Estudio de interfases en óxidos complejos por técnicas avanzadas de microscopía electrónica

Tesis inédita de la Universidad Complutense de Madrid, Facultad de Ciencias Físicas, leída el 07-05-2015Pequeños cambios a nivel atómico de la estructura, composición o estado electrónico de un material pueden producir sorprendentes efectos macroscópicos. En particular, en óxidos complejos basados en metales de transición, un gran número de fenómenos físicos como transiciones metal-aislante, magnetorresistencia colosal o multiferroicidad son extremadamente sensibles a estas variaciones. Por tanto, para abordar el estudio de sistemas con tales características, técnicas experimentales con capacidad de analizar materiales a escala atómica y en el espacio real se hacen indispensables. La microscopía electrónica de transmisión con barrido combinada con la espectroscopia de pérdida de energía de electrones (EELS) forman una pareja con posibilidades únicas para estos estudios. Estas técnicas han crecido enormemente desde el desarrollo del corrector de aberración esférica en la última década y su alta resolución espacial nos permite ahora estudiar átomos individuales. El uso de estos equipos supone una herramienta única para el estudio de sistemas complejos, más aún cuando la dimensionalidad se reduce a pocos nanómetros como en películas delgadas o interfaces. En estos casos, técnicas de difracción promediadas macroscópicamente pueden no ser suficientemente sensibles a los parámetros que rigen la física relevante y por tanto, la gran sensibilidad espacial de la microscopía electrónica supone una gran ventaja. El objetivo principal de este trabajo será precisamente establecer la conexión entre los mecanismos a nivel atómico y las propiedades físicas de una serie de sistemas basados en óxidos complejos cuidadosamente escogidos. Analizaremos en el espacio real fluctuaciones mínimas, casi por debajo del umbral de detectabilidad, responsables últimas del comportamiento macroscópico.En primer lugar, se ha estudiado como pequeñas concentraciones de vacantes de oxígeno, tanto inducidas mediante irradiación como intrínsecas al material, pueden determinar las propiedades físicas macroscópicas del sistema. Se ha observado cómo procesos de irradiación dan lugar a la formación de una capa de TiO con alto grado cristalino en la superficie de monocristales de TiO2 y como además pueden producir estados metálicos superficiales en un aislante de bandas como es el SrTiO3. Se ha analizado además como la reestructuración electrónica debida a la presencia de vacantes de oxígeno estructurales explica por primera vez el origen electroestático del bloqueo iónico en fronteras de grano de materiales con importantes aplicaciones energéticas. Se ha abordado también el estudio de pequeñas variaciones estructurales, en particular, distorsiones colectivas de la red de oxígeno en heteroestructuras de óxidos complejos y su relación con la aparición de estados físicos inexistentes en los materiales masivos. Se ha encontrado una correlación entre rotaciones del octaedro de oxígenos producidas por tensiones epitaxiales y la estabilización de una fase interfacial ferromagnética y conductora en superredes formadas por óxidos aislantes. Además, se ha extendido este análisis a sistemas más complejos como uniones túnel multiferroicas donde se ha obtenido la configuración de dominios ferroeléctricos midiendo las distorsiones en la red de oxígenos para cada celda unidad. Este estudio muestra una de las primeras observaciones experimentales de una configuración de dominios ferroeléctricos tipo head-to-head en capas ultra-delgadas. Se ha encontrado además la presencia de una carga de apantallamiento confinada a la pared de dominio que genera estados electrónicos accesibles en el interior de la barrera ferroeléctrica, proporcionando los mecanismos para estabilizar un tuneleamiento cuántico resonante.El continuo desarrollo de estas técnicas experimentales hace vislumbrar un futuro prometedor tanto para la ciencia de materiales como para la microscopía electrónica. La exploración a escala atómica de fenómenos físicos aún por desvelar está ahora, más que nunca a nuestro alcance.Fac. de Ciencias FísicasTRUEunpu

