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Luminescence Properties of Eu(III) Complex/Polyvinylpyrrolidone Electrospun Composite Nanofibers Xiaoping Zhang,† Shipeng Wen,*,† Shui Hu,‡ Qi Chen,§ Hao Fong,§ Liqun Zhang,*,†,‡ and Li Liu*,†,‡ Key Laboratory of Carbon Fiber and Functional Polymer, Ministry of Education, Beijing UniVersity of Chemical Technology, Beijing 100029, China, Key Laboratory of Beijing City on Preparation and Processing of NoVel Polymer Materials, Beijing UniVersity of Chemical Technology, Beijing 100029, China, and Department of Chemistry, South Dakota School of Mines and Technology, Rapid City, South Dakota 57701 ReceiVed: December 18, 2009; ReVised Manuscript ReceiVed: January 28, 2010

Herein, we report that an innovative type of 1D nanomaterials, electrospun composite nanofibers (diameters ∼175 ( 25 nm) containing a complex of Eu(TTA)3phen in a matrix of polyvinylpyrrolidone (PVP), was prepared and characterized, and their luminescent properties were evaluated. The study revealed that, when the content of Eu(TTA)3phen was low (i.e., less than 23 wt %), the Eu(TTA)3phen was predominantly distributed in the composite nanofibers as molecular clusters and/or nanoscaled particles with sizes smaller than 10 nm; this substantially enhanced intermolecular interactions between individual Eu(TTA)3phen molecules and the neighboring chain segments of PVP. The Judd-Ofelt theory was used to study the effect of the dispersion of Eu(TTA)3phen on the local chemical environment of Eu3+ ions. The composite nanofibers demonstrated a significant improvement of luminescent efficiency compared with neat Eu(TTA)3phen. Introduction One-dimensional (1D) nanomaterials, including nanowires, nanorods, nanotubes, and nanofibers, have attracted enormous interests in recent years due to large specific surface areas and quantum/confinement effects;1 these nanomaterials are expected to play vital roles in the development of innovative electronic and/or optoelectronic devices.2,3 A variety of synthesis and fabrication methods have been reported to successfully develop 1D nanomaterials;1 among them, the technique of electrospinning is of particular interest. Electrospun nanofibers possess extraordinary properties, including small diameters (ranging from tens to hundreds of nanometers) and the concomitant large specific surface areas, high degree of structural perfection, and the resulting superior mechanical and electronic properties. Unlike other 1D nanomaterials, such as nanowires and nanorods, most of which are produced by expensive bottom-up synthetic methods, the electrospun nanofibers are produced through a lowcost, top-down nanomanufacturing process; additionally, they are continuous and easy to align, assemble, and process into applications.4-7 Some coordination complexes containing rare earth elements, such as europium (Eu) β-diketones, possess distinct and desired luminescent characteristics, including sharp emission bands, long lifetimes, and high quantum efficiencies;8 nonetheless, the complexes have low processing capability, poor thermal stability, and weak mechanical strength, which hinder their applications in the fabrication of optical switches and fluorescent probes.9-11 One potential solution is to incorporate the com* To whom correspondence should be addressed. E-mail: liul2001cn@ yahoo.com.cn (L.L.), [email protected] (S.W.), [email protected] (L.Z.). Tel: (+86)-10-6443-4860. Fax: (+86)-10-6443-3964. † Key Laboratory of Carbon Fiber and Functional Polymer, Ministry of Education, Beijing University of Chemical Technology. ‡ Key Laboratory of Beijing City on Preparation and Processing of Novel Polymer Materials, Beijing University of Chemical Technology. § South Dakota School of Mines and Technology.

