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liquid system
Optical Materials 72 (2017) 737e742
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Facile synthesis and properties of CdSe quantum dots in a novel two-phase liquid/liquid system Jidong Wang a, *, Xiaoyu Wang a, Hengshan Tang a, Zehua Gao a, Shengquan He a, Dandan Ke a, Yue Zheng c, Shumin Han a, b, ** a b c
Hebei Key Laboratory of Applied Chemistry, School of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, China State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China The First Hospital in Qinhuangdao Affiliated to Hebei Medical University, Qinhuangdao 066004, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 March 2017 Received in revised form 12 May 2017 Accepted 13 June 2017
High-quantity CdSe QDs were synthesized in a novel two-phase liquid/liquid system. This system, ODE/ water was stable and as-used solvents were almost nontoxic. The methodology leading to the successful synthesis of CdSe QDs was a typical, one-pot approach and the obtained CdSe QDs with zinc-blende phase structure exhibited excellent optical properties, narrow size distribution, higher particle uniformity and crystallinity. The mechanism of nucleation and growth of CdSe QDs were discussed by the possible thermodynamic equilibrium existing in ODE/water interface. This two-phase liquid/liquid system would broaden the synthesis of other semiconductor QDs. © 2017 Published by Elsevier B.V.
Keywords: Two-phase system Synthesis CdSe Quantum dots Optical property
1. Introduction Semiconductor quantum dots (QDs), owing to their sizedependent optical properties, have attracted extensive attention in domains of both fundamental research and technical applications, including photocatalysts [1], solar cells [2e5], biomedical imaging [6e8], and LEDs [9,10]. Cadmium selenide (CdSe) QDs, among II-VI semiconductor nanoscale materials, are most widely studied due to their remarkable photoelectric properties. To obtain the high-quantity CdSe QDs, with high crystallinity, narrow emission peak shape, narrow size distribution and high photoluminescence quantum yields (PLQYs), have become an issue which needs to be solved urgently. Many research groups have made a large quantity of effort in the synthesis of CdSe QDs in recent twenty years focusing on the shape controls, surface engineer,
* Corresponding author. ** Corresponding author. College of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, China. E-mail addresses: [email protected] (J. Wang), [email protected] (X. Wang), [email protected] (H. Tang), [email protected] (Z. Gao), [email protected] (S. He), [email protected] (D. Ke), yuezheng1965@ sina.com (Y. Zheng), [email protected] (S. Han). http://dx.doi.org/10.1016/j.optmat.2017.06.023 0925-3467/© 2017 Published by Elsevier B.V.
properties and applications of QDs [11e18]. Among these syntheses, Murray et al. [19] first proposed the organometallic approach to synthesize CdSe QDs in 1993, based on the Cd(CH3)2 and TOP/Se as the precursors. Although as we known, the Cd(CH3)2 is extremely toxic, easily decomposing at room temperature, the organometallic approach still be thought the most popular synthesis method. CdO and other cadmium salts as cadmium resources to replace Cd(CH3)2 accelerated the development of organometallic approach to obtain the high-quality CdSe QDs [20e23]. Despite all this, complicated processes, strict conditions and biological incompatibility limited the progress of the organometallic methods. In 1994, Brust et al. [24] firstly reported a two-phase approach to preparation of alkanethiol-capped Au nanocrystals in toluene/water. From then on, the Brust method was adopted to synthesize oxide nanocrystals [25], semiconductor nanocrystals [26] and noble metal nanocrystals [27]. The two-phase approach was also successfully explored to synthesize high-quality CdSe QDs [28,29]. Compared with the organometallic synthetic routes, the two-phase approach is controllable, low reaction temperature and the reaction reagents are environmental friendly, low toxicity [30]. However, some problems must been considered in this synthesis: (i) the lower particle crystallinity and wider size distribution of QDs. (ii) hydrothermal/solvothermal or microwave/sonication was as the
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assisted technique in some synthesis. (iii) organic/water systems, especially, toluene/water and chloroform/water, are volatile and toxic [30]. Thus, these challenges in two-phase synthesis hindered seriously the development and application of semiconductor quantum dots. Herein, we introduce a novel two-phase liquid/liquid system, octadecene (ODE)/water, to synthesize high-quality CdSe QDs. The advantages in this systems can been provides (1) what have synthesized QDs are with higher particle uniformity and crystallinity, excellent optical properties and narrow size distribution, (2) reaction temperature can adjust in a wide range for the high boiling point of ODE (without using any assisted technique), (3) as-used solvents are stable and almost nontoxic. Additionally, this route is a facile one-pot approach, that is, all chemicals were simply loaded in flask with condenser at room temperature without the presynthesis of precursors. Finally, we explore the possible thermodynamic equilibrium in the ODE/water interfaces to discuss the mechanism of nucleation and growth of CdSe QDs. This novel, facile method will push forward progress in the synthesis of ZnS QDs, CdTe QDs and other semiconductor nano-materials.
