How to tune trap properties of persistent phosphor: Photostimulated persistent luminescence of NaLuGeO4:Bi3+,Cr3+ tailored by trap engineering

Materials Research Bulletin 97 (2018) 251–259

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How to tune trap properties of persistent phosphor: Photostimulated persistent luminescence of NaLuGeO4:Bi3+,Cr3+ tailored by trap engineering ⁎

MARK

⁎

Zehua Zou, Xue Tang, Chen Wu, Deyin Wang, Jiachi Zhang , Zhipeng Ci , Shanshan Du, Yuhua Wang Key Laboratory for Magnetism Magnetic Materials of the Ministry of Education, Lanzhou University, Lanzhou, 730000, PR China

A R T I C L E I N F O

A B S T R A C T

Keywords: Photostimulated persistent luminescence Trap engineering NaLuGeO4:Bi3+,Cr3+

We take the NaLuGeO4:Bi3+,Cr3+ as a typical example to show an effective approach to tune the trap properties of persistent phosphor as needed. An empirical law concerning the trap engineering is summarized based on the experimental results and a large number of references. Guided by this law, the trap properties of this material are effectively tailored by Cr3+ codopants, and consequently the degradation of the infrared-to-ultraviolet persistent luminescence is greatly reduced. The calculations and a serial of thermoluminescence experiments have been conducted to verify the effect of the trap engineering. The infrared-to-ultraviolet persistent luminescence and trap properties are investigated in details. It demonstrates that the proposed empirical law concerning the trap engineering is indeed valid and the NaLuGeO4:Bi3+,Cr3+ material is potential for photodynamic therapy.

1. Introduction Persistent luminescence (PersL) is an interesting optical phenomenon where a material can remain glowing for an appreciable time after stopping the irradiation. [1–4] Due to the characteristic property of “glow-in-the-dark”, the PersL materials have attracted a widespread interest. In the past decade, the researchers mainly focused on the visible PersL for the night-vision security signs. Thanking to the recent greatly advances of the near-infrared PersL materials, which can afford greater penetration depths because the absorbance of most bio-molecules reaches a minimum in NIR window (700–1000 nm), the applications of the PersL materials have been expanded to the in vivo bioimaging. [5–8] Meanwhile. the efficient ultraviolet (UV) emitting PersL phosphor has been reported and it exhibits the potential application in photodynamic therapy (PDT). [9–12] It is well known that the PersL properties are greatly dependant on the properties of trap such as density, stability, kinetics order and the depth. Among them, the trap depth is the most important factor. If the depth of the traps is too shallow, the photo-induced carriers would be emptied very quickly at room temperature (thermal-stimulated PersL) leading to a short PersL time. On the contrary, the carriers in too deep traps can’t thermally escape unless they are stimulated by the infrared laser (photostimulated PersL). Generally, the different trap depths are needed for different applications. For examples, the PersL for the night-vision security signs

⁎

or in vivo bio-imaging welcomes shallower traps distributed in range 0.5–0.8 eV [13,14] However, the much deeper traps with depth more than 1 eV would be better for application in the optical storage or PDT. [9,15–17] Therefore, it is necessary for us to be able to optionally tune the trap depth of the PersL materials according to the practical applications. In this work, in order to develop a photostimulated PersL material for possible PDT application, we have developed a UV PersL phosphor NaLuGeO4:Bi3+. Unfortunately, this material shows serious photostimulated PersL degradation owing to the lack of the deep traps. Accordingly, we take the NaLuGeO4:Bi3+ as a typical example and show an effective approach to tune the depth of trap as needed. In the course of the study, an important empirical law concerning the trap engineering has been summarized. On the basis of this useful law, the trap properties have been successfully tailored as intended, and the obtained NaLuGeO4:Bi3+,Cr3+ exhibits excellent NIR-to-UV photostimulated PersL performance, which is promising for future application in PDT. Significantly, this result also reveals that this trap engineering works and the proposed empirical law on the trap engineering is valid and a useful reference.

