Atsuyuki Fukano

In situ extended x-ray absorption fine structure study of initial processes in CdSe nanocrystals formation using a microreactor

The nucleation process of CdSe nanocrystals is studied by fluorescence-detected extended x-ray absorption fine structure (EXAFS) using a microreactor and synchrotron radiation. Detailed analysis of in situ Se K-edge EXAFS data measured along a microreactor channel revealed a strong position-dependence which displays a rapid increase in the CdSe phase with time at the initial stage. The results indicate that the CdSe nucleation completes within several seconds starting from trioctylphosphine Se solution and dodecylamine surfactant at 240 °C. This shows the promising capability of in situ EXAFS combined with a microreactor to investigate the nucleation process of nanocrystals synthesized in a solvent.

Highly insulating ultrathin SiO2 film grown by photooxidation

Highly insulating ultrathin SiO2 films (<5 nm) were grown by means of vacuum ultraviolet photooxidation, by use of 126, 172, and 222 nm wavelength photons generated by excimer lamps. The ultrathin silicon dioxide films were grown onto the Si(001) substrate, at low temperature (<500 °C). We found that the densities of photooxidized SiO2 films were 13%–35% higher than that of thermally oxidized SiO2 film. The density profile was obtained by x-ray reflectivity and showed a remarkable decrease in the SiOx (suboxide) layer thickness at the SiO2−Si interface, in comparison with thermally oxidized conventional oxides. The Si 2p photoelectron spectrum confirmed that the SiOx layer was negligibly thin. High insulating performance of the SiO2 film was demonstrated.

Nanoclusters Synthesized by Synchrotron Radiolysis in Concert with Wet Chemistry

Wet chemical reduction of metal ions, a common strategy for synthesizing metal nanoparticles, strongly depends on the electric potential of the metal, and its applications to late transition metal clusters have been limited to special cases. Here, we describe copper nanoclusters grown by synchrotron radiolysis in concert with wet chemistry. The local structure of copper aggregates grown by reducing Cu(II) pentanedionate using synchrotron x-ray beam was studied in situ by x-ray absorption spectroscopy. A detailed analysis of the XANES and EXAFS spectra, compared with DFT calculations and full-potential non-muffin-tin multiple scattering calculations, identified the nanocluster as Cu13 with icosahedral symmetry. The novel “charged” nanoclusters tightly bound to electron-donating amido molecules, which formed as a result of photo-induced deprotonation of ligand amines, were stabilized by irradiation. Monodispersive deposition of nanoclusters was enabled by controlling the type and density of “monomers”, in remarkable contrast to the conventional growth of metallic nanoparticles.

Enhancement of Si-O hybridization in low-temperature grown ultraviolet photo-oxidized SiO2 film observed by x-ray absorption and photoemission spectroscopy

The dielectric properties associated with the electronic and bonding structures of SiO2 films were examined using the Si L3,2- and O K-edge x-ray absorption near-edge structures (XANES) and valence-band photoemission spectroscopy (VB-PES) techniques. The Si L3,2- and O K-edge XANES measurements for the low-temperature grown UV-photon oxidized SiO2 (UV-SiO2) and the conventional high-temperature thermal-oxidized SiO2 (TH-SiO2) suggest enhancement of O 2p–Si 3p hybridization in UV-SiO2. VB-PES measurements reveal enhancement of nonbonding O 2p and O 2p–Si 3p hybridized states. The enhanced O 2p and Si 3p hybridization implies a shortening of the average Si–O bond length, which explains an increase of the density and the improvement of the dielectric property of UV-SiO2.

Small copper clusters studied by x-ray absorption near-edge structure

The local structure of copper nanoparticles grown in organic solution by reducing Cu(II) hexafluoroacetylacetonate [Cu(hfac)2] was studied as-grown by the Cu K-edge x-ray absorption near-edge structure (XANES). Comparison of the experimental XANES spectra with reference materials indicated small copper clusters are formed by ligand-exchange with oleylamine and subsequent reducing by diphenylsilane. The multiple-scattering (MS) calculation for various model clusters consisting of 13–135 atoms suggests that small (13–19 atom) Cu clusters are stabilized without a large deformation.

In-situ EXAFS study of nucleation process of CdSe nanocrystals

An in-situ EXAFS method is developed to study the nucleation and growth processes of nanocrystals by using a microfluidic reactor which converts the time-dependent kinetics to position-dependence. As an example, we measured the Se K-edge EXAFS spectra for CdSe nanocrystals along a microfluidic reactor channel and indicated strong time-dependence of the nucleation and growth at the beginning of the reaction. A rapid increase of the reaction yield within several seconds was observed. It is found that after injection of starting materials, the nucleation occurs abruptly and the CdSe nuclei concentration reaches the maximum and then declines rapidly.

UV Photo‐Oxidation of Silicon: A Novel Growth Method of Ultra‐Thin SiO2 Films

Superior insulating performance is found for dense silicon dioxide ultra‐thin films (∼3 nm) grown by UV photo‐oxidation of silicon. Density profile obtained by glazing incidence x‐ray reflectivity shows that the high density (2.32 g/cm3) SiO2 is formed on Si(100) surface at much lower temperature (<450 °C) than thermal oxidation, using 126 nm photons. The sharp and flat interface within 1–2 monolayers is revealed by high resolution transmission electron microscopy. The film density is strongly dependent on wavelength around 172–126 nm, suggesting that the specific excited species of oxygen is involved in the growth mechanism. Unique properties of photo‐oxidized silicon dioxide are related to the modified Si‐O network structure and ring statistics.

Modification of YBa2Cu3Oy Thin Films by Exposure to Argon Plasma and Ozone: A Structural Study

The effects of argon plasma bombardment and subsequent heat treatments on the structure of YBa2Cu3Oy thin films were studied. We find that the argon plasma processing modifies the yttrium composition through the formation of Y2O3 islands (serving as an yttrium reservoir) and solid-state diffusion. The scanning electron microscopy (SEM) images show that the argon plasma bombardment accelerates the growth of Y2O3 islands which are thinned and washed away during the subsequent annealing in vacuum. Complimentary techniques (X-ray diffraction and X-ray absorption spectroscopy) show that the structural disorder caused by argon plasma processing is recovered by heat treatment in vacuum while oxygen vacancies are compensated by the final ozone treatment. The results reveal the individual roles of sequential treatments of YBa2Cu3Oy thin films which improve the electrical properties of ramp-edge Josephson junctions.