Luminescent Monodisperse Nanocrystals of Lanthanide Oxyfluorides

J. Phys. Chem. C 2008, 112, 405-415

405

Luminescent Monodisperse Nanocrystals of Lanthanide Oxyfluorides Synthesized from Trifluoroacetate Precursors in High-Boiling Solvents Ya-Ping Du, Ya-Wen Zhang,* Ling-Dong Sun, and Chun-Hua Yan* Beijing National Laboratory for Molecular Sciences, State Key Lab of Rare Earth Materials Chemistry and Applications and PKU-HKU Joint Lab in Rare Earth Materials and Bioinorganic Chemistry, Peking UniVersity, Beijing 100871, China ReceiVed: August 21, 2007; In Final Form: October 18, 2007

Monodisperse lanthanide oxyfluoride nanocrystals (LaOF:Ce and/or Tb, LaOF:Eu, LaOF:Eu/LaOF (core/ shell), GdOF:Ce,Tb, GdOF:Eu, GdOF:Yb,Er) with cubic structure were prepared by codecomposing the lanthanide trifluoroacetate precursors (Ln(CF3COO)3) in oleic acid/oleylamine. The nanocrystals show controlled size (2-7 nm) and shape (nanopolyhedra and elongated nanocrystals) and can form a large-area superlattice on copper grids via self-assembly. They were characterized by means of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and photoluminescence (PL) and upconversion (UC) spectroscopies, along with PL lifetime measurement. The formation of well-doped LnOF nanocrystals has been demonstrated by the combined results of XRD, XPS, and luminescence measurements. The luminescent behaviors of differently sized and shaped nanocrystals have been revealed to be largely dependent upon the small size effect in terms of surface-to-volume ratio and the surface-structure effects in terms of the surface site of the luminescent ions and surface bonding of the capping ligands. It seems that LaOF is a better host material for the energy transfer from Ce3+ to Tb3+ ions in emitting green light than GdOF, whereas GdOF is a better host material for the red emissions from Eu3+ ions than LaOF. Under a 980 nm near-IR laser excitation, the GdOF:Yb,Er nanocrystals can emit UC green light through a two-photon process.

1. Introduction The luminescence of trivalent lanthanide ions originates from the transitions within the 4f shell of ions with low absorption cross sections and long luminescence lifetimes. The special luminescence properties of lanthanide ion-doped nanomaterials with various inorganic hosts have made them be applied in many fields, including displays,1 optical telecommunication,2 lasers,3 infrared-to-visible-light upconversion,4 and optoelectronic devices.5 More recently, their unique advantages have been envisaged in biosensing and medical diagnosis.6 As an important category of lanthanide ion-activated luminescent materials, rare earth fluorides have been recognized as the highly efficient fluorescent host materials owing to their high light transparency originating from the low phonon vibrational energy and high ionicity of the rare earth to fluorine bond, which lead to the minimal quenching probability of the excited state of lanthanide ions.7 It is widely documented that the actualization of nanosized materials has opened doors for finding new properties with respect to their macroscopic counterparts. As a consequence, interest in lanthanide-doped fluoride luminescent nanomaterials is always on the upswing. However, so far, few studies have been conducted to uncover the luminescent properties of size/shape-controlled nanocrystals of doped lanthanide oxyfluorides (LnOF), due to the fact that the synthetic strategies toward these nanocrystals have not been well established.8,9 As we know, the majority of research on LnOF has been done on their bulk crystals or thin films obtained by some limited chemical routes, such as high-temperature-based * Corresponding authors. Fax: +86-10-6275-4179. E-mail: yan@ pku.edu.cn (C.-H.Y.); [email protected] (Y.-W.Z.).

solid-state reaction,10 the sol-gel method,11 and chemical vapor deposition (CVD),12 while quite few reports touch upon the fabrication of LnOF nanocrystals in solutions.13,14 More recently, we have demonstrated that monodisperse undoped LnOF nanocrystals can be efficiently synthesized via controlled fluorination in oleic acid/oleylamine using Ln(CF3COO)3 complexes as the precursors.14 In this article, we comprehensively present the controlled synthesis, characterization, and luminescent properties of monodisperse Ln3+-doped LnOF nanocrystals (LaOF:Ce and/or Tb, LaOF:Eu, LaOF:Eu/ LaOF (core/shell), GdOF:Ce,Tb, GdOF:Eu, GdOF:Yb,Er), which can form large-area superlattice on copper grids via selfassembly. 2. Experimental Section 2.1. Chemicals. Oleic acid (OA; 90%; Alpha), oleylamine (OM; >80%, Acros), absolute ethanol and cyclohexane were used as received without further purification. Lanthanide trifluoracetate (Ln(CF3COO)3) precursors were prepared from the corresponding lanthanide oxides and trifluoroacetic acid according to the literature method.15 2.2. Synthesis of LaOF:Ce, LaOF:Tb, LaOF:Eu, and GdOF:Eu Nanocrystals. In a typical procedure, a stoichiometric amount of Ln(CF3COO) (1 mmol in total) was loaded into 40 mmol of OM in a three-necked flask at room temperature (Table 1). The slurry was heated to 100 °C to remove water and oxygen, with vigorous magnetic stirring under vacuum for 20-40 min in a temperature-controlled electromantle. At this stage, a transparent solution was formed. Then, the solution was heated to 330 °C for 60 min at a heating rate of around 20 Κ

10.1021/jp076717r CCC: $40.75 © 2008 American Chemical Society Published on Web 12/15/2007

406 J. Phys. Chem. C, Vol. 112, No. 2, 2008

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TABLE 1: Crystal Structure, Lattice Parameter, Morphology, and Size of the As-Synthesized Ln3+-Doped LnOF Nanocrystals via the Thermolysis of Ln(CF3COO)3 (1 mmol) in Oleic Acid (OA)/Oleylamine (OM) (40 mmol) OA/OM

