Artificial [(Nd2Ti2O7)4/(SrTiO3) n ]10 superlattices (n = 4 and 8) were successfully epitaxially grown on SrTiO3 substrates by pulsed laser deposition using the in situ high energy electron diffraction reflection diagnostic. The crystallographic relationships between Nd2Ti2O7 (NTO) and SrTiO3 (STO) (layers and substrate) were: [100]NTO//[001]STO, [010]NTO//[1Ì10]STO, and (00l)NTO//(110)STO. Nanoscale current variation was detected on both superlattices, with the (NTO4/STO4)10 heterostructure showing a higher density. The (NTO4/STO4)10 sample did not show a piezoelectric response when measured by piezo-force microscopy (PFM), while ambiguous piezoactivity was observed on the (NTO4/STO8)10 superlattice. Scanning transmission electron microscopy energy dispersive spectroscopy analysis showed the diffusion of Nd3+ cations on Sr2+ sites in SrTiO3 structure into the multilayers, which was more pronounced when the value of n was lower. These particular nanoscale electrical behaviors, evidenced by electrical conducting channels and misleading PFM signals, were mainly attributed to the presence of oxygen vacancies in the SrTiO3 layers at higher concentrations near the interface and to the mixed valence state of the titanium (Ti3+/Ti4+). This work showed the strong influence of interface structure on nanoscale electrical phenomena in complex oxide superlattices.
Yttrium-doped barium zirconate (BZY) thin films recently showed surprising electric transport properties. Experimental investigations conducted mainly by electrochemical impedance spectroscopy suggested that a consistent part of this BZY conductivity is of protonic nature. These results have stimulated further investigations by local unconventional techniques. Here, we use electrochemical strain microscopy (ESM) to detect electrochemical activity in BZY films with nanoscale resolution. ESM in a novel cross-sectional measuring setup allows the direct visualization of the interfacial activity. The local electrochemical investigation is compared with the structural studies performed by state of art scanning transmission electron microscopy (STEM). The ESM and STEM results show a clear correlation between the conductivity and the interface structural defects. We propose a physical model based on a misfit dislocation network that introduces a novel 2D transport phenomenon, whose fingerprint is the low activation energy measured.
A systematic study by reversible and hysteretic electrochemical strain microscopy (ESM) in samples of cerium oxide with different Sm content and in several working conditions allows disclosing the microscopic mechanism underlying the difference in electrical conduction mechanism and related surface activity, such as water adsorption and dissociation with subsequent proton liberation. We have measured the behavior of the reversible hysteresis loops by changing temperature and humidity, both in standard ESM configuration and using the first-order reversal curve method. The measurements have been performed in much smaller temperature ranges with respect to alternative measuring techniques. Complementing our study with hard X-ray photoemission spectroscopy and irreversible scanning probe measurements, we find that water incorporation is favored until the doping with Sm is too high to allow the presence of Ce3+. The influence of doping on the surface reactivity clearly emerges from all of our experimental results. We find that at lower Sm concentration, proton conduction is prevalent, featured by lower activation energy and higher electrical conductivity. Defect concentrations determine the type of the prevalent charge carrier in a doping dependent manner.
Bias dependent mechanisms of irreversible cathodic and anodic processes on a pure CeO2 film are studied using modified atomic force microscopy (AFM). For a moderate positive bias applied to the AFM tip an irreversible electrochemical reduction reaction is found, associated with significant local volume expansion. By changing the experimental conditions we are able to deduce the possible role of water in this process. Simultaneous detection of tip height and current allows the onset of conductivity and the electrochemical charge transfer process to be separated, further elucidating the reaction mechanism. The standard anodic/cathodic behavior is recovered in the high bias regime, where a sizable transport current flows between the tip and the film. These studies give insight into the mechanisms of the tip-induced electrochemical reactions as mediated by electronic currents, and into the role of water in these processes, as well as providing a different approach for electrochemical nano-writing.
We report the momentum-resolved measurement of a two-dimensional electron gas at the LaTiO(3)/SrTiO(3) interface by angle-resolved photoemission spectroscopy (ARPES). Thanks to an advanced sample preparation technique, the orbital character of the conduction electrons and the electronic correlations can be accessed quantitatively as each unit cell layer is added. We find that all of these quantities change dramatically with distance from the interface. These findings open the way to analogous studies on other heterostructures, which are traditionally a forbidden field for ARPES.
Variable temperature electrochemical strain microscopy has been used to study the electrochemical activity of Sm-doped ceria as a function of temperature and bias. The electrochemical strain microscopy hysteresis loops have been collected across the surface at different temperatures and the relative activity at different temperatures has been compared. The relaxation behavior of the signal at different temperatures has been also evaluated to relate kinetic process during bias induced electrochemical reactions with temperature and two different kinetic regimes have been identified. The strongly non-monotonic dependence of relaxation behavior on temperature is interpreted as evidence for water-mediated mechanisms.
SrTiO(3)/LaAlO(3) interfaces show an unprecedented photoconductivity effect that is persistent even at room temperature and giant as it gives rise to a conductivity increase of about 5 orders of magnitude at room temperature. The persistent photoconductivity effects play a paramount role in the still controversial intrinsic behavior of the SrTiO(3)/LaAlO(3) interfaces, as even a limited exposure to visible light is able to strongly modify the electrical transport properties of the interface even above room temperature, while only an appropriate thermal treatment in a dark environment can completely suppress the persistent photoconductivity effect unveiling the intrinsic conduction mechanism of the interface. Moreover, our study demonstrates that the origin of the high conductivity, revealed at the STO/LAO interface at room temperature, is purely electronic.
Reducing the operating temperature in the 500-750 °C range is needed for widespread use of solid oxide fuel cells (SOFCs). Proton-conducting oxides are gaining wide interest as electrolyte materials for this aim. We report the fabrication of BaZr(0.8)Y(0.2)O(3-δ) (BZY) proton-conducting electrolyte thin films by pulsed laser deposition on different single-crystalline substrates. Highly textured, epitaxially oriented BZY films were obtained on (100)-oriented MgO substrates, showing the largest proton conductivity ever reported for BZY samples, being 0.11 S cm(-1) at 500 °C. The excellent crystalline quality of BZY films allowed for the first time the experimental measurement of the large BZY bulk conductivity above 300 °C, expected in the absence of blocking grain boundaries. The measured proton conductivity is also significantly larger than the conductivity values of oxygen-ion conductors in the same temperature range, opening new potential for the development of miniaturized SOFCs for portable power supply.
