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Title

Hole detrapping type persistent phosphors of
RE2O2S (RE= La, Gd, Y, Lu) doped with Eu3+-Pr3+
and Eu3+-Tb3+

Author(s)Hashimoto, Atsunori; Ueda, Jumpei; Aoki, Yasushi;
Dorenbos, Pieter; Tanabe, Setsuhisa

Citation

The Journal of Physical Chemistry C, 127(31):15611-15619

Issue Date

2023-07-26

Type

Journal Article

Text version

author

URL

http://hdl.handle.net/10119/19326

RightsAtsunori Hashimoto, Jumpei Ueda, Yasushi Aoki,
Pieter Dorenbos, Setsuhisa Tanabe, The Journal of
Physical Chemistry C, 2023, 127, 31, 15611–15619.
This document is the Accepted Manuscript version
of a Published Work that appeared in final form in
The Journal of Physical Chemistry C, copyright (c)American Chemical Society after peer review andtechnical editing by the publisher. To access the
final edited and published work see
https://doi.org/10.1021/acs.jpcc.3c03251.

Description

Japan Advanced Institute of Science and Technology

Hole detrapping type persistent phosphors of
RE2O2S (RE= La, Gd, Y, Lu) doped with Eu3+-Pr3+
and Eu3+-Tb3+
Atsunori Hashimoto1, 2‡, Jumpei Ueda1,3 *‡, Yasushi Aoki2, Pieter Dorenbos4, Setsuhisa Tanabe3
1

Graduate School of Advanced Science and Technology, Japan Advanced Institute of Scienceand Technology, 1-1 Asahidai, Nomi, 923-1292 Ishikawa, Japan
2

NEMOTO & CO., LTD, 4-1 Shinmachi, Hiratsuka, 254-0076 Kanagawa, Japan

3

Graduate School of Human and Environmental Studies, Kyoto University, 606-8501 Kyoto,

Japan
4

Faculty of Applied Sciences, Delft University of Technology, Mekelweg 15, 2629 JB Delft,

The Netherlands
persistent phosphors, lanthanide ions, Hole trap, Eu3+

1

ABSTRACTRE2O2S:Eu3+-Ln3+ (RE=La, Gd, Y, Lu; Ln=Pr, Tb) samples were prepared by a solid-state reaction
method to develop new red persistent phosphors and to demonstrate the hole de-trapping
mechanism. All Eu3+-singly doped RE2O2S show very weak thermoluminescence (TL) glow peaks,
while by co-doping Pr3+ or Tb3+ ions additional strong TL peaks were observed. In the TL spectra
and persistent luminescence (PersL) spectra, only Eu3+ luminescence lines were observed, butthere is no Pr3+ and Tb3+ luminescence. From the PersL excitation spectra, it is found that the
PersL is caused after the excitation to the charge transfer state of Eu2+-S- in which the hole is in
the valence band. These results show that the Eu3+ acts as a recombination center and the Pr3+ andTb3+ ion act as a hole trap center. The deeper hole trap depth of Pr3+ compared with Tb3+ and the
RE dependence of hole trap depth are explained using a vacuum referred binding energy diagram
considering the nephelauxetic effect. The La2O2S:Eu3+-Pr3+ was the best composition among thesamples as a persistent phosphor at the ambient temperature, showing the strong red persistentluminescence in a short time range (>100 mcd/m2 for a few seconds).

2

1.

Introduction

Persistent phosphors, which show continuing luminescence even after the ceasing of excitation
light, have been widely used as luminous paint in many products such as an emergency sign, adial plate of a watch, etc1, 2. In 1993, Nemoto company developed the brightest and longest
green persistent phosphor, SrAl2O4:Eu2+-Dy3+, and published a paper in 1996 3. In thisphosphor, the persistent luminescence is caused by the 4f65d14f7 transition of Eu2+. At thattime, the persistent luminescence mechanism was not well understood and a hole transfer model(Eu2+-Dy3+⇔Eu+-Dy4+) was proposed. Nowadays, it is considered that the persistentluminescence in SrAl2O4:Eu2+-Dy3+ is caused by the electron transfer process (Eu2+-Dy3+⇔Eu3+Dy2+)4, 5 .
On the other hand, Nichia company successfully developed a bright red persistent phosphor,
Y2O2S:Eu3+-Ti4+-Mg2+ in 19986. The persistent luminescence center is Eu3+ which shows astrong red 4f-4f luminescence (5D07F2). Opposite to SrAl2O4:Eu2+-Dy3+, it is suggested that
the Y2O2S:Eu3+-Ti4+-Mg2+ persistent luminescence is caused by the hole transfer process (Eu3+⇔
Eu2+ + h+) although the hole trapping center is not identified7. To develop a new persistentphosphor based on the hole transfer model and to demonstrate its persistent luminescencemechanism in Eu3+-doped rare earth oxysulfide compounds, the hole traps and their trap depth
must be managed. The vacuum referred binding energy (VRBE) diagram of RE2O2S (RE=La,
Gd, Y, Lu), demonstrates a possibility that Pr3+ and Tb3+ can act as a hole trap7. If the Pr3+ andTb3+ ions act as traps in RE2O2S:Eu3+ red phosphor, the mechanism of persistent luminescence issupported to be based on the hole transfer model. In the hole transfer model, the hole trap depth
can be engineered by either selecting Pr3+ or Tb3+ or by altering the top of valence band energy

