Blue–green color-tunable emissions in novel transparent Sr2LuF7:Eu/Tb glass-ceramics for WLEDs

Xiaoman Li; Shaoshuai Zhou; Rongfei Wei; Xueyun Liu; Bingqiang Cao; Hai Guo

doi:10.3788/COL202018.051601

Abstract

To introduce ordered nano-structures inside a transparent amorphous matrix with superior optical and mechanical properties bears scientific and technological importance, yet limited success has been achieved. Here, via simple melting-quenching and subsequent thermal activation, we report the successful preparation of transparent nano-structured glass-ceramics embedded in Sr₂LuF₇ nano-crystals (～26 nm), as evidenced by X-ray diffraction, transmission electron microscopy (TEM), and high resolution TEM. The successful incorporation of dopants into formed Sr₂LuF₇ nano-crystals with low phonon energy results in highly tunable blue–green photoemission, which depends on excitation wavelength, dopant type, and temperature. We found that Eu³⁺ and Eu²⁺ ions co-exist in this hybrid optical material, accompanied by the broadband blue emission of Eu²⁺ and sharp red emissions of Eu³⁺. A series of optical characterizations are summoned, including emission/excitation spectrum and decay curve measurement, to reveal the reduction mechanism of Eu³⁺ to Eu²⁺. Furthermore, near green–white photoemission is achieved via the enrichment of Tb³⁺/Eu³⁺ into crystallized Sr₂LuF₇ nano-crystals. The temperature-dependent visible photoemission reveals thermal activation energy increases with the precipitation of Sr₂LuF₇ nano-crystals in a glass matrix, suggesting better thermal stability of glass-ceramics than precursor glasses. These results could not only deepen the understanding of glass-ceramics but also indicate the promising potential of Eu³⁺/Tb³⁺-ions-doped Sr₂LuF₇ glass-ceramics for UV pumped white light emitting diodes (WLEDs) with good thermal stability.

Keywords

glass-ceramics luminescent materials Sr2LuF7 nano-crystals WLED

White light emitting diodes (WLEDs), on account of their energy-saving and environment-friendly features, present diverse applications in general lighting, equipment indicators, backlights and car headlights, etc.^[1–7]. In particular, high power ultraviolet (UV) pumped WLEDs are attracting much attention due to un-matched advantages, including high color rendering index and low color temperature^[8,9]. However, resins are usually used to package phosphors when we prepare WLEDs, and the inherently short-lived properties of organic compounds have severely limited the long-term use of WLEDs, especially under the action of high power. The aging of the resins also inevitably causes color drift, decline in efficiency, etc.^[10].

As an alternative, inorganic oxide glasses offer a promising route for the realization of WLEDs with excellent stability, low cost, and an organic resin free assembly process. So far, typical investigations have been conducted in doped bulk oxide glasses to obtain excellent optical performance for WLEDs^[11–13]. Yet, oxide glasses usually bear higher phonon energy, which limits the probability of radiative transitions, resulting in weak emission intensity^[14]. Although fluoride glasses present a low phonon energy, the improved emission efficiency is often at the expense of mechanical properties, especially compared to that of oxide glasses^[15].

Interestingly, by introducing ordered fluoride nano-crystals inside transparent amorphous oxide glasses, superior optical performance of fluoride and mechanical properties of oxide glass can be preserved at the same time^[16,17]. The doped rare earth ions will be incorporated into formed fluoride nano-crystals with low phonon energy, resulting in improved optical properties compared to parent glasses^[13,17–24]. Furthermore, the protection of oxide glass networks to the active fluoride nano-crystals can effectively isolate the outside air and the corrosion of moisture to the fluoride nano-crystals, so as to preserve the excellent mechanical properties of oxide glasses^[25–27]. Thus, rare earth ions activated oxyfluoride glass-ceramics^[28–31] present huge potential in high power WLEDs.

