Surface Morphology Analysis of Thin Films of Perfluoroalkanes Using Atomic Force Microscopy-Infrared (AFMIR) with Respect to Surface Phonon-Polariton Modes.

Although atomic force microscopy-infrared (AFM-IR) spectroscopy enables nanoscale infrared characterization, the dielectric origin of its spectral contrast, particularly the role of collective vibrational modes, remains incompletely understood. Here, we investigate the correlation between nanoscale infrared spectra and surface morphology using thin films of perfluorotetracosane (C24F50). By comparing low-temperature vapor-deposited films and their aged counterparts, we track aging-induced morphological evolution from lying-oriented sub-micrometer aggregates to standing-oriented crystalline grains. For discussing fine features in the nano-scale spectrum, infrared p-polarized multiple-angle incidence resolution spectroscopy (pMAIRS) is also employed as a macroscopic reference in advance. The pMAIRS analysis reveals pronounced longitudinal-transverse optical (LO-TO) splitting, which increases upon aging due to an enhanced Berreman-type longitudinal field contribution associated with surface flattening and improved crystallinity. Nanoscale AFM-IR measurements in a resonance-enhanced-mode with top-down illumination configuration demonstrate that the local spectral response is predominantly governed by the transverse optical (TO) dielectric function, while surface phonon-polariton (SPP) modes are strongly enhanced, reflecting localized electromagnetic field confinement at crystalline grains. Through rigorous cross-scale comparisons of macroscopic and nanoscale spectroscopy, this study has experimentally established the dominant TO-controlled contrast mechanism in top-down AFM-IR and revealed the characteristics of SPP-driven nanoscale spectra. These results demonstrate that AFM-IR is a powerful probe for investigating both the chemical identity of surface-active organic materials and nanoscale surface morphology.