Ligand Dynamics of Aqueous CdTe Nanocrystals at Room

Apr 2, 2008 - Ding Zhou , Haoyang Zou , Min Liu , Kai Zhang , Yu Sheng , Jianli Cui , Hao ... Ding Zhou , Min Lin , Xun Liu , Jing Li , Zhaolai Chen ,...
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J. Phys. Chem. C 2008, 112, 6330-6336

Ligand Dynamics of Aqueous CdTe Nanocrystals at Room Temperature ChunLei Wang, Hao Zhang, JunHu Zhang, Na Lv, MinJie Li, HaiZhu Sun, and Bai Yang* State Key Lab of Supramolecular Structure & Materials, College of Chemistry, Jilin UniVersity, Changchun 130012, People’s Republic of China ReceiVed: December 30, 2007; In Final Form: February 24, 2008

On the basis of dilution and pH experiments, in this paper we demonstrated the possible ligand dynamics of aqueous CdTe nanocrystals (NCs) at room temperature. During storage, the excess ligands and cadmiumligand compounds in solution were preferentially adsorbed on NC surface before they diffused into the ligand shell. Though the adsorption layer could not directly determine the photoluminescence (PL) property of NCs, it affected the diffusion process of free ligands and cadmium-ligand compounds into the ligand layer. The treatments of dilution, concentration, storage, and pH adjustment were believed to affect the thickness of adsorption layer and the diffusion behavior of ligands and hence brought different PL quantum yields.

Introduction Semiconductor NCs have attracted great fundamental and technical interests in recent years. Compared with organic dyes, semiconductor NCs possess broader emission tunability, superior photostability, and longer PL lifetime1-6 and thus be extensively exploited as potential candidates for light-emitting diodes, solar cells, biomedical tags, etc.7-11 For practical application, preparation of NCs with high PL quantum yields (QYs) and surface functionality is the prerequisite.12-17 On the basis of the investigation of growth mechanism, NCs with high PL QYs can be obtained now.18-20 The prevalent viewpoint considers NCs with zero growth rate tend to restructure surface matters from higher energy sites (defects, kinks, etc.) to lower ones, making their QYs higher than other NC samples either with negative or with positive growth rate.19,21 Ligands are also important factors, which not only affect NCs optical property but also influence their nucleation and growth process.18-20,22 In the past two decades, the ligand effect has been gradually recognized. By tuning the species and ratios of ligands, NCs with different shapes, sizes, and optical properties can be designed.22-30 Most recently, the ligand dynamics in growth process of NCs (150 °C) has also been investigated based on nonaqueous CdSe NCs.31 At room temperature (the common preserving condition of NCs), however, the ligand and growth dynamics of NCs are usually neglected since previous works mainly focus on NC surface treatments, such as photoassisted or chemical assisted etching, oxygen-induced oxide shells, pH, and dispersion media dependent ligand modification, etc.20,32-37 In parallel with organic synthesis routes, aqueous methods are also used to prepare II-VI semiconductor NCs, especially CdTe NCs.38-43 Thiol molecules with diverse functional groups, such as thioglycolic acid, 1-thioglycerol, and 2-mercaptoethylamine, are usually used as ligands, which facilitate the conjugation of aqueous NCs with other ions, molecules, or polymers for various applications.44-46 In preparation of aqueous NCs, some interesting experimental phenomena have been observed by several research groups. For instance, it has been found that CdTe NCs synthesized at low precursor concentration * To whom correspondence should be addressed: e-mail byangchem@ jlu.edu.cn; Tel +86-431-85168478; Fax +86-431-85193423.

