Ion-related phenomena in nanoscale oxide systems: from fundamentals to applications

Resume : With the latest technological progress and methodological developments in scanning transmission electron microscopy (STEM) fascinating phenomena have been revealed at the atomic scale in functional complex oxides. Simultaneous acquisition of high-angle annular dark-field (HAADF-) and annular bright-field (ABF-) STEM images opened up an efficient way for combined imaging of light and heavy elements. High-quality NdBa2Cu3O7 (NBCO) ultra-thin films have been deposited on TiO2-terminated SrTiO3 (STO) substrates by high oxygen pressure diode sputtering. The stacking sequence along the growth direction parallel to the c-axis of NBCO is BaO-CuO2-Nd-CuO2-BaO-CuO. In NBCO, Cu atoms are located in CuO2 planes and in parallel CuO chains. Physical properties of NBCO can be drastically influenced due to oxygen content fluctuations, where the charge balance between the planes and the chains strongly affects and dictates Tc [1]. We have applied atomically resolved quantitative analytical STEM imaging combined with extensive STEM image simulations to investigate the local cation and anion sub-lattices and their distortions, as well as the chemical identity of NBCO films and NBCO/STO interfaces [2]. In addition, surface steps on the STO substrate dictate the defects in NBCO films and will be discussed in detail. [1] RJ Cava et al., Physica C 165 (1990) 419. [2] V Srot et al., Microscopy and Microanalysis 24 (2018) 76.

Resume : Recent development of the noncontact atomic force microscopy (nc-AFM) has opened new possibilities in different fields ? imaging of organic molecules [1], controlling the charge state of adsorbed species [2], or enhanced chemical resolution of surface atoms [3]. The emerging possibilities and opportunities in the field of oxide surfaces and their surface chemistry will be outlined in this contribution. The limits of atomic resolution will be illustrated on binary oxides like TiO2, In2O3. The enhanced chemical resolution of nc-AFM offers a unique opportunity for approaching complex materials with ternary chemical composition: In particular, we have succeeded in imaging bulk-terminated (001) surfaces of SrTiO3 and KTaO3 perovskites. A dedicated cleaving procedure [4,5] allows preparing flat regions terminated by domains of SrO/TiO2 (or KO/TaO2) with a well-defined, bulk-like atomic structure. The stability, point defects, electronic structure, and chemical properties of these surfaces will be discussed and linked to the incipient-ferroelectric character of these materials. [1] Gross, L.; Mohn, F.; Moll, N.; Liljeroth, P.; Meyer, G., Science 2009, 325, 1110 [2] Gross, L.; Mohn, F.; Liljeroth, P.; Repp, J.; Giessibl, F. J.; Meyer, G., Science 2009, 324, 1428 [3] Sugimoto, Y.; Pou, P.; Abe, M.; Jelinek, P.; Perez, R.; Morita, S.; Custance, O., Nature 2007, 446, 64 [4] I. Sokolovic, M. Schmid, U. Diebold, M. Setvin, Phys. Rev. Materials 3, 034407 (2019) [5] M. Setvin, M. Reticcioli, F. Poelzleitner, J. Hulva, M. Schmid, L. A. Boatner, C. Franchini, U. Diebold, Science 359, 572-575 (2018)

Resume : The growth of 3D spintronic structures, such as self-assembled ferromagnetic nanowires (NWs) embedded in an insulating matrix (e.g. Ni in SrTiO3 (STO)), has been proposed as a promising route to overcome the limits of current data storage technologies. This interest is mainly motivated by the possibility to tune the magnetic anisotropy either through the control of the chemical composition of the NWs or through the presence of interfacially-induced strain giving rise to magneto-elastic anisotropy. The detection of Ni oxidation, which would be highly detrimental, is of interest and remains a challenge. This is achieved by investigating the Ni-L23, O-K and Ti-L23 fine structures from the matrix to the Ni:STO interface. Here, the local chemistry of the Ni:STO interface is probed at the atomic scale in the monochromated aberration-corrected scanning transmission electron microscope (STEM). Electron energy-loss spectroscopy (EELS) investigations of the structurally abrupt Ni:STO interface indicate the presence of a weak interdiffusion over a few atomic planes only. This leads essentially to a metallization of the last TiOx (x < 2) atomic plane of the matrix, and to the formation of non-stoichiometric mixed Ni-Ti oxide on the external part of the NW. These results demonstrate the absence of Ni oxidation, confirmed by x-ray absorption spectroscopy, and highlight that Ni:STO is a model system for strain engineering of the magnetic anisotropy in vertically aligned nanocomposites.

