Showing papers by "Zhen Li published in 2004"

Enhancement Effect of Illumination on the Photoluminescence of Water-Soluble CdTe Nanocrystals: Toward Highly Fluorescent CdTe/CdS Core−Shell Structure

Abstract: A strong photoluminescence enhancement effect of illumination on CdTe nanocrystals stabilized by thioglycolic acid (TGA) was observed. Under optimized conditions, the fluorescence quantum yield of CdTe nanocrystals in aqueous solution reached 85% at room temperature. Optical measurements indicated that the photodegradation of TGA rather than NCs was the main reason for the PL enhancement effect since sulfide ions released from TGA during illumination coordinated with cadmium ions on the surface of CdTe and formed a shell structure of CdS there. X-ray photoelectron spectroscopic results further proved the formation of CdS shell on the CdTe core during illumination. The colloidal solution of CdTe/CdS core−shell nanocrystals with photoluminescence quantum yield higher than 80% turned out to be very stable. Therefore, the current investigations not only reveal the PL enhancement effect of illumination, but also provide a useful synthetic route for producing water-soluble and highly fluorescent CdTe/CdS core−shell nanocrystals

One-Pot Reaction to Synthesize Water-Soluble Magnetite Nanocrystals

Abstract: Water-soluble magnetite nanocrystals of different sizes have been prepared by a one-pot reaction through thermal decomposition of ferric triacetylacetonate in 2-pyrrolidone.

Effect of the properties of the polymer materials on the quality of selective laser sintering parts

Abstract: The effect of the properties of the polymer materials, such as molecular weight, molten viscosity, crystallization rate and the particle size of the powder, on the quality of selective laser sintering (SLS) parts is researched. The results indicate that the molecular weight affects the quality of the SLS parts through the melting viscosity. SLS parts of higher density can be fabricated with polymer materials of lower melting viscosity. Crystallinity largely affects the precision of the SLS part—shrinkage is more serious with increasing crystallinity. SLS parts sintered with polymer powder materials, whose melting peak and crystalline peak differ greatly, have high dimensional precision. The particle size of the powder affects not only the precision but also the density of the SLS part. The appropriate particle size is about 75-100 μm.

A New Postfunctional Approach To Prepare Second-Order Nonlinear Optical Polyphophazenes Containing Sulfonyl-Based Chromophore

Abstract: A new synthetic strategy was developed to prepare polyphosphazenes with second-order nonlinear optical chromophore in which sulfonyl groups are the acceptors. Thus, polyphosphazenes P1 and P2 that contain aniline or indole groups as side chains were obtained from a highly reactive macromolecular intermediate, poly(dichlorophosphazene), by nucleophilic substitution reaction. Then a post-azo coupling of p-ethylsulfonylbenzenediazonium fluoroborate or p-octylsulfonylbenzenediazonium fluoroborate toward the aniline or indole ring in P1 and P2 afforded the sulfonyl-based chromophore-functionalized polyphosphazenes P3, P4, and P5. The polymers exhibit good solubility in common organic solvents and are thermally stable. The maximum absorption appeared at about 440 nm in P3, while that of P4 was at about 393 nm, which were blue-shifted about 52 and 32 nm, respectively, compared to the corresponding chromophores with nitro acceptor and resulted in a wider transparency window. The poled films of P3 and P4 exhibits ...

Enhancement Effect of Illumination on the Photoluminescence of Water-Soluble CdTe Nanocrystals: Toward Highly Fluorescent CdTe/CdS Core—Shell Structure.

