Formation of Deep Electron Traps by Yb3+ Codoping Leads to Super

Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Formation of Deep Electron Traps by Yb3+ Codoping Leads to SuperLong Persistent Luminescence in Ce3+-Doped Yttrium Aluminum Gallium Garnet Phosphors Jumpei Ueda,* Shun Miyano, and Setsuhisa Tanabe Graduate School of Human and Environmental Studies, Kyoto University, Yoshida-nihonmatsu-cho, Sakyo-ku, Kyoto 606-8501, Japan S Supporting Information *

ABSTRACT: The Y3Al2Ga3O12:Ce3+−Cr3+ compound is one of the brightest persistent phosphors, but its persistent luminescence duration is not so long because of the relatively shallow Cr3+ electron trap. To compare the vacuum referred binding energy of the electron trapping state by Cr3+ and lanthanide ions, we selected Yb3+ as a deeper electron trapping center. The Y3Al2Ga3O12:Ce3+−Yb3+ phosphors show Ce3+:5d → 4f green persistent luminescence after blue light excitation. The formation of Yb2+ was confirmed by the increased intensity of absorption due to Yb2+:4f−5d at 585 nm during the charging process. This result indicates that the Yb3+ ions act as electron traps by capturing an electron. From the thermoluminescence glow curves, it was found that the Yb3+ trap makes a much deeper electron trap with a 1.01 eV depth than the Cr3+ electron trap with a 0.81 eV depth. This deeper Yb3+ trap provides a much slower detrapping rate of filled electron traps than the Cr3+-codoped persistent phosphor. In addition, by preparing transparent ceramics and optimizing Ce3+ and Yb3+ concentrations, the Y3Al2Ga3O12:Ce3+(0.2%)−Yb3+(0.1%) as-made transparent ceramic phosphor showed super-long persistent luminescence for over 138.8 h after blue light charging. KEYWORDS: persistent luminescence, trap engineering, Ce3+, valence-state change, garnet electron trap depth formed by Cr3+ can be controlled by conduction band (CB) engineering with different Ga contents, x.19 For the efficient blue light charging process, it was found that the Ga content, x, in YAGG should be more than x = 3.0. The blue light charging efficiency increases with increasing x because the energy gap between the lowest 5d level (5d1) and the bottom of the CB becomes closer. On the other hand, the trap depth by the Cr3+ electron trap is changed from 1.02 eV (x = 0) to 0.41 eV (x = 5.0) by increasing x in the YAGG host. In addition, with increasing x, the quantum efficiency of Ce3+:5d1−4f luminescence at room temperature decreases significantly above x = 3.0.19,20 At higher Ga content, instead of the high blue light charging efficiency, the trap depth by Cr3+ becomes much shallower and the quantum efficiency also decreases. Thus, for instance, the YAGG:Ce3+−Cr3+ phosphors with high Ga content (x > 4.0) are no longer persistent phosphors with bright and long PersL.19 From the viewpoint of the blue light charging efficiency, trap depth, and quantum efficiency properties, YAGG:Ce3+−Cr3+ with x = 3.0 is the most balanced persistent phosphor in the series of YAGG:Ce3+−Cr3+ persistent phosphors. However, YAGG:Ce3+−Cr3+ with x = 3.0 shows a little bit shorter persistent duration, whereas the initial

1. INTRODUCTION Persistent luminescence (PersL) is continuous luminescence for long duration after excitation. Until now, many persistent phosphors have been reported in various compounds with different luminescence centers such as Eu2+,1,2 Cr3+,3−5 and so on.6 On the other hand, host materials with a garnet crystal structure have attracted a great deal of interest for persistent phosphors as well as for scintillators, light-emitting diode (LED) phosphors, or laser materials. Some Ce3+ singly doped garnets such as (Y,Gd)3Al5−xGaxO12:Ce3+,7−9 Y 3 Sc 2 Al 3−x Ga x O 12 :Ce 3+ , 10−12 Mg 3 Y 2 Ge 3 O 12 :Ce 3+ , 13 and Lu2CaMg2(Si1−xGex)3O12:Ce3+14 have been known to show short PersL because of limited and shallow electron traps originating from intrinsic defects. However, to improve the PersL properties, it is necessary to engineer the electron traps for persistence. In 2014, we found that Cr3+ acts as an excellent electron trap to dramatically improve the PersL intensity and duration in Ce3+-doped Y3Al5−xGaxO12 (yttrium aluminum gallium garnet, YAGG).15 YAGG:Ce3+−Cr3+ shows the 5d−4f green PersL for several hours after blue light excitation as well as white LED illumination. After the discovery of the YAGG:Ce3+−Cr3+ phosphors, we also developed transparent ceramics of YAGG:Ce3+−Cr3+ with super-long persistent duration16 and yellow persistent phosphors of Gd3Al5−xGaxO12:Ce3+−Cr3+.17,18 In YAGG:Ce3+−Cr3+, the threshold energy for charging and the © XXXX American Chemical Society

