Enhanced long afterglow of SrAl₂Si₂O₈:Eu²⁺ by codoping Dy³⁺

Abstract

Eu²⁺/Dy³⁺ doped SrAl₂Si₂O₈ phosphors were synthesized by a solid state reaction. The phase and luminescent properties of the synthesized phosphors were investigated by the X-ray powder diffraction, photoluminescence spectra, decay curves and the thermo-luminescence glow curves. The XRD results show that the doped Eu²⁺/Dy³⁺ has no influence on the phase of SrAl₂Si₂O₈. The incorporation of Dy³⁺ could significantly enhance the intensity and prolong the afterglow duration of SrAl₂Si₂O₈:Eu²⁺. The co-doped Dy³⁺ ions act as trap centers and trap the electrons generated during exposure of the phosphor to an excitation source, which induces the longer afterglow comparing with Eu²⁺ singly doped SrAl₂Si₂O₈ phosphor.

1 Introduction

Long afterglow phosphors are a special type of luminescent material with persistent luminescence that lasts for several minutes or hours after the removal of the activated light source. It is generally accepted that the persistent luminescence involves luminescence and trap centers in a long afterglow phosphor [1]. Charge carriers are generated by the excitation in the luminescence centers and subsequently trapped in the trap centers. If the traps are deep, the carriers may reside there for an infinite time when no external stimulation is provided. Their de-trapping is thermally activated, which can cause a delay in the spectral emission and result in the persistent luminescence. Long afterglow phosphors have wide applications in various fields, such as displays, lighting devices, traffic signs, emergency signages, textile printing, decorations and watch dials [2]. Since the first report of SrAl₂O₄:Eu²⁺, Dy³⁺ persistent phosphor [3], rare earth ions doped persistent phosphors have continuously gained popularity. Among various materials, alkaline earth metal (Ca, Sr, Ba) aluminates or silicates have attracted great attention because there are relatively wide band gaps. It has been found that they are suitable for persistent luminescence centers of Eu²⁺ and Ce³⁺ [4].

Aluminosilicate compounds have been extensively studied as an important family of host materials for rare earth ions because of their excellently physical and chemical properties, high efficiency and thermal stability [5]. Among various aluminosilicate compounds, (Ca, Sr, Ba)Al₂Si₂O₈ have been reported to act as hosts for various rare earth ions [6,7,8,9,10,11,12]. Moreover, it has been found that it can show long afterglow properties after the dopant of Eu²⁺ [12]. In this work, we report on the synthesis and long afterglow luminescence of Eu²⁺/Dy³⁺ doped SrAl₂Si₂O₈ phosphors. The role of Dy³⁺ in SrAl₂Si₂O₈:Eu²⁺ phosphor for the extension of luminescence has been investigated.

2 Materials and methods

SrAl₂Si₂O₈:0.02Eu²⁺, SrAl₂Si₂O₈:0.02Dy³⁺ and SrAl₂Si₂O₈:0.02Eu²⁺/0.02Dy³⁺ phosphors were synthesized by a solid state reaction. SrCO₃ (99.5%), α-Al₂O₃ (99.5%), SiO₂ (99.0%), Eu₂O₃ (99.99%) and Dy₂O₃ (99.99%) were used as starting materials. In the synthesis, stoichiometric amounts of raw materials were weighted and mixed thoroughly in an agate mortar by grinding for 60 min. The obtained mixture subsequently was transferred to an alumina crucible and calcined at 1400 °C for 6 h in a muffle furnace. The SrAl₂Si₂O₈:0.02Dy³⁺ was calcined in air, but SrAl₂Si₂O₈:0.02Eu²⁺ and SrAl₂Si₂O₈:0.02Eu²⁺/0.02Dy³⁺ were calcined under a reducing atmosphere (5% H₂/95% N₂). After that, the products were cooled to room temperature in the furnace, collected and re-grinded for the next measurements.