In-Plane Anisotropic Optical and Mechanical Properties of Two-Dimensional MoO3

Molybdenum trioxide (MoO3) in-plane anisotropy has increasingly attracted the attention of the scientific community in the last few years. Many of the observed in-plane anisotropic properties stem from the anisotropic refractive index and elastic constants of the material but a comprehensive analysis of these fundamental properties is still lacking. Here we employ Raman and micro-reflectance measurements, using polarized light, to determine the angular dependence of the refractive index of thin MoO3 flakes and we study the directional dependence of the MoO3 Young's modulus using the buckling metrology method. We found that MoO3 displays one of the largest in-plane anisotropic mechanical properties reported for 2D materials so far.This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement n degrees 755655, ERC-StG 2017 project 2D-TOPSENSE), the European Commission, under the Graphene Flagship (Core 3, grant number 881603), the Spanish Ministry of Economy, Industry and Competitiveness through the grant MAT201787134-C2-2-R. R.F. acknowledges the support from the Spanish Ministry of Economy, Industry and Competitiveness (MINECO) through a Juan de la Cierva-formacion fellowship 2017 FJCI-2017-32919. S.P. acknowledges the fellowship PRE2018-084818. R. D'A. acknowledges financial support from the grant Grupos Consolidados UPV/EHU del Gobierno Vasco (Grant No. IT1249-19), the support of the MICINN through the grant "SelectDFT" (Grant No. FIS2016-79464-P) and travel support from the MINECO grant "TowTherm" (Grant No. MINECOG17/A01). G.S.-S. acknowledges financial support from Spanish MICIU RTI2018-099054-J-I00 and MICINN IJC2018-038164-I. Electron microscopy observations were carried out at the Centro Nacional de Microscopia Electronica, CNME-UC

Magnetic phase diagram, magnetotransport and inverse magnetocaloric effect in the noncollinear antiferromagnet Mn5Si3

This Accepted Manuscript will be available for reuse under a CC BY-NC-ND licence after 24 months of embargo periodThe antiferromagnet Mn5Si3 has recently attracted attention because a noncollinear spin arrangement has been shown to produce a topological anomalous Hall effect and an inverse magnetocaloric effect. Here we synthesize single crystals of Mn5Si3 using flux growth. We determine the phase diagram through magnetization and measure the magnetoresistance and the Hall effect. We find the collinear and noncollinear antiferromagnetic phases at low temperatures and, in addition, a third magnetic phase, in between the two antiferromagnetic phases. The latter magnetic phase might be caused by strain produced by Cu inclusions. This suggests that fluctuations of the mixed character magnetic ordering in this compound can be easily quenched by stressThis work was supported by the Spanish MINECO (Consolider Ingenio Molecular Nanoscience CSD2007-00010 program, FIS2017-84330-R, MDM-2014-0377, MAT2014-52405-C2-2-R, FJCI-2015-25427 and CSD2009-00013), by the Comunidad de Madrid through program NANOMAGCOST-CM (S2018 NMT-4321) and MAD2D-CM (S2013/MIT-3007) and by EU (Graphene Core1 contract No. 696656, Nanopyme FP7-NMP-2012 SMALL-6 NMP3-SL-2012 310516 and COST CA16218

Formation of titanium monoxide (001) single-crystalline thin film induced by ion bombardment of titanium dioxide (110)

© 2015 Macmillan Publishers Limited. All rights reserved. A plethora of technological applications justify why titanium dioxide is probably the most studied oxide, and an optimal exploitation of its properties quite frequently requires a controlled modification of the surface. Low-energy ion bombardment is one of the most extended techniques for this purpose and has been recently used in titanium oxides, among other applications, to favour resistive switching mechanisms or to form transparent conductive layers. Surfaces modified in this way are frequently described as reduced and defective, with a high density of oxygen vacancies. Here we show, at variance with this view, that high ion doses on rutile titanium dioxide (110) induce its transformation into a nanometric and single-crystalline titanium monoxide (001) thin film with rocksalt structure. The discovery of this ability may pave the way to new technical applications of ion bombardment not previously reported, which can be used to fabricate heterostructures and interfaces.Peer Reviewe

