Finding New Optical Materials

The method, developed at Purdue University, works by positioning tiny particles onto a silicon template containing precisely spaced holes that are about one-hundredth the width of a human hair. The template is immersed in water on top of which particles are floating, and the particles automatically fill in the holes as the template is lifted.

The researchers have used the technique to create a "nearly perfect two-dimensional colloidal crystal," or a precisely ordered layer of particles. 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.

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. Omni-directional coatings would increase how much light is transmitted by fiber-optics and could possibly be used in future sensor technology and "optical computers" and circuits that use light instead of electronic signals to process information. It might be possible to use the method to create special crystals with particles arranged in the same pattern as carbon atoms in diamonds. The first layer could be a starting point for growing the crystals.

To produce the single-layer structure, the engineers used a process called Langmuir-Blodgett monolayer deposition, a standard technique used in physical chemistry, primarily to create lipid membranes for research. The method developed at Purdue is faster and would be far less expensive than a competing method for creating the crystals, a technique called "nano-robotics.”
Researchers found that it was essential to control three conditions to successfully create the layer of particles: humidity, how fast the template is lifted out of the solution and the initial density of particles in the solution.
The researchers discovered that defects form when the air is too dry.

"When we suppressed water evaporation by humidifying the area, we created a completely flat, horizontally uniform structure," they said. "Water evaporation causes a non-uniform structure formation on the surface. This is a huge problem because our goal is to make two-dimensional crystal structures as uniform as possible over the entire patterned region. By controlling humidity, we proved that we can solve that problem."The engineers were able to precisely control the particle density, or how many particles occupy a given space, by using two Teflon bars like bookends on either end of the particle layer formed on the water surface to compress the particles before being deposited. The particles in the research had a diameter of about one micron, or millionth of a meter. Producing a high-quality single layer of micron-size particles has proven difficult for researchers until the new technique was developed.

The researchers used their technique to make layers in various patterns, such as square or hexagonal. "We can make whatever structure we want," they said.

COMPAMED.de; Source: Purdue University