2630
OPTICS LETTERS / Vol. 40, No. 11 / June 1, 2015
Plasmon enhanced near-infrared quantum cutting of KYF4: Tb3+, Yb3+ doped with Ag nanoparticles Biao Zheng,1 Senyuan Xu,1 Lin Lin,1,2,3 Zhezhe Wang,1,2 Zhuohong Feng,1,2 and Zhiqiang Zheng1,2,* 1
2
College of Physics and Energy, Fujian Normal University, Fuzhou 350117, China Fujian Provincial Key Laboratory of Quantum Manipulation and New Energy Materials, Fuzhou 350117, China 3
e-mail: [email protected] *Corresponding author: [email protected] Received March 18, 2015; revised May 11, 2015; accepted May 11, 2015; posted May 12, 2015 (Doc. ID 235704); published June 1, 2015 Novel quantum cutting (QC) phosphor KYF4 : Tb3 , Yb3 doped Ag nanoparticles (NPs) was prepared by using the sol-gel method. Plasmon enhanced near-infrared (NIR) QC involving Yb3 ion at 975 nm (2 F5∕2 → 2 F7∕2 ) emission was achieved under the excitation of 374 nm (7 F6 → 5 D3 ) and 485 nm (7 F6 → 5 D4 ) of Tb3 ions, respectively. The effect of Ag NPs on NIR QC luminescence was investigated, and the results show that QC luminescence intensity first increases, then decreases with the increase of the Ag NPs concentration. The maximum enhancement factor is about 1.9 when the concentration of Ag NPs is 0.5%. Our study may have potential application in the field of silicon-based solar cells. © 2015 Optical Society of America OCIS codes: (160.5690) Rare-earth-doped materials; (240.6680) Surface plasmons; (300.6280) Spectroscopy, fluorescence and luminescence. http://dx.doi.org/10.1364/OL.40.002630
The increasing demand for solar energy due to energy crisis and environmental pollution of fossil-fuel, has put how to improve the photoelectric conversion efficiency of solar cells at the forefront of research [1]. In order to make full use of the solar energy, there have been increasing efforts on the research of QC [2–12]. The QC process can convert one ultraviolet (UV)-visible solar photon (300–500nm) into two NIR photons (∼1000 nm) [11], which can be efficiently absorbed by silicon-based solar cells. Therefore, the solar spectrum modulation through QC process can improve the photoelectric conversion efficiency in the silicon-based solar cells. On the other hand, noble metal NPs have been introduced to enhance the upconversion luminescence of rare-earth (RE) doped phosphors [13–16].The incorporation of noble metal NPs in RE doped phosphors can enhance the luminescence of RE ions when the excitation or emission wavelength is near the surface plasmon resonance (SPR) wavelength of noble metal NPs. The enhancement is mainly attributed to local field enhancement effect (LFE) [17,18], However, there is little report about plasmon enhanced QC luminescence of co-doped RE ions and noble metal NPs. In the previous report, the plasmon enhanced QC luminescence wasn’t observed in the Tb3 and Yb3 co-doped glass containing Ag NPs [19]. In this Letter, the plasmon enhanced QC luminescence was observed for the first time to our knowledge. It will provide a better candidate for application as QC layer on the top of silicon-based solar cells to improve the photoelectric conversion efficiency. Tb3 , Yb3 and Ag NPs co-doped KYF4 phosphors were prepared. We chose KYF4 as the host of QC phosphors due to its low phonon energy, good chemical and thermal stability [20]. The preparation of KYF4 : Tb3 , Yb3 phosphors has been introduced thoroughly in the article of our group [21], which mentioned that the optimal doped concentration of the Tb3 and Yb3 ions was 15% and 10%, respectively. The silver solutions (1 ml containing 1.5625 × 10−5 mol Ag NPs) were prepared by 0146-9592/15/112630-04$15.00/0
