Spectroscopic and Photoluminescence Properties of Dy3+ ions in

[email protected] Current Physical Chemistry, 2017, 7, 000-000

1

RESEARCHARTICLE

Spectroscopic and Photoluminescence Properties of Dy3+ ions in Different Alkaline-Earth Oxide Modified Lead Fluoroborate Glasses G. Anjaiah1, T. Sasikala2 and P. Kistaiah1,* 1

Department of Physics, Osmania University, Hyderabad-500007, Telangana, India and 2Department of Physics, Sri Venkateswara University, Tirupati, A.P., India

ARTICLEHISTORY Received: December 23, 2016 Revised: February 07, 2017 Accepted: April 03, 2017 DOI: 10.2174/1877946807666170420110827

Abstract: A new series of Dy3+ ions doped lead fluoroborate glasses containing different alkaline earth oxides of composition 20PbF2-10Li2O-5RO-5ZnO-59B2O3-1Dy2O3 (R = Ba, Sr and Ca; abridged as PLRZFB: Dy) were prepared by conventional melt quenching method. The non-crystalline nature and the glass transition temperatures (T g) of the prepared glass samples were confirmed through XRD studies and DSC analysis respectively. The EDS analysis was performed to know the elements present in the prepared glass compositions. The Judd-Ofelt (J-O) intensity parameters determined from the absorption spectra were used to investigate the symmetry orientation of the Dy- ligand field environment. Various radiative parameters were determined using J-O parameters for different Dy3+ emission transitions. Photoluminescence spectra exhibit emission bands in the blue, yellow and red regions. Stimulated emission cross section ( 𝜎𝑃𝐸 ) branching ratio (βexp), yellow to blue intensity (Y/B) ratio and Dy - O bond covalence were estimated from the luminescence spectra. The nature of the decay curves of 4F9/2 level of Dy3+ ions was analyzed. Finally the effect of different alkaline earth metal oxide modifiers on the luminescence properties of PLZFB: Dy glass system was analyzed. Among the prepared glasses, PLSrZFB: Dy glass possesses higher values of AR, βr, 𝜎𝑃𝐸 and η indicating its usefulness for the development of optical, display and LED devices in the visible region.

Keywords: Lead fluoroborate glass, Alkaline earth oxide, Optical absorption, Photoluminescence, Rareearth ion. 1. INTRODUCTION In solid state lighting and lasing device fabrication technology rare earth (RE) ions are superior activators due to their characteristic 4f 4f or 5d – 4f emission transitions. Due to the shielding of outermost electrons of the rare-earth ions, sharp absorption and emission transitions are observed which are useful in the solid state laser field [1, 2]. The rare earth ions play an important role in the field of photonics and optoelectronics [3, 4]. Among different rare earth ions, the Dy3+ ion is an attractive laser ion with optical transitions in the visible as well as infrared region due to its *Address correspondence to this author at the Department of Physics, Osmania University, Hyderabad-500007, Telangana, India; E-mail: [email protected]

1877-9468/17 $ 58.00+.00

intricate energy level structure. Many researchers studied the emission properties of Dy3+ ions in different glass hosts and found that the selection of proper host glass composition is important for efficient luminescence of Dy3+ ion. Oxide glasses are well suited hosts for obtaining efficient luminescence of rare earth ions [5]. Among different glass formers, borate glass is one of the conventional glass formers due to its high transparency and thermal stability, low melting point and good RE ion solubility [6]. Borate glasses are structurally more intricate as compared to silicate or phosphate glasses due to two types of coordination of boron atoms with oxygens (3 and 4) and the structure of vitreous B2O3 consists of a random network of boroxyl rings and BO3 triangles connected by B-O-B linkages. Pure borate glass has lower stability and devitrification ©2017 Bentham Science Publishers

