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Optical and luminescence characteristics of Eu3+ doped zinc bismuth borate (ZBB) glasses for red emitting device
Materials Research Bulletin 71 (2015) 37–41
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Optical and luminescence characteristics of Eu3+ doped zinc bismuth borate (ZBB) glasses for red emitting device J. Kaewkhaoa,* , K. Boonina , P. Yasakaa , H.J. Kimb a b
Center of Excellence in Glass Technology and Materials Science (CEGM), Nakhon Pathom Rajabhat University, Nakhon Pathom 73000, Thailand Department of Physics, Kyungpook National University, Deagu 702-701, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 19 March 2015 Received in revised form 6 June 2015 Accepted 3 July 2015 Available online 8 July 2015
Zinc bismuth borate (ZBB) glasses with chemical composition of (60 x) B2O3: 30Bi2O3: 10ZnO: xEu2O3 for different Eu2O3 concentrations of x = 1, 3, 5, 7 and 9 mol% were prepared by the conventional melt quenching technique. The density and molar volume of the glasses have been found to increase with Eu2O3 concentration. The IR studies indicated that these glasses were made up of [BiO3], [BO3], [BO4] and [BO] basic structural units. The absorption spectra had revealed two intense bands due to 7F0 ! 7F6 (2076 nm) and 7F1 ! 7F6 (2198 nm) transitions in the NIR regions. The fractional populations of the (7F0) and (7F1) ground state were 65% and 35%, respectively. Five luminescence bands were observed at 579 nm (5D0 ! 7F0), 589 nm (5D0 ! 7F1), 613 nm (5D0 ! 7F2), 651 nm (5D0 ! 7F3) and 696 nm (5D0 ! 7F4), when excited by a 465 nm source, the most intense red emission was found at 613 nm. A shorter decay time of 5 D0 excited state was found with increasing of Eu3+ concentration as more asymmetries were created around Eu3+. For comparison, further investigations were carried out with two other glass structures; bismuth borate glass (BB) and zinc borate glass (ZB), doped with Eu3+ in the same range. The ZBB glasses have exhibited with the highest emission intensity especially at 613 nm; the intensity for the ZBB was around 7 times of the ZB and twice of the BB glasses. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: Glasses Luminescence Oxides Optical materials Emission
1. Introduction In recent years, luminescence materials have emerged as a fascinating field of research since its characteristics encompasses a wide variety of scientific applications including the design and development of new optical devices such as solid state lasers, white light emitting diodes (WLEDs) and other flat panel technologies like plasma display panels (PDP) [1–4]. Distinct visible emission of glasses doped with rare earth (RE) ions has attracted researchers into developing glasses for various optical devices such as lasers, upconverters, stimulated phosphors and optical amplifiers [5]. The luminescence and absorption properties of RE ions in glasses depend on the chemical composition, structure and nature of bonds of the host glass [6]. Borate glasses are good hosts for RE ions and having flexibility for both size and composition of the materials [7]. The Eu3+ ion; a well known spectroscopic probe, is commonly employed to study local structure around the RE ions in glasses because its relative simplicity of energy level structure with non-degenerate ground (7F0) and excited (5D0) states [8–15]. A narrow and monochromatic
* Corresponding author. Fax: +66 34 261 065. E-mail address: [email protected] (J. Kaewkhao). http://dx.doi.org/10.1016/j.materresbull.2015.07.002 0025-5408/ ã 2015 Elsevier Ltd. All rights reserved.
emitting nature of 5D0 ! 7F2 transition around 613 nm of Eu3+ doped red phosphors are frequently used in field emission technology. ZnO–Bi2O3–B2O3 (ZBB) glass systems are expected to be a new kind of forming glass because of their low melting temperature. In the presence of conventional glass formers (such as B2O3, SiO2, etc.), Bi2O3 may builds a glass network of [BiOn] (n = 3 and 6) pyramids [16] and could be a suitable substitute of PbO in the preparation of lead-free glass compositions [17–19]. Zinc oxide (ZnO) in particular is one of the important constituent components in the formation of oxide glasses. The zinc bismuth borate (ZBB) glasses combine the strong properties of constituent possess low melting temperature, large refractive index, good physical and chemical properties [20,21]. The present work focuses study on effects of Eu3+ on the densities, molar volume, FTIR transmittance, absorption, photoluminescence and decay time in ZBB glass with compositions (60 x) B2O3: 30Bi2O3: 10ZnO: xEu2O3 (where x = 1, 3, 5, 7 and 9 mol%). Moreover; the bismuth borate (BB) and zinc borate (ZB) glass structures doped with Eu3+ in the same range of concentration were prepared in parallel for comparison of emission spectra with the zinc bismuth borate glass (ZBB).
