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Author's personal copy Journal of Luminescence 147 (2014) 63–71
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Analysis of energy transfer based emission spectra of (Sm3 þ , Dy3 þ ): Li2O–LiF–B2O3–CdO glasses V. Naresh n, S. Buddhudu Department of Physics, Sri Venkateswara University, Tirupati-517502, India
art ic l e i nf o
a b s t r a c t
Article history: Received 14 June 2013 Received in revised form 27 September 2013 Accepted 14 October 2013 Available online 6 November 2013
The present paper brings out the results concerning the preparation and optical properties of Sm3 þ and Dy3 þ each ion separately in four different concentrations (0.1, 0.5, 1.0 and 1.5 mol%) and also together doped (1 mol% Dy3 þ þx mol% Sm3 þ ): Li2O–LiF–B2O3–CdO (where x ¼0.1, 0.5, 1.0 and 1.5 mol%) glasses by a melt quenching method. Sm3 þ doped base glasses have displayed an intense orange emission at 602 nm (4G5/2-6H7/2) with an excitation at 403 nm and Dy3 þ doped glasses have shown two emissions located at 486 nm (4F9/2-6H15/2; blue) and 577 nm (4F9/2-6H13/2; yellow) with λexci ¼ 387 nm. The co-doped (Dy3 þ þ Sm3 þ ) lithium fluoro-boro cadmium glasses have been excited with an excitation at 387 nm of Dy3 þ which has resulted in with a significant reduction in Dy3 þ emission, at the same time there exists an increase in the reddish-orange emission of Sm3 þ due to an energy transfer from Dy3 þ to Sm3 þ . The non-radiative energy transfer from Dy3 þ to Sm3 þ is governed by dipole–quadrupole interactions as is explained in terms of their emission spectra, donor lifetime, energy level diagram and energy transfer characteristic factors. & 2013 Elsevier B.V. All rights reserved.
Keywords: Glasses Energy transfer luminescence
1. Introduction Trivalent rare earth ions doped glasses have generated a great deal of interest in studying their luminescence, lasing and sensing properties because of their wide range of applications like optoelectronic devices, fibre optics, fibre amplifiers, Q-switched devices, solid state lasers and multicolour emitting devices [1,2]. Though rare earth ions possess unique nature of exhibiting sharp and distinct spectral lines of absorption and emission, sometimes they fail to absorb the excitation energy and result in exhibiting low emission intensity. In order to overcome this, the host matrix containing a luminescent rare earth ion (acts as activator/acceptor) has been co-doped with another luminescent rare earth ion (acts as sensitiser/donor). Co-dopant ion as sensitiser absorbs excitation energy and transfers it to the acceptor ion in enhancing its luminescence performance [3]. The energy transfer based luminescence has quite significant applications in research and development of novel materials by means of pumping schemes in enhancing luminescence for achieving lasing action with an improved efficiency at a reduced threshold energy of laser oscillations in solid state lasers, in producing multicolour emitting devices, and in up-converters (IR to visible) [4,5]. Host plays a vital role in exhibiting efficient and intense luminescence, thus in the present study, alkali fluoro-borate glass with a divalent oxide (CdO) has n
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been considered because of its combined optical advantages of fluorides and oxides. Fluorides having low phonon energy (300– 600 cm 1) reduce non-radiative decay losses due to multiphonon relaxations and increase the quantum efficiencies of rare earth ions related emission and do possess higher IR cutoff edge towards higher wavelength side, which can have transmission ability from UV to IR, low non linear refractive index and also decreases the OH absorption because fluorine might react with OH group and forms HF and oxides impart high mechanical and thermal stability and chemical durability [6–8]. Samarium (Sm3 þ ) ion has been chosen here to analyse its luminescence properties in visible region (orange) with potential applications in high-dose measurements in medical radiation dosimetry, spectral hole burning, high-density optical storage, colour displays, under sea communication and alongside samarium ion doped glasses can also be used as a cladding for Nd-glass laser rods [9–11]. Dysprosium (Dy3 þ ) ion possesses emission in visible (yellow/blue) and NIR regions, its visible emission is used in white light emitting diodes, possible with mixing good ratio of blue and yellow colours, blue emission can be used in the development of blue laser diodes and midinfrared (1.3 mm) emission of Dy3 þ ion for the optical amplification telecommunication systems and its visible up-conversion emission can be used as a solid state lasers [11–13]. In the present work, we have undertaken both the Dy3 þ and Sm3 þ ions glass alongside single ion doped glasses, to understand the energy transfer based emission and also the interaction which governs energy migration in terms of their emission spectra, emission transition decay-curves in evaluating their lifetimes.
