Critical photonic, electronic, and magnetic applications of two dimensional (2D) nanoparticle superlattices often require special structural states such as the often desired perfect single crystal phase with long range orderings and limited defects. Self-assembly of nanoparticles at liquid/air and liquid/liquid interfaces has emerged as a simple yet efficient way to create 2D superlattices with tunable properties. Although microscopic mechanisms for the self-organization of ligand stabilized nanoparticles have been widely explored, the macroscopic patterns as a consequence of the inter-particle interactions on large length scales vary drastically under different assembly environments. Even for a simple case of evaporating a colloidal droplet of nanoparticles on a surface, a variety of patterns have been reported, ranging from ordered 2D, three dimensional (3D) nanoparticle superlattices, and fractal-like aggregates to percolated networks. Having such vastly different structures of nanoparticle superlattices hinders the identification of their physical properties thus creating obstacles for their device applications. It is therefore essential to further understand and control the nanoparticle self-assembly mechanism. X-ray scattering based techniques provide a unique <i>in-situ</i> and time-resolved tool to study the nucleation and growth kinetics of these superlattices at the assembly interfaces. With X-ray scattering, we were able to track the superlattice phase evolution of the droplet evaporated nanoparticle colloidal suspensions. Recently, we also showed that nanoparticle membranes formed at air/water interfaces exhibit a small (sub-nm) but significant ligand distribution asymmetry on nanoparticle surface. Molecular dynamics simulations elucidate the roles of ligand coverage and mobility in producing and maintaining this asymmetry. Understanding this Janus-like membrane asymmetry opens up new avenues for designing nanoparticle superstructures.

Self-assembly of two-dimensional nanoparticle superlattice membranes: A study by synchrotron x-ray scattering

253rd National Meeting (2017)

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