Infrared-to-visible upconversion emission in Er3+ doped TeO2-WO3-Bi2O3 glasses with silver nanoparticles Vitor P. P. de Campos, Luciana R. P. Kassab, Thiago A. A. de Assumpção, Diego S. da Silva, and Cid B. de Araújo Citation: J. Appl. Phys. 112, 063519 (2012); doi: 10.1063/1.4754468 View online: http://dx.doi.org/10.1063/1.4754468 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v112/i6 Published by the American Institute of Physics.
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JOURNAL OF APPLIED PHYSICS 112, 063519 (2012)
Infrared-to-visible upconversion emission in Er31 doped TeO2-WO3-Bi2O3 glasses with silver nanoparticles ~o,3 Vitor P. P. de Campos,1 Luciana R. P. Kassab,2 Thiago A. A. de Assumpc¸a jo4,a) Diego S. da Silva,3 and Cid B. de Arau 1
Departamento de Engenharia Metal urgica e de Materiais, Escola Polit ecnica da USP, 05508-010 S~ ao Paulo, SP, Brazil 2 Faculdade de Tecnologia de S~ ao Paulo, 01124-060 S~ ao Paulo, SP, Brazil 3 Departamento de Engenharia de Sistemas Eletr^ onicos, Escola Polit ecnica da USP, 05508-010 S~ ao Paulo, SP, Brazil 4 Departamento de Fısica, Universidade Federal de Pernambuco, 50670-901 Recife, PE, Brazil
(Received 12 July 2012; accepted 24 August 2012; published online 24 September 2012) The influence of silver nanoparticles (NPs) on the frequency upconversion luminescence in Er3þ doped TeO2-WO3-Bi2O3 glasses is reported. The effect of the NPs on the Er3þ luminescence was controlled by appropriate heat-treatment of the samples. Enhancement up to 700% was obtained for the upconverted emissions at 527, 550, and 660 nm, when a laser at 980 nm is used for excitation. Since the laser frequency is far from the NPs surface plasmon resonance frequency, the luminescence enhancement is attributed to the local field increase in the proximity of the NPs and not to energy transfer from the NPs to the emitters as is usually reported. This is the first time that the effect is investigated for tellurite-tungstate-bismutate glasses and the enhancement observed is the largest reported for a tellurium C 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4754468] oxide based glass. V
I. INTRODUCTION
Photoluminescence (PL) enhancement in rare-earth (RE) doped heavy-metal oxide glasses containing metallic nanoparticles (NPs) has been investigated by many authors.1–18 The studies are motivated by the glasses’ applications in photonic devices, such as lasers, optical amplifiers, displays, and solar cells. The influence of the NPs on the PL efficiency of the RE ions is larger when the incident light and/or the PL wavelengths are near-resonance with the resonance frequency, xSP , of the localized surface plasmon (LSP), the quanta of coherent free-electrons oscillations in the NPs. The NPs’ dielectric function, their shape and sizes distribution, the host environment, and the relative distances between the ions and the NPs are important parameters that influence the PL efficiency. The optimum distance between a silver NP and a RE ion for larger PL enhancement is 15 nm;19 when the distance NP-RE ion is smaller, PL quenching due to energy transfer from the RE ion to the NP is dominant. One successful way to grow silver or gold NPs inside heavy-metal oxide glasses is based on the melt-quenching method. The appropriate concentration of NPs as well as their average size and shapes depend on the glass viscosity and the growth process is controlled by the diffusion of the metal atoms and ions. Therefore, one of the important steps to obtain large PL enhancement is the efficient nucleation of the metallic NPs through controlled heat-treatment of the samples. Different mechanisms may contribute for the PL of a RE ion located in the vicinity of a metallic NP. In the a)
Author to whom correspondence should be addressed. Electronic mail:
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0021-8979/2012/112(6)/063519/4/$30.00