2.3. Characterization The CdSe QDs aliquot was diluted with hexane using for optical measurements without any treatment. The ultraviolet-visible (UVevis) absorptions were recorded on a Hitachi UV-2550 spectrophotometer (Hitachi, Japan). The photoluminescence (PL) spectra were recorded on a Hitachi F-7700 spectrofluorometer (Hitachi, Japan) with the excitation wavelength of 350 nm. The morphology and size of the QDs were characterized using a transmission electron microscope with an accelerating voltage of 160 kV (TEM, Ht-7700, and Hitachi, Japan). After washing by nhexane, the CdSe QDs solutions were dropped onto carbon-coated copper grids and the solvent evaporated slowly. Powder X-Ray diffraction measurements were analyzed by an Ultima IV (Rigaku, Japan) powder X-ray diffraction (XRD) (D/max-
2. Experimental procedure 2.1. Materials Cadmium acetate (Cd(OA)2, 95%), dodecylamine (DDA, AR), and octadecene (ODE, 90%) were purchased from Aladdin. Se powder, sodium borohydride (NaBH4, 98%), sodium citrate (SC, 99%) were obtained from Alfa Aesar. All chemicals were used as received without further purification. 2.2. Synthesis of CdSe QDs The successful preparation method of CdSe QDs was a typical and facile one-pot approach. In this method, all chemicals and reagents, including 0.533 g (2 mmol) cadmium acetate (Cd(OA)2), 5 mL dodecylamine (DDA), 10 mL ODE, 0.0198 g (0.25 mmol) Se powder, 0.0378 g (1 mmol) sodium borohydride (NaBH4), 0.1000 g (3.875 mmol) sodium citrate (SC) and 15 mL of deionized water, were loaded simply in a glass three-neck flask with condenser at room temperature, stirred and heated to 150 C under an N2 atmosphere. The molar ratio of Cd precursor and Se precursor was 8:1.
Fig. 1. Absorption and emission spectra of CdSe QDs at 150 C.
Fig. 2. (A) TEM image of CdSe QDs, (B) size distribution of CdSe QDs (mean: 2.4 nm, SD: 0.4 nm).
J. Wang et al. / Optical Materials 72 (2017) 737e742
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is the difference between positions of the band maxima of the absorption and emission spectra, indicated the CdSe QDs with the well-defined structure and minimum surface defects. As a result, the CdSe QDs exhibited superb fluorescence properties and higher crystallinity.
D ¼ 1:6122 109 l4 2:6575 106 l3 2 þ 1:6242 103 l ð0:4277Þl þ 41:57
Fig. 3. Powder X-ray diffraction (XRD) pattern of CdSe QDs.
(1)
where D (nm) is the particle diameter of CdSe QDs and l (nm) is the absorption peak position of the corresponding sample. The empirical equation was from the relationship curves between the size and first absorption peak position of nanocrystals in Peng's report [31]. As we known, to determine the actual concentration of colloidal semiconductor nanocrystals (or QDs) is essential to study the synthesis and growth of QDs. However, for colloidal QDs, a lot of ligands capped on their surfaces, which led to the concentration unidentified using conventional methods. Therefore,
rA, Cu Ka 40 kV, 20 mA, l ¼ 1.5406 Å). The XRD sample was prepared by dropping CdSe QDs solution on the glass plate. FTIR spectral analysis was performed by a Nicolet iS10 spectrometer (Thermo Fisher Scientific Co., Madison, WI). 3. Results and discussion The UVevis absorption and PL spectra of the as-synthesized CdSe QDs at 150 C were shown in Fig. 1. A distinct excitonic absorption peak of CdSe QDs at 504 nm was exhibited, corresponding to the HOMO-LUMO bandgap of 2.46 eV calculating from the extended theoretical approach [31]. Compared with the HOMO-LUMO bandgap of bulk materials (1.74 eV), the as-synthesized CdSe QDs were larger. The particle size of CdSe QDs is about 2.4 nm estimating from empirical formula [31] based on the first excitonic absorption peak as following Equ. (1). The sharp PL emission peak of CdSe QDs was at about 523 nm and the full width at half maximum (FWHM) was about 26 nm, which represented CdSe QDs with a narrow size distribution. Furthermore, about 13 nm Nonresonant Stokes Shift (NRSS), which
Fig. 4. FTIR spectra of the free DDA and the DDA-capped CdSe QDs.
