System Bits: Oct. 9

Bringing plasmonic color to solid materials
Researchers at the University of California, Riverside, used silver nanoparticles (AgNPs) to produce plasmonic color-switchable films for solid materials. This effect was previously achieved only in liquids.

Rapid and reversible tuning of plasmonic color in solid films, a challenge until now, holds great promise for a number of applications,” said Yadong Yin, a professor of chemistry, who led the research team. “Our new work brings plasmonic metal nanoparticles to the forefront of color-converting applications.”

Study results appear in Angewandte Chemie International Edition. The research paper has been designated a VIP paper by the journal.

Image credit: Yin lab, UC Riverside.

Plasmonic metal nanoparticles, such as gold and silver, have special optical properties because they efficiently absorb and scatter light at particular wavelengths. Their colors can be altered by changing the distance between their individual particles — a feature that Yin’s research team took advantage of to develop their plasmonic color-switching film.

The researchers coated a glass substrate with a layer of sodium borate, or borax. Then they sprayed AgNPs over the borax to form a film. Yin explained that each AgNP has capping ligands on its surface that introduce distance between the AgNPs. Without the buffer provided by the ligands, the nanoparticles would clump together.

In the presence of water or moisture, borax turns to boric acid and releases hydroxyl ions. These ions “deprotonate” a chemical group of the ligands, resulting in the loss of a proton and the addition of a negative charge on the AgNPs. Repulsion forces push the negatively charged nanoparticles away from each other. The nanoparticles, which are pink, acquire new interparticle distances, causing them to reflect a different color: yellow.

When the moisture is removed, the boric acid converts back to borax by capturing hydroxyl ions, initiating a protonation of the ligand’s chemical group. This causes a reduction in surface charges on the ligand, weakening the repulsion forces between the AgNPs and causing them to draw closer to each other and aggregate. With interparticle distances now reduced, the color of the AgNP film switches back from yellow to pink, demonstrating full reversibility.

“Through this mechanism, we could rapidly achieve plasmonic color switching of the AgNP film in the presence or absence of moisture,” Yin said. “In our experiments, we exposed the AgNP film to moisture of 80% relative humidity and found the film changed colors from pink to red, orange, and finally yellow.”

Making use of the relative humidity around human fingers — as high as 100% — Yin’s team found AgNP films can change color in response to the proximity of a fingertip.

“This allows for a convenient, rapid, and touchless method that can be used in information encryption and product authentication,” Yin said. “Various high-resolution patterns can be effectively encrypted in the AgNP films through a lithography process and then decrypted when exposed to moisture in human breath or from fingertips. Other foreseeable applications include secure communication and calorimetric real-time environment or health monitoring.”

Yin’s team found that the moisture-responsive AgNP films showed reversibility and repeatability in plasmonic color switching for more than 1,000 cycles.

The research was supported by a grant to Yin from the National Science Foundation. He was joined in the research by Rashed Aleisa and Ji Feng of UC Riverside; and Luntao Liu, Yun Zhang, Yiqun Zheng, and Wenshou Wang of Shandong University, China.

Printing electronics on paper, human skin
Electrical engineers at Duke University have devised a fully print-in-place technique for electronics that is gentle enough to work on delicate surfaces, including paper and human skin. The advance could enable technologies such as high-adhesion, embedded electronic tattoos and bandages tricked out with patient-specific biosensors.

The techniques are described in a series of papers published online July 9 in the journal Nanoscale and on October 3 in the journal ACS Nano.

“When people hear the term ‘printed electronics,’ the expectation is that a person loads a substrate and the designs for an electronic circuit into a printer and, some reasonable time later, removes a fully functional electronic circuit,” said Aaron Franklin, the James L. and Elizabeth M. Vincent Associate Professor of Electrical and Computer Engineering at Duke.

“Over the years there have been a slew of research papers promising these kinds of ‘fully printed electronics,’ but the reality is that the process actually involves taking the sample out multiple times to bake it, wash it or spin-coat materials onto it,” Franklin said. “Ours is the first where the reality matches the public perception.”

The concept of so-called electronic tattoos was first developed in the late 2000s at the University of Illinois by John A. Rogers, who is now the Louis Simpson and Kimberly Querrey Professor of Materials Science and Engineering at Northwestern University. Rather than a true tattoo that is injected permanently into the skin, Rogers’s electronic tattoos are thin, flexible patches of rubber that contain equally flexible electrical components.

The thin film sticks to skin much like a temporary tattoo, and early versions of the flexible electronics were made to contain heart and brain activity monitors and muscle stimulators. While these types of devices are on their way to commercialization and large-scale manufacturing, there are some arenas in which they’re not well suited, such as when direct modification of a surface by adding custom electronics is needed.

“For direct or additive printing to ever really be useful, you’re going to need to be able to print the entirety of whatever you’re printing in one step,” said Franklin. “Some of the more exotic applications include intimately connected electronic tattoos that could be used for biological tagging or unique detection mechanisms, rapid prototyping for on-the-fly custom electronics, and paper-based diagnostics that could be integrated readily into customized bandages.”

In the July paper, Franklin’s lab and the laboratory of Benjamin Wiley, professor of chemistry at Duke, developed a novel ink containing silver nanowires that can be printed onto any substrate at low temperatures with an aerosol printer. It yields a thin film that maintains its conductivity without any further processing. After being printed, the ink is dry in less than two minutes and retains its high electrical performance even after enduring a 50% bending strain more than a thousand times.

