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.     

Observation of hydrogen adsorption on 6H-SiC(0 0 0 1) surface

We have used coaxial impact-collision ion scattering spectroscopy (CAICISS) and time-of-flight elastic recoil detection analysis (TOF-ERDA) to investigate the adsorption of atomic hydrogen on the 6H-SiC(0 0 0 1)√3×√3 surface. It has been found that the saturation coverage of hydrogen on the 6H-SiC(0 0 0 1)√3×√3 surface is about 1.7 ML. Upon saturated adsorption of atomic hydrogen, the √3×√3 surface structure changes to the 1×1 structure. The data of the CAICISS measurements have indicated that as a result of the hydrogen adsorption, Si adatoms on the √3×√3 surface move from T₄ to on-top sites.     

Density functional calculations for Ni adsorption on Al(1 1 0)

The adsorption of 0.25, 0.5 and 1 monolayer (ML) of the transition metal Ni on the metal substrate Al(1 1 0) was studied using first-principles calculations at the level of density functional theory. The metal–metal system was analyzed with the generalized gradient approximation. Four stable atomic configurations were considered, and the optimized geometries and adsorption energies of different Ni adsorption sites on the Al(1 1 0) surface at selected levels of coverage were calculated and compared. The four-fold hollow site was determined to be the most stable adsorption site with adsorption energy of 5.101 eV at 0.25 ML, 3.874 eV at 0.5 ML and 3.665 eV at 1 ML. The adsorption energies of the four sites slightly decreased as the Ni coverage increased. Work function analysis showed that when Ni is adsorbed on the Al(1 1 0) surface, the work function decreased as the coverage increased due to depolarization. The Mulliken population and density of states were calculated to determine the charge distribution of the adsorption site, confirming that a chemisorption interaction exists between the adsorbed Ni atom and Al(1 1 0) surface atoms.     

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.     

Reaction of tert-butyl isocyanate and tert-butyl isothiocyanate at the Ge(100) − 2 × 1 Surface

The adsorption of tert-butyl isothiocyanate and tert-butyl isocyanate at the Ge(100) ? 2 × 1 surface was probed using multiple internal reflection Fourier transform infrared (FTIR) spectroscopy X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) modeling. Results indicate that there are multiple surface products for each molecule. FTIR studies of tert-butyl isothiocyanate reveal adsorption through an S-dative bonded state, while XPS studies further suggest a reactive desorption product which leaves excess sulfur atoms at the surface. Studies of tert-butyl isocyanate indicate that the molecule dissociatively adsorbs at the surface, resulting in tert-butyl and germyl isocyanate groups, as the major pathway, in addition to forming several minor products, including a [2 + 2] cycloaddition product across the C=N bond. DFT was used to simulate vibrational spectra and map the reaction pathways, and confirms that the assigned products are energetically favorable.     

XPS and SRUPS study of oxygen adsorption on Cd0.9Zn0.1Te (1 1 1)A surface

Synchrotron radiation ultraviolet photoemission spectroscopy (SRUPS) and X-ray photoelectron spectroscopy (XPS) have been applied to investigate oxygen adsorption on a cadmium zinc telluride (CZT) (1 1 1)A surface. The surface chemical composition and the surface oxidation process were monitored by recording the Te 3d, O 1s, Zn 2p, Cd 4d core level peaks, and the Cd MNN Auger peak. The CZT (1 1 1)A surface was effectively oxidized by dosing oxygen directly. The typical surface state of the clean CZT (1 1 1)A surface was identified. After oxygen exposure, this surface state disappeared and a signal due to the formation of O–CZT appeared. In addition, the work function of CZT decreased with the increasing oxygen exposure.     

Sulphur adsorption on Au{1 1 0}: DFT and LEED study

The adsorption of sulphur on clean reconstructed Au{1 1 0}-(1 × 2) surface was studied using density functional theory (DFT) and quantitative low energy electron diffraction (LEED) calculations. The results show that the sulphur atoms form a (4 × 2) ordered structure which preserves the missing row reconstruction of the clean surface. The sulphur atom is found to adsorb on threefold hollow sites, on the {1 1 1} microfacets that border the trenches of the missing rows.     

