Engineering the Lattice Site Occupancy of Apatite-Structure Phosphors for Effective Broad-Band Emission through Cation Pairing

Unithrattil, Sanjith,Kim, Ha Jun,Gil, Kyeong Hun,Vu, Ngoc Hung,Hoang, Van Hien,Kim, Yoon Hwa,Arunkumar, Paulraj,Im, Won Bin American Chemical Society 2017 Inorganic chemistry Vol.56 No.10

<P>A series of britholite compounds were synthesized by simultaneous introduction of trivalent La3+ and Si4+ ions into an apatite structure. The variations in the average structure, electronic band structure, and microstructural properties resulting from the introduction of cation pairs were analyzed as a function of their concentration. The effects of the structural variance and microstructural properties on the broad-band-emitting activator ions were studied by introducing Eu2+ ions as activators. For the resulting compound, which had dual emission bands in the blue and yellow regions of the spectrum, the emission peak position And strength were dependent upon the concentration of La3+-Si4+ pairs. By engineering the relative sizes of the two possible activator sites in the structure, 4f and 6h, through the introduction of a combination of trivalent La3+ and a polyanion, the preferential site occupancy of the activator ions was favorably altered. Additionally, the activator ions responsible for the lower-Stokes-shifted blue component of the emission functioned as a sensitizer of the larger-Stokes-shifted yellow emitting activators, and predominantly yellow-emitting phosphors were achieved. The feasibility of developing a white light emitting Sad-state device using the developed phosphor was also demonstrated.</P>

Phase formation and luminescence properties of ternary solid-solution among tetragonal systems

Unithrattil, Sanjith,Kim, Ha Jun,Im, Won Bin ELSEVIER SCIENCE 2019 JOURNAL OF ALLOYS AND COMPOUNDS Vol.798 No.-

<P><B>Abstract</B></P> <P>A new strategy for developing solid-solution compounds from three constituent compounds was analyzed. Using this strategy, a series of yellow-light-emitting phosphors derived from ternary solid-solution host compositions were prepared using solid-state synthesis. Three vertex members, Sr<SUB>3</SUB>SiO<SUB>5</SUB> (space group <I>P</I>4/<I>ncc</I>), Sr<SUB>3</SUB>AlO<SUB>4</SUB>F (space group <I>I</I>4/<I>mcm</I>), and LaSr<SUB>2</SUB>AlO<SUB>5</SUB> (space group <I>I</I>4/<I>mcm</I>), were selected from similar crystal structures with slightly different symmetries. The crystallization space group of the composition was found to be dependent on the end members, and from the vertex of the ternary plot to the centroid, the composition assumed the space group that was more tolerant toward substitutions. All the compositions, except those lying at the periphery of the ternary plot, formed a single phase and emitted yellow light when activated with Ce<SUP>3+</SUP> ions. The luminescence properties of the 28 selected compositions on the ternary plot were analyzed. The compound that was formed from a 1:2:1 ratio of end members assumed a <I>I</I>4/<I>mcm</I> structure and showed the highest-intensity emission. Phosphor derived from the developed host lattice was analyzed for application in solid-state lighting devices by preparing a white-light emitting device.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Phase formation in ternary solid-solutions of tetragonal systems. </LI> <LI> Tetragonal structure Sr<SUB>3</SUB>SiO<SUB>5</SUB>, LaSr<SUB>2</SUB>AlO<SUB>3</SUB> and Sr<SUB>3</SUB>AlO<SUB>4</SUB>F are investigated in the preparation of ternary solid solution. </LI> <LI> Structural analysis of all the selected compositions was carried out using the Le Bail method. </LI> <LI> The detailed structural analysis of selected compositions was performed via Rietveld refinement. </LI> <LI> Ternary solid solution present great color tunability in wLED fabrication. </LI> </UL> </P>

A Phosphosilicate Compound, NaCa₃PSiO₈: Structure Solution and Luminescence Properties

Unithrattil, Sanjith,Arunkumar, Paulraj,Kim, Yoon Hwa,Kim, Ha Jun,Vu, Ngoc Hung,Heo, Jaeyeong,Chung, Woon Jin,Im, Won Bin American Chemical Society 2017 Inorganic Chemistry Vol.56 No.24

<P>NaCa<SUB>3</SUB>PSiO<SUB>8</SUB> was synthesized in a microwave-assisted solid-state reaction. The crystal structure of the synthesized compound was solved using a least-squares method, followed by simulated annealing. The compound was crystallized in the orthorhombic space group <I>Pna</I>2<SUB>1</SUB>, belonging to Laue class <I>mmm</I>. The structure consisted of two layers of cation planes, each of which contained three cation channels. The cation channels in each of the layers ran antiparallel to that of the adjacent layer. All the major cations together constituted four distinct crystallographic sites. The Rietveld refinement of the powder X-ray diffraction data, followed by the maximum-entropy method analysis, confirmed the obtained structure solutions. The electronic band structure of the compound was analyzed through density function theory calculations. Luminescence properties of the compound, upon activating with Eu<SUP>2+</SUP> ions, were analyzed through photoluminescence measurements and decay profile analysis. The compound was found to exhibit green luminescence centered at ∼502 nm, with a typical broadband emission due to the transition from the crystal-field split 4<I>f</I><SUP>6</SUP>5<I>d</I> to 4<I>f</I><SUP>7</SUP> levels.</P><P>A new phosphosilicate composition with diverse cation sites for activator substitution leads to a broadband emitting phosphor with peak emission wavelengths located in the green region of the spectrum.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/inocaj/2017/inocaj.2017.56.issue-24/acs.inorgchem.7b02456/production/images/medium/ic-2017-024568_0007.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/ic7b02456'>ACS Electronic Supporting Info</A></P>

