1. Introduction

Over the past two decades, there has been a growing focus on the research into phosphor-converted white-light emitting diodes (pc-WLEDs) owing to their economical and technological advantages with energy-efficient and environment-friendly characteristics [1–6]. High power pc-WLEDs fabricated by combining an ultraviolet (UV) LED chip with tri-color phosphors have particularly attracted more attention in recent years for the provision of high color quality, such as full gamut, higher color rendering index, lower color temperature, etc [7–9]. However, one of the major drawbacks for pc-WLEDs is the deterioration and yellowing of organic-resins used as encapsulating materials when the chip temperature of LED increases at higher driving current, which will lead to the luminous efficiency degradation, shift of chromaticity, and reduction of long-term reliability [6,10,11]. In addition, different luminescence responses of multiple phosphors packed in pc-WLEDs to thermal impact can also result in poor white light luminous brightness and color variation of the device [12–14].

Aiming to solve these problems, single-phase white light-emitting glasses are of great interest as an alternative strategy for WLEDs because of their excellent optical properties, better mechanical resistance, lower production cost and simpler package schemes. More importantly, the luminescent glasses show an organic-resin free assembly process and thus can effectively avoid aforementioned shortcomings [15,16]. Up to now, many studies regarding the white light luminescent glasses have been carried out by codoping multiple activators and managing the energy transfer between them, such as Ce³⁺/Eu³⁺ [17], Ce³⁺/Sm³⁺ [18], Ce³⁺/Tb³⁺/Mn²⁺ [15], Eu²⁺/Tb³⁺ [19], Eu²⁺/Dy³⁺ [20], Ag/Eu³⁺ [21], Cu/Eu³⁺ [16], and Cu/Sm³⁺ [22].

Trivalent bismuth (Bi³⁺), as one other kind of activator, has the outer 6s² electronic configuration, enabling photoluminescence in different color region (from ultraviolet to blue, green, yellow and even red spectral range) which depends strongly on the type of host for Bi³⁺ doping [23,24]. These outstanding luminescent properties make Bi³⁺ not only a good candidate for spectra conversion but a fascinating class of sensitizers as well. Co-doping of Bi³⁺ with rare earth (RE) ions, such as Eu³⁺ and Sm³⁺, has been carried out extensively on phosphors as a means of acquiring white light emission via ET [14,25–27]. However, this strategy has rarely been utilized in glasses that could be efficiently excited by UV LED chips.

Herein, the glass-forming system GeO₂-B₂O₃-Gd₂O₃-La₂O₃ was chosen as host due to the specific suitability for optically active Bi³⁺ species which generally occurs in germanates and (or) borates. Gd₂O₃ and La₂O₃ were introduced as modifiers to broaden the excitation wavelength of Bi³⁺ around 350-380 nm for matching well with the UV LED chips. Moreover, the high solubility for rare earth ions, such as terbium (Tb³⁺) has been reported in this glass system previously [28]. In the present study, Bi³⁺ emission centering at about 480 nm was observed under 345 nm light excitation. Because the lack of red emission in Bi³⁺ singly doped glass, a red emitter Eu³⁺ was intentionally introduced into this system for the white light tunable emission by controlling the energy transfer from Bi³⁺ to Eu³⁺. Different techniques including transmission spectra, photoluminescence excitation and emission spectra and lifetime decay curves have been utilized to investigate the luminescent properties of Bi³⁺/Eu³⁺ co-doped germanium-borate glasses, and the energy transfer mechanism was thoroughly discussed. To understand the thermal quenching behavior of the optimized white light-emitting sample, temperature-dependent photoluminescence spectra in the range of 298-573 K was also performed.