Resume : Multiferroic materials are of current technological interest. They are expected to become key components of new devices in important technological fields like spintronics, sensors, multiple state memory cells, energy harvesting etc. Within this framework, magneto-electric multifunctional multiferroics having simultaneous ferromagnetic and ferroelectric long range orderings are particularly relevant. Artificial multiferroics obtained by combining ferroelectric and ferromagnetic materials are a seductive route to overcome the lack of intrinsic single phase multiferroics. We considered nanometric combinations of epitaxial thin films of BaTiO3, which is the prototypical ferroelectric material, and ferrimagnetic ferrites over-layers (NiFe2O4, MnFe2O4 and CoFe2O4), which are respectively low, moderately and highly magnetostrictive oxides. These combinations are of practical interest since all ordering temperatures are above room temperature and the compounds are fully oxidized and highly stable. All films were elaborated by atomic oxygen plasma assisted molecular beam epitaxy in conditions providing high quality single crystalline stacks. The layers were electrically polarized by using local pattern writing with Piezo-Force Microscopy (PFM) and we studied the ferroelectric, chemical and magnetic responses of the individual layers thanks by synchrotron radiation X-ray spectromicroscopy (XPEEM) exploiting photon energy and light polarization tuning. For all systems, electrical local polarization revealed specific spectroscopic responses evidencing chemical reduction and likely ion migration. Our observations show that the chemical, morphologic, magnetic and ferroelectric contributions are highly entangled.

Resume : Energy storage through the electrocatalytic generation of chemical fuels such as hydrogen is an attractive pathway for storing intermittent renewable energies. However, the high overpotentials resulting from the slow kinetics of the oxygen evolution reaction (OER) must be overcome, requiring a detailed understanding of the underlying relationships between catalytic activity, stability and atomic-level surface structure. Such an understanding has proven challenging to obtain due to the practical difficulties in controlling surface areas and structures during study. To overcome this limitation, we employ single crystalline model systems: Thin, epitaxial perovskite films are used as electrocatalysts for the OER, providing a versatile and atomically defined platform for the investigation of activity and stability trends. We focus our study on LaNiO3, because it was predicted to possess one of the highest catalytic activities of all ternary oxides yet reported activities for polycrystalline samples vary significantly. As even nominally single crystalline surfaces may undergo chemical and structural changes due to dissolution processes and self-(re)assembly under OER conditions, we study these model catalysts using a diverse suite of advanced in situ and ex situ analysis techniques for structural, morphological, electronic and chemical understanding of the processes occurring under OER conditions. We find that the surface chemistry and morphology of LaNiO3 critically depend on the fabrication temperature, resulting in selectively tunable Ni redox chemistry and OER activity. These findings underline the importance of well-defined surface structures and chemistries for catalyst design.

Resume : Oxides can dissociatively incorporate water into oxygen vacancies, but the thermodynamic feasibility of this reaction varies greatly. The individual contributions from proton affinity of lattice oxide ions and hydroxide affinity of oxygen vacancies to the hydration enthalpy are experimentally not accessible. This impedes an in-depth understanding of the hydration trends for different materials. We present a thermochemical cycle involving first-principles calculations that allows for the calculation of proton and hydroxide affinities, and apply this scheme to a large variety of oxides ranging from binary oxides, e.g. Cs2O to SiO2, to ternary oxides, e.g. BaZrO3. Band alignment with respect to vacuum ensures the comparability of the calculated ion affinities.[1] Although they describe purely ionic reactions, the proton and hydroxide affinities correlate with the oxide?s electronic structure, in particular with the ionization potential (position of O2p states relative to vacuum). The variation is more pronounced for the proton affinity, naturally explaining the often found phenomenological trend between basicity of the oxide and more favorable hydration.[1] Having established a general understanding of the interplay between the oxide?s electronic structure and the thermodynamics of water incorporation, we proceed to open-shell proton and electron conducting perovskites (e.g. cathode materials). [1] T.S. Bjorheim, M.F. Hoedl, R. Merkle, E.A. Kotomin, J. Maier, to be submitted

Resume : Large out-of-plane electrical polarization, which is perpendicular to free surface of electrodeless perovskite oxide ferroelectrics, can enable advanced applications in solar water splitting, artificial photosynthesis, catalysis, and cell biology. However, due to natural coexistence of ferroelectric domains with different orientations of polarization, the desired polarization is difficult to obtain without electrical poling and electrodes. Here, we demonstrate out-of-plane self-polarization of 0.2 C/m2, which is achieved by defect engineering in thin films of environmentally friendly ferroelectric barium titanate. The films are prepared by pulsed laser deposition. The self-polarized epitaxial films are grown using reduced oxygen pressure during deposition. Another necessary condition is the presence of large biaxial in-plane compressive misfit strain. The films? crystal and electronic structures and electrical responses are analyzed by several complementary techniques. It is shown that the in-built field and self-polarization originate from the formation and ordering of dipolar defect complexes. The complexes comprise charged oxygen vacancies and trivalent titanium cations. It is argued that the peculiar defects? configuration is caused by lattice strain.