plexes into zeolites or mesoporous materials,12,13 organically modified silicates,14 and/or polymers.15,16 For example, research efforts have shown that the polymer-capped complexes have much improved luminescent properties, thermal stability, and mechanical flexibility.17-19 Wang and co-workers synthesized and evaluated luminescent copolymers containing Eu complexes, and their study revealed that the relative luminescent intensities enhanced proportionally with the increases of Eu-complex amounts in the copolymers.20 The uniform distribution of the complex units along macromolecular chains was the fundamental reason that accounted for the enhancement of luminescent efficiencies. It is noteworthy that the syntheses of such copolymers are often difficult; this is due to reasons such as the different polymerization activities between the monomers containing the complex components and other organic monomers as well as the complicated polymerization processes. Recently, Zhang and co-workers reported that composite nanofibers containing rare earth complexes in a polymer matrix were developed by electrospinning, and the thermal stability of the photoluminescence in the composite fibers was improved considerably over the pure complex.8,21 As compared with the synthetic method of copolymerization, the technique of electrospinning is more straightforward for convenient fabrication of composites containing complexes; therefore, if the similar distribution of complexes could be achieved by electrospinning, the technique would be a viable method for the preparation of innovative composites in the form of nanofibers with high luminescent properties. Herein, we report that composite nanofibers of Eu(TTA)3phen/ PVP (TTA ) 2-thenoyltrifluoroacetone, phen ) 1,10-phenanthroline, and PVP ) polyvinylpyrrolidone) with diameters of ∼175 ( 25 nm were successfully prepared through electrospinning. When the content of Eu(TTA)3phen was low (i.e., less than 23 wt %), the majority of Eu(TTA)3phen was uniformly distributed in the nanofibers as molecular clusters and/or nanoscaled particles with sizes smaller than 10 nm. The study

10.1021/jp9119843  2010 American Chemical Society Published on Web 02/12/2010

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Figure 1. SEM images (A, C, E) and TEM images (B, D, F,) showing the representative morphological structures of neat PVP nanofibers (A, B) and composite nanofibers with Eu(TTA)3phen contents of 11 wt % (C, D) and 23 wt % (E, F), respectively.

revealed that the luminescent efficiency for the 5D0 f 7F0-4 transitions of the Eu3+ ions in the composite nanofibers was distinguishably higher than that in the neat complex of Eu(TTA)3phen. This was because the uniform distribution of the Eu(TTA)3phen in the composite nanofibers substantially enhanced the interactions between individual Eu(TTA)3phen molecules and neighboring chain segments of PVP. Experimental Section A photoactive, anhydrous, rare earth organic complex of Eu(TTA)3phen was synthesized and characterized; the detailed synthesis and characterization methods and conditions are provided in the Supporting Information. The electrospinning solution was prepared by first dissolving PVP (Mw ) 1.3 × 106 g mol-1) in a solvent mixture of N,N-dimethylformamide and dichloromethane (volume ratio of 8/2) at the concentration of 12 wt %; subsequently, varied amounts of Eu(TTA)3phen were added, making the contents of Eu(TTA)3phen to PVP equal to 3, 7, 11, 15, 19, 23, 27, 31, and 34 wt %. This was followed by stirring for 24 h to acquire uniform solutions. The solutions were then placed into 10 mL plastic syringes having 90° blunt end stainless-steel needles with an inner diameter of 0.37 mm. The electrospinning setup included a high-voltage power supply (model no. ES30P), purchased from the Gamma High Voltage Research, Inc. (Ormond Beach, FL). During electrospinning, a positive high voltage of 15 kV was applied to the needle, and the flow rate of 0.1 mL h-1 was maintained using a syringe pump (model no. KDS 100) purchased from the KD Scientific Inc. (Holliston, MA). The distance between the plate and the tip of the needle was set at 25 cm. The nanofibers were collected as randomly overlaid mats on an electrically grounded copper plate wrapped with aluminum foil. After electrospinning, the composite nanofiber mats were dried in a vacuum oven at room temperature for 12 h before characterizations. An Hitachi S-4700 scanning electron microscope (SEM) was employed to examine the morphologies of the electrospun nanofibers. Prior to SEM examination, the specimens were sputter-coated with gold to avoid charge accumulation. A JEM3010 transmission electron microscope (TEM) equipped with a GENESIS 307 energy-dispersive X-ray spectroscope (EDS) was used to examine the dispersion of Eu(TTA)3phen in the composite nanofibers. X-ray diffraction (XRD) patterns were acquired from a Rigaku D/Max-2500 X-ray diffractometer. A rotating X-ray generator (40 kW, 20 mA) with Cu KR radiation

