Synthesis and Characterization of Upconversion Fluorescent Yb3+

Apr 8, 2010 - A compendium concerning the crystal chemistry of inorganic fluorides is given by Babel and Tressaud.4 A large number of reports have bee...
0 downloads 0 Views 3MB Size
Download PDF
DOI: 10.1021/cg901441p

Synthesis and Characterization of Upconversion Fluorescent Yb3þ, Er3þ Doped RbY2F7 Nano- and Microcrystals

2010, Vol. 10 2202–2208

Helmut Sch€afer,* Pavel Ptacek, Benjamin Voss, Henning Eickmeier, J€ org Nordmann, and Markus Haase University of Osnabr€ uck, Institute of Chemistry, Barbarastrasse 7, D-49069 Osnabr€ uck, Germany Received November 18, 2009; Revised Manuscript Received March 2, 2010

ABSTRACT: Lanthanide-doped RbY2F7 nanocrystals with a mean diameter of approximately 10 nm were synthesized at 185 °C in the high boiling organic solvent N-(2-hydroxyethyl)-ethylenediamine (HEEDA) using ammonium fluoride, rare earth chlorides, and a solution of rubidium alkoxide of N-(2-hydroxyethyl)-ethylenediamine in HEEDA as precursors. Transmission electron microscopy images of the particles reveal that they are separated but have a broad size distribution ranging from 6 to 22 nm. Heat-treatment of these nanocrystals (600 °C for 45 min) led to bulk material which shows highly efficient light emission upon continuous wave (CW) excitation at 978 nm. As an alternative to the synthesis procedure carried out in the organic solvent HEEDA, we performed a microwave synthesis under hydrothermal conditions. This procedure led to the same composition of the material, but we obtained a marked increase of the particle size (60 nm). Apart from the optical properties, the structure and the morphology as well as the constitution of the three products were investigated by means of powder X-ray diffraction and X-ray fluorescence spectroscopy.

Introduction In recent years, lanthanide-doped phosphors, especially those that are able to convert long wave radiation into radiation with higher energy by so-called photon upconversion (UC), became quite popular. In contrast to second harmonic generation (SHG) and simultaneous two photon absorption (STPA), both of which involve a nonstationary (“virtual”) quantum mechanical state, the upconversion process is based solely on metastable quantum mechanical states. As a consequence, the efficiency at low excitation densities is orders of magnitude higher than for SHG and simultaneous two photon absorption. Typical excitation densities occur in the range of 1-103 W cm-2 for upconversion, compared to 106-109 W cm-2 for STPA.1-3 The use of cheap continuous wave diode laser sources is then possible. Different host lattices were doped with the UC ion couple Yb3þ/Er.3þ In these codoped compounds, the excitation light is primarily absorbed by Yb3þ. Acting as a so-called sensitizer, it shows a comparatively high absorption cross section at 978 nm. The energy of two excited Yb3þ states is then transferred to one Er3þ ion, resulting in emission mainly in the green and the red spectral region. Fluoride host lattices have some benefits. Low phonon energies of these lattices minimize nonradiative multiphonon relaxation processes in the dopant. A compendium concerning the crystal chemistry of inorganic fluorides is given by Babel and Tressaud.4 A large number of reports have been published on phosphors based on (cubic or hexagonal) NaYF4 host lattices since especially these materials display a very strong effect of NIR-to-Vis photon upconversion. More recently, synthesis procedures for lanthanide-doped nanocrystals of R-NaYF45-14 and β-NaYF46,7,13-19 have been developed. LiYF4, a promising candidate for many applications (laser material, lamp phosphor), was prepared for example by the sol gel technique.20 In 2007, we investigated *To whom correspondence should be addressed. Tel.: þ49 54 19692382; fax: þ49 5419693323; e-mail: [email protected]. pubs.acs.org/crystal