Fig. 5. Temporal evolution of UVevis absorption (A) and normalized (B) PL spectra of CdSe QDs at different reaction time.
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Peng et al. proposed the concentration calculation of QDs by simply taking the extinction coefficient and UVevis absorption spectrum. Their experimental results also demonstrated the extinction coefficient per mole of QDs increasing with the growth of their size in a square to a cubic function. So, how to obtain the size data of QDs by simpler method would be the key problems. In their research, the size data of the QDs were determined by both TEM measurements and literature. The first absorption peak positions of QDs were recorded by UVevisible spectrometer. By the functions fitting and calculation, the empirical formula were built using for the calculation of QDs size only by the first excitonic absorption peak of UVevis spectrogram as shown in Equ. (1). After the size of QDs were achieved by Equ. (1), the extinction coefficient was calculated easily. Thus, a simple and convenient approach was explored to determine the concentration of CdTe, CdSe, and CdS QDs based on the extinction coefficient, which was depended on the size calculation of QDs. The empirical equation is also widely employed in the study of the characterization, nucleation and growth mechanisms of QDs. The particle size distribution and size of CdSe QDs were measured by TEM as shown in Fig. 2. The results of TEM analysis revealed that the average particle diameter of CdSe QDs obtained by the ODE/water two-phase approach was also about 2.4 nm. CdSe QDs were dot-shaped, uniform. Compared with the Bohr radius of bulk CdSe (5.4 nm), the radius of the synthesized QDs was so small that they would exhibit the blue-shift of maximum absorption resulting from the strong quantum confinement effect. Also, it's worth noting that although there was not an obvious separation between nucleation and growth stages by precursor injection technique in this synthesis, the as-prepared CdSe QDs exhibited a fairly good monodispersity as shown in Fig. 2. It was maybe that the interfaces between ODE/water made particle growth slowly leading to the break of the focusing of the size distribution [32].
The crystalline structures of CdSe QDs were investigated by power XRD measurement as shown in Fig. 3. Although reflections were broadened due to the small particle size of CdSe QDs, three reflections of (111), (220) and (311) planes at 2 q angles of 25.3 , 42.2 and 49.8 appeared distinctly. Thus, the diffraction pattern of CdSe QDs synthesized in the octadecene (ODE)/water system exhibited a zinc blende (ZB) structure in accordance with JCPDS (no. 19019). The structural features of free DDA and DDA-capped CdSe QDs were investigated by FTIR as showed in Fig. 4. For the free DDA, the signal around 3331 cm1 could be attributed to NeH stretching vibration modes. The absorptions near to 1489, 1569 and 1648 cm1 were assigned to the flexural vibration modes of eNH2. However, for the DDA-capped CdSe QDs, the corresponding stretching peak of NeH in free DDA became wider and moved to 3441 cm1. Besides that, the new flexural vibration modes at 1635 cm1 appeared and the old ones disappeared, which demonstrated that the N atoms of DDA had been bound to Cd atoms of QDs surface through the CdeN bond. Moreover, from Fig. 4, we could also draw the conclusion that sodium citrate was not a ligand in the preparation of CdSe QDs but only a pH buffer regulator, because sodium citrate was the small molecular with high valence and weak acidity. Fig. 5 showed the CdSe QDs evolution of the UVevis absorption and PL spectra what had synthesized at different reaction time in ODE/water systems. The absorption peaks moved from 550 nm to 582 nm with the reaction time and their sizes increased gradually, from 2.4 nm to 2.7 nm. The increase of QDs particle size led the UVevis absorption to remarkably red-shifts. The PL peaks the CdSe QDs were sharp and had the same growth tendency. Furthermore, the FWHM of PL spectra became broader significantly (from 26 nm to 29 nm), which indicated the larger CdSe QDs appeared. Thus, as the quantum size effects, the optical properties of the CdSe QDs
Fig. 6. Schematic illustration of the formation of CdSe QDs in the ODE/water system.
