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Journal of Nanomaterials

Volume 2014 (2014), Article ID 903208, 11 pages

http://dx.doi.org/10.1155/2014/903208
Research Article

Upconversion luminescence and Visible-Infrared Properties of β-NaLuF4:Er3+ Microcrystals Synthesized by the Surfactant-Assisted Hydrothermal Method

1College of Science, Nanjing University of Aeronautics and Astronautics, Nanjing 211106, China

2School of Electronic Science and Engineering, Nanjing University of Posts and Telecommunications, Nanjing 210046, China

Received 21 February 2014; Revised 20 May 2014; Accepted 22 May 2014; Published 18 June 2014

Academic Editor: Alireza Talebitaher

Copyright © 2014 Han Lin et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

We report the obtention of β-NaLuF4 microcrystals doped with Er3+ ions by the surfactant-assisted hydrothermal method. It was found that shape modulation could be realized by changing the surfactants (ethylenediaminetetraacetic acid, polyvinylpyrrolidone, and trisodium citrate) introduced into the reaction system. The surfactants can strongly control the size and shape of as-prepared samples through absorbing on the surface of primary particles and/or coordinating with rare earth ions. Hexagonal prism-like β-NaLuF4:Er3+ microcrystals demonstrate intense upconverted luminescence (UCL) pumped by 1.54 μm infrared laser in comparison with hexagonal tube-like, disk-like, and sphere-like microcrystals, which exhibit great distinction. More interestingly, a synergistic effect combined dual mode (i.e., downconversion and upconversion) with 8% absolute enhancement rate of the red emission centered at 659 nm (4F9/2 4I15/2) is witnessed in hexagonal prisms β-NaLuF4:Er3+ phosphors by employing the dual wavelength 416 nm and 1.54 μm excitation source for the first time.

1. Introduction

Rare earth fluorides including REF3 and AREF4 (A = alkali; RE = rare earth) have been regarded as excellent downconversion (DC) and upconversion (UC) luminescent hosts for various optically active Ln3+ ions [16]. They normally possess low phonon energy, low probability of nonradiative decay, and high chemical stabilities, and the luminescent quantum yields higher than that in oxide hosts and most inorganic matrices. Thus, rare earth fluorides have attracted much attention of scientific community due to their potential applications in the fields of solid-state lasers, multicolor three dimensional displays, optical storage, and biological fields including fluorescent labels, therapy, and drug delivery [710]. One example for this class of fluoride compounds is NaLuF4, which is an ideal UC host material [1116]. Shi et al. [11] synthesized hexagonal nanoplates β-NaLuF4:Yb3+/Tm3+ crystals by hydrothermal method using oleic acid as the surfactant and demonstrated that β-NaLuF4 nanocrystals might be a better kind of upconversion material than their β-NaYF4 counterpart. Li et al. [15] prepared multiform morphologies β-NaLuF4 by changing the solution pH values, F sources, and organic additives and studied the morphological evolvement and the growth mechanism for the synthesized lutetium fluorides under different conditions in detail.

In this work, we present our recent efforts on the fabrication of NaLuF4 crystals with controllable sizes and enhanced PL properties. Emission intensity and energy efficiency are used as measures of the phosphors performance. They are important performance characteristics that determine which applications are appropriate. In most cases, the thermalization losses and subbandgap light transmission are the major bottleneck effect on energy efficiency [1720]. Erbium (Er) possesses several long-lived intermediate levels and metastable high-energy levels, and the energy gap between the ground level 4I15/2 and the first excited level 4I13/2 is matched well with the absorption of 1.54 μm photon, so it can be considered as a promising candidate among REs for UC of photons [2125]. The current researches mainly focus on the single mode UC or DC. No matter which mode both can enhance and improve the spectral response characteristics of materials. If we combine the two kinds of conversion mechanisms, UC and DC can be achieved in specific materials at the same time. This will make the high-energy and low-energy photons convert to middle-energy photons ultimately, and the middle-energy photon is needed for photovoltaic materials. Therefore, our group proposed a new mechanism named photon-excited synergistic effect [20].

