Eur. Phys. J. D (2012) 66: 237 DOI: 10.1140/epjd/e2012-30089-1
THE EUROPEAN PHYSICAL JOURNAL D
Regular Article
Enhanced frequency upconversion in Er3+-doped sodium lead tellurite glass containing silver nanoparticles M. Reza Dousti1,2,a , M.R. Sahar1 , Raja J. Amjad1 , S.K. Ghoshal1 , A. Khorramnazari2, A. Dordizadeh Basirabad3, and A. Samavati1 1 2 3
Advanced Optical Material Research Group, Department of Physics, Faculty of Science, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor Department of Physics, Tehran-North Branch, Islamic Azad University, Tehran, Iran Department of Physics, Science and Research Branch, Islamic Azad University, Tehran, Iran Received 3 February 2012 / Received in final form 6 June 2012 c EDP Sciences, Societ` Published online 12 September 2012 – a Italiana di Fisica, Springer-Verlag 2012 Abstract. A series of silver nanoparticle embedded in erbium-doped tellurite glasses were synthesized using a one step melt-quenching method. Density and refractive index of glasses were measured. Thermal and optical characterizations were performed and plasmon bands of elliptical nanoparticles were observed. An enhancement of green (525 and 550 nm) and red (632 nm) lines in luminescence spectra of Er3+ doped silver-embedded tellurite glass was recorded and explained by energy transfer mechanism from silver nanoparticles to erbium ion in addition to enhanced local field in vicinity of metallic nanoparticles in the glass. The presence of nanoparticles was confirmed by transmission electron microscopy imaging and reduction of silver ions to silver neutral particles discussed through the redox potential estimation in probable reactions. Silver-erbium co-doped tellurite glass exhibits strong novel optical properties which nominate it as the promising glass for laser, color displays, and photonic applications.
1 Introduction Rare-earth (RE) ions doped glasses are found to be great source of lasers in visible and infrared (IR) range. Moreover, they are nominated as the transporter in fiber glasses, which are themselves applicable to optical fibers. Among the RE, trivalent Eu, Sm, Tm, Yb and Er were investigated so often due to their brilliant energy levels. Absorption of electrons in ground state and different excited states, near-energy excited states, suitable non-radiative emissions and upconversion through energy transfer processes, are the most common routes to yield the desirable emissions from RE ions. A large number of research has been done on Er3+ -ions due to its potentiality of blue, green, and red emissions, in addition to an IR broadband line around 1.5 μm. Improvement of quantum efficiency in such glasses brings the need to find the host matrices with low cut-off energy. Among the oxide glasses, tellurite and antimony present the lowest maximum phonon energy about 700 [1] and 600 cm−1 [2], respectively. However, tellurite glasses are more appropriate in optical applications owing their high linear refractive index (>2), high nonlinear refractive index, wide range of transparency (0.4– 6 μm), good solubility of REs, chemical durability and thermal stability [1,3–7]. a
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Recently, the study on effect of metallic nanoparticles (NP) on fluorophore centers in dielectric environments has attracted a large interest on implement and improvement of the photoluminescence lines. During last decades, introduction of the metallic particles as the second dopants in a wide area of research showed a promising application of nanoparticles to develop the properties and characteristics of new materials. One the other hand, optical properties of new glasses presented them to be the superior candidate in laser application and development. Surface Plasmon Resonance (SPR) is known as the result of interaction of light by metallic NPs [8] which leads to a coherent oscillation of the free conduction electrons [9]. SPR is localized in vicinity of NP in the host matrix and its consequent enhanced electric field affects the neighboring emitters. For two spherical NPs, electric field of localized SPR (LSPR) is maximized between the two absorbents, called as “hot-spots” [9]. However in practice, this field is more enhanced due to the presence of nonspherical (elliptical, pyramid, tetrahedron, and so on) NPs (lightening rod effect) in different sizes [10,11]. Although, Malta [12] showed the effect of silver particles in enhancement of emissions in Eu3+ -doped borate glass in 1985, it is just recent that many researchers are using metallic NPs in new glasses to improve and optimize the luminescence of RE-doped glasses. Lately, Kassab and co-workers [13,14] reported an enhancement up to 6 times
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Eur. Phys. J. D (2012) 66: 237 Table 1. Glass composition, their labels, optical, inter-nuclear and thermal properties. Topic Glass label Glass Composition (mol%)
Optical properties
Concentration of dopants (×1020 ) Inter-nuclear distance between dopants (˚ A) Thermal characterizations (◦ C)
TeO2 PbO Na2 O Er2 O3 AgNO3 Density, ρ (g cm−3 ) Refractive index, n Optical band gap, Eg (eV) NEr (ions cm−3 ) NAg (atoms cm−3 ) dEr−Er dNP−NP dNP−Er Glass transition temperature, Tg Crystallization temperature, Tc Melting point, Tm
Corresponding values TPE TPEA* TPA*** 74.3 73.3 74 18 18 8 7 7 7 0.7 0.7 0 0 1 1 5.899 5.885 5.895 1.91 2.03 2.00 (221 nm) (258 nm) (247 nm) 5.60 4.81 5.02 1.515 1.510 – – 2.157 2.182 18.76 18.77 – – 16.67 16.61 – 17.91 – 325 362** – 491, 501 495** – 507 509** –
*Four different samples with the same composition were prepared for heat treatment. ** The data given are collected from the sample after 20 h annealing time interval. *** Sample TPA is annealed for 20 h at 340 ◦ C.
