liquid system

Colloids and Surfaces A: Physicochem. Eng. Aspects 506 (2016) 378–382

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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa

Facile synthesis of CdSe quantum dots in a high-boiling two-phase liquid/liquid system Ying Zhang a , Xiaobo Nie b,∗ a b

University of South China, Hengyang 421001, Hunan, PR China College of Chemistry and Chemical Engineering, University of South China, Hengyang 421001, Hunan, PR China

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• High-quality CdSe quantum dots (QDs) are prepared in a high boiling two-phase liquid/liquid system. • The growth of CdSe nanocrystals can be controlled to obtain different sized CdSe QDs. • The two-phase approach may be scaled and broadened to synthesis other semiconductor nanocrystals.

a r t i c l e

i n f o

Article history: Received 26 April 2016 Received in revised form 2 July 2016 Accepted 6 July 2016 Available online 6 July 2016 Keywords: Cadmium selenide Quantum dots Nanocrystals Synthesis Two-phase approach

a b s t r a c t We introduce a facile way to synthesize high-quality CdSe quantum dots (QDs) in a high-boiling twophase liquid/liquid system. The cadmium myristic acid salt (Cd-MA) dispersed in the dimethylbenzene and selenourea dissolved in the ethylene glycol (EG) reacted at the two-phase interface for the nucleation and growth of nanocrystals. The n-trioctylphosphine oxide (TOPO) was used as capping agent of the CdSe QDs. In this route, the selenourea used as the precursor was dissolved in viscous ethylene glycol (EG), which facilitated the control of the nanocrystal growth by virtue of the slow decomposition and diffusion of selenourea in EG. The size, shape, crystalline structure and optical properties of CdSe QDs were investigated by transmission electron microscopy (TEM), X-ray powder diffraction (XRD), UV–vis spectrometer and photoluminescence spectroscopy. This route could be exploited to synthesize nanocrystals with a large scale and could broaden to synthesis of metal or other semiconductor nanocrystals. © 2016 Elsevier B.V. All rights reserved.

1. Introduction

∗ Corresponding author. E-mail addresses: [email protected], [email protected] (X. Nie). http://dx.doi.org/10.1016/j.colsurfa.2016.07.013 0927-7757/© 2016 Elsevier B.V. All rights reserved.

Owing to the quantum confinement effect, semiconductor nanocrystals, especially the II–VI semiconductor nanocrystals, exhibit remarkable size-dependent optical properties [1]. Research

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on II–VI semiconductor nanocrystals has attracted considerable attention in the past two decades for both fundamental research and technical applications such as light-emitting diodes (LED) [2–5], solar cells [6–8], photocatalysts [9] and biological labels [10–13]. Among various kinds of II–VI semiconductor materials, cadmium selenide (CdSe) nanocrystals are most widely studied because of their tunable emission in the visible range. Tremendous efforts have been made on the synthesis of high-quality CdSe nanocrystals in the past two decades [14–25]. Also, the size and shape controls, properties and applications of the CdSe nanocrystals were focused on by the scientists. Among the methods employed for synthesizing CdSe quantum dots (QDs), the organometallic approach [14,15] and its variations [16–18] have been proved to be the most popular, although other methods [19–25] have also been very successful. The organometallic approach was first developed by Murray [14] in 1993, which involved the Cd(CH3 )2 and TOP/Se as the reaction precursors. It is well known that Cd(CH3 )2 is extremely toxic, pyrophoric, expensive, unstable at room temperature and explosive at elevated temperatures. Peng’s group [16,17], Li’s group [18,19] and Pan et al. [20,21] proved that CdO and some cadmium salts with an anion of a weak acid can be a replacement of Cd(CH3 )2 as cadmium resources. The selenium resources were supplied by Se powder, selenourea, SeSO3 2− , H2 Se, HSe− , and many others [18–20]. The organometallic approach and its variations can obtain high-quality semiconductor nanocrystals but with disadvantages of complicated preparation processes, strict and harsh conditions. Brust et al. [26] firstly used a two-phase approach to synthesize gold nanoparticles. Then, other noble metal nanoparticles such as Pt, Pd, Ag and so forth have been obtained [22]. Moreover, the two-phase approach has been successfully used by Ji’s group to synthesize II–VI semiconductor nanocrystals and magnetic nanoparticles [20–22]. In their works, cadmium precursors and surfactants were dissolved in toluene, selenium precursors were dissolved in water, the reaction between cadmium and selenium precursors occurred at the organic-water interface which provided nano-sized confiniments for the nucleation and growth of CdSe nanocrystals. SeSO3 2− , H2 Se and HSe− were commonly used as the selenium precursors, all of which were unstable and should be prepared immediately at a low temperature before using. Thus, the two-phase approach is facile and convenient for the synthesis of kinds of nanomaterials. Herein, we introduce a convenient, controllable synthetic method to synthesize CdSe QDs based on the two-phase thermal approach. In this approach, cadmium myristic acid salt (Cd-MA) as cadmium precursor and n-trioctylphosphine oxide (TOPO) as surfactant are dissolved in the dimethylbenzene, selenourea used as selenium precursor is dissolved in the ethylene glycol (EG). The precursors collided and reacted at the high-boiling two-phase liquid/liquid interface for the nucleation and growth of CdSe QDs. Comparable to the toluene-water system, the reaction temperature in the dimethylbenzene-EG two phase system can reach to 140 ◦ C at normal pressure, which is helpful for the formation of CdSe QDs with perfect crystallites. Therefore, we can obtain relatively highquality CdSe QDs. Additionally, the viscosity of EG is bigger than that of water. Thus, the diffusion rate of the selenourea is slower, by virtue of which we can effectively control the reaction rate of precursors. Owing to the simple reaction apparatus used and facile processes taken place, this novel method will have a good future to synthesize ZnSe, CdS, CdTe, and many other II–VI semiconductor nanocrystals. 2. Experimental 2.1. Materials Cadmium oxide (CdO, purity 99.95%), myristic acid (MA, purity ≥ 99%), and n-trioctylphosphine oxide (TOPO, purity 98%)

