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Different SrTiO3 thin films are investigated to unravel the nature of ultra‐low conductivities recently found in SrTiO3 films prepared by pulsed laser deposition. Impedance spectroscopy reveals electronically pseudo‐intrinsic conductivities for a broad range of different dopants (Fe, Al, Ni) and partly high dopant concentrations up to several percent. Using inductively‐coupled plasma optical emission spectroscopy and reciprocal space mapping, a severe Sr deficiency is found and positron annihilation lifetime spectroscopy revealed Sr vacancies as predominant point defects. From synchrotron‐based X‐ray standing wave and X‐ray absorption spectroscopy measurements, a change in site occupation is deduced for Fe‐doped SrTiO3 films, accompanied by a change in the dopant type. Based on these experiments, a model is deduced, which explains the almost ubiquitous pseudo‐intrinsic conductivity of these films. Sr deficiency is suggested as key driver by introducing Sr vacancies and causing site changes (FeSr and TiSr) to accommodate nonstoichiometry. Sr vacancies act as mid‐gap acceptor states, pinning the Fermi level, provided that additional donor states (most probably TiSr••\[{\rm{Ti}}_{{\rm{Sr}}}^{ \bullet \bullet }\]) are present. Defect chemical modeling revealed that such a Fermi level pinning also causes a self‐limitation of the Ti site change and leads to a very robust pseudo‐intrinsic situation, irrespective of Sr/Ti ratios and doping. [ABSTRACT FROM AUTHOR]

The defect chemistry and electronic trapping energies in undoped single crystalline SrTiO 3 were examined by electrochemical impedance spectroscopy at low (25–160 °C) and intermediate (500–700 °C) temperatures. Electronic and ionic conductivity as well as chemical capacitance values were obtained with a transmission line equivalent circuit. Impedance spectroscopy at low temperatures was used to quantify trapping energies of main ionic defects. Particularly the chemical capacitance is shown to be a highly valuable, though hardly used tool for establishing a defect model based solely on electrochemical measurements. It is very sensitive for minority charge carriers and can thus unveil otherwise hardly accessible defect concentrations. The chemical capacitance analysis yields a valence dependent acceptor concentration in the ppm range for the investigated samples. Complementary positron annihilation lifetime spectroscopy (PALS) suggests existence of Ti vacancies and both methods (chemical capacitance and PALS) agree in their quantification of the corresponding vacancy concentration (6 ppm). Beyond successfully predicting acceptor defect concentrations in undoped SrTiO 3 , the method is sensitive for electronically relevant defects in sub-ppm concentrations. [ABSTRACT FROM AUTHOR]

Strontium titanate (SrTiO3) has received much attention due to its wide range of potential applications, including in electrochemical devices such as solid oxide fuel cells and capacitors. The stability and safety features of SrTiO3 led to the development of promising electrodes for Li-ion batteries. Here, we use density functional theory simulations to examine the incorporation of lithium from its gas-phase and bulk forms. The results show that a single Li atom is thermodynamically stable in bulk SrTiO3 with respect to its gas-phase and slightly unfavorable compared to its bulk. Multiple Li incorporation up to six is also considered, and the incorporation is exoergic with respect to both gas-phase and bulk forms. Charge analysis confirmed the presence of Li+ ions in the lattice. Li incorporation turns the insulating nature of SrTiO3 into metallic and non-magnetic into magnetic. Lithium incorporation facilitates the formation of Sr, Ti, and O vacancies. The loss of Li2O is exoergic, suggesting that oxygen vacancy mediated self-diffusion will be promoted. [ABSTRACT FROM AUTHOR]

Abstract This paper describes a specific route for the complete integration of a novel planar thermoelectric microgenerator (μTEG) that can operate under environmental conditions using commercial miniaturized heat exchangers. The proposed heat exchanger integration process is compatible with the fragility of planar micromachined silicon structures. The main structure of the μTEG is built around a micromachined silicon platform defined by silicon microfabrication technologies. Different silicon-based materials, such as bottom-up grown silicon and silicon-germanium nanowire arrays as well as top-down fabricated silicon microbeams are used as thermoelectric materials. μTEGs with those materials are characterized both before and after heat exchanger integration. The presence of the heat exchanger increases the μTEG performance significantly and power densities around 40 μW cm−2 are obtained when placed on a heat source at 100 °C and exposed under natural convection to a surrounding ambient at room temperature. Graphical abstract Image 1 Highlights • Energy harvesting is in high demand for powering IoT devices. • Managing the thermal match is crucial to enhance the performance of a thermogenerator. • Different Si-based materials are successfully integrated into thermal generators. • The heat exchanger integration route is compatible with microdevice fragility. • Power density attained is in accordance with the requirements of IoT microsensors. [ABSTRACT FROM AUTHOR]

High temperature solid oxide solar cells based on SrTiO 3 (STO) are investigated. Schottky contacts between STO and different materials, including La 1-x Sr x CrO 3 , La 0.8 Sr 0.2 MnO 3 , La 0.6 Sr 0.4 CoO 3 and metals like Au and Pt are illuminated by UV light with a wavelength of 365 nm at 350 °C in air and the resulting voltages and currents are measured. With certain material combinations, e.g. LaCrO 3 /STO or Au/STO heterojunctions, high photovoltages of more than 1.0 V are obtained. Current measurements reveal changes in the bulk STO due to stoichiometry polarization, leading to a self-enhancement effect of the photovoltaic cell. Mechanistic insight in the processes under UV light is gained by electrochemical impedance spectroscopy, especially regarding the space charge region at the heterojunction and bulk changes due to stoichiometry polarization. In addition to the electronic effects upon illumination, also photo-ionic effects of oxygen transport caused by chemical potential changes were observed. • SrTiO 3 based high temperature solar cells yield up to 1 V photo voltages. • Robust, high photo-voltages with varying oxide and metal hetero junctions. • Self enhancing effect under operation due to stoichiometry polarization. • Oxygen exchange affects photo-voltages on the timescale of minutes. [ABSTRACT FROM AUTHOR]

The thermoelectric performance of nanostructured low dimensional silicon and silicon-germanium has been functionally compared device-wise. The arrays of nanowires of both materials, grown by a VLS-CVD (Vapor-Liquid-Solid Chemical Vapor Deposition) method, have been monolithically integrated in a silicon micromachined structure in order to exploit the improved thermoelectric properties of nanostructured silicon-based materials. The device architecture helps to translate a vertically occurring temperature gradient into a lateral temperature difference across the nanowires. Such thermocouple is completed with a thin film metal leg in a unileg configuration. The device is operative on its own and can be largely replicated (and interconnected) using standard IC (Integrated Circuits) and MEMS (Micro-ElectroMechanical Systems) technologies. Despite SiGe nanowires devices show a lower Seebeck coefficient and a higher electrical resistance, they exhibit a much better performance leading to larger open circuit voltages and a larger overall power supply. This is possible due to the lower thermal conductance of the nanostructured SiGe ensemble that enables a much larger internal temperature difference for the same external thermal gradient. Indeed, power densities in the μW/cm2 could be obtained for such devices when resting on hot surfaces in the 50–200 °C range under natural convection even without the presence of a heat exchanger. [ABSTRACT FROM AUTHOR]