Monatomic Iodine Dielectric Layer for Multimodal Optical Spectroscopy of Dye Molecules on Metal Surfaces

Fluorescence and Raman scattering spectroscopies have been used in various research fields such as chemistry, electrochemistry, and biochemistry because they can easily obtain detailed information about molecules at interfaces with visible light. In particular, multimodal fluorescence and Raman scattering spectroscopy have recently attracted significant attention, which enables us to distinguish chemical species and their electronic states that are important for expressing various functions. However, a special strategy is required to perform simultaneous measurements because the cross sections of fluorescence and Raman scattering differ by as much as ∼10¹⁴. In this study, we propose a method for the simultaneous measurement of dye molecules on a metal surface using a monatomic layer of iodine as the dielectric layer. The method is based on adequately quenching the photoexcited state of the molecules near the metal surface to weaken the fluorescence intensity and using the resonance effect to increase the Raman signal. We have validated this concept by experiments with insulating layers of different thicknesses and dye molecules of different chemical structures. The proposed multimodal strategy paves the way for various applications such as catalytic chemistry and electrochemistry, where the adsorption structure and electronic states of molecular species near the metal surface determine functionalities.