N-Doped ZnS Nanoparticles Prepared through an Inorganic−Organic

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N-Doped ZnS Nanoparticles Prepared through an Inorganic-Organic Hybrid Complex ZnS · (piperazine)0.5 Ji Wook Jang,† Sun Hee Choi,*,‡ Jum Suk Jang,† Jae Sung Lee,*,† Seungho Cho,† and Kun-Hong Lee† Department of Chemical Engineering, Pohang Accelerator Laboratory, Pohang UniVersity of Science and Technology (POSTECH), San 31, Hyojadong, Namgu, Pohang 790-784, Korea ReceiVed: August 5, 2009; ReVised Manuscript ReceiVed: October 12, 2009

N-doped ZnS nanoparticles with wurtzite phase was synthesized at 150 °C, derived from an inorganic-organic complex, ZnS · (piperazine)0.5. The metastable ZnS · (piperazine)0.5 nanohybrid materials could be described as the layered structure where wurtzite ZnS layers are connected to each other through the bondings of nitrogen atoms in piperazine. It was found that with the progress of the synthetic reaction, the interlayer piperazine molecules were moved out of the layers and the phase was transformed into wurtzite ZnS. Interestingly, nitrogen atoms in piperazine could be doped into ZnS in the extraction of the interlayer molecules. Phase transition was studied by using various techniques, including powder X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and high-resolution scanning transmission electron microscopy (HRSTEM). The N-doping was characterized with UV-vis spectroscopy, and experimental and theoretical analyses of X-ray absorption structure (XANES). The N-doped ZnS was applied to the photocatalytic degradation of dye under visible light irradiation. Introduction It is a breakthrough in materials science to create hybrid materials holding both inorganic and organic properties in a single structure. Inorganic compounds that exhibit useful optical, electrical, and magnetic properties are improved in their processing by incorporation of organic molecules with good mechanical flexibility.1,2 In another point of view, functional organic molecules such as biomolecules show better stability and performance if introduced in an inorganic matrix.3 Whichever the desired function is delivered from, the combination of two counterparts could be done at mild conditions and the geometrical control such as size and shape is facilitated. Organic species are connected to inorganic ones through diverse interactions of van der Waals, H-bonding, ionic, coordinative, or covalent bonding. The last two interactions are strong enough to form a stable structure and consequently, applied to the synthesis of hybrid semiconductors. After Huang at al. reported about the covalent networks of hybrid chalcogenides,4 the hybrid materials of II-VI family have been much studied for their functionalities in terms of structural diversity.2,5-7 As an important II-VI material, zinc sulfide (ZnS) is either sphalerite or wurtzite phase, which is the high-temperature polymorph of sphalerite.8 It is generally used as a phosphor in electroluminescent devices, and is recently applied in the solar cells.9 ZnS nanocrystals also show activity for the photocatalytic decomposition of organic materials due to trapped holes arising from surface defects on the sulfides.10 The hybrid materials with wurtzite ZnS fragments could be synthesized at low temperatures below 200 °C via a wet chemical route and their thermal decomposition at 250-500 * To whom correspondence should be addressed. E-mail: (S.H.C.) [email protected]; (J.S.L.) [email protected]. Tel: +82-54-279-1552. Fax: +82-54-279-1599. † Department of Chemical Engineering. ‡ Pohang Accelerator Laboratory (PAL).

