Judd–Ofelt and luminescence properties of Dysprosium and Terbium doped bismuth-borate glass system

Absorption spectra of Tb3+ ions from the ground state 7F6 revealed three absorption bands. In addition to the six absorption bands for Dy3+ ions observed in the visible and near-infrared ranges. The optical band gape of sample free from Dy2O3 (4 mol% Tb4O7) smaller than other samples containing Dy2O3. The oscillator strengths and Judd–Ofelt parameters (Ω2, Ω4, Ω6) for reported Dy3+ and Tb3+ absorption transitions are estimated, and they do not follow a same trend depend on the substitution between Dy and Tb. The photoluminescence excitation of the Dy3+ at λem = 575 nm reveals the presence of a Tb3+ peak at 375 nm. The emission spectra of the glasses show that only the λem = 575 nm increases due to the excitation of the Tb3+ at λem = 545 nm, indicating effective energy transfer (ET) from the Tb3+ to Dy3+ in sample containing Dy2O3 up to 2 mol% and then changed from the Dy3+ to Tb3+. An efficient yellow luminescence arises from the activator's emitting centers. ET predicts the sensitizer's luminescence degradation and the activator's luminescence growth. The color coordinates and the correlated color temperature (CCT) indicate that the glass samples are suitable for white, yellow-green, and blue-light emission depend on the energy of excitation.


Introduction
The scientific community plays a vital role in increasing color rendering and service life and eliminating the use of traditional incandescent and fluorescent lamps (Ravita and Rao 2021). The W-LED draws attention to the fact that it is not limited in the light it emits by the liquid crystal found in phone and digital camera manufacturing (Sun et al. glasses for green photonic device applications. Juarez-Batalla et al. (2017) studied green to white tunable light-emitting phosphors: Dy 3+ /Tb 3+ in zinc phosphate glasses. Ramachari et al. (2014) studied energy transfer and photoluminescence properties of Dy 3+ /Tb 3+ co-doped oxyfluoride silicate glass-ceramics for solid-state white lighting.
Our main goal in the current work is to understand the suitability of the glass samples under anesthetic study using Dy 3+ and Tb 3+ to obtain effective white, green, blue, and red emissions useful for the application of photovoltaic device. The visible emission, age and energy transfer of the glasses were determined using photoluminescence and lifetime decay profiles. The effect of Tb ion substitution at the expense of Dy ions on optical properties and emissions was studied.
The chemical's purity grades were mixed and annealed for 15 min at 400 °C in a porcelain crucible, then melted at 1000 °C in an electric programmable and automatic temperature control oven (Lenton thermal designs of type VAF15/10). The molten material was cooled and poured between two copper plates in the air. All samples are examined using the Philips Analytical X-ray analysis system (type PW3710) with Cu Kα = 1.54°A.
The density of glasses is determined by the Archimedes method in which the sample is weighed more than once in the air and when immersed in toluene at room temperature. Density calculation uses the formula where ρ is the density of the glass sample, Wa is the weight of the glass sample in air, Wb is the weight of the glass sample in toluene, and 0.865 is the density of toluene.
The optical properties of glass samples (transmission, absorption, excitation, emission, and lifetime decay) were measured using a JASCO V-570 spectrophotometer (with a precision of 1 nm) and a JASCO FP-8300 spectrofluorometer (with a 150 W Xenon arc lamp). Figure 1 shows the absence of peaks and the presence of a hump that represents the amorphous nature of glass.

Results and discussion
The application of light on a sample doped with RE was mainly affected by the UV-Vis spectrum as each RE has significant transitions from the ground state to the different excited states. Figure 2 shows the UV-Vis-near IR spectra of glass samples, the first addition of Dy 2 O 3 with a concentration of 4 mol% indicates a significant energy level of third ionization of Dy, reported as the transition from 6 H 15/2 (ground state) to 6 H 11/2 , 6 F 11/2 + 6 H 9/2 , 6 F 9/2 , 6 F 7/2 , 6 F 5/2 , 6 F 3/2 , 4 F 9/2 , and 4 I 15/2 at 1696, 1274, 1102, 906, 810, 778, 470 and 458 nm (Babu and Cole 2018Karki et al. 2019;Shaaban et al.2018;Kashif and Ratep 2022a, b). The hypersensitive transition is sensitive to the ligand environment of Dy 3+ sites selected at high intensity at 1274 nm and follows the selection rule ΔS = 0, |ΔL|≤ 2, and |ΔJ|≤ 2 (Babu and Cole 2018;Divina et al. 2021). The disappearance of peaks below 600 nm reflects the strong absorption of the host matrix under UV illumination (Divina et al. 2021).
