Kee-Sun Sohn

SnO2/Graphene Composites with Self‐Assembled Alternating Oxide and Amine Layers for High Li‐Storage and Excellent Stability

An alternating stack (SG/GN) consisting of SnO₂-functionalized graphene oxide (SG) and amine-functionalized GO (GN) is prepared in solution. The thermally reduced SG/GN (r(SG/GN)) with decreased micro-mesopores and completely eliminated macropores, results in a high reversible capacity and excellent capacity retention (872 mA h g⁻¹ after 200 cycles at 100 mA g⁻¹) when compared to a composite without GN.

Discovery of a Phosphor for Light Emitting Diode Applications and Its Structural Determination, Ba(Si,Al)5(O,N)8:Eu2+

Most of the novel phosphors that appear in the literature are either a variant of well-known materials or a hybrid material consisting of well-known materials. This situation has actually led to intellectual property (IP) complications in industry and several lawsuits have been the result. Therefore, the definition of a novel phosphor for use in light-emitting diodes should be clarified. A recent trend in phosphor-related IP applications has been to focus on the novel crystallographic structure, so that a slight composition variance and/or the hybrid of a well-known material would not qualify from either a scientific or an industrial point of view. In our previous studies, we employed a systematic materials discovery strategy combining heuristics optimization and a high-throughput process to secure the discovery of genuinely novel and brilliant phosphors that would be immediately ready for use in light emitting diodes. Despite such an achievement, this strategy requires further refinement to prove its versatility under any circumstance. To accomplish such demands, we improved our discovery strategy by incorporating an elitism-involved nondominated sorting genetic algorithm (NSGA-II) that would guarantee the discovery of truly novel phosphors in the present investigation. Using the improved discovery strategy, we discovered an Eu(2+)-doped AB5X8 (A = Sr or Ba, B = Si and Al, X = O and N) phosphor in an orthorhombic structure (A21am) with lattice parameters a = 9.48461(3) Å, b = 13.47194(6) Å, c = 5.77323(2) Å, α = β = γ = 90°, which cannot be found in any of the existing inorganic compound databases.

Luminescence quenching in thermally-treated barium magnesium aluminate phosphor

BaMgAl10O17:Eu2+ (BAM) phosphors used for plasma display panels (PDP) are compelled to be exposed to an oxidizing environment at about 500 °C, which is currently unavoidable in the actual manufacturing process of PDP. We investigated the mechanism of the luminance degradation of BAM caused by the annealing at 500 °C, using photoluminescence (PL), decay measurement, and synchrotron light source x-ray absorption and diffraction measurements. The annealing treatment altered the valence state of Eu ions, whereas no new Eu compounds were detected. By estimating the exact fraction of divalent Eu ions and by comparing it with the luminance data, we found that more than 30% degradation of luminance was induced at the expense of only a few percent of divalent Eu. This finding led us to suggest that the origin of the dramatic decrease in PL intensity is not due to the valence state change but due to the local structure change surrounding the Eu2+ ions.

Mechanoluminescent SrAl2O4:Eu,Dy phosphor for use in visualization of quasidynamic crack propagation

The propagation of a macroscale crack was visualized in Mg partially stabilized zirconia ceramic using the mechanoluminescence (ML) of SrAl2O4:Eu,Dy. The transformation zone around the crack was also clearly detected by the ML. The ML technique permitted on a relatively fast time frame the quasidynamic R curve to be measured for crack speeds up to 15m∕s. The transformation zone width grew to a maximum in the initial transient part of the quasidynamic R curve. The applied stress intensity factor then commenced to increase to 16MPam.

Eu2+ luminescence from 5 different crystallographic sites in a novel red phosphor, Ca15Si20O10N30:Eu2+

Because it is impossible to realize broadband-type, red-light-emitting phosphors in the oxide composition pool, both pure nitrides and oxynitrides have been considered as potential red phosphors for use in light emitting diode (LED) applications. However, only a limited number of red phosphors have been discovered. For instance, only a few red phosphors such as CaAlSiN3:Eu2+, Sr2Si5N8:Eu2+, and their variations are worthy of notice. We examined the CaO–Si3N4 binary system to discover novel oxynitride red phosphors. We finally identified a novel oxynitride phosphor, Ca15Si20O10N30:Eu2+, with a cubic structure in the Pa space group. This phosphor showed a broadband-type, red-light emission at UV and blue-light excitations. A continuous-wave-excited, steady-state photoluminescence (PL) and a pulsed-laser-excited, time-resolved PL for the discovered phosphor were investigated in conjunction with a structural analysis of Ca15Si20O10N30:Eu2+. The variation in decay rate with respect to the detection wavelength was examined in detail, and the energy transfer among activators located at 5 different crystallographic sites proved to be responsible.

