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Ligand stabilized metal cluster compounds and colloids consist of nanometer-size metal cores surrounded by ligand shells. We study the dielectric properties of randomly packed aggregates of these compounds as a function of temperature T, electric field E and frequency ω. Cores were formed by Pd-atoms and stabilized by a selection of ligands. The core size varied between 2.4 and 3.6 nm (monodisperse) and 18 nm (5-10% dispersion). The dielectric properties of all compounds show scaling: the same functional dependence of the normalized conductivity on normalized frequency irrespective of temperature. The results imply that at room temperature at all frequencies below 1 THz exclusively hopping between the particles is seen. More details of the hopping processes are obtained by varying E. The observed dependences are consistent with stochastic models that use random packing of clusters inside the sample. [ABSTRACT FROM AUTHOR]

With decreasing size the density of electronic states of a metal particle becomes different from that of the bulk. This significantly changes the behaviour of the thermodynamic properties of such a particle. We present specific heat and spin susceptibiblity measurements on a series of chemically synthesized Pd particles with diameters of 2.4-15 nm and magnetic measurements on a Ni colloid with an average diameter of about 2.8 nm. Pronounced size dependence of the electronic magnetic properties was found which is accompanied by only weak size dependence of the electronic specific heat. This result can be understood in terms of a size-dependence of the Stoner enhancement factor, which only enters into the susceptibility and not into the specific heat. [ABSTRACT FROM AUTHOR]

The technique of Au Mössbauer absorption spectroscopy has been used to study four varieties of the high nuclearity gold molecular cluster compounds abbreviated as Au, all having a cuboctahedral structure, with 12 PPh-ligands or modifications of PPh. The technique of emission spectroscopy developed in our laboratory [1] has been applied to four different molecular platinum carbonyl cluster compounds of varying cluster nuclearity, abbreviated as Pt, Pt, Pt, and Pt [2]. The four Au55 compounds were studied, both as dry materials and as frozen solutions. The M6ssbauer parameters of the chlorine ligated site of the water soluble version differ strongly due to the effect of Na on the Au-Cl distances. In the frozen solution, the effect of the ionic charge on this cluster can be clearly seen. The similarity of the spectra of all the Pt compounds, especially the occurrence of a substantial singlet contribution in all spectra, is explained by a coalescing mechanism for the low-nuclearity clusters, induced by the neutron irradiation damage inflicted while preparing the sample as M6ssbauer sources. The observed decrease of the absorption intensity with increasing temperature is evidence for a cluster character of the sample remaining after neutron irradiation. [ABSTRACT FROM AUTHOR]

Au cluster compounds are investigated by optical spectroscopy and TEM. The optical spectra appear to be rather structureless, neither showing a collective excitation resonance nor exhibiting distinct absorption bands known from lower nuclearity clusters. We discuss changes of the electronic properties compared to larger Au clusters affecting both, 6 sp electrons and 5d- 6sp interband transitions, the cluster-ligand-interaction being considered as a charge transfer process. We additionally report on a low temperature instability of the cluster compound, which results in changed optical extinction spectra. A characteristic absorption feature at λ=400 nm is attributed to small, ligand-free Au cluster fragments. [ABSTRACT FROM AUTHOR]

The optical response of many-cluster systems (cluster matter) depends strongly on the mean filling factor and on its local variations. This is shown from Au clusters of various sizes which form coagulation systems. Measured spectra of the extinction, the absorbance, the integral scattering, the angle resolved scattering, the regular reflection and corresponding TEM micrographs are presented. It is demonstrated that by varying f from 10 to 0.5 the incoherent Mie scattering is transformed into regular geometric-optical transmission and reflection ('Oseen-transition') while the spectral features even of the most densely packed samples still differ strongly from those of a bulk Au film. [ABSTRACT FROM AUTHOR]

The giant metal cluster molecule PdPhenO is the largest member of a series that has recently been prepared by Schmid [1]. Pronounced size effects are expected to show up in the magnetic properties of metal particles of the size of the Pd-561 core of the molecule. We have measured the susceptibility on four different batches of the material, with quite satisfactory reproducibility. There is a low temperature tail that is attributed to small amounts of impurities. The nearly temperature-independent paramagnetism that is observed is most probably due to an exchange enhanced Pauli spin susceptibility and is already close to the bulk value. The absence of size effects, i.e. a drop at low temperatures, is either due to interactions between the clusters or a result of a very small energy level spacing. [ABSTRACT FROM AUTHOR]

