Cyan long lasting phosphorescence in green emitting phosphors Ba₂Gd₂Si₄O₁₃:Eu²⁺, RE³⁺ (RE³⁺ = Dy³⁺, Ho³⁺, Tm³⁺, Nd³⁺ and Tb³⁺)

Abstract

Novel long lasting phosphorescence materials Ba₂Gd₂Si₄O₁₃:Eu²⁺, RE³⁺ (RE³⁺ = Dy³⁺, Ho³⁺, Tm³⁺, Nd³⁺ and Tb³⁺) are designed and prepared by a solid state reaction. Their photoluminescence emission spectra show a broad asymmetric green emitting band peaking at 517 nm, attributed to the 5d–4f transitions of Eu²⁺ ion occupying Ba²⁺ and Gd³⁺ site respectively. The Eu²⁺ ion occupying the Gd³⁺ site is not involved in the long lasting phosphorescence process, resulting in an interesting phenomenon that the green emitting phosphors show cyan phosphorescence. The experimental results show that co-doping the rare earth ions can improve the long lasting phosphorescence performance effectively, and with the moderate introduction of Nd³⁺ ion the most persistent cyan emission can last for 3 h approximately. This work shows up an interesting phosphorescence phenomenon, and provides a new and efficient candidate for long lasting phosphorescence materials.

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1. Introduction

Long lasting phosphorescence (LLP) materials relate to an interesting optical phenomenon whereby luminescence can be observed for several seconds to hours after the end of the excitation period.^1,2 They can absorb and store energy under excitation (such as sunlight and some artificial light source) and then release the stored energy in the form of light at room temperature under thermal stimulation. Due to their environmentally friendly, energy saving and recyclable properties, LLP materials can be applied in extensive important fields, e.g. emergency signs, decorations, electronic displays, solar energy utilization, imaging or optical memory storage, medical diagnostics and vivo bio-imaging, etc.^3–7

In recent years, the LLP material has attracted hectic researches. Most of the reported researches are mainly focused on blue, green and red LLP materials for practical application,^8–11 because in theory, any color can be available by mixing the three primary colors in different proportions. However, this method is hard to actualize, because although LLP materials exhibiting blue and green phosphorescence with high brightness are commercially available, none of red LLP materials is good enough to be applied to practical applications. Thus, the exploitation of excellent phosphors with different emission colors are still one of the most critical and urgent challenges in the field of LLP materials. In fact, especially for cyan, because of the narrower spectral region (490–515 nm) of wavelength, cyan phosphor with decent LLP performance is scarce.^12–14 Therefore, the development of new cyan LLP materials would be necessary to meet the multicolor demand of practical utilization.

It is well known that the silicate-based LLP materials recently have attracted much attention for excellent chemical stability, thermal stability, weather resistance and low synthetic temperature.^15–17 In 2010, the silicate Ba₂Gd₂Si₄O₁₃ compound was first reported in the form of a single crystal by Maria Wierzbicka-Wieczorek et al.¹⁸ Generally, Eu²⁺ ion acting as an efficient activator is a common ion widely studied in the field of LLP materials, while the trivalent europium is difficult to be reduced in the Ba₂Gd₂Si₄O₁₃ compound. So that in the following years, only a few research studies focus on the luminescence properties of some trivalent rare earth (RE) ion doped Ba₂Gd₂Si₄O₁₃ phosphors.^19–23 Until 2015, the bivalent Eu²⁺ doped Ba₂Gd₂Si₄O₁₃ phosphor was prepared and proposed by Zhou et al., but there still exist intense Eu³⁺ ion characteristics emission under ultraviolet excitation.²⁴ Even so, this phosphor with high luminescence efficiency attracts our intense interest and none of its LLP properties has been studied yet upon reviewing the literature. In addition, co-doping RE³⁺ ion is one of the most common methods used to produce new traps due to nonequivalent substitution or at least modify the intrinsic trap properties,^25,26 which may improve the LLP performance. Accordingly, Eu²⁺ and RE³⁺ ions co-doped Ba₂Gd₂Si₄O₁₃ phosphors were selected as the silicate-based candidate in our work for the sake of a basic scientific study and demand for LLP materials. In this paper, novel cyan LLP materials, Ba₂Gd₂Si₄O₁₃:Eu²⁺, RE³⁺ (RE³⁺ = Dy³⁺, Ho³⁺, Tm³⁺, Nd³⁺ and Tb³⁺), were successfully prepared by a solid state reaction and identified by XRD refinement. The photoluminescence, phosphorescence and thermoluminescence properties of the phosphors were investigated in detail.

