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Park S.,Silla University | Vogt T.,University of South Carolina
Journal of Physical Chemistry C | Year: 2010

We show that a family of anion ordered oxyfluorides with the general composition Sr3-xAxMO4F with A = Ca, Ba and 0 ≤ x ≤ 1 and M = Ga1-zAlz and 0 ≤ z ≤ 1, which are promising new host lattices for near-UV excited phosphors with potential use in light-emitting devices, develop defect structures during exposure to reducing gases. These defects in anion-deficient nonstoichiometric (Sr 3-xAx)1-α-2δMO 4-αF1-δ, can be monitored by broad-band photoluminescence emissions centered near 500 nm when excited with 254 nm UV light. We show how the broad-band photoluminescence emissions of these materials respond to synthesis parameters such as temperature, time, and reducing gas flow rate and argue that they could be used to monitor the long-term stability of these phosphors under UV light. Substituting indium into the oxyfluoride lattices we discovered a class of single phase materials with the general formula (Sr3-xBax)1-α-2δAl 1-cIncO4-α F1-δ (x = 0-0.6, c = 0-0.2, and α, δ = 0-0.05) which show self-activating luminescence with an emission maximum near 600 nm when excited in the near-UV region near 400 nm. © 2010 American Chemical Society.

Luminescent materials composed of Sr 2.5-3x/2Ba 0.5Sm xAl 1-yIn yO 4F (0.001≤x, y≤0.1) were prepared by the solid-state reaction method. After the replacement of Sr 2 and Al 3 ions by Sm 3 and In 3 ions in Sr 2.5Ba 0.5AlO 4F host structure, a novel charge-transfer band centered around 304 nm shifted from 240 nm is monitored; moreover, sharp and well-resolved emission peaks in the 4G 5/2→ 6H J transitions of the Sm 3 activator are observed. The diverse excitation and emission photoluminescence spectra of Sr 2.5-3x/2Ba 0.5Sm xAl 1-yIn yO 4-αF 1-δ (0.001≤x, y≤0.1) phosphors originated by the charge-transfer of the host to the Sm 3 activator, the ff transitions in the Sm 3 ions, and the defect-induced self-activation are also introduced. © 2011 Elsevier Inc.

The nitrogen and aluminium-codoped ZnO (NAZO) thin films are grown by rf magnetron sputtering system with the nitrogen flow ratio changing from 0% to 30%. All of the NAZO films show a strongly c-axis preferred orientation, while the values of the electron concentration are found to decrease with the increase of the nitrogen flow ratio, and they are determined as 1.5-27 × 10 19 cm-3. The optical transmittance spectra indicate an average transmittance of about 95% in the visible range, and a red shift of the band gap energy is observed with increasing the nitrogen flow ratio. These results suggest that the nitrogen flow ratio is an important parameter to grow the high-quality NAZO films for the application of solar cells. © 2010 Elsevier B.V. All rights reserved.

Ahn S.I.,Silla University
Chemical Physics Letters | Year: 2012

The effects of Ag surface plasmons (SPs) on the defects of ZnS are investigated. The photoluminescence (PL) spectra of ZnS on an Ag film show well resolved, enhanced, and red-shifted emissions compared to reference samples. The enhancement of the PL by the Ag layer is attributed to an enhanced energy transfer between overlapping band energies of ZnS by Ag SP. Based on the results, it is suggested that the PL emissions from defects can be enhanced and distinguished by SPs, which provide a direction for the application of SPs to characterize inorganic materials. © 2012 Elsevier B.V. All rights reserved.

Aluminum nitride (AlN) films have been deposited on glass substrates at various nitrogen flow ratios by rf reactive magnetron sputtering. The AlN film deposited at 10% of nitrogen flow ratio shows a strongly c-axis preferred orientation with a crystalline size of 100 nm, thickness of 1100 nm, and band gap energy of 4.38 eV. The optimum crystallographic structure occurs at a nitrogen flow ratio of 100%, where a considerable crystallinity enhancement of the AlN film is observed. The band gap energies E g calculated by the Tauc model and parabolic bands are well described by a relationship, E g=10 -3X4.39, where X is the nitrogen flow ratio. The results suggest that the nitrogen flow ratio plays a key role in growing the high-quality AlN films. © 2011 Elsevier B.V. All rights reserved.