A persistent decrease in the concentration of 1 mol% Dy was replaced by 1 mol% Tb 4 O 7 in each sample, causing the intensity of Dy peaks to decrease.
In addition to the appearance of amplitude peaks in the NIR region characterizing Tb 3+ ions at 2228, 1944, 1898, and 1812, representing the forbidden spin transition from the ground state 7 F 6 to the excited states 7 F 3 and 7 F 2 , the appearance of peaks at UV-Vis 392 and 488 nm represents the 7 F 6 → 5 L 10 + 5 G 5 ( 5 D 3 ) and 7 F 6 → 5 D 4 ( 5 D 4 ) (Quang et al. 2020;Kumar and Rao 2021;Chen et al. 2017). The appearance of a new band indicates the high solubility of RE (Hegde et al. 2019).
The absorption spectra were used to determine the bonding parameters (δ) of RE with its surrounding ligand by the nephelauxetic ratio (β) according to the where β = υ c /υ a as υ c is the transition of RE ion in the host matrix, υ a is the aqua ion transition (Table 1).
The value obtained decomposes the type of bond because it is ionic when negative and covalent when positive. The determined values were detected in the negative charge that detected the ionic bond, and the value decreased with the increase of Tb 4 O 7 as the trend of bonding was directed to increase the covalency. The interaction of light with electrons in the valance band passing through the band gap to the conduction band decides their suitability in optoelectronics devices (Hegde et al. 2019). In addition to the determination of material type as becoming a semiconductor in the range of 0-4 eV for Eg, while an insulator at Eg > 4 eV (Shaaban et al.2018), it is sensitive mainly to the RE addition through the glass preparation.
Based on the relation, Mott and Davis proposed an Eg determination for direct and indirect allowed and forbidden transitions (Selvi et al. 2014).
where α is the absorption coefficient, is constant, and n = 2, 1/2, 3, and 3/2 is the type of transition (direct, indirect, allowed, and forbidden).  Figure 3 represents the best straight line for the relation ( hv) 2 and ( hv) 2∕3 . Chosen n = 1/2 and 3/2 for the calculation and plotting of band gap graph because the tangential line of both n = 1/2 and n = 3/2 passes through the largest number of points, unlike n = 2 and n = 3, so they were excluded. The Eg values are calculated by extrapolating the previous relation to a value = 0 at the x-axis. All energy gap values are tabulated in Table 2.
Many researchers (Babu and Cole 2018;Aljewaw et al. 2020;Guo et al. 2016;Kashif and Ratep 2022a, b;Chen et al. 2017) study the effect of adding the RE as Dy or Tb on different glass formers. Figure 4 shows the density calculated from the Archimedes method represented in two parts. The first part is the former doped with one of the RE, and the other part is the former doped with mixed RE.
In the part of adding one type of RE to glass, it noticed the greater density of Tb 4 O 7 glass than Dy 2 O 3 glass. The higher density for glass doped with Tb 4 O 7 than with Dy 2 O 3 is explained by the higher MW t (molecular weight) of Tb 4 O 7 than with Dy 2 O 3 .
The density change is explained by the state of structural compression of the glass that occurs with the addition of different RE and increases with increasing rare-earth content (Babu and Cole 2018;Zaman et al. 2021;Guo et al. 2016;Yang and Zhu 2021;Hegde et al. 2019;Kashif and Ratep 2021).
The probability of transitions between the ground state and different energy levels of rare-earth ions expresses the oscillator strength.  In other words, the oscillator strength is the indicating tool of the absorption intensity measured as the integrated area of the peak, which comprises information about electronic transitions and radiative properties according to the relation The theoretical method derived by Judd-Ofelt's theory calculates an electric dipole transition from the ground state ΨJ to an excited state Ψ′J′ is given by The root mean square deviation given by the relation Examine the quality fit between the experimental and calculated oscillator strength, while the result is present in Table 3.
From the results obtained, noticed the low value, which shows the validity of the Judd-Ofelt (JO) theory and the agreement between the experimental and theoretical oscillator power.
