Teresa Puig, Patrick Krkotic, Artur Romanov, Joan O’Callaghan, Danilo Andrea Zanin, Holger Neupert, Pedro Costa Pinto, Pierre Demolon, Ângelo Rafael Granadeiro Costa, Mauro Taborelli, Francis Perez, Montse Pont, Joffre Gutierrez and Sergio Calatroni. Superconductor Science and Technology
Hailin Wang, Jaume Gazquez, Carlos Frontera, Matthew F. Chisholm, Alberto Pomar, Benjamin Martinez & Narcis Mestres. NPG Asia Materials volume 11, Article number: 44 (2019).
Ferroelectric perovskite oxides are emerging as a promising photoactive layer for photovoltaic applications because of their very high stability and their alternative ferroelectricity-related mechanism for solar energy conversion that could lead to extraordinarily high efficiencies. One of the biggest challenges so far is to reduce their band gap toward the visible region while simultaneously retaining ferroelectricity. To address these two issues, herein an elemental composition engineering of BiFeO3 is performed by substituting Fe by Co cations, as a means to tune the characteristics of the transition metal–oxygen bond. We demonstrate by solution processing the formation of epitaxial, pure phase, and stable BiFe1–xCoxO3 thin films for x ≤ 0.3 and film thickness up to 100 nm. Importantly, the band gap can be tuned from 2.7 to 2.3 eV upon cobalt substitution while simultaneously enhancing ferroelectricity. As a proof of concept, nonoptimized vertical devices have been fabricated and, reassuringly, the electrical photoresponse in the visible region of the Co-substituted phase is improved with respect to the unsubstituted oxide.
A fast and single‐step preparation of patchy LnF3 faceted‐charge nanocrystals are described. These hexagonal faceted nanocrystals allow the spontaneous selective adsorption of cations or anions in the different faces, producing stable and well‐defined patches of different charge. The mechanism for the formation of the patches and the properties of the obtained nanocrystals were characterized by a combination of experimental techniques and all‐atomic molecular dynamics simulations. The spontaneous dual‐charged surface as well as the luminescence effects that can be achieved by doping host–LaF3 systems make these new nanocrystals interesting both from a fundamental point of view and for a wide range of applications.
Modulation of carrier concentration in strongly correlated oxides offers the unique opportunity to induce different phases in the same material, which dramatically change their physical properties, providing novel concepts in oxide electronic devices with engineered functionalities. This work reports on the electric manipulation of the superconducting to insulator phase transition in YBa2Cu3O7−δ thin films by electrochemical oxygen doping. Both normal state resistance and the superconducting critical temperature can be reversibly manipulated in confined active volumes of the film by gate-tunable oxygen diffusion. Vertical and lateral oxygen mobility may be finely modulated, at the micro- and nano-scale, by tuning the applied bias voltage and operating temperature thus providing the basis for the design of homogeneous and flexible transistor-like devices with loss-less superconducting drain–source channels. We analyze the experimental results in light of a theoretical model, which incorporates thermally activated and electrically driven volume oxygen diffusion.
We use an original water-based chemical method, to grow pure epitaxial BiFeO3 (BFO) ultra-thin films with excellent piezoelectric properties. Particularly, we show that this novel chemical route produces a higher natural ferroelectric domain size distribution and coercive field compared to similar BFO films grown by physical methods. Moreover, we measured the d33 piezoelectric coefficient of 60 nm thick BFO films with a direct approach, using Direct Piezoelectric Force Microscopy (DPFM). As a result, first piezo-generated charge maps of a very thin BFO layer were obtained applying this novel technology. We also performed a comparative study of the d33 coefficients between standard PFM analysis and the DPFM microscopy showing similar values i.e. 17 pm/V and 22 pC/N respectively. Finally, we proved that the directionality of the piezoelectric effect in BFO ferroelectric thin films is preserved at low thickness dimensions demonstrating the potential of chemical processes for the development of low cost functional ferroelectric and piezoelectric devices.
DOI:https://doi.org/10.1103/PhysRevMaterials.2.063607
In this work we evaluate the defects and the associated distortions present in tensile and compressive-strained chemical solution deposition–derived NdNiO3 (NNO) and LaNiO3 (LNO) thin films by means of aberration corrected scanning transmission electron microscopy. We elucidate a fundamental link between strain and the most common defect observed in nickelate films, the Ruddlesden-Popper fault (RPF), which will ultimately impinge on the electrical properties of the films. Overall, the concentration of RPF defects increases with the lattice mismatch. More specifically, LNO films are always metallic, although transitioning from compressive to tensile strain results in the appearance of RPFs and an increase of the resistivity. On the other hand, NNO films always behave as insulators under tensile strain, whereas under compressive strain the increase of the thickness makes the onset of the metal-to-insulator transition shift to higher temperatures.
