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RE3+:LaALO3 doped luminescent polymer composites
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RE3+:LaALO3 doped luminescent polymer composites Ryszard Piramidowicza,∗, Anna Juszaa, Ludwika Lipińskab, Małgorzata Gilc, Paweł Mergoc a
Institute of Microelectronics and Optoelectronics, Faculty of Electronics and Information Technology, Warsaw University of Technology, Koszykowa 75, Warsaw, 00-662, Poland Institute of Electronic Materials Technology, Wolczynska 133, Warsaw, 01-919, Poland c Faculty of Chemistry, Maria Curie-Sklodowska University, M.C. Skłodowska Sq. 2, Lublin, 20-031, Poland b
A R T I C LE I N FO
A B S T R A C T
Keywords: Polymer composites Rare-earth ions Luminescence LaAlO3
In this work we summarize the results of our research on design and development of the PMMA-based composites doped with lanthanum aluminum oxide (LaAlO3) nanocrystals activated with selected rare-earth ions dysprosium, holmium, thulium and praseodymium. A set of rare-earth doped nanopowders, differing in activators' concentration was manufactured and carefully characterized with respect of luminescent properties in the visible spectral range. The samples of optimal luminescent features were introduced into PMMA polymer, constituting the luminescent composite materials with the active centers shielded from the parasitic interactions with polymer phonons. These, in turn, were subjected to optical characterization covering luminescence spectra and kinetics measurements, which enabled comparison of the luminescent features and assessment of the quality of developed materials as well as correctness of proposed approach.
1. Introduction Previous decade has witnessed the significant progress in the field of polymer-based composite materials, deployed readily in all those fields, where excellent mechanical properties combined with low weight and easiness of manufacturing are of concern. The numerous applications in aviation and automotive industry or civil engineering constructions are excellent examples of their omnipresence. It should be noted, however, that apart from the formidable mechanical properties, polymer-based composite materials may offer also very attractive luminescent features, which may open completely new areas of their applications. Light emitting polymers have a long history in research – they have been tested for luminescence and laser properties practically from the very beginning of the laser technology era. Among the main paths of research conducted for nearly five decades, three main can be enumerated: (1) active organic dye doped materials [1], (2) electroluminescent polymers [2] and (3) polymers doped with rare-earth compounds [3]. The latter group includes polymers activated with rare earth ions, typically introduced in the form of metal-organic (M-O) complexes [4] or, alternatively, as dopants in inorganic nanocrystals, which are dispersed in the volume of the polymer host, thus forming the composite material of luminescent properties determined mostly by the features of RE3+ doped active crystals [5,6]. In general, introducing rare earths as M-O complexes, compatible in
∗
a natural way with polymer host, enable achieving better homogeneity of resulting material, better mechanical and thermal properties as well as higher optical transparency, however at the cost of exposing the active ions to parasitic interactions with highly energetic phonons inherent to polymer hosts. These may result in very efficient depopulating of excited levels via non-radiative multiphonon processes leading to luminescence quenching. For that reasons the choice of appropriate combination of active ion and M-O complex providing both efficient excitation (via “antenna effect”) and shield from matrix’ vibration is not trivial. The majority of results demonstrated so far were obtained using europium and terbium complexes [7–9], with sparse reports on other ions [10,11]. The second, less popular approach is based on the concept of introducing into polymer host rare earth elements encapsulated within nanocrystalline structures of dimensions at least one order of magnitude lower than emitted wavelength. In this approach, the crystalline environment shields the active ions from parasitic influence of polymer host, which enables keeping the original luminescent features practically unchanged. Simultaneously, polymer provides the isolation from interactions of nanopowders with all contaminating factors present in atmosphere, and specifically OH− groups. Defined in such way, luminescent composite materials may potentially enable development of entirely new class of optical materials, combining the excellent luminescent (and lasing) properties of solid state active media and unique
Corresponding author. Institute of Microelectronics and Optoelectronics, Koszykowa 75, 00-662 Warsaw, Poland. E-mail address: [email protected] (R. Piramidowicz).
https://doi.org/10.1016/j.optmat.2018.06.018 Received 15 March 2018; Received in revised form 16 May 2018; Accepted 11 June 2018 0925-3467/ © 2018 Elsevier B.V. All rights reserved.
Please cite this article as: Piramidowicz, R., Optical Materials (2018), https://doi.org/10.1016/j.optmat.2018.06.018
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Fig. 1. Attenuation spectrum of commercial PMMA optical fiber (Super ESKA).
advantages of polymers i.e. mechanical strength, flexibility and low cost of manufacturing. In this work we summarize the results of our research on design and development of the PMMA-based composites doped with lanthanum aluminum oxide (LaAlO3) nanocrystals activated with selected rareearth ions - dysprosium, holmium, thulium and praseodymium.
Fig. 2. XRD patterns for LaAlO3 nanocrystalline samples doped with different rare-earth ions.
