1. Introduction

The long lasting phosphorescence is an interesting optical phenomenon, which is a special case of thermally stimulated luminescence after the stoppage of the excitation source at room temperature. Persistent luminescence phosphors exhibit particular properties of storing light energy such as sunlight or artificial light sources [1,2]. To date, long persistent phosphors have received much attention and interests due to their wide application, e.g., such as emergency escape routes and exit signs, optical energy media, thermal sensors [1–3]. Besides the traditional applications, new fields of interest emerged in medical field such as cancer photodynamic therapy, cancer diagnoses and in vivo imaging [4,5].

Interestingly, the long lasting phosphors (LLPs) have been attracted extensive research interests and attention to in vivo imaging [3–5]. Especially, LLPs in the near infrared (NIR) region have been developed as a new category of luminescent labels which have been becoming promising alternatives to the organic fluorophores and quantum dots applied in biological assays and medical imaging due to their unique optical properties, such as low background autofluorescence, sharp emission band, long luminescent lifetimes, good photostability and low toxicity [6,7]. As a result, the long persistent nature of LLP particles allows optical excitation before bioimaging, and permits detection and imaging without external illumination, thereby avoiding the background noise from in situ excitation [8–10]. Moreover, it is reported that the use of NIR emitting photons is more suitable for biomedical imaging and detection than that of ultraviolet (UV) or visible ones as optical probes [11].

In design of NIR phosphors, interest in Cr³⁺ activated phosphors is widespread, such as ZnGa₂O₄:Cr³⁺ [12], Ca₃Ga₂Ge₄O₁₄:Cr³⁺ [13], La₃Ga₅GeO₁₄:Cr³⁺ [14], La₃Ga_5.5Nb_0.5O₁₄:Cr³⁺ [15], Zn₃Ga₂GeO₈:Cr³⁺ [16], etc. These showed that Cr³⁺ ions would experience the disorder-induced distributions of strong and weak crystal field strengths, resulting in two types of luminescence being observed: sharp R-line (600-700nm) and broad NIR (900-1300 nm) emissions. However, NIR bio-imaging window is composed of two ranges (NIR I, 650-950nm and NIR II, 1000-1400nm) because there is a strong absorption at 980 nm due to water absorption in bio-tissues. It’s no doubt that Cr³⁺ doped persistent phosphors have attracted extremely attentions for bio-imaging since they are matching well with the NIR bio-imaging window. The past few years have witnessed great progress in the development of the LPP probe in in vivo bioimaging, with the main focus on the Cr³⁺-activated gallate materials [12–16].

Recently, Xia et al. reported the multi-color emission evolution and energy transfer behavior of La₃GaGe₅O₁₆:Tb³⁺,Eu³⁺ phosphors [17]; La₃GaGe₅O₁₆:Cr³⁺ phosphor, reported also by Xia et al., showed a strong NIR emission, only about the photoluminescence properties [18]. In our previous work, we discussed the optical properties of La₃GaGe₅O₁₆ host and Pr³⁺-doped La₃GaGe₅O₁₆ phosphor [19]. However, the long persistent properties of La₃GaGe₅O₁₆:Cr³⁺ phosphor was still not studied. In our work, La₃GaGe₅O₁₆:Cr³⁺ phosphor was prepared via the solid state reaction and the LLP properties were investigated in detail. La₃GaGe₅O₁₆:Cr³⁺ phosphor gives a new near-infrared persistent luminescence, which is potentially suitable for in vivo imaging due to its 650nm-750nm emission range.

2. Experimental

2.1 Synthesis

The samples were prepared by a simple solid state reaction. La₂O₃(99.99%), Ga₂O₃ (99.99%), GeO₂ (99.99%), H₃BO₃ (99.99%) and Cr₂O₃ (99.99%) were used as starting materials. After the raw materials were weighed according to the composition of La₃Ga_1-xGe₅O₁₆:xCr³⁺(x = 0, 0.005, 0.01, 0.02, 0.03, 0.04, 0.05 and 0.07, etc), the powders were mixed and milled thoroughly for 1h in an agate mortar and prefired at 900°Cfor 8h in air. After ground again, the mixtures were sintered at 1150°Cfor 6h. After being cooled down to room temperature naturally, the as-prepared powder samples were obtained.

