Two-component optical spectra in thin PTCDA films due to the coexistence of α- and β-phase

Thin crystalline PTCDA films deposited under UHV conditions on various substrates at temperatures 10–373 K were investigated in-situ by emission and excitation spectroscopy after being annealed. Morphology and crystal structure of these films were examined by atomic force microscopy and X-ray measurements. In optical and structural investigations two spectral components were distinguished and correlated with the two known crystal phases of perylene-3,4:9,10-tetracarboxylic-dianhydride (PTCDA) in any annealed film. The relative amount of the phases depends on substrate temperature and film thickness. These effects can be made responsible for previously reported thickness-dependent energy shifts and spectral changes, which were attributed to quantum confinement.