J. Phys. Chem. C 2008, 112, 1885-1889
1885
Influence of Interparticle Electrostatic Repulsion in the Initial Stage of Aqueous Semiconductor Nanocrystal Growth Hao Zhang, Yi Liu, Junhu Zhang, Chunlei Wang, Minjie Li, and Bai Yang* State Key Laboratory of Supramolecular Structure and Materials, Jilin UniVersity, Changchun 130012, People’s Republic of China ReceiVed: October 17, 2007; In Final Form: NoVember 19, 2007
In this study, we demonstrated a general but ignored principle for the preparation of highly luminescent semiconductor nanocrystals (NCs) in aqueous media; namely, interparticle electrostatic repulsion greatly affected the growth rate of NCs, especially in the initial stage. Because both aqueous NCs and their monomers possessed charges, dispersion and aggregation of them were mainly determined by the interparticle electrostatic repulsion. The aggregation of NCs and/or their monomers facilitated the agglomeration growth of NCs. Current work demonstrated that the growth rate of aqueous NCs in the initial stage was accelerated through the deliberate decrease of electrostatic repulsion, represented by a pH effect and salt effect. According to the DerjaguinLandau-Verwey-Overbeek (DLVO) model, moreover, electrostatic repulsion was more dependent on NC size than other interparticle interactions, thus leading to size-dependent aggregation and disaggregation of NCs in the initial stage.
Introduction Semiconductor nanocrystals (NCs) are promising materials in optoelectronic and biological applications because of their unique size-dependent properties.1-4 Among many physical and chemical routes of NC preparation, the colloid chemical method is generally used for the synthesis of various high-quality NCs with desired sizes, shapes, and surface chemistries.5-10 The colloidal method also makes it possible to synthesize NCs alternatively in aqueous solution and various nonaqueous media.11-13 In any terms, the growth rate of NCs in a colloidal solution is a dominant parameter that significantly governs the quality of the resultant NCs, for instance, their photoluminescent (PL) quantum yield, full width at half-maximum of PL peak, and size distribution,14,15 thus continuously leading to academic and technical interest. On the basis of the studies concerning nonaqueous synthesized NCs, the underground mechanism of NC growth has been well-revealed.14-16 The growth of NCs is mainly dominated by a diffusion-limited Ostwald ripening (OR) process, which is strongly dependent on the diffusion rate of monomers. In this context, a high temperature and weak bonding ligand are theoretically and experimentally found to be favorable in the acceleration of NC growth.17 Likewise, the growth rate of NCs in aqueous solution is mainly governed by a diffusioncontrolled OR process.18 Quasi-spherical nanoparticles were prepared under a moderate growth rate,18-20 whereas onedimensional nanowires are favorable under a high growth rate simultaneously in the presence of proper ligands.21-25 Note that in comparison to nonaqueous synthesis, the growth of NCs in aqueous media is more environmentally dependent. Variations of solution pH, concentration of precursor, and ratio of various monomers significantly alter the NC growth rate, implying that their growth may involve a more complex mechanism.26-30 Consequently, improved understanding is required to reveal the lost process in NC growth. * To whom correspondence should be addressed. E-mail: byangchem@ jlu.edu.cn; fax: (+86) 431-85193423.
