Enhanced green emission of terbium- ions-doped

Enhanced green emission of terbiumions-doped phosphate glass embedding metallic nanoparticles Mohammad Reza Dousti Raja Junaid Amjad

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Enhanced green emission of terbium-ions-doped phosphate glass embedding metallic nanoparticles Mohammad Reza Doustia,b,* and Raja Junaid Amjadc,* a

Laboratório de Espectroscopia de Materiais Funcionais (LEMAF), Instituto de Fisica de Sao Carlos, Universidade de Sao Paulo, Av. Trabalhador Saocarlense 400, 13566-590 Sao Carlos-SP, Brazil b Islamic Azad University, Department of Physics, Tehran-North Branch, Tehran, Iran c Department of Physics, COMSATS Institute of Information Technology, Lahore 54000, Pakistan

Abstract. Tb3þ -doped glasses are promising materials for green lasers working at a UV-excitation light. It is essential to find a commercially low-cost host with high quantum efficiency in the visible region. We report the preparation and optical characterization of a thermally stable and optically transparent phosphate glass containing silver nanoparticles with nominal composition 57P2 O5 -40ðZnO-PbOÞ-2Tb2 O3 -1AgNO3 (mol%) obtained by a melt-quenching technique and subsequent heat-treatments. The glasses are transparent in UV to near-infrared region and are not hygroscopic at ambient environment. The optical absorption and luminescence excitation spectra of the samples with and without silver nanoparticles are identical, and no sign of any silver species is revealed, while the luminescence spectrum is enhanced up to 1.7 times after heat-treating the samples for 15 h. The transmission electron microscopy and selected area diffraction pattern of glasses show the presence of silver nanoparticles with an average size of ∼15 nm and growth at [200] crystallographic direction. The lifetime of the 5 D4 state of Tb3þ ions decreases from 3.06 to 2.85 ms by increasing the heat-treatment duration, which is indicative of the plasmonic effect of nanoparticles on the radiative rates of Tb3þ -doped glasses. © 2015 Society of Photo-Optical Instrumentation Engineers (SPIE) [DOI: 10.1117/1.JNP.9.093068]

Keywords: glass; nanophotonics; photoluminescence; solid-state lasers; electron microscopy; absorption. Paper 15016 received Mar. 13, 2015; accepted for publication Apr. 29, 2015; published online Jun. 25, 2015.

1 Introduction Rare earth (RE) doped glasses have recently received a lot of attention due to their significant optical properties with various applications such as laser hosts, tunable waveguides, upconvertors, optical fibers, sensors, etc.1–3 Such glasses are known as interesting materials due to the f-f transitions of the RE ions in visible and near-infrared (NIR) regions of the spectrum with high quantum efficiency and selective working wavelengths. The low cost and ease of fabrication are two major factors of glasses that nominate them as the most comfortable compact media for various optoelectronic devices. However, the selection of the glassy host is an important aspect since they present different physical, structural, and optical properties, such as glass transition temperature, refractive index, density, phonon energy, chemical durability, electrical conductivity, etc. Among them, phosphate glasses have attracted much attention and versatile laser sources have been fabricated to date.4 In comparison to silica, phosphate glasses possess lower working temperatures, a larger refractive index, and higher RE solubility, which make them a suitable host for laser gain media either as bulk or optical fibers, providing a favorable emission cross-section and excited state lifetime.4–6 The optical damage threshold and thermal

*Address all correspondence to: Mohammad Reza Dousti, E-mail: [email protected]; Raja Junaid Amjad, rajajunaid25@ gmail.com 1934-2608/2015/$25.00 © 2015 SPIE

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Dousti and Amjad: Enhanced green emission of terbium-ions-doped phosphate glass embedding. . .

