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Enhancement of the up-conversion luminescence from NaYF4:Yb3+,Tb3+
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Enhancement of the up-conversion luminescence from NaYF4:Yb3 þ ,Tb3 þ Q1
Jorma Hölsä a,b,c,n, Tero Laihinen a, Taneli Laamanen a,b, Mika Lastusaari a,b, Laura Pihlgren a,d, Lucas C.V. Rodrigues a,c, Tero Soukka e a
University of Turku, Department of Chemistry, FI-20014 Turku, Finland Turku University Centre for Materials and Surfaces (MatSurf), Turku, Finland c Universidade de São Paulo, Instituto de Química, São Paulo-SP, Brazil d Graduate School of Materials Research (GSMR), Turku, Finland e University of Turku, Department of Biochemistry and Food Chemistry, FI-20520 Turku, Finland b
art ic l e i nf o
Keywords: Up-conversion Mechanism NaYF4 Ytterbium Terbium Erbium
a b s t r a c t The synthesis conditions of the Yb3 þ and Tb3 þ co-doped NaYF4 were optimized by reducing the number of washings to include only ethanol. The avoidance of the loss of amorphous NaF prior to post-annealing of the as-prepared materials resulted in the enhancement of the otherwise rather weak up-conversion from Tb3 þ by 1–2 orders of magnitude. At the same time, the temperature of formation of the hexagonal NaRF4 phase with high up-conversion could be lowered by 100 1C down to 350 1C. This improvement in up-conversion was concluded to result from the better stoichiometry of the material without washing with water. The deficit of Na þ would result in the excess of fluoride which, although not as fatal to the luminescence as the fluoride vacancies, has serious implications to the up-conversion intensity. A further enhancement in the up-conversion luminescence was observed to be due to the Er3 þ ion impurity frequently associated with high-concentration Yb3 þ containing materials. The mechanism involving the unintentional Er3 þ sensitizer and the resonance energy transfer in the Yb3 þ –Er3 þ –Tb3 þ co-doped NaYF4 were discussed based on the energy level schemes of the Yb3 þ , Er3 þ , and Tb3 þ ions in NaYF4. & 2013 Published by Elsevier B.V.
1. Introduction Photon up-conversion is a unique type of luminescence in which low-energy radiation (NIR) is converted to higher energy radiation using certain combinations of a sensitizer (e.g. Yb, Er and Sm) and an activator (e.g. Er, Ho, Pr and Tm) [1]. The up-conversion phosphors have many potential applications in optoelectronic devices as lasers and displays as well as in inks for security printing (e.g. bank notes, bonds) [2–4]. As a novel field of application, materials with efficient up-conversion luminescence may be used in homogeneous label technology for quantitative all-in-one whole blood immunoassay [5–7]. The NaYF4 host lattice has been found among the best hosts for up-conversion luminescence. This is somewhat surprising since NaYF4 has not only two crystal structures: cubic α-NaYF4 for materials prepared at low and high temperatures and hexagonal β-NaYF4 at intermediate ones [8–10] but also the cation sites are randomly occupied by the Na þ and Y3 þ ions. In the cubic form,
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n Corresponding author at: University of Turku, Department of Chemistry, FI-20014 Turku, Finland. Tel.: þ 358 2 3336737; fax: þ358 2 3336730. E-mail address: jholsa@utu.fi (J. Hölsä).
the single one cation site and in the hexagonal form, one of the two sites [11–13] show such an occupation order, or, perhaps one should say disorder since the different valences of the two cations request charge compensation resulting in either an excess or deficit of the F ions for excess or deficit of Y3 þ , respectively. The highest up-conversion luminescence intensity has been observed from the hexagonal Yb3 þ and Er3 þ co-doped NaYF4 material due to the very low phonon energy of the NaYF4 lattice [14] and very efficient energy transfer from Yb3 þ and Er3 þ . Although some chloride and bromide lattices show intense upconversion luminescence, most of them are sensitive to moisture [15]. They might not be suitable for labeling biomolecules which are used mostly in aqueous solutions. In contrast to the Yb3 þ –Er3 þ combination, the up-conversion luminescence from the Yb3 þ and Tb3 þ co-doped materials is several orders of magnitude weaker, and thus every possibility to enhance the luminescence intensity would be welcome. In this work, the up-converting NaYF4:Yb3 þ ,Tb3 þ materials were obtained with the co-precipitation synthesis [16] by using different washing conditions to enhance the up-conversion luminescence. This approach was used in addition to the usual optimization of the morphology and/or concentrations of the NaYF4 host and the dopants, respectively. The thermal behavior
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Please cite this article as: J. Hölsä, et al., Physica B (2013), http://dx.doi.org/10.1016/j.physb.2013.11.020i
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of the as-prepared materials was studied with DSC measurements. The crystal structure and phase purity of the materials were analyzed with the X-ray powder diffraction. The up-conversion luminescence was studied with NIR laser excitation at 976 nm (10246 cm 1).
