CdxZn1–xS Quantum Dots with Narrow

ARTICLE pubs.acs.org/JPCC

Highly Luminescent CdSe/CdxZn1 xS Quantum Dots with Narrow Spectrum and Widely Tunable Wavelength Ping Yang, Masanori Ando, Takahisa Taguchi, and Norio Murase* Health Research Institute, National Institute of Advanced Industrial Science and Technology, Midorigaoka, Ikeda-city, Osaka 563-8577, Japan

bS Supporting Information ABSTRACT: We exploit the synthesis of CdSe core and CdSe/ CdxZn1 xS core shell quantum dots (QDs) with spherical and rod morphologies through a controlled core formation and single coating step by using different growth kinetics of CdS and ZnS. The size and morphology of CdSe cores depended strongly on the injection speed of trioctylphosphine selenium (TOPSe), reaction temperature, and time. The cores with spherical morphology exhibited tunable photoluminescence (PL) wavelength from 544 to 623 nm. CdSe cores with rod morphology and a PL peak wavelength of 623 nm were prepared when TOPSe was injected slowly. The molar ratio of Cd/Zn upon preparation was adjusted for investigating the effect of the CdxZn1 xS shell on the properties of the resulting core shell QDs. The coated QDs exhibited a controlled red shift from the initial PL of CdSe cores with much improved PL efficiency (a maximum value of 89%). CdSe/CdxZn1 xS core shell QDs prepared by using those rod cores exhibited a PL peak wavelength of 652 nm and a PL efficiency of 61%. All resulting core shell QDs exhibited narrow PL spectra (full-width at halfmaximum of 23 29 nm). High PL efficiency and narrow PL spectra in the wide visible range make these core shell QDs attractive for applications.

’ INTRODUCTION In the last 20 years, an explosion in research dealing with the synthesis and characterization of semiconductor quantum dots (QDs) has been done.1 3 To improve the luminescent efficiency and colloidal stability of these QDs, surface modification of colloidal nanocrystals (NCs) becomes necessary. The growth of a shell of a higher band gap inorganic material on a core of another lower band gap material to form a heterostructure has been one of the successful routes in the surface modification. Both the “bare” quantum dots such as CdSe and CdS and core shell QDs including CdS on CdSe,4 ZnS on CdSe,5,6and ZnS on CdS7,8 have been extensively studied due to their novel properties. Researchers have used unique and interesting reaction conditions to prepare QDs with different sizes, shapes, and compositions.8 12 Certain reaction conditions restrict the polarity, surface chemistry, and solubility of the QDs. The main method for preparing the QDs is by classical colloidal chemistry, where various researchers have employed organometallic and/or metal organic compounds under anaerobic conditions. For example, CdSe QDs was prepared by reacting dimethylcadmium (CdMe2) with trioctylphosphine selenium (TOPSe) in TOP/ trioctylphosphine oxide (TOPO) at high temperatures.13 Other researchers used single-molecule precursors such as bis(diethyldithio-/diselenocarbamato) Cd(II)/Zn(II) for the preparation of CdSe/ZnS QDs.14,15 In this paper, we prepared CdSe r 2011 American Chemical Society

and its core shell QDs by an organic synthesis using cadmium acetate dihydrate, zinc acetate, octadecylphosphonic acid (ODPA), trioctylamine (TOA), oleic acid (OA), TOPSe, and TOPS. The preparation method presented here shows significant advantage over the conventional TOPO method because of its reproducibility and widely tunable PL wavelength range together with high emission efficiency and narrow spectral width. CdSe QDs are attractive for use in emissive applications due to their size-dependent photoluminescence (PL) tunable across the visible range, and core/shell CdSe/CdS, CdSe/ZnS, and CdSe/ Zn0.5Cd0.5S/ZnS QDs have been synthesized.4,16,17 ZnS is a nontoxic, chemically stable wide band gap (3.8 eV for the bulk material) semiconductor. Potentially, the ZnS shell should provide the best passivation of the CdSe core. However, the large mismatch (ca. 12%) between CdSe and ZnS lattice parameters induces the strain at the interface between the core and the shell.18 In the case of CdSe/CdS and CdSe/ZnSe core shells, the lattice mismatch between the core and the shell materials is relatively small. However, the band gaps of CdS and ZnSe are not large enough to provide the potential barrier necessary to block both electrons and holes inside CdSe core.19

