Origin of tunable emission wavelengths for (Sr 1-xCax) 0.95Ga2S4:0.05Eu and Sr1-yGa2S4:yEu phosphors

Highlights: • (Sr_1-xCa_x)_0.95Ga₂S₄:0.05Eu and Sr_1-yGa₂S₄:yEu phosphors were synthesized. • PL spectra and luminescence parameters of these phosphors were examined. • These phosphors showed promise as color-tunable phosphors for use in pc-LEDs. (Sr_1-xCa_x)_0.95Ga₂S₄:0.05Eu (x = 0, 0.25, 0.50, 0.75, 1) and Sr_1-yGa₂S₄:yEu (y = 0.05, 0.25, 0.50, 0.75, 1) phosphors were synthesized from CaS, SrS, EuS, and Ga₂S₃ by carrying out solid-state reactions. X-ray diffraction (XRD) results indicated that the (Sr_1-xCa_x)_0.95Ga₂S₄:0.05Eu and Sr_1-yGa₂S₄:yEu phosphors formed solid solutions between SrGa₂S₄ and CaGa₂S₄, and between SrGa₂S₄ and EuGa₂S₄, respectively. The photoluminescence excitation (PLE) and photoluminescence (PL) spectra of these phosphors were examined. The wavelength at maximum intensity (λ_max) of the PL spectra changed gradually from 537 to 559 nm as the calcium ion mole fraction (x) increased in the (Sr_1-xCa_x)_0.95Ga₂S₄:0.05Eu phosphors. λ_max changed from 537 to 545 nm as the europium mole fraction (y) in the Sr_1-yGa₂S₄:yEu phosphors increased. The luminescence parameters, such as the zero phonon line energy (E₀), Stokes shift (ΔS), and red shift (D), were calculated from the PLE and PL spectra of these phosphors. The origins of the wavelength changes were deduced from the luminescence parameters of these phosphors. The three-band white light emitting diode (WLED) was fabricated by coating the (Sr_0.5Ca_0.5)_0.95Ga₂S₄:0.05Eu as a greenish yellow-emitting phosphor and K₂SiF₆:Mn⁴⁺ as a red-emitting phosphor onto a commercial blue LED. The photoluminescence property of the three-band WLED was investigated.