J. Wang et al. / Optical Materials 72 (2017) 737e742
could be regulated and controlled conveniently, only by reaction time. The nucleation and growth procession of CdSe QDs, was illustrated in Fig. 6. The possible thermodynamic equilibrium equations drove the growth of the CdSe QDs in ODE/water interfaces as following:
Cd precursor þ Se precursor/ðCd SeÞmonomer
(2)
ðCd SeÞ monomer%ðCdSeÞ nuclei
(3)
ðCdSeÞ nuclei%ðCdSeÞn ðregularÞ%ðCdSeÞm ðregularÞ ðn < mÞ (4) The Cd source was from the reaction between Cd (OA)2 with DDA. Power Se, reduced by NaBH4, as the Se source, formed the Se precursor. Sodium citrate, as the protective reagent, prevented the Se precursor being oxidized. At the interfaces, the random collision of precursor accelerated with the increase of reaction temperature. Se precursor moved from water phase into ODE phase leading to the emergence of the CdSe monomers, as described in Equ. (2). Then, the thermodynamic equilibrium was formed between the monomers of CdSe and the nuclei of CdSe as shown in Equ. (3). In the typical “hot injection” approach, Se precursor was injected into high boiling Cd precursor mixture leading to the burst growth of nuclei resulted of the supersaturation of monomers. However, for the nucleation of CdSe QDs in the ODE/water system, this process was mild because of the existence of interfaces, which controlled the speed of nucleation. The continuous growth of nuclei led to the formation of regular QDs, as described in Equ. (4). QDs could grow with different size distributions, which also depended on the monomer concentration and reaction speed present in the liquid/ liquid system. The unique interface kept the continue increase of monomer concentration in a lower speed. As Alivisatos et al. [33,34] reported that when the monomer concentration was relatively low, a wider size distribution would appear, because of the dissolution of smaller particles and the growth of larger particles continuously. This is the size “defocusing” or “Ostwald ripening”. With the monomer concentration increased, the as-synthesis QDs were still with a narrower size distribution, because the growth of smaller particles was faster than larger. That was the monodispersed QDs ensemble [35,36]. 4. Conclusions In summary, we have developed a novel ODE/water two-phase liquid/liquid system to synthesize high-quantity CdSe QDs. The reaction system is stable and as-used solvents are almost nontoxic. Additionally the methodology leading to the successful synthesis of CdSe QDs is a typical and facile one-pot approach. The obtained CdSe QDs exhibit excellent optical properties and narrow size distribution. The synthesized CdSe QDs with zinc-blende phase structure show higher particle uniformity and crystallinity. The temporal evolution of the absorption and PL spectra of CdSe QDs at various reaction times show distinguished quantum size effects. Furthermore, we have discussed the mechanism of nucleation and growth of CdSe QDs by the possible thermodynamic equilibrium existing in ODE/water interface. This two-phase liquid/liquid system, ODE/water, would broaden to synthesize other optical materials. Acknowledgements This work was supported by the Natural Science Foundation of Hebei Province (H2015203231), Youth Foundation Project
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supported by the Hebei Education Department of China (QN2014004), the Key Research and Development Program of Qin Huangdao (201602A110), Doctoral Fund of Yanshan University (B821). References [1] V.I. Klimov, S.A. Ivanov, J. Nanda, M. Achermann, I. Bezel, J. McGuire, A. Piryatinski, Single-exciton optical gain in semiconductor nanocrystals, Nature 447 (2007) 441e446. [2] I. Gur, N.A. Fromer, M.L. Geier, A.P. Alivisatos, Air-stable all-inorganic nanocrystal solar cells processed from solution, Science 310 (2005) 462e465. [3] X. Lan, O. Voznyy, A. Kiani, F.P. García de Arquer, A.S. Abbas, G.H. Kim, M. Liu, Z. Yang, G. Walters, J. Xu, M. Yuan, Z. Ning, F. Fan, P. Kanjanaboos, I. Kramer, D. Zhitomirsky, P. Lee, A. Perelgut, S. Hoogland, E.H. Sargent, Passivation using molecular halides increases quantum dot solar cell performance, Adv. Mater. 