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The optical materials showing thermally-activated delayed fluorescence (TADF), have been considered to be promising light emitters for developing high-efficiency light-emitting devices due to their unique capability of harvesting both singlet and triplet states. Biocompatible gold nanoclusters (AuNCs) also exhibit TADF in the pristine aqueous solution. It would be useful by further exploiting TADF emission in the solid state for novel bio-friendly light-emitting materials. To this end, here, a facile, low-cost but effective method was used to generate stable and enhanced TADF emission in the solid state under ambient conditions by embedding biocompatible AuNCs into inorganic salt matrices. Temperature-dependent transient photoluminescence (PL) measurements were used to investigate TADF behavior of AuNCs embedded into the solid matrices. As compared with AuNCs in an aqueous solution, the TADF was enhanced for solid AuNCs even under ambient environment. In addition, by using waterproof salt matrices with polymer encapsulants, stable and broadband emission was demonstrated based on all biocompatible nanomaterials under UV-light illumination.

In order to analyze the boiling and capillary limitations of two-phase heat transport devices, the existing models developed by Chi and Peterson and the existing experimental data carried out with various micro channel wick structures from literature were collected for benchmark. It was found that the dominant parameters for boiling and capillary limitations were the nucleation sites and structure geometries of the micro channels, and important parameters were considered to modify the models empirically. It was also found that for micro channel structures the inclined angle is sensitive to the capillary limitations and not to boiling limitations. By properly estimating the nucleation sites and empirical coefficients for micro channels needed by the newly modified models, the boiling and capillary limitations can be accurately predicted, and hence the applicability of the modified models is confirmed. Based on this, a numerical analysis was then carried out to investigate the trends of boiling and capillary limitations of the micro channel wick structures. Effects of the channel geometries and arrangement were taken into account, including the aspect ratio and structure size of the micro channels. Furthermore, the effects of inclined angle and contact angle were also analyzed. The present results can provide a design reference of performance trends of micro channel wick structures.

Vortex motion in superconductors of high-Tc superconducting maglev system is studied by a computational simulation. The vortex system is treated in a similar fashion, as a system where defects are arranged in periodic arrays and driven by an alternating current (AC) field. Computational simulation reveals that due to the high nonlinearity of the vortex-defect interaction, the system shows typical chaotic characters. Effects of the vortex-vortex coupling, the driving frequency, and the vortex viscosity on the vortex motion have been studied to reveal the interaction between the spatial and temporal chaos. It is found that the mean-field approach is a good approximation to describe the vortex motion in one dimensional vortex system under an AC driving field. The vortex motion under the damping mode is a kind of self-organized motion. Lastly, the spatial chaos can dominate the chaotic behavior of the system.

Ac loss behavior in HTS bulk materials is studied by developing an analytic method. The self-heating effect on the distributions of temperature in HTS bulks has been calculated. A critical frequency which sets the upper limit for driving frequency of the external field in sustaining a superconducting state is obtained. Our model provides useful information of understanding the experimental phenomena, including an anomalous ac loss behavior, a retardation of the inner field respect to the external ac field, etc., in HTS bulk materials.

Alloys with the nominal composition Mo-12Si-8.5B (at. %) were prepared by arc-melting and drop casting. They consisted of approximately 38 vol% α-Mo in a brittle matrix of Mo3Si and Mo5SiB2. Screening tests were carried out to assess their oxidation resistance. Their high temperature strength was investigated in tensile creep (constant applied stress) and in compression (constant applied crosshead speed) at temperatures ranging from about 1200 to 1400°C and stresses ranging from 50 to 540 MPa. A phenomenological creep equation describing the creep strength of this material was derived.

Multiple steady-state solutions of natural convection in an inclined enclosure with a fluid layer and a heat-generating porous bed is investigated numerically by the finite volume method. The conservation equations for the porous layer are based on a general flow model which includes both the effects of flow inertia and friction. The flow in fluid layer is modeled by Navier–Stokes equations. The method of pseudo arc-length continuation is adapted in studying the effects of tilt angle on flow pattern and heat transfer. It is found that, in the whole domain of tilt angle, there exist two groups of solutions with quite different flow pattern and heat transfer behavior. The effects of aspect ratio on flow pattern and heat transfer have also been studied.

