Segregation of Pr3+ ions in Y2SiO5:Pr3+ nanocrystals was revealed by means of spectroscopic techniques.
Increase of doped ions concentration in the near-surface layer of Y2SiO5:Pr3+ nanocrystals was confirmed
by modification of luminescence spectra with the heat treatment temperature. Relaxation of excess elastic
stresses created by Pr3+ ions with volumes greater than volume of regular Y3+ ion was determined to be the
main cause of observed effects. Theoretical estimations clearly confirm the preliminary predictions.
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3543

Malyukin, Yu.V.

Masalov, A.A.

Seminko, V.V.

SocioEconomic Challenges Journal (Sumy State University)

PROCEEDINGS OF THE INTERNATIONAL CONFERENCE NANOMATERIALS: APPLICATIONS AND PROPERTIES 
Vol. 1 No 4, 04RES04(3pp) (2012) 
 
 
2304-1862/2012/1(4)04RES04(6) 04RES04-1  2012 Sumy State University 
Spectroscopically detected segregation of Pr3+ ions in Y2SiO5 nanocrystals  
  
V.V. Seminko*, A.A. Masalov, Yu.V. Malyukin 
 
Institute for Scintillation Materials, 60, Lenin ave., 61001  Kharkiv, Ukraine 
 
(Received 12 June 2012; published online 21 August 2012) 
 
Segregation of Pr3+ ions in Y2SiO5:Pr3+ nanocrystals was revealed by means of spectroscopic techniques. 
Increase of doped ions concentration in the near-surface layer of Y2SiO5:Pr3+ nanocrystals was confirmed 
by modification of luminescence spectra with the heat treatment temperature. Relaxation of excess elastic 
stresses created by Pr3+ ions with volumes greater than volume of regular Y3+ ion was determined to be the 
main cause of observed effects. Theoretical estimations clearly confirm the preliminary predictions.  
 
Keywords: Segregation, Luminescence, Nanocrystals, Energy transfer. 
 
  PACS numbers: 78.55. – m, 78.67. – n 
 
 
______________ 
* seminko@isma.kharkov.ua 
1. INTRODUCTION 
 
Segregation of impurities near the defects of the 
crystal lattice can sufficiently change the local proper-
ties of solids. The peculiarities of mechanical proper-
ties and diffusion processes for grain boundaries and 
near-surface domains have been shown in a wide 
range of theoretical and experimental works [1-3]. 
Recently was revealed that for nanocrystals the 
change of local properties can manifest itself also in 
change of bulk properties (such as luminescent prop-
erties) because the range of segregation effects be-
comes comparable with the average size of nanocrys-
tal. Such pattern was revealed for YAG:Nd and 
YAG:Ce nano-ceramics [4, 5], SnO2:Eu3+ nanocompo-
sites [6] and Y2O3:Eu3+ phosphors [7]. In [8] the ab-
normal low threshold of luminescence concentration 
quenching was revealed for  Y2SiO5:Pr3+ nanocrystals. 
The preliminary interpretation was given [9], accord-
ing to which the low concentration quenching thresh-
old in these nanocrystals was caused by non-uniform 
distribution (segregation) of doped ions within the 
volume of nanocrystal. The main mechanism of this 
segregation is the relaxation of elastic stresses creat-
ed by Pr3+ ion, which atomic radius (1.01 Å) is greater 
than atomic radius of regular Y3+ ion (0.9 Å). In this 
paper we confirm the preliminary interpretation and 
trace the processes of doped ions segregation in 
Y2SiO5:Pr3+ nanocrystals. 
 
2. EXPERIMENTAL METHODS  
 
Y2SiO5:Pr3+ nanocrystals were synthesized by the 
sol–gel technique [10]. The average size of nanocrys-
tals was about 50 nm. The steady state luminescence 
was measured with a spectrofluorimeter on the base 
of a grating monochromator. The luminescence decay 
was taken using the time-correlated single-photon 
counting (TCSPC) technique [11]. Luminescence 
spectra and decay curves were collected in the confo-
cal geometry [12]. The 1D2→3H4 luminescence was 
excited by argon laser (λexc  488 nm) modulated with 
AOM. 
 
