Gold etchant

The procedure to transfer the films, we employed a technique which we have reported previously^{17 (link)}: After the CVD growth of the GNRs, a thin layer of poly(methyl methacrylate) (PMMA) was spun onto the GNR/gold/mica stack which provided additional mechanical stability and facilitated the transfer of intact films over a large area. Carefully, the resulting stack was floated on concentrated HF for several hours to delaminate the PMMA/GNR/gold film from the mica slab. After the delamination was complete, the gold was etched away in a gold etchant (Sigma-Aldrich). We then transferred the PMMA/GNR film to the target substrate with Au electrodes. To dissolve the PMMA, the PMMA/GNR/substrate stack was immersed in an acetone bath. Finally, we rinsed the chip with isopropanol and dry blow the GNR film.

A sapphire wafer (University wafer) was spin-coated with poly(methyl methacrylate) resist (MicroChem, 950 PMMA C4) at 3000 r.p.m. for 60 s, followed by thermal curing on a hot plate at 180 °C for 15 min. After a 20-nm-thick Au layer was sputtered on the electron-beam resist to prevent electrons from piling up, a nanohole array (250 nm diameter and 500 nm period) was patterned via electron-beam lithography (VISTEC, EBPG5000+) at 100 KeV beam energy and 1000 μC/cm² exposure dose. After removal of Au in gold etchant (Sigma-Aldrich) and developing with a solution of MIBK:IPA (1:3) for 90 s, 3-nm Ti and 200-nm Au film were then directionally deposited on the patterned substrate using electron-beam evaporator (CHA, SEC 600). After a lift-off process in acetone and an oxygen plasma cleaning (STS, 320PC) at 100 W for 30 s to remove resist residue, the resultant Au nanodisk array was coated with an Al₂O₃ film using ALD (Cambridge Nano Tech Inc., Savannah) at a typical deposition rate of 1 Å per cycle at 250 °C, followed by conformal deposition of a 400 nm Au film using an electron-beam evaporator (CHA, SEC 600) with a planetary fixture. Finally, the structures were planarized via ion milling (Intlvac, Nanoquest) with an Ar beam of 130 mA and 36 V incident at 10º from the horizontal plane.