2. Experiment

2.1. Synthesis

The investigated powders Ba₂Gd₂Si₄O₁₃:Eu²⁺, RE³⁺ (RE³⁺ = Dy³⁺, Ho³⁺, Tm³⁺, Nd³⁺ and Tb³⁺) in this study were synthesized by conventional solid state reaction method. The raw materials BaCO₃ (99%), Gd₂O₃ (99.99%), SiO₂ (99.9%), Eu₂O₃ (99.99%), Dy₂O₃ (99.99%), Ho₂O₃ (99.99%), Tm₂O₃ (99.99%), Nd₂O₃ (99.99%) and Tb₄O₇ (99.99%) were stoichiometrically weighted out. After the ingredients were mixed thoroughly, the mixtures were placed into an alumina crucible filled with a moderate amount of carbon powder, and then sintered at 1350 °C for 6 h under a reductive atmosphere (5% H₂ + 95% N₂) in an electric tube furnace. Finally, after calcination, the samples were cooled to room temperature in the furnace and ground again into powder for subsequent use.

2.2. Characterization

The phase purity of samples was analyzed by X-ray powder diffraction (XRD) using a Bruker D2 PHASER X-ray diffractometer with graphite monochromator using Cu Kα radiation (λ = 1.54184 Å), operating at 30 kV and 15 mA with a scanning step of 0.02° in the 2θ range from 10° to 80°. Diffuse reflectance spectra (DRS) were measured on a PE lambda950 UV-vis spectrophotometer, and the BaSiO₄ white power was used as the reference. Photoluminescence emission (PL), excitation (PLE), LLP spectra were carried out by a FLS-920T spectrometer with Xe 900 (450 W xenon arc lamp) as the light source. The ex-slit and the em-slit for the PL and PLE spectra were 1 nm and the em-slit for the LLP spectra was set to 5 nm. Afterglow decay curve measurements were performed with a PR305 long afterglow instrument after the samples were irradiated with standard artificial daylight (the color temperature 6500 K and the power 18 W) for 15 min. Thermoluminescence (TL) curves were measured with a FJ-427A TL meter (Beijing Nuclear Instrument Factory) with a heating rate β = 1 K s⁻¹. The sample weight was kept constant (5 mg). Before the measurements, the samples were irradiated with ultraviolet light (254 nm) for 15 min. All the data were measured at room temperature except for TL curves. The results are repeatable.

3. Results and discussion

3.1. Phase purity analysis

Fig. 1 shows Rietveld structural refinement of the XRD pattern of Ba₂Gd₂Si₄O₁₃ host, obtained using Materials Studio program. Red solid line, black crosses and blue solid line are the calculated pattern, experimental pattern and background, respectively. Pink short vertical lines show the positions of Bragg reflections of the calculated pattern. The difference between experimental and calculated pattern is plotted by dark cyan line at the bottom. For the structure refinement, initial structural model is constructed with the single crystal data (ICSD-260737).¹⁸ The final refinement residual factors are R_wp = 7.53% and R_p = 5.78%. The refinement results confirm the single-phase nature of the compound in the monoclinic space group C2/c (no. 15) with cell parameters a = 12.884(5) Å, b = 5.203(6) Å and c = 17.529(8) Å, which almost coincide with the unit cell parameters of the reported Ba₂Gd₂Si₄O₁₃ host.¹⁸ In the Ba₂Gd₂Si₄O₁₃ structure, there are two different cations, namely [9 + 1]-coordinated Ba²⁺ (r = 1.52 Å)²⁷ and 7-coordinated Gd³⁺ (r = 1.00 Å)²⁷ respectively, as shown in the inset of Fig. 1. Generally, an acceptable percentage difference in ionic radii between the doped and substituted ions must not exceed 30%,²⁸ which suggests that when the rare earth ions are doped in this structure, they could substitute for both Ba²⁺ and Gd³⁺ site. Therefore, the general formula of the investigated phosphors is described as Ba_2−xGd_2−xSi₄O₁₃:xEu²⁺, xRE³⁺, where x denote the substitution ratio of the RE^2+/3+ ion for the cation.

Fig. 1 XRD refinement results of Ba₂Gd₂Si₄O₁₃ host. The inset shows the coordination environment of Ba²⁺ and Gd³⁺ site.

All the prepared samples were characterized by XRD to verify their phase purity. Fig. 2 shows the XRD patterns of Ba₂Gd₂Si₄O₁₃:Eu²⁺, RE³⁺ (RE³⁺ = Dy³⁺, Ho³⁺, Tm³⁺, Nd³⁺ and Tb³⁺) phosphors as well as the calculated XRD pattern of Ba₂Gd₂Si₄O₁₃ host according to the refinement results. The concentrations of Eu²⁺ ion and all RE³⁺ ions in the phosphors are fixed at 1 mol% here. In order to further investigate the effect of Nd³⁺ ion on the luminescence properties of Ba₂Gd₂Si₄O₁₃:Eu²⁺ phosphor in detail, a series of Ba_2−xGd_2−xSi₄O₁₃:xEu²⁺, xNd³⁺ (0.001 ≤ x ≤ 0.06) phosphors with different doping concentrations were prepared. Fig. 3 shows their XRD patterns and the calculated XRD pattern of Ba₂Gd₂Si₄O₁₃ host. It is obviously found that in Fig. 2 and 3 all XRD profiles are well fitted with the calculated XRD pattern of Ba₂Gd₂Si₄O₁₃ host and the diffraction peaks of these phosphors can be exactly assigned to Ba₂Gd₂Si₄O₁₃ host. No detectable impurity phase is observed in the obtained phosphors, indicating that all samples are single phase and the RE^2+/3+ ions are successfully incorporated in Ba₂Gd₂Si₄O₁₃ host without changing the crystal structure noticeably.

