Investigation of luminescence and laser transition of Dy3+ ion in P2O5PbOBi2O3R2O3 (R = Al, Ga, In) glasses

Optical Materials 66 (2017) 189e196

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Investigation of luminescence and laser transition of Dy3þ ion in P2O5ePbOeBi2O3eR2O3 (R ¼ Al, Ga, In) glasses G. Chinna Ram, T. Narendrudu, S. Suresh, A. Suneel Kumar, M.V. Sambasiva Rao, V. Ravi Kumar, D. Krishna Rao* Department of Physics, Acharya Nagarjuna University, Nagarjuna Nagar, 522510, A.P., India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 November 2016 Received in revised form 11 January 2017 Accepted 3 February 2017

P2O5ePbOeBi2O3eR2O3 (R ¼ Al, Ga, In) glasses doped with Dy2O3 were prepared by melt quenching technique. The prepared glasses were characterized by XRD, optical absorption, FTIR, luminescence studies. Judd-Ofelt parameters have been evaluated for three glass systems from optical absorption spectra and in turn radiative parameters for excited luminescent levels of Dy3þ ion are also calculated. Emission cross section and branching ratio values are observed to high for 6H13/2 level for Dy3þ ion. The yellow to blue intensity ratios and CIE chromaticity coordinates were calculated. Decay curves exhibit non exponential behavior. Quantum efficiency of prepared glasses was measured by using radiative and calculated life times. IR studies, J-O parameters and Y/B ratio values indicate that more asymmetry around Dy3þ ions in Ga2O3 mixed glass was observed. Chromaticity coordinates lie near ideal white light region. These coordinates and CCT values have revealed that all the prepared glasses emit quality white light especially the glasses mixed with Ga2O3 are suitable for development of white LEDs. © 2017 Elsevier B.V. All rights reserved.

Keywords: Lead bismuth phosphate glasses Dy3þ ions Optical absorption Photoluminescence White LEDs

1. Introduction Inorganic glasses are right choice over polymer sealant for potential phosphor candidate for futuristic applications as they are highly durable and possess excellent emission property and also they have added advantages such as homogeneous light emission, excellent thermal and mechanical stabilities, simple manufacturing technique, lower production cost [1,2]. Especially rare earth doped glasses have attracted persistent research interest as they are easy to shape with high luminescence efficiency, free from halo effect and they have been playing significant role for the development of optical devices such as compact micro chip lasers, communication fibers, amplifiers, visible and infrared solid state lighting (SSL) devices like W-LED, wave guides, display devices, scintillators and sensors. In modern applications these glasses have found applications in Faraday rotation effect, an optical attenuators, circulator and magnetic field sensor [3e7]. These applications are mainly due to unidirectional light propagation in optical isolators which result in large Verdet constants. Among various trivalent rare earth ions, Dy3þ is one of the

* Corresponding author. E-mail address: [email protected] (D.K. Rao). http://dx.doi.org/10.1016/j.optmat.2017.02.004 0925-3467/© 2017 Elsevier B.V. All rights reserved.

interesting ion with closely packed energy levels, because Dy3þ doped glasses have been considered as promising luminescent materials in the blue(4F9/2 / 6H15/2) and yellow(4F9/2 / 6H13/2) regions. The yellow transition is hyper sensitive transition which obeys the selection rule DL ¼ 0, DJ ¼ 0 and is strongly affected by environment around Dy3þ ion leads to changes in Y/B ratio. This ratio is also suggests the local symmetry in the environment of dysprosium ions and indicates the degree of covalence between Dy3þ and O2.Therefore by fine-tuning to appropriate Y/B ratio, the chromaticity coordinates of the glasses contain Dy3þ ions can be attuned to the white light zone and can be used for SSL applications [8e10]. A proper selection of the host will facilitate the extraction of primary colors because even at ordinary excited wavelengths Dy3þ ions have large absorption cross section. The degree of depolymerization around dysprosium ion in the glass host affects the luminescence intensities [11,12]. In the midst of different glass hosts such as silicate, borate etc., phosphate glasses have their unique characteristics that include high transparency, low melting point, high thermal stability, high gain density that is high solubility for rare earth ions [13]. Lead oxide is heavy metal oxide which plays dual role one as modifier with PbO6 structural units and other as network former with PbO4 structural units. Its incorporation in phosphate glass improves the chemical stability due to the formation of PeOePb bonds makes it

