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Emission properties of polymer composites doped with Er3+:Y2O3 nanopowders
Optical Materials 34 (2012) 1964–1968
Contents lists available at SciVerse ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Emission properties of polymer composites doped with Er3+:Y2O3 nanopowders Krzysztof Anders a, Anna Jusza a, Magdalena Baran b, Ludwika Lipin´ska b, Ryszard Piramidowicz a,⇑ a b
Institute of Microelectronics and Optoelectronics, Warsaw University of Technology, ul. Koszykowa 75, 00-662 Warsaw, Poland Institute of Electronics Materials Technology, ul. Wolczynska 131, 01-919 Warsaw, Poland
a r t i c l e
i n f o
Article history: Available online 19 December 2011 Keywords: Nanopowder Polymer Erbium ion Luminescence Upconversion
a b s t r a c t In this work we report the recent results of our investigation on visible emission properties of the PMMAbased polymer nanocomposites doped with Er3+:Y2O3 nanopowders. The set of active nanopowders, and polymer films, differing in active ions concentration, was characterized with respect of their luminescent properties in the green spectral range, available to a limited extent for semiconductor lasers. In particular – the concentration dependent emission spectra and fluorescence dynamics profiles were measured under direct (single photon) and up-converted excitation, enabling the comparison of luminescent properties of developed nanocomposite materials and original nanopowders, optimization of erbium dopant concentration as well as discussion of excitation mechanisms and analysis of the efficiency of depopulation processes. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Recent years have observed continuous effort focused on the development of novel, low-cost photonic materials, which is driven by the dynamically expanding range of their applications, covering the areas of light sources and lightning systems, full-color displays, optical communications, data processing and storage, photovoltaics and many others. Since recently, the specific attention is focused on polymer-based composite materials, which allow emission and amplification of the visible light [1,2]. The polymer materials dedicated to photonic applications, like e.g. poly(methyl methacrylate) (PMMA), are known to have the excellent mechanical properties and relatively good transmission over the visible spectral range, which in combination with the easiness of fabrication and low cost of mass production make them extremely attractive matrices for rare-earth doped light sources, lasers and amplifiers. Unfortunately, when directly doped with active ions the typical polymer matrices exhibit strong luminescence quenching, which results from interactions of the active ions with highly energetic phonons of the polymer matrix. The polymer-based composite materials doped with active nanopowders are considered as one of the most promising solutions of this problem, potentially offering both advantages of low cost and excellent mechanical properties of polymer matrices and the attractive luminescent properties of the embedded active nanocrystallites (which spectroscopic features are often better than bulk crystals) [3–5]. This work is focused on the emission properties of PMMA-based polymer composite doped with Er3+:Y2O3 nanopowders. Apart ⇑ Corresponding author. Tel.: +48 22 234 50 47; fax: +48 22 628 87 40. E-mail address: [email protected] (R. Piramidowicz). 0925-3467/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2011.11.011
from their excellent and well known emission properties in the infra-red, erbium doped materials (mainly low phonon crystals and glasses) have been also discussed in the context of the emission and lasing in green spectral range, being a result of either direct or up-converted excitation [6,7]. In particular, the green emission have been observed in oxide and fluoride nanopowders [8,9] and in polymer-based composite structures [10]. However, up to now there is a limited number of reports on short wavelength luminescence in PMMA polymers doped with oxide nanopowders, like yttria (Y2O3), which has proven lasing potential, good thermomechanical properties and relatively low phonon energy of 600 cm 1 [11]. 2. Manufacturing of the samples The nanopowders were prepared using sol–gel method in the Institute of Electronic Materials Technology (Warsaw), by dissolving the Y2O3 and Er2O3 in acetic acid and solution of nitric acid, mixed together and stirred for 1 h at 65 °C. At the end of this procedure, ethylene glycol was added as cross-linking agent and the solution was stirred again for 2 h. During slow evaporation of the mixture the obtained sol transformed into sticky gel, which, in turn, after drying in 120 °C for 12 h changed into solid xerogel. The grinded powders were calcined in the air atmosphere at 1000 °C for 7 h. The obtained active nanocrystals were added into PMMA (poly(methyl methacrylate)), dissolved in suitable solvent (ethyl acetate). The homogenous dispersion of nanopowder was obtained by ultrasonification applied for 12 h. The resulting polymer foils were cast on glass substrate and dried in air at room temperature during 20 h. Finally, we disposed of the set of the erbium doped
K. Anders et al. / Optical Materials 34 (2012) 1964–1968
1965
Y2O3 nanopowders of different dopant concentrations (0.1, 1 and 5 at.%) and the PMMA-based active nanocomposites differing in the concentration of both RE3+ ions and nanopowder introduced into PMMA matrix (1, 5, 10 and 20 wt.%) as well. 3. Structural characterization To determine the fundamental structural properties all manufactured nanopowders were subjected to X-ray diffraction (XRD) and scanning electron microscopy (SEM) measurements. The crystal structure of the samples was characterized using a Siemens D500 diffractometer with Cu Ka radiation at 1.548 Å. The size and morphology of the samples were analyzed with Carl Zeiss SMT AURIGATM CrossBeam Workstation apparatus. The most stable form of Y2O3 at room temperature is a cubic type, belonging to space group Ia3 [12] and the erbium ions can be located in two distinct sites, substituting the yttrium ions with approximately equal probability [13]. The unit cell contains 24 yttrium ions in non-centro-symmetric sites C2 and eight Y3+ ions in centro-symmetric sites S6, so the site occupation is 3(C2):1(S6). The Y2O3 structure can be described as distorted cube with one Y3+ ion in the center and six O2 ions occupying the corners with vacancy diagonals shown in Fig. 1. The pure electronic transitions are electric dipole forbidden for rare earth in inversion symmetric sites [14–16] and only magnetic dipole transitions should be observed at lattice site of S6 symmetry, which are ineffective in the case of erbium in Y2O3 [16]. Therefore it is expected that optical transitions will occur purely from the erbium ions at the C2 sites.