Resume : We have performed ab initio calculations for the F-center in the BaTiO3 and SrZrO3 bulk and on the BaO-terminated and ZrO2-terminated (001) surface using a supercell model and a hybrid B3PW exchange-correlation functional [1,2]. We find that two Ti atoms nearest to the bulk F-center in BaTiO3 are repulsed, while nearest eight oxygen and four barium atoms relax towards the oxygen vacancy (by 1.06, 0.71 and 0.08% of the lattice constant a0, respectively). The magnitudes of atomic displacements around the F-center located on the BaO-terminated (001) surface in most cases (except for Ti) are larger than those around the bulk F-center (0.1, 1.4 and 1.0% of a0, respectively). The bulk and BaO-terminated (001) surface F-center bands in BaTiO3 matrix are located only 0.23 and 0.07 eV under the conduction band (CB) bottom, indicating that the F-center is a shallow donor. The F-center in the BaTiO3 bulk contains charge of 1.103e, whereas slightly less charge, only 1.052e, are localized inside the F-center on the BaO-terminated (001) surface. Our calculations demonstrate considerable increase of the chemical bond covalency between the BaTiO3 bulk F-center and its two nearest Ti atoms equal to 0.320e, and even larger increase for BaO-terminated (001) surface F-center and its nearest Ti atom 0.480e, in comparison to the relevant Ti-O chemical bond covalency in the perfect BaTiO3 bulk 0.100e. The difference between F-center formation energy in BaTiO3 bulk (10.3 eV) and on the BaO-terminated (001) surface (10.2 eV) trigger the segregation of the F-center from the bulk towards the BaO-terminated (001) surface. We also performed ab initio calculations for BaTiO3/SrTiO3 and PbZrO3/SrZrO3 (001) interfaces as well as WO3 polar (001) surfaces [3]. For BTO/STO and PZO/SZO (001) heterostructures, we found that the number of interface layers do not influence much the electronic structure of studied structures, while termination of deposited BTO and PZO (001) thin film atop of STO and SZO substrates considerably shift the band edges with respect to the vacuum level and thus reduce the band gap. References: 1. R.I. Eglitis, S. Piskunov, Comput. Condens. Matter 7, 1-6 (2016) 2. M. Sokolov, R.I. Eglitis, S. Piskunov, Y.F. Zhukovskii, Int. J. Mod. Phys. B 31, 1750251 (2017) 3. S. Piskunov, R.I. Eglitis, Solid State Ionics 274, 29-33 (2015)

Resume : Stability and functional properties of doped nanoscale metal oxides are determined by defect chemistry and interfaces. For vapor phase grown non-equilibrium solids, ion diffusion provides efficient means to adjust the functional properties of the material. Insights into the underlying transformation process are essential for both nanomaterial design and material applications at elevated temperatures. In the present contribution we will discuss the transformation behaviour of diluted transition metals (e.g. Fe, Co) in MgO nanocrystals, which were prepared by metallocene injection into a metal combustion flame. Vacuum annealing provides means to control impurity localization and to trigger phase separation in these non-equilibrium solids.[1] By combining structure characterization with X-ray absorption and X-ray photoelectron spectroscopy investigations, we tracked valence state and local chemical environment changes of transition metal ions inside the nanoparticles. For Fe-Mg-O nanoparticles we obtained evidence for the formation of impurity-Mg vacancy complexes which ultimately undergo surface migration to enable magnesioferrite nucleation.[2] Differences in the behaviour between Co and Fe impurities will be discussed and rationalized by means of ab initio calculations. [1] A. R. Gheisi, Part. Part. Syst. Char. 2017, 34, 1700109. [2] M. Niedermaier, J. Phys. Chem. C 2017, 121, 24292.

