Publications
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A generalized approach for evaluating the mechanical properties of polymer nanocomposites reinforced with spherical fillers
Martinez-Garcia J.C., Serraïma-Ferrer A., Lopeandía-Fernández A., Lattuada M., Sapkota J., Rodríguez-Viejo J. Nanomaterials; 11 (4, 830) 2021. 10.3390/nano11040830. IF: 5.076
In this work, the effective mechanical reinforcement of polymeric nanocomposites containing spherical particle fillers is predicted based on a generalized analytical three-phase-series-parallel model, considering the concepts of percolation and the interfacial glassy region. While the concept of percolation is solely taken as a contribution of the filler-network, we herein show that the glassy interphase between filler and matrix, which is often in the nanometers range, is also to be considered while interpreting enhanced mechanical properties of particulate filled polymeric nanocomposites. To demonstrate the relevance of the proposed generalized equation, we have fitted several experimental results which show a good agreement with theoretical predictions. Thus, the approach presented here can be valuable to elucidate new possible conceptual routes for the creation of new materials with fundamental technological applications and can open a new research avenue for future studies. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
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Characterization of charge states in conducting organic nanoparticles by X-ray photoemission spectroscopy
Fraxedas J., Vollmer A., Koch N., de Caro D., Jacob K., Faulmann C., Valade L. Materials; 14 (8, 2058) 2021. 10.3390/ma14082058. IF: 3.623
The metallic and semiconducting character of a large family of organic materials based on the electron donor molecule tetrathiafulvalene (TTF) is rooted in the partial oxidation (charge transfer or mixed valency) of TTF derivatives leading to partially filled molecular orbital-based electronic bands. The intrinsic structure of such complexes, with segregated donor and acceptor molecular chains or planes, leads to anisotropic electronic properties (quasi one-dimensional or two-dimensional) and morphology (needle-like or platelet-like crystals). Recently, such materials have been synthesized as nanoparticles by intentionally frustrating the intrinsic anisotropic growth. X-ray photoemission spectroscopy (XPS) has emerged as a valuable technique to characterize the transfer of charge due to its ability to discriminate the different chemical environments or electronic configurations manifested by chemical shifts of core level lines in high-resolution spectra. Since the photoemission process is inherently fast (well below the femtosecond time scale), dynamic processes can be efficiently explored. We determine here the fingerprint of partial oxidation on the photoemission lines of nanoparticles of selected TTF-based conductors. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
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Critical Role of Electrical Resistivity in Magnetoionics
De Rojas J., Salguero J., Quintana A., Lopeandia A., Liedke M.O., Butterling M., Attallah A.G., Hirschman E., Wagner A., Abad L., Costa-Krämer J.L., Sort J., Menéndez E. Physical Review Applied; 16 (3, 034042) 2021. 10.1103/PhysRevApplied.16.034042. IF: 4.985
The utility of electrical resistivity as an indicator of magnetoionic performance in stoichiometrically and structurally similar thin-film systems is demonstrated. A series of highly nanocrystalline cobalt nitride (Co-N) thin films (85 nm thick) with a broad range of electrical properties exhibit markedly different magnetoionic behaviors. Semiconducting, near stoichiometric CoN films show the best performance, better than their metallic and insulating counterparts. Resistivity reflects the interplay between atomic bonding, carrier localization, and structural defects, and in turn determines the strength and distribution of applied electric fields inside the actuated films. This fact, generally overlooked, reveals that resistivity can be used to quickly evaluate the potential of a system to exhibit optimal magnetoionic effects, while also opening interesting challenges. © 2021 American Physical Society.
