NIMP| Raluca NEGREA, Author at NIMP

Publications

1. Synthesis of TiO₂-(B) Nanobelts for Acetone Sensing

Authors: Kumarage, GWC; Panamaldeniya, SA; Maddumage, DC; Moumen, A; Maraloiu, VA; Mihalcea, CG; Negrea, RF; Dassanayake, BS; Gunawardhana, N; Zappa, D; Galstyan, V; Comini, E

Published: OCT 2023, SENSORS, 23, 8322, DOI: 10.3390/s23198322

Titanium dioxide nanobelts were prepared via the alkali-hydrothermal method for application in chemical gas sensing. The formation process of TiO2-(B) nanobelts and their sensing properties were investigated in detail. FE-SEM was used to study the surface of the obtained structures. The TEM and XRD analyses show that the prepared TiO2 nanobelts are in the monoclinic phase. Furthermore, TEM shows the formation of porous-like morphology due to crystal defects in the TiO2-(B) nanobelts. The gas-sensing performance of the structure toward various concentrations of hydrogen, ethanol, acetone, nitrogen dioxide, and methane gases was studied at a temperature range between 100 and 500 C-degrees. The fabricated sensor shows a high response toward acetone at a relatively low working temperature (150 C-degrees), which is important for the development of low-power-consumption functional devices. Moreover, the obtained results indicate that monoclinic TiO2-B is a promising material for applications in chemo-resistive gas detectors.

2. Ferroelectricity modulates polaronic coupling at multiferroic interfaces

Authors: Husanu, MA; Popescu, DG; Bisti, F; Hrib, LM; Filip, LD; Pasuk, I; Negrea, R; Istrate, MC; Lev, L; Schmitt, T; Pintilie, L; Mishchenko, A; Teodorescu, CM; Strocov, VN

Published: AUG 15 2022, COMMUNICATIONS PHYSICS, 5, 209, DOI: 10.1038/s42005-022-00983-3

A deeper understanding of the coupling at the interface of multiferroics heterostructures is being achieved by the use of synchrotron radiation techniques. Here, the authors use k-resolved soft X-ray photoemission spectroscopy and first principles calculations to investigate the band structure of several multiferroic heterostructures, isolating the distinct signature of the interface. Physics of the multiferroic interfaces is currently understood mostly within a phenomenological framework based on screening of the polarization field and depolarizing charges. Additional effects still unexplored are the band dependence of the interfacial charge modulation and the associated changes of the electron-phonon interaction, coupling the charge and lattice degrees of freedom. Here, multiferroic heterostructures of the colossal-magnetoresistance manganite La1-xSrxMnO3 buried under ferroelectric BaTiO3 and PbZrxTi1-xO3 are investigated using soft-X-ray angle-resolved photoemission. The experimental band dispersions from the buried La1-xSrxMnO3 identify coexisting two-dimensional hole and three-dimensional electron charge carriers. The ferroelectric polarization modulates their charge density, affecting the coupling of the 2D holes and 3D electrons with the lattice which forms large Frohlich polarons inherently reducing mobility of the charge carriers. Our k-resolved results on the orbital occupancy, band filling and electron-lattice interaction in multiferroic oxide heterostructures modulated by the ferroelectric polarization disclose most fundamental physics of these systems needed for further progress of beyond-CMOS ferro-functional electronics.

3. Ferroelectric properties of ZrO2 films deposited on ITO-coated glass

Authors: Silva, JPB; Sekhar, KC; Negrea, RF; Ghica, C; Dastan, D; Gomes, MJM

Published: MAR 1 2022, CERAMICS INTERNATIONAL, 48, DOI: 10.1016/j.ceramint.2021.11.152

In this work, the ferroelectric characteristics of ZrO2 thin films grown on ITO-coated glass have been investigated. The ferroelectric nature of the ZrO2 films has been studied by polarization-electric field (P-E) hysteresis loops and found to be optimum for the films processed by rapid thermal annealing at 600 degrees C. The increase in the annealing temperature improves the ferroelectric properties through the increase of the in-plane strain that causes the formation of the ferroelectric orthorhombic phase. The formation of the orthorhombic phase was confirmed through high-resolution transmission electron microscopy. The effect of the electric field on the polarization switching kinetics of ZrO2 films has been investigated revealing that the switching kinetics follows the nucleation limited switching (NLS) model. The activation fields estimated from the peak values of the polarization currents (im) and the time (tm) at which im occurs are in good agreement with the values obtained from the switching characteristic time of the NLS model. This work paves the way towards the integration of (pseudo)binary oxide thin films on cheap substrates like glass for the next-generation of non-volatile memories.

4. Progress and perspective on different strategies to achieve wake-up-free ferroelectric hafnia and zirconia-based thin films

Authors: Silva, JPB; Sekhar, KC; Negrea, RF; MacManus-Driscoll, JL; Pintilie, L

Published: MAR 2022, APPLIED MATERIALS TODAY, 26, 101394, DOI: 10.1016/j.apmt.2022.101394

In the last decade orthorhombic hafnia and zirconia films have attracted tremendous attention arising from the discovery of ferroelectricity at the nanoscale. However, an initial wake-up pre-cycling is usually needed to achieve a ferroelectric behaviour in these films. Recently, different strategies, such as microstructure tailoring, defect, bulk and interface engineering, doping, NH3 plasma treatment and epitaxial growth, have been employed to obtain wake-up free orthorhombic ferroelectric hafnia and zirconia films. In this work we review recent developments in obtaining polar hafnia and zirconia-based thin films without the need of any wake-up cycling. In particular, we discuss the rhombohedral phase of hafnia/ zirconia, which under a constrained environment exhibits wake-up-free ferroelectric behaviour. This phase could have a strong impact on the current investigations of ferroelectric binary oxide materials and pave the way toward exploiting ferroelectric behaviour for next-generation memory and logic gate applications. Crown Copyright (c) 2022 Published by Elsevier Ltd. All rights reserved.

5. Wake-up Free Ferroelectric Rhombohedral Phase in Epitaxially Strained ZrO2 Thin Films

Authors: Silva, JPB; Negrea, RF; Istrate, MC; Dutta, S; Aramberri, H; Iniguez, J; Figueiras, FG; Ghica, C; Sekhar, KC; Kholkin, AL

Published: NOV 3 2021, ACS APPLIED MATERIALS & INTERFACES, 13, 51392, DOI: 10.1021/acsami.1c15875

Zirconia- and hafnia-based thin films have attracted tremendous attention in the past decade because of their unexpected ferroelectric behavior at the nanoscale, which enables the downscaling of ferroelectric devices. The present work reports an unprecedented ferroelectric rhombohedral phase of ZrO2 that can be achieved in thin films grown directly on (111)-Nb:SrTiO3 substrates by ion-beam sputtering. Structural and ferroelectric characterizations reveal (111)-oriented ZrO2 films under epitaxial compressive strain exhibiting switchable ferroelectric polarization of about 20.2 mu C/cm(2) with a coercive field of 1.5 MV/cm. Moreover, the time-dependent polarization reversal characteristics of Nb:SrTiO3/ZrO2/Au film capacitors exhibit typical bell-shaped curve features associated with the ferroelectric domain reversal and agree well with the nucleation limited switching (NLS) model. The polarization-electric field hysteresis loops point to an activation field comparable to the coercive field. Interestingly, the studied films show ferroelectric behavior per se, without the need to apply the wake-up cycle found in the orthorhombic phase of ZrO2. Overall, the rhombohedral ferroelectric ZrO2 films present technological advantages over the previously studied zirconia- and hafnia-based thin films and may be attractive for nanoscale ferroelectric devices.

6. Degenerated TiO(2)Semiconductor Modified with Ni and Zn as Efficient Photocatalysts for the Water Splitting Reaction

Authors: Neatu, F; Abramiuc, LE; Trandafir, MM; Negrea, RF; Florea, M; Teodorescu, CM; Neatu, S

Published: SEP 17 2020, CHEMCATCHEM, 12, DOI: 10.1002/cctc.202000691

The development of robust, safe, cost-effective and efficient photocatalytic systems for water splitting should take into account the presence of a proper and powerful photon absorber and an efficient, low-cost and earth-abundant electrocatalyst to perform the reaction at high conversions. In this study, Ni-Zn/TiO(2)ternary composites with high photocatalytic activity for water splitting under UV irradiation were successfully prepared via a simple and low-cost deposition-precipitation route. Thus, different Ni : Zn molar ratios (1 : 0, 1 : 1, 3 : 1, 6 : 1, 9 : 1, and 0 : 1) were deposited on TiO(2)in order to reach a total metal loading of 50 wt. %. The obtained composites were characterized using several techniques, such as: X-ray diffraction, UV-Vis spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy. The most active synthesized composite, namely Ni-Zn/TiO2(9 : 1), exhibits H(2)generation rate above 17 mmols g(-1) h(-1), which is nearly one thousand times higher than that obtained with TiO(2)Evonik P25. Our study demonstrates that TiO(2)becomes a degenerated semiconductor in the presence of Ni and ZnO, with remarkable photocatalytic properties. Thus, the obtained results can open new opportunities in the preparation of very active materials for hydrogen production based on the optimization of three-component structures.

7. Degenerated TiO2 Semiconductor Modified with Ni and Zn as Efficient Photocatalysts for the Water Splitting Reaction

Authors: Neatu, F; Abramiuc, LE; Trandafir, MM; Negrea, RF; Florea, M; Teodorescu, CM; Neatu, S

Published: SEP 17 2020, CHEMCATCHEM, 12, 4651, DOI: 10.1002/cctc.202000691

8. Interface chemistry of pristine TiN/La: Hf 0.5 Zr 0.5 O 2 capacitors

Authors: Hamouda, W; Lubin, C; Ueda, S; Yamashita, Y; Renault, O; Mehmood, F; Mikolajick, T; Schroeder, U; Negrea, R; Barrett, N

Published: JUN 22 2020, APPLIED PHYSICS LETTERS, 116, DOI: 10.1063/5.0012595

We present a hard and soft x-ray photoelectron spectroscopy study of the interface chemistry in pristine TiN/La-doped Hf 0.5 Zr 0.5 O 2/TiN capacitors. An oxynitride phase (similar to 1.3nm) is formed at the top interface, while a TiO 2 - delta phase was detected near the bottom interface. The oxygen vacancy ( V O) concentration is higher at the top interface than in the film due to oxygen scavenging by the top electrode. The V O concentration was also found to increase from similar to 1.5 to 1.9 x 10 20 cm - 3 when increasing La doping from 1.7 to 2.7mol. %. Two La dopants are compensated by the formation of one positively charged V O.

9. Energy storage performance of ferroelectric ZrO(2)film capacitors: effect of HfO2:Al(2)O(3)dielectric insert layer

Authors: Silva, JPB; Silva, JMB; Sekhar, KC; Palneedi, H; Istrate, MC; Negrea, RF; Ghica, C; Chahboun, A; Pereira, M; Gomes, MJM

Published: JUL 28 2020, JOURNAL OF MATERIALS CHEMISTRY A, 8, 14177, DOI: 10.1039/d0ta04984k

The present work reports for the first time, the employment of ferroelectric ZrO(2)films as energy storage capacitors utilized in pulsed power systems. Furthermore, the effect of insertion of a low permittivity dielectric HfO2:Al2O3(HAO) layer, with a thickness ranging from 2 to 8 nm, on the tunability of ferroelectric and energy storage characteristics of ZrO(2)films is assessed. The increase in thickness of the HAO layer gave rise to distorted ferroelectric loops with decreased polarization, coercive field, and hysteresis loss of the films. These results are correlated with the depolarization field induced by the insertion of the dielectric HAO layer. An optimum combination of high energy density of 54.3 J cm(-3)and good storage efficiency of 51.3% are obtained for the ZrO(2)film capacitors with 2 nm-thick HAO insert layer. These values correspond to an increase of similar to 55% and similar to 92%, from the respective values of pure ZrO(2)film capacitors. In addition, the HAO/ZrO(2)films showed a good fatigue endurance of energy storage performance over 10(9)electric field cycles. The energy storage density obtained from HAO(2 nm)/ZrO(2)film capacitor is found to be higher than that reported for several Pb-based as well as Pb-free ferroelectric ceramic films with complex compositions. The present study demonstrates that simple binary oxides such as ZrO(2)with ferroelectric behavior could be potential candidates for developing high performance energy storage capacitors.

