Spacer Layer Thickness Dependence of the Giant Magnetoresistance in Electrodeposited Ni-Co/Cu Multilayers.

Electrodeposited Ni65Co35/Cu multilayers were prepared with Cu spacer layer thicknesses between 0.5 nm and 7 nm. Their structure and magnetic and magnetoresistance properties were investigated. An important feature was that the Cu layers were deposited at the electrochemically optimized Cu deposition potential, ensuring a reliable control of the spacer layer thickness to reveal the true evolution of the giant magnetoresistance (GMR). X-ray diffraction indicated satellite reflections, demonstrating the highly coherent growth of these multilayer stacks. All of the multilayers exhibited a GMR effect, the magnitude of which did not show an oscillatory behavior with spacer layer thickness, just a steep rise of GMR around 1.5 nm and then, after 3 nm, it remained nearly constant, with a value around 4%. The high relative remanence of the magnetization hinted at the lack of an antiferromagnetic coupling between the magnetic layers, explaining the absence of oscillatory GMR. The occurrence of GMR can be attributed to the fact that, for spacer layer thicknesses above about 1.5 nm, the adjacent magnetic layers become uncoupled and their magnetization orientation is random, giving rise to a GMR effect. The coercive field and magnetoresistance peak field data also corroborate this picture: with increasing spacer layer thickness, both parameters progressively approached values characteristic of individual magnetic layers. At the end, a critical analysis of previously reported GMR data on electrodeposited Ni-Co/Cu multilayers is provided in view of the present results. A discussion of the layer formation processes in electrodeposited multilayers is also included, together with a comparison with physically deposited multilayers.

Graphene-Encapsulated Silver Nanoparticles for Plasmonic Vapor Sensing.

Graphene-covered silver nanoparticles were prepared directly on highly oriented pyrolytic graphite substrates and characterized by atomic force microscopy. UV-Vis reflectance spectroscopy was used to measure the shift in the local surface plasmon resonance (LSPR) upon exposure to acetone, ethanol, 2-propanol, toluene, and water vapor. The optical responses were found to be substance-specific, as also demonstrated by principal component analysis. Point defects were introduced in the structure of the graphene overlayer by O2 plasma. The LSPR was affected by the plasma treatment, but it was completely recovered using subsequent annealing. It was found that the presence of defects increased the response for toluene and water while decreasing it for acetone.

A Rational Fabrication Method for Low Switching-Temperature VO2.

Due to its remarkable switching effect in electrical and optical properties, VO2 is a promising material for several applications. However, the stoichiometry control of multivalent vanadium oxides, especially with a rational deposition technique, is still challenging. Here, we propose and optimize a simple fabrication method for VO2 rich layers by the oxidation of metallic vanadium in atmospheric air. It was shown that a sufficiently broad annealing time window of 3.0-3.5 h can be obtained at an optimal oxidation temperature of 400 °C. The presence of VO2 was detected by selected area diffraction in a transmission electron microscope. According to the temperature dependent electrical measurements, the resistance contrast (R30 °C/R100 °C) varied between 44 and 68, whereas the optical switching was confirmed using in situ spectroscopic ellipsometric measurement by monitoring the complex refractive indices. The obtained phase transition temperature, both for the electrical resistance and for the ellipsometric angles, was found to be 49 ± 7 °C, i.e., significantly lower than that of the bulk VO2 of 68 ± 6 °C.

Synthesis and Characterization of Graphene-Silver Nanoparticle Hybrid Materials.

Silver nanoparticles (Ag NPs) play important roles in the development of plasmonic applications. Combining these nanoparticles with graphene can yield hybrid materials with enhanced light-matter interaction. Here, we report a simple method for the synthesis of graphene-silver nanoparticle hybrids on highly oriented pyrolytic graphite (HOPG) substrates. We demonstrate by scanning tunneling microscopy and local tunneling spectroscopy measurements the electrostatic n-type doping of graphene by contact with silver. We show by UV-Vis reflectance investigations that the local surface plasmon resonance (LSPR) of Ag NPs partially covered with graphene is preserved for at least three months, i.e., three times longer than the LSPR of bare Ag NPs. The gradual loss of LSPR is due to the spontaneous sulfurization of non-covered Ag NPs, as revealed by scanning electron microscopy and energy-dispersive X-ray spectroscopy. We show that the Ag NPs completely sandwiched between graphene and HOPG do not sulfurize, even after one year.