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moisture resistant. Also the chemical stability of lead phosphate glasses has been further improved by creating more non bridging oxygen atoms by incorporating another heavy metal oxide such as Bi2O3 to the glass system [14,15]. Earlier Saddek et al. [16], Reddy et al. [17] reported the incorporation of Al2O3 to the dysprosium phosphate glasses enhances its luminescent properties of these glasses. The increase of asymmetry around Dy3þ ion is caused by Al2O3 may be one of the reasons for enhancement of luminescent properties of these phosphate glass systems. The objective of the present investigation is to characterize the optical absorption, FTIR and fluorescence spectra of Dy3þ ions in lead bismuth phosphate glasses mixed with three different metal oxides viz. Al2O3, Ga2O3 and In2O3 and to throw some light on the relationship between structural modifications and luminescence efficiencies.

transition probabilities respectively and are given by

   2 X   Sed jJ; j0 J 0 ¼ e2 Ul jJ U l j0 J 0 and

  Smd jJ; j0 J 0 ¼

fexp

2:303mc2 ¼ pe2 N

Z

9

aðnÞdn ¼ 4:318 x 10

Z

aðnÞdn

(1)

where m is the mass of the electron, c is the velocity of light, e is the charge of electron and N is rare earth concentration per unit volume. aðnÞ is molar absorptivity of the related transition. The theoretical oscillator strength of f-f transition can be evaluated using J-O theory [20,21]. From this theory the calculated oscillator strength of an induced electric dipole transition from ground state jJ to an excited state j0 J 0 is given by the following equation

fcalðjJ;j0 J0 Þ ¼

2  2    2 X n þ2 8p2 mcn  U l j J U l j 0 J 0 3hð2J þ 1Þ 9n

(2)

l¼2;4;6

where n is the refractive index of the medium, n is the energy of the transition in cm1 and h is Planck's constant, J is the angular momentum of the ground state, Ul (l ¼ 2, 4, 6) are J-O intensity pa    rameters and U l  are doubly reduced matrix elements of the unit tensor operator evaluated from the intermediate coupling scheme for a transition jJ to j0 J 0 . A least square fit method is then applied to equation (2) to obtain good fit between the experimental and calculated oscillator strengths. The quality fit is known as root mean square deviation (s) between Fexp and Fcal is given by

8 2 912 > = < fexp  fcal > srms ¼ S > > N ; :

(3)

where N refers to the total number of levels included in the fit. The radiative transition probability (A) for a transition jJ to j0 J 0 can be calculated from the following relation.



0 0

AR jJ; j J

# "  2 64p4 n3 e2 n n2 þ 2 3 Sed þ n Smd ¼ 3hð2J þ 1Þ 9

(4)

where Sed and Smd are the electric and magnetic dipole radiative



   

2 e2 Ul  jJ L þ 2Sj0 J 0  4m2 c2

(6)

The total radiative transition probability

  X   AT j0 J 0 ¼ AR jJ; j0 J 0

(7)

The radiative life time (tcal) is related to the total radiative transition probability AT of an excited state given by the following equation

2. Theory In the present work, the f-f intensity model described by Carnall et al. [18] and Gorller-Walrand and K. Binnemans [19] has been used. The experimental oscillator strengths of absorption transitions estimated by using area under the absorption peaks and following relations.

