National Institute Of Materials Physics - Romania

Random sampling provides several benefits for multi-band signals compared to uniform sampling case. These benefits include greater sample frequency flexibility, fewer restrictions on signal filtering, and, in the case of stationary sampling sequences, a decrease or suppression of spectrum aliases. This makes random sampling a preferred option in Software Defined Radio (SDR) systems, which face the challenge of supporting different standards with different sampling frequencies. However, the task of reconstruction becomes more complex with random sampling. In the literature, various reconstruction techniques are provided. In this paper, we focus on employing the ADPW-CG iterative method to address the reconstruction issue with randomly sampled signals within an SDR system context. The effectiveness of the proposed algorithm is evaluated in terms of complexity, reconstructed signal quality, and robustness, and robustness and the findings is compared with the performance of the SVD direct algorithm and other iterative methods. Based on the obtained results, we observed that the proposed approach demonstrates promising performance in comparison to other algorithms.

The synthesis of carbon quantum dots (CQDs) from high molecular weight biomass-derived precursors poses a significant challenge due to the complex molecular structures and low conversion efficiency. This work demonstrates a green, rapid, and sustainable continuous hydrothermal flow synthesis (CHFS) approach for nitrogen-doped carbon quantum dots (NCQDs) from various biomass-derived precursors, including high molecular weight polymeric sources like chitosan, lignin, and humic acid. We find that the precursor structure significantly impacts the size of the fabricated NCQDs and their optical properties. Citric acid, a low molecular weight precursor, yields NCQDs with excitation-independent emission, higher quantum yields, and low non-radiative losses, while NCQDs derived from polymeric precursors exhibit excitation-dependent, red-shifted, and lower efficiency emission. Theoretical calculations, performed to understand the configuration and distribution of nitrogen dopants within the NCQD structure, show that pyridinic and graphitic nitrogen atoms exhibit a strong preference to aggregate near the centre of the edge of the NCQD and not in the vertices nor in the graphitic core, thus affecting the HOMO and LUMO, bandgap, and light absorption and emission wavelengths. The life cycle assessment (LCA) analysis highlights the green and scalable advantages of the CHFS process for producing NCQDs compared to batch methods, making it a sustainable and economically viable approach for large-scale NCQD synthesis from high molecular weight biomass-derived precursors. Hence, the combination of experimental data and theoretical calculations provides a comprehensive understanding of the structure-property relationships in these NCQDs.

In the present work, we report the synthesis and investigations of La0.9K0.1MnO3 and La0.8K0.1Pb0.1MnO3 bulk samples which could be potential magnetocaloric materials for magnetic refrigeration close to room temperature. A flash combustion reaction and sintering at 1200 degrees C for 10 h are used to prepare the bulk materials. Both compounds crystallized into a rhombohedral structure with R3\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\overline{3 }$$\end{document}c space group confirmed by X-ray powder diffraction results. Scanning electron microscopy analysis, combined with XRD peak profiles is performed to estimate the particle/crystallite size of the samples. Moreover, the Curie temperature, TC, is found to be higher in lead-rich sample due to the enhancement of the grain size and the Mn3+-O-Mn4+ double exchange (DE) interaction. Therefore, the bulk sample La0.8K0.1Pb0.1MnO3 shows a room temperature phase transition of 289 K as well as a higher saturation magnetization. The La0.8K0.1Pb0.1MnO3 bulk compound exhibits a high and sharp peak in magnetic entropy change up to 5.5 Jkg-1 K-1 under 5 T at the magnetic transition temperature TC. To compare the magnetocaloric performances of the studied compounds, relative cooling power (RCP) was employed. The obtained experimental results revealed that the increase in particle size influences severely the magnetocaloric properties.