Corresponding authors. E-mail addresses: [email protected] (J. Zhang), [email protected] (Z. Ci).

http://dx.doi.org/10.1016/j.materresbull.2017.09.011 Received 14 June 2017; Received in revised form 8 August 2017; Accepted 6 September 2017 Available online 07 September 2017 0025-5408/ © 2017 Elsevier Ltd. All rights reserved.

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CN = 6) is closer to Bi3+ (r = 1.03 Å, CN = 6) than that of Lu3+ (r = 0.86 Å, CN = 6). Therefore, the Bi3+ emitter is expected to substitute the six-coordinated Na+ site when the content of Bi3+ is low, and the Lu3+ can be replaced by Cr3+ codopants (r = 0.62 Å, CN = 6) [18].

2. Experimental procedures 2.1. Materials and synthesis All materials were synthesized by the solid state method. The raw materials were Na2CO3·10H2O (A.R.), Lu2O3 (99.99%), GeO2 (99.99%), Bi2O3 (99.99%) and Cr(NO3)3·9H2O (99.999%), which were used directly without any further treatment. The stoichiometric stating materials were thoroughly homogenized in an agate pestle, and the mixture was transferred into an alumina crucible and then loaded into a muffle furnace. Then the mixed samples were sintered at 1488 K for 6 h in air. The obtained samples were cooled to room temperature and then ground again in an agate mortar. Finally, the powders were obtained.

3.2. Serious NIR-to-UV PersL degradation of NaLuGeO4:Bi3+ Fig. 3a gives the photostimulated PersL emission spectrum of typical NaLuGeO4:5%Bi3+ sample stimulated by a NIR (980 nm) laser. Before these measurements, all samples have been charged by an UV (254 nm) lamp for 15 min. It is found that the emission spectrum shows a very broad band (330–500 nm) with maximum at 400 nm, and it matches well with the Soret band of photosensitizers for a 94.6% covering of the spectral area. [19] And the UV emission is attributed to the 3P0,1 →1S0 transitions of Bi3+ ions. [20] Although the emission spectrum can meet the requirement of PDT, however, it is found that the photostimulated PersL intensity has been greatly degraded after a delay even without any NIR stimulation. Fig. 3b shows the photostimulated PersL intensities of the sample recorded after different delay time (t = 24, 96, 240 h). It is found that, as the increase of the delay time, the photostimulated PersL intensity has been sharply reduced and it only remains 0.3% of its initial intensity after delay for 96 h. This result indicates that the photo-induced carriers can not be steadily stored in the traps of the materials, and thus the photostimulated PersL intensity of the sample would degrade quickly even without NIR stimulation. In order to reveal the essential reason for this degradation and find a solution, Fig. 3c shows the thermoluminescence (TL) glow curves of the NaLuGeO4 (green line) and NaLuGeO4:Bi3+ samples (blue line), respectively. By comparison, it can be concluded that the TL peak at 309 K is attributed to the intrinsic traps (T1), and the TL band at 375 K is reasonably ascribed to the traps related to Bi3+ dopants (T2). Particularly, it is important to note that both the TL peaks of NaLuGeO4:Bi3+ sample are located in the low temperature region (Tm < 400 K). Since the maximum of TL peaks generally corresponds to the depth of traps, it suggests that the depth of traps in the present NaLuGeO4:Bi3+ sample is not sufficiently deep. As a consequence, the carriers stored in the shallow traps can be thermally emptied at room temperature, and thus few carriers stay behind before NIR stimulation, resulting in the serious degradation of the photostimulated PersL. According to the results, if we want to decrease the degradation fundamentally, it is an emergency for us to decrease the density of shallow traps and at the same time induce sufficiently deep traps in the present material.

