Morphology of CdS Nanocrystals Synthesized in a Mixed Surfactant

NANO LETTERS

Morphology of CdS Nanocrystals Synthesized in a Mixed Surfactant System

2002 Vol. 2, No. 4 263-268

Blake A. Simmons,† Sichu Li,† Vijay T. John,*,† Gary L. McPherson,*,‡ Arijit Bose,§ Weilie Zhou,| and Jibao He| Department of Chemical Engineering, and Department of Chemistry, Tulane UniVersity, New Orleans, Louisiana 70118, and Department of Chemical Engineering, UniVersity of Rhode Island, Kingston, R.I. 02881, and AdVanced Materials Research Institute, UniVersity of New Orleans, New Orleans, Louisiana Received October 10, 2001; Revised Manuscript Received December 17, 2001

ABSTRACT High aspect ratio cadmium sulfide (CdS) quantum rods were synthesized at room temperature in the environment of water-in-oil microemulsions using a combination of two surfactants: the anionic bis(2-ethylhexyl) sulfosuccinate (AOT) and the zwitterionic phospholipid L-rphosphatidylcholine (lecithin). These highly acicular particles, obtained from a water-in-oil microemulsion containing an equimolar mixture of AOT and lecithin, possess an average width of 4.1 nm ± 0.6 nm, with lengths ranging from 50 to 150 nm. In contrast, conventional spherical CdS quantum dots are obtained from the AOT water-in-oil microemulsion system, with an average particle diameter of 5.0 nm ± 0.6 nm. X-ray and electron diffraction analyses reveal that the quantum dots have the face centered cubic structure of zinc blende, while the quantum rods predominantly have the hexagonal structure of wurtzite. Luminescence spectrophotometry of the samples indicates a blue-shift in the emission spectra when quantum rods are synthesized. It is hypothesized that this change in particle morphology is directly related to the shape of the reversed micelle in which it was synthesized and is evidence of surfactant templating.

Introduction The synthesis of inorganic nanocrystals in a controllable fashion has been the goal of much active research over the past decade. Semiconductor nanocrystals have been studied extensively as their optical properties are highly dependent on size and morphology.1 Several different synthesis techniques have been employed, and in the case of CdSe nanoparticles there have been remarkable advances in controlling the shape and size of these particles.2 Typically, such controlled synthesis schemes have involved the thermal decomposition of organometallic precursors of cadmium and selenium dissolved in tributylphosphine or trioctylphosphine and injected into mixtures of phosphine surfactants such as tiroctylphosphine oxide and hexylphosphonic acid maintained at a temperature of 300 °C.2 The surfactants adsorb to the growing crystal, and, depending upon the precursor concentrations, can moderate the growth rate of crystal faces. In a recent paper by Peng and Peng,3 it is stated that in contrast to CdSe synthesis there is no active method to * Corresponding author. E-mail: [email protected]. Phone: (504) 865-5883. Fax: (504) 865-6744. † Department of Chemical Engineering, Tulane University. ‡ Department of Chemistry, Tulane University. § University of Rhode Island. | University of New Orleans. 10.1021/nl010080k CCC: $22.00 Published on Web 03/07/2002

© 2002 American Chemical Society

control the shape characteristics of CdS. These authors have proceeded to develop an elegant, but high-temperature route to the synthesis of CdS, CdSe, and CdTe quantum rods using phosphine ligands to control specific crystal facet growth rates. We also address the issue in this work but describe a simple ambient temperature process for the synthesis of CdS nanocrystals with high shape anisotropy as well as ones that are nearly spherical, using the environment of water-in-oil microemulsions. Inorganic materials synthesis in water-inoil microemulsions (or reverse micelles as they are commonly called) has been widely studied, as it is a relatively easy method to generate nanoparticles. The size of the nanoparticle is restricted by the size of the microaqeuous core of the reversed micelle in which it is formed, with polydispersity values usually in the range of 10-15%. Examples of nanocrystals synthesized in reversed micelles include metallic particles such as Cu and Ag,4-6 the sulfides of Pb, Zn, and Cd,7-10 and metal oxides such as the superparamagnetic ferrites.11,12 In most prior work with nanoparticle synthesis in reversed micelles, the anionic surfactant bis(2-ethylhexyl) sulfosuccinate, or AOT (Figure 1a), has been used to form the reversed micellar system. Particles generated in these systems are typically spherical in morphology. A notable exception is the work by Pileni and co-workers who have shown transformations from