Optimization of Gd2 O 3-Based Red Phosphors Using Combinatorial Chemistry Method

Combinatorial chemistry was applied to the optimization of (Gd 2-x-y M x )O 3 :Eu y 3+ Phosphors for display. The (Gd 2-x-y M x )O 3 :Eu y 3+ Phosphor is a promising candidate for red field emission display (FED) phosphors. Screening was per formed in terms of cathodoluminescence at low voltage exciation and photoluminescence. We developed a ternary material library of a triangle-type composition array to obtain the optimum composition fo co-dopants. Three co-dopants (Al, Ca, Mg) were introduced into Gd 2 O 3 :Eu 3+ . Gd 2 O 3 was a good host material for red FED phosphors at a low voltage excitation, and the cathodoluminescent efficiency of Al co-doped phosphor, (Gd 1.83 Al 0.05 )O 3 :Eu 3+ 0.12, was superior to that of the commercial red phosphor Y 2 O 3 :Eu 3+ . We also investigated the energy transfer between Eu 3+ Ions by analyzing the decay curves of emissions originating from 5 D 0 energy states in terms of multipolar interactions. We can explain the quenching of emissions from 5 D 0 in terms of a newly proposed cross relaxation in association with an intercenter diffusion.

Photoluminescence Behavior of BaAl12 O 19 : Mn Phosphor Prepared by Pseudocombinatorial Chemistry Method

A relatively detailed search for the optimum composition of BaAl 12 O 19 :Mn phosphor was achieved by the pseudocombinatorial chemistry method (PCCM). The optimum ratio of BaO to Al 2 O 3 was first found in terms of Mn 2+ emission. As a result, the maximum luminance was obtained at the ratio of 1.3/6. Based on the host composition determined as Ba 1.3 Al 12 O 19.3 , the optimum Mn 2+ content was also obtained to be 0.3 mol. Consequently the final composition was determined to be Ba 1.3 All 1.7 Mn 0.3 O 18.85 , In parallel, Eu 2+ was also taken into consideration as a co-dopant, which enhances UV absorption efficiency, and simultaneously the energy transfer between Eu 2+ and Mn 2+ was investigated in terms of Eu 2+ and Mn 2+ concentration. The decay analysis based on a diffusion-controlled quenching scheme also carried out to get an insight into the exact quenching mechanism of Mn 2+ centers in BaAl 12 O 19 host.

Rate-equation model for the loading-rate-dependent mechanoluminescence of SrAl2O4:Eu2+ Dy3+

The mechanoluminescence (ML) of SrAl2O4:Eu2+,Dy3+ (SAO) phosphors was monitored as a function of its instantaneous loading rate, on which it was found to be strongly dependent. The effect of the loading rate on the ML of SAO was investigated in a systematic manner using rate equations involving the loading rate term. The rate equations and the experimental data matched well. We confirmed that the ML of SAO was created by the change in load rather than by the static load, and that the loading rate determined the shape of the ML versus the time curve in the transient regime.

Effect of Local Structures on the Luminescence of Li2 ( Sr , Ca , Ba ) SiO4 : Eu2 +

The baricenter and crystal field splitting of the 5d transition of the Eu 2+ activator were approximated from the experimental spectrum data for Li 2 SrSiO 4 , Li 2 CaSiO 4 , and Li 2 BaSiO 4 hosts and examined in conjunction with the ionic radii of neighboring alkali earth ions and the local structure around the divalent europium activator. In addition, Li2(Sr,Ca,Ba)SiO 4 :Eu 2+ ternary phosphors were screened in terms of photoluminescence (PL) intensity and color chromaticity at an excitation of 400 nm to determine their suitability for use in white-light-emitting diodes. As a result, several intermediate compounds (mixtures or solid solutions) were pinpointed in a specific composition range in the Li 2 (Sr,Ca,Ba)SiO 4 :Eu 2+ ternary composition library. In particular, Li2(Ba 1-x Sr x )SiO 4 :Eu 2+ (0.3 < x < 0.6) showed the best performance in terms of both PL intensity and color chromaticity.

Photoluminescent and decay behaviors of Mn2+ and Ce3+ coactivated MgSiN2 phosphors for use in light-emitting-diode applications.

We examined the photoluminescent behaviors of MgSiN2:Mn2+ and MgSiN2:Ce3+,Mn2+ phosphors for use in white-light-emitting diodes. The red emission from MgSiN2:Mn2+ phosphors consisted of two Gaussian components, P1 from a single Mn2+ ion and P2 from either Mn2+ pairs or clusters. Decay analysis based on the Yokota and Tanimoto equation identified long decay for P1 and fast decay for P2. Most importantly, Ce3+ codoping enhanced Mn2+ emission intensity; in particular, emission at 460 nm excitations was promoted by the Ce3+ codoping.