English

• 1. INTRODUCTION

  Persistent luminescence materials (PLMs) can absorb excitation energy and continue to emit light after the excitation is stopped^[1-7]. In ancient China, natural minerals with luminous properties were made into luminous cups and night pearls. In 1866, Sidot et al. first prepared the sulfide PLMs ZnS: Cu²⁺, making PLMs enter the vision of researchers^[8]. In 1968, Pailil et al. observed the persistent luminescence of SrAl₂O₄: Eu²⁺, pushing the researches of PLMs into a new stage^{[9, 10]}. Subsequently, Matsuzawa et al. found that the phosphorescence intensity and phosphorescence time of SrAl₂O₄: Eu²⁺, Dy³⁺ phosphors were more than ten times those of early PLMs^[11]. Since then, many PLMs have been developed and widely used in transportation, military facilities, biosensing and other fields^[12-15].

  The persistent luminescence inherence makes PLMs proper candidates in material applications. Given this, many researchers are devoted to finding ways to improve the phosphorescence intensity and phosphorescence time of PLMs. The introduction of doped ions provides a possibility to improve the optical performance of PLMs^[16-23]. For example, Yamamoto et al. discovered that the phosphorescence intensity and phosphorescence time of the materials were improved after introducing Dy³⁺ ion into SrAl₂O₄: Eu^2+[11]. Liu et al. enhanced the phosphorescence intensity and prolonged the phosphorescence time exceeding 13 hours by adjusting the contents of Nd³⁺ ion in Zn₂Ga_3–x–yGe_0.75O₈: Cr_x, Nd_y^[24]. Studying the mechanisms of ion doping to enhance the optical performance of PLMs can help design PLMs with stronger phosphorescence intensity and longer phosphorescence time, thereby expanding the applcations of PLMs.

  Recently, reviews focus on synthesis methods, persistent luminescence mechanisms and applications of PLMs have been published. Li et al. systematically summarized the synthesis techniques, persistent luminescence mechanisms, characterizations and applications of PLMs^[9]. Wang et al. summarized the persistent luminescence mechanisms, synthesis methods and biomedical applications of PLMs^[25]. Singh et al. outlined the surface modifications, bioimaging applications of PLMs, and future research directions^[26]. Doped ions play a vital role in improving the optical performance of PLMs. However, the influence of doped ions on the structure and optical performance of PLMs is seldom summarized. In this review, we mainly focus on how the optical performance of PLMs is affected by the type of doped ions, the number of types of doped ions, and the content of doped ions. Furthermore, the opportunities and challenges of PLMs in ion doping are also presented.

  2. INFLUENCE OF DOPED IONS ON THE STRUCTURE AND OPTICAL PERFORMANCE OF PLMs

  With the introduction of doped ions, different energy levels are produced in PLMs, resulting in different structures and optical performance. Herein, we classify and discuss how doped ions affect the structure and optical performance of PLMs based on the number of types of doped ions, the type of doped ions and the content of doped ions. To better demonstrate the influence of doped ions on PLMs, a summary of the main influence of doped ions on some common ion-doped PLMs is shown in Table 1.

  Table 1

  

  Table 1.  Details of Host, Emitter, Co-dopants, Emission Region and Afterglow Decay of PLMs

  DownLoad: CSV

  Host	Emitter	Co-dopants	Emission region	Afterglow decay	Ref.
  SrAl2O4	Eu2+		450~700 nm	> 2 h	4
  GdAlO3	Mn2+	Ge4+	600~840 nm	> 1200 s	64
  Y3Al5–xGaxO12 (x = 0~4)	Ce3+	Bi3+	505 nm	54~68 min	67
  ZnGa2O4	B3+		502, 695 nm	600 s	18
  Zn1.25Ga1.5Ge0.25O4	Cr3+	Yb3+, Er3+	650~850 nm	> 480 h	23
  Li2ZnGeO4	Mn2+		550~800 nm	> 960 h	62
  MgGeO3	Mn2+	Bi3+	680 nm	100 min	68
  Y3Sc2Ga3O12	Ce3+		500 nm	100 min	69
  CaMgSi2O6	Eu2+	Dy3+, Mn2+	468, 600, 670 nm	> 600s	66
  Ca9Bi(PO4)7	Ce3+	Tb3+, Mn2+	300~500 nm	5 h	70
  Ca2BO3Cl	Eu2+	Dy3+	580 nm	48 h	71
  Gd2O2S	Eu3+	Ti4+, Mg2+	580~720 nm	120 s	20
  Lu2O3	Tb3+	Ca2+	543 nm	20~30 h	72
  KY3F10	Tb3+		542 nm	108 s	73
  AlN	Mn2+		570~700 nm	1 h	74

