J. Phys. Chem. C 2007, 111, 5661-5666

5661

Low-Temperature Synthesis of Oil-Soluble CdSe, CdS, and CdSe/CdS Core-Shell Nanocrystals by Using Various Water-Soluble Anion Precursors Daocheng Pan, Qiang Wang, Shichun Jiang, Xiangling Ji,* and Lijia An* State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Graduate School of the Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, People’s Republic of China ReceiVed: NoVember 23, 2006; In Final Form: February 2, 2007

Colloidal CdSe and CdS quantum dots were synthesized at low temperatures (60-90 °C) by a two-phase approach at a toluene-water interface. Oil-soluble cadmium myristate (Cd-MA) was used as cadmium source, and water-soluble Na2S, thiourea, NaHSe, Na2SeSO3, and selenourea were used as sulfur and selenium sources, respectively. When a cadmium precursor in toluene and a selenium precursor in water were mixed, CdSe nanocrystals were achieved at a toluene-water interface in the range of 1.2-3.2 nm in diameter. Moreover, we also synthesized highly luminescent CdSe/CdS core-shell quantum dots by a two-phase approach using poorly reactive thiourea as sulfur source in an autoclave at 140 °C or under normal pressure at 90 °C. Colloidal solutions of CdSe/CdS core-shell nanocrystals exhibit a photoluminescence quantum yield (PL QY) up to 42% relative to coumarin 6 at room temperature.

Introduction Metal and semiconductor nanocrystals play an important role in the development of functional nanoscale materials and devices.1 Synthesis of such nanocrystals is an important topic in the field of material science. Through a two-phase approach, some noble metal nanocrystals such as Au,2 Ag,3 Pt,4 and Pd5 nanocrystals have been synthesized at low temperatures. As two important group II-VI semiconductor materials, CdSe and CdS nanocrystals have received considerable interest of researchers because of potential applications in light-emitting diode (LED),1c,d solar cells,1f and biological labeling.1g Numerous approaches such as homogeneous phase precipitation,6 reverse micelle,7 and organometallic approach8 and its variants9 have been applied to prepare CdSe and CdS nanocrystals. Moreover, highly luminescent CdSe/CdS and CdSe/ZnS core-shell nanocrystals have also been prepared through organometallic approach10 and its variants.11 However, these reactions mentioned above were all carried out in organic phase or aqueous phase, and both nucleation and growth of the nanocrystals only happened in a homogeneous system. It is very difficult for organic-phase approaches to synthesize oil-soluble nanocrystals by using various water-soluble precursors. Recently, we developed a versatile two-phase approach to synthesize highly luminescent CdS,12 extremely small CdSe,13 TiO2,14 ZrO2,15 and so forth nanocrystals. The oil-soluble CdSe and CdS nanocrystals can be synthesized by using water-soluble thiourea and selenourea as sulfur source and selenium source, respectively. The nanocrystals have a very long nucleation and growth stage because of a slow decomposition of thiourea and selenourea. It was found that a slow nucleation does not always lead to polydisperse nanocrystals if the growth time is long enough. In this paper, we prepared CdSe nanocrystals with a relatively narrow size distribution through a rapid nucleation and a rapid * To whom correspondence should be addressed. Phone: +86-43185262876 and +86-431-85262206. E-Mail: [email protected] (L.A.) and [email protected] (X.J.).

growth using highly reactive NaHSe as selenium source or a slow nucleation and a slow growth using poorly reactive Na2SeSO3 or selenourea as selenium source via a two-phase approach at a toluene-water interface. Oil-soluble cadmium myristate was used as cadmium source, and n-trioctylphosphine oxide (TOPO) and n-trioctylphosphine (TOP) were used as capping agents. Cadmium myristate, TOPO, and TOP can be readily dissolved in toluene above 60 °C. The CdSe nanocrystals were achieved at a liquid-liquid interface when a toluene solution of cadmium myristate and an aqueous solution of NaHSe were mixed under stirring. The surface of nanocrystals was capped by a monolayer of n-trioctylphosphine oxide (TOPO) and n-trioctylphosphine (TOP) as the nanocrystals start to grow at a liquid-liquid interface. The size of resulting nanocrystals is very small in the range of 1.2-3.2 nm in diameter, because the duration of nucleation and growth for the nanocrystals is very short. Scheme 1 illustrates a proposed mechanism for formation of CdSe nanocrystals at a toluene-water interface by using watersoluble NaHSe, Na2SeSO3, and selenourea as selenium precursors, respectively. Moreover, we also synthesized highly luminescent CdSe/CdS core-shell nanocrystals using thiourea as sulfur source through a two-phase approach in an autoclave at 140 °C or under normal pressure at 90 °C. Experimental Section I. Chemicals. Cadmium oxide (99.5%), sodium borohydride (99%), myristic acid (MA, 99.5%), oleic acid (OA, 90%), TOP, and TOPO (tech, 90%) were purchased from Aldrich. Thiourea (99%) and selenourea (98%) were obtained from Alfa, and 1-dodecanethiol (98%) and coumarin 6 (98%) were purchased from Acro¨s. II. Synthesis of Cd-MA. CdO (1.926 g, 15 mmol) and myristic acid (7.5 g, 33 mmol) were loaded into a flask and were heated to 210 °C for 10 min. An optically clear solution was obtained. The crude product was recrystallized twice from

