Oxide Nanophysics

Group Leader: Gustau Catalán

Publications

2024

Coexistence of ferroelectric and ferrielectric phases in ultrathin antiferroelectric PbZrO₃ thin films
Ying Liu; Ranming Niu; Roger Uriach; David Pesquera; José Manuel Caicedo; José Santiso; Julie M. Cairney; Xiaozhou Liao; Jordi Arbiol; Gustau Catalán Microstructures; 4 (4) 2024. 10.20517/microstructures.2024.12.
Flexoelectric Enhancement of Strain Gradient Elasticity Across a Ferroelectric-to-Paraelectric Phase Transition
Harbola, Varun; Pesquera, David; Xu, Ruijuan; Ashby, Paul D; Martin, Lane W; Hwang, Harold Y Nano Letters; 2024. 10.1021/acs.nanolett.4c02946. IF: 9.600
Flexophotovoltaic Effect and Above-Band-Gap Photovoltage Induced by Strain Gradients in Halide Perovskites
Wang, Zhiguo; Shu, Shengwen; Wei, Xiaoyong; Liang, Renhong; Ke, Shanming; Shu, Longlong; Catalan, Gustau Physical Review Letters; 132 (8): 086902. 2024. 10.1103/PhysRevLett.132.086902. IF: 8.100
Metal poles around the bend
Catalan, Gustau Nature Physics; 2024. 10.1038/s41567-024-02402-6. IF: 17.600
Non-Destructive Tomographic Nanoscale Imaging of Ferroelectric Domain Walls
He, Jiali; Zahn, Manuel; Ushakov, Ivan N; Richarz, Leonie; Ludacka, Ursula; Roede, Erik D; Yan, Zewu; Bourret, Edith; Kezsmarki, Istvan; Catalan, Gustau; Meier, Dennis Advanced Functional Materials; 2024. 10.1002/adfm.202314011. IF: 18.500
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. IF: 27.400
Switchable tribology of ferroelectrics
Cho, Seongwoo; Gaponenko, Iaroslav; Cordero-Edwards, Kumara; Barcelo-Mercader, Jordi; Arias, Irene; Kim, Daeho; Lichtensteiger, Celine; Yeom, Jiwon; Musy, Loic; Kim, Hyunji; Han, Seung Min; Catalan, Gustau; Paruch, Patrycja; Hong, Seungbum Nature Communications; 15 (1): 387. 2024. 10.1038/s41467-023-44346-0. IF: 14.700
Switching Dynamics and Improved Efficiency of Free-Standing Antiferroelectric Capacitors
Saeed, Umair; Pesquera, David; Liu, Ying; Fina, Ignasi; Ganguly, Saptam; Santiso, Jose; Padilla-Pantoja, Jessica; Roque, Jose Manuel Caicedo; Liao, Xiaozhou; Catalan, Gustau Advanced Electronic Materials; 2024. 10.1002/aelm.202400102. IF: 5.300

2023

Competition between Carrier Injection and Structural Distortions in Electron-Doped Perovskite Nickelate Thin Films
Hadjimichael, M; Mundet, B; Dominguez, C; Waelchli, A; De Luca, G; Spring, J; Johr, S; Walker, SM; Piamonteze, C; Alexander, DTL; Triscone, JM; Gibert, M Advanced Electronic Materials; 9 (5) 2023. 10.1002/aelm.202201182. IF: 6.200
Translational Boundaries as Incipient Ferrielectric Domains in Antiferroelectric PbZrO3
Liu, Y; Niu, RM; Majchrowski, A; Roleder, K; Cordero-Edwards, K; Cairney, JM; Arbiol, J; Catalan, G Physical Review Letters; 130 (21): 216801. 2023. 10.1103/PhysRevLett.130.216801. IF: 8.600
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
Weak low-temperature polarity in a PbZrO3 single crystal
Roleder, K; Catalan, G; Glazer, AM; Baker, JS; Ko, JH; Naqvi, FH; Junaid, SB; Majchrowski, A; Trybula, Z; Zareba, J; Lazar, I; Kajewski, D; Koperski, J; Soszynski, A Physical Review b; 107 (14): L140102. 2023. 10.1103/PhysRevB.107.L140102. IF: 3.700

2022

Enabling ultra-low-voltage switching in BaTiO3
Jiang Y., Parsonnet E., Qualls A., Zhao W., Susarla S., Pesquera D., Dasgupta A., Acharya M., Zhang H., Gosavi T., Lin C.-C., Nikonov D.E., Li H., Young I.A., Ramesh R., Martin L.W. Nature Materials; 21 (7): 779 - 785. 2022. 10.1038/s41563-022-01266-6.

Single crystals of BaTiO3 exhibit small switching fields and energies, but thin-film performance is considerably worse, thus precluding their use in next-generation devices. Here, we demonstrate high-quality BaTiO3 thin films with nearly bulk-like properties. Thickness scaling provides access to the coercive voltages (<100 mV) and fields (<10 kV cm−1) required for future applications and results in a switching energy of <2 J cm−3 (corresponding to <2 aJ per bit in a 10 × 10 × 10 nm3 device). While reduction in film thickness reduces coercive voltage, it does so at the expense of remanent polarization. Depolarization fields impact polar state stability in thicker films but fortunately suppress the coercive field, thus driving a deviation from Janovec–Kay–Dunn scaling and enabling a constant coercive field for films <150 nm in thickness. Switching studies reveal fast speeds (switching times of ~2 ns for 25-nm-thick films with 5-µm-diameter capacitors) and a pathway to subnanosecond switching. Finally, integration of BaTiO3 thin films onto silicon substrates is shown. We also discuss what remains to be demonstrated to enable the use of these materials for next-generation devices. © 2022, The Author(s), under exclusive licence to Springer Nature Limited.

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Freestanding complex-oxide membranes
Pesquera D., Fernández A., Khestanova E., Martin L.W. Journal of Physics Condensed Matter; 34 (38, 383001) 2022. 10.1088/1361-648X/ac7dd5.

Complex oxides show a vast range of functional responses, unparalleled within the inorganic solids realm, making them promising materials for applications as varied as next-generation field-effect transistors, spintronic devices, electro-optic modulators, pyroelectric detectors, or oxygen reduction catalysts. Their stability in ambient conditions, chemical versatility, and large susceptibility to minute structural and electronic modifications make them ideal subjects of study to discover emergent phenomena and to generate novel functionalities for next-generation devices. Recent advances in the synthesis of single-crystal, freestanding complex oxide membranes provide an unprecedented opportunity to study these materials in a nearly-ideal system (e.g. free of mechanical/thermal interaction with substrates) as well as expanding the range of tools for tweaking their order parameters (i.e. (anti-)ferromagnetic, (anti-)ferroelectric, ferroelastic), and increasing the possibility of achieving novel heterointegration approaches (including interfacing dissimilar materials) by avoiding the chemical, structural, or thermal constraints in synthesis processes. Here, we review the recent developments in the fabrication and characterization of complex-oxide membranes and discuss their potential for unraveling novel physicochemical phenomena at the nanoscale and for further exploiting their functionalities in technologically relevant devices. © 2022 IOP Publishing Ltd.

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Intrinsic flexoelectricity of van der Waals epitaxial thin films
Shu L., Wang Z., Liang R., Zhang Z., Shu S., Tang C., Li F., Zheng R.-K., Ke S., Catalan G. Physical Review B; 106 (2, 024108) 2022. 10.1103/PhysRevB.106.024108.

The direct measurement of flexoelectric coefficients in epitaxial thin films is an unresolved problem, due to the clamping effect of substrates which induces a net strain (and hence parasitic piezoelectricity) in addition to strain gradients and flexoelectricity. Herein, we propose and demonstrate the use of van der Waals epitaxy as a successful strategy for measuring the intrinsic (clamping-free = flexoelectric coefficients of epitaxial thin films. We have made, measured, and compared BaTiO3 and SrTiO3 thin film capacitor heterostructures grown both by conventional oxide-on-oxide epitaxy and by van der Waals oxide-on-mica epitaxy, and found that, whereas the former is dominated by parasitic piezoelectricity, the response of the latter is truly flexoelectric. The results are backed by theoretical calculations of the film-substrate mechanical interaction, as well as by direct measurements that confirm the strain-free state of the films. van der Waals epitaxy thus emerges as powerful new tool in the study of flexoelectricity and, in particular, they finally allow exploring flexoelectric phenomena at the nanoscale (where strain gradients are highest) with direct experimental knowledge of the actual flexoelectric coefficients of thin films. © 2022 American Physical Society.

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Progress on Emerging Ferroelectric Materials for Energy Harvesting, Storage and Conversion
Wei X.-K., Domingo N., Sun Y., Balke N., Dunin-Borkowski R.E., Mayer J. Advanced Energy Materials; 12 (24, 2201199) 2022. 10.1002/aenm.202201199.

Since the discovery of Rochelle salt a century ago, ferroelectric materials have been investigated extensively due to their robust responses to electric, mechanical, thermal, magnetic, and optical fields. These features give rise to a series of ferroelectric-based modern device applications such as piezoelectric transducers, memories, infrared detectors, nonlinear optical devices, etc. On the way to broaden the material systems, for example, from three to two dimensions, new phenomena of topological polarity, improper ferroelectricity, magnetoelectric effects, and domain wall nanoelectronics bear the hope for next-generation electronic devices. In the meantime, ferroelectric research has been aggressively extended to more diverse applications such as solar cells, water splitting, and CO2 reduction. In this review, the most recent research progress on newly emerging ferroelectric states and phenomena in insulators, ionic conductors, and metals are summarized, which have been used for energy storage, energy harvesting, and electrochemical energy conversion. Along with the intricate coupling between polarization, coordination, defect, and spin state, the exploration of transient ferroelectric behavior, ionic migration, polarization switching dynamics, and topological ferroelectricity, sets up the physical foundation ferroelectric energy research. Accordingly, the progress in understanding of ferroelectric physics is expected to provide insightful guidance on the design of advanced energy materials. © 2022 The Authors. Advanced Energy Materials published by Wiley-VCH GmbH.

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Terahertz displacive excitation of a coherent Raman-active phonon in V2O3
Giorgianni F., Udina M., Cea T., Paris E., Caputo M., Radovic M., Boie L., Sakai J., Schneider C.W., Johnson S.L. Communications Physics; 5 (1, 103) 2022. 10.1038/s42005-022-00882-7.

Nonlinear processes involving frequency-mixing of light fields set the basis for ultrafast coherent spectroscopy of collective modes in solids. In certain semimetals and semiconductors, generation of coherent phonon modes can occur by a displacive force on the lattice at the difference-frequency mixing of a laser pulse excitation on the electronic system. Here, as a low-frequency counterpart of this process, we demonstrate that coherent phonon excitations can be induced by the sum-frequency components of an intense terahertz light field, coupled to intraband electronic transitions. This nonlinear process leads to charge-coupled coherent dynamics of Raman-active phonon modes in the strongly correlated metal V2O3. Our results show an alternative up-conversion pathway for the optical control of Raman-active modes in solids mediated by terahertz-driven electronic excitation. © 2022, The Author(s).

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The emancipation of flexoelectricity
Arias I., Catalan G., Sharma P. Journal of Applied Physics; 131 (2, 020401) 2022. 10.1063/5.0079319.

[No abstract available]

View abstract
Top-Layer Engineering Reshapes Charge Transfer at Polar Oxide Interfaces
De Luca G., Spring J., Kaviani M., Jöhr S., Campanini M., Zakharova A., Guillemard C., Herrero-Martin J., Erni R., Piamonteze C., Rossell M.D., Aschauer U., Gibert M. Advanced Materials; 2022. 10.1002/adma.202203071.

Charge-transfer phenomena at heterointerfaces are a promising pathway to engineer functionalities absent in bulk materials but can also lead to degraded properties in ultrathin films. Mitigating such undesired effects with an interlayer reshapes the interface architecture, restricting its operability. Therefore, developing less-invasive methods to control charge transfer will be beneficial. Here, an appropriate top-interface design allows for remote manipulation of the charge configuration of the buried interface and concurrent restoration of the ferromagnetic trait of the whole film. Double-perovskite insulating ferromagnetic La2NiMnO6 (LNMO) thin films grown on perovskite oxide substrates are investigated as a model system. An oxygen-vacancy-assisted electronic reconstruction takes place initially at the LNMO polar interfaces. As a result, the magnetic properties of 2–5 unit cell LNMO films are affected beyond dimensionality effects. The introduction of a top electron-acceptor layer redistributes the electron excess and restores the ferromagnetic properties of the ultrathin LNMO films. Such a strategy can be extended to other interfaces and provides an advanced approach to fine-tune the electronic features of complex multilayered heterostructures. © 2022 The Authors. Advanced Materials published by Wiley-VCH GmbH.

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Voltage control of magnetism with magneto-ionic approaches: Beyond voltage-driven oxygen ion migration
De Rojas J., Quintana A., Rius G., Stefani C., Domingo N., Costa-Krämer J.L., Menéndez E., Sort J. Applied Physics Letters; 120 (7, 070501) 2022. 10.1063/5.0079762. IF: 3.791

Magneto-ionics is an emerging field in materials science where voltage is used as an energy-efficient means to tune magnetic properties, such as magnetization, coercive field, or exchange bias, by voltage-driven ion transport. We first discuss the emergence of magneto-ionics in the last decade, its core aspects, and key avenues of research. We also highlight recent progress in materials and approaches made during the past few years. We then focus on the "structural-ion"approach as developed in our research group in which the mobile ions are already present in the target material and discuss its potential advantages and challenges. Particular emphasis is given to the energetic and structural benefits of using nitrogen as the mobile ion, as well as on the unique manner in which ionic motion occurs in CoN and FeN systems. Extensions into patterned systems and textures to generate imprinted magnetic structures are also presented. Finally, we comment on the prospects and future directions of magneto-ionics and its potential for practical realizations in emerging fields, such as neuromorphic computing, magnetic random-access memory, or micro- and nano-electromechanical systems. © 2022 Author(s).

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2021

Approaching ultrathin VO2films on sapphire (001) substrates by biased reactive sputtering: Characteristic morphology and its effect on the infrared-light switching
Okimura K., Sakai J., Kuwahara M., Zaghrioui M., Uehara Y. Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films; 39 (4, 043401) 2021. 10.1116/6.0001023. IF: 2.427

Ultrathin VO2 films with insulator-metal transition (IMT) were successfully fabricated on sapphire (001) substrates by utilizing radio frequency-biased reactive sputtering. We realized a 6 nm-thick VO2 film that shows resistance change over 2 orders of magnitude. Microscopic observations combined with energy dispersive x-ray analyses revealed characteristic networking morphology in VO2 films with thickness up to around 10 nm. It was found through micro-Raman analyses that a 30 nm-thick film possessed flat surface and ordered lattice with strong in-plane tensile stress. We evaluated the thickness dependence of optical switching performance for infrared-light. The results suggest that the thickness of the VO2 films should be carefully selected for realizing required performances of optical switching, which depends on not only IMT but also characteristic morphological aspects. © 2021 Author(s).

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Avalanche criticality during ferroelectric/ferroelastic switching
Casals, B; Nataf, GF; Salje, EKH Nature Communications; 12 (1) 2021. 10.1038/s41467-020-20477-6. IF: 14.919
Correlating Surface Crystal Orientation and Gas Kinetics in Perovskite Oxide Electrodes
Gao R., Fernandez A., Chakraborty T., Luo A., Pesquera D., Das S., Velarde G., Thoréton V., Kilner J., Ishihara T., Nemšák S., Crumlin E.J., Ertekin E., Martin L.W. Advanced Materials; 33 (20, 2100977) 2021. 10.1002/adma.202100977. IF: 30.849

Solid–gas interactions at electrode surfaces determine the efficiency of solid-oxide fuel cells and electrolyzers. Here, the correlation between surface–gas kinetics and the crystal orientation of perovskite electrodes is studied in the model system La0.8Sr0.2Co0.2Fe0.8O3. The gas-exchange kinetics are characterized by synthesizing epitaxial half-cell geometries where three single-variant surfaces are produced [i.e., La0.8Sr0.2Co0.2Fe0.8O3/La0.9Sr0.1Ga0.95Mg0.05O3−δ/SrRuO3/SrTiO3 (001), (110), and (111)]. Electrochemical impedance spectroscopy and electrical conductivity relaxation measurements reveal a strong surface-orientation dependency of the gas-exchange kinetics, wherein (111)-oriented surfaces exhibit an activity >3-times higher as compared to (001)-oriented surfaces. Oxygen partial pressure ((Formula presented.))-dependent electrochemical impedance spectroscopy studies reveal that while the three surfaces have different gas-exchange kinetics, the reaction mechanisms and rate-limiting steps are the same (i.e., charge-transfer to the diatomic oxygen species). First-principles calculations suggest that the formation energy of vacancies and adsorption at the various surfaces is different and influenced by the surface polarity. Finally, synchrotron-based, ambient-pressure X-ray spectroscopies reveal distinct electronic changes and surface chemistry among the different surface orientations. Taken together, thin-film epitaxy provides an efficient approach to control and understand the electrode reactivity ultimately demonstrating that the (111)-surface exhibits a high density of active surface sites which leads to higher activity. © 2021 Wiley-VCH GmbH

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Deconvolution of Phonon Scattering by Ferroelectric Domain Walls and Point Defects in a PbTiO3Thin Film Deposited in a Composition-Spread Geometry
Bugallo D., Langenberg E., Ferreiro-Vila E., Smith E.H., Stefani C., Batlle X., Catalan G., Domingo N., Schlom D.G., Rivadulla F. ACS Applied Materials and Interfaces; 13 (38): 45679 - 45685. 2021. 10.1021/acsami.1c08758. IF: 9.229

We present a detailed analysis of the temperature dependence of the thermal conductivity of a ferroelectric PbTiO3 thin film deposited in a composition-spread geometry enabling a continuous range of compositions from ∼25% titanium deficient to ∼20% titanium rich to be studied. By fitting the experimental results to the Debye model we deconvolute and quantify the two main phonon-scattering sources in the system: ferroelectric domain walls (DWs) and point defects. Our results prove that ferroelectric DWs are the main agent limiting the thermal conductivity in this system, not only in the stoichiometric region of the thin film ([Pb]/[Ti] ≈ 1) but also when the concentration of the cation point defects is significant (up to ∼15%). Hence, DWs in ferroelectric materials are a source of phonon scattering at least as effective as point defects. Our results demonstrate the viability and effectiveness of using reconfigurable DWs to control the thermal conductivity in solid-state devices. © 2021 American Chemical Society.

