Suchergebnisse

López Varo, Pilar et al. ; Solar energy conversion using semiconductors to fabricate photovoltaic devices relies on efficient light absorption, charge separation of electron-hole pair carriers or excitons, and fast transport and charge extraction to counter recombination processes. Ferroelectric materials are able to host a permanent electrical polarization which provides control over electrical field distribution in bulk and interfacial regions. In this review, we provide a critical overview of the physical principles and mechanisms of solar energy conversion using ferroelectric semiconductors and contact layers, as well as the main achievements reported so far. In a ferroelectric semiconductor film with ideal contacts, the polarization charge would be totally screened by the metal layers and no charge collection field would exist. However, real materials show a depolarization field, smooth termination of polarization, and interfacial energy barriers that do provide the control of interface and bulk electric field by switchable spontaneous polarization. We explore different phenomena as the polarization-modulated Schottky-like barriers at metal/ferroelectric interfaces, depolarization fields, vacancy migration, and the switchable rectifying behavior of ferroelectric thin films. Using a basic physical model of a solar cell, our analysis provides a general picture of the influence of ferroelectric effects on the actual power conversion efficiency of the solar cell device, and we are able to assess whether these effects or their combinations are beneficial or counterproductive. We describe in detail the bulk photovoltaic effect and the contact layers that modify the built-in field and the charge injection and separation in bulk heterojunction organic cells as well as in photocatalytic and water splitting devices. We also review the dominant families of ferroelectric materials that have been most extensively investigated and have provided the best photovoltaic performance. ; The research leading to these results has received funding from the European Union Seventh Framework Program [FP7/2007–2013] under grant agreement 316494 and from MINECO of Spain under project TEC2013-47283-R and Grant FPU12/02712. F.R. acknowledges funding and partial salary support for work on multiferroic materials from NSERC, through an EWR Steacie Memorial Fellowship. ; Peer reviewed

Mesoporous Ni and Cu-Ni (Cu20Ni80 and Cu45Ni55 in at. %) films, showing a three-dimensional (3D) porous structure and tunable magnetic properties, are prepared by electrodeposition from aqueous surfactant solutions using micelles of P-123 triblock copolymer as structure-directing entities. Pores between 5 and 30 nm and dissimilar space arrangements (continuous interconnected networks, circular pores, corrugated mesophases) are obtained depending on the synthetic conditions. X-ray diffraction studies reveal that the Cu-Ni films have crystallized in the face-centered cubic structure, are textured, and exhibit certain degree of phase separation, particularly those with a higher Cu content. Atomic layer deposition (ALD) is used to conformally coat the mesopores of Cu20Ni80 film with amorphous Al2O3, rendering multiphase "nano-in-meso" metal-ceramic composites without compromising the ferromagnetic response of the metallic scaffold. From a technological viewpoint, these 3D nanoengineered composite films could be appealing for applications like magnetically actuated micro/nanoelectromechanical systems (MEMS/NEMS), voltage-driven magneto-electric devices, capacitors, or as protective coatings with superior strength and tribological performance. ; Financial support by the European Research Council (SPIN-PORICS 2014-Consolidator Grant, Agreement No 648454), the Spanish Government (Project MAT2017-86357-C3-1-R and associated FEDER) and the "Severo Ochoa" Programme for Centres of Excellence in R&D (SEV-2015-0496), the Generalitat de Catalunya (2017-SGR-292 and 2017-SGR-1519) and the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement no 665919 is acknowledged. E.P. and M. C. are grateful to MINECO for the "Ramón y Cajal" contract (RYC-2012-10839 and RYC-2013-12448). ; Peer reviewed

