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Langmuir 2007, 23, 850-854
Synthesis of Nanocrystal-Polymer Transparent Hybrids via Polyurethane Matrix Grafted onto Functionalized CdS Nanocrystals Su Chen,* Jia Zhu, Yongfeng Shen, Chunhui Hu, and Li Chen College of Chemistry and Chemical Engineering and Key Laboratory of Material-Oriented Chemical Engineering of Jiang Su ProVince and Ministry of Education, Nanjing UniVersity of Technology, No. 5 Xin Mofan Rd., Nanjing 210009, PR China ReceiVed July 27, 2006. In Final Form: September 30, 2006 We reported the first synthesis of CdS nanocrystal-polymer transparent hybrids by using polyurethane (PU) grafted onto quantum dots (QDs) CdS nanocrystals. In a typical run, the appropriate amounts of cadmium chloride (CdCl2) and sodium sulfide (Na2S) in the presence of 2-mercaptoethanol (ME) as the organic ligand are well dispersed in H2O/DMF solution without any aggregation. From a combination of transmission electron microscopy (TEM) and a computing method of Brus’s model according to UV-vis absorption spectra, the particle size of as-prepared hydroxylcoated CdS nanocrystals is about 5 nm. Then, PU-CdS transparent nanocomposites hybrids were synthesized by a two-step reaction. The effect of the different ratios of ME/Cd2+ and H2O/DMF on the resulting particle size of CdS nanocrystals was investigated by UV-vis absorption measurements. FT-IR and TGA characterizations indicate the formation of robust bonding between CdS nanocrystals and the organic ligand. The fluorescence measurement shows that CdS-PU hybrids exhibit good optical properties.
Introduction Recently, nanocrystals have been a rapidly growing research area due to their strong size-dependent properties and special optical and electronic features.1 In particular, semiconductor nanocrystals exhibit novel thermodynamic, optical, electrical, and magnetic properties, which are strongly dependent on the cluster shape and size.2 As a typical semiconductor material, cadmium sulfide (CdS) nanocrystal is one of the important nanostructured semiconductor materials, which has been widely studied due to its nonlinear optical property, luminescent property, quantum size effect, and other important physical and chemical properties.3-11 A variety of methods (both physical and chemical) have been explored for preparing CdS nanocrystals, such as solgel,12,13 electrostatic deposition,14 gas evaporation,15 micelles,16 and solvent growth.17-20 Among them, the solvent growth * To whom correspondence should be addressed. E-mail: prcscn@ yahoo.com.cn. (1) Hu, L.; Brust, M.; Bard, A. J. Chem. Mater. 1998, 10, 1160. (2) Alivisatos, A. P. J. Phys. Chem. 1996, 100, 13226. (3) Ghosh, S.; Mukherjee, A.; Kim, H.; Lee, C. Mater. Chem. Phys. 2003, 78, 726. (4) Yan, B.; Chen, D.; Jiao, X. Mater. Res. Bull. 2004, 39, 1655. (5) Beecroft, L. L.; Ober, C. K. Chem. Mater. 1997, 9, 1302. (6) Murry, C. B.; Kagan C. R.; Bawendi, M. G. Science 1995, 270, 1335. (7) Rosseti, R.; Hill, R.; Gibson, J. M.; Brus, L. E. J. Chem. Phys. 1995, 82, 552. (8) Wang, W.; Germanenko, I.; El-Shall, M. S.Chem. Mater. 2002, 14, 3028. (9) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425. (10) Mahapatra, P. K.; Roy, C. B. Electrochim. Acta 1984, 29, 1439. (11) Herron, N.; Wang, Y.; Eddy, M. M.; Stucky, G. D.; Cox, D. E.; Moller, K.; Bein, T. J. Am. Chem. Soc. 1989, 111, 530. (12) Mathieu, H.; Richard, T.; Allegre, J.; Lefebvre, P.; Arnaud, G. J. Appl. Phys. 1995, 77, 287. (13) Rao, A. V.; Pajonk, G. M.; Parvathy, N. N. Mater. Chem. Phys. 1997, 48, 234. (14) Salata, O. V.; Dobson, P. J.; Hull, P. J.; Hutchinson, J. L. Thin Solid Films 1994, 251, 1. (15) Arai, T.; Yoshida, T.; Ogawa, T. J. Appl. Phys. 1987, 26, 396. (16) Lianos, P.; Thomas, J. K. Chem. Phys. Lett. 1986, 125, 299. (17) Spanhel, L.; Hause, N.; Weller, H.; Henglein, A. J. Am. Chem. Soc. 1987, 109, 5649. (18) Nayar, S.; Sinha, A.; Das, S.; Das, S. K.; et al. J. Mater. Sci. Lett. 2001, 20, 2099. (19) Ravindran, T. R.; Arora, A. K.; Balamurugan, B.; Mehta, B. R. Nanostruc. Mater. 1999, 11, 603. (20) Khosravi, A. A.; et al. Appl. Phys. Lett. 1995, 67, 2506.
