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Compound 2-D Material Leads to a Practical Electronic Device

Researchers have found it effective to combine different two-dimensional (2D) materials to create compound materials that have properties that no single 2D material would have on its own.  For instance, hexagonal boron nitride with its enormous band gap—which gives it pseudo-insulator characteristics—has been combined with a pure conductor like graphene to create a semiconductor material for a host of applications, especially in flexible electronics.

Now researchers at the University of California, Riverside and the University of Georgia have taken this practice one step further and added a third 2D material to boron nitride and graphene: tantalum sulfide. The result is a compound material that the researchers used to make a voltage-controlled oscillator (VCO). These VCOs are ubiquitous, and are found in applications such as clocks, radios, and computers.

In research described in the journal Nature Nanotechnology, the researchers fabricated for the first time a practical device that exploits charge-density waves to modulate an electrical current through a 2D material. A charge-density wave is an ordered array of electrons in either a linear chain compound or layered crystal.

The VCO that the researchers made could be used as an ultralow power alternative to conventional devices that are now based on silicon. Because this VCO is flexible, it could be also used in wearables.

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Map of crystal facets in a perovskite photovoltaic.

Secret Hidden in Grains of Perovskite Could Boost Solar Cell Efficiency

The rise of the crystal perovskite as a potential replacement for silicon in photovoltaics has been impressive over the last decade, with its conversion efficiency improving from 3.8 to 22.1 percent over that time period. Nonetheless, there has been a vague sense that this rise is beginning to peter out of late, largely because when a solar cell made from perovskite gets larger than 1 square centimeter the best conversion efficiency had been around 15.6 percent. This figure has been improved recently in the work of Michael Grätzel to an average of 19.6 percent.  However, the bright prospects for perovskite have dimmed in eyes of some.

Now researchers at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) may have discovered something hidden in perovskite crystals that could boost the conversion efficiency of this material to as high as 31 percent.

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Microscope image of a carbon nanobrush

Nanobrushes: A New Carbon Material to Boost Batteries and Capacitors

NEC says it has produced a new addition to its portfolio of nanomaterial discoveries—the carbon nanobrush. The company says the new material will boost the performance of batteries, sensors, and capacitors among other devices.

The nanobrush is a fibrous aggregate of single-walled carbon nanohorns measuring several microns in length. These thimble-shaped nanohorns measure 2 to 5 nm in diameter, and between 40 to 50 nm in length.

Like nanohorns, which NEC discovered in 1998, nanobrushes have disperse well in water and allow other molecules to stick to their surface, which makes them suitable for use as conductive additives and compounds. Despite the similarities between the two materials, the nanobrush boasts more than 10 times the electrical conductivity of spherical aggregate of nanohorns of the same length.

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A red octopus floats underwater, with two rows of white suckers on the underside of each tentacle

Octopus-Inspired Smart Adhesive Boosts Flexible Electronics

Octopuses are widely admired for their intelligence, their skill as escape artists, and their extraordinary ability to camouflage themselves. Materials scientists are also taken by the adhesive properties of their tentacles. By controlling the pressure in their suckers, the cephalopods can rapidly grab onto and then release themselves from just about any surface they come across as they make their way along the ocean floor.

Now researchers led by materials scientist Hyunhyub Ko at the Ulsan National Institute of Science and Technology (UNIST) in South Korea have made octopus-inspired adhesives. Described online in Advanced Materialsthe adhesives can be used in the transfer printing of nanomaterials to make flexible electronics and compound-semiconductor devices on silicon: adhesive pads can pick-and-place a broad range of nanomaterials, including as indium gallium arsenide nanoribbons, and place them on a wafer or flexible substrate.

Nature has long offered many examples of adhesives that put chemical engineers to shame—including tree-frog toes and the gecko’s foot. Attempts to mimic them have yielded flurries of research papers. But in just about every case, engineers have trouble matching the natural adhesives’ high on/off strength ratio, says Ko, who came into this area through his work on electronic skin.

Flexible pressure sensors, circiuts, and other electronics might give future prosthetics and robots a better sense of touch. But they can be a pain to build. Making such flexible electronics involves a lot of laborious transferring of nano- and microribbons of inorganic semiconductor materials onto polymer or rubber sheets. In search of an easier way to do this transfer printing, Ko turned to the octopus for inspiration.

Each of the animal’s suckers contains a cavity whose pressure is controlled by surrounding muscles. When one set of muscles contracts, the wall of the sucker thins. This increases the volume of the cavity, reducing the pressure in the sucker relative to the surroundings and creating suction. When it's time to let go, other muscles contract to thicken the wall, increasing the pressure and releasing the sucker.

