Structure and reaction pathways of methyl lactate on Pd(1 1 1)

The adsorption and reaction of methyl lactate (CH₃CH(OH)COOCH₃) is studied in ultrahigh vacuum on a Pd(1 1 1) surface using temperature-programmed desorption (TPD) and reflection–absorption infrared spectroscopy (RAIRS). Methyl lactate reacts at relatively low temperatures (220 K) by O–H bond scission. This intermediate can either react with hydrogen to reform methyl lactate at 280–300 K or undergo β-hydride elimination to form flat-lying methyl pyruvate. This decomposes to form acetyl and methoxy carbonyl species as found previously following methyl pyruvate adsorption on Pd(1 1 1). These species predominantly react to form carbon monoxide, methane and hydrogen.     

Cu dimer diffusion on strained Cu(0 0 1)

Cu dimer diffusion energy barrier on strained Cu(0 0 1) surfaces has been studied with nudged elastic band method (NEB) and embedded atom method (EAM). Dimer exchange and hopping mechanisms are chosen as the initial diffusion paths in the NEB method. It is shown here that the dimer exchange is dominant on tensile surfaces and the dimer hopping is dominant on compressive surfaces. For most strain conditions Cu dimer diffusion energy barrier is lower than Cu monomer diffusion barrier. The concerted movement of the remaining adatom toward the hopping adatom lowers the dimer hopping barrier. The adsorption induced relaxation makes the dimer exchange barriers lower than the monomer exchange barriers on tensile surfaces. Transition state theory is used to calculate the diffusion frequencies as a function of temperature. No surface crowdion is observed on the shear strained surfaces for the dimer diffusion.     

Interaction of atomic H and CO with Cu(1 1 0) and bimetallic Ni/Cu(1 1 0)

Co-adsorption of hydrogen and CO on Cu(1 1 0) and on a bimetallic Ni/Cu(1 1 0) surface was studied by thermal desorption spectroscopy. Hydrogen was exposed in atomic form as generated in a hot tungsten tube. The Ni/Cu surface alloy was prepared by physical vapor deposition of nickel. It turned out that extended exposure of atomic hydrogen leads not only to adsorption at surface and sub-surface sites, but also to a roughening of the Cu(1 1 0) surface, which results in a decrease of the desorption temperature for surface hydrogen. Exposure of a CO saturated Cu(1 1 0) surface to atomic H leads to a removal of the more strongly bonded on-top CO (α₁ peak) only, whereas the more weakly adsorbed CO molecules in the pseudo threefold hollow sites (α₂ peak) are hardly influenced. No reaction between CO and H could be observed. The modification of the Cu(1 1 0) surface with Ni has a strong influence on CO adsorption, leading to three new, distinct desorption peaks, but has little influence on hydrogen desorption. Co-adsorption of H and CO on the Ni/Cu(1 1 0) bimetallic surface leads to desorption of CO and H₂ in the same temperature regime, but again no reaction between the two species is observed.     

An investigation of the surface chemistry of methyl pyruvate on Cu(1 1 1)

The surface chemistry of an α-ketoester, methyl pyruvate, has been studied on a model Cu(1 1 1) single crystal surface. Monolayers of methyl pyruvate at 180 K consist predominately (ca. 66%) of a chemisorbed methyl pyruvate moiety, with its keto-carbonyl bonded to the surface in a η² configuration, this moiety desorbs intact at 365 K. The rest of the monolayer contains weakly adsorbed methyl pyruvate, which desorbs at 234 K, which interacts with the surface through the lone pair electrons of the oxygen atoms of the CO groups, adopting a η¹ configuration. Previous studies of simple ketones on model noble metal surfaces have only observed weakly bonded η¹ configurations. The observation of a strongly chemisorbed moiety in the present study is attributed to the activation of the keto-carbonyl by the electron withdrawing ester group. This behaviour is consistent with the homogeneous inorganic chemistry of ketones. Given both the formation of a η² bonded methyl pyruvate moiety on Cu(1 1 1) and the known activity of Cu as a selective hydrogenation catalyst, it is suggested that it maybe worthwhile considering the possibility of testing the effectiveness of chirally modified supported Cu as an enantioselective catalyst.     

Electronic surface states of Cu(1 1 0) surface

Surface states are a unique and important class of quantum states that shave an important effect on the electronic properties of Cu(1 1 0) surface. The Cu(1 1 0) surface has been studied using ultraviolet photoemission spectroscopy (PES), inverse photoemission spectroscopy (IPES), and reflection anisotropy spectroscopy (RAS), and shows a resonance in the RAS spectra at 2.1 eV due to a transition between occupied and unoccupied surface states. The unoccupied surface state involved in the RAS transition at an energy of 1.7 eV at the point of the surface Brillouin zone has been investigated using IPES and the occupied surface state is seen in PES spectra at 0.45 eV below the Fermi level. The energy difference of the surface states, 2.15 eV, is a good match to the transition energy found in the RAS experiments.     