2

Current Physical Chemistry, 2017, Vol.7, No. 2

tendency. Its main drawback is the reduction in the rare earth ion emission due to its high phonon energy [7]. However the addition of PbF2 to borate glass changes its topological nature and decreases the phonon energy and it also reduces the nonradiative loss due to multi phonon relaxation [8 10]. The addition of a modifier oxide causes a progressive change of some BO3 triangles to BO4 tetrahedra and this leads to the formation of various cyclic stable units like diborate, triborate and tetraborate groups [11]. ZnO participates as a modifier in the glass network and is expected to shorten the time taken for solidification of glass during quenching process and increase its optical quality [12]. The addition of alkali and alkaline metal oxides act as network modifiers and converts the glass into more stable form by forming non-bridging oxygens [13]. Particularly, as important as the appropriate host materials, the doped rare-earth ions are also the key components for the laser materials. Trivalent dysprosium ion (Dy3+) is attracting much attention because it usually exhibits two main intense emission bands in the blue (460 – 510 nm) and yellow (560 – 590 nm) regions which are assigned to the 4F9/2 → 6H15/2 magnetic dipole (MD) transition and the 4F9/2 → 6H13/2 electric dipole (ED) transition, respectively. A feeble red emission band is also observed in the wavelength region 660 - 670 nm corresponding to the 4F9/2 → 6 H11/2 transition of Dy3+ ions [14- 16]. The yellow to blue (Y/B) intensity ratio indicates the information about symmetry orientations around the Dy3+ ion site. Hence Dy3+ ions doped materials find attractive blue and yellow light sources for visible solid state laser applications [17]. Optical methods like UV-visible and infrared absorption spectroscopy can give the average coordination number, bond lengths, local symmetry or covalence of bonds between ion and the first shell neighbors. Recently there has been a considerable interest in the fabrication of Dy3+ doped glasses for white light emitting diodes [5]. However, to the best of our knowledge, there is no detailed analysis from the available literature, on the structural and luminescence behavior on Dy3+ doped fluoroborate glasses modified by the alkaline earth metal oxides. The heavy metal lead fluoroborate glass systems are chosen in this work to incorporate the Dy3+ ions because they have good glass forming region, a high refractive index, good physical and chemical stability and are easily

Anjaiah et al.

prepared in different shapes and sizes. The present work reports a comparative study of the spectroscopic and luminescence properties of Dy3+ doped lead fluoroborate glasses modified with alkaline earth metal oxides and to identify a suitable glass host for visible laser applications. The Y/B ratios have been calculated and compared with those reported in other Dy3+ doped glass matrices. The Dy3+ doped multi-component glasses developed in the present work are expected to be useful in designing new optical devices in the visible region. 2. EXPERIMENTAL PROCEDURE 2.1. Materials and Synthesis The lead fluoroborate glasses doped with Dy3+ ions of the chemical composition (all in mol %) 20PbF2-10Li2O-5RO-5ZnO-59B2O3-1Dy2O3 (R = Ca, Sr and Ba; PLRZFB:Dy) were prepared using a conventional melt-quenching method. For the preparation of different glass compositions, high purity chemicals (Sigma Aldrich); H3BO3, PbF2, Li2CO3, CaO, SrO, BaO, ZnO and Dy2O3 were used. About 15g of batch compositions were ground in an agate mortar to obtain homogenize the mixture. The mixture was melted in an electric furnace at 950 oC in porcelain crucible. The melt was then poured into pre-heated cylindrical brass split mold to get flat shaped 0.3 cm thick samples. These glasses thus obtained were annealed at 350 o C for 6h before cooled down to room temperature to remove thermal strains in the glasses. The glass samples were grounded and optically polished to have suitable dimensions for measuring their physical and optical properties. 2.2. Characterization Methods The amorphous nature of these glasses has been tested through XRD profiles which are recorded using Bruker D8 advanced powder X-ray diffractometer. The SEM with EDS spectra were recorded using Carl Zeiss EVO-MA 15 scanning electron microscope. The DSC curves of powdered glasses were recorded in the temperature range 50 – 550 0C using TA Instrument, DSC2910. All the samples were heated at the rate of 10 0C / min. The absorption spectra were recorded with 0.1 nm resolution using JASCO V-670 double beam UVVis-NIR spectrometer in the wavelength range 200 - 2400 nm. The excitation, emission and decay

