Keiko Onishi

Reconstruction of atomic force microscopy image by using nanofabricated tip characterizer toward the actual sample surface topography

The atomic force microscopy (AFM) image is a dilation of the sample surface topography due to the finite-sized AFM tip. We accurately estimated the tip apex shape with a nanofabricated Si tip characterizer and applied the estimated tip shape function to a dilation-erosion algorithm for image reconstruction. The reconstructed images from the original AFM images attained with different AFM tips show consistent surface features and closely match the high-resolution field-emission scanning electron microscope image. The results demonstrate the reliability of our method and suggest the importance of AFM image reconstruction for a variety of technologies requiring new strategies of measuring, interpreting, manipulating, and positioning in the submicrometer and nanometer range.

Unique synthesis of few-layer graphene films on carbon-doped Pt(83)Rh(17) surfaces.

We report a unique synthesis of single- and few-layer graphene films on carbon-doped Pt(83)Rh(17) surfaces by surface segregation and precipitation. The ultrathin graphene films were characterized by atomic force microscopy, Auger electron spectroscopy, and micro-Raman spectroscopy measurements, providing evidence of graphene film thickness and structural quality. The G and 2D band intensity images from micro-Raman spectroscopy measurements confirm that the graphene films with different coverage have very limited defects. Additionally, the 2D band peak can be well-fitted by a single Lozentian peak, indicating that graphene films are characteristic of single layer graphene. Graphene film thickness can be determined by analysis of Auger spectra, indicating that graphene films after 850 degrees C annealing mainly consist of monolayer graphene. By precise adjustment of annealing temperature, graphene film thickness and area size can be controlled and uniform large-area single-layer and double-layer graphene can be achieved.

Discovery of Carbon Nanowires Formed on a Carbon-Doped Ni(111) Substrate by a Bulk-to-Surface Precipitation Process

We have discovered a synthesis method for carbon nanowires on a flat single-crystal graphite (0001) surface on a carbon-doped Ni(111) substrate using only heat treatment in ultrahigh vacuum. It should be noted that this new carbon nanowire synthesis method requires no external carbon-containing source. The growth mechanism utilizes a bulk-to-surface precipitation process of internal carbon atoms that were doped in a pure Ni(111) substrate in advance. Nanometer-scale morphology and chemistry of the carbon nanowires have been clarified by low-energy electron diffraction combined with Auger electron spectroscopy (LEED/AES), scanning tunneling microscopy (STM) and field-emission scanning Auger microcopy (FE-SAM).

An atomic resolution scanning tunneling microscope that applies external tensile stress and strain in an ultrahigh vacuum

We have developed an ultrahigh vacuum scanning tunneling microscope with an in situ external stress application capability in order to determine the effects of stress and strain on surface atomistic structures. It is necessary to understand these effects because controlling them will be a key technology that will very likely be used in future nanometer-scale fabrication processes. We used our microscope to demonstrate atomic resolution imaging under external tensile stress and strain on the surfaces of wafers of Si(111) and Si(001). We also successfully observed domain redistribution induced by applying uniaxial stress at an elevated temperature on the surface of a wafer of vicinal Si(100). We confirmed that domains for which an applied tensile stress is directed along the dimer bond become less stable and shrink. This suggests that it may be feasible to fabricate single domain surfaces in a process that controls surface stress and strain.

Silver Nanostructures Formation on Si(111)-(7×7) Surfaces by the Tip of a Scanning Tunneling Microscope

We have succeeded in the reproducible fabrication of nanometer-scale silver dots, wires, characters and figures on Si(111) in ultrahigh vacuum by scanning tunneling microcopy (STM). The silver nanodots were deposited by the application of voltage pulses between the silver-coated tip and the surface. The dominant mechanism of nanodot formation can be attributed to the spontaneous formation of a nanometer-scale point contact between the tip and the surface due to field-enhanced diffusion of silver atoms to the tip apex. Scanning tunneling spectroscopy over the silver nanodots on Si(111) showed metallic property for the nanodots and carrier depletion in the neighboring near-surface regions.

Light emission induced by tunnelling electrons from a p-type GaAs(110) surface observed at near-field by a conductive optical fibre probe

An ultrahigh-vacuum low-temperature scanning tunnelling microscope with a near-field optical detection system using conductive and optically transparent probes was used to study tunnelling-electron-induced photon emission from a cleaved p-type GaAs(110) surface. Photons generated in a nanometre-scale area just under the probe were collected within the optical near-field region into the core of the optical fibre tip. We observed a strong photon emission at positive sample biases by injecting electrons into the surface, where radiative recombination of electron–hole pairs is a reasonable explanation for the STM-induced photon emission. A drop in photon intensity at the Zn dopant regions was observed, which can be explained by the local band potential change around the Zn acceptor atoms located at sub-surface layers.