(λ ) 1.54 Å) was used in the XRD experiments, and the XRD profiles were recorded from 5° to 90°. The excitation and emission spectra were recorded at room temperature using an Hitachi F-4500 spectrophotometer equipped with a 450 W xenon lamp as the excitation source. The slit widths for both excitation and emission were set at 2.5 nm. The fluorescence dynamics of the samples were measured with an FLS920 instrument (Edinburgh). During the measurements, the 345 nm incident light generated from a microsecond flash-lamp was used as the excitation source, and an oscillograph was used to record the decay dynamics. Results and Discussion A. Morphology of Composite Nanofibers and Dispersion of Eu(TTA)3phen. Composite nanofibers consisting of Eu(TTA)3phen and PVP with an average diameter of 175 ( 25 nm were successfully prepared (Figure 1C,E). As compared with the nanofibers of neat PVP (Figure 1A), the composite nanofibers had slightly smaller diameters; this was because the presence of Eu(TTA)3phen led to an increase of excess charge density in the electrospinning filaments/fibers.22,23 The surface of the composite nanofibers was smooth without microscopically identifiable particles, suggesting that the Eu(TTA)3phen might be uniformly dispersed in the nanofibers. The dispersion and morphological structures of Eu(TTA)3phen in the composite nanofibers were further characterized by TEM. The results indicated that almost no particles could be observed when the Eu(TTA)3phen content was low (i.e., less than 11 wt %, Figure 1B). When the content of Eu(TTA)3phen was increased gradually, a few nanoparticles with sizes of 40-150 nm could be occasionally identified. The nanoparticles were aggregates of Eu(TTA)3phen, and they were distributed unevenly in the PVP matrices. It is noteworthy that only a small amount of nanoparticles could be observed even when the Eu(TTA)3phen content reached 23 wt % (Figure 1F). The TEM-EDS analyses of the composite nanofibers with Eu(TTA)3phen contents of 11 and 23 wt % (Figure 2A, insets I and II) were then carried out. By examination of the chemical compositions at five different locations in the samples, it was evident that the europium element existed in the nanoparticles as well as in the PVP matrix. The above results and analyses suggested that the majority of Eu(TTA)3phen was uniformly dispersed in the PVP matrix as molecular clusters and/or nanoparticles with sizes smaller than 10 nm, while a small

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Figure 2. (A) High-resolution TEM images of composite nanofibers with Eu(TTA)3phen contents of 11 wt % (I) and 23 wt % (II), and the insets are the EDS spectra collected at different locations. The scale bars in the TEM images are 30 nm. (B) XRD patterns of PVP nanofibers without Eu(TTA)3phen, composite nanofibers with Eu(TTA)3phen contents of 11 and 23 wt %, and the neat Eu(TTA)3phen complex.

