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Dy3+: Li2O-LiF-B2O3-ZnO glasses for photonic applications
Optical Materials 72 (2017) 781e787
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Tunable color emission via energy transfer in co-doped Ce3þ/Dy3þ: Li2O-LiF-B2O3-ZnO glasses for photonic applications L. Vijayalakshmi a, *, K. Naveen Kumar b, **, K. Srinivasa Rao c, Pyung Hwang a, *** a
Dept. of Automotive Engineering, Yeungnam University, Gyeongsan, Gyeongbuk, 38541, Republic of Korea Clean Energy Priority Research Center, Yeungnam University, Gyeongsan, Gyeongbuk, 38541, Republic of Korea c Dept. of Physics and Computer Science, V.S.M. College, Ramachandrapuram, East Godavari, 533255, A.P., India b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 May 2017 Received in revised form 13 July 2017 Accepted 16 July 2017
A set of co-doped (Ce3þ/Dy3þ): LBZ glasses were prepared by standard melt quenching technique. The pertinent absorption bands were observed in the optical absorption spectrum of co-doped Ce3þ/Dy3þ: LBZ glasses. We have been observed a prominent blue and yellow emission pertaining to Dy3þ ions at 0.5 mol % under the excitation of 385 nm doped glasses. However, the photoluminescence intensities were remarkably enhanced by co-doping with Ce3þ ions to Dy3þ: LBZ glasses due to energy transfer from Ce3þ to Dy3þ. The emission spectra of co-doped (Ce3þ/Dy3þ): LBZ glass exhibits three strong emissions at 440 nm, 480 nm and 574 nm which are assigned with corresponding electronic transitions of 4I15/ 6 4 6 4 6 2 / H15/2, F9/2 / H15/2 and F9/2 / H13/2 respectively. The Commission International de E'clairage coordinates were calculated from their emission spectra of single doped Dy3þ and co-doped (Ce3þ/Dy3þ): LBZ glasses. The obtained CIE chromaticity coordinates for optimized co-doped glass are found to be very close to the standard white region. Based on the concentration of Ce3þ, the emitting color of the codoped glass can be changed from blue to white color. The transformation of the color from blue to white region due to energy transfer from Ce3þ to Dy3þ. The energy transfer mechanism was substantiated by various fluorescence dynamics such as overlapped spectral profiles, photoluminescence, lifetime decay and CIE color coordinate analysis. These results could be suggested that the obtained co-doped (Ce3þ/Dy3þ): LBZ glasses are promising candidates for commercial white light applications. © 2017 Elsevier B.V. All rights reserved.
Keywords: Ce3þ/Dy3þ Energy transfer Photoluminescence White light emission
1. Introduction The current attention on the energy saving and environmental safety have motivated the replacement of white light emitting diodes (w-LEDs) for traditional fluorescent and incandescent lamps because of their unique advantages like long operating time, reliable, high performance, small size, cost effectiveness, low pollution, portal compactness and so on. The excessive versatility of these LEDs is a verdict for the solid state lasers, sensors, flash lights, optical storage, automobile headlights, solar cells, biological labels, backlighting of LCDs [1e3]. Rare earth doped materials are most often used in sensing, displaying and lighting because of their
* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (L. Vijayalakshmi), knaveenphy@gmail. com (K. Naveen Kumar), [email protected] (P. Hwang). http://dx.doi.org/10.1016/j.optmat.2017.07.034 0925-3467/© 2017 Elsevier B.V. All rights reserved.
luminescence characteristics having sharp emission peaks and long lifetimes. They can show multiple colors by varying the concentrations of dopants [4,5]. Dysprosium (Dy3þ) ion is one of the lanthanide ions which can be widely used as an activator in the preparation of LED based white light sources. Dy3þ ion exhibit two dominant emissions pertinent to the corresponding electronic transitions 4F9/2 / 6H15/2 and 4F9/2 / 6H13/2 in blue and yellow regions respectively. Generally, Dy3þ can emit white light of an appropriate yellow to blue intensity ratio. However, singly doped Dy3þ ion based materials shows much poor luminescence intensity because of their parity forbidden f-f transitions and low absorption cross section of Dy3þ ion. Moreover, the hypersensitive transition 4 F9/2 / 6H13/2 (yellow) is strongly influenced by outside environment and magnetic dipole transition 4F9/2 / 6H15/2 (blue) is insensitive and hardly changes with the ligand around Dy3þ ion. The intensity ratio of yellow to blue emissions can change greatly with the host [6e8]. Hence, a suitable co-doped sensitizer is necessary to enhance the luminescence efficiency of single doped Dy3þ ion materials. Because of parity and spin allowed 4f-5d
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electric dipole transition, Ce3þ ion can act as a dynamic sensitizer for certain rare earth ions. Further, Ce3þ ions exhibit intense and broad f-d absorption and emission bands varying from ultraviolet to visible region due to the large energy gap between their ground state (4f) and excited state (5d). Moreover, Ce3þ ions can effectively excite Dy3þ ions because there is strong overlapping between the emission of Ce3þ ion and 4f9-4f9 absorption of Dy3þ ion. So, there could be possibility of energy transfer from Ce3þ to Dy3þ and it can enhance the emission intensity of Dy3þ by compensating the blue emission and also combine blue and yellow emission of Dy3þ to achieve white light emission [9e12]. Selection of host material plays pivotal role in achieving white light emission. Among distinct host matrices, borate glasses have been deliberate as promising luminescent host lattices for various optically active lanthanide ions, because they possess low phonon energy, high optical transparency and good enough thermal stability and chemical durability. Especially, oxide glasses containing zinc have been used in solid state laser hosts, glass-polymer composites, optical displays, optical amplifiers etc. because zinc acts as network modifier/former. It can support large glass forming compositional range, low rate of crystallization, good chemical durability and low glass transition temperature [13,14]. The diameter of lithium ions is small enough so that these can easily accommodate the interstitial sites in a glassy matrix. Li2O, an alkali metal oxide, acts as a modifier in glasses and changes the glass network structure by creating nonbridging oxygens (NBOs). They create a new environment around atoms or molecules in glassy matrix and changes the properties related to structure. The addition of modifier salt (LiF) improves the transparency and stability of the glass [15,16]. In our present work, we prepare lithium fluoro zinc borate (LBZ) glasses doped with Ce3þand Dy3þ ions individually and combinedly to study their luminescence properties. Tunable luminescence is obtained by changing the concentration of Ce3þ ions in the host (LBZ) glass. Transfer of energy from Ce3þ to Dy3þ also has been discussed in detail. CIE chromaticity coordinates have been compared between single Dy3þ: LBZ glass and co-doped Ce3þ/Dy3þ: LBZ glass. 2. Experimental section 2.1. Materials and preparation of the sample The molar composition of the prepared glasses is as follows:
principle. By conventional optical method, the refractive indices of the obtained glass samples were measured with sodium as a light source. The values of densities are found to be in the range of 2.626e2.697 (gm/cm3) and refractive index values are found to be in the range of 1.652e1.659 for (Ce3þ/Dy3þ) co-doped LBZ glasses. Optical absorption spectra of Ce3þ: LBZ, Dy3þ: LBZ and (Ce3þ/Dy3þ): LBZ glasses were measured on a Varian-Cary-Win Spectrometer (JASCO V-570). The photoluminescence spectra of 0.5 mol % Ce3þ: LBZ, 0.5 mol % Dy3þ: LBZ and (Ce3þ/Dy3þ): LBZ glasses have been recorded on an YVON Fluorolog-3 Fluorimeter with Xe-arc lamp of power 450 W. Decay-curve data was recorded from the same instrument by a phosphorimeter and a Xe-flash lamp. Based on the equidistant wavelength method the CIE color coordinates (x, y) were computed from photoluminescence emission spectra. 3. Results and discussion 3.1. Absorption spectra The absorption spectrum of the co-doped (Ce3þ/Dy3þ): LBZ glass is shown in Fig. 1. This co-doped glass reveals the characteristic peaks of both Ce3þ and Dy3þ ions. The absorption bands of Dy3þ are observed at 355 nm, 747 nm, 801 nm, 894 nm, 1083 nm, 1263 nm and 1668 nm which are assigned with corresponding electronic transitions from ground state 6H15/2 to various excited states (4I15/2, 4 M15/2), 6F3/2, 6F5/2, 6F7/2, (6F9/2, 6H7/2), (6F11/2, 6H9/2) and 6H11/2 respectively [17]. The broad and intense band at 350 nm is also pertinent to the 4f (2F5/2) / 5d (2A1g) transition of Ce3þ ions. Due to spin orbit interaction, 2F ground state splits up into two J levels. The degeneracy of these states is reduced by the surrounding ligand field. Because the 4f electron is shielded from the ligand field by the closed 5s and 5p electron shells, the overall splitting of the 2FJ states is relatively small. When the 4f electron is excited to the outer excited 5d state, depending on the site symmetry the degeneracy of the 5d state is partially or completely removed due to the effect of the ligand fields. Electric-dipole transitions between the 4f ground state and the 5d excited state of Ce3þ are parity allowed [18]. Therefore, the absorption band at 350 nm is attributed to mixed effect of Ce3þ and Dy3þ ions within the glass. Hence, there is strong overlapping of f-d transition of Ce3þ ion with those of Dy3þ ion in the ultraviolet range. This result could be support for the effective energy transfer from Ce3þ to Dy3þ which is confirmed from the