10. Synthesis of Core-Double Shell Nylon-ZnO/Polypyrrole Electrospun Nanofibers

Authors: Beregoi, M; Preda, N; Costas, A; Enculescu, M; Negrea, RF; Iovu, H; Enculescu, I

Published: NOV 2020, NANOMATERIALS, 10, DOI: 10.3390/nano10112241

Core-double shell nylon-ZnO/polypyrrole electrospun nanofibers were fabricated by combining three straightforward methods (electrospinning, sol-gel synthesis and electrodeposition). The hybrid fibrous organic-inorganic nanocomposite was obtained starting from freestanding nylon 6/6 nanofibers obtained through electrospinning. Nylon meshes were functionalized with a very thin, continuous ZnO film by a sol-gel process and thermally treated in order to increase its crystallinity. Further, the ZnO coated networks were used as a working electrode for the electrochemical deposition of a very thin, homogenous polypyrrole layer. X-ray diffraction measurements were employed for characterizing the ZnO structures while spectroscopic techniques such as FTIR and Raman were employed for describing the polypyrrole layer. An elemental analysis was performed through X-ray microanalysis, confirming the expected double shell structure. A detailed micromorphological characterization through FESEM and TEM assays evidenced the deposition of both organic and inorganic layers. Highly transparent, flexible due to the presence of the polymer core and embedding a semiconducting heterojunction, such materials can be easily tailored and integrated in functional platforms with a wide range of applications.

11. Performant Composite Materials Based on Oxide Semiconductors and Metallic Nanoparticles Generated from Cloves and Mandarin Peel Extracts

Authors: Zgura, I; Enculescu, M; Istrate, C; Negrea, R; Bacalum, M; Nedelcu, L; Barbinta-Patrascu, ME

Published: NOV 2020, NANOMATERIALS, 10, DOI: 10.3390/nano10112146

In this work, the metal and semiconducting nanoparticles (AgNPs, ZnONPs and AgZnONPs) were phyto-synthesized using aqueous vegetal extracts from: Caryophyllus aromaticus L. (cloves) and Citrus reticulata L. (mandarin) peels. The morphological, structural, compositional, optical and biological properties (antibacterial activity, and cytotoxicity) of the prepared composites were investigated. The most effective sample proved to be AgZnONPs, derived from cloves, with a minimum inhibitory concentration (MIC) value of 0.11 mg/mL and a minimum bactericidal concentration (MBC) value of 2.68 mg/mL. All the other three composites inhibited bacterial growth at a concentration between 0.25 mg/mL and 0.37 mg/mL, with a bactericidal concentration between 3 mg/mL and 4 mg/mL. The obtained composites presented biocidal activity against Staphylococcus aureus, and biocompatibility (on human fibroblast BJ cells) and did not damage the human red blood cells. Additionally, an important result is that the presence of silver in composite materials improved the bactericidal action of these nanomaterials against the most common nosocomial pathogen, Staphylococcus aureus.

12. HfO2-Al2O3 Dielectric Layer for a Performing Metal-Ferroelectric-Insulator-Semiconductor Structure with a Ferroelectric 0.5Ba(Zr0.2Ti0.8)O-3-0.5(Ba0.7Ca0.3)TiO3 Thin Film

Authors: Silva, JPB; Sekhar, KC; Veltruska, K; Matolin, V; Negrea, RF; Ghica, C; Oliveira, MJS; Moreira, JA; Pereira, M; Gomes, MJM

Published: SEP 22 2020, ACS APPLIED ELECTRONIC MATERIALS, 2, 2787, DOI: 10.1021/acsaelm.0c00480

In this work, the ferroelectric and fatigue characteristics of Au/0.5Ba(Zr0.2Ti0.8)O-3-0.5(Ba0.7Ca0.3)TiO3(BCZT)/Si metal-ferroelectric-semiconductor (MFS) structures are investigated. Moreover, the effect of introducing a thin dielectric HfO2-Al2O3 (HAO) layer with different thicknesses between the BCZT layer and the Si substrate on the ferroelectric characteristics in the metal-ferroelectric-insulator-semiconductor (MFIS) configuration is evaluated. It is evidenced that the insertion of the HAO layer with a thickness of 8 nm improves the memory window of the capacitance-voltage (C-V) curves by 106% compared to the value obtained in the MFS structure and reduces the leakage currents. Furthermore, the Au/BCZT/HAO (8 nm)/Si structure shows a remarkable remnant polarization (P-r) of 7.8 mu C/cm(2), with a coercive voltage of 1.9 V. The obtained value for P-r corresponds to a six times enhancement when compared to the value obtained in the Au/BCZT/Si structure. In addition, the fatigue studies reveal that the P-r obtained in the Au/BCZT/HAO/Si structure slightly decreases (3%) with continuous cycling, up to 10(9) cycles. The present work evidences that Au/BCZT/HAO/Si structures are promising for nonvolatile memory applications.

13. Phase Control in Hafnia: New Synthesis Approach and Convergence of Average and Local Structure Properties

Authors: Cojocaru, B; Avram, D; Negrea, R; Ghica, C; Kessler, VG; Seisenbaeva, GA; Parvulescu, VI; Tiseanu, C

Published: MAY 2019, ACS OMEGA, 4, 8891, DOI: 10.1021/acsomega.9b00580

Technologically relevant tetragonal/cubic phases of HfO2 can be stabilized at room temperature by doping with trivalent rare earths using various approaches denoted generically as bulk coprecipitation. Using in situ/ex situ X-ray diffraction (XRD), Raman spectroscopy, high-resolution transmission electron microscopy, and in situ/ex situ site-selective, time-gated luminescence spectroscopy, we show that wet impregnation of hafnia nanoparticles with 10% Eu oxide followed by mild calcination in air at 500 degrees C produces an efficient stabilization of the cubic phase, comparable to that obtained by bulk precipitation. The physical reasons behind the apparently conflictual data concerning the actual crystallographic phase and the local symmetry around the Eu stabilizer and how these can be mediated by luminescence analysis are also discussed. Apparently, the cubic crystal structure symmetry determined by XRD results in a pseudocubic/tetragonal local structure around Eu determined by luminescence. Considering the recent findings on wet impregnated CeO2 and ZrO2, it is concluded that CeO2, ZrO2, and HfO2 represent a unique case of a family of oxides that is extremely tolerant to heavy doping by wet impregnation. In this way, the same batch of preformed nanoparticles can be doped with different lanthanide concentrations or with various lanthanides at a fixed concentration, allowing a systematic and reliable investigation of the effect of doping, lanthanide type, and lanthanide concentration on the various functionalities of these technologically relevant oxides.

14. Unveiling the double-well energy landscape in a ferroelectric layer

Authors: Hoffmann, M; Fengler, FPG; Herzig, M; Mittmann, T; Max, B; Schroeder, U; Negrea, R; Pintilie, L; Slesazeck, S; Mikolajick, T

Published: JAN 24 2019, NATURE, 565, +, DOI: 10.1038/s41586-018-0854-z

The properties of ferroelectric materials, which were discovered almost a century ago(1), have led to a huge range of applications, such as digital information storage(2), pyroelectric energy conversion(3) and neuromorphic computing(4,5). Recently, it was shown that ferroelectrics can have negative capacitance(6-11), which could improve the energy efficiency of conventional electronics beyond fundamental limits(12-14). In Landau-Ginzburg-Devonshire theory(15-17), this negative capacitance is directly related to the double-well shape of the ferroelectric polarization-energy landscape, which was thought for more than 70 years to be inaccessible to experiments(18). Here we report electrical measurements of the intrinsic double-well energy landscape in a thin layer of ferroelectric Hf0.5Zr0.5O2. To achieve this, we integrated the ferroelectric into a heterostructure capacitor with a second dielectric layer to prevent immediate screening of polarization charges during switching. These results show that negative capacitance has its origin in the energy barrier in a double-well landscape. Furthermore, we demonstrate that ferroelectric negative capacitance can be fast and hysteresis-free, which is important for prospective applications(19). In addition, the Hf0.5Zr0.5O2 used in this work is currently the most industry-relevant ferroelectric material, because both HfO2 and ZrO2 thin films are already used in everyday electronics(20). This could lead to fast adoption of negative capacitance effects in future products with markedly improved energy efficiency.

15. Very large remanent polarization in ferroelectric Hf1-xZrxO2 grown on Ge substrates by plasma assisted atomic oxygen deposition

Authors: Zacharaki, C; Tsipas, P; Chaitoglou, S; Fragkos, S; Axiotis, M; Lagoyiannis, A; Negrea, R; Pintilie, L; Dimoulas, A

Published: MAR 18 2019, APPLIED PHYSICS LETTERS, 114, DOI: 10.1063/1.5090036

Plasma assisted atomic oxygen deposition was used to grow polycrystalline ferroelectric Hf1-xZrxO2 (x = 0.5-0.7) on technologically important (100) Germanium substrates showing sharp crystalline interfaces free of interfacial amorphous layers and strong evidence for the presence of a predominately orthorhombic phase. The electrical properties, evaluated using metal-ferroelectric-semiconductor (MFS) capacitors, show symmetric and robust ferroelectric hysteresis with weak or no wake-up effects. The MFS capacitors with x = 0.58 show very large remanent polarization up to 34.4 mu C/cm(2) or 30.6 mu C/cm(2) after correction for leakage and parasitics, combined with good endurance reaching 10(5) cycles at a cycling field of 2.3 MV/cm. The results show good prospects for the fabrication of Ge ferroelectric field effect transistors (FeFETs) for use in 1 T FeFET embedded nonvolatile memory cells with improved endurance. (C) 2019 Author(s).

16. Palladium/palladium oxide coated electrospun fibers for wearable sweat pH-sensors

Authors: Diculescu, VC; Beregoi, M; Evanghelidis, A; Negrea, RF; Apostol, NG; Enculescu, I

Published: JUN 20 2019, SCIENTIFIC REPORTS, 9, DOI: 10.1038/s41598-019-45399-2

The work describes the development of a flexible, hydrogel embedded pH-sensor that can be integrated in inexpensive wearable and non-invasive devices at epidermal level for electrochemical quantification of H+ ions in sweat. Such a device can be useful for swift, real time diagnosis and for monitoring specific conditions. The sensors' working electrodes are flexible poly(methyl methacrylate) electrospun fibers coated with a thin gold layer and electrochemically functionalized with nanostructured palladium/palladium oxide. The response to H+ ions is investigated by cyclic voltammetry and electrochemical impedance spectroscopy while open circuit potential measurements show a sensitivity of aprox. -59 mV per pH unit. The modification of the sensing interface upon basic and acid treatment is characterized by scanning and transmission electron microscopy and the chemical composition by X-ray photoelectron spectroscopy. In order to demonstrate the functionality of the pH-sensor at epidermal level, as a wearable device, the palladium/palladium oxide working electrode and silver/silver chloride reference electrode are embedded within a pad of polyacrylamide hydrogel and measurements in artificial sweat over a broad pH range were performed. Sensitivity up to -28 mV/pH unit, response time below 30 s, temperature dependence of approx. 1 mV/degrees C as well as the minimum volume to which the sensor responses of 250 nanoliters were obtained for this device. The proposed configuration represents a viable alternative making use of low-cost and fast fabrication processes and materials.

17. Dramatic impact of pressure and annealing temperature on the properties of sputtered ferroelectric HZO layers

Authors: Bouaziz, J; Romeo, PR; Baboux, N; Negrea, R; Pintilie, L; Vilquin, B

Published: AUG 2019, APL MATERIALS, 7, DOI: 10.1063/1.5110894

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/).