Hexagonal Hybrid Bismuthene by Molecular Interface Engineering

[EN] High-quality devices based on layered heterostructuresare typicallybuilt from materials obtained by complex solid-state physical approachesor laborious mechanical exfoliation and transfer. Meanwhile, wet-chemicallysynthesized materials commonly suffer from surface residuals and intrinsicdefects. Here, we synthesize using an unprecedented colloidal photocatalyzed,one-pot redox reaction a few-layers bismuth hybrid of "electronicgrade" structural quality. Intriguingly, the material presentsa sulfur-alkyl-functionalized reconstructed surface that preventsit from oxidation and leads to a tuned electronic structure that resultsfrom the altered arrangement of the surface. The metallic behaviorof the hybrid is supported by ab initio predictionsand room temperature transport measurements of individual nanoflakes.Our findings indicate how surface reconstructions in two-dimensional(2D) systems can promote unexpected properties that can pave the wayto new functionalities and devices. Moreover, this scalable syntheticprocess opens new avenues for applications in plasmonics or electronic(and spintronic) device fabrication. Beyond electronics, this 2D hybridmaterial may be of interest in organic catalysis, biomedicine, orenergy storage and conversion.This work has been supported by the European Union (ERC-2018-StG 804110-2D-PnictoChem & and ERC Proof of Concept Grant 101101079-2D4H2 to G.A.; ERC-2021-StG 101042680 2D-SMARTiES awarded to J.J.B.), the Spanish MICINN (PID2019-111742GA-I00, PID2020-115100GB-I00, MRR/PDC2022-133997-I00, TED2021-131347B-I00, and Excellence Unit Maria de Maeztu CEX2019-000919-M), and the Generalitat Valenciana (CIDEGENT/2018/001, CIDEGENT/2018/005, and CDEIGENT/2019/022). Financial support by Severo Ochoa centre of excellence program (CEX2021-001230-S) is gratefully acknowledged. M.K. and H.B.W. acknowledge support by the Deutsche Forschungsgemeinschaft (DFG), under Projektnummer 182849149 (SFB 953, projects B08 and B13). Electron microscopy work carried out at UCM (M.V., G.S.S.) sponsored by MICINN PID2021-122980OB-C51 and Comunidad de Madrid MAD2D-CM-UCM3. G.S.S. acknowledges financial support from Spanish MCI Grant Nos. RTI2018-099054-J-I00 (MCI/AEI/FEDER, UE) and IJC2018-038164-I. C.D. and Y.M.E. thank the cluster of excellence 3DMM2O funded by DFG under Germany's Excellence Strategy - 2082/1 - 390761711 for financial support. The authors thank Lukas Grunwald and Erich Muller for helpful discussions. A.M.R. thanks the Spanish MIU (Grant No FPU21/04195). A.S.-D. thanks the Universidad de Valencia, for an Atraccion del talento' predoctoral grant. F.G.-P. thanks ITQ, UPV-CSIC for concession of a contract (PAID 01-18).Dolle, C.; Oestreicher, V.; Ruiz, AM.; Kohring, M.; Garnes-Portoles, F.; Wu, M.; Sánchez-Santolino, G.... (2023). Hexagonal Hybrid Bismuthene by Molecular Interface Engineering. Journal of the American Chemical Society. 145(23):12487-12498. https://doi.org/10.1021/jacs.2c1303612487124981452

Hexagonal Hybrid Bismuthene by Molecular Interface Engineering

High-quality devices based on layered heterostructures are typically built from materials obtained by complex solid-state physical approaches or laborious mechanical exfoliation and transfer. Meanwhile, wet-chemically synthesized materials commonly suffer from surface residuals and intrinsic defects. Here, we synthesize using an unprecedented colloidal photocatalyzed, one-pot redox reaction a few-layers bismuth hybrid of “electronic grade” structural quality. Intriguingly, the material presents a sulfur-alkyl-functionalized reconstructed surface that prevents it from oxidation and leads to a tuned electronic structure that results from the altered arrangement of the surface. The metallic behavior of the hybrid is supported by ab initio predictions and room temperature transport measurements of individual nanoflakes. Our findings indicate how surface reconstructions in two-dimensional (2D) systems can promote unexpected properties that can pave the way to new functionalities and devices. Moreover, this scalable synthetic process opens new avenues for applications in plasmonics or electronic (and spintronic) device fabrication. Beyond electronics, this 2D hybrid material may be of interest in organic catalysis, biomedicine, or energy storage and conversion