amount of Eu(TTA)3phen existed as aggregates. This was mainly due to the following reasons: during electrospinning of solutions containing uniformly dispersed Eu(TTA)3phen molecules, the rapid evaporation of the solvent and the concomitant fast solidification of the filaments (within tens of milliseconds24) hindered the aggregation of Eu(TTA)3phen, resulting in that the Eu(TTA)3phen predominantly existed as molecular clusters and/ or nanoparticles with sizes of 10 nm or smaller in the composite nanofibers. With increasing the Eu(TTA)3phen content, aggregates could be probably formed even in the electrospinning solutions, leading to the nanoparticles in the resultant composite nanofibers (see Figure 1F). The above explanations were further supported by the results acquired from the XRD analyses of the composite nanofibers. As shown in Figure 2B, no diffraction peaks associated with Eu(TTA)3phen could be identified when its content was 11 wt %, indicating that, when the Eu(TTA)3phen content was low, almost all of the Eu(TTA)3phen existed in the PVP matrix as molecular clusters. When the Eu(TTA)3phen content was 23 wt %, the diffraction peaks attributed to the Eu(TTA)3phen aggregates started to appear. B. Luminescent Properties. The luminescent properties of the Eu(TTA)3phen/PVP composite nanofibers were then investigated, and the results were compared to those of the neat Eu(TTA)3phen. The excitation spectra for various samples are shown in Figure 3A around the band associated with the intense 5 D0 f 7F2 transition of Eu3+ ions. In the neat complex of Eu(TTA)3phen, there was a broad excitation band ranging from 250 to 450 nm, which was assigned to the π f π* electron transition of the ligands. Intriguingly, such an excitation for the Eu(TTA)3phen/PVP composite nanofibers blue shifted distinguishably and split into two bands centered at 270 and 345 nm, respectively. This suggested that the site symmetry of Eu3+ ions became lower in the composite nanofibers due to the influences of the neighboring chain segments of PVP.21 Additionally, both 7 F0 f 5D2 and 7F1 f 5D1 excitations for the composite nanofibers disappeared, suggesting that the f f f inner-shell transitions for the composite nanofibers were quenched through nonradiative energy transfers.8 In the emission spectra (Figure 3B), the bands at 580, 591, 612, 651, and 702 nm were assigned to the 5D0 f 7FJ (J ) 0, 1, 2, 3, 4) transitions, respectively; and the 5D0 f 7F2 hypersensitive transition at 612 nm was very intense, indicating that there was a highly polarized chemical environment around the Eu3+ ions. It is noteworthy that the integrated intensity ratios of 5D0 f 7F2 to 5D0 f 7F1 (I02/I01), which indicated the asymmetry of the coordination sphere of Eu3+ ions,25 were considerably higher for the composite nanofibers than that for the neat complex. The ratios of the neat

complex and composite nanofibers with Eu(TTA)3phen contents of 3, 11, 23, and 34 wt % were calculated to be 9.31, 16.21, 15.25, 14.22, and 13.88, respectively. The results suggested that the local environment around the Eu3+ ions was more disordered, and the degree of polarization for Eu(TTA)3phen was higher when the complex was impregnated into the PVP matrix; this further led to higher probability for the electronic dipole allowed transitions.26 Figure 4 shows the integrated emission intensity of I02 for the 5D0 f 7F2 transition as a function of the Eu(TTA)3phen content in the composite nanofibers. The emission intensity increased with increasing the Eu(TTA)3phen content and reached its maximal value at ∼23 wt %; this was followed by typical emission concentration quenching with further increasing of the Eu(TTA)3phen content. Such a quenching phenomenon was caused by the deactivation of the 5D0 and/ or 5D1 states through the electrostatic multipolar interactions and/or by the exciton migration via the Fo¨rster dipole-dipole mechanism,20,27 the mechanism of which has been illustrated in the Supporting Information. Compared with the composites prepared via copolymerization,20 the electrospun composite nanofibers achieved even higher Eu-complex contents without inducing the concentration quenching. This was attributed to the desired dispersion of Eu(TTA)3phen in the matrix of the composite nanofibers; arguably, such a dispersion might be even more preferred than the distribution of complex units along macromolecular chains. In the composite nanofibers, most of the complex dispersed in the nanofiber matrix as molecular clusters and/or nanoparticles with sizes smaller than 10 nm when the content of Eu(TTA)3phen was low; the exciton migration via diffusion induced collision among Eu3+ ions was negligible. When the content of Eu(TTA)3phen was high, on the other hand, some aggregates with sizes of tens of nanometers formed. Such aggregates led to high Eu3+ concentrations locally and were responsible for the emission concentration quenching.27 C. Temperature Dependence of Emission Intensity. To investigate the thermal stability of the photoluminescence, the temperature dependence of the luminescent intensity was measured under 345 nm excitation in the temperature range of 73K-293 K. Figure 5 shows the dependence of the integrated PL intensity of the 5D0 f 7F2 transitions on temperature for the neat Eu(TTA)3phen complex and composite nanofibers. It was evident that the PL intensity decreased with increasing temperature. Compared with the neat complex, the composite fibers showed a different variation of the PL intensity for Eu3+. The integrated PL