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Direct Visualization of Anti-Ferroelectric Switching Dynamics via Electrocaloric Imaging
Vales-Castro P., Vellvehi M., Perpiñà X., Caicedo J.M., Jordà X., Faye R., Roleder K., Kajewski D., Perez-Tomas A., Defay E., Catalan G. Advanced Electronic Materials; 7 (12, 2100380) 2021. 10.1002/aelm.202100380. IF: 7.295

The large electrocaloric coupling in PbZrO3 allows using high-speed infrared imaging for visualizing anti-ferroelectric switching dynamics via the associated temperature change. It is found that in ceramic samples of homogeneous temperature and thickness, switching is fast due to the generation of multiple nucleation sites, with devices responding in the millisecond range. By introducing gradients of thickness, however, it is possible to change the dynamics to propagation limited, whereby a single-phase boundary sweeps across the sample like a cold front, at a speed of ≈20 cm s−1. Additionally, introducing thermostatic temperature differences between two sides of the sample enables the simultaneous generation of a negative electrocaloric effect on one side and a positive one on the other, yielding a Janus-like electrocaloric response. © 2021 Wiley-VCH GmbH

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Effect of Humidity on the Writing Speed and Domain Wall Dynamics of Ferroelectric Domains
Spasojevic I., Verdaguer A., Catalan G., Domingo N. Advanced Electronic Materials; 8 (6, 2100650) 2021. 10.1002/aelm.202100650. IF: 7.295

The switching dynamics of ferroelectric polarization under electric fields depends on the availability of screening charges in order to stabilize the switched polarization. In ferroelectrics, thin films with exposed surfaces investigated by piezoresponse force microscopy (PFM), the main source of external screening charges is the atmosphere and the water neck, and therefore relative humidity (RH) plays a major role. Here, it is shown how the dynamic writing of domains in BaTiO3 thin films changes by varying scanning speeds in the range of RH between 2.5% and 60%. The measurements reveal that the critical speed for domain writing, which is defined as the highest speed at which electrical writing of a continuous stripe domain is possible, increases non-monotonically with RH. Additionally, the width of line domains shows a power law dependence on the writing speed, with a growth rate coefficient decreasing with RH. The size of the written domains at a constant speed as well as the creep-factor μ describing the domain wall kinetics follow the behavior of water adsorption represented by the adsorption isotherm, indicating that the screening mechanism dominating the switching dynamics is the thickness and the structure of adsorbed water structure and its associated dielectric constant and ionic mobility. © 2021 The Authors. Advanced Electronic Materials published by Wiley-VCH GmbH.

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Energy exponents of avalanches and Hausdorff dimensions of collapse patterns
Casals B., Salje E.K.H. Physical Review E; 104 (5, 054138) 2021. 10.1103/PhysRevE.104.054138. IF: 2.529

A simple numerical model to simulate athermal avalanches is presented. The model is inspired by the "porous collapse"process where the compression of porous materials generates collapse cascades, leading to power law distributed avalanches. The energy (E), amplitude (Amax), and size (S) exponents are derived by computer simulation in two approximations. Time-dependent "jerk"spectra are calculated in a single avalanche model where each avalanche is simulated separately from other avalanches. The average avalanche profile is parabolic, the scaling between energy and amplitude follows E∼Amax2, and the energy exponent is ϵ = 1.33. Adding a general noise term in a continuous event model generates infinite avalanche sequences which allow the evaluation of waiting time distributions and pattern formation. We find the validity of the Omori law and the same exponents as in the single avalanche model. We then add spatial correlations by stipulating the ratio G/N between growth processes G (linked to a previous event location) and nucleation processes N (with new, randomly chosen nucleation sites). We found, in good approximation, a power law correlation between the energy exponent ϵ and the Hausdorff dimension HD of the resulting collapse pattern HD-1∼ϵ-3. The evolving patterns depend strongly on G/N with the distribution of collapse sites equally power law distributed. Its exponent ϵtopo would be linked to the dynamical exponent ϵ if each collapse carried an energy equivalent to the size of the collapse. A complex correlation between ϵ,ϵtopo, and HD emerges, depending strongly on the relative occupancy of the collapse sites in the simulation box. © 2021 American Physical Society.

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Local and correlated studies of humidity-mediated ferroelectric thin film surface charge dynamics
Gaponenko I., Musy L., Domingo N., Stucki N., Verdaguer A., Bassiri-Gharb N., Paruch P. npj Computational Materials; 7 (1, 163) 2021. 10.1038/s41524-021-00615-4. IF: 12.241

Electrochemical phenomena in ferroelectrics are of particular interest for catalysis and sensing applications, with recent studies highlighting the combined role of the ferroelectric polarisation, applied surface voltage and overall switching history. Here, we present a systematic Kelvin probe microscopy study of the effect of relative humidity and polarisation switching history on the surface charge dissipation in ferroelectric Pb(Zr0.2Ti0.8)O3 thin films. We analyse the interaction of surface charges with ferroelectric domains through the framework of physically constrained unsupervised machine learning matrix factorisation, Dictionary Learning, and reveal a complex interplay of voltage-mediated physical processes underlying the observed signal decays. Additional insight into the observed behaviours is given by a Fitzhugh–Nagumo reaction–diffusion model, highlighting the lateral spread and charge passivation process contributors within the Dictionary Learning analysis. © 2021, The Author(s).

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Low-Voltage Magnetoelectric Coupling in Fe0.5Rh0.5/0.68PbMg1/3Nb2/3O3-0.32PbTiO3 Thin-Film Heterostructures
Zhao W., Kim J., Huang X., Zhang L., Pesquera D., Velarde G.A.P., Gosavi T., Lin C.-C., Nikonov D.E., Li H., Young I.A., Ramesh R., Martin L.W. Advanced Functional Materials; 31 (40, 2105068) 2021. 10.1002/adfm.202105068. IF: 18.808

The rapid development of computing applications demands novel low-energy consumption devices for information processing. Among various candidates, magnetoelectric heterostructures hold promise for meeting the required voltage and power goals. Here, a route to low-voltage control of magnetism in 30 nm Fe0.5Rh0.5/100 nm 0.68PbMg1/3Nb2/3O3-0.32PbTiO3 (PMN-PT) heterostructures is demonstrated wherein the magnetoelectric coupling is achieved via strain-induced changes in the Fe0.5Rh0.5 mediated by voltages applied to the PMN-PT. We describe approaches to achieve high-quality, epitaxial growth of Fe0.5Rh0.5 on the PMN-PT films and, a methodology to probe and quantify magnetoelectric coupling in small thin-film devices via studies of the anomalous Hall effect. By comparing the spin-flop field change induced by temperature and external voltage, the magnetoelectric coupling coefficient is estimated to reach ≈7 × 10−8 s m−1 at 325 K while applying a −0.75 V bias. © 2021 Wiley-VCH GmbH

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Mechanical reading of ferroelectric polarization
Stefani C., Langenberg E., Cordero-Edwards K., Schlom D.G., Catalan G., Domingo N. Journal of Applied Physics; 130 (7, 0059930) 2021. 10.1063/5.0059930. IF: 2.546

Flexoelectricity is a property of dielectric materials whereby they exhibit electric polarization induced by strain gradients; while this effect can be negligible at the macroscale, it can become dominant at the nanoscale, where strain gradients can turn out to be tremendous. Previous works have demonstrated that flexoelectricity coupled with piezoelectricity enables the mechanical writing of ferroelectric polarization. When considering ferroelectric materials with out-of-plane polarization, the coupling of piezoelectricity with flexoelectricity can insert a mechanical asymmetry to the system and enable the distinction of oppositely polarized domains, based on their nanomechanical response. Using atomic force microscopy and, more specifically, contact resonance techniques, the coupling of flexoelectricity to piezoelectricity can be exploited to mechanically read the sign of ferroelectric polarization in a non-destructive way. We have measured a variety of ferroelectric materials, from a single crystal to thin films, and domains that are polarized down always appear to be stiffer than oppositely polarized domains. In this article, we demonstrate experimentally that the phenomenon is size-dependent and strongly enhanced when the dimension of the material is reduced to nanoscale in thin films. Ultimately, we demonstrate how the sensitivity in mechanical reading of ferroelectric polarization can be improved by appropriately tuning the mechanical stiffness of the cantilevers. © 2021 Author(s).

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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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Non-linear nanoscale piezoresponse of single ZnO nanowires affected by piezotronic effect
Lozano H., Catalán G., Esteve J., Domingo N., Murillo G. Nanotechnology; 32 (2, 025202) 2021. 10.1088/1361-6528/abb972. IF: 3.874

Zinc oxide (ZnO) nanowires (NWs) as semiconductor piezoelectric nanostructures have emerged as material of interest for applications in energy harvesting, photonics, sensing, biomedical science, actuators or spintronics. The expression for the piezoelectric properties in semiconductor materials is concealed by the screening effect of the available carriers and the piezotronic effect, leading to complex nanoscale piezoresponse signals. Here, we have developed a metal-semiconductor-metal model to simulate the piezoresponse of single ZnO NWs, demonstrating that the apparent non-linearity in the piezoelectric coefficient arises from the asymmetry created by the forward and reversed biased Schottky barriers at the semiconductor-metal junctions. By directly measuring the experimental I-V characteristics of ZnO NWs with conductive atomic force microscope together with the piezoelectric vertical coefficient by piezoresponse force microscopy, and comparing them with the numerical calculations for our model, effective piezoelectric coefficients in the range d 33eff ∼ 8.6 pm V-1-12.3 pm V-1 have been extracted for ZnO NWs. We have further demonstrated via simulations the dependence between the effective piezoelectric coefficient d 33eff and the geometry and physical dimensions of the NW (radius to length ratio), revealing that the higher d 33eff is obtained for thin and long NWs due to the tensor nature proportionality between electric fields and deformation in NW geometries. Moreover, the non-linearity of the piezoresponse also leads to multiharmonic electromechanical response observed at the second and higher harmonics that indeed is not restricted to piezoelectric semiconductor materials but can be generalized to any type of asymmetric voltage drops on a piezoelectric structure as well as leaky wide band-gap semiconductor ferroelectrics. © 2020 IOP Publishing Ltd.

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Origin of large negative electrocaloric effect in antiferroelectric PbZr O3
Vales-Castro P., Faye R., Vellvehi M., Nouchokgwe Y., Perpiñà X., Caicedo J.M., Jordà X., Roleder K., Kajewski D., Perez-Tomas A., Defay E., Catalan G. Physical Review B; 103 (5, 054112) 2021. 10.1103/PhysRevB.103.054112. IF: 4.036

We have studied the electrocaloric response of the archetypal antiferroelectric PbZrO3 as a function of voltage and temperature in the vicinity of its antiferroelectric-paraelectric phase transition. Large electrocaloric effects of opposite signs, ranging from an electrocooling of -3.5 K to an electroheating of +5.5K, were directly measured with an infrared camera. We show by calorimetric and electromechanical measurements that the large negative electrocaloric effect comes from an endothermic antiferroelectric-ferroelectric switching, in contrast to dipole destabilization of the antiparallel lattice, previously proposed as an explanation for the negative electrocaloric effect of antiferroelectrics. © 2021 American Physical Society.

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Oxidation processes at the surface of BaTiO3 thin films under environmental conditions
Spasojevic I., Sauthier G., Caicedo J.M., Verdaguer A., Domingo N. Applied Surface Science; 565 (150288) 2021. 10.1016/j.apsusc.2021.150288. IF: 6.707

Dissociation and adsorption of water on ferroelectric oxide surfaces play important role in the processes of screening and switching dynamics of ferroelectric polarization, as well as in catalytic processes which can be additionally coupled with light, temperature or vibration stimuli. In this work, we present XPS study of ferroelectric BaTiO3thin films and determine the entanglement between surface chemistry, polarization direction and stability, by observing changes upon time exposure to environmental conditions, heating in O2atmosphere and irradiation in vacuum. We devote special attention to Ba 3d spectral region and identify two different oxidation states of O atoms in the compounds of Ba. While this second specie was generally attributed to Ba in surface compounds where it has different oxygen coordination than in the bulk, based on the XPS results of oxygen annealed thin films, we demonstrate that this so far neglected component, corresponds to barium peroxide (BaO2) and identify it as important active specie for the study of screening mechanisms closely related with catalytic activity present in this ferroelectric material. Finally, we report on chemically assisted polarization switching in thin films induced by heating in vacuum or exposure to X-Ray radiation due to the formation of positive surface electric field created by oxygen or electron vacancies, respectively. © 2021 The Authors

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Piezoelectricity in nominally centrosymmetric phases
Aktas O., Kangama M., Linyu G., Catalan G., Ding X., Zunger A., Salje E.K.H. Physical Review Research; 3 (4, 043221) 2021. 10.1103/PhysRevResearch.3.043221. IF: 0.000

Compound phases often display properties that are symmetry forbidden relative to their nominal, average crystallographic symmetry, even if extrinsic reasons (defects, strain, or imperfections) are not apparent. Specifically, breaking the macroscopic inversion symmetry of a centrosymmetric phase can dominate or significantly change its observed properties while the detailed mechanisms and magnitudes of the deviations of symmetry breaking are often obscure. Here, we choose piezoelectricity as a tool to investigate macroscopic inversion-symmetry breaking in nominally centrosymmetric materials as a prominent example and measure resonant piezoelectric spectroscopy (RPS) and Resonant Ultrasound Spectroscopy (RUS) in 15 compounds, 18 samples, and 21 different phases, including unpoled ferroelectrics, paraelectrics, relaxors, ferroelastics, incipient ferroelectrics, and isotropic materials with low defect concentrations, i.e., NaCl, fused silica, and CaF2. We exclude the flexoelectric effect as a source of the observed piezoelectricity yet observe piezoelectricity in all nominally cubic phases of these samples. By scaling the RPS intensities with those of RUS, we calibrate the effective piezoelectric coefficients using single-crystal quartz as standard. Using this scaling we determine the effective piezoelectric modulus in nominally nonpiezoelectric phases, finding that the "symmetry-forbidden"piezoelectric effect ranges from ∼1 to 10-5 pm/V (∼0.5% to ∼2×10-5% of the piezoelectric coefficient of poled ferroelectric lead zirconate titanate). The values for the unpoled ferroelectric phase are only slightly higher than those in the paraelectric phase. The extremely low coefficients are well below the detection limit of conventional piezoelectric measurements and demonstrate RPS as a convenient and ultrahighly sensitive method to measure piezoelectricity. We suggest that symmetry-breaking piezoelectricity in nominally centrosymmetric materials and disordered, unpoled ferroelectrics is a common phenomenon. © 2021 authors. Published by the American Physical Society.

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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.

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Pyroelectric thin films - Past, present, and future
Velarde G., Pandya S., Karthik J., Pesquera D., Martin L.W. APL Materials; 9 (1, 010702) 2021. 10.1063/5.0035735. IF: 5.096

Pyroelectrics are a material class that undergoes a change in polarization as the temperature of the system is varied. This effect can be utilized for applications ranging from thermal imaging and sensing to waste-heat energy conversion to thermally driven electron emission. Here, we review recent advances in the study and utilization of thin-film pyroelectrics. Leveraging advances in modeling, synthesis, and characterization has provided a pathway forward in one of the more poorly developed subfields of ferroelectricity. We introduce the complex physical phenomena of pyroelectricity, briefly explore the history of work in this space, and highlight not only new advances in the direct measurement of such effects but also how our ability to control thin-film materials is changing our understanding of this response. Finally, we discuss recent advances in thin-film pyroelectric devices and introduce a number of potentially new directions the field may follow in the coming years. © 2021 Author(s).

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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

2020

Beyond Expectation: Advanced Materials Design, Synthesis, and Processing to Enable Novel Ferroelectric Properties and Applications
Kim J., Lupi E., Pesquera D., Acharya M., Zhao W., Velarde G.A.P., Griffin S., Martin L.W. MRS Advances; 2020. 10.1557/adv.2020.344. IF: 0.000

Ferroelectrics and related materials (e.g., non-traditional ferroelectrics such as relaxors) have long been used in a range of applications, but with the advent of new ways of modeling, synthesizing, and characterizing these materials, continued access to astonishing breakthroughs in our fundamental understanding come each year. While we still rely on these materials in a range of applications, we continue to re-write what is possible to be done with them. In turn, assumptions that have underpinned the use and design of certain materials are progressively being revisited. This perspective aims to provide an overview of the field of ferroelectric/relaxor/polar-oxide thin films in recent years, with an emphasis on emergent structure and function enabled by advanced synthesis, processing, and computational modeling. Copyright © The Author(s), 2020, published on behalf of Materials Research Society by Cambridge University Press.

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Beyond Substrates: Strain Engineering of Ferroelectric Membranes
Pesquera D., Parsonnet E., Qualls A., Xu R., Gubser A.J., Kim J., Jiang Y., Velarde G., Huang Y.-L., Hwang H.Y., Ramesh R., Martin L.W. Advanced Materials; 32 (43, 2003780) 2020. 10.1002/adma.202003780. IF: 27.398

Strain engineering in perovskite oxides provides for dramatic control over material structure, phase, and properties, but is restricted by the discrete strain states produced by available high-quality substrates. Here, using the ferroelectric BaTiO3, production of precisely strain-engineered, substrate-released nanoscale membranes is demonstrated via an epitaxial lift-off process that allows the high crystalline quality of films grown on substrates to be replicated. In turn, fine structural tuning is achieved using interlayer stress in symmetric trilayer oxide-metal/ferroelectric/oxide-metal structures fabricated from the released membranes. In devices integrated on silicon, the interlayer stress provides deterministic control of ordering temperature (from 75 to 425 °C) and releasing the substrate clamping is shown to dramatically impact ferroelectric switching and domain dynamics (including reducing coercive fields to <10 kV cm−1 and improving switching times to <5 ns for a 20 µm diameter capacitor in a 100-nm-thick film). In devices integrated on flexible polymers, enhanced room-temperature dielectric permittivity with large mechanical tunability (a 90% change upon ±0.1% strain application) is demonstrated. This approach paves the way toward the fabrication of ultrafast CMOS-compatible ferroelectric memories and ultrasensitive flexible nanosensor devices, and it may also be leveraged for the stabilization of novel phases and functionalities not achievable via direct epitaxial growth. © 2020 Wiley-VCH GmbH

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.

