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The resistive switching (RS) properties of strontium titanate (Sr1+xTi1+yO3+(x+2y), STO) based metal-oxide-metal structures prepared from industrial compatible processes have been investigated focusing on the effects of composition, microstructure, and device size. Metastable perovskite STO films were prepared on Pt-coated Si substrates utilizing plasma-assisted atomic layer deposition (ALD) from cyclopentadienyl-based metal precursors and oxygen plasma at 350 °C, and a subsequent annealing at 600 °C in nitrogen. Films of 15 nm and 12 nm thickness with three different compositions [Sr]/([Sr]+[Ti]) of 0.57 (Sr-rich STO), 0.50 (stoichiometric STO), and 0.46 (Ti-rich STO) were integrated into Pt/STO/TiN crossbar structures with sizes ranging from 100 μm2 to 0.01 μm2. Nano-structural characterizations revealed a clear effect of the composition of the asdeposited STO films on their crystallization behavior and thus on the final microstructures. Local current maps obtained by local-conductivity atomic force microscopy were in good agreement with local changes of the films' microstructures. Correspondingly, also the initial leakage currents of the Pt/STO/TiN devices were affected by the STO compositions and by the films' microstructures. An electroforming process set the Pt/STO/TiN devices into the ON-state, while the forming voltage decreased with increasing initial leakage current. After a RESET process under opposite voltage has been performed, the Pt/STO/TiN devices showed a stable bipolar RS behavior with non-linear current-voltage characteristics for the high (HRS) and the low (LRS) resistance states. The obtained switching polarity and nearly area independent LRS values agree with a filamentary character of the RS behavior according to the valence change mechanism. The devices of 0.01 μm2 size with a 12 nm polycrystalline stoichiometric STO film were switched at a current compliance of 50 lA with voltages of about ±1.0V between resistance states of about 40 kΩ (LRS) and 1 MΩ (HRS). After identification of the influences of the films' microstructures, i.e., grain boundaries and small cracks, the remaining RS properties could be ascribed to the effect of the [Sr]/([Sr]+[Ti]) composition of the ALD STO thin films. [ABSTRACT FROM AUTHOR]

In this work, the results of the electrocoloration of strontium titanate single crystals with different iron concentrations are presented. The samples of SrTiO3(100) doped with 0.06 at. % and 0.13 at. % of iron were electroreduced at low pressure (10-8 mbar) and elevated temperature (250 °C) using a DC voltage of 200 V. This led to the migration of oxygen vacancies and subsequent electrocoloration of the samples, which was confirmed by optical analysis and electrical measurements. Evolution of the color front was compared with finite element calculations of electric potential indicating good agreement. Both macroscopic and nanoscopic measurements showed insulator-metal transition at several hundreds of seconds (0.06%Fe) and resistive switching behavior. We found that the resistive switching is clearly modified by the oxygen partial pressure of the ambient atmosphere. Moreover, after electroreduction, in the region between the electrodes, stripes can be found following simple crystallographic directions connected with the extended defects and easy diffusion paths also observed in the single crystals of undoped strontium titanate. Furthermore, migration of negatively charged oxygen ions towards the anode led to the formation of oxygen bubbles trapped between the surface of the crystal and the electrode. Using atomic force microscopy, we were able to measure the geometry of a bubble and calculate the oxygen pressure necessary for the formation of such bubbles and the total amount of oxygen ions trapped within. [ABSTRACT FROM AUTHOR]

The resistive switching effect in transition metal oxides allows for a dedicated manipulation of the oxide resistance via electrical stimuli. Here, we perform local-conductivity atomic force microscopy simultaneously with the Kelvin probe force microscopy under ultra-high vacuum conditions using the very same tip investigating the very same sample area to monitor the surface conductivity and surface potential of thermally reduced TiO2 single crystals. We show that the resistance of confined surface areas can be switched by applying a voltage of several volts to the tip during scanning in the contact mode. By conducting in situ oxidation experiments, we present that this surface switching is related to a local redox reaction, which can be controlled electrically allowing for surface nanopatterning and illustrates the capability of transition metal oxides for multilevel resistive switching being a prerequisite for neuromorphic computing. We discuss that the features of the electrically engraved nanopattern can be scaled down to a lower boundary at several tens of nanometers. The observed limit around 25 nm is determined by the presence of intrinsic local variations in electrical surface properties appearing as a common phenomenon of slightly reduced metal oxide surfaces. [ABSTRACT FROM AUTHOR]

