Recrystallization-Enabled\nFabrication of Single-Crystalline\nGold Flakes for Plasmonic Applications

Gold\nfilms with a single-crystalline structure and an ultrasmooth\nsurface are highly desired for fabricating plasmonic nanostructures\nwith low loss. Here, we report a recrystallization-based approach\nto synthesize on-substrate single-crystalline gold flakes with high\nefficiency. By dissolving a multicrystalline gold film with tetraoctylammonium\nbromide at ∼140 °C and then recrystallizing at ∼160\n°C for 2 h, high-density (>1000 pieces per cm<sup>2</sup>)\ngold\nflakes can be obtained directly on a substrate, which have a thin\nthickness concentrated around 30 nm and a maximum lateral size up\nto 0.12 mm. The as-synthesized gold flakes have a face-centered cubic\ncrystalline structure and smooth surface with a surface root-mean-square\nroughness as low as ∼0.3 nm. To demonstrate their advantage\nfor fabricating high-quality plasmonic nanostructures, gold nanodisk\narrays that can support surface lattice resonance mode are fabricated\nusing focused ion beam milling from a single-crystalline gold flake\nand a multicrystalline gold film, respectively, for comparison. Their\ntransmission spectra show that the single-crystalline gold nanodisk\narray has a higher quality factor (23.3 vs 15.1) and a deeper transmission\ndip (41.3% vs 62.2% for the minimum transmittance) of the plasmonic\nresonance, which can be attributed to the reduction in electron scattering\nloss caused by surface roughness and grain boundaries.