Nanotechnology can change color of gold

Gold doesn't have to appear gold in color, and in fact a new nanotechnology technique can turn it red or even green, British researchers say. A University of Southampton team has found they can change the color of the world's most iconic precious metal by embossing tiny raised or indented patterns onto the metal's surface that can change the way it absorbs and reflects light, so to the human eye it doesn't appear "golden" in color at all. The technique, equally applicable to other metals such as silver and aluminum, opens up the prospect of coloring metals without having to coat or chemically treat them, providing valuable economic, environmental and other benefits, a Southampton release said Wednesday. The method could be useful for purposes ranging from manufacturing jewelry to making banknotes and documents harder to forge, the researchers said. "The colors of the objects we see all around us are determined by the way light interacts with those objects. For instance, an object that reflects red light but absorbs other wavelengths will appear red to the human eye," Southampton researcher Nikolay Zheludev said. "This is the fundamental principle we have exploited in this project. By embossing metals with patterns only around 100 nanometers across, we've found that we can control which wavelengths of light the metal absorbs and which it reflects." The precise shape and height or depth of the patterns determine the behavior of light when it strikes the metal and therefore what color is created, and a wide range of colors on a given metal can be created, researchers said.