Probing polymorphs of organic semiconductors by lattice phonon Raman microscopy

Abstract

Using micro-Raman techniques to investigate crystal polymorphism is an efficient method, capable of monitoring physical modifications and phase inhomogeneities in crystal domains at the micrometre scale. In the presence of polymorphism, phase mixing is a common occurrence which becomes a crucial issue in structured organic materials tailored for applications in molecular electronics and photonics. A good phase homogeneity is, in fact, required for optimal and reproducible device performance. We tackle the problem of polymorphism in organic semiconductors by combining experimental and theoretical methods. Experimentally we have found that different crystalline polymorphs may be conveniently investigated using their Raman spectra in the region of the lattice phonons, whose frequencies probe intermolecular interactions and are very sensitive to differences in molecular packing. We propose lattice phonon confocal micro-Raman mapping as a fast and reliable diagnostic tool for in-situ characterization of the phase purity. The theoretical approach aims to predict crystal structures and possible coexistence of polymorphs by ranking them in energy and proving that the deepest calculated minima actually correspond to the experimental X-ray diffraction structures of bulk crystals. This combined spectroscopic and theoretical approach to the dynamical properties of a crystal lattice provides a unique body of information on crystal structure recognition of molecular crystals.