O2S6
The XRD spectra, shown in Fig. 2 confirm that obtained Er3+:Y2O3 powders contain only one, cubic phase (ICDD card No. 41-1105), the small distortions of the lines reflect the distortion of the lattice due to introduced active ions. The average size of the crystallites, calculated according to the Scherrer’s formula [17] is of the order of 18.4 nm, 19.5 nm and 19.5 nm for 0.1, 1 and 5 at.% Er3+, respectively. The SEM picture on an exemplary sample, presented in Fig. 3, shows that the average size of the crystallites is bigger than those estimated from broadening of diffraction peaks, but it is still well below 100 nm. 4. Optical characterization
Vacancy diagonal
Er3+
Fig. 3. The exemplary SEM picture of the 1 at.% Er3+:Y2O3 sample.
Er3+
C2 Fig. 1. Schematic presentation of the two Er3+ crystallographic sites in cubic Y2O3.
All the samples (both nanopowders and PMMA-based nanocomposites) were carefully characterized by means of optical spectroscopy. In particular – the excitation and emission spectra together with the profiles of fluorescence dynamics from excited states were measured and analyzed. Also the up-converted emission spectra were recorded under excitation by laser diode operating at 980 nm. The PTI QuantaMaster-based spectrofluorimetric system, equipped in double monochromators in the excitation and emission paths was used as a primary measuring setup, enabling both continuous wave (CW) and pulsed excitations over a wide spectral range (200–2000 nm) as well as signal detection within the required spectral range. All measurements were taken at room temperature and all spectra were corrected for spectral characteristic of the detector. 4.1. Er3+:Y2O3 nanopowders
3+
0.1% Er :Y2O3 3+
1.0% Er :Y2O3 3+
intensity [a.u.]
5.0% Er :Y2O3
10
20
30
2
40
50
60
Fig. 2. The XRD spectra of the Er3+:Y2O3 samples differing in dopant concentration.
The investigations of optical properties of Er3+:Y2O3 nanoparticles were focused mainly on their luminescent behavior and its sensitivity to the polymer matrix parameters. Due to specific (powdered) type of original active medium, the typical measurements of absorption characteristics were replaced by excitation spectra measurements. The excitation spectra of the green emission recorded for all investigated concentrations does not exhibit significant differences, therefore in Fig. 4 only the single trace is shown together with the attribution of particular transitions. All the spectra are dominated by two strong, broadband transitions to the excited 4G11/2 and thermally coupled 2H11/2 + 4S3/2 manifolds in the UV and green spectral range, respectively, with less intense contribution of much weaker transitions to the 4H9/2, 4F3/2 + 4F5/2 and 4F7/ 2 levels, extending over violet, blue and blue–green part of the spectrum. The emission spectra, shown in Fig. 5, confirm that luminescence intensity depends strongly on concentration of the activator
1966
K. Anders et al. / Optical Materials 34 (2012) 1964–1968
4
G11/2
em. @ 565 nm
3+
Er :Y2O3
3+
1.0% Er :Y2O3
= 379 nm
exc
2
4
em
= 565 nm
intensity [a.u.]
intensity [a.u.]
H11/2 + S3/2
4
H9/2
250 275 300
325 350
375
0.1% 1.0% 5.0%
4 4
4
400 425
F3/2+ F5/2
450 475
F7/2
100
500 525 550
200
300
400
500
600
700
800
900
1000
time [us]
wavelength [nm]
Fig. 6. Fluorescence decay profiles of green emission in Er3+:Y2O3.
Fig. 4. Excitation spectrum of green emission of 1 at.% Er3+:Y2O3 nanopowder (the emission was monitored at 565 nm).
Table 1 Spectroscopic properties of the Er3+:Y2O3 nanopowders. Emitting level 4
2
S3/2 + H11/2
exc. @ 379 nm 3+
intensity [a.u.]
0.1% Er :Y2O3
WNR (s
1
)
8.06 10
3
1
Calculated radiative lifetime (ls)
WCR (s
79 ± 4 [14]
3.22 103 (1 at.%) 5.63 104 (5 at.%)
)
3+
1.0% Er :Y2O3 3+
5.0% Er :Y2O3
400 425 450 475 500 525 550 575 600 625 650 675 700
wavelength [nm] Fig. 5. Emission spectra of Er3+:Y2O3 under direct 4G11/2 excitation.