°C generates wurizite ZnS with further oxidation into ZnO.5,11,12 Organic linear-type diamine molecules used as a liquid medium act as a structure-directing, coordinative template that leads to a two-dimensional platelet morphology of the product.13 However, the hybrid is usually produced to be bulk, that is, several micrometers of lateral dimension in a plate. In this study, we paid attention to organic-inorganic interaction in a solution to synthesize nanosized ZnS particles with wurtizite phase at 150 °C. Inherited from organic molecules, the product would have trace nitrogen atoms in its lattice. Detailed characterization was carried out to examine the electronic structures and the optical properties, and the product was applied to the photocataytic degradation of Orange II dye under visible light irradiation. Experimental Methods Synthesis of Hybrid Materials. ZnS-based nanohybrid materials were synthesized by incorporating sulfur into a metal organic framework. The metal organic framework was organized by mixing 4 mmol zinc chloride (ZnCl2, Aldrich, 98%) and 40 mmol piperazine (C4H10N2, Aldrich, 99%) in 140 mL of dimethylformamide (HCON(CH3)2, Mallinckrodt, 99.8%) at room temperature. After the transparent solution changed to white, 12 mmol thiourea (NH2CSNH2, Aldrich, 99%) was added. The solution temperature was then increased to 150 °C. After synthetic reaction was performed in reflux, the final precipitates were filtered and washed with deionized water and ethanol, and dried at 80 °C for 3 h in vacuum. To eliminate organic residuals in obtained products, the dried sample was treated at 400 °C for 2 h in air under ambient pressure. Depending on the reaction time and post-treatment, the samples were designated as Z-3 (3 h of reaction time), Z-24 (24 h of reaction time), and Z-24400 (24 h of reaction time; heat treatment at 400 °C). Also, the reference material ZnS(en)0.5 (en ) ethylenediamine) was solvothermally synthesized by the same procedure in our previous work.12 Thus 16.2 mmol Zn(NO3)2 · 6H2O and 48.6 mmol thioura (NH2CSNH2) were added into a Teflon-lined

10.1021/jp907526e CCC: $40.75  2009 American Chemical Society Published on Web 10/28/2009

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stainless steel autoclave that had been filled with ethylenediamine to 70% of its volume. The autoclave reactor was maintained at 160 °C for 12 h and allowed to cool to room temperature. Precipitates were filtered and washed with deionized water and absolute ethanol and then were dried in a vacuum oven at 70 °C for 2 h. Characterization. The crystalline phase of obtained products was identified with powder X-ray diffraction (XRD). The XRD patterns were collected by using Cu KR radiation (PANalytical X’Pert) The morphologies were examined with high-resolution field emission scanning electron microscopy (HR-SEM) (JEOL JSM-7401F, 8 kV) and the Cs-corrected high-resolution scanning transmission electron microscope (Cs-corrected HR-STEM) (JEOL JEM 2200FS, 200 kV). Before the measurement of Cscorrected HR-STEM, the powder materials were dispersed in ethanol using sonic treatment, dropped onto a holey carbon film on a copper grid, and dried in an oven at 80 °C. The optical properties of the products were measured by a UV-vis spectrophotometer (Shimadzu UV-2401PC) at room temperature. X-ray absorption fine structure (XAFS) was applied to investigate the local electronic and the local geometric structures of nanostructured materials. Synchrotron X-ray measurements were performed on 5A wiggler beamline of Pohang Accelerator Laboratory (2.5 GeV; 150-180 mA), Korea. The incident beam was monochromatized using a Si(111) double crystal monochromator. The spectra for K-edge of Zn (Eo ) 9659 eV) were taken at room temperature in a transmission mode with separate He-filled IC Spec ionization chambers for incident and transmitted beam. The energy scan was performed in five regions for good energy resolution in a steep absorption and measurement of XANES and EXAFS spectra at a time, 5 eV-step in the region of 9459-9609 eV, 1 eV-step in 9609-9649 eV, 0.2 eV-step in 9649-9689 eV, 0.03 k-step in 9689-10199 eV, and 0.04 k-step in 10199-10659 eV. The obtained data were processed in the usual way to obtain the absorbance and analyzed with ATHENA in the IFEFFIT suite of software programs.14 To investigate the physical texture of the nanostructures, measurements of N2 adsorption-desorption isotherms at 77 K were performed in a constant volume adsorption apparatus (Micrometrics ASAP 2010) at relative pressures (P/P0) ranging from 10-4 to 0.995. Before the measurement, the samples were degassed for 4 h at 120 °C under 10-4 torr. The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method.15 The C, N, and H contents of the products were analyzed by an elemental analyzer (Analysennsysteme GmBH, Vario EL II). Photocatalytic Reaction. Visible light-induced photocatalytic activities of obtained products were investigated by using degradation of Orange II dye purchased from Aldrich. Thus 50 µM of Orange II dye was stirred with 0.1 g of catalyst for 30 min before illumination. A 500-W Hg arc lamp (Oriel 61945) was used as a light source. The light was passed through an IR filter and a cutoff filter (λ g 400 nm), and then the filtered light was focused onto the reactor. Sample aliquots were withdrawn with 1.5 mL and filtered through 0.45 µm PTTE filter. The degradation of Orange II was monitored by measuring the maximum absorbance around 484 nm as a function of irradiation time with a UV-vis spectrometer. Result and Discussion Preparation of Nanohybrid Materials and Phase Transition. The X-ray diffraction patterns of ZnS-based hybrid materials are shown in Figure 1. While Z-24 and Z-24-400 have the diffraction peaks at the same positions as those for bulk

