Wide visible-range fluorescence of Eu³⁺ located in the macroscopic bi-layer ceramic/glass composite

Abstract

The Eu³⁺ doped fluoride bi-layer ceramic/glass composite (GC_ZBL-Eu) was prepared by a one-step method and the effective wide visible-range fluorescence was recorded. The de-population rates of the ⁵D₀, ⁵D₁, ⁵D₂, and ⁵D₃ multi-levels in the glass layer (G_ZBL-Eu) were estimated to be 214, 746, 1163, and 680 s⁻¹, respectively, and that in the ceramic layer (C_ZBL-Eu) were 211, 730, 1075, and 654 s⁻¹, which implies multi-channel radiative transitions due to the non-radiative relaxation limitation of low OH content and low phonon energy. Simultaneously, the quantum efficiencies of the ⁵D₀ levels in G_ZBL-Eu and C_ZBL-Eu were as high as 98.5% and 94.8%, respectively, thus demonstrating the effectiveness of the radiative transition emissions from Eu³⁺. Besides, GC_ZBL-Eu with the glass forming layer increases the emission intensity by 24% compared to C_ZBL-Eu, which is attributed to the multiple-cycle reflection in the composite structure of the glass–ceramic transition region, and the color coordinates of C_ZBL-Eu (0.483, 0.385) and GC_ZBL-Eu (0.469, 0.389) show that they can release yellowish-white light. The hetero-structured GC_ZBL-Eu provides a new approach for laser lighting, fluorescent display, and up-conversion applications.

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Introduction

White light-emitting diodes (WLEDs) have recently attracted considerable attention as next generation light sources and as alternatives for conventional incandescent and fluorescent lamps.^1–4 One of the most popular ways to obtain WLEDs is to combine near UV-LEDs with a glass or crystal phosphor emitting in the blue, green, and red spectral regions.^5–7 For this purpose, multiple luminescence centers in the phosphors are widely adopted to achieve high brightness and high color rendering index of white light emission.^8–12 However, rare earth (RE) co-doping leads to complex cross relaxation processes, which cannot release photons efficiently. In view of the high fluorescence quantum efficiency in visible white light illumination, the exploration focusing on the compound material with single RE³⁺ ions doping and multi-channel emission becomes urgent.

The emission spectrum of Eu³⁺ located in the hosts, characterized by the existence of high energy phonons such as the oxides, is strong only for the case of the bands produced by the radiative relaxations from ⁵D₀.^13–19 The emissions from ⁵D₀ are favored mostly because the energy difference between the ⁵D₃, ⁵D₂, ⁵D₁, and ⁵D₀ levels of the Eu³⁺ cation can be covered by non-radiative transitions from the higher levels to ⁵D₀. The resulting emission profile, with intense bands mostly confined to a narrow range, is not suitable for the preparation of white-light phosphors if the intention is to use only a single RE³⁺ dopant. However, in hosts having weak phonons, such as fluorides, the non-radiative transitions from the higher electronic levels to ⁵D₀ become difficult and the electronic transitions originating in those levels acquire higher intensity than in the oxide hosts. The result is a fluorescence spectrum exhibiting bands of significant intensity (albeit emission at wavelength higher than 500 nm remains strongest) at various positions of the visible range; such a spectral profile makes Eu³⁺, in fluorides, a possible candidate for the fabrication of white light phosphors based on a single RE³⁺. In this work, for the first time, it has been examined whether the emission of Eu³⁺, introduced in a Zr–Ba–La–F host (macroscopic ceramic/glass bi-layer), can be used to obtain such phosphors. In addition, the transitions ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂ have different dependences on the crystal field environment belonging to the magnetic dipole and electric dipole transitions.^20–25 Also, Eu³⁺ ions have similar ionic radius and crystalline properties as other RE³⁺ ions, which can be used as a probe of the local structure to explore the influence of the micro-environment of RE³⁺ ions on the luminescence of materials.

