National Institute Of Materials Physics - Romania

Spectroscopic characteristics of Pr3+ ions doped CNGG and CLNGG single crystals were investigated in order to assess their potential as laser materials for visible emission. The Judd-Ofelt intensity parameters for the f-f transitions of Pr3+ ions were used to determine spectroscopic and laser emission features. The temperature dependence of the absorption spectra for the H-3(4) -> P-3(0) transition was used to highlight the multicenter structure, different Stark levels, hot bands, and also the connection between the vibronic and electronic lines. The presence of electron-phonon interaction was also observed in the emission spectra corresponding to the P-3(0) -> H-3(4) transition under different excitation wavelengths. Based on low temperature absorption and emission spectra, partial energy level diagrams of Pr3+ ions doped in CNGG and CLNGG single crystals were obtained. The emission cross-sections for different transitions of Pr3+ ions were evaluated by the Fuchtbauer-Ladenburg formula. The fluorescence decay curve of the D-1(2) level was measured under selective excitation at different concentrations and temperatures. The concentration quenching process for the D-1(2) state was also studied. (C) 2019 Elsevier B.V. All rights reserved.

Multiple nonvolatile and well-separated capacitive states can be obtained in a two-terminal ferroelectric capacitor setup by fine tuning the polarization switching process. This approach allows for the implementation of memcomputing (same platform for storage and computing) capable ferroelectric structures. Digital and analog storage modes are exemplified in this work together with an algorithm for simple binary computation functions such as OR/NOR and AND/NAND for data processing on the same device. Results are obtained by controlling the polarization switching process in ferroelectric multi-layers such as Pb (Zr0.2Ti0.8)O-3/SrTiO3/Pb (Zr0.2Ti0.8)O-3 and Pb (Zr0.2Ti0.8)O-3/BaTiO3/Pb (Zr0.2Ti0.8)O-3. Besides memcomputing, these results can be used for nondestructive capacitive reading of information in simple ferroelectric capacitors or can open the way toward applications such as neuromorphic and chaotic circuits.

The present investigation deals with the development, characterization and application of nano-structured Pd doped Ni electrodes (Pd@Ni), which uses the electrochemical properties of Pd in synergy with the magnetic properties of Ni for biosensors development. The Pd@Ni electrodes have been characterized by X-ray diffraction, scanning electron microscopy with energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy. It has been shown that palladium presented spherical assemblies ranging 150-200 nm medium diameter size that covers large areas of the electrode surface while metallic nickel, which confers magnetic properties, showed a uniform granular structure with sizes between 20 and 50 nm. Cyclic voltammetry and electrochemical impedance spectroscopy were performed to understand the electrochemical process at the Pd@Ni electrodes in neutral media. The Pd@Ni electrodes were applied for the electrochemical detection of H2O2. Finally, Ni nanoparticles (NiNP) functionalized with the model enzyme glucose oxidase (GOx-NiNP) have been attached to the Pd@Ni electrode solely through magnetic interactions, and the obtained GOx-NiNP/Pd@Ni biosensor applied for glucose determination in aqueous solutions by fixed potential amperometry at -0.05 V (vs Ag/AgCl) with reduced interferences. (C) 2019 Elsevier Ltd. All rights reserved.

Adsorption processes of 1,4-phenylene diisothiocyanate (PDITC) on two new platforms of the type graphene oxide (GO) sheets and GO layers functionalization with polydiphenylamine (PDPA) are studied by Raman scattering and photoluminescence (PL). An interaction in solid state phase of the two constituents, i.e. PDITC and GO sheets, and a deposition of PDITC onto the PDPA functionalized GO layers, respectively, by the drop casting method, were performed. In the first case, it is shown that interaction in solid state phase of GO with PDITC leads to an intercalation of the organic compound between GO sheets simultaneously with the appearance of the o-thiocarbamate groups, that induces: (i) an enhancement of the PDITC Raman lines situated in the 400-800 and 1000-1300 cm(-1) spectral ranges, (ii) a change in the ratio between the relative intensities of the two Raman lines peaked at 1585 and 1602 cm(-1) accompanied by an up-shift in the case of the second line and (iii) a down-shift of the PDTIC PL band from 502 to 491 nm. Using cyclic voltammetry, an electrochemical functionalization of the GO layers with PDPA doped with H3PMo12O40 heteropolyanions takes place, as demonstrated by Raman scattering and FTIR spectroscopy. The presence of the amine groups in the molecular structure of the doped PDPA functionalized GO layers induces a chemical adsorption of PDITC on this platform, when the thiourea groups appear simultaneously with o-thiocarbamate groups. A chemical mechanism is proposed to take place at the interface of the GO sheets and the doped PDPA functionalized GO layers, respectively, with PDITC.

A carbon-based layer was deposited by spraying on top of a ferroelectric layer grown by sol-gel on Si (001) substrate and its properties as electrode and absorber for pyroelectric detection were tested. It was found that the electric properties of the ferroelectric capacitor with top carbon-based sprayed electrode (CBSE) are comparable with those of the capacitors with standard top SrRuO3 (SRO)/Au electrode. Pyroelectric measurements show that the pyroelectric signal recorded on ferroelectric capacitors with top CBSE electrode is 2.5 times greater than for top SRO/Au electrode for low frequency range. The value of the pyroelectric coefficient was estimated to 9.73.10(-4) C/m(2)K for CBSE electrodes and 3.36.10(-4) C/m(2)K for SRO/Au respectively. The fabrication process of CBSE is of low cost, easy to implement and with high throughput making it attractive for manufacturing various devices like pyroelectric detector, thermal imaging, solar cells, etc.