  2.1 Influence of doped ions type on the structure and optical performance of PLMs

  Due to the difference in the atomic structure and energy levels, various doped ions have different effects on the structure and optical performance of PLMs. Thus, the relationship between the type of doped ions and the structure and optical performance of PLMs is introduced in this section.

  2.1.1   Cr³⁺-doped PLMs

  Cr³⁺ ion is usually introduced into PLMs to obtain red or near-infrared persistent luminescence resulting from its unique electronic transitions. Among them, Cr³⁺-doped ZnGa₂O₄ is one of the research hotspots in PLMs. The ZnGa₂O₄ crystal exhibits a cubic spinel structure, in which the Zn²⁺ ion occupies the A-sites of the tetrahedron, and the Ga³⁺ ion the B-sites of the octahedron^{[27, 28]}. Cr³⁺ ion tends to replace Ga³⁺ ion in the distorted octahedral coordination for the reason that Cr³⁺ ion has the same valence electron and ion radius as Ga³⁺ ion, which makes the Cr³⁺-doped ZnGa₂O₄ materials produce strong near-infrared phosphorescence at 696 nm^{[2, 29, 30]}. Hao et al. found that the Cr³⁺-doped ZnGa₂O₄ exhibits excellent optical performance, which can maintain phosphorescence for 10 hours after stopping the ultraviolet light irradiation^[31]. Moreover, the author observed that the excitation spectrum of Cr³⁺-doped ZnGa₂O₄ has three excitation bands of 250~350, 350~487, and 487~650 nm, which are caused by the host excitation band of ZnGa₂O₄, the charge transfer between ZnGa₂O₄ and Cr³⁺ ion, and the electronic transitions of Cr³⁺ ion, respectively. Based on previous researches, Yan et al. further comprehensively studied the persistent luminescence mechanism of Zn_2.94Ga_1.96Ge₂O₁₀: Cr^3+[19]. As shown in Fig. 1, after the Zn_2.94Ga_1.96Ge₂O₁₀ absorbs incident photons, the electrons of Zn_2.94Ga_1.96Ge₂O₁₀ move to the conduction band and are trapped by native defects by means of nonradiative relaxation. When the ultraviolet light excitation is finished, the combination of holes and electrons released by native defects produces short phosphorescence. Since the absorption spectrum of Cr³⁺ ion fully overlaps with the emission spectrum of Zn_2.94Ga_1.96Ge₂O₁₀, the energy absorbed by Zn_2.94Ga_1.96Ge₂O₁₀ is transferred to Cr³⁺ ion by means of nonradiative energy^[32]. The continuous energy transfer causes the electrons of Cr³⁺ ion to transform into three different excited states. Then, the electrons are trapped by the shallow electron traps or deep electron traps by means of nonradiative relaxation^[33]. At the same time, the electron (t²e) fills the energy-matched traps in the form of ⁴T₁ and ⁴T₂ through the tunneling process. When the continuous energy transfer is stopped, the electron traps release the electrons which recombine with Cr³⁺ ion, thereby producing a strong or ultra-long phosphorescence^[19].

