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The introduction of silver into the Eu3+-doped sodium–aluminosilicate glasses prepared by Ag+-Na+ ion exchange leads to the formation of different ionic silver species. Under 350nm excitation, effective enhancement of Eu3+ luminescence is ascribed to energy transfer (ET) from silver aggregates to Eu3+ and White light emission was realized by combining red light emission of Eu3+ with blue/green light emission of silver aggregates. The formation of silver aggregates such as Ag2+ was promoted by Eu2+ in the heat treatment process. The local field enhancement effect due to SPR of Ag NPs was not observed. Our research will help to understand the interaction between different types of Ag species and europium ions.
©2013 Optical Society of America
1. Introduction
The use of noble metals as a means for improving the spectroscopic properties of rare-earth (RE)-doped materials for functional applications has recently attracted persistent attention [1–8]. It was first reported by Malta in [1] where samples of Eu3+-doped fluoroborate glasses containing Ag NPs were investigated. Large enhancement of the Eu3+ luminescence was observed due to the local field effect (LFE) induced by surface plasmon resonance (SPR) of Ag NPs. Sequentially many investigations on enhancement of REI emissions in Ag-REI co-doped glasses were reported [2–14]. In fact, both enhancement and quenching of luminescence have been reported. On a practical perspective, understand the interactions between metal and RE is more importation. Recently, some researchers [9–11,13,14] ascribe this enhancement effect to energy transfer (ET) from very small molecule-like, non plasmonic silver particles (ML-Ag) or isolated Ag+ to REI, but not to the LFE induced by SPR. Rademann et al [9] developed a technique to produce molecule-like noble metal particles in soda-lime silicate glasses by synchrotron X-ray activation and ascribe the enhancement effect to ET from ML-Ag to REI. Hai Guo also reported an energy transfer process from ML-Ag-particles to Eu3+ in the Ag and Eu3+ co-doped oxyfluoride glasses [10]. We can conclude that the enhancements of REI luminescence were caused by either LFE induced by SPR of Ag NPs or ET from ML-Ag and isolated Ag+. However, given the difficulties in controlling the fabrication of different types of Ag species in Ag-REI co-doped glasses, it is important to discriminate when the effects observed are due to a different, coexistent process.
Silver-sodium ion exchange in silicate glasses is one of the most exploited approaches for the production of passive and active optical devices on glass substrates [15–17]. Different ionic silver species are introduced into the glass by silver-sodium ion exchange. Optical properties of these different ionic silver species in silver ion-exchanged glasses have been widely studied because they lead to a novel broad emission, covering 350nm to 610nm [18–28].
In this article, the introduction of silver into the europium-doped sodium–aluminosilicate glasses prepared by Ag+-Na+ ion exchange leads to the formation of complex optical centers involving europium ions and different ionic silver species. After heat treatment, silver nanoparticles can be formed in the silver ion-exchanged glasses. Upon silver ion-exchanged and heat treatment silver glasses, this enabled us to analyze the sensitization of the luminescence of rare earth ions by different ionic silver species and the influence of silver nanoparticles on the intensity of optical transitions of rare earth ions under investigation. By this way, the enhancement mechanisms may be distinguished from each other without doubt in same system.
2. Experimental
Glasses of the 10 g batch of the composition of 55SiO2-5Al2O3-10Na2O-10MgO-20ZnO-1Eu2O3 were prepared by using SiO2 (99.99%), Al2O3 (99.99%), Na2CO3 (99.99%), MgO (99.99%), ZnO (99.99%) and Eu2O3 (99.99%). Then the sample was melted at 1600°C in an alumina crucible in air for an hour. After the synthesis the crucibles were extracted from the furnace, the melt was poured onto a stainless steel and cooled down to room temperature. The obtained glasses were annealed at 600°C for 4 h to remove thermal strains. The bulk glass samples were cut to a thickness of 3mm and polished to optical quality before subjecting them to the optical measurements. After polished, the glasses were in a molten salt bath formed by a mixture of 95mol% NaNO3 and 5mol% AgNO3 in a crucible of Al2O3. The ion exchanges took place at a temperature of 360°C with a processing time of 30min or 120min. The reference sample (indicated as GEu) did not have ion-exchanged. The Al2O3 crucible held in a vertical furnace, in which the temperature was controlled to within ± 1°C. After inter-diffusion, samples were removed from the molten bath and washed with distilled water and alcohol to remove any silver nitrate adhering to their surface. To promote the Ag NPs formation, the samples were further heat treatment at a temperature of 500°C during 4 h in a furnace.
