Achieving Optical Refractive Index of 10‐Plus by Colloidal Self‐Assembly

This study demonstrates the developments of self‐assembled optical metasurfaces to overcome inherent limitations in polarization density (P) and high refractive indices (n) within naturally occurring materials. The Maxwellian macroscopic description establishes a link between P and n, revealing a static limit in natural materials, restricting n to ≈4.0 at optical frequencies. Previously, it is accepted that self‐assembly enables the creation of nanogaps between metallic nanoparticles (NPs), boosting capacitive enhancement of P and resultant exceptionally high n at optical frequencies. The work focuses on assembling gold (Au) NPs into a closely packed monolayer by rationally designing the polymeric ligand to balance attractive and repulsive forces, in that polymeric brush‐mediated self‐assembly of the close‐packed Au NP monolayer is robustly achieved over a large‐area. The resulting monolayer of Au nanospheres (NSs), nanooctahedras (NOs), and nanocubes (NCs) exhibits high macroscopic integrity and crystallinity, sufficiently enough for pushing n to record‐high regimes. The systematic comparisons between each differently shaped Au NP monolayers elucidate the significance of capacitive coupling in achieving an unnaturally high n. The achieved n of 10.12 at optical frequencies stands as a benchmark, highlighting the potential of polyhedral Au NPs in advancing optical metasurfaces. This work demonstrates that achieving an unnaturally high refractive index at optical frequencies is possible through colloidal self‐assembly. By utilizing polymeric brush‐assisted balancing between attractive and repulsive potentials, polyhedral gold nanoparticles (i.e., nanocubes) are self‐assembled into a close‐packed monolayer with high macroscopic integrity and crystallinity. This represents the first realization of an optical metasurface with a refractive index of 10‐plus, which is out of reach thus far.