Concentration dependent luminescence studies on Eu3+ doped telluro fluoroborate glasses

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Concentration dependent Luminescence studies on Eu3 þ doped Telluro fluoroborate glasses R. Vijayakumar, K. Maheshvaran, V. Sudarsan, K. Marimuthu

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S0022-2313(14)00261-0 http://dx.doi.org/10.1016/j.jlumin.2014.04.022 LUMIN12663

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Journal of Luminescence

Received date: 30 January 2014 Revised date: 26 March 2014 Accepted date: 18 April 2014 Cite this article as: R. Vijayakumar, K. Maheshvaran, V. Sudarsan, K. Marimuthu, Concentration dependent Luminescence studies on Eu3 þ doped Telluro fluoroborate glasses, Journal of Luminescence, http://dx.doi.org/10.1016/j. jlumin.2014.04.022 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Concentration dependent Luminescence studies on Eu3+ doped Telluro fluoroborate glasses R. Vijayakumar1, K.Maheshvaran2, V.Sudarsan3, K.Marimuthu1,∗ 1 2

Department of Physics, Gandhigram Rural UniveFrsity, Gandhigram – 624 302, India

Department of Physics, K.S.Rangasamy College of Technology, Trichengode – 637 215, India 3

Radiochemistry Division, Bhabha Atomic Research Centre, Mumbai – 400 085, India

Abstract Eu3+ doped telluro fluoroborate glasses with the chemical composition (40–x)B2O3+ 15TeO2+15Li2CO3+15LiF+15NaF+xEu2O3 (where x = 0.05, 0.1, 0.25, 0.5, 1 and 2 in wt%) have been prepared following the melt quenching technique. Optical properties of the prepared glasses were studied through the absorption, excitation, luminescence and decay spectral measurements. Nephelauxetic’s ratios (βR) and bonding parameters (δ) were calculated from the absorption spectra to examine the nature of the bonding between the Eu3+ ion and its surrounding ligands. The optical band gap and Urbach’s energy of the title glasses were determined from the absorption spectra to study the electronic band structure and it is observed that, the band gap values vary inversely with the Urbach’s energy values. The Judd-Ofelt intensity parameters (Ω2, Ω4 and Ω6) determined from the emission spectra provides valuable information about the Eu−O bonding nature and the structure of the local environment around the Eu3+ ion site. The luminescence intensity ratio (R) of the 5D0→7F2/5D0→7F1 transition has been calculated from the emission spectra to determine the degree of asymmetry around the Eu3+ ion site and to identify the strength of the covalence of Eu3+ ion with its surrounding ligands. The chromaticity coordinates have been calculated from the emission spectra and analyzed with Commission International de I’Eclairage (CIE) 1931 diagram. The JO intensity parameters have been used to determine the radiative properties such as radiative transition probability (AR), stimulated emission cross- section ( σ PE ) and branching ratios (βR) to estimate the lasing power of the emission transitions and optical gain band width due to their practical importance in laser applications. Keywords: Absorbance; Urbach’s energy; Luminescence; CIE color chromaticity; Judd–Ofelt parameters; Stimulated emission cross-section. 1

PACS: 78.30.Ly, 78.40.Pg, 78.47.Cd, 78.55.Qr. *

Corresponding author. Tel.: +91 451 2452371; Fax: +91 451 2454466.

E-mail address: [email protected] 1. Introduction For the past few decades, in order to find new optical materials considerable work have been carried out on rare earth (RE) ion doped glass materials due to their potential applications in optoelectronics, solid state lasers, fiber amplifiers, infrared to-visible up-converters, phosphors and field emission displays [1−4]. Materials doped with trivalent RE ions give much devotion for the development of optical devices due to the fact that the 4fn electrons are effectively shielded by the 5s2 and 5p6 electrons which offer sharp lines in the absorption and emission spectra due to its f−f transition. Many researchers paid much attention on rare earth doped glasses due to their inhomogeneously broadened linewidth because of site to site variation of local environment, probability of getting bulk samples and easy fabrication which are difficult to obtain with crystals [5]. Interestingly, among the several host materials tellurite glass is the best choice for RE ion doping because it possess high refractive index, low phonon energy (707 cm−1), high transparency in the mid-IR region and capable of incorporating large number of rare earth ions than the other hosts. However, pure TeO2 is a conditional glass former which does not form glass on its own, but it does with the help of modifier oxides like B2O3, Li2O and CdO etc., Here, borate is added as a network modifier which forms the disordered network through the change of boron coordination into BO3, BO4 units along with the formation of non-bridging oxygens, facilitating the occupancy of large number of RE ions. Also, it possesses the shielding property against infrared radiation, so that it can be used as a dielectric media and enhances the glass stability. Still, it has some drawbacks like low absorption coefficient and high phonon energy leads to non-radiative transitions [6]. In order to reduce the non-radiative losses, the low phonon energy fluoride content is added which decreases the multiphonon non-radiative de-excitation which in turn results intense radiative emission [7–12]. Furthermore, it is moisture resistance and gives good transparency in the visible region. The migration of fluoride ion with fast moving anion significantly increases the electrical conductivity and is suitable for the development of advanced batteries and electrochemical devices [13]. Eu3+ ions are more attractive than any other RE ions because of its narrow band emission, nearly monochromatic nature of the 5D0→7F2 transition gives red emission around 616 nm which 2

is suitable for laser applications and for the preparation of red phosphors and also it provides higher luminescence efficiency. The sufficient information about the local structure around the Eu3+ ion site can be easily obtained from the f−f transition spectra and therefore, it may be used as a probe to estimate the local structure around the Ln3+ ions in various matrices. The ground (7F0) and excited state (5D0) of the Eu3+ ion are non-degenerate and give valuable information about asymmetry as well as inhomogeneity of the surrounding ligands [14]. The splitting of the 5

