Scanning Angle Total Internal Reflection Raman Spectroscopy of Thin Polymer Films
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Scanning Angle Total Internal Reflection Raman Spectroscopy of Thin Polymer Films Matthew W. Meyer1,2 and Emily A. Smith1,2 1 Ames Laboratory, U.S. Department of Energy, Ames, IA 50011-3111, U.S.A. 2 Department of Chemistry, Iowa State University 1605 Gilman Hall, Ames, IA 50011-3111, U.S.A. ABSTRACT Plasmon waveguide resonance (PWR) Raman spectroscopy provides chemical content information with interface or thin film selectivity. Near the plasmon waveguide interface, large increases in the interfacial optical energy density are generated at incident angles where plasmon waveguide resonances are excited. When a polymer of sufficient thickness is deposited on a gold film, the interface acts as a plasmon waveguide and large enhancements in the Raman signal can be achieved. This paper presents calculations to show how polymer thickness and excitation wavelength are predicted to influence PWR Raman spectroscopy measurements. The results show the optical energy density (OED) integrated over the entire polymer film using 785 nm excitation are 1.7× (400 nm film), 2.17× (500 nm film), 2.48× (600 nm film), 3.08× (700 nm film) and 3.62× (800 nm film) higher compared to a 300 nm film. Accounting for the integrated OED and frequency to the fourth power dependence of the Raman scatter, a 532 nm excitation wavelength is predicted to generate the largest PWR Raman signal at the polymer waveguide interface. This work develops a foundation for chemical measurements of numerous devices, such as solar energy capturing devices that utilize conducting metals coated with thin polymer films. INTRODUCTION Plasmon waveguide resonance (PWR) Raman spectroscopy is a label free spectroscopic method for interfacial analysis with chemical specificity [1-2]. In a standard PWR Raman spectroscopy experiment the illuminating light is directed onto a prism/metal/dielectric/bulk interface at a specific incident angle. Raman scatter and reflectivity data are collected simultaneously for each incident angle above the critical angle for total internal reflection [3]. Conventional Raman spectroscopy, when the illuminating light is not under total internal reflection, suffers in studying interfaces and thin films due to a lack of interfacial selectivity and the small signals that result from reduced analysis volumes. Plasmon waveguides operate by coupling surface plasmon modes in the metal to guided modes within the dielectric layer or waveguide [4]. Recent work has shown that the amplified electric field at a waveguide interface can be used for generating Raman spectra with large signal-to-noise ratios for solutions, polymer films and monolayers [1-2]. Two recently reported sample geometries suitable to measure PWR Raman spectra of thin polymer films are shown in Figure 1. The interface shown in Figure 1A is suitable for measuring polymer films down to a couple nanometers in thickness using 785 nm excitation. The interface contains a 600 nm silica waveguide layer with the polymer deposited on top of the
silica. The electric field within the polymer layer
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