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Photoluminescence properties of Pr3+ doped Bi2ZnOB2O6 microcrystals and PMMA-based composites
Optical Materials 62 (2016) 72e79
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Photoluminescence properties of Pr3þ doped Bi2ZnOB2O6 microcrystals and PMMA-based composites K. Jaroszewski a, M. Chrunik b, P. Głuchowski c, E. Coy d, B. Maciejewska d, e, R. Jastrzab f, A. Majchrowski b, D. Kasprowicz a, * , Poland Faculty of Technical Physics, Poznan University of Technology, Piotrowo 3, 60-965 Poznan Institute of Applied Physics, Military University of Technology, Kaliskiego 2, 00-908 Warsaw, Poland c lna 2, 50-422 Wroclaw, Poland Institute of Low Temperature and Structure Research of Polish Academy of Sciences, Oko d , Poland NanoBioMedical Centre, Adam Mickiewicz University, Umultowska 85, 61-614 Poznan e , Poland Faculty of Physics, Adam Mickiewicz University, Umultowska 85, 61-614 Poznan f , Poland Faculty of Chemistry, Adam Mickiewicz University, Umultowska 89b, 61-614 Poznan a
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 August 2016 Accepted 19 September 2016
Photoluminescence properties of red-emitting Pr3þ-doped Bi2ZnOB2O6 microcrystalline powder and PMMA-based composite materials were reported. Bi2ZnOB2O6:Pr3þ powders were synthesized by means of the modified Pechini method. The morphology and crystallographic structure of Bi2ZnOB2O6:Pr3þ microcrystals were investigated by XRD and HRTEM. The PMMA-based composite materials were prepared by embedding of Bi2ZnOB2O6:Pr3þ powder in the PMMA matrix. The vibrational properties of the powder and composite systems were investigated by m-Raman spectroscopy. Emission spectra of the samples were measured under blue (451.6 nm) and UV (320 nm) excitation. Both, the powder and composite samples show enhancement of red emission (1D2 / 3H4) and quenching of greenish-blue luminescence from 3P0 level of Pr3þ ions after excitation in UV and VIS caused by the 3P0 ~ 1D2 nonradiative relaxation of Pr3þ ion by low-lying charge transfer state and non-radiative de-excitation through Pr3þeBi3þ energy transfer. The response of the powder and composite samples to pulsed excitation at 451.6 nm was measured by monitoring emission from the 1D2 level (595 nm/3P0 / 3H6 transition). The determined lifetimes t1 and t2 of the red emission at 595 nm (1D2 / 3H4 transition) of the composite are significantly longer in comparison to powder samples. Moreover, because of the good nonlinear optical properties of the Bi2ZnOB2O6 crystals and effective luminescence of the Pr3þ-doped Bi2ZnOB2O6 powders and composites, they can be very useful as bi-functional materials in the new generation of optoelectronic devices. © 2016 Elsevier B.V. All rights reserved.
Keywords: Bi2ZnOB2O6 Praseodymium Microcrystals Polymer composites Raman spectroscopy Photoluminescence
1. Introduction Significant progress in the research of the luminescent and nonlinear optical properties of nano- and microcrystals is the result of intense investigations of new multifunctional systems [1,2]. There is still the demand for novel, compact, efficient and low cost materials for many optoelectronic applications including optical switching devices [3], integrated optical circuits [4] as well as widely used light emitters [5]. Among various new materials the polymer-based composites containing optically active and
* Corresponding author. E-mail address: [email protected] (D. Kasprowicz). http://dx.doi.org/10.1016/j.optmat.2016.09.059 0925-3467/© 2016 Elsevier B.V. All rights reserved.
nonlinear optical (NLO) nano- or microcrystals have very desirable features, which make them a good alternative for presently used materials [6,7]. In some bi-functional materials the luminescence effect of optically active ions and NLO phenomena occur in the same host [8]. The polymer-based composite systems obtained on the basis of such a bi-functional materials present an excellent light propagation properties as a result of the polymer matrix and microor nanoparticles optical properties, which makes them very promising for new optoelectronic applications. Among various optically active materials rare earth (RE) doped systems arise great interest because of their unique luminescence properties mainly due to sharp and narrow f-f transitions on the manifold energy for which shifting of the band position depends slightly on the host matrix. On the basis of these properties such
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materials can be used for a variety of applications such as lasers and optical amplifiers [9], multicolor displays [10], scintillators [11], biological labeling [12] and optical frequency converters [13]. The triply ionized praseodymium ions (Pr3þ) were used as an activator in a variety of host materials useful in many phosphors [14,15] for different types of light emitting devices. In particular, the luminescence properties of Pr3þdoped materials were extensively investigated for many potential application including fiber optical communication [16], LEDs [17], field-emission display devices and photoluminescence devices [18]. Depending on the concentration of Pr3þ ions and the host matrix properties the Pr3þ ions can emit in the visible (VIS) and the near infrared (NIR) spectral range [19]. Pr3þ ions possess the specific electronic energy level structure with several optical transitions in VIS spectral range. The 4f2 electronic configuration of Pr3þ ions consists of: 3H4,5,6, 3F4,3,2, 1G4, 1D2, 1I6, 3 P2,1,0 and 1S0 electronic energy levels. The greenish-blue and red emission of the Pr3þ ions originates from excited 3P0 and 1D2 levels upon the UV or VIS excitation [20]. In many oxide-based host materials, the Pr3þ ions exhibit a prominent red luminescence from 1D2 level under blue or UV excitation accompanied by a total or partial quenching of the emission from the 3P0 level. Several non-radiative pathways were proposed to explain quenching of 3P0 luminescence of Pr3þ ion in oxide-based lattices: multiphonon relaxation, cross-relaxation, intersystem crossing through low-lying 4f15d1 levels, energy transfer between Pr3þ and metal ions as well as charge transfer (CT) mechanism [21]. The CT between active ions and the host lattice results in the formation of the charge transfer state CTS. The energetic position of the CTS depends on the nature of the host lattice and for the oxide-based crystals is typically located around excited states of rare earths. Partial or total quenching of 3P0 emission of Pr3þ ions depend on the relative energetic position of the CTS with respect to the 3P0 and 1D2 levels. The possible CT mechanism, very probable for oxide-based host lattices is ligand-to-metal charge transfer (LMCT) [19]. The LMCT state was previously reported for BZBO:Eu3þ phosphors [22]. Recently new RE-doped Bi2ZnOB2O6 materials (abbreviated as BZBO) in the form of single crystals [23], powders [23] as well as glasses [24,25] and polymer-based composites [7] are gathering important attention because of their promising optical properties and consequently numerous potential applications. BZBO crystals are also known as effective nonlinear optical materials [26], which possess high laser damage tolerance. Moreover, they may be efficiently doped with rare earth ions resulting in effective luminescence [22,27e29]. Depending on the host material, the structure of the energetic levels of the Bi3þ ions may change. Particularly, for oxide-based lattices the structure of electronic levels of Bi3þ ions strongly depends on configuration of the nearest oxygen surrounding of Bi3þ ions [30]. In the BZBO structure the Bi3þ ions are located at 4c positions (C1 symmetry) and occur in octahedral coordination BiO6 being six-coordinated by the oxygen atoms forming the BiO6 octahedra [31] and the lowering of the excited states energy is typically observed in comparison to the structure of electronic levels of isolated Bi3þ ions [30]. The BZBO is an optically positive biaxial optical crystal [40]. The experimentally determined band gap of BZBO single crystal is equal to 3.444 eV (27777.72 cm1) [32]. The ultraviolet cutoff absorption edge of the BZBO:Pr3þ single crystal was reported to be about 350 nm [27]. The vibrational and some spectroscopic properties of BZBO:Pr3þ single crystals have been already reported [31]. This system is particularly interesting because it is characterized with the large values of nonlinear optical coefficients as well as the effective luminescence of excited Pr3þ ions, which make this system an excellent candidate for NIR to VIS laser converters [27]. In this work we report for the first time the result of structural
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and spectroscopic investigation of the BZBO:Pr3þ microcrystalline powder and the BZBO:Pr3þ/PMMA polymer composite. The synthesis of pure and Pr3þ-doped BZBO microcrystals and their XRD, HRTEM and spectroscopic characterization are presented. In particular the result of the Raman spectroscopy measurements, photoluminescence (PL) and photoluminescence excitation (PLE) spectra and the fluorescence decay profiles for BZBO:Pr3þ microcrystals and BZBO:Pr3þ/PMMA composites are presented and discussed. 2. Experiment 2.1. Synthesis of BZBO:Pr3þ powder Polycrystalline samples of pure and Pr3þ-doped BZBO phases were synthesized by means of modified Pechini method [33]. The starting materials were: Bi2O3 (ALFA, 99.999%), Pr6O11 (ALFA, 99.95%), ZnO (REACHIM, 99.5%), H3BO3 (STENMARK, 99.5%), C6H8O7$H2O (POCH, 99.4%, complexing agent) and C6H8(OH)6 (CHEMDREAMLAND, 99.5%, polymeric agent). First, the stoichiometric amounts of Bi, Zn metal precursors were dissolved in the minimal amount concentrated HNO3 in the presence of citric acid monohydrate (C6H8O7$H2O) under continuous heating (75 C) and stirring to complex the cations. In case of Pr-doped powder, the equivalent amounts of Pr6O11 were additionally dissolved in HNO3 solution. The overall molar ratio between metal ions and citric acid was set to be 1:3, respectively. When the homogeneous and clarified solution was obtained, an excess (10% molar) of boric acid (H3BO3) was charged into solution. This excess of boron source is needed due to volatilization of B2O3 at higher temperatures during the synthesis. After all the reagents were completely dissolved, the pH value of the solution was set to about 8 ÷ 9 using ammonia solution. Next, the proper amount of mannitol (C6H8(OH)6) was added into vessel to enable creation of rigid and volumetric polyester. Citric acid and mannitol were taken in the molar ratio 1:2, respectively. The temperature was then raised up to 90 C and kept under constant stirring for 2 h to initiate the polyesterification reaction. Then the solution was transferred into alumina crucible, put inside furnace and kept at 140 C for 20 h to evaporate the solvents. In consequence of polyesterification the brown foamy-like viscous gel was formed. The temperature was then raised to 350 C for 3 h to eliminate the residuals of solvents and to dry the gel until it turns into black resin. To start the pyrolysis process and burn out the organic part, the temperature was rapidly increased to 640 C. After 72 h the crucible content was crushed and grounded in agate mortar and very fine, white powders of undoped BZBO and doped BZBO:Pr3þ were obtained. The content of Pr3þ ions in BZBO:Pr3þ powder was equal to 2.5% at. 2.2. BZBO:Pr3þ/PMMA composite Polymeric matrix was prepared of methyl methacrylate (C5O2H8)n purchased from Sigma-Aldrich. In the first step methyl methacrylate was purified via distillation performed at 130 C in oil bath and inhibitor of polymerization (monomethyl ether hydroquinone - MEHQ) was removed. Appropriate amount of investigated powder were added to purified (C5O2H8)n. Polymerization process was initiated by addition of benzoyl peroxide (SigmaAldrich) to the pure MMA, 10 mg of benzoyl peroxide to 10 ml MMA were added. Polymerization process is described with the equation of chemical reaction showed in Fig. 1. Polymerization was performed at 80 C with mechanical shaking thus uniform samples of powders in PMMA matrix were obtained. The content of BZBO:Pr3þ microcrystals in PMMA was equal to 20% at.
