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For first time the magnetic porphyrin molecule has been directly connected to an electronic circuit

Graphene Nanoribbons Reach Out to the Molecular World

A collaboration among Spanish research institutes—led by the nanoGUNE Cooperative Research Center (CIC)—has made a significant breakthrough in so-called molecular electronics by devising a way to connect magnetic porphyrin molecules to graphene nanoribbons. These connections may be another example of how graphene could enable the potential of molecular electronics.

Porphyrin is a hemogloblin-like molecule that is responsible for making photosynthesis possible in plants and transporting oxygen in our blood. But recently, researchers have been experimenting with so-called magnetic porphyrins and discovered that they can form the basis of spintronic devices.

Spintronics involves manipulating the spin of electrons and in this way differs from conventional electronics that manipulates their movement. It is this spin that is responsible for magnetism: When a majority of electrons in a material have their spins pointing in the same direction, the material is magnetized. If you can move all the spins up or down and can read that direction, you can create the foundation of the “0” and “1” of digital logic.

Spintronic devices based on the porphyrin molecule exploit the magnetic atom—typically iron, which has spin-polarized states—that is in the middle of each molecule. There are a number of ways of exploiting the spin of these magnetic atoms to polarize the transported current. If magnetic molecules with a larger spin are used—the so-called a single-molecule magnet—a “1” or “0” state could be stabilized by a magnetic field and read by currents.

The Spanish researchers have taken a unique approach to setting this up. They’ve created direct connections to the molecules with atomically precise graphene wires, which covalently bond to specific sites of the molecules.

“This allows the injection of electronic currents into the molecule,” says Nacho Pascual, Ikerbasque Professor and leader of the Nanoimaging Group at nanoGUNE. “We further show that even after the connection, the molecule maintains its magnetic property.”

Pascual adds that the Spanish collaborators have demonstrated that small variations in the way the graphene nanoribbons are attached to a molecule can alter its magnetic properties. Further, a  molecule’s spin can be manipulated via the injected currents.

“We tested the magnetization by performing tunneling spectroscopy,” says Pascual. “We saw that the iron ion maintained its spin and its preferred direction after connection to the graphene nanoribbons, but in a few cases where the bonding was different it completly vanished. So the way of contacting is crucial.”

Pascual sees this work as bringing spintronics into molecular electronics, and has unofficially dubbed it “Molecular Spintronics.” 

In future research, Pascual and his colleagues aim to use single-molecule magnets, and improve the magnetic function in transport experiments by injecting current though the ribbons. “This will be closer to the real usage of these devices,” he added.

Microscopy image of a nanostructured photodetector looks like the letter C with a dark black center.

Silicon Nanostructures Bend Light to Make Faster Photodiodes

There are some things silicon doesn’t do well. It neither absorbs nor emits light efficiently. So, silicon photonics systems, which connect racks of servers in data centers via high-data rate optical fiber links, are dependent on photodiode receivers made of germanium or other materials to turn optical signals into electronic ones. Saif Islam, professor of electrical and computer engineering at University of California Davis, and colleagues have come up with a way for silicon photodiodes to do the job, potentially driving down the cost of optical computer-to-computer communications.

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Diagram showing annihilation or creation of a single magnetic skyrmion

Swirly Skyrmions Could Be the Future of Data Storage

About five years ago, researchers at the University of Hamburg demonstrated that tiny, swirling magnetic spin patterns on thin films—known as skyrmions—could be used to store and erase data on magnetic media.

At that time, these spinning magnetic swirls that had been proposed over 60 years ago by British physicist Tony Skyrme—from whom the name derives—had suddenly become a potentially game changing magnetic data storage system. And what a change it represented: skyrmions are 10 times smaller than the magnetic regions used on traditional hard drives.

Now a team of researchers from CNRS/Thales joint lab in France with European Funding under the MAGicSky program have taken a critical step in the commercial realization of this technology by electrically detecting for the first time a single small skyrmion at room temperature.