Previous investigations have demonstrated that the interparticle interaction of nonaqueous synthesized NCs is mainly van der Waals interactions.31 This short-range interaction has little effect on the growth of NCs; therefore, NC growth is consistent with the diffusion-limited model. For aqueous NCs, however, both NCs and their monomers possess charges due to the capping of charged ligands on the NC surface. Thus, the NC colloidal stability is improved through long-range electrostatic repulsion.32 A decrease of electrostatic repulsion is thought to induce the aggregation of NCs and/or monomers, and therefore, agglomeration and fusion of them may occur.33,34 Although electrostatic interaction has been widely used in the selfassembly and multiple assembly of aqueous NCs,35,36 it is usually ignored in the process of NC growth. Instead, most attention is paid to the effects of temperature, microwaves, sonochemistry, etc.37-39 Here, we demonstrate that interparticle electrostatic repulsion greatly affects the agglomeration growth of aqueous NCs in the initial stage. Through a deliberate decrease of interparticle electrostatic repulsion, it becomes possible to obtain aqueous NCs with desired sizes in a shortened preparation duration. Experimental Procedures Materials. NaBH4 (99%), tellurium powder (-200 mesh, 99.8%), selenium powder (-100 mesh, 99.5+ %), cadmium chloride hemi(pentahydrate) (CdCl2, 99+ %), zinc nitrate hexahydrate (Zn(NO3)2, 98%), 3-mercaptopropionic acid (MPA, 99%), thioglycolic acid (TGA, 97+ %), and 2-mercaptoethylamine (MA, 98%) were purchased from Aldrich. Preparation of Thiol-Stabilized NCs. Thiol-stabilized aqueous CdTe, CdSe, and ZnSe NCs were prepared according to the methods described elsewhere.40-42 Typically, aqueous precursor solutions of CdTe NCs were obtained by injecting a freshly prepared solution of NaHTe into 12.5 mM N2-saturated CdCl2 solutions in the presence of various thio-ligands at different pH values. The molar ratio of Cd2+/thiol/HTe- was set as 1:2.4:0.5. The resultant precursor solutions were refluxed
10.1021/jp710110n CCC: $40.75 © 2008 American Chemical Society Published on Web 01/23/2008
1886 J. Phys. Chem. C, Vol. 112, No. 6, 2008
Zhang et al.
at 100 °C to maintain the growth of CdTe NCs; their sizes increased with the reflux period. Following a similar procedure, except for using NaHSe and Zn(NO3)2 or CdCl2 instead of NaHTe and CdCl2, ZnSe and CdSe precursors and corresponding NCs were prepared. In the investigation of pH effect, the concentration of CdTe NCs was 12.5 mM referring to Cd2+. The pH of the Cd-MA mixtures was, respectively, adjusted to 5.0, 6.0, and 7.0 using 1 M NaOH before NaHTe injection, and the pH values of the resulting precursors were 5.2, 6.2, and 7.8. In the study of the salt effect, the concentration of CdTe NCs was 1.25 mM referring to Cd2+. Different volumes of a 2 M NaCl aqueous solution was added to the Cd-MPA mixtures, before which the pH of the Cd-MPA mixture was adjusted to 7.0. After NaHTe injection, moreover, the pH values of the CdTe precursors were 8.6 (0 mM NaCl), 7.78 (100 mM NaCl), and 8.15 (200 mM NaCl), respectively. In the study of the temporal evolution of NC size, the first electronic transition in the absorption spectra of the NCs was used to analyze the diameters of NCs.12 Characterization. UV-vis absorption spectra were obtained using a Shimadzu 3100 UV-vis spectrophotometer. On the basis of the method reported by Nozik and co-workers, the first electronic transition of the absorption spectra was used to evaluate the sizes of the CdTe NCs.12 Fluorescence spectroscopy was performed with a Shimadzu RF-5301 PC spectrophotometer. The excitation wavelength was 400 nm. All optical measurements were performed at room temperature under ambient conditions. Transmission electron microscopy (TEM) was conducted using a JEOL-2010 electron microscope at an acceleration voltage of 200 kV. Dynamic light scattering and zeta potential measurements were performed using a Zetasizer NanoZS (Malvern Instruments). Results and Discussion Typically, the preparation of aqueous NCs involved two steps: the preparation of precursors at room temperature by fast injection of hydrogen chalcogenide into an aqueous solution of metal salt simultaneously in the presence of thio-ligands and thermal growth of precursors to obtain NCs.40-42 Figure 1a shows the optical photograph of a TGA-stabilized CdTe precursor and corresponding NCs after a short duration of growth (7 min). It is clearly seen that the CdTe precursor exhibited a red color, whereas the corresponding NC solution was brown. The darker color of the precursor indicates that its size might be larger than that of NCs since larger NCs absorb light with longer wavelengths. However, the size of the precursor calculated from the first electronic transition of the UV-vis absorption spectra was smaller than the NCs. Similarly, the color of the CdSe and ZnSe precursors was also darker than the corresponding NCs; for CdSe, the color turned from brown to yellow, whereas it turned from slight yellow to colorless for ZnSe (Figure 1b,c). On the contrary, the calculated precursor sizes of CdSe and ZnSe from the first electronic transition were smaller than those of the corresponding NCs. These interesting results imply the existence of an ignored process in the initial stage of NC growth from precursors.43 Using 2-(dimethylamino)ethanethiol hydrochloride and MA as stable ligands to prepare CdTe NCs, moreover, significant aggregation was even observed for freshly prepared precursors,8,35 although the aggregates disappeared in the following duration of NC thermal growth (Figure 2a,b). Besides, TEM proved that the aggregates were composed of very small CdTe particles, the diameter of which was within 1 nm (Figure S1). These results clearly reveal that aqueous precursors generally have the tendency to form
Figure 1. UV-vis absorption spectra of aqueous solutions of TGAstabilized NC precursors and corresponding NCs obtained by heating precursors to 100 °C (it took 7 min for a 100 mL solution). (a) CdTe, (b) CdSe, and (c) ZnSe. The insets are the corresponding optical photographs. Before NaHTe or NaHSe injection, the pH of the CdTGA and Zn-TGA mixtures was adjusted to 11.0.
