Controlled Synthesis of Hyperbranched Cadmium Sulfide Micro

Jan 10, 2008 - fabricate cadmium sulfide (CdS) hyperbranched micro/nano- structures. ..... S2- and Cd2+ by high temperature decomposition in hydro-...
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Controlled Synthesis of Hyperbranched Cadmium Sulfide Micro/ Nanocrystals Minghai Chen, Yong Nam Kim, Cuncheng Li, and Sung Oh Cho* Department of Nuclear and Quantum Engineering, Korea AdVanced Institute of Science and Technology (KAIST), 373-1 Guseong-dong, Yuseong, Daejeon 305-701, Republic of Korea

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 2 629–634

ReceiVed August 28, 2007; ReVised Manuscript ReceiVed October 7, 2007

ABSTRACT: A series of novel wurtzite cadmium sulfide (CdS) hyperbranched structures, including dendrites, multipetal flowers, and multipods, were controllably synthesized by a N,N-dimethylformamide (DMF) assisted hydrothermal route. CdS micro/ nanostructures of a certain morphology could be selectively produced by only varying the concentration of DMF in the reaction system. The crystal structure, compositional information, and morphological structures were carefully characterized by X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), field emission scanning electronic microscopy (FESEM), transmission electronic microscopy (TEM), and selected area electronic diffraction (SAED). On the basis of the characterization, we propose a formation mechanism of the hyperbranched CdS crystals. This route provides a facile strategy to fabricate complex hierarchical CdS structures. Introduction Driven by interesting size- and shape-dependent properties of nanocrystals, several methods have been developed for the morphology-controlled synthesis of nanorods, nanowires, nanotubes, and nanocubes.1a In addition, synthesis of more complex micro/nanostructures, including fractal patterns, dendrites, and other hierarchical structures, has attracted great attention because studies on these hyperbranched structures are useful to understand their formation mechanism and to fabricate electronic or photonic nanodevices.1b-d Although hyperbranched structures have been successfully prepared from metal,2 oxides,3,4 and inorganic semiconductors,5,6 most of these structured materials were limited to zinc oxide3 and lead sulfide5 due to the ease of preparation. In this study, we present an effective approach to fabricate cadmium sulfide (CdS) hyperbranched micro/nanostructures. CdS is one of the most important II-VI group semiconductors that are widely used for photoelectric conversion devices, lightemitting diodes, biological labeling, and other optical devices based on its nonlinear properties.7–15 Tremendous effort has been paid to synthesize CdS micro/nanostructures with various morphologies by chemical or physical methods, including laser ablation,7 template,8 vapor deposition,9,10 colloidal,11 and solvothermal methods.12 Among them, the hot-inject colloid method is considered as the most successful approach to control the final morphology; the method utilizes a thermal decomposition of organometallic precursors in a hot mixture of binary surfactants.11a,b Moreover, the thermal evaporation method with or without catalysis showed its superiorities in synthesizing onedimensional (1D) CdS crystals through vapor-solid (VS)9 or vapor–liquid–solid (VLS)10 mechanisms. In contrast, threedimensional (3D) complex hyperbranched CdS structures were mostly based on the solution method. Among them, the solvothermal approach showed great success in the morphology control of CdS structures. 3D CdS structures, such as multiarmed crystals,13 3D nanoflowers,14a,b and star-shaped structures,14c have been successfully prepared by the solvothermal method with the help of a complexing agent such as a copolymer and biomolecule. Xie et al.15a reported branched CdS micropatterns with polycrystalline structure using thiosemicarbazide * Corresponding author. E-mail: [email protected]. Tel.: +82-42-869-3823.

as both a sulfur source and a capping ligand in ethanol–water mixtures. Su et al.15b reported a mild solvothermal method to prepare single crystal dendritic CdS using ammonium thiocyanate as the sulfur source in ethanol–water mixtures. Han et al.15c reported a hydrothermal method to prepare 3-fold symmetric hierarchical dendritic CdS nanocrystals using poly(ethylene glycol) (PEG) as a capping agent. However, it is still challenging to control the morphologies of 3D CdS micro- and nanostructures over a wide range. Here, we present a facile approach to controllably synthesize a series of hyperbranched CdS micro- and nanocrystals using a hydrothermal process. By only tuning the N,N-dimethylformamide (DMF) concentration in the reaction system, the morphology of the produced CdS crystals can be manipulated from a dendrite to a flower and then to a multipod structure. The growth mechanism of the crystals was also proposed on the base of the characterization of their crystal structures and morphology analysis. This strategy shows high flexibility in preparing complex hyperbranched micro/nanostructures.