Structural and dynamical properties of Ru(0 0 0 1) surface

I performed density functional theory (DFT) calculations combined with MD simulations to study the structural relaxation of Ru(0001) surface. The surface relaxation of the topmost layer is found to be about ?4% at absolute zero temperature. Using MD simulations in the temperatures range of 50 K and 900 K, the effect is found to be minor on the surface relaxation as compared to Pd (1 1 1) clean surface. The effect of surface vibration is also investigated using a LEED code and shows no effect of the vibrational level on the IV curves, which rules out any disagreement between proper theory and LEED results of well prepared surfaces.     

Surface structure and morphology of InAs(1 1 1)B with/without gold nanoparticles annealed under arsenic or atomic hydrogen flux

We have investigated the structure and morphology of the InAs(1 1 1)B surface using Low Energy Electron Diffraction (LEED), Scanning Tunneling Microscopy (STM) and Scanning Electron Microscopy (SEM). The surface was prepared by annealing in the presence of an arsenic or atomic hydrogen pressure. A (2 × 2) reconstruction that changes into a (1 × 1) unreconstructed surface after prolonged annealing was observed irrespective of preparation method, while the surface morphology was distinctly different in the two cases. Detailed atomic scale models are proposed to explain the behavior. Deposition of Au aerosol nanoparticles on the sample prior to annealing was found to have no effect on the surface reconstruction. The Au particles were found to sink into the surface.     

Chemisorption of isocyanate (NCO) on the Pd(1 0 0) surface at different coverages

The chemisorption of isocyanate (NCO) species on Pd(1 0 0) was studied within the density functional formalism (DFT) using a periodic slab model. The NCO was adsorbed on top, bridge and hollow sites of the metal surface at different coverages. At low coverages, the adsorption energies are in the range of ?2.5/?3.0 eV, indicating an important adsorbate–substrate interaction for the three sites studied. The lateral repulsive interaction between neighboring NCO species is almost negligible or weak at lower and intermediate coverages, and very strong at complete monolayer. At low coverages, both bridge and hollow sites are energetically preferred; yet the bridge site becomes the only favoured site at intermediate and complete coverages. Work function and dipole moment calculations can be interpreted by an important charge transfer from the metal surface to NCO. Interestingly, while on hollow site the charge taken by NCO is essentially the same over all the range of coverage, an increasing depolarization is observed on bridge and top sites as the coverage increases. Symmetric and asymmetric NCO stretching modes were also calculated and compared with recent infrared spectroscopy measurements.     

Interaction of NO with the O-rich RuO2(1 1 0) surface at 300 K

The interaction of NO with the O-rich RuO₂(1 1 0) surface, exposing coordinatively unsaturated O-bridge, O-cus, and Ru-cus atoms, was studied at 300 K by thermal desorption spectroscopy (TDS) and high-resolution electron energy-loss spectroscopy (HREELS). The conclusions are validated by isotope substitution experiments with ¹⁸O. During exposure to NO an O···N–O surface group (NO₂-cus) is formed with O-cus. Additionally, a smaller number of empty Ru-cus sites are filled by NO-cus. If one warms the sample to 400 K, NO₂-cus does not desorb but decomposes into O and NO again, the latter being either released into gas phase or adsorbed as NO-cus. With O-bridge such a surface group is not stable at 300 K. Our experiments further prove that O-cus is more reactive than O-bridge.     

