Nucleation of Aqueous Semiconductor Nanocrystals: A Neglected

Dec 9, 2010 - Size dependence of upconversion photoluminescence in MPA capped CdTe quantum dots: Existence of upconversion bright point...
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J. Phys. Chem. C 2010, 114, 22487–22492

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Nucleation of Aqueous Semiconductor Nanocrystals: A Neglected Factor for Determining the Photoluminescence Ding Zhou, Jishu Han, Yi Liu, Min Liu, Xue Zhang, Hao Zhang,* and Bai Yang State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin UniVersity, Changchun 130012, P. R. China ReceiVed: September 13, 2010; ReVised Manuscript ReceiVed: NoVember 23, 2010

The nucleation of aqueous semiconductor nanocrystals (NCs) was identified by investigating the evolution of precursor solution and the subsequent N2H4-promoted growth at room temperature. Current synthesis of NCs at room temperature allowed for distinguishing the nucleation and growth processes, which were inseparable in the previous aqueous synthetic methods. Experimental results indicated that the highly crystalline nucleus contributed greatly to the fluorescence of the as-prepared NCs, which was particularly important for synthesizing highly luminescent NCs with small size and alloyed structure. Thus, our finding helped to repeatably synthesize aqueous NCs, and at the same time offered an opportunity to perfect the synthetic route and further structural design of aqueous NCs. Introduction Because of the quantum confinement effect, semiconductor nanocrystals (NCs) (i.e., colloidal quantum dots) show unique size-dependent luminescent properties and are becoming the alternatives for common luminescent materials.1-6 For example, in comparison to conventional organic dyes, NCs possess many advantages, including high photoluminescence quantum yields (PLQYs), photobleaching stability, and a continuous absorption band.7-10 Particularly, the capability for preparing NCs with desired size, shape, and surface chemistry makes them excellent building blocks for the applications in photovoltaic and optoelectronic devices,11-13 intelligent materials,14 and biomedicine labeling.4,10,15 At present, colloidal methods for synthesizing NCs is one of the most successful approaches for obtaining NCs with high PLQYs, narrow size distribution, and tunable sizes and shapes.7-9,16-34 In terms of the polarity of the reaction media, NCs can be prepared either in organic media or in aqueous media.7,8,29-34 The former has been practiced for two decades, and many high-quality NCs have been synthesized by the pyrolysis of organometallic precursors in high boiling point organic solvents.7,9,31 On the basis of this synthesis, the mechanism of NC growth has also been well-revealed. The formation of NCs must go through nucleation and growth stages.35-38 Nucleation occurs rapidly after the injection of precursors at higher temperature, whereas the subsequent growth undergoes at lower temperature. They are two dominant factors in determining the quality and morphology of as-prepared NCs. For controllable synthesis of NCs, most efforts are focused on the growth stage, because fine-tuning the process of NC growth provides the opportunity to control the size, size distribution, shape, and PLQYs of NCs.5,7,38,39 The nucleation stage has been thought to determine the size distribution of NCs. Through control of the nucleation process, magic-sized NCs can be obtained, which are expected to produce single-size band gap emission.28,40 Most recently, Peng and co-workers investigated the nucleation mechanism of NCs to further reveal the charac* To whom correspondence should be addressed. Fax: +86 431 85193423. E-mail: [email protected].