“We believe this is an important advance because it demonstrates one of the unavoidable functions for any type of future concept of devices: electrical detection,” said Vincent Cros, a researcher at CNRS and co-author of the paper published in Nature Nanotechnology.

While the electrical signal of skyrmion lattices or an ensemble of skyrmions has been measured before, mostly at low temperatures, this is the first time that measuring an electrical signal has been demonstrated for a single skyrmion and at room temperature.

As one might imagine, the electrical signals associated with these 100 nanometer skyrmions remain relatively small. These signals are so small, according to Cros, that they had to be sure that the measured electrical signal is actually associated with the presence of a skyrmion. “That is exactly what we demonstrate here by a concomitant electrical measurement and magnetic imaging on the very same devices,” said Cros.

While it was necessary to use magnetic imaging to ensure that they were measuring a skyrmion for their research, in future memory devices the only possible reading procedure will be through electrical measurement and not by imaging the magnetic configuration of the skyrmion.

Nonetheless, the ability to make electrical measurements of the sample while imaging it magnetically at the same time using magnetic force microscopy is extremely significant, according to Cros, and had never been done before.

As for the actual device, this goes back to work Cros and his colleagues did in 2013 that suggested the best memory device for exploiting skyrmions would be what’s known as “racetrack memory.” 

Nearly a decade ago, Stuart Parkin and his colleagues at IBM Almaden Research demonstrated a three-bit version of so-called “racetrack memory,” which is a solid-state non-volatile memory that promises much higher storage density than conventional solid-state memory devices.

When Parkin first envisioned racetrack memory, they were based on magnetic features known as domain walls, which essentially separate the magnetic direction of a material into different areas. Electric currents could push those domain walls around the track and a sensor could detect the changes, leading to the “0” and “1” of digital memory.

What Cros and his colleagues suggested five years ago was that the skyrmions could replace the domain walls and they could move along the track and their presence or absence could be detected electrically, leading to a digital memory device.

By using a basic skyrmion-race-track memory, the researchers designed electrical contacts on both ends of the tracks. In order to detect the electrical skyrmion signal, they also designed lateral contacts. The electrical signal is simply detected by measuring the associated electrical voltage using a commercial voltmeter.

It sounds all pretty straight forward, however, controlling the position and the density of the skyrmions remained a challenge. The main obstacle revolved around the creation of the skyrmions in the material where prior to their formation it had all been in a uniform magnetized state. The traditional method for producing the skyrmions was based on the use of a magnetic field.

“In the present work, we have employed a new approach in which we inject short current pulses into the materials, which allows us to create isolated skyrmions located in a strip (or track) designed by electron-beam lithography,” explained Cros.

The result is that Cros and his colleagues can now adjust the total number of nucleated skyrmions by tuning different parameters, such as the current pulse width or the intensity of the external magnetic field.

While all of this will certainly go down as a significant step towards using skyrmions in memory devices, Cros concedes that commericialization is still a ways down the road.

“We are not yet at the stage where skyrmion devices can be used and implemented as a real new electronic device,” said Cros. “The standard and reasonable time scale between fundamental discoveries and consumer electronics is often between 10 and 15 years.”

To realize this 15 year time line, Cros believes that more efforts are needed to further decrease the skyrmion size, targeting sub 10-nm diameter, to increase the skyrmion speed, better understand and control the interaction of skyrmions with material grains (typically of the same sizes) and to increase the electrical signal.

Aledia's gallium nitride nanowire LEDs on 200-mm silicon wafers.

Cash Comes in for Nanowire Display Startups

While much of the near-term innovation of future TVs will come from the processing horsepower behind the screen, farther out you can expect a big change in the pixels themselves: micro-LEDs. These displays would be made up of pixels made of miniaturized gallium-nitride LEDs, which are so efficient that displays would consume half or even one-third of the energy used by OLED or LCD displays while being considerably brighter than both.

