Synthesis of TiO2 nanoparticles on the Au(111) surface

The growth of titanium oxide nanoparticles on reconstructed Au(111) was investigated by scanning tunneling microscopy and x-ray photoelectron spectroscopy. Ti was deposited by physical-vapor deposition at 300 K. Regular arrays of titanium nanoparticles form by preferential nucleation of Ti at the elbow sites of the herringbone reconstruction. The titanium oxide nanoclusters were synthesized by subsequent exposure to O(2) at 300 K. Two-and three-dimensional titanium oxide nanocrystallites form during annealing in the temperature range from 600 to 900 K. At the same time, the Au(111) surface assumes a serrated 110-oriented step-edge morphology suggesting step-edge pinning by titanium oxide nanoparticles. The oxidation state of the titanium oxide nanoparticles varies with annealing temperature. Specifically, annealing to 900 K results in the formation of stoichiometric TiO(2) nanocrystals as judged by the Ti(2p) binding energies measured in the x-ray photoelectron data. The nanodispersed TiO(2) on Au(111) is an ideal system to test the various models proposed for the enhanced catalytic reactivity of supported Au nanoparticles.     

Molecular beam measurements and Monte Carlo simulations of the kinetics of N2O decomposition on Rh(111) single-crystal surfaces

The kinetics of N2O decomposition on Rh(111) single-crystal surfaces were investigated both experimentally by isothermal molecular beam measurements and theoretically using a Monte Carlo algorithm. The present work was directed to the understanding of two unusual observations derived from our previous work on this system, namely, (1) the lower rates for N2O decomposition seen at higher reaction temperatures, and (2) the lower total nitrogen yields and final oxygen surface coverages that accompany that behavior. Experimentally, it was determined here that after the rhodium surface is rendered inactive by N2O decomposition at high (520 K) temperatures, significant activity is still possible at lower (350 K) temperatures. The Monte Carlo simulations explain these observations by assuming that the surface sites required for the activation of adsorbed N2O increase in size with increasing reaction temperature.     

Dipolar and nonpolar altitudinal molecular rotors mounted on an Au(111) surface

We describe the preparation of a compound whose molecules consist of two metal sandwich stands carrying tentacles with affinity to metal surfaces and holding an axle that carries a dipolar or a nonpolar rotator. The dipolar rotor exists as three pairs of enantiomers, rapidly interconverting at room temperature. When mounted on a gold surface, each molecule represents a chiral altitudinal rotor, with the rotator axle parallel to the surface. The surface-mounted rotor molecules are characterized by several spectroscopic and imaging techniques. At any one time, in about one-third of the dipolar rotors the rotator is free to turn and the direction of its dipole can be flipped by the electric field applied by an STM tip, as revealed by differential barrier height imaging. Molecular dynamics calculations suggest that electric field normal to the surface causes members of one pair of enantiomers to rotate unidirectionally.     

Au adatoms in self-assembly of benzenethiol on the Au(111) surface

Self-assembly of benzenethiol at low coverage on Au(111) was studied using low-temperature scanning tunneling microscopy. Phenylthiolate species (PhS), formed by thermal dehydrogenation of the parent PhSH molecule, was found to self-assemble into surface-bonded complexes with gold adatoms. Each complex involves two PhS species and one gold adatom. The PhS species form either cis- or trans-geometry relative to each other. At a higher coverage, the complexes coalesce, most likely due to the formation of weak C-H...S hydrogen bonds facilitated by the spatial arrangement of the PhS groups. Our findings thus establish that the self-assembly of arenethiols on the Au(111) surface is driven by gold adatom chemistry, which has recently been found to be the key ingredient in the self-assembly of alkanethiols on gold.     

Decoupling surface reconstruction and perchlorate adsorption on Au(111)

On Au(111) electrodes, the investigation of ClO₄⁻ adsorption is hampered by a simultaneous surface reconstruction. We demonstrate that these two processes can be decoupled in cyclic voltammograms by a proper choice of the scan rate and of the initial potential. Our approach allowed the establishment of a relation between potentials of zero charge for the reconstructed and unreconstructed Au(111) surfaces.     

High-density DNA alignment on an Au(111) surface starting from folded DNA

High-density uniform DNA alignment on a metal substrate is essential for creating sensitive DNA devices. We develop a self-sensing DNA alignment process starting from folded DNA to achieve high-density, uniform DNA alignment on an Au(111) surface. We demonstrate that folded DNA plays a critical role in avoiding DNA aggregation and distributing the DNA uniformly on an Au(111) surface at the greatest density and quality ever attained. We also verify that the distributed, folded DNA can be stimulated to align only when the appropriate buffer flow is applied. This selective self-sensing DNA alignment on an Au surface will be a key technology for creating dynamic DNA sensors and switches.     