Vapour sensing properties of graphene-covered gold nanoparticles.

We investigated the vapour sensing properties of different graphene-gold hybrid nanostructures. We observed the shifts in the optical spectra near the local surface plasmon resonance of the gold nanoparticles by changing the concentration and nature of the analytes (ethanol, 2-propanol, and toluene). The smaller, dome-like gold nanoparticles proved to be more sensitive to these vapours compared to slightly larger, flat nanoparticles. We investigated how the optical response of the gold nanoparticles can be tuned with a corrugated graphene overlayer. We showed that the presence of graphene increased the sensitivity to ethanol and 2-propanol, while it decreased it towards toluene exposure (at concentrations ≥ 30%). The slope changes observed on the optical response curves were discussed in the framework of capillary condensation. These results can have potential impact on the development of new sensors based on graphene-gold hybrids.

Scanning tip measurement for identification of point defects.

Self-assembled iron-silicide nanostructures were prepared by reactive deposition epitaxy of Fe onto silicon. Capacitance-voltage, current-voltage, and deep level transient spectroscopy (DLTS) were used to measure the electrical properties of Au/silicon Schottky junctions. Spreading resistance and scanning probe capacitance microscopy (SCM) were applied to measure local electrical properties. Using a preamplifier the sensitivity of DLTS was increased satisfactorily to measure transients of the scanning tip semiconductor junction. In the Fe-deposited area, Fe-related defects dominate the surface layer in about 0.5 µm depth. These defects deteriorated the Schottky junction characteristic. Outside the Fe-deposited area, Fe-related defect concentration was identified in a thin layer near the surface. The defect transients in this area were measured both in macroscopic Schottky junctions and by scanning tip DLTS and were detected by bias modulation frequency dependence in SCM.

Half-magnitude extensions of resolution and field of view in digital holography by scanning and magnification.

Digital holography replaces the permanent recording material of analog holography with an electronic light sensitive matrix detector, but besides the many unique advantages, this brings serious limitations with it as well. The limited resolution of matrix detectors restricts the field of view, and their limited size restricts the resolution in the reconstructed holographic image. Scanning the larger aerial hologram (the interference light field of the object and reference waves in the hologram plane) with the small matrix detector or using magnification for the coarse matrix detector at the readout of the fine-structured aerial hologram, these are straightforward solutions but have been exploited only partially until now. We have systematically applied both of these approaches and have driven them to their present extremes, over half a magnitude in extensions.

Gold nanoparticles deposited on SiO2/Si100: correlation between size, electron structure, and activity in CO oxidation.

Nanosize gold particles were prepared by Ar(+) ion implantation of 10-nm thick gold film deposited onto a SiO(2)/Si(100) wafer possessing no catalytic activity in the CO oxidation. Along with size reduction the valence band of the gold particles and the actual size were determined by ultraviolet- and X-ray photoelectron spectroscopy (UPS, XPS) and by transmission electron microscopy (TEM) as well as atomic force microscopy (AFM), respectively. The catalytic activity was determined in the CO oxidation. Energy distribution of the photoelectrons excited from 5d valence band of gold was strongly affected by Ar(+) implantation. This variation was interpreted by the redistribution of the valence band density of states (DOS). The intrinsic catalytic activity of the gold particles increased with decreasing size. When an Au/FeO(x) interface was created by FeO(x) deposition on large gold nanoparticles, a significant increase in the rate of the CO oxidation was observed. These data can be regarded as an experimental verification of the correlation between the catalytic activity and valence band density of states of gold.