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Regulation of gold nanoparticles for the rare earth luminescence enhancement based on nanoporous silica glass
Journal of Luminescence 204 (2018) 104–109
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Regulation of gold nanoparticles for the rare earth luminescence enhancement based on nanoporous silica glass
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Yunxiu Ma, Zhangru Chen, Yingbo Chu, Yu Yang, Yongguang Liu, Haiqing Li, Jinggang Peng, ⁎ Nengli Dai, Jinyan Li, Luyun Yang Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Nanoporous silica glass Rare earth ions Nanoparticles Luminescence intensity
A silica composite system with rare earth ions and gold nanoparticles was fabricated based on nanoporous silica glass. The formation and optical properties of the systems were controlled by the nanoparticles doping concentration and heat treatment. The luminescence intensity dependence on the HAuCl4 doping concentration and heat-treated temperature were investigated. At the optimized concentration of HAuCl4 and heat-treated temperature, the maximal luminescence intensity can be enhanced up to 14.33-fold for Yb2+ and 10.52-fold for Eu2+. It is indicated that the rare earth ions luminescence can be regulated by interactions distance between different types of RE ions and metal NPs as well as the state of nanopores.
1. Introduction The unique electron shell structure of rare earth (RE) ions allows them to be widely utilized in illumination, displaying, data storage and fiber devices [1–3]. In the electron shell structure of RE ions, the 4f electron shell is affected slightly by external atmosphere under the shielding of outer 5s and 5p electrons. There are two transitions of RE ions, one is the forbidden transition in the 4f configuration while the other is the 4f-5d transition usually occurring in the two valence RE ions. The forbidden transition between 4f configuration is less affected by the external crystal field and express sharp emission. The lifetime of the excited state is generally from microseconds to milliseconds. The 4f5d transition occurs between the 4f shell and outer 5d shell. The electrons located at 5d shells do not get shelter from the outer shell and the emission peak of the RE ions can be easily affected by the external matrix environment. The luminescence of these transitions is abundant with broadband emission and the lifetime is at microsecond scale. Recently, metal nanoparticles (NPs) have promoted infinite potential in RE ions luminescence enhancement because of their distinctive properties in maneuvering the interactions between NPs and local electromagnetic field [3–7]. This characteristic has made it possible to be ultilized in illumination display, fluorescent probe and optical fiber amplification. The luminescence emission of RE ions, such as Er3+ [8,9], Yb3+ [10,11], Pr3+ [12], Sm3+ [13], Eu2+, and Eu3+ [1,14], has been demonstrated to be greatly enhanced by metallic NPs. The enhancement mechanism can be partly attributed to the energy transfer
⁎
from NPs to RE ions and the enhanced local optical field induced by Surface Plasmon Resonance (SPR) from NPs, especially when the wavelength of incident light is close to the SPR absorption peak [15,16]. When the excitation wavelength matches well with the specific incidence conditions, the free electrons of the metal will be excited and the intense resonance is occurred, which is well known as SPR. SPR exhibits great ability in altering the luminescence intensity of RE ions by enhancing surrounding electromagnetic field. The SPR effect is closely related to the NPs characters, which refers to the composite, shape, size and surrounding environment of the nanostructures as well as the interaction distance with RE ions [17]. Some approaches have been reported in preparing NPs with great performance, such as sol-gel method [18,19], thermal annealing [20], ion implantation [21], electron beam lithography [22], template method [23,24], photochemical reducing technics [25]. Among these methods, thermal annealing method is competitive to prepare NPs in a more efficient and convenient way. In terms of the characteristics of NPs, they can be defined and controlled by changing the heat-treated temperature and atmosphere in thermal annealing method. The application aspect strongly depends on the prescribed properties of NPs embedded in glass matrix. Considering the great optical performance, it's of great significance to achieve a balance between the physical parameters and other competing factors. Regulating the size and geometrical shape of NPs is the simplest way to engineer the optical function of them. Nanoporous silica glass (NPSG) [26], which possesses plentiful non-bridging oxygen as well as network broken bond and
Corresponding author. E-mail address: [email protected] (L. Yang).
https://doi.org/10.1016/j.jlumin.2018.07.041 Received 26 February 2018; Received in revised form 24 July 2018; Accepted 25 July 2018 Available online 26 July 2018 0022-2313/ © 2018 Elsevier B.V. All rights reserved.
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destroys the glass network continuity in physical way, shows a great promising in adsorbing more ions. Moreover, NPSG is equivalent to a kind of molecular sieve, that is, RE ions and NPs can be confined in the nanopores and nanochannels of the glasses. Such flexible interconnected nanostructures have certain effects on the size, shape, distribution as well as the interaction distance with RE ions, which can regulate the growth of NPs in a more accurate way. Accordingly, the emission intensity of RE ions can be adjusted in this way. The trivalent Ytterbium ions are widely used as gain ions in high power fiber laser, which has been deeply investigated. While the divalent Ytterbium ions are rarely studied. Yb2+ is the potential factors contributing to the photodarking in Yb3+ doped fiber laser [27]. As a sensitizer, Yb2+ can enhance the luminescent properties and chromogenic properties of Eu2+, which has widely application prospects in UVLED [28]. Europium is a kind of rare earth element which is widely ultilized in visible luminescent materials. The divalent charge state of Eu2+ shows broad-band emission in the blue and green spectral range [29]. Such materials express great potential in preparing scintillators, white light source, X-ray storage, and etc [30,31]. However, the emission band of divalent charge state of rare earth ions are usually associated with 4f-5d transition which is sensitive to the glass host. Yb2+ radiation shows a very broad band emission nearly covers the whole visible range with ultra-violet or blue laser excitation in the different glass matrix while the Eu2+ shows blue emission under ultra-violet excitation. Thus, it is of great significance to study on the Yb2+ and Eu2+ radiation in NPSG. In this work, NPSG was used as the matrix material to regulate metallic NPs and their interaction with RE ions. The different doping concentration and heat treatments were carried out to control physical features of NPs. The spectroscopic properties were systematically investigated to understand the regulation of RE ions luminescence. The results showed that the luminescence intensity of RE ions could be greatly enhanced by SPR effect induced by metallic NPs in NPSG.
