Nanoscale Imaging of Heterogeneity in Local Chemical Environments

The nanoscale morphology of molecular materials and the associated relationships between structure, inter- and intra-molecular coupling, and dynamics define material properties and macroscopic response. Vibrational spectroscopy can be used as a structurally sensitive probe of local chemical environment. However, the limited spatial resolution of conventional far-field spectroscopy results in ensemble averaged measurements of inhomogeneous samples. We use IR scattering scanning near-field optical microscopy (s-SNOM) with a tunable femtosecond laser source to probe local chemical environments on the natural length scales of ordering. Broadband IR s-SNOM spectroscopy measures the molecular response of a small homogeneous sub-ensemble within a 20 nm sample volume with sensitivity of a few vibrational oscillators approaching the single molecule limit. We image and spectroscopically resolve the vibrational response of characteristic reporter modes of molecules as a measure of intra- and intermolecular coupling. We demonstrate spatio-spectral imaging in polytetrafluoroethylene films, porphyrin nano-crystals, and self-assembled monolayers of 4-nitrobenzenethiol with few-wavenumber spectral resolution and few-nm spatial resolution and relate this to molecular disorder and crystallinity.