pubs.acs.org/Langmuir © 2010 American Chemical Society
Determining the Morphology of Polystyrene-block-poly(2-vinylpyridine) Micellar Reactors for ZnO Nanoparticle Synthesis Osman El-Atwani,†,§ Taner Aytun,† Omer Faruk Mutaf,† Vesna Srot,‡ Peter A. van Aken,‡ and Cleva W. Ow-Yang*,† †
Materials Science and Engineering Program, Sabanci University, Orhanli, Tuzla, 34956 Istanbul, Turkey, and ‡ Stuttgart Center for Transmission Electron Microscopy, Max Planck Institute for Metals Research, Heisenbergstrasse 3, D-70569 Stuttgart, Germany. §Current address: College of Engineering, Purdue University, West Lafayette, Indiana, 47905 Received November 1, 2009. Revised Manuscript Received December 16, 2009
We report the use of reverse PS-b-P2VP diblock copolymer micelles as true nanoscale-sized reactor vessels to synthesize ZnO nanoparticles. The reverse micelles were formed in toluene and then sequentially loaded with zinc acetate dihydrate and tetramethylammonium hydroxide reactants. Moreover, high spatial resolution Z-contrast imaging and EDX spectroscopy techniques were used to confirm the segregation of the Zn cation to the core of the loaded micelles. Determining the chemical distribution with high nanoscale spatial resolution is shown to complement the less direct characterization by AFM, DLS and FTIR, thus demonstrating broader implications for the characterization of hybrid nanocomposite systems.
In past decade, much attention has been devoted to using amphiphilic diblock copolymer reverse micelles as reactors to synthesize noble metal and semiconductor nanoparticles, fueled by the high degree of monodispersity achievable and the ability to assemble the particles into 2-D periodic arrays.1-10 Our contribution to this rapidly evolving field is 2-fold, as described in this report. First, we demonstrate the use of polymeric micelles truly as vessels for controlling the nucleation and growth of nanosized particles inside the micelle core. Moreover, we present the adaptation of scanning transmission electron microscopy techniques to elucidate the elemental distribution and morphology inside the reactant-loaded micelle reactors. In polymeric micelles, it is the solvent-selective nature of one block of the unimer molecule, as well as the ability of the other block to coordinate with metal compounds, that enables the functionality of these nanoscale chemical reaction vessels. Located at the core of a reverse micelle are functional groups of soft basic character that could bond with an acidic species, inducing nanoparticle precipitation and growth. Because the final stable structure of diblock copolymer micelles has been shown to scale *Corresponding author. Telephone: þ90 216 483 9592. Fax: þ90 216 483 9550. E-mail:
[email protected]. (1) Foerster, S.; Antonietti, M. Adv. Mater. 1998, 10, 195–217. (2) Antonietti, M.; Heinz, S. Nachr. Chem. Lab. Tech. 1992, 40, 308–314. (3) Antonietti, M.; Wenz, E.; Bronstein, L.; Seregina, M. Adv. Mater. 1995, 7, 1000–1005. (4) Spatz, J. P.; Roescher, A.; Sheiko, S.; Krausch, G.; Moeller, M. Adv. Mater. 1995, 7, 731–735. (5) Nguyen, D.; Williams, C. E.; Eisenberg, A. Macromolecules 1994, 27, 5090– 5093. (6) Foerster, S.; Zisenis, M.; Wenz, E.; Antonietti, M. J. Chem. Phys. 1996, 104, 9956–9968. (7) Moeller, M.; Spatz, J. P. Curr. Opin. Colloid Interface Sci. 1997, 2(Part 2), 177–187. (8) Bronstein, L.; Antonietti, M.; Valetsky, P. Metal Colloids in Block Copolymer Micelles: Formation and Material Properties. In Nanoparticles and nanostructured films: preparation, characterization and applications; Fendler, J., Ed. Wiley-VCH: Weinheim, Germany, 1998. (9) Spatz, J. P.; Moessmer, S.; Moeller, M. Chem.;Eur. J. 1996, 2, 1552–1555. (10) Spatz, J. P.; Sheiko, S.; Moeller, M. Macromolecules 1996, 29, 3220–3226. (11) Marques, C.; Joanny, J. F.; Leibler, L. Macromolecules 1988, 21, 1051– 1059. (12) Borisov, O.; Zhulina, E. B. Macromolecules 2002, 35, 4472–4480.
