The thickness of vanadium dioxide (VO2) films is a crucial parameter for the study of their optical and thermal properties. In this paper we studied the effect of the film thickness on the thermal hysteresis loop during the phase transition of VO2 deposited on a sapphire substrate by pulsed laser deposition (PLD), by the application of the Infrared Thermography technique. We measure the main thermal hysteresis parameters of VO2 samples with different thicknesses in the LWIR range (8-14 mu m) showing how the transition temperature during the heating and cooling cycles, and the width of the hysteresis loop, may change with thickness. We analyzed and compared the obtained results with, in situ Grazing Incidence X-Ray Diffraction (GI-XRD). A good agreement between the results obtained with the two techniques was found demonstrating the reliability of the IR Thermography as a quantitative characterization tool.The results show that the structural and IR emissivity properties of the VO2 layer exhibit a dynamic range dependent on the layer thickness due to a correlation with the crystalline grain size. This has important effects in view of a tailored energy management for the use of those materials as smart radiators or smart windows.

Vanadium dioxide has attracted much interest due to the drastic change of the electrical and optical properties it exhibits during the transition from the semiconductor state to the metallic state, which takes place at a critical temperature of about 68 degrees C. Much study has been especially devoted to developing advanced fabrication methodologies to improve the performance of VO2 thin films for phase-change applications in optical devices. Films structural and morphological characterisation is normally performed with expensive and time consuming equipment, as x-ray diffractometers, electron microscopes and atomic force microscopes. Here we propose a purely optical approach which combines Polarized Raman Mapping and Phase-Transition by Continuous Wave Optical Excitation (PTCWE) to acquire through two simple measurements structural, morphological and thermal behaviour information on polycrystalline VO2 thin films. The combination of the two techniques allows to reconstruct a complete picture of the properties of the films in a fast and effective manner, and also to unveil an interesting stepped appearance of the hysteresis cycles probably induced by the progressive stabilization of rutile metallic domains embedded in the semiconducting monoclinic matrix.

Thin layers of MoOx have been deposited by thermal evaporation followed by post-deposition annealing. The density of states distributions of the MoOx films were extracted deconvoluting the absorption spectra, measured by a photothermal deflection spectroscopy setup, including the small polaron contribution. Results revealed a sub-band defect distribution centered 1.1 eV below the conduction band; the amplitude of this distribution was found to increase with post-deposition annealing temperature and film thickness.

A cheap, compact and customisable characterisation system for memristor devices, working between +/- 10 V, is presented. SPICE (Simulation Program with Integrated Circuit Emphasis) simulations are performed to verify the circuit feasibility and a proper software is developed to drive the system. The potentiality of the realised system is tested by performing several electrical measurements on both Cu/HfO2/Pt memristors and two-terminals com-mercial devices.

We report the photocatalytic properties of ZnO and Cu2O thin films deposited on glass substrates at room temperature by DC sputtering and pulsed laser deposition. The photoactivity of the films was investigated through the degradation of rhodamine B (RhB) and methyl orange (MO) under solar light. In order to select the most suitable film of ZnO for the of RhB and MO degradation, the relationship between the characteristics (e.g. energy levels and defects concentration) of ZnO films and their effectiveness in the photocatalytic yield of RhB and MO been studied, where several films were deposited by using different oxygen partial pressures (PO2: 0.05–1.3 mbar), while Cu2O films were grown under a pressure of 0.01 mbar. The XRD patterns show that all ZnO films have (002) preferential orientation, and crystallite size increases from 73 to 122 nm raising PO2. The gap Eg of ZnO (3.26 and 4.15 eV) depends on PO2, and the films present photoluminescence emission in the UV–Vis-near IR region. On the basis of structural, optical and electrical characterizations of both films, a comparative study was carried out on the dyes degradation. Cu2O films exhibit a high photoactivity with MO (81.69%) under solar light (6 h), whilst for RhB the best elimination rate (60.85%) was achieved with ZnO films deposited at 0.1 mbar, which were also the ones exhibiting the highest PL peak intensity at the characteristic absorption wavelength of RhB (553 nm).

Plasmonic nanoantennas (NAs) have received a growing attention in recent years due to their ability to confine light on sub-wavelength dimensions [1]. More recently, this property has been exploited in the terahertz (THz) frequency range (0.1-10 THz) for enhanced sensing and spectroscopy [2], as well as for more fundamental investigations [3]. These applications typically require high local electric fields that can be achieved by concentrating THz radiation into deeply sub-wavelength volumes located at the NAs extremities. However, the achievable near-field enhancement values are severely limited by the poor resonance quality factor of traditional rod-shaped THz NAs. Unlike what is commonly assumed in the infrared domain [4], here we show that an optimal NA tapering angle can be effectively introduced to obtain higher quality factors and, at least, twofold higher local near-field enhancement in comparison with standard (wire-like) dipolar THz NAs. To evaluate how the tapering angle affects the NA performance, a simplified quasi-analytical model was first developed. Each NA is considered as a truncated cone constituted by a sequence of gold cylinders of increasing radii, so that the effective refractive index of the surface mode propagating along the NA changes gradually along the main axis. Once the reflection coefficients for the surface mode at both extremities are retrieved [5], a NA can be interpreted as a Fabry-Perot resonator and its resonances can be analytically calculated. This model reveals a trade-off between large tapering angles (resulting in a low reflection coefficient at the large extremity) compared to small tapers (which are affected by high propagation losses for the surface mode), leading to an optimal taper angle. FEM-based simulations (COMSOL Multiphysics) were then used to confirm this prediction. 60-nm-thick gold tapered NA dimers were designed with 45-μm-long arms (in order to resonate at around 1 THz) and with their facing tips (100-nm-wide) separated by a 30 nm gap, thus realizing a bowtie geometry (Fig. 1a). We numerically investigated the near-field enhancement in the gap between the NAs, varying the tapering angle α from 0° to 10°.