3

by changing the type of host compound rare earth ion. Also, recently, strong
mechanoluminescence was reported in RE2O2S:Ln3+ (RE = Y, Lu, La, or Gd; Ln = Eu, Pr, Nd,
Sm, Tb, Dy, Ho, Er, Tm, or Yb)8. Thus, it is important to investigate carrier trapping anddetrapping in RE2O2S:Ln3+.
In this study, Eu3+-Ln3+ (Ln = Pr and Tb) codoped RE2O2S (RE=La, Gd, Y, Lu) red persistentphosphors were prepared and its optical properties were investigated. From the results ofthermoluminescence and persistent luminescence measurements, it is successfully demonstrated
that the Pr3+ and Tb3+ ions act as hole traps. Also, variations in hole trap depths by Pr3+ and Tb3+
ions in RE2O2S host were explained using a newly constructed vacuum referred binding energy
diagram that takes the nephelauxetic effect into account.

2.

Experimental Procedure

RE1.99Eu0.01O2S and RE1.988Eu0.01Ln0.002O2S (RE=La, Gd, Y, Lu; Ln=Pr, Tb) were prepared by asolid state reaction method. La2O3, Gd2O3, Y2O3, Lu2O3, Eu2O3, Pr6O11, Tb4O7, S were used as
raw materials. In order to prevent oxidation, an excess amount of sulfur was added. Also,
Na2CO3 and K3PO4 were used to facilitate the solid-state reaction. These chemicals were mixed
by ball milling and sintered at 1200°C for 4 h in air. The obtained materials were washed bydeionized water several times to remove remnants. The crystal phases of samples were analyzed
by an XRD measurement system (XRD-6100, Shimadzu). Photoluminescence (PL) andphotoluminescence excitation (PLE) spectra were measured by a self-built spectrometer;monochromatic excitation light was generated by a 500 W Xe short arc lamp (OPM2-502XQ,
Ushio Inc.) and a double monochromator setup using two single monochromators (SpectraPro-

4

300i, Acton Research Corporation) and luminescence was detected by a photomultiplier tube
(R10699, Hamamatsu Photonics) equipped with a single monochromator (SP-2300i, Princeton
Instruments). The measured PLE spectra were calibrated by a photon flux spectrum of excitation
light source which is measured using a standard Si (S1337-1010BQ, Hamamatsu Photonics).
The PL spectra were calibrated by a standard halogen lamp (DH-2000CAL, Ocean Photonics) tothe photon flux spectrum. Thermoluminescence (TL) glow curves were measured using aphotomultiplier tube, PMT (R3896, Hamamatsu Photonics) with a bandpass filter of 640 nmwith 100 nm FWHM. The samples were fixed into cryostat (VPF-800, Janis) to control thetemperature. The samples were charged by UV excitation (250 ~ 400 nm) from a 300 W Xelamp (MAX-302, Asahi Spectra) at 80 K for 10 min and kept for 10 min after ceasing excitation
at the same temperature. The sample was then heated with the heating rate of 10 K/min up to400 K. The TL spectra were measured simultaneously by a Si CCD spectrometer (QE65-Pro,
Ocean Optics). Persistent luminescence (PersL) spectra were detected by a Si CCD spectrometer(PMA-12, Hamamatsu Photonics). PersL decay curves were measured by the luminance meter(LS-100, KONICA MINOLTA) after UV irradiation. For PersL excitation (PersLE) spectra, thesample was irradiated for 5 min by the monochromatic light and 1 min after stopping irradiation
PersL spectra were measured by the spectrophotometer (RF-5000, Shimadzu).
3.

