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MYU Tokyo
S & M 3537

Photoluminescence and Radioluminescence Propertiesof Y3Al5O12 Doped with 3d-transition Metal Ions
Noriaki Kawaguchi,1* Naoki Hayashi,2 Toshiaki Kunikata,1 Kensei Ichiba,1
Takumi Kato,1 Daisuke Nakauchi,1 and Takayuki Yanagida1
1Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara 630-0192, Japan
2Yamato University, 2-5-1 Katayama-cho, Suita-shi, Osaka 564-0082, Japan(Received November 5, 2023; accepted January 18, 2024)

Keywords: scintillator, 3d-transition metal ions, single crystal

The photoluminescence and radioluminescence properties of Y3(Al0.99TM0.01)5O12 (TM = Ti,
V, Mn, and Cu) single crystals were investigated. Single crystalline rods of approximately 5 mmdiameter were grown by the optical floating zone method. The photoluminescence quantum
yields of the obtained single crystals were 1.3–3.8%. Both the intrinsic luminescence ofY3Al5O12 (YAG) and the extrinsic luminescence due to each dopant were observed in theradioluminescence spectra. The radioluminescence decay times of YAG doped with either Ti, V,
Mn, or Cu were in the range from 6.9 μs to 10.2 ms.

1.

Introduction

Sensing techniques, such as radiation detection,(1) bio-imaging,(2) and thermometry,(3) aresome of the applications of luminescent materials. In particular, luminescent materials forradiation detection, which emit light under radiation excitation and are called scintillators, have
many applications including medical, industrial, and scientific ones. Although many scintillators
have been studied in the last few decades,(4–7) fundamental studies on novel scintillators still
continue to achieve better performance than ever before. There have been reports on various
types of scintillator, such as single crystals,(8–21) nanocrystals,(22) crystalline films,(23)ceramics,(24–27) glasses,(28–37) plastics,(38) and organic–inorganic hybrid materials,(39–42) evenin the last few years. In terms of the luminescent center, specific dopants such as Ce, Eu, and Tl
have been mainly used for commercial scintillators; however, other dopants are also of ourinterest.
In this study, we investigated the photoluminescence (PL) and radioluminescence (RL)
properties of Y3Al5O12 (YAG) doped with 3d-transition metal ions. We chose Ti, V, Mn, andCu as 3d-transiton metals because they have been known or studied as dopants ofluminescent materials for laser lighting and displays, such as Ti-doped Al 2O3,(43) V-doped
YAG,(44) Mn-doped ZnS,(45) and Cu-doped glasses.(46,47) Since garnet-type single crystals*Corresponding author: e-mail: n-kawaguchi@ms.naist.jp

https://doi.org/10.18494/SAM4768

ISSN 0914-4935 © MYU K.K.
https://myukk.org/

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have been one of the most well-known host crystals for scintillators, we chose YAG as thehost crystal.

2.

Materials and Methods

Y3(Al0.99TM0.01)5O12 (TM = Ti, V, Mn, and Cu) single crystals were grown by the opticalfloating zone (FZ) method. Because the ionic radii of the dopants were closer to that of Al than
to that of Y, we expected that dopants would replace Al. Raw powders of Y2O3 (4N; Furuuchi
Chemical), Al 2O3 (4N; Kojundo Chemical Laboratory), TiO2 (4N; Kojundo Chemical
Laboratory), V2O5 (4N; Kojundo Chemical Laboratory), MnO (3N; Furuuchi Chemical), andCu2O (2N; Kojundo Chemical Laboratory) were mixed and ground using an agate mortar andpestle. Each mixed powder was shaped into a rod by hydrostatic pressure and sintered at 1400 °C
for 8 h in air. The sintered rods were crystallized using an optical FZ furnace (FZD0192; Canon
Machinery). Each rod was rotated at 7 rpm and partially melted using two halogen lamps facing
each other. The melted zone moved gradually at 3 mm/h. All the obtained single crystals wereconfirmed to be single phases of YAG by X-ray diffraction analysis.
PL quantum yields (QYs) and excitation-emission maps were measured using a PL
spectrometer (Quantaurus-QY; Hamamatsu Photonics). PL decay curves were recorded using
Quantaurus-τ C11367 (Hamamatsu Photonics). The obtained PL decay curves were fitted withexponential functions to estimate decay times. RL spectra were measured using our customized
measurement system.(48) RL decay curves were measured using our other customized
measurement system(49) and analyzed in the same manner as PL.

