Searching for New Polymorphs by Epitaxial Growth

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).

衬底表面结晶过程中，未知晶型（polymorphs）的形成现象屡见不鲜。然而针对外延生长的分子晶体，这类现象的研究却相对匮乏，究其原因在于，确凿证明新晶型的存在本身便是一项极具挑战性的工作。微晶存在多种外延取向，且不同晶型同时共存，这使得相鉴定难以通过简易手段完成。本研究针对通过物理气相沉积（physical vapor deposition）在Ag(111)、Cu(111)及石墨烯等单晶表面生长的共轭分子，开展了掠入射X射线衍射（grazing incidence X-ray diffraction）研究，所涉分子包括苝四甲酸二酐（perylenetetracarboxylic dianhydride, PTCDA）、并五苯（pentacene）、二苯并并五苯（dibenzopentacene, trans-DBPen）以及二氰基乙烯基四噻吩（dicyanovinylquater-thiophene, DCV4T-Et2）。本文提出了一种可对观测到的布拉格峰（Bragg peaks）进行索引标定的新方法，借此可确定晶体学晶胞，进而明确甄别晶体学相的类型。即便单个样品中同时存在多种晶型，该方法依然适用，例如在Ag(111)表面生长的DCV4T-Et2即可验证这一点。此外，本研究还确定了外延生长微晶与单晶衬底之间的外延取向关系。后续步骤中，实验数据可用于未知晶型的晶体结构解析（crystal structure solution），以在Cu(111)表面生长的trans-DBPen为例，即可展示该流程的可行性。