Researchers from North Carolina State University and China’s Suzhou Institute of Nano-Science and Nano-Biotics have developed an inexpensive technique called “microcombing” to align carbon nanotubes (CNTs), which can be used to create large, pure CNT films that are stronger than any previous such films. The technique also improves the electrical conductivity that makes these films attractive for use in electronic and aerospace applications (, "Strong and Conductive Dry Carbon Nanotube Films by Microcombing"). 
“It’s a simple process and can create a lightweight CNT film, or ‘bucky paper,’ that is a meter wide and twice as strong as previous such films – it’s even stronger than CNT fibers,” says Yuntian Zhu, Distinguished Professor of Materials Science and Engineering at NC State and corresponding author of a paper describing the work. The researchers begin by growing the CNTs on a conventional substrate in a closely packed array. The CNTs are tangled together, so when researchers pull on one end of the array the CNTs form a continuous ribbon that is only nanometers thick. This ribbon is attached to a spool, which begins winding the ribbon up. As the spool pulls, the CNT ribbon is dragged between two surgical blades. While the blades appear straight to the naked eye, they actually have micrometer-scale fissures on their cutting edge. These fissures create a kind of “microcomb” that pulls the CNTs into alignment – just as a regular comb sorts through tangled hair. When the ribbon of aligned CNTs is being wound onto the spool, the researchers apply an alcohol solution. This pulls the CNTs closer together, strengthening the bonds between CNTs. 
The micrometer-scale fissures on the edge of a surgical blade act as a "microcomb" to align carbon nanotubes. (Image: Yuntian Zhu) The CNT ribbon wraps around itself as it winds around the spool, creating a layered film of pure CNTs. Researchers can control the thickness of the film by controlling the number of layers. The CNT films made using the microcombing technique had more than twice the tensile strength of the uncombed CNT films – greater than 3 gigapascals for the microcombed material, versus less than 1.5 gigapascals for the uncombed material. The microcombed CNT film also had 80 percent higher electrical conductivity than the uncombed film. “This is a significant advance, but we want to find ways to make CNT alignment even straighter,” Zhu says. “It’s still not perfect. “In addition, the technique would theoretically be easy to scale up for large-scale production. We’d like to find an industry partner to help us scale this up and create a material for the marketplace.”
read more "'Microcombing' creates stronger, more conductive carbon nanotube films"
“It’s a simple process and can create a lightweight CNT film, or ‘bucky paper,’ that is a meter wide and twice as strong as previous such films – it’s even stronger than CNT fibers,” says Yuntian Zhu, Distinguished Professor of Materials Science and Engineering at NC State and corresponding author of a paper describing the work. The researchers begin by growing the CNTs on a conventional substrate in a closely packed array. The CNTs are tangled together, so when researchers pull on one end of the array the CNTs form a continuous ribbon that is only nanometers thick. This ribbon is attached to a spool, which begins winding the ribbon up. As the spool pulls, the CNT ribbon is dragged between two surgical blades. While the blades appear straight to the naked eye, they actually have micrometer-scale fissures on their cutting edge. These fissures create a kind of “microcomb” that pulls the CNTs into alignment – just as a regular comb sorts through tangled hair. When the ribbon of aligned CNTs is being wound onto the spool, the researchers apply an alcohol solution. This pulls the CNTs closer together, strengthening the bonds between CNTs. The micrometer-scale fissures on the edge of a surgical blade act as a "microcomb" to align carbon nanotubes. (Image: Yuntian Zhu) The CNT ribbon wraps around itself as it winds around the spool, creating a layered film of pure CNTs. Researchers can control the thickness of the film by controlling the number of layers. The CNT films made using the microcombing technique had more than twice the tensile strength of the uncombed CNT films – greater than 3 gigapascals for the microcombed material, versus less than 1.5 gigapascals for the uncombed material. The microcombed CNT film also had 80 percent higher electrical conductivity than the uncombed film. “This is a significant advance, but we want to find ways to make CNT alignment even straighter,” Zhu says. “It’s still not perfect. “In addition, the technique would theoretically be easy to scale up for large-scale production. We’d like to find an industry partner to help us scale this up and create a material for the marketplace.”
