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Ho Chi Minh City, Vietnam

Bui B.T.,Hanoi University of Science | Dang B.A.T.,Ho Chi Minh City University of Education | Than T.C.,Hanoi University of Science | Nguyen M.H.,Hanoi University of Science | And 6 more authors.
Journal of Electronic Materials | Year: 2016

Aluminum-doped zinc oxide was deposited on silicon and glass substrates by magnetron sputtering methods with various conditions starting from conventional, then to oxygen reactive technique, and finally to cosputtering of aluminum and 2% Al2O3 in ZnO target. The results showed that the optical transmittance of the thin films in the visible range was above 80%. The direct bandgap of the aluminum-doped zinc oxide thin films, depending on the deposition conditions, was estimated to lie in the range between 3.2 eV and 4.2 eV. Good electrical properties with resistivity of about 2.5 × 10−3 Ω cm were also observed when doping more Al using the cosputtering method. Optimization of the synthesis conditions for aluminum-doped zinc oxide thin films was performed using a newly proposed dimensionless figure of merit based on analysis of the efficiency of solar cells. This study highlights that the optimized deposition condition was to cosputter aluminum during a total additional time of 2 min. The results show that such transparent conductive thin films could be applied in future low-cost heterojunction oxide-based solar cells with CuFeO2 delafossite p-type absorber. © 2016 The Minerals, Metals & Materials Society

Tan L.M.,Sudan University of Science and Technology | Thu N.N.,Geophysical Center | Ha T.Q.,Ho Chi Minh City University of Education | Marbouti M.,Amirkabir University of Technology
Indian Journal of Radio and Space Physics | Year: 2014

About 26 solar flare events from C2.56 to X3.2 classes were obtained and analyzed at Tay Nguyen University, Vietnam (12.56°N, 108.02°E) during May - December 2013 using very low frequency remote sensing to understand the responses of low latitude D-region ionosphere during solar flares. The observed VLF amplitude perturbations are used as the input parameters for the simulated Long Wavelength Propagation Capability (LWPC) program, using Wait's model of lower ionosphere, to calculate two Wait's parameters, viz. the reflection height (H') and the sharpness factor (β). The results reveal that when X-ray irradiance is increased, β increased from 0.3 to 0.506 km-1, while H' decreased from 74 to 60 km. The electron density increased at the height of 74 km with 1-3 orders of magnitude during solar flares. These phenomena can be explained as: the ionization due to X-ray irradiance becomes greater than that due to cosmic rays and Lyman-α radiation, which increases the electron density profile. The present results are in agreement with the earlier results. The 3D representation of the electron density changes with altitude and time supports to fully understand the shape of the electron density changes due to X-ray flares. The shape variation of electron density is roughly followed to the variation of the amplitude perturbation and keeps this rule for different altitudes. It is also found that the electron density versus the height in lower latitude D-region ionosphere increases more rapidly during solar flares.

Thu N.N.,Geophysical Center | Ha T.Q.,Ho Chi Minh City University of Education | Nguyen-Luong Q.,Japan National Astronomical Observatory
Earth, Planets and Space | Year: 2015

We present the observation of tweek atmospherics with harmonics m = 1-8 during the solar maximum year, 2013, at Tay Nguyen University, Vietnam (Geog. 12.65°N, 108.02°E). The analysis of 33,690 tweeks on ten international quiet days during 2 months each season, summer (May, August), winter (February, November), and equinox (March, September), shows that tweeks occur about 51 % during summer, 22 % during winter, and 27 % during equinox. The D-region ionosphere is more sharply bounded for harmonics m = 5-6 around an altitude of 85.5 km. The environment of the D-region is more inhomogeneous during winter and equinox seasons. The mean electron density varies from 28.4-225 cm -3, which corresponds to the harmonics m = 1-8 at the mean reflection height of 81.5-87.7 km. The results reveal that the lower reference height in our work as compared to other works is due to the higher level of solar activity. The equivalent electron density profile of the nighttime D-region ionosphere using tweek method during summer, equinox, and winter seasons shows lower values of electron density by 12-58 %, 3-67 %, and 24-76 % than those obtained using the International Reference Ionosphere (IRI-2012) model. © 2015 Tan et al.

Cong N.T.,Ho Chi Minh City University of Education | Nhan H.T.,Ho Chi Minh City University of Education | Van Hung L.,Ho Chi Minh City University of Education | Thang T.D.,Vinh University | Kuo P.-C.,National formosa University
Molecules | Year: 2014

In an effort to develop new antimicrobial agents, 3-(4-methylcoumarin-7- yloxyacetylamino)-2-thioxo-1,3-thiazolidin-4-one (4) was synthesized by reaction of thiocarbonylbisthioglycolic acid with ethyl (4-methyl-2-oxo-2H-chromen-7-yloxy)acetohydrazide (3), which was prepared in turn from 7-hydroxy-4-methylcoumarin (1). The condensation of compound 4 with different aromatic aldehydes afforded a series of 5-(arylidene)-3-(4-methylcoumarin-7-yloxyacetyl-amino)-2-thioxo-1,3-thiozolidin-4-one analogs 5a-h. The structures of these synthetic compounds were elucidated on the basis of IR, 1H-NMR and 13C-NMR spectral data and ESI-MS spectrometric analysis. Compounds 5a-h were examined for their antibacterial activity against several strains of Gram-positive and Gram-negative bacteria. © 2014 by the authors; licensee MDPI Basel Switzerland.

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