technique is very effective in yielding a size distribution and optimization of the properties in preparation of nanocrystals. Several research efforts have focused on using various stabilizers to control architecture of nanocrystals.21-24 Especially, the highquality CdS nanocrystals have been synthesized via arrested precipitation from simple inorganic ions using polyphosphate17 and low molecular weight thiols21,22 as stabilizers, from dimethylcadmium in trioctylphosphine using trioctylphosphine oxide as a stabilizer23 and from cadmium 2-ethylhexanoate in dimethyl sulfoxide (DMSO) using ethylhexanoate as a stabilizer.24 To allow CdS nanocrystals to embed in a polymer matrix, many research groups have reported on the preparation and applications of CdS-polymer nanocomposites. The CdS-poly(N-vinylcarbazole) (PVK) nanocomposite was simply prepared by mixing PVK and CdS nanoclusters.25 Qian and co-workers26 produced CdS-PVK nanocomposites via an in situ microwave irradiation method at atmospheric conditions. Chin and coworkers27 fabricated the nonlinear optical hybridized CdSpolystyrene nanocomposites by embedding CdS nanoparticles into sulfonated polystyrene matrix. Olshavsky and Allcock28 prepared the CdS-polyphosphazene nanocomposite using a guest-host approach. Although a number of investigations on CdS nanocrystalpolymer hybrid materials have been carried out, there are few reports on preparation of CdS nanocrystal-polymer hybrids by using polyurethane (PU) as a polymer matrix. PUs provide a wide range of versatile properties from a variety of starting materials. Tailor-made properties of these materials can be (21) Vossmeyer, T.; Katsikas, L.; Giersig, M.; Popovic, I. G.; Diesner, K.; Chemseddine, A.; Eychmul¨ler, A.; Weller, H. J. Phys. Chem. 1994, 98, 7665. (22) Rockenberger, J.; Tro¨ger, L.; Kornowski, A.; Vossmeyer, T.; Eychmu¨ller, A.; Feldhaus, J.; Weller, W. J. Phys. Chem. B 1997, 101, 2691. (23) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (24) Diaz, D.; Rivera, M.; Ni, T.; Rodriguez, J. C.; Castillo-Blum, S.-E.; Nagesha, D.; Robles, J.; Alvarez-Fregoso, O. J.; Kotov, N. A. J. Phys. Chem. B 1999, 103, 9854. (25) Wang, Y.; Herron, N. J. Lumin. 1996, 70, 48. (26) He, R.; Qian, X. F.; Yin, J.; Bian, L. J.; Xi, H. A.; Zhu, Z. K. Mater. Lett. 2003, 57, 1351. (27) Du, H.; Xu, G. Q.; Chin, W. S. Chem. Mater. 2002, 14, 4473. (28) Olshavsky, M. A.; Allcock, H. R. Chem. Mater. 1997, 9, 1367.
