National Institute Of Materials Physics - Romania

Photo-induced effects on charging and discharging of nanocrystals (NCs) in capacitor memories with Ge NCs in an HfO2 matrix as a floating gate layer are studied. The sequence of layers HfO2/Ge-HfO2/ HfO2 was deposited on a p-Si substrate using magnetron sputtering. Well separated Ge NCs are obtained after rapid thermal annealing at 600 degrees C. The optoelectric capacitor memories were fabricated with a semi-transparent electrode on top of the structure and an Al electrode on the back side of the Si substrate. Light illumination effects on hysteresis curves were investigated using different operation modes. The hysteresis window increases by increasing the light exposure time. The spectral dependence of the hysteresis window shows the maximum contribution of the light in the wavelength range of 950-1000 nm, corresponding to both contributions from the Si substrate and from Ge NCs. The stored information about the electrical and optical pulses is also investigated in the regime of the flat band potential measurements (retention measurements). It is shown that in our memory structure, the photo-effect on the memory retention corresponds to a tunnelling transfer of negative charges from the Si substrate to Ge NCs, up to a mean value of 1.6 electrons per NC. Published by AIP Publishing.

Electrospun sub-micrometer polymer fiber mats represent an interesting substrate which can be employed as a transparent conducting electrode. Functionalization by using nanostructures represents a convenient way of increasing the range of applications. The present paper describes an electrodeposition process which can be applied for preparing ZnO nanostructures covered fibers in a straightforward manner. Poly(methyl methacrylate) fiber mats were obtained by electrospinning using metal frame collectors. Subsequent metallization by DC sputtering was used, these microstructured electrodes being thermally transferred onto glass substrates and further employed as working electrodes for the electrochemical deposition of ZnO. The transparency of the metal covered webs, a function of fiber density, is comparable to that of conventional transparent conductive oxides electrodes such as ITO. The same enhanced control of the ZnO electrodeposition process was observed for the case of the web electrodes as for the classic case of deposition on transparent conducting oxides or on metallic substrates. Structural, optical, morphological and wetting properties were investigated and correlated with the electrodeposition conditions. The photocatalytic properties of ZnO covered fibers were tested through the decomposition of methylene blue thin films under UV irradiation.

Two ZnO materials of spherical hierarchical morphologies, with hollow (ZnOHS) and solid cores (ZnOSS), were obtained through the hydrolysis of zinc acetylacetonate in 1,4-butanediol. The nature of the defects and surface reactivity for the two ZnO materials were investigated through photoluminescence, X-ray photoelectron spectroscopy, and electron paramagnetic resonance (EPR) spectroscopy proving the coexistence of shallow and deep defects and, also, the presence of polyol byproducts adsorbed on the outer layers of the ZnO samples. The EPR spectroscopy coupled with the spin-trapping technique showed that the surface of the ZnO samples generates reactive oxygen species (ROS) like hydroxyl (OH) and singlet oxygen (O-1(2)) as well as carbon-centered radicals. The ZnO materials exhibited a wide spectrum of antimicrobial activity, being active against Gram-positive, Gram-negative, and fungi strains, both in planktonic and, more importantly, adherent groywth states. The decrease of antimicrobial efficiency in the presence of a ROS scavenger (mannitol) and the decrease of the cell viability with the ROS level suggest that one of the mechanisms that governs both the antimicrobial and cytotoxic activities on human liver cells is ROS-mediated. However, at active antimicrobial concentrations, the biocompatibility of the tested materials is very good.

Highly defected tubular SiO2 is found to outperform the activity of TiO2-P25 and the SiO2-TiO2 composite in photocatalytic H-2 generation from methanol-water solution under simulated solar light AM 1.5. The enhanced performances of SiO2 come from the particularities of the reaction mechanism and ROS (reactive oxygen species) generation. The SiO2 exposed to light generates solely O-1(2) (singlet oxygen). The TiO2-P25 produces uniquely large quantities of OH radicals whereas the formation of O-2(-) is evidenced only over SiO2-TiO2, along with small amounts of OH. The TiO2 works as a photocatalyst by intermediation of OH radicals. In contrast, the organic substrate is activated on the surface of SiO2 by the intra-band gap, isolated, surface quantum defects. Distinct reaction mechanisms, involving the participation of photogenerated charges and ROS, are proposed. The material-related ROS production can be of great practical importance in fields such as biology (germ inactivation), medicine (photodynamic therapy by O-1(2)), and synthesis of oxygenated organic compounds of great added value.

Chalcogenide amorphous materials, such as GeTe, are known to exhibit deposition dependent optical and structural properties. The formation of a single and homogeneous amorphous GeTe (a-GeTe) phase is questionable since the deposited films can be mixtures of monoelemental nanoclusters. In this work, we employed two deposition techniques, pulsed laser deposition from a polycrystalline GeTe target and co-sputtering from two distinct Ge and Te targets, respectively, to obtain a-GeTe films. To improve the homogeneity of the amorphous phase obtained by magnetron sputtering, the substrate temperature was varied from room temperature up to 180 degrees C. The samples were investigated by X-ray diffraction, X-ray reflectometry, X-ray photoelectron spectroscopy and spectroscopic ellipsometry. It was found that the film mass density, optical bandgap, refractive index and absolute reflectivity become progressively larger with increasing substrate temperature, due to the minimization of voids fraction and the number of dangling bonds in the amorphous structure. Moreover, X-ray photoelectron spectroscopy results prove the formation of Ge-Te bonds and therefore of the GeTe alloy at the optimal substrate temperature of 180 degrees C. This study reveals the importance of optimizing the deposition conditions for obtaining a specific amorphous phase, which enables the atomic rearrangements responsible for fast phase-change needed in memory applications.

