Luis Yate

Tailoring mechanical properties and electrical conductivity of flexible niobium carbide nanocomposite thin films

Flexible NbC nanocomposite thin films with carbon content ranging from 0 to 99 at.% were deposited at room temperature on Si (100) and polystyrene substrates by non-reactive magnetron sputtering from pure Nb and C targets without applying bias voltage to the substrates. HRTEM images reveal that the films exhibit a nanocomposite structure consisting of NbC nanocrystals (2 to 15 nm in size) embedded in an amorphous carbon matrix. By simply adjusting the Nb flux in the plasma, we can monitor the nanocrystal size and the percent of free-carbon phase in the films, which in turn allows for the tailoring of both mechanical properties and electrical conductivity of the films. It was found that the films composed of ∼8–10% free-carbon exhibited a relatively high hardness and elastic recovery, around 23 GPa and 85%, respectively, and an electrical conductivity of 2.2 × 106 S m−1 at 22 °C. This study indicates the potential of this non-reactive sputtering approach in depositing hard, elastic and electrically conductive nanocomposite films at low temperatures, which is especially useful for preparation of films on temperature sensitive polymers or plastic substrates for nano- and micro- electronics applications.

Stability of polyelectrolyte multilayers in oxidizing media: a critical issue for the development of multilayer based membranes for nanofiltration

Polyelectrolyte multilayers (PEMs) for nanofiltration and reverse osmosis are fabricated by means of the Layer by Layer technique from the negatively charged poly(sodium 4-styrenesulfonate) (PSS) and polycations with primary, secondary, tertiary, and quaternary amines. PEMs stability is studied after treatment with the oxidizing agent sodium hypochlorite (NaOCl), in the same conditions as in membrane modulus cleaning. PEM assembly and mass changes after treatment with NaOCl are studied with the Quartz Crystal Microbalance. The chemical composition of the PEMs after the treatment with NaOCl is studied by X-ray photoelectron spectroscopy. The oxidation of polycations in bulk is studied by UV–vis. The PEMs fabricated with poly(diallydimethylammonium chloride) (PDADMAC) and poly(vinylbenzyltrimethylammonium chloride) (PVBTMAC), bearing quaternary amines, show the highest chemical stability and smallest mass variations after oxidation. PEMs including other polycations bearing quaternary amines and polycations with primary, secondary, and tertiary amines are either fully removed or significantly changed chemically.

Tuning the photodynamic efficiency of TiO2 nanotubes against HeLa cancer cells by Fe-doping

In this study Fe-doped TiO2 (0.35 to 3.50 wt% Fe) nanotubes (NTs) were prepared as the potential photosensitizer for near-visible light driven photodynamic therapy (PDT) against cervical cancer cells (HeLa). Characterization of the prepared nanotubes by X-ray diffraction (XRD), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) confirmed the successful incorporation of Fe3+ as a dopant into the TiO2 matrix, which was mainly composed of an anatase phase, while elemental mapping using energy dispersive X-ray spectroscopy (EDX) showed homogenous distribution of the dopant ions in TiO2 for both low and high doping levels. UV-Vis studies showed that Fe doping in TiO2 increases the light absorption within the visible range, particularly in the case of 0.70 and 1.40 wt% Fe–TiO2 and provides additional energy levels within the band gap, which promotes the photo-excited charge transport towards the conduction band. Photo-cytotoxic activity of the prepared Fe-doped TiO2 NTs was investigated in vitro against cervical cancer cells (HeLa) and compared with human normal fibroblasts (GM07492). Fe-doped TiO2 NTs exhibited no or lower dark cytotoxicity than un-doped TiO2 NTs, which confirms their superior biocompatibility. Under the near-visible light irradiation (∼405 nm) Fe-doped TiO2 NTs showed higher photo-cytotoxic efficiency than un-doped TiO2 NTs, which was found to be dependent on the NTs concentration, but not on the incubation time of cells after near-visible light irradiation. The highest activity was observed for 0.70 and 1.40 wt% Fe–TiO2 NTs. Fluorescent labeling of treated HeLa cells showed distinct morphological changes, particularly in the perimitochondrial area suggesting a mitochondria-involved apoptosis of cells, but also the nuclei and cytoskeleton were subject to Fe–TiO2 NTs induced photo-damage. Apoptosis of PDT treated HeLa cells was also confirmed using ethidium homodimer (EthD-1).

