White-light emission and chromaticity characterization of Dy3+ doped fluoride glass for standard white light source

Journal of Non-Crystalline Solids 526 (2019) 119697

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White-light emission and chromaticity characterization of Dy3+ doped fluoride glass for standard white light source

T

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Yumian Ye, Shuangbao Wang , Hao An School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan, 430074, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Dy3+: ZBAN glass Absorption spectrum White light emission Chromaticity coordinates

In this paper, we reported the white light emission properties of Dy3+ doped ZrF4-BaF2-AlF3-NaF (ZBAN) glass. Physical properties of the host glass were examined. The intensity parameters were determined from Judd–Ofelt theory based on the absorption spectrum. The chromaticity coordinates calculated using emission spectrum are located in the white area of the CIE chromaticity diagram. Judd–Ofelt theory was used to analyze the relationship between the Dy3+ doping concentration and the yellow/blue ratio of the fluorescence. Furthermore, the change of chromaticity coordinates in the CIE chromaticity diagram was predicted. Anomalous change caused by fluorescence quenching at high concentrations was also discussed. In order to find the fluorescence closest to the equal energy light, the chroma difference between the fluorescence and the equal energy point was calculated. These results demonstrate that Dy3+ doped fluoride glass is a suitable material for use as a standard white light source.

1. Introduction Color sensing is widely used in industrial applications such as dyes, textiles, and paper production [1]. The colorimeter can only calculate the color difference between two samples instead of absolute color parameters, limiting its application range. The RGB color sensor can accurately acquire the RGB parameters of the tested sample [1,2].However, since the parameters are referenced by the ambient light or the white LED, the accuracy of the color sensor is greatly influenced by the light source. In fact, the white LEDs currently used need to be calibrated to standard light sources using software [2], which increases the complexity of the instrument. If a standard white light source is used as the reference for color evaluation, the accuracy of the color sensor can be improved. More importantly, a colorimeter with standard light source can determine the RGB parameters of the sample by measuring the color difference between the sample and the standard light source, performing the same function as a RGB color sensor. However, the principal white LED techniques, such as phosphor-converted luminescence and red, green and blue combination of LEDs [3–6] have difficulties emitting standard white light. This is due to disadvantages such as a lack of red light, complicated structure, immature production process, and poor stability [4,7,8]. Therefore, it is necessary to find a candidate light source with suitable white light luminescent properties close to that of standard white light.

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Rare earth ions show high potential as luminescent materials due to their special fluorescent properties in the visible waveband [9]. Dy3+ ions emit three characteristic bands at blue (483 nm, 4F9/2→6H15/2), yellow (575 nm, 4F9/2→6H13/2) and red (666 nm, 4F9/2→6H11/2) region under the excitation of ultraviolet light [10,11]. The yellow emission of Dy3+ is a hypersensitive transition and is greatly affected by the surrounding environment of Dy3+, so the Yellow/Blue(Y/B) emission intensity ratio of Dy3+ is adjustable. Using different Y/B ratios, the white light fluorescence can be tuned [3,10–12]. Furthermore, the weak red light can compensate for the low color rendering index (CRI) of white light that consists only of yellow and blue light components [13] and reaches the requirements of standard light source. The adjustability of the Y/B ratio makes the fluorescence of Dy3+ suitable for various types of standard white light sources. Due to its adjustable white light fluorescence properties, Dy3+ has aroused great interest in the field [14,15]. Compared with conventional phosphor materials (predominantly crystalline), fluorescent glass has the advantages of easy manufacturing, low cost and fine structure [16]. In addition, it can be integrated into the device as both a light source and an optical instrument, reducing the complexity and improving the stability of the device. Although fluorescent glass has lower luminous efficiency than fluorescent crystals and still exhibits certain issues in regarding illuminstion, it is acceptable for use as a standard light source. Since the Y/

Correspondence author. E-mail address: [email protected] (S. Wang).

https://doi.org/10.1016/j.jnoncrysol.2019.119697 Received 6 August 2019; Received in revised form 15 September 2019; Accepted 16 September 2019 Available online 31 October 2019 0022-3093/ © 2019 Elsevier B.V. All rights reserved.

