Direct Determination of Plasmon Enhancement Factor and Penetration Depths in Surface Enhanced IR Absorption Spectroscopy

Abstract

Surface Enhanced Infrared Absorption Spectroscopy (SEIRAS) is a powerful tool for studying a wide range of surface and electrochemical phenomena. For most electrochemical experiments the evanescent field of an IR beam partially penetrates through a thin metal electrode deposited on top of an attenuated total reflection (ATR) crystal to interact with molecules of interest. Despite its success, a major problem that complicates quantitative interpretation of the spectra from this method is the ambiguity of the enhancement factor due to plasmon effects in metals. We developed a systematic method for measuring this, which relies upon independent determination of surface coverage by Coulometry of a surface-bound redoxactive species. Following that, we measure the SEIRAS spectrum of the surface bound species, and from the knowledge of surface coverage, retrieve the effective molar absorptivity, ε SEIRAS . Comparing this to the independently determined bulk molar absorptivity leads us to the enhancement factor f = ε SEIRAS /ε bulk . We report enhancement factors in excess of 1000 for the C−H stretches of surface bound ferrocene molecules. We additionally developed a methodical approach to measure the penetration depth of the evanescent field from the metal electrode into a thin film. Such systematic measure of the enhancement factor and penetration depth will help SEIRAS advance from a qualitative to a more quantitative method.

Oxygen Vacancies Alter Methanol Oxidation Pathways on NiOOH

“…The AFM measurements gave an estimated Ni film thickness of ∼150 nm. The surface roughness and thickness of the metal film are critical factors determining the quality and extent of signal enhancement in ATR-SEIRAS because only noncontinuous thin films give rise to strong signal amplification. , In terms of structures, the XRD pattern for the Ni film in Figure S4 shows three distinctive diffraction peaks at 44.5, 52 and 76.5° corresponding to the (111), (200) and (220) facets of face-centered cubic Ni (ICDD 00–001–1258), providing evidence that the film is primarily made up of Ni metal. , Further evidence of metallic Ni as the dominant species in the electrodeposited films is provided by XPS (see below).…”

Gold-diamond Nanocomposites Efficiently Generate Hydrated Electrons upon Absorption of Visible Light

“…The simplest mechanism by which the AuNPs can enhance a process in a resonant manner (cases in which there is a spectral overlap between the SPR excitation and the transition to be enhanced) is by enhancement of the local electric field of the light on the surface of an NP due to oscillations of the conduction electrons. Absorption at the surface of the NP can be greatly enhanced by this mechanism, with enhancement factors in excess of 1000 being observed . More complex mechanisms, such as hot electron injection from the Au to the diamond, chemical enhancement (in analogy to SERS chemical enhancement), or resonant energy transfer, cannot be ruled out a priori but are not needed to explain the enhancement observed.…”

“…Absorption at the surface of the NP can be greatly enhanced by this mechanism, with enhancement factors in excess of 1000 being observed. 49 More complex mechanisms, such as hot electron injection from the Au to the diamond, 50 chemical enhancement (in analogy to SERS chemical enhancement), 48 or resonant energy transfer, 51 cannot be ruled out a priori but are not needed to explain the enhancement observed.…”

Lipid Landscapes: Vibrational Spectroscopy for Decoding Membrane Complexity

“…the IR enhancement is general for nonvolatile molecules; the enhancement increases for molecules strongly chemisorbed on the metal film; and the effect is observed only for the first few adsorbed monolayers (82)(83)(84). More recent models provide a surface-average enhancement factor and IR penetration depth from experiment as the ratio of molar absorptivities of SEIRAS to the bulk (85).…”