Highly ordered C60 monolayer self-assembled by using an iodine template on an Au(111) surface in solution

An iodine-modified Au(111) surface, (I/Au(111)), was used as a substrate to prepare a C 60 adlayer by self-organization in a benzene solution. A highly ordered C 60 adlayer was successfully prepared due to the moderate C 60-I/Au(111) interaction. Two lattice structures, (2 square root 3 x 2 square root 3) R30 degrees and p(2 x 2), were imaged for this C 60 adlayer. For the first structure, a featureless ball-like molecular shape was imaged, ascribed to the molecular rotation resulting from a symmetrical location between C 60 and iodine atoms. For the p(2 x 2) structure, the asymmetrical location of C 60 with respect to the iodine atoms freezes the C 60 molecules on the substrate, leading to a clear image of intramolecular structure. The intermediate iodine atoms in the C 60/I/Au(111) adlayer can be desorbed by electrochemically reduction without significantly affecting the ordering of the C 60 adlayer. However, the internal pattern of C 60 disappears in the absence of iodine.     

MP2 study on adsorption of hydrated Na+ and Au+ cations on the Au(111) surface

The interactions of Na+ and Au+ cations with an Au(111) surface in the presence and absence of water molecules were investigated using Au18 and Au22 cluster models and the MP2 method with a triple-zeta valence basis set. The interactions between Na+ ions and the Au(111) surface were found to be primarily electrostatic, contrary to the much stronger Au+-Au(111) interactions that were dominated by orbital contributions. The largest CP-corrected MP2 adsorption energies were -156.9 kJ/mol for Na+ and -478.7 kJ/mol for Au+. When hydrated, Na+ prefers to be completely surrounded by water molecules rather than adsorbed to the surface, whereas Au+ remains adsorbed to the surface as water molecules bond with each other and with the Au surface. CP correction did not change the relative adsorption energy trends of Na+ or Au+ ions, but it had an effect on the interaction energy trends of the hydrated cations because of the weak water-surface and water-water interactions.     

Density functional theory investigation of benzenethiol adsorption on Au(111)

We have studied the adsorption of benzenethiol molecules on the Au(111) surface by using first principles total energy calculations. A single thiolate molecule is adsorbed at the bridge site slightly shifted toward the fcc-hollow site, and is tilted by 61 degrees from the surface normal. As for the self-assembled monolayer (SAM) structures, the (2 square root of 3 x square root of 3)R30 degrees herringbone structure is stabilized against the (square root 3 x square root 3)R30 degrees structure by large steric relaxation. In the most stable (2 square root 3 x square root 3)R30 degrees SAM structure, the molecule is adsorbed at the bridge site with the tilting angle of 21 degrees, which is much smaller compared with the single molecule adsorption. The van der Waals interaction plays an important role in forming the SAM structure. The adsorption of benzenethiolates induces the repulsive interaction between surface Au atoms, which facilitates the formation of surface Au vacancy.     

An efficient Monte Carlo algorithm for the fast equilibration and atomistic simulation of alkanethiol self-assembled monolayers on a Au(111) substrate

A new Monte Carlo algorithm is presented for the simulation of atomistically detailed alkanethiol self-assembled monolayers (R-SH) on a Au(111) surface. Built on a set of simpler but also more complex (sometimes nonphysical) moves, the new algorithm is capable of efficiently driving all alkanethiol molecules to the Au(111) surface, thereby leading to full surface coverage, irrespective of the initial setup of the system. This circumvents a significant limitation of previous methods in which the simulations typically started from optimally packed structures on the substrate close to thermal equilibrium. Further, by considering an extended ensemble of configurations each one of which corresponds to a different value of the sulfur-sulfur repulsive core potential, sigmass, and by allowing for configurations to swap between systems characterized by different sigmass values, the new algorithm can adequately simulate model R-SH/Au(111) systems for values of sigmass ranging from 4.25 A corresponding to the Hautman-Klein molecular model (J. Chem. Phys. 1989, 91, 4994; 1990, 93, 7483) to 4.97 A corresponding to the Siepmann-McDonald model (Langmuir 1993, 9, 2351), and practically any chain length. Detailed results are presented quantifying the efficiency and robustness of the new method. Representative simulation data for the dependence of the structural and conformational properties of the formed monolayer on the details of the employed molecular model are reported and discussed; an investigation of the variation of molecular organization and ordering on the Au(111) substrate for three CH3-(CH2)n-SH/Au(111) systems with n=9, 15, and 21 is also included.     

Scanning tunneling microscopy investigation of a supramolecular self-assembled three-dimensional chiral prism on a Au(111) surface

Spontaneous symmetry-breaking of a racemic mixture of supramolecular triginal prisms into chiral domains on a Au(111) surface is observed by scanning tunneling microscopy (STM). High-resolution STM analysis enables the structural aspects of each enantiomeric domain to be elucidated. The ability to resolve chirality on achiral surfaces has potential applications in heterogeneous stereoselective synthesis and catalysis.     

Adsorption of multivalent alkylthiols on Au(111) surface: Insights from DFT

The adsorption of multivalent thiols on gold (111) surface was investigated using density functional theory applying the Perdew–Burke–Ernzerhof functional. Through the comparison of differences in energetics, structure and charge density distribution of a set of monodentate and polydentate thiols, we have described in detail the factors affecting the adsorption energy and the role played by the multivalence, which causes a decreasing of adsorption energy because of both electronic and steric hindrance effects. Finally, the comparison between the adsorption of 1,2‐ and 1,3‐disulfides revealed how the chain length may affect the cleavage of the S? S bond when they adsorb on Au(111) surface. © 2013 Wiley Periodicals, Inc.     