 
3. RESULTS AND DISCUSSION 
 
For a wide range of Pr3+ doped bulk and nanocrys-
tals the strong dependence of  1D2→3H4 luminescence 
intensity on the Pr3+ concentration was shown [13, 14]. 
Namely, increase of Pr3+ concentration from 0.1 to 
5 at. % manifested itself in 1D2→3H4 luminescence 
quenching. At the same time 3P0→3H6 luminescence 
does not exhibit any observable quenching in this con-
centration range, so the ratio between 1D2→3H4 and 
3P0→3H6 spectral bands depends on concentration of 
doped ions.  
The luminescence spectra of Y2SiO5:Pr3+ nanocrystals 
for different doped ions concentration are shown in 
Fig. 1.  
 
 
 
Fig. 1 – Concentration dependence of 1D2→3H4 luminescence 
intensity for Y2SiO5:Pr3+ nanocrystals (0.2, 1 and 2 at.% are 
shown) 
 
Increase of doped ion concentration leads to strong 
quenching of 1D2→3H4 luminescence in relation to 
3P0→3H6 luminescence. Moreover, this quenching is 
more strong than in the case of bulk Y2SiO5:Pr3+ crys-
 
V.V. SEMINKO, A.A. MASALOV, YU.V. MALYUKIN PROC. NAP 1, 04RES04 (2012)s 
 
 
04RES04-2 
tals [13] and the threshold of luminescence concentra-
tion quenching is lower. 1D2→3H4 luminescence decay 
curves (Fig. 2) for Y2SiO5:Pr3+ nanocrystals exhibit so-
called ordered stage with linear dependence of slope 
angle from the time.  
 
 
 
Fig. 2 – Quenching functions for Y2SiO5:Pr3+ bulk (1) and 
nanocrystals (2) at 2 at.%  
 
This stage occurs when in the crystal there are a 
lot of doped ion pairs (i.e. doped ions separated by 
minimal permissible distances) [15]. For the bulk 
Y2SiO5:Pr3+ crystals this stage of quenching was 
never observed. Occurrence of the ordered stage in 
the decay curves as well as the low threshold of lu-
minescence concentration quenching can be consid-
ered as an evidence of non-uniform doped ions dis-
tribution within the volume of nanocrystal. In an-
other words, in Y2SiO5:Pr3+ nanocrystals regions 
with high doped ion concentration are present. The 
most reliable explanation of this non-uniformity is 
the segregation of doped Pr3+ ions near the surface of 
nanocrystal. The same effect is observed for bulk 
crystals too, but, as the quota of segregated ions as 
compared to total doped ions quantity in this case is 
rather small, these segregated ions cannot manifest 
itself in change of luminescent properties of the crys-
tal. 
To confirm the segregation nature of this effect 
and to trace the segregation processes in Y2SiO5:Pr3+ 
nanocrystals the concentration and heat treatment 
temperature were varied. As the segregation process 
is the temperature activated one, the increase of 
heat treatment time must lead to the increase of 
doped ions concentration near the surface of nano-
crystal. The luminescence spectra for Y2SiO5:Pr3+ 
nanocrystals with 0.2, 1 and 2 at.% are shown in 
Fig. 3.  
 
 
 
Fig. 3 – Luminescence spectra for Y2SiO5:Pr3+ nanocrystals (a 
– 0.2 at.%, b – 1 at.%, c – 2 at.%) for 0 (curve 1) and 4 (curve 2) 
hours of heat treatment 
 
 SPECTROSCOPICALLY DETECTED SEGREGATION OF PR3+ IONS IN … PROC. NAP 1, 04RES04 (2012) 
 
 
04RES04-3 
For concentration of 0.2 at.% the heat treatment 
does not change the relative intensity of 1D2→3H4 lu-
minescence band (3a), so in this case segregation of 
doped ions does not have any visible influence on the 
luminescence quenching processes in the nanocrystal. 
For concentrations of 1 and 2 at.% increase of heat 
treatment time from 0 to 4 hours leads to sufficient 
quenching of 1D2→3H4 luminescence band relative to 
3P0→3H6 band. Namely, at doped ions concentration of 
1 at.% relation between intensities of 1D2→3H4 for 0 
and 4 hours of heat treatment is ≈ 1,2 (Fig. 3b), while 
for 2 at.% ≈1,9 (Fig. 3c). These results correspond well 
with theoretical estimations of average distances be-
tween impurity centers in the volume of nanocrystal 
and near its surface for different doped ions concentra-
tions. Average distances between doped ions were es-
timated as dvol  (V/N)1/3 – for doped ions in the bulk of 
nanocrystal and dsurf  (S/N)1/2 – for ions placed near 
the surface. Here N  C·4/3πR3 – total quantity of 
doped Pr3+ ions in the nanocrystal, R, V and S – radius, 
volume and surface area of nanocrystal, respectively. 
Dependence of dvol/dsurf ratio on the concentration of 
doped ions is shown in Fig. 4.  
 