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Figure 1. XRD patterns of ZnS-based nanohybrid and its derivative materials. The pattern of bulk ZnS(wurtzite) is displayed as a reference.

Figure 2. Fourier-transforms of Zn K-edge EXAFS for the products. (A) Magnitude function, (B) imaginary function.

wurtzite ZnS (JCPDS 36-1450), a new pattern is observed for Z-3. The pattern can be compared with that of ZnS(en)0.5 with a layered structure where ZnS inorganic layers are connected to each other through the bondings of nitrogen atoms in ethylendiamine.16 While an extraordinarily strong peak due to the interplanar distance of the layered structure appears at 10.22° for ZnS(en)0.5,17 a similar peak is observed at 9.78° for Z-3 product. The local structure around Zn was characterized with Fouriertransform analyses of extended X-ray absorption fine structure (EXAFS) as shown in Figure 2. The magnitude function of Fourier transforms reveals the neighboring atoms arranged according to distance from a central absorber atom, and the imaginary function characterizes the kind of the neighbor atom at a distance. The first shell at 0.7-2.3 Å in both Figure 2A and 2B are all similar, but the second shell at ca. 2.9-4.6 Å shows different characteristics, depending on the involved

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TABLE 1: Particle Size Calculated by Sherrer Equation plane Z-3 Z-24 Z-24-400

a

[110] [112] [110] [112]

particle size (nm) 7.5 7.6 3.3 6.9 5.5

a The plane corresponding to the diffraction peak at 9.78° was used for this calculation.

