Direct Deposition of Fluorescent Emission-Tunable CdSe on

Apr 22, 2009 - blue-shift was ascribed to the lowering of activation energy in ... CdSe quantum dots would directly deposit on seed nanocrystals. 1...
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Direct Deposition of Fluorescent Emission-Tunable CdSe on Magnetite Nanocrystals Ke Tao, Huirui Zhou, Hongjing Dou, Bin Xing, Wanwan Li, and Kang Sun* State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong UniVersity, Shanghai 200240, People’s Republic of China ReceiVed: February 13, 2009; ReVised Manuscript ReceiVed: April 1, 2009

Fluorescent color-tunable Fe3O4-CdSe hybrid nanocrystals were synthesized by a facile hot-injection method. Heterogeneous nucleation was found directly happening on starting Fe3O4 seeds, leading to the controllable emission peaks by changing reaction time, temperature, and dose of oleic acid. In comparison with pure CdSe quantum dots, it was found that the emission peaks of hybrid nanocrystals were obviously blue-shifted and responsible for the amount of seed nanocrystals. By evaluating the concentration of CdSe nuclei, the blue-shift was ascribed to the lowering of activation energy in heterogeneous nucleation and explained why CdSe quantum dots would directly deposit on seed nanocrystals. 1. Introduction Multicomponent hybrid nanocrystals consisting of two or more functional inorganic domains have attracted intense attention in recent years because they usually possess bifunctional or multifunctional properties, and therefore their potential applications in biomedical fields are expanded.1-3 Efforts have been devoted to the synthesis of variant kinds of hybrid nanostructures, such as noble metal-noble metal,4 noble metal-quantum dots (QDs),5 and metal oxide-metal nanocrystals.6 Among these hybrid nanocrystals, iron oxide-quantum dots, known as magnetic quantum dots (MQDs), are of great interest. Magnetic nanocrystals exhibit superparamagnetism and have been widely used in biomedical fields, such as magnetic resonance imaging, drug delivery, and cell separation,7-9 while size-tunable optical properties and high photoluminescence quantum yields have made QDs a novel and ideal optical material for biological fluorescent labels, light-emitting diodes, and lasers.10-12 By combining magnetic nanocrystals and QDs together, MQDs are possible to simultaneously show magnetic and optical properties and, therefore, have shown a promising future or revealed some novel applications in biomedical fields.13,14 Some MQDs, such as Fe3O4-CdS, Fe2O3-CdS, FePt-CdSe, or Co-CdSe,15-19 have been synthesized by using magnetic nanocrystals as seeds. In these approaches, an amorphous metastable QDs shell structure is presented as intermediate and then transformed to crystalline QDs domains by an annealing process. However, the ability to tune separately the size of their single domain has not been demonstrated, and therefore the emission-tunable property of MQDs, which is significant for their applications, could not be achieved. Recently, Selvan et al.20 applied a hot-injection approach and succeeded in the synthesis of MQDs (Fe2O3-CdSe) with tunable emission peaks by changing the reaction time. However, in this case, a purification procedure is required for iron oxide seed nanocrystals before further reaction and the expensive hazardous solvent tri-n-octylphosphine oxide (TOPO) is used. Furthermore, in the mostly used seed-mediated growth strategy of MQDs synthesis, how the seed particles participated in the heteroge* To whom correspondence should be addressed. Tel.: 86-21-34202743. Fax: 86-21-34202745. E-mail: [email protected].