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Figure 3. (A) Excitation spectra (λem ) 612 nm) and (B) emission spectra (λex ) 345 nm) for the 5D0 f 7FJ transitions of Eu3+ ions in the neat Eu(TTA)3phen complex (curve b) and in the composite nanofibers with Eu(TTA)3phen contents of 11 wt % (curve c) and 23 wt % (curve d). (C) Schematic representation of the intermolecular interactions between Eu3+ ions in the complex cluster and the neighboring chain segments of PVP.

Figure 4. Correlation of the integrated emission intensity for 5D0 f 7 F2 transitions of Eu3+ ions versus the Eu(TTA)3phen content in the composite nanofibers.

intensity as a function of temperature could be well-fitted by the well-known thermal activation function.11,28

I0 I(T) ) 1 + R exp(-∆E/KBT)

(1)

where I0 is the emission intensity at 0 K, R is the proportional coefficient, ∆E is the activation energy of the thermal

Figure 5. Dependence of the integrated PL intensity of the 5D0 f 7F2 transitions on temperature: (a) the neat Eu(TTA)3phen complex and (b) the composite nanofibers with Eu(TTA)3phen content of 11 wt %. The data points are the experimental results, and the solid lines are the fitting curves to the thermal activation equation.

quenching process, KB is the Boltzmann constant, and T is the absolute temperature. By fitting the experimental data using eq 1, the values of ∆E in the neat Eu(TTA)3phen complex and the composite nanofibers with a Eu(TTA)3phen content of 11 wt % were obtained to be 98.1 and 201.4 meV, respectively. The improved value of ∆E in the composite nanofibers suggested that the thermal stability of the photo-

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TABLE 1: Solid-State Spectroscopic Parameters for the 5D0 Luminescence of the Eu(TTA)3phen Complex and the Eu(TTA)3phen/PVP Composite Nanofibers samples Eu(TTA)3phen Aa B C

Ω2 Ω4 (×10-20 cm2) (×10-20 cm2) 15.91 26.04 24.02 23.69

4.64 5.81 5.40 5.30

ARAD (s-1)

ANR (s-1)

τobs (µs)

Φ (%)

600.77 926.88 858.66 847.52

593.77 403.79 623.15 666.39

837.14 751.50 674.85 660.54

50.29 69.66 57.95 55.98

a The samples A, B, and C are the composite nanofibers with Eu(TTA)3phen contents of 11, 23, and 34 wt %, respectively.