In this paper, we report on the effect of different surfactants, such as ethylenediaminetetraacetic acid (EDTA), polyvinylpyrrolidone (PVP-K30), and trisodium citrate (hereinafter shortened form Cit3−), on the growth process and PL properties at room temperature of β-NaLuF4 microcrystals synthesized by the hydrothermal method. The as-prepared hexagonal prisms β-NaLuF4 crystals have highly efficient UC luminescence and synergistic effect by employing the 416 nm and 1.54 μm coexcitation source.

2. Experimental Details

2.1. Synthesis of β-NaLuF4 Microcrystals Doped with Er3+ Ions

Rare earth oxides Lu2O3 (99.99%) and Er2O3 (99.99%) were purchased from Beijing Lansu Co., China. Rare earth chlorides (LnCl3, Ln:Lu/Er) were prepared by dissolving the corresponding metal oxide in 10% HCl solution at elevated temperature and then evaporating the water completely. NaF (98% purity), NaOH (96% purity), EDTA (99% purity), PVP (95% purity), and Cit3− (99% purity) were purchased from Sinopharm Chemical Reagent Co., China. All chemicals were of analytical grade and were used without further purification. Deionized water (H2O) was used throughout.

In the typical experiment of NaLuF4:Er3+ micromaterials, a predetermined amount of EDTA was first dissolved in 18 mL distilled water under magnetic stirring to form a solution of desired molar concentration and then stoichiometric amounts of LuCl3 and ErCl3 were added to the solution. Under vigorous stirring for 30 min, 10 mL aqueous solution containing NaF was added dropwise into the above mixture. Then, the pH value of the suspension was adjusted to 7 through the addition of 2 M NaOH solution. The precursor solution was stirred magnetically for 10 min and then transferred to a Teflon lined stainless steel autoclave for hydrothermal treatment maintained at 190°C for 24 h. After the hydrothermal treatment, the autoclave was cooled naturally to room temperature. A light pink colored precipitate settled at the bottom, which was collected and washed repeatedly with distilled water and ethanol to remove the impurities and excessive surfactant. The sample was finally dried at 50°C for 12 h for further characterization. When PVP and Cit3- were selected as chelators, the synthesis procedure was basically identical to the above description. Table 1 shows the parameters of hydrothermal synthesis and characteristics of the powders. And the samples prepared under the specific reaction conditions were denoted as S1–S9, respectively.

tab1
Table 1: Summary of morphologies and their corresponding detailed experimental conditions of samplesa.
2.2. Characterizations

X-ray powder diffraction (XRD) measurements were performed on an Ultima-III (Rigaku) diffractometer at a scanning rate of 10° min−1 in the 2θ range from 10° to 70°, with graphite monochromatic Cu Kα radiation ( ). The morphologies were obtained using scanning electron microscopy (SEM, S-3400N II, Hitachi, Japan). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were recorded on a JEOL JEM-200CX with a field emission gun operating at 200 kV. For the UC emission measurement, all the samples were pressed into pellets with 1 cm diameter and 5 mm thickness. The excitation spectra and emission spectra of samples are measured by an Omni-λ3007 spectrophotometer with a CW Xe lamp and infrared lasers with the wavelength at 1.54 μm employed as the excitation source. The dependence of upconverted emission intensity on pumping powers for different samples was obtained by changing the excitation powers. All the measurements above were performed at room temperature.

3. Results and Discussions

3.1. XRD Patterns Analyses

Figure 1 exhibits XRD patterns of all the prepared products corresponding to Table 1. The regularity indicated from the results is described as follows. For the samples prepared in the presence of EDTA (Figure 1(a)) and PVP (Figure 1(b)) with different agents/Ln3+ molar ratio, the strong and sharp diffraction peaks suggest that the pure hexagonal structure of NaLuF4 (JCPDS 27-0726) is successfully achieved through the proposed direct hydrothermal process and the crystal planes for each peak are marked. Besides, the enhanced intensity of peak at ( ) and ( ) can be observed in comparison with the standard value, in which the diffraction peaks ( ) and ( ) are particularly strong. This result implies that the samples tend to be preferentially oriented. As shown in Figure 1(c), unexpectedly, the as-synthesized samples prepared in the presence of Cit3− consist of two different phases, that is, the orthorhombic structure (space group Pnma) of LuF3 (S7) and the hexagonal structure of NaLuF4 (S8 and S9), which are in good agreement with the standard literature data. This result demonstrates that Cit3− plays an important role in the phase for the formation of β-NaLuF4, and it may provide a Na+ source. When the concentration of Cit3− is relatively low, there is no sufficient Na+ source to form NaLuF4, but LuF3. The calculated cell lattice constants of the samples are summarized in Table 2, and the standard data for hexagonal structure of β-NaLuF4 and orthorhombic structure of LuF3 are also given for comparison. Obviously, the calculated cell lattice constants of the samples are consistent with the standard data.

