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J. Phys. Chem. B 2006, 110, 9034-9040
Microwave-Assisted Aqueous Synthesis: A Rapid Approach to Prepare Highly Luminescent ZnSe(S) Alloyed Quantum Dots Huifeng Qian, Xin Qiu, Liang Li, and Jicun Ren* College of Chemistry & Chemical Engineering, Shanghai Jiaotong UniVersity, Shanghai 200240, People’s Republic of China ReceiVed: July 17, 2005; In Final Form: October 5, 2005
In this paper, we present a new procedure for the rapid synthesis of luminescent ZnSe nanocrystals in aqueous phase by microwave irradiation with controllable temperature. The effects of microwave irradiation and experimental conditions on the synthesis of nanocrystals were investigated systematically. It was found that there were significant effects of pH value of reaction solutions, molar ratio of precursors, and heating time of microwave irradiation on the optical properties of the ZnSe nanocrystals. A series of nanocrystals with different size was prepared in 1 h, and the photoluminescence quantum yield reached up to 17% at the optimal reaction condition. The results of HRTEM and XRD showed that the as-prepared nanocrystals had high crystallinity. The characterizations of EDS spectra and elemental analysis showed that the sulfur content of nanocrystals increased with the growth of nanocrystals. We speculated that the structure of nanocrystals was an alloy ZnSe(S) shell on the surface of the ZnSe particles core. Furthermore, we found that the oxygen from air in the reaction vessel played an important role in the decomposition of the thiol group under microwave irradiation.
Introduction High-quality colloidal semiconductor nanocrystals (also referred to quantum dots, QDs) have drawn significant attention due to their size-dependent properties and flexible processing chemistry over the past decade. Compared to organic dyes, QDs have unique optical properties and are characterized as narrow emission spectra, continuous absorption band, high chemical and photobleaching stability, and surface functionality.1-3 They have been successfully used in optoelectronics, nonlinear optical devices, quantum-dot lasers, solar cells, and bio-tagging recently.4-9 So far, great progress has been made in preparation of QDs,10-13 and a large number of high-quality QDs, such as CdSe, CdTe, and some alloy nanocrystals, were successfully synthesized by the organometallic approach and the aqueous approach, whose emission were at visible and near-infrared ranges. However, few reports were found on successful synthesis of strong UV-blue-emitting QDs. ZnSe (bulk band gap 2.7 eV) is a kind of wide band gap material that is suitable for UVblue-emitting laser diodes and biolabeling,14 passivation of shell for semiconductor core/shell nanocrystals,15 and hosts for the formation of doped nanocrystals.16 Hines and Guyot-Sionnest first reported that high luminescent ZnSe was prepared with organometallic diethylzinc in a trioctylphosphine oxide (TOPO)-hexadecylamine (HAD) mixture.17 ZnSe QDs prepared by this method had high quantum yield (QY) up to 20-50% and high crystallinity and monodispersity. However, key chemicals used in this route are toxic, expensive, pyrophoric, and even explosive.17 Recently, Peng and co-workers18 and Chen et al.19 improved this synthesis method using ZnO instead of Zn(CH3)2. But this procedure also needed very high reaction temperature (even up to 350 °C).18 Furthermore, as-prepared products could not be directly dis* Corresponding author. Tel: +86-21-54746001. Fax: +86-21-54741297. E-mail:
[email protected].