Samsung, seemingly at great cost, assembled a huge microLED display for CES that it called “The Wall,” but the technology is likely to make its mark much sooner in small displays for augmented reality and smartwatches. Apple, for example, acquired micro-LED display startup LuxVue in 2014, which reportedly had raised $43-million to that point. MicroLED displays still haven’t appeared in the Apple Watch, though.

In the past four months, venture capital groups have poured cash into two microLED startups whose particular take on the technology could speed up its adoption. Both rely on growing nanometer-wide wires that each comprise an LED. In August, Glo, founded by Lund University nanowires expert Lars Samuelson and based in Sweden and Silicon Valley, got SEK241 million (US $15 million), with Google leading the investment. And in January Aledia, a spinoff of CEA in Grenoble, France, took in €30 million (US $37 million), adding Intel Capital to its investors.

“This is going to be a generational shift in technology,” says Aledia’s CEO, Georgio Anania. The main advantage of using gallium-nitride LEDs as pixels is efficiency. Today’s technologies, LCDs and OLED displays, are only around 5- to 7-percent efficient. But the efficiency of gallium-nitride LEDs for lighting is closer to 70 percent. Efficiency degrades as you make the LEDs tinier and tinier, Analia points out, but even a 15-percent-efficient display “would be a revolution.”

But gallium-nitride is expensive, costing multiples of silicon, so you have to limit how much of the material is used. Even for LEDs destined for lighting applications, the gallium nitride is grown as a thin layer atop a wafer of sapphire and in some cases silicon. Sapphire is used because its crystal lattice matches that of gallium nitride pretty well. That matchup means the gallium nitride grown atop it has few defects. In larger LEDs, the defects can sap power. But in the tiny ones needed for displays, it can kill the device entirely.

In an effort to make LEDs cheaper by using a more plentiful starting wafer that comes in larger sizes, LED makers have worked hard on ways to grow gallium nitride on silicon. Silicon isn’t a natural fit, so there are bound to be more defects. Part of Aledia’s allure is that those defects don’t matter much to its nanowire LEDs.

The company grows fields of gallium nitride nanowire LEDs on 200-millimeter silicon wafers. Each nanowire has an inner and outer core of gallium nitride sandwiching a series of what are called quantum wells—very thin layers of material that confine charges and have the effect of enhancing the recombination of electrons and holes to produce light. Doping the structure with specific types and concentrations of atoms makes LEDs that shine in either red, green, or blue.

In gallium nitride–on-silicon systems defects can occur because the two materials expand at different rates when heated. This stresses the gallium nitride, creating dislocations in its crystal structure. But nanowires have such a small footprint that the resulting stresses across one of them are pretty small. Even if a defect does occur, there are potentially hundreds of nanowires in each pixel, so one dud doesn’t make a difference.

Still, duds are a big problem for a display that’s supposed to be made up of thousands of individual LEDs. Such displays would be made by placing each tiny LED onto the screen substrate. If one LED doesn’t work, the whole screen is a waste. Samsung’s Wall demo is a 4K TV; meaning it had, at minimum, a preposterous 8.3 million perfectly operating, perfectly placed LEDs.

For large displays, Aledia’s advantage is that there will be no duds. But for small displays, such as smartwatches or the microdisplays that will enable future AR and VR systems—and someday even contact-lens systems—Aledia can take further advantage of its silicon base. It can build the whole display out of a single nanowire-studded silicon chip.

Such monolithic displays can have the silicon portion fully processed into the needed circuits to drive the pixels, and then the gallium nitride LEDs can be grown right on top, says Anania. The company’s first target is monolithic displays for smart watches and other small forms. It’s closest to producing such a display using only blue pixels, which would then be converted by phosphorescent chemicals to produce green and red.

