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Shu Fen Tan1 Geeta Bisht1 Utkur Mirsaidov1

1, National University of Singapore, Singapore, , Singapore

Hierarchically organized nanoparticles are good candidates for plasmonics [1-2] and catalysis [3] studies due to its distinctive properties than its isolated forms. Self-assembly serves as a robust ‘bottom-up’ method to organize nanoparticles into desired arrays. The assembly process is often characterized by the ‘quench-and-look’ approach or indirect dynamic spectroscopic methods [4]. As such, the intermediate steps and the how the nanoparticle morphology, surface molecules participate in controlling the final assembled structures still remains largely unresolved.

Here, we use of in situ liquid cell electron microscopy to directly probe the self-assembly of chemically-synthesized nanoparticles of different morphologies; Au nanocubes, nanorods, nanospheres and nanobypiramids with surfactant molecules; cetyltrimethylammonium bromide (CTAB) at high temporal and spatial resolution. Our real-time observations reveal that there are different attachment pathways for side-to-side assembly of Au nanocubes: pre-alignment attachment and post-attachment re-orientation. We further investigate how the shape of nanoparticle and chemical surfactants can affect the nanoparticle assemblies to attain the lowest total energy state by monitoring the nanoparticle dynamics as a function of time. Understanding the physical and chemical interactions that govern the self-assembly could potentially lay the foundation for rational design of desired assembled nanostructures for application in catalysis, opto-electronics and drug delivery.

Acknowledgements
We acknowledge the support from Ministry of Education of Singapore (award No. MOE2015-T2-1-007).
References
S. F. Tan et al., Quantum plasmon resonances controlled by molecular tunnel junctions. Science 343: 1496-1499, 2014.
K. J. Savage et al., Revealing the quantum regime in tunnelling plasmonics. Nature 491: 574-577, 2012.
G. Prieto et al., Towards stable catalysts by controlling collective properties of supported metal nanoparticles. Nature Mater. 12: 34-39, 2013.
M. Grzelczak et al., Shape control in gold nanoparticle synthesis. Chem. Soc. Rev. 37: 1783-1791, 2008.

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