persed in the water phase due to its hydrophobic surface. As compared to many other organic solvents, water is cheap, nontoxic, nonflammable, and readily available.20 The use of water as a solvent for nanocrystal synthesis has attracted considerable research interest.21-23 More recently, Murase and co-workers developed an alternative method to synthesize blueemitting ZnSe nanocrystals in aqueous solution.24,25 Compared with organometallic methods, this method had low toxicity, was inexpensive, had good reproducibility, and had good water solubility. However, this procedure was time-consuming (about 1-2 days), and the product had low quantum yield (110%).24,25 To improve the luminescent properties, an inorganic shell material with a wider band gap was used to passivate a core material, which could lead to significant reduction of surface-related defect states and certain improvement for the quantum yield of QDs.26-28 ZnSe nanocrystals synthesized in the water phase had poor luminescence; however, its quantum yield could be dramatically enhanced by forming the ZnSe(S) core shell structure by post-illumination.29 Microwave irradiation is an attractive method for synthesis of nanocrystals. The synthesis of nanocrystals by microwave irradiation was first introduced by the Kotov group.30 Some studies showed that the synthesis of nanocrystals by microwave irradiation was generally quite faster, simpler, and very energy efficient as compared to conventional hydrothermal synthesis.31 Recently, our group reported a new method for rapid synthesis of high-quality CdTe QDs and alloy CdSe-CdS nanocrystals using microwave irradiation with a controllable temperature.32,33 Water is an excellent solvent for microwave-assisted synthesis since its tan δ is equal to 0.127. The aqueous solvent will be rapidly heated above 100 °C in a sealed vessel by microwave irradiation. At the elevated temperature, the synthesis rate will be improved dramatically, and some reactions that cannot occur at room temperature will rapidly take place. The combination of using water as a solvent to synthesize nanocrystals and
10.1021/jp0539324 CCC: $33.50 © 2006 American Chemical Society Published on Web 04/18/2006
Microwave-Assisted Aqueous Synthesis applying microwave irradiation as an efficient heating source is a very desirable way to make the synthesis of nanocrystals cleaner and more environment-friendly.20 In this paper, we extended this microwave irradiation method to the synthesis of water-soluble ZnSe QDs in aqueous phase. We found the significant effects of microwave irradiation and experimental conditions on the synthesis of ZnSe QDs. A series of nanocrystals with different sizes and optical properties were prepared within one hour, which was several times faster than conventional aqueous synthesis. ZnSe QDs prepared by microwave irradiation were water-soluble and had high crystallinity, and their photoluminescence (PL) quantum yield was up to 17%. These properties were remarkable improvements as compared with ZnSe QDs prepared by conventional aqueous synthesis method. Meanwhile, the reaction did not need rigorous reaction conditions, and some hazardous, expensive, environmentally unfriendly solvents such as trioctylphosphine (TOP), tributylphosphine (TBP), and trioctylphosphine oxide (TOPO). Experimental results showed that the as-prepared nanocrystal was an alloy ZnSe(S) shell formed on the surface of the ZnSe core, whose sulfur ions were from the decomposition of stabilizer by microwave irradiation. Experimental Section Chemicals. 3-Mercaptopropionic acid (MPA, 99%) was a product of Fluka, and Rhodamine 6G (95%) was from SigmaAldrich. ZnCl2 (99%), NaBH4 (96%), and selenium powder (99.999%, about 200 mesh) were obtained from Shanghai Reagent Company. Other chemicals were of analytical grade. The ultrapure water with 18.2 ΩM (Millipore Simplicity, USA) was used in all synthesis. Synthesis of ZnSe QDs by Microwave Irradiation. A microwave digestion system (WX-3000) made by Shanghai YiYao Instruments (Shanghai) was used for the preparation of ZnSe QDs, which was equipped with controllable temperature units. The system can operate at 2450 MHz frequency and work at 0-1000 W power. The reaction temperature and time can be programmed by users. The synthesis of nanocrystals was preformed in lined digestion vessels that are double-walled vessels consisting of a Teflon inner liner and cover surrounded by a high-strength vessel shell of Ultem polyetherimide.The volume of vessel used in the reaction was 60 mL. In general, ZnSe monomers occupied 1/6 volume of the vessel, and air filled the rest of the space. MicrowaVe-Assisted High-Speed Synthesis of ZnSe QDs. The preparation of NaHSe was performed according to the method described in ref 34. ZnSe monomers were synthesized according to the method described in ref 29. Briefly, colloidal ZnSe