Scientists at the University of Manchester develop revolutionary method for graphene printed electronics – Impact for IoT

This visualisation shows layers of graphene used for membranes. Credit: University of Manchester

 

A team of researchers based at The University of Manchester have found a low cost method for producing graphene printed electronics, which significantly speeds up and reduces the cost of conductive graphene inks.

Printed electronics offer a breakthrough in the penetration of information technology into everyday life. The possibility of printing electronic circuits will further promote the spread of Internet of Things (IoT) applications.

The development of printed conductive inks for electronic applications has grown rapidly, widening applications in transistors, sensors, antennas RFID tags and wearable electronics.

Current conductive inks traditionally use metal nanoparticles for their high electrical conductivity. However, these materials can be expensive or easily oxidised, making them far from ideal for low cost IoT applications.

The team have found that using a material called dihydro-levo-gucosenone known as Cyrene is not only non-toxic but is environmentally- friendly and sustainable but can also provide higher concentrations and conductivity of graphene ink.

Professor Zhiurn Hu said: “This work demonstrates that printed graphene technology can be low cost, sustainable, and environmentally friendly for ubiquitous wireless connectivity in IoT era as well as provide RF energy harvesting for low power electronics”.

Professor Sir Kostya Novoselov said: “Graphene is swiftly moving from research to application domain. Development of production methods relevant to the end-user in terms of their flexibility, cost and compatibility with existing technologies are extremely important. This work will ensure that implementation of graphene into day-to-day products and technologies will be even faster”.

Kewen Pan, the lead author on the paper said: “This perhaps is a significant step towards commercialisation of printed graphene technology. I believe it would be an evolution in printed electronics industry because the material is such low cost, stable and environmental friendly”.

The National Physical Laboratory (NPL), who were involved in measurements for this work, have partnered with the National Graphene Institute at The University of Manchester to provide a materials characterisation service to provide the missing link for the industrialisation of graphene and 2-D materials. They have also published a joint NPL and NGI a good practice guide which aims to tackle the ambiguity surrounding how to measure graphene’s characteristics.

Professor Ling Hao said: “Materials characterisation is crucial to be able to ensure performance reproducibility and scale up for commercial applications of graphene and 2-D materials. The results of this collaboration between the University and NPL is mutually beneficial, as well as providing measurement training for Ph.D. students in a metrology institute environment.”

Graphene has the potential to create the next generation of electronics currently limited to science fiction: faster transistors, semiconductors, bendable phones and flexible wearable electronics.

 Explore further: Fully integrated circuits printed directly onto fabric

Provided by: University of Manchester

 

Researchers make major breakthrough in smart printed electronics

Prof Jonathan Coleman has fabricated printed transistors consisting entirely of 2-dimensional nanomaterials for the first time. Credit: AMBER, Trinity College Dublin

Researchers in AMBER, the Science Foundation Ireland-funded materials science research centre hosted in Trinity College Dublin, have fabricated printed transistors consisting entirely of 2-dimensional nanomaterials for the first time. These 2D materials combine exciting electronic properties with the potential for low-cost production. This breakthrough could unlock the potential for applications such as food packaging that displays a digital countdown to warn you of spoiling, wine labels that alert you when your white wine is at its optimum temperature, or even a window pane that shows the day’s forecast. The AMBER team’s findings have been published today in the leading journal Science.

This discovery opens the path for industry, such as ICT and pharmaceutical, to cheaply print a host of electronic devices from solar cells to LEDs with applications from interactive smart food and drug labels to next-generation banknote security and e-passports.

Prof Jonathan Coleman, who is an investigator in AMBER and Trinity’s School of Physics, said, “In the future, printed devices will be incorporated into even the most mundane objects such as labels, posters and packaging. Printed electronic circuitry (constructed from the devices we have created) will allow consumer products to gather, process, display and transmit information: for example, milk cartons could send messages to your phone warning that the milk is about to go out-of-date.