Resume : Oxide electrodes can form highly dynamic surface states during the oxygen evolution reaction. Understanding the transition from reversible adatom dynamics to irreversible defect chemical reactions is essential for understanding catalyst activity and stability. We present environmental Transmission Electron Microscopy (ETEM) studies of different manganese oxide electrodes in H2O at positive electric potentials, involving manganite perovskites, Ruddlesden-Popper phases and birnessites. Depending on the atomic and electronic structure of the studied systems, a quite different behavior is observed: In addition to reversible surface dynamics of Mn and O atoms, irreversible surface defect reactions due to oxygen, manganese vacancy or pair defect formation can evolve, influencing the stability of the surface. This defect chemistry seems to be correlated to the involvement of metal centers or lattice oxygen as redox active sites, critically depending on the nature of the electronic structure, i.e. the nature of the acceptor states for electrons. Correlating trends in ETEM with electrochemical studies by cyclovoltammetry, post mortem surface analysis and electronic structure calculations allow progress toward understanding atomic scale mechanisms of activity and stability of the electrodes. S. Raabe, D. Mierwaldt, J. Ciston, M. Uijttewaal, H. Stein, J. Hoffmann, Y. Zhu, P. Blöchl, and Ch. Jooss, Adv. Funct. Mater. 22 (2012) 3378. S. Mildner M. Beleggia, D. Mierwaldt Th. W. Hansen, J. B. Wagner, S. Yazdi, T. Kasama, J. Ciston, Y. Zhu, and Ch. Jooss, J. Phys. Chem. C, 119 (2015) 5301. D. Mierwaldt, S. Mildner, R. Arrigo, A. Knop-Gericke, E. Franke, A. Blumenstein, J. Hoffmann, C. Jooss, Catalysts 4 (2014) 129. J. Scholz, M. Risch, K.A. Stoerzinger, G. Wartner, Y. Shao-Horn, C. Jooss, J. Phys. Chem. C 120 (2016) 27746. M. Sotoudeh, S. Rajpurohit, P.E. Blöchl, D. Mierwaldt, J. Norpoth, V. Roddatis, S. Mildner, B. Ifland, C. Jooss, Phys. Rev. B 95 (2017) 235150. D. Mierwaldt, V. Roddatis, M. Risch, J. Scholz, J. Geppert, M. E. Abrishami, and C. Jooss, Adv. Sustainable Syst. 1 (2017) 1700109.

Resume : Nanoparticles catalytic performance is determined by the atomically precise architecture of structure, composition and morphology of their surface and near-surface area. In the catalysis community, this is commonly referred to as the structure-activity relationship. However, predicting, understanding and finally, designing wanted nanocrystal structure at the atomic-scale is far from straightforward. Much progress has been achieved concerning the synthesis procedure of the so-called as-synthesized nanoparticles. However, when particles get exposed to real conditions and a process of activation and degradation, forming the actual active centers of electrocatalysts, much is still to be learned, especially in the case of Pt-alloys. Advancements in the scanning transmission electron microscopy together with identical location approach have made it possible to track the electrochemical changes of the individual Pt-based nanocrystals. Such atomic-scale mechanisms are supported by Monte-Carlo simulations and very sensitive online dissolution measurements (ICP-MS). Our novel advanced multidisciplinary approach paves the way to the atomic-scale understanding of the dynamic structural and morphological evolution of nanostructures and thus deeper understanding of the structure-stability relationship. Pt is one of the most studied electrocatalysts. However, degradation mechanisms have not been explored sufficiently. For instance, Pt nanoparticles are known to dissolve and also to detach from the catalyst support due to carbon corrosion and for agglomerates. Insight into these mechanisms can provide us twofold information: (1) how to stabilize Pt nanoparticles (new ceramic-based supports) and (2) completely new Pt hydrometallurgical recycling protocols.

Resume : Microstructure tailoring has been attempted to improve the electrochemical reactivity of solid oxide fuel cell cathodes operated at intermediate temperatures. Cathode surface modification using electro-chemically reactive nano-catalysts has induced the increment of triple phase boundaries and electrode reactivity to oxygen reduction reaction. It has been also reported that Sr segregation may be inhibited by a microstructural modification on a conventional Sr- and Fe-doped LaCoO3 (LSCF) cathode, which results in durability enhancement. In this study, an in situ sol?gel process was applied to modify the cathode surface with an oxide ion conductor of Sm- and Nd-doped ceria (SNDC), in which the precursors of SNDC were ultrasonically infiltrated into the cathode and subsequently heated up while increasing temperature for measurement. The influence of surface microstructure tailored using SNDC particles is presented on the electro-catalytic property and electrochemical performance of conventional LSCF composite cathode. Sr segregation is also investigated when the oxide ion conducting particles are decorated at the LSCF cathode.

