Scientists from the NUST MISIS Laboratory of Inorganic Nanomaterials together with their international colleagues have proved it possible to change the structural and conductive properties of nanotubes by stretching them. This can potentially expand nanotubes’ application into electronics and high-precision sensors such as microprocessors and high-precision detectors. The research article has been published in Ultramicroscopy.

Carbon nanotubes can be represented as a sheet of graphene rolled in a special way. There are different ways of «folding» it, which leads to the graphene edges interconnecting at different angles, forming either armchair, zigzag or chiral nanotubes (Pic.1).

Nanotubes are considered to be promising materials for use in electronics and sensors because they have high electrical conductivity, which would work well in things like microprocessors and high-precision detectors. However, when producing carbon nanotubes it is hard to control their conductivity. Nanotubes with metallic and semiconducting properties can grow into a single array while microprocessor-based electronics require semiconducting nanotubes that have the same characteristics.

Scientists from the NUST MISIS Laboratory of Inorganic Nanomaterials jointly with a research team from Japan, China and Australia, led by Professor Dmitri Golberg, have proposed a method that allows for the modification of the structure of ready-made nanotubes and thus changes their conductive properties.

«The basis of the nanotube – a folded layer of graphene – is a grid of regular hexagons, the vertices of which are carbon atoms. If one of the carbon bonds in the nanotube is rotated by 90° degrees, a pentagon and a heptagon are formed at this [junction] instead of a hexagon, and a so-called Stone-Wales defect is obtained in this case. Such a defect can occur in the structure under certain conditions. Back in the late 90s, it was predicted that the migration of this defect along the walls of a highly heated nanotube with the application of mechanical stress could lead to a change in its structure – a sequential change in the chirality of the nanotube, which leads to a change in its electronic properties. No experimental evidence for this hypothesis has previously been obtained, but our research paper has presented convincing proof of it», said Associate Professor Pavel Sorokin, Doctor of Physical & Mathematical Sciences and head of the «Theoretical Materials Science of Nanostructures» infrastructure project at the NUST MISIS Laboratory of Inorganic Nanomaterials.

Scientists from the NUST MISIS Laboratory of Inorganic Nanomaterials have conducted simulations of the experiment at the atomic level. At first, the nanotubes were lengthened to form the first structural defect consisting of two pentagons and two heptagons (a Stone-Wales defect, pic.2a), where the prolonged lengthening of the tube began to «spread» to the sides, rearranging other carbon bonds (pic.2b). It was at this stage that the structure of the nanotubes changed. With further stretching, more and more Stone-Wales defects began to form, eventually leading to a change in the nanotubes’ conductivity (Pic. 2).

«We were responsible for the theoretical modeling of the process on a supercomputer in the NUST MISIS Laboratory for Modeling and Development of New Materials for the experimental part of the work. We are glad that the simulation results [support] the experimental data», added Dmitry Kvashnin, co-author of the research work, Candidate of Physical & Mathematical Sciences and a researcher at the NUST MISIS Laboratory of Inorganic Nanomaterials.

The proposed technology is capable of helping in the transformation of «metallic» nanotubes’ structure for their further application in semiconductor electronics and sensors such as microprocessors and ultrasensitive detectors.

Graphene Flagship researchers have shown in a paper published in Science Advanceshow heterostructures built from graphene and topological insulators have strong, proximity induced spin-orbit coupling which can form the basis of novel information processing technologies.

Scanning Electron Microscope micrograph of a fabricated device showing the graphene topological insulator heterostructure channel. Credit: Dmitrii Khokhriakov, Chalmers University of Technology

Spin-orbit coupling is at the heart of spintronics. Graphene’s spin-orbit coupling and high electron mobility make it appealing for long spin coherence length at room temperature. Graphene Flagship researchers from Chalmers University of Technology (Sweden), Catalan Institute of Nanoscience and Nanotechnology – ICN2 (Spain), Universitat Autònoma de Barcelona (Spain) and ICREA Institució Catalana de Recerca i Estudis Avançats (Spain) showed a strong tunability and suppression of the spin signal and spin lifetime in heterostructures formed by graphene and topological insulators. This can lead to new graphene spintronic applications, ranging from novel circuits to new non-volatile memories and information processing technologies.