But “Silicon Valley wanted native RGB,” he says. So the company is working on making pixels containing nanowires that have different chemical doping profiles to produce all three colors. “We’re still working through the tech challenges,” says Anania. “A display is a complicated subsystem.”

Aledia’s might not be the only path to monolithic displays, of course. Calls to Glo were not returned by press time; however, Google’s investment speaks to its progress. And indeed, developers of 2D (non-nanowire) microLED displays—both those that have been swallowed by giants like Apple and Facebook and startups such as Ostendo—are surely in the race as well. Significantly, Plymouth, England–based Plessey Semiconductor recently pledged to be first to market a microLED display in the first half of 2018 using its own gallium-nitride-on-silicon technology.

DNA origami nanostructure shapes.

Novel Lithography Technique Combines Speed With Accuracy

What happens when you combine DNA origami techniques with conventional lithography? You get a novel lithography technique dubbed DNA-assisted lithography (DALI) that has the resolution of electron beam lithography with the speed of conventional lithography.

In research described in the journal Science Advances, an international team of scientists from Finland, Denmark, and the United States have combined the programmable and accurate shapes made possible with DNA origami with conventional lithography to fabricate structures that are accurate below 10 nanometer resolution and are tens of nanometers in size.

The resulting method offers a unique example of combining bottom-up based approaches (i.e. the self-assembled DNA structures) with top-down techniques (conventional lithography), according to Jussi Toppari, a senior lecturer at University of Jyväskylä in Finland and co-author of the research. “It will extend the possibilities of both standard lithography as well as DNA origami techniques, said Toppari.

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Illustrations showing the basic operation of NIST’s artificial synapse, which could connect processors and store memories in future neuromorphic computers operating like the human brain.

Superconducting Synapse Could Let Neuromorphic Chips Beat Brain’s Energy Efficiency

Scientists at the U.S. National Institute of Standards and Technology, in Boulder, Colo., have developed a superconducting device that acts like a hyperefficient version of a human synapse.

Neural synapses are the connections between neurons, and changes in the strength of those connections are how neural networks learn. The NIST team has come up with a superconducting synapse made with nanometer-scale magnetic components that is so energy efficient, it appears to beat human synapses by a factor of 100 or more.

“The NIST synapse has lower energy needs that the human synapse, and we don’t know of any other artificial synapse that uses less energy,” NIST physicist Mike Schneider said in a press release.

The heart of this new synapse is a device called a magnetic Josephson junction. An ordinary Josphson junction is basically a “weak link between superconductors,” explains Schneider. Up to a certain amperage, current will flow with no voltage needed through such a junction by tunneling across the weak spot, say a thin sliver of non-superconducting material. However, if you push more electrons through until you pass a “critical current,” the voltage will spike at an extremely high rate—100 gigahertz or more.

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Illustration of graphene, with shining light.

Here's How Graphene Makes Photodetectors 100,000 Times More Responsive Than Silicon

Two years ago, we covered research out of the University of Manchester that demonstrated that graphene-based membranes could serve as a filter for cleaning up nuclear waste at nuclear power plants.

While it’s not clear that this particular application for the graphene membranes ever made much headway in nuclear waste cleanup, they did discover an interesting phenomenon about these graphene membranes in the ensuing two years: protons can transport through graphene.

Based on that knowledge, Andre Geim’s team at the University of Manchester began to investigate whether light could be used to enhance proton transport through graphene by the addition of other light sensitive materials, such as titanium dioxide (TiO2). Turns out that graphene did the job quite effectively on their own.

“We were not expecting that graphene on its own – without the addition these light sensitive ingredients – would show any response,” said Marcelo Lozada-Hidalgo of the University of Manchester and co-author of this research and the work from two years ago. “We were very surprised by our results.”

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Test tubes and other objects forming a pattern of 0s and 1s.