3. Manufacturing the samples The set of investigated samples contained lanthanum aluminum oxide (LaAlO3) nanocrystals doped with dysprosium, holmium, thulium and praseodymium ions, differing in dopant concentration of the activator (0.1, 1, 3, and 5 at.% in each case). All nanocrystalline samples were manufactured using sol-gel method, described in our previous papers [27,28]. The crystal structure of the samples was checked using a Siemens D500 diffractometer with Cu Kα radiation at 1.548 Å. The obtained XRD spectra (shown in Fig. 2) confirm that all investigated samples are single-phased and contain only rhombohedral phase (ICDD card no. 310022). The size and morphology of the samples were analyzed with Carl Zeiss SMT AURIGATM CrossBeam Workstation apparatus. The exemplary SEM picture of LaAlO3 nanocrystals, presented in Fig. 3 for praseodymium doped sample, shows that the nanopowder has agglomerated coral-like form, with the average size of obtained nanocrystals of order of 50–100 nm. All remaining samples exhibit exactly the same structural properties, irrespective of the dopant type. The set of PMMA-based composite bulk samples doped with nanocrystals of the best luminescence properties (1% Dy3+:LaAlO3, 5% Ho3+:LaAlO3, 1% Tm3+:LaAlO3 and 1% Pr3+:LaAlO3) were prepared using the technology mastered at Maria Curie-Sklodowska University,
2. Active ions and matrix The choice of activator ions for luminescent polymer composite depends on the optical properties of polymer host – the wavelengths of radiative transitions should correspond to transmission characteristics of the host. In our experiments we decided to use one of the most popular optical polymer - poly (methyl methacrylate), PMMA, a typical material for manufacturing polymer optical fibers (POFs) for applications in optical communication and sensing. From the absorption characteristics, measured for commercially available Super ESKA fiber and presented in Fig. 1 it is clearly seen that optimal range of wavelengths extends between 420 nm and 600 nm, for which the attenuation coefficient does not exceed 0.2 dB/m. Among all available rare-earth elements several offer emission of light within the desired spectral range. We have chosen dysprosium, holmium, thulium and praseodymium due to their interesting luminescent and lasing properties in visible spectral range. Dysprosium offers strong emission and lasing in the unique yellow range, at ca. 575 nm (attributed to 4F9/2 → 6H13/2 transition) [12], complemented by additional transitions from the same energy level in blue (ca. 480 nm) and red (ca. 660 nm) spectral range, which may result in white light emission [13]. Holmium enables efficient emission and lasing in green (ca. 540 nm) and red (ca. 650 nm) spectral range, corresponding to 5 S2→5I8 and 5F5→5I8 transitions, respectively [14], while thulium transitions cover violet-blue range (ca. 450 nm and 480 nm, for 1 D2→3F4 and 1G4→3H4 transitions) [15,16]. At last, praseodymium ion has a number of emission (and laser) lines (like ca. 490 nm, 522 nm, 545 nm, 604 nm, 613 nm) related with transitions from 3PJ levels and covering the whole visible spectral range [17,18]. Lanthanum aluminum oxide (LaAlO3) was considered as an attractive crystalline matrix for lanthanides since middle 1960s [19,20] but the significant interest in optical properties of this perovskite is observed only from the beginning of 21st century (for materials both in bulk [21,22] and nanopowder form [23,24]). LaAlO3 is a pseudo-cubic perovskite and the rare earth ions replace lanthanum in D3 symmetry, which is coordinated by 12 oxygens. Wide band gap energy (5–6 eV [25]), thermal stability and low phonon energy (707 cm−1 [26]) comparing to other oxides makes lanthanum aluminum oxide a very attractive nanocrystalline matrix for polymer composite materials.
Fig. 3. Exemplary SEM image for LaAlO3 nanocrystalline sample. 2
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Emission spectrum of Dy3+:LaAlO3 nanopowder is dominated by a strong yellow line with maximum at 573.5 nm which corresponds to 4 F9/2 → 6H13/2 transition, typical for this material [29]. Less intense emission line in blue spectral range (481 nm) and weak red emission (at ca. 663 nm) were also recorded, corresponding to transitions to 6H15/2 and 6H11/2 levels, respectively. For holmium doped samples transitions from 5F3, 5S2 and 5F5 manifolds to the ground state were observable in the visible spectral range (under excitation of 5G6 at 451 nm) with evident dominance of green emission from 5S2, weak presence of red luminescence (at ca. 650 nm) and almost imperceptible contribution of blue emission (at ca. 488 nm). Similar results were reported for Ho3+:LaAlO3 phosphor in Ref. [30]. Strong blue emission at 452 nm, corresponding to 1D2→3H6 transition was recorded for Tm3+:LaAlO3 powders under UV 1D2 excitation (359 nm), which was also reported as a main transition for LaAlO3 bulk monocrystals [31]. Vestigial luminescence bands localized around 514 nm and 660 nm originating from the same energy level were detected as well. Luminescence spectrum of praseodymium doped sample is also dominated by intense blue emission at 492 nm (corresponding to 3P0→3H4 transition), complemented with several weaker lines in green (with maxima at 529 nm and 545 nm), orange (with maxima at 607 nm and 613 nm) and red (with maximum at 654 nm) spectral range. The majority of observed lines was attributed to transitions from 3P0 level, however there is also observable a trace of emission from 1D2 level, passed over in our previous paper [27]. All attributions have been confirmed by fluorescence kinetics measurements. The visible transitions observed for optimal excitation wavelengths are presented on simplified energy levels diagrams for all investigated RE3+ ions in Fig. 6. For all nanocrystalline samples (and for all concentrations of the rare earth dopants) the fluorescence decay profiles were recorded for the most intensive lines as presented in Fig. 7. All decays are exponential or nearly exponential – only for higher concentrations a slight deviation from the linear profile shape is observed. For all cases the concentration quenching is clearly seen, caused by the cross relaxation (CR) processes, manifesting themselves by shortening the fluorescence lifetime and distortions of the profile shape for high dopant levels. For all samples the cross relaxation rates (WCR) were calculated using the formula:
Fig. 4. Photograph of manufactured bulk PMMA-based composites doped with LaAlO3 nanocrystals.
