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Institute of Solid State Physics
Polymorphism is a widely observed phenomenon in molecular crystals, it describes the appearance of different crystal structures from one type of molecule. The project focuses on one hardly considered origin of polymorphism: the influence of a solid surface to the crystallisation. The formation of surface induced crystal structures will be investigated on a specific series of rod-like conjugated molecules based on benzothieno-benzothiophene. Thin films will be prepared from the monolayer regime up to thick films by different methods ranging from physical vapour deposition under ultra-high vacuum conditions up to solution processing by drop casting and spin coating. Several crystallisation parameters will be varied ranging from the temperature of deposition, the supersaturation during the crystallisation process and the total deposited amount of material. Disordered surfaces will be used for the crystallisation, silicon oxide and polymer surfaces with different roughnesses, surface energies and polarities will be used. The prepared films will be characterised in terms of their structural properties by a variety of methods including surface science techniques but also atomic force microscopy, different methods of x-ray diffraction and infrared absorption spectroscopy. A part of the project is associated with modelling to consider molecular packing including force field calculations and molecular dynamics. The project will answer questions related to the origin and stability of surface induced crystal structures. The variation of the growth conditions will reveal important preparation parameters which cause the formation of a surface induced crystal structure. Studies of monolayer formation will give answers, if a first initial wetting layer is an important prerequisite for the formation of surface induced phases. The experimental investigations together with the modelling will reveal characteristic structural features of surface induced phases. In-situ temperature measurements will give answer if the surface induced phases are thermodynamically stable or if they represent a metastable state originated by growth kinetics.