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Investigation of The Cellular Response to Bone Fractures: Evidence for Flexoelectricity
Núñez-Toldrà R., Vasquez-Sancho F., Barroca N., Catalan G. Scientific Reports; 10 (1, 254) 2020. 10.1038/s41598-019-57121-3. IF: 3.998

The recent discovery of bone flexoelectricity (strain-gradient-induced electrical polarization) suggests that flexoelectricity could have physiological effects in bones, and specifically near bone fractures, where flexoelectricity is theoretically highest. Here, we report a cytological study of the interaction between crack stress and bone cells. We have cultured MC3T3-E1 mouse osteoblastic cells in biomimetic microcracked hydroxyapatite substrates, differentiated into osteocytes and applied a strain gradient to the samples. The results show a strong apoptotic cellular response, whereby mechanical stimulation causes those cells near the crack to die, as indicated by live-dead and caspase staining. In addition, analysis two weeks post-stimulation shows increased cell attachment and mineralization around microcracks and a higher expression of osteocalcin –an osteogenic protein known to be promoted by physical exercise. The results are consistent with flexoelectricity playing at least two different roles in bone remodelling: apoptotic trigger of the repair protocol, and electro-stimulant of the bone-building activity of osteoblasts. © 2020, The Author(s).

View abstract
James F. Scott (1942-2020)
Catalan G., Dawber M., Gregg M., Morrison F., Ramesh R., Zubko P. Nature materials; 19 (6): 580. 2020. 10.1038/s41563-020-0692-x. IF: 38.663

[No abstract available]

View abstract
Mechanical Softness of Ferroelectric 180° Domain Walls MECHANICAL SOFTNESS of FERROELECTRIC 180 DEGREE ... STEFANI CHRISTINA et al.
Stefani C., Ponet L., Shapovalov K., Chen P., Langenberg E., Schlom D.G., Artyukhin S., Stengel M., Domingo N., Catalan G. Physical Review X; 10 (4, 041001) 2020. 10.1103/PhysRevX.10.041001. IF: 12.577

Using scanning probe microscopy, we measure the out-of-plane mechanical response of ferroelectric 180° domain walls and observe that, despite separating domains that are mechanically identical, the walls appear mechanically distinct-softer-compared to the domains. This effect is observed in different ferroelectric materials (LiNbO3, BaTiO3, and PbTiO3) and with different morphologies (from single crystals to thin films), suggesting that the effect is universal. We propose a theoretical framework that explains the domain wall softening and justifies that the effect should be common to all ferroelectrics. The lesson is, therefore, that domain walls are not only functionally different from the domains they separate, but also mechanically distinct. © 2020 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the "https://creativecommons.org/licenses/by/4.0/"Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

View abstract
Photoflexoelectric effect in halide perovskites
Shu L., Ke S., Fei L., Huang W., Wang Z., Gong J., Jiang X., Wang L., Li F., Lei S., Rao Z., Zhou Y., Zheng R.-K., Yao X., Wang Y., Stengel M., Catalan G. Nature Materials; 19 (6): 605 - 609. 2020. 10.1038/s41563-020-0659-y. IF: 38.663

Harvesting environmental energy to generate electricity is a key scientific and technological endeavour of our time. Photovoltaic conversion and electromechanical transduction are two common energy-harvesting mechanisms based on, respectively, semiconducting junctions and piezoelectric insulators. However, the different material families on which these transduction phenomena are based complicate their integration into single devices. Here we demonstrate that halide perovskites, a family of highly efficient photovoltaic materials1–3, display a photoflexoelectric effect whereby, under a combination of illumination and oscillation driven by a piezoelectric actuator, they generate orders of magnitude higher flexoelectricity than in the dark. We also show that photoflexoelectricity is not exclusive to halides but a general property of semiconductors that potentially enables simultaneous electromechanical and photovoltaic transduction and harvesting in unison from multiple energy inputs. © 2020, The Author(s), under exclusive licence to Springer Nature Limited.

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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
Strain-Engineered Ferroelastic Structures in PbTiO3 Films and Their Control by Electric Fields
Langenberg E., Paik H., Smith E.H., Nair H.P., Hanke I., Ganschow S., Catalan G., Domingo N., Schlom D.G. ACS Applied Materials and Interfaces; 12 (18): 20691 - 20703. 2020. 10.1021/acsami.0c04381. IF: 8.758

We study the interplay between epitaxial strain, film thickness, and electric field in the creation, modification, and design of distinct ferroelastic structures in PbTiO3 thin films. Strain and thickness greatly affect the structures formed, providing a two-variable parameterization of the resulting self-assembly. Under applied electric fields, these strain-engineered ferroelastic structures are highly malleable, especially when a/c and a1/a2 superdomains coexist. To reconfigure the ferroelastic structures and achieve self-assembled nanoscale-ordered morphologies, pure ferroelectric switching of individual c-domains within the a/c superdomains is essential. The stability, however, of the electrically written ferroelastic structures is in most cases ephemeral; the speed of the relaxation process depends sensitively on strain and thickness. Only under low tensile strain - as is the case for PbTiO3 on GdScO3 - and below a critical thickness do the electrically created a/c superdomain structures become stable for days or longer, making them relevant for reconfigurable nanoscale electronics or nonvolatile electromechanical applications. Copyright © 2020 American Chemical Society.

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Stress-induced in situ modification of transition temperature in vo2 films capped by chalcogenide
Sakai J., Kuwahara M., Okimura K., Uehara Y. Materials; 13 (23, 5541): 1 - 11. 2020. 10.3390/ma13235541. IF: 3.057

We attempted to modify the monoclinic–rutile structural phase transition temperature (Ttr) of a VO2 thin film in situ through stress caused by amorphous–crystalline phase change of a chalcogenide layer on it. VO2 films on C-or R-plane Al2 O3 substrates were capped by Ge2 Sb2 Te5 (GST) films by means of rf magnetron sputtering. Ttr of the VO2 layer was evaluated through temperature-controlled measurements of optical reflection intensity and electrical resistance. Crystallization of the GST capping layer was accompanied by a significant drop in Ttr of the VO2 layer underneath, either with or without a SiNx diffusion barrier layer between the two. The shift of Ttr was by ~30◦ C for a GST/VO2 bilayered sample with thicknesses of 200/30 nm, and was by ~6◦ C for a GST/SiNx /VO2 trilayered sample of 200/10/6 nm. The lowering of Ttr was most probably caused by the volume reduction in GST during the amorphous–crystalline phase change. The stress-induced in in situ modification of Ttr in VO2 films could pave the way for the application of nonvolatile changes of optical properties in optoelectronic devices. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.

View abstract
Temperature-independent giant dielectric response in transitional BaTiO3 thin films
Everhardt A.S., Denneulin T., Grünebohm A., Shao Y.-T., Ondrejkovic P., Zhou S., Domingo N., Catalan G., Hlinka J., Zuo J.-M., Matzen S., Noheda B. Applied Physics Reviews; 7 (1, 011402) 2020. 10.1063/1.5122954. IF: 17.054

Ferroelectric materials exhibit the largest dielectric permittivities and piezoelectric responses in nature, making them invaluable in applications from supercapacitors or sensors to actuators or electromechanical transducers. The origin of this behavior is their proximity to phase transitions. However, the largest possible responses are most often not utilized due to the impracticality of using temperature as a control parameter and to operate at phase transitions. This has motivated the design of solid solutions with morphotropic phase boundaries between different polar phases that are tuned by composition and that are weakly dependent on temperature. Thus far, the best piezoelectrics have been achieved in materials with intermediate (bridging or adaptive) phases. But so far, complex chemistry or an intricate microstructure has been required to achieve temperature-independent phase-transition boundaries. Here, we report such a temperature-independent bridging state in thin films of chemically simple BaTiO3. A coexistence among tetragonal, orthorhombic, and their bridging low-symmetry phases are shown to induce continuous vertical polarization rotation, which recreates a smear in-transition state and leads to a giant temperature-independent dielectric response. The current material contains a ferroelectric state that is distinct from those at morphotropic phase boundaries and cannot be considered as ferroelectric crystals. We believe that other materials can be engineered in a similar way to contain a ferroelectric state with gradual change of structure, forming a class of transitional ferroelectrics. Similar mechanisms could be utilized in other materials to design low-power ferroelectrics, piezoelectrics, dielectrics, or shape-memory alloys, as well as efficient electro- and magnetocalorics. © 2020 Author(s).

View abstract

2019

Converse flexoelectricity yields large piezoresponse force microscopy signals in non-piezoelectric materials
Abdollahi A., Domingo N., Arias I., Catalan G. Nature Communications; 10 (1, 1266) 2019. 10.1038/s41467-019-09266-y. IF: 11.878

Converse flexoelectricity is a mechanical stress induced by an electric polarization gradient. It can appear in any material, irrespective of symmetry, whenever there is an inhomogeneous electric field distribution. This situation invariably happens in piezoresponse force microscopy (PFM), which is a technique whereby a voltage is delivered to the tip of an atomic force microscope in order to stimulate and probe piezoelectricity at the nanoscale. While PFM is the premier technique for studying ferroelectricity and piezoelectricity at the nanoscale, here we show, theoretically and experimentally, that large effective piezoelectric coefficients can be measured in non-piezoelectric dielectrics due to converse flexoelectricity. © 2019, The Author(s).

View abstract
Disentangling Highly Asymmetric Magnetoelectric Effects in Engineered Multiferroic Heterostructures
Menéndez E., Sireus V., Quintana A., Fina I., Casals B., Cichelero R., Kataja M., Stengel M., Herranz G., Catalán G., Baró M.D., Suriñach S., Sort J. Physical Review Applied; 12 (1, 014041) 2019. 10.1103/PhysRevApplied.12.014041. IF: 4.532

One of the main strategies to control magnetism by voltage is the use of magnetostrictive-piezoelectric hybrid materials, such as ferromagnetic-ferroelectric heterostructures. When such heterostructures are subjected to an electric field, piezostrain-mediated effects, electronic charging, and voltage-driven oxygen migration (magnetoionics) may simultaneously occur, making the interpretation of the magnetoelectric effects not straightforward and often leading to misconceptions. Typically, the strain-mediated magnetoelectric response is symmetric with respect to the sign of the applied voltage because the induced strain (and variations in the magnetization) depends on the square of the ferroelectric polarization. Conversely, asymmetric responses can be obtained from electronic charging and voltage-driven oxygen migration. By engineering a ferromagnetic-ferroelectric hybrid consisting of a magnetically soft 50-nm thick Fe75Al25 (at. %) thin film on top of a (110)-oriented Pb(Mg1/3Nb2/3)O3-32PbTiO3 ferroelectric crystal, a highly asymmetric magnetoelectric response is obtained and the aforementioned magnetoelectric effects can be disentangled. Specifically, the large thickness of the Fe75Al25 layer allows dismissing any possible charge accumulation effect, whereas no evidence of magnetoionics is observed experimentally, as expected from the high resistance to oxidation of Fe75Al25, leaving strain as the only mechanism to modulate the asymmetric magnetoelectric response. The origin of this asymmetric strain-induced magnetoelectric effect arises from the asymmetry of the polarization reversal in the particular crystallographic orientation of the ferroelectric substrate. These results are important to optimize the performance of artificial multiferroic heterostructures. © 2019 American Physical Society.

View abstract
Enhancing the intrinsic p-type conductivity of the ultra-wide bandgap Ga2O3 semiconductor
Chikoidze E., Sartel C., Mohamed H., Madaci I., Tchelidze T., Modreanu M., Vales-Castro P., Rubio C., Arnold C., Sallet V., Dumont Y., Perez-Tomas A. Journal of Materials Chemistry C; 7 (33): 10231 - 10239. 2019. 10.1039/c9tc02910a. IF: 6.641

While there are several n-type transparent semiconductor oxides (TSO) for optoelectronic applications (e.g. LEDs, solar cells or display TFTs), their required p-type counterpart oxides are known to be more challenging. At this time, the n-type TSO with the largest bandgap (∼5 eV) is Ga2O3 that holds the promise of extending the light transparency further into the deep ultraviolet. In this work, it is demonstrated that strongly compensated Ga2O3 is also an intrinsic (or native) p-type TSO with the largest bandgap for any reported p-type TSO (e.g. NiO, SnO, delafossites, oxychalcogenides). The achievement of hole mobility in excess of 10 cm2 V-1 s-1 and (high temperature) free hole concentrations in the ∼1017 cm-3 range challenges the current thinking about achieving p-type conductivity in Ga2O3 being "out of the question". The results presented in this paper therefore further clarify that p-type Ga2O3 is possible, although more research must be conducted to determine what are the real prospects for Ga2O3 solar blind bipolar optoelectronics and ultra-high power electronics based on p-n homojunctions. © The Royal Society of Chemistry 2019.

View abstract
Ferroelectric Domain Walls in PbTiO3 Are Effective Regulators of Heat Flow at Room Temperature
Langenberg E., Saha D., Holtz M.E., Wang J.-J., Bugallo D., Ferreiro-Vila E., Paik H., Hanke I., Ganschow S., Muller D.A., Chen L.-Q., Catalan G., Domingo N., Malen J., Schlom D.G., Rivadulla F. Nano Letters; 19 (11): 7901 - 7907. 2019. 10.1021/acs.nanolett.9b02991. IF: 12.279

Achieving efficient spatial modulation of phonon transmission is an essential step on the path to phononic circuits using "phonon currents". With their intrinsic and reconfigurable interfaces, domain walls (DWs), ferroelectrics are alluring candidates to be harnessed as dynamic heat modulators. This paper reports the thermal conductivity of single-crystal PbTiO3 thin films over a wide variety of epitaxial-strain-engineered ferroelectric domain configurations. The phonon transport is proved to be strongly affected by the density and type of DWs, achieving a 61% reduction of the roomerature thermal conductivity compared to the single-domain scenario. The thermal resistance across the ferroelectric DWs is obtained, revealing a very high value (≈5.0 × 10-9 K m2 W-1), comparable to grain boundaries in oxides, explaining the strong modulation of the thermal conductivity in PbTiO3. This low thermal conductance of the DWs is ascribed to the structural mismatch and polarization gradient found between the different types of domains in the PbTiO3 films, resulting in a structural inhomogeneity that extends several unit cells around the DWs. These findings demonstrate the potential of ferroelectric DWs as efficient regulators of heat flow in one single material, overcoming the complexity of multilayers systems and the uncontrolled distribution of grain boundaries, paving the way for applications in phononics. Copyright © 2019 American Chemical Society.

View abstract
Flexoelectric Fracture-Ratchet Effect in Ferroelectrics
Cordero-Edwards K., Kianirad H., Canalias C., Sort J., Catalan G. Physical Review Letters; 122 (13, 135502) 2019. 10.1103/PhysRevLett.122.135502. IF: 9.227

The propagation front of a crack generates large strain gradients and it is therefore a strong source of gradient-induced polarization (flexoelectricity). Herein, we demonstrate that, in piezoelectric materials, a consequence of flexoelectricity is that crack propagation is helped or hindered depending on whether it is parallel or antiparallel to the piezoelectric polar axis. The discovery of crack propagation asymmetry proves that fracture physics cannot be assumed to be symmetric in polar materials, and indicates that flexoelectricity should be incorporated in any realistic model. © 2019 American Physical Society.

