Embedding silver nanoparticles in a thin polymer sheet can produce transparent displays based on narrow-band resonance scattering phenomena, according to research reported in the January 21 issue of Nature Communications (DOI: 10.1038/ncomms4152) by scientists at the Massachusetts Institute of Technology (MIT) and the US Army Edgewood Chemical Biological Center. The frequency-selective properties of these displays, which can be tuned to scatter light of a desired, single wavelength, and their wide viewing angle, make them attractive possible alternatives to currently available transparent displays. Eventually, they might be manufactured inexpensively on a large scale using roll-to-roll polymer processing, which would give them a cost advantage over other display types.
As an example, the head-up display in aircraft that projects flight data on the cockpit window for easy viewing by the pilot works by specular reflection of the images off the glass. This reflection limits the angle of viewing and the images can be seen only from the pilot's seat; they are not visible to someone standing behind the pilot or off to one side. In contrast, the scattering of an image from the new silver-nanoparticle-embedded plastic film can be seen over a wide range of viewing angles. “This makes it useful for viewing by multiple audience members,” said Chia Wei Hsu of MIT and Harvard University, the lead author of the article.
The proof-of-concept display developed by Hsu and his colleagues is also more transparent to ambient light than most other types of transparent displays because of its frequency selectivity. By changing the diameter and volume fraction of the silver nanoparticles, it is possible to tune the frequency of light that is scattered. Most head-up displays are not frequency-selective, so basically all of the light is either reflected or transmitted, Hsu said; to reflect more light, it has to be made less transparent. “In our case we can make the display scatter this one particular color but keep the transparency at other colors,” he said, “so we won’t have to decrease the transparency as much.” The research team’s display was measured at 60% transparency in the visible range, compared to 20–40% for liquid-crystal displays or organic light-emitting diodes.
The research team chose silver nanoparticles for this initial trial because silver provided the best performance among the materials they had at hand. They mixed hydrolyzed polyvinyl alcohol with an aqueous solution of approximately 64-nm-diameter silver nanoparticles (concentration 0.01 mg/ml), and allowed the solution to dry on a square glass plate over 40 hours. The resulting polymer film had a thickness of 0.46 nm, with nanoparticle density of 7 μg Ag/cm2. Such a display scatters blue light of 458 nm wavelength.
Hsu and his colleagues see this material as simply a plastic foil that can be placed on glass surfaces, transforming an existing window into a transparent display. For example, an office window with a layer of this foil could be used as a data projection surface while retaining the ability to see through it. Shop windows could be enhanced by projecting product data or branding information on the window’s surface to complement the products that are clearly visible behind the screen. Because the scattered light retains the polarization of the incident light, three-dimensional viewing might be achieved by simultaneously projecting two images with different circular polarities and viewing the image through polarized lenses.
Plans for future research include making the resonance band narrower to render the screen more transparent, and to enable the production of three-color displays, instead of the monochromatic display demonstrated in this early work. “In order to enable three-color displays,” Hsu said, “we need each of the three resonances [red-green-blue] to be narrow enough so that the three resonances don’t overlap each other.”
