Noriaki Itamura

Superlubricity of Fullerene Intercalated Graphite Composite

A novel superlubric system of fullerene intercalated graphite composite is reported. First, it is clarified that fullerene intercalated graphite films exhibit an ultralow average friction force and an excellent friction coefficient µ<0.001 smaller than µ<0.002 for MoS2 and µ\cong0.001 for graphite. Next, it is demonstrated that superlubricity can be controlled by changing the intercalant species. The C60 intercalated graphite film shows much less maximum static friction force than the C70 intercalated graphite film. Finally, we propose one of the simple guidelines on designing a practical superlubric system–reduction in the contact area between the intercalated fullerene and the graphite sheet to the pointlike contact. Our newly developed superlubric system will contribute to solving energy and environmental problems.

Simulation of Scan-Directional Dependence of Superlubricity of C60 Molecular Bearings and Graphite

The scan-directional dependence of the superlubricity of a C60 molecular bearing system (graphite/C60/graphite interface) is studied and compared with that of a graphite system (graphite/graphite/graphite interface) by molecular mechanics simulation. The mean lateral force reaches a maximum within a narrow region approximately in the [1010] direction. For other regions, has a nearly constant value of less than 1 pN. In particular, in the [1230] direction, reaches a minimum of nearly zero. It is clarified that reflects the following types of C60 motion: sliding above the carbon bond and a discrete slip to the neighboring AB-stacking position. The load dependence of also exhibits marked anisotropy. The orders of magnitude of the simulated friction coefficients are comparable to those obtained in our previous experiments.

Simulation of Atomic-Scale Ultralow Friction of Graphite/C60/Graphite Interface along [101̄0] Direction

The graphite/C60/graphite interface plays an important role in the atomic-scale ultralow friction of C60 intercalated graphite. In this study, the ultralow frictional feature along the [100] direction of the graphite/C60/graphite interface is numerically investigated and compared with that of the graphite/graphite/graphite interface. Simulated interlayer distances of about 1.3 nm are in good agreement with previous experimental results. The atomic-scale friction coefficient of the graphite/C60/graphite interface decreases to about 30% of that of the graphite/graphite/graphite interface. It is clarified that the three-dimensional degree of freedom of intercalated C60 motion is one of the origins of the ultralow friction of the graphite/C60/graphite interface along the [100] direction.

Simulated nanoscale peeling process of monolayer graphene sheet: effect of edge structure and lifting position

The nanoscale peeling of the graphene sheet on the graphite surface is numerically studied by molecular mechanics simulation. For center-lifting case, the successive partial peelings of the graphene around the lifting center appear as discrete jumps in the force curve, which induce the arched deformation of the graphene sheet. For edge-lifting case, marked atomic-scale friction of the graphene sheet during the nanoscale peeling process is found. During the surface contact, the graphene sheet takes the atomicscale sliding motion. The period of the peeling force curve during the surface contact decreases to the lattice period of the graphite. During the line contact, the graphene sheet also takes the stick-slip sliding motion. These findings indicate the possibility of not only the direct observation of the atomic-scale friction of the graphene sheet at the tip/surface interface but also the identification of the lattice orientation and the edge structure of the graphene sheet.

Analysis of Mechanism of Low Lateral Stiffness of Superlubric C60 Bearing System

The mechanism of the low lateral stiffness of the superlubric C60 bearing system along the [1010] direction, kC60 , is studied and compared with that of the graphite system by static molecular mechanics simulation. It is clarified that the C60 rotation and the elastic contact at the C60/graphite interface contribute to a decrease in kC60 . Under low and high loading conditions, the elastic contact and C60-rotation plays a major role for the low kC60 , respectively. Particularly effect of the C60-rotation on the decrease of kC60 becomes markedly enhanced as the loading force increases.

Atomic-scale ultralow friction – simulation of superlubricity of C60 molecular bearing

Simulation of superlubricity of C60 molecular bearing is performed based on molecular mechanics. Atomic-scale frictional feature along [10 0] direction of the graphite/C60/graphite interface is numerically investigated compared with that of the graphite/graphite/graphite interface. Simulated interlayer distances of about 1.3nm are in good agreement with previous experimental results[1-3]. Atomic-scale friction coefficient of graphite/C60/graphite interface decreases to about 30% of that of the graphite/graphite/graphite interface. It is clarified that three-dimensional degree of freedom of intercalated C60 motion is one of origins of ultralow friction of graphite/C60/graphite interface along [10 0] direction.

Nanomechanical Studies of Superlubricity

We briefly review the nanomechanical studies of ultralow friction in the following carbon hybrid systems: atomic force microscopy (AFM) tip on graphite surface, AFM tip on C60/graphite, graphite on graphite surface, graphite/C60/graphite, and C60 intercalated graphite. For the atomic and flake frictions, frictional force maps are compared between simulations and experiments, which can be explained by stick-slip motion of the tip apex atom and flake. For the graphite/C60/graphite system, superlubricity appears, where the maximum static frictional forces have finite values but denote that dynamical frictional forces are zero within the resolution of the experiment. Furthermore, for the C60 intercalated graphite system, greater superlubricity appears. It is clarified that fullerene intercalated graphite films exhibit ultralow average friction force, and excellent friction coefficients µ < 0.001. Our results propose one of the simple guidelines of designing a practical superlubric system - reduction of the contact area between intercalated C60 and graphite sheet to the point contact. Clearly, the C60 intercalated graphite system will contribute to solving energy and environmental problems in the future. graphite system. Several possible mechanisms to induce superlubricity of C60 intercalated graphite system are proposed and discussed. It can be expected that the superlubricity is induced by internal sliding between close-packed C60 monolayers and graphite layers. Our results propose a simple guideline for designing practical superlubric system - reduc- tion of the contact area between intercalated C60 and graphite sheet to the point contact. We anticipate our novel lubrication system to be a starting point for developing more practical superlubricants using intercalated graphite, which will contribute to solving the energy and environmental problems.

Atomic-Scale Peeling of Graphene

We report the atomic-scale peeling of a single-layer graphene on a graphite substrate, in which stick-slip sliding of the single-layer graphene occurs at the atomic scale while maintaining AB-stacking registry with the graphite substrate. The peeling force curve clearly exhibits a transition from surface-contact to line-contact between the graphene and graphite surfaces. The amplitude of the peeling force depends on the lattice orientation of the surface, which is affected by the sliding force at the interface between the graphene and graphite surfaces. This study of peeling at the atomic scale will clarify the relationship among peeling, friction, adhesion, and superlubricity.

Unique Near-Zero Friction Regime of C60 Molecular Bearings Along [12bar 30] Direction

We numerically analyzed unique near-zero friction regime of the C60 molecular bearings, graphite/C60/graphite interface, for the lateral scan along the [120] direction under the relatively low loading condition. Here the C60 molecule slides, facing its six-membered ring nearly parallel to both the upper and lower graphene sheets. The sinusoidal motion of the C60 molecule along the carbon bond is continuous and reversible during the forward and backward scans. As a result, the hysteresis loop of the lateral force curve nearly disappears, which leads to a mean frictional force of nearly zero, (FL) 0. The mechanism of this conservative motion is clarified by comparing the structural optimization of the C60 molecular bearing system with the direct calculation of the local minimum position located on the total potential energy surface Vtotal. The energy barrier between the neighboring minimum positions always exists, which prevents the C60 molecule from taking stick-slip motion.