Experimental Section Chemicals and Synthesis. CdCl2 · 1.5H2O, thiourea, and N,Ndimethylformamide (DMF), purchased from Junsei Chemical Co., Ltd., Japan, were used without further purification. In a typical experiment, 2 mmol of CdCl2, 2 mmol of thiourea, and various amounts of DMF (1–20 mL) were put into a Teflon-lined stainless steel autoclave with 80 mL capacity, and then, the distilled water was added up to 90% of the total volume. The autoclave was loaded into an electrical oven and maintained at 200 °C for 10 h. After cooling to room temperature, the precipitate was washed with water and alcohol repeatedly. Finally, the products were dried at 60 °C in vacuum for 4 h. Characterization. The crystal structures of the as-prepared samples were characterized by X-ray diffraction (XRD; Philips X’pert PRO), using Cu KR radiation. The morphologies and microstructures of the samples were observed using an FEI Tecnai F20 transmission electron microscope (TEM) and a Hitachi S4300 field emission scanning electron microscope (FESEM). Selected area electron diffraction (SAED) patterns and high-resolution transmission electron microscopy (HRTEM) images were taken by a JEOL-2100F transmission electron microscope at the Korea Basic Science Institute (KBSI). X-ray photoelectron spectra (XPS) were recorded on a VG ESCA2000

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Figure 2. XRD patterns of the as-prepared samples synthesized at 200 °C for 10 h at different addition amounts of DMF: (a) 0, (b) 1, (c) 5, (d) 10, (e) 20, and (f) 72 mL of DMF.

Figure 1. FESEM images of the CdS structures evolved at different addition amounts of DMF: (a) dendrites at 0 mL, (b) flowers at 1 mL, (c) flowers at 5 mL, (d) multipods at 10 mL, (e) multipods at 20 mL, and (f) nanoparticle aggregations at 72 mL of DMF. spectrometer, using a non-monochromatized Mg KR X-ray as the excitation source and choosing C1s as the reference line.

Results and Discussions Figure 1 shows the morphology evolution with increasing the DMF concentration in the hydrothermal system. When no DMF was added in the solution, well-defined dendrite structures with a pronounced trunk and multibranches were created in high yield (Figure 1a). When 1 mL of DMF was introduced into the solution, flowerlike structures with multiple petals were formed (Figure 1b). Each petal of the flowerlike structure consists of ordered veins as in the dendrite structure, suggesting that growth processes of the two different structures are similar. Occasionally, a 6-fold symmetric flower was observed in the product (inset of Figure 1b). This 6-fold symmetric flower structure is not in the same plane but had a 3D structure, which is clearly different from the usually reported flower structures.4a,6b,15b Upon increasing the DMF amount to 5 mL, flowerlike structures were still the main product although the petal thickness was slightly incrassated (Figure 1c). When 10 mL of DMF was added, the flowerlike patterns disappeared and instead conical multipod structures with ordered corrugations became the main products (Figure 1d) The corrugations were composed of many small triangular nanoparticles surrounding a main trunk, forming a towerlike hierachical structure. Increase in the DMF concentration shortened the length of the conical trunk (Figure 1e), and finally, only irregular particulates were generated when the addition amount of DMF was above 40 mL (Figure 1f). XRD patterns of the as-prepared samples at different concentration of DMF are shown in Figure 2. The sharp diffraction peaks indicate the good crystallization of the products. All the peaks can be well-indexed to wurtizite CdS with lattice constants of a ) 4.14 Å and c ) 6.71 Å, which are in good agreement with the JCPDS card no. 41-1049 (a ) 4.14 Å and c ) 6.719 Å). These indicate that an addition of DMF has no effect on