T (°C)

t (h)

structure

a (nm)

morphology

size (nm)a

0:1 0:1 0:1 0:1 0:1 0:1 0:1 0:1 5:3

330 330 330 330 330 330 330 330 305

1 1 1 1/4 1 2 1 1 1

cubic cubic cubic cubic cubic cubic cubic cubic cubic

5.56(1) 5.54(7) 5.50(3)

GdOF:5%Eu

0:1 5:3

330 310

1 1

cubic cubic

5.54(1) 5.53(8)

GdOF:20%Yb,2%Er

0:1 5:3

330 305

1 1

cubic cubic

5.50(5) 5.49(3)

polyhedron polyhedron polyhedron polyhedron polyhedron polyhedron polyhedron polyhedron elongated nanocrystals polyhedron elongated nanocrystals polyhedron elongated nanocrystals

3.9 ( 0.2 3.8 ( 0.4 2.9 ( 0.2 2.4 ( 0.3 4.2 ( 0.3 5.5 ( 0.4 6.5 ( 0.4 3.8 ( 0.3 (2.3 ( 0.2)× (5.0 ( 0.4) 6.3 ( 0.4 (3.1 ( 0.4)× (6.4 ( 0.6) 3.9 ( 0.2 (2.3 ( 0.3)× (6.2 ( 0.6)

LaOF:5%Ce LaOF:5%Tb LaOF:45%Ce,15%Tb LaOF:5%Eu LaOF:5%Eu/LaOF GdOF:45%Ce,15%Tb

a

5.55(0) 5.59(6) 5.55(9) 5.54(8)

The standard deviation statistic from at least 100 particles.

min-1 under an Ar atmosphere. As the temperature approached 330 °C, the solution became somewhat turbid with producing some bubbles in it. When the reaction was completed, excess ethanol was poured into the solution at room temperature to form a white precipitate. The as-precipitated nanocrystals without any size sorting were washed several times with cyclohexane and ethanol and dried (yields: 60-75%). The asformed nanocrystals were easily redispersed in various apolar organic solvents (e.g., cyclohexane). 2.3. Synthesis of LaOF:45%Ce,15%Tb Nanocrystals. The synthetic procedure was the same as that used to synthesize LaOF:Ce and LaOF:Tb nanocrystals, except that 0.40 mmol of La(CF3COO)3, 0.45 mmol of Ce(CF3COO)3, and 0.15 mmol of Tb(CF3COO)3 were added to 40 mmol of OM in a threenecked flask at room temperature (Table 1). 2.4. Synthesis of GdOF:45%Ce,15%Tb Nanocrystals. The synthetic procedure was the same as that used to synthesize LaOF:45%Ce,15%Tb nanocrystals, except that 0.40 mmol of Gd(CF3COO)3 was used instead of La(CF3COO)3 (Table 1). 2.5. Synthesis of GdOF:20%Yb,2%Er Nanocrystals. The synthetic procedure was the same as that used to synthesize GdOF:45%Ce,15%Tb nanocrystals, except that the stoichiometric amounts of Gd(CF3COO)3, Yb(CF3COO)3, and Er(CF3COO)3 were added to 40 mmol of OM in a three-necked flask at room temperature (Table 1). 2.6. Synthesis of LaOF:5%Eu/LaOF (Core/Shell) Nanocrystals. A typical two-step synthetic procedure was adopted here for the preparation of LaOF:5%Eu/LaOF (core/shell) nanocrystals. The first step was the same as that for the synthesis of LaOF:5%Eu nanocrystals; then the as-obtained nanocrystals and 0.67 mmol of La(CF3COO)3 were added to 40 mmol of OM in a three-necked flask for a reaction at 330 °C for 30 min. 2.7. Instrumentation. The crystal structures of the asobtained samples were identified by powder X-ray diffraction (XRD) analysis with a Rigaku D/max-2000 diffractometer (Japan) using Cu KR radiation (λ ) 1.5418 Å) in the 2θ ranging from 20° to 80°. With the use of the software “LAPOD” of least-squares refinement of cell dimensions from powder data by Cohen’s method,16 the lattice constants of the nanocrystals were calculated (Table 1). The metal contents in the Ln3+-doped LnOF nanocrystals were determined by a Leeman Labs Profile spec (U.S.A.) inductively coupled plasma atomic emission spectrometer (ICP-AES). The size, shape, and lattice structure of the nanocrystals were observed on a JEOL 200CX (Japan) low-resolution transmission electron microscope (TEM) and a