structures. The first shell of ZnS consists of four nearest S atoms with the bond distance of 2.35 Å, and ZnS(en)0.5 has one N at 2.17 Å and three S atoms at 2.34 Å as nearest neighbors. Little change in the first shell results in little variation in both spectra. In the case of the second shell, the next nearest 12 Zn at 3.81 Å and 10 S atoms at 3.89-4.47 Å make up the shell of ZnS. On the contrary, the second shell of ZnS(en)0.5 comprises 6 Zn at 3.56-3.70 Å, four N at 3.91-4.42 Å, four S atoms at 3.93-4.57 Å, three C atoms at 3.02-4.56 Å, and fifteen H atoms at 3.09-4.80 Å. These identifications are done with the aid of FEFF calculations for the corresponding theoretical structures.16,18,19 In Figure 2, the product Z-3 has an imaginary function similar to that of ZnS(en)0.5 for the shell at 2.9-4.2 Å, suggesting a structural similarity between the two material. Difference is observed in their magnitude functions. The magnitude function of Z-3 exhibits two peaks at 2.9-4.2 Å, but it seems that ZnS(en)0.5 has one peak. As multiple scattering is prominent in large particles, the magnitude function of bulk ZnS(en)0.5 is significantly affected and the peaks associated with specific bonds, that is single scatterings, are superimposed by multiple scatterings of Zn-N-H-N and Zn-N-C with scattering lengths comparable to the bond distances. However, Z-3 has less developed peaks, which is indicative of nanoparticles. The second shells of ZnS and the products Z-24 and Z-24400 extend up to 4.6 Å and their respective imaginary functions are almost same one another. The relative peak intensities are a little different in the magnitude functions of Z-24 and Z-24400. It is due to improved crystallinity and increased particle size by heat treatment at 400 °C. According to XRD patterns in Figure 1, Z-24-400 has stronger (002) and (100) peaks compared to Z-24. Materials with better crystallinity exhibit more multiple scatterings. The effect of multiple scattering is also observed in the magnitude function of bulk ZnS whose intensity is much higher than Z-24 and Z-24-400. The elemental analysis for the product Z-3 indicates that the material includes 9.20% N, 18.17% C, and 4.12% H. Accordingly, Z-3 can be expressed by the formula ZnS · (piperazine)0.5 where the theoretical content of its organic components is 30.65%. The difference between the experimental (31.49%) and the theoretical contents is attributed to organic molecules adsorbed strongly on the surface, not contributing to the formation of a structure. With close structural similarities with ZnS(en)0.5 in short and long-range orders, it is reasoned that Z-3 has the layered structure in which piperazine acts as interlayer molecules connecting ZnS layers. It is worth noticing that all XRD patterns of the products show broad peaks with very small intensities, representative characteristic of nanoparticles. The particle sizes calculated by Sherrer equation are given by Table 1. Nitrogen atoms in piperazine enables combination of organic properties with inorganic ZnS layers through coordinative covalent bondings, but steric hindrance derived from piperazine molecules would prevent particles from being clumped in the synthesis. The electron microscopy images in Figure 3 show that the product Z-3 almost consists of echinoid nanohybrids. The

Figure 3. Electron microscopy images of Z-3 product. (a) FE-SEM image of overall nanostructures, (b) magnified FE-SEM image of echinoid nanostructure, (c) HR-TEM image of echinoid nanostructures, and (d) HR-STEM image for a circle region in (c) showing some specific planes.

nanorods with ca. 7 nm diameter aggregate in a circular shape. A detailed examination of the echinoid structures by HR-STEM (Figure 3d) reveals that the nanostructures have some specific planes. The structural closeness between ZnS(en)0.5 and Z-3 implies that the shown planes represents interlayer distance, like (200) plane in ZnS(en)0.5. According to the progress of synthetic

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Figure 5. Nitrogen adsorption-desorption isotherms measured at 77 K.

Figure 6. Schematic view of Z-3 and Z-24 structures.

Figure 4. Electron microscopy images of Z-24 product. (a) FE-SEM image of the nanostructures, (b) HR-STEM image of the nanostructure, and (c) magnified HR-STEM image for a circle region in (b), showing hexagonal shape and (100) planes.

reaction, the nanohybrid phase of ZnS · (piperazine)0.5 get transformed into wurtzite ZnS nanoparticles. However, the echinoid morphology is preserved as shown in FE-SEM images of Figure 4. It appears that nanostructures with ca. 3.5 nm dimension are embedded in the overall structure (Figure 4b). In the magnified HR-STEM image of Figure 4c, a honeycomblike shape is observed and the interplanar distance of the structures is identified to be 0.330 nm, corresponding to that of (100) plane of hexagonal ZnS. N2 adsorption-desorption isotherms in Figure 5 reveal the physical texture of the materials. Z-3 does not develop a hysteresis loop. But, Z-24 exhibits similar isotherm patterns corresponding to Type IV hysteresis, which is associated with slit-shaped pores or the space between parallel plates.20 The specific surface area obtained by the Braunauer-Emmett-Teller (BET) method is 0.306, 75, 226, and 86.4 m2/g for ZnS, Z-3, Z-24, and Z-24-400, respectively. Correlated with the XRD

Figure 7. UV-vis diffuse reflectance spectra of the products. Those of ZnS and ZnS(en)0.5 are compared.