Published on Web 04/08/2010

the optical and structural properties of Yb, Er doped KYF4 core particles and Yb, Er, doped KYF4 core particles modified with undoped KYF4.21,22 As a result of the Fullprof refinement, we found that KYF4, generated in HEEDA at 185 °C, crystallizes in the cubic (alpha) NaYF4 structure. In particular, the surface-modified particles showed intensive upconversion emission upon excitation in the NIR.21 In the 1970s Aleonard et al. synthesized undoped RbM2F7 (M = Y, Eu, Gd, Dy, Er) and investigated the structural properties.23 You et al. generated RbM2F7 (M = Y, Er, Yb) and examined the structural properties as well as the optical specifications of the Eu3þ doped species.24 Gd-doped RbY2F7 as well as RbGd2F7 were synthesized by Khaidukov et al.25, and the optical properties of these compounds were investigated in detail by Blasse, Sytsma, and Ellens et al.25 Hebecker and L€ osch achieved under fluorination of RbCl/Ln2O3 mixtures at 550 °C, the generation of bulk RbMF4 with M = Y, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, and Lu and determined the lattice constants on the basis of single X-ray diffraction data.26 The quaternary species Rb2KYF6 and Rb2KInF6 were gained by Grannec et al. Via NMR study they found out that in these matters temperature-dependent phase transitions take place.27 All these compounds of the type RbMF4, RbM2F7, or Rb2KMF6 described in the literature were synthesized under drastic conditions by solid-state reactions or by hydrothermal techniques. Drastic reaction conditions (pressure, temperature) respectively the lack of size limiting compounds very often lead to bulk material or at least to material with a particle size above 200 nm, which do not form transparent colloids and are much too large to replace molecular dyes in biological tagging applications. Anyway, as far as we know, these materials have not been produced in the nanoscale up to now. In this report, we describe two synthesis procedures for the generation of Yb, Er doped RbY2F7. In addition, a heat treatment of one of the products is presented. The procedures presented herein are suitable in order to achieve rare earth doped crystalline material of RbY2F7 in a crystallite size range r 2010 American Chemical Society

