Abstract-Tunable Graphene Metasurface Reflectarray for Cloaking, Illusion and Focusing

Sudipta Romen Biswas, Cristian E Gutiérrez, Andrei Nemilentsau, In-Ho Lee, Sang-Hyun Oh, Phaedon Avouris, Tony Low

https://arxiv.org/abs/1712.04111

We present a graphene-based metasurface that can be actively tuned between different regimes of operation, such as anomalous beam steering and focusing, cloaking and illusion optics, by applying electrostatic gating without modifying the geometry of the metasurface. The metasurface is designed by placing graphene nano-ribbons (GNRs) on a dielectric cavity resonator, where interplay between geometric plasmon resonances in the ribbons and Fabry-Perot resonances in the cavity is used to achieve 2π phase shift. As a proof of the concept, we demonstrate that wavefront of the field reflected from a triangular bump covered by the metasurface can be tuned by applying electric bias so as to resemble that of bare plane and of a spherical object. Moreover, reflective focusing and change of the reflection direction for the above-mentioned cases are also shown.

Abstract-Polaritons in layered 2D materials

Tony Low, Andrey Chaves, Joshua D. Caldwell, Anshuman Kumar, Nicholas X. Fang, Phaedon Avouris, Tony F. Heinz, Francisco Guinea, Luis Martin-Moreno, Frank Koppens

(Submitted on 14 Oct 2016 (v1), last revised 7 Jul 2017 (this version, v3))

https://arxiv.org/abs/1610.04548

In recent years, enhanced light-matter interactions through a plethora of dipole-type polaritonic excitations have been observed in two-dimensional (2D) layered materials. In graphene, electrically tunable and highly confined plasmon-polaritons were predicted and observed, opening up opportunities for optoelectronics, bio-sensing and other mid-infrared applications. In hexagonal boron nitride (hBN), low-loss infrared-active phonon-polaritons exhibit hyperbolic behavior for some frequencies, allowing for ray-like propagation exhibiting high quality factors and hyperlensing effects. In transition metal dichalcogenides (TMDs), reduced screening in the 2D limit leads to optically prominent excitons with large binding energy, with these polaritonic modes having been recently observed with scanning near field optical microscopy (SNOM). Here, we review recent progress in state-of-the-art experiments, survey the vast library of polaritonic modes in 2D materials, their optical spectral properties, figures-of-merit and application space. Taken together, the emerging field of 2D material polaritonics and their hybrids provide enticing avenues for manipulating light-matter interactions across the visible, infrared to terahertz spectral ranges, with new optical control beyond what can be achieved using traditional bulk materials.

Researchers explore new 2D materials that could make devices faster, smaller, and efficient

http://www.nanowerk.com/nanotechnology-news/newsid=45205.php

(Nanowerk News) A new study by an international team of researchers led by the University of Minnesota highlights how manipulation of 2D materials could make our modern day devices faster, smaller, and better.

The findings are now online and will be published in Nature Materials ("Polaritons in layered two-dimensional materials").

Two-dimensional materials allow strong light-matter interactions through polaritons.

Two-dimensional materials are a class of nanomaterials that are only a few atoms in thickness. Electrons in these materials are free to move in the two-dimensional plane, but their restricted motion in the third direction is governed by quantum mechanics. Research on these nanomaterials is still in its infancy, but 2D materials such as graphene, transition metal dichalcogenides and black phosphorus have garnered tremendous attention from scientists and engineers for their amazing properties and potential to improve electronic and photonic devices.

In this study, researchers from the University of Minnesota, MIT, Stanford, U.S. Naval Research Laboratory, IBM, and universities in Brazil, UK and Spain, teamed up to examine the optical properties of several dozens of 2D materials. The goal of the paper is to unify understanding of light-matter interactions in these materials among researchers and explore new possibilities for future research.

They discuss how polaritons, a class of quasiparticles formed through the coupling of photons with electric charge dipoles in solid, allow researchers to marry the speed of photon light particles and the small size of electrons.

“With our devices, we want speed, efficiency, and we want small. Polaritons could offer the answer,” said Tony Low, a University of Minnesota electrical and computer engineering assistant professor and lead author of the study.

