Transitioning from Ionic Liquids to Deep Eutectic Solvents - Huan Zhang, Jose Manuel Vicent-Luna, Shuxia Tao, Sofia Calero, Rafael J. Jimenez Rioboo, Maria Luisa Ferrer, Francisco del Monte, Maria Concepcion Gutierrez, Acs Sustainable Chem. Eng. 10, 1231 (2022).
Looking at the "Water-in-Deep-Eutectic-Solvent" System: A Dilution Range for High Performance Eutectics - Nieves Lopez-Salas, Jose Manuel Vicent-Luna, Silvia Imberti, Elena Posada, Maria Jesus Roldan, Juan A. Anta, Salvador R. G. Balestra, Rafael M. Madero Castro, Sofia Calero, Rafael J. Jimenez-Rioboo, Maria Concepcion Gutierrez, Maria Luisa Ferrer, Francisco del Monte, Acs Sustainable Chem. Eng. 7, 17565 (2019).
Magnon-mediated magnetoresistance in layered manganites - A. Hernando, R. Cortes-Gil, D. Gonzalez-Merchante, M. Hernando, JM. Alonso, MA. Garcia, JL. Martinez, L. Ruiz-Gonzalez, JM. Gonzalez-Calbet, Phys Rev B 99, 024403 (2019).
On the magnetic structure and magnetic behaviour of the most distorted member of the series of RNiO3 perovskites (R = Lu)
Federico Serrano-Sanchez, Maria Teresa Fernandez-Diaz, Jose Luis Martinez, Jose Antonio Alonso,
Dalton T 51, 2278 (2022).
Abstract
The crystal structure of LuNiO3 perovskite has been examined below RT and across T-N = 125 K by neutron powder diffraction. In this temperature region (2-298 K), well below the metal-insulator transition this oxide exhibits at T-MI = 599 K, this material is insulating and characterized by a partial charge disproportionation of the Ni valence. In the perovskite structure, defined in the monoclinic P2(1)/n space group, there are two inequivalent Ni sites located in alternating octahedra of different sizes. The structural analysis with high-resolution techniques (lambda = 1.594 angstrom) unveils a subtle increase of the charge disproportionation as temperature decreases, reaching delta(eff) = 0.34 at 2 K. The magnetic structure has been investigated from low-T NPD patterns collected with a larger wavelength (lambda = 2.52 angstrom). Magnetic peaks are observed below T-N; they can be indexed with a propagation vector k = (1/2, 0, 1/2), as previously observed in other RNiO3 perovskites for the Ni sublattice. Among the three possible solutions for the magnetic structure, the first one is discarded since it would correspond to a full charge ordering (Ni2+ + Ni4+), with magnetic moments only on Ni2+ ions, not compatible with the structural findings assessing a partial charge disproportionation. The best agreement is found for a non-collinear model with two different moments in Ni1 and Ni2 sites, 1.4(1) mu(B), and m 0.7(1) mu(B) at 2 K, the ordered magnetic moments lying on the a-c plane. This is similar to that found for YNiO3. In complement, the magnetic and thermal properties of LuNiO3 have been investigated. AC susceptibility curves exhibit a clear peak centered at T-N = 125 K, corresponding to the establishment of the Ni antiferromagnetic structure. This is corroborated by DC susceptibility and specific heat measurements. Magnetization vs. field measurements confirm that the system is antiferromagnetic down to 2 K, without any further magnetic change. This linear behavior is also observed in the paramagnetic regime (T > T-N).
Dehydroxylation processing and lasing properties of a Nd alumino-phosphate glass
Monica Munoz-Quinonero, Jon. Azkargorta, Ignacio Iparraguirre, Rafael J. Jimenez-Rioboo, Gregory Tricot, Chongyoun Shao, Francisco Munoz, Joaquin Fernandez, Rolindes Balda,
J Alloy Compd 896, 163040 (2022).
Abstract
In this work, we have studied the preparation and properties of an alumino-phosphate glass with composition 13Na2O-13K2O-16BaO-4Al2O3-54P2O5 (mol%). The first part of the work deals with the study of the processing conditions of the dehydroxylation of the phosphate glass, which was performed by remelting under N2 flow using graphite crucibles. Glass samples from 5 to 50 g and Nd2O3 doped were submitted to dehydroxylation and the influence of temperature, time, mass of glass and viscosity were correlated with the content of water in the glasses through the coefficient of absorption of OH ions. The network structure of the glasses was also determined by means of 31P and 27Al 1D/2D nuclear magnetic resonance and the local environment of Nd3+ ions was probed by electron paramagnetic resonance. The optimized conditions of processing were then used to obtain a dehydroxylated glass with a 2.5 wt% Nd2O3 whose spectroscopic and laser emission properties were studied. The spectroscopic properties of Nd3+ ions which include, JuddOfelt calculation, stimulated emission cross-section of the laser transition, lifetime, and quantum efficiency are presented. Site-selective laser spectroscopy and stimulated emission obtained under selective wavelength pumping along the 4I9/2 -> 4F5/2 absorption band were performed to determine the distribution of crystal field in which the rare earth is located, together with its influence in the pump wavelength dependence of the spontaneous and laser emissions of Nd3+ in this glass matrix. (c) 2021 The Author(s). Published by Elsevier B.V. CC_BY_4.0
Transitioning from Ionic Liquids to Deep Eutectic Solvents
Huan Zhang, Jose Manuel Vicent-Luna, Shuxia Tao, Sofia Calero, Rafael J. Jimenez Rioboo, Maria Luisa Ferrer, Francisco del Monte, Maria Concepcion Gutierrez,
Acs Sustainable Chem. Eng. 10, 1231 (2022).
Abstract
Ionic liquids (ILs) and deep eutectic solvents (DESs) have been lately the solvents of choice in a number of processes because they offer a valid alternative to conventional solvents. Despite main interactions in ILs differ from those in DESs (e.g., electrostatic-type in the former and H-bond-type in the latter), these two neoteric solvents are more closely related that appeared and can be seen as the two sides, the face and the cross, of the same coin. Herein, we hypothesized about a way for transitioning from one to the other. In particular, we promoted the transition from 1-ethyl-3-methylimidazolium chloride (EMIMCI) to EMIMCI-nAcOH-based DESs by the simple addition of stoichiometric amounts of acetic acid (AcOH) to EMIMCI. H-1 NMR spectroscopy and DSC studies confirmed the occurrence of such a transition. Molecular dynamics (MD) simulations revealed the capability of the Cl anion to fully accommodate up to 4 AcOH molecules (e.g., EMIMCl center dot 1AcOH, EMIMCl center dot 2AcOH, EMIMCl center dot 3AcOH, and EMIMCl center dot 4AcOH) without signs of H-bond self-interactions between AcOH molecules. These DESs also exhibited quite different solvent properties, with alpha and beta Kamlet-and-Taft parameters that differed from those of EMIMCl and 1-ethyl-3-methylimidazolium acetate (EMIMOAc). Interestingly, excess molar volume and excess viscosity measurements as well as Brillouin spectroscopic experiments indicated that aqueous dilutions of EMIMCl center dot AcOH-based DESs deviated from ideality as a consequence of the formation of HBs between water molecules and the anion, as observed by H-1 NMR spectroscopy.
Atomic Structure and Lattice Dynamics of CoSb3 Skutterudite-Based Thermoelectrics
Joao Elias F. S. Rodrigues, Javier Gainza, Federico Serrano-Sanchez, Carlo Marini, Yves Huttel, Norbert M. Nemes, Jose Luis Martinez, Jose Antonio Alonso,
Chem Mater , (2022).
Abstract
The local atomic structure of skutterudite-type compounds derived from CoSb3 plays a pivotal role in tuning their electronic and thermal properties in thermoelectric applications. For instance, the shape of the occurring [Sb-4] rings has direct consequences on the band convergence and, then, the possible enhancement of the thermoelectric efficiency. In this work, both local and electronic structures of the CoSb3 skutterudite were evaluated by the X-ray absorption technique. Extended X-rayabsorption fine-structure oscillations at the Sb K-edge were fitted in good agreement to the body-centered cubic phase. The edge shift values were taken referencing the Co and Sb foils. The standard samples were used, namely, CoO (Co2+), Co3O4 (Co2.5+), Sb(2O)3 (Sb3+), and Sb2O5 (Sb5+). Based on the valence state dependence of the edge shift, the valences of Co and Sb in CoSb3 were estimated as +0.8(5) for Co and -2.2(3) for Sb, which suggests a partial charge transfer from the metal to the pnictide element. From the bonding distances of Co-Sb, Sb-Sb (short), and Sb-Sb (long), the lattice parameter and fractional coordinates (y, z) were evaluated and, then, compared to those extracted from synchrotron X-ray diffraction. From temperature-dependent X-ray absorption spectroscopy data at 80-350 K, the Einstein temperatures and local coefficients of thermal expansion of those pair-bonds were properly estimated. Comparing these values with those obtained from diffraction, we have established the boundaries of both shortand long-range order techniques for structural characterization of skutterudite-based thermoelectrics.
Wide Dynamic Range Thermometer Based on Luminescent Optical Cavities in Ga2O3:Cr Nanowires
Manuel Alonso-Orts, Daniel Carrasco, Jose M. San Juan, Maria Luisa No, Alicia de Andres, Emilio Nogales, Bianchi Mendez,
Small 18, 2105355 (2022).