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2. Experimental studies Glasses in general composition of Li2O–LiF–B2O3–CdO (LFBCd) doped individually with Sm3 þ and Dy3 þ in different concentrations (0.1, 0.5, 1, 1.5 mol%) and also co-doped with fixed 1 mol% concentration of Dy3 þ and Sm3 þ concentration varied from 0.1 to 1.5 mol% were prepared by a quenching method as listed out here: i. 30Li2O–20LiF–46B2O3–4CdO (reference glass) ii. 30Li2O–20LiF(46 x)B2O3–4CdO–xSm2O3 (where x¼0.1, 0.5, 1.0, 1.5 mol%) iii. 30Li2O–20LiF–(46 y)B2O3–4CdO–yDy2O3 (where y¼ 0.1, 0.5, 1.0, 1.5 mol%) iv. 30Li2O–20LiF–(46 x y)B2O3–4CdO–xDy2O3–ySm2O3 (where x¼ 1.0 mol% and y¼0.1, 0.5, 1.0, 1.5 mol%) Reagent grade chemicals of H3BO3, Li2CO3, LiF, CdO, Sm2O3, and Dy2O3 were used in the preparation of both host and rare-earth ions doped glasses. All these chemicals were weighed separately in 10 g batch each, thoroughly mixed and finely powdered using an agate mortar and pestle. Each batch of chemicals mix was transferred into porcelain crucibles and each of those was sintered separately in electric furnace for an hour at 950 1C in order to ensure homogenous melts and then these melts were quenched in between two smooth surfaced brass plates to obtain circular glass discs of 2–3 cm in diameter and 0.3 cm as thickness. By incorporating the rare earth ions (Sm3 þ and Dy3 þ ), each ion separately and also together in (LFBCd) glasses have exhibited orange, blue/yellow and reddish-orange colours under UV lamp illumination of these glasses.
3. Measurements Glass densities were measured using xylene as an immersion liquid from Archimedes’ principle. Abbe refractometer was used to measure the glass refractive indices with a sodium (589.3 nm) lamp. The values of density are found to be in the range of 3.288– 3.672 (g/cm3) with an error of 70.001 and refractive index values are found to be in the range of 1.652–1.653 with an error of 70.001 for Dy3 þ (1.0 mol%) and Sm3 þ (0.1–1.5 mol%) co-doped glasses. The XRD profiles of rare-earth ions (Sm3 þ and Dy3 þ ) doped LFBCd glasses were measured on a Seifert X-ray Difractometer (model 3003TT) with CuKα radiation (λ ¼1.5406 Å) at 40 KV as the applied voltage and 20 mA as the anode current using a Si detector in the range of 2θ¼ 10–601 at the scanning rate of 21/min. The optical absorption spectra of LFBCd glasses doped with (Sm3 þ þDy3 þ ) were recorded on a Varian-Cary-Win Spectrometer (JASCO V-570). The excitation and emission spectra of singly doped Sm3 þ , Dy3 þ and co-doped Sm3 þ and Dy3 þ glasses were recorded at room temperature on a SPEX Flurolog-3 (Model-II) spectrophotometer, attached with an Xe-arc lamp (450 W) as the excitation source. This system is employed with a Datamax software package for acquiring the spectral data and emission decay-curve (lifetime measurement) data using a phosphorimeter attached with a Xe-flash lamp.
4. Results and discussion 4.1. XRD spectrum The measured XRD features of LFBCd glasses with dopant ions clearly reveal the amorphous nature as shown in Fig. 1.
Fig. 1. XRD profiles of singly doped 1Sm3 þ , 1Dy3 þ : Li2O–LiF–B2O3–CdO glasses, and together doped (1 mol%) Sm3 þ þ(1 mol%) Dy3 þ : Li2O–LiF–B2O3–CdO glasses.
4.2. Optical spectra of Sm3 þ : LFBCd and Dy3 þ : Li2O–LiF–B2O3–CdO glasses In Fig. 2(a) and (b), vis–NIR absorption spectra of 1.0 mol% Sm3þ : LFBCd and 1.0 mol% Dy3 þ : LFBCd glasses are shown. In Sm3 þ doped glass the intra-configurational transitions in the absorption spectrum originate from ground state 6H5/2 to various excited states. The bands are assigned to 6H5/2-4F7/2 (402 nm), 6H5/2-4I11/2 (471 nm), 6H5/26 F11/2 (942 nm), 6H5/2-6F9/2 (1078 nm), 6H5/2-6F7/2 (1228 nm), 6 H5/2-4F5/2 (1374 nm), 6H5/2-4F3/2 (1449 nm), 6H5/2-6H15/2 (1524 nm), 6H5/2-6F1/2 (1584 nm), and 6H5/2-6H13/2 (1920 nm) respectively. Most of the absorption transitions of Sm3þ originate based on the selection rules of |ΔL|Z0, |ΔJ|Z0 and |ΔS|¼0 [14]. The high intense transitions in the infrared region are spin allowed (ΔS¼0) and the transitions in the visible region are spin-forbidden. In case of the absorption spectrum of Dy3þ doped glass, the transitions originate from the ground state 6H15/2 to various excited states 4K17/2 (376 nm), 4 I13/2 (386 nm), 4G11/2 (425 nm), 4I15/2 (452 nm), 4F9/2 (476 nm), 6F3/2 (742 nm), 6F5/2 (801 nm), 6F7/2 (898 nm), 6F9/2, 6H7/2 (1088 nm), 6F11/2, 6 H9/2 (1261 nm), 6H11/2 (1325 nm), and 6H13/2 (1674 nm) respectively. The transitions 6H5/2-6F1/2, 6F3/2 of Sm3þ and 6H15/2-6F11/2, 6H9/2 of Dy3þ are hypersensitive transitions as they follow the selection rules, | ΔS|¼0, |ΔL|r2, and |ΔJ|r2 and are found to be more intense than other transitions [11,15]. 4.3. Photoluminescence of Sm3 þ : Li2O–LiF–B2O3–CdO glasses The excitation spectrum of (1 mol%) Sm3 þ : LFBCd glass is shown in Fig. 3(a). From this figure we could notice the presence of excitation bands 6H5/2-4H9/2 (345 nm), 4D5/2,6P5/2 (363 nm), 4D1/2 (376 nm), 4 F7/2 (403 nm), 4M19/2 (418 nm), 4G9/2 (439 nm), 4I13/2 (463 nm), 4I11/2 (471 nm), 4G7/2 (501 nm), 4F3/2 (528 nm), 4G5/2 (563 nm) attributed to 4f–4f transitions of Sm3þ . Of all the transitions, the prominent excitation 403 nm (6H5/2-4F7/2) has been chosen for the measurement of emission spectra of Sm3þ glasses as shown in Fig. 3(b) for four concentrations (0.1, 0.5, 1, and 1.5 mol%) of Sm3þ (inset figure shows the dependence of emission intensity as a function of Sm3þ concentration). At 403 nm, Sm3þ ions are excited to upper energy state 4H9/2 and from where those decay to 4F7/2 state and these excited ions cascades rapidly to the 4G5/2 metastable state by populating it. The energy states between 4F7/2 and 4G5/2 are very close and their energy differences are small, which encourages an efficient and fast non-radiative relaxation to 4G5/2 state. Upon reaching 4G5/2 state these unstable ions relaxes radiatively by emitting fluorescence to the
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3þ
Fig. 2. (a) Vis–NIR absorption spectrum of 1.0 mol% Sm : Li2O–LiF–B2O3–CdO glass. (b) Vis–NIR absorption spectrum of 1.0 mol% Dy3 þ : Li2O–LiF–B2O3–CdO glass.
nearest lower lying multiplet 6HJ (J¼ 5/2, 7/2, 9/2, and 11/2) energy state separated at a distance of 7500 cm 1. The photoluminescence spectra consists of four emission bands assigned to their electronic transitions 4G5/2-6H5/2 (565 nm: yellow), 4G5/2-6H7/2 (602 nm: orange), 4G5/2-6H9/2 (647 nm: orange reddish), and 4G5/2-6H11/2 (709 nm: red) are observed. Of all these transitions, 4G5/2-6H7/2 (602 nm) is the most dominant transition with intense orange emission which appears to be suitable for laser emission. Sharp and narrow emission peaks have observed due to the shielding effect of 4f6 electrons by the outer ligands. 4G5/2-6H5/2 (565 nm) transition is a forbidden magnetic dipole transition because of ΔJ¼0, i.e., having same J values. The transition 4G5/2-6H7/2 (602 nm) is magnetic dipole allowed but electric dipole nature is more dominant with selection rule ΔJ¼ 71, therefore it can be considered as partially MD- and partially ED-allowed, the other transitions 4G5/2-6H9/2 (647 nm) is purely electric dipole transition with ΔJ¼ 72 having moderate intensity and 4G5/2 -6H11/2 (709 nm) is forbidden transition with ΔJ¼ 73 having feeble intensity [16]. Fluorescence lines of Sm3 þ are parity forbidden, but they can be observed with the violation of parity for dipole transitions (4G5/2-6H5/2, 6H5/2) special case of ΔJ¼0, 71, this breakdown of selection rule could be due to hybridisation of rare earth 4f state with the nearest neighbour shell [17,18]. The intensity ratio (R) between ED and MD transitions gives the asymmetry of the
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Fig. 3. (a) Excitation spectrum of 1 mol% Sm3 þ : Li2O–LiF–B2O3–CdO glass monitored at λemi ¼ 602 nm. (b) Emission spectra of (0.1, 0.5, 1.0, and 1.5 mol%) Sm3 þ : Li2O–LiF–B2O3–CdO glasses excited at λexci ¼ 403 nm (inset figure shows the variation of emission intensity as function of Sm3 þ concentration).