We report on the systematic exploration of electronic and structural changes of Nb-doped rutile TiO2(110) single crystal surfaces due to the thermoreduction under ultra-high vacuum conditions (without sputtering), with comparison to undoped TiO2(110) crystals. It has been found that the surface of the doped sample undergoes a previously unknown transition during reduction above 850 °C, as provided by LEED, STM and LC-AFM. This transition involves a change from heterogeneous conductivity (due to the presence of conducting filaments) to homogeneous conductivity, connected with a new (4 × 2) reconstruction of rows parallel to the [001] direction. DFT calculations suggest substitution of Ti by Nb atoms in the first atomic layer. Due to the strong reducing conditions during annealing, oxygen is released from the crystal and Nb diffuses from the subsurface into the bulk, agglomerating however on the surface, as shown by SIMS depth profiling. We present that 0.5% Nb doping significantly influences the reduction process and in turn the structural properties of the surface by supporting the evolution of the new reconstruction. It is shown that the thermal treatment of TiO2:Nb under low oxygen partial pressure gives an opportunity to tune the electrical conductivity and work function of the surface. [ABSTRACT FROM AUTHOR]

We investigate the thermal reduction of TiO2 in ultra-high vacuum. Contrary to what is usually assumed, we observe that the maximal surface reduction occurs not during the heating, but during the cooling of the sample back to room temperature. We describe the self-reduction, which occurs as a result of differences in the energies of defect formation in the bulk and surface regions. The findings presented are based on X-ray photoelectron spectroscopy carried out in-operando during the heating and cooling steps. The presented conclusions, concerning the course of redox processes, are especially important when considering oxides for resistive switching and neuromorphic applications and also when describing the mechanisms related to the basics of operation of solid oxide fuel cells. [ABSTRACT FROM AUTHOR]

The ever-increasing utility of imaging technology in proton exchange membrane water electrolyzer research raises the demand for rapid and precise image analysis. In particular, for optical video recordings, the challenge primarily lies in the large number of frames that impede the delineation of bubble dynamics with standard methods. In order to address this problem, the present study supports the automation of data analysis to facilitate swift, comprehensive, and measurable insights from captured imagery. We present a deep learning-based framework to perform high-throughput analyses of bubble dynamics using optical images of proton exchange membrane water electrolyzers. Leveraging a relatively small annotated imaging dataset of just 35 images, various configurations of the U-Net architecture were trained to perform bubble segmentation tasks. The best model achieved a precision of 95%, a recall of 78%, and an F1-score of 86% on the validation set. Subsequent to segmentation, the methodology enabled the rapid extraction of parameters such as time-resolved bubble area, size distributions, bubble position probability density, and individual bubble shape analytics. The findings underscore the potential of deep learning to enhance the analysis of polymer electrolyte membrane water electrolyzer imaging, offering a path toward more efficient and informative evaluations in electrochemical research.

The electrochemical reaction kinetics, especially the oxygen reduction reaction (ORR) at the cathode, is crucial for the performance of a fuel cell. In this study, the electrochemical processes on a polycrystalline Pt electrode in the presence of protic ionic liquid (PIL) electrolyte diethylmethylammonium triflate [Dema][TfO] are investigated by means of cyclic voltammetry and electrochemical impedance spectroscopy. Since water is continually produced during fuel cell operation, the effect of the water content in the PIL has been intensively analyzed. In order to reveal the dependence of the interfacial reaction characteristics on the electrode potential, the impedance spectra were simulated by an equivalent circuit whose parameters can be related to both Faradaic and capacitive processes. Two interfacial resistances were identified, which differ by about 3 orders of magnitude. The larger one is a charge transfer resistance that can be associated with slow Faradaic processes like the ORR and platinum oxidation/oxide reduction. The smaller resistance is probably linked with fast processes that involve water molecules, such as hydrogen deposition and oxidation. The high- and midfrequency capacitive processes are attributed to "classical" double layer and pseudocapacitive behavior, similar to those identified under nitrogen atmosphere.

In situ monitoring of the electrolyte/electrode interfacial processes, such as the oxygen reduction reaction (ORR), is crucial for the design of electrolytes for fuel cells. In this study, we investigate the electrochemical behavior of platinum electrodes in protic ionic liquids (PILs) by means of in situ Fourier-transform infrared spectroscopy coupled with cyclic voltammetry. The result provides direct evidence of the change of water at the Pt electrode surface due to Pt oxide formation and reduction. A decrease in the interfacial water was observed in the spectra upon the formation of the Pt oxide. Conversely, the local water concentration at the electrode surface increases if the Pt oxide is reduced and the ORR takes place. At the same time, more cations replace anions on the electrode. The ORR kinetics in the [TFSI]-based PILs is slower than in the [TfO]-based ones, which could result from a blockage of catalytic sites by the adsorbed [TFSI] anions. It suggests that reducing the anion adsorption on the platinum surface could provide an opportunity to enhance the ORR activity.

