National Institute Of Materials Physics - Romania

Publications

5043 articles found

1. Structural, electrical properties and photoluminescence analyses of the terbium doped barium titanate

Authors: Cernea, M; Secu, M; Radu, R; Ganea, P; Surdu, VA; Trusca, R; Vasile, ET; Secu, EC

Published: OCT 15 2021, JOURNAL OF ALLOYS AND COMPOUNDS, 878, 160380, DOI: 10.1016/j.jallcom.2021.160380

Multifunctional materials integrating optical and electric properties into a composite or into a doped single-phase material have attracted much attention from scientists for future optoelectronic devices. In this work, polycrystalline Ba1-xTbxTiO3, x = 0.00, 0.01 and 0.05 materials were synthesized by hydrothermal route and their photoluminescence and dielectric properties were studied. The as-prepared powders showed a microstructure consisting of grains with the shape of cube; and average particle size ranging from 75 nm to 150 nm. The investigated temperature dependence of dielectric permittivity of Ba1-xTbxTiO3 ceramics revealed a decrease of the Curie temperature (Tc) with increasing terbium content, and a more pronounced degree of diffuseness of phase transition. The resistivity at room temperature was discussed in terms of positive temperature coefficient of resistance (PTRC) effect. UV-vis reflectance spectra of Ba1-xTbxTiO3 samples showed a decrease of the band gap energy with the increase of Tb amount, due to the band-gap narrowing effect. The photoluminescence spectra recorded for Tb doped BaTiO3 materials, at 80 K, evidenced the typical f-f luminescence lines of the Tb3+ accompanied the broad luminescence band at about 425 nm due to self-trapped exciton recombination. These results demonstrated that the terbium enhances the dielectric properties and photoluminescence simultaneously, being suitable for electronic applications. (C) 2021 Elsevier B.V. All rights reserved.

2. Enhancing stability of hybrid perovskite solar cells by imidazolium incorporation

Authors: Tomulescu, AG; Leonat, LN; Neatu, F; Stancu, V; Toma, V; Derbali, S; Neatu, S; Rostas, AM; Besleaga, C; Patru, R; Pintilie, I; Florea, M

Published: AUG 1 2021, SOLAR ENERGY MATERIALS AND SOLAR CELLS, 227, 111096, DOI: 10.1016/j.solmat.2021.111096

Hybrid perovskites based solar cells have demonstrated high conversion efficiency but poor long-term stability. This study reports on the results obtained after doping the CH3NH3PbI2.6Cl0.4 mixed halide perovskite with imidazolium (C3N2H5+, denoted IM) on the "A site" position of a perovskite, to improve photovoltaic performances and stability of hybrid perovskite solar cells. The perovskite films were investigated exhaustively by different characterization techniques: X-ray diffraction, Atomic Force Microscopy, Scanning Electron Microscopy, UV-Vis, X-ray Photoelectron Electron Paramagnetic Resonance spectroscopies, Impedance Spectroscopy and Incident Photon-to-Electron Conversion Efficiency. The photovoltaic parameters were determined by measuring the IV curves of the corresponding solar cells. The amount of IM inserted in the perovskite play a key role on the film properties. The calculated new tolerance factors according to the "globularity factor" are experimentally proved and thus at doping concentrations greater than 20% for CH3NH3PbI2.6Cl0.4 perovskite the 3D structure is no longer obtained. However, below this value, the IM substituted perovskite film possesses an improved film quality and crystallinity as compared to the pristine film. Substituting MA+ with IM+ provides a favorable way to reduce recombination processes and shows great potential to achieve high stability, and an improved charge generation, resulting in increased PCE values. We find that the optimal percentage of imidazolium incorporation to achieve better stability of solar cells is 6%.

3. Magnesium-silver cathodes for efficient charge injection into Organic Light Emitting Diodes deposited by LTVA method

Authors: Vladoiu, R; Mandes, A; Dinca, V; Kudrna, P; Tichy, M; Polosan, S

Published: JUL 15 2021, JOURNAL OF ALLOYS AND COMPOUNDS, 869, 159364, DOI: 10.1016/j.jallcom.2021.159364

The Laser-induced Thermionic Vacuum Arc method was applied to optimize magnesium-silver (Mg:Ag) alloys, which can be potentially used as stable metallic cathodes for optoelectronic devices. Besides lowering the cathode work function given by the magnesium that improves the electron injection, Mg:Ag alloys induce a higher electrical conductivity, estimated here to be 3.42 x 10(7) S m(-1) for AgMg3, compared with 2.64 x 10(7) S m(-1), estimated here for Mg thin films. Mg:Ag alloys avoid the critical issue of Mg oxidation for better charge injection in optoelectronic active layers. By improving the Thermionic Vacuum Arc technique with the laser beam, this method enables the control of the silver concentration in these alloys due to photonic processes. The uniformity of metallic thin films, compactness, and high purity are the primary advantages of the Laser-induced Thermionic Vacuum Arc method. (C) 2021 Elsevier B.V. All rights reserved.

