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Open Access Publications from the University of California

Scholarly Works (3 results)

  • Thesis
  • Peer Reviewed
UC San Diego Electronic Theses and Dissertations (2019)

White-emitting light sources based on light-emitting diodes (white-emitting LEDs) are considered to be the next generation in lighting. A common approach to create a white-emitting LED is to combine a blue-emitting InGaN LED with a yellow-emitting phosphor, but it has a low color rendering index (CRI) value due to the rack of red emission. An alternative approach is to combine a near UV-LED (nUV-LED) with blue-, green-, and red-emitting phosphors. However, the development of phosphors for the white-emitting LEDs is still challenging due to the numerous factors that influence on the luminescent properties. In this research, five approaches have been investigated to design and synthesize phosphors for solid state lighting:

First, the Debye temperature (DT) was used as a descriptor for phosphors with high quantum efficiency (QE) (> 80%). Compositions with DT > 500K were considered. CaMgSi2O6:Eu2+ and Ca7Mg(SiO4)4:Eu2+ were synthesized through a co-precipitation and a sol-gel reactions, respectively, due to the high DT. However, a correlation between high DT and high QE was not found.

Second, a flux (NH4F, NH4Cl, or H3BO3) was introduced during co-precipitation synthesis of CaMgSi2O6:Eu2+ to increase the QE. The flux affected both the crystallite size and the QE of the submicron sized phosphors. It was found that the QE increased 3X compared to no flux.

Third, newly identified phosphors, green-emitting Sr2LiAlO4:Eu2+ and blue-emitting Sr2LiAlO4:Ce3+ were discovered by data mining unexplored chemistries for potentially high QE. The new, Sr2LiAlO4:Eu2+ and Sr2LiAlO4:Ce3+ were successfully predicted and synthesized.

Fourth, color tunable single phase phosphors were developed using the Eu2+ and Ce3+ co-activated Sr2LiAlO4 for improvement of QE. The QE increased by 40% with Eu2+ and Ce3+ co-activation compared to the singly activated phosphors.

Fifth, the synthesis of Na2SiF6 was demonstrated through a green synthetic method without toxic HF that is typically used for fluorides synthesis. The red-emitting phosphor, Na2SiF6:Mn4+ was prepared in a low concentration HF solution using the synthesized Na2SiF6 as a host.

This study provides strategies to design and synthesize phosphors with improved quantum efficiency and thermal stability for white-emitting near-UV LEDs for potential applications in solid state lighting.

    Cover page: Design and synthesis of phosphors to improve efficiency for solid state lighting application
    • Article
    • Peer Reviewed
    LBL Publications (2017)
    Crassulacean acid metabolism (CAM) is a water-use efficient adaptation of photosynthesis that has evolved independently many times in diverse lineages of flowering plants. We hypothesize that convergent evolution of protein sequence and temporal gene expression underpins the independent emergences of CAM from C3 photosynthesis. To test this hypothesis, we generate a de novo genome assembly and genome-wide transcript expression data for Kalanchoë fedtschenkoi, an obligate CAM species within the core eudicots with a relatively small genome (~260 Mb). Our comparative analyses identify signatures of convergence in protein sequence and re-scheduling of diel transcript expression of genes involved in nocturnal CO2 fixation, stomatal movement, heat tolerance, circadian clock, and carbohydrate metabolism in K. fedtschenkoi and other CAM species in comparison with non-CAM species. These findings provide new insights into molecular convergence and building blocks of CAM and will facilitate CAM-into-C3 photosynthesis engineering to enhance water-use efficiency in crops.
      Cover page: The Kalanchoë genome provides insights into convergent evolution and building blocks of crassulacean acid metabolism
      • Article
      • Peer Reviewed
      LBL Publications (2015)
      Crassulacean acid metabolism (CAM) is a specialized mode of photosynthesis that features nocturnal CO2 uptake, facilitates increased water-use efficiency (WUE), and enables CAM plants to inhabit water-limited environments such as semi-arid deserts or seasonally dry forests. Human population growth and global climate change now present challenges for agricultural production systems to increase food, feed, forage, fiber, and fuel production. One approach to meet these challenges is to increase reliance on CAM crops, such as Agave and Opuntia, for biomass production on semi-arid, abandoned, marginal, or degraded agricultural lands. Major research efforts are now underway to assess the productivity of CAM crop species and to harness the WUE of CAM by engineering this pathway into existing food, feed, and bioenergy crops. An improved understanding of CAM has potential for high returns on research investment. To exploit the potential of CAM crops and CAM bioengineering, it will be necessary to elucidate the evolution, genomic features, and regulatory mechanisms of CAM. Field trials and predictive models will be required to assess the productivity of CAM crops, while new synthetic biology approaches need to be developed for CAM engineering. Infrastructure will be needed for CAM model systems, field trials, mutant collections, and data management.
        Cover page: A roadmap for research on crassulacean acid metabolism (CAM) to enhance sustainable food and bioenergy production in a hotter, drier world
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