10.1021/jp0678047 CCC: $37.00 © 2007 American Chemical Society Published on Web 03/24/2007

5662 J. Phys. Chem. C, Vol. 111, No. 15, 2007 SCHEME 1: A Proposed Mechanism for a Formation of Oil-Soluble CdSe Nanocrystals at a Toluene-Water Interface by Using Water-Soluble NaHSe, Na2SeSO3, and Selenourea as Selenium Precursors, Respectively

toluene. The Cd-MA was dried in an oven and was used for further reaction. III. Synthesis of CdSe and CdS Nanocrystals using NaHSe and Na2S as Selenium and Sulfur Precursors. Some reactions were run under a nitrogen atmosphere to prevent oxidation of the oxygen-sensitive selenide ions. As a typical example, Cd-MA (0.2268 g, 0.4 mmol), TOPO (1 g), TOP (1 mL), and toluene (10 mL) were placed in a flask and the mixture was heated to 100 °C for 10 min until an optically clear solution was obtained. A freshly prepared aqueous solution (10 mL, 0.02 M) of NaHSe16 or Na2S was swiftly injected into the above flask under stirring. Immediately, crimson and green colloids were produced for CdSe and CdS nanocrystals. The system was kept at 80 °C for 20 min for the growth of CdSe or CdS nanoparticles. At 20 min of reaction time, the second injection of aqueous solution (10 mL, 0.02 M) of NaHSe or Na2S was carried out dropwise (approximately one drop per second) to the flask. When the reaction lasted for 60 min, organic phase was separated from the crude solution. Insoluble solid was separated by centrifugation and decantation prior to further purification. The resulting nanocrystals in toluene solution were precipitated with methanol and were further isolated by centrifugation and decantation. The purified nanocrystals were redispersed in toluene for UV-vis, photoluminescence (PL), transmission electron microscopy (TEM), and X-ray diffraction (XRD) measurements without any size sorting. IV. Synthesis of CdSe/CdS Core-Shell Nanocrystals in an Autoclave. A two-step method was exploited to synthesize CdSe/CdS core-shell nanocrystals. At first, the CdSe core nanocrystals were prepared by a rapid nucleation and growth using NaHSe as selenium source. Cd-MA (0.2 mmol), TOPO (0.5 g), and toluene (10 mL) were added to a flask, and the reaction mixture was heated to 60 °C for 10 min. An optically clear solution was obtained. A freshly prepared aqueous solution (10 mL, 0.02 M) of NaHSe was swiftly injected into the flask under stirring. The reaction lasted for 30 min for the growth of CdSe cores. In the second step, about 8 mL of the crude solution of CdSe nanocrystals, Cd-MA (0.2 mmol), and TOPO (0.5 g) were put into a 30 mL Teflon-lined stainless steel autoclave, and it was heated to 80 °C for 5 min and then was cooled down to room temperature. Thiourea (0.5 mmol) was dissolved in 10 mL of water and the solution was transferred into the autoclave without stirring. The autoclave was sealed and

