Pigmenting Plastics, Plasmonically

When white light hits the plasmonic metasurface, absorption resonances in the disk-hole system allow reflection in a different color (left), which does not vary with viewing angle (right). Credit: Nano Lett., doi:10.1021/nl5014986.

A team of scientists from Denmark and Israel has developed a method for creating bright, angle-insensitive reflective colors on plastics, using localized surface plasmonic resonances, or LSPR (Nano Lett., doi:10.1021/nl5014986). The group believes that the method could provide a practical alternative to sometimes costly dye-based methods commonly used for pigmenting consumer plastics, which can create problems in plastics recycling.

 

The new method focuses on achieving color structurally—through surface patterning, analogous to the butterfly’s wing or the peacock’s feather—rather than through added bulk pigments. A number of research groups have worked toward that end, using techniques such as dielectric structures based on photonic crystals, and plasmonic cavity resonances. For structural coloration to be practical for consumer products, however, it needs to be insensitive to viewing angle, durable and scalable to mass manufacturing.

 

To meet those three criteria using LSPR, the team devised a fabrication process that begins with creation of a nanohole array in silicon (using electron-beam lithography) to serve as a master mold for the pattern on the plastic surface. The mold is then used to create a polymer sheet featuring nubby polymer “pillars” based on the master mold, using hot embossing or injection molding. A thin film of aluminum is then evaporated on top of the nubby polymer surface, leaving aluminum nanodisks at the pillar tops hovering over a “holey” aluminum film beneath. Finally, the entire surface is covered with a protective coating for durability.

 

When incident white light hits the plasmonic metasurface, the absorption resonances in the coupled disk-hole system modify the wavelength and lead to colored reflections that don’t vary with the angle of incident light or viewing. The colors can be tuned by varying the diameter of the nanodisks. And the team notes that details of the production process—such as the use of cheap, abundant aluminum rather than the usual suspects in plasmonic experiments, gold and silver—could help make the process work in large-scale manufacturing.