1. Introduction

Recently, the rapid development and enormous success of LEDs operating in the near-UV to visible spectrum have provided alternative excitation sources for lighting and displays that are environment friendly and energy saving. The interest in LED-excited luminescent materials has increased drastically during the past decades [1, 2]. The luminescent materials are mainly composed of powders of polycrystals doped with rare-earth ions or transition metal ions. Compared with crystalline powders, glasses possess specific advantages such as easy formation of any shape, transparency, and low cost. Therefore, luminescent glasses play an indispensable role in many LED applications. However, despite the extensive research on numerous glass systems doped with rare-earth ions, the number of commercially available luminescent glasses is quite limited [3], because rare-earth ions tend to cluster and quench luminescence. Until recently, researchers mainly focused on rare-earth ion-doped borate and phosphate glasses, because these types of host matrices can dissolve much more rare earth ions without heavily concentration quenching [4–7]. Considering the glass matrix, the silicate glasses especially the high silica glasses are attractive for luminescence due to their excellent optical and mechanical properties. However, the lower solubility of rare-earth ions in high-silica glass can lead to segregation and phase separation even at ppm concentration. Recently, our group developed a new method to overcome the above problems associated with doping rare-earth ions in high-silica glass [8–10]. The method is based primarily on sintering of porous glass with nano-sized pores impregnated with rare-earth ions. Our experiments demonstrated that rare-earth ions can be doped into high-silica glass matrices up to high concentration without encountering the above mentioned problems. By implementing this method, we obtained a series of transparent and colorless blue luminous glasses with high emission yields; in particular, we obtained Eu²⁺-doped high-silica glass, which can even be effectively excited by near-UV LEDs [10]. In this paper, we report on near-UV excited green and red luminous high-silica glasses. To the best of our knowledge, this is the first observation of intense green and red luminescence in high-silica glasses under long-wavelength UV excitation. These two types of luminous glasses, along with the blue luminous glass previously developed by us [10], constitute optical units for producing the three primary colors, and provide new candidates for glass lighting and displays.

2. Experiment

Tb³⁺ and Eu³⁺ were chosen as emitters in our experiments; Ce³⁺ and the vanadate group were chosen to sensitize Tb³⁺ and Eu³⁺, respectively. Some optically inert rare-earth ions were also introduced into the glasses. The procedure for preparing glass was described in our previous reports [8–10]. Tb³⁺, Ce³⁺, La³⁺, Eu³⁺, V⁵⁺, and Y³⁺ were introduced into the glasses by immersing the porous glass into 0.2–1.5 M solutions containing corresponding nitrate or sulfate compounds. After drying in air for 1 hour, the glasses were sintered at 1120°C in a reducing atmosphere (for green luminous glass) or an air atmosphere (for red luminous glass) for 2 hours. Finally, a series of compact, transparent, and colorless glasses were fabricated.

Then the large surfaces of glasses were optically polished for subsequent measurements. All of the samples have thickness of 1mm. In order to reduce the artificial error, we put every sample on same position and keep them on same angle related to excitation light. The fluorescence spectra and excitation spectra were measured by using a Hitachi F-4500 fluorescence spectrophotometer equipped with a 150W Xenon lamp source. PL decay measurements were performed by using the 248nm laser from KrF excimer laser (Lambda Physik, COMPex 102) with a pulse width of 200nm and a repetition rate of 2Hz. The emission was detected by a photomultiplier tube and the signal was fed to a digital oscilloscope (Iwatsu-LeCroy LC564A). All of measurements were carried out at room temperature.

3. Results and discussion

Figure 1 shows the emission (upper) and excitation (lower) spectra of Tb³⁺-doped, Tb³⁺−Ce³⁺-codoped, and Tb³⁺−Ce³⁺−La³⁺-codoped sintered porous glasses. The excitation and monitoring wavelengths are 350 and 543 nm, respectively.

 

Fig. 1. Emission spectra (upper) and excitation spectra (lower) of Tb³⁺doped (a), Tb³⁺−Ce³⁺-codoped (b), and Tb³⁺−Ce³⁺-La³⁺-codoped (c) sintered porous glasses. The excitation and monitoring wavelengths are 350 and 543 nm, respectively. Tb³⁺-doped glass has a Tb³⁺ concentration of 0.3mol%. Tb³⁺−Ce³⁺-codoped glass has a Tb³⁺ concentration of 0.3mol% and a Ce³⁺ concentration of 0.08mol%. Tb³⁺−Ce³⁺-La³⁺-codoped glass has a Tb³⁺ concentration of 0.3mol%, a Ce³⁺ concentration of 0.07mol% and a La³⁺ concentration of 0.95mol%.

