Mapping in-plane orientational order and correlation lengths in molecular films using azimuthal-scanning vibrational sum-frequency generation microscopy

Self-assembled molecular films are omnipresent in nature, where their highly ordered anisotropic packing structures play a crucial role in governing the macroscopic properties and functional behavior of interfaces. Beyond their local anisotropy, these molecular structures can also often display pronounced heterogeneity and long-range in-plane packing order - from the molecular-to-microscopic scale. Accessing this complex structural information experimentally, however, is a veritable challenge. Phase-resolved sum-frequency generation (SFG) microscopy has recently emerged as a powerful technique for elucidating these structural aspects of thin films, but many properties have so far remained inaccessible such as details about the width and shape of the microscopic orientational distribution. In this work, we show how implementing an azimuthal-scanning approach in SFG microscopy can be used to overcome this limitation by yielding the full in-plane orientational distribution, going far beyond extracting the average molecular orientation. Specifically, by analyzing the complete set of rotational frequencies that arise from the azimuthal dependency, we show through simulated data how they are differently affected by any in-plane orientational disorder or deviation from perfect crystallinity, as well as more complex packing such as bimodal arrangements. This hence offers a route to elucidate the details of the orientational distribution. We then apply this concept to a model membrane comprised of a phase-separated mixed phospholipid monolayer, demonstrating that the molecules within the condensed domains possess micron-scale orientational correlations but nevertheless display substantial diversity in their in-plane orientation, showing both a non-negligible spread in the molecular orientational distribution, as well as profoundly different orientations for their two tail-groups. Overall, this showcased example highlights the potential for this method in future investigations on the role packing structure plays in the functional behavior in lipid membranes. Beyond this, the theoretical concepts presented in this work can be extended to a wide range of systems, from molecular samples to phononic materials, and thus has potential to open-up new directions in the structural elucidation at interfaces.