2.2. Measurements and characterization A Rigaku D/Max-2400 X-ray diffractometer (XRD) was employed to check the crystalline phases of the obtained samples. The photostimulated luminescence (PSL) spectra were recorded using a FLS-920T spectrophotometer (Edinburgh Instruments Ltd, Edinburgh, U.K.) with a 500 mW NIR (980 nm) laser. Before the PSL measurement, the sample has been charged by an UV (254 nm) lamp for 15 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 (254 nm) lamp for 15 min and then heated from the room temperature at a rate of 1 Ks−1. All measurements were carried out at room temperature except for TL measurements. 3. Results and discussion 3.1. Structural analysis Fig. 1 exhibits the XRD patterns of the typical NaLuGeO4:xBi3+,yCr3+ (x = 0 y = 0, x = 5% y = 0 and x = 5% y = 0.5%) samples. All the samples can be well indexed to the NaLuGeO4 phase (JCPDS No. 88-1178). Fig. 2a depicts the original absorption spectrum of the NaLuGeO4 host. The edge of the absorption exhibits a remarkable drop around 280 nm due to host absorption. Based on the Tauc relation as shown in the inset of Fig. 2a, the optical band gap energy of the NaLuGeO4 host is calculated to be 4.396 eV by extrapolating the linear portion to the photon energy. Moreover, Fig. 2b shows the crystal structure of the NaLuGeO4 host and the coordination of atoms. It notes that the NaLuGeO4 belongs to the orthorhombic phase and the space group Pnma (62), and significantly there are two six-coordinated cation sites including Na+ and Lu3+. The ionic radii of Na+ (r = 1.02 Å,

3.3. Empirical law concerning traps engineering But how to tailor the trap properties as needs? Although there is not any universally accepted law on the trap engineering, it is reasonable that the understanding of trap mechanisms can enlighten us. In general, there are many defects in actual materials including the intrinsic vacancies and impurities, and the defects can act as the traps to hold carriers. At room temperature, if the carriers can be easily released, it means that the depth of traps is shallow. On the contrary, if the traps can hold the carriers more steadily, the depth can be regarded as “deep”. Since the holes are generally immovable, it is reasonable that the depth of traps can be evaluated by the ability to hold electrons. Based on the definition of IE, it is well known that the larger the ionization energy (IE) of a cation, the greater the energy of the cation to lose an electron. Accordingly, the IE of a cation (such as Bi3+ → Bi4+ + e−) may be an appropriate parameter to gain insight into the energy of the trap depth. [21] Therefore, an empirical law concerning the influences of dopants on traps properties can be proposed: the depth of traps can be tailored by dopants, and the dopants with larger IE can increase the density of deeper traps or make the present trap deeper and vice versa.

Fig. 1. the XRD patterns of the typical NaLuGeO4:xBi3+,yCr3+ (x = 0 y = 0, x = 5% y = 0 and x = 5% y = 0.5%) samples.

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Fig. 2. The absorption spectrum of NaLuGeO4 host (a), and the crystal structure of NaLuGeO4 host and coordinations of positive ions (b). Inset (i): the plot of (αhν)1/2 vs photon energy of NaLuGeO4 host.

Fig. 3. The photostimulated PersL spectrum of NaLuGeO4:Bi3+ stimulated by 980 nm laser (a), the photostimulated PersL intensity of NaLuGeO4:Bi3+ different delay time (b) and the TL glow curves of the NaLuGeO4 and NaLuGeO4:Bi3+ (c).

than that of emitters, the intensity of TL peak at low temperature is increased or the maximum of TL peak shifts to a lower temperature, due to the increase of shallow traps. Oppositely, the codopants with larger IE would increase the density and depth of the deep traps corresponding

However, it is well known that practice is the sole criterion for testing truth. Table 1 shows a summary of the IE of cations and the influences of codopants on TL peaks previously reported for some phosphors. [22–64] It is found that when the IE of codopants are lower 253

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Table 1 A summary of the ionization energy of ions and the influences of co-dopants on the TL peaks previously reported for some phosphors. Phosphors host