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Except for the selection of host, suitable dopants are essential to achieving high performance oxyfluoride glass-ceramics. ${Eu}^{3 +}$ ions with orange–red emission have been well accepted as excellent emitters and optical probes due to the sensitive features to surrounding environment^[32], while ${Eu}^{2 +}$ ions in glasses normally present a broad emission covering the blue–green spectral region^[33]. Therefore, white emission can be achieved by using the red emission of ${Eu}^{3 +}$ with the broad blue emission of ${Eu}^{2 +}$ in appropriately designed glass-ceramics^[34,35]. Single phased glass-ceramics with dual emission from ${Eu}^{2 +} / {Eu}^{3 +}$ should be promising in WLEDs; yet, they are rarely reported due to lack of a proper glass-ceramics host for the co-existence of ${Eu}^{3 +}$ and ${Eu}^{2 +}$ .

Here, novel ${Sr}_{2} {LuF}_{7}$ nano-crystals were successfully prepared in situ via thermal activation inside alumino-silicate glass-ceramics. Under a flexible excitation wavelength, ${Sr}_{2} {LuF}_{7}$ glass-ceramics could present color-tunable photoemission by using the merits of the dual emission feature of ${Eu}^{3 +}$ and ${Eu}^{2 +}$ , which cover the whole visible emission range, presenting great potential in WLEDs. The Eu/Tb co-doped glass-ceramics were also prepared to adjust the emission color. The structure and luminescence properties have been systematically studied based on detailed optical and structural characterization. Our results indicate that rare-earth-ion-doped ${Sr}_{2} {LuF}_{7}$ glass-ceramics with tunable emission and excellent stability could be excellent candidates for plant lighting or WLEDs.

Via the melting-quenching method, samples with a nominal composition (Table 1) were prepared. ${SiO}_{2}$ , ${Al}_{2} O_{3}$ , ${SrF}_{2}$ , NaF (A.R.), and high purity ${LuF}_{3}$ , ${EuF}_{3}$ , and ${TbF}_{3}$ (99.99%) were chosen as raw materials. Typically, 25 g of well mixed raw materials were melted in a crucible (1550°C for 1 h) and quenched in a plate to form a solid bulk glass sample, that is precursor glass (PG), which is denoted as PGEu, PGTb, and PGEuTb. All PGs were isothermal treated (750°C for 5 h) to form corresponding glass-ceramics (GC) samples (labeled as GCEu, GCTb, and GCEuTb, respectively). For further measurement, these PG and GC samples were optically polished.

Samples	Host	Eu	Tb
PGEu/GCEu	SiO2-Al2O3-SrF2-LuF3	0.5EuF3	0
PGEuTb/GCEuTb	SiO2-Al2O3-SrF2-LuF3	0.5EuF3	0.5TbF3
PGTb/GCTb	SiO2-Al2O3-SrF2-LuF3	0	0.5TbF3

Table 1. Compositions of Glasses Samples (in mol%)

View all Tables

X-ray diffraction (XRD) patterns were obtained via a Philips X’Pert PRO SUPER XRD apparatus. The microstructure was evaluated by JEM-2010 transmission electron microscopy (TEM) (JOEL Ltd.). Transmittance of PG and GC samples was recorded on a U-3900 UV-visible (UV-VIS) spectrophotometer (Hitachi). Optical characterizations were done on an Edinburgh FS920 fluorescence spectrometer. Temperature control of the sample in the temperature-dependent spectra was realized through heat conduction with a copper plate, whose temperature was controlled using a heating tube and a temperature controller (OMRON E5CC-800).

Figure 1(a) demonstrates the XRD patterns of glass samples including PGEuTb, GCEu, GCTb, and GCEuTb. The standard diffraction data for tetragonal ${Sr}_{2} {LuF}_{7}$ are also given for comparison (JCPDS card No. 39-0926). All PG samples are amorphous, and no diffraction peaks are typically seen for PGEuTb. After thermal activation, sharp diffraction peaks of all GC samples corresponding to tetragonal ${Sr}_{2} {LuF}_{7}$ emerge, indicating that GC samples are crystallized during thermal treatment. Moreover, different dopants do not cause phase changes. The crystalline size D can be acquired by the following Scherer’s equation^[36], and it is about 28 nm in diameter for GC samples: $D = k λ / β \cos θ .$ (1)