View abstract
Functional Oxides for Photoneuromorphic Engineering: Toward a Solar Brain
Pérez-Tomás A. Advanced Materials Interfaces; 6 (15, 1900471) 2019. 10.1002/admi.201900471. IF: 4.713

New device concepts and new computing principles are needed to balance our ever-growing appetite for data and information with the realization of the goals of increased energy efficiency, reduction in CO2 emissions, and the circular economy. Neuromorphic or synaptic electronics is an emerging field of research aiming to overcome the current computer's Von-Neumann bottleneck by building artificial neuronal systems to mimic the extremely energy efficient biological synapses. The introduction of photovoltaic and/or photonic aspects into these neuromorphic architectures will produce self-powered adaptive electronics but may also open new possibilities in artificial neuroscience, artificial neural communications, sensing, and machine learning which would enable, in turn, a new era for computational systems owing to the possibility of attaining high bandwidths with much reduced power consumption. This perspective is focused on recent progress in the implementation of functional oxide thin-films into photovoltaic and neuromorphic applications toward the envisioned goal of self-powered photovoltaic neuromorphic systems or a solar brain. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

View abstract
Giant bulk photovoltaic effect in solar cell architectures with ultra-wide bandgap Ga2O3 transparent conducting electrodes
Pérez-Tomás A., Chikoidze E., Dumont Y., Jennings M.R., Russell S.O., Vales-Castro P., Catalan G., Lira-Cantú M., Ton –That C., Teherani F.H., Sandana V.E., Bove P., Rogers D.J. Materials Today Energy; 14 (100350) 2019. 10.1016/j.mtener.2019.100350. IF: 0.000

The use of ultra-wide bandgap transparent conducting beta gallium oxide (β-Ga2O3) thin films as electrodes in ferroelectric solar cells is reported. In a new material structure for energy applications, we report a solar cell structure (a light absorber sandwiched in between two electrodes - one of them - transparent) which is not constrained by the Shockley–Queisser limit for open-circuit voltage (Voc) under typical indoor light. The solar blindness of the electrode enables a record-breaking bulk photovoltaic effect (BPE) with white light illumination (general use indoor light). This work opens up the perspective of ferroelectric photovoltaics which are not subject to the Shockley-Queisser limit by bringing into scene solar-blind conducting oxides. © 2019 Elsevier Ltd

View abstract
In Situ Electrochemical Oxidation of Cu2S into CuO Nanowires as a Durable and Efficient Electrocatalyst for Oxygen Evolution Reaction
Zuo Y., Liu Y., Li J., Du R., Han X., Zhang T., Arbiol J., Divins N.J., Llorca J., Guijarro N., Sivula K., Cabot A. Chemistry of Materials; 31 (18): 7732 - 7743. 2019. 10.1021/acs.chemmater.9b02790. IF: 10.159

Development of cost-effective oxygen evolution catalysts is of capital importance for the deployment of large-scale energy-storage systems based on metal-air batteries and reversible fuel cells. In this direction, a wide range of materials have been explored, especially under more favorable alkaline conditions, and several metal chalcogenides have particularly demonstrated excellent performances. However, chalcogenides are thermodynamically less stable than the corresponding oxides and hydroxides under oxidizing potentials in alkaline media. Although this instability in some cases has prevented the application of chalcogenides as oxygen evolution catalysts and it has been disregarded in some others, we propose to use it in our favor to produce high-performance oxygen evolution catalysts. We characterize here the in situ chemical, structural, and morphological transformation during the oxygen evolution reaction (OER) in alkaline media of Cu2S into CuO nanowires, mediating the intermediate formation of Cu(OH)2. We also test their OER activity and stability under OER operation in alkaline media and compare them with the OER performance of Cu(OH)2 and CuO nanostructures directly grown on the surface of a copper mesh. We demonstrate here that CuO produced from in situ electrochemical oxidation of Cu2S displays an extraordinary electrocatalytic performance toward OER, well above that of CuO and Cu(OH)2 synthesized without this transformation. © 2019 American Chemical Society.

View abstract
Optical Fiber Humidity Sensor Based on Polyvinylidene Fluoride Fabry-Perot
Vaz A., Barroca N., Ribeiro M., Pereira A., Frazao O. IEEE Photonics Technology Letters; 31 (7, 8653388): 549 - 552. 2019. 10.1109/LPT.2019.2901571. IF: 2.553

An optical fiber Fabry-Perot (FP) for relative humidity (RH) sensing is proposed. The FP cavity is fabricated by splicing a short length of hollow silica tube in a single mode fiber. The fiber is then coated with a polyvinylidene fluoride (PVDF) thin film to work as a mirror. The fabrication process of the FP interferometer with a dip coating process in a PVDF/dimethyl formamide solution is presented. The pattern fringes of the FP suffer a wavelength shift due to the change in the PVDF's refractive index with the ambient RH variation. A short overview of the cavity's formation and stability is presented. The RH response of the FPI cavity is tested. The sensor presented a sensitivity of 32.54 pm/%RH at constant temperature and -15.2 pm/°C for temperature variation. © 1989-2012 IEEE.

View abstract
PbZrTiO 3 ferroelectric oxide as an electron extraction material for stable halide perovskite solar cells
Pérez-Tomas A., Xie H., Wang Z., Kim H.-S., Shirley I., Turren-Cruz S.-H., Morales-Melgares A., Saliba B., Tanenbaum D., Saliba M., Zakeeruddin S.M., Gratzel M., Hagfeldt A., Lira-Cantu M. Sustainable Energy and Fuels; 3 (2): 382 - 389. 2019. 10.1039/c8se00451j. IF: 4.912

State-of-the-art halide perovskite solar cells employ semiconductor oxides as electron transport materials. Defects in these oxides, such as oxygen vacancies (O vac ), act as recombination centres and, in air and UV light, reduce the stability of the solar cell. Under the same conditions, the PbZrTiO 3 ferroelectric oxide employs O vac for the creation of defect-dipoles responsible for photo-carrier separation and current transport, evading device degradation. We report the application of PbZrTiO 3 as the electron extraction material in triple cation halide perovskite solar cells. The application of a bias voltage (poling) up to 2 V, under UV light, is a critical step to induce charge transport in the ferroelectric oxide. Champion cells result in power conversion efficiencies of ∼11% after poling. Stability analysis, carried out at 1-sun AM 1.5 G, including UV light in air for unencapsulated devices, shows negligible degradation for hours. Our experiments indicate the effect of ferroelectricity, however alternative conducting mechanisms affected by the accumulation of charges or the migration of ions (or the combination of them) cannot be ruled out. Our results demonstrate, for the first time, the application of a ferroelectric oxide as an electron extraction material in efficient and stable PSCs. These findings are also a step forward in the development of next generation ferroelectric oxide-based electronic and optoelectronic devices. © 2019 The Royal Society of Chemistry.

View abstract
Periodicity-Doubling Cascades: Direct Observation in Ferroelastic Materials
Everhardt A.S., Damerio S., Zorn J.A., Zhou S., Domingo N., Catalan G., Salje E.K.H., Chen L.-Q., Noheda B. Physical Review Letters; 123 (8, 087603) 2019. 10.1103/PhysRevLett.123.087603. IF: 9.227

Very sensitive responses to external forces are found near phase transitions. However, transition dynamics and preequilibrium phenomena are difficult to detect and control. We have observed that the equilibrium domain structure following a phase transition in ferroelectric and ferroelastic BaTiO3 is attained by halving of the domain periodicity multiple times. The process is reversible, with periodicity doubling as temperature is increased. This observation is reminiscent of the period-doubling cascades generally observed during bifurcation phenomena, and, thus, it conforms to the "spatial chaos" regime earlier proposed by Jensen and Bak [Phys. Scr. T 9, 64 (1985)PHSTER0281-184710.1088/0031-8949/1985/T9/009] for systems with competing spatial modulations. © 2019 American Physical Society.

View abstract
Persistent M2 phase in strongly strained (011)-oriented grains in VO2 films grown on sapphire (001) in reactive sputtering
Matsuoka K., Okimura K., Azhan N.H., Zaghrioui M., Sakai J. Journal of Applied Physics; 125 (16, 165304) 2019. 10.1063/1.5068700. IF: 2.328

We report on the first observation of the persistent M2 phase in strongly strained (011)-oriented grains in VO2 films grown on Al2O3 (001) substrates by means of conventional rf reactive sputtering under adequate deposition conditions. Spatially resolved micro-Raman spectra clearly showed that (011)-oriented large crystalline grains with the cR-axis parallel to the substrate resulted in the appearance of the M2 phase over a wide temperature range of 30 °C. A close correlation of the appearance range of the M2 phase with the in-plane tensile stress of (011)-oriented grains was revealed by X-ray diffraction. We present a phase diagram for the M1, M2, and R phases in relation to the stress of (011)-oriented grains and temperature. It was shown that (011)-oriented micrometer-sized long grains play a crucial role in the emerging structural phase transition (SPT) via an M2 phase even in a film grown on Al2O3 (001), which is ordinarily reserved for the (020)-oriented VO2 growth. The results shown here will contribute to make clear the conditions for obtaining VO2 films with the appearance of the M2 phase in their SPT process. © 2019 Author(s).

View abstract
Puzzling robust 2D metallic conductivity in undoped β-Ga 2 O 3 thin films
Chikoidze E., Rogers D.J., Teherani F.H., Rubio C., Sauthier G., Von Bardeleben H.J., Tchelidze T., Ton-That C., Fellous A., Bove P., Sandana E.V., Dumont Y., Perez-Tomas A. Materials Today Physics; 8: 10 - 17. 2019. 10.1016/j.mtphys.2018.11.006. IF: 0.000

Here, we report the analogy of an extremely stable topological-like ultra-wide bandgap insulator, a solid that is a pure insulator in its bulk but has a metallic conductive surface, presenting a two-dimensional conductive channel at its surface that challenges our current thinking about semiconductor conductivity engineering. Nominally undoped epitaxial β-Ga 2 O 3 thin films without any detectable defect (after a range of state-of-the-art techniques) showed the unexpectedly low resistivity of 3 × 10 −2 Ωcm which was found to be also resistant to high dose proton irradiation (2 MeV, 5 × 10 15 cm −2 dose) and was largely invariant (metallic) over the phenomenal temperature range of 2 K up to 850 K. The unique resilience and stability of the electrical properties under thermal and highly ionizing radiation stressing, combined with the extended transparency range (thanks to the ultra-wide bandgap) and the already known toughness under high electrical field could open up new perspectives for use as expanded spectral range transparent electrodes (e.g., for UV harvesting solar cells or UV LEDs/lasers) and robust Ohmic contacts for use in extreme environments/applications and for novel optoelectronic and power device concepts. © 2018 Elsevier Ltd

View abstract
Signal enhancement on gold nanoparticle-based lateral flow tests using cellulose nanofibers
Quesada-González D., Stefani C., González I., de la Escosura-Muñiz A., Domingo N., Mutjé P., Merkoçi A. Biosensors and Bioelectronics; 141 (111407) 2019. 10.1016/j.bios.2019.111407. IF: 9.518

Lateral flow paper-based biosensors merge as powerful tools in point-of-care diagnostics since they are cheap, portable, robust, selective, fast and easy to use. However, the sensitivity of this type of biosensors is not always as high as required, often not permitting a clear quantification. To improve the colorimetric response of standard lateral flow strips (LFs), we have applied a new enhancement strategy that increases the sensitivity of LFs based on the use of cellulose nanofibers (CNF). CNF penetrate inside the pores of LFs nitrocellulose paper, compacting the pore size only in the test line, particularly near the surface of the strip. This modification retains the bioreceptors (antibodies) close to the surface of the strips, and thus further increasing the density of selectively attached gold nanoparticles (AuNPs) in the top part of the membrane, in the test line area, only when the sample is positive. This effect boosts in average a 36.6% the sensitivity of the LFs. The optical measurements of the LFs were carried out with a mobile phone camera whose imaging resolution was improved by attaching microscopic lens on the camera objective. The characterization of CNF into paper and their effect was analyzed using atomic force microscope (AFM) and scanning electron microscope (SEM) imaging techniques. © 2019 Elsevier B.V.

View abstract
Strain-induced resistance change in V2O3 films on piezoelectric ceramic disks
Sakai J., Bavencoffe M., Negulescu B., Limelette P., Wolfman J., Tateyama A., Funakubo H. Journal of Applied Physics; 125 (11, 115102) 2019. 10.1063/1.5083941. IF: 2.328

We prepared a stacked structure consisting of a quasi-free-standing functional oxide thin film and a ceramic piezoelectric disk and observed the effect of the piezoelectric disk deformation on the resistance of the thin film. Epitaxial V2O3 films were grown by a pulsed laser deposition method on muscovite mica substrates, peeled off using Scotch tapes, and transferred onto piezoelectric elements. In this V2O3/insulator/top electrode/piezoelectronic disk/bottom electrode structure, the resistance of the V2O3 film displayed a variation of 60% by sweeping the piezoelectronic disk bias. With support from x-ray diffraction measurements under an electric field, a huge gauge factor of 3 × 103 in the V2O3 film was inferred. The sizeable resistance change in the V2O3 layer is ascribed to the piezo-actuated evolution of c/a ratios, which drives the material towards an insulating phase. A memory effect on the resistance, related to the hysteretic displacement of the piezoelectric material, is also presented. © 2019 Author(s).

View abstract
Surface charged species and electrochemistry of ferroelectric thin films
Domingo N., Gaponenko I., Cordero-Edwards K., Stucki N., Pérez-Dieste V., Escudero C., Pach E., Verdaguer A., Paruch P. Nanoscale; 11 (38): 17920 - 17930. 2019. 10.1039/c9nr05526f. IF: 6.970

The combination of scanning probe microscopy and ambient pressure X-ray photoelectron spectroscopy opens up new perspectives for the study of combined surface chemical, electrochemical and electromechanical properties at the nanoscale, providing both nanoscale resolution of physical information and the chemical sensitivity required to identify surface species and bulk ionic composition. In this work, we determine the nature and evolution over time of surface chemical species obtained after water-mediated redox reactions on Pb(Zr0.2,Ti0.8)O3 thin films with opposite as-grown polarization states. Starting with intrinsically different surface chemical composition on the oppositely polarized films (as a result of their ferroelectric-dominated interaction with environmental water), we identify the reversible and irreversible electrochemical reactions under an external electric field, distinguishing switching and charging events. We find that while reversible ionic displacements upon polarization switching dominate screening in the bulk of the sample, polarization dependent irreversible redox reactions determine surface chemical composition, which reveals itself as a characteristic fingerprint of the ferroelectric polarization switching history. © 2019 The Royal Society of Chemistry.

View abstract
Surface polarization feels the heat
Gustau Catalan, Beatriz Noheda Nature; 575 (7784): 600 - 602. 2019. 10.1038/d41586-019-03494-4.

A crystal’s surface has been found to behave as a distinct material that has temperature-dependent electrical polarization — despite the rest of the crystal being non-polar.

View abstract
Vehicle Classification System Based on Ferroelectric Materials
Fina I., Martí X., Catalan G. 2019 IEEE International Symposium on Applications of Ferroelectrics, ISAF 2019 - Proceedings; (9034960) 2019. 10.1109/ISAF43169.2019.9034960.

Vehicle classification taking into account the weight of the vehicle and its number of shafts can be relevant for adequate control of vehicles mobility. Piezoelectric materials can be used to infer both parameters in a reliable manner. Also piezoelectric sensors can be cost-effective. In the present work, we focus our attention in the characterization of a piezoelectric cable, where the piezoelectric active material is PVDF. We show that the piezoelectric cable can be used to classify vehicles and count their number of shafts, using a simply signal processing. © 2019 IEEE.

View abstract
Water adsorption, dissociation and oxidation on SrTiO 3 and ferroelectric surfaces revealed by ambient pressure X-ray photoelectron spectroscopy
Domingo N., Pach E., Cordero-Edwards K., Pérez-Dieste V., Escudero C., Verdaguer A. Physical Chemistry Chemical Physics; 21 (9): 4920 - 4930. 2019. 10.1039/c8cp07632d. IF: 3.567

Water dissociation on oxides is of great interest because its fundamental aspects are still not well understood and it has implications in many processes, from ferroelectric polarization screening phenomena to surface catalysis and surface chemistry on oxides. In situ water dissociation and redox processes on metal oxide perovskites which easily expose TiO 2 -terminated surfaces, such as SrTiO 3 , BaTiO 3 or Pb(Zr,Ti)O 3 , are studied by ambient pressure XPS, as a function of water vapour pressure. From the analysis of the O1s spectrum, we determine the presence of different types of oxygen based species, from hydroxyl groups, either bound to Ti 4+ and metal sites or lattice oxygen, to different peroxide compounds, and propose a model for the adsorbate layer composition, valid for environmental conditions. From the XPS analysis, we describe the existing surface redox reactions for metal oxide perovskites, occurring at different water vapour pressures. Among them, peroxide species resulting from surface oxidative reactions are correlated with the presence of Ti 4+ ions, which are observed to specifically promote surface oxidation and water dissociation as compared to other metals. Finally, surface peroxidation is enhanced by X-ray beam irradiation, leading to a higher coverage of peroxide species after beam overexposure and by ferroelectric polarization, demonstrating the enhancement of the reactivity of the surfaces of ferroelectric materials due to the effect of internal electric fields. © 2019 the Owner Societies.

View abstract

2018

A Solar Transistor and Photoferroelectric Memory
Pérez-Tomás A., Lima A., Billon Q., Shirley I., Catalan G., Lira-Cantú M. Advanced Functional Materials; 28 (17, 1707099) 2018. 10.1002/adfm.201707099. IF: 13.325

This study presents a new self-powered electronic transistor concept “the solar transistor.” The transistor effect is enabled by the functional integration of a ferroelectric-oxide thin film and an organic bulk heterojunction. The organic heterojunction efficiently harvests photon energy and splits photogenerated excitons into free electron and holes, and the ferroelectric film acts as a switchable electron transport layer with tuneable conduction band offsets that depend on its polarization state. This results in the device photoconductivity modulation. All this (i.e., carrier extraction and poling) is achieved with only two sandwiched electrodes and therefore, with the role of the gating electrode being taken by light. The two-terminal solar-powered phototransistor (or solaristor) thus has the added advantages of a compact photodiode architecture in addition to the nonvolatile functionality of a ferroelectric memory that is written by voltage and nondestructively read by light. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

View abstract
Flexoelectricity in antiferroelectrics
Vales-Castro P., Roleder K., Zhao L., Li J.-F., Kajewski D., Catalan G. Applied Physics Letters; 113 (13, 132903) 2018. 10.1063/1.5044724. IF: 3.495

Flexoelectricity (coupling between polarization and strain gradients) is a property of all dielectric materials that has been theoretically known for decades, but only relatively recently it has begun to attract experimental attention. As a consequence, there are still entire families of materials whose flexoelectric performance is unknown. Such is the case of antiferroelectrics: materials with an antiparallel but switchable arrangement of dipoles. These materials are expected to be flexoelectrically relevant because it has been hypothesised that flexoelectricity could be linked to the origin of their antiferroelectricity. In this work, we have measured the flexoelectricity of two different antiferroelectrics (PbZrO3 and AgNbO3) as a function of temperature, up to and beyond their Curie temperature. Although their flexocoupling shows a sharp peak at the antiferroelectric phase transition, neither flexoelectricity nor the flexocoupling coefficients are anomalously high, suggesting that it is unlikely that flexoelectricity causes antiferroelectricity. © 2018 Author(s).