tab2
Table 2: Unit cell lattice constants and crystallite sizes for hexagonal phase of NaLuF4 and orthorhombic phase of LuF3 prepared using different surfactants, respectively.
fig1
Figure 1: XRD patterns of the as-synthesized products obtained in the presence of different surfactants (EDTA: S1 to S3, PVP: S4 to S6, and Ct3−: S7 to S9). The standard pattern of hexagonal NaLuF4 (JCPDS card 27-0726) and orthorhombic LuF3 (JCPDS card 17-0796) is also presented for comparison.

The SEM images of microcrystals prepared using different surfactants are given in Figure 2. It can be seen that all samples exhibit relatively uniform, well-dispersed morphology within micrometer size range, yet the specific shapes and sizes of the samples are much different. This distinction is derived from the surfactant, since it is the only difference during the synthesis process. For the samples prepared with EDTA (sample S1–S3), uniform hexagonal prisms are obtained with 9 μm, 3.5 μm, and 1 μm in length, respectively, except that both ends of S2 are sharp. Samples prepared with PVP (S4–S6) consist of similar tube-shaped aggregates. The diameters of the three tubes are much closer ( 0.5 μm), while the lengths differ greatly. As for the samples prepared with Ct3− (S7–S9), they are different. Sphere-like β-NaLuF4: Er3+ microcrystals are obtained in S7-S8, while S9 consists of fairly uniform and smooth microdisks. The peripheral surface of S8 is rough due to the composition of many small nanoparticles. This finding indicated that any change may result in different morphology and uniformity even when keeping other parameters the same. The morphological difference is caused by the physical and chemical properties of different ligands, and the effects of different surfactants will be discussed thoroughly in the following section.

fig2
Figure 2: SEM images of as-prepared microcrystals using different surfactants with different agents/Ln3+ molar ratio: (a) S1, (b) S2, (c) S3, (d) S4, (e) S5, (f) S6, (g) S7, (h) S8, and (i) S9. Insets are corresponding high magnification SEM images of the microstructures.

TEM images, HRTEM images, and SAED patterns of the NaLuF4:Er3+ microcrystals (S3, S6, S8, and S9) are demonstrated in Figure 3, which provide an insight of different structures. TEM image of sample S3 (Figure 3(a)) confirm it as a prism-like structure, which is in good agreement with SEM image. The ED patterns insets in Figure 3(a) show clear and regular diffraction spots, reveal the single crystalline nature of the microprism, and can be indexed as the pure hexagonal structure. The HRTEM image of a single particle confirms the distance of 0.50 nm between the adjacent lattice planes, ascribed to that of (110) crystal planes. Unlike sample S3, the tube-like structure of sample S6 is irregular (Figure 3(b)), and ED pattern exhibits the single crystalline nature of the microtube. HRTEM image recorded from the tip of an individual tube verified the lattice fringe separation of 0.52 nm. This plane coincides well with the distances between (100) crystal planes. Figures 3(c) and 3(d) exhibit the TEM images of samples prepared with Ct3− (samples S8 and S9). We can see that both TEM images show the obvious spherical shape with a uniform size distribution. It should be noted that the ED patterns are the polycrystalline. The lattice fringes of the sphere-like sample cannot be seen clearly due to the desultory accumulation of particles. The interplanar distance of the disk-like sample is 0.37 nm and 0.32 nm, which corresponds to the d-spacing value of the (101) and (201) planes, respectively.

fig3
Figure 3: The TEM images, HRTEM images, and ED patterns of as-prepared β-NaLuF4 microcrystals using different surfactants: (a) S3, (b) S6, (c) S8, and (d) S9.
3.2. Growth Mechanism of β-NaLuF4 Microcrystals