Received: February 14, 2018 Accepted: May 23, 2018 Published: May 23, 2018 A

DOI: 10.1021/acsami.8b02758 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

≈ 3.0) and successfully developed long persistent phosphors in YAGG:Ce3+−Yb3+. Also, for improving the PersL intensity and for new unique applications, we prepared transparent ceramic phosphors of YAGG:Ce3+−Yb3+. Transparent ceramics have many advantages such as high transparency, high density, low light scattering, and good crystallinity and attract a great deal of attention as optical material hosts. Actually, the transparent ceramics were already reported and commercialized as laser host materials33 and white LED phosphors34 in garnet ceramics. Recently, we reported that the transparent ceramic persistent phosphors show better PersL performance compared with powder-type persistent phosphors.16,18 This is because low light scattering enables charging inside of the persistent phosphors. For the powder of persistent phosphors, the inside of the powder compact cannot be charged because of the strong light scattering. In addition, recently, for transparent persistent phosphors with deep electron traps, the possibility to be data storage materials was reported.35 Thus, we also investigated the PersL properties of YAGG:Ce3+−Yb3+ transparent ceramic phosphors.

PersL intensity is high. To improve the PersL duration at room temperature with keeping the blue light charging efficiency and quantum efficiency, the trap depth should be deeper in the YAGG with x ≅ 3 composition to avoid quick electron detrapping. For the management of electron trap depth, lanthanide (Ln) codoping can also be a smart method by taking into account the zigzag curve of vacuum referred binding energy, VRBE, of divalent lanthanide (Ln2+) ions. So far, a number of persistent phosphors codoped with Ln3+ were developed.2,6 In these phosphors, Ln3+ works as an electron trap and the trap depth can be controlled by a type of Ln3+. This is because some Ln3+ ions can be changed into the Ln2+ states after capturing one electron and the relative energy level of the Ln2+ ground state with respect to the bottom of the CB is different by a type of lanthanide ions. Bos et al. reported the systematic energy shift of the electron trap depth by changing Ln3+ codopants in YPO4:Ce3+ phosphors and the good agreement with the energy gap between the bottom of the CB and Ln2+ ground state with the zigzag curve in a host referred binding energy diagram as well as the VRBE.21−23 Thus, for selecting a suitable lanthanide codopant for the PersL at ambient temperature, the VRBE diagram including the CB, valence band, and ground states of Ln2+ and Ln3+ becomes a strong tool. By investigating the energy gap between the bottom of the CB and the level of Ln2+, the trap depth can be predicted. Also, the trap depth by the Cr3+ codopant corresponds to the energy gap between the bottom of the CB and Cr2+. In YAGG with x = 3.0, the VRBE of the Cr2+ level and the bottom of the CB are located at −2.80 and −1.99 eV, respectively.19 Because the CB energy is not largely changed by doping with low concentration in the same host material, the deeper electron trap in YAGG with the x = 3.0 host can be realized by selecting a codopant whose VRBE is slightly lower than the Cr2+ level. When we check the VRBE diagram for YAGG with x = 3.0,19,24−27 the VRBE of Yb2+ is located at −3.52 eV. Therefore, we expected that Yb3+ can be a suitable deeper trap in the YAGG host with x = 3.0 compared with the electron trap of Cr3+ for long PersL, as shown in Figure 1a,b. Actually, it is already known that Yb3+ works well as

