G. Wedler

Bio: G. Wedler is an academic researcher from University of Erlangen-Nuremberg. The author has contributed to research in topics: Hall effect & Electrical resistivity and conductivity. The author has an hindex of 8, co-authored 8 publications receiving 224 citations.

Growth of Fe on Si (100) at room temperature and formation of iron silicide

Abstract: Low-energy electron diffraction (LEED), Auger electron spectroscopy and X-ray photoelectron spectroscopy (XPS) investigations of both the growth of an iron film on silicon (100) at room temperature and the subsequent formation of iron silicide are the subjects of this paper. An in-situ cleaned silicon (100) wafer without carbon or oxygen contamination exhibiting the known 2 × 1 reconstruction in the LEED pattern served as the substrate. Iron was deposited on this reconstructed surface at 300 K. The comparison of theoretical calculations based on three growth mechanisms with XPS data obtained with take-off angles of 0° and 50° clearly demonstrates a layer-by-layer growth of the iron film on silicon (100). At 300 K no formation of iron silicide was observed, although an interaction between iron and silicon could be detected at the interface. The formation of iron silicide was observed at annealing temperatures of 630–730 K. Quantitative XPS analysis yields the presence of FeSi2, when the thickness is large enough. Neither the iron film on silicon nor the silicide shows any LEED pattern.

Resistivity and Hall effect of copper films before and after adsorption of carbon monoxide

Abstract: The dependence of the resistivity and the Hall effect of copper films on the method of film preparation and the film thickness has been studied before and after adsorption of carbon monoxide. It is shown that the Fuchs-Sondheimer theory does not adequately predict the dependence of resistivity on film thickness, because thickness-dependent disorder must be considered. It is further shown in the discussion of the Hall effect that scattering anisotropy must be taken into account. The influence of the adsorbed carbon monoxide on the resistivity and Hall coefficient can be ascribed to the scattering effect at the adsorbed molecules.

The influence of thickness on the resistivity, the temperature coefficient of resistivity and the thermoelectric power of evaporated palladium films at 77 K and 273 K

Abstract: The resistivity, the temperature coefficient of resistivity and the thermoelectric power of palladium films were measured at 77 K and at 273 K in the thickness range 6.7–97 nm. Existing theories were used to describe the thickness dependence of the experimental data and the relationship between these electrical properties.

The influence of annealing on the resistivity and the thermoelectric power of evaporated palladium films

Abstract: Palladium films in the thickness range from 5 to 60 nm were evaporated at 80 K and annealed at various temperatures up to 440 K. The resistivity and the thermoelectric power of the films were measured as functions of the thickness, the annealing temperature and the measuring temperature. It is necessary to use annealing temperatures as high as 400–440 K in order to obtain films in which the resistivity (ϱ 0 (280 K ) = 11.5 μΩ cm ) and the thermoelectric power (S0(280 K) = −9.3 μV K−1) approach, when extrapolated to infinite thickness, the values of well-ordered bulk palladium (ϱ = 10.03 μΩ cm, S = −9.30 > μV K−1 at 280 K).

Galvanomagnetic and magnetic properties of evaporated thin nickel films II. Thickness dependence of the Hall coefficients, the magnetoresistivity and the saturation magnetization

Abstract: The film thickness dependence of the Hall coefficients, the magnetoresistivity and the saturation magnetization was studied in the range 10–2000 A. An oscillation of the ordinary Hall coefficient as a function of thickness was observed. These oscillations were interpreted as magnetomorphic oscillations. Similar oscillations of the extraordinary Hall coefficient and of the magnetoresistivity were also observed. The saturation magnetization approaches a value of 1.8 kG for very thin films ( d

