Resonant Coupling between Molecular Vibrations and Localized Surface Plasmon Resonance of Faceted Metal Oxide Nanocrystals

Doped metal oxides are plasmonic materials that boast both synthetic and postsynthetic spectral tunability. They have already enabled promising smart window and optoelectronic technologies and have been proposed for use in surface enhanced infrared absorption spectroscopy (SEIRA) and sensing applications. Herein, we report the first step toward realization of the former utilizing cubic F and Sn codoped In₂O₃ nanocrystals (NCs) to couple to the C-H vibration of surface-bound oleate ligands. Electron energy loss spectroscopy is used to map the strong near-field enhancement around these NCs that enables localized surface plasmon resonance (LSPR) coupling between adjacent nanocrystals and LSPR-molecular vibration coupling. Fourier transform infrared spectroscopy measurements and finite element simulations are applied to observe and explain the nature of the coupling phenomena, specifically addressing coupling in mesoscale assembled films. The Fano line shape signatures of LSPR-coupled molecular vibrations are rationalized with two-port temporal coupled mode theory. With this combined theoretical and experimental approach, we describe the influence of coupling strength and relative detuning between the molecular vibration and LSPR on the enhancement factor and further explain the basis of the observed Fano line shape by deconvoluting the combined response of the LSPR and molecular vibration in transmission, absorption and reflection. This study therefore illustrates various factors involved in determining the LSPR-LSPR and LSPR-molecular vibration coupling for metal oxide materials and provides a fundamental basis for the design of sensing or SEIRA substrates.