Spectroscopic data in our investigation, combined with those in references, are used to construct an energy diagram of the typical Lu3Al2Ga3O12:Ln2+/Ln3+ phosphors. Based on the diagram, the persistent luminescence properties of Lu3Al2Ga3O12: Ln2+/Ln3+ (Ln = La–Yb) phosphors are theoretically predicted. We have shown that the position of the 4f ground level of Ln2+ in the band gap can estimate the ability of Ln3+ codopants as foreign electron traps, and it is confirmed by our experimental results of the typical Lu3Al2Ga3O12:Tb3+,Ln3+ samples. Finally, the critical roles of the Yb3+ codopants as efficient foreign traps in the typical Lu3Al2Ga3O12:Tb3+ phosphor is revealed. The requirements for efficient foreign traps and the fundamental persistent luminescence mechanism of this phosphor are systematically summarized.
© 2016 Optical Society of America
Persistent luminescence, also known as long-lasting phosphorescence or afterglow luminescence, of materials is based on the transient storage of radiation in the form of trapped electrons and holes, followed by the slow detrapping and radiative recombination of carriers [1–6 ]. This gives rise to visible-light emission lasting for minutes or hours, suitable for many applications such as emergency lighting, safety signage , optical storage media [8,9 ], in vivo imaging [1,10 ], drug carriers , solar energy , and photocatalysis [13,14 ]. The persistent luminescence field of research began to attract much interest because of the discovery of the highly bright SrAl2O4:Eu2+,Dy3+ phosphor . Unfortunately, despite the intensive search in the past decades, only a handful of efficient persistent luminescence phosphors have been developed.
One of the main reasons for the slow development of persistent luminescence phosphors is the trial and error nature of previous research. Researchers must often select a previously unreported host to ensure the so-called “novelty” and then dope ions to obtain persistent luminescence phosphors through a trial and error method. Obviously, this is an inefficient approach to research that relies on serendipity, and positive results are not guaranteed. Thus, we recently proposed an approach to develop highly efficient persistent luminescence phosphors by tailoring commercial phosphors, and a highly efficient green persistent luminescence phosphor was obtained by codoping Yb3+ into Zn2SiO4:Mn2+ . However, the Yb3+ codopants do not always work to enhance persistent luminescence properties of all persistent luminescence phosphors such as SrAl2O4:Eu2+,Dy3+ or CaAl2O4:Eu2+,Nd3+. On the contrary, the Dy3+ or Nd3+ codopants are totally useless for persistent luminescence improvement of Zn2SiO4:Mn2+ . This is a very interesting phenomenon, and the underlying cause is still an open question. The roles of codopants must be strongly dependent on the electronic structures of a given host. Regrettably, thus far, the details of this association and fundamental persistent luminescence mechanism remain unclear. Therefore, our understanding limits the further development of persistent luminescence phosphors, which is a critical bottleneck for this field.
Recently, Dorenbos reported a method to construct an energy diagram containing important information about electronic structure such as conduction, valence bands, and energy-level positions of Ln2+ and Ln3+ in inorganic compounds (which is very helpful for understanding the photoluminescence (PL) mechanism in Ln-doped compounds) [17–21 ]. Based on this diagram, some groups proposed a method of band-gap engineering to improve persistent luminescence properties of typical Ln3Al5O12:Ce3+ (Ln = Y, Gd, Lu) phosphors by substituting Al3+ with Ga3+ [22–25 ]. In their studies, Ga3+ ions are mainly used to tailor the band-gap energy, but this is significantly different from the critical roles of Dy3+ and Nd3+ codopants as foreign electron traps for SrAl2O4:Eu2+ or CaAl2O4:Eu2+ . Persistent luminescence mechanisms and natures of foreign electron traps are highly linked to the details of the electronic structure (energy diagram), in particular, the host band-gap energy and energy-level positions of emitters and codopants capable of storing energy .Therefore, from a fundamental and applied viewpoint, the study of the critical role of codopants based on an energy diagram is highly desired.