Article

Crystal Growth & Design, Vol. 10, No. 5, 2010

2203

from less than 10 nm to more than 5 μm. The optical properties as well as the structural properties were investigated based on powder X-ray diffraction (PXRD) data and Rietveld refinements. As the luminescence of the synthesized nanomaterial was very weak, the heat-treated sample, consisting of micrometer-sized grains, turned out to be a good upconversion emitter. Experimental Section Synthesis. Synthesis of the Yb3þ, Er3þ Doped RbY2F7 Particles in HEEDA. RbY2F7:20% Yb, 2% Er nanocrystals were prepared in the coordinating solvent N-(2-hydroxyethyl) ethylenediamine (HEEDA) similar to the method described previously.28 The following three solutions were used in the synthesis: (A) Solution of the lanthanide chlorides:A clear solution of 3.55 g (11.7 mmol) of YCl3 3 6H2O (99.9%, Treibacher Industries), 1.16 g (3 mmol) of YbCl3 3 6H2O (99.9%, Treibacher Industries), and 0.115 g of (0.3 mmol) ErCl3 3 6H2O (99.9%, Treibacher Industries) in about 25 mL of methanol was combined with 35 mL of N-(2-hydroxyethyl)-ethylenediamine (99%, Sigma-Aldrich). The methanol was removed with a rotary evaporator, and the water was distilled off in high vacuum at 75 °C. The remaining slightly cloudy suspension was allowed to cool down to 60 °C and kept at this temperature under dry nitrogen. (B) Preparation of the rubidium alkoxide solution:A solution of the rubidium alkoxide of N-(2-hydroxyethyl)ethylenediamine (HEEDA) was prepared by dissolving 0.78 g (7.5 mmol) of rubidium metal (Sigma-Aldrich) in 10 mL of HEEDA at 20 °C under dry nitrogen. (C) Preparation of the solution containing fluoride:1.96 g (52 mmol) of NH4F (98% Fluka) were dissolved in about 25 mL of methanol and combined with 25 mL of HEEDA. The methanol was removed with a rotary evaporator at 45 °C and subsequently in high vacuum at 60 °C. The remaining solution was kept at 45 °C under dry nitrogen. Solution A and solution B were combined and heated to 60 °C. Subsequently, the fluoride containing solution (C), which had a temperature of 45 °C, was added under stirring, and the resulting mixture was degassed at 80 °C under a vacuum. The reaction mixture was heated to 185 °C under dry nitrogen and kept at this temperature for 13 h. After the transparent solution had cooled down to room temperature, the nanocrystals were precipitated by adding a mixture of 200 mL of water and 200 mL of 2-propanole. The precipitate was separated by centrifugation and washed several times by repeatedly resuspending the solid in 2-propanol and centrifuging the suspension. Usually, the purified precipitate was directly redispersed in 2-propanole without drying the powder (see below). For XRD measurements, the precipitate was dried in air (white powder, yield: 2.24 g (69%)). Heat Treatment of the Particles. The particles were heated under air for 45 min at 600 °C. Microwave-Assisted Synthesis. A solution containing 2.36 g (7.8 mmol) of YCl3 3 6H2O (99.9%, Treibacher Industries), 0.78 g (2 mmol) of YCl3 3 6H2O (99.9%, Treibacher Industries), 76 mg (0.2 mmol) of ErCl3 3 6H2O (99.9%, Treibacher Industries), 1.48 g (40 mmol) of NH4F (98% Fluka), and 52 mL of water was filled into the 80 mL reaction vessel of the microwave system. Subsequently, the amount of 0.73 g (7 mmol) of RbF was added and the process was started. The maximum radiation power was set to 300 W, the maximum temperature was set to 185 °C, and the maximum pressure was set to 13.8 bar. After radiation for 2 h, the mixture was allowed to cool down. The precipitate was separated by centrifugation and washed several times with water. For XRD measurements, the precipitate was dried in air (white powder, yield: 1.78 g (82%)). Measurements. X-ray diffraction data were recorded at room temperature on an X’Pert Pro diffractometer (Panalytical) with Bragg-Brentano geometry using Cu KR1 radiation (40 kV, 40 mA) λ = 1.5406 A˚. The average apparent crystallite size as well as the lattice parameters are evaluated by structure profile refinements of

Figure 1. (a) Line pattern of orthorhombic KEr2F7 file number PDF 28-01-086-2454. (b) Observed X-ray powder diffraction profile of the heat treated sample of RbY2F7:78% Y, 20% Yb, 2.0% Er (gray line), Rietveld fit (black line), and residuum. (c) Observed X-ray powder diffraction profile of the microwave generated sample of RbY2F7:78% Y, 20% Yb, 2.0% Er (gray line), Rietveld fit (black line), and residuum. (d) Observed X-ray powder diffraction profile of RbY2F7:78% Y, 20% Yb, 2.0% Er generated in HEEDA (gray line), Rietveld fit (black line), and residuum. X-ray powder diffraction data collected at constant steps in scattering angle 2Θ using FullProf program29 (version Feb. 2007. LLB, J. Rodrı´ guez-Carvajal, Saclay, France).29 An Y2O3 powder standard was used to determine the instrumental resolution function of the diffractometer. The measurement was done with a step size of 0.334 2Θ. For the correction of refinement, the Modified March’s Function is used. Emission spectra of colloidal solutions of the nanocrystals and of the pure crystals were measured with a Fluorolog 3 spectrometer (Jobin Yvon) combined with a continuous wave 978 nm laser diode (LYPE 30-SG-WL978-F400). Quartz cuvettes (Hellma, QX) containing solutions of the samples or tubes with powder samples were placed inside the spectrometer and excited by the 978 nm light via an optical fiber. All spectra were corrected for the sensitivity of the monochromators and the detection system. The upconversion emission spectra of powder samples were measured with the same instrument but in front-face geometry. Transmission electron microscopy (TEM) images were taken with a 200 kV JEOL JEM-2100 microscope equipped with a charged-coupled device (CCD)-camera (Gatan). X-ray fluorescence spectroscopy (XFA) measurements were performed with an X-ray fluorescence (XRF) sequential spectrometer ARL Advant’ XP. FTIR spectra have been taken on a Bruker (Rheinst€ atten, Germany) Vertex 70 equipped with a Golden Gate Single Reflection Diamond ATR system. The microwave synthesis was performed in a CEM Discover, 47456 Kamp-Lintfort, Germany.

