The Journal of Physical Chemistry C 128, 2052

In this study, we utilize scattering-type scanning near-field optical microscopy combined with a tunable infrared quantum cascade laser to achieve nanoscale imaging of vibrational modes within colloidal nanocrystal films under ambient conditions. We focus on the characterization of the bending mode of water and the stretching modes of carboxylates in films composed of gold nanocrystals (AuNCs) coated with trisodium citrate (Na₃Cit) ligands. Our approach enables the spatial resolution of vibrational spectra, revealing a line width for the water bending mode of less than 3 cm^–1, a value notably consistent with that found in gas-phase spectra. Intriguingly, our detailed analysis of the near-field spectra discriminates among three overlapping domains: the film–air interface, the internal layers of the film, and the film–substrate boundary. Further investigation into the citrate stretching modes elucidates three structural domains differentiated by their spatial relation to the AuNCs and the film’s thickness. Our results challenge previous understandings by demonstrating that a particular vibrational band, formerly attributed to surface-bound carboxylates, is in fact a signature of higher-order structures in dense Na₃Cit aggregates. Complementary surface-enhanced Raman spectroscopy measurements indicate that citrate’s interaction with the nanocrystal surface is intricately modulated by sodium ions and water molecules. These experimental observations are further supported and explained by theoretical insights from density functional theory calculations. Collectively, our findings not only present a more nuanced view of the interactions within these complex nanoscale systems but also pave the way for a deeper understanding of surface chemistry and interfacial phenomena at the molecular level.