conductivity of the phosphate glasses are lower than that of silicate glasses. However, the low glass transition temperature of these glasses (∼350°C) is a detrimental factor for fabrication of high-power fiber lasers and amplifiers but could be improved by incorporation of modifiers or glass formers, such as Al2 O3 , WO3 , etc.7,8 The hygroscopic properties of phosphate glasses result in a low chemical durability and water resistivity, which could also be solved by the addition of some modifiers, such as PbO, Al2 O3 , etc.8 Therefore, phosphate glasses are excellent hosts for RE-doping. Tb3þ -doped glasses show strong green emission, which could be applied in different devices such as displays, projection television tubes, x-ray intensifying screens, scintillators, and biological probes. The emissions of Tb3þ ions are generated due to the radiative decays from 5 D4 and 5 D3 excited states to 7 FJ (J ¼ 0 − 6) lower-lying levels. However, the current challenge is to improve the quantum efficiency and increase the intensity and cross-section of the latter emissions of Tb3þ ions. Several methods have been presented to overcome the quenching in the luminescence spectrum of RE ions.9 One of the most successful proposals is to embed the metallic nanoparticles (NPs) in the RE-doped glasses. In this theme, several reports showed enhancement in the luminescence intensity of RE ions thanks to the plasmonic effects of the silver or gold NPs.10–19 However, only a few reports are available on the effect of metallic NPs on the luminescence of Tb3þ -doped glasses. The emission of Tb3þ -doped silicate glasses was improved due to the presence of silver NPs, although it quenches at higher concentrations of NPs due to energy transfer from the RE ions to metallic particles.20 Moreover, Kassab et al.21 reported on the enhancement of the luminescence of Tb3þ ions in Tb3þ -Eu3þ codoped tellurite glasses by heat-treating the silver NPs. Due to the technological importance of the aforementioned research on the luminesce of RE ions in the presence of metallic NPs, in this work, we aimed to study the influence of silver NPs on the luminescence emissions of Tb3þ -doped phosphate glasses prepared by the conventional melt-quenching technique and subsequent heat-treatments near the glass transition temperature. The glasses show good thermal stability and the green emission of Tb3þ ions are enhanced by incorporation of silver NPs.

2 Experimental Procedures Phosphate glasses doped with Tb3þ ions having the chemical composition xAgNO3 -yTb2 O3 20ZnO-20PbO-ð60-x-yÞP2 O5 (where x ¼ 0;1 and y ¼ 0, 2 mol%) were prepared by the conventional melt-quenching method. About 10 g batches of the analytical grade samples were measured, weighted, and mixed to achieve a homogenous mixture of powders, the powders were then placed in a platinum crucible and melted at 900°C for 30 min. Then the molten substance was air-quenched in between two stainless steel molds. The glasses were subsequently annealed at 300°C for 1 h and again at 340°C for various periods of 5, 10, and 15 h. The glasses are labeled according to their heat-treatment durations and dopants, as given in Table 1. Finally, the annealed glass samples were cut and polished before taking spectral measurements. The optical absorption spectra for the prepared glasses were recorded using Perkin Elmer Lambda 25 UV-Vis-NIR spectrophotometer in the range of 300 to 500 nm. A Shimadzu RF-5301 PC spectrofluorophotometer Table 1 Nominal compositions, heat-treatment durations below and above glass transition temperature, and corresponding labels of studied Tb3þ -doped phosphate glasses. P2 O5

(ZnO-PbO)

Tb2 O3

AgNO3

HT < T g

HT > T g

PT

58

40

2

0

1

0

PTA5

57

40

2

1

1

5

PTA10

57

40

2

1

1

10

PTA15

57

40

2

1

1

15

PA10

59

40

0

1

1

10

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is used to record the excitation and emission spectra. The JEOL 2100 transmission electron microscope (TEM) working at 240 kV and a commercial x-ray diffractometer (XRD) were used to characterize the samples and obtain the TEM and high-resolution TEM (HR-TEM) images of NPs in addition to the selected area electron diffraction (SAED) pattern. All the aforementioned measurements were performed at room temperature. The differential thermal analysis has been performed for sample PT using a Diamond DTA-TG thermo-analyzer under N2 atmosphere and a heating rate of 15°C∕ min, in the 50 to 900°C region. The characteristic glass transition temperature, crystallization temperature, and melting point were determined to be at 330, 462, and 752° C, respectively, as reported earlier.22

3 Results and Discussion The obtained glasses are colorless and are not hygroscopic at the ambient temperature and atmosphere. Figure 1 shows the XRD pattern of a representative glass sample containing silver NPs. The broad humps in the 20 to 40 and 40 to 60 deg regions are the characteristics of the amorphous lead zinc phosphate glass. Figure 2 presents the TEM image of the sample PTA15 as a representative sample in which a few silver nanoparticles are indicated by arrows. The majority of NPs present a spherical shape with an average diameter of ∼15 nm. The insets of Fig. 2 show the SAED and size distribution of the NPs in the same glass. The SAED pattern designates the formation of silver NPs in the [200] crystallized bulk direction. The HR-TEM image of a silver NP also shows a lattice constant of ∼2.15 Å, a second confirmation for the crystal structure of the observed NPs. The lattice constant of bulk silver is d200 ¼ 2.05 Å as given in JCPDS NO. 030931.23 The optical absorption spectrum of the sample PTA15 in the UV-Vis region is shown in Fig. 3 as the representative spectrum. All the other spectra show the identical band positions in this region. There is no band observed due to the silver NPs, likely due to the small volume fraction of the particles. In order to look for the SPR band of silver NPs in this glassy system, a Tb3þ free glass has also been prepared with the same methodology (labeled as PA10), and its absorption spectrum is given in the inset of Fig. 3. However, this spectrum has features similar to that of undoped lead-zinc phosphate glass, which could be indicative of the small fraction of silver NPs in this glassy system. However, Piasecki et al.24 reported the plasmon band of silver NPs in the Tb3þ -doped aluminosilicate glass at ∼420 nm, which progressively intensified by the addition of silver concentration or increasing the heat-treatment duration. Although the microscopy measurements of PTAx samples revealed the presence of NPs, silver may also exist in different forms, such as Ag ions, dimers, trimers, etc. Figure 4(a) shows the excitation spectrum of the Tb3þ -doped phosphate glass. The excitation bands centered as 484, 376, 368, 357, 350, 339, 316, and 302 nm could be attributed to the