2. EXPERIMENTAL PROCEDURE 2.1. Synthesis of YAGG:Ce 3+−Yb3+ Normal Ceramics. Polycrystalline ceramics of Y3Al5−xGaxO12 garnet, YAGG(x = 3.0), doped with 0.5% Ce3+ and y% (y = 0.05, 0.1, 0.5, and 1) Yb3+ for the dodecahedral site (A site in {A}3[B]2(C)3O12) were synthesized by solid-state reactions. To avoid concentration quenching for Ce3+ luminescence, 0.5% was selected first. 36 Chemicals of Y 2 O 3 (99.99%), Al2O3 (99.99%), Ga2O3 (99.99%), CeO2 (99.99%), and Yb2O3 (99.9%) were used as the starting materials. The powder was mixed by a ball milling system (Premium Line P-7, Fritsch) with ethanol. The obtained slurry was dried and pulverized. The dried powders were pressed at 50 MPa into 10 mm ϕ × 2 mm thick pellets. The pellets were sintered at 1600 °C for 10 h in air. 2.2. Synthesis of YAGG:Ce3+−Yb3+ Transparent Ceramics. Transparent ceramics of YAGG(x = 3.0) doped with z% (z = 0.5 and 0.2) Ce3+ and 0.1% Yb3+ were prepared by solid-state reactions. The starting chemicals are the same as those used in the synthesis of conventional ceramics. The powder with stoichiometric compositions was mixed by the ball milling system with ethanol, tetraethyl orthosilicate, and dispersant. The mixed powder was dried and pulverized and then compacted to form a ceramic green body (20 mm ϕ × approximately 3 mm thick) under uniaxial pressing of 50 MPa. The green body was sintered at 1600 °C for 10 h under a vacuum atmosphere. Some of the as-prepared samples were annealed at 1250 °C for 5 h under air. The both sides of all the obtained transparent ceramics were polished simultaneously by using a copper plate and diamond slurry, and the final thickness was 2.1 mm. 2.3. X-ray Diffraction and Optical Measurements. The crystalline phases of ceramic samples were identified as garnet crystals using an X-ray diffraction (XRD) equipment (Ultima IV, Rigaku) (see Figure S1). For photoluminescence (PL) spectra in the range of visible to near-infrared (NIR), the sample was excited by a 365 nm LED, and the luminescence was detected with two charge-coupled device (CCD) spectrometers (USB 2000+, Ocean Optics) of different gratings with a bifurcated fiber. The PL excitation (PLE) spectra were measured using a fluorescence spectrophotometer (RF-5300, Shimadzu). For PersL spectra, the sample was charged by 450 nm monochromatic light by using a Xe lamp and a 450 nm bandpass filter, and the PersL was detected using a CCD spectrometer (QE65PRO, Ocean Optics). After the sample was charged by the 450 nm monochromatic light mentioned above, the PersL decay curve was detected by a photomultiplier tube (PMT, R3896, Hamamatsu Photonics), which was covered with a 475 nm shortcut filter and a 600 nm long-cut filter to filter out all but the Ce3+ luminescence. The persistent luminance (mcd/m2) was measured by using a luminance

Figure 1. (a) Schematic energy diagram of the relationship between Ce3+:4f,5d levels and Cr3+ and Yb3+ electron traps and (b) schematic image of the PersL decay profile controlled by trap depth management.

an electron trap in some phosphors, such as the persistent phosphor of MgGeO3:Mn2+−Yb3+28,29 and the persistent and thermoluminescence (TL) phosphor of (Y,Gd)3Al5O12:Ce3+− Yb3+,30−32 where Yb3+ acts as a much deeper electron trap. In this study, we investigated the possibility of Yb3+ as a deeper electron trap than the Cr3+ electron trap in the YAGG hosts (x B


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addition to the Ce3+:5d1 → 4f band, sharp PL lines due to the Yb3+:2F5/2 → 2F7/2 transitions were observed at around 1 μm by UV excitation. With an increasing Yb3+ concentration, y, the Ce3+:5d1 → 4f PL intensity decreases monotonically and the Yb3+:2F5/2 → 2F7/2 PL intensity increases up to y = 0.5% and the Yb3+ PL intensity of y = 1% is almost the same as that of y = 0.5%. From the PLE spectra of the Ce3+ luminescence, two PLE bands were observed at around 440 and 350 nm in all the samples of Y3Al2Ga3O12:Ce3+(0.5%)−Yb3+(y%). These PLE bands are originated from the transitions of Ce3+ ion from the ground 4f level to the 5d1 level and the second lowest 5d (5d2) level, respectively. In the PLE spectra monitoring Yb3+ luminescence between 900 and 1100 nm, similar PLE bands were observed at around 425 and 360 nm, which are attributed to Ce3+:4f−5d1 and 5d2 (see Figure S2). These results show that the energy transfer occurs from Ce3+ to Yb3+ in the YAGG(x = 3.0) host. It was suggested that the energy transfer from Ce3+ to Yb3+ is caused through the metal-to-metal chargetransfer state of Ce4+−Yb2+ in the Y3Al5O12 garnet doped with Ce3+ and Yb3+ but not through the down conversion (quantum cutting) process.37,38 The energy-transfer efficiency can be estimated by the lifetime of the donor-excited level (Ce3+:5d1). The obtained lifetime of Ce3+:5d1 in YAGG:Ce3+(0.5%)− Yb3+(y%) decreases with increasing Yb3+ concentration in the order of 39.8 (y = 0), 37.3 (y = 0.05), 34.0 (y = 0.1), 27.7 (y = 0.3), 24.5 (y = 0.5), and 18.5 ns (y = 1), as shown in Figure S3. From these obtained lifetime values of the Ce3+:5d1 level, the energy-transfer efficiency (ηET) from Ce3+ to Yb3+ increases with increasing Yb3+ concentration and reaches to approximately 53.5% at Yb3+ (1%) (see Figure S4). The quantum efficiency (η) of the Ce3+:5d1−4f luminescence can be roughly estimated by η = 100 − ηΕΤ (%). Thus, the observed decrease of the Ce3+:5d1 → 4f PL intensity with the Yb3+ concentration is caused by the decrease of quantum efficiency because of the strong energy transfer. 3.2. PersL Spectra of YAGG(x = 3.0):Ce3+−Yb3+(y%). Figure 2b shows the PersL spectra of Y3Al2Ga3O12:Ce3+−Yb3+ samples with different Yb3+ concentrations after 450 nm blue light charging. All the YAGG(x = 3.0):Ce3+−Yb3+ samples exhibit the PersL of Ce3+ at around 520 nm because of the Ce3+:5d → 4f transition, but they do not exhibit the PersL of Yb3+ at around 1 μm. With increasing Yb3+ concentration, the PersL intensity of Ce3+:5d1−4f first increases up to y = 0.1 and decreases. From the observation of Ce3+ PersL and the absence of Yb3+ PersL, it is expected that Yb3+ acts only as an electron trap. The lack of Yb3+ emission in PersL spectrum could be related to the absence of persistent energy transfer from Ce3+ to Yb3+ only in PersL. However, some persistent phosphors containing Yb, such as MgGeO3:Yb3+, Ca2SnO4:Yb3+, and (Ba1−xSrx)AlSi5O2N7:Yb2+−Yb3+, show NIR PersL at around 1 μm because of the electron transfer or energy transfer.39−41 To clarify the absence of Yb3+ PersL in YAGG:Ce3+−Yb3+, further study is needed, but it may be possibly related to the different Ce3+−Yb3+ pairs which show PL and PersL. 3.3. TL Glow Curves of YAGG(x = 3.0):Ce3+−Yb3+(y%). Figure 2c shows the TL glow curves monitoring the Ce3+ luminescence of the YAGG(x = 3.0):Ce3+−Yb3+ samples and the reference samples of YAGG(x = 3.0):Ce3+ and YAGG(x = 3.0):Ce3+−Cr3+ reported by our group.15 The Ce3+ singly doped YAGG(x = 3.0) sample exhibits some weak TL glow peaks originated from the intrinsic defects at 206 and 286 K, whereas the YAGG(x = 3.0):Ce3+−Yb3+ samples exhibit only single TL glow peak from 343 to 355 K. The TL glow peak