Activators

IEa (kJmol−1)

Codopants

IEa (kJmol−1)

TLsb (K)

TLcc(K)

References

NaLuGeO4

Bi3+

4372

Ca3(PO4)2 CaMgSi2O6

Mn2+ Mn2+

3248 3248

3839 3248 3248 2404 2404

CaAl2Si2O8

Eu2+

2404

Sr3Al2O5Cl2 LaAlO3 MgGeO3 Zn11B10Si4O34 BaSiO3

Eu2+ Cr3+ Mn2+ Mn2+ Eu2+

2404 4740 3248 3248 2404

Sr6Al18Si2O37

Eu2+

2404

Ca6BaP4O17

Eu2+

2404

439 350 350 216 353 460 480 400 309 426 360 367 327 345 347 348 455 372 347 349/388 317 360 320 369 309 315 323 308 381 358 317 330 310 335 325 394 333 358 623 373 250 271 250 375 323 340 360 305# 152 190 340 580 330 582 340 350 300 250 > 560 523 523# 369 335 375 398 351 507 274 270/405 276 280 295 325 360

This work [22] [23] [24]

Tb3+ Mn2+ Mn2+ Eu2+ Eu2+

4740 4110 4001 3547 3761 3899 4001 4372 3051 4001 4119 4119 4001 3761 3899 4001 4100 4115 3761 3547 2415 2415 3899 4119 4250 4100 4001 3761 4001 3899 3547 4250 3839 4100 4119 4001 4372 7298 4740 3232 4115 3899 4001 4119 3839 3833 5200 4115 4001 3899 4119 4910 2415 5963 3899 4001 4100 4115 4119 3126 3216 3248 6200 3761 4250 3761 9573 4210 7860 – 4115 3899 4100 4001

375 450 225 150

NaAlSiO4 α-Zn3(PO4)2 β-Zn3(PO4)2 Sr2Al2SiO7 Ca2Si5N8

Cr3+ Eu3+ Dy3+ Ce3+ Pr3+ Nd3+ Dy3+ Bi3+ K+ Dy3+ Tm3+ Tm3+ Dy3+ Pr3+ Nd3+ Dy3+ Ho3+ Er3+ Pr3+ Ce3+ Yb2+ Yb2+ Nd3+ Tm3+ Gd3+ Ho3+ Dy3+ Pr3+ Dy3+ Nd3+ Ce3+ Gd3+ Tb3+ Ho3+ Tm3+ Dy3+ Bi3+ Li+ Cr3+ Co2+ Er3+ Nd3+ Dy3+ Tm3+ Tb3+ Zn2+ In3+ Er3+ Dy3+ Nd3+ Tm3+ Ca2+ Yb2+ Y3+ Nd3+ Dy3+ Ho3+ Er3+ Tm3+ Hf4+ Hf4+ Mn2+ Ga3+ Pr3+ Gd3+ Pr3+ Ti4+ Sr2+ Zr4+ Sr2+/Zr4+ Er3+ Nd3+ Ho3+ Dy3+

Ca3Ga2Ge3O12 ZnS Sr3SiO5

Eu2+ Eu2+ Cr3+ Ga3+ Nd3+ Cu+ Ce3+

2404 2404 4740 6200 3899 1958 3547

SrMg2(PO4)2 Ga2O3

Eu2+ Cr3+

2404 4740

Ba4(Si3O8)2 Ca2Si5N8

Eu2+ Eu2+

2404 2404

Zn2SiO4

Mn2+

3248

SrSi2AlO2N3

Eu2+

2404

Lu2O3

Tb3+ Pr3+ Ce3+ Eu2+ – – Mn2+ Eu3+ Tb3+

3839 3761 3547 2404 – – 3248 4115 3839

Ce3+

3547

Ba13Al22Si10O66 Ca2MgSi2O7 ZnGa2O4

SrAl2O4 β-Ba3P4O13 CdSiO3 Zn2GeO4 Y2O2S

YPO4

380 400 313 350 313 341

347 398 406 410 – – 309

306

320 361 333 333 383 173 245

388 350 350 330 –

507

200

443 613 369/434 330 – – 335 383 – – – 102

[25] [26] [27] [28] [29] [30]

