W. Daum

CO stretching vibrations on Pt(111) and Pt(110) studied by sumfrequency generation

Abstract Optical sum-frequency generation (SFG) is used to characterize CO stretching vibrations on Pt(111) and Pt(110) surfaces. Different adsorption sites (terminal, bridge and step sites) are identified in the SFG spectra of CO on Pt(111), in good quantitative agreement with previous infrared reflection-absorption experiments on this system. For CO on Pt(110) we only observe CO molecules on terminal sites. The measured CO stretching vibration frequencies on Pt(110), both for low and high coverages, are at variance with the results of previous infrared studies. Our SFG results for CO on Pt(110) are confirmed by independent EELS measurements which, in addition, also reveal the frustrated rotational mode and the metal-CO vibration. The measured frequency of 2065 cm −1 for low CO coverage on Pt(110)-(1 × 2) is consistent with a previously proposed empirical relation between the frequency of an isolated adsorbed CO molecule and the coordination number of the binding Pt surface atom.

Surface phonon dispersion and adsorbate induced reconstruction of Ni(100)

Abstract The Ni(100) surface reconstructs upon adsorption of half a monolayer of nitrogen. This observation is at variance with a previous report. The reconstructed phase consists of a c(2 × 2) overlayer of nitrogen with a clockwise and counterclockwise rotation of the nickel surface atoms around two nearest neighbor nitrogen atoms, respectively. The same p4g(2 × 2) reconstruction was also observed with carbon overlayers. Recent total energy calculations and lattice dynamic analysis have shown that this reconstruction is driven by internal stresses caused by a repulsive interaction between the first layer nickel atoms. We have measured the phonon dispersion branches of the p4g(2 × 2) nitrogen covered surface along the [110] direction. Two adsorbate induced phonon branches above the bulk phonon branch of nickel are found. The branches are similar to those found for the carbon covered surface and indicate that nitrogen sits close to the surface in the fourfold hollow site. In addition to the adsorbate associated modes surface resonances inside the bulk band and the Rayleigh phonon have been measured. The various modes are identified by comparing experimental results to lattice dynamical calculations.

Superstructures and Order−Disorder Transition of Sulfate Adlayers on Pt(111) in Sulfuric Acid Solution

The surface structure of Pt(111) in a 0.1 M H2SO4 electrolyte was investigated in the potential range of sulfate adsorption with electrochemical scanning tunneling microscopy (STM) and cyclic voltammetry. Two ordered anion structures were observed coexisting in the potential range between 0.49 and 0.79 V (vs RHE): the well-known (radical3xradical7)R19.1 degrees superstructure with an anion coverage of 0.20 monolayer and a new, high-density (3x1) superstructure with a coverage of 0.33 monolayer. Both superstructures undergo a reversible order-disorder transition at 0.8 V. Simultaneous imaging of the adsorbed ions and of topographic details of the Pt substrate lattice allows us to study the local adsorption geometry of the sulfate. In the (radical3xradical7)R19.1 degrees, structure the sulfate ions are adsorbed close to depressions in the STM image of the Pt substrate which may be identified with face-centered cubic (fcc) hollow sites. In addition to the sulfate ions, a coadsorbed species, possibly water molecules, is observed in the unit cell of the (radical3xradical7)R19.1 degrees superstructure. Preliminary potentiodynamic STM data indicate that the transformation of the ordered sulfate adlayer into a disordered structure at 0.8 V is not directly related to adsorption/desorption features in the voltammogram commonly attributed to the adsorption/desorption of OH, and that the sulfate adlayer remains on the surface for potentials well above the adsorption potentials of OH.

In-situ spectroscopy of cyanide vibrations on Pt(111) and Pt(110) electrode surfaces: potential dependencies and the influence of surface disorder

The stretching vibrations of cyanide (CN-) ions adsorbed on single-crystalline Pt electrodes in aqueous electrolytes were measured in-situ by optical sum-frequency generation (SFG). Adsorption of CN- on well-ordered Pt(111) and Pt(110) surfaces is characterized by a stretching vibration with a strongly potential-dependent frequency, ranging from about 2090 cm -1 near the potential of hydrogen evolution to values of about 2150 cm -1 (Pt(111)) and 2130 cm -1 (Pt(110)) for potentials close to Pt oxidation. At negative potentials, a second band with a frequency around 2090 cm -1 is resolved on Pt(111). A vibrational band around 2147 cm -1 with very small potential dependence is related to cyanide on microscopically disordered surfaces. This band, previously observed on polycrystalline Pt electrodes as well as on Pt(110) and Pt(100), appears after application of oxidation-reduction cycles to single-crystalline electrode surfaces and is assigned to a surface intermediate state of [Pt(CN) 4 ] 2- complex formation.

A Highly Selective and Self‐Powered Gas Sensor Via Organic Surface Functionalization of p‐Si/n‐ZnO Diodes

Selectivity and low power consumption are major challenges in the development of sophisticated gas sensor devices. A sensor system is presented that unifies selective sensor-gas interactions and energy-harvesting properties, using defined organic-inorganic hybrid materials. Simulations of chemical-binding interactions and the consequent electronic surface modulation give more insight into the complex sensing mechanism of selective gas detection.