Spectroscopic and Photoluminescence Properties of Dy3+ ions

measurements were carried out using Horiba Jobin Yvon Fluorolog-3 spectrofluorimeter with 0.2 nm and 0.01 ms resolutions respectively and xenon flash lamp as light source. All the measurements were carried out at room temperature. The Abbe refractometer was used to obtain the refractive index (n) of all the glass samples at sodium wavelength using 1-bromonapthalene as a contact liquid. The density (d) of the glass samples was measured at room temperature using Archimedes’ principle and Xylene as an immersive liquid. From the measured values of refractive index, sample thickness and density, the rare-earth ion concentration was estimated. The various measured and calculated physical properties of PLRZFB: Dy glasses are presented in Table 1. 3. RESULTS AND DISCUSSION 3.1. X-ray Diffraction and Energy Dispersive Spectral Analysis Fig. (1) shows the results of X-ray diffraction (XRD) patterns for PLRZFB:Dy glasses as a function of 2Ɵ. It can be seen that there are no sharp peaks present in the spectra. The broad diffuse scattering at lower angles ( 10 o  2  80 o ) indicates long range disorder of the prepared glasses. Hence, the overall feature of the X-ray patterns confirms the amorphous nature of the

Current Physical Chemistry, 2017, Vol. 7, No. 2 3

prepared glasses under investigation. As a representative of the prepared glass compositions, the energy dispersive spectrum (EDS) of PLCaZFB:Dy glass is shown in Fig. (2), which exhibits the presence of B, O, F, Zn, Ca, Pb and Dy ions in the prepared glasses. Similar evolution was observed for all the glass samples, though the rest are not shown. The micro structure of this glass was studied from its SEM picture (not shown). There is no nucleation part in the SEM image even at a maximum magnification. This indicates that the prepared glass possesses high homogeneity without cluster formation, absence of cracks and unmelted portion. 3.2. Differential Scanning Calorimetric Analysis The DSC curves of Dy3+ ions in PLRZFB glasses scanned at the rate of 10 oC / min in the temperature range between 200 - 550 oC are shown in Fig. (3). In the present study the endothermic humps of DSC curves are used to determine the glass transition temperature (Tg) and the observed values are 442 0C, 450 0C and 456 0C for Ba, Sr and Ca glasses respectively. The variation of Tg value in PLRZFB:Dy glasses are affected by two factors; (1) metal-oxygen (R-O) bond strength and (2) cross linked density of borate network [18]. The cross linked density and R - O bond strength may increase from Ba to Ca oxide based glass due to the progressive decrease

Table 1. Physical properties of Dy3+ ions doped PLRZFB glasses. Physical Parameter

Ca

Sr

Ba

Sample thickness, l (cm)

0.222

0.215

0.211

Refractive index, n

1.654

1.655

1.656

cm3)

3.695

3.878

3.819

Rare-earth ion concentration (mol / lit)

0.258

0.272

0.263

Rare-earth ion concentration

1.555

1.638

1.586

Avg. molecular weight,M (g)

139.88

142.34

144.82

Dielectric constant, ε

2.735

2.739

2.742

Molar volume, V m (cm3/ mol)

38.65

36.70

37.92

Polaron radius, Rp (Å)

7.29

7.36

7.44

Interionic distance, Ri (Å)

18.59

18.27

18.47

5.34

5.53

5.41

456

450

442

Density, d (g /

(x1020 ions / cm3)

Field strength, Fi (x1014 cm-2) Glass transition temperature, Tg

(oC)

4


in their ionic radius (rCa < rSr < rBa). This causes an increase in the network rigidity resulting in the increase of Tg value from Ba to Ca oxide based PLZFB: Dy glass matrix.

Fig. (1). X-ray diffraction patterns of Dy3+ ions doped PLRZFB glasses.

Anjaiah et al.

