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Abstract
Molecular-like silver nanoclusters (ML-Ag NCs) with size dependent tunable luminescence properties and high-quantum yield has been explored as a new type of sensitizer for rare earth (RE) ions in glasses recently. In this research, the borosilicate glasses containing ML-Ag NCs and RE3+ (RE = Sm, Eu, Tb) ions were prepared with melt-quenching method. The absorption, TEM and steady spectra measurements indicated that compare with Sm3+ and Tb3+, the introduction of Eu3+ can more effectively promote the formation of luminescent ML-Ag NCs and their further aggregation. Besides the predictable efficient energy transfer from ML-Ag NCs to a single type of RE3+ ion in the codoped glasses, the simultaneously sensitization of Sm3+/Eu3+ and Sm3+/Tb3+ couples by ML-Ag NCs were further realized in the tri-doped glasses. Benefited from the excitation wavelength dependence of energy transfer from ML-Ag NCs to Sm3+/Eu3+ and Sm3+/Tb3+ couples and excitation efficiency on ML-Ag NCs and RE3+ ions, the tri-doped glasses exhibit broad tunable emission simply by changing the excitation wavelength, and the white light emission was achieved in GAgSmEu under UV excitation.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
ML-Ag NCs are nanoscale aggregates that contain only a few Ag atoms and/or ions with sizes commensurate to the Fermi wavelength of electrons. In this size regime, the strong quantum confinement of free electrons leads to discrete electronic states and thus the ML-Ag NCs exhibit size dependent optical properties [1–3]. On the other hand, because of high surface-to-volume ratio, the ML-Ag NCs are extremely easy to aggregate into bigger particles unless they are stabilized in proper host. Besides the organic scaffolds such as polymers and DNA, the glasses are desirable inorganic matrix to stabilize ML-Ag NCs due to the immobility of the glass network [4].
Benefiting from brightness, photostability and tunable luminescence properties, the glasses that contain ML-Ag NCs are now increasingly investigated, and the preparation methods include sol-gel, melt-quenching, ion-exchange, ion implantation and so on [4–9]. When dispersed in glasses, the ML-Ag NCs give off intense broadband emission, and their emission property can be modified by codoping of RE ions through energy transfer. So far, the luminescence kinetics of ML-Ag NCs dispersed in glasses have been studied with quantum chemistry methods [10,11], and the energy transfer from ML-Ag NCs to a single type of RE ions such as Sm3+, Yb3+, and Eu3+ have been widely reported to realized diverse potential applications. For example, the efficient energy transfer from ML-Ag NCs to Yb3+ ions was realized in the borosilicate glasses and the oxyfluoride glasses respectively to benefit the improvement of photovoltaic conversion efficiency [12–14]. The tunable emission can be obtained in the ML-Ag NCs /Eu3+ codoped and ML-Ag NCs /Sm3+ codoped glasses by changing the excitation wavelength [15–17]. Besides, the generation of white light emission in the ML-Ag NCs single-doped glasses was also reported in the SiO2-Al2O3-CdF2-PbF2-ZnF2 glasses [18,19]. However, it is not easy to obtain white light emission by using ML-Ag NCs as the only emission center, because the broad emission band of ML-Ag NCs was generally limited in the blue-to-green region. Therefore, the red emission centers is necessary to be introduced together with ML-Ag NCs. It was reported that the white light emission can be generated through the combined emissions from ML-Ag NCs and Eu3+ in the Ag+-Na+ exchanged glasses [20]. This is possible since the Ag NCs were only formed in the glass surface while the Eu3+ was doped inside the whole glass matrix, and meanwhile the UV irradiation can directly excite Eu3+ ions. However, the emission intensity of Ag NCs in the blue-green region can be very strong with optimized doping level and host design when prepared with melt-quenching method, due to the large amount of ML-Ag NCs luminescent centers distributed inside the whole glass matrix. Therefore, very strong red emission is required to balance the intense blue-green emission for efficient white light generation. While simply increase Eu3+ doping will lead to concentration quenching and/or the aggregation of ML-Ag NCs into non luminescent silver nanoparticles (Ag NPs) [16,21–23], which will eventually result in reduced emission intensity. In this case, it is necessary to employ other red emission centers that can be simultaneously sensitized by ML-Ag NCs together with Eu3+. Furthermore, in order to flexibly turn the emission color of the phosphors, it is desirable to introduce more than one luminescence centers that can be effectively sensitized by ML-Ag NCs.
