Nucleation and Growth Kinetics in Solution-Processed Organic Molecular Crystalline Thin Films.

We report in-situ, real-time observation of early stage nucleation and growth kinetics in submonolayer crystalline films of the organic semiconductor tetracene grown in ultrathin liquid solvent layers. Films are prepared using a vapor-liquid-solid deposition technique in which tetracene monomers are delivered at a constant rate via a vapor-phase flux to a substrate coated with a sub-micron thick layer of an organic liquid solvent, causing crystals to nucleate and grow. ~~Using fluorescence videomicroscopy we follow the formation and growth of individual crystals, including simultaneous mapping of spatial variations in monomer concentration and depletion zones. A unified theoretical treatment accurately describing the time- and flux-dependent nucleation rate, limiting nucleation density, steady-state growth rates, and crystal spacing statistics is developed by modifying the Walton relation to account for the presence of the liquid solvent, with the critical nucleus size treated as a concentration-dependent variable. We discuss the differences between molecular crystallization in quasi-2D liquid films, versus traditional growth on a bare substrate in vacuum by physical vapor deposition (PVD).