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

## TL;DR

This paper explores how the unique structure of squaraine thin films affects their optical properties and how specific surface features might enhance these effects.

## Contribution

The study introduces a novel investigation into how nanodunes and other morphological features in squaraine films influence excitonic properties.

## Key findings

- SQIB forms two polymorphs with distinct optical and electronic properties.
- Nanodunes in the orthorhombic polymorph may enhance excitonic effects through local field confinement.
- Polarized Raman and AFM techniques reveal structural and alignment details of the polymorphs.

## Abstract

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-(N,N-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 c-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.