10.1021/la062210g CCC: $37.00 © 2007 American Chemical Society Published on Web 11/17/2006
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Scheme 1. Synthesis of Hydroxyl Group Coated CdS Nanocrystals and CdS/PU Nanocomposite Hybrids
designed from well-designed combinations of monomeric materials to meet the diverse requirements of many materials such as elastomers, coatings, adhesives, and foams.29,30 The key point for preparing high-performance nanocrystal-polymer hybrids is how to produce good fluorescent materials, along with excellent transparency. In this work, we describe how we produced CdS nanocrystal-PU hybrids. Considering that nanocrystals have high surface area and surface energy and are easy to aggregate in polymer matrixes, we use 2-mercaptoethanol as the organic ligand to obtain hydroxyl-coated functional CdS nanocrystals, allowing them to react with toluene 2,4-diisocyanate (TDI) reagent and then to fabricate well-defined transparent CdS nanocrystalPU nanocomposite hybrids in situ. The optical properties of the functional CdS nanocrystals and CdS nanocrystal-PU nanocomposite hybrids were thoroughly investigated. Experimental Section Materials. 2-Mercaptoethanol (ME), cadmium chloride (CdCl2‚ 2.5H2O), sodium sulfide (Na2S‚9H2O), N,N-dimethylformamide (DMF), TDI, and 1,4-butylene glycol (BG) were supplied by Aldrich and used as received. DMF was dried over 4 Å molecular sieves and used without further purification. Poly(propylene glycol) (PPG) (hydroxyl number is 56 mg of KOH/g of N220, the average molecule weight of 2000) was supplied by Jinling Petrochemical Co. Synthesis of Hydroxyl-Coated CdS Nanocrystals. In the first step, 2.5 mmol of cadmium chloride (CdCl2‚2.5H2O) was dissolved in 3 mL of deionized water. After the cadmium salt completely dissolved, the cadmium chloride solution was mixed with 5.0 mmol of 2-mercaptoethanol (ME) in 30 mL of DMF and stirred vigorously for 10 min. In the second step, 3 mL of aqueous solution of sodium sulfide (Na2S: 1.67 mmol) was slowly added dropwise into the above solution with stirring. Once added, the color of the above solution turned yellow immediately, and further the solution turned cloudy. Then the reaction was carried out for an additional 4 h at room temperature. Finally, the yellow solution gradually turned transparent. The amounts of water and salts in this CdS nanocrystal suspension thus obtained were removed, respectively, and then washed by fresh DMF solvent for several times. Synthesis of CdS Nanocrystal-PU Nanocomposite Hybrids. The CdS nanocrystal-PU nanocomposite hybrid was synthesized by a stepwise procedure, with BG, PPG, and TDI (BG:PPG:TDI ) 1:1:1.6 mol/mol/mol), the appropriate amounts of CdS nanocrystal suspensions, stannous caprylate catalyst (as the catalyst), and 50 wt % DMF (as the solvent) carried out in a three-necked glass reactor equipped with a stirrer, a reflux condenser, and thermocouples. In the first step, the appropriate amounts of CdS nanocrystal suspensions (in DMF) and TDI were stirred under nitrogen at 85 °C for 3 h. In the second step, a DMF solution of BG and PPG was added dropwise at 85 °C for 0.5 h, along with stannous caprylate catalyst. To produce the NCO-terminated prepolymer, the temperature was increased to 90 °C, which was maintained for 4 h. The schematic synthesis of hydroxyl-coated functional CdS nanocrystal and CdS-PU nanocomposite hybrids is shown in Scheme 1. (29) Chen, S.; Tian, Y.; Chen, L.; et al. Chem. Mater. 2006, 18, 2159. (30) Chen, S.; Sui, J.; Chen, L. Colloid Polym. Sci. 2004, 283 (1), 66.