To mimic this approach, Ko made an array of microscale “suckers” using the rubbery material PDMS. Sheets of PDMS riddled with pores are coated with a thermally responsive polymer to create sucker-like walls. At ambient temperatures, the polymer is hydrophilic, swollen up with water. When a pad of the material is heated to 32 degrees Celsius, the polymer undergoes a phase transition, becoming hydrophobic. It shrinks dramatically, creating suction. The adhesive strength spikes from .32 kilopascals to 94 kilopascals at high temperature.

“This temperature control makes it very easy to transfer semiconductor nanoribbons,” says Ko. “We can transfer any nanomaterial to any substrate.” (Transfer printing is pretty much the only way to get high quality, inorganic semiconducting materials like silicon or gallium arsenide onto flexible or stretchy substrates. Typically, matching the material, the stamp, and the substrate can be tricky.)

So far Ko has used this universal smart adhesive to make indium gallium arsenide transistors on a silicon dioxide substrate. They’ve also transferred nanomaterials to flexible polymers. And they’ve made stacks of crossed silicon ribbons, and silicon criss-crossed on compound semiconductor ribbons—something that’s quite laborious to do with traditional adhesive stamps, which adhere weakly using van der Waals forces.

Ko says these adhesives could also be used as the substrate for wearable health sensors, eliminating the need for any extra glues. The adhesive is very sticky at body temperature, 37 degrees; it could be removed by rinsing under cold water—with no painful band-aid tear. 

An illustration styled to look like a semiconductor micrograph. A sheet of hexagons with an electrode on either side is stretched over a circular depression.

Resonating Drums Made Out of Graphene Could Lead to Better Sensors

Researchers at Cornell University have fabricated graphene membranes into micrometer-scale drums that can be stimulated by a voltage to transfer one type of mechanical vibration into another. The researchers believe this capability can lead to new applications in telecommunications where graphene-based mechanical resonators could be used as frequency mixers, which are often used, for example, to modulate carrier waves with signals.

In research described in the journal Nature Nanotechnology, the researchers found that graphene could produce three different frequencies of vibration in circular drums made of graphene. Each frequency, or mode, of vibration was coupled to the other and could exchange energy.

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Nanophotonic Crystals Separate Real Luxury Watches From the Fakes

Researchers at Ecole Polytechnique Fédérale de Lausanne (EPFL) in Switzerland have developed a technique for embedding nanoscale photonic crystals into the glass face of a watch so that when it is exposed to UV light, the design made by the crystals becomes visible. The team at Nanoga, a new startup that has been spun off to market this research, believes that this technology can be used to offer luxury watch makers an anti-counterfeiting method that is quick and easy to adopt.

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Nanowires Offer Real-Time Monitoring and Control of Heart Tissue

A key treatment for heart disease is a procedure known as myocardial resection, wherein a section of diseased or damaged heart tissue is removed. Researchers have been trying to figure out the best way to fill the void where the tissue has been removed. There has been research into creating 3-D scaffolds made from the patient’s own skin cells—structures on which the heart muscle can be regenerated. However, this approach doesn’t provide for a way to continuously monitor and control the development of these tissue patches.

Now, researchers out of Harvard University, led by Charles Lieber, may have found a way to create tissue scaffolds that can be controlled and monitored. The Harvard researchers added this capability by mimicking 3-D nanoelectronic arrays; the results show a way towards real-time mapping and control of electrophysiology in tissues.

In research described in the journal Nature Nanotechnology, Lieber and his team employed a bottom-up approach that started with the fabrication of doped p-type silicon nanowires. Lieber has been spearheading the use of silicon nanowires as a scaffold for growing nerve, heart, and muscle tissue for years now.

In this latest work, Lieber and his team fabricated the nanowires, applied them onto a polymer surface, and arranged them into a field-effect transistor (FET). The researchers avoided an increase in the device’s impedance as its dimensions were reduced by adopting this FET approach as opposed to simply configuring the device as an electrode. Each FET, along with its source-drain interconnects, created a 4-micrometer-by-20-micrometer-by-350-nanometer pad. Each of these pads was, in effect, a single recording device.

“Our latest work shows for the first time a nanoelectronics-enabled scaffold with both size scale and mechanical properties on the same order of magnitude as conventional cardiac tissue scaffold made of FDA-approved biomaterials,” said Lieber in an email interview with IEEE Spectrum. “Compared to previous work, we have reduced the device dimensions by 10x and [made it, mechanically] 1000x softer—which is critical for being matched to the tissue.”

Each tissue scaffold consists of four layers, each of which contains a four-by-four array of the pads described above, along with four circular palladium-platinum microelectrodes that provide electrical stimulation. Sandwiched between each of the four layers are polymer fibers that led to the final scaffold having bending stiffness on par with current technology.

