Polymorphic Assembly from Beveled Gold Triangular Nanoprisms.

The shape anisotropy of nanoparticle building blocks is of critical importance in determining their packing symmetry and assembly directionality. While there has been extensive research on the effect of their overall geometric shapes, the importance of nanometer morphology details is not well-recognized or understood. Here we draw on shape-anisotropic gold triangular nanoprism building blocks synthesized based on a method we recently developed; besides the "large-scale" triangular prism shape (79.8 nm in side length and 22.0 nm in thickness), the prisms are beveled with their sides convexly enclosed by two flat {100} facets. We engineer the balance between electrostatic repulsion and entropically driven depletion attraction in the system to generate self-assemblies without or with the effect of the nanoscale beveling detail. A conventional, planar honeycomb (p-honeycomb) lattice forms with the triangular basal planes packed on the same plane at low depletion attraction, whereas an unexpected interlocking honeycomb (i-honeycomb) lattice and its "supracrystal" forms are assembled with additional close-paralleling of side facets at high depletion attraction. The i-honeycomb lattice renders all the metallic surfaces in close proximity and leads to a surface-enhanced Raman scattering signal nearly 5-fold higher than that in the p-honeycomb lattice and high sensitivity for detecting the model molecule Rhodamine 6G at a concentration as low as 10-8 M. Our study can guide future work in both nanoparticle synthesis and self-assembly; nanoscale geometrical features in anisotropic nanoparticles can be used as an important handle to control directional interactions for nonconventional ordered assemblies and to enrich diversity in self-assembly structure and function.