The Eu2+^{2+}-doped chlorometasilicate (Ca1−x−ySrxEuy)7(SiO3)6Cl2(Ca_{1-x-y}Sr_xEu_y)_7(SiO_3)_6Cl_2 (abbreviated Cl_MS:Eu2+^{2+}) shows yellow 580-nm emission from Eu2+^{2+} at the M(3) site with an internal quantum efficiency of 94% and, remarkably, no observable concentration quenching of that band up to roughly 20 atom% Eu, while the 480-nm band from Eu2+^{2+} at the M(2) site of the same host is drastically concentration-quenched. The discoverers stated explicitly that they could not explain, on the basis of their experiments, why the M(3) site escapes concentration quenching, and only suggested that local site asymmetry and the low-dimensional arrangement of the dopant sites control the energy-transfer processes. The open question is to establish microscopically and quantitatively why Eu2+^{2+} at the M(3) site of this layered mixed-ligand host avoids concentration quenching at dopant levels roughly an order of magnitude above conventional optima, while the M(2) site in the same crystal does not. Existing work on this host is application-oriented, and general frameworks for concentration quenching (energy migration to quenchers, activator-site separation) have not been applied to the M(2)/M(3) contrast in this material.

Background

Phosphor-converted white light-emitting diodes (pc-LEDs) use a semiconductor chip (typically blue or near-ultraviolet) to excite a luminescent powder, the phosphor, whose broad-band emission combines with the chip light to make white light. The most common activator ion, Eu2+^{2+}, emits via parity-allowed transitions from relaxed 4f65d14f^65d^1 excited states to the 4f74f^7 ground state; because the 5d electron interacts strongly with the host lattice, the emission wavelength and the absorption (excitation) band depend sensitively on the crystal site the ion occupies. Concentration quenching is the standard loss channel that limits how heavily a phosphor can be doped: as the activator concentration grows, resonant energy transfer between neighboring activator ions (usually electric multipolar, i.e. dipole-dipole or higher-order, interaction as described by Dexter's theory) allows the excitation to migrate from ion to ion until it reaches a quenching center or 'killer' site (a defect, an unreduced Eu3+^{3+} ion, or the surface) where it decays non-radiatively. In most practical Eu2+^{2+} phosphors this restricts the optimum dopant level to a few mol%, above which the internal quantum efficiency (IQE, the fraction of absorbed photons re-emitted) falls steeply. Thermal quenching, the other main loss channel, is the thermally activated non-radiative decay of the excited ion itself. In 2012, Daicho and co-workers designed a layered chlorometasilicate host, (Ca1−x−ySrxEuy)7(SiO3)6Cl2(Ca_{1-x-y}Sr_xEu_y)_7(SiO_3)_6Cl_2 ('Cl_MS'), consisting of alternating metal-silicate and metal-chloride layers, with three distinct divalent-cation crystallographic sites: M(1) inside the silicate layer, M(2) in the metal-chloride layer, and M(3) at the boundary between the layers, where the Eu2+^{2+} ion is coordinated by a mixed ligand sphere of one bridging oxygen, four non-bridging oxygen ions and two chloride ions. The asymmetric coordination at M(3) produces a large crystal-field splitting of the Eu2+^{2+} 5d levels, giving strong violet excitation, weak blue absorption and a large Stokes shift (the gap between excitation and emission energies, here about 6.9×103 cm−16.9\times10^{3}\,\mathrm{cm}^{-1}) — properties that made Cl_MS:Eu2+^{2+} useful for low-glare, large-area white LEDs. The two inequivalent Eu2+^{2+} sites give two emission bands: a 480-nm band assigned to Eu2+^{2+} at M(2), and a 580-nm band assigned to Eu2+^{2+} at M(3), an assignment supported by temperature-dependent decay lifetimes and synchrotron X-ray diffraction bond-length analysis. Strikingly, the 580-nm M(3) band showed no concentration quenching up to about 20 atom% Eu2+^{2+}: its emission intensity is maximal near 10 atom% Eu2+^{2+} (the optimum composition, IQE 94%), yet its efficiency does not decline steeply out to roughly 20 atom%, whereas the 480-nm M(2) band was drastically concentration-quenched. Density-functional-theory calculations of the electric-field-gradient (EFG) asymmetry parameter at the two sites indicated a substantially more asymmetric local environment at M(3), and the authors noted prior reports that phosphors whose dopant ions occupy a low-dimensional (layered- or chain-like) arrangement show little concentration quenching, including fully stoichiometric phosphors. They offered this structural argument only as a consistency observation, not an established mechanism, and explicitly stated that they could not explain the phenomenon on the basis of their experimental results.

Problem Statement

Explain, microscopically and quantitatively, why Eu2+^{2+} ions at the M(3) crystallographic site of the layered chlorometasilicate (Ca1−x−ySrxEuy)7(SiO3)6Cl2(Ca_{1-x-y}Sr_xEu_y)_7(SiO_3)_6Cl_2 show essentially no concentration quenching of their 580-nm 4f65d1→4f74f^65d^1\rightarrow4f^7 emission up to roughly 20 atom% Eu2+^{2+} (internal quantum efficiency 94% at 10 atom%), while Eu2+^{2+} ions at the M(2) site of the very same host are drastically concentration-quenched in their 480-nm emission. Concretely: identify the radiative and non-radiative decay pathways and the inter-site and intra-site Eu2+^{2+}-Eu2+^{2+} energy-transfer (migration) channels and their spatial range as functions of dopant concentration; determine the site-specific Eu2+^{2+} occupancy distribution on M(1), M(2) and M(3) as Eu content increases; establish whether the large local asymmetry of the M(3) site and the low-dimensional arrangement of M(3) dopant positions genuinely suppress the non-radiative energy-transfer network (and with what quantitative effect on the transfer rates or critical distances), or whether the absence of quenching instead stems from another mechanism such as a low density of quenching centers in these samples or the energetics of the Eu2+^{2+} 5d levels relative to the host bands; and quantitatively reproduce the observed site contrast (no quenching of the 580-nm band versus drastic quenching of the 480-nm band).

References

  1. Hisayoshi Daicho, Takeshi Iwasaki, Kiminori Enomoto, Yasutaka Sasaki, Yuzo Maeno, Yu Shinomiya, Shinobu Aoyagi, Eiji Nishibori, Makoto Sakata, Hiroshi Sawa, Satoru Matsuishi, Hideo Hosono, 'A novel phosphor for glareless white light-emitting diodes', Nature Communications 3, 1132 (2012), DOI 10.1038/ncomms2138, https://doi.org/10.1038/ncomms2138
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