Hitachi H-9000 (Japan) high-resolution TEM (HRTEM), operated at 160 and 300 kV, respectively. The samples were prepared by slowly vaporizing a drop of nanocrystal dispersion in cyclohexane on carbon-coated copper grids. With the use of Al KR radiation (BE ) 1486.6 eV) as the X-ray excitation source, X-ray photoelectron spectroscopic (XPS) measurements were carried out in an ion-pumped chamber (evacuated to 2 × 10-9 Torr) of an Escalad5 (U.K.) spectrometer so as to determine the atomic ratios of as-prepared nanocrystals and the valence states of the corresponding lanthanide ions in the nanocrystals. The atomic ratio of La/Eu/O/F was determined as 1.0:0.05:1.1: 1.0 for a LaOF:5%Eu nanopolyhedron sample, and that of Gd/ Eu/O/F was determined as 1.0:0.04:1.1:1.2 for a GdOF:5%Eu nanopolyhedron sample and 1.0:0.04:1.1:1.1 for the GdOF: 5%Eu elongated nanocrystals, indicating the formation of stoichiometric lanthanide oxyfluorides in this work. The fluorescence spectra were recorded on a Hitachi F-4500 spectrophotometer (Japan) equipped with a 150 W Xe arc lamp at room temperature. The upconversion luminescence spectra were performed on a modified Hitachi F-4500 spectrophotometer with a tunable 2 W 980 nm laser diode (Beijing) as the excitation source. The lifetime measurements were performed with an Edinburgh Instruments FLS920 transient/steady-state fluorescence spectrometer (U.K.) at room temperature. The samples for spectra and lifetimes measurements were obtained from the nanocrystal dispersions in cyclohexane with an approximate concentration of 0.05 mol L-1. 3. Results and Discussion 3.1. Crystal Structure, Stoichiometry, Shape, and Size of the Nanocrystals. 3.1.1. X-ray Diffraction. The stoichiometric LnOF usually appears in two phases: the low-temperature β-form (rhombohedral; space group, R3hm) and the hightemperature R-form (cubic; space group, Fm3h m).17 Figure 1 depicts the XRD patterns of doped LaOF and GdOF nanocrystals. The significant broadening of all of the reflections indicates the formation of LnOF nanocrystals but makes it a little difficult to determine the exact crystal structures of the nanocrystals. Since the peaks at 2θ ) 38.1° typical for tetragonal LaOF polymorph,12a and at 2θ ) 55.5° typical for rhombohedral LaOF polymorph,11a are unobservable from Figure 1a, we sequentially assign the well-resolved four peaks between 20° and 60° in 2θ value to (111), (200), (220), and (311) crystal planes of cubic R-LnOF for all the as-obtained doped LaOF nanocrystals.14 Analogously, the doped GdOF nanocrystals are concluded to

Nanocrystals of Lanthanide Oxyfluorides

J. Phys. Chem. C, Vol. 112, No. 2, 2008 407

Figure 1. (a) XRD patterns of LaOF:5%Ce, LaOF:5%Tb, LaOF:45%Ce,15%Tb, LaOF:5%Eu, and LaOF:5%Eu/LaOF (core/shell) nanocrystals. (b) XRD patterns of GdOF:5%Eu, GdOF:45%Ce,15%Tb, and GdOF:20%Yb,2%Er nanocrystals.

TABLE 2: ICP Analysis Results of As-Obtained Ln3+-Doped LaOF and GdOF Nanocrystals samples

Ln (µg/mL)

dopants (µg/mL)

exptl (molar ratio)

LaOF:5%Eu LaOF:45%Ce,15%Tb GdOF:5%Eu GdOF:20%Yb,2%Er

797 (La) 192 (La) 698 (Gd) 256 (Gd)

50.5 (Eu) 196 (Ce), 62.2 (Tb) 42.0 (Eu) 69.1 (Yb), 10.3 (Er)

1:0.06 1:1.02:0.32 1:0.06 1:0.27:0.04

hold the same cubic structure (see Figure 1b). For the Ln3+doped LaOF nanocrystals, the as-calculated lattice constant a is in the range of 5.50-5.56 Å, which is distinctly smaller than that of the reported value for R-LaOF (5.756 Å, JCPDS: 77204), indicating the formation of complex LaOF solid solutions. For LaOF:5%Eu/LaOF nanocrystals, the calculated lattice parameter is a ) 5.59(6) Å, which lies between those of LaOF and LaOF:5%Eu nanocrystals (5.55(0) Å), suggesting that a LaOF layer was possibly coated onto the surfaces of the LaOF: 5%Eu nanocrystals.14 For the Ln3+-doped GdOF nanocrystals, the as-calculated lattice constant a is in the range of 5.49-5.55 Å, which is a bit smaller than that of the reported value for R-GdOF (5.573 Å), presumably suggesting the formation of complex GdOF solid solutions.17 The successful doping of various lanthanide ions into the crystal lattices of LaOF and GdOF nanocrystals has also been demonstrated by the empirical metal molar ratios determined by the ICP for the doped nanocrystals (Table 2). 3.1.2. X-ray Photoelectron Spectroscopy. Figure 2a depicts the typical XPS survey spectra of LaOF:45%Ce,15%Tb and GdOF:5%Eu nanopolyhedra. Several intense peaks attributable to the core levels of La 3d, Ce 3d, Tb 4d, Gd 3d, Eu 3d, O 1s, and F 1s, can be observed for the LaOF:45%Ce,15%Tb and GdOF:5%Eu nanopolyhedra, again demonstrating the formation of doped LnOF compounds. The appearance of the peaks ascribed to the core levels of N 1s and C 1s further indicates the presence of OM ligands on the surfaces of the doped LnOF nanocrystals. By XPS analysis, we have determined that the oxidation states of the multivalent lanthanides such as cerium and terbium are trivalent for the as-obtained nanocrystals. As an example, Figure 2b shows the La 3d, Ce 3d, and Tb 4d signals recorded for the LaOF:45%Ce,15%Tb nanopolyhedra. Double satellite peaks belonging to the core levels of La 3d5/2 and La 3d3/2 appear at 836.11 and 840.63 eV, and 852.92 and 857.00 eV, respectively, for La3+ ions in the LaOF:45%Ce,15%Tb nanopolyhedra. Another set of double satellite peaks associated with the core levels of Ce 3d5/2 and Ce 3d5/2 are observed to lie at 883.30 and 886.86 eV, and 901.79 and 905.52 eV, respectively, for Ce3+ ions doped in the lattice of LaOF