2. Experimental

Glasses with nominal molar composition of 40GeO₂-25B₂O₃-25Gd₂O₃-10La₂O₃ (Host) were prepared using a conventional melt-quenching technique. The doping species were 1 mol% Bi₂O₃ and x mol% Eu₂O₃ (where x = 0, 0.1, 0.3, 0.5, 1.0, 2.0, 2.5 and 3.5, labeled as GB1.0 and GBEx, respectively). Another sample doped with 0.3 mol% Eu₂O₃ alone (GE0.3) was also prepared for comparison. GeO₂ (≥99.999%, Lincang Xinyuan Germanium Industrial Co., Ltd., China), H₃BO₃ (A.R. purity, Sinopharm Chemical Reagent Co., Ltd., China), Gd₂O₃, La₂O₃ (99.99%, Shanghai Yuelong Nonferrous Metals Co., Ltd., China), Bi₂O₃ and Eu₂O₃ (99.99%) from Aladdin reagents were used as raw materials. Approximately 10 g batches of raw materials were fully mixed and melted in a covered corundum crucible at 1350 °C for 1h in air. The melt was poured onto a 300 °C preheated stainless-steel plate and pressed by another plate, and then annealed at 450 °C to release the inner stress. Finally, all the obtained glasses were sliced and polished with thickness of 2 mm for optical measurements.

The glass density was calculated according to the Archimedes principle, and distilled water was used as an immersion liquid. The refractive index of the glasses was measured directly by a Metricon Model 2010 prism coupler. Transmittance spectra were measured using a Hitachi U-3900 Ultraviolet-visible (UV-vis) spectrophotometer. Photoluminescence (PL) and photoluminescence excitation (PLE) spectra were recorded on an Edinburgh FLS920 spectrofluorometer equipped with a continuous wave 450 W Xe lamp as excitation source. To study the thermal quenching properties of glasses, high-temperature PL spectra were measured between 298 and 573 K using a TAP-02 high-temperature heating instrument (Tian Jin Orient-KOJI instrument Co., Ltd.) connected to the FLS920 spectrofluorometer. A Hamamatsu R928 photomultiplier was used for signal detection. All PL results were corrected for the spectral response of the detection system. Luminescence decay lifetimes (ns) were determined by a Hamamatsu Quantaurus-Tau C11367 compact fluorescence lifetime spectrometer. Unless otherwise specified, all the measurements were performed at room temperature.

3. Results and discussion

3.1 UV-vis transmission spectra and optical band gap

Figure 1 displays the UV-vis transmission spectra of the Host, GB1.0, GBEx and GE0.3 samples in the spectral range of 300 to 800 nm. All samples are highly transparent (about 82%) and colorless. Two weak absorption peaks at 273 and 312 nm associated with the ⁸S_7/2$\to$⁶I_J and ⁸S_7/2$\to$⁶P_J characteristic transitions of Gd³⁺ ions [29], respectively, can be detected in the host glass (Host). Compared with the Host sample, the GB1.0 and GBEx samples show a noticeable red-shift in the glass absorption edge from 300 to 350 nm, which is generally from the characteristic absorption of Bi³⁺, corresponding to the ¹S_0$\to$³P₁ transition [30,31]. Meanwhile, two weak absorption peaks at 273 and 312 nm completely vanish. For all the Eu³⁺ doped samples (GE0.3 and GBEx), several characteristic absorption peaks located at 393, 464 and 529 nm are observed due to the 4f-4f electric transitions of Eu³⁺ ions from the ground state ⁷F₀ to the excited states ⁵L₆, ⁵D₂ and ⁵D₁, respectively [32], as labeled in Fig. 1. Notably, the intensity of all Eu³⁺-related absorption peaks increases gradually with the elevated Eu₂O₃ concentration.

 

Fig. 1 UV-vis transmission spectra of the Host, GB1.0, GBEx (x = 0.3, 1.0) and GE0.3 samples.

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Normally, the absorption edge is caused by the electronic transitions (as discussed above), which also reflects the optical band gap of glass. As developed by Tauc and Menth [33], the relationship between the absorption coefficient ($\alpha$) and the optical band gap (E_g) can be expressed through the following equation:

Table 1. Density, refractive index, and indirect optical band gap of glasses samples.

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In order to evaluate the physical properties, the density and refractive index of all glasses samples are investigated and presented in Table 1.