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were purchased from Aladdin. Selenourea (SeC(NH2 )2 , purity 99%) were obtained from Alfa Aesar. Dimethylbenzene, ethylene glycol (EG) and toluene were purchased from Beijing Chemical Reagent Company (Beijing, China). 2.2. Synthesis of CdSe QDs The cadmium myristic acid salt (Cd-MA) was synthesized by the reaction of cadmium oxide (CdO) and myristic acid (MA) according to the reference [20]. Typically, CdO (1.926 g, 15 mmol) and MA (7.500 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 toluene. Then, the CdMA was dried in an oven and was used for further reaction. In a typical synthesis of CdSe QDs, Cd-MA (113.4 mg, 0.2 mmol), TOPO (618.6 mg, 1.6 mmol), and dimethylbenzene (10 mL) were placed in a three-necked reaction flask. The vessel was heated to 140 ◦ C for a few minutes until an optically clear solution was obtained. Meanwhile, 12.3 mg of selenourea (0.1 mmol) was dissolved in 10 mL of N2 -saturated ethylene glycol by heating and ultrasonic treatment. The freshly prepared selenium precursor in EG was swiftly injected to the three-necked flask. Under N2 atmosphere, the cadmium and selenium precursors reacted in the flask at 140 ◦ C for one hour. After the heating treatment, the system was cooled to room temperature. Then, the organic phase was separated from the crude solution. The resulting nanocrystals in dimethylbenzene solution were precipitated with methanol and were further isolated by centrifugation and decantation. The process of precipitation-centrifugation-decantation was repeated for 3–5 times to completely removed the redundant TOPO, unreacted precursors and impurities. The purified CdSe QDs were well dispersed in toluene for further tests. 2.3. Characterization TEM measurement was performed on a JEOL JEM-1011 transmission electron microscopy operated at an accelerating voltage of 100 kV. The sample was prepared by depositing one drop of the purified CdSe QDs solution in toluene onto a 300 mesh copper grid coated with a carbon film. The sample was allowed to dry in air and at room temperature before observation. UV-vis absorption spectra and PL spectra of samples solution were recorded on a Shimadzu UV-2450 PC spectrometer and a Shimadzu RF-5301 PC fluorometer with a resolution of 1.0 nm, respectively. For the tests of PL spectra, the excitation wavelength was 380 nm, the excitation slit and emission slit were both 3 nm. X-ray powder diffraction (XRD) patterns of the prepared samples were recorded on a Philip PW1700 diffractometer equipped with graphite monochromatized Cu K␣ (␭ = 1.54178 Å) radiation at a scanning speed of 4◦ /min in range from 10◦ to 90◦ . XRD samples were prepared by evaporating a drop of concentrated CdSe QDs solution on a glass plate. 3. Results and discussion 3.1. Size and morphology We used a facile and mild two-phase approach to prepare CdSe nanocrystals, as illustrated in Fig. 1. A solution of Cd-MA and TOPO in dimethylbenzene and a solution of selenourea in ethylene glycol (EG) were mixed and heated with stirring. The Cd-MA and selenourea touched and reacted at the two-phase interface for the nucleation and growth of CdSe nanocrystals, as the selenourea slowly decomposed. The surfaces of the CdSe nanocrystals were capped by TOPO, which facilitated the well dispersion of CdSe nanocrystals in the dimethylbenzene. Compared with the