Abstract
Remote temperature sensing at the micro- and nanoscale is key in fields such as photonics, electronics, energy, or biomedicine, with optical properties being one of the most used transducing mechanisms for such sensors. Ga2O3 presents very high chemical and thermal stability, as well as high radiation resistance, becoming of great interest to be used under extreme conditions, for example, electrical and/or optical high-power devices and harsh environments. In this work, a luminescent and interferometric thermometer is proposed based on Fabry-Perot (FP) optical microcavities built on Cr-doped Ga2O3 nanowires. It combines the optical features of the Cr3+-related luminescence, greatly sensitive to temperature, and spatial confinement of light, which results in strong FP resonances within the Cr3+ broad band. While the chromium-related R lines energy shifts are adequate for low-temperature sensing, FP resonances extend the sensing range to high temperatures with excellent sensitivity. This thermometry system achieves micron-range spatial resolution, temperature precision of around 1 K, and a wide operational range, demonstrating to work at least in the 150-550 K temperature range. Besides, the temperature-dependent anisotropic refractive index and thermo-optic coefficient of this oxide have been further characterized by comparison to experimental, analytical, and finite-difference time-domain simulation results.
SnSe:K-x intermetallic thermoelectric polycrystals prepared by arc-melting
Javier Gainza, Sergio Molto, Federico Serrano-Sanchez, Oscar J. Dura, Maria Teresa Fernandez-Diaz, Neven Biskup, Jose Luis Martinez, Jose Antonio Alonso, Norbert M. Nemes,
J Mater Sci , (2022).
Abstract
Neutron powder diffraction and thermoelectric characterization of SnSe:K-x intermetallic alloys are presented. Nanostructured ingots were prepared by arc-melting elemental tin and selenium along with potassium hydride. Up to x = 0.1 of K can be incorporated into SnSe. Rietveld refinement of the diffractograms locates potassium on the Sn site in the high-temperature Cmcm structure. However, in the low-temperature Pnma structure, K cannot be localized by difference Fourier maps, indicating the incorporation of K in a disordered form in the interlayer space. STEM-EELS indicates the incorporation of K into the SnSe grains. The resistivity upon K-doping at intermediate temperatures decreases by 1-2 orders of magnitude, but at high temperature is higher than the undoped SnSe. The Seebeck coefficient of K-doped SnSe remains p-type and almost temperature independent (400 mu V/K for x = 0.1). The ultralow thermal conductivity of undoped SnSe decreases further upon K-doping to below 0.3 W/m K.
Forbidden and Second-Order Phonons in Raman Spectra of Single and Few-Layer MoS2 Close to C Exciton Resonance
Sandra Cortijo-Campos, Patrick Kung, Carlos Prieto, Alicia de Andres,
J Phys Chem C 125, 23904 (2021).
Abstract
The complex Raman spectra of single- to few-layer MoS2 obtained by different methods and substrates are analyzed and compared to bulk. Depending on the Raman process that originates the phonons, these are sensitive to different variables such as the number of layers (N), the growth method, or the substrate. The behavior of forbidden 150 and 190 cm(-1) peaks is explained by a double resonant mechanism close to C exciton assisted by intrinsic defects, and their strong dependence on N is due to electronic structure modifications. The band around 450 cm(-1), in off-resonance conditions, is found to be little sensitive to the number of layers or to the obtaining method but strongly influenced by the coupling to the substrate and by temperature; it is thus conjectured to be an excellent indicator of strain. Several peaks are found to occur solely for vapor deposition methods, indicating the presence of specific defects.
Stress-mediated solution deposition method to stabilize ferroelectric BiFe1-xCrxO3 perovskite thin films with narrow bandgaps
Ricardo Jimenez, Jesus Ricote, Inigo Bretos, Rafael J. Jimenez Rioboo, Federico Mompean, Ana. Ruiz, Haibing Xie, Monica Lira-Cantu, M. Lourdes Calzada,
J Eur Ceram Soc 41, 3404 (2021).
Abstract
Ferroelectric oxides with low bandgaps are mainly based on the BiFeO3 perovskite upon the partial substitution of iron with different cations. However, the structural stability of many of these perovskites is only possible by their processing at high pressures (HP, >1GPa) and high temperatures (HT, >700?C). Preparation methods under these severe conditions are accessible to powders and bulk ceramics. However, transferring these conditions to the fabrication of thin films is a challenge, thus limiting their use in applications. Here, a chemical solution deposition method is devised, which overcomes many of these restrictions. It is based on the application of an external compressive-stress to the film sample during the thermal treatment required for the film crystallization, promoting the formation and stabilization of these HP perovskites. We demonstrate the concept on BiFe1-xCrxO3 (BFCO) thin films deposited on SrTiO3 (STO) substrates and with large chromium contents. The resulting BFCO perovskite films show narrow bandgaps (Eg-2.57 eV) and an excellent ferroelectric response (remnant polarization, PR- 40 ?C cm-2). The polarized thin films under illumination present a large out-put power of -6.4 ?W cm-2, demonstrating their potential for using in self-powered multifunctional devices. This stress-mediated solution deposition method can be extended to other perovskite films which are unviable under conventional deposition methods.
Mitochondria selective trackers for long-term imaging based on readily accessible neutral BODIPYs
Agata Ramos-Torres, Edurne Avellanal-Zaballa, Fernando Garcia-Garrido, Ana B. Fernandez-Martinez, Alejandro Prieto-Castaneda, Antonia R. Agarrabeitia, Jorge Banuelos, Inmaculada Garcia-Moreno, Francisco-Javier Lucio-Cazana, Maria J. Ortiz,
Chem Commun 57, 5318 (2021).
Abstract
We report the design of a new model based on a small neutral 8-aryl-3-formylBODIPY and its suitability to develop privileged highly bright and photostable fluorescent probes for selective and, more importantly, covalent staining of mitochondria.
Tools for extending the dilution range of the "solvent-in-DES" regime
Huan Zhang, M. Luisa Ferrer, Rafael J. Jimenez-Rioboo, Francisco del Monte, Maria C. Gutierrez,
J Mol Liq 329, 115573 (2021).
Abstract
Deep eutectic solvents (DESs) and dilutions thereof are currently gaining increased interest in sustainable processes. DES dilutions are particularly interesting in the so-called "solvent-in-DES" regime where the intriguing features of DESs remain (coming from hydrogen bonding among the components) and some typical drawbacks (e.g., high viscosity, low electrical conductivity, etc.) are mitigated. Actually, DES dilutions may exhibit excellent performances (more so than the original neat DESs) in certain applications. Knowing about the tools that allow predicting (and eventually extending) the dilution range of the "solvent-in-DES" regime is obviously of interest. With this aim, we herein studied two sets of DES dilutions. In the first set, we used the DES composed of choline chloride (ChCl) and urea (U) and its dilutions in H2O, methanol (MeOH) and ethanol (EtOH). In the second set, we studied two DESs composed of ethylene glycol (EG) and either tetraethylammonium bromide (TEABr) or tetrabutylammonium bromide (TBABr), and their dilutions in H2O. Data coming from DSC, H-1 NMR spectroscopy, and Brillouin spectroscopy revealed how the dilution range of the "solvent-in-DES" regime in ChClU dilutions in H2O, MeOH and EtOH increased in the order H2O < MeOH < EtOH while in aqueous dilutions of TEABrEG and TBABrEG increased in the order TEABrEG < TBABrEG. Our results suggest that the extension of the dilution range of the "water-in-DES" regime could be tuned by both the DES components solubility in the different solvents, and the strength and/or number of the hydrogen bonds established between DES components and solvent molecules. (C) 2021 Elsevier B.V. All rights reserved.
Impact of Tb4+ and morphology on the thermal evolution of Tb-doped TiO2 nanostructured hollow spheres and nanoparticles
MT. Colomer, E. Rodriguez, M. Moran-Pedroso, F. Vattier, A. de Andres,
J Alloy Compd 853, 156973 (2021).
Abstract
Tb-doped TiO2 hollow spheres (HSs) in the range 0.0-2.0 at.% have been synthesized by the first time to the best of our knowledge. The HSs are compared with nanoparticles (NPs) to evaluate the impact of morphology on their physicochemical and photoluminescence (PL) behavior upon increasing calcination temperature. After calcination at 550 degrees C, the particles are anatase with a primary average size of 10.0 +/- 0.2 nm for the NPs and 12.0 +/- 0.2 nm for those that form the micron sized hollow spheres of 1.8 +/- 0.2 mu m diameter and ca. 64 nm shell thickness. The temperature of the anataseerutile transition is found to be strongly dependent on the presence of Tb as well as on morphology. Contrarily to the usual stabilization of anatase when doping with trivalent rare-earth ions, the transition temperature is reduced when doping with Tb. The rutile phase is further favored for the HSs compared to the NPs probably related to the low density of the HSs and/or a more efficient packing density and/or a bigger crystal size of the nanoparticles that form those spheres with respect to the packing and the size of the NPs and/or the crystal size of the nanoparticles of the HSs with respect to the size of the NPs. Only a slight unit-cell volume increase for the anatase structure is observed upon Tb doping, in both the NPs and in the HSs, contrary to the expected increment due to the larger ionic radius of Tb3+ compared to Ti4+. In addition, the intensity of the characteristic f-f Tb3+ emission bands is extremely weak both in the anatase and rutile phases. The transition is accompanied with the emergence of an infrared emission band centered at 810 nm related to the formation of defects during the structural transformation providing deep levels in the gap that partly quench the f-f emissions in the rutile phase. The results are consistent with the presence of Tb in both +3 and +4 valence states. XPS measurements confirmed the presence of Tb3+ as well as of Tb4+ in both HSs and NPs. The large fraction of Tb4+ present in the samples originates the weak f-f emission intensity, an only slight increase of the cell parameters and the destabilization of the anatase phase. (c) 2020 Elsevier B.V. All rights reserved.
