Surface Enhancement Factors for Ag and Au Surfaces Relative to Pt Surfaces for Monolayers of Thiophenol

Abstract

Raman signal intensities from the 997 cm⁻¹ ring breathing mode of thiophenol monolayers adsorbed at Ag, Au, and Pt surfaces were employed for determination of absolute surface enhancement factors (SEFs). Unlike previous estimations of SEFs, these SEFs are determined by referencing the surface-enhanced Raman scattering (SERS) intensities to the unenhanced Raman scattering at Pt surfaces. The surfaces studied include those commonly prepared in a laboratory ambient and those prepared in vacuum. Surfaces prepared in ambient include polycrystalline Ag electrochemically roughened by an oxidation-reduction cycle (ORC), mechanically polished (MP) polycrystalline Ag, chemically polished (CP) polycrystalline Ag, Ag (111), MP polycrystalline Au, and MP polycrystalline Pt. Vacuum environment surfaces include coldly deposited Ag films (cold Ag) and room temperature-deposited (RT) “thick” Ag films. Each of these thiophenol/metal systems was sampled with an excitation wavelength (λ_ex) of 514.5 nm; MP Au surfaces were also studied with λ_ex of 720 nm. SEFs of 2.0 × 10⁴ for ORC Ag, 5.3 × 10³ for MP Ag, 160 for cold Ag, 64 for MP Au₇₂₀ (λ_ex = 720 nm), 69 for Ag (111), 39 for CP Ag, 7.9 for RT “thick” Ag, and 2.2 for MP Au_514.5 (λ_ex 514.5 nm) are observed relative to the SEF of MP Pt, which is assigned as 1. For practical purposes, the significance of the magnitudes of these SEFs is discussed in terms of estimated surface Raman limits of detection (LODs).