Organic crystal needle formation via solvent-annealing: coarsening, wetting, and crystallization in a thin liquid film

Single-crystal, organic semiconductors are attractive because of their highly-nonlinear optical properties and, relative to their amorphous counterparts, they exhibit higher charge mobilities and improved morphological stability. But for such materials to be practical, researchers must develop methods for controlled growth of organic single-crystals. Here we investigate the growth of high-aspect-ratio ``needles'' of single-crystal Alq$_3$---tris(8-hydroxyquinoline) aluminum---from amorphous Alq$_3$ films. During deposition, thin films of amorphous Alq$_3$ are locked in an energetically-unfavorable state. When exposed to solvent, these films evolve into three distinct states: Alq$_3$-rich droplet regions, solvent-rich wetting regions, and Alq$_3$ needles. To understand the experimental results, we model the dynamics of a thin fluid film driven by capillary pressure and intermolecular forces. The resulting flow, coupled with diffusion, transports Alq$_3$ to a growing needle, which is represented by a moving, absorbing interface bounding the fluid.