Correlated effects of fluorine and hydrogen in fluorinated tin oxide (FTO) transparent electrodes deposited by sputtering at room temperature
M. Moran-Pedroso, R. Gago, J. Julin, E. Salas-Colera, I. Jimenez, A. de Andres, C. Prieto,
Appl Surf Sci 537, 147906 (2021).
Abstract
The optical and electrical properties of fluorinated tin oxide (FTO) films deposited at room temperature by sputtering have been investigated varying the fluorine content and the hydrogen atmosphere. The complex behavior of the obtained films is disclosed using a wide set of characterization techniques that reveals the combined effects of these two parameters on the generated defects. These defects control the electrical transport (carrier density, mobility and conductivity), the optical properties (band gap and defects-related absorption and photoluminescence) and finally promote the amorphization of the samples. H-2 in the sputtering gas does not modify the H content in the films but induces the partial reduction of tin (from Sn4+ to Sn2+) and the consequent generation of oxygen vacancies with shallow energy levels close to the valence band. A variation of up to four orders of magnitude in electrical conductivity is reported in samples with the appropriate fluorine doping and hydrogen fraction in the sputtering gas, maintaining excellent optical transparency. Optimized room temperature grown electrodes reach sheet resistance similar to 20 Omega/square and transparency > 90%. This room temperature deposition process enables film preparation on flexible organic substrates, such as polyethylene terephthalate (PET), with identical performance of doubtless interest in flexible and large scale electronics.
On the lack of monoclinic distortion in the insulating regime of EuNiO3 and GdNiO3 perovskites by high-angular resolution synchrotron X-ray diffraction: a comparison with YNiO3
F. Serrano-Sanchez, JL. Martinez, F. Fauth, JA. Alonso,
Dalton T 50, 7085 (2021).
Abstract
Rare-earth nickelates RNiO3 (R = Y, LaMIDLINE HORIZONTAL ELLIPSISLu) are electron-correlated perovskite materials where the interplay between charge and spin order results in a rich phase diagram, evolving from antiferromagnetic insulators to paramagnetic metals. They are well-known to undergo metal-insulator (MI) transitions as a function of temperature and the size of the rare-earth ion. For intermediate-size Eu3+ and Gd3+ ions, the MI transitions are described to happen at T-MI = 463 K and 511 K, respectively. We have investigated their structural evolution across T-MI with the excellent angular resolution of synchrotron X-ray diffraction, using high-crystalline quality samples prepared at elevated hydrostatic pressures. Unlike YNiO3, synthesized and measured under the same conditions, exhibiting a characteristic monoclinic phase (space group P2(1)/n) in the insulating regime (below T-MI), the present EuNiO3 and GdNiO3 samples do not exhibit such a symmetry, but their crystal structures can be defined in an orthorhombic superstructure of perovskite (space group Pbnm) in all the temperature interval, between 100 and 623 K for Eu and 298 K and 650 K for Gd. Nevertheless, an abrupt evolution of the unit-cell parameters is observed upon metallization above T-MI. A prior report of a charge disproportionation effect by Mossbauer spectroscopy on Fe-doped perovskite samples seems to suggest that the distribution of two distinct Ni sites must not exhibit the required long-range ordering to be effectively detected by diffraction methods. An abrupt contraction of the b parameter of EuNiO3 in the 175-200 K range, coincident with the onset of antiferromagnetic ordering, suggests a magnetoelastic coupling, not described so far in rare-earth nickelates. The magnetic susceptibility is dominated by the paramagnetic signal of the rare-earth ions; however, the AC susceptibility curves yield a Neel temperature corresponding to the antiferromagnetic ordering of the Ni moments of T-N = 197 K for EuNiO3, corroborated by specific heat measurements. For GdNiO3, a chi T vs. T plot presents a clear change in the slope at T-N = 187 K, also consistent with specific heat data. Magnetization measurements at 2 K under large fields up to 14 T show a complete saturation of the magnetic moments with a rather high ordered moment of 7.5 mu B per f.u.
Enhancing the Neel temperature in 3d/5d R2NiIrO6 (R=La, Pr and Nd) double perovskites by reducing the R3+ ionic radii
P. Kayser, A. Munoz, JL. Martinez, F. Fauth, MT. Fernandez-Diaz, JA. Alonso,
Acta Mater 207, 116684 (2021).
Abstract
Double perovskites containing Ir4+ were synthesised by a citrate technique, followed by an annealing treatment in air at 1100 degrees C. The crystal structure of the three compounds, with formula R2NiIrO6 (R= La, Pr and Nd), were determined using a combined refinement against neutron powder diffraction (NPD) and synchrotron x-ray powder diffraction (SXRPD) data sets. At room temperature, all the samples were indexed in the space group P2(1)/n and the monoclinic symmetry remains in the 300 to 1273 K temperature range. Magnetization measurements suggest competitive antiferromagnetic and ferromagnetic interactions, with an unexpected increment of the ordering temperature (T-N) along the series. The magnetic structures of all the samples were defined with the propagation vector k = 0; the Ni2+ and Ir4+ moment arrangement, different for each compound, shows a strong dependence on the nature of the rare-earth ion. (C) 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Large Enhancement of Critical Current in Superconducting Devices by Gate Voltage
M. Rocci, D. Suri, A. Kamra, G. Vilela, Y. Takamura, NM. Nemes, JL. Martinez, MG. Hernandez, JS. Moodera,
Nano Lett 21, 216 (2021).
Abstract
Significant control over the properties of a high-carrier density superconductor via an applied electric field has been considered infeasible due to screening of the field over atomic length scales. Here, we demonstrate an enhancement of up to 30% in critical current in a back-gate tunable NbN micro- and nano superconducting bridges. Our suggested plausible mechanism of this enhancement in critical current based on surface nucleation and pinning of Abrikosov vortices is consistent with expectations and observations for type-II superconductor films with thicknesses comparable to their coherence length. Furthermore, we demonstrate an applied electric field-dependent infinite electroresistance and hysteretic resistance. Our work presents an electric field driven enhancement in the superconducting property in type-II superconductors which is a crucial step toward the understanding of field-effects on the fundamental properties of a superconductor and its exploitation for logic and memory applications in a superconductor-based low-dissipation digital computing paradigm.
Strongly reduced lattice thermal conductivity in Sn-doped rare-earth (M) filled skutterudites MxCo4Sb12-ySny, promoted by Sb-Sn disordering and phase segregation
J. Gainza, F. Serrano-Sanchez, N. M. Nemes, O. J. Dura, J. L. Martinez, F. Fauth, J. A. Alonso,
Rsc Advances 11, 26421 (2021).
Abstract
CoSb3 thermoelectric skutterudite has been filled with rare-earth metals (M = La, Ce, Yb) and partially doped with Sn in specimens of MxCo4Sb12-ySny stoichiometry. This has been achieved under high-pressure conditions at 3.5 GPa in a piston-cylinder hydrostatic press. A structural investigation using synchrotron X-ray diffraction data reveals a phase segregation in twin skutterudite phases with filling fraction fluctuation and different unit-cell sizes. As a result of three effects acting as phonon scatterers, namely the rattling effect of M at the wide 8a cages of the cubic Im3 structure, the phase segregation, and the intrinsic disorder introduced by Sn substitution at the Sb sublattice, the total thermal conductivity (kappa) dramatically falls to reach minimum values under 2 W m(-1) K-1, well below those typically exhibited by other thermoelectric materials based upon single-filled skutterudites. The power factor is substantially enhanced to 1.11 mW m(-1) K-2 in Yb0.5Co4Sb11.6Sn0.4 with respect to the unfilled composition, as a result of the charge transfer promoted by the filler.
Novel insights into the magnetic behavior of non-stoichiometric LaMnO3+delta nanoparticles
Raquel Cortes-Gil, Antonio Hernando, Jose M. Alonso, Kevin M. Sigcho-Villacis, Alberto Azor-Lafarga, Jose L. Martinez, M. Luisa Ruiz-Gonzalez, Jose M. Gonzalez-Calbet,
J Mater Chem C 9, 10361 (2021).
Abstract
LaMnO3+delta is one of the classical-formal nonstoichiometric systems in materials science that, for delta = 0, shows antiferromagnetic ordering by super-exchange interactions between Mn3+ ions. Nevertheless, the chemical reactivity of solids goes through different pathways when the particle size is reduced to the nanometric range, leading to the modification of the physico-chemical properties of materials. It is demonstrated here that octagonal LaMnO3+delta nanoparticles of average size 20 nm significantly modify their non-stoichiometry, microstructure and magnetotransport properties and are thus able to tune their behavior, as a function of the cationic vacancy concentration. For delta = 0, LaMnO3 nanoparticles, even with only Mn3+, become ferromagnetic, whereas for delta = 0.23, i.e. La0.93Mn0.93O3, due to a cationic diffusion mechanism, weaker ferromagnetic interactions appear promoting the emergence of magnetoresistance. This accommodation of compositional variations triggers Mn3+ -> Mn4+ oxidation and facilitates, as observed by atomically resolved scanning transmission electron microscopy and electron energy loss spectroscopy, the displacement of some La atoms around their normal site that can introduce restrictions in double exchange Mn3+-O2--Mn4+ interactions.
