- Email: [email protected]
Praseodymium doped nanocrystals and nanocomposites for application in white light sources
Optical Materials 95 (2019) 109247
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Praseodymium doped nanocrystals and nanocomposites for application in white light sources
T
Anna Juszaa,*, Ludwika Lipińskab, Magdalena Baranb, Andrzej Olszynac, Agnieszka Jastrzębskac, Małgorzata Gild, Paweł Mergod, Ryszard Piramidowicza a
Warsaw University of Technology, Institute of Microelectronics and Optoelectronics, Koszykowa 75, 00-662 Warsaw, Poland Institute of Electronic Materials Technology, Wolczynska 133, Warsaw, 01-919, Poland c Warsaw University of Technology, Faculty of Material Science and Engineering, Woloska 141, Warsaw, 02-507, Poland d Maria Curie-Sklodowska University, Faculty of Chemistry, M.C. Skłodowska Sq. 2, Lublin, 20-031, Poland b
ARTICLE INFO
ABSTRACT
Keywords: Praseodymium Visible light Luminescence Polymer composites PMMA
In this work we report the results of our investigations on luminescence properties in the visible spectral range of composite materials based on polymer matrices doped with nano-sized crystals activated by Pr3+ ions. In particular, we investigated the visible emission of YF3, LaAlO3, Al2O3 and Y2O3 nanopowders activated with Pr3+ ions, as well as PMMA-based composites doped with particles of the best luminescent properties. The basic structural characterization of the developed samples was performed, followed by versatile optical characterization, focused on measurements and analysis of excitation and luminescence spectra and fluorescence dynamics profiles. The comparison of spectroscopic features of developed materials enabled discussion of the influence of crystalline and polymer environment on luminescent properties of resulting materials as well as consideration of potential applicability of developed materials in white light emitters.
1. Introduction The recent years observed an intensive development of optoelectronic devices and systems using a compact, highly efficient short-wavelength radiation sources, both coherent and incoherent. Demand for emission of light from the visible part of the spectrum (red, green and blue as well as white, obtained from a combination of these three colors) results from a number of application areas, covering among others lighting systems, imaging, recording and information processing, medical diagnostic techniques as well as optical telecommunication and sensing. Among the best known and most common applications, the large scale video displays (laser or OLED based), white LEDs for lighting systems, high-capacity disk storage media like DVD or blu-ray and optical transmitters compatible with polymer optical fibers transmission windows might be enumerated. Such a wide range of applications stimulates looking for new active materials which would combine the advantages of optical and thermo-mechanical properties of polymer matrices with well known (and low-cost) manufacturing technology and excellent luminescence properties of rare-earth ions, typically deployed as active centers in majority of “classical” solid state lasers. Polymer materials have been tested for luminescence and laser properties practically from the very beginning of the development of *
laser technology [1]. Among the main paths of research conducted for nearly five decades, the three main should be enumerated: active organic dye doped materials [2,3], electroluminescent polymers [4–6], and polymers doped with rare-earth compounds [7]. It should be mentioned, that doping polymers with rare earth ions is not trivial – in general, the lanthanide salts are insoluble in organic compounds [8]. Besides, the polymer matrices are susceptible to adsorption of water molecules, which are transported to the volume of material by diffusion, resulting in the extreme increase of luminescence quenching by vibrionic OH modes coupling with excited levels of active ions. One of possible solutions was introduced at the beginning of the 60's by Crosby, who proposed to use chelates to isolate centrally located active ion from the influence of OH bonds [9,10]. It is worth to mention also the reports of Matthews and Knobbe [11], Zhang [12], Kobayashi [13] and Lin [14]. Also noteworthy are research, carried out since the late 90's, by the Dutch research team of L.H. Slooff and dedicated mainly to applications of polymer materials in optical amplifiers operating in infrared part of the spectrum, which are summarized in paper [15], with conclusion of the need for searching the new compounds, which could provide more effective isolation of the active ion from vibronic interactions with polymer matrix. This can be potentially obtained in nanocrystalline materials in which the active ion is insulated from the
Corresponding author. Institute of Microelectronics and Optoelectronics, Koszykowa 75, 00-662, Warsaw, Poland. E-mail address: [email protected] (A. Jusza).
https://doi.org/10.1016/j.optmat.2019.109247 Received 13 May 2019; Received in revised form 4 July 2019; Accepted 10 July 2019 0925-3467/ © 2019 Elsevier B.V. All rights reserved.
Optical Materials 95 (2019) 109247
A. Jusza, et al.
external factors by crystalline environment. The first reports of research on active nanocrystals doped with rare earth ions and dispersed in liquid polymer matrix can be find in R.S. Meltzer works from 2001 [16,17]. In the following years luminescent properties of polymer composites doped with active nanocrystals were studied, among others, by teams of Kumar [18], Gruber [19] and Zhang [20]. Polymer composites activated with RE3+ doped nanocrystals seem to be an excellent candidate for visible and especially white light sources as they potentially may offer luminescent features comparable to crystalline materials. Recently, a number of attempts have been made to obtain the efficient emission of white light, being a combination of three fundamental color components, from multi-doped nanocrystalline materials. In particular, Er3++Tm3++Yb3+, Er3++Tm3+ +Yb3++Li3+ [21], Ho3++Tm3++Yb3+ [22], Tb3++Tm3++Sm3+ [23], Er3++Eu3+ [24], Er3++Ho3++Tm3++Yb3+ [25], Tm3+ +Gd3++Yb3++Tb3++Eu3+ core-shell [26], and Tm3++Yb3+ +Nd3++Er3+ core-shell [27] configurations have been proposed. It seems however, that the level of complexity of these materials is too high to compete efficiently on the market of white light emitters. The reports published by A. S. Gouveia-Neto's team [28,29] indicate praseodymium ion as a promising activator for single-doped phosphors for white-light sources. Among various rare earth elements, which may be used as active dopants for visible and white light emitters, praseodymium ion seems to be the best probe for investigation the possibilities of shaping luminescent properties by changing the structure of the crystalline environment. The energy level scheme of 4f2 of Pr3+ contains a number of metastable levels which enable obtaining many optical transitions in ultraviolet (UV), visible (VIS), near- and mid-infrared (NIR and MIR) spectral range. Visible emission in praseodymium doped materials results mainly from 4f intraconfigurational transitions originating from 3PJ and 1D2 metastable levels with energies about 21000 cm−1 and 17000 cm−1. For majority of praseodymium compounds typical is the presence of many emission lines in the blue, green and red spectral range [30,31], which opens attractive possibilities for developing white light sources (including “white lasers” [32]). It should be noted, however, that emission properties of praseodymium ions depend largely on the crystalline surrounding, which determines the distance between the dopant ions, their relative spatial position and the type of anions coordinating active centers. Therefore, in certain materials the typical, rich emission spectrum of praseodymium ion, resulting from optical transitions from 3P0 level and characterized by multiple lines in blue, green and red (like in YF3 and LaAlO3 hosts) is not observable, being replaced by a 1D2-related emission concentrated in red spectral range. In particular, it has been shown that in several cubic sesquioxides Pr3+ ions exhibit efficient red luminescence from 1D2 level, while the 3P0 emission is quenched (e.g. in Y2O3, Gd2O3) [33,34]. The main concept of the research reported here was to develop a polymer-based composite material doped with optically active nanocrystals, which would be able to emit blue, green and red light under single wavelength excitation. This paper is focused on investigations and analysis of the visible luminescence properties of selected nanocrystals doped with praseodymium ions, considered as a promising dopant for abovementioned application, and polymer composites activated with these nanocrystals. The set of the nanocrystalline activators has been chosen in the way enabling analysis of luminescent properties of praseodymium ions in hosts differing in structural features, chemical composition and corresponding phonon energies. In particular, both oxide (Y2O3. LaAlO3, Al2O3) and fluoride (YF3) nanocrystals have been prepared using technologies mastered previously by Institute of Electronic Materials Technology and Warsaw University of Technology teams. The detailed information is presented in the following section.