By exciting the polaritons in 2D materials, electromagnetic energy can be focused down to a volume a million times smaller compared to when its propagating in free space.

“Layered two-dimensional materials have emerged as a fantastic toolbox for nano-photonics and nano-optoelectronics, providing tailored design and tunability for properties that are not possible to realize with conventional materials,” said Frank Koppens, group leader at the Institute of Photonic Sciences at Barcelona, Spain, and co-author of the study.

 “This will offer tremendous opportunities for applications.”Others on the team from private industry also recognize the potential in practical applications.“

The study of the plasmon-polaritons in two-dimensions is not only a fascinating research subject, but also offers possibilities for important technological applications,” said Phaedon Avoruris, IBM Fellow at the IBM T. J. Watson Research Center and co-author of the study. “For example, an atomic layer material like graphene extends the field of plasmonics to the infrared and terahertz regions of the electromagnetic spectrum allowing unique applications ranging from sensing and fingerprinting minute amounts of biomolecules, to applications in optical communications, energy harvesting and security imaging.

”The new study also examined the possibilities of combining 2D materials. Researchers point out that every 2D material has advantages and disadvantages. Combining these materials create new materials that may have the best qualities of both.

“Every time we look at a new material, we find something new,” Low said. “Graphene is often considered a ‘wonder’ material, but combining it with another material may make it even better for a wide variety of applications.”

In addition to Low, Avoruris and Koppens, other researchers involved in the study include Andrey Chaves, Universidade Federal do Cearán (Brazil) and Columbia University; Joshua D. Caldwell, U.S. Naval Research Laboratory; Anshuman Kumar, University of Minnesota and Massachusetts Institute of Technology; Nicholas X.Fang, MIT; Tony Heinz, Stanford University; Francisco Guinea, IMDEA Nanociencia and University of Manchester; and Luis Martin-Moreno, University of Zaragoza (Spain).

IBM assigned Patent for Generation of Terahertz Electromagnetic Waves in Graphene By Coherent Photon-mixing

http://technews.tmcnet.com/news/2014/08/19/7978040.htm
*** International Business Machines Assigned Patent for Generation of Terahertz Electromagnetic Waves in Graphene By Coherent Photon-mixing ALEXANDRIA, Va., Aug. 18 -- International Business Machines, Armonk, New York, has been assigned a patent (8,805,148) developed by four co-inventors for the "generation of terahertz electromagnetic waves in graphene by coherent photon-mixing." The co-inventors are Phaedon Avouris, Yorktown Heights, New York, Chun-Yung Sung, Poughkeepsie, New York, Alberto Valdes Garcia, Hartsdale, New York, and Fengnian Xia, Plainsboro, New Jersey.

The patent application was filed on July 14, 2011 (13/182,621). The full-text of the patent can be found athttp://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=8,805,148.PN.&OS=PN/8,805,148&RS=PN/8,805,148 Written by Balkishan Dalai; edited by Jaya Anand.

Abstract-Graphene Plasmonics for Terahertz to Mid-Infrared Applications

Tony Low, Phaedon Avouris

(Submitted on 12 Mar 2014)

http://arxiv.org/abs/1403.2799

In recent years, we have seen a rapid progress in the field of graphene plasmonics, motivated by graphene's unique electrical and optical properties, tunabilty, long-lived collective excitation and their extreme light confinement. Here, we review the basic properties of graphene plasmons; their energy dispersion, localization and propagation, plasmon-phonon hybridization, lifetimes and damping pathways. The application space of graphene plasmonics lies in the technologically significant, but relatively unexploited terahertz to mid-infrared regime. We discuss emerging and potential applications, such as modulators, notch filters, polarizers, mid-infrared photodetectors, mid-infrared vibrational spectroscopy, among many others.