 
 
Fig. 4 – Dependence of distances ratio between doped ions in 
the volume and near the surface of 20 nm nanocrystal on 
doped ions concentration 
At doped ions concentration of 0.2 at.% even at full 
segregation of doped ions the decrease of average dis-
tance near the surface of nanocrystal as compared to 
the average distance inside the nanocrystal is about 
1.15 (from 3.7 to 3.2 nm). Such a small decrease cannot 
lead to any obvious intensification of quenching pro-
cesses in the nanocrystal, as the energy transfer proba-
bility wda ~ 1/r6 [15] would be negligibly small as in the 
first, so in the second case. The situation is rather dif-
ferent for concentration of 1 and 2 at.%. According to 
calculated dependence (Fig. 4) the average distances 
between subsurface ions for these concentrations will 
be 1.5 and 1.7 times smaller than for ions resided in the 
bulk. Such shortening (from 2.2 nm to 1.45 nm at 1 
at.% and from 1.75 nm to 1 nm at 2 at.%) leads in turn 
to sufficient quenching of 1D2→3H4 luminescence in 
Y2SiO5:Pr3+ nanocrystals at Pr3+ concentrations of 1 
and 2 at.%. 
To estimate a local concentration of doped ions near 
the surface of Y2SiO5:Pr3+ nanocrystal we have used  
the theory of surface segregation [1 - 3]. According to 
[1], the main characteristic of this process is so-called 
enrichment factor   Csurf·(1  Cbulk)/Cbulk·(1  Csurf), 
where Cbulk and Csurf are the bulk and surface concen-
trations of the segregated component, respectively. In 
equilibrium state, the enrichment factor can be written 
as the Boltzman term:   exp( Eel/kT), where 
Eel  2G2(ΔΩ)2/3πKr13 is the total elastic energy created 
by the impurity ion, G and K are the shear and the 
bulk modulus of the host matrix, T is the temperature.  
For Y2SiO5:Pr3+ nanocrystals G  47 GPa; 
K  108 GPa (the elastic constants for Y2SiO5:Pr3+ were 
taken from [16]); r1  1.01 Å and r2  0.9 Å. At room 
temperature the enrichment factor χ is equal to 4.26. In 
other words, for bulk dopant concentration Cbulk  1 
at.% the surface dopant concentration Csurf must be 
equal to about 4 at.%, while for Cbulk  2 at.% - about 8 
at.%, i.e. four times higher. The high concentration of 
doped ions near the surface is the source of the effects 
described earlier (such as occurrence of ordered stage 
in decay curves and low threshold of concentration 
quenching) and it provides difference between bulk 
crystal and nanocrystal luminescence properties.  
 
REFERENCES 
 
1. D. McLean, Grain Boundaries in Metals (London: Oxford 
University Press: 1957). 
2. W.M.H. Sachtler, R.A.Van Santen, Applications of Surface 
Science 3, 121 (1979). 
3. P. A. Dowben, A. Miller, Surface Segregation Phenomena 
(New York: CRC Press: 1990). 
4. M. O. Ramirez, J. Wisdom, H. Li et al., Opt. Exp. 16, 5965 
(2008). 
5. V. I. Chani, G. Boulon, W. T. Zhao et al., Jpn. J. Appl. 
Phys. 49, 075601 (2010). 
6. E. A. Morais, L.V. A. Scalvi, A. Tabata et al. J Mater Sci. 
43, 345 (2008). 
7. J. Amami , D. Hreniak , Y. Guyot et al. J. Lumin. 130, 603 
(2010). 
8. P.N. Zhmurin, N.V. Znamenskii, T.G. Yukina, Yu.V. Mal-
yukin, phys. status solidi (b) 244, 3325 (2007). 
9. A. Masalov, O. Viagin, I. Ganina et al., J. Lumin. 129, 
1695 (2009). 
10. W. Zhang, P. Xie, C. Duan et al., Chem. Phys. Lett. 292, 
133 (1998). 
11. T. Basche, W.E. Moerner, M. Orrit, Single Molecule Opti-
cal Detection, Imagening and Spectroscopy (Weinheim: 
VCH: 1996).  
12. D.V. O’Connor, D.Phillipe, Time-Correlated Single Photon 
Counting (New York: Academic Press: 1984). 
13. Yu.V. Malyukin, A.A. Masalov, P.N. Zhmurin, et al., phys-
ica status solidi (b) 240, 655 (2003). 
14. H. Chen, R. Lian, M. Yin, et al., J. Phys.: Condens. Matter. 
13, 1151 (2001). 
15. Yu.K. Voron'ko, T.G. Mamedov, V.V. Osiko, JETP 44, 251 
(1976).   
16. Z. Sun, J. Wang, M. Li, J. Europ. Cer. Soc. 28, 2895 
(2008). 


