Accepted Manuscript Structural and Luminescence behaviour of Er3+ doped Telluro-fluoroborate glasses P. Karthikeyan, P. Suthanthirakumar, R. Vijyakumar, K. Marimuthu PII: DOI: Reference:
S0022-2860(14)01206-X http://dx.doi.org/10.1016/j.molstruc.2014.12.003 MOLSTR 21153
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Journal of Molecular Structure
Received Date: Revised Date: Accepted Date:
19 September 2014 30 November 2014 1 December 2014
Please cite this article as: P. Karthikeyan, P. Suthanthirakumar, R. Vijyakumar, K. Marimuthu, Structural and Luminescence behaviour of Er3+ doped Telluro-fluoroborate glasses, Journal of Molecular Structure (2014), doi: http://dx.doi.org/10.1016/j.molstruc.2014.12.003
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Structural and Luminescence behaviour of Er3+ doped Telluro-fluoroborate glasses P. Karthikeyan, P. Suthanthirakumar, R. Vijyakumar, K. Marimuthu* Department of Physics, Gandhigram Rural University, Gandhigram - 624 302, India Abstract The Er3+ doped telluro-fluoroborate glasses with the chemical composition (30 ̶ x)B2O3 +30TeO2+16ZnO+10ZnF2+7CaF2+7BaF2+xEr2O3 (x = 0.05 ,0.1, 0.5, 0.75, 1.0 and 3 in wt%) have been prepared by melt quenching technique and characterized through XRD, SEM, FTIR, Raman, absorption and luminescence spectral analysis. The XRD and SEM measurements were made to examine the amorphous nature. The presence of various stretching and bending vibration modes of functional groups have been investigated through FTIR and Raman spectra. The bonding parameters ( and d) were calculated from the absorption spectra to claim the
covalent/ionic nature of the metal-ligand bond in the prepared glasses. From the absorption
spectra, optical band gap energies (Eopt) corresponding to the direct and indirect allowed transitions were calculated to analyze the electronic band structure. The Urbach energy values have also been estimated and discussed. The Judd-Ofelt (JO) intensity parameters (Wl (l = 2, 4, 6)) were determined from the absorption spectra in order to study the symmetry around the RE ion site and used to compute the radiative properties such as transition probability (AR), stimulated emission cross-section (
) and branching ratios (bR) for the different emission
transitions. The emission intensities of the prepared glasses were characterized through CIE 1931 chromaticity diagram and the results were discussed and compared with the reported literature. Keywords: FTIR spectra; Absorption; Judd-Ofelt theory; Luminescence; Stimulated emission cross-section *
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Email address: [email protected]
1. Introduction Recently glasses doped with trivalent rare earth (RE) ions have been widely investigated by many researchers due to their potential applications in solid state lasers, optical amplifiers, 1
color display devices and fibre optical communication systems etc., [1-3]. The complete knowledge on spectroscopic properties of optical materials such as transition probabilities stimulated emission cross-section, branching ratios are essential to find new optical devices or to enhance the performance of the existing devices [4, 5]. The trivalent RE doped materials offer sharp absorption and emission spectra due to its incomplete 4f electronic shell which is shielded by the 5s and 5p shells. Among the several RE 3+ ions, the Er3+ doped glass materials are useful for the design of visible lasers due to the fact that the emission bands 2H11/2→4I15/2, 4S11/2→4I15/2 and 4F9/2→4I15/2 lies in the visible region. Further, the Er3+ doped fibre amplifiers (EDFA) improves the transmission capacity of the Wavelength Division Multiplexing (WDM) systems. Among the several hosts, telluro-fluoroborate glass is most suitable for the doping of Er3+ ions due its peculiar properties like large RE ion solubility, high refractive index, high thermal and mechanical stabilities etc., . It is a well known fact that, pure TeO2 does not form a glass on its own without adding other elements to it but it does when combined with the network modifiers. TeO2 based ternary glasses possess good transparency and moisture resistant property. Here, the borate was added as a network modifier to obtain the glassy nature by changing the structural network along with the formation of BO 3 and BO4 units and also it increases the stability of the tellurite glasses. The higher phonon energy of the telluro-borate glasses leads to have non-radiative emission by multiphonon relaxation and narrow down its applications . In order to overcome this difficulty low phonon energy fluoride content was additionally added into the telluro-borate glasses which in turn enhance the radiative emission of the Er3+ ions by reducing the non-radiative relaxation . Addition of ZnO into the telluro-borate glasses increases the glass forming ability and non-hygroscopic nature . Therefore, in the present study telluro-fluoroborate glass has been chosen as a promising host material for the doping of Er3+ ions. Many Researchers reported the spectroscopic properties of Er3+ doped glasses. L.R.P. Kassab et al.  reported Er3+ laser transitions in lead fluoroborate glasses for laser action. The role of PbO on structural and luminescence behaviour of rare earth-doped lead borate glass have been studied and reported by W. A. Pisarski et al. . M.V.VijayaKumar et al.  explored the luminescence and gain characteristics of 1.53 µm broad band of Er3+ ions in lead telluroborate glasses. Further, they have reported the influence of hydroxyl groups and the nature 2