Pan et al. maintained at 140 °C for 4 h. Finally, the autoclave was cooled to the room temperature with tap water. V. Synthesis of CdSe Nanocrystals Using Selenourea as Selenium Precursor. Loaded into a Teflon-lined stainless steel autoclave, 0.1134 g of Cd-MA, 1.0 mL of oleic acid, and 10 mL of toluene were heated until Cd-MA was dissolved. Then, the solution was cooled to room temperature. After the 0.013 g of selenourea was dissolved in 10 mL of N2-saturated water, the solution was transferred to the autoclave. The autoclave was sealed and maintained at 180 °C for 15-90 min. Finally, the autoclave was cooled to room temperature with tap water. The CdSe crude solution was precipitated with methanol and was further isolated by centrifugation and decantation. VI. Synthesis of CdSe/CdS Core-Shell Nanocrystals under Normal Pressure. First, CdSe cores were obtained by a two-phase approach in the autoclave using selenourea as selenium precursor. The autoclave was maintained at 180 °C for 18 min. Then, the CdSe crude solution was precipitated with methanol and was further isolated by centrifugation and decantation. The purified nanocrystals were redispersed in 10 mL of toluene. Finally, 10 mL of purified CdSe solution, 1.0 mL of oleic acid, and 0.1134 g of Cd-MA were added to a flask, and the mixture was heated to 100 °C for 10 min. Forty milligrams of thiourea was dissolved in 10 mL of water, and this solution was injected into the flask under magnetic stirring condition. The reaction mixture was kept at 90 °C for 150 min. Aliquot solutions (0.20 mL) were taken from organic phase at different reaction times for UV-vis and PL measurements. The crude solution was purified with methanol prior to measurements. VII. Synthesis of CdSe Nanocrystals Using Na2SeSO3 as Selenium Precursor. Added to a flask were 0.1134 g of Cd-MA, 0.5 g of TOPO, and 10 mL of toluene, and the mixture was heated to 90 °C for 10 min. A 0.02 M aqueous solution (10 mL) of Na2SeSO3 was injected into the flask for the nucleation and growth of CdSe nanocrystals. The system is kept at 90 °C for 180 min. UV-vis absorbance and PL spectra were recorded at different reaction times. The Na2SeSO3 solution was prepared according to the following reaction: Na2SO3 + Se f Na2SeSO3. Placed into a flask were 0.39 g of selenium, 1.58 g of Na2SO3, and 20 mL of water, and the mixture was heated to 90 °C for 10 h. After cooling to room temperature, the solution was filtered off and diluted to 250 mL in a volumetric flask with deionized water. The Na2SeSO3 solution was stored in dark and was used within 3 days. VIII. Synthesis of CdS Nanocrystals Using Thiourea as Sulfur Precursor. Added to a flask were 0.2268 g of Cd-MA, 1.0 mL of OA, and 10 mL of chlorobenzene, and the mixture was heated to 90 °C for 10 min. A 0.1 M aqueous solution (10 mL) of thiourea was injected into the flask for the nucleation and growth of CdS nanocrystals. The system was kept at 90 °C for 150 min. UV-vis absorbance and PL spectra were recorded at different reaction times. IX. Synthesis of DDT-Capped CdS Nanocrystals. Added to a flask were 0.2268 g of Cd-MA, 1.0 mL of 1-dodecanethiol, and 10 mL of toluene, and the mixture was heated to 100 °C for 10 min. A 0.02 M aqueous solution (10 mL) of Na2S was injected into the flask. The system was kept at 90 °C for 24 h. The reaction did not happen. Then, a 0.1 M toluene solution of phase-transfer reagent tetra-n-octylammonium bromide was injected into the flask. X. Characterization of Samples. UV-vis absorption and PL spectra were recorded on a Shimadzu UV-2450 PC spectrometer and a Shimadzu RF-5301 PC fluorometer with a

Synthesis of Oil-Soluble Core-Shell Nanocrystals

Figure 1. UV-vis absorption and PL spectra of TOPO-TOP capped CdSe nanocrystals using NaHSe as selenium precursor.

resolution of 1.0 nm, respectively. Room-temperature PL quantum yields (PL QYs) were calculated against coumarin 6 in ethanol as a standard sample (QY ) 0.78).17 The absorbances of nanocrystal samples and standard sample at the excitation wavelength (400 nm) are similar and small (about 0.05) to avoid a self-absorbance. The XRD patterns were obtained using a Rigaku D/MAX2500 using Cu KR1 radiation and employing a scanning speed of 0.02°/s in the range of 10-60°. XRD samples were prepared by evaporating a drop of concentrated nanocrystal solution on a glass plate.

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Figure 2. UV-vis absorption and PL spectra of the CdSe cores synthesized under normal pressure at 60 °C using NaHSe as selenium precursor and the corresponding CdSe/CdS core-shell nanocrystals prepared in an autoclave at 140 °C using thiourea as sulfur precursor.