2. Experimental 2.1. Materials preparation The NaYF4:Yb3 þ ,Tb3 þ materials were prepared with the coprecipitation method [16]. This involved the dissolution of solid NaF to distilled water which was further mixed with aqueous solution of RCl3 (R: Y, Yb and Tb) to attain the stoichiometry desired. After stirring for 1 h at room temperature, the precipitate thus formed was centrifuged and divided into parts which were washed according to different washing schemes. The washing was carried out either by three times with water and once with ethanol or only once with ethanol. The precipitates were then dried in a vacuum desiccator at room temperature. The subsequent postannealing was carried out in a slightly reducing static N2 þ 10% H2 gas sphere at 500 1C for 5 h. After annealing, the materials were cooled down freely to the room temperature under the same gas sphere. 2.2. Characterization The thermal behavior of the materials was studied with a TA Instruments SDT Q600 Simultaneous TGA–DSC apparatus between 25 and 720 1C in a flowing N2 gas sphere (flow rate: 100 cm3 min 1). A heating rate of 5 1Cmin 1 and sample weight of ca. 10 mg was used. The sample pan was aluminum oxide and the reference material was an empty Al2O3 pan. The crystal structures and phase purities of the materials were analyzed with the X-ray powder diffraction (XPD) measurements. The patterns were collected at room temperature between 41 and 1001 (in 2θ) with a Huber G670 image plate Guinier camera (CuKα1 radiation, 1.5406 Å). The data collection time was 30 min. The asymmetry of the diffraction reflections is due to the apparatus used. For e.g. Rietveld refinements, this effect can still be corrected with success, however. The up-conversion luminescence spectra of the materials inside a capillary tube were measured at room temperature with an Ocean Optics PC2000-CCD spectrometer. The NIR excitation (λexc: 976 nm/10246 cm 1) source was a Hamamatsu L9418-04 NIR laser diode. The emission was collected at a 901 angle to the excitation. The emission was directed to the spectrometer with an optical fiber (diameter: 200 μm). Lenses were used to focus both the exciting and emitted radiation.
3. Results and discussion 3.1. Thermal behavior The DSC curves of the as-prepared NaYF4:Yb3 þ ,Tb3 þ materials show exothermic (325-475 1C) and endothermic signals (ca. 670 1C) due to the formation of the hexagonal and high temperature cubic phases of NaRF4, respectively (Fig. 1). Both of these structural chances are irreversible. Since the luminescence from the hexagonal phase is much stronger than that from the cubic one, the crystalline materials should be obtained in the former form and temperatures as low as possible should be used to avoid crystal growth and sintering of the product during post-annealing. The signals at ca. 630 1C, just prior to the hexagonal to cubic phase
Fig. 1. DSC curves of the as-prepared NaYF4:Yb3 þ ,Tb3 þ materials with different washing schemes and Tb3 þ concentrations.
transition signal, are so far unidentified. However, tentatively these signals could be attributed to the presence of amorphous NaF and/or its reaction with non-stoichiometric NaRF4. More work is needed to clarify this issue. The cubic to hexagonal phase transition of NaYF4:Yb3 þ ,Tb3 þ occurs at a significantly lower temperature when the material is not washed with water (Fig. 1). With terbium concentration equal to 4 mol%, the cubic to hexagonal DSC signal is barely visible when the product is washed with water suggesting that there are both cubic and hexagonal phases present in this material after annealing. The signal is much stronger with the same Tb3 þ concentration without water washing. It is obvious that using water washing hinders the formation of the hexagonal form both by retarding and weakening the reaction. An evident explanation to this behavior is the removal of amorphous NaF needed for the phase transformation. The presence of the amorphous NaF phase during post-annealing drastically lowers the cubic to hexagonal phase transition temperature making it possible to obtain smaller crystal size of the hexagonal material with the use of a lower annealing temperature (exact results are not presented here due to space restrictions). When eventually formed, the hexagonal to cubic phase transition temperature shows remarkable independency of the washing scheme remaining practically constant at ca. 670 1C. 3.2. Crystal structure and phase purity According to the XPD patterns, the crystal structure of the asprepared NaYF4:Yb3 þ ,Tb3 þ materials was cubic (Fm3m, space group #225, Z: 2). Other rare earth impurities were not observed. The NaF impurity was amorphous and could only be seen as diffuse scattering. The XPD patterns of the annealed NaYF4:Yb3 þ , Tb3 þ materials (Fig. 2) confirmed the hexagonal crystal structure (P6, space group #174, Z: 1.5) [17]. Also the cubic phase was observed in the materials washed with water as expected based on the DSC curves. A small crystalline NaF impurity was observed in the annealed materials without water washing. The existence of NaF during the annealing is essential, since it acts as a source for additional Na needed for the formation of the stoichiometric and highly luminescence hexagonal form [18]. The use of water should thus be avoided during the synthesis because NaF is dissolved in water and the lack of NaF during annealing hinders the formation of the hexagonal form. The presence of the NaF impurity might explain the unknown DSC signals (Fig. 1), although these signals cannot be explained e.g. by the melting of NaF which starts at a much higher temperature, at 996 1C [19]. To clarify the origin of these additional DCS signals, high temperature
Please cite this article as: J. Hölsä, et al., Physica B (2013), http://dx.doi.org/10.1016/j.physb.2013.11.020i
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Fig. 2. XPD patterns of the annealed NaYF4:Yb3 þ ,Tb3 þ materials with different washing schemes and Tb3 þ concentrations. The reference patterns of the cubic NaYF4 and the hexagonal NaY0.57Yb0.39Er0.04F4 [17] are given as references.