Figure 1. UV-vis absorption spectra of CdS nanocrystals prepared with different molar ratios of ME/Cd2+: (a) ME/Cd2+ ) 0.5/1 mol/ mol; Cd2+/S2- ) 1/0.67 mol/mol, solvent H2O/DMF ) 0.2/1.0 w/w, reaction time 2 h; (b) ME/Cd2+ ) 1/1 mol/mol, Cd2+/S2- ) 1/0.67 mol/mol, solvent H2O/DMF ) 0.2/1.0 w/w, reaction time 2 h; (c) ME/Cd2+ ) 2/1 mol/mol, Cd2+/S2- ) 1/0.67 mol/mol, solvent H2O/ DMF ) 0.2/1.0 w/w, reaction time 2 h. Characterizations. Ultraviolet-visible (UV-vis) absorption spectra were taken with a Perkin-Elmer Lambda 900 UV-vis spectrometer with the scan range 300-500 nm using DMF as solvent, and all the UV-vis samples were diluted 60 times with DMF for analysis. Transmission electron microscopic (TEM) observation was performed with a JEOL JEM-2100 transmission electron microscope. The samples were dispersed in DMF, and a drop of the solution was placed on a copper grid that was left to dry before transferring into the TEM sample chamber. The thermogravimetric analyses (TGA) were obtained with a NETZSCH STA 409 PC in N2 flow in the temperature range 40-450 °C, the heating rate being 10 °C/min, and the TGA samples were prepared by drying under vacuum for solvent removal. The powder X-ray diffraction (XRD) patterns were conducted on a Bruker-AXS D8 ADVANCE X-ray diffractometer at a scanning rate of 6°/min in 2θ ranging from 5 to 75° with Cu KR radiation (λ ) 0.1542 nm). Fourier transform infrared (FT-IR) spectra were recorded on a NICOLET-NEXUS670 spectrometer. The samples were ground with KBr crystals, and the mixture was pressed into a flake for IR measurement. Photoluminescence (PL) spectra were measured on an Aminco Bowman series 2 spectrofluorometer at room temperature operating with a 325 nm laser beam as a light source.
Results and Discussion The synthesis of hydroxyl-coated CdS nanocrystals involves the reaction between cadmium and sulfur ions in the presence of mercapto-group-containing ligands as the organic ligands. The electron-deficient atoms of cadmium on the surface of the semiconductor serve as binding sites to anchor organic ligands and to hinder the further growth of crystal grains, which results in the formation of nanosized crystals. In the report of Carrot,31 ME was used to prepare CdS nanocrystals by cadmium acetate and thiourea as raw materials. However, the reaction had to be performed at elevated temperature over 10 h. In our case, preparation of CdS nanocrystals by cadmium chloride, sodium sulfide, and ME (as the ligand) was done in only 20 min even at room temperature. To investigate the effect of the ligand concentration on the particle size of CdS nanocrystals, we measured UV-vis spectra of CdS nanocrystals prepared with different molar ratios of ligand to Cd2+. As seen in Figure 1, a weak blue shift in the maximum absorption band from 410 to 396 nm occurs as the concentration of organic ligand increased. The absorption peaks of the three samples of different molar ratios of ligand to Cd2+ center at 396 (31) Tang, W.; Farries, R. J.; Macknight, W. J.; et al. Macromolecules 1994, 27, 2814.
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Figure 2. TEM images of CdS nanocrystal dispersed in DMF. ME/Cd2+/S2- ) 2/1/0.67 mol/mol, H2O/DMF ) 0.2/1.0 w/w, and reaction time ) 2 h.
nm (3.13 eV), 400 nm (3.10 eV), and 410 nm (3.03 eV), respectively. The result of the blue shift is in agreement with quantum confinement effects due to decreasing particle size since the higher concentration ligand is unfavorable to the nucleation growth of CdS nanocrystals. According to Brus’s model,32 the exited energy of nanocrystals is in reverse of their particle size. The higher energy makes the position of the maximum absorption peak of its UV-vis spectrum blue-shifted. The calculated particle size of CdS nanocrystals prepared with the 1/1 of ME/Cd2+ molar ratio is 4.34 nm according to Brus’s model, while the other two samples are 3.88 nm (ME/Cd2+ ) 0.5/1 mol/mol) and 5.02 nm (ME/Cd2+ ) 2/1 mol/mol). The value is similar to the particle size observed directly from TEM images shown in Figure 2, which demonstrates that the mean size of the CdS nanocrystals is about 5 nm and the particles are dispersed well in solution. The results also show that CdS nanocrystals as-prepared behave as quantum dots (QDs). DMF was used as an organic solvent to enhance the solubility of the CdS nanocrystals, allowing CdS nanocrystals to behave QDs, since CdS nanocrystals are unavailable in aqueous solution without the addition of DMF. The weight ratio of H2O to DMF in the solution also has an effect on the particle size, which can be easily observed through the transparency of the solution. When the weight ratio (H2O/DMF) exceeded 1.5, CdS precipitation was obviously observed. The UV-vis absorption spectra of CdS nanocrystals prepared with different weight ratios of H2O/DMF ranged from 0.2/1.0 to 0.9/1.0 are given in Figure 3. With the same condition of reagents concentration (Cd2+/S2- ) 1/0.67 mol/mol), the absorption peaks of weight ratios of H2O/DMF 0.2/1.0 (a), 0.45/1.0 (b), and 0.9/1.0 (c) (seen in Figure 3) center at 377 nm (3.29 eV), 388 nm (3.20 eV), and 400 nm (3.10 eV), respectively, and the corresponding particle diameters of CdS nanocrystals calculated by Brus’s model32 are 3.71, 3.98, and 4.34 nm, respectively. Under higher concentration of DMF, the absorption band is blue-shifted, which indicates that DMF concentration affects the particle size of the CdS nanocrystals. To investigate the effect of ME ligand on CdS crystal structure, we characterized CdS nanocrystal samples containing ME ligands and control samples by XRD (seen in Figure 4). The XRD patterns of CdS with ME and without ME ligand show strong (111) peaks, (220) peaks, and (311) peaks, suggesting that there are (32) Brus, L. E. J. Phys. Chem. 1986, 90, 2555.
