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Hydrogen is chemically flexible revealing three aspects; protium (H $$^0$$ ), proton (H $$^+$$ ), and hydride (H $$^-$$ ), which are active toward hydrogenation reactions. While the reactivity of protium is characterized by the potential energy at a catalyst surface, electron transfer is of importance as an additional degree of freedom in the reactions with proton and hydride. A promising strategy is to make full use of these active hydrogen species for hydrogenation of stable molecules and achieve chemical transformation of these molecules, which we call “Hydrogenomics”. By reviewing recent studies on the hydrogenation of hydrocarbon, carbon dioxide, dinitrogen, and so on using these active hydrogen species on model metal surfaces, nanoscale electrocatalysts, and multimetallic polyhydride clusters, we discuss the fundamental concepts behind the reactions and possible control of the efficient and selective hydrogenation reactions.

The adsorption and interaction of methane on Pd(110) were investigated by vibrational spectroscopy, synchrotron-based X-ray spectroscopy, and first-principles density functional theory (DFT) calcul...

We investigated mass transports during hydrogen adsorption and absorption processes of PdCu alloys that has advantages of higher hydrogen diffusivity and economically lower-cost than the other Pd-alloys. The research was made with a comparison of the well-known ordered phase of the bcc structure (the B2 phase) and a mixed phase of the fcc and B2 structures. In-situ ultrahigh vacuum X-ray photoelectron spectroscopy (UHV-XPS) and ambient pressure X-ray photoelectron spectroscopy (AP-XPS) using synchrotron radiation were carried out to trace the chemical states of the Pd and Cu atoms as a function of temperatures. It is newly elucidated that the initial adsorption and absorption processes were similar in the two phases, but a hydrogen diffusion rate to the bulk was apparently higher in the ordered phase than in the mixed one. We found the dynamics of the Pd and Cu atoms during the hydrogen adsorption/absorption processes largely depends on temperature. In the hydrogen atmosphere, the Pd atoms segregate at the surface below 373 K and Cu atoms segregate at the surface above 373 K, showing a reversal behavior. The present results agree well with the previous theoretical calculations and, thus, provide appropriate inputs toward developments of the industrial hydrogen permeation materials.

The reaction of carbon dioxide (CO2) on Zn-deposited copper surfaces was systematically investigated by ambient-pressure X-ray photoelectron spectroscopy (AP-XPS). In the presence of 0.8 mbar CO2 a...

We investigate the polymeric adsorption of formic acid (HCOOH) on the Cu(111) surface by means of density functional theory. We present structural models for the polymeric form of HCOOH on Cu(111) and characterize their stability, electronic, and vibrational properties. Based on the energetics and dynamics, as well as simulated scanning tunneling microscopy and atomic force microscopy images, we propose that the $\ensuremath{\alpha}$ polymorph is likely to be formed on the Cu(111) surface and can explain the experimental findings. We also study the initial step of the catalytic dehydrogenation of HCOOH and find that the O-H bond dissociation at the edge of the polymer is facilitated, rather than that forming hydrogen bonding, agreeing well with the experiments.

Ge atoms segregating on zirconium diboride thin films grown on Ge(111) were found to crystallize into a two-dimensional bitriangular structure, which was recently predicted to be a flat band material through an embedded kagome lattice. Angle-resolved photoelectron emission experiments together with theoretical calculations pointed out the existence of a nearly flat band in spite of non-negligible in-plane long-range hopping and interactions with the substrate. This provides an experimental verification for the fact that a flat band can emerge from the electronic coupling between atoms and not from the geometry of the atomic structure.

The ceria-based catalyst incorporated with Cr and a trace amount of Rh (Cr0.19Rh0.06CeOz) was prepared and the reversible redox performances and oxidation catalysis of CO and alcohol derivatives with O2 at low temperatures (

In single-atom catalysis, the atomic structure and local electronic states of single atoms on a supporting material remain a fundamental question. We experimentally and theoretically solved these problems for single Pt atoms dispersed on freestanding graphene using plasma sputtering. Electron microscopy revealed the atomic arrangements of Pt atoms binding to carbon atoms at the step edges of nanographene flakes. X-ray photoelectron spectroscopy revealed a large binding energy shift of the Pt 4f state of a single Pt atom. First-principles calculations elucidated that the Pt 5dxy-orbital in the step edge plays a crucial role in the formation of chemical bonds to C atoms and in the considerable charge transfer from Pt to C atoms, resulting in the large binding energy shift of the Pt 4f state.

Complete H–D exchange of butene was reported on a D-absorbed Pd–Au alloy surface. However, its reaction mechanism has not been clarified yet. Here we investigate the reaction of cis-2- and 1-butene on Pd70Au30(110) by means of high-resolution electron energy loss spectroscopy. We demonstrate that both butene isomers are in the π-bonded state and that 1-butene irreversibly converts to cis-2-butene around 250 K. This isomerization accounts for previously observed similar product distributions in the H–D exchange reaction of cis-2- and 1-butene on the D-absorbed Pd70Au30(110) surface. Since the reverse change from cis-2- to 1-butene is not observed at any temperature, we conclude that the H–D exchange proceeds by the dissociative mechanism via vinylic and π-allylic intermediates rather than the Horiuti–Polanyi mechanism.

