Ligand Effects of Amine on the Initial Nucleation and Growth

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Ligand Effects of Amine on the Initial Nucleation and Growth Processes of CdSe Nanocrystals Z. H. Sun,†,‡ H. Oyanagi,*,†,‡ H. Nakamura,§ Y. Jiang,†,‡ L. Zhang,§ M. Uehara,§ K. Yamashita,§ A. Fukano,† and H. Maeda§,|,⊥ Photonics Research Institute, National Institute of AdVanced Industrial Science and Technology, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan; National Synchrotron Radiation Laboratory, UniVersity of Science and Technology of China, Hefei, Anhui 230029, People’s Republic of China; Micro- and NanoSpace Chemistry Group, Nanotechnology Research Institute, National Institute of AdVanced Industrial Science and Technology, 807-1, Shuku-machi, Tosu, Saga 841-0052, Japan; Department of Molecular and Material Sciences, Interdisciplinary Graduate School of Engineering Sciences, Kyushu UniVersity, 6-1 Kasuga-koen, Kasuga, Fukuoka 816-8580, Japan; and CREST, Japan Science and Technology Agency, 4-1-8, Hon-chou, Kawaguchi, Saitama 332-0012, Japan ReceiVed: February 12, 2010; ReVised Manuscript ReceiVed: March 29, 2010

The role of dodecylamine (DDA) as surface ligand in the initial nucleation and growth processes of CdSe nanocrystals was studied by in-situ extended X-ray absorption fine structure and a microfluidic reactor. The results combined with ex-situ UV-vis absorbance spectra provided the kinetic information of the nanocrystals grown in a noncoordinating solvent octadecene. We found that the initial nucleation stage is significantly accelerated by the presence of DDA: without addition of DDA the nucleation took about 5 s, while it was shortened to within 1 s with the presence of 5 and 10 wt % DDA. Increasing the DDA concentration from 5 to 10 wt % increased the starting particle size but reduced the growth rate of the nanocrystals. The results may indicate two competitive roles of DDA in the current system, namely, activating reagent of the precursors in the initial nucleation stage and passivating ligand of the nanocrystals surfaces during the subsequent growth process. 1. Introduction The past two decades have witnessed an explosive expansion of research on semiconductor nanocrystals, with CdSe being a model system in this field.1–4 One of the most important routes of synthesizing CdSe nanocrystals is chemical growth in colloidal solutions.5 In general, these colloidal nanocrystals are small and are not thermodynamically stable for crystal growth kinetically. To produce stable nanocrystals by arrested precipitation during a colloidal chemical reaction, a critical and practical way is to add appropriate surface passivating ligands which adsorb rapidly on the nascent particle surface and act to minimize the van der Waals interactions between nearby crystallites.3,6 Besides, the surface ligands also play key roles in determining the size, size distribution, electronic structure, optical properties, and shape of the nanocrystal products.2–4,7 The most efficient capping ligands appear to be small organic molecules that contain metal coordinating groups and solvophilic groups. A lot of organic capping ligands, including phosphines, fatty acids, and amines, have been attempted to synthesize highquality CdSe nanocrystals and to tune their properties. It has been found that different ligands may have distinct roles in the nanocrystals synthesis, depending on the bond strength of the ligand-monomer complex and its solubility.6 Amine, in particular primary amine, is one type of the most important and widely used surface ligands for synthesis of * To whom correspondence should be addressed. † National Institute of Advanced Industrial Science and Technology. ‡ University of Science and Technology of China. § National Institute of Advanced Industrial Science and Technology. | Kyushu University. ⊥ CREST, Japan Science and Technology Agency.

