1. Introduction

Persistent luminescence is a phenomenon whereby luminescence can last for a long time ranging from hours to days after the stoppage of the excitation light [1–4]. Since its discovery in 17th century, phosphors of long-persistent luminescence have gained a great deal of interest due to their use as night-vision materials in numerous important fields such as, security sign, emergency route sign, traffic signage, and medical diagnostics [5–9]. Persistent luminescence phosphors typically involve two kinds of active centers (i.e., emitters and traps) in an appropriate host lattice to create afterglow luminescence. Emitters are the doped metallic ions capable of emitting radiation at designated wavelength, while the traps (typically defined by the host lattice) store the excitation energy and then gradually transfer it to the emitters owing to thermal or other physical stimulations. Persevere investigations of the combination of the inorganic host lattices and the doped metallic emitting ions, have resulted in successful development and marketing of multicolor persistent phosphors in the visible range [10, 11]. The representative ones include CaAl₂O₄:Eu²⁺/Nd³⁺ (blue), SrAl₂O₄:Eu²⁺/Dy³⁺ (green), and Y₂O₂S:Eu³⁺, Mg²⁺, Ti⁴⁺ (red) phosphors of three primary colors [10, 12, 13]. These phosphors are able to emit strong and long-lasting (>10 h) luminescence under excitation with sunlight or room light within minutes. However, the development of bright long-lasting persistent phosphors in the near infrared (NIR) range remains rather limited.

Recently, a set of spinel gallate materials doped with trivalent chromium (Cr³⁺) (e.g., La₃Ga₅GeO₁₄:Cr³⁺, and ZnGa₂O₄:Cr³⁺) are emerging as long-lasting NIR persistent substance [14–18]. The emission wavelength is about 700 nm originating from the ²E→⁴A₂ transition of Cr³⁺ emitters, while the afterglow time is over hours that are mainly determined by the type and distribution of formed traps during the growth of the spinel crystal. Among them, zinc gallogermanate phosphors with an optimized formula of Zn₃Ga₂Ge₂O₁₀:0.5%Cr³⁺ are of great interest, which are reported to exhibit persistent luminescence with the longest afterglow time over 360 h, observed with the aid of a night-vision monocular in a dark room [17]. This compound also possesses the AB₂O₄ spinel structure, whereas Zn²⁺ occupies the tetrahederal sites, Ga³⁺ occupies the octahedral sites, and Ge⁴⁺and Cr³⁺ replace Ga³⁺ to occupy the octahedral sites [3, 19]. Though the brightness of NIR long-lasting luminescence can be optimized through optimizing the content of the composition (Zn_1+xGa_2−2xGe_xO₄, 0≤x≤0.5) or through substitution of the occupants at the octahedral sites with trivalent tin (Sn³⁺) [3], there lack routes to further improve the optical properties of the established spinel compound with a defined formula.

In this work, we show that the brightness of persistent luminescence from the established nominal Zn₃Ga₂Ge₂O₁₀:0.5%Cr³⁺ materials can be improved further through divalent calcium (Ca²⁺) doping. A slight substitution of zinc ions with Ca²⁺ (ca. 3%) at the tetrahedral sites can improve the persistent luminescence brightness by 15 fold after 5 min stoppage of the excitation light.

2. Method

Material Synthesis. The powder samples of Zn₃Ga₂Ge₂O₁₀:0.5%Cr³⁺, x%Ca²⁺ (x = 0, 1, 3, 5) and zin-deficient ZGGO:0.5%Cr,d3%Zn (with 3% zinc deficient) were synthesized using a conventional solid-state reaction method. In a typical procedure, stoichiometric ZnO (Aladdin, 99.99%), Ga₂O₃ (Aladdin, 99.99%), GeO₂ (Aladdin, 99.99%), Cr₂O₃ (Aladdin, 99.95%), CaO (Aladdin, 99.9%) were first mixed thoroughly, and ground to form a homogeneous fine powder in an agate mortar. Subsequently, the mixed powders were transferred to a corundum crucible, and then heated at 900 °C in a muffle furnace for 2 hours. After this, the prefired material was ground again to a fine powder suitable for sintering. Finally, the powder samples were sintered at 1100 °C for 2 hours in air.