Size dependence of nanostructures: Impact of bond order deficiency

Abstract: This report deals with the mechanism behind the unusual behavior of nanostructures in mechanical strength, thermal stability, acoustics (lattice dynamics), photonics, electronics, magnetism, dielectrics, and chemical reactivity and its indication for designing and fabricating nanostructured materials with desired functions. A bond-order-length-strength (BOLS) correlation mechanism has been initiated and intensively verified, which has enabled the tunability of a variety of properties of a nanosolid to be universally reconciled to the effect of bond order deficiency of atoms at sites surrounding defects or near the surface edges of the nanosolid. The BOLS correlation indicates that atomic coordination imperfection causes the remaining bonds of the under-coordinated atom to contract spontaneously associated with bond strength gain and the intraatomic trapping potential well depression. Consequently, localized densification of charge, energy and mass occurs to the surface skin, which modify the atomic coherency (the product of bond number and the single bond energy), electroaffinity (separation between the vacuum level and the conduction band edge), work function, and the Hamiltonian of the nanosolid. Therefore, any detectable quantity can be functionalized depending on the atomic coherency, electroaffinity, work function, Hamiltonian or their combinations. For instances, the perturbed Hamiltonian determines the entire band structure such as the band-gap expansion, core-level shift, Stokes shift (electron-phonon interaction), and dielectric suppression (electron polarization); The modified atomic coherency dictates the thermodynamic process of the solid such as self-assembly growth, atomic vibration, phase transition, diffusitivity, sinterbility, chemical reactivity, and thermal stability. The joint effect of atomic coherency and energy density dictates the mechanical strength (surface stress, surface energy, Young's modulus), and compressibility (extensibility, or ductility) of a nanosolid. Most strikingly, a combination of the new freedom of size and the original BOLS correlation has allowed us to gain quantitative information about the single energy levels of an isolated atom and the vibration frequency of an isolated dimer, and the bonding identities in the metallic monatomic chains and in the carbon nanotubes. A survey and analysis of the theoretical and experimental observations available to date demonstrated that the under-coordinated atoms in the surface skin of 2-3 atomic layers dictate the performance of nanostructures yet atoms of the interior remain as they are in the bulk counterpart. Further extension of the BOLS correlation and the associated approaches to atomic defects, impurities, liquid surfaces, junction interfaces, and amorphous states and to the temperature domain would be more challenging, fascinating, and rewarding.

Characterization of Alumina, Silica, and Titania Supported Cobalt Catalysts

Abstract: A multiple technique approach was used to characterize the structural, chemical, and electronic properties of Co catalysts supported on Al2O3, SiO2, and TiO2 as well as Co/Mn catalysts on TiO2. The morphology of the catalysts was studied by transmission electron microscopy followed by X-ray diffraction to determine the phase composition and the distribution of the crystallite size. The electronic properties of the calcined catalysts were investigated by X-ray photoelectron spectroscopy. Comparison with reference data allows identification of the cobalt-containing species in the surface. In agreement with adsorption experiments, the signal intensities yield the dispersion of the applied catalysts in the sequence Co/Al2O3>Co/TiO2>Co/SiO2. Temperature-programmed reduction and oxidation reveal the formation of various oxides in dependence on temperature as well as, in case of the alumina- and titania-supported cobalt catalysts, the formation of high-temperature compounds CoAl2O4 and CoTiO3, respectively. Dynamic and static adsorption studies and BET measurements complete the characterization of the supported catalysts.

Saturation magnetization of ferromagnetic and ferrimagnetic nanocrystals at room temperature

Abstract: The size-dependent saturation magnetization Ms(D) of ferromagnetic and ferrimagnetic nanocrystals at room temperature, without free parameters, has been predicted in terms of a size-dependent cohesive energy model, where D denotes the diameter of nanoparticles or the thickness of thin films. The given Ms(D) functions, which are also a function of interface conditions for substrate supported nanocrystals, drop as D decreases, which correspond to the available experimental and theoretical results for ferromagnetic Ni films, Fe, Co, Ni nanoparticles, and ferrimagnetic γ-Fe2O3, Fe3O4, MnFe2O4 and CoFe2O4 nanoparticles.

Influence of iron–silicon interaction on the growth of carbon nanotubes produced by chemical vapor deposition

Abstract: Carbon nanotubes are often grown by chemical vapor deposition on silicon substrates covered with an iron catalyst. Photoemission and scanning electron microscopy studies presented here reveal how the iron silicide interface phase formed at elevated temperatures influences the catalytic efficiency of the iron. Moreover, we will show how the deposition of a thin layer of dense titanium nitride between the silicon substrate and the iron catalyst effectively prevents the formation of the silicide phase and consequently improves the carbon nanotubes growth.

The design and operation of a MEMS differential scanning nanocalorimeter for high-speed heat capacity measurements of ultrathin films

Abstract: A MEMS sensor has been developed for use as a calorimetric cell in an ultra-sensitive, thin-film, differential scanning calorimetric technique. The sensor contains a freestanding, thin (30 nm to 1000 nm), low-stress silicon nitride membrane with lateral dimensions of a few millimeters. This membrane, along with a thin (50 nm) metallization layer, forms a calorimetric cell with an exceptionally small addenda. This small addenda creates a very sensitive calorimetric cell, able to make heat capacity measurements of nanometer-thick metal and polymer films. The sensor fabrication and various design considerations are discussed in detail. The calorimetric technique and examples of applications are described.