Test Tube Hard Drives Compute with Chemicals

A group of scientists and engineers at Brown University is planning to use chemicals in a droplet of fluid to store huge amounts of data and, eventually, get them to do complex calculations instantly. They’ve just received US $4.1 million from the Defense Advanced Research Projects Agency to get started, and plan to borrow robots and automation from the pharmaceutical industry to speed their progress.

“We’re hoping that at the end of this we’ll have a hard drive in a test tube,” says Jacob Rosenstein, assistant professor of electrical engineering, who is co-leading the project with theoretical chemist Brenda Rubenstein.

There’s been a big push recently to store data as molecules of DNA, but the Brown chemical computing project will do things differently, potentially ending up with greater data density and quicker readouts.

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Illustration of a voltage-induced memory effect in monolayer nanomaterials, which layer to create "atomristors," the thinnest memory storage device that could lead to faster, smaller and smarter computer chips.

Atom-Thin Memristors Discovered

Two-dimensional atom-thin materials are good for a lot of things, but they don’t make good memory devices. At least that’s what everyone thought until  Ruijing Ge, a first-year graduate student at the University of Texas, Austin, persuaded her mentor—flexible electronics guru Deji Akinwande—to let her try. They sandwiched an atom-thick layer of molybdenum disulfide between two electrodes and found that, contrary to expectation, the structure displayed memristance; it can be set to a high resistance or low resistance state by particular voltages and remain stable long after the voltage is removed.

It’s not completely clear why it works, but these “atomristors,” as Akinwande has christened them, could have a big impact—and not just as memory devices. They could serve as switches in radios of 5G smartphones and Internet-of-Things gadgets, and as computational elements in brain-inspired artificial intelligence circuits.

Ordinary memristors are made of oxide materials sandwiched between two conductors. The resistance across the oxide changes when a high current in one direction moves oxygen atoms vertically through the oxide. The original resistance is restored by switching the direction of the current, putting the oxygen back in its place.

But that can’t be what’s happening in an atomristor. There is neither oxygen nor a vertical direction for it to move. Instead, Akinwande hypothesizes that defects in the 2D crystal lattice—the holes left by occasional missing sulfur atoms, for instance—are what are moving around. Voltage of one polarity attracts the defects, bunching them together in a way that decreases the resistance across the material. Switching the polarity scatters the defects, ramping the resistance back up.

That’s the theory, at least. Akinwande says his group is collaborating with one of the U.S. National Labs, which have the kind of microscope that should allow them to see defects moving during operation.

Importantly, this memristance property seems to be common to the whole class of 2D materials to which molybdenum disulfide belongs. That class is called transition metal dichalcogenides, and they are “the premier 2D semiconductor,” says Akinwande. Collaborating with materials scientists in Yanfeng Zhang’s group at Peking University, which supplied several of the materials, they tested molybdenum diselenide, tungsten disulfide, and tungsten diselenide as well (all less than 1 nanometer thick). In every case, it worked. “We tried four or five, and there are possibly hundreds of them,” says Akinwande.

In addition to clarifying how atomristors work and determining how many types there are, his lab plans to work on some interesting applications for them. They’ve already demonstrated an ultra-thin 2D version of the memristor cell by building the conductive parts out of graphene. And Akinwande believes it should be possible to integrate 3D arrays of memristors atop finished silicon logic chips, because the materials can be transferred at room temperature. The end result could be neuromorphic systems with a brain-like density of connections. “The sheer density of memory storage that can be made possible by layering these synthetic atomic sheets onto each other, coupled with integrated transistor design, means we can potentially make computers that learn and remember the same way our brains do,” Akinwande said in a press release.

But atomristors might have a much more down-to-earth use. The RF switches used to channel radio signals into and out of wireless devices are best when they consume little power when not switching but can pass a lot of current through them. Atomristors seem to fit that job quite well, as they don’t need any power to hold their states. Akinwande’s lab has shown that the atomristors can channel frequencies as high as 50 GHz—which includes the key 5G bands—through them.  

