title: Thermally driven smoothening of molecular thin films: Structural transitions in n-alkane layers studied in real-time.
abstract: We use thermal annealing to improve smoothness and to increase the lateral size of crystalline islands of n-tetratetracontane (TTC, C44H90) films. With in situ x-ray diffraction, we find an optimum temperature range leading to improved texture and crystallinity while avoiding an irreversible phase transition that reduces crystallinity again. We employ real-time optical phase contrast microscopy with sub-nm height resolution to track the diffusion of TTC across monomolecular step edges which causes the unusual smoothing of a molecular thin film during annealing. We show that the lateral island sizes increase by more than one order of magnitude from 0.5 μm to 10 μm. This desirable behavior of 2d-Ostwald ripening and smoothing is in contrast to many other organic molecular films where annealing leads to dewetting, roughening, and a pronounced 3d morphology. We rationalize the smoothing behavior with the highly anisotropic attachment energies and low surface energies for TTC. The results are technically relevant for the use of TTC as passivation layer and as gate dielectric in organic field effect transistors.
authors.affiliation: Institut für Physik, Humboldt-Universität zu Berlin, 12489 Berlin, Germany.
authors.forename: Linus
authors.initials: L
authors.lastname: Pithan
authors.affiliation: Institut für Physik, Universität Augsburg, 86135 Augsburg, Germany.
authors.forename: Eduard
authors.initials: E
authors.lastname: Meister
authors.affiliation: Max-Planck-Institut für Kolloid- und Grenzflächenforschung, 14476 Potsdam-Golm, Germany.
authors.forename: Chenyu
authors.initials: C
authors.lastname: Jin
authors.affiliation: Institut für Physik, Humboldt-Universität zu Berlin, 12489 Berlin, Germany.
authors.forename: Christopher
authors.initials: C
authors.lastname: Weber
authors.affiliation: Institut für Physik, Humboldt-Universität zu Berlin, 12489 Berlin, Germany.
authors.forename: Anton
authors.initials: A
authors.lastname: Zykov
authors.affiliation: Institut für Physik, Humboldt-Universität zu Berlin, 12489 Berlin, Germany.
authors.forename: Katrein
authors.initials: K
authors.lastname: Sauer
authors.affiliation: Institut für Physik, Universität Augsburg, 86135 Augsburg, Germany.
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Brütting
authors.affiliation: Max-Planck-Institut für Kolloid- und Grenzflächenforschung, 14476 Potsdam-Golm, Germany.
authors.forename: Hans
authors.initials: H
authors.lastname: Riegler
authors.affiliation: Institut für Physik, Humboldt-Universität zu Berlin, 12489 Berlin, Germany.
authors.forename: Andreas
authors.initials: A
authors.lastname: Opitz
authors.affiliation: Institut für Physik, Humboldt-Universität zu Berlin, 12489 Berlin, Germany.
authors.forename: Stefan
authors.initials: S
authors.lastname: Kowarik