18. Structural, electric and pyroelectric properties of up and down graded PZT multilayers

Authors: Botea, M; Hrib, L; Pasuk, I; Iuga, A; Trupina, L; Negrea, R; Becherescu, N; Pintilie, L

Published: JUL 2019, CURRENT APPLIED PHYSICS, 19, 810, DOI: 10.1016/j.cap.2019.04.010

Multi-layered structures, composed of thin films from materials with different compositions or physical properties, represents a way to obtain enhanced properties or even new functionalities. In this work, lead zirconate titanate PbZrxTi1-xO3 (PZT; x = 0.20, 0.52, 0.80) multilayers were grown by pulsed laser deposition (PLD) on a single crystal strontium titanate (SrTiO3, STO) substrate, using a strontium ruthenate (SrRuO3, SRO) film as buffer layer for epitaxial growth, and also as back electrode. Up and down multi-layers were grown and their physical and structural properties were compared, up being the structure in which Zr concentration was varied from 20% near the STO substrate to 80% at the surface, while down is for the structure in which the Zr concentration starts with 80% near the substrate and ends with 20% at the surface. It was found that the electric and pyroelectric properties of the two graded structures are significantly different. The up structure presents electric properties that are comparable with those of single composition PZT films while the properties of the down structure are deteriorated, especially in terms of the leakage current magnitude. Pyroelectric signal could be measured only for the up structure. These differences were attributed to larger density of structural defects in the down structure compared to the up one. This is due to the different growth sequence: Lop structure starts with tetragonal PZT on cubic substrate (lower lattice mismatch, 1.1%) while down structure starts with rhombohedral PZT on cubic substrate (larger lattice mismatch, almost 5%).

19. Low value for the static background dielectric constant in epitaxial PZT thin films

Authors: Boni, GA; Chirila, CF; Hrib, L; Negrea, R; Filip, LD; Pintilie, I; Pintilie, L

Published: OCT 11 2019, SCIENTIFIC REPORTS, 9, DOI: 10.1038/s41598-019-51312-8

Ferroelectrics are intensively studied materials due to their unique properties with high potential for applications. Despite all efforts devoted to obtain the values of ferroelectric material constants, the problem of the magnitude of static dielectric constant remains unsolved. In this article it is shown that the value of the static dielectric constant at zero electric field and with negligible contribution from the ferroelectric polarization (also called static background dielectric constant, or just background dielectric constant) can be very low (between 10 and 15), possibly converging towards the value in the optical domain. It is also found that the natural state of an ideal, mono-domain, epitaxial ferroelectric is that of full depletion with constant capacitance at voltages outside the switching domain. The findings are based on experimental results obtained from a new custom method designed to measure the capacitance-voltage characteristic in static conditions, as well from Rayleigh analysis. These results have important implications in future analysis of conduction mechanisms in ferroelectrics and theoretical modeling of ferroelectric-based devices.

20. Akermanite-based coatings grown by pulsed laser deposition for metallic implants employed in orthopaedics

Authors: Negrea, R; Busuioc, C; Constantinoiu, I; Miu, D; Enache, C; Iordache, F; Jinga, SI

Published: JAN 15 2019, SURFACE & COATINGS TECHNOLOGY, 357, 1026, DOI: 10.1016/j.surfcoat.2018.11.008

The aim of the present paper is to develop ceramic thin films by laser ablation in order to improve the biological behaviour of metallic implants dedicated to hard tissue restoration. The composition of the coatings was selected within SiO2-P2O5-CaO-MgO-ZnO-CaF2 system, while their processing has gone through two stages: target preparation via a wet chemistry approach and films deposition through a physical deposition method, on titanium substrates. The characteristics of the final layered structures were evaluated by X-ray diffraction, Fourier-transform infrared spectroscopy, scanning and transmission electron microscopy, energy-dispersive X-ray spectroscopy and selected area electron diffraction. In vitro investigation techniques were employed for the bioactivity and biocompatibility assessment. The results indicated the growth of nanostructured akermanite-based thin films with an excellent bioactivity and a good effect on stem-type cells, which validates the suitability of such structures for medical implant applications.

21. Hysteretic Characteristics of Pulsed Laser Deposited 0.5Ba(Zr0.2Ti0.8)O-3-0.5(Ba0.7Ca0.3)TiO3/ZnO Bilayers

Authors: Silva, JPB; Wang, J; Koster, G; Rijnders, G; Negrea, RF; Ghica, C; Sekhar, KC; Moreira, JA; Gomes, MJM

Published: MAY 2 2018, ACS APPLIED MATERIALS & INTERFACES, 10, 15249, DOI: 10.1021/acsami.8b01695

In the present work, we study the hysteretic behavior in the electric-field-dependent capacitance and the current characteristics of 0.5Ba(Zr0.2Ti0.8)O-3-0.5(Ba0.7Ca0.3)TiO3 (BCZT)/ZnO bilayers deposited on 0.7 wt % Nb-doped (001)-SrTiO3 (Nb:STO) substrates in a metal-ferroelectric-semiconductor (MFS) configuration. The X-ray diffraction measurements show that the BCZT and ZnO layers are highly oriented along the c-axis and have a single perovskite and wurtzite phases, respectively, whereas high-resolution transmission electron microscopy revealed very sharp Nb:STO/ BCZT/ZnO interfaces. The capacitance-electric field (C-E) characteristics of the bilayers exhibit a memory window of 47 kV/cm and a capacitance decrease of 22%, at a negative bias. The later result is explained by the formation of a depletion region in the ZnO layer. Moreover, an unusual resistive switching (RS) behavior is observed in the BCZT films, where the RS ratio can be 500 times enhanced in the BCZT/ZnO bilayers. The RS enhancement can be understood by the barrier potential profile modulation at the depletion region, in the BCZT/ZnO junction, via ferroelectric polarization switching of the BCZT layer. This work builds a bridge between the hysteretic behavior observed either in the C-E and current-electric field characteristics on a MFS structure.

22. Ferroelectric photovoltaic characteristics of pulsed laser deposited 0.5Ba (Zr0.2Ti0.8)O-3-0.5(Ba0.7Ca0.3)TiO3/ZnO heterostructures

Authors: Silva, JPB; Sekhar, KC; Cortes-Juan, F; Negrea, RF; Kuncser, AC; Connolly, JP; Ghica, C; Moreira, JA

Published: JUN 2018, SOLAR ENERGY, 167, 23, DOI: 10.1016/j.solener.2018.03.072

In this work, we investigate the photovoltaic response of Pt/0.5Ba(Zr0.2Ti0.8)O-3-0.5(Ba0.7Ca0.3)TiO3(0.5BZT-0.5BCT)/ITO structures through the insertion of a semiconductor ZnO layer at different positions. The values of short-circuit photocurrent density (J(sc)) of the Pt/ZnO/0.5BZT-0.5BCT/ITO, Pt/0.5BZT-0.5BCT/ZnO/ITO and Pt/ZnO/0.5BZT-0.5BCT/ZnO/ITO capacitors are around 5.31, 0.0034 and 0.052 mA/cm(2), respectively. The enhanced photovoltaic (PV) effect is observed when ZnO layer is inserted between Pt and the 0.5BZT-0.5BCT layer. The built-in field developed at the ZnO/ferroelectric interface in the same direction of the depolarizing field, provides a favorable electric potential for the efficient separation and transportation of photo generated e-h pairs. Furthermore, the polarization-dependent interfacial coupling effect enhances PV effect, which is confirmed by investigating the role of polarization flipping on switchable photo response. This work provides an efficient pathway in tuning the PV response in ferroelectric-based solar cells.

23. Triggering surface ferroelectric order in Pb(Zr, Ti)O-3(001) by deposition of platinum

Authors: Bucur, IC; Tanase, LC; Abramiuc, LE; Lungu, GA; Chirila, C; Trupina, L; Apostol, NG; Costescu, RM; Negrea, RF; Pintilie, L; Teodorescu, CM

Published: FEB 28 2018, APPLIED SURFACE SCIENCE, 432, DOI: 10.1016/j.apsusc.2017.04.238

By platinum deposition on a 150 nm thick film of lead zirco-titanate oriented PZT(001), grown on strontium titanate (001) single crystals with a strontium ruthenate buffer layer, which did not show initial preferential out-of-plane orientation of its ferroelectric polarization, a band bending near the interface towards lower energies is observed using photoelectron spectroscopy, by following all core levels from the substrate (Pb 4f, Zr 3d, Ti 2p, O 1s). This is unexpected given the fact that platinum has a larger work function than PZT and a rectifying contact for electrons is expected to be built at the interface. This observation may have two explanations: (i) platinum forms an alloy with elements from PZT yielding a metal with considerable lower work function; (ii) platinum provides electrons to the substrate which are able to compensate the depolarization field generated by the outwards polarization state. Several arguments are brought in favor of the second hypothesis, especially the attenuation of core levels from the substrate which is well described by exponential functions with reasonable values of the photoelectron inelastic mean free path, suggesting the formation of a sharp interface. High resolution transmission electron microscopy confirmed the sharpness of the interface. (C) 2017 Elsevier B.V. All rights reserved.

24. Zinc Oxide Spherical-Shaped Nanostructures: Investigation of Surface Reactivity and Interactions with Microbial and Mammalian Cells

Authors: Visinescu, D; Hussien, MD; Moreno, JC; Negrea, R; Birjega, R; Somacescu, S; Ene, CD; Chifiriuc, MC; Popa, M; Stan, MS; Carp, O

Published: NOV 13 2018, LANGMUIR, 34, 13651, DOI: 10.1021/acs.langmuir.8b02528

Two ZnO materials of spherical hierarchical morphologies, with hollow (ZnOHS) and solid cores (ZnOSS), were obtained through the hydrolysis of zinc acetylacetonate in 1,4-butanediol. The nature of the defects and surface reactivity for the two ZnO materials were investigated through photoluminescence, X-ray photoelectron spectroscopy, and electron paramagnetic resonance (EPR) spectroscopy proving the coexistence of shallow and deep defects and, also, the presence of polyol byproducts adsorbed on the outer layers of the ZnO samples. The EPR spectroscopy coupled with the spin-trapping technique showed that the surface of the ZnO samples generates reactive oxygen species (ROS) like hydroxyl (OH) and singlet oxygen (O-1(2)) as well as carbon-centered radicals. The ZnO materials exhibited a wide spectrum of antimicrobial activity, being active against Gram-positive, Gram-negative, and fungi strains, both in planktonic and, more importantly, adherent groywth states. The decrease of antimicrobial efficiency in the presence of a ROS scavenger (mannitol) and the decrease of the cell viability with the ROS level suggest that one of the mechanisms that governs both the antimicrobial and cytotoxic activities on human liver cells is ROS-mediated. However, at active antimicrobial concentrations, the biocompatibility of the tested materials is very good.

25. Full Tetragonal Phase Stabilization in ZrO2 Nanoparticles Using Wet Impregnation: Interplay of Host Structure, Dopant Concentration and Sensitivity of Characterization Technique

Authors: Colbea, C; Avram, D; Cojocaru, B; Negrea, R; Ghica, C; Kessler, VG; Seisenbaeva, GA; Parvulescu, V; Tiseanu, C

Published: DEC 2018, NANOMATERIALS, 8, DOI: 10.3390/nano8120988

Here, we show that wet impregnation of ZrO2 nanoparticles with 10% and 20% Eu oxide followed by thermal anneal in air above 500 degrees C produces full stabilization of the tetragonal phase of ZrO2 without evidencing any phase separation. The bare ZrO2 nanoparticles were obtained using three synthetic methods: oil in water microemulsion, rapid hydrothermal, and citrate complexation methods. The homogeneity of the solid solutions was assessed using X-ray diffraction, Raman spectroscopy, high resolution transmission electron microscopy, and advanced luminescence spectroscopy. Our findings show that wet impregnation, which is a recognized method for obtaining surface doped oxides, can be successfully used for obtaining doped oxides in the bulk with good homogeneity at the atomic scale. The limits of characterization technique in detecting minor phases and the roles of dopant concentration and host structure in formation of phase stabilized solid solutions are also analyzed and discussed.