Topologically induced surface electron state on Si(1 1 1) surfaces

First-principle electronic structure calculation reveals the appearance of a new class of surface state on hydrogenated and clean Si(1 1 1) surfaces. The states are found to exhibit different characteristics to conventional surface electron states in terms of the peculiar distribution of the wavefunction depending on the wavenumber. In addition, the state results in flat dispersion bands in a part of the surface Brillouin zone having energy of about 8 eV below the top of the valence band. An analytic expression based on the tight-binding approximation corroborates the surface electron state results from the delicate balance of the electron transfer among the atoms situated near the surface. The obtained results give a possible extension and generalization of the edge state in graphite ribbons with zigzag edges.     

The surface chemistry of methyl and methylene radicals adsorbed on Cu(111)

The interactions of methyl and methylene radicals on Cu(111) were investigated with XPS, AES and HREELS under various exposure conditions. The CH₂ and CH₃ radicals are generated through a hot nozzle source with ketene and azomethane gases. It is shown that with substrate at 300 K, the impinging CH₃ radicals are trapped mainly as CH₃(ads), while a part of the adsorbate decomposes to form CH₂(ads) and H(ads). H atoms are found to desorb at about 380 K, while the chemisorbed hydrocarbon adspecies desorb at about 420 K. In drastic contrast, exposing the clean Cu surface to methylene radicals results not only in the trapping of CH₂(ads), but also in the formation of complex aromatic species. The adlayer is sensitive to annealing at elevated temperatures. Desorption and partial conversion to methylidyne take place at around 420 K. The CH(ads) species can survive up to 700 K and then decomposes to form residual carbon above 800 K. In both radical-Cu(111) systems, surface coverage appears to saturate near one monolayer. The relative concentrations of different surface species in the adlayer, however, depend on the amount of radical exposure. The reaction properties of the two systems are compared and discussed.     

Laser-induced photo-desorption in the Na/Cu(1 1 1) system

Desorption in the Na/Cu(1 1 1) system induced by an electronic excitation is studied using a quantal approach. The system is excited by a laser pulse in the fs range to the Na¹ state corresponding to the transient capture of an electron by the alkali adsorbate. The present quantal approach describes on an equal footing the laser-induced vibrational excitation of the adsorbate in the adsorption well and the photo-desorption process. It confirms earlier results using a semi-classical input. It also allows a discussion of the photo-desorption probability with the photon energy: the maximum of the desorption probability per absorbed photon occurs off-resonance in the high-energy wing of the electronic transition. This feature is related to the dynamics of the laser-induced process.     

Formate stability and carbonate hydrogenation on strained Cu overlayers on Pt(1 1 1)

Formate (HCOO) synthesis, decomposition and the hydrogenation of carbonate (CO₃) on Cu overlayers deposited on a Pt(1 1 1) single crystal are investigated to examine the reactivity of a Cu surface under tensile strain with defects present.Formate is synthesized from a 0.5 bar mixture of 70% CO₂ and 30% H₂ at varying temperatures, and the evolution is followed with polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). Subsequent TPD reveals decomposition of the formate species into CO₂ and H₂ at 420 ± 5 K for strained Cu at sub-monolayer to monolayer coverages. This is a significantly lower decomposition temperature than obtained earlier on pristine Cu(1 1 1) (460 K), as well as for thicker Cu layers where we assign an observed decomposition peak at 440 ± 5 K to relaxed, but defect-rich Cu(1 1 1). However, the thermal stability of formate on strained and defect-rich Cu is similar to previous results obtained for supported, and lattice-strained, Cu nanoparticles.The hydrogenation of carbonate produced by 0.3 bar CO₂ exposure at room temperature was monitored with XPS and TPD showing a significant loss of carbonate when subjected to 0.2 bar H₂ at room temperature. However, the presence of formate on the surface, or any other hydrogenation product, could not be established during or after H₂ exposure by PM-IRRAS, EELS or TPD. Even so, the results suggest that carbonate and its hydrogenation may constitute a relevant pathway to methanol production.     