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deficit of Na þ may lead either to the introduction of R3 þ ions into the exclusively Na þ site [20] or to a low Na þ occupancy of the mixed site. Both possibilities lead to the formation of interstitial F ions which, however, are less of a threat to high luminescence efficiency. Formation of F vacancies would be a much more unpleasant possibility. In any case, achieving the rather strict NaYF4 stoichiometry of the hexagonal structure [18] may not be realized or it may be slow, if not enough NaF is present at the annealing stage. The present results show that water washing does not necessarily prevent the formation of the hexagonal form (Fig. 1) or result in smaller crystallites when materials are annealed at a constant temperature (Fig. 2). However, the lower annealing temperature to be used yields smaller nanocrystals. A change in the occupation of the Na þ and Na þ /R3 þ sites, i.e. when some R3 þ also occupies the Na þ site, results in the change in the intensity ratio between the (1 1 0) and (1 0 1) reflections [20] in the XPD patterns of the NaYF4:Yb3 þ , Tb3 þ materials (Fig. 2). This shortens the average R–R distances and may, in fact, enhance the up-conversion emission intensity by improving the energy transfer between the R3 þ ions. On the other hand, improvement in energy transfer may open new pathways and actually lead to the loss of excitation energy due to extensive energy migration – eventually to a quenching site. In the present case, it is evident that such an extensive energy migration stage has not yet been achieved and only the beneficial effects of the Na þ shortage can be observed. 3.4. Up-conversion luminescence mechanisms with Er3 þ impurity
Fig. 3. Up-conversion luminescence spectra of the annealed NaYF4:Yb3 þ , Tb3 þ materials with different washing schemes and Tb3 þ concentrations.
X-ray powder diffraction measurements should be carried out in the first place. 3.3. Up-conversion luminescence The up-conversion luminescence of Tb3 þ in the NaYF4:Yb3 þ , Tb3 þ materials was obtained in both the UV and visible ranges (Fig. 3). The UV (3 8 0), violet (4 3 8), blue (457, 472 and 490), green (5 4 2), yellow (5 8 5) and orange (620 nm) emission is due to the 5D3-7FJ (J: 6, 4, 3 and 2) and 5D4-7FJ (J: 6-3) transitions of Tb3 þ . Also green (525 and 545) and red (660 nm) up-conversion luminescence due to an Er3 þ impurity was observed originating from the 2H11/2, 4S3/2-4I15/2 and 4F9/2-4I15/2 transitions of Er3 þ . The up-conversion luminescence of Tb3 þ was found to be 1–2 orders of magnitude stronger when no water was used for washing (Fig. 3). The foremost reason to the low up-conversion luminescence with the use of water for washing is the fact that NaF dissolves in water. This leads to a shortage of Na þ during annealing and to the non-stoichiometry of the materials. As such, non-stoichiometry is not detrimental to the luminescence intensity if the non-stoichiometry is managed in an appropriate manner. However, the presence of defects formed as a result of unsuccessful or neglected charge compensation has a much more important effect. Since the crystal structure of NaRF4 has a delicate balance between cation sites of both exclusively Na þ and a mixed Na þ /R3 þ occupancy, the chance for non-stoichiometry cannot be ruled out. According to simple charge compensation schemes,