aggregates during their formation.34 In the initial stage of NC growth, however, these aggregated precursors disaggregated simultaneously with the formation of small NCs. The aforementioned results clearly indicate that the growth of aqueous NCs from a precursor must involve two different stages of size evolution; first from aggregated clusters to small NCs and then from small NCs to larger NCs (Figure 2). The second stage was easy to understand following the well-known OR mechanism,18 whereas the first stage has been less understood so far. Taking MA-stabilized CdTe as an example, the disappearance of aggregated precursors during heating indicated that they were only dynamic aggregates of small clusters instead of actual NCs. These clusters attached to each other because their attraction was temporarily stronger than their repulsion.44 As the clusters grew to actual NCs, their repulsion in turn became stronger than the attraction, making the aggregates disaggregate (Figure 3). Moreover, TEM and HRTEM images indicated that the asprepared CdTe NCs were quasi-spherical and well-dispersed (Figure S3), making it possible to calculate interparticle interac-
Aqueous Semiconductor Nanocrystal Growth
J. Phys. Chem. C, Vol. 112, No. 6, 2008 1887
Figure 3. Schematic illustration of the size-dependent aggregation and disaggregation of aqueous NCs, which were also dependent on the relationship of total repulsion and attraction between NCs. a, a1, and a2 present the radius of CdTe NCs. The decrease of electrostatic repulsion facilitated NC aggregation, whereas disaggregation of aggregates was adverse, represented by the pH effect and salt effect on the agglomeration growth of NCs.
TABLE 1: Measured Zeta Potential of Freshly Prepared MA-Stabilized CdTe Precursors and Corresponding NCs Obtained by Heating Precursors to 100 °Ca Figure 2. (a) Optical photographs of freshly prepared aqueous solutions of the precursors of MA-stabilized CdTe NCs. Before NaHTe injection, the pH of the Cd-MA mixtures was, respectively, adjusted to 5.0, 6.0, and 7.0 using 1 M NaOH, and the pH values of the resulting precursors were 5.2, 6.2, and 7.8. (b) Optical photographs of CdTe NCs obtained by heating precursors to 100 °C (it took 20 min for a 400 mL solution). Corresponding photographs of precursors are indicated in panel a. (c) Temporal size evolution of MA-stabilized CdTe NCs obtained during reflux of the precursor solutions at 100 °C. The first electronic transition in the absorption spectra of the NCs was used to analyze the temporal evolution of NC diameters.
tions based on spherical assumptions.45,46 Consequently, this size-dependent evolution of interparticle interactions could be understood according to the Derjaguin-Landau-VerweyOverbeek (DLVO) model.47,48 For aqueous nanoparticles, the interparticle interactions mainly involved electrostatic repulsion, van der Waals attraction, and dipolar attraction. These could be expressed as44,47,48
Velec(r) )
{
4πs0a2
}
2 8 tan h(eΨ0/4kT) × 1/2 2κa + 1 2 1+ 1tan h (eΨ /4kT) 0 (κa + 1)2 kT 2 exp[-κ(R - 2a)] (1) e R
[
Vvdw(r) ) -
[
]
( )
]
AH (R/a)2 - 4 2 2 + + ln (2) 6 (R/a)2 - 4 (R/a)2 (R/a)2
Vdipole(r) ) -
p2 2πs0R(R2 - 4a2)
VT ) Velec + Vvdw + Vdipole
(3) (4)
where s is the relative dielectric constant of water, 0 is the vacuum dielectric constant, e is the electronic charge, Ψ0 is the surface potential of the particle, k is the Boltzmann constant, R is the distance between the centers of the two particles, a is the radius of the CdTe particle, κ is the inverse Debye length, p is the dipole moment, and AH is the Hamaker constant of the CdTe particle. The total interaction potential (VT) was expressed as the sum of electrostatic repulsion potential (Velec), van der Waals
precursor NCs
pH 5.0
pH 6.0
pH 7.0
+41.0 mV +37.4 mV
+39.7 mV +37.3 mV
+35.2 mV +24.9 mV
a It took 20 min for a 400 mL solution. Before precursor preparation, the pH of the Cd-MA mixtures was, respectively, adjusted to 5.0, 6.0, and 7.0. Corresponding optical photographs are indicated in Figure 2a,b.