Mixed convection heat transfer from a vertically moving plate to a flowing free stream is investigated. The plate moves either in parallel or reversely to the free stream; and the buoyancy force accelerates or retards the flow. An universal formulation can be obtained from which similarity and nonsimilarity equations for six limiting cases of forced, natural, and mixed convection can be readily reduced. Accurate finite-difference solutions and comprehensive correlations of heat transfer rate for 0.01 ≤ Pr ≤ 10000 are presented over the entire domains of mixed convection and relative velocity.

The optical properties of (InP)2/(GaP)2 bilayer superlattice (BSL) structures have been examined with linearly polarized cathodoluminescence (CL), time-resolved CL spectroscopy, and cathodoluminescence wavelength imaging. An In and Ga composition modulation of ∼18% forms during the metalorganic chemical vapor deposition growth of short period (InP)2/(GaP)2 bilayer superlattices. Transmission electron microscopy showed a period of ∼800 A along the [110] direction, resulting in coherently strained quantum wires. A strong excitation dependence of the polarization anisotropy and energy of excitonic luminescence from the quantum wires was found. The results are consistent with a phase-space and band filling model that is based on a k⋅p and two dimensional quantum confinement calculation which takes the coherency strain into account. CL images reveal that defects in the BSL originate from the GaAs substrate and/or the initial stages of InGaP growth. The effects of defects on the band filling, carrier relaxation...

We report on (a) the effect of growth interruption on the growth profile evolution in growth on non-planar patterned mesa tops via substrate-encoded size-reducing epitaxy (SESRE) and (b) the optical behavior of isolated 3D-confined GaAs volumes as well as 3D-confined GaAs volumes coupled with 1D-confined quantum wells (QWs) fabricated by SESRE. Steady-state excitation and time-resolved cathodoluminescence (CL) are used for these optical studies.

This paper studies combined heat and mass transfer by laminar natural convection from a vertical plate maintained with uniform surface heat flux and species concentration. Very accurate finite-difference solutions of a set of nonsimilarity equations have been obtained for most practical gaseous solutions (Pr = 0.7, 0.21 ≤ Sc ≤ 2.1) and aqueous solutions (Pr = 7, 140 ≤ Sc ≤ 1400). Variations of heat and mass transfer rates with buoyancy ratio and Lewis number are presented. Precise correlations have been developed for predicting heat and mass transfer rates of natural convection arising from single (solutal or thermal) buoyancy force and dual buoyancy forces.

We have examined carrier thermalization, recombination, and band filling in GaAs/AlGaAs quantum boxes with low-temperature cathodoluminescence (CL). The temperature dependence of the quantum box CL intensity for T⩽ 90 K exhibits an Arrhenius behavior, as a result of carrier thermalization between the quantum box and surrounding barrier regions. The width of the quantum box luminescence is found to increase rapidly with an increasing excitation density and reveals an enhanced phase-space and real-space filling, in comparison to the behavior observed for quantum wells.

This work studied, both theoretically and experimentally, the inclined wall plumes which arise from a line thermal source imbedded at the leading edge of an adiabatic plate with arbitrary tilt angle between 0 and π/12. An appropriate formulation and a very effective numerical scheme are developed to obtain rigorous numerical solutions over the full range of tilt angle from the horizontal to the vertical for fluids of any Prandtl number between 0.001 and 1000. The effects of the tilt angle on the velocity and temperature profiles are presented. A simple, but very accurate, correlation equation is proposed for predicting the wall temperature of the inclined plate over the whole range of tilt angle for 0.001 ⩽ Pr ⩽ 1000. The temperature fields of air over the inclined plates have been visualized by using holographic interferometry. The photographs of interferograms confirm the physical reality of the horizontal and the inclined wall plumes.

The effects of strain‐induced structural defects in a nipi‐doped In0.2Ga0.8As/GaAs multiple‐quantum well sample were studied with time‐resolved electron‐beam‐induced absorption modulation, in which carrier recombination lifetimes and ambipolar diffusion constants are measured with high spatial, spectral, and temporal resolution. Based on a phenomenological model, carrier lifetimes in the limit of weak excitation at room temperature were determined. The lifetime is found to be reduced by a factor of ∼1013 compared to a theoretically calculated value, owing to the presence of strain‐induced defects and alternate recombination channels. By using a two‐dimensional diffusion model, the ambipolar diffusion coefficients Da along high‐symmetry [110], [110], and [100] directions were determined and resulted in an anisotropic behavior such that D[110]a≳D[110]a ≳D[100]a. The anisotropy in diffusion is attributed to corresponding asymmetries in the misfit dislocation density.

