Optical Materials 58 (2016) 14e17

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Optical Materials
journal homepage: www.elsevier.com/locate/optmat

Crystal growth and luminescence properties of Yb2Si2O7 infra-redemission scintillator
Takahiko Horiai a, *, Shunsuke Kurosawa b, **, Rikito Murakami a, Jan Pejchal c,Akihiro Yamaji a, Yasuhiro Shoji a, d, Valery I. Chani a, Yuji Ohashi a, Kei Kamada b, d,Yuui Yokota b, Akira Yoshikawa a, b, daInstitute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi, 980-8577, JapanNew Industry Creation Hatchery Center (NICHe), Tohoku University, 6-6-10 Aoba, Aramaki, Aoba-ku, Sendai, Miyagi, 980-8579, Japan
Institute of Physics CAS, Cukrovamicka 10, Prague 162 00, Czech Republic
dC&A Corporation, 6-6-40 Aoba, Aramaki, Aoba-ku, Sendai, Miyagi 980-8579, Japanbca r t i c l e i n f oa b s t r a c t

Article history:
Received 12 February 2016
Received in revised form
15 April 2016
Accepted 15 April 2016

(CexYb1x)2Si2O7 (x ¼ 0.00, 0.01) single crystals were grown by the micro-pulling-down method to test
the possibility of its application as infra-red scintillator for medical imaging. Powder X-ray diffraction
analysis indicated that the crystals were single-phase materials. The radioluminescence spectra of thecrystals demonstrated presence of two near infra-red emission peaks (at 1010 and 1030 nm). Theemission peaks at 420 and 580 nm ascribed to defects were also observed in the crystals. The humanbody has maximum transmission in wavelength range from 650 to 1200 nm. Therefore, Yb2Si2O7 is
expected to be used as efﬁcient infra-red scintillator for medical applications.
© 2016 Elsevier B.V. All rights reserved.

Keywords:
Scintillator
Pyrosilicate
Charge transfer
Infra-redSingle crystal

1. Introduction
Radiation therapy is one of the widely-used tumor treatmentsthat uses high-energy particles or X/gamma rays to destroy cancer
cells [1]. However, medical accidents such as over- or underexposure occur occasionally when such treatment is applied [2].
Therefore, the real-time monitoring of radiation dose is required toincrease the security of radiation treatment. One of the ideas used
to control the radiation dose is based on application of a probe,
which consists of optical ﬁber with small volume scintillator [3]; Xrays generate considerable noise in the ﬁber.
Recently, implantable real time dose monitoring system using
infra-red scintillators has been also proposed (Fig. 1) [4e7]. In sucha system, the light output from the infra-red scintillator can beproportional to the X-ray dose. Thus, the resulting scintillation
photons can propagate through the human tissue and can be read-* Corresponding author.
** Corresponding author.
E-mail addresses: horiai@imr.tohoku.ac.jp (T. Horiai), kurosawa@imr.tohoku.ac.
jp (S. Kurosawa).
http://dx.doi.org/10.1016/j.optmat.2016.04.032
0925-3467/© 2016 Elsevier B.V. All rights reserved.

out with photon detectors such as Charge-Coupled Device (CCD)
camera or Complementary metaleoxideesemiconductor (CMOS)
camera positioned outside of the patient's body.
In the previous studies, scintillation properties of Yb-doped
Gd3Al2Ga3O12 (Yb:GAGG), Cr-doped Gd3Ga5O12 (Cr:GGG) and Nddoped Lu2O3 as near-infra-red (NIR) scintillators were examined[4e7]. Regarding the application discussed here, the implantable
scintillator must have small size of ~3 mm in diameter. Additionally,
the scintillating material should have a high effective atomic
number to establish acceptable X-ray detection efﬁciency [8].
However, Yb:GAGG and Cr:GGG have low effective atomic numbersof 51 and 50, respectively. On the other hand, Nd:Lu2O3 has relatively high effective atomic number of 67. However, Lu has largeintrinsic gamma-rays background due to 176Lu isotope, which is notsuitable for the patient.
To develop novel scintillator with NIR emission and comparatively high effective atomic number, we focused on Yb2Si2O7(YbPS), because (i) Yb3þ 4fe4f emission wavelength is expected tobe in the NIR region [4], (ii) YbPS has effective atomic number(Zeff(Yb2Si2O7) ¼ 60) which is greater than that of GAGG and GGG,(iii) YbPS melts congruently and can be produced in the single