of the host on the spectroscopic behaviour of Er3+ ions. Y.Yan et al. reported the fact that in Er3+ doped glass materials, more than one half of the OH groups were coupled to the Er3+ ions and become effective quenchers which in turn decreases the luminescence lifetime by two-phonon quenching mechanism . W.A.Pisarski et al. studied and reported that the higher concentration of Er3+ activator ions lead to luminescence quenching due to the efficient energy transfer between nearby Er3+-Er3+ ions and in turn enhances the non-radiative decay . Z.Pan et al. reported the fact that the glasses with higher phonon energy exhibit larger multi-phonon relaxation rate leading to non-radiative decay which in turn decreases the radiative emission . Z.A.S. Mahraz et al.  reported on the structural and concentration dependent luminescence behaviour of Er3+ doped Zinc boro-tellurite glasses. K. Selvaraju et al.  investigated the structural and visible luminescence properties of Er3+ doped boro-tellurite glasses for green laser applications. The visible and near infrared emission spectra of the Er3+ doped glasses were analysed and reported by B. C. Jamalaiah et al. . P. Babu et al.  explored the optical absorption and visible emission properties of Er3+ doped oxyfluorotellurite glasses for solid state green laser applications. The present work reports structural and luminescence properties of Er3+ doped telluorofluoroborate glasses by varying the RE ion content. The prepared glasses were characterized through XRD, FTIR, Raman, SEM, optical absorption, excitation and luminescence spectral measurements. The main objective of the present work is to (i) synthesis Er3+ doped tellurofluoroborate glasses (ii) explore the structural behaviour through XRD, FTIR, Raman and SEM measurements (iii) examine the energy levels and bonding parameters ( and d) to claim
covalent/ionic nature (iv) determine the optical band gap (Eopt) and Urbach energy (ΔE) values to analyze the optical behaviour (v) evaluate the JO parameters (Ω2, Ω4 and Ω6) and to compare the trends of JO parameters with respect to other Er3+ doped glasses (vi) determine the CIE color coordinates for different Er3+ ions concentration and finally, to (vii) derive the radiative
properties for the significant energy levels of the Er3+ ions and to compare the results with reported literature. 2. Experimental Er3+ doped telluro-fluoroborate glasses with the chemical composition (30 ̶ x)B2O3+ 30TeO2+16ZnO+10ZnF2+7CaF2+7BaF2+xEr2O3 (where, x = 0.05, 0.1, 0.5, 0.75, 1 and 3 in wt 3
%) have been prepared by conventional melt quenching technique and labelled as 0.05ETFB, 0.1ETFB, 0.5ETFB, 0.75ETFB, 1ETFB and 3ETFB respectively. The raw materials used for the glass preparation are of high purity (99.99%) analytical grade chemicals purchased from SigmaAldrich. The chemicals were grounded thoroughly in an agate mortar and taken into a porcelain crucible and then melted at 1050 °C for half an hour in an electrical furnace. The melt was then poured onto a preheated brass mould and subsequently annealed at 350 °C for 8 h to remove the thermal strain and air bubbles. Then the samples were slowly brought to room temperature and the glass samples were polished carefully on both sides before further optical measurements. The density of the glass samples was measured by Archimedes principle with xylene as an immersion liquid. The refractive indices of the prepared glasses were measured using Abbe refractometer at sodium wavelength using 1-bromonaphthaline as a contact liquid. The physical properties of the prepared glasses were calculated and presented in table 1. The X-ray diffraction measurements were carried out using JEOL 8530 X-ray diffractometer employing CuKɑ radiation. SEM images and EDX spectra were recorded using VEGA3 TESCAN spectrometer for various scan rates. The IR transmission spectra were measured by using Perkin-Elmer paragon 500 FTIR spectrophotometer in the range 400-4000 cm−1 following KBr Pellet method. The Raman spectra were recorded using SJ-301 Mitutoyo surface Profilometer with Imaging Spectrograph STR 500 mm focal length Laser Raman spectrometer. The absorption spectra were recorded between 350 and 1700 nm by using Perkin–Elmer–Lambda 950 UV/Vis/NIR spectrophotometer with a resolution of ± 0.1 nm. Luminescence spectra of the glasses were measured using Perkin Elmer LS55 spectrophotometer in the wavelength range 500−700 nm with a spectral resolution of ±1.0 nm. All these measurements were carried out at room temperature (RT). 3. Results and discussion 3.1. XRD, SEM and EDX Analysis