Standardization of nanomaterials characterization by scanning probe microscopy for societal acceptance

Novel nanomaterials are expected to play key roles for the promotion of innovations in the various industrial products. In order to make such novel nanomaterials to be socially acceptable and widely used, it is very important and necessary to establish the reliable nano-characterization methodology for the industrial nanomaterials under the authorized international scheme for standardization. Among the nano-characterization methods, scanning probe microscopy (SPM) is the most versatile both in the measurement functions and the operational environments. Whereas there are various nanomaterials of industrial application, fullerene nanomaterials (FNM) have attracted much attention due to their unique physical properties. Here we show the importance of the quantitative analysis and standardization of SPM using FNM as a typical example.

The Effect of Magnesium Ions on Chromosome Structure as Observed by Helium Ion Microscopy

One of the few conclusions known about chromosome structure is that Mg2+ is required for the organization of chromosomes. Scanning electron microscopy is a powerful tool for studying chromosome morphology, but being nonconductive, chromosomes require metal/carbon coating that may conceal information about the detailed surface structure of the sample. Helium ion microscopy (HIM), which has recently been developed, does not require sample coating due to its charge compensation system. Here we investigated the structure of isolated human chromosomes under different Mg2+ concentrations by HIM. High-contrast and resolution images from uncoated samples obtained by HIM enabled investigation on the effects of Mg2+ on chromosome structure. Chromatin fiber information was obtained more clearly with uncoated than coated chromosomes. Our results suggest that both overall features and detailed structure of chromatin are significantly affected by different Mg2+ concentrations. Chromosomes were more condensed and a globular structure of chromatin with 30 nm diameter was visualized with 5 mM Mg2+ treatment, while 0 mM Mg2+ resulted in a less compact and more fibrous structure 11 nm in diameter. We conclude that HIM is a powerful tool for investigating chromosomes and other biological samples without requiring metal/carbon coating.

In situ voltage-application system for active voltage contrast imaging in helium ion microscope

In this study, the authors present a new method for electrical potential observation in which helium ion microscope (HIM) observations can be performed while applying a selected voltage (up to approximately +5 V) to a sample. The in situ voltage-application system is operated in a high vacuum HIM chamber and consists of a transfer rod, tilt joint, x-, y-, and z-axis mechanisms, and two probes at the end of the transfer rod. The new system was used to acquire a secondary electron (SE) images of the cross-sectional surfaces of a multilayer ceramic capacitor (MLCC) with voltages (from 0.5 to 5 V) applied to the internal electrodes of the MLCC. The active voltage contrast corresponding to the electrical potential in the SE images could be observed when the applied voltage was less than or equal to 1.5 V. This technique enables any voltage to be applied to the samples from outside the chamber and can be used to measure the nanometer-scale electrical potential distribution of Li-ion rechargeable batteries, solar cells, etc.In this study, the authors present a new method for electrical potential observation in which helium ion microscope (HIM) observations can be performed while applying a selected voltage (up to approximately +5 V) to a sample. The in situ voltage-application system is operated in a high vacuum HIM chamber and consists of a transfer rod, tilt joint, x-, y-, and z-axis mechanisms, and two probes at the end of the transfer rod. The new system was used to acquire a secondary electron (SE) images of the cross-sectional surfaces of a multilayer ceramic capacitor (MLCC) with voltages (from 0.5 to 5 V) applied to the internal electrodes of the MLCC. The active voltage contrast corresponding to the electrical potential in the SE images could be observed when the applied voltage was less than or equal to 1.5 V. This technique enables any voltage to be applied to the samples from outside the chamber and can be used to measure the nanometer-scale electrical potential distribution of Li-ion rechargeable batteries, sola...

Advanced in situ multi-scale characterization of hardness of carbon-fiber-reinforced plastic

In situ multi-scale characterization of hardness of carbon-fiber-reinforced plastic (CFRP) is demonstrated by a traditional hardness tester, instrumented indentation tester and atomic-force-microscope (AFM)-based nanoindentation. In particular, due to the large residual indentation and nonuniform distribution of the microscale carbon fibers, the Vickers hardness could not be calculated by the traditional hardness tester. In addition, the clear residual microindentation could not be formed on the CFRP by instrumented indentation tester because of the large tip half angle of the Berkovich indenter. Therefore, an efficient technique for characterizing the true nanoscale hardness of CFRP was proposed and evaluated. The local hardness of the carbon fibers or plastic matrix on the nanoscale did not vary with nanoindentation location. The Vickers hardnesses of the carbon fiber and plastic matrix determined by AFM-based nanoindentation were 340 ± 30 and 40 ± 2 kgf/mm2, respectively.