3.3. Optical Absorption Spectra and Bonding Parameters The optical absorption spectra of the PLRZFB: Dy glasses recorded in the visible and near infrared (NIR) region are shown in Fig. (4). The absorption bands in the higher energy side (25,000 cm-1) of the spectra could not be observed and few bands observed are too weak in intensity. This may be due to the overlapping of different 2S+1LJ levels and absorption of the host glass matrix. Due to the presence of heavy metal (Pb) in the prepared borate glasses the fundamental absorption edges considerably shift towards longer wavelength side compared to the alkali borate [3], fluoroborate [19], phosphate [1, 2, 17] and silicate [20] glasses. The absorption spectra of these glasses in near infrared (NIR) range exhibit several absorption bands and they are similar to those reported earlier [1-7] except a few additional bands which are due to change in the host glass matrix. The observed bands are due to f - f transitions from the 6H15/2 ground state to the various excited 6H11/2, 6 F11/2+6H9/2, 6F9/2+6H7/2, 6F7/2+6H5/2, 6F5/2, 6F3/2, 4 F9/2 and 4I15/2 states. Among the various transitions of Dy3+ ions, the spectral intensity of the 6H15/2 → (6F11/2+6H9/2) transition is found to be sensitive to the environment around the RE ion. Such transitions are called hypersensitive transitions and they follow the selection rules ∆S = 0, ∆L ≤ 2 and ∆J ≤ 2.

Fig. (2). EDS spectrum of Dy3+ ions doped PLCaZFB glass.

Fig. (4). Optical absorption spectra of Dy3+ ions doped PLRZFB glasses Fig. (3). DSC curves of Dy3+ ions doped PLRZFB glasses.

To know the nature of Dy3+- (O-2/ F-) coordination bond, nephelauxetic ratio (β) and bonding parameter (  ) have been calculated and



using the equation given in [22]. The calculated spectral intensities ( f cal ) are evaluated using the Judd-Ofelt theory [23, 24]. The oscillator strengths, f exp and f cal were presented along with root mean square (rms) deviation (δrms) in Table 2. The rms deviation is a measure of difference between the values of experimental and calculated oscillator strengths. The rms deviation of Dy3+ ions in PLRZFB glasses is very small and it indicates a good agreement between the experimental and calculated values. Among different transitions, the 6 H15/2 → (6F11/2+6H9/2) transition has higher spectral intensity in all the present glasses and the strontium glass has the highest oscillator strengths for all the transitions. This result suggests that the local coordination environment of Sr - O around the Dy3+ ions is considerably asymmetric in PLSrZFB: Dy glass compared to other two glass systems.

these are presented in Table 2. The nephelauxetic parameter can be calculated using the expression  

c , where  is the ratio between a

the energy corresponding to the observed band position of bonded RE ion and free RE ion. From the average value of β (referred as  ), the bonding parameter can be calculated using the 1   100 [21]. The metal-ligand formula    bond may be covalent or ionic nature depending upon the positive or negative sign of  . The calculated values of  and  of the prepared glasses are presented in Table 2 and it is observed from table that Dy3+- (O-2/ F-) coordination bond is of ionic nature in the present glass system. Similar ionic nature of this ligand bond has been observed in other Dy3+ doped glasses [11, 17, 20].

Judd-Ofelt intensity parameters, Ωλ (λ = 2, 4, 6) are determined by using least squares fit method as explained in [22]. In general, the J-O parameters provide the information on the nature of bonding between RE3+ ions and the surrounding ligands as well as the symmetry of environment around the RE3+ ions. The values of J-O parameters are used to obtain the relevant optical parameters

3.4. Spectral and J-O Intensity Parameters The experimental spectral intensities ( f exp ) of all the absorption bands of Dy3+ ions doped PLRZFB glass matrices are obtained by measuring the integrated area under the absorption bands

Table 2. Transition assignments, observed band positions (in cm-1), experimental and calculated spectral intensities (f ×10-6), nephelauxetic ratio and bonding parameter of Dy3+ ions doped PLRZFB glasses. S.No

Transition

Energy

from 6H15/2 →

(cm-1)

fexp

fcal

fexp

fcal

fexp

fcal

6H 11/2

5,966

2.42

2.74

2.45

2.66

2.28

2.82

1

6 11/2+ H9/2

Ca

Sr

Ba

2

6F

7,886

11.69

11.65

12.58

12.55

11.75

11.67

3

6F

6 9/2+ H7/2

9,174

4.54

4.65

5.15

5.11

4.32

4.56

4

6F

6 7/2+ H5/2

11,123

5.31

4.95

4.04

4.38

5.41

4.52

12,500

2.86

2.17

3.51

2.08

3.06

2.29

5

6

6

6F

3/2

13,315

0.74

0.41

0.13

0.39

0.80

0.43

7

4F

9/2

21,186

0.17

0.38

0.18

0.34

0.06

0.36

8

4I

15/2

22,172

0.92

0.95

0.54

0.93

0.12

0.99

F5/2

δrms = ±

0.331x10-6

δrms = ±

0.572x10-6

δrms = ± 0.586x10-6

𝛽=1.007

𝛽=1.009

𝛽=1.008

δ= - 0.695

δ= - 0.891

δ= - 0.793

6


Anjaiah et al.

