Published: April 21, 2011 r2011 American Chemical Society 8956 dx.doi.org/10.1021/ja201129n | J. Am. Chem. Soc. 2011, 133, 8956–8960 ARTICLE pubs.acs.org/JACS Three-Dimensional Nanocrystal Superlattices Grown in Nanoliter Microfluidic Plugs Maryna I. Bodnarchuk, Liang Li, Alice Fok, Sigrid Nachtergaele, Rustem F. Ismagilov, and Dmitri V. Talapin* Department of Chemistry, University of Chicago, Chicago, Illinois 60637, United States b S Supporting Information 1. INTRODUCTION Many key features of ordinary crystals (e.g, faceting, twinning, polymorphism, etc.) have been observed in nanocrystal (NC) superlattices, suggesting that their assembly follows the same fundamental principles as crystallization of conventional atomic and molecular solids. 1À4 In contrast to individual atoms and molecules, which are very difficult to image in real space, NCs provide a unique chance to study the crystallization of complex structures in real space and real time. Such studies should provide important insights into the fundamental aspects of crystal nucleation and growth, formation of structural defects, etc. 5 Control over the formation of ordered single-component 2,6À8 and binary 9À15 NC arrays is also important for successful development of NC-based electronic and optoelectronic devices such as solar cells, photodetectors, field-effect transistors, and light-emitting diodes. 16 The approaches to growing NC superlattices can be divided in two categories. In “evaporation-driven” methods, the carrier solvent is slowly evaporated from a colloidal solution of NCs. 8,9,12À14 When the NC volume fraction reaches a certain threshold, the system undergoes a transition from a disordered state to an ordered one. 17 In contrast, “destabilization-driven” approaches use slow destabilization of a colloidal solution, which is typically achieved by layering the NC solution with a pre- cipitant that slowly diffuses into the NC solution. This approach is similar to the technique of free interface diffusion used in protein crystallization. 18 It allows the growth of large three-dimensional (3D) faceted crystals of long-range-ordered NCs. 4,6,19À22 Like many other crystallization experiments, NC assembly requires numerous trials to find the optimal conditions, especially in the case of binary nanoparticle superlattices (BNSLs). The availability of techniques for fast combinatorial screening of experimental parameters would greatly accelerate such studies. In protein research, high-throughput crystallization studies often employ robotics 23 and microfluidics. 24À27 The latter case pro- vides a number of unique advantages, such as ultrasmall materials consumption and precise control over molecular diffusion and crystal nucleation. 18 In this work, we applied a microfluidic platform for studies of the self-assembly of colloidal NCs. It allowed us to explore the homo- geneous nucleation and support-free growth of NC superlattices with fast combinatorial screening of the experimental conditions. 2. EXPERIMENTAL DETAILS Figure 1a and Figure S1 in the Supporting Information (SI) show our experimental setup for the formation of nanoliter droplets confined inside a microfluidic capillary. The capillary (thin-wall Teflon tubing) was cut and inserted into lithographically defined polydimethylsiloxane (PDMS) devices. The NC solution and precipitant were simultaneously injected into a stream of immiscible fluorinated carrier fluid (Figure 1a and Figure S1). The injection rates were synchronized by a LabView program controlling several syringe pumps. The width of the plugs was determined by the inner diameter of the capillary (400 μm), while the plug volume could be varied by tuning the flow rates of the solutions entering the capillary, with reproducibility of the plug volumes within 3À20%. 28 The separation between plugs was controlled by the flow rate of the carrier fluid. As the carrier fluid, we used a mixture of perfluoro-tri-n-butylamine and perfluoro-di-n-butylmethylamine (FC-40) or perfluorotripentyla- mine (FC-70). These fluids showed the lowest miscibility with alcohols and toluene. 29 FC-40 and FC-70 have high boiling points (155 and 215 °C, respectively) and preferentially wet the surface of the Teflon capillary. The latter allowed us to minimize the contact area between the Received: February 5, 2011 ABSTRACT: We studied the self-assembly of inorganic nano- crystals (NCs) confined inside nanoliter droplets (plugs) into long-range ordered superlattices. We showed that a capillary microfluidic platform can be used for the optimization of growth conditions for NC superlattices and can provide insights into the kinetics of the NC assembly process. The utility of our approach was demonstrated by growing large (up to 200 μm) three-dimensional (3D) superlattices of various NCs, including Au, PbS, CdSe, and CoFe 2 O 4 . We also showed that it is possible to grow 3D binary nanoparticle superlattices in the micro- fluidic plugs.