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Understanding electron transport in graphene nanoribbons

Posted By Graphene Council, Tuesday, September 15, 2020
Graphene is a modern wonder material possessing unique properties of strength, flexibility and conductivity whilst being abundant and remarkably cheap to produce, lending it to a multitude of useful applications -- especially true when these 2D atom-thick sheets of carbon are split into narrow strips known as Graphene Nanoribbons (GNRs).

New research published in EPJ Plus, authored by Kristians Cernevics, Michele Pizzochero, and Oleg V. Yazyev, Ecole Polytechnique Federale de Lausanne (EPFL), Lausanne, Switzerland, aims to better understand the electron transport properties of GNRs and how they are affected by bonding with aromatics. This is a key step in designing technology such chemosensors.

"Graphene nanoribbons -- strips of graphene just few nanometres wide -- are a new and exciting class of nanostructures that have emerged as potential building blocks for a wide variety of technological applications," Cernevics says.

The team performed their investigation with the two forms of GNR, armchair and zigzag, which are categorised by the shape of the edges of the material. These properties are predominantly created by the process used to synthesise them. In addition to this, the EPFL team experimented p-polyphenyl and polyacene groups of increasing length.

"We have employed advanced computer simulations to find out how electrical conductivity of graphene nanoribbons is affected by chemical functionalisation with guest organic molecules that consist of chains composed of an increasing number of aromatic rings," says Cernevics.

The team discovered that the conductance at energies matching the energy levels of the corresponding isolated molecule was reduced by one quantum, or left unaffected based on whether the number of aromatic rings possessed by the bound molecule was odd or even. The study shows this 'even-odd effect' originates from a subtle interplay between the electronic states of the guest molecule spatially localised on the binding sites and those of the host nanoribbon.

"Our findings demonstrate that the interaction of the guest organic molecules with the host graphene nanoribbon can be exploited to detect the 'fingerprint' of the guest aromatic molecule, and additionally offer a firm theoretical ground to understand this effect," Cernevics concludes: "Overall, our work promotes the validity of graphene nanoribbons as promising candidates for next-generation chemosensing devices."

These potentially wearable or implantable sensors will rely heavily on GRBs due to their electrical properties and could spearhead a personalised health revolution by tracking specific biomarkers in patients.

Tags:  Biosensor  EPFL  Graphene  Graphene Nanoribbons  Healthcare  Kristians Cernevics  Michele Pizzochero  Oleg V. Yazyev  Sensors 

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Bottom-up Synthesis of Atomically Precise Graphene Nanoribbons Directly on Metal Oxide Surfaces

Posted By Graphene Council, Monday, July 13, 2020
Scientific Achievement
Decoupled, atomically precise graphene nanoribbons (GNRs) are obtained by an on-surface synthesis approach on a model metal oxide and exhibit spin-polarized magnetic states.

Significance and Impact
This work demonstrates a path toward forming custom-designed carbon nanostructures by direct on-surface synthesis methods on technologically relevant semiconducting or insulating surfaces.

Research Details
– Highly selective and sequential activation of C-Br, C-F, and C-H bonds are thermally triggered on the oxide surface with rationally designed molecular precursors during multi-step synthesis of GNRs.
– Scanning tunneling microscopy (STM) used to monitor the formation of intermediates and GNRs in situ, revealing electronic and magnetic states and confirming weak interactions between GNRs and rutile TiO2.

Tags:  An-Ping Lee  Graphene  Graphene Nanoribbons  Mark A. Kolmer  Wonhee Ko 

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Discovery of graphene switch

Posted By Graphene Council, Wednesday, June 17, 2020
Researchers at Japan Advanced Institute of Science and Technology (JAIST) have successfully measured the current-voltage curve of graphene nanoribbons (GNRs) that were suspended between two electrodes. Measurements were performed using transmission electron microscopy (TEM) observation. Results revealed that, in contrast to the findings of previous reports, the electrical conductance of GNRs with a zigzag edge structure (zigzag GNRs) abruptly increased above the critical bias voltage. This finding is worth noting because the abrupt change in these GNRs can be applied to switching devices, which are the smallest devices in the world.

The electrical structure of GNRs have been systematically investigated through theoretical calculations. Studies have reported that both zigzag and armchair GNRs exhibit semiconducting behavior below several nm in width, although the origin of the energy gap is different. On the other hand, the electrical transport properties have rarely been calculated owing to the non-equiribrium calculations required. In 2009, Nikolic et al. predicted that sharp increments in electrical conductance would occur for extremely thin and short zigzag GNRs as the magnetic-insulator-nonmagnetic-metal phase transition occurs above a certain bias voltage [Phys. Rev.B 79, 205430 (2009)]. The obtained experimental results correspond closely to the results of this non-equilibrium calculation.