26. Branch-like NiO/ZnO heterostructures for VOC sensing

Authors: Kaur, N; Zappa, D; Ferroni, M; Poli, N; Campanini, M; Negrea, R; Comini, E

Published: JUN 1 2018, SENSORS AND ACTUATORS B-CHEMICAL, 262, 485, DOI: 10.1016/j.snb.2018.02.042

We report for the first time on the synthesis of NiO/ZnO one-dimensional (1D) nanowire (NW) based heterostructures by applying a suitable methodology of transport and condensation. The synthesis involves, firstly the growth of NiO NWs on gold (Au) catalyzed alumina substrates using the vapor-liquid-solid (VLS) mechanism and then the formation of ZnO NWs directly on the NiO NWs using the vapor-solid (VS) mechanism. Sequential evaporation-condensation over Au-seeded alumina promotes the formation of NiO NWs, driven by the VLS growth mechanism. These NiO NWs act as backbones for the condensation of epitaxial ZnO nanostructures. The detailed morphological study of these heterostructures reveals that ZnO nanowires completely cover the whole NiO nanowires completely and growing out in the form of flat leaves from the NiO nanowire branches. The diameters of the NiO NWs have been found to vary from 15 nm to 60 nm. Selected area electron diffraction data (SAED) indicate an epitaxial growth of ZnO nanowires along (101)-planes on the strongly oriented NiO nanowires along (200) crystallographic planes. Finally, NiO NW and NiO/ZnO heterostructure based conductometric gas sensing devices have been fabricated and the comparison between their sensing performances have been compared. Interestingly, NiO/ZnO NWs heterostructure based sensing devices shows superior performance compared to NiO sensors toward volatile organic compounds (VOC). (C) 2018 Elsevier B.V. All rights reserved.

27. Ambiguous Role of Growth-Induced Defects on the Semiconductor-to-Metal Characteristics in Epitaxial VO2/TiO2 Thin Films

Authors: Mihailescu, CN; Symeou, E; Svoukis, E; Negrea, RF; Ghica, C; Teodorescu, V; Tanase, LC; Negrila, C; Giapintzakis, J

Published: APR 25 2018, ACS APPLIED MATERIALS & INTERFACES, 10, 14144, DOI: 10.1021/acsami.8b01436

Controlling the semiconductor-to-metal transition temperature in epitaxial VO2 thin films remains an unresolved question both at the fundamental as well as the application level. Within the scope of this work, the effects of growth temperature on the structure, chemical composition, interface coherency and electrical characteristics of rutile VO2 epitaxial thin films grown on TiO2 substrates are investigated. It is hereby deduced that the transition temperature is lower than the bulk value of 340 K. However, it is found to approach this value as a function of increased growth temperature even though it is accompanied by a contraction along the V4+-V4+ bond direction, the crystallographic c-axis lattice parameter. Additionally, it is demonstrated that films grown at low substrate temperatures exhibit a relaxed state and a strongly reduced transition temperature. It is suggested that, besides thermal and epitaxial strain, growth-induced defects may strongly affect the electronic phase transition. The results of this work reveal the difficulty in extracting the intrinsic material response to strain, when the exact contribution of all strain sources cannot be effectively determined. The findings also bear implications on the limitations in obtaining the recently predicted novel semi-Dirac point phase in VO2/TiO2 multilayer structures.

28. Gold nanoparticles stabilized on SBA-15 functionalized NNN-pincer ligand; highly effective catalyst for reduction of nitroarenes in aqueous medium

Authors: Hosseini, SH; Zohreh, N; Alipour, S; Busuioc, C; Negrea, R

Published: APR 2018, CATALYSIS COMMUNICATIONS, 108, 97, DOI: 10.1016/j.catcom.2018.01.002

Gold nanoparticles were decorated on/into the SBA-15 channels covalently functionalized by new class of symmetrical tridentate NNN-pincer ligand. This provides a unique support for stabilization of Au NPs preventing agglomeration due to the strong ligation of pincer ligand. The catalyst proved to be highly active for the reduction of nitroarenes. The kinetics study showed that the rate constant for the current protocol are comparable or larger than the reported ones in literature. The catalyst also showed high recoverability and reusability up to ten times without significant loss of activity and leaching or aggregation of Au NPs (overall TOF = 8798).

29. Palladium pincer complex incorporation onto the Fe3O4-entrapped cross-linked multilayered polymer as a high loaded nanocatalyst for oxidation'

Authors: Zohreh, N; Hosseini, SH; Tavakolizadeh, M; Busuioc, C; Negrea, R

Published: SEP 15 2018, JOURNAL OF MOLECULAR LIQUIDS, 266, 404, DOI: 10.1016/j.molliq.2018.06.076

Pd nanoparticles are stabilized onto the surface of Fe3O4-entrapped poly(aminoethyl acrylamide) via functionalization of polymer with a new class of NNN-pincer ligand providing a high loading and stable catalyst for aerobic oxidation of alcohols to aldehydes. Varieties of alcohols are oxidized to the corresponding aldehydes with good selectivity using 0.95 mol% Pd catalyst. The catalyst is reusable (5 runs) without loss of activity and leaching or agglomeration of Pd nanoparticles while structural regularity of the catalyst is preserved. Oxidation of benzyl alcohol is also scaled up to a few grams. (C) 2018 Elsevier B.V. All rights reserved.

30. Rolling dopant and strain in Y-doped BiFeO3 epitaxial thin films for photoelectrochemical water splitting

Authors: Haydous, F; Scarisoreanu, ND; Birjega, R; Ion, V; Lippert, T; Dumitrescu, N; Moldovan, A; Andrei, A; Teodorescu, VS; Ghica, C; Negrea, R; Dinescu, M

Published: OCT 25 2018, SCIENTIFIC REPORTS, 8, DOI: 10.1038/s41598-018-34010-9

We report significant photoelectrochemical activity of Y-doped BiFeO3 (Y-BFO) epitaxial thin films deposited on Nb: SrTiO3 substrates. The Y-BFO photoanodes exhibit a strong dependence of the photocurrent values on the thickness of the films, and implicitly on the induced epitaxial strain. The peculiar crystalline structure of the Y-BFO thin films and the structural changes after the PEC experiments have been revealed by high resolution X-ray diffraction and transmission electron microscopy investigations. The crystalline coherence breaking due to the small ionic radius Y-addition was analyzed using Willliamson-Hall approach on the 2 theta-omega scans of the symmetric (00l) reflections and confirmed by high resolution TEM (HR-TEM) analysis. In the thinnest sample the lateral coherence length (L-parallel to) is preserved on larger nanoregions/ nanodomains. For higher thickness values L-parallel to is decreasing while domains tilt angles (alpha(tilt)) is increasing. The photocurrent value obtained for the thinnest sample was as high as J(ph) = 0.72 mA/cm(2), at 1.4 V(vs. RHE). The potentiostatic scans of the Y-BFO photoanodes show the stability of photoresponse, irrespective of the film's thickness. There is no clear cathodic photocurrent observation for the Y-BFO thin films confirming the n-type semiconductor behavior of the Y-BFO photoelectrodes.

31. Limits and Particularities of the Synthesis of Ba1-xCaxTiO3 for Piezoelectric Applications, by Topochemical Conversion from Molten Salt Solutions

Authors: Vlaicu, ID; Maraloiu, AV; Ghica, D; Mercioniu, IF; Stefan, M; Vlaicu, AM; Negrea, RF; Kuncser, AC; Bulat, S; Krzmanc, MM; Ciobanu, R; Plopa, O

Published: 2018, 2018 INTERNATIONAL CONFERENCE AND EXPOSITION ON ELECTRICAL AND POWER ENGINEERING (EPE), 1050, DOI:

Because of the unique properties of the ferroelectric perovskite particles with a well-defined anisotropic form like shape-and size dependent at low dimensions they have all the attention of the scientific world. Extensive morphostructural techniques will be used to characterize the piezoelectric material.

32. Ferroelectric switching dynamics in 0.5Ba(Zr0.2Ti0.8)O-3-0.5( Ba0.7Ca0.3)TiO3 thin films

Authors: Silva, JPB; Kamakshi, K; Negrea, RF; Ghica, C; Wang, J; Koster, G; Rijnders, G; Figueiras, F; Pereira, M; Gomes, MJM

Published: AUG 20 2018, APPLIED PHYSICS LETTERS, 113, DOI: 10.1063/1.5044623

In this work, the ferroelectric characteristics of 0.5Ba(Zr0.2Ti0.8)O-3-0.5(Ba0.7Ca0.3)TiO3 (BCZT) thin films grown on 0.7 wt. % Nb-doped (001)-SrTiO3 (Nb:STO) single-crystal have been investigated. High-resolution transmission electron microscopy and electron energy loss spectroscopy revealed a very sharp Nb:STO/BCZT interface, while selected area electron diffraction revealed the epitaxial growth of the BCZT layer on the Nb:STO substrate. The ferroelectric nature of the BCZT films have been investigated by piezoresponse force microscopy and hysteresis loops. The effect of electric field on polarization switching kinetics has been investigated and has been analyzed by the nucleation limited switching model with a Lorentzian distribution function. The local field variation was found to decrease with the increase in the electric field, and thus, the switching process becomes faster. The peak value of the polarization current and the logarithmic characteristic switching time exhibited an exponential dependence on the inverse of electric field. This model gave an excellent agreement with the experimental polarization reversal transients throughout the whole time range. Published by AIP Publishing.

33. On the threshold for ion track formation in CaF2

Authors: Karlusic, M; Ghica, C; Negrea, RF; Siketic, Z; Jaksic, M; Schleberger, M; Fazinic, S

Published: FEB 10 2017, NEW JOURNAL OF PHYSICS, 19, DOI: 10.1088/1367-2630/aa5914

There is an ongoing debate regarding the mechanism of swift heavy ion (SHI) track formation in CaF2. The objective of this study is to shed light on this important topic using a range of complementary experimental techniques. Evidence of the threshold for ion track formation being below 3 keV nm(-1) is provided by both transmission electron microscopy (TEM) and Rutherford backscattering spectroscopy in the channelling mode, which has direct consequences for the validity of models describing the response of CaF2 to SHI irradiation. Furthermore, information about the elemental composition within the ion tracks is obtained using scanning TEM, electron energy loss spectroscopy, and with respect to the stoichiometry of the materials surface by in situ time of flight elastic recoil detection analysis. Advances in the analyses of the experimental data presented here pave the way for a better understanding of the ion track formation.

34. 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.

35. Enhancement of the electroluminescence of organic light emitting devices based on Ir(ppy)(3) by doping with metallic and magnetic nanoparticles

Authors: Ciobotaru, CC; Ciobotaru, IC; Schinteie, G; Negrea, R; Polosan, S

Published: DEC 2017, MATERIALS SCIENCE IN SEMICONDUCTOR PROCESSING, 72, 84, DOI: 10.1016/j.mssp.2017.09.019

Magnetic nanoparticles embedded in the active layer of the Organic Light Emitting Diodes (OLEDs) significantly increases the electroluminescence and the charge transport without influencing the transparency of these devices. A brief comparison was done in order to identify which parameter influences these properties, by comparing the CoFe2O4 magnetic nanoparticles with CoFe2 metallic magnetic nanoparticles, the latter one being obtained by thermal reduction in hydrogen of cobalt ferrite nanoparticles. CoFe2 have shown a better efficiency of the metallic nanoparticles where probably the main advantage is the higher magnetization property instead of the coercive field. Concerning the charge transport across the OLEDs, these nanoparticles reduce the electron injection, acting as filling traps, which directly increases the electroluminescence and the current at the same voltage.