Journal of Non-Crystalline Solids 526 (2019) 119697

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B ratio of Dy3+ is affected by the host glass [17,18], a proper selection of the host glass affects the standard white light type matching the fluorescence. Fluorozirconate glasses have been extensively studied in RE doping field due to their low melting point, low phonon energy, high transmission and high rare earth solubility [19,20]. Moreover, compared with other glasses, the white fluorescent of Dy3+ fluorozirconate glass has a satisfactory Y/B ratio close to equal energy light. However, the study of chromaticity of Dy3+ doped fluorozirconate glass has not been reported in detail. In this paper, a series of Dy3+ doped ZBAN glasses were prepared and their physical and optical properties were investigated. The absorption spectrum of the glass was measured and Judd–Ofelt theory was used to predict radiative properties of Dy3+ ions in the glass. Chromaticity coordinates of the glass at different doping concentrations are calculated from emission spectrum. The yellow/blue ratio change of Dy3+ doped glass with varying concentrations are predicted using the Judd–Ofelt theory and the track of the fluorescence chromaticity coordinates is explored. In order to find the glass with fluorescence closest to equal energy point, the chroma difference between the glasses at various Dy3+ concentrations and the equal energy point were calculated and analyzed.

Fig. 1. Structure of spectrometer. Table 2 Physical properties of ZBAN glass.

2. Experiment

Samples

Density /g.cm−3

Refractive index /@633nm

Hardness /HV

Host ZBAN0.1 ZBAN0.3 ZBAN0.5 ZBAN0.7 ZBAN1

4.41 4.53 4.55 4.59 4.67 5.02

1.4902 1.4919 1.4946 1.4948 1.4965 1.5025

239 242 229 235 234 229

2.1. Preparation of samples and normal tests A series of glasses were prepared by melt quenching technique. The starting materials ZrF4, BaF2, AlF3, NaF, DyF3, NH4HF2 (99.9%) were weighed and thoroughly mixed. The mixtures packed in an alumina crucible were sintered for pre-treatment at 350 °C for 0.5 h (with the purpose of decomposing NH4HF2 to produce NH3 and HF), then melted at 930 °C for 1 h. The melt was poured into a 200 °C brass mold immediately and cooled to room temperature. All of these processes were conducted in a glove box in N2 atmosphere. The glasses with low stress were optically polished, and finally prepared glass samples had a size of about 20 mm × 10 mm × 2 mm. The composition of the prepared glass samples is shown in Table 1. The density of the glasses was measured using the Archimedes method using water as the immersion medium. The refractive index was recorded using 2WA-J Abbe refractometer and the hardness was measured using a Shore hardometer. The crystalline structure of glasses was investigated with an X'Pert PRO type diffractometer. The crystallization process of the glasses was examined by a PerkinElmer Diamond TG/DTA thermal analyzer at a heating rate of 10 °C/min in a nitrogen atmosphere in the range of 50–600 °C. The absorption spectrum was analyzed with a Shimadzu SolidSpec-3700 ultraviolet-visible (near-infrared) spectrophotometer in the range of 300–1000 nm. Fig. 1 shows the emission spectrum (400–650 nm) measured with a laboratory-made laser fluorescence spectrometer. A 350 nm ultraviolet laser used for excitation light was focused on the sample glass. A lightconcentrated apparatus composed of a number of mirrors was mounted on the side of the optical axis, focused the fluorescence in the lateral direction by reflection. The focused fluorescence is received and measured by a spectrometer placed on the side.