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Fig. 1. Schematic illustration of the formation of CdSe QDs in the two-phase system constituted by dimethylbenzene and ethylene glycol.

toluene-water system by Ji’s group [20,21] and Zhu et al. [27], the reaction temperature in the dimethylbenzene-EG two phase system can reach to 140 ◦ C under the normal pressure (without using an autoclave), which is helpful for the formation of high-quality semiconductor nanocrystals. The high temperature is favorable to the synthesis of monodisperse nanocrystals with perfect crystalline structures and good performances all of which are the necessary properties to define high-quality semiconductor nanocrystals [6,15–17,22]. For example, Peng et al. [16,17] set the growth temperature between 200 and 320 ◦ C to synthesize high-quality CdSe nanocrystals that show narrow size distribution (5–10% relative standard deviation), perfect crystalline feature, and high photoluminescence quantum efficiency up to 20–30%. Here, the viscosity of EG is bigger than that of water. Thus, the diffusion rate of the selenourea or other precursors is slower, by virtue of which we can effectively control the reaction rate of precursors. This is beneficial for us to obtain different sized CdSe QDs related to varied optical properties. In the typical synthesis of CdSe nanocrystals described in Section 2, the purified CdSe nanocrystals were dispersed in toluene for further tests by TEM, XRD, UV–vis absorption and PL spectrophotometer. In order to evaluate the size and morphology of CdSe nanocrystals, TEM measurements were carried out. Fig. 2 represents a typical micrograph and size distribution histogram of the CdSe QDs. It can be clearly seen from Fig. 2(a) that the CdSe nanocrystals are dot-shaped and quite uniform. In Fig. 2(b), the size of the CdSe nanocrystals is 2.7 ± 0.9 nm, and the size distribution agreed with a “Gauss Fit”. Thus, the nanocrystals obtained by the dimethylbenzene-EG two-phase approach are CdSe quantum dots (QDs). The radius of these nanocrystals is far smaller than the Bohr radius of bulk CdSe (5.4 nm), which indicates that the particle dimension is in the strong quantum confinement region. Such effects can be exhibited by the blue-shift of absorption edge as compared to that of bulk materials [27].

Fig. 2. Typical TEM image (a) and size distribution (b) of CdSe QDs.

Fig. 3. UV–vis absorption (black curve) and PL spectra (red curve) of CdSe QDs well dispersed in toluene. Insert is the photograph of the CdSe QDs solution emitting green color under the 365 nm UV light. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).

3.2. Optical properties Fig. 3 shows the typical UV–vis absorption (black curve) and PL (red curve) spectra of the CdSe QDs in toluene. A very well defined excitonic absorption peak at 539.4 nm can be seen, corresponding to an HOMO-LUMO bandgap of 2.3 eV. The energy is obviously greater than that of bulk materials (1.74 eV). It has been found that the blue-shift of the absorption threshold of the QDs can be linked directly to the nanocrystal size [27,28]. The size of the CdSe QDs calculated from the first excitonic absorption peak of UV–vis absorption spectrum [29] is 2.8 nm, which is consistent with the results from TEM analysis and statistics in Fig. 2. The PL spectrum was also shown in Fig. 3, when excitated, the CdSe QDs exhibit a sharp emission peak at 566.7 nm with a small full width at half maximum (fwhm) of only 39 nm, consistent with the narrow distribution of the CdSe QDs as evidenced in TEM measurements. Furthermore, the small difference between the emission peak energy and the absorption edge imply that the particles behave as direct bandgap materials with minimum surface defects. Therefore, the CdSe QDs exhibit excellent fluorescence efficiency. This can also be visualized from the photograph of the CdSe QDs solution irradiated at the 365 nm UV light, as depicted in the insert of Fig. 3. The CdSe QDs emitted pure and bright green luminescence, which is in good accordance with the emission peak at 566.7 nm in the PL spectrum.