Abstract: A strong photoluminescence enhancement effect of illumination on CdTe nanocrystals stabilized by thioglycolic acid (TGA) was observed. Under optimized conditions, the fluorescence quantum yield of CdTe nanocrystals in aqueous solution reached 85% at room temperature. Optical measurements indicated that the photodegradation of TGA rather than NCs was the main reason for the PL enhancement effect since sulfide ions released from TGA during illumination coordinated with cadmium ions on the surface of CdTe and formed a shell structure of CdS there. X-ray photoelectron spectroscopic results further proved the formation of CdS shell on the CdTe core during illumination. The colloidal solution of CdTe/CdS core−shell nanocrystals with photoluminescence quantum yield higher than 80% turned out to be very stable. Therefore, the current investigations not only reveal the PL enhancement effect of illumination, but also provide a useful synthetic route for producing water-soluble and highly fluorescent CdTe/CdS core−shell nanocrystals

Change of bacterial community in the Malan ice core and its relation to climate and environment

Abstract: In order to understand the relationship between the community structure of bacteria in ice core and the past climate and environment, we initiated the study on the microorganisms in the three selected ice samples from the Malan ice core drilled from the Tibetan Plateau. The 16S ribosomal DNA (rDNA) molecules were directly amplified from the melt water samples, and three 16S rDNA clone libraries were established. Among 94 positive clones, eleven clones with unique restriction pattern were used for partial sequence and compared with eight reported sequences from the same ice core. The phylotypes were divided into 5 groups: alpha, beta, gamma proteobacteria; CFB, and other eubacteria group. Among them, there were many “typical Malan glacial bacteria” pertaining to psychrophilies and new bacteria found in the ice core. At a longer time scale, the concentration distribution of “typical Malan glacial bacteria” with depth showed negative correlation with temperature variations and was coincident with dirty layer. It implied the influence of temperature on the microbial record through impact on the concentrations of the “typical Malan glacial bacteria”. In addition, the nutrition contained in ice was another important factor controlling the distribution of microbial population in ice core section. Moreover, the result displayed an apparent layer distribution of bacterial community in the ice core section, which reflected the microbial response to the past climatic and environmental conditions at the time of deposition.

Synthesis and structural characterization of a new polysiloxane with second‐order nonlinear optical effect

Abstract: A new polysiloxane (P2), with a high density of the indole-based chromophore and carbazolyl side groups, was prepared. Thus a polysiloxane (P1), with indole and carbazolyl groups as side chains, was first synthesized through a hydrosilylation reaction, and then the post-azo coupling of p-nitrobenzenediazonium fluoroborate toward the indole ring gave the mulifunctional indole-based chromophore-functionalized polysiloxane (P2). Molecular structural characterization for the polymers was determined by 1H-NMR, IR, and UV–visible spectra, gel permeation chromatography, and differential scanning calorimetry. The polymers were easily soluble in common organic solvents. The poled film of P2 revealed a resonant d33 value of 27 pm/V by second-harmonic generation measurements. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 769–774, 2004

Synthesis of C60‐containing polyphosphazenes from a new reactive macromolecular intermediate: Polyphophazene azides

Abstract: A novel synthetic strategy was developed to prepare polyphosphazenes containing C60 moieties as side chains. Thus, a new reactive macromolecular intermediate, polyphosphazene azides (P1), was obtained from poly(dichlorophosphazene) by the direct nucleophilic substitution reaction. Then the azide group in P1 reacted with C60 molecules to afford the first example of C60-containing polyphosphazenes (P2 and P3). The polymers are soluble in common organic solvents. Molecular structural characterization for the polymers was presented by 1H NMR, 13C NMR, IR, ultraviolet–visible spectra, and gel permeation chromatography. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 194–199, 2004

New approaches for the synthesis of hindered C60-Containing polyphosphazenes via functionalized intermediates

Abstract: Two new approaches were developed to synthesize C60-containing polyphosphazenes. Accordingly, two new reactive macromolecular intermediates (P4 and P8) were obtained from poly(dichlorophosphazene) by the direct nucleophilic substitution reaction. In one approach, a predesigned amimo end–functionalized polyphosphazene (P4) was prepared and then reacted with C60 molecules in chlorobenzene to yield C60-containing polyphosphazene; in the other approach, a polyphosphazene containing 4-methyl phenoxy groups as side chains was first prepared, and then part of the 4-methyl groups were converted to azidomethyl groups (in P8), which reacted with C60 to yield C60-containing polyphosphazene. The polymers were characterized by 1H NMR, 13C NMR, IR, and UV–visible spectra and by gel permeation chromatography. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2877–2885, 2004