28 (2016) 299e304. [4] M. Liu, F. de Arquer, Y. Li, X. Lan, G.H. Kim, O. Voznyy, J.Y. 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Peng, Alternative routes toward high quality CdSe nanocrystals, Nano Lett. 1 (2001) 333e337. [22] 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) 16423e16429. [23] 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) 5022e5028. [24] M. Brust, M. Walker, D. Bethell, D.J. Schiffrin, R.J. Whyman, Synthesis of thiolderivatised gold nanoparticles in a two-phase liquideliquid system, Chem. Soc. Chem. Commun. 7 (1994) 801e802. [25] N.N. Zhao, D.C. Pan, N. Wei, X.L. Ji, Two-phase synthesis of shape-controlled colloidal zirconia nanocrystals and their characterization, J. Am. Chem. Soc. 128 (2006) 10118e10124. [26] D. Pan, Q. Wang, S.C.J. Jiang, L.J. 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Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Facile synthesis and properties of CdSe quantum dots in a novel two-phase liquid/liquid system Jidong Wang a, *, Xiaoyu Wang a, Hengshan Tang a, Zehua Gao a, Shengquan He a, Dandan Ke a, Yue Zheng c, Shumin Han a, b, ** a b c
Hebei Key Laboratory of Applied Chemistry, School of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, China State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China The First Hospital in Qinhuangdao Affiliated to Hebei Medical University, Qinhuangdao 066004, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 March 2017 Received in revised form 12 May 2017 Accepted 13 June 2017
High-quantity CdSe QDs were synthesized in a novel two-phase liquid/liquid system. This system, ODE/ water was stable and as-used solvents were almost nontoxic. The methodology leading to the successful synthesis of CdSe QDs was a typical, one-pot approach and the obtained CdSe QDs with zinc-blende phase structure exhibited excellent optical properties, narrow size distribution, higher particle uniformity and crystallinity. The mechanism of nucleation and growth of CdSe QDs were discussed by the possible thermodynamic equilibrium existing in ODE/water interface. This two-phase liquid/liquid system would broaden the synthesis of other semiconductor QDs. © 2017 Published by Elsevier B.V.
Keywords: Two-phase system Synthesis CdSe Quantum dots Optical property
1. Introduction Semiconductor quantum dots (QDs), owing to their sizedependent optical properties, have attracted extensive attention in domains of both fundamental research and technical applications, including photocatalysts [1], solar cells [2e5], biomedical imaging [6e8], and LEDs [9,10]. Cadmium selenide (CdSe) QDs, among II-VI semiconductor nanoscale materials, are most widely studied due to their remarkable photoelectric properties. To obtain the high-quantity CdSe QDs, with high crystallinity, narrow emission peak shape, narrow size distribution and high photoluminescence quantum yields (PLQYs), have become an issue which needs to be solved urgently. Many research groups have made a large quantity of effort in the synthesis of CdSe QDs in recent twenty years focusing on the shape controls, surface engineer,
* Corresponding author. ** Corresponding author. College of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, China. E-mail addresses: [email protected] (J. Wang), [email protected] (X. Wang), [email protected] (H. Tang), [email protected] (Z. Gao), [email protected] (S. He), [email protected] (D. Ke), yuezheng1965@ sina.com (Y. Zheng), [email protected] (S. Han). http://dx.doi.org/10.1016/j.optmat.2017.06.023 0925-3467/© 2017 Published by Elsevier B.V.