Enhanced pinning due to nanoscale strain is unique to the high-Tc cuprates, where pairing may be modified with lattice distortion. Therefore a comprehensive understanding of the defect landscape is required for a broad range of applications. However, determining the type and distribution of defects and their associated strain constitutes a critical task, and for this aim, real-space techniques for atomic resolution characterization are necessary. Here, we use scanning transmission electron microscopy (STEM) to study the atomic structure of individual defects of solution-derived YBa2Cu3O7 (YBCO) nanocomposites, where the inclusion of incoherent secondary phase nanoparticles within the YBCO matrix dramatically increases the density of Y1Ba2Cu4O8 (Y124) intergrowths, the commonest defect in YBCO thin films. The formation of the Y124 is found to trigger a concatenation of strain-derived interactions with other defects and the concomitant nucleation of intrinsic defects, which weave a web of randomly distributed nanostrained regions that profoundly transform the vortex-pinning landscape of the YBCO nanocomposite thin films. ; Authors acknowledge financial support from Spanish Ministry of Economy and Competitiveness through the "Severo Ochoa" Programme for Centres of Excellence in R&D (SEV-2015-0496), CONSOLIDER Excellence Network (MAT2015-68994-REDC), COACHSUPENERGY project (MAT2014-51778-C2-1-R, co-financed by the European Regional Development Fund), European Research Council Advanced Grant Award ERC-2014-ADG-669504, and from the Catalan Government with 2014-SGR-753 and Xarmae. J.G. and M.C. acknowledge Ramon y Cajal Program (RyC-2012- 11709 and RYC-2013-12448, respectively). The microscopy work were conducted in the Laboratorio de Microscopias Avanzadas at Instituto de Nanociencia de Aragon - Universidad de Zaragoza (Zaragoza, Spain). Authors acknowledge the LMA-INA for offering access to their instruments and expertise. ; Peer reviewed

An optical in situ strategy for the analysis of oxygen diffusion in ultrathin ceria layers with a thickness of 2–10 nm at temperatures between 50 and 200 °C is presented, which allows for the determination of diffusion coefficients. This method is based on the sensitivity of the photoluminescence (PL) intensity of InGaN nanowires to adsorbed oxygen. The oxygen diffusion through an ultrathin CeO2 coating deposited on the InGaN nanowires is monitored by analyzing the transient PL behavior of the InGaN nanowires, which responds to changes of the oxygen concentration in the environment when the corresponding oxygen concentration is established at the CeO2/InGaN interface due to diffusion through the coating. Quantitative evaluation of the oxygen diffusion in CeO2 based on a model considering Langmuir Adsorption and recombination yields a diffusion coefficient D of (2.55 ± 0.05) × 10−16 cm2 s−1 at a temperature of 100 °C. Temperature‐dependent measurements reveal an Arrhenius type behavior of D with an activation energy of (0.28 ± 0.04) eV. In contrast, no oxygen diffusion is detected for an ultrathin layer (≥5 nm) of Al2O3, which is known as a poor oxygen ion conductor within the analyzed temperature regime. ; We thank Herbert Over from the Institute of Physical Chemistry at the JLU Giessen for sharing his knowledge and experience and helping improve the manuscript. Financial support was provided by the DFG via the GrK (Research training group) 2204 "Substitute Materials for sustainable Energy Technologies." M.C. thanks Ramón y Cajal program RYC‐2013‐12448. The authors also acknowledge financial support from the Spanish Ministry of Economy and Competitiveness, through the "Severo Ochoa" Programme for Centres of Excellence in R&D (SEV‐ 2015‐0496) and MAT2017‐83169‐R AEI/FEDER,UE. This article is based upon work from COST Action MP1402 "Hooking together European research in atomic layer deposition (HERALD)," supported by COST (European Cooperation in Science and Technology). S.M.S. acknowledges funding from "Programa Internacional de Becas 'la Caixa"'‐Severo Ochoa." ICN2 members acknowledge funding from Generalitat de Catalunya 2017 SGR 327 and the Spanish MINECO project ENE2017‐85087‐C3. ICN2 acknowledges support from the Severo Ochoa Programme (MINECO, grant no. SEV‐2013‐0295) and is funded by the CERCA Programme /Generalitat de Catalunya. Part of this work was performed in the framework of Universitat Autònoma de Barcelona Materials Science PhD program. This received funding from the European Union's Horizon 2020 Research and Innovation Programme under grant agreement no. 654360 NFFA‐Europe. ; Peer reviewed