nanocrystalline samples manufactured using different synthesis methods. The Y2O3 nanopowder was prepared using sol-gel method in the Institute of Electronic Materials Technology, Warsaw [35]. The Y2O3 dissolved in acetic acid was mixed together with Pr6O11 dissolved in solution of nitric acid and stirred for 1 h at 65 °C. Then, 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 powder was calcined in the air atmosphere at 1100 °C for 7 h. The LaAlO3 nanopowder was prepared using sol-gel method in the Institute of Electronic Materials Technology [36]. Firstly, the La2O3 was dried in 900 °C for 2 h, next it was dissolved in solution of nitric acid. Separately Pr6O11 was dissolved in diluted nitric acid and Al (NO3)3·9H2O in deionized water, then all solutions were mixed together and stirred for 1 h at 65 °C. Then, ethylene glycol was added as crosslinking agent and the solution was stirred during next 2 h. During slow evaporation the obtained sol transformed into sticky gel, which, in turn, after being dried in 150 °C for 12 h changed into solid xerogel. The ground powder was carefully calcined in the air atmosphere in several stages (from 300 °C for 2 h up to 1200 °C for 12 h) to avoid a combustion. New chemical “wet” synthesis developed in the Institute of Electronic Materials Technology was used for manufacturing the YF3 powders [37]. The Y2O3 was dissolved in acetic acid, Pr6O11 in diluted nitric acid and NH4F in deionized water. Next, all solutions were mixed together and stirred for 6 h at 70 °C. During slow evaporation of the mixture the volume was decreased 5 times. Then the concentrated solution was dried in 150 °C to completely remove the remaining water particles. The dried powder was then calcined in air atmosphere in temperature of 400 °C for 2 h. Praseodymium doped Al2O3 nanopowders were prepared using a socalled “dry synthesis”, developed at the Warsaw University of Technology [38]. This method involves the reaction of an organometallic compound, praseodymium ions, in an organic solvent in a gas shielded inert (argon) environment. Thus, prepared mixture was allowed to stand for 72 h in argon with vigorous stirring. After removal of the solvent and drying the mixture, the organic precursor is thermally decomposed in a muffle furnace at 1000 °C for 24 h in air to give the final product (Pr3+:Al2O3 nanopowder). In order to avoid the presence of undesirable OH− groups samples were stored under argon atmosphere. The set of poly(methyl methacrylate) - PMMA-based composite bulk samples doped with nanocrystals with best luminescence properties were prepared using new method mastered at Maria Curie-Sklodowska University. 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. 2,2-Azoisobutyronitrile (AIBN) as an initiator was added to the 20 ml of the nanopowder-MMA solution. The homogenous dispersion of nanopowders was obtained by using ultrasonification 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 obtained samples (8 wt.% (1 at.% Pr3+:LaAlO3):PMMA and 6 wt.% (1 at.% Pr3+:YF3):PMMA) were cut and prepared for optical measurements. The crystal structure of the samples was characterized using a Siemens D-500 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 excitation spectra, emission spectra and fluorescence decay curves were recorded using the Horiba PTI QuantaMaster based spectrofluorimetric system equipped with double computer-controlled autocalibrated Czerny-Turner monochromators in the excitation and emission paths and enabling both CW (Xe 100 W lamp) and pulsed (Xe Flash
2. Materials and methods The set of investigated samples consisted of 1 at.% Pr3+:LaAlO3, 1 at.% Pr3+:YF3, 0.1 at.% Pr3+:Y2O3 and 0.1 at.% Pr3+:Al2O3 2
Optical Materials 95 (2019) 109247
A. Jusza, et al.
Fig. 1. X-ray diffraction patterns of the Pr3+ doped LaAlO3, Y2O3, YF3 and Al2O3 nanocrystals.
Fig. 2. SEM images of the Pr3+ doped LaAlO3, Y2O3, YF3 and Al2O3 nanocrystals.
lamp) excitations over a wide spectral range (200–2000 nm) as well as signal detection within the spectral range from 220 nm to 1.7 μm. All measurements were taken in room temperature and all spectra were corrected for spectral characteristics of the detector's response.
obtained X-ray diffraction patterns (Fig. 1) confirm that all investigated samples were single-phased and contained rhombohedral (ICDD card no. 31–0022), orthorhombic (ICDD card no. 32–1431), cubic (ICDD card no. 41–1105) and corundum (ICDD card no. 46–1212) phase in the case of LaAlO3, YF3, Y2O3 and Al2O3, respectively. The SEM pictures of all manufactured nanocrystalline samples, presented in Fig. 2, show the evident structural differences between investigated nanocrystals, both in the shape and size domain. The shape of LaAlO3 and Al2O3 nanoparticles, having a coral-like form, differs significantly from the shape of yttrium oxide and yttrium fluoride nanocrystals, which are in a form of nearly spherical agglomerated structures. The average sizes of the particles estimated from SEM images were in the range of 40–150 nm
3. Results and discussion 3.1. Structural characterization All manufactured nanopowders were subjected to X-ray diffraction (XRD) and scanning electron microscopy (SEM) measurements, which enabled determination of their fundamental structural properties. The 3
Optical Materials 95 (2019) 109247
A. Jusza, et al.
Fig. 3. Excitation spectra of the Pr3+ doped LaAlO3, Y2O3, YF3 and Al2O3 nanocrystals.
Fig. 4. Emission spectra of the Pr3+ doped LaAlO3, Y2O3, YF3 and Al2O3 nanocrystals.
transitions corresponding to 3PJ+1I6 energy levels are clearly visible, enabling localizing of these levels in different nanocrystalline hosts. Almost imperceptible, additional band at 522.5 nm observed in the case of Al2O3 sample is most probably related with a vibronic sideband accompanying the excitation spectrum. This band is shifted from 3P0 excitation transition by c.a. 750 cm−1 which corresponds well to maximum phonon energy of α-alumina, reported in Ref. [39]. The similar effect was investigated by O.K Moune et al. [40]. Moreover, 1D2 originating transitions are also detectable for yttria and alumina samples.
for all samples. All samples were partially agglomerated in bigger grains of micrometric dimensions. 3.2. Luminescent properties of the praseodymium doped nanocrystals Due to a specific (powdered) type of original active medium, the typical measurements of absorption characteristics were replaced by excitation spectra measurements. The excitation spectra of red emission of all investigated nanopowders are shown in Fig. 3. For all samples 4
Optical Materials 95 (2019) 109247
A. Jusza, et al.
different nanocrystalline host could be explained by different crystalline structures of investigated materials. At the pseudo-cubic perovskite LaAlO3 (with a rhombohedral structure with hexagonal space group R3c) Pr3+ ions replace lanthanum ions and occupy the D3 site symmetry positions [41]. The structure of YF3 is orthorhombic with Pnma space group with four formulae per unit cell [42]. In this structure Pr3+ replace yttrium ions which are coordinated by nine fluorine ions and occupy the CS site symmetry positions. The most stable form of Y2O3 at room temperature is a cubic type, belonging to space group Ia3 [43]. The unit cell contains twenty-four yttrium ions in non-centro-symmetric sites C2 (occupy the Wyckoff position 24d) and eight Y3+ ions in centrosymmetric sites S6 (occupy the position 8b). The trivalent rare earth dopants can substitute yttrium ions in these both types of sites with approximately equal probabilities. α-Al2O3 structure can be described both as a rhombohedral or a hexagonal unit cell belonging to the space group R3c. Oxygen atoms are in a distorted hexagonal structure, while the aluminum atoms occupying 2/3 of the octahedral sites. The hexagonal cell contains 12 aluminum and 18 oxygen atoms [44]. For LaAlO3 and YF3 powders the behavior of luminescence spectra is similar - small differences observed are the consequence of different splitting of the energy levels of praseodymium (see the excitation spectra in Fig. 3) due to different crystal fields of host lattices. The lack of 3P0 originated emission in the cubic Y2O3 and the quenching of this luminescence for the α-Al2O3 phase can be explained using the energy level diagrams in coordinate configuration (Fig. 5, followed by Ref. [45]) which show the path of non-radiative relaxation from 3P0 to 1D2 via 4f5d configuration for sesquioxides. The splitting of the 4f5d energy level is related to the interatomic distance between the Pr3+ and the surrounding O2− ligands an determines the origin of the emission of the 3 P0 or the 1D2 level [34]. It should be noted that all attributions of emission lines were additionally confirmed by fluorescence decays measurements. Apart from praseodymium originating luminescence, for Pr3+:Al2O3 sample also strong parasitic emission with maximum at 694 nm was recorded (marked as artificial in Fig. 4) associated with Cr3+ impurity (confirmed additionally by excitation spectrum with two excitation bands with maxima at 409 nm and 553 nm corresponding to spin-allowed transitions of Cr3+ [46]). The fluorescence dynamics profiles were
Fig. 5. Energy level diagrams in coordinate configuration (followed by Ref. [45]) for Pr3+ states.
The strong ultraviolet transition observed for Y2O3 (the long-wavelength edge of this transition is also detectable for YF3) is a manifestation of interconfigurational excitation to the 4f5d configuration bands and is typical for Pr3+:Y2O3 structure [33]. According to the position of 3 PJ manifolds for each structure it is possible to select a single wavelength appropriate for efficient excitation of praseodymium ions in all examined nanocrystals. Luminescence spectra of all investigated nanopowders under optimal (the most efficient within the disposed range of exciting wavelengths) excitation, which corresponds to 3P2 level pumping for LaAlO3, YF3 and Al2O3, and 4f5d band for Y2O3, are shown in Fig. 4. For Pr3+:LaAlO3 and Pr3+:YF3 the spectrum is dominated by transitions originating from 3PJ manifolds with very weak contribution of transitions from 1D2 to the ground level observed for both materials. For Pr3+:LaAlO3 blue emission attributed to 3P0→3H4 transition is the most intense, while for Pr3+:YF3 transitions in blue (3P0→3H4) and red (3P0→3H6) spectral range are of comparable intensity. Completely opposite situation was observed for Pr3+:Y2O3 – all emission lines in the visible range were originating from 1D2 level. For alumina sample both 3 P0 and 1D2 emissions were noticed, however with evident domination of 1D2→3H4 transition in red spectral range. This behaviour of luminescent properties of praseodymium ions in
Fig. 6. Fluorescence decays of the Pr3+ doped LaAlO3, Y2O3, YF3 and Al2O3 nanocrystals. 5
Optical Materials 95 (2019) 109247
A. Jusza, et al.
Fig. 7. The CIE 1931 chromaticity diagram of the Pr3+ doped LaAlO3, Y2O3, YF3 and Al2O3 nanocrystals.
