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Physicist obtain atomically thin molybdenum disulfide films on large-area substrates

Posted By Graphene Council, Thursday, January 23, 2020
Researchers from the Moscow Institute of Physics and Technology have managed to grow atomically thin films of molybdenum disulfide spanning up to several tens of square centimeters. It was demonstrated that the material's structure can be modified by varying the synthesis temperature. The films, which are of interest to electronics and optoelectronics, were obtained at 900-1,000 degrees Celsius. The findings were published in the journal ACS Applied Nano Materials.

Two-dimensional materials are attracting considerable interest due to their unique properties stemming from their structure and quantum mechanical restrictions. The family of 2D materials includes metals, semimetals, semiconductors, and insulators. Graphene, which is perhaps the most famous 2D material, is a monolayer of carbon atoms. It has the highest charge-carrier mobility recorded to date. However, graphene has no band gap under standard conditions, and that limits its applications.

Unlike graphene, the optimal width of the bandgap in molybdenum disulfide (MoS2) makes it suitable for use in electronic devices. Each MoS2 layer has a sandwich structure, with a layer of molybdenum squeezed between two layers of sulfur atoms. Two-dimensional van der Waals heterostructures, which combine different 2D materials, show great promise as well. In fact, they are already widely used in energy-related applications and catalysis. Wafer-scale (large-area) synthesis of 2D molybdenum disulfide shows the potential for breakthrough advances in the creation of transparent and flexible electronic devices, optical communication for next-generation computers, as well as in other fields of electronics and optoelectronics.

"The method we came up with to synthesize MoS2 involves two steps. First, a film of MoO3 is grown using the atomic layer deposition technique, which offers precise atomic layer thickness and allows conformal coating of all surfaces. And MoO3 can easily be obtained on wafers of up to 300 millimeters in diameter. Next, the film is heat-treated in sulfur vapor. As a result, the oxygen atoms in MoO3 are replaced by sulfur atoms, and MoS2 is formed. We have already learned to grow atomically thin MoS2 films on an area of up to several tens of square centimeters," explains Andrey Markeev, the head of MIPT's Atomic Layer Deposition Lab.

The researchers determined that the structure of the film depends on the sulfurization temperature. The films sulfurized at 500 ? contain crystalline grains, a few nanometers each, embedded in an amorphous matrix. At 700 ?, these crystallites are about 10-20 nm across and the S-Mo-S layers are oriented perpendicular to the surface. As a result, the surface has numerous dangling bonds. Such structure demonstrates high catalytic activity in many reactions, including the hydrogen evolution reaction. For MoS2 to be used in electronics, the S-Mo-S layers have to be parallel to the surface, which is achieved at sulfurization temperatures of 900-1,000 ?. The resulting films are as thin as 1.3 nm, or two molecular layers, and have a commercially significant (i.e., large enough) area.

The MoS2 films synthesized under optimal conditions were introduced into metal-dielectric-semiconductor prototype structures, which are based on ferroelectric hafnium oxide and model a field-effect transistor. The MoS2 film in these structures served as a semiconductor channel. Its conductivity was controlled by switching the polarization direction of the ferroelectric layer. When in contact with MoS2, the La:(HfO2-ZrO2) material, which was earlier developed in the MIPT lab, was found to have a residual polarization of approximately 18 microcoulombs per square centimeter. With a switching endurance of 5 million cycles, it topped the previous world record of 100,000 cycles for silicon channels.

Tags:  2D materials  ACS Applied Nano Materials  Andrey Markeev  Graphene  Moscow Institute of Physics and Technology  Optoelectronics  Semiconductors 

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Chemists have managed to stabilize the 'capricious' phosphorus

Posted By Graphene Council, Tuesday, January 21, 2020
An international team of Russian, Swedish and Ukrainian scientists has identified an effective strategy to improve the stability of two-dimensional black phosphorus, which is a promising material for use in optoelectronics.

The most effective mechanism of fluorination has been revealed. In addition to increased stability compared to previously proposed structures, the materials predicted by the researchers showed high antioxidative stability. The main results of the work have been presented in The Journal of Physical Chemistry Letters.

Black phosphorus is obtained from white phosphorus under conditions of high pressure and elevated temperature. The material has a layered structure and resembles graphite in appearance and properties. However, unlike graphite, it is a good semiconductor.