in germanate glasses. Moreover, Rivera et al. [15,16], Maier and Atwater [9] and Som and Karmakar [17,18] reported many works on tellurite and antimony nanoglasses to achieve strong emissions of erbium co-doped with silver and gold NPs, respectively. Although, a great deal of experiments is available on literature, the hidden potentiality of plasmonic effects and attempts to enhance the luminescent properties of REdoped glasses are taken into account for further investigation on new metallic NPs embedded glasses. In this paper, we report the effect of silver NPs in optical properties of Er3+ -doped lead tellurite glass. To our knowledge, this is the first time that reduction of silver NPs in tellurite glass is investigated from the thermodynamic point of view.
gregate. Samples of thickness about 2.2 mm were grained, polished, and cut for optical measurements. The density of samples was measured using the Archimedes methods using sterilized water as the immersion liquid. The UV-VIS-IR absorption spectra of samples were recorded by a Lambda 20 Perkin-Elmer spectrophotometer. Fluorescence spectra were collected using PerkinElmer luminescence spectrophotometer (LS 55) with an external 980 nm laser diode (LD, 1W output power) as the source instead of its internal xenon lamp. A transmission electron microscope (TEM 2100, JEOL) operating at 200 kV acceleration was used to verify the presence of silver NPs. All presented measurements are performed at regular room temperature.
2 Experimental procedure
3 Results and discussion
The glass samples with composition (75-x-y)TeO217PbO-8Na2O-xEr2 O3 -yAgNO3 were prepared (x = 0, 0.7 and y = 0, 1) by a conventional melt-quench technique and are listed by their labels in Table 1. The batches of 20 g of well-mixed raw materials consist of tellurite oxide (TeO2 , 99%), lead oxide (PbO, 99%), sodium oxide (Na2 O, 99%), erbium oxide (Er2 O3 , 99.8%), and silver nitrate (AgNO3 , 99%) were melted at 900 ◦ C for one hour in a platinum crucible. Then, the melt was poured into a preheated steel mold at 300 ◦ C and was kept for 2 h. All samples were synthesized by this method and were annealed above the glass transition temperature (at 340 ◦ C) for different periods (2.5, 10, and 20 h). Differential thermal analysis (DTA) was performed for based glass sample to determine the glass transition temperature (Tg = 315 ◦ C). Therefore, the viscosity of the glass at annealing temperature is enough to allow the NPs to move, grow, and ag-
Compositions, density, and refractive index of studied samples are collected in Table 1. Reduction in density of glass by introducing NP is due to high density value of tellurite oxide. The refractive index of each glass is obtained by following equation [19], where Eg is the direct optical band gap of the corresponding glass. n2 − 1 Eg =1− . (1) 2 n +2 20 The direct optical band gap (Eg ) of the amorphous materials can be calculated by a fitting method of Tauc [20] model (Eq. (2)) to ultraviolet region of absorption spectrum. α(ω) =
(hν − Eg )1/2 hν
(2)
Eur. Phys. J. D (2012) 66: 237
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(a)
(b)
Fig. 1. Absorption spectra of (a) erbium-doped with and without silver NPs and (b) silver-doped lead tellurite glass in absence of Er3+ ions.