Received: February 7, 2011 Revised: May 23, 2011 Published: July 12, 2011 14455

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Table 1. Preparation Conditions and Properties of CdSe Cores sample

injection speed of TOPSe/(mL/min)

reaction temperature/°C

total reaction time/min

morphology

diameter/nma

PL peak wavelength/nm

core 1

20

300

2

spherical

2.7

543.6

core 2

20

325

3

spherical

4.0

574.0

core 3

1

325

3

spherical

5.9

610.8

core 4

0.25

325

6

rod

8.5

623.0

a

The mean diameters of spherical CdSe cores and the mean length of a rod-shaped CdSe core were obtained by transmission electron microscopy observation. The result is very similar to the values derived from the absorbance at first absorption peak.

There are a few reports about core/multishell structure QDs prepared by inserting an interlayer, such as CdS and ZnSe between the CdSe core and ZnS shell to reduce the lattice parameter difference. Jun and co-workers reported on CdSe/ CdS/ZnS QDs prepared by a one-step process to grow CdS and ZnS shells on a CdSe core.20 The authors used CdO, ODPA, and TOPSe as starting materials and TOA as a solvent to prepare CdSe core. The shell coating was carried out by using cadmium acetate dihydrate, zinc acetate, and TOPS as starting materials, OA as a capping agent, and TOA as a solvent. This method has significant advantages: being less toxic, insensitive to air and moisture, easier to purify, and providing highly monodisperse QDs with a high PL efficiency (70%). However, the PL peak wavelength of the QDs is from 490 to 600 nm. The lack of longer PL peak wavelength is still a problem. The shell growth can eliminate nonradiative defects on the core’s surface, so the PL efficiency is greatly enhanced, but the PL efficiency is not increased monotonically with increasing the shell thickness.16 Because of the lattice mismatch between the core and the shell, the interface strain accumulates dramatically during the shell growth and eventually can be released through the formation of dislocations, indicating that the spatial distribution of defects in core shell NCs can be controlled by the shellgrowth. Therefore, the series of PL spectra from QDs with successive shell-growth can reveal the nature of traps in QDs. Aside from fundamental scientific interest, a better understanding of the shell effect on optical properties of QDs is important for optimizing QD synthesis, and can also lead to improvements in QD performance in applications. In this work, CdSe QDs were successively coated with a CdxZn1 xS shell through a single coating step by using a facile organic route. The experimental results indicated clearly the PL efficiency of core shell QDs was drastically increased when a CdSe core was first coated with a CdS layer. The effect of the molar ratio of Cd/Zn on the PL properties of the core shell QDs was investigated. The tunable PL spectra with peak wavelength from 590 652 nm and a high PL efficiency of 89% with narrow spectral width (e30 nm) were obtained by adjusting the size and morphology of CdSe cores and CdxZn1 xS shells. These results demonstrated current method should be crucial for the preparation and application of semiconductor QDs.

’ EXPERIMENTAL SECTION Chemicals. Cadmium oxide (99.99%), selenium (99.5%, 100 mesh), ODPA (97%), sulfur (99.98%, powder), TOP (90%), OA (90%), cadmium acetate dihydrate (Cd(Ac)22H2O 98%), zinc acetate (Zn(Ac)2, 99.99%), and TOA were purchased from Sigma Aldrich. All the chemicals were used directly without any further purification except for TOP. The pure water was obtained from a Milli-Q synthesis system.

