Phosphor informatics based on confirmatory factor analysis

Exploring new useful phosphors by combining experiments with machine learning

New phosphors are consistently in demand for advances in solid-state lighting and displays. Conventional trial-and-error exploration experiments for new phosphors require considerable time. If a phosphor host suitable for the target luminescent property can be proposed using computational science, the speed of development of new phosphors will significantly increase, and unexpected/overlooked compositions could be proposed as candidates. As a more practical approach for developing new phosphors with target luminescent properties, we looked at combining experiments with machine learning on the topics of emission wavelength, full width at half maximum (FWHM) of the emission peak, temperature dependence of the emission spectrum (thermal quenching), new phosphors with new chemical composition or crystal structure, and high-throughput experiments.

Dissimilarity measure of local structure in inorganic crystals using Wasserstein distance to search for novel phosphors

To efficiently search for novel phosphors, we propose a dissimilarity measure of local structure using the Wasserstein distance. This simple and versatile method provides the quantitative dissimilarity of a local structure around a center ion. To calculate the Wasserstein distance, the local structures in crystals are numerically represented as a bag of interatomic distances. The Wasserstein distance is calculated for various ideal structures and local structures in known phosphors. The variation of the Wasserstein distance corresponds to the structural variation of the local structures, and the Wasserstein distance can quantitatively explain the dissimilarity of the local structures. The correlation between the Wasserstein distance and the full width at half maximum suggests that candidates for novel narrow-band phosphors can be identified by crystal structures that include local structures with small Wasserstein distances to local structures of known narrow-band phosphors. The quantitative dissimilarity using the Wasserstein distance is useful in the search of novel phosphors and expected to be applied in materials searches in other fields in which local structures play an important role.

A data-driven approach to predicting band gap, excitation, and emission energies for Eu2+-activated phosphors

An integrated ML model platform is developed to predict the peak emission wavelength (PEW), excitation band edge wavelength (EBEW), and band gap (Eg) from structural, elemental, chemical, and physical descriptors of Eu2+-activated phosphors.

Adjusting the structure and luminescence properties of Sr2-x Bax MgAl22 O36 :Eu2+ phosphors by Sr:Ba ratio

Europium ion (Eu²⁺ )-doped phosphors exhibit adjustable photoluminescence due to the sensitivity of their luminescence to the local environment. It is of great significance to adjust the luminescence of Eu²⁺ by changing their local environment in the host. In this work, we investigated the effect of strontium/barium (Sr:Ba) ratio on the structure and luminescence properties of Sr_2-x Ba_x MgAl₂₂ O₃₆ :Eu²⁺ phosphors. Our investigation indicates that with the decrease of Sr:Ba ratio, the matrix lattice gradually expands and the peak wavelength for the luminescence of Eu²⁺ presents an obvious blue shift. The occupancy of Eu²⁺ was analyzed and the reason for the blue shift was explained. Thermal stability for the luminescence of Eu²⁺ can also be adjusted by changing the Sr:Ba ratio. This work has a positive effect on the regulation of the emission of phosphors and the improvement of thermal stability, which will promote the application of Sr_2-x Ba_x MgAl₂₂ O₃₆ :Eu²⁺ phosphors in the field of white light emitting diodes.

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.

Factor Analysis of Conformations and NMR Signals of Rotaxanes: AIMD and Polarizable MD Simulations

The interlocked ⟨rod | ring⟩ structures of pseudorotaxanes and [2]rotaxanes are usually maintained by the complex hydrogen-bonding (H-bonding) network between the rod and ring. Ab initio molecular dynamics (AIMD) using generalized energy-based fragmentation approach and polarizable force field (polar FF)-based molecular dynamics (MD) simulations were performed to investigate the conformational changes of mechanically interlocked systems and to obtain the ensemble-averaged NMR chemical shifts. Factor analysis (FA) demonstrates that the ring H-donor (2,6 pyridinedicarboxamide group) plays an important role in the ring-rod recognition. In comparison to the conventional fixed-charge force field, the polarization effect is crucial to account for the H-bonding interactions in supramolecular systems. In the hybrid scheme, the polar FF-based MD simulations are used to generate different initial states for the AIMD simulations, which are able to give better prediction of ensemble-averaged NMR signals for chemically equivalent amide protons. The magnitude of the deshielding shift of NMR signal is correlated with the length of hydrogen bond. The polar FF model with variable charges shows that the dipole-dipole interactions between the flexible diethylene glycol chain of ring and polar solvents induce the upfield shifts of NMR signals of rod H-donors and the directional distribution of the neighboring CH3CN solvents.

Recent developments in the new inorganic solid-state LED phosphors

The emerging new solid-state LED phosphors and the methodologies for their development have been reviewed in this perspective.