semiconductor, metal, and metal oxide nanocrystals. In the colloidal synthesis of CdSe nanocrystals, the addition of amines to the growth solvent leads to significant improvements in both the size distribution and the photoluminescence quantum efficiency (QE).8–14 For example, dodecylamine (DDA)-capped CdSe nanocrystals exhibit remarkably higher QE (50-60%) than that of trioctylphosphine oxide (TOPO)-capped nanocrystals, and the DDA treatment of TOPO-capped CdSe nanocrystals by surface exchange can further improve the QE to be as high as 70%.9 The benefits of amines over phosphines and phosphine oxides as surface ligands in nanocrystals synthesis are believed to be due to higher packing densities, which provide much better passivation of surface traps participating in the nonradioactive recombination processes.9 A recent work by Pradhan et al. has shown that the surface ligand dynamics of amines is strongly dependent on the ligand concentration and the ligand chain length.15 Despite all of these progresses, it is still under debate whether the amine-cadmium binding strength in CdSe nanocrystals is strong or weak.8–11,15,16 Moreover, there are some conflicting viewpoints regarding the role of amine in the nucleation of CdSe nanocrystals. Foos et al.11 and Jose et al.17 reported that the addition of amine to the TOPO coordinating solvent delayed the initial nucleation of the CdSe particles as compared with the TOPO-alone synthesis, while Pradhan et al. suggested that amines act as activation reagents for synthesis of oxide and chalcogenide nanocrystals when fatty acid salts and phosphonic acid salts were used as the precursors.15 Therefore, more research efforts are needed to understand the details of the nucleation and growth processes of CdSe nanocrystals incorporated with amine. This is not only of academic importance but also may provide new opportunities

10.1021/jp101345n  2010 American Chemical Society Published on Web 05/14/2010

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to synthesize nanocrystals with desired size and shape and thus to tailor their structural and physical properties in a controllable manner. In our previous papers,18,19 we have established a method of combining in situ extended X-ray absorption fine structure (EXAFS) technique and a microfluidic reactor system to study the kinetics of CdSe nanocrystals in the colloidal synthesis. Quantitative relationships between the EXAFS-determined Se consumption percentage and the particle size as well as particle concentration have been established. In this work, this combination method is applied to investigate the initial kinetic processes of colloidal CdSe nanocrystals in the presence of DDA with varying concentrations. We aimed to explore separately the role of amine in the initial nucleation and subsequent growth stages of CdSe nanocrystals. The nanocrystals synthesis was carried out in the solvent of octadecene (ODE), which provides a noncoordinating environment for probing the impact of the surface ligand on the produced nanocrystals. 2. Experimental Section The experimental details, including the synthesis of CdSe nanocrystals in the microfluidic reactor and the in situ EXAFS measurement, have been described in our previous papers18,19 and are briefly outlined below. Three raw material stock solutions (Se source, Cd source, and DDA solutions) were prepared. The selenium stock solution (TOP-Se) was prepared by dissolving selenium powder in trioctylphosphine (TOP) and then diluted in octadecene (ODE). The cadmium stock solution was prepared by heating a mixture of Cd(CH3COO)2 · 2H2O, oleic acid, and ODE at 180 °C under Ar flow. The DDA solution was prepared by dissolving DDA in ODE. These stock solutions were loaded into glass syringes separately and then mixed in a mixer connected to a capillary tube with an inner diameter of 0.5 mm. A length of 70 cm of the capillary tube was attached to the heating unit, where the mixed solution injected at a constant velocity of 7.6 mm/s was heated to a target temperature of 240 °C. In the mixed solution, the Se and Cd concentrations were kept at 30 and 12 mM, respectively, while the DDA concentration was varied from 0, 5, to 10 wt %. The results on the 5 wt % DDA case have been reported in our previous paper.19 In-situ EXAFS measurement at Se K-edge was performed at beamline BL13B1 of Photon Factory (PF) of High Energy Accelerator Research Organization, Japan. For the reactions in 5 and 10 wt % DDA, we collected EXAFS data at eight points, corresponding to the reaction time of 0.0, 1.1, 1.4, 1.8, 2.4, 3.0, 5.0, and 8.1 s.19 When no DDA was added, only 4 points with the reaction time of 1.1, 1.8, 3.0, and 5.0 s were measured because of the lack of enough EXAFS beamtime. Under the same reaction conditions as in the in-situ EXAFS experiment, ex-situ UV-vis absorbance measurement was performed using a UV-vis spectrophotometer (QE65000; Ocean Optics Inc.). 3. Results The EXAFS data analysis and fitting strategy were identical to those used in our previous paper.19 The oscillation functions were extracted from the raw experimental spectra using the ATHENA module,20 and the least-squares parameter fitting was performed using the ARTEMIS module.20 Both modules are implemented in the IFEFFIT package.21 Figure 1 shows the EXAFS oscillation χ(k) functions at some typical reaction time for different DDA concentrations. In order to analyze the data quantitatively, the k2-weighted χ(k) functions were Fourier transformed (FT) into R-space using a Hanning window and a