Synthesis of CdSe Cores. All reactions were conducted under N2 atmosphere. CdSe cores were synthesized by modifying the published method.20 In a typical synthesis of the cores, CdO (0.54 mmol), 180 mg of ODPA, and 5 mL of TOA were first placed in a three-neck round-bottom flask under N2 flow and stirred at 300 °C until the CdO completely dissolved. Se powder (1 mmol) was dissolved in 1 mL of TOP. The TOPSe solution was then injected into the cadmium precursor solution with rapid stirring and kept at 300 °C for 2 min, followed by cooling down to room temperature. Hexane (10 mL) and 60 mL of ethanol were added to precipitate the CdSe QDs. The product was then washed with copious ethanol, redispersed in 20 mL of toluene, and centrifuged at 15000 rpm for 15 min to remove the sludge. Next, the CdSe cores were precipitated with ethanol, and redispersed in 10 mL of toluene for subsequent shell coating. To prepare CdSe cores with different size and morphology, the reaction time, temperature, and injection speed of TOPSe were adjusted. The preparation parameters are summarized in Table 1. Coating of CdxZn1 xS Shell. In a typical synthesis of the Cd x Zn 1 x S shell, Cd(Ac)2 2H 2 O (0.05 mmol), Zn(Ac)2 (0.05 mmol), 2 mL of OA, and 5 mL of TOA were placed in a three-neck round-bottom flask under N2 flow and stirred at 300 °C until the Cd and Zn salts were completely dissolved. S powder (0.19 mmol) was dissolved in 0.5 mL of TOP. The toluene solution of CdSe cores (3 mL) was injected with vigorous stirring, followed by the injection of the TOPS solution. The mixture was kept at 300 °C with stirring for further certain time (5 to 70 min), followed by cooling down to room temperature. The products were precipitated, washed twice with ethanol, and redispersed in 10 mL of toluene. Different Cd/Zn molar ratios were used to investigate the effect on the PL properties of CdSe/CdxZn1 xS core shell QDs. Coating of CdS Shell. For CdS shell deposition, cadmium precursor solution was obtained by dissolving 0.08 mmol of Cd(Ac)22H2O in 2 mL of OA and 5 mL of TOA in a 5-mL three-neck flask under N2 by heating to 300 °C while stirring. After the solution became clear, TOPS (0.15 mmol of S) was injected into the cadmium precursor solution. Aliquots of the reaction mixture were taken at 3 60-min intervals for absorption and PL spectral characterization. Apparatus. Transmission electron microscopy (TEM) observation was carried out by mainly using Hitachi EF-1000 electron microscope. The absorption and PL spectra were recorded using conventional spectrometers (Hitachi U-4000 and F-4500, respectively). The PL efficiencies of the emitting beads and the QDs in solution were estimated with a method previously reported.21 Briefly, the PL and absorption spectra of a standard quinine solution (quinine in 0.1 N H2SO4 solution; PL efficiency η0 of 55%) were measured in a 1-cm quartz cell as a function of its concentration. The emission intensity P0 (in units of the number of photons) is expressed as P0 ≈ Kη0a010 0.5a0, 14456

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Table 2. Reaction Time and Properties of CdSe/CdxZn1 xS QDs Prepared by Using Core 1 and the Cd/Zn Molar Ratio of 1/1

Figure 1. Absorption and PL spectra of CdSe cores and CdSe/ CdxZn1 xS core shell QDs (samples 1 1, 1 2, 1 5, and 1 6 shown in Table 2) with different reaction times.

where a0 is absorbance at the excitation wavelength (365 nm), and K is the apparatus function. After measurement of the absorbance and PL intensity of the sample using the same apparatus parameters, the PL efficiency of the sample was derived by comparing the PL intensity of samples with that of the standard quinine solution. The error in the PL efficiency is estimated to be within 10% by comparing the results using two standards including quinine and R6G.

’ RESULTS AND DISCUSSION The reaction temperature, time, and the injection speed of TOPSe were extraordinarily critical for the size and morphology of CdSe cores. As summarized in Table 1, the size of CdSe cores increased with increasing the reaction temperature. For example, the mean diameters of cores 1 and 2 were 2.7 and 4.0 nm, respectively. This is ascribed that temperature accelerates the nucleation and growth of CdSe. A maximum PL peak wavelength of 623 nm from CdSe cores with rod morphology can be achieved while the reaction temperature reached 325 °C and injected speed is 0.25 mL/min, respectively. To get the cores with different size and narrow size distribution, several preparation parameters have to be considered at the same time as shown in Table 1. When TOPSe was injected quickly (such as cores 1 and 2) and reaction time became short, a large amount of CdSe nuclei quickly formed and grew into spherical cores. In contrast, a small amount of CdSe nuclei formed and the growth slowed when a slow TOPSe injection speed was used. CdSe has a hexagonal crystal structure (wurtzite),20 and it is now well-established that some growth conditions favor crystallization in the {002} direction.22 CdSe nanorod growth can be promoted by inducing a burst of particle nucleation and then adding more reactant at low supersaturation to alleviate further particle nucleation and favor epitaxial deposition on the {002} surfaces,23 which appears to be the most reactive facet of the nanorods. Figure 1 shows the absorption and PL spectra of CdSe/ CdxZn1 xS core shell QDs prepared by using a molar ratio of Cd/Zn of 1/1. The absorption and PL spectra of CdSe cores are shown for comparison. A significant red-shift was observed in both the absorption and PL spectra of the core shell QDs in contrast to those of CdSe cores. This was an indication of the formation of a CdSe/CdxZn1 xS core shell structure. The thickness of the CdxZn1 xS shell increased with prolonging reaction time, which resulted in a gradual red-shift of PL spectra. The red-shifts of the absorption and PL peaks with increasing shell thickness can be ascribed both to a reduction in confinement