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Figure 3. UV-vis absorption spectra of CdS nanocrystals of different weight ratios of H2O/DMF: (a) H2O/DMF ) 0.2/1.0 w/w, Cd2+/S2- ) 1/0.67 mol/mol, reaction time 2 h; (b) H2O/DMF ) 0.45/1.0 w/w, Cd2+/S2- ) 1/0.67 mol/mol, reaction time 2 h; (c) H2O/DMF ) 0.9/1.0 w/w, Cd2+/S2- ) 1/0.67 mol/mol, reaction time 2 h.
Figure 4. XRD patterns of CdS powders fabricated without (a) addition of ME and with (b) addition of ME: (a) Cd2+/S2- ) 1/0.67 mol/mol, solvent H2O/DMF ) 0.2/1.0 w/w, reaction time 2 h; (b) ME/Cd2+/S2- ) 2/1/0.67 mol/mol, solvent H2O/DMF ) 0.2:1.0 w/w, reaction time 2 h.
Figure 5. TGA diagrams of as-prepared CdS cryatals without (a) and with (b) addition of ME: (a) Cd2+/S2- ) 1/0.67 mol/mol, solvent H2O/DMF ) 0.2/1.0 w/w, reaction time 2 h; (b) ME/Cd2+/S2- ) 2/1/0.67 mol/mol, solvent H2O/DMF ) 0.2/1.0 w/w, reaction time 2 h.
cubic and hexagonal phases in the two kinds of CdS crystal structures. For the comparison of these two kinds of XRD patterns, the XRD peaks of CdS containing ME broadens, indicating that particle size of CdS crystals prepared with ME ligand become smaller. On the other hand, organic ME ligand in the preparation of CdS crystals cannot change the structure of CdS crystal. TGA results for (a) CdS nanocrystal prepared with ME ligand and (b) without ME ligand are shown in Figure 5. There is a single degradation step in the curve, which means that MEtethered CdS nanocrystals are produced. About 95 wt % of the
Nanocrystal-Polymer Transparent Hybrids
Figure 6. FT-IR spectra of CdS nanocrystal coated (a) with hydroxyl group and (b) modified with TDI.