3.2 Photoluminescence of Bi³⁺/Eu³⁺ co-doped glasses

Excitation and emission spectra of the Host, GB1.0, GBE0.3 and GE0.3 samples are compared in Fig. 2. It can be clearly seen that the excitation spectra (λ_em = 480 nm) of the GB1.0 sample covers a broad UV excitation band from 250 to 400 nm with a maximum intensity at about 345 nm corresponding to the ¹S_0$\to$³P₁ transition of Bi³⁺, being consistent with the absorption band of Bi³⁺ in Fig. 1. The high excitation intensity at 320-380 nm suggests that Bi³⁺ doped germanium-borate glasses can be a good candidate for UV-pumping WLEDs. Upon excitation at 345 nm, this sample shows a broad intense bluish-green emitting band spanning the spectral range of 380 to 700 nm, originating from the ³P_1$\to$¹S₀ characteristic transition of Bi³⁺ (Fig. 2(a)). For the Eu³⁺ singly doped sample GE0.3, there are two sets of excitation signals observed in the excitation spectra shown in Fig. 2(b) (λ_em = 612 nm), namely, one broad band at around 290 nm and several sharp excitation peaks centered at 362, 378, 393, 413 and 464 nm. Obviously, the first one arises from the well-known Eu³⁺-O^2- charge transfer (CT) band, and the other sharp peaks are assigned to the 4f-4f transitions of Eu³⁺ from ⁷F₀ to ⁵D₄, ⁵G₄, ⁵L₆, ⁵D₃ and ⁵D₂, respectively. The dominant excitation peak is located at 393 nm, being fully in agreement with the transmittance spectra (Fig. 1). Moreover, two sharp peaks at about 273 and 312 nm, originated from the Gd^{3+ 8}S_7/2$\to$⁶I_J and ⁸S_7/2$\to$⁶P_J transitions, respectively, are also observed in the excitation spectra, which confirms the existence of the energy transfer (ET) from Gd³⁺ to Eu³⁺ in the present GE0.3 glass sample. When monitoring the 612 nm emission for the co-doped sample GBE0.3, both CT band and two sharp peaks from Gd³⁺ disappear in the excitation spectra. Oppositely, an extra broad excitation band centered at 345 nm ranging from 250 to 380 nm can be detected in addition to the aforementioned several characteristic sharp peaks related to 4f-4f transitions of Eu³⁺ ions. The shape and peak position of this broad band resemble those in the Bi³⁺ singly doped sample (Fig. 2(a)), implying an efficient occurrence of ET from Bi³⁺ to Eu³⁺ in the co-doped glass sample. This energy transfer could be attributed to a strong overlap between the emission of Bi³⁺ (Fig. 2(a)) and the excitation of Eu³⁺ (Fig. 2(b)) in the range of 380-480 nm. Accordingly, excited by a 345 nm light, the optimal excitation wavelength for Bi³⁺ but not for Eu³⁺, the emission spectra of GBE0.3 glass not only includes a bluish-green emitting broad band centered about 480 nm from Bi³⁺ but also contains five red characteristic emissions of Eu³⁺, peaking at 580, 591, 612, 652 and 702 nm, respectively. The latter can readily be assigned to the intrinsic 4f-4f transitions from the excited state ⁵D₀ to the index states ⁷F_J (J = 0-4). It is notable that the Eu³⁺-related emissions become significantly enhanced for GBE0.3 compared with that of the GE0.3 sample, and almost no emission is detected in the host glass under the same excitation conditions. All of these results indicate that high light output of Eu³⁺ actually comes from the ET process from Bi³⁺ to Eu³⁺.

 

Fig. 2 Excitation and emission spectra of (a) GB1.0, (b) GE0.3 and GBE0.3 samples.

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In order to obtain a deeper insight into the ET process, photoluminescence and luminescence dynamics of the Bi³⁺/Eu³⁺ co-doped glasses with varying Eu₂O₃ doping concentrations were systemically investigated. Figure 3(a) shows the emission spectra (λ_ex = 345 nm) of GB1.0 and GBEx (0.1, 0.3, 0.5, 1.0, 2.0, 2.5, 3.5) samples. With increasing content of Eu³⁺, the emission intensity of Bi³⁺ decreases continuously, while that of Eu³⁺ increases rapidly. This observation directly verifies the presence of ET process from Bi³⁺ to Eu³⁺. In particular, within the doping concentration range (up to x = 3.5), no quenching of Eu³⁺ emission occurs, indicating that germanium-borate glass could be a suitable host for Eu³⁺ heavy doping.

