Satoru Fujisawa

Spatially quantized friction with a lattice periodicity

Abstract Using a two-dimensional frictional force microscope (2D-FFM), we investigated the two-dimensional nature of spatially quantized friction with a lattice periodicity on an atomic scale. As a result, in addition to the well-known stick-slip behavior along the scanning direction, we found the appearance of friction with square-wave behavior which works across the scanning direction. To explain the observed two-dimensional friction with the lattice periodicity, we proposed a two-dimensional stick-slip model which predicts spatially quantized adhesions and jumps with the lattice periodicity. By calibrating displacements due to frictional forces using the lateral force curve, we obtained quantitative agreements between predicted and experimentally deduced displacements on an atomic scale. We also observed the fluctuation of the spatially quantized friction. Further, the two-dimensional stick-slip model with an effective adhesive region enabled us to explain individual sawtooth and square-wave behaviors in more detail. This model also enabled us to interpret a normal load dependence of the frictional force hysteresis as a normal load dependence of the effective adhesive region. Besides, by comparing the normal load dependence of the frictional force hysteresis on layered materials such as MoS2 with that on the three-dimensional NaF(100) crystal surface, we made clear that the atomic-scale facial friction works on layered materials, in contrast to the near single-atom friction on the three-dimensional one.

Simultaneous Observation of Millisecond Dynamics in Atomistic Structure, Force and Conductance on the Basis of Transmission Electron Microscopy

High-resolution transmission electron microscopy (HRTEM) has been developed to possess functions of atomic force microscopy and scanning tunneling microscopy. Dynamics of subnano Newton-scale force and conductance were simultaneously observed at intervals of 1/30–1/3840 s during HRTEM imaging of contact, deformation and fracture processes between nanometer-sized tips. The experimental basis of the atomic-scale mechanics of materials was developed on the basis of the present microscopy.

Metal-Insulator Transition in Stable One-Dimensional Arrangements of Single Gold Atoms

The atomic arrangement and conductance during the separation process of gold point contacts were simultaneously observed in situ by high-resolution transmission electron microscopy. One-dimensional arrangements of gold single atoms, i.e., atomic wires, appeared between two tips at the point contacts. They were stable when their length was increased up to 2.6 nm. The interatomic distance of the wires was 0.25–0.31 nm. It was found that metal-insulator transition occurs in the wires.

Lateral force curve for atomic force/lateral force microscope calibration

In this letter, a method to calibrate sensitivity for the three‐dimensional displacement of X, Y, and Z directions by using the novel force curve, namely lateral force curve, as well as vertical force curve in atomic force/lateral force microscope (AFM/LFM) measurement is described. Furthermore, from quantized friction measurement based on the two‐dimensional stick–slip model, it is experimentally confirmed that this lateral force curve calibration is reasonable.

Difference between the forces measured by an optical lever deflection and by an optical interferometer in an atomic force microscope

Using a simple model, we investigated the difference between the forces measured by an atomic force microscope (AFM) with an optical lever deflection method and that with an optical interferometer method. Then, using a mica with an atomically flat surface as a test sample, we confirmed experimentally the above difference, which says that the optical lever deflection method detects not only the surface corrugation but also the frictional force, while the optical interferometer method detects only the surface corrugation. Based on the above results, we proposed a method to measure the three‐dimensional force vector by using both the optical lever AFM/LFM and the optical interferometer AFM simultaneously. Furthermore, we mention that the measurement of three‐dimensional spatial distribution of the force vector will implement the computed tomography technique into AFM measurements, which yields a three‐dimensional spatial distribution of the force origin.

The effect of pile-up and contact area on hardness test by nanoindentation

We used atomic force microscopy (AFM) for the indentation test evaluating the indentation hardness of materials in the nanometer range. BK7, fused silica, and single-crystal silicon were used as test sample materials. The data analysis processes used to determine the contact area were important in evaluating the indentation hardness of the materials. The direct measurement of the size of the residual hardness impression was useful in evaluating the contact area even in the nanometer region. The results led us to conclude that AFM indentation using a sharp indenter is a powerful method for estimating the indentation hardness in the nanometer range.

Direct Observation of Electromigration and Induced Stress in Cu Nanowire

We report the direct observation of electromigration and induced stress in the Cu nanowire. During the electromigration, the decrease in pulling stress in the nanowire is observed with the modification of the nanowire. When the nanowire reaches the stable state, the single crystal lattice of Cu appears in the nanowire. This suggests that electromigration could be used as method of a annealing of the nanowire.

Fluctuation in Two-Dimensional Stick-Slip Phenomenon Observed with Two-Dimensional Frictional Force Microscope

We used an atomic force microscope combined with a lateral force microscope (AFM/LFM) as a two-dimensional frictional force microscope (2D-FFM) to investigate the two-dimensional behavior of the atomic-scale friction between the cleaved surface of MoS 2 and the Si 3 N 4 tip apex of the microcantilever based on the two-dimensional stick-slip model. As a result, for the scan direction along the row of the stick-points, we found the unstable state where the tip apex shows fluctuation between two adjacent rows of stick-points. On the other hand, near the row of the stick-points, we also found the stable state where the tip apex takes a straight walk on a row of the stick-points without fluctuation

Analysis of frictional-force image patterns of a graphite surface

We discuss the mechanism of image patterns of the frictional-force microscopy (FFM) of a graphite surface by using a three-dimensional model comprised of a tip connected to a cantilever and a substrate surface. A simulated FFM image is in good agreement with an experimental one. A stable domain of the tip atom position can be defined in an analytic way. In the frictional-force regime, more than one quasistable tip atom position are mapped into a single cantilever basal position. Part of the boundary of the two-dimensional domain of the cantilever basal position appears as a fringe between the bright and the dark areas along the scan direction of the FFM image. General features of FFM images can be completely understood by this analysis.

Two-dimensionally quantized friction observed with two-dimensional frictional force microscope

Using a two-dimensional frictional force microscope, we studied the two-dimensional nature of the atomic scale friction between a Si3N4 sharp tip and a cleaved graphite surface, which is composed of only C atoms and is a good conductor. As a result, we observed the two-dimensionally quantized friction with the lattice periodicity of the graphite surface, similarly to mica, MoS2 and NaF surfaces. Thus quantized friction occurs at material surfaces which are composed of not only some elements but also of a single element, and the quantized friction does not depend on the conductivity of the surface.