Enhancement of the Infrared Absorption from Molecular Monolayers with Thin Metal Overlayers

Abstract

The infrared absorption from molecular monolayers is enhanced a factor of 20 by thin metal overlayers or underlayers with use of the attenuated-total-reflection technique.The total enhancement, including contributions from the attenuated-total-reflection geometry, is almost 10 . This effect is consistent with an electric field enhancement due to collective electron resonances associated with the island nature of the thin metal films. PACS numbers: 78.30.-j The vibration@ states of molecular monolayers can be studied experimentally with inelastic electron-tunneling spectroscopy,~Raman scattering, 2 ' inelastic electron scattering, ' and infrared absorption. " In the inelastic electron-tunneling experiments it is necessary for the molecules to be sandwiched between two conducting media.Inelastic electr on-scattering experiments must be performed in vacuum and have relatively low energy resolution. Both conventional Raman scattering' and conventional infrared absorption' from monolayers must be done with high surfacearea samples, with consequent difficulties in sample char acterization, substr ate absorption and Quorescence. Baman scattering from surfaces can become quite strong with thin overlayers' or underlayers'~of metal, but so far only Ag and Au with a few molecular monolayers have shown a lar ge enhancement. The attenuatedtotal-reflectance (ATR) geometry in infrared absorption' achieves a large effective surface area without the need for powder samples. In this Letter, we report the first observation of the enhancement of infrared absorption from molecular monolayers due to thin metal overlayers and underlayers in the ATB geometry and show that this technique has application to a number of interesting current problems in surface science.The samples used in this study consisted of molecular monolayers of organic acids deposited on silicon substrates, either followed by or preceded by the evaporation of a thin metal layer. The sample preparation technique has been described by Hansma and Kirtley. ' The molecular monolayers used in this study were: 4-nitrobenzoic acid, benzoic acid, and 4-pyridine-COOH. The metal overlayers and underlayers studied were either Ag or Au and were evaporated at room temperature to an average thickness, d 100 I no Ag 80 I-LLI O 60 Z 0 I-40 0 M CO 20 I

Plasmons in nanoscale and atomic-scale systems

“…Also, nanofilms with a random morphology and high surface roughness provide extremely high enhancement of optical absorption by molecular vibrations in a broad spectral range [13,14,[71][72][73][74]. This phenomenon has been known since the discovery of SEIRA in the early 1980s and has recently achieved worldwide success because the broadband plasmon resonance matches the absorption band of the molecular analytes.…”

A New Surface for Surface-Enhanced Infrared Spectroscopy: Tin Island Films

“…Its development and the initial efforts are compiled in two basic reference books. 1,2 An early report by Hartstein et al 3 showed that silver films could also be used to do surface-enhanced infrared (SEIR). However, SEIR did not reach the same level of development of SERS.…”

“…Since SEIR can only benefit from the |A(ω)| 2 factor, the total SEIR enhancement is expected to be orders of magnitude smaller than the enhancement factor observed in SERS. For instance, in the original report of Hartstein et al, 3 the maximum enhancement factor (EF) measured for SEIR was 20 using ATR and silver islands. Osawa and Ikeda 8 also report an electromagnetic EF of 20-30 for transmission IR spectra on silver islands and an enhancement factor of 20-50 for the case of reflectionabsorption SEIR.…”

“…Tin has a density of 7.2984 g/cm 3 and the difference between its melting point 231.9°C and its boiling point 2270°C is one of the widest of any metal. The common phase (white tin) can be easily evaporated to form island films with regular shape.…”

Highly doped semiconductor plasmonic nanoantenna arrays for polarization selective broadband surface-enhanced infrared absorption spectroscopy of vanillin

“…Within the last decade, it has been explored that SEIRA can be improved when resonant, tailored nanoantennas are used [48], instead of rough metallic surfaces where the effect has initially been discovered [49]. The polarization selective resonances of the investigated HDSC nanoantennas are particularly suitable for resonant SEIRA spectroscopy at mid-IR wavelengths.…”