Fig. 1 X-ray diffractometer pattern of the Tb3þ -doped glass containing silver nanoparticles (PTA15). Journal of Nanophotonics

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Fig. 2 Transmission electron microscope (TEM) image, high-resolution TEM image, and selected area electron diffraction pattern of the silver nanoparticles in PTA15 glass sample. A few nanoparticles are shown with arrows. The lattice constant of one nanoparticle matches the latter value corresponding to bulk silver in the 200 direction.

Fig. 3 UV-Vis absorption spectrum of Tb3þ -doped sample (PTA15). Inset shows the absorption spectrum of PA10 glass sample. No surface plasmon band corresponding to nanoparticles is observed.

transitions from the 7 F6 ground state to various 5 D4 , (5 D3 þ 5 G6 ), 5 L10 , 5 G5 , 5 G4 , 5 L7 , 5 H7 , and 5 H6 states, respectively. The excitation wavelength at 376 nm is among the most important excitation sources for this Tb3þ -doped samples since it can be provided by low-cost blue-emitting LEDs. The excitation spectra of the samples with and without heat-treatments are identical and no band is observed due to Agþ ions or the Ag NPs. Again, the hypothesis of the small volume fraction of this species can support the observed particles in the TEM image while no evidence of NPs is observed in performed spectroscopic techniques. The emission spectra of the terbium-doped phosphate glasses with and without silver NPs are presented in Fig. 4(b). Under a 376-nm excitation wavelength, seven emission bands are observed in the 400 to 700 nm region which originate due to the transitions from 5 D3 and Journal of Nanophotonics

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Fig. 4 (a) Excitation spectra of samples PT and PAT15 detecting the signal of emission at 541 nm and (b) emission spectra of the Tb3þ -doped glass samples before and after heat treatments under 376 nm excitation wavelength. 5

D4 excited states to various lower-lying states. The bands centered at 412, 434, and 456 nm are attributed to the transitions from 5 D3 state to 7 F5 , 7 F4 , and 7 F3 states, respectively. Moreover, the emission bands located at 486, 541, 584, and 618 nm are associated to the 5 D4 → 7 F6 , 5 D → 7 F , 5 D → 7 F , 5 D → 7 F transitions, respectively. The green to blue intensity ratio 4 5 4 4 4 3 in the Tb3þ -doped glasses depends strongly on the concentration of the Tb3þ ions. The green-to-blue intensity ratio for this glass system is ∼63, which is similar to 8 mol% Tb3þ doped aluminosilicate glass25 and >2 mol% Tb3þ -doped zinc-borate glasses.26 In principle, the blue-to-green emission intensity ratio is a degree of quenching, which increases by decreasing the concentration of Tb3þ ions. This should be due to the concentration quenching of the 5 D3 state through a cross-relaxation process, as discussed elsewhere.27 Therefore, we can conclude that the cross-relaxation process in the current glass is more efficient than zinc-borate glass doped with 2 mol% of Tb3þ ions and is comparable to aluminosilicate glass doped with 8 mol% of Tb3þ ions. The latter intensity ratio does not change drastically by increasing the heat-treatment duration of the samples. The intensity of the emission bands in the 400 to 700 nm region is enhanced by addition of silver NPs into the host glass and subsequent heat-treatments up to 15 h. The observed bands in the blue to red regions of the emission spectra are enhanced up to 1.5 to 1.8 times. The enhancement in the emission intensity in the Tb3þ -doped glasses can be Journal of Nanophotonics

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Fig. 5 Schematic partial energy level diagram of Tb3þ ions. Excitation, possible emission channels, nonradiative, and cross-relaxation mechanism are shown.