neous nucleation has not been fully elucidated yet. It is also unclear whether the optical properties of QDs could be influenced by magnetic nanocrystals. Herein we designed a facile approach to yield Fe3O4-CdSe hybrid nanocrystals based on the similar conditions of two classic synthetic approaches for Fe3O4 and CdSe nanocrystals, respectively, and therefore the syntheses of both Fe3O4 seeds and MQDs were preceded in the same noncoordinating octadecene (ODE) solution. By this method, heterogeneous nucleation directly happens on a starting Fe3O4 seed by a facile injection of cadmium precursor, and the position of fluorescent peaks can be easily controlled by changing reaction conditions. Besides, it was also found that the emission wavelength of MQDs was obviously blue-shifted in comparison with pure CdSe QDs synthesized under the same reaction conditions. The elucidation of this reaction process may be helpful for understanding the role of inorganic seed nanocrystals in the synthesis of multicomponent hybrid nanocrystals. 2. Experimental Section The 8 nm Fe3O4 nanocrystals were synthesized by Hyeon’s method21 using ODE as solvent. In a typical experiment, 0.812 g of FeCl3 (5 mmol) and 4.563 g of sodium oleate (15 mmol) were dissolved in a mixture solvent composed of 20 mL of ethanol, 15 mL of distilled water, and 35 mL of hexane. The resulting solution was heated to 70 °C and stirred at this temperature for 4 h. When the reaction was finished, the upper organic layer containing the iron-oleate complex was separated and washed three times with 30 mL of distilled water in a separatory funnel. After washing, the resultant iron-oleate complex was dried in a vacuum oven at 70 °C for 24 h. An amount of 5 mmol of iron-oleate complex synthesized as described above and 1.636 mL of oleic acid (OA) (5 mmol) were dissolved into 30 mL of ODE at room temperature. The reaction mixture was heated to 320 °C under nitrogen flow and kept at this temperature for 30 min. The resultant solution was then cooled down to the room temperature for further reaction without any separation or purification. The resultant Fe3O4 solution (without rinsing or separation; contains 0.1-0.3 mmol of Fe3O4), 0.2 mmol of Se powder, and 18 mL of ODE were mixed and heated to 200-230 °C under vigorous stirring and a nitrogen atmosphere. Meanwhile, a

10.1021/jp901335s CCC: $40.75  2009 American Chemical Society Published on Web 04/22/2009

Fluorescent Color-Tunable Fe3O4-CdSe Nanocrystals

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Figure 1. TEM image (A) and XRD pattern (B) of Fe3O4 seeds nanocrystals.

Figure 2. TEM (A) and HRTEM (B) images of Fe3O4-CdSe MQDs.

cadmium oleate stock solution was prepared by heating a mixture of 51.2 mg of CdO (2 mmol), 1.875 mL of OA (6 mmol), and 9 mL of ODE to 160 °C until a clear solution was obtained. Then, 2 mL of cadmium oleate stock solution (containing 0.4 mmol of CdO and 1.2 mmol of OA) was injected into the ODE solution of Fe3O4 and Se. The color of the mixture changed from black to red-black immediately, indicating the formation of CdSe domains. The resulting MQDs were allowed to grow for different times to yield CdSe domains corresponding to different optical emissions. After that, the reaction mixture was cooled to room temperature by removing the heat source. Acetone was added to the mixture, and the red-black product was precipitated via centrifugation. Then the particles were redispersed in hexane/ acetone solution and subsequently collected by a magnet at the wall side of the flask. The supernate was poured. This process was repeated for five times to remove the free QDs. The final MQDs were dissolved in hexane or freeze-dried for further characterization. UV-vis absorption and fluorescence data were obtained by using a UV-2550 Shimadzu UV-vis spectrophotometer and RF5301PC Shimadzu spectrofluorophotometer at room temperature, respectively. The photoluminescence (PL) spectra were collected between 400 and 800 nm using an excitation wavelength of 400 nm with a slit width of 5.0 nm. The UV-vis absorption spectra were collected between 300 and 800 nm with a slit width of 2.0 nm. The PL quantum yields of the as-prepared CdSe QDs were obtained by comparing the integrated PL intensities of the CdSe QDs with that of rhodamine 6G (quantum yield ) 95%). High-resolution transmission electron microscopy (HRTEM) images were obtained on a JEOL JEM-2100F transmission electron microscope. X-ray diffraction (XRD) patterns were recorded with a Bruker-AXS X-ray diffractometer.