Results and Discussion The chemical composition of the three samples was determined via XFA measurements and differs slightly. The detailed data which can be taken from Table 3 reveals the stoichiometry RbY2Fx. The crystal structures of the three products were determined by means of powder XRD and the resulting pattern was analyzed with the help of Rietveld refinements. Figure 1 presents the powder diffraction patterns of the RbY2F7 samples together with the Rietveld refinements and diffraction lines of KEr2F7 (PDF 01-086-2454, ICSD 040450). Of course, different crystallite sizes lead to different line broadening; this can be concluded in detail from Figure 1

2204

Crystal Growth & Design, Vol. 10, No. 5, 2010

Sch€ afer et al.

(right figure). The refinements were performed using FullProf starting with (1) the structure model of KEr2F7 - ICSD 40450 (Kþ and Er3þ was replaced by Rbþ and Y3þ, respectively) and (2) the instrumental resolution file (obtained after measurement of bulk Y2O3) including the instrumental broadening of the diffraction lines. The scale factor, lattice parameters, and the background parameters were refined first, followed by the peak profile, atomic positions, and anisotropic temperature Table 1. Atomic Coordinates and Anisotropic Temperature Factors of Sample 1c Gained from the Microwave Assisted Synthesis atom

x

y

z

occ.

Y1 Y2 Y3 Rb1 Rb2 F1 F2 F3 F4 F5 F6 F7 F8 F9 F10

0.0021 0.2557 0.2286 0.0728 0.4391 0.2007 0.1803 0.2811 0.2491 0.3647 0.3124 0.4270 0.0869 0.3764 0.0245

0.7512 0.3421 0.0691 0.0468 0.0624 0.5220 0.1667 0.5020 0.2341 0.4044 0.3374 0.2670 0.3777 0.1245 0.0886

0.2500 0.0240 0.7500 0.2500 0.2500 0.2500 0.9286 0.5085 0.2500 0.2500 0.7500 0.0034 0.9230 0.7500 0.7500

0.5500 1.1470 0.5290 0.3680 0.4220 1.0130 1.6230 0.9180 0.6310 0.5000 0.5000 1.0000 1.0000 0.5000 0.5000

atom

B11

B22

B33

B12

B13

B23

Y1 Y2 Y3 Rb1 Rb2 F1 F2

91.2 88.8 479.6 140.0 267.0 165.9 498.6

35.0 72.1 167.4 247.0 947.7 839.9 481.2

187.0 168.6 86.8 -92.1 53.2 -256.3 722.2

2.1 -40.8 -108.2 141.1 -601.3 567.3 -536.9

0.0 0.0 0.0 0.0 0.0 0.0 -137.8

0.0 0.0 0.0 0.0 0.0 0.0 451.3

Table 2. Atomic Coordinates and Anisotropic Temperature Factors of Sample 1d Gained from the HEEDA Synthesis atom

x

y

z

occ.

