Nickel Nanopillar Arrays Electrodeposited on Silicon Substrates Using Porous Alumina Templates.

Nickel nanopillar arrays were electrodeposited onto silicon substrates using porous alumina membranes as a template. The characterization of the samples was done by scanning electron microscopy, X-ray diffraction, and alternating force gradient magnetometry. Ni nanostructures were directly grown on Si by galvanostatic and potentiostatic electrodeposition techniques in three remarkable charge transfer configurations. Differences in the growth mechanisms of the nanopillars were observed, depending on the deposition method. A high correlation between the height of the nanopillars and the charge synthesis was observed irrespective of the electrochemical technique. The magnetization measurements demonstrated a main dependence with the height of the nanopillars. The synthesis of Ni nanosystems with a controllable aspect ratio provides an effective way to produce well-ordered networks for wide scientific applications.

Optical switching in hybrid VO2/Si waveguides thermally triggered by lateral microheaters.

The performance of optical devices relying in vanadium dioxide (VO2) technology compatible with the silicon platform depends on the polarization of light and VO2 properties. In this work, optical switching in hybrid VO2/Si waveguides thermally triggered by lateral microheaters is achieved with insertion losses below 1 dB and extinction ratios above 20 dB in a broad bandwidth larger than 30 nm. The optical switching response has been optimized for TE and TM polarizations by using a homogeneous and a granular VO2 layer, respectively, with a small impact on the electrical power consumption. The stability and reversibility between switching states showing the possibility of bistable performance is also demonstrated.

Experimental demonstration of a tunable transverse electric pass polarizer based on hybrid VO2/silicon technology.

A tunable transverse electric (TE) pass polarizer is demonstrated based on hybrid vanadium dioxide/silicon (VO2/Si) technology. The 20-µm-long TE pass polarizer exploits the phase transition of the active VO2 material to control the rejection of the unwanted transverse magnetic (TM) polarization. The device features insertion losses below 1 dB at static conditions and insertion losses of 5.5 dB and an attenuation of TM polarization of 19 dB in the active state for a wavelength range between 1540 nm and 1570 nm. To the best of our knowledge, this is the first time that tunable polarizers compatible with Si photonics are demonstrated.

Nanoscale self-organization and metastable non-thermal metallicity in Mott insulators.

Mott transitions in real materials are first order and almost always associated with lattice distortions, both features promoting the emergence of nanotextured phases. This nanoscale self-organization creates spatially inhomogeneous regions, which can host and protect transient non-thermal electronic and lattice states triggered by light excitation. Here, we combine time-resolved X-ray microscopy with a Landau-Ginzburg functional approach for calculating the strain and electronic real-space configurations. We investigate V2O3, the archetypal Mott insulator in which nanoscale self-organization already exists in the low-temperature monoclinic phase and strongly affects the transition towards the high-temperature corundum metallic phase. Our joint experimental-theoretical approach uncovers a remarkable out-of-equilibrium phenomenon: the photo-induced stabilisation of the long sought monoclinic metal phase, which is absent at equilibrium and in homogeneous materials, but emerges as a metastable state solely when light excitation is combined with the underlying nanotexture of the monoclinic lattice.

Magnetic orders and origin of exchange bias in Co clusters embedded oxide nanocomposite films.

Magnetic nanoparticles embedded oxide semiconductors are interesting candidates for spintronics in view of combining ferromagnetic (FM) and semiconducting properties. In this work, Co-ZnO and Co-V2O3 nanocomposite thin films are synthesized by Co ion implantation in crystalline thin films. Magnetic orders vary with the implantation fluence in Co-ZnO, where superparamagnetic (SPM) order appears in the low-fluence films (2 × 1016 and 4 × 1016 ions cm-2) and FM order co-exists with the SPM phase in high-fluence films (1 × 1017 ions cm-2). Exchange bias (EB) appears in the high-fluence films, with an EB field of about 100 Oe at 2 K and a blocking temperature of around 100 K. On the other hand, Co-V2O3 thin films with an implantation fluence of 3.5 × 1016 ions cm-2 exhibit a clear antiferromagnetic (AFM) coupling at low temperatures without the EB effect. The different magnetic behavior of the Co-implanted films with different Co content leads us to conclude that the observed EB effect in the Co-ZnO films results from the FM/AFM coupling between sizable Co nanoparticles and their CoO/Co3O4 surroundings in the (Zn,Co)O matrix. On the other hand, the absence of EB effect in Co-V2O3 appears to be due to the small size of the FM Co nanoparticles in spite of an AFM magnetic order. Detailed studies of magnetic orders and EB effect in magnetic nanocomposite semiconductors can pave the way for their application in spintronics.

Spontaneous formation of nanopatterns in velocity-dependent dip-coated organic films: from dragonflies to stripes.

We present an experimental study of the micro- and mesoscopic structure of thin films of medium length n-alkane molecules on the native oxide layer of a silicon surface, prepared by dip-coating in a n-C32H66/n-heptane solution. Electron micrographs reveal two distinct adsorption morphologies depending on the substrate withdrawal speed v. For small v, dragonfly-shaped molecular islands are observed. For a large v, stripes parallel to the withdrawal direction are observed. These have lengths of a few hundred micrometers and a few micrometer lateral separation. For a constant v, the stripes' quality and separation increase with the solution concentration. Grazing incidence X-ray diffraction and atomic force microscopy show that both patterns are 4.2 nm thick monolayers of fully extended, surface-normal-aligned alkane molecules. With increasing v, the surface coverage first decreases then increases for v > v(cr) ∼ 0.15 mm/s. The critical v(cr) marks a transition between the evaporation regime, where the solvent's meniscus remains at the bulk's surface, and the entrainment (Landau-Levich-Deryaguin) regime, where the solution is partially dragged by the substrate, covering the withdrawn substrate by a homogeneous film. The dragonflies are single crystals with habits determined by dendritic growth in prominent 2D crystalline directions of randomly seeded nuclei assumed to be quasi-hexagonal. The stripes' strong crystalline texture and the well-defined separation are due to an anisotropic 2D crystallization in narrow liquid fingers, which result from a Marangoni flow driven hydrodynamic instability in the evaporating dip-coated films, akin to the tears of wine phenomenology.

Selective growth of palladium and titanium dioxide nanostructures inside carbon nanotube membranes.

Hybrid nanostructured arrays based on carbon nanotubes (CNT) and palladium or titanium dioxide materials have been synthesized using self-supported and silicon-supported anodized aluminum oxide (AAO) as nanoporous template. It is well demonstrated that carbon nanotubes can be grown using these membranes and hydrocarbon precursors that decompose at temperatures closer to 600°C without the use of a metal catalyst. In this process, carbonic fragments condensate to form stacked graphitic sheets, which adopt the shape of the pores, yielding from these moulds' multi-walled carbon nanotubes. After this process, the ends of the tubes remain open and accessible to other substances, whereas the outer walls are protected by the alumina. Taking advantage of this fact, we have performed the synthesis of palladium and titanium dioxide nanostructures selectively inside carbon nanotubes using these CNT-AAO membranes as nanoreactors.