Fig. 1. The SEM image of the nanopores of NPSG.
graphite monochromator with 2θ from 10° to 80°. The absorption spectrum of glass sample in the wavelength range of 400–1100 nm was measured with a Perkin-Elmer-Lambda 950 UV/Vis/NIR spectrophotometer. The luminescence spectrum was measured by Jasco FP6500 Spectrofluorometer. All measurements were taken at room temperature. 3. Results and discussion 3.1. Shape, size and SPR peak Fig. 1 displays the SEM images of the prepared NPSG. The micropore structure labeled shows that the size of nanopores are centrally distributed from 20 nm to 30 nm. The size of the nanopores can be adjusted in a certain range by changing the matrix composite and phase separation technology. To analyze the crystal structure of the Au NPs in NPSG, the X-ray diffraction (XRD) patterns of the sample containing nanoscale Au is measured (Fig. 2). Four different diffraction peaks located at 33.86°, 44.65°, 64.6°, 76.6° can be seen in the Fig. 2, which correspond to (111), (200), (220), (311) crystal face in the gold crystals respectively (JCPDS Card File No. 04-0784). With the increasing of the HAuCl4 concentration, more of Au NPs can be formed, that is, more crystal faces can be exposed in this case. Thus, the diffraction peaks become sharper with higher doping concentration. The size of formed Au NPs can be estimated by the Scherrer equation [33],
2. Experimental Firstly, SiO2, Na2CO3 and H3BO3 were used as the raw materials to fabricate the sodium borosilicate glass with the composition (in mol%) of 66SiO2–26H3BO3–8Na2CO3. All reagents were purchased from Alladin reagent with the purity of 99.99%. The melted bulk glass to the size of 10 mm*10 mm*1 mm could be obtained by mechanically processing. The phase separation and acid treatment were carried out to create interconnected nanopores in NPSG [32]. Secondly, the phase separation of borosilicate glass was carried out at 590 °C for 24 h. After such heat treatment, the glass was separated into two different phases: one phase rich in sodium and boron and the other phase rich in silica. Different phase showed distinct tolerance ability to inorganic acid. The phase rich in sodium and boron could be easily dissolved in hot acid like nitric acid, hydrochloric acid and sulphuric acid, while the other phase, rich in silica, could not be dissolved. Hence, immersed in hot diluted hydrochloric acid for 10 h at 90 °C, the soluble phase rich in sodium and boron of the glass sample could be washed out and left the interconnected SiO2 skeleton. The nanopore size of the NPSG can be adjusted by the host composite and subsequent treatment in a wide range, which determines the absorption degree and homogeneity of doping ions. Then the noble metal ions were introduced into the NPSG by the solution doping method. After soaking the NPSG samples into the mixture solution of × mol% Eu(NO3)3·6H2O or Yb(NO3)3·5H2O and y mol% HAuCl4 for 2 days, the glass samples were heat-treated under different temperatures (600 °C,700 °C, 800 °C, 900 °C, 1000 °C) in 95%Ar/5%H2 reducing atmosphere, respectively. The pore structure of the NPSG was measured by field emission scanning electron microscope (FESEM) Sirion 200 (FEI, NL). The morphology of Au NPs was taken by transmission electron microscope (TEM). Powder X-ray diffraction (XRD) spectrum was recorded by a powder diffractometer with Cu Kα radiation (40 kV × 25 mA) and a
D = Kλ / β cos θ
(1)
Κ- a constant, value is 0.89; λ-X ray wavelength, value is 0.15405 nm; β- the diffraction peak half width; θ- diffraction angle. From the XRD patterns, we can get different diffraction angle θi (i = 1, 2, 3, 4). The size of metallic NPs can be calculated by the Eq. (1). The average particle diameters of Au NPs is around 21.21 nm, which corroborates the regulation function of nanopores on the size of metallic NPs. Fig. 2(b) shows the XRD pattern of the composite system with rare earth element Yb and gold nanoparticles. It can be seen that just four peaks related to the gold nanoparticles exist in the XRD pattern. There is no crystal of rare earth element in the glass. Fig. 3 shows the morphology of the Au NPs in the composite system with NPs and rare earth ions. Fig. 3(a) shows composite system with Au NPs and element Yb with heat-treated temperature of 800 °C while Fig. 3(b) presents the dimension distribution of Au NPs with 1000 °C. It can be seen that more small size Au NPs around 5 nm exist in the visual field of Fig. 3(a). High temperature accelerates the decomposing of 105
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Fig. 2. XRD patterns (a) with HAuCl4 content of 0–0.05 mol%. (b) Au NPs with rare earth ions.
HAuCl4 and more uniform Au NPs can be obtained in this way. Furthermore, the nanopores start to close at high temperature and the shape of Au NPs will be defined in this way. When the temperature continues to increase, many small nanoparticles accumulate together and form a large one. The Au NPs is around 20 nm in the right one.
3.2. Yb-Au co-doped in nanoporous glass Fig. 4 presents the absorption spectrum of Yb-Au co-doped NPSG sample with the heat-treated temperature of 700 °C. Such temperature is not enough to collapse the nanopores which still remain opened. The red shift of SPR peak starts from 510 nm to 523 nm with the increasing HAuCl4 concentration. When the HAuCl4 content increases within a certain range, the number of Au NPs in per unit volume will become larger. This is in favor of more Au NPs accumulating together and forming larger NPs. Accordingly, the SPR peak moves to longer wavelength [34]. The weak absorption peak ascribed to the Yb3+ transition of 2F7/2→2F5/2 can be observed around 1 µm. Most of the Yb3+ ions are reduced to Yb2+ and small amounts of 3+ Yb existed in the fabricated NPSG sample, which contribute slightly to the absorption peak of Yb3+ in Fig. 4. Broad emission of Yb2+ under blue or green excitation is a matter of occurrence in glass ceramics and oxide matrix [35]. Series of silica composite system with Yb and Au NPs were prepared to investigate the influence of the presence of nano-sized Au particles on the luminescence of Yb2+. The emission spectra of Yb-Au NPs co-doped NPSG with different treatment under 532 nm excitation are displayed in Fig. 5. The similar emission shape in Fig. 5(a) and (b) indicates that the emission peak located at 590 nm is contributed to energy level 4f135d1→4f14
Fig. 4. The absorption spectra of 0.2 mol%Yb-xmol%HAuCl4(x = 0.01, 0.02, 0.03) NPSG with heat-treated temperature of 700 °C.
transition of Yb2+ [36]. It is evident that radiation of Yb2+ can be sharply increased when the content of HAuCl4 is fixed at 0.02 mol%. To further determine the optimal doping concentration, 0.2 mol%Yb-xmol % HAuCl4(x = 0.01, 0.02, 0.03) NPSG were prepared and the emission spectrum was measured (in Fig. 5(b)). The maximal luminescence enhancement can reach up to 5.22 folds when the HAuCl4 reaches 0.02 mol%. The 4f135d1→4f14 transition of Yb2+ is sensitive to the nearby crystal field environment, which can be easily affected by the local electromagnetic field induced by SPR effect. Excitation wavelength 532 nm is exactly the resonance wavelength of Au NPs, the nearfield NPs enhance the local electromagnetic field strongly and affects
Fig. 3. The TEM of composited system glass sample with Au NPs and rare earth ions. (a) with the heat-treated temperature of 800 °C. (b) with the heat-treated temperature of 1000 °C. 106
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Fig. 5. The emission spectra of Yb-Au NPs co-doped NPSG with different treatment under 532 nm excitation. (a) Emission spectrum of 0.02 mol% Au NPs-x mol% Yb with heat-treated temperature of 700 °C (b) Emission spectrum of 0. 2 mol% Yb-x mol% Au NPs with heat-treated temperature of 700 °C (c) 0.2 mol% Yb-0.02 mol% Au NPs with different heat-treated temperature (d) The dependence of luminescence enhancement factor on HAuCl4 concentration ranging from 0.01 mol% to 0.03 mol% with different heat-treated temperature.
