Self-Assembled Nanoprisms Form Smectic Crystal

Richard Gaughan

Building nanometer-scale devices such as photonic crystals requires assembly techniques of unprecedented accuracy. Developing these techniques can be as difficult as designing the devices and, thus, self-assembly methods — in which the components of nanoscale devices put themselves into the desired configuration — are highly desirable. A new technique creates self-assembled hollow hexagonal nanoprisms that can collect into a smectic phase that could function as a photonic crystal.

At the Chinese Academy of Sciences in Beijing, researchers have found that, when zinc meso-tetra (4-pyridyl) porphyrin (ZnTPyP) is suspended in solution with a surfactant molecule, cetyltrimethylammonium bromide (CTAB), the molecules assemble into hollow tubes with a hexagonal cross section, a wall thickness of 30 nm and an outer diameter of 95 nm. The length of the tubes varies with the relative concentrations of ZnTPyP and CTAB. In their initial experiments, three solutions resulted in hollow hexagonal nanoprisms with lengths of 340, 530 and 720 nm.
When the solution is allowed to evaporate, the structures align themselves parallel to each other — first in a single plane and then in additional well-defined planes. Such a smectic crystal is intriguing for several reasons. For one, it represents one of the few examples of an organic material that can self-assemble into a secondary structure. Also, porphyrins are interesting because they are biologically compatible and exhibit photochemical activity. The self-assembled structure could be useful for photonic crystals, drug-delivery devices and molecular filters.

Li-Jun Wan, the lead researcher on the project, said that the scientists are working to understand the properties of single hollow hexagonal nanoprisms and trying to find another method to control the secondary structure.