The image depicts a neuronal network growing on a novel nanotextured gold electrode coating. The topographical cues presented by the coating preferentially favor spreading of neurons as opposed to scar tissue. This feature has the potential to enhance the performance of neural interfaces. (Image: Ryan Chen/LLNL) Neural interfaces (e.g., implantable electrodes or multiple-electrode arrays) have emerged as transformative tools to monitor and modify neural electrophysiology, both for fundamental studies of the nervous system, and to diagnose and treat neurological disorders. These interfaces require low electrical impedance to reduce background noise and close electrode-neuron coupling for enhanced recording fidelity. Designing neural interfaces that maintain close physical coupling of neurons to an electrode surface remains a major challenge for both implantable and in vitro neural recording electrode arrays. An important obstacle in maintaining robust neuron-electrode coupling is the encapsulation of the electrode by scar tissue. Typically, low-impedance nanostructured electrode coatings rely on chemical cues from pharmaceuticals or surface-immobilized peptides to suppress glial scar tissue formation over the electrode surface, which is an obstacle to reliable neuron-electrode coupling. However, the team found that nanoporous gold, produced by an alloy corrosion process, is a promising candidate to reduce scar tissue formation on the electrode surface solely through topography by taking advantage of its tunable length scale. “Our results show that nanoporous gold topography, not surface chemistry, reduces astrocyte surface coverage,” said Monika Biener, one of the LLNL authors of the paper. Nanoporous gold has attracted significant interest for its use in electrochemical sensors, catalytic platforms, fundamental structure-property studies at the nanoscale and tunable drug release. It also features high effective surface area, tunable pore size, well-defined conjugate chemistry, high electrical conductivity and compatibility with traditional fabrication techniques. “We found that nanoporous gold reduces scar coverage but also maintains high neuronal coverage in an in vitro neuron-glia co-culture model,” said Juergen Biener, the other LLNL author of the paper. “More broadly, the study demonstrates a novel surface for supporting neuronal cultures without the use of culture medium supplements to reduce scar overgrowth.”
Illustration of electron spin in a graphene lattice. (Image: Bart van Wees) Electron spin and quantum logic Before looking at spintronics in graphene, it is worth noting that spintronics is already established in one critical area of digital electronics, namely data storage. Spin can be thought of as the rotation of the electron around its own axis. It is a form of intrinsic angular momentum, and can be detected as a magnetic field with one of two orientations: up and down. Combine these magnetic orientations with the on/off current states in binary logic, and we have a system of four states, with the two magnetic orientations forming a quantum bit, or qubit. In computing technology terms, four states rather than two provides for higher data transfer speeds, increased processing power and memory density, and added storage capacity. Electron spin provides an additional degree of freedom to store and manipulate information. The read heads of modern magnetic hard drives exploit the spin-related effects known as Giant Magnetoresistance (GMR) and Tunnel Magnetoresistance (TMR). In GMR devices, two or more layers of ferromagnetic materials are separated by a spacer. When the magnetisation vectors of the magnetic layers are aligned, the electrical resistance is lower than when the vectors are in the opposite sense. A device based on such a configuration is known as a spin valve. In TMR, electron transport is achieved by quantum mechanical tunnelling of the particles through an insulator separating ferromagnetic layers. In both cases, the result is a magnetic field sensor that may be used to read data magnetically encoded on hard drive platters. And not only hard drives. Two types of computer memory – Magnetoresistive Random Access Memory and racetrack memory – also exploit electron spin. Spin transport in graphene Graphene, an atomic monolayer of graphitic carbon, is a promising material for spintronics applications owing to its capacity for room-temperature spin transport over relatively long diffusion lengths of several micrometres. Graphene also has high electron mobility, and a tuneable charge carrier concentration. Interest in room-temperature spin transport in graphene goes back to 2007, with experiments performed by the research group of Groningen University physicist and leading Graphene Flagship scientist Bart van Wees. A discussion of that first practical demonstration of spin transport, together with a detailed technical overview of graphene spintronics in theory and practice, can be found in an article published last year in the academic journal ("Graphene spintronics"). One of the review authors is Regensburg-based flagship scientist Jaroslav Fabian. The van Wees group experiments and subsequent studies showed a relatively low spin injection efficiency of around 10%, which was attributed to either a conductance mismatch between the ferromagnetic metals and graphene, or other contact-related effects. Considerably higher efficiencies were achieved by using magnesium oxide thin films as the tunnel barrier. Further approaches were also employed, including pinhole contacts across an insulating barrier, transparent contacts, in which the ferromagnetic electrodes are in direct contact with the graphene layer, and the use of non-magnetic metals such as copper. In the case of tunnelling across an insulating barrier, the largest magnetoresistance measured was 