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2. Experimental Zinc bismuth borate (ZBB) glass samples doped with Eu3+ in chemical composition of (60 x)B2O3: 30Bi2O3: 10ZnO: xEu2O3 (where x varied from 1, 3, 5, 7 and 9 mol%) were prepared by conventional melt quenching technique. Homogeneous powders of 20 g for each batch composition of all the glass samples were melted at 1100 C in alumina crucibles for about 3 h in an electrical furnace. The melts were air quenched by pouring onto a preheated stainless steel mould and annealed at 500 C for 3 h to remove thermal strains. The glass samples were cut and polished into 1.0 1.5 0.3 cm3 shape for optical measurements. The densities (r) were measured by Archimedes method using distilled water as an immersion liquid. Infrared spectra of the glass samples were recorded at room temperature in the range 650–4000 cm1 using an Agilent-Cary-630 FT-IR spectrometer. The optical absorption spectra of the glasses were recorded in the NIR regions in the range of 1800–2400 nm using a UV-3600 Shimadzu UV–VIS–NIR spectrophotometer. The photoluminescence spectra and decay time measurements were carried out using Cary Eclipse Fluorescence Spectrophotometer using a 465 nm wavelength excitation source from xenon flash lamp. 3. Result and discussions 3.1. Density and molar volume The densities and molar volumes of the investigated ZBB glasses are shown in Fig. 1. Both the density and molar volume of the glass samples are seen to increase with increasing Eu2O3 concentration. The values of density are in the range of 4.06–4.33 g/cm3 while their molar volumes (VM) are in the range of 47.41–49.73 cm3/mol. The increase in density of the glass with the increasing Eu3+ doping content is due to a higher molecular weight of Eu2O3 as compared to other components in the glasses. While the increase in the molar volume, indicates the increasing of inter-atomic spacing in the glass network. Eu2O3 acts as a network modifier and produces more non-bridging oxygen (NBOs) in glass matrices. 3.2. FTIR transmittance spectra The infrared spectrum of ZBB glass doped Eu3+ at 5 mol% is displayed in Fig. 2. According to Krogh-Moe model, the structure of borate glass consists of BO3 triangles with certain fraction of six membered (boroxal) rings [22]. In B2O3 glasses, boron [B3+] ions
Fig. 2. Typical IR transmission spectra of the 5 mol% Eu3+ doped ZBB glass.
are triangularly coordinated by oxygen to easily form glasses. The BO3 triangles are corner bonded in a random network [23]. Inclusion of transition metal ions in these glasses helps the boron to form tetraborate groups and progressive substitution of boroxal rings by triborate and tetraborate groups [24]. The boroxal ring shows its characteristic frequency at 806 cm1 and the presence or absence of this band decides the existence or absence of boroxal rings in the structure. In the present set of glasses, no band was observed at or around 806 cm1 indicating that boroxal ring was not present in these glasses. The peaks in the IR spectra of the glasses under studied are listed in Table 1. The present set of glasses shows transmission bands at 3750, 2904, 2830, 2287, 2080, 1268, 996, 848, 766 and 651 cm1. The band around 3750 cm1 is due to stretching vibration of OH group and bands around 2287 and 2080 cm1 are due to OH group [25]. The bands observed at 2904 cm1 and 2830 cm1 are also attributed to hydrogen bonds in OH group [26]. It has been reported that the band observed at 1268 cm1 was due to the asymmetrical stretching relaxation of the B-O bond of trigonal BO3 units [27]. The band around 996 cm1 originates from B O bond stretching of the tetrahedral BO4 units and is due to the vibration of some boron atoms attached to the non-bridging oxygen in the form of BO4 vibrations [28]. The shoulder around 840 cm1 is related to the symmetrical stretching vibration of Bi O in [BiO3] group [29]. The band around 766 cm1 is the bending mode of ¼B O BR bonds in which oxygen bridges between one tetrahedral and one trigonal boron atom [30]. The band observed around 651 cm1 is the bending bond mode of BO B vibrations [31,32]. In the IR spectra of the present glass system, it was found that the incorporation of ZnO did not show much effect on the structure of the glasses studied. The addition of ZnO into the present glass system produces a very small change in the IR bands that does not account for the major structural changes. However, in many glass systems in which ZnO is a major constituent the possibility of ZnO4 formation may be higher [33]. Bale et al. [34] had reported the formation of ZnO4 units with the increase of zinc oxide content. 3.3. Absorption spectrum and energy levels
Fig. 1. The densities and molar volumes of the Eu3+: ZBB glass system.