(i) 30 Li2O-20 LiF-5ZnO-45B2O3 (LBZ) host glass (ii) 30 Li2O-20 LiF-5ZnO-(45-x)B2O3-x mol% Ce2O3 (x ¼ 0.5mol %)(Ce3þ: LBZ) (iii) 30 Li2O-20 LiF-5ZnO-(45-y)B2O3-y mol% Dy2O3 (y ¼ 0.3, 0.5, 1.0 and 1.5 mol%)(Dy3þ: LBZ) (iv) 30 Li2O-20 LiF-5ZnO-(44.5-x) B2O3-0.5 Dy2O3-x mol% Ce2O3 (x ¼ 0.1, 0.5, 1.0 and 1.5 mol%) (Ce3þ/Dy3þ: LBZ) An appropriate amount of reagent powders with analytical quality of Li2O, LiF, H3BO3, ZnO, Dy2O3 and Ce2O3 were mixed and properly grounded in an agate mortar about one hour. After this homogeneous mixture was taken into the crucible and placed in a furnace at a temperature of 1050 C for one hour. The obtained melt was quickly poured between two flat brass plates to get the glasses in the circular design having thickness 0.3 cm and diameter 2e3 cm. All the obtained glass samples are amorphous in nature. These glasses are taken for further optical investigation. 2.2. Characterizations The density of the prepared glassy materials was determined with water as an immersion liquid by using the Archimedes
Fig. 1. Optical absorption spectrum of co-doped (0.5 Ce3þ/0.5 Dy3þ): LBZ glasses.
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photoluminescence spectra in a later section. 3.2. Photoluminescence of 0.5 Ce3þ: LBZ and (0.3,0.5,1.0 & 1.5) Dy3þ: LBZ glasses The excitation and emission spectral profile of single doped Ce3þ: LBZ glass is shown in Fig. 2. This spectrum consists of a broad excitation and emission bands centered at 350 nm and 447 nm respectively, which are pertinent to 4f/ 5d transition of Ce3þ ions. By absorbing the incident radiation Ce3þ ions are excited from ground state 4f1 (2F5/2, 2F7/2) to the excited state 5d1 (2D). By exciting with 350 nm, a broad asymmetric blue emission band centered at 447 nm can be observed, which can assign to the 5d1/ 4f1 transition of Ce3þ ions. Because of spin orbit coupling in two levels of ground state leads to the asymmetry in the emission band [19]. Fig. 3 (a & b) shows the excitation and emission spectral profile of single doped Dy3þ: LBZ glass. The excitation spectrum of Dy3þ exhibits excitation bands monitored at 325 nm, 350 nm, 365 nm, 385 nm, 425 nm and 452 nm which are assigned with corresponding electronic transitions from ground state 6H15/2 to several excited states 4P3/2, 4M15/2, 4I11/2, 4I13/2, 4G11/2 and 4I15/2 respectively. Among all these excitation bands, the prominent band at 385 nm (6H15/2 / 6I13/2) is observed and it was used to record the emission spectrum of single doped Dy3þ: LBZ glass. Under the excitation of 385 nm, the emission spectra of Dy3þ: LBZ glass reveals three emission peaks in the regions of blue, yellow and red resulting from 4 I15/2 / 6H15/2, 4F9/2 / 6H15/2,6H13/2 and 6H11/2 transitions located at 440 nm, 480 nm, 574 nm and 664 nm respectively. From the emission spectra of Dy3þ: LBZ glasses, it is observed that the positions of the different emission bands do not change with increasing Dy3þ ion concentration. But, the relative emission intensity of individual peaks varies remarkably and it is strongly depends on Dy3þ ion concentration. This could be due to the screening effect of the electron clouds of the 5s and 5p orbitals protects the 4f- 4f transitions from any variation in the local crystal field environment. This provides the electrons in the 4f orbitals are relatively insensitive to the glassy matrix, hence the positions of the bands (which is strongly correlated with the configuration of electrons in the 4f orbital) does not change with the concentration of Dy3þ ion [20]. However, as the concentration increased, the average distance between Dy3þ- Dy3þ ions is decreased. This could
Fig. 2. Excitation and Emission spectral profile of 0.5 Ce3þ: LBZ glass under the excitation of lexci ¼ 350 nm.
Fig. 3. (a) Excitation and (b) Emission spectra of single doped Dy3þ: LBZ glass under the excitation of lexci ¼ 385 nm.
be increases the probability of non-radiative energy transfer between the ions through the cross relaxation process beyond the critical distance. It could affect the relative intensity of the individual peaks. The excited Dy3þ ions may relax to their ground state by means of radiative transitions or non-radiative transitions via the emission of photons and phonons respectively. A vibration of atoms or molecular groups constitutes phonons which can mediate relaxation in the Dy3þ ions. Initially, Dy3þ ions are excited to a higher energy level 4I13/2 by absorbing the incident photon. When an excited Dy3þ ion loses part of its excitation energy to a nearby Dy3þ ion at the ground state and promoting it to some meta-stable state, then Cross-Relaxation takes place. In this way both of the participating Dy3þ ions enter into meta-stable states. Hence, they decays nonradiatively to the metastable state 4F9/2 from the higher energy state 4I13/2 through the levels 4G11/2 and 4I15/2. The energy difference between the energy states lying above 4F9/2 level is very small, thus the level 4F9/2 receives many electrons via non radiative relaxation and highly populated. Moreover, the large energy gap between level 4F9/2 and the lower energy level 6F3/2 decreases the probable non-radiative relaxations and results the radiative transitions. Hence, Dy3þ ions are finally relaxed radiatively from the level 4F9/2 to several lower levels by emitting fluorescence in three regions. The low phonon energy of LBZ glass plays a critical role in the high production of the visible bands [21,22]. The yellow emission (4F9/ 6 2 / H13/2) is a magnetic dipole transition which is strongly influenced by the ligand field around the rare earth ion and is
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dominated when Dy3þ ions engaged in non-inversion symmetry site. However, blue emission (4F9/2 / 6H15/2) is an electric dipole transition which hardly varies with the crystal field strength and is dominating when Dy3þ ions are engaged at inversion symmetry site. The yellow to blue intensity ratio (Y/B) is useful to determine the coordination environment around the Dy3þ ion and is related to symmetry around the Dy3þ ion site in the luminescent glasses. The relative intensities of the these emissions are strongly dependent on the host lattice and the Dy3þ ion concentration. In the present work, blue emission is dominating over the yellow emission which indicates the symmetric nature of Dy3þ ions in the host glass [23]. From the emission spectra it has been found that the emission intensities are increasing with the concentration of Dy3þ ions from 0.3 to 0.5 mol %. Above 0.5 mol % the emission intensities are found to be decreasing with the increase of Dy3þ concentration due to concentration quenching. At higher concentrations, the average distance between neighboring Dy3þ ions decreases which leads to cross-relaxation of 4F9/2þ6H15/2 / 6H9/ 6 2 þ F3/2 levels. Due to this cross relaxation process, non-radiative energy transfer occurs between adjacent Dy3þ ions which results to concentration quenching [24]. Among all concentrations of Dy3þ: LBZ glasses, the 0.5 mol % concentration of Dy3þ doped glass exhibits prominent emission features. Hence 0.5 mol% concentration has been found to be optimized concentration for the Dy3þin the LBZ glasses. From the above photoluminescence analysis of Ce3þ and Dy3þ ions, the co-doped sample has an intense and broad absorption band in UV region which is a combination of both Ce3þ and Dy3þ ions. Moreover, single doped Dy3þ: LBZ glass exhibits less luminescence intensity and emitting only a single color. As stated, that the energy of Ce3þ ion of 5d1/4f1 transition is higher than that of the Dy3þ ions, so it can transfer some of its energy to Dy3þ ions which can enhance the luminescence intensity of Dy3þ ions and changes the emitting color of co-doped sample from blue to white depending upon the energy transfer from Ce3þ to Dy3þ.