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For the Tb³⁺-doped porous glass (a), there is almost no emission under 350nm excitation and only a broad excitation band near 238 nm by monitoring 543nm emission. However, for the Tb³⁺−Ce³⁺-codoped glass (b) and the Tb³⁺−Ce³⁺−La³⁺-codoped glass (c), there are broad emission bands around 400 nm, and several narrow peaks centered at 488, 543, 588, and 622 nm. Furthermore, from their excitation spectra, it is shown that there are broad excitation bands centered at 238 and 305 nm. The broad emission band and narrow emission peaks are due to the 5d→4f transition of Ce³⁺ and ⁵D₄→⁷F_J transitions of Tb³⁺, respectively [6–8]. The broad excitation band near 238 and 305 nm corresponds to the 4f→5d transition of Tb³⁺ and f→d transition of Ce³⁺, respectively [6–8]. It was shown that by codoping of a small amount of Ce³⁺ with Tb³⁺ into glass, the emission of Tb³⁺ can be significantly increased due to energy transfer from Ce³⁺ to Tb³⁺ [6-8]. It is amazing that, compared with the Tb³⁺−Ce³⁺-codoped glass, the Tb³⁺ ions in the Tb³⁺−Ce³⁺-La³⁺-codoped glass have stronger excitation bands and stronger emission whose intensity is enhanced at least 20 times than that in Tb³⁺−Ce³⁺-codoped glass. It is obvious that La³⁺ plays an important role in increasing the emission of Tb³⁺.

To investigate the role of La³⁺ in the glass, we prepared glass samples with the same Tb³⁺ concentration and different La³⁺ concentrations. Figure 2(a) and (b) exhibit the ⁵D₃ and ⁵D₄ emission spectra of Tb³⁺ under 254nm irradiation.

 

Fig. 2. (a) and (b) are ⁵D³ and ⁵D⁴ emission spectra of Tb³⁺-La³⁺-codoped glasses under 254-nm excitation. (c) and (d) are the excitation spectra of ⁵D³ and ⁵D⁴ emission of Tb³⁺ obtained by monitoring 379nm and 543nm emissions, respectively. All of the glasses have Tb³⁺ concentration of 0.3mol%. The concentration of La³⁺ is 0mol% (A), 0.55mol% (B) and 0.95mol% (C), respectively.

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For the ⁵D³ emission, there are several emission peaks at 379, 415, 438, and 458 nm corresponding to ⁵D³→7F_J (J = 6, 5, 4, and 3) transitions, respectively. It is shown that the emission intensity of ⁵D³ gradually decreases with increasing La³⁺ concentration. However, for the ⁵D⁴ emission (⁵D⁴→7F_J) of Tb³⁺, the intensity can be greatly increased with increasing La³⁺. Corresponding to the ⁵D³ and ⁵D⁴ emission spectra, Figure 2 (c) and (d) show the excitation spectra of Tb³⁺ obtained by monitoring 379nm (⁵D³→7F₆) and 543nm (⁵D⁴→7F₅) emissions, respectively. The excitation band of 379nm decreases when increasing of La³⁺ concentration. However, the excitation band of 543nm increases with the increasing of La³⁺ concentration. All above results implies the occurrence of cross-relaxation processes among Tb³⁺ ions [11–13].

A well-investigated cross-relaxation process involving energy transfer between two Tb³⁺ ions deexcites the ⁵D³ level and populates the ⁵D⁴ levels. This process prevents the ⁵D³ emission and facilitates the ⁵D⁴ emission; it can be expressed as [11–13]:

Assuming that the interaction scheme is multipolar interaction type, the ⁵D³ decay curves could be analyzed using the well-known direct quenching mechanism. There is no energy migration effect or less energy migration effect in the ⁵D³ level because it is obviously suppressed by faster cross relaxation. Usually, the interaction scheme of 5D³ emission involes both the dipole-dipole and the dipole-quadrupole interactions, which can be described by Inokuti-Hirayama formula [11–14]. We measured the fluorescence decay from 5D³ level of Tb³⁺ ions in these samples, which is shown in Figure 3. It is found that the fluorescence decay can not be exactly simulated by dipole-dipole or the dipole-quadrupole interaction on their own. Doping of rare-earth ions into porous glass leads to their accumulation at the internal surface of the pores and channels of porous glass. During subsequent sintering process, parts of rare-earth ions tend to cluster because the ions have high coordination numbers and must share the limited non-network oxygen atoms in the glass matrix [14, 15]. It is reasonable that single doping of Tb³⁺ into porous glass results in clustered (or quenched) as well as non-clustered Tb³⁺ ions. The clustered Tb³⁺ ions have no or less contribution to the emission. The non-clustered Tb³⁺ ions can efficiently emit fluorescence from the ⁵D³ and ⁵D⁴ levels, and can be classified into two types due to the distance between Tb³⁺ ions, one isolated and with no interaction with other ions, another with cross-relaxation by multipolar interaction. Therefore, the fluorescence from the former should decay exponentially and the latter should decay according to Inokuti-Hirayama formula.