Lublin. The nanopowders were degassed under vacuum at temperature of 300 °C for 24 h, to avoid the undesired presence of O-H groups in the host material and were added into MMA - purified methyl methacrylate monomer. As an initiator a 2,2-Azoisobutyronitrile (AIBN) was added to the 20 ml of the nanopowder-MMA solution. The homogenous dispersion of nanopowders was obtained by ultrasonification carried out for 2 h. The nanopowder-MMA solutions were pre-polymerized at 80 °C for 70 min and poured into glass vessels where the polymerization was carried out at 50 °C for 48 h. The finally obtained samples (presented in Fig. 4) were cut and prepared for optical measurements. Depending on the concentration of active nanocrystals, the manufactured bulk samples have milky white color (for dopant content higher than 0.5 wt.%) or stay semi-transparent (up to 0.2 wt.%). 4. Results and discussion
WCR = 1 τ − 1 τ0
All nanocrystalline and polymer samples were carefully characterized by means of classical optical spectroscopy. The excitation spectra, emission spectra and fluorescence decay profiles were recorded using the Horiba PTI QuantaMaster-based modular spectrofluorimetric system equipped with double monochromators in the excitation and emission paths and enabling both CW and pulsed excitations over a wide spectral range (200–2000 nm) and optical signals detection within the spectral range from 220 nm to 10 μm. All measurements were taken at room temperature and all spectra were corrected for spectral characteristics of the detector's response.
(1)
where τ is the measured lifetime of the excited level and τ0 is the lifetime of the isolated ion, approximated by the measured time constant for the sample of the lowest activator's concentration. The proposed CR paths for all ions are presented as dotted arrows in Fig. 6. It should be noted here, that the excited levels are depopulated not only by CR processes, but also due to multiphonon transitions, which efficiency depends on the maximal phonon energy of the host (707 cm−1) and the energy gap to the next lower lying energy level. Multiphonon transitions probabilities can be calculated using energy gap law formulated by M. Weber [32]:
4.1. Luminescent properties of the RE3+:LaAlO3 nanocrystals
WMNR = B⋅exp (−α⋅ΔE )
Excitation and emission spectra as well as fluorescence decay profiles were recorded for all samples, doped with all investigated activators, and for all concentrations of the dopant (i.e. 0.1, 1, 3, and 5 at. %). Emission spectra of the samples of dopant concentrations considered as optimal (using the product of intensity and fluorescence lifetime as the assessment criterion) are presented in Fig. 5, together with corresponding excitation spectra shown in insets. Excitation characteristics of investigated materials allowed determination of the optimal excitation wavelengths of each ion in short-wavelength spectral range. These were 351 nm, 471 nm, 359 nm and 449 nm, respectively for Dy3+, Ho3+, Tm3+ and Pr3+. In the case of praseodymium doped sample, the main emission line was localized close to the excitation band (492 nm vs. 471 nm), therefore less efficient excitation via 3P2 level at 449 nm was used in the following measurements.
(2)
where B and α are material constants, determined for this host in Ref. [33]. Calculated WMNR and WCR ratios are set together in Table 1. It is clearly seen, that for holmium doped sample the multiphonon nonradiative transitions are the main mechanisms responsible for depopulating excited 5S2 level, while for dysprosium, thulium and praseodymium cross relaxation processes play the dominant role in luminescence quenching of 4F9/2, 1D2 and 3P0 levels, respectively. 4.2. Luminescent properties of the PMMA-based composites Nanocrystalline samples of dopant concentrations considered as optimal were introduced into polymer host. Depending on the individual character and disposed amount of the powder different 3
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Fig. 5. Emission spectra of 1% Dy3+:LaAlO3, 5% Ho3+:LaAlO3, 1% Tm3+:LaAlO3 and 1% Pr3+:LaAlO3 under optimal short-wavelength excitation (351 nm, 451 nm, 359 nm and 449 nm, respectively) together with excitation spectra (as insets).
presented results show, that nanopowder additive may result in the reduction of the polymer crystallinity for the high concentration of dopants. The deterioration of mechanical properties (defined as the average tensile strength) of obtained composite materials is of order of 45% for 8 wt.% of the nanocrystalline dopant, and of order of 14% only for 2 wt.%.
concentrations of the powders were used for manufacturing the individual composites, which does not affect significantly the luminescent features, influencing only the optical transparency of the samples, and, to some extent, their mechanical strength. The influence of dopant concentration on mechanical properties of polymer composites doped with oxide nanocrystals were presented and discussed in Ref. [34]. The
Fig. 6. Simplified energy levels diagrams for all investigated RE ions together with excitation and observed emission transitions (color solid arrows) and proposed cross relaxation paths (grey dotted arrows). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) 4
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Fig. 7. Fluorescence decays of 4F9/2, 5S2, 1D2, and 3P0 energy levels of Dy3+:LaAlO3, Ho3+:LaAlO3, Tm3+:LaAlO3 and Pr3+:LaAlO3 nanocrystals, respectively.
