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Photographic image of ultrathin AM-OLED display on the human wrist

Stick-on Displays: Bendy 2D Semiconductor is Fast Enough to Drive OLED Pixels

One of the great things about 2D semiconductors like molybdenum disulfide is that they bend easily. They also allow electrons to zip through them pretty quickly. And, unsurprisingly, since they are only about an atom thick, they are transparent. That combination makes them perfect for flexible OLED displays.

However, if display makers try to build MoS2 into transistors needed to control OLED pixels, the resistance between the MoS2 and the transistors’ source and drain electrodes is so high that it puts the 2D wonder material out of contention for the job.

But now, engineers in South Korea have come up with a way to build MoS2 transistors that can work in bendable OLED displays. They used the transistors to construct a simple 6 x 6-pixel array on a 7 micrometer-thick plastic sheet you can stick on your skin. The display is so flexible it could be repeatedly bent to a radius smaller than one millimeter without breaking.

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“Experiment (left) and simulation (right) of a diamond nanoneedle being bent by the side surface of a diamond tip, showing ultralarge and reversible elastic deformation.”

How to Bend Diamonds

It’s commonly known that diamonds are the hardest natural material. However, with that hardness comes brittleness: they may be hard but they’re not very flexible.

Now an international team of researchers has demonstrated that diamonds, which are commonly believed to be inflexible, can be bent and stretched significantly. The researchers showed that the maximum tensile elastic strain of a diamond can reach nearly 9 percent, close to the theoretical limit of the material.

The researchers believe that these enhanced mechanical properties make nanodiamonds much more durable than expected, and therefore could lead to applications that involve mechanical loading, making them candidates for applications such as diamond needle-based intracellular delivery. But it is what this flexiblity does to diamonds’ optical and electrical properties may prove to be the most significant in the long run.

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Magnified images revealing the internal structure of nanoporous graphene.

Graphene is Grown With the Same Band Gap as Silicon

For all of graphene’s amazing electronic capabilities, it has not made much of an impact as a replacement for silicon in digital logic applications. This shortcoming is largely due to its lack of an inherent band gap that’s needed in computing applications to start and stop the flow of electrons.

While methods for engineering a band gap into graphene have been around for years, these approaches have been recognized as imperfect solutions. They have added critical costs and complications to using the material and compromised the attractive electronic properties that made graphene a desirable replacement for silicon in the first place.

Now researchers in Spain have devised an inexpensive way to grow graphene with the same band gap that exists in silicon (1 eV), and in so doing, may have reopened graphene’s potential as an alternative to silicon for digital logic.

In research described in the journal Science, a team from throughout Spain and led by the Catalan Institute of Nanoscience and Nanotechnology (ICN2) has employed bottom-up manufacturing techniques to assemble nanopororus graphene in such a way that the pores have the size, density, and morphology to create a perfect band gap for digital electronics. The researchers then made a field-effect transistor (FET) device using this material.

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Schematic structure of the 2D SFG memory

"Quasi-Non-Volatile" Memory Looks to Fill Gap Between Volatile and Non-Volatile Memory

Researchers at Fudan University in Shanghai, China have leveraged two-dimensional (2D) materials to fabricate a relatively new gate design for transistors that may fill the gap between volatile and non-volatile memory.

The result is what the researchers are dubbing a “quasi-non-volatile” device that combines the benefits of static random access memory (SRAM) and dynamic random access memory (DRAM). The new device will make up for DRAM’s limited data retention ability and its need to be frequently refreshed and SRAM’s high cost.

In research described in Nature Nanotechnology, the Chinese researchers leveraged a gate design that has been gaining popularity, recently called semi-floating gate (SFG) memory technology. The SFG gate design is similar to a typical field effect transistor except that SFG transistor can “remember” the applied voltage from the gate.

The researchers have shown that the 2D SFG memory they have fabricated has 156 times longer refresh time (10 seconds) than DRAM (64 milliseconds), which saves power, and ultrahigh-speed writing operations on nanosecond timescales (15 nanoseconds), which puts it on par with DRAM (10 nanoseconds). This new device also boosts the writing operation performance to approximately 106 times faster than other memories based on 2D materials.

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“Rocking” Brownian Motor Pushes Nanoparticles Around

The constant movement of atoms and molecules as they bounce off one another in a liquid or a gas is known as Brownian motion and it seems to have held back molecular nanotechnology (MNT) in which molecular-scale machines work in unison to create macroscale objects.