3.

Results and Discussion

Figure 1 shows the PL excitation–emission maps, PL decay curves, and appearances of YAG
single crystals doped with either Ti, V, Mn, or Cu. The obtained YAG samples were transparent,
and no inclusions were found in them by visual observation. In addition, the Ti-, V-, Mn-, andCu-doped YAG samples looked colorless, green, red, and light brown, respectively. Differencesin the colors of the samples might be caused by the absorption of each dopant. The maximum PL
QYs of Ti-, V-, Mn-, and Cu-doped YAG samples were 1.3, 3.6, 3.8, and 3.7%, respectively. The
PL QYs of the samples were low; however, they might increase if the dopant concentration is
optimized.
Two emission peaks at around 470 and ~790 nm were observed in the PL excitation–emission
map of the Ti-doped YAG sample upon 280 nm excitation. The PL decay times of the Ti-doped
YAG sample at around 470 and ~790 nm were estimated to be approximately 31.5 µs and 8.0 ms,
respectively. The emission peak at around 470 nm would be attributed to the charge transfer(CT) transition of Ti4+ ions.(50,51) Various decay times of this emission have been reported, forexample, that of Ti, Si-doped Al2O3 has been reported to be 42.9 µs,(52) which was roughly
consistent with that of the Ti-doped YAG sample. Ti-doped materials can show luminescence inthe red wavelength region owing to the d–d transitions of Ti3+ ions.(53,54) This is not an origin of
the emission of our Ti-doped YAG sample at ~790 nm since the decay time is significantly

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501

(a)

(b)

(c)

(d)
Fig. 1. (Color online) From top to bottom, PL excitation–emission maps, PL decay curves, and appearances of
YAG single crystals doped with either (a) Ti, (b) V, (c) Mn, or (d) Cu.

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different. For example, the decay time of the d-d luminescence of Ti-doped Al2O3 is 3.2 µs,(43)whereas that of our sample is approximately 8.0 ms. The origin of the emission at ~790 nm could
be explained by the 4T1– 6A1 transition of Fe3+ ions,(55–58) which was an unexpectedcontaminant.
In the case of V-doped YAG, one emission peak at 630 nm was observed and its excitation
peak wavelength was ~ 340 nm. The decay curve of the V-doped YAG sample was approximatedby a single exponential function and its decay time was ~6.9 µs. A possible origin of thisemission is the CT transition between V5+ and O2−, which can show decay times of a few µs.
This CT emission has various excitation and emission wavelengths, which change depending on
the V–O distance.(59) For example, the excitation and emission wavelengths of VAlO4 are around
330–360 and 620 nm, respectively,(60) which are consistent with those of the V-doped YAG
sample. In a previous study, the excitation and emission wavelengths of the CT emission of
V-doped YAG were reported to be 266 and 570 nm, respectively.(44) The reason for the slightly
different wavelengths may be due to the difference in the sensitivities of the detectors.
Three emission peaks at around 600, 640, and 660 nm were observed in the Mn-doped YAG,and excitation peak wavelengths were at around 470, 340, and 340 nm, respectively. The decay
curves of emissions at 600 and 640–660 nm were successfully fitted by a single exponentialfunction, and their decay times were estimated to be approximately 1.1 and 1.0 ms, respectively.
Results were consistent with the previous report on Mn-doped YAG.(61) The emission at 600 nm
was ascribed to the 4T1– 6A1 transitions of Mn2+ ions, and the emissions at 640 and 660 nm wereascribed to the 4T2– 4A2 transitions of Mn4+ ions.
The Cu-doped YAG sample showed an emission peak at ~800 nm and weak emissions at
640–660 nm. The decay time at 800 nm was ~8.6 ms. The decay curve of the weak emissions at
640–660 nm was not clearly observed because of the low emission intensity. These emissions
were not consistent with those originating from the Cu dopant. Possible origins would be theunexpected impurities containing Fe3+ and Mn2+ ions. The emission at ~800 nm was consistent
with that due to the 4T1– 6A1 transition of Fe3+ ions.(55,56) The emission due to Fe3+ contamination
was observed also in the Ti-doped YAG sample. Unfortunately, our single crystals seemed to beeasily contaminated by Fe3+ ions because we often used iron oxide powders, which were put on
raw material rods as a light absorber in heating during the crystal growth using the optical FZ
furnace. The weak emission at 640–660 nm was consistent with those due to the 4T2– 4A2
transitions of Mn4+ ions.(61) The Mn4+ ions seem to be present in the Cu-doped YAG single
crystal as contamination. Because the Mn-doped YAG single crystal was also prepared, the Cudoped YAG sample could be contaminated by Mn4+ ions from experimental apparatus forsample preparation.
Figure 2 shows the X-ray-excited RL spectra and RL decay curves of the Ti-, V-, Mn-, andCu-doped YAG samples. In the RL spectra of all the samples, emission in a broad wavelength
range from 300 to 400 nm was observed. This is the significant difference between PL and RL.
These broad emissions can be ascribed to the intrinsic luminescence of YAG owing to defectrelated transitions(62) or self-trapped excitons.(63) Most of the emission peaks in the PL spectra
were observed also in the RL; however, the emission peaks of the Ti- and Cu-doped YAG
samples at ~800 nm were not detected even when using a red-sensitive CCD. The emission peakSensors and Materials, Vol. 36, No. 2 (2024)