local environment around trivalent rare earth ions in the glass matrix. The intensity ratios are mostly affected by the charge differences and polarizabilities of the ligand atoms in the axial and equatorial positions even though their co-ordination geometries are similar. The higher is the intensity of the ED transition greater is the asymmetric nature. In the present work, 4G5/2-6H9/2 (ED) transition is less intense compared to 4G5/2-6H5/2 (MD) transition suggesting the symmetric nature of Sm3 þ in the host glass. 4.4. Photoluminescence of Dy3 þ : Li2O–LiF–B2O3–CdO glasses Fig. 4(a) and (b) presents the excitation and emission spectra of Dy3 þ : LFBCd glasses for various concentrations (0.1, 0.5, 1, and 1.5 mol%) (inset figure in Fig. 4(b) shows the variation emission intensity as a function of Dy3 þ concentration). The excitation spectra consists of 6 bands at 325 nm, 350 nm, 365 nm, 387 nm, 425 nm, and 452 nm attributed to the transitions from 6H15/2 to 4 P3/2, 4I15/2, 4I11/2, 4I13/2, 4G11/2 and 4I15/2 of Dy3 þ .Of these transitions 6H15/2-6I13/2 (387 nm) is more prominent, it is therefore used for the measurement of emission spectra of Dy3 þ : LFBCd glass. Dy3 þ ions are excited to (4f85d) upper energy level under an excitation with 387 nm and from where these excited ions cascade rapidly towards 4F9/2 state through 4G11/2, 4I15/2 levels and then
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finally relaxes non-radiatively by populating 4F9/2 metastable state. The non-radiative decay is very fast because of closely spaced 4f9 levels between 4F9/2 and 4f85d levels. On reaching 4F9/2 level these unstable ions relax radiatively by emitting fluorescence to the nearest lower lying multiplet 6HJ (J¼ 15/2, 13/2, and 11/2) energy level. The fluorescence spectrum exhibited two main peaks at 486 nm, 577 nm and with a weak band at 665 nm assigned to the transitions from 4F9/2 to 6H15/2, 6H13/2, and 6H11/2. The intensity of 4 F9/2-6H15/2 (486 nm; Blue) emission is stronger than that of 4 F9/2-6H13/2 (577 nm; Yellow) and 4F9/2-6H11/2 (665 nm; Red) emissions. The blue emission at 4F9/2-6H15/2 is a magnetic dipole
(ΔJ ¼0, 71 but 020 is forbidden) transition which hardly varies with the host glass environment around Dy3 þ ion. The yellow emission due to 4F9/2-6H13/2 transition is a forced electric dipole transition (hypersensitive) with the selection rule ΔJ ¼ 72 and it is strongly influenced by the crystal field strength around the rare earth ion and red emission at 665 nm assigned to 4F9/2-6H11/2 transition is an electric dipole allowed. When Dy3 þ occupies a non-inversion symmetry site (low local symmetry site) yellow (6H13/2; ED) emission is more dominant and if it occupies an inversion symmetry site (high symmetry local site) blue (6H15/2; MD) emission is more dominant in the emission spectrum [19]. In the present work, 4F9/2-6H13/2 (ED) transition is less intense compared to 4F9/2-6H15/2 (MD) transition suggesting the symmetric nature of Dy3 þ in the host glass. The intensity ratio of yellow/blue emission can be used to analyse the distortion around the Dy3 þ ion in the glass matrices. The Sm3 þ and Dy3 þ excitation and emission bands are assigned based on the earlier reports. The electric/magnetic transitions pertaining to Sm3 þ and Dy3 þ are given in Table 1. 4.5. Activator ions (Sm3 þ , Dy3 þ ) concentration quenching From Figs. 3(b) and 4(b), it is observed that variation of the dopant ions (Sm3 þ , Dy3 þ ) concentrations have affected the luminescence intensity in the host glass. Emission intensity increases in concentrations up to 1.0 mol% and thereafter intensity quenches further for higher concentration. This suggests that at lower concentrations the interaction between ions is negligible, i.e., until 1.0 mol% and for higher concentration the distance between ions decreases and they commence interacting with each other due to enhanced non-radiative coupling between ions resulting in quenching of luminescence intensity. Migration of excitation energy takes from one ion to another ion of the same species in a random walk manner and finally transfers to acceptor ions leading to concentration quenching. Various interactions have also been reported to be responsible for the ion–ion relaxations in (Sm3 þ , Dy3 þ ) that lead to quenching of fluorescence. Dipole–dipole interactions are considered to be more responsible for the self quenching. 4.6. Fluorescence decay analysis and cross-relaxation mechanism The mono- and bi-exponential decay curve fittings were used in order to compute the lifetime for single doped Sm3 þ and Dy3 þ in the glass matrix. Ι ¼ Ι 0 expð t=τÞ
ðmonoexponential decayÞ
I ¼ A1 expð t=τ1 Þ þ A2 expð t=τ2 Þ
Fig. 4. (a) Excitation spectrum of 1 mol% Dy3 þ : Li2O–LiF–B2O3–CdO glass monitored at λemi ¼577 nm. (b) Emission spectra of (0.1, 0.5, 1.0, and 1.5 mol%) Dy3 þ : Li2O–LiF–B2O3–CdO glasses excited at λexci ¼ 387 nm (inset figure shows the variation of emission intensity as function of Dy3 þ concentration).