The temperature dependence of the cathodoluminescence (CL) originating from In0.21Ga0.79As/GaAs multiple quantum wells has been studied between 86 and 250 K. The CL intensity exhibits an Arrenhius‐type dependence on temperature (T), characterized by two different activation energies. The influence of misfit dislocations and point defects associated with strain relaxation on the thermal quenching of luminescence has been investigated, and the spatial variation in the activation energies has been examined. The CL intensity dependence on temperature for T≲150 K is controlled by thermally activated nonradiative recombination. For T≳150 K the decrease in CL intensity is largely influenced by thermal re‐emission of carriers out of the quantum wells.

Simultaneous heat and mass transfer in buoyancy-induced laminar boundary-layer flow along a vertical plate is studied for any ratio of the solutal buoyancy force to the thermal buoyancy force by using a new similarity transformation. The effects of the buoyancy ratio and Lewis number on the rates of heat and mass transfer are presented explicitly for most practical gaseous solutions (Pr=0.7, 0.21≤Sc≤2.1) and aqueous solutions (Pr=7, 140≤Sc≤1400). Very accurate correlations of the mass transfer and heat transfer rates are developed for the cases of single and combined buoyancy forces.

The nonlinear optical and transport properties of a nipi‐doped InxGa1−xAs/GaAs multiple‐quantum well sample (x=0.23) has been studied using a novel approach called electron‐beam‐induced absorption modulation (EBIA). The absorption in the sample is modulated as a result of screening of the built‐in electric field in the nipi structure due to excess carrier generation. The change in field causes a Stark shift of the first quantized optical transitions in QWs which are situated in the intrinsic layers. In EBIA, a scanning electron probe is used to locally generate an electron–hole plasma that is used to study the spatial distribution of defects that impede excess carrier transport and reduce the lifetime of spatially separated carriers. The Stark shift in the MQW structure is imaged with micrometer‐scale resolution and is compared with cathodoluminescence imaging results which show dark line defects resulting from strain‐induced misfit dislocations. Theoretical calculations using Airy functions in the transf...

Three-dimensionally confined GaAs/AlGaAs and InAs/GaAs structures on 〈100〉 oriented square mesas patterned onto GaAs(001) substrates are realized, in-situ, via size-reducing molecular beam epitaxy. Two stages of mesa top pinch-off involving ∼ {103} and subsequently {101} side facets are revealed. GaAs and InAs quantum boxes with lateral linear dimensions down to 40 nm and confined by AlGaAs and GaAs, respectively, are reported. For InAs, the strain relief in mesas is found to enhance the well known ∼ 2 ML thickness for three-dimensional island formation on unpatterned substrates to, remarkably, > 5 ML for mesa size ∼ 75 nm. Cathodoluminescence emission from the InAs on the mesa top attests to its good optical quality.

We have studied the planar Hall effect (PHE) in a Mn $\ensuremath{\delta}$-doped GaAs-based heterostructure consisting of Mn $\ensuremath{\delta}$-doped GaAs and $p$-type AlGaAs. We observe a distinct and large PHE and a specific in-plane [110] uniaxial magnetic anisotropy below the ferromagnetic transition temperature $({T}_{\mathrm{C}})$. This uniaxial in-plane magnetic anisotropy is found dominant over the biaxial cubic anisotropy, and is clearly identified by the angular dependence of PHE. This observation is quantitatively discussed in terms of two models of magnetization reversal: the coherent rotation model and the domain-wall motion model. The model calculation fit to the experimental data makes it possible to determine the uniaxial and cubic anisotropy fields and examine the behavior of magnetization reversal.