measurement setup (BW-L1, Konica Minolta) composed of a CCD spectrometer (Glacier X, B&W Tek Inc.), a fiber, and a collimator lens. Using the obtained persistent luminance, the persistent decay curves were calibrated. For the persistent luminance measurement of YAGG:Ce3+−Yb3+ transparent ceramics, the back side and edge of the sample were covered with an aluminum foil to collect PersL. TL glow curves monitoring the Ce3+ luminescence were measured using the same photomultiplier and the filters as the PersL decay curve measurement. The sample was mounted on a cryostat (Helitran LT3, Advanced Research Systems) to control the temperature. The samples were excited by the UV light of the Xe lamp (250−400 nm) at 100 K for 10 min and kept for another 10 min after excitation, and then the sample temperature was increased with the heating rate of 10 K/min up to 600 K. For the transparent ceramics, in-line transmittance spectra were measured by a UV−vis−NIR spectrometer (UV-3600, Shimadzu). The time course of in-line transmittance spectra during the charging process and the detrapping process was also measured by the UV−vis−NIR spectrometer. For the charging process, the sample was illuminated by a 455 nm LED (LLS-455, OceanOptics) for different times (1, 3, 5, 10, 30, and 60 min). For the detrapping process, the spectrum was measured at different times (5 min, 60 min, 24 h, and 4 days) after blue light charging. Images of PersL were taken by a digital camera (NEX-7, Sony) with ISO 6400, F-number 5, and different shutter speeds, SSs, after blue LED charging for 5 min.

3. RESULTS AND DISCUSSION 3.1. PL Spectra of YAGG(x = 3.0):Ce3+−Yb3+(y%). Figure 2a shows the PL spectra excited by 365 nm and the PLE spectra of 520 nm for YAGG(x = 3.0):Ce3+−Yb3+ with different Yb3+ concentrations. An intense luminescence band peaked at 510 nm was observed. This band is attributed to the transitions from the 5d1 energy level to the 4f energy levels of Ce3+. In

Figure 2. Spectroscopic results of YAGG(x = 3.0):Ce3+−Yb3+ samples with different Yb3+ concentrations. (a) PL spectra excited by 365 nm and PLE spectra monitoring the luminescence at 520 nm. (b) PersL spectra at 10 sec after 450 nm excitation. (c) TL glow curves at a heating rate of 10 K/min after UV charging for 10 min. The black and brown TL glow curves are the reference data of YAGG(x = 3.0):Ce3+(0.5%)−Cr3+(0.05%) and YAGG(x = 3.0):Ce3+(0.5%), respectively.15 (d) PersL decay curves after 450 nm excitation for 5 min. The gray PersL decay curve is the reference data of YAGG(x = 3.0):Ce3+(0.5%)−Cr3+(0.05%).15 C


