Advances in colloidal chemistry techniques have enabled self-assembly of ligand-stabilized nanoparticles (NPs) into highly ordered arrays (termed superlattices) with exotic collective properties that are entirely different from those of bulk phase crystals, isolated nanocrystals and even disordered nanocrystal assemblies. The exceptional thermal, mechanical, electronic, and optical properties of these superlattices (SLs) make them promising for numerous optoelectronics, energy harvesting, and sensing applications. However, precise engineering of SLs to realize their full potential remains challenging due to lack of fundamental understanding of the molecular mechanisms controlling their collective properties. Here, we integrate coarse-grained molecular dynamics simulations with small/wide angle X-ray spectroscopy, and electron microscopy experiments to identify the crucial role played by ligand coverage density, and surface dynamics of ligands on the structure, thermo-mechanical, and high-pressure behavior of SLs. We find that ligand coverage density dictates (a) the extent of diffusion of ligands over NP surfaces, (b) spatial distribution of the ligands in the interstitial spaces between neighboring NPs, and (c) the fraction of ligands that interdigitate across different nanoparticles. These inter-dependent processes lead to a critical ligand coverage density (1.8 nm−2 for 7 nm PbS NPs capped with oleic acid) below which, the SLs collapse via sintering of individual NPs. Above the critical coverage, the SLs can preserve their crystallinity crystalline order even under high applied pressures (∼40–55 GPa), and show a completely reversible pressure behavior.1 Such coverage-dependent processes also govern exceptional thermo-mechanical properties of 2D SLs. These results will be discussed in the context of designing superlattices with prescribed functionality for various energy applications.

Molecular insights into surface ligand-dynamics and their impact on functionality of self-assembled nanoparticle superlattices

2019 Fall National Meeting

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