attraction potential (Vvdw), and dipolar attraction potential (Vdipole). Aggregation or dispersion of aqueous nanoparticles was determined by VT. If the total repulsion was weaker than the attraction (VT < 0), particles aggregated. If the repulsion was stronger than the attraction (VT > 0), particles dispersed.49,50 Accordingly, the evolution of aqueous NCs from aggregated clusters to individual ones should involve two aspects. On one hand, the precursors of aqueous NCs were in the form of small clusters. Their size was within 1 nm (Figure S1). Under this condition, Velec was not strong enough to screen Vvdw and Vdipole, leading to particle aggregates. On the other hand, according to the expressions of various interparticle interactions,44 Velec was long-range repulsive and was more size-dependent than Vvdw and Vdipole; thus, Velec increased more significantly than Vvdw and Vdipole as the particle size increased.44,47,48 Thus, with the increase of NC size during growth, Velec increased more significantly than the sum of Vvdw and Vdipole.44 Thus, the strong compensation of Velec to Vvdw and Vdipole made VT more repulsive and therefore caused the disaggregation of aggregated NCs (Figure 3). Note that for aqueous synthesized CdTe NCs, they were not monomorphous. Twinning planes or stacking faults were usually observed under HRTEM (Figure S3). This also implied the agglomeration growth mechanism of NCs at the initial stage.50 The effect of electrostatic interaction on precursor aggregation was further investigated by varying the pH of the precursor solution. As shown in Figure 2a, MA-stabilized CdTe precursors exhibited a more obvious aggregation at pH 7.0 than at pH 5.0 and 6.0. Zeta potential measurement indicated a decrease of the surface potential of the precursor solution from +41.0, to +39.7, to +35.2 mV as the pH increased from 5.0, to 6.0, to 7.0 (Table 1). This result revealed the more positive charges of MA-stabilized NCs at acidic range than at basic range, thus providing a stronger electrostatic repulsion at low pH range. We also measured the zeta potential of the aggregates spontaneously separated from the pH 7.0 solution, which was +41.6 mV. The dramatically higher surface potential of aggregates
1888 J. Phys. Chem. C, Vol. 112, No. 6, 2008
Zhang et al.
Figure 5. UV-vis absorption spectra of aqueous solutions of MPAstabilized CdTe NCs by heating precursors to 100 °C (it took 7 min for a 100 mL solution). Before NaHTe injection, different volumes of a 2 M NaCl aqueous solution were added to the Cd-MPA mixtures. The corresponding optical photograph is indicated in Figure 4b.
Figure 4. (a) Optical photographs of freshly prepared aqueous solutions of the precursors of MPA-stabilized CdTe NCs in the presence of different concentrations of NaCl. From left to right: 0, 30, 100, and 200 mM NaCl. (b) Optical photographs of MPA-stabilized CdTe NCs obtained by heating precursors to 100 °C (it took 7 min for a 100 mL solution). Corresponding photographs of precursors are indicated in panel a. (c) PL spectra of MPA-stabilized CdTe NCs with 2 h reflux at 100 °C in the presence of different concentrations of NaCl. From left to right: 0, 100, and 200 mM NaCl. (d) Temporal size evolution of MPA-stabilized CdTe NCs in the presence of different concentrations of NaCl obtained during reflux of the precursor solutions at 100 °C.
