J.F. Felix

Inkjet Printing of Lanthanide–Organic Frameworks for Anti-Counterfeiting Applications

Photoluminescent lanthanide-organic frameworks (Ln-MOFs) were printed onto plastic and paper foils with a conventional inkjet printer. Ln-MOF inks were used to reproduce color images that can only be observed under UV light irradiation. This approach opens a new window for exploring Ln-MOF materials in technological applications, such as optical devices (e.g., lab-on-a-chip), as proof of authenticity for official documents.

Deep-level transient spectroscopy of interfacial states in “buffer-free” p-i-n GaSb/GaAs devices

A systematic study was carried out on defect states in Interfacial Misfit (IMF) unpassivated and Te-passivated IMF in p-i-n GaSb/GaAs devices using Deep Level Transient Spectroscopy (DLTS) and Laplace DLTS. Additionally, Current-Voltage (I–V) measurements were performed, which showed that the turn-on voltage (Von) of passivated samples is lower than that for unpassivated samples; an effect which can be explained by the introduction of new defects states near to the interface of GaSb/GaAs, where Te was incorporated to passivate the IMF. The Capacitance-Voltage (C-V) analysis demonstrates that these new states are the consequence of adding Te at the misfit of GaSb/GaAs. Furthermore, DLTS measurements reveal a distribution of states including a main midgap energy level, namely the well documented EL2 trap, with some peculiar behaviour. Most of these levels are related to interface states that are generated by the mismatch between GaAs and GaSb. Originally, the addition of Te atoms was thought to passivate these interface states. On the contrary, this paper, which attempts at correlating the current-voltage and capacitance-voltage characteristics to the DLTS results, shows clearly that Te atoms increase the density of interface states.

Investigation of deep-level defects in conductive polymer on n-type 4H- and 6H-silicon carbide substrates using I-V and deep level transient spectroscopy techniques

The current-voltage (I–V) characteristics of Au/sulfonated polyaniline (SPAN)/n-SiC heterojunctions have been investigated in detail over a wide range of temperatures between 20 and 440 K. The measured I–V characteristics of all devices show a good rectification behavior at all temperatures. The room temperature rectification ratios (forward to reverse currents ratio, IF/IR) at 0.6 V for SPAN/n-type 4H-SiC and SPAN/n-type 6H-SiC heterojunctions are 2 × 104 and 7 × 106, respectively. The value of rectification of SPAN/6H-SiC heterojunction is four orders of magnitutude higher than the state-of-the art sulfonated polyaniline thin films deposited on n-type silicon substrates. A self-assembly technique and copolymerization were used to fabricate a self-doped polyaniline films on SiC substrates. The experimental I–V data were analysed using the Werner model, which includes the series resistance of the heterojunctions. The diode parameters such as the ideality factor and the barrier height are determined from t...

Zinc oxide thin films on silicon carbide substrates (ZnO/SiC): electro-optical properties and electrically active defects

The electrical and optical properties of heterojunctions formed by thermally deposited ZnO thin films on n-type 4H-SiC substrates have been investigated. Current–voltage characteristics of the fabricated light emitting devices revealed excellent rectifying behaviors with a typical leakage current lower than 1 nA at a reverse bias of −3 V, and with a forward current at 3 V in the range of 2 mA. A study of the electroluminescent characteristics of ZnO/SiC heterojunctions over the temperature range of 50–450 K showed an emission peak around 410 nm and a broad defect-related electroluminescence at room temperature in the visible range for a forward current of 300 mA. Electrically active deep level centers in ZnO and n-type 4H-SiC epilayers have been investigated by deep level transient spectroscopy (DLTS) and high-resolution Laplace DLTS (LDLTS). Additionally, LDLTS has successfully been employed to resolve the closely spaced hole trap energy levels.

One step fabrication of glass–silver@core–shell fibers: silver-doped phosphate glasses as precursors of SERS substrates

In this study we report on the fabrication, morphological characterization, and surface enhanced Raman scattering (SERS) activity of innovative glass–silver@core–shell substrates obtained by an inexpensive and simple approach. Silver-doped glass fibers were obtained from the phosphate glass system NaH2PO4–GeO2–Al2O3 (3% AgNO3) using the conventional melt quenching technique. The subsequent thermal treatment of the silver-doped glass fibers, in the range of 150 to 350 °C, for annealing times up to 30 min under reducing hydrogen atmosphere, induces migration of the as-dispersed silver ions toward the surface, where they are reduced to form silver-based shell nanostructures on the glass core. We found the SERS activity of the as-produced glass–silver@core–shell fibers can be easily controlled by setting both the annealing temperature and time. Field-emission scanning electron microscopy (FE-SEM) was used to characterize the morphology of the as-produced SERS active substrates, which were correlated with the SERS signal using cresyl violet (CV) as the model. The as-produced glass substrates decorated with silver-based nanostructures revealed to be a robust candidate and a very efficient SERS substrate, providing Raman signal enhancement of about 105 fold orders of magnitude while compared with the silver-undoped glass substrate. Finally, based on the morphological and optical data we proposed a one-step mechanism for the growth of the silver-based nanostructures under the employed synthesis condition.