Unveiling the Structural Behavior under Pressure of Filled M0.5Co4Sb12 (M = K, Sr, La, Ce, and Yb) Thermoelectric Skutterudites
Joao Elias F. S. Rodrigues, Javier Gainza, Federico Serrano-Sanchez, Mateus M. Ferrer, Guilherme S. L. Fabris, Julio R. Sambrano, Norbert M. Nemes, Jose L. Martinez, Catalin Popescu, Jose A. Alonso,
Inorg Chem 60, 7413 (2021).
Abstract
Skutterudite-type compounds based on square Co4Sb12 pnictide are promising for thermoelectric application due to their good Seebeck values and high carrier mobility. Filling the 8a voids (in the cubic space group Im (3) over bar) with different elements (alkali, alkali earth, and rare earth) helps to reduce the thermal conductivity and thus increases the thermoelectric performance. A systematic characterization by synchrotron X-ray powder diffraction of different M-filled Co4Sb12 ( M = K, Sr, La, Ce, and Yb) skutterudites was carried out under high pressure in the range similar to 0-12 GPa. The isothermal equations of state (EOS) were obtained in this pressure range and the Bulk moduli (B-0) were calculated for all the filled skutterudites, yielding unexpected results. A lattice expansion due to the filler elements fails in the description of the Bulk moduli. Topochemical studies of the filler site environment exhibited a slight disturbance and an increased ionic character when the filler is incorporated. The mechanical properties by means of Bulk moduli resulted in being sensitive to the presence of filler atoms inside the skutterudite voids, being affected by the covalent/ionic exchange of the Co-Sb and Sb-Sb bonds.
Further Extending the Dilution Range of the "Solvent-in-DES" Regime upon the Replacement of Water by an Organic Solvent with Hydrogen Bond Capabilities
N. Lopez-Salas, J. M. Vicent-Luna, E. Posada, S. Imberti, R. M. Madero-Castro, S. Calero, C. O. Ania, R. J. Jimenez-Rioboo, M. C. Gutierrez, M. L. Ferrer, F. del Monte,
Acs Sustainable Chem. Eng. 8, 12120 (2020).
Abstract
Aqueous dilutions of deep eutectic solvents (DESs) have lately allowed exploring new and more demanding applications where neat DESs are not able to perform well. However, the use of DES dilutions with nonaqueous hydrogen-bond-forming solvents remains basically unexplored. It is worth noting the obvious interest of using organic solvents in those cases where any reagent or byproduct is nonsoluble, nonmiscible, or unstable in water, the presence of water might alter the reaction kinetics (for instance, when water is a byproduct), or a cosolvent with low vapor pressure allows exploring reaction processes (high temperatures or solvothermal conditions, among others) not suitable for water. Herein, we investigated benzyl alcohol (BA) dilutions of RUChCl, a DES composed of resorcinol (R), urea (U), and choline chloride (ChCl). In particular, neutron scattering, nuclear magnetic resonance, and Brillouin experiments revealed how BA was accommodated within the hydrogen-bond (HB) complex structure of RUChCl for BA contents of up to 67 wt % whereas this HB complex structure basically disappeared for higher BA contents. This behavior somehow resembled that found in aqueous solutions, with two well-differentiated regimes-e.g., the "solvent-in-DES" and the "DES-in-solvent"-depending on the DES content. However, the "solvent-in-DES" regime was preserved for much higher solvent contents of BA than H2O-e.g., 60-65 versus 18-20 wt %, respectively. Interestingly, the specific BA dilution where transition from one regime to the other occurs was particularly well suited to develop a spinodal decomposition process when used as the precursor for the preparation of polymer resins (by polycondensation between R and p-phthalaldehyde).
Raman amplification in the ultra-small limit of Ag nanoparticles on SiO2 and graphene: Size and inter-particle distance effects
Sandra Cortijo-Campos, Rafael Ramirez-Jimenez, Esteban Climent-Pascual, Montserrat Aguilar-Pujol, Felix Jimenez-Villacorta, Lidia Martinez, Rafael Jimenez-Rioboo, Carlos Prieto, Alicia de Andres,
Mater Design 192, 108702 (2020).
Abstract
Size, shape and hot spots are crucial to optimize Raman amplification from metallic nanoparticle (NPs). The am-plification from radius = 1.8 +/- 0.4 nm ultra-small silver NPs was explored. Increasing NP density redshifts and widens their plasmon that, according to simulations for NPs arrays, is originated by the reduction of the interpar-ticle distance, d, becoming remarkable for d <= R. Inter-particle interaction red-shifts (>130 nm) and widens (>90 nm) the standard plasmon of non-interacting spherical particles. Graphene partly delocalizes the carriers enhancing the NIR spectral weight. Raman amplification of graphene phonons is moderate and depends smoothly on d while that of Rhodamine 6G (R6G) varies almost exponentially due to their location at hot-spots that depend strongly on d. The experimental correlation between amplification and plasmon position is well reproduced by simulations. The amplification originated by the ultra-small NPs is compared to that of larger particles, granular silver films with 7 < R < 15 nm grains, with similar extinction values. The amplification is found to be larger for the 1.8 nm NPs due to the higher surface/volume ration that allows higher density of hot spots. It is demonstrated that Raman amplification can be efficiently increased by depositing low density layers of ultra -small NPs on top of granular films. (c) 2020 The Authors. Published by Elsevier Ltd.
Aqueous-Eutectic-in-Salt Electrolytes for High-Energy-Density Supercapacitors with an Operational Temperature Window of 100 degrees C, from-35 to+65 degrees C
Xuejun Lu, Rafael J. Jimenez-Rioboo, Donal Leech, Maria C. Gutierrez, M. Luisa Ferrer, Francisco del Monte,
Acs Appl. Mater. Interfaces 12, 29181 (2020).
Abstract
Water-in-salt (WIS) electrolytes are gaining increased interest as an alternative to conventional aqueous or organic ones. WIS electrolytes offer an interesting combination of safety, thanks to their aqueous character, and extended electrochemical stability window, thanks to the strong coordination between water molecules and ion salt. Nonetheless, cost, the tendency of salt precipitation, and sluggish ionic transfer leading to poor rate performance of devices are some intrinsic drawbacks of WIS electrolytes that yet need to be addressed for their technological implementation. It is worth noting that the absence of "free'' water molecules could also be achieved via the addition of a certain cosolvent capable of coordinating with water. This is the case of the eutectic mixture formed between DMSO and H2O with a molar ratio of 1:2 and a melting point as low as -140 degrees C. Interestingly, addition of salts at near-saturation conditions also resulted in an increase of the boiling point of the resulting solution. Herein, we used a eutectic mixture of DMSO and H2O for dissolution of LiTFSI in the 1.1-8.8 molality range. The resulting electrolyte (e.g., the so-called aqueous-eutectic-in-salt) exhibited excellent energy and power densities when operating in a supercapacitor cell over a wide range of extreme ambient temperatures, from as low as -35 degrees C to as high as +65 degrees C.
Brillouin Spectroscopy as a Suitable Technique for the Determination of the Eutectic Composition in Mixtures of Choline Chloride and Water
Huan Zhang, M. Luisa Ferrer, Maria J. Roldan-Ruiz, Rafael J. Jimenez-Rioboo, Maria C. Gutierrez, Francisco del Monte,
J Phys Chem B 124, 4002 (2020).
Abstract
Deep eutectic solvents (DESs) resulting from the right combination between a hydrogen-bond donor (HBD) and a hydrogen-bond acceptor (HBA) are becoming quite popular in number of applications. More recently, natural DESs (NADESs) containing sugars, natural organic acids, and amino acids as HBDs and ChCl as HBA have received great attention because of their further environmental sustainability as compared to regular DESs. Within this context, mixing water in controlled amounts has been widely accepted as a simple and practical way of altering DES chemical and thermodynamic properties, with viscosity and conductivity experiencing the most significant changes. However, the number of papers describing eutectic mixtures with water as the only HBD is scarce and basically none has been done in fundamental terms. Herein, we investigated mixtures composed of water as the only HBD and ChCl as the HBA using differential scanning calorimetry (DSC) as well as H-1 nuclear magnetic resonance (NMR) and Brillouin spectroscopies. We found the aqueous dilution of ChCl/2H2O with a ChCl/2H2O content of ca. 80 wt % was an eutectic. Interestingly, this mixture could be considered a NADES according to its eutectic distance (Delta T-me), in range to eutectics obtained in aqueous dilutions of salt hydrates.
Carbon and carbon composites obtained using deep eutectic solvents and aqueous dilutions thereof
Gaspar Carrasco-Huertas, Rafael J. Jimenez-Rioboo, Maria Concepcion Gutierrez, Maria Luisa Ferrer, Francisco del Monte,
Chem Commun 56, 3592 (2020).