ðbiexponential decayÞ
ð1Þ ð2Þ
where I0 is the initial emission intensity at t ¼0,τ is the time required for the intensity to decay to 1/e of initial lifetime value (37% of I0) in the case of mono-exponential decay equation and for bi-exponential decay Eq. (1) is the photoluminescence intensity at any time ‘t’ after switching off the excitation illumination, τ1 and τ2 are the slow and fast decay components (long and short lifetimes),
Table 1 Electric/magnetic dipole transitions related to Sm3 þ and Dy3 þ ions in LFBCd glass. Ion
Excitation transition
Emission transitions
Peak position (nm)
Assigned to
Sm3 þ
6
4
G5/2-6H5/2 H7/2 6 H9/2 6 H11/2
565 601 647 709
Forbidden magnetic dipole transition (ΔJ¼ 0) Partially MD and partially ED allowed (ΔJ¼ 7 1) Electric dipole allowed transition (ΔJ¼ 7 2) Forbidden transition (ΔJ¼ 7 3)
4
486 577 665
Magnetic dipole (ΔJ ¼0, 7 1) Electric dipole (ΔJ ¼ 72) Electric dipole (ΔJ ¼ 72)
H5/2-4G5/2 (403 nm)
6
Dy3 þ
6
H15/2-4F9/2 (387 nm)
F9/2-6H15/2 H13/2 6 H11/2 6
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A1, A2 are the fitting (weighing factors) parameters. Using the above two lifetime values (τ1 and τ2), the average lifetimes o τ4 have been calculated for Sm3 þ and Dy3 þ emission states from: oτ 4 ¼
A1 τ21 þ A2 τ22 A1 τ1 þ A2 τ2
ð3Þ
The lifetimes for the decay curves of (Sm3þ , Dy3þ ): LFBCd glasses are shown in Fig. 5(a) and (b). The emission decay curves for Sm3 þ doped glasses are obtained from the 4G5/2 state by monitoring the 4 G5/2-6H7/2 emission line of Sm3þ , whereas for Dy3 þ doped glasses, from 4F9/2 state by monitoring the 4F9/2-6H15/2 emission line of the Dy3þ in the glass matrices. The lifetime decay curve for Sm3þ for 0.1 mol% is fitted by a single exponential function and with increase in concentration from 0.5 to 1.5 mol% the decay curve nature has deviated from single-exponential to double-exponential. The lifetimes are found to be 1.78 ms, 1.43 ms, 1.29 ms and 0.86 ms. In the case of Dy3þ doped glasses, decay curves are fitted with bi-exponential fitting for all concentrations (0.1–1.5 mol%). The emission decay lifetime values are computed to be 0.88 ms, 0.64 ms, 0.57 ms, and 0.39 ms for Dy3þ doped LFBCd glasses. The nature of emission decay curves for (Sm3 þ , Dy3 þ ) is observed to be non-exponential for higher concentrations. At lower concentrations the interaction between ions are negligible but at higher concentrations the distance between optically
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active ions (donors and acceptors) decreases resulting in an energy transfer between same ions (Sm3 þ 2Sm3 þ /Dy3þ 2Dy3 þ ) revealing that more than one relaxation process may occur. The decrease in emission decay lifetimes with increase in concentration is explained based on the energy transfer through cross-relaxations in the present glasses. The cross-relaxation channels responsible for non-exponential nature of the decay curves and luminescence quenching are explained from the partial energy level diagrams that are separately shown in Fig. 6(a) and (b). Generally, cross-relaxation occurs between two neighbouring (similar/dissimilar) and closely spaced rare earth ions, whose energy levels are separated by same amount. The excitation energy from an ion decaying from a highly excited state promotes a nearby ion from the ground state to metastable state from where both the ions relax to ground state by means of multiphonon relaxation. Two important mechanisms are mainly considered to be responsible for such luminescence quenching and shorter lifetimes at higher concentrations, firstly cross relaxation through energy transfer between neighbouring rare earth ions i.e., from Sm3þ (donor) to Sm3 þ (acceptor) ions (or Dy3 þ to Dy3 þ ), secondly migration of excitation energy. In Sm3þ doped glasses, the gap between energy levels is close, during energy transfer from Sm3þ ions in 4G5/2 excited state to nearby lower lying ground state 6H5/2, the first ion is left in the intermediate 6F5/2 (1381 nm) level, the second ion is left in 6F7/2 (1234 nm), third one at 6F9/2 (1081 nm) and the fourth one at 6F11/2 (947 nm) which are in resonance or closely matched with the ions promoted by absorption from the ground state to metastable states 6H5/2 (6FJ, J¼11/2, 9/2, 7/2, and 5/2) respectively, later on these ions decay non-radiatively to ground state. The energy transfers through cross relaxation channels are as given below [20,21]: Donor emission (Sm3 þ ) F11/2 (7383 cm 1) F9/2 (8700 cm 1) 6 F7/2 (9900 cm 1) 6 F5/2 (10,800 cm 1)
4
6
4
6
G5/2, G5/2, 4 G5/2, 4 G5/2,
Acceptor absorption (Sm3 þ ) -
F5/2 (7294 cm 1) F7/2 (8668 cm 1) 6 F9/2 (9764 cm 1) 6 F11/2 (10,481 cm 1)
6
6
6
6
H5/2, H5/2, 6 H5/2, 6 H5/2,
In the case of Dy3 þ doped glasses, energy transfer takes place from the excited 4F9/2 state to nearby ground state of 6H15/2, during this process ions exist in the intermediate states of 6F1/2, 6F3/2, 6F5/2 and 6 F11/2 þ 6H9/2, 6F9/2 þ 6H11/2 which are in resonance (matching of their energies), and finally de-excite to the ground state 6H15/2 non-radiatively. The cross-relaxation channels due to nonradiative energy transfer are [22,23] Donor emission (Dy3 þ ) 4
F9/2, 6F9/2 þ 6H7/2 (12057 cm 1) 4 F9/2, 6F11/2 þ 6H9/2 (13408 cm 1) 4 F9/2, 6F1/2 (7584 cm 1)
Fig. 5. (a) Emission decay curves of (0.1, 0.5, 1.0, and 1.5 mol%) Sm3 þ : Li2O–LiF– B2O3–CdO glasses at an excitation of 403 nm for λemi ¼ 602. (b) Emission decay curves of (0.1, 0.5, 1.0, and 1.5 mol%) Dy3 þ : Li2O–LiF–B2O3–CdO glasses at an excitation of 387 nm for λemi ¼577 nm.