Depending upon the extent of overlapping between the emission of sensitizer and excitation of activator energy transfer could
be taking place between sensitizer and activator. Hence, there is a significant overlap between the emission spectrum of Ce3þ and excitation spectrum of Dy3þ which has been shown in Fig. 4. As per Dexter theory, Ce3þ ions can sensitize the Dy3þ ions and enhance the luminescence intensity of Dy3þ ions effectively. Moreover, when compare the excitation spectra of all the glass samples such as 0.5Ce3þ: LBZ, 0.5Dy3þ: LBZ and 0.5Ce3þ/0.5Dy3þ: LBZ glasses, we have observed a common intense excitation band at 350 nm has been shown in Fig. 5. This indicates that there is possibility of effective resonance energy transfer from Ce3þ to Dy3þ ions at an excitation of 350 nm. The emission spectral profile of co-doped Ce3þ/Dy3þ: LBZ glass at different concentrations of Ce3þ ions under the excitation of 350 nm has been shown in Fig. 6 (a). The emission spectral profile consists of a broad emission band centered at 440 nm which could be a combination of 5d/4f transition of Ce3þ and 4I15/2 / 6H15/2 transition of Dy3þ and two sharp characteristic peaks of Dy3þ of electronic transitions 4F9/2 / 6H15/2 and 4F9/2 / 6H13/2 centered at 480 nm and 574 nm respectively [25]. As compared with the single doped Dy3þ: LBZ glass, the luminescence intensity of co-doped Ce3þ/Dy3þ: LBZ glass has been found to be enhanced by the addition of Ce3þ ions. This could be occurred due to an efficient energy transfer from Ce3þ to Dy3þ. At lower concentrations of Ce3þ ions, there is possibility of energy transfer from Ce3þ to Dy3þ ions which leads to enhance the intensity of the yellow emission (4F9/2 / 6H13/ 4 6 2) than the intensity of the blue emission ( F9/2 / H15/2) up to 3þ 0.5 mol % Ce ions. This could result to emitting white color from the co-doped sample which is explained from CIE diagram. But as concentration of Ce3þ ions increases, instead of energy transfer from Ce3þ to Dy3þ there is a possibility of back transfer from Dy3þ to Ce3þ ions which leads to enhancement of blue emission intensity than the intensity of yellow emission. Further increase of concentration of Ce3þ ions, the luminescence intensity has been found to be decreased due to concentration quenching at higher concentrations [26] as shown in Fig. 6 (b). The energy transfer mechanism between Ce3þ to Dy3þ is further explained through the partial energy level diagram which is shown in Fig. 7. By the strong absorption of UV radiation of 350 nm, Ce3þ ions are excited to a higher energy state (5d) from the ground state (4f). Then it relaxes non-radiatively to the lowest crystal field splitting of 5d state and finally returns to the ground state by producing the emission of 447 nm. At this stage partial amount of
Fig. 4. Overlapped emission spectrum of Ce3þ and excitation spectrum of Dy3þ.
Fig. 5. Excitation spectra of 0.5 Ce3þ, 0.5 Dy3þ and (0.5Ce3þ/0.5Dy3þ) co-doped LBZ glasses.
3.3. Energy transfer and luminescence of co-doped (Ce3þ/Dy3þ): LBZ glass
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Fig. 7. Energy level scheme diagram for the energy transfer mechanism from Ce3þ to Dy3þ ions in LBZ glasses.
I (t) ¼ A1 exp (t/t1) þ A2 exp (t/t2)
3þ
3þ
Fig. 6. (a) Emission spectra of co-doped (Ce /Dy ): LBZ glasses at different concentration of Ce3þ ions (lexci ¼ 350 nm). (b) Variation of the emission intensity with respect Ce3þ ion Concentration.
excitation energy is transferred to a higher energy state (4F7/2) of Dy3þ ions from which they relaxes non-radiatively to the metastable state 4F9/2. Finally, Dy3þ ions relax relatively from this 4F9/2 level to lower energy levels by emitting blue (4F9/2 / 6H15/2) and yellow (4F9/2 / 6H13/2) emissions. 3.4. Decay curve analysis Photoluminescence decay curves of the co-doped glassy matrices clearly elucidates the energy transfer between Ce3þ ions and Dy3þ ions. Decay curves are obtained by monitoring emission of Ce3þ at 447 nm and Dy3þ at 574 nm under the excitation of 350 nm for the co-doped glass as shown in Fig. 8. The luminescence decay of Ce3þ in single doped Ce3þ: LBZ and co-doped Ce3þ/Dy3þ: LBZ glasses have been found to be 0.017 ms and 0.012 ms respectively which are shown in Fig. 8 (a & c). In co-doped sample, Ce3þ ions decays much faster than in single doped glass. At the same time the luminescence decay of Dy3þ in single doped Dy3þ: LBZ and co-doped Ce3þ/Dy3þ: LBZ glassy matrices have been found to be 0.020 ms and 0.613 ms respectively as shown in Fig. 8 (b & d). The decay curves of both Ce3þ and Dy3þ in single ion doped glass displays single exponential nature, but in co-doped glass they exhibit non-exponential nature and are fitted to second order exponential function which can be calculated by using the relation
(1)
where ‘I’ is the luminescence intensity, A1 and A2 are constants, t is time, and t1 and t2 are the fast and slow lifetime values for exponential components, respectively. By fitting the decay curves we can calculate the values of A1, A2, t1 and t2. The effective lifetime constant (t*) can be calculated by:
t ¼ A1 t21 þ A2 t22 =ðA1 t1 þ A2 t2 Þ
(2)
The calculated lifetime values are tabulated in Table 1. This distinctive nature in co-doped glass because inter ionic distance decreases due to the presence of more number of donor and acceptor ions. This could result in a multi-polar interactions and migration of energy among the sensitizer and activator [27]. Moreover, the decrement lifetime of Ce3þ and increasing lifetime of Dy3þ in co-doped glassy matrices confirm the process of energy transfer from Ce3þ to Dy3þ in host glass unambiguously. The energy transfer efficiency from donor to acceptor can be calculated by using the expression given by
hT ¼ 1 IS/ISO
(3)
where hT is the energy transfer efficiency and ISO and IS are the luminescence intensity of a sensitizer in the absence and presence of an activator, respectively. From the decay curve analysis, the efficiency of energy transfer for the optimized glass has been found to be 29.41%. Based on all these results we could appreciate the possibility of effective energy transfer from Ce3þ to Dy3þ in the host LBZ glassy material.