We believe that 2D nanomaterials can compete with the materials currently used for printed electronics. Compared to other materials employed in this field, our 2D nanomaterials have the capability to yield more cost effective and higher performance printed devices. However, while the last decade has underlined the potential of 2D materials for a range of electronic applications, only the first steps have been taken to demonstrate their worth in printed electronics. This publication is important because it shows that conducting, semiconducting and insulating 2D nanomaterials can be combined together in complex devices. We felt that it was critically important to focus on printing transistors as they are the electric switches at the heart of modern computing. We believe this work opens the way to print a whole host of devices solely from 2D nanosheets.”

Prof Jonathan Coleman and team have fabricated printed transistors consisting entirely of 2-dimensional nanomaterials for the first time. Credit: AMBER, Trinity College Dublin

Led by Prof Coleman, in collaboration with the groups of Prof Georg Duesberg (AMBER) and Prof. Laurens Siebbeles (TU Delft, Netherlands), the team used standard printing techniques to combine graphene nanosheets as the electrodes with two other nanomaterials, tungsten diselenide and boron nitride as the channel and separator (two important parts of a transistor) to form an all-printed, all-nanosheet, working transistor.

Printable electronics have developed over the last thirty years based mainly on printable carbon-based molecules. While these molecules can easily be turned into printable inks, such materials are somewhat unstable and have well-known performance limitations. There have been many attempts to surpass these obstacles using alternative materials, such as carbon nanotubes or inorganic nanoparticles, but these materials have also shown limitations in either performance or in manufacturability. While the performance of printed 2D devices cannot yet compare with advanced transistors, the team believe there is a wide scope to improve performance beyond the current state-of-the-art for printed transistors. 

The ability to print 2D nanomaterials is based on Prof. Coleman’s scalable method of producing 2D nanomaterials, including graphene, boron nitride, and tungsten diselenide nanosheets, in liquids, a method he has licensed to Samsung and Thomas Swan. These nanosheets are flat nanoparticles that are a few nanometres thick but hundreds of nanometres wide. Critically, nanosheets made from different materials have electronic properties that can be conducting, insulating or semiconducting and so include all the building blocks of electronics. Liquid processing is especially advantageous in that it yields large quantities of high quality 2D materials in a form that is easy to process into inks. Prof. Coleman’s publication provides the potential to print circuitry at extremely low cost which will facilitate a range of applications from animated posters to smart labels.

Explore further: Scalable 100 percent yield production of conductive graphene inks

More information: “All-printed thin-film transistors from networks of liquid-exfoliated nanosheets” Science (2017). science.sciencemag.org/cgi/doi/10.1126/science.aal4062

Journal reference: Science

Stanford University: Flawed “nanodiamonds” could produce next-generation tools for imaging and communications

Close-up of purified diamondoids on a lab bench. Too small to see with the naked eye, diamondoids are visible only when they clump together in fine, sugar-like crystals like these.

 

Stanford and SLAC National Accelerator Laboratory jointly run the world’s leading program for isolating and studying diamondoids—the tiniest possible specks of diamond. Found naturally in petroleum fluids, these interlocking carbon cages weigh less than a billionth of a billionth of a carat (a carat weighs about the same as 12 grains of rice); the smallest ones contain just 10 atoms.

Over the past decade, a team led by two Stanford-SLAC faculty members—Nick Melosh, an associate professor of materials science and engineering and of photon science, and Zhi-Xun Shen, a professor of photon science and of physics and applied physics – has found potential roles for diamondoids in improving electron microscope images, assembling materials and printing circuits on computer chips. The team’s work takes place within SIMES, the Stanford Institute for Materials and Energy Sciences, which is run jointly with SLAC.

Before they can do that, though, just getting the diamondoids is a technical feat. It starts at the nearby Chevron refinery in Richmond, California, with a railroad tank car full of crude oil from the Gulf of Mexico. “We analyzed more than a thousand oils from around the world to see which had the highest concentrations of diamondoids,” says Jeremy Dahl, who developed key diamondoid isolation techniques with fellow Chevron researcher Robert Carlson before both came to Stanford—Dahl as a physical science research associate and Carlson as a visiting scientist.

