JOURNAL OF INTERFACES, THIN FILMS AND LOW DIMENSIONAL SYSTEMS
Year:2019 | Volume:2 | Issue:2|Papers Count:6

Multi-walled carbon nanotubes (MWCNT)-doped TiO2 thin films were synthesized by the dip-coating method. The obtained products were characterized by SEM, EDX, XRD, and UV-vis absorption spectroscopy. The XRD patterns showed the presence of anatase phase. Absorption spectrum of MWCNT-doped TiO2 revealed a red shift in the optical absorption edge due to carbon doping. The photocatalytic properties were investigated using methylene blue (MB) degradation under visible light (400-700 nm). MWCNT/TiO2 thin films exhibited significantly higher photocatalytic activity as compared to undoped TiO2 thin films due to reduction of the rate of electron-hole recombination by doping carbon. Photocatalytic activity studies with different light source (white light and blue light) indicated a significant enhancement in photocatalytic removal under white light irradiation.

In this research, we aim to clarify the relationship between the structural distortion due to doping and the superconductivity existence in the FeAs4 structure. For this, we have prepared polycrystalline of NdFeAsO0. 8F0. 2, NdFeAs0. 95Sb0. 05O0. 8F0. 2 and Nd0. 99Ca0. 01FeAsO0. 8F0. 2 samples by one-step solid state reaction method. The structural and electrical properties of the samples were characterized through the X-ray diffraction pattern and the 4-probe method, respectively. The XRD patterns, refined using the MAUD software and Rietveld’ s method, have indicated formation of the tetragonal structure with the space group P4/nmm: 2. The variation of the lattice structural parameters for the NdFeAsO0. 8F0. 2 doping with Sb and Ca ions were obtained by the Rietveld refinement. The changes of bond lengths and the α , β bond angles have been specified from the corresponding value of α and β regular FeAs4-tetrahedron with the Sb/As and Ca/Nd doping, where they affect the superconductivity existence. The α and β bond angles increase and decrease with the substitution of Sb/As and Ca/Nd doping, which leads to the contraction of the FeAs layer, and therefore the lattice parameter “ c” decreases. The superconducting transition temperature was reduced from 56K for NdFeAsO0. 8F0. 2 sample to 48K and 46K for Ca/Nd and Sb/As doping, respectively. It can be concluded from the structural and electrical properties that the superconducting transition temperature decreases with increasing the distortion of FeAs4-tetrahedron from the regular one. So, there is a relation between the structural properties and the superconductivity in the iron based superconductors-1111 regardless of the doping procedures.

In this study, a set of self-consistent coupled-integral equations for the local magnetic flux and current distributions in a finite superconducting hollow cylinder under an axial magnetic field has been directly derived by using the Biot-Savart law within the framework of the critical-state model. The equations were first solved numerically in the three-dimensional space before obtaining the hysteresis loops for the Kim and Exponential models. We have assumed the contribution of the flux penetration from the inner surface of the sample to be higher than that of other surfaces. It is found that the variation in the area of lateral surface changes the magnitude of the magnetic moment of the finite hollow cylinder in the applied magnetic field. The obtained results are in good agreement with calculates. The formalism presented here can be used for an arbitrary shape of the superconducting system in the presence of any magnetic field dependence of the critical current density
    in an external magnetic field of arbitrary direction.

The nonlinear optical properties of carbon nanotubes (CNTs) have been studied by using the Z-scan technique. Experiments are performed by CW second harmonic of Nd-YAG laser at 532 nm wavelength with a power of 40 mW. The samples are synthesized by the chemical vapor deposition (CVD) method in the presence of Nickel and Nickel-Cobalt catalysts. In this work, for the first time, effect of the type of catalyst has been investigated with CNTs being a set of series single-walled and multi-walled nanotubes with different diameters. Nonlinear refractive index (n2) and nonlinear absorption coefficient (β ) were measured by a closed and open aperture experimental configuration of Z-scan. The experimental results show that the two-photon absorption effect is the dominant nonlinear process in the samples. The results approve the capability of the samples to be used as an optical limiter device. The nonlinear optical refractive index and nonlinear absorption coefficient of samples are in the order of 10-6 cm2/W and 10-1 cm/W, respectively. The sign of nonlinear refractive index of the samples is negative which indicates that the sample has acted as a divergent lens.

In this paper, the drain-source current, transconductance, and cutoff frequency in AlGaN/GaN high electron mobility transistors have been investigated. In order to obtain the exact parameters of AlGaN/GaN high electron mobility transistors such as the electron density, wave function, band gap, polarization charge, effective mass, and dielectric constant, the hydrostatic pressure and temperature effects are taken into account. It has been found that the drainsource current decreases as the temperature increases and increases as the hydrostatic pressure increases. The increase in temperature is equivalent to a negative virtual gate while an increase in the hydrostatic pressure is equivalent to the positive virtual gate voltage. Moreover, the temperature, hydrostatic pressure, and effective mass dependence in high electron mobility transistor structures are investigated. It is observed that the increase of hydrostatic pressure decreases the effective mass as the wave function penetrates through the quantum barrier AlGaN. In general, the process of increasing and decreasing the cutoff frequency and transconductance is similar to the variations in the drain-source current. The calculated results are in good agreement with the existing experimental data.

In this work, we have studied the miniband and minigaps of GaN/AlN constant total effective radius multi-shells quantum dots (CTER-MSQDs) and Rings (CTERMSQRs). We have investigated effects of the Hydrogenic donor impurities, quantum dots and rings radii, and the number of wells on miniband formation by sub-band energy calculations. We show that in these systems, minigaps can be created and then disappeared when the number of wells increases. We observe that cylindrical CTER-MSQDs have one miniband more than the spherical CTER-MSQD. However, minibands of the cylindrical CTER-MSQD are wider. For cylindrical systems, the first minigap position can undertake a blue shift but for spherical systems, we can not observe this fact. The first minigap position undertakes more blue shifts when the inner QD radius R1 increases. Thus, the number of wells, Hydrogenic donor impurities, and quantum dot & ring radii can be used as tuning tools to have a system with the typical number of minibands and minigaps with desired widths.