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ACS Applied Materials & Interfaces temperature of the samples with Yb3+ (0.05, 0.1, and 0.5%) is similar to each other, but the sample with Yb3+ (1%) shows a slightly higher TL glow peak temperature of 355 K. This is probably because the effect to the host electronic band structure by the Yb3+ doping with slightly higher concentration is not negligible. The doping of smaller Yb3+ ions (0.98 Å) in 8fold coordination42 can increase the band gap as is observed in the composition of Y3Al5O12 and Lu3Al5O12. When Y3+ (1.015 Å) ions in 8-fold coordination are replaced by smaller Lu3+ (0.97 Å) ions,42 the band gap energy is known to be increased.43 Compared with the YAGG(x = 3.0):Ce3+−Cr3+ sample, the TL glow peak of the YAGG(x = 3.0):Ce3+−Yb3+ sample clearly shifts to higher temperatures. The peak temperatures of YAGG(x = 3.0):Ce3+(0.5%)−Cr3+(0.05%) and YAGG(x = 3.0):Ce3+(0.5%)−Yb3+(0.05%) samples are 294 and 346 K, respectively. These results show that Yb3+ acts as a new electron trap, which is distinguished from the traps by intrinsic defects and the electron trap by Cr3+. From the observed TL peak temperature, we estimated the trap depth using the first-order kinetics equation44−46 as given below: ⎛ E ⎞ βE s exp = × ⎟ ⎜− kTm 2 ⎝ kTm ⎠

electron trap depth. The PersL duration on 0.32 mcd/m2 after light excitationis listed in column 4 of Table 1. With increasing Yb3+ concentration, y, the PersL intensity as well as the duration increases up to y = 0.1 and then decreases. The PersL intensity corresponds to the electron trap concentration; thus, it is expected that the PersL intensity will increase monotonically with increasing Yb3+ concentration. However, the optimum Yb3+ concentration for the PersL intensity was observed. The decrease of PersL intensity with higher Yb3+ concentration is probably due to the decrease of the Ce3+:5d1−4f quantum efficiency as discussed in Section 3.1 and the increase of nonradiative recombination probability increases. From the viewpoint of long PersL duration, 0.1% of Yb3+ concentration is the best among the YAGG(x = 3.0):Ce3+(0.5%)−Yb3+(y%) samples with different y. To improve the PersL intensity furthermore, we tried to prepare transparent ceramic persistent phosphors. 3.5. PersL of YAGG(x = 3.0):Ce3+−Yb3+ Transparent Ceramics. Figure 3a shows the in-line transmittance spectra of the as-made and annealed YAGG(x = 3.0):Ce3+(0.5%)− Yb3+(0.1%) transparent ceramics. The inset of Figure 3a shows the images of the as-made and annealed YAGG(x = 3.0):Ce3+(0.5%)−Yb3+ (0.1%) transparent ceramics. The asmade and annealed samples show green and yellow colors, respectively, and both of them possess high transparency. The thickness of both as-made and annealed samples is 2.1 mm. Both the samples show over 50% transmittance at 1200 nm. In the transmittance spectrum of the as-made sample before charging, four absorption bands were observed at around 950, 585, 425, and 340 nm. The sharp absorption lines at around 950 nm are attributed to Yb3+:4f−4f transitions from 2F7/2 to 2F5/2, while the broad absorption bands at around 425 and 340 nm are attributed to the transitions of Ce3+ from 4f to 5d1 and 5d2, respectively. The broad absorption band at 585 nm can be originated from the 4f14−4f135d1 spin allowed transition of a small amount of Yb2+, which was generated by the reduction of Yb3+ under vacuum sintering. On the other hand, in the transmittance spectrum of the annealed samples, the absorption band of Yb2+:4f−5d was not observed because the minor amount of Yb2+ ions was oxidized to be Yb3+. The observed small bump at 690 nm in all the transmittance spectra is due to the small change of instrument response by switching a detector from a InGaAs photodiode to a PMT. It is already known that Yb2+ can be stabilized in the garnet compounds (Y3Al5O12,48−51 Y3Ga5O12,52 Lu3Al5O12,53 and Lu3Ga5O1252) by vacuum sintering with proper charge compensators or the reduction with γ-irradiation in Yb3+doped compounds. In Y3Al5O12:Yb2+ and Y3Ga5O12:Yb2+, the first absorption bands due to the spin allowed 4f14−4f135d1 transition (4f−5d), which were reported to be approximately 650 and 555 nm, respectively. The obtained 4f−5d absorption band (585 nm) of a small amount of Yb 2 + in Y3Al2Ga3O12:Ce3+(0.5%)−Yb3+ (0.1%) transparent ceramics is located between the Yb2+:4f−5d absorption wavelengths of Y3Al5O12 and the Y3Ga5O12 host. Thus, the Yb2+:4f−5d absorption wavelength is blue-shifted monotonically with increasing Ga content. This tendency of Yb2+:4f−5d absorption wavelength shifting for the Ga content corresponds to that of Ce3+:4f−5d1. Dorenbos summarized the Ce3+:4f−5d1 absorption wavelength in RE3Al5−xGaxO12 (RE = Gd, Y, Lu) hosts and reported that the Ce3+:4f−5d1 absorption wavelength is blueshifted with increasing Ga content because of the crystal field.