36. Tungsten nanoparticles with controlled shape and crystallinity obtained by magnetron sputtering and gas aggregation

Authors: Acsente, T; Negrea, RF; Nistor, LC; Matei, E; Grisolia, C; Birjega, R; Dinescu, G

Published: AUG 1 2017, MATERIALS LETTERS, 200, 124, DOI: 10.1016/j.matlet.2017.04.105

We report the synthesis of tungsten nanoparticles with two distinct shapes, concave hexapods and faceted cube-octahedral nanocrystals. These types of nanoparticles were obtained with a cluster source based on magnetron sputtering and gas aggregation, by supplying the magnetron discharge with continuous or pulsed wave radiofrequency power. Detailed morphological and microstructural studies were performed. The hexapod particles present a dendritic growth while the faceted ones have a single crystal structure. The alpha-W and beta-w crystalline phases are present in both types of nanoparticles; nevertheless, the beta-W structure is dominant for the hexapod ones. Although the beta-w phase is usually metastable, we observed its unexpected long term preservation in hexapod nanoparticles. The results, added to previously reported nanoflower-like particles, point out the ability to control the shape and structure of the tungsten nanoparticles using the magnetron sputtering gas aggregation technique. (C) 2017 Elsevier B.V. All rights reserved.

37. Tunable dielectric properties in polyacrylonitrile/multiwall carbon nanotube composites

Authors: Palade, S; Pantazi, A; Vulpe, S; Berbecaru, C; Tucureanu, V; Oprea, O; Negrea, RF; Dragoman, D

Published: AUG 2017, POLYMER COMPOSITES, 38, 1748, DOI: 10.1002/pc.23744

Composites containing polyacrylonitrile (PAN) and different mass contents of multiwall carbon nanotubes (MWCNTs) were prepared and structurally investigated. X-ray Diffraction of pristine PAN reveals the presence of crystalline and amorphous phases which change their ratio under thermal annealing and addition of MWCNT. For as prepared samples, thermal analysis reveals two glass transition temperatures, which support the hypothesis that unoriented PAN is a two-phase material. Infrared spectrum of as prepared PAN suggests that the polymer is not stabilized. Dielectric investigations of PAN/MWCNT composites show that permittivity has a strong increase as the MWCNT mass content increases, while the dielectric losses are comparable in all samples. These results suggest that PAN/MWCNT composites could find important applications in electronics. POLYM. COMPOS., 38:1741-1748, 2017. (c) 2015 Society of Plastics Engineers

38. Facile, high yield ultrasound mediated protocol for ZnO hierarchical structures synthesis: Formation mechanism, optical and photocatalytic properties

Authors: Carp, O; Tirsoaga, A; Ene, R; Ianculescu, A; Negrea, RF; Chesler, P; Ionita, G; Birjega, R

Published: MAY 2017, ULTRASONICS SONOCHEMISTRY, 36, 335, DOI: 10.1016/j.ultsonch.2016.12.005

Hierarchical flowers-like zinc oxide structures have been successfully obtained by a simple and fast ultrasound-assisted method performed in a ordinary ultrasonic bath using an ammonia solution and zinc acetate, in the absence of any surfactant or template. The composition, structure, crystallinity, morphology and optical properties of the materials obtained at different ultrasound irradiation times were characterized by infrared, UV-Vis and photoluminescence spectroscopy, X-ray diffraction, scanning and transmission electron microscopy investigations. It was proved that the ultrasound irradiation time manipulates both the defect content (implicit the photoluminescent properties) and morphology of the ZnO materials: shorter irradiation times leads to the synthesis of high-defected ZnO structures of flower morphology with triangular-shaped petals, while higher irradiation times favours the formation of low-defected ZnO structures with tipped rod-like petals. A plausible growth mechanism of the architectures that implies aggregation via oriented attachment followed by an Ostwald ripening is advanced based on these results. The ZnO flower-like structures present high photocatalytic activities, a total phenol mineralization being registered in the case of visible light experiments. Electron-spin resonance measurements demonstrate the generation of reactive oxygen species, namely hydroxyl radicals but also C centred radicals adducts derived most probable from the residual acetate adsorbed on ZnO surface. (C) 2016 Elsevier B.V. All rights reserved.

39. Steplike Switching in Symmetric PbZr0.2Ti0.8O3/CoFeO4/PbZr0.2Ti0.8O3 Heterostructures for Multistate Ferroelectric Memory

Authors: Boni, AG; Chirila, C; Pasuk, I; Negrea, R; Pintilie, I; Pintilie, L

Published: SEP 28 2017, PHYSICAL REVIEW APPLIED, 8, DOI: 10.1103/PhysRevApplied.8.034035

A hysteresis loop with three polarization states is obtained in the case of a symmetric epitaxial ferroelectric-interlayer-ferroelectric structure with bottom and top SrRuO3 electrodes. The ferroelectric layers are of PbZr0.2Ti0.8O3, while the interlayer is CoFe2O4. It is shown that the three polarization states can be separately accessed, suggesting that this type of structure can be used as building element for a three-state nonvolatile ferroelectric random-access memory (FERAM). The presence of the three-state memory effect is explained through a simple phenomenological model based on Landau-Ginzburg-Devonshire theory. The findings of this study can pave the way to multistate all-oxide FERAM devices, resulting in a 50% increase in the storage density compared to actual nonvolatile memories.

40. Joining Chemical Pressure and Epitaxial Strain to Yield Y-doped BiFeO3 Thin Films with High Dielectric Response

Authors: Scarisoreanu, ND; Craciun, F; Birjega, R; Ion, V; Teodorescu, VS; Ghica, C; Negrea, R; Dinescu, M

Published: MAY 9 2016, SCIENTIFIC REPORTS, 6, DOI: 10.1038/srep25535

BiFeO3 is one of the most promising multiferroic materials but undergoes two major drawbacks: low dielectric susceptibility and high dielectric loss. Here we report high in-plane dielectric permittivity (epsilon' similar to 2500) and low dielectric loss (tan delta < 0.01) obtained on Bi0.95Y0.05FeO3 films epitaxially grown on SrTiO3 (001) by pulsed laser deposition. High resolution transmission electron microscopy and geometric phase analysis evidenced nanostripe domains with alternating compressive/tensile strain and slight lattice rotations. Nanoscale mixed phase/domain ensembles are commonly found in different complex materials with giant dielectric/electromechanical (ferroelectric/relaxors) or magnetoresistance (manganites) response. Our work brings insight into the joined role of chemical pressure and epitaxial strain on the appearance of nanoscale stripe structure which creates conditions for easy reorientation and high dielectric response, and could be of more general relevance for the field of materials science where engineered materials with huge response to external stimuli are a highly priced target.

41. Dielectric characterization of BaxSr1-xFe12O19 (x=0.05-0.35) ceramics

Authors: Cernea, M; Negrea, RF; Ciuchi, IV; Baldisserri, C; Trusca, R; Galassi, C

Published: JAN 2016, CERAMICS INTERNATIONAL, 42, 1056, DOI: 10.1016/j.ceramint.2015.09.029

BaxSr1-xFe12O19 (x=0.05, 0.15, 0.25, 0.35) ferrites were prepared by both the sol gel and conventional solid state reaction techniques. The structural characteristics and dielectric properties of these ferrites were investigated and compared. BaxSr1-xFe12O19 ceramics obtained by the sol gel method show homogeneous morphology and single crystalline phase while those prepared by solid state reaction route exhibit less homogeneous morphology, being accompanied by a secondary crystalline phase (BaFe2O4). The size of the grains varies in the range 200-300 nm for x=0.05-0.35 when the powders are prepared by sol gel method and, 600-700 nm for the powders prepared by solid state reaction. For the same value of x, the BaxSr1-xFe12O19 ferrites obtained via sol-gel display higher permittivity and lower dielectric losses compared to those prepared by solid state reaction route. (C) 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

42. High atomic diffusivity during pulsed laser irradiation of TiON quasi-amorphous films

Authors: Teodorescu, VS; Maraloiu, AV; Negrea, RF; Ghica, D; Scarisoreanu, ND; Dinescu, M; Gartner, M; Blanchin, MG

Published: JUN 30 2016, APPLIED SURFACE SCIENCE, 374, 251, DOI: 10.1016/j.apsusc.2015.11.188

Quasi-amorphous titanium oxynitride (TiON) films were obtained by annealing sol-gel anatase TiO2 films in NH3 atmosphere at 600 degrees C. These films were irradiated with 50 laser pulses using the fourth harmonic (266 nm) radiation of the Nd-YAG laser, with an average fluence of 20 mJ/cm(2). HRTEM observations of the pulsed laser irradiated films evidenced the rutile TiO2 nanocrystallites formation. The rutile structure was not present either in the TiON films before the laser irradiation, or in the initial sol-gel anatase TiO2 films. During the laser irradiation, the film structure remains in the solid state phase, as it results from the temperature estimation and microscopic observations. For the rutile nanocrystals formation, the atomic diffusion length of the oxygen and titanium atoms should be in the nanometric range during the laser pulse action, which implies a diffusivity close to the values observed in the liquid phase. We consider that the rutile phase formation is a proof of the fast atomic diffusion in the solid amorphous matrix, during the laser irradiation. (C) 2015 Published by Elsevier B.V.

43. Atomic scale elemental mapping of light elements in multilayered perovskite coatings

Authors: Negrea, RF; Teodorescu, VS; Ghica, C

Published: NOV 15 2015, APPLIED SURFACE SCIENCE, 355, 255, DOI: 10.1016/j.apsusc.2015.07.106

Spherical aberration corrected transmission electron microscopes offer unprecedented capabilities in materials structural characterization down to atomic resolution. Electron energy loss spectroscopy (EELS) - spectrum imaging (SI) and annular bright field (ABF) imaging allow to simultaneously identify both the position and nature of the atomic species in a crystalline material. These techniques, along with conventional high-resolution transmission electron microscopy are particularly useful in heterostructures interfaces like epitaxial multilayers characterization, for identifying possible atomic interdiffusion at sub-nanometric scale. This paper presents the structural and compositional microanalysis down to atomic resolution of an epitaxial BaTiO3/SrRuO3/SrTiO3 ferroelectric heterostructure using complex complementary analytical electron microscopy techniques. The atomic arrangement of both heavy and light atomic species across the interfaces in the BaTiO3/SrRuO3/SrTiO3 heterostructures is revealed. (C) 2015 Elsevier B.V. All rights reserved.

44. Electrical properties of single CuO nanowires for device fabrication: Diodes and field effect transistors

Authors: Florica, C; Costas, A; Boni, AG; Negrea, R; Ion, L; Preda, N; Pintilie, L; Enculescu, I

Published: JUN 1 2015, APPLIED PHYSICS LETTERS, 106, DOI: 10.1063/1.4921914

High aspect ratio CuO nanowires are synthesized by a simple and scalable method, thermal oxidation in air. The structural, morphological, optical, and electrical properties of the semiconducting nanowires were studied. Au-Ti/CuO nanowire and Pt/CuO nanowire electrical contacts were investigated. A dominant Schottky mechanism was evidenced in the Au-Ti/CuO nanowire junction and an ohmic behavior was observed for the Pt/CuO nanowire junction. The Pt/CuO nanowire/Pt structure allows the measurements of the intrinsic transport properties of the single CuO nanowires. It was found that an activation mechanism describes the behavior at higher temperatures, while a nearest neighbor hopping transport mechanism is characteristic at low temperatures. This was also confirmed by four-probe resistivity measurements on the single CuO nanowires. By changing the metal/semiconductor interface, devices such as Schottky diodes and field effect transistors based on single CuO p-type nanowire semiconductor channel are obtained. These devices are suitable for being used in various electronic circuits where their size related properties can be exploited. (c) 2015 AIP Publishing LLC.

