A. Kayani

Enhanced cathodoluminescence from an amorphous AlN:holmium phosphor by co-doped Gd +3 for optical devices applications

Sputter-deposited thin films of amorphous AlN:Ho (1 at.%) emits in the green (549 nm) region of the visible spectrum under electron excitation. The addition of Gd (1 at.%) in the film enhances the green emission linearly after thermal activation at 900°C for 40 min in a nitrogen atmosphere. The luminescence enhancement saturates when the gadolinium concentration reaches four times the holmium concentration. The optical bandgap of amorphous AlN is about 210 nm, so that the film is transparent in the ultraviolet, allowing us to observe the ultraviolet emission at 313 nm from Gd. No significant quenching of the Gd emission is observed. Energy dispersive x-ray (EDX) spectra confirm the increasing concentration of Gd. X-ray diffraction (XRD) analysis shows no peaks other than those arising from the Si (111) substrate, confirming that the films are amorphous. The enhanced luminescence can be used to make high-efficiency optical devices.

Influence of growth rate on the epitaxial orientation and crystalline quality of CeO2 thin films grown on Al2O3(0001)

Growth rate-induced epitaxial orientations and crystalline quality of CeO2 thin films grown on Al2O3(0001) by oxygen plasma-assisted molecular beam epitaxy were studied using in situ and ex situ characterization techniques. CeO2 grows as three-dimensional (3D) islands and two-dimensional layers at growth rates of 1–7 A/min and ≥9 A/min, respectively. The formation of epitaxial CeO2(100) and CeO2(111) thin films occurs at growth rates of 1 A/min and ≥9 A/min, respectively. Glancing-incidence x-ray diffraction measurements have shown that the films grown at intermediate growth rates (2–7 A/min) consist of polycrystalline CeO2 along with CeO2(100). The thin film grown at 1 A/min exhibits six in-plane domains, characteristic of well-aligned CeO2(100) crystallites. The content of the poorly aligned CeO2(100) crystallites increases with increasing growth rate from 2 to 7 A/min, and three out of six in-plane domains gradually decrease and eventually disappear, as confirmed by XRD pole figures. At growth rates ≥9...

Rapid doubling of the critical current of YBa2Cu3O7-δ coated conductors for viable high-speed industrial processing

We demonstrate that 3.5-MeV oxygen irradiation can markedly enhance the in-field critical current of commercial second generation superconducting tapes with an exposure time of just 1 s per 0.8 cm2. The speed demonstrated here is now at the level required for an industrial reel-to-reel post-processing. The irradiation is made on production line samples through the protective silver coating and does not require any modification of the growth process. From TEM imaging, we identify small clusters as the main source of increased vortex pinning.

Subsurface synthesis and characterization of Ag nanoparticles embedded in MgO.

Metal nanoparticles exhibit a localized surface plasmon resonance (LSPR) which is very sensitive to the size and shape of the nanoparticle and the surrounding dielectric medium. The coupling between the electromagnetic radiation and the localized surface plasmon in metallic nanoparticles results in a sizable enhancement of the incident fields, making them possible candidates for plasmonic applications. In particular, partially exposed metallic nanoparticles distributed in a dielectric matrix can provide prime locations for LSPR spectroscopy and sensing. We report the synthesis and characterization of a plasmonic substrate consisting of Ag nanoparticles partially buried in MgO. Ag nanoparticles of different shapes and size distributions were synthesized below the surface of MgO by implanting 200 keV Ag(+) ions followed by annealing at 1000 °C for 10 and 30 h. A detailed optical and structural characterization was carried out to understand the evolution of the Ag nanoparticle and size distribution inside the MgO matrix. Micro x-ray diffraction (Micro-XRD) was employed to investigate the structural properties and estimate the crystallite size. The nanoparticles evolved from a spherical to a faceted morphology with annealing time, assuming an octahedral shape truncated at the (001) planes, as visualized from aberration-corrected transmission electron microscopy (TEM) images. The nanoparticles embedded in MgO were shown to be pure metallic Ag using atom probe tomography (APT). The nanoparticles were partially exposed to the surface by employing plasma etch techniques to remove the overlaying MgO. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were employed to study the surface morphology and obtain a height distribution for the partially exposed nanoparticles.

Thickness Dependency of Thin-Film Samaria-Doped Ceria for Oxygen Sensing

High-temperature oxygen sensors are widely used for exhaust gas monitoring in automobiles. This particular study explores the use of thin-film single crystalline samaria-doped ceria as the oxygen sensing material. Desired signal-to-noise ratio can be achieved in a material system with high conductance. From previous studies, it is established that 6 atomic percent samarium doping is the optimum concentration for thin-film samaria-doped ceria (SDC) to achieve high ionic conductivity. In this study, the conductance of the 6 atomic percent samaria-doped ceria (SDC) thin film is measured as a function of the sensing film thickness. Hysteresis and dynamic response of this sensing platform are tested for a range of oxygen pressures from 0.001 to 100 torr for temperatures above 673 K. An attempt has been made to understand the physics behind the thickness-dependent conductance of this sensing platform by developing a hypothetical operating model and through COMSOL simulations. This study can be used to identify the parameters required to construct a fast, reliable, and compact high-temperature oxygen sensor.

Doping- and irradiation-controlled pinning of vortices in BaFe 2 (As 1-x P x ) 2 single crystals

We report on the systematic evolution of vortex pinning behavior in isovalent doped single crystals of BaFe{sub 2}(As{sub 1-x}P{sub x}){sub 2}. Proceeding from optimal doped to overdoped samples, we find a clear transformation of the magnetization hysteresis from a fishtail behavior to a distinct peak effect, followed by a reversible magnetization and Bean-Livingston surface barriers. Strong point pinning dominates the vortex behavior at low fields whereas weak collective pinning determines the behavior at higher fields. In addition to doping effects, we show that particle irradiation by energetic protons can tune vortex pinning in these materials.

Engineered Pinning Landscapes for Enhanced 2G Coil Wire

We demonstrate a twofold increase in the in-field critical current of AMSCs standard 2G coil wire by irradiation with 18-MeV Au ions. The optimum pinning enhancement is achieved with a dose of 6 × 1011 Au ions/cm2. Although the 77 K, self-field critical current is reduced by about 35%, the in-field critical current (H//c) shows a significant enhancement between 4 and 50 K in fields > 1 T. The process was used for the roll-to-roll irradiation of AMSCs standard 46-mm-wide production coated conductor strips, which were further processed into standard copper laminated coil wire. The long-length wires show the same enhancement as attained with short static irradiated samples. The roll-to-roll irradiation process can be incorporated in the standard 2G wire manufacturing, with no modifications to the current process. The enhanced performance of the wire will benefit rotating machine and magnet applications.

Robust odd-parity superconductivity in the doped topological insulator NbxBi2Se3

We present resistivity and magnetization measurements on proton-irradiated crystals demonstrating that the superconducting state in the doped topological superconductor Nb

Effect of chromium underlayer on the properties of nano-crystalline diamond films

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Silver nanorod arrays for photocathode applications

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