1. Introduction

Recently, white light-emitting diodes (W-LEDs) have attracted considerable attention due to their excellent advantages such as compact size, longer lifetime, energy saving, better reliability and environment protection. The W-LEDs have been regarded as the fourth generation of solid-state light sources [1–3]. At present, the W-LEDs consisting of a blue InGaN LED chip and yellow-emitting YAG: Ce³⁺ phosphors have been commercialized due to their easy fabrication, low cost, and high brightness. However, there are some inherent drawbacks such as poor color rendering index, poor emission homogeneity, and lower reproducibility in these white-LEDs. In addition, the emission color is largely determined by the input power, the luminescence properties of the phosphors and the phosphors coating thickness. Considerable investigations have been directed to tricolor phosphors for W-LEDs due to the recent development of near-UV LED chips [2–5]. Compared with the conventional phosphors, luminescent glasses are promising phosphor candidates for W-LEDs due to their specific advantages such as easy formation of any shape, higher transparency, simpler manufacture procedure and environment protection. Researches on luminescent glasses primarily focused on phosphate and borate based glasses in that rare earth and transition metal ions can be dissolved much more without heavily concentration quenching [6–8]. However, the poor chemical stability as well as the physical properties of these glasses limited their application. Borosilicate glasses are appropriate host materials for rare earth ions and transition metal ions due to their easy access of raw materials, lower melting temperature, excellent chemical stability and physical properties and higher transparency in UV and visible region [9, 10]. In this manuscript, we report on a new kind of Ce-Tb-Mn co-doped long-wavelength ultraviolet (346nm) excited white light emitting borosilicate luminescent glass. The color coordinate (x = 0.318, y = 0.333) is very close to the ideal white light. In these co-doped glasses, Ce³⁺ acts as a blue emitter as well as a donor transferring part of its excitation energy to Tb³⁺ and Mn²⁺ acceptors, which can emit green and red fluorescence, respectively.

2. Experiment

The borosilicate glass samples have the composition of 50SiO₂-14B₂O₃-8MgO-8ZnO-3.5ZrO₂-3Na₂O-0.4Gd₂O₃-0.03Sb₂O₃-10Al₂O₃-xCeO₂-yTb₂O₃-zMnO. The raw materials are analytical grade and chosen from corresponding oxides and carbonates. The reduction atmosphere is necessary to keep the doped ions in the appropriate valence state, which is realized by addition of carbon powder around the batch. The batches are put in corundum crucible and melted for two hours at 1580°C in a reduction atmosphere. Then, the melts were poured on the pre-heated steel mold and annealed for 2h at 550°C to obtain the final glasses.

The large surfaces of glasses were optically polished for subsequent measurements. Every sample was put at the same position and kept on the same angle related to excitation light. Excitation and emission spectra in the ultra-violet and visible wavelength were recorded on a JASCO FP-6500 fluorescence spectrophotometer equipped with a tunable excitation source range from 200nm to 750nm. The absorption spectra were performed by a Lambda 35 UV-Vis spectrophotometer equipped with a tunable wavelength range from 190nm to 2000nm. Photoluminescence decay measurements were performed by using an FLS920 fluorescence spectrophotometer. All of measurements were carried out at room temperature.

3. Results and discussion

Figure 1 shows the UV-visible absorption spectra of the host (SiB), the Tb³⁺ single doped (Tb), the Ce³⁺ single doped (Ce), the Mn²⁺ single doped (Mn) and the Ce³⁺– Tb³⁺–Mn²⁺ co-doped (CTM) borosilicate glasses. The inset shows the absorption spectra around 415nm. The host has only a strong absorption band near 200nm corresponding to its intrinsic absorption. It is shown from the spectra that the UV absorption cut-off wavelength of Ce³⁺ doped glass is obviously red-shifted compared with the host glass due to the allowed 4f→5d transition of Ce³⁺ ions. As for the Tb³⁺ doped glass, the absorption spectra is similar to the host and the UV cut-off wavelength is a little red-shifted corresponding to the electronic transitions from the ⁷F₆ ground state to the higher excited state. For the Mn²⁺ doped glass, the UV absorption cut-off wavelength is very close to the host glasses and the inset suggested that there is a Mn²⁺ absorption band around 415nm ascribed to its intrinsic absorption. It shows that there is a high absorbance in the near ultraviolet region by comparing the absorption spectra of the host and the sample co-doped with Ce³⁺, Tb³⁺ and Mn²⁺ ions (CTM). The co-doped sample has an absorption band centered at 415nm corresponding to the intrinsic absorption of Mn²⁺ ions [7, 11, 12]. Moreover, these glasses have high optical transparency due to their low absorptivity in the visible region, which is essential for the realization of high fluorescent quantum yields.

