Non-radiative energy transfer from trivalent dysprosium to praseodymium in calibo glass

Journal of Non-CrystallineSolids 45 (1981) 39-45 North-Holland Publishing Company

39

NON-RADIATIVE ENERGY TRANSFER FROM TRIVALENT D Y S P R O S I U M T O P R A S E O D Y M I U M IN CALIBO GLASS Bharat Chandra JOSHI Department of Physics, Kumaun Universi(v (AImora Campus), AImora (U.P.), India

Received 8 May 1980

Non-radiative energy transfer from trivalent dysprosium to praseodymium in calibo glass has been observed by measuring the decay time and emission intensity of Dy3+ with varying Pr 3~ concentration and found to occur from the 4F9/2 level of Dy3. to various levels of Pr 3+ . The interaction mechanism of donor (Dy3+) and acceptor (Pr 3+) ions is found to be electric dipole-quadrupole and due to this transfer is weak. Various important parameters such as the critical transfer distance Ro, transfer efficiencyPda, and transfer probability have been computed.

1. Introduction In recent years there has been much interest in the study of energy transfer because of its importance in improving the efficiency of laser materials and phosphors. The mechanism involved in the process of energy transfer from one ion (donor) to another ion (acceptor) was first given by Forster [1] and then extended by Dexter [2] but was not ver~ satisfactory for all ranges of concentration. Inokuti-Hirayama [3] successfully explained the transfer mechanism by observing the decay pattern of emission. Fong and Diestler [4] explained the mechanism by treating it as a many body interaction problem. Rare-earth i o n s , d u e to their shielded 4f shell, play an important role in energy transfer phenomena, energy transfer, with Dy 3+ as an energy donor, has been studied by Joshi et al. [5]. This paper presents the mechanism involved in the interaction between excited Dy 3+ ions and Pr 3+ ions in calibo glass and various parameters involved in the interaction have also been computed.

2. Experimental Calcium oxide (Analar Riedel, FRG), lithium carbonate (Analar Lab Chemie Industry, Bombay, India) and boric acid (Analar, BDH) were used to prepare the host matrix "calibo glass" having a density of 2.2 g / c m 3. Dy203 obtained from G T E sylvania Owanda, USA and Pr203 99.9% from Indian Rare Earth Ltd, Kerala, were used as activators. The rare-earth doped glass pellets of almost equal dimensions and desired concentration were prepared by melting 0022-3093/81/0000- 0000/$02.50 © 198 l North-Holland

40

B.C. Joshi / Non-radiative energy transfer

the homogeneous mixture at about 1000°C in a platinum crucible. For details see refs. 6 and 7. The emission spectra were scanned by using a SPM-2 Carl Zeiss Jena monochromator with quartz optics, an EMI 9558Q/B photomultiplier tube and multiflex galvanometer assembly. The decay times were found using the single flash technique with a high pressure mercury lamp (BH-6 Hg). Both the emission spectra and decay times were taken by exciting the samples with the 365 nm group of mercury lines. The observations were taken at 80 K and 500 K as well as at room temperature (300 K).

200

160 u') I-:D IX.

<

120

>: I-U3

Z

IJJ

I'Z 80 t~

z LU

O3 IJJ Z :D

--

40

I,

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=

0

460

480

~

500

___~

. . . . . . . . . .

520 WAVELENGTH

.--

540

|

560

580

600

~ nm

Fig. 1. Fluorescence spectra of: (A) Dy 3+ (2.0 wt%); (B) Dy 3+ (2.0 wt%)+ Pr 3+ (2.0 wt%); in calibo glass.

B.C. Joshi / Non-radiative energy transfer

41

3. Results and discussion

The emission spectrum of the dysprosium ion (2 wt%) in calibo glass is shown in fig. 1A. The two bands shown therein arise due to the transitions 4F9/2 --~6H]5/2 (482 rim), 4F9/2 ---~6H13/2 (575 nm) and confirm the trivalent state of dysprosium in this matrix. The broadening of the spectral lines (bands) is a characteristic of all glassy matrices caused by the inhomogeneous broadening in the energy levels of the doped RE ion. Incorporation of praseodymium ions along with Dy 3÷ ions reduces the emission intensity of the later ions (fig. 1B) and the overall reduction in the intensity can be attributed mainly due to the non-radiative energy transfer from Dy 3÷ to Pr 3÷ [8]. The emission decay curve of Dy 3÷ (3 wt.%) when plotted on a semilog scale shows an exponential pattern of decay indicating the absence of self-quenching of Dy 3~ emission [9]. Addition of Pr 3+ ions makes the curve non-exponential at the initial part which increases with an increasing concentration of Pr 3÷ ions. Also, the decay time (~-) of Dy 3+ decreases with increasing Pr 3+

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Fig. 2. T h e decay curves A a n d B are theoretical curves f o r s = 6 a n d s - 8 and the p o i n t s are

experimentally obtained values for the decay curve.

42

B.C. Joshi / Non-radiative energy transfer

t

+

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~

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~

gg

B.C. Joshi

/

Non.radiative energv transfer

43

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,

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Fig. 3. Variation of transfer probability concentration.

