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Electrochemical doping: researchers improve carbon nanotube transparent conductors

Posted By Graphene Council, Wednesday, July 29, 2020
Skoltech researchers and their colleagues from Aalto University have discovered that electrochemical doping with ionic liquid can significantly enhance the optical and electrical properties of transparent conductors made of single-walled carbon nanotube films. The results were published in the journal Carbon.

A single-walled carbon nanotube (SWCNT) is a seamless rolled sheet of graphene, a list of graphite that is one atom thick. Just as other new carbon allotropes, SWCNTs demonstrate unique properties which can be employed in novel electronic devices that we use in our everyday life. One of the most promising applications is transparent conductors, which can be useful in medicine, green energy, and other fields: here, SWCNT films can replace the industrial standard indium-tin oxide (ITO). They are highly conductive, flexible, stretchable and can be easy doped due to the fact that all atoms in the nanotube are located on its surface.

Doping of SWCNTs allows to significantly increase film conductivity by eliminating the Schottky barriers between the tubes with different nature and increase the concentration of charge carriers. Moreover, the doping process leads to an increase in the transmittance of the films due to supersession of optical transitions.

While adsorption doping remains one of the most promising techniques for SWCNT modification, this method lacks uniformity and reversibility. In the new study, researchers propose a new reversible method to fine-tune the Fermi level of SWCNTs, dramatically increasing the conductivity while the optical transitions are suppressed. For this, they used electrochemical doping with an ionic liquid with a large potential window, which facilitates a high level of doping.

“We placed the SWCNT thin film into electrochemical cell and used standard three electrode scheme to apply potential to the nanotubes. With applying the negative/positive potential to the SWCNT film, an electrical double layer is formed at the SWCNT/ionic liquid interface. The latter acts as parallel plate capacitor causing positive/negative charge injection to SWCNT film surface and consequently the Fermi level shift,” explains Daria Kopylova, the first author of the study and senior research scientist at Skoltech.

The scientists were able to show that their electrochemical method can help achieve extremely high doping levels, comparable to the best results for doped SWCNTs films recently published in the field.

“The process is fully reversible so that it can be used to fine-tune the electronic structure of the single-walled carbon nanotubes in real time. Operating with the gate voltage, you can drive both optical transmittance and electrical conductivity of the films. The results open new avenues for future electronics, electrochromic devices, and ionotronics,” says Albert Nasibulin, head of Laboratory of Nanomaterials at the Skoltech Center for Photonics and Quantum Materials.

Tags:  Aalto University  Albert Nasibulin  carbon nanotube  Daria Kopylova  Graphene  Skoltech 

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Stopping the unstoppable with atomic bricks

Posted By Graphene Council, Monday, June 29, 2020
Graphene's unique 2D structure means that electrons travel through it differently to most other materials. One consequence of this unique transport is that applying a voltage to them doesn't stop the electrons like it does in most other materials. This is a problem because to make useful applications out of graphene and its unique electrons like quantum computers, it is necessary to be able to stop and control graphene electrons.

An interdisciplinary team of scientists from the Universidad Autonoma de Madrid (Spain), Université Grenoble Alpes (France), International Iberian Nanotechnology Laboratory (Portugal) and Aalto University has managed to solve this long-standing problem. They combined experimental researchers including Eva Cortés del Río, Pierre Mallet, Héctor González‐Herrero, José María Gómez‐Rodríguez, Jean‐Yves Veuillen and Iván Brihuega with theorists, including Joaquín Fernández-Rossier and Jose Lado, assistant Professor in the department of Applied Physics at Aalto.

The experimental team used atomic bricks to build walls capable of stopping the graphene electrons. This was achieved by creating atomic walls that confined the electrons, leading to structures whose spectrum was then compared with theoretical predictions, demonstrating that electrons were confined. In particular, it was obtained that the engineered structures gave rise to nearly perfect confinement of electrons, as demonstrated from the emergence of sharp quantum well resonances with a remarkably long lifetime.

The work, published this week in Advanced Materials, demonstrates that impenetrable walls for graphene electrons can be created by collective manipulation of a large number of hydrogen atoms. In the experiments, a scanning tunnelling microscope was used to construct artificial walls with sub nanometric precision. This led to graphene nanostructures of arbitrarily complex shapes, with dimensions ranging from two nanometres to one micron.