View abstract
Flexoelectricity in Bones
Vasquez-Sancho F., Abdollahi A., Damjanovic D., Catalan G. Advanced Materials; 30 (9, 1705316) 2018. 10.1002/adma.201705316. IF: 21.950

Bones generate electricity under pressure, and this electromechanical behavior is thought to be essential for bone's self-repair and remodeling properties. The origin of this response is attributed to the piezoelectricity of collagen, which is the main structural protein of bones. In theory, however, any material can also generate voltages in response to strain gradients, thanks to the property known as flexoelectricity. In this work, the flexoelectricity of bone and pure bone mineral (hydroxyapatite) are measured and found to be of the same order of magnitude; the quantitative similarity suggests that hydroxyapatite flexoelectricity is the main source of bending-induced polarization in cortical bone. In addition, the measured flexoelectric coefficients are used to calculate the (flexo)electric fields generated by cracks in bone mineral. The results indicate that crack-generated flexoelectricity is theoretically large enough to induce osteocyte apoptosis and thus initiate the crack-healing process, suggesting a central role of flexoelectricity in bone repair and remodeling. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

View abstract
Flexoelectricity in Bones (vol 30, 1705316, 2018)
Vasquez-Sancho, Fabian; Abdollahi, Amir; Damjanovic, Dragan; Catalan, Gustau; Advanced Materials; 30 (21): e1801413. 2018. 10.1002/adma.201801413. IF: 21.950
Interfacial Engineering of Metal Oxides for Highly Stable Halide Perovskite Solar Cells
Mingorance A., Xie H., Kim H.-S., Wang Z., Balsells M., Morales-Melgares A., Domingo N., Kazuteru N., Tress W., Fraxedas J., Vlachopoulos N., Hagfeldt A., Lira-Cantu M. Advanced Materials Interfaces; 5 (22, 1800367) 2018. 10.1002/admi.201800367. IF: 4.834

Oxides employed in halide perovskite solar cells (PSCs) have already demonstrated to deliver enhanced stability, low cost, and the ease of fabrication required for the commercialization of the technology. The most stable PSCs configuration, the carbon-based hole transport layer-free PSC (HTL-free PSC), has demonstrated a stability of more than one year of continuous operation partially due to the dual presence of insulating oxide scaffolds and conductive oxides. Despite these advances, the stability of PSCs is still a concern and a strong limiting factor for their industrial implementation. The engineering of oxide interfaces functionalized with molecules (like self-assembly monolayers) or polymers results in the passivation of defects (traps), providing numerous advantages such as the elimination of hysteresis and the enhancement of solar cell efficiency. But most important is the beneficial effect of interfacial engineering on the lifetime and stability of PSCs. In this work, the authors provide a brief insight into the recent developments reported on the surface functionalization of oxide interfaces in PSCs with emphasis on the effect of device stability. This paper also discusses the different binding modes, their effect on defect passivation, band alignment or dipole formation, and how these parameters influence device lifetime. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

View abstract
Local Piezoelectric Behavior of Potassium Sodium Niobate Prepared by a Facile Synthesis via Water Soluble Precursors
Senes N., Iacomini A., Domingo N., Enzo S., Mulas G., Cuesta-Lopez S., Garroni S. Physica Status Solidi (A) Applications and Materials Science; 215 (16, 1700921) 2018. 10.1002/pssa.201700921. IF: 1.795

Due to the ever-increasing restrictions connected to the use of toxic lead-based materials, the developing of lead-free piezoceramics has become one of the most urgent tasks. In this context, potassium sodium niobate materials (KNN) have attracted a lot of interest as promising candidates due to their excellent piezo properties. For this reason, many efforts have been addressed to optimize the synthesis process now suffering by several drawbacks including the high volatilization of potassium and sodium at the conventional high temperature treatments and the use of expensive metal precursors. To overcome these issues, a new modified Pechini method to synthesize single phase K0.5Na0.5NbO3 powders, from water soluble metal precursors, is presented. Microstructural and structural parameters are characterized by X-ray diffraction (XRD). Depending on the amount of citric acid added to the starting reagents, two pure single-phase K0.5Na0.5NbO3 (2 g citric acid) and K0.3Na0.7NbO3 (0.2 g citric acid), respectively, are obtained with a good crystallinity at a moderate temperature of 500 °C. The piezo responses of the as calcined systems are tested by piezoresponse force microscopy (PFM). K0.5Na0.5NbO3 exhibits a much higher response with respect to the other phase, which relates to the larger crystallinity and to the chemical composition. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

View abstract
Piezoelectric Mimicry of Flexoelectricity
Abdollahi A., Vásquez-Sancho F., Catalan G. Physical Review Letters; 121 (20, 205502) 2018. 10.1103/PhysRevLett.121.205502. IF: 8.839

The origin of "giant" flexoelectricity, orders of magnitude larger than theoretically predicted, yet frequently observed, is under intense scrutiny. There is mounting evidence correlating giant flexoelectriclike effects with parasitic piezoelectricity, but it is not clear how piezoelectricity (polarization generated by strain) manages to imitate flexoelectricity (polarization generated by strain gradient) in typical beam-bending experiments, since in a bent beam the net strain is zero. In addition piezoelectricity changes sign under space inversion but giant flexoelectricity is insensitive to space inversion, seemingly contradicting a piezoelectric origin. Here we show that, if a piezoelectric material has its piezoelectric coefficient asymmetrically distributed across the sample, it will generate a nonzero bending-induced polarization impossible to distinguish from true flexoelectricity even by inverting the sample. The effective flexoelectric coefficient caused by piezoelectricity is functionally identical to, and often larger than, intrinsic flexoelectricity: our calculations show that, for standard perovskite ferroelectrics, even a tiny gradient of piezoelectricity (1% variation of piezoelectric coefficient across 1 mm) is sufficient to yield a giant effective flexoelectric coefficient of 1 μC/m, three orders of magnitude larger than the intrinsic expectation value. © 2018 American Physical Society.

View abstract
Substrate Dependence of the Freezing Dynamics of Supercooled Water Films: A High-Speed Optical Microscope Study
Pach E., Rodriguez L., Verdaguer A. Journal of Physical Chemistry B; 122 (2): 818 - 826. 2018. 10.1021/acs.jpcb.7b06933. IF: 3.146

The freezing of supercooled water films on different substrates was investigated using a high-speed camera coupled to an optical microscope, obtaining details of the freezing process not described in the literature before. We observed the two well known freezing stages (fast dendritic growth and slow freezing of the water liquid left after the dendritic growth), but we separated the process into different phenomena that were studied separately: two-dimensional dendrite growth on the substrate interface, vertical dendrite growth, formation and evolution of ice domains, trapping of air bubbles and freezing of the water film surface. We found all of these processes to be dependent on both the supercooling temperature and the substrate used. Ice dendrite (or ice front) growth during the first stage was found to be dependent on thermal properties of the substrate but could not be unequivocally related to them. Finally, for low supercooling, a direct relationship was observed between the morphology of the dendrites formed in the first stage, which depends on the substrate, and the roughness and the shape of the surface of the ice, when freezing of the film was completed. This opens the possibility of using surfaces and coatings to control ice morphology beyond anti-icing properties. © 2017 American Chemical Society.

View abstract
Wide and ultra-wide bandgap oxides: Where paradigm-shift photovoltaics meets transparent power electronics
Pérez-Tomás A., Chikoidze E., Jennings M.R., Russell S.A.O., Teherani F.H., Bove P., Sandana E.V., Rogers D.J. Proceedings of SPIE - The International Society for Optical Engineering; 10533 ( 105331Q) 2018. 10.1117/12.2302576. IF: 0.000

Oxides represent the largest family of wide bandgap (WBG) semiconductors and also offer a huge potential range of complementary magnetic and electronic properties, such as ferromagnetism, ferroelectricity, antiferroelectricity and high-temperature superconductivity. Here, we review our integration of WBG and ultra WBG semiconductor oxides into different solar cells architectures where they have the role of transparent conductive electrodes and/or barriers bringing unique functionalities into the structure such above bandgap voltages or switchable interfaces. We also give an overview of the state-of-the-art and perspectives for the emerging semiconductor β- Ga2O3, which is widely forecast to herald the next generation of power electronic converters because of the combination of an UWBG with the capacity to conduct electricity. This opens unprecedented possibilities for the monolithic integration in solar cells of both self-powered logic and power electronics functionalities. Therefore, WBG and UWBG oxides have enormous promise to become key enabling technologies for the zero emissions smart integration of the internet of things. © Copyright 2018 SPIE.

View abstract

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.

View abstract
Epitaxial Growth of SrTiO3 Films on Cube-Textured Cu-Clad Substrates by PLD at Low Temperature Under Reducing Atmosphere
Padilla J.A., Xuriguera E., Rodríguez L., Vannozzi A., Segarra M., Celentano G., Varela M. Nanoscale Research Letters; 12 (1, 226) 2017. 10.1186/s11671-017-1997-9. IF: 2.833

The growth of epitaxial {001}<100> SrTiO3 (STO) on low-cost cube-textured Cu-based clad substrate at low temperature was carried out by means of pulsed laser deposition (PLD). STO film was deposited in one step under a reducing atmosphere (5% H2 and 95% Ar mixture) to prevent the oxidation of the metal surface. The optimization of PLD parameters leads to a sharpest biaxial texture at a temperature as low as 500 °C and a thickness of 500 nm with a (100) STO layer. The upper limit of highly textured STO thickness was also investigated. The maximum thickness which retains the best quality {001}<100> texture is 800 nm, since the texture is preserved not only through the layer but also on the surface. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) measurements showed that STO films are continuous, dense, and smooth with very low roughness (between 5 and 7 nm). This paper describes the development of STO layer by means of PLD in absence of oxygen throughout the process, suggesting an alternative and effective method for growing highly {001}<100> textured STO layer on low-cost metal substrates. © 2017, The Author(s).

View abstract
Ferroelectrics as Smart Mechanical Materials
Cordero-Edwards K., Domingo N., Abdollahi A., Sort J., Catalan G. Advanced Materials; 29 (37, 1702210) 2017. 10.1002/adma.201702210. IF: 19.791

The mechanical properties of materials are insensitive to space inversion, even when they are crystallographically asymmetric. In practice, this means that turning a piezoelectric crystal upside down or switching the polarization of a ferroelectric should not change its mechanical response. Strain gradients, however, introduce an additional source of asymmetry that has mechanical consequences. Using nanoindentation and contact-resonance force microscopy, this study demonstrates that the mechanical response to indentation of a uniaxial ferroelectric (LiNbO3) does change when its polarity is switched, and use this mechanical asymmetry both to quantify its flexoelectricity and to mechanically read the sign of its ferroelectric domains. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

View abstract
Functional oxide as an extreme high-k dielectric towards 4H-SiC MOSFET incorporation
Russell S.A.O., Jennings M.R., Dai T., Li F., Hamilton D.P., Fisher C.A., Sharma Y.K., Mawby P.A., Pérez-Tomás A. Materials Science Forum; 897 MSF: 155 - 158. 2017. 10.4028/www.scientific.net/MSF.897.155. IF: 0.399

MOS Capacitors are demonstrated on 4H-SiC using an octahedral ABO3 ferroic thin-film as a dielectric prepared on several buffer layers. Five samples were prepared: ABO3 on SiC, ABO3 on SiC with a SiO2 buffer (10 nm and 40 nm) and ABO3 on SiC with an Al2O3 buffer (10nm and 40 nm). Depending on the buffer material the oxide forms in either the pyrochlore or perovskite phase. A better lattice match with the Al2O3 buffer yields a perovskite phase with internal switchable dipoles. Hysteresis polarization-voltage loops show an oxide capacitance of ~ 0.2 μF/cm2 in the accumulation region indicating a dielectric constant of ~120. © 2017 Trans Tech Publications, Switzerland.

View abstract
Heteroepitaxial Beta-Ga2O3 on 4H-SiC for an FET with Reduced Self Heating
Russell S.A.O., Perez-Tomas A., McConville C.F., Fisher C.A., Hamilton D.P., Mawby P.A., Jennings M.R. IEEE Journal of the Electron Devices Society; 5 (4, 7932063): 256 - 261. 2017. 10.1109/JEDS.2017.2706321. IF: 3.141

A method to improve thermal management of ${\beta }$ -Ga2O3 FETs is demonstrated here via simulation of epitaxial growth on a 4H-SiC substrate. Using a recently published device as a model, the reduction achieved in self-heating allows the device to be driven at higher gate voltages and increases the overall performance. For the same operating parameters an 18% increase in peak drain current and 15% reduction in lattice temperature are observed. Device dimensions may be substantially reduced without detriment to performance and normally off operation may be achieved. © 2013 IEEE.

View abstract
Hidden Magnetic States Emergent under Electric Field, in A Room Temperature Composite Magnetoelectric Multiferroic
Clarkson J.D., Fina I., Liu Z.Q., Lee Y., Kim J., Frontera C., Cordero K., Wisotzki S., Sanchez F., Sort J., Hsu S.L., Ko C., Aballe L., Foerster M., Wu J., Christen H.M., Heron J.T., Schlom D.G., Salahuddin S., Kioussis N., Fontcuberta J., Marti X., Ramesh R. Scientific Reports; 7 (1, 15460) 2017. 10.1038/s41598-017-13760-y. IF: 4.259

The ability to control a magnetic phase with an electric field is of great current interest for a variety of low power electronics in which the magnetic state is used either for information storage or logic operations. Over the past several years, there has been a considerable amount of research on pathways to control the direction of magnetization with an electric field. More recently, an alternative pathway involving the change of the magnetic state (ferromagnet to antiferromagnet) has been proposed. In this paper, we demonstrate electric field control of the Anomalous Hall Transport in a metamagnetic FeRh thin film, accompanying an antiferromagnet (AFM) to ferromagnet (FM) phase transition. This approach provides us with a pathway to "hide" or "reveal" a given ferromagnetic region at zero magnetic field. By converting the AFM phase into the FM phase, the stray field, and hence sensitivity to external fields, is decreased or eliminated. Using detailed structural analyses of FeRh films of varying crystalline quality and chemical order, we relate the direct nanoscale origins of this memory effect to site disorder as well as variations of the net magnetic anisotropy of FM nuclei. Our work opens pathways toward a new generation of antiferromagnetic - ferromagnetic interactions for spintronics. © 2017 The Author(s).

View abstract
High-Temperature Electrical and Thermal Aging Performance and Application Considerations for SiC Power DMOSFETs
Hamilton D.P., Jennings M.R., Perez-Tomas A., Russell S.A.O., Hindmarsh S.A., Fisher C.A., Mawby P.A. IEEE Transactions on Power Electronics; 32 (10, 7776925): 7967 - 7979. 2017. 10.1109/TPEL.2016.2636743. IF: 7.151

The temperature dependence and stability of three different commercially-available unpackaged SiC Dmosfets have been measured. On-state resistances increased to 6 or 7 times their room temperature values at 350 °C. Threshold voltages almost doubled after tens of minutes of positive gate voltage stressing at 300 °C, but approached their original values again after only one or two minutes of negative gate bias stressing. Fortunately, the change in drain current due to these threshold instabilities was almost negligible. However, the threshold approaches zero volts at high temperatures after a high temperature negative gate bias stress. The zero gate bias leakage is low until the threshold voltage reduces to approximately 150 mV, where-after the leakage increases exponentially. Thermal aging tests demonstrated a sudden change from linear to nonlinear output characteristics after 24-100 h air storage at 300 °C and after 570-1000 h in N2 atmosphere. We attribute this to nickel oxide growth on the drain contact metallization which forms a heterojunction p-n diode with the SiC substrate. It was determined that these state-of-the-art SiC mosfet devices may be operated in real applications at temperatures far exceeding their rated operating temperatures. © 1986-2012 IEEE.

View abstract
Lateral Magnetically Modulated Multilayers by Combining Ion Implantation and Lithography
Menéndez E., Modarresi H., Petermann C., Nogués J., Domingo N., Liu H., Kirby B.J., Mohd A.S., Salhi Z., Babcock E., Mattauch S., Van Haesendonck C., Vantomme A., Temst K. Small; 13 (11, 1603465) 2017. 10.1002/smll.201603465. IF: 8.643

The combination of lithography and ion implantation is demonstrated to be a suitable method to prepare lateral multilayers. A laterally, compositionally, and magnetically modulated microscale pattern consisting of alternating Co (1.6 µm wide) and Co-CoO (2.4 µm wide) lines has been obtained by oxygen ion implantation into a lithographically masked Au-sandwiched Co thin film. Magnetoresistance along the lines (i.e., current and applied magnetic field are parallel to the lines) reveals an effective positive giant magnetoresistance (GMR) behavior at room temperature. Conversely, anisotropic magnetoresistance and GMR contributions are distinguished at low temperature (i.e., 10 K) since the O-implanted areas become exchange coupled. This planar GMR is principally ascribed to the spatial modulation of coercivity in a spring-magnet-type configuration, which results in 180° Néel extrinsic domain walls at the Co/Co-CoO interfaces. The versatility, in terms of pattern size, morphology, and composition adjustment, of this method offers a unique route to fabricate planar systems for, among others, spintronic research and applications. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

View abstract
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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P-type beta-gallium oxide: A new perspective for power and optoelectronic devices
Chikoidze E., Fellous A., Perez-Tomas A., Sauthier G., Tchelidze T., Ton-That C., Than Huynh T., Phillips M., Russell S., Jennings M., Berini B., Jomard F., Dumont Y. Materials Today Physics; 3: 118 - 126. 2017. 10.1016/j.mtphys.2017.10.002.

Wide-bandgap semiconductors (WBG) are expected to be applied to solid-state lighting and power devices, supporting a future energy-saving society. Here we present evidence of p-type conduction in the undoped WBG β-Ga2O3. Hole conduction, established by Hall and Seebeck measurements, is consistent with findings from photoemission and cathodoluminescence spectroscopies. The ionization energy of the acceptor level was measured to be 1.1eV above the valence band edge. The gallium vacancy was identified as a possible acceptor candidate based on thermodynamic equilibrium Ga2O3 (crystal) – O2 (gas) system calculations (Kroger theory) which revealed a window without oxygen vacancy compensation. The possibility of fabricating large diameter wafers of β-Ga2O3 of p and n type nature, provides new avenues for high power and deep UV-optoelectronic devices.