Acceptor absorption (Dy3 þ ) -
6
H15/2, 6F5/2 (12390 cm 1)
-
6
H15/2, 6F3/2 (13172 cm 1)
-
6
H15/2, 6F11/2 þ 6H9/2 (7690 cm 1)
As the energy separation between intermediate energy levels for Sm3 þ , Dy3 þ ions are small; the donor emission and acceptor absorption energy levels are in resonance with a small energy difference which could be balanced by host phonon energy. Therefore energy transfer can occur by means of dipole–dipole/ quadrupole interactions resulting in a decrease in emission peak intensities of all transitions due to depopulation of 4F9/2 and 4G5/2 of Sm3 þ , Dy3 þ . The latter mechanism is connected to the migration of excitation energy, which can accelerate the decay of (Sm3 þ , Dy3 þ ) ions by an energy transfer to the structural defects acting as
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Fig. 6. Energy level schemes of (a) Sm3 þ and (b) Dy3 þ ions in Li2O–LiF–B2O3–CdO glasses.
Fig. 7. Spectral overlap of Dy3 þ (sensitiser) emission and Sm3 þ (acceptor) absorption in Li2O–LiF–B2O3–CdO glasses. (Inset figure shows the partial energy level diagram showing energy transfer process in co-doped (Sm3 þ þDy3 þ ) ions.)
energy sinks along these two factors presence of OH also plays an important role in quenching the excited lifetimes. 4.7. Energy transfer between Dy3 þ and Sm3 þ ions in Li2O–LiF–B2O3–CdO glass The phenomenon of excitation energy transfer among rare earth ions take place from ion with high fluorescent states to low lying fluorescent states (excited state of the first ion above the fluorescent state of the second ion). Based on Dieke’s energy level diagram, energy transfer could proceed from Dy3 þ to Sm3 þ (as Dy3 þ fluorescence state or excited state lies above the fluorescence state of Sm3þ ). In the present study, based on the results obtained from the luminescence intensities of singly doped rare earth ions, the optimised concentrations of ((1.0 mol%) Sm3þ and (1.0 mol%) Dy3 þ ) are taken together to evaluate the possibility of energy transfer between them. The primary criterion for energy transfer is the spectral overlap of Dy3 þ
fluorescence and Sm3þ absorption in the glass studied as shown in Fig. 7 [24,25]. From the figure it is observed that the Dy3þ emission band (487 nm) and absorption band of Sm3 þ (472 nm) overlap at 480 nm in the visible region of 450–500 nm. This gives a confirmation that Dy3þ acts as a donor (sensitiser) and Sm3 þ acts as an acceptor (activator) and further partial energy transfer takes place from Dy3 þ to Sm3þ . The radiative/non-radiative transfer of energy also depends on the distance between sensitiser and activator ions, therefore the average distance (R) between Dy3 þ and Sm3 þ found to vary from 14.1 Å to 9.1 Å which is sufficient enough for energy transfer to take place. The process of energy transfer occurring from Dy3þ to Sm3 þ is further explained from the photoluminescence spectra, energy level diagram and decay lifetimes of the donor. In Fig. 8(a) and (b), the emission spectra of together doped (Sm3 þ þDy3 þ ): LFBCd glasses with fixed Dy3 þ concentration (1.0 mol%) and varied Sm3 þ concentration (0.5–1.5 mol%) at 387 nm and 403 nm excitation wavelengths are shown. Fig. 8(a) shows the
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emission spectra of Dy3 þ þSm3 þ together doped LFBCd glasses, under near UV excitation wavelength (λexci ¼ 387 nm) of Dy3 þ has promoted the unexcited ions of Sm3 þ to higher excited levels and in turn resulted in a pronounced energy transfer from Dy3 þ to Sm3 þ . The luminescence spectra exhibited emission bands (4G5/2-6H5/2, 6 H7/2, 6H9/2, 6H11/2) of Sm3 þ besides (4F9/2-6H15/2, 4F9/2-6H13/2) Dy3 þ emission bands. The 4F9/2-6H13/2 emission transition at 577 nm attributed to Dy3 þ and 4G5/2-6H5/2 emission transition of Sm3 þ at 565 nm are merged together and evolved as a single peak at 567 nm in the (Sm3 þ þDy3 þ ) together doped glass attributed to both Sm3 þ and Dy3 þ emission bands. From Fig. 7(a) it is also noticed that, reddish-orange emission intensity of Sm3 þ has been