Staff directory José Santiso López

José Santiso López

CSIC Tenured Scientist, Nanomaterials Growth Unit Leader and X-Ray Diffraction Expert
jose.santiso(ELIMINAR)@icn2.cat
Nanomaterials Growth Unit - X-Ray Diffraction Facility

ORCID: 0000-0003-4274-2101

Publications

2024

Impact of Oxygen Stoichiometry on the Thermoelectric Properties of Bi₂Sr₂Co₂O_y Thin Films
Arindom Chatterjee; Alexandros El Sachat; C. M. Sotomayor Torres; J. Santiso; Emigdio Chávez‐Ángel Acs Applied Energy Materials; 2024. 10.1021/acsaem.4c00551.
On the Electrochemical Activation of Nanoporous Reduced Graphene Oxide Electrodes Studied by In Situ/Operando Electrochemical Techniques
Bernicola, Maria del Pilar; Lounasvuori, Mailis; Padilla-Pantoja, Jessica; Santiso, Jose; Debiemme-Chouvy, Catherine; Perrot, Hubert; Petit, Tristan; Garrido, Jose A; del Corro, Elena Advanced Functional Materials; 2024. 10.1002/adfm.202408441.
Photostrictive Actuators Based on Freestanding Ferroelectric Membranes
Saptam Ganguly; David Pesquera; Daniel Moreno Garcia; Usman Saeed; Nona Mirzamohammadi; José Santiso; Jessica Padilla; José Manuel Caicedo Roque; Claire Laulhé; Felisa Berenguer; Luis Guillermo Villanueva; Gustau Catalán Advanced Materials; 2024. 10.1002/adma.202310198.

2023

Antisite Defects and Chemical Expansion in Low-damping, High-magnetization Yttrium Iron Garnet Films
Santiso, J; Garcia, C; Romanque, C; Henry, L; Bernier, N; Bagues, N; Caicedo, JM; Valvidares, M; Sandiumenge, F Chemnanomat; 9 (2) 2023. 10.1002/cnma.202200495. IF: 3.800
Band Alignment and Photoresponse of LaFeO3-Based Heterojunctions
Sheng, YW; Mirjolet, M; Villa, M; Gazquez, J; Santiso, J; Klein, A; Fraxedas, J; Fontcuberta, J Physical Review Applied; 19 (2): 24001. 2023. 10.1103/PhysRevApplied.19.024001. IF: 4.600
Enhanced Thermoelectric Properties of Misfit Bi2Sr2-xCaxCo2Oy: Isovalent Substitutions and Selective Phonon Scattering
Chatterjee, A; Banik, A; El Sachat, A; Roque, JMC; Padilla-Pantoja, J; Torres, CS; Biswas, K; Santiso, J; Chavez-Angel, E Materials (Basel); 16 (4): 1413. 2023. 10.3390/ma16041413. IF: 3.400
Improved High Temperature Thermoelectric Properties in Misfit Ca3Co4O9 by Thermal Annealing
Chatterjee, A; El Sachat, A; Banik, A; Biswas, K; Castro-Alvarez, A; Torres, CMS; Santiso, J; Chavez-Angel, E Energies; 16 (13): 5162. 2023. 10.3390/en16135162. IF: 3.200
Native defects association enabled room-temperature p-type conductivity in β-Ga2O3
Chi, ZY; Sartel, C; Zheng, YL; Modak, S; Chernyak, L; Schaefer, CM; Padilla, J; Santiso, J; Ruzin, A; Gonçalves, AM; von Bardeleben, J; Guillot, G; Dumont, Y; Pérez-Tomás, A; Chikoidze, E Journal Of Alloys And Compounds; 969: 172454. 2023. 10.1016/j.jallcom.2023.172454. IF: 6.200
Tunable Molecular Electrodes for Bistable Polarization Screening
Spasojevic, I; Santiso, J; Caicedo, JM; Catalan, G; Domingo, N Small; 19 (30): e2207799. 2023. 10.1002/smll.202207799. IF: 13.300

2022

Effects of exsolution on the stability and morphology of Ni nanoparticles on BZY thin films
Jennings D., Ricote S., Santiso J., Caicedo J., Reimanis I. Acta Materialia; 228 (117752) 2022. 10.1016/j.actamat.2022.117752. IF: 8.203

Yttria doped barium zirconate (BZY) is of interest for use as a catalyst support material, supporting exsolved Ni nanoparticles. Exsolution has been hypothesized to impart catalytic nanoparticles with exceptional resistance to particle coarsening, a known degradation mechanism in catalysts. However, the mechanisms and kinetics of Ni nanoparticle coarsening in BZY are unknown. This work analyzes the kinetics of the coarsening of exsolved Ni nanoparticles on epitaxial BZY thin films at three temperatures (600, 700, and 800 ∘C) over a time span of 150 h. It is demonstrated that Ni coarsening transitions from an Ostwald ripening process to particle migration and coalescence after Ni particles reach a critical size. The coarsening behavior of BZY/Ni is shown to be dependent on the BZY surface orientation, with Ni particles on (111) oriented thin films coarsening the least. The preferred orientation relationships between Ni and BZY on (100), (110), and (111) oriented films are determined. Additionally, the morphology of Ni particles produced through exsolution and thin film dewetting are compared, showing that the socketing behavior and interfacial energy are independent of the Ni particle preparation method. © 2022

View abstract
Role of pO2 and film microstructure on the memristive properties of La2NiO4+δ/LaNiO3−δ bilayers
Maas K., Wulles C., Caicedo Roque J.M., Ballesteros B., Lafarge V., Santiso J., Burriel M. Journal of Materials Chemistry A; 10 (12): 6523 - 6530. 2022. 10.1039/d1ta10296f. IF: 12.732

LaNiO3/La2NiO4 bilayers deposited at varying pO2 conditions resulted in remarkable differences in film microstructure and cell parameters, directly affecting the electrical behaviour of Pt/LaNiO3/La2NiO4/Pt devices. The devices deposited at low pO2 showed the largest memristance. We propose this is due to the formation of a p-type Schottky contact between LaNiO3 and La2NiO4, where the extent of its carrier depletion width can be modulated by the electric-field induced drift of interstitial oxygen ions acting as mobile acceptor dopants in La2NiO4 © 2022 The Royal Society of Chemistry

View abstract
The impact of Mn nonstoichiometry on the oxygen mass transport properties of La0.8Sr0.2MnyO3 +/-delta thin films
Chiabrera, FM; Baiutti, F; Borgers, JM; Harrington, GF; Yedra, L; Liedke, MO; Kler, J; Nandi, P; Sirvent, JD; Santiso, J; Lopez-Haro, M; Calvino, JJ; Estrade, S; Butterling, M; Wagner, A; Peiro, F; De Souza, RA; Tarancon, A Jphys Energy; 4 (4) 2022. 10.1088/2515-7655/ac98df. IF: 7.528

2021

A high-entropy manganite in an ordered nanocomposite for long-term application in solid oxide cells
Baiutti F., Chiabrera F., Acosta M., Diercks D., Parfitt D., Santiso J., Wang X., Cavallaro A., Morata A., Wang H., Chroneos A., MacManus-Driscoll J., Tarancon A. Nature Communications; 12 (1, 2660) 2021. 10.1038/s41467-021-22916-4. IF: 14.919

The implementation of nano-engineered composite oxides opens up the way towards the development of a novel class of functional materials with enhanced electrochemical properties. Here we report on the realization of vertically aligned nanocomposites of lanthanum strontium manganite and doped ceria with straight applicability as functional layers in high-temperature energy conversion devices. By a detailed analysis using complementary state-of-the-art techniques, which include atom-probe tomography combined with oxygen isotopic exchange, we assess the local structural and electrochemical functionalities and we allow direct observation of local fast oxygen diffusion pathways. The resulting ordered mesostructure, which is characterized by a coherent, dense array of vertical interfaces, shows high electrochemically activity and suppressed dopant segregation. The latter is ascribed to spontaneous cationic intermixing enabling lattice stabilization, according to density functional theory calculations. This work highlights the relevance of local disorder and long-range arrangements for functional oxides nano-engineering and introduces an advanced method for the local analysis of mass transport phenomena. © 2021, The Author(s).