than that of solution clearly revealed that the formation of larger NC aggregates led to stronger electrostatic repulsion, which was consistent with our model (Figure 3). Moreover, the duration of aggregate disappearance was also found to be dependent on the pH of the precursor solution. Aggregates disappeared within 10 min of heating when the initial pH was 5.0, 10 min when the pH was 6.0, but 15 min when the pH was 7.0. The corresponding surface potentials of these samples were +37.4, +37.3, and +24.9 mV, respectively (Table 1). These results are consistent with our speculation that the aggregation of precursors resulted from low electrostatic repulsion. Although aggregated clusters or NCs finally disaggregate to individual units when their sizes are large enough, the initial size of the CdTe NCs increased with the increase of pH from 5.0 to 7.0 (Figure 2b,c). This result means that the aggregation of NC precursors actually accelerated NC growth at this stage. The agglomeration and fusion of small clusters and/or NCs was greatly favored in the aggregates, leading to more rapid growth of NCs beyond the diffusion-controlled process. At the following growth stage, moreover, NCs indicated similar growth rates, indicating that NC growth was through a diffusion-controlled process (Figure S4). From these results, it can be safely concluded that low electrostatic repulsion induced the aggregation of NC precursors and therefore the rapid agglomeration growth of NCs in the aggregates. The rapid increase of NC size in return enhanced the electrostatic repulsion, finally leading to disaggregation of NC aggregates. On the basis of the aforementioned discussion, the growth rate of aqueous NCs was predictably controlled through tuning the interparticle electrostatic repulsion. Figure 4 presents the effect of NaCl concentration on the aggregation and growth rate of MPA-stabilized CdTe NCs. With an increase of NaCl
concentration from 0 to 200 mM, the initial size of the CdTe NCs significantly increased, which is represented both by optical photographs and by UV-vis spectra (Figures 4a,b and 5). According to classical colloidal chemistry principles,47,48 an increase of electrolyte concentration above the millimol level would greatly reduce the Debye layer thickness of the aqueous particles from several tens of nanometers to just several nanometers. Consequently, the particle electrostatic repulsion decreased from several hundred kT to several tens or even just several kT values, making it impossible to compensate for van der Waals and dipolar attraction and therefore the aggregation of particles. The aggregation of particles in turn accelerated the agglomeration growth of NCs. Figure 4c,d shows the typical PL spectra and temporal size evolution of NCs obtained in the presence of different concentrations of NaCl in solution. It can be clearly observed with the same growth duration that different emission color NCs were obtained just through tuning the NaCl concentration. This difference was kept within the whole duration of NC growth. Note that although both salt effect and pH effect could accelerate NC growth, only the salt effect completely altered the ionic atmosphere around the NCs, and therefore, the salt effect was present for the whole duration of NC size evolution (Figure 4d), whereas the pH effect was present only in the initial stage (Figure 2c). Besides, although an increase in NaCl concentration accelerated NC growth both at initial stage and following the growth stage, the mechanism is different. At the initial stage, the salt concentration influenced precursor aggregation through the variation of interparticle interactions, whereas at the growth stage, the salt concentration was primarily