A luminescent tag, contactless printed onto a plastic foil. The light emitting layer is thinner than a human hair. The imprint can be erased and replaced by another pattern.

Credit: M. Gmelch and H. Thomas, TU Dresden

A luminescent tag, contactless printed onto a plastic foil. The light emitting layer is thinner than a human hair. The imprint can be erased and replaced by another pattern.

Credit: M. Gmelch and H. Thomas, TU Dresden

Close

Prof. Reineke and his LEXOS team work with simple plastic foils with a thickness of less than 50 ?m, which is thinner than a human hair. In these transparent plastic foils, they introduce organic luminescent molecules. In the beginning, these molecules are in an inactive, dark state. By locally using ultraviolet irradiation, it is possible to turn this dark state into an active, luminescent one. By mask illumination or laser writing, activated patterns can be printed onto the foil with a resolution that is comparable to common laser printers. Similar to glow-in-the-dark stickers, the patterns can be brought to shine and the imprinted information can be read. By illuminating with infrared light, the tag is erased completely and new data can be written into it.

Carbon fibers, already known as a high-performing engineering material, are widely used in the aerospace and automotive industries. One application is the shells of luxury cars like Mercedes-Benz, BMW, or Lamborghini.

Carbon fibers, thin hair-like strands of carbon, possess multiple prime material properties: they are mechanically strong, chemically resistant, electrically conductive, fire retardant, and perhaps most importantly, lightweight. The weight of carbon fibers improves fuel and energy efficiency, producing faster jets and vehicles.

Designing materials for structure and function

Guoliang “Greg” Liu, an assistant professor in the Department of Chemistry, conceived the idea of creating carbon fibers that wouldn’t only be structurally useful; they would also be functionally useful.

“What if we can design them to have functionality, such as energy storage?” said Liu, also a member of the Macromolecules Innovation Institute. “If you want them to store energy, you need to have sites to put ions in.”

Liu said ideally the carbon fibers could be designed to have micro-holes uniformly scattered throughout, similar to a sponge, that would store ions of energy.

After tweaking a longtime conventional method of chemically producing carbon fibers, Liu now has developed a process to synthesize porous carbon fibers for the first time with uniform size and spacing. He details this work in a recently published article in the high impact journal Science Advances.

“Making porous carbon fibers is not easy,” Liu said. “People have tried this for decades. But the quality and the uniformity of the pores in the carbon fibers were not satisfactory.

“We designed, synthesized, and then processed these polymers in the lab, and then we made them into porous carbon fibers.”

Using block copolymers to create porous carbon fibers

Liu used a multistep chemical process using two polymers — long, repeating chains of molecules — called polyacrylonitrile (PAN) and poly(acrylonitrile-block-methyl methacrylate) (PMMA).

PAN is well-known in the polymer chemistry field as a precursor compound to carbon fibers, and PMMA acts as a place-holding material that is later removed to create the pores.

But in the past, other chemists had typically mixed PAN and PMMA separately into a solution. This created porous carbon fibers but with differently sized and spaced pores. Energy storage can be maximized with greater surface area, which occurs with smaller, uniform pores.

Liu came up with the new idea of bonding PAN and PMMA, creating what is known as a block copolymer. One half of the compound polymer is PAN, and the other half is PMMA, and they’re covalently bonded in the middle.

“This is the first time we utilize block copolymers to make carbon fibers and the first time to use block copolymer-based porous carbon fibers in energy storage,” Liu said. “Often, we’re only thinking from the process point-of-view, but here we’re thinking from the materials design point-of-view.”

After synthesizing the block copolymer in the lab, the viscous solution then underwent three chemical processes to achieve porous carbon fibers.

The first step is electrospinning, a method that uses electric force to create fibrous strands and harden the solution into a paper-like material. Next, Liu put the polymer through an oxidation heating process. In this step, the PAN and PMMA naturally separated and self-assembled into the strands of PAN and uniformly scattered domains of PMMA.

In the final step, known as pyrolysis, Liu heated the polymer to an even higher temperature. This process solidified PAN into carbon and removed PMMA, leaving behind interconnected mesopores and micropores throughout the fiber.

New possibilities in energy storage

Although this breakthrough improves an already high-performing engineering material, perhaps the greater breakthrough is the ability to use block copolymers to create uniform porous structures for energy storage possibilities.

“It opens the way we think about designing materials for energy storage,” Liu said. “Now we can also start to think about functionality. We not only use (carbon fibers) as a structural material but also a functional material.”

Liu had been toying with this idea since he joined Virginia Tech in 2014, but he started formal research on this idea after submitting a winning proposal through the Air Force Young Investigator Program (YIP) in 2016.