In this work, Eu³⁺-doped fluoride ceramics with a glass layer (GC_ZBL-Eu) have been prepared and intense yellowish-white fluorescence, attributed to multi-peak emission, was observed. With the formation of the thin glass layer on the ceramic surface, the GC_ZBL-Eu increases its emission intensity by 24% compared to C_ZBL-Eu, which is ascribed to multiple-cycle reflection in the complex structure of the glass–ceramic transition region. The micro-environment symmetry of G_ZBL-Eu and C_ZBL-Eu is judged according to the value of the J-O parameter Ω₂ with different emission intensity ratios between ⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₁. The differential fluorescence characteristics, such as the radiative lifetimes of the multi-levels and the quantum efficiency of the ⁵D₀ levels from Eu³⁺ in G_ZBL-Eu and C_ZBL-Eu, were analyzed. In addition, the color coordinates of C_ZBL-Eu and GC_ZBL-Eu are located in the yellowish-white lighting region, which indicates that the hetero-structured materials can be well applied to optical devices.

Preparation and measurements of GC_ZBL-Eu

The fluoride bi-layer ceramic/glass composites were prepared according to the molar host composition of 60ZrF₄–30BaF₂–10LaF₃ (ZBL) via the melt-quench method and 0.2 wt% EuF₃ was introduced into the ZBL matrix as the dopant. The well-mixed high-purity fluoride raw materials were melted at 900 °C for 5 min in a platinum crucible using an electric furnace at the rate of 5 °C min⁻¹ from room temperature to 900 °C. In addition, NH₄F on the raw materials provides a reducing atmosphere and thus, the molten glass was quenched onto an aluminum plate. Herein, the bottom molten glass, when in contact with the aluminum plate, takes away a lot of heat rapidly and forms an ultrathin glass layer owing to the process of efficient heat conduction. Correspondingly, the upper liquid starts to crystallize at the interface with air because air cannot take away a lot of heat. Subsequently, the molded samples were annealed at 260 °C for 2 h and then cooled down slowly to room temperature. The schematic diagram of the systematic preparation procedure is exhibited in Fig. 1. Relevant tests were performed on the glass surface of the bi-layer ceramic/glass composite, the ceramic layer, and the glass layer, which were named as GC_ZBL-Eu, C_ZBL-Eu, and G_ZBL-Eu, respectively (C_ZBL-Eu and G_ZBL-Eu samples were ground to the glass or ceramic layer by the GC_ZBL-Eu composite).

Fig. 1 The schematic diagram of the systematic preparation procedure.

The differential thermal analysis (DTA) scan was carried out on a WCR-2D differential thermal analyzer at the rate of 10 °C min⁻¹ from room temperature to 900 °C. X-ray diffraction (XRD) measurements for the powders of G_ZBL-Eu and C_ZBL-Eu were carried out on a Shimadzu XRD-7000 diffractometer (40 kV, 30 mA). The morphological behavior of the section of GC_ZBL-Eu was observed by a field-emission scanning electron microscope (SEM instrument, JEOL JSM-7800F). The glass layer thickness of GC_ZBL-Eu was measured by a fluorescence microscope (Imaging system CK-500). The refractive index of G_ZBL-Eu was measured by using the Metricon 2010 prism coupler. The visible fluorescence spectra and the fluorescence decay curves were obtained using a Hitachi F-7000 fluorescence spectrophotometer equipped with an R928 photomultiplier tube (PMT) as the detector and a commercial Xe-lamp as the excitation source.

Results and discussion

The transition temperature T_g = 305 °C, the onset crystallization temperature T_x = 364 °C, and the peak temperature T_c = 379 °C of the DTA curve are shown in Fig. 1(a). The parameters including the temperature difference value ΔT = T_x − T_g = 59 °C, the thermal stability parameter H = ΔT/T_g = 0.19, and the Saad–Poulain criterion S = ΔT(T_c − T_x)/T_g = 2.9 °C demonstrate that this sample is more effortless to crystallize than other oxide glasses or oxyfluoride glasses, and is suitable for the preparation of the fluoride ceramic-based composite glass. The inset of Fig. 2(a) shows the cross-sectional morphology of GC_ZBL-Eu under an optical microscope. The shiny part of this picture is the glass layer, whose thickness was identified to be ∼0.81 mm, the dark part is ceramic-based, and the intersection part of the two layers is called the GC transition region.