Characterization Methods. The morphology of the resulting powders was characterized by a field emission scanning electron microscope (SEM) (Zeiss Supra 55) at an acceleration voltage of 15 kV. The powder X-ray diffraction (XRD) patterns were recorded by a Siemens D500 diffractometer, using Cu Kα radiation (λ = 0.15418 nm). The 2θ angle of the XRD spectra was recorded at a scanning rate of 5°/min. Photoluminescence excitation spectra, emission spectra, afterglow spectra, and afterglow decay curves were all recorded on FLS980 spectrometer (Edinburgh instruments). Thermoluminesence spectra was acquired using a home-made setup with a temperature-controlled sample holder and a monochromator for afterglow luminescence detection. To acquire the excitation spectra, the powders were excited by light in the wavelength range of 330-500 nm while monitoring the emission intensity at 696 nm. Emission spectra were recorded under the excitation at 410 nm. Light excitation was provided by the 450 W ozone-free xenon arc lamp with optical filtering defined by the excitation monochormator of FLS 980. After charging the samples with ultraviolet light at 320 nm for 5 mins, photographic images of persistent luminescence were recorded by iPhone 6s (Apple Inc.) at varying time points after the stoppage of excitation light.

3. Results and discussions

To investigate the influence of Ca²⁺ doping on the morphology change of the resulting particles, we acquired scanning electron microscopic (SEM) images of Zn₃Ga₂Ge₂O₁₀:0.5%Cr³⁺, x%Ca²⁺ (x = 0, 1, 3, 5) powder samples prepared under the identical condition (Fig. 1). As one can see in this figure, the size and morphology of the sample without Ca²⁺ doping are diversified, with a large amount of small particles with size of 200-700 nm, and also rod-shaped particles with length over 2 µm. Along with the increase of Ca²⁺ doping content, the resulting shape becomes irregular, and the resulting size grows significantly into an average size of 2-4 µm for Ca²⁺ 1%, and over 4-5 µm for both Ca²⁺ 3 and 5%, respectively. Note that sub-micrometer sized particles observed for the sample with null Ca²⁺ doping almost disappear for all the samples with Ca²⁺ doping. Moreover, the escalated Ca²⁺ content tends to evoke the growth of the resulting particles into one single crystal due to the increased grain size and the smeared particle boundaries. The improved crystallization generally can help reduce the luminescence quenching centers, thus favoring higher photoluminescence and persistent luminescence intensity.

 

Fig. 1 Scanning electron microscopic (SEM) images of the resulting Zn₃Ga₂Ge₂O₁₀:0.5%Cr³⁺, x%Ca²⁺ (x = 0, 1, 3, 5) powder samples. Scale bar, 2 µm.

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We further acquired the x-ray diffraction (XRD) patterns of Zn₃Ga₂Ge₂O₁₀:0.5%Cr³⁺, x% Ca²⁺ (x = 0, 1, 3, 5) powder samples to investigate the impact of calcium doping on the crystallographic structure (Fig. 2). A dominant spinel phase was observed for the resulting sample of Zn₃Ga₂Ge₂O₁₀:0.5%Cr³⁺ (null Ca²⁺ doping), but accompanied with impurity crystal phases of Zn₂GeO₄ (Fig. 2(b)) and GeO₂ (Fig. 2(c)). Whereas Ca²⁺-doped samples all have identical peak positions that agree well with the standard PDF 38-1240 pattern of spinel ZnGa₂O₄. No impurity diffraction peaks were observed. This observation implies that the doped Ca²⁺ ion substitutes the Zn²⁺ ion at the tetrahedral sites to enter the host lattice of a spinel structure. This is reasonable as the ionic radii of Ca²⁺ (114 pm) and Zn²⁺ (88 pm) are similar, and no charge compensation is needed for such substitution. However, the ionic radii difference will create impact on the existing structure disorder (the number and distribution of antisite defect) and the local environment of Cr³⁺-occupied octahedral site, thus influencing their luminescent properties. The well-defined peaks in Fig. 2 are indicative of the high crystallinity of the resulting sample of Zn₃Ga₂Ge₂O₁₀:0.5%Cr³⁺, x% Ca²⁺ (x = 0, 1, 3, 5). Moreover, the disappearance of impurity XRD peaks in Ca²⁺-doped sample suggests that Ca²⁺ is able to help form samples with improved crystallinity, in good agreement with SEM result in Fig. 1.

