Spatially resolved x-ray fluorescence maps are presented that show the introduction and the evolution of oxygen vacancies in chromium-doped strontium titanate during an electric-field-driven insulator-to-metal transition. The vacancies are introduced at the anode and diffuse through the crystal toward the cathode. The spatial distribution of vacancies is explained by a model describing the electrical breakdown as a percolation process. Strong differences in the vacancy distribution were found when the transition took place in air and in a hydrogen-enriched atmosphere. In air, the vacancies disappeared from the surface, whereas in the reducing hydrogen atmosphere, they remained at the surface. © 2009 American Institute of Physics.

X-ray absorption spectroscopy was used to study the microscopic origin of conductance and resistive switching in chromium-doped strontium titanate (Cr:SrTiO₃). Differences in the X-ray absorption near edge spectroscopy (XANES) at the Cr K-edge indicate that the valence of Cr changes from 3+ to 4+ underneath the anode of our sample device after the application of an electric field. Spatially resolved X-ray fluorescence microscopy (μ-XRF) maps show that the Cr⁴⁺ region retracts from the anode-Cr:SrTiO₃ interface after a conducting state has been achieved. This interface region is studied with extended X-ray absorption fine structure (EXAFS) and the results are compared with structural parameters obtained from density functional theory (DFT) calculations. They confirm that oxygen vacancies which are localized at the octahedron with a Cr at its center are introduced at the interface. It is proposed that the switching state is not due to a valence change of chromium but caused by changes of oxygen vacancies at the interface. © 2007 Elsevier B.V. All rights reserved.

The resistive switching state in Cr-doped SrTiO₃ was induced by applying an electric field. This was done in ambient air and in an atmosphere of H₂/Ar. The distribution of the thereby introduced oxygen vacancies was studied by spatially resolved X-ray fluorescence images. It was concluded that the oxygen vacancies were introduced in the interface between the SrTiO₃ and the positively biased electrode. © 2009 IOP Publishing Ltd.

X-ray absorption experiments on 3d transition-metal tracer ions in SrTiO₃ are presented. The absorption spectra of the tracer-ion changed upon reduction in the SrTiO₃. This change is due to an oxygen vacancy created at the tracer-ion site. This finding is supported by density-functional theory calculations, which prove that the oxygen vacancies preferentially are created at the tracer-ion sites. Using the chemical sensitivity of x-ray absorption spectroscopy, tracer ions can be used to detect oxygen vacancies in SrTiO₃ and possibly in other oxide systems. © 2009 The American Physical Society.

Background: The purpose of this study was to determine if 3D printed lenses with wavelength specific anti-reflective (AR) surface structures would improve beam intensity and thus radar efficiency for a Printed Circuit Board (PCB)-based 60 GHz radar. This would have potential for improved low-cost radar lenses for the consumer product market. Methods: A hyperbolic lens was designed in 3D Computer Aided Design (CAD) software and was then modified with a wavelength specified AR structure. Electromagnetic computer simulation was performed on both the ‘smooth’ and ‘AR structure’ lenses and compared to actual 60 GHz radar measurements of 3D printed polylactic acid (PLA) lenses. Results: The simulation results showed an increase of 10% in signal intensity of the AR structure lens over the smooth lens. Actual measurement showed an 8% increase in signal of the AR structure lens over the smooth lens. Conclusions: Low cost and readily available Fused Filament Fabrication (FFF) 3D printing has been shown to be capable of printing an AR structure coated hyperbolic lens for millimeter wavelength radar applications. These 3D Printed AR structure lenses are effective in improving radar measurements over non-AR structure lenses.

This paper describes an iterative approach to determine quasi-optical properties of standard 3D printer filament material to, in an inexpensive and fast way, construct focusing lenses for millimetre wave systems. Results from three lenses with different focal lengths are shown and discussed. The real part of the permittivity at 60GHz for polylactic acid (PLA) is in this paper determined to be ε_r=2.74. © 2018 Institution of Engineering and Technology. All rights reserved.

A high density of oxygen vacancies has been found in an experiment to determine the path of electrical conduction in Cr-doped SrTiO₃ memory cells. The Cr acts as a seed for the localization of oxygen vacancies, leading to a statistically homogeneous distribution of charge carriers within the path. This warrants a controllable doping profile and improved device scaling down to the nanometer scale. The combination of laterally resolved micro-X-ray absorption spectroscopy and thermal imaging concludes that the resistance switching in Cr-doped SrTiO₃ originates from an oxygen-vacancy drift to/from the electrode that was used as anode during the conditioning process. The experiments shows that this oxygen vacancy concept is crucial for the entire class of transition-metal-oxide-based bipolar resistance-change memory.

The modified phonon dispersion is of importance for understanding the origin of the reduced heat conductivity in nanowires. We have measured the phonon dispersion for 50 nm diameter InSb (111) nanowires using time-resolved X-ray diffraction. By comparing the sound speed of the bulk (3880 m/s) and that of a classical thin rod (3600 m/s) to our measurement (2880 m/s), we conclude that the origin of the reduced sound speed and thereby to the reduced heat conductivity is that the C44 elastic constant is reduced by 35% compared to the bulk material. © 2014 American Chemical Society.

