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Choongnam, South Korea

SrY2- x(MoO4)4:Er3+/Yb3 phosphors with doping concentrations of Er3+ and Yb3+ (x = Er3+ + Yb3+, Er3+ = 0.05, 0.1, 0.2 and Yb3+ = 0.2, 0.45) have been successfully synthesized by a cyclic microwave-modified sol-gel method, and the upconversion photoluminescence properties have been investigated. Well-crystallized particles showed a fine and homogeneous morphology with particle sizes of 1-3 μm. Under excitation at 980 nm, SrY2(MoO4)4:Er3+/Yb3+ particles exhibited a strong 525-nm, weak 550-nm emission bands in the green region, and a very weak 655-nm emission band in the red region. The possible mechanism of the green and red emissions was discussed in detail under consideration of a two-photon process. The Raman spectra of the particles indicated the presence of strong peaks at both higher and lower frequencies. © 2014 Elsevier B.V. All rights reserved. Source

Sung Lim C.,Hanseo University
Journal of Physics and Chemistry of Solids | Year: 2015

Double tungstate KGd1-x(WO4)2:Ho3+/Yb3+ phosphors with doping concentrations of Ho3+ and Yb3+ (x=Ho3++Yb3+, Ho3+=0.05, 0.1, 0.2 and Yb3+=0.2, 0.45) were successfully synthesized by the microwave sol-gel method, and the upconversion mechanisms were investigated in detail. The synthesized particles formed after heat-treatment at 900 °C for 16 h showed a well crystallized morphology with particle sizes of 2-5 μm. Under excitation at 980 nm, the UC intensities of KGd0.7(WO4)2:Ho0.1Yb0.2 and KGd0.5(WO4)2Ho0.05Yb0.45 particles exhibited yellow emissions based on a strong 550-nm emission band in the green region and a strong 655-nm emission band in the red region, which were assigned to the 5S2/5F4→5I8 and 5F5→5I8 transitions, respectively. The Raman spectra of the doped particles indicated the presence of strong peaks at higher frequencies of 764, 812, 904, 984, 1050, 1106, 1250 and 1340 cm-1 induced by the disorder of the [WO4]2- groups with the incorporation of the Ho3+ and Yb3+ elements into the crystal lattice or by a new phase formation. © 2014 Elsevier Ltd. All rights reserved. Source

Oh E.,Hanseo University | Son K.,University of Southern California | Krishnamachari B.,University of Southern California
IEEE Transactions on Wireless Communications | Year: 2013

In this paper, we investigate dynamic base station (BS) switching to reduce energy consumption in wireless cellular networks. Specifically, we formulate a general energy minimization problem pertaining to BS switching that is known to be a difficult combinatorial problem and requires high computational complexity as well as large signaling overhead. We propose a practically implementable switching-on/off based energy saving (SWES) algorithm that can be operated in a distributed manner with low computational complexity. A key design principle of the proposed algorithm is to turn off a BS one by one that will minimally affect the network by using a newly introduced notion of network-impact, which takes into account the additional load increments brought to its neighboring BSs. In order to further reduce the signaling and implementation overhead over the air and backhaul, we propose three other heuristic versions of SWES that use the approximate values of network-impact as their decision metrics. We describe how the proposed algorithms can be implemented in practice at the protocol-level and also estimate the amount of energy savings through a first-order analysis in a simple setting. Extensive simulations demonstrate that the SWES algorithms can significantly reduce the total energy consumption, e.g., we estimate up to 50-80% potential savings based on a real traffic profile from a metropolitan urban area. © 2002-2012 IEEE. Source

Wang F.,Beijing Institute of Technology | Zhang K.,Hanseo University
Journal of Molecular Catalysis A: Chemical | Year: 2011

Reduced graphene oxide-TiO 2 (RGO-TiO 2) nanocomposites have been successfully synthesized through a facile hydrothermal reaction with minor modification using graphene oxide (GO) and commercial P25 as starting materials in an ethanol-water solvent, followed by calcining temperature at 400 °C for 2 h in Ar. These nanocomposites prepared with different ratios of graphene oxide (GO) were characterized by BET surface area, X-ray diffraction (XRD), Raman spectroscopy, UV-vis diffuse reflectance spectroscopy (UV-vis DRS), Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), Transmission Electron Microscopy (TEM) and ultraviolet-visible (UV-vis) absorption spectroscopy. The RGO-TiO 2 nanocomposites exhibited much higher photocatalytic activity than bare P25 for the degradation of rhodamine B (Rh.B) in an aqueous solution. The improved photocatalytic activities may be attributed to increased adsorbability for Rh.B molecular, light absorption levels in visible region and charge transfer rate in the presence of a two-dimensional graphene network. Source

Lim C.S.,Hanseo University
Materials Chemistry and Physics | Year: 2013

Er3+-doped BaMoO4 (BaMoO4:Er3+) and Er3+/Yb3+ co-doped BaMoO4 (BaMoO 4:Er3+/Yb3+) particles were successfully synthesized by a cyclic microwave-assisted metathetic (MAM) method, and show fine and homogeneous morphology with particle sizes of 0.5-1 μm. At 980-nm excitation, BaMoO4:Er3+ and BaMoO4:Er 3+/Yb3+ particles exhibited a strong 525-nm emission band and a weak 550-nm emission band in the green region. The Raman spectrum of BaMoO4:Er3+/Yb3+ particles indicated the appearance of additional peaks at higher frequencies (390 and 505 cm -1) and at lower frequencies (218 and 255 cm-1). © 2013 Elsevier B.V. All rights reserved. Source

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