Helium was implanted in type 316 stainless steel, through tritium decay, to levels of 0. 18, 2. 5, 27, 105, and 256 atomic parts per million (appm). Bead-on-sheet welds were then made using the gas tungsten arc (GTA) process. Intergranular cracking occurred in the heat-affected zones (HAZs) of specimens with helium concentrations equal to or greater than 2.5 appm. No such cracking was observed in helium-free control specimens or in specimens containing the lowest helium concentration. In addition to the HAZ cracking, brittle, centerline cracking occurred in the fusion zone of specimens containing 105 and 256 appm helium. Transmission and scanning electron microscopy results indicated that both the HAZ cracking and centerline cracking in the fusion zone resulted from the stress-induced growth and coalescence of cavities initiated at helium bubbles on interfaces. For the HAZ case, the cavity growth rate is modeled and shown to predict the experimentally measured 1-second time lag between peak weld temperature and the onset of cracking.

We use the scanning SQUID microscope (SSM) to image the vortices in superconducting FeSe0.3Te0.7 (FST) thin films. The observed peak flux value of FST is nearly a quarter of that of an accompanying Nb film. We developed a method for quantitatively determining the London penetration depth of the FST film from the known value of Nb. The obtained value, 0.88 μm, is significantly larger than those obtained from single crystals of similar compositions by using other methods. The methodology developed for this study is useful for measuring London penetration depths of thin-film superconductors in general.

The influence of strain‐induced defects on the ambipolar diffusive transport of excess electrons and holes in the δ‐doped InGaAs/GaAs multiple quantum well system has been examined with a new technique called electron‐beam‐induced absorption modulation (EBIA). The excess carrier lifetime and diffusion coefficient are obtained by a one‐dimensional diffusion experiment that utilizes EBIA. An anisotropy in the ambipolar diffusion along both high‐symmetry 〈110〉 directions is found, and this is seen to correlate with the distribution of dark line defects observed in cathodoluminescence.

Linearly polarized cathodoluminescence (LPCL) imaging and spectroscopy techniques have been employed to examine the optical properties and homogeneity of (InP)2/(GaP)2 bilayer superlattice (BSL) structures which exhibit a lateral composition modulation that leads to the formation of quantum wires. LPCL spectra were measured for various temperature and electron beam excitation densities. The magnitude of the polarization anisotropy and spectral lineshape are found to depend sensitively on the excitation conditions, revealing large nonlinear optical effects in these samples. CL images reveal that defects in the bilayer superlattice structure originate from the GaAs substrate or the initial stages of InGaP growth.

We have examined the influence of strain relaxation on the excitonic recombination and diffusion in In0.2Ga0.8As/AlxGa1−xAs quantum‐well (QW) samples designed for high‐electron‐mobility transistors, using spectrally and spatially resolved polarized cathodoluminescence (CL). Six molecular‐beam epitaxial grown samples, with varying channel thicknesses ranging from 75 to 300 A, were examined at various temperatures between 87 and 300 K. An increase in misfit dislocation density occurred with increasing channel thicknesses and resulted in changes in the dark line defect (DLD) density, polarization anisotropy, QW excitonic luminescence energy, and luminescence activation energy, as observed in CL. The influence of misfit dislocations on the ambipolar diffusion of excess carriers in a direction parallel to the dislocation line, in varying proximity to the DLDs, was examined with a CL‐based diffusion experiment. The temperature dependence of the CL imaging was examined, enabling a study of the spatial variation ...

Local variations in the optical properties of thick In0.13Ga0.87As films grown on GaAs(001) substrates misoriented toward {111} planes have been studied with polarized and spectrally‐resolved cathodoluminescence (CL) imaging. The degree of anisotropic relaxation and density of dark line defects (DLDs) in CL was found to depend on the choice of the substrate miscut orientation. An enhanced anisotropy in DLD density and strain relaxation was found for a misorientation towards (111)A relative to that for a misorientation towards (111)B. Local variations and spatial correlations in polarization anisotropy, band‐gap energy shifts, luminescence efficiency, and defect‐induced long‐wavelength luminescence were examined.

The effects of excess carrier generation on excitonic absorption phenomena in nipi‐doped In0.2Ga0.8As/GaAs multiple quantum wells (MQWs) was examined using a novel technique called electron beam‐induced absorption modulation imaging. The nipi‐doping induced barrier height is determined by measuring the frequency response of the absorption modulation as a function of temperature and employing a model which is based on thermal excitation of carriers in the limit of Boltzmann statistics. Spatial steps in the absorption modulation which correlate with the positions and orientation of dark line defects imaged in cathodoluminescence are observed. These results indicate the existence of defects in the MQWs which impede the ambipolar diffusive transport of the spatially separated electron‐hole plasma.