Tag Archives: Jay Guo

Transparent silver

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This March 21, 2017 news item on Nanowerk is the first I’ve heard of transparent silver; it’s usually transparent aluminum (Note: A link has been removed),

The thinnest, smoothest layer of silver that can survive air exposure has been laid down at the University of Michigan, and it could change the way touchscreens and flat or flexible displays are made (Advanced Materials, “High-performance Doped Silver Films: Overcoming Fundamental Material Limits for Nanophotonic Applications”).

It could also help improve computing power, affecting both the transfer of information within a silicon chip and the patterning of the chip itself through metamaterial superlenses.

A March 21, 2017 University of Michigan  news release, which originated the news item, provides details about the research and features a mention about aluminum,

By combining the silver with a little bit of aluminum, the U-M researchers found that it was possible to produce exceptionally thin, smooth layers of silver that are resistant to tarnishing. They applied an anti-reflective coating to make one thin metal layer up to 92.4 percent transparent.

The team showed that the silver coating could guide light about 10 times as far as other metal waveguides—a property that could make it useful for faster computing. And they layered the silver films into a metamaterial hyperlens that could be used to create dense patterns with feature sizes a fraction of what is possible with ordinary ultraviolet methods, on silicon chips, for instance.

Screens of all stripes need transparent electrodes to control which pixels are lit up, but touchscreens are particularly dependent on them. A modern touch screen is made of a transparent conductive layer covered with a nonconductive layer. It senses electrical changes where a conductive object—such as a finger—is pressed against the screen.

“The transparent conductor market has been dominated to this day by one single material,” said L. Jay Guo, professor of electrical engineering and computer science.

This material, indium tin oxide, is projected to become expensive as demand for touch screens continues to grow; there are relatively few known sources of indium, Guo said.

“Before, it was very cheap. Now, the price is rising sharply,” he said.

The ultrathin film could make silver a worthy successor.

Usually, it’s impossible to make a continuous layer of silver less than 15 nanometers thick, or roughly 100 silver atoms. Silver has a tendency to cluster together in small islands rather than extend into an even coating, Guo said.

By adding about 6 percent aluminum, the researchers coaxed the metal into a film of less than half that thickness—seven nanometers. What’s more, when they exposed it to air, it didn’t immediately tarnish as pure silver films do. After several months, the film maintained its conductive properties and transparency. And it was firmly stuck on, whereas pure silver comes off glass with Scotch tape.

In addition to their potential to serve as transparent conductors for touch screens, the thin silver films offer two more tricks, both having to do with silver’s unparalleled ability to transport visible and infrared light waves along its surface. The light waves shrink and travel as so-called surface plasmon polaritons, showing up as oscillations in the concentration of electrons on the silver’s surface.

Those oscillations encode the frequency of the light, preserving it so that it can emerge on the other side. While optical fibers can’t scale down to the size of copper wires on today’s computer chips, plasmonic waveguides could allow information to travel in optical rather than electronic form for faster data transfer. As a waveguide, the smooth silver film could transport the surface plasmons over a centimeter—enough to get by inside a computer chip.

Here’s a link to and a citation for the paper,

High-Performance Doped Silver Films: Overcoming Fundamental Material Limits for Nanophotonic Applications by Cheng Zhang, Nathaniel Kinsey, Long Chen, Chengang Ji, Mingjie Xu, Marcello Ferrera, Xiaoqing Pan, Vladimir M. Shalaev, Alexandra Boltasseva, and Jay Guo. Advanced Materials DOI: 10.1002/adma.201605177 Version of Record online: 20 MAR 2017

© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

This paper is behind a paywall.

Peacocks and their structural colour inspire better resolution in e-readers

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Thank goodness birds, insects, and other denizens of the natural world have not taken to filing patents otherwise we’d be having some serious problems in the courts as I have hinted in previous postings including this March 29, 2012 posting titled, Butterflies give and give … .

This time, it’s the peacock which is sharing its intellectual property as per this Feb. 5, 2013 news item on ScienceDaily,

Now, researchers at the University of Michigan have found a way to lock in so-called structural color, which is made with texture rather than chemicals. A paper on the work is published online in the current edition of the Nature journal Scientific Reports.

