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Emil S. Koteles B. Elman P. Melman J. Y. Chi C. A. Armiento 《Optical and Quantum Electronics》1991,23(7):S779-S787
The effect of rapid thermal annealing (RTA) on the shapes of GaAs/AlGaAs quantum wells (QWs) has been investigated by monitoring exciton energies using low temperature photoluminescence and photoluminescence excitation spectroscopies. After RTA, large changes in exciton energies were observed only in regions of the samples in which excess surface vacancies were generated, either by capping with a thin layer of SiO2 or by low-energy ion implantation. These changes were interpreted as resulting from modifications of the shapes of the as-grown QWs from abrupt or square to gradual (rounded) due to enhanced interdiffusion of well/barrier atoms. For single QWs there was an increase in exciton energy whose magnitude depended on the width of the well, its distance from the surface of the wafer, the annealing temperature and the total number of surface vacancies available. From studies of coupled QWs, there was clear evidence of asymmetry in the heterostructure after RTA. Although both techniques of vacancy generation yield substantial QW shape modifications, the ion implantation technique has the advantages of being highly reproducible and of being compatible with any material system. 相似文献
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Shams El-Dine Karin Faust Theodor W. Güntert Emil Hauser Horst H. A. Linde Sigrid Spengel 《Helvetica chimica acta》1979,62(4):1283-1287
Synthesis of 3β-acetoxy-24-aza-24-desoxa-xymalogenin Starting from the pregnenolone-derivative 1 we report the synthesis of the possibly cardiac-active compound 24-aza-24-desoxa-xysmalogenin 15 . 相似文献
187.
Two new correlation coefficients are introduced, that characterize the correlations between the spatial frequency components of the source distribution and of the field distribution respectively, in the plane of a two-dimensional source. It is then shown that except possibly for a simple phase factor depending on the magnitudes of the spatial frequencies, the two correlation coefficients are equal to each other. It is further shown that the degree of spatial coherence of the far field generated by the source is equal, except possibly for a simple geometrical phase factor, to either of the two new correlation coefficients relating to the distributions in the source plane. 相似文献
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Schmidt JA Lobkovsky EB Coates GW 《Journal of the American Chemical Society》2005,127(32):11426-11435
The development of a highly active and selective porphyrin-based epoxide carbonylation catalyst, [(OEP)Cr(THF)2][Co(CO)4] (1; OEP = octaethylporphyrinato; THF = tetrahydrofuran), is detailed. Complex 1 is a separated ion pair composed of a tetracarbonylcobaltate anion and an octahedral chromium porphyrin complex axially ligated by two THF ligands. Regarding the carbonylation of epoxides to beta-lactones, catalyst 1 exhibits excellent turnover numbers (up to 10,000) and turnover frequencies (up to 1670 h(-1)), with regioselective carbonyl insertion occurring between the oxygen and the sterically less hindered carbon of the epoxide substrate. Complex 1 is highly tolerant of nonprotic functional groups, carbonylating an array of aliphatic and cycloaliphatic epoxides, as well as epoxides with pendant ethers, esters, and amides. With careful control of reaction conditions in the carbonylation of glycidyl esters, the exclusive production of either the beta- or gamma-lactone isomer was achieved. Through analysis of reaction stereochemistry, a mechanism for the formation of gamma-lactone products was proposed. Overall, a broad array of synthetically useful lactones has been synthesized in a rapid and selective fashion by catalytic carbonylation using [(OEP)Cr(THF)2][Co(CO)4]. 相似文献
190.
Emil Hilb 《Mathematische Annalen》1964,78(1):137-170
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