D0→7F1 transition into three components [15] can also be used to confirm the lower symmetry

possessed by the Eu3+ ions, which is further confirmed through the luminescence intensity ratio (R) of the (5D0→7F2 )/(5D0→7F1) transitions, widely known as asymmetric ratio. Luminescence behavior of Eu3+ ions doped borate and fluoroborate glasses containing lithium, zinc and lead oxides has been studied and reported by Venkatramu et al. [6]. B.Deva Prasad Raju et al. [13] investigated the structural and optical properties of Eu3+ ions in alkali fluoroborate glasses containing lead oxide. Spectroscopic studies of Eu3+ ions in LBTAF glasses have been reported by B.C. Jamalaiah et al. [14]. S.Surendra Babu et al. [15] studied and reported the optical absorption and photoluminescence behavior of Eu3+ doped phosphate and fluorophosphate glasses. Absorption and emission properties of Eu3+ ions in sodium fluoroborate glasses have been reported by S. Balaji et al. [16]. The present work reports, luminescence properties of Eu3+ doped telluoro fluoroborate glasses by varying the RE ion concentration. The prepared glasses were characterized through optical absorption, excitation, luminescence and decay measurements. The main objective of the present work is to (i) synthesis Eu3+ doped telluro fluoroborate glasses (ii) examine the energy levels and bonding parameters (βR and δ) to claim covalent/ionic nature (iii) determine the optical band gap (Eopt) and Urbach energy (ΔE) values to analyze the optical behavior (iv) evaluate the JO parameters (Ω2, Ω4 and Ω6) and to compare the trends of JO parameters with respect to other Eu3+ doped glasses (v) determine the CIE color coordinates for different Eu3+ ions concentration and finally, to (vi) derive the radiative properties for the significant energy levels of the Eu3+ ion and to compare the results with reported literature. 2. Experimental Eu3+ doped telluro fluoroborate glasses were prepared by following conventional melt quenching technique by taking high purity (99.99%) analytical grade chemicals such as B2O3, 3

TeO2, Li2CO3, LiF, NaF and Eu2O3 from Sigma Aldrich following the procedure reported in literature [17]. The sample codes and the chemical compositions (in wt%) of the prepared glasses are as follows: TFB0.05Eu

:

39.95 B2O3+15TeO2+15Li2CO3+15LiF+15NaF+0.05Eu2O3

TFB0.1Eu

:

39.9 B2O3+15TeO2+15Li2CO3+15LiF+15NaF+0.1Eu2O3

TFB0.25Eu

:

39.75 B2O3+15TeO2+15Li2CO3+15LiF+15NaF+0.25Eu2O3

TFB0.5Eu

:

39.5 B2O3+15TeO2+15Li2CO3+15LiF+15NaF+0.5Eu2O3

TFB1Eu

:

39 B2O3+15TeO2+15Li2CO3+15LiF+15NaF+1Eu2O3

TFB2Eu

:

38 B2O3+15TeO2+15Li2CO3+15LiF+15NaF+2Eu2O3

The absorption spectra have been recorded using CARY 500 UV-VIS-NIR spectrometer in the wavelength region 380–2500 nm with a resolution of ±0.1 nm. The luminescence spectra were recorded in the wavelength range between 550 and 725 nm using Perkin Elmer LS55 spectrophotometer with a spectral resolution of ±1.0 nm. The decay measurements were made employing sciencetech modular spectrometer using xenon flash lamp as an excitation source. All these measurements were carried out at RT only. The densities of the prepared glasses have been determined following the Archimedes principle using xylene as an immersion liquid. Refractive index of the title glasses were found employing Abbe refractometer at sodium wavelength (5893 Ǻ) using the monobromonapthalene as a contact liquid. The physical properties of the title glasses have been calculated and presented in table 1. 3. Results and discussion 3.1 Absorption spectral analysis The optical absorption spectra of the Eu3+ doped telluro fluoroborate glasses recorded in the wavelength range 380 – 2500 nm is shown figure 1. The absorption transitions arise from both 7F0 and 7F1 states to various higher excited states and are due to 4f−4f transitions [18,19]. The band assignments along with the band positions are presented in table 2 and it is observed from the table that, the band position occurs almost at the same wavelength irrespective of the 4

change in RE ion concentration. The absorption spectra exhibit eight bands at around 395, 402, 415, 465, 526, 534, 588 and 612 nm corresponding to the transitions 7F0→5L6, 7F1→5L6, 7

F1→5D3, 7F0→5D2, 7F0→5D1, 7F1 →5D1, 7F0→5D0 and 7F1 →5D0 in the visible region and two

more bands at 2091 nm, 2205 nm corresponding to the transitions 7F0→7F6, 7F1→7F6 in the NIR region. The absorption bands arise due to the 7FJ →5DJ transitions are found to be weak compared with other transitions because they are spin forbidden but partially allowed due to J mixing by the crystal field. The intensity of the 7F0→5L6 transition is found to be higher than all other transitions and the induced electric dipole transition 7F0→5D2 is hypersensitive in nature. The 7F0→5D1 transition follows magnetic dipole selection rule and possess less intensity than the electric dipole transition 7F0→5D2. No transition is observed below 390 nm and is due to the rapid increase in the absorption edge of the glass host [17]. The nature of the bonding between the Eu3+ ion and its surrounding ligands can be obtained by calculating the bonding parameter (δ) values using the below given relation [20], δ=

1− β

β

(1)

×100

where, β is average value of the Nephelauxetic ratios (β) calculated from the relation β=νc/νa, where νc is the wavenumber (in cm−1) of a particular RE ion transition and νa is the wavenumber (in cm−1) of the corresponding aquo-ion transition. The bonding between RE ion and its surrounding ligands may be covalent or ionic depending upon positive or negative sign of δ and it provides information about the local structure around the RE ion site. The calculated β and δ  values of the title glasses were presented in table 2 and it is observed from the tabulated results that, the Nephelauxetic ratios (β) were found to decrease with the increase in Eu3+ ion concentration in the prepared glasses. The positive values of the bonding parameter (δ) indicate the covalent nature of the Eu−O bond and are found to increase in the order TFB0.05Eu
5