Y1 Y2 Y3 Rb1 Rb2 F1 F2 F3 F4 F5 F6 F7 F8 F9 F10

0.00890 0.25635 0.22736 0.09030 0.43110 0.18905 0.17427 0.28109 0.24912 0.36470 0.31240 0.42700 0.08690 0.37640 0.02450

0.74797 0.34466 0.06787 0.07713 0.07051 0.52169 0.19206 0.50199 0.23407 0.40440 0.33740 0.26700 0.37770 0.12450 0.08860

0.25000 0.01884 0.75000 0.25000 0.25000 0.25000 0.91773 0.50845 0.25000 0.25000 0.75000 0.00340 0.92300 0.75000 0.75000

0.511 1.088 0.530 0.584 0.384 0.620 1.267 0.918 0.631 0.500 0.500 1.000 1.000 0.500 0.500

atom

B11

B22

B33

B12

B13

B23

Y1 Y2 Y3 Rb1 Rb2 F1 F2

68.9 72.8 181.7 7.9 123.5 389.0 1684.8

28.8 -52.4 5.3 460.9 7.7 3282.7 5443.3

8.3 410.3 186.1 -67.0 118.2 423.7 1711.6

-63.8 23.5 -44.0 97.3 -75.1 1694.0 -2933.6

0.0 0.0 0.0 0.0 0.0 0.0 -2216.4

0.0 0.0 0.0 0.0 0.0 0.0 3087.3

factors. The site occupancies were not refined and were used as given in the CIF file from ICSD. Crystallite size determination within the FullProf is described in detail by Carvajal and Roisnel.30 Atomic coordinates and anisotropic temperature factors of the nanosized samples can be taken from Tables 1 and 2. In a parallelly performed investigation regarding CsY2F7, we also decided to use the crystallographic data of orthorhombic KEr2F7 to create the input control file (PCRfile) for FullProf refinements and there the diffractogram was already well fitted by the Rietveld method.31 These findings can be easily explained by the investigations of Karbowiak et al.32 who found CsGd2F7 to be isostructural with KEr2F7 (orthorhombic, space group Pnam). The structure of the heattreated sample 1b (heat treatment of the sample from the HEEDA synthesis; see Figure 1b) was indexed with the following lattice constants a = 11.99 A˚, b = 13.51 A˚, and c = 7.75 A˚, and we deduced an estimated crystallite size of >5 μm. TEM images of the sample also show particles of this size (Figure 2). The sample gained from the microwave synthesis apparatus (sample 1c) showed an average crystallite size of 60 nm and the following lattice constants: a = 11.97 A˚, b = 13.41 A˚, and c = 7.79 A˚ after Rietveld refinement of the corresponding powder data (Figure 1c). The particle size can also be extracted from the TEM images, though the particle size distribution was quite broad (see Figure 3). The powder pattern from the nanosized RbY2F7 sample (sample 1d) gained by the HEEDA synthesis is shown in Figure 1d. The best agreement between observed and calculated profiles was obtained by the predefinition of spherical crystallite shapes. The diffractogram is well fitted by the Rietveld method yielding a value of a = 11.97 A˚, b = 13.46 A˚, and c = 7.76 A˚ for the orthorhombic unit cell and a mean crystallite size of 10 nm. Obviously, some particles are not single crystallites and contain more than one crystallite; hence, the average particle size extracted from the TEM image (Figure 4) is higher than the corresponding size of the crystallites. Regarding the Rietveld method it is surely possible to determine crystal structures

Figure 2. TEM image of RbY2F7:78% Y, 20% Yb, 2.0% Er nanocrystals prepared in HEEDA after heat-treatment.

Table 3. Calculated and Obtained Stoichiometry of the Samples 1b, 1c, and 1d expected as-prepared sample (1d) microwave sample (1c) heat-treated sample (1b)

Rb (wt %)

Y (wt %)

Yb (wt %)

Er (wt %)

stoichiometry

19.73 21.6 23.8 25.58

32.02 31.7 36.2 37.68

18.51 16.2 15.97 15.98

1.54 1.89 1.95 1.96

RbY1.56Yb0.4Er0.04 RbY1.411Yb0.37Er0.044 RbY1.462Yb0.33Er0.042 RbY1.474Yb0.321Er0.041

Article

Crystal Growth & Design, Vol. 10, No. 5, 2010

2205

Figure 5. Characterization by dynamic light scattering: particle size and size distribution of RbY2F7:78% Y, 20% Yb, 2.0% Er nanocrystals (HEEDA synthesis) in 2-propanole. Figure 3. TEM image of RbY2F7:78% Y, 20% Yb, 2.0% Er nanocrystals generated in the microwave synthesis apparatus.

