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Physics Seminar Series 9/30/16

September 30, 2016 - 10:30am

Flyer File

Ram Seshadri, University of California, Santa Barbara


In the first part, I will address phosphors that play a key role in the now almost-mature solid-state white-lighting technologies based on combining a III-nitride-based near-UV or blue solid-state light source with down-conversion to longer wavelengths.[1] Almost all widely used phosphors comprise a crystalline oxide, nitride, or oxynitride host that is appropriately doped with either Ce3+ or Eu2+. Optical excitation into these states and concomitant reemission can be tuned into the appropriate regions of the visible spectrum by the crystal these ions are hosted in. Experimental studies of some of the best phosphor materials, employing state-of-the-art structural tools, have yielded guidelines for what are desirable structural features. We find that a useful sorting diagram for efficient hosts with high quantum yield has the band gap of the host – readily calculated with high reliability using hybrid functionals in DFT – as one of the axes, and the calculated Debye temperature as the other axis.[2]

In the second part, I will describe the creation of a large database of thermoelectric materials prepared by abstracting information from over 100 publications. Appropriate visualization of the data immediately allows certain insights to be gained with regard to the property space of plausible thermoelectric materials. The Herfindahl–Hirschman index – a commonly accepted measure of market concentration and monopoly – has been calculated from geological data (known elemental reserves) and geopolitical data (elemental production) for much of the periodic table. The visualization strategy employed allows rapid sorting of thermoelectric compositions with respect to important issues of elemental scarcity and supply risk.[3]

[1] N. C. George, K. A. Denault, and R. Seshadri, Phosphors for solid-state white lighting, Annu. Rev. Mater.   Res. 43 (2013) 481–501.

[2] J. Brgoch, S. P. DenBaars, and R. Seshadri, Proxies from ab-initio calculations for screening efficient Ce3+ phosphor hosts, J. Phys. Chem. C 117 (2013) 17955–17959.

[3] M. W. Gaultois, T. D. Sparks, C. K. H. Borg, R. Seshadri, W. D. Bonificio, and D. R. Clarke, A data-driven review of thermoelectric materials: Performance and resource considerations, Chem. Mater. 25 (2013) 2911–2920.


Classroom and Office Building 267

Contact Information

Linda Hirst
School of Natural Sciences, Physics