Long-time electric field action on perovskite piezoelectric ceramic leads to chemical degradation. A new way to accelerate the degradation is the exposure of the ceramic to DC electric fields under a vacuum. A high-quality commercial piezoelectric material based on PbZr 1-x Ti x O 3 is used to study such impacts. To avoid the influence of ferroelectric properties and possible removal of oxygen and lead oxides during the degradation process, the experiments are in the temperature interval of 500 °C > T > T C . Changes in resistance during the electrodegradation process is an electrically-induced deoxidation, transforming the ceramic into a metallic-like material. This occurs with an extremely low concentration of effused oxygen of 10 16 oxygen atoms per 1 cm 3 . Due to this concentration not obeying the Mott criterion for an isolator-metal transition, it is stated that the removal of oxygen mostly occurs along the grain boundaries. It agrees with the first-principle calculations regarding dislocations with oxygen vacancies. The decrease in resistivity during electrodegradation follows a power law and is associated with a decrease in the dislocation dimension. The observed reoxidation process is a lifeline for the reconstructing (self-healing) properties of electro-degraded ceramics in harsh cosmic conditions. Based on all of these investigations, a macroscopic and nanoscopic model of the electrodegradation is presented.

Protic ionic liquids are promising electrolytes for fuel cell applications. They would allow for an increase in operation temperatures to more than 100 °C, facilitating water and heat management and, thus, increasing overall efficiency. As ionic liquids consist of bulky charged molecules, the structure of the electric double layer significantly differs from that of aqueous electrolytes. In order to elucidate the nanoscale structure of the electrolyte-electrode interface, we employ atomic force spectroscopy, in conjunction with theoretical modeling using molecular dynamics. Investigations of the low-acidic protic ionic liquid diethylmethylammonium triflate, in contact with a platinum (100) single crystal, reveal a layered structure consisting of alternating anion and cation layers at the interface, as already described for aprotic ionic liquids. The structured double layer depends on the applied electrode potential and extends several nanometers into the liquid, whereby the stiffness decreases with increasing distance from the interface. The presence of water distorts the layering, which, in turn, significantly changes the system's electrochemical performance. Our results indicate that for low-acidic ionic liquids, a careful adjustment of the water content is needed in order to enhance the proton transport to and from the catalytic electrode.

Mixed ionic-electronic-conducting perovskites such as SrTiO 3 are promising materials to be employed in efficient energy conversion or information processing. These materials exhibit a self-doping effect related to the formation of oxygen vacancies and electronic charge carriers upon reduction. It has been found that dislocations play a prominent role in this self-doping process, serving as easy reduction sites, which result in the formation of conducting filaments along the dislocations. While this effect has been investigated in detail with theoretical calculations and direct observations using local-conductivity atomic force microscopy, the present work highlights the optical properties of dislocations in SrTiO 3 single crystals. Using the change in optical absorption upon reduction as an indicator, two well-defined arrangements of dislocations, namely a bicrystal boundary and a slip band induced by mechanical deformation, are investigated by means of scanning near-field optical microscopy. In both cases, the regions with enhanced dislocation density can be clearly identified as regions with higher optical absorption. Assisted by ab initio calculations, confirming that the agglomeration of oxygen vacancies significantly change the local dielectric constants of the material, the results provide direct evidence that reduced dislocations can be classified as alien matter embedded in the SrTiO 3 matrix.

Controlling the work function of transition metal oxides is of key importance with regard to future energy production and storage. As the majority of applications involve the use of heterostructures, the most suitable characterization technique is Kelvin probe force microscopy (KPFM), which provides excellent energetic and lateral resolution. In this paper, we demonstrate precise characterization of the work function using the example of artificially formed crystalline titanium monoxide (TiO) nanowires on strontium titanate (SrTiO 3 ) surfaces, providing a sharp atomic interface. The measured value of 3.31(21) eV is the first experimental work function evidence for a cubic TiO phase, where significant variations among the different crystallographic facets were also observed. Despite the remarkable height of the TiO nanowires, KPFM was implemented to achieve a high lateral resolution of 15 nm, which is close to the topographical limit. In this study, we also show the unique possibility of obtaining work function and conductivity maps on the same area by combining noncontact and contact modes of atomic force microscopy (AFM). As most of the real applications require ambient operating conditions, we have additionally checked the impact of air venting on the work function of the TiO/SrTiO 3 (100) heterostructure, proving that surface reoxidation occurs and results in a work function increase of 0.9 eV and 0.6 eV for SrTiO 3 and TiO, respectively. Additionally, the influence of adsorbed surface species was estimated to contribute 0.4 eV and 0.2 eV to the work function of both structures. The presented method employing KPFM and local conductivity AFM for the characterization of the work function of transition metal oxides may help in understanding the impact of reduction and oxidation on electronic properties, which is of high importance in the development of effective sensing and catalytic devices.

