1. Introduction

Long lasting phosphors (LLPs) are known for their persistent luminescence, which can emit for hours after terminating irratation. LLPs exhibit potential in night-vision assistants, security signs, and dial displays [1–5 ]. In recent years, transition metal-activated (such as Cr³⁺or Mn²⁺) LLPs have attracted much interests because of their perfect near-infrared (NIR) persistent luminescence, excellent chemical stability, and suitable emission range [6–10 ]. In particular, NIR LLPs can be used as long-term optical probes for in vivo imaging systems because these phosphors feature high signal-to-noise ratio and deep penetration and do not require in situ excitation [11–13 ].

The majority of studies on NIR LLPs have focused on Cr³⁺. The 3d³ electron configuration of Cr³⁺ can feature different types of emission depending on the crystal-field environment of the host; Cr³⁺ exhibits narrow-band emission at around 700 nm because of the spin-forbidden ²E→⁴A₂ transition or broadband emission ranging from 650 nm to 1600 nm because of the spin-allowed ⁴T₂→⁴A₂ transition. Cr³⁺ presents strong ability to substitute for Ga³⁺ (with a similar ionic radius) in octahedral coordination; as such, gallate materials used as host have been intensively investigated [14–17 ]. Pan et al. developed a series of Cr³⁺-doped zinc gallogermanate NIR persistent phosphors, which exhibit a super-long afterglow of more than 360 h [16]. Allix et al. demonstrated a considerable improvement on NIR persistent luminescence by partially substituting Ge and Sn to form the Zn_1+xGa_2−2x(Ge/Sn)_xO₄ solid solution [17]. Recently, Li et al. reported an earth-abundant and inexpensive NIR phosphorescent Cr³⁺-doped non-gallate material; the light emission (650–1200 nm) and persistence time of this material can be precisely controlled by tailoring local crystal field around the activated Cr³⁺ and controlling trap distribution by altering the composition of the Zn_2-xAl_2xSn_1-xO₄ solid solution [18]. On the other hand, co-doping with different rare earth ions in Cr³⁺-activated LLPs can be used to tailor and improve the persistent luminescence property of LLPs. Abdukayum et al. demonstrated the remarkably improved NIR persistent luminescence property of Pr³⁺/Cr³⁺ co-doped with activated Zn_1+xGa_2-2xGe_xO₄ nanoparticles; the fabricated material exhibits super long-lasting afterglow (> 360 h) for in vivo bioimaging [11]. Although great progress has been achieved in co-doping LLPs with different rare earth ions, tailoring LLPs for specific applications remains a challenge. Ueda et al. developed bright green persistent phosphors by incorporating Ce³⁺ and Cr³⁺ into Y₃Al_5-xGa_xO₁₂ ceramics; the phosphors show persistent emission that lasts for several hours after blue-light excitation [19]. It is worth noticing that in that case the Ce³⁺ ions act as luminescence center and electron donor, whereas the Cr³⁺ ions serve as electron acceptor. In fact, Ce³⁺ could be a potential sensitizer activator in hosts for NIR luminescence because its broad absorption bands in the UV and blue regions and relatively broad emission in the blue and green regions can be tuned by local ligand field [20, 21 ].

In this regard, we prepared Ce³⁺/Cr³⁺ co-doped LaAlO₃ phosphors for NIR LLPs based on thorough analysis of pre-existing persistent phosphors (Cr³⁺-doped LaAlO₃ and Ce³⁺-doped LaAlO₃). Incorporation of Ce³⁺ ions into Cr³⁺-doped LaAlO₃ significantly enhanced the NIR persistent luminescence by more than one order of magnitude compared with LaAlO₃ doped with Cr³⁺. Basing on the detailed analysis, we propose a model of charge trapping and energy transfer between Ce³⁺ and Cr³⁺ ions in LaAlO₃.