this case). The fluorescence decays are practically identical for original nanocrystals and composite polymer, confirming that the active ions are efficiently shielded from parasitic influence of polymer host vibrations. The determined time constants are considerably long, of order of 1100 μs in both cases. In Fig. 9 the analogous characteristics are presented for holmium doped samples. Again, all spectroscopic features are very similar, differing only in the luminescence intensity. Unfortunately, due to technological problems, the only available composite sample has the concentration of nanopowder of 0.5 wt.%. This results from very limited amount of nanopowder disposed (while the process requires 1–2 g at minimum) and affects only the luminescence intensity. Apart from this, the obtained material has all desired luminescent features enabling optical characterization and comparison of its
In the figures below, the fundamental spectroscopic features (luminescence spectra and kinetics) of nanocrystals and polymer composites are compared. Specifically, the shape of the fluorescence kinetics profile may provide important information on potential parasitic influence of polymer environment on rare-earth ions luminescence characteristics. In the Fig. 8, the emission spectra and fluorescence kinetics of dysprosium doped LaAlO3 original nanopowder and composite material are presented. It is clearly seen, that the main spectroscopic features are very well preserved in composite material – the emission lines are at exactly the same spectral positions and are of the same width. The only difference is broadband emission extending from UV up to red spectral range, identified as a luminescence of the polymer matrix, typical for PMMA under short wavelength excitation (at 351 nm in
Table 1 Calculated WMNR and WCR ratios together with measured decay time constants for all investigated nanocrystalline samples. Material
Emitting level
WMNR (s−1)
Dopant concentration (at. %)
Τ (μs)
WCR (s−1)
Dy3+:LaAlO3
4
4,97 × 10−5
0.1 1 3 5
1180 1180 890 790
2,76 × 102 4,18 × 102
0.1 1 3 5
62,0 60,0 53,0 43,0
5,38 × 102 2,74 × 103 7,13 × 103
0.1 1 3 5
30,0 29,0 24,0 22,0
1,15 × 103 8,33 × 103 1,21 × 104
0.1 1 3 5
40,0 30,0 18,0 7,5
8,33 × 103 3,06 × 104 1,08 × 105
Ho3+:LaAlO3
Tm3+:LaAlO3
Pr3+:LaAlO3
F9/2
5
S2
1
D2
3
P0
3,33 × 104
5,03 × 10−2
1,50 × 103
5
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Fig. 10. Emission spectra and fluorescence decays (inset) of the Tm3+:LaAlO3 (as reference) and (Tm3+:LaAlO3):PMMA samples. Fig. 8. Emission spectra and fluorescence decays (inset) of the Dy3+:LaAlO3 (as reference) and (Dy3+:LaAlO3):PMMA samples.
Fig. 11. Emission spectra and fluorescence decays (inset) of the Pr3+:LaAlO3 (as reference) and (Pr3+:LaAlO3):PMMA samples. Fig. 9. Emission spectra and fluorescence decays (inset) of the Ho3+:LaAlO3 (as reference) and (Ho3+:LaAlO3):PMMA samples.
considerably longer (449 nm) and does not excite the PMMA matrix, there is no parasitic luminescence observed. Both spectra are identical with respect of all spectroscopic features. Also, the luminescence decay profiles are the very same, with the considerably long (for praseodymium materials) time constants of 30 μs. It should be emphasized, that in this case the intensity of the luminescence of polymer composite is impressively high, further confirming the excellent quality of developed material.
properties with original nanopowder. The measured luminescence dynamics profiles are exactly the same for both nanopowder and polymer composite (with time constants of 43 μs), again confirming the excellent isolation of active ions from interaction with polymer matrix phonons. In Fig. 10 the characteristics recorded for thulium doped samples are compared. Due to UV excitation at 359 nm, apart from thulium transitions (dominated by blue emission line at 453 nm) also the parasitic broadband luminescence of the PMMA host is clearly visible. The decays are again very similar, with slightly shorter time constant of 24 μs recorded for composite material. In Fig. 11 the spectra and kinetics profiles of 3PJ originating luminescence of praseodymium doped samples are presented. As in this case the excitation wavelength is
5. Summary and conclusions In this work we have presented the results of our research and development works on PMMA-based luminescent composites activated with rare-earth ions introduced as dopants of inorganic LaAlO3 nanocrystals. We have mastered the technology of manufacturing both 6
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active nanocrystals and bulk polymer composites of very good optical and mechanical quality, with uniform distribution of active nanopowders in the volume of the samples. In general, the obtained results confirm the correctness of the proposed approach – all developed composite materials exhibit the same spectroscopic features as original powders. Encapsulation of active ions within the nanocrystals provides isolation from the influences of phonons of polymer matrix - total shielding was observed in the case of all investigated materials, which was confirmed by the luminescence kinetics profiles, which remain practically unchanged for all composites. Also, the luminescence spectra were identical for original nanocrystals and polymer composites doped with them (apart from the presence of parasitic broadband luminescence of the PMMA matrix, observable under UV excitation for dysprosium and thulium doped samples). The obtained results prove the great potential of luminescent polymer composite materials with respect of applications in new generation of light sources, and specifically in polymer optical fibers (POFs) of a new kind, combining the excellent luminescent and lasing properties of rare-earth doped crystalline materials and unique mechanical and optical properties of polymer hosts, however under condition of mastering the technology of active POFs drawing. As the research is in progress, the following results will be reported soon.
