In situ Si doping of heteroepitaxially grown c-BN thin films at different temperatures

Phase pure cubic boron nitride (c-BN) films have been epitaxially grown on (001) diamond substrates at 420 °C, 600 °C and 900 °C.

Super spin-glass state and exchange bias in Fe/CoO hybrid nanostructures

Fe/CoO heterostructures were realized by depositing Fe thin films on CoO nanoparticle arrays. Magnetization measurements revealed that 1 nm Fe exhibits a superparamagnetic behavior at 300 K and a super spin-glass state at temperatures below 80 K. The superparamagnetic as well as super spin-glass state vanishes for higher Fe film thicknesses once Fe starts to form a continuous layer across the CoO nanoparticle arrays. Furthermore, all samples exhibit an exchange bias effect at 6 K after field cooling, with a maximum exchange bias field of about 60 Oe for a Fe thickness of 2 nm. M-H loops of thicker Fe samples show a two-step magnetization reversal where Fe in the area in between CoO nanoparticles reverses at low fields, while, in proximity to the CoO nanoparticles, Fe switches at substantially higher fields. Both reversals are exchange biased.

Analysis of rich inelastic electron tunneling spectra: case study of terthiophene on Au(111)

Even moderately small molecules like 2,2':5',2"-terthiophene exhibit quite rich vibrational spectra. Detection and assignment of vibronic transitions of such a single adsorbed molecule in inelastic electron tunneling spectroscopy (IETS) using scanning tunneling microscopy are notoriously hampered by noise and the low efficiency of inelastic channels of typically well below 1%. We demonstrate by a thorough statistical analysis that detection of almost all predicted transitions can be determined experimentally within the energy range 0-120 meV with an estimated detection limit for the efficiency of inelastic channels of ∼0.15%. The maximum accuracy of our transition energies is 2 meV and thus smaller than the thermal broadening at 5 K. On short time scales up to some hours, that accuracy appears to be limited by tunneling current noise. The present analysis confirms earlier results which showed that IETS obeys propensity rules rather than selection rules as observed for optical transitions. Furthermore, the previous indications that anharmonic components in the interaction potentials are important for calculating properties of molecular vibrations were corroborated.

Monodispersed NiO nanoflowers with anomalous magnetic behavior

Nickel oxide (NiO) nanoflowers, prepared by thermal decomposition, exhibit anomalous magnetic properties far below the blocking temperature, i.e., a cusp in both the zero-field-cooled and field-cooled curves at about 21 K. Detailed characterization discloses that the individual NiO nanoflower consists of porous crystals with holes (1.0-1.5 nm in size) inside. We believe that the low temperature magnetic feature observed here could be a new kind of spin transition for the uncompensated spins around the holes and will trigger more studies in other nanostructured antiferromagnetic materials.

Controlling the interparticle spacing of Au-salt loaded micelles and Au nanoparticles on flat surfaces

The self-organization of diblock copolymers into micellar structures in an appropriate solvent allows the deposition of well ordered arrays of pure metal and alloy nanoparticles on flat surfaces with narrow distributions in particle size and interparticle spacing. Here we investigated the influence of the materials (substrate and polymer) and deposition parameters (temperature and emersion velocity) on the deposition of metal salt loaded micelles by dip-coating from solution and on the order and inter-particle spacing of the micellar deposits and thus of the metal nanoparticle arrays resulting after plasma removal of the polymer shell. For identical substrate and polymer, variation of the process parameters temperature and emersion velocity enables the controlled modification of the interparticle distance within a certain length regime. Moreover, also the degree of hexagonal order of the final array depends sensitively on these parameters.

Nanostructured Pt/GC model electrodes prepared by the deposition of metal-salt-loaded micelles

Novel, nanostructured, carbon-supported Pt model electrodes with homogeneously distributed Pt nanoparticles of uniform size were fabricated and analyzed with respect to their electrochemical properties. For this purpose, Pt-salt-loaded micelles were deposited on a glassy carbon substrate and subsequently exposed to an oxygen plasma and a H2 atmosphere for removal of the polymer carriers and reduction of the Pt salt. The morphology of the resulting nanoparticles and their electrochemical/electrocatalytic properties were characterized by high-resolution scanning electron microscopy, X-ray photoelectron spectroscopy, cyclic voltammetry, and differential electrochemical mass spectrometry for CO electrooxidation. The data demonstrate that this method is generally suited to the production of nanostructured model electrodes with well-defined and independently adjustable particle size and interparticle distance distributions, which are specifically suited for quantitative studies of transport processes in electrocatalytic reactions.