results and electron microscopy images, the products can be depicted in Figure 6. Optical Properties and Local Electronic Structures. The materials with echinoid morphology display interesting optical properties. In the UV-vis diffuse reflectance spectra in Figure 7, the product Z-3 has the absorption peak near 250 nm, similar to ZnS(en)0.5. The large blue-shift with respect to bulk ZnS is attributed to strong quantum confinement effect due to the novel structure, similar to other II-VI hybrid materials.2,4-6 The extremely thin thickness of inorganic layer in the hybrid structure (2.5-2.6 Å) contributes to the increased band gap energy by about 1.3 eV compared to its parent inorganic compound. On the contrary, a weak blue-shift of absorption is observed for Z-24 whose phase is identified to wurtzite ZnS. It

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Figure 9. Proposed mechanism of N-doping in the synthesis.

Figure 8. (A) Experimental Zn K-edge XANES spectra of the products and (B) theoretical absorption spectra of ZnS1-xNx (x ) 0, 0.03, 0.06, 0.15). The inset plot included in A shows magnified spectra for the peak a.

is totally induced from the dimensions of nanoparticles. The Bohr radius of bulk ZnS is about 2.5 nm,21 which is comparable to the particle size of Z-24. Moderate quantum confinement comes into effect due to the thickness (ca. 3.5 nm) rather than lateral direction of the sheet in the Z-24 sample. Absorption bands at 400-600 nm of Z-3 and Z-24 are mostly attributed to piperazine molecules adsorbed on the surface. But, Z-24 exhibits a higher absorbance at 400-600 nm compared to Z-3. When ZnS(piperazine)0.5 changes to wurtzite ZnS, all interlayer piperazine molecules will not be removed from the layered structure and some Zn-piperazine are produced from Znpiperazine-Zn structural units. The absorbance at 400-600 nm of Z-3 is only due to piperazine adsorbed on the surface of the layered structure, while that of Z-24 is enhanced by additional Zn-piperazine surface species formed during the phase transformation. The heat treatment for Z-24 removes the absorption band, but a weak absorption shoulder is still observed. The electronic structure accompanied during the transformation from ZnS · (piperazine)0.5 into ZnS was investigated by X-ray absorption near-edge structure (XANES) analyses in Figure 8. The Zn K-edge absorption reflects an electric dipole transition from 1s core level to unoccupied states of p type. Impurity-free ZnS and ZnS(en)0.5 have the same intensity at peak a1 as 1.23, but different shape resonances. This suggests that although Zn atoms in the two compounds have different coordination environments, they have same valence electrons in the outermost levels. In the case of the product Z-3, the a1 peak intensity is also 1.23 and the shape resonance (feature around peak a2) is different from those of pure ZnS and ZnS(en)0.5. However, Z-24 exhibits increased a1 peak intensity (1.28) although its feature in the continuum resonance is almost same as that of wurtzite ZnS. The heat treatment (Z-24-400) increases the peak intensity with preserving the structure. The increase is probably due to N-doping into the product ZnS.