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RE3+:LaALO3 doped luminescent polymer composites Ryszard Piramidowicza,∗, Anna Juszaa, Ludwika Lipińskab, Małgorzata Gilc, Paweł Mergoc a
Institute of Microelectronics and Optoelectronics, Faculty of Electronics and Information Technology, Warsaw University of Technology, Koszykowa 75, Warsaw, 00-662, Poland Institute of Electronic Materials Technology, Wolczynska 133, Warsaw, 01-919, Poland c Faculty of Chemistry, Maria Curie-Sklodowska University, M.C. Skłodowska Sq. 2, Lublin, 20-031, Poland b
A R T I C LE I N FO
A B S T R A C T
Keywords: Polymer composites Rare-earth ions Luminescence LaAlO3
In this work we summarize the results of our research on design and development of the PMMA-based composites doped with lanthanum aluminum oxide (LaAlO3) nanocrystals activated with selected rare-earth ions dysprosium, holmium, thulium and praseodymium. A set of rare-earth doped nanopowders, differing in activators' concentration was manufactured and carefully characterized with respect of luminescent properties in the visible spectral range. The samples of optimal luminescent features were introduced into PMMA polymer, constituting the luminescent composite materials with the active centers shielded from the parasitic interactions with polymer phonons. These, in turn, were subjected to optical characterization covering luminescence spectra and kinetics measurements, which enabled comparison of the luminescent features and assessment of the quality of developed materials as well as correctness of proposed approach.
1. Introduction Previous decade has witnessed the significant progress in the field of polymer-based composite materials, deployed readily in all those fields, where excellent mechanical properties combined with low weight and easiness of manufacturing are of concern. The numerous applications in aviation and automotive industry or civil engineering constructions are excellent examples of their omnipresence. It should be noted, however, that apart from the formidable mechanical properties, polymer-based composite materials may offer also very attractive luminescent features, which may open completely new areas of their applications. Light emitting polymers have a long history in research – they have been tested for luminescence and laser properties practically from the very beginning of the laser technology era. Among the main paths of research conducted for nearly five decades, three main can be enumerated: (1) active organic dye doped materials [1], (2) electroluminescent polymers [2] and (3) polymers doped with rare-earth compounds [3]. The latter group includes polymers activated with rare earth ions, typically introduced in the form of metal-organic (M-O) complexes [4] or, alternatively, as dopants in inorganic nanocrystals, which are dispersed in the volume of the polymer host, thus forming the composite material of luminescent properties determined mostly by the features of RE3+ doped active crystals [5,6]. In general, introducing rare earths as M-O complexes, compatible in
∗
a natural way with polymer host, enable achieving better homogeneity of resulting material, better mechanical and thermal properties as well as higher optical transparency, however at the cost of exposing the active ions to parasitic interactions with highly energetic phonons inherent to polymer hosts. These may result in very efficient depopulating of excited levels via non-radiative multiphonon processes leading to luminescence quenching. For that reasons the choice of appropriate combination of active ion and M-O complex providing both efficient excitation (via “antenna effect”) and shield from matrix’ vibration is not trivial. The majority of results demonstrated so far were obtained using europium and terbium complexes [7–9], with sparse reports on other ions [10,11]. The second, less popular approach is based on the concept of introducing into polymer host rare earth elements encapsulated within nanocrystalline structures of dimensions at least one order of magnitude lower than emitted wavelength. In this approach, the crystalline environment shields the active ions from parasitic influence of polymer host, which enables keeping the original luminescent features practically unchanged. Simultaneously, polymer provides the isolation from interactions of nanopowders with all contaminating factors present in atmosphere, and specifically OH− groups. Defined in such way, luminescent composite materials may potentially enable development of entirely new class of optical materials, combining the excellent luminescent (and lasing) properties of solid state active media and unique
Corresponding author. Institute of Microelectronics and Optoelectronics, Koszykowa 75, 00-662 Warsaw, Poland. E-mail address: [email protected] (R. Piramidowicz).
https://doi.org/10.1016/j.optmat.2018.06.018 Received 15 March 2018; Received in revised form 16 May 2018; Accepted 11 June 2018 0925-3467/ © 2018 Elsevier B.V. All rights reserved.
Please cite this article as: Piramidowicz, R., Optical Materials (2018), https://doi.org/10.1016/j.optmat.2018.06.018
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Fig. 1. Attenuation spectrum of commercial PMMA optical fiber (Super ESKA).
advantages of polymers i.e. mechanical strength, flexibility and low cost of manufacturing. In this work we summarize the results of our research on design and development of the PMMA-based composites doped with lanthanum aluminum oxide (LaAlO3) nanocrystals activated with selected rareearth ions - dysprosium, holmium, thulium and praseodymium.
Fig. 2. XRD patterns for LaAlO3 nanocrystalline samples doped with different rare-earth ions.