4. Cyclophane with eclipsed pyrene units enables construction of spin interfaces with chemical accuracy

Authors: Metzelaars, M; Schleicher, S; Hattori, T; Borca, B; Matthes, F; Sanz, S; Burgler, DE; Rawson, J; Schneider, CM; Kogerler, P

Published: JUN 28 2021, CHEMICAL SCIENCE, 12, DOI: 10.1039/d1sc01036k

Advanced functionality in molecular electronics and spintronics is orchestrated by exact molecular arrangements at metal surfaces, but the strategies for constructing such arrangements remain limited. Here, we report the synthesis and surface hybridization of a cyclophane that comprises two pyrene groups fastened together by two ferrocene pillars. Crystallographic structure analysis revealed pyrene planes separated by similar to 352 pm and stacked in an eclipsed geometry that approximates the rare configuration of AA-stacked bilayer graphene. We deposited this cyclophane onto surfaces of Cu(111) and Co(111) at submonolayer coverage and studied the resulting hybrid entities with scanning tunnelling microscopy (STM). We found distinct characteristics of this cyclophane on each metal surface: on non-magnetic Cu(111), physisorption occurred and the two pyrene groups remained electronically coupled to each other; on ferromagnetic Co(111) nanoislands, chemisorption occurred and the two pyrene groups became electronically decoupled. Spin-polarized STM measurements revealed that the ferrocene groups had spin polarization opposite to that of the surrounding Co metal, while the pyrene stack had no spin polarization. Comparisons to the non-stacked analogue comprising only one pyrene group bolster our interpretation of the cyclophane's STM features. The design strategy presented herein can be extended to realize versatile, three-dimensional platforms in single-molecule electronics and spintronics.

5. Engineering hydrogenation active sites on graphene oxide and N-doped graphene by plasma treatment

Authors: Magureanu, M; Mandache, NB; Rizescu, C; Bucur, C; Cojocaru, B; Man, IC; Primo, A; Parvulescu, VI; Garcia, H

Published: JUN 15 2021, APPLIED CATALYSIS B-ENVIRONMENTAL, 287, 119962, DOI: 10.1016/j.apcatb.2021.119962

Graphene oxide (GO) and N-doped graphene [(N)G] graphenes were submitted to H-2 glow discharge under different discharge regimes, in both the negative glow and positive column plasma regions. The resulted catalysts were fully characterized using several techniques such as Raman, DRIFT and XPS spectroscopy, powder X-ray diffraction, H-2 pulse chemisorption and H-2-, CO2- and NH3-TPD experiments. Density functional theory calculations were performed taking a slab model of graphene sheet with an optimized C-C bond length (1.426 angstrom) and a 16 angstrom vacuum layer between sheets. An overview of these characterizations showed that the O/C atomic ratio of GO is influenced by the plasma regime, indicating the occurrence of O removal, as also predicted by DFT calculations. In the case of (N)G, the plasma treatment also removes pyridinic N with an increase of the C/N ratio. The efficiency of the plasma modification has been checked through catalytic tests in hydroisomerization of 1-octene and hydrogenation of alpha-methyl-styrene. Contrarily to classical thermal activation requiring high temperatures, the generation of the defects by treating with plasma occurs at voltages in the range of 2 5 kV. In consequence, the hydrogenation and isomerization of alkenes resulted with high yields and good selectivities. Graphene prepared from sodium alginate from brown algae was considered as reference in these investigations.

6. Electronic and transport properties of the dual-emitter organometallic compound IrQ(ppy)(2)

Authors: Polosan, S

Published: JUN 14 2021, JOURNAL OF ORGANOMETALLIC CHEMISTRY, 942, 121814, DOI: 10.1016/j.jorganchem.2021.121814

The structure of the IrQ(ppy)(2) organometallic dual phosphorescent compound was experimentally and theoretically investigated. The Metal-to-Ligand Charge Transfer toward the phenylpiridine and quinoline ligands explains its dual phosphorescence property. The IrQ(ppy)(2) organometallic's electronic properties extend the absorption spectra toward the lower energies than Ir(ppy)(3) due to the bandgap reduction induced by the quinoline ligand. The magneto-optical measurements reveal A-terms, which are produced by splitting the triplet states and B-terms, which arise when the electronic levels are mixed in the magnetic field. The IrQ(ppy)(2) molecule's transport properties confirm an ambipolar character, consisting of a higher hole transport character induced by the phenyilpiridine ligands and a lower electron transport character induced by the quinoline ligand. (C) 2021 Elsevier B.V. All rights reserved.