Importantly, the developed method is non-destructive, allowing to erase and rebuild the nanostructures at will, providing an unprecedented degree of control to create artificial graphene devices. The experiments demonstrate that the engineered nanostructures are capable of perfectly confining the graphene electrons in these artificially designed structures, overcoming the critical challenge imposed by Klein tunnelling. Ultimately, this opens up a plethora of exciting new possibilities, as the created nanostructures realize graphene quantum dots that can be selectively coupled, opening ground-breaking possibilities for artificially designed quantum matter.

Tags:  2D materials  Aalto University  Graphene  Jose Lado  nanostructures  quantum materials  Universidad Autonoma de Madrid 

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Lightning in a (nano)bottle: new supercapacitor opens door to better wearable electronics

Posted By Graphene Council, Friday, June 12, 2020
Researchers from Skoltech, Aalto University and Massachusetts Institute of Technology have designed a high-performance, low-cost, environmentally friendly, and stretchable supercapacitor that can potentially be used in wearable electronics. The paper was published in the Journal of Energy Storage.

Supercapacitors, with their high power density, fast charge-discharge rates, long cycle life, and cost-effectiveness, are a promising power source for everything from mobile and wearable electronics to electric vehicles. However, combining high energy density, safety, and eco-friendliness in one supercapacitor suitable for small devices has been rather challenging.

"Usually, organic solvents are used to increase the energy density. These are hazardous, not environmentally friendly, and they reduce the power density compared to aqueous electrolytes with higher conductivity," says Professor Tanja Kallio from Aalto University, a co-author of the paper.

The researchers proposed a new design for a "green" and simple-to-fabricate supercapacitor. It consists of a solid-state material based on nitrogen-doped graphene flake electrodes distributed in the NaCl-containing hydrogel electrolyte. This structure is sandwiched between two single-walled carbon nanotube film current collectors, which provides stretchability. Hydrogel in the supercapacitor design enables compact packing and high energy density and allows them to use the environmentally friendly electrolyte.

The scientists managed to improve the volumetric capacitive performance, high energy density and power density for the prototype over analogous supercapacitors described in previous research. "We fabricated a prototype with unchanged performance under the 50% strain after a thousand stretching cycles. To ensure lower cost and better environmental performance, we used a NaCl-based electrolyte. Still the fabrication cost can be lowered down by implementation of 3D printing or other advanced fabrication techniques," concluded Skoltech professor Albert Nasibulin.

Tags:  Aalto University  Albert Nasibulin  Electronics  Graphene  Massachusetts Institute of Technology  Supercapacitor  Tanja Kallio 

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Twisting 2D materials uncovers their superpowers

Posted By Graphene Council, Saturday, May 2, 2020
Two-dimensional (2D) materials, which consist of a single layer of atoms, have attracted a lot of attention since the isolation of graphene in 2004. They have unique electrical, optical, and mechanical properties, like high conductivity, flexibility and strength, which makes them promising materials for such things as lasers, photovoltaics, sensors and medical applications.

When a sheet of 2D material is placed over another and slightly rotated, the twist can radically change the bilayer material's properties and lead to exotic physical behaviours, such as high temperature superconductivity - exiting for electrical engineering; nonlinear optics - exciting for lasers and data transmission; and structural super-lubricity- a newly discovered mechanical property which researchers are only beginning to understand. The study of these properties has given birth to a new field of research called twistronics, so-called because it's a combination of twist and electronics.

Aalto University's researchers collaborating with international colleagues have now developed a new method for making these twisted layers on scales that are large enough to be useful, for the first time. Their new method for transferring single-atom layers of molybdenum disulfide (MoS2) allows researchers to precisely control the twist angle between layers with up to a square centimetre in area, making it record-breaking in terms of size. Controlling the interlayer twist angle on a large scale is crucial for the future practical applications of twistronics.

'Our demonstrated twist method allows us to tune the properties of stacked multilayer MoS2 structures on larger scales than ever before. The transfer method can also apply to other two-dimensional layered materials', says Dr Luojun Du from Aalto University, one of the lead authors of the work.

A significant advancement for a brand-new field of research

Since twistronics research was introduced only in 2018, basic research is still needed to understand the properties of twisted materials better before they find their ways to practical applications. The Wolf Prize in Physics, one of the most prestigious scientific awards, was awarded to Profs. Rafi Bistritzer, Pablo Jarillo-Herrero, and Allan H. MacDonald this year for their groundbreaking work on twistronics, which indicates the game-changing potential of the emerging field.