Piezoelectricity in La0.85Ce0.15MnO3 layer of BiFeO3/ La0.85Ce0.15MnO3 based ferroelectric/semiconductor oxide superlattice

황승현,Sanjith Unithrattil,이현준,송재선,이혜정,아눕,이상한,조지영 한국물리학회 2019 Current Applied Physics Vol.19 No.8

Superlattice comprising of ferroelectric (FE) and non-ferroelectric (non-FE) material has been developed to induce a ferroelectric response in the non-ferroelectric layer. In this study, a superlattice thin film comprising of BiFeO3 (BFO) as FE oxide and La0.85Ce0.15MnO3 (LCMO) as semiconducting non-FE oxide is fabricated and investigated the FE and piezoelectric responses in LCMO. We observed the piezoelectric response of individual layers using time-resolved X-ray microdiffraction experiment under an external electric field. Piezoelectric response of individual layers was resolved by comparing it with the values obtained from kinematic diffraction calculation. Piezoelectric coefficient (d33) of LCMO monolayer is found to be approximately 27 pm/V, which is a similar value as that of the BFO layer.

Stacked Quantum Dot Embedded Silica Film on a Phosphor Plate for Superior Performance of White Light-Emitting Diodes

Sohn, In Seong,Unithrattil, Sanjith,Im, Won Bin American Chemical Society 2014 ACS APPLIED MATERIALS & INTERFACES Vol.6 No.8

<P>Application of quantum dots as a color converter in white light-emitting diodes (WLEDs) has been highly restrained because of its lower stability under the operating conditions of LEDs. The feasibility of using quantum dots in WLEDs has been studied and demonstrated by developing a non-conventional packing technique. Multiple core shell CuInS<SUB>2</SUB>/ZnS quantum dots were coated by silica, and the silica-coated quantum dots were dispersed in ethoxylated trimethylolpropane triacrylate to form a color conversion film. This along with phosphor in a glass plate made of Y<SUB>3</SUB>Al<SUB>5</SUB>O<SUB>12</SUB>:Ce<SUP>3+</SUP> phosphor was stacked in different configurations, and its effect on color rendering of WLEDs was studied. In addition, the configuration developed here protects the color converter from thermal strain and moisture.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/aamick/2014/aamick.2014.6.issue-8/am500429c/production/images/medium/am-2014-00429c_0007.gif'></P>

Color-tunable binary solid-solution phosphor, (Sr₃SiO₅)_1-x(Sr₃AlO₄F)_x, for white LEDs: Energy transfer mechanism between Ce³⁺ and Tb³⁺

Lee, J.S.,Unithrattil, S.,Im, W.B. Elsevier Sequoia 2013 Journal of alloys and compounds Vol.555 No.-

The possibility of using a solid solution as a light convertor in white LEDs was studied. A solid solution of Sr<SUB>3</SUB>AlO<SUB>4</SUB>F and Sr<SUB>3</SUB>SiO<SUB>5</SUB> was prepared by solid-state reaction and was activated with Ce<SUP>3+</SUP>. Phosphor exhibits an asymmetric yellow-orange emission band which is broadened and red-shifted when coactivated with Tb<SUP>3+</SUP>. Decay profile analysis using the Inokuti-Hirayama model revealed that the mechanism involved was electric dipole-quadrupole interaction. The critical distance was calculated from concentration quenching data, multipolar interaction, and exchange interaction. The thermal stability of photoluminescence was investigated by assuming an Arrhenius temperature dependence, and the activation energy for thermal quenching was calculated. When analyzed in combination with an InGaN LED, the phosphor exhibited high color-rendering index and excellent correlated color temperature. These results demonstrate that this solid solution phosphor with its superior properties is highly suitable for use in white LEDs.

Preparation of electrospun pyrochlore-structure KGdTa2O7:Eu3+ phosphor: the optical and structural properties for white light emitting diode applications.