Fig. 2 XRD patterns of Ba₂Gd₂Si₄O₁₃:Eu²⁺, RE³⁺ (RE³⁺ = Dy³⁺, Ho³⁺, Tm³⁺, Nd³⁺ and Tb³⁺) and the calculated XRD pattern from the refinement results.

Fig. 3 XRD patterns of Ba_2−xGd_2−xSi₄O₁₃:xEu²⁺, xNd³⁺ (0.001 ≤ x ≤ 0.06) and the calculated XRD pattern from the refinement results.

3.2. UV-visible DRS analysis

The UV-visible DRS of Ba₂Gd₂Si₄O₁₃ host, Ba₂Gd₂Si₄O₁₃:Eu²⁺ and Ba₂Gd₂Si₄O₁₃:Eu²⁺, RE³⁺ (RE³⁺ = Dy³⁺, Ho³⁺, Tm³⁺, Nd³⁺ and Tb³⁺) phosphors are shown in Fig. 4. As can be seen in the DRS of Ba₂Gd₂Si₄O₁₃ host, the absorption in the ultraviolet region from 200 to 400 nm can be attributed to the valence-to-conduction band. The addition of Eu²⁺ ion accompanies the production of a broad valley from 220 to 480 nm attributable to the strong 4f–5d transitions of Eu²⁺ ion, which indicates that the doping Eu²⁺ ion creates localized energy levels within the band-gap of Ba₂Gd₂Si₄O₁₃ host.^29,30 Compared with that of Ba₂Gd₂Si₄O₁₃:Eu²⁺ phosphor, almost all the DRS of Ba₂Gd₂Si₄O₁₃:Eu²⁺, RE³⁺ phosphor have an overall drop, which means that the co-doped phosphors can absorb more energy than the singly doped one and the additional absorbed energy could have a great influence upon the LLP performance. By the way, in the co-doped phosphors (RE³⁺ = Ho³⁺, Tm³⁺ and Nd³⁺), there obviously exist new valleys in the long wavelength region from 500 to 700 nm, which are attributable to the intra-4f transitions of each RE³⁺ ion.³¹

Fig. 4 The UV-visible DRS of Ba₂Gd₂Si₄O₁₃ host, Ba₂Gd₂Si₄O₁₃:Eu²⁺ and Ba₂Gd₂Si₄O₁₃:Eu²⁺, RE³⁺ (RE³⁺ = Dy³⁺, Ho³⁺, Tm³⁺, Nd³⁺ and Tb³⁺).

The inset shows the plot of [F(R) × hν]² vs. photoenergy hν for Ba₂Gd₂Si₄O₁₃ host, where F(R) is the Kubelka–Munk function, with F(R) = (1 − R)²/2R, and R is the observed reflectance in the DRS.³² By adopting these methods, the optical bandgap energy of Ba₂Gd₂Si₄O₁₃ is determined to be 5.68 eV by extrapolation to F(R) = 0.

3.3. Photoluminescence analysis

Fig. 5a illustrates the PLE and PL spectra of Ba₂Gd₂Si₄O₁₃ host, Ba₂Gd₂Si₄O₁₃:Eu²⁺ and Ba₂Gd₂Si₄O₁₃:Eu²⁺, RE³⁺ (RE³⁺ = Dy³⁺, Ho³⁺, Tm³⁺, Nd³⁺ and Tb³⁺) phosphors, respectively. The Ba₂Gd₂Si₄O₁₃ host shows strong characteristic excitation (275 nm) and emission (311 nm) peaks, attributed to ⁸S_7/2 → ⁶I_J and ⁶P_7/2 → ⁸S_7/2 transitions of Gd³⁺ ion, respectively.³³ When monitored at 517 nm, the PLE spectrum of Ba₂Gd₂Si₄O₁₃:Eu²⁺ phosphor exhibits two distinct bands at 292 nm and 343 nm, which are attributed to the 4f⁷ → 4f⁶5d¹ transition of Eu²⁺ ion. Under excitation at 343 nm, the PL spectrum shows a broad asymmetric green emitting band peaking at 517 nm, corresponding to the allowed 4f⁶5d¹ → 4f⁷ transition of Eu²⁺ ion.³⁴ When incorporate the RE³⁺ ions into Ba₂Gd₂Si₄O₁₃:Eu²⁺ phosphor, the PLE and PL spectra of all co-doped phosphors are below that of Ba₂Gd₂Si₄O₁₃:Eu²⁺ phosphor. Compared with Ba₂Gd₂Si₄O₁₃:Eu²⁺ phosphor, the co-doped phosphors have a larger absorption (Fig. 4), whereas the co-doped phosphors do not show stronger emission, illustrating that co-doped phosphors should be storing energy in this process and the stored energy may have an influence upon the LLP performance.