 

Fig. 2 X-ray diffraction (XRD) patterns of Zn₃Ga₂Ge₂O₁₀:0.5%Cr³⁺ codoped with Ca²⁺ of (d) null, (e) 1%, (f) 3%, and (g) 5%, in reference to the standard ones of (a) ZnGa₂O₄ (PDF 38-1240), (b) Zn₂GeO₄ (PDF 11-0687), and (c) GeO₂ (PDF 65-8052)

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Figure 3(a) shows the luminescence spectra of the samples of Zn_3(1-x)Ga_1.99Ge₂O₁₀: 0.5%Cr³⁺, xCa²⁺ (x = 0,1%, 3%, 5%) and Zn_2.91Ga_1.99Ge₂O_9.91: 0.5%Cr³⁺ under excitation at 410 nm. A broadband emission peaked at 698 nm (extending from 650 to 850 nm) is observed for all the samples, which originates from the ²E→⁴A₂ transition of Cr³⁺ emitter. This is in marked contrast to the reported luminescence spectra of Cr-doped spinel persistent phosphors (ZnGa₂O₄:Cr³⁺), which consists of zero phonon (ZP) lines (687.0 nm), N2 line (695 nm, structure-dependent line) and phonon side bands (PSB) (708 nm, 715 nm and 722 nm for Stokes PSB, and 670 nm and 680 nm for Anti-Stokes PSB) [16]. It has been established that the ZP lines (687 nm) and PSB lines are characteristic of Cr³⁺ occupying an octahedral site (D_3h symmetry) of ideal spinel environment, while the N2 line (695 nm) corresponds to an environment distorted by an antisite defect located in the first cationic neighbors of Cr³⁺ (inversion spinel structure) [16]. The observation of the intense peak at 698 nm (i.e., the N2 line) from our samples implies that the luminescence primarily comes from the Cr³⁺ ions located at antisite-defect-distorted octahedral sites. It is striking that the luminescence intensity increases by more than two-fold along with an escalation of Ca²⁺ content from 0 to 3%. However, the luminescence intensity drops significantly with a further increase of Ca²⁺ concentration to 5%. A small amount of zinc deficiency in the lattice was reported to be able to enhance the brightness and lengthen afterglow time of persistent luminescence from a spinel structure [20, 21]. We, therefore, prepared the Zn₃Ga₂Ge₂O₁₀:0.5%Cr³⁺, d3%Zn sample with 3% zinc deficiency, and include its spectrum in Fig. 3. The observation of decreased luminescence from the Zn₃Ga₂Ge₂O₁₀:0.5%Cr³⁺, d3%Zn than from the Zn₃Ga₂Ge₂O₁₀:0.5%Cr³⁺ sample rules out the mechanisms that the observed luminescence enhancement comes from the reduction of Zn content in the host lattice due to the replacement of Zn²⁺ by Ca²⁺. Indeed, when normalizing the luminescence spectra to the PSB peak at 716 nm (the R line), the intensity of N2 line at 695 nm in reference to the R line at 716 nm increases with Ca²⁺ content, and reaches an apex at 3% Ca²⁺. This demonstrates the introduction of Ca²⁺ is able to increase the antisite defects in the host lattice that are essential for persistent luminescence. Note that the spectra for samples with Ca²⁺ 1% and 5% are similar in Fig. 3 (b), but quite different in intensity in Fig. 3(a). This difference implies that the Ca²⁺ dopant is able to enhance the energy trapping sites, but its high concentration can introduce quenching effects. Moreover, the Ca²⁺-induced alternation of luminescence intensity in Fig. 3(a) corresponds well to the change of the excitation spectra in Fig. 3(c). The excitation spectrum covers a spectral region from 330 to 500 nm, which consists of two main excitation bands originating from the ⁴A₂→⁴T₁ (te2) transition (330 nm), and the ⁴A₂→⁴T₁ (t2e) transition (410 nm) of Cr³⁺, respectively. Taken together, the results in Fig. 3 indicate that Ca²⁺ renders photoluminescence enhancement through engineering the structural disorder of the host lattice.

 

Fig. 3 (a) Photoluminescence, (b) normalized photoluminescence (normalized to the peak at 716 nm), and (c) excitation spectra of the samples Zn_3(1-x)Ga_1.99Ge₂O₁₀: 0.5%Cr³⁺, xCa²⁺ (x = 0,1%, 3%, 5%) and Zn_2.91Ga_1.99Ge₂O_9.91: 0.5%Cr³⁺. The emission intensity at 698 nm was monitored to acquire the excitation spectra of Fig. 3 (c).

