Solvent‐driven symmetry of self‐assembled nanocrystal superlattices—A computational study

Abstract

The preference of experimentally realistic sized 4-nm facetted nanocrystals (NCs), emulating Pb chalcogenide quantum dots, to spontaneously choose a crystal habit for NC superlattices (Face Centered Cubic (FCC) vs. Body Centered Cubic (BCC)) is investigated using molecular simulation approaches. Molecular dynamics simulations, using united atom force fields, are conducted to simulate systems comprised of cube-octahedral-shaped NCs covered by alkyl ligands, in the absence and presence of experimentally used solvents, toluene and hexane. System sizes in the 400,000-500,000-atom scale followed for nanoseconds are required for this computationally intensive study. The key questions addressed here concern the thermodynamic stability of the superlattice and its preference of symmetry, as we vary the ligand length of the chains, from 9 to 24 -CH(2) groups, and the choice of solvent. We find that hexane and toluene are "good" solvents for the NCs, which penetrate the ligand corona all the way to the NC surfaces. We determine the free energy difference between FCC and BCC NC superlattice symmetries to determine the system's preference for either geometry, as the ratio of the length of the ligand to the diameter of the NC is varied. We explain these preferences in terms of different mechanisms in play, whose relative strength determines the overall choice of geometry.

Energy landscape of self-assembled superlattices of PbSe nanocrystals

“…Of the three kinds of capping ligands (OA, DPP, and TOP), OA has the longest hydrocarbon chain, so interactions between neighboring OA chains probably play a dominant role in the NC assembly process. Although both hexane and toluene are considered "good" solvents for OA in terms of full permeation of their ligand coronas to PbSe NC surfaces (17), the nonpolar hydrocarbon chain of OA is solvated more effectively in nonpolar hexane than in the slightly polar toluene (28,29). More specifically, as the inter-NC surface distance decreases to less than twice the OA length, the use of hexane as the solvent may result in a relatively large degree of ligand penetration.…”

“…In addition to the unique properties of individual NCs, these NC superlattices manifest new collective behavior [such as electronic, plasmonic, magnetic, and catalytic properties (4)] through near-field coupling of neighboring NCs (5). In the past two decades, various NC superlattices were developed successfully via the controlled assembly of colloidal nanoparticles (2,(6)(7)(8)(9)(10)(11)(12)(13)(14), and, at the same time, extensive simulation efforts have been devoted to understanding this complicated process and to predict the assembly patterns (15)(16)(17). To achieve rational design of NC superlattices, however, it is essential to have a fundamental understanding of the mechanisms underlying their formation, including their energetics of formation and polymorphism, which form the focus of this study.…”

Pair and many-body interactions between ligated Au nanoparticles

“…A substantial number of molecular dynamics simulation studies of the ligand structure of isolated NPs and of the interactions between NPs that utilize these models have been reported . The more sophisticated of these investigations involve simulations that use full atom [10][11][12][13][14][15][16][17][18] and pseudo-atom force fields [19][20][21][22][23][24][25][26][27][28][29], amongst which are a few investigations of the importance of the three NP interaction level deviation from additivity of the pair potential of mean force [23,24,[38][39][40]. The results obtained from these simulations establish that the interaction between the ligands of the dressed nanoparticles, not the core-core interaction, determines the NP monolayer mechanical properties.…”

Temperature effects on nanostructure and mechanical properties of single-nanoparticle thick membranes

“…Both united atom [12][13][14][15][16][17][18][19][20] and atomistic force elds [21][22][23][24][25][26] have been used. Both united atom [12][13][14][15][16][17][18][19][20] and atomistic force elds [21][22][23][24][25][26] have been used.…”