F. Vallès, A. Palau, V. Rouco, B. Mundet, X. Obradors & T. Puig. Scientific Reports, volume 8, Article number: 5924(2018).
doi:10.1038/s41598-018-24392-1
Flux magnetic relaxation (flux creep) causes logarithmic decay on the critical currents in superconductors, especially at high temperatures, in detriment of applications for high temperature superconductors. In this work, we present a novel methodology to measure the flux creep rate in YBCO from electrical transport measurements instead of using traditional magnetic relaxation measurements. This new methodology provides a faster way to analyze creep and enables to expand the analysis to any orientation of the magnetic field. In particular, we have applied this analysis to study the creep rate in chemical solution deposited nanocomposites (YBCO with included nanoparticles), revealing that emerging stacking faults provide flux pinning and additionally reduce the flux magnetic relaxation.
A Palau, F Vallès, V Rouco, M Coll, Z Li, C Pop, B Mundet, J Gàzquez, R Guzman, J Gutierrez, X Obradors and T Puig. , ,
In-field angular pinning performances at different temperatures have been analysed on chemical solution deposited (CSD) YBa2Cu3O7−x (YBCO) pristine films and nanocomposites. We show that with this analysis we are able to quantify the vortex pinning strength and energies, associated with different kinds of natural and artificial pinning defects, acting as efficient pinning centres at different regions of the H–T phase diagram. A good quantification of the variety of pinning defects active at different temperatures and magnetic fields provides a unique tool to design the best vortex pinning landscape under different operating conditions. We have found that by artificially introducing a unique defect in the YBCO matrix, the stacking faults, we are able to modify three different contributions to vortex pinning (isotropic-strong, anisotropic-strong, and isotropic-weak). The isotropic-strong contribution, widely studied in CSD YBCO nanocomposites, is associated with nanostrained regions induced at the partial dislocations surrounding the stacking faults. Moreover, the stacking fault itself acts as a planar defect which provides a very effective anisotropic-strong pinning at H//ab. Finally, the large presence of Cu–O cluster vacancies found in the stacking faults have been revealed as a source of isotropic-weak pinning sites, very active at low temperatures and high fields.
Jordi Martínez-Esaín, Jordi Faraudo*, Teresa Puig, Xavier Obradors, Josep Ros, Susagna Ricart, and Ramón Yáñez*. J. Am. Chem. Soc., Article ASAP. DOI: 10.1021/jacs.7b09821
Ligand-to-surface interactions are critical factors in surface and interface chemistry to control the mechanisms governing nanostructured colloidal suspensions. In particular, molecules containing carboxylate moieties (such as citrate anions) have been extensively investigated to stabilize metal, metal oxide, and metal fluoride nanoparticles. Using YF3 nanoparticles as a model system, we show here the self-assembly of citrate-stabilized nanostructures (supraparticles) with a size tunable by temperature. Results from several experimental techniques and molecular dynamics simulations show that the self-assembly of nanoparticles into supraparticles is due to ionic bridges between different nanoparticles. These interactions were caused by cations (e.g., ammonium) strongly adsorbed onto the nanoparticle surface that also interact strongly with nonbonded citrate anions, creating ionic bridges in solution between nanoparticles. Experimentally, we observe self-assembly of nanoparticles into supraparticles at 25 and 100 °C. Interestingly, at high temperatures (100 °C), this citrate-bridge self-assembly mechanism is more efficient, giving rise to larger supraparticles. At low temperatures (5 °C), this mechanism is not observed, and nanoparticles remain stable. Molecular dynamics simulations show that the free energy of a single citrate bridge between nanoparticles in solution is much larger than the thermal energy and in fact is much larger than typical adsorption free energies of ions on colloids. Summarizing our experiments and simulations, we identify as key aspects of the self-assembly mechanism the requirement of NPs with a surface able to adsorb anions and cations and the presence of multidentate ions in solution. This indicates that this new ion-mediated self-assembly mechanism is not specific of YF3 and citrate anions, as supported by preliminary experimental results in other systems.