The adsorption and reaction of methanol on Zn-modified Cu(111) surfaces were investigated by synchrotron-radiation X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD). Dehydrogenation of methanol is not observed on clean Cu(111) or clean Zn–Cu(111) surfaces. The amount of more stable methanol molecules at defect sites is increased on a Zn–Cu(111) surface because surface roughening occurs as a result of surface-alloy formation, which leads to an increase in step density. All of the methanol is desorbed intact by heating to ∼220 K. On the other hand, when a Zn–Cu(111) surface is preoxidized by oxygen, methanol is partially dehydrogenated to methoxy species at 130–160 K. The reactivity of methoxy depends significantly on the adsorption sites. In particular, some reactive methoxy species are found, which are dehydrogenated to formaldehyde at 220–290 K, on a preoxidized Zn–Cu(111) surface. Methoxy species on the surface of a zinc oxide island are dehydrogenated to formaldehyde at...

The vibrational modes of chemisorbed CO on a Si(001) surface are investigated by means of transmission Fourier-transform infrared absorption spectroscopy. We observed the three components corresponding to the stretching vibration of the terminal-site CO adsorbed on the down-dimer sites of a Si(001) surface. The symmetric stretching vibration and asymmetric stretching vibration are observed separately. This assignment is consistent with the polarization dependence of the incident light and with the vibrational modes obtained by performing theoretical calculations. We found that both the intradimer row and interdimer row coupling should be considered to explain the appearance of the three components.

Formic acid (HCOOH) can be catalytically decomposed into H2 and CO2 and is a promising hydrogen storage material. As H2 production catalysts, Cu surfaces allow selective HCOOH decarboxylation; however, the on-surface HCOOH decomposition reaction pathway remains controversial. In this study, the temperature dependence of the HCOOH/Cu(111) adsorption structures is elucidated by scanning tunneling microscopy and non-contact atomic force microscopy, establishing the adsorbate chemical species using density functional theory. 2D HCOOH islands at 80 K, linear chains of HCOOH and monodentate formate at 150 K, chain-like assemblies of monodentate and bidentate formate at 200 K, and bidentate formate clusters at 300 K are observed. At each temperature, the adsorbates experience attractive interactions among themselves. Such aggregation stabilizes them against desorption and decomposition. Thus, accurate evaluation of intermolecular interactions is essential to understand catalytic reactivity.

The adsorption of CO2 molecules on monolayer epitaxial graphene on a SiC(0001) surface at 30 K was investigated by temperature-programmed desorption and X-ray photoelectron spectroscopy. The desorption energy of CO2 on graphene was determined to be (30.1–25.1) ± 1.5 kJ/mol at low coverages and approached the sublimation energy of dry ice (27–25 kJ/mol) with increasing the coverage. The adsorption of CO2 on graphene was thus categorized into physisorption, which was further supported by the binding energies of CO2 in core-level spectra. The adsorption states of CO2 on graphene were theoretically examined by means of the van der Waals density functional (vdW-DF) method that includes nonlocal correlation. The experimental desorption energy was successfully reproduced with high accuracy using vdW-DF calculations; the optB86b-vdW functional was found to be most appropriate to reproduce the desorption energy in the present system.

The chemistry of simple alkene molecules on the Si(100) surface is reviewed with the newly-produced visual presentation by theoretical calculations. The early pioneering studies by the Kyoto Group and Pittsburgh group reported the di–σ bond formation and the precursor-mediated chemisorption for acetylene and ethylene on Si(100), respectively. Thereafter, these studies have been stimulating various studies of organic molecules on Si surfaces. Our recent studies have observed the precursor states for alkene chemisorption and elucidated the microscopic mechanisms of the di–σ bond formation (cycloaddition) with the help of theoretical calculations; the site-, stereo- and regio-selective chemisorption of simple alkene molecules on Si(100)c(4 × 2) has been established.

The Pd-catalyzed hydrogenation of C═C double bonds is one of the most important synthetic routes in organic chemistry. This catalytic surface reaction is known to require hydrogen in the interior of the Pd catalyst, but the mechanistic role of the Pd-dissolved H has remained elusive. To shed new light into this fundamental problem, we visualized the H distribution near a Pd single crystal surface charged with absorbed hydrogen during a typical catalytic conversion of butene (C4H8) to butane (C4H10), using H depth profiling via nuclear reaction analysis. This has revealed that the catalytic butene hydrogenation (1) occurs between 160 and 250 K on a H-saturated Pd surface, (2) is triggered by the emergence of Pd bulk-dissolved hydrogen onto this surface, but (3) does not necessarily require large stationary H concentrations in subsurface sites. Even deeply bulk-absorbed hydrogen proves to be reactive, suggesting that Pd-dissolved hydrogen chiefly acts by directly providing reactive H species to the surface ...

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.

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.

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.

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.

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.

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...

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)...