Fig. 8. Fluorescence decays and emission spectra (inset) of the Pr3+:LaAlO3 (as reference) and (Pr3+:LaAlO3):PMMA samples.
Fig. 9. Fluorescence decays and emission spectra (inset) of the Pr3+:YF3 (as reference) and (Pr3+:YF3):PMMA samples.
recorded for most intensive emissions, corresponding to 3P0→3H4 transition for Pr3+:LaAlO3 and Pr:YF3 samples and to 1D2→3H4 transition for Pr3+:Y2O3 and Pr3+:Al2O3 (see Fig. 6). The time constants determined for the samples were of order of 30.4 μs (LaAlO3), 25.9 μs (YF3), 113 μs (Y2O3) and 152 μs (Al2O3). Obtained lifetime values are in a good agreement with those reported earlier for similar materials [47–49], except Al2O3, for which 1D2 time constant is reported here for the first time. All recorded emission spectra were converted to the CIE chromaticity coordinates and are presented at the CIE 1931 chromaticity diagram (Fig. 7). The chromaticity coordinates were calculated from the spectral power distribution of the light and the CIE colourmatching functions [50]. White light emission has been confirmed for LaAlO3 and YF3 samples with chromaticity coordinates (CIE 1931) x = 0.3588, y = 0.4306 and x = 0.4406, y = 0.3900, respectively. Summarizing this part, it can be concluded that we have successfully manufactured praseodymium doped oxide and fluoride nanocrystals of good luminescent properties, providing efficient emission within the visible spectral range. It should be noted, that among four
investigated samples, two can be considered as potential materials for white light emitters. These are Pr3+:LaAlO3 and Pr3+:YF3 nanocrystals, offering luminescent transitions at several wavelengths within blue (ca. 480–490 nm), green (ca. 525–545 nm) and red (ca. 600–650 nm) spectral range, originating from 3PJ levels. The simple comparison of the intensities of individual emission lines of these two materials evidently shows that luminescent features of Pr3+:YF3 may easier satisfy requirements defined for white light emitters - the material offers comparable levels of blue and red luminescence and considerably high (comparing to Pr3+:LaAlO3) intensity of emission in green. Pr3+:Y2O3 and Pr3+:Al2O3 sesquioxides suffer from efficient quenching of emission from 3PJ levels and offer visible luminescence limited to the red part of the spectrum related with transitions from 1D2 level, and may be considered only as additional additives to materials emitting from 3P0 level, enabling enhancement of red luminescence. The excellent luminescent properties (and specifically RGB emission), combined with reasonably good structural properties and well mastered manufacturing technology have had a decisive influence on 6
Optical Materials 95 (2019) 109247
A. Jusza, et al.
choosing the Pr3+:LaAlO3 and Pr3+:YF3 nanopowders as dopants for PMMA-based polymer composites.
emitters, specifically when doped with a combination of nanocrystals optimized with respect of the shape of luminescence characteristics.
3.3. Luminescent properties of the PMMA-based composites
Declaration of competing interest
As it was mentioned before - the main benefit expected from the concept of polymer nanocomposite doped with optically active nanopowder was the combination of excellent luminescent properties of crystalline hosts doped with RE3+ ions with extraordinary mechanical features and low cost of manufacturing of polymer materials. In the developed composite the polymer surrounding shall not only provide the mechanical strength but also protect the nanocrystals from the influence of OH− groups and other contaminations, while the encapsulation of active ions in nanocrystalline environment should result in shielding the ions from parasitic interaction with highly energetic phonons of polymer host. The results of optical characterization of both composite materials have confirmed the correctness of proposed approach. The main evidence of efficient isolation of active ions from the polymer host influence is the shape and time constant of fluorescence decay. Both decays, presented in Fig. 8 for Pr3+:LaAlO3 nanopowder and composite material remain practically identical, with time constant of order of 30 μs, irrespective of the material type. This means, that there are no additional processes, related with polymer host phonons which would speed up depopulation of the excited 3P0 state. Also, the luminescence spectra, shown in the inset, remain identical, exhibiting the same spectroscopic features both for nanopowder and polymer composite. A similar situation can be observed in the case of Pr3+:YF3 doped composite. The fluorescence decay of composite, shown in Fig. 9, is only slightly shorter than decay of original nanopowder (22 μs vs. 26 μs, respectively) and somewhat distorted, which is a manifestation of some weak (definitely not critical) interaction with polymer matrix. The main features of luminescence spectrum – position of the maxima, linewidth and line intensity ratios remain practically unchanged.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work has been supported by the National Science Centre, Poland, grant number: UMO-2013/09/N/ST8/04347. References [1] B.H. Soffer, B.B. McFarland, Continuously tunable, narrow-band organic dye lasers, Appl. Phys. Lett. 10 (1967) 266–267. [2] A. Costela, I. García-Moreno, R. Sastre, Polymeric solid-state dye lasers: recent developments, Phys. Chem. Chem. Phys. 5 (2003) 4745–4763. [3] S. Chenais, S. Forget, Recent advances in solid-state organic lasers, Appl. Opt. 36 (27) (2011) 6760–6763. [4] M. Pope, H.P. Kallmann, P. Magnante, Electroluminescence in organic crystals, J. Chem. Phys. 38 (1963) 2042–2043. [5] S. Reineke, M. Thomschke, B. Lüssem, K. Leo, White organic light-emitting diodes Status and perspective, Rev. Mod. Phys. 85 (3) (2013) 1245–1293. [6] S.Z. Bisri, T. Takenobu, Y. Iwasa, The pursuit of electrically-driven organic semiconductor lasers, J. Mater. Chem. C 2 (2014) 2827–2836. [7] K. Binnemans, Lanthanide-based luminescent hybrid materials, Chem. Rev. 109 (2009) 4283–4374. [8] L.H. Slooff, A. Polman, S.I. Klink, G.A. Hebbink, L. Grave, F.C.J.M. van Veggel, D.N. Reinhoudt, J.W. Hofstraat, Optical properties of lissamine functionalized Nd3+ complexes in polymer waveguides and solution, Opt. Mater. 14 (2000) 101–107. [9] G.A. Crosby, Intramolecular energy transfer in rare earth chelates. Role of the triplet state, J. Chem. Phys. 34 (1961) 743–747. [10] R.E. Whan, G.A. Crosby, Luminescence studies of rare earth complexes: benzoylacetonate and dibenzoylmethide chelates, J. Mol. Spectrosc. 8 (1–6) (1962) 315–327. [11] L.R. Matthews, E. Knobbe, Luminescence behavior of europium complexes in sol-gel derived host materials, Chem. Mater. 5 (1993) 1697–1700. [12] Q.J. Zhang, P. Wang, X.F. Sun, Y. Zhai, P. Dai, B. Yang, M. Hai, J.P. Xie, Amplified spontaneous emission of an Nd3+-doped poly(methyl methacrylate) optical fiber at ambient temperature, Appl. Phys. Lett. 72 (1998) 407–409. [13] T. Kobayashi, S. Nakatsuka, T. Iwafuji, K. Kuriki, N. Imai, T. Nakamoto, ChD. Claude, K. Sasaki, Y. Koike, Y. Okamoto, Fabrication and superfluorescence of rare-earth chelate-doped graded index polymer optical fibers, Appl. Phys. Lett. 71 (1997) 2421–2423. [14] S. Lin, R. Feuerstein, A.R. Mickelson, A study of neodymium-chelate-doped optical polymer waveguides, J. Appl. Phys. 79 (1996) 2826–2874. [15] L.H. Slooff, A. van Blaaderen, A. Polman, G.A. Hebbink, S.I. Klink, F.C.J.M. van Veggel, D.N. Reinhoudt, Rare-earth doped polymers for planar optical amplifiers, J. Appl. Phys. 91 (2002) 3955–3979. [16] R.S. Meltzer, W.M. Yen, H. Zheng, S.P. Feofilov, M.J. Dejneka, B. Tissue, H.B. Yuan, Effect of the matrix on the radiative lifetimes of rare earth doped nanoparticles embedded in matrices, J. Lumin. 94–95 (2001) 217–220. [17] R.S. Meltzer, W.M. Yen, H. Zheng, S.P. Feofilov, M.J. Dejneka, B. Tissue, H.B. Yuan, Interaction of rare earth ions doped in nanocrystals embedded in amorphous matrices with two-level systems of the matrix, J. Lumin. 