We report on the preparation of transparent and colorless green-and red-emitting luminous glasses by sintering high-silica porous glass impregnated with rare-earth ions. These kinds of glasses can be efficiently excited by near-UV sources. The fluorescence of the glasses under near-UV excitation is dependent on energy transfer processes. In order to obtain strong visible emission, it is necessary to co-dope some optically inert rare-earth ions into the glasses. The roles of the optically inert rare-earth ions are discussed.
© 2009 Optical Society of America
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 , 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 . 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 , constitute optical units for producing the three primary colors, and provide new candidates for glass lighting and displays.
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.
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→7FJ 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.
For the 5D3 emission, there are several emission peaks at 379, 415, 438, and 458 nm corresponding to 5D3→7FJ (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→7FJ) 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.
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 , 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.
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+.
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.
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).
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.
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.
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