the ions excitation rate and efficiency [37]. More ions will be excited to the upper level to boost the excitation rate and luminescence efficiency. The emission intensity is proportional to the square of the E-field near the NPs [38]. As the Yb3+ emission region is far away from the Au NPs SPR peak, the formed Au NPs have slight effect on Yb3+ luminescence enhancement. Fig. 5(c) plots the emission spectrum of 0.2 mol% Yb0.02 mol% Au NPs NPSG with varied heat-treated temperature. It is indicated that the luminescence intensity increases firstly with the higher heat-treated temperature and then decreases slightly. Heat treated temperature of 800 °C is the appropriate choice of the prepared NPSG sample. Considering the combined effect of heat treatment and amount of HAuCl4, Fig. 5(d) reveals the relationship between HAuCl4 concentration and luminescence enhancement factor under different heat-treated temperature. Compared with Yb2+ singly doped NPSG, the maximum luminescence enhancement factor can reach up to 14.33 times when the composite doping concentration is 0.2 mol% Yb0.02 mol% Au with the heated temperature of 800 °C.
Fig. 6. The absorption spectra of 0.02 mol% Au-x mol% Eu(x = 0.1, 0.2, 0.3) co-doped NPSG sample with the heat-treated temperature of 600 °C.
3.3. Eu-Au co-doped in nanoporous glass the photoluminescence spectra of composite system of NPSG with Eu and Au NPs under different treatment. The influences of doping concentration on Eu2+ luminescence properties were studied by changing the content of Eu (NO3)3 and HAuCl4. As is shown in Fig. 7(a) and (b), 0.2 mol% Eu3+-0.01 mol% Au is the optimal concentration with 600 °C heat-treated temperature. It is obvious that the main emission peak located at 425 nm is derived from 4f65d1→4f7 transition of Eu2+. The inset of Fig. 7(a) exhibits the excitation spectrum monitored at 425 nm
Fig. 6 plots the absorption spectra of 0.02 mol% Au-x mol% Eu (x = 0.1, 0.2, 0.3) co-doped NPSG sample with the heat-treated temperature of 600 °C when the nanopores are not collapsed. The absorption peak sites at 525 nm corresponding to the SPR peak of Au NPs [39]. The SPR absorption peaks are so strong that no other absorption peaks derived from Eu can be observed in the spectrum. Eu3+ ions can be reduced at a reduction atmosphere under hightemperature conditions and Eu2+ emits blue light [40]. Fig. 7 presents 107
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Fig. 7. The emission spectra of Eu-Au NPs codoped NPSG with different doping concentration and heat treatment under 325 nm excitation. (a) Emission spectrum of 0.02 mol% Au NPs-x mol% Eu with heat-treated temperature of 600 °C, the inset shows the excitation spectra of prepared sample monitored at 425 nm. (b) Emission spectrum of 0. 2 mol% Eu-x mol% Au NPs with heat-treated temperature of 600 °C. (c) 0.2 mol% Yb-0.03 mol% Au NPs with different heat-treated temperature. (d) The dependence of luminescence enhancement factor on HAuCl4 concentration ranging from 0.01 mol% to 0.05 mol% with different heat-treated temperature.
luminescence which can be carefully engineered by the characteristics of the NPs. As the doping concentration is in a certain range, the NPs nucleate to a proper size to achieve strongest luminescence. However, the excess Au NPs either cluster together or shorten the distance with RE ions, the benefit of SPR effect can be weakend and the luminescence of RE ions begins to decrease [42]. As for the heat-treated temperature, higher heat-treated temperature is in favor of the shaping of Au NPs due to the regulation of the state of nanopores in NPSG. Temperature accelerates the decomposing of HAuCl4 and more uniform Au NPs can be obtained in this way. When the heat-treated temperature is 700 °C, the luminescence intensity increased with a relatively lower enhancement factor. The state of nanopores starts to close with the higher heattreated temperature. The closing state nanopores define the shape and distribution of Au NPs, therefore the luminescence intensity is higher. However, when the temperature exceeds a certain range, the distributed Au NPs tend to accumulate together to form larger NPs and the distance between Au NPs and RE ions get restricted, the advantages of SPR effect will be eliminated. The detailed interaction mechanism between RE ions and Au NPs needs to be further studied.
ranging from 250 nm to 400 nm, which confirms that 325 nm is the proper excitation wavelength. The Eu2+ emission can be 2.47 times stronger with the existence of Au NPs than that of Eu2+ singly doped glass sample. It is known that the 4f65d1→4f7 transition of Eu2+ belongs to electric dipole transition, which can be remarkably affected by the varied electromagnetic field induced by SPR effect. When the emission wavelength overlaps with the SPR absorption peak of Au NPs, the excited RE ions transfer their energy to Au NPs and form SP Polaritons (SPP). The SPPs absorb the energy of excitation light and emit the light with same frequency. Thus, the luminescence intensity is expected to be stronger with the increased transition rate of the RE ions [41]. In Fig. 7(a) and (b), there is a weak peak at 592 nm which is ascribed to Eu3+ 5D0→7F1 transition. Fig. 7(c) plots the emission spectra of fabricated 0.2 mol% Eu-0.03 mol% Au NPs NPSG samples in accordance with different heat-treated temperature. The luminescence intensity of Eu2+ can be enhanced drastically with proper heat-treated temperature, as shown in the Fig. 7(c). The 0.2 mol% Eu-x mol% Au NPs (x = 0.1, 0.3, 0.5) NPSG sample with different heat-treated temperature were prepared to investigate the comprehensive results of the doping content and heat treatment. Fig. 7(d) demonstrates the variation of enhancement factor under such mixture effects. It is obvious that the luminescence enhancement factor reaches its maximum when the HAuCl4 content is 0.01 mol% of 1000 °C heat-treated temperature. With such treatment, the luminescence intensity can be 10.52 times stronger than that of Eu2+ singly doped glass. It is universally known that the near-field interactions between RE ions and metallic NPs is the main reason resulting in metal enhanced luminescence. The SPR properties determine the surface-enhanced
4. Conclusions The silica composite system with RE ions and Au NPs was fabricated based on nanoporous silica glass. The doping concentration and heattreated temperature were adjusted to regulate Au NPs for RE ions luminescence enhancement in NPSG. luminescence investigation has demonstrated that higher heat-treated temperature and relatively low doping concentration are in favor of the emission enhancement. When 108
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the 0.2 mol% Eu-0.01 mol% Au NPs doped NPSG was annealed at 1000 °C, the luminescence intensity of Eu2+ can be enhanced up to 10.52 times stronger than that of Eu2+ singly doped sample. Furthermore, the emission intensity of Yb2+ can be enhanced up to 14.33 folds when the NPSG was doped with 0.2 mol% Yb-0.02 mol% Au NPs under heat-treated temperature of 800 °C. The results show that RE ions luminescence intensity can be significantly enhanced by Au NPs in NPSG. A balance between quenching and a local field enhancement can improve the luminescence of RE ions. This study gives us a new orientation to regulate the emission of RE ions.