Based on the above analysis, a possible growth mechanism for the microcrystals was proposed to explain the effect of different surfactants on the shape and size in detail (Figure 4). Firstly Figure 4(a) shows the coordination structures of EDTA, PVP, and Ct3−. EDTA molecule possesses four carboxyl groups (–COOH) and two lone pairs of electrons on two nitrogen atoms which can act as binding sites and help to form hydrogen bonds [26]. PVP-K30 is a type of nonionic surfactant with long carbon chains and strong selective adsorption, and lanthanide ions were coordinated with the pyrrolidone groups of PVP [27]. A Ct3− molecule has four binding sites, including one hydroxyl group and three COO, among which three sites can be bound with lanthanide ions. After formation of the nuclei (Figure 4(b)), the hydrothermal conditions performed at 190°C for 24 h inside the stainless autoclave intensify the effective collision frequency involving the anisotropic nanoparticles in suspension, producing a mutual aggregation between them. The self-assembly process can occur in a spontaneous way under hydrothermal conditions, where several nanocrystals are aggregated in a same or different crystallographic plane which can drive the growth of oriented aggregate [28]. The Ln3+-EDTA complex can significantly decrease the concentration of free RE3+ ions and reduce the crystal growth rate, leading to the effective separation of nucleation and growth steps and thus facilitating the synthesis process of crystals. PVP-K30 can be adsorbed onto the surface of nuclei particles to further control its morphology. Furthermore, PVP-K30 can facilitate the nucleation and growth of rare earth compounds on each surface to form tube-like crystals. The Ct3− molecule can also control both the crystal nucleation and the growth in the reaction system and it can cap on the side of precursor particles and lead to the selective growth rate of various crystallographic facets.

fig4
Figure 4: (a) Newman representations for coordination compound structure of EDTA, PVP, and Ct3−. (b) Schematic illustration of the main stages involved in the growth mechanism of β-NaLuF4 microcrystals.
3.3. UC Luminescence Properties

The upconversion (UC) luminescence spectra for the samples (S3, S6, S8, and S9) were recorded upon 1.54 μm pumping with power density of 16 mW/mm2. Obviously, the four samples exhibit quite different emission intensities and peak positions. Figure 5 shows the upconversion emission spectra of β-NaLuF4:  mol%Er3+ ( ) microcrystals. As can be observed, in hexagonal prisms (S3; Figure 5(a)) and hexagonal tubes (S6; Figure 5(b)), there were three well-known intense emission bands centered at 541 nm, 659 nm, and 805 nm, which are associated with the transition from 4S3/2, 4F9/2, and 4I9/2 levels to ground state of Er3+, respectively. The intensities of emission bands were so strong that they can be clearly seen by the naked eyes even though they were under a low excitation power. The emission intensities are highly dependent on the Er3+ concentration. Initially, increasing Er3+ ion doping concentration, the intensity from all regions of the spectra increases rapidly and reaches a maximum. Then, a decrease appears at higher Er3+ concentrations due to the concentration quenching effect. As for the red emission, it is determined by the population of the 4F9/2 level. The 4F9/2 level is mainly populated by energy transfer between Er3+ ions when the doping concentration is increased, 4I11/2(Er3+)+4I9/2(Er3+) 4I13/2(Er3+)+4F9/2(Er3+) [26]. According to our experimental data, the optimum Er3+ concentration is 12 mol%Er3+ ions in S3 and 24 mol%Er3+ ions in S6. It is interesting to point out that the typical 541 nm green and 659 nm red emissions of Er3+ disappear in the spheres (S8; Figure 5(c)) and disks (S9; Figure 5(d)) and the intensities of emission bands of the two samples are poor. Comparing various morphologies, the emission intensity of hexagonal prism-like β-NaLuF4: Er3+ microcrystals was found to be 105 more efficient and brighter than others. The ranking of the emission intensities is .

fig5
Figure 5: UCL spectra pumped by 1.54 μm laser diode with power density of 16 mW/mm2 for β-NaLuF4:  mol%Er3+ ( ) synthesized using EDTA as the chelator for EDTA/Ln3+ ratio of 3 : 1 (a), using PVP as the chelator for PVP/Ln3+ ratio of 3 : 1 (b), and using Ct3- as the chelator for Ct3−/Ln3+ ratio of 2 : 1 (c) and 3 : 1 (d). Insets are corresponding high magnification SEM images of the samples. All of the samples were hydrothermally treated with the same condition except for the molar ratio of chelators/Lncl3.

