ARTICLE pubs.acs.org/crystal
Flux Growth of Highly Crystalline NaYF4:Ln (Ln = Yb, Er, Tm) Crystals with Upconversion Fluorescence Katsuya Teshima,*,† SunHyung Lee,‡ Nobutaka Shikine,† Toshiko Wakabayashi,† Kunio Yubuta,§ Toetsu Shishido,§ and Shuji Oishi† †
Department of Environmental Science and Technology, Faculty of Engineering, Shinshu University, Nagano 380-8553, Japan Faculty of Engineering, Shinshu University, Nagano 380-8553, Japan § Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan ‡
ABSTRACT: Highly crystalline, NaYF4 and NaYF4:Ln (Ln = Yb, Er, Tm) crystals with upconversion fluorescence were successfully grown by the cooling of the solo NaF flux at a holding temperature of 1100 °C and cooling rate of 5 °C 3 h-1. The basic forms of NaYF4 and NaYF4:Ln crystals were a sphere. The crystal system, form, and size were affected by cooling rate. The red, green, and blue upconversion fluorescence of NaYF4:Ln crystals were clearly observed under 980 nm laser irradiation by a two- or three-photon upconversion process. The upconversion fluorescence of NaYF4:Ln crystals was successfully controlled by changing the cooling rate and type of dopant. Furthermore, the NaYF4:Yb,Er crystals were successfully grown using a mixed NaF-KF flux cooling method at a relatively low temperature of 800 °C.
’ INTRODUCTION Rare-earth-doped fluorescence materials have received much attention for their potential application in phosphors,1-4 solar cells,5 flat-panel displays,6,7 scintillators,8 and solid-state lasers9 due to their high fluorescence properties. Recently, they have also been studied in the fields of biology and biomedicine, where they have been used as biosensors,3 such as fluorescent labels and biological probes for the sensitive detection of biomolecules.10-15 In particular, near-infrared-to-visible (NIR-vis) upconversion fluorescent materials are expected to be next-generation emission products. The upconversion process is defined as the generation of higher-energy photons from lower-energy radiation via multiple absorptions or energy transfers. Various oxides doped with Er3þ and Tm3þ ions, often in combination with Yb3þ as a sensitizer, are frequently used as highly efficient upconversion materials.1-5,7,9,11-17 We successfully fabricated a green upconversion fluorescence crystal of ytterbium phosphate (YbPO4) doped with Er3þ using a two-step process, that is, the synthesis of spherical YbPO4:Er 3 nH2O precursors in naturally derived gel matrices and their subsequent annealing.18 Fluoride phosphor is well-known for its high photon efficiency, which is higher than that of oxide upconversion materials.1-4,9,12-15,19-24 NaYF4:Ln (Ln = Yb, Er, Tm) phosphors, especially hexagonal-system NaYF4:Yb,Er and NaYF4:Yb, Tm, are well-known as the most efficient NIR-vis upconversion fluorescence materials.1-4,12-15,19-24 The crystals of NaYF4 belong to the cubic (R-NaYF4) or hexagonal (β-NaYF4) system. The phase transformation of NaYF4 has been reported; the cubic phase could transfer into the hexagonal phase under heat treatment. The cubic phase is high-temperature metastable, r 2011 American Chemical Society
whereas the hexagonal phase is thermodynamically stable. The NaYF4 crystal with the hexagonal system has lattice parameters of a = 0.596 and c = 0.353 nm and is composed of edge-linked Na.25 In addition, the hexagonal-system NaYF4:Ln crystal exhibits a higher fluorescence efficiency than the cubic-system NaYF4:Ln crystal.26 Therefore, control of the NaYF4:Ln crystal phase and fabrication of pure, hexagonal-system NaYF4:Ln crystal are very important for high fluorescence performance. Several studies have investigated the preparation of the hexagonal-system NaYF4:Ln crystal using different techniques, such as coprecipitation,15,27 solvothermal,2 and hydrothermal methods. 24,28 Unfortunately, these methods have several inherent disadvantages, such as expensive equipment, high total costs, high environmental loads, and poor crystallinities. These issues direct the need for a simple, low cost, and environmentally friendly fabrication method of NaYF4:Ln crystals. Until now, we have successfully fabricated various inorganic functional crystals by the flux cooling method.29-31 The flux growth is particularly preferred because it readily allows crystal growth at a temperature well below the melting point of the solute. Other important advantages of flux growth are that the grown crystals have an idiomorphic or enhedral habit and a reasonably lower degree of dislocation density. For the flux method, the most important thing is to choose a proper flux. The purpose of this study is the fabrication of high-quality NaYF4:Ln crystals, which belong to the hexagonal system, by the cooling of solo NaF and mixed NaF-KF fluxes. The melting Received: July 15, 2010 Revised: December 17, 2010 Published: February 25, 2011 995
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point of NaF flux is 990 °C, and the eutectic temperature of the mixed NaF-KF flux (at a mixing rate of 4:6) is approximately 710 °C; thus, the desired crystals can be grown at a relatively low temperature. Furthermore, the upconversion fluorescence property of NaYF4:Ln crystals was investigated under 980 nm laser irradiation.