3.2 Optical bandgap and Urbach’s energy studies Photons with energy greater than the optical band gap energy will get absorbed and based on this principle the electronic band structure of an amorphous material can be studied through the absorption spectra [23]. The band gap in electronic band structure of solids generally refers to the energy difference between the top of the valence band and the bottom of the conduction band in insulators and semiconductors. This is equivalent to the energy required to free an outer shell electron from its orbit and to become a mobile charge carrier move freely within the solid material. The band gap is an important factor which determines the electrical conductivity of a solid material. The absorption coefficient near the fundamental absorption edge gives information about the optically induced transitions and the same can be calculated from the relation α( ) = (1/d)ln(I0/It)

(2)

where, d is the thickness, I0 and It are the intensities of the incident and transmitted radiations respectively. For the higher absorption region (α( )>104 cm−1), Mott and Davis [24,25] proposed the relation between the absorption coefficient and the optical band gap as (3) where h is the plank’s constant, Eopt is the optical band gap energy, B is the band tailing parameter and m denotes the index number m = 1/2, 2 depending upon the direct and indirect allowed transitions [26]. The optical band gap ( expression plot of (

) values were determined using the

by extrapolating the linear region of the curve in the Tauc’s )m against

at (

)m = 0 for the corresponding direct (m=1/2) and indirect (m=2)

allowed transitions [27]. Figure 2 shows the Tauc’s plot of (

)1/2 vs incident photon energy

for the indirect allowed transitions and it is observed that the indirect band gap values vary from 2.76 eV to 2.81 eV and the direct band gap values vary from 3.04 eV to 3.17 eV for all the prepared glasses. The optical band gap and band tailing parameter values were calculated and presented in table 3 and from the results it is observed that the direct and indirect band gap values are found to increase with the increase in Eu3+ ion concentration upto 1wt% and then decreases for higher concentration. The increasing band gap values are due to continuous 6

reduction of B2O3 content in the prepared glasses which decreases the formation of non-bridging oxygen and produce structural changes in the title glasses. Further, it increases the donor centers which in turn lead to increase the degree of localization of electrons within the host matrix and the absorption edge shift towards the lower wavelength region [23,26]. Urbach’s energy is the width of the tail of the localized states extends into the energy gap between the lowest state of conduction band and upper edge of the valance band. Urbach’s suggested that in the lower absorption region (<104), absorption coefficient (ν) varies exponentially with the incident photon energy

and obeys the empirical relation given by

Urbach [28],

⎛ hν ⎞ ⎟ ⎝ ΔE ⎠

α (ν ) = α 0 exp⎜

(4)

where 0 is a constant and ΔE is the Urbach energy which is obtained from the Urbach’s plot of lnα vs

by taking the reciprocal of slope of the curves [23]. Urbach energy is due to the

formation of defects associated with the width of the tail of localized states. Inset of figure 2 shows the Urbach’s plot of the prepared glasses and the Urbach energy values for the prepared glasses are presented in table 3. The Urbach energy values are found to decrease with the increase in Eu3+ ion concentration upto 1wt% and after that increases. Materials with larger Urbach energy would have greater tendency to convert weak bonds into defects.The decreasing Urbach’s energy values indicate the presence of minimum number of defects in the prepared glasses. The Urbach’s energy values vary inversely with the optical band gap values. Therefore, the lower Urbach energies indicate the lower extension of width of the tail of localized states into the forbidden band gap. 3.3 Judd-Ofelt intensity parameters and Radiative properties The radiative transitions of rare earth ions in different host matrices have been studied using Judd-Ofelt (JO) theory [29,30]. The JO intensity parameters (Ω2, Ω4, Ω6) provide valuable information about the bonding nature of the RE ion with its surrounding ligands and the local structure around the RE ion site. The application of JO theory to Eu3+ ion possess serious difficulties due to the presence of some of the transitions such as 5D1, 5D2 and 5L6 from 7F0 and 7F1 ground state in the optical absorption spectra with good intensity. Further, the ground state 7

(7F0) and first excited state (7F1) are being close to each other so that the JO model requires thermal correction to the oscillator strength of the corresponding transitions. In order to solve these drawbacks, the JO intensity parameters were calculated from the emission spectra following the procedure reported in literature [31] by using the doubly reduced nonzero matrix elements such as <|| Uλ||>, (λ = 2, 4, 6) corresponding to the 5D0 →7Fλ (λ = 2, 4, 6) electric dipole emission transitions of the Eu3+ ions, because the emission intensities of the 5D0 →7F2,4,6 transitions are mainly depend on the JO intensity parameters (Ω2, Ω4, Ω6). The calculated JO intensity parameter values (Ω2, Ω4, Ω6) of the prepared glasses are presented in table 4 along with the other reported Eu3+ doped glasses [13,23,32–34]. The larger Ω2 values claim the existence of covalent bond between the Eu3+ ion and their surrounding ligands which is confirmed through the bonding parameter and luminescent intensity ratio values. Further, the larger Ω2 values indicate the higher asymmetry around the Eu3+ ion site in all the prepared glasses. The observed Ω2 values of the present TFB1Eu glass is (6.9228×10–20 cm2) higher than the reported fluoroborate (2.548) [23], silicate (5.61) [32], PTBEu05 (3.72) [33] and fluorophophate (3.64) [34], LLiFB (3.62), LNaFB (3.62), LKFB (3.81) [13] glasses. The Ω4 values are found to be 1.3018, 1.0923, 1.9757, 1.6455, 3.4339 and 2.5916 (×10–20 cm2) corresponding to the TFB0.05Eu, TFB0.1Eu, TFB0.25Eu, TFB0.5Eu, TFB1Eu and TFB2Eu glasses respectively. In the present work, the value of Ω6 intensity parameter is taken as zero because the 5D0 →7F6 emission transition correspond to the Ω6 parameter could not be obtained due to instrument limitations and also Balakrishnaiah et al. [35] reported that Ω6 shows negligible contribution in the determination of lifetime values of Eu3+ ions doped glasses. The Ω4 and Ω6 parameters are related to the bulk properties like viscosity and rigidity of the host matrix. The JO parameters of all the prepared glasses follow the trend as Ω2 >Ω4 >Ω6 and is similar to the reported Eu3+ doped glasses [15,31,33,36,37]. The JO intensity parameters (Ω2, Ω4, Ω6) of the title glasses have been used to obtain the radiative properties such as radiative transition probability (AR), stimulated emission crosssection ( σ PE ), branching ratios (βR), radiative lifetime for the 5D0→7FJ (J=0,1,2,3,4) transitions of the Eu3+ ions following the procedure reported in literature [33,36,37] and the same were presented in table 5. Of all the emission transitions, the magnitude of the radiative transition probability for the 5D0→7F2 transition is found to be higher with the values 187.67, 179.15, 8

