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We report significant enhancements in Er3+ luminescence as well as in Raman intensity in silver nanoparticles embedded zinc-tellurite glass. Surface enhanced Raman scattering effect is highlighted for the first time in tellurite glass containing silver NPs resulting in an enhanced Raman signal (~10 times). SAED manifest the growth of Ag0 nanoparticles along the (111) and (200) crystallographic planes having average diameter in the range 14-36 nm. Surface plasmon resonance bands are observed in the range 484-551 nm. Furthermore, four prominent photoluminescence bands undergo significant enhancements up to 3 times. The enhancement is majorly attributed to the local field effect of silver NPs.
©2013 Optical Society of America
1. Introduction
Due to the distinctive characteristic of resonant absorption displayed by different glasses when nano-sized structures (usually metals or semiconductors) are incorporated are getting large interest recently [1–3]. It is well known that emission cross-section for Er3+ is small hence there is ways to enhance it for different applications. One way is to co-doped with another rare-earth (RE) like Yb [4], another strategy is to embed with semiconductor nanostructures like Si [2] and more lately embedding metallic nanoparticles (NPs) like Ag or Au [5]. Collective oscillation of free electrons is called surface Plasmon resonance (SPR). SPR gives origin to large and highly confined local electric field within the proximity of metal NPs [6, 7]. These metallic NPs (plasmonic nanostructures) when lie within the vicinity of the RE ion can enhance the luminescence yield by several order of magnitude due to the local field effect (LFE) induce by SPR [8, 9]. This mechanism is known as metal enhanced luminescence (MEL) which is a powerful tool not only in biotechnology [8, 9] but also in the novel field of nanophotonics [10]. For conducting fluorescence experiments, the geometry of the specimen should be large enough when comparing with the dimension of fluorophores and relative to the emission and absorption wavelengths [9]. Fluorophores emits radiations into free space in this case which is the usual observation. However, the existence of small metallic NPs can modify the free space situation and can cause remarkable optical alteration. Surprisingly, there may be increase or decrease in the radiative decay rates of fluorophores due to metal surfaces which can enhance the rate of resonance energy transfer [11, 12]. When fluorophores in excited states interact with free electrons of the metal (so called surface Plasmon electrons) induce modifying effect on the fluorophore. The interaction between fluorophore and metal is referred as metal-enhanced fluorescence (MEF) in the field of biotechnology. In telecommunications, for data transfer optical fibres are used, however due to scattering, absorption and impurities optical signal can be attenuated. To overcome these problems Erbium doped fibre amplifiers were developed and other possibility is Raman Amplifier utilizing stimulated Raman scattering [13].
The aim of the present study is to investigate the effect of SPR of metallic NPs on the photoluminescence and Raman signal in erbium-zinc-tellurite glass. To our knowledge only few studies are found in literature with enhancement in Raman signal when embedding metallic NPs inside the glass host [14,15].
2. Experimental work
A series of tellurite glass are prepared by melt-quenching technique.TeO2, ZnO, Er2O3 and AgCl are used as starting materials in the powder form. Glass composition and their corresponding labels are given in Table 1. The ingredient chemicals are weighed and mixed thoroughly and then placed into the furnace at 900 °C. The mixture is kept at this temperature for 15 minutes in order to achieve complete melting. Next the melt is poured into a square stainless steel mold kept inside a second furnace at 340 °C and annealed for 24 Hrs. By the thermal analysis of a primarily sample the glass transition temperature (Tg) was measured and the annealing temperature is selected above Tg in order to grow and nucleate silver NPs inside the glass matrix. Table 1 shows the compositions and glass title for all the studied glasses. The amorphous nature of the host glass is confirmed by X-ray diffraction (XRD) measurements using Bruker D8 Advance diffractometer using CuKα radiations (λ = 1.54 Å) at 40 KV and 100 mA. The 2θ range is 0-60° with step size of 0.02° and resolution of 0.01°. Transmission electron microscope (TEM) Phillips CM12 with Docu Version 3.2 image analysis is used to investigate the nucleation of silver NPs. Specimens for TEM are prepared by dispersing the powder sample in acetone using ultrasonic bath. The solution is then placed onto copper grid and then allowed to dry before it is ready for characterization. The Raman spectrum was taken using a confocal Horbia Jobin Yvon (Model HR800 UV) in the range of 200-2000 cm−1. The argon ion laser operating at 514.55 nm was used as the excitation source with 5 mW power. Shimadzu UV-3101PC scanning spectrophotometer is employed to measure the visible and near infra-red absorption spectra in the range (190-2000) nm. The emission spectra are recorded by a Perkin Elmer LS-55 luminescence spectrometer in which a pulsed Xenon lamp operates as a source of excitation. The emitted light is dispersed by Monk-Gillieson monochromators and is detected with the standard photomultiplier tube.
