Minseuk Kim

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+.

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.

A smart load-sensing system using standardized mechano-luminescence measurement.

The mechano-luminescence (ML) of phosphors has stirred a great deal of interest for its potential application in inexpensive, non-destructive load sensors. However, the most serious drawback of ML phosphors has been responses that differ according to the loading conditions. This has led to a lack of standardization in realizing smart ML sensor applications. We improved the applicability of ML phosphors to that of a smart, standardized load sensor by detecting ML based on the UV excitation above the threshold power density during the entire loading process. The ML behavior under these conditions was completely different from that of conventional ML behavior with UV excitation turned off. The ML output was clearly represented as a simple linear function of the applied load under conditions that could be either static or dynamic. In addition, neither a ML loss angle nor hysteresis behavior was observed under these ML measurement conditions.

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.

Metaheuristics-Assisted Combinatorial Screening of Eu2+-Doped Ca–Sr–Ba–Li–Mg–Al–Si–Ge–N Compositional Space in Search of a Narrow-Band Green Emitting Phosphor and Density Functional Theory Calculations

A metaheuristics-based design would be of great help in relieving the enormous experimental burdens faced during the combinatorial screening of a huge, multidimensional search space, while providing the same effect as total enumeration. In order to tackle the high-throughput powder processing complications and to secure practical phosphors, metaheuristics, an elitism-reinforced nondominated sorting genetic algorithm (NSGA-II), was employed in this study. The NSGA-II iteration targeted two objective functions. The first was to search for a higher emission efficacy. The second was to search for narrow-band green color emissions. The NSGA-II iteration finally converged on BaLi2Al2Si2N6:Eu2+ phosphors in the Eu2+-doped Ca-Sr-Ba-Li-Mg-Al-Si-Ge-N compositional search space. The BaLi2Al2Si2N6:Eu2+ phosphor, which was synthesized with no human intervention via the assistance of NSGA-II, was a clear single phase and gave an acceptable luminescence. The BaLi2Al2Si2N6:Eu2+ phosphor as well as all other phosphors that appeared during the NSGA-II iterations were examined in detail by employing powder X-ray diffraction-based Rietveld refinement, X-ray absorption near edge structure, density functional theory calculation, and time-resolved photoluminescence. The thermodynamic stability and the band structure plausibility were confirmed, and more importantly a novel approach to the energy transfer analysis was also introduced for BaLi2Al2Si2N6:Eu2+ phosphors.

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

Correction for ‘Radiative and non-radiative decay rate of K2SiF6:Mn4+ phosphors’ by Minseuk Kim et al., J. Mater. Chem. C, 2015, 3, 5484–5489.