Fig. 2 (a) The DTA curve of G_ZBL-Eu. Inset: the cross-section morphology of the GC_ZBL-Eu sample under an optical microscope. (b) The XRD patterns of G_ZBL-Eu and C_ZBL-Eu.

The typical XRD pattern of G_ZBL-Eu in Fig. 2(b) exhibits a broad diffuse scattering at lower angles without the narrow diffraction peaks of the crystal phase and the amorphous state of the prepared glass layer is well-identified. At the same time, by comparing the sharp diffraction peaks in the XRD spectrum of C_ZBL-Eu, the crystal phase of C_ZBL-Eu is identified to be pure BaZrF₆. The slight differences in the cell parameters between C_ZBL-Eu (a = 7.744 Å, b = 11.691 Å, c = 5.404 Å, α = β = γ = 90°) and the standard BaZrF₆ phase (a = 7.681 Å, b = 11.357 Å, c = 5.511 Å, α = β = γ = 90°) indicate the possibility of lattice deformation caused by the introduction of Eu³⁺. The cell volumes of orthorhombic C_ZBL-Eu and BaZrF₆ judged by the cell parameters were calculated to be 489.25 and 480.74 ų by the equation V = a × b × c, indicating that the doping of Eu³⁺ has little effect on the cell volume before and after substitution. In order to maintain electrical neutrality, Eu³⁺, whose radius of 0.95 Å is between that of Zr⁴⁺ (0.80 Å) and Ba²⁺ (1.35 Å), is more likely to replace Zr⁴⁺ in the lattice and forms F⁻ vacancies.

Molten glass appears to be disordered owing to increased interatomic amplitudes and irregular diffusion of the molecules, and the disordered state is fixed to form a glass on sudden cooling. When molten glass cools down slowly, each atom returns to the lowest energy state and forms crystals. In this matrix, BaZrF₆ is a crystal with lower lattice energy and is easier to precipitate; Fig. 3 shows the schematic diagram of the crystal structure of BaZrF₆.

Fig. 3 The schematic illustration of the crystal structure of BaZrF₆.

Spectrum 1 and spectrum 2 with the same element type were chosen from the SEM images of the sample cross-section in Fig. 4(a) for elemental analysis, as shown in Fig. 4(c) and (d). In comparison, the element contents in G_ZBL-Eu and C_ZBL-Eu have little difference within the allowable error range, indicating that there is no obvious element migration during the crystallization process. In addition, the absence of oxygen and other elements in the EDS spectra indicates that there is no foreign matter in the prepared G_ZBL-Eu and C_ZBL-Eu and the surface of the samples is not oxidized. The SEM image of the ceramic region displays that the BaZrF₆ crystalline grain covered by glass in C_ZBL-Eu grows in an ordered direction and arrangement, as shown in Fig. 4(b). The elemental mapping in the regions contained within the red frames of Fig. 4(e–i) exhibits that Zr and Ba are densely distributed whereas it is the opposite for La, which indicates that the red frame region represents the cross-section of the BaZrF₆ grains coated with glass.

Fig. 4 (a and b) SEM images of the cross-section of GC_ZBL-Eu. (c and d) EDS spectra and the elemental content of G_ZBL-Eu and C_ZBL-Eu. (e–i) Elemental mapping of C_ZBL-Eu.

Based on the FT-IR spectrum of the glass layer in Fig. 5(a), the molar absorption coefficient α_OH can be used to evaluate the residual OH content in the glass samples and was found to be 0.91 cm⁻¹ in the 75TeO₂–10ZnO–10Na₂O–5GeO₂ glasses,²⁶ while the value in this work is as low as 0.57 cm⁻¹. The low OH content of the samples is helpful in achieving the anticipated photon emission by reducing the fluorescence loss. In view of the high sensitivity of the multi-channel transition to the phonon energy, the maximum phonon energy (E) of the glass samples was estimated through the empirical formula E = 92.9 + 0.4257R, where R corresponds to the wavenumber at 10% transmittance of the infrared transmission sideband and the maximum phonon energy of G_ZBL-Eu is identified to be ∼483 cm⁻¹.