teristics of nucleation. They found that the nucleation process matched a reaction-controlled model rather than classic crystallization one, thus giving a new insight into the nucleation stage of NCs.37 As an alternative route to afford high-quality NCs, the synthesis of NCs in aqueous media is close to the “green chemistry” concept, because environment- and user-friendly solvents of water are adopted in aqueous synthesis.8,18,41-43 Accordingly, numerous efforts are devoted to study the growth process of aqueous NCs and perfect the preparation techniques.3,43-45 With the consideration of the surface property of NCs, the previous efforts are mainly concerned to perfect the surface structure of NCs, namely eliminate the surface defects.46 The relationship between NC surface quality and PLQYs has been intensely investigated by Talapin et al., contributing to a better understanding of experimental variables in controlling PLQYs and size distribution.44 Besides, many techniques are applied to promote the growth of aqueous NCs, such as hydrothermal synthesis, ultrasonic irradiation, microwaveassistant synthesis, and so forth.47-57 However, the quality of the as-prepared NCs is usually worse than those synthesized through the organometallic method (or the derivative methods), because of the complex ionic environment and the low boiling point of water.58,59 Moreover, versatile strategies are applied to optimize the quality of as-prepared NCs through postpreparative treatments, including size-selective precipitation, selective photochemical etching, and surface modification.18,43,46 In all, the aforementioned preparation and postpreparative treatment methods mainly considered the surface property of NCs, whereas little attention has been paid to the impact of the nucleus quality of NCs. In this context, it is believed that the nucleation of aqueous NCs proceeds during reflux process, which is inseparable from the growth process.43 Namely, in the aqueous synthesis route the separate nucleation and its time frame have not been experimentally identified yet.43,58 It has been observed in our previous investigations that the storage of precursors of aqueous NCs at room temperature or reflux at 100 °C for a short duration led to the apparent color variation from dark red to pale yellow,60 implying that a structural change of precursors might occur. In addition, the

10.1021/jp108708n  2010 American Chemical Society Published on Web 12/09/2010

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1s-1s absorption band of NCs became sharper and more symmetric,58 which seemed to indicate the size distribution and the quality of NCs were improved. These phenomena were similar to the initial nucleation of NCs prepared in organic media. To better reveal the underground mechanism of aqueous NC formation, in this work the nucleation and its time frame of aqueous NCs were identified by investigating the evolution of precursors at room temperature and the subsequent N2H4promoted growth. The results indicated that the storage of precursors at room temperature led to a separate nucleation, and the highly crystalline nucleus significantly improved the fluorescence of the as-prepared NCs. Experimental Section Materials. Tellurium powder (-200 mesh, 99.8%), selenium powder (-100 mesh, 99.5+ %), and 3-mercaptopropionic acid (MPA, 99+ %) were purchased from Aldrich. NaBH4 (96%), CdCl2 (99%), NaOH (99%), and N2H4 · H2O (85%) were commercially available products and used as received. Preparation of Aqueous CdTe Precursors. Aqueous precursors of CdTe NCs were prepared by injecting freshly prepared NaHTe solution to N2-saturated CdCl2 solution at pH 9.5 in the presence of MPA.47 The concentration of the precursors was 20 mmol/L with reference to the concentration of Cd2+, whereas the molar ratio of Cd2+/MPA/HTe- was fixed at 1:2.0:0.2. N2H4-Promoted Growth of Aqueous CdTe NCs at Room Temperature. Samples of the precursors of 20 mmol/L stored for some intervals (0, 1, 3, 5, 8, 9, and 22 h) at room temperature were directly diluted by a factor of 20 in deionic water for UV-vis absorption and PL measurements. On the other hand, the proper amount of deionic water was mixed with the precursors and was diluted to the concentration of 1 mmol/L using N2H4 · H2O, whereas the molar ratio of Cd2+/N2H4 was 1:5000. The resulting mixture was still stored at room temperature to maintain the growth of CdTe NCs. As precursors were stored for the specific time, the precipitate of NC solution could be gained by using 2-propanol and centrifugation. The collected precipitates of NCs were ready for XRD measurement. Differently Sized Nucleus. Differently sized nucleus was prepared by varying precursor concentrations (1 and 20 mmol/ L) and stored at room temperature for a specific duration. Synthesis of the Precursors of Ternary CdSexTe1-x NCs. Aqueous precursors of CdSexTe1-x NCs were prepared by injecting freshly prepared NaHSexTe1-x solution, which was prepared by the reaction of NaBH4 with a mixture of Se and Te powders, to N2-saturated CdCl2 solution at pH 9.5 in the presence of MPA. The concentration of the precursors was 20 mmol/L with reference to the concentration of Cd2+, whereas the molar ratio of Cd2+/MPA/HSe-/HTe- was fixed at 1:2.0: 0.05:0.15. Effect of Nucleation on the Composition and PL Intensity of CdSexTe1-x NCs. Method 1: As soon as the precursors were achieved, N2H4 was added into the solution to promote the growth of NCs, and the molar ratio of Cd2+/N2H4 was 1:5000. Method 2: N2H4 was not added into the solution until the nucleation process was accomplished. And the concentration of NCs and the ratio between Cd2+ and N2H4 were parallel to those in Method 1. Characterization. UV-visible absorption spectra were obtained using a Lambda 800 UV-vis spectrophotometer. 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