In this research, we selected the quaternary glass with composition of SiO2-B2O3-Na2O-SrO as host to stabilize ML-Ag NCs and meanwhile dope RE ions, benefited from the simultaneous energy transfer from ML-Ag NCs to Sm3+/Eu3+ couple, the white light emission was obtained under 360 nm UV excitation. Besides, the simultaneous energy transfer of ML-Ag NCs to more than one RE ions was demonstrated in the ML-Ag NCs/Sm3+/Tb3+ tri-doped glasses. For the first time, we realized the simultaneous energy transfer from ML-Ag NCs to two emission centers that give off visible emission and therefore achieved white light and broader tunable emissions.
2. Experimental
The AgNO3/SmF3/Eu2O3 and AgNO3/SmF3/Tb4O7 tri-doped glasses with composition and mole ratio of 64SiO2-10B2O3-15Na2O-11SrO were prepared by the melting-quenching method, in which the active ion concentrations were set as 0.5 mol% for Ag+, Sm3+, Eu3+, and Tb3+, respectively. Analytical SiO2, B2O3, Na2CO3, SrCO3, and 99.99% purity SmF3, Tb4O7 and Eu2O3 were used as starting materials, which were mixed homogeneously according to the doping conditions. After melting at 1450°C for 1h in a covered corundum crucible in air, the melts were poured on a copper plate and then pressed by another plate. The obtained glass samples were then annealed at 450°C for 2h to release inner stress. All the glasses were cut into square pieces of 1.5 mm thick and polished. Besides, a series of AgNO3, SmF3, Tb4O7, Eu2O3 single-doped, and AgNO3/SmF3, AgNO3/Tb4O7, AgNO3/Eu2O3 co-doped glasses were also prepared with the same methods for detail spectral analysis. The glass samples were named as GAgSmEu, GAgSmTb, GAg, GSm, GTb, GEu, GAgSm, GAgEu and GAgTb, respectively, according to their doping conditions.
The optical absorption spectra were recorded by a U-4000 spectrophotometer. The photoluminescence (PL) and photoluminescence-excitation (PLE) spectra were recorded with a FLS920 fluorescence spectrophotometer. The microstructure of the glasses were studied using a JEM 2100F transmission electron microscopy (TEM).
3. Results and discussion
The absorption spectra for the AgNO3 single-doped and the AgNO3/RE codoped/tridoped glasses were firstly measured to study the effect of RE ions doping on the formation of ML-Ag NCs and Ag NPs inside the glass matrix. As can be seen from Fig. 1, for all the AgNO3 single-doped and AgNO3-RE codoped glasses, no SPR absorption originated from Ag NPs can be observed, indicating only ML-Ag NCs may be formed inside these glasses. While the characteristic SPR absorption of Ag NPs located at 418 nm was seen in the tri-doped glass of GAgSmEu, but not in GAgSmTb, suggesting Eu3+ can more effectively promote the formation of Ag NPs than Tb3+. As we know that the reduction of Ag+ ions into Ag0 is a necessary step for the formation of ML-Ag NCs and Ag NPs, and it has been reported that the spontaneous redox of Eu3+ can provide the electrons necessary for Ag+ reduction. In this case, the extra introduction of Eu3+ can accelerate the further growth of ML-Ag NCs into Ag NPs in glass matrix, as describe below [21,22,24,25]:
Fig. 1. Absorption spectra of GAg (a), GAgSm (b), GAgTb (c), GAgEu (d), GAgSmEu (e) and GAgSmTb (f), respectively.