Now researchers at IBM Zurich have developed a new technology that uses Brownian motion to create a motor for sorting, separating and moving nanoparticles without the need for a flowing fluid. The IBM scientists described their research in the journal Science, and they believe that it could eventually lead to lab-on-a-chip applications for environmental sciences or biochemistry.

While Brownian motors have been developed in the past, they have mainly focused on what is known as Brownian “flashing” ratchets. These so-called flashing motors get their name because they “flash” the energy landscape (or the potential). In other words, they switch it off for a certain amount of time and then switch it on again for another duration.

If the potential is off, the particles diffuse in all directions. When switched on, the asymmetry of the potential leads to an asymmetric collection of particles onto the potential minimum. If, for instance, more particles are collected from the left side than from the right, this leads to an effective motion of particles to the right. 

What the IBM scientists have done is make what’s termed a “rocking” Brownian motor. It gets this name because the force on the particles via an electric field can be interpreted as a “rocking” or tilting of the energy landscape. Most importantly, with this kind of Brownian motor the energy landscape is fixed.

Most previous implementations of rocking Browning motors used dielectric forces between pre-patterned electrodes on a sample surface to create the switchable asymmetric potential. Unfortunately for this design, these forces are too weak if particles become smaller than one micron. This limited their use to plastic particles larger than one micron.

“Our motor is the first rocking Brownian motor that works for nanoparticles,” said Christian Schwemmer, a post-doc physicist at IBM Research-Zurich and co-author of the paper. “Our implementation works with the electrostatics of charged surfaces, and operates down to particle sizes of 5 nanometers at least. Therefore, particles like DNA, proteins and other biologically relevant entities become accessible.”

The key to device is its 3D patterned surface, according to Armin Knoll a nanoscale systems scientist at IBM Research Zurich and co-author of the research.

Knoll describes the pattern as a 3D saw-tooth pattern with the teeth in the vertical direction. The researchers employed scanning probe lithography to produce the saw-tooth pattern down 1 nm in depth precision.

After patterning is completed, a droplet of a water-based nanoparticle suspension is deposited on the pattern and afterwards covered by a glass coverslip. The distance of the glass to the surface of the patterned film is adjusted to be ~150nm.

Knoll explained that in water all surfaces are negatively charged (glass, pattern, particles) and the particles are repelled from the surfaces. “It costs them more energy to squeeze in between the two surfaces where the gap is small (at the top ridge of the teeth),” he added.

This is the surface pattern’s energy landscape, according to Knoll, and because of it the nanoparticles follow the 3D pattern. While this energy landscape has the shape of a saw tooth (a ratchet), Knoll cautions that the particles would not start to move in one direction, even if there is Brownian motion. To get this movement an oscillating electric field has to be applied.

“The field induces a flow of the water in the nanochannel that oscillates with the field,” said Knoll. “By drag forces the water pushes the particles back and forth in a rocking motion. The saw-tooth energy landscape rectifies this motion because it is much harder for the nanoparticles to move over the steep slope than over the shallow slope of the saw-tooth energy landscape.”

In this design, the particles start to flow along the shallow slope direction. In other words, the oscillating electric field moves the particles back and forth and the saw-tooth landscape hinders the particles from moving backwards.

While the device the IBM researchers have come up with looks to have lab-on-a-chip applications, the technology has two features that differentiate it from other lab-on-chip devices.

“The first feature is that our motor allows for a directed particle transport without net fluid flow,” said Schwemmer. “The second one is that it reaches unprecedented resolution in particle separation.”

In the model the researchers developed to predict the capability of the device, the researchers found that it was capable of separating two nanoparticle populations with merely 1 nm difference in size. So, in practice, 40 nm and 41 nm particles could be sorted into different directions. Because of this the researchers believe it can be used to detect ultra small quantities, such as nanoscale pollutants in drinking water.

Knoll believes the ability of the motors to size-selectively transport nanoparticles without a net fluid flow will make the devices ideal for particle sorting and separation. “The fact that our motor does not require a fluid flow is a huge advantage because flow-based devices need to be operated at high pressures,” added Knoll.

As a separation device, it has an extremely small footprint and requires only a few volts of electric potential, which makes it ideal for mobile or handheld lab-on-chip devices, according to Knoll. In contrast, electrophoresis—which is commonly used to separate proteins or nucleic acids—requires well above 100 Volts.

To become a viable commercial device, the device will have to be integrated into a microfluidic chip that will enable control of the input and output ports. It will also have to be bigger, in the centimeter range in order to reach interesting throughputs for applications. Finally, for real word applications, surfaces will need to be covered to guard against unwanted deposition of biomaterials.