Figure 4. TEM image of RbY2F7:78% Y, 20% Yb, 2.0% Er nanocrystals prepared in HEEDA.

from powder data. In this case, the possibility to succeed is probably limited by the fact that the refinement would include too many lines and/or better diffraction data would be needed. The average hydrodynamic diameter of these particles, extracted from dynamic light scattering measurements in 2-propanole, is about 32 nm, probably because of agglomeration of the particles (see Figure 5). Upon excitation in the NIR, the samples show visible light emission and the emitted light was analyzed. Sample 1d formed a transparent solution in 2-propanole. Figure 6 shows the light emission of a 1 wt % colloidal solution of RbY2F7: Yb, Er nanoparticles in 2-propanole upon excitation in the NIR. The laser power was about 20 W mm-2, and the overall laser power was 3 W. The emitted light appears green to the eye; the corresponding emission spectrum can be taken from Figure 7. The spectrum is similar to those of Yb, Er doped NaYF4, but there are differences concerning the splittings of the emission lines which correspond to different local structures of emitting ions. In order to evaluate the upconversion quality of the particles, we used hexagonal bulk NaYF4: 18% amer (University of Yb3þ, 2% Er3þ synthesized by K. Kr€ Bern) as a comparison compound. The green to red ratios of the compared samples differ distinctly, and the corresponding

Figure 6. Image of upconversion luminescence in 1 wt % colloidal solutions of RbY2F7:78% Y, 20% Yb, 2.0% Er nanocrystals in 2-propanole. Excitation at 978 nm with a power density of about 20 W mm-2. Overall laser power: 3 W. The laser is positioned on the right side.

Figure 7. Emission spectrum of 1 wt % colloidal solution of RbY2F7:78% Y, 20% Yb, 2.0% Er nanocrystals in methanol.

values can be taken from the emission spectra (Figure 8a-d). Figure 9 displaying double logarithmic plots of the emitted

2206

Crystal Growth & Design, Vol. 10, No. 5, 2010

Sch€ afer et al.

Figure 8. Emission spectra of the pure crystals of a: RbY2F7:78% Y, 20% Yb, 2.0% Er nanocrystals generated in HEEDA. b: RbY2F7:78% Y, 20% Yb, 2.0% Er nanocrystals generated in the microwave synthesis apparatus. c: RbY2F7:78% Y, 20% Yb, 2.0% Er nanocrystals generated in HEEDA and heat treated for 45 min at 600 °C. d: Hexagonal bulk NaYF4: 18% Yb3þ, 2.0% Er3þ.

Figure 10. FTIR spectrum of the solvent N-(2-hydroxyethyl)-ethylenediamine (HEEDA) a: RbY2F7:78% Y, 20% Yb, 2.0% Er generated in HEEDA a0 : RbY2F7:78% Y, 20% Yb, 2.0% Er generated in HEEDA treated in a vacuum at 240 °C b: RbY2F7:78% Y, 20% Yb, 2.0% Er generated in the microwave synthesis apparatus c: RbY2F7:78% Y, 20% Yb, 2.0% Er generated in HEEDA and heat treated for 45 min at 600 °C. Figure 9. Double logarithmic plots of emitted light intensity versus power density of exciting light for pure crystals of a: RbY2F7:78% Y, 20% Yb, 2.0% Er generated in HEEDA. b: RbY2F7:78% Y, 20% Yb, 2.0% Er generated in the microwave synthesis apparatus. c: RbY2F7:78% Y, 20% Yb, 2.0% Er generated in HEEDA and heat treated for 45 min at 600 °C. d: Hexagonal bulk NaYF4: 18% Yb3þ, 2% Er.3þ.