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Fig. 8. Decay curve analysis of (a) 0.5 Ce3þ: LBZ glass, (b) 0.5 Dy3þ: LBZ glass, (c) Ce3þ in Ce3þ/Dy3þ: LBZ glass and (d) Dy3þ in Ce3þ/Dy3þ: LBZ glass.
Table 1 Calculated lifetime values. Glass Composition 0.5 0.5 0.5 0.5
Ce3þ: LBZ Dy3þ: LBZ Ce3þ/0.5 Dy3þ: LBZ Ce3þ/0.5 Dy3þ: LBZ
Excitation wavelength (nm)
Emission wavelength (nm)
Lifetime (ms)
350 385 350 350
447 574 447 (Ce3þ) 574 (Dy3þ)
0.017 0.020 0.012 0.613
3.5. Photometric analysis Commission International de I'Eclairage (CIE) chromaticity coordinates of single doped Dy3þ: LBZ glass by exciting with 385 nm and co-doped Ce3þ/Dy3þ: LBZ glass under the excitation of 350 nm has been shown in Fig. 9. The calculated chromaticity coordinates of both the glassy matrices are tabulated in Table 2. In general, we expect white light from the single doped Dy3þ: LBZ glass by varying the ratio of yellow and blue emissions. But, this single doped Dy3þ: LBZ glass exhibits nearer to bluish white color only for the optimized concentration (0.5 mol %) and for all other concentrations it emits only dark blue color. When Ce3þ ions are co-doped into this Dy3þ: LBZ glassy matrix, it exhibits white color for lower concentrations (0.1 and 0.5) of Ce3þ ions. The concentration of Ce3þ ions can affect the emitting color of the co-doped sample because it changes the Y/B ratio of Dy3þ. From the emission spectra, it has been observed that the intensity of yellow emission is more than that of the intensity of the blue emission for 0.1 and 0.5 mol % of Ce3þ ions. The emitting color changing from blue to white is attributed to the presence of energy transfer from Ce3þ ions to Dy3þ
ions [28]. As the concentration of Ce3þ ions increases again, it exhibits a blue color because at higher concentrations of Ce3þ instead of energy transfer from Ce3þ to Dy3þ there is back transferred from Dy3þ to Ce3þ. Moreover, this blue emission is a combination of 5d/4f transition of Ce3þ and 4F9/2 / 6H15/2 transition of Dy3þ. Hence, by adding appropriate concentrations of Ce3þ and Dy3þ, the emitting color of the sample can be tuned from blue to bluish white and ultimately white color based on the extent of energy transfer from Ce3þ to Dy3þ ions. It is an appealing to state that the 0.5 mol % concentration of the (Ce3þ/Dy3þ) ions co-doped LBZ glass sample exhibited white light luminescence coordinates (0.2987, 0.2915) compared to other samples and these glasses can be deliberated as a promising aspirant for the fabrication of w-LEDs. 4. Conclusion In summary, co-doped (Ce3þ/Dy3þ): LBZ glasses with different concentrations of Ce3þ ions have been successfully prepared and characterized by using their optical studies. The optical absorption spectrum of the co-doped (Ce3þ/Dy3þ): LBZ glass exhibits high
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to be enhanced remarkably by increasing the concentration of Ce3þ ions. At 1.0 mol % Ce3þ co-doped (Ce3þ/Dy3þ): LBZ glass shows prominent emission features. The CIE chromaticity coordinates of all the obtained glasses were determined from their emission spectra. At 0.5 mol % of co-doped (Ce3þ/Dy3þ): LBZ glass exhibit dazzling white light emission. Based on all these features the obtained co-doped (Ce3þ/Dy3þ): LBZ glass matrices are suggested as potential luminescent material for the commercial white lighting applications. Acknowledgement The authors are delighted to acknowledge the research 2017 financial support from the Yeungnam University (YU), South Korea. References
Fig. 9. CIE chromaticity coordinates of single doped Dy3þ: LBZ glass and co-doped (Ce3þ/Dy3þ): LBZ glasses.
Table 2 CIE chromaticity coordinates of single doped Dy3þ and co-doped (Ce3þ/Dy3þ): LBZ glasses under the excitation of 385 nm and 350 nm respectively. Sample. No
Sample composition
1 2 3 4 5 6 7 8
0.3 0.5 1.0 1.5 0.1 0.5 1.0 1.5
Dy3þ: LBZ Dy3þ: LBZ Dy3þ: LBZ Dy3þ: LBZ Ce3þ/0.5 Dy3þ: Ce3þ/0.5 Dy3þ: Ce3þ/0.5 Dy3þ: Ce3þ/0.5 Dy3þ:
LBZ LBZ LBZ LBZ
CIE coordinates (x, y) (0.1980, (0.2195, (0.1962, (0.1951, (0.2718, (0.2987, (0.2002, (0.1720,
0.1780) 0.2061) 0.1772) 0.1753) 0.2618) 0.2915) 0.1782) 0.1374)
intensity at 350 nm which is the combination of both the ions in UV region. Overlapped spectra between emission of Ce3þ and excitation of Dy3þ which can prove the possibility of energy transfer from Ce3þ to Dy3þ ions undoubtedly. Photoluminescence spectra of codoped (Ce3þ/Dy3þ): LBZ glass exhibits a well-known peaks of Dy3þ in blue (480 nm) and yellow (574 nm) regions corresponding to the electronic transitions 4F9/2 / 6H15/2 and 4F9/2 / 6H13/2 respectively. By co-doping with Ce3þ ions to the single doped Dy3þ: LBZ glass, the emission intensities pertaining to Dy3þ ions are found
[1] Kai Li, Mengmeng Shang, Hongzhou Lian, Jun Lin, J. Mater. Chem. C 4 (2016) 5507. [2] Honglan Li, Guixia Liu, Jinxian Wang, Xiangting Dong, Wensheng Yu, New J. Chem. 41 (2017) 1609. [3] Yongqiang Han, Pengfei Yan, Jingwen Sun, Guanghui An, Xu Yao, Yuxin Li, Guangming Li, Dalton Trans. 46 (2017) 4642. [4] Xiao-Fei Wang, Qi Yang, Gui-Gen Wang, Xin-Zhong Wang, Jie-Cai Hana, RSC Adv. 6 (2016) 96263. [5] Sk. Khaja Hussain, Jae Su Yu, RSC Adv. 7 (2017) 13281. [6] Yue Guo, Byung Kee Moon, Byung Chun Choi, Jung Hyun Jeong, Jung Hwan Kim, RSC Adv. 7 (2017) 23083. [7] Yiyang Zhou, Weiren Zhao, Junhua Chen, Zifeng Liao, RSC Adv. 7 (2017) 17244. [8] Anna M. Kaczmarek, Dorine Ndagsi, Rik Van Deun, Dalton Trans. 45 (2016) 16231. [9] Mingye Ding, Yuting Li, Daqin Chen, Hongwei Lu, Junhua Xi, Zhenguo Ji, J. Alloy. Compd. 658 (2016) 952. [10] Zihan Xu, Zhiguo Xia, Bingfu Lei, Quanlin Liu, J. Mater. Chem.C 4 (2016) 9711. [11] R. Zheng, D. Luo, Y. Yuan, Z. Wang, Y. Zhang, W. Wei, L.B. Kong, D. Tang, J. Am. Ceram. Soc. 98 (2015) 3231. [12] U. Manik, S.C. Gedam, S.J. Dhoble, Lumin 30 (2015) 910. [13] P. Liang, J.W. Liu, Z.H. Liu, RSC Adv. 6 (2016) 89113. ~ o, A. Lira, A.N. Meza-Rocha, E. Pasquini, S. Pelli, A. Speghini, [14] U. Caldin M. Bettinelli, G.C. Righini, J. Lumin. 167 (2015) 327. [15] Arti Yadav, Manjeet S. Dahiya, A. Hooda, Prem Chand, S. Khasa, Solid State Sci. 70 (2017) 54. [16] M.G. Dong, M.I. Sayyed, G. Lakshminarayana, M. Çelikbilek Ersundu, A.E. Ersundu, Priyanka Nayar, M.A. Mahdi, J. Non Cryst. Solids 468 (2017) 12. [17] Saisudha B. Mallur, Hio Giap Ooi, Stewart K. Ferrell, Panakkattu K. Babu, Mater. Res. Bull. 92 (2017) 52. [18] N.Ch. Ramesh Babu, Ch. Srinivasa Rao, G. Naga Raju, V. Ravi Kumar, I.V. Kityk, N. Veeraiah, Opt. Mater. 34 (2012) 1381. [19] X. Zhang, Y. Huang, M. Gong, Chem. Engg. J. 307 (2017) 291. [20] Lokesh Mishra, Anchal Sharma, Amit K. Vishwakarma, Kaushal Jha, M. Jayasimhadri, B.V. Ratnam, Kiwan Jang, A.S. Rao, R.K. Sinha, J. Lumin 169 (2016) 121. [21] P.P. Pawar, S.R. Munishwar, R.S. Gedam, Solid State Sci. 64 (2017) 41. [22] Valluri Ravi kumar, G. Giridhar, N. Veeraiah, Opt. Mater. 60 (2016) 594. [23] Luxiang Wang, Mengjiao Xu, Hongyang Zhao, Dianzeng Jia, New J. Chem. 40 (2016) 3086. [24] Huaixin Liu, Ziying Guo, J. Lumin 187 (2017) 181. [25] Ting Li, Panlai Li, Nian Fu, Zhijun Wang, Shuchao Xu, Qiongyu Bai, Zhiping Yang, J. Solid State Chem. 248 (2017) 26. [26] K. Naveen Kumar, R. Padma, J.L. Rao, Misook Kang, RSC Adv. 6 (2016) 54525. [27] Mengjiao Xu, Luxiang Wang, Dianzeng Jia, Lang Liu, Mater. Res. Bull. 70 (2015) 691. [28] Meng Jian-Xin, Yang Chuang-Tao, Chen Qing-Qing, J. Lumin 130 (2010) 1320.