Figure 1. Chemical structures of (a) bis (2-ethylhexyl) sodium sulfosuccinate (AOT) and (b) L-R-phosphatidylcholine (lecithin).

spherical to rod-shaped Cu nanocrystals by conducting synthesis in appropriate regions of the Cu(AOT)2 + water + hydrocarbon phase diagram, or by varying the salt concentrations in the Cu(AOT)2 reversed micellar system.13,14 Our approach to generating nonspherical nanoparticles of CdS is based on the premise that surfactant mixtures may generate nonspherical templates for nanoparticle growth. We have used the combination of AOT and phosphatidylcholine to form water-in-oil microemulsions where the droplets may be nonspherical in nature. AOT has a surfactant packing parameter, P, of 1.1 (P ) V/al, where V is the volume occupied by the tails, a the effective headgroup crosssectional area, and l the maximum effective tail length). The surfactant thus tends to have a spontaneous curvature that is concave toward water,15,16 leading to the formation of spherical reversed micelles above the critical micelle concentration (cmc).17 On the other hand, the zwitterionic surfactant phosphatidylcholine (lecithin) has a significantly larger headgroup and a smaller packing parameter of 0.6.15,18 In aqueous systems, lecithin tends to form interfaces with minimal curvatures, such as flexible bilayers and vesicles.15 In nonpolar solvents, lecithin forms wormlike cylindrical reverse micelles that entangle to form gellike systems that can incorporate water up to a water/lecithin molar ratio of 20.19 Recent work in this laboratory has shown that AOT + lecithin act synergistically to form crystalline mesophases with viscosities of the order of 106 Pa-s in the presence of equal volumes of water and a liquid hydrocarbon (isooctane) and surfactant concentrations of 0.4 M and higher.20 Small angle neutron scattering studies have shown that these systems undergo well-defined transitions from hexagonal to lamellar microstructures with variations in temperature and water content.21 Our hypothesis was that the addition of a surfactant (lecithin) that tends to form surfaces of lower curvature about water, to a surfactant (AOT) with a high curvature about water, would result in a modification of droplet shape in systems where the total surfactant concentration is low enough to maintain a low viscosity liquid solution. The 264

objective was to conduct CdS synthesis in such mixed surfactant systems to determine if there was a change in particle morphology as a consequence. Our results indicate that there is a dramatic change in CdS morphology when a 1:1 molar ratio of a lecithin to AOT is used at a total surfactant concentration of 0.1 M in isooctane. In the synthesis, all chemicals used were purchased from commercial sources,22 and stock solutions were prepared containing the surfactants dissolved in isooctane.23 For CdS synthesis, the procedure followed is relatively simple and just involves contacting microemulsions containing Cd2+ to microemulsions containing S2- in their water pools to initiate CdS precipitation.24 Following this step, the resulting solution is kept stirred for 2 h, after which it is dried in a vacuum oven at a temperature of 50-55 °C for 12 h to remove all water and solvent. The samples are then resuspended with isooctane for further analysis through luminescence spectroscopy,25 X-ray diffraction (XRD),26 and transmission electron microscopy (TEM).27 TEM images for the CdS synthesized in the AOT reversed micellar system and in the AOT + lecithin reversed micellar system are shown in Figures 2a and 2b. A typical EDAX spectrum for the synthesized CdS (Figure 2c), verifies that the particles are indeed composed of Cd2+ and S2-, with no remaining precursor salt elements. Average particle diameters and standard deviations were calculated from measuring large TEM fields of approximately 70 particles for each sample. For the control sample of CdS synthesized in the 0.1 M AOT system (Figure 2a) the particles possess the expected spherical morphology. Particle size obtained by direct measurement through the high-resolution TEMs of Figure 4a are of the order of 5 ( 0.6 nm. For the 0.05 M AOT/0.05 M lecithin system, Figure 2b, a dramatic change in particle morphology is observed. The CdS particles are highly acicular in appearance, with widths of 4.2 nm ( 0.6 nm. The large aspect ratio is evidenced by the length dimension, which can exceed 100 nm. The elongated CdS particles are typically tapered at both ends. In many instances, it appears that there is a curvature to the particles. It is noteworthy that all particles are highly elongated, and there is no evidence of the coexistence of spherical particles. High-resolution TEM (HRTEM) images and electron diffraction patterns of particles synthesized in these systems are shown in Figures 3a and 3b, respectively. The clear lattice plane observations are indicative of relatively good crystallinity. The electron diffraction pattern of the spherical particles (inset to Figure 3a) indexes to the face centered cubic structure of zinc blende (sphalerite). On the other hand, the electron diffraction pattern for the acicular particles from the mixed surfactant system (inset to Figure 3b) indexes clearly to the hexagonal structure of wurtzite. It is reasonable that the elongation occurs through growth along the c axis. To confirm this observation, we carried out X-ray powder diffraction (XRD) analysis of the particles with the results in Figure 4 and analysis in Table 1. For the CdS quantum dots synthesized in the 0.1 M AOT system, shown in Figure 4a, the pattern obtained indexes to a cubic (sphalerite) crystal Nano Lett., Vol. 2, No. 4, 2002