2.2 Measurement

The phase purity of the prepared phosphors was measured by an X-ray diffractometer with Cu Ka radiation (wavelength = 0.15406 nm) at 36 kV tube voltage and 20 mA tube current.The morphology of the samples was characterized by S3400N scanning electronic microscope (SEM) with accelerating voltage of 10 kV. Diffuse reflection spectra were obtained by an ultraviolet (UV)-visible spectrophotometer (Shimadzu UV-2450) using BaSO4 as a reference. The excitation and emission spectra of all the samples were measured by a Hitachi F-7000 Fluorescence Spectrophotometer equipped with a 150 W xenon lamp as excitation source. The persistent luminescence spectrum was also measured using the spectrophotometer by shutting off the Xe lamp. The decay curves were measured by a GFZF-2A single-photo-counter system. The thermoluminescence (TL) spectrum was measured with a FJ-427A1 thermoluminescence meter. Prior to the persistent luminescence spectra, decay curve and TL glow curve measurements, the samples were excited for 3 min by a 15 W low-pressure mercury discharge lamp (254 nm). For the TL glow curve measurements, the heating rate was 1°C/s and the range of the measurement is from room temperature to 300°C. A delay for 3 min was used between the irradiation and measurement. All measurements were carried out at room temperature except for the TL spectrum.

3. Results and discussion

3.1 XRD phase

The XRD patterns of the samples, La₃Ga_1-xGe₅O₁₆:xCr³⁺, are shown in Fig. 1. A single phase of La₃Ga_1-xGe₅O₁₆:xCr³⁺ is obtained and all the diffraction peaks are in good agreement with with the standard data of La₃Ga₁Ge₅O₁₆ (ICSD#50521), indicating that Cr³⁺ ions are successfully dissolved into the La₃Ga₁Ge₅O₁₆ host lattice while maintaining the crystal structure intact. For achieving crystallographic data for La₃GaGe₅O₁₆:Cr³⁺ sample, we analyzed the XRD of La₃GaGe₅O₁₆:Cr³⁺ with the classic Rietveld refining technique in Fig. 1 (b). The refinement started with the crystallographic data of La₃GaGe₅O₁₆ (ICSD#50521) and all the refinement factors (R_p, R_wp and R_exp are less than 10%) reveal a good quality of fitting. The refining results confirms Cr³⁺ prefers to substitute for the the Ga³⁺ site, as shown in Fig. 1(b).

 

Fig. 1 (a) The XRD pattern of La₃Ga_1-xGe₅O₁₆:xCr³⁺ phosphor and that of ICSD#50521 given for comparison (bottom); (b) Experimental and calculated of the XRD refinement of La₃GaGe₅O₁₆:0.01Cr.

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3.2 Luminescence properties

In Fig. 2, the photoluminesence excitation (PLE) spectrum consists of three broad excitation band located at 310 nm, 413 nm and 570 nm, which originate from the inner transitions of Cr³⁺, namely ⁴A₂→⁴T₁(4P), ⁴A₂→⁴T₁(4F) and ⁴A₂→⁴T₂(4F) transitions. Under such different excitation wavelengths, La₃Ga_1-xGe₅O₁₆:xCr³⁺ phosphor exhibits the same shapes and positions of the emission band besides the intensity of ²E→⁴A₂ transition. The photoluminesence properties of Cr³⁺ doped La₃GaGe₅O₁₆ were discussed in Ref [18], so it is discussed on longer in this article. But the PL spectrum (Fig. 2(b)) is a little different from that in Ref [18], the reason for this might be ascribed to the synthesis condition.

 

Fig. 2 The PLE and PL spectra of La₃Ga₁Ge₅O₁₆:Cr³⁺ at room temperature.

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3.2 Phosphorescence properties

To investigate the persistent luminescence characteristics, the afterglow spectra of La₃Ga_0.99Ge₅O₁₆:0.01Cr³⁺ and host were recorded after ceasing the irradiation. As shown in Fig. 3(a)-3(b), La₃Ga_0.99Ge₅O₁₆:0.01Cr³⁺ sample shows a bright NIR persistent emission, which is similar in shape and position to the emission spectrum. For better understanding of the LPP of La₃GaGe₅O₁₆:Cr³⁺ phosphor, La₃GaGe₅O₁₆ host was also observed a cyan afterglow luminescence. At the bottom in Fig. 3(a)-3(b), the images were recorded by a classic digital camera varying with the different afterglow times. We can see that La₃Ga_0.99Ge₅O₁₆:0.01Cr³⁺ gives a bright red afterglow emission. In order to study the afterglow decay behaviors and the concentration quenching of La₃Ga_1-xGe₅O₁₆:xCr³⁺ (x = 0,0.005, 0.01, 0.02, and 0.03) phosphors in detail, the afterglow decay curves of samples are measured at room temperature. Figure 3(c) gives the afterglow decay curves of La₃Ga_1-xGe₅O₁₆:xCr³⁺ (x = 0,0.005, 0.01, 0.02,and 0.03) after irradiation by a 15 W low-pressure mercury discharge lamp for 3 min. For higher concentration (x = 0.03), the persistent emission is too weak to be observed by human naked eyes. As usual, the afterglow decay processes can be considered to be two parts: one part is a fast decay process and the other is a slow decay part. However, the afterglow duration is largely dependent on the slow decay part. From Fig. 3, we can see that the doping concentration of Cr³⁺ has an influence on the persistent luminescence properties and the optimal doping concentration of Cr³⁺ in La₃GaGe₅O₁₆ host is about 0.01 due to the highest intensity and slowest decay process.