The preparation and manipulation of crystalline yet bendable functional complex oxide membranes has been a long-standing issue for a myriad of applications, in particular, for flexible electronics. Here, we investigate the viability to prepare magnetic and crystalline CoFe2O4 (CFO) membranes by means of the Sr3Al2O6 (SAO) sacrificial layer approach using chemical deposition techniques. Meticulous chemical and structural study of the SAO surface and SAO/CFO interface properties have allowed us to identify the formation of an amorphous SAO capping layer and carbonates upon air exposure, which dictate the crystalline quality of the subsequent CFO film growth. Vacuum annealing at 800 °C of SAO films promotes the elimination of the surface carbonates and the reconstruction of the SAO surface crystallinity. Ex-situ atomic layer deposition of CFO films at 250 °C on air-exposed SAO offers the opportunity to avoid high-temperature growth while achieving polycrystalline CFO films that can be successfully transferred to a polymer support preserving the magnetic properties under bending. Float on and transfer provides an alternative route to prepare freestanding and wrinkle-free CFO membrane films. The advances and challenges presented in this work are expected to help increase the capabilities to grow different oxide compositions and heterostructures of freestanding films and their range of functional properties. ; This work was funded by MICIN/AEI/10.13039/501100011033/FEDER through the projects Severo Ochoa FUNFUTURE CEX2019-00917-S and PID2020-114224RB-I00. We also acknowledge the financial support from the 2020 Leonardo Grant for Researchers and Cultural Creators BBVA Foundation, the i-link A20346-CSIC project, the National Key RD Program of China (2018YFA0305800), and the Beijing Outstanding Young Scientist Program (BJJWZYJH01201914430039). The project that gave rise to these results received the support of a fellowship from the "la Caixa" Foundation LCF/BQ/DI19/11730026. D.Z. acknowledges the funding and financial support of the Fund of Chemical Industries (Kekulé fellowship) for his Ph.D. research. The authors are thankful to R. Solanas for his help with the RHEED measurements. This work has been done in the framework of the doctorate in material science of the Autonomous University of Barcelona. ; With funding from the Spanish government through the 'Severo Ochoa Centre of Excellence' accreditation (CEX2019-000917-S). ; Peer reviewed

Compositional engineering of BiFeO3 can significantly boost its photovoltaic performance. Therefore, controlling site substitution and understanding how it affects the optical and electronic properties while achieving robust and stable phases is essential to continue progressing in this field. Here the influence of cation co-substitution in BiFeO3 on phase purity, optical and electronic properties is investigated by means of X-ray diffraction, spectroscopic ellipsometry and X-ray absorption spectroscopy, respectively. Piezoelectric force microscopy and ferroelectric characterization at room temperature has been carried out in co-doped BiFeO3 films. First-principles calculations are also performed and compared to the experimental observations. It is shown that the incorporation of La3+ in Bi(Fe,Co)O3 films improves phase purity and stability while preserving the reduced band gap achieved in metastable Bi(Fe,Co)O3. Moreover, it is suggested that the changes in the optoelectronic properties are mainly dictated by the hybridisation between unoccupied Co 3d and O 2p states along with the presence of Co3+/Co2+ species. This thorough study on (Bi,La)(Fe,Co)O3 thin films coupled with the use of a cost-effective and facile solution deposition synthesis increases the motivation to continue exploiting the potential of these perovskite materials. ; This research was supported by the Spanish Ministerio de Ciencia, Innovación y Universidades ("Severo Ochoa" Programme for Centres of Excellence in R&D CEX2019-000917-S, MAT2017-83169-R, RTI2018-093996-B-C32, PID2019-107727RB-I00 (AEI/FEDER, EU)). P. M. thanks financial support from FPI fellowship (PRE2018-084618). C. M. and C. C. acknowledge computational resources and technical assistance from the Australian Government and the Government of Western Australia through the National Computational Infrastructure (NCI) and Magnus under the National Computational Merit Allocation Scheme and The Pawsey Supercomputing Centre. The authors acknowledge Prof. J. Fontcuberta for providing access to his experimental facilities and the support of ALBA staff for the successful performance of the measurements at CIRCE beamline of the ALBA Synchrotron Light Source. I. F. and C. C. acknowledge support from the Spanish Ministry of Science, Innovation and Universities under the "Ramón y Cajal" fellowship RYC2017-226531 and RYC2018-024947-I, respectively. M. C and I. F acknowledge Beca Leonardo from fundación BBVA. H. T is financially supported by China Scholarship Council (CSC) with no. 201906050014. The work of P. M. and H. T have been done in the framework of the doctorate in Materials Science of the Autonomous University of Barcelona. ; Peer reviewed

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 acknowledge the financial support from Spanish Ministry of Economy and Competitiveness through the Severo Ochoa Programme for Centres of Excellence in R&D (SEV-2015-0496), CONSOLIDER Excellence Network (MAT2015-68994-REDC), COACHSUPENERGY project (MAT2014-51778-C2-1-R), co-financed by the European Regional Development Fund. We also thank the support from the European Union for NanoSC Cost Action (MP1201), NANOCOHYBRI (CA 16218) and ERC-2014-ADG-669504, and from the Catalan Government with 2014-SGR-753 and Xarmae. A.F.-R. and J.C.G.-R. thank Spanish Ministry of Economy for his FPI Spanish grants (BES-2016-077310, BES-2012-053814). ; Peer reviewed