Self-Healing Self-Assembly of Aspect-Ratio-Tunable Chloroplast

Two particles collide because of Brownian motion and form a larger crystal (see .... self-healing self-assembly of novel ZnS 3D chloroplast-shaped arc...
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DOI: 10.1021/cg9006026

Self-Healing Self-Assembly of Aspect-Ratio-Tunable Chloroplast-Shaped Architectures

2009, Vol. 9 4745–4751

Bo Peng,†,‡,§ Zhengtao Deng,†,§ Fangqiong Tang,*,† Dong Chen,† Xiangling Ren,† and Jun Ren† †

Laboratory of Controllable Preparation and Application of Nanomaterials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, and ‡Graduate University of the Chinese Academy of Sciences, Beijing 100190, P. R. China. §These authors contributed equally to this work

Received June 4, 2009; Revised Manuscript Received July 27, 2009

ABSTRACT: Challenges are emerging that demand facile approaches to control the self-assembly of nanocrystal morphologyspecific higher-order architectures. Herein, we show a novel reversible wet-chemical approach to control the self-assembly of ZnS nanocrystals into well-defined, uniform, three-dimensional, aspect-ratio-tuned, micrometer-scale, self-healing architectures, which resemble chloroplast in shape. We can control the aspect ratio and the chemical potential of the 3D chloroplastshaped architectures by adjusting the concentration of ammonia and the ratio of S2-ions to Zn2þ ions. Possible mechanism of the controllable self-assembly is proposed. This self-assembly concept can also be applicable to a wide range of other transitionmetal sulfide chloroplast-shaped architectures, such as CdS, CuS, and Ag2S.The present study could open a new avenue to understand the important nanocrystal self-assembly phenomenon.

Introduction Many phenomena in nature, such as Fibonacci sequence of sunflowers, the patterns on seashells, the whorls of our fingerprints, the colorful patterns of fish and even the spatial pattern of stars in a spiral galaxy, inspire the human to explore the nature and the rule of the natural phenomena. Most scientists nowadays focus on uncovering nature’s mysteries and man have paid great attention to simulating the biological systems involving components of the system at the individual level.1-4 Nowadays, chemical self-assembly has become an important method in the field of biomimetic research. And what is more, it is accepted as one of the top 25 big questions facing science over the next quarter-century and the only practical approach for building a wide variety of nanostructures.5-15 Researchers have paid great attention to investigating the law of self-assembly in self-assembling systems. The past few years have been witness to an unprecedented revolution in understanding the controllable self-assembly of nanoscale building blocks into complex and regular higherorder architectures, such as ZnO nanorods,16 ZnS nanorods,17 CdTe nanowires and sheets,18,19 SnO2 nanospheres,20 Sb2O3 multisegmented coaxial nanowires,21 CuO ellipsoids,22 and In2O3 nanoflowers.23 However, the law of the self-assembly in complex self-assembling systems has never been clear exactly. Up to the present, scientists is still designing the systems with complexity and exploring the rule of the selfassembly. In this article, our intention is to solve synthesis challenges by a simple means with the help of the further understanding the mechanism of the self-assembly. We first develop a novel simple template-free wet-chemical approach to control the self-assembly of zero-dimensional transition-metal sulfide nanocrystals into well-defined aspect-ratio-tunable chloroplast-shaped three-dimensiona aggregation architectures,

which consist of many individual small nanoparticles. What is most important is the self-assembly processes in our experiment are reversible and chloroplast-shaped three-dimensiona superstructures can be self-healing, which is different from the unidirectional processes in other reports before. In our work, we investigate emphatically the mechanism of the self-assembly and the relation of the chemical potential and the surface atom ratio of a 3D chloroplast-shaped architecture. We calculate the ammonia concentration dependence of the aspect ratio and shape-dependent chemical potential of 3D chloroplast-shaped architectures in theory. And our experimental data and results unambiguously reveal that the nucleation in the system is the key factor for the self-assembly of zero-dimensional nanocrystals and the aspect ratio and the chemical potential of the 3D chloroplast-shaped architectures lie on the size and the number of the nucleus. Therefore, studies of the shape control of chloroplast-like architectures may further provide some insight for understanding the important nanocrystal self-assembly phenomenon. Experimental Section

*To whom correspondence should be addressed. E-mail: tangfq@mail. ipc.ac.cn.