Fig. 5 (a) PLE and PL spectra of Ba₂Gd₂Si₄O₁₃ host, Ba₂Gd₂Si₄O₁₃:Eu²⁺ and Ba₂Gd₂Si₄O₁₃:Eu²⁺, RE³⁺ (RE³⁺ = Dy³⁺, Ho³⁺, Tm³⁺, Nd³⁺ and Tb³⁺). (b) PLE spectra of Ba₂Gd₂Si₄O₁₃ host and Ba₂Gd₂Si₄O₁₃:Eu²⁺, RE³⁺ (RE³⁺ = Dy³⁺ and Tb³⁺) monitored at different wavelengths.

It can been seen that Ba₂Gd₂Si₄O₁₃:Eu²⁺, Dy³⁺ and Ba₂Gd₂Si₄O₁₃:Eu²⁺, Tb³⁺ phosphors show the additional sharp emission peaks attributed to the characteristic intra-4f transitions of the RE³⁺ ions, which correspond to the ⁴F_9/2 → ⁶H_13/2 transition of Dy³⁺ ion (574 nm) and the ⁵D₄ → ⁷F₅ transition of Tb³⁺ ion (543 nm).^35–37 Accordingly, the PLE spectra of Ba₂Gd₂Si₄O₁₃:Eu²⁺, Dy³⁺ and Ba₂Gd₂Si₄O₁₃:Eu²⁺, Tb³⁺ phosphors monitored at different wavelengths are given in Fig. 5b. The PLE spectrum of Ba₂Gd₂Si₄O₁₃:Eu²⁺, Dy³⁺ phosphor monitored at 574 nm is composed of three parts: a prominent band from 250 nm to 450 nm attributed to the 4f⁷ → 4f⁶5d¹ transition of Eu²⁺ ion, three sharp peaks in the range from 270 to 315 nm, which are attributed to the Gd³⁺ typical transitions, corresponding to the ⁸S_7/2 → ⁶I_J (275 nm) and ⁸S_7/2 → ⁶P_J (307 and 313 nm) respectively, and several sharp peaks in the range from 325 to 475 nm attributed to the transitions from the ground state of ⁶H_15/2 to the excited states of the 4f⁹ electronic configurations of Dy³⁺, which are located at 326 nm (⁶H_15/2 → ⁴M_17/2), 351 nm (⁶H_15/2 → ⁶P_7/2), 365 nm (⁶H_15/2 → ⁴I_11/2), 386 nm (⁶H_15/2 → ⁴I_13/2), 425 nm (⁶H_15/2 → ⁴G_11/2), 454 nm (⁶H_15/2 → ⁴I_15/2) and 474 nm (⁶H_15/2 → ⁴F_9/2), respectively. The result indicates that Dy³⁺ ion serves as an emission center and the energy transfer from Gd³⁺ to Dy³⁺ ion occurs in the phosphor. The PLE spectrum of Ba₂Gd₂Si₄O₁₃:Eu²⁺, Tb³⁺ phosphor monitored at 543 nm only contains two parts: a prominent band attributed to Eu²⁺ ion and three sharp peaks attributed to Gd³⁺ ion, indicating that the characteristic emission peak of Tb³⁺ ion should mainly be derived from the energy transfer from Gd³⁺ to Tb³⁺ ion.

Due to the outstanding effect of Nd³⁺ ion on the LLP performance in this work, the photoluminescence properties of a series of Ba_2−xGd_2−xSi₄O₁₃:xEu²⁺, xNd³⁺ (0.001 ≤ x ≤ 0.06) phosphors with different Nd³⁺ ion co-doping concentrations are studied in detail. As shown in Fig. 6, none of the typical excitation and emission of Nd³⁺ ion is observed and the broad bands are all assigned to the transition between the ground-state 4f⁷ and the crystal-field split 4f⁶5d¹ configuration of Eu²⁺ ion. The introduction of Nd³⁺ ion has little influence on the PLE and PL spectra. With different Eu²⁺/Nd³⁺ doping concentration, there is no variation of the profile of the PLE and PL spectra but the intensity. The inset shows the dependence of the emission intensity at 517 nm on Eu²⁺/Nd³⁺ doping concentration. The emission intensity increases with the increasing Eu²⁺/Nd³⁺ ion concentration, until it reaches a maximum at x = 0.04, and then the emission intensity declines when the concentration of Eu²⁺/Nd³⁺ ions goes beyond 4 mol% because of concentration quenching.³⁸ Thus, the optimal Eu²⁺/Nd³⁺ doping concentration for the photoluminescence property of the phosphors is 4 mol%.

Fig. 6 PLE and PL spectra of Ba_2−xGd_2−xSi₄O₁₃:xEu²⁺, xNd³⁺ (0.001 ≤ x ≤ 0.06). The inset shows the dependence of the emission intensity at 517 nm on Eu²⁺/Nd³⁺ doping concentration x.