Trivalent Tb3 þ does not have intermediate 4f energy levels that could absorb NIR excitation photons (Fig. 4) [22]. The upconversion of the Tb3 þ -Yb3 þ combination is thus usually presented to occur due to a co-operative sensitization of Ybβ þ whose efficiency is very weak [1]. However, it is well known that there is some erbium (and other lanthanides as Tm3 þ ) as impurity in the rare earth oxides. In addition, the Yb3 þ -Er3 þ energy transfer is very efficient when compared to the co-operative sensitization of Yb3 þ as can be readily seen in the up-conversion spectra of nominally pure Yb3 þ compounds [23]. In the first place, the high efficiency is due to the energy levels in resonance between these two ions, i.e. between the 2F5/2 (Yb3 þ ) and 4I11/2 (Er3 þ ) levels (Fig. 4). To stay on this track, there is a similar resonance between the 4F7/2 (Er3 þ ) and 5D4 (Tb3 þ ) levels. Although the green upconversion luminescence from Er3 þ originates from the 2H11/2, 4 S3/2-4I15/2 and 4F9/2-4I15/2 transitions, one should not forget that the final level of energy transfer from Yb3 þ is actually 4F7/2
Fig. 4. Energy level schemes of Yb3 þ , Er3 þ and Tb3 þ [22]. The red, dashed and blue Q5 arrows represent the energy of the laser excitation (976 nm/10246 cm 1), R3 þ –R3 þ energy transfer and energy of co-operative sensitization of Yb3 þ , respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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which is then thermalized to the levels 2H11/2, 4S3/2 for the green emission. Despite the rapid de-excitation of the 4F7/2 level there is a possibility to energy transfer from Er3 þ to Tb3 þ because of the favorable resonant level positions. Obviously, one should not forget the prospect of the three photon up-conversion process in the Yb3 þ -Er3 þ system responsible for the albeit weak blue emission of Er3 þ at 402 nm. This three photon process is in an excellent resonant condition with the Tb3 þ energy levels due to the high density of 2S þ 1LJ levels above the 5D3 level of Tb3 þ . After de-excitation to the 5D3 level, also 5D3-7FJ emission in UV and blue can be observed. As a conclusion, it is possible that the two photon upconversion process involving Yb3 þ and the Er3 þ impurity might contribute to the up-conversion luminescence of Tb3 þ by the Yb3 þ -Er3 þ -Tb3 þ energy transfer. Tb3 þ is then excited to the 5D4 level which yields strong green emission at ca. 545 nm with lower intensity emission in blue–green as well as yellow and even red. Unfortunately, the green emission of Er3 þ overlaps rather efficiently with the green Tb3 þ emission though the higher-energy green emission of Er3 þ is still visible at ca. 525 nm (Fig. 3). The best opportunity to observe the Er3 þ emission in the visible range is at ca. 650 nm. However, it was found that with increasing Er3 þ and Tb3 þ concentrations (ca. 4 and 6 mol%, respectively), the Er3 þ emission vanishes and the Tb3 þ emission is enhanced and dominates completely the visible emission of the Yb3 þ -Er3 þ Tb3 þ co-doped materials (not shown here). Of course, the IR emission of Er3 þ above the excitation of Yb3 þ may unveil the presence of Er3 þ but one must both be aware of this emission and to be able to measure this IR emission. Either of them may not be evident to a spectroscopist who has ignored the possibility of Er3 þ impurity in high concentration Yb3 þ materials. The contribution of the Er3 þ impurity may not be restricted only to the 5D4 emission since a three photon process in the Yb3 þ -Er3 þ system may contribute to emission of the 5D3 level of Tb3 þ . This is in addition to the energy transfer up-conversion (ETU) or excited state absorption (ESA) process based on the 5D4 level of Tb3 þ . The up-conversion luminescence of Tb3 þ yielding both the UV radiation at 360 nm as well as blue and green in visible, may be obtained as a result of the contribution of the Er3 þ impurity in Yb3 þ . A further, more detailed study of the effect the Er3 þ impurity is out of the scope of this short report.
removal of the NaF impurity as well as the origin of the DSC signals just prior to the hexagonal to cubic phase transition temperature will be studied in detail. Eventually, further effort will be focused to optimize the effect of the Er3 þ ion as an additional sensitizer to boost the still too weak up-conversion from Tb3 þ in the Yb3 þ , Tb3 þ co-doped materials and elucidate the Yb3 þ –Er3 þ –Tb3 þ energy transfer mechanism.
Uncited reference [21].