Electroreduction experiments on metal oxides are well established for investigating the nature of the material change in memresistive devices, whose basic working principle is an electrically-induced reduction. While numerous research studies on this topic have been conducted, the influence of extended defects such as dislocations has not been addressed in detail hitherto. Here, we show by employing thermal microscopy to detect local Joule heating effects in the first stage of electroreduction of SrTiO 3 that the current is channelled along extended defects such as dislocations which were introduced mechanically by scratching or sawing. After prolonged degradation, the matrix of the crystal is also electroreduced and the influence of the initially present dislocations diminished. At this stage, a hotspot at the anode develops due to stoichiometry polarisation leading not only to the gliding of existing dislocations, but also to the evolution of new dislocations. Such a formation is caused by electrical and thermal stress showing dislocations may play a significant role in resistive switching effects.

The electrical properties of SrTiO 3 (100) single crystals were investigated in-situ at different stages of thermal reduction by means of a 4-tip STM. Using the tips of the STM as electrical probes, distance-dependent four-point measurements were performed at the surface of the crystal at room temperature after reduction by thermal treatment. For annealing temperatures T ≤ 700 °C, charge transport is confined to a surface region <3 μm below the surface. For reduction at T ≥ 900 °C a transition from a conducting 2D sheet with insulating bulk to a system with dominant 3D bulk conductivity is found. At an intermediate reduction temperature of T = 800 °C, a regime with mixed 2D/3D contributions is observed in the distance-dependent resistance measurements. Describing the depth-dependent conductivity with an analytical N-layer model, this regime of mixed 2D/3D conductivity is evaluated quantitatively under the assumption of an exponentially decaying conductivity profile, correlated with the previously observed depth-dependent dislocation density in the sample. A non-monotonous temperature dependence of the 3D conductivity in the respective conducting layer is found and possible underlying mechanisms are discussed, particularly with regard to non-intrinsic material properties depending on details of the sample preparation.

Reduced titanium oxide structures are regarded as promising materials for various catalytic and optoelectronic applications. There is thus an urgent need for developing methods of controllable formation of crystalline nanostructures with tunable oxygen nonstoichiometry. We introduce the Extremely Low Oxygen Partial Pressure (ELOP) method, employing an oxygen getter in close vicinity to an oxide during thermal reduction under vacuum, as an effective bottom-up method for the production of nanowires arranged in a nanoscale metallic network on a SrTiO3 perovskite surface. We demonstrate that the TiO nanowires crystallize in a highly ordered cubic phase, where single nanowires are aligned along the main crystallographic directions of the SrTiO3 substrate. The dimensions of the nanostructures are easily tunable from single nanometers up to the mesoscopic range by varying the temperature of reduction. The interface between TiO and SrTiO3 (metal and insulator) was found to be atomically sharp providing the unique possibility of the investigation of electronic states, especially since the high conductivity of the TiO nanostructures is maintained after room temperature oxidation. According to the growth model we propose, TiO nanowire formation is possible due to the incongruent sublimation of strontium and crystallographic shearing, triggered by the extremely low oxygen partial pressure (ELOP). The controlled formation of conductive nanowires on a perovskite surface holds technological potential for implementation in memristive devices, organic electronics, or for catalytic applications, and provides insight into the mechanism of nanoscale phase transformations in metal oxides. We believe that the ELOP mechanism of suboxide formation is suitable for the formation of reduced suboxides on other perovskite oxides and for the broader class of transition metal oxides.

We report on the systematic exploration of electronic and structural changes of Nb-doped rutile TiO 2 (110) single crystal surfaces due to the thermoreduction under ultra-high vacuum conditions (without sputtering), with comparison to undoped TiO 2 (110) crystals. It has been found that the surface of the doped sample undergoes a previously unknown transition during reduction above 850 °C, as provided by LEED, STM and LC-AFM. This transition involves a change from heterogeneous conductivity (due to the presence of conducting filaments) to homogeneous conductivity, connected with a new (4 × 2) reconstruction of rows parallel to the [001] direction. DFT calculations suggest substitution of Ti by Nb atoms in the first atomic layer. Due to the strong reducing conditions during annealing, oxygen is released from the crystal and Nb diffuses from the subsurface into the bulk, agglomerating however on the surface, as shown by SIMS depth profiling. We present that 0.5% Nb doping significantly influences the reduction process and in turn the structural properties of the surface by supporting the evolution of the new reconstruction. It is shown that the thermal treatment of TiO 2 :Nb under low oxygen partial pressure gives an opportunity to tune the electrical conductivity and work function of the surface.