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Fig. 1. Scheme of the polymerization process of PMMA.
2.3. Experimental methods Diffraction data patterns for BZBO and BZBO:Pr3þ(2.5%) were collected by means of the BRUKER D8 Discover diffractometer equipped with standard CuKa radiation (l ¼ 1.54056 Å) 40 kV voltage and 40 mA current in operating mode. The BraggeBrentano geometry diffraction for 2q range 15e65 was used, with step size 0.0151821, acquisition time 2 s per step. The XRD measurements were done at room temperature. High-resolution transmission electron microscopy (HRTEM, JEOL ARM 200F) analyses were performed with 200 kV acceleration voltage for BZBO and BZBO:Pr3þ powders. The samples were placed on a commercially available Lacey Carbon Cu grid (TEDPELLA INC 01824G). The Raman study was carried out in back-scattering geometry using a Renishaw. In Via Raman spectroscope equipped with a confocal DM 2500 Leica optical microscope, a CCD camera as a detector and Arþ ion laser as a source of exciting light working at the 514.5 nm wavelength. The unpolarized Raman spectra of BZBO Pr3þ powders and BZBO:Pr3þ/PMMA composites were recorded in single scan with a 100 s exposure time of the CCD detector. The position of the Raman peaks was calibrated before collecting data using a Si reference sample as an internal standard. The Raman spectra were recorded in the spectral range 70e1800 cm1. The instrumental resolution of the system was better than 2 cm1. Raman mapping technique was used for imaging the surface of composite samples. The spatial resolution of the system for the numerical aperture NA 0.75 of the objective 50 and used excitation wavelength 514.5 nm was approximately 0.9 mm. The luminescence emission spectra of the BZBO:Pr3þ powder, BZBO:Pr3þ/PMMA composite and PMMA matrix were measured under 456.1 and 320 nm excitation. The excitation spectra were detected for monitoring emission at 605.2 nm. The emission spectra excited in VIS, the excitation spectra and decay times were measured using an FLS980 fluorescence spectrometer from Edinburgh Instruments with monochromators in Czerny Turner configuration and a Hamamatsu 928 PMT as a detector. The emission spectra excited in UV were measure using Gilden Photonics FluoroSENS fluorometer. 3. Result and discussion 3.1. XRD analysis The phase uniformity and purity of undoped and Pr3þ-doped BZBO powders were confirmed by XRD measurements (Fig. 2). For both samples there was no evidence of secondary phases. Diffraction patterns are in agreement with ICDD PDF-2 database No. 01074-7878 (space group Pba2 (No. 32), Z ¼ 4). Lattice constants were calculated using FullPROF EdPCR ver. 2.00 program and are given as follows: for undoped BZBO they are equal to a ¼ 10.8212 Å, b ¼ 11.0268 Å, c ¼ 4.8871 Å (V ¼ 583.15 Å3) and for BZBO:Pr3þ(2.5%) it is given as a ¼ 10.8193 Å, b ¼ 11.0185 Å, c ¼ 4.8826 Å (V ¼ 582.06 Å3). As it is seen, when the BZBO is doped with mentioned content of Pr3þ ions (substituting the Bi3þ) the unit
cell shrinks. This effect can be explained according to the effective ionic radius of introduced Pr3þ dopant (0.99 Å) that is smaller than Bi3þ one (1.03 Å), what allows Pr3þ to be incorporated into Bi3þ positions [34]. As a result of Pr3þ-doping, the characteristic BZBO Bragg's lines are slightly shifted towards higher 2q values. The distinctive diffraction peaks in XRD pattern are also slightly broadened in case of BZBO:Pr3þ powder, what means the crystallite sizes of Pr3þ-doped BZBO are smaller than for the undoped BZBO sample. 3.2. HRTEM measurements The crystallographic structure, composition and shape of undoped and Pr3þ-doped BZBO powders were analyzed using HRTEM technique. Powders of pure (Fig. 3a) and Pr3þ-doped (Fig. 3b) samples showed a clear polycrystalline structure with overall size distribution of z3 mm. Moreover, HRTEM images showed that the polycrystalline particles are composed from monocrystalline nanoparticles with the size around 180 nm. In the inserts of Fig. 3 the crystallographic planes of the selected nanocrystals pure (Fig. 3a) and Pr3þ-doped BZBO (Fig. 3c) are visible. The determined interplanar distances are equal to 0.487 and 0.298 Å, respectively. Selected Area Electron Diffraction (SAED) patterns were collected from single nanocrystals of undoped (Fig. 3c) and Pr3þ-doped BZBO (Fig. 3d) powders. The highly polycrystalline image was treated with DiffTools of DigitalMicrograph™ [35] in order to enhance the general distribution of peaks and analyze the general distribution of planes observed, results are congruent with the XRD data. 3.3. Raman measurements The unpolarized Raman spectra of BZBO:Pr3þ crystalline powder, BZBO:Pr3þ/PMMA composite and PMMA matrix were recorded in the 70-1500 cm1 spectral range at room temperature (Fig. 4). The Raman bands related with internal and external vibrations of BO3, BO4, BiO6 and ZnO4 groups as well as Bi and Zn atoms were detected in the spectra of BZBO:Pr3þ powder and BZBO:Pr3þ/PMMA composite. The modes detected in the spectral range of 1500e700 cm1 correspond to the stretching vibrations of BeO bonds in BO3 and BO4 groups and BieO bonds in BiO6 octahedra. The most intensive mode in this spectral range is at 700 cm1 and modes at 738, 712 cm1 were assigned to the symmetric stretching vibrations of BO4 tetrahedra. Raman bands located at 1265 and 869 cm1 arise from the BieO stretching vibrations. The other Raman bands occurring in this spectral range can be assigned to triply degenerated and doubly degenerated stretching modes of BO4 and BO3 groups respectively. In spectral range of 660e423 cm1 the bending modes of BO3 and BO4 groups and vibrations of BieO and ZneO bonds in BiO6 and ZnO4 polyhedra were recorded. The highest intensity modes in this range were detected at 634 and 581 cm1. The Raman band at 634 cm1 corresponds to the bending vibration of BO3 group. The modes located at 590 and 581 cm1 belongs to the double structure band and can be assigned to the stretching vibrations of BieO bonds of BiO6 octahedra and ZneO bonds of ZnO4 tetrahedra, respectively. The low intensity Raman bands in the range 539e423 cm1 appear mainly from bending vibrations of BO4 tetrahedral and stretching vibrations of BieO bonds of BiO6 octahedra. The lattice modes assigned to translational and librational motions of BO3 and BO4 groups and Bi and Zn atoms are detected in the spectral range below 390 cm1. The highest intensity modes in this range are centered at 222, 155 and 139 cm1. The Raman bands centered at 350 and 390 cm1 are assigned to translational motions of BO3 and BO4 groups, while modes appearing in 250e140 cm1 spectral range can be assigned
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Fig. 2. XRD patterns of as-synthesized BZBO and BZBO:Pr3þ(2.5%) powders.
Fig. 4. Raman spectra of the BZBO:Pr3þ powder, BZBO:Pr3þ/PMMA composite and PMMA matrix at 300 K.
Fig. 3. HRTEM images and SAED patterns for undoped (a, b) and Pr powders (c, d).
