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Creating 2D heterostructures for future electronics

Posted By Graphene Council, The Graphene Council, Thursday, November 7, 2019
Updated: Thursday, November 7, 2019
While many nanomaterials exhibit promising electronic properties, scientists and engineers are still working to best integrate these materials together to eventually create semiconductors and circuits with them.

Northwestern Engineering researchers have created two-dimensional (2D) heterostructures from two of these materials, graphene and borophene, taking an important step toward creating intergrated circuits from these nanomaterials.

"If you were to crack open an integrated circuit inside a smartphone, you'd see many different materials integrated together," said Mark Hersam, Walter P. Murphy Professor of Materials Science and Engineering, who led the research. "However, we've reached the limits of many of those traditional materials. By integrating nanomaterials like borophene and graphene together, we are opening up new possibilities in nanoelectronics."

Supported by the Office for Naval Research and the National Science Foundation, the results were published October 11 in the journal Science Advances. In addition to Hersam, applied physics PhD student Xiaolong Liu co-authored this work.

Creating a new kind of heterostructure

Any integrated circuit contains many materials that perform different functions, like conducting electricity or keeping components electrically isolated. But while transistors within circuits have become smaller and smaller -- thanks to advances in materials and manufacturing -- they are close to reaching the limit of how small they can get.

Ultrathin 2D materials like graphene have the potential to bypass that problem, but integrating 2D materials together is difficult. These materials are only one atom thick, so if the two materials' atoms do not line up perfectly, the integration is unlikely to be successful. Unfortunately, most 2D materials do not match up at the atomic scale, presenting challenges for 2D integrated circuits.

Borophene, the 2D version of boron that Hersam and coworkers first synthesized in 2015, is polymorphic, meaning it can take on many different structures and adapt itself to its environment. That makes it an ideal candidate to combine with other 2D materials, like graphene.

To test whether it was possible to integrate the two materials into a single heterostructure, Hersam's lab grew both graphene and borophene on the same substrate. They grew the graphene first, since it grows at a higher temperature, then deposited boron on the same substrate and let it grow in regions where there was no graphene. This process resulted in lateral interfaces where, because of borophene's accommodating nature, the two materials stitched together at the atomic scale.

Measuring electronic transitions

The lab characterized the 2D heterostructure using a scanning tunneling microscope and found that the electronic transition across the interface was exceptionally abrupt -- which means it could be ideal for creating tiny electronic devices.

"These results suggest that we can create ultrahigh density devices down the road," Hersam said. Ultimately, Hersam hopes to achieve increasingly complex 2D structures that lead to novel electronic devices and circuits. He and his team are working on creating additional heterostructures with borophene, combining it with an increasing number of the hundreds of known 2D materials.

"In the last 20 years, new materials have enabled miniaturization and correspondingly improved performance in transistor technology," he said. "Two-dimensional materials have the potential to make the next leap."

Tags:  2D materials  Graphene  Mark Hersam  nanoelectronics  nanomaterials  Northwestern University  Xiaolong Liu 

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Laser-induced graphene composites are eminently wearable

Posted By Graphene Council, The Graphene Council, Monday, June 24, 2019
Graphene has a unique combination of properties that is ideal for next-generation electronics, including mechanical flexibility, high electrical conductivity, and chemical stability. The burgeoning field of wearable electronics – 'smart' fabrics with invisibly integrated energy harvesting, energy storage, electronics and sensor systems – benefits from graphene in numerous ways. Graphene materials, be they pristine or composites, will lead to smaller high-capacity and fast-charging supercapacitors, completely flexible and even rollable electronics and energy-storage devices, and transparent batteries.

To realize the commercial potential of graphene, it is necessary to develop reliable, cost-effective and facile processes for the industry-scale fabrication of graphene-based devices.

One possible route is inkjet printing, already extensively demonstrated with conductive metal nanoparticle inks. Although liquid-phase graphene dispersions have been demonstrated, researchers are still struggling with sophisticated inkjet printing technologies that allow efficient and reliable mass production of high-quality graphene patterns for practical applications.

