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    Enhanced crystallinity, optical conductivity and terahertz cutoff frequency of stacked layers of FeSe2 by Al nanosheets
    (Elsevier, 2023) Qasrawi, Atef Fayez; Toubasi, Ahmad J.
    Herein a 250 nm thick two stacked layers of FeSe2 (abbreviated as FF) thin films and FF stacks comprising aluminum nanosheets of thicknesses of 50 nm (FAF) are studied. Amorphous FeSe2 thin films are deposited using thermal evaporation technique under a vacuum pressure of 10-5 mbar. It is observed that insertion of Al nanosheets induced the evolution of orthorhombic phase of FeSe2. nanowire and elongated rectangular grains are also formed. Optically Al nanosheets enhanced the light absorption by 4.1 times and redshifted the indirect/direct energy band gaps from 2.80/2.49 eV to 2.42/1.85 eV, respectively. It also improved the optical conductivity and terahertz cutoff frequency as well. In addition the optical conductivity parameters of FF and FAF terahertz resonators are determined by the Drude-Lorentz approach. It is observed that in the infrared range of light Al nanosheets improved the drift mobility of FF from 2.06 cm2/Vs to 12.55 cm2/Vs. The hole density is reduced and the scattering time constant at femtosecond level is increased. FF and FAF terahertz resonators performed as optical filters suitable for terahertz technology applications with terahertz cutoff frequency varying in the range of 17.2-300 THz.
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    Iron selenide nanowire bundles for microwave communication technology
    (Wiley, 2023) Toubasi, Ahmad J.; Qasrawi, Atef Fayez
    Herein stacked layers of FeSe2 deposited onto ytterbium substrates using the thermal evaporation technique under a vacuum pressure of 10-5 mbar are observed to form nanowire bundles (NB). Structural investigations on these NBs have shown the preferred growth of orthorhombic FeSe2 in addition to hexagonal ytterbium selenide as a minor phase in the structure of FeSe2. Impedance spectroscopy analyses conducted in the frequency domain of 0.01-1.80 GHz shown that FeSe2 stacks can exhibit a negative capacitance effect and band stop filter characteristics. For these band stop filters, the refection coefficient (S11), the voltage standing wave ratios (VSWR) and the return loss (Lr) spectra displayed optimum values at a notch frequency of 0.87 GHz. In addition, computational analysis using Lorentz Oscillator (LO) Model has shown that the negativity of the capacitance is dominated by two oscillators centered at 0.08 GHz and 1.25 GHz. On the other hand, the insertion of aluminum nanosheets between the stacked layers of FeSe2 has been observed to decrease the negativity of the capacitance, increase the values of S11 and Lr and bring the VSWR closer to ideality. FeSe2 nanowire bundles resulting from FeSe2 stacks comprising Al nanosheets are considered suitable for microwave communication technology.