Abstract-Graphene Plasmonics for Terahertz to Mid-Infrared Applications

Tony Low * and Phaedon Avouris *

IBM T.J. Watson Research Center, 1101 Kitchawan Road, Yorktown Heights, New York 10598, United States

http://pubs.acs.org/doi/abs/10.1021/nn406627u

In recent years, we have seen a rapid progress in the field of graphene plasmonics, motivated by graphene’s unique electrical and optical properties, tunability, long-lived collective excitation and its extreme light confinement. Here, we review the basic properties of graphene plasmons: their energy dispersion, localization and propagation, plasmon–phonon hybridization, lifetimes and damping pathways. The application space of graphene plasmonics lies in the technologically significant, but relatively unexploited terahertz to mid-infrared regime. We discuss emerging and potential applications, such as modulators, notch filters, polarizers, mid-infrared photodetectors, and mid-infrared vibrational spectroscopy, among many others.

IBM Demonstrates a Competitive Graphene Infrared Detector

Image: Tony Low

In the image, plasmon dispersion in graphene on silicon dioxide substrate, reveals coupling with long-lived substrate phonons. They can be excited by patterning graphene into nanoribbons.

By Dexter Johnson

Posted 24 Jul 2013 | 20:43 GMT

http://spectrum.ieee.org/nanoclast/semiconductors/nanotechnology/ibm-demonstrates-a-competitive-graphene-infrared-detector

Earlier this year, researchers at IBM’s Nanoscale Science and Technology group revealed some of the fundamental photoconductivity mechanisms of graphene.

The IBM researchers demonstrated that graphene can either be positive or negative depending on its gate bias. The positive is due to a photovoltaic effect and the negative is due to a bolometric effect.

The bolometric effect involves photo-generated carriers that, while propagating across graphene, emit quanta of lattice vibrations called phonons and thereby transfer their energy into the lattice. Heating up the lattice implies enhancing the electron-phonon scattering process and reducing the carrier’s mobility. The IBM researchers discovered this effect was dominant in the photo response of graphene and is what leads to the photocurrent flowing in the opposite direction of the source-drain current.

In new research, which was published both in Nature Communications(“Photocurrent in graphene harnessed by tunable intrinsic plasmons”) andNature Photonics (“Damping pathways of mid-infrared plasmons in graphene nanostructures”), the IBM team has begun to explore ways to amplify this bolometric effect in graphene.

The research team, which includes Hugen Yan, Tony Low, Wenjuan Zhu, YanqingWu, Marcus Freitag, Xuesong Li, Francisco Guinea, Phaedon Avouris, and Fengnian Xia, began by first studying the fundamental property of plasmons in graphene metamaterials by purely optical methods, revealing important information about its dispersion and damping mechanisms. This knowledge guided them in their design of graphene photodetectors, leading to the first demonstration of a graphene infrared detector driven by intrinsic plasmons.

Graphene’s high mobility and zero gap nature gives it fast optoelectronic response and detection in an extremely broad spectral range from the visible over the infrared and into the terahertz range.

In the visible and near-IR, semiconductors are more efficient in detecting light than graphene because they can have matched bandgaps to a particular spectral window, and because a single layer of graphene absorbs only a small fraction of the incoming light. So it is very unlikely that we will some day be able to buy a cell-phone or camera with a graphene photodetector in it.

However, at lower energies, for example in the mid-IR or terahertz regime, graphene could be much more competitive and provide a unique technology solution. Currently, superconducting transition-edge detectors and bolometers are state of the art in these regimes, and these detectors are very expensive. The absorption in a single layer of graphene can be as high as 40 percent in the terahertz, and the window of high absorption can be moved into the mid-IR by patterning the graphene and harvesting graphene plasmons.

The graphene-based photodetectors, which utilize their intrinsic plasmons, have been demonstrated to yield an order of magnitude improvement in the device’s photo-responsivity in comparison to its non-plasmonic counterpart.

The graphene used in the photodetectors were first grown by CVD on copper foil. Copper was then dissolved in etchant, and finally graphene was transferred to a silicon/silicon oxide chip. The researchers built the graphene photodetector itself by patterning graphene into superlattices of graphene nanoribbons using e-beam lithography. The ribbons widths range from 80 to 200 nm and lateral confinement in ribbons provides the necessary momentum to couple with the graphene plasmons. It is then illuminated with a chopped CO2 infrared laser beam.

The researchers believe that graphene plasmonics could potentially provide a natural platform for a range of technologies in the infrared regime such as light detection and modulation, optical communications, photovoltaics, and spectroscopy.