possess higher QYs.2,41-43 Besides, the PL intensity and stability of freshly prepared NCs greatly increase during storage at room temperture.20,39,47 Using NCs as building blocks to fabricate hybrid materials moreover, such as compositing or assembling with other NCs, biomaterials, or polymers, etc., the use of fresh or stored NCs directly determines the PL QYs preserved in hybrids.38-47 Stored NCs usually preserve high PL QYs. Since the storage process alters NC optical property so much, it inspires us to explore what happens during storage. In this work, we find the storage-induced PL enhancement depends on NCs concentration rather than the materials of core NCs. On the basis of dilution and pH experiments, we reveal the ligand dynamics according to a multilayer model of NCs. Namely, from inside to outside, NC composes of core, ligand shell, and adsorption layer.48 The treatments of dilution, storage, and pH adjustment are believed to affect the adsorption behavior of ligands and hence the PL QYs of NCs. Our model reveals the intrinsic reason for lots of peculiarities of aqueous NCs that have been observed but puzzled people for one decade. Experimental Section Materials. All materials used in this work were AR reagents. CdCl2, NaBH4, HCl, and NaOH were purchased from the Beijing Chemical Factory, China. Thioglycolic acid (TGA) and Te powder were purchased from Aldrich. NaHTe solution was prepared by using Te and NaBH4 in accordance with the reference method.49 Synthesis of CdTe NCs. CdTe NCs were prepared according to a previous method.49 Typically, freshly prepared NaHTe solution was injected into the solutions of CdCl2 and TGA after degassed with N2 for 30 min at pH 9.5. The concentration of CdCl2 was 1.25 × 10-3 mol/L, and the molar ratio of Cd2+/ TGA/NaHTe was 1:2.4:0.2. To obtain CdTe NCs, the crude solution was refluxed at 100 °C and maintained for specific hours. CdTe NCs with green, yellow, or orange emission were obtained by refluxing for different time. In concentration experiments, the concentrated CdTe NCs were prepared by a similar process, except 1.25 × 10-2 mol/L CdCl2 and pH 11.5 were used. Dilution Experiments. Freshly prepared concentrated CdTe NCs (1.25 × 10-2 mol/L referring to the concentration of CdCl2)

10.1021/jp7121664 CCC: $40.75 © 2008 American Chemical Society Published on Web 04/02/2008

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were separated into three portions. Two portions were diluted to 1.25 × 10-3 and 2.5 × 10-4 mol/L (referring to CdCl2) immediately after the solution was cooled to room temperature (25 °C in our experiments). Stirring was employed in dilution process. Then NCs were stored for specific days before measurement (brief as dilution and then storage samples or DS samples). The third portion was first stored for specific days and then measured immediately after diluted to the same concentration with above portions (brief as storage and then dilution sample or SD sample). It should be emphasized that all samples were sealed in bottles covering with silver paper in order to avoid the photoillumination in the whole process of storage. Before measurement, the solution was stirred to make it homogeneous. pH Experiments. Freshly prepared green, yellow, and orange emitted CdTe NCs (1.25 × 10-2 mol/L referring to the concentration of CdCl2) were used in the pH experiment. Each NCs solution was divided into three aliquots. One aliquot kept original pH value without any treatment, and the other two samples were adjusted to pH 7.2 or 11.2 by using 0.1 mol/L HCl or NaOH. After specific days storage at room temperature (25 °C under dark), the samples were measured. Similarly, photoillumination was also avoided by sealing samples in silver paper covered bottles. PL QYs Measurement. The PL QYs were evaluated according to a previous method.21 Quinine in 0.5 mol/L H2SO4 aqueous solution was used as the PL reference. For comparison, SD samples were diluted to the same concentration with DS samples before measurement. Then the QYs of CdTe NCs were calculated by following equation:

φ s ) φs

( )( )( ) As Sx Ns Ss Ax Ns

2

where φ, A, S, and N respectively represent the PL QYs, optical density at excitation wavelength, integrated PL intensity, and the refractive index of solution. The subscripts s and x denoted the standard (quinine) and measured samples (CdTe). The PL QYs of quinine was 54.6% when the excited wavelength was set as 365 nm. The split values in PL measurements were kept the same in the course of testing CdTe and quinine. Characterization. UV-vis absorption spectra were recorded with a Shimadzu 3100 UV-vis near-infrared spectrophotometer. Fluorescence experiments were performed with a Shimadzu RF5301 PC spectrofluorimeter. X-ray photoelectron spectroscopy (XPS) was investigated by using a VG ESCALAB MK II spectrometer with a Mg KR excitation (1253.6 eV). Binding energy calibration was based on C 1s at 284.6 eV. For XPS measurement, silicon substrate with purified CdTe solution was used. According to the previous method,21,43 CdTe was purified by centrifugal separation the mixture of CdTe and 2-propanol. After dryness, the powder of CdTe was redispersed in water, and then the purified CdTe solution was dropped on silicon substrate. Before XPS measurement, the substrate was preserved in vacuum. For comparison, SD samples were diluted to the same concentration with DS samples before purification process. Results and Discussion For aqueous synthesized NCs, it was always observed that after storing the freshly prepared NCs at room temperature they displayed much higher PL.39,47 Moreover, it was general for both TGA-capped CdTe NCs and TGA-capped ZnSe NCs (Figure 1 and Figure S1).

