Useful structural and mechanical properties of nanoparticle superlattices (NPSLs) are shown to originate from the interplay of ligand coverage, mobility and molecular-scale dynamics in the spaces between neighboring nanoparticles. Highly ordered arrays of ligand-stabilized colloidal nanoparticles, or NPSLs, have exceptional thermal, mechanical, electronic and optical properties. Coarse-grained molecular dynamics (CGMD) simulations were combined with in situ SAXS experiments at the Advanced Photon Source in a diamond anvil cell (DAC) to explore dynamic processes at the molecular level. The structural response of NPSLs to applied pressure, their mechanical properties, as well as NP–NP spacing are strongly correlated with the surface mobility of ligands, their spatial distribution between neighboring NPs, and the extent of ligand-interdigitation. A certain critical density of ligand coverage on the constituent NPs is required to form stable NPSLs. Above the critical coverage, NPSLs preserve their crystalline order even up to high applied pressures (∼40 GPa); more importantly, the pressure-induced changes in NP–NP spacing are completely recovered upon release. The PbS NPs (7 nm) were synthesized at CNM by colloidal synthesis. The predictions of CGMD simulations are in good accordance with electron microscopy and SAXS characterization of NPSLs. The fundamental knowledge of ligand dynamics and its connection to the assembly process is not only crucial for gaining precise control over the physical properties of NPSLs, but is also essential to design pathways to synthesize these hybrid crystals with desired crystallinity.
Significance and Impact
Knowledge of the impact of ligand dynamics on behavior of NPSLs can help scientists to finely tune their electronic, optical, thermo-mechanical and magnetic properties. Their unique sets of properties make NPSLs promising for numerous optoelectronics, energy harvesting, and sensing applications.
- Faceted NPSL analogues of atomic crystals were synthesized in a toluene solution, characterized using scanning electron microscopy (SEM) and in situ SAXS X-ray techniques, and were modeled by coarse-grained molecular dynamics simulations.
- Synthesis and microscopy of the nanoparticle superlattice and part of the computational modeling were performed at CNM. SAXS measurements were performed at APS.
Work was performed in part at the Center for Nanoscale Materials and the Advanced Photon Source.
About Argonne’s Center for Nanoscale Materials
The Center for Nanoscale Materials is one of the five DOE Nanoscale Science Research Centers, premier national user facilities for interdisciplinary research at the nanoscale supported by the DOE Office of Science. Together the NSRCs comprise a suite of complementary facilities that provide researchers with state-of-the-art capabilities to fabricate, process, characterize and model nanoscale materials, and constitute the largest infrastructure investment of the National Nanotechnology Initiative. The NSRCs are located at DOE’s Argonne, Brookhaven, Lawrence Berkeley, Oak Ridge, Sandia and Los Alamos National Laboratories. For more information about the DOE NSRCs, please visit https://science.osti.gov/User-Facilities/User-Facilities-at-a-Glance.
About the Advanced Photon Source
This research used resources of the Advanced Photon Source, a U.S. DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.
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