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Open Access Publications from the University of California

Scholarly Works (5 results)

  • Thesis
  • Peer Reviewed
UC San Diego Electronic Theses and Dissertations (2018)

The discovery of novel phosphors is key to the development of highly efficient and environmental friendly light-emitting diodes (LED)-based solid-state lighting. This discovery, however, has largely taken place through painstaking experiments in an Edisonian fashion. In this thesis, we use first-principles calculations to explore new phosphor materials and advance our understanding of photoluminescence (PL) properties.

Phosphors with narrow-band emission are a critical component for high brightness LEDs and liquid crystal display (LCD) backlighting with wide color gamut. In the first part, we discovered a quantitative descriptor for narrow-band Eu2+-activated emission by comparing electronic structures of known narrow-band and broad-band phosphors. Incorporating this descriptor into a high throughput first principles screening of 2,259 nitride compounds, we identified five promising new nitride hosts for Eu2+-activated red-emitting phosphors that are predicted to exhibit good chemical stability, thermal quenching resistance and quantum efficiency, as well as narrow-band emission.

In the second part, we performed a systematic investigation of structure-composition-property relationships in Eu2+-activated β-SiAlON, one of the most promising narrow-band green phosphors. Using first-principles calculations, we first identified the most energetically favorable structure for β-SiAlON:Eu2+ and then studied the effect of oxygen content and Eu2+ activator concentrations on the local EuN9 activator environment, and its impact on important PL properties such as emission peak position, bandwidth, and thermal quenching resistance. The insights obtained provide a constructive means to optimize the PL performance of β-SiAlON in experiment.

In the third part, we developed an approach to discover new oxide phosphors by mining unexplored chemistries with data-driven structure prediction and high-throughput screening. This approach was demonstrated by the prediction and experimental validation of a novel, earth-abundant Eu2+ and Ce3+-activated Sr2LiAlO4 phosphors. The Sr2LiAlO4:Eu2+/Ce3+ phosphors exhibit broad emissions at λmax ~ 512 nm (green-yellow) and λmax ~ 434 nm (blue), respectively, with excellent thermal quenching resistance of > 88% intensity at 150 C. A prototype white LED utilizing Sr2LiAlO4-based phosphors yields an excellent color rendering index exceeding 90.

    Cover page: Design and Optimization of Phosphors for Solid-State Lighting using First-Principles Calculations
    • Article
    • Peer Reviewed
    UC San Diego Previously Published Works (2016)
    Narrow-band red-emitting phosphors are a critical component of phosphor-converted light-emitting diodes for highly efficient illumination-grade lighting. In this work, we report the discovery of a quantitative descriptor for narrow-band Eu2+-activated emission identified through a comparison of the electronic structures of known narrow-band and broad-band phosphors. We find that a narrow emission bandwidth is characterized by a large splitting of more than 0.1 eV between the two highest Eu2+ 4f7 bands. By incorporating this descriptor in a high-throughput first-principles screening of 2259 nitride compounds, we identify five promising new nitride hosts for Eu2+-activated red-emitting phosphors that are predicted to exhibit good chemical stability, thermal quenching resistance, and quantum efficiency, as well as narrow-band emission. Our findings provide important insights into the emission characteristics of rare-earth activators in phosphor hosts and a general strategy to the discovery of phosphors with a desired emission peak and bandwidth.
      Cover page: Electronic Structure Descriptor for the Discovery of Narrow-Band Red-Emitting Phosphors
      • Article
      • Peer Reviewed
      UC San Diego Previously Published Works (2018)
      Predicting the stability of crystals is one of the central problems in materials science. Today, density functional theory (DFT) calculations remain comparatively expensive and scale poorly with system size. Here we show that deep neural networks utilizing just two descriptors-the Pauling electronegativity and ionic radii-can predict the DFT formation energies of C3A2D3O12 garnets and ABO3 perovskites with low mean absolute errors (MAEs) of 7-10 meV atom-1 and 20-34 meV atom-1, respectively, well within the limits of DFT accuracy. Further extension to mixed garnets and perovskites with little loss in accuracy can be achieved using a binary encoding scheme, addressing a critical gap in the extension of machine-learning models from fixed stoichiometry crystals to infinite universe of mixed-species crystals. Finally, we demonstrate the potential of these models to rapidly transverse vast chemical spaces to accurately identify stable compositions, accelerating the discovery of novel materials with potentially superior properties.
        Cover page: Deep neural networks for accurate predictions of crystal stability
        • Article
        • Peer Reviewed
        UC San Diego Previously Published Works (2018)
          Cover page: Correction to Insights into the Performance Limits of the Li7P3S11 Superionic Conductor: A Combined First-Principles and Experimental Study
          • Article
          • Peer Reviewed
          LBL Publications (2022)
          Guided by computational Pourbaix screening and high-throughput experiments aimed at the development of precious-metal-free fuel cells, we investigate rutile CoSb2O6 as an electrocatalyst for oxygen reduction in 1 M sulfuric acid. Following 4 h of catalyst conditioning at 0.7 V vs RHE, operation at this potential for 20 h yielded an average current density of −0.17 mA cm-2 with corrosion at a rate of 0.04 nm hour-1 that is stoichiometric with catalyst composition. Surface Pourbaix analysis of the (111) surface identified partial H coverage under operating conditions. The Sb active site has an HO* binding free energy of 0.49 eV, which is near the peak of the kinetic 4e- ORR volcano for transition-metal oxides in acidic conditions. The experimental demonstration of operational stability and computational identification of a reaction pathway with favorable energetics place rutile CoSb2O6 among the most promising precious-metal-free electrocatalysts for oxygen reduction in acidic media.
            Cover page: Stability and Activity of Cobalt Antimonate for Oxygen Reduction in Strong AcidCreative Commons 'BY-NC' version 4.0 license
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