luminescence was much higher than that in the neat complex. In the composite, the majority of the Eu(TTA)3phen molecules distributed in the PVP nanofiber matrix as molecular clusters and/or nanoscaled particles; the exciton migration from one excited Eu3+ to another Eu3+ and the energy vibration transitions between the complexes were restrained, owing to the effective capping of PVP on the Eu(TTA)3phen complex, resulting in the improved thermal stability of the photoluminescence.26 D. Judd-Ofelt Analysis and Luminescent Quantum Efficiency. The Judd-Ofelt theory is a useful tool for analyzing f-f inner shell electronic transitions.29 Interaction parameters of ligand fields are given by the Judd-Ofelt parameters Ωλ (λ ) 2, 4, and 6), in which Ω2 is sensitive to the symmetry and sequence of ligand fields.30 The experimental values of Ω2 and Ω4 were determined, respectively, from the 5D0 f 7F2 and 5D0 f 7F4 transitions (Figure 3B) using the magnetic dipole transition of 5D0 f 7F1 as the reference.31-33 The detailed principles and specific calculations were provided in the Supporting Information, and the calculated values of Ω2 and Ω4 for the neat complex and the composite nanofibers were listed in Table 1. It was evident that the composite nanofibers had higher Ω2 values than the neat complex, suggesting an increase of the covalence degree in the first coordination shell of Eu3+ ions and an enhancement of the 5D0 f 7F2 hypersensitive transition.34 This was also due to the change of the chemical environment surrounding Eu3+ ions, which was induced by the intermolecular interactions between Eu(TTA)3phen and neighboring chain segments of PVP, as schematically shown in Figure 3C. The higher values of Ω4 for the composite nanofibers as compared with that of the neat complex indicated a perturbation on the coordination effect of the bidentate TTA by the steric factors from the surrounding PVP.35 Furthermore, the values of Ω2 and Ω4 decreased with increasing the complex content, suggesting that the effect of neighboring PVP chain segments on the ligands fields of Eu3+ was gradually weakened. Such a phenomenon was also attributed to the uniform dispersion of Eu(TTA)3phen in the nanofiber matrix. When the content of Eu(TTA)3phen was lower than 11 wt %, most of the complex existed as molecular clusters and the chemical environment around the Eu3+ ions was significantly affected by the surrounding PVP, whereas such an influence reduced gradually with the increase of the complex content and the resulting formation of aggregates. The luminescent quantum efficiency (Φ) for the 5D0 f 7F0-4 transitions of Eu3+ ions fundamentally determines the luminescent properties of the complex, and Φ is defined as ARAD/(ARAD + ANR),36 where ARAD is the total radiative transition rate of the 5 D0 f 7F0-4 transitions and ANR is the nonradiative transition rate. The ARAD, ANR, and Φ values of the neat complex and the composite nanofibers were calculated based upon the Judd-Ofelt theory, and the detailed calculations are provided in the Supporting Information. The typical luminescence decay curves

Figure 6. Luminescence decay curves of the excited Eu3+ ions at the 5 D0 level for the neat Eu(TTA)3phen complex and composite nanofibers with Eu(TTA)3phen contents of 11 wt % (A), 23 wt % (B), and 34 wt % (C).

for the excited Eu3+ ions at the 5D0 state under 345 nm excitation are illustrated in Figure 6, and they were described in terms of a single-exponential function within the experimental errors. The obtained lifetime values (τobs) and ARAD, ANR, and Φ values of the neat complex and the composite nanofibers are listed in Table 1. The ARAD values of Eu3+ for the composite nanofibers increased approximately 250-330 cm-1 as compared with that for the neat complex due to the enhanced electronic dipole transition, making the Φ values of Eu3+ significantly higher. Intriguingly, both the ARAD and the Φ values of Eu3+ in the composite nanofibers decreased with increasing the complex content. This was mainly due to the following two reasons. (1) The status of the uniform dispersion of Eu(TTA)3phen in the nanofiber matrix: when the complex content was low, most of the Eu(TTA)3phen complex dispersed uniformly in the nanofiber matrix as molecular clusters and/or nanoparticles, which maximized the luminescence of individual Eu(TTA)3phen molecules and considerably enhanced the luminescent efficiency. With the increase of the Eu(TTA)3phen content, some aggregates formed in the fiber matrix. The exciton migration between the Eu(TTA)3phen molecules resulted in the luminescence quenching of the complex, which further led to the decrease of ARAD and Φ values, as shown in Table 1. (2) The interaction between the Eu(TTA)3phen molecules and neighboring chain segments of PVP led to the increased polarization degree of Eu3+ ions, which significantly enhanced the electronic dipole allowed transitions of Eu3+ ions. Accordingly, the composite nanofibers exhibited higher values of ARAD and Φ as compared with the neat Eu(TTA)3phen. As the Eu(TTA)3phen content increased, the Eu(TTA)3phen aggregates formed in the fiber matrix. The interaction between the particles and neighboring chain segments of PVP became weaker, which led to the decrease of the ARAD and Φ values of Eu3+ ions. The above results demonstrated that the composite nanofibers with a low content of the Eu(TTA)3phen complex uniformly distributed in the PVP matrix significantly outperformed the neat complex in achieving a high luminescent efficiency, and the composite nanofibers could be an innovative type of nanomaterial for applications in the fields of optical switches and fluorescent probes. Conclusion In summary, Eu(TTA)3phen/PVP composite nanofibers with an average diameter of 175 ( 25 nm were prepared by electrospinning. TEM and EDS analyses indicated that the Eu(TTA)3phen complex predominantly existed as molecular clusters and/or nanoparticles with sizes smaller than 10 nm when the content of Eu(TTA)3phen was low (i.e., less than 23 wt %).