Hydrogen-Induced Reduction Improves the Photoelectrocatalytic Performance of Titania

One of the main challenges to expand the use of titanium dioxide (titania) as a photocatalyst is related to its large band gap energy and the lack of an atomic scale description of the reduction mechanisms that may tailor the photocatalytic properties. We show that rutile TiO2 single crystals annealed in the presence of atomic hydrogen experience a strong reduction and structural rearrangement, yielding a material that exhibits enhanced light absorption, which extends from the ultraviolet to the near-infrared (NIR) spectral range, and improved photoelectrocatalytic performance. We demonstrate that both magnitudes behave oppositely: heavy/mild plasma reduction treatments lead to large/negligible spectral absorption changes and poor/enhanced (×10) photoelectrocatalytic performance, as judged from the higher photocurrent. To correlate the photoelectrochemical performance with the atomic and chemical structures of the hydrogen-reduced materials, we have modeled the process with in situ scanning tunneling microscopy measurements, which allow us to determine the initial stages of oxygen desorption and the desorption/diffusion of Ti atoms from the surface. This multiscale study opens a door toward improved materials for diverse applications such as more efficient rutile TiO2-based photoelectrocatalysts, green photothermal absorbers for solar energy applications, or NIR-sensing materials.1. Introduction ARTICLE SECTIONSJump To Titanium dioxide (titania) is one of the most widely used materials in the industry, reaching a world production capacity of more than eight million metric tons in 2020, with a potential total value in the market of several billion USD. (1) The reason for such a high consumption resides in its versatility and interesting properties such as high chemical stability, photoreactivity, UV light absorption, and biocompatibility, properties that are usually enhanced at the nanoscale, making it suitable for a plethora of industrial applications. (2) In some of these applications, defects that appear upon TiO2 reduction play a pivotal role, as they constitute the active sites of the material, conferring its catalytic properties toward photoreduction of target molecules, such as water or CO2, for solar fuel production, or even the photodegradation of organic pollutants. (2,3) However, one of the main challenges of TiO2 for its application as an efficient photocatalyst is related to its large band gap energy (∼3.1 eV) that limits the light absorption in the visible and infrared regions of the electromagnetic spectrum, which constitutes more than 90% of the total solar radiation reaching the Earth. (4,5) Different strategies to increase titania light absorption through band gap engineering have been explored, such as doping with metallic and nonmetallic species, (6) or generation of defects such as oxygen vacancies (Ovac), interstitial titanium atoms (Tiint), hydrogenation, or disorder. (7) In the past decade, the use of black titanium dioxide, obtained through strong hydrogenation, has been extended to improve light absorption in the visible range. (8,9) Some pioneering works have used hydrogenation to introduce disorder or cover oxygen vacancies as a way of enhancing solar light absorption, catalytic properties, (10,11) or solar hydrogen conversion via photoelectrochemical (PEC) water splitting. (12) Many of these approaches involve complex alloys and metal–organic heterostructures whose combined properties are required to obtain a tuned absorption response. Thus, hydrogenation appears to provide a clean, economical, and versatile alternative to obtain materials with efficient light absorption over a broad energy range. In this direction, here we propose the use of hydrogen on rutile TiO2 (110) samples as a reducing agent to increase the vacancies and active sites, leading to an enhancement in the photoelectrochemical performance. To better understand the atomic-scale interaction of atomic hydrogen with the titania surfaces, Surface Science model studies under UHV have been undertaken, (2) providing access to the structural, chemical, and electronic properties of the surfaces. Despite the huge interest in the interaction of hydrogen with titania, the adsorption and thermal evolution of hydrogen species on rutile TiO2 (110) surfaces is still an open question. It has been reported that only atomic hydrogen adsorbs on the rutile TiO2 (110) surfaces, (13) preferentially at Obr sites, (14) where it can undergo four different thermally triggered competing processes: (i) desorption as H2, (ii) desorption as H2O, (iii) surface migration, and (iv) diffusion into the bulk to form interstitial subsurface OH groups. (14,15) Interestingly, the reduction level of the substrate will affect hydrogen diffusion and H2O or H2 desorption. (16,17) However, the interaction of the rutile TiO2 (110) surface with atomic hydrogen as a function of sample temperature has been scarcely investigated, most of the studies being focused on its desorption from initially hydrogenated surfaces close to room temperature. It is important to note that temperature can play a pivotal role in the etching mechanisms, as will be demonstrated below. Even under these constrained conditions, a possible etching effect of the surface as a consequence of H2O desorption has been suggested. (18) However, a detailed and comprehensive