Jun Yamasaki

Junctions in Axial III-V Heterostructure Nanowires Obtained via an Interchange of Group III Elements

We present an investigation of the morphology and composition of novel types of axial nanowire heterostructures where Ga(x)In(1-x)As is used as barrier material in InAs nanowires. Using aberration-corrected scanning transmission electron microscopy and energy dispersive X-ray analysis we demonstrate that it is possible to grow junctions by changing the group III elements, and we find that a substantial fraction of Ga can be incorporated in axial InAs/Ga(x)In(1-x)As/InAs, retaining straight nanowire configurations. We explain how the adatoms are transferred to the incorporation site at the growth interface via two different routes, (1) interface diffusion and (2) volume diffusion through the catalyst particle.

Diffractive imaging of the dumbbell structure in silicon by spherical-aberration-corrected electron diffraction

The dumbbell structure in crystalline silicon as known with the separation of 0.136 nm has been reconstructed clearly by diffractive imaging using an electron beam. The spatial resolution in the result is estimated at about 0.1 nm. By utilizing the selected area diffraction technique in a spherical-aberration-corrected transmission electron microscope, one can reconstruct nanostructures with atomic resolution, even if they are not surrounded by empty space such as localized structures embedded in thin film samples. This means that the present method has a unique potential to expand the versatility of diffractive imaging by electron beams drastically.

In situ high-resolution transmission electron microscopy observation of photodecomposition process of poly-hydrocarbons on catalytic TiO2 films

Photocatalytic TiO2 materials have been one of the strong subjects of investigation for these ten years among various kinds of advanced functional materials. For studying the catalytic reaction by physical science techniques, a kind of transmission electron microscope technique is presented and applied to the study of photocatalysis on TiO2 films by UV light. Using the technique, we visualized in situ the decomposition process of hydrocarbons deposited on the TiO2 films in atomic level and elucidated a mechanism of the decomposition process.

First observation of In(x)Ga(1-x)As quantum dots in GaP by spherical-aberration-corrected HRTEM in comparison with ADF-STEM and conventional HRTEM.

In(x)Ga(1-x)As quantum dots in GaP(100) crystals prepared by the OMVPE technique are observed along the [011] direction with a newly developed 200-kV spherical aberration(Cs)-corrected HRTEM, a 200-kV annular dark-field (ADF)-STEM, and a 200-kV conventional HRTEM equipped with a thermal field-emission gun. The dots are 6-10 nm in size and strongly strained due to the misfit of about 9% with the GaP substrate and GaP cap layer. All of the cross-sectional high-resolution electron micrographs show dumbbell images of Ga and P atomic columns separated by 0.136 nm in well-oriented and perfect GaP areas, but the interpretable images are limited to those taken with the Cs-corrected HRTEM and ADF-STEM with Fourier filtering of the images. The Cs-corrected HRTEM and ADF-STEM are comparable from the viewpoint of interpretable resolution. A detailed comparison between the Cs-corrected HRTEM images and the simulated ones with electron incidence tilted by 1 degree to 5 degrees from the [011] zone axis gives information on local lattice bending in the dots from the images around 0.1 nm resolution. This becomes one of the useful techniques newly available from electron microscopy with sub-angstrom resolution.

Development of microcapsules for electron microscopy and their application to dynamical observation of liquid crystals in transmission electron microscopy

Microcapsules made of hard carbon films were fabricated for electron microscopic observations of liquid-like samples such as biological cells/molecules in water solution and liquid crystals. 4-(1-methylheptyloxycarbonyl) phenyl--octyloxybiphenyl-4-carboxylate (MHPOBC) liquid crystals housed in the capsule were observed by transmission electron microscopy at 120 kV accelerating voltage. From the series of electron diffraction patterns, the rate of electron irradiation damage was estimated for the cases of changing sample temperature, beam current density and accelerating voltage. Phase transitions from a crystalline phase to liquid crystalline phases such as an isotropic-A phase of a smectic state (Sm-IA) and a chiral-A phase of a smectic state (Sm-CA*) were successfully observed and the rotation fluctuation of the molecules in the phase was measured by using the electron diffraction patterns.