3. Manufacturing the samples The set of investigated samples contained lanthanum aluminum oxide (LaAlO3) nanocrystals doped with dysprosium, holmium, thulium and praseodymium ions, differing in dopant concentration of the activator (0.1, 1, 3, and 5 at.% in each case). All nanocrystalline samples were manufactured using sol-gel method, described in our previous papers [27,28]. The crystal structure of the samples was checked using a Siemens D500 diffractometer with Cu Kα radiation at 1.548 Å. The obtained XRD spectra (shown in Fig. 2) confirm that all investigated samples are single-phased and contain only rhombohedral phase (ICDD card no. 310022). The size and morphology of the samples were analyzed with Carl Zeiss SMT AURIGATM CrossBeam Workstation apparatus. The exemplary SEM picture of LaAlO3 nanocrystals, presented in Fig. 3 for praseodymium doped sample, shows that the nanopowder has agglomerated coral-like form, with the average size of obtained nanocrystals of order of 50–100 nm. All remaining samples exhibit exactly the same structural properties, irrespective of the dopant type. The set of PMMA-based composite bulk samples doped with nanocrystals of the best luminescence properties (1% Dy3+:LaAlO3, 5% Ho3+:LaAlO3, 1% Tm3+:LaAlO3 and 1% Pr3+:LaAlO3) were prepared using the technology mastered at Maria Curie-Sklodowska University,
2. Active ions and matrix The choice of activator ions for luminescent polymer composite depends on the optical properties of polymer host – the wavelengths of radiative transitions should correspond to transmission characteristics of the host. In our experiments we decided to use one of the most popular optical polymer - poly (methyl methacrylate), PMMA, a typical material for manufacturing polymer optical fibers (POFs) for applications in optical communication and sensing. From the absorption characteristics, measured for commercially available Super ESKA fiber and presented in Fig. 1 it is clearly seen that optimal range of wavelengths extends between 420 nm and 600 nm, for which the attenuation coefficient does not exceed 0.2 dB/m. Among all available rare-earth elements several offer emission of light within the desired spectral range. We have chosen dysprosium, holmium, thulium and praseodymium due to their interesting luminescent and lasing properties in visible spectral range. Dysprosium offers strong emission and lasing in the unique yellow range, at ca. 575 nm (attributed to 4F9/2 → 6H13/2 transition) [12], complemented by additional transitions from the same energy level in blue (ca. 480 nm) and red (ca. 660 nm) spectral range, which may result in white light emission [13]. Holmium enables efficient emission and lasing in green (ca. 540 nm) and red (ca. 650 nm) spectral range, corresponding to 5 S2→5I8 and 5F5→5I8 transitions, respectively [14], while thulium transitions cover violet-blue range (ca. 450 nm and 480 nm, for 1 D2→3F4 and 1G4→3H4 transitions) [15,16]. At last, praseodymium ion has a number of emission (and laser) lines (like ca. 490 nm, 522 nm, 545 nm, 604 nm, 613 nm) related with transitions from 3PJ levels and covering the whole visible spectral range [17,18]. Lanthanum aluminum oxide (LaAlO3) was considered as an attractive crystalline matrix for lanthanides since middle 1960s [19,20] but the significant interest in optical properties of this perovskite is observed only from the beginning of 21st century (for materials both in bulk [21,22] and nanopowder form [23,24]). LaAlO3 is a pseudo-cubic perovskite and the rare earth ions replace lanthanum in D3 symmetry, which is coordinated by 12 oxygens. Wide band gap energy (5–6 eV [25]), thermal stability and low phonon energy (707 cm−1 [26]) comparing to other oxides makes lanthanum aluminum oxide a very attractive nanocrystalline matrix for polymer composite materials.
Fig. 3. Exemplary SEM image for LaAlO3 nanocrystalline sample. 2
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Emission spectrum of Dy3+:LaAlO3 nanopowder is dominated by a strong yellow line with maximum at 573.5 nm which corresponds to 4 F9/2 → 6H13/2 transition, typical for this material [29]. Less intense emission line in blue spectral range (481 nm) and weak red emission (at ca. 663 nm) were also recorded, corresponding to transitions to 6H15/2 and 6H11/2 levels, respectively. For holmium doped samples transitions from 5F3, 5S2 and 5F5 manifolds to the ground state were observable in the visible spectral range (under excitation of 5G6 at 451 nm) with evident dominance of green emission from 5S2, weak presence of red luminescence (at ca. 650 nm) and almost imperceptible contribution of blue emission (at ca. 488 nm). Similar results were reported for Ho3+:LaAlO3 phosphor in Ref. [30]. Strong blue emission at 452 nm, corresponding to 1D2→3H6 transition was recorded for Tm3+:LaAlO3 powders under UV 1D2 excitation (359 nm), which was also reported as a main transition for LaAlO3 bulk monocrystals [31]. Vestigial luminescence bands localized around 514 nm and 660 nm originating from the same energy level were detected as well. Luminescence spectrum of praseodymium doped sample is also dominated by intense blue emission at 492 nm (corresponding to 3P0→3H4 transition), complemented with several weaker lines in green (with maxima at 529 nm and 545 nm), orange (with maxima at 607 nm and 613 nm) and red (with maximum at 654 nm) spectral range. The majority of observed lines was attributed to transitions from 3P0 level, however there is also observable a trace of emission from 1D2 level, passed over in our previous paper [27]. All attributions have been confirmed by fluorescence kinetics measurements. The visible transitions observed for optimal excitation wavelengths are presented on simplified energy levels diagrams for all investigated RE3+ ions in Fig. 6. For all nanocrystalline samples (and for all concentrations of the rare earth dopants) the fluorescence decay profiles were recorded for the most intensive lines as presented in Fig. 7. All decays are exponential or nearly exponential – only for higher concentrations a slight deviation from the linear profile shape is observed. For all cases the concentration quenching is clearly seen, caused by the cross relaxation (CR) processes, manifesting themselves by shortening the fluorescence lifetime and distortions of the profile shape for high dopant levels. For all samples the cross relaxation rates (WCR) were calculated using the formula:
Fig. 4. Photograph of manufactured bulk PMMA-based composites doped with LaAlO3 nanocrystals.