Resume : Protonic ceramic fuel cells (PCFC) attract growing interest. Proton-conducting ceramics offer a higher ionic conductivity compared to oxide ion conductors, in particular at intermediate temperature (300-600°C). Finding optimum cathode materials with mixed protonic and electronic conductivity is crucial for PCFC performance. Proton concentrations were measured for several cathode materials [1], but an atomistic understanding of the parameters determining proton uptake is of great importance. We employed density functional calculations on the basis of the Hubbard (DFT U) approach and hybrid PBE0 exchange-correlation functional as implemented in VASP and CRYSTAL. The latter calculations for La(1-x)SrxFeO3-delta [2] yield protonic defects (hydroxide ions on regular oxygen sites) with similar geometry as in BaZr(1-x)YxO3-x/2 electrolytes, but much less negative hydration enthalpy. In the present study, we extend the calculations to nonstoichiometric Ba(1-x)SrxFeO3-delta and compare to La(1-x)SrxFeO3-delta. We present a detailed analysis of the density of states, volume and local geometry changes, oxidation and hydration energies as functions of oxygen deficiency (nominal Fe oxidation state). Thus, we strive for a comprehensive understanding of water incorporation in mixed protonic-electronic conductors. [1] R. Zohourian, R. Merkle, G. Raimondi, J. Maier, Adv. Func. Mater. 28 (2018), 1801241. [2] D. Gryaznov, R. Merkle, E. A. Kotomin, J. Maier, J. Mater. Chem. A 4 (2016), 13093.

Resume : Oxygen defective metal oxides are electric charge-carriers ceramics that are rapidly defining a new generation of ionic-electronic systems, for energy, electro-mechanics, sensing and advanced electronics. In energy technologies, they are especially emerging for their wide range of functions and tunability that can be achieved by chemical strategies and nano-scaling. Nano-sizing has especially received great attention due to the possibility of getting properties that are very different from the bulk. In this presentation, I will show how oxygen defects can be the real player not only in controlling the electrical properties but also in tuning microstructure, mass diffusion and in generating new unexpected states of metastability at the nanoscale.

Resume : There is a growing interest in the development of fast oxide ion conductors for their application in energy and information technologies. Among the explored materials, Bi4V1.8Cu0.2O10.7 (BICUVOX) and, in general, stabilized bismuth vanadates are known to display one of highest ionic conductivity ever reported at intermediate temperature. Unfortunately, their poor chemical stability at T>500 ºC has traditionally hampered their use in bulk electrochemical devices. In this study, we unveil the possibility of employing BICUVOX in thin film form for low temperature applications, where it keeps its superior performance and, at the same time, shows a good stability over a wide range of oxygen partial pressure. BICUVOX thin films were deposited by pulsed laser deposition (PLD) method. First, a study on the effect of the deposition conditions on the structural and chemical properties of these films was conducted, evidencing the tremendous capability of PLD in tuning the film?s stoichiometry. Then, the ionic conductivity of the material was investigated by impedance spectroscopy. Since oxygen diffusion in BICUVOX is characterized by a strong anisotropy, we studied the conduction properties along the different atomic directions by preparing epitaxial films on SrTiO3 (001) and Nb-doped SrTiO3 (001) and measuring their conductivity in orthogonal directions (in-plane and out-of-plane configurations). The results showed that epitaxial thin films measured along the {VO3.5?0.5}2- layers possess the highest oxide ion conductivity ever reported at this temperature range (T < 300 ºC) and good electrochemical stability, demonstrating their potential use in solid state electrochemical devices at temperatures below 200ºC.

Resume : Dielectric oxide films in electronic devices undergo significant structural changes during device operation under bias. These changes are usually attributed to aggregation of oxygen vacancies resulting in formation of oxygen depleted regions and conductive filaments. However, neutral oxygen vacancies have high diffusion barriers in ionic oxides and their interaction and driving forces for aggregation are still poorly understood. We will present the results of Density Functional Theory (DFT) calculations on the structure and properties of neutral dimers and trimers of oxygen vacancies in technologically relevant amorphous SiO2 and HfO2. These results demonstrate weak interaction between neutral O vacancies, which does not explain their quick aggregation. We propose that trapping of electrons, injected from an electrode, by the vacancies may result in creation of new neutral vacancies in the vicinity of pre-existing vacancies. The electron localization weakens Me?O bonds, which can be broken upon thermal activation, creating an O2- interstitial ion and a neutral O vacancy [1]. We describe this mechanism in a-SiO2 and demonstrate that this process becomes more efficient as the vacancy clusters grow larger [2]. These results demonstrate how the interaction between ionic and electronic species leads to structural and electrical changes in amorphous dielectric films. [1] D. Z. Gao, A.-M. El-Sayed, A. L. Shluger, Nanotechnology, 27, 505207 (2016). [2] D. Z. Gao, J. Strand, M. S. Munde and A. L. Shluger, Front. Phys. 7:43 (2019)

