G.-X. Yang's research while affiliated with China Academy of Engineering Physics and other places

The effects of stacking periodicity on the electronic and optical properties of GaAs/AlAs superlattice have been explored by density functional theory calculations. Among the (GaAs)m/(AlAs)m, (GaAs)1/(AlAs)m and (GaAs)m/(AlAs)1 (m = 1 to 5) superlattices, the band gaps of (GaAs)m/(AlAs)1 superlattices decrease significantly as the layer of GaAs increases, and the cut-off wavelengths are found to locate in the near infrared region. For (GaAs)m/(AlAs)1 SLs, the conduction bands shift toward Fermi level, resulting in the smaller band gap, while conduction bands of (GaAs)1/(AlAs)n SLs slightly shift to higher energy, which lead to comparable band gaps. The layer number of GaAs shows negligible effects on the reflectivity spectra of superlattice structures, while the absorption coefficient shows a red-shift with the increasing layer of GaAs, which is beneficial for the application of GaAs/AlAs superlattice in the field of near infrared detector. These results demonstrate that controlling the number of GaAs layers is a good method to engineer the optoelectronic properties of GaAs/AlAs superlattice.

Ferroelectric BiFeO3 is promising for photovoltaic applications, especially in regard to the exploitation of ferroelectric photovoltaic effects for charge separation. However, its large band gap limits efficient sunlight absorption. Here, we demonstrate a new strategy to effectively tune the band gap of tetragonal BiFeO3 via superlattice structuring with the ferroelectric BiCrO3. The (BiCrO3)m/(BiFeO3)n superlattices are found to exhibit conventional ferroelectric properties, but low fundamental band gaps, smaller than either of the parent materials. First-principles calculations reveal that the unexpected band-gap reduction is induced by charge reconstruction due to lattice strain, octahedral distortion, and polarization discontinuity at the BiCrO3-BiFeO3 interfaces. Ultimately, these results provide a new strategy, in the form of superlattice structuring, which could open the door to the creation of efficient ferroelectric photovoltaics.

In this study, the low energy radiation responses of AlAs, GaAs and GaAs/AlAs superlattice are simulated and the radiation damage effects on their electronic structures are investigated. It is found that the threshold displacement energies for AlAs are generally larger than those for GaAs, i.e., the atoms in AlAs are more difficult to be displaced than those in GaAs under radiation environment. As for GaAs/AlAs superlattice, the Ga and Al atoms are more susceptible to the radiation than those in the bulk AlAs and GaAs, whereas the As atoms need comparable or much larger energies to be displaced than those in the bulk states. The created defects are generally Frenkel pairs, and a few antisite defects are also created in the superlattice structure. The created defects are found to have profound effects on the electronic properties of GaAs/AlAs superlattice, in which charge transfer, redistribution and even accumulation take place, and band gap narrowing and even metallicity are induced in some cases. This study shows that it is necessary to enhance the radiation tolerance of GaAs/AlAs superlattice to improve their performance under irradiation.

Sapphire samples have been implanted successively by helium ions with a series of energies to obtain a uniform layer of impurities in the range of 80-200 nm beneath the surface at room temperature. After helium ion implantation, the surface morphology has been greatly changed. In addition, two broad absorption bands at 360 nm and 780 nm are observed and their intensities significantly increase. An infrared band shifts from 782 cm-1 to 760 cm-1 and the band obviously broadens. Moreover, a luminescence band at 330 nm (3.8 eV) is associated with the 2P?1S∗ transition of the F+ centers. After laser irradiation, the laser damage morphologies of samples before and after ion implantation are presented. An obvious degradation of laser induced damage threshold (LIDT) is observed and the mechanism for the degradation of LIDT is discussed.

Objective: To separate and purify an isomer(Rap-T) which is company with rapamycin observed on HPLC analysis and its conversion product. Their chemical structures were identified. Methods: Rap-T and its conversion product were separated by preparative liquid chromatography. The conversion of Rap-T was in a 50% acetonitrile - water solution at 25°, which was traced by HPLC. The chemical structures were determined by physico-chemical properties and spectral analyses. Results: Rap-T was revealed with oxepane isomer changing from pyrano form of rapamycin. Its conversion product was identified as rapamycin. However, Rap-T and rapamycin can be mutual conversion in a 50% acetonitrile-water solution at 25°C. Conclusion: Rap-T is rapamycin tautomer and both chemicals can be mutual transformation.

The effects of γ-irradiation on potassium dihydrogen phosphate crystals containing arsenic impurities are investigated with different optical diagnostics, including UV-VIS absorption spectroscopy, photo-thermal common-path interferometer and photoluminescence spectroscopy. The optical absorption spectra indicate that a new broad absorption band near 260 nm appears after γ-irradiation. It is found that the intensity of absorption band increases with the increasing irradiation dose and arsenic impurity concentration. The simulation of radiation defects show that this absorption is assigned to the formation of AsO4⁴⁻ centers due to arsenic ions substituting for phosphorus ions. Laser-induced damage threshold test is conducted by using 355 nm nanosecond laser pulses. The correlations between arsenic impurity concentration and laser induced damage threshold are presented. The results indicate that the damage performance of the material decreases with the increasing arsenic impurity concentration. Possible mechanisms of the irradiation-induced defects formation under γ-irradiation of KDP crystals are discussed.

In the research of microbial conversion of rapamycin by 55 strains of Micromonospora sp., among them, 15 strains were found to be capable of transforming rapamycin into various products. By HPLC analyse, it showed of 13 strains could transform rapamycin into 7 products with the same relative retention time. Micromonospora sp. FIM02-848 was selected for further study and classified as Micromomospora echinospora FIM02-848, the strain could transform rapamycin into 7 products. One converted product(RRT 0.92) with molecular weight of 916 was identified with 14-hydrorapamycin. The other converted product(RRT 1.78) with molecular weight of 900 showed it was the same as 14-deoxorapamycin in HPLC, UV and MS. Thus, it was inferred to be 14-deoxorapamycin.

In the course of searching for new rapamycin derivatives by microbial conversion, a microorganism, which was able to biotransform rapamycin into four products, was obtained and classified as Micromonospora chersina FIM03-712. One conversion product from the converted broth of the strain was isolated and purified. By means of physico-chemical properties and spectral analyses, it was identified with 14-deoxorapamycin.

Bacterium strain 287 isolated from the soil samples in Fuzhou, China was identified as Bacillus megaterium based on the taxonomic properties and 16s rRNA gene sequence. Bacillus megaterium 287 biotransformed sirolimus to three compounds: 287-P1, 287-P2 and 287-P3. Among them, 287-P1 was identified as 29,42-bis-O-demethylrapamycin by physico-chemical properties and spectral analyses.