(branching ratios, radiative life times of excited states and induced emission cross-sections) in order to optimize the best configuration of the ionhost to improve the luminescence efficiency of a specific transition. The covalency of the rare earth ion and the the symmetry of the rare earth ion site is related to the parameter  2 which is strongly dependent on short range effect whereas the parameters  4 and  6 are related to the bulk properties of the glass such as rigidity and viscosity [25]. The calculated J-O parameters (  2 ,  4 ,  6 ) and their trend for the present glasses are given in Table 3 along with those reported in other Dy3+ doped glass systems [26-31]. The higher magnitude of  2 parameter in the present glasses indicates the higher degree of covalency of Dy-O bond and higher asymmetry around the Dy3+ ions [25]. In the present investigation, the higher value of  2 for PLSrZFB: Dy glass indicates a stronger covalency and higher asymmetry of Dy-O bond than the other two glasses. 3.5. Emission Covalency

Spectra

and

Dy-O

Bond

The excitation spectra of PLRZFB:Dy glasses at a wavelength 389 nm and by monitoring the emission spectra at an emission wavelength of 485 nm are shown in Fig. (5). The excitation spectra consist of sharp f - f excitation bands corresponding to Dy3+ ions. The excitation spectra exhibit six excitation bands from the 6H15/2 ground state to various excited (4M,4I)15/2, 4P5/2, 4I13/2, 4 G11/2, 4I15/2 and 4F9/2 levels. Among the various transitions, the H15/2 → 4I13/2 (389 nm) transition

possesses higher intensity and is used as an excitation wavelength to obtain emission spectra. After the excitation to the (4K17/2+4I13/2) level, Dy3+ ions relax non-radiatively to the lower metastable 4 F9/2 level and emit intense emissions. The visible emission spectra exhibit two intense bands at 486 nm (blue) and 577 nm (yellow) corresponding to the 4F9/2 → 6H15/2 and the 4F9/2 → 6H13/2 transitions, respectively. Another band is observed at 666 nm (red) with feeble intensity corresponding to the 4 F9/2 → 6H11/2 transition. Further, interestingly a very weak intense emission band is observed at 457 nm corresponding to the (4F7/2) 4I15/2 → 6H15/2 transition [32] and all the emission bands are shown in Fig. (6). Among the three emission transitions, the 4F9/2 → 6H13/2 transition is electric dipole (ED) transition and its intensity is influenced by the environment around the Dy3+ ion. When the Dy3+ ions are located in the higher symmetry site the intensity of the 4F9/2 → 6H13/2 (ED) transition dominates over the 4F9/2 → 6H15/2 (MD) transition. When the Dy3+ ions are located in the lower symmetry site the 4F9/2 → 6H15/2 (MD) transition dominates over the 4F9/2 → 6H13/2 (ED) transition [33]. In the present investigation, the 4 F9/2 → 6H15/2 (MD) blue band is much influenced by the host environment around the Dy3+ ions in calcium oxide based glass an indication of lower symmetry around the Dy3+ ion site. In other two glasses the 4F9/ 2 → 6H13/2 (yellow) ED transitions are stronger than the 4F9/2 → 6H15/2 (blue) MD transitions indicating higher degree of symmetry around Dy3+ ion sites.

