H. Mori

In situ atomic-scale observation of melting point suppression in nanometer-sized gold particles

Phase stabilities of nanometer-sized materials are quite different from those of the corresponding bulk materials. Among the phase stabilities, melting point suppression is one of the most fundamentally important issues. In this work, real-time, atomic-scale direct observation of melting point suppression in nanometer-sized Au particles, along with simple size reduction, was carried out by means of in situ high resolution electron microscopy. Namely, it was confirmed in real space on an atomic scale that a solid-to-liquid transition occurred when the size of a particle, placed on a graphite substrate maintained at 1100 K, decreased to 5 nm during diminution. Furthermore, a monolayer-thick hole was formed on the substrate at the position of the liquid Au particle, probably due to carbon dissolution into the liquid Au particle.

Quantitative analysis on size dependence of eutectic temperature of alloy nanoparticles in the Ag–Pb system

The eutectic transition of Ag–Pb alloy nanoparticles was investigated using in situ transmission electron microscopy. It was found that the eutectic temperature of Ag–Pb alloy nanoparticles decreased with decreasing particle size. A linear relationship was obtained for the eutectic temperature as a function of the reciprocal of the particle radius. Theoretical calculations based on the theory of thermodynamics also suggested a linear relationship between the eutectic temperature and the inverse radius. The calculated results were in good agreement with the experimental observations.

In situ observations of crystalline-to-liquid and crystalline-to-gas transitions of substrate-supported Ag nanoparticles

The phase transitions of silver nanoparticles on graphite and alumina substrates have been investigated using in situ high-resolution transmission electron microscopy. It was revealed that no crystalline-to-liquid transition, only a crystalline-to-gas transition, took place in graphite-supported silver nanoparticles prior to the complete disappearance of the nanoparticles; crystalline-to-liquid transition did take place in alumina-supported silver nanoparticles. The substrate-induced difference in the phase transition behavior is discussed on the basis of a thermodynamic model.

Two-dimensional metallic tungsten nanowire network fabricated by electron-beam-induced deposition

One-dimensional tungsten nanowires and two-dimensional tungsten nanowire networks were fabricated on a tungsten substrate by using electron-beam-induced deposition (EBID) without precursor injection. The as-prepared tungsten nanostructures were studied using a combination of energy-dispersive x-ray spectroscopy and selected area electron diffraction. It was revealed that the tungsten nanostructures were composed of pure metallic tungsten. The WO(3) oxide formed in the chemical preparation of tungsten foils was probably the source of tungsten for the fabrication of tungsten nanostructures by EBID.

In situ TEM observation of the growth and decomposition of monoclinic W18O49 nanowires

The growth of monoclinic W(18)O(49) nanowires by heat treatment of a tungsten filament at approximately 873 K and the decomposition of these nanowires under 200 keV electron irradiation at approximately 1023 K have been investigated using in situ transmission electron microscopy (TEM). In situ TEM observation of the growth confirmed the vapor-solid growth mechanism of the monoclinic W(18)O(49) nanowires. In situ irradiation experiments revealed the formation of metallic bcc tungsten from monoclinic W(18)O(49) nanowires under 200 keV electron irradiation.

Silver nanowires with a monoclinic structure fabricated by a thermal evaporation method

Silver nanowires with a monoclinic structure (mono-Ag NWs) were fabricated by a thermal evaporation method for the first time. The crystal lattice parameters of the mono-Ag NWs were calculated using the UnitCell program. They are as follows: a = 0.303 nm, b = 1.140 nm, c = 0.292 nm, and beta = 118.5 degrees. In situ annealing experiments revealed that the as-prepared mono-Ag NWs transited to fcc-Ag NWs during annealing at approximately 1173 K for 60 s.

Comparative study on size dependence of melting temperatures of pure metal and alloy nanoparticles

A comparative study on the size dependence of the melting temperatures of pure metal and alloy nanoparticles has been carried out. It was found that the melting temperatures of Bi-Sn, In-Sn, and Pb-Sn alloy nanoparticles decreased more rapidly with decreasing particle size than those of the constituent metal nanoparticles (Bi, In, Pb, Sn). Namely, the size dependence of the melting temperature was stronger for the alloy nanoparticles than that for the constituent metal nanoparticles. Results calculated with a thermodynamic model were in good agreement with the experimental observations.

In situ TEM observation of decomposition of high-purity sapphire

The decomposition of α-Al2O3 under 200 keV electron irradiation has been investigated by in situ high-resolution electron microscopy (HREM). It was confirmed that aluminium precipitated from α-Al2O3 under 200 keV electron irradiation for less than 1 min over the temperature range from 700 to 1273 K. The electron dose rate was of the order of 1023 e m−2 s−1 and the vacuum level of the microscope was better than 10−6 Pa. The mechanisms of α-Al2O3 decomposition were discussed based on two possible decomposition models: the thermally activated atom movement and the forced atom displacement.

Effects of temperature and electron energy on the electron-irradiation-induced decomposition of sapphire

The effects of temperature and electron energy on the electron-irradiation-induced decomposition of sapphire have been investigated by in situ transmission electron microscopy (TEM). It was found that the decomposition rate of α-Al2O3 increased with increasing irradiation temperature and decreased with increasing acceleration voltage. The core-hole Auger decay process (K–F model), rather than knock-on displacement, is responsible for the decomposition of α-Al2O3 under electron irradiation.

In-situ transmission electron microscopy studies on the dynamic behaviors of materials

Topics of in-situ transmission electron microscopy studies on the dynamic behaviors of materials are described. Estimation of the activation energy for the migration of self-interstitial atoms is successfully carried out. The coalescence between nm-scale prismatic dislocation loops with different Burgers vectors, as well as the one-dimensional glide diffusion of the loops, has been confirmed. A new electron-beam-induced deposition method using no precursor gas, has been developed for the fabrication of tungsten nanostructures. Size dependence of the eutectic temperature of alloy particles was examined. A linear relationship between the eutectic temperature and the inverse radius of the particle was obtained. In-situ experiments with TEM are useful in revealing the dynamic behaviors of materials.