Figure 1 shows the XRD pattern of the 0.5ETFB glass and it is observed from the figure that the XRD profile exhibits broad diffused scattering at lower angles suggesting the long range structural disorder confirming the amorphous nature . The EDX spectrum of the 0.5ETFB glass shown in figure 2 confirms the presence of heavy metal oxides such as Zn, Ba, Ca and F in the prepared glasses. The surface morphology of the prepared Er3+ doped telluro-fluoroborate 4
glasses have been investigated through scanning electron microscope (SEM) technique. The inset of figure 2 shows the SEM images of the 0.5ETFB glass for the various magnifications such as 10 µm, 5 µm and 500 nm respectively. All the prepared glasses exhibit same surface morphology and there is no cluster formation or nucleation growth found on the smooth glassy surface. 3.2. FTIR spectral analysis The FTIR spectra of the prepared Er3+ doped telluro-fluoroborate glasses recorded in the wave number region 400 to 4000 cm-1 are shown in figure 3. The band positions and their assignments are presented in table 2. The FTIR spectra of all the title glasses follow the similar pattern and contain many peaks specifying the local structure of the prepared glasses . The broad shoulder observed at around 3450 cm-1 in all the glasses are mainly due to the fundamental stretching vibration of hydroxyl groups [17, 20, 21]. The band around 1650 cm-1 is due to the stretching vibrations of borate triangles with non-bridging oxygens . The presence of B–O stretching vibration of BO3 units was identified by observing a peak at 1359 cm-1 . The broad peak observed at 1225 cm–1 is due to the stable tetrahedral BO4 units . The band around 1010 cm-1 corresponds to the stretching vibration of BO4 group and a band around 690 cm-1 is due to the oxygen bridge between two trigonal boron atoms forming the B–O–B bending vibration . The band at around 450 cm-1 is attributed to the Te-O-Te or O-Te-O linkage bending vibrations [25-27]. 3.3. Raman spectral analysis The Raman spectroscopy is an effective tool to investigate the structure of the amorphous materials. Figure 4 represents the deconvoluted Raman spectrum of the 0.5ETFB glass and the observed Raman spectral peak positions along with their band assignments are listed in table 3. The peak at 371 cm−1 can be contributed to the axial bending vibration mode due to O-Te-O . The band assignment at around 522 cm−1 represents the isolated diborate group [29, 30]. The Raman band around 682 cm−1 probably originates from the vibrations of ring and chain type metaborate groups . The peak position around 1013 cm−1 is due to the vibrations of pentaborate and tetraborate groups . The band at around 1094 cm−1 indicate the presence of diborate group and B-O stretching vibration of BO4 units in tri–, tetra– and penta– borate groups . The band around 1197 cm−1 is assigned to the symmetric stretching vibrations of BO2O− 5
triangles linked with BO4- tetrahedral units . The strong band observed around 1310 cm−1 correspond to the B-O stretching vibrations involving non-bridging oxygens . The Raman bands observed at around 1392 cm−1 and 1463 cm−1 indicate the BO2O− triangles linked with other borate triangular units . 3.4. Absorption spectra and bonding parameters Figure 5 shows the absorption spectrum of the 3wt% Er3+ doped telluro-fluoroborate glass recorded in the wavelength region 350-1700 nm. The spectrum consists of 10 absorption bands at around 1531, 977, 798, 651, 543, 522, 488, 450, 442 and 405 nm corresponds to the transition from 4I15/2 ground state to the various excited states 4I13/2, 4I11/2, 4I9/2, 4F9/2, 4S3/2, 2H11/2, 4
F7/2, 4F5/2, 4F3/2 and 2H9/2 respectively. Among all the transitions, the absorption band observed at
522 nm corresponding to the 4I15/2→2H11/2 transition exhibit higher intensity and follows the selection ruleôDLô£ 2 and ôDJô£ 2. This transition is known as hypersensitive transition due to its strong dependency on the environment of the active Er3+ ions. The bonding nature of the Er3+ligand can be evaluated from the nephelauxetic ratios and bonding parameter values because the RE ions produce nephelauxetic effect when doped into the glass matrix. The nephelauxetic ratio (β) can be defined as the ratio between wave number of a particular transition of RE ion in the host matrix (vc) and the wave number (va ) of the same transition of aqua-ion. The nephelauxetic ratio is given by  b=
From the mean values of b , the bonding parameter (δ) values can be calculated using the following expression .