The typical absorption spectrum of the ZBB glass doped Eu3+ at 5 mol% is shown in Fig. 3 along with their assignments. The assignment of energy levels has been done according to Carnall et al. [15]. There is no absorption peak observed in visible region of studied glass. The absorption spectrum reveals two intense bands
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Table 1 FTIR analysis of the 5 mol% Eu3+ doped ZBB glass system. Wavenumber (cm1)
Assignment
651 766 848 996 1268 2080–3750
The bending vibrations of BOB linkages of BO3 units The bending mode of ¼BOB¼bonds Symmetrical stretching vibration of the BiO bonds in the BiO3 groups BO bond stretching of the tetrahedral BO4 units Asymmetrical stretching of the BO bond in the trigonal units OH group
Fig. 4. Typical excitation spectrum of the 5 mol% Eu3+ doped ZBB glass. Fig. 3. Typical absorption spectrum of the 5 mol% Eu3+ doped ZBB glass.
at 7F0 ! 7F6 (2076 nm) and 7F1 ! 7F6 (2198 nm) due to transitions in the NIR regions. Except for Eu3+ ion, it is well known that all the other trivalent RE3+ ions possess only a single populated ground state. A close examination of absorption band positions of 7F0 ! 7F6 and 7F1 ! 7F6 transitions clearly indicates that the energy gap between the ground state (7F0) and the first excited state (7F1) is about 360 cm1. Due to the effect of thermalization, the fractional population of any excited level is given by Deun et al. [35] gJ ðEJ E0 Þ=kT CJ (1) ¼ e C0 g0
transitions originating not only from the ground state (7F0), but also from the first excited state (7F1) to 5D1. As seen from Fig. 4, the 7 F0 ! 5D2 transition is the more intense compared to other transitions. Hence, the emission measurements were performed by exciting the samples at 465 nm wavelength using a xenon flash lamp. Fig. 5 shows the room temperature emission spectra of Eu3+: ZBB glasses. All the spectra exhibited with five emission bands corresponding to the 5D0 ! 7F0 (579 nm; yellow), 5D0 ! 7F1 (589 nm; orange), 5D0 ! 7F2 (613 nm; red), 5D0 ! 7F3 (651 nm; red) and 5D0 ! 7F4 (696 nm; deep-red) transitions. The luminescence intensity of luminescence materials is known to be dependent on the doping concentration of luminescent ions.
where {C 0 ,C J }, {E0 , EJ } and {g0 , gJ (2J + 1)} are the thermal correction factors, energies and degeneracies of the ground state (7F0) and the first excited state (7F1), respectively. The magnitude of kT is in the order of 208 cm1, where k is the Boltzmann’s constant and T is the absolute room temperature (300 K). For the ZBB glass doped with 5 mol% of Eu3+, the evaluated thermal correction factors for the ground state (7F0) and the first excited state (7F1) are: C 0 = 0.65 and C 1 = 0.35, respectively. Owing to the population of 7F0 (65%) and 7 F1 (35%) states, the absorption spectrum exhibited the transitions originating not only from the ground level (7F0) but also from the thermally excited level (7F1). 3.4. Excitation and emission spectra The excitation spectrum of ZBB glasses doped Eu3+ at 5 mol% obtained by monitoring the emission with 613 nm is shown in Fig. 4. The spectrum contains seven bands at 363, 383, 394, 414, 465, 485 and 533 nm corresponding to the transitions from the ground state (7F0) to 5D4, 5L7, 5L6, 5D3, 5D2, and 7F1 ! 5D1 excited states, respectively. The excitation spectrum exhibited the
Fig. 5. Emission spectrum of the ZBB glasses doped at different Eu3+concentrations. (For interpretation of the references to color in the text, the reader is referred to the web version of this article.)
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It is also noticed that the relative intensity of 5D0 ! 7F2 transition of Eu3+ various as a function of Eu3+ concentration (x = 1, 3, 5, 7 and 9 mol%). The maximum luminescence intensity was observed at x = 5 mol% of Eu2O3. The decrease in emission intensities with the increase of 7% and 9% mol of Eu2O3 concentration is due to the rise in non-radiative decay channels, which are enhanced due to the concentration quenching effect. If the concentration of an activator is higher than an appropriate value (typically several mol%), the emission intensity will be decreased. This is called concentration quenching [36]. The emission intensity of 5D0 ! 7F2 transition is electric-dipole allowed and depends strongly on the local symmetry around the Eu3+ ion, whereas the transition 5D0 ! 7F1 is magnetic-dipole allowed and is independent of the local symmetry. The fluorescence intensity ratio (R) of 5D0 ! 7F2 to 5D0 ! 7F1 transitions is used to establish the degree of asymmetry in the vicinity of Eu3+ ions and Eu–O covalence for various Eu3+-doped systems which is used to explain the short-range effects. The R values are found to be 2.80, 2.78, 2.76, 2.73 and 2.65 for 1, 3, 5, 7 and 9 mol% of Eu2O3, respectively. Fig. 6 shows the room temperature emission spectra of 55B2O3: 30Bi2O3: 10ZnO: 5Eu2O3 (ZBB) glass compared with 65B2O3: 30Bi2O3: 5Eu2O3 (BB) glass and 85B2O3: 10ZnO: 5Eu2O3 (ZB) glass. All the spectra exhibited with five emission bands corresponding to the 5D0 ! 7F0 (579 nm; yellow), 5D0 ! 7F1 (589 nm; orange), 5 D0 ! 7F2 (613 nm; red), 5D0 ! 7F3 (651 nm; red) and 5D0 ! 7F4 (696 nm; deep-red) transitions. It is noted that, all spectra were measured under the same condition. Relatively very high intensities are exhibited by ZBB glass in the emission spectra especially at 613 nm, the intensity for the ZBB is about 7 times of the ZB and around twice of the BB glasses. The emission intensity increases in the ZBB glass compared to BB and ZB may be explained in term of 5D0 ! 7F2 transition. It is well-known that the intensity of the electric-dipole 5D0 ! 7F2 transition is significantly affected by the degree at the center of symmetry in the environments around Eu3+ ions, and that the intensity of an emission transition is proportional to the radiative decay of these transitions. When Eu3+ ions are situated at low-symmetry sites, there is a high probability of the electric-dipole transition to occur. From this result indicated that the Eu3+ in ZBB site is lower symmetry sites compared with others. Fig. 7 shows the partial energy levels of Eu3+ ions in the ZBB glasses along with emission and non-radiative (NR) channels. When the Eu3+ ions are excited to any level above the 5D0, there is a
Fig. 7. The diagram of partial energy levels of Eu3+: ZBB glasses.