High Electrocatalytic Response of a Mechanically Enhanced NbC Nanocomposite Electrode Toward Hydrogen Evolution Reaction

Resistant and efficient electrocatalysts for hydrogen evolution reaction (HER) are desired to replace scarce and commercially expensive platinum electrodes. Thin-film electrodes of metal carbides are a promising alternative due to their reduced price and similar catalytic properties. However, most of the studied structures neglect long-lasting chemical and structural stability, focusing only on electrochemical efficiency. Herein we report on a new approach to easily deposit and control the micro/nanostructure of thin-film electrodes based on niobium carbide (NbC) and their electrocatalytic response. We will show that, by improving the mechanical properties of the NbC electrodes, microstructure and mechanical resilience can be obtained while maintaining high electrocatalytic response. We also address the influence of other parameters such as conductivity and chemical composition on the overall performance of the thin-film electrodes. Finally, we show that nanocomposite NbC electrodes are promising candidates toward HER and, furthermore, that the methodology presented here is suitable to produce other transition-metal carbides with improved catalytic and mechanical properties.

Synthesis, structure, EPR studies and up-conversion luminescence of ZnO:Er3+–Yb3+@Gd2O3 nanostructures

ZnO:Er3+–Yb3+@Gd2O3 nanostructures were obtained by “wet” chemistry methods – the sol–gel technique for the preparation of ZnO and ZnO:Er3+–Yb3+ nanoparticles (NPs), and the seed deposition method for obtaining Gd2O3. The crystal structure, morphology, phase and elemental composition, resonant microwave absorption of rare earth ions, point defects in the ZnO:Er3+–Yb3+@Gd2O3 crystal structure and up-conversion luminescence were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive X-ray (EDX), X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR) spectroscopy, and optical spectroscopy. The crystallization temperature (600 °C) of the Gd2O3 phase on the ZnO surface was found. As-obtained ZnO:Er3+–Yb3+ NPs (with size ∼7 nm) are highly crystalline and monodispersed. ZnO:Er3+–Yb3+ NPs annealing at 900 °C leads to the formation of highly polydispersed ZnO:Er3+–Yb3+ NPs, covered by a Gd2O3 shell. The process of the incorporation of the rare earth ions into the ZnO structure, as well as the effect of Gd2O3 content on the morphology and visible up-conversion (UC) luminescence in ZnO:Er3+–Yb3+ matrices were studied.

OPTICAL PROPERTIES DEPENDENCE WITH GAS PRESSURE IN AlN FILMS DEPOSITED BY PULSED LASER ABLATION

AlN films were deposited by pulsed laser deposition technique (PLD) using an Nd: YAG laser (λ = 1064 nm). The films were deposited in a nitrogen atmosphere as working gas; the target was an aluminum high purity (99.99%). The films were deposited with a laser fluence of 7 J/cm2 for 10 minutes on silicon (100) substrates. The substrate temperature was 300 °C and the working pressure was varied from 3 mtorr to 11 mtorr. The thickness measured by profilometer was 150 nm for all films. The crystallinity was observed via XRD pattern, the morphology and composition of the films were studied using scanning electron microscopy (SEM) and Energy Dispersive X-ray analysis (EDX), respectively. The optical reflectance spectra and color coordinates of the films were obtained by optical spectral reflectometry technique in the range of 400 cm-1- 900 cm-1 by an Ocean Optics 2000 spectrophotometer. In this work, a clear dependence of the reflectance, dominant wavelength and color purity was found in terms of the applied pressure to the AlN films. A reduction in reflectance of about 55% when the pressure was increased from 3 mtorr to 11 mtorr was observed. This paper deals with the formation of AlN thin films as promising materials for the integration of SAW devices on Si substrates due to their good piezoelectric properties and the possibility of deposition at low temperature compatible with the manufacturing of Si integrated circuits.

Robust tribo-mechanical and hot corrosion resistance of ultra-refractory Ta-Hf-C ternary alloy films

In this work we report the hot corrosion properties of binary and ternary films of the Ta-Hf-C system in V2O5-Na2SO4 (50%wt.-50%wt.) molten salts at 700 °C deposited on AISI D3 steel substrates. Additionally, the mechanical and nanowear properties of the films were studied. The results show that the ternary alloys consist of solid solutions of the TaC and HfC binary carbides. The ternary alloy films have higher hardness and elastic recoveries, reaching 26.2 GPa and 87%, respectively, and lower nanowear when compared to the binary films. The corrosion rates of the ternary alloys have a superior behavior compared to the binary films, with corrosion rates as low as 0.058 μm/year. The combination and tunability of high hardness, elastic recovery, low nanowear and an excellent resistance to high temperature corrosion demonstrates the potential of the ternary Ta-Hf-C alloy films for applications in extreme conditions.

