Color tuneability behaviour and energy transfer analysis on Dy3+-Eu3+ co-doped glasses for NUV-WLEDs application

A series of Dy3+ and Eu3+ co-doped zinc aluminoborosilicate (ZABS) glasses were synthesized by a high-temperature melt-quenching method. Visible and NIR transitions of Dy3+-Eu3+ ions are observed through absorption spectra. A reverse trend in the optical band gap values and Urbach energy are seen with addition of Eu3+ ions. Photoluminescence studies recorded under different excitation wavelengths showed a variation in the emission intensities and prevailed the color tuneability behaviour of dopants. The energy transfer between Dy3+ and Eu3+ ions are studied through emission profiles, energy level diagram, and decay curves. The type of multipolar interaction between Dy3+ and Eu3+ are understood via Inokuti-Hirayama (IH) model and Dexter energy model. The CIE chromaticity coordinates, and correlated color temperature (CCT) values suggest that the prepared glasses can be used for light emitting diode application when excited at near-ultraviolet region.


Introduction
White light emission from single light emitting components like phosphor played a significant role in the lighting industry due to its peculiar property of giving high brightness and color quality [1]. An attempt to obtain white light with tri-color-based phosphors fascinated researchers in recent times [2]. However, such a method resulted a variation in the color of phosphors as time goes on and needs dif-zinc alumino borosilicate glasses, with varying Eu 3? concentrations, under different excitation wavelengths is analyzed. Also, the type of energy transfer interaction from the sensitizer (Dy 3? ) to the activator (Eu 3? ) are reported using Inokuti-Hirayama (IH) fitting and Dexter energy transfer model. The obtained results showed the suitability of prepared glasses for near-ultraviolet W-LEDs applications.

Experimental details
Transparent glasses with glass matrix formula given as 20SiO 2 -(20-x-y)B 2 O 3 -10Al 2 O 3 -10ZnO-30NaF-10ZnF 2 -xDy 2 O 3 -yEu 2 O 3 (where x = 0.5 mol% and y = 0, 0.1, 0.5, 1.0, 1.5, 2.0 and 2.5 mol%) were synthesized using high-temperature melt-quenching method. The starting materials were initially taken by proper weighing of them to get a total of 10-gram quantity of glass. After a constant grinding of raw materials, they are melted in an alumina crucible at 1320°C for 2 h. The formed melt is cascaded quickly on a pre-heated brass plate at 350°C. Once the melt is released or quenched on the brass plate, it immediately forms a solid glass. The solid glass is further annealed for 2 h at the quenched temperature to reduce thermal stresses, avoid breaking of glass and to maintain transparency. The glasses were polished to get a smooth surface and their thicknesses was reduced to * 2 mm for optical studies. The prepared glasses were labelled as ZABSDE0, ZABSDE1, ZABSDE2, ZABSDE3, ZABSDE4, ZABSDE5 and ZABSDE6, respectively.
In the Eq. (1), the terms a; h, m, E g denote the absorption coefficient, Planck's constant, photon frequency, and bandgap energy. The term B is a constant known as band-tailing parameter and n is the power factor that determines the nature of the electronic transition. A value of n ¼ 1 2 shows a direct bandgap and while n ¼ 2 indicates an indirect one. For amorphous and disordered materials such as glasses, n takes up the value as 2 because of the indirect transitions of rare earth ions. The band gap values are given in Table 