Magnetic properties of ferromagnetic ultrafine particles prepared by vacuum evaporation on running oil substrate

Ultrafine particles of ferromagnetic metals (Fe, Co and Ni) were prepared by the vacuum evaporation onto a running oil substrate. Particles thus obtained were suspended in the oil and their average diameter was about 25 Å. An electron diffraction analysis indicated that the particles were oxidized and the main component was Fe₃O₄, CoO and NiO for Fe, Co and Ni fine particles, respectively. From magnetic measurements in the temperature range between 4.2 and 300 K, the main magnetic behavior of the fine particles can be explained in terms of the superparamagnetism or superantiferromagnetism. Furthermore, it was concluded that the Fe₃O₄ particles may be covered with several atomic layers of α-Fe₂O₃. For CoO and NiO particles, existence of an imperfect oxide structure was required besides the main components in order to explain the magnetic behavior.     

Crystal structure and habit of fine metal particles formed by gas-evaporation technique; bcc metals (V,Fe, Cr,Mo and W)

Fine particles of several kinds of bcc metals were prepared by evaporation in an argon atmosphere. They were collected at various growth zones in the smokes and their structures and habits were studied by electron microscopy. Vanadium and iron particles grown in all zones have the ordinary bcc structure. However, chromium and molybdenum particles grown in the inner part of the smokes have the bcc structure, and those in the outer part have the A-15 type structure. Habits of particles with the bcc structure can be generally described in terms of rhombic dodecahedra truncated by six (100) faces with various degrees of truncation from 0 to 100%. The truncated rhombic dodecahedron is a common habit to the bcc metals, though the degree of truncation is dependent on the metal. For vanadium it varies from zone to zone systematically. Molybdenum particles of rounded cube and {211} icositetrahedron are found to have the A-15 type structure in addition to the well known {211} icositetrahedron and rhombic dodecahedron found in chromium.     

Mechanisms of chemical vapor deposition of silicon

The distribution in the silicon epitaxial growth from SiCl₄ and hydrogen are observed in situ by IR absorption spectroscopy. Two methods are used complementarily, one is IR spectroscopy of reactants extracted from the reactor by a fine quartz tube which is not disturbing the reactions, and gives knowledge about the local distribution, the other is direct IR spectroscopy of hot reactants in the reactor which is useful to ascertain the results at the real high temperature situation. The intermediate species are SiHCl₃, SiH₂Cl₂ which is estimated from the induced emission bands at 500 and 570 cm^-1. HCl is a dominant waste product and contributes to reverse reactions. To investigate the reaction, HCl is intentionally injected into the reacting gas. This kind of injection method may also be very effective to analyze the reactions using other reactants such as SiCl₄, SiHCl₃ and SiH₂Cl₂.     

Deep levels and growth conditions of LPE GaAs crystals

Deep levels in LPE GaAs were studied in relation to growth conditions. The residual deep levels in LPE layers are hole traps 0.50 and 0.65 eV above the top of the valence band. Their concentrations are of the order of 10¹³ cm^-3 for growth temperatures between 650 and 840°C, and increase with increasing growth temperature. The activation energy of incorporation is about 0.7 eV, which is very close to that of an arsenic vacancy. It is also noted that their concentrations near the grown surface are independent of the growth temperature. This indicates that the hole traps undergo an annealing effect after the growth period. From the experimental results for Fe and oxygen doping, each impurity acts as a deep level only when it makes a complex defect with a gallium vacancy, otherwise they are shallow levels, The distribution coefficients at 750°C are 1 × 10^-7 and 1.1 × 10^-5 for the deep and Fe and the shallow acceptor, respectively.     

Preparation and optical properties of silver nanoparticles induced by a femtosecond laser irradiation

Silver nanoparticles were prepared by a focused femtosecond laser irradiation in a 5 mM AgNO₃ solution in the presence of TiO₂ sol. TEM analysis revealed that the size of silver nanoparticles was less than 20 nm. A mechanism for the precipitation of silver nanoparticles was proposed. Nonlinear absorptions and optical limiting properties of silver nanoparticles contained solution were also measured. It is observed that the composite material showed strong self-focused effect and significant optical limiting property.     

Transmission electron microscopy study of an AlN nucleation layer for the growth of GaN on a 7-degree off-oriented (0 0 1) Si substrate by metalorganic vapor phase epitaxy

We have investigated the morphology of the high-temperature-grown AlN nucleation layer and its role in the early stage of GaN growth, by means of transmission electron microscopy. The nitride was selectively grown on a 7-degree off-oriented (0 0 1) patterned Si substrate by metalorganic vapor phase epitaxy. AlN was deposited on the inclined unmasked (1 1 1) facet in the form of islands. The size of the islands varied along the slope, which is attributable to the diffusion of the growth species in the vapor phase. The GaN nucleation occurred at the region where rounded AlN islands formed densely. The threading dislocations were observed to generate in the GaN nucleated region.