"Phosphorene is a monolayer of black phosphorus with interesting physical properties (high anisotropic electrical and thermal conductivity, flexible band gap variability depending on the number of layers), which makes it a promising material for use in various fields of optoelectronics (transistors, inverters, flexible electronics, solar panels). Unfortunately, one of its main problems is instability in the environment. Unlike its volumetric analogue, which is almost immune to external conditions, phosphorene quickly begins to attach oxygen from the air and degrades within a few hours. As one of the strategies for improving the stability of phosphorene, mechanism of fluorination was proposed. Over the past five years, scientists have proposed several theoretically possible options for such a "coupling". An experiment was conducted that showed a significant increase in the stability of phosphorus in ambient conditions after fluorination. However, the features of the obtained material structure remained unexplained.

Using various theoretical approaches, my colleagues and I showed that the previously proposed structures of "stabilized" phosphorus were actually unstable. It is known that phosphorus is able to form compounds with 3 or 5 fluorine atoms. Our calculations also confirmed that the characteristic coordination of the phosphorus atom in the PF system is 3 or 5. By sequential addition of atoms, it was possible to identify the most effective and really working mechanism by which fluorine atoms should attach to the surface of phosphorene. Thus, we have determined the type of structures that are likely to have been obtained by our predecessors in the above-mentioned experiment," -- said Artem Kuklin, a research fellow of SibFU.

Scientists note that the materials formed by the predicted mechanism are really stable and have increased antioxidant ability (that is, they are not quickly degradable) and their electronic properties, which do not differ much from the properties of pure phosphorus, provide the possibility of their practical application in optoelectronic devices, i.e. transistors, solar panels, flexible electronics, LEDs, photosensors, biomedical devices, optical devices for storing and transmitting information, etc.

Tags:  Artem Kuklin  Graphene  optoelectronics  photonics  Semiconductor  Siberian Federal University 

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Energy levels in electrons of 2D materials are mapped for the first time

Posted By Graphene Council, Thursday, January 9, 2020
Researchers based at the National Graphene Institute at The University of Manchester have developed an innovative measurement method that allows, for the first time, the mapping of the energy levels of electrons in the conduction band of semiconducting 2D materials.

Writing in Nature Communications, a team led by Dr Roman Gorbachev reports the first precise mapping of the conduction band of 2D indium selenide (InSe) using resonant tunnelling spectroscopy, to access the previously unexplored part of the electronic structure. They observed multiple subbands for both electrons and holes and tracked their evolution with the number of atomic layers in InSe.

Many emerging technologies rely on novel semiconductor structures, where the motion of electrons is restricted in one or more directions. Such confinement is in the nature of 2D materials and it is responsible for many of their new and exciting properties.

For instance, the colour of the emitted light shifts towards shorter wavelengths as they get thinner, analogous to quantum dots changing colour when their size is varied. As another consequence, the allowed energy available for the electrons in such materials, called conduction and valence bands, split into multiple subbands.

We hope this study will pave the way for exploration of intersubband transitions and lead to development of prototype optoelectronic devices with tuneable emission in the challenging terahertz range, Dr Roman Gorbachev.

Optical transitions between such subbands present a large potential for real-life applications as they provide optically active in terahertz and far-infrared ranges, which can be employed for security and communication technologies as light emitters or detectors.

Dr Roman Gorbachev said: “We hope this study will pave the way for exploration of intersubband transitions and lead to development of prototype optoelectronic devices with tuneable emission in the challenging terahertz range.”

Tags:  2D materials  Graphene  optoelectronics  Roman Gorbachev  Semiconductor  University of Manchester 

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Graphene and layered materials boost silicon technologies

Posted By Graphene Council, Saturday, November 16, 2019
Updated: Friday, November 8, 2019
Silicon semiconductor technology has done marvels for the advancement of our society, which has benefited tremendously from its versatile use and amazing capabilities. The development of electronics, automation, computers, digital cameras and smartphones based on this material and its underpinning technology has reached skyrocket limits, downscaling the physical size of devices and wires to the nanometre regime. 

Although this technology has been growing since the late 1960s, the miniaturization of circuits seems to have reached a possible halt, since transistors can only be shrunk down to a certain size and not further beyond. Thus, there is a pressing need to complement Si CMOS technology with new materials and fulfil the future computing requirements as well as the needs for diversification of applications.

Graphene and related materials offer prospects of advances in device performance at the atomic limit.  They provide a possible solution to overcome the limitations of silicon technology, where the combination of layered materials with silicon chips promises to surpass the current technological limitations.