English

Spectroscopically detected segregation of Pr3+ ions in Y2SiO5 nanocrystals

Electronic Sumy State University Institutional Repository

PROCEEDINGS OF THE INTERNATIONAL CONFERENCE NANOMATERIALS: APPLICATIONS AND PROPERTIES Vol. 1 No 4, 04RES04(3pp) (2012)   2304-1862/2012/1(4)04RES04(6) 04RES04-1  2012 Sumy State University Spectroscopically detected segregation of Pr3+ ions in Y2SiO5 nanocrystals    V.V. Seminko*, A.A. Masalov, Yu.V. Malyukin  Institute for Scintillation Materials, 60, Lenin ave., 61001  Kharkiv, Ukraine  (Received 12 June 2012; published online 21 August 2012)  Segregation of Pr3+ ions in Y2SiO5:Pr3+ nanocrystals was revealed by means of spectroscopic techniques. Increase of doped ions concentration in the near-surface layer of Y2SiO5:Pr3+ nanocrystals was confirmed by modification of luminescence spectra with the heat treatment temperature. Relaxation of excess elastic stresses created by Pr3+ ions with volumes greater than volume of regular Y3+ ion was determined to be the main cause of observed effects. Theoretical estimations clearly confirm the preliminary predictions.   Keywords: Segregation, Luminescence, Nanocrystals, Energy transfer.    PACS numbers: 78.55. – m, 78.67. – n   ______________ * seminko@isma.kharkov.ua 1. INTRODUCTION  Segregation of impurities near the defects of the crystal lattice can sufficiently change the local proper-ties of solids. The peculiarities of mechanical proper-ties and diffusion processes for grain boundaries and near-surface domains have been shown in a wide range of theoretical and experimental works [1-3]. Recently was revealed that for nanocrystals the change of local properties can manifest itself also in change of bulk properties (such as luminescent prop-erties) because the range of segregation effects be-comes comparable with the average size of nanocrys-tal. Such pattern was revealed for YAG:Nd and YAG:Ce nano-ceramics [4, 5], SnO2:Eu3+ nanocompo-sites [6] and Y2O3:Eu3+ phosphors [7]. In [8] the ab-normal low threshold of luminescence concentration quenching was revealed for  Y2SiO5:Pr3+ nanocrystals. The preliminary interpretation was given [9], accord-ing to which the low concentration quenching thresh-old in these nanocrystals was caused by non-uniform distribution (segregation) of doped ions within the volume of nanocrystal. The main mechanism of this segregation is the relaxation of elastic stresses creat-ed by Pr3+ ion, which atomic radius (1.01 Å) is greater than atomic radius of regular Y3+ ion (0.9 Å). In this paper we confirm the preliminary interpretation and trace the processes of doped ions segregation in Y2SiO5:Pr3+ nanocrystals.  2. EXPERIMENTAL METHODS   Y2SiO5:Pr3+ nanocrystals were synthesized by the sol–gel technique [10]. The average size of nanocrys-tals was about 50 nm. The steady state luminescence was measured with a spectroﬂuorimeter on the base of a grating monochromator. The luminescence decay was taken using the time-correlated single-photon counting (TCSPC) technique [11]. Luminescence spectra and decay curves were collected in the confo-cal geometry [12]. The 1D2→3H4 luminescence was excited by argon laser (λexc  488 nm) modulated with AOM.   3. RESULTS AND DISCUSSION  For a wide range of Pr3+ doped bulk and nanocrys-tals the strong dependence of  1D2→3H4 luminescence intensity on the Pr3+ concentration was shown [13, 14]. Namely, increase of Pr3+ concentration from 0.1 to 5 at. % manifested itself in 1D2→3H4 luminescence quenching. At the same time 3P0→3H6 luminescence does not exhibit any observable quenching in this con-centration range, so the ratio between 1D2→3H4 and 3P0→3H6 spectral bands depends on concentration of doped ions.  The luminescence spectra of Y2SiO5:Pr3+ nanocrystals for different doped ions concentration are shown in Fig. 1.     