Results

3.1. Crystal structure
Figure 1 shows the X-ray diffraction (XRD) patterns of Eu3+ doped RE2O2S (RE2O2S:Eu3+;
RE=La, Gd, Y, Lu). The observed XRD patterns of Eu3+-doped La2O2S, Gd2O2S, Y2O2S andLu2O2S correspond to the reference XRD patterns of RE2O2S (RE=La, Gd, Y, Lu) in the ICDD

5

(International Centre for Diffraction Data) database with the card number #01-071-2098, #01079-5662, #00-024-1424 and #00-026-1445, respectively. Thus, all samples are identified as asingle crystalline phase of RE2O2S (RE=La, Gd, Y, Lu) with the space group of 𝑃𝑃3� 𝑚𝑚1. The

observed XRD peaks are shifted to the higher diffraction angle in the order of La2O2S, Gd2O2S,
Y2O2S and Lu2O2S because the cation size of RE ion becomes smaller in the same order.
Consequently, the lattice constant decreases in the order of La2O2S, Gd2O2S, Y2O2S and Lu2O2S.

Figure 1. X-ray diffraction (XRD) patterns of the RE2O2S:Eu3+ (RE = La, Gd, Y, Lu).

3.2. PL and PLE properties
Figure 2 shows the PL spectra of RE2O2S:Eu3+ (RE=La, Gd, Y, Lu). All samples show strong red
luminescence mainly due to the typical Eu3+ 4f-4f luminescence (5D07F2) at around 625 nm.
The peak wavelength of the 5D07F2 in La2O2S, Gd2O2S, Y2O2S and Lu2O2S were 624.6nm,
626.4nm, 627.0nm, 627.6nm, respectively, at ambient temperature. In addition to the 5D0-7FJ

6luminescence lines from 590 nm to 720 nm, several PL peaks were observed in the range from
450 nm to 575 nm in the Gd2O2S, Y2O2S and Lu2O2S samples. On the other hand, for
La2O2S:Eu3+, the luminescence peaks in the range from 450 nm to 520 nm are almost quenchedat ambient temperature. These PL peaks can be attributed to the 5D1 –7FJ and 5D2 -7FJ
luminescence as shown in Figure 2.

Figure 2. (a) PLE and (b) PL spectra at room temperature of the RE2O2S: Eu3+ (RE = La, Gd, Y,
Lu).
In the PLE spectra, sharp lines at around 395 nm, 470 nm, 530 nm and 580 nm and two broad
bands in the range between 250 nm and 290 nm and between 290 nm and 400 nm were observed.
The sharp lines are attributed to 4f-4f transitions of Eu3+ from 7F0 to 5L6, 5D2, 5D1 and 5D0. The
broad band in the longer wavelength range originates from electron transfer from the valenceband to Eu3+, which is called the charge transfer (CT) band. The broad band in the shorter
wavelength at around 270 nm originates from the host exciton creation bands. The energy of theCT band and the host exciton band varied by depending rare earth ion of the host compound. To
clarify the host exciton energy and the CT energy in each host compound, the PLE spectra from7the VUV to UV were also measured at 15 K (see Figure S1 in the supporting information). The
host exciton energy is shifted to higher energy in the order of La2O2S (4.62 eV), Gd2O2S (4.66
eV), Y2O2S (4.81 eV) and Lu2O2S (5.19 eV). The exciton creation energy seems to be very highonly for the Lu2O2S:Eu3+ although the exciton creation energy of Lu2O2S:Eu3+ at room
temperature is similar to other RE2O2S compounds (Figure 2). Compared with the PLE spectraof Lu2O2S:Eu3+ in VUV region at 300 K and 15 K (Figure S2), the spectral shape is dramaticallychanged. Because the Lu2O2S is a mixed anion compound, the host exciton absorption may
occur from oxide 2p and sulfide 3p orbitals. At 15K the energy transfer from the exciton
absorption due to the S(3p)-Lu(5d) to Eu3+ may not take place efficiently in Lu2O2S:Eu3+(FigureS2). Consequently, the host excitation energy at 15K was overestimated in Lu2O2S. Thus, thehost exciton energy of 4.82 eV, which was estimated from the exciton peak at room temperature
with a correction of 0.15 eV by temperature effect, was used only for Lu2O2S.
3.3. TL and PersL properties
Figure 3 shows TL glow curves of La2O2S:Eu3+ and La2O2S:Eu3+-Ln3+ (Ln=Pr, Tb). By codoping with Pr3+ and with Tb3+, additional strong TL peaks were observed. The TL peaktemperature of La2O2S:Eu3+-Pr3+ and La2O2S:Eu3+-Tb3+ are 242 K and 233 K, respectively. The
additional TL peaks by co-doping with Pr3+ and with Tb3+ were also observed in other RE2O2S
hosts (See Figure S3, S4 and S5 in the supporting information). The TL glow curves in
RE2O2S:Eu3+-Ln3+ (RE=La, Gd, Y, Lu; Ln=Pr, Tb) are summarized in the series of Pr3+ codopant
and Tb3+ codopant as shown in Figure 4a and 4b, respectively. The TL glow peak is shifted byvarying the rare earth ion of host compounds. The main TL glow peak temperature in La2O2S isthe highest, followed by Y2O2S, Gd2O2S, Lu2O2S in both series of RE2O2S:Eu3+-Pr3+ andRE2O2S:Eu3+-Tb3+. The TL glow peak temperatures for RE2O2S:Eu3+-Ln3+ (RE=La, Gd, Y, Lu;8