3þ
-doped BZBO
mainly to the librational motions of these groups. The position of the bands in Raman spectra of BZBO:Pr3þ crystalline powders and the position most of Raman bands of composites correspond to the modes detected for BZBO:Pr3þ single crystal presented previously [31]. As it is seen from Fig. 4 the spectrum of PMMA matrix was also detected in the considered spectral range. The relatively low intensity mode located at 1736 cm1 can be assigned to the stretching mode n(C]O) of the CeCOO carbonyl group. The multimode band in the spectral range 1500e1435 cm1 corresponding to the
bending vibrations d(CeH) of CH2 and CH3 groups. In particular, the mode recorded at 1487 cm1 is associated with the bending vibration d(CeH) of CH2 group, subsequently modes at 1460 cm1 and 1448 cm1 are assigned to the asymmetric bending vibrations da(CeH) of OeCH3 and a-CH3 groups, respectively. In the 12481060 cm1 and 1000-965 cm1 spectral ranges several very weak bands arising from CeO, CeC and CeH vibrations of particular molecular groups occur. The highest intensity band of PMMA in the investigated range is located at 813 cm1 and corresponds to the symmetric stretching vibration of ns(CeCeO) group. The remaining Raman modes arise from vibrations of CeCOO ester group: the stretching modes n(CeCOO) at 600 and 490 cm1 and symmetric bending mode ds(CeCeO) at 370 cm1. The detailed assignment of Raman active modes of PMMA, investigated in the 300 to 3100 cm1 spectral range, has been already presented by other
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authors [36]. As it is seen from Fig. 4 in the Raman spectra of the BZBO:Pr3þ/ PMMA composite the bands assigned to the vibrations of BZBO:Pr3þ microcrystals as well as vibrations of PMMA matrix also occur. The bands derived from vibrations of mentioned above molecular groups of PMMA are located at 1736 cm1, in the spectral ranges: 1500-1435 cm1, 1248-1060 cm1 and 1000-965 cm1 and at 800 cm1. Moreover, in the BZBO:Pr3þ/PMMA spectra the band located at 600 cm1 originating from PMMA is overlapping with the band at 590 cm1 assigned to BieO vibrations of BiO6 octahedra in BZBO:Pr3þ microcrystals. To determine the distribution of the BZBO:Pr3þ microcrystals in the BZBO:Pr3þ/PMMA composite the Raman mapping technique was used. Optical microscope image (Fig. 5a) and corresponding Raman map (Fig. 5b) show particles distribution in the PMMA matrix. As it is seen the distribution of the microcrystals is locally homogeneous. Raman map of the specimen surface was acquired over an area of 50 mm 50 mm with the resolution 0.8 mm. During the Raman mapping process the spectra were recorded sequentially from the series of positions on the sample surface. In each position the Raman spectrum was detected in the 80e1050 cm1 spectral range. The Raman map presented in Fig. 5b consists of 3844 points and was created from the integral intensity of the selected 698 cm1 Raman band of BZBO:Pr3þ microcrystals. 3.4. The energy transfer in BZBO:Pr3þ system The energy of the electronic levels of Pr3þ ions in BZBO lattice is affected by the Bi3þ ions and influences the luminescence properties of this system. Basically, there are two types of emission transitions from Pr3þ ion that can occur under the high energetic excitation (UV or VUV): inter-configurational 4f5d / 4f2 or intraconfigurational 4f2 / 4f2 transitions. The position of the 4f5d states relative to the 1S0 state strongly depends on the host matrix. The broad band from the lowest 4f5d state located below 1S0 state, can be seen in UV region on the emission spectra. When the 1S0 state lies below the lowest 4f5d state, typically a narrow emission peak from 1S0 state may be observed [20]. The ground state of Bi3þ ion with 6s2 electronic configuration is expressed as 1S0. The 6s6p excited states consist of 1P1, 3P2, 3P1 and 3P0. The transition from 1P1 to the ground state generally occurs in the UV region, whereas the location of the emission band originating from 3P1 and 3P0 can be observed for the lower wavelengths (in UV or VIS range) depending on the host lattice. Although the transition from the ground state to the first excited level (1S0 / 3P0) of Bi3þ is strongly forbidden, the transitions from the 1S0 ground state to the 3P1, 3P2 and 1P1 states are observed in optical absorption spectra. There are many reports
available on the use of Bi3þ as a sensitizer for RE3þions [30]. If the energy level of the 3P1 and 3P0 states are located close to the energy compared to the excited level of the RE3þ ions, it allows an overlap between the orbitals for energy transfer from the RE3þ to the Bi3þ level. For BZBO:Pr3þ microcrystals the energy transfer from the Pr3þ(3P0) to the Bi3þ (3P1) ion results in the quenching of the greenish-blue luminescence [30]. Moreover, in the oxide-based host lattices doped with RE atoms the possible energy transfer by the CT mechanism/ligand-to-metal charge transfer (LMCT) may occur [19]. In the scheme of coordinate diagram (Fig. 6) the energy levels 1D2 and 3P0 of Pr3þ ions with additional CTS were shown. The possible non-radiative energy transfer and radiative transitions related to greenish-blue and red emission is proposed. Fig. 6a shows the possible non-radiative energy transfer from 3P0 level to CTS and then to the 1D2 resulting to the total or partial quenching of greenish-blue emission from 3 P0 of Pr3þ ion. If the CTS is located at higher energy than 3P0, the UV excitation relaxes to this state enhancing the greenish-blue emission from 3P0 (Fig. 6b). The most probable mechanism in the BZBO:Pr3þ system is partial quenching of greenish-blue emission from 3P0 level with enhance of red emission from 1D2 level to 3H4 ground state. LMCT involves promotion of an electron from the ligand atom (O2) to the unfilled 4f shell of Pr3þ ion, that changes its valence state: Pr3þ / Pr2þ þ h (h - the hole). The hole, although delocalized over the ligand, is bound to the RE2þ core, resulting in the creation of Pr2þ þ h charge transfer excited state. If the CTS is located close in energy to the 3P0 and 1D2 energy levels, fast intersystem crossing to 4f states of Pr3þ ions will occur and will be followed by red emission from 1D2 level. The energetic position of the LMCT state sct depends on the nature of the host lattice and can be calculated from the following equation:
h
i
sct ¼ 30000cm1 copt ðXÞ cuncorr ðMÞ
(1)
where copt(X) and cuncorr(M) are the optical electronegativity of the ligand and uncorrected optical electronegativity of the central ion [37]. In BZBO:Pr3þ crystal Pr3þ ion is located in octahedral coordination, with different values of inter-atomic distances between Pr3þ and each of six oxygen atoms, which are equal 2.168, 2.217, 2.297, 2.442, 2.548 and 2.669 Å respectively [31]. For O2 and Pr3þ, copt(O2) ¼ 3.1 [38] and cuncorr(Pr3þ) ¼ 1.96 [21]. It is evident that values of inter-atomic distances play crucial role in the orbital overlaps of ligand and active ion and therefore on the energy of the LMCT. It is clear that for different inter-atomic distances in sixcoordinated oxygen atoms the energetic position of LCTS will be different and band broadening will take place.
Fig. 5. Optical microscope image (a) and Raman map of the BZBO:Pr3þ/PMMA composite (b).
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oscillator strength f for transition between two manifold of any rare earth ion jA〉 and jB〉 can be given by the following expressions:
f ¼
Fig. 6. Scheme of coordinate diagram showing mechanism of total or partial quenching (a) and enhancement (b) of 3P0 emission of Pr3þ ions. The arrows show the possible non-radiative energy transfer and radiative transition related with greenishblue and red emission. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
3.5. Luminescence of BZBO:Pr3þ and BZBO:Pr3þ/PMMA In the luminescence emission spectra of BZBO:Pr3þ powder and BZBO:Pr3þ/PMMA composite excited with 451.6 nm wavelength (Fig. 7) mainly red emission from 1D2 and 3P0 levels is observed. The peaks in the emission spectra are assigned to the transition from the 3P0 level to the 3H4 (480 nm), 3H5 (580 nm), 3H6 (620 nm) and 3F2 (650 nm) levels and from the 1D2 level to the 3H4 one (595 nm). The weak emission in the greenish-blue range (3P0 / 3H4 transition) is due to the quenching of luminescence from 3P0 level caused mainly by non-radiative energy transfer from the Pr3þ (3PJ) to the Bi3þ (3P1) ion and by the 3P0 ~ 1D2 non-radiative relaxation of Pr3þ ion due to low-lying CTS. It is worthy to note, that emission from the BZBO:Pr3þ microcrystals embedded in the PMMA polymer matrix is stronger comparing to the powder. This effect was observed also in case of Eu3þ emission in (Eu3þ:YVO4) PMMA nanocomposite [6]. Increase of the emission intensity in the composite samples is related to the change of the refractive index of the medium surrounding the microcrystals: air for powder and PMMA for composite samples. This may be explained after taking into consideration the oscillator strength relation (1), expressing the probability of the transition between two manifold related with the new defined effective refractive index neff of the sample. The
Fig. 7. Emission spectra of BZBO:Pr3þ powder and BZBO:Pr3þ/PMMA composite upon 452 nm excitation at 300 K.
h i 4p2 8p2 mc 2 2 c þ c j〈AjPjB〉j j〈AjMjB〉j ED MD e2 3hl ð2J þ 1Þn2
(2)
Where cED is2Lorentz field corrections for electric dipole transitions: 2 cED ¼ nðn 9þ2Þ and cMD is magnetic dipole transitions: cMD ¼ n3 (for emission transition), m is the electron mass, c is the speed of light, e is the elementary charge, h is the Planck's constant, l is the wavelength of electromagnetic wave, n is refractive index of the sample, J is the total angular momentum, P is the electric dipole moment operator and M is the magnetic dipole moment operator [39]. On the other hand, following to the model of Meltzer [40] and taking into account relation expressing an effective refractive index:
neff ðxÞ ¼ xnmat þ ð1 xÞnmed
(3)
where neff is an effective refractive index, nmat is a refractive index of microcrystals, nmed is a refractive index of medium and x is the socalled filling factor the effective refractive index of powder neff and composite sample n0 eff can be calculated from the following formulas:
neff ðxÞ ¼ xnBZBO þ ð1 xÞnair
(4)
n0eff ðxÞ ¼ x0nBZBO þ ð1 x0ÞnPMMA :
(5)
For the powder and PMMA-based composite, n in Eq. (2) is replaced by neff or n0 eff respectively. The effective refractive index neff and n0 eff calculated from Eqs. (4) and (5) are equal to 1.15 and 1.64 for powder and composite samples, respectively. The filling factors used in the calculations were equal to 0.5 for powder and 0.2 for composite. The refractive index of BZBO powder was determined as the average value of the three principal refractive indices nx ¼ 2.13, nx ¼ 2.17, nx ¼ 2.23, calculated for wavelengths 450 nm from the Sellmeier equations [23] and is equal to nBZBO ¼ 2.18. Finally, it is clear that changing of the refraction index from 1 (for air) for powder to about 1.5 (for PMMA) [41] for composite sample strongly influences the f-f transitions. The emission spectra of BZBO:Pr3þ powder and BZBO:Pr3þ/ PMMA composite and PMMA polymer matrix under 320 nm excitation are shown in Fig. 8.