excitation stage, the beam may be coupled into confined LSP modes enhancing the energy density near the RE ion position that increases the absorption rate. In the emission stage, the same LSP modes may enhance the PL efficiency through the Purcell effect.20 Normally, the more confined modes that have larger Purcell enhancement present larger nonradiative losses;20 therefore, for a given environment, there is an optimum NP size in order to achieve large PL enhancement. When the excitation frequency is smaller than xSP , the LSP influence on the absorption rate is not large; the most important contribution from the NPs is due to the Purcell effect for the emissions with frequencies near to xSP . A theoretical model applied for Er3þ doped tellurite glasses containing silver NPs showed increased upconversion (UC) efficiency with the increase of the NPs’ diameters.21 The selection of materials with appropriate parameters is a hard task when one is performing experiments with NPs ensembles such as the ones performed with glasses. Also, it is very important the selection of glasses with large transparency window and small cutoff-phonon energy to reduce the probability of nonradiative relaxation of the RE ions. Tellurite glasses are excellent materials for UC experiments because of their high refractive index (2.0), large transmittance window (360–1000 nm), low cutoff-phonon energy (700 cm1), large mechanical resistance, high chemical durability, high vitreous stability, and high solubility for RE ions doping species.22 Recently, we demonstrated the possibility of obtaining enhanced UC efficiency in tellurite glasses containing silver and gold NPs. In all cases, the presence of NPs contributed for the PL efficiency either due to energy transfer from the NPs to the RE ions or by the influence of the large local field on the RE ions positioned in the vicinity of the NPs. PL quenching
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was observed when the NPs concentration was above certain values. Indeed, the presence of silver nanostructures in TeO2PbO-GeO2 glasses improved the luminescence efficiency of Pb2þ clusters.5 Enhanced UC was observed in Pr3þ doped TeO2-PbO-GeO2 glasses containing silver NPs.12,13 PL experiments with TeO2-PbO-GeO2 glasses doped with Eu3þ and containing gold NPs14 and Tb3þ doped TeO2-ZnO-Na2OPbO glass with silver NPs15 were reported. Also experiments with tellurium based glasses containing metallic NPs and codoped with two different RE ions, Eu3þ/Tb3þ (Ref. 16) and Yb3þ/Tm3þ (Ref. 17), were studied. The present experiments demonstrate, for the first time, the influence of silver NPs in the green and red emissions of Er3þ doped TeO2-WO3-Bi2O3 glasses excited by a laser operating at 980 nm. The UC efficiency was controlled heattreating the samples during various time intervals, sHT . Enhancement of up to 700% was measured for the Er3þ emissions at 527, 550, and 660 nm with respect to the samples without silver NPs. This is the largest infrared-to-visible UC enhancement observed in tellurite glasses doped with Er3þ ions and silver NPs. II. EXPERIMENTAL
The samples were prepared adding 1.0 wt. % of Er2O3 and 2.0 wt. % of AgNO3 to the glass composition [54.6 TeO2–22.6 WO3–22.8 Bi2O3 (in wt. %)] using the meltquenching method. A batch of 10 g of high purity (99.999%) compounds were fully mixed in a pure platinum crucible and melted at 760 C for 45 min. The melts were then poured into pre-heated brass molds, in air, and annealed at 360 C for 1 h to release internal stresses. Finally, the glasses were cooled to room temperature following the cooling inertia of the furnace. Afterwards the samples were polished, cut, and heat-treated at 360 C during 24, 48 (two steps of 24 h), and 72 h (three steps of 24 h) to thermally reduce Agþ to Ag0 and consequently to nucleate and to grow silver NPs. Details of the heat-treatment procedure used in our laboratory are given in Ref. 17. A 200 kV transmission electron microscope (TEM) was used to investigate the presence of silver NPs inside the glass matrix. Optical absorption spectra were measured at room temperature in the 360–1000 nm range using a commercial spectrophotometer. The UC emission spectra were obtained by exciting the samples with a continuous-wave diode laser operating at 980 nm and the signals were analyzed using a monochromator fitted with a photomultiplier connected to a lock-in and a computer. All measurements were made at room temperature. III. RESULTS AND DISCUSSIONS
Figure 1 shows a TEM image of a sample heat-treated during 24 h. Silver NPs with average size of 35 nm were observed. Electron diffraction patterns showed the crystalline structure of the NPs. Figure 2 presents the absorption spectra of the Er3þ doped TeO2-WO3-Bi2O3 glass with silver NPs for different values of sHT . The spectrum of a sample without NPs is also shown to be used as reference. Absorption bands attributed
FIG. 1. (a) Transmission electron microscope image of a sample heattreated during 24 h. (b) Histogram of the nanoparticles sizes.
