Mapping Molecular/Nanocrystal Orientation with Nano-FTIR

Molecular orientation plays a critical role in determining the optoelectronic and mechanical properties of organic thin films and functional interfaces, yet the orientations are often heterogeneous, leading to nanometer-scale domain structures. Characterizing three-dimensional molecular orientations with nanoscale spatial resolution remains a significant challenge for conventional ensemble-averaged spectroscopic techniques. Here, we report a quantitative method to resolve local molecular orientation by combining scattering-type scanning near-field optical microscopy (s-SNOM) with dipole reorientation mapping. By analyzing hyperspectral nano-FTIR data of Cu(Open-^MesN₆)(OTf)₂ films, we developed an experiment-to-simulation workflow that extracts molecular orientation from relative vibrational peak intensities through transition dipole moment (TDM) projections, with a spatial resolution of ∼40 nm. Our analysis reveals that while molecules maintain a uniform alignment within domains, they undergo a sharp reorientation at grain boundaries, structural details that are invisible to topographic imaging alone. Further transformation allows us to extract the orientation of the nanocrystals that host the molecules. We found that the crystal structure exhibits an ∼55° tilt difference relative to the surface normal at the grain boundary, while maintaining a constant in-plane rotation angle. This methodology provides a robust, generalizable framework for decoding complex molecular/nanocrystal anisotropic organization in organic semiconductors, catalytic surfaces, and biological assemblies beyond the diffraction limits.