Showing papers by "Fritz Haber Institute of the Max Planck Society published in 1980"

Observation of the Surface State Resonance Effect by the Convergent Beam RHEED Technique

Abstract: The angular dependence of the specular spot intensity reflected from a ZnS (110) surface has been analyzed by using Bethe's: dynamical theory. Many-beam calculations have been carried out with simple surface potential models. The calculations showed that the angular dependence of the intensity changed very sensitively with the surface potential shape at the surface state resonance positions. One of the models was in very good qualitative agreement with the present experimental result.

The Applicability of Semiconducting Layered Materials for Electrochemical Solar Energy Conversion

Abstract: Semiconducting transition metal dichalcogenides with layer structure belong to the most stable electrode materials in electrochemical solar cells. The photoelectrochemical reactions however, are complicated by the surface anisotropy. Results of photoelectrochemical cells with n-type MoSe2 and WSe2 demonstrate the importance of surface morphology for the energy conversion efficiency. — The optimal energy conversion yield of these layered semiconductors has turned out being lower than theoretically expected from their band gaps. It is shown that this is connected with the existence of an indirect band gap which restricts light absorption in a considerable energy quantum range above the band gap. Besides this, surface defects and inhomogeneities reduce the quantum yield by increased surface recombination. — Photoelectrochemical hydrogen evolution has been investigated with p-type WSe2. It is shown that a considerable energy gain can be reached by the catalytic action of platinum deposits on the surface of the electrode. This is explained in terms of surface state catalysis. Halbleitende Ubergangsmetalldichalkogenide mit Schichtgitterstruktur gehoren zu den bestandigsten Elektrodenmaterialien fur elektrochemische Solarzellen. Photoelektrochemische Reaktionen werden jedoch durch die Oberflachenanisotropie kompliziert. Ergebnisse von photoelektrochemischen Zellen mit n-leitendem MoSe2 und WSe2 zeigen die Bedeutung der Oberflachenmorphologie fur den Wirkungsgrad der Energieumwandlung. — Der optimale Energieumwandlungswirkungsgrad dieser Schichtgitterhalbleiter erwies sich als geringer, als man gemas der Bandlucke theoretisch erwarten konnte. Es wird gezeigt, das dies mit der Existenz einer indirekten Bandlucke in Zusammenhang steht, welche eine sehr schwache Lichtabsorption in einem betrachtlichen Energiebereich oberhalb der Bandkante bedingt.

Photoelectrochemical Reactions and Formation of Inversion Layers at n-Type MoS2-, MoSe2-, and WSe2-Electrodes in Aprotic Solvents

Abstract: Capacity measurements and cyclic voltammetry have been used to conduct a systematic investigation of the potential distribution at the semiconductor-organic electrolyte interface in the dark and under illumination with the n-type synthetic layered compounds MoS2, MoS2, and WSe2. From the obtained flat band potentials it could be concluded that the valence band edges vs. Ag/Ag+ are located in acetonitrile at ca. 1.0 V (MoS2), 0.7 V (MoSe2), and 0.4 V (WSe2). As theoretically expected, the oxidized component of redox systems with Fermi levels near or below the valence band edge can inject holes thus inducing an inverted region and generating a p-n junction in the solid. The cathodic current in the cyclic voltammograms of such redox systems occurs via hole injection with consecutive recombination of the injected holes while less oxidizing redox systems can only be reduced via the conduction band. — Since the semiconductors used in these experiments do not corrode in presence of a high surface concentration of holes, such a contact with an oxidizing redox electrolyte offers most favourable conditions for the application in a photoelectrochemical solar cell. The inversion layer has a maximal band bending and promises to obtain the highest possible photovoltage, which is however limited by the width of the band gap. Mit Hilfe von Kapazitatsmessungen und zyklischer Voltametrie wurde die Potentialverteilung an der Grenzflache Halbleiter/organischer Elektrolyt im Dunkeln und unter Belichtung untersucht. Aus der Lage der Flachbandpotentiale synthetischer Schichtgitterkristalle von MoS2, MoSe2 und WSe2 in Acetonitril-Elektrolyten konnten die Lagen der Valenzbandkanten ermittelt werden. Sie liegen bei 1,0 V (MoS2), 0,7 V (MoSe2) und 0,4 V (WSe2) gegenuber einer Ag/Ag+-Elektrode in Acetonitril. Entsprechend der theoretischen Erwartung konnen Redoxsysteme mit Fermi-Niveaus nahe oder unterhalb der Valenzbandkante Defektelektronen in der Oberflache dieser Halbleiter injizieren. Dadurch entsteht eine Inversionsschicht und somit ein p-n-Ubergang im Festkorper. Der kathodische Strom in den zyklischen Voltamogrammen mit solchen Redoxsystemen erfolgt durch Injektion von Defektelektronen, wonach diese in der Inversionsschicht mit Elektronen rekombinieren. Dagegen konnen weniger oxidierende Redoxsysteme nur uber das Leitungsband reduziert werden. — Die Halbleiter, die hier untersucht wurden, korrodieren in diesem Medium nicht, obwohl die Oberflache stark p-leitend ist. Solch ein Kontakt mit einem oxidierenden Redoxsystem bietet daher besonders gunstige Voraussetzungen fur die Verwendung in photoelektrochemischen Solarzellen. Die Inversionsschicht besitzt eine maximale Bandverbiegung und fuhrt daher zur Erzeugung der grostmoglichen Photospannung, die allerdings durch die Weite der Bandlucke begrenzt wird.