monomers were prepared by adding freshly NaHSe solution to ZnCl2 solution at pH 6.5 in the presence of 3-mercaptopropionic acid (MPA) as the stabilizer. The molar ratios of Zn2+:NaHSe: MPA is equal to 1:0.5:2.4. Then ZnSe QDs grew at microwave irradiation (140 °C) or by water-bath heating (100 °C). The onset of absorption spectra was size-dependent, and we could evaluate the growth rate from the shift of the onset of absorption spectra. ImproVement of PL Properties by Optimizing Synthesis Conditions. The pH influences of reaction solutions, molar ratio of precursors, and heating time of microwave irradiation on optical properties were systematically studied. In an optimal synthesis, aqueous colloidal ZnSe monomers were prepared by adding freshly prepared NaHSe solution to ZnCl2 solution at pH 7.3 in the presence of MPA as stabilizer. The molar ratio of Zn2+:NaHSe:MPA used was 8:0.5:28.8. ZnSe monomers were put into the vessel and placed into the microwave digestion
J. Phys. Chem. B, Vol. 110, No. 18, 2006 9035 furnace. Under microwave irradiation (200 W), the reaction was maintained at 140 °C for 55 min. Highly luminescent ZnSe QDs were obtained according to this procedure. Characterization of As-Prepared Nanocrystals. No postpreparative treatment was performed on any as-prepared samples for optical characterization. The QY of nanocrystals was measured according to the methods described in ref 35 using Rhodamine 6G as a reference standard (QY: 95%). The asprepared nanocrystals were precipitated by adding 2-propanol to the solution. The precipitate was isolated by centrifugation and decantation. The wet precipitate was dried at vacuum. The as-prepared powders of nanocrystals were characterized by XRD and elemental analysis. TEM and HRTEM samples were prepared by dropping the aqueous nanocrystals onto carboncoated copper grids with excess solvent evaporated. UV-vis absorption spectra were obtained using a Lambda 20 UV-visible spectrophotometer (Perkin/Elmer). Fluorescence measurements were performed using a Varian Cary fluorescence spectrometer. All optical spectra were measured at room temperature. X-ray powder diffraction (XRD) spectra were taken on a Bruker AXS D8-advance X-ray diffractometer with Cu KR radiation (λ )1.5418 Å). TEM images were recorded on a JEM-100CX with an accelerating voltage of 100 kV. HRTEM images were recorded on a JEM-2010F with an accelerating voltage of 200 kV. Elemental analysis performed by VARIO EL III was to characterize the contents of sulfur and carbon. Results and Discussion Although aqueous synthesis exhibited certain advantages described above, preparation of ZnSe QDs in the water phase usually took a very long reaction time (more than 1 day). Importantly, the product possessed very low QY (less than 1%) and a broad trap emission band (400-600 nm). Only an additional very weak band edge emission was observed.25 Therefore, a direct method is necessary for rapid synthesis of highly luminescent ZnSe QDs in water phase. Rapid Synthesis of ZnSe QDs by Microwave Irradiation. A conventional external heat source, such as oil bath and water bath, was a comparatively slow and inefficient method for transferring energy into the system.36 These heating techniques would bring a temperature gradient within the sample. In addition, local overheating would easily occur near the vessel wall. These drawbacks of a conventional heat source would cause a slow and inhomogeneous nucleation process, which would facilely bring out broad distribution of nanocrystals.37 On the contrary, microwave irradiation was internal heating by a coupling of microwave energy and the molecules interaction in reaction solution. It was a fast and highly efficient method for transferring energy into the system. The temperature increased uniformly throughout the sample. In our system, the water was suitable for serving as a solvent of microwave synthesis. High temperature (140 °C) could be facilely obtained in few minutes (less than 2 min) by microwave irradiation. The good crystallinity and high quality of the as-prepared nanocrystals may contribute to the uniformity and fast heating under microwave irradiation. Figure 1 presented the temporal evolution of UV-vis absorption spectra of ZnSe QDs prepared with microwave irradiation at 140 °C (A) and prepared by conventional aqueous synthesis (refluxing at 100 °C) (B). As shown in Figure 1, by conventional refluxing at 100 °C, the growth rate of ZnSe QDs was very slow, and generally, it took 1-2 days to prepare the ZnSe QDs. The growth rate of ZnSe QDs was very fast using microwave irradiation at 140 °C, and the preparation of same
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Figure 2. Comparison of PL spectra of ZnSe QDs prepared with microwave irradiation at the optimal procedure (A) and with waterbath heating (100 °C) (B). The excited wavelength was 300 nm.