Fig. 5 (a) The FT-IR spectrum of the glass layer. (b and c) The excitation spectra of G_ZBL-Eu and C_ZBL-Eu monitored at 615 nm emission.

The energy difference between the phonon sideband spectrum and the zero-phonon line is the coupled phonon energy for the electron transition of the RE³⁺ ions. The phonon energies of G_ZBL-Eu and C_ZBL-Eu were estimated to ∼452 and ∼424 cm⁻¹ according to the fitting of the two curves shown in Fig. 5(b) and (c), respectively. The local vibrational modes with the corresponding RE³⁺ ions are generally considered to be slightly less than the maximum phonon energy of the material.²⁷ G_ZBL-Eu and C_ZBL-Eu with low phonon energies, which have been confirmed by the two methods mutually, can reduce the possibility of non-radiative transitions and facilitate fluorescence emission.

The emission spectra of G_ZBL-Eu and C_ZBL-Eu were recorded under 395 nm excitation as shown in Fig. 6(a). The emission intensity of C_ZBL-Eu is much higher than that of G_ZBL-Eu owing to the ordered arrangement of BaZrF₆ particles and another factor is that Eu³⁺ maybe be retained in C_ZBL-Eu more than in G_ZBL-Eu. Eleven emission peaks of G_ZBL-Eu and C_ZBL-Eu are located at 429, 445, 464, 488, 509, 525, 535, 554, 590, 615, and 698 nm, which originate from the ⁵D₃ → ⁷F_2,3,4, ⁵D₂ → ⁷F_2,3, ⁵D₁ → ⁷F_0,1,2, and ⁵D₀ → ⁷F_1,2,4 transitions of Eu³⁺, respectively. The energy level diagram in Fig. 6(b) shows the radiation and relaxation processes of multiple channels and the values of energy gaps among the ⁵D₀, ⁵D₁, ⁵D₂, and ⁵D₃ levels. In the G_ZBL-Eu and C_ZBL-Eu systems, the multiphonon relaxation processes from the ⁵D₃ and ⁵D₂ levels are accomplished by six-phonon bridging and that from ⁵D₁ level is completed by four-phonon bridging. All of them belong to high order processes (more than 3 phonons) and thus, the probability of energy loss by non-radiative transitions is lowered. Therefore, the blue and green light emission peaks from ⁵D₃, ⁵D₂, and ⁵D₁ levels cooperating with the red-light emission peaks from the ⁵D₀ level can be well applied to luminescent color tuning, thus expanding the application direction of single RE³⁺ ions.

Fig. 6 (a) Emission spectra of G_ZBL-Eu and C_ZBL-Eu under 395 nm excitation. (b) The schematic diagram of multimodal emission energy levels of Eu³⁺ under 395 nm excitation.

By referring to the halogen lamp, the relative spectral power distribution is converted from the emission spectrum and is shown in Fig. 7(a). Depending on the relative spectral power distribution, photon distribution N(ν) can be deduced as where ν, λ, h, c, and P(λ) represent the wavenumber, wavelength, Planck constant, vacuum light velocity, and spectral power distribution, respectively.^28–30 Under 395 nm excitation, the emission photon number distribution curves of G_ZBL-Eu and C_ZBL-Eu are derived as shown in Fig. 7(b).

Fig. 7 The relative spectral power distributions (a) and the relative emission photon distributions (b) of G_ZBL-Eu and C_ZBL-Eu under 395 nm excitation.

