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Study of Magic-Size-Cluster Mediated Formation of CdS Nanocrystals: Properties of the Magic-Size Clusters and Mechanism Implication Qiyu Yu†,‡ and Chun-Yan Liu*,† Key Laboratory of Photochemical ConVersion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, P.R. China and Graduate UniVersity of Chinese Academy of Sciences, Beijing 100049, P.R. China ReceiVed: April 7, 2009; ReVised Manuscript ReceiVed: May 21, 2009
The magic-size clusters (MSCs) of semiconductor nanocrystals (NCs) have attracted much attention the last few years. In this work, we studied the synthesis of CdS NCs in a liquid-paraffin/glycerol biphasic system at different temperatures. The properties of a 323-nm-absorbing MSC species, an intermediate in the synthesis, were investigated using an ultraviolet-visible (UV-vis) spectroscopic technique. Alcohol-induced transformations of the MSCs into a ∼309-nm-absorbing and/or a ∼348-nm-absorbing cluster/crystal species were studied in detail. Based on the studies, a plausible nucleation-growth mechanism was proposed for the MSC-mediated formation of CdS NCs. Introduction Colloidal semiconductor nanocrystals (NCs) have attracted tremendous attention for their significance in fundamental studies and technical applications, mainly due to their interesting size-dependent properties and flexible processing chemistry.1 Recently, significant interest has arisen in molecule-like precursors2,3 to semiconductor NCs, called magic-size clusters (MSCs), in part because of the ability to control the size of the nanoparticle with atomic precision, something not currently possible with the regular semiconductor NCs. Extremely sizestable MSCs have been reported for several II-VI4-10 and IV-VI11 semiconductor materials. Also, MSC species as intermediate was encountered at the early stage of quite a few syntheses of CdTe,12,13 CdSe,2,14 and CdS NCs.15-18 The knowledge of nucleation and growth is important to the development of synthetic chemistry for high-quality semiconductor NCs.19-21 However, the understanding of NC nucleation is very limited, mainly because nucleation usually occurs too quickly to access.22 When MSC species were involved in the crystallization systems, the nucleation and growth mechanism may become even more complicated. Peng’s group2 proposed a forward/backward-tunneling mechanism for the fate of MSCs during the synthesis of anisotropic CdSe NCs. Chikan and coworkers13 proved that the MSCs could evolve into regular CdTe NCs by particle-particle aggregation, namely quantized growth. Pradhan et al.14 observed that the CdSe MSCs could form nanowires by orient attachment. Pan et al.16,17 found that the MSCs acted as nuclei in a low-temperature synthesis of CdS NCs and the nuclei size was independent of monomer concentration, capping agent concentration, solvent, or even reaction temperature. However, due to the lack of information in the literature and the difficulty of observing MSCs using electron microscopy, the knowledge about the properties of MSCs is rather limited. The nucleation and growth of NCs in the presence of MSCs, especially the evolution from MSCs to regular NCs, * To whom correspondence should be addressed. E-mail: cyliu@ mail.ipc.ac.cn. † Chinese Academy of Sciences. ‡ Graduate University of Chinese Academy of Sciences.
was quite elusive.2,17 Therefore, a related study should be rather interesting and desirable. Experimental Section Chemicals. Thiourea, oleic acid, glycerol, hexane, methanol, absolute ethanol, propanol and butanol were A.R. grade and purchased from Beijing Chemical Regent Ltd., China. Cadmium oxide (g99%), and paraffin liquid (chemical grade, with boiling point higher than 300 °C) were obtained from other kinds of commercial sources, and used as received. Cadmium Stock Solution. 1.28 g of CdO (10 mmol) and 20 mL of oleic acid (OA) were loaded into a three-neck flask. Under N2 flow, the mixture was vigorously stirred, quickly heated to 140 °C, and kept at this temperature for about 10 min to dissolve CdO. Into this solution was added 30 mL of paraffin to dilute and cool the solution. Synthesis of OA-Capped CdS NCs. The synthesis method was adapted from the one recently reported by our group.18 Typically, 38 mg of thiourea (0.5 mmol) was dissolved in 20 mL of glycerol by stirring and heating at about 40 °C. Five mL of cadmium stock solution was diluted to 20 mL with paraffin. The solution was transferred into a 100 mL reaction flask. Under N2 flow, the mixture was vigorously stirred, quickly heated to a desired temperature (taking about 10 min), and maintained for a given time. Aliquots were taken at different time intervals when the desired temperature was reached (the time was taken as t ) 0) and diluted in hexane for absorption and emission measurements. Study of the Properties of the MSCs. Effect of Alcohols on the MSCs. The synthetic reaction was stopped at a suitable stage to achieve the coexistence of the MSCs and regular NCs. The reaction mixture was cooled to room temperature. A MSC/ NC aliquot was collected from the paraffin phase (the upper layer), diluted to 4 mL with hexane, and then further diluted with 1 mL of alcohol. The solution was fairly stable. After a given time (up to tens of minutes), the solution was characterized by UV-vis absorption spectroscopy. Methanol was immiscible with hexane at this ratio and two layers formed; the upper hexane layer was taken for absorption measurement.
