Optical, Structural and Paramagnetic Properties of Eu-Doped Ternary Sulfides ALnS2 (A = Na, K, Rb; Ln = La, Gd, Lu, Y)

Abstract

Eu-doped ternary sulfides of general formula ALnS2 (A = Na, K, Rb; Ln = La, Gd, Lu, Y) are presented as a novel interesting material family which may find usage as X-ray phosphors or solid state white light emitting diode (LED) lighting. Samples were synthesized in the form of transparent crystalline hexagonal platelets by chemical reaction under the flow of hydrogen sulfide. Their physical properties were investigated by means of X-ray diffraction, time-resolved photoluminescence spectroscopy, electron paramagnetic resonance, and X-ray excited fluorescence. Corresponding characteristics, including absorption, radioluminescence, photoluminescence excitation and emission spectra, and decay kinetics curves, were measured and evaluated in a broad temperature range (8–800 K). Calculations including quantum local crystal field potential and spin-Hamiltonian for a paramagnetic particle in D3d local symmetry and phenomenological model dealing with excited state dynamics were performed to explain the experimentally observed features. Based on the results, an energy diagram of lanthanide energy levels in KLuS2 is proposed. Color model xy-coordinates are used to compare effects of dopants on the resulting spectrum. The application potential of the mentioned compounds in the field of white LED solid state lighting or X-ray phosphors is thoroughly discussed.

Circadian Light Source Based on KxNa1-xLuS2:Eu2+ Phosphor

“…As both do not change significantly within a certain temperature range, which is our case (see 25,36 ), quantum yield in the first approximation can be estimated to be close to unity. Certainly, during rather complicated sample preparation, some parasitic absorption lowering the real quantum yield could be expected and therefore additional experiments will have to be provided in future research.…”

“…Based on these comments we believe we can, very conservatively, consider the quantum efficiency of our Eu 2+ -doped K x Na 1-x LuS 2 to be comparable to that of YAG:Ce 3+ , at least up to 500 K. 25 Naturally, the Stokes losses of Eu 2+ -doped K x Na 1-x LuS 2 for nearUV excitation (390 nm) will increase compared to that in the blue spectral range. However, YAG:Ce 3+ cannot be excited in this wavelength region at all due to the absence of Ce 3+ absorption bands.…”

“…However, taking into account the Stokes shift of YAG:Ce 3+ , which is approximately 500 meV, and the Stokes shift of our phosphor for limited composition KLuS 2 :Eu 2+ , which can be estimated roughly as 430 meV (if we consider the lowest Eu 2+ 4f-5d absorption band to be at around 440 nm 36 ), we can guess that thermal recombination losses will be comparable for both these phosphors. 25,36 The quantum efficiency of YAG:Ce 3+ under blue LED excitation is >85% even at 200…”

Tunable Eu2+ emission in KxNa1−xLuS2 phosphors for white LED application

“…Recently, a very interesting group of sulfide-based materials has been given attention as well [34][35][36][37][38][39][40][41][42][43][44][45][46]. Especially Eu 2+ -doped ALnS 2 (A_Na, K, Rb; Ln_La, Gd, Lu, Y), which can be produced in the form of transparent hexagonal single crystalline platelets, deserve attention as these compounds allow to cover emission range from 498 nm (RbLuS 2 :Eu 2+ ) to 776 nm (NaGdS 2 :Eu 2+ ) while having a broad absorption in the region of 360-460 nm due to 4f-5d transition of Eu 2+ , which matches both near-UV and blue diode very well [46]. Temperature stability of these phosphors up to at least 150°C has also been confirmed as it is one of the crucial parameters for white LED operation [46].…”

Nonequivalent lanthanide defects: Energy level modeling

“…Up to now, the described empirical rules and techniques for constructing charge-state transition level schemes have been applied to many host materials [16,17,18,19,20,21,22,23]. This can be unambiguously done for host materials in which all lanthanides act identically in chemical and crystallographic terms.…”