Solutions containing diamondoids await purity analysis in a SLAC lab. Credit: Christopher Smith, SLAC National Accelerator Laboratory

The original isolation steps were carried out at the Chevron refinery, where the selected crudes were boiled in huge pots to concentrate the diamondoids. Some of the residue from that work came to a SLAC lab, where small batches are repeatedly boiled to evaporate and isolate molecules of specific weights. These fluids are then forced at high pressure through sophisticated filtration systems to separate out diamondoids of different sizes and shapes, each of which has different properties.

The diamondoids themselves are invisible to the eye; the only reason we can see them is that they clump together in fine, sugar-like crystals. “If you had a spoonful,” Dahl says, holding a few in his palm, “you could give 100 billion of them to every person on Earth and still have some left over.”

Recently, the team started using diamondoids to seed the growth of flawless, nano-sized diamonds in a lab at Stanford. By introducing other elements, such as silicon or nickel, during the growing process, they hope to make nanodiamonds with precisely tailored flaws that can produce single photons of light for next-generation optical communications and biological imaging.

Jeremy Dahl holds clumps of diamondoid crystals. Credit: Christopher Smith, SLAC National Accelerator Laboratory

Early results show that the quality of optical materials grown from diamondoid seeds is consistently high, says Stanford’s Jelena Vuckovic, a professor of electrical engineering who is leading this part of the research with Steven Chu, professor of physics and of molecular and cellular physiology.

“Developing a reliable way of growing the nanodiamonds is critical,” says Vuckovic, who is also a member of Stanford Bio-X. “And it’s really great to have that source and the grower right here at Stanford. Our collaborators grow the material, we characterize it and we give them feedback right away. They can change whatever we want them to change.”

Nano-scale diamondoid crystals, seen above, are derived from petroleum. They have potential for applications in energy, electronics, and molecular imaging. Credit: Nick Melosh

Explore further: Forces within molecules can strengthen extra-long carbon-carbon bonds

Provided by: Stanford University

 

Ink with Carbon Nanodots Fights Counterfeting: Via 3 Different Mechanisms

Banknotes, documents, branded products, and sensitive goods like pharmaceuticals or technical components are often marked to distinguish them from imitations. However, some counterfeiters have learned to copy conventional fluorescent tags. In the journal Angewandte Chemie, Chinese scientists have now introduced a new, exceptional anti-counterfeit ink made with carbon nanodots. Their ingenious composite material emits three different types of luminescence.

A material that emits light in three different ways at room temperature would be a first. The team led by Hengwei Lin at the Ningbo Institute of Materials Technology & Engineering of Chinese Academy of Sciences, the University of Chongqing, and Southeast University in Nanjing, has successfully produced such a substance based on carbon nanodots—luminescent nanomaterials, which have attracted much attention in recent years due to their unique optical properties and extremely low toxicity.

The researchers used a facile process to make carbon nanodots from m-phenylenediamine. These were then dispersed in water with polyvinyl alcohol and dispensed as ink from a gel pen onto a banknote and a document. After drying, the result was a transparent film of carbon nanodots in a polyvinyl alcohol matrix. This film is colorless under ordinary light, but has three tricks up its sleeve: 1) Irradiation with a UV lamp (365 nm) causes the mark to emit blue light (photoluminescence); 2) the UV irradiation also results in a green afterglow that continues for several seconds after the UV lamp is switched off (room temperature phosphorescence); and 3) irradiation with an infrared femtosecond pulse laser (800 nm) induces a blue-green glow (two-photon luminescence).

