Takazumi Kawai

Nanoporous Carbon Tubes from Fullerene Crystals as the π‐Electron Carbon Source

Here we report the thermal conversion of one-dimensional (1D) fullerene (C60) single-crystal nanorods and nanotubes to nanoporous carbon materials with retention of the initial 1D morphology. The 1D C60 crystals are heated directly at very high temperature (up to 2000 °C) in vacuum, yielding a new family of nanoporous carbons having π-electron conjugation within the sp(2)-carbon robust frameworks. These new nanoporous carbon materials show excellent electrochemical capacitance and superior sensing properties for aromatic compounds compared to commercial activated carbons.

Phase Control of Graphene Nanoribbon by Carrier Doping: Appearance of Noncollinear Magnetism

The zigzag graphene nanoribbon (ZGNR) has an antiferromagnetic property, that is, the relative spin angle theta between the two edges is 180 degrees . By using noncollinear first-principles calculations, we find that the magnetic phase of the ZGNR can be controlled by injecting either electrons or holes: as the carrier density increases, theta continuously decreases from 180 to 0 degrees , which indicates that the net magnetization is possible. Either FET doping or chemical doping is found to be possible.

Electronic Structure of Graphene with a Topological Line Defect

Using the first-principle total-energy procedure within the framework of density functional theory, we study the electronic structure of graphene with a topological line defect that is comprised of a pair of fused pentagons and an octagon. We find that introduction of the topological line defect effectively terminates the sp 2 network of graphene leading the flat dispersion band around the Γ point. A detailed investigation of the wave function of this flat-band state reveals its edge-state nature, which is particular to graphene nanoribbons with zigzag edges. Our tight-binding molecular dynamics simulation also reveals that the topological line defect is spontaneously formed from defects in the graphene sheet when held at a temperature of 1000 K.

Mechanically-induced transport switching effect in graphene-based nanojunctions

We report a theoretical study suggesting a novel type of electronic switching effect, driven by the geometrical reconstruction of nanoscale graphene-based junctions. We considered junction structures which have alternative metastable configurations transformed by rotations of local carbon dimers. The use of external mechanical strain allows a control of the energy barrier heights of the potential profiles and also changes the reaction character from endothermic to exothermic or vice-versa. The reshaping of the atomic details of the junction encode binary electronic ON or OFF states, with ON/OFF transmission ratio that can reach up to 10 4 -10 5 . Our results suggest the possibility to design modern logical switching devices or mechanophore sensors, monitored by mechanical strain and structural rearrangements.

Fusion of ultra thin carbon nanotubes: tight-binding molecular dynamics simulations

Abstract We investigated the reactivity of ultra thin carbon nanotubes (UTCNTs) with diameter nm , which were synthesized in experiment. Tight-binding molecular dynamics (TBMD) simulations showed that isolated UTCNTs were thermally stable up to about 3000 K . By performing first principles calculations and TBMD simulations, we found that all pairs of UTCNTs coalesced into a single tube, without necessitating the presence of atomic defects.

Li atom adsorption on graphene with various defects for large-capacity Li ion batteries: First-principles calculations

We investigated the fundamental properties of the interaction between a Li atom and a graphene surface with various defect structures by first-principles electronic state calculations to improve the capacity and charge rate of a graphitic anode for Li ion battery applications. The adsorption energy tends to decrease as the number of deficit carbon atoms at a neighboring defect increases even for adsorption at a hexagonal ring (HR) away from defects, although the interaction between a Li adatom and an HR is similar independent of the defect structure. The reason for the change in adsorption energy is the electronic charge transfer from the Li 2s-like state to the defect-induced state near the Fermi level. We also found that a Li atom diffuses through a V6 defect without a diffusion barrier practically.

Formation of Multi-Walled Nanotubes from Diamond Nanowires

Based on classical molecular dynamics simulation, we show the possibility of the formation of multi-walled carbon nanotubes from diamond nanowires at elevated temperatures. Since the outermost shell of a diamond nanowire can be regarded as a corrugated graphene sheet bound via elongated intershell bonds, thermal annealing of the nanowire causes successive peeling of the outermost shell and results in a structural transformation from an sp3 structure into an sp2 rich nanostructure. The resultant structures could be classified as multi-walled carbon nanotubes with some stacking faults.

Orientation dependence of magnetic moment on double-walled nanotubes with topological line defects

The authors propose a double-walled structure of carbon nanotubes with topological line defects as a structure that exhibits magnetic moment depending on the mutual orientation of constituent nanotubes. In a short distance between the defects on each nanotube, the hybridization between the localized states on each nanotube results in a low-spin state as the ground state. In other orientations, the polarized electron spins localized around the defect on each tube are in parallel direction to each other resulting in a high-spin state. Local density of states around the Fermi level indicates that the double-walled nanotube exhibits spin-dependent transport properties.

Ab initio calculations of polycyclic aromatic hydrocarbons adsorbed on graphite edge for molecular-scale surface coatings of lithium-ion battery anodes

We have investigated the atomic configurations and electronic structures of graphite edges adsorbed by polycyclic aromatic hydrocarbons, aiming for application to lithium-ion-battery anodes, where thin and robust coating layers are required for fast charge/discharge processes and long lifetime. In particular, we examined the adsorption of a perylene molecule on an edge of graphite. We considered that the edge of perylene connects two layers of graphite with σ-bonds. We found that the optimized atomic structures are stable and the planar surface of perylene is parallel to the insertion direction of lithium ions similar to the original graphite edges. Therefore, the coating does not disturb the diffusion of lithium ions during charge/discharge processes. Furthermore, there is no hybridization between the graphite edge and the perylene molecule near the Fermi level, even though they have chemical bonds between them. Thus, the electronic states of the chemisorbed perylene molecule are insensitive to the change in the electric potential of the graphite. Consequently, we expect them to suppress the electrochemical decomposition of electrolytes and solvents on the anode surface. Hence, polycyclic aromatic hydrocarbons are one of the best candidates for the coating materials of graphite anodes for realizing a higher charge/discharge rate and longer lifetime.

First Principles Calculations for Diffusion Barriers of Lithium Intercalation into Graphite with Various Edge Terminations

Lithium atom intercalation into a graphite layer from the step edge to the interlayer region with various edge terminations of graphite is investigated by first-principles electronic structure calculations. The interactions between the step edge and a Li atom depend on the functional group. There is little interaction of the edge with hydrogen termination, while a large interaction traps diffusing Li atoms for termination by oxidative functional groups. The hydroxylic termination would reduce the initial diffusion barrier for the intercalation. Therefore, the controlled edge termination would successfully enhance the charge and discharge properties of Li ion batteries; on the other hand, the progress of oxidative termination would decrease capacity and charge and discharge rates. The diffusion barrier heights of Li is comparable to or larger than those of other rate-limiting factors during charge and discharge, such as desolvation of Li from electrolyte.