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Physical characterisation of 3C-SiC(001)/SiO2 interface using XPS
Li F., Vavasour O., Walker M., Martin D.M., Sharma Y., Russell S., Jennings M., Pérez-Tomás A., Mawby P.A. Materials Science Forum; 897 MSF: 151 - 154. 2017. 10.4028/www.scientific.net/MSF.897.151. IF: 0.399

Normally-on MOSFETs were fabricated on 3C-SiC epilayers (Si face) using high temperature (1300 °C) wet oxidation. XPS analysis found little carbon at the MOS interface yet the channel mobility (60 cm2/V.s) is considerably low. Si suboxides (SiOx, x<2) exist at the wet oxidised 3C-SiC/SiO2 interface, which may act as interface traps and degrade the conduction performance. © 2017 Trans Tech Publications, Switzerland.

View abstract
Production of biofunctionalized MoS2 flakes with rationally modified lysozyme: A biocompatible 2D hybrid material
Siepi M., Morales-Narváez E., Domingo N., Monti D.M., Notomista E., Merkoçi A. 2D Materials; 4 (3, 035007) 2017. 10.1088/2053-1583/aa7966. IF: 6.937

Bioapplications of 2D materials embrace demanding features in terms of environmental impact, toxicity and biocompatibility. Here we report on the use of a rationally modified lysozyme to assist the exfoliation of Mos2 bulk crystals suspended in water through ultrasonic exfoliation. The design of the proposed lysozyme derivative provides this exfoliated 2D-materail with both, hydrophobic groups that interact with the surface of Mos2 and hydrophilic groups exposed to the aqueous medium, which hinders its re-Aggregation. This approach, clarified also by molecular docking studies, leads to a stable material (ζ-potential, 27 ?} 1 mV) with a yield of up to 430 μg ml-1. The bio-hybrid material was characterized in terms of number of layers and optical properties according to different slots separated by diverse centrifugal forces. Furthermore the obtained material was proved to be biocompatible using human normal keratinocytes and human cancer epithelial cells, whereas the method was demonstrated to be applicable to produce other 2D materials such as graphene. This approach is appealing for the advantageous production of high quality Mos2 flakes and their application in biomedicine and biosensing. Moreover, this method can be applied to different starting materials, making the denatured lysozyme a promising bio-Tool for surface functionalization of 2D materials. © 2017 IOP Publishing Ltd.

View abstract

2016

3C-SiC Transistor with Ohmic Contacts Defined at Room Temperature
Li F., Sharma Y., Walker D., Hindmarsh S., Jennings M., Martin D., Fisher C., Gammon P., Pérez-Tomás A., Mawby P. IEEE Electron Device Letters; 37 (9, 7518645): 1189 - 1192. 2016. 10.1109/LED.2016.2593771. IF: 2.528

Among all SiC polytypes, only 3C-SiC has a cubic structure and can be hetero-epitaxial grown on large area Si substrate, thus providing an alternative choice for fabricating cheap wide bandgap power devices. Here, we present a low resistivity (~3 × 10-5Ω cm2) ohmic contact formed by directly depositing a Ti/Ni metal stack on n-type 3C-SiC without any extra annealing. For the first time, 3C-SiC lateral MOSFETs with asdeposited ohmic contacts were fabricated, and it turned out not only the ohmic contact is free from any interface voids, but also a higher field-effect mobility value (~80 cm2/V · s) was achieved compared with the annealed devices. © 1980-2012 IEEE.

View abstract
A flexoelectric microelectromechanical system on silicon
Bhaskar U.K., Banerjee N., Abdollahi A., Wang Z., Schlom D.G., Rijnders G., Catalan G. Nature Nanotechnology; 11 (3): 263 - 266. 2016. 10.1038/nnano.2015.260. IF: 35.267

Flexoelectricity allows a dielectric material to polarize in response to a mechanical bending moment and, conversely, to bend in response to an electric field. Compared with piezoelectricity, flexoelectricity is a weak effect of little practical significance in bulk materials. However, the roles can be reversed at the nanoscale. Here, we demonstrate that flexoelectricity is a viable route to lead-free microelectromechanical and nanoelectromechanical systems. Specifically, we have fabricated a silicon-compatible thin-film cantilever actuator with a single flexoelectrically active layer of strontium titanate with a figure of merit (curvature divided by electric field) of 3.33MV â '1, comparable to that of state-of-the-art piezoelectric bimorph cantilevers. © 2016 Macmillan Publishers Limited.

View abstract
Above-Bandgap Photovoltages in Antiferroelectrics
Pérez-Tomás A., Lira-Cantú M., Catalan G. Advanced Materials; 28 (43): 9644 - 9647. 2016. 10.1002/adma.201603176. IF: 18.960

The closed circuit photocurrent and open circuit photovoltage of antiferroelectric thin films were characterized both in their ground (antipolar) state and in their polarized state. A sharp transition happens from near zero to large photovoltages as the polarization is switched on, consistent with the activation of the bulk photovoltaic effect. The AFE layers have been grown by a solution processing method (sol?gel synthesis followed by spin coating deposition) onto fluorine-doped tin oxide (FTO), a transparent conducting oxide with low sheet resistance and a higher resilience to high-temperature processing than indium tin oxide and a standard for solar cells such as organometal trihalide perovskites. Light absorption confirmed that the PZO films are, as expected, wide-band gap semiconductors with a gap of 3.7.8 eV and thus highly absorbing in the near-ultraviolet range. On a virgin sample, there is no shortcircuit photocurrent, consistent with the antipolar nature of the ground state. As an external bias voltage is applied, the current remains negligible until suddenly, at the coercive voltage, a spike is observed, corresponding to the transient displacement current caused by the onset of polarization.

View abstract
Conductance of Threading Dislocations in InGaAs/Si Stacks by Temperature-CAFM Measurements
Couso C., Iglesias V., Porti M., Claramunt S., Nafría M., Domingo N., Cordes A., Bersuker G. IEEE Electron Device Letters; 37 (5, 7422696): 640 - 643. 2016. 10.1109/LED.2016.2537051. IF: 2.528

The stacks of III-V materials, grown on the Si substrate, that are considered for the fabrication of highly scaled devices tend to develop structural defects, in particular threading dislocations (TDs), which affect device electrical properties. We demonstrate that the characteristics of the TD sites can be analyzed by using the conductive atomic force microscopy technique with nanoscale spatial resolution within a wide temperature range. In the studied InGaAs/Si stacks, electrical conductance through the TD sites was found to be governed by the Poole-Frenkel emission, while the off-TDs conductivity is dominated by the thermionic emission process. © 1980-2012 IEEE.

View abstract
Enhanced flexoelectric-like response in oxide semiconductors
Narvaez J., Vasquez-Sancho F., Catalan G. Nature; 538 (7624): 219 - 221. 2016. 10.1038/nature19761. IF: 38.138

Flexoelectricity is a property of all dielectric materials whereby they polarize in response to deformation gradients such as those produced by bending. Although it is generally thought of as a property of dielectric insulators, insulation is not a formal requirement: in principle, semiconductors can also redistribute their free charge in response to strain gradients. Here we show that bending a semiconductor not only generates a flexoelectric-like response, but that this response can in fact be much larger than in insulators. By doping single crystals of wide-bandgap oxides to increase their conductivity, their effective flexoelectric coefficient was increased by orders of magnitude. This large response can be explained by a barrier-layer mechanism that remains important even at the macroscale, where conventional (insulator) flexoelectricity otherwise tends to be small. Our results open up the possibility of using semiconductors as active ingredients in electromechanical transducer applications. © 2016 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.

View abstract
Ferroelectric Domain Structures in Low-Strain BaTiO3
Everhardt A.S., Matzen S., Domingo N., Catalan G., Noheda B. Advanced Electronic Materials; 2 (1, 1500214) 2016. 10.1002/aelm.201500214. IF: 0.000

Epitaxial strain in ferroelectric films offers the possibility to enhance the piezoelectric performance utilizing low crystal symmetries and high density of domain walls. Ferroelectric BaTiO3 has been predicted to order in a variety of phases and domain configurations when grown under low strain on low mismatched substrates, but little experimental evidence of that region of the phase diagram exist. Here, epitaxial BaTiO3 thin films are grown on NdScO3 substrates under ≈0.1% strain. A monoclinic ca1/ca2 phase, with 90° periodic in-plane domain configuration and small additional out-of-plane component of polarization, is stabilized at room temperature and investigated using piezoelectric force microscopy and X-ray diffraction. Above 50 °C, this phase is transformed into an a/c phase with alternating in-plane and out-of-plane polarizations and forming zigzag domain walls between up-polarized and down-polarized superdomains. Both types of domain patterns are highly anisotropic, giving rise to very long domain walls. Above 130 °C, the paraelectric phase is observed. The occurrence of a phase transition close to room temperature, a low symmetry ca1/ca2 phase, and the formation of periodic domains make of this material a promising candidate for high piezoelectric response. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

View abstract
Flexoelectric MEMS: Towards an electromechanical strain diode
Bhaskar U.K., Banerjee N., Abdollahi A., Solanas E., Rijnders G., Catalan G. Nanoscale; 8 (3): 1293 - 1298. 2016. 10.1039/c5nr06514c. IF: 7.760

Piezoelectricity and flexoelectricity are two independent but not incompatible forms of electromechanical response exhibited by nanoscale ferroelectrics. Here, we show that flexoelectricity can either enhance or suppress the piezoelectric response of the cantilever depending on the ferroelectric polarity and lead to a diode-like asymmetric (two-state) electromechanical response. © 2016 The Royal Society of Chemistry.

View abstract
Freezing the Nonclassical Crystal Growth of a Coordination Polymer Using Controlled Dynamic Gradients
Rubio-Martinez M., Imaz I., Domingo N., Abrishamkar A., Mayor T.S., Rossi R.M., Carbonell C., deMello A.J., Amabilino D.B., Maspoch D., Puigmartí-Luis J. Advanced Materials; : 8150 - 8155. 2016. 10.1002/adma.201506462. IF: 18.960

An experiment was conducted to show that diffusion-limited and kinetically controlled growth regimes occurring in microfluidic devices can provide valuable insights into crystallization processes. The microfluidic channels employed in this study were structured in PDMS master form fabricated by standard photolithographic techniques. Before attaching the cured and structured PDMS mould to a glass coverslip through plasma activation, inlet holes connecting the microfluidic channels were punched with a Biopsy puncher. The cross-sectional dimensions of the microchannels were 50 μm into 50 μm for the four input microchannels, and 250 μm into 50 μm for the main reactor channel. The total length of the main reactor channel was 9 mm. Data were indexed, integrated, and scaled using HKL2000 software. The H atoms were included in theoretical positions but not refined. The low max value was due to the data collection process, which was performed in the BM16 line with only a phi scan. The structure was solved by direct methods using the program SHELXS-97. The AFM results suggest that the early stage isolated seeds organize at a single level and in a perpendicular fashion, leading to the final plate-like crystalline morphologies observed in bulk and at an FFR of 0.1.

View abstract
Identification of HIV-1–Based Virus-like Particles by Multifrequency Atomic Force Microscopy
González-Domínguez I., Gutiérrez-Granados S., Cervera L., Gòdia F., Domingo N. Biophysical Journal; 111 (6): 1173 - 1179. 2016. 10.1016/j.bpj.2016.07.046. IF: 3.632

Virus-like particles (VLPs) have become a promising platform for vaccine production. VLPs are formed by structural viral proteins that inherently self-assemble when expressed in a host cell. They represent a highly immunogenic and safe vaccine platform, due to the absence of the viral genome and its high protein density. One of the most important parameters in vaccine production is the quality of the product. A related bottleneck in VLP-based products is the presence of cellular vesicles as a major contaminant in the preparations, which will require the set up of techniques allowing for specific discrimination of VLPs from host vesicular bodies. In this work novel, to our knowledge, multifrequency (MF) atomic force microscopy (AFM) has permitted full structural nanophysical characterization by its access to the virus capsid of the HIV-based VLPs. The assessment of these particles by advanced amplitude modulation-frequency modulation (AM-FM) viscoelastic mapping mode has enhanced the imaging resolution of their nanomechanical properties, opening a new window for the study of the biophysical attributes of VLPs. Finally, the identification and differentiation of HIV-based VLPs from cellular vesicles has been performed under ambient conditions, providing, to our knowledge, novel methodology for the monitoring and quality control of VLPs. © 2016 Biophysical Society

View abstract
Improved channel mobility by oxide nitridation for n-channel MOSFET on 3C-SiC(100)/Si
Li F., Sharma Y.K., Jennings M.R., Pérez-Tomás A., Shah V.A., Rong H., Russell S.A.O., Martin D.M., Mawby P.A. Materials Science Forum; 858: 667 - 670. 2016. 10.4028/www.scientific.net/MSF.858.667. IF: 0.000

In this work we studied the gate oxidation temperature and nitridation influences on the resultant 3C-SiC MOSFET forward characteristics. Conventional long channel lateral MOSFETs were fabricated on 3C-SiC(100) epilayers grown on Si substrates using five different oxidation processes. Both room temperature and high temperature (up to 500K) forward IV performance were characterised, and channel mobility as high as 90cm2/V.s was obtained for devices with nitrided gate oxide, considerable higher than the ones without nitridation process (~70 cm2/V.s). © 2016 Trans Tech Publications, Switzerland.

View abstract
Monolithic integration of room-temperature multifunctional BaTiO 3 -CoFe 2 O 4 epitaxial heterostructures on Si(001)
Scigaj M., Dix N., Gázquez J., Varela M., Fina I., Domingo N., Herranz G., Skumryev V., Fontcuberta J., Sánchez F. Scientific Reports; 6 ( 31870) 2016. 10.1038/srep31870. IF: 5.228

The multifunctional (ferromagnetic and ferroelectric) response at room temperature that is elusive in single phase multiferroic materials can be achieved in a proper combination of ferroelectric perovskites and ferrimagnetic spinel oxides in horizontal heterostructures. In this work, lead-free CoFe 2 O 4 /BaTiO 3 bilayers are integrated with Si(001) using LaNiO 3 /CeO 2 /YSZ as a tri-layer buffer. They present structural and functional properties close to those achieved on perovskite substrates: the bilayers are fully epitaxial with extremely flat surface, and exhibit robust ferromagnetism and ferroelectricity at room temperature. © The Author(s) 2016.

View abstract
Performance and stability of mixed FAPbI3(0.85)MAPbBr3(0.15) halide perovskite solar cells under outdoor conditions and the effect of low light irradiation
Reyna Y., Salado M., Kazim S., Pérez-Tomas A., Ahmad S., Lira-Cantu M. Nano Energy; 30: 570 - 579. 2016. 10.1016/j.nanoen.2016.10.053. IF: 11.553

We demonstrate for the first time, the real lifetime response of mixed halide perovskite solar cells (PSCs) for >1000 h under outdoor conditions and the exceptional photoresponse observed at low-light intensities attributed to the ionic-electronic nature of the material. The investigated devices were fabricated by utilizing mixed perovskites containing formamidinium (FA) and methylammonium (MA) cations, in a one step solution-process method through a solvent engineering approach. The devices’ architecture is FTO/TiO2 (blocking layer) TiO2 (mesoporous)/FAPbI3(0.85)MAPbBr3(0.15)/Spiro-OMeTAD/Au. Notably, low short circuit current (Jsc) was observed at low light intensities (<50 W/m2) together with high open circuit potential build-up, which resulted in high PCEs. This response is in agreement with a “double electronic-ionic transport” model of the halide perovskite where the ionic component dominates at low light intensities and the electronic component dictates at high light irradiances. Our results highlight the exceptional stability of mixed MA/FA mesoscopic PSCs when operated for >1000 h under real outdoor conditions and the strong ionic component observed at low light irradiation. © 2016 Elsevier Ltd

View abstract
Piezoelectric Templates - New Views on Biomineralization and Biomimetics
Stitz N., Eiben S., Atanasova P., Domingo N., Leineweber A., Burghard Z., Bill J. Scientific Reports; 6 ( 26518) 2016. 10.1038/srep26518. IF: 5.228

Biomineralization in general is based on electrostatic interactions and molecular recognition of organic and inorganic phases. These principles of biomineralization have also been utilized and transferred to bio-inspired synthesis of functional materials during the past decades. Proteins involved in both, biomineralization and bio-inspired processes, are often piezoelectric due to their dipolar character hinting to the impact of a template's piezoelectricity on mineralization processes. However, the piezoelectric contribution on the mineralization process and especially the interaction of organic and inorganic phases is hardly considered so far. We herein report the successful use of the intrinsic piezoelectric properties of tobacco mosaic virus (TMV) to synthesize piezoelectric ZnO. Such films show a two-fold increase of the piezoelectric coefficient up to 7.2 pm V-1 compared to films synthesized on non-piezoelectric templates. By utilizing the intrinsic piezoelectricity of a biotemplate, we thus established a novel synthesis pathway towards functional materials, which sheds light on the whole field of biomimetics. The obtained results are of even broader and general interest since they are providing a new, more comprehensive insight into the mechanisms involved into biomineralization in living nature.

View abstract
Spontaneous formation of spiral-like patterns with distinct periodic physical properties by confined electrodeposition of Co-In disks
Golvano-Escobal I., Gonzalez-Rosillo J.C., Domingo N., Illa X., López-Barberá J.F., Fornell J., Solsona P., Aballe L., Foerster M., Surinãch S., Baró M.D., Puig T., Pané S., Nogués J., Pellicer E., Sort J. Scientific Reports; 6 ( 30398) 2016. 10.1038/srep30398. IF: 5.228

Spatio-temporal patterns are ubiquitous in different areas of materials science and biological systems. However, typically the motifs in these types of systems present a random distribution with many possible different structures. Herein, we demonstrate that controlled spatio-temporal patterns, with reproducible spiral-like shapes, can be obtained by electrodeposition of Co-In alloys inside a confined circular geometry (i.e., in disks that are commensurate with the typical size of the spatio-temporal features). These patterns are mainly of compositional nature, i.e., with virtually no topographic features. Interestingly, the local changes in composition lead to a periodic modulation of the physical (electric, magnetic and mechanical) properties. Namely, the Co-rich areas show higher saturation magnetization and electrical conductivity and are mechanically harder than the In-rich ones. Thus, this work reveals that confined electrodeposition of this binary system constitutes an effective procedure to attain template-free magnetic, electric and mechanical surface patterning with specific and reproducible shapes.