calcd (molar ratio) 1:0.05 1:1.13:0.37 1:0.05 1:0.26:0.03

nanocrystals. The absence of the peak at around 915.00 eV characteristic of Ce4+ proposes that Ce3+ ions predominately exist in the LaOF:45%Ce,15%Tb nanopolyhedra.18,19a In addition, the appearance of the peak at 147.17 eV instead of 156.00 eV for the core levels of Tb 4d indicates that terbium cations adopted in the LaOF:45%Ce,15%Tb nanopolyhedra are also trivalent in nature (Figure 2b).19 Furthermore, the peaks at 684.14 and 531.29 eV can be ascribed to the core levels of F 1s and O 1s of the lattice fluorine and oxygen in the LaOF: 45%Ce,15%Tb nanopolyhedra, respectively (see Figure 2, parts c and d).18a 3.1.3. Transmission Electron Microscopy. The TEM and HRTEM measurements have revealed that the as-obtained cubic LnOF:Ln3+ nanocrystals are single-crystalline and monodisperse and display two kinds of typical shapes: nanopolyhedra and elongated nanocrystals (Figures 3 and 4).14 Panels a-d of Figure 3 show the TEM images of (3.8 ( 0.4) nm LaOF:5%Tb, (2.9 ( 0.2) nm LaOF:45%Ce,15%Tb, (4.2 ( 0.3) nm LaOF:5%Eu, and (6.5 ( 0.4) nm LaOF:5%Eu/LaOF (core/shell) nanopolyhedra, respectively. Panels a-d of Figure 4 depict the TEM images of (6.3 ( 0.4) nm GdOF:5%Eu nanopolyhedra, (3.1 ( 0.4) nm × (6.4 ( 0.6) nm GdOF:5%Eu elongated nanocrystals, (3.9 ( 0.2) nm GdOF:20%Yb,2%Er nanopolyhedra, (2.3 ( 0.3) nm × (6.2 ( 0.6) nm GdOF:20%Yb,2%Er elongated nanocrystals, (3.8 ( 0.3) nm GdOF:45%Ce,15%Tb nanopolyhedra, (2.3 ( 0.2) nm × (5.0 ( 0.4) nm GdOF:45%Ce,15%Tb elongated nanocrystals, respectively. The intriguing highly ordered arrangement of the nanocrystals on copper grids in a large area has demonstrated that the nanocrystals surfaces are well passivated by the capping ligands.14 The clear-cut lattice fringes shown in the HRTEM images of LnOF:Ln3+ nanocrystals (insets in Figure 3a and Figure 4, parts a and b) verify that they are of a high nature of crystallization. As seen from the HRTEM image inset in Figure 3a, the LaOF:5%Tb nanopolyhedra show the (111) facet with the interplanar spacing of 0.32 nm. The HRTEM image inset in Figure 3d indicates that the LaOF:5%Eu/LaOF (core/shell) nanocrystals are also highly crystallized, and the interplanar spacing of 0.32 nm is attributable to the (111) facet. The size of the LaOF:5%Eu/LaOF


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Figure 2. (a) XPS survey spectra of LaOF:45%Ce,15%Tb and GdOF:5%Eu nanopolyhedra. (b) La 3d, Ce 3d, Tb 4d, (c) F 1s, and (d) O 1s signals recorded for LaOF:45%Ce,15%Tb nanopolyhedra.

nanocrystals is (6.5 ( 0.4) nm (Figure 3d), which is considerably larger than that ((4.2 ( 0.3) nm) of the LaOF:5%Eu nanocrystals (Figure 3c), presumably suggesting the formation of LaOF: 5%Eu/LaOF core/shell structure. If we suppose the nanocrystals are spherically shaped, the average thickness of LaOF shells surrounding LaOF:5%Eu cores is estimated to be about 1.1 nm. The HRTEM image inset in Figure 4a shows that the GdOF: 5%Eu nanopolyhedra expose the (200) facet with the interplanar spacing of 0.28 nm. For the GdOF:5%Eu elongated nanocrystals, they show the (200) and (111) facet with the interplanar distances of 0.28 and 0.32 nm, respectively, and the preferred growth direction along [100] (inset in Figure 4b). 3.2. Photoluminescence Properties of Ce3+- and/or Tb3+Doped LaOF and GdOF Nanocrystals. 3.2.1. LaOF:Ce and LaOF:Tb Nanocrystals. Figure 5a shows the excitation and emission spectra of 3.9 nm LaOF:5%Ce nanocrystals dispersed in cyclohexane (0.05 mol L-1). Only one broad and intense peak is observed at 336 nm (monitored under λem ) 395 nm), which is assignable to the electric-dipole-allowed 4f5d transitions of Ce3+ ions.20,21 Presumably due to the fact that the size of the LaOF:5%Ce nanocrystals is as small as 3.9 nm, the transitions from the ground state 2F5/2 of Ce3+ ions to the different components of the excited Ce3+ 5d states split by the crystal field are unobservable.22 As λex ) 336 nm, the appearance of a strong and broad band ranging from 350 to 650 nm centering at 395 nm confirms that the emissions are due to 5d f4f transitions of Ce3+ ions.20,23 Figure 5b depicts the luminescence decay curve of the 395 nm emission. The curve can be well fitted using a double-exponential function as I ) I1 exp(-t/τ1)

+ I2 exp(-t/τ2) (τ1, τ2 correspond to the two different lifetimes of Ce3+ ions). The calculated values for the lifetimes are τ1 ) 10.31 ns (79%) for the long component and τ2 ) 2.65 ns (21%) for the short component (Figure 5b and Table 3). The luminescence decay exhibits a biexponential decay, presumably due to surface effects.4a Both the lifetimes for long and short components are considerably shorter than that of the reported lifetimes of Ce3+ ions,21,23 implying the high disorder of 3.9 nm LaOF:5%Ce nanocrystals which have a large surface-tovolume ratio.4a,21 The short lifetimes of Ce3+ ions is attributed to the characteristic of 5d4f transitions.24 Figure 5c depicts the excitation and emission spectra of 3.8 nm LaOF:5%Tb nanocrystals dispersed in cyclohexane (0.05 mol L-1). Also due to the small size effect, only one broad and intense peak is observable at 294 nm (monitored under λem ) 545 nm), attributed to the 4f f 5d transitions of Tb3+ ions (see the left part of Figure 5c).20a,22,25,26 The emission spectrum (λex ) 294 nm) between 450 and 650 nm shows typical Tb3+ ion emissions corresponding to the transitions from 5D4 to 7FJ (J ) 6-3) (see the right part of Figure 5c). Figure 5d shows the luminescence decay curve of the 545 nm emission. This decay curve can be well fitted into a single-exponential function I ) I0 exp(-t/τ) (τ is the lifetime of Tb3+ ions) (Figure 5d) with a lifetime of 3.15 ms (Figure 5d and Table 3), consistent with the values observed for LaPO4:Ce,Tb nanoparticles25a and bulk material.25b 3.2.2. LaOF:Ce,Tb and GdOF:Ce,Tb Nanocrystals. Because of the suitable electronic transition levels between Ce3+ and Tb3+ ions,20a,22,25,26 Ce3+ ions can absorb irradiative light and