the crystal phase of the products and that CdS was the only product after hydrothermal treatment at 200 °C for 10 h. Although the as-prepared samples had well-defined morphologies as shown in Figure 1a-e, the relative intensities of XRD peaks (Figure 2a-e) corresponding to different crystal planes did not show obvious variation. These may be attributed to the facts that the samples randomly lay on the substrate and their corresponding preferential crystal planes were randomly distributed. The compositional information of the as-prepared products was analyzed by XPS spectra (Figure 3). The peaks of XPS spectra appeared at 161.2, 405.1, and 411.8 eV, which correspond to S2p, Cd3d5/2, and Cd3d3/2, respectively. These further confirm that the products are composed of high purity CdS. The detailed morphologies of the CdS micro- and nanostructures were further analyzed with FESEM and TEM. When no DMF was added in the solution, dendrite structures were formed. Figure 4a shows a magnified FESEM image of a single dendrite, displaying that the dendrite consists of one main trunk and 3-fold symmetric rows of branches that are separated by 120°. All the branches in the same row are parallel with each other, and these branches extend at ∼42° with respect to the trunk (Figure 4b). The SAED pattern (inset in Figure 4b) taken from the entire dendrite reveals that the dendrite is a single crystal. The HRTEM images show that the interplanar distances of the trunk and the branch are 0.335 and 0.245 nm, which agree well with those of (002) and (102) of wurtzite CdS, respectively (Figure 4c and d). Furthermore, the measured angles between the main trunk and the branches are about 42°, which also corresponds well with the angle of ∼43° between (001) and (102) of the hexagonal CdS crystals. Consequently, these results reveal that the created dendrites are CdS single crystals and that the main trunk of the dendrite grows along the [001] direction while the branches grow in the [102] direction. A simulacrum of this 3-fold symmetric dendrite is shown in Figure 4e. We note here that the CdS dendrites produced in our study have different growth direction from the dendrites reported elsewhere,15c where the growth directions of both trunks and branches are [001]. Interestingly, when the DMF concentration was slightly increased, one row of the branches grows more slowly than the other two rows. Finally, 3-fold dendrite structures became 2-fold dendrite structures and, moreover, the dendrites assembled to form a multipetal flowerlike structure while sharing the same

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Figure 3. XPS patterns of the CdS sample: (a) survey spectrum and (b and c) high-resolution spectra for (b) S2p and (c) doublet Cd3d5/2 and Cd3d3/2.

Figure 4. Microstructure characterizations of dendritic crystals: (a) SEM image of a single of 3-fold symmetric dendrite, (b) TEM image (the inset shows its SAED pattern) and the corresponding HRTEM images taken from the (c) trunk and (d) branch; (e) simulacrum of this dendritic structure.

core (Figure 5a and b) when 1 mL of DMF was added into the solution. The SAED pattern taken from the entire flowerlike structure indicates that the structure is a polycrystal (Figure 5c). However, the SAED pattern of single petal displays that each petal is a single crystal (Figure 5d). These results suggest that many dendrites started to grow from nearly the same point to arbitrarily different directions without having preferred directions. When the amount of DMF inserted into the solution was increased to 10 mL, the multipod structures displayed in Figure 6 were generated. The multipod structures show several different

Figure 5. Microstructure characterizations of multipetal flower structure: (a) SEM image of multipetal flower composed by several dendritic petals, (b) TEM image, and its SAED patterns taken from (c) the entire structure and (d) a single petal.

morphologies. First, Figure 6a-c exhibits a very unique tetrapod structure, which consists of three arms meeting at 120° with each other on the same plane and one vertical arm at a right angle to the other three arms. Furthermore, each arm of the tetrapod had a hierarchical structure comprising a conical trunk and many triangular nanoparticles (Figure 6b). The small triangular particles grew vertically to the axis of the trunk (Figure 6b), and these particles formed a well-defined hexagram

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Figure 6. Microstructure characterizations of multipods: (a) a top view SEM image of a tetrapod structure; (b) a lateral SEM image showing the longitudinal distributions of triangle particles around the trunk; (c) TEM image of a tetrapod and (inset) its SAED pattern; (d) HRTEM image of the lying arm; (e) TEM image of a vertical arm and (f) its HRTEM image; (g) SEM and (h) TEM image of a multipod; (i) HRTEM image taken at any arm.

