2, University of California, Santa Barbara, Santa Barbara, California, United States
Fluorescent materials have attracted increasing interest for their use in new generation of low energy lighting.[1,2] However, the vast majority of the fluorescent materials currently employed in commercial LED lighting are based on rare earth elements (REE)– particularly in the yellow, orange and red regions of the emissive visible spectrum. The US Department of Energy currently has the development of new low 600 nm wavelengths fluorescent materials as a priority. Furthermore the location and the extraction of rare earth elements remain major geopolitical, environmental, and economic issues of the near future. One solution to these issues would be to develop cheap, sustainable, REE-free, fluorescent materials that exhibit excellent durability and reasonably high emission efficiencies in the key 590-620 nm region.
The species we have identified for this purpose is the simple [S2−] anion. This anion occurs naturally, trapped in certain minerals, including sodalites and scapolites. The low level of [S2−] present produces a very strong orange/yellow/red fluorescence when irradiated with wavelengths between 350 and 450 nm. [3–6] Our work concerns the synthesis of new materials containing [S2−] and optimization of their fluorescence efficiencies to levels equivalent to, and better than, those of the natural minerals. In particular we have investigated the synthesis and properties of sodalite, Na8[AlSiO4]6(Cl,SO4,S2), with various ratio of sulfate, disulfide and chloride anions. The results of this systematic optimization of the fluorescent properties of Na8[AlSiO4]6(Cl,SO4,S) compositions will be presented.
Optimized materials show a very strong fluorescence, following 390 nm excitation, centred on 615 nm with quantum yields up to 40% - comparable to natural mineral samples. Modifications to the exact sodalite composition also allow the emission frequency maximum to be tuned between 585 and 620 nm. We continue to develop our understanding of the [S2−] host structures and thus the relationships that exist between the different crystallographic structures, composition and their light emission properties. Full development of these materials should allow the design of very cheap, sustainable, fluorescent materials with high efficiencies and the additional ability to tune the emission over a large wavelength range in the yellow–orange-red region.
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 A. Sidike, I. Kusachi, S. Kobayashi, K. Atobe, N. Yamashita, Phys. Chem. Miner. 2008, 35, 137–145.
 M. Kaiheriman, A. Maimaitinaisier, A. Rehiman, A. Sidike, Phys. Chem. Miner. 2014, 41, 227–235.