This photograph shows a side-by-side comparison between Purdue's structure (right) and a structure that results when a template is not used. Researchers at Purdue have developed a "self-assembling" technique to create a "nearly perfect two-dimensional colloidal crystal," or a precisely ordered layer of particles, a critical step toward growing three-dimensional crystals for use in optical communications and other technologies. The method works by positioning tiny particles onto a silicon template containing precisely spaced holes that are about one-hundredth the width of a human hair. (Credit: You-Yeon Won and Jaehyun Hur, Purdue University School of Chemical Engineering)
The researchers have used a new technique to create a "nearly perfect two-dimensional colloidal crystal," or a precisely ordered layer of particles. This is a critical step toward growing three-dimensional crystals for use in optical technologies, said You-Yeon Won, an assistant professor of chemical engineering.
"Making the first layer is very difficult, so we have taken an important step in the right direction," Won said. "Creating three-dimensional structures poses a big challenge, but I think it's feasible."
The single-layer structures might be used to form "micro lenses" to improve the performance of optical equipment, such as cameras and scientific instruments, and to control the color and other optical properties of materials for consumer products.
More importantly, the technique represents one of several possible approaches to create "omni-directional photonic band gap materials." Unlike conventional mirrored materials, which reflect light hitting the mirror at certain angles, the omni-directional materials would be "perfect mirrors," reflecting certain wavelengths of light coming from all directions.
The materials would dramatically improve the performance of optical fibers, which contain a mirrored coating to keep light from escaping.
"We envision that this self-assembly method will open a new possibility for mass fabricating complicated 3-D colloid crystal structures for various applications," Won said.
The Purdue-developed technique takes about 20 minutes to create a structure that would take weeks to produce using nano-robotics.
It was the first time researchers had demonstrated how to create a uniform structure over the relatively large area of such a templated region, which measured about 9 square millimeters, or large enough to contain about 1.7 million particles. Other researches have created self-assembling layers of particles without controlling the spacing between particles, resulting in "close-packed structures," which cannot be used to build three-dimensional, high-quality photonic crystals. Using a template enabled the researchers to create the precisely controlled pattern of particle spacing, a "non-close-packed" first layer, which is critical to building up to a three-dimensional crystal with an arbitrary, desired optical property.