Electrodes constituted by nitrogen-doped reduced graphene oxide (NrGO) in combination with NiO nanostructures were fabricated by means of reactive inverse matrix assisted pulsed laser evaporation technique. The structure-composition of the electrode composites was tailored by laser-inducing chemical reactions of graphene oxide (GO) flakes with different precursor molecules (citric acid, ascorbic acid and imidazole) during GO deposition. Structural characterizations reveal the formation of wrinkles and nanoholes in the NrGO sheets, besides their coating with NiO nanostructures. Compositional studies disclose that imidazole precursor promotes the synthesis of NrGO with the largest degree of reduction and nitrogen doping (mainly with graphitic and pyridinic N). Electrochemical analyses of the obtained electrodes reveal that NiO nanostructures increase surface charge storage processes (double layer - pseudocapacitive) over diffusive ones, being the imidazole-based electrodes the ones exhibiting the best performance (up to 114 F cm(-3) at 10 mV s(-1)). Symmetric and asymmetric electro-chemical capacitors were also fabricated showing excellent robustness over 10,000 charge-discharge cycles at high specific currents.

We recall theoretical studies on transient transport through interacting mesoscopic systems. It is shown that a generalized master equation (GME) written and solved in terms of many-body states provides the suitable formal framework to capture both the effects of the Coulomb interaction and electron-photon coupling due to a surrounding single-mode cavity. We outline the derivation of this equation within the Nakajima-Zwanzig formalism and point out technical problems related to its numerical implementation for more realistic systems which can neither be described by non-interacting two-level models nor by a steady-state Markov-Lindblad equation. We first solve the GME for a lattice model and discuss the dynamics of many-body states in a two-dimensional nanowire, the dynamical onset of the current-current correlations in electrostatically coupled parallel quantum dots and transient thermoelectric properties. Secondly, we rely on a continuous model to get the Rabi oscillations of the photocurrent through a double-dot etched in a nanowire and embedded in a quantum cavity. A many-body Markovian version of the GME for cavity-coupled systems is also presented.

We have investigated the structural and electronic properties of the alpha-GeSe surface using atomic force microscopy, scanning tunneling microscopy and density functional theory calculations. GeSe belongs to the group-VI transition metal monochalcogenides and occurs in two polymorphs, alpha-GeSe and beta-GeSe. The most redundant polymorph, alpha-GeSe, has a structure that is very similar to black phosphorene. The alpha-GeSe surface has a centered rectangular unit cell with dimensions a = 3.8 angstrom and b = 4.4 angstrom, respectively. In scanning tunneling microscopy images only the Se atoms are resolved owing to the substantial transfer of electrons from the Ge to the Se surface atoms. This experimental finding is fully in line with density functional theory calculations. Scanning tunneling spectroscopy reveals that the alpha-GeSe surface is a p-type semiconductor with a band gap of 1.0 eV. The GeSe surface is stable at ambient conditions, which makes this material very appealing for technological applications.

The crystallization of ferroelectric (Hf,Zr)O-2 thin films is achieved by playing on the deposition pressure during reactive magnetron sputtering from a Hf/Zr metallic target. Postdeposition annealing was tried at different temperatures in order to optimize the quality of the samples. Structural characterizations are performed by transmission electron microscopy (TEM) and electrical characterizations are carried out. TEM analyses reveal that the samples deposited at a low working pressure show no orthorhombic phase, and thus are not ferroelectric, whereas the samples deposited at higher working pressure show the orthorhombic ferroelectric phase. The maximum remnant polarization is 6 mu C/cm(2) and is obtained for the sample annealed at 600 degrees C. The maximum cycles to breakdown is higher than 2 x 10(10) cycles and is reached for the sample annealed at 400 degrees C. These results are discussed in the matter of phase transition and oxygen vacancies redistribution. (c) 2019 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

N-doped defective graphene [(N)G] obtained by pyrolysis at 900 degrees C of chitosan contains about 3.7% of residual N atoms, distributed as pyridinic, pyrrolic and graphitic N atoms. It has been found that (N)G acts as basic catalyst promoting two classical C-C bond forming nucleophilic additions in organic chemistry, such as the Michael and the Henry additions. Computational calculations at DFT level of models corresponding to the various N atoms leads to the conclusion that N atoms are more stable at the periphery of the graphene sheets and that H adsorption on these sites is a suitable descriptor to correlate with the catalytic activity of the various sites. According to these calculations the most active sites are pyridinic N atoms at zig-zag edges of the sheets. In addition, N as dopant changes the reactivity of the neigh. bour C atoms. Water was found a suitable solvent to achieve high conversions in both reactions. In this solvent the initial distribution of N atoms is affected due to the easy protonation of the N-py to N-pyH sites. As an effect, C edge sites adjacent at N-PyH with an appropriate reactivity towards the alpha-C-H bond breaking are formed. The present results show the general activity of N-doped graphene as base catalysts and illustrate the potential of carbocatalysis to promote reactions of general interest in organic synthesis. (C) 2019 Elsevier Inc. All rights reserved.