All of the chemical reagents used in the experiments were purchased from Beijing Chemical Reagents Company. The chloroplastshaped ZnS architectures were obtained by aging of zinc acetate and sodium sulfide in double distilled water in present of ammonia. In a typical preparation for synthesis of chloroplast-shaped architectures with an aspect ratio of 3.5, 10 mL of 0.1 M Zn(CH3COO)2 3 2H2O aqueous solution was added to 4 mL of NH3 3 H2O (28%) aqueous solution. After 30 min, 10 mL of 0.1 M Na2S aqueous solution was added dropwise into the solution. The mixed solution remained for another 4 h before characterization. Double-distilled water was added to keep the total volume remained at 50 mL. For synthesis of other chloroplast-shaped architectures with different aspect ratio, the volume of the NH3 3 H2O aqueous solution was changed correspondingly. The scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were performed on a HITACHI S-4300 scanning electron microscope with an accelerating voltage of 5 kV. The transmission electron microscopy (TEM) images and selected-area

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Figure 1. (a), Low- and (c, e) high-magnification SEM images of the as-synthesized 3D chloroplast-shaped architectures. (b, d) TEM images of the 3D chloroplast-shaped architectures; (f) SEM image of the tip of chloroplast-shaped architectures; (g) HRTEM of marked part in (f); inset, the corresponding SAED pattern. electron diffraction (SAED) were performed on a JEM 200CX transmission electron microscope with an accelerating voltage of 160 kV and a JEM 2010 Electron Microscope with an accelerating voltage of 200 kV. Samples for TEM and HRTEM were prepared by directly putting a drop of the original reaction solution of architectures onto a Formvar substrate or an amorphous carbon substrate supported on a copper grid and then allowing the solvent to evaporate at room temperature. Ultraviolet and visible absorption (UV-vis) spectra were recorded using a JASCO V-570 spectrophotometer at room temperature. Photoluminescence (PL) spectra were measured using a PTI-C-700 fluorescence spectrometer at room temperature. Raman spectra were measured using a Renishaw Confocal Raman Microscope excited with a 532 nm laser. X-ray photoelectron spectroscopy (XPS) was performed using an achromatic Al KR source (1486.6 eV) and a double pass cylindrical mirror analyzer (Physical Electronics 549). Survey and high resolution spectra were recorded at pass energies of 200 and 50 eV, respectively. Surface charging was corrected by referencing the spectra to the C-C state of the C 1s peak at a binding energy at 284.5 eV. Peak fitting was performed using multi peak fit packages included with Igor Pro (WaveMetrics, Inc., v.6). X-ray powder diffraction (XRD) measurement was employed a Japan Regaku D/max γA X-ray diffractometer equipped with graphite monochromatized Cu KR radiation (λ = 1.5418 A˚) irradiated with a scanning rate of 0.02°/s Photoluminescence (PL) spectra were measured using a PTI-C-700 fluorescence spectrometer at room temperature.

Results and Discussion The low-magnification scanning electron microscopy (SEM) image shown in Figure 1a clearly reveals that the shapes of the products are almost uniform chloroplast-shaped architectures. The transmission electron microscopy (TEM) images b and d in in Figure 1 also clearly show the formation

Figure 2. (a) Raman spectrum of ZnS chloroplast-shaped architectures. (b) Survey X-ray photoelectron spectrum and (c) highresolution X-ray photoelectron spectra of ZnS chloroplast-shaped architectures of S 2p regions.

of uniform chloroplast-shaped architectures with average major axis of 1.7-2 μm and minor axis of 300-400 nm. The high-magnification SEM image in Figure 1c demonstrates that the chloroplast-shaped architectures are micrometerscale particles. Furthermore, as seen from Figure 1e, we find that the architectures are made from nanoflakes with a width of about 20 nm. The image of a tip of a chloroplast-shaped architecture in Figure 1f shows the obvious nanoflake-assembled structure, which is consistent with the SEM observations. Figure 1g is a higher-resolution TEM image of the marked place in Figure 1f; it clearly shows the nanocrystalaggregated structures. The selected area electron diffraction (SAED) pattern of the chloroplast-shaped architectures inset Figure 1g can also be indexed to the cubic phase of ZnS, which is consistent with the result obtained from X-ray powder diffraction (see Figure S1 in the Supporting Information). Figure 2a shows the Raman spectrum of ZnS chloroplastshaped architectures, which indicates two peaks at 262 and 346 cm-1. Raman spectra of bulk cubic zinc-blende phase of

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Figure 3. TEM images of the time-depended shape evolution of the as-synthesized architectures at different times: (a) 0.5 min; (b) 3 min; (c) 10 min; (d) 4 h. (e) Schematic illustration of three distinctive stages involved in the growth of chloroplast-shaped architectures. Three distinctive stages, nanocrystals nucleation, aggregation, and further aggregation, were observed in the chloroplast-shaped architecture formation.