Results and Discussion I. Synthesis of CdSe Nanocrystals Using Differently Reactive NaHSe, Selenourea, and Na2SeSO3 as Selenium Precursors. Figure 1 shows typical UV-vis absorption and PL spectra of CdSe nanocrystals obtained through a two-phase approach by using NaHSe as selenium precursor. The luminescence of TOPO-TOP capped CdSe nanocrystals is dominated by near-band-edge luminescence. The Stokes shift is very small (15 nm) and the fwhm (full width at half-maximum) is so narrow (30 nm) which suggested a regular surface of particles and a narrow size distribution. The CdSe nanocrystals is about 3.2 nm in diameter calculated from the first excitonic absorption peak of UV-vis absorption spectrum.18 Definitely, the size of CdSe nanoparticles is smaller than those obtained by organometallic route or its variants resulting from the more rapid nucleation and growth, that is, NaHSe has a faster reaction rate than selenium powder. To prepare CdSe nanocrystals with sizes more than 2.0 nm, additional injections of precursor were required. For comparison, the preparation of CdSe nanocrystals was carried out at a lower temperature while keeping the other conditions the same. The size of CdSe nanocrystals was smaller (∼1.5 nm) when the reaction temperature was at 60 °C (Figure 2). In general, the CdSe nanocrystals synthesized by using NaHSe as precursor exhibit quantum yields of 8-10% relative to coumarin 6 at room temperature. Figure 2 shows UV-vis absorption and PL spectra of the CdSe cores and corresponding CdSe/CdS core-shell nanocrystals. At room temperature, PL spectrum of the CdSe cores shows a shallow trap emission because of the incomplete surface passivation. The PL for CdSe/CdS core-shell nanocrystals is dominated by near-band-edge emission, and quantum yield can reach as high as 42% relative to coumarin 6, which indicates the surface traps are markedly removed. The growth of the CdS shell causes about 50 nm of red-shift in both UV-vis absorption and PL spectra. Similar shifts due to the leakage of the exciton into the shell have previously been reported for CdSe/ZnS10a,b

Figure 3. Temporal evolution of UV-vis absorption and PL spectra of OA-capped CdSe nanocrystals using selenourea as selenium source.

or CdSe/CdS11c systems. It was found that TOP is not necessary to prepare high-quality CdSe and CdSe/CdS core-shell nanocrystals. So, TOPO is used as sole capping agent to synthesize CdSe and CdSe/CdS core-shell nanocrystals. Herein, we took advantage of an autoclave to obtain a high temperature and pressure for the growth of CdSe/CdS core-shell nanocrystals. Meanwhile, CdSe/CdS core-shell nanocrystals under normal pressure at 90 °C were also prepared. Measurements proved that the two-phase synthesis in the autoclave is more reproducible than in a normal pressure because the stirring is not necessary in the autoclave. Figure 3 shows temporal evolution of UV-vis absorption and PL spectra of CdSe nanocrystals in the presence of OA in the size range of 1.2-3.0 nm using selenourea as selenium source in the autoclave. CdSe nanocrystals cannot be obtained for 15 min. During the first 20 min, the width of PL spectrum decreases, and the width of PL spectrum markedly increases with broadening of the size distribution during the reaction time from 25 min to 90 min. The room-temperature PL QY of asprepared CdSe nanocrystals is in the range of 7-35% and has a tendency to increase with increasing the nanocrystal size and reaction time. Figure 4 shows UV-vis absorption and PL spectra of OAcapped CdSe cores synthesized at 180 °C under high pressure and corresponding CdSe/CdS core/shell nanocrystals synthesized under normal pressure at 90 °C and temporal evolution of PL spectra of CdSe/CdS core-shell nanocrystals during growth of CdS shell. The CdSe cores are about 1.6 nm in diameter18 and show a band-edge emission as well as a shallow trap emission because of the incomplete surface passivation. It was found that

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Pan et al.

Figure 6. UV-vis absorption and PL spectra of TOPO-TOP capped CdS nanocrystals using Na2S as sulfur source.

Figure 4. UV-vis absorption and PL spectra of OA-capped CdSe cores synthesized under high pressure at 180 °C and corresponding CdSe/CdS core/shell nanocrystals synthesized under normal pressure at 90 °C and temporal evolution of PL spectra of CdSe/CdS coreshell nanocrystals during growth of CdS shell.