45. Comparison between the ferroelectric/electric properties of the PbZr0.52Ti0.48O3 films grown on Si (100) and on STO (100) substrates

Authors: Chirila, C; Boni, AG; Pasuk, I; Negrea, R; Trupina, L; Le Rhun, G; Yin, S; Vilquin, B; Pintilie, I; Pintilie, L

Published: JUN 2015, JOURNAL OF MATERIALS SCIENCE, 50, 3894, DOI: 10.1007/s10853-015-8907-2

Ferroelectric/electric properties of PbZr0.52Ti0.48O3 (PZT) thin films grown by pulsed laser deposition (PLD) on two different substrates, Si (001) and SrTiO3 (STO) (001), were comparatively analyzed. The structural characterization has revealed the epitaxial relationship between the grown layers and the two types of substrates, with larger density of structural defects for the films deposited on Si (001) with buffer STO layer. The ferroelectric/electric properties are also different, with lower remnant polarization (about half of the value obtained on STO substrate), higher dielectric constant (about two times larger), and lower leakage current (about two orders of magnitude lower) for the PZT films deposited on Si (001) compared to those deposited on (001) STO substrates. Nevertheless, the results show that the use of a STO buffer layer on Si can be a solution to obtain good quality PZT capacitor structures without using expensive single-crystal oxide substrates. In this way, applications based on PZT capacitors (e.g. non-volatile memories, pyroelectric detectors, light switches, etc.) would be more easily integrated directly on Si wafers.

46. Ferromagnetic behaviour of bismuth germanate oxides glass-ceramic materials

Authors: Polosan, S; Negrea, R; Ciobotaru, IC; Schinteie, G; Kuncser, V

Published: FEB 25 2015, JOURNAL OF ALLOYS AND COMPOUNDS, 623, 196, DOI: 10.1016/j.jallcom.2014.10.104

The content of defects in Bi4Ge3O12 (BGO) glass-ceramic materials together with their ordering during crystallization induces ferromagnetic behaviours in these materials. The observed ferromagnetism has to be associated strictly with the GeO4 related defects in the atypical amorphous phase. The photoluminescence of BGO glass-ceramic materials can give useful information about the nature of defects, especially the oxygen vacancies from the GeO4 tetrahedra and Bi4Ge3O12 cubic structures. The concentration of these defects depends on annealing between 600 degrees C and 700 degrees C where orthorhombic and cubic phases coexist. The magnetization at saturation is higher in the samples dominated by the amorphous phase and the strength of the exchange interaction is higher in the samples dominated by the cubic phase, obtained over 700 degrees C, when the GeO4 tetrahedra are rearranged during crystallization together with their oxygen vacancies. (C) 2014 Elsevier B.V. All rights reserved.

47. High Permittivity (1-x)Ba(Zr0.2Ti0.8)O-3 - x(Ba0.7Ca0.3)TiO3 (x=0.45) Epitaxial Thin Films with Nanoscale Phase Fluctuations

Authors: Scarisoreanu, ND; Craciun, F; Moldovan, A; Ion, V; Birjega, R; Ghica, C; Negrea, RF; Dinescu, M

Published: NOV 4 2015, ACS APPLIED MATERIALS & INTERFACES, 7, 23992, DOI: 10.1021/acsami.5b06745

Epitaxial (1 - x)Ba(Ti0.8Zr0.2)TiO3 - x(Ba0.7Ca0.3)TiO3, x = 0.45 (BCZT 45), thin films have been deposited on (001) SrTiO3 (STO) and (001/100) SrLaAlO4 (SLAO) substrates by pulsed laser deposition. X-ray diffraction and high-resolution transmission electron microscopy (HRTEM) confirmed the epitaxial growth of the films. A high structural quality has been evidenced for the BCZT/STO films. Geometric phase analysis (GPA) associated with the HRTEM enabled us to obtain microstrain analysis and the in-plane and out-of-plane lattice parameter variation on different areas. Tetragonality ratio fluctuations at nanoscale level which are relevant for the existence of nanodomains have been evidenced on the BCZT/STO films. The in-plane dielectric constant has been measured on interdigital electrodes deposited by lift-off technique on the top of the films. High values of dielectric permittivity (>3000) combined with low dielectric loss (<0.01) are obtained for BCZT 45 film deposited on STO substrate, showing nearly constant values between 1 kHz and 10 MHz. The high dielectric permittivity of BCZT thin films was attributed to their high structural quality and to the loss of rotation stability of the polarization associated with the presence of nanodomains. This results into a divergence of fluctuations of polarization direction and a peak of dielectric susceptibility. The enhanced switching of such nanodomain configuration was probed by piezoforce microscopy, by writing and reading domains during topography scanning.

48. Band bending at copper and gold interfaces with ferroelectric Pb(Zr,Ti)O-3 investigated by photoelectron spectroscopy

Authors: Apostol, NG; Stoflea, LE; Tanase, LC; Bucur, IC; Chirila, C; Negrea, RF; Teodorescu, CM

Published: NOV 1 2015, APPLIED SURFACE SCIENCE, 354, 468, DOI: 10.1016/j.apsusc.2015.02.167

Interfaces formed by gold and copper on single crystal layers of (0 0 1) PbZr0.2Ti0.8O3 (PZT) produced by pulsed laser deposition and exhibiting outwards polarization are analyzed by X-ray photoelectron spectroscopy. The stoichiometry of the layers reproduces reasonably that of the PZT target. The band bending occurring at the interface between PZT and the metals is investigated by analyzing the core level shifts as function on the metal deposition. It is found that for Au/PZT(0 0 1) the gold layer is not continuous and the observed band bendings can be attributed to a Schottky mechanism, whereas for Cu/PZT(0 0 1) the copper layer is continuous; in this latter case, the observed band bendings towards higher energies (lower binding energies) can be attributed to a concomitant bending due to the Schottky effect together with the disappearance of the initial bending due to the outwards polarization of the samples. Metal Pb is observed to segregate only in the case of Cu/PZT(0 0 1), therefore the surface self-reduction might also be connected to the presence of a metal with lower work function, which for larger coverage forms a continuous metal layer, able to provide electrons to the surface. High resolution transmission electron spectroscopy yielded the disappearance of the tetragonal distortion in the case of Cu/PZT(0 0 1), in line with the assumption of disappearance of the polarization-induced band bending. (C) 2015 Elsevier B.V. All rights reserved.

49. Polarization induced self-doping in epitaxial Pb(Zr0.20Ti0.80)O-3 thin films

Authors: Pintilie, L; Ghica, C; Teodorescu, CM; Pintilie, I; Chirila, C; Pasuk, I; Trupina, L; Hrib, L; Boni, AG; Apostol, NG; Abramiuc, LE; Negrea, R; Stefan, M; Ghica, D

Published: OCT 8 2015, SCIENTIFIC REPORTS, 5, DOI: 10.1038/srep14974

The compensation of the depolarization field in ferroelectric layers requires the presence of a suitable amount of charges able to follow any variation of the ferroelectric polarization. These can be free carriers or charged defects located in the ferroelectric material or free carriers coming from the electrodes. Here we show that a self-doping phenomenon occurs in epitaxial, tetragonal ferroelectric films of Pb(Zr0.2Ti0.8)O-3, consisting in generation of point defects (vacancies) acting as donors/acceptors. These are introducing free carriers that partly compensate the depolarization field occurring in the film. It is found that the concentration of the free carriers introduced by selfdoping increases with decreasing the thickness of the ferroelectric layer, reaching values of the order of 10(26) m(-3) for 10 nm thick films. One the other hand, microscopic investigations show that, for thicknesses higher than 50 nm, the 2O/(Ti+Zr+Pb) atomic ratio increases with the thickness of the layers. These results suggest that the ratio between the oxygen and cation vacancies varies with the thickness of the layer in such a way that the net free carrier density is sufficient to efficiently compensate the depolarization field and to preserve the outward direction of the polarization.

50. Synthesis of flower-like tungsten nanoparticles by magnetron sputtering combined with gas aggregation

Authors: Acsente, T; Negrea, RF; Nistor, LC; Logofatu, C; Matei, E; Birjega, R; Grisolia, C; Dinescu, G

Published: JUN 23 2015, EUROPEAN PHYSICAL JOURNAL D, 69, DOI: 10.1140/epjd/e2015-60097-4

We present the synthesis of tungsten nanoparticles using a gas aggregation cluster source based on a magnetron sputtering discharge. The nanoparticles have sizes between 70 and 100 nm and a dendritic morphology, with branches emerging from the center and evolving in a flower-like pattern. Structural investigations revealed the presence of alpha-W and residual beta-W crystalline phases. Post deposition oxidation of the nanoparticles is also investigated.

51. Additive-free 1,4-butanediol mediated synthesis: a suitable route to obtain nanostructured, mesoporous spherical zinc oxide materials with multifunctional properties

Authors: Visinescu, D; Scurtu, M; Negrea, R; Birjega, R; Culita, DC; Chifiriuc, MC; Draghici, C; Moreno, JC; Musuc, AM; Balint, I; Carp, O

Published: 2015, RSC ADVANCES, 5, 99989, DOI: 10.1039/c5ra20224h

A family of mesoporous, self-aggregated zinc oxide materials with spherical morphologies of high crystalline quality, is obtained through a facile, additive-free polyol procedure. The forced hydrolysis of zinc acetylacetonate in 1,4-butanediol (BD), in various reaction conditions, affords ZnO materials with versatile morphologies and optical properties. The reaction parameters (temperature, time and zinc source concentration) modulate the ZnO nanocrystallites size (from 8.1 to 13.2 nm), the spheres diameter (ranging from 50 up to 250 nm), the internal structure of the spherical aggregates (hollow or solid) and their specific surface area (from 31 to 92 m(2) g(-1)). Polycrystalline spheres with hollow cores are obtained at the lowest temperature (90 degrees C) and zinc cation concentration (0.1 M), while at higher reaction temperatures (140-180 degrees C), solid spherical aggregates are developed. A reaction mechanism for ZnO formation via zinc layered hydroxide (LDH-Zn) is proposed based on nuclear magnetic resonance (H-1 NMR and C-13 NMR) and powder X-ray diffraction (XRD) studies. The obtained ZnO materials have a functionalized surface, derived from the polyol and act as a nitrogen selective photocatalyst in the reduction reaction of NO3-. The organic residual attached on the ZnO surface plays a crucial role in the denitrification reaction, since the photo-cleaned sample showed negligible photocatalytic activity. The ZnO materials also exhibited microbicidal and anti-biofilm activity against reference and clinical strains, highlighting their potential for the development of novel antimicrobial formulations.

52. Electrode interface controlled electrical properties in epitaxial Pb(Zr0.52Ti0.48)O-3 films grown on Si substrates with SrTiO3 buffer layer

Authors: Boni, AG; Chirila, C; Pasuk, I; Negrea, R; Trupina, L; Le Rhun, G; Vilquin, B; Pintilie, I; Pintilie, L

Published: OCT 30 2015, THIN SOLID FILMS, 593, 130, DOI: 10.1016/j.tsf.2015.09.028

Electrical properties of ferroelectric capacitors based on PbZr0.52Ti0.48O3 thin films grown by pulsed laser deposition on silicon substrate with SrTiO3 buffer layer grown by molecular beam epitaxy were studied. A SrRuO3 layer was deposited as bottom electrode also by pulse laser deposition and Pt, Ir, Ru, SrRuO3 were used as top contacts. Electrical characterization comprised hysteresis and capacitance-voltage measurements in the temperature range from 150 K to 400 K. It was found that the macroscopic electrical properties are affected by the electrode interface, by the choice of the top electrode. However, even for metals with very different work functions (e.g. Pt and SrRuO3) the properties of the top and bottom electrode interfaces remain fairly symmetric suggesting a strong influence from the bound polarization charges located near the interface. (C) 2015 Elsevier B.V. All rights reserved.

53. Polarization-Control of the Potential Barrier at the Electrode Interfaces in Epitaxial Ferroelectric Thin Films

Authors: Pintilie, I; Teodorescu, CM; Ghica, C; Chirila, C; Boni, AG; Hrib, L; Pasuk, I; Negrea, R; Apostol, N; Pintilie, L

Published: FEB 26 2014, ACS APPLIED MATERIALS & INTERFACES, 6, 2939, DOI: 10.1021/am405508k

Electrode interface is a key element in controlling the macroscopic electrical properties of the ferroelectric capacitors based on thin films. In the case of epitaxial ferroelectrics, the electrode interface is essential in controlling the leakage current and the polarization switching, two important elements in the read/write processes of nonvolatile memories. However, the relation between the polarization bound charges and the electronic properties of the electrode interfaces is not yet well understood. Here we show that polarization charges are controlling the height of the potential barriers at the electrode interfaces in the case of Pb(Zr,Ti)O-3 and BaTiO3 epitaxial films. The results suggest that the height is set to a value allowing rapid compensation of the depolarization field during the polarization switching, being almost independent of the metals used for electrodes. This general behavior open a new perspective in engineering interface properties and designing new devices based on epitaxial ferroelectrics.