Comparative dynamical study of the HD and isotopomers interacting with Pd(1 1 1) and Cu(1 1 0) surfaces

We study the dynamics of HD and H₂ molecules interacting with Pd(1 1 1) and Cu(1 1 0) using the classical trajectory method based on potential energy surfaces obtained from Density Functional Theory calculations. Our results predict a negligible isotopic effect on the dissociative adsorption probability on Pd(1 1 1) whereas on Cu(1 1 0), the adsorption probability for HD(ν_i=0) is slightly lower than for H₂(ν_i=0), mainly due to its lower initial vibrational zero point energy. The final rotational energy distribution of scattered HD and H₂ molecules are very similar. This shows that the asymmetric mass distribution of HD, barely affects the fraction of initial translational energy transferred to rotation during the scattering process. Our calculations point to the larger number of open rotational excitation channels for HD, as the main cause of rotational excitation probabilities larger than for H₂. The theoretical apparent rotational temperature, T_rot, of HD molecules scattered from Pd(1 1 1) at impact energy , is in good agreement with the experimental value. In contrast, for Cu(1 1 0) the theoretical T_rot is much lower than the value measured for Cu(1 0 0). Possible reasons for such a discrepancy between theory and experiments are discussed.     

Fabrication of InGaN/GaN stripe structure on (1 1 1)Si and stimulated emission under photo-excitation

InGaN/GaN QW laser structure was investigated on a GaN trapezoid grown on (1 1 1)Si substrate by selective MOVPE. The dislocation density in the active layer was reduced by a two-step growth method adopting facet controlled epitaxial lateral overgrowth (FACELO). A sample with 350 μm long cavity length showed narrowing of the spectral peak under optical excitation. The compositional non-uniformity originating from the ridge growth on the trapezoid is removed by adopting re-evaporation phenomenon under heat treatment during the growth process.     

Theoretical study of hydrogenation process of formate on clean and Zn deposited Cu(1 1 1) surfaces

We have studied the effect of Zn on hydrogenation of formate to dioxomethylene on the Cu(1 1 1) surface by using a density functional theory–generalized gradient approximation (DFT–GGA)-pseudopotential method. We show that substitutionally adsorbed Zn changes the stability of intermediate states and the activation barrier of the hydrogenation process only slightly. On the other hand, the Zn atom adsorbed on the Cu surface stabilizes all formate, transition state, and dioxomethylene relative to the gas-phase molecules. Our results support a previously proposed reaction scheme that the adsorption state of Zn changes from substitutional to on-surface adsorption during the methanol synthesis.     

Surface phonons of GaP(1 1 0) and InAs(1 1 0)

The dispersions of low energy surface phonon modes of GaP(1 1 0) and InAs(1 1 0) measured with inelastic He-atom scattering along the and 0 0 1 directions are presented. Aside from the Rayleigh mode, additional distinct acoustic modes are observed as well as indications of optical modes. Contrary to results for GaAs(1 1 0), a rocking mode was not observed. The experimentally determined phonon dispersions are in excellent agreement with recent ab initio calculations by C. Eckl, et al. [1].     

Carbon-doped high-mobility hole gases on (0 0 1) and (1 1 0) GaAs

Since Stormer and Tsang have introduced the first two-dimensional hole gas (2DHG) in the GaAs/AlGaAs heterosystem, the choice of suitable dopants was limited to beryllium and silicon over the last 20 years. Both acceptor atoms have significant disadvantages, i.e. either high-diffusion rates or a limitation to specific growth directions. Utilizing a carbon filament-doping source, we prepared high-quality 2DHGs in the (0 0 1) and the nonpolar (1 1 0) crystal plane with carrier mobilies beyond 10⁶ cm²/Vs in quantum well and single interface structures. Low-temperature magnetoresistance measurements recover a large number of fractional QHE states and show a pronounced beating pattern from which the Rashba induced spin-splitting has been determined. In addition, 2DHGs have been grown on cleaved edges of (1 1 0) and (0 0 1) wafers with transport features in qualitative agreement to our findings on (1 1 0) substrates.     

A PdO(1 0 1) thin film grown on Pd(1 1 1) in ultrahigh vacuum

We present experimental results demonstrating that a high quality PdO(1 0 1) thin film can be grown on Pd(1 1 1) in ultrahigh vacuum by oxidizing the metal at 500 K using an oxygen atom beam, followed by annealing to 675 K. Low energy electron diffraction (LEED) images show that the [0 1 0] direction of the PdO(1 0 1) thin film aligns with the [−1 1 0] direction of the Pd(1 1 1) substrate, and that the PdO film grows in three degenerate domains, rotated 120° relative to one another. Based on excellent agreement between the experimental and simulated LEED patterns, we conclude that the surface structure of the PdO thin film deviates minimally from bulk-terminated PdO(1 0 1). Recent temperature programmed desorption (TPD) experiments also provide evidence that the PdO(1 0 1) thin film on Pd(1 1 1) is terminated by the stoichiometric surface in which half of the Pd atoms are coordinatively unsaturated (cus), corresponding to a cus-Pd atom density equal to about 35% of the surface density of Pd(1 1 1). The ability to generate a well-defined PdO(1 0 1) surface in ultrahigh vacuum should provide new opportunities for conducting model surface science studies of PdO, particularly studies aimed at elucidating the reactivity of PdO(1 0 1) toward species important in commercial applications of Pd catalysis.     

