Novel Fe₃O₄@GNF@SiO₂ nanocapsules fabricated through the combination of an in situformation method and SiO₂ coating process for magnetic resonance imaging

Changyong Lu,* Stefania Sandoval, Teresa Puig, Xavier Obradors, Gerard Tobias, Josep Rosa and Susagna Ricart. RSC Adv., 2017, 7, 24690. DOI: 10.1039/c7ra04080f

An in situ approach for the synthesis of Fe₃O₄ nanoparticles combined with a SiO₂ coating process was employed to prepare Fe₃O₄@GNF@SiO>₂ nanocapsules. Graphitised nanofibres (GNF) were initially filled with iron(III) acetylacetonate, and used as a precursor for the synthesis of ultrasmall Fe₃O₄ nanoparticles (4.6 nm in diameter) inside the cavities of GNF (Fe₃O₄@GNF) with a high density. By using a silica coating process, Fe₃O₄@GNF@SiO>₂ nanocapsules were obtained. The presence of the silica shell not only prevented leakage of the nanoparticles from inside the GNF but also protected the magnetite nanoparticles from dissolution, even in harsh acidic conditions. Furthermore, the silica coating resulted in an increased dispersability of the nanocomposites in water. Magnetic resonance imaging (MRI) studies indicate relatively high  relaxivities for Fe₃O₄@GNF nanocomposites and Fe₃O₄@GNF@SiO>₂ nanocapsules revealing the potential application of these hybrid materials for bioimaging. Therefore, the coating of filled GNF with silica is as an excellent strategy for the protection of encapsulated payloads.

Hybrid YBa₂Cu₃O₇ Superconducting–Ferromagnetic Nanocomposite Thin Films Prepared from Colloidal Chemical Solutions

Elena Bartolomé, Pablo Cayado, Eduardo Solano,Cristian Mocuta, Susagna Ricart, Bernat Mundet, Marionna Coll, Jaume Gázquez, Alexander Meledin, Gustaaf van Tendeloo, Manuel Valvidares, Javier Herrero-Martín, Pierluigi Gargiani, Eric Pellegrin, Cesar Magén, Teresa Puig, Xavier Obradors. Adv. Electron. Mater. 2017, 1700037. DOI: 10.1002/aelm.201700037

High Tc superconductor–ferromagnetic heterostructures constitute an appealing playground to study the interplay between flux vortices and magnetic moments. Here, the capability of a solution-derived route to grow hybrid YBa2Cu3O7-ferromagnetic nanocomposite epitaxial thin films from preformed spinel ferrite (MFe2O4, M = Mn, Co) nanoparticles (NPs) is explored. The characterization, performed using a combination of structural and magnetic techniques, reveals the complexity of the resulting nanocomposites. Results show that during the YBCO growth process, most of the NPs evolve to ferromagnetic double-perovskite (DP) phases (YBaCu2−x−yFexCoyO5/YBaCoFeO5), while a residual fraction of preformed ferrite NPs may remain in the YBCO matrix. Magnetometry cycles reflect the presence of ferromagnetic structures associated to the DPs embedded in the superconducting films. In addition, a superparamagnetic signal that may be associated with a diluted system of ferromagnetic clusters around complex defects has been detected, as previously observed in standard YBCO films and nanocomposites. The hybrid nanocomposites described in this work will allow studying several fundamental issues like the nucleation of superconductivity and the mechanisms of magnetic vortex pinning in superconducting/ferromagnetic heterostructures.

P. Cayado, C. F. Sánchez-Valdés,  A. Stangl,  M. Coll,  P. Roura,  A. Palau,  T. Puig  and  X. Obradors*. Phys. Chem. Chem. Phys., 2017,19, 14129-14140. DOI: 0.1039/C7CP01855J0.1039/C7CP01855J
The kinetics of oxygen incorporation (in-diffusion process) and excorporation (out-diffusion process), in YBa₂Cu₃O_6+x (YBCO) epitaxial thin films prepared using the chemical solution deposition (CSD) methodology by the trifluoroacetate route, was investigated by electrical conductivity relaxation measurements. We show that the oxygenation kinetics of YBCO films is limited by the surface exchange process of oxygen molecules prior to bulk diffusion into the films. The analysis of the temperature and oxygen partial pressure influence on the oxygenation kinetics has drawn a consistent picture of the oxygen surface exchange process enabling us to define the most likely rate determining step. We have also established a strategy to accelerate the oxygenation kinetics at low temperatures based on the catalytic influence of Ag coatings thus allowing us to decrease the oxygenation temperature in the YBCO thin films.

