Surface-enhanced Raman scattering (SERS) attracts wide attention in trace analysis. However, its practical application is still limited by insufficient hotspot density, poor signal reproducibility, and the difficulty of balancing low cost with large-area uniformity. Random plasmonic nanostructures formed by thin-film dewetting offer a promising route for scalable substrate fabrication, but conventional one-step annealing usually cannot precisely control particle morphology, interparticle spacing, or hotspot distribution. To address these issues, this study proposes an on-chip strategy combining ion sputtering with stepwise thermal annealing to regulate the nanoparticle evolution in Ag/Au alloy films and establish a low-cost, highly reproducible SERS platform with potential for industrial-scale fabrication.

First, Ag/Au bilayer films were deposited onto the substrate surface by ion sputtering. High-temperature thermal annealing was then employed to induce film rupture, atomic diffusion, and alloying transformation, thereby forming random nanoparticle structures with SERS activity. Microscopic morphology characterization and spectral measurements were performed to systematically investigate the effects of the annealing process on nanoparticle morphology evolution and plasmonic properties. Based on the optimized first-step annealing condition, an ultrathin Au film was further deposited on the surface of the preformed nanoparticle structures, followed by a second low-temperature annealing treatment to reconstruct particle surfaces and interparticle gap regions. The finite-difference time-domain (FDTD) method was employed for simulation analysis. Subsequently, the morphology, size distribution, and structural characteristics of the prepared substrates were analyzed and correlated with their Raman enhancement behavior. BPG and MB were selected as probe molecules to evaluate the analytical performance of the optimized substrates. Raman measurements at different concentrations were conducted to assess signal enhancement, detection sensitivity, and limits of detection.

The results showed that high-temperature annealing effectively transformed the sputtered Ag/Au bilayer films into random alloy nanoparticle structures through dewetting, elemental interdiffusion, and alloy formation, thereby significantly changing their plasmonic response and SERS activity (Fig.2 and Fig.3). The two-step annealing process optimized the nanoparticle evolution in Ag/Au alloy films, overcame the technical limitations of the conventional one-step method, and significantly increased the density of nanoparticles and SERS hotspots (Fig.4). Compared with the conventional one-step method, the two-step annealing method increased the SERS signal intensity approximately sevenfold, while the enhancement factor increased by about two orders of magnitude (Fig.5). The optimized SERS substrate enabled highly sensitive detection of benzylpenicillin potassium (BPG) and methylene blue (MB), with limits of detection reaching 10⁻⁶ mol/L and 10⁻¹⁰ mol/L, respectively (Fig.6 and Fig.7).

This study proposes a method combining ion sputtering with stepwise thermal annealing. High-temperature annealing induces atomic diffusion and alloying in the Ag/Au bilayer, thereby improving the plasmonic properties of the system by combining the strong plasmonic response of Ag with the chemical stability of Au. After the first annealing step, secondary Au sputtering and low-temperature annealing further increase nanoparticle density and hotspot abundance, leading to stronger SERS signals. This surfactant-free process is compatible with CMOS manufacturing processes and shows good potential for the scalable fabrication of low-cost, highly reproducible SERS substrates. This study provides a feasible technical solution for low-cost and highly reproducible SERS sensing.