3. Results and Discussion Magnetite seed nanocrystals were synthesized by Hyeon’s method21 by using ODE as the solvent. The TEM image (Figure 1A) demonstrates their unique size at about 8 nm. The XRD pattern (Figure 1B) illustrates the phase of magnetite due to the same position and intensity of diffractive peaks with pure magnetite. Because of the similar environment (OA/ODE), a subsequent step to grow CdSe on the magnetite nanocrystals was performed by simply synthesizing CdSe QDs in the original Fe3O4/OA/ODE solution. HRTEM images shown in Figure 2 demonstrate successful connection between magnetic seed nanocrystals and CdSe QDs. The heterojunction structure of as-prepared nanocrystals was obtained similarly to the literatures,15-18,20 which is caused by the lattice mismatch between Fe3O4 and CdSe. The domains of seed nanocrystals and subsequent grown QDs can be relatively identified according to their appropriate size, and clear lattice fringes indicate their good crystallinity. It can also be found that almost all seed nanoparticles were connected with CdSe, with some individual seed particles attaching by several CdSe domains. Crystallinity is very important for the functional properties of the particle domains. In order to directly compare the fluorescent property of the hybrid particles with those of individual CdSe nanoparticles, the semiconductor domain in the hybrid particles should be crystalline. In the case of synthesis of hybrid particles the CdSe domain is formed through heterogeneous nucleation followed by growth and dewetting, and therefore the peculiar crystal-formation pathway needs to be monitored in order for the crystallinity to be assessed. XRD patterns clearly indicate the cystallinity of Fe3O4 seed nanoparticles and Fe3O4-CdSe MQDs during the whole process in the reaction, as shown in Figure 3. After the addition of Se

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Figure 3. XRD patterns of Fe3O4 after adding Se and the resultant Fe3O4-CdSe MQDs.

(followed by rinsing), amorphous background cannot be observed on the XRD pattern. Meanwhile, the peak positions of the synthesized MQDs are, respectively, well consistent with those calculated for the spinel Fe3O4 (JCPDS PDF 89-4319) and zinc blende CdSe (PDF 88-2346), as well as similar to the presynthesized seed nanocrystals (Figure 1B) and phosphinefree synthesized CdSe QDs,22 respectively, confirming the composition and crystalline structure of the Fe3O4 -CdSe MQDs. The fluorescence spectra of Fe3O4-CdSe MQDs in a typical synthesis are shown in Figure 4A. With growth time increasing, the PL peaks of MQDs varied from 488 to 527 nm and are corresponding to the size of CdSe domains in a similar manner to pure colloidal CdSe nanocrystals. The full width at halfmaximum (fwhm) of the PL peak was kept at about 30 nm during the whole reaction, which is narrower than previously reported MQDs and comparable to pure CdSe QDs. A weak shoulder peak at about 460 nm was found on the fluorescent spectrum, which should be ascribed to the solvent (hexane) in Fe3O4 solution (Figure 4A inset). The corresponding UV-vis spectra are shown in Figure 4B and indicated that the Stoke’s shift is about 7 nm. Similar to the phosphine-free synthesis of pure CdSe QDs,22,23 the optical emission peak positions are redshifted with the increase of reaction temperature and concentration of OA. By changing these conditions, PL peaks of Fe3O4-CdSe MQDs can be controlled in the range from 486 to 568 nm with fwhm still keeping at about 30 nm. Their quantum yields are located in the range of 2-9%, which are comparable to the reported FePt-CdSe core-shell nanocrystals and higher than FePt-CdS core-shell nanocrystals.18 Additionally, MQDs in this work show high sensitivity of fluorescence and express cyan to yellow color under 365 nm UV excitation, as is shown in Figure 5. By the current method, red color can also be obtained by increasing the amount of OA or decreasing the number of seed nanocrystals. However, in the red color window, PL intensity is extremely weak (comparable to the PL of Fe3O4 solution) and fwhm is relatively wide (about 50 nm). In previous syntheses of Fe2O3-CdS, FePt-CdSe, or FePt-CdS,16-19,24 by mixing precursors at 80-120 °C and

