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The present article describes the size induced changes in the structural arrangement of intrinsic defects present in chemically synthesized ZnO nanoparticles of various sizes. Routine x-ray diffraction and transmission electron microscopy have been performed to determine the shapes and sizes of the nanocrystalline ZnO samples. Detailed studies using positron annihilation spectroscopy reveals the presence of zinc vacancy. Whereas analysis of photoluminescence results predict the signature of charged oxygen vacancies. The size induced changes in positron parameters as well as the photoluminescence properties, has shown contrasting or nonmonotonous trends as size varies from 4 to 85 nm. Small spherical particles below a critical size (∼23 nm) receive more positive surface charge due to the higher occupancy of the doubly charge oxygen vacancy as compared to the bigger nanostructures where singly charged oxygen vacancy predominates. This electronic alteration has been seen to trigger yet another interesting phenomenon, described as positron confinement inside nanoparticles. Finally, based on all the results, a model of the structural arrangement of the intrinsic defects in the present samples has been reconciled. [ABSTRACT FROM AUTHOR]

Defects present in ZnO nanocrystals prepared by a wet chemical method have been characterized by photoluminescence (PL) and positron annihilation spectroscopy (PAS) techniques. The as-prepared sample was heat treated at different temperatures to obtain nanocrystals in the size range of 19–39 nm. X ray diffractograms confirmed the single-phase wurtzite structure formation. Photoluminescence measurements showed a strong violet band at 434 nm, which has been identified as due to electronic transitions from the zinc interstitial defect level to the top of the valence band. A marked decrease in the intensity of the violet emission with increasing heat-treatment temperature has been observed, which is attributed to recombination of zinc interstitials with zinc vacancies. Positron annihilation spectroscopy has been employed to understand the dynamics of the vacancy-type defects and their annealing behavior. The observed variation of the defect related lifetime components with heat-treatment temperature has been successfully explained by using a three-state trapping model. The results of PL and PAS studies in the present case are found to be complementary to each other. [ABSTRACT FROM AUTHOR]

This paper introduces novel potential device applications based on the nonlinear phase shifting properties of high-temperature superconductor (HTS) transmission lines. It demonstrates how the nonlinear characteristics of HTS materials can be used to build devices that improve the risetime of pulses as well as exhibit power-dependent phase shifting properties. The nonlinear transmission-line circuit layouts consist of a linear transmission-line periodically loaded with HTS stubs operating in the nonlinear region. Theoretical results are presented using a lumped-element SPICE model to verify the pulse-shaping concept presented in this paper. The power-dependent phase shifting properties are demonstrated with the measurements of two prototype circuits. Index Terms--High-temperature superconductor (HTS), nonlinear transmission line (NLTL), pulse sharpening, risetime improvement, power-dependent phase shifter, shockline.