around the trunk (Figure 6a). The SAED pattern (inset of Figure 6c) clearly indicates that the whole tetrapod is a single-crystalline wurtzite CdS along the [0001] zone axis. The HRTEM image (Figure 6d) of an arm shows that interplanar distances are 0.18 nm, which corresponds to the distance of (200) of wurtzite CdS, in all six directions. The lattice image combined with the SAED pattern reveals that the three arms grow along the [101j0], [1j100], and [01j10] directions, respectively. HRTEM images show that the small triangle particles on the trunk grow along [2110] directions (the images are not shown here). Since this direction is not the preferential growth direction of a hexagonal crystal, the small particles cannot sufficiently grow. In contrast to the three planar arms, the remaining vertical arm of the tetrapod grew along the [0001] direction, which was identified from both the HRTEM image (Figure 6f) and its fast fourier transform (FFT) pattern (inset of Figure 6f). Therefore, this tetrapod has a crystalline structure that is completely different from the tetrapod crystals grown on blende seeds,2b,13 where no three arms exist on the same plane and thus the tetrapod is not a single crystal. To the best of our knowledge, this is the first report on the CdS tetrapod single crystal structure with conical arms, and furthermore, such a hierarchical structure consisting of microscale trunk and nanoscale particles is also very unique. Figure 6g and h shows FESEM and TEM images of multipod structures that are composed of more than four arms. HRTEM images (Figure 6i) and SAED patterns (insert of Figure 6i) show that all of the arms grow along the [0001] direction and do not show a geometrical symmetry.

In addition to the tetrapod, another kind of multipod structure, 6-fold symmetric CdS crystals resembling snow crystals were also found (Figure 7a.) Figure 7b shows the backside of the 6-fold CdS crystal, which shows the well-developed hexagonal structure with six trunks extending outward, which are completely different from their top view images. The growth stages (shown by arrows) in the trunks can be clearly seen, which result from the fast growth of (100) dragged by the slow growth of (120). Figure 7c shows the TEM image of a single snowlike particle. Its SAED pattern (Figure 7d) confirms that it is a single crystal with an orientation parallel to (0001) and six branches that grow along [100] directions. This CdS single crystalline structure composed of a snowlike plane with a vertical arm is unique and has never been reported. All of the above results reveal that the arms of multipod structure can grow along two directions: [100] and [001]. All the arms of the multipod structures without high geometric symmetry preferentially grow along the [001] direction. Whereas, for the highly symmetric multipod structures, such as a tetrapod (Figure 5a) and a snowlike shape (Figure 7a), one vertical arm grows along the [001] direction and the other three or six planar arms grow along [100] directions. As shown in the FESEM and TEM images, all the hyperbranched CdS crystals fabricated by the presented method have micro- and nanometer combined hierarchical structures. It is generally accepted that hierarchical structures are formed in situations far from thermodynamic equilibrium, where high driving forces lead to generation of rough crystallites and

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Figure 7. Microstructure characterizations of novel snowlike CdS: (a) top view, (b) back, and lateral image SEM images; (c) TEM image and (d) its corresponding SAED pattern.

Scheme 1. Sketch Map of the Growth Procedure in the System at Different DMF Concentrations: (I) Dendrites, Without DMF; (II) Multipetal Flowers, Low DMF Concentration; (III) Multipods, Medium DMF Concentration; (IV) Particles, High DMF Concentration. The Circle Inset Shows the Dissolution and Reprecipitation Mechanism Between Dendritic Tips and Conjunctions. The SEM Micrographic Images Show the Products Prepared at Different Stages (200 °C for 3 and 10 h)

random association.1 Such necessary thermodynamic nonequilibrium microenvironments can be easily achieved in wet chemical processes if complexing agents or surfactants that are widely used to control the shapes and structures of produced crystals are inserted in the reaction solutions. Accordingly, complexing soft template theory12,13 and selected adsorption theory4 are widely used to explain the mechanisms of crystal growth. Although the exact growth mechanism of the hyperbranched CdS crystals presented here is not clear, a possible growth mechanism is proposed. Scheme 1 illuminates the growth procedure of CdS crystals in the reaction system with different DMF concentrations. The inserted SEM images show the samples prepared at different stages. It has been reported that

