Origins of polarization-dependent anisotropic X-ray scattering from organic thin films

Abstract

Organic thin films that have no overall in-plane directional ordering often nonetheless produce anisotropic scattering patterns that rotate with the polarization of incident resonant X-rays. Isotropic symmetry is broken by local correlations between molecular orientation and domain structure. Such examples of molecular alignment at domain interfaces and within the bulk of domains, which are both critical to fields such as organic electronics, are simulated and compared with experimental scattering. Anisotropic scattering patterns are found to allow unambiguous identification of the mechanism of local molecular orientation correlations and, as such, promise to be both distinct and complementary to isotropic scattering intensity as a general measure of thin film microstructure.

Combining theory and experiment for X-ray absorption spectroscopy and resonant X-ray scattering characterization of polymers

“…The orientation of the polymer chains relative to the fullerene domain at the polymerfullerene interface was also varied. The simulations alone can reproduce similar anisotropic scattering patterns ( Figure 4) and help understand experimental observations [75]. This demonstrates how scattering patterns can be predicted based on a known morphology, but determining the precise morphology of a sample with unknown structure remains much more challenging.…”

“…This allows RSoXS to go beyond RSoXR and investigate in-plane orientation with polarized incident Xrays. A unique feature in polarized scattering that is often seen, especially in semicrystalline polymers, is anisotropic two-dimensional scattering patterns (varying intensity as a function of azimuthal angle for a given Q), which have been attributed to molecular alignment at domain interfaces or fibrillar bulk ordering [75]. This anisotropic scattering occurs in samples that are globally isotropic in-plane and the direction of scattering intensity anisotropy changes with incident X-ray polarization direction, suggesting this feature arises due to core-level transitions with specific dipole moment directions and local molecular arrangement.…”

“…This anisotropic scattering occurs in samples that are globally isotropic in-plane and the direction of scattering intensity anisotropy changes with incident X-ray polarization direction, suggesting this feature arises due to core-level transitions with specific dipole moment directions and local molecular arrangement. Several cases of scattering anisotropy have been observed in blends of conjugated polymers and polymer-fullerene blends [68,71,70,76,72,75], but it can also be observed in a single semicrystalline polymer, as shown in Energy (eV)…”

“…Furthermore, resonant scattering can uncover the spatial periodicity in a twist-bend liquid crystal phase that has no modulation in electron density [79]. A full understanding of the origins of this phenomenon is still underway [75], and challenges exist due to the many details present, ranging from an understanding of the core-level transitions and orientation dependence on the level of individual atoms and functional groups, to intermolecular interactions and local and global phase separation and structure. A complicated problem such as this highlights the need for theoretical developments to help understand subtle connections between spectroscopy and scattering, and push the capabilities of soft X-ray characterization forward.…”

“…Simulations of scattering patterns in the hard X-ray regime, where scattering contrast is dominated by differences in electron density, have become prevalent [66,[95][96][97], but very little has been done to predict soft X-ray scattering for polymer and soft material systems. The origins of anisotropic scattering seen with polarized incident X-rays has been explored for a rigid-rod type conjugated polymer system [75]. Gann et al simulated twodimensional scattering patterns for various morphologies for a blend of a rigid-rod polymer, such as poly(3-hexylthiophene) (P3HT), and an isotropic fullerene-like molecule (PCBM).…”