Lijia Yu

Influence of different types of threading dislocations on the carrier mobility and photoluminescence in epitaxial GaN

High-resolution x-ray diffraction has been used to analyze the type and density of threading dislocations in a series (0001)-oriented GaN epitaxial film. Photoluminescence (PL) and carrier mobility of the films are measured at room temperature. The intensities of both the band edge (3.42 eV) peak and yellow luminescence (YL) are strongly related to the threading dislocation density of the GaN films. But different types of dislocations show different relationship with the intensities of PL and YL. The fundamental correlation is found not only between the interaction of edge- and screw-type dislocations and the carrier mobility but also between the interaction and the intensities of both the band edge peak and the YL.

Relationship between combinative energy and superconductivity in Tl2Ba2Can−1CunO2n+4

Abstract The non-monotonic relationship of T c with the number of Cu–O planes ( n ) per unit cell for compounds of Tl 2 Ba 2 Ca n −1 Cu n O 2 n +4 ( n =1, 2, 3, and 4) is investigated from the perspective of cohesive energy. The unit cell of the Tl superconductors is treated as two blocks: the perovskite block where the Cu–O planes are located and the rock salt block, which is considered as a charge-reservoir. We proposed a model to calculate the combinative energy of the two blocks. It is found that the combinative energy between the two blocks is closely related to the value of T c . The relation demonstrates an interesting way to understand the nonlinear change of T c with the number of Cu–O planes in the layered superconductors. This means that the interaction between the two blocks plays an important role in superconductivity.

Interaction of YBCO thin film and substrates and its effects on the quality of the film

Abstract YBCO thin films grown on SrTiO 3 , yttria-stabilized zirconia (YSZ), and YSZ with a CeO 2 buffer layer, by pulsed laser deposition, were studied by high resolution X-ray diffraction, including rocking curves, and Φ-scans. It is shown that the distortion of the thin films is different in the different substrates. The lattice mismatch between them can partially explain this. The larger mismatch corresponds to the larger distortion of the films, which can be determined by the full width at half maximum (FWHM) in the rocking curve and Φ-scan curve. As the mismatch between the film and substrate increases, the FWHM of both rocking curve and Φ-scan curve increases simultaneously. But, the YBCO on the YSZ with CeO 2 buffer layer does not obey this regulation. The careful calculation of the cohesive energy of the thin films and substrates, especially considering the effect of the interaction of the YBCO and substrates, indicates there is a close relationship between the cohesive energy and the distortion of the films. Using this to explain the distortion is better than using mismatch.

Two phase transitions in Zn-doped YBCO superconductor

Abstract A series polycrystalline samples of YBa 2 Cu 3− x Zn x O 7− δ ( x =0.05, 0.10, 0.15, 0.20, and 0.25, respectively) were studied by means of differential scanning calorimetry, thermogravimetry, X-ray diffraction, and resistance–temperature measurement. It is found that there are two unknown subtle phase transitions in each compound before melting. At about 430°C and 800°C the samples show an endothermal anomaly and a weight loss, respectively. The amount of Zn-dopant affects the phase transitions obviously. Careful analysis of the experimental results demonstrates that two-phase transitions come out of different structural blocks, perovskite and rock salt, in the compounds.

Circular siRNAs for Reducing Off-Target Effects and Enhancing Long-Term Gene Silencing in Cells and Mice

Circular non-coding RNAs are found to play important roles in biology but are still relatively unexplored as a structural motif for chemically regulating gene function. Here, we investigated whether small interfering RNA (siRNA) with a circular structure can circumvent off-target gene silencing, a problem often observed with standard linear duplex siRNA. In the present work, we, for the first time, synthesized a series of circular siRNAs by cyclizing two ends of a single-stranded RNA (sense or antisense strand) to construct circular siRNAs that were more resistant to enzymatic degradation. Gene silencing of GFP and luciferase was successfully achieved using these circular siRNAs with circular sense strand RNAs and their complementary linear antisense strand RNAs. The off-target effect of sense strand RNAs was evaluated and no cross off-target effects were observed. In addition, we successfully achieved longer gene-silencing efficiency in mice with circular siRNAs than with linear siRNAs. These results indicate the promise of circular siRNAs for overcoming off-target effects of siRNAs and enhancing the possible long-term effect of siRNA gene silencing in basic research and drug development.

Synthesis and Evaluation of Caged siRNA with Terminal Single Vitamin E Modification.

RNA‐induced gene silencing has been widely applied as a powerful research tool in drug development due to its sequence‐specific degradation of target mRNA. Conditional regulation of gene functions with small interfering RNAs (siRNAs) is highly useful, especially when specific gene expression regulation with spatiotemporal resolution and amplitude is desired. Here, the synthesis of a series of new caged siRNAs with vitamin E (vitE) modification and/or a single photolabile linker at the 5′ terminal is described. Their capability of photolysis was investigated by PAGE gel analysis. Then, a dual reporter firefly/renilla luciferase assay with siQuant vectors and GFP/RFP reporter genes was applied to show the effect of vitE‐modified caged and non‐caged siRNAs on gene expression. The intracellular distribution and cellular uptake pathways of caged siRNAs are also discussed.