Development of A Novel Dual-Layer Thick Ag Substrate for Surface-Enhanced Raman Scattering (SERS) of Self-Assembled Monolayers

Abstract

A dual-layer, thick (∼70 nm) vapor-deposited Ag substrate has been developed that gives enhancement factors on the order of 104 for surface-enhanced Raman scattering (SERS) experiments. This substrate has a total thickness of ∼70 nm but also has an outermost surface morphology that approximates that of a thin Ag island film (AgIF) substrate. This is accomplished using a dual overlayer/underlayer structure in which a thick underlayer of 45-nm Ag is vapor-deposited onto a treated glass slide. This Ag underlayer is exposed to ambient conditions under which the surface chemisorbs oxygen, leading to the thermodynamically favorable formation of an active Ag2O interface. An overlayer of 25-nm Ag is vapor-deposited on top of this structure. The first Ag/Ag2O underlayer produces an active interface that decreases the diffusion of the Ag atoms from the second vapor-deposited overlayer, thereby forming Ag particles with shapes favorable for SERS enhancement. Atomic force microscopy results show that the Ag overlayer particles have ideal shapes for SERS enhancement with morphology comparable to thin, vapor-deposited AgIFs. X-ray photoelectron spectroscopy in the O(1s) region showed the presence of multiple forms of oxygen in the Ag/Ag2O underlayer; the main forms were identified as dissolved bulk oxygen and chemisorbed oxygen. Using self-assembled monolayers (SAMs) of 1-dodecanethiol, a SERS intensity increase of ∼400% was obtained for this new overlayer/underlayer Ag substrate when compared to SAMs formed on traditional thick Ag substrates. Enhancement factors calculated from the Raman intensity of trans-1,2-bis(4-pyridyl)ethane showed a SERS enhancement of approximately 104 for this new Ag/Ag2O/Ag substrate compared with the bulk. This compares favorably with the SERS enhancements obtained using thin AgIF substrates. The Ag/Ag2O/Ag substrates showed reproducible SERS intensities (RSD = 0.45−5%). The mechanisms responsible for the overall enhancement on this new substrate are proposed to be the ideal surface morphology of the Ag overlayer particles as well as a combined enhanced electromagnetic field produced by both the Ag/Ag2O underlayer and the Ag overlayer.

In situ nucleation and growth of silver nanoparticles in membrane materials: a controllable roughened SERS substrate with high reproducibility

“…Silver was subsequently deposited onto the etched probe resulting in a highly uniform and reproducible SERSactive surface (<1.9% RSD across the image surface). There are two other supported substrates in the literatures, [5,10] which are also found to be beyond our work. One is a dual-layer vapordeposited Ag substrate (Ag/Ag 2 O/Ag substrate) that shows the RSD of SERS intensities is 0.45-5%.…”

“…[1,2] In 1997, studies on single-molecule SERS detection have further caught the researchers' attention for the ultra low detection limit and super high enhancement factor. [3,4] Recently, many different methods have been employed to fabricate SERS substrates with surface roughness features, including vapor-deposited metal island films, [5] electrochemically roughened electrodes, [1,6,7] metal colloidal suspensions [8,9] and their support-based substrates. [10] However, the development of an SERS-based surface chemicalanalysis application requires an effective SERS-active substrate that can not only provide strong enhancement factor and greater reproducibility, but is also facile and relatively inexpensive to fabricate and store.…”

Recent advances in plasmonic nanostructures for sensing: a review

“…6). 57,[77][78][79][80] Such multilayer geometries have been employed on silver island films 77 and MeFON surfaces, 57,78,79,83 demonstrating the versatility of this enhancement mechanism. In particular, by applying (10) Strobbia, Languirand, and Cullum: Recent advances in plasmonic nanostructures for sensing: a review (10) Strobbia, Languirand, and Cullum: Recent advances in plasmonic nanostructures for sensing: a review this multilayer geometry to MeFON substrates, it is possible to further enhance the original SERS signal of these substrates by over two orders of magnitude.…”

“…These mixed material substrates include (i) dielectric materials coated with metals, [62][63][64]70 (ii) metals coated with dielectrics, 53,[71][72][73][74] (iii) semiconductors coated with metals, 75,76 and (iv) multilayered metal structures separated by dielectrics. 57,[77][78][79][80] One of the earliest and most prominent classes of these mixed material nanostructures for SERS sensing is known as core-shell nanoparticles, which consist of a dielectric core structure coated with a metallic shell [see Fig. 5(a)].…”

Rapid Detection and Recognition of Organic Pollutants at Trace Levels by Surface-Enhanced Raman Scattering

“…In particular, multilayer structures can improve SERS enhancement, such as "sandwich" structures with silver oxide or carbon inside and Ag or Au as both underlayer and overlayer. [24][25][26][27][28] Other researchers found that multilayer structures of Ag/Au nanostructures on the smooth metallic underlayer exhibited better SERS sensitivity compared to those without metallic underlayer (EF = 5 × 10 8 ). [29][30][31] However, the factor that governs the enhancement for multilayer structures is not very clear.…”