here, α is the absorption coefficient of glass in frequency ω. Therefore, by a linear fit on the curve of (αhν)2 versus hν (photon energy), one can determine the optical band gap by extrapolating the fit until x-intercept. The obtained Eg for each glass is listed also in Table 1. No change was found in refractive index and color of sample TPEA after annealing for 2.5, 10 and 20 h. Figure 1a shows the absorption spectra of erbium-doped lead tellurite glass with and without presence of NPs. The bands centered at 1532, 980, 800, 652, 545, 522, and 489 nm are ascribed to absorption of light from 4 I15/2 ground state to 4 I13/2 , 4 I11/2 , 4 I9/2 , 4 F9/2 , 4 S3/2 , 2 H11/2 , and 4 F7/2 excited energy states, respectively. Presence of Ag NPs changes the absorption spectra weakly, but it is clear that after heat-treatment for 20 h, the spectra relative intensity is decreased due to the scatter and absorption of the light by NPs and surface plasmon resonance. The Plasmonic absorption band cannot be observed in erbium absorption spectra due to the overlapping bands of two different dopants. Therefore, an identical glass without erbium ions (TPA) was prepared to monitor the plasmon band of metallic NP. Similar method is performed lately in literature to measure the absorption bands of metallic NPs [9,21–23]. Figure 1b shows the absorption band of silver in lead tellurite glass annealed for 20 h. Two intense bands are noticeable at 438 and 472 nm (23 831 and 21 186 cm−1 ) due to non-spherical shapes of NPs. Figure 2 shows the DTA curve of the sample TPE. Glass transition, crystallization, and melting points are measured at a rate of 10 ◦ C/min, and are found to be at 325, 491, and 507 ◦ C, respectively. The difference of sample temperature and system temperature is plotted versus sample temperature. The second crystallization peak at 501 ◦ C is probably appeared due to different constituents in the sample [24]. The glass transition temperature of the TPEA sample shifted to 362 ◦ C after 20 h annealing. Thermal properties are listed also in Table 1. Figure 3 presents the upconversion luminescence spectrum of the samples under excitation of 980 nm. Three common emission bands at 525, 550, and 632 are ob-
Fig. 2. Differential thermal anaslysis of the tellurite glass by a rate of 10 ◦ C/min.
served and ascribed to the 2 H11/2 →4 I15/2 , 4 S3/2 →4 I15/2 , and 4 F9/2 →4 I15/2 transitions, respectively. The glass with maximum annealing time (20 h) shows the largest intensity in the whole range of this spectrum. An enhancement in order of 3.2, 2.5, and 3 times for the bands centered at 525, 550, and 632 are achieved for the glass with 20 h heat treatment, respectively. The red emission is more affected by the NPs with a blue shift due to the strong localized electric field around the NPs. Figure 4a shows the TEM imaging of the silver NPs in lead tellurite glass after 20 h annealing. Analyzing the image, average sizes of NPs were estimated to be 14–16 nm. The size distribution of the NPs due to growth after 20 h annealing time intervals is shown in Figure 4b, where a Gaussian curve is well-fitted to abundance histogram. However, the NPs are not thoroughly spherical and some of them are detected in an elliptical shape. Accordingly, this fact is in good conformity with results of plasmon bands which was shown in UV-VIS spectra previously.
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Eur. Phys. J. D (2012) 66: 237
(a)
(b)
Fig. 3. (Color online) Upconversion spectrum of the lead tellurite glassed doped with 0.5% Er2 O3 (TPE) and 0.5% Er2 O3 : 0.7% AgNO3 (TPEA) for 2.5, 10, and 20 h annealing time under 980 nm laser excitation.
Two plasmon bands are attributed to two different modes: a longitudinal mode is aligned to the longer axis of ellipse; and another band, located in higher wavelength, is perpendicular to the first one [3]. The aspect ratio of elliptical NPs is estimated to be 1.86. Silver NPs are made during the melting procedure. These particles are grown and aggregated in annealing time interval. The reduction of Ag+ particles to Ag0 NPs can be discussed by the reduction potential (E 0 ) of redox system elements. The E 0 values of each component in this system are [25]: Te
6+
4+
/Te
+
0
3+
0
= 1.02 V,
(3)
Ag /Ag = 0.7996 V, Er
3+
Er
/Er = −2.331 V, 2+
/Er
= −3.0 V.
(4) (5) (6)
Probable reduction process and their total potentials are as following: Te4+ + 2Er3+ → Te6+ + 2Er2+ 3Te
4+
Te
3+
+ 2Er
4+
+
→ 3Te
+ 2Ag → Te
6+
6+
0
+ 2Er + 2Ag
0
ΔE ◦ = −7.02, ◦
ΔE = −7.722,
(7) (8)
◦
ΔE = +0.5792. (9)
Therefore, from the thermodynamic point of view, only the last redox reaction (9) is feasible; and this fact verifies the presence of silver NPs in the samples by taking the results of both UV-VIS and TEM into account. Besides, it should be noticed that still the existence of the large portion of silver in the form of ions, atoms, charged or neutral dimmers and multimers is likely [26,27].