The X-ray diffraction (XRD) patterns were performed on a Rigaku D/max-RA X-ray diffractometer with Cu Kα radiation (λ = 1.5406 Å) at 40 kV and 30 mA. The data were recorded in a 2θ angle of 10°–55° with a scan speed of 10°/min. The excitation and emission spectra were measured by an Edinburgh Instrument FLS920 spectrophotometer equipped with a 150 W xenon lamp as the excitation source and a grating to select a suitable excitation wavelength with excitation and emission slit widths of 2.5 nm, as well as a scan rate of 600 nm/min. Afterglow decay curves were measured by a PR305 long-lasting luminescence detector after the samples were irradiated by ultraviolet light at 365 nm for 10 min. The thermo-luminescence measurements were carried out in the temperature range of 300–500 K with a heating rate of 1 K/s.

3 Results and discussion

Figure 1 shows the XRD patterns of SrAl₂Si₂O₈:0.02Eu²⁺, SrAl₂Si₂O₈:0.02Dy³⁺ and SrAl₂Si₂O₈:0.02Eu²⁺/0.02Dy³⁺ phosphors. All of diffraction peaks are well accordance with the standard data of JCPDs 38-1454, indicating that the doped Eu²⁺/Dy³⁺ ions have no significant influence on the structure of SrAl₂Si₂O₈ host. There are no diffractions peaks corresponding to other materials, suggesting that Eu²⁺/Dy³⁺ ions have doped into SrAl₂Si₂O₈ host entirely. There is only one Sr²⁺ crystallographic site in the monoclinic SrAl₂Si₂O₈ unit cell and the Sr²⁺ is bonded to eight oxygen atoms forming an irregular hexahedron, but the Al³⁺ and Si⁴⁺ connect four O atoms to form three-dimensional (Al/Si)O₄ frameworks and creates an infinite net structure by corner-sharking. Due to similar ionic radii of Eu²⁺ (1.25 Å, CN = 8), Dy³⁺ (1.027 Å, CN = 8) and Sr²⁺ (1.26 Å, CN = 8), Eu²⁺ and Dy³⁺ ions substitute the Sr²⁺ sites in the synthesized phosphors.

Fig. 1

XRD patterns of SrAl₂Si₂O₈:0.02Eu²⁺, SrAl₂Si₂O₈:0.02Dy³⁺ and SrAl₂Si₂O₈:0.02Eu²⁺/0.02Dy³⁺

Figure 2 presents the excitation and emission spectra of SrAl₂Si₂O₈:0.02Eu²⁺. By monitoring the emission at 415 nm, the excitation spectrum of SrAl₂Si₂O₈:0.02Eu²⁺ exhibits a strong excitation band with a peak at about 365 nm, which results from the 4f⁷ → 4f⁶ 5d transitions of Eu²⁺ [13]. Under the excitation at 365 nm, the emission spectrum of SrAl₂Si₂O₈:0.02Eu²⁺ shows a strong emission band with a peak at about 415 nm, which originates from the spin-allowed transition from the 5d to the 4f state of Eu²⁺ [4]. Figure 3 gives the excitation and emission spectra of SrAl₂Si₂O₈:0.02Dy³⁺. The excitation spectrum monitored at 573 nm consists of a series of excitation bands in the range of 300–500 nm with peaks at about 322 nm, 346 nm, 361 nm, 383 nm, 420 nm, 446 nm and 451 nm, which come from the ⁶H_15/2 → ⁴M_17/2, ⁶H_15/2 → ⁴M_15/2, ⁶H_15/2 → ⁴I_11/2, ⁶H_15/2 → ⁴I_13/2, ⁶H_15/2 → ⁴G_11/2 and ⁶H_15/2 → ⁴G_15/2 transitions of Dy³⁺ [14]. Among these excitation bands, the excitation band peaking at 346 nm is much stronger than other excitation bands. Under the excitation at 346 nm, SrAl₂Si₂O₈:0.02Dy³⁺ exhibits three emission bands peaking at 480 nm, 573 nm and 664 nm, which originate from the ⁴F_9/2 → ⁶H_15/2, ⁴F_9/2 → ⁶H_13/2 and ⁴F_9/2 → ⁶H_11/2 transitions of Dy³⁺, respectively [15]. It is known that the yellow emission originating from the ⁴F_9/2 → ⁶H_13/2 transition belongs to a hypersensitive (forced dielectric dipole) transition with the selection rule ∆J = 2, which is influenced by the surrounding environment. However, the blue emission corresponding to the ⁴F_9/2 → ⁶H_15/2 transition is a magnetic dipole transition, which hardly varies with the crystal field around Dy³⁺ ions. The yellow emission is predominant only when Dy³⁺ ions locate at low-symmetry sites without inversion centers, but the blue emission will be predominant if the ligand-field deviates from inversion symmetry in the host [16]. The stronger yellow emission indicates that Dy³⁺ ions are in the low symmetry sites without inversion. That is consistent with coordination surrounding of Sr²⁺ ions.

