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We have investigated the adsorption and thermal reaction processes of NO with silicene spontaneously formed on the ZrB2/Si(111) substrate using synchrotron radiation x-ray photoelectron spectroscopy (XPS) and density-functional theory calculations. NO is dissociatively adsorbed on the silicene surface at 300 K. An atomic nitrogen is bonded to three Si atoms most probably by a substitutional adsorption with a Si atom of silicene (N≡Si3). An atomic oxygen is inserted between two Si atoms of the silicene (Si—O—Si). With increasing NO exposure, the two-dimensional honeycomb silicene structure gets destroyed, judging from the decay of typical Si 2p spectra for silicene. After a large amount of NO exposure, the oxidation state of Si becomes Si4+ predominantly, and the intensity of the XPS peaks of the ZrB2 substrate decreases, indicating that complicated silicon oxinitride species have developed three-dimensionally. By heating above 900 K, the oxide species start to desorb from the surface, but nitrogen-bonded species still exist. After flashing at 1053 K, no oxygen species is observed on the surface; SiN species are temporally formed as a metastable species and BN species also start to develop. In addition, the silicene structure is restored on the ZrB2/Si(111) substrate. After prolonged heating at 1053 K, most of nitrogen atoms are bonded to B atoms to form a BN layer at the topmost surface. Thus, BN-covered silicene is formed on the ZrB2/Si(111) substrate by the adsorption of NO at 300 K and prolonged heating at 1053 K.

The functionalization of graphene is important in practical applications of graphene, such as in catalysts. However, the experimental study of the interactions of adsorbed molecules with functionalized graphene is difficult under ambient conditions at which catalysts are operated. Here, the adsorption of CO2 on an oxygen-functionalized epitaxial graphene surface was studied under near-ambient conditions using ambient-pressure X-ray photoelectron spectroscopy (AP-XPS). The oxygen-functionalization of graphene is achieved in situ by the photo-induced dissociation of CO2 with X-rays on graphene in a CO2 gas atmosphere. The oxygen species on the graphene surface is identified as the epoxy group by XPS binding energies and thermal stability. Under near-ambient conditions of 1.6 mbar CO2 gas pressure and 175 K sample temperature, CO2 molecules are not adsorbed on the pristine graphene, but are adsorbed on the oxygen-functionalized graphene surface. The increase in the adsorption energy of CO2 on the oxygen-functionalized graphene surface is supported by first-principles calculations with the van der Waals density functional (vdW-DF) method. The adsorption of CO2 on the oxygen-functionalized graphene surface is enhanced by both the electrostatic interactions between the CO2 and the epoxy group and the vdW interactions between the CO2 and graphene. The detailed understanding of the interaction between CO2 and the oxygen-functionalized graphene surface obtained in this study may assist in developing guidelines for designing novel graphene-based catalysts.

A rigid and planar ethynylene-bridged pentacene dimer (PenD) was synthesized from pentacenequinone in two steps, skipping the conventional stepwise approach. A brickwork motif in the single crystal shows two-dimensionally extended electronic interaction in the solid state. Highly crystalline dip-coated films exhibited average hole mobility of 0.24 cm2 V-1 s-1 , comparable to that of the single-crystal organic field-effect transistors. This discovery and understanding of the reaction for the facile synthesis of ethynylene-bridged π-conjugated systems enables to the synthesis of a wide range of organic semiconducting materials.

The reaction of CO2 on the vicinal Cu(997) surface at 340 K under CO2 gas pressure of 0.8 mbar was investigated by ambient pressure X-ray photoelectron spectroscopy. A main reaction product on the surface was identified as carbonate (CO3), based on estimation of the composition ratio of oxygen to carbon. CO3 was produced on the surface through the reaction of CO2 with oxygen formed from CO2 dissociation. The amount of adsorbed CO3 was increased and saturated as time elapsed. After saturation of adsorbed CO3, atomic oxygen appeared on the surface, indicating that CO2 dissociation into CO and O continued to take place. The present study shows the importance of CO3 intermediate in the CO2 chemistry on stepped Cu surfaces.

