Members: Michael Leung, George Baxter, Benjamin Greenlay, Elwin Ha, Matthew Haines
Abstract: It has been demonstrated that randomly distributed, sub-wavelength nanostructures are able to decrease reflection, and hence increase transmission, of broadband wavelengths over a large angular range. The unacceptable drawback of these structures, however, is that they are fragile and environmentally sensitive. One particularly promising solution involves the combination of such nanostructures with single layer anti-reflective coatings, which have been proven to minimize reflectance in a wavelength-selective manner for narrow angular ranges. They also possess extensive passivation properties. This project investigates a hybrid anti-reflective coating through the use of a patterning process in which the surface profile of black silicon is transferred to optical grade polymers, which are subsequently passivated utilizing a thin film consisting of mesoporous silica nanoparticles.
This project focuses on a two-layered passivated anti-reflective coating, by combining randomly distributed nanostructures created on a transparent substrate with with a mesoporous silica coating. The nanostructures are subwavelength in size and create a gradient refractive index gradient between the mesoporous silica and the substrate and the mesoporous silica has a low refractive index which reduces the reflection at the surface. The fabrication process for both components is simple and inexpensive. Together these factors create an inexpensive resilient coating that is also anti-reflective to reduce glare for applications with touch screen technology.
The best prototype was constructed using a combination of nanostructures, imprinted into Norland Optical Adhesive 61, and mesoporous silica nano particles, dip coated with a 35% binder solution at 120 mm / minute. It was tested to determine the transmission and hardness characteristics. This prototype exhibited less then 2% combined reflection and absorption across most visible wavelengths. This is an improvement over the 8-9% reflection observed on baseline polished glass slides. It was also able to withstand mild contact, however adhesions issues with the mesoporous silica coating prevented accurate hardness measurements from being made.
In conclusion the prototype made outperforms both plain glass samples, plain nano structured samples and thin mesoporous silica anti-reflective coatings. While the hardness doesn’t current compete with industry standards there is room for future research into better adhesion and stronger mesoporous silica coatings. In addition more research can be done to better mass produce the samples, through hot embossing and injection moulding of the nanostructures and spray coating the mesoporous silica solution.