study on the structural and electronic modification of the surfaces at the atomic level upon exposure to atomic hydrogen, as well as the correlation of the photoabsorption and photoelectrochemical performance to these changes is still missing. In this work, the photoelectrocatalytic performance of hydrogen-exposed model single crystal rutile TiO2 (110) samples is evaluated and correlated with the modifications in the structural, chemical, and light absorption properties, thanks to a multitechnique approach, including an unprecedented surface science methodology. Our results indicate that the samples that exhibit the best photoelectrochemical performance are those that have undergone a superficial reduction localized at the topmost layers, while heavy reduction reaching the bulk is detrimental to their performance, in good agreement with recent results. (19,20) Furthermore, the combination of mesoscopic and nanoscopic measurements allows rationalizing the hydrogen-induced etching mechanism, demonstrating that model studies performed on single crystalline substrates using surface science characterization techniques under highly controlled UHV conditions constitute a privileged framework to access the atomic-scale properties of the treated materials and achieve a valuable structure-performance correlation. This work will contribute to the comprehension and control of the hydrogenation process as a clean, economical, and versatile method for the development of broadband absorbers from the UV to the IR regions, efficient photocatalysts, and photothermal energy conversion devices. 2. Results and Discussion ARTICLE SECTIONSJump To Rutile TiO2 (110) single crystals (sample ref. in Figure 1a) exhibit drastic color changes after hydrogen plasma etching treatments (see section 1 in ESI for more details on the plasma etching setup and process), from gray (samples S5 and S6, after 30 and 60 min at 500 K in Figure 1a, respectively) to dark blue (sample S7, 730 K, 30 min) and black (S4, S3, S2, and S1, 950 K, 1, 30, 60, and 180 min, respectively). These changes are accompanied by an improvement in the light absorption in the visible and NIR regions, reaching an almost flat absorption above 80% from 300 to 900 nm for the treatments performed in samples S1–S4 (Figure 1b). This absorption increase is attributed to the appearance of ingap states as a consequence of the formation of reduced Ti species that reduce the effective gap (see Figure S2). Similar band gap reduction has already been reported, for example, in ref (7). The observed modifications in the optical properties give rise to significant changes in the photoelectrochemical performance. Figure 1c shows the linear sweep voltammetry (LSV) performed on selected samples (S1, S3–S5, and S7) and on pristine TiO2 for comparison, which shows almost negligible photocurrent under simulated solar irradiation values. It is interesting to note that the sample with the mildest treatment (S5) presents the highest photocurrent. Contrarily, those that have undergone the more severe H-plasma treatment present a much higher light absorption in the visible/NIR regions (S1 and S3) but exhibit a poor photocurrent. Finally, intermediate treatments, either for high temperature and short time (S4) or at moderate temperature (S7), show a halfway behavior, with an improved photocurrent for voltages below 0.3 V. These results are consistent with recent literature that highlights the pivotal role played by oxygen vacancies in solar energy conversion applications. (21) While these can increase the optical absorption in the visible range, an excess can induce a metal-like behavior (degenerate semiconductor), leading to charge transfer recombination and concomitant deactivation of the photoactivity of the material. However, not only the density of vacancies is important but also their location. Surface oxygen vacancies have been reported to be beneficial to the performance of photoanodes as they improve the charge separation by narrowing the space charge layer, (22,23) while bulk oxygen vacancies are disadvantageous, as they increase recombination dynamics and activate loss channels, with an associated decrease in photocurrent. (24) Figure 1 Figure 1. (a) Images showing the color change of the different TiO2 samples after each hydrogen plasma treatment. (b) Light adsorption curves in the UV–NIR regions for the TiO2 samples after different hydrogen plasma treatments. The pristine TiO2 sample is included as a reference. (c) LSV curves for selected treated samples covering the whole range of temperatures used in the treatments. (d) Cumulative hydrogen production vs reaction time of TiO2 S5 sample under solar simulated irradiation at 0.6 V (vs Ag/AgCl) during 40 min. Considering our experimental setup where platinum (almost 100% faradaic efficiency for HER) is used as a reference photocathode, we have chosen the sample with the higher photocurrent, S5, to be used as photoelectrode in a photoelectrochemical cell connected to a gas chromatograph to quantify the hydrogen evolution reaction (HER) produced in the counter electrode by the generated photocurrent. It must be noted that, as the HER will be directly related to the photocurrent, only this sample has been considered. In this experiment, where the reaction was carried out under conditions of 0.6 V versus Ag/AgCl, the sample is illuminated and biased during 40 min (from minute 10 to 50) and then it is let