Interaction of HNCO with Au(111) surfaces

The surface chemistry of isocyanic acid, HNCO, and its dissociation product, NCO, was studied on clean, O-dosed and Ar ion bombarded Au(111) surfaces. The techniques used are high resolution energy loss spectroscopy (HREELS) and temperature-programmed desorption (TPD). The structure of Ar ion etched surface is explored by scanning tunneling microscopy (STM). HNCO adsorbs molecularly on Au(111) surface at 100 K yielding strong losses at 1390, 2270 and 3230 cm^? 1. The weakly adsorbed HNCO desorbs in two peaks characterized by T_p = 130 and 145 K. The dissociation of the chemisorbed HNCO occurs at 150 K to give NCO species characterized by a vibration at 2185 cm^? 1. The dissociation process is facilitated by the presence of preadsorbed O and by defect sites on Au(111) produced by Ar ion bombardment. In the latter case the loss feature of NCO appeared at 2130 cm^? 1. Isocyanate on Au(111) surface was found to be more stable than on the single crystal surfaces of Pt-group metals. Results are compared with those obtained on supported Au catalysts.     

Interfacial structure of Co porphyrins on Au(111) electrode: Interaction of porphyrin molecules with substrate

Interfacial structures of cobalt(II) porphine (CoP) and [2,3,7,8,12,13,17,18‐octaethyl‐21H,23-H-porphine]cobalt(II) (CoOEP) have been studied on Au(111) electrode using electrochemical scanning tunneling microscopy (EC-STM), in-situ X-ray diffraction, and density functional theory (DFT) calculations. The adsorption of porphyrins affects the reconstruction of Au(111) surface. The adsorption of CoP causes a lifting of the reconstruction to a complete 1 × 1 structure of Au(111). On CoOEP modified Au(111), the unit cell periodicity of the reconstructed substrate structure expands compared with the √3 × 23 structure of bare Au(111). The same expanded substrate structure was observed on Au(111) modified with OEP without the coordinated Co ion; the coordinated metal ion of the adsorbed porphyrin molecule does not affect the substrate structure. This result indicates that the interaction of conjugated π electrons of porphyrin with the substrate is stronger than that of the coordinated Co ion. In-situ X-ray diffraction and DFT calculation support non-covalent interaction of porphyrins with the Au(111) surface.     

Effects of Immersion Temperature on Self-Assembled Monolayers of Octanethiol on Au(111)

Self-assembled monolayers (SAMs) of 1-octanethiol (OT) on Au(111) surfaces, prepared at immersion temperatures between 300 K and 363 K in a sealed stainless steel chamber, were studied by scanning tunneling microscopy (STM). An oblique (√3 × √3)R30° OT-SAM structure was observed below 348 K, whereas a superstructure (3×√7)R11° covered the gold surfaces at 363 K. The highly resolved STM images permitted assignment of four symmetry-inequivalent OT molecules per 8.7 × 7.6 Å² unit cell at 363 K. Differences in the topographical heights of the molecules were attributed to the binding of OT sulfur head groups at different adsorption sites on Au(111). This structure was not observed in stirring reflux at a high temperature, which indicates a higher pressure (> 1 atm) in the chamber may be one of crucial factors for structural transition. As the immersion temperature increased, a lower density of vacancy islands and a higher fraction of island area were observed.     

Medium energy ion scattering investigation of methylthiolate-induced modification of the Au(111) surface

100 keV H⁺ scattering has been used to investigate the structure of the methylthiolate/Au(111) interface in the Au(111)(√3 × √3)R30° phase. Adsorption of the thiolate onto the clean Au(111) surface leads to a large drop in the scattered ion yield due to the lifting of the clean surface ‘herring-bone’ reconstruction, but the thiolate-covered surface shows an ion yield higher than that of an unreconstructed Au(111) surface, providing direct evidence of a significant number of Au atoms that are displaced from their bulk-terminated positions at the buried interface. Simulations for two different Au adatoms models at the interface, namely, the Au-adatom-monothiolate (AAM) and Au-adatom-dithiolate (AAD) models, show significant sensitivity to the exact values of interlayer spacings and atomic vibrational amplitudes, but the comparison with experimental results appears to favour the AAD model with 0.17 ML Au adatoms in bridging sites at the interface.     