A research team led by Ms. Chumeng LIU, Professor Yoshifumi OSHIMA and Associate Professor Xiaobin ZHANG (now of Shibaura Institute of Technology) has developed a special in situ TEM holder and a GNR device for TEM observation. This combination is aimed at clarifying the relationship between the edge structure of GNRs and electrical transport properties. Ms. Liu, the doctoral student of JAIST, said, "The fabrication process of our GNR device is much more difficult than the conventional one because we need to make very narrow GNR which should be stably suspended between both electrodes." She reviewed the literature focused on the fabrication process of GNR devices and verified their process en route to establishing her original fabrication method. Assoc. Prof. ZHANG said, "We were really happy to see that the I-V curve obviously changed when changing the edge structure to zigzag. I suppose we have encountered new possibilities for graphene nanoribbons." The team has successfully performed the in situ TEM observation of extremely narrow GNRs, and they plan to continue identifying electrical transport properties that are sensitive to the edge structure of these GNRs.

Tags:  Graphene  graphene nanoribbons  Japan Advanced Institute of Science and Technology  Xiaobin ZHANG 

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Engineering Edge States of Graphene Nanoribbons for Narrow-Band Photoluminescence

Posted By Graphene Council, Monday, May 11, 2020
A bottom-up approach for coupling graphene nanodots (GND) covalently at the edges of graphene nanoribbons (GNR) to create quantum-well-like states for well-defined narrow-band light emission.

Significance and Impact
This work establishes a new strategy to achieve narrow-band emission by engineering interface states of mixed-dimensional GNR-GND heterojunctions with atomic precision.

Research Details
– Covalent heterostructures are formed by fusing GNDs to the edges of GNRs via controlled on-surface reactions of molecular precursors.

– Scanning tunneling microscopy (STM) reveals the quantum-well-like electronic states and photoluminescence (PL) spectra show a defined optical transition energy with an ultra-narrow linewidth.

Tags:  Graphene  Graphene Nanoribbons  quantum materials 

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New 'brick' for nanotechnology: Graphene Nanomesh

Posted By Graphene Council, Wednesday, April 22, 2020
Researchers at Japan advanced institute of science and technology (JAIST) have successfully fabrication the suspended graphene nanomesh in a large area by the helium ion beam microscopy. 6nm diameter nanopores were pattern on the 1.2 um long and 500 nm wide suspended graphene uniformly. By systematically controlling the pitch (nanopore's center to nanopore's center) from 15 nm to 50 nm, a series of stable graphene nanomesh devices were achieved. This provides a practical way to investigate the intrinsic properties of graphene nanomesh towards the application for gas sensing, phonon engineering, and quantum technology.

Graphene, with its excellent electrical, thermal and optical properties, is promising for many applications in the next decade. It is also a potential candidate instead of silicon to build the next generation of electrical circuits. However, without a bandgap, it is not straightforward to use graphene as field-effect transistors (FETs). Researchers tried to cut the graphene sheet into a small piece of graphene nanoribbon and observed the bandgap opening successfully. However, the current of graphene nanoribbons is too low to drive the integrated circuit. In this case, the graphene nanomesh is pointed out by introducing periodical nanopores on the graphene, which is also considered as very small graphene nanoribbon array.

A research team led by Dr Fayong Liu and Professor Hiroshi MIZUTA has demonstrated in collaboration with researchers at the National Institute of Advanced Industrial Science and Technology (AIST) that large area suspended graphene nanomesh is quickly achievable by the helium ion beam microscopy with sub-10 nm nanopore diameter and well-controlled pitches. Comparing to slow speed TEM patterning, the helium ion beam milling technique overcomes the speed limitation, and meanwhile, provides a high imaging resolution. With the initial electrical measurements, it has found that the thermal activation energy of the graphene nanomesh increased exponentially by increasing the porosity of the graphene nanomesh. This immediately provides a new method for bandgap engineering beyond the conventional nanoribbon method. The team plans to continue exploring graphene nanomesh towards the application of phonon engineering.

"Graphene nanomesh is a kind of new 'brick' for modern micromachine systems. Theoretically, we can generate many kinds of periodical patterns on the original suspended graphene, which tunes the property of the device to the direction for a special application, in particular nanoscale thermal management" says Prof. Hiroshi Mizuta, the Head of MIZUTA Lab. The MIZUTA lab is currently developing the electrical and thermal properties of graphene-based devices for fundamental physics and potential applications such as gas sensors and thermal rectifier. The aim is to use graphene to build a green world.

Tags:  Fayong Liu  Graphene  graphene nanoribbons  Hiroshi MIZUTA  Japan Advanced Institute of Science and Technology 

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Graphene nanoribbons lay the groundwork for ultrapowerful computers

Posted By Graphene Council, Friday, September 13, 2019
Smaller, better semiconductors have consistently allowed computers to become faster and more energy-efficient than ever before.

But the 18-month cycle of exponential increases in computing power that has held since the mid 1960s now has leveled off. That’s because there are fundamental limits to integrated circuits made strictly from silicon—the material that forms the backbone of our modern computer infrastructure.

As they look to the future, however, engineers at the University of Wisconsin-Madison are turning to new materials to lay down the foundations for more powerful computers.

They have devised a method to grow tiny ribbons of graphene—the single-atom-thick carbon compound—directly on top of silicon wafers.