light intensity versus the power density of the exciting light, shows the upconversion efficiency of the synthesized probes in detail. Hexagonal NaYF4 is considered to be the most efficient host material for the UC ion couple Yb3þ, Er3þ, and it was no surprise to us that the NaYF4 reference sample showed the most intensive light emission. The upconversion efficiency of the synthesized rubidium probes increases with growing particle size (a < b < c), and the efficiency ratios between the reference sample and the Rb containing compounds was at 1.78 W laser power 5 (c), 1690 (b), and 7.0  104 (a). In the case of lanthanide-doped materials, the quantum yield and, hence, also the UC efficiency can be strongly reduced, if groups with vibrational modes of high energy for example OH, NH2 from ligands or solvent molecules are located in close proximity to the light-emitting lanthanide ions. We therefore investigated the surface of the particles via FTIR (Figure 10). On the surface of the RbY2F7 nanocrystals, the presence of NH2 and OH groups of HEEDA molecules was proven (curve a). In order to check the quenching properties of the solvent HEEDA, we performed an additional experiment in which HEEDA was distilled off from this sample in a vacuum at 240 °C. And indeed the corresponding FTIR spectrum (curve Figure 10a0 ) shows no absorption bands. This procedure does not influence the crystallographic properties. However, after removal of the HEEDA by

Figure 11. Fourier transform electron diffraction pattern of the nanocrystalline sample generated in the solvent HEEDA.

vacuum distillation, the integrated emission intensity increases by a factor of approximately 2. This experiment proves that HEEDA acts as a quenching compound and decreases the optical efficiency. Together with the low particle size and the resulting high surface to volume ratio, it explains the lowest UC efficiency to be found by the particles (curve a). Sample 1c showed a stronger luminescence, probably because of the lack of quenching components on the surface of the particles (Figure 10b) respectively a better crystallinity of the low energy phonon host (RbY2F7) (see also corresponding Fourier transform electron diffraction pattern, Figures 11 and 12). Within the series of rubidium containing compounds sample 1b

Article

Crystal Growth & Design, Vol. 10, No. 5, 2010

2207

Figure 12. Fourier transform electron diffraction pattern and high resolution TEM image of the sample generated in the microwave synthesis apparatus.

Figure 13. Fourier transform electron diffraction pattern and high resolution TEM image of the heat treated sample (45 min, 600 °C) generated in HEEDA.

showed the highest UC efficiency. The FTIR spectra showed no absorption bands like in the case of the microwave-generated compound (Figure 10c). A better crystallinity of the matrix can be taken from the corresponding Fourier transform electron diffraction pattern (Figure 13) and also a better electronic environment of the ions responsible for the light emission could cause the increase of the light emission.

today, hexagonal bulk NaYF4: 18% Yb3þ, 2% Er3þ, the upconversion efficiency, especially of the sample generated in HEEDA, was very low. Acknowledgment. The authors are grateful to Dr. Karl Kr€ amer (University of Bern) for synthesizing the bulk reference material.

References Conclusion IR-to-visible UC fluorescent nanocrystals of orthorhombic phase RbY2F7: 78%Y, 20% Yb, 2% Er particles were synthesized via different routes. Heat treatment of the nanocrystals led to bulk material (microcrystals) of the same composition but with much better crystallinity. FullProf refinements of the powder diffraction data of the probes led us to the conclusion that RbY2F7, generated in the way presented in this article, crystallizes in an orthorhombic structure known from KEr2F7. The average crystallite size calculated by FullProf refinement was 10 nm for the RbY2F7 sample, generated in HEEDA, about 60 nm for the sample gained from the synthesis carried out in microwave synthesis apparatus, and more than 5 μm for the particles after heat treatment. The nanoparticles from the HEEDA synthesis form transparent colloidal solutions in polar organic solvents. These solutions as well as the pure crystals show visible light emission upon excitation in the NIR. Nevertheless, in comparison with the most efficient upconversion phosphor known

(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18)