to 4f–4f transitions of Er3þ ions corresponding to the transitions starting from the ground state (4I15/2) to the excited states are observed. The bands centered at 490, 522, 545, 654, 800, and 973 nm are due to the transitions: 4I15/2!4F7/2, 4 I15/2!2H11/2, 4I15/2!4S3/2, 4I15/2!4F9/2, 4I15/2!4I9/2, and 4 I15/2!4I11/2, respectively. The absorption band associated with the LSP modes is not clearly observed because of the small amount of silver NPs, because the LSP associated with large NPs presents broadband resonances with small amplitudes, and because the glass matrix absorbs strongly in the blue region. However, the spectra of Fig. 2 reveal the contribution of the LSP absorption band as a tail in the blue-green region. With the basis on the dielectric function of silver23 and the glass refractive index (2), we estimate the LSP resonance wavelength between 420 and 500 nm in agreement with the tail observed in Fig. 2.
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FIG. 2. Absorption spectra of the samples heat-treated during various times. The spectrum of a sample without silver NPs is shown for reference. Samples’ thickness: 2.0 mm.
Figure 3(a) shows the UC emission spectra of the samples. The PL bands centered at 527, 550, and 660 nm correspond to the 2H11/2!4I15/2, 4S3/2!4I15/2, and 4F9/2!4I15/2 transitions of Er3þ ions, respectively. The spectrum of a
FIG. 3. (a) UC luminescence spectra obtained for the heat-treated samples and for a sample without silver NPs. (b) Dependence of the UC signals at 550 and 670 nm versus the laser intensity. Sample heat-treated during 24 h.
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FIG. 4. Dependence of the green and red luminescence signals on the heattreatment times.
sample without silver NPs is included for comparison. Notice that the PL bands in the green-red region increase with the increase of sHT because the concentration of the NPs grows. The spectra are not corrected for the photomultiplier spectral response, which exhibits reduced quantum efficiency of about 140% from green to red wavelengths. Figure 3(b) shows the dependence of the UC intensities versus the laser intensity. The quadratic slopes of the lines indicate that two photons are contributing to generate each UC photon. The excitation pathway 4I15/2!4I13/2!4F7/2 is the same identified in previous experiments: based on leadgermanate glasses.7 It can be seen in Fig. 3(a) that the intensities corresponding to the transitions 2H11/2!4I15/2, 4S3/2!4I15/2, and 4F9/2 !4I15/2, for the sample heat-treated during 24 h, increased by almost eight times with respect to the sample without NPs. On the other hand, the UC efficiency shown by the samples heat-treated during 48 and 72 h is reduced. Figure 4 summarizes the dependence of the UC signals on the heat-treatment time. It is shown that the value of sHT for maximum enhancement of the emission at 550 nm is 24 h. For sHT ¼ 48 h and sHT ¼ 72 h the UC signal is reduced, probably because the concentration of silver NPs is too large and then the average distance NP-RE ion is smaller than the critical distance for PL enhancement. For such small distances, energy transfer from the excited Er3þ ions to the NPs may be the most probable process and nonradiative relaxation is dominant. Since the 980 nm excitation wavelength is far from the LSP resonance wavelength, the probability of direct excitation of the plasmon band is very small and the intensity enhancement observed is attributed to the increased local field in the vicinities of the silver NPs. Accordingly, the probability of energy transfer from the NPs to the Er3þ ions is negligible. As reported before in the case of Er3þ doped PbO-GeO2 glasses,7 the proximity between the PL wavelengths and the plasmon absorption band favors the enhancement of the UC emissions due to the intensified local field effect. The same behavior was observed for other tellurite glass composition18 but in that case the observed
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enhancement was only 30% of the value obtained in the present experiment. IV. SUMMARY
We reported the nucleation and growth of silver NPs with average diameter of 40 nm in Er3þ doped TeO2-WO3Bi2O3 glasses. Intensified infrared-to-visible upconversion was observed using for excitation a laser operating at 980 nm. Due to the presence of the NPs, the luminescence intensity in the green-red region exhibited an increase of 700% with respect to the intensity emitted by the same glass without silver NPs. Considering the large detuning of the excitation wavelength with respect to the LSP resonance, we attribute the enhancement observed to the increase of the local optical field amplitude in the proximities of the NPs. ACKNOWLEDGMENTS
This work was supported by the National Institute of Photonics (INCT Project) granted by the Conselho Nacional de Desenvolvimento Cientıfico e Tecnologico-CNPq. The authors also thank P. K. Kiyohara and S. P. de Toledo (Laborat orio de Microscopia Eletr^onica, Instituto de Fısica, USP) for TEM measurements. 1
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