In view of the strong crystal field dependence of the 5d levels in Eu²⁺ ion, the emission band of Eu²⁺ ion is strongly affected by its coordination environment.³⁹ In this work, the incorporated Eu²⁺ ions could occupy both Ba²⁺ and Gd³⁺ sites in Ba₂Gd₂Si₄O₁₃ host, so that they should have two types of emission centers. Thereby, the asymmetric broad emission spectrum of Ba_1.98Gd_1.98Si₄O₁₃:0.02Eu²⁺, 0.02Nd³⁺ phosphor under excitation at 343 nm are dealt with by Gaussian deconvolution and can be fitted well into two peaks at 510 nm and 564 nm, respectively. The fitting curves are shown in Fig. 7a. To further understand the site occupancy of Eu²⁺ ion in Ba₂Gd₂Si₄O₁₃ host, the position of the Eu²⁺ emission is estimated according to Van Uitert's reported empirical relation.⁴⁰ The equation is given as follows.

Fig. 7 (a) Gaussian deconvolution of the PL spectrum of Ba_1.98Gd_1.98Si₄O₁₃:0.02Eu²⁺, 0.02Nd³⁺. (b) PLE and PL spectra of Ba₂Gd₂Si₄O₁₃ host and Ba_1.94Gd_1.94Si₄O₁₃:0.06Eu²⁺, 0.06Nd³⁺.

Q relates to the energy position of the lower d-band edge for the free ion (Q = 34 000 cm⁻¹ for Eu²⁺ ion); V represents the valence state of the activator Eu²⁺ ion; N is equivalent to the number of O²⁻ of a surrounding Eu²⁺ ion; r is the ionic radius of the host cation of which the activator Eu²⁺ ion substitutes for; E_a represents the electron affinity of the anions around Eu²⁺ ion (in this case the anions are O²⁻, and its value of E_a is 1.46 eV). Ba²⁺ site is coordinated with ten oxygen ions and Gd³⁺ site is coordinated with seven oxygen ions. The ionic radii of Ba²⁺ and Gd³⁺ ion are 1.52 and 1.00 Å, respectively. Thus, according to the above calculation, we can draw a conclusion that the peak 1 centered at 510 nm is attributed to the 5d–4f transitions of Eu²⁺ ion occupying Ba²⁺ site (Eu_Ba), and the peak 2 centered at 564 nm is attributed to the 5d–4f transitions of Eu²⁺ ion occupying the Gd³⁺ site (Eu_Gd).

Generally, in some compounds the europium is difficult to be completely reduced, especially when the europium is incorporated to substitute for a trivalent cation. Thereby, Eu³⁺ ion often exists alongside Eu²⁺ ion in phosphors. Previously, Zhou et al. reported that under excitation at 266 nm, the PL spectrum of Ba₂Gd₂Si₄O₁₃:Eu²⁺ phosphor prepared with graphite sticks as a reducing agent was mainly composed of a series of strong sharp peaks, which are associated with the ⁵D₀ → ⁷F_j (j = 0, 1, 2, 3, 4) transitions of Eu³⁺ ion, indicating that many Eu³⁺ ions exist in the phosphor.²⁴ In our work, in order to strengthen the reduction effect, not only the carbon powder as a reducing agent but also a reductive atmosphere were utilized in the production process of the samples. The PL spectrum of the phosphor with the largest europium concentration among them under the same excitation at 266 nm is shown in Fig. 7b. In order to facilitate comparison, they are plotted with an amplification of three times. Clearly, although there still exist weak sharp peaks at 613 and 703 nm attributed to ⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₄ transitions of Eu³⁺ ion respectively (labeled with triangle), it is safe to say that the reduction effect is greatly improved in our samples due to the intensity ratio of Eu²⁺/Eu³⁺ ion. When the emission of Eu³⁺ ion is monitored, except for the prominent band from 250 to 450 nm attributed to the 4f⁷ → 4f⁶5d¹ transition of Eu²⁺ ion and two sharp peaks at 395 and 468 nm attributed to the ⁷F₀ → ⁵L₆ and ⁷F₀ → ⁵D₂ transitions of Eu³⁺ ion respectively, the PLE spectra also exhibit sharp peaks in the range from 270 to 315 nm attributed to Gd³⁺ ion, indicating the energy transfer from Gd³⁺ to Eu³⁺ ion.

3.4. Phosphorescence and thermoluminescence characteristic

Fig. 8 depicts the afterglow decay curves of Ba₂Gd₂Si₄O₁₃:Eu²⁺ and Ba₂Gd₂Si₄O₁₃:Eu²⁺, RE³⁺ (RE³⁺ = Dy³⁺, Ho³⁺, Tm³⁺, Nd³⁺ and Tb³⁺) phosphors. Without co-doping RE³⁺ ions, the LLP duration of Ba₂Gd₂Si₄O₁₃:Eu²⁺ phosphor is merely about 3 min above recognizable intensity level (≥0.32 mcd m⁻²). By co-doping the RE³⁺ (RE³⁺ = Dy³⁺, Ho³⁺, Tm³⁺, Nd³⁺ and Tb³⁺) ion, the LLP duration of all phosphors have a great improvement, especially that of the Eu²⁺/Nd³⁺ co-doped phosphor. The Eu²⁺/RE³⁺ (RE³⁺ = Dy³⁺, Ho³⁺, Tm³⁺ and Tb³⁺) co-doped Ba₂Gd₂Si₄O₁₃ phosphors can last 9, 5, 11 and 6 min respectively, and the Eu²⁺/Nd³⁺ co-doped phosphor can last more than 80 min.