Ω₂, which is influenced by the local structure symmetry of Eu³⁺ in G_ZBL-Eu and C_ZBL-Eu, is usually used to describe the emission distribution of the ⁵D₀ level and can be calculated based on the emission photon number distributions instead of the general absorption spectra. In the calculation, the ordinary refractive index n value of G_ZBL-Eu is measured to be 1.555 and the value of C_ZBL-Eu is estimated to be 1.590.³¹ The Ω₂ value of Eu³⁺ in C_ZBL-Eu is 1.75 × 10⁻²⁰ cm², which is lower than 2.49 × 10⁻²⁰ cm² in G_ZBL-Eu. A smaller Ω₂ value indicates a higher symmetry of the local structure of Eu³⁺ ions and weaker covalency around the metal–donor interaction. Also, compared with C_ZBL-Eu, G_ZBL-Eu, and other substrates,^32–37 a smaller Ω₂ value (higher symmetry) corresponds to lower red-light emission ratio of the ⁵D₀ → ⁷F₂ transition. In other words, at a higher symmetry site, the intensity ratio of the ⁵D₀ → ⁷F₂ emission is reduced and thus, it becomes more similar to that of the ⁵D₀ → ⁷F₁ transition. This means that C_ZBL-Eu can better limit the emission of red light and using the shorter-wavelength orange light generated in the ⁵D₀ → ⁷F₁ transition, it is easier to synthesize white light with blue and green light from the ⁵D₃, ⁵D₂, and ⁵D₁ level transitions. Such a change towards more equal intensity emission at various wavelengths is more suitable for realizing an overall white emission of the phosphor and the excitation source system.

To better express the capacity of the potential radiative transition among the levels, the experimental average lifetimes and the de-population rate of Eu³⁺ in G_ZBL-Eu and C_ZBL-Eu can be derived from the fluorescence decay curves, as shown in Fig. 8, and the related results are calculated and listed in Table 1. In the fluoride system with low phonon energy, photons can be effectively released and the low de-population rate represents that the radiative transition occupies a major proportion. In order to evaluate the effective radiation of C_ZBL-Eu and G_ZBL-Eu, the quantum efficiencies of the ⁵D₀ level was calculated. The data of theoretical radiative fluorescent lifetimes (including the relevant calculation data), experimental average lifetimes, and quantum efficiencies are listed in Table 2. The quantum efficiencies of Eu³⁺ from the ⁵D₀ level in G_ZBL-Eu and C_ZBL-Eu were calculated to be 98.5% and 94.8%, which demonstrate the effectiveness of radiative transition emission from Eu³⁺ in G_ZBL-Eu and C_ZBL-Eu. The difference in the quantum efficiencies of G_ZBL-Eu and C_ZBL-Eu is consistent with the conclusion that the luminous efficiency of Eu³⁺ decreases in a more symmetrical crystal field environment. The high quantum efficiencies of G_ZBL-Eu and C_ZBL-Eu demonstrate the effectiveness of radiative transition emissions from Eu³⁺ and show the application prospects in energy conversion and energy saving.

Fig. 8 Fluorescence decay curves from the ⁵D₀, ⁵D₁, ⁵D₂, and ⁵D₃ level of Eu³⁺ in G_ZBL-Eu (a–d) and C_ZBL-Eu (e–h) monitored at 615, 535, 509, and 464 nm, respectively.

Table 1 The experimental lifetimes and de-population rates of ⁵D₀, ⁵D₁, ⁵D₂, and ⁵D₃ in G_ZBL-Eu and C_ZBL-Eu

Energy level	GZBL-Eu		CZBL-Eu
	Experimental lifetime (ms)	De-population rate (s^−1)	Experimental lifetime (ms)	De-population rate (s^−1)
^5D0	4.67	214	4.74	211
^5D1	1.34	746	1.37	730
^5D2	0.86	1163	0.93	1075
^5D3	1.47	680	1.53	654

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Table 2 Photon number ratios, spontaneous transition probabilities A_ij, fluorescence branching ratios β_ij, calculated radiative fluorescent lifetimes τ_rad, experimental average lifetimes τ_exp, and quantum efficiencies of ⁵D₀ in G_ZBL-Eu and C_ZBL-Eu

Sample	Transition	Energy (cm^−1)	Photon number ratio	Aij (s^−1)	βij (%)	τrad (ms)	τexp (ms)	Quantum efficiency (%)
GZBL-Eu	^5D0 → ^7F1 (N1)	16 949	—	64.3	30.5	4.74	4.67	98.5
	^5D0 → ^7F2 (N2)	16 271	N2/N1 = 1.30	83.8	39.7
	^5D0 → ^7F4 (N4)	14 318	N4/N1 = 0.98	63.0	29.8
CZBL-Eu	^5D0 → ^7F1 (N1)	16 938	—	68.7	34.4	5.00	4.74	94.8
	^5D0 → ^7F2 (N2)	16 207	N2/N1 = 0.98	63.3	31.7
	^5D0 → ^7F4 (N4)	14 351	N4/N1 = 1.06	67.9	33.9