The optical absorption spectra of different glasses were measured on a HITACHI U–4100 type spectrophotometer. The photoluminescence (PL) spectra were measured with a HITACHI F-7000 fluorescence spectrophotometer, using a static 150 W Xe lamp as the excitation source. All the measurements were taken under room temperature.
3. Results and discussion
3.1 Silver ion-exchanged glasses
Figure 1 shows the excitation spectra of silver ion-exchanged glasses, monitored at 615nm emission due to the 5D0-7F2 transition of Eu3+ ions. It can be clearly seen that excitation spectra of GEu sample consist of two parts: one broad band at 260 nm and several sharp peaks in 300-550 nm region. Obviously, the first one results from the well-known O2−-Eu3+ charge transfer (CT) band, and the other sharp peaks are related to f-f transitions of Eu3+ ions. The strongest excitation peak at 393 nm is ascribed to7F0→5L6 transition of Eu3+ ions. In addition to the characteristic excitation peaks of Eu3+, excitation spectra of silver ion-exchanged glasses show two surprising broad excitation bands centered around 280 and 350 nm, respectively.
The PL spectra under 350nm excitation of silver ion-exchange samples are shown in Fig. 2. Emission spectra of GE consist of a broad band the maximum at 436nm and some weak sharp emission peaks corresponding to 5D0-7F0-4 transitions of Eu3+. The influence of glass matrix on the broad emission can be excluded, because no emission can be detected excited by 350 nm light for basic glass without Eu2O3 dopants and silver ion exchange. A similar broad emission around 430 nm from Eu2+ was also reported [29,30]. Therefore, the broad emission at 430nm may be assigned to Eu2+ 5d-4f transition. The intensity of Eu3+ emissions in GEuAg30 and GEuAg120 samples increases obviously than single-doped sample. Besides, an additional strong broad emission at 400–700 nm was also observed. In order to confirm the origin of the additional emission bands (400-700 nm), silver ion exchange GAg sample was prepared. A broad band at 400-700 nm with a peak at 450nm is observed in emission spectra, which was commonly attributed to silver aggregates such as Ag2+. The amounts of silver aggregates increase with increasing ion exchange time, while the intensity of Eu3+ emissions monotonously increases with increasing ion exchange time. Such phenomena may prove that the enhancement of Eu3+ emission under 350nm excitation is due to ET from silver aggregates to Eu3+. This ET process is also supported by the excitation spectra in Fig. 1.
A novel emission band about 410nm is observed in ion-exchanged glasses excited by 280nm in Fig. 3. Based on literatures [21,23,31], excitation at 280 nm and emission at 410 nm are related to 4d10 (1S0)–4d95s1 (3D1) transition of isolated Ag+. In addition, it is worth noting that the emission intensity of Ag+ decreases largely in Eu3+-doped silver ion-exchanged glasses. Only weak blue/green light observed. Therefore, we assume that the following reactions may occur in the process of silver ion exchange:
So in the process of silver ion exchange, a large number of Ag+ ions were reduced to Ag0. According to the emission intensity of blue/green light, the degree of reaction (2) is weak. What’s more, under 280nm excitation, the intensity of Eu3+ emissions in silver ion-exchanged samples increases slightly. It may prove efficient ET from isolated Ag+ to Eu3+.It is surprising to notice that the emission band of silver aggregates covering blue and green regions and the emission band of Eu3+ ions located at red region. Combing their emissions, white emission may be achieved. Luminescence colors of glasses are characterized by Commission International de I’Eclairage (CIE) chromaticity diagram and shown in Fig. 4. The inset of Fig. 4 gives luminescent photos (λex = 350nm) for GEu, GEuAg30 and GEuAg120 samples. The color shifts from blue to white with increasing ion exchange time. The color is white for GEuAg30 (X = 0.2705, Y = 0.2718) and GEuAg120 (X = 0.3077, Y = 0.313). The excitation source that could be used to generate white emission is 350nm UV light, matching well with the emission of UV LED chips. This characteristic indicates that silver ion-exchanged europium doped glasses can act as promising white-emitting phosphors for UV LED chips.
