Spectral Tuning and Characterization of Cuboid Nitride Phosphors

• Main Authors: Julius Jr. Liclican Leano, 李岸歐
• Other Authors: Ru-Shi Liu
• Format: Others
• Language: en_US
• Published: 2018
• Online Access: http://ndltd.ncl.edu.tw/handle/kv2d67

博士 === 國立臺灣大學 === 化學研究所 === 106 === Nitride phosphors emerged in the forefront of phosphor research and development due to the thermal and chemical stability derived from the various structural motifs that it could form. Doping with rare earth elements such as Eu2+ gives rise to red emission that is vital in improving the color rendering capacity of white light emitting diodes. The UCr4C4-type nitrides are interesting phosphors with condensed framework (host) that gives rise to eight-nitrogen symmetric cuboid coordination site occupied by a cation and to which rare earth elements (activator) could be doped into. The optimization of the synthesis and preparation of these cuboid nitride phosphors via solid state reaction approach takes off from the rather long and multi-step radiofrequency approach. Using all-nitride starting materials, synthesis was done at high pressure and temperature affording pure-phase luminescent products. Spectral tuning was achieved by chemical tuning of the phosphors through the partial or full substitution of the activator (activator substitution) in the cuboid site, and/or the nitride framework (framework editing); and/or the cation that constitutes the host (cation tuning and co-doping). These three approaches were the gateways in unraveling the chemical, spectral and thermal behavior of these nitride phosphors. Activator substitution with use of Ce3+ instead of Eu2+ reveals completely different spectral properties. The spin-orbit coupled ground state of Ce3+ gives rise to a broad emission band that spans up to the red region offering interesting spectral properties towards practical lighting applications. a green-light excitable property with a broad emission band peaking at 580 and 620 nm. The assembly of an LED package through the sequential coating of a green phosphor (β-SiAlON), followed by Ce3+-doped nitride phosphor generated white light. Framework editing towards the improvement of thermal properties pertains to the chemical composition of the host. While maintaining the structure and electrical neutrality, (Mg2+-Al3+) couple is partially substituted with (Li+-Si4+) on Sr[Mg2Al2N4]. The solid solution generation through high-pressure solid state reaction demonstrated how thermal and photoluminescence properties can be improved by this chemical tuning strategy. Relative band broadening has also been correlating structural and photoluminescence behavior despite a single emitting crystallographic site. The cation tuning whereby the Sr2+ cation that occupies the cuboid site by substituting with the larger Ba2+ bringing along smaller thermal vibration frequency, enhanced the emission and systematically shifted it to the red region between 620-690 nm. This enables the development of tunable deep red phosphor for agricultural applications. Further insight into the enhanced thermal stability has been investigated. The unusual redshift with increasing Ba in the same structure reveals how the reduction of symmetry due to size mismatch explained the unusual redshift. Co-doping was explored whereby Tm3+ showed that Ba[Mg2Al2N4]:Eu2+could be tailored for near-infrared applications. The energy transfer from Eu2+ to Tm3+ extended the emission to the first biological window (~800 nm) thereby demonstrating potential use beyond LED lighting. The simplified preparation of this cuboids nitrides via solid-state approach enhances their upscale production, investigation, and use. Through this chemical tuning approaches, these cuboid phosphors have revealed interesting new luminescence properties, and mechanisms and insights that accounts for these have been offered. The systematic investigation of the effects of several compositional changes redounds to structural changes, spectral tuning and thermal improvement which are collectively vital in the design and development of phosphors for lighting applications and beyond.