Table 3. Comparison of Judd-Ofelt intensity parameters (Ω λ×10-20cm2), their trends and spectroscopic quality factor (Ω4/ Ω6) of Dy3+ ions doped PLRZFB glasses with other reported glass hosts. Glass system

Ω2

Ω4

Ω6

Trend

Ω4/ Ω6

References

PLCaZFB:Dy

11.58

6.17

4.93

Ω2> Ω4 >Ω6

1.25

Present work

PLSrZFB:Dy

12.61

3.57

4.71

Ω2> Ω6 >Ω4

0.75

Present work

PLBaZFB:Dy

12.46

1.95

5.19

Ω2> Ω6 >Ω4

0.37

Present work

SFBiBDy10

46.92

14.60

11.89

Ω2> Ω4 >Ω6

1.22

[27]

PKAZFDy

14.11

3.07

1.95

Ω2> Ω4 >Ω6

1.57

[28]

PKAlCaFDy10

13.30

3.38

2.83

Ω2> Ω4 >Ω6

1.19

[29]

Phosphate

4.62

0.77

0.79

Ω2> Ω6 >Ω4

0.97

[30]

Tellurite

5.66

0.84

2.17

Ω2> Ω6 >Ω4

0.38

[31]

Silicate

2.69

1.64

1.64

Ω2> Ω6 = Ω4

1.00

[26]



Fig. (5). Excitation spectra of PLRZFB:Dy glasses, monitoring the emission at 485 nm.

In general, the ratio of electric dipole transition (ED) to magnetic dipole transition (MD) is a measure of degree of symmetry of the local environment around the RE3+ ion site. Generally, the yellow to blue (4F9/2 → 6H13/2 / 4F9/2→6H15/2) intensity ratio indicates the nearest environment site symmetry and electronegativity of the surrounding ligands [34]. In the present study, the values of (Y/B) intensity ratio are found to be 0.53, 1.18 and 1.15 for Ca, Sr and Ba oxide based glasses, respectively. The variation in the (Y/B) ratio is an indication of the change in the local environment of Dy3+ ions [35].

Fig. (6). Visible emission spectra of PLRZFB:Dy glasses.

3.6. Radiative Properties Radiative properties, such as radiative transition probability (AR), radiative lifetime (  r ), branching ratio (  r ) and peak stimulated emission cross section (  pE ) are calculated for the 4F9/2 → 6H15/2, 3+ ion in 13/2, 11/2 emission transitions of Dy PLRZFB glasses using the formulae given in [4]. The experimental branching ratio (βexp) of the emission transitions was calculated by measuring the relative area of the respective emission bands. All the radiative parameters (AR,  r ) and emission parameters (  p ,  p ,  pE and βexp) obtained in the

Table 4. Emission band peak positions (λp), effective band widths (∆λp), radiative transition probabilities (AR), peak stimulated emission cross sections (𝝈𝑬𝑷 ), experimental (βexp) and calculated (βr) branching ratios, gain band widths (𝝈𝑬𝑷 × ∆𝝀𝒑) and optical gain parameters (𝝈𝑬𝑷 × 𝝉𝒓 ) of 4F9/2 state for Dy3+ ions doped PLRZFB glasses. Glass system

PLCaZFB:Dy

PLSrZFB:Dy

PLBaZFB:Dy

Transition

4F

4F

4F

𝝈𝑬𝑷

λp (nm)

∆λp

AR

βr

(nm)

485

18.72

7.26

507

0.22

6H 13/2

576

17.23

48.4

1563

6H 11/2

664

11.06

11.2

485

17.74

6H 13/2

577

6H 11/2

βexp

𝝈𝑬𝑷 × ∆𝝀𝒑

𝝈𝑬𝑷 × 𝝉𝒓

(10-28cm3)

(10-25cm2s)

0.63

13.59

2.79

0.70

0.34

83.39

18.63

161

0.07

0.007

12.38

4.31

6.84

453

0.18

0.45

12.13

3.03

16.51

57.25

1547

0.63

0.52

94.51

25.42

665

13.87

11.23

165

0.06

0.012

15.57

4.98

484

17.74

7.07

473

0.19

0.45

12.54

3.06

6H 13/2

577

16.13

52.0

1561

0.64

0.53

83.87

22.56

6H 11/2

666

13.38

11.73

165

0.06

0.008

15.69

5.09

6H 15/2

9/2→

6 9/2→ H15/2

6 9/2→ H15/2

(×

10-22cm2)

(s-1)