’ EXPERIMENTAL SECTION Reagent-grade NaF, YF3, YbF3, ErF3, and TmF3 powders (Wako Pure Chemical Industries) were used for the growth of NaYF4 and NaYF4:Ln (Ln = Yb, Er, Tm) crystals. First, a stoichiometric mixture of reagentgrade NaF and YF3 powders, and solo NaF or mixed NaF-KF were used as a solute and flux, respectively, for the growth of NaYF4 crystal. In addition, in order to grow NaYF4:Ln crystals, mixed dopants [YbF3ErF3 or YbF3-TmF3] were added to a stoichiometric mixture of reagent-grade NaF and YF3 powders. Dopants were added at a molar ratio of Y:Yb:Er (or Y:Yb:Tm) = 78:20:2. Each of the mixtures was put into 30 cm3 capacity crucibles. After the lids were loosely closed, the crucibles were placed in an electric furnace. The crucibles were heated to 800-1100 °C at a rate of 45 °C 3 h-1 and held at this temperature for 10 h. They were then cooled to 500 °C at a rate of 5 °C 3 h-1 (slow cooling) or >120000 °C 3 h-1 (water quenching). The cooling rate of 5 °C 3 h-1 was controlled by a cooling program, and the crucibles were allowed to cool to room temperature in the furnace. A cooling rate of >120000 °C 3 h-1 could not be attained by the cooling program in the furnace. For >120000 °C 3 h-1, the crucibles were held at each holding temperature for 10 h and then removed from the high-temperature furnace. They were then immersed in water and cooled extremely rapidly. This cooling rate, >120000 °C 3 h-1, is an approximate estimated value. In all cases, the crystal products were separated from the remaining flux in warm water. The obtained crystals were observed by use of a field emission scanning electron microscope (FESEM, JEOL, JSM-7000F). Crystal phases were studied by X-ray diffraction (XRD, SHIMADZU, XRD6000). An energy-dispersive X-ray spectrometer (EDS, JEOL, JSM7000F) was used to study any variation in the concentration of the major constituents in the grown crystals. The high-resolution transmission electron microscopy (HRTEM) and electron diffraction observations were carried out with JEM-2010 and JEM-2000EXII (JEOL) instruments operated at 200 kV to analyze the crystallinity and developed faces of the grown crystals. The vis-NIR light diffuse reflectance spectra of the grown products were obtained with a spectrophotometer (SHIMADZU, UV3150). To simply examine their upconversion fluorescence, an NIR laser (980 nm, 10 mW) was used to irradiate the obtained crystals under an atmosphere. The upconversion fluorescence spectrum of the grown products was obtained with a spectrofluorometer (JASCO, FP-6600) at an exciting-wavelength of 980 nm.
Figure 1. FESEM images of (a) NaYF4 and (b) NaYF4:Yb,Er crystals grown at a holding temperature of 1100 °C and a cooling rate of 5 °C 3 h-1. (c) Low and (d) high magnification FESEM images of NaYF4: Yb,Er crystals at a holding temperature of 1100 °C and a cooling rate of >120000 °C 3 h-1.