(2015) 203–207. [13] N.M. Yusoff, M.R. Sahar, S.K. Ghoshal, J. Mol. Struct. 1079 (2015) 167–172. [14] L. Li, Y. Yang, D. Zhou, Z. Yang, X. Xu, J. Qiu, Opt. Mater. Express 3 (2013) 806–812. [15] L.R.P. Kassab, C.B. de Araújo, R.A. Kobayashi, R. de Almeida Pinto, D.M. da Silva, J. Appl. Phys. 102 (2007) 103515. [16] M. Eichelbaum, B.E. Schmidt, H. Ibrahim, K. Rademann, Nanotechnology 18101 (2007) 1366–1370. [17] F. Magnan, J. Gagnon, F.-G. Fontaine, D. Boudreau, Chem. Commun. 49 (2013) 9299–9301. [18] Q. Wang, Q. Shi, S. Li, D. Zhang, W. Wang, J. Fluoresc. 177 (2016) 295–298. [19] M. Fukushima, N. Managaki, M. Fujii, H. Yanagi, S. Hayashi, J. Appl. Phys. 98 (2005) 275. [20] L.R.P. Kassab, F.A. Bomfim, J.R. Martinelli, N.U. Wetter, J.J. Neto, Cid B. de Araujo, Appl. Phys. B 94 (2009) 239–242. [21] A.L. Stepanov, V.N. Popok, Surf. Sci. 566–568 (2004) 1250–1254. [22] L. Wang, W. Xiong, Y. Nishijima, Y. Yokota, K. Ueno, H. Misawa, G. Bi, Jianrong Qiu, Opt. Express 19 (2011) 10640–10646. [23] E. Prodan, C. Radloff, N.J. Halas, P. Nordlander, Science 302 (2003) 419. [24] Y. Wang, P. Chen, M. Liu, Nanotechnology 19 (2008) 045607. [25] C. Xue, G.S. Métraux, J.E. Millstone, C.A. Mirkin, J. Am. Chem. 130 (2008) 8337–8344. [26] Y. Chu, Y. Ma, Y. Yang, L. Liao, Y. Wang, X. Hu, J. Peng, H. Li, N. Dai, J. Li, L. Yang, Opt. Lett. 41 (2016) 1225–1228. [27] A.D.C. Guzman, A.V. Kiryanov, Y.O. Barmenkov, N.N. Ilichev, Laser Phys. Lett. 4 (2007) 734–739. [28] J. Ruan, R.J. Xie, N. Hirosaki, J. Electrochem. Soc. 159 (2012) 66–71. [29] W. Chewpraditkul, D. Chen, B. Yu, Phys. Status Solidi-R 5 (2011) 40–42. [30] T. Nakanishi, S. Tanabe, IEEE J. Sel. Top. Quant. 15 (2009) 1171–1176. [31] J. Manam, P. Kumari, S. Das, Appl. Phys. A 104 (2011) 197–203. [32] Y. Chu, Y. Yang, Z. Liu, L. Liao, Y. Wang, J. Li, L. Yang, Appl. Phys. A 118 (2015) 1429–1435. [33] B.D. Cullity, Answers to Problems: Elements of X-ray Diffraction, Addison-Wesley Publishing Company, 1978. [34] C.M. Cobley, M. Rycenga, F. Zhou, Z.Y. Li, Y. Xia, J. Phys. Chem. C 113 (2009) 16975–16982. [35] A.A. Assadi, A. Herrmann, K. Damak, C. Rüssel, R. Maalej, Int. J. Appl. Glass Sci. (2017) 1–7. [36] M. Engholm, L. Norin, D. Åberg, Opt. Lett. 32 (2007) 3342. [37] F. Lü, H. Zheng, Y. Fang, Prog. Chem. 19 (2007) 256–266. [38] J. Zhang, Y. Fu, M.H. Chowdhury, J.R. Lakowicz, Nano Lett. 7 (2007) 2101. [39] J. Zheng, C. Zhou, Yu M, J. Liu, Nanoscale 4 (2012) 4073–4083. [40] W. Chewpraditkul, D. Chen, B. Yu, Q. Zhang, Y. Shen, M. Nikl, Phys. Status Solidi-R 5 (2011) 40–42. [41] A.A. Kinkhabwala, K. Mullen, S. Fan, W.E. Moerner, Y. Avlasevich, Z. Yu, Nat. Photon. 3 (2009) 654–657. [42] A.L. Feng, M. Lin, L. Tian, H.Y. Zhu, H. Guo, S. Singamaneni, Z. Duan, T.J. Lu, F. Xu, RSC Adv. 5 (2015) 76825–76835.
Acknowledgments Yunxiu Ma and Zhangru Chen contributed equally to this work and should be considered co-first authors. This study was supported by the National Natural Science Foundation of China (Grant No. 51302092). The authors would also like to thank the Huazhong University of Science and Technology, Analytical and Testing Center for sharing their equipment. References [1] G. Schider, J.R. Krenn, W. Gotschy, B. Lamprecht, J. Appl. Phys. 90 (2001) 3825–3830. [2] O.L. Malta, P.A. Santa-Cruz, G.F. De Sá, F. Auzel, J. Fluoresc. 33 (1985) 261–272. [3] R. Das, P. Phadke, N. Khichar, S. Chawla, J. Mater. Chem. C 2 (2014) 8880–8885. [4] M.E. Stewart, C.R. Anderton, L.B. Thompson, J. Maria, S.K. Gray, J.A. Roger, Chem. Rev. 108 (2008) 494–521. [5] J. Wang, H. Huang, D. Zhang, M. Chen, Y. Zhang, X. Yu, L. Zhou, Q. Wang, Nano Res. 8 (2015) 2548–2561. [6] M. Runowski, S. Goderski, J. Paczesny, M. Księżopolska-Gocalska, A. Ekner-Grzyb, T. Grzyb, J.D. Rybka, M. Giersig, S. Lis, J. Phys. Chem. C 120 (2016) 23788–23798. [7] A. Priyam, N.M. Idris, Y. Zhang, Gold nanoshell coated NaYF4 nanoparticles for simultaneously enhanced upconversion luminescence and darkfield imaging, J. Mater. Chem. 22 (2012) 960–965. [8] W. Ge, X.R. Zhang, M. Liu, Z.W. Lei, R.J. Knize, Y. Lu, Distance dependence of goldenhanced upconversion luminescence in Au/SiO2/Y2O3:Yb3+, Er3+ nanoparticles, Theranostics 3 (2013) 282–288. [9] H. Fares, H. Elhouichet, B. Gelloz, M. Ferid, J. Appl. Phys. 117 (2015) 205. [10] M. Runowski, J. Lumin. 186 (2017) 199–204. [11] S.Q. Wang, J.B. Qiu, X.H. Xu, Q. Wang, D.C. Zhou, Z.W. Yang, Z.G. Song, Opt. Mater. Express 6 (2016) 1065–1078. [12] Y.Y. Du, B.J. Chen, E.Y.B. Pun, Z.Q. Wang, X. Zhao, H. Lin, Opt. Commun. 334
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Regulation of gold nanoparticles for the rare earth luminescence enhancement based on nanoporous silica glass
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Yunxiu Ma, Zhangru Chen, Yingbo Chu, Yu Yang, Yongguang Liu, Haiqing Li, Jinggang Peng, ⁎ Nengli Dai, Jinyan Li, Luyun Yang Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Nanoporous silica glass Rare earth ions Nanoparticles Luminescence intensity
A silica composite system with rare earth ions and gold nanoparticles was fabricated based on nanoporous silica glass. The formation and optical properties of the systems were controlled by the nanoparticles doping concentration and heat treatment. The luminescence intensity dependence on the HAuCl4 doping concentration and heat-treated temperature were investigated. At the optimized concentration of HAuCl4 and heat-treated temperature, the maximal luminescence intensity can be enhanced up to 14.33-fold for Yb2+ and 10.52-fold for Eu2+. It is indicated that the rare earth ions luminescence can be regulated by interactions distance between different types of RE ions and metal NPs as well as the state of nanopores.