Hydrogen-bonding heterogeneous bilayers on substrates have been studied as a base for new functions of molecular adlayers by means of atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), infrared reflection absorption spectroscopy (IRAS), and density functional theory (DFT) calculations. Here, we report the formation of the catechol-fused bis(methylthio)tetrathiafulvalene (H2Cat-BMT-TTF) adlayer hydrogen bonding with an imidazole-terminated alkanethiolate self-assembled monolayer (Im-SAM) on Au(111). The heterogeneous bilayer is realized by sequential two-step immersions in solutions for the individual Im-SAM and H2Cat-BMT-TTF adlayer formations. In the measurements by AFM, a grained H2Cat-BMT-TTF adlayer on Im-SAM is revealed. The coverage and the chemical states of H2Cat-BMT-TTF on Im-SAM are specified by XPS. On the vibrational spectrum measured by IRAS, the strong hydrogen bonds between H2Cat-BMT-TTF and Im-SAM are characterized by the remarkably red-shifted OH stretching mode at 3140 cm-1, which is much lower than that for hydrogen-bonding water (typically ∼3300 cm-1). The OH stretching mode frequency and the adsorption strength for the H2Cat-BMT-TTF molecule hydrogen bonding with imidazole groups are quantitatively examined on the basis of DFT calculations.

Infrared reflection–absorption spectroscopy (IRAS) is a spectroscopic method for measuring an infrared reflection absorption spectrum due to vibrations of adsorbed atoms and molecules (Hoffmann in Surf. Sci. Rep. 3: 107, 1983 [1]; Chabal in Surf. Sci. Rep. 8: 21, 1988 [2]; Bradshaw and Schweizer in Adv. Spectrosc. 16: 413, 1988 [3]; Madey and Yates (ed.) in Vibrational Spectroscopy of Molecules on Surfaces (Methods of Surface Characterization). Springer, 1987. [4]), where infrared light is incident to an optically flat metal surface at a grazing angle and reflected light is detected.

We investigated the adsorption of CO

The adsorption, desorption, and decomposition of formic acid (HCOOH) on Cu(111), Cu(997), Zn-Cu(111), and Zn-Cu(997) were systematically studied by high-resolution x-ray photoelectron spectroscopy, temperature programmed desorption, and infrared reflection absorption spectroscopy. On the clean Cu(111) surface, 13% of formic acid molecules adsorbed at 83 K were dissociated to form bidentate formate species by heating at 300 K; however, on the Zn-Cu(111) surface, only 4% of adsorbed HCOOH molecules were dissociated into the bidentate formate species. On the contrary, 13% of adsorbed HCOOH molecules were already dissociated into monodentate formate species on Cu(997) even at 83 K and 17% of adsorbed formic acid molecules were transformed to bidentate formate species by heating at 300 K, indicating that the stepped Cu surface has higher reactivity for HCOOH dissociation at low temperature. On the Zn-Cu(997) surface, 20% of formic acid became bidentate formate species in contrast to the case with Zn-Cu(111). Thus, the Zn deposited Cu step surface shows special activity for adsorption and dissociation of formic acid. The desorption peak maxima of the formate decomposition products (CO2 and H2) on Zn-Cu(997) were shifted to higher temperatures than those on Cu(997). Zn on Cu surfaces plays an important role in the stabilization of formate species, which probably leads to the decrease in the activation barrier for hydrogenation on the Zn-Cu alloyed surface.

One of the real challenges in realization of many of graphene’s anticipated applications is the development of a common chemical route for modifying graphene with varieties of functionalities. Here, we successfully demonstrate the organic modification of epitaxial graphene (EG) grown on the Si-face of SiC substrate through two-step chlorination–alkylation reactions. Pristine and chemically modified graphene are characterized by scanning tunneling microscope and spectroscopy, X-ray photoelectron spectroscopy, and Raman measurements. The first-step photochlorination is found to occur very selectively on the monolayer graphene region leaving the bi- and trilayer graphene regions clean. Consequently, the CH3-functionalized graphene is observed only in the monolayer graphene regions after the chlorinated EG was treated with CH3MgBr in air-free condition. Both Cl and CH3 are observed to be chemically bonded to the basal plane of the graphene. The CH3-functionalized graphene is thermally more stable than that of...

The effect of iridium valence in Ir:SrTiO3 on the electronic structure and the photocatalytic activity in a water splitting reaction was studied. Epitaxial thin film photoelectrodes were grown with controlled Ir valence and used to measure the electrochemical efficiency of Ir:SrTiO3. The positions of the in-gap Ir impurity levels were determined by optical and X-ray photoelectron spectroscopies. Comparison with ab initio calculations was used to assign the observed electronic states to either Ir4+ or Ir3+ dopants in SrTiO3. The measurements show that Ir3+:SrTiO3 forms a single midgap impurity state that is strongly localized, completely quenching the photoelectrochemical response. An anodic photoresponse was seen in Ir4+:SrTiO3 under visible-light illumination up to a wavelength of 600 nm (hν = 2.0 eV). Ir4+:SrTiO3 contains an in-gap state close to the top of the SrTiO3 valence band. The performance of Ir4+:SrTiO3 in electrochemical reactions is compared with cathodic Rh3+:SrTiO3, clearly illustrating the...