Electrical characterizations of single Zn₂SnO₄ (ZTO) nanowire devices are presented. These include resistivity, mobility, and photosensing measurements. The resistivity and the mobility of the Zn₂SnO₄ nanowire were measured to be 5.6 cm and 0.2 cm²/Vs, respectively. These values were found to be strongly dependent on the amount of electron-donating defects and less dependent on the thickness of the nanowires. An increase in the resistivity when changing the ambient atmosphere is observed. This change is caused by defect states lying in the bandgap, as shown by photoluminescence. The results imply the potential of ZTO nanowires as phototransistors and other photosensitive devices. © 2010 The Electrochemical Society.

Oxide nanowire networks or oxide nanonets leverage some of the exceptional functionalities of one-dimensional nanomaterials along with the fault tolerance and flexibility of interconnected nanowires to creating exciting opportunities in large-area electronics as well as green energy systems. This paper reviews the electronic and optoelectronic properties of these networks and highlights their potential applications in field-effect transistors, optoelectronic devices, and solar cells. Techniques to grow nanowires and their subsequent integration into networks using contact printing and electrospinning are described. Electrical properties of field-effect transistors fabricated from contact printed nanowire networks are discussed, and means of integration of the nanowire networks of heterogenous materials that enable ambipolar device operation are outlined. Photocurrent properties of these nanowires are described, including the dye sensitization of large-bandgap SnO2 nanowires. The final section deals with the advantages of employing nanowire networks in dye-sensitized solar cells and the dependence of solar cell performance on morphology and surface area. © The Royal Society of Chemistry 2010.

The spectrum of laser-generated acoustic phonons in indium antimonide coated with a thin nickel film has been studied using time-resolved x-ray diffraction. Strain pulses that can be considered to be built up from coherent phonons were generated in the nickel film by absorption of short laser pulses. Acoustic reflections at the Ni-InSb interface leads to interference that strongly modifies the resulting phonon spectrum. The study was performed with high momentum transfer resolution together with high time resolution. This was achieved by using a third-generation synchrotron radiation source that provided a high-brightness beam and an ultrafast x-ray streak camera to obtain a temporal resolution of 10 ps. We also carried out simulations, using commercial finite element software packages and on-line dynamic diffraction tools. Using these tools, it is possible to calculate the time-resolved x-ray reflectivity from these complicated strain shapes. The acoustic pulses have a peak strain amplitude close to 1%, and we investigated the possibility to use this device as an x-ray switch. At a bright source optimized for hard x-ray generation, the low reflectivity may be an acceptable trade-off to obtain a pulse duration that is more than an order of magnitude shorter. © 2015 Author(s).

Copper oxides (Cu_xO) thin films were deposited using radio frequency (RF) sputtering on glass substrates. By tuning the argon (Ar) partial pressure during deposition, cuprous oxide (Cu₂O), cupric oxide (CuO), or their mixed phase could be achieved. Drastic variations in the Hall mobility, hole density, and resistivity of the samples were observed due to the presence of different phases in the films. Kelvin probe studies indicate that the photo-generated carriers have lower recombination rate in pure Cu ₂O phase. This was further validated by transient absorption measurements, where the estimated carrier lifetime for Cu₂O was much larger that other phases. © 2013 American Institute of Physics.

Developing Cyber-Physical Systems requires methods and tools to support simulation and verification of hybrid (both continuous and discrete) models. The Acumen modeling and simulation language is an open source testbed for exploring the design space of what rigorous-but-practical next-generation tools can deliver to developers of Cyber-Physical Systems. Like verification tools, a design goal for Acumen is to provide rigorous results. Like simulation tools, it aims to be intuitive, practical, and scalable. However, it is far from evident whether these two goals can be achieved simultaneously.

This paper explains the primary design goals for Acumen, the core challenges that must be addressed in order to achieve these goals, the "agile research method" taken by the project, the steps taken to realize these goals, the key lessons learned, and the emerging language design. © ICST Institute for Computer Sciences, Social Informatics and Telecommunications Engineering 2016.

In phase-change memory devices, a material is cycled between glassy and crystalline states. The highly temperature-dependent kinetics of its crystallization process enables application in memory technology, but the transition has not been resolved on an atomic scale. Using femtosecond x-ray diffraction and ab initio computer simulations, we determined the time-dependent pair-correlation function of phase-change materials throughout the melt-quenching and crystallization process. We found a liquid–liquid phase transition in the phase-change materials Ag₄In₃Sb₆₇Te₂₆ and Ge₁₅Sb₈₅ at 660 and 610 kelvin, respectively. The transition is predominantly caused by the onset of Peierls distortions, the amplitude of which correlates with an increase of the apparent activation energy of diffusivity. This reveals a relationship between atomic structure and kinetics, enabling a systematic optimization of the memory-switching kinetics. © 2019 American Association for the Advancement of Science.