and the best properties among investigated samples exhibits that of concentration of 1.0 at.%. In general, all observed spectra reflect the typical luminescence behavior of erbium doped bulk crystals, except a slightly broadened lines. Like a bulk crystals, the Er3+:Y2O3 powders emits mainly in the green spectral range, with main peaks at 524 nm, 540 nm, 555 nm and 565 nm (which is the strongest one), all being related with transitions from thermally coupled 2 H11/2 and 4S3/2 levels. The samples exhibit also a weak luminescence in the violet (at ca. 410 nm) and red (at ca. 663 nm) spectral range. These emission lines correspond to 4H9/2 ? 4I15/2 and 4F9/ 4 2 ? I15/2 transitions, respectively. Fig. 6 illustrates the fluorescence dynamics profiles recorded for the green emission, monitored at 565 nm, for all investigated samples as a function of dopant concentration. It should be mentioned, that due to the thermal coupling of both green-emitting levels the fluorescence dynamics profiles were identical for all particular lines of the investigated green band. In all samples the shortening of the fluorescence lifetime is observed when dopant’s concentration increases, which is a manifestation of cross-relaxation processes resulting in concentration quenching of the fluorescence. The calculated cross-relaxation rate for the sample doped with
5 at.% of erbium is of order of 104 s 1 (see Table 1) while the respective values of multiphonon decays are of order of 103 s 1, which suggests that the cross-relaxation is the dominant depopulating mechanism of 4S3/2 + 2H11/2 multiplet of Er3+ in this host. All profiles are nearly exponential, except the initial part of a trace. This may suggest the slight influence of ions at different lattice locations. The measured values of fluorescence lifetimes are slightly shorter than those reported for bulk samples (e.g. the reported in [14] time constant of green emission is equal to 88 ± 7 ls for 0.2% Er3+:Y2O3 sample, which is still longer than 80.9 ± 0.7 ls measured in this work for the sample with 0.1% of activator), but simultaneously much longer than lifetimes reported for Y2O3 nanopowders (collected in Table 2). It should be noted that the lifetimes, reported by different authors differ significantly – this applies specifically to the results obtained for nanopowders and may result from the differences in manufacturing technology. Apart from the direct excitation the fundamental up-conversion properties were investigated, under excitation at the most typically deployed in erbium doped systems infra-red wavelength of 980 nm. All investigated nanopowders exhibit efficient IR-to-visible up-conversion, with spectral features compared in Fig. 7, where normalized emission characteristics of all samples are shown. It is evident, that with the increasing dopant level the contribution of the red emission increases, which is the result of cross relaxation processes, populating the 4F9/2 red-emitting level and simultaneously depopulating the 4 S3/2 manifold. Nevertheless, when the intensity level is taken into account, the best luminescent properties has the sample doped with 1 at.% of erbium. It should be also noted that there are practically no traces of violet emission, observable in the directly excited samples. The analysis of energy level scheme and observed luminescent properties allow to propose the excitation pathway as the sequential absorption of two photons (GSA to the 4I11/2 level followed by an ESA from the 4I11/2 to the 4F7/2, which relaxes nonradiatively to the manifold 2H11/2 + 4S3/2).
1967
K. Anders et al. / Optical Materials 34 (2012) 1964–1968 Table 2 Time constants of green emission in Er3+:Y2O3. Dopant content (at.%)
Type of sample
Measured time constant of (ls)
4
0.02 0.04 0.1 0.1
Nanopowder Nanopowder Nanopowder Nanopowder in polymer Nanopowder Nanopowder Bulk Bulk Nanopowder Nanopowder Nanopowder Nanopowder Nanopowder in polymer Nanopowder Nanopowder Nanopowder in polymer
34 [6] 31 [6] 80.9 ± 0.7 [this work] 97.74 ± 1.2 [this work]
S3/2 + 2H11/2
0.1 0.1 0.2 0.2 0.2 0.3 0.5 1.0 1.0 1.0 5.0 5.0
12 [6] 11.0 [18] 88 ± 7 [14] 56 [6] 3.8 [6] 16.5 [18] 11.8 [18] 64.2 ± 0.6 [this work] 71.5 ± 1.2 [this work] 11.6 [18] 14.56 ± 0.1 [this work] 16.86 ± 0.3 [this work]
exc. @ 379 nm 3+
20%(0.1% Er :Y2O3):PMMA
intensity [a.u.]
Emitting level
3+
20%(1.0% Er :Y2O3):PMMA 3+
20%(5.0% Er :Y2O3):PMMA
400 425 450 475 500 525 550 575 600 625 650 675 700 725 750
wavelength [nm] Fig. 8. Emission spectra of Er3+:Y2O3 doped PMMA under direct 4G11/2 excitation.
exc. LD @ 980 nm 3+
1.0% Er :Y2O3
3+
0.1% Er :Y2O3
1000
3+
1.0% Er :Y2O3
3+
20%(1.0% Er :Y2O3):PMMA
3+
intensity [a.u]
intensity [a.u.]
5.0% Er :Y2O3
100
10
400 425 450 475 500 525 550 575 600 625 650 675 700 725 750
100
Fig. 7. Normalized 980 nm excitation.
upconversion
emission
spectra
200
300
400
500
600
700
time [us]
wavelength [nm] of
Er3+:Y2O3
under
Fig. 9. Fluorescence decay profiles of green emission in 1% Er3+:Y2O3 nanopowder and the same powder doped PMMA.
4.2. PMMA-based nanocomposites
exc.LD @ 980 nm 3+
20%(0.1% Er :Y2O3):PMMA 3+
20%(1.0% Er :Y2O3):PMMA 3+
20%(5.0% Er :Y2O3):PMMA
intensity [a.u.]