Fig. 2

Excitation and emission spectra of SrAl₂Si₂O₈:0.02Eu²⁺

Fig. 3

Excitation and emission spectra of SrAl₂Si₂O₈:0.02Dy³⁺

Figure 4 exhibits the excitation and emission spectra of SrAl₂Si₂O₈:0.02Eu²⁺/0.02Dy³⁺. There is only one excitation band peaking at about 365 nm in the excitation spectrum of SrAl₂Si₂O₈:0.02Eu²⁺/0.02Dy³⁺, which does not show any significant changes from the SrAl₂Si₂O₈:0.02Eu²⁺ except for the higher intensity. And this excitation band is also induced by the 4f⁷ → 4f⁶ 5d transitions of Eu²⁺. Under the excitation at 365 nm, SrAl₂Si₂O₈:0.02Eu²⁺/0.02Dy³⁺ shows an emission spectrum consisting of a strong emission band with a peak at about 416 nm. The luminescence of SrAl₂Si₂O₈:0.02Eu²⁺/0.02Dy³⁺ tells us that there is no energy transfer between Eu²⁺/Dy³⁺ ion pairs although the emission spectrum of SrAl₂Si₂O₈:0.02Eu²⁺ overlaps with the excitation spectrum of SrAl₂Si₂O₈:0.02Dy³⁺. Figure 5 presents the afterglow decay curves of SrAl₂Si₂O₈:0.02Eu²⁺ and SrAl₂Si₂O₈:0.02Eu²⁺/0.02Dy³⁺. The initial afterglow intensity shows a rapid decay process firstly and then it displays slow decay with lasting decay time. The decay times of SrAl₂Si₂O₈:0.02Eu²⁺ and SrAl₂Si₂O₈:0.02Eu²⁺/0.02Dy³⁺ are about 20.6 min and 33.7 min, respectively. It can be seen that the decay time of SrAl₂Si₂O₈:0.02Eu²⁺/0.02Dy³⁺ is longer than that of SrAl₂Si₂O₈:0.02Eu²⁺, which is induced by the hole traps corresponding to the codoped Dy³⁺ ions [12]. The co-doped Dy³⁺ ions act as trap centers and trap the electrons generated during exposure of the phosphor to an excitation source and the afterglow mechanism could be described as producing more traps and stabilizing trapping electrons. On one hand, parts of the divalent alkaline earth cation (Sr²⁺) were replaced aliovalently by the trivalent rare earth cation (Dy³⁺) in SrAl₂Si₂O₈. In order to keep charge balance, a strontium vacancy (V_Sr″) and two Dy_Sr sites were generated. This process can be express by the formula of 3Sr_Sr → 2Dy_Sr + V_Sr″ [17]. Therefore, new traps would capture more charge carrying. This result means the concentration of trapped charge will increase. When these carriers were released by thermal agitation at room temperature, SrAl₂Si₂O₈:0.02Eu²⁺/0.02Dy³⁺ can emit a stronger afterglow than before. On the other hand, the optical electronegativity of the Dy³⁺ cation (1.21 eV) is very suitable for stabilizing the captured electrons around traps.

