600 and 650 K.

Preface. Introduction. 1 Surfaces: An Introduction. 2 The Structure of Surfaces. 3 Thermodynamics of Surfaces. 4 Dynamics at Surfaces. 5 Electrical Properties of Surfaces. 6 Surface Chemical Bond. 7 Mechanical Properties of Surfaces. 8 Polymer Surfaces and Biointerfaces. 9 Catalysis by Surfaces. Index.

The thermal chemistry of iodomethane, iodoethane, 1-iodopropane, 1-iodobutane, and 2-iodohexane on copper (100), (110), and (111) single-crystal surfaces was characterized in this and previous studies by temperature-programmed desorption (TPD) spectroscopy. The main decomposition pathway available to the methyl surface moiety that results from C-I bond activation in adsorbed iodomethane is a-hydride elimination to methylene, a step that occurs around 460-470 K on all three surfaces. Some methylene dimerization to ethane is also seen at higher coverages, at a rate that depends significantly on surface structure; ethane desorption peaks at 400 K on Cu(110), but only above 440 K on Cu(100) and Cu(111). Ethyl groups produced by iodoethane decomposition react at much lower temperatures and mostly undergo beta-hydride elimination to ethylene. The ethyl dehydrogenation reaction is structure sensitive as well, a fact illustrated by the different ethylene desorption peak maxima observed in the TPD experiments, at 225, 247, and 255 K on Cu(110), Cu(111), and Cu(100), respectively (at saturation). Perhaps the more telling observations are the difference in feasibility of H-D scrambling in the ethylene resulting from conversion of a 1:1 mixture of normal and perdeutero iodoethane, a reaction viable on Cu(100) but not on Cu(110), and the 10-fold difference in ethane yield between those two crystals. Additional studies with 1-iodopropane and 1-iodobutane provided some information on the effect of chain length on reactivity, and experiments with 2-iodohexane attested to the high selectivity for removal of internal hydrogen atoms during beta-hydride elimination from alkyl species.

The adsorption geometries of iodomethane, iodoethane, 1-iodopropanel and 2-iodopropane on Cu(110) single-crystal surfaces were characterized by using reflection-absorption infrared spectroscopy. At 100 K adsorption is molecular in all cases, but with adsorption geometries that change with increasing coverages. All alkyl iodides adsorb with the C-I bond perpendicular to the surface at low coverages and tilted at saturation. The hydrocarbon chains in iodoethane, 1-iodopropane, and 2-iodopropane follow the expected behavior, namely, the first molecules chemisorb flat on the surface and those added above about half a monolayer adopt a vertical orientation. All the adsorbed alkyl iodides decompose by 140 K via the scission of their C-I bond and generate alkyl groups on the surface. Those surface alkyls also change configuration with coverage, aligning themselves at saturation in a fashion reminiscence of that seen in self-assembled monolayers.

We report the use of near-edge X-ray absorption fine structure (NEXAFS) to unambiguously characterize the oxidation state of copper in copper oxides. The method is based on the analysis of the oxygen K-edge and copper L-edge features of Cu2O and CuO powder materials. To further demonstrate its utility, NEXAFS was also used to follow spectroscopically the oxidation of a Cu(100) single crystal surface by NO2, and the thermal decomposition of the resulting oxides. In agreement with previous studies [J.A. Rodriguez, J. Hrbek, J. Vac. Sci. Technol. 12 (1994) 2140], our NEXAFS results indicate that NO2 dissociates on the metal surface, generating NO and atomic oxygen which subsequently reacts with the Cu(100) surface to produce Cu2O (300K) or a mixture of Cu2O and CuO (250K). Upon heating, the thermodynamically unstable CuO transforms into Cu2O.

The dehydrogenation and thermal decomposition mechanisms of cyclohexene and 1,3-cyclohexadiene on clean Mo(110) and carbide-modified (4 x 4)-C/Mo(110) surfaces have been studied using temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). On the clean Mo(110) surface, partial dehydrogenation of a fraction of the cyclohexene molecules occurs at temperatures as low as 80 K. When the surface is heated to 150 K, the HREEL spectra obtained are characteristic of a C6H9 intermediate, as seen by a comparison with HREEL spectra reported for C6H9 On Pt(111).(1,2) At higher temperatures, competing C-C and C-H bond cleavage reactions lead to the formation of surface carbon and the evolution of hydrogen. In contrast, on the carbide-modified surface, the primary reaction pathway for cyclohexene is selective dehydrogenation to form benzene and hydrogen. In the case of 1,3-cyclohexadiene, the HREEL results suggest that dehydrogenation to form benzene occurs at 80 K on the clean Mo(110) surface, based on a comparison with the HREEL spectrum for benzene directly dosed onto Mo(110) at 80 K. However, upon heating, most of the benzene decomposes to form surface carbon and hydrogen, as shown by TPD studies. On the carbide-modified surface, the primary reaction pathway for 1,3-cyclohexadiene is selective dehydrogenation to form benzene, which desorbs at 313 K. Furthermore, the HREEL results also indicate that a competing reaction pathway occurs to form a surface intermediate which most likely has an tilted aromatic c-C-6 ring, such as a surface phenyl species.

