Controlling Surface-Enhanced Raman Scattering and Metal-Enhanced Fluorescence in Silver Nanofilms Using Ultrathin Aluminum Oxide Spacers via Atomic Layer Deposition

Atomic layer deposition (ALD) enables simultaneous passivation of silver and nanometer-scale tuning of the near-field landscape that controls surface-enhanced Raman scattering (SERS) and metal-enhanced fluorescence (MEF). Here, sputtered ∼16 nm Ag films were conformally coated with 1–20 ALD cycles of Al₂O₃ (≈0.17–1.76 nm) and analyzed by atomic force microscopy (AFM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), spectroscopic ellipsometry, UV–Vis spectroscopy, time-resolved fluorescence, large-area Raman mapping of Rhodamine 6G and finite-difference time-domain (FDTD) modeling. Morphology evolves from isolated oxide nuclei after one cycle through a conformal roughness-amplifying shell at 5–15 cycles to vertically elongated outgrowths at 20 cycles; ellipsometry confirms self-limiting growth with 0.10 ± 0.02 nm cycle^–1. Optical measurements reveal three thickness regimes: ≤1 cycle (<0.2 nm) yields SERS-dominated behavior with picosecond quenching and intense Raman hotspots; ∼5 cycles (∼0.5 nm) provides the hybrid optimum, giving the highest Raman enhancement (EF ≈ 2 × 10³) together with a 4-fold fluorescence-lifetime extension (⟨τ⟩ ≈ 26 ns) that signals strong MEF; whereas >10 cycles (>1 nm) attenuate both SERS and MEF as the evanescent field decays. FDTD maps based on AFM topographies reproduce the heavy-tailed hotspot distribution and identify the 0.5–1.0 nm window as the sweet spot for co-optimizing field confinement and radiative efficiency. Stability tests show that five-cycle coatings endure solvent rinsing and cotton-swab abrasion while retaining─or even increasing─SERS activity, whereas thicker oxides guarantee mechanical integrity at the cost of weaker near-fields. These combined results show an experimentally validated framework for engineering reusable, dual-mode plasmonic substrates by angstrom-level control of dielectric spacer thickness.