Figure 5. Temporal evolution of UV-vis absorption and PL spectra of TOPO-capped CdSe nanocrystals using Na2SeSO3 as selenium source.

TABLE 1: Sizes, FWHMs, and QYs for CdSe Nanocrystals Synthesized Using Different Sources precursors

reactivity

sizes (nm)

fwhm (nm)

QYs (%)

NaHSe selenourea Na2SeSO3

high medium low

1.2-2.0 1.2-3.0 2.0-3.2

30 28-45 30

8-10 7-35 1-3

the trap emission could be removed completely if the reaction time is more than 30 min when CdS shell is formed on the CdSe cores, and the continuous red-shift was observed in the UV-vis absorption and PL spectra of CdSe/CdS core-shell nanocrystals with the growth of CdS shell. The quantum yields of CdSe/CdS core-shell nanocrystals can be as high as 3040%. Although these quantum yields of CdSe/CdS core-shell nanocrystals synthesized under normal pressure are markedly lower than those synthesized under high pressure in the autoclave (60-80%),13 the growth CdS shell under normal pressure is more convenient to observe temporal evolution of absorption, PL spectra, and quantum yields of nanocrystals, because it is difficult to take aliquot solution from the autoclave at high temperature. Moreover, the growth CdS shell under normal pressure has a lower reaction temperature and can markedly eliminate the isolated nucleation of CdS, that is, forming CdS nanocrystals instead of CdSe/CdS core-shell nanocrystals.

Figure 5 shows temporal evolution of UV-vis absorption and PL spectra of TOPO-capped CdSe nanocrystals using poorly reactive Na2SeSO3 as selenium source. The nanocrystals have a very long nucleation and growth stage because the Na2SeSO3 was slowly decomposed. The size of nanocrystals was easily controlled by varying reaction time. The reaction is more reproducible and controllable than that using highly reactive NaHSe as selenium source. The fwhm in the PL spectra of CdSe nanocrystals is at about 30 nm during the reaction time from 20 min to 180 min. This suggests that no evident defocusing and focusing of the size distribution is happening. In general, a fast nucleation and a slow growth are favorable for preparation of monodisperse nanocrystals under diffusion control because the small nanocrystals have a faster rate than the big nanocrystals.19 When highly reactive NaHSe was used as selenium source, the nanocrystals with a narrow size distribution could be obtained because of a very fast nucleation rate. When poorly reactive Na2SeSO3 was used as selenium source, the nanocrystals with a narrow size distribution could also be obtained because of a very slow growth rate. In case of medium reactive selenourea as selenium source, the CdSe nanocrystals do not also show a broad size distribution. So, the narrow size distributions that are independent of precursors always can be obtained by a two-phase approach. However, the Ostwald ripening is more evident for those reactions using highly reactive precursors, such as NaHSe and selenourea. For CdSe nanocrystals using differently reactive NaHSe, Na2SeSO3, and selenourea as selenium sources, we compared their sizes, fwhm, and quantum yields in Table 1. II. Synthesis of CdS Nanocrystals Using Differently Reactive Na2S and Thiourea as Sulfur Precursors. In this part, we made two CdS nanocrystal samples by using highly reactive Na2S and poorly reactive thiourea as sulfur source. In Figure 6, PL spectrum of TOPO-TOP capped CdS nanocrystals using Na2S as sulfur source shows a shallow trap emission resulting from lower capping densities, and similar trap emission has been observed for TOPO-capped CdS using thiourea as sulfur precursor.12a A sharp first excitonic absorption peak at 400 nm and a narrow PL band of CdS nanocrystals indicate that nanocrystals have a narrow size distribution. This reaction has a poor reproducibility when Na2S is used as sulfur source resulting from a too fast reaction rate. For each time, the size of CdS nanocrystals might be different at the same temperature and reaction time. It is difficult to control the nanocrystal sizes by reaction time. To control the nanocrystal sizes by reaction time, the reaction rate must be slowed down. Herein, the poorly reactive thiourea was used as sulfur source to synthesize CdS nanocrystals in a water/chlorobenzene two-phase system. Figure 7 shows the

Synthesis of Oil-Soluble Core-Shell Nanocrystals

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Figure 7. Temporal evolution of UV-vis absorption and PL spectra of OA-capped CdS nanocrystals using thiourea as sulfur source. The reaction time is 30, 35, 40, 45, 60, 75, 90, 110, 130, and 150 min from bottom to top, respectively.