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Tunable color emission via energy transfer in co-doped Ce3þ/Dy3þ: Li2O-LiF-B2O3-ZnO glasses for photonic applications L. Vijayalakshmi a, *, K. Naveen Kumar b, **, K. Srinivasa Rao c, Pyung Hwang a, *** a
Dept. of Automotive Engineering, Yeungnam University, Gyeongsan, Gyeongbuk, 38541, Republic of Korea Clean Energy Priority Research Center, Yeungnam University, Gyeongsan, Gyeongbuk, 38541, Republic of Korea c Dept. of Physics and Computer Science, V.S.M. College, Ramachandrapuram, East Godavari, 533255, A.P., India b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 May 2017 Received in revised form 13 July 2017 Accepted 16 July 2017
A set of co-doped (Ce3þ/Dy3þ): LBZ glasses were prepared by standard melt quenching technique. The pertinent absorption bands were observed in the optical absorption spectrum of co-doped Ce3þ/Dy3þ: LBZ glasses. We have been observed a prominent blue and yellow emission pertaining to Dy3þ ions at 0.5 mol % under the excitation of 385 nm doped glasses. However, the photoluminescence intensities were remarkably enhanced by co-doping with Ce3þ ions to Dy3þ: LBZ glasses due to energy transfer from Ce3þ to Dy3þ. The emission spectra of co-doped (Ce3þ/Dy3þ): LBZ glass exhibits three strong emissions at 440 nm, 480 nm and 574 nm which are assigned with corresponding electronic transitions of 4I15/ 6 4 6 4 6 2 / H15/2, F9/2 / H15/2 and F9/2 / H13/2 respectively. The Commission International de E'clairage coordinates were calculated from their emission spectra of single doped Dy3þ and co-doped (Ce3þ/Dy3þ): LBZ glasses. The obtained CIE chromaticity coordinates for optimized co-doped glass are found to be very close to the standard white region. Based on the concentration of Ce3þ, the emitting color of the codoped glass can be changed from blue to white color. The transformation of the color from blue to white region due to energy transfer from Ce3þ to Dy3þ. The energy transfer mechanism was substantiated by various fluorescence dynamics such as overlapped spectral profiles, photoluminescence, lifetime decay and CIE color coordinate analysis. These results could be suggested that the obtained co-doped (Ce3þ/Dy3þ): LBZ glasses are promising candidates for commercial white light applications. © 2017 Elsevier B.V. All rights reserved.
Keywords: Ce3þ/Dy3þ Energy transfer Photoluminescence White light emission
1. Introduction The current attention on the energy saving and environmental safety have motivated the replacement of white light emitting diodes (w-LEDs) for traditional fluorescent and incandescent lamps because of their unique advantages like long operating time, reliable, high performance, small size, cost effectiveness, low pollution, portal compactness and so on. The excessive versatility of these LEDs is a verdict for the solid state lasers, sensors, flash lights, optical storage, automobile headlights, solar cells, biological labels, backlighting of LCDs [1e3]. Rare earth doped materials are most often used in sensing, displaying and lighting because of their
* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (L. Vijayalakshmi), knaveenphy@gmail. com (K. Naveen Kumar), [email protected] (P. Hwang). http://dx.doi.org/10.1016/j.optmat.2017.07.034 0925-3467/© 2017 Elsevier B.V. All rights reserved.