As presented in the transmittance spectra of PGEu, GCEu, PGEuTb, and GCEuTb [Fig. 1(b)], we observed the characteristic absorption peaks of ${Eu}^{3 +}$ at ∼393 and ∼464 nm, which could be assigned to the ${{}^{7}F}_{0} - {{}^{5}L}_{6}$ and ${{}^{5}D}_{2}$ transitions of ${Eu}^{3 +}$ , respectively, while the typical absorption of ${Tb}^{3 +}$ with peaks at 377 nm ( ${{}^{7}F}_{6} - {{}^{5}D}_{3}$ ) can also be observed in ${Tb}^{3 +}$ -doped samples. Moreover, the cut-off wavelength of GC shows a clear red shift compared to that of PG, which is due to the scattering effect of particles and further confirms the formation of ${Sr}_{2} {LuF}_{7}$ nano-crystals. Even so, the transparency of GC samples reaches 60% in the GCEuTb sample, which reveals the small size of the formed nano-particles.

Figure 1.(a) XRD patterns of PGEuTb, GCEu, GCTb, and GCEuTb and the standard data for ${Sr}_{2} {LuF}_{7}$ (JCPDS No. 39-0926); (b) transmission spectra of PGEu, GCEu, PGTb, PGEuTb, and GCEuTb; (c) TEM and (d) HRTEM images of nano-structured GCEu. The inset is corresponding SAED patterns.

To directly confirm the formation of ${Sr}_{2} {LuF}_{7}$ nano-crystals, the TEM and high resolution TEM (HRTEM) images of GCEu samples were measured, as shown in Figs. 1(c) and 1(d), respectively. From the TEM image displayed in Fig. 1(c), it is very clear to see dark uniform nano-crystals on the amorphous glass matrix, which directly reveal the crystallization of ${Sr}_{2} {LuF}_{7}$ in alumino-silicate glasses. It is worth mentioning that the formed ${Sr}_{2} {LuF}_{7}$ nano-crystals are homogeneous in grain size (∼26 nm), which is consistent with results from XRD patterns. With the suitable crystalline size to this glass matrix, the glass samples of both PGEu and GCEu show high transparency^[35] of up to 72%. The corresponding selected area electron diffraction (SAED) patterns of the GCEu sample, as presented in Fig. 1(c), reveal the polycrystalline feature of ${Sr}_{2} {LuF}_{7}$ nano-crystals, suggesting this hybrid optical material is composed of poly-nano-crystals and amorphous alumino-silicate glasses. Besides, as presented in Fig. 1(d), the lattice structure can be observed in the HRTEM image. The interplanar spacing $d$ from HRTEM is $\sim 3.330 Å$ , which can be attributed to (213) of tetragonal ${Sr}_{2} {LuF}_{7} (d_{(213)} = 3.284 Å)$ .

After detailed structural characterizations, the optical properties of ${Sr}_{2} {LuF}_{7}$ glass-ceramics were investigated. As shown in the excitation ( $λ_{em} = 613 nm$ ) spectra [Fig. 2(a)], characteristic excitation features (350–550 nm) of ${Eu}^{3 +}$ are observed in GCEu, which can be attributed to $4 f - 4 f$ transitions of ${Eu}^{3 +}$ ions^[35–38]. The excitation peak at 393 nm should be from the ${{}^{7}F}_{0} \to {{}^{5}L}_{6}$ transition of ${Eu}^{3 +}$ . The emission ( $λ_{ex} = 393 nm$ ) spectra [Fig. 2(b)] show typical red and green emission peaks from ${{}^{5}D}_{0} \to {{}^{7}F}_{J}$ ( $J = 0 - 4$ ) and ${{}^{5}D}_{1} \to {{}^{7}F}_{J}$ ( $J = 1 - 3$ ) transitions of ${Eu}^{3 +}$ ions (500–720 nm), respectively. Compared to PGEu, the 613 nm and 702 nm emissions of ${Eu}^{3 +}$ ions of GCEu are highly improved after thermal treatment. Furthermore, Stark splitting for ${{}^{5}D}_{1} \to {{}^{7}F}_{1} / {{}^{7}F}_{2}$ transitions of ${Eu}^{3 +}$ ions is observed after crystallization. These phenomena suggest the successful enrichment of ${Eu}^{3 +}$ into ${Sr}_{2} {LuF}_{7}$ nano-crystals. Moreover, the broadband excitation and emission of ${Eu}^{2 +}$ were also observed in ${Sr}_{2} {LuF}_{7}$ glass-ceramics, which will be discussed in detail in the following.