Cu2O/ZnO:Al (AZO) and Cu2O/ZnO/AZO heterojunctions have been deposited on glass substrates by a unique three-step pulsed laser deposition process. The structural, optical, and electrical properties of the oxide films were investigated before their implementation in the final device. X-ray diffraction analysis indicated that the materials were highly crystallized along the c-axis. All films were highly transparent in the visible region with enhanced electrical properties. Atomic force and scanning electron microscopies showed that the insertion of a ZnO layer between the Cu2O and AZO films in the heterojunction enhanced the average grain size and surface roughness. The heterojunctions exhibited remarkable diode behavior and good rectifying character with low leakage current under reverse bias. The presence of the ZnO interlayer film significantly reduced the parasitic and leakage currents across the barrier, improved the quality of the heterostructure, made the energy band between AZO and Cu2O layers smoother, and eliminated the possibility of interface recombination, leading to much longer electron lifetime.

The paper presents the resistive switching of electroforming-free Ti/anodic-TiO 2 /Cu memristors. Anodic TiO 2 thin films were prepared by anodizing Ti layers. Microscale devices were fabricated by direct laser-assisted photolithography. Experimental results showed a bipolar and self-rectifying behavior of the devices, which could be useful for crossbar array configurations. Moreover, a gradual resistive switching of the devices in both directions was observed, indicating the presence of multi-level resistance states.

ZnO thin films were deposited by home-made DC sputtering of zinc target under mixed gases (Argon, Oxygen) plasma on glass substrates. Films were deposited by varying oxygen partial pressure (PO2) from 0.09 to 1.3 mbar in the deposition chamber, at a fixed substrate temperature of 100 degrees C. The samples were characterized by photoluminescence (PL), X-ray diffraction (XRD), optical transmissions (UV-vis), scanning electron microscopy (SEM) and electrical (Hall effect) measurements. The results indicate that by varying the oxygen pressure in the deposition chamber, the films show a precise and well defined photoluminescence emissions for each range of pressure covering almost the entire visible domain (UV, UV-Violet, Violet, Blue, and Red) with high intensities. Moreover, the deposited films have different defects levels. The XRD analysis indicates that the films are well grown along the c-axis peak, but with different crystalline quality. Optical measurements reveal a high transmission, up to 90%, in the spectral region between 400 and 2500 nm and a large variation of the optical band gap (3.16-4.34 eV). As an application of the deposited ZnO films, the photo-catalytic degradation of a synthetic solution of Rhodamine B (RhB) poured on a ZnO thin film was successfully achieved and an elimination rate of 38% was obtained after exposing the film to solar light for 3 h.

A way to grow and characterize isolated and coalesced ZnO nanorods on p-GaN/sapphire structure is presented. Chemical bath deposition can be used to grow ZnO nanorods of device-quality, simply controlling the duration time of the growth process and the concentration of the nutrient solution in the bath. Increasing the duration of the process, as well as the concentration of the solution, leads to compact and sound layers instead of separated nanorods. However, too high concentrations stop the growth process. Light-emitting diodes fabricated on these ZnO-p-GaN heterostructure have a peak of electroluminescence at 400 nm and exhibit interesting electrical and optical properties. Optical power of 225 μW at 0.425 A/cm 2 and a related wall-plug efficiency of 0.23% are obtained with 2 mm-diameter LEDs fabricated on a ZnO layer grown for 30 h in a solution of concentration 70 mM. The performances shown by these LEDs can be used as a way to test the quality of the ZnO growth.

The potentiality of anodic TiO2 as an oxide material for the realization of resistive switching memory cells has been explored in this paper. Cuianodic-TiO2/Ti memristors of different sizes, ranging from 1 x 1 mu m(2) to 10 x 10 mu m(2) have been fabricated and characterized. The oxide films were grown by anodizing Ti films, using three different process conditions. Measured IV curves have shown similar asymmetric bipolar hysteresis behaviors in all the tested devices, with a gradual switching from the high resistance state to the low resistance state and vice versa, and a R-OFF/R-ON ratio of 80 for the thickest oxide film devices. (C) 2017 Elsevier Ltd. All rights reserved.

