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Photo of Stefano Cabrini, head of nanofabrication at The Molecular Foundry

Novel Technique Stamps Out Nanoprobes

One of the cornerstones of the U.S. National Nanotechnology Initiative was the establishment of nanotechnology research facilities that would be available to outside users.  One of the five user facilities operated by the U.S. Department of Energy (DOE) is located at Berkeley National Lab and is known as The Molecular Foundry.

In a visit to The Molecular Foundry, we had the opportunity to discuss the state of nanofabrication with Stefano Cabrini, who is the director of its nanofabrication facility. “The mission of The Molecular Foundry is dedicated to nanoscience in general, but it is a user facility,” Cabrini told IEEE Spectrum. “So we are hosting here people from all over the world, from companies and from academia, from Europe, from Asia, from everywhere. They come here to work with us, to use or our instruments and facilities, but also to have access to our expertise.”

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SEM image of patterned PHCs

Photonic Hypercrystals Are Now a Reality, and Light Will Never Be the Same

In 2014, researchers  predicted the theoretical existence of a strange new material that altered the interaction between light and matter. The material, dubbed photonic hypercrystal, has now become a reality.

Researchers at the City College of New York (CCNY) say they have actually created some, and the realization of these materials could yield big changes in applications including light-based technologies such as solar cells and Li-Fi, where visible light from light emitting diodes provides a means of communication (essentially doing what Wi-Fi does, but with flashes of light instead of radio waves) and quantum information processing.

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The newly-developed smart lenses with built-in pressure-sensing and glucose-monitoring sensors.

Smart Contact Lens Detects Diabetes and Glaucoma

While tech giant Google continues to struggle to make a contact lens for monitoring diabetes, researchers at Ulsan National Institute of Science and Technology (UNIST) in South Korea have offered up at least one part of the puzzle: better wearability. Through the use of a hybrid film made from graphene and silver nanowires, the UNIST researchers have made contact lenses for detecting multiple biomarkers that are clear and flexible.

In research described in the journal Nature Communications, the UNIST researchers used graphene-nanowire hybrid films to serve as conducting, transparent, and stretchable electrodes. While the hybrid film alone does not perform any detection, the electrodes do ensure that the electrodes in the contact lenses don’t obscure vision and that they’re flexible enough to make wearing the lenses comfortable.

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New research allows sound frequencies to be mixed together, amplified and equalized - all within the same millimeter-sized device.

Graphene Speaker Produces Sound and Mixes Frequencies Simultaneously

The history of using nanomaterials such as magnetic nanoparticles or carbon nanotubes in audio speakers has mainly been to demonstrate the capabilities of these materials rather than to yield speakers that will actually be listened to. That changed last year when South Korean researchers used graphene to produce a speaker that does not require an acoustic box to produce sound.

Now researchers from the University of Exeter in the UK are turning again to graphene to make a speaker that produces sound thermoacoustically. Instead of depending on vibrations of a material inside of an acoustic box, thermoacoustics leverages a century-old idea that sound can be produced by the rapid heating and cooling of a material. Where the Exeter researchers’ work departs from that of their Korean counterparts is that this newest device serves not only as a speaker, but also as an amplifier and graphic equalizer—all on a thumbnail-size chip.

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A schematic showing a focused electron beam (green) shining through a polymeric film (grey: carbon atoms; red: oxygen atoms; white: hydrogen atoms).

Lithographic Feature Sizes Reduced to One Nanometer

Scientists at the U.S. Department of Energy’s (DOE) Center for Functional Nanomaterials (CFN) at Brookhaven National Laboratory have established a new record in reducing lithographic feature sizes using electron-beam lithography (EBL), which involves exposing an electron-sensitive material to a focused beam of electrons.

In their latest research, described in the journal Nano Letters, the CFN team performed electron beam lithography with a scanning transmission electron microscope (STEM) to bring individual feature sizes of patterns on polymer poly (methyl methacrylate)—or PMMA—down to one nanometer. The spaces between these features were only 11 nanometers. This brings areal density—a measure of the quantity of information bits that can be stored on a given area of surface—to nearly one trillion per square centimeter.

