Full-color tuning in europium doped phosphosilicate phosphors via adjusting crystal field modulation or excitation wavelength

Abstract

A series of Eu ions activated in Sr9-xLa1+x(PO4)7-x(SiO4)x (x = 1, 3, 5, 7) prepared from powder-based precursors have been investigated firstly. The luminescence evolution in response to crystal structure variations was investigated using X-ray diffraction, Rietveld refinement, photoluminescence (PL) and PL excitation spectra, and CIE coordination. As x increased from 1 to 7, the crystal structure changed from whitlockite to apatite, and the ligand bonds between the Eu2+ and O2− ions became shorter. Both crystal structures include two different kinds of Eu sites. The peak positions of the PL bands caused by the Eu2+ ions shifted from 416 nm to 505 nm as x increased with 365 nm excitation. This shift is mainly attributed to the fact that Eu2+ ions prefer higher energy sites in the whitlockite structure and lower energy sites in the apatite structure. The red PL originating from the trivalent europium ions appeared due to the increase in the La3+/Sr2+ ratio with increasing x. Under the various excitation wavelengths, the PL spectra changed tremendously. CIE coordination measurements and 2D PL contour map profiles demonstrated that the phosphor series could be tuned from blue to red by adjusting x, and a tunable phosphor covering full color could be achieved by adjusting the excitation wavelength. Moreover, the PL emission from the x = 1 sample provided an internal quantum efficiency of up to 100%.