ZnS shows the transverse optical (TO) phonon mode and longitudinal optical (LO) phonons mode zone center phonons at 276 and 351 cm-1, respectively. The Raman peak observed at 346 cm-1 can be assigned as the LO mode of cubic ZnS and that observed at 262 cm-1 can be attributed to the LO phonon-plasmon coupled modes.24 Figure 2b shows the survey X-ray photoelectron spectroscopy (XPS) spectrum. Both core level photoemission and Auger peaks are observed for the constituent atoms of Zn and S. The N signal can also be observed, because ammonia is absorbed on the surfaces of the ZnS nanocrystals, which is due to the binding affinity of ammonia to zinc ions, and left in the obtained ZnS chloroplast-shaped architectures. The carbon tape support contributed to the C and O signals. The quantitative analysis shows the atomic content of S in ZnS is about 49%, which is consistent with result form the energy-dispersive X-ray spectroscopic pattern (see Figure S2 in the Supporting Information). As shown in Figure 2c, the S 2p region contained two states: one at 160.9 eV, which is assigned to S 2p3/2, and one at 162.1 eV, which is assigned to S 2p1/2, which is consistent with the value reported in the literature.25 The size and shape evolution of the chloroplast-shaped architectures are investigated as shown in Figure 3a-d. First, uniform nanocrystals are formed at 0.5 min; the nanocrystals then interconnect with each other to form nanoflakes at 10 min. Finally, the nanoflakes assemble into uniform 3D chloroplastshaped architectures at 4 h. It is documented that 0D nanocrystals can aggregate into 2D nanosheets instead of bulk under basic conditions.26 However, we report here that the 2D nanostructures can further assemble into well-defined 3D nanostructures. Possible mechanisms of the controlled selfassembly in this study are tentatively studied, as shown in Figure 3e. There generally exist two concepts in particle growth: Ostwald ripening mechanism27,28 vs oriented attachment aggregation mechanism,14,15,29,30 i.e., atom-by-atom addition growth vs particle-by-particle growth. In our experiments, three distinctive stages, nucleation, aggregation, and further aggregation, are observed in the chloroplast-shaped architectures formation. First, nanocrystals form as the individual primary particles. Next, nanocrystals aggregate into secondary particles-nanoflakes. And finally, the secondary particles achieve self-assembly to form complex higherorder architectures with an aspect ratio finely controlled. Aggregation of nanocrystals is energetically favored because the formation of larger crystals can greatly reduce the interfacial energy of small primary nanoparticles. Semiconductor

Figure 4. TEM images of (a) the original ZnS chloroplast-shaped architectures, (b) after sonication, and (c) after restirring of the sonicated samples for another 4 h; (d) Raman spectra of the above three samples.

nanocrystals with zinc blend structure have been determined to have a dipole moment contributing to the self-assembly of these particles.19,31 Additionally, ammonia can be absorbed on different crystallographic planes of nanocrystals and there is also an attraction force between nanocrystals due to the binding affinity of ammonia to zinc ions, which can partially initiate the particle aggregation.22 Therefore, we think the anisotropic electrostatic interactions arising from the dipole moment, combined with the binding affinity are responsible with the self-assembly of nanocrystals. In our experiment, it is found that the architectures can be dissociated to individual ZnS nanocrystals by sonication, indicating that the architectures are formed by aggregation of nanocrystals, and the bonding energy between the nanocrystals is may mainly van der Waals force because it is much weaker than chemical bonding force. Images a and b in Figure 4 show the TEM images of the as-synthesized ZnS chloroplast-shaped architectures before sonication and after sonication. After sonication, the chloroplast-shaped architectures change into irregular shaped small ZnS nanocrystals. The HRTEM images of the small ZnS nanocrystals are shown in Figure S3a and 3b in the Supporting Information. The circle marks a single ZnS nanocrystal. UV-vis absorption and photoluminescence (PL) spectra of the as-synthesized ZnS chloroplast-shaped architectures before and after sonication are shown in Figure S3c and 3d in the Supporting

