Keywords: CaF2, Microspheres, Upconversion, Nanoparticles, Size Effect. 1. INTRODUCTION. Rare earth ions doped inorganic micro- and nano- structures with ...
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Journal of Nanoscience and Nanotechnology Vol. 11,9916–9919, 2011
Synthesis and Upconversion Luminescence Properties of CaF2:Yb3+/Tm3+ Microspheres Tao Jiang, Dan Zhao, and Weiping Qin∗ State Key Laboratory on Integrated Optoelectronics, College of Electronic Science & Engineering, Jilin University, Changchun 130012, P. R. China CaF2 :Yb3+ /Tm3+ microspheres were synthesized by using a simple hydrothermal method. Their structures and morphologies were characterized by X-ray diffraction (XRD) and field-effective scanning electron microscopy (FESEM). The microspheres were formed from a large number of nanospheres with almost the same diameter after annealing. The growth mechanism of the microspheres was discussed. Strong ultraviolet (UV) and blue upconversion (UC) emission from the CaF2 microspheres was observed under 980 nm excitation. The enhancement of the UV UC emission was attributed to the size effect of the nanoscale particles.
Keywords: CaF2 , Microspheres, Upconversion, Nanoparticles, Size Effect.
1. INTRODUCTION
∗
Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2011, Vol. 11, No.11
2. EXPERIMENTAL DETAILS 2.1. Sample Preparation The starting materials were all analytical reagents. All chemicals were directly used as received without further purification. In a typical preparation, certain amount of Ca(CH3 COO)2 · H2 O was dissolved in hydrochloric acid to form clear solution. NaOH solution in excess was dissolved in the above solution. After that the upper clear solution was discarded the precipitate was separated via centrifugation. Then trifluoroacetic acid (TFA) was added dropwise into the precipitate while stirring. Trifluoroacetic calcium was obtained through evaporation in vacuum. Trifluoroacetic ytterbium and Trifluoroacetic thulium were synthesized through the same processes, respectively.
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Rare earth ions doped inorganic micro- and nanostructures with controlled shape and composition are of fundamental and technological interest for wide range applications in microelectronic, optoelectronic, catalysis, and bio-sensing.1 2 In rare earth activated inorganic materials, Upconversion (UC) micro- and nano-crystals (NCs) have attracted much attention due to their frequency UC luminescence properties from infrared (IR) to visible/UV. From various factors that influence UV UC luminescence properties, the morphology of the NC and the aggregation of the NCs are dominant. In order to achieve more intense UV UC luminescence and promote the applications, UC materials with various morphologies, such as nanorods,3–7 nanotubes,8 9 nanowires,10–13 and nanobelts,14–16 have been synthesized, and the effects of those morphologies on UC luminescence were investigated. In our group, some unusual UC emission have been observed in Yb3+ –Tm3+ codoped YF3 octahedral NCs, and these unusual UC emission have been attribute to the size effect.17 In comparison with the other systems, fluorides possess optical transparency over a wide wavelength range and very low phonon energies which minimize UC luminescence quenching of the rare earth ions in them.19–21 In this paper, Yb–Tm codoped CaF2 microspheres composed of large amount of nanospheres were prepared by a facial hydrothermal method. Lanthanide fluorides are often synthesized by liquid reactions between metal
nitrates/chlorides and HF/NaF/NH4 F in hydrothermal, coprecipitation, or combustion methods. There are fewer types of precursor and less impurities in the single-source molecular precursor method than in other methods. The method of using a single-source molecular precursor has been initially reported by Trindade et al. in synthesizing high quality semiconductor nanoparticles.22 23 Herein, such a single-source molecular precursor method was used. Rare-earth ions doped CaF2 microspheres were formed directly by Ca(CF3 COO)2 and La(CF3 COO)3 decomposition. The microspheres were uniform in size and well dispersed. Intense UV, visible, and near infrared (NIR) UC luminescence was observed and studied under the excitation of a 980 nm laser.
Intensity (a.u.)
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Synthesis and Upconversion Luminescence Properties of CaF2 :Yb3+ /Tm3+ Microspheres 1.0 0.8 0.6 0.4 0.2 0.0 10 1.0 0.8 0.6 0.4 0.2 0.0 10
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diffractometer (XRD) using a nickel-filtered Cu K radiation ( = 15418 Å) in the 2 range of 10 to 60 . The morphology of the annealed sample was investigated by scanning electron microscope (SEM, Hitachi TM1000). The UC luminescence spectra were recorded with a Hitachi F-4500 fluorescence spectrophotometer (2.5 nm for spectral resolution and 400 V for PMT voltage) under a 980-nm continuous wave diode laser (CW LD 2 W) excitation at room temperature.
3. RESULTS AND DISCUSSION 20
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Fig. 1. XRD pattern of the Ca0795 Yb02 Tm0005 F2 microspheres annealed at 600 C.
