Quantitative Raman Spectroscopic Analysis of Crystallographic and Molecular Orientation in Rubrene Films

Understanding and controlling the structure of molecular crystals is essential for advancing high-performance molecular electronics and biocompatible devices. Among organic semiconductors, rubrene, a tetraphenyl derivative of tetracene, stands out due to its exceptional charge carrier mobility in single-crystal form. In this work, we investigate the crystallographic and molecular orientation of rubrene thin films grown by hot-wall epitaxy on mica substrates using ex-situ polarized Raman spectroscopy.

Polarized Raman spectroscopy provides a powerful, non-destructive approach to probe both molecular anisotropy and chemical identity. By exploiting the symmetry properties of Raman tensors and the polarization dependence of scattered light, we determine the orientation of rubrene molecules within crystalline films. Measurements were performed using a 784 nm polarized laser in a backscattering configuration, with systematic in-plane rotation of the sample to resolve angular-dependent Raman responses.

Rubrene films exhibit a characteristic evolution from an amorphous phase to spherulitic structures and finally to a coalesced crystalline film. Across all growth stages, the Raman breathing mode at 1003 cm⁻¹ remains invariant, serving as a robust spectral fingerprint. To extract molecular orientation, additional internal vibrational modes associated with phenyl groups were analyzed under different polarization conditions. The resulting dataset enabled the construction and iterative solution of Raman tensor elements through a back-and-forth computational approach.

The analysis reveals clear anisotropic behavior and provides quantitative insight into molecular alignment and dipole–dipole interactions within the films. These results demonstrate the capability of polarized Raman spectroscopy to resolve crystallographic properties of organic molecular solids using relatively low-energy excitation, offering a versatile tool for the structural characterization of functional molecular materials.