Oxygen release and structural changes in TiO2 films during photocatalytic oxidation

Changes in the crystal structure and grain modifications in titanium oxide (TiO2) thin films were observed during the photocatalytic oxidation of hydrocarbons. When the hydrocarbon and collodion films were irradiated, single crystalline titanium oxide transformed into polycrystals. The titanium oxide films gradually became network aggregates. These changes were analyzed with a dedicated in situ transmission electron microscope and observed three dimensionally by electron tomography. A detailed analysis of electron energy loss spectra of the samples also revealed that the changes were associated with the loss of oxygen atoms in the TiO2 crystal lattice. Correlations between the polycrystalline grain size of TiO2 and its catalyst activity were discussed based on the measured data.

Mild Hydrogenation of Amides to Amines over a Platinum-Vanadium Bimetallic Catalyst

Hydrogenation of amides to amines is an important reaction, but the need for high temperatures and H2 pressures is a problem. Catalysts that are effective under mild reaction conditions, that is, lower than 30 bar H2 and 70 °C, have not yet been reported. Here, the mild hydrogenation of amides was achieved for the first time by using a Pt-V bimetallic catalyst. Amide hydrogenation, at either 1 bar H2 at 70 °C or 5 bar H2 at room temperature was achieved using the bimetallic catalyst. The mild reaction conditions enable highly selective hydrogenation of various amides to the corresponding amines, while inhibiting arene hydrogenation. Catalyst characterization showed that the origin of the catalytic activity for the bimetallic catalyst is the oxophilic V-decorated Pt nanoparticles, which are 2 nm in diameter.

Quantitative phase imaging of electron waves using selected-area diffraction

A method for quantitative phase imaging of electron waves was developed based on diffractive imaging. Phase images over field of views of more than 100 nm were reconstructed from pairs of a selected-area diffraction pattern and a transmission electron microscopy image. The illumination wave field was reconstructed uniformly with a phase fluctuation of less than 0.1 rad and a spatial resolution of 2-3 nm. The phase image for wedge-shaped silicon was converted to a thickness map, which agreed quantitatively with electron energy-loss spectroscopy. The present method is also valid for arbitrary-shaped samples even if dynamical diffraction effects are significant.

Annealing effects on a high-k lanthanum oxide film on Si "001… analyzed by aberration-corrected transmission electron microscopy/scanning transmission electron microscopy and electron energy loss spectroscopy

The annealing behavior of a lanthanum oxide thin film deposited on a silicon (001) substrate by electron-beam evaporation has been studied by aberration-corrected transmission electron microscopy (TEM), scanning TEM (STEM), and electron energy loss spectroscopy (EELS). We have developed a procedure for the precise measurement of thickness and interfacial roughness by taking advantage of features of aberration correction combined with the statistics of fluctuating crystalline edge positions. The results of the measurements and quantitative elemental analyses by STEM-EELS have revealed atomic diffusion and reactions during deposition and postdeposition annealing (PDA) at 300 and 500 °C. The channel mobility could be limited by Coulomb scattering before PDA, and by remote roughness scattering and remote phonon scattering after PDA at 500 °C. When we consider the large leakage current caused by oxygen defects in the as-deposited sample and the large equivalent oxide thickness of the thick Si-rich layer in the...

Observation of silicon surface nanoholes by scanning tunneling microscopy

Abstract We have studied electron-irradiation-induced defects created on an electron exit surface of a Si thin film by means of scanning tunneling microscopy (STM). Several electron-irradiated areas with different electron doses are provided for STM observation. Transmission electron microscopy (TEM) observation reveals a number of silicon-surface-nanoholes of 2–3 nm in diameter and about 5 nm apart in an irradiated area whenever it receives the dose larger than 1.5×10 24 e/cm 2 , while no distinctive TEM contrast of defects is observed in an area with lower dose. STM observation has shown that electron-irradiated surfaces are rougher than a nonirradiated surface. Examining the depth distribution of the areas with different doses, we have found that each irradiated surface exhibits two depth levels which are attributed to a rough surface and a bottom of surface nanoholes, respectively. Even in an area with the lowest dose (1.5×10 22 e/cm 2 ) in this experiment we have observed distinctive STM contrasts, the arrangement and sizes of which are similar to those of the well-developed surface nanoholes observable by TEM. This STM observation shows that the arrangement of nanoholes on an electron exit surface is set up at the very early stage, followed by the excavating of nanoholes under prolonged electron irradiation. We suggest that nanoholes exist in the early stage when only a few atomic layers are removed from the initial surface.