attributed to the enlarged localized electric field in the vicinity of the silver NPs and RE ions.28,29 Moreover, such enhancements could be attributed to the energy transfer from the surface of the NPs to the RE ions, as a minor effect.30,31 However, such an energy transfer is negligible since the lifetime of the NP is much shorter than that of the RE ions.32 On the other hand, there is no quench observed due to the increased heat-treatment duration, which could be a result of the small volume fraction of the NPs in the system having small diameters, which drastically decreases the self-absorption in the host and scattering of incident and emitted lights. Figure 5 illustrates a schematic partial energy level diagram of the Tb3þ ions. The Tb3þ ions can be excited at 376 nm to obtain an intense green emission [see Fig. 4(a)]. This wavelength also has another importance. Such a wavelength could be provided by commercial blue LED sources. At such efficient excitations, Tb3þ ions can emit a strong green emission thanks to the fast nonradiative decay from 5 L10 and 5 D3 excited states to the 5 D4 lower state. The gap between the 5 D3 and 5 D4 state is ∼5900 cm−1 , which could only be bridged by at least five phonons in the glass network having the maximum phonon energy of ∼1100 to 1200 cm−1 .33 The probability of such a multiphonon relaxation process involving five phonons is negligible. However, at higher

Fig. 6 Excited state intensity decay profile of 5 D4 → 7 F5 transition of Tb3þ -doped glasses without silver nanoparticles and with nanoparticles after heat-treated for various 5, 10, and 15 h. Journal of Nanophotonics

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concentration of Tb3þ ions, the following cross-relaxation process is more likely to populate the 5 D4 state. 5 D ðAÞ 3

þ 7 F0 ðBÞ → 5 D4 ðAÞ þ 7 F6 ðBÞ;

where A and B represent two neighboring Tb3þ ions. Figure 6 shows the decay profile of the 5 D4 → 7 F5 transition (541 nm) of the Tb3þ ions doped in the phosphate glass with and without silver NPs, excited at 376 nm. The excited state lifetimes are evaluated by fitting the experimental data to a mono-exponential decay function. The lifetime of the 5 D4 state decreases when increasing the heat-treatment durations from 3.06 to 2.85 ms for samples PT and PTA15, respectively. The decrease in the lifetime of the excited state is indicative of the influence of the plasmonic contributions of the silver NPs on the rates of different radiative emission channels of Tb3þ ions and the absence of any energy transfer from NPs to RE ions, where an increase in lifetime is expected.

4 Conclusion The optical absorption, luminescence excitation, and emission spectra of the phosphate glasses doped with 2 mol% terbium ions were characterized in the absence and presence of silver NPs. The silver NPs were formed by heat-treating the glasses at 10°C above the glass transition temperature. The NPs with an average diameter of 15 nm, which are formed along the [200] crystallographic direction, were observed by TEMs. The presence of silver NPs enhances the luminescence emissions of Tb3þ ions up to 1.8 times. The glasses present good thermal stability and strong green emission, which could suggest this material for solid-state laser hosts.

Acknowledgments The authors would like to thank the financial supports from COMSATS and Higher Education Commission (HEC) of Pakistan through the research grant IPFP/HRD/HEC/2014/1641, partial support program I-8/HEC/HRD/2014 and support through the Interim program, FAPESP-Brazil (2013/24064-8), and Iranian Nanotechnology Council (Codes # 68867 and 69548).