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Figure 5. TEM, SEM images, and XPD patterns of the chloroplast-shaped architectures with different aspect ratios synthesized under different conditions. The corresponding aspect ratios are (a-c) 6.0, (d-f) 3.5, (g-i) 2.2, (j-l) 1.5, and (m-o) 1.1, respectively. (p) Effect of the amine-to-Zn cation ratio on the aspect ratio of the chloroplast-shaped architectures is illustrated and typical TEM images recorded are shown. (q) the corresponding XRD patterns of ZnS chloroplast-shaped architectures built by nanocrystals, bottom to top, for a, d, g, j, and m, respectively.

Information. The sharp absorption band at 303 nm is blueshift from the bulk band gap wavelength of 340 nm. With the excitation wavelength of 290 nm, the photoluminescence spectrum shows intense peak maximum at 321 nm with a full width at half-maximum of 25 nm, revealing a well-defined band-edge emission feature with little trap-state defect emission. The positions of the UV-vis and PL emission peaks are almost unchanged, indicating that optical properties are independent of the morphologies of the nanocrystals aggregates, which is consistent with what we have expected. It is surprise that the isolated ZnS can be reorganized to chloroplast-shaped architectures again after 4 h by reself-assembly (see Figure 4c). But the aspect ratio decreases. The positions of the Raman peaks are blue-shifted a little for isolated nanocrystals and then red-shifted back after the reself-assembly, indicating the dissociation and reself-assembly process (see Figure 4d). Though previous reports show the observation of the nucleation and aggregation of the nanocrystals,17,32 we demonstrate a new self-healing process for the first time and ZnS nanocrystals can achieve self-assembly, dissociation and reself-assembly to form well-defined 3D architectures. TEM and SEM images of five types of well-defined uniform chloroplast-shaped architectures with the aspect ratio of 6.0, 3.5, 2.2, 1.5, and 1.1 are obtained respectively as shown in Figure 5a-o, in which the aspect ratio control of the architectures is achieved by altering the concentration of ammonia. It is shown that chloroplast-shaped architectures change from “slim” to “moderate”, further to “obese” gradually with the increasing ammonia concentration as shown in Figure 5p. Thus, we can finely tune the aspect

ratio of the chloroplast-shaped architectures. The crystal structures of five types of well-defined chloroplast-shaped architectures are confirmed by XRD. Figure 5q shows the XRD patterns of the ZnS architectures with different aspect ratio. All the reflections of the XRD patterns can be indexed to a pure cubic zinc blende phase of ZnS, which are in agreement with the report data (JCPDS File 05-0566). All the XRD peaks are considerably broadened compared to those of bulk ZnS because of the finite size of the crystallites, indicating that the architectures are made from small subunits. These XRD patterns indicate that the crystallized ZnS architectures can be obtained under current synthetic conditions. And under similar synthetic conditions, when we use ZnCl2, Zn(NO3)2, ZnSO4, and Zn3(PO4)2 in place of Zn(AC)2, 3D chloroplast-shaped architectures can also be obtained (see Figure S4a-d in the Supporting Information). Our experimental results indicate that it is the key factor for the formation of chloroplast-shaped architectures that it is the NH3 rather than the acetate. The concentration of ammonia in the reaction system plays an important role on the aspect ratio control of the ZnS chloroplast-shaped architectures. In the nucleation process, the reaction equations can be written as ½ZnðNH3 Þ4 2þ h Zn2þ þ 4NH3

ð1Þ

Zn2þ þ S2 - h ZnS

ð2Þ

The concentration of Zn (NH3)42þ is supposed to be a constant because the concentration of ammonia is tens of

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times larger than that of Zn2þ. So we can obtain the concentration of Zn2þ. ½Zn2þ  ¼ cK a ½NH3 4 -

ð3Þ

(NH3)42þ

and Ka is the where c is the concentration of Zn instability constant of Zn (NH3)42þ. Thus, describe the nucleation rate as -d½Zn2þ  dt

