Time-Resolved VUV Spectroscopy of Ce3+-doped
Yttrium and Gadolinium Oxyorthosilicates
K. Ivanovskikh1, V. Babin2, Z. Tikhonova2, D. Wang1, A. Meijerink1
1 Debye
Institute, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The Netherlands
2 Institute of Physics, University of Tartu, Riia 142, 51014 Tartu, Estonia
Ce3+-doped yttrium and gadolinium oxyorthosilicates (Y,Gd)2SiO5:Ce3+ attract an increasing interest as promising scintillation materials with a high light yield and high stopping power for gamma
and X rays [1-6]. These materials exhibit rather complicated luminescence properties caused by the
existence of several energy transfer and dissipation mechanisms as well as by occurrence of two
crystallographic sites (noted as Ce1 and Ce2) occupied by Ce3+ impurity ions. In this work, we present initial results of our systematic investigation on spectroscopic properties and dynamics of impurity and intrinsic electronic excitations.
The powder samples of Gd2SiO5:Ce and Y2SiO5:Ce with different Ce3+ concentration were synthesized and provided by Philips Research Aachen. The spectroscopic investigations in VUV-UV excitation range and in a wide temperature region were carried out at the SUPERLUMI station. The
time-resolved emission and excitation spectra were recorded in two independent time gates 6.533 ns and 301-457 ns (further called as "fast" and "slow" components respectively) with respect to
the onset of sub-nanosecond synchrotron radiation pulse.
The samples studied exhibit strong dependence of the emission spectra on Ce3+ concentration. For
further detailed investigation, the samples of both Y and Gd oxyorthosilicates containing 0.2% of
Ce3+ were selected, as they reveal most clearly all kinds of luminescence peculiarities. In Fig. 1, the
emission spectra of Y2SiO5:Ce3+ demonstrating two groups of emission bands, located in the range
of 210-360 nm and 360-570 nm wavelength regions, are shown.
Figure 1: Y2SiO5:0.2%Ce3+. a) Emission and excitation spectra. b) Decay kinetics of Ce3+ 420 nm emission.
In the first group, two broad emission bands with maxima at 325 nm (dominating component) and
270 nm (weak component) are evident. The emission bands, located in this wavelength region,
were ascribed to the intrinsic emission bands, and are also observed for the undoped samples [7].
The dominating component at 325 nm is completely quenched at 300 K while the 270 nm band is
already quenched at 100K. These bands most effectively excited in the wavelength range below
190 nm. At 8 K, an intense excitation band is located near 187 nm (6.63 eV) (Fig.1, a). At shorter
wavelength range in the band-to-band absorption region, the excitation spectrum demonstrates the
decrease of intensity. Further rise of excitation efficiency for the 325 nm emission band is observed
at ex < 93 nm. Thus, the dominating emission band possesses the properties of intrinsic luminescence caused by relaxation of self-trapped excitons [6]. The second emission band located at around
270 nm can most probably be ascribed to intrinsic luminescence as well. The sharp onset of the exci889
tation region for this emission is appeared at 176 nm (7.05 eV) i.e. notably shifted towards higher
energy with respect to the excitation region of dominating emission at 325 nm. Such kind of emission excited in the range of creation of separated electron-hole pairs can be considered in terms of
model of radiative relaxation of excitons with large radii typically occurred in complex oxides
[6,8]. In our case, a fairly big Stokes shift (2.2 eV) observed is probably caused by strong excitonphonon interaction and by high relaxation probability of excitons on surface and internal defects of
powder material.
The second group of Y2SiO5:Ce3+ emission bands is located in the range of 360-570 nm and is assigned to radiative 5d4f (2F7/2,5/2) relaxation of Ce3+ ions [1-5]. A presence of two kinds of Ce3+
centres is observed in the spectra recorded near 8 K. Under selective excitation at 323 nm a new
emission component corresponding to emission of Ce2 centres becomes evident as a long wavelength shoulder 460-570 nm. A shorter wavelength intense emission band, connected with Ce1 centres shows almost temperature-independent behaviour in the range of 8-300 K. The excitation spectra of both types of Ce3+ emission have known [1,3] differences in the position of 4f5d bands. In
the range of fundamental absorption region, these spectra demonstrate high efficiency of matrix-toimpurity energy transfer mechanism (see Fig.1, a, bottom). The decay kinetics monitoring most intensive Ce1 emission band is characterised by the strong dependence on the temperature and excitation wavelength (Fig.1, b). At 8 K the decay time of cerium Ce1 emission band under intra-centre
excitation is 36 ns.
Comparative analysis of the time-resolved emission spectra of Gd2SiO5:Ce3+ obtained at 8 K and
300 K allows to conclude that unlike the Y2SiO5:Ce3+ we observe an overlapping of intrinsic and
impurity emission bands in the range of 330-600 nm (Fig.2, a). Time-resolved excitation spectra
recorded monitoring luminescence at 440 nm show two types of excitation, via direct population of
cerium 5d-levels (fast component), and via non-radiative energy transfer from the higher excited
6
PJ, 6IJ, 6DJ and 6GJ multiplets of Gd3+ lattice sites (slow component). The first type of excitation
results in single exponential shape of decay curve with decay constant of 32 ns. Selective excitation
of Gd3+ f-levels leads to appearance of initial build-up of about 28 ns and considerable increase of
the decay time (Fig.2, b). In the band-to-band excitation region, the decay curves also demonstrate
an increase of the decay time constant and a temperature dependent build-up and decay time due to
exciton energy transfer and energy migration from populated 4f-levels [5].
Figure 2: Gd2SiO5:0.2%Ce3+.a) Emission and excitation spectra. b) Decay kinetics of Ce3+ 440 nm emission.
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