Resume : In some instances, barium strontium titanate (BST) [1,2] and titanium dioxide [4] capacitors have exhibited transients in their conductance as a response to step potentials. These transients involve an initial increase in conductance followed by a slower decrease in conductance, resulting in a distinctive peak in device current. They have previously been used to estimate the drift mobilities of oxygen vacancies and in our other work we have shown similar transients in amorphous silicon-oxide have uses in computation. It is proposed transients are the result of charged oxygen vacancies drifting under the applied bias. [3,4] However, having observed similar behaviour in our amorphous silicon-oxide devices, we question one of the key assumptions in this previous work and propose an alternative approach to investigating such transients. The analysis of these transients has historically involved comparing the voltage being applied and the time at which the current peak occurs - specifically the proportionality of the two. [1,2,5,6] Peaks in current are said to indicate the point in time at which vacancies have traversed across the device and begun collecting at the opposite electrode. This timing is therefore dependent on the drift properties of the vacancies and has been used to determine their mobilities. In addition to this, the same proportionality of peak timing and applied voltage has also been used to argue between causes of the behaviour. However, underlying this analysis is an assumption that transients are the result of a single cause. That both the rise and fall in conduction are the result of a redistribution of oxygen vacancies. Vacancies which act as donors, forming an n-type region at the negative electrode thus modulating the electronic conduction. [3] In this way, the two processes may be described with a single state variable ? the distribution of vacancies at a given point in time. If this held true, we would expect the two changes in conductance to have similar characteristics, particularly with respect to relaxation times. As vacancies relax to their thermal equilibrium position both the rise and fall in conduction should reset on similar timescales owing to them both being functions of vacancy distribution. However, our analysis finds these relaxation times differ significantly. It seems that to describe this system we require not just one state variable but two - one describing the increase in conductance and the second the decrease. We take an alternative approach and treat the rise and fall in conductance as two separate processes occurring in parallel. Although both cause a change in conductance, how long these changes persist varies between the two. For example, the increase in conductance is volatile on the order of seconds, its changes in conduction are lost between subsequent tests when left with no applied potential for more than a second. In contrast, the latter reduction in conductance is more permanent. These changes persist for minutes and the original state is only recovered after waiting for over half an hour at room temperature. Therefore, we find the relaxation times of the two processes differ significantly. If the assumption of a single cause is incorrect and only one of the changes in conductance is the result of vacancy drift, then the timing of the peak in current is no longer solely defined by the drift properties of the vacancies. It must also depend on a second unaccounted for effect. This puts any analysis assuming a common cause at risk of being inaccurate. By acknowledging that two effects may be occurring in parallel we have approached the problem from a different perspective. Rather than attempting to determine motilities from the timing of the current peak, we suggest to first characterise the processes independently in order to identify their respective mechanisms. We find the combination of two decaying exponentials fit the transients well allowing us to separate the two processes. With this approach, we have characterised the voltage dependency, activation energies and relaxation times of these processes. We believe this approach has potential to uncover further information regarding the mechanisms behind these current transients. [1] S. Zafar, R. E. Jones, B. Jiang, B. White, P. Chu, D. Taylor, and S. Gillespie, ?Oxygen vacancy mobility determined from current measurements in thin Ba0.5Sr0.5TiO3 films,? Appl. Phys. Lett., vol. 73, no. 2, p. 175, Jul. 1998. [2] S. Saha and S. B. Krupanidhi, ?Transient analysis in Al-doped barium strontium titanate thin films grown by pulsed laser deposition,? J. Appl. Phys., vol. 90, no. 3, pp. 1250?1254, Aug. 2001. [3] R. Meyer, R. Liedtke, and R. Waser, ?Oxygen vacancy migration and time-dependent leakage current behavior of Ba0.3Sr0.7TiO3 thin films,? Appl. Phys. Lett., vol. 86, no. 11, p. 112904, Mar. 2005. [4] N. Zhong, H. Shima, and H. Akinaga, ?Transient Current Study on Pt/TiO 2- x /Pt Capacitor,? Jpn. J. Appl. Phys., vol. 49, no. 4, p. 04DJ15, Apr. 2010. [5] J. Wang and S. Trolier-McKinstry, ?Oxygen vacancy motion in Er-doped barium strontium titanate thin films,? Appl. Phys. Lett., vol. 89, no. 17, p. 172906, Oct. 2006. [6] S. Zafar, H. Jagannathan, L. F. Edge, and D. Gupta, ?Measurement of oxygen diffusion in nanometer scale HfO2 gate dielectric films,? Appl. Phys. Lett., vol. 98, no. 15, p. 152903, Apr. 2011.2011.