Organometallic compounds could be an excellent alternative to the organic active layers for solar cells due to several better properties like thermal and chemical stability. These organometallic compounds are electron donor materials which are easily used in donor-acceptor heterojunctions in these solar cells. One of the main problems for the active media in the solar cells is connected with the exciton diffusion length which limits the thickness of the donor layer in these donor-acceptor heterojunctions. A way to improve the exciton diffusion length is better charge transfer between the donor and acceptor facilitating the exciton diffusion towards the electrodes of the solar cells. The adding of electronegative ions or chemical groups could also influence the band alignment between the donor and acceptor smoothing the charge transport across the solar cells.

Sensors based networked mesoporous SnO2 nanostructures templated by non-ionic surfactant - Brij (R) 35 were prepared via hydrothermal chemistry route. Specific patterns of the structure, morphology, surface chemistry and sensing properties were obtained by pH and autogenous pressure tuning. Consequently, the as-obtained SnO2 powders were subjected to extensive BET, Raman, TEM, HRTEM and XPS complementary investigations. The sensitive films were obtained by screen printing deposition of the powders onto commercial Al2O3 substrates. The gas sensing properties were assessed towards different hazardous gas species: CO, CH4, NH3, NO2, SO2 and H2S over a wide range of operating temperatures. Our particular SnO2 HP sensor synthesized at high autogenous pressure showed the highest selective-sensitivity to H2S (< 5 ppm) under 50% relative humidity (RH). The enhancement in the H2S sensitivity at low operating temperature under infield conditions was found to be closely connected to the morphological aspects and surface chemistry, peculiarities that can be assessed as consequences of the chemical tuning.

Short powder-in-tube tapes of MgB2 in the Fe sheath were fabricated by ex situ route from a commercial powder containing some free Mg and MgO impurity phases. The final heat treatment was performed by spark plasma sintering (SPS). Tapes were with open (OT) or closed (CT) endings. Closed endings were made by folding and pressing. The MgB2 core of the OT sample has shown a higher low-field critical current density, a higher maximum pinning force, a slightly higher disorder, smaller average MgB2 crystallite size, a weak contact between Fe and MgB2 core, and more macro-flux jumps. The upper and irreversibility fields were similar for OT and CT samples. In the center of the MgB2 cores, the detected impurity phase is MgO, while at the interface with Fe, MgB4 also occurs. Impurity phases found at interface, MgO and MgB4, are present in the center of the bulk SPSed samples. Reactions and pinning-force-related parameters are discussed with respect to Mg behavior influenced by condition of endings. It is inferred that the presence of free Mg in the raw MgB2 powder has an important contribution to observed differences, and its removal or control is recommended.

Contamination of water with heavy metals such as lead is a major worldwide problem because they affect the physiological functions of living organisms, cause cancer, and damage the immune system. Hydroxyapatite, (Ca-5(PO4)(3)OH) is considered one of the most effective materials for removing heavy metals from contaminated water. The hydroxyapatite nanopowders (N-HAp) obtained by a co-precipitation method were used in this research to determine the effectiveness in removing lead ions from contaminated solutions. In this study, we have investigated the structure and morphology of N-HAp nanopowders using X-ray diffraction (XRD), electronic transmission microscopy (TEM), and scanning electron microscopy (SEM). The structure information was also obtained by spectroscopy measurements. The Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy measurements revealed the presence of peaks corresponding to the phosphate and hydroxyl groups. The ability of N-HAp nanopowders to adsorb lead ions from aqueous solutions were established. The results of the kinetic and equilibrium studies on the removal of Pb (II) from aqueous solution revealed that the adsorption of lead (II) cations is due to the surface reaction with the hydroxyl terminal groups on the adsorbent and the combination of the positive charges of the metal cations with the negative charges on the adsorbent surfaces. These observations could validate the use of these ceramic nanopowders in ecological remediation strategies.

High-performance bioceramics are required for preventing failure and prolonging the life-time of bone grafting scaffolds and osseous implants. The proper identification and development of materials with extended functionalities addressing socio-economic needs and health problems constitute important and critical steps at the heart of clinical research. Recent findings in the realm of ion-substituted hydroxyapatite (HA) could pave the road towards significant developments in biomedicine, with an emphasis on a new generation of orthopaedic and dentistry applications, since such bioceramics are able to mimic the structural, compositional and mechanical properties of the bone mineral phase. In fact, the fascinating ability of the HA crystalline lattice to allow for the substitution of calcium ions with a plethora of cationic species has been widely explored in the recent period, with consequent modifications of its physical and chemical features, as well as its functional mechanical and in vitro and in vivo biological performance. A comprehensive inventory of the progresses achieved so far is both opportune and of paramount importance, in order to not only gather and summarize information, but to also allow fellow researchers to compare with ease and filter the best solutions for the cation substitution of HA-based materials and enable the development of multi-functional biomedical designs. The review surveys preparation and synthesis methods, pinpoints all the explored cation dopants, and discloses the full application range of substituted HA. Special attention is dedicated to the antimicrobial efficiency spectrum and cytotoxic trade-off concentration values for various cell lines, highlighting new prophylactic routes for the prevention of implant failure. Importantly, the current in vitro biological tests (widely employed to unveil the biological performance of HA-based materials), and their ability to mimic the in vivo biological interactions, are also critically assessed. Future perspectives are discussed, and a series of recommendations are underlined.