evolve, observing adequate stability (current density in the Figure S3) and yielding a production of 14 micromoles of H2. These changes in the photoelectrocatalytic behavior as a function of the reduction level can be explained on the basis of the structural, chemical, and electronic modifications of the treated samples. Figure 2 shows the XPS spectra of the Ti 2p core level for S1, S3–S5, and S7 samples. The region below the Ti4+ peak (459.3 eV) is characteristic of reduced Ti species, from Ti3+ to Ti2+. Sample S5, presenting the higher photocurrent, shows an almost negligible amount of reduced Ti species (red curve), as indicated by the absence of any shoulder at ∼457.7 eV, which corresponds to Ti3+ species. This can be understood by a very superficial and subtle etching of the surface, in agreement with the light gray color exhibited by the sample (see Figure 1a). Our assumption on the formation of only superficial defects is empirically supported by the fact that, despite the very low reduction level of S5, XPS measurements could be carried out without any problem, i.e., no charging effects were observed, as known to occur in pristine TiO2 due to its large band gap. When the temperature of the sample is increased to 730 K during the etching (S7), the XPS spectrum starts developing a small shoulder at lower binding energies (BE) (see ESI for the complete XPS analysis including Ti 2p, C 1s, and N 1s peak deconvolution), compatible with the appearance of reduced Ti3+ species as a consequence of surface reduction. If the temperature is further increased up to 950 K, a much more severe reduction of the sample is observed, as judged by the development of lower BE components down to 455.4 eV. It is interesting to note that there is no evidence for the formation of Ti1+ and/or Ti0 species even after a heavy etching of the surface. Instead, the component at 455.4 eV suggests the formation of TiN and TiOxNy species, probably as a consequence of air exposure of the plasma-etched samples (it must be noted that highly reduced Ti species are very reactive toward both N and O). (25) Thus, the XPS results indicate the possibility to tune the reduction level of atomic species in the samples by tuning the sample temperature and duration of the plasma treatment, allowing for the control of the light absorption and amount of reduced species, which critically influence their photoelectrocatalytic performance. Figure 2 Figure 2. Waterfall representation of the Ti 2p core-level XPS spectra for treated samples. The y-axes of the spectra have been offset for clarity. The structure of the surface region seems to be crucial in determining the properties of titania. The surface roughness of the plasma-treated samples has been studied by atomic force microscopy (AFM) (see ESI, Figure S7) and it can be concluded that S5 (soft etching, short-absorption range, and high photoelectrocatalytic performance) shows a very small RMS roughness (0.2 nm), with a value slightly higher than that observed for the pristine TiO2 surface (0.7 Å), in good agreement with the XPS spectrum shown above (soft etching = low reduction level = low rugosity). However, increasing the reduction temperature has a dramatic effect on the surface rugosity, that grows by a factor of ∼10. This fact indicates that the hydrogen plasma etching removes atomic species from the surface. Scanning transmission electron microscopy and electron energy loss spectroscopy (STEM-EELS) measurements (see ESI, Figure S8) of the cross-section of the S3 sample corroborate a profound sample etching, which extends approximately 180 nm into the sample. So far, our results present clear evidence for the possibility to tune the photoelectrochemical properties of TiO2 single crystals by the rational selection of the hydrogen plasma etching parameters. However, little can be said about the etching mechanism yielding this behavior and, more specifically, the surface structure of the softly etched S5 sample, as AFM cannot produce the necessary resolution. In order to comprehend the etching mechanism at the atomic level, model UHV experiments with single crystal rutile TiO2 (110) samples have been carried out. In this respect, new samples were produced by exposing them to a flux of atomic and molecular hydrogen produced by a hydrogen cracker in equivalent conditions to those in the plasma procedure (see Methods section for further details). In this way, the hydrogen dose can be fine-tuned with high precision, allowing for the characterization of the initial etching stages via high-resolution STM images. In this regard, two analyses were performed: (i) analysis of the surface structure upon a variable hydrogen dose (achieved by changing the dosing time at a fixed sample temperature), and (ii) evolution of surface structure with sample temperature at a fixed dose (i.e., dosing time). Figure 3a) shows a schematic representation of the rutile TiO2 (110)-(1 × 1) surface. This surface is characterized by the presence of in-plane Ti and protruding O rows running along the [001] surface direction (Ti5c and Obr rows, respectively). The corresponding STM image of the clean surface is presented in Figure 3b), where bright rows are associated with Ti5c rows and not the protruding Obr rows due to a well-known electronic effect. (26) Reduced (1 × 1) surfaces prepared under UHV conditions typically present two types of defects as revealed by STM, bright protrusions over the dark