Posted: Aug 27, 2017

Conformal metasurface coating eliminates crosstalk and shrinks waveguides (Nanowerk News) The properties of materials can behave in funny ways. Tweak one aspect to make a device smaller or less leaky, for example, and something else might change in an undesirable way, so that engineers play a game of balancing one characteristic against another. Now a team of Penn State electrical engineers have a way to simultaneously control diverse optical properties of dielectric waveguides by using a two-layer coating, each layer with a near zero thickness and weight.

„Imagine the water faucet in your home, which is an essential every-day device,” said Douglas H. Werner, John L. and Genevieve H. McCain Chair Professor of Electrical Engineering. „Without pipes to carry the water from its source to the faucet, the device is worthless. It is the same with ‘waveguides.’ They carry electromagnetic or optical signals from the source to the device – an antenna or other microwave, millimeter-wave or terahertz device. Waveguides are an essential component in any electromagnetic or optical system, but they are often overlooked because much of the focus has been on the devices themselves and not the waveguides.”

According to Zhi Hao Jiang, former postdoctoral fellow at Penn State and now a professor at Southeast University, Nanjing, China, metasurface coatings allow researchers to shrink the diameter of waveguides and control the waveguiding characteristics with unprecedented flexibility.

The researchers developed a material that is so thin it is almost 2-dimensional, with characteristics that manipulate and enhance properties of the waveguide.

Rod shaped waveguide with two quasi-two dimensional conformal coatings that sheild the waveguide from crosstalk and blocking and allow the waveguide to be smaller. (Image: Werner Lab, Penn State)

They developed and tested two conformal coatings, one for guiding the signal and one to cloak the waveguide. They created the coatings by judiciously engineering the patterning on the surfaces to enable new and transformative waveguide functionality. The coatings are applied to a rod-shaped, Teflon waveguide with the guiding layer touching the Teflon and the cloaking layer on the outside.

This quasi 2-dimensional conformal coating that is configured as a cloaking material can solve the crosstalk and blockage problem. Dielectric waveguides are not usually used singly, but in bundles. Unfortunately, conventional waveguides leak, allowing the signal from one waveguide to interfere with those located nearby.

The researchers also note in their paper in Nature Communications („Conformal metasurface-coated dielectric waveguides for highly confined broadband optical activity with simultaneous low-visibility and reduced crosstalk”) that „the effectiveness of the artificial coating can be well maintained for waveguide bends by properly matching the dispersion properties of the metasurface unit cells.” Although the coating can be applied to a bend in the waveguide, the waveguide cannot be bent after the coating is applied.

Improving the properties of the waveguide to carefully control polarization and other attributes allows the waveguides to be smaller, and alleviating crosstalk allows these smaller waveguides to be more closely bundled. Smaller waveguides more closely bundled could lead to increased miniaturization.

„In terms of applications these would include millimeter-wave/terahertz/infrared systems for sensing, communications, and imaging that need to manipulate polarization, squeeze signals through waveguides with a smaller cross-section, and/or require dense deployment of interconnected components,” said Jiang.

Read more: Conformal metasurface coating eliminates crosstalk and shrinks waveguides

Posted: Aug 25, 2017

No batteries required: Energy-harvesting nanotechnology yarns generate electricity (Nanowerk News) An international research team led by scientists at The University of Texas at Dallas and Hanyang University in South Korea has developed high-tech yarns that generate electricity when they are stretched or twisted.

In a study published in the Aug. 25 issue of the journal Science („Harvesting electrical energy from carbon nanotube yarn twist”), researchers describe „twistron” yarns and their possible applications, such as harvesting energy from the motion of ocean waves or from temperature fluctuations. When sewn into a shirt, these yarns served as a self-powered breathing monitor.

Coiled carbon nanotube yarns, created at the University of Texas at Dallas and imaged here with a scanning electron microscope, generate electrical energy when stretched or twisted. (Image: University of Texas at Dallas)

„The easiest way to think of twistron harvesters is, you have a piece of yarn, you stretch it, and out comes electricity,” said Dr. Carter Haines, associate research professor in the Alan G. MacDiarmid NanoTech Institute at UT Dallas and co-lead author of the article. The article also includes researchers from South Korea, Virginia Tech, Wright-Patterson Air Force Base and China.

Yarns Based on Nanotechnology

The yarns are constructed from carbon nanotubes, which are hollow cylinders of carbon 10,000 times smaller in diameter than a human hair. The researchers first twist-spun the nanotubes into high-strength, lightweight yarns. To make the yarns highly elastic, they introduced so much twist that the yarns coiled like an over-twisted rubber band.

