Scanning tunneling microscopy study on initial stage of atomic hydrogen adsorption on the Si(1 1 1) 7 × 7 surface

Initial hydrogen adsorption on the Si(1 1 1) 7 × 7 surface was studied by scanning tunneling microscopy (STM) in an ultrahigh vacuum. Room temperature adsorbed hydrogen on the adatom in the 7 × 7 reconstruction led to depression of adatoms in the STM images. The hydrogen uptake curve at the adatom site as a function of hydrogen exposure time was well represented by Langmuir adsorption. No preferential adsorption was seen among four inequivalent adatoms in the 7 × 7 reconstruction. Adsorption of the adjacent center and corner adatoms respectively showed ∼10% higher adsorption. Even though the number of reacted adatoms in the half unit of the 7 × 7 reconstruction was statistically random, the number of reacted adatoms in the nearest neighbor half unit was enhanced as the number of reacted sites increased in the half unit.     

An STM study of the localized atomic reaction of 1,2- and 1,4-dibromobenzene at Si(1 1 1)-7 × 7

A comparative study is reported of the thermal reaction of 1,2- and 1,4-dibromobenzene (1,2- and 1,4-diBrPh) on Si(1 1 1)-7 × 7, investigated by STM. Some results are given for the intermediate case of 1,3-diBrPh. The STM images gave evidence of a different pattern of reaction to yield pairs of Br-Si for 1,2-, 1,3- and 1,4-diBrPh. The ratio of pairs of Br-Si to single bromination events was 1:2 for 1,2-diBrPh and 1:3 for 1,4-diBrPh. In many cases organic residue from the bromination reaction, R-Si, was evident in the STM image. The products R-Si and Br-Si were found to be bound to adjacent Si, for both 1,2- and 1,4-diBrPh. The mean Br?Br pair separation at the surface depended on the parent molecule, being 7.6 Å for 1,2-diBrPh, 10.3 Å for 1,3-diBrPh, and 11.3 Å for 1,4-diBrPh. These separations are, in each case, about 4 Å greater than the separation of the Br-atoms in the intact parent molecule, which increases systematically down the series. There was a marked decrease in the percentage of R-Si accompanying the Br-Si in going down the series, decreasing from 70% for 1,2- to 20% for 1,4-diBrPh; this was interpreted as being due to a decrease in the percentage of `benzene-mediated' reaction dynamics, in which the benzene ring was bound to the surface. At moderately increased surface temperature (45 °C) the reaction of 1,2- and also 1,4-diBrPh no longer resulted in R-Si formation, suggesting that the dynamics had altered from benzene-mediated to `bromine-mediated'.     

Interaction of hydrogen with InN thin films elaborated on InP(1 0 0)

III-V semiconductor compound structures are widely applied in technology of advanced microelectronics, optoelectronics, and gas sensors. In this paper, we report on the use of XPS to characterize in situ the interaction of thermally activated hydrogen atoms and hydrogen molecules with InP(1 0 0) surfaces covered by thin InN overlayers. XPS spectra were taken with an ESCALAB-210 spectrometer after repeated hydrogenation cycles at temperatures up to 350 °C. The evolution of the In 3d, In 4d, P 2p, N 1s, O 1s and C 1s photoelectron spectra was carefully monitored. The XPS spectra of the hydrogen exposed surface revealed significant differences compared to those from the non-hydrogenated surface. InN films were found to be weakly reactive to hydrogen under experimental conditions explored. The behavior of P atoms at the hydrogenated surface was dependent on the parameters characterizing each hydrogenation (exposure, hydrogen species used, annealing temperature). Moreover, the heavily hydrogenated surface exhibited a phosphorus enrichment.     