[31] [32] [33] [34] [35] [36]

[37]

[38] [37] [39] [40] [41] [42] [43] [44] [45,46] [47]

[48] [49] [50] [51] [52]

[53]

[54]

[55] [56] [57] [58] [59] [60] [61] [62]

[63]

(continued on next page)

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Table 1 (continued) Phosphors host CaWO3

a b c

Activators 3+

Eu

IEa (kJmol−1) 4110

Codopants 4+

Ti Mg2+

IEa (kJmol−1)

TLsb (K)

TLcc(K)

References

9573 7732

381

386 389

[64]

IE: ionization energy of ions. TLs: Positions of the TL main peaks for the single doped phosphor. TLc: position of the TL peak greatly improved by codopants.

compositions of the CB, and it shows that the Cr orbits can be divided into two parts: one is distributed in the impurity levels as deep traps, and the other is merged into the CB.

to the TL bands in a higher temperature region. It is interesting to note that almost all of previous reports that have given the experimental TL curves can demonstrate this law well. Therefore, our empirical law concerning the influences of codopants on traps properties is actually reliable at this point. Of course, there is no rule but has exceptions, and a few exceptions can be also found to this law as labeled a superscript # in Table 1, and the reason is an open question at this stage. Significantly, what's the option in this work? On the basis of our empirical law, if we want to induce deep traps, it should codope some cations with high IE. However, the IE is clearly not the larger the better. Too large IE may result in too deep depth of traps, and probably the carriers stored in traps can not be promoted even by NIR laser. Here, we can refer to the experiences of the thermal-stimulated PersL. In many cases, the Nd3+, Dy3+ ions are usually codoped into phosphors for the improvement of PersL properties, and the most representative examples are these commercial tricolor PersL materials such as CaAl2O4:Eu2+,Nd3+ and SrAl2O4:Eu2+,Dy3+. The IE of Nd3+ and Dy3+ ions are 3899 kJ/mol and 4001 kJ/mol, while the depth of traps for the CaAl2O4:Eu2+,Nd3+ and SrAl2O4:Eu2+,Dy3+ are in range of 0.6-0.8 eV. It suggests that, if we want to induce traps with a depth larger than 1 eV suitable for NIR (980 nm) stimulation, the IE of the codopants should be correspondingly larger than those of Nd3+ and Dy3+ ions. After deep consideration and careful selection, we finally notice the Cr3+ ions, whose IE is about 4740 kJ/mol. Moreover, the ionic radii of Cr3+ (r = 0.76 Å, CN = 6) is close to that of Lu3+ (r = 0.86 Å, CN = 6), and thus the formation of solid solutions and the soft substitutions at Lu3+ sites in NaLuGeO4 can be expected. But, do the Cr3+ codopants really work for the trap engineering?

3.5. Effect of trap engineering based on thermoluminescence investigations In order to experimentally verify the effect of trap engineering, Fig. 5a shows the TL curves of the NaLuGeO4, NaLuGeO4:Bi3+ and NaLuGeO4:Bi3+,Cr3+ samples. It can be clearly seen that the TL of the NaLuGeO4:Bi3+,Cr3+ sample consists of three bands at about 309 K, 375 K and 439 K, respectively. The formers two have been attributed to the intrinsic and Bi3+ related traps, and the latter at 439 K is believed to be associated with the Cr3+ codopants (T3). To gain insight into the trap properties, a classical multi-peak fitting method developed by Chen [65], Kitis [66] and Nakazawa [67] is introduced in the following equations：

E T − Tm ) kT Tm b T2 E T − Tm × [(b − 1)(1 − Δ) 2 exp ( ) + Zm ]− b − 1 , kT Tm Tm b