  Figure 1

  

  Figure 1.  Persistent energy transfer mechanism of Cr³⁺-doped ZnGa₂O₄ materials

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  2.1.2   Mn²⁺-doped PLMs

  Mn²⁺ ion has a typical 3d⁵ electronic configuration. The energy transitions of Mn²⁺ ions are quite sensitive to the ligand/crystal field due to the participation of the d shell. Mn²⁺ ions surrounded by anions can have different geometric structures, such as linear, octahedron, spherical, tetrahedron, or square plane. The environment of Mn²⁺ ion at different crystal fields leads to different emission energy distributions and produces an emission band of 450~750 nm. For example, tetrahedral coordination of Mn²⁺ ion produces green emission^[34], while octahedral coordination of Mn²⁺ ion produces orange to red emission^{[35, 36]}. Tan et al. researched the size, crystal structure and optical performance of Mn²⁺-doped ZnGa₂O₄ with a typical rod-shaped structure in different environments^[21]. The size of Mn²⁺-doped ZnGa₂O₄ rapidly decreases as the pH increases from 6 to 7.5. In comparison, when the pH further increases to 9.5, the size of the Mn²⁺-doped ZnGa₂O₄ materials slightly increases to 80 nm. The detailed crystal structure of the Mn²⁺-doped ZnGa₂O₄ shows that the materials have lattice fringe (110) parallel to the direction of the crystal rod and lattice fringe (113) at an angle of 66° to the direction of the crystal rod^[37]. Their interplanar spacing is 0.71 and 0.29 nm, indicating that the Mn²⁺-doped ZnGa₂O₄ grows along the c axis^[38]. The optical performance of ZnGa₂O₄: Mn²⁺ materials was further researched by researchers^[39]. The photoluminescence spectrum of Mn²⁺-doped ZnGa₂O₄ materials shows that two emission peaks are detected at 450 and 480 nm, which are resulted from the native defects, for example, interstitial Zn and oxygen vacancies (Fig. 2a). When the pH is below 7.0, the intrinsic luminescence of the materials and the emission of Mn²⁺ ions have a serious overlap. As the pH increases, the intrinsic luminescence intensity gradually decreases, and the emission band intensity of the Mn²⁺ ion increases. The intrinsic luminescence almost disappears and is dominated by the green emission band of Mn²⁺ when the pH increases to 9.5. In this case, the phosphorescence time of Mn²⁺-doped ZnGa₂O₄ materials exhibits more than 100 s (Fig. 2b). According to the above results, a possible persistent luminescence mechanism of Mn²⁺-doped ZnGa₂O₄ materials is proposed (Fig. 2c)^{[40, 41]}. The excited electrons and holes produced under ultraviolet light excitation are trapped by electron traps and hole traps, respectively^[42]. Then, some electrons get away from the electron traps under thermal stimulation and transfer to native defects, thereby resulting in the formation of emission peaks from ZnGa₂O₄ materials^[43]. The other part of the electrons escape and transfer to the excitation level of the Mn²⁺ ion^[44]. Subsequently, the recombination of holes and electrons causes Mn²⁺ ion to emit green light^[45].

  Figure 2

  

  Figure 2.  (a) Photoluminescence spectrum, (b) phosphorescence decay curve, (c) persistent energy transfer mechanism of ZnGa₂O₄: Mn²⁺ materials

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  2.1.3   Eu²⁺-doped PLMs

  Eu²⁺ ion is a common doped ion in PLMs. The introduction of Eu²⁺ ion usually produces yellow, orange and red emissions for the reason that the crystal field reduces the emission energy of 4f⁶5d¹ electron configuration of Eu²⁺ ion^[46]. Yang et al. researched the structure and optical performance of Eu²⁺-doped SrAl₂O₄ materials^[4]. It is found that the crystal phase of the Eu²⁺-doped SrAl₂O₄ materials corresponds to the standard pattern, indicating that the structure of the SrAl₂O₄ materials has not obviously changed after the introduction of Eu²⁺ ion. Moreover, Eu²⁺-doped SrAl₂O₄ shows a strong emission band of 450~700 nm and exhibits a phosphorescence time that exceeds 2 hours. Considering the structure and characteristics of SrAl₂O₄: Eu²⁺, a possible persistent luminescence mechanism of SrAl₂O₄: Eu²⁺ is deduced. The SrAl₂O₄ material as the host lattice absorbs the soft X-ray photon energy and stores it in electron traps. Then, the energy gets away from the electron traps and transfers to the 4f⁶5d¹ energy level in the Eu²⁺ ion, which causes the Eu²⁺ ion to produce radioactive transition luminescence.