Figure 1. Typical Se K-edge EXAFS oscillation function χ(k) at different reaction time in the microfluidic reactor with the addition of (a) 0 wt % DDA, (b) 5 wt % DDA, and (c) 10 wt % DDA.

typical k-range of 2.5-10.5 Å-1 (a shorter k-range of 2.5-8.5 Å-1 was used for the 3.0 and 5.0 s data for 10 wt % DDA due to worse statics of the data). The FT magnitudes are plotted in Figure 2 as solid lines. It can be readily observable from Figure 2 that the addition of DDA with varying concentrations causes distinguished temporal evolutions of the EXAFS spectral features. In the absence of DDA, the FT curves exhibit the same spectral shapes before 5.0 s in that only a prominent peak at 1.6 Å ascribed to the Se-P bond of TOP-Se is visible. When the reaction time reaches 5 s, a weak peak at 2.5 Å attributed to the Se-Cd bonds becomes apparent, suggesting the formation of CdSe nanocrystals. However, the FT for 5 and 10 wt % DDA shows much faster formation of CdSe particles after heating the reaction solution because the Se-Cd peak appears even at the reaction time as short as 1.1 s. One may suspect that the Se-Cd peak at such a short reaction time might come from some kind of magicsize CdSe nanoclusters.17,22,23 But this possibility can be excluded by the absence of any visible UV-vis absorption peak in the wavelength range of 350-450 nm, since these nanoclusters can only be stable at relatively low temperatures of 90-150 °C.22 Upon further increasing the reaction time, the Se-Cd peak intensity is enhanced, and at the same time, the Se-P peak intensity decreases gradually. Furthermore, it is noticeable that the Se-Cd peak intensity increase (and the Se-P peak intensity decrease) is quicker when the added DDA concentration is higher. For instance, at 1.1 s the Se-Cd peak intensity for 10 wt % DDA is only slightly larger than that for 5 wt % DDA, while at 5.0 s this difference in intensity becomes pronounced. This indicates that higher DDA concentration facilitated the conversion of TOP-Se precursors into CdSe nanocrystals. Least-squares parameter fitting was performed to extract quantitative results using the same fitting strategy as that in our previous paper.19 An adjustable parameter, the Se consumption percentage p, was also used in the fitting. The curve-fitting

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Figure 2. Magnitude of the Fourier transform (FT) of the k2-weighted oscillation function χ(k) shown in Figure 1 with the addition of (a) 0 wt % DDA, (b) 5 wt % DDA, and (c) 10 wt % DDA. The solid lines represent the experimental data, and the open circles show the fitting results.

Figure 3. Temporal evolution of Se consumption percentage p(t) for different DDA concentrations. The dotted lines are guides to the eyes. The error bars of p(t) were generated from the EXAFS fitting, while error bars of time were estimated from the beam size and the flowing velocity of the reaction solution.

results are plotted in Figure 2 by open circles showing the satisfactory fitting quality. The obtained bond lengths (RSe-P ) (2.11-2.13) ( 0.02 Å, RSe-Cd ) (2.59-2.64) ( 0.04 Å) did not show obvious dependence on reaction time or DDA concentration within the error bars. The pileup of CdSe nanocrystals on the capillary wall was observable for the 5 wt % DDA and 10 wt % DDA case; therefore, the obtained results were corrected.18 On the contrary, pileup was very weak for the DDA-free experiment within the first 5 s of reaction time. After correction of the pileup effect, the time evolution of p is shown in Figure 3 for different DDA concentrations. Evidently, at the same reaction time the consumption percentage of Se increases rapidly with DDA concentration. Figure 3 also indicates that for the 10 wt % DDA case, at the reaction times of 5.0 and 8.1 s, the Se consumption

Sun et al.

Figure 4. UV-vis absorbance spectra of the CdSe nanocrystals in the presence of (a) 0 wt % DDA, (b) 5 wt % DDA, and (c) 10 wt % DDA. There are no visible absorption peak in the spectra with reaction time shorter than 2.4 s. For clarity these spectra are not shown.

