Abstract: Thin films of the superconducting Nd1+xBa2−xCu3O7−δ (NBCO) and Y1(NdxBa2−x)Cu3O7−δ (YNBCO) compounds have been deposited by dc magnetron sputtering and diode high pressure oxygen sputtering. Different techniques, including scanning tunneling microscopy, atomic force microscopy, X-ray diffraction and transmission electron microscopy (TEM) have been used to study the film microstructures and the surface morphology. It is shown that the Nd/Ba exchange promotes a 2D heteroepitaxial growth in the case of the NBCO films deposited on (1 0 0) SrTiO3 substrates, with a suppression of the characteristic 3D spirals structures. On the contrary in YNBCO the Nd/Ba exchange does not provide a driving force for the suppression of screw-dislocations, since spirals or towers originated by screw and half loop dislocations are observed on the surface. TEM reveals that anti-phase boundaries are eliminated during the growth of NBCO films thanks to the Nd/Ba exchange while this mechanism is not operating in the case of YNBCO films.

Abstract: Nickel foams were used as catalysts to dissociate acetylene and deposit carbon atoms. Graphite nanofibers with distinct structures were developed at 550degreesC with nickel foams pretreated with hydrogen. HREM observations showed that the graphite layers of the nanofibers were aligned at a certain angle to the fiber axis. It is suggested that hydrogen treatment and metal catalysts have a tremendous impact on the yields and microstructures of the graphite nanofibers. The growth mechanism of these fish-bone graphite nanofibers is also discussed.

Abstract: The atomic structure of single-wall carbon nanotube bundles produced by three different techniques has been characterized by electron diffraction and microscopy. Small bundles produced by Catalytical Chemical Vapor Deposition (CCVD) exhibit only one or two tube chiralities within a single bundle while bundles produced by arc-discharge or laser-ablation exhibit more chiralities. A detailed analysis of the central line of diffraction is also presented. The CCVD nanotubes present more intense spots around 1.7 Angstrom(-1) < k < 2Angstrom(-1) (k is the momentum transfer) compared to what is observed for nanotubes produced by other methods. Amongst the possible explanation for such an anomaly, we put forward that in this range of momentum transfer, the relative tube orientations and translations are important for what concerns the interpretation of the diffraction peaks intensities.

Abstract: The present paper describes the synthesis, characterization of mixed-valence bismuthates with three- or two-dimensional perovskite-like structures and structural criteria that influence superconductivity in these compounds. Single-phase samples of Sr1-xKxBiO3 were prepared for the broad range of K-content: 0.25 less than or equal to x less than or equal to 0.65. For these bismuthates the symmetry of the structure changes from monoclinic to orthorhombic and finally to tetragonal upon increasing the K-content thus resulting in the decrease of the Bi-O distances and reduction of the network distortions. Superconductivity with maximum T-c = 12 K exists in the narrow range (x approximate to 0.5-0.6) within the stability field of the tetragonal phase (0.33 less than or equal to x less than or equal to 0.65), when the three-dimensional octahedral framework has close to the ideal perovskite structure arrangement. The layered type (Ba,K)(3)Bi2O7 and (Ba,K)(2)BiO4 bismuthates belonging to the A(n+1)B(n)O(3n+1) homologous series were investigated. Buckling of the (BiO2) layers in the structure of the n = 2 member occurs due to the ordering of alkaline- and alkaline-earth cations between two independent positions. The formation of the one-layer bismuthate was revealed by Electron Microscopy and XRPD studies. Both types of compounds are considered to be possible candidates for new superconducting materials among bismuthates. (C) 2002 Published by Elsevier Science B.V.

Abstract: Epitaxially grown heterostructures consisting of alternating layers of LaMnO(3) (LMO, 9 or 15 unit cells) and SrMnO(3) (SMO, 4 or 6 unit cells) on a SrTiO(3)(100) (STO(100)) substrate have been studied by a combination of high resolution transmission electron microscopy (HRTEM), electron diffraction, quantitative electron energy loss spectroscopy (EELS) with model fitting, energy filtered TEM (EFTEM) and imaging spectroscopy on an atomic scale. The combination of these techniques is necessary for the structural, chemical, and electronic characterization of these heterostructures. A model is proposed containing chemically and structurally sharp interfaces. The SrMnO(3) layers are stabilized in a Pm3m structure between two LMO layers. Tensile stress causes oxygen deficiency in the SMO layers increasing the number of 3d electrons on the Mn sites to resemble the Mn(3+) sites in LMO. The energy loss near edge structure (ELNES) of O and Mn is compared for both LMO and SMO layers and shows that the Mn-O bonds have a partially covalent character. The absence of a strong valency effect in the Mn ELNES is due to the oxygen vacancies in SMO.

Abstract: Results are presented on the photoluminescence (PL) characterization of heavily doped p(+) Czochralski silicon, which has been subjected to a two-step, oxygen precipitation heat treatment. It will be shown that the presence of oxygen precipitates gives rise to the D1, D2 and D5 lines, where the energy of the D1 line shifts to lower values for a stronger degree of precipitation. The occurrence of these PL features is also a function of the boron concentration in the p(+) material. The PL results are compared with Fourier transform infrared absorption data and with transmission electron microscope, results. From this, it is concluded that PL has a good potential for use in the assessment of oxygen precipitation in heavily doped silicon.