  2.1.4   B³⁺-doped PLMs

  B³⁺ ion can replace Ga³⁺ ion in ZnGa₂O₄ materials to improve the optical performance of ZnGa₂O₄ materials because of the similarity of valence and chemical properties between Ga³⁺ and B³⁺ ions. Wang et al. researched the changes in the optical performance of ZnGa₂O₄ materials after B³⁺-doping^[18]. Both ZnGa₂O₄ and ZnGa₂O₄: B³⁺ can detect phosphorescence signal peaks at 502 and 695 nm. However, the phosphorescence decay curves indicate that the phosphorescence intensity and the phosphorescence time of ZnGa₂O₄: B³⁺ have been significantly improved compared with ZnGa₂O₄. According to previous reports, the proper trap depth is essential to achieve a better optical performance of PLMs^{[1, 47, 48]}. There is a bold inference that the introduction of B³⁺ ion can also increase the content of electron traps $ {\mathrm{G}\mathrm{a}}_{\mathrm{Z}\mathrm{n}}^{\mathrm{\text{'}}} $, thus leading to the enhancement of optical performance of ZnGa₂O₄ after B³⁺-doping.

  2.2 Influence of the number of types of doped ions on the structure and optical performance of PLMs

  As above mentioned, the structure and optical performance of PLMs are closely related to the type of doped ions. Most PLMs used in practical applications need to be doped with multiple ions to enhance optical performance. The simultaneous introduction of multiple ions will affect not only the host of PLMs, but also the interaction among different ions. Therefore, we introduce the influence of the number of types of doped ions on the structure and optical performance of PLMs in this section.

  2.2.1   Ge⁴⁺ and Cr³⁺ co-doped PLMs

  The introduction of Ge⁴⁺ ion will change the width of the material bandgap due to its unique electronic structure. Thus, the change of structure and optical performance of Ge⁴⁺, Cr³⁺ co-doped PLMs is discussed^[5]. Wang et al. found that the crystal phases of the Cr³⁺-doped ZnGa_1.995O₄ and Ge⁴⁺, Cr³⁺ co-doped ZnGa_1.995O₄ materials are corresponding to the standard pattern, indicating that the structure of the materials has not obviously changed after Ge⁴⁺, Cr³⁺ co-doping. In addition, the emission spectra of ZnGa_1.995O₄: Cr³⁺ and Zn_1.25Ga_1.5Ge_0.25O₄: Cr³⁺ both present a near-infrared emission band of 650~900 nm^{[49, 50]}. The phosphorescence decay curves show that the optical performance of Zn_1.25Ga_1.5Ge_0.25O₄: Cr³⁺ is better than that of ZnGa_1.995O₄: Cr^3+[5]. Based on the above results, the introduction of Ge⁴⁺ improves the filling efficiency of electron traps, thus increasing the phosphorescence time of the materials^[23].

  2.2.2   Pr³⁺, Cr³⁺ co-doped PLMs

  The number and depth of the trap energy levels of PLMs are changed after the introduction of Pr³⁺ ion because of the special 4f5d electronic structure and abundant electronic transition types of Pr³⁺ ion. Therefore, the changes in structure and optical performance of Zn_2.94Ga_1.96Ge₂O₁₀ after the introduction of Pr³⁺ and Cr³⁺ ions are discussed^[19]. By analyzing the XRD pattern, the structure of Zn_2.94Ga_1.96Ge₂O₁₀: Cr³⁺, Pr³⁺ has not obviously changed, and the pure spinel phase is still maintained. Later, the author does in-depth studies on the optical performance of Zn_2.94Ga_1.96Ge₂O₁₀: Pr³⁺, Cr³⁺. The electrons recombine with holes in the natural defects, thereby leading to a wide emission band of 350~660 nm. In addition, the Cr³⁺ ion provides a near-infrared emission band of 695 nm through the ²E→⁴A₂ transition^[27], while the Pr³⁺ ion increases the phosphorescence time by adjusting the depth and density of traps based on the energy levels of 4f electron configurations^[51-55]. Similarly, Yu et al. did further research on the persistent luminescence mechanisms of Zn₃Ga₂GeO₈: Cr³⁺, Pr³⁺ to clarify the function of Pr³⁺ ion^[56]. The introduction of Pr³⁺ ion deepens the trap energy level and increases the number of traps. Given this, a possible persistent luminescence mechanism of Zn₃Ga₂GeO₈: Cr³⁺, Pr³⁺ is proposed (Fig. 3). Trap A forms more traps than trap A' by introducing Pr³⁺ ion, thereby improving the phosphorescence intensity and the phosphorescence time of Zn₃Ga₂GeO₈: Cr³⁺, Pr³⁺ materials.