Fig. 8 The afterglow decay curves of Ba₂Gd₂Si₄O₁₃:Eu²⁺ and Ba₂Gd₂Si₄O₁₃:Eu²⁺, RE³⁺ (RE³⁺ = Dy³⁺, Ho³⁺, Tm³⁺, Nd³⁺ and Tb³⁺). The inset shows the TL curves of Ba₂Gd₂Si₄O₁₃ host, Ba₂Gd₂Si₄O₁₃:Eu²⁺ and Ba₂Gd₂Si₄O₁₃:Eu²⁺, RE³⁺ (RE³⁺ = Dy³⁺, Ho³⁺, Tm³⁺, Nd³⁺ and Tb³⁺).

Generally, LLP is governed by the slow liberation of trapped charge carriers by thermo.⁴¹ Its performance should be closely related to the distribution of traps. Usually, the information about the traps can be obtained by the TL curve.⁴² The inset of Fig. 8 shows the TL curves of Ba₂Gd₂Si₄O₁₃ host, Ba₂Gd₂Si₄O₁₃:Eu²⁺ and Ba₂Gd₂Si₄O₁₃:Eu²⁺, RE³⁺ (RE³⁺ = Dy³⁺, Ho³⁺, Tm³⁺, Nd³⁺ and Tb³⁺) phosphors. Ordinarily, the TL band at somewhere between 50 and 120 °C is suitable for LLP materials to free the trapped carriers slowly by thermal energy at room temperature.^43,44 In Ba₂Gd₂Si₄O₁₃ host, there exists a TL band peaking at 53 °C, corresponding to a kind of traps. It can been seen more clearly in Fig. 10b. The corresponding traps could be ascribed to the intrinsic defects, such as oxygen vacancies in the host.⁴⁵ With the introduction of RE^2+/3+ ions, the profiles of the TL bands peaking at 53 °C are similar while their intensities rise, indicating that more intrinsic defects arise in the doped phosphors. In particular, the TL intensity of the Eu²⁺/Nd³⁺ co-doped phosphor is enhanced most greatly, resulting in a greatest improvement on the LLP performance. For the Eu²⁺/Tm³⁺ co-doped phosphor, new TL bands corresponding to new defects created by the introduction of Tm³⁺, appear in the range from 100 to 250 °C. However, they almost have no contribution to the LLP performance due to that the trapped electrons in the deep traps cannot readily be released to generate LLP through thermal relaxation at room temperature, thereby it will not be discussed in detail here.

Fig. 9 (a) The afterglow decay curves of Ba_2−xGd_2−xSi₄O₁₃:xEu²⁺, xNd³⁺ (0.001 ≤ x ≤ 0.06). (b) The dependence of the LLP duration on Eu²⁺/Nd³⁺ doping concentration x. (c) The PL spectrum and phosphorescence spectra of Ba_1.98Gd_1.98Si₄O₁₃:0.02Eu²⁺, 0.02Nd³⁺ (solid) and Ba₂Gd₂Si₄O₁₃:Eu²⁺ (dash). (d) CIE chromaticity coordinates for the PL and phosphorescence of Ba_1.98Gd_1.98Si₄O₁₃:0.02Eu²⁺, 0.02Nd³⁺. Photos of Ba_1.98Gd_1.98Si₄O₁₃:0.02Eu²⁺, 0.02Nd³⁺ before (e), under (f) and after (g) UV irradiation.

Fig. 10 (a) TL curves of Ba_2−xGd_2−xSi₄O₁₃:xEu²⁺, xNd³⁺ (0.001 ≤ x ≤ 0.06). The inset shows the dependence of the TL intensity on Eu²⁺/Nd³⁺ doping concentration x. (b) The normalized TL curves of Ba₂Gd₂Si₄O₁₃ host and Ba_1.98Gd_1.98Si₄O₁₃:0.02Eu²⁺, 0.02Nd³⁺.

In view of the outstanding effect of Nd³⁺ ion on the LLP performance, the afterglow decay curves and TL curves of the co-doped phosphors with different Eu²⁺/Nd³⁺ doping concentrations were measured and studied in detail. Fig. 9a shows the afterglow decay curves of Ba_2−xGd_2−xSi₄O₁₃:xEu²⁺, xNd³⁺ (0.001 ≤ x ≤ 0.06) phosphors, and Fig. 9b shows the dependence of the LLP duration on Eu²⁺/Nd³⁺ doping concentration. Generally, for the majority of LLP materials, the doping concentration for the optimal LLP behavior is different from that for the optimal photoluminescence property. In this work, the experimental results show that the phosphor with the longest LLP duration is Ba_1.98Gd_1.98Si₄O₁₃:0.02Eu²⁺, 0.02Nd³⁺ phosphor rather than Ba_1.96Gd_1.96Si₄O₁₃:0.04Eu²⁺, 0.04Nd³⁺ phosphor which possesses the strongest emission intensity. The persistent emission of Ba_1.98Gd_1.98Si₄O₁₃:0.02Eu²⁺, 0.02Nd³⁺ phosphor can last for 3 h approximately.