94–95 (2001) 221–224. [18] G.A. Kumar, C.W. Chen, J. Ballato, R.E. Riman, Optical characterization of infrared emitting rare-earth-doped fluoride nanocrystals and their transparent nanocomposites, Chem. Mater. 19 (2007) 1523–1528. [19] D.K. Sardar, S. Chandra, J.B. Gruber, W. Gorski, M. Zhang, J.H. Shim, Preparation and spectroscopic characterization of Nd3+:Y2O3 nanocrystals suspended in polymethyl methacrylate, J. Appl. Phys. 105 (2009) 093105 1–8. [20] P.-P. Zhang, Y.-B. Lang, J.-J. Guo, H. Chen, W.-P. Qin, D. Zhao, Up-conversion NaYF4-PMMA nanocomposites prepared by copolymerization, Chin. J. Lumin. 37 (8) (2016) 919–926. [21] J. Sun, B. Xue, H. Du, White upconversion emission in Li+/Yb3+/Tm3+/Er3+doped Gd6MoO12 phosphors, Opt. Commun. 298–299 (2013) 37–40. [22] V. Jambunathan, X. Mateos, M.C. Pujol, J.J. Carvajal, F. Diaz, M. Aguilo, Control of the cool/warm white light generation from lanthanide ions in monoclinic double tungstate crystals, J. Lumin. 131 (2011) 2212–2215. [23] C. Zhu, X. Liang, Y. Yang, G. Chen, Luminescence properties of Tb doped and Tm/ Tb/Sm co-doped glasses for LED applications, J. Lumin. 130 (2010) 74–77. [24] G. Lin, F. Luo, H. Pan, Q. Chen, D. Chen, J. Qiu, Q. Zhao, Three primary colors emitting from Er3+-Eu3+ co-doped oxygen-deficient glasses, J. Alloy. Comp. 509 (2011) 6462–6466. [25] T.V. Gavrilović, D.J. Jovanović, K. Smits, M.D. Dramićanin, Multicolor upconversion luminescence of GdVO4:Ln3+/Yb3+ (Ln3+ = Ho3+, Er3+, Tm3+, Ho3+/Er3+/
4. Summary and conclusions In this work we have investigated and compared the luminescent properties in the visible of the set of nanopowders based on fluoride and oxide hosts (YF3, LaAlO3, Al2O3 and Y2O3) activated with Pr3+ ions, as well as PMMA-based composites doped with those nanocrystals which could potentially enable obtaining white light emission. Spectroscopic characterization was focused on luminescent properties in the visible spectral range and, in particular, on possibility of obtaining white (or nearly white) light emission. All investigated samples were characterized by strong emission in the visible spectral range, originating either from excited 3PJ or 1D2 levels. Unequivocally, among all investigated samples the Pr3+:YF3 nanopowder offers the best luminescent properties in terms of spectral distribution of the emitted light and potential applicability in white light emitters, although Pr3+:LaAlO3 also offers interesting channels of visible luminescence, specifically in blue spectral range. For that reasons these both nanocrystalline materials have been chosen as dopants for polymer composites, developed to combine the excellent luminescent properties of active nanocrystals and optomechanical properties of polymer material. Optical characterization of developed composites has proved that the resulting material keeps the original properties of the nanocrystals both with respect of spectral distribution of luminescence intensity and fluorescence dynamics, which remained practically unchanged. This, in turn, confirms that the nanocrystalline environment provides sufficiently good isolation of active ions from parasitic influence of highly energetic phonons of the polymer matrix. Although the work is still in progress, it can be summarized that despite some technological challenges which still have to be overcome, the developed polymer composites doped with active nanocrystals may be considered as a promising material for white light 7
Optical Materials 95 (2019) 109247
A. Jusza, et al. Tm3+) nanorods, Dyes Pigments 126 (2016) 1–7. [26] L. Tao, L. Yan, Y. Lou, Y. Li, Y. Zhao, B. Zhou, J. Li, Bright white-light upconversion from core-shell nanocrystals through interfacial energy transfer, Dyes Pigments 154 (2018) 87–91. [27] S. Hao, G. Chen, C. Yang, W. Shao, W. Wei, Y. Liu, P.N. Prasad, Nd3+-Sensitized multicolor upconversion luminescence from a sandwiched core/shell/shell nanostructure, Nanoscale 9 (2017) 10633–10638. [28] A.S. Gouveia-Neto, N.P.S.M. Rios, L.A. Bueno, Spectroscopic study and white-light simulation using praseodymium-doped fluorogermanate glass as single phosphor for white LEDs, Opt. Mater. 35 (2012) 126–129. [29] A.S. Gouveia-Neto, N.P.S.M. Rios, L.A. Bueno, Generation of white-light through frequency upconversion in praseodymium-ytterbium codoped nano-structured fluorogermanate glass, Proc. SPIE 8240 (2012) 82401C. [30] R. Balda, J. Fernandez, A. de Pablos, J.M. Fdez-Navarro, Spectroscopic properties of Pr3+ ions in lead germanate glass, J. Phys. Condens. Matter 11 (1999) 7411–7421. [31] T. Danger, A. Bleckmann, G. Huber, Stimulated emission and laser action of Pr3+doped YAlO3, Appl. Phys. B Laser Opt. 58 (1994) 413–420. [32] A.A. Kaminskii, H.J. Eichler, White light creation by using visible stimulated emission of praseodymium crystalline lasers, Phys. Status Solidi 185 (1994) K85–K88. [33] G.C. Aumueller, W. Kostler, B.C. Grabmaier, R. Frey, Luminescence properties of Pr3+ in cubic rare earth oxides, J. Phys. Chem. Solids 55 (1994) 767–772. [34] M. Okumura, M. Tamatani, A.K. Albessard, N. Matsuda, Luminescence properties of rare earth ion-doped monoclinic yttrium sesquioxide, Jpn. J. Appl. Phys. 36 (1997) 6411–6415. [35] A. Rzepka, W. Ryba-Romanowski, R. Diduszko, L. Lipińska, A. Pajączkowska, Growth and characterization of Nd, Yb - yttrium oxide nanopowders obtained by sol-gel method, Cryst. Res. Technol. 42 (12) (2007) 1314–1319. [36] M. Dudek, A. Jusza, K. Anders, L. Lipińska, M. Baran, R. Piramidowicz, Luminescent properties of praseodymium doped Y2O3 and LaAlO3 nanocrystallites and polymer composites, J. Rare Earths 29 (12) (2011) 1123–1129. [37] A. Skuta, E. Talik, L. Lipińska, M. Michalska, A. Guzik, P. Zajdel, Investigations of YF3:1% Er nanocrystals, J. Cryst. Growth 401 (2014) 480–483.
[38] A. Jusza, K. Anders, A. Jastrzębska, P. Polis, A. Olszyna, M. Kuś, A. Kunicki, R. Piramidowicz, Luminescent and structural properties of Yb3+-doped Al2O3 nanopowders, Opt. Mater. 33 (10) (2011) 1487–1491. [39] J. Thapa, B. Liu, S.D. Woodruff, B.T. Chorpening, M.P. Buric, Raman scattering in single-crystal sapphire at elevated temperatures, Appl. Optic. 56 (2017) 8598–8606. [40] O.K. Moune, M.D. Faucher, N. Edelstein, Spectroscopic investigations and configuration-interaction assisted crystal field analysis of Pr3+ in YPO4 single crystal, J. Lumin. 96 (2002) 51–68. [41] P.J. Dereń, K. Lemański, A. Gągor, A. Watras, M. Małecka, M. Zawadzki, Symmetry of LaAlO3 nanocrystals as a function of crystallite size, J. Solid State Chem. 183 (2010) 2095–2100. [42] A. Zalkin, D.H. Templeton, The crystal structures of YF3 and related compounds, J. Am. Chem. Soc. 75 (1953) 2453–2458. [43] K. Lonsdale, International Tables for X-Ray Crystallography. Volume III. Physical and Chemical Tables, Kynoch, Birmingham, 1962. [44] A. Tougerti, Ch Methivier, S. Cristol, F. Tielens, M. Che, X. Carrier, Structure of clean and hydrated α-Al2O3 (1102) surfaces: implication on surface charge, Phys. Chem. Chem. Phys. 13 (2011) 6531–6543. [45] H. Retot, A. Bessiere, B. Viana, B. LaCourse, E. Mattmann, 5d–4f and 4f–4f emissions in Ln-doped sesquioxide ceramics, Opt. Mater. 33 (2011) 1008–1011. [46] D. Liu, Effects of Cr content and morphology on the luminescence properties of the Cr-doped alpha-Al2O3 powders, Ceram. Int. 39 (2013) 4765–4769. [47] K. Lemański, P. Dereń, Luminescent properties of LaAlO3 nanocrystals, doped with Pr3+ and Yb3+ ions, J. Lumin. 146 (2014) 239–242. [48] F. Pelle, M. Dhaouadi, L. Michely, P. Aschehoug, A. Toncelli, S. Veronesi, M. Tonelli, Spectroscopic properties and upconversion in Pr3+:YF3 nanoparticles, Phys. Chem. Chem. Phys. 13 (2011) 17453–17460. [49] D. Avram, B. Cojocaru, M. Florea, C. Tiseanu, Advances in luminescence of lanthanide doped Y2O3: case of S6 sites, Opt. Mater. Express 6 (2016) 1635–1643. [50] T. Smith, J. Guild, The C.I.E. colorimetric standards and their use, Trans. Opt. Soc. 33 (3) (1931-32) 73–134.
