The synthesis of 6,6-difluoroshikimic acid (4) has been achieved in nine steps from the enantiopure diol 9, which is derived from microbial dihydroxylation of iodobenzene. The synthetic strategy has also been demonstrated to be applicable to the preparation of other 6-substituted analogues of shikimic acid. 相似文献
Nonpolar (11-20) GaN films with different basal-plane stacking fault (BSF) densities (determined using transmission electron microscopy) were investigated using X-ray diffraction. Diffuse streaking from I1 and I2 BSFs was observed in reciprocal space maps of the 10-10 and 20-20 reflections. X-ray calibration curves for BSF density determination can be plotted using the diffusely scattered intensity of open detector 10-10 or 20-20 ω-scans measured at a fixed, large separation from the peak maximum. However, ab initio determination of stacking fault densities is not possible due to additional broadening from other defects. Similarly, ω-scan peak widths are poor indicators of BSF densities. 相似文献
High-pressure gas-jet injection of neon and argon is shown to be a simple and robust method to mitigate the deleterious effects of disruptions on the DIII-D tokamak. The gas jet penetrates to the central plasma at its sonic velocity. The deposited species dissipates >95% of the plasma by radiation and substantially reduces mechanical stresses on the vessel caused by poloidal halo currents. The gas-jet species-charge distribution can include >50% fraction neutral species which inhibits runaway electrons. The favorable scaling of this technique to burning fusion plasmas is discussed. 相似文献
An improved method for the assay of trifluoroacetic acid (TFA) in a cyclosporin-like drug substance is presented, based on ion chromatography with suppressed conductivity detection. Column fouling by the drug molecule is avoided by use of a sample preparation method in which the drug substance is precipitated at alkaline pH whilst the TFA remains in solution. The new method requires a smaller sample mass than a previous method based on headspace-GC-FID whilst achieving an improvement in sensitivity. During validation, the method's performance was found to be consistent with usual acceptance criteria, and the method was found to be robust in routine use. 相似文献
We have investigated the optical properties of a series of non‐polar InGaN/GaN single quantum wells (QWs) grown on r‐plane sapphire with QW widths ranging from 7 to 45 Å. The emission is attributed to recombination in regions of the QW intersected by basal plane stacking faults. For QW widths < 30 Å, the linewidth of both the photoluminescence (PL) spectrum and the QW exciton measured in PL excitation reduces with decreasing QW width, effects which are attributed to the increasing penetration of the exciton wavefunction into the GaN barriers. Furthermore, the associated increase in confinement energy is proposed to induce a delocalisation of the exciton within the plane of the QW/basal plane stacking fault (BSF), which results in an increased temperature sensitivity of the integrated emission intensity for QWs narrower than 22 Å. 相似文献
The simulation, fabrication and optical characterization of InGaN/GaN MQW‐LEDs grown by MOVPE over embedded photonic quasi‐crystals (PQCs) are reported. Fully coalesced GaN layers as thin as 140 nm were grown over 1200 nm high air‐gap PQCs using an intermittent pulsed/normal growth method. Simulations and angle‐resolved photoluminescence measurements reveal there are strong interactions between the embedded PQC and a wide range of trapped modes as well as the dominant low order mode. The interaction with a dominant low order mode is most pronounced when the LED cavity above the embedded PQC has fewer guided modes.
The effects of a KOH treatment on the properties of n-type GaN surfaces and associated Au/n-GaN contacts have been investigated by X-ray photoelectron spectroscopy, atomic force microscopy, reflection high-energy-electron diffraction, current–voltage and electron-beam-induced current characterization. Ga-polar surfaces grown by molecular beam epitaxy and metal–organic chemical vapour deposition were compared. A decrease in electron barrier height and an increase in non-radiative recombination properties of Au/n-GaN contacts were found with KOH treatment, correlated with an increase of surface Ga vacancies, an increase in surface N–H2 content and a decrease in surface C contamination. A 0.3-eV shift in the Ga3d peak position towards the valence band and a reduction in the dislocation contrast were observed for the case of molecular-beam-epitaxy-grown GaN only, demonstrating that surface Ga vacancies and threading dislocations play only a limited role in defining the resultant metal/GaN contact properties. Accordingly, the surface atomic content and the resulting surface states, following KOH treatment, should be taken into consideration when appraising the electrical properties of n-GaN surfaces and the performance of associated metallic contacts. 相似文献
