Structure and dynamics of pentacene on SiO2 : From monolayer to bulk structure

We have used confocal micro Raman spectroscopy, atomic force microscopy (AFM) and X-ray diffraction (XRD) to investigate pentacene films obtained by vacuum deposition on SiO2 substrates. These methods allow us to follow the evolution of lattice structure, vibrational dynamics and crystal morphology during the growth from monolayer, to thin film and, finally, to bulk crystal. The Raman measurements, supported by the AFM and X-ray data, indicate that the film morphology depends on the deposition rate. High deposition rates yield 2D nucleation and quasi layer-by-layer growth of the thin film form only. Low rates yield 3D nucleation and growth, with phase mixing occurring in sufficiently thick films, where the thin film form is accompanied by the ``high temperature'' (HT) bulk phase. Our general findings are consistent with those of previous works. However, the Raman measurements, supported by lattice dynamics calculations, provide additional insight into the nature of the thin films, showing that their characteristic spectra originate from a loss of dynamical correlation between adjacent layers.