In this work, we report on a new, efficient and low cost process of silicon carbide (SiC) powder purification intended to be used in photovoltaic applications. This process consists on the preparation of porous silicon carbide layers followed by a photo-thermal annealing under oxygen atmosphere and chemical treatment. The effect of etching time on impurities removal efficiency was studied. Inductively coupled plasma atomic emission spectrometry (ICP-AES) results showed that the best result was achieved for an etching time of 10 min followed by gettering at 900 degrees C during 1 h. SiC purity is improved from 3N (99.9771%) to 4N (99.9946%). Silicon carbide thin films were deposited onto silicon substrates by pulsed laser deposition technique (PLD) using purified SiC powder as target. Significant improvement of the minority carrier lifetime was obtained encouraging the use of SiC as a passivation layer for silicon. (C) 2016 Elsevier Ltd. All rights reserved.

Photoelectrochemical polymerization of poly(3,4-ethylenedioxythiophene), PEDOT, was successfully realized on anodic film grown to 50 V on magnetron sputtered Ti–6 atom % Si alloys. Scanning electron microscopy allowed us to evidence formation of compact and uniform polymer layers on the oxide surface. Photoelectrochemical and impedance measurements showed that photopolymerization allows one to grow PEDOT in its conducting state, while a strong cathodic polarization is necessary to bring the polymer in its p-type semiconducting state. Information on the optical and electrical properties of metal/oxide/polymer/electrolyte junctions proves that PEDOT has promising performance as an electrolyte in metal/insulator/metal structures, while its use in metal/oxide/semiconductor-based devices is negatively influenced by the occurrence of lithium intercalation phenomena during the dedoping process.

The resistive switching behaviour observed in microscale memristors based on laser ablated ZnO and VO2 is reported. A comparison between the two materials is reported against an active device size. The results show that devices up to 300 × 300 μm2 exhibit a memristive behaviour regardless of the device size, and 100 × 100 μm2 ZnO-based memristors have the best resistance off/on ratio.

We report on incorrect carrier type identification achieved by Hall effect measurements performed on ZnO films grown by pulsed laser deposition on InP substrates and subsequently annealed for 1 h at 600 °C in air. While Hall measurements, after post-growth annealing, reveal a change in the electrical properties of the films, from n-type to p-type, both photocurrent-based and standard C−V measurements performed on the same samples show no change in the native n-type doping of the ZnO films. A possible interpretation of the two results is reported. In particular, p-type conductivity observed by Hall effect may be ascribed to a highly conductive thin layer formed during the annealing process at the ZnO/InP interface, which dominates the Hall effect measurements and does not influence the photo-electrochemical behavior of ZnO as well as the measured differential capacitance. The conflicting results here reported show that for this kind of samples, Hall effect measurement can be misleading with respect to the real nature of the analyzed material, instead both C−V and photocurrent-based characterization techniques are more reliable and therefore could be alternatively used when particularly ambiguous results are expected by Hall effect measurements.

We report on ZnO epitaxial growth by pulsed-laser deposition (PLD) on different substrates, such as quartz, sapphire, and GaN template. Approximately 1 μm-thick films were grown under different substrate temperatures and background oxygen conditions. X-ray diffraction analysis indicated preferential growth along the c-axis direction with a full-width at half maximum (FWHM) of the rocking curve as narrow as 230 arcs in the case of the GaN template. Low-temperature photoluminescence showed A-excitonic emission near 3.36 eV and a FWHM of D0XA emission as small as 2.89 meV at 9 K. Atomic force microscope measurements showed that roughness as low as 18 nm could be obtained. These results prove that PLD is a low-cost technique suitable to grow heteroepitaxial ZnO layers with excellent properties in terms of luminescence, crystallinity and morphology.

A high efficiency white light emitting diode (LED) was fabricated by generation of frequency downconversion from a GaN/InGaN blue LED. In place of conventional inorganic phosphors, a perylene-based dye was used for colour conversion. The resulting hybrid structure is analysed by focusing on the visual performance of the realised LEDs employing the most relevant photometric parameters of a light source. Preparation of the organic polymer is described as well. The thermal stability of the dye was investigated and a simple structure which avoids colour degradation is proposed.

We present a feasibility study for loading cold atomic clouds into magnetic traps created by single-wall carbon nanotubes grown directly onto dielectric surfaces. We show that atoms may be captured for experimentally sustainable nanotube currents, generating trapped clouds whose densities and lifetimes are sufficient to enable detection by simple imaging methods. This opens the way for a different type of conductor to be used in atomchips, enabling atom trapping at submicron distances, with implications for both fundamental studies and for technological applications.

We present experimental results on 40 Gb/s large-signal modulation performance of 1.31 Am monolithic integrated laser-modulator in the InGaAlAs/InP material system, exploiting the gain and absorption properties of an identical multiple quantum well (MQW) active layer. In continuous wave operation, at 15 degrees C, the devices achieved threshold currents < 28 mA, fiber coupled optical power levels up to + 0.4 dBm. The measured small-signal modulation bandwidth was about 32 GHz. An air-cavity based Fabry-Perot interferometer has been realized to characterize the spectral chirp of the integrated structures in the time domain up to 40 Gb/s.