The addition reaction of gas-phase D atoms to olefin monolayers adsorbed on a Cu(100) surface is studied, with a focus on the regioselectivity of deuterium addition onto monolayer 1-butene and 1-pentene molecules. Both 1- and 2-alkyl groups are generated from D atom additions to 2 degrees and 1 degrees carbons, respectively. The alkyl groups are separated based on a difference in their beta-hydride elimination kinetics, with the rate of 2-alkyl groups losing beta-hydrogens about 2 orders of magnitude faster than 1-alkyl groups on the copper surface. The results suggest that D addition to terminal (1 degrees) carbon is favored by a factor of similar to 3 for 1-butene and similar to 4 for 1-pentene molecules adsorbed on the surface.

Near-edge X-ray absorption fine structure (NEXAFS) and temperature-programmed desorption (TPD) studies have been performed to establish the relationship between adsorbate structure and binding energy in a monolayer of hydrocarbons on a Cu(100) surface. Fourteen different saturated and monounsaturated hydrocarbons were studied. The activation energy for desorption of these compounds has been found to be dependent on the following factors: (1) the length of the saturated hydrocarbon linear chain; (2) the presence and location of a double bond; (3) the cyclic versus acyclic nature of the hydrocarbons; and (4) the accessibility of the CHn groups for creating van der Waals interactions with the surface. Similar to previous observations on other surfaces, our results show that an increase of the linear hydrocarbon chain length by one methylene group increases the binding energy of a hydrocarbon by 1.5 kcal/mol. Our results also indicate that the presence of a double bond in a position where overlap between π-orbitals of a hydrocarbon and d-orbitals of the metal is significant (double bond parallel to the Cu(100) surface) increase the binding energy of an olefin molecule by 0.75 kcal/mol with respect to that of the corresponding saturated hydrocarbon.

We report here ultrahigh vacuum studies of the dehydrocyclization reaction of submonolayer coverages of l-hexene to benzene on a Cu3Pt(lll) single crystal surface, using reflection-absorption infrared spectroscopy (RAIRS), near edge X-ray absorption fine structure (NEXAFS) studies, and temperature-programmed reaction/desorption (TPR/D) spectrometry. As discussed in a previous TPR/D paper, at surface coverages up to 13% of monolayer saturation, l-hexene forms benzene on a Cu3Pt(lll) surface. Selectivity to benzene formation is 70 +/- 10%, with the remaining 30 +/- 10% of the adsorbed 1-hexene dehydrogenating irreversibly to surface carbon and H-2. For higher coverages, molecular desorption commences. Spectroscopic identification of the intermediates of the reaction of l-hexene and other model compounds, such as a 1,3,5-hexatriene, with a Cu3Pt(111) surface suggests that l-hexene and 1,3,5-hexatriene have a common intermediate, and this intermediate has been identified as a rehybridized hexatriene species. Other model compounds, such as trans-3-hexene, have also been used to provide further understanding of the mechanism of the aromatization reaction.

Motivated by a controversy about the proper interpretation of x-ray photoelectron spectra of Si/SiO 2 interfaces derived from the adsorption of H 8Si8O12 spherosiloxane clusters on Si ~100! surfaces, we have studied the adsorption geometry of the H 8Si8O12 clusters on deuterium-passivated and clean Si~100! surfaces by using external reflection infrared spectroscopy. Access to frequencies below 1450 cm was made possible through the use of specially prepared Si ~100! samples which have a buried metallic CoSi 2 layer that acts as an internal mirror. A comparison of the infrared spectrum of the clusters on a deuterium-passivated Si ~100! surface at 130 K with an infrared spectrum of the clusters in a carbon tetrachloride solution reveals that the clusters are only weakly physisorbed on the D/Si ~100! surface and also provides evidence for the purity of the cluster source. We also present infrared spectra of clusters directly chemisorbed on a clean Si ~100! surface and show evidence that the clusters are adsorbed on the Si ~100! via attachment by one vertex. A complete assignment of the observed vibrational features, for both physisorbed and chemisorbed clusters, has been made based upon comparisons with the results obtained in ab i itio calculations using gradient-corrected density functional methods. © 1998 American Institute of Physics. @S0021-9606 ~98!02820-7#

The Direct Synthesis of methylchlorosilanes from methyl chloride and silicon, catalyzed by copper and minor promoter elements was reviewed with respect to use of ultra-high vacuum (UHV) surface reaction techniques to uncover the mechanism of the reaction. In particular, recent results were presented for sequentially adsorbing methyl radicals and chlorine on polycrystalline Cu3Si alloy under ultra-high vacuum conditions. Methyl monolayers in the absence of chlorine produced primarily trimethylsilane, and chlorine monolayers in the absence of methyl produced SiCl4. However, mixed monolayers of methyl groups with chlorine atoms abandoned these separate pathways and instead reacted at similar temperatures on the surface to produce methylchlorosilanes with selectivities to 85% Me2SiCl2 with Zn, Sn, and Al as promoters.

