Stefan A. Quaiser

Visualization of Surfactants on Nanostructured Palladium Clusters by a Combination of STM and High-Resolution TEM

Scanning tunneling microscopy (STM) and high-resolution transmission electron microscopy (TEM) have been used to determine the dimensions of a series of palladium clusters stabilized by tetraalkylammonium salts. Electrochemically prepared colloids were used in which the average diameter of the inner metal core was varied between 2 and 4 nanometers, and the size of the ammonium ions was adjusted in the series +N(n-C4H9)4 < +N(n-C8H17)4 < +N(n-C18H37)4. The difference between the mean diameter determined by STM and that measured by TEM allows the determination of the thickness of the protective surfactant layer. On the basis of these studies, a model of the geometric properties of ammonium-stabilized palladium clusters has been proposed. Suggestions for the mechanism of the STM imaging process are also made.

ELECTROCHEMICAL GROWTH OF SUPERPARAMAGNETIC COBALT CLUSTERS

The magnetization of stabilized cobalt colloids in tetrahydrofuran has been studied by a superconducting quantum interference device (SQUID) and magnetic balance measurements in dependence of applied magnetic field and temperature. The colloids are generated by a newly developed electrochemical method which allows one to generate clusters containing about 1000 atoms with a narrow size distribution. The final size distribution of the clusters is examined by high resolution transmission electron microscopy and small angle x‐ray scattering. The magnetization curves have been determined with special emphasis on changes at the freezing point of the solution. The curves of the liquid phase can be reasonably described by the Langevin function and the magnetic moments of isolated cobalt clusters that have been recently measured by Stern–Gerlach experiments. Deviations that appear at the freezing point can be understood in terms of magnetic anisotropy effects. It is shown that the cluster sizes and the susceptibil...

MAGNETIC PROPERTIES OF COBALT-CLUSTER DISPERSIONS GENERATED IN AN ELECTROCHEMICAL CELL

Magnetization curves of stabilized cobalt-cluster dispersions in tetrahydrofuran with a narrow size distribution have been studied by SQUID and Gouy balance measurements. The cobalt colloids are generated by a newly developed electrochemical method which allows one to generate clusters with mean cluster sizes of about 1000 atoms. The final size distribution of the clusters is examined by small-angle x-ray scattering (SAXS) and high-resolution transmission electron microscopy (HRTEM) measurements. The magnetization curves have been measured with special emphasis on changes at the freezing point of the solution. The liquid phase shows typical superparamagnetism whereas strong deviations from Langevin behavior occur in the solid dispersion which can be understood in terms of magnetic anisotropy effects. These observations are discussed with regard to recent Stern-Gerlach experiments on isolated ferromagnetic clusters in molecular beams. It is shown that cluster size and susceptibility of the dispersions are related. This permits the control of the growth of the clusters during the electrolysis by measuring the susceptibility as a function of the charge that is converted in the cell.