Figure 3. (a) TEM and HRTEM (inset) images of (3.8 ( 0.4) nm LaOF:5%Tb nanopolyhedra. TEM images of (b) (2.9 ( 0.2) nm LaOF:45%Ce,15%Tb nanopolyhedra and (c) (4.2 ( 0.3) nm LaOF:5%Eu nanopolyhedra. (d) TEM and HRTEM (inset) images of (6.5 ( 0.4) nm LaOF:5%Eu/ LaOF nanopolyhedra.

transfer the energy to Tb3+ ions, which produces green emission in the end. Figure 6a gives the excitation and emission spectra of 2.9 nm LaOF:45%Ce,15%Tb nanopolyhedra dispersed in cyclohexane (0.05 mol L-1). The excitation peak centering at around 260 nm (monitoring with λem ) 545 nm) is attributable to the absorption of 4f f 5d transitions of Ce3+ ions.20,22,23,28 After an energy transfer from Ce3+ to Tb3+ ions, the Tb3+ ions emission of green light resulting from 5D4 f 7FJ relaxation takes place. The emission spectrum shows a typical green emission band between 450 and 650 nm corresponding to the 5D4 f 7FJ (J ) 6-3) transitions of Tb3+ ions. Interestingly, the emission band within 350-650 nm for Ce3+ ions observed for 3.9 nm LaOF:5%Ce nanopolyhedra (see Figure 5a) has disappeared for the LaOF:45%Ce,15%Tb nanocrystals, suggesting that the emission of Ce3+ ions is completely quenched in this case.20,22,23 Figure 6b depicts the luminescence decay curve of the 545 nm emission of Tb3+ ions in the LaOF:45%Ce,15%Tb nanocrystals. The curve is fitted into a single-exponential function. The lifetime of Tb3+ ions emission (545 nm) is determined to be 3.89 ms (Figure 6b and Table 3), consistent with the values observed for LaPO4:Ce,Tb nanoparticles25a and bulk material.25b Figure 6c exhibits the room-temperature excitation and emission spectra of 3.8 nm GdOF:45%Ce,15%Tb nanocrystals dispersed in cyclohexane (0.05 mol L-1). The GdOF:45%Ce,15%Tb

nanocrystals show an additional Ce3+ ion broad band lying at around 380 nm in the emission spectrum, as compared with the LaOF:45%Ce,15%Tb nanopolyhedra (Figure 6a). The lifetime time of Tb3+ ions emission (545 nm) decreases apparently from 3.89 ms for LaOF:45%Ce,15%Tb to 1.58 ms for GdOF:45%Ce,15%Tb (Figure 6, parts b and d, and Table 3). These results strongly suggest that LaOF should be a better host material for the emission of Tb3+ ions in this work. The quantum yields (QY) of the as-obtained LaOF:45%Ce,15%Tb and GdOF:45%Ce,15%Tb nanocrystals dispersed in cyclohexane (0.05 mol L-1) were determined by comparing the luminescence intensity of an ethanol (spectroscopic grade) solution of rhodamine B (Lambda Physics, laser grade) solution with approximately the same absorption at 256 nm. The quantum yield is calculated from the following equation, QY ) QYR(I/ IR)(ODR/OD)(n2/nR2), where QY and QYR are the quantum yields of the sample and rhodamine B, I is the emission intensity, OD is the optical density, and n is the refraction index. The quantum yield of rhodamine B is 95%. The QY for LaOF: 45%Ce,15%Tb is determined as 40% (Table 3), which is much higher than that reported (19%) for LaF3:Ce,Tb nanocrystals in dichloromethane.27 Therefore, under 254 nm UV lamp excitation, the LaOF:45%Ce,15%Tb nanocrystals exhibit bright green emission (see Figure 7a). On the other hand, the QY for the


Du et al.

Figure 4. TEM and HTEM (inset) images of (a) (6.3 ( 0.4) nm GdOF:5%Eu nanopolyhedra and (b) (3.1 ( 0.4) nm × (6.4 ( 0.6) nm GdOF: 5%Eu elongated nanocrystals. TEM images of (c) (3.9 ( 0.2) nm GdOF:20%Yb,2%Er nanopolyhedra and (d) (2.3 ( 0.3) nm × (6.2 ( 0.6) nm GdOF:20%Yb,2%Er elongated nanocrystals.

GdOF:45%Ce,15%Tb nanopolyhedron and elongated nanocrystals are both 18% (Table 3), which is much lower than that of the LaOF:45%Ce,15%Tb nanocrystals, further demonstrating the more efficient energy transfer between Ce3+ and Tb3+ ions in the LaOF host. 3.3. Photoluminescence Properties of Eu3+-Doped LaOF and GdOF Nanocrystals. 3.3.1. Differently Sized LaOF:Eu and LaOF:Eu@LaOF (Core/Shell) Nanocrystals. As is known, Eu3+ ion is a sensitive structure probe to detect the lattice symmetry of the host materials.20a,27,28 Figure 8a depicts the excitation spectrum (monitored under λem ) 611 nm) of 5.5 nm LaOF: 5%Eu nanopolyhedra dispersed in cyclohexane (0.05 mol L-1). The broad excitation band centering at about 270 nm is attributed to O2--Eu3+ charge transfer (CT), indicating the formation of LaOF:Eu nanocrystals.11b To support this deduction, the excitation spectrum of LaF3:5%Eu nanocrystals is also shown in Figure 8a for comparison. Obviously, the CT band at 270 nm is not observed for LaF3:5%Eu, suggesting the absence of oxygen ions in its crystal lattice.11b,29 Figure 8b shows the emission spectrum (λex ) 270 nm) of 5.5 nm LaOF:5%Eu nanocrystals dispersed in cyclohexane (0.05 mol L-1) (QY ) 5%). The peaks at 580, 591, 611, 651, and 695 nm are ascribed to the 5D0 f 7FJ line emissions (J ) 0, 1, 2, 3, 4) of the Eu3+ ions, respectively.11,30 The 5D0 f 7F2 transition at 611 and 620