(5)

l¼2;4;6

tcal ¼

1   A T j0 J 0

(8)

The branching ratio (bR) of emission transition from excited state to ground state is given by





bR jJ; j0 J 0 ¼

  AR jJ; j0 J 0  0  AT j J 0

(9)

The stimulated emission cross section probability (sEP) having radiative transition probability AR is given by

 



s lp jJ; j0 J 0 ¼

l4p   AR jJ; j0 J 0 8pcn2 Dleff

(10)

where lP refer to the transition band wavelength and Dleff is effective line width and it is calculated by dividing the area of the emission peak by its average height. The quantum efficiency h of the excited state of Dy3þ is given by

h¼

texp tcal

(11)

texp is calculated from decay curve profile. The color of any light source can be described by color matching functions x(l), y(l), z(l) which are dimensionless quantities [22]. The International de I'Eclairage (CIE) 1931 chromaticity color coordinates are determined from following equations:

x¼

X XþY þZ

(12)

y¼

Y XþY þZ

(13)

z¼

Z XþY þZ

(14)

where X, Y, Z are the tristimulus values, which are related to the three basic colors red, green and blue needed to match the color wavelength P(l). The quality of light emitted is represented by the color correlated temperature (CCT) and is calculated by using McCamy empirical formula [23] shown below.

CCT ¼ 449n3 þ 3525n2  6823n þ 5520:33

(15)

e where n ¼ XX YYe and the chromaticity epicenter is Xe ¼ 0.332 and

Ye ¼ 0.186.

G.C. Ram et al. / Optical Materials 66 (2017) 189e196

3. Materials and methods The glass composition 60P2O5-17.5PbO-10Bi2O3-10R2O32.5Dy2O3 (R ¼ Al, Ga and In) is chosen for the present study. Glass samples with different compositions are prepared by melt quenching technique at temperature 1200 C for 10 min appropriate amounts of AR grade material powders viz, P2O5, PbO, Bi2O3, Al2O3, Ga2O3, In2O3 and Dy2O3 with 99.9% purity are homogenized mechanically and they were melted in silica crucible (Infusil make) in a programmable electrical furnace. The resultant melt was swirled to ensure the homogeneity and then poured on a pre heated brass mould at room temperature and subsequently annealed at 350oc in order to eliminate internal mechanical stress. The detailed composition (all in mol %) of the prepared glasses in the present study are as follows.

191

Table 1 Physical parameters of Dy3þ doped P2O5ePbOeBi2O3eR2O3 (R ¼ Al, Ga, In) glasses. Physical parameters

PPBAlDy

PPBGaDy

PPBInDy

Density, d (g/cm3) Molar volume (cm3/mol) Dopant ion concentration, Ni (1023ions/cm3) Polaron radius, Rp (A0) Field strength (1013cm2) Energy band gap (eV) Urbach energy (eV) Inert ionic separation, Ri (A0) Refractive index, n

4.011 0.779 1.269 0.372 21.66 4.088 0.126 0.923 1.705

4.210 0.677 1.274 0.371 21.72 4.110 0.105 0.922 1.712

4.339 0.598 1.256 0.373 21.52 4.061 0.110 0.926 1.718

PPBAlDy: 60P2O5-17.5PbO-10Bi2O3-10Al2O3-2.5 Dy2O3 PPBGaDy: 60P2O5-17.5PbO-10Bi2O3-10Ga2O3-2.5 Dy2O3 PPBInDy: 60P2O5-17.5PbO-10Bi2O3-10In2O3-2.5 Dy2O3 The physical appearance of the prepared glass samples is shown in Fig. 1. Density of these samples was measured according to Archimedes principle with O-xylene as immersion liquid using VIBRA HT density measurement kit to an accuracy ±0.001 g/cm3. Refractive index of the prepared samples was measured by using Abbe refractometer with monobromo naphthalene as the contact layer between the glass and refractometer prism. The optical absorption spectra of prepared samples were recorded at room temperature using double beam UV-VIS-NIR spectrophotometer JASCO V-670 in the wavelength range 300e1800 nm with a spectral resolution of 0.1 nm. The infrared absorption spectra of these samples were recorded at room temperature in the range 400e1600 cm1 with spectral resolution of 0.85 cm1 using SHIMADZU FTIR 8400 S spectrometer. The luminescence spectra and life time measurements were carried out at room temperature using JOBINYVON FLUOROLOG-3 spectro fluorimeter using Xenon arc lamp as radiation source.