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The above three trifluoroacetates were further dissolved in water to form solutions in calculated proportion. Each solution was then vigorously stirred for 1 h, resulting the formation of a colorless solution. The clear solution was transferred to 25 ml Teflon-lined stainless steel autoclaves and treated at 180 C for 24 h. After cooling to room temperature naturally, samples were separated by centrifugation. The precipitate was purified with ethanol and dried in vacuum, and then annealed in HF atmosphere at 600 C for 1 h. Finally, the Ca0795 Yb02 Tm0005 F2 samples were obtained. 2.2. Characterization The crystal structure and phase purity of the products were analyzed by a Rigaku RU-200b X-ray powder
Figure 1 presents the XRD pattern of the as-prepared micro-crystals. Most of the diffraction peaks can be readily indexed to those of cubic CaF2 phase (JCPDS 77-2245, space group: F m3m225. The mole ratio of Ca and Yb ions is approximately 4 to 1, so the peak at 33.6 degree is due to the existence of YbF3 embedded into the matrix. No other phases have been found in the synthesized sample. This result indicates that this single-source molecular precursor method is an appropriate method to prepare CaF2 micro-crystals with purely cubic phase. The morphology and size of the products were characterized by the SEM observations, as shown in Figure 2. All the micro-crystals have a spherical shape and a diameter of 2–4 m. In their nucleation stage, Ca(CF3 COO)2 and La(CF3 COO)3 decomposed to form rare-earth doped CaF2 NCs, and then in their growing stage, the NCs self-assembled into microspheres in hydrothermal process. In order to confirm this process, several experiments were carried out by changing the hydrothermal time. It can be seen that the samples are spheres with a diameter of ∼200 nm, as shown in Figure 3(a), and no larger particles exist after 1 h hydrothermal reaction. When the
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Fig. 2. SEM images of the Ca0795 Yb02 Tm0005 F2 microspheres.
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Synthesis and Upconversion Luminescence Properties of CaF2 :Yb3+ /Tm3+ Microspheres
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Fig. 3. SEM images of the Ca0795 Yb02 Tm0005 F2 microspheres in different hydrothermal time (a) 1 h, (b) 2 h, (c) 5 h, (d) 6 h.
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Fig. 4. UC luminescence spectra of CaF2 :Yb3+ (20%)/Tm3+ (0.5%) microsphere (a) and bulk sample (b) under 980-nm excitation (the excitation current density is 1.60 mA).
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Fig. 5. Energy level diagrams of Yb3+ ions and Tm3+ ions and UC emission mechanism in CaF2 :Yb3+ (20%)/Tm3+ (0.5%) nanocrystals.
may alternatively play an important role in populating the 1 D2 level. The radiative transitions of the populated 1 D2 level to the lower states cause 362 nm and 452 nm emissions. On the other hand, the Tm3+ ions in the 1 D2 state may be excited to the 3 P2 level via another energy transfer process, and then they relax to the 1 I6 level, nonradiatively. Finally, the transitions of 1 I6 → 3 H6 and 1 I6 → 3 F4 produce 291 nm and 347 nm UC emissions, respectively. According to the UC mechanism described above, populating 1 D2 and 3 P2 levels of Tm3+ ions in our microspheres is more efficient than that in bulk CaF2 sample.
4. CONCLUSION Yb3+ –Tm3+ codoped CaF2 microspheres were synthesized by a simple single-source molecular precursor method. The doped CaF2 microspheres were formed as the decomposition of Ca(CF3 COO)2 and La(CF3 COO)3 directly.
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hydrothermal time increased to 5–6 h, more and more large particles appeared, as shown in Figures 3(c and d). Strong UV and blue UC emissions from the sample were observed under 980 nm excitation, as shown in Figure 4(a). UV emission peaks at 291 nm, 347 nm, and 362 nm are attributed to the 1 I6 → 3 H6 , 1 I6 → 3 F4 , and 1 D2 → 3 H6 transitions of Tm3+ , respectively. Blue emissions centered at 452 nm and 476 nm came from the 1 D2 → 3 F4 and 1 G4 → 3 H6 transitions, respectively. Red emissions centered at 644 nm and 699 nm are attributed to the 1 G4 → 3 F4 and 3 F3 →3 H6 transitions, respectively. The increase of UV emissions in these microspheres is obvious when compared with those from a bulk sample (Fig. 4(b)). Presumably, the enhanced UV emissions could be attributed to the aggregated nanoparticles although these microspheres were micron-sized. In a previous paper, we have compared UC luminescence between CaF2 bulk sample and CaF2 nanoscaled thin films.24 We proposed that the enhanced UV UC emissions from the thin films arose from their nanoscaled sizes. Considering that the microspheres were composed of a large number of nanospheres, the intense UV UC luminescence is attributed to the size effect of the buildup nanoparticles. Figure 5 describes schematically possible UC processes in energy level diagrams of Yb3+ and Tm3+ . The pump light excites only Yb3+ ions, the 3 H5 , 3 F2 and 1 G4 levels are populated by three successive energy transfers from Yb3+ to Tm3+ . The populated 3 F2 may nonradiatively relax to 3 F3 levels, which produce 699 nm emissions. The 1 G4 level may radiatively relax to 3 H6 and 3 F4 levels, which cause 476 nm and 644 nm emissions, respectively. The 1 D2 level of Tm3+ can not be populated by the fourth photon from Yb3+ via energy transfer due to the large energy mismatch (about 3500 cm−1 between them. The cross relaxation process of 3 F2 + 3 H4 → 3 H6 + 1 D2 between Tm3+ ions
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From the SEM images, we confirmed that the microspheres were composed of a large number of nanoparticles. The UC luminescence properties of the sample were studied. Under 980 nm laser excitation, intense UV, visible, and near infrared (NIR) UC emissions were observed. The mechanism of UC luminescence was also discussed. In comparison with other micro materials, the enhancement of UV UC luminescence was attributed to the size effect of the buildup nanoparticles in our microspheres. Acknowledgments: This work was supported by the National High Technology Research and Development Program of China (863 Program: 2009AA03Z309) and the National Natural Science Foundation of China (NNSFC) (grants 10874058, 51072065, and 60908031).
References and Notes
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Received: 1 November 2010. Accepted: 11 April 2011.
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