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We report the formation processes of the self-assembled layer of 1,3,5-tris(4-bromophenyl)benzene (TBB) molecules on the Ag(111) and Si(111) (√3 × √3)-Ag surfaces by STM measurements and density functional theory (DFT) calculations. The self-assembled layers on the surfaces show characteristic structures controlled by the interplay between the intermolecular interaction and the molecule-substrate interaction. Through the cooperative interplay between the molecule-substrate interaction and the intermolecular halogen bond (XB), the periodic arrangement of TBB molecules appears on the Ag(111) surface. On the other hand, the two types of TBB arrangement appear on the Si(111) (√3 × √3)-Ag surface (phases 1 and 2). Phase 1 is the periodic arrangement of the TBB molecules and is derived from the cooperative interplay between the molecule-substrate interaction and the intermolecular van der Waals (vdW) interaction and the hydrogen bond (HB), and phase 2 is a random arrangement and is derived from the competitive interplay between the molecule-substrate interaction and the intermolecular XB and HB. Our present study specifies the role of the substrate in the molecular self-assembly of the substrate. Although the structure of the molecular self-assembly is controlled by the choice of the substrate, the cooperative interplay between the molecule-substrate interaction and the intermolecular interaction is necessary to realize the ideal periodic arrangement.

The electromagnetic response of Dirac semimetals in the infrared and terahertz frequency ranges is attracting growing interest for potential applications in optoelectronics and nonlinear optics. The interplay between the free-carrier response and interband transitions in the gapless, linear dispersion relation plays a key role in enabling novel functionalities. Here we investigate ultrafast dynamics in thin films of a photoexcited Dirac semimetal Cd$_3$As$_2$ by probing the broadband response functions as complex quantities in the multiterahertz region (10-45 THz, 40-180 meV, or 7-30 $\mu$m), which covers the crossover between the inter and intraband response. We resolve dynamics of the photoexcited nonthermal electrons which merge with originally existing carriers to form a single thermalized electron gas and how it is facilitated by high-density excitation. We also demonstrate that a large reduction of the refractive index by 80% dominates the nonequilibrium infrared response, which can be utilized for designing ultrafast switches in active optoelectronics., Comment: 20 pages

GaN is an excellent candidate for photocatalytic, optoelectronic, and high-power devices, and the interaction between the GaN surface and ambient species, especially H2O and O2, has drawn exception...

The interactions of CO2 with terrace, step, and defect or kink sites on Pt surfaces were investigated using temperature-programmed desorption, X-ray photoelectron spectroscopy, and density function...

Hydrogen spillover is a crucial process in the selective hydrogenation reactions on Pd/Cu single atom alloy catalysts. In this study, we report the atomic-scale perspective of these processes on the single atom alloy catalyst Pd/Cu(111) based on the experimental and theoretical results, including infrared reflection absorption spectroscopy (IRAS), temperature programmed desorption (TPD), high-resolution X-ray photoelectron spectroscopy (HR-XPS), and density functional theory (DFT) calculations for core-level excitation. The hydrogen spillover onto Cu(111) was successfully observed in real time using time-resolved IRAS measurements at 80 K. The chemical shifts of Pd 3d

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.

To improve the performance of semiconductor photoelectrodes for water splitting, the amount of band bending in the depletion layer of a semiconductor should be accurately ascertained, since it dete...

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...

Photocatalytic water splitting takes place at the semiconductor/electrolyte interface. Although the reactions are strongly affected by the subtle changes in the interface structure, little is known...

We have investigated the adsorption and thermal reaction processes of NO with silicene spontaneously formed on the ZrB

We investigated ultrafast broadband (12-45 THz) response functions of a photoexcited Dirac semimetal Cd3As2. Under periodic driving at 30 THz, we observed coherent nonlinear response dominated by stimulated Rayleigh scattering between the photon-dressed states.