Figure 2. X-ray diffraction patterns (Cu KR) of (a) the NaYF4 crystals and (b) NaYF4:Yb,Er crystals grown at a cooling rate of 5 °C 3 h-1 and (c) NaYF4:Yb,Er crystals grown at a cooling rate of >120000 °C 3 h-1.
crystals grown at a cooling rate of 5 °C 3 h-1 was also spherical in shape, as shown in Figure 1b. The spheres of NaYF4:Yb,Er crystals were densely aggregated, and their size increased more than that of the NaYF4 crystals. On the other hand, the effect of cooling rate on the form change of the crystals was shown in Figure 1c and d. Figure 1c and d shows high and low magnification FESEM images of NaYF4:Yb,Er crystals grown at a cooling rate of >120000 °C 3 h-1. The basic form of NaYF4:Yb,Er crystals grown at high cooling rate was included of spherical, cubic, and bulklike in shape. Figure 2 shows the XRD profiles of data for the pulverized crystallites of (a) NaYF4 crystals and (b) NaYF4:Yb,Er crystals grown at a cooling rate of 5 °C 3 h-1, and (c) NaYF4:Yb,Er crystals grown at a cooling rates of >120000 °C 3 h-1. The NaYF4 crystals grown at a cooling rate of 5 °C 3 h-1 were found to contain highly crystalline hexagonal- and cubic-system NaYF4 crystals, as shown in Figure 2a. The NaYF4:Yb,Er crystals grown at a cooling rate of 5 °C 3 h-1 were found to be mainly the hexagonal phase, as shown in Figure 2b. When a very small amount of dopant is added, however, the reason why phase
’ RESULTS AND DISCUSSION First, the NaYF4 crystals were grown from NaF and YF3 powders using the solo NaF flux (i.e., self-flux) cooling method at a holding temperature of 1100 °C. Figure 1a exhibits the FESEM images of typical NaYF4 crystals without Ln ions, which were grown at a cooling rate of 5 °C 3 h-1. The basic form of the grown crystals, which were transparent and colorless, was spherical in shape. The NaYF4 crystals consisted of an aggregation of numerous, dense nanocrystals. Spherical crystals up to approximately 900 nm in diameter on average were grown using a NaF flux. In addition, the NaYF4:Yb,Er crystals were grown using a NaF flux at cooling rates of 5 or >120000 °C 3 h-1 (Figure 1b and c). The effect of dopant addition on the form change of the crystals was not observed. The basic form of the NaYF4:Yb,Er 996
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Figure 3. SAED patterns of (a) NaYF4 crystals and (b) NaYF4:Yb,Er crystals grown at a cooling rate of 5 °C 3 h-1, and (c) lattice image of NaYF4:Yb,Er crystals.
Figure 4. EDS spectra of (a) NaYF4:Yb,Er crystals and (b) NaYF4:Yb, Tm crystals grown from a NaF flux at a cooling rate of 5 °C 3 h-1.
transformation occurs is not understood. By contrast, the NaYF4: Yb,Er crystals grown at a cooling rate of >120000 °C 3 h-1 were found to be mainly the cubic phase, as shown in Figure 2c. It is considered that not the hexagonal phase but the cubic phase of the NaYF4:Yb,Er crystals is formed in the high-temperature solution, and the cubic phase cannot transfer into the hexagonal phase by water quenching. On the other hand, for the NaYF4:Ln crystals grown at the holding temperature of 1100 °C and the cooling rate of 5 °C 3 h-1, the cubic phase is thought to transform to the hexagonal phase during the cooling process (Figure 2b). However, no signal originating from the Yb3þ and Er3þ ions was detected in these XRD patterns, since the ionic radii of Y3þ, Yb3þ, and Er3þ, which are respectively 0.121, 0.118, and 0.120 nm, are almost the same. The effect of the dopant ions (Yb3þ and Er3þ) on the crystallinity of NaYF4:Ln crystals can be easily confirmed by selected area electron diffraction (SAED) patterns. Figure 3 shows SAED patterns of (a) undoped NaYF4 crystals and (b) NaYF4:Yb,Er crystals grown at a holding temperature of 1100 °C and a cooling rate of 5 °C 3 h-1. It was found that the ring pattern apparently changed to a spot pattern by doping with Yb and Er; that is, the crystallinity of NaYF4:Yb,Er crystals increased in comparison to that of NaYF4 crystals, though a weak streak, which is related to the local disorder of the atomic arrangement, is seen. However, the formation of single-phase, hexagonalsystem NaYF4:Ln crystals was confirmed from this pattern when doped with Ln. The minute differences in ionic radii among Y3þ, Yb3þ, and Er3þ were thought to affect the crystallinity of NaYF4 and NaYF4:Yb,Er crystals. Furthermore, the lattice image obtained from NaYF4:Yb,Er crystals grown at a cooling rate of
Figure 5. (a) Diffuse reflection spectra of NaYF4 and NaYF4:Ln crystals; (b) red, green, and blue upconversion emission of the NaYF4:Ln crystals under 980 nm laser irradiation.