The nature of C-H···M (M = metal surface) interactions is reviewed based mainly on our recent investigations of cyclohexane on Rh(111). Infrared reflection-absorption spectroscopy measurements at low temperature (∼20 K) have shown that the softened CH stretching band consists of several sharp peaks. At temperatures above 80 K, each peak is broadened, most probably by anharmonic coupling with thermally excited low-energy frustrated translational modes. The origin of fine structure in this band and its similarity to that in hydrogen bond systems are discussed. In addition, novel isotope effects were observed in desorption kinetics and adsorption geometry of cyclohexane on Rh(111) using temperature programmed desorption, ultraviolet photoelectron spectroscopy, and spot profile analysis low-energy electron diffraction. The desorption energy of deuterated cyclohexane (C6 D12 ) is lower than that of C6 H12 (inverse kinetic isotope effect). In addition, the work function change by adsorbed C6 D12 is smaller than that by adsorbed C6 H12 . These results indicate that C6 D12 molecules are slightly more distant from the surface than C6 H12 molecules (vertical geometric isotope effect). A lateral geometric isotope effect was also observed for the two-dimensional cyclohexane superstructures as a result of the repulsive interaction between interfacial dipoles (= work function change). These isotope effects are ascribed to the quantum nature of hydrogen involved in the C-H···M interaction.

We have investigated the valence electronic states of α-sexithiophene (α-6T) on three passivated Si(0 0 1) surfaces, oxidized Si(0 0 1), water-adsorbed Si(0 0 1) and ethylene-adsorbed Si(0 0 1), using ultraviolet photoelectron spectroscopy (UPS). At a thickness of α-6T layer below 0.5 nm, clear features of the π states are observed for water-adsorbed Si(0 0 1) and ethylene-adsorbed Si(0 0 1), whereas broad features are observed for oxidized Si(0 0 1). This difference is attributed to the formation of well-ordered stacking on water-adsorbed Si(0 0 1) and ethylene-adsorbed Si(0 0 1).

The electronic states of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) molecules sandwiched between a pentacene (Pn) film and the ethylene-terminated Si(1 0 0) surface were investigated using X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS), and their effects on the morphology and the energy level alignment of the Pn film were investigated by atomic force microscopy (AFM) and polarization dependent ultraviolet photoelectron spectroscopy (UPS), respectively. The XPS and NEXAFS spectra show that F4-TCNQ molecules are involved in charge transfer with the surface, and they adsorb with tilt or random orientation. They consist of both neutral and anionic C ≡ N groups, and the neutral groups are still observed after Pn deposition, indicating the absence of charge transfer between F4-TCNQ and the Pn overlayer. We find that the Pn molecules take a standing-upright orientation on the ethylene/Si(1 0 0) surface. F4-TCNQ deposition on the ethylene/Si(1 0 0) surface induces some disorder in the Pn overlayer at the Pn thickness lower than 0.60 nm. F4-TCNQ deposition leads to an increase in the vacuum level by 1 eV, and thus a decrease in the hole injection barrier by 0.6–0.8 eV. Thus, the decrease is mainly attributed to the charge transfer between the F4-TCNQ and the ethylene/Si(1 0 0) surface.

To explore the chemical and electronic states of oxidized epitaxial graphene (EG) grown on the Si face of SiC(0001), we employ the Hummers oxidizing agents (H2SO4 + NaNO3 + KMnO4) under different reaction conditions that oxidize the graphene layer. The resulting material is characterized with scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). A mild “drop-cast” procedure at 60 °C is found to be equally effective at oxidizing EG as the conventional Hummers procedure. This aqueous-phase oxidation reaction appears to proceed in an autocatalytic manner as indicated by the concurrent observation of patches of oxidized and clean graphene areas in atomically resolved STM images on partially oxidized EG. STS further reveals substantial changes in electronic structure for oxidized EG including the opening of a local band gap of ∼0.4 eV. The oxidation is confined to the graphene layers as verified by XPS characterization of the...

We fabricated self-assembled monolayers (SAMs) composed of aromatic molecules with different anchor groups on Si(111) surfaces by wet chemical reactions. We investigated the bonding structures and transport properties by spectroscopic and electrical measurements, respectively. By using simple aromatic molecules (phenol, styrene, and phenylacetylene) as initial precursors, we successfully fabricated aromatic SAMs covalently bonded to Si(111) surfaces through different anchor structures (Si–O–, Si–CH2–CH2–, and Si–CH═CH−). Transmission infrared spectroscopy clarify that the phenyl rings in the SAMs are oriented almost perpendicular to the Si surfaces. High-resolution X-ray photoelectron spectroscopy reveals that the aromatic molecules attach to the Si surface with the surface coverage of ∼0.5. The experimental results of these spectroscopies lead to a conclusion that the aromatic SAMs form densely packed monolayers on Si(111). Current density–voltage measurements of Hg/aromatic SAM–Si(111) sandwiched struct...