Fig. 8. The decay time profiles of 5D0 level of Eu3+: ZBB glasses.
fast non-radiative relaxation to 5D0 level [37] and the emission takes place from 5D0 level to lower level, 7FJ. Fig. 8 shows the decay time profiles of 5D0 level for different Eu3 + concentrations in the ZBB glasses obtained by exciting the samples with 465 nm wavelength and monitoring the emission at 613 nm. All the decay profiles are well fitted to single exponential function, It = Ioet/t , where Io is the fluorescence intensity when t = 0 and (t ) represents the decay time of the excited state. The measured decay time (t) obtained by taking the first e-folding times of the decay time curves are 1.034, 0.984, 0.907, 0.807 and 0.733 ms for 1, 3, 5, 7 and 9 mol% of Eu3+ doped ZBB glasses, respectively. The t values slightly decrease with Eu3+ concentration. The decease of t may be due to the increase of the electronic transition probability. 3.5. CIE chromaticity coordinates
Fig. 6. Emission spectra of the ZBB glass comparing with the BB and ZB glasses. (For interpretation of the references to color in the text, the reader is referred to the web version of this article.)
The Commission International De I-Eclairage (CIE) 1931 chromaticity coordinates for ZBB glasses doped with different of Eu3+ concentrations for the luminous color was determined from the
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concentration due to the asymmetry around the Eu3+ in the ZBB glass host. For all the studied glasses, the strongest peak was found at 613 nm (red emission) and the ZBB glass doped with 5 mol% Eu3+ had shown the highest emission. The decay time curve exhibited a single exponential nature for all the glasses and the measured decay time was found to decrease with the increase of Eu3+ concentration. Systematic analysis of the results suggested that the glass doped with Eu3+ concentration near 5 mol% is suitable for LED and display device applications. Acknowledgements The authors would like to thank National Research Council of Thailand (NRCT) and Nakhon Pathom Rajabhat University for support of this research. J. Kaewkhao would like to thank Prof. Dr. HongJoo Kim for luminescence mechanism discussion. References
Fig. 9. The CIE diagram of 5 mol% Eu3+ doped ZBB glass.