5 °C 3 h-1 is shown in Figure 3c. The crystals were of very good crystallinity in this image. Figure 4 shows the EDS spectra of (a) 997
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Figure 7. FESEM images of (a) NaYF4 and (b) NaYF4:Yb,Er crystals grown at a holding temperature of 800 °C and a cooling rate of >120000 °C 3 h-1.
Figure 8. X-ray diffraction patterns (Cu KR) of (a) the NaYF4 crystals and (b) NaYF4:Yb,Er crystals grown at a cooling rate of 5 °C 3 h-1 and (c) NaYF4:Yb,Er crystals grown at a cooling rate of >120000 °C 3 h-1.
Figure 6. Upconversion fluorescence spectra under 980 nm excitation of (a) NaYF4:Yb,Er crystal grown at a cooling rate of >120000 °C 3 h-1 (red fluorescence), (b) NaYF4:Yb,Er crystal grown at a cooling rate of 5 °C 3 h-1 (green fluorescence), and (c) NaYF4:Yb,Tm crystal with blue upconversion fluorescence.
green and red upconversion emission, respectively. Because the crystal system of the NaYF4:Yb,Er crystals grown at cooling rates of 5 and >120000 °C 3 h-1 was hexagonal and cubic, respectively, the upconversion emission was influenced by the cooling rate (i.e., the crystal system). Furthermore, the NaYF4:Yb,Tm crystals showed blue upconversion emission regardless of cooling rate, as shown in Figure 5b. From the fluorescence results, the upconversion fluorescence characteristics of NaYF4:Ln crystals depend on the growth conditions and the type of dopant. Red, green, and blue upconversion fluorescence of NaYF4:Ln crystals were clearly observed under 980 nm excitation and were successfully controlled by changing the growth conditions and type of dopant. The upconversion fluorescence spectra of NaYF4:Ln crystals under 980 nm excitation are shown in Figure 6, which shows a NaYF4:Yb,Er crystal grown at (a) a cooling rate of >120000 °C 3 h-1 (red fluorescence) and (b) a cooling rate of 5 °C 3 h-1 (green fluorescence), and (c) a NaYF4:Yb,Tm crystal with blue upconversion fluorescence. When using the NaYF4:Yb,Er crystals with red fluorescence, two strong peaks were observed at the wavelength between 630 and 680 nm in Figure 6a and were attributed to the 4 F9/2 f 4I15/2 radiative transition. These peaks might originate the cubic-system NaYF4:Ln crystals. Three emissions were observed in Figure 6b using a NaYF4:Yb,Er crystal with green fluorescence, which were assigned to the transitions 2H11/2 f 4I15/2, 4S3/2 f 4 I15/2, and 4F9/2 f 4I15/2 at the wavelengths 521 nm, 541 nm, and 655 nm, respectively, for the Er3þ ions. The emission peak of 4 S3/2 f 4I15/2 was mostly strong, and the dominant green-emission band was clearly around 541 nm. When using the NaYF4:Yb,Tm
NaYF4:Yb,Er crystals and (b) NaYF4:Yb,Tm crystals grown from a NaF flux. These EDS spectra indicate that fluorine, sodium, ytterbium, and yttrium atoms were detected in both crystals. Furthermore, erbium and thulium atoms can clearly be observed in the NaYF4:Yb,Er and NaYF4:Yb,Tm crystals in Figure 4a and b, respectively. In addition, it was also found from EDS analysis that these component atoms were homogeneously distributed in both crystals. From XRD, SAED, and EDS analyses and SEM and TEM observations, it was found that highly crystalline and well-developed NaYF4 and NaYF4:Ln (Ln = Yb, Er, Tm) crystals were successfully fabricated by the NaF flux cooling method. The vis-NIR light diffuse reflectance spectra of the NaYF4 and NaYF4:Ln crystals were measured in order to investigate the presence of Yb3þ ions. In addition, their upconversion fluorescence was observed under 980 nm laser irradiation. No absorbance spectra in the NIR region were observed in the NaYF4 and NaYF4:Er diffuse reflection spectra, as shown in Figure 5a. By contrast, the diffuse reflection spectra of NaYF4:Yb, NaYF4:Yb, Er, and NaYF4:Yb,Tm crystals exhibit maximum absorption at around 970-980 nm, as shown in Figure 5a. These results agree well with previous studies and show that Yb3þ ions are included in the grown crystals.32 In addition, the NaYF4:Ln crystals obviously demonstrated bright red, green, and blue upconversion emission under 980 nm laser irradiation, as shown in Figure 5b. The upconversion emission in the NaYF4:Ln crystal was dependent on the cooling rate and type of dopant. The NaYF4:Yb,Er crystals grown at cooling rates of 5 and >120000 °C 3 h-1 show 998
