This Grant Opportunity for Academic Liaison with Industry (GOALI) project is designed to test the hypothesis that a coining process can be used to rapidly create a large mold containing billions of miniature features in a high-precision machine tool. These molds would then be used to micro-replicate these features onto plastic sheets creating large volumes of low-cost optical elements. Because these features are smaller than the wavelength of light, the functionality and applicability of these optical elements will be improved. For example, small features can create a non-reflective surface mimicking the nanostructures in moth eyes or can efficiently couple light from an organic light emitting diode. This project will combine mechanical engineering and material science faculty and students along with fabrication experts from 3M to create the miniature die features and the dynamic process needed to replicate these features onto a large mold. Material selection, feature shape replication, actuator design, process control will be topics of research.
If successful, this project will revolutionize micro-replication processes used for applications such as reflective films and anti-glare laptop screens. It also will have application for improving the light capture efficiency in organic solar cells In addition, the research project provides the opportunity for graduate and undergraduate students at North Carolina State University to learn the details and extend the capability of nanoscale manufacturing. The results of the work will be presented at technical meetings and published in scientific journals. The research experience provided to each of these groups will improve the quality of the science and engineering education
NANOCOINING If you have seen a presentation using a liquid crystal display (LCD) projector, used a laptop computer, watched a large screen television or seen a reflective sign or cone at night, you have seen examples of large area micro-structured optical components for non-imaging applications. The pyramids, ridges and other surface shapes on the plastic film can alter the physical, chemical or optical properties of surfaces. A sheet of microscopic pyramids can make road signs brighter by reflecting the light back to the source even if the beam is not perpendicular to the surface of the sign. As a result, they return more than two-thirds of the light that falls on them. This technology has been applied to signs since the 60s but has expanded to computers and cell phones allowing amazingly thin laptop screens where light from sources around the perimeter is turned to create uniform illumination over the whole area of the screen. This process is accomplished by precision machining the outside of a drum mold using a sharp diamond tool to cut a series of overlapping grooves. A sheet of plastic is then pressed between a pair of rotating drums, one with and one without features. The result is a thin sheet of plastic with sub-millimeter features on the surface. If pyramidal shapes are machined, the resulting film will become an optical turn into a corner-cube where the incident light beam is turned180º and return it to its source. The drum can be meters long and miles of this reflective film can be created in minutes. But the diamond turning process has limitations based on the feature size that can be machined on the drum mold. Features less than 10 µm (1/10 of a millimeter) are difficult to produce. Smaller features are desirable as they add functionality to the surfaces created. Biological examples include moth eyes that have sub-wavelength feature that make them non-reflective and a lotus leaf with small features that keep water droplets from wetting the surface. These examples require sub-micrometer features (0.5 µm or 1/5000 of a millimeter) that can change the optical or chemical characteristics of the surface. To add this functionality, a new method of creating the features on the drum is needed. Nanocoining is a way to create small features on the drum mold that is not machining. It takes a page from a process dating back to the Roman Empire that was (and is) used to make coins. A die with say Caesar’s likeness was pressed into a metal coin to create money. The mold in the nanocoining case is the metal mold discussed above but the die is a small diamond (diameter of a hair or 20 µm wide) that has a series of focused-ion beam machined grooves that make it look like a miniature waffle iron. This small die has 6400 posts – each smaller than the wavelength of light. The die is attached to an ultrasonic actuator (runs at a frequency that is beyond what a human can hear - 40,000 cycles/sec) which presses the die into the same drum roll making 256,000,000 features/sec. The result is a drum covered with tiny sub-wavelength features that can be transferred to a sheet of plastic to enhance the functionality of the surface. The process to create the plastic film is the same, but the tools used to make the mold are very different.