Figure 3. High-resolution TEM images of CdS synthesized in (a) 0.1 M AOT, WT ) 10 water-in-oil microemulsion, (b) 0.05 M AOT + 0.05 M lecithin, WT ) 10 water-in-oil microemulsion. The corresponding electron diffraction patterns are shown in the insets.

Figure 2. Transmission electron micrographs (TEM) of CdS synthesized in (a) 0.1 M AOT, WT ) 10 water-in-oil microemulsion, (b) 0.05 M AOT + 0.05 M lecithin, WT ) 10 water-in-oil microemulsion. A typical EDAX spectrum of the synthesized CdS is shown in (c). Nano Lett., Vol. 2, No. 4, 2002

structure.28 For the rod shaped CdS particles synthesized in the 0.05 M AOT/0.05 M lecithin system, shown in Figure 4b, the XRD pattern obtained clearly indexes to a hexagonal structure of wurtzite.3 While we cannot conclusively exclude polycrystallinity through the XRD data obtained, it is clear that the quantum dots are predominantly cubic in structure, while the quantum rods are predominantly hexagonal. It is interesting to note that for the hexagonal CdS, the diffraction peaks are noticeably sharper for the l ) 0 reflections. This suggests that crystallographic ordering within the nanorods is most pronounced parallel to the c-axis. The TEM image of the quantum rods also shows the ordering to be most pronounced along their length axis (presumably the c-axis). 265

Figure 4. X-ray diffraction patterns for CdS synthesized in (a) 0.1 M AOT, WT ) 10 water-in-oil microemulsion with the cubic crystal structure indices shown, (b) 0.05 M AOT + 0.05 M lecithin WT ) 10 water-in-oil microemulsion with the hexagonal crystal structure indices shown. Table 1: XRD Analysisa face-centered cubic CdS (zinc blende structure)b plane index (hkl)

d-1 (Å-1)

2θ

2θ (observed)

(111) (220) (311)

0.2976 0.4835 0.5699

26.490 43.71 52.06

26.3 44.0 52.1

hexagonal close-packed CdS (wurtzite structure) plane index (hk.l)

d-1 (Å-1)

2θ

2θ (observed)

(10.0) (00.2) (10.1) (10.2) (11.0) (10.3) (11.2)

0.2789 0.2958 0.3158 0.4069 0.4822 0.5247 0.5667

24.88 26.33 28.15 36.52 43.69 47.66 51.74

24.7 26.3 28.0 36.4 43.7 47.7 51.8

a

λ ) 2d sin(θ); λ ) 1.54 (Cu KR). b Lattice constant a ) 5.82 Å;

1 h2 + k2 + l2 ) d2 a2 c Lattice constants a ) 4.14 Å, c ) 6.75 Å;

1 4 h2 + hk + k2 l2 ) + 2 d2 3 a2 c

The change in particle morphology from spherical quantum dots to high aspect ratio quantum rods, along with a change in the crystal structure of the nanoparticles obtained, is an interesting example of surfactant templating of nanoparticle morphology during synthesis. Figure 5 illustrates a possible templating mechanism. In the mixed surfactant system, there would be a nonuniform distribution of the two surfactants, with AOT primarily occupying regions of higher curvature (around water), and lecithin occupying regions of lower curvature. AOT would thus tend to cap the ends of wormlike micelles formed by lecithin thus leading to the highly ellipsoidal droplet structure that might template particle synthesis. We are carrying out small angle neutron scattering (SANS) experiments (with D2O to provide scattering contrast) on these systems at extremely dilute surfactant 266

Figure 5. Proposed model of templating through modification of microemulsion droplet shape: (a) schematic of particle synthesis in the AOT reversed micellar system, (b) schematic of particle synthesis in the mixed surfactant system of AOT + lecithin in a 1:1 ratio.