Fig. 1 – Concentration dependence of 1D2→3H4 luminescence intensity for Y2SiO5:Pr3+ nanocrystals (0.2, 1 and 2 at.% are shown)  Increase of doped ion concentration leads to strong quenching of 1D2→3H4 luminescence in relation to 3P0→3H6 luminescence. Moreover, this quenching is more strong than in the case of bulk Y2SiO5:Pr3+ crys- V.V. SEMINKO, A.A. MASALOV, YU.V. MALYUKIN PROC. NAP 1, 04RES04 (2012)s   04RES04-2 tals [13] and the threshold of luminescence concentra-tion quenching is lower. 1D2→3H4 luminescence decay curves (Fig. 2) for Y2SiO5:Pr3+ nanocrystals exhibit so-called ordered stage with linear dependence of slope angle from the time.     Fig. 2 – Quenching functions for Y2SiO5:Pr3+ bulk (1) and nanocrystals (2) at 2 at.%   This stage occurs when in the crystal there are a lot of doped ion pairs (i.e. doped ions separated by minimal permissible distances) [15]. For the bulk Y2SiO5:Pr3+ crystals this stage of quenching was never observed. Occurrence of the ordered stage in the decay curves as well as the low threshold of lu-minescence concentration quenching can be consid-ered as an evidence of non-uniform doped ions dis-tribution within the volume of nanocrystal. In an-other words, in Y2SiO5:Pr3+ nanocrystals regions with high doped ion concentration are present. The most reliable explanation of this non-uniformity is the segregation of doped Pr3+ ions near the surface of nanocrystal. The same effect is observed for bulk crystals too, but, as the quota of segregated ions as compared to total doped ions quantity in this case is rather small, these segregated ions cannot manifest itself in change of luminescent properties of the crys-tal. To confirm the segregation nature of this effect and to trace the segregation processes in Y2SiO5:Pr3+ nanocrystals the concentration and heat treatment temperature were varied. As the segregation process is the temperature activated one, the increase of heat treatment time must lead to the increase of doped ions concentration near the surface of nano-crystal. The luminescence spectra for Y2SiO5:Pr3+ nanocrystals with 0.2, 1 and 2 at.% are shown in Fig. 3.     Fig. 3 – Luminescence spectra for Y2SiO5:Pr3+ nanocrystals (a – 0.2 at.%, b – 1 at.%, c – 2 at.%) for 0 (curve 1) and 4 (curve 2) hours of heat treatment   SPECTROSCOPICALLY DETECTED SEGREGATION OF PR3+ IONS IN … PROC. NAP 1, 04RES04 (2012)   04RES04-3 For concentration of 0.2 at.% the heat treatment does not change the relative intensity of 1D2→3H4 lu-minescence band (3a), so in this case segregation of doped ions does not have any visible influence on the luminescence quenching processes in the nanocrystal. For concentrations of 1 and 2 at.% increase of heat treatment time from 0 to 4 hours leads to sufficient quenching of 1D2→3H4 luminescence band relative to 3P0→3H6 band. Namely, at doped ions concentration of 1 at.% relation between intensities of 1D2→3H4 for 0 and 4 hours of heat treatment is ≈ 1,2 (Fig. 3b), while for 2 at.% ≈1,9 (Fig. 3c). These results correspond well with theoretical estimations of average distances be-tween impurity centers in the volume of nanocrystal and near its surface for different doped ions concentra-tions. Average distances between doped ions were es-timated as dvol  (V/N)1/3 – for doped ions in the bulk of nanocrystal and dsurf  (S/N)1/2 – for ions placed near the surface. Here N  C·4/3πR3 – total quantity of doped Pr3+ ions in the nanocrystal, R, V and S – radius, volume and surface area of nanocrystal, respectively. Dependence of dvol/dsurf ratio on the concentration of doped ions is shown in Fig. 4.     Fig. 4 – Dependence of distances ratio between doped ions in the volume and near the surface of 20 nm nanocrystal on doped ions concentration At doped ions concentration of 0.2 at.