Illustration of ultrasensitive optical interrogation of the motions of drumhead-structured MoS2 resonators.

Dynamic Range of Nanoresonators on Par With Animal Hearing

Researchers at Case Western University have fabricated nanoelectromechanical system (NEMS) resonators made from molybdenum disulfide that have an extraordinarily broad dynamic range. These nanoscale devices’ range is roughly comparable to that of human and animal hearing. The advance could lead to ultra-low-power signal processing and sensing functions in future electronic and optoelectronic chips, according to the researchers.

Dynamic range—the ratio between the highest level of an undistorted signal (“signal ceiling”) over its lowest detectable signal (“noise floor”)—is essential to all sensing and communication, whether it be sensory organs in animals or engineered devices. It’s usually measured in decibels (dB). 

In research described in the journal Science Advances, the Case Western researchers, led by Philip Feng, demonstrated that their NEMS resonators have a dynamic range of up to ~110 dB at radio frequencies up to over 120 megahertz. This dynamic range represents the highest reported to date for vibrating resonators made of two-dimensional (2D) materials and other nanoscale structures, says Feng.

These 2D resonators function like the skin of a drum that vibrates at certain frequencies. When struck by something, such as a molecule or photon, those frequencies change. Measuring those frequency changes makes it possible to identify what has hit the device. You can see a visual representation of how this technology operates in the video below.

In this research, Feng and his team demonstrated that they can also tune the device by stretching the drumhead membranes using electrostatic forces induced by controlling the number of electrons accumulated in the devices, thus providing it with a fairly wide tunability. 

“When fully leveraged, this broad dynamic range will translate into high performance in sensitivity for various physical sensors and transducers enabled by such devices, toward real-time sensing applications,” said Feng.  “The unique combination of broad dynamic range and frequency tunability also holds promise for making ultralow-power oscillators that could have both low noise performance and excellent tuning capability.”

This is not the first time NEMS resonators have been produced using molybdenum disulfide. In fact, Feng and his research team fabricated just such a device five years ago. However, at that time, no tests were performed to measure the resulting device’s dynamic range, mainly because in order to take measurements at frequencies above 100 MHz at room temperature, ultrasensitive equipment is required.

“These are very ‘quiet’ devices and we have to ‘listen’ to them very carefully and ‘talk’ to them very gently,” said Feng. 

When researchers have bothered to measure the dynamic range of nanoscale resonators, the results have been pretty poor. The reason one-dimensional (1D) nanomaterials such as carbon nanotubes (CNTs) and nanowires have such relatively poor dynamic ranges stems from their high thermomechanical fluctuations due to large Brownian motion, the random movement of particles resulting from them bumping into each other. As a result, these materials have high noise floors that narrow their dynamic ranges from the lower boundary side.

The upper boundary of dynamic range, the signal ceiling, is usually based on the onset of signal distortion. The 2D devices also experience distortion later that helps to improve the dynamic range of 2D materials as well.  

“2D membranes have broader dynamic ranges than 1D devices because they benefit from both ends–lower noise floor, and higher signal ceiling,” said Feng. 

In order to commercialize this technology, Feng foresees that some demanding engineering still needs to be addressed, because 2D NEMS technology is still a quite new area that is in its early stage. 

Feng added: “Exciting and challenging topics for us include wafer-scale fabrication and testing of such devices, further engineering on improving device performance, and developing prototype chips for targeted applications in relevant frequency bands.”

A schematic diagram of a microscopic chemical detection machine depicting a micrometer-sized polymer particle coated with a nanoelectronic circuit.

2-D Materials Enable Swarms of Floating Microbots

Often when science fiction has envisioned nanotechnology, it takes the form of some miniature devices floating around in a swarm performing either a nefarious or miraculous act. Now, this vision is no longer relegated to thinly plotted sci-fi.

Researchers at the Massachusetts Institute of Technology (MIT) have devised a way to graft nanoscale electronic devices onto floating microscale particles to monitor everything from gases in a range of environments to the inner workings of the human digestive system.

Last week at the American Chemical Society National Meeting & Exposition, Michael Strano, a chemical engineering professor at MIT, and Volodymyr Koman, a research fellow in Strano's group at MIT, presented work in which they were able to exploit Van der Waals forces (forces that create a bond between atoms and molecules that are close together) to get electronic devices made from two-dimensional (2D) materials to stick to floating microparticles.

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Photograph of the sensor, with the top electrode removed. It looks like a black and white cube with a drop of water bulging from the top. The dimensions are 3 × 3 × 3 mm.