Figure 2.(a) Excitation ( $λ_{em} = 613 nm$ ) and (b) emission ( $λ_{ex} = 393 nm$ ) spectra of PGEu and GCEu.

As is known to us, the intensity ratio R of ${{}^{5}D}_{0} \to {{}^{7}F}_{2}$ to ${{}^{5}D}_{0} \to {{}^{7}F}_{1}$ emissions is recognized as a sensitive probe to evaluate the symmetry of the local crystal field around ${Eu}^{3 +}$ . A low value of R reveals high symmetry of the surrounding environment^{[19–21,35,36,39]}. Here, the R is decreased from 2.04 (PGEu) to 0.96 (GCEu), which suggests the enrichment of ${Eu}^{3 +}$ into formed ${Sr}_{2} {LuF}_{7}$ nano-crystals with higher symmetry.

Furthermore, the broadband emission of ${Eu}^{2 +}$ ( ${4 f}^{6} 5 d \to {4 f}^{7}$ transition)^[34,35], which is centered at about 450 nm, is also observed in both PGEu and GCEu, suggesting ${Eu}^{2 +}$ and ${Eu}^{3 +}$ co-exist in the hybrid materials, and partial ${Eu}^{3 +}$ is reduced to ${Eu}^{2 +}$ during melting-quenching, considering the raw material of ${EuF}_{3}$ . Figure 3(a) illustrates the typical emission of ${Eu}^{2 +}$ in PGEu and GCEu samples upon the excitation of 350 nm. Compared with PGEu, the broadband ${Eu}^{2 +}$ emission in GCEu is enhanced obviously after the crystallization of ${Sr}_{2} {LuF}_{7}$ nano-crystals. In addition, the emission peak at 422 nm in PGEu presents a red shift to 430 nm in GCEu after crystallization, which is caused by the strong vibration around ${Eu}^{2 +} - {Eu}^{2 +}$ ions with a shortened distance. Figures 3(b) and 3(c) show the normalized typical emission and excitation spectra in the GCEu sample. Clearly, the emission peak shows an obvious red shift from 420 to 445 nm with an increase in the excitation wavelength from 280 to 390 nm. Simultaneously, the emission color of glass-ceramics is tunable from bluish violet to blue, as shown in following Commission International de I’Eclairage (CIE) figure. This broadband and color-tunable emission of ${Eu}^{2 +}$ ions in the stable GCEu sample could be a potential candidate for a UV pumped light source, such as a lamp for plants.

The conversion of ${Eu}^{3 +}$ to ${Eu}^{2 +}$ in air atmosphere may be understood by two models^[40,41]: one is the reduction caused by volatilization of fluoride, and the other one is the charge compensation model. For the former, ${Eu}^{3 +}$ is reduced in the melting process, with the volatilization of fluoride: ${2 Eu}^{3 +} + 2 F^{-} \to {2 Eu}^{2 +} + F_{2} ↑$ , which corresponds to the existence of ${Eu}^{2 +}$ emission in the PGEu sample [shown in Fig. 2(b)]. For the latter, when doped into ${Sr}_{2} {LuF}_{7}$ nano-crystals, ${Eu}^{3 +}$ will replace ${Sr}^{2 +} (1.18 Å)$ ions. At the same time, two ${Eu}^{3 +}$ ions are essential to substituting three ${Sr}^{2 +}$ ions, so as to keep the charge balance. As a result, two defects [ ${Eu}_{Sr}^{\cdot}$ ] and one cation vacancy [ ${V^{″}}_{Sr}$ ] would be created simultaneously. Here, ${Eu}_{Sr}^{\cdot}$ presents one positive charge, while ${V^{″}}_{Sr}$ holds two negative charges. ${V^{″}}_{Sr}$ is a donor of electron, while ${Eu}_{Sr}^{\cdot}$ acts as the acceptor. Via thermal treatment, the electrons in ${V^{″}}_{Sr}$ are released and then trapped by ${Eu}_{Sr}^{\cdot}$ , finally resulting in the reduction of ${Eu}^{3 +}$ . This charge compensation model can well illustrate the enhanced emissions of ${Eu}^{2 +}$ in Figs. 2(b) and 3(a).