In order to generate electricity, the yarns must be either submerged in or coated with an ionically conducting material, or electrolyte, which can be as simple as a mixture of ordinary table salt and water.

„Fundamentally, these yarns are supercapacitors,” said Dr. Na Li, a research scientist at the NanoTech Institute and co-lead author of the study. „In a normal capacitor, you use energy – like from a battery – to add charges to the capacitor. But in our case, when you insert the carbon nanotube yarn into an electrolyte bath, the yarns are charged by the electrolyte itself. No external battery, or voltage, is needed.”

When a harvester yarn is twisted or stretched, the volume of the carbon nanotube yarn decreases, bringing the electric charges on the yarn closer together and increasing their energy, Haines said. This increases the voltage associated with the charge stored in the yarn, enabling the harvesting of electricity.

Stretching the coiled twistron yarns 30 times a second generated 250 watts per kilogram of peak electrical power when normalized to the harvester’s weight, said Dr. Ray Baughman, director of the NanoTech Institute and a corresponding author of the study.

„Although numerous alternative harvesters have been investigated for many decades, no other reported harvester provides such high electrical power or energy output per cycle as ours for stretching rates between a few cycles per second and 600 cycles per second.”

Lab Tests Show Potential Applications

In the lab, the researchers showed that a twistron yarn weighing less than a housefly could power a small LED, which lit up each time the yarn was stretched.

To show that twistrons can harvest waste thermal energy from the environment, Li connected a twistron yarn to a polymer artificial muscle that contracts and expands when heated and cooled. The twistron harvester converted the mechanical energy generated by the polymer muscle to electrical energy.

„There is a lot of interest in using waste energy to power the Internet of Things, such as arrays of distributed sensors,” Li said. „Twistron technology might be exploited for such applications where changing batteries is impractical.”

The researchers also sewed twistron harvesters into a shirt. Normal breathing stretched the yarn and generated an electrical signal, demonstrating its potential as a self-powered respiration sensor.

„Electronic textiles are of major commercial interest, but how are you going to power them?” Baughman said. „Harvesting electrical energy from human motion is one strategy for eliminating the need for batteries. Our yarns produced over a hundred times higher electrical power per weight when stretched compared to other weavable fibers reported in the literature.”

Electricity from Ocean Waves

„In the lab we showed that our energy harvesters worked using a solution of table salt as the electrolyte,” said Baughman, who holds the Robert A. Welch Distinguished Chair in Chemistry in the School of Natural Sciences and Mathematics. „But we wanted to show that they would also work in ocean water, which is chemically more complex.”

In a proof-of-concept demonstration, co-lead author Dr. Shi Hyeong Kim, a postdoctoral researcher at the NanoTech Institute, waded into the frigid surf off the east coast of South Korea to deploy a coiled twistron in the sea. He attached a 10 centimeter-long yarn, weighing only 1 milligram (about the weight of a mosquito), between a balloon and a sinker that rested on the seabed.

Every time an ocean wave arrived, the balloon would rise, stretching the yarn up to 25 percent, thereby generating measured electricity.

Even though the investigators used very small amounts of twistron yarn in the current study, they have shown that harvester performance is scalable, both by increasing twistron diameter and by operating many yarns in parallel.

„If our twistron harvesters could be made less expensively, they might ultimately be able to harvest the enormous amount of energy available from ocean waves,” Baughman said. „However, at present these harvesters are most suitable for powering sensors and sensor communications. Based on demonstrated average power output, just 31 milligrams of carbon nanotube yarn harvester could provide the electrical energy needed to transmit a 2-kilobyte packet of data over a 100-meter radius every 10 seconds for the Internet of Things.”

Read more: No batteries required: Energy-harvesting nanotechnology yarns generate electricity

Posted: Aug 23, 2017

2D van der Waals heterostructures with tunable optical-electronic-magnetic coupling