Fig. 4. (Color online) (a) Transmission electron microscopy (TEM) of sample TPEA after 20 h annealing. (b) Histogram of silver nanoparticles size distribution.
Figure 5 shows a schematic representation of energy levels of Er3+ ion in vicinity of Ag NP embedded in tellurite glass. By and large, the interactions in the Er3+ :Ag co-doped heavy tellurite glass can be discussed through three different channels. The interaction of (i) excitation light with Er3+ ion, (ii) excitation light with Ag NPs and (iii) Er3+ ions with Ag NPs [28]. In the first mechanism, a 980 nm excitation beam stimulates the erbium ions and the 4 I11/2 level is populated through the ground state absorption (GSA). Absorption of another 980 nm laser photon excites the ion from 4 I11/2 to 4 F7/2 excited state absorption (ESA) and subsequently 4 F7/2 → 2 H11/2 → 4 S3/2 → 4 F9/2 non-radiative (NR) decays lead to populate corresponding levels and emit the 525, 550 (green), and 632 (red) lines [29]. The 4 I13/2 excited state is further populated by NR decay from 4 I11/2 due to multiphonon relaxation process (MRP) [30]. Energy transfer (ET) between two neighboring Er3+ ions is also probable through two different mechanisms: (1) where a non-radiative decay from 4 I11/2 to ground state excites the neighboring ion from 4 I13/2 to 4 F9/2 ; (2) another ET process may occur where two electrons in 4 I11/2 excited state of different ions
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4 Conclusion Significant enhancement in green and red emissions of Er3+ -doped tellurite glass containing silver NPs has been achieved due to the enhanced local field in the vicinity of metallic NPs in addition to an energy transfer from surface of Ag NP to Er3+ ions. The plasmon absorption peaks are observed in a separate sample in absence of erbium ion. Reduction of silver ions with Te4+ was discussed by considering the probable reductive reactions and corresponding potentials. Such a method is applicable for any other tellurite glass. Er3+ :Ag co-doped lead tellurite glass presents remarkable optical properties and can be optimized for optical devices, color displays, and nanophotonic applications. Fig. 5. (Color online) Schematic energy level diagram of Er3+ ion in sodium lead tellurite glass presenting the red and green upconversion emissions by the ground state absorption (GSA), excited state absorption (ESA), non-radiative decays (NR), multiphonon relaxation process (MRP) and energy transfer (ET) between two neighboring Er3+ ions under the 980 nm laser diode. Energy transfer from Ag NP to Er3+ ions (ET) is also shown in addition to enhanced local field (LFE) near the metallic NP.
contribute to one non-radiative emission from 4 I11/2 to ground state, while transfer the energy to another one to jump to 4 F7/2 excited state. It is worth to mention that another ESA process is likely from 4 I13/2 to 4 F9/2 excited state [30]. In the second channel, interaction of NIR excitation light (here 980 nm) with Ag NPs causes emissions from asymmetric metallic NPs in the visible region. Such photoluminescence (PL) emissions are attributed to a recombination of electron-hole pairs after exciting the electrons from d-bands to unoccupied sp-conduction band, and moving them to Fermi level through a phonon-electron interaction [28,31,32]. Since the metallic NPs in the heavy metal oxide glass shows PL emissions at NIR radiations, energy transfer from surface of Ag NPs to Er3+ ions are also likely to occur, as the third mechanism [33]. Som and Karmakar [28] showed that the corresponding florescence of Ag NPs is so weak that such large enhancement in Er3+ luminescence cannot be attributed as a major factor. The PL from asymmetric Ag NPs plays a minor role which firstly increases the photonic density around the Er3+ ions and thus, enlarges the excitation rate of RE ion [34,35]. On the other hand, the energy transfer from Er3+ ions to Ag0 NPs surface is also likely to happen in high concentration of NPs, resulting to quench in emissions of erbium ions [28], which is not observed in this study. In the present study, considering the energy transfer between two erbium ions and from Ag0 NPs to Er3+ ions, the enhancement of emission in visible range is observed. However, enhanced local field due to presence of elliptical NPs along with energy transfer from silver NP to Er3+ ion are attributed to be the main reasons to achieve the enhancement in green and red emission.
Financial supports by Islamic Azad University, (Tehran North, and Science and Research Branches) are greatly acknowledged. M.R. Dousti and M.R. Sahar are thankful for grants and financial supports through RMC, MOHE and IDF by UTM (Q.J130000.7126.00J39/GUP and 4D005). Moreover, authors are very thankful to Mr. A. Jaleb for performing the TEM.
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