properties and applications of QDs [11e18]. Among these syntheses, Murray et al. [19] first proposed the organometallic approach to synthesize CdSe QDs in 1993, based on the Cd(CH3)2 and TOP/Se as the precursors. Although as we known, the Cd(CH3)2 is extremely toxic, easily decomposing at room temperature, the organometallic approach still be thought the most popular synthesis method. CdO and other cadmium salts as cadmium resources to replace Cd(CH3)2 accelerated the development of organometallic approach to obtain the high-quality CdSe QDs [20e23]. Despite all this, complicated processes, strict conditions and biological incompatibility limited the progress of the organometallic methods. In 1994, Brust et al. [24] firstly reported a two-phase approach to preparation of alkanethiol-capped Au nanocrystals in toluene/water. From then on, the Brust method was adopted to synthesize oxide nanocrystals [25], semiconductor nanocrystals [26] and noble metal nanocrystals [27]. The two-phase approach was also successfully explored to synthesize high-quality CdSe QDs [28,29]. Compared with the organometallic synthetic routes, the two-phase approach is controllable, low reaction temperature and the reaction reagents are environmental friendly, low toxicity [30]. However, some problems must been considered in this synthesis: (i) the lower particle crystallinity and wider size distribution of QDs. (ii) hydrothermal/solvothermal or microwave/sonication was as the
738
J. Wang et al. / Optical Materials 72 (2017) 737e742
assisted technique in some synthesis. (iii) organic/water systems, especially, toluene/water and chloroform/water, are volatile and toxic [30]. Thus, these challenges in two-phase synthesis hindered seriously the development and application of semiconductor quantum dots. Herein, we introduce a novel two-phase liquid/liquid system, octadecene (ODE)/water, to synthesize high-quality CdSe QDs. The advantages in this systems can been provides (1) what have synthesized QDs are with higher particle uniformity and crystallinity, excellent optical properties and narrow size distribution, (2) reaction temperature can adjust in a wide range for the high boiling point of ODE (without using any assisted technique), (3) as-used solvents are stable and almost nontoxic. Additionally, this route is a facile one-pot approach, that is, all chemicals were simply loaded in flask with condenser at room temperature without the presynthesis of precursors. Finally, we explore the possible thermodynamic equilibrium in the ODE/water interfaces to discuss the mechanism of nucleation and growth of CdSe QDs. This novel, facile method will push forward progress in the synthesis of ZnS QDs, CdTe QDs and other semiconductor nano-materials.
2.3. Characterization The CdSe QDs aliquot was diluted with hexane using for optical measurements without any treatment. The ultraviolet-visible (UVevis) absorptions were recorded on a Hitachi UV-2550 spectrophotometer (Hitachi, Japan). The photoluminescence (PL) spectra were recorded on a Hitachi F-7700 spectrofluorometer (Hitachi, Japan) with the excitation wavelength of 350 nm. The morphology and size of the QDs were characterized using a transmission electron microscope with an accelerating voltage of 160 kV (TEM, Ht-7700, and Hitachi, Japan). After washing by nhexane, the CdSe QDs solutions were dropped onto carbon-coated copper grids and the solvent evaporated slowly. Powder X-Ray diffraction measurements were analyzed by an Ultima IV (Rigaku, Japan) powder X-ray diffraction (XRD) (D/max-
2. Experimental procedure 2.1. Materials Cadmium acetate (Cd(OA)2, 95%), dodecylamine (DDA, AR), and octadecene (ODE, 90%) were purchased from Aladdin. Se powder, sodium borohydride (NaBH4, 98%), sodium citrate (SC, 99%) were obtained from Alfa Aesar. All chemicals were used as received without further purification. 2.2. Synthesis of CdSe QDs The successful preparation method of CdSe QDs was a typical and facile one-pot approach. In this method, all chemicals and reagents, including 0.533 g (2 mmol) cadmium acetate (Cd(OA)2), 5 mL dodecylamine (DDA), 10 mL ODE, 0.0198 g (0.25 mmol) Se powder, 0.0378 g (1 mmol) sodium borohydride (NaBH4), 0.1000 g (3.875 mmol) sodium citrate (SC) and 15 mL of deionized water, were loaded simply in a glass three-neck flask with condenser at room temperature, stirred and heated to 150 C under an N2 atmosphere. The molar ratio of Cd precursor and Se precursor was 8:1.
Fig. 1. Absorption and emission spectra of CdSe QDs at 150 C.
Fig. 2. (A) TEM image of CdSe QDs, (B) size distribution of CdSe QDs (mean: 2.4 nm, SD: 0.4 nm).
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is the difference between positions of the band maxima of the absorption and emission spectra, indicated the CdSe QDs with the well-defined structure and minimum surface defects. As a result, the CdSe QDs exhibited superb fluorescence properties and higher crystallinity.
D ¼ 1:6122 109 l4 2:6575 106 l3 2 þ 1:6242 103 l ð0:4277Þl þ 41:57
Fig. 3. Powder X-ray diffraction (XRD) pattern of CdSe QDs.
