Raman Crystallography as a Spectroscopic Probe of Structure in Single Crystal Organic Semiconductors

Raman spectroscopy is a powerful technique to nondestructively investigate the vibrational modes of organic crystals. These phonons are spatially localized within the lattice and correlate to the underlying crystallographic structure of the material. This provides a straightforward means to investigate the crystallinity, orientation, composition, and phase with high spatial resolution and no sample preparation. The structure can also be related to important properties such as carrier mobility. Intermolecular motion is theorized to strongly affect the inherent charge transport of organic semiconductors, influencing the anisotropic character. Here we present a polarization-orientation Raman spectroscopy analysis of single-crystalline organic semiconductor tetracene. We find five distinct vibrational modes in the low-energy regime that are ascribed to intermolecular lattice phonons. Polar plots indicate that these motions are A_g modes, in agreement with group theory, directed within the ab plane of tetracene (001), as confirmed by diffraction and density functional theory modeling. Collectively, these data provide a clear picture of the Raman-active lattice motion of tetracene, which we subsequently relate to its anisotropic charge transport behavior.