emission spectra. According to our knowledge, the CIE coordinates of red emission from Eu3+ ions not only depend upon the asymmetric ratio but also the emission from other higher energy (5D1, 5D2 and 5D3) emission levels [38]. The corresponding emission spectra were converted to the CIE 1931 chromaticity diagram and shown in Fig. 9. The color coordinates of the ZBB glasses doped with Eu3+ at 5 mol% are x = 0.645 and y = 0.353; which is very near to the orange-red light emission. Therefore, it is concluded that among the studied 1,3,5,7 and 9 mol% Eu3+ doped glasses, the 5 mol% Eu3+ doped glass has exhibited excellent color tunability of orange to red light emission. 4. Conclusions ZBB glasses doped with different Eu3+ concentrations were prepared and their red emitting efficiencies were studied for the development of phosphors device. The densities of the glasses were found to increase with the increasing of Eu2O3 concentration, due to higher molecular weight of Eu2O3. The increases in the molar volume with Eu2O3 doping concentrations are due to the increases in the bond length or inter-atomic spacing between atoms as the Eu2O3 acts as network modifier and creates more nonbridging oxygen (NBOs) in the glass matrices. The structure of ZBB glasses consists of randomly connected BO3 and BO4 groups. No boroxal ring formation was observed in the structure of the studied glasses. The structure of the present glass system is independent on change in composition though very small changes in IR bands have been noticed. The absorption spectrum revealed transitions from both the ground state (7F0) and the first excited state (7F1). The intensities of emission bands increase with the increase of Eu3+
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Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Optical and luminescence characteristics of Eu3+ doped zinc bismuth borate (ZBB) glasses for red emitting device J. Kaewkhaoa,* , K. Boonina , P. Yasakaa , H.J. Kimb a b
Center of Excellence in Glass Technology and Materials Science (CEGM), Nakhon Pathom Rajabhat University, Nakhon Pathom 73000, Thailand Department of Physics, Kyungpook National University, Deagu 702-701, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 19 March 2015 Received in revised form 6 June 2015 Accepted 3 July 2015 Available online 8 July 2015
Zinc bismuth borate (ZBB) glasses with chemical composition of (60 x) B2O3: 30Bi2O3: 10ZnO: xEu2O3 for different Eu2O3 concentrations of x = 1, 3, 5, 7 and 9 mol% were prepared by the conventional melt quenching technique. The density and molar volume of the glasses have been found to increase with Eu2O3 concentration. The IR studies indicated that these glasses were made up of [BiO3], [BO3], [BO4] and [BO] basic structural units. The absorption spectra had revealed two intense bands due to 7F0 ! 7F6 (2076 nm) and 7F1 ! 7F6 (2198 nm) transitions in the NIR regions. The fractional populations of the (7F0) and (7F1) ground state were 65% and 35%, respectively. Five luminescence bands were observed at 579 nm (5D0 ! 7F0), 589 nm (5D0 ! 7F1), 613 nm (5D0 ! 7F2), 651 nm (5D0 ! 7F3) and 696 nm (5D0 ! 7F4), when excited by a 465 nm source, the most intense red emission was found at 613 nm. A shorter decay time of 5 D0 excited state was found with increasing of Eu3+ concentration as more asymmetries were created around Eu3+. For comparison, further investigations were carried out with two other glass structures; bismuth borate glass (BB) and zinc borate glass (ZB), doped with Eu3+ in the same range. The ZBB glasses have exhibited with the highest emission intensity especially at 613 nm; the intensity for the ZBB was around 7 times of the ZB and twice of the BB glasses. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: Glasses Luminescence Oxides Optical materials Emission
1. Introduction In recent years, luminescence materials have emerged as a fascinating field of research since its characteristics encompasses a wide variety of scientific applications including the design and development of new optical devices such as solid state lasers, white light emitting diodes (WLEDs) and other flat panel technologies like plasma display panels (PDP) [1–4]. Distinct visible emission of glasses doped with rare earth (RE) ions has attracted researchers into developing glasses for various optical devices such as lasers, upconverters, stimulated phosphors and optical amplifiers [5]. The luminescence and absorption properties of RE ions in glasses depend on the chemical composition, structure and nature of bonds of the host glass [6]. Borate glasses are good hosts for RE ions and having flexibility for both size and composition of the materials [7]. The Eu3+ ion; a well known spectroscopic probe, is commonly employed to study local structure around the RE ions in glasses because its relative simplicity of energy level structure with non-degenerate ground (7F0) and excited (5D0) states [8–15]. A narrow and monochromatic
* Corresponding author. Fax: +66 34 261 065. E-mail address: [email protected] (J. Kaewkhao). http://dx.doi.org/10.1016/j.materresbull.2015.07.002 0025-5408/ ã 2015 Elsevier Ltd. All rights reserved.
emitting nature of 5D0 ! 7F2 transition around 613 nm of Eu3+ doped red phosphors are frequently used in field emission technology. ZnO–Bi2O3–B2O3 (ZBB) glass systems are expected to be a new kind of forming glass because of their low melting temperature. In the presence of conventional glass formers (such as B2O3, SiO2, etc.), Bi2O3 may builds a glass network of [BiOn] (n = 3 and 6) pyramids [16] and could be a suitable substitute of PbO in the preparation of lead-free glass compositions [17–19]. Zinc oxide (ZnO) in particular is one of the important constituent components in the formation of oxide glasses. The zinc bismuth borate (ZBB) glasses combine the strong properties of constituent possess low melting temperature, large refractive index, good physical and chemical properties [20,21]. The present work focuses study on effects of Eu3+ on the densities, molar volume, FTIR transmittance, absorption, photoluminescence and decay time in ZBB glass with compositions (60 x) B2O3: 30Bi2O3: 10ZnO: xEu2O3 (where x = 1, 3, 5, 7 and 9 mol%). Moreover; the bismuth borate (BB) and zinc borate (ZB) glass structures doped with Eu3+ in the same range of concentration were prepared in parallel for comparison of emission spectra with the zinc bismuth borate glass (ZBB).