(1)

where β is the heating rate, k is the Boltzmann constant (eV/ K), s is the frequency factor (s−1), and Tm is the TL glow peak temperature (K). It is known that the first-order kinetics equation is dominant rather than the non-first-order kinetics in many compounds.47 Here, we assume the frequency factor to be 1 × 1013 s−1 because we already estimated it from the heating rate plot for the YAGG(x = 3.0):Ce3+−Cr3+ sample19 and also because the frequency factor depends on the host material but not on the dopants. The estimated trap depths are shown in column 3 of Table 1. The trap depth values of Table 1. TL Glow Peak Temperature, Trap Depth, and Persistent Luminescence Duration for YAGG(x = 3.0):Ce3+−Yb3+(y%) Yb3+ concentration (%)

TL peak temperature (K)

trap depth (eV)

duration until 0.32 mcd/m2 (min)

0.05 0.1 0.5 1

346 343 349 355

1.01 1.00 1.02 1.04

343.2 >830.0 404.2 47.3

YAGG(x = 3.0):Ce3+(0.5%)−Yb3+(y%) with y = 0.05, 0.1, 0.5, and 1 samples range within 1.00−1.04 eV and are nearly similar to each other. Thus, the deeper electron trap was realized by the Yb3+ codoping compared with the Cr3+ electron trap of 0.81 eV, as is predicted from the VRBE diagram. 3.4. PersL Decay Curves of YAGG(x = 3.0):Ce3+−Yb3+(y %). Figure 2d shows the PersL decay curves of the YAGG(x = 3.0):Ce3+−Yb3+ samples with different Yb3+ concentrations after 450 nm blue light charging for 5 min. The PersL of all the samples is detectable by human eyes in a dark room. The PersL decay rate which corresponds to the slope does not change much by the Yb3+ concentration because those trap depths are almost the same. However, the PersL decay rate of Yb3+codoping samples becomes slower compared with that of the Cr3+-codoping sample. This is because the deeper electron trap of Yb3+ causes a slower detrapping rate. Thus, as we expected, the PersL decay rate was successfully controlled by the deeper D


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Figure 3. (a) In-line transmittance spectra of as-made and annealed YAGG(x = 3.0):Ce3+(0.5%)−Yb3+(0.1%) transparent ceramics (the spectrum of the annealed sample was added by 5% to avoid overlapping) and (inset) images of the as-made and annealed samples. Variation of in-line transmittance spectra of as-made YAGG(x = 3.0):Ce3+(0.5%)−Yb3+(0.1%) transparent ceramics for the (b) charging process by 455 nm LED for different charging times and (c) detrapping process at different times after excitation. Time dependence of the integrated absorption coefficient for Yb2+:4f−5d and Yb3+:4f−4f in the (d) charging process and (e) detrapping process.

Y3Al5O12 and Y3Ga5O12 garnets and suggested that it is possibly related to the large hardness of the garnets combined with the exceptionally large crystal field splitting.43 Taking into account the 0.6 eV correction, the Yb2+:4f−5d absorption wavelength in Y3Al2Ga3O12 was calculated to be 636 nm from the reported Ce3+:4f−5d absorption wavelength (435 nm) in Y3Al2Ga3O12.43 The estimated Yb2+:4f−5d absorption wavelength is roughly in line with the observed absorption band. This result also shows that the absorption band at 585 nm in Y3Al2Ga3O12:Ce3+(0.5%)−Yb3+ (0.1%) transparent ceramics is originated from the Yb2+:4f−5d transition. Figure 3b,c shows the variation of in-line transmittance spectra of the as-made YAGG(x = 3.0):Ce3+(0.5%)−Yb3+ (0.1%) transparent ceramics during the charging process by the 455 nm LED light and the detrapping process at room temperature. During the charging process, the Yb2+:4f−5d absorption intensity clearly increases with increasing charging time, and the absorption intensity was almost saturated after 1 h charging. Because this absorption intensity is proportional to the Yb2+ concentrations, some of the Yb3+ ions were changed into the Yb2+ state after charging. In addition, the Yb2+:4f−5d absorption intensity decreases with time after excitation, as shown in Figure 3c. The Yb2+ ions created by the charging process gradually change back to the Yb3+ state by releasing the trapped electron. To visualize the changing of Yb2+ and Yb3+ concentrations by charging and detrapping processes, the integrated absorption coefficient of Yb2+:4f−5d and Yb3+:4f−4f was plotted as a function of charging time as shown in Figure 3d and as a function of time after excitation as shown in Figure 3e. The Yb2+:4f−5d-integrated absorption coefficient clearly increases