Lublin. The nanopowders were degassed under vacuum at temperature of 300 °C for 24 h, to avoid the undesired presence of O-H groups in the host material and were added into MMA - purified methyl methacrylate monomer. As an initiator a 2,2-Azoisobutyronitrile (AIBN) was added to the 20 ml of the nanopowder-MMA solution. The homogenous dispersion of nanopowders was obtained by ultrasonification carried out for 2 h. The nanopowder-MMA solutions were pre-polymerized at 80 °C for 70 min and poured into glass vessels where the polymerization was carried out at 50 °C for 48 h. The finally obtained samples (presented in Fig. 4) were cut and prepared for optical measurements. Depending on the concentration of active nanocrystals, the manufactured bulk samples have milky white color (for dopant content higher than 0.5 wt.%) or stay semi-transparent (up to 0.2 wt.%). 4. Results and discussion
WCR = 1 τ − 1 τ0
All nanocrystalline and polymer samples were carefully characterized by means of classical optical spectroscopy. The excitation spectra, emission spectra and fluorescence decay profiles were recorded using the Horiba PTI QuantaMaster-based modular spectrofluorimetric system equipped with double monochromators in the excitation and emission paths and enabling both CW and pulsed excitations over a wide spectral range (200–2000 nm) and optical signals detection within the spectral range from 220 nm to 10 μm. All measurements were taken at room temperature and all spectra were corrected for spectral characteristics of the detector's response.
(1)
where τ is the measured lifetime of the excited level and τ0 is the lifetime of the isolated ion, approximated by the measured time constant for the sample of the lowest activator's concentration. The proposed CR paths for all ions are presented as dotted arrows in Fig. 6. It should be noted here, that the excited levels are depopulated not only by CR processes, but also due to multiphonon transitions, which efficiency depends on the maximal phonon energy of the host (707 cm−1) and the energy gap to the next lower lying energy level. Multiphonon transitions probabilities can be calculated using energy gap law formulated by M. Weber [32]:
4.1. Luminescent properties of the RE3+:LaAlO3 nanocrystals
WMNR = B⋅exp (−α⋅ΔE )
Excitation and emission spectra as well as fluorescence decay profiles were recorded for all samples, doped with all investigated activators, and for all concentrations of the dopant (i.e. 0.1, 1, 3, and 5 at. %). Emission spectra of the samples of dopant concentrations considered as optimal (using the product of intensity and fluorescence lifetime as the assessment criterion) are presented in Fig. 5, together with corresponding excitation spectra shown in insets. Excitation characteristics of investigated materials allowed determination of the optimal excitation wavelengths of each ion in short-wavelength spectral range. These were 351 nm, 471 nm, 359 nm and 449 nm, respectively for Dy3+, Ho3+, Tm3+ and Pr3+. In the case of praseodymium doped sample, the main emission line was localized close to the excitation band (492 nm vs. 471 nm), therefore less efficient excitation via 3P2 level at 449 nm was used in the following measurements.
(2)
where B and α are material constants, determined for this host in Ref. [33]. Calculated WMNR and WCR ratios are set together in Table 1. It is clearly seen, that for holmium doped sample the multiphonon nonradiative transitions are the main mechanisms responsible for depopulating excited 5S2 level, while for dysprosium, thulium and praseodymium cross relaxation processes play the dominant role in luminescence quenching of 4F9/2, 1D2 and 3P0 levels, respectively. 4.2. Luminescent properties of the PMMA-based composites Nanocrystalline samples of dopant concentrations considered as optimal were introduced into polymer host. Depending on the individual character and disposed amount of the powder different 3
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Fig. 5. Emission spectra of 1% Dy3+:LaAlO3, 5% Ho3+:LaAlO3, 1% Tm3+:LaAlO3 and 1% Pr3+:LaAlO3 under optimal short-wavelength excitation (351 nm, 451 nm, 359 nm and 449 nm, respectively) together with excitation spectra (as insets).
presented results show, that nanopowder additive may result in the reduction of the polymer crystallinity for the high concentration of dopants. The deterioration of mechanical properties (defined as the average tensile strength) of obtained composite materials is of order of 45% for 8 wt.% of the nanocrystalline dopant, and of order of 14% only for 2 wt.%.
concentrations of the powders were used for manufacturing the individual composites, which does not affect significantly the luminescent features, influencing only the optical transparency of the samples, and, to some extent, their mechanical strength. The influence of dopant concentration on mechanical properties of polymer composites doped with oxide nanocrystals were presented and discussed in Ref. [34]. The
Fig. 6. Simplified energy levels diagrams for all investigated RE ions together with excitation and observed emission transitions (color solid arrows) and proposed cross relaxation paths (grey dotted arrows). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) 4
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Fig. 7. Fluorescence decays of 4F9/2, 5S2, 1D2, and 3P0 energy levels of Dy3+:LaAlO3, Ho3+:LaAlO3, Tm3+:LaAlO3 and Pr3+:LaAlO3 nanocrystals, respectively.