Figure 1. Temporal evolution of the UV-vis absorption spectra of ZnSe QDs in aqueous prepared at 140 °C by microwave irradiation (A) and prepared by conventional aqueous synthesis (refluxing at 100 °C) (B).
size ZnSe QDs as in conventional refluxing for 2 days only needed bout 30 min. Meanwhile, the fast and homogeneous nucleation process and crystal growth under microwave irradiation would narrow the size distribution and improve the crystallinity of ZnSe nanocrystals. In our procedure, the rapid growth of ZnSe QDs was mainly attributed to the effects of higher reaction temperature and microwave irradiation. Increasing reaction temperature was an efficient way to accelerate the growth rate of nanocrystals. In traditional aqueous synthesis and normal microwave-assisted synthesis, the synthetic temperature generally did not exceed 100 °C. In our microwave system, the reaction temperature could be controlled from 100 to 200 °C, which was suited to the rapid synthesis of nanocrystals. Additionally, microwave irradiation also was another factor affecting the growth rate of nanocrystals, but its effects are not yet understood enough so far. Worth noticing was that ZnSe QDs obtained from both microwave irradiation and conventional aqueous synthesis had poor PL properties, which were mainly attributed to traps on the surface of ZnSe QDs. Improvement of PL Properties of ZnSe QDs. To improve the PL properties of the ZnSe QDs, that is, improvement of the band edge and suppression of the trap emission, we systematically investigated the influences of synthesis conditions on the PL properties. Figure 2 showed the PL spectra of ZnSe QDs prepared with microwave irradiation and conventional procedure with water-bath heating (100 °C). The ZnSe QDs prepared according to conventional aqueous synthesis had poor luminescence, their QYs were less than 0.1%, and their luminescence was mainly attributable to trap emission. However, the ZnSe QDs obtained from microwave irradiation at the optimal
Figure 3. PL spectra of ZnSe QDs prepared at different pH values of Zn2+-MPA precursors (a, pH 6.5; b, pH 6.9; c, pH 7.3; d, pH 7.7; and e, pH 8.0). Heating temperature was 140 °C, and heating time was 35 min ([Zn] ) 5 mmol/L, [Se] ) 0.675 mmol/L, and [MPA] ) 18 mmol/ L). The excited wavelength was 350 nm.
procedure showed high luminescence, the QY was about 17%, and excitionic emission was dominant in luminescence. Figure 3 showed the PL spectra of ZnSe QDs prepared at pH 6.5 to pH 8.7 of Zn2+-MPA precursor solutions. The heating temperature and time were 140 °C and 35 min, respectively. We found that when the pH value of the Zn2+-MPA precursor was 6.5, the band edge emission was slightly stronger than the trap emission. When the pH of the Zn2+-MPA precursor was 7.3, the as-prepared ZnSe QDs showed a very strong band edge emission and a relatively weak trap emission. The intensity of both band edge emission and trap emission were enhanced dramatically. However, when the pH value of the Zn2+-MPA precursors was over 7.7, the PL properties of as-prepared solutions became very weak. The improvement of ZnSe QDs at lower pH value (the pH value of Zn2+-MPA precursors was 7.3, and after injection of NaHSe the pH value of ZnSe precursors increased to 7.5) probably was attributed to the formation of the Zn2+-MPA complex shell on the surface of the ZnSe core. This result was similar to the synthesis of CdTe QDs.38,39 We fixed the pH value of Zn2+-MPA at 7.3 in latter experiments. To get an optimal molar ratio of Zn2+:NaHSe:MPA, we investigated PL properties of various molar ratios of Zn2+: NaHSe:MPA with prolonging the heating time. We set the molar ratio of Zn2+/MPA as a constant (Zn2+:MPA ) 3.6) and then changed the molar ratio of Zn2+/NaHSe from 4 to 24. The reaction temperature was 140 °C. We found that when Zn2+/
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Figure 4. PL spectra of ZnSe nanocrystals with different molar ratios of Zn2+:NaHSe:MPA (A, 12:0.5:43.2; B, 8:0.5:28.8; C, 4:0.5:14.4; and D, 2:0.5:7.2) at different reaction time (5, 15, 25, 35, and 55 min). The reaction temperature was 140 °C. The excited wavelength was 300 nm.
