CMSS Colloquium | High Pressure Study Of LED Phosphors Doped With Ln³⁺ And Ln²⁺, Oct. 20

The Condensed Matter & Surface Sciences Colloquium Series presents Marek Grinberg of University of Gdansk, Poland on “High Pressure Study Of LED Phosphors Doped With Ln³⁺ And Ln²⁺,” on Thursday, Oct. 20, at 4:10 p.m. in Walter 245.

Marek Grinberg

Abstract: Doubly and triply ionized lanthanide ions (Ln²⁺ and Ln³⁺) create the luminescence centers in solids that emit due to the internal 4fⁿ® 4fⁿ and 4f^n-15d¹® 4fⁿ transitions. The most important for performance such materials as phosphors for light emitting diodes (LED) is the high quantum efficiency, that means that most of the absorbed UV or blue photons generated by semiconductor p-n junction after absorption by phosphor is transformed and emitted as green, yellow and red light. This is a reason why the efficient phosphor lattice should be characterized by the energy of the forbidden gap, E_g, much larger than the energy of Ln ions luminescence and additionally the excited emitting state of Ln ions should be located much below of the bottom of the host conduction band.  The Ln doped phosphor emission can be excited due to internal absorption (parity allowed 4fⁿ ®4f^n-15d¹ transition), by band –to- band excitation, by charge transfer transition (CT), and by ionization process. In the last three cases the fundamental for effective excitation energy transfer from lattice to the impurity is existence of the intermediate states; impurity trapped exciton (ITE).

The talk presents the spectroscopic investigation of luminescence materials doped with several ions: Ce³⁺, Pr³⁺, Tb³⁺, Dy³⁺ Eu³⁺ and Eu²⁺. The leading, unique experimental technique was high pressure spectroscopy where high hydrostatic pressure is applied in diamond anvil cell (DAC). High hydrostatic pressure compresses the materials and leads to increase of interaction of ion with lattice. This changes the energies localized states related to the Ln²⁺⁽³⁺⁾ ions and energies of the band edges. Influence of pressure on luminescence related to 4f^n-15d¹®4fⁿ transition in Eu²⁺ and Ce³⁺ and anomalous luminescence related to ITE recombination in the case of Eu²⁺ doped fluorides is presented and discussed. Also, influence of pressure on the ITE states and energy transfer processes in phosphors doped Ln³⁺ and Ln²⁺ ions is analyzed. Specifically observed pressure induced quenching of the f-f luminescence in Pr³⁺ and Tb³⁺ and pressure induced activation of the luminescence of Eu³⁺ ions is discusses using the model of ITE. It is also elucidated how obtained high pressure data allows to determine locations the ground states of the Ln²⁺⁽³⁺⁾ ions with respect to band edges and with respect to the vacuum level.

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