(1)
where D (nm) is the particle diameter of CdSe QDs and l (nm) is the absorption peak position of the corresponding sample. The empirical equation was from the relationship curves between the size and first absorption peak position of nanocrystals in Peng's report [31]. As we known, to determine the actual concentration of colloidal semiconductor nanocrystals (or QDs) is essential to study the synthesis and growth of QDs. However, for colloidal QDs, a lot of ligands capped on their surfaces, which led to the concentration unidentified using conventional methods. Therefore,
rA, Cu Ka 40 kV, 20 mA, l ¼ 1.5406 Å). The XRD sample was prepared by dropping CdSe QDs solution on the glass plate. FTIR spectral analysis was performed by a Nicolet iS10 spectrometer (Thermo Fisher Scientific Co., Madison, WI). 3. Results and discussion The UVevis absorption and PL spectra of the as-synthesized CdSe QDs at 150 C were shown in Fig. 1. A distinct excitonic absorption peak of CdSe QDs at 504 nm was exhibited, corresponding to the HOMO-LUMO bandgap of 2.46 eV calculating from the extended theoretical approach [31]. Compared with the HOMO-LUMO bandgap of bulk materials (1.74 eV), the as-synthesized CdSe QDs were larger. The particle size of CdSe QDs is about 2.4 nm estimating from empirical formula [31] based on the first excitonic absorption peak as following Equ. (1). The sharp PL emission peak of CdSe QDs was at about 523 nm and the full width at half maximum (FWHM) was about 26 nm, which represented CdSe QDs with a narrow size distribution. Furthermore, about 13 nm Nonresonant Stokes Shift (NRSS), which
Fig. 4. FTIR spectra of the free DDA and the DDA-capped CdSe QDs.
Fig. 5. Temporal evolution of UVevis absorption (A) and normalized (B) PL spectra of CdSe QDs at different reaction time.
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J. Wang et al. / Optical Materials 72 (2017) 737e742
Peng et al. proposed the concentration calculation of QDs by simply taking the extinction coefficient and UVevis absorption spectrum. Their experimental results also demonstrated the extinction coefficient per mole of QDs increasing with the growth of their size in a square to a cubic function. So, how to obtain the size data of QDs by simpler method would be the key problems. In their research, the size data of the QDs were determined by both TEM measurements and literature. The first absorption peak positions of QDs were recorded by UVevisible spectrometer. By the functions fitting and calculation, the empirical formula were built using for the calculation of QDs size only by the first excitonic absorption peak of UVevis spectrogram as shown in Equ. (1). After the size of QDs were achieved by Equ. (1), the extinction coefficient was calculated easily. Thus, a simple and convenient approach was explored to determine the concentration of CdTe, CdSe, and CdS QDs based on the extinction coefficient, which was depended on the size calculation of QDs. The empirical equation is also widely employed in the study of the characterization, nucleation and growth mechanisms of QDs. The particle size distribution and size of CdSe QDs were measured by TEM as shown in Fig. 2. The results of TEM analysis revealed that the average particle diameter of CdSe QDs obtained by the ODE/water two-phase approach was also about 2.4 nm. CdSe QDs were dot-shaped, uniform. Compared with the Bohr radius of bulk CdSe (5.4 nm), the radius of the synthesized QDs was so small that they would exhibit the blue-shift of maximum absorption resulting from the strong quantum confinement effect. Also, it's worth noting that although there was not an obvious separation between nucleation and growth stages by precursor injection technique in this synthesis, the as-prepared CdSe QDs exhibited a fairly good monodispersity as shown in Fig. 2. It was maybe that the interfaces between ODE/water made particle growth slowly leading to the break of the focusing of the size distribution [32].