Fig. 4. Powder X-ray diffraction (XRD) pattern of CdSe QDs. The inset shows the unit cell of CdSe crystallites with typical zinc-blende phase.

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Fig. 5. Temporal evolution of UV–vis absorption (left) and normalized PL (right) spectra of CdSe QDs well dispersed in toluene after purification. In the two figures, the curves from left to right marked by a-d are corresponding to the samples reacted at different time, i.e. 1 h, 2 h, 3 h and 4 h, respectively.

3.3. Crystalline structure The crystalline structures of CdSe QDs were investigated by powder X-ray diffraction (XRD) analysis. A typical XRD pattern is shown in Fig. 4. Apart from strongly broadened reflections due to small CdSe crystallite size, three diffraction peaks at 25.3◦ , 42.2◦ and 49.8◦ appear, well corresponding to the (111), (220) and (311) planes of the zinc-blende phase of CdSe (JPDS No.19-019) [19,20,27,30], which is different from those obtained from organic phase route at higher temperature (above 200 ◦ C) [14,17,31]. The main reason for the formation of the zinc-blende crystallites is that our reaction mostly occurred at a comparative low temperature (90–160 ◦ C). The crystallite sizes of CdSe QDs calculated using the Sherrer formula [27,32] are 2.8 nm, which is consistent with the results determined from TEM image in Fig. 2 and calculated according to the first excitonic absorption peak of UV–vis absorption spectrum in Fig. 3. 3.4. Quantum size effects The size-dependent optical properties of CdSe nanocrystals are predominantly attractive because of their quantum size effects as well as high photoluminescence quantum efficiency [30,31]. Fig. 5 shows the UV–vis absorption and PL spectra of toluene solutions of CdSe QDs at different reaction time. The samples of CdSe QDs were synthesized at the following condition: the molar ratio of CdMA, selenourea and TOPO was 1:0.5:5; the original concentration of Cd-MA in dimethylbenzene was 0.05 mol/L; the reaction proceeded at 140 ◦ C in the N2 atmosphere for 1–4 h. Aliquots of the CdSe nanocrystals at different reaction time were taken out and purified for UV–vis and PL spectra tests. In Fig. 5, evident excitonic absorption and narrow emission peaks of the corresponding samples could be seen respectively. The absorption peaks are tunable in the 550–582 nm range and emission peaks red-shift from 577 nm to 614 nm along the reaction time course from one hour to four hours. With the growth of CdSe nanocrystals by reaction time, their sizes increased gradually, leading to remarkably red-shifts of the UV–vis absorption and PL peaks. Thus, the optical properties of the CdSe QDs with noticeable quantum size effects can be easily tuned by reaction time and many other effects, such as reaction temperature, precursors’ ratio and surfactants (see the Supplementary content) [19,20,30]. With improvement of reaction temperature or increase of Cd-MA:SeC(NH2 )2 , the absorption and PL spectra showed remarkably red shifts indicating the enlargement of the CdSe QDs’ size (Figs. S1 and S2). The amounts of surfactants, the molar ratio of selenourea and TOPO in the range of 1:4 to 1:10, had no significant effects on the qualities of CdSe QDs. Excessive

