Woon Bae Park

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.

Radiative and non-radiative decay rate of K2SiF6:Mn4+ phosphors

Mn4+-activated K2SiF6 phosphors for use in light emitting diode (LED) applications have recently attracted a great deal of attention since they exhibit an advantage over conventional wide band-type red-light-emitting phosphors. K2SiF6:Mn4+ phosphors have shown extremely narrow emission peaks in wavelengths ranging from 620 to 630 nm, leading to a higher color-rendering index and larger color gamut for the final LED applications. We examined the decay behavior in terms of radiative and non-radiative rates along with a reliable evaluation of Mn4+ concentrations. Inter-activator energy transfer played a significant role in the luminescence process in this well-known narrow peak emission type of the red phosphor K2SiF6:Mn4+.

Mechanoluminescence of SrAl 2 O 4 :Eu 2+ , Dy 3+ under cyclic loading

The mechanoluminescence (ML) of SrAl2O4:Eu(+), Dy(3+) (SAO) has been of particular interest based on the possibility that these materials could be used as nondestructive, reproducible stress (or load) sensors. However, there has been no in-depth study of ML under a cyclic load. It was found that a cyclic load generated harmonics in the ML response. The second harmonic term exhibiting a doubled frequency was significant, but the others could be ignored. In addition, hysteresis behavior was observed in the ML and was examined by comparison with the hysteresis that is typical in piezoelectricity.

Combinatorial Screening of Luminescent and Structural Properties in a Ce3+-Doped Ln-Al-Si-O-N (Ln = Y, La, Gd, Lu) System: The Discovery of a Novel Gd3Al3+xSi3–xO12+xN2–x:Ce3+ Phosphor

The discovery of novel phosphors for use in light emitting diodes (LED) has gained in significance because LED-based solid-state lighting applications now attract a great deal of attention for energy savings and environmental concerns. Recent research trends have centered on the discovery of novel phosphors, not on slight variations of well-known phosphors. In a real sense, novelty goes beyond simple variations or improvements in existing phosphors. A brilliant strategy for the discovery of novel phosphors is to introduce an appropriate activator to existing inorganic compounds. These compounds have structures that are well-defined in crystallographic structure databases, but they have never been considered as a phosphor host. Another strategy is to discover new host compounds with structures that cannot be found in existing databases. We have simultaneously pursued both strategies by employing metaheuristics-assisted combinatorial material search techniques. In the present investigation, we screened a search space consisting of Ln-Al-Si-O-N (Ln = Y, La, Gd, Lu), and thereby we discovered a blue-light-emitting novel phosphor, Gd3Al(3+x)Si(3-x)O(12+x)N(2-x):Ce(3+), with a monoclinic system in the C2 space group--a potential candidate for UV-LED applications.

Nonradiative energy transfer between two different activator sites in La 4−x Ca x Si 12 O 3+x N 18−x :Eu 2+

Energy transfer, which affects the entire performance of luminescent material, has been generally treated as an averaged parameter by assuming the host material to be a homogeneous continuum. However, energy transfer should be investigated in association with the crystallographic local structure around an activator site. To accomplish this, we established an analytical model and derived comprehensive rate equations, elucidating the relationship between the local structure and energy transfer behavior of La(4-x)Ca(x)Si12O(3+x)N(18-x):Eu2+, which is a recently discovered luminescent material for use in light-emitting diodes. Using the rate-equation model with the assistance of particle swarm optimization, the full-scale decay curves of donors and acceptors located at different crystallographic sites was computed.

Solid-State Combinatorial Screening of ARSi4N7:Eu2+ (A = Sr, Ba, Ca; R = Y, La, Lu) Phosphors

A double-ternary combinatorial chemistry (combi-chem) library was visualized in terms of structure, PL intensity, and color chromaticity for a nitride phosphor system, ARSi4N7:Eu(2+) (A = Sr, Ca, Ba; R = Y, La, Lu), so as to obtain a quantitative structure and property relationship (QSPR) in a systematic manner. Most of the samples constituting the double-ternary combi-chem library turned out to have ARSi4N7 structures with a P63mc space group. However, several phases such as Ca2Si5N8 with a Cc space group, LaSi3N5 with a P212121 space group, R6Si11N20O with a P31c space group, etc., coexisted. Aside from the green luminescence from the well-known SrYSi4N7:Eu(2+) and BaYSi4N7:Eu(2+) phosphors, their solid solutions (Sr,Ba)Si4N7:Eu(2+) proved to possess better PL properties. In addition, novel phosphors with an acceptable green PL intensity and color chromaticity were discovered in the ALuSi4N7:Eu(2+) side of the double-ternary combi-chem library. The Ca-rich side did not constitute a single-phase ARSi4N7 structure with a P63mc space group, and therefore the red emission in the Ca-rich side proved to originate from well-known Ca2Si5N8:Eu(2+) phosphors, which resided in the sample as a minor phase.