NaHSe ratio was less than 4, as-prepared nanocrytals possessed poor photoluminescence and were facile to precipitate under heating conditions. As shown in Figure 4D, both band edge and trap emission of as-prepared nanocrytals were very weak, and there was almost no emission when the heating time was more than 35 min due to its precipitation. Figure 4A-C indicate that when the Zn/Se ratio was more than 8, the photoluminescence of as-prepared nanocrystals improved dramatically with prolonging heating time from 5 to 55 min. Usually, the band edge emissions increased continually with prolonged time until the reactant solution precipitated. Furthermore, the trap emissions increased gradually to the maximum at 35 min and then decreased with the reaction progress. We found that the optimal molar ratios of Zn2+:NaHSe:MPA was 8:0.5:28.8. In this molar ratios of Zn2+:NaHSe:MPA, the band edge emission and the ratios of band edge emission/trap emission increased significantly with prolonged heating time, and the band edge emission reached a maximum at 55 min. However, if we continued increasing the molar ratio of Zn/Se to 24, the band edge emissions decreased slightly, and the trap emissions showed no obvious change. We sum up that the optimal procedure to synthesize ZnSe QDs was as follows: First, aqueous colloidal ZnSe monomers were prepared by adding freshly NaHSe solution to ZnCl2 solution at pH 7.3 in the presence of MPA as stabilizer. The molar ratio of Zn2+:NaHSe:MPA used was 8:0.5:28.8. Second, ZnSe monomers were put into the vessel and placed into the microwave digestion furnace. Under microwave irradiation (200 W), the reaction was maintained at 140 °C for 55 min. The QY of as-prepared nanocrystals could be up to 17% under this condition. The nanocrystals prepared at the optimal condition had an excitonic peak at 395 nm, and they were characterized by TEM
and HRTEM. Figure 5A shows the TEM image of as-prepared nanocrystals, and Figure 5B shows a size distribution histogram. The average size was about 3.5 ( 0.6 nm, which was larger than the corresponding ZnSe QDs because it probably formed ZnSe-ZnS alloy nanocrystals.19 The inset shows the HRTEM image of one nanocrystal, which demonstrated its high crystallinity. Formation of ZnSe(S) Alloy Nanocrystals. In the optimal procedure for synthesis of ZnSe QDs, as shown in Figure 4B, the PL spectra improved dramatically with prolonging heating time from 5 to 55 min at 140 °C. The as-prepared nanocrystals in the different stages of growth were characterized by XRD, EDS, and elemental analysis in order to know the structure and composition of nanocrystals. Figure 6 shows the temporal evolution of the power XRD patterns of as-prepared nanocrystals. The as-prepared nanocrystals belonged to a cubic structure (zinc blende). When the nanocrystals were prepared by heating for 5 min at 140 °C, the diffraction peaks were located at the positions of the corresponding peaks for pure ZnSe. Meanwhile, the broad peaks implied that the nanocrystals were very small. With a prolonged heating time, the diffraction peaks gradually shifted from cubic ZnSe to cubic ZnS phase, which indicated that sulfur entered the ZnSe structure gradually and a ZnSe(S) alloy structure was formed. The diffraction peaks also became narrower with a longer heating time, which implied that the size of corresponding nanocrystals become bigger. The details about the sizes of nanocrystals prepared under different heating time are shown in the Table 1, which was calculated according to the DebyeScherrer formula.40 The size of as-prepared nanocrystals by heating 55 min at 140 °C was about 3.4 nm, which was in accord with the results of TEM.
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Figure 5. TEM image (A) and size distribution (B) of ZnSe QDs. The scale bar in the inset of HRTEM image was 5 nm. The ZnSe nanocrystals were prepared according to the optimal procedure. The size distribution histograms were obtained by averaging the sizes of 150 particles from the TEM images.