Yoon J.H.,Silla University
Microwave and Optical Technology Letters | Year: 2015

In this article, a novel coplanar waveguide (CPW)-fed monopole antenna with three branch strips for wireless local area network (WLAN)/worldwide interoperability of microwave access (WiMAX) triple-band applications is proposed. The proposed antenna is simply composed of three branch strips. By carefully selecting the positions and lengths of these branch strips, the desired resonant modes excited at 2.46/3.6/5.4 GHz with less than -10 dB frequency responses to the impedance can be achieved. In addition, an acceptable agreement is obtained between the simulation and the experiment. Also, the compact design of the proposed antenna, which has enough impedance bandwidth, makes the antenna suitable for WLAN and WiMAX operations. This article further presents and discusses the 2D radiation patterns and 2D gains according to the experiment results. © 2015 Wiley Periodicals, Inc.

Cho S.-W.,Silla University
Bulletin of the Korean Chemical Society | Year: 2013

Red phosphors Ca(1-1.5x)EuxWO4 and Ca(1-2x)EuxNaxWO4 were synthesized with various concentrations x of Eu 3+ ions by using a solid-state reaction method. The crystal structure of the red phosphors were found to be a tetragonal scheelite structure with space group I41/a. X-ray diffraction (XRD) results show the (112) main diffraction peak centered at 2θ = 28.71o, and indicate that there is no basic structural deformation caused by the vacancies VCa″ or the Eu3+ (and Na+) ions in the host crystals. Densities of Ca(1-1.5x)EuxWO4 were measured on a (helium) gas pycnometer. Comparative results between the experimental and theoretical densities reveal that Eu3+ (and Na+) ions replace the Ca2+ ions in the host CaWO4. Also, the photoluminescence (PL) emission and photoluminescence excitation (PLE) spectra show the optical properties of trivalent Eu3+ ions, not of divalent Eu2+. Raman spectra exhibit that, without showing any difference before and after the doping of activators to the host material CaWO4, all the gerade normal modes occur at the identical frequencies with the same shapes and weaker intensities after the substitution. However, the FT-IR spectra show that some of the ungerade normal modes have shifted positions and different shapes, caused by different masses of Eu3+ ions (or Na+ ions, or V Ca″ vacancies) from Ca2+.

Park S.,Silla University | Vogt T.,University of South Carolina
Journal of the American Chemical Society | Year: 2010

A near UV excitable phosphor Sr2.85Eu0.1Al 0.9In0.1O4-αF1- was made by reducing as-made Sr2.85Eu0.1Al0.9In 0.1O4F. This material reveals defect-induced broad-band photoluminescence emissions centered at 600 nm and a line emission at 619 nm due to the5D0 → 7F2 transition of the Eu activator (curve 1). Such combined line and broad-band emitters have the potential when combined with another broad-band emitting phosphor with emissions near 500 nm to create white light by converting near-UV light from a Ga 1-xInxN light-emitting device. © 2010 American Chemical Society.

Park S.,Silla University
Journal of Luminescence | Year: 2012

Effective orange Sm 3-doped Sr 2.5Ba 0.5AlO 4F phosphors excited at 254 and 408 nm excitation were prepared by the solid-state method. The excitation and emission spectra of Sr 2.5-3x/2Ba 0.5Sm xAlO 4F and Sr 2.5-3x/2Ba 0.5Sm xAlO 4-αF 1-δ (x=0.001∼0.1) based on photoluminescence spectroscopy are investigated. The defects in anion-deficient Sr 2.5-3x/2Ba 0.5Sm xAlO 4-αF 1-δ (x=0.001, 0.01) are monitored by broad-band photoluminescence emission centered near 480 nm along with the orange emission transitions of Sm 3. CIE values and relative luminescent intensities of Sr 2.5-3x/2Ba 0.5Sm xAlO 4F and Sr 2.5-3x/2Ba 0.5Sm xAlO 4-αF 1-δ by changing the Sm 3 content (x=0.001∼0.1) are discussed. © 2011 Elsevier B.V. All rights reserved.

The present invention relates to a method for producing carbon micro- and nano-coils using sulfur hexafluoride gas, wherein the carbon micro- and nano-coils are synthesized and grown on a ceramic substrate and sulfur hexafluoride is introduced during the synthesis of the carbon coils to control the geometry of the carbon coils. The invention also discloses a method of producing carbon micro- and nano-coils by synthesizing and growing the carbon coils on a substrate using a chemical vapor deposition system, wherein sulfur hexafluoride (SF_(6)), acetylene (C_(2)H_(2)) and hydrogen (H_(2)) gases are introduced into a chamber during synthesis of the carbon coil, and wherein the sulfur hexafluoride and acetylene gases are introduced alternately for predetermined amounts of time, or any one or more of the flow rate, time or time point of introduction of the sulfur hexafluoride, thereby controlling the shape, length and geometry of the carbon coils.

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