(271d) Tuning Molecular Vibration through Resonant Coupling with Tin-Doped Indium Oxide Nanocrystals

Resonant coupling between plasmons and molecular vibrations in nanocrystals (NCs) offers a promising approach for tailoring the optoelectronic and chemical reactivity of molecules. While metallic NC-based superlattices can induce resonant coupling, their effectiveness is limited by the fixed electron concentration within the metal framework and inefficient hot spot creation due to the large mode volume. Doped metal oxide NCs, such as tin-doped indium oxide (ITO), overcome these limitations by enabling broad tunability of resonance frequencies in the mid-infrared range through independent adjustments of size and doping concentration. This study investigates the potential of ITO NC monolayers for coupling between plasmons and various molecular vibrations. We demonstrate a correlation between synthetically tunable parameters and the intensity of molecular vibrations, achieving maximum intensity in monolayers composed of large, highly Sn-doped ITO NCs, where the plasmon peak is higher in frequency than the molecular vibration. Using finite element and mutual polarization methods, we show that near-field enhancement from the NC monolayer is stronger on the lower frequency side of the plasmon spectrum and in NCs with a reduced charge carrier depletion layer, thus rationalizing the design rules we experimentally found to maximize intensity enhancement. Our study provides a design principle for a metal oxide NC-based superstructure system with carefully tuned synthetic parameters for sensing certain molecules or modifying the chemical properties of molecules through their vibrations.