Co‑Sputtering and ALD Fabricated Ag Nanoparticle SERS Substrate: High Sensitivity and 30‑Day Stability for Glycerol Detection

Introduction

Since its first observation, surface‑enhanced Raman scattering (SERS) has become a cornerstone for detecting trace analytes thanks to its high sensitivity, rapid response, and non‑destructive fingerprinting capabilities. Recent advances in in‑situ and real‑time SERS have opened new avenues for surface science, enabling detailed interfacial investigations. Various metals—including Au, Ag, Cu, and Pt—have been explored as plasmonic substrates, but Ag nanostructures remain the most effective due to their superior plasmonic resonance.

Efforts to enhance Ag‑based SERS performance focus on controlling nanoparticle (NP) size, shape, density, and arrangement. Spherical, cubic, octahedral, and wire‑like Ag NPs have been fabricated via electron‑beam lithography, reactive ion etching, immersion plating, and chemical reduction. However, few studies address Ag‑NP SERS substrates tailored for super‑lubricating liquids, where rapid oxidation and aggregation limit performance and lifetime. Moreover, humidity fluctuations in lubricants can attenuate Raman enhancement, compromising interfacial analysis during friction.

Here we introduce a facile co‑sputtering/ALD method to produce a highly sensitive and stable Ag‑NP SERS substrate optimized for glycerol detection—a key component in super‑lubricants. By co‑sputtering Ag with Al, then removing Al to yield a uniform Ag film, we tune NP size and distribution through deposition time and power ratios. Subsequent TiO₂ ALD encapsulation not only preserves Ag against oxidation but also enhances the Raman signal, enabling >30‑day stability in air.

Methods

Fabrication of Ag NPs on Glass by Co‑sputtering

Glass slides (15 × 15 mm, Sail Brand) were sequentially cleaned in acetone, ethanol, and deionized water (15 min each) to remove contaminants. Ag and Al (≥ 99.99 % purity, 60 mm diameter) were co‑sputtered at room temperature using a LLJGP‑450 Magnetron Sputtering System. The base pressure was < 4 × 10⁻⁴ Pa; argon was maintained at 0.8 Pa. The Ag target radio‑frequency power was fixed at 30 W while the Al DC power varied to adjust the Ag:Al ratio. After deposition, Al was etched away by immersing the slides in 0.5 M phosphate solution for 4 h, followed by five rinses in deionized water and nitrogen drying. The resulting Ag NP layer served as the SERS active surface.

Preparation of Protective TiO₂ Layer via ALD

An ultrathin TiO₂ film was deposited over the Ag NPs using a Picson‑100 ALD reactor. TiCl₄ (99.99 %) and H₂O were alternately pulsed at 300 °C under 10 hPa pressure. Pulse/purge times were 400 ms/5 s for TiCl₄ and 200 ms/8 s for H₂O. The growth rate was 0.04 nm per cycle; the number of cycles was chosen to achieve the desired TiO₂ thickness (e.g., 2 nm).

Characterization of Substrates and SERS Measurements

Field‑emission SEM (Hitachi S‑4800) and EDS (ORAN System SIX) assessed surface morphology and elemental composition. UV‑Vis absorption (Perkin Elmer Lambda2) probed plasmonic features. SERS spectra were acquired with a Renishaw Invia‑reflex confocal Raman microscope, 532‑nm laser, 1800 lines/mm grating, × 50 objective. Glycerol solutions (0.1 mL of 10 % glycerol) were drop‑cast for testing.

Results and Discussion

Co‑sputtering followed by Al removal yields a uniform Ag NP film that acts as the SERS active moiety. Figure 1 illustrates the fabrication workflow.

Scheme of highly sensitive SERS substrate fabricated by co‑sputtering and atomic layer deposition on glass slides.

Glycerol Raman signals increased with co‑sputtering time, peaking at 60 s with a 1:1 Ag:Al power ratio. Beyond 60 s, the enhancement factor (EF) decreased sharply (Fig. 2a‑b). The deposition rate was 0.14 nm/s. Varying the Al content (fixed 60 s) revealed that EF rose with Al until a 2:1 Ag:Al ratio, after which it declined (Fig. 2c‑d). This trend reflects the optimal NP size and spacing that maximize hot‑spot density.

The SERS spectra of glycerin collected on substrates prepared with different (Ag, Al) co‑sputtering time (a, b) and power ratio (c, d) without TiO₂ layer.

SEM images (Fig. 3) confirm that the optimal substrate (1:1 ratio, 60 s) exhibits the most uniform NP distribution, directly correlating with its superior EF. EDS (Fig. 4a) verifies the presence of Ag and absence of residual Al. UV‑Vis spectra (Fig. 4b) show plasmon peaks shifting from 404 nm (30 s, 1:1) to 468 nm (60 s, 4:1), reflecting size variations.

a EDS characterization of the sample prepared by co‑sputtering (Ag, Al) targets for 60 s at 1:1 power rate. b UV‑visible absorption spectrum of Ag NPs prepared with different co‑sputtering time and power ratio.

Uniformity was demonstrated by Raman spectra from ten random spots, all showing consistent intensity (Fig. 5a). However, the uncoated substrate suffered rapid signal decay in air, confirming Ag oxidation as the primary degradation mechanism (Fig. 5b).

The SERS spectra of glycerin collected from a 10 random points on the substrate as soon as well‑prepared. b The same position on the substrate after different time left in air condition.

Coating TiO₂ via ALD not only protects against oxidation but also enhances the Raman signal. A 2 nm TiO₂ layer yields the strongest glycerin response (Fig. 6a). Thicker TiO₂ attenuates the signal due to exponential decay of the electromagnetic field with spacer thickness. Importantly, the 2 nm TiO₂‑protected substrate retains >70 % of its initial EF after 30 days in air (Fig. 6b).

Comparison of SERS glycerin spectra collected from a uncoated Ag NPs on glass sides and coated with TiO₂ of different thickness. b The substrate coated a 2‑nm TiO₂ film in different duration left in air condition.

Conclusion

We have developed a straightforward co‑sputtering/ALD protocol that yields Ag‑NP SERS substrates with exceptional sensitivity and >30‑day air stability for glycerol detection. Fine control over deposition time and Ag:Al power ratio generates well‑distributed NPs, while an ultrathin TiO₂ shell provides both chemical and electromagnetic enhancement and prevents oxidation. This platform is poised to enable reliable interfacial studies in advanced liquid lubricants.

Abbreviations

ALD:

Atomic layer deposition

FE‑SEM:

Field emission scanning electron microscopy

NPs:

Nanoparticles

SERS:

Surface‑enhanced Raman scattering