Downing, E.; Macfarlane, R. M. Science 1996, 273, 1185. Scheps, R. Prog. Quantum Electron. 1996, 20, 271. Bruchez, M.; Moronne, Jr.; Gin, M.; Weiss, P. Science 1998, 281, 2013. Heer, S.; Kompe, K.; Gudel, H. U.; Haase, M. Adv. Mater. 2004, 16, 2102. Babel, D.; Tressaud, A. In Inorganic Solid Fluorides; Hagenmuller, P., Ed.; Academic Press, Inc.: New York, 1985; pp 77-203. Mai, H.-X.; Zhang, Y.-W.; Yan, C.-H. J. Am. Chem. Soc. 2006, 128, 6426. Wang, L.; Li, Y. Chem. Mater. 2007, 19, 727. Wang, Y.; Qin, W.; Wang, L. Chem. Lett. 2007, 36 (7), 912. Sun, Y.; Chen, Y.; Kong, X.; Zhang, H. Nanotechnology 2007, 18, 1. Yi, G.; Lu, H.; Guo, L.-H. Nano Lett. 2004, 4, 2191. Wang, X.; Zhuang, J.; Peng, Q.; Li, Y. D. Nature 2005, 437, 121. Boyer, J.-C.; Cuccia, L. A.; Capobianco, J. A. Nano Lett. 2007, 7 (3), 847. Wie, Y.; Lu, F.; Zhang, X.; Chen, D. Chem. Mater. 2006, 18, 5733. Patra, A.; Ghosh, P. J. Phys. Chem. C 2008, 112, 3223–3231. Zeng, J.-H.; Su, J.; Li, Y.-D. Adv. Mater. 2005, 17, 2119. Chow, G. M.; Yi, G. S. Adv. Funct. Mater. 2006, 26, 2324. Yi, G.-S.; Chow, G.-M. Chem. Mater. 2007, 19, 341. Wang, L.; Li, Y. Nano Lett. 2006, 6 (8), 1645.

2208

Crystal Growth & Design, Vol. 10, No. 5, 2010

(19) Sch€ afer, H.; Ptacek, P.; Eickmeyer, H.; Haase, M. Adv. Funct. Mater. 2009, 19, 3091. (20) Boyer, D.; Mahiou, R. Chem. Mater. 2004, 16, 2518. (21) Sch€ afer, H.; Ptacek, P.; Zerzouf, O.; Haase, M. Adv. Funct. Mater. 2008, 18, 2913. (22) Sch€ afer, H.; Ptacek, P.; Hickmann, K.; Prinz, M.; Neumann, M.; Haase, M. Russ. J. Inorg. Chem. 2009, 54 (12), 1914. (23) Aleonard, S.; Gonzales, O.; Gorius, M. F.; Roux, M. T. Mater. Res. Bull. 1975, 10, 1185. (24) You, F.; Huang, S.; Liu, S.; Tao, Y. J. Solid State Chem. 2004, 177, 2777. (25) Ellens, A.; Kroes, S. J.; Sytsma, J.; Blasse, G.; Khaidukov, N. M. Mater. Chem. Phys. 1991, 30, 127.

Sch€ afer et al. (26) Hebecker, C.; L€ osch, R. Naturwissenschaften 1975, 62, 37. (27) Faget-Guengard, H.; Bobe, J. M.; Senegas, J.; Tressaud, A.; Grannec, J. J. Alloys Comp. 1995, 238, 49. (28) Sch€afer, H.; Ptacek, P.; K€ ompe, K.; Haase, M. Chem. Mater. 2007, 19, 1396. (29) Rodriguez-Carvajal, J. Physica B 1993, 192, 55. (30) Rodrı´ guez-Carvajal, J.; Roisnel, T. Mater. Sci. Forum 2004, 443-444, 123. (31) Sch€afer, H.; Ptacek, P.; Eickmeier, H.; Haase, M. J. Nanomater. 2009, 2009, Article ID 685624, doi:10.1155/2009/685624. (32) Karbowiak, M.; Mech, A.; Romanowski, W. R. J. Lumin. 2005, 114, 65.