National Institute Of Materials Physics - Romania

Publications

5714 articles found

1321. Multiple polarization states in symmetric ferroelectric heterostructures for multi-bit non-volatile memories

Authors: Boni, GA; Filip, LD; Chirila, C; Pasuk, I; Negrea, R; Pintilie, I; Pintilie, L

Published: DEC 28 2017, NANOSCALE, 9, 19278, DOI: 10.1039/c7nr06354g

Here we report a ferroelectric capacitor structure obtained by alternating ferroelectric and insulator thin-film layers which allows an increase of up to 2(n) polarization states, with n the number of ferroelectric layers. Four and up to eight distinct, stable and independently addressed polarization states are experimentally demonstrated in this work. The experimental findings are supported by a theoretical model based on the Landau-Ginzburg-Devonshire theory. The key parameter is the change in the strain conditions of ferroelectric layers induced by the insulating separator. Notably, the 2(n) increase in the storage capacity can be achieved without major changes in the present technology used for FeRAM devices. The test structures demonstrate very good memory characteristics such as retention and fatigue, opening the way towards the design of high density ferroelectric memories.

1322. Dimensionality-Driven Metal-Insulator Transition in Spin-Orbit-Coupled SrIrO3

Authors: Schutz, P; Di Sante, D; Dudy, L; Gabel, J; Stubinger, M; Kamp, M; Huang, Y; Capone, M; Husanu, MA; Strocov, VN; Sangiovanni, G; Sing, M; Claessen, R

Published: DEC 22 2017, PHYSICAL REVIEW LETTERS, 119, DOI: 10.1103/PhysRevLett.119.256404

Upon reduction of the film thickness we observe a metal-insulator transition in epitaxially stabilized, spin-orbit-coupled SrIrO3 ultrathin films. By comparison of the experimental electronic dispersions with density functional theory at various levels of complexity we identify the leading microscopic mechanisms, i.e., a dimensionality-induced readjustment of octahedral rotations, magnetism, and electronic correlations. The astonishing resemblance of the band structure in the two-dimensional limit to that of bulk Sr2IrO4 opens new avenues to unconventional superconductivity by "clean" electron doping through electric field gating.

1323. Anodic deposition of nanostructured hematite film using agarose gel as template. Application in water splitting

Authors: Sima, M; Vasile, E; Sima, A; Logofatu, C

Published: DEC 20 2017, ELECTROCHIMICA ACTA, 258, 1462, DOI: 10.1016/j.electacta.2017.12.013

The present study shows the results obtained in anodic electrodeposition of hematite precursor films in agarose gel template and photoelectrochemical characterization of the prepared hematite films. The peak current in cyclic voltammetry increases after each scan of the potential between -0.1 and 1.3 V for FTO electrode covered with a film of agarose gel in 0.1 M iron sulfate solution. The hematite samples with different thicknesses were prepared using template electrodeposition and annealing in air. The obtained nanostructured films were characterized by field emission scanning electron microscopy, X-ray diffraction, UV-vis spectroscopy, X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy. The results show that a synergistic improvement of photocurrent density (1.14 mAcm(-2) at 1.23 V vs RHE) is induced by Sn-doping and slight surface modification of hematite with thin and discontinuous NiO film. The results indicate that thicker hematite films and hematite films strongly modified with NiO nanostructures show a reduced photoactivity. (C) 2017 Elsevier Ltd. All rights reserved.

1324. Structural and optical properties of un-doped and doped Sr3Al2O6 obtained through the tartarate precursor method

Authors: Mindru, I; Gingasu, D; Patron, L; Marinescu, G; Culita, DC; Calderon-Moreno, JM; Preda, S; Secu, M

Published: DEC 15 2017, CERAMICS INTERNATIONAL, 43, 16675, DOI: 10.1016/j.ceramint.2017.09.057

Un-doped and doped strontium aluminates (Sr3Al2O6, Sr3Al2O6:Tb3+, Sr3Al2O6:Tb3+,Eu3+ and Sr3Al2O6:Tb3+,Eu3+/Eu2+) were synthesized through a soft chemical method - the tartarate precursor route. The tartarate precursors were characterized by infrared spectroscopy (IR) and thermal analysis. The strontium aluminates were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and infrared spectroscopy (IR). The XRD patterns confirmed the formation of single-phase cubic structure Sr3Al2O6 with average crystallite size of 18 nm. SEM micrographs showed porous polycrystalline powder microstructures and EDX analysis confirmed the incorporation of the dopant in the Sr3Al2O6 powders. The photoluminescence spectrum recorded on Sr3Al2O6: Tb3+,Eu3+ (obtained from tartarate precursor calcined in air) showed the typical f-f luminescences of the RE-ions. Subsequent calcination of this sample in reducing atmosphere is accompanied by new photoluminescence and thermoluminescence features associated to the Eu2+ ions.