4. Results and discussion 4.1. Structural analysis Some important physical parameters like density (d), rare earth ion concentration (Ni), inter ionic separation (Ri), polaron radius (Rp), refractive index (n) etc., of the prepared glasses are estimated by means of conventional formulae [24] and are mentioned in Table 1. The XRD patterns of prepared glasses are shown in Fig. 2. These patterns indicate the amorphous nature of prepared glasses.

Fig. 1. Physical appearance of Dy3þ doped P2O5ePbOeBi2O3eR2O3 (R ¼ Al, Ga, In) glasses.

Fig. 2. XRD profile of Dy3þ doped P2O5ePbOeBi2O3eR2O3 (R ¼ Al, Ga, In) glasses.

The non destructive method for analyzing nature of chemical bonds in glass network is FTIR spectroscopy. Fig. 3 shows the FTIR spectra of the titled glasses and the band positions are given in Table 2. The P2O5 is well known network former with PO4 structural units with one of the four oxygen atoms in PO4 is doubly bonded to the phosphorus atom with the substantial P-bond character to account for penta valance of phosphorous [25]. The PO4 tetrahedrons are linked together with covalent bonding in chains or rings by bridging oxygens. Neighboring phosphate chains are linked together by cross bonding between the metal cation and two non bridging oxygens atoms of each PO4 tetrahedron. In general the PeOeP bond between PO4 tetrahedra is much stronger than the cross bond between chains via the metal cation [26]. PbO may also

Fig. 3. FTIR spectra of Dy3þdoped P2O5ePbOeBi2O3eR2O3 (R ¼ Al, Ga, In) glasses.

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G.C. Ram et al. / Optical Materials 66 (2017) 189e196

Table 2 Data on FTIR spectra of Dy3þ doped P2O5ePbOeBi2O3eR2O3 (R ¼ Al, Ga, In) glasses. (Band positions are in cm1). PPBAlDy

PPBGaDy

PPBInDy

Assignment

454 610 773 933 1061 1255

460 608 778 924 1058 1248

453 611 773 934 1062 1251

Vibrations of PbO4 structural units BieO vibrations of distorted BiO6 units PeOeP symmetric vibrations PeOeP asymmetric vibrations PO3 4 stretching vibrations PO2 asymmetric vibrations

participate in the glass network with PbO4 structural units when lead ion is linked to four oxygens in a covalency bond configuration. In such a case the network structure is considered to be built up from PbO4 units that alternate with PO4 structural units [27]. The presence of such PbO4 units in the network is evident from the band observed in the IR spectrum at 454 cm1. It may be worth noting here in case of IR spectra of Ga2O3 mixed glasses a vibrational band in the region 400e600 cm1 due to GaO6 structural units is reported to be possible [28]. Such structural units similar to modifier oxides are reported to induce structural defects like, non bridging oxygens in the glass network. The bands at 1255 cm1 were attributed to asymmetric stretching vibrations of PO4 tetrahedral with one non-bridging and two bridging oxygen atoms attached to phosphorus. The vibrational bands at 773 cm1 and 933 cm1 are due to PeOeP symmetric and asymmetric stretching vibrations respectively. Band at 1061 cm1 may be assigned to PO3 4 group stretching vibrations. The addition of intermediate oxide such as Bi2O3 in to the phosphate network results in the formation of PeOeBi bonds which further enhances chemical durability of phosphate glasses. Bi2O3 exists mostly in BiO6 octahedral units, The presence of such BiO6 units in glass network is evident from band observed at 610 cm1 in IR spectrum of the present glasses. Incorporation of Bi3þ ions makes the structural changes which result the depolymerization of the phosphate network. Further incorporation of Al2O3 to the lead bismuth phosphate glasses aluminum ions enter gradually in to network as network former replacing some of lead ions thus the non-bridging oxygens of P]O bonds may be converted in to bridging oxygens upon the formation of PeOeAl bonds and the links OeAleO replace with OePeO as aluminum enters the network [16]. Some of AlO4 structural units enter in to the phosphate network as former and the characteristic bonds of phosphate glass will be modified by the excess of these oxygens. Such oxygens can be considered as bridging oxygens i.e., Al2O3 polymerize the phosphate chains. Whereas the incorporation of Ga2O3 to the lead bismuth phosphate glass leads to the depolymerization of phosphate network by disruption of PeOeP bridges to phosphate network fragments, which are connected by PeOeGa bridges [29]. Such structural changes improve the physical properties of the prepared glasses. Normally the two indium ions in In2O3 exist in trivalent state and occupy distorted octahedral coordination by six oxygen atoms. Each octahedron, In3þO6 shares four corners with four PO4 tetrahedra. The two remaining corners are occupied by oxygen ions and are linked to other In3þO6 octahedral oxygens. Indirectly it leads to the conclusion that incorporation of In2O3 to the glass system may increase the polymerization [27]. Therefore the incorporation of Al2O3, In2O3 may enhance the asymmetry to the lesser extent of the titled glass and Ga2O3 depolymerizes the network and gives more asymmetry in the local environment. The simple iso-exchange substitution of Al3þ with Dy3þ ion does not occur easily because there is a large ionic radii difference between Dy3þ and Al3þ ions. Hence Dy3þ ions preferentially participate with Al3þ forming AleOeDy bonds rather than DyeOeDy bonds. This participation results in larger spacing among rare earth ions and decreases the