8
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Praseodymium doped nanocrystals and nanocomposites for application in white light sources
T
Anna Juszaa,*, Ludwika Lipińskab, Magdalena Baranb, Andrzej Olszynac, Agnieszka Jastrzębskac, Małgorzata Gild, Paweł Mergod, Ryszard Piramidowicza a
Warsaw University of Technology, Institute of Microelectronics and Optoelectronics, Koszykowa 75, 00-662 Warsaw, Poland Institute of Electronic Materials Technology, Wolczynska 133, Warsaw, 01-919, Poland c Warsaw University of Technology, Faculty of Material Science and Engineering, Woloska 141, Warsaw, 02-507, Poland d Maria Curie-Sklodowska University, Faculty of Chemistry, M.C. Skłodowska Sq. 2, Lublin, 20-031, Poland b
ARTICLE INFO
ABSTRACT
Keywords: Praseodymium Visible light Luminescence Polymer composites PMMA
In this work we report the results of our investigations on luminescence properties in the visible spectral range of composite materials based on polymer matrices doped with nano-sized crystals activated by Pr3+ ions. In particular, we investigated the visible emission of YF3, LaAlO3, Al2O3 and Y2O3 nanopowders activated with Pr3+ ions, as well as PMMA-based composites doped with particles of the best luminescent properties. The basic structural characterization of the developed samples was performed, followed by versatile optical characterization, focused on measurements and analysis of excitation and luminescence spectra and fluorescence dynamics profiles. The comparison of spectroscopic features of developed materials enabled discussion of the influence of crystalline and polymer environment on luminescent properties of resulting materials as well as consideration of potential applicability of developed materials in white light emitters.
1. Introduction The recent years observed an intensive development of optoelectronic devices and systems using a compact, highly efficient short-wavelength radiation sources, both coherent and incoherent. Demand for emission of light from the visible part of the spectrum (red, green and blue as well as white, obtained from a combination of these three colors) results from a number of application areas, covering among others lighting systems, imaging, recording and information processing, medical diagnostic techniques as well as optical telecommunication and sensing. Among the best known and most common applications, the large scale video displays (laser or OLED based), white LEDs for lighting systems, high-capacity disk storage media like DVD or blu-ray and optical transmitters compatible with polymer optical fibers transmission windows might be enumerated. Such a wide range of applications stimulates looking for new active materials which would combine the advantages of optical and thermo-mechanical properties of polymer matrices with well known (and low-cost) manufacturing technology and excellent luminescence properties of rare-earth ions, typically deployed as active centers in majority of “classical” solid state lasers. Polymer materials have been tested for luminescence and laser properties practically from the very beginning of the development of *
laser technology [1]. Among the main paths of research conducted for nearly five decades, the three main should be enumerated: active organic dye doped materials [2,3], electroluminescent polymers [4–6], and polymers doped with rare-earth compounds [7]. It should be mentioned, that doping polymers with rare earth ions is not trivial – in general, the lanthanide salts are insoluble in organic compounds [8]. Besides, the polymer matrices are susceptible to adsorption of water molecules, which are transported to the volume of material by diffusion, resulting in the extreme increase of luminescence quenching by vibrionic OH modes coupling with excited levels of active ions. One of possible solutions was introduced at the beginning of the 60's by Crosby, who proposed to use chelates to isolate centrally located active ion from the influence of OH bonds [9,10]. It is worth to mention also the reports of Matthews and Knobbe [11], Zhang [12], Kobayashi [13] and Lin [14]. Also noteworthy are research, carried out since the late 90's, by the Dutch research team of L.H. Slooff and dedicated mainly to applications of polymer materials in optical amplifiers operating in infrared part of the spectrum, which are summarized in paper [15], with conclusion of the need for searching the new compounds, which could provide more effective isolation of the active ion from vibronic interactions with polymer matrix. This can be potentially obtained in nanocrystalline materials in which the active ion is insulated from the
Corresponding author. Institute of Microelectronics and Optoelectronics, Koszykowa 75, 00-662, Warsaw, Poland. E-mail address: [email protected] (A. Jusza).
https://doi.org/10.1016/j.optmat.2019.109247 Received 13 May 2019; Received in revised form 4 July 2019; Accepted 10 July 2019 0925-3467/ © 2019 Elsevier B.V. All rights reserved.
Optical Materials 95 (2019) 109247
A. Jusza, et al.
external factors by crystalline environment. The first reports of research on active nanocrystals doped with rare earth ions and dispersed in liquid polymer matrix can be find in R.S. Meltzer works from 2001 [16,17]. In the following years luminescent properties of polymer composites doped with active nanocrystals were studied, among others, by teams of Kumar [18], Gruber [19] and Zhang [20]. Polymer composites activated with RE3+ doped nanocrystals seem to be an excellent candidate for visible and especially white light sources as they potentially may offer luminescent features comparable to crystalline materials. Recently, a number of attempts have been made to obtain the efficient emission of white light, being a combination of three fundamental color components, from multi-doped nanocrystalline materials. In particular, Er3++Tm3++Yb3+, Er3++Tm3+ +Yb3++Li3+ [21], Ho3++Tm3++Yb3+ [22], Tb3++Tm3++Sm3+ [23], Er3++Eu3+ [24], Er3++Ho3++Tm3++Yb3+ [25], Tm3+ +Gd3++Yb3++Tb3++Eu3+ core-shell [26], and Tm3++Yb3+ +Nd3++Er3+ core-shell [27] configurations have been proposed. It seems however, that the level of complexity of these materials is too high to compete efficiently on the market of white light emitters. The reports published by A. S. Gouveia-Neto's team [28,29] indicate praseodymium ion as a promising activator for single-doped phosphors for white-light sources. Among various rare earth elements, which may be used as active dopants for visible and white light emitters, praseodymium ion seems to be the best probe for investigation the possibilities of shaping luminescent properties by changing the structure of the crystalline environment. The energy level scheme of 4f2 of Pr3+ contains a number of metastable levels which enable obtaining many optical transitions in ultraviolet (UV), visible (VIS), near- and mid-infrared (NIR and MIR) spectral range. Visible emission in praseodymium doped materials results mainly from 4f intraconfigurational transitions originating from 3PJ and 1D2 metastable levels with energies about 21000 cm−1 and 17000 cm−1. For majority of praseodymium compounds typical is the presence of many emission lines in the blue, green and red spectral range [30,31], which opens attractive possibilities for developing white light sources (including “white lasers” [32]). It should be noted, however, that emission properties of praseodymium ions depend largely on the crystalline surrounding, which determines the distance between the dopant ions, their relative spatial position and the type of anions coordinating active centers. Therefore, in certain materials the typical, rich emission spectrum of praseodymium ion, resulting from optical transitions from 3P0 level and characterized by multiple lines in blue, green and red (like in YF3 and LaAlO3 hosts) is not observable, being replaced by a 1D2-related emission concentrated in red spectral range. In particular, it has been shown that in several cubic sesquioxides Pr3+ ions exhibit efficient red luminescence from 1D2 level, while the 3P0 emission is quenched (e.g. in Y2O3, Gd2O3) [33,34]. The main concept of the research reported here was to develop a polymer-based composite material doped with optically active nanocrystals, which would be able to emit blue, green and red light under single wavelength excitation. This paper is focused on investigations and analysis of the visible luminescence properties of selected nanocrystals doped with praseodymium ions, considered as a promising dopant for abovementioned application, and polymer composites activated with these nanocrystals. The set of the nanocrystalline activators has been chosen in the way enabling analysis of luminescent properties of praseodymium ions in hosts differing in structural features, chemical composition and corresponding phonon energies. In particular, both oxide (Y2O3. LaAlO3, Al2O3) and fluoride (YF3) nanocrystals have been prepared using technologies mastered previously by Institute of Electronic Materials Technology and Warsaw University of Technology teams. The detailed information is presented in the following section.
