Berlin 2015 – scientific programme

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CPP: Fachverband Chemische Physik und Polymerphysik

CPP 44: P9: Crystallization, Nucleation and Self Assembly

CPP 44.6: Poster

Wednesday, March 18, 2015, 10:00–13:00, Poster A

Enhanced phase purity in sexithiophene thin films through laser illumination — •Linus Pithan¹, Caterina Cocchi^1,2, Hannes Zschiesche¹, Christopher Weber¹, Anton Zykov¹, Sebastian Bommel^1,3, Peter Schäfer¹, Steven Leake⁴, Claudia Draxl^1,2, and Stefan Kowarik¹ — ¹Instiut für Physik, Humboldt Universität zu Berlin — ²IRIS Adlershof, HU Berlin — ³DESY, Hamburg — ⁴Swiss Light Source, PSI, present address: ESRF, France

We study the influence of light on the phase coexistence in α-sexithiophene (6T) thin films. We focus on vacuum deposited films grown on potassium chloride (KCl) that exhibit a bimodal growth with two co-existing crystal phases, the low-temperature (LT) and high-temperature (HT) 6T polymorphs. We find that laser illumination (532 nm, 1.5 W/cm²) during growth suppresses the formation of HT crystallites, thus the phase purity is enhanced compared to the bimodal growth without light. To understand the mechanism behind this optical control, we use in situ x-ray diffraction, atomic force microscopy (AFM), optical absorption measurements, as well as first-principles calculations for the optical absorption spectra of the HT and LT phase. We deduce that the phase purification is due to optical heating of the molecular film and lower cohesive energy of the HT phase compared to the LT phase. This finding demonstrates that light can serve as an additional control parameter in molecular crystal growth to optimize the structural quality of molecular thin films.