The luminescence spectra of PMMA-based nanocomposites doped with 20% of Er3+:Y2O3 nanopowders are presented in Fig. 8. The Y2O3 nanopowder doped with 1 at.% of erbium ions, which was considered as the optimal value, was used as an activator for PMMA polymer and added in the different amounts (from 1% to 20%). The dependence between the amount of powder and the intensity of the signal was found to be linear, therefore for further experiments the maximal dopant level of 20% has been used, being a compromise between the intensity of the signal and mechanical properties of the doped polymer. In general, except the intensity level, all the samples keep the luminescent properties of original nanopowders. The intensity of the signal is certainly weaker, but the lines are at the same spectral positions and are characterized by very similar spectral width. It should be noted, however, that the emission is affected by strong, broadband blue–green luminescence of the PMMA matrix, manifesting itself specifically under short wavelength excitation. The fluorescence dynamics measurements, presented in Fig. 9 for original nanopowder and polymer-based composite material, recorded for the green emission and dopant concentrations consid-
400 425 450 475 500 525 550 575 600 625 650 675 700 725 750
wavelength [nm] Fig. 10. Normalized upconversion emission spectra of Er3+:Y2O3 doped PMMA under 980 nm excitation.
1968
K. Anders et al. / Optical Materials 34 (2012) 1964–1968
ered as optimal, exhibit very similar character of the both profiles. Surprisingly, the fluorescence lifetime of the composite is slightly longer, taking a value of 71 ls (value recorded for the nanopowder was 64 ls). The same tendency was observed for the composites doped with 0.1 and 5 at.% of erbium. The respective values of fluorescence lifetimes were equal 98 ls and 17 ls. The lack of any shortening of the lifetime confirms that the active nanopowders are effectively shielded from the influence of the polymer matrix. The PMMA-based composites exhibit also the similar up-conversion features – the results are presented in Fig. 10. The character of the spectra, concentration-dependent behavior and relations between intensities of the lines are practically identical to these obtained for the original powders. This further confirms the excellent optical properties of developed composites and, simultaneously, the potential of developed technology of their manufacturing.
5. Conclusions The set of active nanopowders, obtained by sol gel method and differing in activator’s concentrations has been carefully characterized by means of highly-resolved laser spectroscopy. In particular – the emission and excitation spectra have been investigated together with fluorescence dynamics profiles, which in turn enabled optimization of developed manufacturing technology. First polymer composites, based on PMMA films doped with Er3+:Y2O3 have been obtained and characterized in respect of their luminescent properties in the visible spectral range. What is more – the IR-togreen and IR-to-red up-conversion effects have been observed
under IR excitation at 980 nm and preliminarily discussed in the context of possible excitation scheme. In general the obtained results have confirmed that nano-crystallites incorporated into polymer matrix tend to keep their original optical properties, thus enabling designing of new class of optically active polymer components for applications in photonics. References [1] L.H. Slooff, A. van Blaaderen, A. Polman, G.A. Hebbink, S.I. Klink, F.C.J.M. Van Veggel, D.N. Reinhoudt, J.W. Hofstraat, J. Appl. Phys. 91 (2002) 3955. [2] L. Peng, Y. Luo, Y. Dan, L. Zhang, Q. Zhang, S. Xia, X. Zhang, Colloid Polym. Sci. 285 (2006) 153. [3] D.K. Sardar, S. Chandra, J.B. Gruber, W. Gorski, M. Zhang, J.H. Shim, J. Appl. Phys. 105 (2009) 093105. [4] S. Chandra, J.B. Gruber, G.W. Burdick, D.K. Sardar, J. Appl. Polym. Sci. 122 (2011) 289. [5] J.C. Boyer, N.J.J. Johnson, F.C.J.M. van Veggel, Chem. Mater. 21 (2009) 2010. [6] J.A. Capobianco, F. Vetrone, T. D’Alesio, G. Tessari, A. Speghini, M. Bettinelli, Phys. Chem. Chem. Phys. 2 (2000) 3203. [7] J. Zhang, S. Wang, L. An, M. Liu, L. Chen, J. Lumin. 122–123 (2007) 8. [8] S. Xiao, Xiaoliang Yang, Zhengwei Liu, X.H. Yan, J. Appl. Phys. 96 (2004) 3. [9] F. Zhang, J. Li, J. Shan, L. Xu, D. Zhao, Chem. – Eur. J. 15 (2009) 11010. [10] R. Chai, H. Lian, Z. Hou, C. Zhang, C. Peng, J. Lin, J. Phys. Chem. C 114 (2010) 610. [11] H. Guo, Y.M. Qiao, Opt. Mater. 31 (2009) 583. [12] K. Lonsdale: International Tables for X-ray Crystallography, vol. III, Physical and Chemical Tables, Birmingham, Kynoch, 1962. [13] Y. Guyot, R. Moncorgé, L.D. Merkle, A. Pinto, B. McIntosh, H. Verdun, Opt. Mater. 5 (1996) 127. [14] M.J. Weber, Phys. Rev. 171 (1968) 283. [15] P.A. Tanner, X. Zhou, F. Liu, J. Phys. Chem. A 108 (2004) 11521. [16] J. Heber, K.H. Hellwege, U. Köbler, H. Murmann, Z. Physik 237 (1970) 189. [17] P. Klug, L.E. Alexander, X-ray Diffraction Procedure, Wiley, New York, 1954. [18] S. Chandra, F.L. Deepak, J.B. Gruber, D.K. Sardar, J. Phys. Chem. C 114 (2010) 874.
