Coarsening and solidification via solvent-annealing in thin liquid films

Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use. The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. We examine solidification in thin liquid films produced by annealing amorphous Alq 3 (tris-(8-hydroxyquinoline) aluminium) in methanol vapour. Micrographs acquired during annealing capture the evolution of the film: the initially-uniform film breaks up into drops that coarsen, and single crystals of Alq 3 nucleate randomly on the substrate and grow as slender 'needles'. The growth of these needles appears to follow power-law behaviour, where the growth exponent, γ , depends on the thickness of the deposited Alq 3 film. The evolution of the thin film is modelled by a lubrication equation, and an advection–diffusion equation captures the transport of Alq 3 and methanol within the film. We define a dimensionless transport parameter, α, which is analogous to an inverse Sherwood number and quantifies the relative effects of diffusion-and coarsening-driven advection. For large α-values, the model recovers the theory of one-dimensional, diffusion-driven solidification, such that γ → 1/2. For low α-values, the collapse of drops, i.e. coarsening, drives flow and regulates the growth of needles. Within this regime, we identify two relevant limits: needles that are small compared to the typical drop size, and those that are large. Both scaling analysis and simulations of the full model reveal that γ → 2/5 for small needles and γ → 0.29 for large needles. 1. Introduction An amorphous solid, i.e. a glass, can be 'melted' by raising its temperature above its glass-transition temperature. Alternatively, introducing solvents, or other additives, to the amorphous solid phase can reduce its glass-transition temperature below the ambient temperature, a process known as plasticization. During plasticization, the amorphous solid becomes a liquid at an ambient temperature that is above its glass-transition temperature, but below its melting temperature. Applying this process to organic semiconductors enables the growth of single crystals of organic electronics at room temperature.