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We also investigated the doping influence of Ca²⁺ on the persistent luminescence from the Zn₃Ga₂Ge₂O₁₀:0.5%Cr³⁺ sample, and presented afterglow decay curves of all samples in Fig. 4(a). To acquire the decay profiles, the samples were irradiated with 410 nm light for 60 s, and then recorded luminescence at 698 nm for a period of 300 s post the stoppage of light stimulus. As one can see, the persistent luminescence intensity of Zn₃Ga₂Ge₂O₁₀:0.5%Cr³⁺ sample was significantly improved due to the introduction of Ca²⁺ ions into the host lattice. The observed afterglow spectra (Fig. 4(c)) is similar to the photoluminescence spectra in Fig. 3(a). However, unlike the observation of 4-fold lower luminescence intensity for Ca²⁺ 5% sample than Ca²⁺ 0% sample, the Ca²⁺ 5% sample exhibits about 7-fold higher persistent luminescence than the Ca²⁺ 0% sample at 300 s post the stoppage of the light excitation (Fig. 4(b)). This confirms that the Ca²⁺ doping is able to engineer the structural disorder of the spinel host lattice. Moreover, the sample doped with 3% Ca²⁺ concentration exhibits the highest persistent luminescence intensity for the whole period of recording. After the cease of excitation for 300s, the persistence luminescence intensity of this sample is calculated to be about 15 fold higher than the powder Zn₃Ga₂Ge₂O₁₀:0.5%Cr³⁺ sample with null Ca²⁺ doping, which was reported to emit observed afterglow luminescence over 360 h with the aid of a night-vision monocular in a dark room [17]. We have also measured thermoluminesence (TL) spectra for the samples of Zn₃Ga₂Ge₂O₁₀: 0.5%Cr³⁺ codoped with null and 3% Ca²⁺ (Fig. 4(d)). The TL spectrum for the sample with 3% Ca²⁺ is about 14.5-fold higher than the one with null Ca²⁺, agreeing well with the result of Fig. 3(b). Moreover, the TL spectrum for 3%Ca²⁺-doped sample is extremely broad (over a range of 25-250 °C), indicating that the traps distribute over a wide range of energies (thus, a wide range of depths). This broad spectrum is in marked contrast to the narrow one for the Ca²⁺-free sample with a range of 25-150 °C, further supporting that Ca²⁺ doping can enhance the trapping sites of the lattice, especially those deep traps with high energies. Moreover, since light irradiation was performed at 400 nm that has energy only able to populate the ⁴T₁(t²e) state instead of the ⁴T₁(te²) close to the bandgap, the TL spectra substantiate the occurrence of quantum tunneling effect between the ⁴T₁(t²e) state and the deep trap sites.

 

Fig. 4 (a) Afterglow decay curves of the samples Zn_3(1-x)Ga_1.99Ge₂O₁₀: 0.5%Cr³⁺, xCa²⁺ (x = 0,1%, 3%, 5%) and Zn_2.91Ga_1.99Ge₂O_9.91: 0.5%Cr³⁺. The samples were charged with 410 nm light for 60s, and then the time-lapsed emission intensity at 698 nm was recorded. (b) The persistent luminescence peak intensity at 698 nm for all the samples after 300 s stoppage of excitation light at 410 nm. (c) Afterglow spectra of Zn₃Ga_1.99Ge₂O₁₀:0.5%Cr³⁺, 3%Ca²⁺ (charged with 410 nm for 60s, recorded after 10 s stoppage of light excitation). (d) Thermoluminescence curves monitored at 695 nm over 25–250 °C for Zn_3(1-x)Ga_1.99Ge₂O₁₀: 0.5%Cr³⁺, xCa²⁺ (x = 0 and 3%). The samples were irradiated with 400 nm light for 5 min, and the curves were measured at 2 min after the stoppage of light irradiation.

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The significantly enhanced persistent luminescence due to Ca²⁺ doping can also be clearly observed from photographic images of afterglow luminescence (Fig. 5). The persistent luminescence from 0% Ca²⁺ sample can barely be seen for 10 s, while the 3% Ca²⁺ sample is able to exhibit clear and discernible persistent luminescence (by iphone 6S) for up to 80 s. It has been demonstrated that divalent magnesium (Mg²⁺) doping can replace Zn²⁺ ions in the spinel structure (ZnGa₂O₄:0.5%Cr³⁺), and the increase of Mg²⁺/Zn²⁺ ratio can promote the formation of antisite defects [22–25]. In analogy, the introduction of Ca²⁺ ions in the spinel structure of Zn₃Ga₂Ge₂O₁₀:0.5%Cr³⁺ also favors the formation of antisite defects (the occupation of tetrahedral Zn²⁺ site with Ga³⁺ or Ge⁴⁺ ions), i.e., the trapping centers, which are beneficial for long-lasting luminescence creation. Moreover, the enhanced structural disorder at the tetrahedral sites also render more distorted octahedral sites, enhancing the number of persistent luminescence emitting centers of Cr³⁺.