Fig. 8. Emission spectra of BZBO:Pr3þ powder and BZBO:Pr3þ/PMMA composite upon 320 nm excitation at 300 K.
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As can be seen in Fig. 8 in the emission spectrum of BZBO:Pr3þ powder excited in UV the broad fluorescence band in the range 350e450 nm with maximum at 368 nm was detected. This emission band arises from the 1P1/1S0 transition of Bi3þ ions and 4f5d / 4f2 transition of Pr3þ ions. The emission spectrum of BZBO:Pr3þ/PMMA composite contains the broad emission band in the range 350e575 nm with the maximum at 375 nm. This band is a cooperative emission of the PMMA matrix and BZBO:Pr3þ powder. Moreover, in the BZBO:Pr3þ powder and BZBO:Pr3þ/PMMA composite spectra the very intensive feature in the range 590e660 nm (red emission) assigned to the 1D2 / 3H4 and 3P0 / 3H6 transitions and the weak bands assigned to 3P0 / 3H4, 3P0 / 3H5, 3P0 / 3F2 and 3P0 / 3F3 transitions occur. All detected bands in spectra excited in UV correspond to the emission bands in the spectra excited with 451.6 nm (Fig. 7) with expectation the high energy fluorescence band at 368 nm. It was observed the increase of the red emission intensity in BZBO:Pr3þ powder and BZBO:Pr3þ/PMMA composite samples under UV excitation compared to intensity of emission under blue excitation. This effect is connected with quenching of greenish-blue 3P0 emission due to the 3P0 ~ 1D2 nonradiative relaxation of Pr3þ ions with CTS assist. Also in luminescence excitation spectra it can be observed that f-f transitions are more intense for composite sample (Fig. 9). Fig. 9 shows the excitation spectra monitored at 605.2 nm for the BZBO:Pr3þ powder and BZBO:Pr3þ/PMMA composite, recorded in spectral range 250e550 nm. A broad bands located in the range 285e350 nm (3508728571 cm1) is the charge-transfer band (CTB) and can be assigned to ligand-to-Pr3þ charge-transfer (LMCT). The energetic position of the LMCT state sct calculated from the Eq. (1) is equal to 34200 cm1 (292 nm) that correspond to the band (285e350 nm) detected in the excitation spectra. It should be noted, that no band was detected in this spectral range (285e350 nm) in the excitation spectra of undoped BZBO sample, so we can conclude that the detected CTB for BZBO:Pr3þ originates from CTS of Pr3þ ions. Moreover, in the spectra we observe the bands characteristic for Pr3þ ions emission: at 450 nm (3H4 / 3P2 transition), 475 nm (3H4 / 3P1 transition) and at 485 nm (3H4 / 3P0 transition). As can be also seen in Fig. 8 the spectra of the BZBO:Pr3þ and BZBO:Pr3þ/ PMMA composite are almost identical, however the intensity all of detected bands are evidently higher for BZBO:Pr3þ/PMMA composite in comparison with the powder sample.
Fig. 10. Decay curves monitored for 1D2 level collected for BZBO:Pr3þ powder and BZBO:Pr3þ/PMMA composite.
3.6. Decay kinetics The decay times measurements of 1D2 / 3H4 transition (595 nm emission) of Pr3þ ions in BZBO:Pr3þ powder and BZBO:Pr3þ/PMMA composite were performed using pulsed excitation at 451.6 nm wavelength. Fig. 10 shows the result of lifetime measurements. The photoluminescence decay curves of Pr3þ ions for both samples have non-exponential character and were approximated as sum of two exponential functions [27]. The decay of the 1D2 level is governed by three processes: radiative decay (1D2 / 3H4), multiphonon relaxation (1D2 / 1G2), and energy transfer via a cross relaxation (1D2 / 3H4/3H4 / 3F3,4). Non-exponential character of the decays is mainly the effect of cross-relaxation and luminescence quenching. The process becomes more probable as the concentration of Pr3þ increases and because here concentration was low the curves are not single exponential. The determined lifetimes of the red emission at 595 nm (1D2 / 3H4 transition) are equal to t1 ¼ 0.57 ms and t2 ¼ 2.50 ms for powder sample and t1 ¼ 0.69 ms and t2 ¼ 2.69 ms for composite sample. Decrease of the decay times observed for composite sample is mainly due to surface effects (OH groups luminescence quenching, change of refractive index in the grain boundaries, defects etc.) and structural distortions present in the microcrystals that are generally much stronger for smaller grains compared to the single crystals. 4. Conclusions
Fig. 9. Excitation spectra of BZBO:Pr3þ powder and BZBO:Pr3þ/PMMA composite at 300 K.
The synthesis of undoped and Pr3þ- doped BZBO microcrystalline powders and BZBO:Pr3þ/PMMA polymer composite as well as study of the structure and spectroscopic properties by XRD, HRTEM, Raman and photoluminescence spectroscopy are reported. XRD analysis confirmed the phase uniformity and purity of undoped and Pr3þ-doped BZBO powders. From HRTEM measurements it was found a clear polycrystalline structure of pure and Pr3þ-doped BZBO powders, with overall size distribution of z3 mm. Moreover HRTEM images showed that the polycrystalline particles are composed of single-crystalline nanoparticles with sizes around 180 nm. The vibrational properties of BZBO:Pr3þ powder, PMMA and BZBO:Pr3þ/PMMA composite were investigated for the first time by Raman spectroscopy. Both, powder as well the composite show enhancement of red emission (1D2 / 3H4 and 3P0 / 3H6 transitions) of Pr3þ ions after excitation in UV region in comparison
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to emission excited in VIS (blue excitation). This red emission enhancement may be caused by quenching of luminescence from 3 P0 level due to non-radiative de-excitation through Pr3þeBi3þ energy transfer and the 3P0 ~ 1D2 non-radiative relaxation of Pr3þ ion by low-lying CTS. The results show that BZBO:Pr3þ/PMMA polymer composite is a very promising candidate for new redemitting phosphors. Acknowledgements This work was supported by the Research Project of the Polish Ministry of Sciences and Higher Education: 06/65/DSPB/0516. DK and KJ thanks for collaboration to Prof. S. Jurga from Nano. BioMedical Centre Adam Mickiewicz University in Poznan References [1] D. Cotter, M.G. Burt, R.J. Manning, Phys. Rev. Lett. 68 (1992) 1200e1203. [2] H. Liu, L. Yu, W. Chen, Y. Li, J. Nanomater. 2012 (2012) 1e9. [3] V. Sreeramulu, Krishna Kanta Haldar, Amitava Patra, D. Narayana Rao, J. Phys. Chem. C 118 (2014) 30333e30341. [4] B. Karmakar, T. Som, S.P. Singh, M. Nath, Trans. Indian Ceram. Soc. 69 (2010) 1e16. [5] G. Blasse, J. Alloys Compd. 192 (1993) 17e21. [6] D. Hreniak, J. Doskocz, P. Głuchowski, R. Lisiecki, W. Stre˛ k, N. Vu, D.X. Loc, T.K. Anh, M. Bettinelli, A. Speghini, J. Lumin. 131 (2011) 473e476. [7] A. Majchrowski, A. Wojciechowski, L.R. Jaroszewicz, M. Chrunik, A. Fedorchuk, B. Sahraoui, I.V. Kityk, J. Mater. Sci. Mater. Electron. 25 (2014) 2426e2434. [8] A. Brenier, D. Jaque, A. Majchrowski, Opt. Mater. 28 (2006) 310e323. [9] A.A. Kaminskii, Crystalline Lasers: Physical Processes and Operating Schemes, CRC, Boca Raton, 1996. €, J. Alloys Compd. 275e277 (1998) 702e708. [10] M. Leskela [11] J. Pejchal, K. Fukuda, S. Kurosawa, Y. Yokota, A. Yoshikawa, Opt. Mater. 41 (2015) 58e62. [12] D. Giaume, M. Poggi, D. Casanova, G. Mialon, K. Lahlil, A. Alexandrou, T. Gacoin, J.-P. Boilot, Langmuir 24 (2008) 11018e11026. [13] R.T. Wegh, H. Donker, K.D. Oskam, A. Meijerink, Science 283 (1999) 663e666. [14] V.K. Rai, Opt. Mater. 29 (2007) 873e878.