54. Significant enhancement of the critical current density for cubic BN addition into ex situ spark plasma sintered MgB2

Authors: Badica, P; Aldica, G; Burdusel, M; Popa, S; Negrea, RF; Enculescu, M; Pasuk, I; Miu, L

Published: SEP 2014, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, 27, DOI: 10.1088/0953-2048/27/9/095013

High density (above 93%) superconducting bulks of MgB2 with addition of hexagonal BN (h-BN) and cubic BN (c-BN) with compositions ((MgB2) + (BN)(x), x = 0.01, 0.03, 0.05) were obtained by ex situ spark plasma sintering. All the investigated samples have the critical temperature T-c = 38.8 K. The variation of the critical current density J(c) with the external magnetic field H for h-BN added sample is almost overlapping the J(c)(H) dependence for the pristine MgB2 sample. On the other hand, J(c) for the samples added with c-BN is larger at high magnetic fields, while the decrease of J(c) at low H is very small. At T = 20 K, a J(c) of 10(2) A cm(-2) is determined for the sample with x(c-BN) = 0.005 at H = 58 kOe, and for the sample with x(c-BN) = 0.01 at 54 kOe. Magnetic relaxation measurements indicate a significant flux pinning enhancement in MgB2 samples added with c-BN. It is proposed that the disorder at the interface caused by the convenient lattice matching relationship between the lateral plane of the MgB2 crystal prism and the face of the c-BN crystal cube is responsible for the observed vortex pinning increase.

55. Strain-induced long range ferroelectric order and linear electro-optic effect in epitaxial relaxor thin films

Authors: Scarisoreanu, ND; Craciun, F; Birjega, R; Andrei, A; Ion, V; Negrea, RF; Ghica, C; Dinescu, M

Published: AUG 21 2014, JOURNAL OF APPLIED PHYSICS, 116, DOI: 10.1063/1.4893364

Relaxor ferroelectrics have neither long range ferroelectric order nor structural transformation down to the lowest temperatures, and display isotropic optical properties like quadratic electro-optic effect. However, if an anisotropy is forced through an external agent, like electric field or uniaxial strain, a ferroelectric and structural long range order can be induced in these materials. Here, we show that epitaxial strain in relaxor ferroelectric thin films can be employed to induce a linear electro-optic effect, opening the path to new strain-controlled electro-optic materials. Epitaxial thin films with Pb1-3x/2LaxZr0.2Ti0.8O3 (x = 0.22) composition grown by pulsed laser deposition on (001) SrRuO3/SrTiO3 single crystal heterostructures become tetragonal below the susceptibility peak, which occurs at a temperature 140K higher as compared to bulk. These films show piezoelectric properties and almost linear electro-optic behaviour. (C) 2014 AIP Publishing LLC.

56. Nanoscale monoclinic domains in epitaxial SrRuO3 thin films deposited by pulsed laser deposition

Authors: Ghica, C; Negrea, RF; Nistor, LC; Chirila, CF; Pintilie, L

Published: JUL 14 2014, JOURNAL OF APPLIED PHYSICS, 116, DOI: 10.1063/1.4889932

In this paper, we analyze the structural distortions observed by transmission electron microscopy in thin epitaxial SrRuO3 layers used as bottom electrodes in multiferroic coatings onto SrTiO3 substrates for future multiferroic devices. Regardless of the nature and architecture of the multilayer oxides deposited on the top of the SrRuO3 thin films, selected area electron diffraction patterns systematically revealed the presence of faint diffraction spots appearing in forbidden positions for the SrRuO3 orthorhombic structure. High-resolution transmission electron microscopy (HRTEM) combined with Geometric Phase Analysis (GPA) evidenced the origin of these forbidden diffraction spots in the presence of structurally disordered nanometric domains in the SrRuO3 bottom layers, resulting from a strain-driven phase transformation. The local high compressive strain (-4% divided by -5%) measured by GPA in the HRTEM images induces a local orthorhombic to monoclinic phase transition by a cooperative rotation of the RuO6 octahedra. A further confirmation of the origin of the forbidden diffraction spots comes from the simulated diffraction patterns obtained from a monoclinic disordered SrRuO3 structure. (C) 2014 AIP Publishing LLC.

57. Schottky barrier versus surface ferroelectric depolarization at Cu/Pb(Zr, Ti)O-3 interfaces

Authors: Stoflea, LE; Apostol, NG; Chirila, C; Trupina, L; Negrea, R; Pintilie, L; Teodorescu, CM

Published: MAY 2014, JOURNAL OF MATERIALS SCIENCE, 49, 3351, DOI: 10.1007/s10853-014-8041-6

The band bending at Cu/PZT(001) interfaces is investigated by X-ray photoelectron spectroscopy (XPS) for a PZT(001) layer which exhibits initial outwards ferroelectric polarization. Two competitive processes are identified: (a) formation of the Schottky barrier between the ferroelectric and unconnected Cu islands, and (b) coalescence of the Cu islands, realisation of an electrical contact to the ground of the system, inducing the apparent loss of the component of the ferroelectric polarization perpendicular to the sample surface, at least as it manifests in band bending. Three mechanisms are proposed to explain this loss of band bending when a full metal layer connected to ground is formed on the surface: (i) over-compensation of depolarization field in the sub-surface region, (ii) formation of domains with in-plane orientation of the polarization vector and (iii) loss of polarization in the near-surface layers of the ferroelectric due to electrons provided by the metal. These result in a non-monotonous variation of binding energies with the amount of Cu deposited. High resolution transmission electron microscopy and piezoresponse force microscopy confirmed these hypotheses. The XPS data allowed also to derive the surface PZT composition, its evolution with the deposition of copper and the formation of surface compounds.

58. High magnetic field enhancement of the critical current density by Ge, GeO2 and Ge2C6H10O7 additions to MgB2

Authors: Batalu, D; Aldica, G; Popa, S; Miu, L; Enculescu, M; Negrea, RF; Pasuk, I; Badica, P

Published: JUL 1 2014, SCRIPTA MATERIALIA, 82, 64, DOI: 10.1016/j.scriptamat.2014.03.024

Ge, GeO2 and Ge2C6H10O7 additions to MgB2 obtained by ex situ spark plasma sintering significantly enhance the critical current density J(c) in high magnetic fields. A J(c)(T = 20 K) of 102 A cm(-2) is obtained at 3.9 T in the pristine sample and at 5.8 T in the MgB2(Ge2C6H10O7)(0.0014) sample. The decrease in the critical temperature for added samples is less than 1 K and T-c(20 K, H = 0) shows a small decrease from 5.5 x 10(5) A cm(-2) in the pristine sample to 3.9 x 10(5) A cm(-2) in MgB2(Ge2C6H10O7)(0.0014) sample. Ge does not substitute into the MgB2 lattice. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

59. Synthesis and exceptional thermal stability of Mg-based bimetallic nanoparticles during hydrogenation

Authors: Krishnan, G; Negrea, RF; Ghica, C; ten Brink, GH; Kooi, BJ; Palasantzas, G

Published: 2014, NANOSCALE, 6, 11970, DOI: 10.1039/c4nr03885a

Here we report the extraordinary thermal stability of Mg rich bimetallic nanoparticles (NPs), which is important for hydrogen storage technology. The enhanced NP stability is accomplished because of two critical improvements: (i) no void development within NPs (nanoscale Kirkendall effect) during their formation and (ii) suppressed Mg evaporation and NP hollowing during Mg hydrogenation at elevated temperature. The mechanism leading to the improved thermal stability of Mg-based bimetallic NPs is shown to be due to MgH2 hydride formation before evaporation can take place. These findings were tested for various compositions of Mg with Ni, Cu, and Ti, which are interesting combinations of materials for hydrogen storage systems. To achieve this we first demonstrate the synthesis mechanism of Mg-Ni and Mg-Cu NPs, which is well controlled at the single particle level, in order to accomplish multi-shell, alloy and intermetallic structures of interest for hydrogen storage tests. Aberration corrected transmission electron microscopy was carried out to unravel the detailed atomic structure and composition of the bimetallic NPs after production, processing, and hydrogenation. Finally, a simple and effective methodology is proposed for tuning the composition of the Mg-based bimetallic NPs based on the temperature-dependent nucleation behavior of NPs in the gas-phase.

60. MICROSTRUCTURE OF THE a-C: H/W LAYERS DEPOSITED BY PLASMA ASSISTED SEQUENTIAL DEPOSITION METHOD

Authors: Acsente, T; Negrea, RF; Lazea-Stoyanova, A; Nistor, LC; Dinescu, G

Published: APR-JUN 2014, DIGEST JOURNAL OF NANOMATERIALS AND BIOSTRUCTURES, 9, 450, DOI:

A study regarding the microstructure of a-C: H/W mixed materials deposited by plasma assisted sequential deposition method is presented. The deposition of layers was performed by cyclic exposing a substrate, for predefined time intervals, to two deposition processes, working in alternative sequences: Magnetron Sputtering (MS) for metal deposition and Plasma Enhanced Chemical Vapor Deposition (PECVD) for carbon deposition. During the deposition the plasma parameters were kept constant while the durations of the individual MS and PECVD steps were systematically modified. The deposited samples were investigated using both transmission and scanning electron microscopy methods. Nanocomposite structures are obtained if each metal deposition step is interrupted in the island stage of the layer formation followed by coverage with a continuous layer of carbon. Multilayer structures are obtained when the full development of a continuous metallic layer is allowed. The spatial periodicity of the multilayers can be adjusted in between tens and hundreds of nanometers by proper setting the duration of substrate exposure to plasma sources.

61. Influence of co-dopants average valence on microstructural and electromechanical properties of lead titanate ceramics

Authors: Amarande, L; Miclea, C; Cioangher, M; Grecu, MN; Pasuk, I; Negrea, RF

Published: MAY 2014, JOURNAL OF THE EUROPEAN CERAMIC SOCIETY, 34, 1200, DOI: 10.1016/j.jeurceramsoc.2013.12.002

Effects of Ni, Nb and Mn co-doping on microstructural, dielectric and electromechanical properties of lead titanate (PT) dense ceramics, obtained as a result of decreasing lattice anisotropy with increasing amount of co-dopants, were investigated. Different values of the average valence (AV) of Ni-Nb co-dopants were obtained by mutual valence compensation between these acceptor donor ions of various relative concentrations. Significantly higher amounts of Mn2+ (resulted by reduction of Mn4+ in presence of donor co-dopants) were found in materials with Ni-Nb AV >4+. These materials show higher coupling factors, correlated with higher mechanical quality factors, compared to the rest of materials. Hard-like piezoelectric behavior was manifested in materials with Ni-Nb AV <4+, whereas ceramics with Ni-Nb AV >4+, and without manganese, turned out to have a soft-like behavior. Electromechanical properties of PT ceramics are discussed in terms of average valence of Ni-Nb co-dopants and multivalence state of manganese. (C) 2013 Elsevier Ltd. All rights reserved.

