Polymer chain transport investigated using surface enhanced Raman spectroscopy: monitoring of diffusion kinetics on meso-structured plasmonic substrates

Abstract

We utilize the results of surface-enhanced Raman spectroscopy (SERS)-based interdiffusion experiments on meso-structured substrates to independently validate direct observations of plasmonic enhancements on these elements. The plasmonic enhancement function (PEF) is crucial for accurately determining interdiffusion coefficients using this newly proposed SERS-based methodology. The substrates feature a microscale inverted pyramid geometry, coated with nanoscale sputtered gold. Interdiffusion experiments involve the sequential deposition of polymer bilayers, with deuterated polystyrene (dPS) at the bottom and polystyrene (PS) on top, followed by annealing while periodically acquiring Raman spectra. The temporal evolution of the PS Raman signal reflects not only the interdiffusion process but also plasmonic effects, as the Raman scattering primarily arises from the substrate's plasmonic hotspots. High-resolution finite element (FE) diffusion simulations, combined with experimental SERS data, are used to infer the PEF of the substrate. The derived PEF is consistent with two hotspots located at the apex and vertices of the pyramidal cavity, extending along the edges and spreading into the molecular layer in direct contact with the substrate. This finding is tested against experiments conducted at various diffusion rates, showing excellent agreement. It corroborates recent observations by Steuwe et al. regarding the localization of hotspots on this specific substrate but contradicts other studies that attribute hotspots solely to the micron-scale geometry. This analysis establishes a solid foundation for reliably determining diffusion coefficients using this SERS-based methodology.