sample begins to decompose at 220 °C, which is higher than the boiling point of pure ME (157 °C). It clearly shows that robust bonding between ME and CdS is formed. The actual organic ME content in this CdS nanocrystal is about 25 wt %. The FT-IR spectra of the CdS nanocrystal prepared in the presence of the organic ligand and the CdS nanocrystal modified with TDI are presented in Figure 6a,b, respectively. As can be seen in Figure 6a, strong absorption peaks at 1105 and 1020 cm-1 (νC-O) show that abundant hydroxyl groups are tethered on the surface of CdS nanocrystals. The absorption peaks at 2926, 2850 (νC-H), and 1440 cm-1 (νCH2) indicate the existence of a methylene group. In this case, there are no characteristic peaks of the mercapto group in Figure 6a, indicating the free mercapto groups almost disappeared and the formation of robust bonding between Cd2+ and ME organic ligand. Figure 6b shows that the characteristic absorption peaks of -NCO (2262 cm-1) and carbamate groups (1712 cm-1) exist, indicating that the hydroxyl-coated CdS nanocrystals have been successfully modified with TDI and the modified CdS nanocrystals contain an -NCO group. The isocyanate-capped CdS nanocrystals may further react with PPG and BG to produce PU-CdS hybrids in situ. To confirm that the CdS nanocrystals were well-dispersed and embedded in the PU matrix, the PU-CdS hybrids with the different concentration of CdS nanocrystals were measured by UV-vis absorption compared with the pure PU polymer. Figure 7 shows UV-vis absorption spectra of pure PU, the PU-CdS hybrid, and a CdS nanocrystal diluted with DMF ((a) pure PU; (b) PU-CdS hybrid, CdS wt % ) 1.5 wt %; (c) PU-CdS hybrid, CdS wt % ) 3.0 wt %; (d) pure CdS nanocrystal dispersed in DMF). As seen in Figure 7, there are no absorption peaks in pure PU, and the absorption peak at 400 nm can be observed in PUCdS hybrids and pure CdS nanocrystals, indicating that the particle size of CdS nanocrystals in the hybrids does not change and is still kept at about 5 nm. Also, the strength of absorption peak is enhanced with the increase of CdS concentration. As a typical semiconductor, CdS nanocrystals exhibit interesting optical properties. Figure 8 shows the fluorescence emission spectra of the ligand-modified hydroxyl-coated CdS nanocrystal suspension and PU-CdS nanocomposite hybrid film with excitation at 325 nm. As seen in Figure 8, the characteristic emissions for CdS nanocrystals are about 425 and 600 nm. The emission at 600 nm is assigned to electron-hole recombination at surface traps, while the higher energy emission at 425 nm is attributed to recombination from the excitonic state in the
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Figure 7. UV-vis absorption spectra of pure PU, PU-CdS hybrid, and CdS nanocrystals diluted with DMF: (a) pure PU dispersed in DMF; (b) PU-CdS hybrid, CdS wt % ) 1.5 wt %, dispersed in DMF; (c) PU-CdS hybrid, CdS wt % ) 3.0 wt % dispersed in DMF; (d) CdS nanocrystals dispersed in DMF.
Figure 8. Fluorescence emission spectra of (a) hydroxyl-coated CdS nanocrystals suspension and (b) CdS wt % ) 1.5 wt %, PUCdS hybrid film composite with excitation at 325 nm.
crystallite interior.33,34 Figure 8b shows that an increase in the emission peak strength and bandwidth occurs in PU-CdS hybrid film. Usually, the energy and bandwidth of CdS PL bands are related to the size and the nature of carrier trap states located at the surface of nanocrystals. It may be explained that an effect of band broadening is that the surface structure of CdS nanocrystals and the particle size of CdS change when CdS nanocrystals are embedded in the PU matrix.
Conclusion The first facile synthesis of PU-CdS nanocomposites has been achieved. We have found that CdS nanocrystals are obtained by the reaction between cadmium chloride (CdCl2) and sodium sulfide (Na2S) in the presence of ME as the organic ligand. The hydroxyl-ending alkyl group thus introduced onto the surface of CdS nanocrystals enhances their dispersibility in solvent, allowing the particle size of nanocrystals to be controlled. The effect of concentration of the reactants and the solvent on the particle size of as-prepared CdS nanocrystals has been thoroughly investigated. We have found that the particle size of the CdS nanocrystal characterized by TEM is about 5 nm, in agreement with the calculated data from UV-vis absorption spectra according to Brus’s model. Also, for the higher concentration of ME ligand and DMF, respectively, the UV-vis absorption band is blueshifted, indicating that ME and DMF concentration is a function of the particle size of the CdS nanocrystals. FT-IR and TGA characterizations indicate the formation of robust bonding between CdS nanocrystals and the organic ligand. (33) Premachandran, R.; Banerjee, S.; John, V. T.; McPherson, G. L. Chem. Mater. 1997, 9, 1342. (34) Noglik, H.; Pietro, W. J. Chem. Mater. 1994, 6, 1593.
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The PL measurement shows that PU-CdS hybrids exhibit good optical properties. Acknowledgment. This work was supported by the National Science Foundation of China (Grant No. 20576053), the Natural
Chen et al.
Science Foundation of Jiangsu province, China (Grant No. BK2005119), and the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (Grant No. 04KJB430038). LA062210G