rows and dark depressions on the bright rows. The former is known to be due to Obr vacancies (Ovac) and/or hydroxyl groups, (27) while the origin of the latter is still not clear but could be associated with missing Ti atoms, as will be shown. Figure 3 Figure 3. Model characterization of the H-induced etching of the TiO2 surface by STM. (a) Schematic representation of the rutile TiO2 (110)-(1 × 1) surface termination, composed of alternating rows of protruding Obr atoms and in-plane Ti5c atoms. Red and gray atoms correspond to oxygen and titanium, respectively. (b) STM image of the clean TiO2 surface after several sputtering and annealing cycles under UHV conditions. Bright rows correspond to in-plane Ti5c atoms. (26) STM parameters: (25 × 25 nm), I = 36 pA, V = 1.5 V. (c,d) STM images of the TiO2 surface after exposure to atomic hydrogen during 2 and 30 min, respectively (substrate temperature during etching: 500 K). The blue ring in panel (d) highlights the remaining patch of the (1 × 1) surface termination. STM parameters: (15 × 15 nm), I = 77 pA, V = 1.5 V; and (35 × 35 nm), I = 122 pA, V = 1.5 V, respectively. The STM images in Figure 3c,d show the evolution of two UHV-prepared rutile TiO2 (110)-(1 × 1) samples upon exposure to different doses of atomic hydrogen (2 and 30 min of atomic hydrogen, respectively) while being heated at 500 K. After exposure, a series of trenches appeared on the surface aligned along the [001] surface direction. At low etching times (Figure 3c), the height of the trenches corresponds to one TiO2 atomic layer (∼ 3.2 Å) and they extend over several unit cells along the [001] surface direction but only 1–3 unit cells in the [11̅0] direction. Interestingly, on some occasions, it was possible to distinguish individual bright dots inside the trenches (see green arrows in Figure 3c). Given their bright appearance and their location at the expected position of the Ti5c rows (see dashed blue lines), these can be assigned to highly undercoordinated Ti atoms that appear as a consequence of oxygen removal in their vicinity. Their assignment to TiH species can be ruled out as it has been shown that these species are not stable at 500 K. (18) This can be understood considering the rather low diffusion barrier (0.99 eV) to transform hydride hydrogen into hydroxyl groups. (28) The STM image for long-term etching (30 min) shows a TiO2 surface completely restructured, with a corrugation of 2 versus 0.5 Å of the clean surface (see Figure S9). The surface maintains a strong directionality along the [001] surface direction. Although the surface etching is extended over the vast majority of the surface, it is still possible to observe some patches of the (1 × 1) surface structure, such as that highlighted with a blue circle. To investigate the TiO2 surface reconstruction mechanism, a series of experiments modifying the atomic hydrogen dose, i.e., exposure time (1, 1.5, 2, 3, 10, and 30 min) at 500 K were performed (Figure S10). For short exposures, the creation of trenches involving both Obr and Ti rows was observed, while the (1 × 1) surface termination was preserved. This etching of the surface increased homogeneously with exposure time until no (1 × 1) areas were observed after 30 min (panel f). Considering the STM and XPS results, the proposed reduction mechanism is as follows: in the first stage, atomic hydrogen is adsorbed on the Obr atoms of the surface giving rise to the formation of surface hydroxyl groups. After saturation of the Obr sites, extra hydrogen atoms will interact with the hydroxyl groups yielding H2O rather than adsorbing on the Ti5c atoms. (29) As a result, Obr atoms will desorb as H2O, leading to the formation of Ti3+ sites (either originated by the loss of O atoms or by the hydroxylation of Ti atoms), as shown by XPS. However, the STM images reveal that, given the dimensions of the trenches appearing on the surface, not only Obr atoms are removed but Ti atoms are also affected, probably diffusing into the bulk, occupying interstitial positions. STM simulations (Figure S11) confirm our assignation of individual bright spots inside the trenches to highly reactive undercoordinated Ti sites formed during surface reconstruction. In addition, the existence of a non-negligible energy barrier in the process is corroborated by studying the evolution of the TiO2 sample with the surface temperature during the H exposure (see Figure S12). This study shows that a threshold temperature in the order of 500 K is required to initiate the H-induced etching of the surface. Finally, to establish a correlation between the etching methodology, i.e., surface restructuration and reduction with the photoelectrochemical performance of such samples, electrochemical impedance spectroscopy (EIS) measurements were performed, which show a direct correlation between surface reconstruction and the photogenerated charge transfer. Figure 4 presents Nyquist plots obtained under dark and illuminated conditions for all measured samples. Using the Randles circuit, (30,31) the acquired semicircles can be fitted to obtain the equivalent electrical circuit composed by a series resistance RS (that comprises the electrical contacts and electrolyte resistances) and a resistance–capacitance (RCT–CCT) in parallel (Figure S13), accounting for the TiO2/electrolyte interface (see Table XV in ESI). As observed, there is a strong influence of the plasma treatment on the photogenerated