  Figure 3

  

  Figure 3.  Persistent luminescence mechanism of Zn_2.94Ga_1.96Ge₂O₁₀: Pr³⁺, Cr³⁺ materials

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  2.2.3   Yb³⁺, Er³⁺, Cr³⁺ co-doped PLMs

  Yb³⁺ ion and Er³⁺ ion are rare earth element ions used as luminescence centers, which are usually introduced into PLMs to change the density and depth of the trap levels resulting from their unique electronic structure. Consequently, the structure and optical performance of Zn_1.25Ga_1.5Ge_0.25O₄: Yb³⁺, Er³⁺, Cr³⁺ (ZGGO: Cr³⁺, Yb³⁺, Er³⁺) with a pure spinel phase is discussed^[23]. Yan et al. found that the structure of Zn_1.25Ga_1.5Ge_0.25O₄ has not obviously changed after doping with Yb³⁺, Er³⁺ and Cr³⁺ ion. Moreover, ZGGO: Cr³⁺, Yb³⁺, Er³ presents an excellent optical performance, and the phosphorescence time exceeds 20 days. Later, the author studied the persistent luminescence mechanism of ZGGO: Yb³⁺, Er³⁺, Cr³⁺ (Fig. 4). The excited electrons are captured by electron traps under ultraviolet light irradiation, and then moves to deep traps by means of nonradiative relaxation. Subsequently, the combination of the excited electrons and the Cr³⁺ ion leads to the formation of a persistent phosphorescence signal after the ultraviolet radiation is stopped. The introduction of Yb³⁺ and Er³⁺ ions can not only provide the materials with additional electrons and energy levels, but also adjust the density and depth of traps^{[51, 57, 58]}. The excited electrons are trapped in the deepest 4f energy levels of Yb³⁺ and Er³⁺ ions, thereby extending the time for the trapped electrons to return to the ²E energy level of Cr³⁺ ion (Fig. 4a, b)^[58], which also obviously prolongs the phosphorescence time of materials.

  Figure 4

  

  Figure 4.  Persistent luminescence mechanism of (a) ZGGO: Cr³⁺, (b) ZGGO: Cr³⁺, Yb³⁺, Er³⁺, (c) ZGO: Cr³⁺, Yb³⁺, Er³⁺. ZGO refers to a zinc gallate matrix

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  2.2.4   Eu³⁺, Ti⁴⁺ and Mg²⁺ co-doped PLMs

  Viana et al. researched the structure and optical performance of Eu³⁺, Ti⁴⁺, Mg²⁺ co-doped Gd₂O₂S materials with a pure hexagonal Gd₂O₂S phase^[20]. The study found that Ti⁴⁺ and Mg²⁺ ions are trapping centers, and Eu³⁺ ions are the emission centers. The Gd₂O₂S: Eu³⁺, Ti⁴⁺, Mg²⁺ has an emission band of 580~720 nm due to the electronic transitions of Eu³⁺ ion. Therefore, it can be inferred that the Eu³⁺ ion plays the role of the luminescence center in the materials. Moreover, Gd₂O₂S: Eu³⁺, Ti⁴⁺, Mg²⁺ has a longer phosphorescence time compared with Gd₂O₂S: Eu³⁺ for the reason that Ti⁴⁺ and Mg²⁺ ions play an important in improving the density of trap energy levels. However, the particle size and crystal structure of Gd₂O₂S materials have not obviously changed after the introduction of Eu³⁺, Ti⁴⁺ and Mg²⁺ ions.