“The advantage of using heterostructures built from two Dirac materials is that, graphene in proximity with topological insulators still supports spin transport, and concurrently acquires a strong spin-orbit coupling,” said Associate Professor Saroj Prasad Dash, from Chalmers University of Technology.

“We do not just want to transport spin we want to manipulate it,” said Professor Stephan Roche from ICN2 and deputy leader of the Graphene Flagship’s spintronics Work-Package, “the use of topological insulators is a new dimension for spintronics, they have a surface state similar to graphene and can combine to create new hybrid states and new spin features. By combining graphene in this way we can use the tuneable density of states to switch on/off – to conduct or not conduct spin. This opens an active spin device playground.”

The Graphene Flagship, from its very beginning, saw the potential of spintronics devices made from graphene and related materials. This paper shows how combining graphene with other materials to make heterostructures opens new possibilities and potential applications.

“This paper combines experiment and theory and this collaboration is one of the strengths of the Spintronics Work-Package within the Graphene Flagship,” said Roche.

“Topological insulators belong to a class of material that generate strong spin currents, of direct relevance for spintronic applications such as spin-orbit torque memories. As reported by this article, the further combination of topological insulators with two-dimensional materials like graphene is ideal for enabling the propagation of spin information with extremely low power over long distances, as well as for exploiting complementary functionalities, key to further design and fabricate spin-logic architectures,” said Kevin Garello from IMEC, Belgium who is leader of the Graphene Flagships Spintronics Work-Package.

Professor Andrea C. Ferrari, Science and Technology Officer of the Graphene Flagship, and Chair of its Management Panel added “This paper brings us closer to building useful spintronic devices. The innovation and technology roadmap of the graphene Flagship recognises the potential of graphene and related materials in this area. This work yet again places the Flagship at the forefront of this field, initiated with pioneering contributions of European researchers.”

Organic semiconductor materials have the potential to be used in innovative applications such as transparent and flexible devices, and their low cost makes their potential use particularly attractive. The properties of organic semiconductor materials can be tuned by controlling their structure at the molecular level through parts of the structure known as electron-accepting units. A group of researchers centered at Osaka University has specifically tailored an electron-accepting unit that was then successfully used in an organic semiconductor applied in solar cell device that showed high photovoltaic performance. Their findings were published in NPG Asia Materials.

Chemical structures and photovoltaic characteristics. Credit: Osaka University

“Electron-accepting units are important elements of organic semiconductors,” study corresponding author Yoshio Aso says. “Through the controlled addition of electronegative fluorine groups to a widely used electron-accepting material, we were able to show precise control of the energy levels within the resulting semiconductor. This ability to tune the band gap translates to selectivity over the injection and transport of holes and/or electrons within the material, which is important in potential applications.”

The fluorinated electron-acceptor unit was used to prepare a thin film solar cell that was compared with a cell based on a non-fluorinated analogue. The researchers found that the fluorinated material showed enhanced power conversion efficiency, up to 3.12%. The morphology of the fluorinated film was also found to be good, which supported the efficient charge generation and transport that is necessary for successful application.

“The more we are able to fine tune organic semiconductor behavior on the molecular level, the more possibilities there will be for demonstrating their macroscopic applications,” co-author Yutaka Ie says. “It is our hope that the band gap control and high photovoltaic performance we have demonstrated will lead to our material being applied in devices such as organic light-emitted diodes, field-effect transistors, and thin film solar cells.”

The straightforward demonstration of the link between high electronegativity, greater electron-accepting tendency, and enhanced semiconductor performance, highlights both the potential and versatility of organic semiconductors. Further elegant solutions such as this one could substantially broaden the range of ƒÎ-conjugated materials, and reinforce the case for organic electronics.