Figure 1. UV-vis absorption and PL spectra (λex ) 400 nm) of SD (black solid line) and DS (red dash line) samples after 3 days storage. Inset: images of DS (left) and SD (right) samples under irradiation of UV lamp (365 nm).

Concentration-Dependent PL Enhancement during Storage at Room Temperature. Freshly prepared CdTe NCs exhibited different optical behavior when they were stored at room temperature with different concentration. As shown in Figure 1, DS sample was diluted solution of freshly prepared CdTe NCs after 3 days storage (25 °C under dark). The concentration of cadmium was 1.25 × 10-3 mol/L after dilution. Bright green PL of DS sample could be observed from the inset photo of Figure 1. Its PL QY was 13.9%. In comparison, the SD sample was concentrated CdTe solution (1.25 × 10-2 mol/ L) which was first stored for 3 days and then immediately measured after being diluted to 1.25 × 10-3 mol/L. Obviously, the SD sample displayed much weaker PL than that of the DS sample. Though the PL QY of the SD sample was only 6.3%, it was still larger than that of freshly prepared solution (5.5%). UV-vis spectra displayed only neglectable difference between SD and DS samples, indicating changeless particle size.50 X-ray powder diffractions of SD and DS samples were similar, implying no difference in crystalline structure after dilution. Therefore, the PL enhancement during storage was more likely relevant to the dynamics of surface ligands rather than the alteration of NCs size or intrinsic crystalline. The synthesis of aqueous CdTe NCs began with a large excess of cadmium and ligands, existing mainly in form of cadmiumligand compounds in solution.38-43 Taking the concentrated CdTe NCs in current work for example, the concentration of NCs and cadmium-ligand compounds in solution was estimated as 10-5 and 10-2 mol/L. Since both thiol and carboxylic groups of ligand could interact with cadmium,49 the excess cadmiumligand compounds in solution were liable to adsorb on NCs. According to the reference, aqueous nanoparticles were mainly composed of core, ligand shell, and adsorption layer from inside to outside.48 Because of the ligand shell directly bonded the core, it took charge of the optical property of CdTe NCs as reported elsewhere.20,38-43 After NC growth at elevated temperature, though major surface sites of NC were capped by ligands, some vacancies and defects still existed,20,49,51 providing the possibility for further ligand modification. As for adsorption layer, the excess cadmium-ligand compounds in solution could adsorb on ligand shell either by physical adsorption or by chemical adsorption through coordination of carboxylic cadmium.49 Because of the separation of ligand shell, the matters in adsorption layer, referring to adsorbed cadmium, ligand, and their compounds, seemed unavailable to determine the optical

