Self‐assembled Supraparticles by Spherical Confinement
Authors: Wang Da , de Nijs Bart , Tasios Nick , Dussi Simone , Smallenburg Frank , Filion Laura , Liu Yang , Altantzis Thomas , Zanaga Daniele , Wu Yaoting , Najmr Stan , Gaulding E. Ashley , Meeldijk Johannes D. , Groenendijk Dirk , Murray Christopher B. , Bals Sara , van Huis Marijn A. , Imhof Arnout , Dijkstra Marjolein , van Blaaderen Alfons 
Published: December 2016

Colloidal supraparticles [1,2] which are assembled from size‐ and morphology‐ controlled nanoparticles, combine multi scale properties of the single particles such as quantum confinement and localized plasmon resonances with collective effects resulting from being arranged in near proximity on a 3D lattice. In addition, properties on longer length scales, e.g. photonic, are affected by the supraparticle size and its effective refractive index [3] and additionally can be controlled by a subsequent self‐assembly step. One way to realize such colloidal supraparticles is by suspending nanocrystals in emulsion droplets of a low boiling point solvent in water and slowly evaporating the solvent [3,4] (or vice versa for particles dispersible in a polar solvent). In this way the nanocrystals are forced to self‐assemble into supraballs and surprises already were found in the structure of hard spherical particles crystallized in a spherical confinement where icosahedral crystals were found for supraparticles containing several thousands of spheres or less [4]. By tuning the concentrations and types of nanocrystals, supraballs and differently shaped supraparticles with different structures and sizes can be obtained.

The goal of this research is to extend the spherical confinement method to binary particles systems and anisotropic particles systems. For instance, spherical 8 nm sized (including stabilizer) PbSe nanocrystals and 10 nm PbSe nanocrystals were used to synthesize binary crystalline supraballs with a bulk AB
2
binary colloidal crystal phase [5]. We further found 17.5 nm sized Ag/10.7 nm sized Fe
3
O
4
and 8 nm PbSe/11 nm Au binary systems will self‐assemble into Janus/patchy shape binary supraballs (where segregation into the two pure components was found) or core‐shell binary supraballs where one of the components ends exclusively in a shell on the outside, respectively. From computer simulations between hard particles we infer that the core‐shell morphology is most likely caused by the presence of attractions, but this preliminary conclusion needs further study. EuF
3
round platelets nanocrystals were also found to self‐assemble into highly ordered supraballs with a liquid‐crystal‐like interior structure. In addition, crystalline supraparticles were successfully assembled by using rounded edged FeO/CoFe
2
O
4
nanocubes as building blocks. After freeze drying, the structure of the supraballs was studied with high‐angle annular dark‐field scanning transmission electron microscopy (HAADF‐STEM) tomography and secondary electron scanning transmission electron microscopy (SE‐STEM). By coating the supraballs with a thin (~50 nm) layer of (meso)porous or non‐porous silica, the particles become more robust and do not deform by drying on a substrate. To study how the structure of the more complex supraballs is affected by the spherical confinement, work is in progress by advanced electron microscopy [6,7] and 3D particle fitting techniques [8,9]