Figure 3.(a) Emission spectra of PGEu and GCEu excited by 350 nm; normalized (b) emission and (c) excitation spectra of ${Eu}^{2 +}$ ions in the GCEu sample.

Figure 4 shows the excitation ( $λ_{em} = 543 nm$ ) and emission ( $λ_{ex} = 377 nm$ ) spectra of GCEuTb and GCTb. By monitoring the ${{}^{5}D}_{4} \to {{}^{7}F}_{5}$ emission for ${Tb}^{3 +}$ at 543 nm, typical excitation peaks at 283, 302, 316, 340, 350, 368, 377, and 483 nm are observed, and they can be attributed to ${{}^{7}F}_{6} \to {{}^{5}I}_{8}$ , ${{}^{5}H}_{6}$ , ${{}^{5}H}_{7}$ , ${{}^{5}L}_{7}$ , ${{}^{5}L}_{9}$ , ${{}^{5}D}_{2}$ , ${{}^{5}D}_{3}$ , and ${{}^{5}D}_{4}$ transitions for ${Tb}^{3 +}$ , respectively^[13]. As presented in Fig. 4(b), upon the excitation of 377 nm ( ${{}^{7}F}_{6} \to {{}^{5}D}_{3}$ ) light, emission bands with peaks at 488, 543, 585, and 621 nm can be observed, which are attributed to ${{}^{5}D}_{4} \to {{}^{7}F}_{J}$ ( $J = 6 - 3$ ) of ${Tb}^{3 +}$ , respectively. Bright green emissions of the GCTb sample can be obtained when excited by 377 nm.

Figure 4.(a) Excitation ( $λ_{em} = 543 nm$ ) and (b) emission ( $λ_{ex} = 377 nm$ ) spectra of ${Tb}^{3 +}$ -doped samples (PGTb and GCTb).

The emission intensity is highly enhanced with Tb/Eu co-doping on account of the energy transfer (ET) from ${Eu}^{2 +}$ to ${Tb}^{3 +}$ . The ET is relatively apparent when excited by 350 nm, which can be certified by the decay lifetimes^[25] of ${Eu}^{2 +}$ ions in GCEu and GCEuTb samples [Fig. 5(b)]. The lifetime of ${Eu}^{2 +}$ in the GCEu sample was about 838 μs, while that in GCEuTb was slightly reduced by 52 μs. Simultaneously, as is shown in Fig. 5(a), when excited by 350 nm, the ${Eu}^{2 +}$ emission is enhanced in GCEuTb due to better crystallization and larger crystallization size of ${Sr}_{2} {LuF}_{7}$ nano-crystals, which means the phonon energy around ${Eu}^{2 +}$ is further lowered. This phenomenon is in accordance with the sharper diffraction peaks of GCEuTb than GCEu in XRD patterns in Fig. 1.

Figure 5.(a) Emission spectra ( $λ_{ex} = 350 nm$ ) and (b) decay lifetime of ${Eu}^{2 +}$ ions ( $λ_{ex} = 350 nm$ , $λ_{em} = 427 nm$ ) of GCEu and GCEuTb samples.

Besides, as mentioned in Fig. 3, when excited by different excitation wavelengths, these glass-ceramics can exhibit tunable color from bluish violet to blue. Figure 6 shows the CIE of the GCEu sample excited by 280, 300, 350, and 393 nm, respectively. Simultaneously, the green emission of ${Tb}^{3 +}$ can be also obtained in the GCTb sample excited by 377 nm, as presented in Fig. 6. Combined with the blue ( ${Eu}^{2 +}$ ), green ( ${Tb}^{3 +}$ ), and red ( ${Eu}^{3 +}$ ) emission, near white emission can be expected under UV excitation in Eu/Tb co-doped ${Sr}_{2} {LuF}_{7}$ glass-ceramics. This indicates that Eu/Tb co-doped ${Sr}_{2} {LuF}_{7}$ glass-ceramics have potential application in WLED fields.

