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

High Resolution Image Download MS PowerPoint Slide 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 Al 2 O 3 (≈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 × 10 3 ) 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.