¼ K 1 ½Zn2þ ½S2 -  ¼ cK 1 K a ½S2 - ½NH3 4 ¼ K 2 ½S2 - ½NH3 4 -

ð4Þ

Where K1 is the rate constant of the nucleation, [S2-] is a constant, K2= cK1Ka. Equation 4 tells us that the nucleation rate of nanocrystals is strongly dependent on the ammonia concentration. The lower the concentration of ammonia, the faster the rate of nucleation. The initial average particle number (N0) increases and the initial average diameter of particle (D0) decreases as the ammonia concentration decreases. Therefore, the concentration of ammonia determines N0 and D0. Two particles collide because of Brownian motion and form a larger crystal (see Figure S5 in the Supporting Information). The aggregation rate, the number of initial particles reacted in a unit time, can be written as dN t ¼ K3N t2 ð5Þ dt By integrating eq 5, we have 1 1 ¼ K3t Nt N0

ð6Þ

where k3 is the rate constant of aggregation. Suppose that Nt is the number of initial particles at t, Dt is the size of the semiminor axis at t, R is the aspect ratio, N2(t) is the number of the chloroplast-shaped architectures, and t is the reaction time. According to the principle of mass balance 1 1 4 ΠFD30 N 0 ¼ ΠFD30 N ðtÞ þ ΠFD3t N 2 ðtÞR ð7Þ 6 6 3 The number of coalesced nanoparticles indicates the time dependence.33 k4t N 2 ðtÞ ¼ ð8Þ k4t þ 1 where k4 is a constant. Combining eqs 67-8, we have 1D30 K 3 N 20 ð1 þ K 4 tÞ R ¼ 8D3t K 4 ðK 3 N 0 t þ 1Þ

ð9Þ

In our experiment, we find that the aspect ratio should be inversely proportional to ammonia concentration. Their relative aspect ratio is plotted in Figure 6a, which indicates strong concentration dependence. We suggest that the aspect ratio of nanocrystals is related to the number and the size of initial particles, which is consistent with the conclusion proposed by Zhang and Banfield.34 For the 3D chloroplast-shaped architectures, the number of surface atoms and the total atoms should be proportional to the surface area and the volume, respectively.35 If we define the surface atom ratio as δ, then  1=2 2k 5 πb 73 a2 þ 23 ab þ b2 ð10Þ δ¼ 2 4 3 k 6 πab

Figure 6. (a) Concentration dependence of the aspect ratio of 3D chloroplast-shaped architectures. (b) Shape-dependent chemical potential of 3D chloroplast-shaped architectures. The volume of all the nanocrystals is set to the value of chloroplast-shaped architectures with the aspect ratio of 6.0. (c) The aspect ratio of the chloroplast-shaped architectures obtained at different S2-:Zn2þ ratios.

where k5 and k6 are proportional constant, a is the size of the semimajor axis, b is the size of the semiminor axis. We assume that the volume of all the nanocrystals is set to the value of chloroplast-shaped architecture with the aspect ratio of 6.0. So eq 10 can be changed to eq 11.  1=2 7 2 2 2 ð11Þ δ ¼ 4:75k 7 b a þ ab þ b 3 3 where k7 is the proportional constant. The chemical potential of a 3D chloroplast-shaped architecture should be proportional to the surface atom ratio, δ. As the surface atom ratio increases, the chemical potential increases. Figure 6b shows the nonlinear relation and the strong shape dependence of the relative chemical potential of crystals. Interestingly, we find

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chloroplast-shaped architectures might attract broad attention in many disciplines, including material science, biomedicine, atmospheric science, remote sensing, and astronomy, which represent a wide range of nonspherical particle length scales. Nonspherical dielectric particles such as ellipsoids offer excellent possibilities as building blocks to create photonic crystals with a three-dimensional photonic band gap (PBG).36-38 Blaaderen and co-workers developed a highenergy ion irradiation method to deform inorganic spherical particles such as ZnS and ZnS-core-SiO2-shell particles into ellipsoid-like particles.39 Conclusion

Figure 7. TEM images of (a, b) CdS, (c, d) CuS, and (e, f)Ag2S chloroplast-shaped architectures.