Besides, a very weak absorption peak located at 402 nm can be seen for GAgSm, GAgSmEu and GAgSmTb, which can be well attributed to Sm3+: 6H5/2 →4F7/2 transition. However, Eu3+ and Tb3+ absorption peaks can not be observed for GAgEu, GAgTb, GAgSmEu and GAgSmTb due to their low doping levels.
In order to further manifest the effect of Tb3+ and Eu3+ doping on the evolution of silver species from ML-Ag NCs to Ag NPs, the TEM measurement was carried on GAgSm, GAgSmTb and GAgSmEu, respectively, as shown in Fig. 2. It can be clearly observed from Figs. 2(a) and 2(b) that the first introduction of 0.5 mol% Sm3+ and the additional doping of 0.5mol% Tb3+ didn’t obviously lead to the aggregation of ML-Ag NCs, and only the ML-Ag NCs with diameter around 1-2 nm can be found in the TEM images of GAgSm and GAgSmTb. In contrast, after the extra codoping of equal amount Eu3+ instead of Tb3+, some Ag NPs with diameter even bigger than 8 nm were formed besides the small ML-Ag NCs, as shown in Figs. 2(c) and 2(d), these ML-Ag NCs and Ag NPs are randomly distributed inside the glass matrix. This is in accordance with the absorption spectra measurement result.
The PLE spectra of Sm3+, Eu3+ and Tb3+ in their respective single-doped glasses were measured by monitoring Sm3+ emission at 600 nm, Eu3+ emission at 612 nm and Tb3+ emission at 542 nm, respectively, which are corresponding to Sm3+: 4G5/2→6H7/2, Eu3+: 5D0→7F2, and Tb3+:5D4→7F5 transitions. As shown in Fig. 3, from which the characteristic Sm3+ absorption peaks assigned to 6H5/2→4H9/2 (341 nm), 4D3/2 (358 nm), 6P7/2 (372 nm),4F7/2 (400 nm), 4I13/2 (466 nm) transitions, Eu3+ absorption peaks due to 7F0→5HJ (J = 3, 4, 5, 7) (319 nm), 5D4 (362 nm), 5L6 (393 nm), 5D3 (412 nm), 5D2 (462 nm) transitions and its charge transfer absorption band (around 254 nm), as well as Tb3+ absorption peaks attributed to 7F6→5D1 (316 nm), 5L10 (350 nm), 5D3 (375 nm), 5D4 (482 nm) transitions and its broad 4f6→4f75d (around 268 nm) transition band can be clearly observed in GSm, GEu and GTb, respectively [17,26,23]. While for GAgSm, GAgEu, and GAgTb, very broad excitation band that ranging from 250 to 400 nm can be simultaneously observed together with those sharp characteristic excitation peaks when monitoring Sm3+ emission at 600 nm, Eu3+ emission at 612 nm, and Tb3+ emission at 542 nm, respectively. This broad excitation band can be well assigned to ML-Ag NCs in glasses [23,27], and thus suggested the occurrence of energy transfer from ML-Ag NCs to those codoped RE ions.
Fig. 3. (a) PLE spectra of Sm3+ in GSm (red line) and GAgSm (blue line), and PLE spectrum of ML-Ag NCs in GAgSm (grey line); (b) PLE spectra of Eu3+ in GEu (red line) and GAgEu (blue line), and PLE spectrum of ML-Ag NCs in GAgEu (grey line); (c) PLE spectra of Tb3+ in GTb (red line) and GAgTb (blue line), and PLE spectrum of ML-Ag NCs in GAgTb (grey line).
