Yuji Kunisada

Three-dimensional analysis of Eu dopant atoms in Ca-α-SiAlON via through-focus HAADF-STEM imaging

Three-dimensional (3D) distributional analysis of individual dopant atoms in materials is important to development of optical, electronic, and magnetic materials. In this study, we adopted through-focus high-angle annular dark-field (HAADF) imaging for 3D distributional analysis of Eu dopant atoms in Ca-α-SiAlON phosphors. In this context, the effects of convergence semi-angle and Eu z-position on the HAADF image contrast were investigated. Multi-slice image simulation revealed that the contrast of the dopant site was sensitive to change of the defocus level. When the defocus level matched the depth position of a Eu atom, the contrast intensity was significantly increased. The large convergence semi-angle greatly increased the depth resolution because the electron beam tends spread instead of channeling along the atomic columns. Through-focus HAADF-STEM imaging was used to analyze the Eu atom distribution surrounding 10nm cubes with defocus steps of 0.68nm each. The contrast depth profile recorded with a narrow step width clearly analyzed the possible depth positions of Eu atoms. The radial distribution function obtained for the Eu dopants was analyzed using an atomic distribution model that was based on the assumption of random distribution. The result suggested that the Ca concentration did not affect the Eu distribution. The decreased fraction of neighboring Eu atoms along z-direction might be caused by the enhanced short-range Coulomb-like repulsive forces along the z-direction.

Two-dimensional quantum dynamics of O2dissociative adsorption on Ag(111)

We have investigated the quantum dynamics of O2 dissociative adsorption on a Ag(111) surface. We performed the calculations with a Hamiltonian where the O2 translational motion is perpendicular to the surface and for O2 vibrational energy. We found that dissociative adsorption occurs with an incident translational energy below the expected activation barrier, while the translational-energy dependence for adsorption probabilities is a smooth sigmoid. Thus, there are non-negligible tunneling effects in the dissociative adsorption that are affected by the activation barrier width. Moreover, the incident translational energies at the inflection points of the adsorption probabilities shift lower with an increase in vibrational quantum numbers of the incident O2. Thus, there is significant energy transfer and coupling from vibration to translational motion. The vibrational energy assists the O2 dissociative adsorption via a vibrationally assisted sticking effect.

Stability of {111}Pd/{0002}ZnO polar interface formed by internal oxidation of Pd–Zn alloys

We investigated the stability of Pd/ZnO polar interfaces formed by internal oxidation of Pd-Zn alloys by using high-resolution transmission electron microscopy, electron energy-loss spectroscopy and convergent-beam electron diffraction. At 1273 K, a [Formula: see text] polar interface defaceted and transformed into a curved interface, while another (111)Pd/(0002)ZnO polar interface retained its flatness. The [Formula: see text] polar interface lost some stability over non-polar interfaces at 1273 K, while the (111)Pd/(0002)ZnO polar interface remained stable.

Measurement of the dielectric function of α-Al2O3 by transmission electron microscopy – Electron energy-loss spectroscopy without Cerenkov radiation effects

The dielectric function of α-Al2O3 was measured by electron energy-loss spectroscopy (EELS) coupled with the difference method. The influence of Cerenkov radiation was significant in measurements using a 200kV transmission electron microscope (TEM) and the correct dielectric function could not be obtained using the conventional EELS procedure. However, a good agreement between the optical data and EELS for the dielectric functions was obtained via a 60kV TEM. Combining EELS and the difference method, however, provided an accurate measurement of the dielectric function for α-Al2O3 even at an accelerating voltage of 200kV. The combination of EELS and the difference method in the nano-beam diffraction mode could derive an accurate dielectric function with superior spatial resolution regardless of the occurrence of Cerenkov radiation.

Enhanced hydrogen permeability of hafnium nitride nanocrystalline membranes by interfacial hydride conduction

Hydrogen permeability based on mixed hydride ion electron conduction was demonstrated for hafnium nitride HfNx (film thickness of 100–500 nm, x = 0.8 and 1.0) nanocrystalline membranes. Nanocrystalline films with a (100) orientation and crystallite sizes of a few tens of nanometers were prepared on porous alumina supports by radio frequency (RF) reactive sputtering. Combined spectroscopic, permeability, and microbalance analysis suggests that the nanocrystalline matrices were readily hydrogenated by the formation of Hf–H terminal groups on the internal grain surfaces at ambient temperature and thus efficient hydrogen permeation took place due to an enhanced diffusion of hydridic defects through the grain boundaries; this was further aided by the Hf–H bond exchange process. Hence, membranes with an average crystallite size of 11 nm yielded a hydrogen flux of 6 × 10−7 mol cm−2 s−1 at 25 °C at an applied hydrogen partial pressure of 50 kPa; this value is higher than those exhibited by the current state-of-the-art Pd membranes. These findings establish a new concept for Pd alternatives based on the pronounced hydric conductivity of transition metal nitride nanomaterials.