“Our latest work demonstrates an array of more than 100 incorporated sensing devices (64 of which act as monitors), versus only 3 previously,” explained Lieber. “This larger-scale integration of active devices allows for true 3-D mapping of action potentials. Due to the fast response time of nanoelectronics, such 3-D activity mapping has sub-millisecond single-shot time resolution, which greatly exceeds capabilities of optical methods.”

In tests, the researchers put the scaffold into a petri dish containing ventricular cardiomyocyte cells taken from rats. Over the subsequent eight days, the scaffold was able to monitor the electrical activity of the cells.

“Thanks to the 3-D mapping capability, we are now able to use our nanoelectronics-innervated cardiac tissues to study critical problems, including 3-D tissue development, arrhythmia, and drug effectiveness,” said Lieber. “We have also achieved simultaneous mapping and regulation of cardiac activity by incorporating both sensor and stimulating devices, which allows us to ‘smartly’ control the abnormal cardiac activities with continuous feedback from the tissues.”

While the researchers were impressed that the scaffold layout they developed displayed only a 10 percent failure rate in the FET sensors over a two-week period, they concede that the silicon nanowires will eventually fail. In order to overcome this time limitation, the strategy would be to perform a metal passivation of the silicon nanowires in which a light coating of a an oxide is applied as a protective shell that would allow them to provide stable recordings for several months.

This work is best described as proof-of-principal research. However, it does offer huge potential impact in the fields of regenerative medicine and electronic therapeutics.

Lieber added: “The next steps towards implantation would be validation of the capabilities of the smart cardiac patch through implantation in animal models of myocardial infarction.”

Lithium-Sulfur Batteries Overcome Another Limitation: High Temperatures

Lithium-sulfur (Li-S) batteries have been pursued as an alternative to lithium-ion (Li-ion) batteries for powering electric vehicles due to their ability to hold up to four times as much energy per unit mass as Li-ion. However, Li-S batteries don’t come without some problems. For instance, the sulfur in the electrode can become depleted after just a few charge-discharge cycles, or polysulfides can pass through the cathode and foul the electrolyte.

Another issue Li-S batteries face is the difficulty of ensuring that they operate safely at high temperatures due to their low boiling and flash temperatures. Now, researchers at the University of Western Ontario, in collaboration with a team from the Canadian Light Source, have leveraged a relatively new coating technique dubbed molecular layer deposition (MLD) that promises to lead to safe and durable high-temperature Li-S batteries.

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Nanocones Boost Efficiency of Solar-based Water Splitting

The long list of attempts to use the sun to split water, and thus isolate hydrogen, has always had one big issue: energy conversion efficiency. Sure, solar-based water-splitting processes don’t yield carbon dioxide byproducts like those based on natural gas. But their conversion efficiencies are so notoriously low that many have never seen a way for them to make economic sense as a replacement for those based on natural gas. While headlines heralded tenfold increases in solar splitting efficiency, the result was only conversion efficiencies of 2.9 percent.

That is, until now. Researchers at Stanford University have made what they believe to be a significant step in realizing the kind of photovoltaic energy conversion efficiencies that will eventually make solar water-splitting economically competitive with natural gas processes. If they can realize their lofty ambitions for the technique, it could lead to the emission-free hydrogen economy that has for so long been promised.

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Flexible Nanogenerators Offer Dependable Energy Source for Flexible Electronics

Ever since 2012, when Zhong Lin Wang and his colleagues at Georgia Tech developed the first triboelectric nanogenerator (TENG), Wang and his team have been making continual progress in updating the technology so it can better deliver power to small electronic devices. TENGs essentially harvest static electricity from friction.

TENGs consist of two different materials that are rubbed together. In this way, materials that like to give off electrons, such as glass or nylon, will donate them to materials that like to absorb them, such as silicon or teflon. Since rubbing generally results in wear, what Wang and his Georgia Tech colleagues did back in 2012 was arrange the component materials so that they generate electricity when they are pressed together. By corrugating the contact surfaces of the materials, and by pressing them together, the structures enmesh, causing the friction that leads to electricity generation.

Such devices have impressive energy conversion efficiency, but they have thus far been limited to use with rigid electronics. Now Wang and his team have adapted them for use in flexible electronics, thus providing a dependable energy source for bendable, stretchable gadgets that has been sorely lacking.

In research described in the journal Science Advances, the Georgia Tech researchers adapted the TENG device by combining a conductive liquid electrode and a flexible, rubber elastic cover. So successful have the researchers been at making the nanogenerator flexible that they have given it a new name: shape-adaptive TENG, or “saTENG”.

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IEEE Spectrum’s nanotechnology blog, featuring news and analysis about the development, applications, and future of science and technology at the nanoscale.

Dexter Johnson
Madrid, Spain
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