255.28, 238.23, 270.55 and 237.69 corresponding to the TFB0.05Eu, TFB0.1Eu, TFB0.25Eu, TFB0.5Eu, TFB1Eu and TFB2Eu glasses respectively. The ratio between the emission areas of the respective transition and the total area of the various transitions under the emission peaks are called as branching ratios βR. The experimental branching ratios (βR) determined from the relative areas of the 5D0 → 7FJ emission transitions are in close agreement with those predicted using JO theory. It is observed from the tabulated results that, the branching ratio values are found to follow the order as 5D0→7F2>7F1>7F4>7F3 for all the prepared glasses. The magnitude of βR, σ PE and Δλeff describes the lasing power of the emission transitions. The stimulated emission cross-section values of the prepared glasses are found to be on the higher side compared to the reported Eu3+ glasses [15,17,23]. The higher stimulated emission cross section value is the attractive factor for low threshold and high gain laser applications which are used to claim good laser action. Of the prepared glasses, TFB1Eu glass possess higher σ PE , βR, Δλeff values corresponding to the 5D0→7F2 transition and is suitable for red laser applications and also useful for the development of color display devices. 3.4 Excitation and Luminescence spectral analysis The excitation spectra of the Eu3+ doped telluro fluroborate glasses have been recorded by monitoring an emission at 612 nm and as a representative case excitation spectrum of the TFB1Eu glass is shown in figure 3. The excitation spectrum exhibit bands at 367, 387, 399, 420, 469, 538 and 557 nm corresponding to the transitions 7F0→5D4, 7F1→5L7 ,7F0→5L6 ,7F1→5D3 ,7F0→5D2 ,7F1→5D1 and 7F0→5D0 respectively. Among the observed bands the 7F0→5D2 transition located at 469 nm is more prominent than all other transitions and the same is used as a source of excitation to measure the luminescence spectra of the prepared glasses. The broad band observed at around 267 nm correspond to the charge transfer band (CTB) which indicates the electronic charge transfer of O2−−Eu3+ corresponding to the electron transfer from 2p orbital of oxygen to the unfilled orbital of the Eu3+ ion. The energy of the CTB depends on the optical electro-negativities, polarizability, coordination number of the ligand fields and the distance between the Eu–O bonds. Also, the energy of this process depends on the covalency of the Eu–O bond, if the covalency increases then the electron energy transfer decreases. In the present study, covalency increases with increasing Eu3+ ion concentration and the same may be the reason for 9

getting less intense dipole allowed CTB compared to the dipole disallowed f-f transitions [34,38,39]. Figure 4 shows the luminescence spectra of the title glasses recorded by monitoring an excitation at 469 nm and are similar to the reported Eu3+ doped glasses [13,15,23]. All the spectra exhibit emission bands at around 578, 590, 612, 652 and 700 nm corresponding to the 5D0→7F0, 5

D0→7F1, 5D0→7F2, 5D0→7F3 and 5D0→7F4 transitions respectively. Due to the experimental

limitations, 5D0→7F6 transition found to occur in the near infrared region could not be observed. It is observed from the luminescence spectra that, the 5D0→7F2 electric dipole transition holds higher intensity than other transitions and is sensitive to the environment around the Eu3+ ion site and follows the selection rule ∆J=2. The 5D0→7F1 transition is magnetic dipole allowed and is allowed by the selection rule ∆J=1 which is independent of the local symmetry. The 5D0 →7F2,4 transitions are electric dipole in nature and their existence is due to the absence of center of symmetry and mixing of 4f orbitals with opposite parity orbital. Generally, luminescence quenching occurs in the RE doped materials due to the efficient energy transfer between the Eu– Eu ions at higher RE ion content in the prepared glasses. It is also expected that, the distance between the RE ions decreases while increasing the concentration of the Eu3+ ions which in turn leads to enhance the non-radiative transitions otherwise quenching center for luminescence. It is observed from the luminescence spectra that, the luminescence intensity of the present glasses gradually increase up to 1% of Eu3+ ion. Beyond this concentration the luminescence output shows a monotonous decrease due to luminescence quenching in the prepared glasses when the concentration of the Eu3+ ion becomes more than 1 wt%. Luminescence quenching has been observed at considerably higher concentrations of Eu3+ ions and reported in literature [ 17,22,23]. The energy level diagram of Eu3+ ions consists of various emission transitions corresponding to the excitation wavelength 469 nm is shown in figure 5. The shielding effect of 4fn shell by 5s2 and 5p6 orbitals and the weak influence of the crystal field arise from the 4fn configuration have made the energy levels of Eu3+ ions appear to be parallel parabolas (ΔK=0) in the coordination diagram. It is observed from the luminescence spectra that, the absence of emission transition from the higher excited states 5DJ (J = 1,2,3) indicates the fast non-radiative transitions through the multi-phonon relaxation due to the higher phonon energy in the prepared