chemical reduction method in which Ag was reduced to Ag by ethylene glycol in the presence of poly (vinyl pyrrolidone) (PVP) and a trace amount of Na2 S [22]. Next, the Ag NPs were doped into the KYF4 host homogeneously by the following method: a proper amount of as-prepared KYF4 : 15%Tb3 , 10%Yb3 phosphors and silver solution were mixed uniformly and dried out at 80°C for 8 h. Then, the mixture was sintered at 600°C for 2 h under Ar atmosphere to obtain the Tb3 , Yb3 and Ag NPs co-doped KYF4 phosphors. Sintering at 600°C can prompt KYF4 phosphors well crystallized and reduce the defect, which will improve the QC luminescence. Ar atmosphere can avoid the oxidation of Ag NPs. In order to research the optimal doped concentration of Ag NPs in the QC luminescence process, a series of KYF4 : 15%Tb3 , 10%Yb3 , x% Ag (x 0, 0.25, 0.5, 1, 1.5, 2) (Tb/Yb/Ag with mole ratio) phosphors was prepared. The amount of KYF4 phosphors was 2.5 mmol (0.5573 g). The volume of silver solutions was 0.4 ml (0.25% Ag), 0.8 ml (0.5% Ag), 1.6 ml (1% Ag), 2.4 ml (1.5% Ag) and 3.2 ml (2% Ag), respectively. As-prepared samples were characterized by X-ray diffraction (XRD, MiniFlex II, Rigaku), fluorescence spectra (Fluorolog 3-22 spectrofluorometer, Horiba Jobin Yvon), scanning electron microscope (SEM) attached with energy dispersive x-ray analysis (SU8010, Hitachi), transmission electron microscope (TEM, G2F20, Tecnai), and absorption spectra (Lambda 950, PerkinElmer). Figure 1 illustrates the XRD pattern of KYF4 : 15%Tb3 , 10%Yb3 sample. All major diffraction peaks match well with those of KYF4 phase (JCPDS 79-1688). The XRD results indicate that the sample is well crystallized under 600°C and doped RE ions can substitute Y3 ions without disturbing the crystal lattice. The UV-visible absorption spectrum and the corresponding morphology of Ag NPs solution are shown in Figure 2. There is a broad absorption band from 320 to 700 nm approximately and the absorption peak is about 420 nm. It indicates that the SPR wavelength of © 2015 Optical Society of America
June 1, 2015 / Vol. 40, No. 11 / OPTICS LETTERS
Fig. 1.
XRD patterns of the KYF4 : 15%Tb3 , 10%Yb3 sample.
the Ag NPs is around 320 to 700 nm. The inset shows that the size distribution of Ag NPs is from 20 to 150 nm. Figure 3 contrasts the absorption spectra of KYF4 : 15%Tb3 , 10%Yb3 phosphors doped with/without Ag NPs. Compared with these two absorption spectra, the absorption peak of Tb3 and Yb3 ions is observed in both spectra. What’s more, a strong and broad absorption band from 320 to 700 nm is observed in the spectrum of doping 2% Ag NPs sample. This absorption band is the characteristic SPR wavelength of Ag NPs, which indicates that Ag NPs have been doped into the KYF4 phosphors successfully. However, compared with the absorption peak at 420 nm of Ag NPs solution (See Fig. 2), the absorption peak at 400 nm of KYF4 phosphors doped 2% Ag NPs has a blue-shift, which is due to the secondary sintering. Specifically, the absorption peak of Ag NPs depends on the morphology of the Ag NPs [23]. According to the TEM image in Figure 4, sintering at a high temperature makes the morphology of the Ag NPs close to sphere, whose SPR wavelength is located at about 400 nm [24]. Figure 5 shows the images of X-ray surface scanning analysis of KYF4 : 15%Tb3 , 10%Yb3 , 2% Ag samples. We can observe the distributions of the Ag, Tb and Yb elements in the samples, which indicates that Ag NPs, Tb3 , Yb3 are doped into the KYF4 phosphors homogenously. The excitation spectra of KYF4 : 15%Tb3 , 10%Yb3 , x% Ag monitored at 975 nm are shown in Figure 6. From the
Fig. 2. UV-visible absorption spectrum of Ag NPs solution. Inset: corresponding SEM image of Ag NPs solution.