View abstract
Back-to-Basics tutorial: X-ray diffraction of thin films
Harrington G.F., Santiso J. Journal of Electroceramics; 2021. 10.1007/s10832-021-00263-6. IF: 1.785

X-ray diffraction (XRD) is an indispensable tool for characterising thin films of electroceramic materials. For the beginner, however, it can be a daunting technique at first due to the number of operation modes and measurements types, as well as the interpretation of the resultant patterns and scans. In this tutorial article, we provide a foundation for the thin-film engineer/scientist conducting their first measurements using XRD. We give a brief introduction of the principle of diffraction and description of the instrument, detailing the relevant operation modes. Next, we introduce five types of measurements essential for thin film characterisation: 2 θ/ ω scans, grazing-incidence scans, rocking curves, pole figures, and azimuth scans (or ϕ scans). Practical guidelines for selecting the appropriate optics, mounting and aligning the sample, and selecting scan conditions are given. Finally, we discuss some of the basics of data analysis, and give recommendations on the presentation of data. The aim of this article is to ultimately lower the barrier for researchers to perform meaningful XRD analysis, and, building on this foundation, find the existing literature more accessible, enabling more advanced XRD investigations. © 2021, The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.

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Carbon Incorporation in MOCVD of MoS2Thin Films Grown from an Organosulfide Precursor
Schaefer C.M., Caicedo Roque J.M., Sauthier G., Bousquet J., Hébert C., Sperling J.R., Pérez-Tomás A., Santiso J., Del Corro E., Garrido J.A. Chemistry of Materials; 33 (12): 4474 - 4487. 2021. 10.1021/acs.chemmater.1c00646. IF: 9.811

With the rise of two-dimensional (2D) transition-metal dichalcogenide (TMD) semiconductors and their prospective use in commercial (opto)electronic applications, it has become key to develop scalable and reliable TMD synthesis methods with well-monitored and controlled levels of impurities. While metal-organic chemical vapor deposition (MOCVD) has emerged as the method of choice for large-scale TMD fabrication, carbon (C) incorporation arising during MOCVD growth of TMDs has been a persistent concern-especially in instances where organic chalcogen precursors are desired as a less hazardous alternative to more toxic chalcogen hydrides. However, the underlying mechanisms of such unintentional C incorporation and the effects on film growth and properties are still elusive. Here, we report on the role of C-containing side products of organosulfur precursor pyrolysis in MoS2 thin films grown from molybdenum hexacarbonyl Mo(CO)6 and diethyl sulfide (CH3CH2)2S (DES). By combining in situ gas-phase monitoring with ex situ microscopy and spectroscopy analyses, we systematically investigate the effect of temperature and Mo(CO)6/DES/H2 gas mixture ratios on film morphology, chemical composition, and stoichiometry. Aiming at high-quality TMD growth that typically requires elevated growth temperatures and high DES/Mo(CO)6 precursor ratios, we observed that temperatures above DES pyrolysis onset (â 600 °C) and excessive DES flow result in the formation of nanographitic carbon, competing with MoS2 growth. We found that by introducing H2 gas to the process, DES pyrolysis is significantly hindered, which reduces carbon incorporation. The C content in the MoS2 films is shown to quench the MoS2 photoluminescence and influence the trion-To-exciton ratio via charge transfer. This finding is fundamental for understanding process-induced C impurity doping in MOCVD-grown 2D semiconductors and might have important implications for the functionality and performance of (opto)electronic devices. ©

View abstract
Metallic Diluted Dimerization in VO2 Tweeds
Sandiumenge F., Rodríguez L., Pruneda M., Magén C., Santiso J., Catalan G. Advanced Materials; 33 (9, 2004374) 2021. 10.1002/adma.202004374. IF: 30.849

The observation of electronic phase separation textures in vanadium dioxide, a prototypical electron-correlated oxide, has recently added new perspectives on the long standing debate about its metal–insulator transition and its applications. Yet, the lack of atomically resolved information on phases accompanying such complex patterns still hinders a comprehensive understanding of the transition and its implementation in practical devices. In this work, atomic resolution imaging and spectroscopy unveils the existence of ferroelastic tweed structures on ≈5 nm length scales, well below the resolution limit of currently used spectroscopic imaging techniques. Moreover, density functional theory calculations show that this pretransitional fine-scale tweed, which on average looks and behaves like the standard metallic rutile phase, is in fact weaved by semi-dimerized chains of vanadium in a new monoclinic phase that represents a structural bridge to the monoclinic insulating ground state. These observations provide a multiscale perspective for the interpretation of existing data, whereby phase coexistence and structural intermixing can occur all the way down to the atomic scale. © 2021 Wiley-VCH GmbH

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Orbital occupancy and hybridization in strained SrV O3 epitaxial films
Mirjolet M., Vasili H.B., Valadkhani A., Santiso J., Borisov V., Gargiani P., Valvidares M., Valentí R., Fontcuberta J. Physical Review Materials; 5 (9, 095002) 2021. 10.1103/PhysRevMaterials.5.095002. IF: 3.989

Oxygen packaging in transition metal oxides determines the metal-oxygen hybridization and electronic occupation at metal orbitals. Strontium vanadate (SrVO3), having a single electron in a 3d orbital, is thought to be the simplest example of strongly correlated metallic oxides. Here, we determine the effects of epitaxial strain on the electronic properties of SrVO3 thin films, where the metal-oxide sublattice is corner connected. Using x-ray absorption and x-ray linear dichroism at the VL2,3 and O K edges, it is observed that tensile or compressive epitaxial strain change the hierarchy of orbitals within the t2g and eg manifolds. Data show a remarkable 2p-3d hybridization, as well as a strain-induced reordering of the V3d(t2g,eg) orbitals. The latter is itself accompanied by a consequent change of hybridization that modulates the hybrid π∗ and σ∗ orbitals and the carrier population at the metal ions, challenging a rigid band picture. © 2021 American Physical Society.