concentrations (to minimize intermicellar interactions that lead to structure factor contributions to the scattering intensity). The initial results clearly show an increase in the radius of gyration of these micelles from Rg ) 3.5 nm with AOT micelles to Rg ) 4.8 nm for micelles with a 1:1 molar ratio of AOT and lecithin. This work is in progress, and a full analysis will be reported when all the data have been acquired and complete model fitting carried out. A complementary interpretation is that lecithin plays the same role that the phosphine surfactants do in aspherical CdSe synthesis2, i.e., they dynamically adsorb to specific crystal faces modifying their growth rates. This is perhaps less likely as the oxidized phosphate groups of lecithin are not expected to bind as strongly to CdS as the phosphine surfactants. Room-temperature fluorescence emission spectra (excitation at 400 nm) for the three samples are shown in Figure 6. The CdS nanocrystals synthesized in the 0.1 M AOT reversed micelles exhibit a broad feature with a maximum intensity at 550 nm. A significant shift in peak position relative to the 0.1 M AOT system is observed in the spectra of the CdS synthesized in the 0.05 M AOT/0.05 M lecithin system. Here, the maximum intensity is at 492 nm (2.73 eV) with a secondary feature (shoulder) present at 467 nm (2.82 eV). The cause for this blue-shift in the emission spectra is thought to be an indication that the quantum confinement length is primarily defined by the particle width and its differences between the widths of the sphere and rods. It is also possible that surface state saturation is modified by particle-surfactant interactions, which may differ between the single surfactant case and the mixed surfactant case. In summary, the current study has demonstrated that it is possible to control the shape of CdS nanocrystals from Nano Lett., Vol. 2, No. 4, 2002

Acknowledgment. We are grateful for support from the National Science Foundation (Grant Nos. 9909912 and 0092001), and DARPA/AMRI (Grant No. MDA 972-97-10003). References

Figure 6. Fluorescence spectra of nanoparticles synthesized in the two microemulsion systems.

spheres to needles by varying the AOT:lecithin ratio in a mixed surfactant reversed micelle system. In typical spherical AOT reversed micelles, the CdS obtained is spherical and relatively monodisperse in terms of size. In an equimolar mix of AOT and lecithin, CdS needles are obtained with an aspect ratio easily exceeding 30. It is speculated that this change in morphology can be related to the shape of the reversed micelle in which the CdS was synthesized, and may be direct evidence of surfactant templating. Additionally, there is a remarkable switch in crystal structure with a transformation from the cubic structure for the quantum dots to the hexagonal structure for the quantum rods. The observation that such high aspect ratios can be obtained through room temperature synthesis with essentially benign surfactants is of importance. In particular, the AOT + lecithin micellar system can be effectively used in the synthesis of polymer-ceramic functional nanocomposites following procedures that have been well established with the AOT micellar system.29-31 Similarly, incorporation of silica and titania precursors in the organic phase can lead to the preparation of highly acicular CdS encapsulation in SiO2 or TiO2.32 The AOT micellar system can also be converted to a series of novel temperature-sensitive organogels using selective hydrogen-bonding dopants.33 These quantum dots and quantum rods can be incorporated into the strands of these organogels to provide new field-responsive materials. The ability to conduct multiple synthesis in the same systems may thus facilitate the development of devices such as highefficiency lasers, photocatalytic systems, and light-emitting diodes.34 Continuing research seeks to address these questions and potential applications. Since the particle precipitation is a bimolecular reaction, there are interesting opportunities to further control particle morphology. For example spherical particles can be initially generated in AOT micelles with an excess of one of the precursors. The other precursor can then be added together with lecithin to observe if the spherical particles then elongate to form tear drop shapes. The reverse method of generating a spherical endcap to quantum rods can also be visualized. To complete these templating studies it is also essential to understand droplet shape as a function of surfactant composition. To this end, we are carrying out rigorous light and neutron scattering studies. Nano Lett., Vol. 2, No. 4, 2002