Acknowledgments Financial support is acknowledged from the Nordic Energy Research “AQUAFEED” project, the Academy of Finland projects “Energy Storage Luminophors 2” and “Novel Rare Earth Optical Sensors and Materials” as well as the Graduate School of Materials Research (GSMR), Turku, Finland. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
4. Conclusions 3þ
3þ
The hexagonal up-converting NaYF4:Yb ,Tb materials were prepared with different Tb3 þ concentrations and washing schemes. The syntheses were optimized by reducing the loss of Na þ due to washing with water. The cubic to hexagonal phase transition of NaYF4:Yb3 þ ,Tb3 þ occurs now at a record low temperature, 350 1C. Moreover, the up-conversion luminescence is now 1–2 orders of magnitude stronger than that of a material washed with water. The enhancement of the up-conversion luminescence was concluded to result from adjusting the Na þ / R3 þ ratio close to the stoichiometric one and the detrimental effects of non-stoichiometry could be avoided. In the future, the
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[16] [17]
[18] [19] [20] [21] [22] [23]
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Physica B journal homepage: www.elsevier.com/locate/physb
Enhancement of the up-conversion luminescence from NaYF4:Yb3 þ ,Tb3 þ Q1
Jorma Hölsä a,b,c,n, Tero Laihinen a, Taneli Laamanen a,b, Mika Lastusaari a,b, Laura Pihlgren a,d, Lucas C.V. Rodrigues a,c, Tero Soukka e a
University of Turku, Department of Chemistry, FI-20014 Turku, Finland Turku University Centre for Materials and Surfaces (MatSurf), Turku, Finland c Universidade de São Paulo, Instituto de Química, São Paulo-SP, Brazil d Graduate School of Materials Research (GSMR), Turku, Finland e University of Turku, Department of Biochemistry and Food Chemistry, FI-20520 Turku, Finland b
art ic l e i nf o
Keywords: Up-conversion Mechanism NaYF4 Ytterbium Terbium Erbium
a b s t r a c t The synthesis conditions of the Yb3 þ and Tb3 þ co-doped NaYF4 were optimized by reducing the number of washings to include only ethanol. The avoidance of the loss of amorphous NaF prior to post-annealing of the as-prepared materials resulted in the enhancement of the otherwise rather weak up-conversion from Tb3 þ by 1–2 orders of magnitude. At the same time, the temperature of formation of the hexagonal NaRF4 phase with high up-conversion could be lowered by 100 1C down to 350 1C. This improvement in up-conversion was concluded to result from the better stoichiometry of the material without washing with water. The deficit of Na þ would result in the excess of fluoride which, although not as fatal to the luminescence as the fluoride vacancies, has serious implications to the up-conversion intensity. A further enhancement in the up-conversion luminescence was observed to be due to the Er3 þ ion impurity frequently associated with high-concentration Yb3 þ containing materials. The mechanism involving the unintentional Er3 þ sensitizer and the resonance energy transfer in the Yb3 þ –Er3 þ –Tb3 þ co-doped NaYF4 were discussed based on the energy level schemes of the Yb3 þ , Er3 þ , and Tb3 þ ions in NaYF4. & 2013 Published by Elsevier B.V.
1. Introduction Photon up-conversion is a unique type of luminescence in which low-energy radiation (NIR) is converted to higher energy radiation using certain combinations of a sensitizer (e.g. Yb, Er and Sm) and an activator (e.g. Er, Ho, Pr and Tm) [1]. The up-conversion phosphors have many potential applications in optoelectronic devices as lasers and displays as well as in inks for security printing (e.g. bank notes, bonds) [2–4]. As a novel field of application, materials with efficient up-conversion luminescence may be used in homogeneous label technology for quantitative all-in-one whole blood immunoassay [5–7]. The NaYF4 host lattice has been found among the best hosts for up-conversion luminescence. This is somewhat surprising since NaYF4 has not only two crystal structures: cubic α-NaYF4 for materials prepared at low and high temperatures and hexagonal β-NaYF4 at intermediate ones [8–10] but also the cation sites are randomly occupied by the Na þ and Y3 þ ions. In the cubic form,
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n Corresponding author at: University of Turku, Department of Chemistry, FI-20014 Turku, Finland. Tel.: þ 358 2 3336737; fax: þ358 2 3336730. E-mail address: jholsa@utu.fi (J. Hölsä).
the single one cation site and in the hexagonal form, one of the two sites [11–13] show such an occupation order, or, perhaps one should say disorder since the different valences of the two cations request charge compensation resulting in either an excess or deficit of the F ions for excess or deficit of Y3 þ , respectively. The highest up-conversion luminescence intensity has been observed from the hexagonal Yb3 þ and Er3 þ co-doped NaYF4 material due to the very low phonon energy of the NaYF4 lattice [14] and very efficient energy transfer from Yb3 þ and Er3 þ . Although some chloride and bromide lattices show intense upconversion luminescence, most of them are sensitive to moisture [15]. They might not be suitable for labeling biomolecules which are used mostly in aqueous solutions. In contrast to the Yb3 þ –Er3 þ combination, the up-conversion luminescence from the Yb3 þ and Tb3 þ co-doped materials is several orders of magnitude weaker, and thus every possibility to enhance the luminescence intensity would be welcome. In this work, the up-converting NaYF4:Yb3 þ ,Tb3 þ materials were obtained with the co-precipitation synthesis [16] by using different washing conditions to enhance the up-conversion luminescence. This approach was used in addition to the usual optimization of the morphology and/or concentrations of the NaYF4 host and the dopants, respectively. The thermal behavior
0921-4526/$ - see front matter & 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.physb.2013.11.020
Please cite this article as: J. Hölsä, et al., Physica B (2013), http://dx.doi.org/10.1016/j.physb.2013.11.020i
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J. Hölsä et al. / Physica B ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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of the as-prepared materials was studied with DSC measurements. The crystal structure and phase purity of the materials were analyzed with the X-ray powder diffraction. The up-conversion luminescence was studied with NIR laser excitation at 976 nm (10246 cm 1).
