Search

For decades, problems of parasitic emissions have been ubiquitously encountered in nearly all deep ultraviolet light-emitting diodes (DUV-LEDs). In this work, 450 nm parasitic peaks in 275 nm AlGaN DUV-LEDs have been studied in details. Upon careful comparisons and analyses on the electroluminescence and photoluminescence spectra at various injection levels and different temperatures, we have discovered a mechanism of exciton-assisted radiative recombination, namely, the reinforcement on radiative recombination via other impurity-trap levels (ITLs) by excitons that are generated in the midst of the band gap. For DUV-LED samples under investigation herein, a system of radiative ITLs within the band gap cannot be neglected. It includes two types of impurities located at two different energy levels, 3.80 eV and 2.75 eV, respectively. The former, establishing a sub-band edge, which behaves like an energy entrance to this system, contains a series of hydrogen-like excitons at a temperature lower than 100 K, which behaves like an energy entrance to this system. On the one hand, these excitons absorb carriers from band-edge and reduce the band-edge recombination. On the other hand they transfer the energy to lower impurity levels, enhancing the radiative recombination and giving rise to the 450 nm parasitic peak. [ABSTRACT FROM AUTHOR]

During the past several decades, inexpensive compact nuclear magnetic resonance (NMR) instruments have been widely used for on-site detections of chemical identity and sample analyses in industrial applications. In general, via shim coils and control strategies, automatic search shimming methods are capable of improving the magnetic field uniformity so that optimized signals can be acquired and particularly suited for rapid detections in compact NMR instruments. However, because these methods inherently endure the inefficiency in multidimensional shims and the sensitivity of starting points, it is time-consuming to obtain acceptable results in a field infected with distortions and strays. Here, two remedies that improve the shimming speed for the search shimming method are proposed. First, analyzing and compensating for magnetic susceptibility effects, we relocate starting points closely to optima and reduce the complexity of global exploration. Second, modifying the simplex movement with adaptive scale factors and gradient-like centroid, we have improved the convergence of the simplex shimming algorithm in a multidimensional search space. Via theoretical analyses and lab experiments, we find that the proposed shimming method outperforms regular shimming methods in terms of handling complex magnetic fields and reducing shimming times and hope that our study can help further improve NMR shimming methods in the future. [ABSTRACT FROM AUTHOR]

Asymmetric island sidewall growth (AISG) is employed to reduce the threading defect density and to modify the surface/interface properties of semipolar GaN templates and InGaN/GaN quantum wells (QWs). We have found that the microstructures and optical properties of the semipolar InGaN/GaN QWs are well correlated with the threading defect density and surface properties of semipolar growth templates. Without AISG, surface undulation, indium fluctuation, and a relatively high indium content within InGaN layers on conventional GaN templates are observed and correlated with high-density threading defects and the formation of specific microfacets with more N dangling bonds on the surface. With AISG, these semipolar InGaN/GaN QWs on modified GaN templates exhibit a uniform indium distribution in the absence of threading defects. The optical properties of InGaN/GaN QWs are significantly improved, and this improvement is attributed to the surface modification of GaN templates, the reduction in threading defect density, and the higher uniformity of indium distribution within InGaN layers. [ABSTRACT FROM AUTHOR]

The tradeoff between the color rendering index (CRI) and the luminous efficacy (LE) for candle-like three-hump white light-emitting diodes (LEDs), which consist of blue InGaN chip, yellow aluminate phosphors, and red AlGaInP chip, has been investigated. Spectral models have been separately developed for the optimization process after experimentally measuring spectral power distributions of phosphor materials and semiconductor chips. By filtering billions of defective spectral data with a nonlinear program, optimal values of LE of 272 lm/W and CRI of 96 (CRI No.9 of 98) at a correlated color temperature of 2000 K can be achieved, while LE and CRI are 0.1 lm/W and 83 for candles. These three-humps LEDs are capable of achieving excellent photometric and chromatic performances of candle-like white light. Finally, this study can serve as a critical guide for manufacturing high-CRI candle-like white LEDs. [ABSTRACT FROM PUBLISHER]

We propose an experimental method that determines junction temperatures in light-emitting diodes by measuring currents while holding the low forward voltages constant. In this procedure, we first calibrate current-temperature-rela- tionship parameters under the condition of negligible thermal generation. With one of the two parametric values, we discover the existence of a forward voltage peak that yields most sensitive measurements of the junction temperature. Results show a nearly linear relationship between the algorithmic currents and temperature reciprocals with high testing precision. [ABSTRACT FROM PUBLISHER]

Epitaxially-oriented large-scale monolayer molybdenum disulfide (MoS 2 ) grown directly on graphene (Gr) with an atomic-clean interface draws intensive interest in both areas of fundamental sciences and electronic-device applications. In general, however, synthetic characteristics and bandgap-related properties of MoS 2 crystals can be varied by the selection of substrates that support Gr, and mechanisms that govern these phenomena remain unclear. Here, by means of scanning tunneling microscope and spectroscopy (STM/STS) and photoluminescence (PL) measurements, we have discovered that considerably-strained Gr can strongly affect the epitaxy of MoS 2 crystals and enhance the interlayer electronic coupling between monolayer MoS 2 and its Gr-platinum (Pt) substrate, leading to the stack of MoS 2 layers and a narrowing direct bandgap of monolayer MoS 2 . Our studies shed light on growing large-scale MoS 2 films on Gr and the bandgap modulation in these heterostructures. [ABSTRACT FROM AUTHOR]

Other than the well-known sulfurization of molybdate compound to synthesize molybdenum disulfide (MoS 2 ) layers, the dynamic process in the whole crystalline growth from nuclei to triangular domains has been rarely experimentally explored. Here, a competing sulfur-capture principle jointly with strict epitaxial mechanism is first proposed for the initial topography evolution and the final intrinsic highly oriented growth of triangular MoS 2 domains with Mo or S terminations on the graphene (Gr) template. Additionally, potential distributions on MoS 2 domains and bare Gr are presented to be different due to the charge transfer within heterostructures. The findings offer the mechanism of templated growth of 2D transition metal dichalcogenides, and provide general principles in syntheses of vertical 2D heterostructures that can be applied to electronics., (© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)