“Delocalized π-bond” guided pyramidal nanocrystal superstructures for excellent light trapping in SERS

Two-dimensional (2D) self-assembly presents significant advantages for optical applications; however, challenges side due to the lack of the z-direction and weak driving force for assembling large particles, making it extremely difficult to achieve the self-assembly of nanoparticles in xy-directions. Herein, we introduce a novel self-assembly route that mimics delocalized π-bonds to construct a 2D CuI pyramidal superstructure, which demonstrates excellent sensitivity and reproducibility for surface enhanced Raman scattering (SERS). After the formation of CuI quasi-octahedra, CuI₂ ions facilitate the assembly of these octahedra into a 2D superstructure, similar to the behavior of delocalized π-bonds. Ultimately, all CuI₂ ions are converted to CuI, effectively immobilizing the neighboring CuI octahedra. The obtained CuI pyramidal superstructures not only trap light effectively but also enhance the scattering length through multiple light scattering. Moreover, a large number of copper and iodide defects were generated during the self-assembly process, which endowed CuI superstructures with excellent SERS performance, achieving a metal-comparable EF (1.2 × 10⁵), a low limit of detection (1 × 10⁻⁷ M) and remarkable reproducibility. The comprehensive strategy broadens the applicability of self-assembly for the guided construction of assemblies, offering a straightforward, rapid, and cost-effective method to prepare highly sensitive and reproducible SERS substrates.