nanocrystalline samples manufactured using different synthesis methods. The Y2O3 nanopowder was prepared using sol-gel method in the Institute of Electronic Materials Technology, Warsaw [35]. The Y2O3 dissolved in acetic acid was mixed together with Pr6O11 dissolved in solution of nitric acid and stirred for 1 h at 65 °C. Then, 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 powder was calcined in the air atmosphere at 1100 °C for 7 h. The LaAlO3 nanopowder was prepared using sol-gel method in the Institute of Electronic Materials Technology [36]. Firstly, the La2O3 was dried in 900 °C for 2 h, next it was dissolved in solution of nitric acid. Separately Pr6O11 was dissolved in diluted nitric acid and Al (NO3)3·9H2O in deionized water, then all solutions were mixed together and stirred for 1 h at 65 °C. Then, ethylene glycol was added as crosslinking agent and the solution was stirred during next 2 h. During slow evaporation the obtained sol transformed into sticky gel, which, in turn, after being dried in 150 °C for 12 h changed into solid xerogel. The ground powder was carefully calcined in the air atmosphere in several stages (from 300 °C for 2 h up to 1200 °C for 12 h) to avoid a combustion. New chemical “wet” synthesis developed in the Institute of Electronic Materials Technology was used for manufacturing the YF3 powders [37]. The Y2O3 was dissolved in acetic acid, Pr6O11 in diluted nitric acid and NH4F in deionized water. Next, all solutions were mixed together and stirred for 6 h at 70 °C. During slow evaporation of the mixture the volume was decreased 5 times. Then the concentrated solution was dried in 150 °C to completely remove the remaining water particles. The dried powder was then calcined in air atmosphere in temperature of 400 °C for 2 h. Praseodymium doped Al2O3 nanopowders were prepared using a socalled “dry synthesis”, developed at the Warsaw University of Technology [38]. This method involves the reaction of an organometallic compound, praseodymium ions, in an organic solvent in a gas shielded inert (argon) environment. Thus, prepared mixture was allowed to stand for 72 h in argon with vigorous stirring. After removal of the solvent and drying the mixture, the organic precursor is thermally decomposed in a muffle furnace at 1000 °C for 24 h in air to give the final product (Pr3+:Al2O3 nanopowder). In order to avoid the presence of undesirable OH− groups samples were stored under argon atmosphere. The set of poly(methyl methacrylate) - PMMA-based composite bulk samples doped with nanocrystals with best luminescence properties were prepared using new method mastered at Maria Curie-Sklodowska University. 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. 2,2-Azoisobutyronitrile (AIBN) as an initiator was added to the 20 ml of the nanopowder-MMA solution. The homogenous dispersion of nanopowders was obtained by using ultrasonification 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 obtained samples (8 wt.% (1 at.% Pr3+:LaAlO3):PMMA and 6 wt.% (1 at.% Pr3+:YF3):PMMA) were cut and prepared for optical measurements. The crystal structure of the samples was characterized using a Siemens D-500 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 excitation spectra, emission spectra and fluorescence decay curves were recorded using the Horiba PTI QuantaMaster based spectrofluorimetric system equipped with double computer-controlled autocalibrated Czerny-Turner monochromators in the excitation and emission paths and enabling both CW (Xe 100 W lamp) and pulsed (Xe Flash
2. Materials and methods The set of investigated samples consisted of 1 at.% Pr3+:LaAlO3, 1 at.% Pr3+:YF3, 0.1 at.% Pr3+:Y2O3 and 0.1 at.% Pr3+:Al2O3 2
Optical Materials 95 (2019) 109247
A. Jusza, et al.
Fig. 1. X-ray diffraction patterns of the Pr3+ doped LaAlO3, Y2O3, YF3 and Al2O3 nanocrystals.
Fig. 2. SEM images of the Pr3+ doped LaAlO3, Y2O3, YF3 and Al2O3 nanocrystals.
lamp) excitations over a wide spectral range (200–2000 nm) as well as signal detection within the spectral range from 220 nm to 1.7 μm. All measurements were taken in room temperature and all spectra were corrected for spectral characteristics of the detector's response.
obtained X-ray diffraction patterns (Fig. 1) confirm that all investigated samples were single-phased and contained rhombohedral (ICDD card no. 31–0022), orthorhombic (ICDD card no. 32–1431), cubic (ICDD card no. 41–1105) and corundum (ICDD card no. 46–1212) phase in the case of LaAlO3, YF3, Y2O3 and Al2O3, respectively. The SEM pictures of all manufactured nanocrystalline samples, presented in Fig. 2, show the evident structural differences between investigated nanocrystals, both in the shape and size domain. The shape of LaAlO3 and Al2O3 nanoparticles, having a coral-like form, differs significantly from the shape of yttrium oxide and yttrium fluoride nanocrystals, which are in a form of nearly spherical agglomerated structures. The average sizes of the particles estimated from SEM images were in the range of 40–150 nm
3. Results and discussion 3.1. Structural characterization All manufactured nanopowders were subjected to X-ray diffraction (XRD) and scanning electron microscopy (SEM) measurements, which enabled determination of their fundamental structural properties. The 3
Optical Materials 95 (2019) 109247
A. Jusza, et al.
Fig. 3. Excitation spectra of the Pr3+ doped LaAlO3, Y2O3, YF3 and Al2O3 nanocrystals.
Fig. 4. Emission spectra of the Pr3+ doped LaAlO3, Y2O3, YF3 and Al2O3 nanocrystals.
transitions corresponding to 3PJ+1I6 energy levels are clearly visible, enabling localizing of these levels in different nanocrystalline hosts. Almost imperceptible, additional band at 522.5 nm observed in the case of Al2O3 sample is most probably related with a vibronic sideband accompanying the excitation spectrum. This band is shifted from 3P0 excitation transition by c.a. 750 cm−1 which corresponds well to maximum phonon energy of α-alumina, reported in Ref. [39]. The similar effect was investigated by O.K Moune et al. [40]. Moreover, 1D2 originating transitions are also detectable for yttria and alumina samples.
for all samples. All samples were partially agglomerated in bigger grains of micrometric dimensions. 3.2. Luminescent properties of the praseodymium doped nanocrystals Due to a specific (powdered) type of original active medium, the typical measurements of absorption characteristics were replaced by excitation spectra measurements. The excitation spectra of red emission of all investigated nanopowders are shown in Fig. 3. For all samples 4
Optical Materials 95 (2019) 109247
A. Jusza, et al.
different nanocrystalline host could be explained by different crystalline structures of investigated materials. At the pseudo-cubic perovskite LaAlO3 (with a rhombohedral structure with hexagonal space group R3c) Pr3+ ions replace lanthanum ions and occupy the D3 site symmetry positions [41]. The structure of YF3 is orthorhombic with Pnma space group with four formulae per unit cell [42]. In this structure Pr3+ replace yttrium ions which are coordinated by nine fluorine ions and occupy the CS site symmetry positions. The most stable form of Y2O3 at room temperature is a cubic type, belonging to space group Ia3 [43]. The unit cell contains twenty-four yttrium ions in non-centro-symmetric sites C2 (occupy the Wyckoff position 24d) and eight Y3+ ions in centrosymmetric sites S6 (occupy the position 8b). The trivalent rare earth dopants can substitute yttrium ions in these both types of sites with approximately equal probabilities. α-Al2O3 structure can be described both as a rhombohedral or a hexagonal unit cell belonging to the space group R3c. Oxygen atoms are in a distorted hexagonal structure, while the aluminum atoms occupying 2/3 of the octahedral sites. The hexagonal cell contains 12 aluminum and 18 oxygen atoms [44]. For LaAlO3 and YF3 powders the behavior of luminescence spectra is similar - small differences observed are the consequence of different splitting of the energy levels of praseodymium (see the excitation spectra in Fig. 3) due to different crystal fields of host lattices. The lack of 3P0 originated emission in the cubic Y2O3 and the quenching of this luminescence for the α-Al2O3 phase can be explained using the energy level diagrams in coordinate configuration (Fig. 5, followed by Ref. [45]) which show the path of non-radiative relaxation from 3P0 to 1D2 via 4f5d configuration for sesquioxides. The splitting of the 4f5d energy level is related to the interatomic distance between the Pr3+ and the surrounding O2− ligands an determines the origin of the emission of the 3 P0 or the 1D2 level [34]. It should be noted that all attributions of emission lines were additionally confirmed by fluorescence decays measurements. Apart from praseodymium originating luminescence, for Pr3+:Al2O3 sample also strong parasitic emission with maximum at 694 nm was recorded (marked as artificial in Fig. 4) associated with Cr3+ impurity (confirmed additionally by excitation spectrum with two excitation bands with maxima at 409 nm and 553 nm corresponding to spin-allowed transitions of Cr3+ [46]). The fluorescence dynamics profiles were
Fig. 5. Energy level diagrams in coordinate configuration (followed by Ref. [45]) for Pr3+ states.
The strong ultraviolet transition observed for Y2O3 (the long-wavelength edge of this transition is also detectable for YF3) is a manifestation of interconfigurational excitation to the 4f5d configuration bands and is typical for Pr3+:Y2O3 structure [33]. According to the position of 3 PJ manifolds for each structure it is possible to select a single wavelength appropriate for efficient excitation of praseodymium ions in all examined nanocrystals. Luminescence spectra of all investigated nanopowders under optimal (the most efficient within the disposed range of exciting wavelengths) excitation, which corresponds to 3P2 level pumping for LaAlO3, YF3 and Al2O3, and 4f5d band for Y2O3, are shown in Fig. 4. For Pr3+:LaAlO3 and Pr3+:YF3 the spectrum is dominated by transitions originating from 3PJ manifolds with very weak contribution of transitions from 1D2 to the ground level observed for both materials. For Pr3+:LaAlO3 blue emission attributed to 3P0→3H4 transition is the most intense, while for Pr3+:YF3 transitions in blue (3P0→3H4) and red (3P0→3H6) spectral range are of comparable intensity. Completely opposite situation was observed for Pr3+:Y2O3 – all emission lines in the visible range were originating from 1D2 level. For alumina sample both 3 P0 and 1D2 emissions were noticed, however with evident domination of 1D2→3H4 transition in red spectral range. This behaviour of luminescent properties of praseodymium ions in
Fig. 6. Fluorescence decays of the Pr3+ doped LaAlO3, Y2O3, YF3 and Al2O3 nanocrystals. 5
Optical Materials 95 (2019) 109247
A. Jusza, et al.
Fig. 7. The CIE 1931 chromaticity diagram of the Pr3+ doped LaAlO3, Y2O3, YF3 and Al2O3 nanocrystals.