When nitrogen with one less valence electrons replaces sulfur in forming the structure, the 4p orbital of Zn atom gets unoccupied, resulting in the enhancement of the absorption peak. The electronic structure is further studied by the theoretical calculations for pure and N-doped ZnS in Figure 8B. Theoretical Zn K-edge absorption spectra were simulated using FEFF 8.2 code, which adopted the Hedin-Lundqvisit exchange potential for excited states.19 A 46 atom-cluster was used to calculate the self-consistent field (SCF) muffin-tin potential, and a 136 atom-cluster for the full-multiple scattering (FMS) XANES calculations. For the calculations of ZnS1-xNx (x ) 0.03, 0.06, 0.15), N atoms were stoichiometrically situated in the lattice of ZnS. The theoretical data was shifted in energy to obtain better agreement between the experimental and theoretical ZnS spectra, and all other theoretical data were shifted by this same value. Inspection on the main features of the absorption spectra shows good agreement between the experimental (A) and theoretical (B) ZnS data. In the theoretical spectra of ZnS1-xNx, the peaks a1 and a2 increase with increasing x. The enhancement of the absorption peak is accordance with the band structure calculation by Yamamoto et al., which showed that N-doping generates a hole at the top of the valence band for ZnS.22 The mechanism of N-doping is proposed in Figure 9. After one Zn-piperazine bond in ZnS · (piperazine)0.5 is broken, a neighboring Zn-piperazine is bonded to that piperazineremoved Zn through nitrogen atom in piperazine. Another piperazine-removed Zn would interact with Zn-piperazine-Zn species carrying a positive charge momentarily and thus, only nitrogen can be doped into the ZnS lattice as the majority of piperazine-removed ZnS builds up. Photocatalytic Performance. Change in the band structure of ZnS by N-doping can extend the application of ZnS up to a visible light regime. Figure 10A shows the degradation of Orange II dye under visible light illumination (λ g 400 nm). While bulk ZnS with the band gap energy of 3.80 eV shows no photocatalytic activity, both Z-24 and Z-24-400 exhibit considerable activities. The dye was almost degraded on Z-24-400 catalyst after 4.2 h. The increased activity of Z-24-400 by a factor of ca. 8 compared to Z-24 is attributed to removal of organic residuals and increased doping. We also investigated the wavelength-dependence on the photocatalytic decomposition of organic dye. By using of different cutoff filters (λ g 299, 400, and 499 nm), we measured the whole photocatalytic activity over Z-24-400 catalyst and calculated the activity at the particular section of wavelengths in Figure 10B. The catalyst decomposes 99.5% of the dye with the filter of λ g 299 after irradiation for 1.5 h. It should be noted that the activities at 400-499 nm and g499 nm are considerable as 22.2 and 23.2%, respectively. The photocatalytic ability under UV light irradia-

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Figure 10. (A) Photocatalytic activity as a function of reaction time under visible light irradiation (λ g 400 nm) and (B) photocatalytic decomposition of Orange II dye at different sections of wavelengths of cutoff filters. The reaction for (B) was performed over 0.1 g of Z-24400 catalyst for 1.5 h.

tion is attributed to the natural band gap (326 nm) of ZnS-type materials, while visible light-induced activity is connected with photoabsorption shoulder and its tail in Figure 7 derived from the doping. It is interesting that the dye on Z-24 catalyst is not degraded in the beginning of photocatalytic reaction (1.5 h),but it appears to be slightly generated. It is because the piperazine molecules remaining on the surface of the catalyst, which cause mainly the absorption at 400-600 nm (Figure 7), would be desorbed into a solution phase. Repetition experiments for Z-24 catalyst in Figure 11 support this explanation. Intensity of the absorption peak at 484 nm (0.56) increases to 0.62 after 1.5 h in the first run, while the second run exhibits decrease in the peak intensity from 0.89 to 0.46. As two experiments were performed at the same amount ratio of dye to catalyst, it is evident that the organic residuals were completely detached from the surface of the catalyst in the first run. Figure 11 also shows that Z-24 catalyst has a good stability in the photocatalytic reaction. The catalyst recovered from the first experiment degraded 91% of the dye after 6 h in the consecutive experiment. Figure 12 proves the removal of surface piperazine species during first-run reaction. The distinct absorption at 400-600 nm of fresh Z-24 catalyst is not observed for the catalyst after reaction and the scarlet catalyst changes to ivory, which is similar to Z-24-400. And, the absorption peak of the catalyst after reaction is slightly shifted to a higher wavelength, the position of bulk ZnS. It is necessary to recall that the weak blue-shift property of fresh Z-24 compared to bulk ZnS is derived from the dimensions of nanoparticles. The increase of particle size after the removal of surface piperazine species suggests that piperazine adsorbed on the surface as well as contributing to interlayer molecules suppress the growth of particle in the synthesis. The Z-24

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Figure 11. Absorption spectra of a solution of Orange II dye in the presence of Z-24 catalyst under exposure to visible light (λ g 400 nm). The catalyst applied in the first run (A) was recovered and used in the second run (B).