In this study, spectroscopic data in our investigation, combined with those in references, are used to construct the theoretical energy diagram of a representative host: Lu3Al2Ga3O12 because of its possible efficient persistent luminescence properties [17–21 ] and the presence of more complete data of the electronic structures of Ln3Al5O12 (Ln = Y, Gd, Lu) in references [22–25 ]. According to the obtained energy diagram, the persistent luminescence properties of Lu3Al2Ga3O12:Ln3+ (Ln = La–Yb) phosphors, in particular, the critical roles of the Yb3+ codopants as efficient foreign electron traps, were theoretically predicted. It is very exciting that almost all theoretical predictions are consistent with the experimental results. This interesting discovery is sufficient for encouraging a thorough investigation. Finally, the requirement of an efficient foreign electron trap and fundamental persistent luminescence mechanism was summarized systematically. Obviously, this report is potentially helpful for the future development of highly efficient persistent luminescence phosphors.
2.1 Materials and synthesis
All samples were synthesized by a traditional solid-state method. Raw materials were Al2O3 (A.R.),Ga2O3(A.R.) and rare earths (99.99%), which were used directly without any further treatment. The optimal Tb3+ single doped content is 1.1mol% and the codoped content of Yb3+ is about 0.3mol%. Stoichiometric starting materials were thoroughly homogenized (grinding was performed using an agate pestle and mortar) and the mixture was transferred into an alumina crucible and then loaded into a muffle furnace. The mixed samples were then sintered at 1550 °C for 5 h in air. The obtained samples were cooled to room temperature and then ground again in an agate mortar.
2.2 Measurements and characterization
A Rigaku D/Max-2400 X-ray diffractometer was employed to check the phases of the obtained samples. PL and persistent luminescence spectra were obtained using a FLS-920T spectrophotometer (Edinburgh Instruments Ltd., Edinburgh, U.K.) with a 450 W xenon arc lamp (Xe900) as the light source. The decay curves were measured using a PR305 persistent luminescence instrument (Zhejiang University Sensing Instrument Co. Ltd., Hangzhou, China). This instrument can directly provide the absolute intensity of persistent luminescence (mcd/m2) measured by an integral sphere and the measurement was finished when the absolute intensity drops below 0.32 mcd/m2. The samples were irradiated with ultraviolet light (254 nm) for 10 min. The thermoluminescence (TL) curves were measured using a FJ-417A TL meter (Beijing Nuclear Instrument Factory, Beijing, China). All samples were first exposed to radiation using an UV lamp (254 nm) for 10 min and then heated from room temperature to 673 K at a rate of 1 K s−1. All measurements were taken at room temperature, except for TL measurements.
3 Results and discussion
3.1 Construction of the energy diagram (electronic structure)
Table 1 lists the main parameters required to construct the electronic structure diagram of Lu3Al2Ga3O12:Ln3+/Ln2+ phosphors. We estimated the mobility band-gap energy 8% larger than the host exciton creation energy (Eex) according to the paper by Dorenbos  and the value of Ec (6.5 eV) originates from the references [24,27 ]. The ECT defines the energy difference between Ev and the ground state energy E4f(7,2 + ,A) of Eu2+, which can be calculated based on the CT band of a Eu3+ single doped sample as shown in Fig. 1 . Since the centroid shift is about the same for the entire Ln3Al5-xGaxO12 (Ln = Y, Gd, Lu) family, the U(6,A) value of 6.8 eV for YAG is also adopted for Lu3Al2Ga3O12 . Accordingly, the E4f(6,3 + ,A) level for Eu3+ is 6.8 eV lower than E4f(7,2 + ,A) of Eu2+. Once the position of the 7F0 ground level of Eu3+was known, following the host independent evolution of the R3+ energy levels, it was possible to determine the position of the 4f n ground levels of all other R3+s relative to the host's valence band. The level position of the first spin allowed 4f–5d transition of Tb3+ can be added by employing the equation: Efd(8, + 3,A) = Efd(8, + 3,free)–D( + 3,A). The value of the Efd(8, + 3,free) is always 7.78 eV for free Tb3+ and that of D( + 3,A) is 3.22 eV for Lu3Al2Ga3O12 . The exchange splitting Eexch(8,3 + ,garnets), showing the energy difference between the excitation to the first high spin [HS] and first low spin [LS] 5d-level of Tb3+ as shown in Fig. 2 . The dominant peaks of the [HS] and [LS] transitions are located at 265 nm (4.68 eV) and 314 nm (3.95 eV). The energy difference (0.73 eV) between the [HS] and [LS] transitions means the value of the exchange splitting (0.73). The value of Eexch(8,3 + ,garnets) is calculated based on the excitation spectrum of the as-prepared Lu3Al2Ga3O12:Tb3+ sample and is consistent with the datum from references. Finally, the Efd(6, + 2,A) of Eu2+ can be determined by adding the redshift for Eu2+, which can be also determined by an empirical equation: D(6, + 2,A) = 0.64D(1,3 + ,A) − 0.833, where the D(1,3 + ,A) is always the redshift for Ce3+ in the same host and is 3.22 eV in this case.