10

glasses. Therefore, the Eu3+ ions reach the 5D0 excited energy level when they are excited to any of the higher excited levels [16]. The luminescence intensity ratio (R) of the 5D0→7F2 /5D0→7F1 transitions has been calculated from the emission spectra for all the prepared glasses and is used to determine the degree of asymmetry around the Eu3+ ion site and further to estimate the degree of covalence of Eu3+ ion with its surrounding ligands. The higher asymmetry is confirmed through the presence of 5D0→7F0 non-degenerate transition. The calculated luminescence intensity ratio (R) values are found to be 2.82, 3.09, 4.01, 4.39, 4.54 and 4.08 corresponding to the TFB0.05Eu, TFB0.1Eu, TFB0.25Eu, TFB0.5Eu, TFB1Eu and TFB2Eu glasses respectively. The luminescence intensity ratio values of the title glasses were presented in table 4 along with the other reported Eu3+ doped glasses and it is clearly observed that the present glass possess higher R values compared to the reported glasses [13,23,32,33,34]. The R values are found to have similar variation as that of Ω2 JO intensity parameter values with the increase in Eu3+ ion concentration. The increase of R value specifies the increase of covalency between the Eu3+ ion and its surrounding ligands which are confirmed earlier through the Ω2 values and bonding parameter results. Of the studied glasses, the TFB1Eu glass possess higher R value (4.54) which in turn indicates that, the Eu3+ ions are situated in higher asymmetrical environment and is potential for laser applications. 3.5 CIE color chromaticity coordinates The CIE 1931 (Commission International d’Eclairage) diagram [34] is a universal method to represent all the possible colors by combining the three primary colors and is used to quantify the tunability of emission wavelength and the change in intensity of the emission bands. The standard x, y coordinates (x=0.33, y=0.33) corresponding to the location of the white light emission is always situated at the center of the CIE 1931 chromaticity diagram. The x,y chromaticity coordinates were calculated by converting emission spectra of the prepared glasses with different concentration of Eu3+ ions into the CIE 1931 chromaticity diagram and the same is shown in figure 6. The values of the color chromaticity coordinates (x, y) are found to be (0.543, 0.422), (0.603, 0.382), (0.628, 0.365), (0.631, 0.363), (0.647, 0.352) and (0.641, 0.356) corresponding to the TFB0.05Eu, TFB0.1Eu, TFB0.25Eu, TFB0.5Eu, TFB1Eu and TFB2Eu glasses respectively. These chromaticity coordinates x and y are useful to determine the exact emission color of the prepared glasses. In the present study the x, y chromaticity coordinates of 11

the prepared glasses are found to occur mostly in the reddish orange region in the CIE 1931 color chromaticity diagram and confirms that the prepared Eu3+ ion doped telluro fluoroborate glasses are suitable for red laser applications and display devices. 3.6 Decay Analysis The 5D0→7F2 excited state lifetime measurements have been made monitoring an excitation at 612 nm for the present glasses at room temperature and the same is shown in figure 7. It is clearly observed from figure that, all the decay curves of the prepared Eu3+ ion doped glasses exhibit single exponential behavior and the excited state lifetime is calculated from the slope of the fitting line using the below given expression I=I e

(5)

where It and I0 are luminescence intensities at time t and at t=0 respectively, τ is the lifetime of the excited state energy level. The obtained experimental lifetime (

) values are found to be 1.452, 1.553, 1.557,

1.598, 1.601 and 1.554 ms corresponding to the TFB0.05Eu, TFB0.1Eu, TFB0.25Eu, TFB0.5Eu, TFB1Eu and TFB2Eu glasses respectively. The radiative lifetime (

) values obtained

following the JO theory are found to be 3.703, 3.880, 2.910, 2.513 and 2.871 ms corresponding to the prepared TFB0.05Eu, TFB0.1Eu, TFB0.25Eu, TFB0.5Eu, TFB1Eu and TFB2Eu glasses respectively. The calculated and experimental lifetime values of the present TFB1Eu glass have been presented in table 5 along with the reported Eu3+ doped glasses [15,23,34]. It is observed from the τexp and τcal values of the Eu3+ doped telluro fluoroborate glasses that, the experimental lifetime values are found to be lower than the calculated lifetime values and is due to the non-radiative relaxation (WNR) of the 5D0 energy level of the Eu3+ ions. The total decay rate of the 5D0 level is the combination of both radiative and non-radiative processes. The radiative transition is attributed to the ion-ion interaction of the Eu3+ ions and it includes all the emission transitions. The non-radiative decay is due to the interaction of Eu3+ ions with the vibrations of the host matrix and it reaches the lower energy states with the emission of multiple phonons called as multiphonon relaxation process. The multiphonon relaxation rate can be calculated using the below given expression 12

WMPR

(6)

The calculated WMPR values of the present glasses are found to be 418.65, 386.59, 299.03, 289.70, 226.68 and 295.16 S−1 corresponding to the prepared TFB0.05Eu, TFB0.1Eu, TFB0.25Eu, TFB0.5Eu, TFB1Eu and TFB2Eu glasses respectively. The estimation of optical gain parameter (

×

) is an essential property to find new stable laser active materials with

high efficiency. The calculated optical gain parameter values corresponding to the