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Progressively thinner flakes of a van der Waals material

2D Materials Go Ferromagnetic, Creating a New Scientific Field

Researchers at the Lawrence Berkeley National Laboratory have successfully demonstrated that two-dimensional (2D) layered crystals held together by van der Waal forces—these include graphene and molybdenum disulfide—can exhibit intrinsic ferromagnetism. Not only did the team demonstrate that it exists in these materials, but the researchers also demonstrated a high degree of control over that ferromagnetism. The discovery could have a profound impact for applications including magnetic sensors and the developing use of spintronics for encoding information.

In research described in the journal Nature, the Berkeley scientists worked with a 2D chalcogenide layered material called chromium germanium telluride (CGT), a layered ferromagnetic insulator that has garnered interest because of its potential in spintronic devices. While the material has been around in bulk form for decades, only recently has it been made into 2D flakes, joining the list of other van der Waals crystals.

The researchers used an optical technique known as the magneto-optic Kerr effect that involves the use of a scanning Kerr optical microscope to observe the material. This technique detects how the rotation of linearly polarized light is changed when it interacts with electron spins in the material. This made it possible to detect unambiguously that the magnetism was originating from the atomically thin materials. 

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A rubber disk with a reflective copper center

Graphene Makes Infinite Copies of Compound Semiconductor Wafers

Despite graphene’s amazing properties and all the engineering that has gone into giving the wonder material a band gap, its prospects for digital logic remain as much in doubt as they have ever been.

But the list of uses for graphene in electronics outside of digital logic continues to grow. The latest comes from research out of MIT in which graphene could make the use of exotic semiconductors more accessible to industries by preparing semiconductor thin films without the high cost of using bulk wafers of the materials.

In research described in the journal Nature, a thin film of graphene is placed on top of a gallium arsenide (GaAs) wafer. Then compound semiconductors—which are made of more than one element such as gallium arsenide (GaAs), indium phosphide (InP) and indium gallium arsenide (InGaAs)—are grown on top of that graphene layer in an epitaxy process.

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Artistic illustration of a photonic integrated device

Integrated Photonic Circuits Shrunk Down to the Smallest Dimensions Yet

In a major breakthrough for optoelectronics, researchers at Columbia University have made the smallest yet integrated photonic circuit. In the process, they have managed to attain a high level of performance over a broad wavelength range, something not previously achieved.

The researchers believe their discovery is equivalent to replacing vacuum tubes in computers with semiconductor transistors—something with the potential to completely transform optical communications and optical signal processing.

The research community has been feverishly trying to build integrated photonic circuits that can be shrunk to the size of integrated circuits (ICs) used in computer chips. But there’s a big problem: When you use wavelengths of light instead of electrons to transmit information, you simply can’t compress the wavelengths enough to work in these smaller chip-scale dimensions.

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The brass block serves as an electric ground plate ensuring an efficient insertion of the RF currents to the antennae and, on the other hand, microwave connectors mounted to the block allow for the embedding of the device into our microwave setup

Move Over, Spintronics: Here Comes Magnonics to the Rescue of Electronics

As we approach the physical limits of electrical currents performing the same logic computations as previous generations of digital electronics, the question has become: How do we continue to fabricate logic gates when the devices are too small for classical physics?

A European collaborative research center called Spin+X has offered a prototype of a device that leverages something called spin waves, which may offer a way forward. Spin waves are the synchronous waves of electron spin alignment observed in a magnetic system. If the prototype is any indication, then researchers may have another avenue to explore when traditional electronics reaches its physical limits.

Spin+X is funded by the German Research Foundation as well as the European Union–funded project InSpin. It's also been supported by the Belgian nanotechnology research institute Imec. This concentration of expertise has led to the recent development of what the researchers have dubbed a spin-wave majority gate.

Traditional semiconductor-based logic gates—known as majority gates—output current to match either the “0” or “1” state that comprises one of its three input currents, or three voltages. In the spin-wave majority gate described in the journal Applied Physics Letters, the researchers built it out of yttrium-iron-garnet (YIG); its basic operating principle relies on the magnetic material’s atomic magnetic moments, which are essentially the strength of an atom’s magnetism. When these magnetic moments are aligned by an externally applied magnetic field, they interact with each other.

“The interaction can be very well visualized by simply imagining two bar magnets,” explains Tobias Fischer, a doctoral student at the University of Kaiserslautern, in Germany, and lead author of the paper, in an email interview with IEEE Spectrum. “If one brings them together closely and moves one of the magnets, the second magnet will also be influenced by the first magnet's motion.”

