Raluca Florentina Negrea

• PhD
• Research Assistant at National Institute of Materials Physics

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...

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 a...

Physics of the multiferroic interfaces is currently understood mostly within a phenomenological framework including screening of the polarization field and depolarizing charges. Largely unexplored still remains the band dependence of the interfacial charge modulation, as well as the associated changes of the electron-phonon interaction, coupling th...

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 °C. The increase in the annealing tem...

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 func...

Zirconia and hafnia based thin films have attracted tremendous attention in the last decade due to their unexpected ferroelectric behavior at the nanoscale, which facilitates the downscaling of ferroelectric devices. The present work reports a novel ferroelectric rhombohedral phase of ZrO2 that can be achieved in thin films grown on (111)- Nb:SrTiO...

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...

In this work, the ferroelectric and fatigue characteristics of Au/0.5Ba(Zr0.2Ti0.8)O3-0.5(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 ferroelectr...

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/TiO2 ternary composites with high phot...

The present work reports for the first time, the employment of ferroelectric ZrO2 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 characteri...

We present a hard and soft x-ray photoelectron spectroscopy study of the interface chemistry in pristine TiN/La-doped Hf0.5Zr0.5O2/TiN capacitors. An oxynitride phase (∼1.3 nm) is formed at the top interface, while a TiO2−δ phase was detected near the bottom interface. The oxygen vacancy (VO) concentration is higher at the top interface than in the...

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 c...

The crystallization of ferroelectric (Hf,Zr)O2 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 electr...

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 elect...

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-select...

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 str...

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, eval...

The properties of ferroelectric materials, which were discovered almost a century ago¹, have led to a huge range of applications, such as digital information storage², pyroelectric energy conversion³ and neuromorphic computing4,5. Recently, it was shown that ferroelectrics can have negative capacitance6–11, which could improve the energy efficiency...

Here, we show that wet impregnation of ZrO2 nanoparticles with 10% and 20% Eu oxide followed by thermal anneal in air above 500 °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 hydrotherma...

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 we...

Abstract 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 thi...

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 (PL), X-ray photoelectron (XPS) and electronic p...

In this work, the ferroelectric characteristics of 0.5Ba(Zr0.2Ti0.8)O3-0.5(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...

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 us...

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)O3-0.5(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...

In this work, we investigate the photovoltaic response of Pt/0.5Ba(Zr0.2Ti0.8)O3-0.5(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 (Jsc) of the Pt/ZnO/0.5BZT-0.5BCT/ITO, Pt/0.5BZT-0.5BCT/ZnO/ITO and Pt/ZnO/0.5BZT-0.5BCT/ZnO/IT...

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 e...

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 Zn...

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 magnet...

Here we report a ferroelectric capacitor structure obtained by alternating ferroelectric and insulator thin-film layers which allows an increase of up to 2ⁿ 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 wor...

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, suggesti...

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, ther...

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 us...

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...

There is ongoing debate regarding the mechanism of swift heavy ion track formation in CaF2. The objective of this study is to shed light on this important topic using a range of complimentary experimental techniques. Evidence of the threshold for ion track formation being below 3 keV/nm is provided by both transmission electron microscopy and Ruthe...

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 material...

There is ongoing debate regarding the mechanism of swift heavy ion track formation in CaF2. The objective of this study is to shed light on this important topic using a range of complimentary experimental techniques. Evidence of the threshold for ion track formation being below 3 keV/nm is provided by both transmission electron microscopy and Ruthe...

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 (ε’ ∼2500) and low dielectric loss (tan δ < 0.01) obtained on Bi0.95Y0.05FeO3 films epitaxially grown on SrTiO3 (001) by pulsed laser deposition. H...

The macroscopic properties of ferroelectrics such as polarization, dielectric response, leakage current, pyroelectricity, and photovoltaic activity are usually studied on samples with capacitor geometry known also as metal-ferroelectric-metal (MFM) structures. In this case the ferroelectric layer is sandwiched between two metal electrodes that can...

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 proper...

Quasi-amorphous titanium oxynitride (TiON) films were obtained by annealing sol-gel anatase TiO2 films in NH3 atmosphere at 600°C. These films were irradiated with 50 laser pulses using the fourth harmonic (266nm) radiation of the Nd-YAG laser, with an average fluence of 20mJ/cm2. HRTEM observations of the pulsed laser irradiated films evidenced th...

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 h...

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 ph...

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...

Silver azide is a primary high explosive that can be initiated by different means. In this work, silver azide nanoparticles were obtained, embedded into silica, and further derivatized with biotin. TEM, DLS, and IR measurements were used to characterize the hybrid energetic nanoparticles. The hybrid nanoparticles are made from an explosive core (si...

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 spec...

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 α-W and r...

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...

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...

In this paper we report on the structural and electrical properties of hafnium silicate films grown by atomic layer deposition on a platinum bottom electrode. The results are compared to previously published results from our laboratory for an aluminum bottom electrode structure, with otherwise identical processing of the dielectric layer. The resul...

Metal insulator metal (MIM) capacitors are fundamental components of the electronic circuitry commonly found in many devices in areas such as analogue to digital conversion, micro-electromechanical systems, medical, automotive and memory applications [1-4]. Current MIMCAPS generally employ optimised low dielectric constant ( k ) materials ( k ~3.9...

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...

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 a...

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 ran...

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 Tc = 38.8 K. The variation of the critical current density Jc with the...

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,...

Ge, GeO2 and Ge2C6H10O7 additions to MgB2 obtained by ex situ spark plasma sintering significantly enhance the critical current density Jc in high magnetic fields. A Jc(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...

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 compe...

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 isla...

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 a...

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 non-vola...

Sol–gel chemistry within the pores of a polycarbonate template membrane was used for the preparation of EosinY–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 sol to trap dye molecules in...

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-ce...

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 t...

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...

The growth of gold layers on Pb(Zr,Ti)O3 (PZT) deposited on SrTiO3 is investigated by X-ray photoelectron spectroscopy in the Au thickness range 2–100 Å. 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...

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 th...

Laser induced ablation of solids situated in liquids (LAL) was used in order to produce nanoparticles. Laser induced breakdown spectroscopy (LIBS) surface analysis correlated with the process of nanoparticle production by LAL was applied to explain the composition of the obtained nanoparticles as determined by Electron Dispersive X-ray Spectroscopy...

We report on the fabrication of magnetite/salicylic acid/silica shell/antibiotics (Fe(3)O(4)/SA/SiO(2)/ATB) thin films by matrix-assisted pulsed laser evaporation (MAPLE) to inert substrates. Fe(3)O(4)-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 St...

We have studied the one-step procedure for simultaneous synthesis and sintering of SiC-doped MgB2 by the spark plasma sintering technique. Two types of composition, one in which Mg is strongly deficient, with the atomic ratio $\mathrm{B/Mg} = 3.75$ , and one in which Mg content is slightly higher than the stoichiometric value, specifically $\ma...

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 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 w...

We present the magnetic properties of magnetic glass ceramics obtained by crystallization of Fe containing borosilicate glass. Two types of nucleators have been used: Cr2O3 and P2O5. The role of the nucleators proved to be crucial in the size and morphology of the crystallites developed within glassy matrix as well in the magnetic response. The for...