thought to determine the electric double-layer structure of NCs and therefore monomer diffusion. To reveal the effect of electrostatics at the growth stage, we are building an electrostatic model of a growing NC system. Conclusion In summary, we clearly demonstrated the effect of electrostatic repulsion on the growth rate of aqueous NCs in the initial stage of their growth. Because the dispersion of aqueous NCs and their monomers was through electrostatic repulsion, the variation of this interaction significantly influenced NC aggregation and their fusion. Besides, electrostatic repulsion was found to be dependent on NC sizes; thus, electrostatic repulsion was more size-dependent than van der Waals and dipolar attraction, so aggregated NCs finally disaggregate when their sizes are large enough. On the basis of this understanding, the agglomeration growth of aqueous NCs was accelerated just
Aqueous Semiconductor Nanocrystal Growth through decreasing the electrostatic repulsion in solution. Our findings benefited the understanding of pH-, ligand-, and concentration-dependent aggregation and/or growth of aqueous NCs.27-30,37-39 Furthermore, the growth rate of NCs was proven to be controllable through tuning the electrolyte concentration in the NC solution, thus providing a new technique to design and prepare NCs in aqueous media. Acknowledgment. This work was supported by the National Basic Research Development Program of China (2007CB936402), the Foundation for the Author of the National Excellent Doctoral Dissertation of P. R. China (FANEDD Grant 200734), and the National Natural Science Foundation of China (Grants 20704014, 20534040, and 20731160002). Supporting Information Available: TEM image of freshly prepared MA-stabilized CdTe precursors, schematic illustration of the electric double layer of aqueous CdTe NCs, TEM and HRTEM images of TGA-stabilized CdTe NCs, and whole temporal size evolution of MA-stabilized CdTe NCs. This information is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Yin, Y.; Alivisatos, A. P. Nature (London, U.K.) 2005, 437, 664. (2) Mokari, T.; Rothenberg, E.; Popov, I.; Costi, R.; Banin, U. Science (Washington, DC, U.S.) 2004, 304, 1787. (3) Klimov, V. I.; Mikhailovsky, A. A.; Xu, S.; Malko, A.; Hollingsworth, J. A.; Leatherdale, C. A.; Eisler, H. J.; Bawendi, M. G. Science (Washington, DC, U.S.) 2000, 290, 314. (4) Chan, W. C. W.; Nie, S. Science (Washington, DC, U.S.) 1998, 281, 2016. (5) Peng, X. G.; Manna, L.; Yang, W.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature (London, U.K.) 2000, 404, 59. (6) Sun, Y.; Xia, Y. Science (Washington, DC, U.S.) 2002, 298, 2176. (7) Yu, W. W.; Peng, X. G. Angew. Chem., Int. Ed. 2002, 41, 2368. (8) Gaponik, N.; Talapin, D. V.; Rogach, A. L.; Hoppe, K.; Shevchenko, E. V.; Komowski, A.; Eychmu¨ller, A.; Weller, H. J. Phys. Chem. B 2002, 106, 7177. (9) Wang, X.; Zhuang, J.; Peng, Q.; Li, Y. Nature (London, U.K.) 2005, 437, 121. (10) Kumar, S.; Nann, T. Small 2006, 2, 316. (11) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (12) Rajh, T.; Miæiæ, O. I.; Nozik, A. J. J. Phys. Chem. 1993, 97, 11999. (13) Rogach, A. L.; Franzl, T.; Klar, T. A.; Feldmann, J.; Gaponik, N.; Lesnyak, V.; Shavel, A.; Eychmu¨ller, A.; Rakovich, Y. P.; Donegan, J. F. J. Phys. Chem. C 2007, 111, 14628. (14) Peng, X. G.; Wickham, J.; Alivisatos, A. P. J. Am. Chem. Soc. 1998, 120, 5343. (15) Peng, Z. A.; Peng, X. G. J. Am. Chem. Soc. 2002, 124, 3343. (16) Yu, W. W.; Wang, Y. A.; Peng, X. G. Chem. Mater. 2003, 15, 4300. (17) Pradhan, N.; Reifsnyder, D.; Xie, R.; Aldana, J.; Peng, X. G. J. Am. Chem. Soc. 2007, 129, 9500. (18) Talapin, D. V.; Rogach, A. L.; Shevchenko, E. V.; Kornowski, A.; Haase, M.; Weller, H. J. Am. Chem. Soc. 2002, 124, 5782.