10.1021/jp903199y CCC: $40.75 2009 American Chemical Society Published on Web 06/22/2009
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Figure 1. Temporal evolution of the UV-vis absorption spectra of the aliquots taken during synthesis. The reaction mixture was heated from room temperature; the time when the mixture reached the desired synthesis temperature was taken as t ) 0.
Additional experiments were conducted to remove the alcohol from the hexane/alcohol solution. Typically, a sample in hexane/ ethanol was distilled under reduced pressure at 25 °C for about 1 h to remove the hexane and ethanol. Then the remained paraffin solution was diluted in hexane for UV-vis absorption measurement. Size-SelectiWe Precipitation of CdS MSCs/NCs. A MSC/NC aliquot was collected, and hexane was added to dilute the solution before adding enough ethanol to precipitate the MSCs/ NCs. A suitable amount of hexane was needed here to facilitate the interfusion of paraffin liquid and ethanol. The mixture was then centrifuged to separate the precipitate and the supernatant solution. Then the precipitate (after redispersed in hexane) and the supernatant were detected with UV-vis absorption. Characterization. The UV-vis absorption and photoluminescence (PL) spectra of the CdS samples were characterized on a Shimadzu UV-1601 PC fluorometer and a Hitachi F-4500 fluorescence spectrometer, respectively. Each aliquot taken during the reaction was diluted with hexane and the upper hexane layer was used for the optical measurements. The absorption onset used the minimum of the second derivative.23 The absorbance of the sample was kept small to avoid selfabsorption. The transmission electron microscopy (TEM) image of CdS NCs was taken on a JEOL JEM-200CX transmission electron microscope, using an accelerating voltage of 160 kV. The highresolution transmission electron microscopy (HRTEM) images of CdS NCs were taken on a JEOL JEM-2010 transmission electron microscope, using an accelerating voltage of 200 kV. Samples for TEM and HRTEM were prepared by dropping dilute solutions of properly washed NCs in toluene onto carboncoated copper grids. Results and Discussion 1. Optical Study of the Synthesis at Different Temperatures. 1.1. Absorption Spectra. Figure 1 shows the temporal evolution of UV-vis absorption spectra for the synthesis at about 110, 130, and 150 °C, respectively. The sharp peaks at ∼323 nm in some of the absorption spectra are due to MSCs of CdS. A congeneric MSC species of CdS absorbing at about 312 nm has been previously reported by Pan et al. and our group.16-18 The persistent absorption features at below 300 nm (∼271 and ∼280 nm) are probably due to some molecular species rather than MSCs.2 The absorption features at longer wavelengths, typically over 350 nm, are assigned to first
Figure 2. (a) Observations of the initial generation of the regular NCs in Figure 1. (b) First absorption peak positions of regular NCs as a function of reaction time for the syntheses at different temperatures.