 

Fig. 3. Normalized fluorescence decay of 5D³→7F₆ emission under 248nm excitation. The excitation source is KrF excimer laser with pulse width of 200ns and repetition rate of 2Hz. All of the glasses have Tb³⁺ concentration of 0.3mol%. The concentration of La³⁺ is 0mol% (A), 0.55mol% (B) and 0.95mol% (C), respectively. The red lines are fitted lines of decay curves corresponding to sample A, B, and C, respectively.

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In such a case, the decay fitting was performed using the sum of exponential decay formula and an Inokuti-Hirayama formula with d-d interaction type [16], that is:

The first term in the right side represents the emission without cross-relaxation and the second term is the rate equation taking account of the cross-relaxation. Where, A is the contributive parameter; τ is the intrinsic lifetime of a single ion (0.85msec for our samples measured for a sample with very low Tb³⁺ concentration of 0.05mol%); C is the number of acceptors per unit volume; C₀^-1 is the volume of donor’s sphere of influence (=4πR₀ ³/3, R₀ is the critical separation between donor and acceptor, at which the nonradiative rate equals that of the internal single ion relaxation); the ratio of C/C₀ means the number of acceptors in donor’s sphere of influence. the fitting result shows that the analysis using the above scheme produces a relatively good fit between the theoretical calculation and the experiment data. The fitting parameters are shown in Table 1. It is shown that the contributive parameter A gradually decrease with the increasing of La³⁺ concentration, which means that with increasing La³⁺ concentration dipole-dipole interaction will dominate the emission. The resultant C/C₀ values were found to be from ~3.33 to ~10.46 with increasing La³⁺ concentration. In a glass system, the C₀ value usually has less variation, in this point, the C value increases greatly. The C is the number of acceptors per unit volume, that is, Tb³⁺ concentration itself in this case. According to our previous assumption, it refers in particular to the concentration of non-clustered Tb³⁺ with cross-relaxation. It is noticeable that the C₀ value is hardly to be determined because we just know the tendency of C value.

Table 1. Parameters for the Inokuti and Hirayama Mechanism for the 5D³→7F₆ transition of Tb³⁺ in sintered porous glass.

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Combined with the fact that introducing La³⁺ into porous glass decreases the ⁵D³ emission and increases the ⁵D⁴ emission, it is reasonable that adding of La³⁺ shortens the distance of emitting Tb³⁺ ions, which implys an increase in the local concentration of emitting Tb³⁺.

In porous glasses, ion clustering occurs because the rare earth ions must accumulate at the surfaces of glass pores to share the limited non-bridging oxygens. Furthermore, the pores are usually connected and elongated with various shape and size, which vary randomly throughout space.

It is suggested that codoping Ce³⁺ and Tb³⁺ into the glass results in Ce³⁺, Tb³⁺, and Ce³⁺-Tb³⁺ clusters coexisting with non-clustered Ce³⁺ and Tb³⁺ ions because the Ce³⁺ has diameter nearly equal to that of Tb³⁺. Ce³⁺ ions can transfer energy to Tb³⁺ ions located in appropriate distance. However, because there are highly clustered Ce³⁺ and Tb³⁺, the energy transfer efficiency between non-clustered ions is low, so that the emission intensity of Tb³⁺ is weak. With regard to the Tb³⁺−Ce³⁺-La³⁺-codoped glass, it is obvious that the La³⁺ ions play an important role of increasing the Tb³⁺ emission. Introducing of La³⁺ ions results in coexistence of Tb³⁺ clusters, La³⁺ clusters, and La³⁺-Tb³⁺ clusters. La³⁺ ions replaced parts of clustered Tb³⁺ ions, and the replaced Tb³⁺ ions become free, therefore the local concentration of non-clustered Tb³⁺ is greatly increased. The same proposal can also be applied in explaining with the Ce³⁺ clusters. That means addition of La³⁺ not only prevent from clustering of Tb³⁺, but also prevent from clustering of Ce³⁺, which will increase the emission intensity of Ce³⁺. A graphical representation is shown in Fig. 4. Therefore, the Ce³⁺−Ce³⁺, Ce³⁺-Tb³⁺, and Tb³⁺- Tb³⁺ distances are greatly shortened by introducing of La³⁺ into the glass. This not only favors the energy transfer between Ce³⁺ and Tb³⁺, but also facilitates the cross-relaxation between the ⁵D³ and ⁵D⁴ levels of Tb³⁺. Therefore, the ⁵D⁴ emission of Tb³⁺ can be enhanced greatly. It seems as if the La³⁺ plays a role of valve controlling the stream of non-clustered Tb³⁺.