Band engineered epitaxial 3D GaN-InGaN core-shell rod arrays as an advanced photoanode for visible-light-driven water splitting.

3D single-crystalline, well-aligned GaN-InGaN rod arrays are fabricated by selective area growth (SAG) metal-organic vapor phase epitaxy (MOVPE) for visible-light water splitting. Epitaxial InGaN layer grows successfully on 3D GaN rods to minimize defects within the GaN-InGaN heterojunctions. The indium concentration (In ∼ 0.30 ± 0.04) is rather homogeneous in InGaN shells along the radial and longitudinal directions. The growing strategy allows us to tune the band gap of the InGaN layer in order to match the visible absorption with the solar spectrum as well as to align the semiconductor bands close to the water redox potentials to achieve high efficiency. The relation between structure, surface, and photoelectrochemical property of GaN-InGaN is explored by transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS), Auger electron spectroscopy (AES), current-voltage, and open circuit potential (OCP) measurements. The epitaxial GaN-InGaN interface, pseudomorphic InGaN thin films, homogeneous and suitable indium concentration and defined surface orientation are properties demanded for systematic study and efficient photoanodes based on III-nitride heterojunctions.

Sum-frequency generation at electrochemical interfaces: Cyanide vibrations on Pt(111) and Pt(110)

We use optical sum-frequency generation to investigate the stretching vibrations of cyanide (CN−) molecules chemisorbed from aqueous electrolytes on single-crystalline Pt(111)- and Pt(110)-electrode surfaces. For clean and well-ordered Pt(111) electrodes, a single vibrational band between 2080 and 2150 cm−1 with a nonlinear frequency dependence on the potential is observed and assigned to the C≡N stretching vibration of chemisorbed cyanide. A second band between 2145 and 2150 cm−1 with very weak potential dependence appears on a surface which was subjected to oxidation-reduction cycles and is attributed to cyanide associated with a microscopically disordered surface. This assignment is supported by preliminary results for a Pt(110) single-crystal electrode. On a well-ordered (110) surface a single and potential-dependent cyanide vibration between 2070 and 2112 cm−1 is observed. After oxidation of the cyanide and readsorption, this band is replaced by a higher frequency band at 2144 cm−1 which is essentially not potential-dependent. Occasionally, additional vibrational bands at lower frequencies not reported in corresponding IR studies are observed on Pt(111).

Nonlinear optical spectroscopy at silicon interfaces

Interface properties of oxidized, clean and hydrogen-terminated silicon surfaces have been studied by optical second-harmonic generation (SHG) and sum-frequency generation (SFG) using tunable laser pulses. A strong resonance band at 3.3eV photon energy is observed in SHG and SFG spectra of clean and various oxidized Si(100) and Si(111) surfaces. In the case of oxidized surfaces, the appearance of the band strongly depends on the treatment of the sample, but not on oxide thickness. Spectra of hydrogen-terminated surfaces prepared by wet chemistry do not reveal appreciable resonant intensity around 3.3eV thus proving the distinct interface character of the band. Chemisorption experiments performed on clean silicon surfaces rule out transitions between surface states as the reason for the observed band. It is concluded that the resonance is caused by direct band gap transitions, derived from the E1 transitions at 3.4eV in bulk silicon, that take place in a thin layer of silicon atoms with strained bonds underneath the surface atoms of clean Si surfaces and at the Si-SiO2 interface. The commonly observed U-shaped distribution of interface defect states in Si-SiO2 structures is discussed in the light of our results.

The lattice vibrations of nearly incommensurate overlayers Ag on Ni(100) and Cu(100)

Abstract A chemisorbed monolayer may form and N × M overlayer, where N or M (or both) may be large. We formulate an approximate description of the lattice vibrations in such structures that includes coupling between the overlayer vibrations, and the substrate phonons. The analysis is a lattice dynamical analogue of Brillouin-Wigner perturbation theory; the scheme is accurate, and readily employed to generate surface spectral densities. A key feature is the ability to analyze the response characteristics for wave vectors in the surface Brillouin zone of the substrate. This greatly simplifies the task of interpreting the output of the numerical computational schemes. We also discuss experimental data on the surface vibrations of a monolayer of Ag on the Ni(100) and the Cu(100) surface, which are closely approximated by a (2 × 8) and (2 × 10) structure, respectively. We achieve a quantitative fit with a simple model, and discuss the physical origin of the interaction between adsorbate and substrate phonons, in regions of the substrate surface Brillouin zone where the adsorbate vibration frequencies lie well below those of the substrate phonons of the same wave vector.

High-quality ZrO2/Si(001) thin films by a sol-gel process: Preparation and characterization

ZrO2 films with a sub-10-nm thickness and a roughness of about 0.2 nm have been prepared on Si(001) by a sol-gel process based on zirconium-(IV)-n-propoxide. The topography of the obtained samples has been controlled by atomic force microscopy. Chemical composition and interface reactions of the deposited films have been studied by x-ray photoemission spectroscopy and Auger electron spectroscopy. The ZrO2 films are stable against heating (up to 700 °C) in a moderate oxygen atmosphere (2×10−5 mbar oxygen partial pressure). Minor changes in the surface composition occur after rapid annealing up to 1000 °C.