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Photoluminescence properties of Pr3þ doped Bi2ZnOB2O6 microcrystals and PMMA-based composites K. Jaroszewski a, M. Chrunik b, P. Głuchowski c, E. Coy d, B. Maciejewska d, e, R. Jastrzab f, A. Majchrowski b, D. Kasprowicz a, * , Poland Faculty of Technical Physics, Poznan University of Technology, Piotrowo 3, 60-965 Poznan Institute of Applied Physics, Military University of Technology, Kaliskiego 2, 00-908 Warsaw, Poland c lna 2, 50-422 Wroclaw, Poland Institute of Low Temperature and Structure Research of Polish Academy of Sciences, Oko d , Poland NanoBioMedical Centre, Adam Mickiewicz University, Umultowska 85, 61-614 Poznan e , Poland Faculty of Physics, Adam Mickiewicz University, Umultowska 85, 61-614 Poznan f , Poland Faculty of Chemistry, Adam Mickiewicz University, Umultowska 89b, 61-614 Poznan a
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 August 2016 Accepted 19 September 2016
Photoluminescence properties of red-emitting Pr3þ-doped Bi2ZnOB2O6 microcrystalline powder and PMMA-based composite materials were reported. Bi2ZnOB2O6:Pr3þ powders were synthesized by means of the modified Pechini method. The morphology and crystallographic structure of Bi2ZnOB2O6:Pr3þ microcrystals were investigated by XRD and HRTEM. The PMMA-based composite materials were prepared by embedding of Bi2ZnOB2O6:Pr3þ powder in the PMMA matrix. The vibrational properties of the powder and composite systems were investigated by m-Raman spectroscopy. Emission spectra of the samples were measured under blue (451.6 nm) and UV (320 nm) excitation. Both, the powder and composite samples show enhancement of red emission (1D2 / 3H4) and quenching of greenish-blue luminescence from 3P0 level of Pr3þ ions after excitation in UV and VIS caused by the 3P0 ~ 1D2 nonradiative relaxation of Pr3þ ion by low-lying charge transfer state and non-radiative de-excitation through Pr3þeBi3þ energy transfer. The response of the powder and composite samples to pulsed excitation at 451.6 nm was measured by monitoring emission from the 1D2 level (595 nm/3P0 / 3H6 transition). The determined lifetimes t1 and t2 of the red emission at 595 nm (1D2 / 3H4 transition) of the composite are significantly longer in comparison to powder samples. Moreover, because of the good nonlinear optical properties of the Bi2ZnOB2O6 crystals and effective luminescence of the Pr3þ-doped Bi2ZnOB2O6 powders and composites, they can be very useful as bi-functional materials in the new generation of optoelectronic devices. © 2016 Elsevier B.V. All rights reserved.
Keywords: Bi2ZnOB2O6 Praseodymium Microcrystals Polymer composites Raman spectroscopy Photoluminescence
1. Introduction Significant progress in the research of the luminescent and nonlinear optical properties of nano- and microcrystals is the result of intense investigations of new multifunctional systems [1,2]. There is still the demand for novel, compact, efficient and low cost materials for many optoelectronic applications including optical switching devices [3], integrated optical circuits [4] as well as widely used light emitters [5]. Among various new materials the polymer-based composites containing optically active and
* Corresponding author. E-mail address: [email protected] (D. Kasprowicz). http://dx.doi.org/10.1016/j.optmat.2016.09.059 0925-3467/© 2016 Elsevier B.V. All rights reserved.
nonlinear optical (NLO) nano- or microcrystals have very desirable features, which make them a good alternative for presently used materials [6,7]. In some bi-functional materials the luminescence effect of optically active ions and NLO phenomena occur in the same host [8]. The polymer-based composite systems obtained on the basis of such a bi-functional materials present an excellent light propagation properties as a result of the polymer matrix and microor nanoparticles optical properties, which makes them very promising for new optoelectronic applications. Among various optically active materials rare earth (RE) doped systems arise great interest because of their unique luminescence properties mainly due to sharp and narrow f-f transitions on the manifold energy for which shifting of the band position depends slightly on the host matrix. On the basis of these properties such
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materials can be used for a variety of applications such as lasers and optical amplifiers [9], multicolor displays [10], scintillators [11], biological labeling [12] and optical frequency converters [13]. The triply ionized praseodymium ions (Pr3þ) were used as an activator in a variety of host materials useful in many phosphors [14,15] for different types of light emitting devices. In particular, the luminescence properties of Pr3þdoped materials were extensively investigated for many potential application including fiber optical communication [16], LEDs [17], field-emission display devices and photoluminescence devices [18]. Depending on the concentration of Pr3þ ions and the host matrix properties the Pr3þ ions can emit in the visible (VIS) and the near infrared (NIR) spectral range [19]. Pr3þ ions possess the specific electronic energy level structure with several optical transitions in VIS spectral range. The 4f2 electronic configuration of Pr3þ ions consists of: 3H4,5,6, 3F4,3,2, 1G4, 1D2, 1I6, 3 P2,1,0 and 1S0 electronic energy levels. The greenish-blue and red emission of the Pr3þ ions originates from excited 3P0 and 1D2 levels upon the UV or VIS excitation [20]. In many oxide-based host materials, the Pr3þ ions exhibit a prominent red luminescence from 1D2 level under blue or UV excitation accompanied by a total or partial quenching of the emission from the 3P0 level. Several non-radiative pathways were proposed to explain quenching of 3P0 luminescence of Pr3þ ion in oxide-based lattices: multiphonon relaxation, cross-relaxation, intersystem crossing through low-lying 4f15d1 levels, energy transfer between Pr3þ and metal ions as well as charge transfer (CT) mechanism [21]. The CT between active ions and the host lattice results in the formation of the charge transfer state CTS. The energetic position of the CTS depends on the nature of the host lattice and for the oxide-based crystals is typically located around excited states of rare earths. Partial or total quenching of 3P0 emission of Pr3þ ions depend on the relative energetic position of the CTS with respect to the 3P0 and 1D2 levels. The possible CT mechanism, very probable for oxide-based host lattices is ligand-to-metal charge transfer (LMCT) [19]. The LMCT state was previously reported for BZBO:Eu3þ phosphors [22]. Recently new RE-doped Bi2ZnOB2O6 materials (abbreviated as BZBO) in the form of single crystals [23], powders [23] as well as glasses [24,25] and polymer-based composites [7] are gathering important attention because of their promising optical properties and consequently numerous potential applications. BZBO crystals are also known as effective nonlinear optical materials [26], which possess high laser damage tolerance. Moreover, they may be efficiently doped with rare earth ions resulting in effective luminescence [22,27e29]. Depending on the host material, the structure of the energetic levels of the Bi3þ ions may change. Particularly, for oxide-based lattices the structure of electronic levels of Bi3þ ions strongly depends on configuration of the nearest oxygen surrounding of Bi3þ ions [30]. In the BZBO structure the Bi3þ ions are located at 4c positions (C1 symmetry) and occur in octahedral coordination BiO6 being six-coordinated by the oxygen atoms forming the BiO6 octahedra [31] and the lowering of the excited states energy is typically observed in comparison to the structure of electronic levels of isolated Bi3þ ions [30]. The BZBO is an optically positive biaxial optical crystal [40]. The experimentally determined band gap of BZBO single crystal is equal to 3.444 eV (27777.72 cm1) [32]. The ultraviolet cutoff absorption edge of the BZBO:Pr3þ single crystal was reported to be about 350 nm [27]. The vibrational and some spectroscopic properties of BZBO:Pr3þ single crystals have been already reported [31]. This system is particularly interesting because it is characterized with the large values of nonlinear optical coefficients as well as the effective luminescence of excited Pr3þ ions, which make this system an excellent candidate for NIR to VIS laser converters [27]. In this work we report for the first time the result of structural
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and spectroscopic investigation of the BZBO:Pr3þ microcrystalline powder and the BZBO:Pr3þ/PMMA polymer composite. The synthesis of pure and Pr3þ-doped BZBO microcrystals and their XRD, HRTEM and spectroscopic characterization are presented. In particular the result of the Raman spectroscopy measurements, photoluminescence (PL) and photoluminescence excitation (PLE) spectra and the fluorescence decay profiles for BZBO:Pr3þ microcrystals and BZBO:Pr3þ/PMMA composites are presented and discussed. 2. Experiment 2.1. Synthesis of BZBO:Pr3þ powder Polycrystalline samples of pure and Pr3þ-doped BZBO phases were synthesized by means of modified Pechini method [33]. The starting materials were: Bi2O3 (ALFA, 99.999%), Pr6O11 (ALFA, 99.95%), ZnO (REACHIM, 99.5%), H3BO3 (STENMARK, 99.5%), C6H8O7$H2O (POCH, 99.4%, complexing agent) and C6H8(OH)6 (CHEMDREAMLAND, 99.5%, polymeric agent). First, the stoichiometric amounts of Bi, Zn metal precursors were dissolved in the minimal amount concentrated HNO3 in the presence of citric acid monohydrate (C6H8O7$H2O) under continuous heating (75 C) and stirring to complex the cations. In case of Pr-doped powder, the equivalent amounts of Pr6O11 were additionally dissolved in HNO3 solution. The overall molar ratio between metal ions and citric acid was set to be 1:3, respectively. When the homogeneous and clarified solution was obtained, an excess (10% molar) of boric acid (H3BO3) was charged into solution. This excess of boron source is needed due to volatilization of B2O3 at higher temperatures during the synthesis. After all the reagents were completely dissolved, the pH value of the solution was set to about 8 ÷ 9 using ammonia solution. Next, the proper amount of mannitol (C6H8(OH)6) was added into vessel to enable creation of rigid and volumetric polyester. Citric acid and mannitol were taken in the molar ratio 1:2, respectively. The temperature was then raised up to 90 C and kept under constant stirring for 2 h to initiate the polyesterification reaction. Then the solution was transferred into alumina crucible, put inside furnace and kept at 140 C for 20 h to evaporate the solvents. In consequence of polyesterification the brown foamy-like viscous gel was formed. The temperature was then raised to 350 C for 3 h to eliminate the residuals of solvents and to dry the gel until it turns into black resin. To start the pyrolysis process and burn out the organic part, the temperature was rapidly increased to 640 C. After 72 h the crucible content was crushed and grounded in agate mortar and very fine, white powders of undoped BZBO and doped BZBO:Pr3þ were obtained. The content of Pr3þ ions in BZBO:Pr3þ powder was equal to 2.5% at. 2.2. BZBO:Pr3þ/PMMA composite Polymeric matrix was prepared of methyl methacrylate (C5O2H8)n purchased from Sigma-Aldrich. In the first step methyl methacrylate was purified via distillation performed at 130 C in oil bath and inhibitor of polymerization (monomethyl ether hydroquinone - MEHQ) was removed. Appropriate amount of investigated powder were added to purified (C5O2H8)n. Polymerization process was initiated by addition of benzoyl peroxide (SigmaAldrich) to the pure MMA, 10 mg of benzoyl peroxide to 10 ml MMA were added. Polymerization process is described with the equation of chemical reaction showed in Fig. 1. Polymerization was performed at 80 C with mechanical shaking thus uniform samples of powders in PMMA matrix were obtained. The content of BZBO:Pr3þ microcrystals in PMMA was equal to 20% at.