magnitude of quenching due to clustering [30]. We cannot expect such case with Ga3þ and In3þ ions with dysprosium ion because of smaller ionic radii difference between Ga3þ, In3þ and Dy3þ. Hence the degree of asymmetry around Dy3þ ion follows the order PPBGaDy > PPBAlDy > PPBInDy in the present investigation. P2O5 is conventional network former with PO4 structural units. If we consider the three metal oxides viz., Al2O3, Ga2O3, In2O3 to be incorporated between the long chain molecules in the vicinity of Dy3þ ion in the phosphate network, then the symmetry and/or covalence of the glass at Dy3þ ions should be different for different metal oxides. Additionally the variation in the concentration of phosphate ions (may be due to volatilization) is also expected to modify the crystal field environment around Dy3þ ions in the glass network. Among these three metal oxides, Ga2O3 enters into the glass network with GaO4 and GaO6 structural units. GaO4 structural units act as network former whereas GaO6 structural units depolymerize the network by disrupting the PeOeP chains. 4.2. Absorption spectra The UV-VIS-NIR absorption spectra of the prepared glasses samples recorded at room temperature in the range 400e1800 nm are shown in Fig. 4(a) and (b). The absorption spectra of three glass samples are similar except feeble change in their peak positions and intensities. Ten absorption peaks observed at 324, 349, 364, 386,754,803,903,1099,1280, and 1690 nm which are attributed to the transitions from ground level 6H15/2 to 6P3/2, 6P7/2, 4P3/2, 4F7/2, 6 F3/2, 6F5/2, 6F7/2,6F9/2,6H9/2þ6F11/2,6H11/2 of the Dy3þ ion respectively [31]. The activator absorption bands in UV region are less intense than bands in VIS-NIR region, which might be due to the strong absorption of the host glass in this region [12] and the assignment of free-ion levels in the UV-VIS region is not easy because of the overlap of different (2sþ1)LJ values. The transitions from the ground 6H15/2 state to 6H9/2 and 6F11/2 are spin-allowed (DS ¼ 0), besides that, the transition to 6H9/2 is also allowed by angular momentum (DL ¼ 0). Hence these two transitions 6H15/2 to 6 H9/2 and 6F11/2 lying in the NIR region are relatively intense than other transitions. The experimental and calculated oscillator strengths along with J-O parameters for the titled samples are tabulated in Table 3. The small values of r.m.s deviation i.e. ±0.587, ±0.627and ± 0.676 for Al2O3, Ga2O3 and In2O3 glasses respectively indicate best quality fit between Fexp and Fcal. The transition intensities are characterized by three sensitive parameters known as J-O intensity parameters U2, U4, and U6. The relative magnitude of these parameters is useful to explain the bonding, symmetry and stiffness of the host matrices. U2 indicates the covalence of the metal-ligand bond, whereas U4 and U6 related to bulk properties such as viscosity and rigidity of the host matrix. In the present investigation U2 values are 8.92  1020, 9.38  1020, 8.67  1020 for Al2O3, Ga2O3, In2O3 mixed glasses respectively. These higher values are consequences of high order of asymmetry around the dysprosium ion. Among these U2 is higher for Ga2O3 mixed glass and these parameters followed the trend U2 > U6 > U4 for all samples. This trend is an evidence for good quality of host glass for optical applications [32]. Such larger value of U2 is probably a direct consequence of more covalent nature of chemical bonds and environment disorder around the dysprosium ion in the present glasses. This may be due to relative larger asymmetry in Ga2O3 mixed glass than other two metal oxide mixed glasses. The J-O parameters have been used to evaluate radiative parameters such as transition probability (AR), radiative life time (tR), branching ratio (bR). The branching ratio is a critical parameter to the laser designer because it characterizes the possibility to attain stimulated emission from any specific transition. The branching