(Nanowerk Spotlight) Ever since the first demonstration of graphene in 2004, the scientific community has pursued the possibility of building man-made materials by artificial stacking of different ultrathin materials one on top of the other. These materials are called van der Waals heterostructures.Due to their unique interlayer coupling and optoelectronic properties, these materials are of considerable interest for the next generation nanoelectronics.Conventional 2D heterostructures usually are composed of two layers of opposite charge carrier type using inorganic materials. One of the challenges when creating 2D heterostructures is the painstaking stacking of the individual components on top of each other.Researchers have now found, for the first time, that there can also be charge transfer (CT) induced interfacial coupling between two different pairs of organic CT layers.Moreover, external stimuli can be harnessed to tune the interfacial coupling to control the physical properties of organic van der Waals heterostructures (vdWHs).”The interface of heterostructures can provide many possibilities to generate new phenomena,” Shenqiang Ren, an Associate Professor in the Mechanical Engineering at Temple University, tells Nanowerk. „Our group has years of investigation on organic CT complexes. As the interface between two layers of organic CT complex is different from single phase, such as intra-layer and inter-layer CT interactions, it is exciting to investigate the interfacial coupling within organic van der Waals heterostructures.”The most significant finding in this work, just published in Nature Communications („Tunable two-dimensional interfacial coupling in molecular heterostructures”), is that the researchers found strong and anisotropic interfacial charge transfer coupling in two-dimensional organic van der Waals heterostructures.Organic van der Waals heterostructures exhibit multi-stimuli controlled physical properties. (Image: Dr. B Xu)As the team (a collaboration between Ren’s group and Prof. Michael L Klein’s High-Performance Computing group at Temple University) points out, such interfacial CT coupling enables external stimuli controlled physical properties in organic vdWHs.To prepare 2D vdWH, researchers widely use a conventional chemical vapor deposition approach. However, the weak interlayer interaction between the monolayer 2D film and the substrate leads to island growth rather than continuous monolayers.The approach demonstrated by the Temple University team allows for large-scale (mm2 range) assembly of free-standing 2D CT heterostructures with controlled orientation and unique physicochemical properties. As they note, the interfacial coupling can be tuned in over a large range by external fields (ferroelectric and magnetic) with strong enhancement of current and capacitance, due to the coupling across two CT pairs along the vertical and horizontal orientations.”As our vdWHs show multifunctional external stimuli dependent response (such as piezoresistance, electroresistive and magnetoconductance), they can be applied for multifunctional sensors,” says Ren. „For example, the vdWHs exhibit excellent pressure dependent sensitivity with a high piezoresistance coefficient of -4.4×10-6 Pa-1.””The organic vdW heterostructures formed by the combination of two pairs of organic CT complex with tunable interfacial coupling show overwhelming advantages over single component,” he adds. „They exhibit unique physicochemical properties covering vast majority of opto-electrical-magnetic-mechanical topics.”The researchers’ next steps will be focused on the synthesis of wafer-scale two-dimensional vdWHs towards artificial intelligence applications.Since the organic vdWHs demonstrate a fast response to external stimuli with the change of conductivity, this opens up the practical applications for multi-stimuli sensors.By Michael Berger – Michael is author of two books by the Royal Society of Chemistry: Nano-Society: Pushing the Boundaries of Technology and Nanotechnology: The Future is Tiny. Copyright © Nanowerk

Read more: 2D van der Waals heterostructures with tunable optical-electronic-magnetic coupling

Posted: Aug 22, 2017

Stretchable biofuel cells extract energy from sweat to power wearable devices (Nanowerk News) A team of engineers has developed stretchable fuel cells that extract energy from sweat and are capable of powering electronics, such as LEDs and Bluetooth radios. The biofuel cells generate 10 times more power per surface area than any existing wearable biofuel cells. The devices could be used to power a range of wearable devices.

The epidermal biofuel cells are a major breakthrough in the field, which has been struggling with making the devices that are stretchable enough and powerful enough. Engineers from the University of California San Diego were able to achieve this breakthrough thanks to a combination of clever chemistry, advanced materials and electronic interfaces. This allowed them to build a stretchable electronic foundation by using lithography and by using screen-printing to make 3D carbon nanotube-based cathode and anode arrays.

The biofuel cells are equipped with an enzyme that oxidizes the lactic acid present in human sweat to generate current. This turns the sweat into a source of power.

The biofuel cell can stretch and flex, conforming to the human body. (Image: University of California San Diego)

Engineers report their results in the June issue of Energy & Environmental Science („Soft, stretchable, high power density electronic skin-based biofuel cells for scavenging energy from human sweat „). In the paper, they describe how they connected the biofuel cells to a custom-made circuit board and demonstrated the device was able to power an LED while a person wearing it exercised on a stationary bike. Professor Joseph Wang, who directs the Center for Wearable Sensors at UC San Diego, led the research, in collaboration with electrical engineering professor and center co-director Patrick Mercier and nanoegnineering professor Sheng Xu, both also at the Jacobs School of Engineering UC San Diego.