enhanced but on the other hand emission due to Dy3 þ has been decreased gradually with an increase in the Sm3 þ concentration. The considerable reduction in Dy3 þ emission intensity with an increase in Sm3 þ concentration is attributed to non-radiative energy transfer from Dy3 þ to Sm3 þ . This decrease in Dy3 þ emission could be due to availability of more Sm3 þ ions per Dy3 þ ion for accepting its excitation energy. This result has been supported by exciting the codoped (Sm3 þ þDy3 þ ): LFBCd glasses at 403 nm for the same concentrations shown in Fig. 8(b). From the two (Fig. 8(a) and (b)) it can be understood that Dy3 þ emission intensity has been reduced with the Sm3 þ incorporation could be due to partial energy transfer
from Dy3 þ to Sm3 þ ions [26–29]. The proposed path for the energy transfer to occur could be 4I13/2 of Dy3 þ to 4F5/2 state of Sm3 þ . At an excitation wavelength 387 nm, the Dy3 þ ions are excited to a higher energy state (4I13/2) and these ions non-radiatively relaxes to 4F9/2 state from where they relax to (6H15/2) ground state by emitting radiations. These emitted radiations of Dy3 þ ions are absorbed by Sm3 þ ions in the ground state to get excited to higher energy states and then relaxes to 4F5/2 state, further they cascade non-radiatively to intermediate states 4I13/2, 4F3/2 through cross-relaxations and therefore finally decays radiatively to 6H5/2 ground state by depopulating 4 G5/2 state with the reddish-orange emission. The energy level diagram for together doped ions is shown in Fig. 7. In the present case, the role of host in energy transfer cannot be completely ruled out, because here the energy states are not resonant with each other, therefore this mismatching of the energy difference between the states is bridged by lattice phonons in the host for energy migration to be carried out. Fig. 9 represents the lifetime of the emission decay curves for together (Sm3 þ þDy3 þ ) doped LFBCd glass at (λemi ¼577 nm) under the excitation of 387 nm. The energy transfer from Dy3 þ to Sm3 þ is further studied from lifetime measurements fitted by a double exponential function given in Eq. (3). From Eqs. (2) and (3) the average lifetime oτ 4 of donor is evaluated to be 0.472 ms, 0.396 ms, 0.234 ms and 0.179 ms respectively. The average lifetime values of the donor (Dy3 þ ) of the together (Dy3 þ þSm3 þ ) doped LFBCd glasses with fixed Dy3 þ (1 mol%) and varied Sm3 þ concentration seems to be reduced by increasing Sm3 þ ion concentration when compared to the lifetime of LFBCd glass with 1 mol% Dy3 þ (0.57 ms) monitored at 577 nm. This shortened lifetime of Dy3 þ in co-doped glass compared to single ion doped glass suggests energy transfer from Dy3 þ to Sm3 þ in the host. From double exponential fitting, the decay time curve for donor appears to be non-exponential due to non-radiative decay in the sample because when the donor ions are excited in the presence of acceptors, at the initial stages the decay will be faster since the donor ions closer to acceptors decay first and donors that are at farther distance to acceptors transfer their excited energy for a long time and then finally decays slowly with its own lifetime suggesting non-exponential nature in the decay curves. From the lifetime measurement, for the together (Sm3 þ , Dy3 þ ) doped glass, the quantum efficiency and energy transfer rate from Dy3 þ to Sm3 þ at this concentration is calculated. The quantum efficiency or energy transfer efficiency from Dy3 þ to Sm3 þ can be
Fig. 8. (a) Emission spectra of co-doped (1.0Dy3 þ þ xSm3 þ ): Li2O–LiF–B2O3–CdO glasses at λexci ¼ 387 nm (where x¼ 0.1, 0.5, 1.0, and 1.5 mol%). (b) Emission spectra of co-doped (1.0Dy3 þ þ xSm3 þ ): Li2O–LiF–B2O3–CdO glass at λexci ¼ 403 nm (where x ¼0.1, 0.5, 1.0, and 1.5 mol%).
Fig. 9. Emission decay curves for together (Sm3 þ þ Dy3 þ ) doped Li2O–LiF–B2O3– CdO glass at an excitation 387 nm for (λemi ¼ 577 nm) emission of Dy3 þ (inset figure shows the I–H curve fit for 1:1 mol% of co-doped (Dy3 þ þ Sm3 þ ): Li2O–LiF–B2O3– CdO glass).
