Posted By Dexter Johnson, IEEE Spectrum,
Tuesday, April 24, 2018
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Photons are faster than electrons. This has lead scientists to see if they can harness light (photons) to operate an integrated circuit. While this should result in faster circuits, there’s a hitch: wavelengths of light are much larger than the dimensions of today’s computer chips. The problem is that you simply can’t compress the wavelengths to the point where they work in these smaller chip-scale dimensions.
Scientists have been leveraging a new tool lately to shrink the wavelengths of light to fit into smaller dimensions: plasmonics. Plasmonics exploits the waves of electrons—known as plasmons—that are formed when photons strike a metallic structure. Graphene has played a large role in this emerging field because it has the properties of a metal—it’s a pure conductor of electrons.
The Institute of Photonic Sciences (ICFO) in Barcelona, which has been a leader in this field for years, is now reporting they have taken the use of graphene for shrinking the wavelengths of light to a new level. In research described in the journal Science, ICFO researchers have managed to confine light down to a space one atom thick in dimension. This is certainly the smallest confinement ever achieved and may represent the ultimate level for confining light.
The way the researchers achieved this ultimate confinement was to use graphene along with one of its two-dimensional (2D) cousins: hexagonal boron nitride, which is an insulator.
By using these 2D cousins together, the researchers created what’s known as van der Waals heterostructures in which monolayers of different 2D materials are by stacked on top of each other and held together by van der Waal forces to create materials with tailored electronic properties—like different band gaps for stopping and starting the flow of electrons. In this case, the layers included hexagonal boron nitride layered on top of the graphene and then involved adding an array of metallic rods on top of that. This structure had the graphene sandwiched between an insulator and a conductor. The graphene in this role served to guide the plasmons that formed when light struck the outer metallic rods.
In the experiment, the ICFO researchers sent infrared light through devices made from the van der Waal heterostructures to see how the plasmons propagated in between the outer metallic rods and the graphene.
To get down to the dimensions of one atom for confining the light, the researchers knew that they had to reduce the gap between the metal and the graphene. But the trick was to see if it was possible to reduce that gap without it leading to additional energy losses.
To their surprise, the ICFO researchers observed that even when a monolayer of hexagonal boron nitride was used as a spacer, the plasmons were still excited by the light, and could propagate freely while being confined to a channel of just on atom thick. They managed to switch this plasmon propagation on and off, simply by applying an electrical voltage, demonstrating the control of light guided in channels smaller than one nanometer of height.
The researchers believe that these results could to lead a new generation of optoelectronic devices that are just one nanometer thick. Down the road, this could lead to new devices such as ultra-small optical switches, detectors and sensors.
Hexagonal boron nitride
Posted By Dexter Johnson, IEEE Spectrum,
Wednesday, November 1, 2017
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Last June, we covered research that brought graphene, quantum dots and CMOS all together into one to change the future of both optoelectronics and electronics.
That research was conducted at the Institute of Photonics (ICFO) located just outside of Barcelona, Spain. The Graphene Council has been speaking to Frank Koppens at ICFO since 2015 about how graphene was impacting photonics and optoelectronics.
Now, in a series of in-person interviews with several researchers at ICFO (the first of which you can find here), we are gaining better insight into how these technologies came to be and where they ultimately may lead.
Gerasimos Konstantatos - group leader at ICFO
The combination of graphene with quantum dots for use in optoelectronics stems in large part from the contributions of Gerasimos Konstantatos, a group leader at ICFO, who worked with Ted Sargent at the University of Toronto, whose research group has been at the forefront of exploiting colloidal quantum dots for use in a range of applications, most notably high-efficiency photovoltaics.
“Our initial expertise and focus was on actually exploiting the properties of solution-process materials particularly colloidal quantum dots as optoelectronic materials for solar cells and photodetectors,” explained Konstantatos. “The uniqueness of these materials is that they give us access to a spectrum that is very rarely reached in the shortwave and infrared and they can do it at a much lower cost than any other technology.”
Konstantatos and his group were able to bring their work with quantum dots to the point of the near-infrared wavelength spectrum, which falls in the wavelength size range of one to five microns. Konstantos is now developing these solution-based quantum dot materials to produce even more sensitive materials capable of getting to 10 microns, putting them squarely in the mid-infrared range.
“My group is now working with Frank Koppens to sensitize graphene and other 2D materials in order to make very sensitive photodetectors at a very low cost that are capable of accessing the entire spectrum, and this cannot be done with any other technology,” said Konstantatos.
What Konstantatos and Koppens have been able to do is to basically eliminate the junction between graphene and the quantum dots and in so doing have developed a way to control the charge transfer in a very efficient way so that they can exploit the very high mobility and transport conductance of graphene.
“We can re-circulate the charges through the materials so that with a single photon we have several billion charges re-circulating through the material and this constitutes the baseline of this material combination,” adds Konstantatos.