Figure 1. UV-vis absorption spectra (a), PL spectra (b), and XRD patterns (c) of CdTe precursors with different storage duration at room temperature. The concentration of precursors was 20 mmol/L, referring to Cd, and the molar ratio of Cd/MPA/Te was 1:2.0:0.2.

under ambient conditions. The PLQYs of NCs were estimated at room temperature using quinine in aqueous 0.5 mol/L H2SO4 as PL reference.44 Transmission electron microscopy (TEM) was conducted using a Hitachi H-800 electron microscope at an acceleration voltage of 200 kV with a CCD camera. Highresolution TEM (HRTEM) imaging was implemented by a JEM2100F electron microscope at 300 kV. X-ray powder diffraction (XRD) investigation was carried out by using Siemens D5005 diffractometer. X-ray photoelectron spectroscopy (XPS) was investigated by using a VG ESCALAB MKII spectrometer with a Mg KR excitation (1253.6 eV). Binding energy calibration was based on C 1s at 284.6 eV. Results and Discussion Nucleation of Aqueous CdTe NCs. The aqueous precursors of MPA-stabilized CdTe were prepared according to the experimental section. Afterward, they were stored in the dark at room temperature under N2 atmosphere to detect the evolution of precursors. As the storage duration was prolonged from 0 to 9 h, the apparent color of precursors gradually changed from dark red to pale yellow, implying that a structural change of precursors might occur.60 Meanwhile, the 1s-1s excitonic peaks around 430 nm became sharp without a red shift, whereas the absorbance shoulder at 500 nm became less obvious (Figure 1a). The variation of absorption peak soundly revealed the size change of nanometer-sized particles.58 The disappearance of the absorbance at 500 nm indicated the decomposition of bigger particles or particle aggregates, and the sharpening of the 430 nm absorption meant smaller particles became the main of particle collectives. According to our previous studies,8,60 this phenomenon exhibited the reorganization of precursors, which did not alter the basic size of particles but transformed aggregated particles to isolated crystals. This consideration was confirmed by the PL spectra, because the peak positions of bandgap emission fixed at 490 nm without any shift, though the QYs were dramatically low (Figure 1b). As shown in Figure 1c, no separated XRD (220) and (311) peak was observed for

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Figure 3. Temperature effect on the nucleation of CdTe NCs. (a) The UV-vis absorption spectra of final nucleus as storage at different temperature. (b) Duration for complete nucleation versus temperature, which was confirmed by monitoring the evolution of PL spectra.

Figure 2. TEM images of the precursors without (a) and with 9 h (b) storage at room temperature. The concentration of precursors was 20 mmol/L, referring to Cd, and the molar ratio of Cd/MPA/Te was 1:2.0: 0.2.

the freshly prepared precursors, indicating the precursors were amorphous, whereas the (220) and (311) peaks appeared and became sharper during storage, which proved the alteration of CdTe from amorphous precursors to approximate zinc blende structure.8 Under TEM observation, the freshly prepared precursors were mainly in the form of aggregates. Only a few isolated particles with the average diameter of 2.2 nm were observable (Figure 2a). After 9 h storage, the aggregates disappeared, and the average diameter of isolated particles was 2.2 nm (Figure 2b). This result was consistent with the analysis of absorption and PL spectra (Figure 1a,b). Note that the formation of aggregates resulted from the weak interparticle electrostatic repulsion due to the small size of initial precursors. During storage, the electrostatic repulsion became stronger, making the aggregated particles disperse.60 The aforementioned results supported that the storage of CdTe precursors at room temperature involved a separate nucleation of NCs, though the form was different from the organic one because of the complex ionic environment of aqueous media. Furthermore, prolonged storage beyond 9 h led to the red shift of UV-vis spectra and an enhancement of PL, showing that the time frame of roomtemperature nucleation was 9 h. The nucleation process was also studied by storing the freshly prepared precursors respectively at 40, 60, and 80 °C. However, the obvious red shift of the 1s-1s excitonic peaks was discovered (Figure 3a). It indicated that simultaneously with the nucleation, the growth of NCs was unavoidable under heating. This result was consistent with the previous observation for NC growth with a reflux at 100 °C.58 Moreover, the duration for a complete nucleation shortened with the increase of heating temperature (Figure 3b), because high temperature promoted the diffusion of various atoms and molecules, thus accelerating the nucleation. The effect of temperature on nucleation revealed