The zigzagged curve is invariable in all compounds, whereas its anchor point relative to the host bands is variable. Therefore, with these collected data listed in Table 1, the theoretical electronic structure of the Lu3Al2Ga3O12:Ln3+/Ln2+ phosphors can be constructed in the form of the host referred binding energy (HRBE), as shown in Fig. 3 .
3.2 Theoretical predictions on optimal codopants
The constructed energy diagram can be used to predict the PL and persistent luminescence properties of Ln2+/Ln3+ in a given host. Because this paper mainly focuses on codopants as foreign electron traps, we only give two typical examples at this stage: (1) For Ln2+, it is found in Fig. 3 that the first 5d state for each divalent lanthanide is within the bottom of conduction band (CB). This means that the Ln2+ in this host is not stable for auto-ionization and the d-f emission of Ln2+ would be quenched. This appears to be a general rule for Ln2+ in trivalent rare earth sites. Moreover, even if the 5d state for each Ln2+ is below the CB, we cannot observe the 5d–4f emission of Ce2+, Pr2+, Nd2+, Tb2+, Ho2+, Er2+, Tm2+ because their 5d states are lower than 4f states (not stable). (2) For Ln3+, as shown in Fig. 3 and Table 2 , because the 5d levels of Ce3+, Pr3+, Tb3+, Dy3+, Ho3+, Er3+, Tm3+, Yb3+ and are close to the bottom of CB (ΔE < 0.7 eV), part of the excited electrons in 5d levels can thermally return to CB and are then re-captured by intrinsic traps many times (i.e., retrapping). Unfortunately, because the 4f levels of Ho3+, Er3+, Tm3+ and Yb3+ are within VB, the photo-induced holes in 4f levels are not stable and would be filled by nearby electrons after excitation (namely, migration of holes). As a consequence, the recombination may occur somewhere far from emitters via a non-radiative approach, resulting in bad persistent luminescence performance. Accordingly, it is expected that we can only observe persistent luminescence in Ce3+, Pr3+, Tb3+ and Dy3+ each individually activated Lu3Al2Ga3O12 samples in theory, and this prediction is consistent with our experimental results. In the following discussion, we will use Lu3Al2Ga3O12:Tb3+ as a typical example to discuss the selection of appropriate codopants as efficient foreign electron traps and the fundamental persistent luminescence mechanism.
The codopants have been widely used to improve the persistent luminescence properties of phosphors, and they can be classified, according to the essential mechanism, into three categories: (1) by energy transfer [28,29 ]; (2) by increasing intrinsic traps ; (3) by foreign traps . The most famous examples of codopants as foreign traps are Dy3+ and Nd3+ for SrAl2O4:Eu2+ and CaAl2O4:Eu2+, respectively. At this stage, we mainly use the typical Lu3Al2Ga3O12:Tb3+ phosphor as an example to discuss the critical Ln3+ codopants as foreign electron traps, and this is the emphasis of this work.