Previous research has demonstrated that it is possible to fabricate the required twist angle by transfer method or atomic force microscope tip manipulation techniques in small scales. The sample size has usually been in the order of ten-microns, less than the size of a human hair. Larger few-layer films have also been fabricated, but their interlayer twist angle is random. Now the researchers can grow large films using an epitaxial growth method and water assistant transfer method.

'Since no polymer is needed during the transfer process, the interfaces of our sample are relatively clean. With the control of twist angle and ultra-clean interfaces, we could tune the physical properties, including low-frequency interlayer modes, band structure, and optical and electrical properties', Du says.

'Indeed, the work is of great significance in guiding the future applications of twistronics based on 2D materials', adds Professor Zhipei Sun from Aalto University.

Tags:  2D materials  Aalto University  Electronics  Graphene  photovoltaics  Sensors 

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Finally, Anyons reveal their exotic quantum properties

Posted By Graphene Council, Monday, April 13, 2020
An Aalto University research fellow, Manohar Kumar is the co-first author on a paper published this week in Science. He and Hugo Bartolomei, the other co-first author, were part of a team at Ecole Normale Supérieure Paris to be the first to directly measure the quantum properties of exotic particles called an “anyons”. Anyons have been explored both theoretically and experimentally; but the true quantum nature of these particles was elusive until now. Anyons are interesting to scientists trying to build quantum computers, and other devices that exploit the properties of quantum physics. The significance of the work on the field of experimental quantum physics is so high, that the work has been selected to appear on the cover of Science. 

What are anyons?
In the three-dimensional world we live in, there are only two types of particles: “fermions”, which repel each other, and “bosons” which like to stick together. A commonly known fermion is the electron, which transports electricity; and a commonly known boson is the photon, which carries light. In the two-dimensional world, however, there is another type of particle, the anyon, which doesn’t behave like either a fermion or a boson. The exact quantum nature of anyons lies in their wave nature, encoded in their quantum statistics. They were first proposed in the late 1970s, but direct experimental evidence of their quantum statistics hasn’t been conclusively shown until now.

‘Other researchers have been able to measure states with fractional charges before, which strongly suggested that anyons existed’ said Professor Gwendal Féve at École Normale Supérieure in Paris, who is in-charge of the research group that carried out the work. ‘However, the definitive proof of the existence of anyons was to prove that they behave like something that’s part way between a fermion and a boson, and that’s what we’ve been able to show for the first time with this experiment.’

Researchers have been trying to create and measure anyons by trapping them in nanosize boxes and measuring how they move around, but the results of those studies have been contentious. The team in this study believe they now have conclusive proof.

‘We created a very tiny particle collider, size of a human hair diameter. In this collider we smashed anyons to reveal their true quantum nature’ said Hugo Bartolomei, a graduate student who worked with Dr. Kumar and Prof. Féve on this project.

‘Our experiment worked like a 4-way junction in a road, with two roads in and two roads out. If you send fermions, which “hate” each other down the two roads in, they meet at the intersect but then leave down separate out roads. If you send bosons, which “like” each other, down the two roads in, they meet at the junction and then leave down the same out road together.’ explained Dr Kumar. ‘However, if you send anyons down the 2 roads in, they behave completely different: sometimes they combine, sometimes they don’t. Though they tend to bunch together like bosons, but the exact degree of togetherness lies in their wave nature’. Dr. Kumar and the team created particles they suspected to be anyons in a 2-dimensional layer of gallium arsenide, where they collided them in the 4-way junction. The experimental results showed this bunching tendency of anyons, replicating perfectly the mathematical model developed by theoretical groups at Leipzig, Harvard and ETH Zurich 4 years ago. 

Any use for anyons?
This result is an important milestone for the field of condensed matter physics, as it provides experimental evidence for a particle that has only existed theoretically for almost a generation. The technique the researchers have used is also important as it allows other experimental scientists to reproduce and extend their research. In terms of real-world applications, anyons will still be confined to the lab for a long time but may have uses someday. ‘Our work until now focused on a type of anyon called “abelian” anyons,’ explains Dr Kumar. ‘However, a more exotic type called non-abelian anyons theoretically exists. These could be very useful because if you can interchange them, you can make them to form a qubit, which is essential for topological quantum computing. Our paper in Science shows a method for interchanging abelian anyons, so if it can be extended to non-abelian anyons then we might be able to open up a new avenue for exploring quantum computers.’  Dr Kumar now works on graphene with Professor Hakonen in the Low Temperature Laboratory at the Department of Applied Physics at Aalto University. ‘Graphene might be useful for make non-abelian anyons, so I’m looking to repeat the experiment in graphene and measure these exotic and exciting new particles.’ said Dr Kumar.

