FWF - Crystal Solution - Solution of surface induced crystal structures

Project: Research project

Project Details

Description

Crystal structures of conjugated molecules which are formed within thin films are partly bulk equilibrium
structures (structures of macroscopic single crystals) as well as metastable crystal phases which are induced due to
the presence of a surface during the thin film growth (surface induced phases). The goal of this project is to
develop a specific method of crystal structure solution of molecular crystals as they are formed within thin films.
The method will consider the strong texture of the molecular crystals within thin films, since they form twodimensional
powders on isotropic surfaces. The developed method will allow the crystal structure determination of
surface induced phases, which is not possible with the currently available methods. The crystal structures will be
solved by experimental techniques which will be partly combined with theoretical approaches. The experimental
technique is grazing incidence x-ray diffraction which will be used for the determination of the crystallographic
unit cell and of the crystallographic structure factors. These structure factors will be used by a Rietveld refinement
procedure to perform the full solution of the crystal structure using the chemical structure of the molecule as input
parameter. The theoretical approaches will be based on force field calculations taking the experimentally
determined lattice constants as input parameters. The energetic stability of the surface induced phases as well as of
the equilibrium structures will be obtained and compared with each other.
By in-situ experiments the influence of temperature to the crystal structures will be investigated and the
temperature stability of the involved crystalline structures will be obtained. These experimental results will be
compared to theoretical calculations performed by quasi-harmonic lattice dynamics. The temperature dependence
of the crystal structures as well as their temperature stability will be obtained.
In a first step of the project the surface induced phase of sexithiophene and the temperature stability of the
pentacene surface induced phase (the so called thin film phase) will be investigated. In both cases the crystal
structure within thin films are of essential importance for transistor applications in organic electronics. In a second
step alkyl-substituted terthiophene and phthalocyanine molecules will be investigated. In a last step of the project it
will be tested if the approach of crystal solution from thin films can be applied also to the polymer poly(3-
hexylthiophene) and to thermotropic liquid crystalline states.
StatusFinished
Effective start/end date15/01/0914/01/12