Figure 12. UV-vis diffuse reflectance spectra of Z-24 catalysts before and after photocatalytic reaction. Inset images show the color of the samples.

catalyst recovered from the photocatalytic experiment exhibits a broad absorption at 400-600 nm, indicating that N-doping is preserved. The elemental analysis for Z-24-400 indicates that C as well N was doped into ZnS lattice, because 0.82% N and 1.61% C remains even after the heat-treatment at 400 °C. In nonmetaldoped TiO2, substitutional doping of N narrows a band gap by mixing with O 2p states at the top of valence band, but C-doping generates too deep states in the gap that they overlap sufficiently with the band states of TiO2.23 It is likely that doped N plays a more decisive role in the degradation of the dye under visible light irradiation.

N-Doped ZnS Nanoparticles Conclusions Nanosized N-doped ZnS with wurtzite phase was synthesized by using organic-inorganic interaction in a solution at the mild temperature of 150 °C. By employing piperazine as an organic source, the metastable ZnS · (piperazine)0.5 nanophybrid materials formed in the initial stage of synthesis. Interlayer piperazine molecules connecting inorganic ZnS layers were moved out of the layered structure with the progress of the synthetic reaction, and wurtzite ZnS nanoparticles with trace nitrogen atoms were finally produced. Nitrogen atoms in piperazine would be doped into ZnS in the phase transition. The organic piperazine molecules played two significant roles in the synthesis; the necleophilic agent and the suppressant to inhibit the growth of particles. The N-doped ZnS had the decreased band gap compared to pure ZnS and it exhibited good photocatalytic activity in the degradation of Orange II dye under visible light irradiation. Acknowledgment. This work was supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD), Basic Research Promotion Fund (KRF-2007-313D00157). Experiments at PLS were supported in part by Ministry of Education, Science, and Technology (MEST) and Pohang University of Science and Technology (POSTECH). References and Notes (1) (a) Kagan, C. R.; Mitzi, D. B.; Dimitrakopoulos, C. D. Science 1999, 286, 945. (b) Li, Z. H.; Yin, C.; Wang, R. J.; Wang, P.; Guo, H. Y. Acta Phys. Chim. Sin. 2003, 19, 1133. (c) Coronado, E.; Palomares, E. J. Mater. Chem. 2005, 15, 3593. (d) Forrest, S. R. J. Phys.-Condens. Mat. 2003, 15, S2599. (e) Houbertz, R.; Domann, G.; Cronauer, C.; Schmitt, A.; Martin, H.; Park, J. U.; Frohlich, L.; R. Buestrick, R.; Popall, M.; Streppel, U.; Dannberg, P.; Wachter, C.; Brauer, A. Thin Solid Films 2003, 442, 194. (f) Sih, B. C.; Wolf, M. O. Chem. Comm. 2005, 3375. (g) Liu, L.; Song, H.; Fan, L.; Wang, F.; Qin, R.; Dong, B.; Zhao, H.; Ren, X.; Pan, G.; Bai, X.; Dai, Q. Mater. Res. Bull. 2009, 44, 1385. (2) Heuling IV, H. R.; Huang, X.; Li, J. Nano. Lett. 2001, 1, 521. (3) (a) Montemagno, C.; Bachand, G. Nanotechnology 1999, 10, 225. (b) Nicole, L.; Boissiere, C.; Gross, D.; Quach, A.; Sanchez, C. J. Mater. Chem. 2005, 15, 3598. (c) Ruiz-Hitzky, E.; Darder, M.; Aranda, P. J. Mater. Chem. 2005, 15, 3650. (d) Munch, E.; Launey, M. E.; Alsem, D. H.; Saiz, E.; Tomsia, A. P.; Ritchie, R. O. Science 2008, 322, 1516.

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