Macromolecular Ligand Engineering for Programmable Nanoprism Assembly

Abstract

Ligands play a central role for the energetics and kinetics of nanocrystal assembly. Yet, the precise and simultaneous manipulation of ligands to dictate assembly outcome has proven difficult. Here, we present macromolecular ligand-engineering strategies to control, characterize, and model four molecular parameters of grafted polymer chains: chain length, chain dispersity, grafting density, and chain distribution. Direct ligand-exchange between nanoprisms and polymers functionalizes facets selectively and produces patchy nanocrystals. We develop a generalizable two-step ligand-exchange approach for the independent control of the two emergent brush parameters, brush thickness and brush softness. The resultant polymer-grafted prismatic nanocrystals with programmable ligand brushes self-assemble into thin-film superstructures of different wallpaper symmetries and faceted supracrystals. Our experiments are complemented by coarse-grained computer simulations of nanoprisms with directional, facet-specific interactions. This work paves the way for the precision synthesis of polymer–nanocrystal hybrid materials and enables the further refinement of theoretical models for particle brush materials.

Structural Diversity in Dimension-Controlled Assemblies of Tetrahedral Gold Nanocrystals

“…40,41 Also, edge-to-edge contacts can better promote ligand interdigitation and superstructure densification in comparison to face-to-face contacts because ligands grafted at regions of high local curvature can tilt away more easily from the axis of contact to lower the osmotic repulsion between opposing polymer brushes. 31,36 Taken together, our results indicate that the preference for face-toface contacts among gold nanotetrahedra diminishes gradually with increasing particle softness, which ultimately triggers the switch to the "edge-up" tetrahedron orientation that facilitates edge-to-edge contacts in 2D superstructures.…”

“…Increasing ligand length causes an effective rounding of the tetrahedral shape and accompanying lattice symmetry transitions. Such in-plane lattice distortion without particle reorientation stems from a collective response to changing effective shapes of nanocrystals, which has been documented in experimental and computational studies of phase behavior of rounded polyhedra. ,, At a high degree of softness (Figure g–i), pronounced shape rounding reduces the preference for face-to-face contacts. , Also, edge-to-edge contacts can better promote ligand interdigitation and superstructure densification in comparison to face-to-face contacts because ligands grafted at regions of high local curvature can tilt away more easily from the axis of contact to lower the osmotic repulsion between opposing polymer brushes. , Taken together, our results indicate that the preference for face-to-face contacts among gold nanotetrahedra diminishes gradually with increasing particle softness, which ultimately triggers the switch to the “edge-up” tetrahedron orientation that facilitates edge-to-edge contacts in 2D superstructures.…”

“…Narrowly dispersed PS-PEHA ligands with number-average molecular weights (M n ) ranging from 1.6 to 39 kDa were synthesized and grafted onto gold tetrahedra through ligand exchange (Figures S5 and S6 and Tables S2− S4). 35,36 We found that the ratio of ligand length to nanotetrahedron edge length (or equivalently, tetrahedron softness) 6 is the key parameter to generate the phase diagram, which includes six distinct 2D superlattices (Figure 2b). Large tetrahedra (l 1 = 57 nm) grafted with the shortest ligands (M n = 1.6 kDa) assemble into a triangular lattice.…”

“…Such in-plane lattice distortion without particle reorientation stems from a collective response to changing effective shapes of nanocrystals, which has been documented in experimental and computational studies of phase behavior of rounded polyhedra. 12,36,39 At a high degree of softness (Figure 2g−i), pronounced shape rounding reduces the preference for face-to-face contacts. 40,41 Also, edge-to-edge contacts can better promote ligand interdigitation and superstructure densification in comparison to face-to-face contacts because ligands grafted at regions of high local curvature can tilt away more easily from the axis of contact to lower the osmotic repulsion between opposing polymer brushes.…”

“…Details regarding the synthesis of pentaethylenehexamine-terminated polystyrene (PS-PEHA) have been described in our previous publications. , Further details on material synthesis and characterization are provided in the Supporting Information (SI).…”