G.C. Ram et al. / Optical Materials 66 (2017) 189e196

193

Table 3 Experimental (fexp) and calculated (fcal) oscillator strengths (106) and J-O parameters (Ulx1020 where l ¼ 2, 4 and 6) of Dy3þ doped P2O5ePbOeBi2O3eR2O3 (R ¼ Al, Ga, In) glasses.

Transition 6 P3/2 6 P7/2 4 P3/2 4 F7/2 6 F3/2 6 F5/2 6 F7/2 6 F9/2 6 H9/2þ6F11/2 6 H11/2 R.M.S

U2 U4 U6 Trend

a

PPBAlDy

PPBGaDy

PPBInDy

fcal fexp 0.347 0.421 0.322 0.838 0.124 0.044 0.204 0.246 1.475 0.515 2.482 1.300 2.106 2.480 2.330 2.278 4.356 4.334 1.272 1.420 ±0.587 8.92 2.55 2.99 U2> U6> U4

fexp fcal 0.341 0.450 0.358 0.885 0.095 0.055 0.187 0.262 1.549 0.543 2.704 1.383 2.449 2.685 2.588 2.574 4.473 4.439 1.242 1.483 ±0.627 9.38 2.85 3.19 U2> U6> U4

fexp fcal 0.316 0.405 0.298 0.811 0.087 0.043 0.181 0.237 1.742 0.498 2.531 1.249 2.012 2.389 2.288 2.203 4.350 4.332 1.256 1.375 ±0.676 8.67 2.48 2.73 U2> U6> U4

results from magnetic dipole (MD) transition, while the transition F9/2 to 6H13/2 shows bright yellow and is due to electric dipole (ED) transition in the visible range spectrum. The magnetic dipole transition results from the interaction of Dy3þion with magnetic field component of light through magnetic dipole and electric dipole transition is the consequence of interaction of Dy3þ ion with electric field vector through electric dipole. Majority of observed optical transitions in lanthanide ions are induced electric dipole transitions [4]. In the present investigation ED transition is more intense than MD transition in all prepared samples. Therefore the present glasses possess more asymmetrical in nature and also it is high in Ga2O3 mixed glass rather than other two glasses. From the measured emission spectra, emission peak position (lP), effective band width (Dl), stimulated emission cross section (sEP), yellow to blue intensity ratio (Y/B) have been determined. The evaluated transition probability (AR), branching ratio (bR), emission peak position (lP), effective band width (Dl), stimulated emission cross section (sEP) are presented in Table 4. The stimulated emission cross section provides information about the potential laser performance and its value signifies the rate of energy excitation from the lasing material which can be evaluated from the emission spectrum [31]. In the present study scattering emission cross section values are found to high for 4F9/2 to 4H13/2 transition than the other transitions. 4

b Fig. 4. (a)Optical absorption spectra of Dy3þ doped P2O5ePbOeBi2O3eR2O3 (R ¼ Al, Ga, In) glasses in UV region. (b) Optical absorption spectra of Dy3þdoped P2O5ePbOeBi2O3eR2O3 (R ¼ Al, Ga, In) glasses in VIS-IR region.