Tags:  Aalto University  École Normale Supérieure in Paris  Graphene  Gwendal Féve  Manohar Kumar  quantum dots 

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New material developed could help clean energy revolution

Posted By Graphene Council, Friday, March 27, 2020
Researchers developed a promising graphene–carbon nanotube catalyst, giving them better control over hugely important chemical reactions for producing hydrogen fuel

Fuel cells and water electrolyzers that are cheap and efficient will form the cornerstone of a hydrogen fuel based economy, which is one of the most promising clean and sustainable alternatives to fossil fuels. These devices rely on materials called electrocatalysts to work, so the development of efficient and low-cost catalysts is essential to make hydrogen fuel a viable alternative.  Researchers at Aalto university have developed a new catalyst material to improve these technologies.

The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are the most important electrochemical reactions that limit the efficiencies of hydrogen fuel cells (for powering vehicles and power generation), water electrolyzers (for clean hydrogen production), and high-capacity metal-air batteries. Physicists and chemists at Aalto collaborating with researchers at CNRS France, and Vienna in Austria have developed a new catalyst that drive these reactions more efficiently than other bifunctional catalysts currently available. The researchers also found that the electrocatalytic activity of their new catalyst can be significantly altered depending on choice of the material on which the catalyst was deposited.

“We want to replace traditional expesive and scarce catalysts based on precious metals like platinum and iridium with highly active and stable alternatives composed of cheap and earth-abundant elements such as transition metals, carbon and nitrogen.” says Dr Mohammad Tavakkoli, the researcher at Aalto who led the work and wrote the paper.

In collaboration with CNRS the team produced a highly porous graphene–carbon nanotube hybrid and doped it with single atoms of other elements known to make good catalysts. Graphene and carbon nanotube (CNT) are the one‐atom‐thick two- and one‐dimensional allotropes of carbon, respectively, which have attracted tremendous interest in both academia and industry due to their outstanding properties compared more traditional materials. They developed an easy and scalable method to grow these nanomaterials at the same time, combining their properties in a single product. “We are one of the leading teams in the world for the scalable synthesis of double-walled carbon nanotubes. The innovation here was to modify our fabrication process to prepare these unique samples,” said Dr Emmanuel Flahut, research director at CNRS.

In this one-step process, they could also dope the graphene with nitrogen and/or metallic (Cobalt and Molybdenum) single-atoms as a promising strategy to produce single-atom catalysts (SACs). In catalysis science, the new field of SACs with isolated metal atoms dispersed on solid supports has attracted wide research attention because of the maximum atom-utilization efficiency and the unique properties of SACs. Compared with rival strategies for making SACs, the method used by the Aalto & CNRS team provides an easy method which takes place in one step, keeping costs down.
Catalyst substrate can boost performance

Catalysts are usually deposited on an underlying substrate. The role this substrate plays on the final reactivity of the catalyst is usually neglected by researchers, however for this new catalyst, the researchers spotted the substrate played an important part in its efficiency. The team found porous structure of their material allows to access more active catalyst sites formed at its interface with the substrate, so they developed a new electrochemical microscopy analysis method to measure how this interface could contribute to catalyze the reaction and produce the most effective catalyst. They hope their study of substrate effects on the catalytic activity of porous materials establishes a basis for the rational design of high-performance electrodes for the electrochemical energy devices and provides guidelines for future studies.

Tags:  Aalto university  carbon nanotube  CNRS  Emmanuel Flahut  energy  Graphene  Mohammad Tavakkoli 

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How to induce magnetism in graphene

Posted By Graphene Council, Wednesday, December 11, 2019
Graphene, a two-dimensional structure made of carbon, is a material with excellent mechanical, electronic and optical properties. However, it did not seem suitable for magnetic applications. Together with international partners, Empa researchers have now succeeded in synthesizing a unique nanographene predicted in the 1970s, which conclusively demonstrates that carbon in very specific forms has magnetic properties that could permit future spintronic applications. The results have just been published in the renowned journal Nature Nanotechnology.

Depending on the shape and orientation of their edges, graphene nanostructures (also known as nanographenes) can have very different properties – for example, they may exhibit conducting, semiconducting or insulating behavior. However, one property has so far been elusive: magnetism. Together with colleagues from the Technical University in Dresden, Aalto University in Finland, Max Planck Institute for Polymer Research in Mainz and University of Bern, Empa researchers have now succeeded in building a nanographene with magnetic properties that could be a decisive component for spin-based electronics functioning at room temperature.