JO parameter determined with the help of f exp , f cal , and refractive index using the root mean square fitting approximation procedure gave deep insight about the symmetry/asymmetric bonding and the nature of the bond between the surrounding ligand around RE. The intensity of parameters affects the chemical composition and the symmetry around RE (Divina et al. 2021). The abrupt modification structure around RE (Babu and Cole 2018;Divina et al. 2021) is demonstrated by the fact that the intensity of a parameter changes with any composition change.
The calculated experimental oscillator strengths are a facility where it is calculated for each peak in a separate calculation while the JO parameters are based on the reduced squared matrix element U2, 4, 6 values. In samples containing double RE, the bands of Tb 3+ and Dy 3+ separated and not overlapped. We are supposed to sum the two RE peaks in the same calculation. With the increase of Tb concentration, it was observed that the intensity of Dy peaks decreased, and the peak of Tb appeared. Among the JO parameters, Ω 2 is the most sensitive to ambient, providing the opportunity to understand the asymmetry around rare-earth ions, the polarization of ligand ions, and the covalency around rare-earth ions.
So, the parameters Ω 4 and Ω 6 are less sensitive to the vicinity and can give information about the bulk properties like rigidity and viscosity (Quang et al. 2020). Figure 5 shows the JO parameters as a function of Tb 4 O 7 concentration. In general, the Ω 2 values are larger than Ω 4 and Ω 6 except in the sample 3Tb-1Dy which has a lower value of Ω 2 than Ω 6 . Higher values of Ω 2 show a higher covalency and are asymmetric around rare-earth ions (Karki et al. 2019). The greater value of Ω 2 in samples (0Tb-4Dy), (2Tb-2Dy), and (4Tb-0Dy) led to the highest green luminescence emission at 543 nm (Quang et al. 2020).
The higher value of Ω 6 in sample 2Tb-2Dy explains the high polarized around RE, the rigidity (Mariyappan et al. 2016).
The increase in Ω 6 values over 4 in all samples explains a host effect with rare-earth ions at different concentrations (Divina et al. 2021).
The JO parameters are useful in determining the highest stimulated emission crosssection used in the photonic device (Mariyappan et al. 2016). The Judd-Ofelt parameter intensities values of the glass samples under study are higher than the other glass  Fig. 6a and b from monitoring the emission at λ em = 575 nm (Dy 3+ ) and λ em = 545 nm (Tb 3+ ) respectively.
The samples doped with Dy-Tb were recorded by monitoring wavelength at 575 and 545 nm are shown in Fig. 7a and b. From Fig. 7a clear that the excitation peaks representing the Dy transitions like the transition shown in Fig. 6a. There was no significant change in the excitation spectra after adding Tb 3+ ions to glass samples.
The recording of excitation bands due to Dy 3+ ions under Tb 3+ emission wavelength of 545 nm in samples doped with both Dy-Tb indicates the occurrence of energy transmission from Dy 3+ to Tb 3+ ions (Ravita and Rao 2021;Pisarska et al. 2014). Figure 7a shows the excitation profile of Dy/Tb co-doped glass samples under 575 nm emission wavelength. The excitation spectrum measured at 575 nm exhibits seven peaks six assigned Dy ions at 354, 367, 389, 427, 454, and 474 nm pertaining to the transition of 6 H 15/2 → 6 p 7/2 , 6 p 5/2 , 4 I 13/2 , 4 g 11/2 and 4 F 9/2 respectively. The remaining band at 488 nm corresponding to 7 F 6 → 5 D 4 transition. As shown in Fig. 7b the excitation spectrum recorder under 543 nm emission wavelength of Tb 3+ ions show eight excitation bands; four are assigned to Tb 3+ ion and four bands are assigned to Dy 3+ ions. The bands at 354, 370, 379, and 486 nm corresponding to 6 F 6 → 5 L 9 , 5 L 10 , 5 G 6 and 5 D 4 transition respectively. The four excitation bands due to the Dy 3+ ions observed at 390, 427, 454, and 474 nm. The presence of excitation band due to Tb 3+ ions while recording under Dy 3+ emission wavelength 575 nm and the presence of excitation band due to Dy 3+ ions while recording under Tb 3+ emission wavelength 543 nm indicate the occurrence of energy transmission from Dy 3+ to Tb 3+ (Bashar et al. 2020) and from Tb 3+ to Dy 3+ ions (Ravita and Rao 2021). The excitation peaks are determined in Fig. 7a Figure 8 shows the emission for the Dy 3+ /Tb 3+ co-doped glass samples under excitation at 351 nm. It consists of six emission peaks: four corresponding to Tb 3+ ions and two corresponding to Dy 3+ ions. The peaks due Tb 3+ are situated at 488, 544, 583 and 621 nm and the peaks due Dy 3+ are situated at 475 and 575 nm.