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The emission spectra and the relative spectral power distributions of C_ZBL-Eu and GC_ZBL-Eu are shown in Fig. 9(a) and (b) under 395 nm excitation, and both them show multi-channel radiative transitions of Eu³⁺ in C_ZBL-Eu and GC_ZBL-Eu. Herein, the CIE-1931 chromaticity coordinates for the luminescence of C_ZBL-Eu and GC_ZBL-Eu were derived to be (0.483, 0.385) and (0.469, 0.389), which fall into the yellowish-white region, as shown in Fig. 10(a). The chromaticity coordinates prove that the emission light of C_ZBL-Eu and GC_ZBL-Eu is yellowish-white, suggesting that the Eu³⁺-activated GC_ZBL-Eu system has application prospect as a single phosphor.

Fig. 9 Comparison of emission intensities (a) and the relative spectral power distributions (b) between C_ZBL-Eu and GC_ZBL-Eu under 395 nm excitation.

Fig. 10 (a) Color coordinates in the CIE 1931 chromaticity diagram of C_ZBL-Eu and GC_ZBL-Eu under 395 nm excitation. (b) The schematic diagram of enhanced GC_ZBL-Eu emission process under 395 nm excitation. Inset: fluorescent photographs of C_ZBL-Eu and GC_ZBL-Eu under 395 nm excitation (left), and the schematic of the photos obtained (right).

The emission intensity of GC_ZBL-Eu at 590 nm is 1.24 times higher than that for C_ZBL-Eu, where the significant improvement in the emission intensity can be attributed to the complex surface morphology of the ceramic matrix and the intense dispersion effect of the glass layer, and the schematic diagram of increasing the incident light absorption efficiency and enhancing the emission is shown in Fig. 10(b). When the incident light shines into GC_ZBL-Eu, the under-utilized light is re-reflected to the ceramic boundary, forming a multiple-cycle effect owing to the specular reflection of the glass layer. Then, the incident light will be absorbed by the glass phase in the reflection process of the glass–ceramic transition region and is converted into other wavelengths for emission.

As shown in the inset of Fig. 10(b), intense lighting is observed in C_ZBL-Eu and GC_ZBL-Eu, and the violet lighting in the picture is illuminated by a 395 nm Xe lamp. Simultaneously, the purple source overlaps with the emitted yellowish-white light to form the pink-white light. Effective fluorescence emission enhancement facilitates better application of GC_ZBL-Eu with warm-white light emission of laser lighting.

Conclusion

Hetero-structured GC_ZBL-Eu with glass region, ceramic region, and GC transition region obtained by the one-step method exhibits yellowish-white light and the crystal phase of C_ZBL-Eu is identified as BaZrF₆. Multi-channel radiative transitions were discovered through multi-peak emission, long fluorescence lifetimes, and de-population rates of the ⁵D₀, ⁵D₁, ⁵D₂, and ⁵D₃ levels in G_ZBL-Eu and C_ZBL-Eu, which is because the low OH content and low phonon energy limit the non-radiative relaxation. The effective radiative transition emission of the ⁵D₀ levels in G_ZBL-Eu and C_ZBL-Eu is demonstrated by the high quantum efficiency of 98.5% and 94.8%, respectively. The release of yellowish-white light is indicated by the color coordinates of C_ZBL-Eu (0.483, 0.385) and GC_ZBL-Eu (0.469, 0.389), and the emission intensity of GC_ZBL-Eu is 1.24 times higher than that of C_ZBL-Eu due to multiple-cycle reflection in the complex structure of the GC transition region. The effective yellowish-white lighting proves that the GC_ZBL-Eu system with fluorescence enhancement has potential in the application direction of laser illumination.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The research work was supported by the Scientific Research Funding Project from the Educational Department of Liaoning Province, China (Grant No. J2019021) and the Research Grants Council of the Hong Kong Special Administrative Region, China (Grant No. CityU 11218018).

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