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Crystal Growth & Design crystals with blue fluorescence, the peak attributed to the 1G4 f 3H6 transition was observed at a wavelength of 474 nm. Next, the NaYF4 and NaYF4:Yb,Er crystals were grown using a mixed NaF-KF flux cooling method to decrease the holding temperature. NaF and KF powders were mixed at a molar ratio of 4:6 because the eutectic temperature of the mixed flux was approximately 710 °C. Therefore, NaYF4 and NaYF4:Yb,Er crystals were successfully grown at a relatively low holding temperature of 800 °C for 10 h. Figure 7 shows the FESEM images of typical (a) NaYF4 and (b) NaYF4:Yb,Er crystals grown at a cooling rate of >120000 °C 3 h-1. The basic forms of the NaYF4 and NaYF4:Yb,Er crystals were spherical in shape. The crystals were relatively uniform, densely aggregated, transparent, and colorless. As shown in Figure 7, crystals up to approximately 700 nm in average size were grown. Figure 8 shows the XRD profiles for the pulverized crystallites of (a) the NaYF4 crystals grown at a cooling rate of >120000 °C 3 h-1, (b) NaYF4:Yb,Er crystals grown at a cooling rate of 5 °C 3 h-1, and (c) NaYF4:Yb, Er crystals grown at a cooling rate of >120000 °C 3 h-1. Strong diffraction lines in all samples can clearly be observed; these patterns corresponded to those of hexagonal-system NaYF4. Additionally, the EDS data showed that fluorine, sodium, and yttrium atoms were homogeneously distributed in the crystals. Potassium atoms from the flux were not detected in the crystals. Therefore, hexagonal-system NaYF4 and NaYF4:Yb,Er crystals were successfully fabricated using a mixed NaF-KF flux cooling method. Since the NaF-KF mixed flux has relatively low eutectic temperatures, it can be applied to grow high quality, functional fluoride crystals under lower temperature conditions.
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program of Advanced Research Center of Metallic Glasses, Institute for Materials Research, Tohoku University.
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’ CONCLUSIONS Environmentally friendly, highly crystalline, and well-formed NaYF4:Ln (Ln = Yb, Er, Tm) crystals with upconversion fluorescence were easily fabricated using a NaF flux cooling method at 1100 °C. The basic forms of the NaYF4:Ln crystals were spherical in shape. Furthermore, their crystal system was affected by the cooling rate. In addition, the upconversion fluorescence properties of NaYF4:Ln crystals also depended on the cooling rate (i.e., the crystal system) and type of dopant. The red, green, and blue upconversion fluorescence of the grown NaYF4:Ln crystals were clearly observed under 980 nm laser irradiation and were successfully controlled by changing the cooling rate and type of dopant. Moreover, the NaYF4:Yb,Er crystals were successfully grown using a NaF-KF flux cooling method at 800 °C. Our crystal growth technique using solo NaF or mixed NaF-KF fluxes can be applied to the growth of a variety of functional fluoride crystals at relatively low temperatures in air, and because they are harmless to humans and the environment, it is an environmentally friendly process. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This research was partially supported by a Grant-in-Aid (No. 20350093) from the Ministry of Education, Culture, Sports, Science and Technology (Japan). This research was partially performed under the interuniversity cooperative research 999
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