Abstract
The aim of this featured article is to illustrate some of the most recent applications of deep eutectic solvents (DESs) in the synthesis of carbon and carbon composites. DESs can be obtained by the complexation of quaternary ammonium salts with hydrogen-bond donors. DESs have typically been referred to as a related class of ionic liquids because they share many properties. However, DESs present the advantage of easier and low-cost preparation. Moreover, their compositional flexibility can eventually be translated into materials that provide advanced functionalities and/or tailored hierarchical structures. Interestingly, the use of the liquid binary mixtures of DESs and H2O for the preparation of carbon materials plays a critical role with regard to the achievement of some particular porous morphologies. Herein, we will also summarize some recent studies performed on DES/H2O liquid binary mixtures, revealing the possibility of obtaining new eutectic mixtures upon the simple addition of water to DESs while keeping the DES contents at a certain pseudo-concentrated range. This finding will pave the way to novel applications, especially in those fields in which the preparation of high-tech products via low-cost processes is critical. We hope that this featured article will encourage scientists to explore the promising perspectives offered by DESs and aqueous dilutions thereof.
Chemical and structural heterogeneities in Nd-doped oxynitride phosphate laser glasses
Francisco Munoz, Rafael J. Jimenez-Rioboo, Rolindes Balda,
J Alloy Compd 816, 152657 (2020).
Abstract
The application of oxynitride glasses in optics has been limited due the issues inherent in their preparation that result in bubbles, metallic inclusions or opacity. Although the synthesis of oxynitride phosphates is easier than in silicates, due to their lower melt viscosity, the remaining water in the form of OH causes, for instance, the neodymium luminescence to be negatively affected. We report on the preparation of oxynitride Nd phosphate glasses that are submitted first to a dehydroxylation, so that their structure and properties can be studied as a function of the nitrogen content taking a completely homogeneous OH-free glass as the starting point of each synthesis. Raman and Brillouin Spectroscopy confirmed the segregation in oxide and N-enriched regions, and NMR of the glasses has been used to calculate the O/P ratio from the distribution of P(O,N)(4) groups. The difference between this and the experimental one obtained from the elemental analyses confirms the segregation in oxide and oxynitride regions that is produced during the ammonolysis of the phosphate glass. The oxynitride glass network could thus be considered a non-homogeneous mixture of the oxide and the nitrided phases that are segregated due to the non-randomness of the nitridation reaction. (C) 2019 Elsevier B.V. All rights reserved.
Breast cancer biomarker detection through the photoluminescence of epitaxial monolayer MoS2 flakes
S. Catalan-Gomez, M. Briones, S. Cortijo-Campos, T. Garcia-Mendiola, A. de Andres, S. Garg, P. Kung, E. Lorenzo, JL. Pau, A. Redondo-Cubero,
Scientific Reports 10, 16039 (2020).
Abstract
In this work we report on the characterization and biological functionalization of 2D MoS2 flakes, epitaxially grown on sapphire, to develop an optical biosensor for the breast cancer biomarker miRNA21. The MoS2 flakes were modified with a thiolated DNA probe complementary to the target biomarker. Based on the photoluminescence of MoS2, the hybridization events were analyzed for the target (miRNA21c) and the control non-complementary sequence (miRNA21nc). A specific redshift was observed for the hybridization with miRNA21c, but not for the control, demonstrating the biomarker recognition via PL. The homogeneity of these MoS2 platforms was verified with microscopic maps. The detailed spectroscopic analysis of the spectra reveals changes in the trion to excitation ratio, being the redshift after the hybridization ascribed to both peaks. The results demonstrate the benefits of optical biosensors based on MoS2 monolayer for future commercial devices.
Enhanced stability and efficiency in inverted perovskite solar cells through graphene doping of PEDOT:PSS hole transport layer
C. Redondo-Obispo, TS. Ripolles, S. Cortijo-Campos, AL. Alvarez, E. Climent-Pascual, A. de Andres, C. Coya,
Mater Design 191, 108587 (2020).
Abstract
Poly(3,4-ethylenedioxythiophene):poly (styrene sulfonate) (PEDOT:PSS) plays a relevant role in the device performance as hole extraction layer (HTL) of inverted perovskite solar cells. Here, we show a simple low-temperature spin coating method for obtaining homogenous graphene-doped thin films of PEDOT:PSS with improved electrical conductivity without decreasing optical transmittance. Moreover, the crystallinity and stability in ambient conditions of the perovskite grown on it are enhanced. The hydrophobic character of graphene probably blocks undesirable reactions hampering degradation. By impedance spectroscopy it is demonstrated better charge extraction and reduction of recombination mechanisms at the doped-HTL/perovskite interface, resulting in improved photovoltaic parameters of the solar cell and greater stability at room operation conditions thus providing a simple and cost-effective method of preparing solar cells based on hybrid perovskites. (C) 2020 The Authors. Published by Elsevier Ltd.
Hydroxyapatites as Versatile Inorganic Hosts of Unusual Pentavalent Manganese Cations
A. Varela, I. Gomez-Recio, L. Serrador, M. Hernando, E. Matesanz, A. Torres-Pardo, MT. Fernandez-Diaz, JL. Martinez, F. Gonell, G. Rousse, C. Sanchez, C. Laberty-Robert, D. Portehault, JM. Gonzalez-Calbet, M. Parras,
Chem Mater 32, 10584 (2020).
Abstract
Contrary to molecular species, only very few solids are reported to host manganese (V) species. Herein, we report three new compounds with a hydroxyapatite structural backbone built on the (MnO43-)-O-V anion: Sr-5[(Mn1-xSix)O-4](3)(OH)(1-3x) (x = 0 and 0.053), Sr-5(Mno(4))(3)(OH)(1-y)F-y(y = 0.90), and Sr-5[(Mn1-xSix)O-4](3)F-1-3(x) (x = 0.058). These solids are fully characterized using powder X-ray and neutron powder diffraction, scanning transmission electron microscopy, electron energy loss spectroscopy (EELS), thermogravimetric analysis, and magnetic measurements. Especially, we report for the first time EELS Mn-L-2,L-3 spectra of manganese with the oxidation state (V). Contrary to other Mn(V) oxides, these solids and the nominal compound Sr-5(MnO4)(3)OH do not comprise Ba2+ cations but rely only on Sr2+ cations, showing that barium is not a required element to stabilize Mn(V) species in inorganic solids. We show that by tuning soft chemistry conditions conditions on the other hand, Mn(V) and hydroxyl groups can be deliver solids with a potentially wide composition range. These suggesting the emergence of new functional materials built from h on the one hand and post-treatment topological transformation substituted by Si(IV) and fluoride ions, respectively. Hence, we compounds show significant oxygen anionic conduction, thus igh-oxidation state manganese cations.
Magnetic properties of Sr0.7R0.3CoO3-delta (R = Tb, Er and Ho) perovskites
V. Cascos, JL. Martinez, MT. Fernandez-Diaz, JA. Alonso,
J Alloy Compd 844, 156121 (2020).
Abstract
The Sr0.7R0.3CoO3-delta (R = Tb, Ho and Er) perovskite derivatives have been previously studied and described as excellent cathode materials for solid-oxide fuel cells (SOFCs) at intermediate temperatures. They were prepared by soft chemistry procedures involving citrates of the different metal ions, followed by thermal treatments in air, and the crystal structure was characterized from neutron powder diffraction (NPD) data in the 300-1100 K temperature range. In this work, these studies have been accompanied with a low-temperature investigation of the magnetic behavior by NPD, in complement with magnetic measurements. For the three members of the series, NPD data unveils a G-type magnetic structure below TN (similar to 300 K), with a propagation vector k = 0. There is an antiferromagnetic coupling between the two types of Co atoms, defined in the I4/mmm space group, with the magnetic moments of Co aligned along the c axis of the tetragonal cell. Even at 4 K, the magnetic moments of the rare-earth elements do not participate in the magnetic structure, given their statistical distribution over the Sr positions, and they remain paramagnetic in all the temperature range. (C) 2020 Elsevier B.V. All rights reserved.
Structural Features, Anisotropic Thermal Expansion, and Thermoelectric Performance in Bulk Black Phosphorus Synthesized under High Pressure
JEFS Rodrigues, J. Gainza, F. Serrano-Sanchez, C. Lopez, OJ. Dura, N. Nemes, JL. Martinez, Y. Huttel, F. Fauth, MT. Fernandez-Diaz, N. Biskup, JA. Alonso,
Inorg Chem 59, 14932 (2020).
Abstract
Black phosphorus (BP) allotrope has an orthorhombic crystal structure with a narrow bandgap of 0.35 eV. This material is promising for 2D technology since it can be exfoliated down to one single layer: the well-known phosphorene. In this work, bulk BP was synthesized under high-pressure conditions at high temperatures. A detailed structural investigation using neutron and synchrotron X-ray diffraction revealed the occurrence of anisotropic strain effects on the BP lattice; the combination of both sets of diffraction data allowed visualization of the lone electron pair 3s(2). Temperature-dependent neutron diffraction data collected at low temperature showed that the a axis (zigzag) exhibits a quasi-temperature-independent thermal expansion in the temperature interval from 20 up to 150 K. These results may be a key to address the anomalous behavior in electrical resistivity near 150 K. Thermoelectric properties were also provided; low thermal conductivity from 14 down to 6 Wm(-1) K-1 in the range 323-673 K was recorded in our polycrystalline BP, which is below the reported values for single-crystals in literature.
Unveiling the Correlation between the Crystalline Structure of M-Filled CoSb3(M = Y, K, Sr) Skutterudites and Their Thermoelectric Transport Properties
J. Gainza, F. Serrano-Sanchez, JE. Rodrigues, J. Prado-Gonjal, NM. Nemes, N. Biskup, OJ. Dura, JL. Martinez, F. Fauth, JA. Alonso,
Adv Funct Mater 30, 2001651 (2020).
