Raman spectra and defect chemical characteristics of Sr(Ti,Fe)O3-y solid solution of bulk pellets vs. thin films.

Sr(Ti,Fe)O3-y perovskite solid solutions are relevant functional materials for energy conversion and electronic devices such as solid oxide fuel and photoelectrochemical cells, electrolyzers, oxygen sensors, resistive random access memories or synaptic transistors. The Raman spectra and vibrational characteristics of the Sr(Ti,Fe)O3-y materials class are suitable for describing their defect chemistry and the iron valence state, which governs a multitude of its mixed ionic-electronic transport and other characteristics. We synthesize a standard range of compositions containing 1-75 mol% of iron including the end members in the form of macrocrystalline bulk pellets, nanocrystalline poly- and single crystalline thin films. Through the change in both iron substitution level and microstructure, we directly see the effect of defect chemistry such as its phase, transition metal ion valence and oxygen nonstoichiometry on the Raman spectra. These are discussed in terms of in and ex situ experiments under oxidizing/reducing atmosphere. In contrast to long range structural X-ray diffraction measurements, Raman spectroscopy provides valuable insights into oxygen vacancy ordering and oxygen nonstoichiometry for the Sr(Ti,Fe)O3-y material class.

Anisotropic Strain in Rare-Earth Substituted Ceria Thin Films Probed by Polarized Raman Spectroscopy and First-Principles Calculations.

Lattice strain in oxygen ion conductors can be used to tune their functional properties for applications in fuel cells, sensors, or catalysis. However, experimental measurements of thin film strain in both in- and out-of-plane directions can be experimentally challenging. We propose a method for measuring strain in rare-earth doped ceria thin films by polarized Raman spectroscopy. We study epitaxial CeO2 films substituted by La, Gd, and Yb grown on MgO substrates with BaZrO3 and SrTiO3 interlayers, where different levels of strain are generated by annealing at distinct temperatures. The films show in-plane compression and out-of-plane expansion, resulting in a lowering from the bulk cubic to tetragonal lattice symmetry. This leads to the splitting of the F2g Raman mode in the cubic phase to B2g and Eg modes in the tetragonal lattice. The symmetry and frequency of these modes are determined by polarized Raman in the backscattering and right-angle scattering geometries as well as by first-principal calculations. The frequency splitting of the two modes is proportional to the strain measured by X-ray diffraction and its magnitude agrees with first-principles calculations. The results offer a fast, nondestructive, and precise method for measuring both in- and out-of-plane strain in ceria and can be readily applied to other ionic conductors.

Toward Controlling Filament Size and Location for Resistive Switches via Nanoparticle Exsolution at Oxide Interfaces.

Memristive devices are among the most prominent candidates for future computer memory storage and neuromorphic computing. Though promising, the major hurdle for their industrial fabrication is their device-to-device and cycle-to-cycle variability. These occur due to the random nature of nanoionic conductive filaments, whose rupture and formation govern device operation. Changes in filament location, shape, and chemical composition cause cycle-to-cycle variability. This challenge is tackled by spatially confining conductive filaments with Ni nanoparticles. Ni nanoparticles are integrated on the bottom La0.2 Sr0.7 Ti0.9 Ni0.1 O3- δ electrode by an exsolution method, in which, at high temperatures under reducing conditions, Ni cations migrate to the perovskite surface, generating metallic nanoparticles. This fabrication method offers fine control over particle size and density and ensures strong particle anchorage in the bottom electrode, preventing movement and agglomeration. In devices based on amorphous SrTiO3 , it is demonstrated that as the exsolved Ni nanoparticle diameter increases up to ≈50 nm, the ratio between the ON and OFF resistance states increases from single units to 180 and the variability of the low resistance state reaches values below 5%. Exsolution is applied for the first time to engineer solid-solid interfaces extending its realm of application to electronic devices.

Duodenal Pressure Necrosis in a Child Caused by a Migrated Percutaneous Endoscopic Gastrostomy.

A two-year-old girl with two weeks of abdominal pain, vomiting, and food refusal, ten months after percutaneous endoscopic gastrostomy insertion because of inadequate peroral intake, was admitted to a tertiary centre hospital. On admission, the extracorporeal part of the gastrostomy was much shortened. X-ray examination revealed migration of the end of the gastrostomy tube with a left-shifted course of the tube through the duodenum. Gastroscopy and subsequently laparotomy were performed. A longitudinal pressure necrosis was identified under the tube, with two perforations in the duodenojejunal region. Ten centimeters of that duodenojejunal region were resected, and end-to-end anastomosis was made. The migration of the gastrostomy was probably caused by insufficient care by the parents. Pathophysiologically, the tube caused the pressure necrosis in the duodenojejunal area; this was supported by histology. This is a hitherto undescribed complication of a percutaneous endoscopic gastrostomy, showing that migration of the gastrostomy to the deeper part of the small bowel can lead to pressure necrosis, a potentially life-threatening condition in children which cannot be treated without invasive procedures.

In Situ Method Correlating Raman Vibrational Characteristics to Chemical Expansion via Oxygen Nonstoichiometry of Perovskite Thin Films.

Effective integration of perovskite films into devices requires knowledge of their electro-chemomechanical properties. Raman spectroscopy is an excellent tool for probing such properties as the films' vibrational characteristics couple to the lattice volumetric changes during chemical expansion. While lattice volumetric changes are typically accessed by analyzing Raman shifts as a function of pressure, stress, or temperature, such methods can be impractical for thin films and do not capture information on chemical expansion. An in situ Raman spectroscopy technique using an electrochemical titration cell to change the oxygen nonstoichiometry of a model perovskite film, Sr(Ti,Fe)O3- y  , is reported and the lattice vibrational properties are correlated to the material's chemical expansion. How to select an appropriate Raman vibrational mode to track the evolution in oxygen nonstoichiometry is discussed. Subsequently, the frequency of the oxygen stretching mode around Fe4+ is tracked, as it decreases during reduction as the material expands and increases during reoxidation as the material shrinks. This methodology of oxygen pumping and in situ Raman spectroscopy of oxide films enables future in operando measurements even for small material volumes, as is typical for applications of films as electrodes or electrolytes utilized in electrochemical energy conversion or memory devices.