The first author of the paper is Zhengping Zhou, Liu’s former postdoctoral associate who now works as an assistant professor at North Dakota State University. Also involved in the research were Tianyu Liu, another postdoctoral associate in the Liu Lab, and Assad Khan, a fifth-year doctoral student in the Department of Chemistry.

In a paper published Friday, Feb. 1, in Science Advances, they outlined advances in creating stretchable rubbery semiconductors, including rubbery integrated electronics, logic circuits and arrayed sensory skins fully based on rubber materials.

Cunjiang Yu, Bill D. Cook Assistant Professor of mechanical engineering at the University of Houston and corresponding author on the paper, said the work could lead to important advances in smart devices such as robotic skins, implantable bioelectronics and human-machine interfaces.

Yu previously reported a breakthrough in semiconductors with instilled mechanical stretchability, much like a rubber band, in 2017.

This work, he said, takes the concept further with improved carrier mobility and integrated electronics.

“We report fully rubbery integrated electronics from a rubbery semiconductor with a high effective mobility … obtained by introducing metallic carbon nanotubes into a rubbery semiconductor with organic semiconductor nanofibrils percolated,” the researchers wrote. “This enhancement in carrier mobility is enabled by providing fast paths and, therefore, a shortened carrier transport distance.”

Carrier mobility, or the speed at which electrons can move through a material, is critical for an electronic device to work successfully, because it governs the ability of the semiconductor transistors to amplify the current.

Previous stretchable semiconductors have been hampered by low carrier mobility, along with complex fabrication requirements. For this work, the researchers discovered that adding minute amounts of metallic carbon nanotubes to the rubbery semiconductor of P3HT — polydimethylsiloxane composite — leads to improved carrier mobility by providing what Yu described as “a highway” to speed up the carrier transport across the semiconductor.

In addition to Yu, the paper’s researchers include first author Kyoseung Sim, and co-authors Zhoulyu Rao, Anish Thukral and Hyunseok Shim, all of UH, and Hae-Jin Kim, a former postdoctoral researcher at UH who is now with Gyeongsang National University in Jinju, Korea.

Future work, Yu said, will involve further raising the carrier mobility and building more complex, hierarchy and high level integrated digital circuits to meet the requirements for integrated circuits, biomedical and other applications.

Story Source:

Materials provided by University of Houston. Original written by Jeannie Kever. Note: Content may be edited for style and length.

In practice, researchers go to great lengths to build highly coherent, near-single-frequency lasers for high-end systems such as atomic clocks. Today, however, because these lasers are large and occupy racks full of equipment, they are relegated to applications based on bench tops in the laboratory.

There is a push to move the performance of high-end lasers onto photonic micro-chips, dramatically reducing cost and size while making the technology available to a wide range of applications including spectroscopy, navigation, quantum computation and optical communications. Achieving such performance at the chip scale would also go a long way to address the challenge posed by the internet’s exploding data-capacity requirements and the resulting increase in worldwide energy consumption of data centers and their fiber-optic interconnects.

In the cover article of the January 2019 issue of Nature Photonics, researchers at UC Santa Barbara and their collaborators at Honeywell, Yale and Northern Arizona University, describe a significant milestone in this pursuit: a chip-scale laser capable of emitting light with a fundamental linewidth of less than 1 Hz — quiet enough to move demanding scientific applications to the chip scale. The project was funded under the Defense Advanced Research Project Agency’s (DARPA) OwlG initiative.

To be impactful, these low-linewidth lasers must be incorporated into photonic integrated circuits (PICs) — the equivalents of computer micro-chips for light — that can be fabricated at wafer-scale in commercial micro-chip foundries. “To date, there hasn’t been a method for making a quiet laser with this level of coherence and narrow linewidth at the photonic-chip scale,” said co-author and team lead Dan Blumenthal, a professor in the Department of Electrical and Computer Engineering at UC Santa Barbara. The current generation of chip-scale lasers are inherently noisy and have relatively large linewidth. New innovations have been needed that function within the fundamental physics associated with miniaturizing these high-quality lasers.

Specifically, DARPA was interested in creating a chip-scale laser optical gyroscope. Important for its ability to maintain knowledge of position without GPS, optical gyroscopes are used for precision positioning and navigation, including in most commercial airliners.

The laser optical gyroscope has a length-scale sensitivity on par with that of the gravitational wave detector, one of the most precise measuring instruments ever made. But current systems that achieve this sensitivity incorporate bulky coils of optical fiber. The goal of the OwlG project was to realize an ultra-quiet (narrow-linewidth) laser on the chip to replace the fiber as the rotation-sensing element and allow further integration with other components of the optical gyroscope.