Albert Queraltó*, María de la Mata, Jordi Arbiol, Ruben Hühne∥, Xavier Obradors, and Teresa Puig. Cryst. Growth Des., 2017, 17 (2), pp 504–516. DOI: 10.1021/acs.cgd.6b01358

 

Self-assembling approaches based on chemical solution deposition (CSD) are ideal methods for the cost-effective production of epitaxial nanostructures with high throughput. Therefore, an in-depth investigation of the nucleation and coarsening processes involved in the self-assembly of nanostructures is mandatory to achieve a good control over nanostructure shape, dimensions, and orientation. Heteroepitaxial Ce0.9Gd0.1O2-y (CGO) is an ideal model system to unveil the underlying nanostructure development mechanisms in addition to their promising properties for catalysis, gas sensors, and ionic conductivity. Rapid thermal annealing furnaces have been used to study separately the thermodynamic and kinetic nucleation and coarsening mechanisms of self-assembled CGO isotropic and anisotropic nanostructures based on strain-engineering and surface energies control. Different CGO nanoislands are obtained: isotropic (001)CGO nanodots are grown on (001)-oriented Y2O3:ZrO2 (YSZ) and LaAlO3 (LAO) substrates, whereas (011)LAO substrates promote the growth of elongated (011)CGO nanowires. HRTEM and RHEED analyses are used to study the early stages of nucleation, as well as the shape and interfacial structure of CGO nanostructures. A systematic study with the heating ramp, annealing temperature and time, and strain in combination with thermally activated theoretical models provides information on the nucleation behavior, nucleation barriers, and atomic diffusion coefficients along in-plane and out-of-plane island orientations. Highly anisotropic atomic diffusion constants have been shown to be at the origin of the high aspect ratios of some of the nanostructures. Overall, our study provides a general method for the evaluation of nucleation and coarsening of multiple CSD-derived oxide nanostructures and understanding the shape development by combining thermodynamic and kinetic approaches.

Katrien De Keukeleere, Pablo Cayado, Alexander Meledin, Ferran Vallès, Jonathan De Roo, Hannes Rijckaert, Glenn Pollefeyt, Els Bruneel, Anna Palau, Mariona Coll, Susagna Ricart, Gustaaf Van Tendeloo, Teresa Puig, Xavier Obradors, Isabel Van Driessche. Advanced Electronic Materials. DOI: 10.1002/aelm.201600161 Although high temperature superconductors are promising for power applications, the production of low-cost coated conductors with high current densities—at high magnetic fields—remains challenging. A superior superconducting YBa2Cu3O7–δ nanocomposite is fabricated via chemical solution deposition (CSD) using preformed nanocrystals (NCs). Preformed, colloidally stable ZrO2 NCs are added to the trifluoroacetic acid based precursor solution and the NCs’ stability is confirmed up to 50 mol% for at least 2.5 months. These NCs tend to disrupt the epitaxial growth of YBa2Cu3O7–δ, unless a thin seed layer is applied. A 10 mol% ZrO2 NC addition proved to be optimal, yielding a critical current density JC of 5 MA cm−2 at 77 K in self-field. Importantly, this new approach results in a smaller magnetic field decay of JC(H//c) for the nanocomposite compared to a pristine film. Furthermore, microstructural analysis of the YBa2Cu3O7–δ nanocomposite films reveals that different strain generation mechanisms may occur compared to the spontaneous segregation approach. Yet, the generated nanostrain in the YBa2Cu3O7–δ nanocomposite results in an improvement of the superconducting properties similar to the spontaneous segregation approach. This new approach, using preformed NCs in CSD coatings, can be of great potential for high magnetic field applications.

Lluís Balcells, Carlos Martínez-Boubeta*, José Cisneros-Fernández, Konstantinos Simeonidis, Bernat Bozzo, Judith Oró-Sole, Núria Bagués, Jordi Arbiol, Narcís Mestres, and Benjamín Martínez*. ACS Appl. Mater. Interfaces, 2016, 8 (42), pp 28599–28606. DOI: 10.1021/acsami.6b08709

 

The fabrication procedure of hollow iron oxide nanoparticles with a large surface to volume ratio by a single-step gas condensation process at ambient temperature is presented. Fe clusters formed during the sputtering process are progressively transformed into hollow cuboids with oxide shells by the Kirkendall mechanism at the expense of oxygen captured inside the deposition chamber. TEM and Raman spectroscopy techniques point to magnetite as the main component of the nanocuboids; however, the magnetic behavior exhibited by the samples suggests the presence of FeO as well. In addition, these particles showed strong stability after several months of exposure to ambient conditions, making them of potential interest in diverse technological applications. In particular, these hierarchical hollow particles turned out to be very efficient for both As(III) and As(V) absorption (326 and 190 mg/g, respectively), thus making them of strong interest for drinking water remediation.