Abstract
Skutterudite-type pnictides based on CoSb(3)are promising semiconductor materials for thermoelectric applications. An exhaustive structural characterization by synchrotron X-ray powder diffraction of different M-filled CoSb3(M = Y, K, Sr, La, Ce, Yb) skutterudites, with a panoply of M atoms with very different chemical nature, allows to better understand the effects of filling from a crystallo-chemical point of view. These analyses focus on the correlation of chemical and structural features with the enhanced thermoelectric properties displayed by certain families of filled-CoSb(3)skutterudites. These are mainly determined by Sb positional parameters, yielding Oftedal plots that depend on the filling fraction, ionic state, and atomic radius of the filler. Together with the distortion of [Sb-4] rings and [CoSb6] octahedra present in the skutterudite structure, these results are linked to the band-convergence concept and its influence on the thermoelectric transport properties. Here, the structural changes observed in the different chemical compositions are relevant to understand the improved thermoelectric performance of single partially filled n-type skutterudites.
Enhanced stability in CH3NH3PbI3 hybrid perovskite from mechano-chemical synthesis: structural, microstructural and optoelectronic characterization
CA. Lopez, C. Abia, JE. Rodrigues, F. Serrano-Sanchez, NM. Nemes, JL. Martinez, MT. Fernandez-Diaz, N. Biskup, C. Alvarez-Galvan, F. Carrascoso, A. Castellanos-Gomez, JA. Alonso,
Scientific Reports 10, (2020).
Abstract
Among the hybrid organic-inorganic perovskites MAPbX(3) (MA: methyl-ammonium CH3-NH3+, X=halogen), the triiodide specimen (MAPbI(3)) is still the material of choice for solar energy applications. Although it is able to absorb light above its 1.6 eV bandgap, its poor stability in humid air atmosphere has been a major drawback for its use in solar cells. However, we discovered that this perovskite can be prepared by ball milling in a straightforward way, yielding specimens with a superior stability. This fact allowed us to take atomic-resolution STEM images for the first time, with sufficient quality to unveil microscopic aspects of this material. We demonstrated full Iodine content, which might be related to the enhanced stability, in a more compact PbI6 framework with reduced unit-cell volume. A structural investigation from neutron powder diffraction (NPD) data of an undeuterated specimen was essential to determine the configuration of the organic MA unit in the 100-298 K temperature range. A phase transition is identified, from the tetragonal structure observed at RT (space group I4/mcm) to an orthorhombic (space group Pnma) phase where the methyl-ammonium organic units are fully localized. Our NPD data reveal that the MA changes are gradual and start before reaching the phase transition. Optoelectronic measurements yield a photocurrent peak at an illumination wavelength of 820 nm, which is redshifted by 30 nm with respect to previously reported measurements on MAPbI(3) perovskites synthesized by crystallization from organic solvents.
Divalent chromium in the octahedral positions of the novel hybrid perovskites CH3NH3Pb1-xCrx(Br,Cl)(3) (x=0.25, 0.5): Induction of narrow bands inside the bandgap
F. Serrano-Sanchez, JC. Conesa, JE. Rodrigues, C. Marini, JL. Martinez, JA. Alonso,
J Alloy Compd 821, 153414 (2020).
Abstract
The hybrid organic-inorganic perovskites, MAPbX(3) (MA: methylammonium CH3-NH3+; X = halogen), are the active light absorbing materials in the new generation of solar cells. Previous theoretical quantum calculations indicated that substituting part of the Pb cations by Cr could introduce a partially filled ingap narrow band, allowing the excitation of electrons from the valence to the conduction band in two steps. This has been realized in this work, where the title compounds were prepared by ball milling from MABr, PbBr2, and CrCl2 in N-2 atmosphere. XRD patterns correspond to pure cubic phases, where Cr2+ ions have entered the octahedral sites of this perovskite framework for the first time. The valence state of Cr ions was probed by Cr K-edge X-ray absorption spectroscopy, confirming the 2+ state and possible local octahedral distortion. Magnetic susceptibility measurements indicate for the x = 0.5 compound a conspicuous antiferromagnetic ordering below T-N = 38 K and a paramagnetic moment in the high-temperature region of mu(eff) = 3.60 mu(B), corresponding to high-spin Cr2+ ions, thus confirming the incorporation of this element at the Pb sublattice. Diffuse reflectance spectra indicate, as predicted, that the presence of Cr induces a narrow band inside the bandgap of those perovskites: the new absorption at ca. 1.6 eV corresponds to the electronic transition from the valence band to the empty part of this partially filled in-gap band. Our result here may open a window of opportunity for new tools in band gap engineering, which can be used in other hybrid lead halide perovskites for improvements in solar cell devices depicting high efficiency. (C) 2019 Elsevier B.V. All rights reserved.
Features of the High-Temperature Structural Evolution of GeTe Thermoelectric Probed by Neutron and Synchrotron Powder Diffraction
J. Gainza, F. Serrano-Sanchez, NM. Nemes, JL. Martinez, MT. Fernandez-Diaz, JA. Alonso,
Metals 10, 48 (2020).
Abstract
Among other chalcogenide thermoelectric materials, GeTe and derivative alloys are good candidates for intermediate temperature applications, as a replacement for toxic PbTe. We have prepared pure polycrystalline GeTe by using arc-melting, and investigated its structural evolution by using neutron powder diffraction (NPD) and synchrotron X-ray diffraction (SXRD), as well as its correlation with the thermal variation of the Seebeck coefficient. Besides a significant Ge deficiency (similar to 7% Ge vacancies), the thermal evolution of the unit-cell volume and Ge-Te bond lengths in the rhombohedral phase (space group R3m), below 700 K, show unexpected anomalies involving the abrupt Ge-Te bond lengthening accompanied by increased Te thermal displacements. Above 700 K, the sample is cubic (space group Fm-3m) and shows considerably larger displacement parameters for Ge than for Te, as a consequence of the random distribution of the lone pair lobes of Ge2+. The Seebeck coefficient, reaching 120 mu V K-1 at 775 K, shows a shoulder in the 500-570 K region that can be correlated to the structural anomaly, modifying the electron-phonon scattering in this temperature range.
BODIPYs revealing lipid droplets as valuable targets for photodynamic theragnosis
Andrea Tabero, Fernando Garcia-Garrido, Alejandro Prieto-Castaneda, Eduardo Palao, Antonia R. Agarrabeitia, Inmaculada Garcia-Moreno, Angeles Villanueva, Santiago de la Moya, Maria J. Ortiz,
Chem Commun 56, 940 (2020).
Abstract
Endowing BODIPY PDT agents with the ability to probe lipid droplets is demonstrated to boost their phototoxicity, allowing the efficient use of highly fluorescent dyes (poor ROS sensitizers) as phototoxic agents. Conversely, this fact opens the way to the development of highly bright ROS photosensitizers for performing photodynamic theragnosis (fluorescence bioimaging and photodynamic therapy) from a single simple agent. On the other hand, the noticeable capability of some of the reported dyes to probe lipid droplets in different cell lines under different conditions reveals their use as privileged probes for advancing the study of interesting lipid droplets by fluorescence microscopy.
Looking at the "Water-in-Deep-Eutectic-Solvent" System: A Dilution Range for High Performance Eutectics
Nieves Lopez-Salas, Jose Manuel Vicent-Luna, Silvia Imberti, Elena Posada, Maria Jesus Roldan, Juan A. Anta, Salvador R. G. Balestra, Rafael M. Madero Castro, Sofia Calero, Rafael J. Jimenez-Rioboo, Maria Concepcion Gutierrez, Maria Luisa Ferrer, Francisco del Monte,
Acs Sustainable Chem. Eng. 7, 17565 (2019).
Abstract
Deep eutectic solvents (DESs) are lately expanding their use to more demanding applications upon aqueous dilution thanks to the preservation of the most appealing properties of the original DESs while overcoming some of their most important drawbacks limiting their performance, like viscosity. Both experimental and theoretical works have studied this dilution regime, the so-called "water-in-DES" system, at near-to stoichiometric amounts to the original DES. Herein, we rather studied the high-dilution range of the "water-in-DES" system looking for enhanced performance because of the interesting properties (a further drop of viscosity) and cost (water is cheap) that it offers. In particular, we found that, in the "water-in-DES" system of a ternary DES composed of resorcinol, urea and choline chloride (e.g., RUChClnW, where n represents mol of water per mole of ternary DES), the tetrahedral structure of water was distorted as a consequence of its incorporation, as an additional hydrogen bond donor or hydrogen bond acceptor, into the hydrogen bond complexes formed among the original DES components. DSC confirmed the formation of a new eutectic, with a melting point below that of its respective components, the original ternary DES and water. This depression in the melting point was also observed in the same regime of reline and malicine aqueous dilutions, thus suggesting the universality of this simple procedure (i.e., water addition to reach the high-dilution range of the "water-in-DES" system) to obtain deeper eutectics eventually providing enhanced performances and lower cost.
Resilience improvement of an isotactic polypropylene-g-maleic anhydride by crosslinking using polyether triamine agents
Adrien Letoffe, Sandrine Hoppe, Richard Laine, Nadia Canilho, Andreea Pasc, Didier Rouxel, Rafael J. Jimenez Rioboo, Sebastien Hupont, Isabelle Royaud, Marc Poncot,
Polymer 179, 121655 (2019).