The bombardment of a two-shell gold complex (Au(PPh)Cl) with 10 keV Xe-ions results in the formation of secondary ion masses up to 140000 u. These are by far the largest secondary ions observed under primary particle bombardment. The detection and identification of these ions with a Time-Of-Flight Secondary Ion Mass Spectrometer (TOF-SIMS) gives important information about the behavior of naked full-shell clusters. Au particles, generated from the Au cluster, serve as building blocks for a series of super-clusters up to (Au). The results for keV-ion bombardment are compared to those for MeV-ion bombardment. [ABSTRACT FROM AUTHOR]

Ac and dc electrical conductivity data are presented for two polynuclear metal cluster compounds Au(PPh))Cl and PdPhenO. The temperature dependence of the low field dc conductivity increases with electrical field, the rate of increase becoming stronger with decreasing temperature. These results are compared with data for granular metal systems, and a close correspondence is observed. The conductivity is found to increase with frequency in analogy to what is observed in amorphous systems. A scaling law for low frequencies that has been recently proposed is shown to describe the data quite well. [ABSTRACT FROM AUTHOR]

Metal cluster compounds are composed of large macromolecules, which consist of a metal core (cluster) containing a certain number ( n=6-560) of metal atoms, to which core a shell of ligands is coordinated. They provide excellent model systems for an assembly of identical metal clusters, embedded in a dielectric matrix. We discuss a number of physical properties of these materials. [ABSTRACT FROM AUTHOR]

Gold cluster compounds were investigated which contain clusters of 55 gold atoms in each unit. The ligands stabilize these clusters and, hence, many-cluster systems can be prepared which mainly show the properties of the single dressed cluster. Experimental results of optical and electron-microscopical investigations are presented and shortly discussed in view of the question whether the clusters are molecular or solid-state like. [ABSTRACT FROM AUTHOR]

The electronic (UV-visible) spectrum of the molecular cluster Au(PPh)Cl shows features corresponding to the 520 nm plasma resonance and the shorter-wavelength interband transition of colloidal gold. These absorptions differ qualitatively from the simpler one-electron transitions of lower-nuclearity cluster molecules. Differential scanning calorimetry has been used to measure the enthalpy of decomposition of Au(PPh)Cl. The Au-Au bonding appears to be substantially stronger than in bulk gold. [ABSTRACT FROM AUTHOR]

We present optical extinction spectra of ligand-stabilized Au-clusters Au with sizes of N=13 and N=55. They are compared to larger, embedded clusters up to N=5·10 (diameter 5nm). The Mie resonance due to excitation of the dipole plasmon polariton vanishes with decreasing cluster size. For N>55, this effect has recently been interpreted as caused by smearing out of the interband transition edge of Au. [ABSTRACT FROM AUTHOR]

C andP solid state NMR measurements on the organic ligands in ligated Au, Ni, Pt, Cu and Cu clusters are reported. The ligands behave like diamagnetic organic molecules, giving rise to relatively narrow lines with excellent cross-polarization efficiency. The resonance lines of the nuclei directly bound to the metal core are systematically broadened in the conducting compounds. No pronounced Knight shifts or evidence of metallic-like relaxation were observed. These results support a model for the electrical conduction involving tunneling between metal cores with the ligands playing the role of a tunneling barrier. [ABSTRACT FROM AUTHOR]

Pt NMR on the organic ligand stabilized metal cluster compound PtPhenO reveals two separate peaks in the lineshape. Ligand-bonded platinum atoms at the surface of the core are thought to be responsible for the peak that does not show any Knight shift. The corresponding spin-lattice relaxation time T is of the order of seconds. The second peak is Knight shifted and is attributed to the other Pt atoms, for which metallic behavior is inferred from the temperature dependence of T. The Korringa relation holds down to 65 K. Below 65 K the relaxation of the magnetization becomes increasingly non-exponential with decreasing temperature. The relaxation process can be successfully modelled under the assumption of a Poisson distribution of the energy levels around the Fermi energy (the electronic quantum size effect). [ABSTRACT FROM AUTHOR]