I (T ) = Im b b − 1 exp (

s=

βE 1 E exp ( ), kTm kTm2 Zm

n0 =

ts =

kTm2 b b Im Zm ( ) b − 1 , βE Zm

1 E exp ( ). s kTn

3.4. Effect of trap engineering based on first-principles calculations

with First-principles calculations can provide information about the band structures and electronic compositions for guidance. On the basis of the 1 × 2 × 3 super cell model in Fig. 2, the calculations of the electronic structures are operated by the Perdew-Burke-Ernzerhof exchange-correlation function within generalized gradient approximation (GGAPBE). [14] The band gap has been corrected by a scissor value of 1.134 eV according to the absorption spectrum owing to the underestimation of GGA-PBE. The top of valance band (VB) is set to 0 eV for convenience. Fig. 4a,b depict the band structure, total densities of states (TDOS) and partial densities of states (PDOS) of the typical NaLuGeO4:Bi3+,Cr3+ sample. It is found that the impurity levels are produced near the CB, which can be attributed to Bi (green) and Cr (red) orbits, respectively. For better references, Fig. 4c,d exhibit the TDOS and PDOS of the NaLuGeO4:Bi3+ (c) and NaLuGeO4 (d) samples. It is found that the band gap of the NaLuGeO4 host is about 4.396 eV, and the Bi3+ doping results in decrease of the gap (3.782 eV). It means that the Bi3+ induce impurity levels, which are 0.614 eV below CB. As mentioned above, the Bi3+ ions can act as both emitters and traps, and thus the depth (0.614 eV) of Bi3+ related traps is too shallow and it may be helpful for PersL but photostimulated PersL performances. However, as shown in Fig. 4b, the Cr3+ induced impurity levels are located at about 1.043 eV below CB. This result indicates that the depth of Cr3+ related traps is much deeper, and thus the introduction of Cr3+ codopants is expected to increase the density of the deep traps (E > 1 eV) suitable for NIR stimulation in theory. Fig. 4e displays the electronic

Δ=

(1)

(2)

(3)

(4)

2kT 2kTm and Zm = 1 + (b − 1) E E

where Im is the glow-peak maximum intensity, b is the kinetics order parameter, E is the trap depth, k is the Boltzmann constant, Tm is the temperature at the maximum, s is the frequency factor, β is the heating rate (1 K/s for this experiment), n0 is the trap concentration of trapped carriers and tn is the theoretical decay time at Tn K (298 K for this experiment). The three TL components are exhibited in Fig. 5b and the fitting results are listed in Table 2. The depths of traps corresponding to three TL components are calculated to be 0.396 eV, 0.609 eV and 1.315 eV, respectively, which are similar to the results of DFT calculations in Fig. 4. This result confirms that not only the Cr3+ codopants indeed induce the deep traps, but also the trap depth is sufficiently deep (1.315 eV) for the NIR stimulation. Meanwhile, it is noted that the theoretical decay time (tn) of the carriers in shallow traps is less than 1.830 × 103 s, but the tn for deep traps (1.315 eV) has been sharply increased to 1.842 × 108 s due to the larger depth. This result indicates that the carriers can be steadily stored in the deep traps for a sufficiently long time as needs. Moreover, this conclusion derived from the TL fitting result is also confirmed by the TL fading experiments. Fig. 6 shows that the TL glow curves of the sample recorded after delay for different time. It can be seen that although the intensity of the overall TL bands is decreasing in the first 30-min, the TL bands no longer decrease obviously due to the elimination of shallow traps corresponding to the TL component at 309 K. Even after delay for 3 h, the TL peak at 255

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Fig. 4. The band structure, TDOS and PDOS of NaLuGeO4:Bi3+,Cr3+ (a), the TDOS and PDOS of NaLuGeO4:Bi3+ (c) and NaLuGeO4 host (d), and the electronic compositions of NaLuGeO4:Bi3+,Cr3+ (e).