this case). The fluorescence decays are practically identical for original nanocrystals and composite polymer, confirming that the active ions are efficiently shielded from parasitic influence of polymer host vibrations. The determined time constants are considerably long, of order of 1100 μs in both cases. In Fig. 9 the analogous characteristics are presented for holmium doped samples. Again, all spectroscopic features are very similar, differing only in the luminescence intensity. Unfortunately, due to technological problems, the only available composite sample has the concentration of nanopowder of 0.5 wt.%. This results from very limited amount of nanopowder disposed (while the process requires 1–2 g at minimum) and affects only the luminescence intensity. Apart from this, the obtained material has all desired luminescent features enabling optical characterization and comparison of its
In the figures below, the fundamental spectroscopic features (luminescence spectra and kinetics) of nanocrystals and polymer composites are compared. Specifically, the shape of the fluorescence kinetics profile may provide important information on potential parasitic influence of polymer environment on rare-earth ions luminescence characteristics. In the Fig. 8, the emission spectra and fluorescence kinetics of dysprosium doped LaAlO3 original nanopowder and composite material are presented. It is clearly seen, that the main spectroscopic features are very well preserved in composite material – the emission lines are at exactly the same spectral positions and are of the same width. The only difference is broadband emission extending from UV up to red spectral range, identified as a luminescence of the polymer matrix, typical for PMMA under short wavelength excitation (at 351 nm in
Table 1 Calculated WMNR and WCR ratios together with measured decay time constants for all investigated nanocrystalline samples. Material
Emitting level
WMNR (s−1)
Dopant concentration (at. %)
Τ (μs)
WCR (s−1)
Dy3+:LaAlO3
4
4,97 × 10−5
0.1 1 3 5
1180 1180 890 790
2,76 × 102 4,18 × 102
0.1 1 3 5
62,0 60,0 53,0 43,0
5,38 × 102 2,74 × 103 7,13 × 103
0.1 1 3 5
30,0 29,0 24,0 22,0
1,15 × 103 8,33 × 103 1,21 × 104
0.1 1 3 5
40,0 30,0 18,0 7,5
8,33 × 103 3,06 × 104 1,08 × 105
Ho3+:LaAlO3
Tm3+:LaAlO3
Pr3+:LaAlO3
F9/2
5
S2
1
D2
3
P0
3,33 × 104
5,03 × 10−2
1,50 × 103
5
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Fig. 10. Emission spectra and fluorescence decays (inset) of the Tm3+:LaAlO3 (as reference) and (Tm3+:LaAlO3):PMMA samples. Fig. 8. Emission spectra and fluorescence decays (inset) of the Dy3+:LaAlO3 (as reference) and (Dy3+:LaAlO3):PMMA samples.
Fig. 11. Emission spectra and fluorescence decays (inset) of the Pr3+:LaAlO3 (as reference) and (Pr3+:LaAlO3):PMMA samples. Fig. 9. Emission spectra and fluorescence decays (inset) of the Ho3+:LaAlO3 (as reference) and (Ho3+:LaAlO3):PMMA samples.
considerably longer (449 nm) and does not excite the PMMA matrix, there is no parasitic luminescence observed. Both spectra are identical with respect of all spectroscopic features. Also, the luminescence decay profiles are the very same, with the considerably long (for praseodymium materials) time constants of 30 μs. It should be emphasized, that in this case the intensity of the luminescence of polymer composite is impressively high, further confirming the excellent quality of developed material.
properties with original nanopowder. The measured luminescence dynamics profiles are exactly the same for both nanopowder and polymer composite (with time constants of 43 μs), again confirming the excellent isolation of active ions from interaction with polymer matrix phonons. In Fig. 10 the characteristics recorded for thulium doped samples are compared. Due to UV excitation at 359 nm, apart from thulium transitions (dominated by blue emission line at 453 nm) also the parasitic broadband luminescence of the PMMA host is clearly visible. The decays are again very similar, with slightly shorter time constant of 24 μs recorded for composite material. In Fig. 11 the spectra and kinetics profiles of 3PJ originating luminescence of praseodymium doped samples are presented. As in this case the excitation wavelength is
5. Summary and conclusions In this work we have presented the results of our research and development works on PMMA-based luminescent composites activated with rare-earth ions introduced as dopants of inorganic LaAlO3 nanocrystals. We have mastered the technology of manufacturing both 6
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active nanocrystals and bulk polymer composites of very good optical and mechanical quality, with uniform distribution of active nanopowders in the volume of the samples. In general, the obtained results confirm the correctness of the proposed approach – all developed composite materials exhibit the same spectroscopic features as original powders. Encapsulation of active ions within the nanocrystals provides isolation from the influences of phonons of polymer matrix - total shielding was observed in the case of all investigated materials, which was confirmed by the luminescence kinetics profiles, which remain practically unchanged for all composites. Also, the luminescence spectra were identical for original nanocrystals and polymer composites doped with them (apart from the presence of parasitic broadband luminescence of the PMMA matrix, observable under UV excitation for dysprosium and thulium doped samples). The obtained results prove the great potential of luminescent polymer composite materials with respect of applications in new generation of light sources, and specifically in polymer optical fibers (POFs) of a new kind, combining the excellent luminescent and lasing properties of rare-earth doped crystalline materials and unique mechanical and optical properties of polymer hosts, however under condition of mastering the technology of active POFs drawing. As the research is in progress, the following results will be reported soon.
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