A novel solution comes from the team at Joseph Wang's Laboratory for Nanobioelectronics at UC San Diego. Reporting their findings in Advanced Materials Technologies ("Laser-Induced Graphene Composites for Printed, Stretchable, and Wearable Electronics"), they demonstrate the synthesis of high-performance stretchable graphene ink using a facile, scalable, and low-cost laser induction method for the synthesis of the graphene component.

As a proof-of-concept, the researchers fabricated a stretchable micro-supercapacitor (S-MSC) demonstrating the highest capacitance reported for a graphene-based highly stretchable MSC to date. This also is the first example of using laser-induced graphene in the form for a powder preparation of graphene-based inks and subsequently for use in screen-printing of S-MSC.

Back in 2014, researchers at Rice University created flexible, patterned sheets of multilayer graphene from a cheap polymer by burning it with a computer-controlled laser, a technique they called laser-induced graphene (LIG). This high-yield and low-cost graphene synthesis process works in air at room temperature and eliminates the need for hot furnaces and controlled environments, and it makes graphene that is suitable for electronics or energy storage.

"LIG can be prepared from a few polymeric substances, such as Kapton polyimide and polyetherimide, as well as various sustainable biomasses, including wood, lignin, cloth, paper, or hydrothermal carbons," Farshad Tehrani, the paper's first author. "On the other hand, LIG has considerably enhanced dispersion in typical solvent and binders due to its inherently abundant defects and surface functional groups."

He points out that the team's novel method, while maintaining the distinct advantages of the direct-written LIG, unlocks untapped potentials of the LIG material in several areas:

Mechanical stretchability: In this study, the inherently brittle and mechanically fragile LIG electrodes are turned into a mechanically robust, highly stretchable electrodes, with the new ink attractive for diverse wearable electronic devices.

Enhanced electrochemical performance: The areal capacitance of the team's S-MSC has far surpassed that of direct-written laser LIG and has produced the highest areal capacitance reported for highly stretchable supercapacitors.

Customized composite formulations: The basic ink formulation is compatible with a wide range of compositions using the LIG as an attractive conductive filler.

Substrate versatility: Unlike direct-laser writing, which is limited to polymeric substrates and several biomasses, the LIG ink can be printed on almost any stretchable and non stretchable substrate, such as polymeric substrates, fabrics, or textiles.

"During the development of our new supercapacitor, we discovered a specific synergic effect between polymeric binders poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) mixed with Polyurethane (PU), PEDOT:PSS-PU and graphene sheets in producing exceptional electromechanical performances," adds Fernando Soto, a co-author of the paper. "We realized that when both sides of the graphene sheets are thoroughly covered with the conductive/elastic PEDOT:PSS/PU polymer, it results in a robust composite that withstands severe shear stresses during stretching."

"Not only that, but it also maintains above 85% of its electrochemical performance such as its charge storing capacitance properties, composite conductivity and electrochemical stability at high charge-discharge cycles," he adds.

In developing wearable electronic devices, researchers need to deal with a range of issues where stretchability and mechanical performance of the device is as important as its electronic properties such as conductivity, charge storage properties and, generally, its high electrochemical performance.

Rather than focusing on one of these specific problems, the team's work addresses a series of challenges that include high mechanical and electrochemical performance while keeping the costs at their lowest possible point for realistic commercialization scenarios.

"From the design to the implementation stages of our study, the primary focus has been devoted to scalability, versatility and cost efficiency of a high performance platform that can potentially spark further innovations using nanocomposite materials in the field of wearable electronics," notes Tehrani.

The next stages of the team's work in this area of wearable applications will see the integration of these high-performance S-MSCs with batteries and energy harvesting systems such as biofuel cells, triboelectrics, and piezoelectrics.

Tags:  Farshad Tehrani  Graphene  Joseph Wang  nanocomposites  nanoelectronics  Sensors  UC San Diego 

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From 2D to 1D: Atomically quasi '1D' wires using a carbon nanotube template

Posted By Graphene Council, The Graphene Council, Wednesday, April 24, 2019
Updated: Tuesday, April 23, 2019
Researchers from Tokyo Metropolitan University have used carbon nanotube templates to produce nanowires of transition metal monochalcogenide (TMM), which are only 3 atoms wide in diameter. These are 50 times longer than previous attempts and can be studied in isolation, preserving the properties of atomically quasi "1D" objects. The team saw that single wires twist when perturbed, suggesting that isolated nanowires have unique mechanical properties which might be applied to switching in nanoelectronics.

