Raman Identification of Polymorphs in Pentacene Films

We use Raman spectroscopy to characterize thin films of pentacene grown on Si/SiOx by Supersonic Molecular Beam Deposition (SuMBD). We find that films up to a thickness of about 781 angstrom (similar to 52 monolayers) all belong to the so-called thin-film (TF) phase. The appearance with strong intensity of some lattice phonons suggests that the films are characterized by good intra-layer order. A comparison of the Raman spectra in the lattice and CH bending spectral regions of the TF polymorph with the corresponding ones of the high-temperature (HT) and low-temperature (LT) bulk pentacene polymorphs provides a quick and nondestructive method to identify the different phases.