Edge engineering is important for both fundamental research and applications as the device size decreases to nanometer scale. This is especially the case for graphene because a graphene edge shows totally different electronic properties depending on the atomic structure and the termination. It has recently been shown that an atomically precise zigzag edge can be obtained by etching graphene and graphite using hydrogen (H) plasma. However, edge termination had not been studied directly. In this study, termination of edges created by H-plasma is studied by high-resolution electron energy loss spectroscopy (HREELS) to show that the edge is $\mathrm{sp}^{2}$ bonded and the edge carbon atom is terminated by only one H atom. This suggests that an ideal zigzag edge, which is not only atomically precise but also $\mathrm{sp}^{2}$ bonding, can be obtained by H-plasma etching. Etching of the graphite surface with plasma of a different isotope, deuterium (D), is also studied by scanning tunneling microscopy (STM) to show that D-plasma anisotropically etches graphite less efficiently, although it can make defects more efficiently, than H-plasma., Comment: 8 pages, 5 figures

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 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.

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...

Electronic states and growth modes of the Zn-deposited Cu(111) surface at 300 K were quantitatively studied using core-level and valence photoelectron spectroscopies. Both Cu 2p and Zn 2p core-levels shifted to higher binding energy with increasing the amount of deposited Zn up to multilayer. The origin of the core-level shift of Cu 2p was further investigated by density functional theory calculations; the shift of the Cu 2p peak results from the change in the effective electrostatic potential (initial state effect) caused by the formation of Zn-Cu surface alloy, and the increase of coordination numbers of surface Cu atoms by Zn overlayer. The observed valence photoelectron spectra show the formation of the two atomic-layer Zn-Cu alloy up to the Zn coverage of 1 ML, followed by the formation of three-dimensional Zn islands on the alloyed surface at 300 K.

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.

Production of chemical fuels by direct solar energy conversion in a photoelectrochemical cell is of great practical interest for developing a sustainable energy system. Various nanoscale designs such as nanowires, nanotubes, heterostructures and nanocomposites have been explored to increase the energy conversion efficiency of photoelectrochemical water splitting. Here we demonstrate a self-organized nanocomposite material concept for enhancing the efficiency of photocarrier separation and electrochemical energy conversion. Mechanically robust photoelectrodes are formed by embedding self-assembled metal nanopillars in a semiconductor thin film, forming tubular Schottky junctions around each pillar. The photocarrier transport efficiency is strongly enhanced in the Schottky space charge regions while the pillars provide an efficient charge extraction path. Ir-doped SrTiO3 with embedded iridium metal nanopillars shows good operational stability in a water oxidation reaction and achieves over 80% utilization of photogenerated carriers under visible light in the 400- to 600-nm wavelength range., Nanoscale designs are known to increase the energy conversion efficiency of photoelectrochemical water splitting. Here, the authors report a self-organized nanocomposite formed by embedding self-assembled metal nanopillars in a semiconductor thin film, for enhanced photocarrier separation efficiency.

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 ...

The structure and electronic structure of a system comprising molecular layers of n-alkane adsorbed on Au(1 1 1) were investigated to understand the origin of the interfacial electric dipole layer. These alkane molecules are inert and thus van der Waals (vdW) interaction is the primary adsorptive force. Although these vdW interfaces are regarded as ideal physical adsorption systems unlikely to chemically interact with the metal surface, a large interfacial electric dipole layer over 1 eV is formed. It is important to elucidate the mechanism underlying the generation of such large dipole layers to understand the electrode interface in organic semiconductor devices. Thus, this paper elucidates the details of the vdW interface, in particular between tetratetracontane (TTC) and Au(1 1 1). First, the mechanism of growth of the TTC layer up to monolayer thickness was revealed using a low-energy electron-diffraction method. Second, the interface states were newly formed, as determined using angle-resolved photoelectron spectroscopy, X-ray photoemission spectroscopy, and near-edge absorption fine-structure spectroscopy. Our findings indicate, unexpectedly, that a relatively strong inter-orbital interaction governs this vdW interface, similar to a weak chemisorption system rather than a physisorption system.