National Institute Of Materials Physics - Romania

Hysteretic effects are investigated in perovskite solar cells in the standard FTO/TiO2/CH3NH3PbI3-xClx/spiro-OMeTAD/Au configuration. We report normal (NH) and inverted hysteresis (IH) in the J-V characteristics occurring for the same device structure, and the behavior strictly depends on the prepoling bias. NH typically appears at prepoling biases larger than the open circuit bias, while pronounced IH occurs for negative bias prepoling. The transition from NH to IH is marked by an intermediate mixed hysteresis behavior characterized by a crossing point in the J-V characteristics. The measured J-V characteristics are explained quantitatively by the dynamic electrical model. Furthermore, the influence of the bias scan rate on the NH/IH hysteresis is discussed based on the time evolution of the accumulated ionic and electronic polarization charge at the interfaces. Introducing a three-step measurement protocol, which includes stabilization, prepoling, and measurement, we put forward the difficulties and possible solutions for a correct photoconversion efficiency evaluation.

The substantial increase in the power conversion efficiency of hybrid perovskite solar cells, to date reaching more than 20% in the laboratory, has strongly motivated research on this class of organic-inorganic materials and related devices, particularly based on CH3NH3PbI3-xXx/TiO2 heterostructures (X = Cl,Br). Taking under consideration that a ferroelectric substrate may act as an efficient electron transporter, positively influencing charge collection across the interface and allowing the tuning of the halide perovskite (HP) - ferroelectric junction, we performed extensive density functional theory calculations on CH3NH3PbI3-xClx layers deposited on tetragonal PbTiO3 (PTO) (001) surfaces, to study their structural and electronic properties. The main findings of this study are as follows. (i) A ferroelectric polarization pointing from the PTO/HP interface to the PTO is favorable for the photogenerated electrons transfer across the interface and their transport to the collecting electrode. (ii) The PTO internal electric field leads to a position dependent energy levels diagram. (iii) The HP gap may be tuned by chlorine concentration at the interface, as well as the by the surface terminations of PbTiO3 and hybrid perovskite layers. (iv) The presence of the PTO ferroelectric surface is likely to have just a slight orientational effect on the (CH3NH3)(+) dipoles.

Two series of cyclometalated palladium(II) complexes based on Schiff bases and N-benzoylthiourea (BTU) ligands having four alkoxy groups at their periphery, with different number of carbon atoms, have been designed and prepared. The liquid crystalline behavior of these palladium(II) complexes was investigated by a combination of polarized optical microscopy (POM), differential scanning calorimetry (DSC) and powder X-ray diffraction while their thermal stability was studied by thermogravimetric analysis (TGA). The palladium(II) complexes are thermally stable on a broad temperature range up to 230 degrees C showing, depending on the alkoxy chain length, either monotropic or, for longest alkyl chain, enantiotropic smectic A and C phases. The luminescent properties of palladium(II) complexes were reported. These complexes show a yellow-orange solid-state emission at room temperature with two emission maxima at lambda(max) around 580 and 650 nm, respectively, with a shoulder around 710 nm, when the samples are irradiated in the 330-380 nm range. (C) 2017 Elsevier B.V. All rights reserved.

Pure BT and BT doped with 1 and 2 mol% Zr, with orthorhombic-tetragonal (O-T) phase transitions near room temperature were prepared, with different grain sizes, by conventional ceramic method, at various sintering temperatures. Intrinsic and extrinsic effects of Zr addition on structural, dielectric, piezoelectric and ferroelectric properties of BT ceramics, near O-T phase transition were studied. In coarse grained ceramics, the intrinsic effects manifested in low field measurements (dielectric and piezoelectric constants), were reduced by Zr addition, while the extrinsic effects which control the high field response (remanent polarization and coercive field) were significantly enhanced. Instead, in fine grains ceramics, of either pure or doped BT, the extrinsic effects related to increased domain wall and grain boundaries density were dominant in both low and high field measurements, overlapping Zr effects. The results were explained in terms of crystal anisotropy correlated with grain size effects.

Aluminum Nitride (AlN) thin films were synthesized on Si (100) wafers at 450 degrees C by pulsed laser deposition. A polycrystalline AlN target was multipulsed irradiated in a nitrogen ambient, at different laser pulse repetition rate. Grazing Incidence X-Ray Diffraction and Atomic Force Microscopy analyses evidenced nanocrystallites with a hexagonal lattice in the amorphous AlN matrix. The thickness and optical constants of the layers were determined by infrared spectroscopic ellipsometry. The optical properties were studied by Fourier Transform Infrared reflectance spectroscopy in polarised oblique incidence radiation. Berreman effect was observed around the longitudinal phonon modes of the crystalline AlN component. Angular dependence of the A(1)LO mode frequency was analysed and connected to the orientation of the particles' optical axis to the substrate surface normal. The role of the laser pulse frequency on the layers' properties is discussed on this basis. (C) 2016 Elsevier B.V. All rights reserved.