(1) Li, L.-S.; Hu, J.; Yang, W.; Alivisatos, A. P. Nano Lett. 2001, 1, 349. Rosetti, R.; Hull, R.; Gibson, J. M.; Brus, L. E. J. Chem. Phys 1985, 82, 552. Alivisatos, A. P. Science 1996, 271, 933. (2) Manna, L.; Scher, E. C.; Alivisatos, A. P. J. Am. Chem. Soc. 2000, 122, 12700-12706. Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. Peng, X.; Manna, L.; Yang, W.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 494, 59. Peng, Z. A.; Peng, X. G. J. Am. Chem. Soc. 2001, 123, 1389. (3) Peng, Z. A.; Peng, X. J. Am. Chem. Soc. 2001, 123, 183. (4) Pileni, M. P.; Gulik-Krzywicki, T.; Tanori, J.; Finalkembo, A.; Dedieu, J. C. Langmuir 1998, 14, 7539-7363. (5) Chiang, C.-L. J. Colloid Interface Sci. 2000, 230, 60-66. (6) Cason, J. P.; Khambaswadkar, K.; Roberts, C. B. Ind. Eng. Chem. Res. 2000, 39, 4749-4755. Motte, L.; Pileni, M. P. J. Phys. Chem. B 1998, 102, 4104-4109. Zhang, Z.; Patel, R. C.; Kothari, R.; Johnson, C. P.; Friberg, S. E.; Aikens, P. A. J. Phys. Chem. B 2000, 104, 1176-1182. (7) Ogawa, S.; Hu, K.; Fan, F.-R.; Bard, A. J. J. Phys. Chem. B 1997, 101, 5707-5711. (8) Tata, M.; Banerjee, S.; John, V. T.; Waguespack, Y.; McPherson, G. L. Colloids Surf., A 1997, 127, 39-46. Curri, M. L.; Agostiano, A.; Manna, L.; Monica, M. D.; Catalano, M.; Chiavarone, L.; Spagnolo, V.; Lugara, M. J. Phys. Chem. B 2000, 104, 8391-8397. (9) De, G. C.; Roy, A. M.; Saha, S. J. Photochem. Photobiol. A 1995, 92, 189-192. (10) Zhou, H. S.; Honma, I.; Komiyama, H.; Haus, J. W. J. Phys. Chem. 1993, 97, 895-901. (11) Lopez-Quintela, M. A.; Rivas, J. J. Colloid Interface Sci. 1993, 158, 446. (12) Kommareddi, N. S.; Tata, M.; John, V. T.; McPherson, G. L.; Herman, M.; Lee, Y. S.; O’Connor, C. J.; Akkara, J. A.; Kaplan, D. L. Chem. Mater. 1996, 8, 801. (13) Pileni, M. P.; Tanori, J.; Filankembo, A. Colloids Surf., A 1997, 123124, 561-573. (14) Filankembo, A.; Pileni, M. P. J. Phys. Chem. B 2000, 104, 58655868. (15) Israelachvili, J. N. Intermolecular and Surface Forces; Academic Press: New York, 1992. (16) De, T. K.; Maitra, A. AdV. Colloid Interface Sci. 1995, 59, 193. (17) Kotlarchyk, M.; Stephens, R. B.; Huang, J. S. J. Phys. Chem. 1988, 92, 1533. (18) Wabel, C. Ph.D. Dissertation, University of Erlangen, 1998. (19) Scartazzini, R.; Luisi, P. L. J. Phys. Chem. 1988, 92, 829. Capitani, D.; Segre, A. L.; Sparapani, R.; Giustini, M.; Scartazzini, R.; Luisi, P. L. Langmuir 1991, 7, 250. Schurtenberger, P.; Scartazzini, R.; Luisi, P. L. Rheol. Acta 1989, 28, 372. Luisi, P. L.; Scartazzini, R.; Haering, G.; Schurtenberger, P. Colloid Polym. Sci. 1990, 268, 356. (20) Li, S.; Irvin, G. C.; Simmons, B.; Rachakonda, S.; Ramannair, P.; Banerjee, S.; John, V. T.; McPherson, G. L.; Zhou, W.; Bose, A. Colloids Surf., A 2000, 174, 275. (21) Simmons, B.; Agarwal, V.; John, V.; McPherson, G.; Bose, A.; Balsara, N. Langmuir 2002, 18, 624. (22) 95% pure L-R-phosphatidylcholine (lecithin) extracted from soybeans was purchased from Avanti Polar Lipids, Inc. Dioctyl sulfosuccinate (AOT), 2,2,4-trimethylpentane (isooctane, 99% purity), cadmium chloride, and sodium sulfide were purchased from Sigma-Aldrich. All chemicals were used without further purification and/or treatment. Distilled water was used in all experiments. (23) Two stock solutions with differing molar ratios of AOT and lecithin were synthesized by adding the measured amount of either surfactant to a specific volume of isooctane and sonicating the mixture until a clear isotropic solution was obtained. The two stock solutions had AOT/lecithin molar compositions of (1) 0.1 M AOT with no lecithin, (2) 0.05 M AOT/0.05 M lecithin. (24) For CdS synthesis the following procedure was normally used. A 0.1 M cadmium chloride solution was prepared with distilled water. A portion of this Cd2+ solution was then added to a vial containing 5 mL of the appropriate stock solution so that the WT () [H2O]/ [surfactant]) was equal to 10. It should be noted that this WT value 267