Ln=Pr, Tb) are listed in Table 1. When we compare the original TL glow peak intensity, theLa2O2S and Y2O2S hosts show better performance than Gd2O2S and Lu2O2S both in Pr3+codoped ones and Tb3+-codoped ones (See Figure3 and Figure S3, S4 and S5 in the supporting
information) .

Figure 3. TL glow curves of the La2O2S:Eu3+, La2O2S:Eu3+-Ln3+ (Ln = Pr, Tb) at heating rate of
10 K/min after UV (250~400 nm) irradiation for 10 min.

9

Figure 4. TL glow curves of the RE2O2S:Eu3+ (RE=La, Gd, Y, Lu) (a) with Pr3+ codopant and (b)with Tb3+ codopant at heating rate of 10 K/min after UV (250~400 nm) irradiation for 10 min.
Table 1. Frequency factor (s)7, temperature maximum (Tm) of TL glow curves at heating rate of
10 K/min and the derived trap depth (Etrap) of RE2O2S:Eu3+-Ln3+ (RE=La, Gd, Y, Lu, Ln=Pr, Tb).
Etrap (eV)

Tm (K)
FrequencyPrfactor (s-1) [7]

Tb

Pr

Tb

RE = La

1.1×1013

242

233

0.70

0.68

RE = Gd

1.3×1013

212

192

0.62

0.56

RE = Y

1.3×1013

228

206

0.66

0.60

RE = Lu

1.4×1013

189

158

0.55

0.46

10

From the well-known equation for the TL glow curve caused by first-order kinetics carriertransportation as presented by Randall and Wilkins,9, 10 one can derive11
𝛽𝛽𝐸𝐸 𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡
𝑘𝑘𝑇𝑇𝑚𝑚2= 𝑠𝑠 ∙ 𝑒𝑒𝑒𝑒𝑒𝑒 �−

𝐸𝐸 𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡

𝑘𝑘𝑇𝑇𝑚𝑚�(1).

Here, β is the heating rate (K/s), Etrap is trap depth, k is the Boltzmann constant, Tm is the

maximum temperature of TL glow peak, s is the frequency factor. Thus, from the obtained TL
glow peak temperature, the trap depths can be estimated by assuming the frequency factor. The
frequency factors were reported in the reference7 as shown in column 2 of Table 1. The
calculated trap depths in the RE2O2S:Eu3+-Ln3+ (RE=La, Gd, Y, Lu; Ln=Pr, Tb) are shown in
Table 1. The trap depth of main peak TL glow peak in RE = La is the deepest, followed by RE=
Y, Gd and Lu both in the series of RE2O2S:Eu3+-Pr3+ and RE2O2S:Eu3+-Tb3+.
Figure 5 shows the luminescence wavelength-temperature (λem-T) contour plots of TL
intensity of the La2O2S:Eu3+-Ln3+ (Ln=Pr, Tb). The observed TL peaks originate only from Eu3+
4f-4f transition. No TL emission from Pr3+ and Tb3+ were observed in the temperature range
from 100 K to 400 K.