 

Fig. 1 The UV-visible absorption spectra of the host, the Tb³⁺ doped, Ce³⁺ doped, Mn²⁺ doped and Ce³⁺– Tb³⁺–Mn²⁺ co-doped melting borosilicate glasses. The inset shows the absorption band about 415nm. The Ce³⁺ doped glass has a Ce³⁺ concentration of 0.8 mol%. The Tb³⁺ doped glass has a Tb³⁺ concentration of 0.8 mol%. The Mn²⁺ doped glass has a Mn²⁺ concentration of 1.5 mol%. The Ce³⁺-Tb³⁺-Mn²⁺ co-doped glass has the same concentration with the single doped glasses.

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To manifest the absorption and the fluorescence of Ce³⁺, Tb³⁺, Mn²⁺ ions, we measured the emission and the excitation spectra of the single doped glasses shown in Fig. 2 . Figure 2(a), Fig. 2(b), and Fig. 2(c) show the emission spectrum (black) and the excitation spectrum (red) of cerium-doped, terbium-doped, and manganese-doped borosilicate glasses, respectively. Figure 2d shows the emission spectrum of cerium-doped (blue), the excitation spectra (green and red) of terbium-doped and manganese-doped borosilicate glasses. The excitation wavelengths for Ce³⁺ doped, Tb³⁺ doped and Mn²⁺ doped glasses are 346nm, 368nm and 417nm, respectively. The monitoring wavelengths for Ce³⁺ doped, Tb³⁺ doped and Mn²⁺ doped glasses are 405nm, 548nm and 607nm, respectively.

 

Fig. 2 Emission spectra (black) and excitation spectra (red) of Ce³⁺-doped (2a), Tb³⁺-doped (2b), Mn²⁺ -doped (2c) and the combination of the excitation spectrum (blue) of Ce³⁺-doped and the emission spectra (green and red) of Tb³⁺-doped and Mn²⁺ doped (2d) borosilicate glasses. For the single doped samples the excitation wavelengths are 346nm, 368nm and 417nm, respectively, and the monitoring wavelengths are 405nm, 548nm and 607nm, respectively. The Ce³⁺ -doped glass has a Ce³⁺ concentration of 0.8 mol%. The Tb³⁺-doped glass has a Tb³⁺ concentration of 0.8 mol%. The Mn²⁺-doped glass has a Mn²⁺ concentration of 1.5 mol%.

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For the Ce³⁺ single doped borosilicate glass (2a), there is a broad asymmetric blue emission band from about 350 nm to 500 nm under an UV excitation at 346 nm, which can be ascribed to the parity-allowed transitions of the lowest component of the 5d state to ²F_5/2 and ²F_7/2 levels of Ce³⁺ ions [6,7,11]. It is shown that there is only a strong excitation band from 250nm to 350nm centered at 340nm due to the 4f→5d transition of Ce³⁺ ions. As for the Tb³⁺ single doped borosilicate glass (2b), there are several emission peaks centered at 415nm, 438nm and 457nm which is corresponding to the ⁵D₃→⁷F_J (J = 5, 4 and 3) transition of Tb³⁺ ions, and there are also lots of narrow emission peaks at 490nm, 546nm, 588nm, and 624nm due to the ⁵D₄→⁷F_J (J = 6, 5, 4 and 3) transitions of Tb³⁺ ions. The ⁵D₄ emission provides strong green fluorescence. For the excitation spectrum of Tb³⁺-doped sample, it is indicated that there are several narrow excitation bands centered at 241, 280 and 313 nm and a broad excitation band centered at 373 nm by monitoring the 548 nm emission [2, 8, 11]. For the Mn²⁺ single doped borosilicate glass (2c), there is only a broad red emission band from 500nm to 700nm centered at 607nm, and there are several excitation bands centered at 280, 309, 355, 416 and 491 nm with a monitoring wavelength at 607 nm. The emission band of Mn²⁺- doped borosilicate glass can be associated with the ⁴T₁→⁶A₁ transition and the excitation bands can be assigned to the transitions from the ground state ⁶A₁ to higher levels of Mn²⁺ ions [6, 8, 11]. It is shown that the Mn²⁺ has weak excitation bands in long wavelength ultraviolet region, and cannot be efficiently excited by long wavelength UV light sources. Figure 2d shows that there is a spectral overlap between the emission spectrum of Ce³⁺ and the excitation spectrum of Tb³⁺. There is also a spectral overlap between the emission spectrum of Ce³⁺ and the excitation spectrum of Mn²⁺. According to Dexter’s theory for energy transfer, there is energy transfer probability between Ce³⁺ and Tb³⁺, and also between Ce³⁺ and Mn²⁺ by nonradiative energy transfer associated with resonance between donor (Ce³⁺) and acceptors (Tb³⁺ and Mn²⁺) [6, 11, 13]. It is shown from Fig. 2 that the Ce³⁺, Tb³⁺ and Mn²⁺ are blue, green and red emitters, respectively. Furthermore, The Ce³⁺ can be efficient sensitizer to transfer energy to Tb³⁺ and Mn²⁺.