Resume : First-principles calculations have been performed on the perfect surface, point-defect surface, step-defect surface, layer-defect surface of Al2O3 film with water molecules (H2O) and chloride (Cl-) ions. The coadsorption mechanism has effect on the reaction and erosion of the surface. The adsorption energies (Eads), stable adsorbed sites, binding of film, charge transfer, reactants and products, activation energies and transition states are calculated and discussed. The results evidence that for the perfect Al2O3 surface, the critical monolayer of Cl- is 3/7, the Eads decrease in three steps, each Eads step only relate to the adsorbed site and the morphology. For point-defect surface, substitution point defects are more sensitive than vacancy point defects for reaction and erosion. The species of products depend on the energy barrier and orientation of water. For step-defect surface, Al1 step-defect and Al3 step-defect surfaces prefer to obtain Al-H2O compounds, while O2 step-defect surface prefers to form Al-Cl products. There is no obvious linear relationship between the number of products and the number of steps. For layer-defect surface, when low concentrations of Cl ions reach the surface, they prefer to erode the Al layer-defect surface with producing Al-Cl compounds, while they prefer to interact with H2O upon the O layer-defect surface.

Resume : Allying epitaxial strain and synthesis conditions allows for the introduction of specific point defects in perovskite oxide films. In ferroelectric films, such defects lead to essential polar and electronic properties, which can enable advanced applications. Here, to elucidate the nature of the defects, optical constants are investigated in the spectral range of (0.7-8.8) eV in epitaxial ferroelectric barium titanate films, which are synthesized by pulsed laser deposition using different pressures of ambient oxygen. It is demonstrated that oxygen-vacancy-related defects are responsible for a peculiar transition below the bandgap at (2.7-2.9 eV) and significant blueshifts of (0.3-0.4) eV of the gap and the main interband transitions. These observations are discussed in terms of the defect-induced in-gap states and internal electic field. It is suggested that the defects are dipolar complexes comprising oxygen vacancies and trivalent titanium cations.

Resume : Optical, electrical and structural properties of Tb and Eu co-doped ZnO films deposited by magnetron sputtering on Si and Al2O3 substrates were investigated by X-ray diffraction, photoluminescence (PL), micro-Raman and IR reflection methods. It is shown that incorporation of rare earth (RE) ions in ZnO host occurs under deposition and is accompanied by the formation of intrinsic defects in oxygen sublattice of ZnO. Both as-deposited and 600°C annealed films demonstrate Tb-related emission and no sign of energy transfer from ZnO host to RE ions and from Tb3 to Eu3 ions. A model of Tb3 emitting centers composed of Tb3 substituting zinc (Tb3 Zn) and interstitial oxygen (Oi) is proposed. Higher intensity of Tb-related emission in the film grown on Al2O3 substrate is found compared with that on Si correlated with larger amount of intrinsic defects and lower concentration of free electrons. It is ascribed to higher content of the emission centers. The annealing at 900°C is found to produce the out-diffusion of RE ions from ZnO grains, annihilation of the intrinsic defects and the increase of free carrier concentration. It leads to the formation of crystalline Tb2O3 phase in both types of the films and crystalline Zn2SiO4 phase in the film on Si substrate. In the PL spectra, the decrease of Tb3 -related PL and the appearance of two sets of Eu3 -related PL bands are found. This is ascribed to the segregation of Tb3 and Eu3 ions in the additional Tb2O3 and Zn2SiO4 phases. An effective energy transfer from Tb3 to Eu3 ions in these phases is assigned to the decrease of distance between these ions.

Resume : The fingerprint identification technology is widely used in mobile devices for biometric authentication because it is more convenient, faster, and safer than entering personal identification number (PIN) or password. As the demand for a mobile device with a full-screen display and one-handed operation while keeping the device flat on the table increases, the methods to put the fingerprint sensor on the front display are attracting great attention. For the on-display fingerprint sensor to be applied, the sensor should be transparent, without decreasing display resolution. Therefore, it is very important to choose proper transparent materials for both electrodes and active channel layers that do not be damaged while etching one from the other during the fabrication process. In this study, a transparent optical fingerprint sensor based on oxide semiconductor thin-film transistors (TFTs) with indium-tin-oxide (ITO) electrodes will be discussed. The triple layer active channel, deposited by sputtering with varying oxygen partial pressures, comprises an aluminum-doped indium-zinc-tin oxide (Al-InZnSnO), an indium-zinc oxide (IZO), and another Al-InZnSnO. The sandwiched IZO layer, having a lower bandgap than Al-InZnSnO, is employed as a light absorbing layer. The commonly known problematic phenomenon of persistent photoconductivity, mainly caused by the light-driven ionization of oxygen vacancy sites, in oxide semiconductors even after the removal of illumination has been successfully resolved by applying a short (< 1 ?s), positive gate bias pulse to the TFTs for electron accumulation. Acknowledgment This work was supported by the ?Development of Core Technologies for Transparent Flexible Display Integrated Biometric Recognition Device (2018-0-00202)? project from the Institute for Information & communications Technology Promotion (IITP) administered by Korea Ministry of Science and ICT (MSIT).