luminescence characteristics having sharp emission peaks and long lifetimes. They can show multiple colors by varying the concentrations of dopants [4,5]. Dysprosium (Dy3þ) ion is one of the lanthanide ions which can be widely used as an activator in the preparation of LED based white light sources. Dy3þ ion exhibit two dominant emissions pertinent to the corresponding electronic transitions 4F9/2 / 6H15/2 and 4F9/2 / 6H13/2 in blue and yellow regions respectively. Generally, Dy3þ can emit white light of an appropriate yellow to blue intensity ratio. However, singly doped Dy3þ ion based materials shows much poor luminescence intensity because of their parity forbidden f-f transitions and low absorption cross section of Dy3þ ion. Moreover, the hypersensitive transition 4 F9/2 / 6H13/2 (yellow) is strongly influenced by outside environment and magnetic dipole transition 4F9/2 / 6H15/2 (blue) is insensitive and hardly changes with the ligand around Dy3þ ion. The intensity ratio of yellow to blue emissions can change greatly with the host [6e8]. Hence, a suitable co-doped sensitizer is necessary to enhance the luminescence efficiency of single doped Dy3þ ion materials. Because of parity and spin allowed 4f-5d
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electric dipole transition, Ce3þ ion can act as a dynamic sensitizer for certain rare earth ions. Further, Ce3þ ions exhibit intense and broad f-d absorption and emission bands varying from ultraviolet to visible region due to the large energy gap between their ground state (4f) and excited state (5d). Moreover, Ce3þ ions can effectively excite Dy3þ ions because there is strong overlapping between the emission of Ce3þ ion and 4f9-4f9 absorption of Dy3þ ion. So, there could be possibility of energy transfer from Ce3þ to Dy3þ and it can enhance the emission intensity of Dy3þ by compensating the blue emission and also combine blue and yellow emission of Dy3þ to achieve white light emission [9e12]. Selection of host material plays pivotal role in achieving white light emission. Among distinct host matrices, borate glasses have been deliberate as promising luminescent host lattices for various optically active lanthanide ions, because they possess low phonon energy, high optical transparency and good enough thermal stability and chemical durability. Especially, oxide glasses containing zinc have been used in solid state laser hosts, glass-polymer composites, optical displays, optical amplifiers etc. because zinc acts as network modifier/former. It can support large glass forming compositional range, low rate of crystallization, good chemical durability and low glass transition temperature [13,14]. The diameter of lithium ions is small enough so that these can easily accommodate the interstitial sites in a glassy matrix. Li2O, an alkali metal oxide, acts as a modifier in glasses and changes the glass network structure by creating nonbridging oxygens (NBOs). They create a new environment around atoms or molecules in glassy matrix and changes the properties related to structure. The addition of modifier salt (LiF) improves the transparency and stability of the glass [15,16]. In our present work, we prepare lithium fluoro zinc borate (LBZ) glasses doped with Ce3þand Dy3þ ions individually and combinedly to study their luminescence properties. Tunable luminescence is obtained by changing the concentration of Ce3þ ions in the host (LBZ) glass. Transfer of energy from Ce3þ to Dy3þ also has been discussed in detail. CIE chromaticity coordinates have been compared between single Dy3þ: LBZ glass and co-doped Ce3þ/Dy3þ: LBZ glass. 2. Experimental section 2.1. Materials and preparation of the sample The molar composition of the prepared glasses is as follows:
principle. By conventional optical method, the refractive indices of the obtained glass samples were measured with sodium as a light source. The values of densities are found to be in the range of 2.626e2.697 (gm/cm3) and refractive index values are found to be in the range of 1.652e1.659 for (Ce3þ/Dy3þ) co-doped LBZ glasses. Optical absorption spectra of Ce3þ: LBZ, Dy3þ: LBZ and (Ce3þ/Dy3þ): LBZ glasses were measured on a Varian-Cary-Win Spectrometer (JASCO V-570). The photoluminescence spectra of 0.5 mol % Ce3þ: LBZ, 0.5 mol % Dy3þ: LBZ and (Ce3þ/Dy3þ): LBZ glasses have been recorded on an YVON Fluorolog-3 Fluorimeter with Xe-arc lamp of power 450 W. Decay-curve data was recorded from the same instrument by a phosphorimeter and a Xe-flash lamp. Based on the equidistant wavelength method the CIE color coordinates (x, y) were computed from photoluminescence emission spectra. 3. Results and discussion 3.1. Absorption spectra The absorption spectrum of the co-doped (Ce3þ/Dy3þ): LBZ glass is shown in Fig. 1. This co-doped glass reveals the characteristic peaks of both Ce3þ and Dy3þ ions. The absorption bands of Dy3þ are observed at 355 nm, 747 nm, 801 nm, 894 nm, 1083 nm, 1263 nm and 1668 nm which are assigned with corresponding electronic transitions from ground state 6H15/2 to various excited states (4I15/2, 4 M15/2), 6F3/2, 6F5/2, 6F7/2, (6F9/2, 6H7/2), (6F11/2, 6H9/2) and 6H11/2 respectively [17]. The broad and intense band at 350 nm is also pertinent to the 4f (2F5/2) / 5d (2A1g) transition of Ce3þ ions. Due to spin orbit interaction, 2F ground state splits up into two J levels. The degeneracy of these states is reduced by the surrounding ligand field. Because the 4f electron is shielded from the ligand field by the closed 5s and 5p electron shells, the overall splitting of the 2FJ states is relatively small. When the 4f electron is excited to the outer excited 5d state, depending on the site symmetry the degeneracy of the 5d state is partially or completely removed due to the effect of the ligand fields. Electric-dipole transitions between the 4f ground state and the 5d excited state of Ce3þ are parity allowed [18]. Therefore, the absorption band at 350 nm is attributed to mixed effect of Ce3þ and Dy3þ ions within the glass. Hence, there is strong overlapping of f-d transition of Ce3þ ion with those of Dy3þ ion in the ultraviolet range. This result could be support for the effective energy transfer from Ce3þ to Dy3þ which is confirmed from the
(i) 30 Li2O-20 LiF-5ZnO-45B2O3 (LBZ) host glass (ii) 30 Li2O-20 LiF-5ZnO-(45-x)B2O3-x mol% Ce2O3 (x ¼ 0.5mol %)(Ce3þ: LBZ) (iii) 30 Li2O-20 LiF-5ZnO-(45-y)B2O3-y mol% Dy2O3 (y ¼ 0.3, 0.5, 1.0 and 1.5 mol%)(Dy3þ: LBZ) (iv) 30 Li2O-20 LiF-5ZnO-(44.5-x) B2O3-0.5 Dy2O3-x mol% Ce2O3 (x ¼ 0.1, 0.5, 1.0 and 1.5 mol%) (Ce3þ/Dy3þ: LBZ) An appropriate amount of reagent powders with analytical quality of Li2O, LiF, H3BO3, ZnO, Dy2O3 and Ce2O3 were mixed and properly grounded in an agate mortar about one hour. After this homogeneous mixture was taken into the crucible and placed in a furnace at a temperature of 1050 C for one hour. The obtained melt was quickly poured between two flat brass plates to get the glasses in the circular design having thickness 0.3 cm and diameter 2e3 cm. All the obtained glass samples are amorphous in nature. These glasses are taken for further optical investigation. 2.2. Characterizations The density of the prepared glassy materials was determined with water as an immersion liquid by using the Archimedes
Fig. 1. Optical absorption spectrum of co-doped (0.5 Ce3þ/0.5 Dy3þ): LBZ glasses.
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photoluminescence spectra in a later section. 3.2. Photoluminescence of 0.5 Ce3þ: LBZ and (0.3,0.5,1.0 & 1.5) Dy3þ: LBZ glasses The excitation and emission spectral profile of single doped Ce3þ: LBZ glass is shown in Fig. 2. This spectrum consists of a broad excitation and emission bands centered at 350 nm and 447 nm respectively, which are pertinent to 4f/ 5d transition of Ce3þ ions. By absorbing the incident radiation Ce3þ ions are excited from ground state 4f1 (2F5/2, 2F7/2) to the excited state 5d1 (2D). By exciting with 350 nm, a broad asymmetric blue emission band centered at 447 nm can be observed, which can assign to the 5d1/ 4f1 transition of Ce3þ ions. Because of spin orbit coupling in two levels of ground state leads to the asymmetry in the emission band [19]. Fig. 3 (a & b) shows the excitation and emission spectral profile of single doped Dy3þ: LBZ glass. The excitation spectrum of Dy3þ exhibits excitation bands monitored at 325 nm, 350 nm, 365 nm, 385 nm, 425 nm and 452 nm which are assigned with corresponding electronic transitions from ground state 6H15/2 to several excited states 4P3/2, 4M15/2, 4I11/2, 4I13/2, 4G11/2 and 4I15/2 respectively. Among all these excitation bands, the prominent band at 385 nm (6H15/2 / 6I13/2) is observed and it was used to record the emission spectrum of single doped Dy3þ: LBZ glass. Under the excitation of 385 nm, the emission spectra of Dy3þ: LBZ glass reveals three emission peaks in the regions of blue, yellow and red resulting from 4 I15/2 / 6H15/2, 4F9/2 / 6H15/2,6H13/2 and 6H11/2 transitions located at 440 nm, 480 nm, 574 nm and 664 nm respectively. From the emission spectra of Dy3þ: LBZ glasses, it is observed that the positions of the different emission bands do not change with increasing Dy3þ ion concentration. But, the relative emission intensity of individual peaks varies remarkably and it is strongly depends on Dy3þ ion concentration. This could be due to the screening effect of the electron clouds of the 5s and 5p orbitals protects the 4f- 4f transitions from any variation in the local crystal field environment. This provides the electrons in the 4f orbitals are relatively insensitive to the glassy matrix, hence the positions of the bands (which is strongly correlated with the configuration of electrons in the 4f orbital) does not change with the concentration of Dy3þ ion [20]. However, as the concentration increased, the average distance between Dy3þ- Dy3þ ions is decreased. This could
Fig. 2. Excitation and Emission spectral profile of 0.5 Ce3þ: LBZ glass under the excitation of lexci ¼ 350 nm.