reaction

PL peak

PL efficiency

sample

time/min

wavelength/nm

(%)

core 1

fwhm/nm

N/A

543.6

9.2

24

1 1

10

588.8

85.7

26

1 2

15

590.6

88.8

27

1 3 1 4

20 40

593.8 596.2

83.1 75.1

28 29

1 5

60

598.2

70.0

29

1 6

70

598.6

66.2

29

associated with the presence of the shell and strain effect.24 In the present case, the lattice mismatch is between core and shell does not large to cause significant strain. Therefore, the main reason for the observed red-shift is due to the reduction in confinement which leads to a delocalization of electron wave functions from the core into the shell. The formation of a CdxZn1 xS shell around CdSe cores results in a drastic improvement of PL efficiency (up to 89% when the reflux time is 15 min). The PL efficiency of the QDs was decreased with further increasing the reflux time. This is ascribed to the lattice mismatch generated by increasing shell thickness. Although the previous report disclosed that more than a monolayer passivating shell caused a decrease in PL efficiency because of the lattice mismatch,6 the intermediate CdS layer could relieve the lattice mismatch between the CdSe core and the ZnS shell. The full width at half-maximum (fwhm) of PL spectra was increased after coating with a CdxZn1 xS shell. Table 2 summarizes the experimental condition and PL properties of CdSe/CdxZn1 xS core shell QDs. Figure 2 shows the evolution of PL peak wavelength and PL efficiency of the core shell QDs during reflux. The CdSe/CdS QD has less lattice mismatch (∼3.9%) than that of CdSe/ZnS QD (∼12%).18 Therefore, an alloy CdxZn1 xS shell can provide better stability by decreasing interfacial strain. In our experiment, the shell layers of CdS and ZnS might not be clearly separated, and it is also possible that the shell layer is a gradient alloy structure of CdS and ZnS. This is ascribed to the growth kinetic of CdS and ZnS. Pons and co-workers confirmed such alloy shell grown on CdTeSe QDs.25 The gradient structure would be beneficial to improve stability by reducing lattice mismatch between CdS and ZnS layers, although it could also cause the charge carriers to leak. Figure 3 shows the TEM images of initial CdSe core and CdSe/ CdxZn1 xS core shell QDs. In Figure 3a, the mean size of CdSe cores is 2.7 nm in diameter. With increasing reflux time, the size of CdSe/CdxZn1 xS core shell QDs increased. The mean sizes of CdSe/CdxZn1 xS core shell QDs for reaction time of 15 and 60 min are 5.0 and 6.7 nm in diameter, respectively. For comparison, we prepared CdSe/CdS core shell QDs. Table S1 (see Supporting Information) summarizes the preparation condition and PL properties of the core shell QDs. The core shell QDs exhibited smaller red-shifted PL peak wavelength, lower PL efficiency, and narrower fwhm of PL spectra compared with those of CdSe/CdxZn1 xS core shell QDs shown in Table 2. The CdSe/CdS core shell QDs after 6 min of the reaction showed 27 nm of red-shift in PL, while after 20 min of the reaction they showed 39 nm of red-shift. 14457

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Figure 2. Evolution of PL peak wavelength and PL efficiency of CdSe/ CdxZn1 xS core shell QDs shown in Table 2 during shell growth.

Figure 4. TEM images of CdSe core (core 3 shown in Table 1) and CdSe/CdxZn1 xS core shell QDs shown in Table S4(see Supporting Information) with different reaction time. (a) Core 3; (b) sample 5 1 (reaction for 5 min); (c) sample 5 4 (reaction for 20 min). Figure 4c (2) shows well-developed lattice fringes of the core shell QDs. Figure S4 shows the size distribution of these samples (see Supporting Information).