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2. Experimental Zinc bismuth borate (ZBB) glass samples doped with Eu3+ in chemical composition of (60 x)B2O3: 30Bi2O3: 10ZnO: xEu2O3 (where x varied from 1, 3, 5, 7 and 9 mol%) were prepared by conventional melt quenching technique. Homogeneous powders of 20 g for each batch composition of all the glass samples were melted at 1100 C in alumina crucibles for about 3 h in an electrical furnace. The melts were air quenched by pouring onto a preheated stainless steel mould and annealed at 500 C for 3 h to remove thermal strains. The glass samples were cut and polished into 1.0 1.5 0.3 cm3 shape for optical measurements. The densities (r) were measured by Archimedes method using distilled water as an immersion liquid. Infrared spectra of the glass samples were recorded at room temperature in the range 650–4000 cm1 using an Agilent-Cary-630 FT-IR spectrometer. The optical absorption spectra of the glasses were recorded in the NIR regions in the range of 1800–2400 nm using a UV-3600 Shimadzu UV–VIS–NIR spectrophotometer. The photoluminescence spectra and decay time measurements were carried out using Cary Eclipse Fluorescence Spectrophotometer using a 465 nm wavelength excitation source from xenon flash lamp. 3. Result and discussions 3.1. Density and molar volume The densities and molar volumes of the investigated ZBB glasses are shown in Fig. 1. Both the density and molar volume of the glass samples are seen to increase with increasing Eu2O3 concentration. The values of density are in the range of 4.06–4.33 g/cm3 while their molar volumes (VM) are in the range of 47.41–49.73 cm3/mol. The increase in density of the glass with the increasing Eu3+ doping content is due to a higher molecular weight of Eu2O3 as compared to other components in the glasses. While the increase in the molar volume, indicates the increasing of inter-atomic spacing in the glass network. Eu2O3 acts as a network modifier and produces more non-bridging oxygen (NBOs) in glass matrices. 3.2. FTIR transmittance spectra The infrared spectrum of ZBB glass doped Eu3+ at 5 mol% is displayed in Fig. 2. According to Krogh-Moe model, the structure of borate glass consists of BO3 triangles with certain fraction of six membered (boroxal) rings [22]. In B2O3 glasses, boron [B3+] ions
Fig. 2. Typical IR transmission spectra of the 5 mol% Eu3+ doped ZBB glass.
are triangularly coordinated by oxygen to easily form glasses. The BO3 triangles are corner bonded in a random network [23]. Inclusion of transition metal ions in these glasses helps the boron to form tetraborate groups and progressive substitution of boroxal rings by triborate and tetraborate groups [24]. The boroxal ring shows its characteristic frequency at 806 cm1 and the presence or absence of this band decides the existence or absence of boroxal rings in the structure. In the present set of glasses, no band was observed at or around 806 cm1 indicating that boroxal ring was not present in these glasses. The peaks in the IR spectra of the glasses under studied are listed in Table 1. The present set of glasses shows transmission bands at 3750, 2904, 2830, 2287, 2080, 1268, 996, 848, 766 and 651 cm1. The band around 3750 cm1 is due to stretching vibration of OH group and bands around 2287 and 2080 cm1 are due to OH group [25]. The bands observed at 2904 cm1 and 2830 cm1 are also attributed to hydrogen bonds in OH group [26]. It has been reported that the band observed at 1268 cm1 was due to the asymmetrical stretching relaxation of the B-O bond of trigonal BO3 units [27]. The band around 996 cm1 originates from B O bond stretching of the tetrahedral BO4 units and is due to the vibration of some boron atoms attached to the non-bridging oxygen in the form of BO4 vibrations [28]. The shoulder around 840 cm1 is related to the symmetrical stretching vibration of Bi O in [BiO3] group [29]. The band around 766 cm1 is the bending mode of ¼B O BR bonds in which oxygen bridges between one tetrahedral and one trigonal boron atom [30]. The band observed around 651 cm1 is the bending bond mode of BO B vibrations [31,32]. In the IR spectra of the present glass system, it was found that the incorporation of ZnO did not show much effect on the structure of the glasses studied. The addition of ZnO into the present glass system produces a very small change in the IR bands that does not account for the major structural changes. However, in many glass systems in which ZnO is a major constituent the possibility of ZnO4 formation may be higher [33]. Bale et al. [34] had reported the formation of ZnO4 units with the increase of zinc oxide content. 3.3. Absorption spectrum and energy levels
Fig. 1. The densities and molar volumes of the Eu3+: ZBB glass system.