A team of researchers including Stijn Goossens and Frank Koppens, based at Graphene Flagship partner ICFO, and industrial leaders from Graphene Flagship partner IMEC and TSMC provided an in-depth and thorough review of opportunities, progress and challenges of integrating atomically thin materials with Si-based technology. They give insights on how and why layered materials could overcome current challenges posed by the existing technology and how they can enhance both device component function and performance, to boost the features of future technologies, in the areas of computational and non-computational applications.

For non-computational applications, they review the possible integration of these materials for future cameras, low power optical data communications and gas and bio-sensors. In particular, in image sensors and photodetectors, graphene and related materials could enable new vision in the infrared and terahertz range in addition to the visible range of the spectrum. These can serve for example in autonomous vehicles, security at airports and augmented reality.

For computational systems, and in particular in the field of transistors, they show how challenges such as doping, contact resistance and dielectrics/encapsulation can be diminished when integrating layered materials with Si technology. Layered materials could also improve memory and data storage devices with novel switching mechanisms for meta-insulator-metal structures, avoid sneak currents in memory arrays, or even push the performance gains of copper wire-based circuitry by adhering graphene to the ultrathin copper barrier materials and thus reduce resistance, scattering and self-heating.

The review provides a roadmap of layered material integration and CMOS technology, pinpointing the stage at which all challenges regarding growth, transfer, interface, doping, contacting, and design are currently standing today and what possible processes are expected to be resolved to achieve such goals of moving from a research laboratory environment to a pilot line for production of the first devices that combine both technologies. The layered materials-CMOS roadmap, as presented in this review, gives an exciting glimpse into the future, with pilot production expected to be just a few years from now.

Frank Koppens, Graphene Flagship Work Package Leader for Photonics and Optoelectronics and lead author of the study, says: "Now we have a clear industry-driven roadmap on layered material-silicon technologies and manufacturing. Complementing the established silicon technology with layered materials is key to combine the best of both worlds and enable a plethora of large volume and low-cost applications."

Marco Romagnoli, Graphene Flagship Work Package Leader for Wafer-Scale System Integration, comments: "This is an interesting paper complementing a previous one focused on graphene photonics for telecommunications that completes the range of applications in which graphene can be exploited for large scale production in CMOS environments. Also interesting is the type of application, in which graphene can best exploit its characteristics, from IR/THz cameras to low-power electronic switching and memories.

Andrea C. Ferrari, Science and Technology Officer of the Graphene Flagship and Chair of its Management Panel, adds: "The integration of graphene and related materials with silicon and CMOS technology is the next goal for the Flagship. For this reason, we will fund the first foundry focussed on the integration of layered materials. This work clearly spells out the vision for the transformative technology that integration will enable."

Tags:  Andrea C. Ferrari  Frank Koppens  Graphene  Graphene Flagship  ICFO  Marco Romagnoli  optoelectronics  photonics  Semiconductor  Stijn Goossens  transistor 

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Scientists Realize High Performance Broadband Graphene Saturable Absorber

Posted By Graphene Council, Sunday, October 6, 2019
The unique mechanical, chemical, electronic and photonic features of graphene are attracting much attention, especially its unusual nonlinear optical (NLO) properties, making it a progressive solution to ultrafast saturable absorption and optical limiting. However, small NLO coefficient in natural materials restricts its applications due to high-intensity thresholds. Embedding NLO materials in a photonic crystal (PC) can yield lower NLO thresholds than bulk materials, making it an exciting NLO device again.

Recently, a study led by Prof. WANG Jun from Shanghai Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences proposed a simple and accessible technology for PC manufacturing and presented a PC1 designed for infrared and visible λ pairs, which is suitable for the two harmonics of a standard laser source. The result was published in Optics Letters.

Their study was based on the polymer dispersion of graphene flakes. To prepare the crystal, graphene was first suspended using the liquid phase exfoliation method. Then the 1D PCs consisting 12 alternating layers of poly(9-vinylcarbazole) (PVK) and of graphene-based poly(vinyl-alcohol) (PVA) nanocomposite was spin coated on a glass substrate.

In order to compare the NLO response of graphene in PC with partially resonant and non-resonant cases, PC2 crystal and a pure PVA-G film were fabricated. In PC1 structure, two bandgaps near 515 nm and 1030 nm were observed. For PC2, the second bandgap fell on 1425 nm, which deprived the possible advantage at 1030 nm.

NLO properties of the three samples were studied using an open-aperture Z-scan setup with 340 fs pulses at the main and second harmonics of a fiber laser. More pronounced saturable behavior with significant enhancement of the nonlinear absorption coefficient and the imaginary part of the third-order nonlinear susceptibility was obtained in the PC structures as compared to the bulk PVA-G film.

