D. van der Putten

The electronic quantum size effect observed by195Pt NMR in the metal cluster compound Pt309Phen 36 * O30

Abstract195Pt NMR on the organic ligand stabilized metal cluster compound Pt309Phen36*O30 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 timeT1 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 ofT1. 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).

Metal Cluster Molecules: From Molecule to Metal

Metal cluster compounds (MCC-’s) present an interesting class of cluster solids which is very suitable to study the transition to “metallic behavior” of a metal cluster as its size increases [1]. An MCC consists of identical macromolecules (the metal cluster molecules), which can be ionic or neutral and which are composed of a metal core (cluster) containing a given number (n) of metal atoms. The core is surrounded by a ligand “shell” formed by ligand atoms (Cl, I, 0,… or ligand molecules (CO, PPh3,… which are chemically bonded to metal atoms at the surface of the metal cores. Since chemical compounds are involved, a sample of a given MCC contains only one particular type of macromolecule (provided that it is pure, of course), and thus presents a macroscopically large collection of identical metal clusters, mutually separated by the ligand shells (plus the counterions in case of the ionic forms). Thus the ligand shells provide a highly effective means of “chemical stabilization” of the small metal particles (analogous to metal colloids). In going from one compound to the other, the type of metal atom in the clusters or the size n of the clusters can be varied. At present a few hundreds of MCC-’s are known already, most of them with cluster sizes ranging up to n = 20 − 30, and with many of the transition metal elements (Fe, Co, Ni, Mo, Ru, Rh, Pd, Ag, Os, Ir, Pt, Au,… For some elements even giant MCC-’s are known, the metal core size becoming as large as n = 309 for Pt and n = 561 for Pd. Clearly, such sizes are already closely approaching those of the smallest metal particles in metal colloids (n ≃ 103 − 104 atoms). Not surprisingly therefore, these giant MCC-’s offer very promising possibilities for the study of the physical and chemical properties of catalysts. In particular the comparison of the properties of the ligated and the bare metal clusters (obtained by removing the ligand shells) are of considerable importance in this regard.

Physical properties of carbon-black/polymer compounds

The conductivity of carbon-black/polymer compounds with a low percolation threshold has been studied as a function of electrical-field strength, carbon-black concentration, temperature, pressure and frequency. In addition ESR experiments have been performed. The results are compared with predictions of classical-percolation and tunneling models.

Low temperature properties of boron carbides

We have investigated boron carbides, B1−xCx with x=0.1, 0.13, and 0.2, especially below 1 K. The samples were characterized by ESR. The integrated intensities are roughly sample independent and correspond to about 1020 spins per gram. The temperature (T) dependences are Curie like. The line shapes were lorentzian and for x=0.1 and 0.13 are strongly temperature dependent, which is suggestive for a coupling of linewidths and charge carriers. The room temperature linewidths are 0.38 mT, 32.6 mT, and 23.5 mT for resp. x=0.2, 0.13 and 0.1. Also the AC susceptibility (χ) measured down to 100 mK points to a similar small number spins. There is no sign of a Pauli susceptibility within experimental error. No phase transition (e.g., to a superconducting state) is observed. For x=0.13 and 0.1 a very small specific heat, C is seen. The T dependence of C can be described by a Tα plus a small cubic term, α about 0.26. The relation between C and χ can be explained with a random exchange model. The C‐value for x=0.2 is b...

NMR in Submicron Particles

The properties of a metal will change if its size is no longer macroscopic. The size regime between large clusters ( 100 nm) is often referred to as mesoscopic condensed matter [1]. If the linear dimensions become comparable to characteristic length scales of the system new phenomena appear, that are not present in the bulk. As an example, if the sample dimensions are lowered to such an extent that the wave coherence length L? becomes comparable with the sample size, the transport properties will bear resemblance with a scattering states problem. If the sample size is reduced still further, the distance between the energy levels around the Fermi level will no longer be small compared to k b T. In this final microscopic or cluster limit quantum size effects rather than intraparticle scattering will dominate the physical properties [1]. Because even between 1 and 10 nm the physics of a cluster changes drastically, one also might (as we will do below) let the mesoscopic regime start for particle sizes above 1 nm (roughly 50 atoms) .

195 Pt NMR of Pt 309 Phen 36 * O 30 Metallic Clusters

195Pt NMR lineshape and spin-lattice relaxation measurements at 77 K are presented for the polynuclear metallic cluster compound Pt309Phen 36 * O30 Two peaks in the NMR lineshape are found centered at Ho/v0=1.110 G/kHz and 1.096 G/kHz. The high field peak is attributed to Pt spins located in the interior of the cluster which are in a metallic environment. The low field peak has its origin in platinum atoms that constitute the surface shell of the cluster. These atoms are involved in chemical bonds with the ligands.

195Pt NMR of Pt309Phen 36 * O30 Metallic Clusters

195Pt NMR lineshape and spin-lattice relaxation measurements at 77 K are presented for the polynuclear metallic cluster compound Pt309Phen 36 * O30 Two peaks in the NMR lineshape are found centered at Ho/v0=1.110 G/kHz and 1.096 G/kHz. The high field peak is attributed to Pt spins located in the interior of the cluster which are in a metallic environment. The low field peak has its origin in platinum atoms that constitute the surface shell of the cluster. These atoms are involved in chemical bonds with the ligands.

195Pt NMR of Pt309Phen36*O30 Metallic Clusters

195Pt NMR lineshape and spin-lattice relaxation measurements at 77 K are presented for the polynuclear metallic cluster compound Pt309Phen 36 * O30 Two peaks in the NMR lineshape are found centered at Ho/v0=1.110 G/kHz and 1.096 G/kHz. The high field peak is attributed to Pt spins located in the interior of the cluster which are in a metallic environment. The low field peak has its origin in platinum atoms that constitute the surface shell of the cluster. These atoms are involved in chemical bonds with the ligands.