The crystalline structures of CdSe QDs were investigated by power XRD measurement as shown in Fig. 3. Although reflections were broadened due to the small particle size of CdSe QDs, three reflections of (111), (220) and (311) planes at 2 q angles of 25.3 , 42.2 and 49.8 appeared distinctly. Thus, the diffraction pattern of CdSe QDs synthesized in the octadecene (ODE)/water system exhibited a zinc blende (ZB) structure in accordance with JCPDS (no. 19019). The structural features of free DDA and DDA-capped CdSe QDs were investigated by FTIR as showed in Fig. 4. For the free DDA, the signal around 3331 cm1 could be attributed to NeH stretching vibration modes. The absorptions near to 1489, 1569 and 1648 cm1 were assigned to the flexural vibration modes of eNH2. However, for the DDA-capped CdSe QDs, the corresponding stretching peak of NeH in free DDA became wider and moved to 3441 cm1. Besides that, the new flexural vibration modes at 1635 cm1 appeared and the old ones disappeared, which demonstrated that the N atoms of DDA had been bound to Cd atoms of QDs surface through the CdeN bond. Moreover, from Fig. 4, we could also draw the conclusion that sodium citrate was not a ligand in the preparation of CdSe QDs but only a pH buffer regulator, because sodium citrate was the small molecular with high valence and weak acidity. Fig. 5 showed the CdSe QDs evolution of the UVevis absorption and PL spectra what had synthesized at different reaction time in ODE/water systems. The absorption peaks moved from 550 nm to 582 nm with the reaction time and their sizes increased gradually, from 2.4 nm to 2.7 nm. The increase of QDs particle size led the UVevis absorption to remarkably red-shifts. The PL peaks the CdSe QDs were sharp and had the same growth tendency. Furthermore, the FWHM of PL spectra became broader significantly (from 26 nm to 29 nm), which indicated the larger CdSe QDs appeared. Thus, as the quantum size effects, the optical properties of the CdSe QDs
Fig. 6. Schematic illustration of the formation of CdSe QDs in the ODE/water system.
J. Wang et al. / Optical Materials 72 (2017) 737e742
could be regulated and controlled conveniently, only by reaction time. The nucleation and growth procession of CdSe QDs, was illustrated in Fig. 6. The possible thermodynamic equilibrium equations drove the growth of the CdSe QDs in ODE/water interfaces as following:
Cd precursor þ Se precursor/ðCd SeÞmonomer
(2)
ðCd SeÞ monomer%ðCdSeÞ nuclei
(3)
ðCdSeÞ nuclei%ðCdSeÞn ðregularÞ%ðCdSeÞm ðregularÞ ðn < mÞ (4) The Cd source was from the reaction between Cd (OA)2 with DDA. Power Se, reduced by NaBH4, as the Se source, formed the Se precursor. Sodium citrate, as the protective reagent, prevented the Se precursor being oxidized. At the interfaces, the random collision of precursor accelerated with the increase of reaction temperature. Se precursor moved from water phase into ODE phase leading to the emergence of the CdSe monomers, as described in Equ. (2). Then, the thermodynamic equilibrium was formed between the monomers of CdSe and the nuclei of CdSe as shown in Equ. (3). In the typical “hot injection” approach, Se precursor was injected into high boiling Cd precursor mixture leading to the burst growth of nuclei resulted of the supersaturation of monomers. However, for the nucleation of CdSe QDs in the ODE/water system, this process was mild because of the existence of interfaces, which controlled the speed of nucleation. The continuous growth of nuclei led to the formation of regular QDs, as described in Equ. (4). QDs could grow with different size distributions, which also depended on the monomer concentration and reaction speed present in the liquid/ liquid system. The unique interface kept the continue increase of monomer concentration in a lower speed. As Alivisatos et al. [33,34] reported that when the monomer concentration was relatively low, a wider size distribution would appear, because of the dissolution of smaller particles and the growth of larger particles continuously. This is the size “defocusing” or “Ostwald ripening”. With the monomer concentration increased, the as-synthesis QDs were still with a narrower size distribution, because the growth of smaller particles was faster than larger. That was the monodispersed QDs ensemble [35,36]. 4. Conclusions In summary, we have developed a novel ODE/water two-phase liquid/liquid system to synthesize high-quantity CdSe QDs. The reaction system is stable and as-used solvents are almost nontoxic. Additionally the methodology leading to the successful synthesis of CdSe QDs is a typical and facile one-pot approach. The obtained CdSe QDs exhibit excellent optical properties and narrow size distribution. The synthesized CdSe QDs with zinc-blende phase structure show higher particle uniformity and crystallinity. The temporal evolution of the absorption and PL spectra of CdSe QDs at various reaction times show distinguished quantum size effects. Furthermore, we have discussed the mechanism of nucleation and growth of CdSe QDs by the possible thermodynamic equilibrium existing in ODE/water interface. This two-phase liquid/liquid system, ODE/water, would broaden to synthesize other optical materials. Acknowledgements This work was supported by the Natural Science Foundation of Hebei Province (H2015203231), Youth Foundation Project
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