The Composite Structure and Two-Peak Emission Behavior of a Ca1.5Ba0.5Si5O3N6:Eu2+ Phosphor

A Ca1.5Ba0.5Si5O3N6:Eu(2+) phosphor with a monoclinic lattice in the Cm space group exhibiting a composite structure consisting of CaSi2O2N2-like and BaSi6N8O-like structures was examined in terms of structure and luminescence. The luminescent properties of the Ca1.5Ba0.5Si5O3N6:Eu(2+) phosphor could be suitable for light-emitting diode applications since it exhibited a promising yellow (or amber) emission peaking at ∼ 570-590 nm at excitations of 450-460 nm. The present investigation was focused on verifying the composite structure by employing quantum mechanical calculations such as the Hartree-Fock ab initio calculation and a density functional theory calculation along with precise structural and compositional analyses. The two-peak emission behavior ascribed to the composite structure was also examined in terms of continuous wave and time-resolved photoluminescence. In addition, the energy transfer between two activator sites ascribed to the composite structure was examined in detail.

Phosphor informatics based on confirmatory factor analysis.

The theoretical understanding of phosphor luminescence is far from complete. To accomplish a full understanding of phosphor luminescence, the data mining of existing experimental data should receive equal consideration along with theoretical approaches. We mined the crystallographic and luminescence data of 75 reported Eu(2+)-doped phosphors with a single Wyckoff site for Eu(2+) activator accommodation, and 32 descriptors were extracted. A confirmatory factor analysis (CFA) based on a structural equation model (SEM) was employed since it has been helpful in understanding complex problems in social sciences and in bioinformatics. This first attempt at applying CFA to the data mining of engineering materials provided a better understanding of the structural and luminescent-property relationships for LED phosphors than what we have learnt so far from the conventional theoretical approaches.

Classification of crystal structure using a convolutional neural network

A deep-machine-learning technique based on a convolutional neural network (CNN) is introduced. It has been employed for the classification of crystal system, extinction group and space group for given powder X-ray diffraction patterns of inorganic materials.

Systematic Approach To Calculate the Band Gap Energy of a Disordered Compound with a Low Symmetry and Large Cell Size via Density Functional Theory

An ab initio calculation based on density functional theory (DFT) was used to verify the disordered structure of a novel oxynitride phosphor host, La4–xCaxSi12O3+xN18–x, with a large unit cell (74 atoms), low level of symmetry (C2), and large band gap (4.45 eV). Several Wyckoff sites in the La4–xCaxSi12O3+xN18–x structure were randomly shared by La/Ca and O/N ions. This type of structure is referred to as either partially occupied or disordered. The adoption of a supercell that is sufficiently large along with an infinite variety of ensemble configurations to simulate such a random distribution in a partially occupied structure would be an option that could achieve a reliable DFT calculation, but this would increase the calculation expenses significantly. We chose 5184 independent unit cell configurations to be used as input model structures for DFT calculations, which is a reduction from a possible total of 20 736 unit cell configurations for C2 symmetry. Instead of calculating the total energy as well as the band gap energy for all 5184 configurations, we pinpointed configurations that would exhibit a band gap that approximated the actual value by employing an elitist nondominated sorting genetic algorithm (NSGA-II) wherein the 5184 configurations were represented mathematically as genomes and the calculated total and band gap energies were represented as objective (fitness) functions. This preliminary screening based on NSGA-II was completed using a generalized gradient approximation (GGA), and thereafter, we executed a hybrid functional calculation (HSE06) for only the most plausible GGA-relaxed configurations with higher band gap energies and lower total energies. Finally, we averaged the HSE06 band gap energy over these selected configurations using the Boltzmann energy distribution and achieved a realistic band gap energy that more closely approximated the experimental measurement.