Effective removal of transfer-induced bubbles in single-crystalline gold flakes for high-performance plasmonics

The transfer of single-crystalline gold flakes is crucial for device integration, yet its impact on optical performance remains unclear. Here, we develop a concentrated sulfuric acid-assisted post-treatment method that effectively removes transfer-induced micro/nano-bubbles on gold flakes, restoring the flake surface to a pre-transfer roughness level of ∼0.26 nm. A scattering-type scanning near-field optical microscope reveals that this treatment significantly improves signal-to-noise ratio and propagation length of propagating surface plasmon polaritons in gold flakes. Moreover, nanoparticle-on-mirror nanocavities constructed on treated flakes exhibit a rise in quality factor from 9.7 (for nanocavities on untreated flakes) to 16.5, owing to suppressed bubble scattering.