surfactants may cause waste and complexity in the purification of nanocrystals (Fig. S3). There is no perceptible change about PL fwhm (ca. 40 nm) of the CdSe QDs, indicating invariable size distribution in the growth of CdSe nanocrystals. Furthermore, to prove the validation issues of synthesis process, we did five parallel experiments at the same conditions but varied batches to synthesize CdSe QDs. The results in Fig. S4 (see the Supplementary content) showed that there were almost no changes among the different synthesized batches of particles, confirming the validation of the synthesis process based on the high-boiling two-phase liquid/liquid system. 4. Conclusions We have succeeded in synthesizing CdSe QDs with high quantity in a high-boiling two-phase liquid/liquid system. The selenourea used as the precursor was dissolved in viscous ethylene glycol (EG), which facilitated the control of the nanocrystal growth by virtue of the slow decomposition and diffusion of selenourea in EG. The size of the CdSe QDs can be readily tuned from 2.8 nm to 17.6 nm just by varying the reaction time, corresponding to green to red emission. The CdSe QDs with narrow size distribution show remarkable quantum size effects. Additionally, they are zinc-blende phase and exhibit excellent fluorescence efficiency. This route could be exploited to synthesize nanocrystals with a large scale and could broaden to prepare other semiconductor nanocrystals, such as CdS, CdTe and ZnSe. Acknowledgement This work was supported by the Startup Foundation for Doctors funded by University of South China (2015XQD10). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.colsurfa.2016.07. 013. References [1] A. Alivisatos, Semiconductor clusters, nanocrystals, and quantum dots, Science 271 (1996) 933–937. [2] Y. Li, A. Rizzo, R. Cingolani, G. Gigli, White-light-emitting diodes using semiconductor nanocrystals, Microchim. Acta 159 (2007) 207–215. [3] A.M. Munro, J.A. Bardecker, M.S. Liu, Y.J. Cheng, Y.H. Niu, I.J.L. Plante, A.K.Y. Jen, D.S. Ginger, Colloidal CdSe quantum dot electroluminescence: ligands and light-emitting diodes, Microchim. Acta 160 (2008) 345–350. [4] A.L. Rogach, N. Gaponik, J.M. Lupton, C. Bertoni, D.E. Gallardo, S. Dunn, N.L. Pira, M. Paderi, P. Repetto, S.G. Romanov, C.O. Dwyer, C.M.S. Torres, A.

382

[5] [6]

[7]

[8]

[9]

[10] [11] [12]

[13]

[14]

[15]

[16]

[17] [18]

Y. Zhang, X. Nie / Colloids and Surfaces A: Physicochem. Eng. Aspects 506 (2016) 378–382 Eychmuller, Light-emitting diodes with semiconductor nanocrystals, Angew. Chem. Int. Ed. 47 (2008) 6538–6549. D. Quanqin, C. Duty, M. Hu, Semiconductor-nanocrystals-based white light-emitting diodes, Small 6 (2010) 1577–1588. L.L. Han, D.H. Qin, X. Jiang, Y.S. Liu, L. Wang, J.W. Chen, Y. Cao, Synthesis of high quality zinc-blende CdSe nanocrystals and their application in hybrid solar cells, Nanotechnology 17 (2006) 4736–4742. S. Shao, F. Liu, Z. Xie, L. Wang, High-efficiency hybrid polymer solar cells with inorganic P- and N-type semiconductor nanocrystals to collect photogenerated charges, J. Phys. Chem. C 114 (2010) 9161–9166. J. Zhang, J. Gao, E.M. Miller, J.M. Luther, M.C. Beard, Diffusion-controlled synthesis of PbS and PbSe quantum dots with in situ halide passivation for quantum dot solar cells, ACS Nano 8 (2013) 614–622. W. Zhang, H. Zhang, X. Zhong, Noninjection ultralarge-scaled synthesis of shape-tunable CdS nanocrystals as photocatalysts, RSC Adv. 3 (2013) 17477–17484. M. Bruchez, M. Moronne, P. Gin, S. Weiss, A. Alivisatos, Semiconductor nanocrystals as fluorescent biological labels, Science 281 (1998) 2013–2016. W.C. Chan, S. Nie, Quantum dot bioconjugates for ultrasensitive nonisotopic detection, Science 281 (1998) 2016–2018. Q. Wang, F. Ye, T. Fang, W. Niu, P. Liu, X. Min, X. Li, Bovine serum albumin-directed synthesis of biocompatible CdSe quantum dots and bacteria labeling, J. Colloid Interface Sci. 355 (2011) 9–14. B. Dong, C. Li, G. Chen, Y. Zhang, Y. Zhang, M. Deng, Q. Wang, Facile synthesis of highly photoluminescent Ag2 Se quantum dots as a new fluorescent probe in the second near-infrared window for in vivo imaging, Chem. Mater. 25 (2013) 2503–2509. C.B. Murray, D.J. Norris, M.G. Bawendi, Synthesis and characterization of nearly monodisperse CdE (E = S, Se Te) semiconductor nanocrystallites, J. Am. Chem. Soc. 115 (1993) 8706–8715. D.V. Talapin, A.L. Rogach, A. Kornowski, M. Haase, H. Weller, Highly luminescent monodisperse CdSe and CdSe/ZnS nanocrystals synthesized in a hexadecylamine-trioctylphosphine oxide- trioctylphospine mixture, Nano Lett. 1 (2001) 207–211. Z.A. Peng, X.G. Peng, Nearly monodisperse and shape-controlled CdSe nanocrystals via alternative routes: nucleation and growth, J. Am. Chem. Soc. 124 (2002) 3343–3353. L.H. Qu, Z.A. Peng, X.G. Peng, Alternative routes toward high quality CdSe nanocrystals, Nano Lett. 1 (2001) 333–337. L.P. Liu, Z.B. Zhuang, T. Xie, Y.G. Wang, J. Li, Q. Peng, Y.D. Li, Shape control of CdSe nanocrystals with zinc blende structure, J. Am. Chem. Soc. 131 (2009) 16423–16429.