Table 3. Reaction Time and Properties of CdSe/CdxZn1 xS QD Prepared by Using Core 4 and the Cd/Zn Molar Ratio of 1/1

Figure 3. TEM images of CdSe cores (core 1 shown in Table 1) and CdSe/CdxZn1 xS core shell QDs shown in Table 2 with different reaction time. (a) Core 1; (b) sample 1 1 (reaction for 10 min); (c) sample 1 2 (reaction for 15 min); (d) sample 1 5 (reaction for 60 min) Figure S3 of shows the size distribution of these samples (see Supporting Information).

PL efficiency started to increase significantly as the CdS shell was coated on the CdSe core and reached 44%. Figure S1 (see Supporting Information) shows the TEM images of CdSe/CdS core shell QDs with different reaction time. The result indicates the QDs exhibited narrow size distribution compared with CdSe/CdxZn1 xS core shell QDs shown in Figure 3. The mean sizes of CdSe/CdS core shell QDs for reaction time of 15 and 60 min are 4.3 and 5.0 nm in diameter, respectively. These results indicate an alloy CdxZn1 xS shell is better to improve PL efficiency and cause a large red-shifted PL peak wavelength. This phenomenon is ascribed to the difference of CdS and CdxZn1 xS alloy shell because the later shell is a gradient alloy structure of CdS and ZnS. The molar ratio of Cd/Zn is an important factor for the formation of CdxZn1 xS shell on CdSe cores because of the

reaction

PL peak

PL efficiency

fwhm

sample

time/min

wavelength/nm

(%)

/nm

core 4

N/A

623

1.1

32

core 6

15

652

60.8

28

different structure and growth kinetic of CdS and ZnS. We investigated the effect of the molar ratio of Cd/Zn (5/1, 2.5/1, 2/ 1, 1.5/1, 1.2/1, 1/1, 1/1.5, and 1/2) on the PL properties of the CdSe/CdxZn1 xS core shell QDs. The result exhibited that the molar ratio of 1/1 is the overall optimum ratio for the core shell QDs with the highest PL efficiency and large red-shifted PL spectra. Table S2 (see Supporting Information) summarizes the experimental conditions and properties of the QDs prepared from Core 1 using the Cd/Zn molar ratio of 1/2. When the amount of Cd precursor was decreased, the red-shift of PL becomes smaller and PL efficiency decreased compared with that of the core shell QDs prepared by using the Cd/Zn molar ratio of 1/1 shown in Table 2. The red shift of PL spectra is large compared with CdSe/CdS core shell QDs shown in Table S1. For example, samples 2 5 (Table S1, d = 4.3 nm) and 3 4 (Table S2, d = 4.2 nm) have similar sizes but different PL peak wavelengths (see Supporting Information). This is ascribed that the Cd/Zn ratio makes CdS and ZnS shell with different alloy structures by interfusion at high reaction temperature.26 The Cd/ Zn ratio also affects the PL efficiency. The ratio can be optimized to maximize the PL efficiency with a maximum value. Figure S2 14458

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Figure 5. Absorbance and PL spectra of CdSe cores and CdSe/ CdxZn1 xS core shell QDs prepared using Core 4 shown in Table 3. The PL efficiencies of CdSe cores and CdSe/CdxZn1-xS core shell QDs are 1.1 and 60.8%, respectively.

Figure 6. TEM images of CdSe cores (core 4 shown in Table 1 (a)) and CdSe/CdxZn1-xS QDs (b) shown in Table 3. Well-developed lattice fringes were observed for the cores and core shell QDs. Figure S5 shows the size distribution of these samples (see Supporting Information).