Ferroionic inversion of spin polarization in a spin-memristor

Magnetoelectric coupling in artificial multiferroic interfaces can be drastically affected by the switching of oxygen vacancies and by the inversion of the ferroelectric polarization. Disentangling both effects is of major importance toward exploiting these effects in practical spintronic or spinorbitronic devices. We report on the independent control of ferroelectric and oxygen vacancy switching in multiferroic tunnel junctions with a La_(0.7)Sr_(0.3)MnO_3 bottom electrode, a BaTiO_3 ferroelectric barrier, and a Ni top electrode. We show that the concurrence of interface oxidation and ferroelectric switching allows for the controlled inversion of the interface spin polarization. Moreover, we show the possibility of a spin-memristor where the controlled oxidation of the interface allows for a continuum of memresistance states in the tunneling magnetoresistance. These results signal interesting new avenues toward neuromorphic devices where, as in practical neurons, the electronic response is controlled by electrochemical degrees of freedom

Enhancement of vortex liquid phase and reentrant behavior in NiBi_(3) single crystals

We investigate the vortex phase diagram of needle shaped high quality NiBi3 single crystals by transport measurements. The current is applied along the crystalline b-axis of this intermetallic quasi-1D BCS superconductor. The single crystals show a Ginzburg-Levanyuk (G (i)) parameter of about 10(-7), larger by two orders of magnitude than G _(i) in elemental low T_(c) BCS superconductors. Vortex phase diagram, critical currents and pinning forces have been extracted from the experimental data. We observe (i) an enhancement of the vortex liquid phase, (ii) a reentrance of the liquid phase at low fields and (iii) an unusual magnetic field dependence of the pinning force. We suggest that these phenomena result from the interplay between pinning due to quenched disorder and the quasi-1D character of the material which could lead, for instance, to more complex pinning mechanisms at play