Fig. 5. The TL glow curves of NaLuGeO4 host, NaLuGeO4:Bi3+ and NaLuGeO4:Bi3+,Cr3+ (a) and the fitting TL glow curves of NaLuGeO4:Bi3+,Cr3+ (b).

Fig. 6. The TL glow curves of NaLuGeO4:Bi3+,Cr3+ measured at different delay time. Inset (i): the TL integral intensity curve of NaLuGeO4:Bi3+,Cr3+ measured at different delay time.

Table 2 The main fitting parameters of TL glow peak of the NaLuGeO4:Bi3+,Cr3+. (the Ec is the trap depth derived from the DTF-calculation.). TL Peak T1 T2 T3

ET (eV)

Ec (eV)

0.396 0.609 1.315

– 0.614 1.043

b 1.044 1.476 2.000

s (s−1)

n0 (cm−3) 5

1.850 × 10 1.050 × 107 8.957 × 1013

photostimulated PersL performances for potential PDT application. Moreover, it is interesting to note that the Cr3+ codopants can influence the shallow traps as well. Fig. 7 shows the TL glow curves of the NaLuGeO4:Bi3+,Cr3+ samples with different Cr3+ contents. It is interesting to find that not only the intensity of TL peak T3 is much stronger but also the peak T1 and T2 become weaker at the same time with the increase of Cr3+ contents. When the content of Cr3+ reaches 0.5 mol%, the TL peaks T1 and T2 corresponding to the shallow traps only remain about 64.6% and 51.4% of the original intensity. This result shows that the density of shallow traps has been greatly reduced by Cr3+ codopants. Particularly, it is noted that the reduction of shallow traps can also restrain the escaping rate of carriers in deep traps as shown in Fig. 6, and thus it is in favor of the photostimulated PersL performances.

tn (s) 6

3.596 × 10 6.554 × 106 3.922 × 107

2.665 × 101 1.830 × 103 1.842 × 108

439 K still remains about 61.9% of its initial intensity. This result confirms that the overall storage ability of carriers in traps of this material has been effectively improved by codoping Cr3+ ions, and the tailoring of the trap engineering is valid. Therefore, the Cr3+ codoped NaLuGeO4:Bi3+ sample is expected to be able to show excellent 256

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Fig. 7. The TL glow curves of NaLuGeO4:Bi3+,Cr3+ with different Cr3+ contents (a and b). Inset (i): the TL intensity curves of different peaks with different Cr3+ contents.

material should mainly contribute to the PersL under NIR laser stimulation. In order to evaluate the NIR stimulated PersL properties of this material, a serial of NIR stimulated PersL measurements have been conducted. Before the measurements, several copies of the optimal NaLuGeO4:Bi3+,Cr3+ samples have been subjected to the UV (254 nm) irradiations for 15 min, and then the samples were placed in the dark for 240 h. Subsequently, the photostimulated PersL spectra of the samples were successively recorded after a same time interval (t = 20 min), and the time of each stimulation has been set to 10 s. Accordingly, Fig. 9a shows the photostimulated PersL intensity dependant on the time, and the original photostimulated PersL spectra have been also depicted in the inset of Fig. 9a. It can be seen that, after the first charging, the photostimulated PersL of the samples can be repeatedly stimulated by NIR laser even after 240 h, and thus it supplies enough time for the preoperative preparations of PDT. However, owing to the discharging, the photostimulated PersL intensity of sample would be successively decreased after the each stimulation. Fig. 9b exhibits the photostimulated PersL intensity of sample dependant on the time under a constant NIR stimulation, and the inset also shows the original photostimulated PersL spectra recorded at 1, 5 and 15 min. It is found that the signals can be recorded continuously. The results indicate that the tailoring of the trap engineering is valid, and the NIR-to-UV PersL of the present NaLuGeO4:Bi3+,Cr3+ material has been greatly improved due to the trap engineering.