T. Horiai et al. / Optical Materials 58 (2016) 14e17

Fig. 1. Schematic diagram of the implantable real-time dosimeter system.

crystal form by the melt growth methods [9,10], and (iv) Yb, Si andO have negligible natural abundance of radio-active isotopes andare good constituents for the material without intrinsic
background.
According to our knowledge, the luminescent properties of YbPS
were not examined in the past. Moreover, Yb2Si2O7 single crystal
growth was not reported except fabrication of nanocrystals andpolycrystalline materials [11]. Thus, the purpose of this research
was to produce relatively large mm-scale Yb2Si2O7 single crystals
and to study their optical and scintillation properties. The crystal
growth was performed from the melt using the micro-pullingdown (m-PD) method [12]. This technique is often considered as
fast crystal growth method that allows production of mm-scale
crystals within one day.
Some Yb-containing materials such as Yb-doped Y3Al5O12(Yb:YAG) have been reported to have charge transfer (CT) emission
around 330 and 490 nm [13]. Since it is the ﬁrst communication on
optical properties of the Yb2Si2O7 crystal, we also evaluated the CT
emission in the interval from ~7.5 K to room temperature (RT).
Additionally, Ce and Yb co-doped YAG demonstrated blue-to-NIR
conversion that is established via the single-step mechanism ofenergy transfer through a Ce4þ-Yb2þ charge transfer state (CTS)[14]. Therefore, the optical properties of Ce-doped YbPS crystals
grown by the m-PD method are also discussed in the current report.
Moreover, the comparison of the light outputs of undoped and Cedoped Yb2Si2O7 crystals excited by X-ray was also performed.

15Since detection of CT emission was expected, temperature dependences of PL emission spectra were evaluated within 7.5e300 K
for both undoped and Ce-doped samples. The samples were irradiated with excitation photons from Beam line 3B of the Ultraviolet
Synchrotron Orbital Radiation Facility III (UVSOR-III) located at theInstitute for Molecular Science in Okazaki, Japan, and the samples
were cooled down to 7.5 K with liquid helium. The emission spectra
were examined with a spectrometer (spectoropro-300i, Actonresearch).
The radioluminescence spectra were measured with two spectrometers with a detection wavelength dynamic range of
190e800 nm and 800e1090 nm (DU420-OE and DU920-BEX-DD,respectively, ANDOR), when the specimens were excited by X-ray
from the same X-ray generator used in XRD analysis (RINT 2000,RIGAKU) with an accelerating voltage of 40 kV and a beam currentof 40 mA.
3. Results and discussions
The obtained bulk crystals of undoped YbPS and Ce:YbPS arecompared in Fig. 2. Fig. 3 displays the results of powder XRD
analysis of the crystals, which were consistent with previous
available data for YbPS (JCPDS No. 25-1345). The crystalline structure of YbPS was monoclinic C2/m and a ¼ 6.7991, b ¼ 8.8734,
c ¼ 4.7084 and b ¼ 101.969 that is also consistent with previous
data [16].
Fig. 4 shows transmission spectra for (Cex Yb1x)2Si2O7(x ¼ 0.00, 0.01) single crystalline specimens with thicknesses of1 mm. The absorption peaks at 910 and 970 nm were ascribed to theabsorption from the Yb3þ 4fe4f transition for both samples. Additionally to these peaks, strong absorption was observed at 300 and350 nm for the Ce-doped Yb2Si2O7 crystal due to the Ce3þ 4fe5dtransitions. Both undoped and Ce-doped YbPS samples had absorption edges below 230 nm, and these absorptions originatedfrom Yb3þ-O2- charge transfer band or the absorption band of theYbPS host. Since the Ce-doped sample had no remarkable emission
peaks originating from Yb3þ fef transition in the NIR region when
excited by ~300 or 350 nm photons (Ce3þ 4fe5d transition), theenergy transfer from Ce to Yb was found to be inefﬁcient in YbPS.
Additionally, (Ce0.01, Yb0.05, Lu0.94)2Si2O7 sintered compact withmuch lower Yb concentration was prepared, and NIR emission
peaks originating from Yb3þ fef transition were observed underexcitation around 350 nm (Fig. 5). This NIR emission would be due
to energy transfer via a Ce4þ-Yb2þ charge transfer state [14]. Thus,
the concentration quenching phenomenon occurring at high Yb