nm (shoulder peak) is the strongest. Figure 8c displays the luminescence decay curve of the 5D0 f 7F2 emission (611 nm) in 5.5 nm LaOF:5%Eu nanocrystals. This curve can be well fitted into a double-exponential function, giving τ1 ) 2.88 ms (80%) and τ2 ) 0.73 ms (20%) (Figure 8c and Table 3). The lifetime of the long component is considerably lower than that (7.7 ms) of LaF3:5%Eu nanocrystals in CH2Cl2,4a and that (6.9 ms) of bulk LaF3:Eu.31 Figure 8d shows the luminescent spectra of (2.4 ( 0.3), (4.2 ( 0.3), and (5.5 ( 0.4) nm LaOF:5%Eu nanopolyhedra dispersed in cyclohexane (0.05 mol L-1). It is well documented that the relative intensity of a typical electric-dipole 5D0 f 7F2 transition depends on the local symmetry of the Eu3+ ions, whereas a typical magnetic dipole 5D0 f 7F1 transition is relatively insensitive to the local symmetry of the Eu3+ ions. Therefore, the crystal field symmetry where the Eu3+ ions are situated can be partially reflected by the ratio of the 5D0 f 7F2 and 5D0 f 7F1 transition probabilities. The intensity ratio of 5D f 7F to 5D f 7F (I /I ) is 1.07, 1.24, and 2.03 for 0 2 0 1 611 589 5.5, 4.2, and 2.4 nm LaOF:5%Eu nanopolyhedra, respectively. The increase of I611/I589 ratio with decreasing size suggests that the symmetry around the Eu3+ ions should decrease with size for our LaOF:5%Eu nanopolyhedra due to the increasing number of Eu3+ ions located near the surface.14,28b,32,33



Figure 5. (a) Excitation and emission spectra of 3.9 nm LaOF:5%Ce nanopolyhedra dispersed in cyclohexane (0.05 mol L-1). (b) Luminescence decay curve of the 395 nm emission of Ce3+ ions in the LaOF:5%Ce nanopolyhedra (λex ) 336 nm). (c) Excitation and emission spectra of 3.8 nm LaOF:5%Tb nanopolyhedra dispersed in cyclohexane (0.05 mol L-1). (d) Luminescence decay curve of the 545 nm emission of Tb3+ ions in LaOF:5%Tb nanopolyhedra (λex ) 294 nm).

TABLE 3: Luminescence Behaviors of As-Obtained Ln3+-Doped LaOF and GdOF Nanocrystals samples

lifetime

λem (nm)

τ1 ) 10.31 ns (79%)a τ2 ) 2.65 ns (21%) τ ) 3.15 ms (100%) τ ) 3.89 ms (100%) τ ) 1.58 ms (100%) τ1 ) 2.88 ms (80%) τ2 ) 0.73 ms (20%) τ1 ) 3.02 ms (77%) τ2 ) 0.91 ms (23%) τ ) 1.72 ms (100%) τ ) 1.94 ms (100%)

395 nm (Ce3+)

QY (%)

3.9 ( 0.2

LaOF:5%Tb LaOF:45%Ce,15%Tb GdOF:45%Ce,15%Tb LaOF:5%Eu

3.8 ( 0.4 2.9 ( 0.2 3.8 ( 0.3 5.5 ( 0.4

35 40 18 5

LaOF:5%Eu/LaOF

6.5 ( 0.4

7

GdOF:5%Eu

a

size (nm)

LaOF:5%Ce

6.3 ( 0.4 (2.3 ( 0.2)× (5.0 ( 0.4)

29 31

545 nm (Tb3+) 545 nm (Tb3+) 545 nm (Tb3+) 611 nm (Eu3+) 611 nm (Eu3+) 611 nm (Eu3+) 611 nm (Eu3+)

The percentages represent the amount of the component contributing to the total lifetime.

Figure 9a displays the emission spectrum of 6.5 nm LaOF: 5%Eu/LaOF (core/shell) nanocrystals (λex ) 270 nm; QY ) 7%). It is noted that, under the same measurement conditions, an almost ca. 21% increase in the Eu3+ ions emission intensity is obtained for the core/shell nanocrystals, compared with that of 4.2 nm core nanocrystals. This improvement may be ascribed to the passivation of the surface defects and corresponding nonradiative decays by the formation of the protective LaOF layer.29,32 Figure 9b displays the luminescence decay curve of 5D f 7F emission (611 nm) in the core/shell nanocrystals. 0 2 The curve can also be well fitted into a double-exponential function, giving the values of τ1 ) 3.02 ms (77%) and τ2 ) 0.91 ms (23%) (Figure 9b and Table 3). In comparison with

the core LaOF:Eu nanocrystals (τ1 ) 2.88 ms, τ2 ) 0.73 ms), the mild increase of the lifetimes of Eu3+ ions emissions in the core/shell nanocrystals also implies the probability that luminescence quenching from the surface of nanocrystals is somewhat suppressed due to the formation of a protective shell around the core.32,34 In addition, the I611/I589 ratio of 5D0 f 7F2 to 5D0 f 7F1 decreases from 1.24 for the core nanocrystals to 1.00 for the core/shell nanocrystals, which indicates the enhanced Eu3+ ion’s symmetry in the core/shell nanocrystals due to the shell formation.29,32,34 3.3.2. Differently Shaped GdOF:Eu Nanocrystals. Figure 10a displays the excitation and emission spectra of 6.3 nm GdOF: 5%Eu nanopolyhedra and 3.1 nm × 6.4 nm GdOF:5%Eu


Du et al.