Fig. 8. The TL glow curves of NaLuGeO4:Bi3+,Cr3+ stimulated by 980 nm laser for different time. Histogram (i): the decrement of different peak.

In addition, when the content of Cr3+ ions is more than 0.5 mol%, all the TL peaks decrease due to the degradation of storage and thus the optimal Cr3+ content can be determined to be 0.5 mol%. 3.6. NIR stimulated TL and PersL properties of NaLuGeO4:Bi3+,Cr3+

3.7. NIR-to-UV PersL mechanisms of NaLuGeO4:Bi3+,Cr3+

Is the introduction of deep traps really in favor of the NIR stimulated PersL properties? Hereby, Fig. 8 gives the TL curves of the NaLuGeO4:Bi3+,Cr3+ sample recorded at different time under a constant NIR (980 nm) stimulation, and the decrements of the TL peaks T2 and T3 for the different time segments are also exhibited in the inset. It is found that the TL intensity of the peak T2 corresponding to shallow traps has been reduced greatly while that of peak T3 only shows a slight decrease, after the constant NIR stimulation for 1 min. It means that the NIR stimulation would firstly promote the carriers stored in shallow traps. However, after the shallow traps have been almost emptied by the NIR stimulation (5 min), the liberation of carriers in the deep traps corresponding to the TL peak T3 would be accelerated. After the NIR stimulation for 10 min, the decrement of the TL peak T3 has increased to 28.0%. Therefore, it shows that the newly induced deep traps in this

Accordingly, the NIR-to-UV PersL mechanism of the NaLuGeO4:Bi3+,Cr3+ material is proposed in Fig. 10. Firstly, the electrons are excited to CB under UV excitation, and some holes stay behind in the ground levels of Bi3+ (process ¢Ù) at the same time. Subsequently, the excited electrons are captured by these intrinsic (T1), Bi3+ (T2) and Cr3+ (T3) related traps below the CB. The electrons in the shallow traps are thermally exhausted at room temperature while the ones in the deep traps will release at a very low rate, and the majority is still steadily stored for a sufficiently long time (process ¢Ú). Under the NIR laser stimulation, the electrons held in the deep traps have to be promoted into the CB again, and then track to the excited levels 3P1 and 3 P0 of Bi3+ emitters (process ¢Û). Finally, the electrons recombine with 257

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Fig. 9. The photostimulated PersL intensity of NaLuGeO4:Bi3+,Cr3+ with the interval stimulation (a) and with the continuous stimulation (b). Inset (i) and (ii): the corresponding original photostimulated PersL spectra of NaLuGeO4:Bi3+,Cr3+.

Fig. 10. The photostimulated PersL mechanism scheme of the NaLuGeO4:Bi3+,Cr3+.

holes in the ground state of 1S0 to generate UV PersL (process ¢Ü).

the National Science Foundation for Fostering Talents in Basic Research of the National Natural Science Foundation of China (Nos. 041105 and 041106).

4. Conclusions In order to induce more deep traps in NaLuGeO4:Bi3+ for possible PDT application, an empirical law is proposed, and it is proved to be reliable for the trap engineering by summarizing the massive previously reported data. The DFT calculations and a serial of TL experiments are conducted to verify the effect of trap engineering. On the basis of this law, we successfully decreased the density of shallow traps and induced the deep traps with depth suitable for NIR stimulation by codoping Cr3+ ions, and consequently reduced the photostimulated PersL degradation. The investigation reveals that the modified traps are suitable for the NIR stimulation, and the tailoring of traps is valid. Finally, it demonstrates that the NIR-to-UV PersL of the NaLuGeO4:Bi3+,Cr3+ material is promising for PDT activation and our empirical law concerning the trap engineering is indeed valid.

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Acknowledgments This work was supported by the National Nature Science Foundation of China (No. 10904057 and 51202099), the Fundamental Research Funds for Central Universities (Nos. Lzjbky-2015-112), and 258

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