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Fig. 1. Scheme of the polymerization process of PMMA.
2.3. Experimental methods Diffraction data patterns for BZBO and BZBO:Pr3þ(2.5%) were collected by means of the BRUKER D8 Discover diffractometer equipped with standard CuKa radiation (l ¼ 1.54056 Å) 40 kV voltage and 40 mA current in operating mode. The BraggeBrentano geometry diffraction for 2q range 15e65 was used, with step size 0.0151821, acquisition time 2 s per step. The XRD measurements were done at room temperature. High-resolution transmission electron microscopy (HRTEM, JEOL ARM 200F) analyses were performed with 200 kV acceleration voltage for BZBO and BZBO:Pr3þ powders. The samples were placed on a commercially available Lacey Carbon Cu grid (TEDPELLA INC 01824G). The Raman study was carried out in back-scattering geometry using a Renishaw. In Via Raman spectroscope equipped with a confocal DM 2500 Leica optical microscope, a CCD camera as a detector and Arþ ion laser as a source of exciting light working at the 514.5 nm wavelength. The unpolarized Raman spectra of BZBO Pr3þ powders and BZBO:Pr3þ/PMMA composites were recorded in single scan with a 100 s exposure time of the CCD detector. The position of the Raman peaks was calibrated before collecting data using a Si reference sample as an internal standard. The Raman spectra were recorded in the spectral range 70e1800 cm1. The instrumental resolution of the system was better than 2 cm1. Raman mapping technique was used for imaging the surface of composite samples. The spatial resolution of the system for the numerical aperture NA 0.75 of the objective 50 and used excitation wavelength 514.5 nm was approximately 0.9 mm. The luminescence emission spectra of the BZBO:Pr3þ powder, BZBO:Pr3þ/PMMA composite and PMMA matrix were measured under 456.1 and 320 nm excitation. The excitation spectra were detected for monitoring emission at 605.2 nm. The emission spectra excited in VIS, the excitation spectra and decay times were measured using an FLS980 fluorescence spectrometer from Edinburgh Instruments with monochromators in Czerny Turner configuration and a Hamamatsu 928 PMT as a detector. The emission spectra excited in UV were measure using Gilden Photonics FluoroSENS fluorometer. 3. Result and discussion 3.1. XRD analysis The phase uniformity and purity of undoped and Pr3þ-doped BZBO powders were confirmed by XRD measurements (Fig. 2). For both samples there was no evidence of secondary phases. Diffraction patterns are in agreement with ICDD PDF-2 database No. 01074-7878 (space group Pba2 (No. 32), Z ¼ 4). Lattice constants were calculated using FullPROF EdPCR ver. 2.00 program and are given as follows: for undoped BZBO they are equal to a ¼ 10.8212 Å, b ¼ 11.0268 Å, c ¼ 4.8871 Å (V ¼ 583.15 Å3) and for BZBO:Pr3þ(2.5%) it is given as a ¼ 10.8193 Å, b ¼ 11.0185 Å, c ¼ 4.8826 Å (V ¼ 582.06 Å3). As it is seen, when the BZBO is doped with mentioned content of Pr3þ ions (substituting the Bi3þ) the unit
cell shrinks. This effect can be explained according to the effective ionic radius of introduced Pr3þ dopant (0.99 Å) that is smaller than Bi3þ one (1.03 Å), what allows Pr3þ to be incorporated into Bi3þ positions [34]. As a result of Pr3þ-doping, the characteristic BZBO Bragg's lines are slightly shifted towards higher 2q values. The distinctive diffraction peaks in XRD pattern are also slightly broadened in case of BZBO:Pr3þ powder, what means the crystallite sizes of Pr3þ-doped BZBO are smaller than for the undoped BZBO sample. 3.2. HRTEM measurements The crystallographic structure, composition and shape of undoped and Pr3þ-doped BZBO powders were analyzed using HRTEM technique. Powders of pure (Fig. 3a) and Pr3þ-doped (Fig. 3b) samples showed a clear polycrystalline structure with overall size distribution of z3 mm. Moreover, HRTEM images showed that the polycrystalline particles are composed from monocrystalline nanoparticles with the size around 180 nm. In the inserts of Fig. 3 the crystallographic planes of the selected nanocrystals pure (Fig. 3a) and Pr3þ-doped BZBO (Fig. 3c) are visible. The determined interplanar distances are equal to 0.487 and 0.298 Å, respectively. Selected Area Electron Diffraction (SAED) patterns were collected from single nanocrystals of undoped (Fig. 3c) and Pr3þ-doped BZBO (Fig. 3d) powders. The highly polycrystalline image was treated with DiffTools of DigitalMicrograph™ [35] in order to enhance the general distribution of peaks and analyze the general distribution of planes observed, results are congruent with the XRD data. 3.3. Raman measurements The unpolarized Raman spectra of BZBO:Pr3þ crystalline powder, BZBO:Pr3þ/PMMA composite and PMMA matrix were recorded in the 70-1500 cm1 spectral range at room temperature (Fig. 4). The Raman bands related with internal and external vibrations of BO3, BO4, BiO6 and ZnO4 groups as well as Bi and Zn atoms were detected in the spectra of BZBO:Pr3þ powder and BZBO:Pr3þ/PMMA composite. The modes detected in the spectral range of 1500e700 cm1 correspond to the stretching vibrations of BeO bonds in BO3 and BO4 groups and BieO bonds in BiO6 octahedra. The most intensive mode in this spectral range is at 700 cm1 and modes at 738, 712 cm1 were assigned to the symmetric stretching vibrations of BO4 tetrahedra. Raman bands located at 1265 and 869 cm1 arise from the BieO stretching vibrations. The other Raman bands occurring in this spectral range can be assigned to triply degenerated and doubly degenerated stretching modes of BO4 and BO3 groups respectively. In spectral range of 660e423 cm1 the bending modes of BO3 and BO4 groups and vibrations of BieO and ZneO bonds in BiO6 and ZnO4 polyhedra were recorded. The highest intensity modes in this range were detected at 634 and 581 cm1. The Raman band at 634 cm1 corresponds to the bending vibration of BO3 group. The modes located at 590 and 581 cm1 belongs to the double structure band and can be assigned to the stretching vibrations of BieO bonds of BiO6 octahedra and ZneO bonds of ZnO4 tetrahedra, respectively. The low intensity Raman bands in the range 539e423 cm1 appear mainly from bending vibrations of BO4 tetrahedral and stretching vibrations of BieO bonds of BiO6 octahedra. The lattice modes assigned to translational and librational motions of BO3 and BO4 groups and Bi and Zn atoms are detected in the spectral range below 390 cm1. The highest intensity modes in this range are centered at 222, 155 and 139 cm1. The Raman bands centered at 350 and 390 cm1 are assigned to translational motions of BO3 and BO4 groups, while modes appearing in 250e140 cm1 spectral range can be assigned
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Fig. 2. XRD patterns of as-synthesized BZBO and BZBO:Pr3þ(2.5%) powders.
Fig. 4. Raman spectra of the BZBO:Pr3þ powder, BZBO:Pr3þ/PMMA composite and PMMA matrix at 300 K.
Fig. 3. HRTEM images and SAED patterns for undoped (a, b) and Pr powders (c, d).