 

Fig. 5 Photographic images for persistent luminescence from the samples of Zn_3(1-x)Ga_1.99Ge₂O₁₀: 0.5%Cr³⁺, xCa²⁺ (x = 0,1%, 3%, 5%) post the stoppage of light excitation at 320 nm at defined time points. Prior to taking these photographs, the samples were exposed to light at 320 nm for 2 mins.

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The mechanisms to generate NIR persistent luminescence can possibly come from two processes (Fig. 6): (i) The electrons of Cr³⁺ ions occupying the octahedral sites of spinel crystals can be excited from the ground state ⁴A₂ to the ⁴T₁(te²) state (process1) under high energy UV irradiation. At this state, the excited electron can transmit to the conduct band (CB) and then get trapped by either shallow (process 4) or deep (process 6) traps. When stopping excitation, the electrons will escape from the shallow traps and go to ⁴T₁(te²) via CB band (process 5). Non-radiative transition (process 2) will result in the population of the ²E state, which subsequently releases the trapped energy through a radiative decay to the ground state ⁴A₂, producing NIR luminescence at 698 nm (process 3). (ii) The deep traps can also release the electrons captured by process 6 through quantum tunneling to the ⁴T₁(t²e) and ²E states (process 7). This tunneling process does not need to go through the CB and thus proceeds at a slow rate. Nonradiative decay can depopulate the electrons at the ⁴T₁(t²e) sate to the ²E state (process 8), from which NIR persistent luminescence is produced. Note that akin to UV light, visible light excitation at 410 nm (corresponding to the ⁴A₂→⁴T₁(t²e) transition of Cr³⁺) is also able to elicit NIR persistent luminescence from our Zn_3(1-x)Ga_1.99Ge₂O₁₀: 0.5%Cr³⁺, xCa²⁺ (x = 0, 1%, 3%, 5%) samples (Fig. 4). This is possibly because the 410 nm light excitation is able to excite electrons to the ⁴T₁(t²e) state, from which the energy can be transferred through quantum tunneling to the deep traps for storing (a reverse process to 7). Followed by the proceeding of process 7, NIR persistent luminescence will be produced through process 3.

 

Fig. 6 A schematic of the energy transfer mechanism of the persistent luminescence material Zn_3(1-x)Ga_1.99Ge₂O₁₀: 0.5%Cr³⁺, xCa²⁺ (x = 1%, 3%, 5%)

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The NIR persistent luminescence intensity and afterglow period are in close relation to the number and distribution of energy trapping centers in the spinel structure. Moreover, both parameters are also affected by the number of luminescence quenching centers existed in the crystal of the resulting powder. Taken together of observed experimental results, the introduction of Ca²⁺ into the Zn₃Ga₂Ge₂O₁₀:0.5%Cr³⁺ crystal might produce two beneficial effects on persistent luminescence: (i) The increase of the resulting particle size but without producing additional crystallographic phases, as confirmed by SEM (Fig. 1) and XRD results (Fig. 2). This will reduce the number of luminescence quenching centers in the crystal, which thus enhance the photoluminescence and persistent luminescence intensity. (ii) The substitution of Zn²⁺ with Ca²⁺ at the tetrahedral site in the Zn₃Ga₂Ge₂O₁₀:0.5%Cr³⁺ crystal promotes the formation of antisite defects (the occupation of tetrahedral Zn²⁺ site with Ga³⁺ or Ge⁴⁺ ions) (Figs. 3 and 4). The antisite defects functionalize as centers to trap electrons and/or holes, preventing from fast recombination of each other. Moreover, the substitution can tailor the local crystal environment surrounding the Cr³⁺ ions occupying the octadehedral site. The Ca²⁺-enhanced structural disorder of a spinel crystal results in the production of enhanced long-lasting persistent luminescence.

3. Conclusion

In this work, we systematically investigated the doping effect of divalent calcium (Ca²⁺) on the NIR persistent phosphors of Cr³⁺-activated spinel zinc gallogermanate (emission at 650–850 nm). An introduction of 3% Ca²⁺ ions into the formulated phosphors was able to enhance its NIR persistent luminescence intensity by about 15 fold (observed 300s posted stoppage of light excitation). The Ca²⁺ doping entails the size increase and the crystallization improvement of the resulting phosphor powder, which can reduce luminescence-quenching centers in the crystal. Moreover, spectral and structural analyses indicate that Ca²⁺ doping promotes the formation tetrahedral antisite trapping defects surrounding the emitting Cr³⁺ ions at the octahedral sites, thus enhancing the NIR persistent luminescence profiles. The described simple route here provides a new paradigm to enhance the performance of persistent phosphors engaged in a number of important technological applications.