We investigated the adsorbed states of CO on Pt(997) using high-resolution X-ray photoelectron spectroscopy (HR-XPS) with synchrotron radiation. Four kinds of adsorbed surfaces were prepared: (1) the saturation at 200 K, (2) the saturation at 310 K, (3) “on-top at terrace” and “on-top at step” CO species, and (4) only step-CO species. HR-XPS measurements of these surfaces were carried out at 90 K or 310 K. In the case of the saturation coverage at 310 K, three sharp peaks were clearly observed at binding energies of 285.91, 286.24, and 286.64 eV in the C 1s region. These peaks were assigned to the “bridge at terrace”, “on-top at step” CO and “on-top at terrace”, respectively. In addition, the Pt 4f 7/2 spectra were measured under the same conditions. In the case of the Pt(997) clean surface, three components, i.e., step, terrace and bulk Pt atoms, were identified at 70.6, 70.8, and 71.1 eV in the Pt 4f 7/2 spectrum, respectively. For the CO saturated Pt(997) surface at 200 K, three shifted surface components were newly observed at 71.5, 72.1 and 71.8 eV. They are related to the “bridge at terrace,” “on-top at terrace,” and “on-top at step” CO species, respectively. We discuss the charge transfer between CO and surface Pt atoms based on the chemical shifts of both C 1s of CO and Pt 4f 7/2 . The charge transfer occurs not only from the step Pt atom but also from surrounding terrace Pt atoms in the case of the “on-top at step” CO species.

Recent studies of core-level X-ray photoelectron spectroscopy (XPS) spectra of silicene on ZrB$_2$(0001) were found to be inconsistent with the density of states (DOS) of a planar-like structure that has been proposed as the ground state by density functional theory (DFT). To resolve the discrepancy, a reexamination of the XPS spectra and direct theoretical access of accurate single-particle excitation energies are desired. By analyzing the XPS data using symmetric Voigt functions, different binding energies and its sequence of Si $2p$ orbitals can be assigned from previously reported ones where asymmetric pseudo-Voigt functions are adopted. Theoretically, we have adopted an approach developed very recently, which follows the sophisticated $\Delta$ self-consistent field ($\Delta$SCF) methods, to study the single-particle excitation of core states. In the calculations, each single-particle energy and the renormalized core-hole charge density are calculated straightforwardly via two SCF calculations. By comparing the results, the theoretical core-level absolute binding energies including the splitting due to spin-orbit coupling are in good agreement with the observed high-resolution XPS spectra. The good agreement not only resolves the puzzling discrepancy between experiment and theory (DOS) but also advocates the success of DFT in describing many-body interactions of electrons at the surface., Comment: 8 pages, 3 figures, 3 tables

We investigated the adsorbed states of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) on Cu(100) using high-resolution electron energy loss spectroscopy (HREELS) and ultraviolet pho...

We investigated the chemical state and molecular orientation of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) adsorbed on a hydrogen-terminated Si(111) (1 × 1) surface using transmission infrared (IR) spectroscopy. We deposited F4-TCNQ molecules on H−Si(111) by a wet chemical method. Similar to evaporated F4-TCNQ molecules on various substrates in vacuum, we observed anionized F4-TCNQ on the H−Si(111) substrate. The incident angle dependence of the IR spectra reveals that this F4-TCNQ anion lies flatly on the surface. On the other hand, minority neutral F4-TCNQ species assume random orientation, judging from the comparison between s- and p-polarized IR spectra. We conclude that the first layer on the H−Si surface is a flat-lying anion species and the upper layers consist of randomly oriented neutral molecules.

We have investigated the valence electronic states of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F 4 -TCNQ) on the 2-methylpropene chemisorbed Si(1 0 0)(2 × 1) surface using valence photoelectron spectroscopy. Since the electron affinity of condensed F 4 -TCNQ is 5.24 eV and the energy from the valence band maximum of the 2-methylpropene saturated Si(1 0 0)(2 × 1) surface to the vacuum level is 4.1 eV, spontaneous charge transfer would be expected in the present system. At sub-monolayer coverage of F 4 -TCNQ, characteristic peaks are observed at 1.1 and 2.5 eV below Fermi energy. The former peak is assigned to a singly occupied affinity level, and the latter is ascribed to a relaxed highest occupied molecular orbital of adsorbed F 4 -TCNQ. The work function change is increased up to +1.3 eV as a function of F 4 -TCNQ coverage. These results support the occurrence of charge transfer into F 4 -TCNQ on the 2-methylpropene saturated Si(1 0 0)(2 × 1) surface.