62. Charge transfer and band bending at Au/Pb(Zr0.2Ti0.8)O-3 interfaces investigated by photoelectron spectroscopy

Authors: Apostol, NG; Stoflea, LE; Lungu, GA; Chirila, C; Trupina, L; Negrea, RF; Ghica, C; Pintilie, L; Teodorescu, CM

Published: MAY 15 2013, APPLIED SURFACE SCIENCE, 273, DOI: 10.1016/j.apsusc.2013.02.056

The growth of gold layers on Pb(Zr,Ti)O-3 (PZT) deposited on SrTiO3 is investigated by X-ray photoelectron spectroscopy in the Au thickness range 2-100 angstrom. Two phases are identified, with compositions close to nominal PZT. The 'standard' phase is represented by all binding energies (Pb 4f, Ti 2p, Zr 3d, O 1s) sensibly equal to the nominal values for PZT, whereas the 'charged' phase exhibits all core levels are shifted by similar to 1 eV toward higher binding energies. By taking into account also scanning probe microscopy images together with recent photoemission results, the 'charged' phase belongs to P(+) regions of PZT, whereas the 'normal' phase corresponds to regions with no net ferroelectric polarization perpendicular to the surface. Au deposition proceeds in a band bending of Phi(PZT) - Phi(Au) similar to 0.4-0.5 eV for both phases, identified as similar shifts toward higher binding energies of all Pb, Ti, Zr, O core levels with Au deposition. The Au 4f core level exhibits also an unusually low binding energy component 1 eV below the 'nominal' Au 4f binding energy position (metal Au). This implies the existence of negatively charged gold, or electron transfer from PZT to Au, although the 'normal' PZT phase have a higher work function, as it is derived from the band bending. Most probably this charge transfer occurs toward Au nanoparticles, which have even higher ionization energies. High resolution transmission electron microscopy evidenced the formation of such isolated nanoparticles. (C) 2013 Elsevier B.V. All rights reserved.

63. Co environment and magnetic defects in anatase CoxTi1-O-x(2) nanopowders

Authors: Grecu, MN; Macovei, D; Ghica, D; Logofatu, C; Valsan, S; Apostol, NG; Lungu, GA; Negrea, RF; Piticescu, RR

Published: APR 22 2013, APPLIED PHYSICS LETTERS, 102, 161909, DOI: 10.1063/1.4802819

Cobalt environment and magnetic defects nature in hydrothermal synthesized anatase CoxTi1-xO2 nanopowders (0 <= x <= 0.1) are investigated by x-ray diffraction and a variety of spectroscopic techniques. One shows that cobalt is partially inserted in the anatase lattice, as Co2+ ions located on substitutional and interstitial sites. The fraction of the diluted Co is limited to 3 at.% for x >= 0.05, while the rest of the Co atoms gather into Co3O4 clusters. As found by electron paramagnetic resonance, the Co doping brings about hole-and electron-excess defects. (C) 2013 AIP Publishing LLC [http://dx.doi.org/10.1063/1.4802819]

64. Ultraviolet and visible up-conversion luminescence of Er3+/Yb3+ co-doped CaF2 nanocrystals in sol-gel derived glass-ceramics

Authors: Georgescu, S; Voiculescu, AM; Matei, C; Secu, CE; Negrea, RF; Secu, M

Published: NOV 2013, JOURNAL OF LUMINESCENCE, 143, 156, DOI: 10.1016/j.jlumin.2013.04.002

Glass-ceramics containing Er3+/Yb3+ codoped CaF2 nanocrystals have been prepared by using the controlled crystallization above 650 degrees C of the (Yb3+/Er3+) codoped CaF2-SiO2 xerogels. X-ray diffraction and scanning electron microscopy data have revealed the formation of CaF2 nanoctystals of about 16 nm size. For pumping at 973 nm, this glass-ceramics shows, besides green and red up-conversion luminescence, violet (at similar to 408 nm) and near-ultraviolet emission (at similar to 380 nm). The mechanisms responsible for the upconversion luminescence are presented and discussed. The fluorescence lifetimes of the metastable levels responsible for violet, green and red luminescence are measured and their quantum efficiencies are estimated. (C) 2013 Elsevier B.V. All rights reserved.

65. Eu3+ probe ion for rare-earth dopant site structure in sol-gel derived LiYF4 oxyfluoride glass-ceramic

Authors: Secu, CE; Negrea, RF; Secu, M

Published: OCT 2013, OPTICAL MATERIALS, 35, 2460, DOI: 10.1016/j.optmat.2013.06.053

Sol-gel route using metal alkoxides and trifluoroacetic acid as precursors has been used to prepare oxyfluoride glass-ceramic containing Eu3+-doped LiYF4 nanocrystals of about tens of nm size embedded in a silica matrix through controlled crystallization at higher temperatures of the xerogel. Photoluminescence spectra and decay curves recorded in the Eu3+-doped LiYF4 polycrystalline pellet and glass ceramic have been discussed using group-theoretical arguments. In the glass-ceramic Eu3+ ions are embedded dominantly inside the LiYF4 nanocrystals most probably as Eu-O center and/or dimer centers in low symmetry (C-2v) sites; oxygen ions were incorporated in their neighborhood during the glass ceramization. (C) 2013 Elsevier B.V. All rights reserved.

66. Structural, electric and magnetic properties of Pb(Zr0.2Ti0.8)O-3-CoFe2O4 heterostructures

Authors: Chirila, C; Ibanescu, G; Hrib, L; Negrea, R; Pasuk, I; Kuncser, V; Pintilie, I; Pintilie, L

Published: OCT 31 2013, THIN SOLID FILMS, 545, 7, DOI: 10.1016/j.tsf.2013.06.033

Electric and magnetic properties of symmetric and asymmetric Pb(Zr-0.2 Ti-0.8)O-3-CoFe2O4 (PZT/CFO) heterostructures, grown by pulsed laser deposition on SrTiO3 (100) substrates with a 25 nm SrRuO3 (SRO) buffer layer as bottom electrode, were investigated by using hysteresis and capacitance measurements. X-ray diffraction, and transmission electron microscopy investigations reveal the high quality crystalline structure and the epitaxial relationship between SRO, PZT and CFO. The electric polarization-voltage hysteresis reveals that the remnant polarization and the coercive field are significantly affected by the CFO layer. The frequency dependence of capacitance suggests a Maxwell-Wagner type relaxation at low frequencies and is also affected by the presence of the PZT/CFO interface(s). The magnetic hysteresis measurements infer the possible presence of another spinel phase (Co3O4) in the CFO film, due to the lattice mismatch at the PZT/CFO interfaces, and with direct influence on the magnetic response of the structure. According to the electric and magnetic characterization, better room temperature multiferroic properties would be expected for the symmetric heterostructure. (C) 2013 Published by Elsevier B. V.

67. Luminescent Eosin Y-SiO2 hybrid nano and microrods prepared by sol-gel template method

Authors: Secu, M; Secu, CE; Sima, M; Negrea, RF; Bartha, C; Dinescu, M; Damian, V

Published: NOV 2013, JOURNAL OF LUMINESCENCE, 143, 92, DOI: 10.1016/j.jlumin.2013.04.043

Sol-gel chemistry within the pores of a polycarbonate template membrane was used for the preparation of Eosin Y-SiO2 hybrid nano- and microrods, using tetraethylorthosilicate [TEOS, Si(OC2H5)(4)] as the precursor in the presence of trifluoroacetic acid (TFA) catalyst. The ethanolic solution of Eosin-Y was added to the silica sal to trap dye molecules inside the SiO2 gel network during the gelation. Structural and morphological characterization using scanning electron microscopy (SEM) and luminescence microscopy have shown the formation of rods with 200 nm and 1.2 mu m diameter and about 30 mu m length, exhibiting luminescence properties. Spectroscopic characterization has shown that the luminescence is due to Eosin-Y molecule in the xerogel porous network, surrounded by a solvation shell given mainly by the water. (C) 2013 Elsevier B.V. All rights reserved.

68. Functionalized magnetite silica thin films fabricated by MAPLE with antibiofilm properties

Authors: Mihaiescu, DE; Cristescu, R; Dorcioman, G; Popescu, CE; Nita, C; Socol, G; Mihailescu, IN; Grumezescu, AM; Gudovan, D; Enculescu, M; Negrea, RF; Ghica, C; Chifiriuc, C; Bleotu, C; Chrisey, DB

Published: MAR 2013, BIOFABRICATION, 5, DOI: 10.1088/1758-5082/5/1/015007

We report on the fabrication of magnetite/salicylic acid/silica shell/antibiotics (Fe3O4/SA/SiO2/ATB) thin films by matrix-assisted pulsed laser evaporation (MAPLE) to inert substrates. Fe3O4-based powder have been synthesized and investigated by XRD and TEM. All thin films were studied by FTIR, SEM and in vitro biological assays using Staphylococcus aureus and Pseudomonas aeruginosa reference strains, as well as eukaryotic HEp-2 cells. The influence of the obtained nanosystems on the microbial biofilm development as well as their biocompatibility has been assessed. For optimum deposition conditions, we obtained uniform adherent films with the composition identical with the raw materials. Fe3O4/SA/SiO2/ATB thin films had an inhibitory activity on the ability of microbial strains to initiate and develop mature biofilms, in a strain-and antibiotic-dependent manner. These magnetite silica thin films are promising candidates for the development of novel materials designed for the inhibition of medical biofilms formed by different pathogenic agents on common substrates, frequently implicated in the etiology of chronic and hard to treat infections.

69. The impact of the Pb(Zr,Ti)O-3-ZnO interface quality on the hysteretic properties of a metal-ferroelectric-semiconductor structure

Authors: Pintilie, I; Pasuk, I; Ibanescu, GA; Negrea, R; Chirila, C; Vasile, E; Pintilie, L

Published: NOV 15 2012, JOURNAL OF APPLIED PHYSICS, 112, DOI: 10.1063/1.4765723

The hysteretic properties of metal-ferroelectric-semiconductor (MFS) structures based on Pb(Zr0.2Ti0.8)O-3 (PZT) and ZnO films were studied with respect of the quality of the PZT-ZnO interface. The films were grown by pulsed laser deposition (PLD) on platinized silicon (Pt/Si) substrate and on single crystal, (001) oriented SrTiO3 (STO) substrates. The structural analysis has revealed that the PZT-ZnO stack grown on single crystal STO is epitaxial, while the structure grown on Pt/Si has columnar texture. The temperature change of the capacitance-voltage (C-V) hysteresis direction, from clockwise at low temperatures to counter clockwise at high temperatures, was observed at around 300K in the case of the MFS structure grown by PLD on Pt/Si substrate. This temperature is lower than the one reported for the case of the PZT-ZnO structure grown by sol-gel on Pt/Si substrate (Pintilie et al., Appl. Phys. Lett. 96, 012903 (2010)). In the fully epitaxial structures the C-V hysteresis is counter clockwise even at 100K. These findings strongly points out that the quality of the PZT-ZnO interface is essential for having a C-V hysteresis of ferroelectric nature, with negligible influence from the part of the interface states and with a memory window of about 5V at room temperature. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4765723]

70. The effect of the top electrode interface on the hysteretic behavior of epitaxial ferroelectric Pb(Zr,Ti)O-3 thin films with bottom SrRuO3 electrode

Authors: Pintilie, L; Pasuk, I; Negrea, R; Filip, LD; Pintilie, I

Published: SEP 15 2012, JOURNAL OF APPLIED PHYSICS, 112, DOI: 10.1063/1.4754318

The hysteretic behavior of the epitaxial Pb(Zr,Ti)O-3 thin films with different top metal electrodes is studied, with emphasis on the influence of the leakage current and trap generation current on the shape of the loop as well as on the magnitude of the measured polarization. Cu, Pt, and SrRuO3 were used as top contacts and important differences were observed for measurements performed in both dynamic and static modes, although the contacts were deposited on the same epitaxial Pb(Zr,Ti)O-3 film grown on SrRuO3/SrTiO3 substrate. A peculiar behavior was observed especially for the static hysteresis loops where, depending of the top contact, the loop is influenced mainly by the leakage current (Pt) or by the trap generation current (Cu and SrRuO3). The last one can contribute with an additive charge, having a linear dependence on the applied voltage, as suggested by the simple model developed to explain the abnormally high values of the dielectric constant extracted from the linear part of the static hysteresis loop. It is concluded that the properties of the top electrode interface can significantly impact the hysteretic behavior of the ferroelectric films. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4754318]

Dr. Raluca NEGREA

Scientific Researcher III