(Pda) with

C 8/3 where C is the donor plus acceptor

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Fig. 4. Energy level diagram of Dy 3+ and Pr 3+

'3H4

B.C. Joshi / Non-radiative energy tramfer

44

concentration. Table 1 shows clearly the non-radiative transfer of energy from Dy 3+ to Pr 3+ . The mechanism of energy transfer i.e. the multipolar term mainly responsible for the transfer is obtained by using the Inokuti-Hirayama equation for transient intensity at time t d~(t) -- exp

-

r0 -

L

r(l

-

3/s)

where ro is the decay time of the donor alone, s is the multipolar term having a value 6 for dipole-dipole, 8 for dipole-quadrupole and 10 for quadrupolequadrupole interactions, CO is the critical transfer concentration [10] and C is the acceptor concentration. Fig. 2 shows the theoretical curves obtained from the above equation for s = 6 and s = 8, alone with the experimental decay curve for the typical sample containing Dy 3+ 2.0 wt% and Pr 3+ 4.0 wt.%. it is clear from the figure that the experimental curve fits best the theoretical curve with s = 8 suggesting mainly the dipole-quadrupole nature of the interaction. This form of interaction is further corroborated by plotting the transfer probabilities, Pda, table 1, against C 8/3 ,.~ 1 / R 8 and C 6/3 ~ 1 / R 6, where C is the acceptor concentration and R is the donor-Acceptor distance. The plot with the former gives a straight line (fig. 3) showing the 1 / R 8 dependence of the transfer probability required by dipole-quadrupole interaction [11]. The critical transfer distance R 0 corresponding to the critical transfer concentration Co (9.5 wt.%) is equal to 11.5 A and can be compared with those values obtained by Nakazawa and Shionya [12] in calcium metaphosphate glass for various rare-earth ion pairs lying between 3.0 and 12.0 A and by Reisfeld et al. [11] for samarium-europium pairs in phosphate glass equal to 14.4 A. The average donor-acceptor distance R varies from 16.8A to l l . I A (table 1) in this study, therefore, the transfer by exchange mechanism [3], which requires a donor-acceptor distance of 3-4 ,~ for rare earths is negligible. The energy level diagram of Dy 3÷ and Pr 3÷ is shown in fig. 4. Pr 3÷ has no energy level corresponding to an energy of 27 300 cm- ] or nearby as given by a 365 nm excitation source, therefore, Pr 3+ ions cannot be excited directly by the source. The energy levels of Pr 3÷ lying close to the 4F9/2 fluorescent level of Dy 3+ in energy are 3Pt, 3P0, ]16 and 3P2, respectively, and hence will receive a considerable portion of excitation energy from Dy 3+ . The slight mismatch in energy of these Pr 3+ levels with the 4F9/2 level of Dy 3+ can be compensated for by the phonons of the host glass having an energy of about 1300-1400 cm-~ [13]. Other levels of Pr 3+ are separated by a large gap of energy from the 4F9/2 level of Dy 3÷ and hence cannot receive energy directly from this level. The observations were also taken at liquid air temperature (80 K) and 500 K but no significant change was observed in the energy parameters (see table 1). The thermally broadened energy levels of the dopant ions slightly increase the parameters at high temperature while at low temperature the reverse is observed.

B.C. Joshi / Non-radiative energy transfer

45

The work was carried out in the Physics Laboratory of D.S.B. (Univ.) College, Nainital. The author is thankful to Dr. J.C. Joshi for helpful discussions and to C.S.I.R. (New Delhi) for financial assistance.

References [1] [2] [3] [4] [5]

Th. Forster, Ann. Phys. 2 (1948) 55. D.L. Dexter, J. Chem. Phys. 21 (1953) 836. M. Inokuti and F. Hirayama, J. chem. Phys. 43 (1965) 1978. F.K. Fong and D.J. Diestler, J. chem. Phys. 56 (1972) 2875. J.C. Joshi, B.C. Joshi, N.C. Pandey, B.C. Pandey and J. Joshi, J. Sol. St. Chem. 26 (1978) no. 2. [6] J.C. Joshi, B.C. Joshi, N.C. Pandey, R. Belwal and J. Joshi, J. Sol. St. Chem. 22 (1977) 439. [7] B.C. Joshi, PhD thesis, Kumaun University, Nainital, India, (1978) unpublished. [8] H.F. Ivey, Proc. Int. Conf. on Luminescence (1966) 2027. [9] W.B. Gandrud and H.W. Moos, J. Chem. Phys. 49 (1968) 2170. [10] L.G. Ven Uitert, J. Lumin. 4 (1971) 1. [11] R. Reisfeld and L. Boehm, J. Sol. St. Chem. 4 (1972) 417. [12] E. Nakazawa and S. Shionoya, J. Chem. Phys. 47 (1967) 321 l. [13] R. Reisfled, Structure and bonding, Vol. 13 (Springer Verlag, Berlin, 1973) p.53.