 

Fig. 3 (a) Emission (λ_ex = 345 nm), (b) Excitation (λ_em = 480 nm) spectra of the GB1.0 and GBEx (x = 0.1, 0.3, 0.5, 1.0, 2.0, 2.5, 3.5) samples. The inset of (b) shows the excitation spectra (λ_em = 612 nm). (c) Luminescence decay curves of GB1.0 and GBEx samples (λ_ex = 345 nm, λ_em = 480 nm). (d) CIE chromaticity coordinates and luminescence photos corresponding to the emission of glass samples.

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Furthermore, it should be noted that a hole located at 464 nm, which just corresponds to the characteristic excitation (absorption) of Eu³⁺, is observed in the Bi³⁺ emission band for all co-doped samples, and this can be clearly seen from the enlarged spectra in the inset of Fig. 3(a)). Such result shows that a portion of the bluish-green emission from Bi³⁺ can also be re-absorbed by Eu³⁺ since there exists a large spectral overlaps between them. Clearly, the re-absorption at 464 nm becomes more efficient with the elevated content of Eu³⁺ ions.

Figure 3(b) depicts the excitation spectra of Bi³⁺ emission monitored at 480 nm (λ_em = 480 nm) for GB1.0 and GBEx (x = 0.1, 0.3, 0.5, 1.0, 2.0, 2.5, 3.5) samples. It is seen that the intensity of excitation band ranging from 250 to 400 nm also decreases all the way, which is similar with the observed luminescence behavior of Bi³⁺ in Fig. 3(a). However, both excitation signals from Bi³⁺ and Eu³⁺ are continuously enhanced when monitoring the strong emission peak of Eu³⁺ at 612 nm (λ_em = 612 nm), as shown in the inset of Fig. 3(b). All the above results definitively corroborate the proposed ET process, i.e., from Bi³⁺ to Eu³⁺.

The luminescence decay curves of Bi³⁺ (λ_ex = 345 nm, λ_em = 480 nm) for the GB1.0 and GBEx samples were measured to give a more convincing evidence of the ET. As shown in Fig. 3(c), all the decay curves fit well only into a double-exponential equation [25,34]:

Table 2. Lifetimes (λ_ex = 345 nm, λ_em = 480 nm), ET efficiency${\eta }_T$, and CIE chromaticity coordinates of glasses samples.

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The Dexter’s energy transfer theory was used to reveal the possible mechanism of the ET from Bi³⁺ to Eu³⁺. For a multipolar-multipolar interaction process, the equation can be expressed as [37]:

 

Fig. 4 Dependence of ($I_0/I$) values on (a) C^6/3, (b) C^8/3 and (c) C^10/3 in GBEx (x = 0.3, 0.5, 1.0, 2.0, 2.5) samples. Correlation efficiencies are 99.66, 97.70 and 94.60% for the fittings to Eq. (5), respectively.

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The energy level scheme of Bi³⁺ and Eu³⁺ ions with optical transitions as well as the possible ET process are schematically depicted in Fig. 5. When excited by the UV light (320-380 nm), electrons of Bi³⁺ are firstly excited from the ¹S₀ ground state to the ³P₁ excited state, and then nonradiatively relax to the lowest vibration level of the ³P₁ state, giving the typical bluish-green emission peaking at 480 nm when they radiatively relax down to the ¹S₀ ground state to. Because the ³P₁ excited state of Bi³⁺ is energetically close to the ⁵L₆ and ⁵D₂ levels of Eu³⁺, a resonance nonradiative ET process can easily proceed. An excited Bi³⁺ relaxes from the excited state to the ground state nonradiatively and transfers the excitation energy to a neighboring Eu³⁺, promoting it from the ⁷F₀ ground state to the ⁵L₆ (393 nm) and (or) ⁵D₂ (464 nm) level, respectively. Such ET process will lead to a decline in the lifetime of Bi³⁺ emission, as shown in Fig. 3(c). After that, the electrons populated in the ⁵L₆ and (or) ⁵D₂ levels of Eu³⁺ undergo multi-phonon relaxation to the emitting ⁵D₀ level and then radiatively relax to the ⁷F_J level, showing enhanced red emissions of Eu³⁺. In addition, a part of the bluish-green emission from Bi³⁺ can also be re-absorbed by Eu³⁺ because of the spectral overlaps (see Figs. 2 and 3), which usually has no effect on the lifetime of Bi³⁺ emission.