The adsorption states and diffusion processes of NO molecules on Pt(997) at low coverage were investigated by using infrared reflection absorption spectroscopy (IRAS), scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. When NO molecules adsorb on the surface, each molecule transiently migrates on the surface from the first impact point to a possible adsorption site at low temperature (11 K). There are four stable adsorption sites for NO on Pt(997); the bridge site of the upper step, the fcc and hcp-hollow sites of the terrace, the on-top site of the terrace and the fcc-hollow site of the lower step. At higher temperature above 45 K, NO molecules start to diffuse thermally to more stable adsorption sites, and finally they are trapped at the bridge sites of the step which are the most stable adsorption sites among the four sites.

The adsorption states of 1,4-cyclohexadiene on Si(100) were studied with low-temperature scanning tunneling microscopy (STM) and ultraviolet photoelectron spectroscopy (UPS). We newly observed two kinds of molecular images by STM, which clearly differed from the previous STM results obtained at room temperature. Molecular protrusion was observed between the down Si dimer atoms of the neighboring dimer rows on Si(100)c(4×2) at 80 K. After heating the substrate, this species converted to a new species that was located on the Si dimer. The UPS results indicate that adsorbed 1,4-cyclohexadiene has two π bonds at 100 K. After annealing at 140 K, one of the two π bonds reacts with the dimer. Thus, we conclude that 1,4-cyclohexadiene is adsorbed on Si(100)c(4×2) as a π-complex species at ∼100 K and that the π-complex species is transformed to a di-σ species after the substrate is heated beyond 140 K. The present results agree with a recent report by Kato et al. [J. Phys. Chem. C 2007, 111, 2557], and we must rev...

Valence and conduction states near the SiO2/Si(100) interface were directly observed using soft x-ray absorption and emission spectroscopy. For the O K-edge absorption spectra, the step-like structures were observed at 531, 533 and 534.5 eV. These step-like structures observed at 531, 533 and 534.5 eV were assigned to an oxygen atom bonding to Si1+, Si2+, and Si3+ of the interface, respectively. In the case of O K-edge emission spectra, with decreasing incident photon energy from 535 to 531 eV so as to shift the conduction band minimum of the interface towards Fermi energy, the corresponding valence band maximum was shifted to Fermi energy direction. Thus, the local band gap became narrower with the decrease of the oxidation number of the suboxide species. Further the interface structure was also discussed from the spectrum features. [DOI: 10.1380/ejssnt.2008.209]

We fabricated iron and iron silicide microstructures on an Si(1 0 0) clean surface via electron beam induced process of Fe(CO)5 multilayer and subsequent annealing. The fabricated microstructures were in situ analyzed by Auger electron spectroscopy (AES) and scanning electron microscopy (SEM). We successfully analyzed the coverage and chemical states of the artificial deposited iron structure area-selectively by AES. The artificial iron structure was fabricated after heating to above 350 K to desorb residual Fe(CO)5 species. The artificial structure was observed even after 1190 K annealing by SEM, but AES measurements showed it to be covered by Si atoms. We concluded that the buried iron silicide microstructure was formed by the present process.

The adsorption states and dissociation process of oxygen on Cu(100) at 40 K were investigated using high-resolution electron energy loss spectroscopy. The dissociation of O2 occurs on the bare Cu(100) surface via a mobile precursor at 40 K. This precursor was not observed as a stable intermediate. The chemisorption state of O2 is a single peroxo species with the O−O stretching energy at 631 cm-1. This peroxo species is stabilized by the interaction with the dissociated oxygen species, and the dissociation barrier from the chemisorbed O2 depends on the coverage of atomic oxygen.

The mechanism of cycloaddition reaction between ethylene and the Si dimer on Si (100) is discussed from our recent experimental results using high-resolution electron energy loss spectroscopy and valence photoelectron spectroscopy at ~50 K. The weakly adsorbed π-complex state is identified as a precursor for di-σ bond formation and the activation barrier is roughly estimated to be ~0.2 eV. A different reaction route may exist since a di-σ species is also observed at low temperature. Using cycloaddition reaction, unsaturated organic molecules are chemically attached to the dimer on Si (100), and the adsorbed species are relatively stable. Thus, scanning tunneling spectroscopy (STS) measurements of a single adsorbed molecule can be carried out at room temperature. We report the STS results of 2-butyne adsorbed on Si (100).

