Understanding and tailoring ligand interactions in the self-assembly of branched colloidal nanocrystals into planar superlattices

Abstract

Colloidal nanocrystals can self-assemble into highly ordered superlattices. Recent studies have focused on changing their morphology by tuning the nanocrystal interactions via ligand-based surface modification for simple particle shapes. Here we demonstrate that this principle is transferable to and even enriched in the case of a class of branched nanocrystals made of a CdSe core and eight CdS pods, so-called octapods. Through careful experimental analysis, we show that the octapods have a heterogeneous ligand distribution, resembling a cone wrapping the individual pods. This induces location-specific interactions that, combined with variation of the pod aspect ratio and ligands, lead to a wide range of planar superlattices assembled at an air-liquid interface. We capture these findings using a simple simulation model, which reveals the necessity of including ligand-based interactions to achieve these superlattices. Our work evidences the sensitivity that ligands offer for the self-assembly of branched nanocrystals, thus opening new routes for metamaterial creation.

Fig. 1

Details of the as-synthesized octapods with different pod length. a Sketches illustrating the geometrical parameters of an octapod nanocrystal and the variation in octapod aspect ratio. L and D represent the octapod length and diameter, respectively. The tip-to-center length of the pods is represented by L/2 and the actual length of the pod is given by P_I. The truncated octahedral CdSe seed is indicated in red and has radius r_{CdSe seed}. b BFTEM images of representative particles for the four octapod aspect ratios L/D studied in this work. Scale bars=20 nm. c Volume-weighted size distribution for D and P_I octapod parameters determined by XRD analysis. Data are presented as the average values and their standard error of the mean, σ_M. d Inverted-contrast HAADF-STEM views of the octapods evidencing the two families of tips observed in both octapods with small and large L/D aspect ratio. That is, four flat-terminated pods (framed in blue) and four pods with sharp tips (framed in cyan). Scale bars=20 nm and 50 nm for the small and large L/D aspect ratio, respectively

Fig. 2

HRSEM images of self-assembled planar superlattices of octapods with different aspect ratio. Octapods with a small L/D stand on four pods with domains of square lattices, while high L/D particles exhibit a packing of interlocked chains. Scale bars=200 nm. The embedded sketches highlight the observed configurations of octapods for each aspect ratio

Fig. 3

Surface characterization of octapods with different aspect ratio. a FTIR spectra collected from the selected octapods; P_n=1…4 indicates the peaks related to P–O(H) and P = O stretching modes of the ligands, mainly the P = O moiety from TOPO and PO₃²⁻ from phosphonic acids. b CH₂/CH₃ asymmetric stretching peak ratio for different L/D proving an increase in the amount of long-tailed ODPA on longer shafts, until the signal covers that of the short ligands, coming from the shaft portion near the core. c P_n/CH₂ and P_n/CH₃ ratios for the studied octapods. Both plots show an increase of the P_n=1…4 compared to the CH₂ and CH₃ asymmetric stretching modes. d Sketch of an octapod pod showing a cone-like ligand distribution predicted from FTIR analysis: short ligand molecules (TOP) wrap the regions near the core, while long ones (ODPA) are preferentially bound toward the tips when increasing octapod aspect ratio. HPA is also present on the shafts of the pods, but there is no evidence of preferential attachment

Fig. 4

Monte Carlo Simulations of 2D superlattices of octapods with different pod length. a–d Snapshots showing a typical top-view configuration of the octapods at the end of our simulation run for different L/D. The value of the shaft interaction parameter ε_S and the tip interaction parameter ε_T are provided to the left of each snapshot, both are given in k_BT. A description of the crystal structure is given in the main text; the insets show a zoom-in to better visualize the structure. e Variation of the parameters ε_S and ε_T for octapods with L/D = 5.0 tune the octapod arrangement from ballerina (for ε_S = −3 and ε_T = 0) to interlocking (for ε_S = 2 and ε_T = −1). f The pressure was increased with respect to (a) and (c)

Fig. 5

Self-assembled 2D superlattices of octapods with L/D of 5.0 stabilized with different ligands. a Square lattice of HT-ligand exchanged octapods at 5 nM and 1 nM. b Interlocked chains of octapods for DDT-exchanged octapods at 5 nM. A tight packing of chains is observed at lower concentrations. c OLAM-exchanged octapods standing on four pods and forming binary square lattice in small domains at 5 nM and even at high concentrations (10 nM). The side representations next to the panels show the atomic structure of the used ligands. The embedded sketches depict the observed octapod configurations. Scales bars: for 5 nM octapod concentration, 200 nm, and for the 1 nM, 0.5 nM, and 10 nM octapod concentration, bottom panels) 100 nm