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with the molecular weight and contrast in polar nature of the two blocks,11,12 the relative size of a reactor chamber could be specified according to the relative block lengths.13 Moreover, the kinetic stability of these unimer aggregates as reverse micelles further complement their suitability as nanoreactors. The use of such micelles as nanosized reactor chambers would find compelling application in the synthesis and 2-D controlled assembly14,15 of semiconductor nanoparticles, which are technologically attractive due to the unique properties endowed by quantum confinement effects, such as the luminescence blue-shift of ZnO. In addition to limiting aggregation, synthesis inside a micelle would enable a high degree of control in size dispersity, by restricting the amount of reactant in each reactor core. In previous work to synthesize ZnO nanoparticles, micelles have been used to control the formation of ZnCl2 nanosized particles. However, an oxygen plasma was necessary to simultaneously etch the polymeric material and oxidize the Zn-containing particles.16 In fact, a similar secondary gas phase-reaction step was also used to form CdS and PbS nanoparticles.17,18 Thus, in order to use micelles as true nanoreactors, we sought to eliminate the plasma-etch step by directing a controlled reaction inside the micelles. To characterize the basic structure of these micellar reactor systems, dynamic light scattering (DLS), small-angle X-ray scattering (SAXS), atomic force microscopy (AFM), and (scanning) transmission electron microscopy ((S)TEM) have been used.3,5-7,10,17-23 Structural information, such as shape, size and polydispersity, could be obtained from micelles suspended in (13) Zhulina, E. B.; Birshtein, T. M. Polymer 1989, 30, 170–177. (14) Kaestle, G.; Boyen, H.-G.; Weigl, F.; Lengl, G.; Herzog, T.; Ziemann, P.; Riethmueller, S.; Mayer, O.; Hartmann, C.; Spatz, J. P.; Moeller, M.; Ozawa, M.; Barnhardt, F.; Garnier, M. G.; Oelhafen, P. Adv. Funct. Mater. 2003, 13, 853–861. (15) Lu, J. Q. J. Phys. Chem. C 2008, 112, 10344–10351. (16) Yoo, S. I.; Sohn, B. H.; Zin, W. C.; An, S. J.; Yi, G. C. Chem. Commun. 2004, 2850–2851. (17) Moffit, M.; Eisenberg, A. Chem. Mater. 1995, 7, 1178–1184. (18) Moffit, M.; McMahon, L.; Pessel, V.; Eisenberg, A. Chem. Mater. 1995, 1995, 1185–1192. (19) Antonietti, M.; Heinz, S.; Schmidt, M.; Rosenauer, C. Macromolecules 1994, 27, 3276–3285. (20) Antonietti, M.; Foerster, S.; Hartmann, J.; Oestreich, S.; Wenz, E. Nachr. Chem. Lab. Tech. 1996, 44, 579–586.
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solution by DLS19 and SAXS.5 However, thin films of loaded micelles could also be characterized, because the structure of the micelles could be kinetically frozen in even though some distortion in the true shape has been shown to occur.4 In the case of AFM, tapping mode was used to obtain topographic images of micelle monolayers.4,10 The presence of inorganic material in the micelle core could introduce inhomogeneity into the mechanical properties of the micelle structure. By mapping the phase angle variation across regions of inhomogenous compliance, further insight could be obtained for the morphology of the loaded micelles.24-28 Furthermore, where TEM has been used for characterizing the composition