We present a method to reduce the size and improve the crystal quality of freestanding nanowires grown by electron beam induced deposition from a platinum metal organic precursor in a dual beam system. By freestanding horizontal growth and subsequent electron irradiation in a transmission electron microscope, sub-10-nm polycrystalline platinum nanowires have been obtained. A combined transmission electron microscopy-electron energy loss analysis has shown that the amorphous carbon, mixed to nanocrystalline platinum in the as-deposited material, is removed from the wires during irradiation. The same treatment progressively transforms nanocrystals dispersed in the amorphous matrix in a continuous polycrystalline platinum wire

We have tested the concept of image charge screening as a new approach to enhance magnetic ordering temperatures and superexchange interactions in ultra thin films. Using a 3 monolayer NiO(100) film grown on Ag(100) and an identically thin film on MgO(100) as model systems, we observed that the Néel temperature of the NiO film on the highly polarizable metal substrate is 390 K while that of the film on the poorly polarizable insulator substrate is below 40 K. This demonstrates that screening by highly polarizable media may point to a practical way towards designing strongly correlated oxide nanostructures with greatly improved magnetic properties.PACS numbers: 75.30.Et, 78.70.Dm Transition metal oxides exhibit many spectacular magnetic and electrical properties including high temperature superconductivity and colossal magnetoresistance [1] making them particularly promising for nanoscience technology applications. An acute issue in the field of nanoscience, however, is the strong reduction of the relevant critical or ordering temperatures due to well known finite size effects [2,3,4,5]. If ways could be found to compensate for these reductions, one would immediately enlarge the materials basis for nano-technology. Current approaches to overcome these problems include the use of chemical doping, pressure, and strain [6,7,8,9,10,11].Here we propose to exploit image charge screening as a new method to compensate finite size phenomena and to enhance magnetic ordering temperatures well beyond the capability of conventional methods [6,7,8,9,10,11]. The basic idea is to bring the material in the close proximity of a strongly polarizable medium. The relevant exchange and superexchange interactions, and thus the related magnetic ordering temperatures, can then be substantially amplified by reducing the energies of the underlying virtual charge excitations as a result of the image-charge-like screening by the polarizable medium [12,13,14].To prove this concept we have chosen to measure the Néel temperature T N of a 3 monolayer (ML) NiO film epitaxially grown on a MgO(100) substrate and of an equally thin film on Ag(100). NiO on MgO and on Ag are ideal model systems for this study because of their simple crystal structure and well characterized growth properties. They have a rock-salt crystal structure with lattice constant a M gO = 4.212Å and a N iO = 4.176Å, respectively, corresponding to a lattice misfit of about 1%. This ensures a perfect layer-by-layer epitaxial growth of NiO(100) on MgO(100), with a NiO(100) film surface roughness of about 0.1Å [15]. Silver has a cubic fcc structure with a lattice constant a Ag = 4.086Å and a mismatch with respect to NiO of about 2%. When misfit dislocations are avoided by keeping the film thickness below the critical thickness for strain relaxation (about 30 ML for NiO/Ag [16]) as done in the present work, then NiO(100) films grow on Ag(100) in a nicely layered and coherent mode with a sharp interface. This was already demonstrated by Kado [17,18], but it has also been verified ...

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A structural study of Fe ultrathin epitaxial films, grown at room temperature on Ni͑111͒, has been performed in the 1.5-18 ML coverage range by angle-scanned photoelectron diffraction. Both backscattering and forward-scattering energy regimes have been employed, in order to enhance the structural sensitivity at lower and higher film thicknesses, respectively. Modeling of the experimental data has been performed with multiple scattering calculations. We found indications that Fe atoms in the first layer occupy fcc hollow sites and stack with a pseudomorphic fcc structure up to 2 ML. Concerning the growth mode at these early stages, data suggest that a good substrate wetting and a sharp Fe/Ni interface take place. Between 3 and 6 ML, transition to a bcc͑110͒ phase develops. By quantitative R-factor analysis, we found that Nishiyama-Wassermann ͑NW͒ in-plane orientation of the bcc͑110͒ cell (͗001͘ bcc ʈ ͗11 0͘ fcc ) is favored over the Kurdjumov-Sachs (͗11 1͘ bcc ʈ ͗11 0͘ fcc ) orientation. The best-fit vertical interlayer distance between bcc͑110͒ planes is d NW ϭ2.11 Å ͑ϩ3.9% expansion͒ at 6 ML and relaxes to d NW ϭ2.05 Å ͑ϩ1.0%͒ at 18 ML, in agreement with the angular shift observed for the forward-focusing features. In the same coverage range, the angle between bcc͑110͒ surface basis vectors changes from 67.7°to 69.0°, corresponding to Ϫ1.7% and Ϫ1.0% contractions of the surface cell area, respectively.

Hole-containing silicon has been regarded as a viable candidate thermoelectric material because of its low thermal conductivity. However, because voids are efficient scattering centers not just for phonons but also for charge carriers, achievable power factors (PFs) are normally too low for its most common form, i.e. porous silicon, to be of practical interest. In this communication we report that high PFs can, indeed, be achieved with nanoporous structures obtained from highly doped silicon. High PFs, up to a huge 22 mW K-2 m(-1) (more than six times higher than values for the bulk material), were observed for heavily boron-doped nanocrystalline silicon films in which nanovoids (NVs) were generated by He+ ion implantation. In contrast with single-crystalline silicon in which He+ implantation leads to large voids, in polycrystalline films implantation followed by annealing at 1000A degrees C results in homogeneous distribution of NVs with final diameters of approximately 2 nm and densities of the order of 10(19) cm(-3) with average spacing of 10 nm. Study of its morphology revealed silicon nanograins 50 nm in diameter coated with 5-nm precipitates of SiB (x) . We recently reported that PFs up to 15 mW K-2 m(-1) could be achieved for silicon-boron nanocomposites (without NVs) because of a simultaneous increase of electrical conductivity and Seebeck coefficient. In that case, the high Seebeck coefficient was achieved as a result of potential barriers on the grain boundaries, and high electrical conductivity was achieved as a result of extremely high levels of doping. The additional increase in the PF observed in the presence of NVs (which also include SiB (x) precipitates) might have several possible explanations; these are currently under investigation. Experimental results are reported which might clarify the reason for this paradoxical effect of NVs on silicon PF