Controlled Growth of Au Nanoislands via Repetitive Sputtering and Rapid Thermal Annealing for Nanogap-Rich SERS Substrates

This study reports a lithography-free route to fabricate nanogap-rich Au nanoislands for reproducible surface-enhanced Raman scattering (SERS) substrates using repetitive DC magnetron sputtering and rapid thermal annealing (RTA). Au was deposited on Si with sputtering times of 10–50 s per cycle and annealed at 500 °C for 10 min, and the cycle was repeated up to seven times. SEM image analysis quantified the effective diameter, covered area fraction (CAF), and mean interisland gap distance as functions of sputtering time and cycle number. As the cycle number increased, the nanoislands progressively coarsened, whereas the mean gap distance generally decreased. At thicker depositions, the 50 s condition remained largely film-like even after repeated cycling. In contrast, the 40 s condition developed a bimodal morphology with abundant secondary particles at higher cycles, providing additional interisland junctions. SERS measurements of methylene blue (785 nm excitation) showed increasing intensities at 1397 and 1623 cm–1 with increasing cycle number, and the 40 s, cycle 7 substrate exhibited the highest signals. The analytical enhancement factors were estimated to be 3.29 × 103 and 3.68 × 103 at 1397 and 1623 cm–1, respectively. Uniformity was confirmed through mapping-based RSD values of 6.64 and 5.06% over a 210 × 210 μm2 area, as well as random-spot RSD values of 6.36 and 5.51%. The practical detection limit for methylene blue was 10–7 M under the selected measurement conditions. These results demonstrate that repetitive sputtering–RTA provides a simple lithography-free strategy to control nanogap characteristics and produce spatially reproducible Au nanoisland SERS substrates.