To elucidate the thermal chemical processes of 1,4-cyclohexadiene (C6H8) on Si(001), the adsorption states were characterized by temperature-programmed desorption (TPD), low-energy electron diffrac...

The interaction of water with solid surfaces plays a crucial role in many areas, e.g., heterogeneous catalysis, electrochemistry, solar energy conversion, and corrosion chemistry. In order to understand microscopic mechanisms in these phenomena, it is important to elucidate how water molecules adsorb, behave, and arrange on metal surfaces. So far, there are many reports concerning the first layer of water on transition metal surfaces. Recently, more detailed microscopic pictures have been obtained from both experiments using synchrotron radiation, scanning tunneling microscopy, etc., and first principles calculations. In this paper, we review the studies about the first layer of water on transition metal surfaces, including our recent study of water on Rh(111).

Quantitative analysis of desorption and decomposition kinetics of formic acid (HCOOH) on Cu(111) was performed by temperature programmed desorption (TPD), X-ray photoelectron spectroscopy, and time-resolved infrared reflection absorption spectroscopy. The activation energy for desorption is estimated to be 53-75 kJ/mol by the threshold TPD method as a function of coverage. Vibrational spectra of the first layer HCOOH at 155.3 K show that adsorbed molecules form a polymeric structure via the hydrogen bonding network. Adsorbed HCOOH molecules are dissociated gradually into monodentate formate species. The activation energy for the dissociation into monodentate formate species is estimated to be 65.0 kJ/mol at a submonolayer coverage (0.26 molecules/surface Cu atom). The hydrogen bonding between adsorbed HCOOH species plays an important role in the stabilization of HCOOH on Cu(111). The monodentate formate species are stabilized at higher coverages, because of the lack of vacant sites for the bidentate formation.

We observed four kinds of adsorbed NO molecules on Pt(9 9 7) at 11 K using infrared reflection absorption spectroscopy (IRAS). The peaks at 1690, 1484 and 1615 cm−1 are assigned to the N–O stretching modes of the on-top site and the hollow site on the terrace and the bridge site at the step, respectively. The 1385 cm−1 peak is observed below ∼70 K. We assign the 1385 cm−1 peak to the hollow site of the (1 1 1) microfacet at the step or the lower-terrace hollow site nearest to the step. By heating, site-to-site hopping to the more stable site occurs and the relative stability of four adsorption sites can be determined.

Using infrared reflection absorption spectroscopy (IRAS) and scanning tunneling microscopy (STM), we investigated the adsorption states of NO on the Pt(9 9 7) step surface. At 90 K, we observe three N–O stretching modes at 1490 cm −1 , 1631 cm −1 and 1700 cm −1 at 0.2 ML. The 1490 cm −1 and 1700 cm −1 peaks are assigned to NO molecules at fcc-hollow and on-top sites of the terrace, respectively. The 1631 cm −1 peak is assigned to the step NO species. In the present STM results, we observed that NO molecules were adsorbed at the bridge sites of the step as well as fcc-hollow and on-top sites of the terrace. To help with our assignments, density functional theory calculations were also performed. The calculated results indicate that a bridge site of the step is the most stable adsorption site for NO, and its stretching frequency is 1607 cm −1 . The interactions between NO species at different sites on Pt(9 9 7) are also discussed.

For today's silicon based devices, understanding the SiO2/Si(111) interface on atomic level is an important subject for fabricating superior devices. However, despite of many studies on the SiO2/Si(111) interface, the interfacial valence electronic states have been typically evaluated as the average and not as individual states. In the present study, we successfully observed valence electronic states of particular atoms at the SiO2/Si(111) interface for the first time using soft x-ray absorption and emission spectroscopy. In addition, comparing the experimental results to first-principles calculations revealed local interfacial properties. [DOI: 10.1380/ejssnt.2006.280]

The chemical reactivity of the buckled dimer on Si(1 0 0) is reviewed from the viewpoint of physical properties of the substrate and the molecules, which have been obtained by several experimental techniques including valence and core-level photoelectron spectroscopy, high-resolution electron energy loss spectroscopy, scanning tunneling microscopy as well as theoretical calculations. The up dimer atom ( S u ) of the buckled dimer on Si(1 0 0) is electron-rich and the down dimer atom ( S d ) is electron-deficient. Thus, S u is nucleophilic and S d is electrophilic, and one can expect that the S u acts as a Lewis base, and the S d acts as a Lewis acid. This is a simple guiding principle and works well to understand and predict Si surface chemistry. Typical adsorption systems including trimethylamine, ammonia, water, BF 3 , ethylene and vinylbromide on Si(1 0 0) c (4 × 2) are discussed mainly based on our recent experimental investigation at the atomic-scale.