[19] L. Liu, Q. Peng, Y. Li, Preparation of CdSe quantum dots with full color emission based on a room temperature injection technique, Inorg. Chem. 47 (2008) 5022–5028. [20] D.C. Pan, Q. Wang, S.C. Jiang, X.L. Ji, L.J. An, Low-temperature synthesis of oil-soluble CdSe, CdS, and CdSe/CdS core–shell nanocrystals by using various water-soluble anion precursors, J. Phys. Chem. C 111 (2007) 5661–5666. [21] D.C. Pan, Q. Wang, S.C. Jiang, X.L. Ji, L.J. An, Synthesis of extremely small CdSe and highly luminescent CdSe/CdS core-shell nanocrystals via a novel two-phase thermal approach, Adv. Mater. 17 (2005) 176–179. [22] N.N. Zhao, W. Nie, J. Mao, M.Q. Yang, D.P. Wang, Y.H. Lin, Y.D. Fan, Z.L. Zhao, H. Wei, X.L. Ji, A general synthesis of high-quality inorganic nanocrystals via a two-phase method, Small 6 (2010) 2558–2565. [23] S. Flamee, M. Cirillo, S. Abe, K. De Nolf, R. Gomes, T. Aubert, Z. Hens, Fast, high yield, and high solid loading synthesis of metal selenide nanocrystals, Chem. Mater. 25 (2013) 2476–2483. [24] Y. Chen, X. Nie, Synthesis and characterization of water-soluble CdSe nanoparticles capped by AOT, Chin. J. Polym. Sci. 31 (2013) 1284–1289. [25] H. Liu, H. Tao, T. Yang, L. Kong, D. Qin, J. Chen, A surfactant-free recipe for shape-controlled synthesis of CdSe nanocrystals, Nanotechnology 22 (2011) 45604–45611. [26] M. Brust, M. Walker, D. Bethell, D.J. Schiffrin, R. Whyman, Synthesis of thiol-derivatised gold nanoparticles in a two-phase liquid–liquid system, J. Chem. Soc. Chem. Commun. 7 (1994) 801–802. [27] H. Zhu, M. Sun, X. Yang, A simple selenium source to synthesize CdSe via the two-phase thermal approach, Colloids Surf. A: Physicochem. Eng. Aspects 320 (2008) 74–77. [28] Q.Y. Yu, C.Y. Liu, Z.Y. Zhang, Y. Liu, Facile synthesis of semiconductor and noble metal nanocrystals in high-boiling two-phase liquid/liquid systems, J. Phys. Chem. C 112 (2008) 2266–2270. [29] W.W. Yu, L.H. Qu, W.Z. Guo, X.G. Peng, Experimental determination of the extinction coefficient of CdTe, CdSe, and CdS nanocrystals, Chem. Mater. 15 (2003) 2854–2860. [30] T. Teranishi, M. Nishida, M. Kanehara, Size-tuning and optical properties of high-quality CdSe nanoparticles synthesized from cadmium stearate, Chem. Lett. 34 (2005) 1004–1005. [31] X. Zhong, Y. Feng, Y. Zhang, Facile and reproducible synthesis of red-emitting CdSe nanocrystals in amine with long-term fixation of particle size and size distribution, J. Phys. Chem. C 111 (2007) 526–531. [32] J. Nanda, S. Sapra, D.D. Sarma, N. Chandrasekharan, G. Hodes, Size-selected zinc sulfide nanocrystallites:synthesis, structure, and optical studies, Chem. Mater. 2 (2000) 1018–1024.