According to Blumenthal, there are two possible ways to build such a laser. One is to tether a laser to an optical reference that must be environmentally isolated and contained in a vacuum, as is done today with atomic clocks. The reference cavity plus an electronic feedback loop together act as an anchor to quiet the laser. Such systems, however, are large, costly, power-consuming and sensitive to environment disturbances.

The other approach is to make an external-cavity laser whose cavity satisfies the fundamental physical requirements for a narrow linewidth laser, including the ability to hold billions of photons for a long time and support very high internal optical power levels. Traditionally, such cavities are large (to hold enough photons), and although they have been used to achieve high performance, integrating them on-chip with linewidths approaching those of lasers stabilized by reference cavities has proved elusive.

To overcome these limitations, the research team leveraged a physical phenomenon known as stimulated Brillouin scattering to build the lasers.

“Our approach uses this process of light-matter interaction in which the light actually produces sound, or acoustic, waves inside a material,” Blumenthal noted. “Brillouin lasers are well known for producing extremely quiet light. They do so by utilizing photons from a noisy ‘pump’ laser to produce acoustic waves, which, in turn, act as cushions to produce new quiet, low-linewidth output light. The Brillouin process is highly effective, reducing the linewidth of an input pump laser by a factor of up to a million.”

The drawback is that bulky optical fiber setups or miniature optical resonators traditionally used to make Brillouin lasers are sensitive to environmental conditions and difficult to fabricate using chip-foundry methods.

“The key to making our sub-Hz Brillouin laser on a photonic integrated chip was to use a technology developed at UC Santa Barbara — photonic integrated circuits built with waveguides that are extremely low loss, on par with the optical fiber,” Blumenthal explained. “These low-loss waveguides, formed into a Brillouin laser ring cavity on the chip, have all the right ingredients for success: They can store an extremely large number of photons on the chip, handle extremely high levels of optical power inside the optical cavity and guide photons along the waveguide much as a rail guides a monorail train.”

A combination of low-loss optical waveguides and rapidly decaying acoustic waves removes the need to guide the acoustic waves. This innovation is key to the success of this approach.

Since being completed, this research has led to multiple new funded projects both in Blumenthal’s group and in those of his collaborators.

Researchers expected to find some gaps between ice and bedrock at Thwaites’ bottom where ocean water could flow in and melt the glacier from below. The size and explosive growth rate of the newfound hole, however, surprised them. It’s big enough to have contained 14 billion tons of ice, and most of that ice melted over the last three years.

“We have suspected for years that Thwaites was not tightly attached to the bedrock beneath it,” said Eric Rignot of the University of California, Irvine, and NASA’s Jet Propulsion Laboratory in Pasadena, California. Rignot is a co-author of the new study, which was published in Science Advances. “Thanks to a new generation of satellites, we can finally see the detail,” he said.

The cavity was revealed by ice-penetrating radar in NASA’s Operation IceBridge, an airborne campaign beginning in 2010 that studies connections between the polar regions and the global climate. The researchers also used data from a constellation of Italian and German spaceborne synthetic aperture radars. These very high-resolution data can be processed by a technique called radar interferometry to reveal how the ground surface below has moved between images.

“[The size of] a cavity under a glacier plays an important role in melting,” said the study’s lead author, Pietro Milillo of JPL. “As more heat and water get under the glacier, it melts faster.”

Numerical models of ice sheets use a fixed shape to represent a cavity under the ice, rather than allowing the cavity to change and grow. The new discovery implies that this limitation most likely causes those models to underestimate how fast Thwaites is losing ice.

About the size of Florida, Thwaites Glacier is currently responsible for approximately 4 percent of global sea level rise. It holds enough ice to raise the world ocean a little over 2 feet (65 centimeters) and backstops neighboring glaciers that would raise sea levels an additional 8 feet (2.4 meters) if all the ice were lost.

Thwaites is one of the hardest places to reach on Earth, but it is about to become better known than ever before. The U.S. National Science Foundation and British National Environmental Research Council are mounting a five-year field project to answer the most critical questions about its processes and features. The International Thwaites Glacier Collaboration will begin its field experiments in the Southern Hemisphere summer of 2019-20.

How Scientists Measure Ice Loss

There’s no way to monitor Antarctic glaciers from ground level over the long term. Instead, scientists use satellite or airborne instrument data to observe features that change as a glacier melts, such as its flow speed and surface height.

Another changing feature is a glacier’s grounding line — the place near the edge of the continent where it lifts off its bed and starts to float on seawater. Many Antarctic glaciers extend for miles beyond their grounding lines, floating out over the open ocean.