In order to improve the emission of Tb³⁺ and Mn²⁺ under the long-wavelength UV excitation, Ce³⁺ was introduced into the Tb³⁺ and Mn²⁺ single doped melting borosilicate glasses [11]. Figure 3 shows the emission spectra (black) and excitation spectra (red) of Ce³⁺-Tb³⁺ co-doped (3a) and Ce³⁺-Mn²⁺ co-doped (3c) melting borosilicate glasses and the emission spectra of single doped (red) and co-doped (black) samples (3b, 3d) under the 346nm excitation. For the Ce³⁺-Tb³⁺ co-doped melting borosilicate glass (3a), the excitation and monitoring wavelengths are 346nm and 547 nm. There is a broad emission band about 400nm and several narrow peaks centered at 491, 547, 589 and 624nm. The broad emission band and narrow emission peaks are assigned to the 5d→4f transition of Ce³⁺ and ⁵D₄→⁷F_J (J = 6, 5, 4 and 3) transitions of Tb³⁺, respectively. Moreover, as for the excitation spectrum, there is a broad excitation band centered at 330nm due to 4f→5d transition of Ce³⁺. It is shown from Fig. 3b that the emission of Tb³⁺ is in the Ce³⁺-Tb³⁺ co-doped glass is approximately twice than that in the Tb³⁺ single doped glass due to the energy transfer from Ce³⁺ to Tb³⁺ [2, 6, 7, 11]. As for the Ce³⁺-Mn²⁺ co-doped melting borosilicate glass (3c), there are broad emission bands around 400nm and 604nm. The former is due to the 5d→4f transition of Ce³⁺ and the latter is due to ⁴T₁→⁶A₁ transition of Mn²⁺. The excitation spectrum consists of a strong band near 319nm and a weak band at 416nm with the monitoring wavelength at 604nm [6, 8, 11]. It is amazing that compared with the Mn²⁺ single doped glass, the Mn²⁺ ions in the Ce³⁺-Mn²⁺ co-doped glass have stronger emission whose intensity is enhanced at least eight times than that in single doped glass under the excitation of 346nm. It is shown that the emission of Mn²⁺ is remarkably improved due to the energy transfer from Ce³⁺.

 

Fig. 3 Emission spectra (black) and excitation spectra (red) of Ce³⁺-Tb³⁺ co-doped (3a) and Ce³⁺-Mn²⁺ co-doped (3c) melting borosilicate glasses. Emission spectra of Tb³⁺ single doped (red) and Ce³⁺-Tb³⁺ co-doped (3b) and Mn²⁺ single doped (red) and Ce³⁺-Mn²⁺ co-doped (3d) melting borosilicate glasses. The excitation wavelength is 346nm for all the glasses, and the monitoring wavelengths are 547nm and 604nm for the co-doped glasses, respectively. The Ce³⁺-Tb³⁺ co-doped glass has a Ce³⁺ concentration of 0.8 mol% and a Tb³⁺ concentration of 0.8 mol%. The Ce³⁺-Mn²⁺ co-doped glass has a Ce³⁺ concentration of 0.8 mol% and a Mn²⁺ concentration of 1.5 mol%.

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The decay curves of Ce³⁺ emission at 395nm are examined for the samples with and without the co-doped Tb³⁺ and Mn²⁺. Figure 4 presents the decay curves of Ce³⁺ ions emission for these samples. It can be seen from Fig. 4 that the decay curves of Ce³⁺ ions are well fitted with a second order exponenitial equation: I(t) = I(0) + Aexp(-t/τ₁) + Bexp (-t/τ₂) where I₀ and I are the luminescence intensity initially and at t, A and B are constants and τ₁ and τ₂ are lifetimes. In the absence of Tb³⁺ and Mn²⁺ ions, the lifetime of Ce³⁺ ions is 46.2ns. With the co-doped Tb³⁺ and Mn²⁺ ions, the lifetime of Ce³⁺ ions have decreased to 44.4ns and 40.4ns, respectively. These phenomena indicate that the excitation energy of Ce³⁺ ions is transferred to Tb³⁺ and Mn²⁺ ions.

