Oct 31 2017
One of the major inconveniences of modern display screens experienced while using a computer underneath overhead lighting or adjacent to a window, watching television in complete darkness, or taking a photo outdoors on a sunny day using a smartphone is the phenomenon called glare.
Majority of the existing electronics include plastic or glass covers as a defense against moisture, dust, and other environmental contaminants. However, light reflection from such surfaces can render it difficult to view information displayed on the screens.
Glass surfaces with etched nanotextures reflect so little light that they become essentially invisible. This effect is seen in the above image, which compares the glare from a conventional piece of glass (right) to that from nanotextured glass (left), which shows no glare at all. CREDIT: Brookhaven National Laboratory.
At present, researchers from the Center for Functional Nanomaterials (CFN), a U.S. Department of Energy Office of Science User Facility at Brookhaven National Laboratory, have developed an innovative technique for minimizing the surface reflections from glass surfaces to almost zero by etching minute nanoscale features into the surfaces.
If there is a sudden alteration of the refractive index in the path of light, a portion of the light is reflected. The refractive index is the extent to which a ray of light bends when it travels from one material to another, for example, between glass and air. The nanoscale aspects make the refractive index to gradually change from that of air to that of glass, thus preventing reflections. The ultra-transparent nanotextured glass is antireflective across a wider range of wavelength—the whole of visible and near-infrared spectrum—and over a broad array of viewing angles. Reflections are minimized to the extent that the glass actually turns invisible.
Apart from enhancing the user’s experience of consumer electronic displays, the “invisible glass” can find many applications. It can improve the energy-conversion efficiency of solar cells by reducing the quantity of sunlight lost due to reflection. It can also be a possible substitute to the damage-susceptible antireflective coatings traditionally used in lasers emitting powerful light pulses, for instance, light pulses used during the production of aerospace components and medical devices.
We’re excited about the possibilities, not only is the performance of these nanostructured materials extremely high, but we’re also implementing ideas from nanoscience in a manner that we believe is conducive to large-scale manufacturing.
Charles Black, CFN Director and corresponding author of the paper
Andreas Liapis, a former Brookhaven Lab postdoc who is at present a research fellow at Massachusetts General Hospital’s Wellman Center for Photomedicine, and Atikur Rahman, who was also a former Brookhaven Lab postdoc and now an assistant professor in the Department of Physics at the Indian Institute of Science Education and Research, Pune, are co-authors of the paper.
For nanoscale texturing of the glass surfaces, the researchers adopted a technique known as self-assembly, that is, the potential of some materials to voluntarily form ordered arrangements by themselves. Here, the way a block copolymer material self-assembled functioned as a template for etching and transforming the glass surface into a “forest” of cone-shaped, nanoscale structures that had sharp tips. Such geometry removes virtually all the surface reflections. Block copolymers are industrial polymers, or continuous chains of molecules, contained in various products such as adhesive tapes, shoe soles, and automotive interiors.
In the recent past, Black and CFN collaborators have adopted a similar nanotexturing process to provide self-cleaning and water-repellent characteristics as well as anti-fogging potential to glass, silicon, and certain plastic materials, and also to render silicon solar cells to be antireflective. The surface nanotextures are similar to those seen in nature—for example, the minute light-trapping posts that render moth eyes dark to assist the insects in preventing detection by predators and the waxy cones that maintain cicada wings clean.
This simple technique can be used to nanotexture almost any material with precise control over the size and shape of the nanostructures, the best thing is that you don’t need a separate coating layer to reduce glare, and the nanotextured surfaces outperform any coating material available today.
Atikur Rahman, assistant professor in the Department of Physics at the Indian Institute of Science Education and Research
“We have eliminated reflections from glass windows not by coating the glass with layers of different materials but by changing the geometry of the surface at the nanoscale,” noted Liapis. “Because our final structure is composed entirely of glass, it is more durable than conventional antireflective coatings.”
For computing the performance of these nanotextured glass surfaces, the team evaluated the amount of light conveyed through and reflected from the surfaces. The results of experimental evaluations of surfaces that had nanotextures of varying heights conformed to their model simulations and demonstrate that lower light levels were reflected by taller cones. For instance, glass surfaces that had with nanotextures with a height of 300 nm reflect less than 0.2% of incoming red-colored light, of 633-nm wavelength. The amount of light transmitted through the nanostructured surfaces is high (i.e. greater than 95% and 90%, respectively) even at the near-infrared wavelength of 2500 nm and viewing angles of 70°, which is very high.
The researchers performed another experiment to compare the performance of a commercial silicon solar cell with no cover, with that of a traditional glass cover, and also with that of a nanotextured glass cover. The nanotextured glass cover-equipped solar cell generated the same electric current as the cell without the cover. Their nanotextured glass was also exposed to short laser pulses in order to ascertain the intensity at which the destruction of the material by laser light is initiated. The evaluations demonstrated that glass can tolerate optical energy per unit area three times more when compared to commercially accessible antireflection coatings that function across a wide range of wavelengths.
Our role in the CFN is to demonstrate how nanoscience can facilitate the design of new materials with improved properties, this work is a great example of that—we’d love to find a partner to help advance these remarkable materials toward technology.
Charles Black, CFN Director and corresponding author of the paper