11

Figure 5. Emission wavelength-temperature (λem-T) contour plots of TL intensity for (a)La2O2S:Eu3+-Pr3+ and (b) La2O2S:Eu3+-Tb3+
Figure 6a shows PersL spectra of La2O2S:Eu3+and La2O2S:Eu3+-Ln3+ (Ln = Pr, Tb). Thesesamples showed red persistent luminescence. By co-doping the lanthanide ions, the intensities of
persistent luminescence become stronger compared with Eu3+ singly-doped La2O2S. In thesesamples, no persistent luminescence from Pr3+ and Tb3+ was observed similar to TL spectra in
Figure 5 which indicates that only the Eu3+ ions act as a recombination center. In the samples of
RE = Gd, Y, and Lu, the PersL cannot be detected by the Si CCD at ambient temperature
because of the too shallow trap depths. Figure 6b shows the PersL decay curves of the
La2O2S:Eu3+and La2O2S:Eu3+-Ln3+ (Ln = Pr, Tb) samples. By the addition of the co-dopant, theluminance is enhanced compared with Eu-singly doped La2O2S. Persistent luminescenceduration of La2O2S:Eu3+-Pr3+ is the longest in these La2O2S-based samples and the duration timeis 60s until the luminance drops 2 mcd/m2.

Figure 6. (a) PersL spectra and (b) PersL decay curves of La2O2S:Eu3+and La2O2S:Eu3+-Ln3+ (Ln= Pr, Tb).

12

Figure 7 shows the PersLE spectra of La2O2S:Eu3+-Pr3+. Two excitation bands were observed;
The PersLE band at 230 nm is attributed to the host exciton creation band and another band at
380 nm is the CT band of Eu3+.

Figure 7. PersL excitation spectra of La2O2S:Eu3+-Pr3+

4.

Discussion

4.1. Quenching process and Nephelauxetic effect of Eu3+ luminescence
As shown in Figure 2b, only La2O2S does not show the luminescence from 5D2 excitedstate. The possible quenching processes are multiphonon relaxation, cross-relaxation andthe thermally activated crossover to the CT state. The relatively low phonon energies(~500 cm-1)12 of the RE2O2S compounds imply no multiphonon relaxation quenching.
Also, the low Eu3+ concentration (0.5 %) means no cross-relation quenching. However, theCT absorption edge of La2O2S:Eu3+ in Fig. 2a shows the longest wavelength among the
RE2O2S:Eu3+ (RE=La, Gd, Y and Lu), indicating that the activation energy to the CT state13

is small. Thus, the 5D2 luminescence quenching in La2O2S:Eu3+ can be caused through the
CT state.
4.2. Hole trap by Pr3+ and Tb3+
As expected, the Pr3+ and Tb3+ co-doping generated additional TL glow peaks (Figure 3, 4).
The persistent luminescence (thermoluminescence) center was identified to be only Eu3+ basedon the PersL and TL spectra (Figure 5, 6). The Eu3+ persistent luminescence was caused after
excitation upon the charge transfer band of Eu3+ and band-to-band absorption (Figure 7). These
results strongly show that the persistent luminescence is caused by the hole transfer process.
When the CT band of Eu3+ is excited, the Eu3+-S2- state is changed into Eu2+-S- because theelectron of 3p orbital of S2- is transferred to Eu3+. One may equally well state that a hole istransferred from Eu3+ to S2- which forms the top of VB. In the hole picture model 13, the CT
excitation is depicted by arrow 1 in Figure 8. This free hole in the valence band can be captured
by Ln3+ (Pr3+, Tb3+), which then changes into a tetravalent state. The tetravalent state is known
as a stable valence state for Pr and Tb in many compounds. This hole trapping process is
illustrated by arrow 2 in Figure 8. On the other hand, the Eu3+ ion keeps the excited electronfrom S2- through the CT absorption, which means that the Eu3+ ion acts as an electron trap.
Thus, the valence state changing of Eu3+-Ln3+⇔Eu2+- Ln4+ (Ln= Pr, Tb) is our proposed carrier
trapping mechanism. So far, these kinds of valence state changing have been demonstrated using
UV-Vis absorption spectroscopy and X-ray absorption spectroscopy5, 14-17. The trapped hole by
Pr3+ or Tb3+ can be released at a lower temperature than the electron from the electron-trappedEu3+ (Eu2+). If the electron of Eu2+ is de-trapped at a lower temperature, the Pr3+ and Tb3+ ionsshould be recombination centers, but the PersL of Pr3+ and Tb3+ was not observed. Thus, the14hole transfer and the hole de-trapping process support the fact that only Eu3+ shows persistentluminescence.

