Sub-micron phase coexistence in small-molecule organic thin films revealed by infrared nano-imaging

Christian Westermeier, Adrian Cernescu, Sergiu Amarie, Clemens Liewald, Fritz Keilmann and Bert Nickel ()
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Christian Westermeier: Fakultät für Physik and Center for NanoScience (CeNS), Ludwig-Maximilians-Universität München
Adrian Cernescu: Fakultät für Physik and Center for NanoScience (CeNS), Ludwig-Maximilians-Universität München
Sergiu Amarie: Neaspec GmbH, Bunsenstrasse 5, 82152 Martinsried, Germany
Clemens Liewald: Fakultät für Physik and Center for NanoScience (CeNS), Ludwig-Maximilians-Universität München
Fritz Keilmann: Fakultät für Physik and Center for NanoScience (CeNS), Ludwig-Maximilians-Universität München
Bert Nickel: Fakultät für Physik and Center for NanoScience (CeNS), Ludwig-Maximilians-Universität München

Nature Communications, 2014, vol. 5, issue 1, 1-6

Abstract: Abstract Controlling the domain size and degree of crystallization in organic films is highly important for electronic applications such as organic photovoltaics, but suitable nanoscale mapping is very difficult. Here we apply infrared-spectroscopic nano-imaging to directly determine the local crystallinity of organic thin films with 20-nm resolution. We find that state-of-the-art pentacene films (grown on SiO2 at elevated temperature) are structurally not homogeneous but exhibit two interpenetrating phases at sub-micrometre scale, documented by a shifted vibrational resonance. We observe bulk-phase nucleation of distinct ellipsoidal shape within the dominant pentacene thin-film phase and also further growth during storage. A faint topographical contrast as well as X-ray analysis corroborates our interpretation. As bulk-phase nucleation obstructs carrier percolation paths within the thin-film phase, hitherto uncontrolled structural inhomogeneity might have caused conflicting reports about pentacene carrier mobility. Infrared-spectroscopic nano-imaging of nanoscale polymorphism should have many applications ranging from organic nanocomposites to geologic minerals.

Date: 2014
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DOI: 10.1038/ncomms5101

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