Photoluminescence is a phenomenon that is widely observed. Irradiation with UV light catapults electrons into a higher energy level. As the electrons return to the ground state, a portion of the energy is re-emitted as visible light. Two-photon luminescence is a significantly less common phenomenon in which two electrons are absorbed simultaneously (in this case in the infrared range) and jumps to a higher level. From this higher level, the electron can return directly to the ground state by emitting light of a shorter wavelength (in the visible range).

Phosphorescence at room temperature is especially rare. It involves a delay in the release of the absorbed energy because quantum mechanically “forbidden”—and therefore unlikely—electronic transitions are involved. The scientists determined that nitrogen-containing groups on the surface of the carbon nanodots are critical to this observed phosphorescence. The embedding of the nanodots in the polyvinyl alcohol matrix is also important, because it inhibits intramolecular motion that works against the phosphorescence.

Explore further: Luminescent ink from eggs

More information: Kai Jiang et al. Triple-Mode Emission of Carbon Dots: Applications for Advanced Anti-Counterfeiting, Angewandte Chemie International Edition (2016). DOI: 10.1002/anie.201602445

Journal reference: Angewandte Chemie Angewandte Chemie International Edition

Provided by: Angewandte Chemie

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Next-Generation Semiconductor Packaging in Printed Electronics: Video

 

Henkel Electronic Materials LLC is a division of global material supplier, Henkel Corporation. Headquartered in Irvine, California with sales, service, manufacturing and advanced R&D centers around the globe.

Henkel is focused on developing next-generation materials for a variety of applications in semiconductor packaging, industrial, consumer, displays and emerging electronics market sectors. With a broad portfolio of silver, carbon, dielectric and clear conductive inks, Henkel is making today’s medical solutions, in-home conveniences, handheld connectivity, RFID and automotive advances reliable and effective. Watch an interview taken at the IDTechEx Printed Electronics event at this link: www.IDTechEx.com/peusa

 

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ECD’s: A New Process for Fully Additive Roll-to-Roll Printing of flexible electrochromic devices

Electrochromic materials exhibit reversible optical change in the visible region when they are subjected to an electric charge. These switchable materials can be used for ‘smart’ windows in buildings, cars and airplanes as well as in information displays and eye wear. An electrochromic device is one of the most attractive candidates for paper-like displays, so called electronic paper, which will be the next generation display, owing to attributes such as thin and flexible materials, low-power consumption, and fast switching times.    By Michael Berger. Copyright © Nanowerk

Electrochromic devices (ECDs) generally consist of a structure where certain material layers, among them an electrolyte, are sandwiched together. A major limitation until now has been the necessity to use the very expensive indium tin oxide (ITO) as transparent electrodes. ITO’s brittleness makes it unsuitable for flexible device applications and its fabrication process – vacuum-coating, high-temperature annealing – is incompatible with plastic-based substrates. “ECD structure and manufacturing is to a wide extent challenged by the electrolyte component,” Frederik C. Krebs, a professor and head of section of Energy Conversion and Storage at the Technical University of Denmark, tells Nanowerk. “As it remains common practice to employ a semisolid adhesive gel electrolyte, fabrication of devices is limited to separately coating of the two electrodes before finalizing the device in a lamination step; a technical challenge in a simple roll-to-roll (R2R) process and an impossibility in advanced R2R processes with 2D registration requirements.” In new work, reported in the September 5, 2014 online edition of Advanced Materials (“From the Bottom Up – Flexible Solid State Electrochromic Devices”), Krebs and first author Dr. Jacob Jensen describe solid state electrochromic devices, manufactured by sequentially stacking layers in one direction using flexographic printing and slot-die coating methods.

