Non-monotonous size-dependent photoluminescence and excitonic relaxations in nanostructured ZnO thin films.

The size dependence of room-temperature photoluminescence (PL) accompanied with near-band-edge emission (NBE) and defect-associated green emission (GE) was investigated using high-quality crystalline nanostructured ZnO thin films with grain sizes varying from 29 nm to 2 nm. The size dependence of correlated intensities of the PL bands was pursued in correlation with structural and defect evolution revealed by X-ray photoelectron spectroscopy (XPS) and previous studies of XRD and Raman scattering. In contrast to the influence of thermally activated reconstruction and changes in defect densities, quantum size effects emerging at grain sizes below a critical value, i.e., d c ∼ 10 nm were inspected in relation to the observed blueshift in the bandgap and correlated variations in the size dependence of the intensity of NBE and GE. Taking into account the geometry-modelled relative emission efficiency, (i) the observed overall linear size dependence of the relative intensity I NBE/I GE was consistent with assuming a 1.05 nm-thick GE-active surface layer, and (ii) a local maximum of I NBE/I GE emerging near grain radius R ≈ 4 nm was identified in relation to the theoretically predicted local minimum in size-dependent exciton radiative lifetime due to the intrinsic quantum nature of excitons confined in ZnO. Our results have provided new insights into non-monotonous size-dependent PL of ZnO, which can benefit future photoelectronic device design by taking advantage of the size-controlled stability of confined excitons in nanostructured thin films and luminescent quantum dots.

Phase Diagram and Superconductivity: New Insight into the Fundamentals of InSn Bimetallic Alloys.

We report the crystal structure and superconducting phase diagram for In pSn1- p (0.01 ≤ p ≤ 0.99) bimetallic alloys. A weak electron-phonon coupling was observed in intergranular linked InSn superconductors over an infinite range mediated by high-energy phonons. An enhanced TC(0) ∼ 6.2 K and critical field HC(0) ∼ 2.7 kOe were determined from intermediate (γ-Sn + ß-InSn) composite alloys attributed to internal strain possibly originating from thermal expansion effect of constituent phases.

New Insights into the Role of Weak Electron⁻Phonon Coupling in Nanostructured ZnO Thin Films.

High-quality crystalline nanostructured ZnO thin films were grown on sapphire substrates by reactive sputtering. As-grown and post-annealed films (in air) with various grain sizes (2 to 29 nm) were investigated by scanning electron microscopy, X-ray diffraction, and Raman scattering. The electron⁻phonon coupling (EPC) strength, deduced from the ratio of the second- to the first-order Raman scattering intensity, diminished by reducing the ZnO grain size, which mainly relates to the Fröhlich interactions. Our finding suggests that in the spatially quantum-confined system the low polar nature leads to weak EPC. The outcome of this study is important for the development of nanoscale high-performance optoelectronic devices.

Tuning the Electron-Phonon and Superconducting Coupling in InPb Bimetallic Alloys.

Electron-phonon coupling is a fundamental inelastic interaction in solid-state physics and superconductors. Here we probe electron-phonon and superconducting coupling strength by tuning the indium composition in In qPb1- q superconducting bimetallic alloys. A crossover from weak to strong coupling strength was observed through the analysis of crystal structure and superconducting phase diagrams. The hole-doped Pb solid solution showed suppressed TC and subverted coupling strength due to decreased density of state N(0) and hardened phonons, while the electron-doped In solid solution revealed enhanced TC and stronger coupling strength because of increased N(0) and softened phonons. Our results are in agreement with the Ginzburg-Landau theory calculations regarding the dirty limits and are well described by the Allen and Dynes formula within the framework of the McMillan formalism.

Short-Range Correlated Magnetic Core-Shell CrO2/Cr2O3 Nanorods: Experimental Observations and Theoretical Considerations.

With the evolution of synthesis and the critical characterization of core-shell nanostructures, short-range magnetic correlation is of prime interest in employing their properties to develop novel devices and widespread applications. In this regard, a novel approach of the magnetic core-shell saturated magnetization (CSSM) cylinder model solely based on the contribution of saturated magnetization in one-dimensional CrO2/Cr2O3 core-shell nanorods (NRs) has been developed and applied for the determination of core-diameter and shell-thickness. The nanosized effect leads to a short-range magnetic correlation of ferromagnetic core-CrO2 extracted from CSSM, which can be explained using finite size scaling method. The outcome of this study is important in terms of utilizing magnetic properties for the critical characterization of core-shell nanomagnetic materials.

In Situ Confocal Raman Mapping Study of a Single Ti-Assisted ZnO Nanowire.

In this work, we succeeded in preparing in-plane zinc oxide nanowires using a Ti-grid assisted by the chemical vapor deposition method. Optical spatial mapping of the Confocal Raman spectra was used to investigate the phonon and geometric properties of a single ZnO nanowire. The local optical results reveal a red shift in the non-polar E2 high frequency mode and width broadening along the growth direction, reflecting quantum-confinement in the radial direction.