Figure 6. Temporal evolution of the power X-ray diffraction patterns of as-prepared ZnSe QDs. The nanocrystals were prepared by heating for 5 min (a), 35 min (b), and 55 min (c) at 140 °C. The molar ratio of Zn2+:NaHSe:MPA is equal to 8:0.5:28.8. The line spectra showed standard diffraction lines of cubic ZnSe (up) and ZnS (down).
TABLE 1: Molar Ratios of Zn/Se, St/Se, SR-/St, S2-/St, and S2-/Se as Well as the Sizes for ZnSe Nanocrystals in Different Stage of Growth under Microwave Irradiationa heating time (min)
size (nm)
Zn/Se
5 35 55
2.34 2.96 3.40
1.99 2.09 2.41
molar ratio of element St/Se SR-/St S2-/St
S2-/Se
1.27 1.96 2.35
0.025 0.49 1.03
0.98 0.75 0.56
0.02 0.25 0.44
The heating temperature is 140 °C. St represents all sulfur in specimens, SR- represents the sulfur contributed by stabilizer MPA, and S2- represents the sulfur coming from ZnS. a
Figure 7 shows the EDS spectra of specimens in different stages of growth for different times of microwave irradiation. The EDS spectra reveal the existence of Zn, S, and Se in the sample, implying that the nanocrystals perhaps had an alloy shell of ZnSe(S). The arrows indicate the positions of sulfur and selenium. It was very obvious that the content of St (total sulfur) increased remarkably with the growth of nanocrystals in comparison with the content of Se. The relative contents of Zn, Se, and St from EDS spectra are also shown in Table 1. With a prolonged heating time, the PL spectra improved remarkably, and the corresponding values of Zn/Se and St/Se increased gradually. The sulfur in the initial sample was mainly from the stabilizer MPA. In principle, with the growth of nanocrystals, the molar ratio of St/Se should decrease because surface area/ volume ratio decreased. But in our procedure, St/Se increased
Figure 7. Temporal evolution of the EDS spectra of specimens. The nanocrystals were prepared by heating for 5 min (a), 35 min (b), and 55 min (c) at 140 °C. The molar ratio of Zn2+:NaHSe:MPA is equal to 8:0.5:28.8.
with the growth of nanocrystals. So we supposed that additional sulfur ions participated in the growth of nanocrystals. The amounts of carbon and sulfur included in the MPAcapped ZnSe nanocrystals were determined by elemental analysis. The molar ratio of carbon to sulfur contributed by MPA (SR-) was equal to 3:1. So we could calculate the percent of S2- (sulfur coming from ZnS) and SR- in the total sulfur (St).41 As shown in Table 1, in the initial stage of growth (5 min),
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SCHEME 1: Potential Reactions of S2- Decomposed from Stabilizer MPA
SR- was the dominant component in the total sulfur, and the content of S2- was so little that it almost need not be considered. However, in prolonging the heating time, the molar ratio of S2-/ St increased greatly. The corresponding PL spectra were also increased dramatically. The nanocrystals that had best the PL properties (55 min at 140 °C) had about 44% S2- in the total sulfur. The molar ratio of S2-/Se in the nanocrystals was calculated according to the results of EDS (St/Se) and elemental analysis (S2-/St). As shown in Table 1, in the initial stage of growth, it was almost pure ZnSe despite the stabilizer MPA. With a prolonged heating time under microwave irradiation, the nanocrystals grew to larger size and the molar ratio of S2-/Se increased obviously, which implied that the growth of nanocrystals was partially attributed to ZnS depositing on the surface of ZnSe core. On the data mentioned above, we supposed that the structure of nanocrystals was a ZnSe(S) alloy shell on the surface of ZnSe particles core. Under heating by microwave irradiation, the temperature of reaction was up to 140 °C. At initial stage, few sulfurs were released by MPA, and only ZnSe nanocrystals were formed because of the intrinsic Se and S reactivity with zinc (Se is more active than S). With prolonged heating time, a lot of sulfurs were released owing to the high concentration of MPA. In the reaction solution, MPA concentration was 57.6 times more than that of NaHSe, which facilely led to form an alloy ZnSe(S) shell on the surface of ZnSe nanocrystals. Meanwhile, with the