Two-dimensional materials have gone from theoretical curiosity to real-life application in the span of less than two decades; the most well-known example of these, graphene, consists of well-ordered sheets of carbon atoms. Though we are far from leveraging the full potential of graphene, its remarkable electrical and thermal conductivity, optical properties and mechanical resilience have already led to a wide range of industrial applications. Examples include energy storage solutions, biosensing, and even substrates for artificial tissue.

Yet, despite the successful transition from 3D to 2D, the barrier separating 2D and 1D has been significantly more challenging to overcome. A class of materials known as transition metal monochalcogenides (TMMs, transition metal + group 16 element) have received particular interest as a potential nanowire in precision nanoelectronics. Theoretical studies have existed for over 30 years, and preliminary experimental studies have also succeeded in making small quantities of nanowire, but these were usually bundled, too short, mixed with bulk material or simply low yield, particularly when precision techniques were involved e.g. lithography. The bundling was particularly problematic; forces known as van der Waals forces would force the wires to aggregate, effectively masking all the unique properties of 1D wires that one might want to access and apply.

Now, a team led by Assistant Professor Yusuke Nakanishi from Tokyo Metropolitan University has succeeded in producing bulk quantities of well-isolated single nanowires of TMM. They used tiny, open-ended rolls of single-layered carbon, or carbon nanotubes (CNTs), to template the assembly and reaction of molybdenum and tellurium into wires from a vapor. They succeeded in producing single isolated wires of TMM, which were only 3-atoms thick and fifty times longer than those made using existing methods. These nanometer-sized CNT "test tubes" were also shown to be not chemically bound to the wires, effectively preserving the properties expected from isolated TMM wires. Importantly, they effectively "protected" the wires from each other, allowing for unprecedented access to how these 1D objects behave in isolation.

While imaging these objects using transmission electron microscopy (TEM), the team found that these wires exhibited a unique twisting effect when exposed to an electron beam. Such behavior has never been seen before and is expected to be unique to isolated wires. The transition from a straight to twisted structure may offer a novel switching mechanism when the material is incorporated into microscopic circuits. The team hope the ability to make well-isolated 1D nanowires might significantly expand our understanding of the properties and mechanisms behind the function of 1D materials.

Tags:  2D materials  Carbon Nanotubes  Graphene  nanoelectronics  Tokyo Metropolitan University  Yusuke Nakanishi 

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New method of synthesising nanographene on metal oxide surfaces

Posted By Graphene Council, The Graphene Council, Tuesday, March 5, 2019
Updated: Tuesday, March 5, 2019

Nanostructures based on carbon are promising materials for nanoelectronics.

However, to be suitable, they would often need to be formed on non-metallic surfaces, which has been a challenge – up to now. Researchers at FAU have found a method of forming nanographenes on metal oxide surfaces. Their research, conducted within the framework of collaborative research centre 953 – Synthetic Carbon Allotropes funded by the German Research Foundation (DFG), has now been published in the journal Science.



Two-dimensional, flexible, tear-resistant, lightweight, and versatile are all properties that apply to graphene, which is often described as a miracle material. In addition, this carbon-based nanostructure has unique electrical properties that make it attractive for nanoelectronic applications. Depending on its size and shape, nanographene can be conductive or semi-conductive – properties that are essential for use in nanotransistors. Thanks to its good electrical and thermal conductivity, it could also replace copper (which is conductive) and silicon (which is semi-conductive) in future nanoprocessors.

Nanographene on metal oxides

The problem: In order to create an electronic circuit, the molecules of nanographene must be synthesised and assembled directly on an insulating or semi-conductive surface. Although metal oxides are the best materials for this purpose, in contrast to metal surfaces, direct synthesis of nanographenes on metal oxide surfaces is not possible as they are considerably less chemically reactive. The researchers would have to carry out the process at high temperatures, which would lead to several uncontrollable secondary reactions.

A team of scientists led by Dr. Konstantin Amsharov from the Chair of Organic Chemistry II have now developed a method of synthesising nanographenes on non-metallic surfaces, that is insulating surfaces or semi-conductors.