In Fig. 8 conspicuous that, with increase in Tb 3+ ion concentration, the intensity of peaks corresponding to Tb 3+ (544 nm) continuously increasing up to 2 mol % Tb 4 O 7 and then slow decrease. It indicates that this concentration is the best energy transfer condition. And the decrease is demonstrating a reverse energy transfer from Tb 3+ to Dy 3+ ions (Ma et al 2021).
As shown in Fig. 8, the emission spectra used to calculate the emission parameters coordinate through the CIE 1931 color chromaticity using the color matching function; X′(y); Y′(y); and Z′(y) as shown in Fig. 9a.
The coordination of glass samples excited at 351 is determined and tabulated in Table 5A. The data obtained explains the appearance of light emission in a different color. Sample 0Tb-4Dy excited at 351 nm (Divina et al. 2021) have a similar standpoint (0.314, 0.301). While the samples doped doubly Tb-Dy and 4Tb-0Dy indicate the greenish color (a) (b) Fig. 6 a The excitation spectra of glass sample free from Tb 3+ at 575 nm. b The excitation spectra of glass sample free from Dy 3+ at 545 nm shifted to blue emission, as it may be explained by the ratio between green to yellow (Table 5A), as the increase of the ratio to reach a value larger than 1 contributes to the yellow-green emission.- The lifetime is important factor for potential laser materials. The fluorescence decay curve of Dy 3+ ion and Tb 3+ excited at 351, 388 and 453 nm and monitored at 575 nm and 543 nm at room temperature of the present samples was measured and is shown in Fig. 9b and c. The decay curves are deviated from single exponential, the decay profile is found to be biexponential for some samples and other follow single exponential decay. The bi-exponential nature may be because of the reason that the energy transfer from excited donor ion to unexcited acceptor ion transpires. The intensity of luminescence can be articulated as follows (Ye et al. 2023): where I 0 is the intensity at t = 0, and I is intensity at time t, A1 and A2 are the amplitudes of decay constants and τ 1 and τ 2 are the luminescence lifetimes for fast and slow channels of decay respectively. The average values of decay time are calculated by using the formula given as (Ye et al. 2023): The average lifetime values for Dy 3+ and Tb 3+ ions doped glass samples are inset in Table 4. It can be seen the average lifetime increases with increase in Tb 3+ ion concentration signifying the lifetime values of the sensitizer [Dy 3+ ) increases while increasing the activator [Tb 3+ ] ions concentration up to 2 mol% (Juarez-Batalla et al. 2017;Vijayakumar et al. 2018).
The experimental lifetime values of the Tb 3+ ions in the co-doped glass samples under study were found to be lower than the Tb 3+ single doped glass samples. And increasing of the Tb 3+ ions lifetimes value up to 2 mol% confirms the energy transfer process from Dy 3+ to Tb 3+ , also the decreases present the possible energy transfer from Tb 3+ to Dy 3+ .
The curve for the samples under study excited at 388 nm shows the sample containing 4 mol% Dy formed emission peaks at 491 nm, and 576 nm. The emission spectra formed as the excitation to the energy level 4 I 15/2 , populated through a rapid non-radiative process to a lower 4 F 9/2 level as the small energy gap ≈ 8000 cm −1 (Babu and Cole 2018) formed two intense emission bands at 491 nm (blue) and 576 nm (yellow) corresponding to 4 F 9/2 → 6 H 15/2 and 4 F 9/2 → 6 H 13/2 transitions, respectively.
The transition centered at 491 nm represents the magnetic dipole (MD) transition and hasn't affected the surroundings following the selection rule (ΔJ = 0, ± 1 but 0 → 0  forbidden) (Babu and Cole 2018). While the transition at 576 nm is a forced electric dipole (ED) transition, which follows (ΔJ = 2 and ΔL = 2) and is hypersensitive to the ligand field environment around the Dy 3+ ion site. The symmetry of the Dy 3+ ions is responsible for the luminous emission intensity. Figure 10 observed the absence of symmetry as the difference in the intensity of yellow and blue. With the help of the ratio Y/B value, it could determine the distortion around Dy as it observed the lower symmetric sites around the Dy 3+ ions to the neighboring atoms, the more covalency around Dy with oxygen ligands, the more intense yellow emission than the blue emission in the photoluminescence spectrum (Ravita and Rao 2021;Rani et al. 2019;Vijayakumar et al. 2018).