With this basic understanding of how graphene plasmon disperses, damps, and generates photocurrent, the IBM team is now more confident about this line of research. The merging of graphene plasmonics with optoelectronics is a field that has essentially just began so there remain fundamental and technological issues to resolve.

Researchers at Georgia Tech develop terabit wireless antennas

by Gareth Halfacree
http://www.bit-tech.net/news/hardware/2013/03/05/georgia-terabit/1
Researchers at the Georgia Institute of Technology have designed a graphene-based antenna that could potentially allow for data transfer rates as high as a terabit per second over a metre range.

The system, developed by a team from the Broadband Wireless Networking Laboratory at Georgia Tech led by director Ian Akyildiz, uses graphene - sheets of carbon just one atom thick arrayed in a honeycomb structure - to create narrow strips between 10 and 100 nanometres wide and one micrometer long, forming terahertz frequency antennas. Electrons oscillating on the surface of each graphene strip, known as plasmonic waves, interact with electromagnetic waves at the terahertz frequency in order to receive or transmit a signal.

According to the team's calculations, such a terahertz radio system could transfer data at a rate of one terabit per second - roughly 2,330 times faster than 802.11n Wi-Fi. While that is only sustainable at a range of a metre or less, as a close-range data-transfer tool the team's graphene-based antenna could prove extremely useful indeed: drop the range to a handful of centimetres and the data transfer rate could reach as high as 100 terabits per second.

Such a system could be a boon for external peripherals that rely on the transfer of large quantities of data. A high-definition video camera, for example, could dump all its footage in under a second just by being placed near a laptop or desktop equipped with the team's antenna, or a smartphone quickly download rented or purchased films for on-the-go viewing.

Sadly, as is often the case with such 'breakthroughs' involving the wonder-material graphene, the technology is far from a commercial reality just yet. '[The team's work] points out and provides a set of classical calculations on estimates of sizes and performance: it points out that there is something worthwhile here' explained Phaedon Avouris, IBM research fellow and graphene expert, in an interview on the subject with MIT Technology Review. It doesn’t solve the whole problem, but points out an opportunity.'

The team's work, which has up to this point been purely theoretical, will need proving with a prototype device - something Akyildiz claims is due for unveiling before the end of the year - and then the antenna will need to be mated to other high-performance hardware in order to reach anywhere near the terabit speeds promised. However, with the research due to appear in the IEEE Journal of Selected Areas in Communication in the coming year, those are problems that are likely to get many eyes eager to help make the next big breakthrough in high-speed wireless communications possible.

Magnetoplasmonics moves on

Image via CrunchBase

http://nanotechweb.org/cws/article/tech/50110

Researchers at IBM have come across an unexpected phenomenon while studying how plasmons in graphene behave in the presence of a magnetic field. The finding could help the new and upcoming field of magnetoplasmonics, with graphene finding its way into terahertz magneto-optical devices, such as modulators and Faraday rotators.

Excitation of magnetoplasmons in graphene disks

The team, headed by Phaedon Avouris of IBM's TJ Watson Research Center in New York and Zhiqiang Li of the National High Magnetic Field Laboratory in Florida, studied graphene that had been patterned into a periodic array of microdisks. The structure absorbs light by confining it into regions that are hundreds of times smaller than the wavelength of the light by exploiting plasmons that occur within the individual microdisks. Plasmons are quantized collective oscillations of electrons – and they interact strongly with light.

Graphene appears to be emerging as a very promising plasmonic material thanks to the material's unusual electronic properties, which result in its electrons moving extremely fast and behaving like relativistic "Dirac" particles with virtually no rest mass. Graphene absorbs light particularly well in the terahertz and infrared parts of the electromagnetic spectrum – something that could lead to novel applications in photonics and quantum optics. "What is more, unlike plasmons in metals, the plasmons in graphene should
be strongly affected by an external magnetic field – all because graphene electrons behave like massless fermions," explained project leader Hugen Yan.

Longer-lived edge plasmons

The researchers obtained their results by measuring the light transmission spectrum of the graphene disk arrays, using a Fourier transform IR spectrometer together with a silicon bolometer, while a magnetic field was applied perpendicular to the disks. To their surprise, they found that plasmons at the edges of the nanostructures appear to last longer when a magnetic field is applied. This is counter-intuitive, Yan tells nanotechweb.org, because there are potentially more defects in the vicinity of an edge plasmon that should, conversely, reduce its lifetime.