(see Supporting Information) shows the TEM images of CdSe/ CdxZn1 xS core shell QDs prepared by a molar ratio of Cd/Zn of 1/2 under different reaction time such as (a) sample 3 1 (reaction time of 5 min) and (b) sample 3 4 (reaction time of 20 min). The mean diameters of the QDs are 3.8 and 4.2 nm after reaction, respectively. To obtain longer PL peak wavelength, CdSe cores with different sizes were used to prepare shells on it. Table S3 (see Supporting Information) summarizes the reaction time and properties of the core shell QDs prepared from core 2 using the Cd/Zn molar ratio of 1/1. Table S4 (see Supporting Information) summarizes the reaction time and properties of the core shell QDs prepared from core 3 using the Cd/Zn molar ratio of 1/1. The PL peak wavelength of the core shell QDs was red-shifted when the size of the core increases as shown in Tables S3 and S4 (see Supporting Information), respectively. The fwhm of PL spectra become very narrow (23 25 nm) when cores 2 and 3 were used. This is ascribed to the narrow size distribution of cores 2 and 3 prepared at 325 °C. Furthermore,

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Figure 4 shows the TEM images of CdSe cores (core 3 shown in Table 1) and CdSe/CdxZn1 xS core shell QDs with different reaction time, such as (a) core 3; (b) sample 5 1 (reaction for 5 min); (c) sample 5 4 (reaction for 20 min). Figure 4c (2) shows well-developed lattice fringes of the core shell QDs. Obviously, the core shell QDs exhibited narrow size distribution which resulted in the narrow PL spectra. The PL properties of CdSe/CdxZn1 xS core shell QDs depended strongly on the properties of CdSe cores. Table 3 shows the reaction time and properties of the core shell QDs prepared from core 4 using the Cd/Zn molar ratio of 1/1. Core 4 with rod morphology and a PL peak wavelength of 623 nm were prepared when TOPSe was injected slowly at 325 °C. Figure 5 shows the absorbance and PL spectra of CdSe cores (Core 4 shown in Table 1) and CdSe/CdxZn1 xS QDs (sample 6 shown in Table 3) prepared by using the Cd/Zn molar ratio of 1/1 and reaction time of 15 min. CdSe/CdxZn1 xS core shell QDs exhibited a PL peak wavelength of 652 nm. The PL efficiencies of CdSe cores and CdSe/CdxZn1 xS QDs are 1.1 and 60.8%, respectively. The extraordinary improvement of PL efficiency indicated current method should be crucial for the preparation of other QDs. Figure 6 shows the TEM images of CdSe cores (core 4 shown in Table 1) and the QDs shown in Figure 5. The cores exhibited rod shape of 8.5 nm in the length with an average aspect ratio of 1.7. However, the CdSe/CdxZn1-xS core shell QDs revealed spherical morphology and had mean size of 10 nm in diameter. This means the shell growth occurred at the sides of the CdSe nanorod and not at the ends. As mentioned before, CdSe has a hexagonal crystal structure and prefers to grow in the {002} direction when well-controlled conditions were used. CdS shells grown preferentially on the {00i} facet of the CdSe core.19 CdxZn1 xS shell did not grow on ̅ the reactive {002} planes at the CdSe nanorod ends. Therefore, the average diameter CdSe/CdxZn1 xS core shell QDs become similar to the length of the CdSe cores. Similar growth kinetics was used to prepare colloidal CdS, CdSe, and CdTe quantum rods and quantum rod heterostructures.23

’ CONCLUSIONS We have synthesized CdSe cores under relatively high temperature reaction conditions using a facile organic route. The resulting CdSe cores exhibited tunable size and morphology. The properties of these cores depended strongly on the injection speed of TOPSe and reaction temperature. Furthermore, these cores were coated with CdxZn1 xS shells by using OA as a capping agent. The CdSe/CdxZn1 xS core shell QDs exhibited high quality such as high PL efficiency (nearly 90%), tunable PL peak wavelength (from green to red), and narrow PL spectra (a fwhm of 23 nm for the best case). The experimental conditions of the core shell QDs were optimized to improve the PL properties. The molar ratio of Cd/Zn plays an important role for the improvement of QD PL efficiency. Because of the extraordinary PL properties, the core shell QDs are crucial for further applications. ’ ASSOCIATED CONTENT

bS

Supporting Information. Tables depicting reaction time and properties of CdSe/CdS QDs prepared by using core 1, reaction time and properties of CdSe/CdxZn1 xS QDs prepared

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The Journal of Physical Chemistry C by using core 1 and the Cd/Zn molar ratio of 1/2, reaction time and properties of CdSe/CdxZn1 xS QDs prepared by using core 2 and the Cd/Zn molar ratio of 1/1, reaction time and properties of CdSe/CdxZn1 xS QD prepared by using core 3 and the Cd/Zn molar ratio of 1/1. Figures depicting the previously mentioned reaction times and properties and size distributions of CdSe cores. This material is available free of charge via the Internet at http://pubs.acs.org.