6332 J. Phys. Chem. C, Vol. 112, No. 16, 2008 property of NCs. Note that low temperature (room temperature) should promote the formation of adsorption layer since the ligands are more dynamic at high temperature.31 Dilution Experiments. To systemically investigate the dilution effect, we monitored the spectra of CdTe NCs at a long time scale. The absorption spectra of SD and DS samples with concentration of 1.25 × 10-3 and 2.5 × 10-4 mol/L were respectively displayed in Figures 2 and 3. NCs size and hwhm (the half-width at half-maximum on the long wavelength side of the first exciton absorption peak, representing the size distribution of NCs) were evaluated according to ref 50. PL QYs were calculated by comparing the integrated PL intensity of CdTe with that of quinine as described in the Experimental Section. No matter from Figure 2 or Figure 3, two different stages could be observed. Taking SD samples in Figure 2, for example, the first stage usually lasted for 10 days. The characteristic of this stage was the changeless NCs size, size distribution (hwhm), and slightly increased QYs. In comparison, the second stage was represented by the decreased hwhm and red-shifted spectra. The QYs quickly increased to the maximum and then decreased. Compared with SD samples, the first stage of DS samples was much shorter. It needed more than or less than 3 days at the concentration of 1.25 × 10-3 mol/L (Figure 2 DS samples) or 2.5 × 10-4 mol/L (Figure 3 DS samples), apparently shorter than that of SD samples (10 days). Besides, large NCs (orange emitted NCs) seemed more inert than small ones (green and yellow emitted NCs). This could be seen from the gentle alterations of QYs, size, and spectra shift of large NCs. We first analyzed what happened at each stage. At the first stage, the changeless NCs size and size distribution indicated CdTe NCs did not grow.50 The slight increase of QYs implied no dramatic change of ligand shell since it directly related to the PL of NCs.20,38-43 Thereby, the mostly possible process at this stage was the formation of adsorption layer. In Figure 4, we also investigated the surface component of NCs at different time scales. The binding energies of 572.8, 405.6, and 162.8 eV respectively corresponded to Te 3d, Cd 3d, and S 2p levels.40-43 The peaks at 576.1 and 168.3 eV were attributed to the oxidation of Te and S during measurement.52 According XPS results, the atomic ratio of S/Te was calculated as 2.0 at 0 day (DS and SD samples shared the same data since they were no difference at 0 day). At 5 days, such ratio was 3.5 for SD sample and 2.3 for DS sample. Since S could only come from the ligand, the time-dependent increase of S/Te atomic ratios indicated the adsorption of ligands and/or cadmiumligand compounds on NCs. Taking into account of the changeless optical property of NCs, it was reasonable to deduce that the first stage mainly involved the adsorption of free ligands and/or cadmium-ligand compounds on NCs. High NCs concentration promoted such adsorption as indicated by the relatively large S/Te atomic ratio and long period of first stage of SD sample. At the second stage, the spectra and QYs of NCs dramatically changed, indicating more compact ligand shell formed.20,38-43 As mentioned above, freshly prepared NCs possessed some vacancies and defects on surface.20,32-37,51 When they were patched by free ligands and/or cadmium-ligand compounds, the PL of NCs could greatly improve. Thus, the diffusion of free ligands and/or cadmium-ligands compounds into the ligand shell became the key, which controlled the ligand occupation of vacancies as well as the PL of NCs. XPS results at 20 days indicated an increased S/Te atomic ratio (3.9 for SD sample and 2.6 for DS sample) than that of 5 days, presenting more ligands attached to NCs (Figure 4). In order to further

Wang et al.

Figure 2. UV-vis spectra of SD (top) and DS (bottom) samples with green (a), yellow (b), and orange (c) emission after different storage duration. The concentration of cadmium after dilution was 1.25 × 10-3 mol/L. Inset table: the size, QYs, and hwhm of NCs with different storage duration. NCs size was calculated by UV-vis spectra. QYs were obtained by comparing with quinine, and hwhm was measured according to UV-vis spectra.

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Figure 4. XPS data of SD (a) and DS (b) samples after storage for 0 (top), 5 (middle), and 20 days (bottom). From left to right, the spectra were S 2p, Cd 3d, and Te 3d, respectively. SD and DS samples shared the same data at 0 day (top of (a) and (b)).

Figure 3. UV-vis spectra of SD (top) and DS (bottom) samples with green (a), yellow (b), and orange (c) emission after different storage duration. The concentration of cadmium after dilution was 2.5 × 10-4 mol/L. Inset table: the size, QYs, and hwhm after different storage duration.

comprehend this PL enhancement, we also monitored the spectra of DS samples at highly diluted solution. Supposing the ligand diffusion-controlled the PL enhancement during storage, it

should relate to the concentration of free ligands or cadmiumligand compounds in solution. In other words, free ligands or cadmium-ligand compounds would difficultly diffuse to NCs if their concentration was low enough; hence the PL of NCs would not improve. As shown in Figure S2, after high dilution (2.5 × 10-6 mol/L), the PL of NCs really kept constant during storage, implying the second stage was controlled by ligand diffusion. Note that the growth of NCs should not be the major process in second stage though it showed similar optical variation and possibly occurred at room temperature. Since the diffusion-controlled NC growth strongly depended on the monomer concentration in solution, large NC grew at the cost of dissolution of small particle at low monomer concentration, thus leading to the defocus of NCs size distribution.53 In our experiments, however, even large NCs with high dilution times (Figure 3c) only displayed focus of size distribution, indicating NC growth was not dominant. In fact, the patching of vacancies by free ligands or cadmium-ligand compounds was quite similar to the growth process, but such occupation might not continue after the vacancies were patched. Therefore, we could say at the current experiment condition, the main process in storage was ligand dynamics rather than NC growth. Besides,