Tao et al. subsequent heating up, amorphous S (Se) and then highly defective shell of QDs were grown on the magnetic seed nanoparticles and consequently coalesced and formed a separate grain upon an annealing process. Probably because of the fixed amount of amorphous QDs shells on individual seed nanocrystals, MQDs fluorescent peak positions are hard to control. Additionally, because of the presence of multicrystalline QDs, their emission peaks are broad with fwhm in the range of 50-70 nm.18 However, in the current work, by injecting cadmium precursor at above 200 °C, instead of 80-120 °C, the highly crystalline QDs heterodimers rather than amorphous Se shell would be directly formed and the annealing process is not required, as shown in Scheme 1. The reason that amorphous Se would not absorb on seed nanocrystals is not clear yet, and we propose that the relatively high temperature (above 200 °C) increases the solubility and activity of Se, leading to the dissociation between Se and Fe3O4. Because an amorphous intermediate does not exist and an annealing process is not required, the emission wavelength can be easily controlled, and fwhm will not be broadened. Additionally, ODE as an inexpensive and green noncoordinating solvent for both magnetite seeds and QDs synthesis allows the reaction to be performed by a facile hot-injection procedure. Although that heterogeneous nucleation and the consequent growth of CdSe domain directly happen on a starting seed in our method is similar to the sizetunable synthesis of the Au-CoPt3 system25 and is possible why CdSe domains could deposit on seed nanocrystals without an intermediate Se absorption should be demonstrated. For comparison, pure CdSe QDs were synthesized under the same conditions, including [Cd]/[Se] molar ratio, concentration of OA, reaction temperature, and time. The PL peak positions of Fe3O4-CdSe MQDs were remarkably found being shifted to the shorter-wavelength direction in comparison with those of pure CdSe QDs, as the examples shown in Figure 6, indicating a lower growth rate for CdSe domains in MQDs. Because simply mixing magnetite nanocrystals and CdSe QDs would not lead to the shift of PL peaks, this result rules out the possibility that our products were predominantly composed of individual Fe3O4 and CdSe nanocrystals. At first sight, the origin of the blue-shift was considered as the control of OA dose in the MQDs synthesizing because OA exists in both Fe3O4 seeds and MQDs preparation. However, the dose of OA to yield CdSe QD domains in Fe3O4-CdSe MQDs is equal to synthesis of pure CdSe QDs, which means that the total dose of OA for MQDs (Fe3O4 seed synthesis plus heterodimers synthesis) had to be more than that for pure CdSe QDs. According to previous literatures,22,23,26 the concentration of the nuclei in the synthesis will decline with the increase of the concentration of capping agents. If higher concentration of OA coming from the Fe3O4 seed synthesis influenced the nucleation and growth of CdSe domains, it would lead to bigger particles corresponding to longer PL peaks. However, the phenomenon was reverse, demonstrating that the observed blue-shift was not caused by the OA dose. In our study, another serial of pure CdSe QDs were synthesized under the condition of keeping the same total dose of OA with MQDs synthesizing. As expected, the similar blue-shift phenomenon was also observed. According to classical nucleation theory,27 the nucleation activation energy for heterogeneous nucleation is lower than that for homogeneous nucleation. Therefore, more nuclei would be produced with the existence of seed nanocrystals, and nucleation on seed nanocrystals is preferred. Actually, the inducing nucleation effect can be caused by macromolecules existing in nanoparticles preparation28 and have been proposed

Fluorescent Color-Tunable Fe3O4-CdSe Nanocrystals

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Figure 4. Photoluminescence (A) and UV-vis (B) spectra of a typical batch of MQDs; the inset in panel A is the PL spectrum of magnetite seed nanocrystals.

Figure 5. Photos of hexane-dispersed MQDs under (A) sunlight and (B) UV excitation at 365 nm, respectively.