1. Introduction The unique electron shell structure of rare earth (RE) ions allows them to be widely utilized in illumination, displaying, data storage and fiber devices [1–3]. In the electron shell structure of RE ions, the 4f electron shell is affected slightly by external atmosphere under the shielding of outer 5s and 5p electrons. There are two transitions of RE ions, one is the forbidden transition in the 4f configuration while the other is the 4f-5d transition usually occurring in the two valence RE ions. The forbidden transition between 4f configuration is less affected by the external crystal field and express sharp emission. The lifetime of the excited state is generally from microseconds to milliseconds. The 4f5d transition occurs between the 4f shell and outer 5d shell. The electrons located at 5d shells do not get shelter from the outer shell and the emission peak of the RE ions can be easily affected by the external matrix environment. The luminescence of these transitions is abundant with broadband emission and the lifetime is at microsecond scale. Recently, metal nanoparticles (NPs) have promoted infinite potential in RE ions luminescence enhancement because of their distinctive properties in maneuvering the interactions between NPs and local electromagnetic field [3–7]. This characteristic has made it possible to be ultilized in illumination display, fluorescent probe and optical fiber amplification. The luminescence emission of RE ions, such as Er3+ [8,9], Yb3+ [10,11], Pr3+ [12], Sm3+ [13], Eu2+, and Eu3+ [1,14], has been demonstrated to be greatly enhanced by metallic NPs. The enhancement mechanism can be partly attributed to the energy transfer
⁎
from NPs to RE ions and the enhanced local optical field induced by Surface Plasmon Resonance (SPR) from NPs, especially when the wavelength of incident light is close to the SPR absorption peak [15,16]. When the excitation wavelength matches well with the specific incidence conditions, the free electrons of the metal will be excited and the intense resonance is occurred, which is well known as SPR. SPR exhibits great ability in altering the luminescence intensity of RE ions by enhancing surrounding electromagnetic field. The SPR effect is closely related to the NPs characters, which refers to the composite, shape, size and surrounding environment of the nanostructures as well as the interaction distance with RE ions [17]. Some approaches have been reported in preparing NPs with great performance, such as sol-gel method [18,19], thermal annealing [20], ion implantation [21], electron beam lithography [22], template method [23,24], photochemical reducing technics [25]. Among these methods, thermal annealing method is competitive to prepare NPs in a more efficient and convenient way. In terms of the characteristics of NPs, they can be defined and controlled by changing the heat-treated temperature and atmosphere in thermal annealing method. The application aspect strongly depends on the prescribed properties of NPs embedded in glass matrix. Considering the great optical performance, it's of great significance to achieve a balance between the physical parameters and other competing factors. Regulating the size and geometrical shape of NPs is the simplest way to engineer the optical function of them. Nanoporous silica glass (NPSG) [26], which possesses plentiful non-bridging oxygen as well as network broken bond and
Corresponding author. E-mail address: [email protected] (L. Yang).
https://doi.org/10.1016/j.jlumin.2018.07.041 Received 26 February 2018; Received in revised form 24 July 2018; Accepted 25 July 2018 Available online 26 July 2018 0022-2313/ © 2018 Elsevier B.V. All rights reserved.
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destroys the glass network continuity in physical way, shows a great promising in adsorbing more ions. Moreover, NPSG is equivalent to a kind of molecular sieve, that is, RE ions and NPs can be confined in the nanopores and nanochannels of the glasses. Such flexible interconnected nanostructures have certain effects on the size, shape, distribution as well as the interaction distance with RE ions, which can regulate the growth of NPs in a more accurate way. Accordingly, the emission intensity of RE ions can be adjusted in this way. The trivalent Ytterbium ions are widely used as gain ions in high power fiber laser, which has been deeply investigated. While the divalent Ytterbium ions are rarely studied. Yb2+ is the potential factors contributing to the photodarking in Yb3+ doped fiber laser [27]. As a sensitizer, Yb2+ can enhance the luminescent properties and chromogenic properties of Eu2+, which has widely application prospects in UVLED [28]. Europium is a kind of rare earth element which is widely ultilized in visible luminescent materials. The divalent charge state of Eu2+ shows broad-band emission in the blue and green spectral range [29]. Such materials express great potential in preparing scintillators, white light source, X-ray storage, and etc [30,31]. However, the emission band of divalent charge state of rare earth ions are usually associated with 4f-5d transition which is sensitive to the glass host. Yb2+ radiation shows a very broad band emission nearly covers the whole visible range with ultra-violet or blue laser excitation in the different glass matrix while the Eu2+ shows blue emission under ultra-violet excitation. Thus, it is of great significance to study on the Yb2+ and Eu2+ radiation in NPSG. In this work, NPSG was used as the matrix material to regulate metallic NPs and their interaction with RE ions. The different doping concentration and heat treatments were carried out to control physical features of NPs. The spectroscopic properties were systematically investigated to understand the regulation of RE ions luminescence. The results showed that the luminescence intensity of RE ions could be greatly enhanced by SPR effect induced by metallic NPs in NPSG.