2. Materials and methods
Undoped and 1%Ce-doepd Yb2Si2O7 single crystals were grownby the m-PD method with a pulling rate of 0.05 mm/min in N2 gas
atmosphere [12]. The starting mixtures were prepared from
Yb2O3, SiO2 (and CeO2) powders with purities of 99.99%. Thecrystals were grown using an Ir crucible and thin Ir rod as a seed.
The phase identiﬁcation of the as produced materials was performed by powder X-ray diffraction (XRD) analysis using a RINT
2000 (RIGAKU) apparatus. The crystals were cut and polished to
produce specimens suitable for investigation of optical and scintillation properties.
Transmittance spectra were recorded with a spectrometer (V670ST, JASCO) at RT. Photoluminescence (PL) emission spectra fromvisible to NIR were measured with an absolute PL quantum yieldmeasurement system consisting of a Xe lamp (150 W) as an excitation lamp and two photonic-multi-channel analyzers withwavelength dynamic rages of 200e950 nm and 350e1100 nm for
C10027-01 PMA-12 and C10027-02 PMA-12 (Hamamatsu),
respectively [15].

Fig. 2. Views of the as-grown (Cex, Yb1x)2Si2O7 (x ¼ 0.00, 0.01) crystals prepared by
the micro-pulling down method (a-1) and (b-1), and corresponding polished specimens (a-2) and (b-2) used for characterizations.

16T. Horiai et al. / Optical Materials 58 (2016) 14e17

Fig. 3. Powder X-ray diffraction data for the (Cex, Yb1x)2Si2O7 (x ¼ 0.00, 0.01) crystals(X-ray Source Target: Cu).

Fig. 6. Photoluminescence spectra of the (Cex, Yb1x)2Si2O7 (x ¼ 0.00, 0.01) crystals
measured at various temperatures.
Fig. 4. Transmittance spectra of (Cex, Yb1x)2Si2O7 (x ¼ 0.00, 0.01) measured between
190 and 2000 nm at room temperature.

Fig. 5. Photoluminescence spectra of the (Ce0.01, Yb0.05, Lu0.94)2Si2O7 sintered compact
excited by 350 nm.

concentration can suppress the Yb emission.
Fig. 6 compares the temperature dependences of the PL

emission spectra for the undoped (a) and Ce-doped (b) YbPS
crystals under 210 nm excitation in the temperature range of
7.5e300 K. Since absorption edge was around 230 nm, the 210 nm
photons were selected as an excitation source. Moreover, variationsof the excitation wavelengths demonstrated that the strongest
emission was observed at 210 nm excitation.
The undoped YbPS had emission peaks at around 420 and
580 nm. Although, the intensity of the 420 nm emission banddecreased until 300 K, this peak still remained with detectable
intensity at RT. On the other hand, intensity of the 580 nm bandremained almost constant and independent on temperature. Thus,
both the 420 nm and the 580 nm bands as being due to defects.
Oppositely, the intensity of the 420 nm emission band of the Cedoped YbPS increased when temperature of the specimen becamehigher. It implied that the same defect as in the undoped YbPS ismodiﬁed by Ce3þ. The 580 nm emission peak was also observed in
the PL spectra of Ce-doped YbPS, while it was suppressed at RT.
The radioluminescence spectra for both crystals are illustratedin Fig. 7. The NIR emission peaks were ascribed to Yb3þ 4fe4f
emission (2F5/2 to 2F7/2). Although radioluminescence intensity ofundoped YbPS was lower than that of the previously studied Ybdoped GAGG by an order of magnitude, YbPS has higher X-raystopping power than Yb:GAGG by a factor of ~1.9 [7]. In addition, Gdis sometimes considered as toxic material that should be avoided[17]. Thus, YbPS is preferable for human body treatment in radiotherapy. As the infrared emission of YbPS was detected within theT. Horiai et al. / Optical Materials 58 (2016) 14e17Fig. 7. Infra-red radioluminescence spectra of the (Cex, Yb1x)2Si2O7 (x ¼ 0.00, 0.01)
crystals.

