KONGENS LYNGBY, Denmark, July 31, 2014 — Nanoscale structures could one day take the place of dyes in the production of colored plastics. A team from the Technical University of Denmark has found a more environmentally friendly way to produce such plastics, using the shapes of materials rather than conventional dyes and pigments. Use of these unnatural materials makes it difficult for that type of plastic to be recycled. The researchers found that aluminum can be made to appear in different colors “purely by designing their surface structures at the nanoscopic level.” The researchers chose aluminum because of its interband absorption properties. Reflected light is modified due to absorption resonances in the plasmonic nanostructure, which leads to colored reflections. These reflected colors can be varied by tuning the surface geometry. Courtesy of Nano Letters. In the study, the team controlled the colors of plastic consumer products based on localized surface plasmon resonances. This bridged the gap between fabricated plasmonic metasurfaces and large-area, structurally colored plastic surfaces, according to the study. Colors produced were based on aluminum metal disks layered “on top of dielectric pillars and hovering above a holey metal film.” The use of ultrathin, high-loss dielectrics on top of metal led to bright colors, the researchers said. The researchers fabricated “bright angle-insensitive colors that may be tuned across the entire visible spectrum,” allowing control of the optical response of aluminum nanodisks and nanoholes in the visible and near-infrared spectral range. The work was funded by the Danish National Advanced Technology Foundation, the Danish Agency for Science, Technology and Innovation and the European Commission. The research was published in Nano Letters (doi: 10.1021/nl5014986). For more information, visit www.dtu.dk.