Perspectives of nanoscience and nanotechnology

Paper Title Page

Authors: M. Godlewski, S. Yatsunenko, A. Opalińska, Witold Łojkowski
Abstract: Nanoparticles of the wide band gap oxides doped with rare earth (RE) ions are prospective materials for application in optoelectronics as phosphors in a new generation of light sources. In this paper the mechanisms of the excitation of efficient 4f-4f intra-shell transitions in RE doped nanoparticles are discussed. These mechanisms either enhance the rate of host to impurity energy transfer or stimulate the intra-shell transitions of RE ions.
Authors: Heiko Paul, Dominik Kessler, Ulrich Herr
Abstract: Due to its high quantum efficiency (QE) for luminescence, conventional coarse-grained YAG:Ce (Y3Al5O12:Ce) finds widespread use in light conversion and scintillator applications. Nanocrystalline YAG:Ce may possess modified optical properties which are advantageous for technological applications, but this will depend on highly efficient energy conversion. In this work, the effect of the particle size and Ce concentration on the quantum efficiency and the optical lifetime of the YAG:Ce emission will be characterized and discussed. Nanocrystalline YAG:Ce with an average particle size of 20 to 50 nm was synthesized by the chemical vapour reaction (CVR) method and subsequently analyzed using various techniques. When comparing the nanocrystalline samples to a coarse-grained reference sample, the particle size and doping concentration was found to have a significant influence on quantum efficiency. It was established that the nanocrystalline samples investigated exhibit a lower QE at ambient temperature than the coarse-grained reference. The results of the optical lifetime measurements are discussed in relation to this reduction in QE.
Authors: Czeslaw Skierbiszewski
Abstract: Recent progress in the growth of nitride based semiconductor structures made by plasma assisted MBE (PAMBE) is reported. The technology is ammonia free and the nitrogen for growth is activated by an RF plasma source from nitrogen molecules. A new approach for the growth of nitrides by PAMBE at temperature range 500 - 600°C is described. The key for this technique is to use a thin, dynamically stable metal (In or Ga) layer on the (0001) GaN surface, which enables a high quality 2D step-flow growth mode to be achieved at temperatures much lower than those determined by thermodynamic considerations. A new perspective for PAMBE in optoelectronics has been opened recently by a demonstration of continuous wave operation of InGaN blue–violet laser diodes. These laser diodes were fabricated on bulk GaN substrates with a low threading dislocation density.
Authors: Giora Kimmel, Jacob Zabicky
Abstract: The following nanocrystalline binary oxide systems were studied: Mg-Ti, Ni-Ti, Zr-Al, as well as some pure and doped unary oxides. The xerogels were heated at a constant T (200 to 1600°C) for 3 to 6 hours. There was a threshold tempearture for oxide formation and in many cases the products were metastable nanocrystalline phases, depending on the grain size and composition, including doping. The oxide phases of Ni-Ti, Mg-Ti, and Zr-Al, formed at 900 °C are different from those formed at higher temperature. New ranges of solid solutions and the formations of higher temperature structures were found. A transition phase can be defined as a structure formed at relative low tempearture, irreversibly transforming at higher temperature into an equilibrium phase of the same elemental composition. Some low temperature transition phases have a structure similar to that of a high temperature equilibrium phase, e.g., (the equilibrium phase is given in parentheses) tetragonal ZrO2 (monoclinic) and low-T qandilite-like solid solutions (qandilite + geikielite). Others are unique with no representation in the equilibrium phase diagram, e.g., gamma-like alumina (corundum) and anatase (rutile), which are formed as nanocrystalline oxides due to a low growth rate caused either by a low temperature of calcination or due to additives. To asses the importance of crystal size in the stabilization of transition phases, the following studies were undertaken: (a) XRPD analysis of all unary, doped and binary compositions; (b) the evolution of transition phases in HT XRPD of the Mg titanates; (c) the phase evolution was studied with time at temperatures were mixtures of transition and equilibrium phases were found; (d) the retention of pure tetragonal ZrO2 on quenching Al-Zr oxides after calcinations at high tempetature; (e) additional evidence from HRTEM, SEM and DTA experiments was also collected. A model, correlating the size effect with the unusual phases and structures is proposed.
Authors: Wu Zong Zhou
Abstract: Using mesoporous silicas as hard templates and facilitating crystal growth of transition metal oxides inside the pores, some mesoporous crystals can be produced after removing the templates. This paper gives a brief review of the research of mesoporous crystals of transition metal oxides in the last five years, including the technical development and potential applications of the new form of oxides.
Authors: Janusz Przewoźnik, T. Tyliszczak, D. Rybicki, Jan Żukrowski, W. Szczerba, M. Sikora, Czesław Kapusta, H. Stepankova, R.F. Pacheco, David Serrate, M.R. Ibarra
Abstract: A combined XRD, Mössbauer, SEM, STXM and NMR study of naturally oxidised, ball milled iron powders is presented. The XRD patterns show the peaks of the bcc-Fe phase with the line widths increasing with the milling time. This corresponds to a flattening of the crystallites, as confirmed by SEM, and increased strain due to the accumulation of defects. The effect is consistent with the variation of the Mössbauer line-widths with the milling time. Scanning Transmission Xray Microscopy (STXM) measurements provided oxygen maps of the particles and revealed that the dominant oxide in the nanometric oxide layer is magnetite. The 57Fe spin echo NMR study reveals a dominant signal corresponding to a bcc-Fe core and a much weaker resonance corresponding to a magnetite amount of less than 1%.
Authors: Tatyana Koutzarova, Svetoslav Kolev, Kornely Grigorov, Chavdar Ghelev, Ivan Nedkov, Marcel Ausloos, Rudi Cloots, Tadeusz Mydlarz, Andrzej Zaleski
Abstract: Barium hexaferrite (BaFe12O19) powders of particle size of 130 and 180 nm were synthesized by a single microemulsion technique. The influence of the concentration of Ba2+ and Fe3+ metallic ions in the aqueous phase in the microemulsion system on the particle size distribution, crystallinity and magnetic properties of BaFe12O19 was studied. The coercive force and saturation magnetization of the sample obtained at a lower concentration of metallic cations in the aqueous phase were higher than those of the sample obtained at higher concentration.

Showing 1 to 10 of 31 Paper Titles