Nanocrystal superlattices are attractive as building blocks for artificial solids in a wide range of applications including optical, electronic, photovoltaic, thermoelectric and biocompatible devices to name a few. A common challenge to commercialize superlattice-based devices is the difficulty associated in controlling the self-assembly over wafer-scales with monolayer thickness control. Most commonly, nanocrystal superlattices are self-assembled through kinetically-driven methods such as solvent drying mediated assembly, or through complex liquid interfaces that are not compatible with large area, high volume manufacturing. Polymer-grafted nanocrystals (PGNCs) in controlled solvent atmospheres (solvent annealed) are particularly attractive because of their dual colloidal and polymeric properties that facilitate self-assembly towards thermodynamic equilibrium. PGNCs share properties with star polymers, and when the inorganic cores are sufficiently screened by the polymer ligands, their thermodynamic properties can be understood in terms of the Daoud-Cotton model developed for star polymers. In this work we study the assembling properties of Fe3O4 nanocrystals tethered with polystyrene ligands. We confirm swelling and assembly properties according to the Daoud-Cotton model. As predicted for star polymers, we confirm a phase transition as a function of particle density into a hexatic phase. We exploit the polymeric properties of PGNCs to perform directed self-assembly to achieve orientation control over large areas. Overall, the polymeric and colloidal duality of PGNCs open interesting opportunities for the self-assembly of superlattices with an ease of processing typical of polymer films.
Ricardo Ruiz is a research technologist at Western Digital Corp. His research interests span alternative nanofabrication techniques for storage and memory devices, block copolymer lithography and colloidal self-assembly. From 2013 to 2016 he managed a Nanopatterning and Self Assembly group at HGST dedicated to block copolymer and colloidal lithography. Prior to that, he was a research staff member at HGST where he helped introducing block copolymer lithography for magnetic bit patterned media technology. Before joining HGST, he was a postdoctoral scientist at IBM T.J. Watson. He received his PhD in Physics from Vanderbilt University in 2003. He has co-authored over 50 publications and holds 35 US Patents. He is a fellow of the American Physical Society and was the recipient of the 2016 ACS Applied Materials and Interfaces Young Investigator Award.