Assignment of surface IR absorption spectra observed in the oxidation reactions: 2H + H2O/Si(1 0 0) and H2O + H/Si(1 0 0)

Infrared reflection absorption spectroscopy that used buried metal layer substrates (BML-IRRAS) and density functional cluster calculations were employed to investigate the water related oxidation reactions of 2H + H₂O/Si(1 0 0)-(2 × 1), 2D + H₂O/Si(1 0 0)-(2 × 1), and H₂O + H/Si(1 0 0)-(2 × 1). In addition to the oxygen inserted coupled monohydrides, which were previously reported in the former reaction system, we report several other oxidized Si hydride species in our BML-IRRAS experiments. Three new pairs of vibrational bands are identified between 900 and 1000 cm⁻¹. These vibrational frequencies were calculated using Si9 and Si10 cluster models that included all possible structures from zero to five oxygen insertions into the top layer silicon atoms using a B3LYP gradient corrected density functional method with a polarized 6-31G** basis set for all atoms. The three pairs of vibrational modes are assigned to the scissoring modes of adjacent and isolated SiH₂ with zero, one, and two oxygen atoms inserted into the Si back bonds. All the other newly observed vibrational peaks related to Si oxidation are also assigned in this study. The Si-O stretching bands observed in the reaction 2D + H₂O/Si(1 0 0)-(2 × 1) show an isotope effect, which suggests that in the system 2H + H₂O/Si(1 0 0)-(2 × 1) also, hydrogen atom tunneling plays an important role for the insertion of oxygen atoms into Si back bonds that form oxidized adjacent dihydrides.     

Prediction of structure-dependent charge transfer rates for a Li atom outside a Si(0 0 1) surface

We investigate the broadening of the 2s energy level of a Li atom outside a Si(0 0 1) surface using a first principles approach. The covalent nature of the Si surface produces large variations in Li energy level widths as a function of lateral position across the surface. The widths above symmetric Si dimers are predicted to be much larger than above buckled Si dimers, suggesting that charge transfer will occur primarily above symmetric dimers. We discuss the ramifications of our results on the controversy surrounding the relative abundance of the buckled vs. symmetric dimers on the Si surface.     

Vibration-induced structures in scanning tunneling microscope light emission spectra of Ni(1 1 0)-streaky (1 × 2)-H

We have measured the scanning tunneling microscope (STM) light emission spectra of Ni(1 1 0)-streaky (1 × 2) surfaces. When the tip was fixed over atomic hydrogen adsorbed on the surfaces, two types of vibration-induced structure were observed in the STM light emission spectra. One is the periodic fine structures that were already reported in our previous paper [Y. Uehara, S. Ushioda, Phys. Rev. Lett. 92 (2004) 066102] and the other newly found in the present experiments is a stepwise structure that is located at the vibrational energy of hydrogen below the cutoff energy of the STM light emission. They are ascribed to different excitation mechanisms of the vibration in the STM light emission process; the periodic fine structures appear when the vibrating motion is directly excited by the electrons injected from the tip. Conversely, the stepwise structure is observed when it is excited by the electromagnetic fields confined in the tip-sample gap, i.e., by localized surface plasmons.     

Morphological change of D2O layers on Ru(0 0 0 1) probed with He atom scattering

The morphological change of D₂O layers on a Ru(0 0 0 1) surface has been investigated on the basis of He atom scattering. With the increase of D₂O exposure on Ru(0 0 0 1) at 111 K, the intensity of a specularly reflected He beam continuously decreases up to the exposure of 1.0 L (Langmuir). At the D₂O coverage of 1.0 adsorbed layer (∼1.5 L), which is characterized by temperature-programmed desorption measurements, the formation of the (√3 × √3)R30° superstructure as a result of the diffusion of D₂O on the surface was confirmed by He atom diffraction. With the further increase of D₂O exposure, at 2-3 adsorbed layers, the disordered structure was found to be on the surface at 111 K. The morphological change of the disordered layers was observed during annealing, and discussed in detail.     

Vacancy charging on Si(1 1 1)-(7 × 7) investigated by density functional theory

The structure and energetics of charged vacancies on Si(1 1 1)-(7 × 7) are investigated using density functional theory calculations supplemented by estimates of ionization entropy. The calculations predict multiple possible charge states for the unfaulted edge vacancy in the adatom layer, although the −2 state is most stable on real Si(1 1 1) surfaces for which the Fermi level lies near the middle of the band gap.     