3. Results and discussion 3.1. Physical properties The physical properties of Dy3+ doped ZBAN glasses are shown in Table 2. It can be seen that the samples have low density and refractive index. The large field strength of Dy3+ ions promotes the accumulation of the glass network, tightening the glass network structure. On the other hand, Dy3+ ions fill the gap of ZrF4 network, which increases the compactness of glass. Therefore, the density of the glass increases with Dy3+ doping concentration. The XRD spectrum of the Dy3+ doped ZBAN glasses is shown in Fig. 2. There is no apparent peak in the spectrum, indicating the amorphous structural nature of the glass; the broad halo of the spectrum indicates the glassy nature without long-range order atomic arrangement [21]. The DTA thermogram of the host glass is shown in Fig. 3. The melting temperature of the sample is 466 °C. The lower melting point on the one hand reduces the firing conditions, while on the other hand limiting the working environment. The glass transition temperature (Tg) of the glass is around 285 °C; in addition, the crystallization peak (Tc) of the glass is observed at 392 °C. The small crystallization peak at 425 °C is considered to be caused by the phase separation of ZBAN glass. The difference between the glass crystallization temperature and the glass transition temperature (Tc–Tg) is an important parameter for measuring the thermal stability of glass [5], which is also critical for evaluating its suitability for industrial production. For the host glass, the thermal stability parameter is 107 °C.

3.2. Absorption spectrum and Judd-Ofelt analysis

Table 1 The composition of Dy3+ doped ZBAN glass. Sample

ZrF4/mol%

BaF2/mol%

AlF3/mol%

NaF/mol%

DyF3/mol%

Host ZBAN0.1 ZBAN0.3 ZBAN0.5 ZBAN0.7 ZBAN1

50 50 50 50 50 50

24 24 24 24 24 24

8 8 8 8 8 8

18 17 17.7 17.5 17.3 17

0 0.1 0.3 0.5 0.7 1

Fig. 4 shows the absorption spectrum of the Dy3+ doped ZBAN glass in the range of 200–2500 nm. There are multiple absorption bands (320, 349, 363, 388, 428, 451, 475, 750, 796, 892, 1082, 1287 and 1675 nm) observed, corresponding to transitions to various excited states (6P3/2, 6P7/2, 4P3/2, 6K17/2, 4G11/2, 4I15/2, 4F9/2, 6F3/2, 6F5/2, 6F7/2, 6 F9/2, 6H9/2 and 6H11/2 transitions) from the ground state 6H15/2. The absorption of the ZBAN glass is denoted by the baseline. The absorption peak of Dy3+ transition from the ground state to the higher excited 2

Journal of Non-Crystalline Solids 526 (2019) 119697

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Fig. 2. XRD spectrum of Dy3+ doped ZBAN glasses.

The oscillator strength fexp of each transition is calculated from the absorption peaks using the expression:

fexp = 4.318 × 10−9

∫ ε (ν) dν

(1)

where ɛ(ν) is the molar absorptivity of the band at a wave number ν (cm−1). The theoretical oscillator strength (fcal) is also calculated by Judd–Ofelt theory:

fcal =

(n2 + 2)2 8π 2mc ·∑ Ωt (ΨJ U t ΨJ ′)2 · t = 2,4,6 3h (2J + 1) λ 9n

(2)

where m is the mass of the electron, c is the velocity of light, h is the Planck's constant, J is the total angular momentum of the ground state, λ is the wavelength from the ground state to the excited state, Ωt is the Judd–Ofelt intensity parameter and ‖Ut‖2 (t = 2,4,6) are the squared reduced matrix elements of the unit tensor operator which were obtained from the data given in [24,25]. The theoretical and experimental oscillator strengths of ZBAN1 glass are presented in Table 3. Small root mean square (R.M.S.) deviation of 0.13×10−6 indicates good agreement between theoretical and experimental oscillator strengths. After subtracting the contribution of the magnetic dipole, intensity parameter Ωt is calculated from the measured oscillator strength values using the least square fit method. The J–O parameters of ZBAN glass and some reported glasses are listed in Table 4. For ZBAN glass, the J–O

Fig. 3. DSC thermogram of ZBAN glass.

states is hidden on the cutoff curve, since the transmission cutoff wavelength of ZBAN is around 300 nm The transition intensity analysis is performed according to the Judd–Ofelt theory with the data of the absorption spectrum [22,23].