2631
Fig. 3. Absorption spectra of KYF4 : 15%Tb3 , 10%Yb3 samples doped (a) 0% and (b) 2% concentration of Ag NPs. Inset: enlarged absorption spectrum of the sample doped 0% Ag.
Fig. 4. Corresponding TEM image of KYF4 samples doped 2% Ag NPs.
excitation spectra, the major excitation peaks are located at 350, 374 and 485 nm, attributed to Tb3 ions 7 F6 → 5 Dj (j 2, 3, 4) transitions, respectively. The excitation intensity increases with increasing Ag NPs concentration from 0% to 0.5%. Then, it decreases with further increase of the Ag NPs concentration for the quenching effect that the energy transfer from excited Tb3 ions to Ag NPs occurs due to larger density of Ag NPs in the sample. The highest excitation enhancement factor is about 1.8.
Fig. 5. Images of X-ray surface scanning analysis of KYF4 : 15%Tb3 , 10%Yb3 , 2% Ag samples.
2632
OPTICS LETTERS / Vol. 40, No. 11 / June 1, 2015
Fig. 6. Excitation spectra (λem 975 nm) of KYF4 : 15%Tb3 , 10%Yb3 , x% Ag (x 0, 0.25, 0.5, 1, 1.5, 2) phosphors.
Figure 7 demonstrates the QC luminescence excited at 374 and 485 nm, respectively. From the emission spectra, the major emission peak is located at 975 nm attributed to the 2 F5∕2 → 2 F7∕2 transitions of Yb3 ions. The emission intensity increases with increasing Ag NPs concentration from 0% to 0.5% and then decreases with the further increase of Ag NPs concentration, which indicates QC luminescence enhancement was achieved. The details of this mechanism will be discussed in the next paragraph. In addition, the energy transfer from Ag NPs to Yb3 ions can be excluded because the resonance band of Ag NPs around 400 nm was not observed in the excitation spectra monitored at 975 nm (See Figure 6). The highest QC luminescence enhancement factor is about 1.6 and 1.9 excited at 374 and 485 nm, respectively. Figure 8 illustrates the NIR QC mechanism of Tb3 , Yb3 ions in terms of the energy level diagrams. Initially the Tb3 ions are excited from the ground level 7 F6 to an excited level 5 D4 (∼485 nm). The cooperative energy transfer occurs from an excited Tb3 ion to two neighbor Yb3 ions in the ground level. Two Yb3 ions are excited to the excited level 7 F5∕2 via a coupled transition 5 D4 → 7 F6 of one Tb3 ion. The NIR QC luminescence at 975 nm is emitted from the transition 2 F5∕2 → 2 F7∕2 of the excited Yb3 ions. In our case, the incorporation of Ag NPs in KYF4 : Tb3 , Yb3 phosphors will enhance the excitation rate of Tb3 ions because the excitation wavelength is near the SPR wavelength of Ag NPs. The enhancement is mainly attributed to the LFE.
Fig. 7. Emission spectra of KYF4 : 15%Tb3 , 10%Yb3 , x% Ag (x 0, 0.25, 0.5, 1, 1.5, 2) phosphors under (a) 374 nm and (b) 485 nm excitation.
Fig. 8. Energy levels diagram of Tb3 , Yb3 ions in the QC energy transfers.