2. Experimental

The La_1-xAl_1-yO₃: xCe³⁺, yCr³⁺ phosphors were synthesized through solid-state reaction. Samples were named by LACr, LACC1, LACC2, LACC3 and LACC4 for the x at 0%, 0.3%, 0.5%, 0.7%, 1% and the y at 0.5%, respectively. For the x at 0.5% and the y at 0%, sample was named as LACe. Pure La₂O₃ (4N), AlO₃ (4N), Ce₂(CO₃)₃⋅5H₂O (4N), and Cr₂O₃ (4N) were used as raw materials. The powders were finely mixed in an agate mortar to form a homogeneous powder for prefiring. After prefiring at 1000 °C for 5 h, the materials were ground again to fine powders. All phosphors were sintered at 1500 °C for 12 h.

The photoluminescence (PL), photoluminescence excitation (PLE), afterglow, and persistent luminescence decay curve spectra were obtained with a Fluorolog-3 spectrofluorimeter (Horiba FL3-22-iHR320) equipped with a photomultiplier (Hamamatsu R928P). A 450 W Xenon lamp was used as the excitation source. Thermo-luminescence (TL) curves were recorded using a spectrofluorometer and a setup consisting of a copper sample holder, a compact furnace with programmable heating and double gratings (1200 gr/mm, 500 nm blaze) in the emission monochromator, and a CCD camera. In the TL measurement, the sample was heated at a linear heating rate of 2 K/s.

3. Results and discussion

Ce³⁺/Cr³⁺ codoped phosphors exhibit improved performance in NIR persistent phosphorescence after the removal of the excitation source. Figure 1(a) presents the afterglow spectra of LACe, LACr, and LACC2 recorded at 10 min after terminating irradiation. Strong sharp peak at 735 nm from LACr and LACC2, labeled by R-line, arises from the spin-forbidden ²E → ⁴A₂ transition of Cr³⁺ ions occupying the octahedral site. The associated broad background emission ranging from 720 nm to 800 nm mostly originated from the phonon sidebands of the ²E → ⁴A₂ transition of Cr³⁺ ions [22]. The characteristic emission band of Cr³⁺ in the octahedral site confirms the substitution of Cr³⁺ ions in the Al³⁺ sites [18, 22 ]. The blue emission band at 415 nm in LACe and LACC2 is associated with the 5d₁-4f transition of Ce³⁺. Compared with the afterglow spectra of the LACe, LACr, and LACC2 samples, the 415 nm band weakens when Cr³⁺ was incorporated into the LACe sample, whereas the 735 nm band intensifies when Ce³⁺ was doped into the LACr sample. Figure 1(b) shows the LPP decay of LACr, LACC1, LACC2 LACC3 and LACC4 samples monitored at 735 nm. Obviously, the incorporation of Ce³⁺ ions into Cr³⁺-doped LaAlO₃ significantly intensifies the NIR persistent luminescence. The starting persistent luminescence intensity of the Ce³⁺/Cr³⁺codoped sample (LACC2) is significantly enhanced by more than one order of magnitude compared to that of the single Cr³⁺-doped sample (LACr). Moreover, LACC2 exhibits the most intense persistent luminescence 60 min after removal of the 315 nm light is four times higher than that of LACr. By contrast, the persistent luminescent intensity at 415 nm caused by the 5d₁-4f transition of Ce³⁺ significantly decreases with the introduction of Cr³⁺ ions (inset of Fig. 1(b)).

 

Fig. 1 (a) The afterglow spectra of LACe, LACr and LACC2. (b) Persistent luminescence decay curves monitoring at the emission of 735 nm for LACe, LACr, LACC1, LACC2, LACC3 and LACC4 measured after irradiation by a 315 nm Xe lamp for 10 min. The inset shows the persistent luminescence decay curves monitoring at the emission of 415 nm for the corresponding phosphors.

