Applications of the discrete variational (DV) Xα molecular orbital method based on the self-consistent Hartree-Fock-Slater model to metal clusters are presented. Numerical basis functions are utilized in the present calculations. Variations of orbital energies and populations with exchange scaling parameter α are investigated. It is proved that the self-consistent-charge (SCC) approximation to the SCF method gives accurate orbital energies. The numerical basis SCC-DV-Xα method is shown to be very efficient for studies of rather large metal clusters such as Ni 13 .

Discrete Variational Xα Cluster Calculations. I. Application to Metal Clusters

A review is presented of the recent advances in the study of oxygen precipitation and of the main properties of oxide precipitates in silicon. After a general overview of the system ‘‘oxygen in silicon,’’ the thermodynamics and the kinetics of the precipitate formation are treated in detail, with major emphasis on the phenomenology; subsequently, the most important techniques for the characterization of the precipitates are illustrated together with the most interesting and recent results. Finally, the possible influence of oxygen precipitation on technological applications is stressed, with particular attention to recent results regarding device yield. Actually, the essential novelty of this review rests on the attempt to give an extended picture of what has been recently clarified by means of highly sophisticated diagnostic methods and of the influence of precipitation on the properties of semiconductor devices.

Oxygen precipitation in silicon

We summarize here the research advances on the reactivity of metal clusters. After a simple introduction of apparatuses used for gas-phase cluster reactions, we focus on the reactivity of metal clusters with various polar and nonpolar molecules in the gas phase and illustrate how elementary reactions of metal clusters proceed one-step at a time under a combination of geometric and electronic reorganization. The topics discussed in this study include chemical adsorption, addition reaction, cleavage of chemical bonds, etching effect, spin effect, the harpoon mechanism, and the complementary active sites (CAS) mechanism, among others. Insights into the reactivity of metal clusters not only facilitate a better understanding of the fundamentals in condensed-phase chemistry but also provide a way to dissect the stability and reactivity of monolayer-protected clusters synthesized via wet chemistry.

Reactivity of Metal Clusters.

Two desorption peaks of hydrogen molecule (mass number=2), starting at about 600 and 950 K, respectively, are observed in thermal desorption mass spectroscopy of nanostructured graphite mechanically milled for 80 h under hydrogen atmosphere. It follows from a combined analysis of thermal desorption mass spectroscopy and thermogravimetry, that ∼6 mass % of hydrogen (corresponding to 80% of the total amount of hydrogen) is desorbed at the first desorption peak as a mixture of pure hydrogen and hydrocarbons. Below the temperature of the second desorption peak, at which recrystallization related desorption occurs, nanostructured graphite is expected to retain its specific defective structures mainly with carbon dangling bonds as suitable trapping sites for hydrogen storage. The formation process of the nanostructures during milling under hydrogen atmosphere is also discussed on the basis of the profile of Raman spectroscopy.

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Hydrogen desorption property of mechanically prepared nanostructured graphite

This paper presents results on the microstructure and physical properties of SiC amorphized by both ion and neutron irradiation. Specifically, 0.56 MeV Si ions have been implanted in single crystal 6H–SiC from ambient through >200°C and the critical threshold for amorphization was measured as a function of the irradiation temperature. From a high resolution transmission electron microscopy (HRTEM) study of the crystalline to amorphous transition region in these materials, elongated pockets of amorphous material oriented parallel to the free surface are observed. Single crystal 6H–SiC and hot pressed and sintered 6H and 3C SiC were neutron irradiated at approximately 70°C to a dose of ∼2.56 dpa causing complete amorphization. Property changes resulting from the crystal to amorphous transition in SiC include a density decrease of 10.8%, a hardness decrease from 38.7 to 21.0 GPa, and a decrease in elastic modulus from 528 to 292 GPa. Recrystallization of the amorphized, single crystal 6H–SiC appears to occur in two stages. In the temperature range of ∼800–1000°C, crystallites nucleate and slowly grow. In the temperature range of 1125–1150°C spontaneous nucleation and rapid growth of crystallites occur. It is further noted that amorphized 6H (alpha) SiC recrystallizes to highly faulted fcc (beta) SiC.

Amorphization of SiC under ion and neutron irradiation

Grazing-incidence electron microscopy of surface blisters in single- and polycrystalline tungsten formed by H+, D+ and He+ irradiation

In order to investigate the mechanism of large volume expansion of graphite due to neutron irradiation we have made in situ observation of damage structure of graphite with a high resolution transmission electron microscope (HRTEM). Both bending and randomizing the orientation of broken basal planes are proposed to be the origin for the large volume expansion of the irradiated graphite. It is also found that graphite easily loses its lattice ordering in the basal planes as demonstrated by halo rings in the (1010) diffraction pattern, while hardly losing its layered structure, i.e., (002) spots are retained.

On the mechanism of dimensional change of neutron irradiated graphite

On the amorphization of neutron-irradiated graphite

The formation of voids and bubbles during ion implantation is an important area of material research. Void and bubble formation can result in swelling and embrittlement of metallic or semiconducting materials, and increase catalytic effects in the nanopores of the bubble. Here, we report the observation of metallic nanoblister formation in GaN nanowires under self-ion implantation using a Ga+ focused ion beam. The mechanism of the blister formation was resolved using high-resolution transmission electron microscopy equipped with electron energy loss spectroscopy and plasmon imaging.

Mechanism of nanoblister formation in Ga+ self-ion implanted GaN nanowires

Self-consistent Hartree-Fock-Slater model cluster calculations are carried out in order to investigate the electronic structure of hydrogen chemisorption on transition metal surfaces. Orbital energies and results of Mulliken's population analysis are presented for HNi5, HNi9, HPd9 and HW9 clusters. The level structures of the clusters are also given by density-of-states (DOS) curves and compared with the experimental UPS and EELS spectra. Hydrogen chemisorption gives rise to bonding levels near the bottom of the valence band and antibonding ones on the top. The energy difference between them in HNi9 is in good agreement with that obtained by EELS. It is concluded that not only localized d electrons but also sp electrons play important roles in hydrogen-metal bondings. Details of hydrogen-metal bond formation, electronic charge transfer and their differences with adsorbate metals are also discussed.

Tetsuo Tanabe

Papers

Grazing-incidence electron microscopy of surface blisters in single- and polycrystalline tungsten formed by H+, D+ and He+ irradiation

On the mechanism of dimensional change of neutron irradiated graphite

On the amorphization of neutron-irradiated graphite

Mechanism of nanoblister formation in Ga+ self-ion implanted GaN nanowires

Hartree-Fock-Slater Model Cluster Calculations. II. Hydrogen Chemisorption on Transition Metal Surfaces