The thermal decomposition pathways of isobutene and 1-butene on both Mo(110) and 4 × 4-C/Mo(110) surfaces have been studied using high-resolution electron energy loss spectroscopy (HREELS) in order to highlight the substantially different activities of these two surfaces towards the cleavage of C–H and C–C bonds. On clean Mo(110), the CH 2 group of isobutene decomposes upon heating to 150 K, producing either a σ/π-bonded isobutenylidene [(CH 3 ) 2 CCH] species or a 1,1-di-σ/π-bonded isobutenyl [(CH 3 ) 2 CC] species. Upon further heating, extensive C–H bond scission occurs to form hydrocarbon fragments which do not contain CH 3 or CH 2 groups, but appear to have largely intact carbon skeletons. By contrast, isobutene is molecularly adsorbed on the carbide-modified surface at 150 K. Further heating produces isobutylidyne [(CH 3 ) 2 HCC] by 300 K, which subsequently decomposes via C–C bond scission to generate surface methyl groups. The different activation sequence of the C–H and C–C bonds of isobutene on clean and carbide-modified Mo(110) surfaces is also qualitatively confirmed by comparative studies of 1-butene on the two surfaces.

The adsorption and thermal decomposition of n-azopropane on Pt(111) were studied by using temperature-programmed desorption (TPD) and reflection–absorption infra-red spectroscopy (RAIRS). At low temperatures, n-azopropane chemisorbs molecularly on Pt(111), but it isomerizes into a cis configuration upon bonding to the surface and changes its adsorption orientation as a function coverage. Multilayer and monolayer molecular desorption occur about 130 and 170K, respectively, and a new surface species – probably dipropyl hydrazine – forms upon annealing the remaining chemisorbed molecules above 175K. Further heating of the sample leads to the desorption of small amounts of propylene and ethylene, at 240 and 275K, respectively. Another surface transformation is identified by RAIRS around 275K, possibly the conversion to an imine-type species, and propionitrile and small amounts of propyl and methyl amines are produced soon after that; they desorb at 320 and 330K, respectively. Finally, a more extensive dehydrogenation of the remaining surface species takes place, and HCN desorbs in two stages around 525 and 600K. The chemistry of propionitrile and propyl amine was also explored briefly by RAIRS and TPD for reference.

Motivated by a controversy about the proper interpretation of x-ray photoelectron spectra of Si/SiO2 interfaces derived from the adsorption of H8Si8O12 spherosiloxane clusters on Si(100) surfaces, we have studied the adsorption geometry of the H8Si8O12 clusters on deuterium-passivated and clean Si(100) surfaces by using external reflection infrared spectroscopy. Access to frequencies below 1450 cm(-1) was made possible through the use of specially prepared Si(100) samples which have a buried metallic CoSi2 layer that acts as an internal mirror. A comparison of the infrared spectrum of the clusters on a deuterium-passivated Si(100) surface at 130 K with an infrared spectrum of the clusters in a carbon tetrachloride solution reveals that the clusters are only weakly physisorbed on the D/Si(100) surface and also provides evidence for the purity of the cluster source. We also present infrared spectra of clusters directly chemisorbed on a clean Si(100) surface and show evidence that the clusters are adsorbed on the Si(100) via attachment by one vertex. A complete assignment of the observed vibrational features, for both physisorbed and chemisorbed clusters, has been made based upon comparisons with the results obtained in ab initio calculations using gradient-corrected density functional methods. (C) 1998 American Institute of Physics. [S0021-9606(98)02820-7].

The bonding and reactivity of allyl groups (CH2CHCH2) on a Cu (100) surface are studied by temperature-programmed desorption and near edge X-ray absorption fine structure measurements. Surface allyl groups are generated by dissociative adsorption of allyl chloride at 110 K and/or a coupling reaction of surface methylene (CH2) and vinyl (C2H5) groups. Allyl groups on a Cu(100) surface coordinate as a nearly flat-lying pi-allyl species. Allyl radical ejection is the dominant reaction pathway upon heating the Cu(100) surface, which is drastically different from the behavior of allyl on Ag(111) and Al(100) surfaces, where allyl groups couple to form 1,5-hexadiene and disproportionate to produce propylene and an unidentified polymeric species, respectively. On a Cu(100) surface precovered with hydrogen atoms, allyl groups are scavenged by hydrogen atoms to produce propylene at similar to 250 K.