Journal of Molecular Structure 1065-1066 (2014) 39–42
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Nano-silver enhanced luminescence of Eu3+-doped lead tellurite glass M. Reza Dousti a,b, M.R. Sahar a,⇑, M.S. Rohani a, Alireza Samavati c, Zahra Ashur Mahraz a, Raja J. Amjad d, Asmahani Awang a, R. Arifin a a
Advanced Optical Materials Research Group, Department of Physics, Faculty of Science, Universiti Teknologi Malaysia, Skudai 81310, Johor, Malaysia Department of Physics, Tehran-North Branch, Islamic Azad University, Tehran, Iran c Ibnu Sina Institute for Fundamental Science Studies, Universiti Teknologi Malaysia, Skudai 81310, Johor, Malaysia d Department of Physics, COMSATS Institute of Information Technology, Lahore 54000, Pakistan b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
3+
Eu -doped tellurite glasses
containing silver nanoparticles are prepared. The surface plasmon resonance peak of silver nanoparticles is observed at 632 nm. The luminescence enhancement is attributed to localized electric field by metal. The quench of the luminescence is described by energy transfer from Eu3+ ions to metal.
a r t i c l e
i n f o
Article history: Received 29 November 2013 Received in revised form 11 February 2014 Accepted 11 February 2014 Available online 24 February 2014 Keywords: Tellurite glass Silver nanoparticles Surface plasmon resonance Eu3+ ions Luminescence enhancement
a b s t r a c t Eu3+-doped lead sodium tellurite glasses containing silver nanoparticles (NPs) were synthesized by melt-quenching technique and annealed for different time intervals at above the glass transition temperature. The glasses were characterized by UV–Vis–IR absorption, photoluminescence spectroscopy and transmission electron microscope imaging. Four absorption peaks of Eu3+ ion were observed due to transitions from ground state to different excited states in 400–600 nm region. The surface plasmon resonance (SPR) peak of silver NPs was probed at 632 nm. Five emission lines were recorded at 568, 587, 614, 650 and 704 nm which were intensified in the order of 1.9 times for heat-treated samples containing silver NPs. The average size of NPs was estimated to be 10 nm. Different mechanisms for interaction of light with metal and luminescent ions are discussed. Such enhancements are attributed to the strong local electric field induced by SPR of silver NPs as the major factor, and energy transfer from surface of silver NP to Eu3+ ion. The glasses show promising properties for optical applications. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Tellurite glasses are nominated as promising materials for a wide range of applications due to their superior optical response, high chemical durability and wide range of thermal stability ⇑ Corresponding author. Tel.: +60 127381709. E-mail address:
[email protected] (M.R. Sahar). http://dx.doi.org/10.1016/j.molstruc.2014.02.032 0022-2860/Ó 2014 Elsevier B.V. All rights reserved.
[1–3]. One of the noble properties of tellurite glasses is their good rare earth (RE) solubility. REs ions are known as extraordinary sources of visible and infrared (IR) emissions due to their mysterious 4f–4f transitions [4]. Europium (Eu3+) ion doped materials present several emissions in the range of yellow (570 nm) to dark-red (700 nm) color. The optical properties of Eu3+ ion in different hosts [2,4,5] are highly sensitive to its environment characteristics and varies strongly from host to host [6]. By and large, the
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luminescence intensity of RE ions quenches by increasing the concentration of dopants [5–7] likely due to the formation of clusters or energy transfer among the dipoles. In order to overcome such disadvantages, several methods have been proposed. The introduction of noble metallic nanoparticles (NPs), addition of second dopant such as alternative RE with a resonant energy level or semiconductor dopants, and introduction of quench reducing agents are some of the efficient techniques to improve the luminescence features of REs [8]. The metal-enhanced fluorescence by silver NPs had initially reported by Malta et al. [9]. They observed several order of amplifications in luminescence emissions of Eu3+-doped borosilicate glass, thanks to the presence of small silver particles. Recently, different authors reported the effect of noble metallic NPs (silver and gold) on the luminescence of various RE ions [10–13]. The major contribution of metallic NPs is to increase the localized electric field by plasmonic phenomena. The interaction of light with metallic NPs with sizes smaller than incident wavelength (d kexc) results in the oscillation of conduction band electrons. Such oscillations induce a confined electromagnetic field in the vicinity of NPs [14]. The stimulated local field affects the luminescence of neighboring emitters by increasing the excitation and radiation rates, which consequently decreases the lifetime of the excited states [15]. Recently, we reported on the effect of silver NPs on the luminescence of Er3+-doped different glasses [11,16,17]. The aim of the current paper is to investigate the influence of heat treatment on the optical properties of Eu3+-doped sodium lead tellurite glass containing silver NPs. 2. Experimental procedure The glass samples with composition (75 x y)TeO2 17PbO 8Na2O xEu2O3 yAgNO3 (where x = 0.7 and y = 0, 1 mol%) have been prepared by melt-quench technique. First, the well-mixed powders of all starting reagents were melted at 1173 K for one hour and quench subsequently between to preheated stainless steel molds at 573 K and annealed for 2 h. Next, the samples were cooled down to ambient temperature. The glasses containing silver NPs were post-heated at 613 K for 3, 9 and 12 h and labeled as TPEu3, TPEu9 and TPEu12, respectively. The silver-free glass is coded as TPEu. Since the glass transition temperature is about 588 K [11], the viscosity of the glass at post-annealing temperature is enough to promote the NPs to aggregate and grow. Finally, the samples were grained and polished carefully to achieve well-transparent glasses, suitable for optical measurements. The absorption spectra were recorded in 400–700 nm region by a Shimadzu UV-3101PC double-beam spectrophotometer. The Perkin–Elmer LS 55 photoluminescence spectrometer was used to detect the emission spectra of Eu3+ ions in tellurite samples under 464 nm excitation wavelength. The internal xenon flash lamp of the spectrophotometer was employed as the excitation source. A JEOL 2100 transmission electron microscope (TEM, working at 200 kV) was utilized to capture the images of the silver NPs. Therefore, a very fine powder of sample was dispersed at acetone and exposed to ultrasonic for 15 min and few drops were placed on the copper grid. The size distribution and average size of silver NPs are estimated by analyzing the TEM results. High resolution TEM (HR-TEM) was also captured to estimate the lattice constant of the bulk silver. 3. Results and discussion Fig. 1 shows the UV–Vis absorption spectra of Eu3+-doped sodium lead tellurite glass, TPEuA9. Three major transitions were recorded at 458, 526 and 582 nm corresponding to the transitions
Fig. 1. UV–Vis absorption spectrum of sample TPEuA9 shows three transition bands of Eu ion and one SPR band of silver NPs.
from 7F0 ground state to 5D2, 5D1 and 5D0 excited states, respectively. In the case of sample TPEuA9, a new small absorption band was also observed at 632 nm which may be attributed to SPR band of silver NPs. However, in our previous study in the same glass matrix, the SPR band was observed at 438 and 472 nm, which is an indicative of formation of elliptical silver NPs after 2 h annealing [11]. Fig. 2(a) shows the TEM image of sample TPEuA9 containing silver NPs with average size of about 10 nm. The histogram of NPs abundance is presented in Fig. 2(b) which fits a Gaussian distribution. The HR-TEM image of a single NP demonstrates the cubic closed packed structure of silver NPs with lattice constant 2 Å. This observation is near to that lattice constant of bulk silver (d200 = 2.05 Å, JCPDS No. 030931). Fig. 3(a) shows the luminescence spectra of the Eu3+ ions doped sodium-lead-tellurite glass exposed to different heat-treatment durations. Five peaks are observed at 568, 587, 614, 650 and 704 nm which are attributed to the transitions from 5D0 excited state to 7F0, 7F1, 7F2, 7F3 and 7F4 lower-lying states. Large enhancement for emissions in the visible region is observed after annealing the samples up to 9 h. The enhancement factor is calculated as the ratio of effective bandwidth of each emission of Eu3+-doped tellurite glass containing silver NPs (TPEux, x = 3, 9, and 12 h) to the TPEu glass.
R
I=I0 dk NP g ¼ R I=I0 dk free
Fig. 3(b) shows the magnified emission spectra of samples in 560–580 nm region. The inset of Fig. 3(b) presents the variation of enhancement factor (g) of each emission by increasing the annealing time interval. Maximum enhancement of the order of 1.91 times is observed for the 5D0 ? 7F4 transitions after 9 h heat-treatment, while further annealing resulted in quenching of all emissions. The enhancement factors for all observed emissions in this study are listed in Table 1. The enhancements of luminescence in presence of NPs are mainly attributed to large localized electric field in vicinity of the REs which is induced by surface plasmon resonance of NPs. At a particular separation of NPs (called as ‘‘hot-spots’’), the electric field is maximized and the largest enhancement may achieve. The aggregations of metallic NPs increase the size of the NPs and result in the formation of the non-spherical particles. Therefore, the effective local field (Eeff)
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Fig. 2. TEM image of sample TPEuA9 (a) showing the NPs with average size of 10 nm (b). HR-TEM image of same sample reveals the (200) crystallography direction of bulk silver (c).