Eu(III) Complex/Polyvinylpyrrolidone Nanofibers Luminescence studies revealed that such a level of dispersion of Eu(TTA)3phen in the composite nanofibers resulted in the distinguishable enhancement of the 5D0 f 7F2 hypersensitive transition, leading to a considerably higher luminescent efficiency of the Eu3+ ions. Spectroscopic parameters (Ω2, Ω4, ARAD, and ANR) of the Eu3+ ions were then calculated based on the Judd-Ofelt theory; the dispersion status of Eu(TTA)3phen on the chemical microenvironment of Eu3+ ions and the reasons that resulted in the enhancement of luminescent efficiency were addressed. This study provided new guidance in the design and fabrication of innovative composites containing rare earth complexes, and the developed 1D nanomaterials could find important applications, particularly in optical switches and fluorescent probes. Acknowledgment. The research project was supported by the Program for New Century Excellent Talents in University (NCET) and the Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT, IRT0807). Supporting Information Available: Experimental details; molecular structure of the Eu(TTA)3phen complex; Judd-Ofelt analysis; table of the calculated and experimental Φ values; and discussion of the fluorescence dynamics, sensitization luminescence, and quenching mechanism. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Xia, Y. N.; Yang, P. D.; Sun, Y. G.; Wu, Y. Y.; Mayers, B.; Gates, B.; Yin, Y. D.; Kim, F.; Yan, H. Q. AdV. Mater. 2003, 15, 353. (2) Duan, X. F.; Hang, Y.; Cui, Y.; Wang, J. F.; Lieber, C. M. Nature 2001, 409, 66. (3) Sui, X. M.; Shao, C. L.; Liu, Y. C. Appl. Phys. Lett. 2005, 87, 113115. (4) Thompson, C. J.; Chase, G. G.; Yarin, A. L.; Reneker, D. H. Polymer 2007, 48, 6913. (5) Greiner, A.; Wendorff, J. H. Angew. Chem., Int. Ed. 2007, 46, 5670. (6) Camposeo, A.; Benedetto, F. D.; Cingolani, R.; Pisignano, D. Appl. Phys. Lett. 2009, 94, 043109. (7) Dzenis, Y. Science 2004, 304, 1917. (8) Zhang, H.; Song, H. W.; Yu, H. Q.; Bai, X.; Li, S. W.; Pan, G. H.; Dai, Q. L.; Wang, T.; Li, W. L.; Lu, S. Z.; Ren, X. G.; Zhao, H. F. J. Phys. Chem. C 2007, 111, 6524. (9) Xu, Q. H.; Li, L. S.; Liu, X. S.; Xu, R. R. Chem. Mater. 2002, 14, 549. (10) Yuan, Y. F.; Cardinaels, T.; Lunstroot, K.; Hecke, K. V.; Meervelt, L. V.; Go1rller-Walrand, C.; Binnemans, K.; Nockemann, P. Inorg. Chem. 2007, 46, 5302.

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