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(26) Klein, D. L.; Roth, R.; Lim, A.; Alivisatos, A. P.; McEuen, P. L. Nature 1997, 389, 699–701.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Fax: +81-72-751-9637.

’ ACKNOWLEDGMENT This work was supported in part by Core Research for Evolutional Science and Technology (CREST) from the Japan Science and Technology Agency (JST). ’ REFERENCES (1) Anderson, R. E.; Chan, W. C. W. ACS Nano 2008, 2, 1341–1352. (2) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. Rev. 2005, 105, 1025–1102. (3) Park, J.; Joo, J.; Kwon, S. G.; Jang, Y.; Hyeon, T. Angew. Chem., Int. Ed. 2007, 46, 4630–4660. (4) Peng, X.; Schlamp, M. C.; Kadavanich, A. V.; Alivisatos, A. P. J. Am. Chem. Soc. 1997, 119, 7019–7029. (5) Hines, M. A.; Guyot-Sionnest, P. J. Phys. Chem. 1996, 100, 468–471. (6) Dabbousi, B. O.; Rodriguez-Viejo, J.; Mikulec, F. V.; Heine, J. R.; Mattoussi, H.; Ober, R.; Jensen, K. F.; Bawendi, M. G. J. Phys. Chem. B 1997, 101, 9463–9475. (7) Youn, H. C.; Baral, S.; Fendler, J. H. J. Phys. Chem. 1988, 92, 6320–6327. (8) Tenne, R. Angew. Chem., Int. Ed. 2003, 42, 5124–5132. (9) Cozzoli, P. D.; Manna, L. Nat. Mater. 2005, 4, 801–802. (10) Duan, H.; Nie, S. J. Am. Chem. Soc. 2007, 129, 3333–3336. (11) Lalatonne, Y.; Richardi, J.; Pileni, M. P. Nat. Mater. 2004, 3, 121–125. (12) Peng, Z. A.; Peng, X. G. J. Am. Chem. Soc. 2001, 123, 183–184. (13) Sheng, W.; Kim, S.; Lee, J.; Kim, S. W.; Jensen, K.; Bawendi, M. G. Langmuir 2006, 22, 3782–3790. (14) Barrelet, C. J.; Wu, Y.; Bell, D. C.; Lieber, C. M. J. Am. Chem. Soc. 2003, 125, 11498–11499. (15) Trindade, T.; O’Brien, P.; Zhang, X. M. Chem. Mater. 1997, 9, 523–530. (16) Xie, R.; Kolb, U.; Li, J.; Basch, T.; Mews, A. J. Am. Chem. Soc. 2005, 127, 7480–7488. (17) Liang, D.; Shen, L.; Wang, Z.; Cui, Y.; Zhang, J.; Ye, Y. Chin. Phys. Lett. 2008, 25, 4431–4434. (18) Talapin, D. V.; Mekis, I.; G€otzinger, S.; Kornowski, A.; Benson, O.; Weller, H. J. Phys. Chem. B 2004, 108, 18829–11831. (19) Talapin, D. V.; Koeppe, R.; G€otzinger, S.; Kornowski, A.; Lupton, J. M.; Rogach, A. L.; Benson, O.; Feldmann, J.; Weller, H. Nano Lett. 2003, 3, 1677–1681. (20) Jun, S.; Jang, E.; Lim, J. E. Nanotechnology 2006, 17, 3892–3896. (21) Murase, N.; Li, C. L. J. Lumin. 2008, 128, 1896–1903. (22) Peng, Z. A.; Peng, X. J. Am. Chem. Soc. 2001, 123, 1389–1395. (23) Shieh, F.; Saunders, A. E.; Korgel, B. A. J. Phys. Chem. B 2005, 109, 8538–8542. (24) Yang, S.; Prendergast, D.; Neaton, J. B. Nano Lett. 2010, 10, 3156–3162. (25) Pons, T.; Lequeux, N.; Mahler, B.; Sasnouski, S.; Fragola, A.; Dubertret, B. Chem. Mater. 2009, 21, 1418–1424. 14460

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