Two commensurate hydrogen-bonded monolayer structures of melamine on Ag(111)

Melamine (1,3,5-triazine-2,4,6-triamine) was deposited on the Ag(111) surface under ultrahigh vacuum conditions. It forms two different monolayer structures, which were investigated by low energy electron diffraction and scanning tunneling microscopy. The α-phase is a honeycomb structure containing two molecules per unit-cell. The molecular orientation within the unit-cell is determined by six hydrogen bonds. The α-phase is kinetically preferred upon deposition at room-temperature and can be transferred to the thermodynamically more stable β-phase by annealing at 333 K. The β-phase has an oblique unit-cell containing four molecules and shows a higher surface density with additional hydrogen bonds between adjacent amino groups. Both structures are commensurate. While the structural motif of the α-phase has been observed before on Au(111) and Ag–Si(111) surfaces, the structure of the β-phase has been so far only theoretically predicted.     

Density functional theory study of the water adsorption at Bi(111) electrode surface

The adsorption of a water molecule on a basal Bi(111) electrode surface, crystallising in the rhombohedral system, has been studied in the framework of cluster model. The quantum chemical calculations were performed at the Density Functional Theory (DFT) level and the electrical double layer effects were analysed by using an external electric field. In contrast to computational predictions reported previously for other metal surfaces, crystallising in the face-centred cubic or hexagonal close-packed systems, a hollow site for Bi(111) was found to be energetically the most preferable; the water adsorption energy amounts to ? 28 kJ mol^? 1. In a wide range of surface charge densities the water molecule is bound preferentially through the O atom in orientation perpendicular to the surface plane. The Bi(111) hydrophilic properties are compared with those for other metals. Some adsorption characteristics of a hydrogen atom and a hydroxyl group at Bi(111) are reported as well, which give evidence in favour of the non-dissociative adsorption of water molecules.     

In-situ surface-sensitive X-ray diffraction study on the influence of iodide over the selective electrochemical etching of Cu3Au (111)

During selective etching (dealloying) surface-sensitive X-ray diffraction employing Synchrotron light has been used to in-situ monitor the potential-controlled formation of Au-rich films on the surface of Cu₃Au (111) in iodide-containing electrolytes. Similar to the case in pure sulfuric acid we observed a sequence of structural transformations starting from a well-prepared pristine surface to a porous film consisting of substrate-oriented Au ligaments. Also stacking-reversed ultrathin Au-rich films and Au islands form as intermediate steps but no passive-like behavior was observed in iodide-containing electrolytes, i.e. the surface quickly developed Au ligaments after reaching the Cu dissolution potential. At low overpotentials comparatively coarse Au islands point to a higher mobility of Au/electrolyte interfaces in iodide-containing solutions. At higher overpotentials and also with higher iodide concentrations an epitaxial Cu-iodide precipitate film showed an orientation relation of CuI (111) || CuAu (111) and two azimuthal domains of < ? 2, 2, 0 > || < ? 2, 2, 0 > and < ? 2, 2, 0 > || < 2, ? 2, 0>. This partially dissolution-inhibiting bulk CuI layer is observed to produce a bimodal pore size instead of usually obtained homogeneous porosity. The X-ray data and supporting ex-situ AFM and SEM images show marked differences in the morphology and connectivity of the forming nanoporous Au layer. Precipitation layers are thus suggested to provide means for controlling the nanoporosity for applications of dealloyed films and surfaces.     