ratios for all prepared glasses are found to be in the order of 4F9/2 to 6 H13/2 > 6H15/2 > 6H11/2 > 6H9/2 of Dy3þion. In broad spectrum the parameter bR  50% typifies the potential lasing transition in that glass matrix. Branching ratios of yellow emission transition 4F9/2 to 6 H13/2 is found to be the highest one, therefore this transition can be considered as lasing transition, which consist major part of emission intensity. High transition probability values for 6H13/2 level for prepared glasses is also indicate that this level is suitable for lasing action. Fig. 5 shows the luminescence spectra of prepared glasses in spectral region 400e800 nm excited at 348 nm consist of four bands centered at around 482, 573, 663, 753 nm which can be assigned to 4F9/2 to 6H15/2,6H13/2,6H11/2,6H9/2 of Dy3þion respectively. The transition 4F9/2 to 6H15/2 shows blue emission which

Fig. 5. Emission spectra of Dy3þ doped P2O5ePbOeBi2O3eR2O3 (R ¼ Al, Ga, In) glasses.

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G.C. Ram et al. / Optical Materials 66 (2017) 189e196

Table 4 Emission band positions (lP nm), effective line width (Dl nm), transition probabilities (A sec1), peak stimulated emission cross section (sEP 1021 cm2) and branching ratio(bR) of emission transitions of Dy3þ doped P2O5ePbOeBi2O3eR2O3 (R ¼ Al, Ga, In) glasses. Transition

Parameter

PPBAlDy

PPBGaDy

PPBInDy

4

lP Dl

481 13.5 294.8 0.172 0.533 573 14.4 1135.8 0.663 3.88 663.5 10.4 111.8 0.0652 0.951 753 8.1 29.5 0.0172 0.534

482 13.8 311.2 0.173 0.551 574 14.8 1187.9 0.6625 3.94 663.5 10.8 116.7 0.0651 0.984 753 8.1 30.8 0.0172 0.553

482 13.4 275.3 0.166 0.49 573 14.3 1107 0.668 3.751 663 10.2 110.2 0.0665 0.9385 753 8.1 28.4 0.0176 0.506

6

F9/2 / H15/2

A

4

bR sEP lP Dl

F9/2 / 6H13/2

A

4

bR sEP lP Dl

6

F9/2 / H11/2

A

4

bR sEP lP Dl

6

F9/2 / H9/2

A

bR sEP

Table 5 Color parameters and Y/B ratio values of Dy3þ doped P2O5ePbOeBi2O3eR2O3 (R ¼ Al, Ga, In) glasses. Sample

Color co-ordinates x

y

PPBAlDy PPBGaDy PPBInDy

0.365 0.331 0.326

0.390 0.373 0.368

CCT value

Y/B ratio

4672 5594 6005

1.92 2.05 2.02

3.88, 3.94 and 3.75(1021cm2) are the scattering emission cross section values for Al2O3, Ga2O3 and In2O3 mixed glasses respectively. The scattering emission cross section value is high for 4F9/2 to 6 H13/2 in Ga2O3 mixed glass. Therefore, such larger emission cross section values are attractive feature for low threshold, high gain applications and are utilized to obtain continuous wave laser action. The integrated fluorescence intensity ratio between ED and MD transitions i.e. Y/B ratio is a criterion to estimate the

Fig. 6. Excitation spectra of Dy3þ doped P2O5ePbOeBi2O3eR2O3 (R ¼ Al, Ga, In) glasses.

Fig. 7. Life time curves of Dy3þ doped P2O5ePbOeBi2O3eR2O3 (R ¼ Al, Ga, In) glasses.

Table 6 Life time values and quantum efficiencies of Dy3þ doped P2O5ePbOeBi2O3eR2O3 (R ¼ Al, Ga, In) glasses. Sample

AT

tcal

texp

s%

PPBAlDy PPBGaDy PPBInDy

1718 1828 1657

584 558 604

420 446 468

71.9 79 77

asymmetry. Higher value of Y/B ratio indicates a higher asymmetry environment around Dy3þions [33]. The evaluated Y/B ratio values are presented in Table 5. Such higher value of Y/B ratio for Ga2O3 mixed glasses indicates that more disorder in the glass network. From FTIR spectra of the titled glasses, the intensity of asymmetric vibrational bands is found to be high in PPBGaDy glasses when compared to that of remaining two glasses. This observation also supports the view point that there is high degree of depolymerization in PPBGaDy glasses. The intensity of both yellow and blue

Fig. 8. CIE diagram of Dy3þ doped P2O5ePbOeBi2O3eR2O3 (R ¼ Al, Ga, In) glasses.