Contents lists available at SciVerse ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Emission properties of polymer composites doped with Er3+:Y2O3 nanopowders Krzysztof Anders a, Anna Jusza a, Magdalena Baran b, Ludwika Lipin´ska b, Ryszard Piramidowicz a,⇑ a b
Institute of Microelectronics and Optoelectronics, Warsaw University of Technology, ul. Koszykowa 75, 00-662 Warsaw, Poland Institute of Electronics Materials Technology, ul. Wolczynska 131, 01-919 Warsaw, Poland
a r t i c l e
i n f o
Article history: Available online 19 December 2011 Keywords: Nanopowder Polymer Erbium ion Luminescence Upconversion
a b s t r a c t In this work we report the recent results of our investigation on visible emission properties of the PMMAbased polymer nanocomposites doped with Er3+:Y2O3 nanopowders. The set of active nanopowders, and polymer films, differing in active ions concentration, was characterized with respect of their luminescent properties in the green spectral range, available to a limited extent for semiconductor lasers. In particular – the concentration dependent emission spectra and fluorescence dynamics profiles were measured under direct (single photon) and up-converted excitation, enabling the comparison of luminescent properties of developed nanocomposite materials and original nanopowders, optimization of erbium dopant concentration as well as discussion of excitation mechanisms and analysis of the efficiency of depopulation processes. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Recent years have observed continuous effort focused on the development of novel, low-cost photonic materials, which is driven by the dynamically expanding range of their applications, covering the areas of light sources and lightning systems, full-color displays, optical communications, data processing and storage, photovoltaics and many others. Since recently, the specific attention is focused on polymer-based composite materials, which allow emission and amplification of the visible light [1,2]. The polymer materials dedicated to photonic applications, like e.g. poly(methyl methacrylate) (PMMA), are known to have the excellent mechanical properties and relatively good transmission over the visible spectral range, which in combination with the easiness of fabrication and low cost of mass production make them extremely attractive matrices for rare-earth doped light sources, lasers and amplifiers. Unfortunately, when directly doped with active ions the typical polymer matrices exhibit strong luminescence quenching, which results from interactions of the active ions with highly energetic phonons of the polymer matrix. The polymer-based composite materials doped with active nanopowders are considered as one of the most promising solutions of this problem, potentially offering both advantages of low cost and excellent mechanical properties of polymer matrices and the attractive luminescent properties of the embedded active nanocrystallites (which spectroscopic features are often better than bulk crystals) [3–5]. This work is focused on the emission properties of PMMA-based polymer composite doped with Er3+:Y2O3 nanopowders. Apart ⇑ Corresponding author. Tel.: +48 22 234 50 47; fax: +48 22 628 87 40. E-mail address: [email protected] (R. Piramidowicz). 0925-3467/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2011.11.011
from their excellent and well known emission properties in the infra-red, erbium doped materials (mainly low phonon crystals and glasses) have been also discussed in the context of the emission and lasing in green spectral range, being a result of either direct or up-converted excitation [6,7]. In particular, the green emission have been observed in oxide and fluoride nanopowders [8,9] and in polymer-based composite structures [10]. However, up to now there is a limited number of reports on short wavelength luminescence in PMMA polymers doped with oxide nanopowders, like yttria (Y2O3), which has proven lasing potential, good thermomechanical properties and relatively low phonon energy of 600 cm 1 [11]. 2. Manufacturing of the samples The nanopowders were prepared using sol–gel method in the Institute of Electronic Materials Technology (Warsaw), by dissolving the Y2O3 and Er2O3 in acetic acid and solution of nitric acid, mixed together and stirred for 1 h at 65 °C. At the end of this procedure, ethylene glycol was added as cross-linking agent and the solution was stirred again for 2 h. During slow evaporation of the mixture the obtained sol transformed into sticky gel, which, in turn, after drying in 120 °C for 12 h changed into solid xerogel. The grinded powders were calcined in the air atmosphere at 1000 °C for 7 h. The obtained active nanocrystals were added into PMMA (poly(methyl methacrylate)), dissolved in suitable solvent (ethyl acetate). The homogenous dispersion of nanopowder was obtained by ultrasonification applied for 12 h. The resulting polymer foils were cast on glass substrate and dried in air at room temperature during 20 h. Finally, we disposed of the set of the erbium doped
K. Anders et al. / Optical Materials 34 (2012) 1964–1968
1965
Y2O3 nanopowders of different dopant concentrations (0.1, 1 and 5 at.%) and the PMMA-based active nanocomposites differing in the concentration of both RE3+ ions and nanopowder introduced into PMMA matrix (1, 5, 10 and 20 wt.%) as well. 3. Structural characterization To determine the fundamental structural properties all manufactured nanopowders were subjected to X-ray diffraction (XRD) and scanning electron microscopy (SEM) measurements. The crystal structure of the samples was characterized using a Siemens D500 diffractometer with Cu Ka radiation at 1.548 Å. The size and morphology of the samples were analyzed with Carl Zeiss SMT AURIGATM CrossBeam Workstation apparatus. The most stable form of Y2O3 at room temperature is a cubic type, belonging to space group Ia3 [12] and the erbium ions can be located in two distinct sites, substituting the yttrium ions with approximately equal probability [13]. The unit cell contains 24 yttrium ions in non-centro-symmetric sites C2 and eight Y3+ ions in centro-symmetric sites S6, so the site occupation is 3(C2):1(S6). The Y2O3 structure can be described as distorted cube with one Y3+ ion in the center and six O2 ions occupying the corners with vacancy diagonals shown in Fig. 1. The pure electronic transitions are electric dipole forbidden for rare earth in inversion symmetric sites [14–16] and only magnetic dipole transitions should be observed at lattice site of S6 symmetry, which are ineffective in the case of erbium in Y2O3 [16]. Therefore it is expected that optical transitions will occur purely from the erbium ions at the C2 sites.
