Researchers have shown why intense, pure red colours in nature are mainly produced by pigments, instead of the structural colour that produces bright blue and green hues.
The researchers, from the University of Cambridge, used a numerical experiment to determine the limits of matt structural colour - a phenomenon which is responsible for some of the most intense colours in nature - and found that it extends only as far as blue and green in the visible spectrum. The results, published in PNAS, could be useful in the development of non-toxic paints or coatings with intense colour that never fades.
Structural colour, which is seen in some bird feathers, butterfly wings or insects, is not caused by pigments or dyes, but internal structure alone. The appearance of the colour, whether matt or iridescent, will depending on how the structures are arranged at the nanoscale.
Ordered, or crystalline, structures result in iridescent colours, which change when viewed from different angles. Disordered, or correlated, structures result in angle-independent matt colours, which look the same from any viewing angle. Since structural colour does not fade, these angle-independent matt colours would be highly useful for applications such as paints or coatings, where metallic effects are not wanted.
"In addition to their intensity and resistance to fading, a matt paint which uses structural colour would also be far more environmentally-friendly, as toxic dyes and pigments would not be needed," said first author Gianni Jacucci from Cambridge's Department of Chemistry. "However, we first need to understand what the limitations are for recreating these types of colours before any commercial applications are possible."
"Most of the examples of structural colour in nature are iridescent - so far, examples of naturally-occurring matt structural colour only exist in blue or green hues," said co-author Lukas Schertel. "When we've tried to artificially recreate matt structural colour for reds or oranges, we end up with a poor-quality result, both in terms of saturation and colour purity."
The researchers, who are based in the lab of Dr Silvia Vignolini, used numerical modelling to determine the limitations of creating saturated, pure and matt red structural colour.
The researchers modelled the optical response and colour appearance of nanostructures, as found in the natural world. They found that saturated, matt structural colours cannot be recreated in the red region of the visible spectrum, which might explain the absence of these hues in natural systems.
"Because of the complex interplay between single scattering and multiple scattering, and contributions from correlated scattering, we found that in addition to red, yellow and orange can also hardly be reached," said Vignolini.
Despite the apparent limitations of structural colour, the researchers say these can be overcome by using other kind of nanostructures, such as network structures or multi-layered hierarchical structures, although these systems are not fully understood yet.