Hidehiro Yasuda

High-resolution electron microscopy study of spontaneous alloying in gold clusters

Abstract The alloying behaviour of copper atoms into gold clusters has been studied by high-resolution electron microscopy. Preparation of nanometre-sized gold clusters and subsequent copper deposition onto the gold clusters has been carried out with the use of a miniature double-source evaporator that has been directly set at the tip of a specimen holder. Upon the deposition of copper, nanometre-sized gold clusters quickly changed into Au-Cu solid solution clusters even at ambient temperature. It has been confirmed that such spontaneous alloying takes place not via a quasi-melting process but via a purely solid-state process.

Cluster-size dependence of alloying behavior in gold clusters

Cluster-size dependence of alloying behavior in nm-sized atom clusters has been studied by transmission electron microscopy, using clusters in the Au-Cu system. It was revealed that occurrence of rapid spontaneous alloying becomes more difficult with increasing cluster size. In gold clusters of approximately 4 nm in the mean size, a rapid dissolution of copper atoms took place and homogeneously mixed Au-Cu alloy clusters were formed. In gold clusters of approximately 10 nm in the mean size, rapid alloying of copper took place only at a shell-shaped region beneath the free surface of individual clusters and pure gold was retained at the central region of clusters. In gold clusters of approximately 30 nm in the mean size, no rapid alloying of copper was induced. The ease with which spontaneous alloying takes place is discussed in terms of the lattice softening in atom clusters.

Synthesis and optical properties of emission-tunable PbS/CdS core–shell quantum dots for in vivo fluorescence imaging in the second near-infrared window

Near-infrared (NIR) fluorescence imaging at wavelengths from 1000 to 1500 nm (2nd-NIR window) is a promising modality for in vivo fluorescence imaging because of the deeper tissue penetration with lower tissue scattering of the 2nd-NIR light. For such in vivo fluorescence imaging, highly fluorescent probes in the 2nd-NIR wavelength region are needed. Although single-walled carbon nanotubes and Ag2S quantum dots (QDs) have recently appeared as 2nd-NIR fluorescent probes, their fluorescence brightness is relatively low (quantum yields <6%). In this study, we developed a synthetic method for preparing highly fluorescent PbS/CdS core–shell QDs (quantum yields, 17% in water) with narrow band widths (<200 nm) that emit in the 2nd-NIR region. By overcoating of a CdS shell onto a PbS QD core, we could easily control the emission wavelengths of the PbS/CdS QDs at 1000 to 1500 nm. To use the QDs for in vivo imaging, we investigated the optical properties of QDs (penetration depth and blurring of fluorescence images in slices of skin, brain, and heart in mice) in the 2nd-NIR region. We found that the 2nd-NIR fluorescence imaging at ca.1300 nm using the PbS/CdS QDs results in the highest signal to background ratio with a low blurring for in vivo imaging. To confirm the capabilities of the PbS/CdS QDs for in vivo imaging, we conducted fluorescence angiography imaging of a mouse head.

Phase diagrams in nanometer-sized alloy systems

Abstract The phase diagram in nm-sized Au–Sb alloy system has been studied by transmission electron microscopy. With decreasing system size, the increase of solid solubilities in solid solution or intermetallic compounds are significantly enhanced. With a further decrease in the system size, the pure substance continuously changes into solid solution, a highly disordered ultra-fine grain nanostructure and then to intermetallic compound as the solute concentration increases. The formation of the highly disordered ultra-fine grain nanostructures may be related to the low heterogeneous nucleation probability.

Deformation-induced amorphization in ball-milled silicon

Using high-resolution electron microscopy the initial stages of deformation induced-crystalline-to-amorphous (c → a) transformation of Si have been observed. It is found that deformation-induced amorphization initiates along dislocation core regions in the interior of grains. It is believed that the energy elevation due to accumulation of defects is high enough to drive the c → a transformation.

Transformation of diamond nanoparticles into onion-like carbon by electron irradiation studied directly inside an ultrahigh-vacuum transmission electron microscope

In situ observation of the transformation of diamond nanoparticles (DNP) into onion-like carbon (OLC) was made during electron-beam irradiation inside an ultrahigh-vacuum transmission electron microscope at 300 kV with 8.5×1023e∕m2. 5 nm DNP changed into OLC within about 10 min of irradiation, while 20 nm DNP did not change to OLC but to graphite. Therefore, the size effect is critical to the formation of OLC. The mechanism of formation of OLC from DNP is discussed.

Multidirectional Observation of Photoluminescence Polarization Anisotropy in Closely Stacked InAs/GaAs Quantum Dots

We studied polarization anisotropy observed in photoluminescence from closely stacked InAs/GaAs quantum dots (QDs). As the number of stacked layers was increased, the anisotropy in the (001) plane became drastically larger and the [001]-polarization component became larger than the [110] component when observed from the [110] direction. However, the polarization intensity of the [110] component remained stronger than that of the [001] component in the stacked QDs. Such varied polarization anisotropies depending on the observation direction have been found to result from the valence-band mixing in the vertically coupled electronic states.

In situ observation of stress-induced martensitic transformation and plastic deformation in TiNi alloy

Stress-induced martensitic transformation and plastic deformation in TiNi alloy have been investigated using transmission electron microscope with an in situ deformation technique. It was found that martensite bands were formed and grew with increasing strain and shrank with decreasing strain in tension. In the second deformation stage, a high density of looped dislocations was formed in the reverse-transformed matrix, while in the first deformation stage dislocations were not formed in the reverse-transformed matrix. The formation of dislocation in the second deformation stage is considered as bearing a relation to reverse martensitic transformation and twinning dislocations lying inside the stress-induced martensite.

Formation of Co filled carbon nanocapsules by metal-template graphitization of diamond nanoparticles

Co filled carbon nanocapsules, which are formed by a heat treatment of the mixture of Co and diamond nanoparticles, have been studied by in situ transmission electron microscopy (TEM), x-ray diffraction, and Raman spectroscopy. Raman studies show that the heat treatment reduces the surface native oxide (Co3O4) of Co nanoparticles. The reduction is accompanied by graphitization of diamond nanoparticles, indicating that diamond nanoparticles being in contact with the metallic Co are transformed into graphitic coating. The in situ TEM studies show that the graphitic coating is formed in the heating process, not in the cooling process. Furthermore, once the coating is completed, the number of the graphitic layers is almost constant on further heating and cooling. These results allow us to conclude that metallic Co particles simply act as templates for graphitic coating.

Alloying behavior of gold atoms into nm‐sized copper clusters

Alloying behavior of gold atoms into copper clusters has been studied using a 200 kV transmission electron microscope (TEM) equipped with a double source evaporator in the specimen chamber. Isolated copper clusters of approximately 4 nm in diameter were first prepared on a supporting film of amorphous carbon, and then gold atoms were deposited onto the same film kept at room temperature. Upon the deposition of gold, the copper clusters quickly changed into homogeneously mixed Cu‐Au alloy clusters. The gold diffusivities in the clusters estimated from the experimental results are at least 19 orders of magnitude faster than those in the bulk copper.