View abstract
Pulsed laser deposition of epitaxial non-doped PbTiO3 thin films from Pbo-TiO2 mosaic targets
Sakai J., Roque J.M.C., Vales-Castro P., Padilla-Pantoja J., Sauthier G., Santiso J. Coatings; 11 (6, 662) 2021. 10.3390/coatings11060662. IF: 2.881

PbTiO3 (PTO) suffers from difficulty in preparing high-density robust bulk ceramics, which in turn has been a bottleneck in thin films growth with physical vapor deposition (PVD) methods. In the present work, we prepared non-doped PTO thin films by a pulsed laser deposition (PLD) method with either a single PTO target or a mosaic target consisting of PbO and TiO2 pie-shaped pieces. On the PTO single target, laser irradiation caused selective ablation of Pb, resulting in Tirich cone-shaped pillar structure on the surface, whereas the irradiated surface of PbO and TiO2 pieces was smoother. Epitaxial PTO films deposited on SrTiO3 (001) substrates from the pie-chart targets with PbO:TiO2 areal ratio from 3:5 to 5:3 resulted in composition, crystallinity, flatness, and ferroelectric properties almost independent of the areal ratio. The averaged composition of each film was close to stoichiometric, suggesting a compositional self-control mechanism. For growing epitaxial and high-quality non-doped PTO films, a PbO-TiO2 pie-chart target is advantageous in easiness of handling and stable surface structure. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.

View abstract
Strong strain gradients and phase coexistence at the metal-insulator transition in VO2 epitaxial films
Rodríguez L., Sandiumenge F., Frontera C., Caicedo J.M., Padilla J., Catalán G., Santiso J. Acta Materialia; 220 (117336) 2021. 10.1016/j.actamat.2021.117336. IF: 8.203

The proximity of a thermodynamic triple point and the formation of transient metastable phases may result in complex phase and microstructural trajectories across the metal-insulator transition in strained VO2 films. A detailed analysis using in-situ synchrotron X-ray diffraction unveils subtle fingerprints of this complexity in the structure of epitaxial films. During phase transition the low-temperature monoclinic M1 phase is constrained along the {111}R planes by the coexisting high-temperature R phase domains, which remain epitaxially clamped to the substrate. This geometrical constraint induces counteracting local stresses that result in a combined tilt and uniaxial in-plane compression of M1 domains, and a concomitant anomalous cR-axis elongation. This mechanism progressively transforms the M1 phase into the transitional triclinic phase (T), and ultimately into the monoclinic M2 phase, generating strong strain and tilt gradients that remain frozen after the complete transformation of the R phase upon cooling to RT. The transformation path of VO2 films, the complex competition between stable and metastable VO2 polymorphs and its impact on the structure of the low temperature monoclinic state, provide essential insights for understanding the electronic and mechanical properties of the films at the nanoscale, as well as to control their use in functional devices. © 2021

View abstract
The effect of Ni and Fe on the decomposition of yttrium doped barium zirconate thin films
Jennings D., Ricote S., Caicedo J.M., Santiso J., Reimanis I. Scripta Materialia; 201 (113948) 2021. 10.1016/j.scriptamat.2021.113948. IF: 5.611

Transition metal dopants like Ni and Fe are known to influence the densification and microstructure evolution of yttrium doped barium zirconate (BZY), and understanding their behavior impacts the use of BZY in applications like protonic ceramic fuel cells and catalysts. This work investigates the effects of Ni and Fe on the evolution of BZY thin films in high temperature (1175°C), reducing environments, where BZY faceting and decomposition is observed. It is shown that the addition of Ni promotes film decomposition, whereas Fe prevents decomposition. The effects of the dopants on film decomposition are consistent with thermodynamic predictions that indicate the addition of Fe to the B-site of the perovskite structure diminishes the Gibbs free energy change for decomposition. © 2021

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2020

Beating the Thermal Conductivity Alloy Limit Using Long-Period Compositionally Graded Si1- xGe xSuperlattices
Ferrando-Villalba P., Chen S., Lopeandía A.F., Alvarez F.X., Alonso M.I., Garriga M., Santiso J., Garcia G., Goñi A.R., Donadio D., Rodríguez-Viejo J. Journal of Physical Chemistry C; 124 (36): 19864 - 19872. 2020. 10.1021/acs.jpcc.0c06410. IF: 4.189

Superlattices with scattering mechanisms at multiple length scales efficiently scatter phonons at all relevant wavelengths and provide a convenient route to reduce thermal transport. Here, we show, both experimentally and by atomistic simulations, that SiGe superlattices with well-established compositional gradients and a sufficient number of interfaces exhibit extremely low thermal conductivity. Our results reveal that the thermal conductivity of long-period (30-50 nm) superlattices with thicknesses below 200 nm is still thickness-dependent and higher than that of the corresponding alloy thin film. Increasing the number of periods up to 16 has a strong impact on heat propagation, leading to thermal conductivity values below the thin-film alloy limit. Lattice dynamics calculations confirm that the reduced thermal conductivity stems from the simultaneous effects of mass scattering, graded interface scattering, and coherent interference from the lattice periodicity. This study provides a significant step forward in understanding the role of compositional gradients in heat transport across nanostructures. The strategy of employing long-period graded superlattices with extremely low thermal conductivities has great potential for micro- and nano-thermoelectric generation and cooling of Si-based devices. Copyright © 2020 American Chemical Society.

View abstract
Control of lateral composition distribution in graded films of soluble solid systems A1-xBx by partitioned dual-beam pulsed laser deposition
Sakai J., Roque J.M.C., Vales-Castro P., Padilla-Pantoja J., Sauthier G., Catalan G., Santiso J. Coatings; 10 (6, 540) 2020. 10.3390/COATINGS10060540. IF: 2.436

Lateral compositionally-graded thin films are powerful media for the observation of phase boundaries aswell as for high-throughputmaterials exploration.We herein propose amethod to prepare epitaxial lateral compositionally-graded films using a dual-beampulsed laser deposition (PLD)method with two targets separated by a partition. Tuning the ambient pressure and the partition-substrate gap makes it possible to control of the gradient length of the deposits at the small sizes (≤ 10 mm) suitable for commercial oxide single crystal substrates. A simple Monte Carlo simulation qualitatively reproduced the characteristic features of the lateral thickness distribution. To demonstrate this method, we prepared (1-x)PbTiO3-xPbZrO3 and (1-x)LaMnO3-xLa0.6Sr0.4MnO3 films with lateral composition gradient widths of 10 and 1 mm, respectively, with the partitioned dual PLD. © 2020 by the authors.

View abstract
Large thermoelectric power variations in epitaxial thin films of layered perovskite GdBaCo2O5.5±δwith a different preferred orientation and strain
Chatterjee A., Chavez-Angel E., Ballesteros B., Caicedo J.M., Padilla-Pantoja J., Leborán V., Sotomayor Torres C.M., Rivadulla F., Santiso J. Journal of Materials Chemistry A; 8 (38): 19975 - 19983. 2020. 10.1039/d0ta04781c. IF: 11.301

This work describes the growth of thin epitaxial films of the layered perovskite material GdBaCo2O5.5±δ(GBCO) on different single crystal substrates SrTiO3(STO), (LaAlO3)0.3(Sr2TaAlO6)0.7(LSAT) and LaAlO3(LAO) as an approach to study changes in the thermoelectric properties by means of the induced epitaxial strain. In addition to strain changes, the films grow with considerably different preferred orientations and domain microstructures: GBCO films on STO are purelyc-axis oriented (c⊥) with an average 0.18% in-plane tensile strain; GBCO on LSAT is composed of domains with a mixed orientation (c‖andc⊥) with an average 0.71% in-plane compressive strain; while on LAO it isb-axis oriented (c‖) with an average 0.89% in-plane compressive strain. These differences result in important cell volume changes, as well as in the orthorhombicity of thea-bplane of the GBCO structure, which in turn induce a change in the sign and temperature dependence of the thermopower, while the electrical conductivity remains almost unchanged. In general, compressively strained films show negativeSthermopower (n-type) while tensile strained films show a positiveS(p-type) at low temperatures, probing the adaptive nature of the GdBaCo2O5.5±δcompound. These results point to the spontaneous generation of oxygen vacancies to partially accommodate the epitaxial stress as the main cause for this effect. © The Royal Society of Chemistry 2020.