of diblock copolymer thin films, negative-staining with a heavy ion, such as osmium tetroxide or iodide, had been used to improve the bright field image contrast.8,22,29,30 However, segregation effects and structural distortion were potential ambiguities introduced by the stain. Conveniently, the use of ionic diblock copolymers enabled the metal salt reactant to function simultaneously as the stain.10,21,31,32 High-angle annular dark field (HAADF) imaging in the scanning transmission microscope (STEM) is particularly useful for revealing contrast variation associated with different atomic number (Z-contrast). Regions with higher Z appear brighter in the image, while those with lower Z number appear darker. With the metal salt-rich regions providing enhanced Z-contrast, HAADF-STEM imaging and energy dispersive X-ray spectroscopy (EDXS) could prove the loading of metal salt into the micelle core. In this work, we have investigated the loading of polystyreneblock-poly(2-vinylpyridine) (PS-b-P2VP) reverse micelle reactors with zinc acetate dihydrate (ZnAc) as the metal salt and demonstrated the nanoreactor concept, by supplying an oxygen source in the form of tetramethylammonium hydroxide (TMA-OH). The subsequent characterization was performed using FTIR, DLS, phase contrast imaging from tapping mode-AFM (TM-AFM), and EDXS in conjunction with HAADF-STEM imaging. The application of these scanning probe microscopy and TEM techniques revealed with unprecedented detail the morphology and elemental distribution of the ZnAc loaded into diblock copolymer micelles. To form the micelle reactor vessels, a relatively symmetric diblock copolymer, composed of a PS block (720 styrene units) and a P2VP block (632 vinylpyridine units) with a Mw/Mn of 1.14 was dissolved in toluene (0.45 wt %), which is a highly selective solvent for the PS blocks only. The resulting morphology would be composed of a core of collapsed P2VP blocks forming a globule, while the PS blocks were elongated into the solvent.6 TM-AFM images in Figure 1, parts a and b, revealed a micelle diameter of 65 ( 5 nm. In order to characterize the unloaded and loaded micellar systems, monolayers of micelles were formed by dip-coating onto mica substrates for AFM analysis and (21) Saito, R.; Okamura, S.; Ishizu, K. Polymer 1992, 33, 1099–1101. (22) Hadjichristidis, N.; Pispas, S.; Floudas, G. Block Copolymers synthetic Strategies, Physical Properties, and Applications; John Wiley & Sons, Inc.: Hoboken, NJ, 2003. (23) Spatz, J. P.; Roescher, A.; Moeller, M. Adv. Mater. 1996, 8, 337–340. (24) Aytun, T.; Mutaf, O. F.; El-Atwani, O.; Ow-Yang, C. W. Langmuir 2008 24(24), 14183–14187. (25) Garcia, R.; Magerle, R.; Perez, R. Nat. Mater. 2007, 6, 405–411. (26) Martinez, N. F.; Garcia, R. Nanotechnology 2006, 17, S167–S172. (27) Tamayo, J.; Garcia, R. Appl. Phys. Lett. 1997, 71, 2394–2396. (28) Buitenhuis, J.; Foerster, S. J. Chem. Phys. 1997, 107, 262–272. (29) Hashimoto, T.; Nagatoshi, K.; Todo, A.; Hasegawa, H.; Kawai, H. Macromolecules 1974, 7, 364–373. (30) Spontak, R. J.; Williams, M. C.; Agard, D. A. Macromolecules 1988, 10, 1377–1387. (31) Bennett, R. D.; Miller, A. C.; Kohen, N. T.; Hammond, P. T.; Irvine, D. J.; Cohen, R. E. Macromolecules 2005, 38, 10728–10735. (32) Moeller, M.; Kuenstle, H.; Kunz, M. Synth. Met. 1991, 41-43, 1159–1162.