Fig. 1. The SEM image of the nanopores of NPSG.
graphite monochromator with 2θ from 10° to 80°. The absorption spectrum of glass sample in the wavelength range of 400–1100 nm was measured with a Perkin-Elmer-Lambda 950 UV/Vis/NIR spectrophotometer. The luminescence spectrum was measured by Jasco FP6500 Spectrofluorometer. All measurements were taken at room temperature. 3. Results and discussion 3.1. Shape, size and SPR peak Fig. 1 displays the SEM images of the prepared NPSG. The micropore structure labeled shows that the size of nanopores are centrally distributed from 20 nm to 30 nm. The size of the nanopores can be adjusted in a certain range by changing the matrix composite and phase separation technology. To analyze the crystal structure of the Au NPs in NPSG, the X-ray diffraction (XRD) patterns of the sample containing nanoscale Au is measured (Fig. 2). Four different diffraction peaks located at 33.86°, 44.65°, 64.6°, 76.6° can be seen in the Fig. 2, which correspond to (111), (200), (220), (311) crystal face in the gold crystals respectively (JCPDS Card File No. 04-0784). With the increasing of the HAuCl4 concentration, more of Au NPs can be formed, that is, more crystal faces can be exposed in this case. Thus, the diffraction peaks become sharper with higher doping concentration. The size of formed Au NPs can be estimated by the Scherrer equation [33],
2. Experimental Firstly, SiO2, Na2CO3 and H3BO3 were used as the raw materials to fabricate the sodium borosilicate glass with the composition (in mol%) of 66SiO2–26H3BO3–8Na2CO3. All reagents were purchased from Alladin reagent with the purity of 99.99%. The melted bulk glass to the size of 10 mm*10 mm*1 mm could be obtained by mechanically processing. The phase separation and acid treatment were carried out to create interconnected nanopores in NPSG [32]. Secondly, the phase separation of borosilicate glass was carried out at 590 °C for 24 h. After such heat treatment, the glass was separated into two different phases: one phase rich in sodium and boron and the other phase rich in silica. Different phase showed distinct tolerance ability to inorganic acid. The phase rich in sodium and boron could be easily dissolved in hot acid like nitric acid, hydrochloric acid and sulphuric acid, while the other phase, rich in silica, could not be dissolved. Hence, immersed in hot diluted hydrochloric acid for 10 h at 90 °C, the soluble phase rich in sodium and boron of the glass sample could be washed out and left the interconnected SiO2 skeleton. The nanopore size of the NPSG can be adjusted by the host composite and subsequent treatment in a wide range, which determines the absorption degree and homogeneity of doping ions. Then the noble metal ions were introduced into the NPSG by the solution doping method. After soaking the NPSG samples into the mixture solution of × mol% Eu(NO3)3·6H2O or Yb(NO3)3·5H2O and y mol% HAuCl4 for 2 days, the glass samples were heat-treated under different temperatures (600 °C,700 °C, 800 °C, 900 °C, 1000 °C) in 95%Ar/5%H2 reducing atmosphere, respectively. The pore structure of the NPSG was measured by field emission scanning electron microscope (FESEM) Sirion 200 (FEI, NL). The morphology of Au NPs was taken by transmission electron microscope (TEM). Powder X-ray diffraction (XRD) spectrum was recorded by a powder diffractometer with Cu Kα radiation (40 kV × 25 mA) and a
D = Kλ / β cos θ
(1)
Κ- a constant, value is 0.89; λ-X ray wavelength, value is 0.15405 nm; β- the diffraction peak half width; θ- diffraction angle. From the XRD patterns, we can get different diffraction angle θi (i = 1, 2, 3, 4). The size of metallic NPs can be calculated by the Eq. (1). The average particle diameters of Au NPs is around 21.21 nm, which corroborates the regulation function of nanopores on the size of metallic NPs. Fig. 2(b) shows the XRD pattern of the composite system with rare earth element Yb and gold nanoparticles. It can be seen that just four peaks related to the gold nanoparticles exist in the XRD pattern. There is no crystal of rare earth element in the glass. Fig. 3 shows the morphology of the Au NPs in the composite system with NPs and rare earth ions. Fig. 3(a) shows composite system with Au NPs and element Yb with heat-treated temperature of 800 °C while Fig. 3(b) presents the dimension distribution of Au NPs with 1000 °C. It can be seen that more small size Au NPs around 5 nm exist in the visual field of Fig. 3(a). High temperature accelerates the decomposing of 105
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Fig. 2. XRD patterns (a) with HAuCl4 content of 0–0.05 mol%. (b) Au NPs with rare earth ions.
HAuCl4 and more uniform Au NPs can be obtained in this way. Furthermore, the nanopores start to close at high temperature and the shape of Au NPs will be defined in this way. When the temperature continues to increase, many small nanoparticles accumulate together and form a large one. The Au NPs is around 20 nm in the right one.
3.2. Yb-Au co-doped in nanoporous glass Fig. 4 presents the absorption spectrum of Yb-Au co-doped NPSG sample with the heat-treated temperature of 700 °C. Such temperature is not enough to collapse the nanopores which still remain opened. The red shift of SPR peak starts from 510 nm to 523 nm with the increasing HAuCl4 concentration. When the HAuCl4 content increases within a certain range, the number of Au NPs in per unit volume will become larger. This is in favor of more Au NPs accumulating together and forming larger NPs. Accordingly, the SPR peak moves to longer wavelength [34]. The weak absorption peak ascribed to the Yb3+ transition of 2F7/2→2F5/2 can be observed around 1 µm. Most of the Yb3+ ions are reduced to Yb2+ and small amounts of 3+ Yb existed in the fabricated NPSG sample, which contribute slightly to the absorption peak of Yb3+ in Fig. 4. Broad emission of Yb2+ under blue or green excitation is a matter of occurrence in glass ceramics and oxide matrix [35]. Series of silica composite system with Yb and Au NPs were prepared to investigate the influence of the presence of nano-sized Au particles on the luminescence of Yb2+. The emission spectra of Yb-Au NPs co-doped NPSG with different treatment under 532 nm excitation are displayed in Fig. 5. The similar emission shape in Fig. 5(a) and (b) indicates that the emission peak located at 590 nm is contributed to energy level 4f135d1→4f14
Fig. 4. The absorption spectra of 0.2 mol%Yb-xmol%HAuCl4(x = 0.01, 0.02, 0.03) NPSG with heat-treated temperature of 700 °C.
transition of Yb2+ [36]. It is evident that radiation of Yb2+ can be sharply increased when the content of HAuCl4 is fixed at 0.02 mol%. To further determine the optimal doping concentration, 0.2 mol%Yb-xmol % HAuCl4(x = 0.01, 0.02, 0.03) NPSG were prepared and the emission spectrum was measured (in Fig. 5(b)). The maximal luminescence enhancement can reach up to 5.22 folds when the HAuCl4 reaches 0.02 mol%. The 4f135d1→4f14 transition of Yb2+ is sensitive to the nearby crystal field environment, which can be easily affected by the local electromagnetic field induced by SPR effect. Excitation wavelength 532 nm is exactly the resonance wavelength of Au NPs, the nearfield NPs enhance the local electromagnetic field strongly and affects
Fig. 3. The TEM of composited system glass sample with Au NPs and rare earth ions. (a) with the heat-treated temperature of 800 °C. (b) with the heat-treated temperature of 1000 °C. 106
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Fig. 5. The emission spectra of Yb-Au NPs co-doped NPSG with different treatment under 532 nm excitation. (a) Emission spectrum of 0.02 mol% Au NPs-x mol% Yb with heat-treated temperature of 700 °C (b) Emission spectrum of 0. 2 mol% Yb-x mol% Au NPs with heat-treated temperature of 700 °C (c) 0.2 mol% Yb-0.02 mol% Au NPs with different heat-treated temperature (d) The dependence of luminescence enhancement factor on HAuCl4 concentration ranging from 0.01 mol% to 0.03 mol% with different heat-treated temperature.
