H. Nienhaus

Electronic excitations by chemical reactions on metal surfaces

Abstract Dissipation of chemical energy released in exothermic reactions at metal surfaces may happen adiabatically by creation of phonons or non-adiabatically by excitation of the electronic system of the metal or the reactants. In the past decades, the only direct experimental evidence for such non-adiabatic reactions has been exoelectron emission into vacuum and surface chemiluminescence which are observed in a special class of very exothermic reactions. The creation of e–h pairs in the metal has been discussed in many theoretical models but it was only recently that a novel experimental approach using Schottky diodes with ultrathin metal films makes direct measurement of reaction-induced hot electrons and holes possible. The chemical reaction creates hot charge carriers which travel ballistically from the metal film surface toward the Schottky interface and are detected as a chemicurrent in the diode. By now, such currents have been observed during adsorption of atomic hydrogen and deuterium on Ag, Cu and Fe surfaces as well as chemisorption of atomic and molecular oxygen, of NO and NO 2 molecules and of certain hydrocarbons on Ag. This paper reviews briefly exoelectron and chemiluminescence experiments and the concept of the Norskov–Newns–Lundqvist model. The major part is devoted to the detection of chemically induced e–h pairs with thin metal film Si Schottky diodes by discussing the different influences on the chemicurrent magnitude and presenting experimental results predominantly with hydrogen and deuterium atoms. The experiments introduce a new method to investigate surface reaction kinetics and dynamics by use of an electronic device. In addition, the diodes may be used as selective reactive gas sensors.

In situ non-DLVO stabilization of surfactant-free, plasmonic gold nanoparticles: effect of Hofmeister's anions.

Specific ion effects ranking in the Hofmeister sequence are ubiquitous in biochemical, industrial, and atmospheric processes. In this experimental study specific ion effects inexplicable by the classical DLVO theory have been investigated at curved water-metal interfaces of gold nanoparticles synthesized by a laser ablation process in liquid in the absence of any organic stabilizers. Notably, ion-specific differences in colloidal stability occurred in the Hückel regime at extraordinarily low salinities below 50 μM, and indications of a direct influence of ion-specific effects on the nanoparticle formation process are found. UV-vis, zeta potential, and XPS measurements help to elucidate coagulation properties, electrokinetic potential, and the oxidation state of pristine gold nanoparticles. The results clearly demonstrate that stabilization of ligand-free gold nanoparticles scales proportionally with polarizability and antiproportionally with hydration of anions located at defined positions in a direct Hofmeister sequence of anions. These specific ion effects might be due to the adsorption of chaotropic anions (Br(-), SCN(-), or I(-)) at the gold/water interface, leading to repulsive interactions between the partially oxidized gold particles during the nanoparticle formation process. On the other hand, kosmotropic anions (F(-) or SO4(2-)) seem to destabilize the gold colloid, whereas Cl(-) and NO3(-) give rise to an intermediate stability. Quantification of surface charge density indicated that particle stabilization is dominated by ion adsorption and not by surface oxidation. Fundamental insights into specific ion effects on ligand-free aqueous gold nanoparticles beyond purely electrostatic interactions are of paramount importance in biomedical or catalytic applications, since colloidal stability appears to depend greatly on the type of salt rather than on the amount.

Selective H atom sensors using ultrathin Ag/Si Schottky diodes

Schottky diodes with ultrathin silver films on n-Si(111) are used for selective detection of atomic hydrogen. The exothermic adsorption of H atoms on the Ag surface creates hot electrons which may travel ballistically through the metal film and traverse the Schottky barrier. The chemically induced current is measurable (≈0.001 electrons/H atom) as a chemicurrent. After saturation of the adsorption sites, this chemicurrent achieves a steady-state value due to a balance of removal of adsorbed hydrogen and readsorption. The detection limit of the sensors is approximately 1010 H atoms cm−2 s−1. The detectors are completely insensitive to H2 molecules.

Direct detection of electron–hole pairs generated by chemical reactions on metal surfaces

Abstract Non-adiabatic energy dissipation during exothermic chemical reactions on metal surfaces occurs by creation of electron–hole pairs in the metal. The excited charge carriers have been directly detected using metal–silicon (Schottky) diodes with ultrathin metal films. The chemically created hot electrons travel ballistically through the metal film, traverse the Schottky barrier and are detected as a chemicurrent in the diode. Three examples are presented, i.e. the adsorption of atomic hydrogen on Ag and Fe and the chemisorption of molecular oxygen on Ag. The chemicurrent transients upon exposure are related to the kinetics of the surface reaction. The large difference in the detection efficiency between Ag/Si and Fe/Si diodes is attributed to different conditions at the metal/silicon interface.