The typical absorption spectrum of the ZBB glass doped Eu3+ at 5 mol% is shown in Fig. 3 along with their assignments. The assignment of energy levels has been done according to Carnall et al. [15]. There is no absorption peak observed in visible region of studied glass. The absorption spectrum reveals two intense bands
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Table 1 FTIR analysis of the 5 mol% Eu3+ doped ZBB glass system. Wavenumber (cm1)
Assignment
651 766 848 996 1268 2080–3750
The bending vibrations of BOB linkages of BO3 units The bending mode of ¼BOB¼bonds Symmetrical stretching vibration of the BiO bonds in the BiO3 groups BO bond stretching of the tetrahedral BO4 units Asymmetrical stretching of the BO bond in the trigonal units OH group
Fig. 4. Typical excitation spectrum of the 5 mol% Eu3+ doped ZBB glass. Fig. 3. Typical absorption spectrum of the 5 mol% Eu3+ doped ZBB glass.
at 7F0 ! 7F6 (2076 nm) and 7F1 ! 7F6 (2198 nm) due to transitions in the NIR regions. Except for Eu3+ ion, it is well known that all the other trivalent RE3+ ions possess only a single populated ground state. A close examination of absorption band positions of 7F0 ! 7F6 and 7F1 ! 7F6 transitions clearly indicates that the energy gap between the ground state (7F0) and the first excited state (7F1) is about 360 cm1. Due to the effect of thermalization, the fractional population of any excited level is given by Deun et al. [35] gJ ðEJ E0 Þ=kT CJ (1) ¼ e C0 g0
transitions originating not only from the ground state (7F0), but also from the first excited state (7F1) to 5D1. As seen from Fig. 4, the 7 F0 ! 5D2 transition is the more intense compared to other transitions. Hence, the emission measurements were performed by exciting the samples at 465 nm wavelength using a xenon flash lamp. Fig. 5 shows the room temperature emission spectra of Eu3+: ZBB glasses. All the spectra exhibited with five emission bands corresponding to the 5D0 ! 7F0 (579 nm; yellow), 5D0 ! 7F1 (589 nm; orange), 5D0 ! 7F2 (613 nm; red), 5D0 ! 7F3 (651 nm; red) and 5D0 ! 7F4 (696 nm; deep-red) transitions. The luminescence intensity of luminescence materials is known to be dependent on the doping concentration of luminescent ions.
where {C 0 ,C J }, {E0 , EJ } and {g0 , gJ (2J + 1)} are the thermal correction factors, energies and degeneracies of the ground state (7F0) and the first excited state (7F1), respectively. The magnitude of kT is in the order of 208 cm1, where k is the Boltzmann’s constant and T is the absolute room temperature (300 K). For the ZBB glass doped with 5 mol% of Eu3+, the evaluated thermal correction factors for the ground state (7F0) and the first excited state (7F1) are: C 0 = 0.65 and C 1 = 0.35, respectively. Owing to the population of 7F0 (65%) and 7 F1 (35%) states, the absorption spectrum exhibited the transitions originating not only from the ground level (7F0) but also from the thermally excited level (7F1). 3.4. Excitation and emission spectra The excitation spectrum of ZBB glasses doped Eu3+ at 5 mol% obtained by monitoring the emission with 613 nm is shown in Fig. 4. The spectrum contains seven bands at 363, 383, 394, 414, 465, 485 and 533 nm corresponding to the transitions from the ground state (7F0) to 5D4, 5L7, 5L6, 5D3, 5D2, and 7F1 ! 5D1 excited states, respectively. The excitation spectrum exhibited the
Fig. 5. Emission spectrum of the ZBB glasses doped at different Eu3+concentrations. (For interpretation of the references to color in the text, the reader is referred to the web version of this article.)
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It is also noticed that the relative intensity of 5D0 ! 7F2 transition of Eu3+ various as a function of Eu3+ concentration (x = 1, 3, 5, 7 and 9 mol%). The maximum luminescence intensity was observed at x = 5 mol% of Eu2O3. The decrease in emission intensities with the increase of 7% and 9% mol of Eu2O3 concentration is due to the rise in non-radiative decay channels, which are enhanced due to the concentration quenching effect. If the concentration of an activator is higher than an appropriate value (typically several mol%), the emission intensity will be decreased. This is called concentration quenching [36]. The emission intensity of 5D0 ! 7F2 transition is electric-dipole allowed and depends strongly on the local symmetry around the Eu3+ ion, whereas the transition 5D0 ! 7F1 is magnetic-dipole allowed and is independent of the local symmetry. The fluorescence intensity ratio (R) of 5D0 ! 7F2 to 5D0 ! 7F1 transitions is used to establish the degree of asymmetry in the vicinity of Eu3+ ions and Eu–O covalence for various Eu3+-doped systems which is used to explain the short-range effects. The R values are found to be 2.80, 2.78, 2.76, 2.73 and 2.65 for 1, 3, 5, 7 and 9 mol% of Eu2O3, respectively. Fig. 6 shows the room temperature emission spectra of 55B2O3: 30Bi2O3: 10ZnO: 5Eu2O3 (ZBB) glass compared with 65B2O3: 30Bi2O3: 5Eu2O3 (BB) glass and 85B2O3: 10ZnO: 5Eu2O3 (ZB) glass. All the spectra exhibited with five emission bands corresponding to the 5D0 ! 7F0 (579 nm; yellow), 5D0 ! 7F1 (589 nm; orange), 5 D0 ! 7F2 (613 nm; red), 5D0 ! 7F3 (651 nm; red) and 5D0 ! 7F4 (696 nm; deep-red) transitions. It is noted that, all spectra were measured under the same condition. Relatively very high intensities are exhibited by ZBB glass in the emission spectra especially at 613 nm, the intensity for the ZBB is about 7 times of the ZB and around twice of the BB glasses. The emission intensity increases in the ZBB glass compared to BB and ZB may be explained in term of 5D0 ! 7F2 transition. It is well-known that the intensity of the electric-dipole 5D0 ! 7F2 transition is significantly affected by the degree at the center of symmetry in the environments around Eu3+ ions, and that the intensity of an emission transition is proportional to the radiative decay of these transitions. When Eu3+ ions are situated at low-symmetry sites, there is a high probability of the electric-dipole transition to occur. From this result indicated that the Eu3+ in ZBB site is lower symmetry sites compared with others. Fig. 7 shows the partial energy levels of Eu3+ ions in the ZBB glasses along with emission and non-radiative (NR) channels. When the Eu3+ ions are excited to any level above the 5D0, there is a
Fig. 7. The diagram of partial energy levels of Eu3+: ZBB glasses.