First-principles study of small palladium clusters on NiAl(1 1 0) alloy surface

Theoretical calculations focused on the geometry, stability, electronic and magnetic properties of small palladium clusters Pd_n (n=1–5) adsorbed on the NiAl(1 1 0) alloy surface were carried out within the framework of density functional theory (DFT). In agreement with the experimental observations, both Ni-bridge and Al-bridge sites are preferential for the adsorption of single palladium atom, with an adsorption energy difference of 0.04 eV. Among the possible structures considered for Pd_n (n=1–5) clusters adsorbed on NiAl(1 1 0) surface, Pd atoms tend to form one-dimensional (1D) chain structure at low coverage (from Pd₁ to Pd₃) and two-dimensional (2D) structures are more stable than three-dimensional (3D) structures for Pd₄ and Pd₅. Furthermore, metal-substrate bonding prevails over metal–metal bonding for Pd cluster adsorbed on NiAl(1 1 0) surface. The density of states for Pd atoms of Pd/NiAl(1 1 0) system are strongly affected by their chemical environment. The magnetic feature emerged upon the adsorption of Pd clusters on NiAl(1 1 0) surface was due to the charge transfer between Pd atoms and the substrate. These findings may shade light on the understanding of the growth of Pd metal clusters on alloy surface and the construction of nanoscale devices.     

Ellipsometric detection of GaAs(0 0 1) surface hydrogenation in H2 atmosphere

Optical properties of MBE-grown GaAs(0 0 1) surfaces have been studied by spectroscopic ellipsometry under dynamic conditions of ramp heating and cooling after desorption of passivating As-cap-layer with low pressure H₂ atmosphere (14 Torr) applied to the surface. The temperature dependence of GaAs pseudo-dielectric function with atomically smooth (0 0 1) surface carrying the fixed Ga-rich (4 × 2) reconstruction was obtained for the temperature range of 160–600 °C. It is shown ellipsometrically that GaAs(0 0 1) heating in the molecular hydrogen atmosphere results in the formation of hydrogenated layer on the surface.     

An investigation of the effect of pre-dosed O atoms on the adsorption of NO on Pt{2 1 1}

Reflection absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD) have been used to investigate the effect of pre-dosed O atoms on the adsorption of NO on Pt{2 1 1} at room temperature. RAIRS experiments show that no new species are formed when NO is adsorbed onto a Pt{2 1 1} surface that has been pre-dosed with oxygen and no species are lost from the spectra, compared to spectra recorded for NO adsorption on the clean Pt{2 1 1} surface. However pre-dosed oxygen atoms do influence the frequency and intensity of several of the observed infrared bands. In stark contrast, pre-dosed O has a large effect on the TPD spectra. In particular N₂ and N₂O desorption, seen following NO adsorption on the clean Pt{2 1 1} surface, is completely inhibited. This effect has been assigned to the blocking of NO dissociation by the pre-adsorbed O atoms. A new NO desorption peak, not seen for NO adsorption on the clean Pt{2 1 1} surface, is also observed in TPD spectra recorded following NO adsorption on an oxygen pre-dosed Pt{2 1 1} surface.     

Preparation and characterization of Fe3O4(1 1 1) nanoparticles and thin films on Au(1 1 1)

Fe₃O₄ nanoparticles and thin films were prepared on the Au(1 1 1) surface and characterized using X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). Fe₃O₄ was formed by annealing α-Fe₂O₃(0 0 0 1) structures on Au(1 1 1) at 750 K in ultrahigh vacuum (UHV) for 60 min. Transformation of the α-Fe₂O₃(0 0 0 1) structures into Fe₃O₄ nanoparticles and thin films was supported by XPS. STM images show that during the growth procedure used, Fe₃O₄ initially appears as nanoparticles at low coverages, and forms thin films at ～2 monolayer equivalents (MLE) of iron. Two types of ordered superstructures were observed on the Fe₃O₄ particles with periodicities of ～50 and ～42 Å, respectively. As the Fe₃O₄ particles form more continuous films, the ～50 Å feature was the predominant superstructure observed. The Fe₃O₄ structures at all coverages show a hexagonal unit cell with a ～3 Å periodicity in the atomically resolved STM images.     