Single layer gold islands at the interface between a self-assembled monolayer and the Au(111) substrate: A high-resolution STM study

Physical vapor deposition of gold onto a self-assembled monolayer (SAM) of octanethiol on Au(111) has been studied at the molecular level in ultra-high vacuum (UHV) using atomic-resolution scanning tunneling microscopy (STM). A specially prepared SAM with not only the usual etch pits but also co-existing phases and domain boundaries is used for the purpose of studying details of the nucleation process. Etch pits are found to be filled by deposited Au atoms. At the same time, preferential nucleation and growth of gold islands at intersections of different domains, as well as inside the domains of the less dense striped phase, is observed. We find no gold islands within the densely-packed (√3 × √3)R30° phase. High-resolution STM imaging shows that the SAM over the newly formed gold islands adopts the same structure as that in the immediate surroundings.     

2-Mercaptopyrimidine as a suitable matrix to trap arenes on Au(111): STM probing of the modified surface

2-Mercaptopyrimidine (2MPy) forms a two-dimensional planar lattice on Au(1 1 1). This structure is able to intercalate guest molecules such as the oligopyridines. To test the ability of the 2MPy lattice as a host able to intercalate a variety of molecules regardless of their geometry and electronic structures, some arenes and polyenes were analyzed as possible guests. Benzene, naphthalene, azulene, phenanthrene, anthracene, biphenyl, fluorobenzene and durene were selected to coadsorb with 2MPy on the Au(1 1 1) surface. These arenes are not known to form stable layers on Au(1 1 1); their adsorption is often studied on Pt and Rh surfaces. The experimental data show that the arenes and polyenes selected coadsorb with 2MPy on Au(1 1 1) under defined experimental conditions, forming a mixed layer with stoichiometry of 2:1 in thiol:guest. The STM contrast observed for the guest molecules is compared to the experimental and theoretical results already published.     

Ultrathin Y2O3(111) films on Pt(111) substrates

The growth of ultrathin films of Y₂O₃(111) on Pt(111) has been studied using scanning tunneling microscopy (STM), X-ray photoemission spectroscopy (XPS), and low energy electron diffraction (LEED). The films were grown by physical vapor deposition of yttrium in a 10^? 6 Torr oxygen atmosphere. Continuous Y₂O₃(111) films were obtained by post-growth annealing at 700 °C. LEED and STM indicate an ordered film with a bulk-truncated Y₂O₃(111)–1 × 1 structure exposed. Furthermore, despite the lattices of the substrate and the oxide film being incommensurate, the two lattices exhibit a strict in-plane orientation relationship with the [11?0] directions of the two cubic lattices aligning parallel to each other. XPS measurements suggest hydroxyls to be easily formed at the Y₂O₃ surface at room temperature even under ultra high vacuum conditions. The hydrogen desorbs from the yttria surface above ~ 200 °C.     

The role of van der Waals interaction in the tilted binding of amine molecules to the Au(111) surface

We present the results of ab initio electronic structure calculations for the adsorption characteristics of three amine molecules on Au(111), which show that the inclusion of van der Waals interactions between the isolated molecule and the surface leads in general to good agreement with experimental data on the binding energies. Each molecule, however, adsorbs with a small tilt angle (between -5 and 9°). For the specific case of 1,4-diaminobenzene (BDA) our calculations reproduce the larger tilt angle (close to 24°) measured by photoemission experiments, when intermolecular (van der Waals) interactions (for about 8% coverage) are included. These results point not only to the important contribution of van der Waals interactions to molecule-surface binding energy, but also that of intermolecular interactions, often considered secondary to that between the molecule and the surface, in determining the adsorption geometry and pattern formation.     

