Recrystallization-Enabled Fabrication of Single-Crystalline Gold Flakes for Plasmonic Applications

Gold films with a single-crystalline structure and an ultrasmooth surface are highly desired for fabricating plasmonic nanostructures with low loss. Here, we report a recrystallization-based approach to synthesize on-substrate single-crystalline gold flakes with high efficiency. By dissolving a multicrystalline gold film with tetraoctylammonium bromide at ∼140 °C and then recrystallizing at ∼160 °C for 2 h, high-density (>1000 pieces per cm²) gold flakes can be obtained directly on a substrate, which have a thin thickness concentrated around 30 nm and a maximum lateral size up to 0.12 mm. The as-synthesized gold flakes have a face-centered cubic crystalline structure and smooth surface with a surface root-mean-square roughness as low as ∼0.3 nm. To demonstrate their advantage for fabricating high-quality plasmonic nanostructures, gold nanodisk arrays that can support surface lattice resonance mode are fabricated using focused ion beam milling from a single-crystalline gold flake and a multicrystalline gold film, respectively, for comparison. Their transmission spectra show that the single-crystalline gold nanodisk array has a higher quality factor (23.3 vs 15.1) and a deeper transmission dip (41.3% vs 62.2% for the minimum transmittance) of the plasmonic resonance, which can be attributed to the reduction in electron scattering loss caused by surface roughness and grain boundaries.