In a peacock’s mother-of-pearl tail, precisely arranged hairline grooves reflect light of certain wavelengths. That’s why the resulting colors appear different depending on the movement of the animal or the observer. Imitating this system—minus the rainbow effect—has been a leading approach to developing next-generation reflective displays.

The University of Michigan Feb. 5, 2013 news release, which originated the news item, provides information about potential applications and more details about the science,

The new U-M research could lead to advanced color e-books and electronic paper, as well as other color reflective screens that don’t need their own light to be readable. Reflective displays consume much less power than their backlit cousins in laptops, tablet computers, smartphones and TVs. The technology could also enable leaps in data storage and cryptography. Documents could be marked invisibly to prevent counterfeiting.

Led by Jay Guo, professor of electrical engineering and computer science, the researchers harnessed the ability of light to funnel into nanoscale metallic grooves and get trapped inside. With this approach, they found the reflected hues stay true regardless of the viewer’s angle.

“That’s the magic part of the work,” Guo said. “Light is funneled into the nanocavity, whose width is much, much smaller than the wavelength of the light. And that’s how we can achieve color with resolution beyond the diffraction limit. Also counterintuitive is that longer wavelength light gets trapped in narrower grooves.”

The diffraction limit was long thought to be the smallest point you could focus a beam of light to. Others have broken the limit as well, but the U-M team did so with a simpler technique that also produces stable and relatively easy-to-make color, Guo said.

“Each individual groove—much smaller than the light wavelength—is sufficient to do this function. In a sense, only the green light can fit into the nanogroove of a certain size,” Guo said.

The U-M team determined what size slit would catch what color light. Within the framework of the print industry standard cyan, magenta and yellow color model, the team found that at groove depths of 170 nanometers and spacing of 180 nanometers, a slit 40 nanometers wide can trap red light and reflect a cyan color. A slit 60 nanometers wide can trap green and make magenta. And one 90 nanometers wide traps blue and produces yellow. The visible spectrum spans from about 400 nanometers for violet to 700 nanometers for red.

“With this reflective color, you could view the display in sunlight. It’s very similar to color print,” Guo said.

Particularly interesting (for someone who worked in the graphic arts/printing industry as I did) are the base colours being used to create all the other colours,

To make color on white paper, (which is also a reflective surface), printers arrange pixels of cyan, magenta and yellow in such a way that they appear to our eyes as the colors of the spectrum. [emphasis mine] A display that utilized Guo’s approach would work in a similar way.

To demonstrate their device, the researchers etched nanoscale grooves in a plate of glass with the technique commonly used to make integrated circuits, or computer chips. Then they coated the grooved glass plate with a thin layer of silver. When light—which is a combination of electric and magnetic field components—hits the grooved surface, its electric component creates what’s called a polarization charge at the metal slit surface, boosting the local electric field near the slit. That electric field pulls a particular wavelength of light in.

The base colours in printing are CMYK (cyan, magenta, yellow, black). At least, that was the case when I worked in the graphic arts industry and quick search on the web suggests that standard still holds.(Have I missed a refinement?) In any event, here’s an image that demonstrates how this new colour scale can be used,

University of Michigan researchers created the color in these tiny Olympic rings using precisely-sized nanoscale slits in a glass plate coated with silver. Each ring is about 20 microns, smaller than the width of a human hair. They can produce different colors with different widths of the slits. Yellow is produced with slits that are each 90 nanometers wide. The technique takes advantage of a phenomenon called light funneling that can catch and trap particular wavelengths of light, and it could lead to reflective display screens with colors that stay true regardless of the viewer's angle. Image credit: Jay Guo, College of Engineering

University of Michigan researchers created the color in these tiny Olympic rings using precisely-sized nanoscale slits in a glass plate coated with silver. Each ring is about 20 microns, smaller than the width of a human hair. They can produce different colors with different widths of the slits. Yellow is produced with slits that are each 90 nanometers wide. The technique takes advantage of a phenomenon called light funneling that can catch and trap particular wavelengths of light, and it could lead to reflective display screens with colors that stay true regardless of the viewer’s angle. Image credit: Jay Guo, College of Engineering

You can find more about this work in the ScienceDaily news item, which includes a link to the abstract, or in the University of Michigan news release, which includes more images from the scientists.