Islands and bridges

To be compatible with wearable devices, the biofuel cell needs to be flexible and stretchable. So engineers decided to use what they call a „bridge and island” structure developed in Xu’s research group. Essentially, the cell is made up of rows of dots that are each connected by spring-shaped structures. Half of the dots make up the cell’s anode; the other half are the cathode. The spring-like structures can stretch and bend, making the cell flexible without deforming the anode and cathode.

The basis for the islands and bridges structure was manufactured via lithography and is made of gold. As a second step, researchers used screen printing to deposit layers of biofuel materials on top of the anode and cathode dots.

Increasing energy density

The researchers’ biggest challenge was increasing the biofuel cell’s energy density, meaning the amount of energy it can generate per surface area. Increasing energy density is key to increasing performance for the biofuel cells. The more energy the cells can generate, the more powerful they can be.

„We needed to figure out the best combination of materials to use and in what ratio to use them,” said Amay Bandodkar, one of the paper’s first authors, who was then a Ph.D. student in Wang’s research group. He is now a postdoctoral researcher at Northwestern University.

To increase power density, engineers screen printed a 3D carbon nanotube structure on top the anodes and cathodes. The structure allows engineers to load each anodic dot with more of the enzyme that reacts to lactic acid and silver oxide at the cathode dots. In addition, the tubes allow easier electron transfer, which improves biofuel cell performance.

Testing applications

The biofuel cell was connected to a custom-made circuit board manufactured in Mercier’s research group. The board is a DC/DC converter that evens out the power generated by the fuel cells, which fluctuates with the amount of sweat produced by a user, and turns it into constant power with a constant voltage.

Researchers equipped four subjects with the biofuel cell-board combination and had them exercise on a stationary bike. The subjects were able to power a blue LED for about four minutes.

Next steps

Future work is needed in two areas. First, the silver oxide used at the cathode is light sensitive and degrades over time. In the long run, researchers will need to find a more stable material.

Also, the concentration of lactic acid in a person’s sweat gets diluted over time. That is why subjects were able to light up an LED for only four minutes while biking. The team is exploring a way to store the energy produced while the concentration of lactate is high enough and then release it gradually.

Read more: Stretchable biofuel cells extract energy from sweat to power wearable devices

Posted: Aug 22, 2017

A traffic cop for molecules (Nanowerk News) Whether making a plastic coating or fuel, saving energy reduces costs. A vital target for cost savings is separating chemicals that are slightly different from each other. Nearly every processing industry must separate chemicals.

Scientists created a rigid material with ultra-small pores with diameter less than 7Å, (less than one-millionth the diameter of a human hair) that are ultra selective based on size. The pores enable scientists to design films that can differentiate between similarly sized molecules based on small differences in a molecule’s shape.

Membranes are separation agents. Scalable, efficient membranes reduce the cost and the environmental impact of producing vital commodities. How? The best membranes use a fraction of the energy of traditional approaches.

This study (Nature Materials, „Materials for next-generation molecularly selective synthetic membranes”) shows how a material with ultra small pores can be scaled to separate large quantities of molecules effectively by allowing an efficient sorting process based on size.

Carbon molecular sieves (CMS) with rigid ultra small pores (pores with diameter less than 7Å) and tailorable size and shape selectivity can enable the membrane-based separation of gas and liquid organic molecules.

Materials with rigid ultra small micropores provide selectivity that controls the motion of individual molecules to improve separation between similarly sized molecules by making it difficult for the “chubby ones” to get through.

Next-generation membranes based on these advanced materials overcome the deficiencies of conventional polymers to separate alkanes and alkenes, natural gases, and isomers. Additionally, these next-generation membranes will achieve solute-solute discrimination in organic solvents, which was also a problem for conventional methods.

Mass-producing molecular-scale separators must be economical. The key is to use hollow fibers with selective thin layers using an advanced version of the process to make textile fibers like those used to make inexpensive clothes.

The advanced version of this technology relies upon hundreds of “spinnerets” to produce the fibers in parallel and then collect them into a holder. When the fibers are extruded and contact air, a skin forms—like it does when glue is in contact with air. Because these advanced materials are scalable, scientists can create molecularly selective membranes that could provide large payoffs in reducing carbon dioxide emissions and energy consumption for a more sustainable future.

Read more: A traffic cop for molecules

Read more: Smart label could let you know when to toss food and cosmetics