Soil moisture measurement and retrieval using Envisat ASAR imagery

Abstract: High resolution data of the ASAR sensor aboard ESA's Envisat satellite offers the opportunity for monitoring surface soil moisture at high spatial resolution with multi-polarization and multi-incidence capabilities. Using Integral Equation Model (IEM), we simulated the backscattering coefficients of C band SAR backscatter at different incidence angles and the alternating polarization mode, and analyzed the sensitivity to soil moisture. The algorithm of inferring surface soil moisture from radar backscatter is developed according to semi-empirical model from the results of simulation and sensitivity analysis. The soil moisture map are retrieved from ASAR backscatter measurements in Hetian, Xinjinag (37/spl deg/15'N, 79/spl deg/48'E) area, where are the arid and semi-arid area of west China. Accuracy in the retrieval of the surface moisture content from ASAR is compared to the spatially distributed ground truth. The results demonstrate the effects of soil moisture monitoring using multi-polarization multi-incidence ASAR data.

New postfunctional method to synthesize C60-Containing poly(ethylene oxide)

Abstract: This article reports the detailed synthesis and structural characterization of two new C60-containing poly(ethylene oxide)s by a new postfunctional method. A predesigned amino end-functionalized poly(ethylene oxide) was prepared, and this reactive macromolecular intermediate was reacted with C60 molecules in chlorobenzene to yield the C60-containing poly(ethylene oxide)s. The molecular structures of the polymers were characterized with 1H-NMR, IR, and ultraviolet–visible spectra. The polymers exhibited good solubility in common organic solvents and water and were stable in air. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 867–870, 2004

Magneto-Optical and Magnetic Properties of DySmCo/Cr Thin Films

Abstract: SmDyCo/Cr films have been prepared by an r.f magnetron sputtering system. The Cr underlayer was found to play an important role in increasing the coercivity (Hc) in the DyCo films because of the formation of granular structure. The effects of Dy substituted by Sm on the magnetic and magneto-optical properties of DyCo/Cr films have been investigated. By analyzing the figure of Ms*Hc vs. Sm concentration, optimum Sm concentration should be found between 16-22 at% to obtain maximum Ms*Hc. With increasing Sm concentration, the Ms is steadily increased, while the He is slightly decreased. With the increasing of Sm concentration, the reflectivity (R) and Kerr rotation angle (θAk) are increased. It is believed that by choosing optimal sputtering conditions and suitable Sm substituting fraction, larger Ms and higher He can be obtained at room temperature for (SmxDy1-x)36Co64 films.

The effects of vegetation in soil moisture retrieval using microwave radiometer data

Abstract: The effects of vegetation are very important in the process of retrieving soil moisture with space borne radiometer. We develop an experimental model, at C-band (about 6.6 GHz), to estimate the effects of vegetation. The developed model is more efficient than the original model and the estimate of the vegetation opacity with the developed model is approximate to the original model. Sets of AMSR-E/Aqua L2A Global Swath Spatially-Resampled Brightness Temperatures (Tb) data, during the SMEX02 period in the watershed region, are used to retrieve soil dielectric constant and the land surface temperature only with the two channels (V- and H-polarization) at 6.92 GHz. The retrieved soil dielectric constant k and land surface temperature Te with the developed model are more approximate to the k and Te measured at WC13/14, and the process takes about 80% CPU-time cost with the developed model than that with the original model. The retrieved results without considering the effects of vegetation vary too significant during the time-sequence, and are not approximate to the synchronic measured data at WC13/14.