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

Thin crystalline PTCDA films deposited under UHV conditions on various subs trates at temperatures 10-373 K were investigated in-situ by emission and e xcitation spectroscopy after being annealed. Morphology and crystal structu re of these films were examined by atomic force microscopy and X-ray measur ements. In optical and structural investigations two spectral components we re distinguished and correlated with the two known crystal phases of peryle ne-3,4:9,10-tetracarboxylic-dianhydride (PTCDA) in any annealed Nm. The rel ative amount of the phases depends on substrate temperature and film thickn ess. These effects can be made responsible for previously reported thicknes s-dependent energy shifts and spectral changes, which were attributed to qu antum confinement. (C) 1999 Elsevier Science B.V. All rights reserved.