Nanoscale redox reactions in transition metal oxides are believed to be the physical foundation of memristive devices, which present a highly scalable, low-power alternative for future non-volatile ...memory devices. The interface between noble metal top electrodes and Nb-doped SrTiO3 single crystals may serve as a prominent but not yet well-understood example of such memristive devices. In this report, we will present experimental evidence that nanoscale redox reactions and the associated valence change mechanism are indeed responsible for the resistance change in noble metal/Nb-doped SrTiO3 junctions with dimensions ranging from the micrometer scale down to the nanometer regime. Direct verification of the valence change mechanism is given by spectromicroscopic characterization of switching filaments. Furthermore, it is found that the resistance change over time is driven by the reoxidation of a previously oxygen-deficient region. The retention times of the low resistance states, accordingly, can be dramatically improved under vacuum conditions as well as through the insertion of a thin Al2O3 layer which prevents this reoxidation. These insights finally confirm the resistive switching mechanism at these interfaces and are therefore of significant importance for the study and application of memristive devices based on Nb-doped SrTiO3 as well as systems with similar switching mechanisms.
Cation stoichiometry has been identified as a major key in establishing 2-dimensional electron gases (2DEGs) in oxide heterostructures. Here, we discuss a 2DEG formation scenario in B-site deficient ...perovskite/perovskite heterostructures, which previously were predicted to show insulating behavior. We elaborate an ionic picture based on oxygen-vacancy-buffered B-site vacancy defects in the polar oxide layer that yields a continuous transition from 2DEG formation to less conducting interfaces to insulating interfaces with increasing B-site deficiency. Experimentally, a corresponding modulation of charge transfer across NdGaO3/SrTiO3 interfaces is inferred using hard x-ray photoelectron spectroscopy analysis and transport experiments. With increasing B-site deficiency, we observe a decrease of the interfacial Ti3+ core level contribution, indicating a reduced charge transfer at the interface. This result is corroborated by temperature-dependent transport measurements, revealing increased low temperature resistance, with a dominant influence of a reduced electron density in the Ga-depleted sample. We consider a redistribution of oxygen vacancies in the B-site deficient polar oxide layer to explain the alleviated interface reconstruction, adding a new perspective on potential built-up in polar-oxide thin films.
Redox-based resistive switching is one of the most-promising concepts in the focus of research to meet the ever-growing demand for faster and smaller non-volatile memory devices. In this work we ...present detailed studies of the impact of cation stoichiometry and surface segregation effects on the performance of the valence change memory model material SrTiO3. In order to clarify if the enhanced switching performance of Sr-rich SrTiO3 devices can be attributed to SrO segregation or to the formation of Sr-rich extended defects, we artificially engineered the formation of SrO islands by depositing additional SrO on top of stoichiometric SrTiO3. We thereby unravel that the enhanced switching performance is solely accounted for by the formation of SrO islands and not influenced by extended defects. Consequently following our findings, we design devices with a further improved retention by tailoring the amount of SrO on the surface.
Some clinical indications require small fields with sharp lateral dose gradients, which is technically challenging in proton beam therapy. This holds especially true for low-range fields applied with ...the spot scanning technique, where large beam profiles entering from the beam-line or the insertion of range shifting blocks lead to large lateral gradients. We regard the latter case and solve it by shifting the range shifting block far upstream in conjunction with a collimating aperture close to the patient. The experiments of the current work are based on a commercial proton therapy treatment head designed for several delivery modes. In a research environment of the spot-scanning delivery mode a range shifter is inserted downstream of the scanning magnets in a slot which is usually employed only in a scattering delivery mode. This configuration is motivated by equations assuming a simple model of proton transport. In the experiments lateral dose planes are acquired with a scintillation screen and radiochromic films. Dose distributions are calculated with the Monte Carlo dose engine of the RayStation treatment planning system. We demonstrate that proton fields with 80%-20% lateral dose fall-off values between 1.4 mm and 4.0 mm can be achieved for water equivalent depths between 0 cm and 10 cm. The simulated lateral dose profiles agree with the experimental dose profiles. The sharpening of the field edges is set off by a broadening of the proton spots towards the center of the fields. This limits the clinical application mainly to small fields for which the distal and proximal conformality is of minor importance.