absorption peaks of regular CdS NCs. The temporal red-shift of the absorption peak indicates the continuous growth of regular CdS NCs during synthesis. In this context, “nanocrystals (NCs)” only refer to those with regular sizes, to distinguish them from so-called MSCs. In the synthesis, the MSCs as intermediate experience an increasing and then decreasing process, as indicated by the intensity variation of the 323 nm peak. At 110 °C, the MSCs exist in the reaction solution over 2 h (Figure 1a). As the reaction temperature increases, the life-span of the MSCs in the reaction solution can be shortened to tens of minutes, and to a few minutes, as depicted in Figure 1, panels b and c. At an early stage of the synthesis, the MSCs are generated and act as nuclei to initiate the growth of NCs.16-18 Figure 2a shows the initially detected formation of the regular NCs (from Figure 1). During the synthesis, the regular NCs form almost at the same time as the MSCs. The 323 nm CdS MSCs seem to possess less stability than the 312 nm CdS MSCs reported previously, which can stay for up to tens of minutes without growing into regular NCs.16-18 The MSCs/NCs form earlier for the synthesis at higher temperatures. At a synthesis temperature of 150 °C, they formed during the temperature climbing process (about 140 °C, 2 min before the desired temperature was reached). Figure 2b shows the temporal shift of the first absorption peak position of regular NCs, demonstrating that the regular NCs grow faster at higher temperatures. 1.2. Photoluminescence Spectra. Figure 3 shows the temporal evolution of the photoluminescence (PL) spectra and
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Figure 3. (a) Normalized photoluminescence (PL) spectra corresponding to Figure 1. (b) The band-edge PL widths as a function of reaction time.
Figure 4. Caculated concentrations of regular CdS NCs as a function of reaction time.
corresponding PL widths of the regular CdS NCs. Two types of emission bands can be detected: the band-edge PL and the deep-trap PL. At higher temperatures, the band-edge PL is more pronounced relative to the deep-trap PL. The full width at halfmaximum (fwhm) of the band-edge PL, which is a measure of the NC size distribution, decreases at first and then keeps almost unchanged during synthesis. Furthermore, the widths of the band-edge PL decrease significantly when the synthesis temperature varies from 110 °C to 130 and 150 °C. This result indicates that a relatively high temperature is favorable for the growth of monodisperse CdS NCs. 1.3. Concentration of CdS NCs. The nucleation and growth details can be studied by following the absorption-derived NC concentration during synthesis.24 Due to the special stability of the MSCs (nuclei), the nucleation process is easy to access. Furthermore, the relatively mild reaction temperature slows down the reaction rate substantially, which also facilitates the study of the nucleation and growth events. Figure 4 plots the temporal evolution of the NC concentrations. At 110 °C, the NC concentration keeps increasing for the entire recorded period of time. This indicates that the nucleation and growth stages are overlapped. Pan et al.17 observed a similar result during the low-temperature synthesis of CdS NCs in toluene/water. At 130 °C, a turning point can be observed for the NC concentration evolution. Before ∼14 min, the NC concentration is increasing; after that, the concentration keeps almost constant. This indicates that the nucleation and growth occur concurrently for the first ∼14 min and then the NC growth begins to dominate. At 150 °C, the NC concentration keeps almost invariable throughout the whole recorded period. This means that only the growth stage is monitored and that the nucleation stage is bypassed quickly.
Yu and Liu
Figure 5. UV-vis absorption spectra of a crude CdS MSC/NC sample before and after alcohol addition. The absorption features are labeled and the corresponding positions are given from left to right. The labeled positions are corresponding to absorption features at approximately 309 (R), 323 (β), 348 (γ), and 390 nm (φ).
The separation of the nucleation and growth stages is often regarded as a prerequisite of producing monodisperse NCs, and a hot-injection method is frequently adopted to achieve this goal.19 In our experiments, no injection-based technique was used to separate the nucleation and growth stages. By increasing the synthesis temperature, the duration of nucleation stage could be controlled to be short enough to separate from the subsequent growth stage. This explains that the CdS NCs prepared at 150 °C possess the narrowest size distribution. 2. Study of the CdS MSCs. 2.1. Alcohol-Induced Forward/ Backward-Tunneling of the MSCs. By quitting the synthetic reaction at an early stage, a mixture of MSCs and regular NCs of CdS can be obtained. We studied the effects of several small alcohols on the hexane-diluted MSC/NC solution using the UV-vis absorption method. As illustrated in Figure 5, after alcohol addition, the absorption of the MSCs decreases or disappears, accompanied by the generation of an absorption feature at ∼309 nm and/or another absorption feature at ∼348 nm. Note that the absorption of the regular CdS NCs is unchanged. Obviously, the two newly generated cluster/crystal species are originated from the MSCs. It is well-known that the MSC species possesses a locally lowest chemical potential; during transforming into the ∼348 nm and ∼309 nm species, it should “tunnel” through the thermodynamic barriers at its bilateral sides.2 The transformations of the MSCs into the ∼348 nm species and the ∼309 nm species can be denoted as “forward-tunneling” and “backward-tunneling”, respectively.2 Further experiments demonstrate that the ∼348 nm and ∼309 nm species will transform into the ∼312 nm MSCs after the alcohol is removed (Figure 6). This result suggests that the two tunneling products are highly unstable. The forward/backward-tunneling of the MSCs may be completed in tens of minutes. Figure 7 shows two examples of the temporal evolution of the MSCs after ethanol addition, one mainly forward-tunneling and another backward-tunneling. The temporal evolution of the absorption spectrum of each sample clearly shows two isosbestic points (at around 315 and 330 nm), suggesting equilibriums between the MSCs and the forward/ backward-tunneling products. El-sayed and co-workers25 also observed an isosbestic point during the transformation of a ∼445 nm CdSe species into a 414 nm MSCs. It may be regarded as the reverse case of the forward-tunneling of the CdS MSCs observed here. 2.2. Colloid Stability. We investigated the colloid stability of the MSC species and its tunneling products by using the size-
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Figure 6. Transformations of the two tunneling products into a 312nm-absorbing MSC species.