Magnetic Exchange Splitting of Electronic Surface States on Ni(110)

Abstract: The temperature-dependent magnetic exchange splitting of an electronic surface state located at the $\overline{S}$ point of the Ni(110) surface Brillouin zone has been measured. The measured value of the splitting is 0.30\ifmmode\pm\else\textpm\fi{}0.02 eV at 100 K which is equal to the exchange splitting of the bulk bands of Ni (0.31\ifmmode\pm\else\textpm\fi{}0.04 eV) from which this surface state is derived. This result directly rules out the existence of a paramagnetic or magnetically "dead" surface layer on Ni(110).

The surface tension of poly(acrylic acid) in aqueous solution

Abstract: Surface tension has been measured of polyacrylic acidwater solutions with pH 3–11. Until approximately pH 4.6 corresponding to 10% neutralization, PAA-solutions are surface active and inactive at higher pH, time dependence of equlibrium surface tension is analysed. Non-ionized “neutral” chain segments are surface active, their diffusion to surface is rate determining.

A New Infrared Luminescence of Nickel-Doped ZnS and Its Interpretation by Means of Absorption Spectroscopy with Ni2+ Ions

Abstract: At low temperatures, a new emission band is detected with Ni-doped ZnS crystals. Zero-phonon transitions at 4379 and 4199 cm−;1 and phonon side bands are resolved. The luminescence is interpreted as a 3T2(F) 3T1(F) transition of substitutional Ni2+(d8) ions in a tetrahedral crystal field. This assignment is verified by the coincidence of (i) the fine structure in the emission and (ii) its excitation spectrum with the known absorption spectrum of the Ni2+ ion. The crystals exhibit all of the known internal absorption transitions of ZnS:Ni2+ and some weaker additional finestructured details which are in accord with the proposed interpretation. An Nickel-dotierten ZnS-Kristallen wird bei tiefen Temperaturen eine neue Emissionsbande nachgewiesen. Es werden Nullphonon-Ubergange bei 4379 und 4199 cm−;1 sowie Phononenseitenbanden aufgelost. Die Lumineszenz wird als 3T2(F) 3T1(F)-Ubergang substitutioneller Ni2+(d8) Ionen in einem tetraedrischen Kristallfeld gedeutet. Diese Zuordnung wird verifiziert durch Ubereinstimmung (1) der Emissions-Feinstruktur und (2) ihres Anregungsspektrums mit dem bekannten Absorptionsspektrum des Ni2+-Ions. Die Kristalle zeigen alle bekannten inneren Absorptionsubergange von ZnS:Ni2+ und weitere schwachere feinstrukturierte Details, die mit der vorgeschlagenen Interpretation ubereinstimmen.