Supersensitive Accelerometer Could Be the Answer to Better Drone Control

Accelerometers are everywhere. You’ve probably got at least one on your person right now. But today’s run-of-the-mill accelerometers—MEMS devices that measure a minute change in capacitance—just aren’t very sensitive. They’re built to fit into smartwatches and smaller things, and that small size hampers how well they can sense changes. Engineers in Florida have now come up with a new take on the accelerometer that is as much as 1 million times as sensitive as a typical smartphone accelerometer, and it maintains that sensitivity up to a car-crash-scale 100 gs.

That combination of high sensitivity and large dynamic range in a cube that’s just 3 millimeters on a side should make the new accelerometer particularly useful in things that move quickly in three-dimensions, such as military drones, microrobots, and self-guided projectiles, according its inventors.

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A hand holds and bends a flexible graphene

Mobile World Congress 2018: The Tangled Fate of Graphene and 5G

5G report logo, link to report landing page

Graphene has been heralded as a “wonder material” for well over a decade now, and 5G has been marketed as the next big thing for at least the past five years. Analysts have suggested that 5G could be the golden ticket to virtual reality and artificial intelligence, and promised that graphene could improve technologies within electronics and optoelectronics.

But proponents of both graphene and 5G have also been accused of stirring up hype. There now seems to be a rising sense within industry circles that these glowing technological prospects will not come anytime soon.

At Mobile World Congress (MWC) in Barcelona last month, some misgivings for these long promised technologies may have been put to rest, though, thanks in large part to each other. For the third year in a row, MWC hosted The Graphene Pavilion organized by The Graphene Flagship, the EU’s €1 billion, 10-year plan to make Europe the “Silicon Valley” of graphene.

In a meeting at MWC with Jari Kinaret, a professor at Chalmers University in Sweden and director of the Graphene Flagship, I took a guided tour around the Pavilion to see some of the technologies poised to have an impact on the development of 5G.

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EUV single patterning of the N5 32-nm metal-2 layer, 32-nm pitch dense lines, and 40-nm hexagonal contact holes and pillars

Getting EUV Ready for 2020

Extreme-ultraviolet lithography looks ready for its debut later this year, making it easier to build huge numbers of chips with even more huge numbers of the tiniest circuits you can buy. But will EUV be ready for the next generation, when circuits are slated to be even tinier? The Belgian microelectronics research house Imec has uncovered some problems with using EUV for the so-called 5-nanometer generation, which is expected to go into full production in late 2020. They are fixable, says Kurt Ronse, program director on advanced patterning at Imec, but there’s quite a lot of work to be done.

EUV uses light with a wavelength of 13.5-nm to define the features that become the transistors and interconnect wiring of a chip. The light is reflected off a pattern onto a photomask and cast onto a chemical coating on the silicon wafer called a photoresist. The photoresist hardens where the light strikes it, transferring the photomask’s pattern onto the silicon.

The 13.5-nm wavelength is considerably smaller than the space between interconnect lines—the 32-nm “pitch”—needed even for the 5-nanometer process. But there’s more to lithography than a slim wavelength. And when Imec engineers began producing experimental features for 5-nm chips last year, they noticed many more defects than they’d expected.

They built rows of trenches of the kind that would form a chip’s wiring once filled with metal and arrays of holes that would become the contacts from the wiring to the parts of the transistors below. But there were “nanobridges” between the trenches, holes that were missing, and holes that had merged with their neighbors, Ronse says. Such random snafus are collectively called stochastic defects.

What causes them? “That’s the million-dollar question,” says Ronse. They can be caused by what’s called photon shot noise. It’s the fact that there are few photons falling on the chip and you just don’t always get the same number at every spot on the chip. “But we started to analyze defects more in detail, and we saw way too many to be explained all through photonic shot noise,” he says.

Simply cranking up the number of photons by exposing an area longer would help, but not enough, he says. Besides, longer exposure means a slower manufacturing process. And nobody wants that.

Another culprit is likely a sort of chemical version of shot noise. The photoactive chemicals in the photoresist may not be perfectly uniformly distributed on the wafer, leaving spots that act as though they are underexposed. And Ronse says there are interactions with the layers beneath the photoresist.

Optimization of all these aspects, as well as layout changes for the metal interconnect layers of logic circuits, should result in a 5-nm processes ready for a 2019 debut, Ronse says. Imec reported its findings at the SPIE Advanced Lithography Conference held this week in San Jose, Calif.

This post was corrected on 1 March 2018.



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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