percentage p is larger than 0.4, which is the upper limit of p in our experiment if the consumed Se:Cd ratio is 1:1. The reason is unclear to us. It might be due to the pileup effect which is not compensated completely by the simple correction. Another possible reason is that the assumption of equal consumption of Se and Cd is not strictly correct; for example, if the consumed Se:Cd ratio is 1.15:1-1.2:1,24 the limit of p can be 0.46-0.48. Since the accurate limit of Se and Cd consumption ratio is unknown, we have to use the 1:1 ratio for a rough estimate. Consequently, for the 10 wt % DDA addition the discussions are mainly focused on the results of t < 5.0 s. To determine the nanocrystals size as a function of time, exsitu UV-vis absorbance experiments were performed under the same reaction conditions as in the in-situ EXAFS experiment. Within the first 2.4 s of the reaction, no excitonic transition peak could be observed, so only the UV-vis spectra at t g 2.4 s are shown in Figure 4 for clarity. In the DDA-free experiment, the exciton absorption peak was almost invisible until the reaction time reached 5.0 s when an absorption peak at 490 nm appeared. The position of this absorption peak was shifted to 512 nm when the reaction proceeded 8.1 s. In the presence of 5 wt % DDA, a weak absorption peak at 456 nm could be detected at 2.4 s, which was rapidly red-shifted to 473, 509, and 524 nm at 3.0, 5.0, and 8.1 s, respectively. For the 10 wt % DDA case, at 2.4 s a very weak absorption peak at 490 nm could be discerned. When the reaction time increased to 3.0 s, the peak position kept almost unchanged. After that, the absorption peak red-shifted to 495 and 512 nm at 5.0 and 8.1 s, respectively. These observations demonstrate that DDA with different concentrations has a significant impact on the growth kinetics of the CdSe nanocrystals, coinciding with the EXAFS results. Using the empirical relation between the UV-vis absorbance wavelength and particle diameter calibrated by Yu

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Figure 5. Temporal evolution of CdSe nanocrystals in the presence of 5 and 10 wt % DDA: (a) diameter, (b) particle concentration. In (a) the dotted lines are guides to the eyes.

et al.25 and re-examined by Jasieniak et al.,26 the average diameters of the CdSe nanocrystals were estimated and are shown in Figure 5a. 4. Discussion We have proposed analytical expressions of the CdSe nanocrystals size D(t) and concentration N(t) in terms of the Se consumption percentage p as a function of reaction time t,19 based on the surface-reaction-driven kinetics equation for describing the nanocrystals growth.27 These expressions for [Cd]0/[Se]0 ) 0.4 are

D(t) ) 6KVm[Se]0

p(t)(0.4 - p(t)) p'(t)

(1)

and

N(t) )

p'(t)2 1 36πK3Vm2[Se]02 p(t)2(0.4 - p(t))3

(2)

where Vm ) 32.99 cm3/mol is the molar volume of CdSe, [Se]0 ) 30 mM is the initial Se precursor concentration, K is an interfacial rate constant that reflects the rate-determining steps during deposition, and p′(t) ) dp(t)/dt. Equation 1 shows that once the sizes of the nanocrystals are known, the surface reaction rate constant K can then be estimated from the p(0.4 - p)/p′ function yielded from the EXAFS analysis. In turn, this allows to evaluate the particle concentrations using eq 2. Since the nanocrystals size can be well determined from the UV-vis absorbance spectrum, the combination of the in-situ EXAFS and UV-vis absorbance spectrum provides a valuable pathway for studying the kinetics of nanocrystals synthesis. Especially, the in-situ EXAFS in conjunction with the microfluidic reactor, although less practical and convenient than the UV-vis absorbance spectrum, fills a gap of the UV-vis absorbance spectra in determining the kinetics information at the very early stage of the nanocrystals nucleation. This is because during this period the excitonic absorption peak has broad width and low intensity due to the wide particle size distribution, like those in Figure 4 for the 2.4 s spectra. In the DDA-free EXAFS experiment, we could only observe the formation of CdSe nanocrystals at one point of t ) 5.0 s. Therefore, we are not able to use eqs 1 and 2 to determine the nanocrystals size and concentration. This is a disadvantage of the presented EXAFS method as compared with UV-vis absorbance that it requires a series of data. For the 10 wt %