the ions excitation rate and efficiency [37]. More ions will be excited to the upper level to boost the excitation rate and luminescence efficiency. The emission intensity is proportional to the square of the E-field near the NPs [38]. As the Yb3+ emission region is far away from the Au NPs SPR peak, the formed Au NPs have slight effect on Yb3+ luminescence enhancement. Fig. 5(c) plots the emission spectrum of 0.2 mol% Yb0.02 mol% Au NPs NPSG with varied heat-treated temperature. It is indicated that the luminescence intensity increases firstly with the higher heat-treated temperature and then decreases slightly. Heat treated temperature of 800 °C is the appropriate choice of the prepared NPSG sample. Considering the combined effect of heat treatment and amount of HAuCl4, Fig. 5(d) reveals the relationship between HAuCl4 concentration and luminescence enhancement factor under different heat-treated temperature. Compared with Yb2+ singly doped NPSG, the maximum luminescence enhancement factor can reach up to 14.33 times when the composite doping concentration is 0.2 mol% Yb0.02 mol% Au with the heated temperature of 800 °C.
Fig. 6. The absorption spectra of 0.02 mol% Au-x mol% Eu(x = 0.1, 0.2, 0.3) co-doped NPSG sample with the heat-treated temperature of 600 °C.
3.3. Eu-Au co-doped in nanoporous glass the photoluminescence spectra of composite system of NPSG with Eu and Au NPs under different treatment. The influences of doping concentration on Eu2+ luminescence properties were studied by changing the content of Eu (NO3)3 and HAuCl4. As is shown in Fig. 7(a) and (b), 0.2 mol% Eu3+-0.01 mol% Au is the optimal concentration with 600 °C heat-treated temperature. It is obvious that the main emission peak located at 425 nm is derived from 4f65d1→4f7 transition of Eu2+. The inset of Fig. 7(a) exhibits the excitation spectrum monitored at 425 nm
Fig. 6 plots the absorption spectra of 0.02 mol% Au-x mol% Eu (x = 0.1, 0.2, 0.3) co-doped NPSG sample with the heat-treated temperature of 600 °C when the nanopores are not collapsed. The absorption peak sites at 525 nm corresponding to the SPR peak of Au NPs [39]. The SPR absorption peaks are so strong that no other absorption peaks derived from Eu can be observed in the spectrum. Eu3+ ions can be reduced at a reduction atmosphere under hightemperature conditions and Eu2+ emits blue light [40]. Fig. 7 presents 107
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Fig. 7. The emission spectra of Eu-Au NPs codoped NPSG with different doping concentration and heat treatment under 325 nm excitation. (a) Emission spectrum of 0.02 mol% Au NPs-x mol% Eu with heat-treated temperature of 600 °C, the inset shows the excitation spectra of prepared sample monitored at 425 nm. (b) Emission spectrum of 0. 2 mol% Eu-x mol% Au NPs with heat-treated temperature of 600 °C. (c) 0.2 mol% Yb-0.03 mol% Au NPs with different heat-treated temperature. (d) The dependence of luminescence enhancement factor on HAuCl4 concentration ranging from 0.01 mol% to 0.05 mol% with different heat-treated temperature.
luminescence which can be carefully engineered by the characteristics of the NPs. As the doping concentration is in a certain range, the NPs nucleate to a proper size to achieve strongest luminescence. However, the excess Au NPs either cluster together or shorten the distance with RE ions, the benefit of SPR effect can be weakend and the luminescence of RE ions begins to decrease [42]. As for the heat-treated temperature, higher heat-treated temperature is in favor of the shaping of Au NPs due to the regulation of the state of nanopores in NPSG. Temperature accelerates the decomposing of HAuCl4 and more uniform Au NPs can be obtained in this way. When the heat-treated temperature is 700 °C, the luminescence intensity increased with a relatively lower enhancement factor. The state of nanopores starts to close with the higher heattreated temperature. The closing state nanopores define the shape and distribution of Au NPs, therefore the luminescence intensity is higher. However, when the temperature exceeds a certain range, the distributed Au NPs tend to accumulate together to form larger NPs and the distance between Au NPs and RE ions get restricted, the advantages of SPR effect will be eliminated. The detailed interaction mechanism between RE ions and Au NPs needs to be further studied.
ranging from 250 nm to 400 nm, which confirms that 325 nm is the proper excitation wavelength. The Eu2+ emission can be 2.47 times stronger with the existence of Au NPs than that of Eu2+ singly doped glass sample. It is known that the 4f65d1→4f7 transition of Eu2+ belongs to electric dipole transition, which can be remarkably affected by the varied electromagnetic field induced by SPR effect. When the emission wavelength overlaps with the SPR absorption peak of Au NPs, the excited RE ions transfer their energy to Au NPs and form SP Polaritons (SPP). The SPPs absorb the energy of excitation light and emit the light with same frequency. Thus, the luminescence intensity is expected to be stronger with the increased transition rate of the RE ions [41]. In Fig. 7(a) and (b), there is a weak peak at 592 nm which is ascribed to Eu3+ 5D0→7F1 transition. Fig. 7(c) plots the emission spectra of fabricated 0.2 mol% Eu-0.03 mol% Au NPs NPSG samples in accordance with different heat-treated temperature. The luminescence intensity of Eu2+ can be enhanced drastically with proper heat-treated temperature, as shown in the Fig. 7(c). The 0.2 mol% Eu-x mol% Au NPs (x = 0.1, 0.3, 0.5) NPSG sample with different heat-treated temperature were prepared to investigate the comprehensive results of the doping content and heat treatment. Fig. 7(d) demonstrates the variation of enhancement factor under such mixture effects. It is obvious that the luminescence enhancement factor reaches its maximum when the HAuCl4 content is 0.01 mol% of 1000 °C heat-treated temperature. With such treatment, the luminescence intensity can be 10.52 times stronger than that of Eu2+ singly doped glass. It is universally known that the near-field interactions between RE ions and metallic NPs is the main reason resulting in metal enhanced luminescence. The SPR properties determine the surface-enhanced
4. Conclusions The silica composite system with RE ions and Au NPs was fabricated based on nanoporous silica glass. The doping concentration and heattreated temperature were adjusted to regulate Au NPs for RE ions luminescence enhancement in NPSG. luminescence investigation has demonstrated that higher heat-treated temperature and relatively low doping concentration are in favor of the emission enhancement. When 108
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the 0.2 mol% Eu-0.01 mol% Au NPs doped NPSG was annealed at 1000 °C, the luminescence intensity of Eu2+ can be enhanced up to 10.52 times stronger than that of Eu2+ singly doped sample. Furthermore, the emission intensity of Yb2+ can be enhanced up to 14.33 folds when the NPSG was doped with 0.2 mol% Yb-0.02 mol% Au NPs under heat-treated temperature of 800 °C. The results show that RE ions luminescence intensity can be significantly enhanced by Au NPs in NPSG. A balance between quenching and a local field enhancement can improve the luminescence of RE ions. This study gives us a new orientation to regulate the emission of RE ions.