Their experiment also showed a remarkable decrease of saturable absorption threshold and saturation intensities of graphene embedded in the PC in comparison with the bulk PVA-G. The saturable absorption enhancement factor reached 7 in the visible and 8 in the infrared, which could be explained by the combination of light field enhancement and absorptive and scattering losses inside the structures.

This work provides a desirable solution for an advanced all-optical laser mode-locking device. This work was supported by the Chinese National Natural Science Foundation, the Strategic Priority Research Program of CAS, the Key Research Program of Frontier Science of CAS, and the Program of Shanghai Academic Research Leader.

Tags:  Graphene  Jun Wang  optoelectronics  Shanghai Institute of Optics and Fine Mechanics 

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Directed evolution builds nanoparticles

Posted By Graphene Council, Thursday, March 7, 2019
Updated: Friday, March 1, 2019

The 2018 Nobel Prize in Chemistry went to three scientists who developed the method that forever changed protein engineering: directed evolution. Mimicking natural evolution, directed evolution guides the synthesis of proteins with improved or new functions.

First, the original protein is mutated to create a collection of mutant protein variants. The protein variants that show improved or more desirable functions are selected. These selected proteins are then once more mutated to create another collection of protein variants for another round of selection. This cycle is repeated until a final, mutated protein is evolved with optimized performance compared to the original protein.

Now, scientists from the lab of Ardemis Boghossian at EPFL, have been able to use directed evolution to build not proteins, but synthetic nanoparticles (Chemical Communications, "Directed evolution of the optoelectronic properties of synthetic nanomaterials").

These nanoparticles are used as optical biosensors – tiny devices that use light to detect biological molecules in air, water, or blood. Optical biosensors are widely used in biological research, drug development, and medical diagnostics, such as real-time monitoring of insulin and glucose in diabetics.

“The beauty of directed evolution is that we can engineer a protein without even knowing how its structure is related to its function,” says Boghossian. “And we don't even have this information for the vast, vast majority of proteins.”

Her group used directed evolution to modify the optoelectronic properties of DNA-wrapped single-walled carbon nanotubes (or, DNA-SWCNTs, as they are abbreviated), which are nano-sized tubes of carbon atoms that resemble rolled up sheets of graphene covered by DNA. When they detect their target, the DNA-SWCNTs emit an optical signal that can penetrate through complex biological fluids, like blood or urine.

Using a directed evolution approach, Boghossian’s team was able to engineer new DNA-SWCNTs with optical signals that are increased by up to 56% – and they did it over only two evolution cycles.

“The majority of researchers in this field just screen large libraries of different materials in hopes of finding one with the properties they are looking for,” says Boghossian. “In optical nanosensors, we try to improve properties like selectivity, brightness, and sensitivity. By applying directed evolution, we provide researchers with a guided approach to engineering these nanosensors.”

The study shows that what is essentially a bioengineering technique can be used to more rationally tune the optoelectronic properties of certain nanomaterials.

Boghossian explains: “Fields like materials science and physics are mostly preoccupied with defining material structure-function relationships, making materials that lack this information difficult to engineer. But this is a problem that nature solved billions of years ago – and, in recent decades, biologists have tackled it as well. I think our study shows that as materials scientists and physicists, we can still learn a few pragmatic lessons from biologists.”

Tags:  Ardemis Boghossian  biosensors  DNA  EPFL  Graphene  nanomaterials  optoelectronics 

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Graphene Changes the Game in Optoelectronics

Posted By Dexter Johnson, IEEE Spectrum, Tuesday, April 24, 2018

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.

Tags:  graphene  Hexagonal boron nitride  optical switches  optoelectronics  plasmonics  sensors 

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How Quantum Dots and Graphene Combined to Change the Landscape for Optoelectronics

Posted By Dexter Johnson, IEEE Spectrum, Wednesday, November 1, 2017

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.”

Tags:  2D materials  graphene  optoelectronics  photodetectors  photonics  photovoltaics  quantum dots 

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Electronics Applications for Graphene Hold Great Promise

Posted By Terrance Barkan, Monday, October 31, 2016

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.

 

 

 

 

 

Tags:  Batteries  Decorated Graphene  Electronics  Flexible Sensors  Graphene  Graphene Nanoribbons  Lasers  Li-ion  optoelectronics  Semiconductor 

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Established Optical Society Sees the Light in Graphene

Posted By Dexter Johnson, IEEE Spectrum, Saturday, October 8, 2016
Updated: Thursday, October 6, 2016

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.

Tags:  corporate members  optoelectronics  photonics  SPIE 

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