Fabrication and electrical characterization of polyaniline/silicon carbide heterojunctions

We report on the fabrication of silicon carbide/polyaniline heterojunctions produced by spin coating of polyaniline films onto n-type 6H-SiC and 4H-SiC substrates. Atomic force microscopy was used to estimate the surface roughness, and their electrical characteristics were investigated by means of current, capacitance and conductance measurements as a function of frequency and voltages. Reproducible characteristics and rectification ratios as high as 2 × 10 6 at ±2V for the 6H-SiC based heterojunctions were obtained. The devices were modelled as Schottky diodes with series resistance and an oxide interfacial oxide layer to account for interface traps. By analysing the forward bias I‐V characteristics, we found that the interface trap density for 4H-SiC/PANI heterojunctions is approximately one order of magnitude higher than for 6H-SiC/PANI heterojunctions, which is consistent with previous studies on SiC/SiO2 interface traps. The average value of interface trap densities for 6H-SiC devices was 8.4 × 10 11 eV −1 cm −2 and for 4H-SiC it was 2.7 × 10 13 eV −1 cm −2 . These values are in the range of previous reports on Schottky diodes with polymer layers. (Some figures in this article are in colour only in the electronic version)

Lead–germanate glasses: an easy growth process for silver nanoparticles and their promising applications in photonics and catalysis

In this study, we report non-conventional silver nanoparticle growth on the surface of lead–germanate oxide glasses. Thermal annealing at around the glass transition temperature (Tg) under a nitrogen atmosphere enables the growth of silver thin films on the glass surface. The nanoparticle growth was monitored by scanning electron microscopy (SEM) and UV-visible spectroscopy as a function of the annealing time. The characteristic temperatures were obtained by differential thermal analysis (DTA) and the influence of the Ag+ ion content on the glass stability (GS) parameters was evaluated. Additionally, the apparent activation energy of crystallization (E) was calculated. The silver thin films obtained after different annealing times were applied as a substrate for luminescence enhancement of the Eu(btfa)3·bipy rare earth europium complex. The catalytic activity of the Ag-doped glasses was tested for the reduction of p-nitrophenol in the presence of NaBH4. The catalytic performance of the unannealed glass demonstrated an unexpected good efficiency compared with the annealed glass samples.

Magnetic vortex crystal formation in the antidot complement of square artificial spin ice

We have studied ferromagnetic nickel thin films patterned with square lattices of elongated antidots that are negative analogues of square artificial spin ice. Micromagnetic simulations and direct current magnetic moment measurements reveal in-plane anisotropy of the magnetic hysteresis loops, and the formation of a dense array of magnetic vortices with random polarization and chirality. These multiply-connected antidot arrays could be superior to lattices of disconnected nanodisks for investigations of vortex switching by applied electric current.We have proposed in this work an original system composed by anti-dots nanopatterned in a ferromagnetic thin film, mimicking negatively the structure of an articial spin ice. In the hysteresis loop we notice the emergency of an anisotropy in the magnetization saturation and in the micromagnetic simulations, in the beginning of the hysteresis loop (relaxation), the formation of a vortex crystal array with vortices in diferent positions possessing random polarization and chirality. The crystal of vortices in this electrically connected sample could be most eficient than those observed in non-connected nanodiscs for current-driven or magnetic vortices switching by electric currents.

Investigation of ferromagnetic resonance and magnetoresistance in anti-spin ice structures.

In this work, we report experimental and theoretical investigations performed in anti-spin ice structures, composed by square lattice of elongated antidots, patterned in nickel thin film. The magnetic vortex crystal state was obtained by micromagnetic simulation as the ground state magnetization, which arises due to the magnetic stray field at the antidot edges inducing chirality in the magnetization of platters among antidots. Ferromagnetic resonance (FMR) and magnetoresistance (MR) measurements were utilized to investigate the vortex crystal magnetization dynamics and magnetoelectric response. By using FMR, it was possible to detect the spin wave modes and vortex crystal resonance, in good agreement with dynamic micromagnetic simulation results. The vortex crystal magnetization configuration and its response to the external magnetic field, were used to explain the isotropic MR behaviour observed.

Down- and Up-Conversion Photoluminescence of Carbon-Dots from Brewing Industry Waste: Application in Live Cell-Imaging Experiments

Simple synthetic procedures have been applied to obtain luminescent carbon quantum dots, also referred as C-dots, from an abundant carbon source, that is, from the brewing industry waste. The synthetic procedures have been conducted aiming to investigate the effects of the oxidation stage on the properties of the nanomaterial. C-dots down- and up-conversion properties, as well as their potential for cellular imaging experiments in live (and adhered) cells, are disclosed herein.