SCHEME 1: Schematic Illustration of the Direct Nucleation That Happened without Amorphous Intermediate in the Current Reaction

Figure 6. Difference between the fluorescence spectra of Fe3O4-CdSe MQDs and CdSe QDs prepared under three different temperatures for 5, 8, 15, and 30 min, respectively. [Fe3O4] ) 10 mM.

in the synthesis of FePt-iron oxide hybrid nanocrystals.29 It is reasonable that the participation of the magnetite seeds should increase the nucleation rate, and the growth rate would be decelerated. To verify this effect of inducing nucleation, various amounts of Fe3O4 seed nanocrystals were controlled to evaluate their influence on CdSe formation. The variation of blue-shift of the PL peaks of MQDs as a function of time for different Fe3O4 concentrations is shown in Figure 7A. In all these reactions, the blue-shift (∆λ) was found to be in existence within 1 min after the injection, followed by an initial rapid increase during the first 10 min and then gradually reached equilibrium when the reactions were terminated at 30 min. It is apparent that the blue-shift should be consistently increased by the addition of the Fe3O4 seed concentrations, which well proves that the difference of PL peak positions between MQDs and pure CdSe QDs is caused by the amount of Fe3O4 seed nanocrystals. To further demonstrate how Fe3O4 seeds affect the nucleation and growth of CdSe domains, the CdSe nuclei concentration in MQDs synthesis was calculated by the extinction coefficient method reported by Yu et al.30 Figure 7B presents the nuclei concentration for CdSe domains in MQDs as a function of time. In all reactions an initial increase in CdSe domains concentration of MQDs is observed, which peaks at a maximum concentration ([CdSe]max), and then the particle concentration declines and

Figure 7. (A) Blue-shift (∆λ) of PL peaks of Fe3O4-CdSe MQDs as a function of time for different concentrations of Fe3O4. (B) Calculated CdSe domains nuclei concentrations as a function of time for different concentrations of Fe3O4 in MQDs synthesizing. The concentration of Cd and Se were fixed as 20 and 10 mM, respectively.

gradually reaches equilibrium. For Fe3O4-CdSe MQDs, [CdSe]max, which directly reflects the amount of initial nuclei, increases with the amount of Fe3O4 seed nanocrystals increasing. This result directly confirms that Fe3O4 seed nanocrystals would induce the nucleation of CdSe caused by lowering nucleation activation energy and also excludes the possibility that Fe3O4 hinders the growth of CdSe domains. Because of the increase of CdSe nuclei, their growth rate would decrease at the same precursors concentration, and therefore blue-shift was observed on the fluorescence spectra for MQDs. In addition, because of

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the lowering nucleation activation energy, it can also explain why CdSe would nucleate and grow on seed nanocrystals without the absorbance of amorphous Se on seed nanocrystals. 4. Conclusion In summary, we showed a facile procedure for synthesizing Fe3O4-CdSe MQDs with tunable optical emission properties in the ODE environment. The optical emission property of MQDs could be easily tuned by changing the reaction conditions for QD domains. By evaluating the optical properties of MQDs prepared under different conditions, the inducing nucleation effect of seed nanocrystals was demonstrated in the synthesis of Fe3O4-CdSe MQDs, which confirmed the mechanism of direct deposition of QDs and would be interesting to extend to the synthesis of other hybrid nanocomposites. Acknowledgment. This work was financially supported by NSFC (No. 30872630), the Science and Technology Committee of Shanghai (No. 08ZR1415700), and a Grant from the Ph.D. Programs Foundation of Ministry of Education of China (No. 200802481131). We thank the Instrumental Analysis Center of SJTU for the assistance on measurements. We also thank Shanghai Sunny New Technology Development Co., Ltd. for their support. References and Notes (1) Cozzoli, P. D.; Pellegrino, T.; Manna, L. Chem. Soc. ReV. 2006, 35, 1195. (2) Zeng, H.; Sun, S. AdV. Funct. Mater. 2008, 18, 391. (3) Jun, Y. W.; Choi, J. S.; Cheon, J. Chem. Commun. 2007, 1203. (4) Gu, H.; Yang, Z.; Gao, J.; Chang, C. K.; Xu, B. J. Am. Chem. Soc. 2005, 127, 34. (5) Mokari, T.; Sztrum, C. G.; Salant, A.; Rabani, E.; Banin, U. Nat. Mater. 2005, 4, 855. (6) Shi, W.; Zeng, H.; Sahoo, Y.; Ohulchanskyy, T. Y.; Ding, Y.; Wang, Z. L.; Swihart, M.; Prasad, P. N. Nano Lett. 2006, 6, 875. (7) Bulte, J. W.; Kraitchman, D. L. NMR Biomed. 2004, 17, 484.