The same holds true for the atomic magnetic moments, according to Fischer. When the researchers locally apply a magnetic radio frequency (RF) field in the input wave guides (which, in this case, is produced by sending RF currents through the copper structures underneath the three inputs of a trident structure), this forces some moments to precess around the direction of the external field. The phenomenon known as “precess” occurs when atoms with unpaired electron spins are placed in a magnetic field and rotate around the magnetic field at a precise frequency. This frequency depends on the field strength and the atom's magnetic moment.

The waves excited in the three input wave guides propagate toward the combiner of the device and interfere with each other, resulting in a wave propagating toward the output. Since a spin wave in a wave guide comes with a stray magnetic field around the wave guide, this can again be picked up inductively by another antenna underneath the output wave guide. The phase of the output wave now depends on the phases of the input wave, which is used to encode and process information.

“The interaction between the magnetic moments also makes neighboring moments start to precess,” says Fischer. “This wave-like excitation begins to propagate through the magnetic material and that is what we call spin wave (or, in the particle picture, magnon).”

The term “magnon” refers to the quasiparticles of spin waves and explains why this field of research is being called "magnonics." In contrast to spintronics, which only makes use of the electric charge as a property of electrons as well as its spin moment, magnonics employs spin-wave excitations in magnetic materials.

“Basically, spintronics still requires electric currents but usually restricts these currents to consisting only of spin-up or spin-down electrons, thus providing an additional degree of freedom to process or encode information,” explains Fischer. “However, magnonics can operate without any electric currents by only relying on the propagation of spin waves in a magnetic material as a carrier of information.”

This ability leads to some pretty clear advantages for magnonics, according to Fischer. Since it avoids electric currents, losses such as Joule heating can be drastically reduced. Also, spin waves can feature wavelengths in the nanometer range and gigahertz frequencies, which allows for downscaling of devices and high clock frequencies.

Nevertheless, there are still some challenges to be overcome, such as the efficient excitation and detection of spin waves in order to couple magnonics to conventional electronics.

While there has been another majority-gate device based on magnons, according to Fischer, that device was based on the excitation of magnons via spin-current injection from adjacent platinum structures and the propagation of magnons in a plain film of magnetic material. “As a consequence, this device would not be suitable to make use of the advantages of a wave-guide-based majority gate such as mode selection in the output wave guide,” he adds.

One of the areas that will need to be addressed in future research will be the material science. While YIG features a very low damping resulting in spin waves propagating long distances, CMOS compatibility of this material is rather limited, according to Fischer. “It would also be nice to have a material which can easily be deposited by conventional sputtering techniques, which is also not the case for YIG,” he adds.

In addition, the device has to be significantly miniaturized. Toward this end, Fischer and his colleagues are looking into fabricating majority gate structures from Heusler thin films, which are mixtures of elements that together have desirable thermoelectric properties.

Fischer adds, “All in all, I think there are still challenges to be overcome until a real implementation of spin-wave devices in information technology comes within reach, but I think we are well on track with investigating the fundamentals of such a concept.”

Image credit:  Photo: Stefan Wachter

The Most Complex 2D Microchip Yet

A three-atom-thick microchip with more than 100 transistors is the most complex microprocessor made from a 2-dimensional material to date, researchers say.

The new device is made of a thin film of molybdenite, or molybdenum disulfide (MoS2), which consists of a sheet of molybdenum atoms sandwiched between two layers of sulfur atoms. A single-molecule layer of molybdenum disulfide is only six-tenths of a nanometer thick. In comparison, the active layer of a silicon microchip is up to about 100 nanometers thick. (A nanometer is a billionth of a meter; the average human hair is about 100,000 nanometers wide.)

Scientists hope two-dimensional materials such as graphene or molybdenite will allow Moore's Law to continue once it becomes impossible to make further progress using silicon. Whereas graphene is an excellent conductor, making it ideal for use in wiring and interconnections, molybdenite is a semiconductor, which means it can serve in the transistor switches that lie at the heart of electronic circuits.

The scientists detailed their findings online April 11 in the journal Nature Communications.

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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
Rachel Courtland
New York City
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