% even at full segregation of doped ions the decrease of average dis-tance near the surface of nanocrystal as compared to the average distance inside the nanocrystal is about 1.15 (from 3.7 to 3.2 nm). Such a small decrease cannot lead to any obvious intensification of quenching pro-cesses in the nanocrystal, as the energy transfer proba-bility wda ~ 1/r6 [15] would be negligibly small as in the first, so in the second case. The situation is rather dif-ferent for concentration of 1 and 2 at.%. According to calculated dependence (Fig. 4) the average distances between subsurface ions for these concentrations will be 1.5 and 1.7 times smaller than for ions resided in the bulk. Such shortening (from 2.2 nm to 1.45 nm at 1 at.% and from 1.75 nm to 1 nm at 2 at.%) leads in turn to sufficient quenching of 1D2→3H4 luminescence in Y2SiO5:Pr3+ nanocrystals at Pr3+ concentrations of 1 and 2 at.%. To estimate a local concentration of doped ions near the surface of Y2SiO5:Pr3+ nanocrystal we have used  the theory of surface segregation [1 - 3]. According to [1], the main characteristic of this process is so-called enrichment factor   Csurf·(1  Cbulk)/Cbulk·(1  Csurf), where Cbulk and Csurf are the bulk and surface concen-trations of the segregated component, respectively. In equilibrium state, the enrichment factor can be written as the Boltzman term:   exp( Eel/kT), where Eel  2G2(ΔΩ)2/3πKr13 is the total elastic energy created by the impurity ion, G and K are the shear and the bulk modulus of the host matrix, T is the temperature.  For Y2SiO5:Pr3+ nanocrystals G  47 GPa; K  108 GPa (the elastic constants for Y2SiO5:Pr3+ were taken from [16]); r1  1.01 Å and r2  0.9 Å. At room temperature the enrichment factor χ is equal to 4.26. In other words, for bulk dopant concentration Cbulk  1 at.% the surface dopant concentration Csurf must be equal to about 4 at.%, while for Cbulk  2 at.% - about 8 at.%, i.e. four times higher. The high concentration of doped ions near the surface is the source of the effects described earlier (such as occurrence of ordered stage in decay curves and low threshold of concentration quenching) and it provides difference between bulk crystal and nanocrystal luminescence properties.   REFERENCES  1. D. McLean, Grain Boundaries in Metals (London: Oxford University Press: 1957). 2. W.M.H. Sachtler, R.A.Van Santen, Applications of Surface Science 3, 121 (1979). 3. P. A. Dowben, A. Miller, Surface Segregation Phenomena (New York: CRC Press: 1990). 4. M. O. Ramirez, J. Wisdom, H. Li et al., Opt. Exp. 16, 5965 (2008). 5. V. I. Chani, G. Boulon, W. T. Zhao et al., Jpn. J. Appl. Phys. 49, 075601 (2010). 6. E. A. Morais, L.V. A. Scalvi, A. Tabata et al. J Mater Sci. 43, 345 (2008). 7. J. Amami , D. Hreniak , Y. Guyot et al. J. Lumin. 130, 603 (2010). 8. P.N. Zhmurin, N.V. Znamenskii, T.G. Yukina, Yu.V. Mal-yukin, phys. status solidi (b) 244, 3325 (2007). 9. A. Masalov, O. Viagin, I. Ganina et al., J. Lumin. 129, 1695 (2009). 10. W. Zhang, P. Xie, C. Duan et al., Chem. Phys. Lett. 292, 133 (1998). 11. T. Basche, W.E. Moerner, M. Orrit, Single Molecule Opti-cal Detection, Imagening and Spectroscopy (Weinheim: VCH: 1996).  12. D.V. O’Connor, D.Phillipe, Time-Correlated Single Photon Counting (New York: Academic Press: 1984). 13. Yu.V. Malyukin, A.A. Masalov, P.N. Zhmurin, et al., phys-ica status solidi (b) 240, 655 (2003). 14. H. Chen, R. Lian, M. Yin, et al., J. Phys.: Condens. Matter. 13, 1151 (2001). 15. Yu.K. Voron'ko, T.G. Mamedov, V.V. Osiko, JETP 44, 251 (1976).   16. Z. Sun, J. Wang, M. Li, J. Europ. Cer. Soc. 28, 2895 (2008). 

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Spectroscopically detected segregation of Pr3+ ions in Y2SiO5 nanocrystals

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