Fig. 3. (a) Excitation and (b) Emission spectra of single doped Dy3þ: LBZ glass under the excitation of lexci ¼ 385 nm.
be increases the probability of non-radiative energy transfer between the ions through the cross relaxation process beyond the critical distance. It could affect the relative intensity of the individual peaks. The excited Dy3þ ions may relax to their ground state by means of radiative transitions or non-radiative transitions via the emission of photons and phonons respectively. A vibration of atoms or molecular groups constitutes phonons which can mediate relaxation in the Dy3þ ions. Initially, Dy3þ ions are excited to a higher energy level 4I13/2 by absorbing the incident photon. When an excited Dy3þ ion loses part of its excitation energy to a nearby Dy3þ ion at the ground state and promoting it to some meta-stable state, then Cross-Relaxation takes place. In this way both of the participating Dy3þ ions enter into meta-stable states. Hence, they decays nonradiatively to the metastable state 4F9/2 from the higher energy state 4I13/2 through the levels 4G11/2 and 4I15/2. The energy difference between the energy states lying above 4F9/2 level is very small, thus the level 4F9/2 receives many electrons via non radiative relaxation and highly populated. Moreover, the large energy gap between level 4F9/2 and the lower energy level 6F3/2 decreases the probable non-radiative relaxations and results the radiative transitions. Hence, Dy3þ ions are finally relaxed radiatively from the level 4F9/2 to several lower levels by emitting fluorescence in three regions. The low phonon energy of LBZ glass plays a critical role in the high production of the visible bands [21,22]. The yellow emission (4F9/ 6 2 / H13/2) is a magnetic dipole transition which is strongly influenced by the ligand field around the rare earth ion and is
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dominated when Dy3þ ions engaged in non-inversion symmetry site. However, blue emission (4F9/2 / 6H15/2) is an electric dipole transition which hardly varies with the crystal field strength and is dominating when Dy3þ ions are engaged at inversion symmetry site. The yellow to blue intensity ratio (Y/B) is useful to determine the coordination environment around the Dy3þ ion and is related to symmetry around the Dy3þ ion site in the luminescent glasses. The relative intensities of the these emissions are strongly dependent on the host lattice and the Dy3þ ion concentration. In the present work, blue emission is dominating over the yellow emission which indicates the symmetric nature of Dy3þ ions in the host glass [23]. From the emission spectra it has been found that the emission intensities are increasing with the concentration of Dy3þ ions from 0.3 to 0.5 mol %. Above 0.5 mol % the emission intensities are found to be decreasing with the increase of Dy3þ concentration due to concentration quenching. At higher concentrations, the average distance between neighboring Dy3þ ions decreases which leads to cross-relaxation of 4F9/2þ6H15/2 / 6H9/ 6 2 þ F3/2 levels. Due to this cross relaxation process, non-radiative energy transfer occurs between adjacent Dy3þ ions which results to concentration quenching [24]. Among all concentrations of Dy3þ: LBZ glasses, the 0.5 mol % concentration of Dy3þ doped glass exhibits prominent emission features. Hence 0.5 mol% concentration has been found to be optimized concentration for the Dy3þin the LBZ glasses. From the above photoluminescence analysis of Ce3þ and Dy3þ ions, the co-doped sample has an intense and broad absorption band in UV region which is a combination of both Ce3þ and Dy3þ ions. Moreover, single doped Dy3þ: LBZ glass exhibits less luminescence intensity and emitting only a single color. As stated, that the energy of Ce3þ ion of 5d1/4f1 transition is higher than that of the Dy3þ ions, so it can transfer some of its energy to Dy3þ ions which can enhance the luminescence intensity of Dy3þ ions and changes the emitting color of co-doped sample from blue to white depending upon the energy transfer from Ce3þ to Dy3þ.
Depending upon the extent of overlapping between the emission of sensitizer and excitation of activator energy transfer could
be taking place between sensitizer and activator. Hence, there is a significant overlap between the emission spectrum of Ce3þ and excitation spectrum of Dy3þ which has been shown in Fig. 4. As per Dexter theory, Ce3þ ions can sensitize the Dy3þ ions and enhance the luminescence intensity of Dy3þ ions effectively. Moreover, when compare the excitation spectra of all the glass samples such as 0.5Ce3þ: LBZ, 0.5Dy3þ: LBZ and 0.5Ce3þ/0.5Dy3þ: LBZ glasses, we have observed a common intense excitation band at 350 nm has been shown in Fig. 5. This indicates that there is possibility of effective resonance energy transfer from Ce3þ to Dy3þ ions at an excitation of 350 nm. The emission spectral profile of co-doped Ce3þ/Dy3þ: LBZ glass at different concentrations of Ce3þ ions under the excitation of 350 nm has been shown in Fig. 6 (a). The emission spectral profile consists of a broad emission band centered at 440 nm which could be a combination of 5d/4f transition of Ce3þ and 4I15/2 / 6H15/2 transition of Dy3þ and two sharp characteristic peaks of Dy3þ of electronic transitions 4F9/2 / 6H15/2 and 4F9/2 / 6H13/2 centered at 480 nm and 574 nm respectively [25]. As compared with the single doped Dy3þ: LBZ glass, the luminescence intensity of co-doped Ce3þ/Dy3þ: LBZ glass has been found to be enhanced by the addition of Ce3þ ions. This could be occurred due to an efficient energy transfer from Ce3þ to Dy3þ. At lower concentrations of Ce3þ ions, there is possibility of energy transfer from Ce3þ to Dy3þ ions which leads to enhance the intensity of the yellow emission (4F9/2 / 6H13/ 4 6 2) than the intensity of the blue emission ( F9/2 / H15/2) up to 3þ 0.5 mol % Ce ions. This could result to emitting white color from the co-doped sample which is explained from CIE diagram. But as concentration of Ce3þ ions increases, instead of energy transfer from Ce3þ to Dy3þ there is a possibility of back transfer from Dy3þ to Ce3þ ions which leads to enhancement of blue emission intensity than the intensity of yellow emission. Further increase of concentration of Ce3þ ions, the luminescence intensity has been found to be decreased due to concentration quenching at higher concentrations [26] as shown in Fig. 6 (b). The energy transfer mechanism between Ce3þ to Dy3þ is further explained through the partial energy level diagram which is shown in Fig. 7. By the strong absorption of UV radiation of 350 nm, Ce3þ ions are excited to a higher energy state (5d) from the ground state (4f). Then it relaxes non-radiatively to the lowest crystal field splitting of 5d state and finally returns to the ground state by producing the emission of 447 nm. At this stage partial amount of
Fig. 4. Overlapped emission spectrum of Ce3þ and excitation spectrum of Dy3þ.
Fig. 5. Excitation spectra of 0.5 Ce3þ, 0.5 Dy3þ and (0.5Ce3þ/0.5Dy3þ) co-doped LBZ glasses.
3.3. Energy transfer and luminescence of co-doped (Ce3þ/Dy3þ): LBZ glass
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Fig. 7. Energy level scheme diagram for the energy transfer mechanism from Ce3þ to Dy3þ ions in LBZ glasses.