With that as their baseline technology, Konstantatos and his colleagues have engineered the quantum dot layer so instead of just having a passive quantum dot layer they have converted it into an electro-diode. In this way they can make much more complex detectors. In the combination of the graphene-based transistor with the quantum dots, it’s not just a collection of quantum dots but is a photodiode made from quantum dots.
“In this way, we kind of get the benefit of both kinds of detectors,” explains Konstantatos. “You have a phototransistor that has a very high sensitivity and a very high gain, but you also get the high quantum efficiency you get in photodiodes. It’s basically a quantum photodiode that activates a transistor.”
In addition to the use of graphene, the ICFO researchers are looking at other 2D materials in this combination, specifically the semiconductor molybdenum disulfide. While this material is a semiconductor and sacrifices somewhat on the electron mobility of graphene, it does make it possible to switch off the material to control the current. As a result, Konstantatos notes that you can have much lower noise in the detector with much lower power consumption.
In continuing research, Konstantatos hinted at yet to be published work on how all of this combination of quantum dots and graphene could be used in solar cell applications.
In the meantime, the work they have been doing with graphene and quantum dots is much further advanced than what they have yet been able to achieve with molybdenum disulfide, mainly because work has advanced much further in making large scale amounts of graphene. But as the processes for producing other 2D materials improves, there will be a real competition between all of the 2D materials to see which provides the best possible performance as well as manufacturability properties.
In any event, Konstantatos sees that the way forward with both quantum dots and 2D materials is using them together.
He adds: “I think we can explore the synergies in between different material platforms. There's no such thing as a perfect material that can do everything right. But there is definitely a group of materials with some unique properties. And if you can actually combine them in a smart way and make hybrid structures, then I think you can have significant added value.”
Posted By Terrance Barkan,
Monday, October 31, 2016
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Applications that have really spurred a huge amount of graphene and other two-dimensional (2D) material research over the years have come from the field of electronics. The fear that complementary metal–oxide–semiconductor (CMOS) technology is quickly nearing the end of its ability to ward off Moore’s Law, in which the number of transistors in a dense integrated circuit doubles approximately every two years, has been the spur for much graphene research.
However, there has always been the big problem for graphene that it does not have an intrinsic band gap. It’s a pure conductor and not a semiconductor, like silicon, capable turning on and off the flow of electrons through it. While graphene can be functionalized in a way that it does have a band gap, research for it in the field of electronics have looked outside of digital logic where an intrinsic band gap is such an advantage.
In the stories below, we see how graphene’s unrivaled conductivity is being exploited to take advantage of its strengths rather than trying to cover up for its weaknesses.
Graphene Comes to the Rescue of Li-ion Batteries
The role of graphene in increasing the charge capacity of the electrodes in lithium-ion (Li-ion) batteries has varied. There’s been “decorated graphene” in which nanoparticles are scattered across the surface of the graphene, and graphene nanoribbons, just to name a few of the avenues that have been pursued.
Another way in which graphene has been looked at is to better enable silicon to serve as the electrode material for Li-ion batteries. Silicon is a great material for increasing the storage capacity of electrodes in Li-ion batteries, but there’s one big problem: it cracks after just few charge/discharge cycles. The aim has been to find a way to make silicon so that it’s not so brittle and can withstand the swelling and shrinking during the charge charging and discharing of lithium atoms into the electrode material In these efforts, like those out Northwestern University, the role of graphene has been to sandwich silicon between layers graphene sheets in the anode of the battery.
Now, Yi Cui from both Stanford University and the Department of Energy’s SLAC National Accelerator Laboratory, who has been at the forefront of research to get silicon to be more flexible and durable for Li-ion batteries, has turned to graphene to solve the issue.
Cui and his colleagues were able to demonstrate in research described in the journal Nature Energy, a method for to encasing each particle of silicon in a cage of graphene that enables the silicon to expand and contract without cracking. In a full-cell electrochemical test, the graphene-infused silicon anodes retained 90 percent of their charge capacity after 100 charge-discharge cycles.
Previous attempts by Cui and many others to create nanostructured silicon has been very difficult, making mass production fairly impractical. However, based on these latest results, Cui believes that this approach is not only technologically possible, but may in fact be commercially viable.
The process involves coating the silicon particles with a layer of nickel. The nickel coating is used as the surface and the catalyst for the second step: growing the graphene. The final step of the process involves using an acid on the graphene-coated silicon particles so that the nickel is etched away.
“This new method allows us to use much larger silicon particles that are one to three microns, or millionths of a meter, in diameter, which are cheap and widely available,” Cui said in a press release. “Particles this big have never performed well in battery anodes before, so this is a very exciting new achievement, and we think it offers a practical solution.”
While a practical manufacturing approach was much needed, the technique also leads to an electrode material with very high charge capacity.
