Shape-directed self-assembly of nanodumbbells into superstructure polymorphs

Abstract

Self-assembly of colloidal nanoparticles into ordered superstructures provides a promising route to create novel/enhanced functional materials. Much progress has been made in self-assembly of anisotropic nanoparticles, but the complexity and tunability of superstructures remain restricted by their available geometries. Here we report the controlled packing of nanodumbbells (NDs) with two spherical lobes connected by one rod-like middle bar into varied superstructure polymorphs. When assembled into two-dimensional (2D) monolayer assemblies, such NDs with specific shape parameters could form orientationally ordered degenerate crystals with a 6-fold symmetry, in which these NDs possess no translational order but three allowed orientations with a rotational symmetry of 120 degrees. Detailed analyses identify the distinct roles of subunits in the ND assembly: the spherical lobes direct NDs to closely assemble together into a hexagonal pattern, and the rod-like connection between the lobes endows NDs with this specific orientational order. Such intralayer assembly features are well maintained in the two-layer superstructures of NDs; however, the interlayer stackings could be adjusted to produce stable bilayer superstructures and a series of metastable moiré patterns. Moreover, in addition to horizontal alignment, these NDs could gradually stand up to form tilted or even vertical packing based on the delicate control over the liquid-liquid interface and ND dimensions. This study provides novel insights into creating superstructures by controlling geometric features of nanoscale building blocks and may spur their novel applications.

Fig. 1. TEM characterization of core–shell β-NaYF₄:Yb/Er@NaGdF₄ NDs. (a) HAADF-STEM image of the as-synthesized core–shell NDs. Inset: schematic of a single ND with several geometric parameters: the distance between the center of two lobes (L), the diameter of the lobe (D) and the width of the middle part (d). (b) HAADF-STEM image and elemental mappings of a representative ND (L/D = 1.08, D/d = 2.08). Scale bar: 10 nm. (c) HR-TEM image of a representative ND. (d and e) HR-TEM images of the lobe and middle part marked with squares in (c), respectively.

Fig. 2. Large-scale monolayer membrane self-assembled from NDs on the EG subphase. (a) Representative TEM image of the ND monolayer membrane, with the lower inset showing the corresponding SAED pattern. (b) TEM image at a higher magnification. (c–e) HR-TEM images (left) of NDs with different orientations captured from (b), scale bar: 10 nm; corresponding FFT patterns (middle); and simulated FFT patterns (right). (f) Schematic of packing of NDs into three possible different bi-dumbbell structures. (g) Schematic of the monolayer membrane captured from (b). (h) FFT pattern of the monolayer TEM image (b). (i and k) Schematics of lobes and middle parts of NDs as marked in (g), respectively. (j and l) FFT patterns of lobes and middle parts of NDs obtained from (i and k), respectively.

Fig. 3. Large-scale bilayer membrane self-assembled on the EG subphase. (a–c) TEM images of the bilayer membrane at different magnifications. The inset of (a) shows the corresponding SAED pattern. (d) Schematic of NDs in the first layer captured from (c). Some cracks in this layer are filled with tilted NDs (white color). (e) Schematic of NDs in the double layer captured from (c), in which the gray color is used to represent NDs in the first layer. (f) Schematic of self-assembly of NDs into six different bilayer structures. (g) Hexagonal packing of ND lobes in this bilayer structure.

Fig. 4. Quasi-12-fold moiré pattern generated from bilayer stacking of NDs on the EG subphase. (a) Representative TEM image of the quasi-12-fold moiré pattern. (b) TEM image at higher magnification. (c) Corresponding FFT pattern of (b). (d) Schematic demonstration of the formation of the quasi-12-fold moiré pattern based on the stacking of two ND monolayers.

Fig. 5. Self-assembly of NDs with different size aspect ratios (L/D) on the DMSO subphase. (a and b) Representative TEM images of tilted NDs (L/D = 1.08, D/d = 2.08) at different magnifications. Upper and lower insets of (a) show the schematics of tilted NDs from the top and front views, respectively. (c) Corresponding SAED pattern of (a). (d and e) Representative TEM images of vertically aligned NDs (L/D = 1.18, D/d = 1.87) at different magnifications. Upper and lower insets of (a) show the schematics of vertically aligned NDs from the top and front views, respectively. (f) Corresponding SAED pattern of (d).