Figure 6. (a) Excitation and emission spectra of 2.9 nm LaOF:45%Ce,15%Tb nanopolyhedra dispersed in cyclohexane (0.05 mol L-1). (b) Luminescence decay curve of the 545 nm emission of Tb3+ ions for LaOF:45%Ce,15%Tb nanopolyhedra (λex ) 256 nm). (c) Excitation and emission spectra of 3.8 nm GdOF:45%Ce,15%Tb nanopolyhedra dispersed in cyclohexane (0.05 mol L-1). (d) Luminescence decay curve of the 545 nm emission of Tb3+ ions for GdOF:45%Ce,15%Tb nanopolyhedra (λex ) 250 nm).

Figure 7. Fluorescence photographs of (a) 2.9 nm LaOF:45%Ce,15%Tb nanopolyhedra and (b) 6.3 nm GdOF:5%Eu nanopolyhedra dispersed in cyclohexane (0.05 mol L-1) under irradiation of a 254 nm UV lamp. (c) Upconversion fluorescence photograph of (2.3 ( 0.3) nm × (6.2 ( 0.6) nm GdOF:20%Yb,2%Er elongated nanocrystals dispersed in cyclohexane (0.20 mol L-1) under a 980 nm laser excitation.

elongated nanocrystals dispersed in cyclohexane (0.05 mol L-1) under λex ) 270 nm. The broad O2--Eu3+ CT band at 270 nm and the narrow f-f excitation bands at longer wavelengths are observed in the excitation spectrum (left part of Figure 10a).11b From the emission spectra, the 5D0 f 7FJ emissions (J ) 0, 1,

2, 3, 4) of the Eu3+ ions are located at 577, 589, 611, 649, and 690 nm, respectively.11,30 It is also noted that, the ratio of I611/ I589 increases from 0.9 for the 3.1 nm × 6.4 nm GdOF:5%Eu elongated nanocrystals to 1.3 for 6.3 nm GdOF:5%Eu nanopolyhedra (Figure 10a), possibly indicating a higher symmetry around the Eu3+ ions in the former nanocrystals.14,28b,32,33 Figure 10b exhibits the luminescence decay curves of 5D0 f 7F2 emission at 611 nm for the nanopolyhedra and elongated nanocrystals. These decay curves can be fitted well into a singleexponential function. The lifetime of the 5D0 f 7F2 emission of Eu3+ ions (611 nm) increases from 1.72 ms for the nanopolyhedra to 1.94 ms for the elongated nanocrystals (Figure 10b and Table 3), indicating a more efficient Eu3+ ion emission in the latter nanocrystals with a higher symmetry around the Eu3+ ions (as revealed by the above I611/I589 ratio analysis). This is possibly due to that the surface defects were better passivated for the elongated nanocrystals (whose surfaces were strongly coordinated by OA as synthesized under OA/OM ) 5:3) than for the nanopolyhedra (whose surfaces were weakly combined by OM as synthesized under OA/OM ) 0:1).32 In comparison with the QY (5-7%) of the LaOF:Eu and LaOF:Eu/LaOF (core/ shell) nanopolyhedra, the GdOF:5%Eu nanopolyhedra show a much higher QY (29%), meaning that GdOF is a better host material for the Eu3+ ion emissions than LaOF. The GdOF: 5%Eu nanopolyhedra show strong red emission under 254 nm UV lamp irradiation (see Figure 7b). 3.4. Upconversion Properties of Yb3+- and Er3+-Doped GdOF Nanocrystals. Figure 11a shows the upconversion (UC)



Figure 8. (a) Excitation spectra (λem ) 611 nm) of the 5.5 nm LaOF:5%Eu (solid line) and the LaF3:5%Eu (dashed line) nanocrystals dispersed in cyclohexane (0.05 mol L-1). (b) Emission spectra of the LaOF:5%Eu nanopolyhedra dispersed in cyclohexane (0.05 mol L-1) under λex ) 270 nm. (c) Luminescence decay curves of the 611 nm emission of Eu3+ ions in LaOF:5%Eu nanopolyhedra under λex ) 270 nm. (d) Emission spectra of the differently sized LaOF:5%Eu nanopolyhedra dispersed in cyclohexane (0.05 mol L-1).

Figure 9. (a) Emission spectrum of 6.5 nm LaOF:5%Eu/LaOF (core/shell) nanocrystals dispersed in cyclohexane (0.05 mol L-1) under λex ) 270 nm. (b) Luminescence decay curves of the 611 nm emission of Eu3+ ions in the LaOF:5%Eu/LaOF (core/shell) nanocrystals under λex ) 270 nm.

fluorescence spectra of 3.9 nm GdOF:20%Yb,2%Er nanopolyhedra and 2.3 nm × 6.2 nm elongated nanocrystals dispersed in cyclohexane (0.05 mol L-1) under 980 nm excitation. For both samples, the green emissions at 521 and 545 nm are attributable to transitions from 2H11/2 to 4I15/2 and from 4S3/2 to 4I 3+ ions, respectively, and the red emission at 659 15/2 of Er nm comes from the 4F9/2 to 4I15/2 transition of Er3+ ions. It is also noted that the UC emission intensity of the GdOF:

20%Yb,2%Er elongated nanocrystals is enhanced by several times with respect to that of the nanopolyhedra. This result perhaps suggests that the nonradiative decays from the surface defects were much better passivated for the elongated nanocrystals (whose surfaces were also strongly coordinated by OA ligands as synthesized under OA/OM ) 5:3) than for nanopolyhedra (whose surfaces were weakly combined by OM as synthesized under OA/OM ) 0:1).35 Figure 7c exhibits the green


Du et al.