1325. New ways to use the red mud waste as raw material for inorganic-organic hybrid hydrogels

Authors: Sandu, T; Sarbu, A; Zavoianu, R; Spatarelu, CP; Florea, M; Bradu, C; Mara, EL; Dragut, DV; Alexandrescu, E; Zaharia, A; Radu, AL

Published: DEC 10 2017, , DOI: 10.1016/j.minpro.2017.11.005

1326. Properties of perovskite ferroelectrics deposited on F doped SnO2 electrodes and the prospect of their integration into perovskite solar cells

Authors: Pintilie, I; Stancu, V; Tomulescu, A; Radu, R; Stan, CB; Trinca, L; Pintilie, L

Published: DEC 5 2017, MATERIALS & DESIGN, 135, 121, DOI: 10.1016/j.matdes.2017.09.013

The integration of ferroelectrics in perovskite solar cells is proposed as possible way to enhance charge collection efficiency. First results on solar cellmanufactured with PbTiO3 (PTO) instead of TiO2 have shown negligible values for the power conversion efficiency (PCE). This is explained by the high serial resistance of sol-gel deposited PTO on F:SnO2 electrodes (FTO). Although PTO layer has remnant polarization of 22 mu C/cm(2), the high potential barrier (0.25 +/- 0.05 eV) at the FTO/PTO interface and lowcarriermobility (10(-8) cm(2) V-1 s(-1)) compared to TiO2 leads to high serial resistance. Better results were obtained with thinner PTO layers grown by pulsed laser deposition, with PCE values up to 0.6%. Further enhancement was obtained by replacing PTO with BaTiO3 (BTO), with PCE value reaching about 0.8% after poling the cell with +3 V. The most important finding was that the magnitude of the short circuit current increases with the amplitude of the poling voltage while the value of the open-circuit voltage remains about the same, around 0.9 V. This is explained through more efficient collection of the charges generated under illumination in the absorber layer due to the polarization that is present in the ferroelectric film. (C) 2017 Elsevier Ltd. All rights reserved.

1327. MAPLE synthesis of reduced graphene oxide/silver nanocomposite electrodes: Influence of target composition and gas ambience

Authors: Queralto, A; del Pino, AP; Logofatu, C; Datcu, A; Amade, R; Alshaikh, I; Bertran, E; Urzica, I; Gyorgy, E

Published: DEC 5 2017, JOURNAL OF ALLOYS AND COMPOUNDS, 726, 1013, DOI: 10.1016/j.jallcom.2017.08.052

Graphene oxide (GO) and reduced GO films with different amounts of silver nanoparticles (Ag NPs) have been deposited on quartz and silicon substrates by matrix assisted pulsed laser evaporation (MAPLE). The morphology, structure, chemical composition, and optical and electrochemical properties were evaluated by scanning electron microscopy, X-ray photoelectron spectroscopy, ultravioletevisible spectrophotometry, and cyclic voltammetry measurements. The properties of the films obtained with two types of GO precursors have been compared. A reduction of the amount of oxygen containing groups is observed with the increase of the Ag concentration, which leads to a decrease of the optical band gap. Moreover, the deposition in nitrogen gas ambience leads to the N-doping of rGO material. The films obtained with the highest amount of Ag and with nitrogen doping show potential to be used in energy storage electrodes. (C) 2017 Elsevier B.V. All rights reserved.

