Stacking of Hexagonal Nanocrystal Layers during Langmuir− Blodgett Deposition Detlef-M. Smilgies,* ,† Andrew T. Heitsch, ‡ and Brian A. Korgel ‡ † Cornell High Energy Synchrotron Source (CHESS), Cornell University, Ithaca New York 14853, United States ‡ Department of Chemical Engineering, Texas Materials Institute, Center for Nano- and Molecular Science and Technology, The University of Texas at Austin, Austin, Texas 78712-1062, United States ABSTRACT: Hexagonally ordered close-packed monolayers of sterically stabilized FePt nanocrystals were deposited on substrates using the Langmuir−Blodgett technique. Mono- layers of nanocrystals were also stacked by sequential Langmuir−Blodgett transfer. The structures of the nanocrystal monolayers and multilayer stacks were examined with scanning electron microscopy (SEM) and grazing-incidence small-angle scattering (GISAXS). An analytical model derived from the quasikinematic approximation provides a convenient description of the GISAXS data of the stacked layers. The transferred monolayers showed good in-plane hexagonal order, even for trilayers. Bilayers exhibited spatial registry with the top layer positioned above the 3-fold coordinated sites of the bottom layer. Trilayers, on the other hand, exhibited significant disorder. ■ INTRODUCTION The advent of methods for nanocrystal synthesis that produce particles with well-controlled size, shape, and ligand shells has enabled the formation of synthetic metamaterials of ordered nanocrystal assemblies. 1−4 The properties of these nanocrystal superlattices derive from the unique size-dependent properties of the nanocrystals combined with interparticle electronic coupling between neighboring nanocrystals arranged on a regular lattice, analogous to atom- and molecule-based solid state materials. Nanocrystals can be deposited onto flat surfaces such as silicon wafers or glass slides with excellent order and orientation by simple solvent evaporation from a nanocrystal dispersion by fine-tuning the drop casting parameters. 5 However, thicker nanocrystal deposits covering large substrate areas are typically rough with extensive cracking. 6 Various other deposition methods can produce much more extended and uniform superlattice films, such as Langmuir−Blodgett (LB) deposition, 7−15 convective self-assembly, 16 inclined plane deposition, 17−19 and hot doctor blading. 20 Due to the size of the nanocrystals (ranging approximately from of 2−20 nm), the structural characterization of these superlattices falls in the realm of small-angle X-ray scattering (SAXS). A convenient technique to characterize nanocrystals superlattices in situ and in real-time is synchrotron-based grazing-incidence small-angle X-ray scattering (GISAXS). 6,21−23 GISAXS provides highly precise and proper statistically averaged information about two- (2D) and three-dimensional (3D) superlattices. In addition, GISAXS is compatible with in situ experiments such as solvent vapor annealing 6 and casting from solution, 16 and provides real-time information about how nanocrystals assemble and order. Well-ordered nanocrystal superlattices can enable detailed studies of collective nanocrystal behavior 1−4,13,24 and are important building blocks in the field of nanostructured artificial solids. 1−4 LB deposition has been established as a route to deposit nanocrystal monolayers covering extensive substrate areas. But it is not known whether sequential stacking of LB monolayers (see Figure 1) can yield spatial registry between stacked planes. Here we find that bilayers exhibit spatial registry, but the stacking of additional layers leads to out-of-plane disorder. ■ EXPERIMENTAL SECTION a. Synthesis. FePt nanocrystals were synthesized by solvent-based arrested precipitation using a modification of a previously published procedure. 25 Typical reactions yield approximately 150 mg of Fe−Pt nanocrystals coated by ligands oleylamine and oleic acid with an average composition of Fe 0.42 Pt 0.58 measured by energy dispersive spectroscopy (EDS) and an average core diameter of 6.9 nm. 13,14 Purified Fe−Pt nanocrystals were stored as 10 mg/mL dispersions in chloroform. Silicon substrates of 1.0 cm 2 area were cleaned by sequential immersion in chloroform, acetone, and then isopropanol with mild sonication for 2 min. The substrates were dried with compressed air between each cleaning step. Received: February 16, 2012 Revised: April 20, 2012 Published: April 26, 2012 Article pubs.acs.org/JPCB © 2012 American Chemical Society 6017 dx.doi.org/10.1021/jp3015436 | J. Phys. Chem. B 2012, 116, 6017−6026