Graphene ribbons have a special advantage over the material when it’s in its more common form of a broad, flat sheet; namely, thin strips of graphene become excellent semiconductors.

“Compared to current technology, this could enable faster, low power devices,” says Vivek Saraswat, a PhD student in materials science and engineering at UW-Madison. “It could help you pack in more transistors onto chips and continue Moore’s law into the future.”

Saraswat and his colleagues published details of their work July 9, 2019, in the Journal of Physical Chemistry.

The advance could enable graphene-based integrated circuits, with much improved performance over today’s silicon chips.

“The main advantage of graphene nanoribbons is that electrons can travel faster through them, compared to silicon so you can make faster chips that use less energy,” says Mike Arnold, a professor of materials science and engineering at UW-Madison and a world expert in graphene growth.

Arnold is pioneer of a strategy to lay down long, thin strips of graphene—structures known as nanoribbons—on top a material called germanium.

That’s useful in many ways. However, since germanium isn’t a widely used semiconductor, it can’t form the basis for computer chips.

Meanwhile, other researchers have not been able to overcome a major barrier in layering graphene nanoribbons onto silicon. Graphene reacts with silicon to form an inert and less useful compound called silicon carbide.

Arnold’s group has developed an ingenious method to avoid that obstacle.

By laying down a thin protective layer of germanium before applying graphene, the researchers could successfully grow graphene nanoribbons on top of silicon wafers. The thin germanium layers protected graphene from reacting with silicon, yet didn’t interfere with the nanoribbons’ semiconducting capabilities.

It’s an important first step toward creating graphene-based integrated circuits. And because the base layer is composed of silicon, the graphene nanoribbon technology can be easily integrated into existing electronic/computing components.

“Our vision is to integrate graphene with existing devices,” says Arnold.

The scientists have patented their technology through the Wisconsin Alumni Research Foundation. One advantage of their synthesis approach is that it takes advantage of a scalable, industry-compatible chemical vapor deposition technique. Now, they’re working to improve the precision with which they lay down their nanoribbons so that they can achieve the complex patterns found in modern computer chips.

“We are using a few strategies to control the thickness and the orientation for the nanoribbons,” says Arnold. “We have a few really cool ideas.”

Tags:  Graphene  Graphene Nanoribbons  Mike Arnold  Semiconductors  University of Wisconsin-Madison  Vivek Saraswat 

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A new 'periodic table' for nanomaterials

Posted By Graphene Council, Monday, February 18, 2019

The approach was developed by Daniel Packwood of Kyoto University's Institute for Integrated Cell-Material Sciences (iCeMS) and Taro Hitosugi of the Tokyo Institute of Technology. It involves connecting the chemical properties of molecules with the nanostructures that form as a result of their interaction. A machine learning technique generates data that is then used to develop a diagram that categorizes different molecules according to the nano-sized shapes they form. This approach could help materials scientists identify the appropriate molecules to use in order to synthesize target nanomaterials.

Fabricating nanomaterials using a bottom-up approach requires finding 'precursor molecules' that interact and align correctly with each other as they self-assemble. But it's been a major challenge knowing how precursor molecules will interact and what shapes they will form.

Bottom-up fabrication of graphene nanoribbons is receiving much attention due to their potential use in electronics, tissue engineering, construction, and bio-imaging. One way to synthesise them is by using bianthracene precursor molecules that have bromine 'functional' groups attached to them. The bromine groups interact with a copper substrate to form nano-sized chains. When these chains are heated, they turn into graphene nanoribbons.

Packwood and Hitosugi tested their simulator using this method for building graphene nanoribbons.

Data was input into the model about the chemical properties of a variety of molecules that can be attached to bianthracene to 'functionalize' it and facilitate its interaction with copper. The data went through a series of processes that ultimately led to the formation of a 'dendrogram'.

This showed that attaching hydrogen molecules to bianthracene led to the development of strong one-dimensional nano-chains. Fluorine, bromine, chlorine, amidogen, and vinyl functional groups led to the formation of moderately strong nano-chains. Trifluoromethyl and methyl functional groups led to the formation of weak one-dimensional islands of molecules, and hydroxide and aldehyde groups led to the formation of strong two-dimensional tile-shaped islands.

The information produced in the dendogram changed based on the temperature data provided. The above categories apply when the interactions are conducted at -73°C. The results changed with warmer temperatures. The researchers recommend applying the data at low temperatures where the effect of the functional groups' chemical properties on nano-shapes are most clear.

The technique can be applied to other substrates and precursor molecules. The researchers describe their method as analogous to the periodic table of chemical elements, which groups atoms based on how they bond to each other. "However, in order to truly prove that the dendrograms or other informatics-based approaches can be as valuable to materials science as the periodic table, we must incorporate them in a real bottom-up nanomaterial fabrication experiment," the researchers conclude in their study. "We are currently pursuing this direction in our laboratories."

Tags:  Daniel Packwood  Graphene  Graphene Nanoribbons  Kyoto University  nanomaterials  Taro Hitosugi  Tokyo Institute of Technology 

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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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