Figure 8. Persistent luminescence mechanism based on the hole picture model. 1. Excitation to
charge transfer state, 2. hole trapping, 3. hole detrapping, 4. 5D0-7F2 luminescence.

4.3. Hole trap depth in VRBE diagram
To understand the hole trap depth by Pr3+ and Tb3+ in RE2O2S, the VRBE (vacuum referredbinding energy diagram) can be useful. The VRBE diagrams of RE2O2S were reported by
Hongde et al. in 2017. Based on the VRBE in 2017, it is considered that the Tb3+ forms deeper

15hole traps compared with Pr3+ in all the RE2O2S hosts and the hole trap depth of both Pr3+ andTb3+ becomes deeper in the order of La2O2S, Gd2O2S, Y2O2S and Lu2O2S with decreasing RE
ionic radius as shown in Table 2. However, the obtained trap depth in the RE2O2S:Eu3+-Ln3+ asshown in Table 1 does not follow this trend. The observed hole trap depth of Tb3+ is always
shallower than that of Pr3+ in all the RE2O2S hosts. In addition, the hole trap depth by Pr3+ andTb3+ in La2O2S is the deepest and that in Lu2O2S is the shallowest among the RE2O2S hosts.
Table 2. Estimated hole trap depth of Pr3+ and Tb3+ in RE2O2S hosts based on the VRBE diagram
reported in reference7 in eV.
Pr3+

Tb3+

La2O2S

0.63

0.81

Gd2O2S

0.74

0.92

Y2O2S

0.77

0.95

Lu2O2S

0.79

0.97

Although the typical errors in VRBE construction are several 0.1 eV, there seems to be a clear
inconsistency in the trends with changing RE and between Pr3+ and Tb3+. In 2019, Dorenbos
improved the model of VRBE diagram construction by considering the Nephelauxetic effect on
VRBE energies18. It was found that the nephelauxetic effect may lower the right hand branch (n=8
to 14) of the zig-zag curves of Ln2+ and Ln3+ by several 0.1 eV with respect to the left hand branch(n=1 to 7). Here, n is the number of 4f electrons of Ln3+. In the RE2O2S hosts, an enhanced
nephelauxetic effect is to be expected due to the smaller electronegativity of sulfide (χs=2.58)
compared with pure oxide materials (χO=3.44). The branch lowering in the zig-zag curve of Ln3+
is crucial for the hole trap depth difference between Pr3+ and Tb3+19. For instance, in the REPO4

16

compounds which show a low nephelauxetic effect by oxygen ligand because of the strong
bonding between P5+ and O2-, the Tb3+ hole trap depth is deeper than the Pr3+ hole trap. On theother hand, in the rare earth aluminates such as Y3Al5O12 and GdAlO3 which show a slightly larger
nephelauxetic effect due to the weaker bonding of Al3+-O2- than P5+-O2-, the Tb3+ hole trap depth
is shallower than the Pr3+ hole trap19. The same appears now for the oxysulfides in this work.
Dorenbos introduced the nephelauxetic ratio β as a parameter to quantify the branch lowering 18,19. β is defined by the ratio of Slater-Condon parameter F2 for the lanthanides in a compound A

with respect to that for lanthanides in vacuum. The β parameter is directly linked to the energy
difference ∆E(8,2,A) between the VRBE of Tb3+ and the VRBE of Pr3+ 19. From the hole trap
depths (Table 1), the ∆E(8,2,A) of La2O2S, Gd2O2S, Y2O2S and Lu2O2S amount -0.02, -0.06, -0.06
and -0.09 eV, respectively, which leads into the decreasing β parameter as like 0.930, 0.927, 0.927
and 0.924. The decreasing tendency of β in the order of La2O2S, Gd2O2S, Y2O2S and Lu2O2S
suggests that the environment at the RE-site becomes more covalent. This tendency cannot be
explained by the electronegativity of RE-cation which increases from La to Lu. In this series,
smaller RE-site size can increase the nephelauxetic effect due to the shorter distance with anion.
Also, the same nephelauxetic effect is apparent in the Eu3+ luminescence peak position. Figure
9 shows host rare earth ion dependence of the Eu3+:5D0-7F0 energy in RE2O2S obtained from PL
spectra in Figure 2 together with data on RE2O3:Eu3+ as a reference. For the RE2O3:Eu3+ series, theEu3+:5D0-7F0 energies at the C2 site of C-type RE2O3 for RE=Gd, Y, Lu20 and at the C3v site of Atype La2O321 are shown. The Eu3+:5D0-7F0 energy shifts to lower energy with decreasing RE ionicradius in the RE2O2S hosts, whereas for RE2O3:Eu3+ the energy remains fairly constant. Generally,
the 4f-4f transition energy can be affected by both the crystal field 22, 23 and the nephelauxetic
effect 24-26. Because the Eu3+:5D0-7F0 does not show any luminescence lines related to the Stark