In a video accompanying the first paper, graduate student Nick Williams prints two electronically active leads along the underside of his pinky finger. Toward the end of his finger, he connects the leads to a small LED light. He then applies a voltage to the bottom of the two printed leads, causing the LED to stay lit even as he bends and moves the finger.

In the second paper, Franklin and graduate student Shiheng Lu take the conductive ink a step further and combine it with two other printable components to create functional transistors. The printer first puts down a semiconducting strip of carbon nanotubes. Once it dries, and without removing the plastic or paper substrate from the printer, two silver nanowire leads that extend several centimeters from either side are printed. A non-conducting dielectric layer of a two-dimensional material, hexagonal boron nitride, is then printed on top of the original semiconductor strip, followed by a final silver nanowire gate electrode.

With today’s technologies, at least one of these steps would require the substrate to be removed for additional processing, such as a chemical bath to rinse away unwanted material, a hardening process to ensure layers don’t mix, or an extended bake to remove traces of organic material that can interfere with electric fields.

But Franklin’s print-in-place requires none of these steps and, despite the need for each layer to dry completely to avoid mixing materials, can be completed at the lowest overall processing temperature reported to date.

“Nobody thought the aerosolized ink, especially for boron nitride, would deliver the properties needed to make functional electronics without being baked for at least an hour and a half,” said Franklin. “But not only did we get it to work, we showed that baking it for two hours after printing doesn’t improve its performance. It was as good as it could get just using our fully print-in-place process.”

Franklin doesn’t see his printing method replacing large-scale manufacturing processes for wearable electronics. But he does see a potential value for applications such as rapid prototyping or situations where one size doesn’t fit all.

“Think about creating bespoke bandages that contain electronics like biosensors, where a nurse could just walk over to a workstation and punch in what features were needed for a specific patient,” said Franklin. “This is the type of print-on-demand capability that could help drive that.”

This work was supported by the Department of Defense Congressionally Directed Medical Research Program, the National Institutes of Health, and the National Science Foundation.

Graphene’s progress at the age of 15
Graphene is light, flexible, conductive, and one of the strongest materials in the world. And it is right on track to deliver on its promises – the Graphene Flagship is confident many applications will be unveiled in the next decade. In a special Nature Nanotechnology issue, celebrating 15 years since the Nobel Prize-winning “ground-breaking experiments on graphene,” the Graphene Flagship analyzed the current graphene landscape and market forecast for graphene over the following decade.

In a world dominated by the immediacy of social media and digital technologies, it is hard to take a step back and think about how long materials take to develop. The silicon transistor, at the heart of all our beloved gadgets, was engineered in 1958. However, scientists had known of silicon for over 120 years – it was discovered in 1824. Although expecting broad market penetration for graphene today would not be realistic, the truth is that one can already find graphene-enabled products on the market.

A number of these commercial applications have been enabled by the Graphene Flagship, a project funded by the European Commission that kicked off in 2013. Bringing together nearly 150 partners from 23 countries, it created the perfect breeding ground for innovation, which could not emerge without an intricate web of collaborations between academics, researchers, and industries. The Graphene Flagship also acted as inspiration for many programs on graphene and related layered materials in many other countries.

The Graphene Flagship expects short-term applications in the materials sector, with graphene-enabled inks, composites, and coatings, for applications ranging from food packaging to textiles and sports goods. In the mid-term, graphene could be crucial for the energy sector, and market analyses agree on a high potential for graphene-enabled batteries and supercapacitors. With the first graphene-enabled solar farm to be installed in Crete next year, the Graphene Flagship will showcase how graphene can enable more sustainable energy generation, in line with Europe’s commitment to renewable energies.

A host of applications for graphene are expected to hit the market 10 to 15 years from now. These are related to optoelectronics, where graphene can deliver performances orders of magnitude higher than current technologies. The developments in this area could trigger the next-generation of optoelectronic devices, bringing the ‘more-than-Moore’ devices to reality.

To secure its most valuable strength – bridging the gap between basic and applied research – the Graphene Flagship has also announced the creation of the first graphene foundry. With a budget of almost €20 million (nearly $22 million) over four years, this experimental pilot line will pave the way towards commercially competitive graphene products, such as transceivers, photodetectors, and sensors. The Graphene Flagship foundry will also develop a process design kit: a set of ‘instructions’ to support product tape-out and guarantee that the finalized designs are high-quality and consistent. The foundry will be accessible by academia and industry stakeholders worldwide.

Kari Hjelt, Graphene Flagship Head of Innovation, stated: “We are now seeing the first wave of graphene-enabled products on the market. The commercialization activities of graphene are moving from materials development towards components and system level integration. In the future we will see a growing number of high-value add products for various application domains.”

Thomas Reiss, Graphene Flagship Work Package Leader for Industrialization, adds: “Key factors facilitating the further commercialization of graphene comprise establishing innovation ecosystems and providing holistic innovation support. This includes elaborating innovation roadmaps and creating trust and confidence in graphene among industry by trusted validation and standardization services.”

Andrea C. Ferrari, Graphene Flagship Science and Technology Officer and Chair of its Management Panel, concludes: “Graphene and related materials are progressing towards commercialization at the expected pace. The Graphene Flagship is not about hype, but about concrete and tangible results and progress. The Flagship Foundry will strengthen the EU position as world leader and pioneer in graphene technology and facilitate incorporation of graphene devices in various industries.”