Enhanced orbital magnetism in Fe(50)Pt(50) nanoparticles

X-ray absorption and magnetic circular dichroism spectra at both the Fe and Pt L(3,2) edges were measured on wet-chemically synthesized monodisperse Fe(50)Pt(50) particles with a mean diameter of 6.3 nm before and after complete removal of the organic ligands and the oxide shell covering the particles by soft hydrogen plasma resulting in a pure metallic state. After thermal treatment of the metallic particles, the coercive field increased by a factor of 6, the orbital magnetic moment at the Fe site increased by 330% and is reduced at the Pt site by 30%, while the effective spin moments did not change. A decrease of the frequency of oscillations in the extended x-ray absorption fine structure at the Pt L(3,2) edges provides evidence for crystallographic changes towards the L1(0) phase.

Alloy formation of supported gold nanoparticles at their transition from clusters to solids: does size matter?

Gold nanoclusters of a size approaching the molecular limit (<3 nm) were prepared on Si substrates in order to study alloy formation on the nanometer scale. For this purpose, indium atoms are deposited on top of the gold particles at room temperature and the formation of AuIn(2) is studied by x-ray photoelectron spectroscopy in situ. It is observed that the alloy formation takes place independent of whether the particles electronically are in an insulating molecular or in a metallic state. Most important, however, closed packed full-shell clusters containing 55 Au atoms are found to exhibit an outstanding stability against alloying despite a large negative heat of formation of the bulk Au-In system. Thus, Au(55) clusters may play a significant role in the design of nanoscaled devices where chemical inertness is of crucial importance.

Epitaxy of cubic boron nitride on (001)-oriented diamond

Cubic boron nitride (c-BN), although offering a number of highly attractive properties comparable to diamond, like hardness, chemical inertness and a large electronic bandgap, up to now has not found the attention it deserves. This mostly has to do with preparational problems, with easy chemical routes not available and, instead, the necessity to apply ion-bombardment-assisted methods. Hence, most of the c-BN samples prepared as thin films have been nanocrystalline, making the prospect of using this material for high-temperature electronic applications an illusion. Although heteroepitaxial nucleation of c-BN on diamond substrates has been demonstrated using the high-pressure-high-temperature technique, none of the low-pressure methods ever succeeded in the epitaxial growth of c-BN on any substrate. Here, we demonstrate that heteroepitaxial c-BN films can be prepared at 900 degrees C on highly (001)-oriented diamond films, formed by chemical vapour deposition, using ion-beam-assisted deposition as a low-pressure technique. The orientation relationship was found to be c-BN(001)[100]||diamond(001)[100]. High-resolution transmission electron microscopy additionally proved that epitaxy can be achieved without an intermediate hexagonal BN layer that is commonly observed on various substrates.

Oxidation-resistant gold-55 clusters

Gold nanoparticles ranging in diameter from 1 to 8 nanometers were prepared on top of silicon wafers in order to study the size dependence of their oxidation behavior when exposed to atomic oxygen. X-ray photoelectron spectroscopy showed a maximum oxidation resistance for "magic-number" clusters containing 55 gold atoms. This inertness is not related to electron confinement leading to a size-induced metal-to-insulator transition, but rather seems to be linked to the closed-shell structure of such magic clusters. The result additionally suggests that gold-55 clusters may act as especially effective oxidation catalysts, such as for oxidizing carbon monoxide.

Chemically induced metal-to-insulator transition in Au55 clusters: effect of stabilizing ligands on the electronic properties of nanoparticles

The cluster compound Au55(PPh3)12Cl6 has been reanalyzed by photoelectron spectroscopy giving direct evidence for a nonmetallic behavior of the individual Au clusters as long as their ligand shell remains intact. The exposure to x-rays during the measurements is found to partly decompose the shell by removal of the chlorine atoms, resulting in a metallic behavior of the clusters as demonstrated by a steplike intensity at the Fermi energy. These observations resolve a long-standing controversy about the metallic behavior of ligated Au clusters emphasizing, in addition, the influence of the local environment on the electronic properties of nanoscaled materials.