that the growth of the surface atom ratio should follow linear growth as the aspect ratio increases during certain range, and the best fit indicates a dependence of the type δ = 17.89 þ 1.77R (the inset in Figure 6b). Combining eqs 4, 9, and 11, we can find that the aspect ratio and the surface atom ratio of a 3D chloroplast-shaped architecture depend on the concentration of ammonia, which has an effect on nucleation. We suggest that the aspect ratio and the chemical potential of the 3D chloroplast-shaped architectures lie on the size and the number of the nucleus. To further verify our explanation, the evolvement of the morphology of the chloroplast-shaped architectures at different ratio of S2- ions to Zn2þ ions is investigated. It is found that the architectures change from “quasi-sphere” to “moderate” as the increasing of the ratio of S2- ions to Zn2þ ions, 0.45:1, 0.6:1, 0.75:1, 1.05:1, respectively, in Figure 6c. When the amount of S2- ions is small, the nucleation reaction is so slow that the number of nucleus is small. Contrarily, when the ratio is high, the fast nucleation reaction leads to small nucleus. Therefore, ZnS nanoparticles preformed aggregate unceasingly into “quasi-sphere”, “moderate”, and future to “slim” architectures as the ratio of S2- ions to Zn2þ ions increases. The chemical potential of the chloroplast-shaped architectures also increases as the ratio of S2- ions to Zn2þ ions increases. The current synthetic procedure turns out to be widely applicable, and many other transition metal sulfides 3D chloroplast-shaped architectures are synthesized from the three-stage nuclear-aggregation self-assembly process. Under similar synthetic conditions, when the Zn2þ cation is changed into Cd2þ, Cu2þ, and Agþ cations, other 3D chloroplastshaped architectures, such as CdS, CuS and Ag2S, can also be obtained (Figure 7a-f). In addition, the recent method can be also extended to synthesis of doped chloroplast-shaped architectures such as ZnS:Mn, CdS:Mn, and ZnS:Cu, which are of especially interest for optical applications. The formation of chloroplast-shaped 3D nanostructure from 0D nanocrystals is still open for further study, and the

In conclusion, a simple template-free solution route is demonstrated to carry out self-healing self-assembly of novel ZnS 3D chloroplast-shaped architectures, which exhibit an unusual structures built from 0D nanocrystals. The aspect ratio and the chemical potential of the 3D chloroplast-shaped architectures lie to the size and the number of the nucleus. It is demonstrated that the self-assembly concept can also be applicable to a wide range of other transition-metal sulfide chloroplast-shaped architectures. These materials are aspectratio-tunable architectures with potential applications as optoelectronic devices. By suitable choice of source and synthetic parameters, it is reasonable to expect that the present study can be extended to self-assembly of other novel 3D nanostructures with unique shapes. Acknowledgment. The current investigation was supported by the National Natural Science Foundation of China (60736001), National Hi-Tech 863 Programme (2007AA021803, 2009AA03z302), Beijing Natural Science Foundation 2093044. We thank Prof. Guoyi Zhang, Prof. Lianmao Peng and Mr. Jueming Yi in Peking University for their valuable suggestions on the manuscript. And we thank Prof. Bingsuo Zou in Institute of Physics, Chinese Academy of Sciences, for the help on the measurement of PL spectra. Supporting Information Available: XRD pattern, EDS pattern, HRTEM images of small ZnS nanocrystal, UV-vis absorption and photoluminescence (λex =290 nm) spectra of the as-synthesized ZnS chloroplast-shaped architectures before sonication and after sonication, TEM images of ZnS chloroplast-shaped architectures obtained by using different zinc salt, and scheme of formation of a large crystal from the collision of two particles (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

References (1) Gao, X. F.; Jiang, L. Nature 2004, 432, 36–36. (2) Blossey, R. Nat. Mater. 2003, 2, 301–306. (3) Huang, J. Y.; Wang, X. D.; Wang, Z. L. Nano Lett. 2006, 6, 2325– 2331. (4) Li, M. Z.; He, F.; Liao, Q.; Liu, J.; Xu, L.; Jiang, L.; Song, Y. L.; Wang, S.; Zhu, D. B. Angew. Chem., Int. Ed 2008, 47, 7258–7262. (5) Zhong, L. S.; Hu, J. S.; Liang, H. P.; Cao, A. M.; Song, W. G.; Wan, L. J. Adv. Mater. 2006, 18, 2426–2431. (6) Jonkheijm, P.; van der Schoot, P.; Schenning, A. P. H. J.; Meijer, E. W. Science 2006, 313, 80–83. (7) Whitesides, G. M.; Grzybowski, B. Science 2002, 295, 2418–2421. (8) Zhang, S. G. Nat. Biotechnol. 2003, 21, 1171–1178. (9) Palermo, V.; Samori, P. Angew. Chem., Int. Ed. 2007, 46, 4428– 4432. (10) Service, R. F. Science 2005, 309, 95–95. (11) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382, 607–609. (12) Li, M.; Schnablegger, H.; Mann, S. Nature 1999, 402, 393–395. (13) Penn, R. L.; Banfield, J. F. Science 1998, 281, 969–971.