The bonding nature of the Er3+ ion with their surrounding ligands can be ionic or covalent depending upon the positive or negative sign of the δ value. The nephelauxetic ratio and the bonding parameter values of the prepared Er3+ doped telluro-fluoroborate glasses have been calculated and presented in table 4. It is observed from the table that the δ values of the present glasses are found to be −0.5319, −0.5312, −0.4675, −0.4674, −0.4618 and −0.3228 6
corresponding to the 0.05ETFB, 0.1ETFB, 0.5ETFB, 0.75ETFB, 1ETFB and 3ETFB glasses respectively. The negative sign of the d values indicates that the Er-O bond is of ionic in nature and the ionicity decreases with the increase in Er3+ ion concentration in the prepared glasses. 3.5. Optical Band gap and Urbach’s energy studies The electronic band structure of the amorphous materials can be studied through the optical absorption spectra and their fundamental absorption edges are not sharply defined which characterize the glassy nature. For higher absorption region (α(ν) ≥ 104 cm-1) Davis and Mott gave the relation between the absorption co-efficient α(ν) as a function of photon energy (hν) and band gap energy and it can be expressed as 
a (n ) =
B(hn - E opt )
where hν is the photon energy, B is the band tailing parameter, Eopt is the optical band gap energy and n is the index number which can have the values as 1/2 and 2 corresponding to the direct and indirect allowed transitions respectively. The optical band gap values were obtained from the Tauc’s plot of (αhν)1/n vs photon energy (hν) by extrapolating the linear portion of the curves at (αhν) 2=0 and (αhν)
=0 for direct and indirect allowed transitions
respectively. Figure 6 shows the Tauc's plot of the prepared Er3+ doped telluro-fluoroborate glasses. The fundamental absorption edge (λedge), optical band gap (Eopt) and the band tailing parameter (B) values corresponding to n=1/2 and 2 of the prepared glasses have been calculated and presented in table 5. The optical band gap energy values of the prepared glasses are found to be in the range 2.852 to 2.899 eV and 2.666 to 2.771 eV corresponding to the direct and indirect allowed transitions respectively. The Eopt values are found to increase with the increase in Er3+ ion content in the prepared glasses and it is due to the reduction of non-bridging oxygen’s (NBO’s). The decrease in B2O3 content reduces the formation of BO4 units and the NBO’s in the prepared glasses. The decrease in NBO’s lowers the valence band maximum (VBM) which in turn leads to have an increase in the band gap values. .
The absorption co-efficient α(ν) near the band edge exhibit an exponential dependence on the photon energy (hν) in the lower absorption region (α(ν)<104 cm-1) and obeys the empirical relation given by Urbach  as,
æ hn ö a (n ) = a 0 expç ÷ è DE ø
where α0 is a constant and ΔE is the Urbach energy defined as width of the tail states which extend into the forbidden band gap. Also, the Urbach energy can be used to identify the degree of disorderliness in amorphous materials. The inset of figure 6 shows the Urbach’s plot of the prepared Er3+ doped telluro-fluoroborate glasses. The ΔE values of the studied glasses have been calculated by using the above expression and the results are presented in table 5. The Urbach energy values are found to be in the range 0.3917 to 0.3095 eV. The Urbach energy values vary inversely with the optical band gap values and decreases linearly with the increase in Er3+ ion content. The lower Urbach energies could not be able to convert weak bonds into defects which indicate that the prepared glasses possess minimum defects and less disorderliness which inturn leads to structural rearrangement facilitating the long range order in the glass network. 3.6. Oscillator strength and Judd-Ofelt parameters The intensity of the absorption bands can be expressed in terms of oscillator strengths. The experimental oscillator strengths can be obtained by integrating the area of each band in the absorption spectra following the below given expression .