We investigated Si 2p spectra for the Si(1 0 0)c(4 × 2) surfaces exposed to CO and CO 2 to clarify how the adsorbed molecules affect the clean surface spectra in ultra-high vacuum (UHV). After a small amount of CO exposure (0.05 L), the spectral shape looks sharper as compared with that of the clean surface due to the reduced intensity of the down dimer peak, because CO is preferentially adsorbed on the down dimer site. Thus the adsorption of CO from the residual gas in UHV might induce a slight change in the spectral shape within a measurement period (∼10 3 s). On the other hand, in the case of adsorbed CO 2 , the spectral shape does not change but the overall intensity decreases, since CO 2 is physisorbed on Si(1 00).

The atomic configuration of C 6 H 8 molecules chemisorbed on Si(0 0 1) surface is elucidated by the first-principles molecular dynamics method. The characteristic pattern in the STM image at ∼0.6 ML coverage is found to be reproduced by considerable thermal fluctuation of the molecules including the change in the molecular conformation. The reaction path and growth mechanism of the chemisorption region is also discussed based on static calculation of the activation barrier.

We investigated the adsorption states and the reaction of vinyl bromide (CH2=CHBr) on Si(100)c(4×2), using high-resolution electron energy loss spectroscopy. We found that the chemisorption of this molecule occurs via a precursor state and a vinyl bromide molecule is di-σ bonded to the Si dimer on Si(100)c(4×2) in the chemisorption state. The precursor species are observed in the temperature range from 58 to 90 K. Judging from the vibrational spectra, we conclude that the structure of the precursor state is a three-atom intermediate state (π complex). The activation energy and the preexponential factor of the reaction from the precursor state to the chemisorption state are estimated to be 27.3 kJ/mol (283 meV) and 1.5 × 1013 s-1, respectively. The activation energy decreases with increasing the vinyl bromide coverage owing to the steric repulsion.

Selective functionalization by switching the adsorption linkage of a bi-functional organic molecule, 1-dimethylamino-2-propyne (DMAP), on Si(1 0 0) has been studied using high-resolution electron energy loss spectroscopy (HREELS) and scanning tunneling microscopy (STM). The HREELS and STM results clearly indicate that DMAP adsorbs at the down dimer atom of Si(1 0 0)c(4 × 2) through an N → Si dative bond leaving the –CCH unit of DMAP free at low temperature (65–90 K). The adsorption linkage is switching from the N → Si dative bond to the Si–C di-σ bonds by annealing the adsorbed surface to 300 K, and thus the tertiary amino group becomes free.

We investigated low-energy electron-stimulated chemical reactions between CO and water molecules in low-temperature ice using infrared reflection absorption spectroscopy. Carbon dioxide, the formyl radical, formaldehyde, and methanol were produced by electron irradiation of the water/CO/water layered ice. The electron energy threshold and temperature dependence for the chemical reactions were investigated to elucidate the reaction mechanisms.

Adsorption of trimethylamine (TMA) on Si(100)c(4×2) has been studied using scanning tunneling microscopy (STM), valence photoelectron spectroscopy (PES), and high-resolution electron energy spectroscopy (HREELS) between 80 and 300 K. The TMA molecule is adsorbed nondissociatively only on the down dimer atom and appears as a triangle shaped bright protrusion in the occupied state STM image. The saturation coverage of TMA on Si(100) is estimated to be ∼0.25 ML (ML = one molecule per surface Si atom). The PES and HREELS results suggest that TMA is adsorbed through the N lone pair by forming an Si−N dative bond. The HREELS spectra indicate that the symmetry of adsorbed TMA is Cs and the molecular C3 axis is tilted from the surface normal. The stretching mode of the SiN dative bond is observed at 525 cm-1. Thus the combined STM, PES, and HREELS results indicate that the chemisorption of TMA on Si(100)c(4×2) occurs selectively on the down dimer atom via dative bonding. Hence, the TMA adsorption on Si(100)c(4×2)...

We have investigated the C 1s photoelectron spectra of ethylene on the Si(1 0 0)(2 × 1) surface, using high resolution photoelectron spectroscopy. The vibrational structure has been observed in the C 1s spectra where the C–H stretching mode is dominant. The vibrational splittings of C 1s ionized state are very similar to those in the ground state. It is found that linear coupling model is applicable in this system and this excitation process is a Frank–Condon process. The curvature of potential energy surface of C–H and C–D bond is almost unchanged upon core ionization.