Subsequently, the emission of Yb3 ions at 975 nm is also enhanced. That is, the plasmon enhanced QC luminescence is achieved. On the other hand, when exciting the 5 D3 (∼374 nm) level of Tb3 ions, the NIR QC luminescence enhancement also occurs due to the crossrelaxation (5 D3 7 F6 → 5 D4 7 F0 ), nonradiative transition (5 D3 → 5 D4 ) [21], and cooperative energy transfer from an excited Tb3 ion to two neighbor Yb3 ions in the ground level. In summary, KYF4 : Tb3 , Yb3 phosphors doped with Ag NPs are prepared and investigated. The plasmon enhanced NIR QC luminescence from one excited Tb3 ion to two Yb3 ions is observed. The maximum enhancement factor is 1.9 when the concentration of Ag NPs is 0.5%. The results all above infer that this phosphor can make full use of solar energy and may raise the photoelectric conversion efficiency of silicon-based solar cells. This work was supported by the National Natural Science Foundation of China (No. 11204039 and No. 51202033), the Natural Science Foundation of Fujian Province of China (No. 2015J01243) and the Science Foundation of the Educational Department of Fujian Province of China (No. JA13084). References 1. B. M. Van der Ende, L. Aarts, and A. Meijerink, Adv. Mater. 21, 3073 (2009). 2. Q. Q. Duan, F. Qin, Z. G. Zhang, and W. W. Cao, Opt. Lett. 37, 521 (2012). 3. H. Lin, D. Q. Chen, Y. L. Yu, and Y. S. Wang, Opt. Lett. 36, 876 (2011). 4. X. T. Wei, S. Huang, M. Yin, and W. Xu, J. Appl. Phys. 107, 103107 (2010). 5. M. M. Smedskjaer, J. R. Qiu, J. Wang, and Y. Z. Yue, Appl. Phys. Lett. 98, 071911 (2011). 6. X. Y. Huang, D. C. Yu, and Q. Y. Zhang, J. Appl. Phys. 106, 113521 (2009). 7. J. D. Chen, H. Guo, Z. Q. Li, H. Zhang, and Y. X. Zhuang, Opt. Mater. 32, 998 (2010). 8. Q. Y. Zhang and X. Y. Huang, Prog. Mater. Sci. 55, 353 (2010). 9. X. P. Chen, X. Y. Huang, and Q. Y. Zhang, J. Appl. Phys. 106, 063518 (2009). 10. K. M. Deng, T. Gong, X. T. Wei, Y. H. Chen, and M. Yin, Opt. Express 19, 1749 (2011). 11. X. J. Zhou, G. C. Wang, K. N. Zhou, and Q. X. Li, Opt. Mater. 35, 600 (2013).
June 1, 2015 / Vol. 40, No. 11 / OPTICS LETTERS 12. Q. Y. Zhang, G. F. Yang, and Z. H. Jiang, Appl. Phys. Lett. 91, 051903 (2007). 13. P. Y. Yuan, Y. H. Lee, Y. Zhang, and Q. H. Xu, Nanoscale 4, 5132 (2012). 14. E. He, H. Zheng, Y. Lu, and H. Tian, Nanotechnol. 25, 045603 (2014). 15. H. Zhang, D. Xu, Y. Huang, and X. F. Duan, Chem. Commun. 47, 979 (2011). 16. M. Saboktakin, X. C. Ye, S. J. Oh, S. H. Hong, and A. T. Fafarman, ACS Nano 6, 8758 (2012). 17. P. Piasecki, A. Piasecki, Z. D. Pan, and S. H. Morgan, J. Nanophotonics 4, 043522 (2010).
2633
18. O. L. Malta, P. A. Santa-Cruz, G. F. De Sa, and F. Auzel, J. Lumin. 33, 261 (1985). 19. Z. Pan, G. Sekar, and S. H. Morgan, Proc. SPIE 8096, 809633 (2011). 20. B. S. Richards, Sol. Energy Mater. Sol. Cells 90, 1189 (2006). 21. L. Lin, H. Lin, Z. Z. Wang, J. X. Chen, and Z. Q. Zheng, Opt. Mater. 36, 1065 (2014). 22. S. E. Skrabalak, L. Au, X. D. Li, and Y. N. Xia, Nat Protoc. 2, 2182 (2007). 23. Y. G. Sun and Y. N. Xia, Science 298, 2176 (2002). 24. R. Jin, Y. W. Cao, and C. A. Mirkin, Science 294, 5548 (2001).