indicates the molar ratio of water to the total surfactant present in the system. This ensures that for any combination of AOT and lecithin the water content remains constant as long as the total surfactant concentration remains constant as well. A 0.05 M sodium sulfide solution was prepared and the same amount added (again to a WT ) 10 level) to a separate vial containing 5 mL of the appropriate stock solution. These two precursor vials were then sonicated until clear isotropic solutions were obtained. The entire S2- micellar solution was then added to the Cd2+ micellar solution to obtain a final [Cd2+]/ [S2-] ) 2. After mixing the two precursor solutions, a yellow color change was observed for all samples, a positive indication of the presence of CdS. No macroscopic precipitation was observed in the reaction vials. (25) Fluorescence emission spectra were recorded with a Perkin-Elmer LS-50 fluorimeter equipped with a Xe discharge lamp with an 8 µs, 20 kW discharge. Samples were transferred via pipet into a quartz cuvette for analysis. The discharge frequency of the fluorimeter is the same as the line frequency of 60 Hz, and the average lamp energy output is 10 W. A slit width of 10 nm was utilized in all experiments. (26) X-ray powder diffraction patterns were obtained with a Scintag diffractometer. Scans were collected at 45 kV and 40 mA with a step size of 0.02° every 0.5 s. Samples were prepared by isolating the CdS nanoparticles by centrifugation at 2500 rpm for 30 min. Samples were then resuspended, centrifuged, and washed with ethanol, methanol, and water. The samples were then dried in a vacuum oven at 40 °C for 4 h, and the resulting powders were placed on glass slides for X-ray data acquisition. (27) TEM images were taken with a JEOL 2010 electron microscope with energy-diffusive electron diffraction (EDAX) capability operating at

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(28) (29)

(30)

(31) (32) (33)

(34)

an acceleration voltage setting of 200 kV. A single drop of the CdS solution was deposited onto a carbon-coated 400-mesh TEM grid (obtained from Ted Pella Associates) and the solvent allowed to evaporate. The TEM grid containing the sample was then washed twice with isooctane and twice with methanol to remove excess surfactant and any remaining precursor salts. Colvin, V. L.; Goldstein, A. N.; Alivisatos, A. P. J. Am. Chem. Soc. 1992, 114, 5221-5230. Pavel, F. M.; Mackay, R. Langmuir 2000, 16, 8658-8574. Leo, G.; Curri, M. L.; Cola, A.; Catalano, M.; Lomascolo, M.; Manna, L.; Quaranta, F.; Agostiano, A.; Farinola, G. M.; Babudri, F.; Naso, F.; Monica, M. D.; Vasanelli, L. Mater. Sci. Eng. 2000 B74, 175-179. Hirai, T.; Komasawa, I. J. Mater. Chem. 2000, 10, 2234-2235. Moffitt, M.; Vali, H.; Eisenberg, A. Chem. Mater. 1998, 10, 10211028. Hirai, T.; Watanabe, T.; Komasawa, I. J. Phys. Chem. B 2000, 104, 8962-8966. Premachandran, R.; Banerjee, S.; John, V. T.; McPherson, G. L. Chem. Mater. 1997, 9, 1342-1347. Hirai, T.; Okubo, H.; Komasawa, I. J. Colloid Interface Sci. 2001, 235, 358-364. Waguespack, Y.; Banerjee, S.; Irvin, G.; John, V. T.; McPherson, G. L. Langmuir 2000, 16, 3036. Simmons, B.; Taylor, C.; Landis, F.; John, V.; McPherson, G.; Schwartz, D.; Moore, R. J. Am. Chem. Soc. 2001, 123, 2414. Kennedy, M. T.; Korgel, B. A.; Monbouquette, H. G.; Zasadzinski, J. A. Chem. Mater. 1998, 10, 2116-2119.

NL010080K

Nano Lett., Vol. 2, No. 4, 2002