Rod-shaped nanocrystals normally arrange themselves parallel to each other. Researchers at Rice University in Houston, Texas now report in the journal Angewandte Chemie completely unexpected behavior of nano-objects: the spontaneous self-assembly of polymer-coated metallic nanorods into ring-shaped structures. These rings, made of tiny gold rods, form within seconds when water droplets condense onto the surface of a solution of the rods in a nonpolar solvent.
Nanoscale objects organized into superstructures are interesting because the properties of such tiny particles depend not only on their composition, shape, and size, but also to a large extent on their spatial distribution and the degree of their ordering within a superstructure.
Bishnu P. Khanal and Eugene R. Zubarev coated gold nanorods with polystyrene. The tiny polymer chains in the coating stick out from the rodlike bristles. Their function is to make the rods soluble in organic solvents but insoluble in water—which is important for the self-assembly process. The nanorods are dissolved in dichloromethane, and then a carbon-coated grid is dipped into this solution. When the grid is pulled out, a thin layer of solution clings to it.
The highly volatile dichloromethane quickly evaporates into the air, which intensely cools the surface of the liquid film. As a consequence, humidity from the air condenses onto the surface. Because water and dichloromethane are immiscible, the condensation forms tiny droplets of water. When the dichloromethane has largely evaporated, the last remnants of the solution form rings around the water drops. Once the dichloromethane has completely evaporated, the substrate warms back up to room temperature and the water droplets also begin to evaporate.
This leaves behind ring-shaped structures made of nanorods. Images obtained with an electron microscope show that the nanorods in the rings are oriented randomly when their concentration in the original solution is high. However, at lower concentrations the result is truly amazing: The nanorods are oriented in a head-to-tail sequence along the edge of the ring.
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