OPTICS LETTERS / Vol. 40, No. 11 / June 1, 2015
Plasmon enhanced near-infrared quantum cutting of KYF4: Tb3+, Yb3+ doped with Ag nanoparticles Biao Zheng,1 Senyuan Xu,1 Lin Lin,1,2,3 Zhezhe Wang,1,2 Zhuohong Feng,1,2 and Zhiqiang Zheng1,2,* 1
2
College of Physics and Energy, Fujian Normal University, Fuzhou 350117, China Fujian Provincial Key Laboratory of Quantum Manipulation and New Energy Materials, Fuzhou 350117, China 3
e-mail: [email protected] *Corresponding author: [email protected] Received March 18, 2015; revised May 11, 2015; accepted May 11, 2015; posted May 12, 2015 (Doc. ID 235704); published June 1, 2015 Novel quantum cutting (QC) phosphor KYF4 : Tb3 , Yb3 doped Ag nanoparticles (NPs) was prepared by using the sol-gel method. Plasmon enhanced near-infrared (NIR) QC involving Yb3 ion at 975 nm (2 F5∕2 → 2 F7∕2 ) emission was achieved under the excitation of 374 nm (7 F6 → 5 D3 ) and 485 nm (7 F6 → 5 D4 ) of Tb3 ions, respectively. The effect of Ag NPs on NIR QC luminescence was investigated, and the results show that QC luminescence intensity first increases, then decreases with the increase of the Ag NPs concentration. The maximum enhancement factor is about 1.9 when the concentration of Ag NPs is 0.5%. Our study may have potential application in the field of silicon-based solar cells. © 2015 Optical Society of America OCIS codes: (160.5690) Rare-earth-doped materials; (240.6680) Surface plasmons; (300.6280) Spectroscopy, fluorescence and luminescence. http://dx.doi.org/10.1364/OL.40.002630
The increasing demand for solar energy due to energy crisis and environmental pollution of fossil-fuel, has put how to improve the photoelectric conversion efficiency of solar cells at the forefront of research [1]. In order to make full use of the solar energy, there have been increasing efforts on the research of QC [2–12]. The QC process can convert one ultraviolet (UV)-visible solar photon (300–500nm) into two NIR photons (∼1000 nm) [11], which can be efficiently absorbed by silicon-based solar cells. Therefore, the solar spectrum modulation through QC process can improve the photoelectric conversion efficiency in the silicon-based solar cells. On the other hand, noble metal NPs have been introduced to enhance the upconversion luminescence of rare-earth (RE) doped phosphors [13–16].The incorporation of noble metal NPs in RE doped phosphors can enhance the luminescence of RE ions when the excitation or emission wavelength is near the surface plasmon resonance (SPR) wavelength of noble metal NPs. The enhancement is mainly attributed to local field enhancement effect (LFE) [17,18], However, there is little report about plasmon enhanced QC luminescence of co-doped RE ions and noble metal NPs. In the previous report, the plasmon enhanced QC luminescence wasn’t observed in the Tb3 and Yb3 co-doped glass containing Ag NPs [19]. In this Letter, the plasmon enhanced QC luminescence was observed for the first time to our knowledge. It will provide a better candidate for application as QC layer on the top of silicon-based solar cells to improve the photoelectric conversion efficiency. Tb3 , Yb3 and Ag NPs co-doped KYF4 phosphors were prepared. We chose KYF4 as the host of QC phosphors due to its low phonon energy, good chemical and thermal stability [20]. The preparation of KYF4 : Tb3 , Yb3 phosphors has been introduced thoroughly in the article of our group [21], which mentioned that the optimal doped concentration of the Tb3 and Yb3 ions was 15% and 10%, respectively. The silver solutions (1 ml containing 1.5625 × 10−5 mol Ag NPs) were prepared by 0146-9592/15/112630-04$15.00/0
