Crystallization in organic thin films
We are interested in understanding and predicting morphologies which emerge during crystallization processes in organic thin films. To this end, we have developed a phase-field model that can capture crystallization of small molecules in different out-of-plane molecular orientations and in multiple polymorphs at the same time. For example, in the figure below, we simulate the application of time-varying treatments to the film. At first, one out-of-plane orientation is energetically favored, and then for the second half of the time, the other out-of-plane orientation is favored. More information can be found in A. Fang, M. Haataja, PRE (2014).
Many polymer and organic small-molecule thin films crystallize with microstructures that twist or curve in a regular manner as crystal growth proceeds. We have developed a phase-field model that energetically favors twisting of the three-dimensional crystalline orientation about and along particular axes, allowing morphologies such as banded spherulites, curved dendrites, and “s”- or “c”-shaped needle crystals to be simulated. When twisting is favored about the axis perpendicular to the plane of the substrate and along the normal growth direction under diffusion-limited single-crystalline growth conditions, crystallization occurs in the form of curved dendrites with uniformly rotating branches. This is illustrated in the animation below. For more details, see Fang and Haataja, PRE (2015).
We have also studied capillary effects in thin films in which crystallization is guided along a narrow channel. We derived an analytical expression for the growth velocity of crystallization as a function of channel width. The equation was validated with phase-field simulations and then fit to experimental data for solvent vapor annealed thin films of the organic semiconductor small molecule TES ADT. This yielded the ratio of interfacial energy to bulk thermodynamic driving force and the minimum feature size that can be patterned with this technique. For more information, see A. Fang, et al, APL Materials (2015).