As for the different UC intensities of the four samples, there may be several factors playing a role. Since the shape and sizes of the microcrystals are different, it will bring a few uncertain factors that influence the UC behaviors through affecting the scattering and absorption of incident light, which decrease the defect concentrations [26, 29]. Moreover, the distinct difference of the emission intensities could be owing to the chelating and capping ability of surfactants. When an EDTA molecule chelates with one Ln3+ ion, all of its six binding sites participate in the reaction to develop a hexagonal structure. Furthermore, the chelate constant for EDTA ( ) is much larger than Ct3− ( ) [7]. The structure stability coefficient of EDTA with Ln3+ ions is larger than Ct3−, owing to its six binding sites (four binding sites in Ct3−). The larger the stability coefficient is, the more closely the chelates combine with rare earth. Based on the above analysis, we can conclude that the difference in chelate structure results in the difference in morphology due to its influence on growth orientation, which can also be related to the difference in the emission intensity. Depending on the UC emission intensity, it is possible to vary particles size and shape and then to choose S3 for further characterization.

Figure 6 shows the UC emission spectra of hexagonal prisms (S3) NaLuF4:12 mol% Er3+ under 1.54 μm laser diode at various pump powers. The 659 nm red emission is predominant, which is attributed to the 4F9/2 4I15/2 transitions of Er3+. Besides, from the spectra, it is obvious that the emission intensities become stronger with increasing pump powers. It is well known that the relationship between the UC emission intensity and the pump power for unsaturated UC processes could be expressed as follows [30]: where is the fluorescent intensity, is the pump power, and is the absorbed photon numbers per visible photon emitted. For the strongest emission peak at 659 nm, a plot of log ( ) versus log ( ) yields a straight line with a slope n for the various power pumps as shown in the inset of Figure 6, and the value obtained is equal to about 2.60. Hence, it is a three-photon process, which agrees with the three-step sequential transitions from the 4I15/2 ground state to the 4I13/2 intermediate state and then to the 4F9/2 state of Er3+ in the excitation process.

903208.fig.006
Figure 6: Power dependence of UC emission intensity of 4F9/2 4I15/2 emission of β-NaLuF4:12 mol%Er3+ submicron crystals using EDTA as surfactant under 1.54 μm laser diode pump. Inset shows dependence of UC emission intensity on excitation powder.
3.4. Synergistic Effect of β-NaLuF4:Er3+ Microcrystals

The excitation spectrum (Figure 7(a)) of β-NaLuF4:12 mol%Er3+ (S3) with hexagonal prisms shape shows that if the 659 nm red emission is monitored, an excitation peak centered at 416 nm is observed corresponding to the 4I15/2 2H9/2 transition of Er3+ ion. Figure 7(b) presents the room temperature photoluminescence spectra under 416 nm single excitation, 1.54 μm single excitation, and 416 nm and 1.54 μm dual excitation. The emission band of Er3+ ions centered at 659 nm is observed under 416 nm and 1.54 μm single excitation, respectively. It is inconceivable that an observable enhancement of the red emission band appears under the dual excitation, which is stronger than the sum of red emission intensity under single excitation. To quantitatively describe the synergistic efficiency, the absolute enhancement rate ( ) of red emission intensity can be defined as follows [20]: where , and are the integrated intensity of red emission bands under 416 nm excitation, under 1.54 μm excitation, and under 416 nm and 1.54 μm dual excitation, respectively, the excitation power of the 416 nm irradiation was fixed at 0.8 mW, and the excitation power ( ) of 1.54 μm laser was adjusted from 2.2 mW to 42.0 mW. The results of absolute enhancement rate ( ) of red emission (659 nm) for the as-prepared β-NaLuF4:12 mol%Er3+ microcrystals with different shapes morphologies are shown in Figure 7(c). There is no synergistic effect on spheres and disks, because the intensities of emission bands are poor there is not exist the 659 nm emission The maximum absolute enhancement rate can be up to 8% in hexagonal prisms with the excitation powers and . It is clear that when it means that certain thermal energy dissipation should be eliminated and transferred to the excitation energy in the dual excitation process. As can be seen, the ranking of absolute enhancement rate values is . The red emission of 4F9/2 4I15/2 shows an unusual enhancement rate using different surfactants. We believe that the surfactants can influence the nonradiative process of these samples and cause these changes. Moreover, the different reflectance losses at the particle-air interface may influence the UC emissions of surface-modified NaYF4:Yb, Er, which has been demonstrated in a recent report by Tan and coworkers [31].