DDA case, as we have mentioned, the data points with t g 3.0 s have to be abandoned. Using the diameter D(2.4 s) ) 2.32 nm determined from the UV-vis absorbance spectra, the surface reaction rate constant K was estimated to be 8.04 × 10-5 cm/s, almost a double of that (4.16 × 10-5 cm/s) for 5 wt % DDA.19 Substituting the obtained K values into eqs 1 and 2, we could evaluate the particle size and concentration at t < 2.4 s. Figure 5a,b compares the time evolutions of the size and concentration of the CdSe nanocrystals in the presence of 5 and 10 wt % DDA, respectively. As seen from Figure 5a, the addition of DDA affects the reaction in two distinct ways. First, in the presence of DDA with higher concentration (10 wt %), the CdSe nanocrystals size is larger within the first 4 s, especially the detectable starting particle size is much larger (1.9 vs 1.4 nm). Second, after 4 s of reaction, the CdSe nanocrystals size for 10 wt % DDA becomes smaller and at the same time the growth rate of the particles is considerably lower. Considering that without the addition of DDA the nucleation of the nanocrystals took longer time (∼5 s) as compared with the much shorter time (∼1 s) in the presence of DDA, it is evident that DDA accelerates the initial surface reaction in the nucleation stage but retards the subsequent particle growth if too much DDA is added. The concentration of DDA affects not only the particle size but also the particle concentration. Figure 5b is a plot of the particle concentration N(t) calculated using eq 2. The obtained N(t) has large variance which mainly comes from the p′3 factor. Nevertheless, some qualitative conclusions can still be drawn. The addition 10 wt % DDA also led to a burst of nucleation28 as judged by the formation of CdSe nanocrystals immediately after (∼1 s) heating the reaction solution. At t e 2.4 s, the particle concentration for 10 wt % DDA was somehow smaller than the value at the same reaction time for 5 wt % DDA, suggesting that too higher DDA concentration reduced the initial nuclei density. In spite of the lack of precise results regarding the particle concentration as the reaction proceeded 3 s or longer, the evolution trend as compared with the 5 wt % DDA case could be inferred. The Se consumption percentage can be expressed as p(t) ) πN(t)D(t)3/6Vm[Se]0 for the spherical nanocrystals,19,29,30 and the particle concentration is then N(t) ∝ p(t)/D(t)3. The EXAFS results (Figure 3) reveal that the overall Se consumption percentage p(t) increased with DDA concentration, and after 4 s of reaction the particle size was smaller for 10 wt % DDA than for 5 wt % (Figure 5a). Accordingly, the particle concentration at t > 4 s is larger as 10 wt % DDA was added. In the absence of DDA, the particle size at the same reaction time of t g 5.0 s was very close to that of the 10 wt % case judged by the similar UV-vis absorption peak position (Figure 4), but with a smaller Se consumption percentage. Analogously, this indicates that the CdSe particle concentration without the addition of DDA was even lower. From these existing experimental observations, the role of DDA in the synthesis of CdSe nanocrystals using the noncoordinating solvent ODE can be inferred. Addition of DDA in the reaction solution plays two competitive roles, namely, accelerating the initial nucleation by activating the precursors and delaying the subsequent growth of the nanocrystals. The reason of the accelerated nucleation by amines is that amines can activate the chemical transform of the precursors into active atomic monomers.31 Our understanding of the role of DDA in the nucleation stage coincides with the viewpoint of Pradhan et al. that amines act as activation reagents15 but does not agree with the results by Foos et al.11 and Jose et al.,17 who reported