According to the team, the applied magnetic field may be suppressing electron backscattering at the edges of the microdisks, so allowing the plasmons to last longer in the samples. The result is doubly unexpected given that the phenomenon has never been observed before in conventional 2D electron gas systems in the terahertz range.

And that is not all: the lifetimes of the edge plasmons can also be tuned by varying the magnitude of the applied magnetic field, with larger fields encouraging longer-lived plasmons.

"A long plasmon lifetime is a big advantage for applications in chemical and biological sensing, as well as in electric field enhancement, and could lead to a variety of magneto-optical applications in the future," said Yan.

The IBM team is now planning to study magnetoplasmons in other graphene microstructures, such as graphene rings, dots, ribbons and elliptical disks.

The current work is detailed in Nano Letters.

About the author

Belle Dumé is contributing editor at nanotechweb.org

Abstract-Magnetic field tuning of terahertz Dirac plasmons in graphene

Hugen Yan, Zhiqiang Li, Xuesong Li, Wenjuan Zhu, Phaedon Avouris, Fengnian Xia

(Submitted on 19 Apr 2012)

Boundaries and edges of a two dimensional system lower its symmetry and are usually regarded, from the point of view of charge transport, as imperfections. Here we present a first study of the behavior of graphene plasmons in a strong magnetic field that provides a different perspective. We show that the plasmon resonance in micron size graphene disks in a strong magnetic field splits into edge and bulk plasmon modes with opposite dispersion relations, and that the edge plasmons at terahertz frequencies develop increasingly longer lifetimes with increasing magnetic field, in spite of potentially more defects close to the graphene edges. This unintuitive behavior is attributed to increasing quasi-one dimensional field-induced confinement and the resulting suppression of the back-scattering. Due to the linear band structure of graphene, the splitting rate of the edge and bulk modes develops a strong doping dependence, which differs from the behavior of traditional semiconductor two-dimensional electron gas (2DEG) systems. We also observe the appearance of a higher order mode indicating an anharmonic confinement potential even in these well-defined circular disks. Our work not only opens an avenue for studying the physics of graphene edges, but also supports the great potential of graphene for tunable terahertz magneto-optical devices.

Comments:	27 pages, 4 figures with supplementary information, submitted
Subjects:	Mesoscale and Nanoscale Physics (cond-mat.mes-hall)
Cite as:	arXiv:1204.4398v1 [cond-mat.mes-hall]

Submission history

From: Hugen Yan Mr [view email]
[v1] Thu, 19 Apr 2012 16:10:28 GMT (622kb)

Image via CrunchBase

IBM demos terahertz graphene photonics

R. Colin Johnson

4/22/2012 1:01 PM EDT

PORTLAND, Ore.—Graphene has been courted as the miracle material of the future, since different formulations have been fabricated into conductors, semiconductors and insulators. Now IBM has added photonic to the list by demonstrating a graphene/insulator superlattice that achieves a terahertz frequency notch filter and a linear polarizer, devices which could be useful in future mid- and far-infrared photonic devices, including detectors, modulators and three-dimensional metamaterials.

"In addition to its good electrical properties, graphene also has exceptional optical properties. In particular, it absorbs light from the far-infrared to to the ultra-violet," said IBM Fellow Phaedon Avouris. "The terahertz range was of particular interest to IBM, because these frequencies can penetrate paper, wood and other solid objects for security applications. Unfortunately, today there are very few ways of manipulating terahertz waves such as polarizing and filtering it, but because graphene operates well at terahertz frequencies we have concentrating on creating these types of devices."

Teraherz frequency oscillations can be carried in graphene by plasmons—the collective oscillation of carriers—to enable low-loss tunable filters. But in single-layer graphene, the carrier concentration and resonant frequency was too weak for photonics applications, according to IBM. However, by going to a multi-layer graphene/instulator superlattice, transparent devices can be patterned into photonic-like crystals that distribute the carriers among the layers effectively enhancing both the carrier density and the resonant frequency.
 
