Photoluminescence properties of Tm³⁺/Tb³⁺/Sm³⁺ tri-doped Na₅Y₉F₃₂ single crystal with high thermal stability for white light-emitting diodes

Highlights

• High quality Na₅Y₉F₃₂:Tm³⁺/Tb³⁺/Sm³⁺ single crystals are prepared.

• The white light emission was obtained by changing the concentration of Sm³⁺ ion.

• The mechanism of energy transfer between Tm³⁺-Tb³⁺-Sm³⁺⁺ was systematically studied.

• The Na₅Y₉F₃₂ single crystal as ion carrier exhibits excellent emission thermal stability.

Abstract

A novel Tm³⁺/Tb³⁺/Sm³⁺ tri-doped Na₅Y₉F₃₂ single crystal was synthesized by a modified Bridgman method for the propose of white light emitting diodes. The fluorescence spectra of various Sm³⁺ ion concentrations and fixed 0.4 mol% Tm³⁺ and 0.5 mol% Tb³⁺ were measured and studied systematically excited by near-ultraviolet light of 355 nm. The Sm³⁺ ion concentration takes apparent effect on the relative intensity of peaks in the visible region and the color coordinate combining from these emission bands. A near pure white light emission with color coordinates (0.3295, 0.3057) and color temperature (5657 K) can be obtained when the concentrations of Tm³⁺, Tb³⁺ and Sm³⁺ ions are 0.4 mol%, 0.5 mol% and 0.8 mol%, respectively. Furthermore, the practical down-conversion internal quantum yield was measured by integrating spheres at about 14.39%. The tri-doped Na₅Y₉F₃₂ single crystal shows a high thermal stability inferring from the temperature dependent emission in which the integrated emission intensities are reduced only by ∼3% with the increase of temperature from 280 to 450 K. The present results demonstrate that the Tm³⁺/Tb³⁺/Sm³⁺ tri-doped Na₅Y₉F₃₂ single crystal may provide a promising candidate for white light-emitting diodes, luminescent materials and fluorescent display devices.

Introduction

Recently, white light-emitting diodes (LEDs), as the substitute of traditional light sources, have received extensive attention due to their excellent characteristics such as high luminous efficiency, long lifetimes, small size, desirable color qualities, environmental friendliness.1, 2, 3, 4 The conventional way for LED to generate white light is to combine the light from blue LED chip (GaN) with appropriate quantities of yellow phosphor (YAG:Ce³⁺), which has been commercialized for the long time.^5,6 This approach has some inherent drawbacks, such as poor heat resistance, high correlated color temperature and low color rendering index (CRI).⁷ Adding red-emitting components is one of the ways to improve CRI, such as red-emitting nitride phosphors or quantum dots. This method is more limited in practical applications, because the nitride phosphors have high manufacturing costs due to harsh synthesis conditions and high patent licensing costs, and red-emitting quantum dots (CdSe) are toxic.^8,9 A new alternative approach combining a near ultraviolet (UV) LED chip with a mixture of red, green and blue phosphors has been proposed.¹⁰ The W-LEDs obtained by this method have a higher CRI and can easily control the luminescent color properties. Nevertheless, the photo-degradation of the phosphor material under short-wavelength excitation and the self-absorption between the phosphors will result in the decrease in luminous efficiency, and then affect the performance of devices. Therefore, it is important to develop an efficient, single-phase and full-color emitting material which can adapt to the excitation of near UV radiation.

Since the 4f-4f or 4f-5d electronic transitions of rare earth (RE) ions can produce almost all spectral ranges from UV to infrared, they are often chosen as a luminescent activator and have extensive application in the field of illumination and display. In fact, many RE ions combinations, such as Eu³⁺/Tb³⁺/Tm³⁺¹¹ and Tb³⁺/Sm³⁺,¹² are used as activators for white LED solid materials. When excited by UV light, Tm³⁺ ion can be observed to have a strong peak (¹D₂→³F₄) at ∼450 nm (blue), and a strong peak (⁵D₄→⁷F₅) for Tb³⁺ ion at ∼545 nm (green). In addition, Sm³⁺ ion has a strong peak (⁴G_5/2 → ⁶H_7/2) at ∼610 nm (orange-red) and a slightly weaker peak (⁴G_5/2 → ⁶H_9/2) at ∼650 nm (red). Combining these three RE ions (Tm³⁺/Tb³⁺/Sm³⁺) at appropriate concentrations and doping in a suitable host may obtain the white light emission.

In this study, the Na₅Y₉F₃₂ single crystal was selected as host material for doping Tm³⁺, Tb³⁺ and Sm³⁺ ions. Compared with frequently-used glass, ceramics, powder hosts, the Na₅Y₉F₃₂ crystal is a better host material because its center ion Y³⁺ has a similar radius with RE (Tm³⁺/Tb³⁺/Sm³⁺) ions and the low phonon energy can promote the radiation emission of doped ions.^13,14 Especially, fluoride crystals have wider transparent area, lower refractive index, weaker multi-phonon relaxation and longer radiative lifetime than oxide crystals.¹⁵ Meanwhile, it can be seen from the temperature dependent emission spectra that Na₅Y₉F₃₂ crystal also has excellent thermal stability. This is the first time to dope Tm³⁺/Tb³⁺/Sm³⁺ ions into the single crystal host, the single crystal prepared under the excitation of near UV light has rich emission from 430 to 680 nm, which is obtained stably and close to white light emission by adjusting the concentration of the doping ions. The results show that Tm³⁺/Tb³⁺/Sm³⁺ tri-doped Na₅Y₉F₃₂ crystal may be a potential material for realizing white LED.