Searching for New Polymorphs by Epitaxial Growth

Abstract

The formation of unknown polymorphs due to the crystallization at a substrate surface is frequently observed. This phenomenon is much less studied for epitaxially grown molecular crystals since the unambiguous proof of a new polymorph is a challenging task. The existence of multiple epitaxial alignments of the crystallites together with the simultaneous presence of different polymorphs does not allow simple phase identification. We present grazing incidence X-ray diffraction studies on conjugated molecules like perylenetetracarboxylic dianhydride (PTCDA), pentacene, dibenzopentacene (trans-DBPen), and dicyanovinylquater-thiophene (DCV4T-Et2) grown by physical vapor deposition on single crystalline surfaces like Ag(111), Cu(111), and graphene. A new method for indexing the observed Bragg peaks allows the determination of the crystallographic unit cells so that the type of crystallographic phase can be clearly identified. This approach even works when several polymorphs are simultaneously present within a single sample as shown for DCV4T-Et2 on Ag(111). Additionally, epitaxial relationships between the epitaxially grown crystallites and the single crystalline surfaces are determined. In a subsequent step, the experimental data are used for the crystal structure solution of an unknown polymorph, as shown for the example trans-DBPen grown on Cu(111).

Correlation between two- and three-dimensional crystallographic lattices for epitaxial analysis. I. Theory

“…the planes with the Miller indices (uvw) and (Àu À v À w). In the case of dicyanovinyl-quaterthiophene (DCV4T-Et2) on Ag(111) we observed three polymorphs with both different crystallographic unit cells as well as contact planes (Simbrunner et al, 2021a). (ii) The crystallites show additionally distinct rotational alignments in the xy plane.…”

“…The analysis of the diffraction pattern relies on indexing the obtained reflections to determine the unit cells formed by the underlying molecules. In a previous work, we described an algorithm which proved effective for the thin film analysis, where unit cells in various orientations and/or with different lattice parameters may occur (Simbrunner et al, 2020(Simbrunner et al, , 2021a. The following general crystallographic features of epitaxially grown films could be observed: (i) the crystallites grow with defined crystallographic planes parallel to the substrate surface (i.e.…”

Bulk and Surface‐Mediated Polymorphs of Bio‐Inspired Dyes Organic Semiconductors: The Role of Lattice Phonons in their Investigation

“…The mechanisms through which the substrate directs the heterogeneous nucleation of a given phase and the role played in the process by kinetics factors are far from being well understood. [71][72][73][74] Generally, phase boundaries produce a lowering in the free energy needed for nucleation. The surface of engineered substrates may act as a template in molecular deposition [75] and the subtle interplay of interactions at the interface may result into altogether new molecular arrangements, which are not observed in the bulk.…”

Singlet exciton fission via an intermolecular charge transfer state in coevaporated pentacene-perfluoropentacene thin films

“…The existence of a PFP:PEN mixed crystal structure with 1:1 stoichiometry has been deduced from XRD and grazing incidence x-ray diffraction (GIXRD) measurements. [28][29][30] Although atomic coordinates could not be obtained from experimental data, the GIXRD diffractogram is sufficient to determine the unit cell parameters 31,32 from the peak positions and to confirm or exclude a given structure by the peak intensities. GIXRD yields a P-1 triclinic unit cell for the PEN:PFP cocrystals (lattice parameters a = 15.7 Å, b = 7.4 Å, c = 7.2 Å, α = 102.8 ○ , β = 68.6 ○ , and γ = 97.6 ○ ).…”