View abstract
Revealing Strain Effects on the Chemical Composition of Perovskite Oxide Thin Films Surface, Bulk, and Interfaces
van den Bosch C.A.M., Cavallaro A., Moreno R., Cibin G., Kerherve G., Caicedo J.M., Lippert T.K., Doebeli M., Santiso J., Skinner S.J., Aguadero A. Advanced Materials Interfaces; 7 (2, 1901440) 2020. 10.1002/admi.201901440. IF: 4.948

Understanding the effects of lattice strain on oxygen surface and diffusion kinetics in oxides is a controversial subject that is critical for developing efficient energy storage and conversion materials. In this work, high-quality epitaxial thin films of the model perovskite La0.5Sr0.5Mn0.5Co0.5O3− δ (LSMC), under compressive or tensile strain, are characterized with a combination of in situ and ex situ bulk and surface-sensitive techniques. The results demonstrate a nonlinear correlation of mechanical and chemical properties as a function of the operation conditions. It is observed that the effect of strain on reducibility is dependent on the “effective strain” induced on the chemical bonds. In-plain strain, and in particular the relative BO length bond, is the key factor controlling which of the B-site cation can be reduced preferentially. Furthermore, the need to use a set of complimentary techniques to isolate different chemically induced strain effects is proven. With this, it is confirmed that tensile strain favors the stabilization of a more reduced lattice, accompanied by greater segregation of strontium secondary phases and a decrease of oxygen exchange kinetics on LSMC thin films. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

View abstract
Self-Pixelation Through Fracture in VO2 Thin Films
Laura Rodríguez, Elena del Corro, Michele Conroy, Kalani Moore, Felip Sandiumenge, Neus Domingo, José Santiso, Gustau Catalan Acs Applied Electronic Materials; 2 (5): 1433 - 1439. 2020. 10.1021/acsaelm.0c00199. IF: 0.000
The formation of oriented barium carbonate from the decomposition of yttria-doped barium zirconate films
Jennings D., Ricote S., Santiso J., Reimanis I. Scripta Materialia; 186: 401 - 405. 2020. 10.1016/j.scriptamat.2020.05.028. IF: 5.079

Yttria doped barium zirconate (BZY) thin films show promise thanks to their high proton conductivities and their possibility for use in studying fundamental processes such as exsolution. This work demonstrates that highly oriented BZY thin (45 nm) films on (100) single crystal strontium titanate decompose into oriented barium carbonate rods and yttria stabilized zirconia when exposed to industrial grade Ar or pure CO2 at 800 °C. It is shown with transmission electron microscopy that the rods nucleate on the BZY surface. The causes and modes of decomposition in these thin films are discussed in detail. © 2020 Acta Materialia Inc.

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2019

Bipolar “table with legs” resistive switching in epitaxial perovskite heterostructures
Bagdzevicius S., Boudard M., Caicedo J.M., Mescot X., Rodríguez-Lamas R., Santiso J., Burriel M. Solid State Ionics; 334: 29 - 35. 2019. 10.1016/j.ssi.2019.01.027. IF: 2.886

We report the experimental investigation of bipolar resistive switching with “table with legs” shaped hysteresis switching loops in epitaxial perovskite GdBaCo 2 O 5+δ /LaNiO 3 bilayers deposited by pulsed laser deposition. The possibility of varying the resistivity of GdBaCo 2 O 5+δ by changing its oxygen content allowed engineering this perovskite heterostructure with controlled interfaces creating two symmetric junctions. It has been proved that the resistance state of the device can be reproducibly varied by both continuous voltage sweeps and by electrical pulses. The symmetric devices show slightly non-symmetric resistance profiles, which can be explained by a valence change resistive switching model, and presented promising multilevel properties required for novel memories and neuromorphic computing. © 2019

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Enhanced thermoelectric properties of lightly Nb doped SrTiO3 thin films
Bhansali S., Khunsin W., Chatterjee A., Santiso J., Abad B., Martin-Gonzalez M., Jakob G., Sotomayor Torres C.M., Chávez-Angel E. Nanoscale Advances; 1 (9): 3647 - 3653. 2019. 10.1039/c9na00361d. IF: 0.000

Novel thermoelectric materials developed for operation at room temperature must have similar or better performance along with being as ecofriendly as those commercially used, e.g., Bi2Te3, in terms of their toxicity and cost. In this work, we present an in-depth study of the thermoelectric properties of epitaxial Nb-doped strontium titanate (SrTi1-xNbxO3) thin films as a function of (i) doping concentration, (ii) film thickness and (iii) substrate type. The excellent crystal quality was confirmed by high resolution transmission electron microscopy and X-ray diffraction analysis. The thermoelectric properties were measured by the three-omega method (thermal conductivity) and van der Pauw method (electrical resistivity), complemented by Seebeck coefficient measurements. A maximum power factor of 8.9 × 10-3 W m-1 K-2 and a thermoelectric figure of merit of 0.49 were measured at room temperature in 50 nm-thick films grown on lanthanum strontium aluminate. The mechanisms behind this high figure of merit are discussed in terms of a possible two-dimensional electron gas, increase of the effective mass of the electrons, electron filtering and change in strain due to different substrates. The overall enhancement of the thermoelectric properties suggests that SrTi1-xNbxO3 is a very promising n-type candidate for room- to high-temperature applications. © 2019 The Royal Society of Chemistry.

View abstract
Independent Tuning of Optical Transparency Window and Electrical Properties of Epitaxial SrVO3 Thin Films by Substrate Mismatch
Mirjolet M., Vasili H.B., López-Conesa L., Estradé S., Peiró F., Santiso J., Sánchez F., Machado P., Gargiani P., Valvidares M., Fontcuberta J. Advanced Functional Materials; 29 (37, 1904238) 2019. 10.1002/adfm.201904238. IF: 15.621

Transparent metallic oxides are pivotal materials in information technology, photovoltaics, or even in architecture. They display the rare combination of metallicity and transparency in the visible range because of weak interband photon absorption and weak screening of free carriers to impinging light. However, the workhorse of current technology, indium tin oxide (ITO), is facing severe limitations and alternative approaches are needed. AMO3 perovskites, M being a nd1 transition metal, and A an alkaline earth, have a genuine metallic character and, in contrast to conventional metals, the electron–electron correlations within the nd1 band enhance the carriers effective mass (m*) and bring the transparency window limit (marked by the plasma frequency, ωp*) down to the infrared. Here, it is shown that epitaxial strain and carrier concentration allow fine tuning of optical properties (ωp*) of SrVO3 films by modulating m* due to strain-induced selective symmetry breaking of 3d-t2g(xy, yz, xz) orbitals. Interestingly, the DC electrical properties can be varied by a large extent depending on growth conditions whereas the optical transparency window in the visible is basically preserved. These observations suggest that the harsh conditions required to grow optimal SrVO3 films may not be a bottleneck for their future application. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Nanostructural changes upon substitutional Al doping in ZnO sputtered films
Landa-Cánovas A.R., Santiso J., Agulló-Rueda F., Herrero P., Navarrete-Astorga E., Ochoa-Martínez E., Ramos-Barrado J.R., Gabás M. Ceramics International; 45 (5): 6319 - 6327. 2019. 10.1016/j.ceramint.2018.12.116. IF: 3.450

Al:ZnO layers, with low and high Al content, 0.2% and 2.1% cat. respectively, have been prepared using the RF magnetron sputtering technique. Noticeable differences in the optical and electrical properties have been detected in these films. With doping, the resistivity decreases and the band-gap increases. The alterations in the films crystalline structure are explained in terms of the nanostructural changes induced by Al substitutional doping, such as a higher concentration of edge dislocation defects and a higher rotation of crystalline nanodomains in the plane of the films (normal to the preferential orientation c-axis) for the high content Al:ZnO layer. A complete description of such effects has been accomplished using several characterization techniques, such as X-ray diffraction, Raman spectroscopy and transmission electron microscopy. The combination of these techniques provides an exhaustive understanding of the films nanostructure. © 2018 Elsevier Ltd and Techna Group S.r.l.