  2.3 Influence of the content of doped ions on the

  2.4 structure and optical performance of PLMs

  As mentioned above, some doped ions play the role of the emission center of PLMs, while other doped ions will affect the number and depth of trap levels. Therefore, it is of great necessity to understand how the content of doped ions influences the structure and optical performance of PLMs. This section will discuss how the content of doped ions affects the structure and optical performance of PLMs.

  2.3.1   Pr³⁺, Cr³⁺ co-doped PLMs

  Normally, Cr³⁺ ion serves as the luminescence center of PLMs, while Pr³⁺ ion affects the number and depth of trap levels. Yan et al. studied how the contents of Pr³⁺ and Cr³⁺ ions influence the structure and optical performance of PLMs. It is found that the increase of Cr³⁺ content promotes the energy release of Zn_2.94Ga_1.96Ge₂O₁₀: Cr³⁺, Pr³⁺. Therefore, as the amount of Cr³⁺ ion doping increases, the phosphorescence intensity and phosphorescence time of the material decrease^{[19, 27, 59]}. Similarly, Zhang et al. studied the changes in particle size of Pr³⁺, Cr³⁺ co-doped Zn₂Ga_2.98–xGe_0.75O₈ materials with different Pr³⁺ contents^[22]. It is found that the average size of the materials decreases with the increase of Pr³⁺ contents. Considering that the doped ions can strongly affect the crystal growth rate through surface charge modification^[60], the size reduction of Zn₂Ga_2.98–xGe_0.75O₈: Cr³⁺, Pr³⁺ may be related to the influence of Pr³⁺ ion on the surface charge of the material. Specifically, the surface charge distribution of the materials will be changed when the Pr³⁺ ion is introduced into the materials, thereby reducing the growth rate. Subsequently, the optical performance of Pr³⁺, Cr³⁺ co-doped Zn₂Ga_2.98–xGe_0.75O₈ materials is researched. As shown in Fig. 5a, the phosphorescence intensity and the phosphorescence time of the materials increase as the increase of Pr³⁺ doping contents due to the changes in the trap levels of the materials caused by the introduction of Pr³⁺ ion (Fig. 5b). When the content of Pr³⁺ ion is too high, the luminescence performance will decrease due to the concentration quenching effect.

  Figure 5

  

  Figure 5.  (a) Phosphorescence decay curves of Pr³⁺, Cr³⁺ co-doped Zn₂Ga_2.98–xGe_0.75O₈ materials with different Pr³⁺ doping contents. The inset presents the phosphorescence intensity of the materials after the ultraviolet light irradiation is stopped, (b) Thermo-luminescence spectra of Pr³⁺, Cr³⁺ co-doped Zn₂Ga_2.98–xGe_0.75O₈ materials with different Pr³⁺ doping contents

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  2.3.2   Bi³⁺, Cr³⁺ co-doped PLMs

  Bi³⁺ ion can significantly reduce the bandgap energy of PLMs and improve the efficiency of electron trap filling resulting from its unique 6s orbital. Tuerdi et al. researched the influence of crystalline structure, emission peak wavelength and phosphorescence time of ZnGa₂O₄: Bi³⁺, Cr³⁺ materials with different Bi³⁺ doping contents^[16]. The Bi³⁺ ion tends to replace Ga³⁺ ion in ZnGa₂O₄ with a similar ion radius, which makes the material produce higher periodic lattice distortion, thereby making the materials exhibit a longer phosphorescence time. When the doped ratio of Bi³⁺ ion is low, the crystal phase of Cr³⁺, Bi³⁺ co-doped ZnGa₂O₄ materials corresponds to its standard spectrum, indicating that the structure of ZnGa₂O₄ materials has not obviously changed after the introduction of Bi³⁺ ion. However, when the doped ratio of Bi³⁺ ion increases to 0.03, a strong peak related to the rhombohedral Bi³⁺ structure at 28.01° is observed in the XRD pattern (Fig. 6a), illustrating that the original crystal structure of the materials is destroyed. Correspondingly, its optical performance is also reduced. The phosphorescence emission spectrum of ZnGa₂O₄: Bi³⁺, Cr³⁺ materials with different Bi³⁺ doping contents shows a red-shift of emission peak wavelength, which indicates that Bi³⁺ ion reduces the crystal field intensity of the Cr³⁺ ion (Fig. 6b).