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onto carbon lacey Formvar-coated copper grid for TEM characterization. Because colloidal ZnO particles are commonly synthesized by precipitating Zn2þ and OH- in an alcoholic environment,33,34 zinc acetate dihydrate (ZnAc) was first predissolved in propanol, then added to the micelle-containing solution. However, the effect of propanol on such a reactant-loaded micelle structure is not known. Therefore, as a control, ZnAc was also loaded directly into micelle-containing solutions. The amount of ZnAc loaded into the micelles was calculated according to the nominal Zn-topyridine (ZnAc/VP) ratios of 0.2, 0.3, 0.4, and 0.5, which are 3.9, 5.8, 7.6, and 9.9 mg, respectively. During characterization of the loaded micelle systems, FTIR spectroscopy showed a broadening in the characteristic vibration resonance band of pyridine from 1590 to 1600-1620 cm-1, indicating that a complex had formed between the pyridine and the zinc cations.15 DLS measurements revealed an increase in the average hydrodynamic diameter, from 78.2 nm for 0.3 ZnAc/VP up to 91.1 nm for 0.5 ZnAc/VP. This trend was consistent with measurements from the TM-AFM images and lateral profiles, which also showed a similar trend of increasing size, despite the associated lateral broadening of the micelles. As the amount of ZnAc was increased, the size of the micelles had also increased, although the effect became smaller at a higher loading concentration; no significant difference was observed in the size of the micelles loaded with 0.4 or 0.5 ZnAc/VP. When ZnAc was predissolved in propanol, the micelles remained intact upon loading and enlarged in diameter, as revealed by AFM imaging. Comparison between parts a and c of Figure 1 (or between parts b and d of Figure 1), reveals that the empty micelle diameter of 65 ( 5 nm (see the line profile in Figure 1e) had increased to 96 ( 8 nm with the addition of the 0.3 ZnAc/VP (calculated ratio) and propanol mixture. In order to characterize the morphology of the loaded micelles, we sought to take advantage of the distinct contrast in atomic number and in mechanical properties between the Zn-containing and polymeric material-only regions. During tapping mode atomic force microscopy (TM-AFM) characterization, the probe tip is oscillated at its natural resonance frequency and scanned across the sample surface. Because interaction of the tip with the sample induces a phase shift in the tip oscillation, the variation in phase shift reveals information concerning the tip-sample interaction. Energy dissipative material properties that perturb this tipsample interaction could then be mapped, and their variations are revealed in the phase image contrast.24,25 When the Zn-containing species have been incorporated into the polymeric micelle core, the mechanical response of the micelle corona above the core is anticipated to change.26-28,35 As the oscillating probe tip is scanned across a single Zn-loaded micelle (illustrated in Figure 1f), changes in phase angle are due to the variation in energy dissipation as the tip interacts first with the micelle corona only, then with the corona above the micelle core, and finally with the micelle corona only again. This variation is reflected in the phase image of a micelle, engendering contrast differences between the core of the micelle and the corona (see Figure 1). In HAADF images (Figure 2), the regions with higher atomic number, Z, appear brighter in the image, while those with lower Z appear darker, as shown in Figure 2, parts b and d. The darkest regions correspond to the lacey Formvar film, while (33) Koch, U.; Fojtik, A.; Weller, H.; Henglein, A. Chem. Phys. Lett. 1985, 122, 507–510. (34) Bahnemann, D. W.; Karmann, C.; Hoffman, M. R. J. Phys. Chem. 1987, 91, 3789–3798. (35) Matsumiya, Y.; Watanabe, H. Macromolecules 2004, 37, 9861–9871.
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Figure 1. AFM images of empty, unloaded PS (720)-b-P2VP (632) micelles, as formed in (a) topographical and (b) phase imaging modes; the same micelles, but loaded with 0.3 ZnAc/VP per 1 mL propanol observed in (c) topographical and (d) phase imaging modes. The imaging conditions were as follows: set point ratio =0.28, driving amplitude =22.5 nm, tip frequency =251.95 kHz, scan rate =3 Hz, integral gain =1.972 and proportional gain =4.027. The blue line in part a represents the location where the line profile in part e was obtained, in order to estimate the empty micelle diameter to be 65 ( 5 nm. These images were processed using the WSxM software program.36 The schematic in part f shows the expected morphology of a single loaded micelle.