Table 1. Glass labels, compositions, SPR position, average NP size and corresponding heat-treatment (HT) durations.
3. Results and discussion
Figure 1 shows the UV-Vis-NIR absorption spectra of all the glass samples. Surface Plasmon resonance (SPR) band due to silver NPs is not observed in the samples containing both Er3+ and Ag, therefore one Er3+ free sample containing 0.5 mol% of AgCl is prepared in order to monitor SPR band. The band around 484 nm belongs to the surface Plasmon absorption of pure 0.5 mol% Ag (inset Fig. 1). The maxima of SPR band is strongly dependent upon the refractive index (RI, n) of the glass according to Mie theory as follows
here c is the speed of light, N is the concentration of free electrons, m is conduction electrons’ effective mass, is the permeability of free space and is the metal optical dielectric function. In case of sodalime silicate glasses with RI~1.5, SPR bands of Ag0 and Au0 NPs are found to be positioned at 410 and 520 nm, respectively [16]. It is well known that if the RI of the matrix is higher, then SPR band will move to longer wavelength [17]. Since, in our case the RI of zinc-tellurite glass is ~2.4, hence SPR is prominently red-shifted to 484 nm.
Fig. 1 (a) Absorption spectra of Er3+ doped tellurite glass for No AgCl (Glass A) and 1.0 mol% AgCl (Glass D). Inset shows surface plasmon resonance (SPR) band located at 484 nm for Glass E. Energy levels are numbered as (1-4I13/2, 2-4I11/2, 3-4I9/2, 4-4F9/2, 5-2H11/2, 6-4F7/2, 7-2G9/2) (b) SPR bands observed for glass samples G, H and I centered at 490, 521, 551 nm.
Altering the annealing time and AgCl concentration modifies the optical properties. It is known that as the inter-particle spacing becomes smaller, interactions (efficient surface plasmon coupling) between the closely spaced metallic NPs significantly generates enhanced localized electric field hence luminescence intensity can be increased by many folds.
The plasmon peak is known to depend upon the refractive index as well as on the concentration of metallic NPs inside the glass host. We have prepared three Er3+ free samples [Glass G, H and I] to monitor the behavior of plasmon peak and its influence upon enhanced luminescence. Consequently, it has been found that this peak is red shifted from 490 nm to 551 nm for samples G to I, respectively, manifesting the formation of larger non-spherical silver NPs [18]. These non-spherical and asymmetric NPs are also evidenced from TEM image (Fig. 2).
Fig. 2 (a) shows the selected area electron diffraction (SAED) of glass C. The high resolution transmission electron microscope (HR-TEM) image of one single NP is shown in Fig. 2(b). Figures 2((c), (d), (e)) shows the TEM images of the glass samples C, D and E respectively. Corresponding histograms for the size distribution is also shown down to each TEM. Average diameter of NPs for glass B, C and D is 14, 24 and 36 nm respectively.
At higher concentration of Ag and maximum annealing time above glass transition temperature (24 Hrs) several NPs approach closely or coalesces (Ostwald’s ripening), so that their induced electric fields are expected to overlie and produce “hot spots” regions (or the regions of stronger electric field) that leads to a stronger surface enhanced fluorescence [7].
Figure 2(a) shows the selected area electron diffraction (SAED) of glass C. The high resolution transmission electron microscope (HRTEM) image of one single NP is shown in Fig. 2 (b). Figures 2(c)-2(e) show the transmission electron microscope (TEM) images of the glass samples C, D and E respectively. Corresponding histograms for the size distribution of the NPs are also given. Average diameter of NPs for glass B, C and D is 14, 24 and 36 nm respectively. It is clear from the size distribution that as the concentration of silver NPs is increased, their average size is also increased.