It’s all about the bond

The researchers’ method involves using a carbon fluorine bond, which is the strongest carbon bond. It is used to trigger a multilevel process. The desired nanographenes form like dominoes via cyclodehydrofluorination on the titanium oxide surface. All ‘missing’ carbon-carbon bonds are thus formed after each other in a formation that resembles a zip being closed.

This enables the researchers to create nanographenes on titanium oxide, a semi-conductor. This method also allows them to define the shape of the nanographene by modifying the arrangement of the preliminary molecules. New carbon-carbon bonds and, ultimately, nanographenes form where the researchers place the fluourine atoms.

For the first time, these research results demonstrate how carbon-based nanostructures can be manufactured by direct synthesis on the surfaces of technically-relevant semi-conducting or insulating surfaces. ‘This groundbreaking innovation offers effective and simple access to electronic nanocircuits that really work, which could scale down existing microelectronics to the nanometre scale,’ explains Dr. Amsharov.

Tags:  Friedrich-Alexander-Universität Erlangen-Nürnberg  Graphene  Konstantin Amsharov  nanoelectronics  nanographene  Semiconductor 

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Large, stable pieces of graphene produced with unique edge pattern

Posted By Graphene Council, The Graphene Council, Wednesday, February 6, 2019
Updated: Wednesday, February 6, 2019

Graphene is a promising material for use in nanoelectronics. Its electronic properties depend greatly, however, on how the edges of the carbon layer are formed. Zigzag patterns are particularly interesting in this respect, but until now it has been virtually impossible to create edges with a pattern like this. Chemists and physicists at FAU have now succeeded in producing stable nanographene with a zigzag edge.

Not only that, the method they used was even comparatively simple. Their research, conducted within the framework of collaborative research centre 953  – Synthetic Carbon Allotropes funded by the German Research Foundation (DFG), has now been published in the journal Nature Communications*.

Bay, fjord, cove, armchair and zigzag – when chemists use terms such as these, it is clear that they are referring to nanographene. More specifically, the shape taken by the edges of nanographene, i.e. small fragments of graphene. Graphene consists of a single-layered carbon structure, where each carbon atom is surrounded by three others. This creates a pattern reminiscent of a honeycomb, with atoms in each of the corners. Nanographene is a promising candidate for use in the field of microelectronics, taking over from silicon which is used today and bringing microelectronics down to the nano scale.

The electronic properties of the material depend greatly on its shape, size and above all, periphery, in other words how the edges are structured. A zigzag periphery is particularly suitable, as in this case the electrons, which act as charge carriers, are more mobile than in other edge structures. This means that using pieces of zigzag-shaped graphene in nanoelectronic components may allow higher frequencies for switches.

The problem currently faced by materials scientists who want to research only zigzag nanographene is that this form makes the compounds rather unstable, and unable to be produced in a controlled manner. This is a prerequisite, however, if the electronic properties are to be investigated in detail.

The team of researchers led by PD Dr. Konstantin Amsharov from the Chair of Organic Chemistry II have now succeeded in doing just that. Not only have they discovered a straightforward method for synthesising zigzag nanographene, their procedure delivers a yield of close to one hundred percent and is suitable for large scale production. They have already produced a technically relevant quantity in the laboratory.

First of all, the FAU researchers produce preliminary molecules, which they then fitt together in a honeycomb formation over several cycles, in a process known as cyclisation. In the end, graphene fragments are produced from staggered rows of honeycombs or four-limbed stars surrounding a central point of four graphene honeycombs, with the sought-after zigzag pattern to their edges. Why is this method able to produce stable zigzag nanographene? The explanation lies in the fact that the product crystallises directly even during synthesis. In their solid state, the molecules are not in contact with oxygen. In solution, however, oxidation causes the structures to disintegrate quickly.

This approach allows scientists to produce large pieces of graphene, whilst maintaining control over their shape and periphery. This breakthrough in graphene research means that scientists should soon be able to produce and research a variety of interesting nanographene structures, a crucial step towards finally being able to use the material in nanoelectronic components.

Tags:  Friedrich–Alexander University  Graphene  Konstantin Amsharov  nanoelectronics  nanographene 

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