The same position peak was observed in Dy after excitation with the same wavelength in a sample containing 4 mol% Tb. In addition, the peak at 543 nm. The emission spectra of Tb represent the excitation to energy level 5 G 6 to populate metastable non-radiatively to 5 D 3 and 5 D 4 excited states. The emission from level 5 D 3 is absent and replaced by only the emission from level 5 D 4 because of cross-relaxation induced by a resonance between the adjacent Tb 3+ ions, which can be described by the following equation: 5 D 3 + 7 F 6 → 5 D 4 + 7 F 0 and 5 D 3 + 7 F 6 → 7 F 0 + 5 D 4 (Zhao et al. 2013;Quang et al. 2020;Dillip et al. 2016). The emission peak at 543 nm represents the 5 D 4 → 7 F 5 transition with a prominent green light. It is a magnetic-dipole transition that follows the selection rule of J = ± 1. The other emission bands centered at 491 nm and 576 nm are attributed to the electronic transitions 5 D 4 → 7 F J (J = 6, 4) (Zhao et al. 2013;Zur et al. 2018). The intensity ratio (blue to green) describes the irregular behavior of doping ions in the host, for all terbium doped glasses, as it explains the ratio Y/B for Dy (Ravita and Rao 2021).
The CIE 1931 color chromaticity and lifetime decay for Dy 3+ and Tb 3+ excited at 388 nm are shown in Fig. 11a, b and c, respectively.
The coordination of glass samples excited at 388B. and lifetime determined and tabulated in Table 5B.
The curve for the sample containing 4 mol% Dy or Tb or both excited at 453 nm shows the sample containing 4 mol% Dy formed emission peaks at 576 nm (Fig. 12). The emission spectra formed as the excitation to the energy level 4 I 15/2 , populated through a rapid non-radiative process to a lower 4 F 9/2 level as the small energy gap ≈ 8000 cm −1 (Babu and Cole 2018) formed intense emission bands at 576 nm (yellow) corresponding to 4 F 9/2 → 6 H 15/2 transitions.
The transition centered at 491 nm represents the magnetic dipole (MD) transition and hasn't affected the surroundings following the selection rule (ΔJ = 0, ± 1 but 0 → 0 forbidden) (Babu and Cole 2018). While the transition at 576 nm is a forced electric dipole (ED) transition, which follows (ΔJ = 2 and ΔL = 2) and is hypersensitive to the ligand field environment around the Dy 3+ ion site. The symmetry of the Dy 3+ ions is responsible for the luminous emission intensity. Figure 12 observed the absence of symmetry as the difference in the intensity of yellow and blue. With the help of the ratio Y/B value, it could determine the distortion around Dy as it observed the lower symmetric sites around the Dy 3+ ions to the neighboring atoms, the more covalency around Dy with oxygen ligands, the more  The CIE 1931 color chromaticity and lifetime decay for Dy 3+ and Tb 3+ excited at 388 nm are shown in Fig. 13a, b and c, respectively. The coordination of glass samples excited at 453 nm. and lifetime determined and tabulated in Table 5C.

Conclusion
Glass samples of bismuth gallium germanium borosilicate containing Tb 4 O 7 and Dy 2 O 3 were prepared using the melt quenching process. At 1270 nm, the 6 H 15/2 , 6 F 11/2 + 6 H 9/2 transitions have the highest strong absorption in the glasses' UV-Vis NIR. When the samples are excited at 351, 388 and 453 nm the emission spectra include Dy 3+ and Tb 3+ emission. This clearly indicates the energy transfer from the Dy 3+ to the Tb 3+ ions. And the lifetime decay confirmed this transfer. The sample containing equal ratio between two rare earth ions is the best energy transfer condition. After this concentration (Tb 3+ greater than 2 mol%) the energy transfer from Tb 3+ to Dy 3+ in sample excited at 351 nm. After doping with Dy 3+ , the prepared glasses emit yellow and blue emissions, with a white light overall. The decreased emission of Dy 3+ and the increased emission of Tb 3+ were reported as evidence of energy transfer in the glasses.