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Superposition of interface and volume type resistive switching in perovskite nanoionic devices
Bagdzevicius S., Boudard M., Caicedo J.M., Rapenne L., Mescot X., Rodríguez-Lamas R., Robaut F., Santiso J., Burriel M. Journal of Materials Chemistry C; 7 (25): 7580 - 7592. 2019. 10.1039/c9tc00609e. IF: 6.641

The microelectronics industry is currently searching for reliable redox-based resistive switching (RS) memories which are filament-free, scale with the electrode size and do not require a high voltage electroforming step. Interface and volume type switching devices are the most promising memristors to achieve these challenging requirements, both for ReRAM (Resistive Random-Access Memories) non-volatile memory and neuromorphic computing applications. Here RS was investigated for the first time in nanoionic memristors based on GdBaCo2O5+δ (GBCO), an oxide with high oxygen mobility. Non-filamentary and non-volatile reproducible RS was obtained when GBCO is sandwiched between Ag and LaNiO3 (LNO) electrodes. The observed bipolar RS could be successfully induced both by voltage sweeps and by pulses, showing asymmetric clock-wise hysteretic R(V) characteristics at room temperature. The temperature dependence of two independent devices in high and low resistance states (HRS and LRS) revealed a gradual decrease of the resistance difference between the two states on cooling from room temperature to 150 K and its increase below 100 K. Similarly, the R(V) switching curves obtained at low temperature showed the disappearance of the hysteresis at 150 K and its reappearance at lower temperatures. The superposition of volume and interface type RS mechanisms have proven to be responsible for the observed non-volatile change of the remnant resistance. The volume-type RS was related to the variation of the GBCO resistivity due to a change in oxygen content. The interface-type RS, on the other hand, was associated to the created electronic and ionic conduction barrier between GBCO and the LNO bottom electrode. © 2019 The Royal Society of Chemistry.

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Thin film oxide-ion conducting electrolyte for near room temperature applications
Garbayo I., Chiabrera F., Alayo N., Santiso J., Morata A., Tarancón A. Journal of Materials Chemistry A; 7 (45): 25772 - 25778. 2019. 10.1039/c9ta07632h. IF: 10.733

Stabilized bismuth vanadate thin films are presented here as superior oxide ionic conductors, for application in solid state electrochemical devices operating near room temperature. Widely studied in the 90s in bulk form due to their unbeatable ionic conduction, this family of materials was finally discarded due to poor stability above 500 °C. Here, we however unveil the possibility of using Bi4V1.8Cu0.2O10.7 at reduced temperatures in thin film-based devices, where the material keeps its unmatched conduction properties and at the same time shows good stability over a wide oxygen partial pressure range. © 2019 The Royal Society of Chemistry.

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2018

An in operando study of chemical expansion and oxygen surface exchange rates in epitaxial GdBaCo2O5.5 electrodes in a solid-state electrochemical cell by time-resolved X-ray diffraction
Chatterjee A., Caicedo J.M., Ballesteros B., Santiso J. Journal of Materials Chemistry A; 6 (26): 12430 - 12439. 2018. 10.1039/c8ta02790k. IF: 9.931

This report explores the fundamental characteristics of epitaxial thin films of a mixed ionic electronic conducting GdBaCo2O5.5±δ (GBCO) material with a layered perovskite structure, relevant for use as an active electrode for the oxygen reduction and evolution reactions in electrochemical devices. Time-resolved X-ray diffraction in combination with voltage step chrono-amperometric measurements in a solid state electrochemical cell provides a deeper insight into the chemical expansion mechanism in the GBCO electrode. The chemical expansion coefficient along the c-axis, αc, shows a negative value upon the compound oxidation contrary to standard perovskite materials with disordered oxygen vacancies. Chemical expansion also shows a remarkable asymmetry from αc = -0.037 to -0.014 at δ < 0 and δ > 0, respectively. This change in chemical expansion is an indication of a different mechanism of the structural changes associated with the variable Co cation oxidation state from Co2+ → Co3+ → Co4+. Since redox reactions are dominated by oxygen surface exchange between the GBCO electrode and gas atmosphere, monitoring the time response of the structural changes allows for direct determination of oxygen reduction and evolution reaction kinetics. The reaction kinetics are progressively slowed down upon reduction in the δ < 0 oxygen stoichiometry region, while they maintain a high catalytic activity in the δ > 0 region, in agreement with the structural changes and the electronic carrier delocalization when crossing δ = 0. This work validates the time-resolved XRD technique for fast and reversible measurements of electrode activity in a wide range of oxygen non-stoichiometry in a solid-state electrochemical cell operating under realistic working conditions. © 2018 The Royal Society of Chemistry.

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Atomic resolution imaging and quantitative elemental mapping of the misfit dislocation core phase in multicomponent oxides
Bagués N., Santiso J., Esser B.D., Williams R.E.A., McComb D.W., Konstantinovic Z., Balcells Ll., Sandiumenge F. Microscopy and Microanalysis; 24 (S1): 24 - 25. 2018. 10.1017/S1431927618000612. IF: 2.124

The relaxation mechanisms of lattice mismatched heteroepitaxial films may accommodate defects, such as misfit dislocations (MDs). Such MDs are located at the interface between two different compounds and can be considered linear defects exhibiting their own distinct structural, chemical, and physical properties. Recent studies have highlighted the strong potential of buried and laterally ordered MD networks in creating a variety of periodic strain-induced functional patterns in oxide thin films [1–3]. These recent findings have motivated analysis of the atomic and chemical structure of the MD phase in complex oxides,in view of the development of functional dislocation-based nanostructures. In this work, we combine imaging and spectroscopic techniques to determine the complex structure of MDs in the perovskite type La0.67Sr0.33MnO3/LaAlO3 (LSMO/LAO) heteroepitaxial system. The atomic structure of the core and surrounding area of the MDs is analyzed by high resolution (HR) transmission electron microscopy (TEM) and aberration-corrected scanning (S)TEM, as well as its chemical and electronic configuration by means of energy dispersive x-ray (EDS) spectral mapping and electron energy-loss (EELS) spectrum images. A high angle annular dark field (HAADF) STEM image of a MD core at the LSMO-LAO interface is shown in Figure 1. This MD is characterized by a a[010] Burgers vector parallel to the LSMO-LAO interface, where a is the LAO lattice parameter. A careful analysis of the HR-TEM and HR-HAADF data reveals that the dislocation core is dissociated and presents two extra half-planes separated by one unit cell. The strain field associated with this dislocation is characterized by tensile and compressive regions extending above and below the dislocation glide plane, which is parallel to LSMO-LAOinterface. The HAADF images of the MD core indicate the formation of a La୑୬୶ anti-site defect in the tensile zone to accommodate the tensile strain. The atomic resolution EDS and EELS analyses reveal that, while the position of the LSMO-LAO interface is blurred by cation intermixing, oxygen vacancies selectively accumulate in the tensile region of the dislocation strain field. Such accumulation of vacancies is accompanied by the reduction of Mn cations in the same area. The imbalance between the concentration of oxygen vacancies and the rate of Sr diffusion out of the core results in a positive net charge q ≈ +0.3 ± 0.1 localized in the tensile region of the dislocation, while the compressive region remains neutral. The basic mechanisms involve in the distribution of cations and oxygen vacancies around the MD core are schematically summarized in the right panel of Figure 1. These results highlight a prototypical core model for perovskite-based heteroepitaxial systems and offer insights into predictive manipulation of MD properties [4, 5].