The increased average size of NPs by further introduction of NPs can be attributed to the growth of NPs during heat treatment by the following mechanisms (A) Ostwald ripening, where atoms or small clusters of atoms diffuse from smaller to larger NPs, and (B) nanoparticle migration followed by coalescence. i.e. at higher concentration of silver NPs (0.5 and 1.0 mol%) quite a few NPs approach closely or coalesces (Ostwald’s ripening) (see Fig. 3). Average diameter of the Ag nanocrystallites is found to be varied in the range of 14–36 nm.
Figure 4 presents the Raman spectra of all the Er3+ doped glass samples. The peaks centered at 281, 367 and 601 cm−1 belongs to the linkages in the bulk matrix. After embedding silver NPs inside the glass matrix the intensity of all these peaks are enhanced drastically (~by a factor of 10 times). These results are quite interesting and relatively new. The peak around at 601 cm−1 may be contributed from TeO2 units [19]. In the lower frequency, the peak at 281 cm−1 is assigned to both TeO3 tp and Er–O bond [20], and the peaks around 336, 367 are assigned to Zn-O [21].
Fleischmann in 1974 [22] observed the phenomenon of SERS for the first time and was elucidated as a consequence of the interaction between a molecule and the metal plasmon three years later [23]. The EM field exaltation and the polarizability tensor exaltation are the two main theories that describe the SERS effect. An EM field is generated when the incident EM field frequency becomes in resonance to the frequency of SP. This allows the strengthening of the incident field and exalts the Raman scattering of the studied molecule. Raman scattering is also enhanced due to its correspondence with SPR frequency and consequently there is a double exaltation effect [24, 25]. In SERS chemical effect (polarizability tensor exaltation) is also involved. Structure of the molecule can be modified by this effect causing a frequency shift of Raman scattering [26–28]. Furthermore, the topological variations of NPs in dielectrics promote the local electric field further by lightening rod effects (LRE) at surface of non-spherical NPs (elliptical, cube, pyramid shapes and so on) which can enhance the intensity of scattered Raman signal up to 1014 times [29].
Figure 5(a) shows the PL of the studied glasses; four prominent emission bands are obtained located at 520 nm, 550 nm, 650 nm and 835 nm attributed to 2H11/2→4I15/2, 4S3/2→4I15/2, 4F9/2→4I15/2 and 4S3/2→4I13/2 transitions, respectively. All the bands are enhanced significantly by factors of 2.5, 2.3, 2 and 1.7 times, respectively (See Fig. 5(b)).
Fig. 5 (a) luminescence spectra of glasses under an excitation of 786 nm i) No AgCl, ii) 0.1 mol% AgCl, iii) 0.5 mol% AgCl, iv) 1.0 mol% AgCl (b) plot of emission intensity vs concentration of Ag (mol%). Maximum amplification for the green and red bands is found to occur at 0.5 mol% Ag (Glass C).
The metallic NPs result in large and highly localized electric field around the Er3+ ions [30–33]. As, surface plasmon polariton generated by electronic oscillations (SPR) move along the surface of metallic NPs focus light in the subwavelength structures due to the difference in relative permittivity of the surrounding host and metal. Furthermore, the local field around the metallic NPs is enhanced by the concentration of light and metallic screening (“Lightning Rod Effect”) with respect to the incident field [34, 35]. Nevertheless, metallic NPs offer a unique prospect of modification of fluorescence due to changes in the rates of excitation and emission of lanthanide ions. If the host is a glass then the effect of metallic NPs on the emission and absorption rates of lanthanide ions is primarily of electronic origin [36, 37]. It can be visualized as an added interaction in the proximity of metallic NPs generated by plasmonic excitation at the Mie resonance frequency [36, 37].
Even though enhanced quantum yield is desirable, however both enhancement and quenching of fluorophore is observed near the metallic NPs [38, 39]. If the distance between metallic NP and fluorophore is 50 Å or more then enhancement in fluorescence can be achieved [37]. Enhancement in fluorescence can be maximized in some regions near metal surface due to the exponential decrease of the local field from the surface [35, 37].