formation of the alloy ZnSe(S) shell, the surface of ZnSe was effectively passivated, which improved the luminescent properties dramatically. The oxygen from air in the reaction vessel played an important role in the decomposition of the thiol group under microwave irradiation. The possible reactions are expressed in Scheme 1. During heating at 140 °C under microwave irradiation, the oxidation of thiols to disulfide occurred with the help of oxygen from air in the reaction reaction (see eq 1). The cleavage of S-S bonds would occur in the presence of OH-. The decomposition products would be thiol and thiocarbonyl compound (see eq 2).42 The sulfur ions would be released under the following degradation of thiocarbonyl compound42,28 (see eq 3). The oxygen from air in the reaction vessel played an important role in the decomposition of thiol group. As shown in Figure 8, the PL spectra of ZnSe nanocrystals prepared according to the optimal procedure just different in heating circumstances (a, air; b, N2; and c, O2) could demonstrate this mechanism indirectly. In the reaction condition mentioned above, ZnSe monomers occupied 1/6 volume of reaction vessel, and air filled the other space when ZnSe monomers were heated at 140 °C under microwave irradiation. The as-prepared nanocrystals showed a strong excitonic emission and a weak trap emission (see spectrum a). However, when ZnSe monomers and other spaces of the vessel were saturated by N2, the trap emission was dominant in the luminescence and the excitionic peak was only a shoulder peak (see spectrum b). It was because the thiol group could hardly decompose without oxygen in this
Figure 8. PL spectra of ZnSe QDs prepared according to the optimal procedure just different in heating circumstances: air (a), N2 (b), and O2 (c). The excited wavelength was 300 nm.
condition. Traps were facilely formed in the surface of ZnSe nanocrystals without coating of larger band gap materials. But when ZnSe monomers and other spaces of the vessel were saturated by O2, the high concentration of O2 would decompose the thiol group rapidly, even the stabilizer on the surface of nanocrystals would be oxidized, causing the precipitation of nanocrystals. There was almost no photoluminescence under this condition (see spectrum c). Conclusion In this paper we described a rapid aqueous synthesis of luminescent ZnSe QDs by microwave irradiation. Our data demonstrated that the growth rates of ZnSe QDs prepared by microwave irradiation were much faster than that prepared by conventional refluxing. The PL properties were dramatically improved by selecting appropriately molar ratios of precursors, the pH value of Zn2+-MPA precursors, and the heating time of microwave irradiation at certain temperature. The as-prepared ZnSe QDs were water-soluble and had high quantum yield (17%) and good crystallinity. According to the results of characterization, the improvement of luminescence was attributed to the sulfur ions released from stabilizer and forming a ZnSe(S) alloy shell on the surface on the ZnSe nanocrystals. We speculated that the oxygen from air in the reaction vessel could cause the release of S2- decomposed from the stabilizer MPA when heated by microwave irradiation and was helpful to the formation of high luminescent ZnSe(S) alloy nanocrystals. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (Nos. 20271033 and 90408014), the key project of National Natural Science Foundation of China the Natural (No. 20335020), and the Nano-Science Foundation of Shanghai (0452NM052, 05NM05002). References and Notes (1) Alivisatos, A. P. J. Phys. Chem. 1996, 100, 13226-13239. (2) Alivisatos, A. P. Science 1996, 271, 933-937. (3) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Annu. ReV. Mater. Sci. 2000, 30, 545-610. (4) Mamedov, A. A.; Belov, A.; Giersig, M.; Mamedova, N. N.; Kotov, N. A. J. Am. Chem. Soc. 2001, 123, 7738. (5) Klimov, V. I.; Mikhailovsky, A. A.; Xu, S.; Malko, A.; Hollingsworth, J. A.; Leatherdale, C. A.; Eisler, H.-J.; Bawendi, M. G. Science 2000, 290, 314. (6) Sundar, V. C.; Eisler, H.-J.; Bawendi, M. G. AdV. Mater. 2002, 14, 739.
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