Fig. 8. The decay time profiles of 5D0 level of Eu3+: ZBB glasses.
fast non-radiative relaxation to 5D0 level [37] and the emission takes place from 5D0 level to lower level, 7FJ. Fig. 8 shows the decay time profiles of 5D0 level for different Eu3 + concentrations in the ZBB glasses obtained by exciting the samples with 465 nm wavelength and monitoring the emission at 613 nm. All the decay profiles are well fitted to single exponential function, It = Ioet/t , where Io is the fluorescence intensity when t = 0 and (t ) represents the decay time of the excited state. The measured decay time (t) obtained by taking the first e-folding times of the decay time curves are 1.034, 0.984, 0.907, 0.807 and 0.733 ms for 1, 3, 5, 7 and 9 mol% of Eu3+ doped ZBB glasses, respectively. The t values slightly decrease with Eu3+ concentration. The decease of t may be due to the increase of the electronic transition probability. 3.5. CIE chromaticity coordinates
Fig. 6. Emission spectra of the ZBB glass comparing with the BB and ZB glasses. (For interpretation of the references to color in the text, the reader is referred to the web version of this article.)
The Commission International De I-Eclairage (CIE) 1931 chromaticity coordinates for ZBB glasses doped with different of Eu3+ concentrations for the luminous color was determined from the
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concentration due to the asymmetry around the Eu3+ in the ZBB glass host. For all the studied glasses, the strongest peak was found at 613 nm (red emission) and the ZBB glass doped with 5 mol% Eu3+ had shown the highest emission. The decay time curve exhibited a single exponential nature for all the glasses and the measured decay time was found to decrease with the increase of Eu3+ concentration. Systematic analysis of the results suggested that the glass doped with Eu3+ concentration near 5 mol% is suitable for LED and display device applications. Acknowledgements The authors would like to thank National Research Council of Thailand (NRCT) and Nakhon Pathom Rajabhat University for support of this research. J. Kaewkhao would like to thank Prof. Dr. HongJoo Kim for luminescence mechanism discussion. References
Fig. 9. The CIE diagram of 5 mol% Eu3+ doped ZBB glass.
emission spectra. According to our knowledge, the CIE coordinates of red emission from Eu3+ ions not only depend upon the asymmetric ratio but also the emission from other higher energy (5D1, 5D2 and 5D3) emission levels [38]. The corresponding emission spectra were converted to the CIE 1931 chromaticity diagram and shown in Fig. 9. The color coordinates of the ZBB glasses doped with Eu3+ at 5 mol% are x = 0.645 and y = 0.353; which is very near to the orange-red light emission. Therefore, it is concluded that among the studied 1,3,5,7 and 9 mol% Eu3+ doped glasses, the 5 mol% Eu3+ doped glass has exhibited excellent color tunability of orange to red light emission. 4. Conclusions ZBB glasses doped with different Eu3+ concentrations were prepared and their red emitting efficiencies were studied for the development of phosphors device. The densities of the glasses were found to increase with the increasing of Eu2O3 concentration, due to higher molecular weight of Eu2O3. The increases in the molar volume with Eu2O3 doping concentrations are due to the increases in the bond length or inter-atomic spacing between atoms as the Eu2O3 acts as network modifier and creates more nonbridging oxygen (NBOs) in the glass matrices. The structure of ZBB glasses consists of randomly connected BO3 and BO4 groups. No boroxal ring formation was observed in the structure of the studied glasses. The structure of the present glass system is independent on change in composition though very small changes in IR bands have been noticed. The absorption spectrum revealed transitions from both the ground state (7F0) and the first excited state (7F1). The intensities of emission bands increase with the increase of Eu3+
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