Publications

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.

Based on simulations using the plane-wave expansion method, we show that the position of the first bandgap for TM polarized electromagnetic radiation in a high-filling ratio two-dimensional square photonic crystal is practically insensitive to the shape of dielectric rods with the same cross-sectional area. The width of this bandgap depends on the rod shape, but the difference between photonic crystals with circular and regular polygonal rod shapes with a number of edges higher than six can be neglected. For higher-order bandgaps the difference is larger. These results suggest that the fabrication tolerances related to the shape of dielectric rods in a PBG are not as strict as previously thought. (C) 2017 Elsevier B.V. All rights reserved.

In: ZnO (IZO) thin films were deposited on flexible plastic substrates by pulsed laser deposition (PLD) method. The obtained layers present adequate optical and electrical properties competitive with those based on indium tin oxide (ITO). The figure of merit (9 X 10(-3) Omega(-1)) calculated for IZO layers demonstrates that high quality coatings can be prepared by this deposition technique. A thermal annealing (150 degrees C for 1 h) or an oxygen plasma etching (6 mbar for 10 min.) were applied to the IZO layers to evaluate the influence of these treatments on the properties of the transparent coatings. Using vacuum evaporation, organic heterostructures based on cooper phthalocyanine (CuPc) and 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) were deposited on the untreated and treated IZO layers. The optical and electrical properties of the heterostructures were investigated by UVVis, FTIR and current-voltage (I-V) measurements. For the heterostructure fabricated on IZO treated in oxygen plasma, an improvement in the current value with at least one order of magnitude was evidenced in the I-V characteristics recorded in dark conditions. Also, an increase in the current value for the heterostructure deposited on untreated IZO layer can be achieved by adding an organic layer such as tris-8-hydroxyquinoline aluminium (Alq(3)).

We report on isochamp magneto-resistivity and ac susceptibility curves obtained in a high-quality single crystal of the isovalent optimally doped pnictide BaFe2(As0.68P0.32)(2) with superconducting temperature T-c = 27.8 K for H||c-axis. Plots of the logarithmic derivative of the resistivity curves allowed the identification of a vortex-glass (VG) phase and to obtain the values of the critical glass temperature T-g, the temperature T* marking the transition to the liquid phase and of the critical exponent s. The presence of the VG phase is confirmed by detailed measurements of the third harmonic signal of the ac magnetic susceptibility. The modified VG model was successfully applied to the data allowing the obtention of the temperature independent VG activation energy U-b. The activation energy U-0 obtained from the Arrhenius plots in the flux-flow region are compared with U-b and with U-0 obtained from flux-creep measurements on a M(H) isothermal in the same sample. A phase diagram of the studied sample is constructed showing the T-g glass line, the T* line representing a transition (melting) to the liquid phase, the mean field temperature T-c(H) line and the H-p line obtained from the peaks in isothermal critical current, J(c)(H) curves, which are explained in terms of a softening of the vortex lattice. The glass line was fitted by a theory presented in the literature which considers the effect of disorder.

Comparative studies between doped conducting polymers and electrochemical deposited organometallic compounds reveals the interplay between crystalline-amorphous phases with significant contributions to the internal quantum efficiency in the OLED devices. The coexistence of the amorphous and crystalline phase in the electrodeposited film is revealed by the minor micro-crystal products which are present in the amorphous phase in thin films, while the many micro-crystals are randomly distributed in the thick films. Concerning the doped conducting polymers, the level of doping induces crystalline effects as a result of the - stacking between molecules, due to the Forester energy transfer processes in which the transfer rate is increased with decreasing of the distances between neighboring molecules. The crystallization processes change the emission properties of the active layers both for the luminance level and all over color, ranging from yellow to red in the case of IrQ(ppy)(2) compounds.