7. One-pot strategy for obtaining magnetic PMMA particles through ATRP using Fe(CO)(5) as co-initiator

Authors: Diacon, A; Rusen, E; Rizea, F; Ghebaur, A; Berger, D; Somoghi, R; Matei, A; Palade, P; Tutunaru, O

Published: JUN 5 2021, EUROPEAN POLYMER JOURNAL, 152, 110446, DOI: 10.1016/j.eurpolymj.2021.110446

The first aim of this study was to develop an ATRP process for methyl methacrylate (MMA) polymerization using Fe(CO)(5) as co-initiator. The kinetics analysis of the reaction in solution confirmed the controlled characteristic of the polymerization process. The process displays a slow initiation step and presents bimolecular termination reactions at higher molecular weights, confirmed by NMR analysis. Also, these observations are sustained by GPC, FT-IR, and XPS analyses, explaining the rather unusual dispersity values (1.4) for an ATRP reaction. The use of formic acid, which shifts the equilibrium towards Fe2+ species affording a quasi-controlled controlled polymerization process, is a key element in achieving control over the reaction. The Fe2+/Fe3+ ratio at the end of the reaction, determined by XPS analysis, suggests the possibility to generate magnetic iron oxide particles. Therefore, the synthesis strategy was adapted for use in the suspension polymerization process of MMA which permitted the production of magnetic PMMA particles through a facile one-pot approach, using only an ammonia treatment stage. The obtained PMMA particles, characterized by SEM, XPS, TEM, TGA, and XRD analyses, have porous characteristics and magnetic properties which makes them good candidates as catalyst supports, separation processes, or biomedical applications.

8. Influence of Ce addition and Pt loading upon the catalytic properties of modified mesoporous PtTi-SBA-15 in total oxidation reactions

Authors: Ciobanu, M; Petcu, G; Anghel, EM; Papa, F; Apostol, NG; Culita, DC; Atkinson, I; Todorova, S; Shopska, M; Naydenov, A; Velinova, R; Parvulescu, V

Published: JUN 5 2021, APPLIED CATALYSIS A-GENERAL, 619, 118123, DOI: 10.1016/j.apcata.2021.118123

A series of PtCeTi-SBA-15 catalysts, with different concentrations of Pt (0.25 %, 0.5 %, 1.0 %), were synthesized. New Ti-SBA-15 material was obtained by direct synthesis and used as support in successive immobilization of Pt and Ce precursors, changing their order of addition. The characterization of the catalysts has been achieved taking an ensemble of techniques which showed preservation of the support porous structure, high dispersion of the amorphous metal oxides and Pt?, the formation of metal crystals for samples with 1wt% Pt and variation of Pt? content on surface in condition of ceria addition. The best conversion (100 %) was obtained in oxidation of hydrocarbons (CH4, C3H8 and C6H14) for the samples with 1% Pt and a significant diminish of activity was obtained for catalyst in which Ce was added on PtTi-SBA-15 sample. Differently of these, CO oxidation was totally irrespective of the catalyst composition.

9. Antioxidant activity of thirty-six peppers varieties and vasorelaxant of selected varieties

Authors: Machado, FB; de Mace, IYL; Campos, HM; Moreno, EKG; Silva, MFB; de Oliveira, JR; Ramalho, RRF; Nascimento, AD; Vaz, BG; da Cunha, LC; Ghedini, PC; Diculescu, VC; Gil, ED

Published: JUN 2021, FOOD BIOSCIENCE, 41, 100989, DOI: 10.1016/j.fbio.2021.100989

Peppers are consumed all over the world, have several benefits to human health, such as antioxidant properties that can prevent diseases related to free radicals such as cardiovascular, inflammatory diseases, cancer, among others. This work aimed to evaluate the antioxidant capacity (AOC) of 36 varieties of peppers through ABTS and DPPH radical scavenging, electroanalytical assays, and to verify the vasorelaxant properties of selected samples. The greater the amount of capsaicin found in the extracts, the higher the AOC the greater the vasorelaxation. Naga had the highest scoring for antioxidant capacity, pout showed the lowest antioxidant capacity, vase pyramid intermediate level, whereas the capsaicin content followed the same trend. Extracts from all pepper varieties studied presented vasorelaxant properties in independent and dependent endothelial pathways.

10. Spin asymmetry originating from densities of states: Criterion for ferromagnetism, structures and magnetic properties of 3d metals from crystal field based DOSs

Authors: Teodorescu, CM

Published: JUN 2021, RESULTS IN PHYSICS, 25, 104241, DOI: 10.1016/j.rinp.2021.104241

Ferromagnetism in 3d metals is re-examined in a simple band model. It is shown that the molecular field model cannot account for the low values of coercive field in ferromagnetic pure metals, and that the standard Stoner theory of band ferromagnetism incorrectly evaluates the total electronic energy and cannot predict reasonable Curie temperatures. A simple band model for magnetism is formulated, yielding a criterion for ferromagnetism even in absence of a Hubbard term, involving only the density of states (DOS) at the Fermi level, its derivative and the filling of the 3d band. By introducing a double-peaked DOS, one may explain the occurrence of ferromagnetism in bcc Fe, hcp-fcc Co and fcc Ni, the stabilization of fcc-hcp or bcc structures across all 3d elements, the occurrence of antiferromagnetism in chromium, and derive reasonable Curie temperatures. 'Re-entrant' ferromagnetism is predicted at ultrahigh temperatures, suggesting an alternate origin for the geomagnetic field.