Chloromethane surface chemistry on Fe3O4(1 1 1)-(2 × 2): A thermal desorption spectrometry comparison of CCl4, CBr2Cl2, and CH2Cl2

The surface chemistry of CBr₂Cl₂ on the Fe₃O₄(1 1 1)-(2 × 2) selvedge of single-crystal α-Fe₂O₃(0 0 0 1) has been investigated using temperature programmed reaction and desorption (TPR/D) measurements. The spectra obtained in this case show that strong chemisorption occurs and that a series of adsorbed halogenated reaction products are present. By comparison, studies of the adsorbed phase of CH₂Cl₂ show that only physisorption occurs. The TPR/D spectra of CBr₂Cl₂ show that dissociative formation of CCl₂ followed by its reaction with lattice oxygen is central to the monolayer reaction chemistry in this chloromethane. The branching ratios of the various desorbed products are compared with those obtained from CCl₄ adsorbed on the same (2 × 2) surface.     

Molecularly intact and dissociative adsorption of water on clean Cu(1 1 0): A comparison with the water/Ru(0 0 1) system

An X-ray photoelectron spectroscopy (XPS) study was undertaken of the water/Cu(1 1 0)-system finding non-dissociative adsorption on clean Cu(1 1 0) at temperatures below 150 K. Thermally induced dissociation of D₂O is observed to occur above 150 K, similar to the H₂O/Ru(0 0 1) system, with an experimentally derived activation barrier of 0.53-0.56 eV which is very close in magnitude to the derived activation barrier for desorption of 0.50-0.53 eV. X-ray and electron induced damage to the water overlayer was quantified and used to rationalize the results of a recent XPS study of the water/Cu(1 1 0)-system where partial dissociation was observed already at 90 K.     

Thermal chemistry of diiodomethane on Ni(1 1 0) surfaces I. Clean and hydrogen-covered

The thermal chemistry of diiodomethane on Ni(1 1 0) single-crystal surfaces was studied by temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). Diiodomethane was chosen as a precursor for the formation of methylene surface species. I 3d and C 1s XPS data indicated that, indeed, adsorbed diiodomethane undergoes the C-I bond dissociations needed for that transformation, and detection of iodomethane production in TPD experiments pointed to the stepwise nature of those reactions. Significant amounts of methane are produced from further thermal activation of the chemisorbed methylene groups. This involves surface hydrogen, both coadsorbed from background gases and produced by dehydrogenation of some of the adsorbed diiodomethane, and can be induced at temperatures as low as about 160 K, right after the C-I bond breaking steps. Unique to this system is the detection of significant amounts, up to 10% of the total CH₂I₂ adsorbed, of heavier hydrocarbons, including ethene, ethane, propene, propane, and butene. Deuterium labeling experiments were used to provide support for a mechanism where the initial hydrogenation of some adsorbed methylene to methyl moieties is followed by a rate-limiting methylene insertion step to yield ethyl intermediates. Facile subsequent β-hydride elimination and reductive elimination with coadsorbed hydrogen account for the formation of ethene and ethane, respectively, while a second and third methylene insertions lead to C₃ and C₄ production. Based on the final product distribution, the methylene insertion was estimated to be approximately 20 times slower than the following hydrogenation-dehydrogenation reactions. Normal kinetic isotope effects were observed for most of the hydrogenation and dehydrogenation reactions involved.     

Evidence for long-range surface state mediated interaction between sodium atoms on copper (0 0 1)

Inelastic helium atom scattering from sodium atoms on the Cu(0 0 1) surface at 50 K reveals a remarkable 15% increase in the frequency of the frustrated translational vibrations (T-mode) from ?ω=5.56 to 6.34 meV with increasing coverage from Θ_Na=0.008 to 0.125. The coverage dependence and the negligible dispersion of the frequency cannot be explained by the direct dipole-dipole coupling but are well-understood in terms of the Lau-Kohn effective long-range interaction via intrinsic surfaces states.     

Si(1 1 1)-H(1 × 1): A mirror for atoms characterized by AFM, STM, He and H2 diffraction

AFM, STM and diffraction of He and H₂ have been used to assess Si(1 1 1)-H(1 × 1) surfaces for their potential as mirrors for matter-waves. The H-passivated samples are produced by wet-chemical methods and delivered to a different laboratory for diffraction measurements. We show that the surface is flat and homogenous over lateral scales of microns and that absolute He and H₂ reflectivities of the order of ∼3% are obtained, even after 20 h storage under Ar and several days’ storage in UHV. These characteristics allow the use of Si(1 1 1)-H(1 × 1) as a highly reflective mirror for atoms and molecules, with application in a future He microscope or focused hydrogen nano-lithography system.     