I (t) ¼ A1 exp (t/t1) þ A2 exp (t/t2)
3þ
3þ
Fig. 6. (a) Emission spectra of co-doped (Ce /Dy ): LBZ glasses at different concentration of Ce3þ ions (lexci ¼ 350 nm). (b) Variation of the emission intensity with respect Ce3þ ion Concentration.
excitation energy is transferred to a higher energy state (4F7/2) of Dy3þ ions from which they relaxes non-radiatively to the metastable state 4F9/2. Finally, Dy3þ ions relax relatively from this 4F9/2 level to lower energy levels by emitting blue (4F9/2 / 6H15/2) and yellow (4F9/2 / 6H13/2) emissions. 3.4. Decay curve analysis Photoluminescence decay curves of the co-doped glassy matrices clearly elucidates the energy transfer between Ce3þ ions and Dy3þ ions. Decay curves are obtained by monitoring emission of Ce3þ at 447 nm and Dy3þ at 574 nm under the excitation of 350 nm for the co-doped glass as shown in Fig. 8. The luminescence decay of Ce3þ in single doped Ce3þ: LBZ and co-doped Ce3þ/Dy3þ: LBZ glasses have been found to be 0.017 ms and 0.012 ms respectively which are shown in Fig. 8 (a & c). In co-doped sample, Ce3þ ions decays much faster than in single doped glass. At the same time the luminescence decay of Dy3þ in single doped Dy3þ: LBZ and co-doped Ce3þ/Dy3þ: LBZ glassy matrices have been found to be 0.020 ms and 0.613 ms respectively as shown in Fig. 8 (b & d). The decay curves of both Ce3þ and Dy3þ in single ion doped glass displays single exponential nature, but in co-doped glass they exhibit non-exponential nature and are fitted to second order exponential function which can be calculated by using the relation
(1)
where ‘I’ is the luminescence intensity, A1 and A2 are constants, t is time, and t1 and t2 are the fast and slow lifetime values for exponential components, respectively. By fitting the decay curves we can calculate the values of A1, A2, t1 and t2. The effective lifetime constant (t*) can be calculated by:
t ¼ A1 t21 þ A2 t22 =ðA1 t1 þ A2 t2 Þ
(2)
The calculated lifetime values are tabulated in Table 1. This distinctive nature in co-doped glass because inter ionic distance decreases due to the presence of more number of donor and acceptor ions. This could result in a multi-polar interactions and migration of energy among the sensitizer and activator [27]. Moreover, the decrement lifetime of Ce3þ and increasing lifetime of Dy3þ in co-doped glassy matrices confirm the process of energy transfer from Ce3þ to Dy3þ in host glass unambiguously. The energy transfer efficiency from donor to acceptor can be calculated by using the expression given by
hT ¼ 1 IS/ISO
(3)
where hT is the energy transfer efficiency and ISO and IS are the luminescence intensity of a sensitizer in the absence and presence of an activator, respectively. From the decay curve analysis, the efficiency of energy transfer for the optimized glass has been found to be 29.41%. Based on all these results we could appreciate the possibility of effective energy transfer from Ce3þ to Dy3þ in the host LBZ glassy material.
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Fig. 8. Decay curve analysis of (a) 0.5 Ce3þ: LBZ glass, (b) 0.5 Dy3þ: LBZ glass, (c) Ce3þ in Ce3þ/Dy3þ: LBZ glass and (d) Dy3þ in Ce3þ/Dy3þ: LBZ glass.
Table 1 Calculated lifetime values. Glass Composition 0.5 0.5 0.5 0.5
Ce3þ: LBZ Dy3þ: LBZ Ce3þ/0.5 Dy3þ: LBZ Ce3þ/0.5 Dy3þ: LBZ
Excitation wavelength (nm)
Emission wavelength (nm)
Lifetime (ms)
350 385 350 350
447 574 447 (Ce3þ) 574 (Dy3þ)
0.017 0.020 0.012 0.613
3.5. Photometric analysis Commission International de I'Eclairage (CIE) chromaticity coordinates of single doped Dy3þ: LBZ glass by exciting with 385 nm and co-doped Ce3þ/Dy3þ: LBZ glass under the excitation of 350 nm has been shown in Fig. 9. The calculated chromaticity coordinates of both the glassy matrices are tabulated in Table 2. In general, we expect white light from the single doped Dy3þ: LBZ glass by varying the ratio of yellow and blue emissions. But, this single doped Dy3þ: LBZ glass exhibits nearer to bluish white color only for the optimized concentration (0.5 mol %) and for all other concentrations it emits only dark blue color. When Ce3þ ions are co-doped into this Dy3þ: LBZ glassy matrix, it exhibits white color for lower concentrations (0.1 and 0.5) of Ce3þ ions. The concentration of Ce3þ ions can affect the emitting color of the co-doped sample because it changes the Y/B ratio of Dy3þ. From the emission spectra, it has been observed that the intensity of yellow emission is more than that of the intensity of the blue emission for 0.1 and 0.5 mol % of Ce3þ ions. The emitting color changing from blue to white is attributed to the presence of energy transfer from Ce3þ ions to Dy3þ
ions [28]. As the concentration of Ce3þ ions increases again, it exhibits a blue color because at higher concentrations of Ce3þ instead of energy transfer from Ce3þ to Dy3þ there is back transferred from Dy3þ to Ce3þ. Moreover, this blue emission is a combination of 5d/4f transition of Ce3þ and 4F9/2 / 6H15/2 transition of Dy3þ. Hence, by adding appropriate concentrations of Ce3þ and Dy3þ, the emitting color of the sample can be tuned from blue to bluish white and ultimately white color based on the extent of energy transfer from Ce3þ to Dy3þ ions. It is an appealing to state that the 0.5 mol % concentration of the (Ce3þ/Dy3þ) ions co-doped LBZ glass sample exhibited white light luminescence coordinates (0.2987, 0.2915) compared to other samples and these glasses can be deliberated as a promising aspirant for the fabrication of w-LEDs. 4. Conclusion In summary, co-doped (Ce3þ/Dy3þ): LBZ glasses with different concentrations of Ce3þ ions have been successfully prepared and characterized by using their optical studies. The optical absorption spectrum of the co-doped (Ce3þ/Dy3þ): LBZ glass exhibits high
L. Vijayalakshmi et al. / Optical Materials 72 (2017) 781e787
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to be enhanced remarkably by increasing the concentration of Ce3þ ions. At 1.0 mol % Ce3þ co-doped (Ce3þ/Dy3þ): LBZ glass shows prominent emission features. The CIE chromaticity coordinates of all the obtained glasses were determined from their emission spectra. At 0.5 mol % of co-doped (Ce3þ/Dy3þ): LBZ glass exhibit dazzling white light emission. Based on all these features the obtained co-doped (Ce3þ/Dy3þ): LBZ glass matrices are suggested as potential luminescent material for the commercial white lighting applications. Acknowledgement The authors are delighted to acknowledge the research 2017 financial support from the Yeungnam University (YU), South Korea. References
Fig. 9. CIE chromaticity coordinates of single doped Dy3þ: LBZ glass and co-doped (Ce3þ/Dy3þ): LBZ glasses.
Table 2 CIE chromaticity coordinates of single doped Dy3þ and co-doped (Ce3þ/Dy3þ): LBZ glasses under the excitation of 385 nm and 350 nm respectively. Sample. No
Sample composition
1 2 3 4 5 6 7 8
0.3 0.5 1.0 1.5 0.1 0.5 1.0 1.5
Dy3þ: LBZ Dy3þ: LBZ Dy3þ: LBZ Dy3þ: LBZ Ce3þ/0.5 Dy3þ: Ce3þ/0.5 Dy3þ: Ce3þ/0.5 Dy3þ: Ce3þ/0.5 Dy3þ:
LBZ LBZ LBZ LBZ
CIE coordinates (x, y) (0.1980, (0.2195, (0.1962, (0.1951, (0.2718, (0.2987, (0.2002, (0.1720,
0.1780) 0.2061) 0.1772) 0.1753) 0.2618) 0.2915) 0.1782) 0.1374)
intensity at 350 nm which is the combination of both the ions in UV region. Overlapped spectra between emission of Ce3þ and excitation of Dy3þ which can prove the possibility of energy transfer from Ce3þ to Dy3þ ions undoubtedly. Photoluminescence spectra of codoped (Ce3þ/Dy3þ): LBZ glass exhibits a well-known peaks of Dy3þ in blue (480 nm) and yellow (574 nm) regions corresponding to the electronic transitions 4F9/2 / 6H15/2 and 4F9/2 / 6H13/2 respectively. By co-doping with Ce3þ ions to the single doped Dy3þ: LBZ glass, the emission intensities pertaining to Dy3þ ions are found
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