Structure and coating of a solid state electrochromic device. The individual layers are successively coated in the A to F direction. All the layers, except the top electrode (F), are slot die coated as depicted in the upper left picture. The electrolyte layer (D) is slot-die coated followed by in-situ photo-curing as depicted in lower right picture. The top silver-grid electrode (F) is deposited by flexographic printing as depicted in the upper right picture. The structure of the primary electrochromic polymer is shown in the lower left side of the figure. (Reprinted with permission by Wiley-VCH Verlag)

The novelty of this bottom-up printing process for electrochromic device fabrication is the use of printed grid structures in combination with printable electrolytes that can be crosslinked in such a way that many layers can be printed on top of each other. Whereas previous processes have employed the lamination of two separately prepared films, this new method provides the ability to constitute multilayer structures with functionality through printing layers consecutively on top of each other. “We show how – using a specially developed ‘curing chamber’ mounted on a mini roll coater – solid state electrochromic devices can be manufactured continuously in one direction, i.e., from the bottom and up, using slot-die coating and flexographic printing,” says Krebs. “This technique eliminates the need for a lamination step and enables fully additive roll-to-roll processes.” This considerably simplified process constitutes an important step towards R2R manufacturing of ECDs without having to employ brittle materials such as ITO.

This new paper extends the team’s previous reports on ECD manufacture such as “Fast Switching ITO Free Electrochromic Devices” in Advanced Functional Materials and “Manufacture and Demonstration of Organic Photovoltaic-Powered Electrochromic Displays Using Roll Coating Methods and Printable Electrolytes” in the Journal of Polymer Science. The ability to cheaply mass-produce ECDs will find applications ranging from light management and shading to large area/low cost displays such as billboards.

Basically, it is a simple way of printing thin, very low cost and low power consumption display devices. The compromises that need to be made with this process are slow switching speed and relatively poor contrast. Both can be improved, notes Krebs, but since these devices rely on a chemical reaction taking place when changing color there are limits to the switching speed that can be reached. Krebs points out that the current version of his team’s ITO- and vacuum-free grid electrodes still require further optimization to achieve the same optical transmission as the brittle ITO.

Rice University: Silicon Oxide Memories Catch Manufacturers’ Eye

Rice’s silicon oxide memories catch manufacturers’ eye: Use of porous silicon oxide reduces forming voltage, improves manufacturability

Houston, TX | Posted on July 10th, 2014

Rice University’s breakthrough silicon oxide technology for high-density, next-generation computer memory is one step closer to mass production, thanks to a refinement that will allow manufacturers to fabricate devices at room temperature with conventional production methods.

First discovered five years ago, Rice’s silicon oxide memories are a type of two-terminal, “resistive random-access memory” (RRAM) technology. In a new paper available online in the American Chemical Society journal Nano Letters, a Rice team led by chemist James Tour compared its RRAM technology to more than a dozen competing versions.

“This memory is superior to all other two-terminal unipolar resistive memories by almost every metric,” Tour said. “And because our devices use silicon oxide — the most studied material on Earth — the underlying physics are both well-understood and easy to implement in existing fabrication facilities.” Tour is Rice’s T.T. and W.F. Chao Chair in Chemistry and professor of mechanical engineering and nanoengineering and of computer science.

Tour and colleagues began work on their breakthrough RRAM technology more than five years ago. The basic concept behind resistive memory devices is the insertion of a dielectric material — one that won’t normally conduct electricity — between two wires. When a sufficiently high voltage is applied across the wires, a narrow conduction path can be formed through the dielectric material.

The presence or absence of these conduction pathways can be used to represent the binary 1s and 0s of digital data. Research with a number of dielectric materials over the past decade has shown that such conduction pathways can be formed, broken and reformed thousands of times, which means RRAM can be used as the basis of rewritable random-access memory.

RRAM is under development worldwide and expected to supplant flash memory technology in the marketplace within a few years because it is faster than flash and can pack far more information into less space. For example, manufacturers have announced plans for RRAM prototype chips that will be capable of storing about one terabyte of data on a device the size of a postage stamp — more than 50 times the data density of current flash memory technology.

The key ingredient of Rice’s RRAM is its dielectric component, silicon oxide. Silicon is the most abundant element on Earth and the basic ingredient in conventional microchips. Microelectronics fabrication technologies based on silicon are widespread and easily understood, but until the 2010 discovery of conductive filament pathways in silicon oxide in Tour’s lab, the material wasn’t considered an option for RRAM.