17

splitting, the Eu3+:5D0-7F0 reflects only nepheraluxetic effect. Also, in the series of RE2O2S withthe same crystal structure, the Eu3+ luminescence energy can be shifted to lower energy mainly byenhancing the nephelauxetic effect. This is probably because of the unique crystal structure of
RE2O2S and the small electronegativity of S2-. The RE2O2S crystal forms a layered structure by
S2-, O2- and RE3+ sheets and the RE site is asymmetrically coordinated by four O2- and three S2- asshown in Figure 10. For this asymmetric coordination, the 4f electron cloud can be expanded to
the S2- ions because of the smaller electronegativity than oxide (χS=2.58 < χO=3.44). These unique
crystal structure and smaller RE site may lead into the increase of nephelauxetic effect. This strong
nephelauxetic effect for Eu3+ applies equally well to all other Ln3+ ions. From the host compound
dependence of the hole trap difference between Pr3+ and Tb3+ and the Eu3+ luminescence redshifting, the decreasing β parameter in the order of La2O2S, Gd2O2S, Y2O2S and Lu2O2S appears
consistent and reasonable.

Figure 9. Host rare earth ion dependence of Eu3+:5D0-7F0 energy in RE2O2S and RE2O3 20, 21.

18

Figure 10. Crystal structure of RE2O2S. (The crystal structure is drawn by VESTA software.27)

4.4. Construction of VRBE diagram considering the nephelauxetic effectBased on the observed trap depth variation of Pr3+ and Tb3+ in RE2O2S, new VRBE diagrams
that take the nephelauxetic effect into account need to be constructed. To make a new VRBE
diagram, the required energetic parameters were also updated based on the newly obtained
spectroscopic data. They are compiled in Table 3. The exciton creation energy was determinedfrom the PLE from the VUV to UV range at 15 K (Figure S1 in the supporting information) as
treated in section 3.2. To obtain the CT energies, the PLE spectra in the energy scale from
Figure 2 were deconvoluted by two Gaussian peaks because we assumed CT absorption bothfrom O2- and from S2-. For example, two clear PLE bands can be seen in Fig. 2a for
Lu2O2S:Eu3+. The electronegativity of S is smaller than that of O, forming the top of valence
state by 3p orbital of S. Thus, the lower CT energies were used for the VRBE diagram. Redshift parameter of Ce3+ was calculated using the 4f-5d1 absorption peak reported in the reference28. The U(6, A) value of RE2O2S is reported to be around 6.37 eV 7. U(6,A) is the energy

19difference between the VRBE of Eu2+ and Eu3+ and it depends on the screening distance in thechemical shift model29. In this study, we also adopted a U(6,A) value of 6.37 eV.
Table 3. Parameters and spectroscopic data utilized to construct the VRBE diagram of Ln in the
RE2O2S host and the estimated VRBE values in eV unit (except for Nephelauxetic ratio β). The
method of the reference12,13 was followed.
U(6,A) Eex

ECTβD(1,3+,A) EV

EC

EEu2+

EPr3+

ETb3+

6.37

4.62

3.58

0.930

3.39

-7.34

-2.72

-3.77

-6.64

-6.67

Gd2O2S 6.37

4.66

3.60

0.927

3.45

-7.43

-2.77

-3.75

-6.57

-6.63

Y2O2S

6.37

4.81

3.58

0.927

3.44

-7.37

-2.56

-3.77

-6.63

-6.68

Lu2O2S

6.37

4.82

3.56

0.924

3.45

-7.34

-2.52

-3.78

-6.67

-6.75

La2O2S

U(6,A): energy difference between the VRBE of Eu2+ and Eu3+ in host A, Eex: host exciton
creatin energy, ECT: charge transfer energy from anion to Eu3+, β: Nephelauxetic ratio, D(1,3+,A):energy difference between the quasi free ion 4f–5d energy and the observed 4f–5d energy of Ce3+
in host A, EV,C, Eu2+, Pr3+ and Tb3+: VRBE of valence band top, conduction band bottom, Eu2+, Pr3+
and Tb3+, respectively.