chemical reduction method in which Ag was reduced to Ag by ethylene glycol in the presence of poly (vinyl pyrrolidone) (PVP) and a trace amount of Na2 S [22]. Next, the Ag NPs were doped into the KYF4 host homogeneously by the following method: a proper amount of as-prepared KYF4 : 15%Tb3 , 10%Yb3 phosphors and silver solution were mixed uniformly and dried out at 80°C for 8 h. Then, the mixture was sintered at 600°C for 2 h under Ar atmosphere to obtain the Tb3 , Yb3 and Ag NPs co-doped KYF4 phosphors. Sintering at 600°C can prompt KYF4 phosphors well crystallized and reduce the defect, which will improve the QC luminescence. Ar atmosphere can avoid the oxidation of Ag NPs. In order to research the optimal doped concentration of Ag NPs in the QC luminescence process, a series of KYF4 : 15%Tb3 , 10%Yb3 , x% Ag (x 0, 0.25, 0.5, 1, 1.5, 2) (Tb/Yb/Ag with mole ratio) phosphors was prepared. The amount of KYF4 phosphors was 2.5 mmol (0.5573 g). The volume of silver solutions was 0.4 ml (0.25% Ag), 0.8 ml (0.5% Ag), 1.6 ml (1% Ag), 2.4 ml (1.5% Ag) and 3.2 ml (2% Ag), respectively. As-prepared samples were characterized by X-ray diffraction (XRD, MiniFlex II, Rigaku), fluorescence spectra (Fluorolog 3-22 spectrofluorometer, Horiba Jobin Yvon), scanning electron microscope (SEM) attached with energy dispersive x-ray analysis (SU8010, Hitachi), transmission electron microscope (TEM, G2F20, Tecnai), and absorption spectra (Lambda 950, PerkinElmer). Figure 1 illustrates the XRD pattern of KYF4 : 15%Tb3 , 10%Yb3 sample. All major diffraction peaks match well with those of KYF4 phase (JCPDS 79-1688). The XRD results indicate that the sample is well crystallized under 600°C and doped RE ions can substitute Y3 ions without disturbing the crystal lattice. The UV-visible absorption spectrum and the corresponding morphology of Ag NPs solution are shown in Figure 2. There is a broad absorption band from 320 to 700 nm approximately and the absorption peak is about 420 nm. It indicates that the SPR wavelength of © 2015 Optical Society of America
June 1, 2015 / Vol. 40, No. 11 / OPTICS LETTERS
Fig. 1.
XRD patterns of the KYF4 : 15%Tb3 , 10%Yb3 sample.
the Ag NPs is around 320 to 700 nm. The inset shows that the size distribution of Ag NPs is from 20 to 150 nm. Figure 3 contrasts the absorption spectra of KYF4 : 15%Tb3 , 10%Yb3 phosphors doped with/without Ag NPs. Compared with these two absorption spectra, the absorption peak of Tb3 and Yb3 ions is observed in both spectra. What’s more, a strong and broad absorption band from 320 to 700 nm is observed in the spectrum of doping 2% Ag NPs sample. This absorption band is the characteristic SPR wavelength of Ag NPs, which indicates that Ag NPs have been doped into the KYF4 phosphors successfully. However, compared with the absorption peak at 420 nm of Ag NPs solution (See Fig. 2), the absorption peak at 400 nm of KYF4 phosphors doped 2% Ag NPs has a blue-shift, which is due to the secondary sintering. Specifically, the absorption peak of Ag NPs depends on the morphology of the Ag NPs [23]. According to the TEM image in Figure 4, sintering at a high temperature makes the morphology of the Ag NPs close to sphere, whose SPR wavelength is located at about 400 nm [24]. Figure 5 shows the images of X-ray surface scanning analysis of KYF4 : 15%Tb3 , 10%Yb3 , 2% Ag samples. We can observe the distributions of the Ag, Tb and Yb elements in the samples, which indicates that Ag NPs, Tb3 , Yb3 are doped into the KYF4 phosphors homogenously. The excitation spectra of KYF4 : 15%Tb3 , 10%Yb3 , x% Ag monitored at 975 nm are shown in Figure 6. From the
Fig. 2. UV-visible absorption spectrum of Ag NPs solution. Inset: corresponding SEM image of Ag NPs solution.