Fig. 9c shows the PL spectrum of Ba_1.98Gd_1.98Si₄O₁₃:0.02Eu²⁺, 0.02Nd³⁺ phosphor and its phosphorescence spectra measured at different times after the excitation source is switched off. The phosphorescence spectra are plotted with an amplification of 20 times. None of Eu³⁺ characteristics emission is observed. It is interesting that different from the PL spectra, the phosphorescence spectra only consist of the Eu_Ba emission. The profiles do not change with the decay time, indicating that the LLP with cyan color originates from the Eu_Ba emission rather than the Eu_Gd emission. Normally, for most LLP materials, phosphorescence color should be the same as PL color due to the same emission center, whereas in this work, the green emitting phosphors show a kind of cyan phosphorescence. In order to explain this interesting phenomenon, the phosphorescence spectrum (dash) of Ba₂Gd₂Si₄O₁₃:Eu²⁺ phosphor is given in Fig. 9c as well and plotted with an amplification of 400 times. It also only consists of the Eu_Ba emission. In combination with Fig. 5, it is found that the introduction of Nd³⁺ ion has no effect on the profiles of the PL and phosphorescence spectra but their intensities. Namely, the Ba²⁺/Gd³⁺ site occupancy ratio of Eu²⁺ ion has no change with the introduction of Nd³⁺ ion in Ba₂Gd₂Si₄O₁₃ host. Thus, in the LLP process, the cyan phosphorescence is not induced by the preferential substitution of Eu²⁺ ion for Ba²⁺ site. Namely, the Eu²⁺ ion occupying the Gd³⁺ site is not involved in the LLP process indeed. As a conclusion, the interesting phenomenon could be explained as follows. Under UV excitation, the 4f⁷ ground-state electrons of Eu²⁺ ions are photoionized to the 4f⁶5d¹ excited state and Eu²⁺ ions are temporarily oxidized to Eu³⁺ ions. A part of excited electrons immediately recombine with the metastable Eu³⁺ ions to generate luminescence, and simultaneously others can move through the conduction band and then are captured by the traps, resulting in that some metastable Eu³⁺ ions stay on the Ba²⁺ and Gd³⁺ sites for a relatively long time. In view of the valence and ionic radii of the europium and substituted cations in Ba₂Gd₂Si₄O₁₃ host, the europium would rather be in the form of Eu²⁺ ion on the Ba²⁺ site while it prefers to be in the form of Eu³⁺ ion on the Gd³⁺ site, so that the metastable Eu³⁺ ions staying on the Gd³⁺ sites would gradually turn into the steady state. Accordingly, when the captured electrons are thermally released at room temperature, they would only recombine with the metastable Eu³⁺ ions on the Ba²⁺ sites rather than those on the Gd³⁺ sites, thereby giving rise to the cyan phosphorescence.

Fig. 9d shows the Commission International de L'Eclairage (CIE) chromaticity diagram for PL and phosphorescence of Ba_1.98Gd_1.98Si₄O₁₃:0.02Eu²⁺, 0.02Nd³⁺ phosphor. It can be seen easily that the PL color is green (0.2861, 0.5026) while the phosphorescence color is cyan (0.2022, 0.3714). As can be seen in the Fig. 9(e–g), the photos of Ba_1.98Gd_1.98Si₄O₁₃:0.02Eu²⁺, 0.02Nd³⁺ phosphor are taken before UV excitation, under UV excitation (PL) and at 30 s after the UV excitation source is switched off (phosphorescence), respectively.

Considering the excellent LLP performance of Ba_2−xGd_2−xSi₄O₁₃:xEu²⁺, xNd³⁺ (0.001 ≤ x ≤ 0.06) phosphors, the TL curves of the phosphors are investigated and depicted in Fig. 10a, and the inset shows the dependence of the TL intensity on Eu²⁺/Nd³⁺ doping concentration. With increasing doping concentration, the intensity of the TL band peaking at 53 °C ascribed to the intrinsic defects, increases at first and decreases when the concentration is more than 2 mol%. In consideration of the different ionic radii between the doped and substituted ions, the introduction of the RE^2+/3+ ions will lead to lattice distortion which results in large generation of the intrinsic defects as energy trap centers for photoenergy storage, such as oxygen vacancies, in the host. As a consequence, with increasing Eu²⁺/Nd³⁺ doping concentration, the number of energy trap centers should rise. And more and more excited electrons would be captured by traps rather than return immediately to emission centers, which is beneficial to the LLP performance. However, when the concentration is more than 2 mol%, the distances of traps to traps or traps to emission centers is so close that the excited electrons prefer to transfer from one center to another with non-radiative transition, such as via thermal energy, leading to the decrease of the direct radiation transition for LLP, which corresponds to the variation tendency of the LLP duration.^46,47