Redox-based resistive switching is one of the most-promising concepts in the focus of research to meet the ever-growing demand for faster and smaller non-volatile memory devices. In this work we ...present detailed studies of the impact of cation stoichiometry and surface segregation effects on the performance of the valence change memory model material SrTiO3. In order to clarify if the enhanced switching performance of Sr-rich SrTiO3 devices can be attributed to SrO segregation or to the formation of Sr-rich extended defects, we artificially engineered the formation of SrO islands by depositing additional SrO on top of stoichiometric SrTiO3. We thereby unravel that the enhanced switching performance is solely accounted for by the formation of SrO islands and not influenced by extended defects. Consequently following our findings, we design devices with a further improved retention by tailoring the amount of SrO on the surface.
•We unravel the enhanced performance of resistive switching Sr-rich SrTiO3 devices.•We engineer the segregation of SrO at the Pt/SrTiO3 interface by PLD.•We thereby mimic the performance of Sr-rich STO.•Following our findings, we design devices with a further improved retention.
Theoretical considerations predicted the feasibility of K-edge x-ray computed tomography (CT) imaging using energy discriminating detectors with more than two energy bins. This technique enables ...material-specific imaging in CT, which in combination with high-Z element based contrast agents, opens up possibilities for new medical applications. In this paper, we present a CT system with energy detection capabilities, which was used to demonstrate the feasibility of quantitative K-edge CT imaging experimentally. A phantom was imaged containing PMMA, calcium-hydroxyapatite, water and two contrast agents based on iodine and gadolinium, respectively. Separate images of the attenuation by photoelectric absorption and Compton scattering were reconstructed from energy-resolved projection data using maximum-likelihood basis-component decomposition. The data analysis further enabled the display of images of the individual contrast agents and their concentrations, separated from the anatomical background. Measured concentrations of iodine and gadolinium were in good agreement with the actual concentrations. Prior to the tomographic measurements, the detector response functions for monochromatic illumination using synchrotron radiation were determined in the energy range 25 keV-60 keV. These data were used to calibrate the detector and derive a phenomenological model for the detector response and the energy bin sensitivities.
Objective
Side effects of radiation therapy may include skin damage. The surface dose is of great interest and contains the buildup effect. In particular, the proton therapy community requires ...further experimental data to quantify doses in the surface region. This specification includes the skin dose, which is defined according to ICRU Report No. 39 at 70 μm water equivalent depth. The aim of this study is to gather more knowledge of the skin dose by varying key parameters defined by the patient treatment plan. This consists of clinical aspects such as the influence of the air gap, the application of a range shifter (RS), or the proton delivery technique.
Material/methods
Skin doses were determined with a PTW 23391 extrapolation chamber with three thin Kapton® entrance windows operated as a conventional ionization chamber. The impact on the skin dose for quasi‐monoenergetic pencil beam scanning (PBS) proton beams was evaluated for clinical air gaps between 3.5 and 51.1 cm. The differences in skin dose were assessed by irradiating equivalent fields with an RS of 51 mm water equivalent thickness (RS51) and without. Furthermore, the delivery techniques PBS, uniform scanning (US), and double scattering (DS) were compared by defining a spread‐out Bragg peak (SOBP). TOPAS (V.3.1.2) was used to model an IBA nozzle with PBS and to score dose to water at the surface of a water phantom.
Results
For the monoenergetic fields without the application of the RS the skin dose was constant down to an air gap of 6.2 cm. A lower air gap of 3.5 cm showed a variation in skin dose by up to 2.4% compared to the results obtained with larger air gaps. With the inserted RS51 an increase in the skin dose was found for air gaps smaller than 11.3 cm. Experimentally, a dose difference of 1.4% was recorded for an air gap of 6.2 cm by inserting an RS and none. With the Monte Carlo calculations the largest dose increase was observed at the air gap of 3.5 cm with 1.7% and 4.0% relative to the skin dose results without the RS and to the largest evaluated air gap of 51.1 cm, respectively. The SOBP comparison of the beam modalities at the measuring plane at the isocenter revealed higher skin doses without RS (including RS) by up to +1.9% (+1.5%) for DS and +1.3% (+1.1%) for US compared to PBS. For all three techniques an approx. 2% rise in skin dose was observed for the largest evaluated air gap of 37.7 cm to an air gap of 6.2 cm when using an RS51.
Conclusion
The study investigated aspects of skin dose of a water equivalent phantom by varying key parameters of a proton treatment plan. Parameters like the RS, the air gap, and the delivery modality have an impact on the order of 4.0% for the skin dose at the depth of 70 μm. The increases in skin dose are the effects of the contribution of the increased electron fluence at small air gaps and the emitted hadronic particles produced by the RS.
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