J. Phys. Chem. C, Vol. 112, No. 6, 2008 1889 (19) Rogach, A. L.; Katsikas, L.; Kornowski, A.; Su, D.; Eychmu¨ller, A.; Weller, H. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 1772. (20) Rogach, A. L.; Kornowski, A.; Gao, M. Y.; Eychmu¨ller, A.; Weller, H. J. Phys. Chem. B 1999, 103, 3065. (21) Bao, H. F.; Wang, E. K.; Dong, S. J. Small 2006, 2, 476. (22) Volkov, Y.; Mitchell, S.; Gaponik, N.; Rakovich, Y. P.; Donegan, J. F.; Kelleher, D.; Rogach, A. L. ChemPhysChem 2004, 5, 1600. (23) Li, J.; Hong, X.; Li, D.; Zhao, K.; Wang, L.; Wang, H.; Du, Z.; Li, J.; Bai, Y.; Li, T. Chem. Commun. 2004, 1740. (24) Zhang, H.; Wang, D. Y.; Mo¨hwald, H. Angew. Chem., Int. Ed. 2006, 45, 748. (25) Sgobba, V.; Schulz-Drost, C.; Guldi, D. M. Chem. Commun. 2007, 565. (26) Piepenbrock, M. O. M.; Stirner, T.; O’Neill, M.; Kelly, S. M. J. Am. Chem. Soc. 2007, 129, 7674. (27) Li, C.; Murase, N. Chem. Lett. 2005, 34, 92. (28) Shavel, A.; Gaponik, N.; Eychmu¨ller, A. J. Phys. Chem. B 2006, 110, 19280. (29) Zhang, H.; Wang, D. Y.; Yang, B.; Mo¨hwald, H. J. Am. Chem. Soc. 2006, 128, 10171. (30) Zhang, H.; Wang, D. Y.; Hartmann, J.; Mo¨hwald, H. J. Phys. Chem. C 2007, 111, 9678. (31) Shevchenko, E. V.; Talapin, D. V.; Kotov, N. A.; O’Brien, S.; Murray, C. B. Nature (London, U.K.) 2006, 439, 55. (32) Tang, Z.; Kotov, N. A.; Giersig, M. Science (Washington, DC, U.S.) 2002, 297, 237. (33) Yaroslavov, A. A.; Sinan, V. A.; Efimova, A. A.; Yaroslavova, E. G.; Rakhnyanskaya, A. A.; Ermakov, Y. A.; Kotov, N. A. J. Am. Chem. Soc. 2005, 127, 7322. (34) Vossmeyer, T.; Katsikas, L.; Giersig, M.; Popovic, I. G.; Diesner, K.; Chemseddine, A.; Eychmu¨ller, A.; Weller, H. J. Phys. Chem. 1994, 98, 7665. (35) Tang, Z.; Zhang, Z.; Wang, Y.; Glotzer, S. C.; Kotov, N. A. Science (Washington, DC, U.S.) 2006, 314, 274. (36) Zhang, H.; Wang, C.; Li, M.; Ji, X.; Zhang, J.; Yang, B. Chem. Mater. 2005, 17, 4783. (37) Gao, J.; Yang, W.; Wang, C. J. Phys. Chem. B 2005, 109, 17467. (38) Li, L.; Qian, H.; Ren, J. Chem. Commum. 2005, 528. (39) Liu, Y.; Chen, W.; Joly, A. G.; Wang, Y.; Pope, C.; Zhang, Y.; Bovin, J. O.; Sherwood, P. J. Phys. Chem. B 2006, 110, 16992. (40) Gao, M.; Kirstein, S.; Mo¨hwald, H.; Rogach, A. L.; Kornowski, A.; Eychmu¨ller, A.; Weller, H. J. Phys. Chem. B 1998, 102, 8360. (41) Zhang, H.; Zhou, Z.; Yang, B.; Gao, M. Y. J. Phys. Chem. B 2003, 107, 8. (42) Zhang, H.; Wang, L.; Xiong, H.; Hu, L.; Yang, B.; Li, W. AdV. Mater. 2003, 15, 1712. (43) Green, M.; Harwood, H.; Barrowman, C.; Rahman, P.; Eggeman, A.; Festry, F.; Dobson, P.; Ng, T. J. Mater. Chem. 2007, 17, 1989. (44) Chow, M. K.; Zukoski, C. F. J. Colloid Interface Sci. 1994, 165, 97. (45) Rogach, A. L.; Nagesha, D.; Ostrander, J. W.; Giersig, M.; Kotov, N. A. Chem. Mater. 2000, 12, 2676. (46) Giersig, M.; Pastoriza-Santos, I.; Liz-Marza´n, L. M. J. Mater. Chem. 2004, 14, 607. (47) Cosgrove, T. Colloid Science: Principles, Methods, and Applications; Blackwell Publishing Ltd.: Oxford, 2005. (48) Kim, T.; Lee, K.; Gong, M. S.; Joo, S. W. Langmuir 2005, 21, 9524. (49) Tang, Z. Y.; Ozturk, B.; Wang, Y.; Kotov, N. A. J. Phys. Chem. B 2004, 108, 6927. (50) Dagtepe, P.; Chikan, V.; Jasinski, J.; Leppert, V. J. J. Phys. Chem. C 2007, 111, 14977.