Figure 6.CIE of GCEu (excited by 280, 300, 350, and 393 nm) and GCTb (excited by 377 nm).

As known to us, thermal quenching performance is vital for the application of glass and glass-ceramics as light sources^[12,42,43], especially for long-term use. The temperature-dependent photoemission spectra of Eu-doped glass and glass-ceramics were recorded to evaluate the quenching behavior. Under 350 nm excitation, the emission spectra of PGEu and GCEu within the range of 300–500 K were obtained and are presented in Figs. 7(a) and 7(b). Clearly, the emission intensities decrease gradually with elevation of the surrounding temperature. The emission intensities in PGEu and GCEu at 500 K are 56% and 52% of the initial intensity at 300 K, respectively. Due to the weakened emission of ${Eu}^{3 +}$ , here, the considered emission intensity is referred to as the whole integrated emission intensity from 375 to 725 nm of both ${Eu}^{2 +}$ and ${Eu}^{3 +}$ .

Figure 7.Emission spectra excited by 350 nm of (a) PGEu and (b) GCEu under different temperatures from 300 to 500 K.

Generally, the thermal quenching behavior is caused by thermally activated multi-phonon non-radiative transition and is determined by activation energy ( $Δ E_{a}$ ). The thermally activated non-radiative transition rate $k_{nr}$ can be written as^[44,45] $k_{nr} = s \exp (- Δ E_{a} / k_{B} T),$ (2)where $s$ stands for frequency factor, $k_{B}$ stands for the Boltzmann constant, and $T$ stands for temperature. The $Δ E_{a}$ can be obtained via the Arrhenius equation^[45,46]: $I_{T} = \frac{I_{0}}{1 + C \exp (- Δ E_{a} / k_{B} T)},$ (3)where $I_{0}$ stands for the initial emission intensity, $I_{T}$ stands for the integral emission intensity at T, and $C$ is a constant. Based on Fig. 7, the values of $Δ E_{a}$ for the emissions of Eu ions in PGEu and GCEu are obtained to be ∼0.1036 and 0.1236 eV, respectively, which suggests PGEu presents a rapid degradation compared with GCEu, and GC shows better stability upon heating^[47]. Moreover, $Δ E_{a}$ was increased by 19.3% in this oxyfluoride ${Sr}_{2} {LuF}_{7}$ GC compared with that in oxyfluoride PG, which is conspicuously larger than that in Eu-doped phospho-alumino-silicate glass-ceramics containing ${Ba}_{3} {AlO}_{3} {PO}_{4}$ nano-crystals with the increased percentage of 1.8%^[48]. These results indicate oxyfluoride ${Sr}_{2} {LuF}_{7}$ glass-ceramics not only show better thermal stability than PG, but also better improvement in thermal stability after crystallization than oxide glass-ceramics, such as ${Ba}_{3} {AlO}_{3} {PO}_{4}$ glass-ceramics, which endows these oxyfluoride glass-ceramics to be good candidates for WLEDs.

Dual valence Eu-doped tetragonal ${Sr}_{2} {LuF}_{7}$ glass-ceramics were fabricated via the melting-quenching method. Detailed structural characterizations including XRD, TEM, and HRTEM suggest the formation of novel ${Sr}_{2} {LuF}_{7}$ nano-crystals. The enhanced photoemission, prolonged decay lifetime, and Stark splitting reveal the enrichment of rare ions into ${Sr}_{2} {LuF}_{7}$ nano-crystals protected by an alumino-silicate glass network. We found that ${Eu}^{3 +}$ , ${Eu}^{2 +}$ , and ${Tb}^{3 +}$ co-exist in these hybrid materials, resulting in highly tunable blue–green photoemission. With the combination of broad blue–green emission of ${Eu}^{2 +}$ , green emission of ${Tb}^{3 +}$ , and sharp orange–red emission of ${Eu}^{3 +}$ , white light emissions can be achieved as excited by proper UV light. Furthermore, the construction of GC favors the improvement of glass thermal stability and makes it promising as a stable light source in practice. These results indicate that such novel Eu/Tb-doped transparent ${Sr}_{2} {LuF}_{7}$ glass-ceramics present potential applications in tunable light sources for display or lighting.