2. Experimental 2.1. Materials preparation The NaYF4:Yb3 þ ,Tb3 þ materials were prepared with the coprecipitation method [16]. This involved the dissolution of solid NaF to distilled water which was further mixed with aqueous solution of RCl3 (R: Y, Yb and Tb) to attain the stoichiometry desired. After stirring for 1 h at room temperature, the precipitate thus formed was centrifuged and divided into parts which were washed according to different washing schemes. The washing was carried out either by three times with water and once with ethanol or only once with ethanol. The precipitates were then dried in a vacuum desiccator at room temperature. The subsequent postannealing was carried out in a slightly reducing static N2 þ 10% H2 gas sphere at 500 1C for 5 h. After annealing, the materials were cooled down freely to the room temperature under the same gas sphere. 2.2. Characterization The thermal behavior of the materials was studied with a TA Instruments SDT Q600 Simultaneous TGA–DSC apparatus between 25 and 720 1C in a flowing N2 gas sphere (flow rate: 100 cm3 min 1). A heating rate of 5 1Cmin 1 and sample weight of ca. 10 mg was used. The sample pan was aluminum oxide and the reference material was an empty Al2O3 pan. The crystal structures and phase purities of the materials were analyzed with the X-ray powder diffraction (XPD) measurements. The patterns were collected at room temperature between 41 and 1001 (in 2θ) with a Huber G670 image plate Guinier camera (CuKα1 radiation, 1.5406 Å). The data collection time was 30 min. The asymmetry of the diffraction reflections is due to the apparatus used. For e.g. Rietveld refinements, this effect can still be corrected with success, however. The up-conversion luminescence spectra of the materials inside a capillary tube were measured at room temperature with an Ocean Optics PC2000-CCD spectrometer. The NIR excitation (λexc: 976 nm/10246 cm 1) source was a Hamamatsu L9418-04 NIR laser diode. The emission was collected at a 901 angle to the excitation. The emission was directed to the spectrometer with an optical fiber (diameter: 200 μm). Lenses were used to focus both the exciting and emitted radiation.
3. Results and discussion 3.1. Thermal behavior The DSC curves of the as-prepared NaYF4:Yb3 þ ,Tb3 þ materials show exothermic (325-475 1C) and endothermic signals (ca. 670 1C) due to the formation of the hexagonal and high temperature cubic phases of NaRF4, respectively (Fig. 1). Both of these structural chances are irreversible. Since the luminescence from the hexagonal phase is much stronger than that from the cubic one, the crystalline materials should be obtained in the former form and temperatures as low as possible should be used to avoid crystal growth and sintering of the product during post-annealing. The signals at ca. 630 1C, just prior to the hexagonal to cubic phase
Fig. 1. DSC curves of the as-prepared NaYF4:Yb3 þ ,Tb3 þ materials with different washing schemes and Tb3 þ concentrations.
transition signal, are so far unidentified. However, tentatively these signals could be attributed to the presence of amorphous NaF and/or its reaction with non-stoichiometric NaRF4. More work is needed to clarify this issue. The cubic to hexagonal phase transition of NaYF4:Yb3 þ ,Tb3 þ occurs at a significantly lower temperature when the material is not washed with water (Fig. 1). With terbium concentration equal to 4 mol%, the cubic to hexagonal DSC signal is barely visible when the product is washed with water suggesting that there are both cubic and hexagonal phases present in this material after annealing. The signal is much stronger with the same Tb3 þ concentration without water washing. It is obvious that using water washing hinders the formation of the hexagonal form both by retarding and weakening the reaction. An evident explanation to this behavior is the removal of amorphous NaF needed for the phase transformation. The presence of the amorphous NaF phase during post-annealing drastically lowers the cubic to hexagonal phase transition temperature making it possible to obtain smaller crystal size of the hexagonal material with the use of a lower annealing temperature (exact results are not presented here due to space restrictions). When eventually formed, the hexagonal to cubic phase transition temperature shows remarkable independency of the washing scheme remaining practically constant at ca. 670 1C. 3.2. Crystal structure and phase purity According to the XPD patterns, the crystal structure of the asprepared NaYF4:Yb3 þ ,Tb3 þ materials was cubic (Fm3m, space group #225, Z: 2). Other rare earth impurities were not observed. The NaF impurity was amorphous and could only be seen as diffuse scattering. The XPD patterns of the annealed NaYF4:Yb3 þ , Tb3 þ materials (Fig. 2) confirmed the hexagonal crystal structure (P6, space group #174, Z: 1.5) [17]. Also the cubic phase was observed in the materials washed with water as expected based on the DSC curves. A small crystalline NaF impurity was observed in the annealed materials without water washing. The existence of NaF during the annealing is essential, since it acts as a source for additional Na needed for the formation of the stoichiometric and highly luminescence hexagonal form [18]. The use of water should thus be avoided during the synthesis because NaF is dissolved in water and the lack of NaF during annealing hinders the formation of the hexagonal form. The presence of the NaF impurity might explain the unknown DSC signals (Fig. 1), although these signals cannot be explained e.g. by the melting of NaF which starts at a much higher temperature, at 996 1C [19]. To clarify the origin of these additional DCS signals, high temperature
Please cite this article as: J. Hölsä, et al., Physica B (2013), http://dx.doi.org/10.1016/j.physb.2013.11.020i
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Fig. 2. XPD patterns of the annealed NaYF4:Yb3 þ ,Tb3 þ materials with different washing schemes and Tb3 þ concentrations. The reference patterns of the cubic NaYF4 and the hexagonal NaY0.57Yb0.39Er0.04F4 [17] are given as references.