Article (14) Banfield, J. F.; Welch, S. A.; Zhang, H. Z.; Ebert, T. T.; Penn, R. L. Science 2000, 289, 751–754. (15) Yang, H. G.; Zeng, H. C. Angew. Chem., Int. Ed. 2004, 43, 5930– 5933. (16) Pacholski, C.; Kornowski, A.; Weller, H. Angew. Chem., Int. Ed. 2002, 41, 1188–1191. (17) Yu, J. H.; Joo, J.; Park, H. M.; Baik, S. I.; Kim, Y. W.; Kim, S. C.; Hyeon, T. J. Am. Chem. Soc. 2005, 127, 5662–5670. (18) Tang, Z. Y.; Kotov, N. A.; Giersig, M. Science 2002, 297, 237–240. (19) Tang, Z. Y.; Zhang, Z. L.; Wang, Y.; Glotzer, S. C.; Kotov, N. A. Science 2006, 314, 274–278. (20) Deng, Z. T.; Peng, B.; Chen, D.; Tang, F. Q.; Muscat, A. J. Langmuir 2008, 24, 11089–11095. (21) Deng, Z. T.; Chen, D.; Tang, F. Q.; Meng, X. W.; Ren, J.; Zhang, L. J. Phys. Chem. C 2007, 111, 5325–5330. (22) Zhang, Z. P.; Sun, H. P.; Shao, X. Q.; Li, D. F.; Yu, H. D.; Han, M. Y. Adv. Mater. 2005, 17, 42–47. (23) Narayanaswamy, A.; Xu, H. F.; Pradhan, N.; Kim, M.; Peng, X. G. J. Am. Chem. Soc. 2006, 128, 10310–10319. (24) Nilsen, W. G. Phys. Rev. 1969, 182, 838–850. (25) Moulder, J. S., W. Sobal, P. Bomber, K. Handbook of X-ray Photoelectron Spectroscopy; Perkin Elmer: Eden Prairie, MN, 1992.

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(26) Vogel, W.; Borse, P. H.; Deshmukh, N.; Kulkarni, S. K. Langmuir 2000, 16, 2032–2037. (27) Matijevic, E. Langmuir 1994, 10, 8–16. (28) Liu, B.; Zeng, H. C. Small 2005, 1, 566–571. (29) Zhang, H. Z.; Banfield, J. F. Nano Lett. 2004, 4, 713–718. (30) Ribeiro, C.; Lee, E. J. H.; Longo, E.; Leite, E. R. Chemphyschem 2005, 6, 690–696. (31) Cho, K. S.; Talapin, D. V.; Gaschler, W.; Murray, C. B. J. Am. Chem. Soc. 2005, 127, 7140–7147. (32) Yu, S. H.; Yoshimura, M. Adv. Mater. 2002, 14, 296–300. (33) Lee, E. J. H.; Ribeiro, C.; Longo, E.; Leite, E. R. J. Phys. Chem. B 2005, 109, 20842–20846. (34) Huang, F.; Zhang, H. Z.; Banfield, J. F. Nano Lett. 2003, 3, 373– 378. (35) Peng, Z. A.; Peng, X. G. J. Am. Chem. Soc. 2002, 124, 3343–3353. (36) Snoeks, E.; van Blaaderen, A.; van Dillen, T.; van Kats, C. M.; Brongersma, M. L.; Polman, A. Adv. Mater. 2000, 12, 1511–1514. (37) Lu, Y.; Yin, Y. D.; Li, Z. Y.; Xia, Y. N. Langmuir 2002, 18, 7722– 7727. (38) Ding, D. S.; K. Clays, K.; Tung, C. H Adv. Mater. 2009, 21, 1–5. (39) Velikov, K. P.; van Dillen, T.; Polman, A.; van Blaaderen, A. Appl. Phys. Lett. 2002, 81, 838–840.