Akinori Tanaka

Electronic structure and surface chemistry of alkyl-passivated Si nanoparticles

We have synthesized the n-butyl-passivated Si nanoparticles with mean diameter of 1.4 nm by the solution routes, and have carried out the various spectroscopic studies in order to investigate their optical properties, electronic structures, and surface chemistry. It is found that photoluminescence (PL) spectra of these Si nanoparticles strongly depend on the surface conditions. Moreover, it is found their electronic features obtained from the synchrotron-radiation valence-band photoemission and PL spectra are reproduced by the recent quantum Monte Carlo calculation. From the synchrotron-radiation Si 2p core-level photoemission spectra, it is found that their interfacial electronic structure of the present Si nanoparticles is significantly different with that of alkyl-terminated bulk Si surface. From these results, we discuss the detailed electronic and surface chemical properties of alkyl-passivated Si nanoparticles.

Formation of nanometer-sized porous GaSb particles by vacancy clustering induced by electronic excitation

Porous materials play an important role for developments of luminescent, catalytic, hydrogen storage materials and so on, in which the properties can be greatly improved by a large surface-to-volume ratio. If we can prepare a porous structure not only in the bulk materials but also in the interior of individual nanoparticles, it is expected that the porous structure increases the surface area of the nanoparticles. We found that phase changes take place by atom displacements when GaSb semiconductor compound nanoparticles are excited by electron beam [1,2]. In the present work, the formation of porous semiconductor GaSb compound nanoparticles by clustering of vacancies introduced efficiently by electronic excitation and the formation mechanism has been studied by in situ TEM.

Process of phase separation induced by low energy electronic excitation in GaSb nanoparticles

The process of electronic-excitation-induced phase separation in GaSb nanoparticles has been studied by transmission electron microscopy from the viewpoint of the relationship between the atomic diffusion and the behaviour of the defects introduced by electronic excitation. When approximately 20 nm-sized GaSb particles kept at 430 K are excited by 25keV electrons, gallium atoms on the lattice points are displaced to form vacancies and gallium interstitials in the crystal. Two-phase separation takes place via the void formation and an increase in lattice constant of GaSb. It is suggested that the vacancy supersaturation near the particle core brings about the void formation and the diffusion of gallium interstitials to the particle surface causes the phase separation.

Electron Dose Rate Dependence of Phase Separation Induced by Electronic Excitation in GaSb Nanoparticles

The effect of electron dose rate on phase separation induced by electronic excitation in GaSb nanoparticles has been studied in order to see whether a nonlinear relation between density of excited states introduced and the efficiency of the phase separation is found or not. The phase separation to two phases consisting of an antimony core and a gallium shell proceeds after incubation time with increasing electron dose and does only at the dose rate above a threshold value. It is suggested that such nonlinear behaviors take place as a cooperative phenomenon among electronic-excitation effect, nano-size effect and temperature.