Introduction
White light emitting diode (LED) is widely used as a high-quality lighting source, and its performance depends critically on the selected light-emitting materials. Early white LEDs were mainly composed of a combination of blue light chips and yellow light emitting phosphors, but suffered from low color rendering index, high color temperature, and blue light hazards. To solve these problems, researchers have turned to improving the color rendering performance of white LEDs by using red phosphors that can be excited by blue light.Professor Zhiguo Xia's group at the State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, has proposed a new material design principle for Eu2+-activated oxide phosphors that are difficult to obtain red light emission: Eu2+ occupying a low coordination number of polyhedra can cause a large crystal field cleavage, which can in turn realize red light or even near-infrared emission. A novel Sr3Ga4O9 phosphor matrix with multiple low-coordination cation lattice sites was designed and synthesized, and broadband red light emission under 450 nm blue light excitation was successfully achieved by doping Eu2+, with the emission peak located at 618 nm.
Methods
Sr3Ga4O9:Eu2+ and Sr3Ga4O9:Eu2+, 0.05Zn2+ phosphors were prepared by high-temperature solid-phase method. According to the proportional content of the different raw materials in the chemical formula, SrCO3 (A.R.: 99.5% analytically pure), Ga2O3 (A.R.), H3BO3 (A.R.) and Eu2O3 (99.99%) were weighed and ground in an agate mortar for half an hour. The mixture was transferred to an alumina crucible and fired at 900 ℃ in an air atmosphere for 6 h, then cooled naturally to room temperature. The mixture was ground again to powder form and refilled into the crucible, which was placed in a tube furnace with a continuous flow of reducing gas ([V(N2):V(H2)] = 80%:20%). The samples were burned at 1 200 ℃ for 10 h, cooled to room temperature naturally, and then ground carefully to obtain the desired phosphor samples.A white light illumination device was fabricated using a blue LED chip stacked with green and red phosphors. The green light was partly provided by Lu3Al5O12:Ce3+ (LuAG:Ce3+) commercial green phosphor, while the red light was contributed by the experimentally prepared Sr3Ga4O9:Eu2+,0.05Zn2+ phosphor.
Results and discussion
The Sr3Ga4O9 cell structure belongs to the P-1 space group of the triclinic crystal system. The results of XRD characterization tests indicate that the prepared samples are in pure phase. A heterovalent substitution strategy was used to introduce Zn2+ into Sr3Ga4O9, and the XRD results showed that this co-doping did not change the structure of the matrix material. Further analysis revealed that Zn2+ ions replaced Ga3+ sites, while Eu2+ may occupy two 6-coordinated Sr2+ lattice sites.Under the excitation of 450 nm, the Sr3Ga4O9:xEu2+ phosphor emits red light at 618 nm, and the emission intensity reaches the maximum at x = 0.04. The low-temperature emission spectra at 78 K can be decomposed into two Gaussian peaks, which indicates that the Eu2+ occupy two different luminescent centers and emit the red light, respectively. The fluorescence lifetime decay curves of the Sr3Ga4O9:0.04Eu2+ sample at low temperature (78 K) can be well fitted by the double-exponential function, which again demonstrates that the Eu2+ ions occupy two different lattice sites.After Zn doping, the luminescence intensity of Sr3Ga4O9:Eu2+,0.05Zn2+ phosphor was enhanced and the thermal stability was also improved significantly. The enhancement of thermal stability is from the presence of defects in the crystal, which compensates for the heat loss. The average decay lifetime of the co-doped Zn2+ is significantly higher than that of the sample without Zn2+, and a clear bulge of new pyroelectric peaks at 210 ℃ and 325 ℃ is clearly seen. Thus the addition of Zn2+ can increase the concentration of traps and contribute to the formation of deeper defect energy levels.For Sr3Ga4O9:0.04Eu2+, the thermal stability is poor due to its shallow trap depth, which causes the electrons to escape from the shallow traps immediately at room temperature. Whereas, Sr3Ga4O9:Eu2+,0.05Zn2+ has deeper trap 2 and trap 3 in it, and thus maintains a good thermal stability even at high ambient temperatures.The white LED device was fabricated to cover the entire visible region of the emission spectrum with a color rendering index of Ra = 87, a color temperature of CCT = 3 500 K, and CIE color coordinates of (0.410, 0.380).
Conclusions
Novel Sr3Ga4O9:Eu2+ red phosphors that can be excited by blue light were designed and prepared, and their crystal structures and spectral properties were investigated. It is found that the Sr3Ga4O9:Eu2+ red light emission comes from two low-coordination polyhedra Sr1O6 and Sr3O6 occupied by Eu2+ with 5d-4f leaps. Zn2+/Ga3+ heterovalent substitution occurs by the introduction of Zn2+ into Sr3Ga4O9, which is proved to introduce a defective energy level through the fluorescence decay lifetime and thermoluminescence to improve the thermal stability of the phosphor. A white LED device prepared using a commercial blue LED chip, Sr3Ga4O9:Eu2+,0.05Zn2+ red phosphor, and a commercial green phosphor, LuAG:Ce3+, has a high color-rendering index, Ra = 87, and a low correlated color temperature, CCT = 3 500 K, which suggests that it is suitable for warm-white-light lighting.