Phonons in 3C-, 4H-, and 6H-SiC

Abstract Silicon carbide epilayers of cubic (3C) and hexagonal (4H and 6H) polytypes were investigated by Auger electron spectroscopy, high-resolution electron energy-loss spectroscopy and Raman spectroscopy to determine the excitation energies of the optical Fuchs-Kliewer surface phonons and their relation to bulk phonon frequencies. The surfaces were treated in a buffered hydrofluoric acid solution. Loss structures attributed to excitation of Fuchs-Kliewer phonons were clearly resolved. Their energies were found at 115.9 ± 1 meV irrespective of the SiC polytype. The experimental data agree with values calculated from the experimental bulk phonon frequencies and tabulated dielectric constants.

Detection of chemically induced hot charge carriers with ultrathin metal film Schottky contacts

Energy dissipation during chemical reactions at metal surfaces may excite electron-hole pairs in the metal. Direct detection of such reaction-induced hot electrons and holes is feasible using solid state barrier devices like Schottky diodes with ultrathin metal films. While exposing the diodes to reactive gases, a chemicurrent is observed in the diodes. The concept of hot charge carrier detection by chemicurrent measurements and the dependence of the current strength on device properties are discussed in detail. Data recorded from thin film Cu/n-Si(1 1 1) and Ag/n-Si(1 1 1) diodes exposed to atomic hydrogen and atomic oxygen are presented. The current detection sensitivity is improved by a factor of 10 if the metal films are annealed to room temperature after low-temperature deposition. This annealing effect is related to a reduced scattering of hot electrons in the metal. Chemicurrents are attenuated exponentially with increasing metal film thickness. Attenuation lengths between 6 and 11 nm are observed. They are much smaller than attenuation lengths for photo- and internal photoemission currents. The results demonstrate that chemicurrents are due to hot charge carrier excitation and transport and are not attributed to surface chemiluminescence and photon reabsorption in the device.

Oxidation stages of clean and H-terminated Si(001) surfaces at room temperature

Abstract The oxygen uptake on clean Si(001)-2 × 1 and H-terminated Si(001)-l × 1 surfaces at room temperature was investigated by Auger electron spectroscopy (AES) and low-energy electron diffraction (LEED). Surfaces were cleaned by Ar + -ion sputtering and annealing at 1200 K. H-terminated surfaces were prepared by etching of thermally oxidized samples in hydrofluoric acid. The samples were then exposed to research grade oxygen in the range from 10 14 to 10 30 O 2 -molecules/cm 2 . During exposures any excitations of the surface or the gas were avoided. The uptake of oxygen on clean surfaces proceeds in two subsequent steps. The first process saturates at about 1 monolayer and may be attributed to dissociative chemisorption. The second process sets in at a dose of 10 19 O 2 -molecules/cm 2 and follows an inverse-logarithmic growth law. It may be described by field-assisted oxidation (Mott-Cabrera mechanism). The results are compared with similar data for Si(111) surfaces. Irrespective of surface orientation and reconstruction, the oxidation process starts always at the same exposure whereas the initial sticking coefficient and the rate of oxidation depend on the orientation of the investigated surfaces. On HF-treated surfaces, the oxygen adsorption is strongly inhibited: the sticking coefficient amounts only to approximately 10 −12 .

Stabilization of mid-sized silicon nanoparticles by functionalization with acrylic acid

We present an enhanced method to form stable dispersions of medium-sized silicon nanoparticles for solar cell applications by thermally induced grafting of acrylic acid to the nanoparticle surface. In order to confirm their covalent attachment on the silicon nanoparticles and to assess the quality of the functionalization, X-ray photoelectron spectroscopy and diffuse reflectance infrared Fourier spectroscopy measurements were carried out. The stability of the dispersion was elucidated by dynamic light scattering and Zeta-potential measurements, showing no sign of degradation for months.

Surface phonons in InP(110)

Abstract The surface phonon dispersion of cleaved InP(110) surfaces was measured by use of high-resolution electron energy-loss spectroscopy (HREELS) along the symmetry directions gGX and gGX′ , i.e., parallel and perpendicular to the surface In-P chains. Besides an acoustic surface phonon branch with energy lower than 9 meV, four optical phonon bands with little dispersion were recorded at 9.5, 19.5, 30.5 to 33.5, and 42.3 meV. The band with excitation energies around 32 meV represents a true surface mode as it is located in the energy gap between the acoustic and optical bulk phonon bands. The results are compared with available calculations of InP(110) surface phonons.

Ultrathin Ag films on H:Si(111)-1×1 surfaces deposited at low temperatures

The growth, structure, and electronic properties of thin Ag films on H-terminated Si(111) surfaces were investigated with Auger electron and photoelectron spectroscopy (and atomic force and secondary electron microscopy). The films were either evaporated at room temperature (RT) or deposited at low temperature (LT) and subsequently annealed to RT in the thickness range between 1 and 50 monolayers (0.2–12 nm). The LT preparation leads to large Ag islands on a wetting monolayer which form a continuous Ag film above a critical thickness of 30 monolayers. Ultraviolet photoelectron spectra and work function measurements reveal a (111) surface orientation of the Ag islands. In constrast, RT deposition results in Stranski-Krastanov growth of smaller and irregularly shaped islands which do not form a continuous layer even up to film thicknesses of 45 monolayers.