Fig. 4. Absorption spectrum of the Dy3+ doped ZBAN glasses at (a) 250–500 nm (b) 750–2000 nm. 3

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Table 3 Calculated and experimental oscillator strength of ZBAN1 glass. Wavelength/nm

Energy level

328 349+363 388 428 451 475 749 796 892 1082 1287 σRMS = 0.13×10−6

6

P3/2 P7/2 + 4P3/2 K17/2 4 G11/2 4 I15/2 4 F9/2 6 F3/2 6 F5/2 6 F7/2 + 6H5/2 6 F9/2 + 6H7/2 6 F11/2 + 6H9/2 6 6

Fexp/10−6

Fcal/10−6

0.49 1.68 0.54 0.05 0.15 0.05 0.24 0.73 0.91 1.07 1.91

0.53 1.62 0.86 0.02 0.20 0.08 0.10 0.50 1.00 1.06 1.91

Fig. 5. Emission spectrum of Dy3+ doped ZBAN glasses (λex = 350 nm). −20,

parameters are found to be Ω2 = 3.29–3.71×10 Ω4 = 0.74–1.36×10−20, Ω6 = 2.38–3.32×10−20 cm−2. The J–O parameter primarily provides information about the nature of the bond between the Dy3+ ions and the surrounding ligands, and the symmetry of the environment surrounding the Dy3+ ions [5,26,27]. Compared with other glasses, fluorine-containing glass such as ZBAN has lower Ω2 value, indicating the higher ionicity of the Dy-F bond and higher symmetry around the Dy3+ ion [27]. At low Dy3+ concentration, as concentration increases, the Ω2/Ω6 value decreases. This is because Dy3+ is distributed in the glass network, causing the glass network accumulated and the polarization intensity decrease. At high Dy3+ concentration, as Dy3+ concentration increases, the distance between Dy3+ ions gradually decreases, and Dy3+ ions compete for non-bridged fluorine in the glass structure, which increases the asymmetry around Dy3+ and increases the polarization, resulting in the increase of Ω2/Ω6 . The ratio Ω2/Ω6 is an important parameter of Dy3+ and has a strong correlation with blue and yellow fluorescence intensity [25,28]. 3.3. Emission spectrum and chromaticity analysis The emission spectrum of Dy3+ doped ZBAN glasses excited by ultraviolet light at 350 nm is shown in Fig. 5. There are two strong emission peaks (480 nm, blue and 575 nm, yellow) and one weak emission peak (660 nm, red) observed in the spectrum. The three peaks respectively belong to 4F9/2→6H15/2, 4F9/2→6H13/2, 4F9/2→6H11/2 transitions. It can be observed form Fig. 5 that there is no change in the fluorescence center for any of ZBAN glasses. Moreover, at low Dy3+ concentrations (<0.7 mol%), the emission intensity is found to increase as Dy3+ concentration increases; however, at high Dy3+ concentrations (>0.7mol%), the emission intensity decreases as Dy3+ concentration increases due to concentration quenching through non-radiative energy transfer processes among Dy3+ ions. The energy transfer process of Dy3+ ions in the energy level diagram is shown in Fig. 6. Some radiative properties of the Dy3+ doped ZBAN glasses, such as peak wavelength, radiative transition probabilities and branching ratio were calculated by using the data in Figs. 5 and 6. The results are listed in Table 5. It can be seen that ZBAN glass has a Y/B ratio of 1.02–1.27,

Fig. 6. Partial energy level diagram of the Dy3+ ions.

which is lower than most other glasses (Y/B ratio of 2 or more). This is due to the hypersensitivity of the 4F9/2→6H13/2 transition (△J = 2, △L = 2), whose intensity is strongly influenced by the surroundings of the Dy3+ ion site [33]. The weak polarization of F leads to low yellow emission intensity of Dy3+ in ZBAN glass. Low Y/B ratio indicates that the white light of Dy3+ doped ZBAN glass is superior to other Dy3+ doped glass. Moreover, the white light can be easily adjusted for use as