TiO2-C (carbon) hybrid materials are promising electrocatalyst supports because the presence of TiO2 results in enhanced stability. Use of new types of carbonaceous materials such as reduced graphene oxide instead of traditional active carbon provides certain benefits. Although the rutile polymorph of TiO2 seems to have the most beneficial properties in these hybrid materials, the anatase type is more frequent in TiO2-rGO composites, especially in graphite oxide (GO) derived ones, as GO has several properties which may interfere with rutile formation. To explore and evaluate these peculiarities and their influence on the composite formation, we compared TiO2-C systems formulated with GO and Black Pearls (BP) carbon. Various physicochemical methods, such as attenuated total reflection infrared (ATR-IR)-, solid state NMR-, Raman- and X-ray photoelectron spectroscopy, X-ray powder diffraction (XRD), electron microscopy, etc. were used to characterize the samples from the different stages of our multistep sol-gel synthesis. Our experiments demonstrated that utilization of GO is indeed feasible for composite preparation, although its sodium contamination has to be removed during the synthesis. On the other hand, high temperature treatment and/or solvothermal treatment during composite synthesis resulted in decomposition of the functional groups of the GO and the functional properties of the final product were similar in case of both composites. However, Pt/TiO2-GO derived sample showed higher oxygen reduction reaction activity than Pt/TiO2-BP derived one. Based on the decrease of electrochemical surface area, the stability order was the following: Pt/C (commercial) < Pt/TiO2-BP derived C < Pt/TiO2-GO derived C.

The tuning sensitivity towards CO2 detection under in-field-like conditions was investigated using SnO2-sensitive material deposited onto Al2O3 substrates provided with platinum electrodes with interdigital gaps of 100 mu m and 30 mu m. X-ray diffraction, low-magnification and high-resolution transmission electron microscopy, and electrical and contact potential difference investigations were employed to understand the sensing mechanism involved in CO2 detection. The morpho-structural analysis revealed that the SnO2 nanoparticles exhibit well-defined facets along the (110) and (101) crystallographic planes. Complex phenomenological investigations showed that moisture significantly affects the gas sensing performance. The experimental results corroborated the literature evidence, highlighting the importance of Pt within the interdigital electrodes subsequently reflected in the increase in the CO2 sensing performance with the decrease in the interdigital gap. The catalytic efficiency is explained by the distribution of platinum at the gas-Pt-SnO2 three-phase boundary, which is critical for enhancing the sensor performance.

This research targets the need for eco-friendly strategies in the synthesis of bioactive materials, addressing the importance of valorization of vegetal waste. This study focuses on developing biohybrids containing biomimetic lipid vesicles and phytosynthesized gold-silver chloride nanoparticles (AuAgCl NPs) derived from Achillea millefolium L. extract. By leveraging the natural antioxidant and antimicrobial properties of the plant, the research proposes a sustainable approach to creating materials with potential biomedical applications. The biomimetic membranes were loaded with chlorophyll a, a natural spectral marker. Three types of bioactive materials (biohybrids) were developed by varying the lipid vesicle/AuAgCl NP ratio. Optical (UV-Vis, fluorescence emission, FTIR), structural (XRD), elemental (EDX, XPS), and morphological (TEM) studies were performed to characterize the bio-developed materials. The hydrophobic/hydrophilic characteristics of the samples were investigated by measuring the water contact angle, and their size was estimated by DLS and TEM. Zeta potential measurements were used to evaluate the physical stability of phyto-developed particles. Antioxidant properties of phyto-particles were investigated through the chemiluminescence technique. The obtained biomaterials exhibited high antioxidant activity and antiproliferative activity against HT-29 and B-16 cancer cells. Therapeutic index values were calculated for each biohybrid. Additionally, the bio-prepared hybrids revealed biocidal action against Staphylococcus aureus and Enterococcus faecalis. The phyto-developed biomaterials are promising in biomedical applications, particularly as adjuvants in cancer therapy.