Figure 9. UV-vis absorption spectrum of DDT-capped CdS nanocrystals.

Figure 8. XRD patterns of TOPO-TOP capped CdSe and CdS nanocrystals synthesized using NaHSe and Na2S as selenium and sulfur precursors. Vertical lines indicate pure CdSe and CdS reflections (top: zinc blende, CdSe; bottom: zinc blende, CdS). The crystallite sizes were calculated from the (111) peak.

The CdSe and CdS nanocrystals could also be synthesized using cadmium acetate as cadmium source instead of Cd-MA. However, their PL QYs are much lower than those synthesized using Cd-MA as cadmium source. If a strong ligand such as 1-dodecanelthiol (DDT) was used as capping agent, the reaction could not happen even at 100 °C for 24 h because long-chain aliphatic thiols/cadmium complexes are more stable than CdSe and CdS and inhibit the nucleation process9b. However, if we added some tetra-n-octylammonium bromide as phase-transfer reagent, the reaction happened immediately. The nucleation and growth could happen if a large number of sulfide ions were transferred into organic phase by the phase-transfer agents, and CdS nanocrystals were possibly formed in organic phase instead of at the toluene-water interface. Figure 9 shows the UV-vis absorption spectrum of DDT-capped CdS nanocrystals. The sharp first excitonic absorption peak also indicates that the size distribution is relatively narrow. The size of DDT-capped CdS nanocrystals is about 2.0 nm calculated from the first excitonic absorption peak of UV-vis absorption spectrum.

temporal evolution of UV-vis absorption and PL spectra of OA-capped CdS nanocrystals during the reaction time from 30 min to 150 min. The trap emission was not observed for OAcapped CdS nanocrystals. The band-edge absorption and PL peaks of CdS nanocrystals show a slow and continuous redshift with the reaction time. The nanocrystals have a very slow growth rate resulting from a slow decomposition rate of thiourea. This nucleation and growth processes are similar to those used Na2SeSO3 to make CdSe nanocrystals. The fwhm in the PL spectra of CdS and CdSe nanocrystals does not change much with reaction time. Ostwald ripening did not happen for these two cases, which indicated the size distributions have no defocusing or focusing in nanocrystal synthesis. The crystal structures of CdSe and CdS nanocrystals are confirmed by the X-ray powder diffraction patterns in Figure 8. For CdSe and CdS nanocrystals, the peak positions match well with the theoretical values of the cubic structure of CdSe (JCPDS No.19-0191) and CdS (JCPDS No.10-0454), respectively. No characteristic peaks of other impurities were observed, and all the reflections could be indexed to the pure cubic phase CdSe and CdS. In general, a high reaction temperature is favorable for formation of hexagonal nanocrystals, and a low reaction temperature usually leads to cubic nanocrystals. The broad diffraction peaks indicate the smaller size of nanocrystals. The crystallite sizes of TOPO-TOP capped CdSe and CdS nanocrystals calculated using the Sherrer formula20 are 2.8 and 2.4 nm, respectively, which are smaller than those determined from TEM images.

Conclusions In summary, a two-phase approach under mild conditions has been developed to synthesize hydrophobic CdSe and CdS quantum dots with a narrow size distribution using various water-soluble precursors. Room-temperature photoluminescence spectra of nanocrystals show the near-band-edge emission for CdSe nanocrystals and the shallow trap emission for CdS nanocrystals. A two-phase approach combined with the autoclave was applied to synthesize highly luminescent CdSe/CdS core-shell nanocrystals. Through our two-phase approach, nanocrystals with narrow size distributions could be obtained by a slow nucleation and a slow growth using poorly reactive Na2SeSO3 or selenourea as selenium source or a rapid nucleation and a rapid growth using highly reactive NaHSe as selenium source. So, it provides a simple and controllable synthetic route to prepare binary oil-soluble nanocrystals under mild conditions by using various water-soluble precursors. Acknowledgment. This work was supported by the National Natural Science Foundation of China for General (General: 90101001, 20674085, 20674086; Key: 50633030; Creative Research Group: 50621302), “863” Project (2006AA03Z224), the Special Pro-Funds for Major State Basic Research Projects (2002CCAD4000), the Special Funds for Major State Basic Research Projects (No. 2003CB615600) and the Distinguished Young Fund of Jilin Province (20050104), the Project (KJCX2SW-H07) from the Chinese Academy of Sciences, and the

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