High temperature continuous flow synthesis of CdSe/CdS/ZnS, CdS/ZnS, and CdSeS/ZnS nanocrystals
Vivek Kumar, Hector A. Fuster, Mathew S. Naughton, Paul J. A. Kenis
Department of Chemical & Biomolecular Engineering, University of Illinois at Urbana-Champaign
Cadmium-based semiconductor nanocrystals have been the subject of intense study for their fluorescent properties. These nanocrystals have manifold applications including solid-state lighting, displays, and fluorescence tagging.[1-2] Traditionally, Cd-based nanocrystals are based around CdSe for emission at green to red wavelengths or CdS for violet-blue emission. Growth of a passivating ZnS shell substantially increases quantum yield for both CdSe and CdS;[2-3] CdSe/CdS/ZnS or CdSe/ZnSe/ZnS particles have the passivating benefits of ZnS combined with the reduced lattice strain between CdSe and the intermediate layer.[3c]
Quantum dot synthesis has been limited by the use of batch systems, which have limited scalability. More recently, flow synthesis has become an increasingly viable option for increased throughput. Here, we discuss results for CdSe/CdS/ZnS, CdS/ZnS, and CdSeS/ZnS nanocrystals synthesized in continuous flow reactors. The CdSe-based systems exhibit quantum yield of up to 60% with elevated temperature and shell growth, which is conducted without the need for cumbersome methods - dropwise addition or SILAR shell growth. CdSe/CdS was formed with a single step inside the continuous flow reactor. CdS-based systems were shown to work more effectively when using Octadecene-S in place of TOP-S, which improved the quality of shell growth. In addition, stoichiometric synthesis of the alternate system CdSeS/ZnS was conducted, removing the need for a large excess of S to offset the lower S reactivity. CdSeS/ZnS showed confinement similar to that of CdSe/ZnS and yielded quantum yields of up to 49%.
Furthermore, we demonstrate a streamlined “heat injection” flow system for core-multishell quantum dot synthesis. This reactor uses liquid-phase chemistry that is not mixing-sensitive at room temperature, eliminating the need for any preheating or in-line mixing, and has channels that are ~1 mm in diameter, thus suitable for rapid heating/cooling. We apply this reactor to the known systems of CdSe/CdS/ZnS and CdS/ZnS; in addition, we demonstrate the ability to synthesize CdSeS/ZnS alloy nanoparticles, which has not been shown in a continuous flow system to our knowledge. Additionally, we are currently working on continuous flow synthesis of Cd-based and Cd-free anisotropic nanoparticles.
 a) D. Bera, L. Qian, T.-K. Tseng, P. H. Holloway, Materials 2010, 3, 2260-2345; b) J. L. West, N. J. Halas, Annual Review of Biomedical Engineering 2003, 5, 285-292.
 H. Yang, W. Luan, Z. Wan, S.-t. Tu, W.-K. Yuan, Z. M. Wang, Crystal Growth & Design 2009, 9, 4807-4813.
 a) T. Trindade, P. O'Brien, X.-m. Zhang, Chemistry of Materials 1997, 9, 523-530; b) J. R. Dethlefsen, A. DÃ¸ssing, Nano Letters 2011, 11, 1964-1969; c) R. Xie, U. Kolb, J. Li, T. Basch, A. Mews, Journal of the American Chemical Society 2005, 127, 7480-7488; d) P. Reiss, J. l. Bleuse, A. Pron, Nano Letters 2002, 2, 781-784; e) P. Laurino, R. Kikkeri, P. H. Seeberger, Nat. Protocols 2011, 6, 1209-1220