Table 4 J–O parameters of ZBAN glass and other reported glasses. Glass

Ω2

Ω4

Ω6

Ω2/Ω6

trend

Reference

ZBAN0.1 ZBAN0.3 ZBAN0.5 ZBAN0.7 ZBAN1 Fluorophosphate Oxyfluoroborate Barium fluoroborate PKAZFDy

3.62 3.48 3.29 3.71 3.35 10.41 2.68 2.90 14.11

0.96 0.74 1.12 1.36 1.07 2.29 2.56 1.09 3.07

2.76 2.38 2.60 3.32 2.77 2.07 0.89 0.98 1.95

1.31 1.45 1.27 1.08 1.44 5.03 3.01 2.96 7.24

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

Present Present Present Present Present [29] [30] [31] [32]

4

work work work work work

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Table 5 The radiative transition probabilities (A/s−1) and calculated and experimental fluorescence branching ratio (βcal/% and βexp/%, respectively) of Dy3+: ZBAN samples. Energy level Radiative properties

4 F9/2→6H15/2 (480 nm) A/s−1 βcal/%

βexp/%

F9/2→6H13/2 (575 nm) A/s−1 βcal/%

βexp/%

F9/2→6H11/2 (660 nm) A/s−1 βcal/%

ZBAN0.1 ZBAN0.3 ZBAN0.4 ZBAN0.7 ZBAN1 Fluorophosphate [29] Oxyfluoroborate [30] Bariumfluoroborate [31] PKAZFDy [32]

163.7 140.9 157.6 200.7 167.4 106.0 283.2 194.0 162.0

42.5 45.4 47.3 47.5 42.9 8.0 47.3b – 55.0

395.1 360.6 372.9 447.2 387.7 910.0 30.1 734.0 1095.0

53.9 52.2 49.2 48.6 50.8 68.0 52.6b – 43.0

33.0 30.9 30.8 35.8 31.7 165.0 10.6 71.0 131.0

a b

27.7 26.5 28.1 29.4 28.5 8.0 66.0a 18.7 10.0

4

66.8 67.7 66.4 65.4 66.1 64.0 7.0a 70.8 69.0

4

5.6 5.8 5.5 5.2 5.4 15.0 3.0 6.8 8.0

βexp/% 3.6 2.4 3.4 3.9 6.3 12.0 – 2.0

It may be a misprint in the text. The data of 480 nm and 575 nm is suspected to have been exchanged according to the text. Calculated according to the peak intensity ratio of 1:0.9 given in the text.

standard white light. The fluorescence spectrum was converted to chromaticity coordinates x, y, z according to the CIE 1931 standard. The tristimulus values can be calculated from the following equation:

⎧ X = k ∑λ φ (λ ) x¯ (λ )Δλ ⎪ Y = k ∑λ φ (λ ) y¯ (λ )Δλ ⎨ ⎪ Z = k ∑λ φ (λ ) z¯ (λ )Δλ ⎩

(3)

where φ(λ) is the spectral distribution of the color stimulus function, x¯ (λ ), y¯ (λ ), z¯ (λ ) are the color-matching function of CIE standard and k is the normalized constant. As for CIE 1931 standard colorimetric system, k is chosen as the maximum spectral luminous efficacy (which is equal to 683 lm/W) and φ(λ) is the spectral concentration of the radiometric quantity corresponding to the photometric quantity required [34]. Chromaticity coordinates are derived from tristimulus values X, Y, Z:

⎧ x = X /(X + Y + Z ) y = Y /(X + Y + Z ) ⎨ ⎩ z = Z /(X + Y + Z )

(4)