Ordered arrays of semi-crown ligands on an Au(111) electrode surface: in situ STM study

Self-organized systems have attracted much at-tention due to their potential applications in nano- technology as a bottom-up?approach for the con-struction of molecule-scale devices and nanostruc-tures[1—4]. Beyond the self-assembly of small molecu-lar building blocks, Schnherr et al. recently suc-ceeded in arranging the rosette supramolecular nanos-tructures in two dimensions on HOPG[5,6]. Moreover, interest has tremendously increased in the su-pramolecular structures via coordination-dr…     

Oxygen island formation on Pt(111) studied by dynamic Monte Carlo simulation

The formation of oxygen islands on the Pt(111) surface has been studied as a function of temperature by low energy electron diffraction (LEED) experiments and dynamic Monte Carlo (DMC) simulations. By raising the temperature, the (2 x 2) LEED spot intensity increases gradually and decays after a peak at around 255 K (T(p)) with full width of half maximum of 160 K. This behavior is interpreted by DMC simulations with the kinematical LEED analysis. In the DMC simulation, an oxygen atom hops to the neighboring site via the activation barrier of the saddle point. The potential energies at initial, saddle, and final points are changed at each hopping event depending on the surrounding oxygen atoms. By comparing the observed T(p) with the simulated one, the interaction energy E of oxygen atoms on Pt(111) was determined to be 25+/-3 meV at 2a(0). The DMC simulations visualize how the oxygen islands are formed and collapse on Pt(111) with increase of the temperature and well reproduce the surface configurations observed by scanning tunneling microscopy.     

Monte Carlo model of the temperature rise at a GaAs surface under an electron beam

Scanning electron microscopy (SEM) has frequently been used to study semiconductor materials. It offers the possibility of obtaining reliable qualitative and quantitative information on relevant local material parameters. The temperature rise due to electron‐beam bombardment can influence some semiconductor parameters, which then will influence the SEM information. In this work we propose a model calculation based on the Monte Carlo (MC) method to calculate the temperature rise due to electron‐beam heating. The results show that the temperature rise increases with increasing numbers of electrons (electron‐beam current), and the inverse behavior is observed with respect to the electron energy (electron‐beam voltage). The decrease in temperature rise with depth is also obtained. Copyright © 2006 John Wiley & Sons, Ltd.     

In situ STM investigation of spinodal decomposition and surface alloying during underpotential deposition of Cd on Au(111) from an ionic liquid

The electrodeposition and anodic dissolution of Cd on Au(111) in an acidic chloroaluminate ionic liquid (MBIC-AlCl(3), 42 : 58 mol%) have been investigated by cyclic voltammetry and in situ STM. In the Cd underpotential deposition region, various nanostructures can be distinguished. At a potential of 0.95 V vs. Al/Al(iii), a transformation from a well ordered AlCl(4)(-) adlayer to a ( radical3 x radical19) superstructure, presumably due to Cd-AlCl(4)(-) coadsorption, is observed. Reducing the potential to 0.45 V, surface alloying of Cd and Au occurs, which is evidenced for the first time by typical spinodal structures occurring both during deposition and dissolution of the surface alloy layer having a hexagonal structure. At still lower potentials below 0.21 V, a layer-by-layer growth of bulk Cd sets in.     

Selective oxidation of styrene on an oxygen-covered Au(111)

For the first time, we demonstrate olefin epoxidation promoted by an extended Au surface. The oxidation of styrene to styrene epoxide, benzoic acid, and benzeneacetic acid is promoted on Au(111) covered with 0.2 ML of oxygen atoms. The estimated selectivity for styrene epoxide formation is approximately 53%. Total combustion to CO2 accounts for approximately 20% of the styrene reaction. We propose that styrene epoxide, benzoic acid, and benzeneacetic acid are produced via two possible oxametallacycle intermediates. Our work demonstrates that extended Au is an effective material for olefin oxidation, which has implications for understanding the activity of nanoscale Au catalysts.     

Dynamic Monte Carlo simulation of the NO + CO reaction on Rh(111)

The kinetics of the catalytic reduction of NO by CO on Rh(111) surfaces was investigated by using dynamic Monte Carlo simulations. Our model takes into account recent experimental findings and introduces relevant modifications to the classical reaction scheme, including an alternative pathway to produce N2 through an (N-NO)* intermediate, the formation of atomic nitrogen islands in the adsorbed phase, and the influence of coadsorbed species on the dissociation of NO. All elementary steps are expressed as activated processes with temperature-dependent rates and realistic values dictated by experiments. Calculated steady-state phase diagrams are presented for the NO + CO reaction showing the windows for the conditions under which the reaction is viable. The model predicts variations in both production rates and adsorbate coverages with temperature consistent with experimental data. The effect of varying the individual kinetic parameters and the importance of each step in the reaction scheme were probed.