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that the addition of amine to the TOPO solvent delayed the initial nucleation of the CdSe. This discrepancy regarding the roles of amines may be arising from the different solvent used by different authors: Foos et al.11 and Jose et al.17 used the coordinating solvent TOPO which also acts as a surface ligand, while Pradhan et al.15 and us used the noncoordinating solvent ODE. When amines are added to the TOPO-contained solution, the ligand-ligand interaction between amines and TOPO would affect the ligand-nanocrystal interaction. This reduces the effective concentration of the ligands, and as a result the precipitation reaction is pushed toward solubilization. On the other hand, DDA also plays the role of passivating ligand which caps the nascent crystallites and hinders their growth and dissolution as the reaction prolongs. In the absence of DDA, the unreacted TOP-Se acted as the ligand and the growth rate of the CdSe nanocrystals was low. This can be understood from two aspects. First, it is known that the binding strength of amines and TOPO (and TOP-Se also) to CdSe nanocrystals can be modeled by Coulombic interactions between partially charged atoms.32 As compared with DDA, the ligand-particle binding strength is stronger for TOPO33 but weaker for TOP-Se.8–10 Second, TOP-Se has a more sterically hindered structure than the strainlike structure of DDA, making it a less effective ligand than DDA. Because of these two factors, the growth peocess with TOP-Se ligand alone was delayed. A similar phenomenon has also been observed for ZnSe nanocrystals.31 In the presence of 5 wt % DDA, the growth of CdSe nanocrystals was greatly accelerated. However, when 10 wt % DDA was added, the growth rate of CdSe nanocrystals was again slowed down. Molecular simulations have shown that the amine capping layer is formed in two stages.32 First, amine molecules bind with the negatively charged N atom to a single surface Cd atom. If the available amine concentration is very high, a second amine may bind to an occupied Cd surface forming hydrogen bonds with the already adsorbed ligands. This therefore provides a very high ligand coverage (even more than the total number of surface Cd atoms) on the nanocrystals surface and severely prohibits their growth. The high surface coverage also prohibits the coalescence and dissolution of the formed particles, making the particle number almost unchanged with time. Another reason for the hindered particle growth at t > 2 s for 10 wt % DDA than for 5 wt % DDA is the lower concentrations of the remaining Cd and Se presursors because more presursors have been consumed in the early time (t < 2 s) reaction. There should be a certain DDA concentration which optimizes the growth of CdSe nanocrystals by balancing the accelerating and decelerating effects, and under current reaction conditions this value seems to be close to 5 wt %. Therefore, varying amine concentration with respect to Se and Cd precursors concentrations provides a practical way to control the sizes and concentrations of the CdSe nanocrystals. In this respect, the role of amine is similar to that of bis(2,2,4trimethylpentyl)phosphinic acid (TMPPA) reported by van Embden and Mulvaney.6 They found that there existed a critical TMPPA concentration above which the addition of TMPPA led to reduced particle concentration and dramatically slowed growth rate. As surface ligands, DDA plays opposite roles as compared with oleic acid,6 which inhibits nucleation and causes a rapid reduction in the number of particles within the first minutes of reaction. 5. Conclusions The role of DDA in the initial nucleation and growth processes of CdSe nanocrystals have been studied by in-situ