1328. Structural and magnetic properties of Ni nanofilms on Ge(001) by molecular beam epitaxy

Authors: Bocirnea, AE; Costescu, RM; Pasuk, I; Lungu, GA; Teodorescu, CM

Published: DEC 1 2017, APPLIED SURFACE SCIENCE, 424, DOI: 10.1016/j.apsusc.2017.03.034

Ni films of 20 nm nominal thickness were grown on Ge(001) substrates by molecular beam epitaxy at several different temperatures from room temperature up to 400 C. X-ray diffraction and X-ray photoelectron spectroscopy reveal the nucleation of Ni-Ge compounds (NiGe, Ni2Ge, Ni5Ge2) as well as a departure from the fcc Ni structure exhibited by the films at and beyond a temperature of 100 C. The binding energy of the Ni 2p peak increases from the RT value (852.7 eV) by 0.51.1 eV for the Ni/Ge(001) samples, while the Ge 2p binding energy changes by 0.60.7 eV after Ni growth compared to a clean Ge(001) substrate (there is only a +/- 0.15 eV shift among the samples grown on substrates at higher temperatures). By increasing substrate temperature, we obtained higher intermixing of Ni and Ge, but rather than both Ni and Ge interdiffusing, we find that Ni diffuses further into the germanium with higher substrate temperature, forming increasingly Ni-rich Ni-Ge compounds diluted into the Ge matrix. Based on Magneto-optic Kerr Effect measurements, Ni/Ge(001) grown on substrates at 100 and 200 C does not exhibit a hysteresis loop, while the samples on 300 and 400 C substrates show magnetic behavior, which we attribute to the magnetic character of hexagonal Ni5Ge2 (which is determined here for the first time to be a ferromagnetic phase). (c) 2017 Elsevier B.V. All rights reserved.

1329. Optimization of the structural configuration of ICBA/P3HT photovoltaic cells

Authors: Nemnes, GA; Iftimie, S; Palici, A; Nicolaev, A; Mitran, TL; Radu, A; Antohe, S

Published: DEC 1 2017, APPLIED SURFACE SCIENCE, 424, 268, DOI: 10.1016/j.apsusc.2017.05.002

We investigate a possible route for optimization of organic P3HT:ICBA photovoltaic cells. In order to ensure a more efficient charge separation and collection at the electrodes, two- and three-layer structures are produced, where additional P3HT and ICBA single layers are placed adjacent to the mixed layer. The J-V characteristics are modeled using Monte-Carlo simulations in a flexible computational framework, reproducing the typical morphologies of the active layers. We discuss the implications of the structural modifications, in particular the enhancement of the open circuit voltage. Qualitative features of the theoretical simulations are validated by experiment. The proposed fabrication technique of using solvents with different boiling points for successive deposition of the individual layers may constitute an accessible route for producing optimized solar cell structures. (C) 2017 Elsevier B.V. All rights reserved.

1330. Chalcogenide thin films deposited by rfMS technique using a single quaternary target

Authors: Prepelita, P; Stavarache, I; Negrila, C; Garoi, F; Craciun, V

Published: DEC 1 2017, APPLIED SURFACE SCIENCE, 424, 427, DOI: 10.1016/j.apsusc.2016.11.071

Thin films of chalcogenide, Cu(In, Ga) Se-2 have been obtained using a single quaternary target by radio frequency magnetron sputtering method, with thickness in the range 750 nm to 1200 nm. X-ray photoelectron spectroscopy investigations showed, that the composition of Cu(In, Ga) Se-2 thin films was very similar to that of the used target CuIn0.75Ga0.25Se2. Identification of the chemical composition of Cu(In, Ga) Se-2 thin films by XPS performed in high vacuum, emphasized that the samples exhibit surface features suitable to be integrated into the structure of solar cells. Atomic Force Microscopy and Scanning Electron Microscopy investigations showed that surface morphology was influenced by the increase in thickness of the Cu(In, Ga) Se-2 layer. From X-Ray Diffraction investigations it was found that all films were polycrystalline, having a tetragonal lattice with a preferential orientation along the (112) direction. The optical reflectance as a function of wavelength was measured for the studied samples. The increase in thickness of the Cu(In, Ga) Se-2 absorber determined a decrease of its optical bandgap value from 1.53 eV to 1.44 eV. The results presented in this paper showed an excellent alternative of obtaining Cu(In, Ga) Se-2 compound thin films from a single target. (C) 2016 Elsevier B.V. All rights reserved.