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deficit of Na þ may lead either to the introduction of R3 þ ions into the exclusively Na þ site [20] or to a low Na þ occupancy of the mixed site. Both possibilities lead to the formation of interstitial F ions which, however, are less of a threat to high luminescence efficiency. Formation of F vacancies would be a much more unpleasant possibility. In any case, achieving the rather strict NaYF4 stoichiometry of the hexagonal structure [18] may not be realized or it may be slow, if not enough NaF is present at the annealing stage. The present results show that water washing does not necessarily prevent the formation of the hexagonal form (Fig. 1) or result in smaller crystallites when materials are annealed at a constant temperature (Fig. 2). However, the lower annealing temperature to be used yields smaller nanocrystals. A change in the occupation of the Na þ and Na þ /R3 þ sites, i.e. when some R3 þ also occupies the Na þ site, results in the change in the intensity ratio between the (1 1 0) and (1 0 1) reflections [20] in the XPD patterns of the NaYF4:Yb3 þ , Tb3 þ materials (Fig. 2). This shortens the average R–R distances and may, in fact, enhance the up-conversion emission intensity by improving the energy transfer between the R3 þ ions. On the other hand, improvement in energy transfer may open new pathways and actually lead to the loss of excitation energy due to extensive energy migration – eventually to a quenching site. In the present case, it is evident that such an extensive energy migration stage has not yet been achieved and only the beneficial effects of the Na þ shortage can be observed. 3.4. Up-conversion luminescence mechanisms with Er3 þ impurity
Fig. 3. Up-conversion luminescence spectra of the annealed NaYF4:Yb3 þ , Tb3 þ materials with different washing schemes and Tb3 þ concentrations.
X-ray powder diffraction measurements should be carried out in the first place. 3.3. Up-conversion luminescence The up-conversion luminescence of Tb3 þ in the NaYF4:Yb3 þ , Tb3 þ materials was obtained in both the UV and visible ranges (Fig. 3). The UV (3 8 0), violet (4 3 8), blue (457, 472 and 490), green (5 4 2), yellow (5 8 5) and orange (620 nm) emission is due to the 5D3-7FJ (J: 6, 4, 3 and 2) and 5D4-7FJ (J: 6-3) transitions of Tb3 þ . Also green (525 and 545) and red (660 nm) up-conversion luminescence due to an Er3 þ impurity was observed originating from the 2H11/2, 4S3/2-4I15/2 and 4F9/2-4I15/2 transitions of Er3 þ . The up-conversion luminescence of Tb3 þ was found to be 1–2 orders of magnitude stronger when no water was used for washing (Fig. 3). The foremost reason to the low up-conversion luminescence with the use of water for washing is the fact that NaF dissolves in water. This leads to a shortage of Na þ during annealing and to the non-stoichiometry of the materials. As such, non-stoichiometry is not detrimental to the luminescence intensity if the non-stoichiometry is managed in an appropriate manner. However, the presence of defects formed as a result of unsuccessful or neglected charge compensation has a much more important effect. Since the crystal structure of NaRF4 has a delicate balance between cation sites of both exclusively Na þ and a mixed Na þ /R3 þ occupancy, the chance for non-stoichiometry cannot be ruled out. According to simple charge compensation schemes,
Trivalent Tb3 þ does not have intermediate 4f energy levels that could absorb NIR excitation photons (Fig. 4) [22]. The upconversion of the Tb3 þ -Yb3 þ combination is thus usually presented to occur due to a co-operative sensitization of Ybβ þ whose efficiency is very weak [1]. However, it is well known that there is some erbium (and other lanthanides as Tm3 þ ) as impurity in the rare earth oxides. In addition, the Yb3 þ -Er3 þ energy transfer is very efficient when compared to the co-operative sensitization of Yb3 þ as can be readily seen in the up-conversion spectra of nominally pure Yb3 þ compounds [23]. In the first place, the high efficiency is due to the energy levels in resonance between these two ions, i.e. between the 2F5/2 (Yb3 þ ) and 4I11/2 (Er3 þ ) levels (Fig. 4). To stay on this track, there is a similar resonance between the 4F7/2 (Er3 þ ) and 5D4 (Tb3 þ ) levels. Although the green upconversion luminescence from Er3 þ originates from the 2H11/2, 4 S3/2-4I15/2 and 4F9/2-4I15/2 transitions, one should not forget that the final level of energy transfer from Yb3 þ is actually 4F7/2