Adsorption of water monomer and clusters on platinum(111) terrace and related steps and kinks: I. Configurations,energies, and hydrogen bonding

Adsorption and rotation of water monomer, dimer, and trimer on the (111) terrace, (221) and (322) stepped, and (763) and (854) kinked surfaces of platinum were studied by density functional theory calculations using the PW91 approximation to the energy functional. On the (111) terrace, water monomer and the donor molecule of the dimer and trimer adsorb at atop sites. The per-molecule adsorption energies of the monomer, dimer, and trimer are 0.30, 0.45, and 0.48 eV, respectively. Rotation of monomers, dimers, and trimers on the terrace is facile with energy barriers of 0.02 eV or less. Adsorption on steps and kinks is stronger than on the terrace, as evidenced by monomer adsorption energies of 0.46 to 0.55 eV. On the (221) stepped surface the zigzag extended configuration is most stable with a per-molecule adsorption energy of 0.57 eV. On the (322) stepped surface the dimer, two configurations of the trimer, and the zigzag configuration have similar adsorption energies of 0.55 ± 0.02 eV. Hydrogen bonding is strongest in the dimer and trimer adsorbed on the terrace, with respective energies of 0.30 and 0.27 eV, and accounts for their increased adsorption energies relative to the monomer. Hydrogen bonding is weak to moderate for adsorption at steps, with energies of 0.04 to 0.15 eV, as the much stronger water–metal interactions inhibit adsorption geometries favorable to hydrogen bonding. Correlations of hydrogen bond angles and energies with hydrogen bond lengths are presented. On the basis of these DFT/PW91 results, a model for water cluster formation on the Pt(111) surface can be formulated where kink sites nucleate chains along the top of step edges, consistent with the experimental findings of Morgenstern et al., Phys. Rev. Lett., 77 (1996) 703.     

Nucleation and growth mechanisms of Fe on Au(111) in the sub-monolayer regime

The growth of Fe on Au(111) at 300 K in the sub-monolayer regime has been investigated using scanning tunneling microscopy, focusing on the mechanisms of nucleation, coalescence and interlayer diffusion. Below a coverage of 0.1 ML, Fe growth proceeds in a well-ordered fashion producing regular arrays of islands, while approaching the island coalescence threshold (above 0.35–0.4 ML), we observed a consistent increasing of random island nucleation. These observations have been interpreted through rate equation models for the island densities in the presence of preferred nucleation sites. The evolution of the second layer fraction has also been interpreted in a rate equation scheme. Our results show that the ordered to random growth transition can be explained by including in the model bond breaking mechanisms due to finite Fe–Fe bond energy. A moderate interlayer diffusion has been inferred from data analysis between the second and the first layer, which has been used to estimate the energy barrier of the adatoms descending process.     

The adsorption of acetic acid on clean and oxygen-covered Au/Pd(100) alloy surfaces

The adsorption of acetic acid is studied on clean and oxygen-covered Au/Pd(100) alloys as a function of gold content by temperature-programmed desorption and reflection–absorption infrared spectroscopy. Au/Pd(100) forms ordered alloys such that, for gold coverages above ~ 0.5 monolayers, only isolated palladium atoms surrounded by gold nearest neighbors are present. Predominantly molecular acetic acid forms on Au/Pd(100) alloy surfaces for gold coverages greater than ~ 0.56 ML, and desorbs with an activation energy of ~ 59 kJ/mol. Heating this surface also forms some η¹-acetate species which decompose to form CO and hydrogen. On alloy surfaces with palladium–palladium bridge sites, η¹-acetate species initially form, but rapidly convert into η²-species. They thermally decompose to form CO and hydrogen, with a small portion rehydrogenating to form acetic acid between 280 and 321 K depending on gold coverage. The presence of oxygen on both Pd(100) and Au/Pd(100) alloys facilitates acetate dehydrogenation so that only η²-acetate species form on these surfaces. The presence of oxygen also serves to stabilize the acetate species.     

Surface interactions of Au(I) cyclo-trimer with Au(111) and Al(111) surfaces: A computational study