The x, y, and z values represent the saturation of red, green, and blue, respectively. The chromaticity coordinates of the fluorescence of Dy3+ doped ZBAN glasses in the CIE 1931 chromaticity diagram is shown in Fig. 7. It can be seen that the chromaticity coordinates of each sample ((0.3451, 0.3707), (0.3320, 0.3540), (0.3246, 0.3348), (0.3199, 0.3209), (0.3298, 0.3437) corresponding to ZBAN0.1 to ZBAN1) are located in the white light area. The chromaticity coordinates of all ZBAN glass are very close to the equal energy point (0.33, 0.33), which is a consequence of the lower Y/B ratio. In addition, at low Dy3+ concentration, as the concentration increases, the chromaticity coordinate shift toward the blue light, which is consistent with the conclusions obtained using the JO theory. At high Dy3+ concentration, as the concentration increases, the chromaticity coordinate shift toward the yellow light. This trend is highly correlated with the Ω2/Ω6 ratio, which is agreement with the validity of the Judd–Ofelt analysis. CIE1931 xyz chromaticity space is non-uniform, thus the chroma difference cannot be obtained intuitively. In order to calculate the chromatic difference between the fluorescence of each sample and the equal energy point, the CIELAB chromaticity space, which is approximately uniform, is required. The LAB coordinates can be transformed from xyz coordinates by the following equation [35]:

( ) ( ( ) ( )) ( ( ) ( ))

⎧ L = 116f Y − 16 Y0 ⎪ ⎪ X Y a = 500 f X − f Y 0 0 ⎨ ⎪ Y Z ⎪ b = 200 f Y0 − f Z0 ⎩

Fig. 7. CIExyz coordinates of the fluorescence of Dy3+ doped ZBAN glasses. The inset is partial enlargement of coordinates of the white area.

where

f(k) =

3 k k > 0.008856 ⎧ 16 ⎨ 7.787k + 116 k ≤ 0.008856 ⎩

(6)

The value of X0, Y0, and Z0 are equal to 100 since the illuminant uses E (equal energy). a value represents the saturation of the color on the green-red axis, the negative part represents green, while positive part represents red; b value represents the saturation on the blue-yellow axis, the negative part represents blue, while the positive part represents yellow. Brightness is denoted by L. Fig. 8 shows the chromaticity coordinates of ZBAN glass in the CIELAB color space. It can be seen that the color difference between the respective chromaticity coordinate points and the performance in the xyz chromaticity space are quite different, indicating that the LAB chromaticity space is used as a relatively uniform chromaticity space. Table 6 gives the LAB chromaticity coordinates of each sample. The CIELAB chroma difference ΔC is used to calculate the chroma difference between two samples:

ΔC = (5)

a12 + b12 −

a22 + b22

(7)

where a1, b1, a2, b2 are a and b coordinate of two samples. The chroma 5

Journal of Non-Crystalline Solids 526 (2019) 119697

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Fig. 8. The CIELAB coordinates of the fluorescence of Dy3+ doped ZBAN glasses.

be improved if the intensity of fluorescence at 660 nm is increased. The results confirm that Dy3+ doped ZBAN glass with a suitable Dy3+ concentration shows promise for use as a standard white light source.

Table 6 The CIELAB chromaticity coordinate and chroma difference of Dy3+doped ZBAN glass.

ZBAN0.1 ZBAN0.3 ZBAN0.5 ZBAN0.7 ZBAN1

A

b

△C

9.290 −8.163 −4.031 −0.415 −5.421

13.374 6.046 −0.886 −6.047 2.702

16.284 10.158 4.127 6.061 6.057

Declaration of Competing Interest None. Acknowledgments The financial support provided by the Fundamental Research Funds for the Central Universities (HUST-2015049) and CETC NO.46 Research Institute in China is appreciated. The author is also grateful for the beneficial discussion with Shouchao Luo.

difference in saturation between each sample and equal energy point (100, 0, 0) is listed in Table 6. It can be seen that for Dy3+ concentration equal to 0.5%, the chroma of fluorescence is closest to the equal energy light. In addition, a is negative for all samples. Therefore, a certain amount of red light is required if a white light source closer to equal energy and improved CRI of the fluorescence is desired.