8


present study are presented in Table 4. The luminescence branching ratio is a crucial parameter to the laser action, since it characterizes the possibility of obtaining the stimulated emission from any specified transition. If the value of experimental branching ratio for a specified transition is  0.5 , then the transition is highly potential transition for laser emission. The stimulated emission cross-section of an emission transition is one of the important parameters used to identify a laser active medium. It is observed that the 4F9/2 → 6H13/2 transition posses higher peak stimulated emission cross section in all the present glass matrices. Among the prepared glasses, strontium oxide based glass exhibits higher  pE value of 57.25 10-22 cm-2 for the 4F9/2 → 6H13/2 transition (Table 5). Strontium oxide is therefore considered to be more appropriate modifying oxide in the PLRZFB glass matrices to produce high intensity yellow/blue emission. The radiative parameters of this transition are comparable with those reported in Dy3+ doped other glass host matrices [17, 33, 36, 37] (Table 5). The optical gain band width (  PE   p ) and optical gain (  PE   r ) are the two important parameters to fabricate optical devices [1, 2, 6, 10, 12]. The higher values of  pE ,  r and (  PE   r ) parameters of PLSrZFB:Dy glass indicates its suitability for yellow emission lasers, color displays and fiber amplifiers. 3.7. Decay Rate Analysis The fluorescence decay curves of the 4F9/2 energy level of the Dy3+ ions in PLRZFB glasses are obtained by exciting at 389 nm and monitoring the 4F9/2 → 6H15/2 (485 nm) emission transition

Anjaiah et al.

(Fig. (7). It clearly exhibits a non-exponential behavior due to the high concentration of Dy3+ ions. It is well known that decay curves of different transitions for RE ions in different glasses have non-single exponential nature due to dispersion of radiation probabilities for different optical centers for in-homogeneously broadened fluorescence and absorption bands. The excited Dy3+ ions in different local environments will relax with different rates and relaxation rate is strongly affected by the energy of the host matrix. The experimental lifetime of Dy3+ ion, considered as the effective decay time for the 4F9/2 level, is evaluated using the following equation [25].

 exp 

  I (t ) dt  I (t ) dt

------- (1)

where I (t ) is emission intensity at time t. The average measured lifetime of 4F9/2 excited level is calculated by the sum of reciprocal of predicted (also known as radiative) lifetimes (  r ) measured from J-O theory, multi-phonon relaxation rate ( WMPR ) and efficient energy transfer rate ( WET ) among the Dy3+ ions as given by;

1

 exp



1

r

 WMPR  WET

------- (2)

The measured and calculated lifetimes of 4F9/2 excited level of Dy3+ ions in the present glasses are given in Table 6 and compared with those reported in Dy3+ doped other glasses [6, 38-40]. Generally, multi-phonon relaxation rate is due to the interaction between Dy3+- Dy3+ ions. It is known that, WMPR depends on the energy gap between two

Table 5. Comparison of radiative parameters of 4F9/2 → 6H13/2 transition of PLSrZFB:Dy glass with those reported in Dy3+ ions doped other glass systems. Glass host

λp (nm)

𝝈𝑬𝑷 (×10-22cm2)

(𝝈𝑬𝑷 × 𝝉𝒓 )(×10-25cm2s)

βr(cal)

Reference

PLSrZFB:Dy10

577

57.25

25.42

0.63

Present work

PbPKANDy10

576

54.52

25.67

0.67

[17]

CZSFBDy10

576

42.60

23.43

0.66

[36]

Tellurite

576

0.86

0.57

---

[37]

TCZNBDy

577

32.25

---

----

[33]



Table 6. Experimental and calculated life times (τexp, τr), non radiative transition rates (WNR) and percentage of quantum efficiencies (η) of 4F9/2 level for Dy3+ ions doped PLRZFB glasses. Glass system

τexp(ms)

τr(ms)

η (%)

WNR (s-1)

References

PLCaZFB:Dy

0.331

0.397

83

503

Present work

PLSrZFB:Dy

0.420

0.451

93

163

Present work

PLBaZFB:Dy

0.390

0.446

87

332

Present work

Ca LFBDy

0.442

0.475

93

157

[6]

0.5DPTFBDy

0.230

0.500

46

2347

[38]