 

Fig. 4 Decay curves of Ce³⁺ emission at 395nm in the single doped and co-doped melting borosilicate glasses under 310nm excitation. Sample a is the Ce³⁺ single doped glass with the concentration of 0.8 mol%. Sample b is the Ce³⁺-Tb³⁺ co-doped glass with the concentration of 0.8 mol% Ce³⁺ and 0.8 mol% Tb³⁺. Sample c is the Ce³⁺-Mn²⁺ co-doped glass with the concentration of 0.8 mol% Ce³⁺ and 1.5 mol% Mn²⁺. The red lines are fitted lines of decay curves corresponding to sample a, b, and c, respectively.

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Because the pure emission colors, the Ce-Tb-Mn co-doped glasses will be good candidates for white light emitting glasses. To obtain the optimal white light emitting, we prepared some samples by adjusting the concentration of different co-dopants. Figure 5 exhibits the emission spectra of Ce³⁺-Tb³⁺-Mn²⁺ co-doped glasses. The sample 5a has the same concentration of Ce³⁺ and Mn²⁺ with sample 5b and they have different Tb³⁺ concentration of 0.6 and 0.8 mol%, respectively. Simple 5c has the same concentration of Ce³⁺ and Tb³⁺ with the sample 5b and they have different Mn²⁺ concentration of 1.6 and 1.5 mol%, respectively. It is shown that there are blue, green and red emissions under 346nm excitation. It can be seen that the blue emission of the cerium decreases as the concentration of terbium and manganese increased. This can be ascribed to the energy transfer from Ce³⁺ to Tb³⁺ and Mn²⁺, the more concentration of Tb³⁺ and Mn²⁺, the more Ce³⁺ emission would be suppressed and facilitated the Tb³⁺ and Mn²⁺ emission. The blue emission of the terbium increases as the concentration of terbium increased, while decreases as the concentration of manganese increased. The red emission of the manganese decreases as the concentration of terbium and manganese increased. The former can be ascribed to the energy transfer from the Ce³⁺ to Tb³⁺, and the latter can be ascribed to the clustering of manganese ions. To further understand the white light emission of these samples in Fig. 5, we calculate their color coordinates according to the standard of CIE 1931, which has been characterized by CIE chromaticity diagram. And we also calculated the correlated color temperature (CCT) and color rendering index (CRI) of the co-doped glasses by the method discussed in the publicly available CIE 13.3-1995. The chromaticity varies with the change of the activator concentrations. The chromaticity points of sample 5a (x = 0.313, y = 0.292; CCT = 6807K, Ra = 82.9), sample 5b (x = 0.318, y = 0.333; CCT = 6202K, CRI = 72.5) and sample 5c (x = 0.308, y = 0.325; CCT = 6805K, CRI = 67.6) have been marked by the asterisks respectively in Fig. 5. All points mentioned above are located within the elliptic region which is the white light emitting region. It demonstrates that the color coordinate of simple 5b is very close to the ideal white light. Although simple 5a has a higher CRI of 82.9 and lower CCT of 6807K compared with a white LED fabricated using YAG: Ce³⁺ phosphor pumped with a blue InGaN chip, the CRI and CCT of the samples in Fig. 5 are generally low. In the follow-up experiments, we will make effects to increase the color rendering index and decrease the correlated color temperature of the Ce-Tb-Mn co-doped melting borosilicate glasses.

 

Fig. 5 Emission spectra of Ce³⁺-Tb³⁺-Mn²⁺ co-doped melting borosilicate glasses under 346nm excitation. Sample 5a has a Ce³⁺ concentration of 0.8 mol%, a Tb³⁺ concentration of 0.6 mol% and a Mn²⁺ concentration of 1.5 mol%. Sample 5b has a Ce³⁺ concentration of 0.8 mol%, a Tb³⁺ concentration of 0.8 mol% and a Mn²⁺ concentration of 1.5 mol%. Sample 5c has a Ce³⁺ concentration of 0.8 mol%, a Tb³⁺ concentration of 0.8 mol% and a Mn²⁺ concentration of 1.6 mol%. The CIE (X, Y) coordinate diagram shows the chromaticity points of sample 5a (1), sample 5b (2) and sample 5c (3), respectively. The inset shows the fluorescence photographs of white-emitting of sample 5a, 5b and 5c under 346nm UV lamp irradiation.

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4. Conclusions

In conclusion, the melting borosilicate glasses co-doped with Ce³⁺, Tb³⁺ and Mn²⁺ ions were successfully prepared. The color coordinate, the correlated color temperature and the color rendering index were calculated to manifest the color properties of these glasses. The color coordinate (x = 0.318, y = 0.333) is very close to ideal white light under long wavelength ultraviolet excitation. The energy transfer of these Ce³⁺-Tb³⁺-Mn²⁺ co-doped borosilicate glasses have been studied. Due to their ideal white light under long wavelength ultraviolet excitation, the Ce³⁺-Tb³⁺-Mn²⁺ co-doped glasses have a potential for application in optical fibers, LED displays and fluorescent lamps.