985 www.advmat.de www.MaterialsViews.com wileyonlinelibrary.com COMMUNICATION © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Him Cheng Wong, Anthony M. Higgins, Andrew R. Wildes, Jack F. Douglas, and João T. Cabral* Patterning Polymer–Fullerene Nanocomposite Thin Films with Light Dr. H. C. Wong, Dr. J. T. Cabral Department of Chemical Engineering and Centre for Plastic Electronics Imperial College London, London SW7 2AZ, UK E-mail: j.cabral@imperial.ac.uk Dr. A. M. Higgins Multidisciplinary Nanotechnology Centre College of Engineering Swansea University Swansea SA2 8PP, UK Dr. A. R. Wildes Institut Laue-Langevin, B.P. 156, 38042 Grenoble, France Dr. J. F. Douglas National Institute of Standards and Technology, Gaithersburg, USA DOI: 10.1002/adma.201203541 Trace amounts of C 60 fullerene impart stability to thin polymer films against dewetting by the combined effects of pinning the contact lines of dewetting holes and by effectively altering the polymer-substrate interaction. [1,2] Polymer nanocomposite thin films of higher molecular mass (M w ) yield well-defined morpholo- gies from uniform to spinodal-like, via spontaneous polymer– fullerene phase separation and crystallization. [3,4] In this paper, we report that UV-visible, and even background, light exposure, is a strong contributing factor to (i) dewetting suppression and (ii) nanoparticle association in polymer-C 60 thin films. We find a coupling of fullerene photo-sensitivity and both self-assembly processes which results in controlled pattern formation, and we illustrate the effect with a model polystyrene–fullerene (PS–C 60 ) circuit pattern. This approach opens new opportunities in soft matter lithography via the directed assembly of polymer nano- composites and underscores their photoactive nature, an effect of great interest to material performance and stability of organic photovoltaics (OPV) and aerospace materials under long-term radiation exposure. The physical mechanism responsible for dewetting suppres- sion in nanoparticle-filled polymer thin films has been associ- ated with an entropically driven substrate segregation [5] of the nanoadditives, including fullerenes and derivatives, dendrimers, and many other nanoscale filler particles. [1,2,6,7] Figure 1 depicts the striking fullerene-induced dewetting suppression of low M w PS(2k)-C 60 thin films cast on a silicon substrate (surface energy σ = 69 ± 1 mN·m -1 , referred to as surface ‘ σ’). Since its dis- covery, both kinetic effects associated with contact line pinning of growing holes and changes of the polymer-surface interac- tion have been suggested as explanations [1,6] of this technologi- cally important phenomenon. Here, we demonstrate that there is an additional highly relevant factor that is responsible for the stability of polymer–fullerene films. In particular, we show a clear correlation between the stability-dewetting transition of low M w polymer films and exposure to light. If the polymer film is kept in the dark, complete film stabilization is inaccessible and, instead, film dewetting is retarded with increasing C 60 concentration, as seen on the left side of each panel of Figure 1 (“dark”). Furthermore, we unequivocally show that the stability of low M w PS(2k) nanocomposite thin films is greatly enhanced when thermal annealing is preceded by a light exposure step, yielding complete stability beyond 2% C 60 loading (indicated as% mass fraction throughout the paper). The left panel of Figure 1 reports the control experiment, confirming that neat PS dewets upon annealing at 180 °C regardless of light exposure and the observed light induced dewetting suppression effect is indeed caused by the fullerenes in the polymer thin film. Both UV and visible light are remarkably effective in dewetting suppression and it is particularly surprising to us that, even low intensity ambient room light suffices, albeit requiring longer times to achieve the same effect. The results shown in Figure 1, corrobo- rated by atomic force microscopy (AFM), correspond to a low intensity UV (365 nm, 0.2 mW/cm 2 , 3 h) exposure according to the schematic on the bottom left panel of Figure 1, followed by step annealing at 100, 140, and 180 °C for 30 min each; similar results are obtained after ≈ 24 h of ambient light exposure. The light driven dewetting suppression effect evidently depends on the fullerene loading in the film, and we find a threshold for complete film stabilization of ≈ 2%, which agrees well with the PS-C 60 miscibility threshold (1-2%) reported earlier. [4,8,9] The reported light induced dewetting suppression effect is robust to a wide variation of substrate properties, illustrated by three silicon substrate treatments (‘ σ’, ‘ σ-’, ‘ σ+’) shown in Figure 2a–c for PS(2k) films with 5% C 60 loading ( h ≈ 35 ± 1 nm). Of course, the stability of low M w nanocomposite films also depends strongly on substrate surface energy (viz., σ = 69 ± 1, σ- = 41 ± 1 and σ+ = 72.5 ± 0.5 mNm -1 ). On substrates σ+, nanocomposite films containing about 5% C 60 do not dewet after annealing for many hours. Instead, we observe a lateral association of the fullerenes (Figure 2a) into circular agglom- erates that eventually crystallize. [4] The magnified view of the small inset images (labelled ∗ and ∗∗) are depicted in Figure 2d. The characteristic lengthscales and coordination of this (spin- odal-like) correlated nucleation morphology [3] scales with film thickness and pins at long annealing times ( > 3 h to 6 h), con- sistent with 2D phase separation of binary mixtures. [10] Similar observations are found for nanocomposite films of various M w , as shown in Figure 2e and 2f for 110-nm PS(2k) and PS(270k) films with 5% C 60 loading. With increasing polymer M w , the Adv. Mater. 2013, 25, 985–991