503

(b)

(a)

(c)

(d)

(e)

(f)

(g)

(h)

(i)

Fig. 2. (Color online) RL spectra of YAG single crystals doped with (a) Ti, (d) V, (f) Mn, or (h) Cu. Their RL decay
curves are also shown in (b), (c), (e), (g), and (i).

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Sensors and Materials, Vol. 36, No. 2 (2024)of the Cu-doped YAG at around 640–660 nm was also not detected owing to the low emission
intensity of impurity Mn4+ ions.
The RL decay curves of the Ti-doped YAG were measured with two time ranges, and eachdecay curve was approximated with a double exponential function. The RL decay curves of theV-, Mn-, and Cu-doped YAG were approximated by single exponential functions. The RL decay
times of the Ti-doped YAG sample were estimated to be 8.5 and 32.8 µs for the 400 µs range
measurement and 0.5 and 4.1 ms for the 50 ms range measurement. The RL decay times of theV-, Mn-, and Cu-doped YAG were estimated to be 6.9 µs, 1.3 ms, and 10.2 ms, respectively. The
estimated values are roughly consistent with those of the PL decay times. Since the wavelength
was not resolved in the RL decay measurements, multiple emission components were included,
and some of the RL decay curves were not approximated by a single exponential function but adouble exponential one. However, no intrinsic luminescence of YAG was detected in the RL
decay curve measurements because we used a photomultiplier tube (PMT) that is sensitive in thevisible to red region. The obtained RL decay times were in the range from 6.9 μs to 10.2 ms.

4.

Conclusions

The PL and RL properties of V-, Mn-, and Cu-doped YAG single crystals were studied.
Emission peaks were ascribed to the intrinsic luminescence of YAG or the extrinsic
luminescence due to dopants or unexpected impurity ions. The RL decay times of the samples
were in the range from 6.9 μs to 10.2 ms. Since the RL decay time of the V-doped YAG sample
was relatively short, it might be used in photon-counting radiation detectors. Other samples
might be used for radiation detectors that are operated in the current mode.

Acknowledgments
This work was supported by Grants-in-Aid for Scientific Research A (22H00309), Scientific
Research B (22H03872, 22H02939, 21H03733, and 21H03736), and Challenging Exploratory
Research (22K18997) from the Japan Society for the Promotion of Science. The Cooperative
Research Project of the Research Center for Biomedical Engineering, A-STEP (JPMJTM22DN)
from JST, Konica Minolta Science and Technology Foundation, Nakatani Foundation, andKazuchika Okura Memorial Foundation are also acknowledged.

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