O2S6
The XRD spectra, shown in Fig. 2 confirm that obtained Er3+:Y2O3 powders contain only one, cubic phase (ICDD card No. 41-1105), the small distortions of the lines reflect the distortion of the lattice due to introduced active ions. The average size of the crystallites, calculated according to the Scherrer’s formula [17] is of the order of 18.4 nm, 19.5 nm and 19.5 nm for 0.1, 1 and 5 at.% Er3+, respectively. The SEM picture on an exemplary sample, presented in Fig. 3, shows that the average size of the crystallites is bigger than those estimated from broadening of diffraction peaks, but it is still well below 100 nm. 4. Optical characterization
Vacancy diagonal
Er3+
Fig. 3. The exemplary SEM picture of the 1 at.% Er3+:Y2O3 sample.
Er3+
C2 Fig. 1. Schematic presentation of the two Er3+ crystallographic sites in cubic Y2O3.
All the samples (both nanopowders and PMMA-based nanocomposites) were carefully characterized by means of optical spectroscopy. In particular – the excitation and emission spectra together with the profiles of fluorescence dynamics from excited states were measured and analyzed. Also the up-converted emission spectra were recorded under excitation by laser diode operating at 980 nm. The PTI QuantaMaster-based spectrofluorimetric system, equipped in double monochromators in the excitation and emission paths was used as a primary measuring setup, enabling both continuous wave (CW) and pulsed excitations over a wide spectral range (200–2000 nm) as well as signal detection within the required spectral range. All measurements were taken at room temperature and all spectra were corrected for spectral characteristic of the detector. 4.1. Er3+:Y2O3 nanopowders
3+
0.1% Er :Y2O3 3+
1.0% Er :Y2O3 3+
intensity [a.u.]
5.0% Er :Y2O3
10
20
30
2
40
50
60
Fig. 2. The XRD spectra of the Er3+:Y2O3 samples differing in dopant concentration.
The investigations of optical properties of Er3+:Y2O3 nanoparticles were focused mainly on their luminescent behavior and its sensitivity to the polymer matrix parameters. Due to specific (powdered) type of original active medium, the typical measurements of absorption characteristics were replaced by excitation spectra measurements. The excitation spectra of the green emission recorded for all investigated concentrations does not exhibit significant differences, therefore in Fig. 4 only the single trace is shown together with the attribution of particular transitions. All the spectra are dominated by two strong, broadband transitions to the excited 4G11/2 and thermally coupled 2H11/2 + 4S3/2 manifolds in the UV and green spectral range, respectively, with less intense contribution of much weaker transitions to the 4H9/2, 4F3/2 + 4F5/2 and 4F7/ 2 levels, extending over violet, blue and blue–green part of the spectrum. The emission spectra, shown in Fig. 5, confirm that luminescence intensity depends strongly on concentration of the activator
1966
K. Anders et al. / Optical Materials 34 (2012) 1964–1968
4
G11/2
em. @ 565 nm
3+
Er :Y2O3
3+
1.0% Er :Y2O3
= 379 nm
exc
2
4
em
= 565 nm
intensity [a.u.]
intensity [a.u.]
H11/2 + S3/2
4
H9/2
250 275 300
325 350
375
0.1% 1.0% 5.0%
4 4
4
400 425
F3/2+ F5/2
450 475
F7/2
100
500 525 550
200
300
400
500
600
700
800
900
1000
time [us]
wavelength [nm]
Fig. 6. Fluorescence decay profiles of green emission in Er3+:Y2O3.
Fig. 4. Excitation spectrum of green emission of 1 at.% Er3+:Y2O3 nanopowder (the emission was monitored at 565 nm).
Table 1 Spectroscopic properties of the Er3+:Y2O3 nanopowders. Emitting level 4
2
S3/2 + H11/2
exc. @ 379 nm 3+
intensity [a.u.]
0.1% Er :Y2O3
WNR (s
1
)
8.06 10
3
1
Calculated radiative lifetime (ls)
WCR (s
79 ± 4 [14]
3.22 103 (1 at.%) 5.63 104 (5 at.%)
)
3+
1.0% Er :Y2O3 3+
5.0% Er :Y2O3
400 425 450 475 500 525 550 575 600 625 650 675 700
wavelength [nm] Fig. 5. Emission spectra of Er3+:Y2O3 under direct 4G11/2 excitation.