Albert Queraltó*, Angel Pérez del Pino, María de la Mata, Jordi Arbiol, Mar Tristany, Xavier Obradors, and Teresa Puig; Chem. Mater., 2016, 28 (17), pp 6136–6145. DOI: 10.1021/acs.chemmater.6b01968 The crystallization process and physical properties of different functional oxide thin films (Ce0.9Zr0.1O2-y, LaNiO3, Ba0.8Sr0.2TiO3, and La0.7Sr0.3MnO3) on single crystal substrates (Y2O3:ZrO2, LaAlO3, and SrTiO3) are studied by pulsed laser annealing (PLA). A Nd:YAG laser source (λ = 266 nm, 10 Hz and τ ∼ 3 ns) is employed to crystallize chemical solution deposited (CSD) amorphous/nanocrystalline films under atmospheric conditions. We provide new insight on the influence of photochemical and photothermal interactions on the epitaxial crystallization kinetics of oxide thin films during the transformation from amorphous/polycrystalline material (i.e., atomic diffusion, epitaxial growth rates, and activation energies of nucleation and crystallization). The epitaxial growth is investigated by varying the laser fluence and the applied number of pulses. The morphology, structure, and epitaxial evolution of films are evaluated by means of atomic force and transmission electron microscopies and X-ray diffraction. Highly epitaxial oriented films of 20–40 nm in thickness are obtained by PLA. The crystallization kinetics of laser treatments is determined to be orders of magnitude faster than thermal treatments with similar activation energies (1.5–4.1 eV), mainly due to the large temperature gradients inducing modified atomic diffusion mechanisms derived mainly from photothermal interactions, as well as a minor contribution of photochemical effects. The fast heating rates achieved by PLA also contribute to the fast epitaxial growth due to reduced coarsening of polycrystalline material. The measurement of the physical properties (electrical resistivity and magnetism) of laser processed CSD films has revealed significantly good functionalities, close to those of thermally grown films, but with much shorter processing times.

Mariona Coll, Jaume Gazquez, Ignasi Fina, Zakariya Khayat, Andy Quindeau, Marin Alexe, Maria Varela, Susan Trolier-McKinstry, Xavier Obradors, and Teresa Puig;
Chem. Mater., 2015, 27 (18), pp 6322–6328

DOI: 10.1021/acs.chemmater.5b02093

In this work, ferroelectricity is identified in nanocrystalline BiFeO3 (BFO) thin films prepared by low-temperature atomic layer deposition. A combination of X-ray diffraction, reflection high energy electron diffraction, and scanning transmission electron microscopy analysis indicates that the as-deposited films (250 °C) consist of BFO nanocrystals embedded in an amorphous matrix. Postannealing at 650 °C for 60 min converts the sample to a crystalline film on a SrTiO3substrate. Piezoelectric force microscopy demonstrates the existence of ferroelectricity in both as-deposited and postannealed films. The ferroelectric behavior in the as-deposited stage is attributed to the presence of nanocrystals. Finally, a band gap of 2.7 eV was measured by spectroscopic ellipsometry. This study opens broad possibilities toward ferroelectric oxides on 3D substrates and also for the development of new ferroelectric perovskites prepared at low temperature.

A. Queraltó, A. Pérez del Pino, M. de la Mata, J. Arbiol, M. Tristany, A. Gómez, X. Obradors and T. Puig. Appl. Phys. Lett. 106, 262903 (2015) http://dx.doi.org/10.1063/1.4923376 Highly crystalline epitaxial Ba0.8Sr0.2TiO3 (BST) thin-films are grown on (001)-oriented LaNiO3-buffered LaAlO3 substrates by pulsed laser irradiation of solution derived barium-zirconium-titanium precursor layers using a UV Nd:YAG laser source at atmospheric conditions. Thestructural analyses of the obtained films, studied by X-ray diffractometry and transmission electron microscopy, demonstrate that laser processing allows the growth of tens of nm-thick BST epitaxialfilms with crystalline structure similar to that of films obtained through conventional thermal annealing methods. However, the fast pulsed nature of the laser employed leads tocrystallization kinetic evolution orders of magnitude faster than in thermal treatments. The combination of specific photothermal and photochemical mechanisms is the main responsible for the ultrafast epitaxial laser-induced crystallization. Piezoresponse microscopy measurements demonstrate equivalent ferroelectric behavior in laser and thermally annealedfilms, being the piezoelectric constant ∼25 pm V⁻¹.    

R. Ortega-Hernandez, M. Coll, J. C. Gonzalez-Rosillo, A. Palau, X. Obradors, E. Miranda, T. Puig and J. Suñé Microelectronic Engineering, 147, 37-40 (2015) DOI:10.1016/j.mee.2015.04.042 The resistive switching of CeO_2−x/La_0.8Sr_0.2MnO₃ bilayer structures has been studied. First, the resistive switching (RS) characteristics of La_0.8Sr_0.2MnO₃ (LSMO) and the CeO_2−x layers are studied separately. Then, the bilayer characteristics are analyzed. It has been demonstrated that inserting a thin CeO_2−x layer between the LSMO film and the metal electrodes deeply modifies the resistive switching characteristics. The metal–insulator transition of the LSMO layer results from the oxygen diffusion in and out of the film. These effects are enhanced through the introduction of the CeO_2−x layer due to the fact it acts as an oxygen reservoir.