Fig. 4 The CIE chromaticity diagram of emission for silver ion-exchanged glasses upon 350nm excitation.
3.2 Heat treatment silver glasses
Figure 5 shows the optical absorption spectra of all heat treatment samples. The obvious absorption bands located at 393 and 464nm in GEu sample can be attributed to the absorptions from7F0 ground state to 5L6 and 5D2 excited states of Eu3+. A strong and broad absorption band peaking at 440nm was obviously observed in GEuAg30 and GEuAg120 samples. This absorption band is the characteristic of SPR of Ag NPs in glass [4,32,33].
Figure 6 shows the excitation spectra of the heat treatment silver glasses, monitored at 615nm emission (5D0-7F2). Excitation spectra in Fig. 6 indicate that the luminescence of Eu3+ ions may increase or decrease in the presence of Ag NPs. Excitation spectra of the heat treatment silver glasses still show a surprising broad excitation band covering from 280nm to 360nm. But its shape is different with that in silver ion exchange samples. Other excitation spectra of Eu3+ almost disappeared.
In order to confirm the difference of the excitation bands (280-360 nm) in silver ion exchange and heat treatment silver glasses, emission spectra (under 350nm excitation) for GEuAg120, 1HT GEuAg120, 2HT GEuAg120 and GEu are presented in Fig. 7. After first heat treatment (indicated as1HT GEuAg120), the emission intensity of Eu3+ increases obviously, and the intensity of broad emission from 400nm to 700nm which ascribed to silver aggregates also increases obviously. Subsequently, a second heat treatment was conducted. After second heat treatment, the dark brown of the glass coloration become deep, indicating more Ag NPs are formed at the cost of silver aggregates reduce. And the intensity of broad band emission and Eu3+ emission simultaneously decreases significantly. It is reasonable for us to assume the Eu3+ emission enhancement mechanism is ET from silver aggregates to Eu3+. A local field enhancement effects due to SPR of Ag NPs has not been observed. In addition, it is worth noting that the reason for the broad band emission intensity increases after the first heat treatment has not been explained and after the second heat treatment, in spite of the broad band emission intensity lower than that in silver ion-exchanged sample, the emission intensity of Eu3+ increases slightly. The most likely reason is that the reactions (1) and (2) were promoted by heat treatment. In this study, the amount of silver aggregates such as Ag2+ and Eu3+ increases obviously in the heat treatment process. So after the second heat treatment, the emission intensity of Eu3+ increasing slightly is attributed to Eu3+ concentration increasing. Further study focusing on the reactions is ongoing.
4. Conclusions
In this article, the introduction of silver into the Eu3+-doped sodium–aluminosilicate glasses prepared by Ag+-Na+ ion exchange leads to the formation of different ionic silver species, which represent Ag+ isolated, silver aggregates, with characteristic luminescence spectra. Under 350 nm excitation, effective enhancement of Eu3+ luminescence is ascribed to ET from silver aggregates to Eu3+ and White light emission was realized by combining red light emission of Eu3+ with blue/green light emission of silver aggregates. The formation of silver aggregates such as Ag2+ was promoted by Eu2+ in the heat treatment process. The local field enhancement effect due to SPR of Ag NPs was not observed. Our research will help to understand the interaction between different types of Ag species and europium ions and may provide a means to distinguish the enhancement mechanisms from different types of Ag species on luminescence of rare earth ions.
Acknowledgments
This work was supported by a grant from the National Natural Science Foundation of China (No. 51002068; 51272097; 61265004), 973 Program (No. 2011CB211708), National Natural Science Foundation for Youths of Yunnan (No. 2012FD009).
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