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In situ XPS analysis of the electronic structure of silicon and titanium thin films exposed to low-pressure inductively-coupled RF plasma
Fraxedas J., Schütte M., Sauthier G., Tallarida M., Ferrer S., Carlino V., Pellegrin E. Applied Surface Science; 542 (148684) 2021. 10.1016/j.apsusc.2020.148684. IF: 6.707
Carbon contamination of synchrotron and free-electron lasers beamline optics continues to be a major nuisance due to the interaction of the intense photon beams with the surfaces of the optical elements in the presence of residual gases even in ultrahigh vacuum (UHV) conditions. Among the available in situ cleaning strategies, low-pressure radio frequency (RF) plasma treatment has emerged as a useful and relatively simple approach to remove such carbon contamination. However, the irreversible damage that the plasma may induce in such critical surfaces has to be carefully characterized before its general application. In this study, we focus on reducing the amount of carbon from UHV chamber inside surfaces via silicon and titanium coatings using a low-pressure inductively-coupled downstream plasma source and we characterize the surface alterations by in situ X-ray photoemission spectroscopy (XPS). The in situ mirror cleaning is simulated by means of silicon wafers. We observe upward band bending, which translates into lower binding energies of the photoemission lines, that we attribute to the generation of vacancies and trapped charges in the oxide layers. © 2020 Elsevier B.V.
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Magneto-Ionics in Single-Layer Transition Metal Nitrides
De Rojas J., Salguero J., Ibrahim F., Chshiev M., Quintana A., Lopeandia A., Liedke M.O., Butterling M., Hirschmann E., Wagner A., Abad L., Costa-Krämer J.L., Menéndez E., Sort J. ACS Applied Materials and Interfaces; 13 (26): 30826 - 30834. 2021. 10.1021/acsami.1c06138. IF: 9.229
Magneto-ionics allows for tunable control of magnetism by voltage-driven transport of ions, traditionally oxygen or lithium and, more recently, hydrogen, fluorine, or nitrogen. Here, magneto-ionic effects in single-layer iron nitride films are demonstrated, and their performance is evaluated at room temperature and compared with previously studied cobalt nitrides. Iron nitrides require increased activation energy and, under high bias, exhibit more modest rates of magneto-ionic motion than cobalt nitrides. Ab initio calculations reveal that, based on the atomic bonding strength, the critical field required to induce nitrogen-ion motion is higher in iron nitrides (≈6.6 V nm-1) than in cobalt nitrides (≈5.3 V nm-1). Nonetheless, under large bias (i.e., well above the magneto-ionic onset and, thus, when magneto-ionics is fully activated), iron nitride films exhibit enhanced coercivity and larger generated saturation magnetization, surpassing many of the features of cobalt nitrides. The microstructural effects responsible for these enhanced magneto-ionic effects are discussed. These results open up the potential integration of magneto-ionics in existing nitride semiconductor materials in view of advanced memory system architectures. © 2021 American Chemical Society. All rights reserved.
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Superconducting super-organized nanoparticles of the superconductor (BEDT-TTF)2Cu(NCS)(2)
Dominique de Caro, Kane Jacob, Marco Revelli-Beaumont, Christophe Faulmann, Lydie Valade, Marine Tassé, Sonia Mallet-Ladeira, Shuxiang Fan, Tadashi Kawamoto, Takehiko Mori, Jordi Fraxedas Synthetic Metals; 278 (116844) 2021. 10.1016/j.synthmet.2021.116844. IF: 3.266
The synthesis of (BEDT-TTF)2Cu(NCS)2 in the presence of poly(ethylene glycol) leads to super-organized nanoparticles of 2–8 nm size. Samples contain crystalline nanoparticles of the κ-(BEDT-TTF)2Cu(NCS)2 phase. The electrical conductivity at room temperature is about 0.08 S · cm–1, a typical value for nanopowders of tetrathiafulvalene-based conducting compounds. The current-voltage characteristic for an individual nanoparticle aggregate is fitted with a Shockley diode model. A saturation current of 4.1 pA and a threshold voltage of 0.45 V are extracted. N1s and S2p lines in X-ray photoelectron spectroscopy evidence a charge transfer, characteristic for tetrathiafulvalene-based conducting salts. Magnetic susceptibility studies show a superconducting transition at 9.1 K, a characteristic value for the κ-(BEDT-TTF)2Cu(NCS)2 phase. The thermoelectric power of the nanopowder is represented by the average //c and //b values for the single-crystal. Finally, resistivity for the nanopowder is nearly flat in the metallic region.