2 . 303 mc 2 N pe 2
ò e (n ) d n
= 4 . 318 ´ 10 - 9 ò e (n ) d n
where Ɛ(ν) is the molar absorptivity (in cm-1) of the corresponding transition, c is the velocity of light, ‘m’ and ‘e’ are the mass and charge of the electron and N is the Avagadro’s number. According to Judd-felt theory, the calculated oscillator strength of an electric dipole transition from ground state to excited state can be calculated by using the equation 2 é 8p 2 mcn ù é (n 2 + 2) ù l ' ' f cal = ê ú ´ å l = 2, 4 , 6 W l Y J U Y J ê ú ë 3h(2 J + 1) û ëê 9n ûú
were n is the refractive index, Ω λ (λ=2,4,6) are the Judd-Ofelt intensity parameters which are used to examine the nature of the metal-ligand bond and ‖ U l ‖ are the reduced square matrix elements of the unit tensor operator of rank λ=2,4 and 6 respectively and J is the angular
momentum of the ground state. The experimental and calculated oscillator strengths for the various absorption transitions of the Er3+ doped telluro-fluoroborate glasses have been calculated and are presented in table 6 along with the root men square deviations. The lower value of root mean square (rms) deviation represents the quality of the fit between experimental and calculated oscillator strengths and confirms the validity of Judd-Ofelt theory. Among all the transitions, 4
I15/2→2H11/2 hypersensitive transition possesses higher oscillator strength and the same is
sensitive to the host matrix since it obeys the selection ruleôDLô£ 2 and ôDJô£ 2. The Judd-Ofelt intensity parameters Ωl (l= 2, 4 and 6) of the prepared glasses were derived from the absorption spectral intensities by least square fitting procedure and the values are presented in table 7. The Ω2 intensity parameter provides information about local structure around the Er3+ ions and bonding nature of the Er-O bond in the host matrix . The covalency of the Er-O bond is inversely proportional to the Ω6 parameter value. For the present glasses, Ω2 parameter values are found to decrease with the increase in Er3+ ion concentration indicating the fact that the Er3+ ions are located in a higher symmetrical environment. The JO parameters follow the trend as Ω2˃Ω6˃Ω4 for all the prepared glasses . The spectroscopic quality factor (Ω4/Ω6) is the measure of emission strength of the glasses and used to identify the optical quality of the prepared glasses. The Ω4/Ω6 values of the prepared glasses are given in table 7 and it is observed from the table that, the Ω4/Ω6 value (0.894) of the 0.75ETFB glass is found to be high compared to the other studied glasses and is useful for different photonic applications . 3.7. Luminescence spectra and Radiative properties Figure 7 shows the excitation spectrum of the 0.5ETFB glass recorded by monitoring an excitation at 548 nm and contains four excitation bands at around 378, 401, 452 and 488 nm corresponding to the
respectively. Among them, the excitation band at 488 nm possesses higher intensity and the same was used as an excitation source to record the emission spectra. The emission spectra of the title glasses have been recorded in the wavelength region 500 nm to 700 nm and are shown in figure 9
8. The emission spectra exhibit three emission bands in the visible region observed at 527 nm (green), 548 nm (green) and 667 nm (red) corresponding to the 2H11/2 →4I15/2 , 4S3/2→4I15/2 and 4
F9/2→4I15/2 transitions respectively. The emission band centred at 548 nm exhibits higher
intensity compared to the other transitions. It is observed that the emission intensity gradually increases with the Er3+ ion concentration upto 0.75 wt% and after that decreases due to efficient energy transfer between Er3+ ions. The JO intensity parameters and the refractive index were used to obtain the radiative properties such as radiative transition probability (A), stimulated emission cross-section (
calculated branching ratios of all the emission transitions of the Er 3+ doped telluro-fluoroborate glasses. The transition probability (A) of a particular transition can be calculated by using the following expression . A( Y J , Y ' J ' ) = Aed + Amd =
ö æ n(n 2 + 2)2 64p 4 çç ´ S ed + n 3 S md ÷÷ 3 3hl ( 2 J + 1) è 9 ø
æ n ( n 2 + 2) 2 ö where Aed and Amd are the electric and magnetic-dipole transition probabilities, çç ÷÷ is 9 ø è