Figure 11 shows the new VRBE diagram for La2O2S using the updated parameters. Different
from the previous VRBE diagram, the VRBE of Tb3+ is located slightly below that of Pr3+. The
electron trap depth by Eu3+ is 1.21 eV and the hole trap depth by Pr3+ and Tb3+ are 0.78 and 0.75
eV, respectively. Because the hole trap depth by Pr3+ and Tb3+ is smaller than the electron trap
depth by Eu3+, the hole trap is released at lower temperature than the electron trap. Thus, theVRBE diagram also supports the hole-detrapping mechanisms for the persistent luminescence.

20

Figure 11. VRBE diagram of La2O2S. VB: valence band, CB: conduction band, GS: ground state,
ES: Excited state, Ln: Lanthanide ionsFigure 12 shows a stacked VRBE diagram of RE2O2S. In the new VRBE diagram, theobserved unusual trend of hole trap depth (the deepest in La2O2S and the shallowest in Lu2O2S)
can be explained partly. The energy difference between the top of VB and Ln3+ (Pr3+, Tb3+) in
La2O2S is much larger than that in Lu2O2S. Figure 13 summarizes the hole trap depth by Pr3+
and Tb3+ estimated from the experimental results and the VRBE diagram. The VRBE diagram
cannot explain the hole trap depth in Gd2O2S while it can explain the decreasing trend of hole
trap depth from La to Lu. For this trend, the La2O2S:Eu3+-Ln3+ show the deep trap depth among
the RE2O2S hosts and it is closest to ambient temperature. For the application demanding the21

longer persistent duration, the La2O2S:Eu3+-Ln3+ is not suitable. However, the strong red
persistent luminance in a short time range (>100mcd/m2 for a few seconds) for the La2O2S:Eu3+Pr3+ can be used as security ink and so on.

Figure 12. Stacked VRBE diagram of RE2O2S with ground state of Eu2+, Pr3+ and Tb3+.

22

Figure13. Hole trap depths of Pr3+ and Tb3+ estimated from the TL glow peak and from thereported VRBE diagram5 and the newly constructed VRBE diagram.

5.

Conclusion
Red RE2O2S:Eu3+-Ln3+ (RE=La, Gd, Y, Lu; Ln=Pr, Tb) thermoluminescence phosphors weredeveloped, where Pr3+ and Tb3+ co-doping generated additional TL glow peaks. The persistentluminescence and thermoluminescence have been identified to arise from only Eu3+. Eu3+
persistent luminescence appears after excitation in the charge transfer band of Eu3+ and by meansof band-to-band excitation. Based on the obtained results, it was concluded that the persistentluminescence of Eu3+ is caused by the hole trapping at Pr3+ and Tb3+ followed by a thermally detrapping process. The vacuum referred binding energy diagrams of RE2O2S taking into accountthe nephelauxetic effect was constructed. Using a new VRBE diagram, the deeper hole trap of23

Pr3+ than that of Tb3+ was explained by a right hand branch lowering of the zig-zag curve of Ln3+
due to an enhanced nephelauxetic effect caused by the sulfide ions. This is consistent with anobserved redshifting of the Eu3+ emission lines. The hole trap depth tendency of Pr3+ and Tb3+
with respect to different host of RE2O2S except Gd2O2S was understood by a new VRBE
diagram. The La2O2S:Eu3+-Pr3+ is among the samples the best composition for a persistentphosphor at ambient temperature, showing a strong red persistent luminescence in a short time
range (>100 mcd/m2 for a few seconds).

ASSOCIATED CONTENT
Supporting Information. The following files are available free of charge.
PLE in vacuum UV region, TL glow curves. (PDF)

AUTHOR INFORMATION
Corresponding Author*E-mail: ueda-j@jaist.ac.jp

Author Contributions
JU conceived the idea of the study. AH and YA prepared the materials. JU and AH investigated
the structure properties and optical properties. AH , JU, and PD constructed the energy
diagrams. AH and JU drafted the original manuscript. JU and ST supervised the conduct of this24

study. All authors reviewed the manuscript draft and revised it critically on intellectual content.
All authors approved the final version of the manuscript to be published. ‡These authors
contributed equally.

Funding Sources
JSPS KAKENHI (18KK0405, 20H02438)

ACKNOWLEDGMENT
This research was financially supported by JSPS KAKENHI (grant number is 18KK0405,
20H02438).

25

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