It is well known that the mechanism of the Eu2+ persistent luminescence assumes that the excited Eu2+ species give up an electron to the CB and further to the traps. This may be interpreted as photoionization of the Eu2+ ion . For the Tb3+ activated persistent luminescence phosphors, it has been demonstrated that the photoionization of Tb3+ would create a [Tb3+-h+] pair, and thus the persistent luminescence of Tb3+ needs the presence of electron traps: [Tb3+-h+] + e- = Tb3+ + hv  The oxygen vacancy is the intrinsic electron traps of the phosphors, which are created during high temperature sintering . However, the amount of Vo is always not enough for efficient persistent luminescence, and therefore some foreign electron traps should be induced. At present, it is reasonable that an Ln3+ codopant as an efficient foreign electron trap should meet at least two requirements explained in the following paragraphs:
- (1) Appropriate ability to hold electron: The ability of Ln3+ to hold an electron can be roughly estimated by its electronegativity, χ, that describes the tendency of an atom to attract electron (Table 3 Fig. 3, the 4f ground states of most Ln2+, except Sm2+, Eu2+, and Yb2+, locate within the CB, showing that the electrons in the 4f levels of Ln2+ are free. As a consequence, the Ln3+ cations (including Nd3+, Dy3+) are not stable to hold electrons and will not function as efficient foreign electron traps to improve persistent luminescence properties of this phosphor. On the contrary, the 4f levels of Sm2+, Eu2+, and Yb2+ are clearly below the CB, meaning that energy is needed to excite an electron from traps, and thus, the Sm3+, Eu3+, and Yb3+ codopants can be considered as stable electron traps (in theory).
- (2) Suitable trap depth distribution: The activation energy required to help electrons escape from traps, which is called the trap depth, is very important for persistent luminescence properties of a phosphor. A trap that is very shallow (i.e., too close to the CB) will result in a very short persistent luminescence; if the trap is too deep, no electrons can escape at room temperature and no persistent luminescence will be observed. Accordingly, the TL peaks of LuAGG:Tb-Yb, LuAGG:Tb-Sm, and LuAGG:Tb-Eu samples were measured and plotted in Fig. 4. The positions of the dominant TL glow peaks shows that codoping Sm3+ provides the shallowest depth of trap while Eu3+ yields the deepest depth. The TL peak located at 460 K results in the weak persistent luminescence, which is too deep for releasing electrons at room temperature; thus, the Eu3+ codopants are not useful for the improvement of persistent luminescence. On the contrary, the dominant TL peak of Sm3+ codoped sample is at about 320K, which is the shallowest. However, its TL intensity corresponding to the density of the traps is too low, and thus it also results in weak persistent luminescence.
A trap depth of around 0.6–0.8 eV is often stated as ideal for LPL . Based on Fig. 3, the lowest 4f ground level of Yb2+ is located 1.1 eV below the CB. Because the trap level is not discrete, but rather a continuum of energy levels around a certain mean value, the value of 1.1 eV should correspond to the theoretical deepest depth of the foreign traps (Yb3+). According to K.V. Eeckhout, the trap depth distribution of Nd3+ in CaAl2O4:Eu2+ covers a broad range of 0.6 eV . Therefore, it is empirically expected that the traps (Yb3+) in a Lu3Al2Ga3O12:Tb3+ sample mainly distribute in the range of 0.5–1.1 eV, which may be very suitable for LPL properties.
Clearly, at this stage, knowing the depth and density of the traps is crucial to understanding the role of Yb3+ codopants on persistent luminescence improvement. The TL is a powerful and versatile tool to investigate the nature of traps present in persistent luminescence phosphors. Unfortunately, a single TL curve cannot provide enough information. It is generally accepted that the trap depth distribution can be obtained by performing the “thermal cleaning” TL experiments, in which the sample is heated to a temperature Tstop after the excitation and before the TL measurements. Figure 5 gives the results of the “thermal cleaning” TL experiments for samples without Fig. 5(a) and with Fig. 5(b) Yb3+ codopants, and the trap depth distributions are also shown below the TL curves. The depth of traps is determined from the corresponding TL curve by initial rise analysis . The majority of traps are very shallow in depth (Ed < 0.6 eV) for the Tb3+ single doped sample; thus, the traps are emptied very quickly, and the persistent luminescence lasts for a very short period of time (t < 600 s). However, for the Yb3+ codoped sample, the trap depth distribution is greatly changed, and the dominant density of traps is at 0.71 eV, a much more suitable value. As a result, the Lu3Al2Ga3O12:Tb3+,Yb3+ phosphor shows much better persistent luminescence properties.