Hybrid ZnPc@TiO2 nanostructures for targeted photodynamic therapy, bioimaging and doxorubicin delivery

In this study ZnPc@TiO2 hybrid nanostructures, both nanoparticles and nanotubes, as potential photosensitizers for the photodynamic therapy, fluorescent bioimaging agents, as well as anti-cancer drug nanocarriers, were prepared via zinc phthalocyanine (ZnPc) deposition on TiO2. In order to provide the selectivity of prepared hybrid nanostructures towards cancer cells they were modified with folic acid molecules (FA). The efficient attachment of both ZnPc and FA molecules was confirmed with dynamic light scattering (DLS), zeta potential measurements and X-ray photoelectron spectroscopy (XPS). It was presented that ZnPc and FA attachment has a strong effect on fluorescence emission properties of TiO2 nanostructures, which can be further used for their simultaneous visualization upon cellular uptake. ZnPc@TiO2 and FA/ZnPc@TiO2 hybrid nanotubes were then employed as doxorubicin nanocarriers. It was demonstrated that doxorubicin can be easily loaded on these hybrid nanostructures via an electrostatic interaction and then released. In vitro cytotoxicity and photo-cytotoxic activity studies showed that prepared hybrid nanostructures were selectively targeting to cancer cells. Doxorubicin loaded hybrid nanostructures were significantly more cytotoxic than un-loaded ones and their cytotoxic effect was even more severe upon irradiation. The cellular uptake of prepared hybrid nanostructures and their localization in cells was monitored in vitro in 2D cell culture and tumor-like 3D multicellular culture environment with fluorescent confocal microscopy. These hybrid nanostructures preferentially penetrated into human cervical cancer cells (HeLa) than into normal fibroblasts (MSU-1.1) and were mainly localized within the cell cytoplasm. HeLa cells spheroids were also efficiently labelled by prepared hybrid nanostructures. Fluorescent imaging of Hela cells treated with doxorubicin loaded hybrid nanostructures showed that doxorubicin was effectively delivered into cells, released and evenly distributed in the cytoplasm. In conclusion, prepared hybrid nanostructures exhibit high potential as selective bioimaging agents next to their photodynamic activity and drug delivery ability.

Mechanical properties of boron nitride thin films prepared by atomic layer deposition

Due to their wide bandgap, boron nitride (BN) thin films are the focus of interest for their potential applications in microelectronic devices. The reliability of these devices is essential and is directly linked to the mechanical properties of the films used for their fabrication. Herein, an atomic layer deposition (ALD) process based on sequential pulses of BBr3 and NH3 at 750 °C is used in order to prepare BN thin films. We report the main physicochemical properties of the films using various analytical methods. We also performed nanoindentation experiments in order to determine the elastic modulus and the hardness. Next, we annealed the films at 1000 and 1350 °C in order to gain understanding on the relation between the annealing temperature, the microstructure obtained and the resulting mechanical properties. Although the hardness of the films presents similar values of 5 ± 1 GPa for all temperatures, it has been found that the elastic modulus increases up to 150 ± 9 GPa when applying an annealing treatment of 1350 °C, which represents a 37% improvement compared to the initial film prepared at 750 °C.

Humic acid attenuation of silver nanoparticle toxicity by ion complexation and the formation of a Ag3+ coating

The use of silver nanoparticles (AgNPs) result in an inevitable contact with aquatic environments. Here we study the behavior of AgNPs and the developmental toxicity in zebrafish embryos exposed to these nanoparticles (0-10 mg/L) with and without the presence of HA (20 mg/L), using zebrafish facility water (ZFW) and zebrafish growing media (ZGM). The presence of cations and HA gave rise to a decrease in Ag ion release and ζ-potential, an increase in the hydrodynamic diameter and oxidation of the AgNP surface. The results show that the presence of HA and cations in the media, as well as the silver speciation, i.e., the unusual presence of Ag3+, decreases the toxicity of AgNPs (LC50AgNPs: 1.19 mg/L; LC50AgNPs + HA: 3.56 mg/L), as well as silver bioavailability and toxicity in zebrafish embryos. Developmental alterations and the LC50 (1.19 mg/L) of AgNPs in ZFW were more relevant (p ≤ 0.05) than for AgNPs in ZGM (LC50 ˃ 10 mg/L). It was demonstrated that the bioaccumulation and toxicity of AgNPs depends on several factors including AgNPs concentration, nanoparticle aggregation, dissolved silver ions, speciation of silver ions, the amount of salt in the environment, the presence of humic substances and others, and different combinations of all of these factors.