“human window,” application of this material in the dose monitors
used during medical radiotherapy is proposed.
4. Conclusions
Successful growth of the (Cex, Yb1x)2Si2O7 (x ¼ 0.00, 0.01)
single crystals from the melt was demonstrated for the ﬁrst time.
The crystals were produced by the micro-pulling-down method.
The photoluminescence and radioluminescence spectra measurements indicated presence of several luminescence peaks such as
Yb3þ 4fe4f and defect luminescence. The Yb3þ 4fe4f emissions
were detected at 1010 and 1030 nm, when the crystals were excitedwith a Xe lamp and X-ray. The NIR emission originating from Yb3þ4fe4f transition energies is expected to be suitable for applicationin real time dose monitoring systems.
Acknowledgements
This work was partially supported by (i) Japan Society for thePromotion of Science KAKENHI Grant Number 14462961,15597934, 15619740 (by Grant-in-Aid for Young Scientists(B), etc.,
S. Kurosawa), (ii) Bilateral AS CReJSPS Joint Research Project17

sported at Czech side by MEYS, KONTAKT LH14266 grant, (iii) Japan
Science and Technology Agency, Development of Systems andTechnology for Advanced Measurement and Analysis (SENTAN),(iv) JST, Adaptable & Seamless Technology Transfer Program
through Target-driven R&D, (v) the Association for the Progress of
New Chemical Technology, (vi) The Murata Science Foundation,(vii) Nippon Sheet Glass Foundation for Materials Science and Engineering, (viii) Tonen General Sekiyu Foundation, (ix) Yazaki Memorial Foundation for Science and Technology, (x) Tokin Scienceand Technology Promotion Foundation, (xi) Intelligent cosmos
research institute, (xii) Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, and (xiii) the UVSOR Facility of
the Institute for Molecular Science. In addition, we would like tothank following persons for their support: Mr. Yoshihiro Nakamuraof Institute of Multidisciplinary Research for Advanced Materials(IMRAM), Tohoku University and Mr. Hiroshi Uemura, Ms. KeikoToguchi, Ms. Megumi Sasaki, Ms. Yuka Takeda and Ms. Kuniko
Kawaguchi of IMR, Tohoku University.

References[1] A. Zerbi, V. Fossati, D. Parolini, M. Carlucci, G. Balzano, G. Bordogna,
C. Staudacher, V.D. Calro, Cancer 73 (1994) 2930.
[2] J.M. Cosset, Radiother. Oncol. 63 (2002) 1.
[3] H.G. Low, W. Bonatz, W. Binder, Radiat. Prot. Dosim. 34 (1990) 283.
[4] A. Suzuki, S. Kurosawa, S. Nagata, T. Yamamura, J. Pejchal, A. Yamaji, Y. Yokota,K. Shirasaki, Y. Homma, D. Aoki, T. Shikama, A. Yoshikawa, Opt. Mater. 36(2014) 1484.
[5] A. Yamaji, V.V. Kochurikhin, S. Kurosawa, A. Suzuki, Y. Fujimoto, Y. Yokota,
A. Yoshikawa, IEEE Trans. Nucl. Sci. 61 (2014) 320.
[6] S. Kurosawa, L. An, A. Yamaji, A. Suzuki, Y. Yokota, K. Shirasaki, T. Yamamura,
A. Ito, T. Goto, G. Boulon, A. Yoshikawa, IEEE Trans. Nucl. Sci. 61 (2014) 316.
[7] A. Suzuki, S. Kurosawa, A. Yamaji, Y. Shoji, J. Pejchal, K. Kamada, Y. Yokota,A. Yoshikawa, Opt. Mater. 36 (2014) 1938.
[8] F.W. Spiers, Br J. Radiol. 19 (1946) 52.
[9] N.A. Toropov, I.A. Bondar, Bull. Acad. Sci. USSR Div. Chem. Sci. 10 (1961) 1278.
[10] The American Ceramic Society and the National Institute of Standards andTechnology, 2016. Figure Numbers 2391; www.nist.gov/srd/nist31.cfm.
[11] A.J. Fernandez-Carrion, M.D. Alba, A. Escudero, A.I. Becerro, J. Solid State Chem.
184 (2011) 1882.
[12] A. Yoshikawa, M. Nikl, G. Boulon, T. Fukuda, Opt. Mater. 30 (2007) 6.
[13] L. van Pieterson, M. Heeroma, E. de Heer, A. Meijerink, J. Lumin. 91 (2000) 177.
[14] D.C. Yu, F.T. Rabouw, W.Q. Boon, T. Kieboom, S. Ye, Q.Y. Zhang, A. Meijerink,Phys. Rev. B 90 (2014) 165126.
[15] S. Kurosawa, Y. Yokota, Y. Yanagida, J. Pejchal, A. Yamaji, Y. Shoji, K. Kamada,
A. Yoshikawa, J. Cryst. Growth 393 (2014) 163.
[16] A.N. Christensen, New Cryst. Struct. 209 (1994) 7.
[17] M.A. Perazella, R.A. Rodby, Semin. Dial. 20 (2007) 179.