Surface induced phases: origin and stability on the example of pentacene
Stefan Pachmajer
15:30 - 16:10 Friday 19 January 2018 PH01150

The presence of a surface during the crystallisation of molecular materials can induce previously
unknown types of polymorphic crystal structures, known as surface induced phases (SIPs) or surface mediated polymorphs. These types of crystal structures are only stable for a limited
crystal size (or limited film thickness), e.g. a critical thickness of 50 nm is reported for thin films of the
molecule pentacene [1], which is one of the most studied organic semiconducting materials. This study focuses on the growth behavior of pentacene within thin films as a function of film thickness ranging from 20 nm to 300 nm by employing various X-ray diffraction methods combined with supporting atomic force microscopy. While many aspects are already identified in very thin films, this study demonstrates some new insight in the behavior of the bulk phase within thin films, which surprisingly was not addressed before, deriving a model to a self-limiting growth of thin film phase and explaining the thickness dependent phase behavior observed in pentacene thin films.
To investigate the molecules behavior on patterned surfaces without changing the surface chemistry, pure glass substrates have been patterned with ripples via ion beam sputtering and pentacene in thicknesses up to 300 nm nominal has been deposited on top. A biaxial oriented growth of pentacene in the thin-film phase and the bulk phase, relative to the ripples orientation, is observed. While the molecules keep the preferred 001 orientations to the surface, the grooves on the surface open the possibility to a certain in-plane orientation by alignment of molecules in different phases.


[1] Kakudate, T.; Yoshimoto, N.; Polymorphism in pentacene thin films on SiO2 substrate. Appl. Phys. Lett. 2007, 90, 81903.