Since then, Tour’s team has raced to further develop its RRAM and even used it for exotic new devices like transparent flexible memory chips. At the same time, the researchers also conducted countless tests to compare the performance of silicon oxide memories with competing dielectric RRAM technologies.

“Our technology is the only one that satisfies every market requirement, both from a production and a performance standpoint, for nonvolatile memory,” Tour said. “It can be manufactured at room temperature, has an extremely low forming voltage, high on-off ratio, low power consumption, nine-bit capacity per cell, exceptional switching speeds and excellent cycling endurance.”

This scanning electron microscope image and schematic show the design and composition of new RRAM memory devices based on porous silicon oxide that were created at Rice University.

Credit: Tour Group/Rice University

In the latest study, a team headed by lead author and Rice postdoctoral researcher Gunuk Wang showed that using a porous version of silicon oxide could dramatically improve Rice’s RRAM in several ways. First, the porous material reduced the forming voltage — the power needed to form conduction pathways — to less than two volts, a 13-fold improvement over the team’s previous best and a number that stacks up against competing RRAM technologies. In addition, the porous silicon oxide also allowed Tour’s team to eliminate the need for a “device edge structure.”

“That means we can take a sheet of porous silicon oxide and just drop down electrodes without having to fabricate edges,” Tour said. “When we made our initial announcement about silicon oxide in 2010, one of the first questions I got from industry was whether we could do this without fabricating edges. At the time we could not, but the change to porous silicon oxide finally allows us to do that.”

Wang said, “We also demonstrated that the porous silicon oxide material increased the endurance cycles more than 100 times as compared with previous nonporous silicon oxide memories. Finally, the porous silicon oxide material has a capacity of up to nine bits per cell that is highest number among oxide-based memories, and the multiple capacity is unaffected by high temperatures.”

Tour said the latest developments with porous silicon oxide — reduced forming voltage, elimination of need for edge fabrication, excellent endurance cycling and multi-bit capacity — are extremely appealing to memory companies.

“This is a major accomplishment, and we’ve already been approached by companies interested in licensing this new technology,” he said.

###

Study co-authors — all from Rice — include postdoctoral researcher Yang Yang; research scientist Jae-Hwang Lee; graduate students Vera Abramova, Huilong Fei and Gedeng Ruan; and Edwin Thomas, the William and Stephanie Sick Dean of Rice’s George R. Brown School of Engineering, professor in mechanical engineering and materials science and in chemical and biomolecular engineering.

Flexible Electronics Market worth $13.23 Billion by 2020

According to a new market research report titled “Flexible Electronics Market by Components (Display, Battery, Sensor, Photovoltaic, Memory), Circuit Structure (Single-Sided, Double-Sided, Rigid), Application (Consumer Electronics, Healthcare, Automotive, Energy and Power), & by Geography – Analysis & Forecast to 2014 – 2020“, published by MarketsandMarkets, the Flexible Electronics Market is expected to reach $13.23 Billion by 2020.

 

 

The development of flexible electronics has spanned the past few years, ranging from the development of flexible solar cell arrays to flexible OLED electronics on plastic substrates. The rapid development of this field has been spurred by consistent technological development in large-area electronics, thereby developing the areas like flat-panel electronics, medical image sensors, and electronic paper. Many factors contribute to the rise of flexible electronics they are more ruggedness, lightweight, portable, and less cost, with respect to production as compared to rigid substrate electronics. Basic electronic structure is composed of a substrate, backplane electronics, a front plane, and encapsulation. To make the structure flexible, all the components must bend up to some degree without losing their function. Two basic approaches have been adopted to make flexible electronics, that is, transfer and bonding of completed circuits to a flexible substrate and fabrication of the circuits directly on the flexible substrate.

