Fabrication of FeSi and Fe3Si compounds by electron beam induced mixing of [Fe/Si]2 and [Fe3/Si]2 multilayers grown by focused electron beam induced deposition

Fe-Si binary compounds have been fabricated by focused electron beam induced deposition by the alternating use of iron pentacarbonyl, Fe(CO)5, and neopentasilane, Si5H12 as precursor gases. The fabrication procedure consisted in preparing multilayer structures which were treated by low-energy electron irradiation and annealing to induce atomic species intermixing. In this way we are able to fabricate FeSi and Fe3Si binary compounds from [Fe=Si]2 and [Fe3=Si]2 multilayers, as shown by transmission electron microscopy investigations. This fabrication procedure is useful to obtain nanostructured binary alloys from precursors which compete for adsorption sites during growth and, therefore, cannot be used simultaneously

Size-dependent resistivity in a micro-processed YBa2Cu3O7-δ superconducting whisker

We report the results of a detailed geometrical and electrical study which has been performed on a YBa2Cu3O7-δ superconducting whisker. This sample has undergone three subsequent steps of micro-machining by means of a focused ion beam (FIB) instrument, in order to progressively decrease its cross-sectional area from ~77 to ~4 µm2, over a length of about 150 µm. A simple analytical model based on the exact shape both of the electrical contacts and of the micro-machined material has been proposed for the voltage drop; besides, an accurate geometrical characterization of all of the sample details has been performed by means of SEM microscopy. This enabled us to extract accurate electrical resistivity curves from the resistance versus temperature characteristics for each of the fabrication steps of the whisker, showing an increase of the sheet resistivity with decreasing cross-sectional area. Among the possible physical reasons for such behaviour, inelastic electron scattering at the sample surfaces has been ruled out because of the very short mean free path of carriers in YBCO. On the other hand, oxygen out-diffusion and Ga ion implantation due to the FIB processing are most likely to be responsible for the observed resistivity trend

Tuning electrical properties of hierarchically assembled Al-doped ZnO nanoforests by room temperature Pulsed Laser Deposition

Large surface area, 3D structured transparent electrodes with effective light management capability may represent a key component in the development of new generation optoelectronic and energy harvesting devices. We present an approach to obtain forest-like nanoporous/hierarchical Al-doped ZnO conducting layers with tunable transparency and light scattering properties, by means of room temperature Pulsed Laser Deposition in a mixed Ar:O2 atmosphere. The composition of the background atmosphere during deposition can be varied to modify stoichiometry-related defects, and therefore achieve control of electrical and optical properties, while the total background pressure controls the material morphology at the nano- and mesoscale and thus the light scattering properties. This approach allows to tune electrical resistivity over a very wide range (10^-1 - 10^6 Ohm*cm), both in the in-plane and cross-plane directions. Optical transparency and haze can also be tuned by varying the stoichiometry and thickness of the nano-forests

Image charge screening: a new approach to enhance magnetic ordering temperatures

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 Neel 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.Comment: 5 pages, 4 figure

Structural evolution and graphitization of metallorganic-Pt suspended nanowires under high-current-density electrical test

We present a real-time investigation of the dramatic structural evolution occurring in metallorganic-Pt suspended nanowires (SNWs) (20 nm size) under high-current-density electrical test. SNWs are fabricated by electron beam-induced deposition and consist of Pt nanograins (2-3 nm) embedded in a carbonaceous matrix. As current increases, the Pt-C granular material transforms into Pt-depleted, graphitized C with a two-stage process. First, Pt coalescence into big grains (10-15 nm) is observed, then, for current density approaching 10(7) A/cm(2), grains are depleted by Pt electro- and thermomigration, leaving a graphitized C matrix. The graphitic-C wire eventually breaks forming a nanosize gap

In-depth structural characterisation of the bct-hcp phase transition in Co epitaxial films

In-depth structural evolution of Co layers grown on Fe(001) has been investigated by modulated electron emission measurements during both film growth, by thermal evaporation, and erosion by mild ion sputtering. Co growth results in an epitaxial body-centered tetragonal (bct) phase which progressively turns to hexagonal close packed (hcp) in the 10–35 monolayers (ML) range. Erosion of a 40 ML thick film, performed under sputtering conditions such as to preserve structural order, shows that the bct/hcp interface is located significantly deeper than expected from measurements during growth. It is concluded that the transition to the hcp phase in the growing film begins on top of the bct layer and afterwards extends both upward and downward, reducing the thickness of the underlying bct layer