View abstract
Water Affinity and Surface Charging at the z-Cut and y-Cut LiNbO3 Surfaces: An Ambient Pressure X-ray Photoelectron Spectroscopy Study
Cordero-Edwards K., Rodríguez L., Calò A., Esplandiu M.J., Pérez-Dieste V., Escudero C., Domingo N., Verdaguer A. Journal of Physical Chemistry C; 120 (42): 24048 - 24055. 2016. 10.1021/acs.jpcc.6b05465. IF: 4.509

Polarization dependence of water adsorption and desorption on LiNbO3 surfaces was demonstrated using X-ray photoelectron spectroscopy (XPS) carried out in situ under near-ambient conditions. Positive and negative (0001) faces (z-cut) of the same crystal were compared for the same temperature and pressure conditions. Our results indicate a preferential adsorption on the positive face of the crystal with increasing water pressure and also higher desorption temperature of the adsorbed molecular water at the positive face. Adsorption measurements on the (1100) face (y-cut) showed also strong affinity to water, as observed for the z-cut positive surface. We found a direct relation between the capacity of the surface to discharge and/or to screen surface charges and the affinity for water of each face. XPS spectra indicate the presence of OH groups at the surface for all the conditions and surfaces measured. © 2016 American Chemical Society.

View abstract

2015

Dielectric relaxation in YMnO3 single crystals
Adem U., Mufti N., Nugroho A.A., Catalan G., Noheda B., Palstra T.T.M. Journal of Alloys and Compounds; 638: 228 - 232. 2015. 10.1016/j.jallcom.2015.02.207. IF: 2.999

We have investigated the origin of the dielectric relaxation in YMnO3 single crystals. Two distinct dielectric relaxation features were observed at low (200≤T≤373K) and high (300≤T≤450K) temperatures. Analysis of our detailed frequency, electrode and thickness dependent dielectric measurements and ac conductivity data as well as the use of single crystals allow us to get a comprehensive picture of these relaxations. The low temperature relaxation is attributed to the Maxwell-Wagner type effects originating from the dipoles at the surface while the high temperature one is suggested to originate from hopping of charge carriers resulting from the second ionization of oxygen vacancies. © 2015 Elsevier B.V. All rights reserved.

View abstract
Electrical activation of nitrogen heavily implanted 3C-SiC(1 0 0)
Li F., Sharma Y., Shah V., Jennings M., Pérez-Tomás A., Myronov M., Fisher C., Leadley D., Mawby P. Applied Surface Science; 353: 958 - 963. 2015. 10.1016/j.apsusc.2015.06.169. IF: 2.711

A degenerated wide bandgap semiconductor is a rare system. In general, implant levels lie deeper in the band-gap and carrier freeze-out usually takes place at room temperature. Nevertheless, we have observed that heavily doped n-type degenerated 3C-SiC films are achieved by nitrogen implantation level of ∼6 × 1020 cm-3 at 20 K. According to temperature dependent Hall measurements, nitrogen activation rates decrease with the doping level from almost 100% (1.5 × 1019 cm-3, donor level 15 meV) to ∼12% for 6 × 1020 cm-3. Free donors are found to saturate in 3C-SiC at ∼7 × 1019 cm-3. The implanted film electrical performances are characterized as a function of the dopant doses and post implantation annealing (PIA) conditions by fabricating Van der Pauw structures. A deposited SiO2 layer was used as the surface capping layer during the PIA process to study its effect on the resultant film properties. From the device design point of view, the lowest sheet resistivity (∼1.4 mΩ cm) has been observed for medium doped (4 × 1019 cm-3) sample with PIA 1375 °C 2 h without a SiO2 cap. Crown Copyright © 2015 Published by Elsevier B.V. All rights reserved.

View abstract
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.

View abstract
Ferroelectrics: Negative capacitance detected
Catalan G., Jimenez D., Gruverman A. Nature Materials; 14 (2): 137 - 139. 2015. 10.1038/nmat4195. IF: 36.503

[No abstract available]

View abstract
Four-state ferroelectric spin-valve
Quindeau A., Fina I., Marti X., Apachitei G., Ferrer P., Nicklin C., Pippel E., Hesse D., Alexe M. Scientific Reports; 5 ( 9749) 2015. 10.1038/srep09749. IF: 5.578

Spin-valves had empowered the giant magnetoresistance (GMR) devices to have memory. The insertion of thin antiferromagnetic (AFM) films allowed two stable magnetic field-induced switchable resistance states persisting in remanence. In this letter, we show that, without the deliberate introduction of such an AFM layer, this functionality is transferred to multiferroic tunnel junctions (MFTJ) allowing us to create a four-state resistive memory device. We observed that the ferroelectric/ferromagnetic interface plays a crucial role in the stabilization of the exchange bias, which ultimately leads to four robust electro tunnel electro resistance (TER) and tunnel magneto resistance (TMR) states in the junction.

View abstract
Fracture toughening and toughness asymmetry induced by flexoelectricity
Abdollahi A., Peco C., Millán D., Arroyo M., Catalan G., Arias I. Physical Review B - Condensed Matter and Materials Physics; 92 (9, 094101) 2015. 10.1103/PhysRevB.92.094101. IF: 3.736

Cracks generate the largest strain gradients that any material can withstand. Flexoelectricity (coupling between strain gradient and polarization) must therefore play an important role in fracture physics. Here we use a self-consistent continuum model to evidence two consequences of flexoelectricity in fracture: the resistance to fracture increases as structural size decreases, and it becomes asymmetric with respect to the sign of polarization. The latter phenomenon manifests itself in a range of intermediate sizes where piezo- and flexoelectricity compete. In BaTiO3 at room temperature, this range spans from 0.1 to 50 nm, a typical thickness range for epitaxial ferroelectric thin films. © 2015 American Physical Society.

View abstract
Giant reversible nanoscale piezoresistance at room temperature in Sr2IrO4 thin films
Domingo N., López-Mir L., Paradinas M., Holy V., Železný J., Yi D., Suresha S.J., Liu J., Rayan Serrao C., Ramesh R., Ocal C., Martí X., Catalan G. Nanoscale; 7 (8): 3453 - 3459. 2015. 10.1039/c4nr06954d. IF: 7.394

Layered iridates have been the subject of intense scrutiny on account of their unusually strong spin-orbit coupling, which opens up a narrow bandgap in a material that would otherwise be a metal. This insulating state is very sensitive to external perturbations. Here, we show that vertical compression at the nanoscale, delivered using the tip of a standard scanning probe microscope, is capable of inducing a five orders of magnitude change in the room temperature resistivity of Sr2IrO4. The extreme sensitivity of the electronic structure to anisotropic deformations opens up a new angle of interest on this material, with the giant and fully reversible perpendicular piezoresistance rendering iridates as promising materials for room temperature piezotronic devices. This journal is © The Royal Society of Chemistry.

View abstract
In-plane tunnelling field-effect transistor integrated on Silicon
Fina I., Apachitei G., Preziosi D., Deniz H., Kriegner D., Marti X., Alexe M. Scientific Reports; 5 ( 14367) 2015. 10.1038/srep14367. IF: 5.578

Silicon has persevered as the primary substrate of microelectronics during last decades. During last years, it has been gradually embracing the integration of ferroelectricity and ferromagnetism. The successful incorporation of these two functionalities to silicon has delivered the desired non-volatility via charge-effects and giant magneto-resistance. On the other hand, there has been a numerous demonstrations of the so-called magnetoelectric effect (coupling between ferroelectric and ferromagnetic order) using nearly-perfect heterostructures. However, the scrutiny of the ingredients that lead to magnetoelectric coupling, namely magnetic moment and a conducting channel, does not necessarily require structural perfection. In this work, we circumvent the stringent requirements for epilayers while preserving the magnetoelectric functionality in a silicon-integrated device. Additionally, we have identified an in-plane tunnelling mechanism which responds to a vertical electric field. This genuine electroresistance effect is distinct from known resistive-switching or tunnel electro resistance.

View abstract
Large Flexoelectric Anisotropy in Paraelectric Barium Titanate
Narvaez J., Saremi S., Hong J., Stengel M., Catalan G. Physical Review Letters; 115 (3, 037601) 2015. 10.1103/PhysRevLett.115.037601. IF: 7.512

The bending-induced polarization of barium titanate single crystals has been measured with an aim to elucidate the origin of the large difference between theoretically predicted and experimentally measured flexoelectricity in this material. The results indicate that part of the difference is due to polar regions (short-range order) that exist above TC and up to T∗≈200-225°C. Above T∗, however, the flexovoltage coefficient still shows an unexpectedly large anisotropy for a cubic material, with (001)-oriented crystals displaying 10 times more flexoelectricity than (111)-oriented crystals. Theoretical analysis shows that this anisotropy cannot be a bulk property, and we therefore interpret it as indirect evidence for the theoretically predicted but experimentally elusive contribution of surface piezoelectricity to macroscopic bending-induced polarization. © 2015 American Physical Society. © 2015 American Physical Society.

View abstract
Mechanical tuning of LaAlO3/SrTiO3 interface conductivity
Sharma P., Ryu S., Burton J.D., Paudel T.R., Bark C.W., Huang Z., Ariando, Tsymbal E.Y., Catalan G., Eom C.B., Gruverman A. Nano Letters; 15 (5): 3547 - 3551. 2015. 10.1021/acs.nanolett.5b01021. IF: 13.592

In recent years, complex-oxide heterostructures and their interfaces have become the focus of significant research activity, primarily driven by the discovery of emerging states and functionalities that open up opportunities for the development of new oxide-based nanoelectronic devices. The highly conductive state at the interface between insulators LaAlO3 and SrTiO3 is a prime example of such emergent functionality, with potential application in high electron density transistors. In this report, we demonstrate a new paradigm for voltage-free tuning of LaAlO3/SrTiO3 (LAO/STO) interface conductivity, which involves the mechanical gating of interface conductance through stress exerted by the tip of a scanning probe microscope. The mechanical control of channel conductivity and the long retention time of the induced resistance states enable transistor functionality with zero gate voltage. © 2015 American Chemical Society.

View abstract
Nanomechanics of flexoelectric switching
Očenášek J., Lu H., Bark C.W., Eom C.B., Alcalá J., Catalan G., Gruverman A. Physical Review B - Condensed Matter and Materials Physics; 92 (3, 035417) 2015. 10.1103/PhysRevB.92.035417. IF: 3.736

We examine the phenomenon of flexoelectric switching of polarization in ultrathin films of barium titanate induced by a tip of an atomic force microscope (AFM). The spatial distribution of the tip-induced flexoelectricity is computationally modeled both for perpendicular mechanical load (point measurements) and for sliding load (scanning measurements), and compared with experiments. We find that (i) perpendicular load does not lead to stable ferroelectric switching in contrast to the load applied in the sliding contact load regime, due to nontrivial differences between the strain distributions in both regimes: ferroelectric switching for the perpendicular load mode is impaired by a strain gradient inversion layer immediately underneath the AFM tip; while for the sliding load regime, domain inversion is unimpaired within a greater material volume subjected to larger values of the mechanically induced electric field that includes the region behind the sliding tip; (ii) beyond a relatively small value of an applied force, increasing mechanical pressure does not increase the flexoelectric field inside the film, but results instead in a growing volume of the region subjected to such field that aids domain nucleation processes; and (iii) the flexoelectric coefficients of the films are of the order of few nC/m, which is much smaller than for bulk BaTiO3 ceramics, indicating that there is a "flexoelectric size effect" that mirrors the ferroelectric one. ©2015 American Physical Society.

View abstract
Nanoscale conductive pattern of the homoepitaxial AlGaN/GaN transistor
Pérez-Tomás A., Catalàn G., Fontserè A., Iglesias V., Chen H., Gammon P.M., Jennings M.R., Thomas M., Fisher C.A., Sharma Y.K., Placidi M., Chmielowska M., Chenot S., Porti M., Nafría M., Cordier Y. Nanotechnology; 26 (11, 115203) 2015. 10.1088/0957-4484/26/11/115203. IF: 3.821

The gallium nitride (GaN)-based buffer/barrier mode of growth and morphology, the transistor electrical response (25-310 C) and the nanoscale pattern of a homoepitaxial AlGaN/GaN high electron mobility transistor (HEMT) have been investigated at the micro and nanoscale. The low channel sheet resistance and the enhanced heat dissipation allow a highly conductive HEMT transistor (Ids>1 A mm-1) to be defined (0.5 A mm-1 at 300 C). The vertical breakdown voltage has been determined to be ∼850 V with the vertical drain-bulk (or gate-bulk) current following the hopping mechanism, with an activation energy of 350 meV. The conductive atomic force microscopy nanoscale current pattern does not unequivocally follow the molecular beam epitaxy AlGaN/GaN morphology but it suggests that the FS-GaN substrate presents a series of preferential conductive spots (conductive patches). Both the estimated patches density and the apparent random distribution appear to correlate with the edge-pit dislocations observed via cathodoluminescence. The sub-surface edge-pit dislocations originating in the FS-GaN substrate result in barrier height inhomogeneity within the HEMT Schottky gate producing a subthreshold current. © 2015 IOP Publishing Ltd.

View abstract
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.

View abstract
Prospect for antiferromagnetic spintronics
Marti X., Fina I., Jungwirth T. IEEE Transactions on Magnetics; 51 (4, 7109970) 2015. 10.1109/TMAG.2014.2358939. IF: 1.386

Exploiting both spin and charge of the electron in electronic micordevices has lead to a tremendous progress in both basic condensed-matter research and microelectronic applications, resulting in the modern field of spintronics. Current spintronics relies primarily on ferromagnets while antiferromagnets (AFMs) have traditionally played only a supporting role. Recently, antiferromagnets have been revisited as potential candidates for the key active elements in spintronic devices. In this paper, we review approaches that have been employed for reading, writing, and storing information in AFMs. © 1965-2012 IEEE.

View abstract
Revealing water films structure from force reconstruction in dynamic AFM
Calò A., Domingo N., Santos S., Verdaguer A. Journal of Physical Chemistry C; 119 (15): 8258 - 8265. 2015. 10.1021/acs.jpcc.5b02411. IF: 4.772

The structure of water films in contact with surfaces has direct implications in many important interfacial processes, from biology to climatology, as well as in ice nucleation. Here we report on the detection of individual ice-like water layers adsorbed on surfaces in ambient conditions. Reconstructed force profiles obtained in amplitude modulation atomic force microscopy (AM-AFM) on top of (111) BaF2 surfaces, with a lattice constant close to the distance of facing water molecules in hexagonal ice (Ih), showed characteristic oscillations in the attractive regime with a periodicity of 3.7 Å. This distance matches the thickness of a bilayer of Ih ice and is absent in force profiles on (111) CaF2 surfaces, which show a different lattice parameter. A thickness of 2.6 Å is measured for the first water layer in contact with the surface, corresponding to a high-density liquid film structure predicted from calculations in the literature. Our results indicate that, although epitaxial Ih growth of the first water layer on BaF2 crystals is not observed, the matching of the lattice parameter between Ih and BaF2 does induce a strong ordering of the water films and the formation of ice-like structures, even at room temperature. © 2015 American Chemical Society.

