A much-revered species that represents beauty and elegance in many cultures, the lotus flower has once again risen to fame. This time, researchers at the Tokyo Institute of Technology have used its leaf as a template to successfully fabricate a material that can absorb virtually the entire spectrum of visible light.
When designing new materials, scientists often seek inspiration from nature for examples of inner structures that lead to specific function. But metamaterials, which are manmade substances specifically engineered to have properties rarely or never seen in nature, seem an unlikely candidate for nature-based inspiration. The lotus leaf, however, has a uniquely structured surface that makes it a founding member of a new class of ‘biometamaterials.’
Cilia, or hair-like structures on the leaves’ surface, serve the primary function of creating a highly water-repellent surface. The cilia take the form of assemblies of randomly-oriented nanotubes, each 100 nm in diameter. The Tokyo team, led by Kotaro Kajikawi, professor in the Department of Electronics and Applied Physics, hypothesized that the nanorods would provide a useful template for a metamaterial designed to absorb light – indeed, good light-absorbing properties of vertically oriented nanorods in other materials have been reported in the literature.
To create a model of the surface structure, the team deposited a liquid gold onto leaf samples using a spray ‘sputtering’ technique, to create films of different thicknesses adhered to separate leaf samples. The exciting result was a 10 nm-thick black-colored gold biometamaterial that was shown in scanning electron microscope images to preserve the nanorods’ structure and that absorbed nearly all the light in the visible spectrum.
The novel black, ultrathin gold film demonstrated reflectivity below 0.01 over the entire visible spectral range. Calculations showed that the low reflectivity, and resulting unparalleled light absorption, does indeed stem from the randomly-oriented, rod-like cilia on the leaf’s surface.
The next step for Kajikawi’s team is to develop an easy and effective chemical process by which to separate the metamaterial from the leaf.
Surfaces that absorb most visible light are used in telescopes and solar panels, and recent technologies that incorporate metamaterials include light absorbers, sensors, optical filters, and cloaking devices. According to the Tokyo Institute of Technology, the study “highlights the potential for using naturally-occurring surfaces to generate very precise materials for specific purposes.” In an email to Cleantech Concepts, Dr. Kajikawa specifically referenced solar energy applications, and that additional research on this front would be published in the future.
The work was published in Nature Scientific Reports in November 2015.