the polymer-only corona regions appear gray. The highest Z regions, those of the micelle core containing Zn, are the brightest. These examples illustrate the power of Z-contrast imaging, providing such chemical composition information that could not be obtained from conventional bright field images alone. Although they are visible when viewed in the BF, the nanoscale-sized particles in the core appear more distinctly in the HAADF image. EDXS measurements were performed in order to verify the location of the Zn-containing species. EDX spectrum (Figure 2e) was recorded while scanning the beam over a rectangular area of 24 nm x 20 nm (labeled in Figure 2d). The Cu signal in the EDX spectrum in Figure 2e originates from the copper TEM specimen grid. The EDXS analyses confirm that the bright contrast regions (in the HAADF images) contain Zn and that Zn was incorporated into the micelle core. In fact, it shows the distribution of heavier Z elements in the core, effectively staining the micelle core and facilitating estimation of the core diameter. In order for the loaded micelle system to achieve the lowest free energy, ionic domains of polar species are expected to form, Langmuir 2010, 26(10), 7431–7436
suspended in a nonpolar medium.4,21,31,37,38 This is consistent with the structural and chemical information revealed by TMAFM and STEM analyses in Figures 1 and 2. In fact, the highest intensity regions correspond to the Zn species, which had segregated to the interior of the micelle. By presolvating the ZnAc, a solution of Zn cations was effectively introduced into the micelle core (see Figure 1f), allowing the Zn2þ cations to coordinate with pyridine. It is expected that these P2VP units would also interact favorably with the polar propanol solvent molecules. As was observed in the ZnAc complex case, the strong interaction with the pyridine in the core would enhance the kinetic stability of the micelles. On the other hand, HAADF images (Figure 2, parts b and d) did show perturbation of the structure of the Zn-complexed (36) Horcas, I.; Fernandez, R.; Gomez-Rodriguez, J. M.; Colchero, J.; Gomez-Herrero, J.; Baro, A. M. Rev. Sci. Instrum. 2007, 78(1), 013705. (37) Nguyen, D.; Varshney, S.; Williams, C. E.; Eisenberg, A. Macromolecules 1994, 27, 5086–5089. (38) Antonietti, M.; Foerster, S.; Oestreich, S.; Hartmann, J. Macromolecules 1996, 29, 3800–3806.
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Figure 2. Z-contrast images of the PS (720)-b-P2VP (632) micelles loaded with 0.3 ZnAc/VP dissolved in 1 mL propanol as observed in a STEM as seen under bright field imaging conditions (a, c) and as seen under Z-contrast (HAADF) imaging conditions (b, d). The Zn-loaded micelle monolayer is shown at low magnification in parts a and b, and at a higher magnification in parts c and d. EDX spectroscopy analysis was performed on the micelles loaded with 0.3 ZnAc/VP per 1 mL propanol. Displayed in part e is an sample spectrum from the region analyzed, indicated by the box in part d. The Zn-containing regions effectively stained the regions where the P2VP blocks had assembled.
micelle core, when propanol was present. The Zn-rich (bright) regions were distributed throughout the interior of the micelles, 7434 DOI: 10.1021/la904143f
rather than merely at the center. Moreover, the micelle shape changed from spherical to oblate. This shape distortion could not Langmuir 2010, 26(10), 7431–7436
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Figure 3. (a) Bright-field TEM image of the ZnO particles (indicated by the white arrows) produced inside a micelle reactor. The micrometer bar corresponds to 5 nm. The inset shows a few of the nanocrystals enlarged to facilitate inspection. (b) Electron diffraction pattern from the ZnO particles produced inside a micelle reactor, indexed to JCPDS card no. 36-1451 for wurtzite ZnO.
be attributed to the monolayer sample preparation, which had occurred well below the glass transition temperature of the constituent polymers. Instead, the structural changes may be due to a change in the solubility contrast between the two copolymer blocks. When this contrast is high, demixing is favored and induces the formation of the core/corona structure. However, the solubility contrast is lowered, when both polar solvent molecules and polar zinc acetate salt are incorporated into the micelle core, thereby screening some of the intra- and interchain Coulomb repulsion.12,37 This is consistent with lowering the chemical contrast of the unimer in toluene by using unimer blocks composed of 25% PS-75% P2VP.15 In such systems, self-organization into micelles could only be obtained by the stabilizing effect of the metal cation forming a coordination complex with P2VP. In fact, an excess amount of propanol would minimize the neutralization of the pyridine groups, and the micelles would be less kinetically stable, which was observed during the preparation of monolayers for AFM analysis. The osmotic pressure of the trapped salt may also contribute to the expanded micelles;for the 0.3 ZnAc/VP loaded micelles, an average diameter of 73 ( 4 nm was determined from the TM-AFM phase images. When the solution of ZnAc predissolved in propanol