Raman Scattering as a Probe of Crystallinity in PTCDA and H_2Pc Single-Layer and Double-Layer Thin Film Heterostructures

摘要

The vibrational properties of single-layer PTCDA (perylene-3,4,9,10-tetracarboxylic dianhydride) and H_2Pc (metal-free phthalocyamine) thin films and PTCDA-H_2Pc double-layer heterostructures are studied by Raman scattering. The evidence of crystallinity of the single-layer films can be supported by the existence of phonons, as well as polarization dependence. Resonance enhancement Raman scattering has been used to selectively measure the individual layer properties in double-layer H_2Pc/PTCDA and PTCDA/H_2Pc heterostructures. When H_2Pc is grown on PTCDA, its structure departs from the herringbone arrangement characteristic of unstrained H_2Pc films, as demonstrated by the different peak intensities in the Raman spectra. Well-defined phonons and the polarization dependence in the H_2Pc top layer are characteristic of long-range order, indicating that the H_2Pc molecular planes in the templated structure are crystalline. No evidence for a new structure was observed for PTCDA deposited on top of a H_2Pc first layer. Here, the relative intensity of the phonons in the PTCDA top layer demonstrates that the PTCDA forms microcrystallites due to strain at the molecular heterojunction, before relaxing to its bulk crystalline form.