Binary nanocrystal superlattices: Self-assembly, structure characterization, collective properties and device fabrication

Self-assembled binary nanocrystal superlattices (BNSLs) represent a versatile bottom-up approach towards designer nanocrystal-based metamaterials and functional devices with well-tailored physical properties. To assign rigorously the three-dimensional crystal structure of BNSLs, we have established a powerful structural characterization framework by employing a tomography TEM holder. The tilt series obtained allow us to differentiate the subtle difference between two AB13 polymorphs: icosahedral AB13 and cuboctahedral AB13. This systematic methodology further allows us to discover a new AB6 phase with the body-centered cubic symmetry in addition to the simple-cubic CaB 6 phase. The bcc-AB6 phase, lacking any atomic analogue, is isomorphic to certain alkali-metal intercalation compounds of fullerene C60 (e.g., K6C60). For the first time, we demonstrate that colloidal inorganic nanocrystals can self-assemble into binary quasicrystalline superlattices. Compositional flexibility of nanocrystal building blocks indicates that the formation of quasicrystalline BNSLs does not require a unique combination of interparticle interactions, but is a general sphere-packing phenomenon governed by maximization of local packing density through nanocrystal clustering and maximization of lattice configurational entropy through aperiodic tiling. The novel liquid-air interfacial assembly approach enables the growth of large-area BNSL membranes with controllable thickness (from monolayer to multiple layers). Furthermore, the ready film transfer allows the integration of BNSLs on arbitrary substrates for device fabrication. For example, we fabricate magnetoresistive devices by incorporating magnetic BNSL membranes and the magnetotransport measurements clearly show that the device magnetoresistance is dependent on the structure, stoichiometry and composition of the BNSLs. In addition, we have found that the combination of FePt and MnO nanocrystals to form BNSLs dramatically enhances the thermal stability of FePt nanocrystals. Fundamentally, the availability of high-quality BNSL membranes allows the investigation of collective interactions between two nanocrystal components. For instance, BNSL membranes self-assembled from two types of magnetic nanocrystals are found to exhibit a single-phase-like magnetization alignment process, which is attributed to the collective interparticle dipolar interactions. The new assembly and structure characterization techniques we have developed will significantly accelerate the exploration of this new class of materials