Resume : We report the hydrogen barrier performance of sputtered La2O3 thin films for the device stability of amorphous indium-gallium-zinc-oxide (a-IGZO) thin-film transistors (TFTs). Hydrogen acts as a shallow donor in a-IGZO films, which makes TFTs conductive, resulting in degradation of their on/off properties. Since hydrogen can be easily incorporated by external environments or post processing, an appropriate hydrogen barrier is essential for enhancing device stability. La2O3, with its extreme electronegativity, can provide excellent hygroscopic characteristics. Because hydrogen exists in the form of -OH groups inside a-IGZO films, La2O3 is expected to be a promising barrier material for preventing hydrogen incorporation. Therefore, we investigate the growth characteristics of sputtered La2O3 thin films as hydrogen barrier layers, focusing on variations in growth rate, refractive index, and film stress, which depend on various process parameters, such as radio-frequency (RF) power, O2 partial pressure, and substrate temperature during reactive magnetron sputtering. The effects of these parameters on hydrogen barrier properties are systematically investigated and correlated with the microstructures of La2O3 films. The results demonstrate that La2O3 films grown with low RF power and low O2 partial pressure have an amorphous phase and provide excellent hydrogen barrier performance. We anticipate that these experimental results will help improve the environmental stability of a-IGZO TFTs.

Resume : Nowadays development of devices for energy storage applications, for the harvesting and mutual transformations of mechanical and electrical energies is of great interest [1]. In this respect (via utilizing the piezoelectric effect) ferroelectric perovskites are important materials for many technological applications. For a long time lead-zirconate-titanate (PZT) is the most widely used piezoelectric material for electromechanical applications. Recently several lead-free piezoelectrics were investigated. BaTiO3-based (BTO) piezoelectric materials e.g. BTO solid solutions with SrTiO3 (STO) are considered as potential substitution for PZT [2]. In this study, we explore the piezoelectric properties of tetragonal STO/BTO superlattice by varying the number of STO and BTO layers. Our theoretical calculations are based on the first principles approach and hybrid (B1WC) exchange-correlation functional as implemented into CRYSTAL computer code. The direct piezoelectric tensor constant (e33) calculations are based on the Berry phase approach using numerical derivatives. On the basis of group theory analysis, we demonstrate that tetragonal heterostructure theoretically could be described using one of two different space groups, that are distinguished by forbidden or allowed TiO6 octahedra rotation around the z-axis. Using different number of relative Ba/Sr layers in STO/BTO heterostructure, we demonstrate that both space groups lead to similar results for small and average STO relative concentrations, when, e33, increases with the concentration of STO, with respect to pure BTO. The different scenarios are suggested for high STO concentrations, when TiO6 octahedra rotation affects the results. [1] C.R. Bowen, et al, Energy Environ. Sci. 2014, 7, 25. [2] L. Rusevich, G. Zvejnieks, et al., J. Phys. Chem. A 2017, 121, 9409.

Resume : Thin films of mixed conducting ionic materials may be used as electrodes for batteries or solid oxide fuel cells, layers used for resistive switching, etc. For deposition of such layers special thin film preparation techniques are necessary. Pulsed laser deposition (PLD) is of particular importance as it allows to deposit even very difficult multi-element compositions almost unchanged from a target material to produce thin films. Still the properties of the thin films may differ from bulk or other thin films due to different parameters such as temperature, gas pressure, laser energy or target to substrate distance. A novel in-situ combination of electrochemical impedance spectroscopy (EIS) and PLD is presented and shown to give direct insight into the electrochemical properties of growing films. Deposition related changes of single crystalline substrates are reported which could be uniquely measured with the described in-situ technique. Development of oxygen transport resistances and defect chemistry (measured via the chemical capacitance of a thin film) with thickness, different grain size or multilayers is investigated for mixed ionic and electronic conducting materials such as La1-xSrxCoO3-?.