STATISTICAL METHODS FOR MULTIVARIATE FAILURE-TIME DATA UNDER COMPETING RISKS

Traditional research on survival analysis often centered on univariate data where the observations are mutually independent. In many modern studies, however, data of interest are observed in clusters, so may be associated. Primary scientific interest often centers on the effect of a treatment on the individuals' outcomes in studies involving multivariate failure time data, but this thesis is mainly concerned with analyses in which the estimation of association between failure times is of interest. A considerable body of literature has addressed this topic, but they have been limited in many ways. They may depend on parametric assumptions that may easily be violated, they may not be flexbile enough, or their interpretations are not intuitive. The primary purpose of this thesis is to investigate the drawbacks of existing methods, and suggest an alternative measure of association that is flexible and interpretable, especially under the competing risks setting. This thesis consists of three main chapters. Chapter 2 discusses a nonparametric estimation of the local version of Kendall's $\tau$. The performance of several smoothing methods are compared, and new methods to deal with censored data are also proposed and assessed. Chapter 3 studies the sensitivity of the Bandeen-Roche and Liang (2002) estimator of the CCSHR to the imposed statistical assumptions and investigate the source of a bias reported in its foundational work. In Chapter 4, novel parametric and nonparametric estimators for the association between failure causes are proposed. Various combinations of existing and new methods for the association between failure times and between failure causes are assessed

Spectroscopic Evidence for Multigap Superconductivity of Y at Megabar Pressures

The recent discovery of room-temperature superconductivity (RTSC) at pressures of several megabars has led to intensive efforts to probe the origin of superconducting (SC) electron pairs. Although the signatures of the SC phase transition have been well established, few reports of the SC properties of RTSCs have been published because of the diamond anvil cell (DAC) environments. Here, we report the first direct evidence of two SC gaps in Y metal via point-contact spectroscopy (PCS) in DAC environments, where a sharp peak at the zero-bias voltage in the differential conductance is overlaid with a broad peak owing to Andreev reflection. Analysis based on the Blonder-Tinkham-Klapwijk (BTK) model reveals the existence of two SC gaps: the larger gap is 3.63 meV and the smaller gap is 0.46 meV. The temperature dependence of the two SC gaps is well explained by the BCS theory, indicating that two-band superconductivity is realized in Y metal. The successful application of PCS to Y in DAC environments is expected to guide future research on the SC gap in megabar high-Tc superconductors.Comment: 17 pages, 4 figure

Tuning of undoped ZnO thin film via plasma enhanced atomic layer deposition and its application for an inverted polymer solar cell

We studied the tuning of structural and optical properties of ZnO thin film and its correlation to the efficiency of inverted solar cell using plasma-enhanced atomic layer deposition (PEALD). The sequential injection of DEZn and O2 plasma was employed for the plasma-enhanced atomic layer deposition of ZnO thin film. As the growth temperature of ZnO film was increased from 100 ??C to 300??C, the crystallinity of ZnO film was improved from amorphous to highly ordered (002) direction ploy-crystal due to self crystallization. Increasing oxygen plasma time in PEALD process also introduces growing of hexagonal wurtzite phase of ZnO nanocrystal. Excess of oxygen plasma time induces enhanced deep level emission band (500 ??? 700 nm) in photoluminescence due to Zn vacancies and other defects. The evolution of structural and optical properties of PEALD ZnO films also involves in change of electrical conductivity by 3 orders of magnitude. The highly tunable PEALD ZnO thin films were employed as the electron conductive layers in inverted polymer solar cells. Our study indicates that both structural and optical properties rather than electrical conductivities of ZnO films play more important role for the effective charge collection in photovoltaic device operation. The ability to tune the materials properties of undoped ZnO films via PEALD should extend their functionality over the wide range of advanced electronic applications.open2

Remote Biosensing with Polychromatic Optical Waveguide Using Blue Light‐Emitting Organic Nanowires Hybridized with Quantum Dots

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/107498/1/adfm201304039.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/107498/2/adfm201304039-sup-0001-S1.pd

Revealing localized excitons in WSe2/β-Ga2O3

We have investigated the optical and magneto-optical properties of monolayer (ML) WSe2 on flakes of β-Ga2O3 under high magnetic fields. Remarkably, sharp emission peaks were observed and associated with localized excitons related to point defects. A detailed study of low-temperature photoluminescence (PL) and magneto-PL under high perpendicular magnetic field up to 9 T was carried out. Several sharp emission peaks have shown valley g-factors values close to −4, which is an unusual result for localized excitons in WSe2. Furthermore, some PL peaks have shown higher g-factor values of ≈−7 and ≈−12, which were associated with the hybridization of strain localized dark excitons and defects. The reported results suggest that β-Ga2O3 is, indeed, a promising dielectric substrate for ML WSe2 and also to explore fundamental physics in view of possible applications in quantum information technology

Near-field optical imaging and spectroscopy of 2D-TMDs

© 2021 Youngbum Kim and Jeongyong Kim, published by De Gruyter, Berlin/Boston.Two-dimensional transition metal dichalcogenides (2D-TMDs) are atomically thin semiconductors with a direct bandgap in monolayer thickness, providing ideal platforms for the development of exciton-based optoelectronic devices. Extensive studies on the spectral characteristics of exciton emission have been performed, but spatially resolved optical studies of 2D-TMDs are also critically important because of large variations in the spatial profiles of exciton emissions due to local defects and charge distributions that are intrinsically nonuniform. Because the spatial resolution of conventional optical microscopy and spectroscopy is fundamentally limited by diffraction, near-field optical imaging using apertured or metallic probes has been used to spectrally map the nanoscale profiles of exciton emissions and to study the effects of nanosize local defects and carrier distribution. While these unique approaches have been frequently used, revealing information on the exciton dynamics of 2D-TMDs that is not normally accessible by conventional far-field spectroscopy, a dedicated review of near-field imaging and spectroscopy studies on 2D-TMDs is not available. This review is intended to provide an overview of the current status of near-field optical research on 2D-TMDs and the future direction with regard to developing nanoscale optical imaging and spectroscopy to investigate the exciton characteristics of 2D-TMDs.11Nsciescopu

Polychromatic ZnO/CdxZn1-xO composite nanorods prepared by simple chemical methods: nanoscale optical characteristics

We report the fabrication and nanoscale optical characteristics of polychromatic ZnO/CdxZn1-xO composite nanorods prepared by simple hydrothermal and sol-gel chemical methods. Hydrothermally grown ~300 nm diameter and ~3.5 μm long ZnO nanorods were coated, using the sol-gel method, with a thin CdxZn1-xO layer having a spatially varying Cd mole fraction, where x ranged from x = 0 to 1. Full control of the emission color, including white emission, was achieved by simply varying the local Cd mole fraction along the single ZnO/CdxZn1-xO nanorod. The continuous variation of the optical band gap energy along the single nanorod was visualized using nanoscale confocal absorption spectral imaging.1111sciescopu