 "We have found that graphene interaction with electromagnetic radiation is particularly strong in the terahertz range, however with a single layer of graphene the interaction was still not strong enough," said Hugen Yan, a member of the Nanoscale Science and Technology Group at IBM's T. J. Watson Research Lab (Yorktown Heights, N.Y.) "But by using a multi-layer stack structure in microdisk arrays we achieved frequency selectivity in the terahertz range, allowing us to tune the desired resonant frequency."

 Scanning Electron Microscope image of, five-layer graphene/insulator superlattice array of two-micron diameter microdisks (purple).

 
I

http://www.eetimes.com/electronics-news/4371446/IBM-demos-terahertz-graphene-photonics

IBM found that by patterning the graphene/insulator microdisks in arrays, it was able to tune their resonant frequency by varying the size of the microdisks, the number of layers, their spacing, and the doping of the graphene layers.  Upon analysis, IBM discovered a unique carrier density scaling law for its graphene/insulator superlattices that, unlike conventional semiconductor superlattices, is based on laws governing Dirac fermions (such as quarks, leptons, baryons and hadrons).

As a result, IBM has been able to demonstrate patterned graphene/insulator stacks implementing a widely tunable notch filters with a 8.2-dB rejection ratio, and a terahertz linear polarizer with 9.5-dB extinction ratio. Implemented by laying down wafer scale alternating layers of graphene and a polymer insulator, then patterning them into microdisks, IBM demonstrated that these graphene/insulator superlattices shielded 97.5 percent of electromagnetic radiation at frequencies below 1.2 terahertz.

For the future, the research group intends to tune its graphene/insulator superlattices for the infrared frequencies used by optical communications equipment today. Fengnian Xia, a member of the Nanoscale Science and Technology Group at T. J. Watson Research Lab (Yorktown Heights, N.Y.), also contributed to the work.

Graphene could be ideal for Terahertz applications

http://nanotechweb.org/cws/article/tech/47974

Graphene absorbs infrared light

Graphene has remarkable optical properties and can absorb more than 2% of incoming visible light – an astonishing fact since the material is only one atom thick. Now, researchers at IBM have discovered that graphene can also absorb up to 40% of light in the far infrared and microwave frequency ranges. The finding confirms that the material could be ideal for terahertz and photonics applications.

Graphene is a 2D sheet of carbon just one atom thick. Since its discovery in 2004, this "wonder material" has continued to amaze scientists with its growing list of unique electronic and mechanical properties. Some believe that graphene could find uses in a number of technological applications – even replacing silicon as the electronic industry's material of choice. This is because electrons whiz through graphene at extremely high speeds, behaving like "Dirac" particles with no rest mass.

Graphene could also be ideal for photonics applications. Thanks to its Dirac electrons, it can also absorb light over a very wide range of wavelengths, ranging from the visible to the infrared. This is unlike III-V semiconductors that do not work over such a wide range.

The infrared part of the electromagnetic spectrum is important for optical telecommunications, for example, and the terahertz range in areas like biological imaging, materials analysis and security screening. Characterizing graphene at these wavelengths is thus crucial for developing graphene optoelectronic devices for such applications.

The IBM researchers, led by Phaedon Avouris of the TJ Watson Research Center in New York, had already analysed the infrared radiation emitted from graphene in previous work. The results from these experiments allowed them to determine the temperature distribution, carrier (electron and hole) densities and the position of the Dirac point in the graphene channel. The Dirac (or charge neutrality) point is the point in graphene's band structure where the valence and conduction bands touch. The Fermi level of undoped (or intrinsic) graphene coincides with the Dirac point, and the position of this point is crucial for defining graphene's properties, explains Avouris.

Now, the same team has gone a step further and has studied few-layer wafer-scale epitaxial and single-layer CVD graphene again using infrared spectroscopy. As well as some of the parameters listed above, the researchers were able to obtain new information on the material. For example, because the absorption of the carriers in the far infrared is directly proportional to the optical conductivity of the graphene sheet, this allowed them to determine the sheet resistance. They also succeeded in calculating the rate at which free carriers are scattered during transport because this is directly related to the frequency dependence of the optical conductivity.