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computed from the equation shown below [30]: ηðDy-SmÞ ¼ 1 ðτd =τdo Þ
ð4Þ
The probability of energy transfer in terms of lifetime is expressed as [29]: P ðDy-SmÞ ¼
1 1 τd τdo
ð5Þ
Where τdo is the intrinsic decay time of donor (Dy3 þ ) in the absence of activator and τd is the lifetime of the donor in the presence of the acceptor (Sm3 þ ). Inokuti and Hirayama model has been employed for the emission decay curve of both (Sm3 þ þ Dy3 þ ) ions together doped glass in 1:1 mol% to analyse the type of multipole (ion–ion) interactions responsible for the energy transfer at λexci ¼387 nm [29,31]. I–H decay curve for Dy3 þ þ Sm3 þ co-doped in equal concentration i.e., (1:1 mol%) fitted from experimental and calculated values of S¼6 and 8 are shown in the inset of Fig. 9. IðtÞ ¼ I 0 exp½ ðt=τ0 Þ Q ðt=τ0 Þ3=S
ð11Þ
Where Q¼
4π 3 Γ 1 N a R30 3 S
R0 ¼
Q ð4π=3Þ Γð1 ð3=SÞ N a
ð12Þ 1=3
1 A
ð13Þ
I(t) is the luminescence intensity of the band at time t, τ0 is the radiative lifetime of the excited ion, Na is the acceptor concentration (rare earth ion concentration), Γ(1 3/S) is Euler’s gamma function, Q is the I–H fitting parameter obtained from the decay fit of the curve, S is the multipole interaction parameter. Based on the type of interactions Γ(13/S) values are given as 1.77 for dipole–dipole type (S¼6), 1.43 for dipole–quadrupole type (S¼8), and 1.30 for quadrupole– quadrupole type (S¼10). R0 is the critical transfer distance calculated from the fitting parameter and the gamma function, defined as the rate of energy transfer to acceptor is equal to the inverse of lifetime τDy. The coupling constant for donor–acceptor (CDA) is given by the expression below: C DA ¼
ðR0 Þ τDy
Table 2 Donor (Dy3 þ ) ion concentration (Nd), acceptor (Sm3 þ ) ion concentration (Na), average distance between donor and acceptor (R). Donor (Dy3 þ ) ion emission lifetimes (τ) for (Dy3 þ þ Sm3 þ ) co-doped glasses monitored at 577 nm under 387 nm excitation, energy transfer efficiency (η), energy transfer probability (Pda). Nd ( 1019 ions/ cm3)
Na ( 1019 ions/ cm3)
R¼ (Nd þ Na) 1/ 3 Å71
τ (ms) 7 0.1
η (%) 71
Pda ( 103 S 1) 7 0.01
32.19
3.29 12.5 34.5 56.4
14.1 13.0 11.4 9.1
0.47 0.39 0.23 0.17
17.5 31.5 59.6 70.1
0.373 0.809 2.593 4.127
Ion concentration N ¼ dsamplemNA/M, where dsample is sample density, m is the molar concentration of the rare earth ion, NA is Avogadro constant, and M is the molar mass.
due to a non-radiative energy transfer to Sm3 þ . The quenching behaviour at higher concentration of Sm3 þ could be attributed to non-radiative decay of two ions to the ground state within the dopant ions. This has been supported by the reduced lifetimes of donor (Dy3 þ ) in co-doped (Dy3 þ and Sm3 þ ) glasses with increasing of the Sm3 þ concentration compared with singly doped (1 mol% Dy3 þ ) in the LFBCd glass matrix monitored under 577 nm. The energy transfer results have demonstrated that an efficient nonradiative energy transfer from Dy3 þ to Sm3 þ could be governed by dipole–quadrupole interaction which is in agreement with I–H luminescence decay curve analysis. Based on the thus obtained results, it could be suggested that these dual ions doped glasses as quite useful multi-colour emitting glassy materials.
Acknowledgement One of us (VN) would like to thank the UGC, New Delhi for the award of a BSR fellowship to him to carry out the present research work at the University. References
6
ð14Þ
From the I–H emission decay curve fit shown in the inset of Fig. 9, it has been observed that S¼8 fits the data best with experimental values (solid line bears the fitting value) in Eq. (12), indicating energy transfer is due to dipole–quadrupole interaction. An exchange interaction plays a major role when average distance between sensitiser and activator ions is less than or equal to 5 Å but in our present study it can be ruled out as the distance between the ions is more. The values of τDy þ Sm ¼0.263, Q¼ 1.1, R0 ¼9.5 Å, and CDA ¼ 2.46 10 42 cm6/s. Therefore from I–H luminescence decay curve fitting, the mechanism existing between donor (Dy3 þ ) and acceptor (Sm3 þ ) is confirmed to be dipole–quadrupole mechanism (Table 2).
5. Conclusion In summary, it is concluded that LFBCd optical glasses doped with Sm3 þ , Dy3 þ ions individually and also together (Sm3 þ þDy3 þ ) have successfully been prepared and analysed. Optical glasses containing Sm3 þ and Dy3 þ ions separately have exhibited prominent orange and blue/yellow emissions respectively, with their lifetimes decreasing eventually due to depopulation of 4G5/2 (Sm3 þ ) and 4F9/2 (Dy3 þ ) states with increase in the dopant ion concentration. Addition of Sm3 þ in four different concentrations (0.1–1.5 mol%) to the 1 mol% of Dy3 þ doped glasses have exhibited a strong decrease in its emission
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