Figure 4. UV-vis absorption spectra (a), PL spectra (b), and PLQYs (c) of CdTe NCs with 2.3 nm in diameter that were prepared by a room-temperature N2H4-promoted growth, before which the precursors were stored for 0, 1, 5, 8, 9, and 22 h, respectively. (d) The durations for NCs to reach 2.3 nm versus the storage time of precursors.

that a separate nucleation was achievable only for the storage of precursors at room temperature. Effect of Nucleation on the PL of as-prepared CdTe NCs. To reveal the relationship between nucleus quality and the PLQYs of the as-prepared NCs, N2H4 was added to the precursors with different nucleation durations (Figure 4), because N2H4 was capable to promote NC growth at room temperature.54-56 The capability of NC growth at room temperature allowed for distinguishing the nucleation and growth processes. By comparing the as-prepared NCs with a same diameter (2.3 nm), represented by the fixed absorption peak at 498 nm and the PL peak at 530 nm, it revealed that the PLQYs of NCs increased with the increase of nucleation durations (Figure 4a,b). The PLQY of NCs with 9 h precursor storage was 4 times stronger than that without storage (Figure 4c). Until 9 h, the PLQYs became a constant. This result again indicated that the nucleation completed after 9 h storage, and meanwhile, highly crystalline nucleus greatly improved the PL of the as-prepared NCs. Note that although the PLQY of the precursors with 9 h storage was dramatically low, it was already 7 times stronger than that without storage (Figure 1b). This relation was kept

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Figure 6. (a) UV-vis absorption spectra of CdTe nucleus (dot and dash dot) and the corresponding NCs (solid and dash) that were prepared by altering precursor concentrations, 1 (dot and solid) and 20 mmol/L (dash dot and dash). (b) PL spectra of NCs with a same PLQY of 7.3%, but from different nucleus, 1 (solid) and 20 mmol/L (dash). Inset: the corresponding PL images of NCs. NCs were prepared by refluxing the precursors at 100 °C, before which the concentrations of precursors were diluted to 1 mmol/L using deionic water.

Figure 5. UV-vis absorption spectra (a), PL spectra (b), and PLQYs (c) of CdTe NCs with 2.8 nm in diameter that were prepared by a room-temperature N2H4-promoted growth, before which the precursors were stored for 0, 1, 5, 8, 9, and 22 h, respectively. (d) The durations for NCs to reach 2.8 nm versus the storage time of precursors.

after N2H4-promoted growth (Figure 4c). Besides, the duration for N2H4 promotion to obtain 2.3 nm NCs also increased from 16 to 97 min, corresponding to the prolonged nucleation from 0 to 9 h (Figure 4d). In this regard, the growth of bigger NCs was supplied by the stepwise decomposition of smaller NCs, namely, the Ostwald ripening process.35,44,61 In comparison to amorphous precursors, the perfect and ordered atomic arrays of the crystalline nucleus inhibited their decomposition and the release of monomers, thus suppressing the diffusion-limited growth of NCs.35,44 Therefore, a slower growth was observed. Once the nucleation completed, the duration for obtaining 2.3 nm NCs became less different (Figure 4d). Under TEM, the appearance of 2.3 nm NCs from the precursors without and with 9 h nucleation was different, though no obvious difference was found in the HRTEM images (Figure S1, Supporting Information). Both the contrast and dispersibility of the NCs without nucleation were worse than those with nucleation, resulting from the less compact crystalline structure. It confirmed that the nucleation process could significantly improve the quality of the as-prepared NCs. A similar relationship was observed for the as-prepared NCs with 550 nm emission (2.8 nm in diameter, Figure 5). NCs exhibited a maximum PLQY as the precursors were stored for 9 h, though the increase was less obvious than that of 2.3 nm NCs (Figures 4 and 5). This result firmly proved that the highly crystalline nucleus improved the PLQY of the as-prepared NCs. Moreover, the durations for NC growth were all around 7 h for the NCs with different nucleation durations, which was different from the 2.3 nm NCs (Figure 4). The detailed dynamic curves further presented that although the growth of the NCs without nucleation was faster than those with 9 h nucleation at the first 7 h, the growth tendency became opposite at the subsequent growth beyond 7 h (Figure S2, Supporting Information). It revealed that highly crystalline nucleus benefited the growth of bigger NCs by providing more compact centers for adsorption and coalescence of monomers, which was consistent with the