Fig. 4. Energy level schemes of Yb3 þ , Er3 þ and Tb3 þ [22]. The red, dashed and blue Q5 arrows represent the energy of the laser excitation (976 nm/10246 cm 1), R3 þ –R3 þ energy transfer and energy of co-operative sensitization of Yb3 þ , respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Please cite this article as: J. Hölsä, et al., Physica B (2013), http://dx.doi.org/10.1016/j.physb.2013.11.020i
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which is then thermalized to the levels 2H11/2, 4S3/2 for the green emission. Despite the rapid de-excitation of the 4F7/2 level there is a possibility to energy transfer from Er3 þ to Tb3 þ because of the favorable resonant level positions. Obviously, one should not forget the prospect of the three photon up-conversion process in the Yb3 þ -Er3 þ system responsible for the albeit weak blue emission of Er3 þ at 402 nm. This three photon process is in an excellent resonant condition with the Tb3 þ energy levels due to the high density of 2S þ 1LJ levels above the 5D3 level of Tb3 þ . After de-excitation to the 5D3 level, also 5D3-7FJ emission in UV and blue can be observed. As a conclusion, it is possible that the two photon upconversion process involving Yb3 þ and the Er3 þ impurity might contribute to the up-conversion luminescence of Tb3 þ by the Yb3 þ -Er3 þ -Tb3 þ energy transfer. Tb3 þ is then excited to the 5D4 level which yields strong green emission at ca. 545 nm with lower intensity emission in blue–green as well as yellow and even red. Unfortunately, the green emission of Er3 þ overlaps rather efficiently with the green Tb3 þ emission though the higher-energy green emission of Er3 þ is still visible at ca. 525 nm (Fig. 3). The best opportunity to observe the Er3 þ emission in the visible range is at ca. 650 nm. However, it was found that with increasing Er3 þ and Tb3 þ concentrations (ca. 4 and 6 mol%, respectively), the Er3 þ emission vanishes and the Tb3 þ emission is enhanced and dominates completely the visible emission of the Yb3 þ -Er3 þ Tb3 þ co-doped materials (not shown here). Of course, the IR emission of Er3 þ above the excitation of Yb3 þ may unveil the presence of Er3 þ but one must both be aware of this emission and to be able to measure this IR emission. Either of them may not be evident to a spectroscopist who has ignored the possibility of Er3 þ impurity in high concentration Yb3 þ materials. The contribution of the Er3 þ impurity may not be restricted only to the 5D4 emission since a three photon process in the Yb3 þ -Er3 þ system may contribute to emission of the 5D3 level of Tb3 þ . This is in addition to the energy transfer up-conversion (ETU) or excited state absorption (ESA) process based on the 5D4 level of Tb3 þ . The up-conversion luminescence of Tb3 þ yielding both the UV radiation at 360 nm as well as blue and green in visible, may be obtained as a result of the contribution of the Er3 þ impurity in Yb3 þ . A further, more detailed study of the effect the Er3 þ impurity is out of the scope of this short report.
removal of the NaF impurity as well as the origin of the DSC signals just prior to the hexagonal to cubic phase transition temperature will be studied in detail. Eventually, further effort will be focused to optimize the effect of the Er3 þ ion as an additional sensitizer to boost the still too weak up-conversion from Tb3 þ in the Yb3 þ , Tb3 þ co-doped materials and elucidate the Yb3 þ –Er3 þ –Tb3 þ energy transfer mechanism.
Uncited reference [21].
Acknowledgments Financial support is acknowledged from the Nordic Energy Research “AQUAFEED” project, the Academy of Finland projects “Energy Storage Luminophors 2” and “Novel Rare Earth Optical Sensors and Materials” as well as the Graduate School of Materials Research (GSMR), Turku, Finland. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
4. Conclusions 3þ
3þ
The hexagonal up-converting NaYF4:Yb ,Tb materials were prepared with different Tb3 þ concentrations and washing schemes. The syntheses were optimized by reducing the loss of Na þ due to washing with water. The cubic to hexagonal phase transition of NaYF4:Yb3 þ ,Tb3 þ occurs now at a record low temperature, 350 1C. Moreover, the up-conversion luminescence is now 1–2 orders of magnitude stronger than that of a material washed with water. The enhancement of the up-conversion luminescence was concluded to result from adjusting the Na þ / R3 þ ratio close to the stoichiometric one and the detrimental effects of non-stoichiometry could be avoided. In the future, the
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[16] [17]
[18] [19] [20] [21] [22] [23]
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Please cite this article as: J. Hölsä, et al., Physica B (2013), http://dx.doi.org/10.1016/j.physb.2013.11.020i