Fig. 3. The emission spectra of Eu3+-doped for different samples in the range of 550–750 nm show transitions from 5D0 excited state to 7F0 (1), 7F1 (2), 7F2 (3), 7F3 (4) and 7F4 (5) lower-lying states (a). Magnified emission spectra of transition 5D0 ? 7F0 (b). Inset shows the variation of enhancement factor by increasing the annealing time duration.
could be boosted thanks to the relatively high concentration of the charges at the sharp edge of a metal. However, at a certain size of NPs, the stress of host material may prevent the further growth process [15]. Therefore, the Ag0 at the surface of NPs starts to disaggregate and the size of NPs decreases. On the other hand, the disaggregated Ag0 grows as a new center. The Plasmonic effect in the dielectric media depends on diverse aspects as size, shape, resonance damping coefficient and dielectric constant of the NPs (which determine the SPR band position) and also on refractive index and dielectric constants of the host. The type of NP indeed
Table 1 Luminescence enhancement factor of studied co-doped tellurite glasses. Emissions (nm)
Ag:Eu3+ co-doped glasses with different heat-treatment duration
5
Without NP PTEu
3h PTEuA3
9h PTEuA9
12 h PTEuA12
1 1 1 1 1
1.31 1.10 1.41 1.30 1.25
1.91 1.80 1.75 1.80 1.90
0.75 0.80 0.78 0.73 0.74
D0 ?
7
F0 F1 F2 7 F3 7 F4 7 7
568 587 614 650 704
alters the optical properties taking the change in all above-mentioned properties into account. The SPR band of gold NPs is usually reported to locate at higher wavelength (500–700 nm) with respect to silver NPs (400–500 nm) [18]. The observed enhancement and quench can be discussed by different interactions of light with dopants through the following approaches: (i) The interaction of light with Eu3+ ions, where a 464 nm photon excites the RE from its ground state 7F0 to 5D2 excited state, and subsequent non-radiative (NR) decays populate the lower states as 5D2 ? 5D1 ? 5D0. Due to the large gap (11,000 cm1) between the meta-stable 5D0 level and its next lower state, radiative decays are more probable than any other loss. Relaxations from this level (5D0) to 7F0, 7F1, 7 F2 and 7F3 states generate yellow, dark-yellow, red and dark-red emissions, as illustrated in Fig. 4. (ii) Interaction of excitation light with silver NPs results in a large localized electric field induced at the surface of the metal. Moreover, excitation of d-band electrons to sp-conduction band and the recombination of electron–hole pairs [19] terminate to the faint emissions from metal in the visible range [20,21].
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investigated. The tellurite glass samples containing 0.7 mol% Eu3+ ions and homogenously dispersed silver NPs (1 mol%) were prepared by the conventional melt-quenching technique and subsequent heat-treatments. SPR band of silver NPs was recorded at 632 nm. The average NP size was determined to be 10 nm by analyzing the TEM image. Five emission peaks in the visible range were observed corresponding to 5D0 ? 7F0, 7F1, 7F2, 7F3 and 7F4 transitions. The emission spectra of samples containing silver NPs revealed that the heat-treatments up to 9 h enhance the visible emissions by a factor of 1.91. The enhancement is interpreted by the localized surface plasmon resonance induced by silver NPs, while the quenches by 12 h annealing time is attributed to disaggregation of NPs and/or energy transfer from RE ion to the surface of metallic NP. Acknowledgements
Fig. 4. Energy band diagram of Eu3+-doped tellurite glass in vicinity of Ag NP.
(iii) The probability of energy transfer (ET) from surface of metal to RE is negligible due to short lifetime of SPR band in compare with the exited states of RE ions [22]. The reverse ET contributes to quench in luminescence peak intensity and depletion of energy by a NR process and heating phenomena in NP. (iv) The interaction between NP–NP. As mentioned above, the presence of ‘‘hot-spots’’ in intermediate separation of two NPs may lead to highly localized places where the effective local field is maximized. (v) The ET mechanism between two REs usually concludes to non-linear properties, such as up-conversion emissions. However, ET at the low concentration of dopants is negligible, since the ion-ion distance is relatively high. In the current system, the large localized electric field by silver NPs is the major explanation for luminescence enhancement. The interaction between NP–NP and the formation of non-spherical NPs are also contributed to improvements of visible emissions. Besides, the ET from RE to Ag NP and/or the disaggregation of massive NPs are responsible factors for quenching phenomena in the luminescence of the heat-treated samples for 12 h. 4. Conclusion In this study, the effect of the heat-treated silver NPs on the optical properties of the Eu3+-doped sodium tellurite glass is
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