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Growth and structural characterization of strained epitaxial H f0.5 Z r0.5 O2 thin films
Torrejón L., Langenberg E., Magén C., Larrea Á., Blasco J., Santiso J., Algarabel P.A., Pardo J.A. Physical Review Materials; 2 (1, 013401) 2018. 10.1103/PhysRevMaterials.2.013401.

Ferroelectricity was recently reported in thin films with several compositions in the HfO2-ZrO2 system with orthorhombic crystal structure. In the present paper we study the growth by pulsed laser deposition and the structural characterization of strained epitaxial Hf0.5Zr0.5O2 films on (001)-oriented yttria-stabilized zirconia (YSZ) substrates. We have determined the conditions for the coherent growth and correlated the deposition parameters with the films structure and microstructure studied through a combination of x-ray diffraction, electron backscatter diffraction, and scanning transmission electron microscopy. In the range of experimental parameters explored, all the films show monoclinic structure with distorted lattice parameters relative to bulk. © 2018 American Physical Society.

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Optimisation of growth parameters to obtain epitaxial Y-doped BaZrO3 proton conducting thin films
Magrasó A., Ballesteros B., Rodríguez-Lamas R., Sunding M.F., Santiso J. Solid State Ionics; 314: 9 - 16. 2018. 10.1016/j.ssi.2017.11.002. IF: 2.751

We hereby report developments on the fabrication and characterization of epitaxial thin films of proton conducting Y-doped BaZrO3 (BZY) by pulsed laser deposition (PLD) on different single crystal substrates (MgO, GdScO3, SrTiO3, NdGaO3, LaAlO3 and sapphire) using Ni-free and 1% Ni-containing targets. Pure, high crystal quality epitaxial films of BZY are obtained on MgO and on perovskite-type substrates, despite the large lattice mismatch. The deposition conditions influence the morphology, cell parameters and chemical composition of the film, the oxygen partial pressure during film growth being the most determining. Film characterization was carried out using X-ray diffraction, transmission electron and atomic force microscopies, wavelength dispersive X-ray spectroscopy and angle-resolved X-ray photoelectron spectroscopy. All films show a slight tetragonal distortion that is not directly related to the substrate-induced strain. The proton conductivity of the films depends on deposition conditions and film thickness, and for the optimised conditions its total conductivity is slightly higher than the bulk conductivity of the target material (3 mS/cm at 600 °C, in wet 5% H2/Ar). The conductivities are, however, more than one order of magnitude lower than the highest reported in literature and possible reasoning is elucidated in terms of local and extended defects in the films. © 2017 Elsevier B.V.

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The Misfit Dislocation Core Phase in Complex Oxide Heteroepitaxy
Bagués N., Santiso J., Esser B.D., Williams R.E.A., McComb D.W., Konstantinovic Z., Balcells L., Sandiumenge F. Advanced Functional Materials; 28 (8, 1704437) 2018. 10.1002/adfm.201704437. IF: 13.325

Misfit dislocations form self-organized nanoscale linear defects exhibiting their own distinct structural, chemical, and physical properties which, particularly in complex oxides, hold a strong potential for the development of nanodevices. However, the transformation of such defects from passive into potentially active functional elements necessitates a deep understanding of the particular mechanisms governing their formation. Here, different atomic resolution imaging and spectroscopic techniques are combined to determine the complex structure of misfit dislocations in the perovskite type La0.67Sr0.33MnO3/LaAlO3 heteroepitaxial system. It is found that while the position of the film–substrate interface is blurred by cation intermixing, oxygen vacancies selectively accumulate at the tensile region of the dislocation strain field. Such accumulation of vacancies is accompanied by the reduction of manganese cations in the same area, inducing chemical expansion effects, which partly accommodate the dislocation strain. The formation of oxygen vacancies is only partially electrically compensated and results in a positive net charge q ≈ +0.3 ± 0.1 localized in the tensile region of the dislocation, while the compressive region remains neutral. The results highlight a prototypical core model for perovskite-based heteroepitaxial systems and offer insights for predictive manipulation of misfit dislocation properties. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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2017

Domain wall magnetoresistance in BiFeO3 thin films measured by scanning probe microscopy
Domingo N., Farokhipoor S., Santiso J., Noheda B., Catalan G. Journal of Physics Condensed Matter; 29 (33, 334003) 2017. 10.1088/1361-648X/aa7a24. IF: 2.678

We measure the magnetotransport properties of individual 71° domain walls in multiferroic BiFeO3 by means of conductive-atomic force microscopy (C-AFM) in the presence of magnetic fields up to one Tesla. The results suggest anisotropic magnetoresistance at room temperature, with the sign of the magnetoresistance depending on the relative orientation between the magnetic field and the domain wall plane. A consequence of this finding is that macroscopically averaged magnetoresistance measurements for domain wall bunches are likely to underestimate the magnetoresistance of each individual domain wall. © 2017 IOP Publishing Ltd.

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On the persistence of polar domains in ultrathin ferroelectric capacitors
Zubko P., Lu H., Bark C.-W., Martí X., Santiso J., Eom C.-B., Catalan G., Gruverman A. Journal of Physics Condensed Matter; 29 (28, 284001) 2017. 10.1088/1361-648X/aa73c3. IF: 2.678

The instability of ferroelectric ordering in ultra-thin films is one of the most important fundamental issues pertaining realization of a number of electronic devices with enhanced functionality, such as ferroelectric and multiferroic tunnel junctions or ferroelectric field effect transistors. In this paper, we investigate the polarization state of archetypal ultrathin (several nanometres) ferroelectric heterostructures: epitaxial single-crystalline BaTiO3 films sandwiched between the most habitual perovskite electrodes, SrRuO3, on top of the most used perovskite substrate, SrTiO3. We use a combination of piezoresponse force microscopy, dielectric measurements and structural characterization to provide conclusive evidence for the ferroelectric nature of the relaxed polarization state in ultrathin BaTiO3 capacitors. We show that even the high screening efficiency of SrRuO3 electrodes is still insufficient to stabilize polarization in SrRuO3/BaTiO3/SrRuO3 heterostructures at room temperature. We identify the key role of domain wall motion in determining the macroscopic electrical properties of ultrathin capacitors and discuss their dielectric response in the light of the recent interest in negative capacitance behaviour. © 2017 IOP Publishing Ltd.