and the best properties among investigated samples exhibits that of concentration of 1.0 at.%. In general, all observed spectra reflect the typical luminescence behavior of erbium doped bulk crystals, except a slightly broadened lines. Like a bulk crystals, the Er3+:Y2O3 powders emits mainly in the green spectral range, with main peaks at 524 nm, 540 nm, 555 nm and 565 nm (which is the strongest one), all being related with transitions from thermally coupled 2 H11/2 and 4S3/2 levels. The samples exhibit also a weak luminescence in the violet (at ca. 410 nm) and red (at ca. 663 nm) spectral range. These emission lines correspond to 4H9/2 ? 4I15/2 and 4F9/ 4 2 ? I15/2 transitions, respectively. Fig. 6 illustrates the fluorescence dynamics profiles recorded for the green emission, monitored at 565 nm, for all investigated samples as a function of dopant concentration. It should be mentioned, that due to the thermal coupling of both green-emitting levels the fluorescence dynamics profiles were identical for all particular lines of the investigated green band. In all samples the shortening of the fluorescence lifetime is observed when dopant’s concentration increases, which is a manifestation of cross-relaxation processes resulting in concentration quenching of the fluorescence. The calculated cross-relaxation rate for the sample doped with
5 at.% of erbium is of order of 104 s 1 (see Table 1) while the respective values of multiphonon decays are of order of 103 s 1, which suggests that the cross-relaxation is the dominant depopulating mechanism of 4S3/2 + 2H11/2 multiplet of Er3+ in this host. All profiles are nearly exponential, except the initial part of a trace. This may suggest the slight influence of ions at different lattice locations. The measured values of fluorescence lifetimes are slightly shorter than those reported for bulk samples (e.g. the reported in [14] time constant of green emission is equal to 88 ± 7 ls for 0.2% Er3+:Y2O3 sample, which is still longer than 80.9 ± 0.7 ls measured in this work for the sample with 0.1% of activator), but simultaneously much longer than lifetimes reported for Y2O3 nanopowders (collected in Table 2). It should be noted that the lifetimes, reported by different authors differ significantly – this applies specifically to the results obtained for nanopowders and may result from the differences in manufacturing technology. Apart from the direct excitation the fundamental up-conversion properties were investigated, under excitation at the most typically deployed in erbium doped systems infra-red wavelength of 980 nm. All investigated nanopowders exhibit efficient IR-to-visible up-conversion, with spectral features compared in Fig. 7, where normalized emission characteristics of all samples are shown. It is evident, that with the increasing dopant level the contribution of the red emission increases, which is the result of cross relaxation processes, populating the 4F9/2 red-emitting level and simultaneously depopulating the 4 S3/2 manifold. Nevertheless, when the intensity level is taken into account, the best luminescent properties has the sample doped with 1 at.% of erbium. It should be also noted that there are practically no traces of violet emission, observable in the directly excited samples. The analysis of energy level scheme and observed luminescent properties allow to propose the excitation pathway as the sequential absorption of two photons (GSA to the 4I11/2 level followed by an ESA from the 4I11/2 to the 4F7/2, which relaxes nonradiatively to the manifold 2H11/2 + 4S3/2).
1967
K. Anders et al. / Optical Materials 34 (2012) 1964–1968 Table 2 Time constants of green emission in Er3+:Y2O3. Dopant content (at.%)
Type of sample
Measured time constant of (ls)
4
0.02 0.04 0.1 0.1
Nanopowder Nanopowder Nanopowder Nanopowder in polymer Nanopowder Nanopowder Bulk Bulk Nanopowder Nanopowder Nanopowder Nanopowder Nanopowder in polymer Nanopowder Nanopowder Nanopowder in polymer
34 [6] 31 [6] 80.9 ± 0.7 [this work] 97.74 ± 1.2 [this work]
S3/2 + 2H11/2
0.1 0.1 0.2 0.2 0.2 0.3 0.5 1.0 1.0 1.0 5.0 5.0
12 [6] 11.0 [18] 88 ± 7 [14] 56 [6] 3.8 [6] 16.5 [18] 11.8 [18] 64.2 ± 0.6 [this work] 71.5 ± 1.2 [this work] 11.6 [18] 14.56 ± 0.1 [this work] 16.86 ± 0.3 [this work]
exc. @ 379 nm 3+
20%(0.1% Er :Y2O3):PMMA
intensity [a.u.]
Emitting level
3+
20%(1.0% Er :Y2O3):PMMA 3+
20%(5.0% Er :Y2O3):PMMA
400 425 450 475 500 525 550 575 600 625 650 675 700 725 750
wavelength [nm] Fig. 8. Emission spectra of Er3+:Y2O3 doped PMMA under direct 4G11/2 excitation.
exc. LD @ 980 nm 3+
1.0% Er :Y2O3
3+
0.1% Er :Y2O3
1000
3+
1.0% Er :Y2O3
3+
20%(1.0% Er :Y2O3):PMMA
3+
intensity [a.u]
intensity [a.u.]
5.0% Er :Y2O3
100
10
400 425 450 475 500 525 550 575 600 625 650 675 700 725 750
100
Fig. 7. Normalized 980 nm excitation.
upconversion
emission
spectra
200
300
400
500
600
700
time [us]
wavelength [nm] of
Er3+:Y2O3
under
Fig. 9. Fluorescence decay profiles of green emission in 1% Er3+:Y2O3 nanopowder and the same powder doped PMMA.
4.2. PMMA-based nanocomposites
exc.LD @ 980 nm 3+
20%(0.1% Er :Y2O3):PMMA 3+
20%(1.0% Er :Y2O3):PMMA 3+
20%(5.0% Er :Y2O3):PMMA
intensity [a.u.]
