Dynamics of Monolayer-Island Transitions in 2,7-Dioctyl-benzothienobenzthiophene Thin Films

• Published in: Chemphyschem Vol. 14; no. 11; pp. 2554 - 2559
• Main Authors: Dohr, Michael, Werzer, Oliver, Shen, Quan, Salzmann, Ingo, Teichert, Christian, Ruzié, Christian, Schweicher, Guillaume, Geerts, Yves Henri, Sferrazza, Michele, Resel, Roland
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 05-08-2013; WILEY‐VCH Verlag; Wiley; Wiley Subscription Services, Inc
• Subjects: Chemistry; Cooling; Exact sciences and technology; General and physical chemistry; liquid crystals; Metallurgy; molecular dynamics; monolayers; Solid-liquid interface; Surface physical chemistry; thin films; X-ray diffraction; Monolayer; monolayers; Molecular dynamics; X-ray diffraction; Liquid crystals; X ray diffraction; Thin film; thin films; liquid crystals; molecular dynamics
• ISSN: 1439-4235; 1439-7641
• DOI: 10.1002/cphc.201300227

The order in molecular monolayers is a crucial aspect for their technological application. However, the preparation of defined monolayers by spin‐coating is a challenge, since the involved processes are far from thermodynamic equilibrium. In the work reported herein, the dynamic formation of dioctyl‐benzothienobenzothiophene monolayers is explored as a function of temperature by using X‐ray scattering techniques and atomic force microscopy. Starting with a disordered monolayer after the spin‐coating process, post‐deposition self‐reassembly at room temperature transforms the initially amorphous layer into a well‐ordered bilayer structure with a molecular herringbone packing, whereas at elevated temperature the formation of crystalline islands occurs. At the temperature of the liquid‐crystalline crystal–smectic transition, rewetting of the surface follows resulting in a complete homogeneous monolayer. By subsequent controlled cooling to room temperature, cooling‐rate‐dependent kinetics is observed; at rapid cooling, a stable monolayer is preserved at room temperature, whereas slow cooling causes bilayer structures. Increasing the understanding and control of monolayer formation is of high relevance for achieving ordered functional monolayers with defined two‐dimensional packing, for future applications in the field of organic electronics.
Keeping order: Highly ordered monolayers of conjugated molecules can be prepared by solution processing and subsequent annealing in the liquid‐crystalline state. The initially disordered layer obtained by spin‐coating can be transferred to two‐dimensional crystals on a surface (see picture). Dewetting of the monolayer from the substrate is prevented by kinetic effects during fast cooling to room temperature.