“Researchers have tried a number of other coatings for silicon anodes, but they all reduced the anode’s efficiency,” said Stanford postdoctoral researcher Kai Yan, in a press release. “The form-fitting graphene cages are the first coating that maintains high efficiency, and the reactions can be carried out at relatively low temperatures.”
Graphene Provides the Perfect Touch to Flexible Sensors
Photo: Someya Laboratory
Flexible sensors are the technological backbone of artificial skin technologies. The idea is that you can impart the sense of touch to a flexible sensor, making it possible to cover a prosthetic device for either a robot or replacement limb so it can feel. Creating materials that tick the boxes of flexibility, durability and sensitivity has been a challenge. Over the years, researchers have increasingly turned to nanomaterials, and graphene in particular, as a possible solution.
Researchers at the University of Tokyo have found that nanofibers produced from a combination of carbon nanotubes and graphene overcomes some of the big problems facing flexible pressure sensors: they’re not that accurate after being bent or deformed. The researchers have suggested that the flexible sensor they have developed could provide a more accurate detection breast cancer.
In research described in the journal Nature Nanotechnology, the scientists produced their flexible sensor by employing organic transistors and a pressure sensitive nanofiber structure.
The researchers constructed the nanofiber structure using nanofibers with diameters ranging between 300 to 700 nanometers. The researchers produced the nanofibers by combining carbon nanotubes and graphene and mixing that into a flexible polymer. The nanofibers entangled with each other to form a thin, transparent structure.
In contrast to other flexible sensors in which the striving for accuracy makes the sensors too sensitive to being deformed in any way, the fibers in this new flexible sensor does not lose their accuracy in measuring pressures. These fibers achieve this because of their ability to change their relative alignment to accommodate the bending. This allows them to continue measuring pressure because it reduces the strain in individual fibers.
Tunable Graphene Plasmons Lead to Tunable Lasers
Illustration: University of Manchester
A few years ago, researchers found that the phenomenon that occurs when photons strike a metallic surface and stir up the movement of electrons on the surface to the point where the electrons form into waves—known as surface plasmons—also occurs in graphene.
This discovery along with the ability to tune the graphene plasmons has been a big boon for the use of graphene in optoelectronic applications. Now research out of the University of Manchester, led by Konstantin Novoselov, who along with Andre Geim were the two University of Manchester scientists who won the Nobel Prize for discovering graphene, has leveraged the ability of tuning graphene plasmons and combined it with terahertz quantum cascade lasers, making it possible to reversibly alter their emission.
This ability to reversibly the alter the emission of quantum cascade lasers is a big deal in optoelectronic applicatiopns, such as fiber optics telecommunication technologies by offering potentially higher bandwidth capabilities.
“Current terahertz devices do not allow for tunable properties, a new device would have to be made each time requirements changed, making them unattractive on an industrial scale,” said Novoselov in a press release. “Graphene however, can allow for terahertz devices to be switched on and off, as well as altering their state.”
In research described in the journal Science, were able to manipulate the doping levels of a graphene sheet so that it generated plasmons on its surface. When this doped graphene sheet was combined with a terahertz quantum cascade laser, it became possible to tune the transmission of the laser by tuning the graphene plasmons, essentially changing the concentration of charge carriers.
Graphene Flakes Speed Up Artificial Brains
Illustration: Alexey Kotelnikov/Alamy
Researchers out of Princeton University have found that graphene flakes could be a key feature in computer chips that aim at mimicking the function of the human brain.
In the human brain, neurons are used to transmit information by passing electrical charges through them. In artificial brains, transistors would take the place of neurons. One approach has been to construct the transistors out of lasers that would turn and off and the time intervals between the on and off states of the lasers would represent the 1s and 0s of digital logic.
One of the challenges that researchers have faced in this design is getting the time intervals between the laser pulses down to picosecond time scales, one trillionth of a second.
In research described in the journal Nature Scientific Reports, the Princeton researchers placed graphene flakes inside a semiconductor laser to act as a kind of “saturable absorber,” that absorbed photons and then was able to emit them in a quick burst.
It turns out graphene possesses a number of properties that makes it attractive for this application. Not only can it absorb and release photons extremely quickly, but it can also work at any wavelength. What this means is that even if semiconductor lasers are emitting different colors, the graphene makes it possible for them to work together simultaneously without interfering with each other, leading to higher processing speeds.
Posted By Dexter Johnson, IEEE Spectrum,
Saturday, October 8, 2016
Updated: Thursday, October 6, 2016
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SPIE—the international society for optics and photonics—has been a society set up to advance light-based technologies since 1955. In this role, it has offered its members conferences, news services and a range of different avenues for exchanging information on this quickly developing field.
