发表文章毕业论文2009-Yang-Crystal Grouthdesign-Continuous Synthesis of Full-Color Emitting Core Shell Quantum Dots via Microreaction

/crystalPublished on Web 09/18/2009 r2009 American Chemical Society DOI: 10.1021/cg900652y 2009, Vol. 9 48074813 Continuous Synthesis of Full-Color Emitting Core/Shell Quantum Dots via Microreaction Hongwei Yang,#Weiling Luan,*,#Zhen Wan,#Shan-tung Tu,#Wei-Kang Yuan,and Zhiming M. Wang #Key Laboratory of Safety Science of Pressurized System, East China University of Science and Technology, Shanghai 200237, China, State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai, 200237, China, and Materials Research Science and Engineering Center, University of Arkansas, Fayetteville, Arkansas 72701 Received June 10, 2009; Revised Manuscript Received September 7, 2009 ABSTRACT: Atwo-step,microreactionsystem,operatingattwodifferenttemperaturesandincorporatingthree-dimensional, serpentine microchannels, was used for the continuous synthesis of CdSe/ZnS and CdS/ZnS nanocrystals. By varying the feed ratiosoftheprecursors,wewereabletocontrolthesizeofbothCdSeandCdSnanocrystals(NCs)ataconstanttemperatureand residence time. The application of zinc diethyl dithiocarbamate as a single molecular source allowed the preparation of high- quality, ZnS-capped NCs at 140 ?C, which is much lower than the temperatures required for the growth of CdS and CdSe NCs. Furthermore, the size-dependent, optimal reaction time for ZnS capping was obtained. The above results facilitated the continuous synthesis of core/shell NCs with photoluminescence wavelength in a wide range. On the basis of the continuous adjustment of the feed ratios of the precursors and the online switching of precursors and serpentine channels for ZnS capping, we obtained the continuous synthesis of full-color-emitting CdS/ZnS and CdSe/ZnS NCs (full-width-at-half-maximum: 25-33 nm; quantum yield: blue - 41%, cyan - 58%, green - 70%, yellow - 78%, orange - 76%, red - 61%). Introduction Nanocrystallinesemiconductorsdemonstratesalientoptical properties and excellent chemical processability,1,2and these properties make them good candidates for applications in various fields, such as light emitting diodes (LED) displays, biological labels, and solar cells.3-5The key factors that govern the applicability of luminescent nanocrystals (NCs) in the biological and LED fields include their high lumines- cence quantum yield (QY), stable luminescent properties under real-world operational conditions, and good solubility in the desired solvents. All of these requires the proper passivation of dangling bonds that are present on the surfaces of the NCs. Organic ligands can endow NCs with high QY of photoluminescence (PL) and good solubility in various solvents. However, these organic ligands are sensitive to the chemical environment, and degradation of PL properties can occur during surface modifications.6Conversely, growing a thin shell of higher band gap inorganic material on the outside of the plain NCs allows substantial improvements of QY and robustness of PL. With well-established synthetic methods and PL covering the visible range of the spectrum, CdSe NCs have been con- sidered to be the most promising light-emitting materials.7,8 Several wide-band gap semiconductors, such as CdS, ZnSe, and ZnS,6,9-14were appliedas the shell fortheCdSe NCs, and ZnS was judged to be the superior choice due to its wide band gap (3.8 eV for the bulk material) and its environmentally benign nature. During the past decade, ZnS-capped CdSe NCs have been investigated extensively, and so-called green, syntheticmethodshavebeendevelopedbyutilizingsourcesfor Cd and ZnS12-14that are stable in the air and less toxic. However, most of the reports related to the preparation of core/shell NCs indicated that discrete or semidiscrete batch methods were used, which require tedious purifications of the core NCs in order to eliminate the cross-contamination of unreacted precursors remaining in each reaction step. Furthermore, the growth of the shells is highly sensitive to changes in reaction conditions, including changes in the tem- peratureandtheconcentrationgradientthatareintrinsictothe large reactor, imposing a barrier for the large-scale production of core/shell CdSe NCs. The high surface-to-volume ratios and short diffusion lengths that exist in a microchannel allow improved efficiency andreproducibilityofthemicrofluidicreaction,andthesuper- iority of the microfluidic reaction for precise control of the synthesis reaction,15-17online sample characterization,18,19 continuous operation,20and parallel operation has been demonstrated based on the synthesis of, for example, CdSe and CdS NCs. In addition, the advantages presented by microfluidic systems are also manifested in their ability to perform multistep reactions in a sequential manner, such as those embodied in the synthesis of CdSe/ZnS NCs and some organic compounds.20-22Potentially, precisely tailored reac- tion conditions, coupled with the properly selected reaction recipes, can eliminate the discrete nucleation of shell material. But the limited reports about the preparation of CdSe/ZnS NCs by the microfluidic method simply combined the well- established synthesis routes for the core and shell in a micro- reactor. As a result, PL QY, as well as PL peak width, cannot satisfytherequirementsofvariousapplications.Furthermore, achieving wide PL tunability for CdSe/ZnS NCs that are continuously synthesized via microfluidic reactions can be challenging, because the parameters for core synthesis and the overcoating process are interrelated during the operation, *To whom correspondence should be addressed. E-mail: Luanecust. . Tel: 86-21-64253513. Fax: 86-21-64253513. 4808Crystal Growth black line: applying con- centrated Cd precursor). Figure 3. (a) Absorption spectra and (b) PL spectra of CdS NCs synthesized using various S/Cd ratios in the precursor (285-260 ?C; 20 s). 4810Crystal Growth (b) CdS NCs; and (c) the dependence of optimal capping time on the size of NCs. 4812Crystal Growth QY:blue,41%;cyan,58%;green,70%;yellow,78%;orange, Figure 7. (a) PL spectra of CdS/ZnS and CdSe/ZnS NCs synthe- sized in the continuous process and (b) Photo illustration of CdS/ ZnS and CdSe/ZnS NCs under indoor light and UV light. Figure 8. XRD pattern of CdSe (4 nm), CdS (4.4 nm), CdSe/ZnS (5.0 nm), and CdS/ZnS NCs (6.2 nm). Figure 9. TEM images of CdSe/ZnS NCs obtained from the con- tinuous reaction, (a) 4.9 nm; (b) 3.7 nm. ArticleCrystal Growth red, 61%) was obtained by the online adjustment of precursors and capping channels. Acknowledgment. Authors appreciate the financial sup- port from the NSFC (50772036), The Focus of Scientific and Technological Research Projects (109063), and the State Key Laboratory of Chemical Engineering at ECUST (SKL-ChE-08C09). Supporting Information Available: TGresults,absorptionspectra, PL spectra, photostability plot, and EDS spectra. This material is available free of charge via the Internet at . References (1) Alivisatos, A. P. J. Phys. Chem. 1996, 100, 13226. (2) Alivisatos, A. P. Science 1996, 271, 933. (3) Moronne,M.; Gin, P.; Shimon, W.; Alivisatos, A. P. Science 1998, 281, 2013. (4) Coe, S.; Woo, W.; Bawendi, M. G.; Bulovic, V. Nature 2002, 420, 800. (5) Klimov, V. I.; Mikhailovsky, A. A.; Xu, S.; Malko, A.; Hollings- worth, J. A.; Leatherdale, C. A.; Eisler, H. J.; Bawendi, M. G. Science 2000, 290, 314. (6) Hines, M. A.; Guyot-Sionnest, P. J. Phys. Chem. 1996, 100, 468. (7) Nirmal, M.; Brus, L. Acc. Chem. Res. 1999, 32, 407. (8) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. 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