Graphene consists only of carbon atoms, but magnetism is a property hardly associated with carbon. So how is it possible for carbon nanomaterials to exhibit magnetism? To understand this, we need to take a trip into the world of chemistry and atomic physics. The carbon atoms in graphene are arranged in a honeycomb structure. Each carbon atom has three neighbors, with which it forms alternating single or double bonds. In a single bond, one electron from each atom – a so-called valence electron – binds with its neighbor; while in a double bond, two electrons from each atom participate. This alternating single and double bond representation of organic compounds is known as the Kekulé structure, named after the German chemist August Kekule who first proposed this representation for one of the simplest organic compound, benzene (Figure 1). The rule here is that electron pairs inhabiting the same orbital must differ in their direction of rotation – the so-called spin – a consequence of the quantum mechanical Pauli’s exclusion principle.

"However, in certain structures made of hexagons, one can never draw alternating single and double bond patterns that satisfy the bonding requirements of every carbon atom. As a consequence, in such structures, one or more electrons are forced to remain unpaired and cannot form a bond," explains Shantanu Mishra, who is researching novel nanographenes in the Empa nanotech@surfaces laboratory headed by Roman Fasel. This phenomenon of involuntary unpairing of electrons is called "topological frustration". But what does this have to do with magnetism?

The answer lies in the "spins" of the electrons. The rotation of an electron around its own axis causes a tiny magnetic field, a magnetic moment. If, as usual, there are two electrons with opposite spins in an orbital of an atom, these magnetic fields cancel each other. If, however, an electron is alone in its orbital, the magnetic moment remains – and a measurable magnetic field results. This alone is fascinating. But in order to be able to use the spin of the electrons as circuit elements, one more step is needed. One answer could be a structure that looks like a bow tie under a scanning tunneling microscope. Two frustrated electrons in one molecule Back in the 1970s, the Czech chemist Erich Clar, a distinguished expert in the field of nanographene chemistry, predicted a bow tie-like structure known as "Clar's goblet" (Figure 1). It consists of two symmetrical halves and is constructed in such a way that one electron in each of the halves must remain topologically frustrated. However, since the two electrons are connected via the structure, they are antiferromagnetically coupled – that is, their spins necessarily orient in opposite directions. In its antiferromagnetic state, Clar's goblet could act as a "NOT" logic gate: if the direction of the spin at the input is reversed, the output spin must also be forced to rotate.

However, it is also possible to bring the structure into a ferromagnetic state, where both spins orient along the same direction. To do this, the structure must be excited with a certain energy, the so-called exchange coupling energy, so that one of the electrons reverses its spin. In order for the gate to remain stable in its antiferromagnetic state, however, it must not spontaneously switch to the ferromagnetic state. For this to be possible, the exchange coupling energy must be higher than the energy dissipation when the gate is operated at room temperature. This is a central prerequisite for ensuring that a future spintronic circuit based on nanographenes can function faultlessly at room temperature. From theory to reality So far, however, room-temperature stable magnetic carbon nanostructures have only been theoretical constructs. For the first time, the researchers have now succeeded in producing such a structure in practice, and showed that the theory does correspond to reality. "Realizing the structure is demanding, since Clar's goblet is highly reactive, and the synthesis is complex," explains Mishra. Starting from a precursor molecule, the researchers were able to realize Clar’s goblet in ultrahigh vacuum on a gold surface, and experimentally demonstrate that the molecule has exactly the predicted properties.

Importantly, they were able to show that the exchange coupling energy in Clar’s goblet is relatively high at 23 meV (Figure 2), implying that spin-based logic operations could therefore be stable at room temperature. "This is a small but important step toward spintronics," says Roman Fasel. Spintronics Spintronics – composed of the words "spin" and "electronics" is a field of research in nanotechnology. The aim is to create electronics in which information is not coded with the electrical charge of electrons, as is the case in conventional semiconductor circuits, but with their magnetic moment caused by the rotation of the electron ("spin"). The electron spin is a quantum mechanical property – a single electron can have not only a fixed state "spin up" or "spin down", but a quantum mechanical superposition of these two states. In the future, spintronics could therefore not only enable further miniaturization of electronic circuits, but could also make electrical switching elements with completely new, previously unknown properties a reality.

Tags:  Aalto University  August Kekule  Graphene  Journal Nature Nanotechnology  magnetism  Max Planck Institute for Polymer Research  nanographene  nanotechnology  Technical University  University of Bern 

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