Berlin 2015 – scientific programme
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CPP: Fachverband Chemische Physik und Polymerphysik
CPP 12: Organic Thin Films
CPP 12.4: Talk
Monday, March 16, 2015, 16:00–16:15, H 2032
Negative thermal expansion based on collective rotational motions of PTCDI-C8 in a crystalline organic thin film — •Sebastian Bommel1,2, Linus Pithan2, Christopher Weber2, Gonzalo Santoro1, Stephan V. Roth1, Edgar Weckert1, and Stefan Kowarik2 — 1Deutsches Elektronen-Synchrotron DESY, Hamburg, Germany — 2Institut für Physik, Humboldt-Universität zu Berlin, Germany
The understanding of the structure-function relationship in molecular thin films for the fabrication of advanced functional nano-materials is still challenging because molecules possess many conformational and rotational degrees of freedom. Here we report on temperature-dependent rotation of the organic semiconductor PTCDI-C8. Highly crystalline organic thin films have been investigated using Grazing Incidence X-ray Diffraction (GIXD), which enables us to determine the temperature-dependent unit cell and the arrangement of molecules within the unit cell. From fitting the intensities of the Bragg reflections at different temperatures we find that the tilt angle of the molecules changes continuously with temperature and does not exhibit hysteresis. A change in molecular tilt of 4° is observed for a 200 K temperature difference. This collective mechanical response of a molecular rotation is connected with a large negative thermal expansion coefficient of the a-axis within the thin film unit cell of -135 ppm/K. Such negative thermal expansion offers highly desirable properties for the design of thermo-mechanical devices such as actuators.