Figure 8. Absorption spectra of two MSC/NC samples before and after size-selective precipitation using hexane/ethanol. Ethanol-induced forward-tunneling (a) and backward-tunneling (b) of the MSCs occurred during the size selection process.
Figure 7. Temporal evolution of the absorption spectra of two MSC/ NC samples after ethanol addition (locally enlarged). (a) 1, 3, 5, 7, 10, and 15 min after ethanol addition; (b) 1, 4, 10, 20, 30, and 40 min after ethanol addition.
selective precipitation method.19,26 Figure 8 shows the results of size-selective precipitation using hexane/ethanol (solvent/ nonsolvent). The 271, 280 nm molecular species and a few smallest regular NCs were removed into the supernatant, while the 349 and 309 nm species, which were resulted from the forward/backward-tunneling of the MSCs, were precipitated out of the solution along with most of the regular NCs. As illustrated in Figure 8b, some unconverted MSCs were also precipitated. In spite of their smaller sizes, the CdS MSCs and the tunneling products exhibit less colloid stability than the regular NCs in the supernatant, suggesting that the MSCs and the tunneling products are quite different from the smallest regular NCs. We also note that the smallest regular NCs have closely similar absorption wavelengths to the ones initially observed in the reaction solution (Figure 2a). This is so because the used samples were arrested at the overlapped nucleation-growth stage when the generation of the regular NCs was still happening. 2.3. Possible Explanations. The CdS MSCs and the tunneling products observed here are extremely small. They possess too few core atoms that the particles are almost all highly curved surfaces. Surface and solvent interactions dominate in the colloid stability.25 The increasing in the average solvent polarity by adding a nonsolvent has much greater influence on these species than the regular CdS NCs. This may lead to the easier
aggregation of these species than the smallest regular NCs. Yu and co-workers also observed the readily precipitation of the rather small MSCs of CdSe.8 We try to give a preliminary explanation for the alcoholinduced transformations of the MSCs as follows. Under the attack of the small alcohol molecules, a few surface-bound capping molecules and even some surface atoms of the MSCs, may be removed.7,19,27 The resulting clusters are extremely unstable: further cracking of the clusters may lead to backwardtunneling into an unstable ∼309 nm species; further aggregation /coalescence of the clusters may lead to forward-tunneling into another unstable ∼348 nm species. More detailed investigation should be carried out to elucidate the properties of the MSCs mentioned above. Nonetheless, these experimental results at least suggest (1) that the MSCs have a strong tendency to aggregate and (2) that the MSCs may undergo aggregation/coalescence or cracking under intense conditions. 3. Possible Nucleation-Growth Mechanism. Experimentally, it is hard to prove whether the mentioned forward/ backward-tunneling events occur or not during NC synthesis (Supporting Information). For one thing, the tunneling products are highly unstable and difficult to detect using the UV-vis absorption method. For another, the development of the regular NCs may complicate the absorption spectral analysis. Nevertheless, there exists a possibility that the forward/backwardtunneling may be induced by the enhanced particle-particle interaction or collision at the elevated synthesis temperatures (above 100 °C). Furthermore, the observed earliest regular NCs (Figure 2a, especially the one at 130 °C) have absorption wavelengths similar to (slightly longer than) that of the forwardtunneling product. It appears that the earliest regular NCs are slightly grown ones from the ∼348 nm species. This also lends some support to the forward-tunneling of the MSCs during synthesis. In this work, we tentatively propose a nucleation-growth mechanism for the formation of the CdS NCs (Scheme 1).
