Ensemble hot-spots in 3D supercrystals of plasmonic octahedral nanoparticles in tip-to-tip configured superlattices

Abstract

Nanoparticle assembly offers promising strategy for harnessing the physicochemical interparticle interactions. Despite its potential for boosting light-matter interaction, achieving nanoparticle assembly with tip-to-tip manner remains a significant challenge. Here we show a synthetic procedure for organizing gold octahedral nanoparticles into a distinct three-dimensional upright superstructure, where the pointed tips are oriented toward neighboring nanoparticles to promote enhanced near-field focusing at these apexes. This arrangement, referred to as the "coupling of the lightning rod effect", facilitates production in the form of "superpowder", which exhibits an extensive assembly order like a powder. Deviating from natural packing principles, this tip-to-tip alignment-the upright octahedral superlattice-optimizes near-field focusing on its vertices while maintaining consistently high porosity, allowing for deep penetration of adsorbates. This configuration is advantageous for enabling surface-enhanced Raman scattering of gaseous molecules with reduced background fluorescence signals, particularly under high-intensity laser excitation, a challenging feat with conventional surface-enhanced Raman scattering techniques.

Fig. 1. Schematic illustration of the stepwise process for the three-dimensional tip-to-tip octahedral supercrystal.

a Au octahedral nanoparticle building blocks were crystallized with tip-to-tip assembly including colloidal Ag shelling (in suspension), templated crystallization (on substrate), and Ag-selective etching (on substrate). b Optical image and (c–d) low-to-high magnified SEM images of three-dimensional tip-to-tip octahedral supercrystals. e Top and (f) side view of the tip-to-tip octahedral nanoparticle assembly.

Fig. 2. Colloidal synthesis and self-assembly of Ag shelling on Au octahedral nanoparticles.

a Scheme of experimental set-up for colloidal Ag shelling controlled by CTAC concentration, injection rate (ml min⁻¹) of Ag precursors, and ascorbic acid. b Figure of cubicity (C_n) of edge length (L), corner diameter (D) for Au octahedra@Ag shell, and (c) TEM image of Ag shells on Au octahedra corresponding to cubicities of 0.31, 0.42, 0.68, and 0.80, respectively (scale bar = 20 nm). d Plots of Au octahedra@Ag outer shell morphology controlled by CTAC concentration and injection rate of Ag precursor (5 mM) and ascorbic acid (0.1 M). SEM images of close-packed Au octahedra@Ag shell assemblies with cubicities of (e) 0.31, (f) 0.42, g 0.68, and h 0.80, and corresponding FFT pattern (scale bar = 0.1 μm⁻¹) and particle distributions of cubicity. i Low-magnified SEM images of Au octahedra@Ag nanocube supercrystals (inset shows the cubic habits). Source data are provided as a Source Data file.

Fig. 3. Ag selective etching on three-dimensional Au octahedra@Ag cube supercrystals.

a Schematic illustration of three-dimensional tip-to-tip octahedral supercrystal. b SEM, c STEM, and EDS image (Au/yellow, Ag/blue) of three-dimensional tip-to-tip octahedral supercrystals. d–g SEM images of tip-to-tip octahedral supercrystals with different edge sizes (32, 45, 60, and 75 nm) of octahedral blocks, corresponding FFT pattern (scale bar = 0.1 μm⁻¹) and figures of the edge size and distance of neighboring particles.

Fig. 4. Optimizing three-dimensional tip-to-tip octahedral supercrystals as a SERS substrate.

Low-to-high magnified SEM images (a–d) 500×, (e–h) 5000 ×, (i–l) 25000× of controlled supercrystal size (inset optical image, scale bar = 10 μm). m SERS measurement of 100 ppm 2-naphthalenethiol molecules according to supercrystal size and (n) diagram of SERS intensity at 1069 cm⁻¹ obtained from different supercrystal size (785 nm laser, integration time = 1 s, laser power = 4 mW). The standard deviations were calculated from N = 10 independent samples. Source data are provided as a Source Data file.

Fig. 5. SERS analysis of gas molecules through close-packed and tip-to-tip octahedral supercrystals.

Surface charge distribution and electromagnetic field distributions of (a, b) close-packed (7 × 3) and (c, d) tip-to-tip (9 × 3) assembly of octahedral building blocks with 3 nm Ag layers under a 785 nm excitation (scale bar = 100 nm). Microscopic images and zoomed-in SEM images of (e) close-packed and (f) tip-to-tip octahedral supercrystals with existence of thin Ag layers. Corresponding SERS measurements of 1000 ppm CEPS gas molecules plotted for different laser powers (0.01, 0.5, 4, 11, and 21 mW) for (g) close-packed and (h) tip-to-tip octahedral supercrystals. Source data are provided as a Source Data file.