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Since persistence time and intensity are mainly determined by trapping states, TL glow curves were determined to investigate traps in the samples. Figure 2(a) shows the normalized TL glow curves of Cr³⁺ emission at 735 nm in the La_1-xAl_0.995O₃:xCe³⁺, 0.5% Cr³⁺ phosphors. The TL curve of LACr shows only one band centered at about 395 K, whereas the TL curves of LACC1, LACC2, LACC3, and LACC4 consist of two bands. Increasing Ce³⁺ concentration in LaAlO₃ can intensify the bands in the low-temperature region. This finding indicates that electron traps responsible for the NIR persistent luminescence are extended to shallow levels after Ce³⁺ doping. As shown in Fig. 2(b), the normalized TL glow curve of LACC2 can be divided into two Gaussian bands centered at about 345 and 395 K; hence, these two trapping states exist in the Ce³⁺/Cr³⁺codoped LACC samples. The trap depth energies (E) for the trapping states can be estimated using the Eq. (1) [23].

 

Fig. 2 Normalized TL curves of LACe, LACr, LACC1, LACC2, LACC3 and LACC4 irradiated by a 315 nm Xenon lamp for 10 min. (a) Monitoring at 735 nm at 2 min after the stoppage of irradiation. (b) The monitoring wavelength of LACr was 415 nm, and the monitoring wavelength of LACr and LACC2 was 735 nm. The TL curve of LACC2 was Gaussian-resolved into two fitted curves. The fitting parameters of T_m1, T_m2, ω₁ and ω₂ are 345 K, 395 K, 54 K and 38 K, respectively.

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where k_B is the Boltzmann constant, T_m is the peak temperature, and ω is the FWHM of the peak. The trap depth energies (E) that correspond to the two trapping states are 0.48 and 0.83 eV, respectively. From Fig. 2(b), one can see that there is a large overlap between the decomposed Gaussian band at about 395 K of the LACC2 TL curve and the LACr TL curve. This coincidence in TL curves indicates that the deep-energy trap states in LACC2 are due to the substitution of Cr³⁺ ions. Thus, the NIR LLP is partially contributed by conduction electrons released from the deep-energy trap states to the Cr³⁺ ions through direct recombination. On the other hand, the decomposed Gaussian bands at about 345 K for the TL curve of LACC2 overlap with the TL curve of LACe. This finding implies that the shallow-energy trap states in LACC2 are created by the introduction of Ce³⁺. Accordingly, the shallow-energy trap states responsible for the blue persistent luminescence in LACe would compensate for NIR persistent luminescence in LACC2, thereby significantly improving the performance of NIR persistent luminescence. This well explains the fact that the blue persistent luminescence almost disappears, and the LPP time seriously decreases after doping Cr³⁺ into LACe (inset of Fig. 1(b)).

To gain more insights on the persistent luminescence process, the PL characteristics in the Ce³⁺/Cr³⁺codoped phosphors were examined, as shown in Fig. 3 . Several discrete emission peaks in the low-energy region related to Cr³⁺ ions are excited by 310 nm UV light in which strong sharp peak at 735 nm (R-lines) is resulted from the spin-forbidden ²E → ⁴A₂ transition of Cr³⁺ ions [24, 25 ]. Meanwhile, the excitation of the corresponding samples at 310 nm yields a blue emission band at 415 nm because of the 5d₁-4f transition of Ce³⁺. Interestingly, the intensity of the R-lines significantly increases with increasing Ce³⁺ concentration, thereby indicating the close correlation between Ce³⁺ and Cr³⁺ ions. To elucidate the enhanced characteristics in NIR PL, we examined the PL and PLE spectra of Ce³⁺ or Cr³⁺ single-doped LaAlO₃ phosphors and Ce³⁺/Cr³⁺codoped phosphors, as shown in Fig. 4 . The emission of single-doped phosphor of LACe exhibits a blue band with peaks at 415 nm because of the 5d₁-4f transition of Ce³⁺. Its PLE spectrum shows one band located at around 315 nm, which is ascribed to the 4f −5d₁ transition. The PLE spectrum of the LACr sample for 735 nm emission exhibits three dominant peaks at 265, 415, and 615 nm, which correspond to the transitions from ⁴A₂ ground state of Cr³⁺ to ⁴T₁(te²), ⁴T₁(t²e), and ⁴T₂ states, respectively [22]. The blue emission band caused by the 5d₁-4f transition of Ce³⁺ in the LACe sample obviously overlaps with the excitation bands of Cr³⁺ in LACr. This finding indicates the possible energy transfer from Ce³⁺ to Cr³⁺ in the LaAlO₃ [26].