the local field correction for the electric dipole transitions and n3 is the local field correction for magnetic dipole transitions. S ed and Smd are the electric and magnetic-dipole line strengths and they can be obtained using the below given expressions,
Sed = e 2 ål =2, 4, 6 Wl YJ U l Y ' J '
and S md =
e2h 2 W l Y J L + 2S Y ' J ' 2 2 2 16 p m c
The total radiative transition probability AT can be obtained from the sum of A(YJ , Y ' J ') calculated over all the terminal levels and is expressed as
AT YJ =S A(YJ , Y ' J ')
The radiative lifetime is defined as the reciprocal of total radiative transition probability and is given by 10
t (YJ) = [AT (YJ)] −1
The radiative branching ratio (bR) can be calculated from the following relation
b R ( YJ , Y ' J ) =
A(YJ , Y ' J ' ) AT YJ
The experimental branching ratios can be obtained from the areas under the emission curves. The peak stimulated emission cross-section ( s PE ) can be found using the expression
l 4p A = 8p cn 2 D l eff
where n is the refractive index, lp is the emission transition peak wavelength and Dleff is the effective line width of the transition and can be expressed as
I max ò
I (l )dl
where I is the fluorescence intensity and Imax is the intensity at band maximum. The calculated radiative properties of the Er3+ doped telluro-fluoroborate glasses are presented in table 8. The radiative properties such as peak emission wavelength (λp), transition probability (A), effective line width (∆λeff), stimulated emission cross-section (
) and branching ratios
corresponding to the H11/2+4S3/2→4I15/2 transition of the present glasses were obtained from the emission spectra and the results are given in table 8. The larger stimulated emission cross-section is the characteristic of efficient laser action. Among the prepared glasses, 0.75ETFB glass exhibits higher
value (42.567×10─22cm2) which is comparatively higher than the reported
glasses such as ZnO-LiF (37×10─22cm2) , sodalime silicate (22.5×10─22cm2) , BNNEr (34.9×10─22cm2)  and N5BEr (41.45×10─22cm2)  glasses respectively. The experimental branching ratio (βExp) gives the probability of obtaining laser action from a particular emission transition. The 0.75ETFB glass possesses higher βExp values among all the studied glasses and is comparatively higher than the reported glasses [49-51]. The higher stimulated emission crosssection and experimental branching ratio values of the H11/2+4S3/2→4I15/2 emission transition corresponding to the 0.75ETFB glass is useful for green laser applications. The optical gain band width (
×∆λeff) values of the H11/2+4S3/2→4I15/2 transition are found to be 12.87, 20.13, 19.73,
34.03, 16.62 and 15.81 (×10─28cm3) corresponding to the 0.05ETFB, 0.1ETFB, 0.5ETFB, 11
0.75ETFB, 1ETFB and 3ETFB glasses respectively. Of all the studied glasses, 0.75ETFB glass exhibit higher (
×∆λeff) value and the same may be suggested as a potential candidate for solid
state green laser applications. 3.8. CIE chromaticity coordinates The emission intensities of the present glasses were characterized through CIE 1931 color chromaticity diagram. Three color matching functions such as ̅ (λ),
(λ) and ̅ (λ) are good
enough to describe the color produced by any light source. The real spectral colors can be obtained by adding three artificial ‘colors’ denoted as X, Y and Z called tristimulus values. The
degree of stimulation necessary to match the color of given power spectral density (p(λ)) can be expressed as = ∫ ̅ (l ) (l ) l
= ∫ (l ) (l ) l
= ∫ ̅ (l ) (l ) l
(15) (16) (17)
where X, Y and Z are the tristimulus values and give the power for each of the three primary colors required to match with the color of P(l). From these tristimulus values, the color chromaticity coordinates x and y can be determined from the following expression , =
The calculated (x, y) coordinates are found to be (0.316, 0.643), (0.314, 0.647), (0.315, 0.663), (0.315, 0.664), (0.321, 0.658) and (0.331, 0.655) corresponding to the 0.05ETFB, 0.1ETFB, 0.5ETFB, 0.75ETFB, 1ETFB and 3ETFB glasses respectively. The location of the chromaticity coordinates (x, y) of the studied glasses were shown in the inset of figure 9 and it is observed that the chromaticity coordinates of the present Er3+ ions doped glasses are mostly passes through the green light region in the CIE 1931 chromaticity diagram.