Autocatalytic partial reduction of FeO(1 1 1) and Fe3O4(1 1 1) films by atomic hydrogen

The interaction of atomic hydrogen with thin epitaxial FeO(1 1 1) and Fe₃O₄(1 1 1) films was studied by TDS, XPS and LEED. On the thin, one Fe-O bilayer thick FeO film, partial reduction occurs in two steps during exposure. It ends after removal of 1/4 monolayer (ML) of oxygen with a 2 × 2 pattern appearing in LEED. This FeO_0.75 film is passive against further reduction. The first reduction step saturates after removal of ∼0.2 ML and shows autocatalytic kinetics with the oxygen vacancies formed during reduction causing acceleration. The second step is also autocatalytic and is related with reduction to the final composition and an improvement of the 2 × 2 order. A structure model explaining the two-step reduction is proposed. On the thick Fe₃O₄ film, irregular desorption bursts of H₂O and H₂ were observed during exposure. Their occurrence appears to depend on the film quality and thus on surface order. Because of the healing of reduction-induced oxygen vacancies by exchange of oxygen or iron with the bulk, a change of the surface composition was not visible. The existence of partially reduced oxide phases resistant even to atomic hydrogen is relevant to the mechanism of dehydrogenation reactions using iron oxides as catalysts.     

An AFM, XPS and electrochemical study of molecular electroactive monolayers formed by wet chemistry functionalization of H-terminated Si(1 0 0) with vinylferrocene

Molecular electroactive monolayers have been produced from vinylferrocene (VFC) via light-assisted surface anchoring to H-terminated n- and p-Si(1 0 0) wafers prepared via wet chemistry, in a controlled atmosphere. The resulting Si-C bound hybrids have been characterized by means of XPS and AFM. Their performance as semiconductor functionalized electrodes and their surface composition have been followed by combining electrochemical and XPS measurements on the same samples, before and after use in an electrochemical cell. White-light photoactivated anchoring at short (1 h) exposure times has resulted in a mild route, with a very limited impact on the initial quality of the silicon substrate. In fact, the functionalized Si surface results negligibly oxidized, and the C/Fe atomic ratio is close to the value expected for the pure molecular species. The VFC/Si hybrids can be described as (η⁵-C₅H₅)Fe²⁺(η⁵-C₅H₄)-CH₂-CH₂-Si species, on the basis of XPS results. Electrochemical methods have been applied in order to investigate the role played by a robust, covalent Si-C anchoring mode towards substrate-molecule electronic communication, a crucial issue for a perspective development of molecular electronics devices. The response found from cyclic voltammograms for p-Si(1 0 0) functionalized electrodes, run in the dark and under illumination, has shown that the electron transfer is not limited by the number of charge carriers, confirming the occurrence of electron transfer via the Si valence band. The hybrids have shown a noticeable electrochemical stability and reversibility under cyclic voltammetry (cv), and the trend in peak current intensity vs. the scan rate was linear. The molecule-Si bond is preserved even after thousands of voltammetric cycles, although the surface coverage, evaluated from cv and XPS, decreases in the same sequence. An increasingly larger surface concentration of Fe³⁺ at the expenses of Fe²⁺ redox centers has been found at increasing number of cv’s, experimentally associated with the growth of silicon oxide. Surface SiO⁻ groups from deprotonated silanol termination, induced by the electrochemical treatments, are proposed as the associated counterions for the Fe³⁺ species. They could be responsible for the observed decrease in the electron transfer rate constant with electrode ageing.     