Figure 4 shows the PL spectra of GAgSm, GAgEu, GAgTb, and GAg under the excitation of UV light from 280-380 nm with a step of 20 nm. It can be seen that GAg shows broad emission band ranging from 300 to 600 nm, and with the increase of excitation wavelength the emission peak gradually red-shifted from 380 to 430 nm. This excitation wavelength dependent PL property is due to the distribution of ML-Ag NCs comprising different number of Ag0 and/or Ag+ inside the glass matrix [17,12,26,21,22]. For the codoped glasses of GAgSm, GAgEu, and GAgTb, their PL spectra show not only a broad ML-Ag NCs emission band, but also the characteristic Sm3+ emissions at 563, 600 and 649 nm due to 4G5/2→6HJ/2 (J = 5, 7, 9) transitions, Eu3+ emissions at 590 and 612 nm corresponding to 5D0→7F1,2 transitions, and Tb3+ emissions at 416, 436, 487, 542 and 550 nm originated from 5D3→7F4, 5D3→7F3, 5D4→7F6, 5D4→7F5 transitions, respectively, which also proved the occurrence of energy transfer from ML-Ag NCs to Sm3+, Eu3+ and Tb3+, respectively. According to the singlet-triplet energy level model of ML-Ag NCs proposed by Tikhomirov et al. [10,11], the PLE spectra of RE3+ and the PL spectra of ML-Ag NCs in Figs. 3 and 4, as well as the previous reports [16,17,22,26], the energy transfer mechanism was tentatively described in Fig. 5. Under UV light excitation, the ML-Ag NCs can be excited to the S1 and T2 energy levels, and then the non-radiative S1→S0 and T2→S0 transitions may transfer the excitation energy to Sm3+ by populate its 4H9/2, 4D3/2, 6P7/2, 4F7/2 energy levels, Eu3+ by populate its 5D4, 5L6, 5D3 energy levels, and Tb3+ by populate its 5D2, 5L10, 5D3 energy levels, respectively. After that, the non-radiative relaxations and subsequent radiative transitions lead to the visible emissions in Fig. 4. Besides, it is worth to mention that the spin-forbidden transition of T2→S0 with relatively longer lifetime may mainly contribute to the above energy transfer processes.
Fig. 4. PL spectra of GAg (grey shadow), GAgSm (red line), GAgEu (blue line) and GAgTb (green line) under the excitation of 280 nm (a), 300 nm (b), 320 nm (c), 340 nm (d), 360 nm (e) and 380 nm (f), respectively.
Fig. 5. Simplified energy level diagrams of ML-Ag NCs, Sm3+, Eu3+ and Tb3+, respectively, and the possible energy transfer routes.
Moreover it is noticed that compare with GAg, the emission intensity and the peak wavelength of GAgTb shows no obvious difference under all the excitations. In contrast, GAgSm exhibits stronger ML-Ag NCs emission under short wavelength excitations of 280-320 nm, while GAgEu shows both stronger and obviously red-shifted ML-Ag NCs emission when the excitation wavelength is longer than 280 nm. It is generally agreed that the size increase of ML-Ag NCs will lead to a red-shift of their excitation and emission peaks [26–28]. Therefore, the emission intensity variation and the peak wavelength shift of ML-Ag NCs in GAgSm, GAgEu, and GAgTb in comparison with in GAg indicated that the introduction of Sm3+ and especially Eu3+ can benefit the formation of ML-Ag NCs and further promote them grow into bigger ones, while Tb3+ has no obvious effect on the evolution of silver species. The previous study also suggested that the valence invariable ions that can be reduced such as Sm3+ and Eu3+ may accelerate the formation of ML-Ag NCs due to the spontaneous redox process of RE2+/3+ ions, as the formation and growth of ML-Ag NCs require the reduction of Ag+ into Ag0, and the different promotion effect is related with their electronegativity [9,28]. In addition, it is noticed that there is an obvious dip located at 402 nm upon the broad emission band of ML-Ag NCs in GAgSm under 280-340 nm excitations, which can be attributed to the characteristic absorption of Sm3+: 6H5/2 →4F7/2. This phenomenon suggested efficient energy transfer from ML-Ag NC to Sm3+. While when excited by 360 and 380 nm, no dips can be observed in the PL spectra of GAgSm, indicating the red emission of Sm3+ is due to the direct population of Sm3+:4G 5/2 excited state under these excitations [17,29]. Actually, the Eu3+ and Tb3+ can also be partially excited when the excitation wavelength matches with their absorption band.