 

Fig. 3 (a-b) the afterglow emission and images of La₃Ga_0.99Ge₅O₁₆:0.01Cr³⁺ at different time; (c) The afterglow decay curves of La₃Ga_1-xGe₅O₁₆:xCr³⁺ (x = 0, 0.005, 0.01, 0.02, 0.03 and 0.05) at room temperature, respectively.

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Since the afterglow emission involves the releasing and trapping of electrons or holes from traps under thermal energy excitation, we can gain insight into some useful information about the traps, such as the evaluating for the depth and density of traps in host material by the TL curves [20]. Figure 4(a) shows the TL glow curves of samples La₃Ga_1-xGe₅O₁₆:xCr³⁺ (x = 0.005, 0.01, 0.02, and 0.03) recorded from 30 to 300°C (the heating rate was 1°C/s). As a rule, each TL peak stands for a kind of trapping centers and the relative trap density or trapping capacity is roughly proportional to the integral TL intensity [7]. In Fig. 4(a), the shape and position of TL curves were found to be almost identical. So, one can see that the depths of TL bands are independent on the doping concentration of Cr³⁺, but the trap density is largely dependent on the Cr³⁺-doped content. Obviously, the integral TL intensity of La₃Ga_0.99Ge₅O₁₆: 0.01Cr³⁺ sample is the largest among the samples. This further confirms that the optimal doping concentration of Cr³⁺ is about 0.01. In order to further investigate the TL curve, we measured the TL curves of the typical sample La₃GaGe₅O₁₆:0.01Cr³⁺ at different delay time in order to clarify the types and kinetic process of traps, as shown in Fig. 4(b). Notably, the position of the TL curve shifts to the higher temperature region with elapse of time. This shift gives us a hint that the estimated trap which varies continuously as a function of delay time is an evidence for the presence of a continuous trap depth distribution. In order to achieve a quantitative analysis of the TL curves, the deconvolution method was used [21,22]. This yields a good agreement between the experimental and calculated glow curves between 0.78 eV and 0.89 eV. The reason for the shift of TL curves is that the trapped electrons escape from the shallower traps much faster than that trapped in the deeper ones. In order to clarify the origin of the defect traps, the TL glow curve of undoped La₃GaGe₅O₁₆ host was measured as well. From Fig. 4(c) and (d), we can see that shape and position between curve A of La₃Ga_0.99Ge₅O₁₆: 0.01Cr³⁺ and TL curve of La₃GaGe₅O₁₆ host are similar and infer that the traps, corresponding to curve A, might originate from the host. The short-wavelength UV radiation can lead to a decrease of the doped Cr³⁺ concentration as a result of change in their valence state to Cr²⁺ and Cr⁴⁺ [22], So, the doped Cr³⁺ ions under illumination not only serve deep acceptor centers, forming Cr⁴⁺, but act as electron traps, forming Cr²⁺ (Cr³⁺-e) [22]. Thus, curve B can be attributed to the traps introduced by the Cr³⁺ ions.

 

Fig. 4 (a) The thermoluminescence (TL) glow curves of La₃Ga_1-xGe₅O₁₆:xCr³⁺ (x = 0, 0.005, 0.01, 0.02, and 0.03) at waiting time of 3 min after the removal of excitation; (b) TL glow curves of La₃Ga_0.99Ge₅O₁₆:0.01Cr³⁺ at different delay times (t = 0.5, 1, 3, 5, 10 and 15min);(c) TL glow curves of the typical sample La₃Ga_0.99Ge₅O₁₆: 0.01Cr³⁺, the square dots are the measured data, the red and blue solid lines are the Gaussian curves;(d) TL glow curves of un-doped La₃GaGe₅O₁₆ sample .