5

D0→7F2

emission transition of the TFB0.05Eu, TFB0.1Eu, TFB0.25Eu, TFB0.5Eu, TFB1Eu and TFB2Eu glasses are found to be 15.84, 20.58, 33.26, 30.08, 31.30, 27.82 (×10–25 cm–2s) respectively. The gain bandwidth ( σ PE ×Δλeff) plays a vital role in optical amplifier applications to enhance the capability of allowing the spectrum of input signals. The calculated gain bandwidth for the prepared Eu3+ doped glasses are found to be 137.08, 131.01, 188.13, 170.36, 193.23 and 169.92 (×10–22 nm cm2) corresponding to the TFB0.05Eu, TFB0.1Eu, TFB0.25Eu, TFB0.5Eu, TFB1Eu and TFB2Eu glasses respectively. Luminescence quantum efficiency is defined as the ratio of number of photons emitted to the number of photons absorbed. But in the case of Eu3+ ion doped glasses, it is defined as the ratio of the experimental lifetime to the calculated lifetime of the 5D0 level and is measured from the following expression,

η=

100

(7)

The obtained luminescence quantum efficiency values are found to be 39%, 40%, 53%, 54%, 63% and 54% corresponding to the TFB0.05Eu, TFB0.1Eu, TFB0.25Eu, TFB0.5Eu, TFB1Eu and TFB2Eu glasses respectively. Among the prepared glasses, TFB1Eu glass possess higher luminescence quantum efficiency value compared to the reported fluoroborate glass (58%) [23] and lower than the PKBFAEu (97%) [15], PKBAEu (95%) [15], PKAlCaFEu10 (98%) [34] glasses and the same is suggested for suitable red laser applications.

13

4. Conclusion In summary, the optical properties of the Eu3+ doped telluro fluoroborate glasses with the chemical composition (40–x)B2O3+ 15TeO2+15Li2CO3+15LiF+15NaF+xEu2O3 (where x = 0.05, 0.1, 0.25, 0.5, 1 and 2 in wt%) have established the following results. (i)

The energy level position exhibit that, the Eu–O metal ligand bond is of covalent in nature and the same is confirmed through the higher luminescence intensity ratio (R) values. The higher R value is due to the higher covalence and higher asymmetry around the Eu3+ ion site. The higher asymmetry is further confirmed through the presence of 5D0→7F0 non-degenerate transition in the emission spectra.

(ii)

It is observed from the Optical band gap and Urbach’s energy studies that, the direct and indirect band gap values are found to increase with the increase of Eu3+ ion concentration and it varies inversely with the Urbach’s energies.

(iii)

The Judd-Ofelt parameters Ω2, Ω4 and Ω6 were determined and the higher Ω2 parameter values indicate the higher covalent nature around the Eu3+ ions and higher asymmetry in the present glasses which is further confirmed through the bonding parameter and luminescence intensity ratio values.

(iv)

The x, y colour chromaticity coordinates of the present glasses are found to occur mostly in the reddish orange region in the CIE 1931 diagram and is suitable for red laser applications and display devices.

(v)

The stimulated emission cross-section, luminescence quantum efficiency, branching ratio, optical gain parameter and gain bandwidth of the present TFB1Eu glass is found to be higher among the prepared glasses and the same is suggested for suitable red laser applications corresponding to the 5D0→7F2 emission transition at 612 nm.

Acknowledgement One of the authors Prof. K. Marimuthu would like to thank the funding agency UGC, New Delhi, Govt. of India for the sanction of financial support in the form of a major research Project No. 41-916/2012(SR), dt. 26-07-2012. Mr.K.Maheshvaran is also thankful to the UGC for the financial support through Project Fellow in the above project.

14

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19) W.T. Carnall, P.R. Fields, K. Rajnak, J. Chem. Phys. 49 (1968) 4424–4442. 20) S.P. Sinha, Complexes of the Rare Earths, Pergamon, Oxford, 1966. 21) P. Babu, C.K. Jayasankar, Physica B 279 (2000) 262–281. 22) R.T. Karunakaran, K. Marimuthu, S. Surendra babu, S. Arumugam, Solid State Sci. 11 (2009) 1882–1889. 23) S. Arunkumar, K. VenkataKrishnaiah, K. Marimuthu, Physica B 416 (2013) 88–100. 24) E.A Davis, N.F Mott, Philos. Mag. 229 (1970) 903–922. 25) N.F Mott, E.A Davis, Electronic Processes in Non-Crystalline Materials, 2nd ed., Clarendon PressOxford, 1979. 26) DariushSouri, Seyed Ali Salehizadeh, J Mater Sci. 44 (2009) 5800–5805. 27) J. Tauc, Amorphous and Liquid Semiconductor, Plenum Press, New York, 1974. 28) F. Urbach, Phys. Rev. 92 (1953) 1324–1326. 29) B.R. Judd, Phys. Rev. 127 (1962) 750–761. 30) G.S. Ofelt, J. Chem. Phys. 37 (1962) 511–520. 31) S. Arunkumar, K. Marimuthu, J. Lumin. 139 (2013) 6–15. 32) S. Hazarika, S. Rai, Opt. Mater. 27 (2004) 173–179. 33) M.V.VijayaKumar, B.C.Jamalaiah, K.RamaGopal, R.R.Reddy, J. Solid State Chem. 184 (2011) 2145–2149. 34) S. N.Rasool, L. R.Moorthy, C. K.Jayasankar, Mater. Express Vol. 3(2013) 231–240. 35) R. Balakrishnaiah, R. Vijaya, P. Babu, C.K. Jayashankar, M.L.P. Reddy, J. Non-Cryst. Solids 353 (2007) 1397–1401. 36) K. Maheshvaran, P.K. Veeran, K. Marimuthu, Solid State Sci. 17 (2013) 54–62. 37) S.A. Saleem, B.C. Jamalaiah, A. Mohan Babu, K. Pavani, L. Rama Moorthy, J. Rare Earths 28 (2010) 189−193. 38) L.N. Puntus, V.F. Zolin, V.A. Kudryashova, V.I. Tsaryuk, J. Legendziewicz,

P.