The luminescence spectra of PMMA-based nanocomposites doped with 20% of Er3+:Y2O3 nanopowders are presented in Fig. 8. The Y2O3 nanopowder doped with 1 at.% of erbium ions, which was considered as the optimal value, was used as an activator for PMMA polymer and added in the different amounts (from 1% to 20%). The dependence between the amount of powder and the intensity of the signal was found to be linear, therefore for further experiments the maximal dopant level of 20% has been used, being a compromise between the intensity of the signal and mechanical properties of the doped polymer. In general, except the intensity level, all the samples keep the luminescent properties of original nanopowders. The intensity of the signal is certainly weaker, but the lines are at the same spectral positions and are characterized by very similar spectral width. It should be noted, however, that the emission is affected by strong, broadband blue–green luminescence of the PMMA matrix, manifesting itself specifically under short wavelength excitation. The fluorescence dynamics measurements, presented in Fig. 9 for original nanopowder and polymer-based composite material, recorded for the green emission and dopant concentrations consid-
400 425 450 475 500 525 550 575 600 625 650 675 700 725 750
wavelength [nm] Fig. 10. Normalized upconversion emission spectra of Er3+:Y2O3 doped PMMA under 980 nm excitation.
1968
K. Anders et al. / Optical Materials 34 (2012) 1964–1968
ered as optimal, exhibit very similar character of the both profiles. Surprisingly, the fluorescence lifetime of the composite is slightly longer, taking a value of 71 ls (value recorded for the nanopowder was 64 ls). The same tendency was observed for the composites doped with 0.1 and 5 at.% of erbium. The respective values of fluorescence lifetimes were equal 98 ls and 17 ls. The lack of any shortening of the lifetime confirms that the active nanopowders are effectively shielded from the influence of the polymer matrix. The PMMA-based composites exhibit also the similar up-conversion features – the results are presented in Fig. 10. The character of the spectra, concentration-dependent behavior and relations between intensities of the lines are practically identical to these obtained for the original powders. This further confirms the excellent optical properties of developed composites and, simultaneously, the potential of developed technology of their manufacturing.
5. Conclusions The set of active nanopowders, obtained by sol gel method and differing in activator’s concentrations has been carefully characterized by means of highly-resolved laser spectroscopy. In particular – the emission and excitation spectra have been investigated together with fluorescence dynamics profiles, which in turn enabled optimization of developed manufacturing technology. First polymer composites, based on PMMA films doped with Er3+:Y2O3 have been obtained and characterized in respect of their luminescent properties in the visible spectral range. What is more – the IR-togreen and IR-to-red up-conversion effects have been observed
under IR excitation at 980 nm and preliminarily discussed in the context of possible excitation scheme. In general the obtained results have confirmed that nano-crystallites incorporated into polymer matrix tend to keep their original optical properties, thus enabling designing of new class of optically active polymer components for applications in photonics. References [1] L.H. Slooff, A. van Blaaderen, A. Polman, G.A. Hebbink, S.I. Klink, F.C.J.M. Van Veggel, D.N. Reinhoudt, J.W. Hofstraat, J. Appl. Phys. 91 (2002) 3955. [2] L. Peng, Y. Luo, Y. Dan, L. Zhang, Q. Zhang, S. Xia, X. Zhang, Colloid Polym. Sci. 285 (2006) 153. [3] D.K. Sardar, S. Chandra, J.B. Gruber, W. Gorski, M. Zhang, J.H. Shim, J. Appl. Phys. 105 (2009) 093105. [4] S. Chandra, J.B. Gruber, G.W. Burdick, D.K. Sardar, J. Appl. Polym. Sci. 122 (2011) 289. [5] J.C. Boyer, N.J.J. Johnson, F.C.J.M. van Veggel, Chem. Mater. 21 (2009) 2010. [6] J.A. Capobianco, F. Vetrone, T. D’Alesio, G. Tessari, A. Speghini, M. Bettinelli, Phys. Chem. Chem. Phys. 2 (2000) 3203. [7] J. Zhang, S. Wang, L. An, M. Liu, L. Chen, J. Lumin. 122–123 (2007) 8. [8] S. Xiao, Xiaoliang Yang, Zhengwei Liu, X.H. Yan, J. Appl. Phys. 96 (2004) 3. [9] F. Zhang, J. Li, J. Shan, L. Xu, D. Zhao, Chem. – Eur. J. 15 (2009) 11010. [10] R. Chai, H. Lian, Z. Hou, C. Zhang, C. Peng, J. Lin, J. Phys. Chem. C 114 (2010) 610. [11] H. Guo, Y.M. Qiao, Opt. Mater. 31 (2009) 583. [12] K. Lonsdale: International Tables for X-ray Crystallography, vol. III, Physical and Chemical Tables, Birmingham, Kynoch, 1962. [13] Y. Guyot, R. Moncorgé, L.D. Merkle, A. Pinto, B. McIntosh, H. Verdun, Opt. Mater. 5 (1996) 127. [14] M.J. Weber, Phys. Rev. 171 (1968) 283. [15] P.A. Tanner, X. Zhou, F. Liu, J. Phys. Chem. A 108 (2004) 11521. [16] J. Heber, K.H. Hellwege, U. Köbler, H. Murmann, Z. Physik 237 (1970) 189. [17] P. Klug, L.E. Alexander, X-ray Diffraction Procedure, Wiley, New York, 1954. [18] S. Chandra, F.L. Deepak, J.B. Gruber, D.K. Sardar, J. Phys. Chem. C 114 (2010) 874.