VNU Journal of Science: Mathematics - Physics

This article presents optical simulation results of planar CH3NH3PbI3 solar cells using a MATLAB script developed by McGehee’s group (Stanford University). The device structure is composed of FTO/HEL/AL/EEL/LiF/Al, where HEL is the hole-extraction layer, AL is the active layer (CH3NH3PbI3), and EEL is the electron-extraction layer. In this MATLAB script, the transfer matrix method was used, where transmission and reflection were calculated for each interface in the stack as well as attenuation in each layer. The wavelength-dependent optical constants (n and k) of each layer were measured by spectroscopic ellipsometry (SE). The exciton generation rates within the active layer were calculated based on the data of optical constants, as well as the thickness of each layer. Considering the Internal Quantum Efficiency (IQE) equal to 100% at all wavelengths, the predicted short-circuit currents (JSC) were also estimated. The obtained results show a good agreement with the experimental values of JSC measured on real devices.

Lead-free Bi0.5(Na0.80K0.20)0.5TiO3 (BNKT) particles were synthesized by using  sol-gel method. The samples were investigated by X-ray diffractometry (XRD), scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDX), and vibrating sample magnetometer (VSM). The absorption of microwaves of the lead-free BNKT powders is determined from the magnetic (permeability) and dielectric (permittivity) properties at the frequency range from 2 to 18 GHz. Absorption characteristics of paraffin(wax)-mixed BNKT compounds at different sample thicknesses were also investigated. The microwave absorption properties show that the maximum reflection loss is -21.72 dB (99.9%) at 13.66 GHz with a thickness of 3.2 mm. BNKT composites are thought to be used as a promising microwave absorption material.  

In this work, we carried out survey on magnetic field strength and gradient in space around arrays of micro-sized parallelepipedic magnets by simulation and calculation. Magnetic field distributions are a function of magnet’s size and position with respect to magnet’s surface. Our purpose is to explain how magnetic interactions evolve while dimensions of magnetic sources are reduced. Firstly, the simulations and calculations were executed for a magnet with a large surface size of 1,000×1,000 µm2, a thickness of 5 µm, and a residual magnetism of 1.6T perpendicular to its surface. Then, the similar works were also performed for arrays of magnets with smaller surface sizes, e.g. 1,000×500 µm2; 1,000×200 µm2; 1,000×100 µm2; 1,000×50 µm2 and 1,000×10 µm2. Consequently, both the magnetic field strength and gradient in the space which is above and near the surface of the magnets, particularly, the space from the surface of the magnets to the height of 100 µm far from the surface of the magnets, were enhanced when the magnets’ size were appropriately reduced. This suggests that the application field of the magnets will be expanded and their integration into microsystems will be grown as the size of the magnets is reduced.  

In this work, electrical simulations of planar CH3NH3PbI3 solar cells using a SCAPS-1D (a Solar Cell Capacitance Simulator) simulation tool were performed to determine the density of defects based on the performance parameters and J-V characteristics of actual experimental planar CH3NH3PbI3 solar cells. Two types of defects (bulk and interface) were introduced into the simulation model. The densities of those defects were found by fitting the J-V characteristics and performance parameters including , , and FF to the experimental data. The methods and results reported in this paper showed a close relationship between the parameters of defects in planar perovskite solar cells.      

Hydroxyapatite (HAp) has been successfully coated on etched titanium substrates by hydrothermal method using solutions of Ca(NO3)2.4H2O, NH4H2PO4 at 200 oC for 12 h. Results obtained from measurements on the modern physical methods of FE-SEM and XRD showedhat the structure of HAp has a regular hexagonal morphology belonging to the space group P63/m. The degree of HAp nucleation and nucleation development on the un-etched titanium plates was lower than that on the etched titanium plates. In vitro bioactivity of HAp-coated titanium testing using SBF solution showed prospective results. Keywords: Hydroxyapatite (HAp), Hydrothermal method, etched, titanium.     

Abstract: In this article carbon quantum dots (Cdots) were prepared by a facile, green, economical hydrothermal method of the mixed deionized water and lemon juice. Optical and structural properties of the Cdots have been extensively studied by UV–visible and fluorescence spectroscopy, x-ray diffraction (XRD) techniques, and high-resolution TEM (HR-TEM). Surface functionality of Cdots has been illustrated by Fourier transform infrared spectroscopy (FTIR). The fabricated Cdots emit bright green light with a broad spectrum. The Cdots exhibit captivating sensitivity and selectivity toward Fe 3+ with a linear range from 0 to 80 ppm and a detection limit of 38.08 ppm. Keywords: Carbon quantum dots, lemon juice, sensing Fe3+

This paper presents studying the properties of sounding curves equivalent to the different electrode arrays by means of simulations for numerical modeling. Following this, the authors proposed a calculation for the synthetical apparent resistivity values in effort to gain higher-resolution sounding curves in electric sounding measurement. Keywords Electric sounding, resistivity surveys, apparent resistivity, focusing array References [1] Электроразведка, Справочник геофизика, Москва «Недра», 1980.[2] Lam Quang Thiep, Le Viet Du Khuong, 1986. A new electric sounding method. Proceeding of the First Conference on geology of Indochina, 5-7 Dec, Ho Chi Minh city.[3] Barker, R.D., 1989. Depth of investigation of collinear symmetrical four-electrode arrays. Geophysics 54, 1031-1037.[4] Michael S.Zhdanov and George V.Keller. The geoelectrical methods in geophysical exploration, Elsevier, Amsterdam-London-NewYork-Tokyo 1994.

This article reports on the synthesis procedure of ZnO nanoparticles/nanofibers structure by electrospinning method using Zinc acetate and polyvinylpyrrolidone (PVP) surfactant as reagents and evaluates their luminescent properties. The microstructure of ZnO nanoparticles/nanofibers was observed by FE-SEM. The phase formation of ZnO nanoparticles/nanofibers was studied by XRD. ZnO nanoparticles/nanofibers structure shows strong luminescence centering at 660 nm, which has potential applications in solid-state lighting.

Abstract: This paper the role of Cu 2+ concentrations in luminescence quenching of Eu3+ / Cu 2+ doped ZrO2 nanoparticles synthesized by co-precipitation method. The synthesized Eu 3+ / Cu 2+ doped ZrO2 nanoparticles were observed to have sphere morphology with a diameter of ~ 25 nm. The XRD patterns of the nanoparticles revealed the peaks that were to be crystalline tetragonal ZrO2. The addition of Cu 2+ to the Eu 3+ doped ZrO2 nanoparticles resulted in a significant suppress luminescence in Eu 3+ / Cu 2+ doped ZrO2 nanoparticles, which was attributed to the spectral overlap occurs between Cu 2+ absorption and Eu 3+ emission (5D0→ 7F2 transition). Keywords: Zirconia; luminescence; precipitation; quenching, nanoparticles.

In this article, micro-/nano urchin-like VO2 particles were successfully synthesized by hydrothermal method. Vanadium pentoxide (V2O5), oxalic acid (C2H2O4) and sodium dodecyl sulfate (SDS) surfactant were used as reagents for the synthesis of VO2. The article reports on the synthesis procedure of VO2 nanorods and micro-/nano urchin-like VO2 structure and evaluates the methylene blue (MB) adsorption properties. Morphology and particle size of VO2 were observed by FE-SEM. The VO2 formation phase was studied by XRD. Raman spectroscopy was also used for characterizing VO2. Micro-/nano urchin-like VO2 structure shows good MB adsorption properties that have potential applications in dye-contaminated water treatment.