 

Fig. 5 Energy level scheme of Bi³⁺ and Eu³⁺ ions as well as the possible ET process

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Through an efficient ET process from Bi³⁺ to Eu³⁺, the intensity ratio of Bi³⁺ bluish-green to Eu³⁺ red emission can be tuned by changing the Eu³⁺ concentration. Combining both of them, therefore, white light emission may be achieved. Figure 3(d) shows the luminescence colors of GB1.0 and GBEx (x = 0.1, 0.3, 0.5, 1.0, 2.0, 2.5, 3.5) samples characterized by the Commission Internationale de L’Eclairage (CIE) chromaticity diagram under 345 nm light excitation. The corresponding chromaticity coordinates are summarized in Table 2. Interestingly, the color gradually shifts from bluish-green to white and finally falls into a red region with the increasing Eu³⁺ content, as vividly shown in the luminescence photographs of the representative glass samples taken under 345 nm excitation in dark (Fig. 3(d)). A perfect white light emission is obtained in GBE0.3 and GBE0.5 samples with the CIE coordinates of (0.309,0.311) and (0.356,0.325), respectively, indicating that Bi³⁺/Eu³⁺ co-doped germanium-borate glasses could be used as white-emitting phosphors for UV LED chips.

3.3 Thermal quenching properties of Bi³⁺/Eu³⁺ co-doped white-emitting glass

The thermal quenching property is an important technological parameter for practical application in the solid-state lighting field because it has a considerable influence on the light output and color rendering index [39–41]. Therefore, it is necessary to evaluate the thermal quenching particularly at temperatures higher than room temperature. Figure 6(a) presents the emission spectra of the white light-emitting GBE0.5 sample measured at different temperatures upon 345 nm light excitation. Clearly, both the emission intensities for Bi³⁺ and Eu³⁺ part decrease with increasing temperature. By plotting the integrated intensity dependence of the Bi³⁺ and Eu³⁺ emissions on temperature in Fig. 6(b), it can be seen that the intensities at 423 K for Bi³⁺ and Eu³⁺ remain only 29.5% and 43.6% of the initial values at 298 K, respectively. This indicates the thermal quenching of Bi³⁺ is much serious than that of Eu³⁺. Generally, the thermal quenching of the emission intensity can be interpreted as the process in which, by active phonon-assistance, the luminescence center is thermally excited through the crossing point between the excited state and the ground state, and then returns to the ground state nonradiatively, thus resulting in a decline of the emission intensity. This nonradiative transition probability by thermal activation is strongly dependent on the energy barrier, that is, activation energy ($E_a$). The smaller activation energy is, the more severe thermal quenching is. According to the Arrhenius equation, the activation energy ($E_a$) of the luminescence center can be evaluated by [42, 43]:

 

Fig. 6 (a) Temperature-dependent emission spectra of the GBE0.5 sample (λ_ex = 345 nm). (b) Relative integrated emission intensity versus temperature upon excitation at 345 nm. (c) Plot of ln(I₀/I_T −1) versus 1/kT and the linear fit of the data through Eq. (6).

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Moreover, the color stability can be quantifiably described by the chromaticity shift (ΔE) using the following equation [44]:

 

Fig. 7 Chromaticity shift of the GBE0.5 sample (λ_ex = 345 nm) as a function of temperature from 298 to 573 K. The inset shows the CIE chromaticity coordinates at different temperatures.

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4. Conclusions

In summary, luminescent properties of novel Bi³⁺/Eu³⁺ co-doped glasses were systematically investigated. Excitation and emission spectra as well as decay lifetime data have demonstrated the efficient energy transfer from Bi³⁺ to Eu³⁺ ions in the Bi³⁺/Eu³⁺ co-doped glasses, and the energy transfer mechanism is confirmed due to a dipole-dipole interaction. By tuning the intensity ratio of Bi³⁺ bluish-green to Eu³⁺ red emission, a tunable white light is obtained under excitation at 345 nm light. Temperature-dependent CIE chromaticity coordinates show excellent color stability in working temperature below 423 K for the glass samples. These excellent characteristics enable the novel Bi³⁺/Eu³⁺ co-doped glasses to be a promising candidate for the application of WLEDs pumped by UV-LED chip.