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2016

Fabrication of phononic crystals on free-standing silicon membranes
Sledzinska M., Graczykowski B., Alzina F., Santiso Lopez J., Sotomayor Torres C.M. Microelectronic Engineering; 149: 41 - 45. 2016. 10.1016/j.mee.2015.09.004. IF: 1.277

Free-standing Si films have been and remain an excellent example to study experimentally the effect of the reduction of the characteristic size on the phonon dispersion relation. A step further in geometrical complexity and, therefore, in increasing the control and manipulation of phonons is achieved by introducing periodicity in the medium to form phononic crystals. Here we report on the development of the fabrication process of large-area, solid-air and solid-solid two-dimensional phononic crystals, directly on free-standing, single crystalline silicon membranes. The patterning of the membranes involved electron-beam lithography and reactive ion etching for holes or metal evaporation and lift-off for pillars. The fabrication was possible due to the external strain induced on the membrane in order to reduce the buckling, which is typically found in large area free-standing structures. As a result, we obtained 250 nm thick structured membranes with patterned areas up to 100 × 100 μm, feature size between 100 and 300 nm and periodicity between 300 and 500 nm. The changes in dispersion relations of hypersonic acoustic phonons due to nanopatterning in free-standing silicon membranes were measured by Brillouin light scattering and the results were compared with numerical calculations by finite elements method. Information on phonon dispersion relation combined with a reliable fabrication process for large-scale structures opens a way for phonon engineering in more complex devices. © 2015 Elsevier B.V. All rights reserved.

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Misfit Dislocation Guided Topographic and Conduction Patterning in Complex Oxide Epitaxial Thin Films
Sandiumenge F., Bagués N., Santiso J., Paradinas M., Pomar A., Konstantinovic Z., Ocal C., Balcells L., Casanove M.-J., Martínez B. Advanced Materials Interfaces; 3 (14, 1600106) 2016. 10.1002/admi.201600106. IF: 3.365

Interfacial dissimilarity has emerged in recent years as the cornerstone of emergent interfacial phenomena, while enabling the control of electrical transport and magnetic behavior of complex oxide epitaxial films. As a step further toward the lateral miniaturization of functional nanostructures, this work uncovers the role of misfit dislocations in creating periodic surface strain patterns that can be efficiently used to control the spatial modulation of mass transport phenomena and bandwidth-dependent properties on a ≈20 nm length scale. The spontaneous formation of surface strain-relief patterns in La0.7Sr0.3MnO3/LaAlO3 films results in lateral periodic modulations of the surface chemical potential and tetragonal distortion, controlling the spatial distribution of preferential nucleation sites and the bandwidth of the epilayer, respectively. These results provide insights into the spontaneous formation of strain-driven ordered surface patterns, topographic and functional, during the growth of complex oxide heterostructures on lengths scales far below the limits achievable through top-down approaches. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Self-Arranged Misfit Dislocation Network Formation upon Strain Release in La0.7Sr0.3MnO3/LaAlO3(100) Epitaxial Films under Compressive Strain
Santiso J., Roqueta J., Bagués N., Frontera C., Konstantinovic Z., Lu Q., Yildiz B., Martínez B., Pomar A., Balcells L., Sandiumenge F. ACS Applied Materials and Interfaces; 8 (26): 16823 - 16832. 2016. 10.1021/acsami.6b02896. IF: 7.145

Lattice-mismatched epitaxial films of La0.7Sr0.3MnO3 (LSMO) on LaAlO3 (001) substrates develop a crossed pattern of misfit dislocations above a critical thickness of 2.5 nm. Upon film thickness increases, the dislocation density progressively increases, and the dislocation spacing distribution becomes narrower. At a film thickness of 7.0 nm, the misfit dislocation density is close to the saturation for full relaxation. The misfit dislocation arrangement produces a 2D lateral periodic structure modulation (Λ≈ 16 nm) alternating two differentiated phases: one phase fully coherent with the substrate and a fully relaxed phase. This modulation is confined to the interface region between film and substrate. This phase separation is clearly identified by X-ray diffraction and further proven in the macroscopic resistivity measurements as a combination of two transition temperatures (with low and high Tc). Films thicker than 7.0 nm show progressive relaxation, and their macroscopic resistivity becomes similar than that of the bulk material. Therefore, this study identifies the growth conditions and thickness ranges that facilitate the formation of laterally modulated nanocomposites with functional properties notably different from those of fully coherent or fully relaxed material. © 2016 American Chemical Society.

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Strain-induced perpendicular magnetic anisotropy in L a2CoMn O6-É thin films and its dependence on film thickness
Galceran R., López-Mir L., Bozzo B., Cisneros-Fernández J., Santiso J., Balcells L., Frontera C., Martínez B. Physical Review B; 93 (14, 144417) 2016. 10.1103/PhysRevB.93.144417.

Ferromagnetic insulating La2CoMnO6-É (LCMO) epitaxial thin films grown on top of SrTiO3 (001) substrates present a strong magnetic anisotropy favoring the out-of-plane (OP) orientation of the magnetization with a large anisotropy field (∼70 kOe for film thickness of about 15 nm). Diminishing oxygen off-stoichiometry of the film enhances the anisotropy. We attribute this to the concomitant shrinkage of the OP cell parameter and to the increasing of the tensile strain of the films. Consistently, LCMO films grown on (LaAlO3)0.3(Sr2AlTaO6)0.7 and LaAlO3 substrates (with a larger OP lattice parameter and compressive stress) display in-plane (IP) magnetic anisotropy. Thus, we link the strong magnetic anisotropy observed in LCMO to the film stress: tensile strain favors perpendicular anisotropy, and compressive stress favors IP anisotropy. We also report on the thickness dependence of the magnetic properties. Perpendicular anisotropy, saturation magnetization, and Curie temperature are maintained over a large range of film thickness. © 2016 American Physical Society.

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Thermodynamic conditions during growth determine the magnetic anisotropy in epitaxial thin-films of La0.7Sr0.3MnO3
Vila-Fungueiriño J.M., Bui C.T., Rivas-Murias B., Winkler E., Milano J., Santiso J., Rivadulla F. Journal of Physics D: Applied Physics; 49 (31, 315001) 2016. 10.1088/0022-3727/49/31/315001.

The suitability of a particular material for use in magnetic devices is determined by the process of magnetization reversal/relaxation, which in turn depends on the magnetic anisotropy. Therefore, designing new ways to control magnetic anisotropy in technologically important materials is highly desirable. Here we show that magnetic anisotropy of epitaxial thin-films of half-metallic ferromagnet La0.7Sr0.3MnO3 (LSMO) is determined by the proximity to thermodynamic equilibrium conditions during growth. We performed a series of x-ray diffraction and ferromagnetic resonance (FMR) experiments in two different sets of samples: the first corresponds to LSMO thin-films deposited under tensile strain on (0 0 1) SrTiO3 by pulsed laser deposition (PLD; far from thermodynamic equilibrium); the second were deposited by a slow chemical solution deposition (CSD) method, under quasi-equilibrium conditions. Thin films prepared by PLD show fourfold in-plane magnetic anisotropy, with an overimposed uniaxial term. However, the uniaxial anisotropy is completely suppressed in the CSD films. This change is due to a different rotation pattern of MnO6 octahedra to accommodate epitaxial strain, which depends not only on the amplitude of tensile stress imposed by the STO substrate, but also on the growth conditions. Our results demonstrate that the nature and magnitude of the magnetic anisotropy in LSMO can be tuned by the thermodynamic parameters during thin-film deposition. © 2016 IOP Publishing Ltd.