  Figure 6

  

  Figure 6.  (a) XRD patterns of ZnGa₂O₄, ZnGa₂O₄: Cr³⁺ and ZnGa₂O₄: Cr³⁺, Bi³⁺, (b) Phosphorescence spectra of ZnGa₂O₄, ZnGa₂O₄: Cr³⁺ and ZnGa₂O₄: Cr³⁺, Bi³⁺

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  2.3.3   B³⁺, Cr³⁺ co-doped PLMs

  As mentioned above, the introduction of B³⁺ ion can improve the optical performance of ZnGa₂O₄. On this basis, the researchers studied the influence of doped ion content on PLMs. For example, Yan et al. synthesized ZnGa₂O₄: B³⁺, Cr³⁺ materials by hydrothermal method, and studied the optical performance of ZnGa₂O₄: Cr³⁺ with different B³⁺ doping contents^[17]. The XRD pattern presents that the structure of the material has not obviously changed after doping with the B³⁺ and Cr³⁺ ions. Besides, the emission wavelength of the materials has not obviously changed when the B³⁺ ion is introduced into the materials, which indicates that the B³⁺ ion does not play the role of luminescence center in the materials. At the same time, the improvement of the optical performance of materials shows that the introduction of B³⁺ ion injects new electron traps into the materials and increases the content of electron traps.

  2.3.4   Eu³⁺, Ti⁴⁺ and Mg²⁺ co-doped PLMs

  Eu³⁺, Ti⁴⁺ and Mg²⁺ ions will act as luminescence centers or trap levels to affect the optical performance of the material. Therefore, studying the optical performance of Gd₂O₂S: Eu³⁺, Ti⁴⁺ and Mg²⁺ materials with different doping contents will help us better understand the role of these ions in the material^[20]. The study found that Eu³⁺ ion provides trap levels for electron-hole recombination, and Ti⁴⁺ ion provides electron traps to achieve the formation of the phosphorescence signal. Since Ti⁴⁺ ion replaces Gd³⁺ ion and destroys the charge conservation of the materials, it is necessary to introduce Mg²⁺ ion to maintain charge balance. In addition, as the doping contents of Mg²⁺ ion increase, the phosphorescence intensity of the material increases. This indicates that the introduction of Mg²⁺ ion can induce the formation of intermediate trap levels in the material, which prolongs the storage time of the carriers in the material, thus realizing the improvement of the persistent performance of the materials^[61].

  Apart from the ion-doped PLMs mentioned above, other doped ions also have an impact on the structure and optical performance. For example, Li₂ZnGeO₄: Mn^2+[62], SrAl₂O₄: Eu²⁺, Dy^3+[63], GdAlO₃: Mn⁴⁺, Ge^4+[64], LaAlO₃: Mn⁴⁺, Ge^4+[65], CaMgSi₂O₆: Eu²⁺, Dy³⁺, Mn^2+[66], Y₃Al_5–xGa_xO₁₂: Ce³⁺, Bi³⁺ (x = 0~4)^[67], MgGeO₃: Mn²⁺, Bi^3+[68], Y₃Sc₂Ga₃O₁₂: Ce^3+[69], Ca₉Bi(PO₄)₇: Ce³⁺, Tb³⁺, Mn^2+[70], Ca₂BO₃Cl: Eu²⁺, Dy^3+[71], Lu₂O₃: Tb³⁺, Ca^2+[72], KY₃F₁₀: Tb^3+[73] and AlN: Mn^2+[74].

