Gao, Yige

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Gao
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Yige
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    NANOPARTICLE ASSEMBLIES AT LIQUID INTERFACES AS REVEALED BY SCANNING ELECTRON MICROSCOPY
    (2020-05) Gao, Yige
    The overarching goal of this work is to obtain a fundamental understanding of the two-dimensional (2D) ordering and dynamics of nanoparticles (NPs) as their packing density approaches jamming. To pursue this goal, an in-situ scanning electron microscopy (SEM) technique was developed, that without damaging the specimen, can track NPs at the single particle level on the surface of an ionic liquid (IL). In addition, a novel sample stage that changes the NP-occupied surface area in a controlled manner was fabricated, facilitating systematic variation of the density of NP assemblies. First, we characterized by SEM the pair interaction potential between PEG-coated silica NPs at IL interfaces. The effects of particle diameter and grafted polymer length on the potential were examined. For a short PEG ligand, a weak, long-range attraction was found, explained by ligand-induced NP menisci that create capillary interactions. However, due to the attraction’s weakness, the overall potential is close to a hard sphere interaction, indicating that the chosen NP-IL pairing comprises a model system to study 2D particle packing structure and dynamics. A SEM liquid cell was designed and fabricated to control the NP areal density, enabling visualization of NP packing in situ under 2D compression and expansion. Upon a slow compression, dense but not jammed NPs of a single size rearranged into more ordered structures with increased crystalline grain sizes. Fast compression of jammed NP layers created wrinkles and buckles. The packing of jammed bidisperse NPs was systematically studied under analogous conditions. Their packing was characterized by two metrics, local orientational and positional order parameters, Ψ6 and T*, respectively. By plotting Ψ6 against T*, a correlation between local orientational and translational order was observed. Statistical local demixing suggested the importance of depletion interactions even in mixtures of NPs with almost equal sizes. Lastly, by the new in-situ SEM imaging method made feasible by IL nonvolatility, we observed electron beam-induced disaggregation of insulating (silica) NPs and attraction/aggregation of metallic (gold) NPs.