Fig. 8. Fluorescence decays and emission spectra (inset) of the Pr3+:LaAlO3 (as reference) and (Pr3+:LaAlO3):PMMA samples.
Fig. 9. Fluorescence decays and emission spectra (inset) of the Pr3+:YF3 (as reference) and (Pr3+:YF3):PMMA samples.
recorded for most intensive emissions, corresponding to 3P0→3H4 transition for Pr3+:LaAlO3 and Pr:YF3 samples and to 1D2→3H4 transition for Pr3+:Y2O3 and Pr3+:Al2O3 (see Fig. 6). The time constants determined for the samples were of order of 30.4 μs (LaAlO3), 25.9 μs (YF3), 113 μs (Y2O3) and 152 μs (Al2O3). Obtained lifetime values are in a good agreement with those reported earlier for similar materials [47–49], except Al2O3, for which 1D2 time constant is reported here for the first time. All recorded emission spectra were converted to the CIE chromaticity coordinates and are presented at the CIE 1931 chromaticity diagram (Fig. 7). The chromaticity coordinates were calculated from the spectral power distribution of the light and the CIE colourmatching functions [50]. White light emission has been confirmed for LaAlO3 and YF3 samples with chromaticity coordinates (CIE 1931) x = 0.3588, y = 0.4306 and x = 0.4406, y = 0.3900, respectively. Summarizing this part, it can be concluded that we have successfully manufactured praseodymium doped oxide and fluoride nanocrystals of good luminescent properties, providing efficient emission within the visible spectral range. It should be noted, that among four
investigated samples, two can be considered as potential materials for white light emitters. These are Pr3+:LaAlO3 and Pr3+:YF3 nanocrystals, offering luminescent transitions at several wavelengths within blue (ca. 480–490 nm), green (ca. 525–545 nm) and red (ca. 600–650 nm) spectral range, originating from 3PJ levels. The simple comparison of the intensities of individual emission lines of these two materials evidently shows that luminescent features of Pr3+:YF3 may easier satisfy requirements defined for white light emitters - the material offers comparable levels of blue and red luminescence and considerably high (comparing to Pr3+:LaAlO3) intensity of emission in green. Pr3+:Y2O3 and Pr3+:Al2O3 sesquioxides suffer from efficient quenching of emission from 3PJ levels and offer visible luminescence limited to the red part of the spectrum related with transitions from 1D2 level, and may be considered only as additional additives to materials emitting from 3P0 level, enabling enhancement of red luminescence. The excellent luminescent properties (and specifically RGB emission), combined with reasonably good structural properties and well mastered manufacturing technology have had a decisive influence on 6
Optical Materials 95 (2019) 109247
A. Jusza, et al.
choosing the Pr3+:LaAlO3 and Pr3+:YF3 nanopowders as dopants for PMMA-based polymer composites.
emitters, specifically when doped with a combination of nanocrystals optimized with respect of the shape of luminescence characteristics.
3.3. Luminescent properties of the PMMA-based composites
Declaration of competing interest
As it was mentioned before - the main benefit expected from the concept of polymer nanocomposite doped with optically active nanopowder was the combination of excellent luminescent properties of crystalline hosts doped with RE3+ ions with extraordinary mechanical features and low cost of manufacturing of polymer materials. In the developed composite the polymer surrounding shall not only provide the mechanical strength but also protect the nanocrystals from the influence of OH− groups and other contaminations, while the encapsulation of active ions in nanocrystalline environment should result in shielding the ions from parasitic interaction with highly energetic phonons of polymer host. The results of optical characterization of both composite materials have confirmed the correctness of proposed approach. The main evidence of efficient isolation of active ions from the polymer host influence is the shape and time constant of fluorescence decay. Both decays, presented in Fig. 8 for Pr3+:LaAlO3 nanopowder and composite material remain practically identical, with time constant of order of 30 μs, irrespective of the material type. This means, that there are no additional processes, related with polymer host phonons which would speed up depopulation of the excited 3P0 state. Also, the luminescence spectra, shown in the inset, remain identical, exhibiting the same spectroscopic features both for nanopowder and polymer composite. A similar situation can be observed in the case of Pr3+:YF3 doped composite. The fluorescence decay of composite, shown in Fig. 9, is only slightly shorter than decay of original nanopowder (22 μs vs. 26 μs, respectively) and somewhat distorted, which is a manifestation of some weak (definitely not critical) interaction with polymer matrix. The main features of luminescence spectrum – position of the maxima, linewidth and line intensity ratios remain practically unchanged.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work has been supported by the National Science Centre, Poland, grant number: UMO-2013/09/N/ST8/04347. References [1] B.H. Soffer, B.B. McFarland, Continuously tunable, narrow-band organic dye lasers, Appl. Phys. Lett. 10 (1967) 266–267. [2] A. Costela, I. García-Moreno, R. Sastre, Polymeric solid-state dye lasers: recent developments, Phys. Chem. Chem. Phys. 5 (2003) 4745–4763. [3] S. Chenais, S. Forget, Recent advances in solid-state organic lasers, Appl. Opt. 36 (27) (2011) 6760–6763. [4] M. Pope, H.P. Kallmann, P. Magnante, Electroluminescence in organic crystals, J. Chem. Phys. 38 (1963) 2042–2043. [5] S. Reineke, M. Thomschke, B. Lüssem, K. Leo, White organic light-emitting diodes Status and perspective, Rev. Mod. Phys. 85 (3) (2013) 1245–1293. [6] S.Z. Bisri, T. Takenobu, Y. Iwasa, The pursuit of electrically-driven organic semiconductor lasers, J. Mater. Chem. C 2 (2014) 2827–2836. [7] K. Binnemans, Lanthanide-based luminescent hybrid materials, Chem. Rev. 109 (2009) 4283–4374. [8] L.H. Slooff, A. Polman, S.I. Klink, G.A. Hebbink, L. Grave, F.C.J.M. van Veggel, D.N. Reinhoudt, J.W. Hofstraat, Optical properties of lissamine functionalized Nd3+ complexes in polymer waveguides and solution, Opt. Mater. 14 (2000) 101–107. [9] G.A. Crosby, Intramolecular energy transfer in rare earth chelates. Role of the triplet state, J. Chem. Phys. 34 (1961) 743–747. [10] R.E. Whan, G.A. Crosby, Luminescence studies of rare earth complexes: benzoylacetonate and dibenzoylmethide chelates, J. Mol. Spectrosc. 8 (1–6) (1962) 315–327. [11] L.R. Matthews, E. Knobbe, Luminescence behavior of europium complexes in sol-gel derived host materials, Chem. Mater. 5 (1993) 1697–1700. [12] Q.J. Zhang, P. Wang, X.F. Sun, Y. Zhai, P. Dai, B. Yang, M. Hai, J.P. Xie, Amplified spontaneous emission of an Nd3+-doped poly(methyl methacrylate) optical fiber at ambient temperature, Appl. Phys. Lett. 72 (1998) 407–409. [13] T. Kobayashi, S. Nakatsuka, T. Iwafuji, K. Kuriki, N. Imai, T. Nakamoto, ChD. Claude, K. Sasaki, Y. Koike, Y. Okamoto, Fabrication and superfluorescence of rare-earth chelate-doped graded index polymer optical fibers, Appl. Phys. Lett. 71 (1997) 2421–2423. [14] S. Lin, R. Feuerstein, A.R. Mickelson, A study of neodymium-chelate-doped optical polymer waveguides, J. Appl. Phys. 79 (1996) 2826–2874. [15] L.H. Slooff, A. van Blaaderen, A. Polman, G.A. Hebbink, S.I. Klink, F.C.J.M. van Veggel, D.N. Reinhoudt, Rare-earth doped polymers for planar optical amplifiers, J. Appl. Phys. 91 (2002) 3955–3979. [16] R.S. Meltzer, W.M. Yen, H. Zheng, S.P. Feofilov, M.J. Dejneka, B. Tissue, H.B. Yuan, Effect of the matrix on the radiative lifetimes of rare earth doped nanoparticles embedded in matrices, J. Lumin. 94–95 (2001) 217–220. [17] R.S. Meltzer, W.M. Yen, H. Zheng, S.P. Feofilov, M.J. Dejneka, B. Tissue, H.B. Yuan, Interaction of rare earth ions doped in nanocrystals embedded in amorphous matrices with two-level systems of the matrix, J. Lumin. 