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Figure 5. UV-vis spectra of green (a), yellow (b), and orange (c) emitted NCs at pH 7.2 (top), original pH without adjusting (middle), and pH 11.2 (bottom). Inset table: PL QYs of NCs after different storage duration.

we also noticed the recent liquid-crystalline model which was another viewpoint about the growth of NCs.54 According to this

Wang et al. model, the limited diameter that NC could grow at room temperature was about 3.0 nm. That meant the NCs used in current experiments could impossibly grow. Moreover, the dilution treatment was considered to lower the concentration of free ligands and cadmium-ligand compounds in solution, hence promoting the formation of thinner adsorption layer.53 As a result, the free ligands and cadmium-ligand compounds could easily traverse the thin adsorption layer and patch the ligand shell, leading to the fast PL enhancement for DS samples. It was easy to comprehend the dilution effect through this context. First, from XPS results in Figure 4, the S/Te atomic ratio of 3.9 for SD sample and 2.6 for DS sample at 20 days, implying thicker adsorption layer formed at high concentration solution. However, concentrated samples (SD samples) displayed much lower QYs than diluted samples (DS samples), indicating the thick adsorption layer was disadvantage for the subsequent diffusion of free ligands or cadmium-ligand compounds into ligand shell. Second, the concentration-dependent period of first stage (10 days for SD sample and 3 days for DS samples in Figure 2) in spectra also revealed thin adsorption layer formed at low concentration. In Figure S2, at cadmium concentration of 1.25 × 10-5 mol/L, the fast PL increase of NCs indicated the extremely short period of first stage, proving thin adsorption layer benefited the diffusion of free ligands or cadmium-ligand compounds into ligand shell. Third, we calculated the occupied solution per NC in Table S1. The results showed each NC possessed plenty of solution even without dilution. Thereby, the dilution effect should not arise from the formation of Cd(OH)2 shell as reported elsewhere.55 Fourth, since the dilution effect was actually dynamics of ligand, such effect should be independent of the materials of NC core. In Figure S1, we also monitored the PL of TGA capped ZnSe NCs after 1 week storage. Likewise, the diluted sample showed much faster PL enhancement than concentrated one. On the basis of above understandings, some experimental observations could be easily comprehended. During storage, no matter whether SD or DS samples were used, their QYs decreased after arriving to the maximum. It was comprehensible with reminiscence of the same observation of core-shell NCs that thicker shells led to decrease of QYs since the crystallization of shell phase brought more interface defects between core and shell.56 In this context, the ligand shell was quite similar to the shell of core-shell NCs,38-43 so it should also have optimal thickness as symbolized by the maximal QYs. The second observation was the inertness of large NCs as referred in Figure 2. Previous investigations had demonstrated the partial hydrolysis of the thiol ligands after long duration of reflux that led to the incorporation of sulfur into the growing NCs as well as the formation of gradient CdS shells.20,43 Such shells protected NCs from the disturbance of environment and hence more inactive than the small ones. Third, after several 10 days storage, DS samples were instable as implied by the nonzero baseline in spectra.57 This should be attributed to the oxidation of NCs. Though the adsorption layer did not directly determine NC optical property, it could affect the diffusion process of free ligands and/or cadmium-ligand compounds into NCs. Likewise, the adsorption layer could also protect NCs from oxidation by affecting the diffusion of oxygen.57 The thin ligand thickness of DS samples made them easily be oxidized. Detailed investigation about the oxidation of NCs was beyond this work, and we will discuss it in the future. pH Experiments. The ligand dynamics of NCs at room temperature was also certified by pH experiments. Freshly

