Jörg August Becker

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...

Analysis of semiconductor cluster beam polarization taking small permanent dipole moments into account

Abstract Deflections of semiconductor cluster beams in an inhomogeneous electric field deliver enhanced polarizabilities for cold isolated GaNAsM clusters with odd number of atoms n=N+M up to n=17. One speculated that the high polarizabilities result from their electronic open shell structure. This was qualitatively confirmed by optical absorption cross section measurements, but the enhancement of polarizability is too large to be explained through a Kramers–Kronig-relation as purely electronic effect. Moreover, quantum chemical calculations gave permanent dipole moments of some tenths of a Debye for certain compositions. The clusters are too heavy to identify these dipole moments by beam broadening effects, but we will show in the present analytic investigation within classical top theory, that a slight reversible-adiabatic alignment of the clusters dipole moment in the electric field increases the average beam deflection and the derived polarizability values. This effect was up to now not taken into account and it offers a consistent explanation of the GaNAsM polarizabilities by small permanent dipole moments of some tenths of a Debye and a reasonable rotational temperature between 2 and 12K.

Electric dipole polarizabilities of isolated gallium arsenide clusters

Abstract The static electric dipole polarizabilities of isolated Ga N As M clusters with N + M = 4–30 atoms and approximatly 1:1 composition have been measured in dependence of the cluster size N + M by means of a molecular beam deflection technique. A striking size-dependent behaviour has been observed for small Ga N As M clusters with up to 15 atoms. Within this size range the polarizability oscillates strongly between low values for even and high values for odd numbers of atoms N + M . For clusters with N + M N + M >12.

Electronic structure of selenium- and tellurium-clusters

The vacuum-UV-photoelectron spectra (hν=10.0 eV or 8.3 eV) of selenium- and tellurium-clusters of different sizes (up to Se25 and Te8) were measured in a molecular beam by a photoion-photoelectron-coincidence experiment. Especially, the photoelectron spectra (4p-π-electrons) of the selenium-clusters converge with growing cluster size to the well known spectra of amorphous solid selenium by taking into account the dielectric constant of the bulk material. The experimental results indicate the first evidence that larger selenium aggregates are van der Waals clusters of the molecules Se5, Se6, Se7 and Se8 which are contained in the equilibrium vapour.

Electric deflection studies on lead clusters

The dielectric response to an inhomogeneous electric field has been investigated for Pb(N) clusters (N=7-38) within a molecular beam experiment. The experiments give clear evidence that lead clusters with 12, 14, and 18 atoms possess permanent dipole moments. For these cluster sizes, the permanent electric dipole moments strongly determine the response to the electric field, leading to a significantly increased apparent polarizability. An adiabatic polarization mechanism allows a semiquantitative explanation of the observed susceptibility anomalies. The beam profiles of most of the lead clusters with N not equal12, 14, and 18 also display a small broadening induced by the electric field, indicating permanent dipole moments of about (0.01-0.02) D/atom. Nearly constant dipole moments per atom for larger lead clusters (N>20) manifest in a linear increase in the polarizability per atom. Also, for lead clusters such as Pb(25), which do exhibit almost no measurable beam broadening, the polarizabilties are increased compared to the bulk value. This could be partially explained by the electronic structure of the lead clusters but might be also a consequence of quenched permanent dipole moments because for highly flexible clusters only an increased beam deflection, but no broadening, will be observed.

Ultraviolet photoelectron studies of the molecules Se5, Se6, Se7 and Se8 with relevance to their geometrical structure

The vacuum-UV-photoelectron spectra (hν=10.0 eV) of Se2, Se5, Se6, Se7 and Se8 were measured. The isolated particles were examined in a supersonic molecular beam employing the photoelectron-photocation-coincidence technique. The structures of the photoelectron spectra of selenium molecules with even and odd numbers of atoms differ in a characteristic manner. Whereas the spectra of Se6 and Se8 show one single broad band, three separated bands with different intensities are observed for Se5 and two for Se7. An attempt to connect these experimental observations with the geometrical structure of the molecules shows that the photoelectron spectra are consistent with theC1h-symmetry of the Se5 and Se7 rings proposed in the literature.

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.

Dielectric properties of semiconductor clusters

The static electric polarizability α of Si N , Ga N As M and Ge N Te M clusters in a molecular beam, with nozzle temperatures ranging from 38 to 300 K, were investigated from their deflections in an inhomogeneous electric field. A striking size dependence is observed for small Ga N As M and Si N clusters. Specifically, for Ga N As M clusters with up to 15 atoms, the polarizability oscillates strongly between low values for even numbers of atoms N + M and high values for odd numbers of atoms N + M. This behavior of α as a function of size can be understood by a widening of the band gap and additional occurrence of defect-like electronic states. The temperature dependence of the polarizability of most Ge N Te M clusters indicates the importance of vibronic (ionic) contributions to α. This may be related to the ferroelectric behavior of bulk GeTe.

Acceleration of neutral silicon cluster isomers in a helium gas expansion

The acceleration of isolated neutral silicon cluster isomers in a helium gas expansion has been studied within a molecular beam experiment. The molecular beam flows through a nozzle tube that can be heated, expands into vacuum and the velocity slip of the clusters against the helium is investigated by a time-of-flight method. The measured mean cluster velocities can be analyzed by use of a simple acceleration model and give evidence for two structural classes of cluster isomers. Temperature depending investigations indicate that the cluster isomer class with an enhanced collision cross-section is metastable and transforms into the stable one if the nozzle tube is heated.

VAN DER WAALS FORCES AND COMPOSITION OF ISOLATED GANASM-CLUSTERS

cross sections in a crossed beam scattering experiment. The measured cross sections of the small clusters are systematically higher for compositions with = M ± 1 than for the compositions with N = M. This effect reflects the high static dipole polarizabilities of the semiconductor clusters with = M ± 1. Polarizabilities and van der Waals effects can be consistently described by a simple model that takes donorand acceptor-like electrons into account. This model relates basic concepts of semiconductor physics to the physical chemistry of intermolecular interactions.