94–95 (2001) 221–224. [18] G.A. Kumar, C.W. Chen, J. Ballato, R.E. Riman, Optical characterization of infrared emitting rare-earth-doped fluoride nanocrystals and their transparent nanocomposites, Chem. Mater. 19 (2007) 1523–1528. [19] D.K. Sardar, S. Chandra, J.B. Gruber, W. Gorski, M. Zhang, J.H. Shim, Preparation and spectroscopic characterization of Nd3+:Y2O3 nanocrystals suspended in polymethyl methacrylate, J. Appl. Phys. 105 (2009) 093105 1–8. [20] P.-P. Zhang, Y.-B. Lang, J.-J. Guo, H. Chen, W.-P. Qin, D. Zhao, Up-conversion NaYF4-PMMA nanocomposites prepared by copolymerization, Chin. J. Lumin. 37 (8) (2016) 919–926. [21] J. Sun, B. Xue, H. Du, White upconversion emission in Li+/Yb3+/Tm3+/Er3+doped Gd6MoO12 phosphors, Opt. Commun. 298–299 (2013) 37–40. [22] V. Jambunathan, X. Mateos, M.C. Pujol, J.J. Carvajal, F. Diaz, M. Aguilo, Control of the cool/warm white light generation from lanthanide ions in monoclinic double tungstate crystals, J. Lumin. 131 (2011) 2212–2215. [23] C. Zhu, X. Liang, Y. Yang, G. Chen, Luminescence properties of Tb doped and Tm/ Tb/Sm co-doped glasses for LED applications, J. Lumin. 130 (2010) 74–77. [24] G. Lin, F. Luo, H. Pan, Q. Chen, D. Chen, J. Qiu, Q. Zhao, Three primary colors emitting from Er3+-Eu3+ co-doped oxygen-deficient glasses, J. Alloy. Comp. 509 (2011) 6462–6466. [25] T.V. Gavrilović, D.J. Jovanović, K. Smits, M.D. Dramićanin, Multicolor upconversion luminescence of GdVO4:Ln3+/Yb3+ (Ln3+ = Ho3+, Er3+, Tm3+, Ho3+/Er3+/
4. Summary and conclusions In this work we have investigated and compared the luminescent properties in the visible of the set of nanopowders based on fluoride and oxide hosts (YF3, LaAlO3, Al2O3 and Y2O3) activated with Pr3+ ions, as well as PMMA-based composites doped with those nanocrystals which could potentially enable obtaining white light emission. Spectroscopic characterization was focused on luminescent properties in the visible spectral range and, in particular, on possibility of obtaining white (or nearly white) light emission. All investigated samples were characterized by strong emission in the visible spectral range, originating either from excited 3PJ or 1D2 levels. Unequivocally, among all investigated samples the Pr3+:YF3 nanopowder offers the best luminescent properties in terms of spectral distribution of the emitted light and potential applicability in white light emitters, although Pr3+:LaAlO3 also offers interesting channels of visible luminescence, specifically in blue spectral range. For that reasons these both nanocrystalline materials have been chosen as dopants for polymer composites, developed to combine the excellent luminescent properties of active nanocrystals and optomechanical properties of polymer material. Optical characterization of developed composites has proved that the resulting material keeps the original properties of the nanocrystals both with respect of spectral distribution of luminescence intensity and fluorescence dynamics, which remained practically unchanged. This, in turn, confirms that the nanocrystalline environment provides sufficiently good isolation of active ions from parasitic influence of highly energetic phonons of the polymer matrix. Although the work is still in progress, it can be summarized that despite some technological challenges which still have to be overcome, the developed polymer composites doped with active nanocrystals may be considered as a promising material for white light 7
Optical Materials 95 (2019) 109247
A. Jusza, et al. Tm3+) nanorods, Dyes Pigments 126 (2016) 1–7. [26] L. Tao, L. Yan, Y. Lou, Y. Li, Y. Zhao, B. Zhou, J. Li, Bright white-light upconversion from core-shell nanocrystals through interfacial energy transfer, Dyes Pigments 154 (2018) 87–91. [27] S. Hao, G. Chen, C. Yang, W. Shao, W. Wei, Y. Liu, P.N. Prasad, Nd3+-Sensitized multicolor upconversion luminescence from a sandwiched core/shell/shell nanostructure, Nanoscale 9 (2017) 10633–10638. [28] A.S. Gouveia-Neto, N.P.S.M. Rios, L.A. Bueno, Spectroscopic study and white-light simulation using praseodymium-doped fluorogermanate glass as single phosphor for white LEDs, Opt. Mater. 35 (2012) 126–129. [29] A.S. Gouveia-Neto, N.P.S.M. Rios, L.A. Bueno, Generation of white-light through frequency upconversion in praseodymium-ytterbium codoped nano-structured fluorogermanate glass, Proc. SPIE 8240 (2012) 82401C. [30] R. Balda, J. Fernandez, A. de Pablos, J.M. Fdez-Navarro, Spectroscopic properties of Pr3+ ions in lead germanate glass, J. Phys. Condens. Matter 11 (1999) 7411–7421. [31] T. Danger, A. Bleckmann, G. Huber, Stimulated emission and laser action of Pr3+doped YAlO3, Appl. Phys. B Laser Opt. 58 (1994) 413–420. [32] A.A. Kaminskii, H.J. Eichler, White light creation by using visible stimulated emission of praseodymium crystalline lasers, Phys. Status Solidi 185 (1994) K85–K88. [33] G.C. Aumueller, W. Kostler, B.C. Grabmaier, R. Frey, Luminescence properties of Pr3+ in cubic rare earth oxides, J. Phys. Chem. Solids 55 (1994) 767–772. [34] M. Okumura, M. Tamatani, A.K. Albessard, N. Matsuda, Luminescence properties of rare earth ion-doped monoclinic yttrium sesquioxide, Jpn. J. Appl. Phys. 36 (1997) 6411–6415. [35] A. Rzepka, W. Ryba-Romanowski, R. Diduszko, L. Lipińska, A. Pajączkowska, Growth and characterization of Nd, Yb - yttrium oxide nanopowders obtained by sol-gel method, Cryst. Res. Technol. 42 (12) (2007) 1314–1319. [36] M. Dudek, A. Jusza, K. Anders, L. Lipińska, M. Baran, R. Piramidowicz, Luminescent properties of praseodymium doped Y2O3 and LaAlO3 nanocrystallites and polymer composites, J. Rare Earths 29 (12) (2011) 1123–1129. [37] A. Skuta, E. Talik, L. Lipińska, M. Michalska, A. Guzik, P. Zajdel, Investigations of YF3:1% Er nanocrystals, J. Cryst. Growth 401 (2014) 480–483.
[38] A. Jusza, K. Anders, A. Jastrzębska, P. Polis, A. Olszyna, M. Kuś, A. Kunicki, R. Piramidowicz, Luminescent and structural properties of Yb3+-doped Al2O3 nanopowders, Opt. Mater. 33 (10) (2011) 1487–1491. [39] J. Thapa, B. Liu, S.D. Woodruff, B.T. Chorpening, M.P. Buric, Raman scattering in single-crystal sapphire at elevated temperatures, Appl. Optic. 56 (2017) 8598–8606. [40] O.K. Moune, M.D. Faucher, N. Edelstein, Spectroscopic investigations and configuration-interaction assisted crystal field analysis of Pr3+ in YPO4 single crystal, J. Lumin. 96 (2002) 51–68. [41] P.J. Dereń, K. Lemański, A. Gągor, A. Watras, M. Małecka, M. Zawadzki, Symmetry of LaAlO3 nanocrystals as a function of crystallite size, J. Solid State Chem. 183 (2010) 2095–2100. [42] A. Zalkin, D.H. Templeton, The crystal structures of YF3 and related compounds, J. Am. Chem. Soc. 75 (1953) 2453–2458. [43] K. Lonsdale, International Tables for X-Ray Crystallography. Volume III. Physical and Chemical Tables, Kynoch, Birmingham, 1962. [44] A. Tougerti, Ch Methivier, S. Cristol, F. Tielens, M. Che, X. Carrier, Structure of clean and hydrated α-Al2O3 (1102) surfaces: implication on surface charge, Phys. Chem. Chem. Phys. 13 (2011) 6531–6543. [45] H. Retot, A. Bessiere, B. Viana, B. LaCourse, E. Mattmann, 5d–4f and 4f–4f emissions in Ln-doped sesquioxide ceramics, Opt. Mater. 33 (2011) 1008–1011. [46] D. Liu, Effects of Cr content and morphology on the luminescence properties of the Cr-doped alpha-Al2O3 powders, Ceram. Int. 39 (2013) 4765–4769. [47] K. Lemański, P. Dereń, Luminescent properties of LaAlO3 nanocrystals, doped with Pr3+ and Yb3+ ions, J. Lumin. 146 (2014) 239–242. [48] F. Pelle, M. Dhaouadi, L. Michely, P. Aschehoug, A. Toncelli, S. Veronesi, M. Tonelli, Spectroscopic properties and upconversion in Pr3+:YF3 nanoparticles, Phys. Chem. Chem. Phys. 13 (2011) 17453–17460. [49] D. Avram, B. Cojocaru, M. Florea, C. Tiseanu, Advances in luminescence of lanthanide doped Y2O3: case of S6 sites, Opt. Mater. Express 6 (2016) 1635–1643. [50] T. Smith, J. Guild, The C.I.E. colorimetric standards and their use, Trans. Opt. Soc. 33 (3) (1931-32) 73–134.
8