130 ohms, corresponding to a spin injection efficiency of over 60%. Moving from small-scale studies to investigations of spin transport in large-area graphene is a key step toward enabling graphene spintronics at the integrated-circuit wafer scale. The focus here has been on spin transport in suspended graphene layers, and graphene deposited on hexagonal boron nitride (hBN) substrates. As the technology progresses, longer spin lengths and lifetimes are observed, and a practical example of such a graphene-hBN heterostructure will be discussed in a follow-up article. Making graphene magnetic Creating magnetic order in graphene, which in its pristine state is a strongly diamagnetic material, is a major challenge. Nonetheless, inducing magnetic moments in graphene is of vital importance if the material is to be used in spintronics. The hope is to have a tuneable magnetism through doping or functionalisation of graphene. This could be achieved through defects in the material's hexagonal crystal structure, or the influence of adsorbed atoms on its surface. Hydrogenated graphene is a benchmark case for graphene magnetism, with hydrogen atoms chemically absorbing onto graphene in a reversible manner. This creates an imbalance in the crystal lattice, inducing a magnetic moment. Another interesting adatom is fluorine, which bonds to carbon, transforming graphene into a wide-gap insulator. As with hydrogen, fluorine can be reversibly chemisorbed on graphene. "Graphene is a promising material for spintronics, given that its spin properties can not only be tailored, but indeed defined by what adatoms and other 2D materials you combine with it," says Fabian. "Once the right materials are identified – and this is what we are investigating in the flagship – a path opens towards specific technological applications." A missing carbon atom, or vacancy in graphene's structure, creates a spin-polarised electron density by stripping four electrons from the bands, three of which form 'dangling bond' states. Two of these dangling bonds contribute magnetic moments, but direct evidence of the predicted -magnetism is missing. Extending spin lifetime Maximising spin lifetime is critical when it comes to applications of graphene spintronics. Theory predicts lifetimes of around a microsecond for pristine graphene, whereas experiment shows values ranging from tens of picoseconds to a few nanoseconds. Only with nanosecond lifetimes and longer will spin transport in graphene prove useful in real-world applications. The more than two orders of magnitude discrepancy is a serious concern, and it suggests that the source of spin relaxation is of extrinsic origin, such as impurities, defects or ripples in the graphene studied. Spin lifetimes of a few nanoseconds have been observed experimentally for graphene spin valves on silicon dioxide substrates with tunnelling contacts, but with pinhole contacts the measured lifetimes are only a fraction of a nanosecond. Contact-induced spin relaxation is a significant factor. This can be minimised by improving the quality of the contacts, and making the distance between ferromagnetic electrodes much larger than the bulk graphene spin-relaxation length. Despite numerous theoretical studies, the origin of spin relaxation in graphene is little understood. Two mechanisms have been put forward to explain experimental trends. Both have their origins in metal and semiconductor spintronics, and they each rely on spin-orbit coupling and momentum scattering. Spin-orbit coupling is the interaction of an electron's spin with its motion, which leads to shifts in the particle's atomic energy levels as a result of the interaction between the spin and the magnetic field generated by the electron's orbit around the atomic nucleus. The problem is that neither of the proposed spin relaxation mechanisms work. Both predict microsecond lifetimes, yet experiments show a few nanoseconds at best. The only mechanism that agrees with experiment for both single and bilayer graphene is based on resonant scattering by local magnetic moments. This model was proposed by Fabian's research group in Regensburg. What recent studies indicate is that electron mobility is not the limiting factor for spin lifetime, and scattering between charged particles and impurities is not primarily responsible for spin relaxation in graphene. That said, determining the primary source of spin relaxation remains an important challenge for graphene researchers. Identifying it should help raise spin lifetime in graphene towards the theoretical limit, which will have important implications for both basic science and technological applications. Future directions In the conclusion to their Nature Nanotechnology review, Fabian and his colleagues consider graphene in spin-transfer torque-based logic devices that use spins and magnets for information processing. Spin-logic devices are now part of the International Technology Roadmap for Semiconductors, with a view to their inclusion in future computers. Examples of spin-logic devices include rewritable microchips, transistors, logic gates, magnetic sensors and semiconductor nanoparticles for quantum computing. These and other opportunities for graphene-based spintronics are discussed in the recently published "Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems". The roadmap was developed within the framework of Europe's Graphene Flagship – an international academic/industrial consortium, part-funded by the European Commission, devoted to the development of graphene and other layered materials. Spintronics may be a relatively young field of research and development, but in recent years we have seen significant progress toward long spin lifetimes and diffusion lengths in graphene and related materials. Graphene Flagship researchers are at the heart of this worldwide effort.