(2015) 203–207. [13] N.M. Yusoff, M.R. Sahar, S.K. Ghoshal, J. Mol. Struct. 1079 (2015) 167–172. [14] L. Li, Y. Yang, D. Zhou, Z. Yang, X. Xu, J. Qiu, Opt. Mater. Express 3 (2013) 806–812. [15] L.R.P. Kassab, C.B. de Araújo, R.A. Kobayashi, R. de Almeida Pinto, D.M. da Silva, J. Appl. Phys. 102 (2007) 103515. [16] M. Eichelbaum, B.E. Schmidt, H. Ibrahim, K. Rademann, Nanotechnology 18101 (2007) 1366–1370. [17] F. Magnan, J. Gagnon, F.-G. Fontaine, D. Boudreau, Chem. Commun. 49 (2013) 9299–9301. [18] Q. Wang, Q. Shi, S. Li, D. Zhang, W. Wang, J. Fluoresc. 177 (2016) 295–298. [19] M. Fukushima, N. Managaki, M. Fujii, H. Yanagi, S. Hayashi, J. Appl. Phys. 98 (2005) 275. [20] L.R.P. Kassab, F.A. Bomfim, J.R. Martinelli, N.U. Wetter, J.J. Neto, Cid B. de Araujo, Appl. Phys. B 94 (2009) 239–242. [21] A.L. Stepanov, V.N. Popok, Surf. Sci. 566–568 (2004) 1250–1254. [22] L. Wang, W. Xiong, Y. Nishijima, Y. Yokota, K. Ueno, H. Misawa, G. Bi, Jianrong Qiu, Opt. Express 19 (2011) 10640–10646. [23] E. Prodan, C. Radloff, N.J. Halas, P. Nordlander, Science 302 (2003) 419. [24] Y. Wang, P. Chen, M. Liu, Nanotechnology 19 (2008) 045607. [25] C. Xue, G.S. Métraux, J.E. Millstone, C.A. Mirkin, J. Am. Chem. 130 (2008) 8337–8344. [26] Y. Chu, Y. Ma, Y. Yang, L. Liao, Y. Wang, X. Hu, J. Peng, H. Li, N. Dai, J. Li, L. Yang, Opt. Lett. 41 (2016) 1225–1228. [27] A.D.C. Guzman, A.V. Kiryanov, Y.O. Barmenkov, N.N. Ilichev, Laser Phys. Lett. 4 (2007) 734–739. [28] J. Ruan, R.J. Xie, N. Hirosaki, J. Electrochem. Soc. 159 (2012) 66–71. [29] W. Chewpraditkul, D. Chen, B. Yu, Phys. Status Solidi-R 5 (2011) 40–42. [30] T. Nakanishi, S. Tanabe, IEEE J. Sel. Top. Quant. 15 (2009) 1171–1176. [31] J. Manam, P. Kumari, S. Das, Appl. Phys. A 104 (2011) 197–203. [32] Y. Chu, Y. Yang, Z. Liu, L. Liao, Y. Wang, J. Li, L. Yang, Appl. Phys. A 118 (2015) 1429–1435. [33] B.D. Cullity, Answers to Problems: Elements of X-ray Diffraction, Addison-Wesley Publishing Company, 1978. [34] C.M. Cobley, M. Rycenga, F. Zhou, Z.Y. Li, Y. Xia, J. Phys. Chem. C 113 (2009) 16975–16982. [35] A.A. Assadi, A. Herrmann, K. Damak, C. Rüssel, R. Maalej, Int. J. Appl. Glass Sci. (2017) 1–7. [36] M. Engholm, L. Norin, D. Åberg, Opt. Lett. 32 (2007) 3342. [37] F. Lü, H. Zheng, Y. Fang, Prog. Chem. 19 (2007) 256–266. [38] J. Zhang, Y. Fu, M.H. Chowdhury, J.R. Lakowicz, Nano Lett. 7 (2007) 2101. [39] J. Zheng, C. Zhou, Yu M, J. Liu, Nanoscale 4 (2012) 4073–4083. [40] W. Chewpraditkul, D. Chen, B. Yu, Q. Zhang, Y. Shen, M. Nikl, Phys. Status Solidi-R 5 (2011) 40–42. [41] A.A. Kinkhabwala, K. Mullen, S. Fan, W.E. Moerner, Y. Avlasevich, Z. Yu, Nat. Photon. 3 (2009) 654–657. [42] A.L. Feng, M. Lin, L. Tian, H.Y. Zhu, H. Guo, S. Singamaneni, Z. Duan, T.J. Lu, F. Xu, RSC Adv. 5 (2015) 76825–76835.
Acknowledgments Yunxiu Ma and Zhangru Chen contributed equally to this work and should be considered co-first authors. This study was supported by the National Natural Science Foundation of China (Grant No. 51302092). The authors would also like to thank the Huazhong University of Science and Technology, Analytical and Testing Center for sharing their equipment. References [1] G. Schider, J.R. Krenn, W. Gotschy, B. Lamprecht, J. Appl. Phys. 90 (2001) 3825–3830. [2] O.L. Malta, P.A. Santa-Cruz, G.F. De Sá, F. Auzel, J. Fluoresc. 33 (1985) 261–272. [3] R. Das, P. Phadke, N. Khichar, S. Chawla, J. Mater. Chem. C 2 (2014) 8880–8885. [4] M.E. Stewart, C.R. Anderton, L.B. Thompson, J. Maria, S.K. Gray, J.A. Roger, Chem. Rev. 108 (2008) 494–521. [5] J. Wang, H. Huang, D. Zhang, M. Chen, Y. Zhang, X. Yu, L. Zhou, Q. Wang, Nano Res. 8 (2015) 2548–2561. [6] M. Runowski, S. Goderski, J. Paczesny, M. Księżopolska-Gocalska, A. Ekner-Grzyb, T. Grzyb, J.D. Rybka, M. Giersig, S. Lis, J. Phys. Chem. C 120 (2016) 23788–23798. [7] A. Priyam, N.M. Idris, Y. Zhang, Gold nanoshell coated NaYF4 nanoparticles for simultaneously enhanced upconversion luminescence and darkfield imaging, J. Mater. Chem. 22 (2012) 960–965. [8] W. Ge, X.R. Zhang, M. Liu, Z.W. Lei, R.J. Knize, Y. Lu, Distance dependence of goldenhanced upconversion luminescence in Au/SiO2/Y2O3:Yb3+, Er3+ nanoparticles, Theranostics 3 (2013) 282–288. [9] H. Fares, H. Elhouichet, B. Gelloz, M. Ferid, J. Appl. Phys. 117 (2015) 205. [10] M. Runowski, J. Lumin. 186 (2017) 199–204. [11] S.Q. Wang, J.B. Qiu, X.H. Xu, Q. Wang, D.C. Zhou, Z.W. Yang, Z.G. Song, Opt. Mater. Express 6 (2016) 1065–1078. [12] Y.Y. Du, B.J. Chen, E.Y.B. Pun, Z.Q. Wang, X. Zhao, H. Lin, Opt. Commun. 334
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