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2015

Enhanced conduction and ferromagnetic order at (100)-type twin walls in L a0.7 S r0.3Mn O3 thin films
Balcells L., Paradinas M., Baguès N., Domingo N., Moreno R., Galceran R., Walls M., Santiso J., Konstantinovic Z., Pomar A., Casanove M.-J., Ocal C., Martínez B., Sandiumenge F. Physical Review B - Condensed Matter and Materials Physics; 92 (7, 075111) 2015. 10.1103/PhysRevB.92.075111. IF: 3.736

There is increasing evidence supporting the strong potential of twin walls in ferroic materials as distinct, spatially tunable, functional elements in future electronic devices. Here, we report an increase of about one order of magnitude in conductivity and more robust magnetic interactions at (100)-type twin walls in La0.7Sr0.3MnO3 thin films. The nature and microscopic origin of such distinctive behavior is investigated by combining conductive, magnetic, and force modulation scanning force microscopies with transmission electron microscopy techniques. Our analyses indicate that the observed behavior is due to a severe compressive strained state within an ∼1nm slab of material centered at the twin walls, promoting stronger Mn 3d-O2p orbital overlapping leading to a broader bandwidth and enhanced magnetic interactions. © 2015 American Physical Society.

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Microstructure and ionic conductivity of LLTO thin films: Influence of different substrates and excess lithium in the target
Aguesse F., Roddatis V., Roqueta J., Garcia P., Pergolesi D., Santiso J., Kilner J.A. Solid State Ionics; 272: 1 - 8. 2015. 10.1016/j.ssi.2014.12.005. IF: 2.561

The deposition of single phase Li3xLa2/3 - xTiO3 (LLTO) thin films remains very challenging. The growth of the perovskite phase is in competition with the insulating La2Ti2O7 phase when prepared at high oxygen pressure by PLD. Nevertheless, we have achieved epitaxial growth of LLTO on different (001) oriented substrates such as LaAlO3, SrTiO3 and MgO despite a large lattice mismatch of up to + 8.8%. We also determined the percentage of lithium excess in the target necessary to reach a maximum ionic conductivity. However, the presence of the blocking La2Ti2O7 phase strongly hinders the lithium ion migration and reduces the total conductivity compared to bulk properties. © 2014 Elsevier B.V.

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Nanostructured Ti thin films by magnetron sputtering at oblique angles
Alvarez R., Garcia-Martin J.M., Garcia-Valenzuela A., Macias-Montero M., Ferrer F.J., Santiso J., Rico V., Cotrino J., Gonzalez-Elipe A.R., Palmero A. Journal of Physics D: Applied Physics; 49 (4, 045303) 2015. 10.1088/0022-3727/49/4/045303. IF: 2.721

The growth of Ti thin films by the magnetron sputtering technique at oblique angles and at room temperature is analysed from both experimental and theoretical points of view. Unlike other materials deposited in similar conditions, the nanostructure development of the Ti layers exhibits an anomalous behaviour when varying both the angle of incidence of the deposition flux and the deposition pressure. At low pressures, a sharp transition from compact to isolated, vertically aligned, nanocolumns is obtained when the angle of incidence surpasses a critical threshold. Remarkably, this transition also occurs when solely increasing the deposition pressure under certain conditions. By the characterization of the Ti layers, the realization of fundamental experiments and the use of a simple growth model, we demonstrate that surface mobilization processes associated to a highly directed momentum distribution and the relatively high kinetic energy of sputtered atoms are responsible for this behaviour. © 2016 IOP Publishing Ltd.

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Persistence of ferroelectricity above the Curie temperature at the surface of Pb(Z n1/3 N b2/3) O3-12%PbTi O3
Domingo N., Bagués N., Santiso J., Catalan G. Physical Review B - Condensed Matter and Materials Physics; 91 (9, 094111) 2015. 10.1103/PhysRevB.91.094111. IF: 3.736

Relaxor-based ferroelectrics have been known for decades to possess a relatively thick surface layer ("skin") that is distinct from its interior. Yet while there is consensus about its existence, there are controversies about its symmetry, phase stability, and origin. In an attempt to clarify these issues, we have examined the surface layer of PZN-12%PT. While the bulk transitions from a ferroelastically twinned tetragonal ferroelectric state with in-plane polarization to a cubic paraphase at Tc=200C, the skin layer shows a robust labyrinthine nanodomain structure with out-of-plane polarization that persists hundreds of degrees above the bulk Curie temperature. Cross-sectional transmission electron microscopy analysis shows that the resilience of the skin's polarization is correlated with a compositional imbalance: lead vacancies at the surface are charge-compensated by niobium enrichment; the excess of Nb5+ - a small ion with d0 orbital occupancy - stabilizes the ferroelectricity of the skin layer. © 2015 American Physical Society.

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Strain-Engineered Ferromagnetism in LaMnO3 Thin Films
Roqueta J., Pomar A., Balcells L., Frontera C., Valencia S., Abrudan R., Bozzo B., Konstantinović Z., Santiso J., Martínez B. Crystal Growth and Design; 15 (11): 5332 - 5337. 2015. 10.1021/acs.cgd.5b00884. IF: 4.891

A systematic study of the growth process of LaMnO3 (LMO) thin films, by pulsed laser deposition, on top of SrTiO3 substrates under different oxygen partial pressures (PO 2) is reported. It is found that the accommodation of the orthorhombic LMO phase onto the cubic STO structure, i.e., the amount of structural strain, is controlled by the background oxygen pressure. We demonstrate that magnetic behavior can be continuously tuned from robust ferromagnetic (FM) ordering to an antiferromagnet. These results strongly point to a strain-induced selective orbital occupancy as the origin of the observed FM behavior, in agreement with recent theoretical calculations. © 2015 American Chemical Society.

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Tailoring thermal conductivity by engineering compositional gradients in Si1−xGe x superlattices
Ferrando-Villalba P., Lopeandía A.F., Alvarez F.X., Paul B., de Tomás C., Alonso M.I., Garriga M., Goñi A.R., Santiso J., Garcia G., Rodriguez-Viejo J. Nano Research; 8 (9): 2833 - 2841. 2015. 10.1007/s12274-015-0788-9. IF: 7.010

The transport properties of artificially engineered superlattices (SLs) can be tailored by incorporating a high density of interfaces in them. Specifically, SiGe SLs with low thermal conductivity values have great potential for thermoelectric generation and nano-cooling of Si-based devices. Here, we present a novel approach for customizing thermal transport across nanostructures by fabricating Si/Si1−xGex SLs with well-defined compositional gradients across the SiGe layer from x = 0 to 0.60. We demonstrate that the spatial inhomogeneity of the structure has a remarkable effect on the heat-flow propagation, reducing the thermal conductivity to ∼2.2 W·m−1·K−1, which is significantly less than the values achieved previously with non-optimized long-period SLs. This approach offers further possibilities for future applications in thermoelectricity. [Figure not available: see fulltext.] © 2015, Tsinghua University Press and Springer-Verlag Berlin Heidelberg.

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2004

Crecimiento de películas delgadas de membranas de conducción iónica mediante la técnica PIMOCVD
Santiso, J.; Santiso J. Boletin de la Sociedad Espanola de Ceramica y Vidrio; 43 (2): 448 - 451. 2004. .