Significant differences are reported for the IR spectra of poly[(2,5-bisoctyloy)-1, 4-phenylenevinylene] (BO-PPV) recorded onto the KBr, Ag and Au supports. In this work, a decrease in the absorbance of the IR spectra of the BO-PPV films deposited onto Au and Ag supports as increasing the macromolecular compound film thickness is reported. A preferential orientation of the BO-PPV molecules and its composite with single-walled carbon nanotubes (SWNTs) versus metallic supports of Ag and Au is assessed using the IR absorption spectroscopy under s and p polarized light. An anti-Stokes photoluminescence (ASPL) is reported to characterize the BO-PPV macromolecular compound. The BO-PPV ASPL spectra intensity dependent both of the thickness of the macromolecular compound layer deposited onto the metallic supports and the SWNTs weight in the BO-PPV/SWNTs composite mass. (C) 2017 Elsevier B.V. All rights reserved.

Using the kp theory and configuration interaction method, we analyze the effect of heavy hole-light hole (HH-LH) mixing in CdTe quantum dots (QDs) with a single manganese (Mn) ion. We find that the hole-Mn exchange switches the coupling between two excitons whose Luttinger spinors have both HH and LH components. If the magnetic dopant is off-centered and the QD is subjected to a single pulse the system periodically bounces between bright and dark mostly HH excitons with opposite HH spins. A pump-and-probe setup allows to estimate the efficiency of this HH spin unpinning from the biexciton response. The biexciton absorption spectrum is also discussed.

Shape memory alloys are known to memorise one -or several-temperatures at which the martensite-austenite transformation was stopped before completion in the past, the memory manifesting as specific dips in subsequent calorimetric scans. Previous studies have shown that this memory can be erased by heating to higher temperatures than the ones previously recorded. In this paper, we study a distinct memory fading effect which takes place by heating to a lower temperature. This effect is reported in NiFeGa as polycrystalline ribbons, the alloy being initially studied as bulk for which the thermal memory effect was not found. If, after an initial incomplete heating up to T-1 one performs a second incomplete heating up to T-2 < T-1, a new calorimetric dip appears at T-2, as expected, while less expected was that the dip corresponding to T-1 reduces in amplitude or even vanishes ( if the arrest at T-2 is repeated). The memory fading effect is more clear for small differences T-1-T-2 and less obvious or absent for large ones. The second part of the paper employs a statistical 2D model, which associates the memorized temperatures with a depletion of certain martensite plates sizes, and also supports the memory fading effect.

The ability to elucidate the elementary steps of a chemical reaction at the atomic scale is important for the detailed understanding of the processes involved, which is key to uncover avenues for improved reaction paths. Here, we track the chemical pathway of an irreversible direct desulfurization reaction of tetracenothiophene adsorbed on the Cu(111) closed-packed surface at the submolecular level. Using the precise control of the tip position in a scanning tunneling microscope and the electric field applied across the tunnel junction, the two carbon-sulfur bonds of a thiophene unit are successively cleaved. Comparison of spatially mapped molecular states close to the Fermi level of the metallic substrate acquired at each reaction step with density functional theory calculations reveals the two elementary steps of this reaction mechanism. The first reaction step is activated by an electric field larger than 2 V nm(-1), practically in absence of tunneling electrons, opening the thiophene ring and leading to a transient intermediate. Subsequently, at the same threshold electric field and with simultaneous injection of electrons into the molecule, the exergonic detachment of the sulfur atom is triggered. Thus, a stable molecule with a bifurcated end is obtained, which is covalently bound to the metallic surface. The sulfur atom is expelled from the vicinity of the molecule.

This paper relates to a method for the preparation of lead acetate film and its use as precursor for twostep solution deposition of CH3NH3PbI3 perovskite film. For the preparation of perovskite film, lead acetate was used both in the form of trihydrate and dehydrated. The effects of lead acetate trihydrate dehydration on the structure of the resulted perovskite films were studied. The prepared perovskite films which consist of cuboid grains, show high surface coverage both on FTO and TiO2 substrates. XRD measurements have not revealed the presence of the PbI2 contaminant in perovskite film. Solvent vapor annealing method was employed to prepare perovskite film that contains lead iodide. Using impedance spectroscopy measurements, we further evaluated the influence of the prepared perovskite films on the performance of the photovoltaic devices. (C) 2017 Elsevier Ltd. All rights reserved.