Abstract
The elaboration of new hybrid polymer/metal composite materials depends on the development of innovative polymer matrices with improved mechanical properties and high effective adhesion behaviour with metal surfaces. This way, a new type of polyolefin was developed based on a peculiar crosslinking reaction. For this elaboration, an iPP-g-MAH with 1 wt% of MAH was crosslinked with polyether triamine molecules by twin screw reactive extrusion. The evolution of the crosslinking reaction, the microstructure and the mechanical behaviour of the different materials were studied as a function of the NH2 :MAH molar ratio in the range of 0:1 and 1.5:1. Gel content determination and FTIR analysis were performed to characterise the material at the molecular scale, when differential scanning calorimetry, wide angle X-rays scattering and self-successive auto-nucleation techniques were used for a deep microstructure analysis. Brillouin spectroscopy showed an interesting evolution of the elastic constants at the microscopic scale and results were in good agreement with those obtained at the macroscopic scale by a video-controlled uniaxial tensile test. Moreover, it was demonstrated that the true mechanical behaviours switch from brittle to ductile illustrating an important improvement of the resilience ability of the synthesized materials as a function of the NH2:MAH molar ratio even for molar ratio other than an equimolar ratio, with remaining MAH or NH2 chemical groups for further chemical reaction.
Brillouin and NMR spectroscopic studies of aqueous dilutions of malicine: Determining the dilution range for transition from a "water-in-DES" system to a "DES-in-water" one
M. J. Roldan-Ruiz, R. J. Jimenez-Rioboo, M. C. Gutierrez, M. L. Ferrer, F. del Monte,
J Mol Liq 284, 175 (2019).
Abstract
Brillouin and NMR spectroscopy were used to explore transitions in the structuring of liquid binary mixtures composed of water and a deep eutectic solvent (DES) known as malicine and composed of malic acid (MA) and choline chloride (ChCl). Malicine/H2O binary mixtures were studied for DES contents ranging from 100 to 40 wt%. Data representation of sound propagation velocities - obtained from Brillouin spectroscopy - versus DES content exhibited two well-differentiated linear regimes above and below ca. 70 DES wt% while transitioning from a "water-in-DES" system to a "DES-in-water" one along with dilution. Two linear regimes intersecting at ca. 70 DES wt% were also observed when representing the self-diffusion coefficients versus malicine contents, with MA and ChCl - and even water - exhibiting nearly identical self-diffusion coefficients for contents above 70 wt%, that diverged for contents below 70 wt%. Interestingly, further insights about this behaviour were provided by spin-lattice relaxation time of MA, also revealing the occurrence of hydrophobic hydration around non-polar moieties as in other aqueous binary mixtures of non-ionic H-bond co-solvents - e.g., DMSO:H2O or dioxane: water. (C) 2019 Elsevier B.V. All rights reserved.
Nanophase separation in aqueous dilutions of a ternary DES as revealed by Brillouin and NMR spectroscopy
E. Posada, M. J. Roldan-Ruiz, R. J. Jimenez Rioboo, M. C. Gutierrez, M. L. Ferrer, F. del Monte,
J Mol Liq 276, 196 (2019).
Abstract
Liquid binary mixtures of deep eutectic solvents (DESs) with H2O are lately attracting the interest of many research works because of the improved performance that these dilutions demonstrated in number of application - e.g., catalysis, separations, or biochemistry, among others - as compared to their respective neat DES counterparts. Interestingly, molecular simulations have revealed that, despite the macroscopically homogeneous appearance of the DES/H2O binary mixtures, phase segregation may occur at the nanoscale. Spectroscopic studies corroborating this event in this sort of mixtures are basically limited to NMR-based ones determining diffusion coefficients and some recent Brillouin spectroscopic studies performed for a quite short list of binary DESs -i.e., those composed of one single hydrogen bond donor (HBD) and one single hydrogen bond acceptor (HBA), being urea-choline-chloride-based DES the most widely studied. Interestingly, NMR data such as chemical shifts and relaxation times have also proved quite useful for the study of liquid binary mixtures different than DES-based ones - e.g., ionic-liquid-based ones. Herein, we have obtained a full set of NMR data - chemical shifts, diffusion coefficients and relaxation times - from liquid binary mixtures of H2O and a ternary DESs, in this particular case one composed of two HBD - e.g., resorcinol and hexylresorcinol - and one HBA - tetraethyl ammonium bromide. Good correlation among different NMR data as well as between NMR and Brillouin data was found in the determination of the DES content at which phase segregation occurs. (C) 2018 Published by Elsevier B.V.
Formation and stability of highly conductive semitransparent copper Meso-grids covered with graphene
L. Alvarez-Fraga, J. Bartolome, MX. Aguilar-Pujol, R. Ramirez-Jimenez, C. Prieto, A. de Andres,
Appl Surf Sci 493, 32 (2019).
Abstract
We report on the formation of highly stable and robust hybrid semitransparent electrodes based on a percolated mesoscale copper grid covered with graphene. Copper films sputtered on transparent substrates are used to synthesize graphene by CVD and annealed to form metallic grids covered with graphene. The copper grid density can be controlled by varying annealing temperature and time, obtaining excellent sheet resistance ( < 0.7 Omega/sq). The single layer graphene that covers the whole sample modifies the preferred crystalline orientation favoring (110) copper grains when treated up to 1050 degrees C. However, when annealed few degrees below bulk copper melting temperature, graphene is partially eliminated, probably due to the high pressure of copper vapor generated underneath, and copper forms isolated dots with mainly (111) orientation. During graphene synthesis, similar to 70% of the copper film is evaporated but, once graphene layer is completed the remaining copper fraction is constant up to almost bulk copper melting temperature demonstrating that graphene is an excellent barrier against copper evaporation. Contrary to the enhanced oxidation of copper foils with graphene, these graphene-Cu grids are extremely stable in ambient conditions maintaining an almost unchanged performance for years. The copper based meso-grids are highly stable and robust and present a suitable surface for organic- and biomolecules provided by the continuous graphene layer.
New Concepts for Production of Scalable Single Layer Oxidized Regions by Local Anodic Oxidation of Graphene
SJ. Quesada, F. Borras, M. Garcia-Velez, C. Coya, E. Climent, C. Munuera, I. Villar, VAD. O'Shea, A. de Andres, AL. Alvarez,
Small 15, 1902817 (2019).
Abstract
A deep comprehension of the local anodic oxidation process in 2D materials is achieved thanks to an extensive experimental and theoretical study of this phenomenon in graphene. This requires to arrange a novel instrumental device capable to generate separated regions of monolayer graphene oxide (GO) over graphene, with any desired size, from micrometers to unprecedented mm(2), in minutes, a milestone in GO monolayer production. GO regions are manufactured by overlapping lots of individual oxide spots of thousands mu m(2) area. The high reproducibility and circular size of the spots allows not only an exhaustive experimental characterization inside, but also establishing an original model for oxide expansion which, from classical first principles, overcomes the traditional paradigm of the water bridge, and is applicable to any 2D-material. This tool predicts the oxidation behavior with voltage and exposure time, as well as the expected electrical current along the process. The hitherto unreported transient current is measured during oxidation, gaining insight on its components, electrochemical and transport. Just combining electrical measurements and optical imaging estimating carrier mobility and degree of oxidation is possible. X-ray photoelectron spectroscopy reveals a graphene oxidation about 30%, somewhat lower to that obtained by Hummers' method.
Huge Photostability Enhancement in Bismuth-Doped Methylammonium Lead Iodide Hybrid Perovskites by Light-Induced Transformation
J. Bartolome, E. Climent-Pascual, C. Redondo-Obispo, C. Zaldo, AL. Alvarez, A. de Andres, C. Coya,
Chem Mater 31, 3662 (2019).
Abstract
The doping strategy of hybrid perovskites is being extensively explored not only for higher efficiency but also to overcome issues in photovoltaic materials such as self-degradation pathways in an ambient atmosphere or under visible irradiation. Here, BiI3 is introduced in the synthesis of MAPbI(3) films (MA: CH3-NH3+) to stabilize the material. Around 25% of nominal Bi3+ is accommodated in the perovskite structure, producing a shrinking of the unit cell and a small increase of the band gap. The presence of empty Bi gap states quenches the 770 nm red interband emission and results in a near-infrared emission at 1100 nm. However, high enough visible irradiation density induces a progressive segregation of Bi3+ out of the perovskite lattice and promotes the re-emergence of the red emission. This emission is blue shifted, and its intensity increases strongly with time until it reaches a saturation value which remains stable in the transformed films for extremely high power densities, around 1000 times higher than for undoped samples. We propose that the underlying processes include the formation of BiI3 and BiOI, probably at the surface of the crystals, hampering the usual decomposition pathways into PbI2 and PbOx for undoped MAPbI(3). These results provide a new path for obtaining highly stable materials which would allow an additional boost of hybrid perovskite-based optoelectronics.
Reduced graphene oxide/polyaniline electrochemical supercapacitors fabricated by laser
A. Ladron-de-Guevara, A. Bosca, J. Pedros, E. Climent-Pascual, A. de Andres, F. Calle, J. Martinez,
Appl Surf Sci 467, 691 (2019).