4. Conclusion The structural and optical properties of the Er3+ doped telluro-fluoroborate glasses have been studied and reported in the present work. The XRD pattern and the SEM images confirm the amorphous nature. The presence of various functional groups such as B ̶ O ̶ B bending linkage vibrations, B ̶ O bond in BO3 units and the symmetric and stretching vibration of Te ̶ O bonds in the TeO3 units were observed from the FTIR and Raman spectral analysis. The band gap values are found to increase with the increase in Er3+ ion content due to the reduction in nonbridging oxygen’s in the prepared glasses. The lower Urbach energies indicate that the present glasses possess minimum defects and less disorderliness. The negative sign of the bonding parameter values indicate the ionic nature of the Er3+ ̶ O bond in the present glasses. The decreasing Ω2 values illustrate the fact that the Er3+ ions are situated in a higher symmetrical environment. Among all the studied glasses, 0.75ETFB glass exhibit higher (
, βExp and
×∆λeff) values compared to the reported glasses and the same may be suggested as a potential
candidate for solid state green laser applications. The chromaticity coordinates of the present glasses are found to lie in the green light region of the CIE 1931 chromaticity diagram illustrating the possibility of obtaining green emission from the studied glasses by exciting with commercially available blue LED’s. Acknowledgement One of the authors Prof. K. Marimuthu would like to thank DAE–BRNS, Mumbai, Govt. of India for the sanction of financial support in the form of a Major Research Project No. 2012/34/49/BRNS/2034, dt. 27/11/2012.
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Figure 1: XRD pattern of the 0.5wt% Er3+ doped Telluro-fluoroborate glass
Figure 2: EDX spectrum of the 0.5ETFB glass [Inset shows the SEM images of the 0.5ETFB glass for the various magnifications (A) 10 µm (B) 5 µm and (C) 500 nm]
0.05ETFB 0.1ETFB 0.5ETFB 0.75ETFB 1ETFB 3ETFB
Figure 3: FTIR spectra of the Er3+ doped Telluro-fluoroborate glasses
Wavenumber (cm )
Figure 4: The experimental (solid line) and deconvoluted (dash line) Raman spectrum of the 0.5ETFB glass
Absorption coefficient (cm-1)
Figure 5: Absorption spectrum of the 3wt% Er3+ doped Telluro-fluoroborate glass
0.05ETFB 0.1ETFB 0.5ETFB 0.75ETFB 1ETFB 3ETFB
(ahn)2 (eV) (cm-1)2
0.05ETFB 0.1ETFB 0.5ETFB 0.75ETFB 1 ETFB 3 ETFB
hn (eV) 3+ Figure 6: Tauc’s plot for the Er doped Telluro-fluoro borate glasses [Inset shows Urbach energy of the Er3+ doped Telluro-fluoroborate glasses]
lemi ® 548nm
Figure 7: The excitation spectrum of the 0.5wt% Er3+ doped Telluro-fluoroborate glass
l® 488nm exc 4F ®4I 15/2 9/2
0.75 ETFB 0.5 ETFB 0.1 ETFB 1 ETFB 3 ETFB 0.05 ETFB
4 ® 4 S3/2 I15/2
0.75 ETFB 0.5 ETFB 0.1 ETFB 1 ETFB 3 ETFB 0.05 ETFB
Figure 8: Luminescence spectra of the Er3+ doped Telluro-fluoroborate glasses
· 0.05ETFB · 0.1ETFB · 0.5ETFB · 0.75ETFB · 1ETFB · 3ETFB
y - chromaticity coordinate
x - chromaticity coordinate Figure 9: The CIE 1931 color chromaticity diagram of the Er3+ doped Tellurofluoroborate glasses
Table 1: Physical properties of the Er3+ doped Telluro-fluoroborate glasses Physical Properties
Density ρ (g/cm3)
Refractive index nd (589.3 nm)
Rare earth ion concentration N (1020 ions/cm3) Polaron radius rp(A°)
Field strength F (1014 cm-2)
Electronic polarizability ae (10-23 cm2) Molar refractivity Rm (cm3)
Dielectric constant (e)
Reflection losses R (%)