consideration of diffusion-limited growth of NCs.44 With respect to the PLQYs, the original dissimilarity in the crystal structure of nucleus decreased after the reorganization of NCs via atom and molecule diffusion, leading to a nearly identical arrangement for the surface atoms of 2.8 nm NCs. As a result, the PLQYs of the as-prepared NCs with different nucleation duration became less different. Nevertheless, a higher crystalline nucleus corresponded to the as-prepared NCs with stronger PL. On the other aspect, it revealed that the difference in PLQYs resulted from nucleus quality rather than surface optimization, because the duration for the growth of 2.8 nm NCs was parallel (Figure 5d). Parallel growth duration led to similar surface quality of NCs. Consequently, the difference in NC PLQYs revealed the influence of nucleus quality. Besides, the influence of the nature of nucleus on the spectral properties of as-prepared NCs was further proved by comparing NCs with a same PLQY but from different nucleus (Figure 6 and Figure S3, Supporting Information). In this context, the differently sized nucleus was foremost prepared by varying precursor concentrations, namely, 1 and 20 mmol/L, and followed through conventional reflux at 100 °C to promote NC growth. It was found that the UV-vis absorption peak positions were 391 and 425 nm corresponded to the original nucleus with the concentration of 1 and 20 mmol/L, respectively (Figure 6a). After dilution of the solutions to a same concentration (1 mmol/ L), the precursors were refluxed at 100 °C to generate NCs. Meaningfully, the size difference in the original nucleus was kept for the as-prepared NCs, represented by the different UV-vis and PL peak positions of these NCs, though both of them possessed the PLQY of 7.3%. Moreover, Figure 7 summarized the total evolution of PLQYs from differently sized nucleus. The difference in PLQYs resulting from nucleus size was kept almost during the whole spectral range, particularly at the original stage. Such dissimilarity gradually reduced after 548 nm. If the growth duration was long enough, after 622 nm, no difference could be observed. This tendency was understandable according to the time dependent thermodynamic equilibrium of NCs and the ligands, which has been discussed in our previous work.60 In all, the aforementioned results indicated that the PLQYs of aqueous CdTe NCs greatly depended on the crystallization of nucleus, which could be tuned by altering the experimental variables, and especially the nucleation duration. This understanding helped to improve the repeatability of NCs synthesized in water.

Nucleation of Aqueous Semiconductor Nanocrystals

Figure 7. PL intensity of CdTe NCs versus emission wavelength. NCs grew from differently sized nuclei that were prepared by altering precursor concentrations, 1 (square) and 20 mmol/L (circle). The growth of NCs was promoted by reflux at 100 °C, before which the concentrations of precursors were diluted to 1 mmol/L using deionic water.

Figure 8. (a) PL spectra of CdSexTe1-x alloyed NCs without (solid) and with (dash) nucleation. Inset: the corresponding PL images of NCs without (b) and with (c) nucleation. (d) PL intensity of CdSexTe1-x NCs versus emission wavelength, with (square) and without (circle) nucleation. The growth of NCs was promoted by N2H4 at room temperature.

Effect of Nucleation on the Composition and PL of CdSexTe1-x Alloyed NCs. Furthermore, a separate nucleation process significantly governed the composition and PL of asprepared ternary NCs, such as CdSexTe1-x alloyed NCs. Similar to binary CdTe NCs, the growth of ternary CdSexTe1-x NCs was also promoted by N2H4 at room temperature. As shown in Figure 8, the PL intensity of CdSexTe1-x NCs with and without a separate nucleation was compared. Meaningfully, the PL intensity of alloyed NCs with a separate nucleation process was 20 times stronger than those without nucleation. If a separate nucleation was adopted, the PLQY of CdSexTe1-x alloyed NCs was up to 3%, whereas the PLQY was only 0.15% for the samples without nucleation. UV-vis absorption spectra indicated that without nucleation, two absorbance peaks respectively at 450 and 530 nm were observed (Figure S4, Supporting Information). It revealed that besides CdSexTe1-x NCs, CdSe NCs formed solely, characterized by the absorbance at 450 nm. In contrast, if a separate nucleation was adopted, only one absorbance peak around 530 nm was observed, suggesting the as-prepared NCs were pure CdSexTe1-x (Figure S4, Supporting Information). The formation of pure CdSexTe1-x alloyed NCs rather than the mixture of CdSe and CdSexTe1-x should be the reason for higher PL intensity of NC collective with a separate nucleation. The removal of CdSe impurity in NC collective avoided the PL quenching resulting from the fo¨rster resonance