References [1] M. Moghavvemi, S.S. Jamuar, E.H. Gan, Y.C. Yap, Design of low cost flexible RGB color sensor, 2012 Int. Conf. Informatics, Electron. Vision, ICIEV 2012, 2012, pp. 1158–1162, , https://doi.org/10.1109/ICIEV.2012.6317416. [2] J.D. Filoteo-Razo, J.M. Estudillo-Ayala, J.C. Hernández-Garcia, M. Trejo-Durán, A. Muñoz-Lopez, D. Jauregui-Vázquez, R. Rojas-Laguna, RGB color sensor implemented with LEDs, Fourteenth Int. Conf. Solid State Light. LED-Based Illum. Syst 9571 (2015) 95710V, https://doi.org/10.1117/12.2188243. [3] P. Haritha, I.R. Martín, K. Linganna, V. Monteseguro, P. Babu, S.F. León-Luis, C.K. Jayasankar, U.R. Rodríguez-Mendoza, V. Lavín, V. Venkatramu, Optimizing white light luminescence in Dy3+-doped Lu3Ga5O12 nano-garnets, J. Appl. Phys. (2014) 116, https://doi.org/10.1063/1.4900989. [4] N. Luewarasirikul, H.J. Kim, P. Meejitpaisan, J. Kaewkhao, White light emission of dysprosium doped lanthanum calcium phosphate oxide and oxyfluoride glasses, Opt. Mater. (Amst) 66 (2017) 559–566, https://doi.org/10.1016/j.optmat.2017.02. 049. [5] P. Haritha, I.R. Martín, C.S. Dwaraka Viswanath, N. Vijaya, K. Venkata Krishnaiah, C.K. Jayasankar, D. Haranath, V. Lavín, V. Venkatramu, Structure, morphology and optical characterization of Dy3+-doped BaYF5 nanocrystals for warm white light emitting devices, Opt. Mater. (Amst) 70 (2017) 16–24, https://doi.org/10.1016/j. optmat.2017.05.002. [6] T. Matsuzawa, Y. Aoki, N. Takeuchi, Y. Murayama, A new long phosphorescent phosphor with high brightness, SrAl2O4:Eu2+,Dy3+, J. Electrochem. Soc. (1996). [7] S. Neeraj, N. Kijima, A.K. Cheetham, Novel red phosphors for solid-state lighting: the system NaM(WO 4)2-x(MoO4)x:Eu3+ (M=Gd, Y, Bi), Chem. Phys. Lett. (2004), https://doi.org/10.1016/j.cplett.2003.12.130. [8] M. Vijayakumar, K. Mahesvaran, D.K. Patel, S. Arunkumar, K. Marimuthu, Structural and optical properties of Dy3+ doped aluminofluoroborophosphate glasses for white light applications, Opt. Mater. (Amst) 37 (2014) 695–705, https://

4. Conclusion In summary, a series of Dy3+ doped ZBAN glasses were successfully synthesized using the melt-quenching method. The amorphous structural nature of glasses was confirmed by XRD and DSC testing. The intensity parameters of the Dy3+ doped ZBAN glass were calculated using J–O theory, with Ω2=3.29–3.71×10−20, −20, −20 Ω4=0.74–1.36×10 Ω6=2.38–3.32×10 cm−2. Three emissions of Dy3+ ions in the visible light region under ultraviolet excitation were observed using emission spectroscopy, and a white light luminescence was determined under the CIE1931 standard. Relations between Y/B ratio of the fluorescence and Ω2/Ω6 ratio are found. The chromaticity coordinates of white light emission are close to equal energy point due to low Y/B ratio of glass. In accordance with by Judd–Ofelt theory, for low Dy3+ doping concentration, as the concentration increases, the chromaticity coordinates shift toward the blue light. At high Dy3+ concentration, as the concentration increases, the chromaticity coordinates shift toward the yellow light. The chroma difference between each samples and equal energy point was calculated, showing that ZBAN0.5 glass is closest to equal energy light. Moreover, the performance of Dy3+: ZBAN glass as an equal energy white light source can 6

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