 

Fig. 4. Microscopic interpreting of the role of La³⁺: (a) Tb-Ce co-doping. There is always coexisting of quenched Tb³⁺ and emitting Tb³⁺. (b) Tb-Ce-La co-doping. The Tb³⁺ clusters are separated by La³⁺. It seems as if the Tb³⁺ clusters release parts of Tb³⁺, which are capable of emitting.

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Fig. 5. Legend for Microscopic schematics

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We also prepared some red-emitting high-silica glass by codoping of Eu³⁺, V⁵⁺, and Y³⁺ into porous glass. As expected, Y³⁺ ions play a key role in increasing the Eu³⁺ emission. Figure 6 compares the emission and excitation spectra of Eu³⁺ single-doped, Eu³⁺, V⁵⁺ double-doped, and Eu³⁺, V⁵⁺-Y³⁺-codoped porous glasses. The excitation and monitoring wavelength is 350nm and 616nm, respectively.

 

Fig. 6. Emission and excitation spectra of Eu³⁺ in (a) Eu³⁺ doped, (b) Eu³⁺, V⁵⁺ codoped, and (c) Eu³⁺, V⁵⁺-Y³⁺ codoped glasses. Excitation and monitoring wavelengths are 350 and 616 nm, respectively. Eu³⁺ doped glass has a Eu³⁺ concentration of 0.7mol%. Eu³⁺, V⁵⁺ codoped glass has a Eu³⁺ concentration of 0.7mol% and a V⁵⁺ concentration of 1.5mol%. Eu³⁺, V⁵⁺-Y³⁺ codoped glass has a Eu³⁺ concentration of 0.7mol% and a V⁵⁺ concentration of 1.44mol% and a Y³⁺ concentration of 1.8mol%.

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It is shown that there is nearly no any emission for Eu³⁺ single-doped glass under 350-nm excitation. Adding of V⁵⁺ can increase the ⁵D₀→⁷F₂ emission of Eu³ due to energy transfer process from the vanadate group to Eu³⁺ by mechanism of dipole-dipole or exchange interaction [17, 18]. However, the transfer process has low efficiency due to highly clustering of Eu³⁺ in porous glass. It shows that this situation can be overcome by introducing Y³⁺ into the Eu³⁺, V⁵⁺ co-doped porous glass. The proposal to explain the Tb³⁺-Ce³⁺-La³⁺-codoped glasses can also be applied in explaining with this kind of glass.

Figure 7 shows the fluorescence photographs of red- (a), green- (b) and blue-emitting (c) luminous glass under near-UV (365 nm) irradiation. As a comparison, Figure 7 also includes fluorescence photographs of Eu³⁺, V⁵⁺-codoped glass (d) and Tb³⁺-Ce³⁺-codoped glass (e).

 

Fig. 7. Photographs of luminous glasses exhibiting red, green, and blue emission under 365-nm irradiation. The a, b, c, d, and e samples correspond to Eu³⁺, V⁵⁺-Y³⁺-codoped glass, Tb³⁺−Ce³⁺−La³⁺-codoped glass, Eu²⁺-doped glass, Eu³⁺, V⁵⁺-codoped glass, and Tb³⁺−Ce³⁺-codoped glass, respectively. All samples were prepared from sintered porous glass impregnated with rare-earth ions.

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It is shown that, under 365-nm excitation, the luminescences of the Eu³⁺, V⁵⁺-Y³⁺-codoped and Tb³⁺-Ce³⁺-La³⁺-codoped glasses are comparable to that of the Eu²⁺-doped glass. Similar results can be obtained by introducing different optically inert rare-earth ions into porous glasses. Thus, this preparation method is very efficient and successful in obtaining luminous glasses suitable for near-UV excitation.

4. Conclusions

In conclusion, we prepared green-emitting and red-emitting luminous glasses, which can be efficiently excited by near-UV sources. It is suggested that optically inert rare-earth ions play a key role in preventing the formation of clusters and facilitating the energy transfer between non-clustered sensitizers and activators. Due to their strong emissions under near-UV excitation, the colorless transparent fluorescence glasses developed in this study have a potential for application in lasers, fiber lasers and amplifiers, solar concentrators, displays, fluorescent lamps, and transparent phosphors used in special conditions such as high-temperature and high-humidity environments.