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J. Phys. Chem. C, Vol. 112, No. 16, 2008 6335 increase of S/Te atomic ratio from 2.8 to 3.0 after 20 days’ storage also implied the formation of thin adsorption layer at high pH, which favored the diffusion of free ligands or cadmium-ligand compounds into the ligand shell; hence the fast increase of PL QYs. In comparison, judging from the spectra, the original samples experienced ligand dynamics like SD samples. The increased S/Te atomic ratio from 3.1 to 3.9 after 20 days storage also supported the spectra conclusion. In addition, large NCs also displayed inertness in Figure 5c as observed in dilution experiments. Herein, it should be emphasized that the mostly striking difference between our experiments and previous reports was the time scale. The results with no storage in pH experiments consisted well with previous observations. However, the long-term spectra variations were quite different, implying ligand dynamics was a slow process, hence leading to diverse optical behavior between fresh and stored NCs. Conclusions

Figure 6. XPS data of green emitted NCs at pH 9.4 (a) and 11.2 (b) after storage for 0 (top), and 20 days (bottom). From left to right, the spectra were S 2p, Cd 3d, and Te 3d, respectively.

prepared NCs were adjusted to pH 7.2 and 11.2, and their spectra were measured at different time scale. According to previous reports, low pH value was proved to benefit the adsorption of free ligands on NCs surface, thus increasing their PL.49,58 In our experiments, we found low pH benefited the diffusion of free ligands or cadmium-ligand compounds directly into the ligand shell. From Figure 5, pH 7.2 samples rapidly reached their maximal QYs, analogously to the second stage in dilution experiments. It was comprehensible due to both NCs and ligands in solution were negative charged. The low electrostatic repulsion of them at near neutral pH facilitated the traverse of free ligands through the adsorption layer,48 thus exhibiting similar optical property with the second stage in dilution experiments. The situation of pH 11.2 samples was slightly complex. Compared with the original samples (pH 9.4, 9.3, and 9.0 respectively for green, yellow, and orange samples), the pH 11.2 samples exhibited decreased PL QYs at beginning, in accord with the previous reports.49,58 After then, their QYs showed rapid increase. Taking the green NCs, for example, the QYs increased from 15.7% to 26.2% for pH 11.2 sample and from 17.8% to 22.9% for pH 9.4 sample after 41 days storage. According to the spectra, we could deduce that pH 11.2 samples experienced homologous process of ligand dynamics as well as DS samples in dilution experiments. From XPS results in Figure 6, the slight

In this work, we demonstrated the ligand dynamics of aqueous CdTe NCs during storage at room temperature. Namely, the excess ligands or cadmium-ligand compounds in solution were preferentially adsorbed on adsorption layer before they diffused into the ligand shell. Though the adsorption layer did not directly determine the optical property of NCs, it could affect the diffusion process of free ligands or cadmium-ligand compounds into the ligand shell. The treatments of dilution, concentration, storage, and pH adjustment affected the thickness of adsorption layer as well as the ligand diffusion into ligand shell, leading to diverse optical behavior of NCs. According to this work, it was easily comprehended the distinction between fresh and stored NCs in compositing or assembling with other NCs, biomaterials, polymers, etc. Our results also revealed the possible reason for the tremendous difference of PL QYs and stability of NCs prepared at different concentration. We expected this work could benefit the deep insight into the alteration of NC structure and property at various chemical, physical, or biological environments in the practical applications whatever from academic or technical interest. Acknowledgment. This work is supported by the National Basic Research Development Program of China (2007CB936402), the Program for Changjiang Scholars and Innovative Research Team in University (No. IRT0422), the Foundation for the Author of National Excellent Doctoral Dissertation of P. R. China (FANEDD Grant No. 200734), and the National Natural Science Foundation of China (Grant No. 20534040, 20704014, and 20731160002). Supporting Information Available: PL spectra of DS and SD samples of ZnSe after 1 week storage, PL variation of DS samples vs storage duration at low concentration, and table of calculated ratios of occupied solution volume per NC (VSPN) to the volume per NC (VNC) at different concentrations. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (2) Rogach, A. L.; Katsikas, L.; Kornowski, A.; Su, D.; Eychmuller, A.; Weller, H. Ber. Bunsenges. Phys. Chem. 1996, 100, 1772. (3) Cao, Y.; Banin, U. J. Am. Chem. Soc. 2000, 122, 9692. (4) Green, M.; O’Brien, P. Chem. Commun. 1999, 2235. (5) Nakamura, H.; Yamaguchi, Y.; Miyazaki, M.; Maeda, H.; Uehara, M.; Mulvaney, P. Chem. Commun. 2002, 2844.

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