An Optical and Structural Study of Pentacene Thin-film Polymorphs


Because of its high conductivity, pentacene is an attractive organic material for electronic applications, but the understanding of its thin-film polymporphism is still lacking. In this work, two pentacene films, 70nm and 2.5um thick, were evaporated onto silicon dioxide substrates. X-ray Diffraction identified two different phases in the 70nm film and four in the 2.5um film. Oxygen and fluorine impurities were detected by X-ray Photoelectron Spectroscopy, suggesting that some of the phases in the films may be impurity-based mixed crystals of pentacene. With improved sensitivity from the resonance Raman effect, low-energy intermolecular phonons were studied and assigned as signature modes to the different polymorphs identified in the films. The 39cm-1 mode is characteristic of the 14.3 Angstrom pure pentacene polymorph, and the 45 and 50cm-1 modes are signatures of the 15.3 Angstrom pure polymorph. This is the first time that the intermolecular modes of pentacene thin films has been studied, and a more thorough analysis can further elucidate the structures of the different thin-film polymorphs.