No abstract

No abstract

No abstract

In this article, we provide a detailed description of the synthesis and properties of Pt dendrimer-encapsulated nanoparticles (DENs) prepared using sixth-generation, hydroxyl-terminated, poly(amidoamine) (PAMAM) dendrimers (G6−OH) and three different PtCl4 2−/G6−OH ratios: 55, 147, and 240. Results obtained from UV−vis spectroscopy, X-ray photoelectron spectroscopy, electron microscopy, X-ray absorption spectroscopy, and high-energy X-ray diffraction show that only a fraction of the Pt2+/dendrimer precursors are reduced by BH4 − and that the reduction process is highly heterogeneous. That is, after reduction each Pt2+/dendrimer precursor complex is either fully reduced, to yield a DEN having a size and structure consistent with the original PtCl4 2−/dendrimer ratio used for the synthesis, or the precursor is not reduced at all. This result is consistent with an autocatalytic process that entails slow formation of a nascent catalytic Pt seed within the dendrimer, followed by rapid, catalytic reduction of nearby Pt2+ ions. Details concerning the formation of the Pt2+/dendrimer precursor are also discussed.

No abstract

No abstract

In recent studies of La 2Ϫx Sr x NiO 4 it has been suggested that ordering of the dopant-induced holes occurs only commensurately at special values of x, such as 1 3 and 1 2 . Commensurate order of both charge and spin densities has also been found in a crystal with xϭ0.20. The present neutron scattering study of an xϭ0.225 crystal demonstrates that the spin and charge order can also be incommensurate. In fact, the incommensurability is temperature dependent, as observed previously in La 2 NiO 4.125 , indicating that such behavior is intrinsic to the doped NiO 2 layers and not dependent on ordering of the dopant ions. A careful analysis of the unusual variation of peak widths as a function of momentum transfer perpendicular to the planes shows that the charge-and spin-density modulations are tied to the lattice, with the shift in phase of the charge modulations from one layer to the next equal to exactly one in-plane lattice spacing. A comparison of results for a number of samples shows that the charge and spin ordering temperatures vary linearly with hole concentration, with charge order always occurring at higher temperature, clearly indicating that the ordering is driven by the charge.

We report a detailed neutron-scattering study of the ordering of spins and holes in oxygen-doped La 2 NiO 4.133 . The single-crystal sample exhibits the same oxygen-interstitial order but better defined chargestripe order than that studied previously in crystals with ␦ϭ0.125. In particular, charge order is observed up to a temperature at least twice that of the magnetic transition, T m ϭ110.5 K. On cooling through T m , the wave vector ⑀, equal to half the charge-stripe density within an NiO 2 layer, jumps discontinuously from 1 3 to 0.2944. It continues to decrease with further cooling, showing several lock-in transitions on the way down to low temperature. To explain the observed lock-ins, a model is proposed in which each charge stripe is centered on either a row of Ni or a row of O ions. The model is shown to be consistent with the l dependence of the magnetic peak intensities and with the relative intensities of the higher-order magnetic satellites. Analysis of the latter also provides evidence that the magnetic domain walls ͑charge stripes͒ are relatively narrow. In combination with a recent study of magnetic-field-induced effects, we find that the charge stripes are all O centered at TϾT m , with a shift towards Ni centering at TϽT m . Inferences concerning the competing interactions responsible for the temperature dependence of ⑀ and the localization of charge within the stripes are discussed. ͓S0163-1829͑98͒00902-3͔