These results support that the absorption band at 585 nm in Y3Al2Ga3O12:Ce3+(0.5%)−Yb3+ (0.1%) transparent ceramics is originated from the Yb2+:4f−5d transition and that the absorption peak wavelength of Y3Al5−xGaxO12:Yb2+ is also blue-shifted with increasing Ga content by the crystal field. In addition, Dorenbos reported that the red-shift parameter of 5d energy level for Ln3+ (D(3+,A)) are correlated with the red-shift parameter for Ln2+ (D(2+,A)) in the same host by Dcal (2 + , A) = 0.64D(3 + , A) − 0.233 eV

(2)

(3+/2+)

The red-shift parameter for Ln is the energy difference between the 5d energy of Ln(3+/2+) as the free ion and that of Ln(3+/2+) in a compound, which includes the 5d energy shifting by the centroid shift and crystal field factors. The red-shift parameters are changed by the valence state of Ln but not changed by the type of lanthanide ions. D(2+,A) and D(3+,A) can be estimated using the below functions with a 4f−5d absorption wavelength of Yb2+ and Ce3+ (λabsYb2+:4f−5d and λabsCe3+:4f−5d (nm)) D(3 + , A) = 6.12 eV − D(2 + , A) = 4.41 eV −

1240 λabs,Ce3+ :4f−5d

(3)

1240 λabs,Yb2+ :4f−5d

(4)

The estimated D(2+,A) and D(3+,A) are listed in Table S1. On the basis of these equations, the Dcal(2+,A) and Yb2+:4f−5d absorption wavelength can be predicted by the Ce3+:4f−5d1 absorption wavelength. However, Dorenbos et al. already reported that Dcal(2+,A) estimated from Ce3+:4f−5d1 is about 0.6 eV smaller than D(2+,A) estimated from Yb2+:4f−5d in E


Research Article


Figure 4. Spectroscopic results of transparent ceramic samples. (a) TL curves of as-made and annealed YAGG(x = 3.0):Ce3+(0.5%)−Yb3+(0.1%) transparent ceramics with 10 K/min heating rate after UV charging for 10 min. PersL decay curves after blue light charging for (b) YAGG(x = 3.0):Ce3+(0.2, 0.5%)−Yb(0.1%) transparent ceramics and YAGG(x = 3.0):Ce3+(0.5%)−Yb(0.1%) opaque ceramics and (c) YAGG(x = 3.0):Ce3+(0.2%)−Yb3+(0.1%) transparent ceramics, YAGG(x = 3.0):Ce3+(0.5%)−Cr3+(0.05%) normal ceramics, and SrAl2O4:Eu2+−Dy3+ powder (GLL-300FFS produced by Nemoto Lumi-Materials Company Limited).

with increasing charging time, whereas the Yb3+:4f−4fintegrated absorption coefficient shows a decreasing tendency. On the other hand, for the detrapping process, Yb2+:4f−5d- and Yb3+:4f−4f-integrated absorption coefficient decreases and slightly increases, respectively. The inverse correlation between Yb2+:4f−4f and Yb3+:4f−5d absorption coefficient for the charging and detrapping processes shows that Yb3+ ions act as electron traps by capturing one electron and changing into Yb2+. In addition, even at 4 days after blue light excitation, the 4f−5d transition of the created Yb2+ was observed, which indicates that a part of electron traps by Yb3+ is still filled. Thus, these transparent ceramics show PersL even 4 days after excitation as discussed later. The reason for the small change of the Yb3+:4f−4f-integrated absorption coefficient during and after charging is because the Yb3+:4f−4f absorption transition probability is lower than the Yb2+:4f−5d transition probability.54 In other words, because the Yb2+ ion has the parityallowed 4f−5d transition with high transition probability, the small amount of change from Yb3+ to Yb2+ can be detected easily by monitoring the Yb2+ absorption intensity in the transmittance spectrum during and after charging. Thus, the YAGG:Ce3+−Yb3+ persistent transparent ceramics is one of the special materials to detect the electron trap species by the transmittance spectrum. Figure 4a shows the TL glow curves of the as-made and annealed transparent ceramics of Y3Al2Ga3O12:Ce3+(0.5%)− Yb3+(0.1%) after UV charging for 10 min. Both TL glow peak temperatures of the as-made and annealed samples are located at approximately 339 K, which is almost the same as that of opaque ceramics of the same composition. However, the width of the TL glow peak of the as-made sample is much broader than that of the annealed samples. There are two possible reasons: one is the formation of additional electron trapping center by oxygen vacancies in the as-made sample because of vacuum sintering. It was known that they act as electron traps, and as-made YAGG:Ce3+ transparent ceramics sintered under

vacuum which has oxygen vacancies also showed some TL glow peaks as shown in Figure S5. Another possible reason is the broadening of trap depth distribution by the inhomogeneity of coordination environments around the Yb3+ electron trapping center. It was known that the TL glow band was broadened by a wider electron trap distribution.55,56 So far, many researchers analyzed the electron trap distribution in some persistent phosphors, which has broad TL glow band.57−60 We also reported that the electron trap depth distribution was broadened when the inhomogeneity increases around the Cr3+ electron trap by the substitution of the Ga ions for the Al sites in Y3Al5−xGaxO12:Ce3+−Cr3+ persistent phosphors.19 Similar to this, the inhomogeneity can be caused by the formation of oxide vacancies by vacuum sintering. In both the cases, its TL glow curves can become much sharper in the annealed sample because the oxide vacancies can be removed after annealing in air, as shown in the below reaction: 2Yb′Y + V O•• +

