Red LaPO4:Eu Nanophosphor for Near UV LED and Field Emission Display Applications
Authors: Yang Su-Hua , Yang Chih-Ka , Yan Jia-Hung 
Published: February 2020

For the phosphor materials, lanthanum phosphate (LaPO4) phosphor is one of potential materials for use in energy-efficient lightings, especially for ultraviolet (UV) light emitting diode (LED) and field emission display (FED) applications. In recent years, with the development of nanotechnologies, the synthesis of LaPO4 phosphor with nanosize particles is of increasing scientific and technical interest.

In this study, the LaPO4:Eu nanophosphor was prepared with the coprecipitation (CP) method using different phosphate sources and anionic surfactants. The phosphor particle size analyzed by the SEM was about 0.5–1.2 μm. The phosphor synthesized by the CP method had improved crystallinity, smooth morphology, and decreased nonradiative transition, consequently, a twofold increase in luminescence intensity was obtained when compared with the phosphor prepared by the solid state reaction method.

Under the excitation at wavelength of 396 nm (the emission wavelength of a UV LED), the LaPO4:Eu phosphor showed a red emission with peak at 594 nm (5
D
0 → 7
F
1). The maximum luminescence intensity of phosphor was obtained when it was doped with 7 mol% Eu3+ and sintered at 1200°C for 2 h. Emission degradation measurement shows that the emission intensity at wavelength of 594 nm was decayed with single-exponential mechanism. The measured decay time was 2.95 ms.

The absorption and emission spectra of the phosphor varied with the kinds of phosphate sources. Using H3PO4 as phosphate source, a strong charge transfer band (CTB) absorption and high 5
D
0 → 7
F
2 emission were measured. When (NH4)2HPO4 and Na5P3O10 were used, a red-shifted and decreased CTB absorption was observed. This shift was caused by the increase in O2- - Eu3+ distance and the crystallinity variation of the phosphor, which led to the decrease in charge transfer energy. On the other hand, for the f-f orbital absorption transitions of Eu3+ inner electrons, the energy absorptions of phosphor prepared with phosphate sources of (NH4)2HPO4 and Na5P3O10 were stronger than that of phosphor prepared with H3PO4. Hence, an increased 5
D
0 → 7
F
1 emission was observed. The highest crystallographic symmetry of Eu3+ in the lattice and a high color-purity red emission were obtained when the (NH4)2HPO4 was used as phosphate source material.

To further enhance the photoluminescence (PL) of the phosphor for LED application, anionic dispersants were used to modify the surface of phosphor particles. The dominant growth plane was changed from (200) to (120) plane when the phosphor was synthesized with dispersants. The decay time of phosphor was 3.02, and 3.15 ms when SDS and (NaPO3)6 dispersants were used, respectively. A certain improvement in the decay time was observed. Apparently, the surface states of phosphor particles were modified, hence, the PL intensity was enhanced. The optimal crystallinity and the maximum PL intensity of phosphor were obtained when 1 and 3 mol% of (NaPO3)6 and SDS were used, respectively, and the corresponding increment in the PL intensity was 10 and 7%.

Moreover, conductive material of carbon nanotubes (CNTs) was doped to improve the cathodoluminescence (CL) of phosphor for FED application. The addition of CNTs reduced the accumulation of electrons on the phosphor surface. Consequently, the CL intensity was increased. The maximum CL intensity was obtained when 1 wt% of CNT was doped. The CL intensity was enhanced by 1.35 times.