3þ
-doped BZBO
mainly to the librational motions of these groups. The position of the bands in Raman spectra of BZBO:Pr3þ crystalline powders and the position most of Raman bands of composites correspond to the modes detected for BZBO:Pr3þ single crystal presented previously [31]. As it is seen from Fig. 4 the spectrum of PMMA matrix was also detected in the considered spectral range. The relatively low intensity mode located at 1736 cm1 can be assigned to the stretching mode n(C]O) of the CeCOO carbonyl group. The multimode band in the spectral range 1500e1435 cm1 corresponding to the
bending vibrations d(CeH) of CH2 and CH3 groups. In particular, the mode recorded at 1487 cm1 is associated with the bending vibration d(CeH) of CH2 group, subsequently modes at 1460 cm1 and 1448 cm1 are assigned to the asymmetric bending vibrations da(CeH) of OeCH3 and a-CH3 groups, respectively. In the 12481060 cm1 and 1000-965 cm1 spectral ranges several very weak bands arising from CeO, CeC and CeH vibrations of particular molecular groups occur. The highest intensity band of PMMA in the investigated range is located at 813 cm1 and corresponds to the symmetric stretching vibration of ns(CeCeO) group. The remaining Raman modes arise from vibrations of CeCOO ester group: the stretching modes n(CeCOO) at 600 and 490 cm1 and symmetric bending mode ds(CeCeO) at 370 cm1. The detailed assignment of Raman active modes of PMMA, investigated in the 300 to 3100 cm1 spectral range, has been already presented by other
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authors [36]. As it is seen from Fig. 4 in the Raman spectra of the BZBO:Pr3þ/ PMMA composite the bands assigned to the vibrations of BZBO:Pr3þ microcrystals as well as vibrations of PMMA matrix also occur. The bands derived from vibrations of mentioned above molecular groups of PMMA are located at 1736 cm1, in the spectral ranges: 1500-1435 cm1, 1248-1060 cm1 and 1000-965 cm1 and at 800 cm1. Moreover, in the BZBO:Pr3þ/PMMA spectra the band located at 600 cm1 originating from PMMA is overlapping with the band at 590 cm1 assigned to BieO vibrations of BiO6 octahedra in BZBO:Pr3þ microcrystals. To determine the distribution of the BZBO:Pr3þ microcrystals in the BZBO:Pr3þ/PMMA composite the Raman mapping technique was used. Optical microscope image (Fig. 5a) and corresponding Raman map (Fig. 5b) show particles distribution in the PMMA matrix. As it is seen the distribution of the microcrystals is locally homogeneous. Raman map of the specimen surface was acquired over an area of 50 mm 50 mm with the resolution 0.8 mm. During the Raman mapping process the spectra were recorded sequentially from the series of positions on the sample surface. In each position the Raman spectrum was detected in the 80e1050 cm1 spectral range. The Raman map presented in Fig. 5b consists of 3844 points and was created from the integral intensity of the selected 698 cm1 Raman band of BZBO:Pr3þ microcrystals. 3.4. The energy transfer in BZBO:Pr3þ system The energy of the electronic levels of Pr3þ ions in BZBO lattice is affected by the Bi3þ ions and influences the luminescence properties of this system. Basically, there are two types of emission transitions from Pr3þ ion that can occur under the high energetic excitation (UV or VUV): inter-configurational 4f5d / 4f2 or intraconfigurational 4f2 / 4f2 transitions. The position of the 4f5d states relative to the 1S0 state strongly depends on the host matrix. The broad band from the lowest 4f5d state located below 1S0 state, can be seen in UV region on the emission spectra. When the 1S0 state lies below the lowest 4f5d state, typically a narrow emission peak from 1S0 state may be observed [20]. The ground state of Bi3þ ion with 6s2 electronic configuration is expressed as 1S0. The 6s6p excited states consist of 1P1, 3P2, 3P1 and 3P0. The transition from 1P1 to the ground state generally occurs in the UV region, whereas the location of the emission band originating from 3P1 and 3P0 can be observed for the lower wavelengths (in UV or VIS range) depending on the host lattice. Although the transition from the ground state to the first excited level (1S0 / 3P0) of Bi3þ is strongly forbidden, the transitions from the 1S0 ground state to the 3P1, 3P2 and 1P1 states are observed in optical absorption spectra. There are many reports
available on the use of Bi3þ as a sensitizer for RE3þions [30]. If the energy level of the 3P1 and 3P0 states are located close to the energy compared to the excited level of the RE3þ ions, it allows an overlap between the orbitals for energy transfer from the RE3þ to the Bi3þ level. For BZBO:Pr3þ microcrystals the energy transfer from the Pr3þ(3P0) to the Bi3þ (3P1) ion results in the quenching of the greenish-blue luminescence [30]. Moreover, in the oxide-based host lattices doped with RE atoms the possible energy transfer by the CT mechanism/ligand-to-metal charge transfer (LMCT) may occur [19]. In the scheme of coordinate diagram (Fig. 6) the energy levels 1D2 and 3P0 of Pr3þ ions with additional CTS were shown. The possible non-radiative energy transfer and radiative transitions related to greenish-blue and red emission is proposed. Fig. 6a shows the possible non-radiative energy transfer from 3P0 level to CTS and then to the 1D2 resulting to the total or partial quenching of greenish-blue emission from 3 P0 of Pr3þ ion. If the CTS is located at higher energy than 3P0, the UV excitation relaxes to this state enhancing the greenish-blue emission from 3P0 (Fig. 6b). The most probable mechanism in the BZBO:Pr3þ system is partial quenching of greenish-blue emission from 3P0 level with enhance of red emission from 1D2 level to 3H4 ground state. LMCT involves promotion of an electron from the ligand atom (O2) to the unfilled 4f shell of Pr3þ ion, that changes its valence state: Pr3þ / Pr2þ þ h (h - the hole). The hole, although delocalized over the ligand, is bound to the RE2þ core, resulting in the creation of Pr2þ þ h charge transfer excited state. If the CTS is located close in energy to the 3P0 and 1D2 energy levels, fast intersystem crossing to 4f states of Pr3þ ions will occur and will be followed by red emission from 1D2 level. The energetic position of the LMCT state sct depends on the nature of the host lattice and can be calculated from the following equation:
h
i
sct ¼ 30000cm1 copt ðXÞ cuncorr ðMÞ
(1)
where copt(X) and cuncorr(M) are the optical electronegativity of the ligand and uncorrected optical electronegativity of the central ion [37]. In BZBO:Pr3þ crystal Pr3þ ion is located in octahedral coordination, with different values of inter-atomic distances between Pr3þ and each of six oxygen atoms, which are equal 2.168, 2.217, 2.297, 2.442, 2.548 and 2.669 Å respectively [31]. For O2 and Pr3þ, copt(O2) ¼ 3.1 [38] and cuncorr(Pr3þ) ¼ 1.96 [21]. It is evident that values of inter-atomic distances play crucial role in the orbital overlaps of ligand and active ion and therefore on the energy of the LMCT. It is clear that for different inter-atomic distances in sixcoordinated oxygen atoms the energetic position of LCTS will be different and band broadening will take place.
Fig. 5. Optical microscope image (a) and Raman map of the BZBO:Pr3þ/PMMA composite (b).
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77
oscillator strength f for transition between two manifold of any rare earth ion jA〉 and jB〉 can be given by the following expressions:
f ¼
Fig. 6. Scheme of coordinate diagram showing mechanism of total or partial quenching (a) and enhancement (b) of 3P0 emission of Pr3þ ions. The arrows show the possible non-radiative energy transfer and radiative transition related with greenishblue and red emission. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
3.5. Luminescence of BZBO:Pr3þ and BZBO:Pr3þ/PMMA In the luminescence emission spectra of BZBO:Pr3þ powder and BZBO:Pr3þ/PMMA composite excited with 451.6 nm wavelength (Fig. 7) mainly red emission from 1D2 and 3P0 levels is observed. The peaks in the emission spectra are assigned to the transition from the 3P0 level to the 3H4 (480 nm), 3H5 (580 nm), 3H6 (620 nm) and 3F2 (650 nm) levels and from the 1D2 level to the 3H4 one (595 nm). The weak emission in the greenish-blue range (3P0 / 3H4 transition) is due to the quenching of luminescence from 3P0 level caused mainly by non-radiative energy transfer from the Pr3þ (3PJ) to the Bi3þ (3P1) ion and by the 3P0 ~ 1D2 non-radiative relaxation of Pr3þ ion due to low-lying CTS. It is worthy to note, that emission from the BZBO:Pr3þ microcrystals embedded in the PMMA polymer matrix is stronger comparing to the powder. This effect was observed also in case of Eu3þ emission in (Eu3þ:YVO4) PMMA nanocomposite [6]. Increase of the emission intensity in the composite samples is related to the change of the refractive index of the medium surrounding the microcrystals: air for powder and PMMA for composite samples. This may be explained after taking into consideration the oscillator strength relation (1), expressing the probability of the transition between two manifold related with the new defined effective refractive index neff of the sample. The
Fig. 7. Emission spectra of BZBO:Pr3þ powder and BZBO:Pr3þ/PMMA composite upon 452 nm excitation at 300 K.
h i 4p2 8p2 mc 2 2 c þ c j〈AjPjB〉j j〈AjMjB〉j ED MD e2 3hl ð2J þ 1Þn2
(2)
Where cED is2Lorentz field corrections for electric dipole transitions: 2 cED ¼ nðn 9þ2Þ and cMD is magnetic dipole transitions: cMD ¼ n3 (for emission transition), m is the electron mass, c is the speed of light, e is the elementary charge, h is the Planck's constant, l is the wavelength of electromagnetic wave, n is refractive index of the sample, J is the total angular momentum, P is the electric dipole moment operator and M is the magnetic dipole moment operator [39]. On the other hand, following to the model of Meltzer [40] and taking into account relation expressing an effective refractive index:
neff ðxÞ ¼ xnmat þ ð1 xÞnmed
(3)
where neff is an effective refractive index, nmat is a refractive index of microcrystals, nmed is a refractive index of medium and x is the socalled filling factor the effective refractive index of powder neff and composite sample n0 eff can be calculated from the following formulas:
neff ðxÞ ¼ xnBZBO þ ð1 xÞnair
(4)
n0eff ðxÞ ¼ x0nBZBO þ ð1 x0ÞnPMMA :
(5)
For the powder and PMMA-based composite, n in Eq. (2) is replaced by neff or n0 eff respectively. The effective refractive index neff and n0 eff calculated from Eqs. (4) and (5) are equal to 1.15 and 1.64 for powder and composite samples, respectively. The filling factors used in the calculations were equal to 0.5 for powder and 0.2 for composite. The refractive index of BZBO powder was determined as the average value of the three principal refractive indices nx ¼ 2.13, nx ¼ 2.17, nx ¼ 2.23, calculated for wavelengths 450 nm from the Sellmeier equations [23] and is equal to nBZBO ¼ 2.18. Finally, it is clear that changing of the refraction index from 1 (for air) for powder to about 1.5 (for PMMA) [41] for composite sample strongly influences the f-f transitions. The emission spectra of BZBO:Pr3þ powder and BZBO:Pr3þ/ PMMA composite and PMMA polymer matrix under 320 nm excitation are shown in Fig. 8.