2631
Fig. 3. Absorption spectra of KYF4 : 15%Tb3 , 10%Yb3 samples doped (a) 0% and (b) 2% concentration of Ag NPs. Inset: enlarged absorption spectrum of the sample doped 0% Ag.
Fig. 4. Corresponding TEM image of KYF4 samples doped 2% Ag NPs.
excitation spectra, the major excitation peaks are located at 350, 374 and 485 nm, attributed to Tb3 ions 7 F6 → 5 Dj (j 2, 3, 4) transitions, respectively. The excitation intensity increases with increasing Ag NPs concentration from 0% to 0.5%. Then, it decreases with further increase of the Ag NPs concentration for the quenching effect that the energy transfer from excited Tb3 ions to Ag NPs occurs due to larger density of Ag NPs in the sample. The highest excitation enhancement factor is about 1.8.
Fig. 5. Images of X-ray surface scanning analysis of KYF4 : 15%Tb3 , 10%Yb3 , 2% Ag samples.
2632
OPTICS LETTERS / Vol. 40, No. 11 / June 1, 2015
Fig. 6. Excitation spectra (λem 975 nm) of KYF4 : 15%Tb3 , 10%Yb3 , x% Ag (x 0, 0.25, 0.5, 1, 1.5, 2) phosphors.
Figure 7 demonstrates the QC luminescence excited at 374 and 485 nm, respectively. From the emission spectra, the major emission peak is located at 975 nm attributed to the 2 F5∕2 → 2 F7∕2 transitions of Yb3 ions. The emission intensity increases with increasing Ag NPs concentration from 0% to 0.5% and then decreases with the further increase of Ag NPs concentration, which indicates QC luminescence enhancement was achieved. The details of this mechanism will be discussed in the next paragraph. In addition, the energy transfer from Ag NPs to Yb3 ions can be excluded because the resonance band of Ag NPs around 400 nm was not observed in the excitation spectra monitored at 975 nm (See Figure 6). The highest QC luminescence enhancement factor is about 1.6 and 1.9 excited at 374 and 485 nm, respectively. Figure 8 illustrates the NIR QC mechanism of Tb3 , Yb3 ions in terms of the energy level diagrams. Initially the Tb3 ions are excited from the ground level 7 F6 to an excited level 5 D4 (∼485 nm). The cooperative energy transfer occurs from an excited Tb3 ion to two neighbor Yb3 ions in the ground level. Two Yb3 ions are excited to the excited level 7 F5∕2 via a coupled transition 5 D4 → 7 F6 of one Tb3 ion. The NIR QC luminescence at 975 nm is emitted from the transition 2 F5∕2 → 2 F7∕2 of the excited Yb3 ions. In our case, the incorporation of Ag NPs in KYF4 : Tb3 , Yb3 phosphors will enhance the excitation rate of Tb3 ions because the excitation wavelength is near the SPR wavelength of Ag NPs. The enhancement is mainly attributed to the LFE.
Fig. 7. Emission spectra of KYF4 : 15%Tb3 , 10%Yb3 , x% Ag (x 0, 0.25, 0.5, 1, 1.5, 2) phosphors under (a) 374 nm and (b) 485 nm excitation.
Fig. 8. Energy levels diagram of Tb3 , Yb3 ions in the QC energy transfers.