Nowadays, the development of devices based on organic materials is an interesting research challenge. The performance of such devices is strongly influenced by material selection, material properties, design, and the manufacturing process. Usually, buckminsterfullerene (C60) is employed as electron transport material in organic photovoltaic (OPV) devices due to its high mobility. However, considering its low solubility, there have been many attempts to replace it with more soluble non-fullerene compounds. In this study, bulk heterojunction thin films with various compositions of zinc phthalocyanine (ZnPc), a perylene diimide derivative, or C60 were prepared by matrix-assisted pulsed laser evaporation (MAPLE) technique to assess the influence of C60 replacement on fabricated heterostructure properties. The investigations revealed that the optical features and the electrical parameters of the organic heterostructures based on this perylene diimide derivative used as an organic acceptor were improved. An increase in the JSC value (4.3 x 10-4 A/cm2) was obtained for the structures where the perylene diimide derivative acceptor entirely replaced C60 compared to the JSC value (7.5 x 10-8 A/cm2) for the heterostructure fabricated only with fullerene. These results are encouraging, demonstrating the potential of non-fullerene compounds as electron transport material in OPV devices.

Among many iron-based superconductors, isovalently substituted BaFe2(As1-xPx)2 displays, for x approximate to 0.3, apart from the quite usual Second Magnetization Peak (SMP) in the field dependence of the critical current density, an unusual peak effect in the temperature dependence of the critical current density in the constant field, which is related to the rhombic-to-square (RST) structural transition of the Bragg vortex glass (BVG). By using multi-harmonic AC susceptibility investigations in three different cooling regimes-field cooling, zero-field cooling, and field cooling with measurements during warming up-we have discovered the existence of a temperature region in which there is a pronounced magnetic memory effect, which we attributed to the direction of the structural transition. The observed huge differences in the third harmonic susceptibility at low and high AC frequencies indicates the difference in the time-scale of the structural transition in comparison with the timescale of the vortex excitations. Our findings show that the RST influence on the vortex dynamics goes beyond the previously observed influence on the onset of the SMP.

This work examines the effect of incorporating a third cation M (M = Mg, Al, Mn, or Fe) into Nb-promoted NiO catalysts (Ni0.85Nb0.15O) on their physicochemical properties and catalytic performance in the oxidative dehydrogenation (ODH) of ethane into ethylene. Therefore, a series of mixed oxides with the composition Ni0.765Nb0.135M0.1O was synthesized, characterized, and analyzed for catalytic behavior. The addition of the third cation markedly modified the redox and semiconductive properties of the catalysts, thus affecting their ODH performance. The selectivity toward ethylene was strongly dependent on the cation modifier, as it modulated the density of nonselective sites on the catalyst surface. In the low-temperature region, Mn-NiNbO demonstrated the highest conversion rates but with the lowest ethylene selectivity, whereas Fe-NiNbO, despite being less active than the undoped NiNbO, exhibited the highest ethylene selectivity. Overall, undoped NiNbO emerged as the best catalyst in terms of both activity and selectivity at isoconversion. All catalysts underwent partial reduction under reaction conditions, with the degree of reduction inversely correlated with the specific reaction rates. Although none of the catalysts remained stable at 400 degrees C, Al-NiNbO showed the least deactivation over time. The deactivation was linked to a reduction in p-type conductivity and a loss of redox functionality during operation.

Rare-earth doped GdF3 nanocrystals embedded in silica glassy matrix have been prepared by controlled crystallization of the xerogel; the influence of rare-earth ion (Pr, Sm, Eu, Tb, Dy, Er, Yb) and additional Li-codopant on structural and optical properties was discussed. The precipitation of RE-doped GdF3 nanocrystalline phase is the result of Gd-trifluoracetate thermolysis revealed as a strong exothermic peak at around 300 degrees C. Structural analysis of RE-doped SiO2-GdF3 glass-ceramic sample calcined at 525 degrees C showed the occurrence of GdF3 nanocrystals of about 25 nm size, showing hexagonal or orthorombic structure depending on the RE-ion. Under UV-light excitation at 273 nm of Gd3+ ions (S-8(7/2) -> I-6(13/2,15/2) transition), photoluminescence spectra showed characteristic RE3+ luminescence due to the non-radiative energy transfer between the excited Gd3+ and acceptor RE ions; the highest energy transfer (congruent to 80 %) was observed for Tb3+ and lowest (congruent to 21 %) for Sm3+.