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We aim to improve the performance of the La₃GaGe₅O₁₆:Cr³⁺ by modified composition around Cr³⁺ by adjusting the Ge/O content, since the intrinsic traps of host has an important influence on its persistent luminescence. Thus, we prepared a series of Cr³⁺-doped La₃Ga_0.99Ge_5+xO_16+4x:0.01Cr³⁺ (−0.05≤x≤0.05) samples. persistent luminescence of La₃Ga_0.99Ge_5+xO_16+4x:0.01Cr³⁺ (−0.05≤x≤0.05) are presented in Fig. 5. From Fig. 5, one can see that Ge/O content deficiency benefits to improve the persistent luminescence and afterglow time, whereas samples with a composition of Ge/O content excess exhibit poor afterglow properties. This one aspect demonstrates the persistent luminescence of La₃GaGe₅O₁₆:Cr³⁺ is closely related to the oxygen or germanium vacancy deficiency in host. In order to evaluate the phosphorescence of La₃GaGe₅O₁₆:Cr³⁺ phosphor, either the persistent luminescence intensity or afterglow time of La₃GaGe₅O₁₆:Cr³⁺ cannot match that of the known deep-red persistent phosphor ZnGa₂O₄:Cr³⁺, as shown in Fig. 6.

 

Fig. 5 Red emission images of La₃GaGe_5+xO_16+4x:0.01Cr³⁺ (−0.05≤x≤0.05) recorded by a classic Reflex digital camera with the same exposure time varying with the different afterglow time (t = 0.5, 3, 5, 10 and 15 min)

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Fig. 6 The afterglow curves of ZnGa₂O₄:Cr³⁺ and La₃GaGe₅O₁₆:Cr³⁺.

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3.3 Mechanism of persistence luminescence

To understand the dynamical process in persistent luminescence, a schematic graph based on the above results is proposed and illustrated in Fig. 7. The relative positions of the traps with respect to the bottom of conduction band can be determined based on the TL peaks. It is difficult to identify the position or the location of different energy levels of Cr³⁺, especially the energy gap between the top of valence band (VB) and the ground state of Cr³⁺ (⁴A₂), and also the energy gap between the bottom of conduct band (CB) and ⁴T₁ state of Cr³⁺. Therefore, just for illustration purpose, the Cr³⁺ energy levels are placed into the middle of the forbidden zone to discuss the afterglow luminescence. After irradiation with the ultraviolet light, the ground electrons of Cr³⁺ are excited along path 1 to the ⁴T₁ excited state, the latter lying in the conduction band. This leads to a separation of the electrons and holes. The majority of excited electrons will be back and then relax to ²E level in non-radiative way. The subsequent jumping of electrons to the ground levels ⁴A₂ of Cr³⁺ and recombination of holes lead to the characteristic emission of Cr³⁺ ions (process numbered 2 and 3). These processes present photoluminescence emission of La₃GaGe₅O₁₆:Cr³⁺. However, The residual minority excited electrons relax to the lower end of the conduction band and then are captured by the traps through the non-radiative way (process numbered 4 and 5). The trapped electrons will jump to the neighboring traps by quantum tunneling due to these traps with continuous distribution and close depths. At last, the electrons captured by the traps escape thermally via the conduction and are transferred to the activators Cr³⁺, followed by the recombination between electrons and holes and resulting in the persistent emission.

 

Fig. 7 Schematic illustration of the trapping and releasing processes of La₃GaGe₅O₁₆:Cr³⁺ phosphor.

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4. Conclusions

In conclusion, a novel NIR persistent phosphor La₃GaGe₅O₁₆:Cr³⁺ was prepared by the simple solid-state reaction. La₃GaGe₅O₁₆:Cr³⁺ phosphor gives a long persistent NIR luminescence, which makes it potential candidates for their application in security, dark/night vision, or medical imaging. We also observed a long afterglow luminescence from the La₃GaGe₅O₁₆ host. The spectroscopic study revealed fundamental information about the persistent luminescence features of Cr³⁺ ions in La₃GaGe₅O₁₆. The optimal concentration of Cr³⁺ in La₃GaGe₅O₁₆ is experimentally about 0.01 and the afterglow time can last more than 30 min. In order to improved the performance of the La₃GaGe₅O₁₆:Cr³⁺, we modified composition around Cr³⁺ by adjusting the Ge/O content.The trapping and releasing processes of trapped electrons were discussed, and a model for the illustration of long persistent luminescence mechanism was proposed.