Squaraine Nanodunes: Structure-Correlated Optical and Vibrational Anisotropy and Morphology-Enhanced Local Field Considerations

Squaraine thin films are emerging as functional optoelectronic elements because of their anisotropic optical properties in the visible to near-infrared spectral range, which include characteristic properties such as large Davydov splitting, hybrid Frenkel-charge transfer excitons, and giant circular dichroism. The prototypical squaraine 2,4-bis-[4-(<i>N,N</i>-diisobutylamino)-2,6-dihydroxyphenyl]-squaraine (SQIB) condenses into two different polymorphs (orthorhombic and monoclinic unit cells), both with distinct optical, electronic, and morphological properties. Polarized Raman microscopy spectra can distinguish between these polymorphs and indicate their crystallographic alignment, similar to polarized transmission spectroscopy, while atomic force microscopy precisely maps all topographical features. During crystallization, periodic nanodunes with cracks and protrusions along the local <i>c</i>-axis can form from the orthorhombic SQIB polymorph without any lithographic steps. The full dielectric tensor is known for this polymorph, and the components of the real part are strongly negative near the absorption bands. For metallic nanoparticles, it is known that a negative dielectric function can lead to localized surface plasmons and field confinement. In this study, we investigate which of the morphological featuresnanodunes, cracks, or protrusionshave the potential to influence the excitonic properties in a similar enhancing fashion.