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11. Y. Wu et al., “Silver nanoparticles enhanced upconversion luminescence in Er3þ ∕Yb3þ codoped bismuth-germanate glasses,” J. Phys. Chem. C 115(50), 25040–25045 (2011). 12. T. Som and B. Karmakar, “Core-shell Au-Ag nanoparticles in dielectric nanocomposites with plasmon-enhanced fluorescence: a new paradigm in antimony glasses,” Nano Res. 2, 607–616 (2009). 13. J. A. Jiménez, S. Lysenko, and H. Liu, “Enhanced UV-excited luminescence of europium ions in silver/tin-doped glass,” J. Lumin. 128, 831–833 (2008). 14. Y. Qi et al., “Enhanced upconversion emissions in Ho3þ ∕Yb3þ codoped tellurite glasses containing silver NPs,” J. Non Cryst. Solids 402, 21–27 (2014). 15. M. R. Dousti, “Efficient infrared-to-visible upconversion emission in Nd3þ -doped PbO-TeO2 glass containing silver nanoparticles,” J. Appl. Phys. 114, 113105 (2013). 16. M. R. Dousti and S. R. Hosseinian, “Enhanced upconversion emission of Dy3þ -doped tellurite glass by heat-treated silver nanoparticles,” J. Lumin. 154, 218–223 (2014). 17. M. R. Dousti et al., “Surface enhanced Raman scattering and up-conversion emission by silver nanoparticles in erbium–zinc–tellurite glass,” J. Lumin. 143, 368–373 (2013). 18. M. R. Dousti et al., “Nano-silver enhanced luminescence of Eu3þ -doped lead tellurite glass,” J. Mol. Struct. 1065–1066, 39–42 (2014). 19. M. R. Dousti, “Plasmonic effect of silver nanoparticles on the upconversion emissions of Sm3þ -doped sodium-borosilicate glass,” Measurements. 56, 117–120 (2014). 20. P. Piasecki et al., “Formation of Ag nanoparticles and enhancement of Tb3þ luminescence in Tb and Ag co-doped lithium-lanthanum-aluminosilicate glass,” J. Nanophotonics 4, 043522 (2010). 21. L. R. P. Kassab et al., “Enhanced luminescence of Tb3þ ∕Eu3þ doped tellurium oxide glass containing silver nanostructures,” J. Appl. Phys. 105, 103505 (2009). 22. M. R. Dousti and R. J. Amjad, “Spectroscopic properties of Tb3þ -doped lead zinc phosphate glass for green solid state laser,” J. Non Cryst. Solids 420, 21–25 (2015). 23. C. Liu and J. Heo, “Local heating from silver nanoparticles and its effect on the Er3þ upconversion in oxyfluoride glasses,” J. Am. Chem. Soc. 93(10), 3349 (2010). 24. P. Piasecki et al., “Plasmon enhanced luminescence of Tb3þ doped Li2 O-LaF3 -Al2 O3 -SiO2 glass containing Ag nanoparticles,” Proc. SPIE 7757, 77572M (2010). 25. A. D. Sontakke, K. Biswas, and K. Annapurna, “Concentration-dependent luminescence of Tb3þ ions in high calcium aluminosilicate glasses,” J. Lumin. 129, 1347–1355 (2009). 26. K. Swapna et al., “b3þ doped zinc alumino bismuth borate glasses for green emitting luminescent devices,” J. Lumin. 156, 180–187 (2014). 27. C. R. Kesavulua et al., “Concentration effect on the spectroscopic behavior of Tb3þ ions in zinc phosphate glasses,” J. Lumin. (2015). 28. L. P. Naranjo et al., “Enhancement of Pr3þ luminescence in PbO-GeO2 glasses containing silver nanoparticles,” Appl. Phys. Lett. 87, 241914 (2005). 29. R. J. Amjad et al., “Enhanced infrared to visible upconversion emission in Er3þ doped phosphate glass: role of silver nanoparticles,” J. Lumin. 132, 2714–2718 (2012). 30. T. Som and B. Karmakar, “Nanosilver enhanced upconversion fluorescence of erbium ions in Er3þ : Ag-antimony glass nanocomposites,” J. Appl. Phys. 105, 013102 (2009). 31. M. Eichelbaum and K. Rademann, “Plasmonic enhancement or energy transfer on the luminescence of gold-, silver-, and lanthanide-doped silicate glasses and its potential for light-emitting devices,” Adv. Funct. Mater. 19, 2045–2052 (2009). 32. O. L. Malta and M. A. C. do Santos, “Theoretical analysis of the fluorescecne yield of rare earth ions in glasses containing small metallic particles,” Chem. Phys. Lett. 174, 13–18 (1990). 33. M. R. Dousti et al., “Structural and optical study of samarium doped lead zinc phosphate glasses,” Opt. Commun. 300, 204–209 (2013). Mohammad Reza Dousti is currently a post-doctoral researcher at Instituto de Fisica de São Carlos, Universidade de São Paulo. He has a PhD on solid state physics from Universtiti Teknologi Malaysia (2010 to 2014) and accomplished his MSc and BSc degrees in TehranNorth Branch of IAU and Guilan University, Iran, respectively. His current research interest Journal of Nanophotonics

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is on the optical properties of the rare earth doped glasses and nanomaterials and spectroscopic studies of various optical materials. Raja Junaid Amjad got his PhD in solid state physics from the Universiti Teknologi Malaysia, 2013. His current research interest lies in nanophotonics, metal enhanced fluorescence and surface enhanced Raman scattering, with particular expertise on the synthesis and characterization of phosphate and tellurite glasses doped with rare earth ions and metallic nanoparticles. Currently, he is working as assistant professor at COMSATS Institute of Information Technology, Lahore, Pakistan, active in both research and teaching.


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