 

Fig. 3 PL spectra of LACC1, LACC2, LACC3 and LACC4 acquired under excitation of 310 nm.

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Fig. 4 PL spectra of LACe excited at 315 nm, and PLE spectra of LACe monitoring at 415nm and LACr, LACC1, LACC2, LACC3 and LACC4 monitoring at 735 nm, respectively.

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The PLE spectra of LACr, LACC1, LACC2, LACC3, and LACC4 monitored at 735 nm in Fig. 4 were determined to further confirm whether the energy transfer from Ce³⁺ to Cr³⁺ exists. The inconspicuous excitation band at 315 nm in LACr intensifies with increasing amount of Ce³⁺ incorporated. The intense excitation band could be attributed to the 4f→5d₁ transition of Ce³⁺ ions. These results provide a clear evidence to support the presence of efficient energy transfer from Ce³⁺ to Cr³⁺ [27]. The energy transfer from Ce³⁺ to Cr³⁺ is a non-radiative process because it is governed by electric dipole–dipole interactions; the strongly enhanced NIR emission from Cr³⁺ is thereby a result of the non-radiative energy transfer from Ce³⁺ and Cr³⁺ ions [27].

Therefore, combining with the PL characteristics in the Ce³⁺/Cr³⁺codoped LACC samples, the remarkably improved performance of NIR persistent luminescence in the Ce³⁺/Cr³⁺ co-doped LACC samples can be attributed to the effective non-radiative persistent energy transfer from Ce³⁺ ions to Cr³⁺ ions. Under this condition, electrons trapped in the shallow traps (0.48 eV) are released from traps into the conduction band and then transferred to Cr³⁺ through Ce³⁺. Our results suggest a possibility that the equivalent substitution of ions at different radii leads to distortions of the substitution site. This substitution mechanism induces site distortion and creation of efficient traps [18]. Thus, schematic of the persistent energy transfer process can be constructed to account for the improved NIR persistent luminescence in Ce³⁺/Cr³⁺ co-doped LACC samples (Fig. 5 ). Under the excitation of 315 nm light from the Xe lamp, the excitation populates the 5d₁ level of the Ce³⁺ ions and then fills the shallow traps through the conduction band. Meanwhile, Cr³⁺ ions are pumped from the ⁴A₂ level to the ⁴T₁ (te²) level, and the excited electrons are captured by deep traps through the conduction band. In the de-trapping process, electrons released from shallow traps with the assistance of thermal activation energy are transferred back to the 5d₁ level of Ce³⁺. Parts of these electrons radiatively relax to the ³F_5/2 level of Ce³⁺, thus producing a weak persistent luminescence centered at 415 nm. However, most of these electrons migrate to the ⁴T₁ (t²e) level of Cr³⁺ through an efficient non-radiative persistent energy transfer. Furthermore, electrons released from deep traps populate the ⁴T₁ (te²) level of Cr³⁺. Thus, the populated electrons of the ⁴T₁ (te²) and ⁴T₁ (t²e) levels relax nonradiatively to the ²E level and then contribute to a notable increase in NIR persistent luminescence through the ²E → ⁴A₂ transition of Cr³⁺ ions.

 

Fig. 5 A schematic representation of persistent energy transfer and persistent luminescence.

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4. Conclusions

We improved the NIR persistent luminescence by introducing Ce³⁺ ions into Cr-doped LaAlO₃ phosphors. A persistent energy transfer from Ce³⁺ to Cr³⁺ was confirmed based on the PL, PLE, and TL spectra. Upon inducing Ce³⁺ ions, extra efficient traps were generated to store the excited electrons. The persistent energy transfer and extra efficient traps were confirmed as the two major components that improve NIR persistent luminescence.