A plane-wave density functional theory (DFT) study on surface interactions of a cyclo-[Au(μ-Pz)]₃ monolayer (denoted as T), Pz = pyrazolate, with Au(111) and Al(111) surfaces (denoted as M′) has been performed. Structural and electronic properties at the M′–T interfaces are determined from individually optimized structures of M′, T and M′–T. Results show that the gold pyrazolate trimer (T) binds more strongly on the Au(111) surface than on Al(111). Charge redistribution has been observed at both M′–T interfaces, where charge is “pushed” back towards the Au(111) surface from the trimer monolayer in Au(111)–T system, while the opposite happens in the Al(111)–T system where the charge is being pushed toward the trimer monolayer from the Al(111) surface. Considerable changes to the work function of Au(111) and Al(111) surfaces upon the trimer adsorption which arise from monolayer vacuum level shifts and dipole formation at the interfaces are calculated. The interaction between cyclo-[Au(μ-Pz)]₃ with metal surfaces causes band broadening of the gold pyrazolate trimer in M′–T systems. The present study aids better understanding of the role of intermolecular interactions, bond dipoles, energy-level alignment and electronic coupling at the interface of metal electrodes and organometallic semiconductor to help design metal–organic field effect transistors (MOFETs) and other organometallic electronic devices.     

Self-assembly of C60 fullerenes on quasi-one-dimensional Si(111)4 × 1-In surface

Using scanning tunneling microscopy observations, self-assembly of C₆₀ fullerenes in the course of room-temperature adsorption onto Si(111)4 × 1-In reconstruction and after subsequent annealing at temperatures ranging from 150 to 450 °C has been studied. Adsorbed C₆₀ fullerenes have been found to occupy off-centered positions on In-atom rows forming linear chains with a maximal length of eight C₆₀ molecules. Intermolecular spacing within the regular chains equals three lattice constants of Si(111) surface. Two energetically different adsorption states of C₆₀ have been detected, one of which is occupied preferentially at room temperature, while occupation of the second (more tight) state dominates at temperature above ~ 150 °C. In the first state, C₆₀ fullerene resides plausibly in a continuous rotation, while in the second state a C₆₀ molecule is fixed tightly in a single orientation with a C₆₀ hexagon pointing upward. Transition of C₆₀ fullerenes to the more stable state is accompanied by expelling In atoms from the Si(111)4 × 1-In reconstruction.     

Infrared spectra of high coverage CO adsorption structures on Pt(111)

The adsorption of carbon monoxide on Pt(111) was studied using polarization modulation infrared reflection absorption spectroscopy (PM-IRAS) and sum frequency generation (SFG) spectroscopy. Two CO on-top signals at 2110 cm^? 1 and 2097 cm^? 1 have been detected under continuous CO exposure in a pressure range from 10^? 7 to 100 mbar and at temperatures between 200 K and 300 K. The formation of the higher wavenumber signal is found to be kinetically limited below 200 K and by the presence of a stable c(4 × 2) adlayer in UHV. On the basis of the results presented in this study and previous experimental findings the two on-top signals are related to different CO compression layers on Pt(111) with θ > 0.5, hexagonal Moiré lattices and rectangular coincident site lattices.     

Ordered structures of two sulfur containing donor molecules on the Au(111) surface

We report experiments by low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) on ordered structures of two sulfur-containing π-conjugated molecules on the Au(111)-surface, namely tetrabenzothianthrene (TBTA) and tetrathiotetracene (TTT). These molecules are candidates for donors in charge transfer salts. On Au(111) both molecules form long-range ordered structures that are commensurate to the top-most surface layer. For TBTA, the reconstruction of the Au(111) surface is maintained, whereas it is lifted by TTT. Both molecules lie flat on the surface. For TBTA, the structure indicates that the molecule is planarized upon adsorption.     

Selective resonance population in high resolution electron energy loss spectroscopy of 4-ethylbenzenethiol self-assembled monolayers on Au(1 1 1)

We have investigated the vibrational frequencies and excitation cross-section of self-assembled monolayers (SAMs) of 4-ethylbenzenethiol (CH₃CH₂(C₆H₄)SH) on Au(1 1 1) by high resolution electron energy loss spectroscopy (HREELS). Negative ion resonances were observed in the energy loss intensities as a function of the incident electron energy. Analysis of the C–H stretching modes indicates resonances of different energies are localised in both the ethyl and phenyl functional groups of the SAM molecules, which regulate the observed vibrational lineshape.