The report segments the Flexible Electronics Market on the basis of the different types of components, circuit structures, applications and geographies. Further, it contains revenue forecast and analyzes the trends in the market. The geographical analysis contains the in-depth classification of Americas, Europe, and APAC, which contains the major countries covering the market. Further, the Middle-East and Africa have been classified under the RoW region. Each of these geographies has been further split by the major countries existing in this market. The sections and the sub-segments in the report would contain the drivers, restraints, opportunities, and current market trends; and the technologies expected to revolutionize the flexible electronics domain.

The Global Flexible Electronics Market is expected to reach $13.23 Billion by 2020, at an estimated CAGR of 21.73%. The emerging consumer electronics market is expected to grow at a CAGR of 44.30%. North America is the biggest flexible electronics market, followed by Europe and APAC.

Related Reports

Flexible Display Market by Application (Smartphone, Tablet, E-reader, Laptop, TV, Smartcard, Wearable Display), Technology (OLED, LCD, E-paper), Component (Emissive &Non-emissive), Material (Polymer, Glass, GRP) & Geography – Forecast & Analysis to 2013 – 2020

Dielectric Material Market by Technology (OLED, LED, TFT-LCD, LED-LCD, Plasma, LCOS, DLP), Application (Conventional, 3D, Transparent, Flexible), Material (Metal Oxide, a-Silicon, LTPS, PET, PEN, Photonic Crystals) & by Geography – Global Forecast to 2013 – 2020

About MarketsandMarkets

MarketsandMarkets is a global market research and consulting company based in the U.S. We publish strategically analyzed market research reports and serve as a business intelligence partner to Fortune 500 companies across the world.

MarketsandMarkets also provides multi-client reports, company profiles, databases, and custom research services. M&M covers thirteen industry verticals, including advanced materials, automotives and transportation, banking and financial services, biotechnology, chemicals, consumer goods, energy and power, food and beverages, industrial automation, medical devices, pharmaceuticals, semiconductor and electronics, and telecommunications and IT.

Will New Transistor Material Replace Silicon?

 

For the ever-shrinking transistor, there may be a new game in town. Cornell researchers have demonstrated promising electronic performance from a semiconducting compound with properties that could prove a worthy companion to silicon.

New data on electronic properties of an atomically thin crystal of molybdenum disulfide are reported online in Science June 27 by Kin Fai Mak, a postdoctoral fellow at the Kavli Institute at Cornell for Nanoscale Science. His co-authors are Paul McEuen, the Goldwin Smith Professor of Physics; Jiwoong Park, associate professor of chemistry and chemical biology; and physics graduate student Kathryn McGill.

Atoms of molybdenum (gray) and sulfur (yellow) are shown in a two-dimensional crystal formation. A laser hits the surface in a spiral, causing a valley current carried by an electron-hole pair, to move through the crystal. Credit: Kathryn McGill

Recent interest in molybdenum disulfide for transistors has been inspired in part by similar studies on graphene – one atom-thick carbon in an atomic formation like chicken wire. Although super strong, really thin and an excellent conductor, graphene doesn’t allow for easy switching on and off of current, which is at the heart of what a transistor does.

Molybdenum disulfide, on the other hand, is easy to acquire, can be sliced into very thin crystals and has the needed band gap to make it a semiconductor. It possesses another potentially useful property: Besides both intrinsic charge and spin, it also has an extra degree of freedom called a valley, which can produce a perpendicular, chargeless current that does not dissipate any energy as it flows.

If that valley current could be harnessed – scientists are still working on that – the material could form the basis for a near-perfect, atomically thin transistor, which in principle would allow electronics to dissipate no heat, according to Mak.

The researchers showed the presence of this valley current in a molybdenum disulfide transistor they designed at the Cornell NanoScale Science and Technology Facility (CNF). Their experiments included illuminating the transistor with circularly polarized light, which had the unusual effect of exciting electrons into a sideways curve. These experiments bolstered the concept of using the valley degree of freedom as an information carrier for next-generation electronics or optoelectronics.