As evidence of its commitment to staying ahead of the latest science and technologies in photonics and optics, SPIE has been offering conferences on the topic of graphene since 2009. SPIE has identified graphene and other two-dimensional materials as a key area of interest for its members because of the properties these new materials are offering in the field.
The Graphene Council certainly shares in SPIE’s interest in how two-dimensional materials, including graphene, will play a key role in optoelectronics and photonics, with our frequent coverage of these two fields.
Now that SPIE has become one of our Corporate Members we took the opportunity to speak to Robert F. Hainsey, Ph.D., the Director of Science and Technology for SPIE to ask him about the role graphene is positioned to play in optics and photonics, how the market is developing and the role of SPIE as these developments evolve.
Q: Graphene has exhibited a number of appealing properties for applications within photonics and optoelectronics, so it’s clear to see why SPIE would become involved with the topic. But could you tell us a little bit about the evolution of how SPIE started getting involved in the topic of graphene?
A: SPIE has a long history of supporting the topic of graphene having launched a volunteer-inspired conference at our Optics + Photonics event held annually in San Diego as early as 2009. The topic appears in a number of other SPIE conferences as well. In 2014, Frank Koppens of ICFO delivered an excellent plenary talk on the subject at our Photonics Europe event in Brussels, and this led, in turn, to Frank Koppens and Nathalie Vermeulen of the B-PHOT team at Vrije Universiteit Brussel organizing and chairing a full-day workshop at this year’s Photonic Europe event on applications and commercialization of graphene. We continue to look for methods to enable the community to best share results and exchange ideas in this rapidly evolving field.
Q: How is SPIE now approaching the topic, i.e. what sort of mediums are you using to get the message out about graphene? How do you see this information serving your members?
A: The information is disseminated in a number of ways. Primary among these methods are our conferences which enable researchers to share and discuss the latest findings in the area of graphene and similar materials. The work shared in those conferences is then packaged into proceedings and made part of the SPIE Digital Library so as to share the results with a wider audience. We also have our journals where researchers can publish their results in a peer-reviewed medium. The “SPIE Professional” magazine, the quarterly magazine for our members, has included articles in this area including one written by Frank Koppens earlier this year. Naturally, we share news about graphene research on our News Room webpage, via Twitter and through our LinkedIn groups. In terms of serving our members, we hope that this diverse set of methods of sharing information keeps our members informed on the latest work in the field and stimulates discussion among researchers to advance the field.
Q: There are a number of different applications within photonic and optoelectronics in which graphene has exhibited promise. In one of your more recent conferences on graphene, communication applications were identified as the most near-term. Has SPIE begun to get a better feel of how graphene applications within photonics and optoelectronics are developing commercially? And could you give us an outline of that development?
A: The workshop you refer to is a positive step towards moving graphene along the commercialization pipeline. This workshop served to bring together academic and industrial researchers as well as entrepreneurs and start-up companies to discuss what is needed to move graphene from a laboratory to a production setting. A look at the program for that event illustrates that large enterprises are investing in the research. In addition, more start-up’s are appearing on the scene at various positions of the value chain. Progress is being made on the road to full-scale production but there is still work to be done.
Q: Is SPIE involved with any of the standards bodies that are attempting to create industry standards for the material? Whether you are involved or not, does SPIE have a position on the role of materials standards as the material becomes increasingly commercialized?
A: At this point we are not actively engaged in the work on developing standards outside of the presentations given in our conferences. That said, one sign of research maturing and preparing to transition to a production environment is the discussion and adoption of standards. Standards are oftentimes crucial since they provide a baseline for methods and performance by which the industry can determine capability and map progress. SPIE supports standards development in other areas through methods such as providing meeting space for standards bodies at our events. We would welcome dialogue with standards bodies in this area to determine if there is a way SPIE can more actively support that work.
Q: How do you see SPIE’s role in graphene education and providing information evolving as the field moves from the lab to the fab? Does the approach to disseminating information on a topic change as it moves from research to commercial interests?
A: Certainly the topic will continue to be a vibrant one in our conferences, our proceedings, the SPIE Digital Library, and our social media outlets. SPIE events also include a set of industry sessions containing presentations, panel discussions, and networking opportunities focused on the commercial aspects of optics and photonics technologies. This combination of conferences, publications, and industry sessions positions SPIE events to track the migration of the technology as it matures. The flexibility we have within our events to include unique offerings such as the dedicated workshop on graphene commercialization at the SPIE Photonics Europe event earlier this year allows SPIE to tailor the forum to best serve the community.
Q: How does partnering with groups, such as The Graphene Council, help or contribute to your strategy in education and providing information on the topic of graphene?
A: SPIE is an organization dedicated to serving the optics and photonics community. Partnering with other organizations to further the sharing of information and enhancing the discussion around technologies not only helps SPIE meet its charter but, more importantly, enables the advancement of research, science, engineering and practical applications in these technologies.