G.C. Ram et al. / Optical Materials 66 (2017) 189e196

195

Fig. 9. Energy level diagram of Dy3þ ion.

transitions is observed to increase as we go from PPBInDy to PPBGaDy. Such behavior indicates more declustering of Dy3þ ions in Ga2O3 mixed glasses when compared with Al2O3 and In2O3 mixed glasses and hence minimal losses of emission due to cross relaxations are anticipated in these glasses. The excitation spectra of PPBAlDy, PPBGaDy and PPBInDy glasses are recorded by choosing the emission wavelength at 573 nm and are shown in Fig. 6.

4.3. Decay curve analysis The luminescence decay curves corresponding to the energy level of dysprosium doped samples are recorded and shown in Fig. 7. The decay measurements exhibit non exponential behavior. From these curves life time values are also calculated and given in Table 6. Ratio of the number of photons emitted to the number of photons absorbed is defined as luminescence quantum efficiency. It is numerically equal to the ratio of experimental life time to the calculated life time for the corresponding energy levels. The quantum efficiencies are found to 71.9%, 79%, and 77% for Al2O3, Ga2O3 and In2O3 mixed glasses respectively. The International de I'Eclairage (CIE) 1931chromaticity color coordinates have been used to determine the emission color of the present glasses. The color chromaticity coordinates (x, y, z) for the titled glasses are calculated by using equations (12)e(14) and are given in Table 5. In the present work the chromaticity coordinates are found to be (0.365, 0.390), (0.331, 0.373), (0.326, 0.368) for Al2O3, Ga2O3, In2O3 mixed glasses respectively. These coordinates lie within the white light region and these coordinates are nearer to the ideal white light illumination (0.330, 0.333) shown in Fig. 8. From these values it is observed that Dy3þ doped P2O5ePbOeBi2O3eR2O3 (R ¼ Al, Ga, In) glasses are suitable for the fabrication of W-LEDs. The quality of light emitted is represented by color correlated temperature (CCT) which is calculated by using equation (15) and given in Table 5. The obtained CCT values are nearer to those commercial and day light W- LED sources

[31]. The observed non-exponential behavior in these glasses can be explained in terms of resonant energy transfer between Dy3þ ions and cross relaxation (denoted as CR1 & CR2 in Fig. 9) process between the donor and acceptor ions. 5. Conclusions In the present study high transparent, moisture resistance and stable Dy3þ doped P2O5ePbOeBi2O3eR2O3 (R ¼ Al, Ga, In) glasses were prepared by melt quenching technique. Optical absorption, FTIR and photoluminescence studies on these glasses have been carried out. The three J-O parameters Ul (l ¼ 2, 4, 6) have been calculated from experimental oscillator strength (fexp) of the absorption spectra. Out of the three J-O parameters, the value of U2, which is related to the structural changes in the vicinity of Dy3þ ion, indicates the highest covalent environment of Dy3þ ion in Ga2O3 mixed glasses. The radiative transition probabilities, branching ratio and scattering emission cross section for various luminescent transitions observed in the luminescence spectra of all the three glasses suggest the highest values for 4F9/2 to 6H13/2 (among various transitions originating from 4F9/2 level) in PPBGaDy glass and this transition is suitable for potential lasing action. The analysis of color coordinates of the titled glasses suggests that comparatively glasses mixed with Ga2O3 are suitable for development of white LEDs. Acknowledgements The authors are thankful to the University Grants Commission (UGC) and the Department of Science and Technology (DST), New Delhi, India for sanctioning DSA and FIST level- I projects respectively to the Physics department of the university. References [1] K. Jha, M. Jayasimhadri, Spectroscopic investigation on thermally stable Dy3þ

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