Epitaxial thin films of NiFe2O4 are fabricated by pulsed laser deposition on SrTiO3 substrate. Symmetrical capacitor-like structures are formed using SrRuO3 as bottom and top electrodes. Electrical characterizations, including current-voltage, capacitance-voltage and capacitance-frequency measurement, reveal a hysteresis-like behaviour for current and capacitance as function of voltage. This could be assigned to a resistive and/or capacitive switching. A "degradation" process takes place after repeated voltage cycling or after heating the sample to 400 K, leading to the stabilization of different resistive states. These features can be related to the changes observed in the capacitance-frequency characteristics, suggesting the presence of a relaxation mechanism at low frequencies, and can be associated with the presence of a deep donor-type level in the band-gap of the NiFe2O4 layer.

A dynamic electrical model is introduced to investigate the hysteretic effects in the J-V characteristics of perovskite based solar cells. By making a simple ansatz for the polarization relaxation, our model is able to reproduce qualitatively and quantitatively detailed features of measured J-V characteristics. Pre-poling effects are discussed, pointing out the differences between initially over- and under-polarized samples. In particular, the presence of the current overshoot observed in the reverse characteristics is correlated with the solar cell pre-conditioning. Furthermore, the dynamic hysteresis is analyzed with respect to changing the bias scan rate, the obtained results being consistent with experimentally reported data: the hysteresis amplitude is maximum at intermediate scan rates, while at very slow and very fast ones it becomes negligible. The effects induced by different relaxation time scales are assessed. The proposed dynamic electrical model offers a comprehensive view of the solar cell operation, being a practical tool for future calibration of tentative microscopic descriptions. (C) 2016 Elsevier B.V. All rights reserved.

Investigations on antenna array for 5.8 GHz frequency band applications are presented in this paper. In order to enhance the characteristics of the dielectric resonator antenna elements, a pair of resonators with different sizes and dielectric constants is used. The antenna elements are connected to four daughter-boards of Universal Software Radio Peripherals, which perform the signal processing required by the sources detection and beamforming.

Synthetic physiological fluids are currently used as a first in vitro bioactivity assessment for bone grafts. Our understanding about the interactions taking place at the fluid-implant interface has evolved remarkably during the last decade, and does not comply with the traditional International Organization for Standardization/final draft International Standard 23317 protocol in purely inorganic simulated body fluid. The advances in our knowledge point to the need of a true paradigm shift toward testing physiological fluids with enhanced biomimicry and a better understanding of the materials' structure-dissolution behavior. This will contribute to "upgrade" our vision of entire cascades of events taking place at the implant surfaces upon immersion in the testing media or after implantation. Starting from an osteoinductive bioglass composition with the ability to alleviate the oxidative stress, thin bioglass films with different degrees of polymerization were deposited onto titanium substrates. Their biomineralization activity in simulated body fluid and in a series of new inorganic-organic media with increasing biomimicry that more closely simulated the human intercellular environment was compared. A comprehensive range of advanced characterization tools (scanning electron microscopy; grazing-incidence X-ray diffraction; Fourier-transform infrared, micro-Raman, energy-dispersive, X-ray photoelectron, and surface-enhanced laser desorption/ionization time-of-flight mass spectroscopies; and cytocompatibility assays using mesenchymal stem cells) were used. The information gathered is very useful to biologists, biophysicists, clinicians, and material scientists with special interest in teaching and research. By combining all the analyses, we propose herein a step forward toward establishing an improved unified protocol for testing the bioactivity of implant materials.

The new opportunities introduced by the large development of the IoT (internet of things) are increasing the demand for sensors to be located as close as possible to the supervised process. The Aluminum Nitride (AIN) is one of the most promising materials for sensors due to its piezoelectric, excellent mechanical properties, chemical inertness and high melting point. Due to these material properties, the AlN sensors are suitable to operate in high temperature and harsh environment conditions and therefore are very promising to be employed in industrial applications. In this article are presented the studies conducted on several Aluminum Nitride-Coated Steel structures with the goal of producing sensors embedded in the ball bearings, bearings and other mobile parts of machine tools. The experiments were conducted on simple coatings structures without lubricating materials and the obtained results are promising, demonstrating that, with some limitations the AIN could be used in such applications.