Tailoring the Morphology and Dewetting of an Organic Thin Film J. M. Topple,* S. A. Burke, † W. Ji, ‡ S. Fostner, A. Tekiel, and P. Gru ¨ tter Department of Physics, McGill UniVersity, Montreal, H3A 2T8, Canada ReceiVed: August 12, 2010; ReVised Manuscript ReceiVed: NoVember 23, 2010 Methods of tailoring molecular thin film morphology and maturation rate were investigated by noncontact atomic force microscopy. Submonolayer coverages of 3,4,9,10-perylenetetracarboxylic diimide deposited on alkali-halides form films of needle-shaped islands, and undergo a dewetting transition when deposited on NaCl. The resulting island surface distribution, size, shape, and rate of dewetting may be varied by changing growth conditions such as temperature and by templating the substrate with single atomic layer deep pits or depositing gold nanoclusters to modify island nucleation. This characterization is an important step in controlling the structure of thin organic films for devices that are sensitive to nanoscale film structure. Introduction The morphology of thin films in organic electronic devices is a crucial parameter, as film structure has a strong influence on application relevant properties. 1-6 Film morphology can be controlled to some degree by growth conditions, 7,8 but post- deposition dynamic processes such as dewetting (Figure 1) and other forms of ripening can alter the film structure and resulting properties, 8-17 potentially leading to problems with device stability and functionality. Recent interest in organic semiconductors has motivated research on a variety of organic materials for electronic and optoelectronic applications. 5,7,18,19 Perylene derivatives are one such class of organic semiconducting molecules which are easily modified, 20-29 and here we investigate the controlled growth of 3,4,9,10-perylenetetracarboxylic diimide (PTCDI). Thin films of a range of organic molecules have been studied on many different substrates (see recent reviews 2,4 and references therein). Insulating substrates provide electrical isolation for deposited structures 30 and prevent leakage currents that might otherwise hamper device characterization. Noncontact atomic force mi- croscopy (NC-AFM) is a nondestructive, high-resolution tech- nique ideal for imaging bulk insulator surfaces. 31-35 A variety of thin molecular films deposited on insulators have been successfully imaged using NC-AFM. 29,36-42 Such studies address the question of which structures and growth modes occur and why, prompting the next question: Can we control the result? Controlling and tailoring the structure of organic thin films, as well as the maturation time to reach a stable device-ready film structure is a desirable ability. This study explores control over thin film morphology by adjusting growth parameters such as temperature and by varying substrate structure in three ways: templating with pits, templating with gold nanoclusters, and with use of two different alkali-halide substrates. When deposited on room temperature NaCl (001), PTCDI molecules undergo postdeposition dewetting in a dynamic process described in a previous work. 16 Heating can induce enhanced Ostwald ripening within a molecular island population. By selectively distributing nucleation sites for island growth (in Volmer-Weber thin films), the resulting film morphology and homogeneity may be altered. Alkali-halide substrates templated with monolayer deep rect- angular pits (created by charge stimulated desorption of the surface prior to molecule deposition), have been used in previous studies 40,41,43,44 as traps for molecules and to promote the formation of molecular structures that may otherwise be unstable or metastable. Gold deposited on KBr has been observed to form nanoclusters 45-47 and the effect of these nanoclusters on subsequent deposition of molecules has been explored. 48,49 The structure of the interface between metal clusters and a molecular layer is of general interest for organic electronic and optoelec- tronic contact geometry properties. 50-52 Here the impact of templating, as well as surface structure and other growth parameters on film growth behavior and morphology are shown to provide means of tailoring of island dimensions and subsequently film continuity, homogeneity, and surface area for films of given coverages. This will consequently affect opto- electronic and other size and structure dependent properties. 1-6 Results and Discussion Growth on Atomically Flat Alkali-Halides. When deposited on NaCl (001) at room temperature, PTCDI molecules initially * To whom correspondence should be addressed. E-mail: topplej@ physics.mcgill.ca. † Also affiliated with the Department of Physics and Astrononomy and Department of Chemistry, University of British Columbia, Vancouver, V6T 1Z1, Canada. ‡ Also affiliated with the Department of Physics, Renmin University of China, Beijing 100872, China. Figure 1. Dewetting of PTCDI on NaCl for two coverages with elapsed times (minutes) since deposition. All images 600 nm × 600 nm, Δf ) (top row) -3.7, -2.2, -4.0 (bottom row) -1.9, -2.4, -3.9 Hz; corresponding normalized frequency shifts Γ )-0.5, -0.3, -0.6 and -0.3, -0.3, -0.6 fNm. J. Phys. Chem. C 2011, 115, 217–224 217 10.1021/jp107644u  2011 American Chemical Society Published on Web 12/14/2010