Sun et al. EXAFS combined with a microfluidic reactor and ex-situ UV-vis absorption spectroscopy. The Se consumption percentage as a function of reaction time extracted from in-situ EXAFS analysis provides a way for studying the growth kinetics of the nanocrystals from viewpoints of chemical bonds. Remarkably different size evolutions of the CdSe nanocrystals with time were observed as the mixed DDA concentrations varied from 0 to 10 wt %. The initial nucleation was significantly delayed (∼5 s) with a slow growth rate of the particles with the absence of DDA. In contrast, with the addition of 5 and 10 wt % DDA, the nucleation stages were accelerated after heating the reaction solution (within ∼1 s), resulting in larger particle size. The fact that high DDA concentration (10 wt %) suppresses subsequent growth rate of nanocrystals may indicate that DDA acts as both activation reagent for the initial nucleation and the capping ligand for prohibiting the growth, dissolution, and coalescence of the nanocrystals. It is demonstrated that the bond kinetics studied by the in-situ EXAFS describes DDA effects on the particle size and concentration providing complementary information with UV-vis spectroscopy. Acknowledgment. This work is supported by the Core Research for Evolutional Science and Technology (CREST) program of the Japan Science and Technology Corporation (JST). The authors thank Photon Factory for the synchrotron radiation beamtime. Z. H. Sun gratefully acknowledges the Japan Society for the Promotion of Science (JSPS) for the financial support. References and Notes (1) Alivisatos, A. P. J. Phys. Chem. 1996, 100, 13226–13239. (2) Yin, Y.; Alivisatos, A. P. Nature 2005, 437, 664–670. (3) Burda, C.; Chen, X. B.; Narayanan, R.; El-Sayed, M. A. Chem. ReV. 2005, 105, 1025–1102. (4) Rosenthal, S. J.; McBride, J.; Pennycook, S. J.; Feldman, L. C. Surf. Sci. Rep. 2007, 62, 111–157. (5) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706–8715. (6) van Embden, J.; Mulvaney, P. Langmuir 2005, 21, 10226–10233. (7) Kilina, S.; Ivanov, S.; Tretiak, S. J. Am. Chem. Soc. 2009, 131, 7717–7726. (8) Talapin, D. V.; Rogach, A. L.; Kornowski, A.; Haase, M.; Weller, H. Nano Lett. 2001, 1, 207–211. (9) Talapin, D. V.; Rogach, A. L.; Mekis, I.; Haubold, S.; Kornowski, A.; Haase, M.; Weller, H. Colloids Surf., A 2002, 202, 145–154. (10) Jasieniak, J.; Mulvaney, P. J. Am. Chem. Soc. 2007, 129, 2841– 2848. (11) Foos, E. E.; Wilkinson, J.; Makinen, A. J.; Watkins, N. J.; Kafafi, Z. H.; Long, J. P. Chem. Mater. 2006, 18, 2886–2894. (12) Sharma, S. N.; Sharma, H.; Singh, G.; Shivaprasad, S. M. Mater. Chem. Phys. 2008, 110, 471–480. (13) Kalyuzhny, G.; Murray, R. W. J. Phys. Chem. B 2005, 109, 7012– 7021. (14) Oluwafemi, S. O.; Revaprasadu, N. Physica B 2009, 404, 1204– 1208. (15) Pradhan, N.; Reifsnyder, D.; Xie, R. G.; Aldana, J.; Peng, X. G. J. Am. Chem. Soc. 2007, 129, 9500–9509. (16) Ratcliffe, C. I.; Yu, K.; Ripmeester, J. A.; Zaman, M. B.; Badarau, C.; Singh, S. Phys. Chem. Chem. Phys. 2006, 8, 3510–3519. (17) Jose, R.; Zhanpeisov, N. U.; Fukumura, H.; Baba, Y.; Ishikawa, M. J. Am. Chem. Soc. 2006, 128, 629–636. (18) Uehara, M.; Sun, Z. H.; Oyanagi, H.; Yamashita, K.; Fukano, A.; Nakamura, H.; Maeda, H. Appl. Phys. Lett. 2009, 94, 063104. (19) Sun, Z. H.; Oyanagi, H.; Uehara, M.; Nakamura, H.; Yamashita, K.; Fukano, A.; Maeda, H. J. Phys. Chem. C 2009, 113, 18608–18613. (20) Ravel, B.; Newville, M. J. Synchrotron Radiat. 2005, 12, 537– 541. (21) Newville, M. J. Synchrotron Radiat. 2001, 8, 322–324. (22) Wang, H. Z.; Tashiro, A.; Nakamura, H.; Uehara, M.; Miyazaki, M.; Watari, T.; Maeda, H. J. Mater. Res. 2004, 19, 3157–3161. (23) Kudera, S.; Zanella, M.; Giannini, C.; Rizzo, A.; Li, Y. Q.; Gigli, G.; Cingolani, R.; Ciccarella, G.; Spahl, W.; Parak, W. J.; Manna, L. AdV. Mater. 2007, 19, 548–552.

CdSe Nanocrystals (24) Taylor, J.; Kippeny, T.; Rosenthal, S. J. J. Cluster Sci. 2001, 12, 571–582. (25) Yu, W. W.; Qu, L. H.; Guo, W. Z.; Peng, X. G. Chem. Mater. 2003, 15, 2854–2860. (26) Jasieniak, J.; Smith, L.; van Embden, J.; Mulvaney, P. J. Phys. Chem. C 2009, 113, 19468–19474. (27) Bullen, C. R.; Mulvaney, P. Nano Lett. 2004, 4, 2303–2307. (28) Park, J.; Joo, J.; Kwon, S. G.; Jang, Y.; Hyeon, T. Angew. Chem., Int. Ed. 2007, 46, 4630–4660. (29) Nakamura, H.; Tashiro, A.; Yamaguchi, Y.; Miyazaki, M.; Watari, T.; Shimizu, H.; Maeda, H. Lab Chip 2004, 4, 237–240.

J. Phys. Chem. C, Vol. 114, No. 22, 2010 10131 (30) Xie, C.; Hao, H. X.; Chen, W.; Wang, J. K. J. Cryst. Growth 2008, 310, 3504–3507. (31) Li, L. S.; Pradhan, N.; Wang, Y. J.; Peng, X. G. Nano Lett. 2004, 4, 2261–2264. (32) Schapotschnikow, P.; Hommersom, B.; Vlugt, T. J. H. J. Phys. Chem. C 2009, 113, 12690–12698. (33) Koole, R.; Schapotschnikow, P.; Donega, C. D.; Vlugt, T. J. H.; Meijerink, A. ACS Nano 2008, 2, 1703–1714.

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