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SCHEME 1: Proposed MSC-Mediated Nucleation and Growth of CdS NCs
As depicted by the scheme, the MSCs form initially, and act as nuclei to forward-tunnel into the ∼348 nm species via particle-particle aggregation and coalescence. The resulted secondary particles then grow quickly into stable regular NCs. The NC concentration increases until nucleation stops due to reduced monomer concentration. In the following growth stage, with the regular NCs growing, the MSCs mainly backwardtunnel into the ∼309 nm clusters and further decompose into monomers until they disappear eventually. This process can be deemed as Ostwald ripening growth, namely the growth of regular NCs at the expense of the extremely small MSCs. The concentration of the regular NCs keeps almost invariable during the growth stage. According to the literature, the evolution of MSCs into regular NCs may be realized in three ways: (1) quantized-growth of MSCs, where the initial formation of regular NCs is the result of particle-particle aggregation of MSCs;13 (2) deposition of a large amount of monomers on the MSCs almost instantaneously;2 (3) oriented attachment of MSCs into nanowires.14 Besides, MSCs and regular NCs may also develop dependently; namely they originate from different nuclei.8 The forwardtunneling scheme proposed in this work resembles the quantizedgrowth scheme recently reported by Chikan and co-workers for the growth of CdTe NCs.13 They found that the domain sizes in the HRTEM images of the regular NCs correlated well with the CdTe MSCs, which clearly suggested a quantized-growth history. Figure 9 shows the representative TEM and HRTEM images of as-synthesized CdS NCs. The NCs are spherical or slightly elongated, suggesting that the oriented attachment of the MSCs into nanowires does not take place.14,28 HRTEM images a, b,
and c are corresponding to (111), (200), and (220) faces of zinc blende CdS NCs, respectively. Unfortunately, no MSC domains can be observed in these images. However, this does not mean that the proposed forward-tunneling event does not happen in the synthesis. Likely, the MSCs observed in this work are extremely small and possess too few atoms to define a core structure. Recrystallization happens easily during the aggregation/coalescence of the MSCs. Thus, the forward-tunneling of the MSCs and subsequent growth lead to CdS NCs with consistent crystallinity. Conclusion In conclusion, we have studied the properties of a 323-nmabsorbing MSC species and the MSC-mediated formation of CdS NCs. In low temperature synthesis, the nucleation stage is overlapped with nuclei growth. In high temperature synthesis, the nucleation stage is bypassed quickly, and thus the nucleation and growth are well separated. The MSCs, induced by an alcohol, can forward (backward) tunnel into a ∼348 nm (∼309 nm) absorbing cluster/crystal species at room temperature. The ∼348 and ∼309 nm species are highly unstable; they can transform into a 312-nm-absorbing MSC species after the alcohol molecules are removed from the system. The MSC species and the tunneling products exhibit unusual instability with respect to aggregation. Based on the studies, we propose a novel MSC-mediated nucleation and growth mechanism for the synthesis of CdS NCs. Our work may provide some understanding of MSC species and the nucleation-growth mechanism of semiconductor NCs in the presence of MSCs. Acknowledgment. This work was financially supported by National Natural Foundation of China (20573126), Chinese Academy of Sciences, and National Basic Research Program of China (973 Program). Prof. Xiangmin Meng and Ms. Xiaoping Yang are thanked for technical assistance and useful discussions. Supporting Information Available: Other experimental results concerning alcohol-induced tunneling of the CdS MSCs. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes
Figure 9. Typical TEM (top) and HRTEM images (bottom) of asprepared CdS NCs. Approximate borders of the particles are marked using dashed lines in the HRTEM images.
