Browsing by Author "Farabi, Bozheyev"

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    MoS2 nanopowder as anode material for lithium-ion batteries produced by self-propagating high-temperature synthesis
    (2017-01-01) Bozheyev, Farabi; Zhexembekova, Anar; Zhumagali, Shynggys; Molkenova, Anara; Bakenov, Zhumabay; Farabi, Bozheyev
    Abstract Due to continuous rise of demand for powerful energy sources for portable applications, high energy density and efficiency rechargeable batteries are under constant development. Currently, the most widely used power source for such applications is rechargeable lithium-ion batteries (LIBs). To increase the energy density, rate capability and cyclability of LIBs, alternative anode materials, such as MoS2, are under intensive investigation. The layered structure of MoS2 resembles graphite, and its theoretical specific capacity is about twice higher than that of graphite (670 mAh g-1 against 372 mAh g-1) due to its higher interlayer spacing (∼0.6 nm) for a Li-ion intercalation. In this work MoS2 nanopowder (MoS2-NP), prepared by self-propagating high-temperature synthesis (SHS), is used as an anode material (MoS2/C composition) for LIBs, and its electrochemical properties were analyzed. The MoS2-NP anode exhibited the initial charge capacity of 567 mAh g-1 at a current density of 50 mAh g-1. This performance will be improved by introduction of MoS2-NP into various carbon-containing composites.
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    Pulsed cathodoluminescence and Raman spectra of MoS2 nanocrystals at different excitation electron energy densities and laser wavelengths
    (Journal of Luminescence, 2017-08-01) Bozheyev, Farabi; Valiev, Damir; Nemkayeva, Renata; Farabi, Bozheyev
    Abstract Molybdenum disulfide nanocrystals exhibit direct dependencies of the Raman peaks' intensities and their positions on the thicknesses of the nanocrystals. This indicates dependencies of the electronic structure on the thickness of the flakes, which scatter differently various laser wavelengths (473nm, 532nm and 633nm). Pulsed cathodoluminescence spectra of MoS2 nanocrystals under the action of high-energy electron pulse show an increase in the luminescence intensities with the increasing input of electron energy densities, where higher electron densities induce greater amount of electron-holes, which afterwards recombining release the energy in the form of luminescence. Irradiation of nanocrystals at maximum electron energy density of 200mJcm−2 leads to a strong luminescence band at 655nm (1.88eV), which is explained by recombination of A exciton. The luminescence decay kinetics of the nanocrystals under the action of different electron energy densities is presented.
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    Pulsed cathodoluminescence of WS2 nanocrystals at various electron excitation energy densities: Defect induced sub-band gap emission
    (Journal of Luminescence, 2017-12-01) Bozheyev, Farabi; Valiev, Damir; Nemkayeva, Renata; An, Vladimir; Tikhonov, Alexander; Sugurbekova, Gulnar; Farabi, Bozheyev
    Abstract Pulsed cathodoluminescence spectra and luminescence decay kinetics of WS2 nanocrystals were studied under a high energy electron pulse excitation. Increasing electron energy density inputs to the WS2 nanocrystals lead to enhancement of the electron-hole number and their lifetimes. The maximum luminescence intensity is reached for the highest electron energy dose. The electrons, interacting with WS2 nanocrystals, form defect vacancies, wherein excited electron-holes create bound excitons which further recombine as a distinct sub-band gap emission. The lifetime of bound excitons does not depend on increasing electron doses due to the limitation of the enhancing number of bound excitons by increasing density of defects, which are radiative recombination active centers. The number of bound excitons, i.e. luminescence intensity, is proportional to the density of defects, which can be tuned by electron doses. Micro-photoluminescence measurements of the WS2 monolayer showed formation of excitons and trions.