According to the above spectral results, the efficient energy transfers from ML-Ag NCs to Sm3+, Eu3+ and Tb3+ were successfully realized in the respective codoped glasses, which is benefited from the broad emission band of ML-Ag NCs that overlaps with the excitation spectra of Sm3+, Eu3+ and Tb3+. Furthermore, the ML-Ag NCs/Sm3+/Eu3+ and the ML-Ag NCs/Sm3+/Tb3+ tri-doped glasses were designed to discuss the possibility of simultaneous sensitization of two RE emission centers by ML-Ag NCs. Figure 6(a) shows the PL spectra of GAgSmEu, from which the broadband emission band of ML-Ag NCs together with the characteristic red emissions from both Sm3+ and Eu3+ can be clearly observed under the excitations of 280-380 nm, confirming the simultaneous energy transfer from ML-Ag NCs to Sm3+and Eu3+. According to the CIE chromaticity diagram of GAgSmEu in Fig. 6(b), the white light emission with the CIE coordinates of (0.335, 0.30) and a correlated color temperature value of 5381K was obtained under 360 nm excitation due to the introduction of Sm3+ and Eu3+ as red-emitting centers to balance the strong blue-green emission from ML-Ag NCs. Besides, the tunable emission from blue to blue-green and then to red can be realized in GAgSmEu by increasing excitation wavelength. It is worth of mention that when turning the excitation wavelength from 280 to 380 nm, the excitation efficiency of ML-AgNCs varies and meanwhile the Sm3+ and Eu3+ can be partially excited under proper excitation wavelength, as shown in Fig. 3. In this case, the excitation wavelength dependence of emission color variation is not only because of the change of energy transfer efficiency from ML-Ag NCs to RE ions, but also the direct excitation efficiency for ML-Ag NCs, Sm3+ and Eu3+.
Moreover, the simultaneous energy transfer from ML-Ag NCs to two RE ions was demonstrated in GAgSmTb. As shown in Fig. 7(a), the visible emission from both Sm3+ and Tb3+ can be observed with the excitations of 280-380 nm. By changing the excitation wavelength, both the energy transfer efficiency and the excitation efficiency for the doped individual RE ions can be modified, which thus leaded to the moving of CIE coordinate in Fig. 7(b). As the excitation wavelength range of the ML-Ag NCs and RE ions co-actived luminescent glasses matches well with the output wavelengths of the common violet LED chips, which therefor have potential application in generating white light and tunable emission by combining with violet LEDs.
4. Conclusion
In this research, the borosilicate glasses containing ML-Ag NCs and RE ions of Sm3+, Eu3+, Tb3+ were prepared with melt-quenching method. The spectral measurements proved the stabilization of luminescent ML-Ag NCs and the occurrence of simultaneous energy transfer from ML-Ag NCs to Sm3+/Eu3+ and Sm3+/Tb3+ couples in the glass matrix. Besides, it was found that compare with Sm3+and Tb3+, the introduction of Eu3+ can more effectively benefit the formation of luminescent ML-Ag NCs and meanwhile promote their further aggregation. Due to the efficient energy transfer from ML-Ag NCs to Sm3+/Eu3+ and Sm3+/Tb3+ couples, as well as the different excitation efficiency and energy transfer efficiency to the doped RE ions, the tri-doped glasses exhibit broad tunable emission simply by changing the excitation wavelength. For the first time, we realized the simultaneous energy transfer from ML-Ag NCs to two emission centers that give off visible emission and therefore achieved white light and broader tunable emissions. This research may benefit the development of ML-Ag NCs and RE ions co-actived luminescence glass with potential applications for optoelectronic devices such as light emitting diode and three-dimensional display.
Funding
National Natural Science Foundation of China (51872200, 51772210); Natural Science Foundation of Shanghai (18ZR1441900).
Disclosures
The authors declare no conflicts of interest.
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