Explore further: Scalable CVD process for making 2-D molybdenum diselenide

Narrow Width Graphene Ribbons for Semi-Conductors

Using graphene ribbons of unimaginably small widths – just several atoms across – a group of researchers at the University of Wisconsin-Milwaukee (UWM) has found a novel way to “tune” the wonder material, causing the extremely efficient conductor of electricity to act as a semiconductor.

In principle, their method for producing these narrow ribbons – at a width roughly equal to the diameter of a strand of human DNA – and manipulating the ribbons’ electrical conductivity could be used to produce nano-devices.

Graphene, a one-atom-thick sheet of carbon atoms, is touted for its high potential to yield devices at nanoscale and deliver computing at quantum speed. But before it can be applied to nanotechnology, researchers must first find an easy method of controlling the flow of electrons in order to devise even a simple on-off switch.

“Nano-ribbons are model systems for studying nanoscale effects in graphene, but obtaining a ribbon width below 10 nanometers and characterizing its electronic state is quite challenging,” says Yaoyi Li, a UWM physics postdoctoral researcher and first author of a paper published July 2 in the journal Nature Communications (“Direct experimental determination of onset of electron–electron interactions in gap opening of zigzag graphene nanoribbons”).

Yaoyi Li (foreground) and Mingxing Chen, UWM physics postdoctoral researchers, display an image of a ribbon of graphene 1 nanometer wide. In the image, achieved with a scanning-tunneling microscope, atoms are visible as ‘bumps’.

By imaging the ribbons with scanning-tunneling microscopy, researchers have confirmed how narrow the ribbon width must be to alter graphene’s electrical properties, making it more tunable.

“We found the transition happens at three nanometers and the changes are abrupt,” says Michael Weinert, a UWM theoretical physicist who worked on the Department of Energy-supported project with experimental physicist Lian Li. “Before this study, there was no experimental evidence of what width the onset of these behaviors is.”

The team also found that the narrower the ribbon becomes, the more “tunable” the material’s behaviors. The two edges of such a narrow ribbon are able to strongly interact, essentially transforming the ribbon into a semiconductor with tunable qualities similar to that of silicon.

The first hurdle

Current methods of cutting can produce ribbon widths of five nanometers across, still too wide to achieve the tunable state, says Yaoyi Li. In addition to producing narrower ribbons, any new strategy for cutting they devised would also have to result in a straight alignment of the atoms at the ribbon edges in order to maintain the electrical properties, he adds.

So the UWM team used iron nanoparticles on top of the graphene in a hydrogen environment. Iron is a catalyst that causes hydrogen and carbon atoms to react, creating a gas that etches a trench into the graphene. The cutting is accomplished by precisely controlling the hydrogen pressure, says Lian Li.

The iron nanoparticle moves randomly across the graphene, producing ribbons of various widths – including some as thin as one nanometer, he says. The method also produces edges with properly aligned atoms.

One limitation exists for the team’s cutting method, and that has to do with where the edges are cut. The atoms in graphene are arranged on a honeycomb lattice that, depending on the direction of the cut produces either an “armchair-shaped” edge or a “zigzag” one. The semiconducting behaviors the researchers observed with their etching method will only occur with a cut in the zigzag configuration.

Manipulating for function

When cut, the carbon atoms at the edges of the resulting ribbons have only two of the normal three neighbors, creating a kind of bond that attracts hydrogen atoms and corrals electrons to the edges of the ribbon. If the ribbon is narrow enough, the electrons on opposite sides can still interact, creating a semiconductive electrical behavior, says Weinert.

The researchers are now experimenting with saturating the edges with oxygen, rather than hydrogen, to investigate whether this changes the electrical behavior of the graphene to that of a metal.

Adding function to graphene nano-ribbons through this process could make possible the sought-after goal of atomic-scale components made of the same material, but with different electrical behaviors, says Weinert.

Source: University of Wisconsin – Milwaukee