Optical dielectric constant (P )
Inter ionic distance ri(A°)
Table 2: Band positions (cm-1) and its corresponding peak assignments of FTIR spectra of Er3+ doped Telluro-fluoroborate glasses 0.05ETFB
3465 2924 2854 1719
3452 2925 2854 1716
3464 2925 2853 1711
3460 2924 2854 1718
3472 2924 2855 1718
3447 2929 2857 1719
Assignments OH stretching vibrations Hydrogen bonding Hydrogen bonding H–O–H bending Stretching vibration of borate triangles B–O Stretching (BO3)– units Stable tetrahedral BO4 units B–O vibrations attached with BO4 units B–O–B bond bending vibrations from pentaborate groups Te–O–Te or O–Te–O linkage bending vibrations
Table 3: Raman spectral peak positions (in cm-1) and band assignments for the 0.5wt% Er3+ doped Telluro-fluoroborate glass Peak positions (cm-1) 371
Peak assignments Axial bending vibration mode (O–Te–O)
Isolated diborate groups
Vibrations of ring and chain-type meta borate units
Vibrations due to pentaborate and tetraborate groups
1094 1197 1310 1392, 1463
Diborate groups and B–O stretching vibration of BO4 units in tri-, tetra- and penta- borate groups Symmetric stretching vibrations due to BO2O– triangles linked with BO4– tetrahedral units Assigned to the B–O vibrations BO2O– triangles linked with other borate BO3 triangular units
Table 4: Observed band positions (cm-1) and bonding parameters ( b and δ) for the Er3+ doped Telluro-fluoroborate glasses
6680 10240 12530 15350 18400 19200 20530 1.005
6690 10245 12525 15299 18367 19235 20564 1.005
6680 10240 12530 15350 18390 19190 20490 22170 1.005
6690 10220 12525 15325 18393 19184 20494 22184 22646 1.005
6680 10230 12500 15360 18410 19200 20480 22190 22610 1.005
6680 10200 12520 15300 18360 19120 20490 22180 22580 24650 1.003
Aqua-ion  6600 10250 12400 15250 18350 19150 20450 22100 22500 24550 -
Energy level 4
I13/2 I11/2 4 I9/2 4 F9/2 2 S3/2 2 H11/2 4 F7/2 4 F5/2 4 F3/2 4 H9/2 4
Table 5: The fundamental absorption edge (λedge), Optical band gap (Eopt), Band Tailing parameter (B) corresponding to the direct (n=½) and indirect (n=2) allowed transitions and Urbach energy (DE) of the Er3+doped Telluro-fluoroborate glasses Sample code
Eopt (eV) 2.852
B -2 (cm eV)2 1054.09
Eopt (eV) 2.666
B -2 (cm eV)-1/2 18.92
Table 6: Experimental and calculated oscillator strengths (×10-6) of the Er3+doped Tellurofluoroborate glasses Energy level 0.05ETFB 4 fexp fcal I15/2 → 4 0.862 0.756 I13/2 4
0.1ETFB fexp fcal
0.5ETFB fexp fcal
0.75ETFB fexp fcal
1ETFB fexp fcal
3ETFB fexp fcal
I9/2 F9/2 2 S3/2 2 H11/2 4
Table 7: Judd-Ofelt (×10-20cm2) parameters, Trends of Ωλ, Spectroscopic quality factors (Ω4/ Ω6) of the Er3+ doped Telluro-fluoroborate glasses
Trends of Ωλ
Table 8: Emission band position (λp, nm), effective bandwidth (∆λeff, nm), radiative transition probability (A, s-1), stimulated emission cross-section ( s PE ×10-22cm2), experimental and calculated branching ratios (βR) of the Er3+ doped Telluro-fluoroborate glasses
H11/2, 4S3/2 → 4I15/2
l® 488nm exc
0.75 ETFB 0.5 ETFB 0.1 ETFB 1 ETFB 3 ETFB 0.05 ETFB
4F ®4I 15/2 9/2
4 2 H11/2 I15/2
4 ® 4 S3/2 I15/2
0.75 ETFB 0.5 ETFB 0.1 ETFB 1 ETFB 3 ETFB 0.05 ETFB
Ø Lower Urbach energy indicate the presence of minimum defects in the glasses Ø Lower Ω2 values illustrate that the Er3+ ions are situated in a higher symmetrical environment Ø 0.75ETFB glass exhibit higher
, βExp and (
×∆λeff) values suitable for laser applications
Ø CIE 1931 diagram illustrate the possibility of obtaining green emission from the present glasses