Ab-initio calculations of interactions between Cu adatoms on Cu(1 1 0): Sensitivity of strong multi-site interactions to adatom relaxations

We have parameterized the various interactions between Cu adatoms on Cu(1 1 0) using density-functional theory based ab-initio calculations. Our results indicate that in addition to pair interactions, 3-adatom and 4-adatom interactions of significant strengths are present in this system. This further stresses the importance of multi-site interactions in constructing a complete lattice–gas picture. Even though adding these multi-site interactions leads to good convergence in interaction energies, we find that some multi-site interactions are very sensitive to adatom relaxations. This makes the application of a simple lattice–gas picture inadequate for such surfaces. We also parameterize adatom interactions on this surface using the recently developed connector model. The connector model parameterization is as efficient as the parameterization using lattice–gas model. Further, we present diffusion barriers for nearest-neighbor (NN) and next-nearest-neighbor (NNN) hops on this surface.     

Optical characterization of methanol adsorption on the bare and oxygen precovered Cu(1 1 0) surface

We have studied the adsorption and reaction of methanol on the bare and oxygen precovered Cu(1 1 0) surface at 200 K using reflectance difference spectroscopy (RDS). On the bare and fully oxygen covered surface, the sticking coefficient is close to zero. In contrast, on the partially oxygen covered surface, a sticking coefficient close to unity is obtained. This observation suggests a high mobility of methanol on both bare and oxygen covered Cu(1 1 0) and of methoxy on Cu(1 1 0). Two reaction regimes, an oxygen supply limited and an adsorption site limited regime are identified. The transition between these two regimes occurs for an oxygen coverage of about 0.2.     

Comparison of sulfur interaction with hydrogen on Pt(1 1 1), Ni(1 1 1) and Pt3Ni(1 1 1) surfaces: The effect of intermetallic bonding

Adsorption strengths of hydrogen and sulfur both individually and together as co-adsorbates were investigated on Pt(1 1 1), Ni(1 1 1) and Pt₃Ni(1 1 1) surfaces using density functional theory in order to determine the effect of metal alloying on sulfur tolerance. The adsorption strengths of H and S singly follow the same trend: Ni(1 1 1) > Pt(1 1 1) > Pt₃Ni(1 1 1), which correlates well with the respective d-band center positions of each surface. We find that the main effect of alloying is to distort both the sub-layer structure and the Pt overlayer resulting in a lowered d-band. For all three surfaces, the d-band shifts downward non-linearly as a function of S coverage. Nearly identical decreases in d-band position were calculated for each surface, leading to an expectation that subsequent adsorption of H would scale with surface type similarly to single species adsorption. In contradiction to this expectation, there was no clearly discernable difference between the energies of coadsorbed H on Pt(1 1 1) and Ni(1 1 1) and only a slightly lowered energy on Pt₃Ni(1 1 1). This provides evidence that coadsorbed species in close proximity interact directly through itinerant mobile electrons and through electrostatic repulsion rather than solely through the electronic structure of the surface. The combination of the lowered d-band position (arising from distorted geometry) and direct co-adsorbate interactions on Pt₃Ni(1 1 1) leads to a lower energy barrier for H₂S formation on the surface compared to pure Pt(1 1 1). Thus, alloying Pt with Ni both decreases the likelihood of S adsorption and favors S removal through H₂S formation.     

Photon- and electron-induced surface voltage in electron spectroscopies on ZnSe(0 0 1)

The surface band bending in ZnSe(0 0 1), as a function of the temperature, is investigated both in the valence band (by photoemission) and in the conduction band (by inverse photoemission and absorbed current spectroscopies). Two different mechanisms are invoked for interpreting the experimental data: the band bending due to surface states, and the surface voltage induced by the incident beam. While the latter is well known in photoemission (surface photovoltage), we demonstrate the existence of a similar effect in inverse photoemission and absorbed current spectroscopies, induced by the incident electrons instead of photons. These results point to the importance of considering the surface voltage effect even in electron-in techniques for a correct evaluation of the band bending.