Butterflies owe much of their charisma to their brilliant colors. And they owe much of their brilliant coloration not to pigments, the molecules that lend color to different inks, but to nanoscale structures on the surfaces of their wings.
Colors that emerge from light reflected from tiny structures, known as structural colors, are common among butterflies, birds and many other creatures, but scientists have only recently figured out how to create such colors in the lab. Now, researchers have refined the process to an unprecedented level of efficiency, allowing them to print with colors extremely bright because of their structure at the nanoscale. They describe the work in a new paper in the journal Scientific Reports.
"We were able to create structures with almost 100 percent absorption of different colors, so our technology produces much brighter colors," senior author Xiaodong Yang told Tech Times.
Pigments are what we generally use in our inkjet printers. Their color is a result of the properties of the pigment molecules themselves, particularly the colors of light that they absorb and reflect.
Hidden within white light are all other colors, which is why holding a prism up to sunlight yields a rainbow. Blue ink looks blue because it reflects blue light and absorbs light of all other colors.
Structural colors also form as a result of which colors are absorbed and which are reflected. But unlike pigments, structural colors occur as a result of nanoscale structures on the surface of a material rather than properties of the molecules that make up the material. In this case, the researchers used beams of ions to carve out extremely tiny holes from a thin layer of silver coating a piece of glass.
"The idea is that when white light such as sunlight comes in, the specific size of the holes allows that area to absorb a specific color of the light," says Yang.
The colors that the researchers produced are so bright because they engineered the holes to be precise size needed to absorb a particular color. By combining holes of different sizes, they were able to create different colors in a way similar to the way an inkjet printer uses only three color cartridges and black to create many other colors.
The researchers stacked thin layers of silver, glass, and another layer of silver to create the material that they carved the nano-holes into, but Yang says that other metals would also work. Each layer is very thin, just dozens of nanometers thick, and could be placed on many different surfaces. One potential application of the technology is to print tiny images on plastic cards such as identification cards.
"Because it's very small, this technology can be used for security markings," Yang says.
Among the technology's less useful, but perhaps more fun, applications are creating "nanoscale paintings" like the one the researchers created of the Missouri S&T athletic logo. Because the most appropriate way to cheer on a sports team for a science and technology school is, of course, to create a highly engineered piece of swag that is too small for anyone to see with the naked eye.