J. Phys. Chem. C, Vol. 114, No. 51, 2010 22491 energy transfer between CdSe and CdSexTe1-x.62 Moreover, XPS investigation revealed that the nucleation process greatly influenced the composition of as-prepared alloyed NCs (Figure S5, Supporting Information). Experimentally, the feed ratio of Se/Te was fixed at 1:3. Without a separate nucleation, the Se/ Te ratio in the as-prepared NCs was 1:2.5. However, the ratio was 1:1 with a separate nucleation. This result revealed that the nucleation process increased the Se amount in alloyed NCs. As reported by Rogach et al., in aqueous media Se possessed higher reactivity than Te in the reaction with Cd-thiolate complexes, increasing the amount of Se in the as-prepared alloyed NCs.63 Thus in the current study, a prolonged nucleation duration favored the reaction between Se and Cd-thiolate complexes, since the nucleation was a reaction-controlled process rather than crystallization.37 The aforementioned results indicated that the nucleation process was dominant for synthesizing pure CdSexTe1-x NCs with higher PLQY. Conclusion In summary, we identified the nucleation process of aqueous semiconductor NCs by studying the structural and spectral evolution of the precursors and the N2H4-promoted NC growth at room temperature. The capability of NC growth at room temperature allowed for distinguishing the nucleation and growth processes, which was difficult to carry out by conventional reflux at 100 °C. By combining UV-vis absorption and PL spectra, TEM observation, and XRD measurement, it could be safely concluded that aqueous NCs possessed a separated nucleation process at room temperature. The time frame of roomtemperature nucleation was 9 h. Moreover, the highly crystalline nucleus greatly improved the PLQY of the as-prepared NCs, which was particularly important for synthesizing highly luminescent NCs with small size and alloyed structure. Our finding improved the repeatability for synthesizing aqueous NCs, and offered further opportunity to perfect the synthetic route and structural design of aqueous semiconductor NCs. Acknowledgment. This work was supported by NSFC (20974038, 20921003, 50973039), the 973 Program of China (2007CB936402, 2009CB939701), the FANEDD of China (200734), the Special Project from MOST of China, and the Program for New Century Excellent Talents in University. Supporting Information Available: Figures showing the influence of nucleation on the PL and TEM appearance of CdTe NCs, the dynamic curves of NC growth with and without nucleation, UV-vis absorption spectra of the CdTe nucleus and NCs, and the comparison of CdSexTe1-x NCs with and without a separate nucleation. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Alivisatos, A. P. Science 1996, 271, 933–937. (2) Klimov, V. I.; Mikhailovsky, A. A.; Xu, S.; Malko, A.; Hollingsworth, J. A.; Leatherdale, C. A.; Eisler, H.-J.; Bawendi, M. G. Science 2000, 290, 314–317. (3) Tang, Z. Y.; Kotov, N. A.; Giersig, M. Science 2002, 297, 237– 240. (4) Han, M. Y.; Gao, X. H.; Su, J. Z.; Nie, S. M. Nat. Biotechnol. 2001, 19, 631–635. (5) Peng, X. G.; Manna, L.; Yang, W. D.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59–61. (6) Rogach, A. L.; Gaponik, N.; Lupton, J. M.; Bertoni, C.; Gallardo, D. E.; Dunn, S.; Pira, N. L.; Paderi, M.; Repetto, P.; Romanov, S. G.; O’Dwyer, C.; Torres, C. M. S.; Eychmu¨ller, A. Angew. Chem., Int. Ed. 2008, 47, 6538–6549.

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