Density, refractive index and sorption capacity of solid CO2 layers

Abstract: The density and refractive index of solid CO2 layers were measured by an interferometric method, as was the sorption capacity of these layers for hydrogen, as a function of the condensation conditions such as temperature, growth rate and layer thickness. These properties, i.e. the density ϱ, refractive index n and sorption capacity, are strongly influenced by the condensation temperature. Above ≈70 K, ϱ and n of solid CO2 layers agree well with those given in the literature for the bulk crystal, but below 70 K they decrease monotonically with decreasing condensation temperature. The charges of ϱ and n for a condensation temperature of 10 K are 40% and 16%, respectively. Below about 50 K, the sorption capacity of solid CO2 layers for H2 becomes measurable and increases considerably with decreasing condensation temperature indicating a porous structure of these layers. At about 20 K the maximum value of 220 mmol H2/mol CO2 has been measured. Below about 10 K drastic changes of the layer properties occur; the sorption capacity reduces considerably and an increase of the density is observed. This behaviour is due to spontaneous transitions of the layer structure during the CO2 condensation, which can be observed by a sudden increase of the layer temperature as well as by an abrupt change of the interference pattern. The frequency of these spontaneous transitions increases with decreasing condensation temperature and increasing deposition speed. The released energy was measured by 7 K and estimated to be ≈ 1% of the total condensation energy of the CO2 quantity condensed after a previous transition.

Monolayers of some liquid-crystal forming compounds

Abstract: A study of monolayer characteristics of some thermotropic liquid-crystal forming molecules on water surfaces has been carried out. Monolayers of horizontal rigid molecules with two polar heads collapse on compression by a “roll over” mechanism. Variation of temperature within an appropriate range gradually changes from liquid-like to a crystal-like collapse. Between these two extremes a combination of both states could be obtained. The state of the corresponding film is attributed to mesogenic properties of the compounds investigated. This investigation proves that some mesogenic compounds are able to form liquid-crystals on the water surface.

Determination of excitonic parameters of the A polariton of CdS from magnetoreflectance spectroscopy

Abstract: The magnetoreflectance of the $A$ exciton of CdS is studied for fields up to 10 tesla for the orientations $\stackrel{\ensuremath{\rightarrow}}{\mathrm{H}}\ensuremath{\perp}\stackrel{\ensuremath{\rightarrow}}{\mathrm{c}}$ and $\ensuremath{\measuredangle}(\stackrel{\ensuremath{\rightarrow}}{\mathrm{H}}, \stackrel{\ensuremath{\rightarrow}}{\mathrm{c}})=45\ifmmode^\circ\else\textdegree\fi{}$. A quantitative line-shape analysis is made of the spectra using the model of polaritons with spatial dispersion and Pekar's $\mathrm{abc}$ and, considering damping and exciton-free surface layer it shows a remarkable good agreement between experiments and calculations. This allows a very distinct investigation of the splitting parameters of the exciton and the corresponding bands. The $g$ values are measured to be ${g}_{e\ensuremath{\perp}}={g}_{e\ensuremath{\parallel}}=1.75\ifmmode\pm\else\textpm\fi{}0.05$ and ${g}_{A\ensuremath{\parallel}}=1.25\ifmmode\pm\else\textpm\fi{}0.1$. The longitudinal-transverse $\mathrm{H}({\ensuremath{\Delta}}_{\mathrm{LT}})$ and, for the first time, the singlet-triplet splitting (${\ensuremath{\Delta}}_{56}$), both resulting from exchange interaction, are determined to ${\ensuremath{\Delta}}_{\mathrm{LT}}=(2.1\ifmmode\pm\else\textpm\fi{}0.03)$ meV and ${\ensuremath{\Delta}}_{56}=(0.2\ifmmode\pm\else\textpm\fi{}0.03)$ meV.

Analysis of the crystal arrangement in collagen fibrils of mineralizing turkey tibia tendon

Abstract: Mineralized pieces of tendons from the tibio-tarsus of turkeys were (i) shock-frozen, freeze-dried, embedded and cut without staining, or (ii) fixed, embedded and stained after sectioning. Micrographs were taken with an electron microscope on longitudinally cut sections. The center-to-center distances of neighboring apatitic needles within collagen fibrils were measured. For shock-frozen and freeze-dried specimens, the average of these distances is 4.7 nm and the most frequent value 4.2 nm; for fixed and stained specimens, 3.8 nm and 3.6 nm, respectively. Laser diffraction of the electron micrographs showed a dumbbell-like intensity pattern (two diffuse maxima of intensity on the equator, one on each side of the central spot), giving an average distance of about 6 nm. This value represents the upper range of the direct measurements. The measurements demonstrate that the arrangement of the collagen microfibrils is mainly preserved during mineralization. However, using laser diffraction, distances of 9–11 nm were also observed. Such large distances can also be demonstrated by X-ray diffraction on collagen fibrils stained under special conditions. This may indicate that special conditions of apatitic mineralization or staining may alter the arrangement of the microfibrils.