Acetaldehyde photochemistry on TiO2(1 1 0)

The ultraviolet (UV) photon induced decomposition of acetaldehyde adsorbed on the oxidized rutile TiO₂(1 1 0) surface was studied with photon stimulated desorption (PSD) and thermal programmed desorption (TPD). Acetaldehyde desorbs molecularly from TiO₂(1 1 0) with minor decomposition channels yielding butene on the reduced TiO₂ surface and acetate on the oxidized TiO₂ surface. Acetaldehyde adsorbed on oxidized TiO₂(1 1 0) undergoes a facile thermal reaction to form a photoactive acetaldehyde–oxygen complex. UV irradiation of the acetaldehyde–oxygen complex initiated photofragmentation of the complex resulting in the ejection of methyl radical into gas phase and conversion of the surface bound fragment to formate.     

Effects of ultrasonic agitation and surfactant additive on surface roughness of Si (1 1 1) crystal plane in alkaline KOH solution

In the silicon wet etching process, the “pseudo-mask” formed by the hydrogen bubbles generated during the etching process is the reason causing high surface roughness and poor surface quality. Based upon the ultrasonic mechanical effect and wettability enhanced by isopropyl alcohol (IPA), ultrasonic agitation and IPA were used to improve surface quality of Si (1 1 1) crystal plane during silicon wet etching process. The surface roughness R_q is smaller than 15 nm when using ultrasonic agitation and R_q is smaller than 7 nm when using IPA. When the range of IPA concentration (mass fraction, wt%) is 5–20%, the ultrasonic frequency is 100 kHz and the ultrasound intensity is 30–50 W/L, the surface roughness R_q is smaller than 2 nm when combining ultrasonic agitation and IPA. The surface roughness R_q is equal to 1 nm when the mass fraction of IPA, ultrasound intensity and the ultrasonic frequency is 20%, 50 W and 100 kHz respectively. The experimental results indicated that the combination of ultrasonic agitation and IPA could obtain a lower surface roughness of Si (1 1 1) crystal plane in silicon wet etching process.     

Coverage effects on the adsorption of sulfur on Co(0 0 0 1): A DFT study

The adsorption of sulfur on Co(0 0 0 1) was studied using density functional theory calculations at coverage from 0.11 ML to 1.0 ML. Calculated results indicate that atomic S favors in hollow sites with bond S–Co dominated at lower coverage and at higher coverage the strong adsorbate S–S interaction leads to the formation of S₂ species. It was shown that the adsorption energy generally increases (gets weaker) with the coverage in a near linear fashion for the most stable configurations. In addition, modification of the surface electronic properties has been discussed and some discrepancy are found between our calculations and the findings of O adsorption on Au(1 1 1) and Pt(1 1 1) surfaces.     

Metallization of Ge(0 0 1)-p(2 × 1) surface as result of thermal fluctuations

We present a theoretical study of the metallization of Ge(0 0 1)-p(2 × 1) surface which is observed in experimental data. We have considered the connection between thermal fluctuation of this surface structure and its metallic properties. To this end we have performed long-time MD-DFT simulations. The obtained results show that thermal fluctuation of the Ge(0 0 1)-p(2 × 1) structure may cause its metallization which in not necessary connected with a flip-flop motion of dimer atoms. It was shown that the metallization of the Ge(0 0 1)-p(2 × 1) surface takes place when the dimer buckling angle is reduced to around 11°. In the case of our simulations the considered surface system remained in the metallic state for 25% of the simulation time. We have also found that the metallic state of the fluctuating Ge(0 0 1)-p(2 × 1) surface is built up by dangling bonds of the dimer atoms shifted up (D_up) and down (D_down).     

Study of CO diffusion on stepped Pt(1 1 1) surface by scanning tunneling microscopy

We have used a time-dependent tunneling current mode based on scanning tunneling microscopy/spectroscopy (STM/STS) to study the tracer diffusion of CO molecules along steps and on terraces of Pt(1 1 1). The results show that the hopping rate of CO molecules along steps is about 10 times faster than that on terraces in the measured temperature range. The diffusion activation energies are 5.1 kcal/mol and 3.8 kcal/mol on terraces and along steps, respectively. The lower activation energy and faster hopping rate for CO molecules diffusing along steps provide evidence that steps provide fast diffusion channels for CO molecules on stepped Pt(1 1 1) surfaces.