Structure and morphology of coevaporated pentacene-perfluoropentacene thin films

Abstract

The structural properties of coevaporated thin films of pentacene (PEN) and perfluoropentacene (PFP) on SiO(2) were studied using x-ray reflectivity and grazing incidence x-ray diffraction. Reciprocal space maps of the coevaporated thin films with different volume fractions reveal the coexistence of two different molecular mixed PEN-PFP phases together with the pure PEN and PFP crystallites. The crystal structure of PEN:PFP blends does not change continuously with volume fraction, instead the proportion of the appropriate phases changes, as seen from the diffraction analysis. Additional temperature dependent experiments reveal that the fraction of the two mixed PEN-PFP phases varies with growth temperature. The λ-phase (molecular plane parallel to the substrate) is metastable and induced by low growth temperature. The σ-phase (molecular plane nearly perpendicular to the substrate) is thermally stable and nucleates predominantly at high growth temperatures.

Photoluminescence spectroscopy of pure pentacene, perfluoropentacene, and mixed thin films

“…For the PL of the mixtures, which is rather redshifted with respect to the PL of the pure samples, we make use of the more intense line at 514 nm. The diameter of the laser spot on the sample was approximately arXiv:1201.4146v1 [cond-mat.mtrl-sci] 19 Jan 2012 1 µm, hence the spectra represent an average over phase separated PEN, PFP, and mixed islands as they occur for non-1:1 mixing ratios 19 . The luminescence spectra were corrected for instrumental sensitivity.…”

“…Concerning PEN:PFP blended thin films, in particular for equimolar mixtures, X-ray diffraction experiments 19 indicate good intermixing of both materials on a molecular level. The disappearance of the S 1 → S 0 transition of PEN and PFP at low temperatures in the blends raises interesting questions.…”

Identification of an organic semiconductor superlattice structure of pentacene and perfluoro-pentacene through resonant and non-resonant X-ray scattering

“…25 A key factor of this desirable property is that blends of PFP and PEN cannot have arbitrary mixing ratios; only pure phases or a 1:1 mixture are possible. 24 This work shows that strong coupling between molecules and formation of co-crystals is not a hindrance for the growth of superlattices consisting of molecular MLs.…”

“…The asymmetry and width of the GIXD Bragg reflections are too large to be able to distinguish between the monolayer and thin film phases of PEN (see Table I). At the positions of mixed phase reflections (from reference, 24 see arrows in Figure 3(b), no significant Bragg reflection intensities are found, indicating that no PEN-PFP co-crystal is formed. In the reciprocal space map in Figure 3(a), PFP and PEN Bragg rods with higher order reflections in q z are present, which indicate crystalline multilayer domains consisting of pure PFP or pure PEN, respectively.…”

“…[20][21][22] The two compounds have been shown to form a 1:1 co-crystal with strong charge transfer and excitonic coupling, 23 but PEN and PFP crystals are not continuously miscible. 24,25 In this paper, we show that stable organic superlattice structures consisting of alternating pentacene (PEN) and perfluoro-pentacene (PFP) monolayers are formed using OMBD with in situ and real-time X-ray thickness monitoring. We use x-ray reflectivity and resonant soft X-ray reflectivity 26 to measure superlattice reflections proving that PEN and PFP MLs alternate.…”