  3. CONCLUSION

  In this review, we mainly focus on the impact of the type of doped ions, the number of types of doped ions, and the content of doped ions on the structure and optical performance of PLMs. In the past decade, although various doped ions have been widely used to improve the optical performance of PLMs, there is still much work to do in the future. First, in the actual preparation process, the optical performance of PLMs will be affected by the symmetry of matrix lattice, the radius of activated ions, and the electronegativity and distribution of external electron cloud. This should be paid more attention to by researchers. Second, exploring the relationship between doped ions and the host lattice and investigating the influence of the defects caused by doped ions on the storage time of the carriers. At present, these issues still have no exact quantitative relationship, and further researches are needed. Third, understanding the role of trap types, trap contents and probability of trapping electrons in the researches of the persistent luminescence mechanisms. Forth, exploring new doped ions and substrates. For example, most current PLMs are based on aluminates, gallates and silicates matrices. It is necessary to find new matrices with excellent properties. Collectively, with the deepening research of PLMs, ion-doped PLMs with higher luminescence intensity and phosphorescence lifetime will have a broad development prospect.

  
  
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  Figure 1  Persistent energy transfer mechanism of Cr³⁺-doped ZnGa₂O₄ materials

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  Figure 2  (a) Photoluminescence spectrum, (b) phosphorescence decay curve, (c) persistent energy transfer mechanism of ZnGa₂O₄: Mn²⁺ materials

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  Figure 3  Persistent luminescence mechanism of Zn_2.94Ga_1.96Ge₂O₁₀: Pr³⁺, Cr³⁺ materials

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  Figure 4  Persistent luminescence mechanism of (a) ZGGO: Cr³⁺, (b) ZGGO: Cr³⁺, Yb³⁺, Er³⁺, (c) ZGO: Cr³⁺, Yb³⁺, Er³⁺. ZGO refers to a zinc gallate matrix

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  Figure 5  (a) Phosphorescence decay curves of Pr³⁺, Cr³⁺ co-doped Zn₂Ga_2.98–xGe_0.75O₈ materials with different Pr³⁺ doping contents. The inset presents the phosphorescence intensity of the materials after the ultraviolet light irradiation is stopped, (b) Thermo-luminescence spectra of Pr³⁺, Cr³⁺ co-doped Zn₂Ga_2.98–xGe_0.75O₈ materials with different Pr³⁺ doping contents

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  Figure 6  (a) XRD patterns of ZnGa₂O₄, ZnGa₂O₄: Cr³⁺ and ZnGa₂O₄: Cr³⁺, Bi³⁺, (b) Phosphorescence spectra of ZnGa₂O₄, ZnGa₂O₄: Cr³⁺ and ZnGa₂O₄: Cr³⁺, Bi³⁺

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  Table 1.  Details of Host, Emitter, Co-dopants, Emission Region and Afterglow Decay of PLMs

  Host	Emitter	Co-dopants	Emission region	Afterglow decay	Ref.
  SrAl2O4	Eu2+		450~700 nm	> 2 h	4
  GdAlO3	Mn2+	Ge4+	600~840 nm	> 1200 s	64
  Y3Al5–xGaxO12 (x = 0~4)	Ce3+	Bi3+	505 nm	54~68 min	67
  ZnGa2O4	B3+		502, 695 nm	600 s	18
  Zn1.25Ga1.5Ge0.25O4	Cr3+	Yb3+, Er3+	650~850 nm	> 480 h	23
  Li2ZnGeO4	Mn2+		550~800 nm	> 960 h	62
  MgGeO3	Mn2+	Bi3+	680 nm	100 min	68
  Y3Sc2Ga3O12	Ce3+		500 nm	100 min	69
  CaMgSi2O6	Eu2+	Dy3+, Mn2+	468, 600, 670 nm	> 600s	66
  Ca9Bi(PO4)7	Ce3+	Tb3+, Mn2+	300~500 nm	5 h	70
  Ca2BO3Cl	Eu2+	Dy3+	580 nm	48 h	71
  Gd2O2S	Eu3+	Ti4+, Mg2+	580~720 nm	120 s	20
  Lu2O3	Tb3+	Ca2+	543 nm	20~30 h	72
  KY3F10	Tb3+		542 nm	108 s	73
  AlN	Mn2+		570~700 nm	1 h	74

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