View abstract

2014

Anisotropic magnetoresistance in an antiferromagnetic semiconductor
Fina, I.; Marti, X.; Yi, D.; Liu, J.; Chu, J.H.; Rayan-Serrao, C.; Suresha, S.; Shick, A.B.; Ž elezný, J.; Jungwirth, T.; Fontcuberta, J.; Ramesh, R. Nature Communications; 5 2014. 10.1038/ncomms5671. IF: 10.742
Chemical strain and oxidation-reduction kinetics of epitaxial thin films of mixed ionic-electronic conducting oxides determined by x-ray diffraction
Moreno, R.; Zapata, J.; Roqueta, J.; Bagués, N.; Santiso, J. Journal of the Electrochemical Society; 161 (11): F3046 - F3051. 2014. 10.1149/2.0091411jes. IF: 2.859
Electrocatalytic tuning of biosensing response through electrostatic or hydrophobic enzyme-graphene oxide interactions
Baptista-Pires, L.; Pérez-López, B.; Mayorga-Martinez, C.C.; Morales-Narváez, E.; Domingo, N.; Esplandiu, M.J.; Alzina, F.; Torres, C.M.S.; Merkoçi, A. Biosensors and Bioelectronics; 61: 655 - 662. 2014. 10.1016/j.bios.2014.05.028. IF: 6.451
Improved performance of 4H-SiC PiN diodes using a novel combined high temperature oxidation and annealing process
Fisher, C.A.; Jennings, M.R.; Sharma, Y.K.; Hamilton, D.P.; Gammon, P.M.; Pérez-Tomás, A.; Thomas, S.M.; Burrows, S.E.; Mawby, P.A. IEEE Transactions on Semiconductor Manufacturing; 27 (3): 443 - 451. 2014. 10.1109/TSM.2014.2336701. IF: 0.977
On the Schottky barrier height lowering effect of Ti3SiC2 in ohmic contacts to p-type 4H-SiC
C. A. Fisher; M. R. Jennings; Y. K. Sharma; A. Sanchez; D. Walker; P. M. Gammon; A. Pérez-Tomás; S. M. Thomas; S. E. Burrows; P. A. Mawby International Journal of Fundamental Physical Sciences (IJFPS); 4 (3): 95 - 100. 2014. 10.14331/ijfps.2014.330071. IF: 0.000
Origin of the enhanced flexoelectricity of relaxor ferroelectrics
Narvaez, J.; Catalan, G. Applied Physics Letters; 104 (16) 2014. 10.1063/1.4871686. IF: 3.515
Spectroscopy methods for molecular nanomagnets
Baker, M.L.; Blundell, S.J.; Domingo, N.; Hill, S. Structure and Bonding; 164: 231 - 292. 2014. 10.1007/430-2014-155. IF: 1.836
Spintronic functionality of BiFeO3domain walls
Lee, J.H.; Fina, I.; Marti, X.; Kim, Y.H.; Hesse, D.; Alexe, M. Advanced Materials; 26 (41): 7078 - 7082. 2014. 10.1002/adma.201402558. IF: 15.409

2013

Anisotropic 18O tracer diffusion in epitaxial films of GdBaCo2O5+δ cathode material with different orientations
Zapata, J.; Burriel, M.; García, P.; Kilner, J.A.; Santiso, J. Journal of Materials Chemistry A; 1: 7408 - 7414. 2013. 10.1039/c3ta10749c. IF: 6.108
Chemical strain kinetics induced by oxygen surface exchange in epitaxial films explored by time-resolved X-ray diffraction
Moreno, R.; García, P.; Zapata, J.; Roqueta, J.; Chaigneau, J.; Santiso, J. Chemistry of Materials; 25: 3640 - 3647. 2013. 10.1021/cm401714d. IF: 8.238
Flexoelectric effect in solids
Zubko, P.; Catalan, G.; Tagantsev, A.K. Annual Review of Materials Research; 43: 387 - 421. 2013. 10.1146/annurev-matsci-071312-121634. IF: 16.179
Local properties of the surface layer(s) of BiFeO3 single crystals
Domingo, N.; Narvaez, J.; Alexe, M.; Catalan, G. Journal of Applied Physics; 113 2013. 10.1063/1.4801974. IF: 2.210
Surface screening of written ferroelectric domains in ambient conditions
Segura, J.J.; Domingo, N.; Fraxedas, J.; Verdaguer, A. Journal of Applied Physics; 113 2013. 10.1063/1.4801983. IF: 2.210
Thickness scaling of ferroelastic domains in PbTiO3 films on DyScO3
Nesterov, O.; Matzen, S.; Magen, C.; Vlooswijk, A.H.G.; Catalan, G.; Noheda, B. Applied Physics Letters; 103 2013. 10.1063/1.4823536. IF: 3.794

2012

Advances on structuring, integration and magnetic characterization of molecular nanomagnets on surfaces and devices
Domingo, N.; Bellido, E.; Ruiz-Molina, D. Chemical Society Reviews; 41: 258 - 302. 2012. 10.1039/c1cs15096k.
Controlled positioning of nanoparticles on graphene by noninvasive AFM lithography
Bellido, E.; Ojea-Jiménez, I.; Ghirri, A.; Alvino, C.; Candini, A.; Puntes, V.; Affronte, M.; Domingo, N.; Ruiz-Molina, D. Langmuir : the ACS journal of surfaces and colloids; 28: 12400 - 12409. 2012. 10.1021/la3023419.
Domain wall nanoelectronics
Catalan, G.; Seidel, J.; Ramesh, R.; Scott, J.F. Reviews of Modern Physics; 84: 119 - 156. 2012. 10.1103/RevModPhys.84.119.
Elastic and anelastic relaxations in the relaxor ferroelectric Pb(Mg 1/3Nb 2/3)O 3: I. Strain analysis and a static order parameter
Carpenter, M.A.; Bryson, J.F.J.; Catalan, G.; Howard, C.J. Journal of Physics Condensed Matter; 24 2012. 10.1088/0953-8984/24/4/045901.
Elastic and anelastic relaxations in the relaxor ferroelectric Pb(Mg 1/3Nb 2/3)O 3: II. Strainorder parameter coupling and dynamic softening mechanisms
Carpenter, M.A.; Bryson, J.F.J.; Catalan, G.; Zhang, S.J.; Donnelly, N.J. Journal of Physics Condensed Matter; 24 2012. 10.1088/0953-8984/24/4/045902.
Magnetotransport at domain walls in BiFeO 3
He, Q.; Yeh, C.-H.; Yang, J.-C.; Singh-Bhalla, G.; Liang, C.-W.; Chiu, P.-W.; Catalan, G.; Martin, L.W.; Chu, Y.-H.; Scott, J.F.; Ramesh, R. Physical Review Letters; 108 2012. 10.1103/PhysRevLett.108.067203.
Mechanical Writing of Ferroelectric Polarization
Lu, H. ; Bark, C.W.; Esque de los Ojos, D.; Alcala, J. ; Eom, C. B.; Catalan, G. ; Gruverman, A. SCIENCE; 336 (6077): 59 - 61. 2012. DOI: 10.1126/science.1218693.
Structural, spectroscopic, magnetic and electrical characterization of Ca-doped polycrystalline bismuth ferrite, Bi 1xCa xFeO 3x/2 (x0.1)
Sardar, K.; Hong, J.; Catalan, G.; Biswas, P.K.; Lees, M.R.; Walton, R.I.; Scott, J.F.; Redfern, S.A.T. Journal of Physics Condensed Matter; 24 2012. 10.1088/0953-8984/24/4/045905.
Structuration and integration of magnetic nanoparticles on surfaces and devices
Bellido, E.; Domingo, N.; Ojea-Jiménez, I.; Ruiz-Molina, D. Small; 8: 1465 - 1491. 2012. 10.1002/smll.201101456.
Surface phase transitions in BiFeO 3 below room temperature
Jarrier, R.; Marti, X.; Herrero-Albillos, J.; Ferrer, P.; Haumont, R.; Gemeiner, P.; Geneste, G.; Berthet, P.; Schülli, T.; Cevc, P.; Blinc, R.; Wong, S.S.; Park, T.-J.; Alexe, M.; Carpenter, M.A.; Scott, J.F.; Catalan, G.; Dkhil, B. Physical Review B - Condensed Matter and Materials Physics; 85 2012. 10.1103/PhysRevB.85.184104.

2011

Flexoelectric rotation of polarization in ferroelectric thin films.
Catalan, G. ; Lubk, A.; Vlooswijk, A. H. G.; Snoeck, E.; Magen, C. ; Janssens, A.; Rispens, G.; Rijnders, G.; Blank, D. H. A.; Noheda, B. Nature Materials; 10: 963. 2011. .
Skin layer of BiFeO3 single crystals
Martí, X.; Ferrer, P.; Herrero-Albillos, J.; Narvaez, J.; Holy, V.; Barrett, N.; Alexe, M.; Catalan, G. Physical Review Letters; 106 2011. 10.1103/PhysRevLett.106.236101.

2010

Comment on "Nanometer resolution piezoresponse force microscopy to study deep submicron ferroelectric and ferroelastic domains" [Appl. Phys. Lett. 94, 162903 (2009)]----NO ES ARTICULO
Vlooswijk, A.H.G.; Catalan, G.; Noheda, B. Applied Physics Letters; 97 2010. 10.1063/1.3467005.
Electric-field control of the metal-insulator transition in ultrathin NdNiO3 films
Scherwitzl, R.; Zubko, P.; Lezama, I.G.; Ono, S.; Morpurgo, A.F.; Catalan, G.; Triscone, J.-M. Advanced Materials; 22: 5517 - 5520. 2010. 10.1002/adma.201003241.
Influence of the microstructure on the high-temperature transport properties of GdBaCo2O5.5+δ epitaxial films
Burriel, M.; Casas-Cabanas, M.; Zapata, J.; Tan, H.; Verbeeck, J.; Solís, C.; Roqueta, J.; Skinner, S.J.; Kilner, J.A.; Van Tendeloo, G.; Santiso, J. Chemistry of Materials; 22: 5512 - 5520. 2010. 10.1021/cm101423z.
Interface exchange coupling in Co nanoparticles dispersed in a Mn matrix
Binns, C. ; Domingo, N. ; Testa, A.M.; Fiorani, D. ; Trohidou, K.N. ; Vasilakaki, M ; Blackman, J.A.; Asaduzzaman, A.M.; Baker, S.; Roy, M.; Peddis, D. Journal of Physics Condensed Matter; 22: 436005 - 6 paginas. 2010. 10.1088/0953-8984/22/43/436005.
Landau theory of domain wall magnetoelectricity
Daraktchiev, M.; Catalan, G.; Scott, J.F. Physical Review B - Condensed Matter and Materials Physics; 81 2010. 10.1103/PhysRevB.81.224118.
Metal-radical chains based on polychlorotriphenylmethyl radicals: Synthesis, structure, and magnetic properties
Roques, N.; Domingo, N.; Maspoch, D.; Wurst, K.; Rovira, C.; Tejada, J.; Ruiz-Molina, D.; Veciana, J. Inorganic Chemistry; 49: 3482 - 3488. 2010. 10.1021/ic100037z.
Neutron diffraction study of the BiFeO3 spin cycloid at low temperature
Herrero-Albillos, J.; Catalan, G.; Rodriguez-Velamazan, J.A.; Viret, M.; Colson, D.; Scott, J.F. Journal of Physics Condensed Matter; 22: 256001 - 256005. 2010. 10.1088/0953-8984/22/25/256001.
The β-to-γ transition in BiFeO 3: A powder neutron diffraction study
Arnold, D.C.; Knight, K.S.; Catalan, G.; Redfern, S.A.T.; Scott, J.F.; Lightfoot, P.; Morrison, F.D. Advanced Functional Materials; 20: 2116 - 2123. 2010. 10.1002/adfm.201000118.
The flexoelectricity of barium and strontium titanates from first principles
Hong, J.; Catalan, G.; Scott, J.F.; Artacho, E. Journal of Physics Condensed Matter; 22 2010. 10.1088/0953-8984/22/11/112201.

2009

Domains in Ferroelectric Nanodots
A. Schilling; D. Byrne; G. Catalan; K. G. Webber; Y. A. Genenko; G. S. Wu; J. F. Scott; J. M. Gregg Nano Letters; 9: 3359. 2009. .
Effect of chemical substitution on the Néel temperature of multiferroic Bi1¿xCaxFeO3
G. Catalan; K. Sardar; N. S. Church; J. F. Scott; R. J. Harrison; S. A. T. Redfern Physical Review B; 79: 212415. 2009. 10.1103/PhysRevB.79.212415.
Effect of wall thickness on the ferroelastic domain size of BaTiO3
G. Catalan; I. Lukyanchuk; A. Schilling; J. M. Gregg; J. F. Scott Journal of Materials Science; 44: 5307. 2009. 10.1007/s10853-009-3554-0.
Elastic and electrical anomalies at low-temperature phase transitions in BiFeO3
S. A. T. Redfern; Can Wang; J. W. Hong; G. Catalan; J. F. Scott Journal of Physics Condensed Matter; 20: 452205. 2009. 10.1088/0953-8984/20/45/452205.
Epitaxial TbMnO3 thin films on SrTiO3 substrates: a structural study
C. J. M. Daumont; D. Mannix; S. Venkatesan; G. Catalan; D. Rubi; B. J. Kooi; J. Th. M. De Hosson; B. Noheda Journal of Physics Condensed Matter; 21: 182001. 2009. 10.1088/0953-8984/21/18/182001.
Origin of ferroelastic domains in free-standing single-crystal ferroelectric films
I. A. Luk¿yanchuk; A. Schilling; J.M. Gregg; G. Catalan; J.F. Scott Physical Review B; 79: 144111. 2009. 10.1103/PhysRevB.79.144111.
Particle-size dependence of magnetization relaxation in Mn12 crystals
Domingo, N.; Luis, F.; Nakano, M.; Muntó, M.; Gómez, J.; Chaboy, J.; Ventosa, N.; Campo, J.; Veciana, J.; Ruiz-Molina, D. Physical Review B - Condensed Matter and Materials Physics; 79 2009. 10.1103/PhysRevB.79.214404.
Physics and applications of BiFeO3
G. Catalan; J. F. Scott Advanced Materials; 21: 2463 - 2485. 2009. 10.1002/adma.200802849.

2008

Conduction at domain walls in oxide multiferroics
J. Seidel; L. W. Martin; Q. He; Q. Zhan; Y.-H. Chu; A. Rother; M. E. Hawkridge; P. Maksymovych; P. Yu; M. Gajek; N. Balke; S. V. Kalinin; S. Gemming; H. Lichte; F. Wang; G. Catalan; J. F. Scott; N. A. Spaldin; J. Orenstein; R. Ramesh Nature Materials; 8: 229 - 234. 2008. 10.1038/nmat2373.
Conformal oxide coating of carbon nanotubes
S. Kawasaki; G. Catalan; H. J. Fan; M. M. Saad; J. M. Gregg; M. A. Correa-Duarte; J. Rybczynski; F. D. Morrison; T. Tatsuta; O. Tsuji; and J. F. Scott Applied Physics Letters; 92 (5): 53109. 2008. 10.1063/1.2841710.
Fractal walls and domain size scaling in thin films of multiferroic BiFeO3
G. Catalan; H. Béa; S. Fusil; M. Bibes; P. Paruch; A. Barthélémy; and J. F. Scott Physical Review Letters; 100: 27602. 2008. 10.1103/PhysRevLett.100.027602.
Landau Theory of Multiferroic Domain Walls
M. Daraktchiev; G. Catalan; J. F. Scott Ferroelectrics; 375: 122 - 131. 2008. 10.1080/00150190802437969.
Progress in perovskite nickelate research
G. Catalan Phase Transitions; 81: 729. 2008. 10.1080/01411590801992463.
ß phase and ¿-ß metal-insulator transition in multiferroic BiFeO3
R. Palai; R. S. Katiyar; H. Schmid; P. Tissot; S. J. Clark; J. Robertson; S. A. Redfern; G. Catalan; J. F. Scott Physical Review B - Condensed Matter and Materials Physics; 77: 14110. 2008. 10.1103/PhysRevB.77.014110.
Solution-process coating of vertical ZnO nanowires with ferroelectrics
S. Kawasaki; H. J. Fan; G. Catalan; F. D. Morrison; T. Tatsuta; O. Tsuji; J. F Scott Nanotechnology; 19: 375302. 2008. 10.1088/0957-4484/19/37/375302.

2007

First-row transition-metal complexes based on a carboxylate polychlorotriphenylmethyl radical: Trends in metal-radical exchange interactions
Maspoch, D.; Domingo, N.; Ruiz-Molina, D.; Wurst, K.; Hernández, J.M.; Lloret, F.; Tejada, J.; Rovira, C.; Veciana, J. Inorganic Chemistry; 46: 1627 - 1633. 2007. 10.1021/ic061815x.
Structural and magnetic modulation of a purely organic open framework by selective guest inclusion
Maspoch, D.; Domingo, N.; Roques, N.; Wurst, K.; Tejada, J.; Rovira, C.; Ruiz-Molina, D.; Veciana, J. Chemistry - A European Journal; 13: 8153 - 8163. 2007. 10.1002/chem.200700353.

2005

An unusually stable trinuclear manganese(II) complex bearing bulk carboxylic radical ligands
Maspoch, D.; Gómez-Segura, J.; Domingo, N.; Ruiz-Molina, D.; Wurst, K.; Rovira, C.; Tejada, J.; Veciana, J. Inorganic Chemistry; 44: 6936 - 6938. 2005. 10.1021/ic050977a.
Carboxylic-substituted polychlorotriphenylmethyl radicals, new organic building-blocks to design nanoporous magnetic molecular materials
Maspoch, D.; Domingo, N.; Ruiz-Molina, D.; Wurst, K.; Tejada, J.; Rovira, C.; Veciana, J. Comptes Rendus Chimie; 8: 1213 - 1225. 2005. 10.1016/j.crci.2005.02.020.
Coexistence of ferro- and antiferromagnetic interactions in a metal-organic radical-based (6,3)-helical network with large channels
Maspoch, D.; Domingo, N.; Ruiz-Molina, D.; Wurst, K.; Hernández, J.-M.; Vaughan, G.; Rovira, C.; Lloret, F.; Tejada, J.; Veciana, J. Chemical Communications; : 5035 - 5037. 2005. 10.1039/b505827a.
Hydrogen-bonded self-assemblies in a polychlorotriphenylmethyl radical derivative substituted with six meta-carboxylic acid groups
Roques, N.; Maspoch, D.; Domingo, N.; Ruiz-Molina, D.; Wurst, K.; Tejada, J.; Rovira, C.; Veciana, J. Chemical Communications; : 4801 - 4803. 2005. 10.1039/b508952b.

2004

A Robust Nanocontainer Based on a Pure Organic Free Radical
Maspoch, D.; Domingo, N.; Ruiz-Molina, D.; Wurst, K.; Tejada, J.; Rovira, C.; Veciana, J. Journal of the American Chemical Society; 126: 730 - 731. 2004. 10.1021/ja038988v.
A robust purely organic nanoporous magnet
Maspoch, D.; Domingo, N.; Ruiz-Molina, D.; Wurst, K.; Vaughan, G.; Tejada, J.; Rovira, C.; Veciana, J. Angewandte Chemie - International Edition; 43: 1828 - 1832. 2004. 10.1002/anie.200353358.
Chiral, single-molecule nanomagnets: Synthesis, magnetic characterization and natural and magnetic circular dichroism
Gerbier, P.; Domingo, N.; Gómez-Segura, J.; Ruiz-Molina, D.; Amabilino, D.B.; Tejada, J.; Williamson, B.E.; Veciana, J. Journal of Materials Chemistry; 14: 2455 - 2460. 2004. 10.1039/b403062a.
Magnetism of isolated Mn12 single-molecule magnets detected by magnetic circular dichroism: Observation of spin tunneling with a magneto-optical technique
Domingo, N.; Williamson, B.E.; Gómez-Segura, J.; Gerbier, Ph.; Ruiz-Molina, D.; Amabilino, D.B.; Veciana, J.; Tejada, J. Physical Review B; 69: 524051 - 524054. 2004. .
Open-shell channel-like salts formed by the supramolecular assembly of a tricarboxylated perchlorotriphenylmethyl radical and a [Co(bpy) 3]2+ cation
Maspoch, D.; Ruiz-Molina, D.; Wurst, K.; Vaughan, G.; Domingo, N.; Tejada, J.; Rovira, C.; Veciana, J. CrystEngComm; 6: 573 - 578. 2004. 10.1039/b410810h.