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thiourea acting as a sulfur source in our reaction system can also work as a complexing agent.15b,c Thiourea can coordinate with Cd2+ to form stable complexing ions, which can release S2- and Cd2+ by high temperature decomposition in hydrothermal conditions. As a result, the concentrations of S2- and Cd2+ are maintained at a stable level, which is in favor of anisotropic growth. Moreover, a thiourea-Cd2+ complex might act as a soft template to direct a CdS nucleus at right sites to form dendrite structures. The key factor to control the morphology of the CdS crystal in our research is the concentration of DMF, which can adjust the coordination ability of thiourea. Some reports have revealed that DMF can effectively increase the dissolvability of CdS through solvation of the oxygen atoms in DMF to the Cd atoms on the CdS crystal surface as well as to the Cd4 clusters in solvent.16 Consequently, an addition of DMF to the hydrothermal system can induce a coordination competition between thiourea and DMF. We speculate that DMF plays two roles in the hydrothermal reaction. First, due to the solvation of the CdS surface, thermodynamically active sites such as surface defects and active crystal surfaces are created on the early formed CdS nuclei. These active sites can then serve as heterogeneous nucleation sites. Once nuclei were formed at these active sites, they can grow along their preferential orientation. As a consequence, many projecting dendrites can develop into a complex flowerlike 3D assembly sharing the same core. If no DMF is added, a nucleus grows along only one direction to form a single dendrite, as shown in Figure 1a. The effect of DMF is very strong, and thus, an addition of DMF with a small amount (for example 1 mL, Figure 1a-c) can change the morphology of CdS crystals from dendrites to flowerlike structures. Second, the solvation process also smoothens the rough surface of CdS crystals during the growth stage, particularly if a large amount of DMF is added. Since the dissolvability of a particle depends on the surface curvature of the particle, the dissolution occurs from the tips of branches that have higher curvature than other regions. The CdS molecules produced by the dissolution of branches might be reprecipitated at the conjunction regions between a trunk and branches, where surface curvature is negative, leading to the formation of multipetal flowerlike structures (Figure 5). This dissolution and reprecipitation process is similar to the wellknown Ostwald ripening process. If a large enough amount of DMF is added, the branches cannot grow and thus multipod structures are generated. When the DMF addition is over 40 mL, the anisotropic growth of the CdS crystals is completely stopped because of the strong interaction between CdS and DMF and instead irregular polycrystalline nanoparticles are produced. We tried to use other complexing agents or surfactants to tune the morphology, such as citrate acid, diethanolamine, cetyltrimethylammonium bromide (CTAB), and poly(diallyldimethylammonium chloride) (PDDA), which have strong coordination or adsorption ability with CdS particles. However, even a small amount of addition of those agents resulted in irregular particle aggregations (not shown here). Conclusion In conclusion, a series of hyperbranched CdS micro/nanostructures were successfully synthesized by a facile DMFassisted hydrothermal route using thiourea as a sulfur source. The morphology of CdS crystals can be controllably changed from dendrites to multipetal flowers and then to multipods only by changing the concentration of DMF in the reaction system. The unique multipod structures (including single crystalline tetrapods and snowlike structures) composed by hierarchical

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conical arms were reported for the first time. A possible mechanism of the crystal formation was proposed on the basis of the understanding of their crystal structures and microstructures. The complexing structure and coordination competition between thiourea and DMF might affect the final morphologies. We believe that this method provides a facile strategy to fabricate complex hierarchical micro/nanostructures. These hyperbranched CdS crystals may possess promising application in microelectronic and photovoltaic devices. Especially, the inorganic networks composed of outstretching branches may enhance the photoelectric conversion efficiency in hybrid solar cells through ensuring a large and distributed surface area for charge separation.17 What’s more, these complex CdS structures can also act as templates to fabricate other hyperbranched chalcogenides solid or core–shell structures through simple replacement reaction, through which compositional dependent properties can be tuned.

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