Just as a grounded boat can float again when the weight of its cargo is removed, a glacier that loses ice weight can float over land where it used to stick. When this happens, the grounding line retreats inland. That exposes more of a glacier’s underside to sea water, increasing the likelihood its melt rate will accelerate.

An Irregular Retreat

For Thwaites, “We are discovering different mechanisms of retreat,” Millilo said. Different processes at various parts of the 100-mile-long (160-kilometer-long) front of the glacier are putting the rates of grounding-line retreat and of ice loss out of sync.

The huge cavity is under the main trunk of the glacier on its western side — the side farther from the West Antarctic Peninsula. In this region, as the tide rises and falls, the grounding line retreats and advances across a zone of about 2 to 3 miles (3 to 5 kilometers). The glacier has been coming unstuck from a ridge in the bedrock at a steady rate of about 0.4 to 0.5 miles (0.6 to 0.8 kilometers) a year since 1992. Despite this stable rate of grounding-line retreat, the melt rate on this side of the glacier is extremely high.

“On the eastern side of the glacier, the grounding-line retreat proceeds through small channels, maybe a kilometer wide, like fingers reaching beneath the glacier to melt it from below,” Milillo said. In that region, the rate of grounding-line retreat doubled from about 0.4 miles (0.6 kilometers) a year from 1992 to 2011 to 0.8 miles (1.2 kilometers) a year from 2011 to 2017. Even with this accelerating retreat, however, melt rates on this side of the glacier are lower than on the western side.

These results highlight that ice-ocean interactions are more complex than previously understood.

Milillo hopes the new results will be useful for the International Thwaites Glacier Collaboration researchers as they prepare for their fieldwork. “Such data is essential for field parties to focus on areas where the action is, because the grounding line is retreating rapidly with complex spatial patterns,” he said.

“Understanding the details of how the ocean melts away this glacier is essential to project its impact on sea level rise in the coming decades,” Rignot said.

The paper by Milillo and his co-authors in the journal Science Advances is titled “Heterogeneous retreat and ice melt of Thwaites Glacier, West Antarctica.” Co-authors were from the University of California, Irvine; the German Aerospace Center in Munich, Germany; and the University Grenoble Alpes in Grenoble, France.

“A cup of coffee is the final product of a complex chain of operations: farming, post-harvest processing, roasting, and brewing,” said principal investigator Luc De Vuyst, M.Sc., Ph.D., Professor of Industrial Microbiology and Food Biotechnology, Vrije Universiteit Brussel, Brussels, Belgium. “There are several variants of post-harvest processing, among which wet processing and dry processing are the most common.” Wet processing — commonly used for Arabica and specialty coffees — is the step that includes fermentation.

“We carried out the research at an experimental farm in Ecuador through a multiphasic approach, encompassing microbiological, metabolomics, and sensory analysis,” said Dr. De Vuyst.

Fermentation was of particular importance. During extended fermentation, leuconostocs — a genus of lactic acid bacteria used in the fermentation of cabbage to sauerkraut and in sourdough starters — declined in favor of lactobacilli, said Dr. De Vuyst. Lactic acid bacteria were already present before fermentation, and these acid tolerant lactobacilli proliferated even more during this process.

However, it is challenging to draw a causal link between the microbiota and the volatile compounds in the beans — those compounds that contribute to the coffee’s smell — since many of these compounds can be of microbial, endogenous bean metabolism, or chemical origin,” said Dr. De Vuyst.

“However, we did see an impact of the microbial communities, in particular the lactic acid bacteria,” said Dr. De Vuyst. They yielded fruity notes, and may have “had a protective effect toward coffee quality during fermentation because of their acidification of the fermenting mass, providing a stable microbial environment and hence preventing growth of undesirable micro-organisms that often lead to off-flavors,” he said.

“Furthermore, there is a build-up of the fermentation-related metabolites onto the coffee beans, which affects the quality of the green coffee beans and hence the sensory quality of the coffees brewed therefrom,” said Dr. De Vuyst.

Dr. De Vuyst emphasized that how each stage of processing influences the taste of coffee remains mostly uncharted. “We were aware of many different micro-organisms during wet coffee fermentation — enterobacteria, lactic acid bacteria, yeasts, acetic acid bacteria, bacilli, and filamentous fungi,” said Dr. De Vuyst, but it is still unknown how most bacteria influence this process.

The work was a collaboration between the Vrije Universiteit Brussel, and the Nestlé Research Center. “Nestlé was interested in the fundamental aspects of coffee processing, in particular, the post-harvest processing chain, in order to correlate it with the roasting process and of course the final cup quality,” said Dr. De Vuyst.