Screen Saver

Sarina Tracy, sarina.tracy@photonics.com

Let’s face it: We spend a large portion of our day looking at screens. The jury is still out on whether that’s an entirely good or bad thing, but it is the reality of modern life. Our ever-evolving arsenal comprises cellphones, tablets, computers, televisions and cameras. And in some ways, the very characteristics that make them so appealing – their sleek glass touch screens – can also become a liability.

The surface reflectivity of glass and polymers (about 4 percent under normal incidence) reduces the performance and lifetime of our glass-covered gadgets. Reflection of sunlight or bright objects can cause optical loss or image corruption in a display, and backreflection can cause a laser cavity to become unstable. Now, nanostructures can provide an accessible, inexpensive way to ward off those effects by averting glare, reflection and fluids on the surfaces of our electronics.

Researchers at the Institute of Photonic Sciences (ICFO), in collaboration with Corning, have demonstrated a highly transparent glass display screen that is said to be the first monolithic, hierarchically structured glass surface that combines micro- and nanoscale surface features to simultaneously generate antiglare, antireflection and superhydrophobic properties.

“We have suppressed reflection from a surface by applying biomimetic micronanostructures,” said Dr. Valerio Pruneri of ICFO. “People can view their screens without any reflection of external light sources and with negligible scattering of the light from the image source.”

Pruneri and his team produced antiglare surfaces by applying a polymer mask to the glass and fusing particles to the surface. A mixture of hydrofluoric and sulfuric acids was applied to the mask, etching and roughening the glass underneath to make it scatter light and ward off glare without hurting the glass’s transparency. The surface was then covered by ultrathin copper films and, using sputtering techniques, nanopillars were fabricated on the surface of the glass to make it antireflective.

A two-tiered structure of 100- to 200-nm nanopillars and 10- to 30-nm branches was created.

A dip in the optical transmission spectrum was related to local surface plasmon resonance effects of the nanoparticles. Nanoparticle geometry can be changed, depending on the initial metal thickness and process parameters.

The researchers found that the textured surface also repels water, mimicking a lotus leaf. Static contact angles of more than 170° for water and 160° for oil were achieved. The rough nature of the antiglare surface led to large, freely suspended water menisci in the air. The contact angle hysteresis on the surface was very low, forcing the droplets to roll off easily. This hydrophilic surface can become superhydrophobic by applying a fluorosilane coating.

While other studies have achieved omniphobicity at the expense of light transparency and through visibility, the ICFO glass averaged a 93.8 percent visible light transmission, as well as low scattering values, with about 1 percent haze. The surface also reduced reflection to <0.5 percent. The antiglare structures and the possibility of glass-substrate ion exchange allow the enhancement of mechanical durability and robustness. This method, relying upon inexpensive lithography-free fabrication techniques, can be industrialized for mass production.

“[People] can view their touch screens in sunlight without any blinding effect,” Pruneri said, “and at the same time increase the quality of the display image.”

The research was published in ACS Applied Materials & Interfaces (doi: 10.1021/am5013062).