Thermally stable photoluminescence and long persistent luminescence of Ca3Ga4O9:Tb3+/Zn2+

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Abstract

A green long persistent luminescence (LPL) phosphor Ca3Ga4O9:Tb3+/Zn2+ was prepared. Ca3Ga4O9 matrix exhibits blue self-activated LPL due to the creation of intrinsic traps. When Tb3+ is doped, the photoluminescence (PL) and LPL colors change from blue to green with their intensities significantly enhanced. The doping of Zn2+ evidently improves the PL and LPL performances of the Ca3Ga4O9 matrix and Ca3Ga4O9:Tb3+. The thermoluminescence (TL) spectra show that a successive trap distribution is formed by multiple intrinsic traps with different depths in the Ca3Ga4O9 matrix, and the incorporation of Tb3+ and Zn2+ effectively increases the densities of these intrinsic traps. The existence of a successive trap distribution makes the Ca3Ga4O9:Tb3+/Zn2+ phosphor exhibit thermally stable PL and LPL. It is indicated that this phosphor shows great promise for the application such as high-temperature LPL phosphor.

Graphical abstract

(a) The temperature dependent emission spectra of the Tb3+/Zn2+ co-doped sample; (b) The lifetime curves of the Tb3+/Zn2+ co-doped sample at different temperatures after being irradiated for 15 min with UV lamp.

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Introduction

As typical optical-storage materials, long persistent luminescence (LPL) phosphors can store charge carriers (electrons and holes) under the excitation at UV or sunlight and then subsequently release at room temperature (∼25–27 °C), owing to the suitable trapping depth. They have attracted extensive attention in multiple fields because of their broad applications, such as safety signage, night-vision surveillance, and decoration.1, 2, 3, 4, 5, 6, 7, 8 Especially, after the SrAl2O4:Eu2+/Dy3+ phosphor was discovered in 1996, the enthusiasm of the researchers has been greatly stimulated.9, 10 However, the process in developing excellent LPL phosphors is quite tortuous due to the lack of effective strategy. Up to now, the trial and error method is still dominant,11, 12 which leads to much consumption in the time. Hence, the use of more effective methods is necessary and meaningful. Because the LPL property is strictly dependent on the generation of suitable traps, simplifying the design with self-activated LPL phosphor as a matrix to effectively realize LPL property seems feasible and meaningful. Recently, alkaline earth metal gallate compounds have received great attention because of low synthesis temperature and high stability.13 Besides, such compounds tend to exhibit self-activated LPL behavior owing to their special structure and semiconducting properties.14 Hence, alkaline earth metal gallate compounds are definitely ideal candidates as LPL phosphor matrix.

Moreover, considering the high sensitivity of the human eye to green light,15 the development of green LPPs is exceedingly meaningful for practical applications. As a rare earth ion, Tb3+ usually presents excellent green luminescence due to the 4f inter-transition 5D47FJ (J = 6, 5, 4, 3).16, 17, 18 More importantly, the radius and ionization potential of Tb3+ are also appropriate for matching the alkaline earth metal ions. It implies that Tb3+ is easily introduced into the lattice to replace alkaline earth ions and create more ample traps.19, 20, 21 For alkaline earth gallate compounds, Ga3+ sites are also often replaced by doping ions with similar radii as luminescent centers or trap centers.22, 23 Zn2+ is adjacent to Ga3+ in the periodic table, which means that they have similar ion radii. And, Zn2+ has a lower ionization potential than Ga3+ ions, which also means that it may further stabilize the traps in the phosphor.

In this research, we selected an alkaline earth metal gallate compound (Ca3Ga4O9) as a matrix. As expected, the matrix shows self-activated LPL behavior related to three intrinsic lattice defects. Then, Tb3+ and Zn2+ were introduced to replace the Ca2+ and Ga3+ sites, respectively. The influence of doping ions on optical properties and traps is discussed. Because of the existence of a successive trap distribution, the thermally stable photoluminescence (PL) and high-temperature-resistance LPL are presented in Ca3Ga4O9:Tb3+/Zn2+. These results refer that this phosphor could provide potential application in a rigorous environment with a high thermal energy than the room temperature, such as in vivo-imaging.

Section snippets

Experimental

Self-activated LPL phosphor (Ca3Ga4O9) and a series of Ca3Ga4O9:Tb3+/Zn2+ phosphors were synthesized through the high-temperature solid–state reaction method. The starting materials are CaCO3 (99.99%), Ga2O3 (99.99%), Tb2O3 (99.99%), and ZnO (99.99%). The starting materials with stoichiometric amounts were mixed thoroughly in an agate mortar by grinding and subsequently sintering at 1210 °C for 10 h in air atmosphere. Finally, the samples were naturally cooled to room temperature in the

Results and discussion

Fig. 1(a) showed the X-ray diffraction patterns of the representative samples of pure Ca3Ga4O9, Ca3Ga4O9:Tb3+, Ca3Ga4O9:Zn2+, and Ca3Ga4O9:Tb3+/Zn2+, which were in good agreement with the reported Ca3Ga4O9 phase (JCPDS No. 26-178). No additional peaks were detected in the compositions, indicating that all samples are of a single phase and all doping ions are incorporated in the Ca3Ga4O9 host. As shown in Fig. 1(b), Ca3Ga4O9 possesses an orthorhombic phase with a space group Cmm2. In the Ca3Ga4O9

Conclusions

The LPL properties of the Ca3Ga4O9 host matrix were investigated, which are related to the existence of the VGa''' and VO. When Tb3+ is doped, the dominate emission center changes from Ga3+ to Tb3+ in the PL and LPL processes, resulting in the variety of corresponding emitting color from blue to green. TL spectra show that the introduction of Tb3+ not only significantly increases the number of intrinsic VO, but also create a new trap (TbCa). On the other hand, Zn2+ exhibits the ability to

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    Foundation item: Project supported by the National Natural Science Foundation of China (11774138), the Society Development Foundation of Yunnan Province (2016FA021) and the Kunming University of Science and Technology (KKSY201632046).

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