Photograph of organic-thin film transistors with a nanostructured gate dielectric under continuous testing on a probe station.

Organic Thin-Film Transistors' New Gate Dielectric Opens Door to Future Electronics

Thin-film transistors (TFT) form the foundation for many of today’s technologies, including smartphones and flat-panel TVs. And these TFTs are made possible by amorphous silicon. While this material has managed to do the trick for the most part, it does have some performance limitations, such as limited carrier mobility, that have sent researchers in search of something better.

Organic thin-film transistors (OTFTs) have proven better than amorphous silicon-based TFTs in some areas of performance, especially carrier mobility. Unfortunately OTFTs have their own problems—in the critical performance parameter of large threshold voltage instabilities, they are poor performers. Threshold voltages—also known as gate voltages—are the minimum voltage differential needed between a gate and the source to create a conducting path between the source and drain terminals.

Now, researchers at Georgia Institute of Technology have developed a nanostructured gate dielectric that overcomes this obstacle of voltage threshold instabilities in OTFTs, and could lead to wider use of organic semiconductors for thin-film transistors.

A gate dielectric is a key part of every thin-film transistor. It is the electrically insulating layer between the gate electrode and the semiconductor. It should have a high dielectric constant, be very thin, and have a high dielectric strength for the transistor to operate at low voltage. 

When a voltage is applied to the gate electrode, the resulting electric field across this insulating layer modulates the density of carriers in the semiconductor layer and controls the current that is flowing between the source and drain electrodes. Chip designers choose from many different materials to form this insulating layer, from single dielectric polymers, to inorganic oxides, to combinations of different organic and inorganic materials.

In research published in the journal Science Advances, the Georgia Tech researchers used atomic layer deposition (ALD) to grow a thin metal oxide layer on top of a perfluorinated dielectric polymer.

They chose atomic layer deposition for its ability to produce layers that are free of defects. “The low defect density reduces the diffusion of moisture into the underlying organic semiconductor layer, preventing its degradation,” said Bernard Kippelen, a professor at Georgia Tech, and leader of the research.

Kippelen notes that ALD was initially developed for traditional CMOS-based technology, but that his research group pioneered its use in organic thin-film transistors.

Initially, the geometry of the gate dielectric was comprised of a perfluorinated polymer layer and alumina grown by ALD. However, under high relative humidity conditions, alumina degrades through corrosion.

“In this work we replaced the alumina layer with a nanolaminate that consists of alternating nanometer-thick layers of alumina and hafnium oxide,” said Kippelen. The nanolainates were less prone to degradation.

The performance of the new organic thin-film transistors appears to exceed that of hydrogenated amorphous silicon technology, both in terms of charge mobility and stability, according to Kippelen.

Kippelen concedes that the charge mobility achieved with the version used in their studies is inferior to that of the best metal oxide semiconductors. However, he argues that new organic materials already exist that yield higher mobility, and which simply need to be tested in the new geometry.

“It is premature and difficult at this stage to provide a direct comparison with what is currently on the market; nevertheless, we believe that the level of stability that is achieved is an important step for printed electronics,” he said.

With this step, Kippelen and his team envision that these advances could make it possible to manufacture large-area products such as displays.

Of course, Kippelen notes that additional engineering must be carried out to address scaling and throughput if these devices are to be used in commercial electronics. But he believes as conventional ALD is replaced with a next-generation ALD that utilizes multiple heads with nozzles to deliver the precursors much faster, combined with direct-write inkjet printing for the other layers, there exists a clear path for devices to be scaled up in size with large throughput.

Before that future is realized, Kippelen and his team will further investigate the mechanical properties of these printed transistors since they are likely to be used in large-area products with flexible form factors. They will also continue to investigate the fundamental mechanisms behind some of the compensation effects they have observed in these devices to gain a better understanding of their operation.

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Nanoclast

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

 
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Dexter Johnson
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