According to team member Hugen Yan, the new work "opens up avenues for applications in transparent terahertz optoelectronics, terahertz and infrared metamaterials, cloaking and transformation optics".

He and his colleagues are now working on further improving the doping in CVD graphene and achieving even higher absorption of light in the far infrared. "Making transparent terahertz devices based on graphene is also on our agenda," he toldnanotechweb.org.

The current work is detailed in ACS Nano.

About the author

Belle Dumé is contributing editor at nanotechweb.org

First Graphene Integrated Circuit

IBM researchers take next step in building graphene-based electronics

By Neil Savage  /  June 2011

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Photo: AAAS/Science

9 June 2011—IBM researchers have built the first integrated circuit (IC) based on a graphene transistor—another step toward overcoming the limits of silicon and a potential path to flexible electronics.
The circuit, built on a wafer of silicon carbide, consists of field-effect transistors (FETs) made of graphene, a highly conductive chicken-wire-like arrangement of carbon that's a single atomic layer thick. The IC also includes metallic structures, such as on-chip inductors and the transistors' sources and drains. The work is described in this week's issue of Science. Researchers say that graphene, which has the potential to make transistors that operate at terahertz speeds, could one day supplant silicon as the basis for computer chips.
Several groups have built transistors out of graphene; the IBM team, led by Phaedon Avouris at the Thomas J. Watson Research Center, demonstrated one last year that operated at 100 gigahertz—more than twice as fast as a silicon transistor of comparable dimensions. But as Keith Jenkins, one of the scientists involved in the new research, points out, "a transistor by itself is no good unless you connect it to something."
The circuit the team built is a broadband radio-frequency mixer, a fundamental component of radios that processes signals by finding the difference between two high-frequency wavelengths. "It's a completely ubiquitous circuit," Jenkins says. The device, which is a proof-of-concept and not designed to be an optimal commercial component, handles frequencies up to 10 GHz. "Ultimately, we should be able to go a lot faster," Jenkins says. "This is not a limit at all."
The tricky part was integrating the graphene FET with other components—"a pretty difficult engineering challenge" that took about a year, Jenkins says. There are two main difficulties: One is that the metals used to make other parts of the circuit—aluminum, gold, and palladium in this instance—don't adhere very well to the graphene. The other is the fact that graphene, being only a single atom thick, is easily damaged by standard semiconductor etching processes. One way the team addressed the damage problem was to grow the graphene on a silicon-carbide wafer, then coat it with a common polymer, PMMA, and a resist that was sensitive to jets of electrons used in electron beam lithography. That allowed them to protect the graphene they needed during processing but also remove it where it wasn't wanted.
One remarkable feature is that the performance of the device didn't change very much when its temperature went from 300 to 400 kelvins (about 27 °C to 127 °C). That means a graphene circuit won't have to be overdesigned to compensate for temperature changes, potentially leading to a less-complex and less-expensive circuit.
Tomás Palacios, an electrical engineer at MIT, called the device "a nice piece of work," adding, "Although there is still a lot of work to be done to improve the device and circuit performance, it represents an important step forward to useful circuits."
The IBM team identified a couple of steps that could improve the performance, such as using a thinner dielectric layer in the transistors. Jenkins says the team is also looking for better materials for the contacts, because anything that touches the graphene has the potential to degrade its electron mobility. The next component he'd like to build is a graphene-based amplifier, though the electronic properties of the material make that challenging.
It will be several years before graphene devices are ready to displace conventional silicon circuits, which are expected to start hitting their limits later this decade. But Jenkins says progress has been remarkably fast with graphene, which was isolated only in 2004. Beyond surpassing the performance of silicon, the material, which is strong, transparent, and bendable, could lead to flexible printed electronics. Applications could include cellphones stitched into clothing or GPS receivers on soldiers' uniforms. Says Palacios: "I think that the exciting opportunity of graphene is to be able to integrate these devices/circuits into arbitrary substrates, from plastics to silicon and glass. This integration will allow us to have graphene-based electronics everywhere. It is what I call 'ubiquitous electronics.' "

About the Author

Neil Savage writes about strange semiconductors and amazing optoelectronics from Lowell, Mass. In May 2011, he wrote about a single-laser system that transmits a record 26 terabits per second of data.