1 × O2 → 2Yb×Y + OO 2

(5)

Figure 4b shows PersL decay curves of YAGG(x = 3.0):Ce3+−Yb3+ transparent ceramics. The PersL of all the transparent ceramic samples also continues over 1000 min after blue light charging. Because the annealed samples also show very high persistent luminance, the contribution to PersL at ambient temperature by oxygen vacancies is negligible. Thus, the main electron trap for PersL is due to the Yb3+ electron traps. The persistent luminance values of the as-made and annealed YAGG(x = 3.0):Ce3+(0.5%)−Yb3+(0.1%) transparent ceramics are much higher than those of the opaque ceramics with the same composition. The higher persistent luminance of the transparent ceramics is ascribed to the “volume effect”, in which the luminescence comes from the deeper interior part of the ceramics because of the low scattering of excitation and emission light.16,18 F


Research Article


4. CONCLUSIONS The green long persistent phosphors chargeable by blue light in Y3Al2Ga3O12:Ce3+−Yb3+ were developed. The Yb3+ ion was chosen as an electron trap based on the Yb2+ energy level location for the zigzag curve of divalent lanthanide ions in the VRBE diagram. The formation of Yb2+ was confirmed by the increased intensity of absorption at 581 nm during the charging process, which decreases after charging. This result indicates that the Yb3+ ions act as electron traps and change into the Yb2+ state. The trap depth of Yb3+ (1.01 eV) estimated from the TL glow peak is much deeper than that of Cr3+ (0.81 eV). The formation of a deeper trap led the super-long persistent transparent ceramic phosphor with Y3Al2Ga3O12:Ce3+(0.2%)− Yb3+(0.1%) composition, which exhibits the PersL for over 138.8 h after blue light charging.

In addition, the persistent luminance was improved by decreasing the Ce3+ concentration to 0.2% in the YAGG(x = 3.0):Ce3+−Yb3+(0.1%) transparent ceramics, as shown in Figure 4b. The slopes of PersL decay in all the annealed YAGG(x = 3.0):Ce3+−Yb3+ samples in Figure 4b are slightly steeper than those of the as-made samples. This is because the mean electron trap depth becomes much shallower by annealing as the TL glow curves become narrower. Among the transparent ceramic samples, the annealed YAGG(x = 3.0):Ce3+(0.2%)−Yb3+(0.1%) transparent ceramics show the brightest PersL before 735 min. After 735 min, the persistent intensity of the as-made YAGG(x = 3.0):Ce3+(0.2%)− Yb3+(0.1%) transparent ceramics overcomes that of the annealed one. Compared with the YAGG(x = 3.0):Ce3+(0.5%)−Cr3+(0.05%) conventional ceramics and SrAl2O4:Eu2+−Dy3+ powder (GLL-300FFS produced by Nemoto Lumi-Materials Company Limited), the as-made YAGG(x = 3.0):Ce3+(0.2%)−Yb3+(0.1%) transparent ceramics show much higher PersL from 40 min after ceasing blue excitation, as shown in Figure 4c. The duration times on 0.32 mcd/m2 of YAGG(x = 3.0):Ce3+(0.5%)−Cr3+(0.05%), SrAl2O4:Eu2+−Dy3+ powder (GLL-300FFS), and as-made YAGG(x = 3.0):Ce3+(0.2%)−Yb3+(0.1%) transparent ceramics are approximately 800, 1500, and 5600 min, respectively. The PersL of the as-made YAGG(x = 3.0):Ce3+(0.2%)−Yb3+(0.1%) transparent ceramics can still be detected by the PMT detector at 138.8 h (8333 min) after excitation. Figure 5 shows the photographs of the samples with PersL at different elapsed times after blue LED excitation. PersL was

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b02758. XRD analysis, energy transfer, red-shift parameters, and TL glow curve (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jumpei Ueda: 0000-0002-7013-9708 Setsuhisa Tanabe: 0000-0002-7620-0119 Author Contributions

The manuscript was written through contributions from all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research was supported by JSPS KAKENHI (grant nos 16K05934 and 16H06441).

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REFERENCES

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Figure 5. Photograph of as-made YAGG(x = 3.0):Ce3+(0.2%)− Yb3+(0.1%) transparent ceramics under white LED and blue LED and at different times after blue LED excitation. (All photos were taken by a Sony digital camera, NEX-7, with ISO 6400, F-number 5, and different shutter speeds, SSs.)

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Formation of Deep Electron Traps by Yb3+ Codoping Leads to Super

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