Abstract
We report on the precise fabrication of low-cost high-performance electrochemical supercapacitors using reduced graphene oxide/polyaniline nanofiber composite electrodes. An infrared laser has been used to reduce the graphene oxide, converting the initial graphene oxide compact layer into a three dimensional open network of exfoliated graphene flakes. This highly conducting porous structure is very well suited for electrodepositing pseudocapacitive materials owing to its large surface area. Polyaniline nanofibers have been controllably electrodeposited on the graphene flake network, not only extending further the electrode surface area and providing it with a strong pseudocapacitance but also preventing the restacking of the graphene sheets during the subsequent device processing and charge-discharge cycling. The composite electrode presents a specific capacitance of 442 F g(-1), as compared to 81 F g(-1) for the bare reduced graphene oxide counterpart, and a capacitance retention of 84% over 2000 cycles.
Experimental Observation of Monoclinic Distortion in the Insulating Regime of SmNiO3 by Synchrotron X-ray Diffraction
F. Serrano-Sanchez, F. Fauth, JL. Martinez, JA. Alonso,
Inorg Chem 58, 11828 (2019).
Abstract
RNiO3 (R = rare-earth element) perovskite materials are well-known to exhibit characteristic metal-insulator transitions. The structural distortion increases as the R member becomes smaller along the series. For SmNiO3, a high-hydrostatic-pressure preparation procedure, yielding samples with much enhanced crystalline quality, combined with the extremely high angular resolution of synchrotron X-ray diffraction (XRD) allowed us to identify a monoclinic phase in the insulating regime (below the metal-insulator transition temperature (T-MI) of 127 degrees C), defined in the space group P2(1)/n. This monoclinic symmetry had not been demonstrated directly using nonresonant XRD or neutron diffraction. This has important repercussions on the electronic nature of this material since the monoclinic structure contains two inequivalent Ni positions, implying a charge disproportionation phenomenon. In the metallic regime (above T-MI), the standard orthorhombic Pbnm structure is observed. Therefore, there is a coupled structural and electronic transition, as happens for the very small rare-earth compounds of the RNiO3 perovskite series. Across T(MI)( )there is a dramatic rearrangement of the lattice parameters, degree of tilting, and distortion of the NiO6 octahedra, showing the convergence of the Ni-O bond lengths upon entering the metallic phase. Brown's valence analysis of the different elements agrees with other reported values in the literature, matching with bond and charge disproportionation models. By magnetization measurements a Neel temperature (T-N) corresponding to the antiferromagnetic ordering of the Ni moments is identified at T-N= 220 K, whereas Sm moments experience long-range ordering below 36 K.
Spin-phonon coupling in uniaxial anisotropic spin-glass based on Fe2TiO5 pseudobrookite
JEFS Rodrigues, WS. Rosa, MM. Ferrer, TR. Cunha, MJM. Zapata, JR. Sambrano, JL. Martinez, PS. Pizani, JA. Alonso, AC. Hernandes, RV. Goncalves,
J Alloy Compd 799, 563 (2019).
Abstract
The ferric pseudobrookite is a rare example of uniaxial anisotropic spin-glass insulator, depicting a multiglass behavior and magnetoelectric coupling. Here, we present Raman spectroscopy results in order to elucidate the spin-phonon coupling in Fe2TiO5 for the first time. The experimental data are supported by computational simulations performed in view of density functional theory, which allowed us to assign the main Raman-active modes. Temperature-dependent phonon behavior exhibited anomalous evolution around 55 and 80-200 K, which was explained as successive coupling between lattice and spin configuration arising from spin freezing and short-range magnetic correlation, respectively. Arguments that the magnetoelectric effect in Fe2TiO5 is mediated by spin-phonon coupling are presented. (C) 2019 Elsevier B.V. All rights reserved.
Evidence of nanostructuring and reduced thermal conductivity in n-type Sb-alloyed SnSe thermoelectric polycrystals
J. Gainza, F. Serrano-Sanchez, M. Gharsallah, F. Carrascoso, J. Bermudez, OJ. Dura, FJ. Mompean, N. Biskup, JJ. Melendez, JL. Martinez, JA. Alonso, NM. Nemes,
J Appl Phys 126, 045105 (2019).
Abstract
SnSe has been recently reported as an attractive thermoelectric material, with an extraordinarily high, positive, Seebeck coefficient. Here, we describe the synthesis, structural, microscopic, and thermoelectric characterization of Sn1-xSbxSe intermetallic alloys prepared by a straightforward arc-melting technique. Sb-doped tin selenide was synthesized as strongly nanostructured polycrystalline pellets. Neutron diffraction studies reveal that Sb is placed at the Sn sublattice in the crystal structure, showing concentrations as high as 30%, and generates a significant number of Sn vacancies, while the increase of the interlayer distances favors the nanostructuration. The material is nanostructured both out-of-plane in nanometer-scale layers and in-plane by similar to 5nm undulations of these layers. This nanostructuring, along with an increased amount of Sn vacancies, accounts for a reduction of the thermal conductivity, which is highly desirable for thermoelectric materials. The phonon mean free path is estimated to be on the order of 2nm from low temperature, thermal conductivity, and specific heat, in accordance with the nanostructuration observed by high-resolution transmission electron microscopy. The thermal conductivity of SnSe is characterized by three independent techniques to assure a room temperature value of Sn0.8Sb0.2Se of kappa similar to 0.6W/mK. The freshly prepared Sb-doped compounds exhibit an abrupt change in the type of charge carriers, leading to large, negative Seebeck coefficients, although the arc-melt synthesized pellets remain too resistive for thermoelectric applications. Cold-pressed pellets evolve to be p-type at room temperature, but reproducibly turn n-type around 500K, with increased electrical conductivity and maximum observed figure of merit, ZT similar to 0.3 at 908K.
Substantial thermal conductivity reduction in mischmetal skutterudites Mm(x)Co(4)Sb(12) prepared under high-pressure conditions, due to uneven distribution of the rare-earth elements
J. Gainza, F. Serrano-Sanchez, J. Prado-Gonjal, NM. Nemes, N. Biskup, OJ. Dura, JL. Martinez, F. Fauth, JA. Alonso,
J Mater Chem C 7, 4124 (2019).
Abstract
Thermoelectric mischmetal-filled Mm(x)Co(4)Sb(12) (Mm: natural cocktail of rare-earth elements, mostly Ce and La) skutterudites have been synthesized and sintered in one step under high-pressure conditions at 3.5 GPa in a piston-cylinder hydrostatic press. Synchrotron X-ray diffraction patterns display a splitting of the diffraction peaks ascribed to purely Ce-, and Mm-filled skutterudite phases, which have been analyzed and confirmed by high-resolution TEM and EELS. A total thermal conductivity () of 1.51 W m(-1) K-1 is measured at 773 K for Mm(0.5)Co(4)Sb(12), below that of other filled skutterudites, which is promoted by the enhanced phonon scattering over a broad range of the phonon spectrum due to the inhomogeneous and nanoscale mischmetal inclusion. Compared to undoped CoSb3 skutterudite synthesized by conventional methods, is reduced by a factor of 3, while the power factor is also substantially enhanced.
Magnon-mediated magnetoresistance in layered manganites
A. Hernando, R. Cortes-Gil, D. Gonzalez-Merchante, M. Hernando, JM. Alonso, MA. Garcia, JL. Martinez, L. Ruiz-Gonzalez, JM. Gonzalez-Calbet,
Phys Rev B 99, 024403 (2019).
Abstract
We describe here a type of magnetoresistance that takes place in naturally layered and outstanding ordered single phase manganites that may be mediated by magnon excitation. In particular, we show the effect for the Ruddlesden-Popper compound, LaSr2Mn2O7 synthesized by ceramic method. This material exhibits, besides the conventional colossal magnetoresistance, another type of magnetoresistance at low temperature, associated with breaking of the A-type antiferromagnetic coupling of Mn-containing planes. Excitation of magnons or application of a magnetic field breaks this antiparallel alignment so that some electrons, initially confined on the planes, become itinerant along the interplain directions through a double exchange mechanism, giving rise to resistance variations of the order of similar to 60% for polycrystalline samples. The effect described here might be present in other types of manganites exhibiting a natural layered structure, opening up the possibility of developing magnetoresistive devices based on antiferromagnetic oxide materials without requiring artificial multilayered structures.
In-situ carboxylation of graphene by chemical vapor deposition growth for biosensing
Sandra Cortijo-Campos, Leo. Alvarez-Fraga, Gil. Goncalves, Mercedes Vila, Patricia Alvarez, Rosa Menendez, Alicia de Andres, Carlos Prieto,
Carbon 141, 719 (2019).
Abstract
A new approach for in-situ specific functionalization of graphene with carboxylic groups through a single-step growth is presented. Depending on the fabrication parameters, it is possible to synthesize functionalized single layer or multilayer graphene. The homogeneity and functionalization degree are evaluated combining micro-Raman and XPS spectroscopies. The obtained COOH content reaches around 5%, similar to the values obtained by the other methods but with significantly lower contents of the other oxygen groups (carbonyl and hydroxyl) and of other sp(3) defects which are detrimental for electronic transport. The obtained COOH-functionalized single-layer graphene presents optimum values of the sheet resistance, around 8 k Omega, and high mobility, around 800 cm(2) V(-1)s(-1). Anchoring of antibodies is demonstrated through the immobilization of IgG1-FITC by the carbodiimide method, showing that these COOH-functionalized single-layer graphene can be very promising materials for electronic sensing applications. (C) 2018 Elsevier Ltd. All rights reserved.
