Custom shapes for organic crystals

As the feature sizes of electronic devices become smaller, the mass production of nanometer- and micron-scale hierarchical structures is a major challenge. (See the article by Matthias Imboden and David Bishop, Physics Today, December 2014, page 45 .) One possible approach is to coax crystals to grow into a desired structure by exploiting chemical interactions between crystal faces and dissolved molecules. Though much used for inorganic systems, that technique has been less explored for organic materials—even though organic molecules offer a far richer range of chemical properties. Now Helmut Cölfen (University of Konstanz, Germany) and colleagues have used a diblock copolymer (see the article by Frank Bates and Glenn Fredrickson, Physics Today, February 1999, page 32 ) to control the morphology of a material called PTCAPS, one of an important class of organic semiconductors. The polymer consists of a block of polyethylene glycol, which is highly soluble in water, bound to a block of polyethyleneimine, which has a strong electrostatic affinity to the (001) face of the nascent PTCAPS crystals. For PTCAPS in a water–polymer solution, therefore, crystal growth on the (001) face is stifled, so the crystals grow in micron-thick plates. The polymer concentration determines whether the plates cluster into irregular aggregates (top panel in the figure), star-shaped structures (middle panel), or round flower-like particles of uniform size (bottom panel). Adjusting the solution’s pH—which affects the polyethyleneimine’s electric charge and thus the strength of the polymer–PTCAPS interaction—also influences the crystals’ final morphology. (M. Huang, M. Antonietti, H. Cölfen, APL Mater. 4, 015705, 2015, doi:10.1063/1.4934704 .)