1. The exponential tail seen at the absorption edge of the glasses indicates that there may be some sort of defect states or disorderness present which are created due to the incident high energy ultra-violet radiation and heavy element doping such as Dy, Eu. The exponential tail is otherwise called Urbach tail, and it determines the number of defects or disorderness present in the glasses. The defects are quantified in terms of Urbach energy (eV) which can be obtained by plotting In (a) against hm (photon energy), and then taking the inverse of the slope value obtained [20] as shown for ZABSDE1 glass in Fig. 3. These values are presented in Table 1. The reverse trend in bandgap energy and Urbach energy values suggest that, the more the localized energy levels/defects in the glass system, the less the bandgap energy becomes. Thus, the glasses with higher bandgap values show lower Urbach energy values.
The emission spectra recorded at k exc = 350 nm is shown in Fig. 5a. Under 350 nm, the Dy 3? singly doped glass features three emission bands from 4 F 9/2 excited level to ground levels lying at 482 ( 6 H 15/2 ), 575 ( 6 H 13/2 ) and 663 nm ( 6 H 11/2 ); whereas the Dy 3? -Eu 3? co-doped glasses show five emission bands in which Eu 3? are located at 613 ( 5 D 0 ? 7 F 2 ) and 701 nm ( 5 D 0 ? 7 F 4 ) along with emissions of Dy 3? at 482 nm, 575 and 663 nm. On exciting Dy 3? ions the emission bands corresponding to Eu 3? are also observed in the spectra. Most importantly, increasing the Eu 3? concentration leads to the decrease in Dy 3? emission intensity and this assures the possibility energy transfer from Dy 3? to Eu 3? . On exciting the glasses under 393 nm wavelength (Fig. 5b), the emission spectra show a steady decrease in the Dy 3? band at 482 nm whereas the Dy 3? peak at 575 nm splits up into two peaks i.e., the original peak at 575 nm (Dy 3? ) and newly formed peak at 590 nm (Eu 3? ). This spectral energy level splitting is seen when increasing the concentration of Eu 3? ions beyond 0.5 mol% (given in inset of Fig. 5b). The intense emission peak of Eu 3? seen at 613 nm ( 5 D 0 ? 7 F 2 ) reaches a maximum height for ZABSDE3 glass i.e., at 1.0 mol% of Eu 3? co-doping. Above this concentration, the emission intensity decreased slowly. Hence, the concentration quenching was achieved for Eu 3? codoping beyond 1.0 mol% under 393 nm excitation [16,17]. This sort of concentration quenching in the co-doped glasses with the excitation source of the activator could be due to the energy transfer between activator ions (i.e., Eu 3? ions), possibly due to radiative re-absorption. Thus, ZABSDE3 glass is regarded as the optimum candidate with excitation in the near ultra-violet region (393 nm). ions lying in the metastable state at 5 D 0 . This is possible because the energy level of Dy 3? ( 4 F 9/2 ) is approximately 21,000 cm -1 which is slightly greater than the energy levels of Eu 3? at 5 D 1 (19,020 cm -1 ) and 5 D 0 (* 17,277 cm -1 ) making the Dy 3? ions a resourceful sensitizer for Eu 3? ions [16]. The green arrow shown in Fig. 6b denotes the transfer of energy from Dy 3? to Eu 3? . After the energy transfer process, Dy 3? ions reach their ground levels at 6 H 15/2 , 6 H 13/2 and 6 H 11/2 by giving visible emissions in blue (482 nm), yellow (575 nm) and red (663 nm) regions, respectively. Similarly, the Eu 3? ions absorb the energy from Dy 3? and undergo emissions to the lower levels at 7 F 1 (590 nm), 7 F 2 (613 nm), 7 F 3 (652 nm), 7 F 4 (701 nm).