Gawryszewska, and R. Szostak, Physics of the Solid State, Vol. 44, No. 8, 2002, pp. 1440–1444. 39) P.S. Duttaa, A. Khanna, ECS Journal of Solid State Science and Technology, 2 (2) R3153–R3167 (2013).

16

Table 1: Physical properties of the Eu3+ doped Telluro fluoroborate glasses Physical Properties Density ρ (g/cm3) Refractive index nd (589.3 nm) RE ion concentration N(1020 ions cm3) Polaron radius rp (A°) Inter ionic distance ri (A°) Field strength F (1014 cm–2) Electronic polarizability αe(10–22 cm3) Molar refractivity Rm (cm3) Dielectric constant (ε) Reflection losses R (%)

TFB0.05E u 2.971 1.596 0.255 13.685 3.396 0.260 31.844 1.719 2.547 5.271

TFB0.1E u 3.222 1.597 0.570 10.472 2.599 0.444 14.285 1.538 2.550 5.285

17

TFB0.25E u 2.965 1.593 1.260 8.038 1.995 0.754 6.426 1.719 2.538 5.230

TFB0.5E u 3.054 1.613 2.570 6.335 1.572 1.213 3.232 1.712 2.602 5.504

TFB1Eu

TFB2Eu

3.066 1.616 5.059 5.057 1.255 1.905 1.649 1.713 2.611 5.545

3.134 1.615 9.964 4.034 1.001 2.993 0.837 1.671 2.608 5.531

Table 2: Observed band positions (in cm−1) and bonding parameters ( β , δ) of the Eu3+ doped Transitions TFB0.05Eu TFB0.1Eu TFB0.25Eu TFB0.5Eu TFB1Eu TFB2Eu 7

F1→7F6 7 F0→7F6 7 F1→5D0 7 F0→5D0 7 F1→5D1 7 F0→5D1 7 F0→5D2 7 F1→5D3 7 F1→5L6 7 F0→5L6 β δ

4545 4808 17126 17296 18723 19033 21519 0.9946 0.5392

4533 4782 17007 17301 18726 19047 21599 0.9932 0.6834

4533 4796 16992 17301 18744 19011 21524 0.9929 0.7198

Telluro fluoroborate glasses

18

4533 4778 16989 17262 18726 18979 21413 24096 25316 0.9928 0.7244

4533 4782 16992 17271 18744 19011 21177 24096 24876 25381 0.9926 0.7438

4533 4777 16989 17262 18744 19010 21164 24096 24875 25381 0.9923 0.7682

Aqua ion [19] 4630 4980 16920 17277 18691 19028 21164 24038 25000 25400 -

Table 3: Fundamental absorption edge (λedge), Direct and indirect band gap energies (Eg), band tailing parameters (B) and Urbach’s energies of the Eu3+ doped Telluro fluoroborate glasses

Sample code

λedge (nm)

TFB0.05Eu TFB0.1Eu TFB0.25Eu TFB0.5Eu TFB1Eu TFB2Eu

399 398 397 386 385 386

Direct allowed transitions Eg (eV) B (eV) 2.76 2.014 2.77 1.999 2.78 1.974 2.79 1.724 2.81 2.249 2.80 2.102

19

Indirect allowed transitions Eg (eV) B (eV) 3.04 7.318 3. 05 10.549 3.07 10.403 3.13 10.086 3.17 10.394 3.14 28.826

Urbach’s energy ΔE (eV) 0.9262 0.9209 0.9184 0.8166 0.6551 1.0690

Table 4: Luminescent intensity ratio values (R) and Comparison of Judd-Ofelt (×10−20cm2) parameters of the Eu3+ doped Telluro fluoroborate glasses with the reported Eu3+ doped glasses Glass code TFBL0.05Eu TFBL0.1Eu TFBL0.25Eu TFBL0.5Eu TFBL1Eu TFBL2Eu LLiFB LNaFB LKFB 1EPBFB Eu:Al(NO3)3–SiO2 PTBEu05 PKAlCaFEu10

R 2.82 3.09 4.01 4.39 4.54 4.08 2.32 3.323 2.434 1.788 3.38 2.59 2.52

JO Parameters Ω2 5.0006 4.7639 6.1329 6.8438 6.9228 6.0945 3.62 3.62 3.81 2.548 5.61 3.72 3.64

Ω4 1.3018 1.0923 1.9757 1.6455 3.4339 2.5916 1.19 1.43 1.42 0.362 3.47 1.78 0.27

20

Ω6 0 0 0 0 0 0 0 0 0 0 2.91 0 0

Trends of Ωλ

References

Ω2> Ω4>Ω6 Ω2> Ω4>Ω6 Ω2> Ω4>Ω6 Ω2> Ω4>Ω6 Ω2> Ω4>Ω6 Ω2> Ω4>Ω6 Ω2> Ω4> Ω6 Ω2> Ω4> Ω6 Ω2> Ω4> Ω6 Ω2> Ω4> Ω6 Ω2> Ω4> Ω6 Ω2> Ω4> Ω6 Ω2> Ω4> Ω6

Present Present Present Present Present Present [13] [13] [13] [23] [32] [33] [34]

Table 5: Emission band position (λp, nm), effective bandwidth (∆λeff, nm), radiative transition probability (A, s–1), stimulated emission cross-section ( σ PE ×10–22 cm2), experimental and calculated branching ratio (βR) of the 5D0→7FJ (J=0, 1, 2, 3 and 4) transitions, experimental (τexp) and calculated (τcal) lifetime and quantum efficiency (η) of the Eu3+ doped Telluro fluoroborate glasses with the reported Eu3+ glasses