Adsorption, ordering, and chemistry of nitrobenzene on Si(1 0 0)-2 × 1

Surface chemistry of nitrobenzene on Si(1 0 0)-2 × 1 has been investigated using multiple internal reflection Fourier-transform infrared spectroscopy (MIR-FTIR), Auger electron spectroscopy (AES) and thermal desorption mass spectrometry. Molecular adsorption of nitrobenzene at submonolayer coverages is dominating at cryogenic temperatures (100 K). As the surface temperature is increased to 160 K, chemical reaction involving nitro group occurs, while the phenyl entity remains intact. Thus, a barrier of approximately 40.8 kJ/mol is established for the interaction of the nitro group of nitrobenzene with the Si(1 0 0)-2 × 1 surface. Further annealing of the silicon surface leads to the decomposition of nitrobenzene. The concentration of nitrogen and oxygen remains constant on a surface within the temperature interval studied here. AES studies also suggest that the majority of carbon-containing products remain bound to the surface at temperatures as high as 1000 K. The only chemical reaction leading to the release of the gaseous products is benzene formation around 670 K. The amount of benzene accounts only for a few percent of the surface species, while the rest of the phenyl groups connected to the silicon surface via a nitrogen linker remain stable even at elevated temperatures, opening an opportunity for stable surface coatings.     

Thermal chemistry of diiodomethane on Ni(1 1 0) surfaces II. Effect of coadsorbed oxygen

The effect of coadsorbed oxygen on the thermal chemistry of diiodomethane on Ni(1 1 0) single-crystal surfaces was studied by temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). I 3d and C 1s XPS data indicated that adsorbed diiodomethane undergoes two sequential C-I bond scission steps to ultimately produce methylene surface species, the same as on clean Ni(1 1 0). Moreover, significant amounts of methane and other heavier hydrocarbons are produced after further thermal activation of those chemisorbed methylene groups. The production of alkanes and alkenes, which is accounted for by a chain growth mechanism where the initial hydrogenation of some adsorbed methylene to methyl moieties is followed by a rate-limiting methylene insertion step to yield ethyl intermediates, is inhibited but not fully blocked by the coadsorbed oxygen. New reaction pathways are also opened up by the presence of oxygen in this system, including a direct coupling of two methylene groups to ethene, the insertion of an oxygen atom into a nickel-methylene group to produce formaldehyde, and a parallel methylene insertion chain growth sequence starting from a CH₂I_ads intermediate to ultimately yield C₃H₅ and C₄H₇ unsaturated gas-phase radicals.     

Theoretical study of hydrogen adsorption on the GaN(0 0 0 1) surface

Ab initio density functional theory, using the B3LYP hybrid functional with all-electron basis sets, has been applied to the adsorption of H on the (0 0 0 1) surface of wurtzite GaN. For bulk GaN, good agreement is obtained with photoemission and X-ray emission data for the valence band and for the Ga 3d and N 2s shallow core levels. A band gap of E_g = 4.14 eV is computed vs the experimental value (at 0 K) of 3.50 eV. A simple model, consisting of a (2 × 2) structure with 3/4-monolayer (ML) of adsorbed H, is found to yield a density of states in poor agreement with photoemission data for H adsorbed on surfaces prepared by ion bombardment and annealing. A new model, consisting of co-adsorbed Ga (1/4 ML) and H (1/2 ML), is proposed to account for these data.     

Chlorinated ethenes at Cu(1 1 0): adsorption, dissociation and reactions of trans-1,2-dichloroethene

The adsorption and reactions of trans-1,2-dichloroethene on Cu(1 1 0) have been investigated using reflection absorption infrared spectroscopy, temperature programmed desorption and molecular beam adsorption reaction spectroscopy. These data reveal that the behaviour of the system is critically affected by temperature and can be described in terms of three main regimes: Regime I, which occurs over 85-165 K, represents molecular adsorption in the monolayer and the multilayer where the trans-1,2-dichloroethene adsorbs with its molecular plane oriented largely parallel to the metal surface; Regime II, extending over the temperature range 170-280 K, is dominated by desorption/dechlorination events which are first initiated at 171 K, then poisoned rapidly and re-initiated at the much higher temperature of 263 K, culminating in the creation of adsorbed acetylene and Cl atoms on the surface; Regime III, spanning the temperature range of 280-500 K, is governed by the behaviour of the C₂H₂ molecular intermediate at the surface which, if created at low temperatures, trimerises to form benzene that subsequently desorbs. However, when created at high temperature, direct desorption of C₂H₂ competes effectively with the trimerisation process.