3.4 Fundamental persistent luminescence mechanism of Lu3Al2Ga3O12:Tb3+,Yb3+
In principle, a TL fading experiment, where the duration between the excitation and the actual TL experiment is varied, could provide real-time information about the persistent luminescence process and is helpful to understand the persistent luminescence mechanism. Accordingly, Fig. 6 exhibits the TL glow curves of typical Lu3Al2Ga3O12:Tb3+ samples without Fig. 6(a) and with Fig. 6(b) Yb3+ codopants after UV (254nm) irradiation for 10 min and then after different delay times (i.e., TL fading experiments). It can be seen that the TL band at low temperature (350 K) of the Tb3+ single doped sample gradually decreases in intensity and slightly shifts to higher temperature, while the high-temperature band (432 K) does not show obvious variation. This result indicates that shallow traps are mainly responsible for the persistent luminescence of Lu3Al2Ga3O12:Tb3+ and the persistent luminescence mechanism should be due to “thermal stimulation” because of the TL peak shift that can be also observed in “thermal cleaning” TL experiments. The trap depths at different delay time are calculated by initial rise analysis and the real-time trap depth distribution is given below the TL curves as well, demonstrating that the shallow traps (E < 0.6 eV) mainly contribute to the persistent luminescence of this phosphor .
After trace Yb3+ is codoped, the TL intensity continues to decrease with an increase of t within the whole range of time investigated, and the TL peak gradually shifts to higher temperature until t > 3600 s. This implies that tunneling decay is dominant in the late slow-decay stage. At this point, the electrons mainly tunnel from the traps into the excited [Tb3+-h+] states and recombine for the subsequent radiative decay (persistent luminescence). We can provide two additional proofs for the tunneling decay:
- (1) For a typical tunneling decay, the persistent luminescence intensity can be expressed in the inverse power-law function I (t) = I 0 t −1 . Accordingly, a linear dependence of the persistent luminescence decay curve with slope = −1 should be expected in the Log-Linear diagram for a tunneling decay phosphor. As shown in Fig. 7(a), a perfect linear dependence is not obtained for the samples without Fig. 7(a) or with Fig. 7(b) Yb3+ codopants in the earlier time (t < 3600 s). However, for the Yb3+ codoped sample, a weak linear dependence can be observed when t > 3600 s at a larger scale (inset of Fig. 7), and the slope of the fitting line is calculated to be −1.01 (standard Error = 0.028), due to the tunneling decay. While the fitting slope of the Tb3+ single doped sample is about −0.00358, which is far different from the characteristic value of −1.
- (2) Figure 8 exhibits the persistent luminescence decay curves of both samples in Log-Log diagrams. It is found that the persistent luminescence decay curve of the Tb3+ single doped sample Fig. 8(a) can be exponentially fitted well, due to the thermal stimulated decay . On the contrary, for the Yb3+ codoped sample Fig. 8(b), although the exponential fitting works in the early rapid-decay stage (t < 3600 s), it is no longer possible when t > 3600 s, due to the dominant tunneling decay. In fact, the tunneling decay is always present, but much weaker than the thermal stimulated decay, and thus, it appears only when the thermal stimulated decay subsides in the late slow-decay stage. Clearly, although the tunneling decay is weak, it is still very significant for extending the persistent luminescence duration time of the phosphor.
An electronic structure diagram of a randomly given phosphor can be constructed by collecting data from experimental spectroscopy and archival literature. The energy-level positions of Ln2+/Ln3+ in the band gap can be used to gain insight into the PL/persistent luminescence properties of Lu3Al2Ga3O12:Ln2+/Ln3+ phosphors, in particular, the ability of Ln3+ codopants as foreign electron traps in the example of Lu3Al2Ga3O12:Tb3+,Ln3+. Accordingly, it proposed that an efficient foreign electron trap must meet at least three requirements: the appropriate ability to hold electron, the inefficient self-emission, and the suitable trap depth distribution. Finally, the critical role of the Yb3+ codopants as efficient foreign traps in the typical Lu3Al2Ga3O12:Tb3+ phosphor is revealed, and the essential persistent luminescence mechanism of this typical phosphor is systematically summarized.
This work was supported by the National Natural Science Foundation of China (Nos. 10904057, 51202099, 61106006 and 61376011), the Fundamental Research Funds for Central Universities (No. Lzjbky-2015-112), and the National Science Foundation for Fostering Talents in Basic Research of the National Natural Science Foundation of China (Nos. 041105 and 041106).
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