There are 5 different Au-sites in the Au core of Au(PPh)Cl. The 13 inner atoms are almost identical to that of bulk gold. At the surface the attachment of the ligands is responsible for further differentiation. We have calculated the low temperature specific heat using the Mössbauer determined electrical quadrupolar splitting of the surface atoms. Due to too long a quadrupolar relaxation time, no such contribution is seen in the zero field results (down to 0.06 K) of Goll et al., Z. Phys. D 20, 329 (1991). The latter zero and in-field data are interpreted as the contribution of an electronic spin, hyperfine coupled to a nuclear spin 3/2. The effective abundance is about one spin per ten clusters. [ABSTRACT FROM AUTHOR]

The low temperature specific heat of the metal cluster molecular compounds Au(PPh)Cl, Pt PhenO and PdPhenO, with resp. 55, 309 and 561 core atoms are compared. For T ∼ 1 K, Pt and Pd show a linear contribution to C/R of about 1/3 of the bulk value. A Poisson distribution of the electronic energy levels is suggested as an explanation. [ABSTRACT FROM AUTHOR]

Specific-heat measurements on the cluster compound Au(P(CH))Cl at temperatures 0.06 K ≤ T≤3 K and in magnetic fields 0≤ B≤6 T are reported. While above 0.6 K the specific heat C is dominated by the inter-cluster vibrational contribution observed previously, an anomalous increase of C towards low T is observed below 0.3 K, with C ∼ T. This contribution develops into a Schottky-like anomaly for B≥0.4 T, indicating that it might be attributed to local moments which are also observed in ESR measurements. From the height of the anomaly one can infer that approximately one tenth of the Au clusters carry a magnetic moment. For 0.6 K≤ T≤1 K and B=0 our data indicate the absence of a linear electronic specific-heat contribution expected for bulk Au. This possibly constitutes the first direct observation of the quantum-size effect on electronic energy levels in the specific heat. [ABSTRACT FROM AUTHOR]

The field and temperature dependence of theP nuclear spin lattice relaxation rate in the metal cluster compound Ru(P( t-Bu))Cl follows a power law: 1/ T ∝ T B, with n=1.5±0.1 at 3.25 T and n=1.3±0.1 at 6.45 T; m ⋍ 1.4. Such dependences have so far only been observed in inorganic glasses and been attributed to two level systems. The correspondence suggests that the relaxation rate is due to interaction of the P-nuclear moment with electronic spins of stochastically moving charge carriers, which are thought to be responsible for the electrical conductivity through hopping between neigboring cluster molecules. [ABSTRACT FROM AUTHOR]

We measured XPS and UPS spectra of gold clusters with 55 atoms, embedded in an electrically isolating phosphine matrix, and of gold-phosphine, from which the clusters were chemically derived. Compared to the spectra of bulk gold the valence band spectrum and the core level spectra of the clusters showed shifts of the peaks and the fermi level to higher binding energies. The shift of the peaks could qualitatively be interpreted by a final state effect. We succeeded in a separation of bulk and surface contributions to the core level spectra and in a reasonable quantitative analysis of the valence band spectrum of the clusters. The Au 4 f core level spectrum of gold-phosphine showed two peaks at 1.5 eV higher binding energies than the corresponding peaks of the clusters. [ABSTRACT FROM AUTHOR]

We present EPR measurements on the polynuclear metal-cluster compound PdPhenO for temperatures ranging from 300 K to 8 K. No temperature dependence was found in either the resonance field or the line-width. The signal intensity shows a Curie-like behavior. The effect of the ligand on this behavior is discussed. We also discuss the possibility that quantum size effects govern the observed temperature dependence of the line-width and resonance field. [ABSTRACT FROM AUTHOR]

In this paper we present measurements of nuclear relaxation times ofP in the polynuclear cluster compounds Au(PPh)Cl and Ru(P(t-Bu))Cl. Above 15 K the data can be described as a Korringa process, while below 15 K the relaxation time appears to be thermally activated. [ABSTRACT FROM AUTHOR]