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ItemPlasmon energy shift in mesoporous and double length-scale ordered nanoporous silica(Georgia Institute of Technology, 1999-05-03) Yin, Jinsong ; Wang, Z. L. (Zhong Lin)Electron energy-loss spectroscopy studies are reported on three different types of structures: solid silica spheres, mesoporous silica, and the double length-scale ordered (DLSO) porous silica. The mesoporous silica has porosity at the length scale of nanometers. The DLSO porous silica has an additional ordering on submicron hollows created by the template polystyrene spheres. The plasmon energy of the porous silica shows a significant shift in comparison to that of the bulk, suggesting that the local density of the bound electrons in the porous structures is lower than that in the bulk. This gives the possibility of tuning the electronic structure of silica by varying its porosity, leading to even lower dielectric loss.
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ItemSynthesis of cobalt oxide nanocrystal self-assembled materials(Georgia Institute of Technology, 1999-02) Yin, Jinsong ; Wang, Z. L. (Zhong Lin)Self-assembling of size-, shape-, and phase-selected nanocrystals into superlattices is a new approach for synthesizing a new generation of advanced materials with functionality. In this paper, high purity and monodispersive tetrahedral nanocrystals of CoO, with edge-lengths of 4.4 ± 0.2 nm, have been synthesized and separated from Co nanocrystals using colloidal chemistry and magnetic separation. The tetrahedral CoO nanocrystals behave like a molecular matter, and their assembling forms superlattices with translational symmetry. The phase transformation of the CoO nanocrystals is examined by ex situ annealing in oxygen, and the results showed the formation of Co₃O₄ with spinel structure.
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ItemAnalysis of Cation Valences and Oxygen Vacancies in Magnetoresistive Oxides by Electron Energy-Loss Spectroscopy(Georgia Institute of Technology, 1998) Wang, Z. L. (Zhong Lin) ; Yin, Jinsong ; Berta, Yolande ; Zhang, JimingMagnetic oxides of (La,A)MnO₃ and (La,A)CoO₃ have two typical structural characteristics: cations with mixed valences and oxygen vacancies, which are required to balance the charge introduced by cation doping. The consequences introduced by each can be different, resulting in different properties. It is important to quantitatively determine the percentage of charges balanced by each, but this analysis is rather difficult particularly for thin films. This paper has demonstrated that electron energy-loss spectroscopy (EELS) can be an effective technique for analyzing Mn and Co magnetic oxides with the use of intensity ratio of white lines, leading to a new technique for quantifying oxygen vacancies in functional and smart materials.
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ItemOrdered Self-Assembling of Tetrahedral Oxide Nanocrystals(Georgia Institute of Technology, 1997-09-29) Yin, Jinsong ; Wang, Z. L. (Zhong Lin)Self-assembling of size, shape, and phase controlled nanocrystals into superlattices with translational and even orientational ordering is a new approach for engineering nanocrystal materials and devices. High purity tetrahedral nanocrystals of CoO, with edge lengths of 4.4±0.2 nm, were synthesized and separated from Co nanocrystals, using a novel magnetic field phase-selection technique. Self-assembling of the faceted CoO nanocrystals forms ordered superlattices, the structures of which are determined.
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ItemStudies of Mn valence conversion and oxygen vacancies in La₁₋ₓCaₓMnO₃₋[subscript y] using electron energy-loss spectroscopy(Georgia Institute of Technology, 1997-06-23) Wang, Z. L. (Zhong Lin) ; Yin, Jinsong ; Jiang, Yongdong ; Zhang, JimingUsing the white line intensities, electron energy-loss spectroscopy in a transmission electron microscope has been employed to characterize the valence conversion and oxygen vacancies in La₁₋ₓCaₓMnO₃₋[subscript y]. For a nominal doping composition x = 0.33, the ratio of Mn³⁺ to Mn⁴ ⁺ is determined to be more than 0.25 but less than 0.5, and the content of oxygen vacancy y is no more than 0.065 (equivalent to 2.2 at. % of the oxygen content). At y[subscript max] = 0.065, 60% of the residual charge introduced by Ca doping is balanced by the conversion of Mn³⁺ to Mn⁴ ⁺and 40% by oxygen vacancy.