Decay analysis of Dy 3? -Eu 3? co-doped glasses
The decay profile of the 4 F 9/2 ? 6 H 15/2 transition of Dy 3? ions under 350 nm excitation and 575 nm emission is given in Fig. 7a. All the glasses show a biexponential behaviour under 350 nm. The lifetime values are drawn out using ExpDec2 Fit as shown in   [20].
where A 1 and A 2 are the constants, t 1 and t 2 are the luminescence decay times. Using the two lifetime values, the average lifetime is determined via the Eq.  [22]. Similarly, the decay study of the 5 D 0 ? 7 F 2 transition of Eu 3? ions under 350 nm excitation and 613 nm emission is given in Fig. 8a. The representative fitting plot is given in Fig. 8b. The bi-exponential behaviour is observed in this case also. Here the excitation of Dy 3? ions improved the lifetime of 5 Table 2. The lifetime values are then used to calculate other parameters such as energy transfer efficiency (g ET ), and the probability of energy transfer (P ET ) applying the following formulas [23].
where s d o and s d are the inherent decay times of donor (Dy) in the presence and absence of acceptor (Eu). The obtained values are listed in Table 2. The increase in energy transfer efficiency is seen from 2 to 15% with increasing the Eu 3? ions.

Inokuti-hirayama fitting
The luminescence quenching via non-radiative energy transfer from 4 F 9/2 (Dy 3? ) level to 5 D 0 (Eu 3? ) level can be explained by Inokuti-Hirayama (I-H) model. Using the I-H model, it is simpler to identify the nature of energy transfer between the donor and the acceptor. The non-exponential decay curves are fitted to the I-H model which implies the following relation, Here, S represents the interaction type such that, S = 6, 8, 10 corresponds to dipole-dipole (d-d), dipolequadrapole (d-q), and quadrapole-quadrapole (q-q) Fig. 7 a Decay curves recorded under 350 nm excitation and 575 nm emission, b Bi-exponential decay fitting shown for ZABSDE6 glass interactions, respectively. The I-H fitting plot for ZABSDE3 glass is shown in Fig. 9. The best linear fit is seen for S = 6, with the R 2 values obtained at 0.99. From the fitting table (inset of Fig. 9) the term Q stands for the energy transfer parameter given as.
where C is the concentration of acceptor ions (Eu 3? ), R O is the critical energy transfer distance or the distance of a donor-acceptor pair, C 1 À 3 S À Á is a constant value which equals to 1.77 for dipole-dipole (S ¼ 6), 1.43 for dipole-quadrapole (S ¼ 8), and 1.30 for quadrapole-quadrapole (S ¼ 10) [22]. From Eq. (7) the R O value is obtained. Using the R O value the donor-acceptor interaction parameter is calculated as follows, The calculated values are grouped in Table 2. The energy transfer (Q), energy transfer efficiency g ET ð Þ, and probability of energy transfer (P ET Þ are all found to increases with increasing Eu 3? concentration, while the critical energy transfer distance (R O ), and donor-acceptor interaction parameter (C DA ) decreases with increasing Eu 3? concentration. Thus, from the I-H fitting, it can be deducted that the type of energy migration between Dy 3? and Eu 3? is 'dipole-dipole' type when the condition S ¼ 6 is satisfied.

Dexter energy transfer model
The Dexter energy transfer model is simple, and it is adopted when there is a spectral overlap between energy levels of donor and acceptor. The Dexter energy transfer is associated with the term 'quenching' such that the emission spectra is wholly considered. The Dexter's energy transfer (ET) formula along with Reisfeld's approximation is used to determine the type of energy migration in Dy 3? ? Eu 3? using the following relation [23,24].
where g o and g represent the quantum efficiency of the Dy 3? in absence and presence of Eu 3? , respectively; C denotes the concentration of sensitizer (Dy) and activator (Eu) in mol% and n stands for the type of interaction i.e., n = 6, 8, 10 for dipole-dipole, dipole-quadrapole, and quadrapole-quadrupole. Equation (9) can be related to luminescence intensities given as Here, I SO and I S denote the luminescence intensity of Dy 3? without Eu 3? and with Eu 3? , respectively when the glasses are excited at 350 nm. By plotting I SO I S versus C n=3 , the best linear fit can be determined when n = 6, 8, 10. From Fig. 10, it is seen that the best linear fit is obtained best for n = 6, with R 2 = 0.9874 suggesting the dipole-dipole type of interaction between Dy 3? and Eu 3? . The latter finding is in accordance with the I-H fitting method.

Color coordinates and correlated color temperatures
The color estimation for the glasses under different excitations were provided by CIE-1931 chromaticity diagram. The CIE plots of glasses under 350 and 393 nm excitations are given in Fig. 11a and Fig. 11b, respectively, and their color coordinates (x, y) are listed in Table 3. Under 350 nm, the CIE chromaticity coordinates are found to move from a neutral white light (0.370, 0.401) to warm white light (0.427, 0.376) with increased Eu 3? content. This change is due to the energy transfer from Dy 3? to Eu 3? . Moreover, the excitation under 393 nm shifts the coordinates from cool white light to reddish region (0.360, 1560). To know about the color tuneability behaviour of the glasses under different excitation sources, ZABSDE3 glass was selected and excited at different wavelengths at 350 nm, 364 nm, 382 nm, and 393 nm. Under all these excitations, it is noted from Fig. 11c that the color emission from the glass moves from white light region to reddish region. The correlated color temperature (CCT) values were evaluated from the equation given as [25,26].  The variation in CCT values is seen with varying the Eu 3? concentration (Table 3). Thus, in the present work, addition of Eu 3? ions played a significant role in color emission from neutral white light to warm white light. Therefore, the glasses can be suitable for color tuneable LEDs and near ultra-violet W-LEDs application.

Conclusion
Dy 3? -Eu 3? co-doped glasses were prepared using a high-temperature melt-quenching method. The UV-Visible-NIR study revealed the presence of Dy 3? and Eu 3? transitions with overlap of Dy-Eu peaks. The bandgap and Urbach energy values followed reverse trend with varying Eu 3? ions. Emission studies revealed that Dy 3? ions exhibit energy transfer to Eu 3? ions through non-radiative process under 350 nm excitation. The dipole-dipole type of interaction between Dy 3? and Eu 3? is determined using the I-H fitting model and dexter energy model. The chromaticity coordinates obtained for lower concentration of Eu 3? is found to be consistent for white light emission compared to higher Eu 3? concentration. The co-doped glass showed color tuneability behaviour with different excitations in the near UV to visible region, favorable for near-ultraviolet light emitting diode applications.

Data availability
The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

Declarations
Conflict of interest The authors declare that they have no known conflicts of interest.
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