Patterned self-assembly of twisted tetrathiafulvalene crystals through solvent vapor annealing

Topographic patterning of materials on the sub-millimeter length scale has applications ranging from optical gratings to adhesive surfaces. Here, we explore the spontaneous reorganization of banded tetrathiafulvalene spherulites into regularly spaced ridges on the tens of microns length scale during solvent vapor annealing. Banded spherulites crystallize from the melt as fibrils grow radially outwards from a single nucleation center, twisting in concert with one another about the growth direction. While these spherulites are topographically flat, concentric rings of interference colors become apparent when viewed between crossed polarizers that arise from rhythmic oscillations in crystal orientations due to twisting. Once these films are exposed to methanol vapor, contrast between alternating bands appears even under linearly polarized light and increases over time. Scanning electron microscopy (SEM) revealed the appearance and darkening of specific bands to be associated with an orientation-dependent reorganization of TTF crystals in which crystalline outgrowths appear on alternating bands at the expense of the adjacent bands. After a period of 24 hours of exposure to methanol vapor, TTF films comprised isolated, concentric circles of crystal outgrowths whose crystal orientations are epitaxially determined by the underlying TTF surface. While the surface of these outgrowths appear smooth, cross-sectional SEM images revealed a porous inner structure, suggesting solvent-vapor induced reorganization to proceed through oriented attachment of TTF nanocrystals onto the surfaces of specific bands. X-ray diffraction patterns collected on these films revealed that TTF crystals maintained the metastable beta-polymorph, typically formed at elevated temperatures, throughout the solvent vapor annealing process. These studies point to the use of crystal twisting as a means of patterning by taking advantage of crystal orientation-dependent properties, such as solubility.