0.5LBTPDy

0.345

0.405

85

423

[39]

TZPbNbDy

0.367

0.453

81

517

[40]

successive levels and phonon energy of the glass host. The energy level diagram representing the excitation and emission transitions of Dy3+ ions in PLSrZFB glass is shown in Fig. (8). The energy level diagram reveals that the energy difference between 4F9/2 level and next lower lying 6F3/2 level is about 7800 cm-1. When this is compared to the phonon energy of the host glass, it is much higher. Hence the phonon energy of the prepared glasses is relatively low and the effect of the multi phonon relaxation is negligible. The energy transfer rate  1 1 W ET     values are found to be 503, 163    exp  r  and 332 s-1 for PLCaZFB:Dy, PLSrZFB:Dy and PLBaZFB:Dy glasses, respectively. It is interesting to observe that the energy transfer rate is lowest for the strontium oxide based glass composition. The luminescence quantum efficiency (η) is defined as the ratio of the number of photons emitted to the number of photons absorbed. In the case of rare-earth ions doped systems; it is equal to the ratio of the measured life time to the predicted lifetime for the corresponding level and is given by [1-2] η = (τexp /τ r) x 100

Fig. (7). Decay profiles for the 4F9/2 excited state of Dy3+ ions in PLRZFB glasses.

-------- (3)

The η values have been determined for the 4F9/2 fluorescence level of Dy3+ ions and are found to be 83, 93 and 87 for PLCaZFB:Dy, PLSrZFB:Dy and PLBaZFB:Dy glasses, respectively. The nonexponential behavior of the prepared glasses is mainly due to the energy transfer process among the neighboring Dy3+ ions. The ion-ion interaction due to the cross-relaxation process between the pair of Dy3+ ions may be a probable reason to have

Fig. (8). Partial energy levels diagram of Dy3+ ion in PLSrZFB glass.

10


Anjaiah et al.

non-exponential behavior in the prepared glasses [47]. The quantum efficiency of an emission level depends on the emission cross section, radiative transition probability and RE ion concentration [29]. Among the three alkaline earth oxide based glass matrices, PLSrZFB:Dy glass possesses higher quantum efficiency (93%) and lower nonradiative transition rate (163 s-1) than the other two glasses. These results demonstrate that this glass composition has promising potential as an efficient laser active material in the visible region.

ACKNOWLEDGEMENTS

4. CONCLUSION

[1]

A new series of Dy3+ ions doped PLRZFB glasses were prepared and characterized for their optical and luminescence properties through XRD, SEM, DSC, optical absorption, emission and decay rate measurements. All the prepared glasses constitute homogeneous amorphous nature, which was confirmed by XRD. DSC profiles are used to determine the glass transition temperatures of the prepared glasses. From optical absorption spectra, Judd-Ofelt intensity parameters   (λ = 2, 4 and 6) were obtained and the bonding parameter, nephelauxetic ratio values indicate the ionic nature of all the prepared glass matrices. The higher value of  2 parameter in strontium oxide based glass matrix indicates the higher covalence of this glass composition. The 4F9/2 → 6H15/2 (MD) transition band is much influenced by the host environment around the Dy3+ ions in calcium oxide glass indicating a lower symmetry around the Dy3+ ion site. The decay curves exhibit non- exponential behavior due to the high concentration of Dy3+ ions in all the prepared glasses. This may be mainly due to the energy transfer process among the neighboring Dy3+ ions. The 4F9/2 → 6H13/2 transition possesses higher value of stimulated emission cross section, quantum efficiency, branching ratio, gain band width, Y/B ratio and optical gain in strontium oxide based lead fluoroborate glass compared to other two glasses. Thus PLSrZFB:Dy glass composition could be considered as a potential material for the development of lighting devices in the visible region. CONFLICT OF INTEREST The authors confirm that this article content has no conflict of interest.

The authors thank Prof.C.K.Jayasankar, Department of Physics, Sri Venkateswara University, Tirupati and Prof.D.Narayana Rao, School of Physics, University of Hyderabad, India for extending the laboratory facilities. One of the authors G.Anjaiah thanks the DST, New Delhi for a fellowship under DST - PURSE program at Osmania University. REFERENCES

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