fig7
Figure 7: (a) Photoluminescence excitation spectra ( ) of β-NaLuF4:12 mol%Er3+ with hexagonal prisms morphology; (b) visible photoluminescence spectra of β-NaLuF4:12 mol%Er3+ under 416 nm excitation (black line), under 1.54 μm excitation (red line), and under 416 nm and 1.54 μm dual excitation (blue line) ( , ); (c) absolute enhancement ( ) for the as-prepared β-NaLuF4:12 mol%Er3+ crystals using different surfactants with increasing 1.54 μm laser power under 416 nm and 1.54 μm dual excitation.
3.5. Synergistic Mechanism Analyses

Considering the slope and energy diagrams of Er3+, the proposed pathways for the synergistic mechanism combined downconversion and upconversion under dual excitation are demonstrated in Figure 8. Under 416 nm excitation, which corresponds to the 4I15/2 2H9/2 transition of Er3+ ion, the 659 nm red emission band also represents the 4F9/2 4I15/2 transition of some Er3+ ions after successive multiphonon nonradiative relaxation from the 2H9/2 to 4F9/2 state. Under 1.54 μm excitation, Er3+ ions can absorb energy by ground state absorption (GSA) and excited state absorption (ESA) consecutively to populate their 4I13/2and 4I9/2 states in succession. Then, some of Er3+ ions at 4I9/2 level will nonradiatively relax (NR) to the 4I11/2 level. NaLuF4 host lattice owns lower phonon energy of less than 400 cm−1, and the NR process ( 2500 cm−1) can be realized according to energy gap law [32]. Then the crossrelaxation process from 4I13/2 to 4I11/2 and ESA process occur simultaneously: Er3+ (4I13/2 4I15/2) and Er3+ (4I11/2 4F9/2), which results in the 659 nm red upconversion emission (4F9/2 4I15/2). Besides, the 4I11/2 and 4I13/2 levels are also populated through NR process in downconversion process. The energy in the 4I11/2 and 4I13/2 levels of Er3+ ions is usually dissipated via thermal energy; however, under the 416 nm and 1.54 μm coexcited, the Er3+ ions in the 4I11/2 and 4I13/2 levels from NR process can be excited again to the 4F9/2 level by absorbing 1.54 μm photons. An energy loop chain can be described as follows: Therefore, the energy of nonradiative relaxation is excited again, enhancing the intensity of red emission ( ) under the dual excitation. As can be concluded, the Er3+ ions from the nonradiative relaxation process create a loop chain in the excitation circuit and offer a reexcited source.

903208.fig.008
Figure 8: Synergistic effect of visible downconversion and IR upconversion in β-NaLuF4:Er3+ phosphors photoexcited with both 416 nm and 1.54 μm photons.

4. Conclusions

In summary, we successfully synthesized β-NaLuF4:Er3+ microcrystals with different surfactants by the hydrothermally method. The organic additives employed in the synthesis process played a significant role. It proved that the chemical nature of the surfactants (EDTA, PVP-K30, and Ct3−) differently acts with Ln3+ ions in solution, influencing the formation and growth process of the microcrystals. The EDTA-modified particles were found to be much more efficient and brighter than others, in which the intense 659 nm red emission band was observed. Under 416 nm and 1.54 μm dual excitation, the 8% absolute enhancement rate of the red emission band originating from the 4I11/2 and 4I13/2 states of Er3+ ion from the nonradiative relaxation process in the DC route can be excited again by absorbing the 1.54 μm IR photons in the UC route.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

This work was supported by the key Project of National High Technology Research and Development Program of China (863 programs) (no. 2011AA050526), National Natural Science Foundation of China (no. 51032002), the Science and Technology Support Plan of Jiangsu Province (BE2011191), and the Funding of Jiangsu Innovation Program for Graduate Education (no. CXZZ12_0137) as well as the Fundamental Research Funds for the Central Universities.

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