Subsequently, the emission of Yb3 ions at 975 nm is also enhanced. That is, the plasmon enhanced QC luminescence is achieved. On the other hand, when exciting the 5 D3 (∼374 nm) level of Tb3 ions, the NIR QC luminescence enhancement also occurs due to the crossrelaxation (5 D3 7 F6 → 5 D4 7 F0 ), nonradiative transition (5 D3 → 5 D4 ) [21], and cooperative energy transfer from an excited Tb3 ion to two neighbor Yb3 ions in the ground level. In summary, KYF4 : Tb3 , Yb3 phosphors doped with Ag NPs are prepared and investigated. The plasmon enhanced NIR QC luminescence from one excited Tb3 ion to two Yb3 ions is observed. The maximum enhancement factor is 1.9 when the concentration of Ag NPs is 0.5%. The results all above infer that this phosphor can make full use of solar energy and may raise the photoelectric conversion efficiency of silicon-based solar cells. This work was supported by the National Natural Science Foundation of China (No. 11204039 and No. 51202033), the Natural Science Foundation of Fujian Province of China (No. 2015J01243) and the Science Foundation of the Educational Department of Fujian Province of China (No. JA13084). References 1. B. M. Van der Ende, L. Aarts, and A. Meijerink, Adv. Mater. 21, 3073 (2009). 2. Q. Q. Duan, F. Qin, Z. G. Zhang, and W. W. Cao, Opt. Lett. 37, 521 (2012). 3. H. Lin, D. Q. Chen, Y. L. Yu, and Y. S. Wang, Opt. Lett. 36, 876 (2011). 4. X. T. Wei, S. Huang, M. Yin, and W. Xu, J. Appl. Phys. 107, 103107 (2010). 5. M. M. Smedskjaer, J. R. Qiu, J. Wang, and Y. Z. Yue, Appl. Phys. Lett. 98, 071911 (2011). 6. X. Y. Huang, D. C. Yu, and Q. Y. Zhang, J. Appl. Phys. 106, 113521 (2009). 7. J. D. Chen, H. Guo, Z. Q. Li, H. Zhang, and Y. X. Zhuang, Opt. Mater. 32, 998 (2010). 8. Q. Y. Zhang and X. Y. Huang, Prog. Mater. Sci. 55, 353 (2010). 9. X. P. Chen, X. Y. Huang, and Q. Y. Zhang, J. Appl. Phys. 106, 063518 (2009). 10. K. M. Deng, T. Gong, X. T. Wei, Y. H. Chen, and M. Yin, Opt. Express 19, 1749 (2011). 11. X. J. Zhou, G. C. Wang, K. N. Zhou, and Q. X. Li, Opt. Mater. 35, 600 (2013).
June 1, 2015 / Vol. 40, No. 11 / OPTICS LETTERS 12. Q. Y. Zhang, G. F. Yang, and Z. H. Jiang, Appl. Phys. Lett. 91, 051903 (2007). 13. P. Y. Yuan, Y. H. Lee, Y. Zhang, and Q. H. Xu, Nanoscale 4, 5132 (2012). 14. E. He, H. Zheng, Y. Lu, and H. Tian, Nanotechnol. 25, 045603 (2014). 15. H. Zhang, D. Xu, Y. Huang, and X. F. Duan, Chem. Commun. 47, 979 (2011). 16. M. Saboktakin, X. C. Ye, S. J. Oh, S. H. Hong, and A. T. Fafarman, ACS Nano 6, 8758 (2012). 17. P. Piasecki, A. Piasecki, Z. D. Pan, and S. H. Morgan, J. Nanophotonics 4, 043522 (2010).
2633
18. O. L. Malta, P. A. Santa-Cruz, G. F. De Sa, and F. Auzel, J. Lumin. 33, 261 (1985). 19. Z. Pan, G. Sekar, and S. H. Morgan, Proc. SPIE 8096, 809633 (2011). 20. B. S. Richards, Sol. Energy Mater. Sol. Cells 90, 1189 (2006). 21. L. Lin, H. Lin, Z. Z. Wang, J. X. Chen, and Z. Q. Zheng, Opt. Mater. 36, 1065 (2014). 22. S. E. Skrabalak, L. Au, X. D. Li, and Y. N. Xia, Nat Protoc. 2, 2182 (2007). 23. Y. G. Sun and Y. N. Xia, Science 298, 2176 (2002). 24. R. Jin, Y. W. Cao, and C. A. Mirkin, Science 294, 5548 (2001).
