Au-Ag controllable composition nanoalloying of hexagonal nanoplates: heterogeneous interfacial nanogaps enhance near-field focusing.

In this study, we present a novel strategy for fabricating binary-array surface-enhanced Raman scattering (SERS) substrates composed of gold (Au) and silver (Ag) plasmonic hexagonal nanoplates (h-NPLs), functioning as a "nanoalloy" system. Using Au h-NPLs as scaffolds, we synthesized Ag h-NPLs of closely identical sizes and shapes, facilitating the construction of a mixed plasmonic system. The flat morphology of h-NPLs enables their face-to-face assembly into parallel "wire-like string" arrays, referred to as "columnar superpowders (SPs)", which expose nanogaps perpendicular to the incident light and maximize near-field focusing. We achieved anisotropic superstructures of Au-Ag core-shell h-NPLs through epitaxial Ag growth on Au surfaces, controlled by the interplay of halide ions and surface crystal energy differences. Free-standing columnar SPs were fabricated an upside-down assembly method, forming dense face-to-face nanogaps that act as hotspots for SERS enhancement. The thickness of the Ag shell was critical in optimizing plasmonic coupling between the Au core and Ag shell, thereby enhancing near-field effects. The observed strong near-field focusing originates from synergistic intra- and inter-interface plasmonic coupling, which induces a mirror charge effect, amplifying near-field polarization and SERS sensitivity. Theoretical simulations and experimental bulk SERS analyses validated these effects, underscoring the potential of binary-component nanoalloy structures for advanced optical sensing technologies. This work highlights the critical role of heterogeneous interfacial nanogaps in enhancing both near- and far-field plasmonic effects, paving the way for next-generation SERS applications.