segregates to the micelle core, the propanol solvent molecules would also participate in the screening. By creating an environment similar to that experienced by the corona of regular micelles as modeled by Borisov and Zhulina,12 the saltcontaining propanol solution could enable the chain extension reflected by the increased average diameter of 96 ( 8 nm. To form the ZnO nanoparticles inside the micellar vessels, tetramethylammonium hydroxide (TMA-OH) was first loaded into the empty micelles, which were then added to the solution of ZnAc and Zn2þ cation-loaded micelles and stirred for an additional 30 h. A stoichiometric amount of TMA-OH, or a slight excess, corresponding to the cation source of the second reactant was used. nZnAc2 þ 2nTMA-OH f nZnO þ 2nTMA-Ac þ nH2 O ð1Þ According to the reaction in eq 1, the ratio of TMA-OH to zinc acetate should be 2:1. However, an excess amount of Langmuir 2010, 26(10), 7431–7436
TMA-OH, 4:1, was used to ensure complete reaction of the ZnAc. For purposes of comparison, colloidal ZnO nanoparticles were also synthesized without micelles or capping agents. The same stoichiometric amount of zinc acetate was dissolved in propanol at 60 °C for 90 min, followed by the addition of TMA-OH under vigorous stirring. To verify if the micelles did indeed serve as nanometer-scale chemical reactors for the formation of ZnO particles, ultraviolet absorption spectroscopy was used to characterize the micelles loaded with both ZnAc and TMA-OH. The formation of ZnO nanosized crystals would be indicated by a significantly blueshifted absorption edge, compared to that of bulk crystalline ZnO at 372 nm, due to quantum confinement effects on the electronic energy levels. The absorption edge appeared at 329 nm for micelles loaded with 0.3 ZnAc/VP and 40 μL of TMA-OH. Using the first approximation of a simple “quantum box” model by Efros and Efros39 such an edge onset would correspond to nanoparticles of 4.5 nm diameter, consistent with high-resolution TEM bright field images and diffraction patterns of the nanosized crystal-filled micelles shown in Figure 3. The values of the effective masses and dielectric constant for ZnO used in this estimation were me = 0.26, mh = 0.59, and ε = 3.2. For comparison, a control experiment was performed reacting ZnAc with TMA-OH in the same stoichiometric ratio but without the presence of micelles, to form ZnO particles with an absorption edge at ∼371 nm, i.e., close to that of bulk ZnO. The difference in chemical potential of the Zn cation, i.e., when introduced as a cation in contrast to as a part of the acetate complex, did not strongly affect the nanoparticle formation, in terms of the optical properties. The absorption edge still had an onset at ∼329 nm. As a final verification, electron diffraction patterns from the particles show polycrystalline rings that are indexed and assigned to wurtzite ZnO. By understanding the effects of reactant-loading on PS-b-P2VP reverse diblock copolymer micelles in toluene, we were able to realize the use of diblock copolymer reverse micelles as reactor vessels for the synthesis of ZnO nanoparticles. The formation of nanoparticles via templating with organic materials has offered a (39) Klimov, V. Los Alamos Sci. 2003, 28, 214–220.
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controlled approach to defining the physical properties of new inorganic-organic hybrid composite materials. Furthermore, owing to the incorporation of the metal salt into the micelle core, the viscoelastic properties were no longer uniform. The consequent variation in mechanical properties enabled TM-AFM phase imaging, which facilitated identification of the core and confirmed loading of the micelle reactors. The loaded micelle reactors also served as model systems for applying HAADFSTEM imaging and EDX spectroscopy to reveal the location of the Zn-containing species in the structure of the diblock copolymer micelles. These results demonstrate the utility of these two nanometer-scale spatial resolution characterization techniques for elucidating the compositional and structural morphology of micelles as nanoparticle reactor vessels. Acknowledgment. The authors acknowledge the European Union under the Framework 6 program, under a contract for an
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Integrated Infrastructure Initiative, Reference 026019 ESTEEM, Transnational Activities (TA7) to MPI and funding from : TUB ITAK Basic Sciences Division, Project No. 106T657. For fruitful discussions, we thank Dr. Mehmet Ali G€ulg€un, Dr. Alpay Taralp, and Ibrahim Inanc at Sabanci University and Prof. Dr. Manfred Ruehle at MPI. C.O. thanks Dr. Metin Usta and Omer Faruk Deniz of Gebze Institute of Advanced Technology and Prof. Dr. Servet Turan and Hilmi Yurdakul of Anadolu University for TEM assistance. O.E. acknowledges financial support from the Yousef Jameel Foundation. C.O. and T.A. acknowledge the help of Murat Eskin. Supporting Information Available: Text giving detailed materials and methods and a figure showing a countor plot of the core region. This material is available free of charge via the Internet at http://pubs.acs.org.
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