In order to further estimate the trap state, the trap depths (E_t) and trap densities (n₀) were calculated by using Chen's equation.⁴⁸

where T_m is the temperature of the TL peak, β is the heating rate, ω, the full width at half maximum (FWHM), is known as the shape parameter and defined as ω = δ + τ, τ is the low-temperature half-width, δ is the high-temperature half-width, the asymmetry parameter is defined as μ_g = δ/(δ + τ), κ_B is the Boltzmann constant (1.38 × 10⁻²³ J K⁻¹) and I_m is the intensity of TL peak. The normalized TL curves of Ba₂Gd₂Si₄O₁₃ host and Ba_1.98Gd_1.98Si₄O₁₃:0.02Eu²⁺, 0.02Nd³⁺ phosphor are shown in Fig. 10b. It is easy to obtain the TL parameters. The calculated results are listed in Table 1. It can be seen from Table 1 that the trap depth and trap density of Ba₂Gd₂Si₄O₁₃ host are about 0.713 eV and 9.547 × 10⁴, respectively, and these of Ba_1.98Gd_1.98Si₄O₁₃:0.02Eu²⁺, 0.02Nd³⁺ phosphor are about 0.670 eV and 6.182 × 10⁶. It is reported that a suitable trap depth in this range 0.65–0.75 eV is essential for phosphors to show long persistence.^49,50 The trap depth of the intrinsic defects of Ba₂Gd₂Si₄O₁₃ host is suitable for the LLP, but the trap density is too low. Therefore, we conclude that the difference in LLP duration only comes from the difference in trap density, and the doping mainly changes the quantity of the intrinsic defects to control the LLP duration. Namely, co-doping RE³⁺ ion indeed has a great effect upon the LLP performance of the phosphors.

Table 1 TL parameters of Ba₂Gd₂Si₄O₁₃ host and Ba_1.98Gd_1.98Si₄O₁₃:0.02Eu²⁺, 0.02Nd³⁺

	Tm (K)	Glow-peak parameters				Et (eV)	n0
		τ	δ	ω	μg
Host	326	19	46	65	0.708	0.713	9.547 × 10^4
x = 0.02	326	22	30	52	0.577	0.670	6.182 × 10^6

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3.5. The mechanism of LLP

To understand the dynamical process of LLP clearly, a model based on the above experimental and calculated results for Ba_1.98Gd_1.98Si₄O₁₃:0.02Eu²⁺, 0.02Nd³⁺ phosphor is proposed and illustrated in Fig. 11. Under UV excitation, the 4f⁷ ground-state electrons of Eu²⁺ ions located on the Ba²⁺ and Gd³⁺ sites are photoionized to the 4f⁶5d¹ excited state and Eu²⁺ ions are temporarily oxidized to metastable Eu³⁺ ions (excitation process 1). A part of excited electrons immediately recombine with the two types of metastable Eu³⁺ emission centers to generate green luminescence (PL process 2), and simultaneously others can move through the conduction band and then are captured by the traps (electron trapping process 3). Because the depth of 0.670 eV for the traps is suitable, the trapped electrons can be thermally released at room temperature and select to recombine with the metastable Eu³⁺ ions on the Ba²⁺ sites rather than those on the Gd³⁺ sites, finally giving rise to the cyan phosphorescence (LLP process 4).

Fig. 11 The schematic diagram of PL and LLP mechanism for Ba_1.98Gd_1.98Si₄O₁₃:0.02Eu²⁺, 0.02Nd³⁺.

4. Conclusions

In summary, novel LLP phosphors Ba₂Gd₂Si₄O₁₃:Eu²⁺, RE³⁺ (RE³⁺ = Dy³⁺, Ho³⁺, Tm³⁺, Nd³⁺ and Tb³⁺) are successfully synthesized by a solid state reaction. The green emitting phosphors show a cyan phosphorescence phenomenon induced by the emission of Eu²⁺ ion only occupying the Ba²⁺ site. Co-doping the RE³⁺ ions can improve the LLP performance effectively, and with the moderate introduction of Nd³⁺ ion the most persistent cyan emission can last for 3 h approximately. It is revealed that the introduction of the RE^2+/3+ ions results in larger generation of the intrinsic defects with a trap depth about 0.713 eV, which exactly contributes to the LLP performance. This work provides a promising cyan LLP material for practical application, and proposes a new approach for the design of LLP materials to control the phosphorescence color by the distinct emission of Eu²⁺ ion at the different crystallographic sites in one host.

Acknowledgements

This work is supported by Specialized Research Fund for the Doctoral Program of Higher Education (Grant No. 20120211130003) and the National Natural Science Funds of China (Grant No. 51372105). Thanks for the support of Gansu Province Development and Reform Commission.

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