580

PKAlCaF Eu10 [34] 579

1.04

4.89

4.00

0

0

0

0

0

0

0

0

0

βR(Exp) βR(Cal) λp ∆λeff Α

0.01 0 590 10.22 60.09

0 0 592 10.83 52

0 0 592 10.78 52.71

0.0231 0 593 10.22 112.04

0.02 0 593 12.96 52

σ PE

3.6

3.25

3.30

4.854

2.80

βR(Exp) βR(Cal) λp ∆λeff Α

0.1389

0.13

0.13

0.3204

0.28

0.1510

0.13

0.14

0.3441

0.29

612

611

611

615

614

9.88 270.55

10.91 238.7

9.44 234.8

8.11 212.39

11.15 122

σ PE

19.6

16.84

19.14

13.463

8.60

βR(Exp) βR(Cal) λp ∆λeff Α

0.6306 0.6800 652 3.08 0

0.62 0.62 653 8.13 0

0.61 0.62 653 7.66 0

0.5915 0.6151 655 6.84 0

0.67 0.68 655 10.04 0

σ PE

0

0

0

0

0

βR(Exp) βR(Cal) λp ∆λeff Α

0.01 0 700 10.34 67.20

0.02 0 702 7.09 86.80

0.03 0 702 7.19 82.27

0.0167 0 702 8.95 25.09

0.01 0 702 9.52 4

σ PE

7.96

16.34

15.82

2.514

0.70

0.2088 0.1689

0.20 0.23

0.21 0.22

0.0541 0.0530

0.02 0.03

2.513

2.6

2.64

2.813

2.45

1.601

2.52

2.51

1.646

2.41

63

97

95

58

98

Transitions Parameters

5

5

5

5

5

D0→7F0

D0→7F1

D0→7F2

D0→7F3

D0→7F4

λp ∆λeff Α

σ

E P

βR(Exp) βR(Cal) τcal (ms) τexp(ms) η(%)

578

PKBFAEu [15] 579

PKBAEu [15] 579

4.44

1.01

0

TFB1Eu

21

1EPbFB [23]

5 L6 7 F0 5 L6

400

500

7 F1

7 F0

5 D0 5 D0 7 F0 7 F1

5 D1 5 D1

7 F6

7 F6

450

7 F0 7 F1

5 D2 7 F0

7 F1

7 F 5 D3 1

−1

Absorption coefficient (cm )

−

550

600

2000

2500

Wavelength (nm) Figure 1: Absorption spectrum of the 1 wt% Eu3+ doped Telluro fluroborate glass

22

1.0

0.35

0.30

TFB0.05Eu TFB0.1Eu TFB0.25Eu TFB0.5Eu TFB1Eu TFB2Eu

0.20

0.15

(αhν) 1/2 (cm -1 eV) 1/2

0.8

ln α (eV)

0.25

0.10

0.05

0.6

2.8

2.9

3.0

3.1

3.2

hν (eV)

TFB0.05Eu TFB0.1Eu TFB0.25Eu TFB0.5Eu TFB1Eu TFB2Eu

0.4

2.6

2.7

2.8

2.9

3.0

3.1

hν (eV) Figure 2: Tauc’s plot of the Eu3+ doped Telluro fluoroborate glasses (Inset shows the Urbach’s plot of the title glasses)

23

5

F0

Intensity (a.u)

5

λEmi

612nm

L6 F1

5

D1

F0

5

L7

D3

7

250

300

D4

350

5

7

5

F1

7

CTB

200

D2

5

7

400

Wavelength (nm)

450

500

D0

550

Figure 3: Excitation spectrum of the 1 wt% Eu3+ doped Telluro fluoroborate glass

24

5

TFB1Eu

D0 → 7 F 2

Exc → 469 nm

TFB0.5Eu TFB0.25Eu TFB0.1Eu

7

D0→ F1

TFB0.05Eu

D0 → 7 F 4

7

D0→ F0

5

5

5

5

Luminescence intensity (a.u)

TFB2Eu

600

D0 → 7 F 3 650

700

Wavelength (nm)

Figure 4: Luminescence spectra of the Eu3+ doped Telluro fluoroborate glasses

25

5

D G 5 4 L6 5 D3

5 4 5

25000

L7

5

D2

NR

20000

5

D1

NR

5

-1 Energy ( cm )

D0

5000

0

Emission

700 nm

652 nm

612 nm

590 nm

578 nm

469 nm

10000

Excitation

15000

}

J=6 J=5 J=4 J=3 J=2 J=1 J=0

7

FJ

Figure 5: Energy level scheme of excitation and emission transitions of the Eu3+ doped Telluro fluoroborate glasses

26

y - chromaticity coordinate

0.8

Green

+ 0.6

0.4

0.2

White

TFB0.05Eu TFB0.1Eu TFB0.25Eu TFB0.5Eu TFB1Eu TFB2Eu

Red

+

+ Blue

0.0 0.0

0.2

0.4

0.6

0.8

x - chromaticity coordinate

Figure 6: CIE color chromaticity diagram of the Eu3+ doped Telluro fluoroborate glasses

27

TFB1Eu TFB0.5Eu TFB0.25Eu TFB2Eu TFB0.1Eu

ln Intensity (a.u)

TFB0.05Eu

0

2

4

Time (ms)

6

8

Figure 7: Decay curves of the 5D0 excited state of the Eu3+ doped Telluro fluoroborate glasses

28

High lights ™ Concentration dependent luminescence behavior of TFBxEu glasses was studied.

™ The lower Urbach’s energy values indicate that, the studied glasses possess minimal defects.

™ Higher Ω2 & R values indicate the higher asymmetry around Eu3+ ion site in all the prepared glasses.

™ Reddish orange color emission of Eu3+ ions was confirmed through CIE 1931 chromaticity diagram.

™ Higher , β, applications.

×λeƒƒ &

×

values of TFB1Eu glass is useful for high gain laser

29