Fig. 8. Emission spectra of BZBO:Pr3þ powder and BZBO:Pr3þ/PMMA composite upon 320 nm excitation at 300 K.
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As can be seen in Fig. 8 in the emission spectrum of BZBO:Pr3þ powder excited in UV the broad fluorescence band in the range 350e450 nm with maximum at 368 nm was detected. This emission band arises from the 1P1/1S0 transition of Bi3þ ions and 4f5d / 4f2 transition of Pr3þ ions. The emission spectrum of BZBO:Pr3þ/PMMA composite contains the broad emission band in the range 350e575 nm with the maximum at 375 nm. This band is a cooperative emission of the PMMA matrix and BZBO:Pr3þ powder. Moreover, in the BZBO:Pr3þ powder and BZBO:Pr3þ/PMMA composite spectra the very intensive feature in the range 590e660 nm (red emission) assigned to the 1D2 / 3H4 and 3P0 / 3H6 transitions and the weak bands assigned to 3P0 / 3H4, 3P0 / 3H5, 3P0 / 3F2 and 3P0 / 3F3 transitions occur. All detected bands in spectra excited in UV correspond to the emission bands in the spectra excited with 451.6 nm (Fig. 7) with expectation the high energy fluorescence band at 368 nm. It was observed the increase of the red emission intensity in BZBO:Pr3þ powder and BZBO:Pr3þ/PMMA composite samples under UV excitation compared to intensity of emission under blue excitation. This effect is connected with quenching of greenish-blue 3P0 emission due to the 3P0 ~ 1D2 nonradiative relaxation of Pr3þ ions with CTS assist. Also in luminescence excitation spectra it can be observed that f-f transitions are more intense for composite sample (Fig. 9). Fig. 9 shows the excitation spectra monitored at 605.2 nm for the BZBO:Pr3þ powder and BZBO:Pr3þ/PMMA composite, recorded in spectral range 250e550 nm. A broad bands located in the range 285e350 nm (3508728571 cm1) is the charge-transfer band (CTB) and can be assigned to ligand-to-Pr3þ charge-transfer (LMCT). The energetic position of the LMCT state sct calculated from the Eq. (1) is equal to 34200 cm1 (292 nm) that correspond to the band (285e350 nm) detected in the excitation spectra. It should be noted, that no band was detected in this spectral range (285e350 nm) in the excitation spectra of undoped BZBO sample, so we can conclude that the detected CTB for BZBO:Pr3þ originates from CTS of Pr3þ ions. Moreover, in the spectra we observe the bands characteristic for Pr3þ ions emission: at 450 nm (3H4 / 3P2 transition), 475 nm (3H4 / 3P1 transition) and at 485 nm (3H4 / 3P0 transition). As can be also seen in Fig. 8 the spectra of the BZBO:Pr3þ and BZBO:Pr3þ/ PMMA composite are almost identical, however the intensity all of detected bands are evidently higher for BZBO:Pr3þ/PMMA composite in comparison with the powder sample.
Fig. 10. Decay curves monitored for 1D2 level collected for BZBO:Pr3þ powder and BZBO:Pr3þ/PMMA composite.
3.6. Decay kinetics The decay times measurements of 1D2 / 3H4 transition (595 nm emission) of Pr3þ ions in BZBO:Pr3þ powder and BZBO:Pr3þ/PMMA composite were performed using pulsed excitation at 451.6 nm wavelength. Fig. 10 shows the result of lifetime measurements. The photoluminescence decay curves of Pr3þ ions for both samples have non-exponential character and were approximated as sum of two exponential functions [27]. The decay of the 1D2 level is governed by three processes: radiative decay (1D2 / 3H4), multiphonon relaxation (1D2 / 1G2), and energy transfer via a cross relaxation (1D2 / 3H4/3H4 / 3F3,4). Non-exponential character of the decays is mainly the effect of cross-relaxation and luminescence quenching. The process becomes more probable as the concentration of Pr3þ increases and because here concentration was low the curves are not single exponential. The determined lifetimes of the red emission at 595 nm (1D2 / 3H4 transition) are equal to t1 ¼ 0.57 ms and t2 ¼ 2.50 ms for powder sample and t1 ¼ 0.69 ms and t2 ¼ 2.69 ms for composite sample. Decrease of the decay times observed for composite sample is mainly due to surface effects (OH groups luminescence quenching, change of refractive index in the grain boundaries, defects etc.) and structural distortions present in the microcrystals that are generally much stronger for smaller grains compared to the single crystals. 4. Conclusions
Fig. 9. Excitation spectra of BZBO:Pr3þ powder and BZBO:Pr3þ/PMMA composite at 300 K.
The synthesis of undoped and Pr3þ- doped BZBO microcrystalline powders and BZBO:Pr3þ/PMMA polymer composite as well as study of the structure and spectroscopic properties by XRD, HRTEM, Raman and photoluminescence spectroscopy are reported. XRD analysis confirmed the phase uniformity and purity of undoped and Pr3þ-doped BZBO powders. From HRTEM measurements it was found a clear polycrystalline structure of pure and Pr3þ-doped BZBO powders, with overall size distribution of z3 mm. Moreover HRTEM images showed that the polycrystalline particles are composed of single-crystalline nanoparticles with sizes around 180 nm. The vibrational properties of BZBO:Pr3þ powder, PMMA and BZBO:Pr3þ/PMMA composite were investigated for the first time by Raman spectroscopy. Both, powder as well the composite show enhancement of red emission (1D2 / 3H4 and 3P0 / 3H6 transitions) of Pr3þ ions after excitation in UV region in comparison
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to emission excited in VIS (blue excitation). This red emission enhancement may be caused by quenching of luminescence from 3 P0 level due to non-radiative de-excitation through Pr3þeBi3þ energy transfer and the 3P0 ~ 1D2 non-radiative relaxation of Pr3þ ion by low-lying CTS. The results show that BZBO:Pr3þ/PMMA polymer composite is a very promising candidate for new redemitting phosphors. Acknowledgements This work was supported by the Research Project of the Polish Ministry of Sciences and Higher Education: 06/65/DSPB/0516. DK and KJ thanks for collaboration to Prof. S. Jurga from Nano. BioMedical Centre Adam Mickiewicz University in Poznan References [1] D. Cotter, M.G. Burt, R.J. Manning, Phys. Rev. Lett. 68 (1992) 1200e1203. [2] H. Liu, L. Yu, W. Chen, Y. Li, J. Nanomater. 2012 (2012) 1e9. [3] V. Sreeramulu, Krishna Kanta Haldar, Amitava Patra, D. Narayana Rao, J. Phys. Chem. C 118 (2014) 30333e30341. [4] B. Karmakar, T. Som, S.P. Singh, M. Nath, Trans. Indian Ceram. Soc. 69 (2010) 1e16. [5] G. Blasse, J. Alloys Compd. 192 (1993) 17e21. [6] D. Hreniak, J. Doskocz, P. Głuchowski, R. Lisiecki, W. Stre˛ k, N. Vu, D.X. Loc, T.K. Anh, M. Bettinelli, A. Speghini, J. Lumin. 131 (2011) 473e476. [7] A. Majchrowski, A. Wojciechowski, L.R. Jaroszewicz, M. Chrunik, A. Fedorchuk, B. Sahraoui, I.V. Kityk, J. Mater. Sci. Mater. Electron. 25 (2014) 2426e2434. [8] A. Brenier, D. Jaque, A. Majchrowski, Opt. Mater. 28 (2006) 310e323. [9] A.A. Kaminskii, Crystalline Lasers: Physical Processes and Operating Schemes, CRC, Boca Raton, 1996. €, J. Alloys Compd. 275e277 (1998) 702e708. [10] M. Leskela [11] J. Pejchal, K. Fukuda, S. Kurosawa, Y. Yokota, A. Yoshikawa, Opt. Mater. 41 (2015) 58e62. [12] D. Giaume, M. Poggi, D. Casanova, G. Mialon, K. Lahlil, A. Alexandrou, T. Gacoin, J.-P. Boilot, Langmuir 24 (2008) 11018e11026. [13] R.T. Wegh, H. Donker, K.D. Oskam, A. Meijerink, Science 283 (1999) 663e666. [14] V.K. Rai, Opt. Mater. 29 (2007) 873e878.
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