Formation of self-assembled gold nanoparticle supercrystals with facet-dependent surface plasmonic coupling

Abstract

Metallic nanoparticles, such as gold and silver nanoparticles, can self-assemble into highly ordered arrays known as supercrystals for potential applications in areas such as optics, electronics, and sensor platforms. Here we report the formation of self-assembled 3D faceted gold nanoparticle supercrystals with controlled nanoparticle packing and unique facet-dependent optical property by using a binary solvent diffusion method. The nanoparticle packing structures from specific facets of the supercrystals are characterized by small/wide-angle X-ray scattering for detailed reconstruction of nanoparticle translation and shape orientation from mesometric to atomic levels within the supercrystals. We discover that the binary diffusion results in hexagonal close packed supercrystals whose size and quality are determined by initial nanoparticle concentration and diffusion speed. The supercrystal solids display unique facet-dependent surface plasmonic and surface-enhanced Raman characteristics. The ease of the growth of large supercrystal solids facilitates essential correlation between structure and property of nanoparticle solids for practical integrations.

Fig. 1

Electron microscopy characterizations of gold NPs and SCs. a TEM image of the synthesized gold NPs. Scale bar is 10 nm. Inset shows statistics of the particle diameter. b SEM image of a SC. Scale bar is 5 μm. c High-resolution SEM image of the top surface of the SC showing hexagonal packing. Scale bar is 50 nm. Inset shows the corresponding FFT pattern

Fig. 2

Supercrystallography analysis of a single gold SC at varying rotational angle phi (ϕ). a–d SAXS patterns in selected projections. Simulated peaks (black dots) from an hcp superlattice are overlaid on top of experimental patterns with Miller indices labeled. Insets of a–d present corresponding schematic illustrations of a rotating SC with X-ray beam shooting perpendicular to paper and an hcp superlattice in the same projections as labeled by SC[hkl]. Three consecutive hexagonal monolayers are shown in two different colors to emphasize the ABA packing for visual aid. WAXS patterns from the same SC with e ϕ = 0° and f ϕ = 90°, collected simultaneously with the SAXS patterns in a, d, respectively. Powder scattering rings from gold atomic lattice are marked with Miller indices. g Integrated azimuthal WAXS spectra of the Au (111) peak. The sharp dips were caused by beam-stop blockage

Fig. 3

Large gold SC of sub-millimeter size and optical characteristics. a Photograph of a gold SC measured 490 μm. Scale bar is 50 μm. The blue and red frame outline SC {011} and SC {001} surface, respectively. b Optical reflectance spectra (normalized) collected from two different facets (blue and red) of the SC and a drop-cast film (black solid line) and absorption from the NP solution (black dashed line). c Anti-Stokes Raman spectra of dodecanethiol ligand collected from the surface of SC (red) and film (black). The peaks are labeled with corresponding vibration modes with (s) = stretching and (b) = bending

Fig. 4

Schematic illustration of the early stage of the SC growth. a The first hexagonal monolayer of NPs. b Cross-section as marked by dashed line in a. c Two consecutive monolayers and cross-sections showing d a TV and e an OV. The red shades outline the free space in these voids

Fig. 5

Simulated solvent composition during counter-diffusion. a Slow and b fast diffusion. The corresponding schematics on the side illustrate the initial configuration of the liquid column (purple: gold NPs in toluene; blue: IPA). c Spatial distribution of diffusion speed when x_IPA reached 0.1