Yim, Chul Jin,Unithrattil, Sanjith,Chung, Woon Jin,Im, Won Bin American Scientific Publishers 2013 Journal of Nanoscience and Nanotechnology Vol.13 No.12

<P>Red emitting nanofibers, KGdTa2O7:Eu3+ were synthesized by electrospinning technique followed by heat treatment. As-prepared uniform fiber precursor with diameter ranging from about 700 nm to about 900 nm were calcined after removing organic species by calcination. The fiber surface become rough and diameter decreased to about 250-340 nm range due to decomposition of organic species and formation of inorganic phase. Morphology, structural and photoluminescent properties of fibers were analyzed using thermogravimetric and differential thermal analysis (TG-DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and photoluminescence (PL). TG-DTA analysis indicates that KGdTa2O7:Eu3+ began to crystalize at 520 degrees C. Fibers annealed at 900 degrees C formed well crystallized uniform fibers. Under ultraviolet excitation KGdTa2O7:Eu3+ exhibits red emission due to transitions in 4f states of Eu3+. The excitation band is dominated by the Eu(3+)--O2-charge transfer band peaked at 289 nm. The emission peak is in the region that is ideal for red light emission.</P>

Template-engaged synthesis of spinel-layered Li_1.5MnTiO_4+<i>δ</i> nanorods as a cathode material for Li-ion batteries

Vu, Ngoc Hung,Unithrattil, Sanjith,Hoang, Van Hien,Chun, Sangeun,Im, Won Bin Elsevier Sequoia 2017 Journal of Power Sources Vol. No.

<P><B>Abstract</B></P> <P>Spinel-layered composites of Li<SUB>1.5</SUB>MnTiO<SUB>4+<I>δ</I> </SUB> were studied for their use as high-energy, low-cost, and environmentally benign cathode materials. The bulk particles showed an attractive specific capacity of up to 250 mAh g<SUP>−1</SUP> at C/10. To improve the performance of this cathode at a high C-rate, a spinel-layered Li<SUB>1.5</SUB>MnTiO<SUB>4+<I>δ</I> </SUB> nanorod was successfully synthesized using a β-MnO<SUB>2</SUB> nanorod template. The nanorod, which had an average diameter of 200 nm and a length of 1 μm, showed specific capacity as high as the bulk particle at C/10. However, owing to a one-dimensional nanostructure with a large effective contact area for Li<SUP>+</SUP> diffusion, the nanorod sample exhibited enhanced capacities 11% (170 mAh g<SUP>−1</SUP>) and 167% higher (80 mAh g<SUP>−1</SUP>) at 1C and 10C rates, respectively, compared to the bulk particles. Moreover, both samples showed good cycle stability and capacity retention of over 85% after 100 cycles at 1C.</P> <P><B>Highlights</B></P> <P> <UL> <LI> A promising Li<SUB>1.5</SUB>MnTiO<SUB>4+<I>δ</I> </SUB> cathode: spinel-layered structure with high capacity. </LI> <LI> A Li<SUB>1.5</SUB>MnTiO<SUB>4+<I>δ</I> </SUB> nanorod was synthesized by using a β-MnO<SUB>2</SUB> nanorod template. </LI> <LI> The Li<SUB>1.5</SUB>MnTiO<SUB>4+<I>δ</I> </SUB> nanorods exhibit improved rate performance at high C-rate. </LI> <LI> The structure and electrochemical performances of Li<SUB>1.5</SUB>MnTiO<SUB>4+<I>δ</I> </SUB> were studied. </LI> </UL> </P>

Robust moisture and thermally stable phosphor glass plate for highly unstable sulfide phosphors in high-power white light-emitting diodes

Lee, J.S.,Unithrattil, S.,Kim, S.,Lee, I.J.,Lee, H.,Im, W.B. OPTICAL SOCIETY OF AMERICA 2013 Optics letters Vol.38 No.17

Comparative study of optical and structural properties of electrospun 1-dimensional CaYAl₃O₇:Eu³⁺ nanofibers and bulk phosphor

Yim, C.J.,Unithrattil, S.,Chung, W.J.,Im, W.B. Elsevier 2014 Materials characterization Vol.95 No.-

We report the optical and structural studies of Eu<SUP>3+</SUP>-doped 1-dimensional CaYAl<SUB>3</SUB>O<SUB>7</SUB> nano-fiber phosphor. CaYAl<SUB>3</SUB>O<SUB>7</SUB>:Eu<SUP>3+</SUP> phosphors were synthesized by electrospinning technique and the pristine nano-fibers were annealed at 900<SUP>o</SUP>C to form well crystallized uniform fibers. Under ultraviolet excitation, the CaYAl<SUB>3</SUB>O<SUB>7</SUB>:Eu<SUP>3+</SUP> exhibited red emission, due to transitions in the 4f states of Eu<SUP>3+</SUP>. In order to explore the difference between the quantum efficiency of nano-fiber and bulk CaYAl<SUB>3</SUB>O<SUB>7</SUB>:Eu<SUP>3+</SUP> phosphor, detailed structural and optical analyses were carried out. The structural analysis of the CaYAl<SUB>3</SUB>O<SUB>7</SUB>:Eu<SUP>3+</SUP> nano-fibers indicates that the structural environment surrounding the dopant Eu<SUP>3+</SUP> ion was more unstable in nano-fiber when compared to a bulk sample. Decay curves for both the samples when fitted with double exponential decay model indicate that the nano-fiber has shorter decay time, arising from the larger contribution from the non-radiative decay, due to defect levels introduced in the host lattice.