(1) For reviews, see: (a) Alivisatos, A. P. Science 1996, 271, 933– 937. (b) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Annu. ReV. Mater. Sci. 2000, 30, 545–610. (2) Peng, Z. A.; Peng, X. J. Am. Chem. Soc. 2002, 124, 3343–3353. (3) Cumberland, S. L.; Hanif, K. M.; Javier, A.; Khitrov, G. A.; Strouse, G. F.; Woessner, S. M.; Yun, C. S. Chem. Mater. 2002, 14, 1576–1584. (4) Herron, N.; Calabrese, J. C.; Farneth, W. E.; Wang, Y. Science 1993, 259, 1426–1428. (5) Vossmeyer, T.; Katsikas, L.; Giersig, M.; Popovic, I. G.; Diesner, K.; Chemseddine, A.; Eychmu¨ller, A.; Weller, H. J. Phys. Chem. 1994, 98, 7665–7673.
Magic-Size-Cluster Mediated Formation of Nanocrystals (6) Kasuya, A.; Sivamohan, R.; Barnakov, Y. A.; Dmitruk, I. M.; Nirasawa, T.; Romanyuk, V. R.; Kumar, V.; Mamykin, S. V.; Tohji, K.; Jeyadevan, B.; Shinoda, K.; Kudo, T.; Terasaki, O.; Liu, Z.; Belosludov, R. V.; Sundararajan, V.; Kawazoe, Y. Nat. Mater. 2004, 3, 99–102. (7) Kudera, S.; Zanella, M.; Giannini, C.; Rizzo, A.; Li, Y.; Gigli, G.; Cingolani, R.; Ciccarella, G.; Spahl, W.; Parak, W. J.; Manna, L. AdV. Mater. 2007, 19, 548–552. (8) Ouyang, J.; Zaman, Md. B.; Yan, F. J.; Johnston, D.; Li, G.; Wu, X.; Leek, D.; Ratcliffe, C. I.; Ripmeester, J. A.; Yu, K. J. Phys. Chem. C 2008, 112, 13805–13811. (9) Kuc¸ur, E.; Ziegler, J.; Nann, T. Small 2008, 4, 883–887. (10) Chen, H. S.; Kumar, R. V. J. Phys. Chem. C 2009, 113, 31–36. (11) Evans, C. M.; Guo, L.; Peterson, J. J.; Maccagnano-Zacher, S.; Krauss, T. D. Nano Lett. 2008, 8, 2896–2899. (12) Brennan, J. G.; Siegrist, T.; Carroll, P. J.; Stuczynski, S. M.; Brus, L. E.; Reynders, P.; Steigerwald, M. L. Chem. Mater. 1990, 2, 403–409. (13) Dagtepe, P.; Chikan, V.; Jasinski, J.; Leppert, V. J. J. Phys. Chem. C 2007, 111, 14977–14983. (14) Pradhan, N.; Xu, H. F.; Peng, X. G. Nano Lett. 2006, 6, 720–724. (15) Yu, W. W.; Peng, X. Angew. Chem., Int. Ed. 2002, 41, 2368–2371. (16) Pan, D. C.; Jiang, S. C.; An, L. J.; Jiang, B. Z. AdV. Mater. 2004, 16, 982–985.
J. Phys. Chem. C, Vol. 113, No. 29, 2009 12771 (17) Pan, D.; Ji, X.; An, L.; Lu, Y. Chem. Mater. 2008, 20, 3560–3566. (18) Yu, Q. Y.; Liu, C. Y.; Zhang, Z. Y.; Liu, Y. J. Phys. Chem. C 2008, 112, 2266–2270. (19) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706–8715. (20) Peng, X. G.; Wickham, J.; Alivisatos, A. P. J. Am. Chem. Soc. 1998, 120, 5343–5344. (21) Peng, X. G. Chem.sEur. J. 2002, 8, 335–339. (22) Qu, L.; Yu, W. W.; Peng, X. G. Nano Lett. 2004, 4, 465–469. (23) Weller, H. Angew. Chem., Int. Ed. Engl. 1993, 32, 41–53. (24) Yu, W. W.; Qu, L. H.; Guo, W. Z.; Peng, X. G. Chem. Mater. 2003, 15, 2854–2860. (25) Landes, C.; Braun, M.; Burda, C.; El-Sayed, M. A. Nano Lett. 2001, 1, 667–670. (26) Chemseddine, A.; Weller, H. Ber. Bunsen-Ges. Phys. Chem. 1993, 97, 636–637. (27) Aldana, J.; Wang, Y.; Peng, X. J. Am. Chem. Soc. 2001, 123, 8844– 8850. (28) Tang, Z.; Kotov, N. A.; Giersig, M. Science 2002, 297, 237–240.
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