Ground state absorption (GSA), excited state absorption (ESA) and energy transfer upconversion (ETU) are the vital involved mechanisms. These mechanisms can be explained as follows
where NR is the non-radiative decay. Moreover, ETU also takes place as follows (See Fig. 7(a)).Downconversion emission spectra of Er3+ doped tellurite glass samples with and without silver NPs are shown in Fig. 6. For all the glass samples the excitation wavelength used is 470 nm. Four prominent emission bands are obtained centered at 520 nm, 550 nm, 650 nm and 835 nm attributed to 2H11/2→4I15/2, 4S3/2→4I15/2, 4F9/2→4I15/2 and4S3/2→4I13/2 transitions, respectively. All the bands are enhanced gradually (by factors of 3, 2.7, 2 and 2.8). Involved mechanisms can be understood similar to upconversion enhancement (see Fig. 7(b)).
Fig. 6 (a) Downconversion luminescence spectra of glasses with i) No AgCl, ii) 0.1 mol% AgCl, iii) 0.5 mol% AgCl, iv) 1.0 mol% AgCl (b) plot of emission intensity vs concentration of Ag (mol%). Maximum amplification for the green and red bands are found to be occur at 0.5 mol% Ag (Glass C).
Fig. 7 (a) Partial energy level diagram of Er3+ ions in zinc-tellurite glass showing UC emissions at 520, 550 and 650 nm through ground state absorption (GSA), excited state absorption (ESA) and energy transfer (ET) between two Er3+ ions. Local field effects due to silver NPs are also shown. (b) Partial energy level diagram of Er3+ ions in zinc-tellurite glass showing downconversion emission at 520, 550, 650 and 835 nm through ground state absorption (GSA) and energy transfer (ET) between two Er3+ ions. Local field effects due to silver NPs are also shown (b).
The effect of metallic NPs on the Eu3+ luminescence in glass and glass ceramics were studied for the first time by Malta et. al. [35] and later on confirmed by Hayakawa et. al. [36] Malta et al. [35] studied the results theoretically and suggested two possible mechanisms: one is the enhanced local field effect in the vicinity of Eu3+ ions and other is the energy transfer between Eu3+ ions and silver NPs.
So it can be concluded that the overall enhancement in the emisission bands of Er3+ is due to the sum of the following two processes
At maximum concentration of AgCl (1.0 mol%), a quench is observed in both up and down conversion fluorescence intensity. The reason for quenching is the energy transfer from Er3+ ions to silver NPs (Er3+→Ag0) and reabsorption by SPR of increased quantity of silver NPs having a plasmon absorption band range extended over the emission peak position of Er3+ [40, 41].
The schematic interaction of the indicent light by Er-Ag couple and visible emissions of the rare earth is presented in Fig. 8.
4. Conclusion
Melt-quench technique has been used to synthesize a series of Ag nanoparticles embedded zinc-tellurite glass. UV-vis absorption spectroscopy shows the plasmon band position in the range of 484-551 nm. The TEM images confirm the existence of spherical as well as anisotropic Ag nanoparticles inside the glass matrices. The growth of Ag0 nanoparticles along the (111) and (200) crystallographic planes is evident from SAED. Under an excitation wavelength of 786 nm, four prominent emission bands are obtained located at 520, 550, 650 and 835 nm attributed to 2H11/2→4I15/2, 4S3/2→4I15/2, 4F9/2→4I15/2 and 4S3/2→4I13/2 transitions respectively. All the bands are enhanced gradually (by factors of 2.47, 2.3, 2 and 1.7 times respectively). The enhancement is mainly attributed to the local field effect of silver NPs. Raman signal is amplified by a factor of 10 times for Glass D relative to Glass A due to the large electric field originated from the silver NPs. Enhanced fluorescence influenced by silver NPs may contribute towards the development of optical displays, laser and optical memory devices whereas amplification in Raman signal is promising for Raman amplifiers.
Acknowledgments
The authors gratefully acknowledge the financial supports from UTM through RUG 06J39 and ministry of higher education through FRGS 4F083.
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