Fig. 4

Excitation and emission spectra of SrAl₂Si₂O₈:0.02Eu²⁺/0.02Dy³⁺

Fig. 5

Afterglow decay curves of SrAl₂Si₂O₈:0.02Eu²⁺ and SrAl₂Si₂O₈:0.02Eu²⁺/0.02Dy³⁺

In order to understand the trapping nature of the synthesized phosphors and study the effect of codoped Dy³⁺ ions in SrAl₂Si₂O₈:Eu²⁺ on the persistent luminescence process, thermo-luminescence spectra were measured after stopping 365 nm radiations, as shown in Fig. 6. It can be seen that the emission intensity of thermo-luminescence for SrAl₂Si₂O₈:0.02Eu²⁺/0.02Dy³⁺ is higher than that of SrAl₂Si₂O₈:0.02Eu²⁺. The higher thermo-luminescence intensity means that there is more traps in SrAl₂Si₂O₈:0.02Eu²⁺/0.02Dy³⁺. As discussed above, the co-doped Dy³⁺ acts as a trap center and traps the electrons generated during exposure of the phosphor to an excitation source, which induce the higher amount of traps in SrAl₂Si₂O₈:0.02Eu²⁺/0.02Dy³⁺. Moreover, the profile of thermo-luminescence spectrum for SrAl₂Si₂O₈:0.02Eu²⁺/0.02Dy³⁺ is wider than that of SrAl₂Si₂O₈:0.02Eu²⁺. The integral area of the thermo-luminescence represents the fact that traps captured the gross amount of charge carriers [4]. The broad thermo-luminescence spectrum covers from 300 to 490 K, which indicates that the traps distribute over a wide range of energies in the bandgap (i.e., traps of various depth were formed).

Fig. 6

Thermo-luminescence spectra of SrAl₂Si₂O₈:0.02Eu²⁺ and SrAl₂Si₂O₈:0.02Eu²⁺/0.02Dy³⁺

On the basis of luminescent properties of SrAl₂Si₂O₈:0.02Eu²⁺ and SrAl₂Si₂O₈:0.02Eu²⁺/0.02Dy³⁺, the possible mechanism of persistent luminescence was speculated. On exposure to an excitation source, an electron of Eu²⁺ (4f⁷) is promoted to the 4f⁶5d¹ band, followed by either direct or phonon-assisted escape of the electron from Eu²⁺ to the host conduction band. The electrons are subsequently captured by shallow traps. During thermal disturbance at room temperature, captured electrons were released from shallow traps and backtracked to the 5d energy levels via a conduction band. Finally, these electrons recombined with emission centers to generate persistent luminescence. Lattice defects close to the bottom of the host conduction band trap the electrons. Huge numbers of electrons are caught by the traps assisted by Dy³⁺ at various depths, highlighting an important role for Dy³⁺ during the long afterglow. After removal of the excitation source, the captured electrons near the host conduction band are released to the conduction band with thermal energy, and their consequent recombination with the emitting Eu²⁺ centers leads to the persistent afterglow.

4 Conclusions

We synthesized Eu²⁺/Dy³⁺ doped SrAl₂Si₂O₈ phosphors through a solid state reaction. Eu²⁺/Dy³⁺ singly doped SrAl₂Si₂O₈ phosphor respectively shows characteristic emission bands of Eu²⁺ or Dy³⁺, and Eu²⁺/Dy³⁺ codoped SrAl₂Si₂O₈ phosphor only exhibits emission band of Eu²⁺ with higher intensity than that of Eu²⁺ singly doped SrAl₂Si₂O₈. Both SrAl₂Si₂O₈:Eu²⁺ and SrAl₂Si₂O₈:Eu²⁺/Dy³⁺ show long afterglow luminescence. In SrAl₂Si₂O₈:Eu²⁺/Dy³⁺, Dy³⁺ acts as a trap center and traps the electrons generated during exposure of the phosphor to an excitation source, which induces the longer afterglow comparing with Eu²⁺ singly doped SrAl₂Si₂O₈ phosphor. The luminescent properties indicate that SrAl₂Si₂O₈:Eu²⁺/Dy³⁺ has potential applications in future opto-electronic devices.