We have investigated the electronic states of clean and BF 3 adsorbed Si(100) surfaces at low temperature by means of high resolution Si 2p photoelectron spectroscopy. The peak intensities of upper atom and lower atom of the asymmetric dimer in Si 2p spectra do not change even at 30 K compared with those at higher temperature up to 300 K, indicating that the dimer is asymmetric in the ground state. In order to investigate chemical reactivity of asymmetric dimer on Si(100), a typical Lewis acid molecule BF 3 is adsorbed on Si(100). We have found that BF 3 molecules are dissociated into BF 2 and F on Si(100) and dissociated species (BF 2 and F) are adsorbed predominantly on the up dimer atoms of the asymmetric dimers.

The adsorbed states of 1,4-cyclohexadiene molecules on Si(1 0 0)(2 × 1) at 300 K have been investigated as a function of coverage in real space by means of scanning tunneling microscopy (STM). Two kinds of intermolecular interaction have been studied (i) between the neighboring molecules on the neighboring dimer rows and (ii) between the neighboring molecules on the same dimer row. By analyzing the observed arrangements of adsorbed molecules, we have concluded that the intermolecular interaction between the molecules on the neighboring dimer rows is very small. On the other hand, the intermolecular interaction between the neighboring molecules on the same dimer row becomes repulsive when the distance between two adsorbates is shorter than 2a (a=3.85 A: the distance between the Si dimers on the same row). Various types of arrangements are discussed, based on the steric repulsion. The proposed models can explain the alternate height variations in consecutive molecular protrusions in STM images and the saturation coverage of 0.6 molecules per Si dimer at 300 K.

The adsorbed states of 1,1,1-trifluoro-2-propanol (TFIP) molecule on Si(1 0 0) c(4 × 2) have been investigated by high resolution electron energy loss spectroscopy. TFIP is dissociatively adsorbed on Si(1 0 0) as CF 3 CH(O–Si)CH 3 and H–Si at 100 K. These species are stable up to 300 K. At 400 K, CF bonds start to be dissociated. At 800 K, all adsorbed species are decomposed into SiH, SiF, SiF 2 and residual carbon species. At 1050 K, the silicon carbide is formed.

We used the energy-selected-photoelectron photoion coincidence (ESPEPICO) method to study site-specific fragmentation caused by C:1s photoionization of 1,1,1-trifluoro-2-propanol-d1 [CF3CD(OH)CH3, TFIP-d1] on a Si(100) surface. High-resolution electron energy loss spectroscopy showed that TFIP-d1 is dissociatively chemisorbed like (CF3)(CH3)CDO–Si(100), and different chemical shifts at the three carbon sites were observed by photoelectron spectroscopy. The site-specific fragmentation evident in the ESPEPICO spectra of the sub-monolayer at room temperature indicates that the TFIP-d1 there has an O–Si bond oriented in the trans position with respect to the C–CF3 bond. Here we discuss the fragmentation processes in light of the results obtained with the ESPEPICO method and the Auger-electron photoion coincidence method.

In order to elucidate the dynamics of coalescence of La 0.6 Sr 0.4 MnO 3 nanostructures on SrTiO 3 (1 0 0), we have investigated the relationship between the electronic states of La 0.6 Sr 0.4 MnO 3 and the surface morphology of nanostructures, by using photoelectron spectroscopy and local probe microscopy. When the La 0.6 Sr 0.4 MnO 3 nanodots are heated from 700 to 800 °C, the average nanodot size increases, and the core level peaks of Mn 2p and La 3d do not change. In addition, the observed nanodot height is always 0.4 nm. Therefore, when the nanodots coalesce, they are laterally grown, maintaining the perovskite manganese structure. On the other hand, when nanowire structures are formed along the steps by heating the sample above 1000 °C, the Mn 2p peaks vanish, although the La 3d spectrum does not change. Therefore, upon this process, Mn atoms of the island dissolve into the bulk of SrTiO 3 (1 0 0), and consequently, the remaining species migrate to steps on the surface, resulting in the formation of step-decorated nanowire perovskite structures.

The interface bonding between adsorbates (ethylene and benzene) and the Si(1 0 0)c(4×2) surface has been investigated by means of angle-dependent Si 2p high resolution photoelectron spectroscopy at 140 K, and several new quantitative properties have been obtained. Upon ethylene adsorption, a drastic decrease of up dimer and down dimer atom components (95%), and the appearance of a new component has been observed. This new component is assigned to the outer most Si atoms which are bonded to ethylene. The originally asymmetric Si dimer atoms become chemically equivalent upon adsorption. In the case of benzene, the decrease of up dimer and down dimer components is about 49% at the saturation coverage, and the unreacted dimers remain asymmetric. The charge transfer between adsorbed molecules and the Si(1 0 0)(2×1) surface has been estimated from the core-level shift and the nature of interface bonding including surface and second layer Si atoms is discussed.