Tao et al. (8) Pankhurst, Q. A.; Connolly, J.; Jones, S. K.; Dobson, J. J. Phys. D: Appl. Phys. 2003, 36, R167. (9) Gupta, A. K.; Gupta, M. Biomaterials 2005, 26, 3995. (10) Alivisatos, A. P. Science 1996, 271, 933. (11) Michalet, X.; Pinaud, F. F.; Bentolila, L. A.; Tsay, J. M.; Doose, S.; Li, J. J.; Sundaresan, G.; Wu, A. M.; Gambhir, S. S.; Weiss, S. Science 2005, 307, 538. (12) 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. (13) Quarta, A.; Di Corato, R.; Manna, L.; Ragusa, A.; Pellegrino, T. IEEE Trans. NanoBiosci. 2007, 6, 298. (14) Gao, J.; Zhang, W.; Huang, P.; Zhang, B.; Zhang, X.; Xu, B. J. Am. Chem. Soc. 2008, 130, 3710. (15) Kwon, K. W.; Shim, M. J. Am. Chem. Soc. 2005, 127, 10269. (16) Kwon, K. W.; Lee, B. H.; Shim, M. Chem. Mater. 2006, 18, 6357. (17) Gu, H.; Zheng, R.; Zhang, X.; Xu, B. J. Am. Chem. Soc. 2004, 126, 5664. (18) Gao, J.; Zhang, B.; Gao, Y.; Pan, Y.; Zhang, X.; Xu, B. J. Am. Chem. Soc. 2007, 129, 11928. (19) Kim, H.; Achermann, M.; Balet, L. P.; Hollingsworth, J. A.; Klimov, V. I. J. Am. Chem. Soc. 2005, 127, 544–546. (20) Selvan, S. T.; Patra, P. K.; Ang, C. Y.; Ying, J. Y. Angew. Chem., Int. Ed. 2007, 46, 2448. (21) Park, J.; An, K.; Hwang, Y.; Park, J. G.; Noh, H. J.; Kim, J. Y.; Park, J. H.; Hwang, N. M.; Hyeon, T. Nat. Mater. 2004, 3, 891. (22) Jasieniak, J.; Bullen, C.; van Embden, J.; Mulvaney, P. J. Phys. Chem. B 2005, 109, 20665. (23) Yu, W. W.; Peng, X. Angew. Chem., Int. Ed. 2002, 41, 2368. (24) Zanella, M.; Falqui, A.; Kudera, S.; Manna, L.; Casula, M. F.; Parak, W. J. J. Mater. Chem. 2008, 18, 4311. (25) Pellegrino, T.; Fiore, A.; Carlino, E.; Giannini, C.; Cozzoli, P. D.; Ciccarella, G.; Respaud, M.; Palmirotta, L.; Cingolani, R.; Manna, L. J. Am. Chem. Soc. 2006, 128, 6690. (26) Bullen, C. R.; Mulvaney, P. Nano Lett. 2004, 4, 2303–2307. (27) Cushing, B. L.; Kolesnichenko, V. L.; O’Connor, C. J. Chem. ReV. 2004, 104, 3893. (28) Lee, J.; Isobe, T.; Senna, M. J. Colloid Interface Sci. 1996, 177, 490. (29) Figuerola, A.; Fiore, A.; Di Corato, R.; Falqui, A.; Giannini, C.; Micotti, E.; Lascialfari, A.; Corti, M.; Cingolani, R.; Pellegrino, T.; Cozolli, P. D.; Manna, L. J. Am. Chem. Soc. 2008, 130, 1477. (30) Yu, W. W.; Qu, L.; Guo, W.; Peng, X. Chem. Mater. 2003, 15, 2854.

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