Figure 10. (a) Excitation and emission spectra of 6.3 nm GdOF:5%Eu nanopolyhedra and 3.1 nm × 6.4 nm elongated nanocrystals dispersed in cyclohexane (0.05 mol L-1). (b) Luminescence decay curves of the 611 nm emission of Eu3+ ions in the GdOF:5%Eu nanopolyhedra and elongated nanocrystals under λex ) 270 nm.

Figure 11. (a) Upconversion fluorescence spectra of 3.9 nm GdOF:20%Yb,2%Er nanopolyhedra and 2.3 nm × 6.2 nm elongated nanocrystals dispersed in cyclohexane (0.05 mol L-1) under λex ) 980 nm. (b) Power density dependence of the upconversion emissions for GdOF:20%Yb,2%Er nanopolyhedra and elongated nanocrystals under λex ) 980 nm.

emission from the GdOF:20%Yb,2%Er elongated nanocrystals dispersed in cyclohexane (0.05 mol L-1) under 980 nm NIR laser excitation. In order to determine the number of photons involved in the UC process of the GdOF:20%Yb,2%Er nanocrystals, the relationships between laser diode power density and emission intensities for the green and red emission are depicted in Figure 11b. The slopes of different curves are found to be approximately 2, suggesting the normal two-photon emission processes are involved in the UC mechanism of the present nanocrystals.36 Under the 980 nm laser excitation, the activated Yb3+ ion absorbs one photon and transfers it to the Er3+ ion, the Er3+ ion receives the energy and its ground state 4I15/2 electron is excited to the 4I11/2 level. A second photon from the Yb3+ ion promotes the electron to the 4F7/2 level. The excited electron decays nonradiatively to 2H11/2, 4S3/2, 4F9/2 levels, and precedently radiative relaxation to the ground state results in the emission of 521, 545, and 659 nm, respectively. At a high excitation density, the slope of the log-log curve is reduced owing to the saturation of the UC processes. The saturation power of 2.3 nm × 6.2 nm GdOF:20%Yb,2%Er elongated nanocrystals is 665 mW for the green emission 4S3/2 f 4I15/2, higher than that (550 mW) of 3.9 nm GdOF:20%Yb,2%Er nanopolyhdedra.

4. Conclusions A general synthesis of orderly aligned monodisperse complex lanthanide oxyfluoride nanocrystals (LaOF:Ce and/or Tb, LaOF: Eu, LaOF:Eu/LaOF (core/shell), GdOF:Ce,Tb, GdOF:Eu, GdOF:Yb,Er) in cubic structure was achieved by the cothermolysis of multiple trifluoroacetate precursors (Ln(CF3COO)3) in OA/ OM, demonstrating the robustness and versatility of the present approach. The nanocrystals display manipulated size (2-7 nm) and shape (nanopolyhedra and elongated nanocrystals) and can be orderly aligned to form large-area nanoarrays on copper grids via self-assembly. Room-temperature luminescent studies have uncovered that the small size effect in terms of surface-tovolume ratio and the surface-structure effects in terms of the surface site of the luminescent ions and surface bonding of the capping ligands have played crucial roles in determining the luminescent behaviors of differently sized and shaped nanocrystals. It seems that LaOF is a better host material for the energy transfer between Ce3+ and Tb3+ ions in emitting green light than GdOF, whereas GdOF is a better host material for Eu3+ ions in emitting red light than LaOF. Under a 980 nm near-IR laser excitation, the GdOF:Yb,Er nanocrystals can emit UC green light through a two-photon process. To get a comprehensive understanding of the corresponding luminescent mech-

Nanocrystals of Lanthanide Oxyfluorides anisms, further work will be done with site-selective excitation spectroscopy. Acknowledgment. We gratefully acknowledge the financial aid from the MOST of China (Grant No. 2006CB601104), the NSFC (Grant Nos. 20571003, 20221101, and 20423005), and the Research Fund for the Doctoral Program of Higher Education of the MOE of China (Grant No. 20060001027). References and Notes (1) (a) Thomas, J.; Hans, N.; Cees, R. Angew. Chem., Int. Ed. 1998, 37, 3084. (b) Park, J. C.; Moon, H. K.; Kim, D. K.; Byeon, S. H.; Kim, B. C.; Suh, K. S. Appl. Phys. Lett. 2000, 77, 2162. (c) Wang, H.; Lin, C. K.; Liu, X. M.; Lin, J. Appl. Phys. Lett. 2005, 87, 181907. (2) Barber, D. B.; Pollock, C. R.; Beecroft, L. L.; Ober, C. K. Opt. Lett. 1997, 22, 1247. (3) (a) O’Connor, J. R. Appl. Phys. Lett. 1966, 9, 407. (b) Klimov, V. I.; Mikhailovsky, A. A.; Xu, S; Malko, A.; Hollingsworth, J. A.; Leatherdale, C. A.; Eisler, H. J.; Bawendi, M. G. Science 2000, 290, 314. (4) (a) Stouwdam, J. W.; van Veggel, F. C. J. M. Nano Lett. 2002, 2, 733. (b) Zeng, J. H.; Su, J.; Li, Z. H.; Yan, R. X.; Li, Y. D. AdV. Mater. 2005, 17, 2119. (5) Heikenfeld, J.; Steckl, A. J. IEEE Trans. Electron DeVices 2002, 49, 1545. (6) (a) Yi, G. S.; Lu, H. C.; Zhao, S. Y.; Yue, G.; Yang, W. J.; Chen, D. P.; Guo, L. H. Nano Lett. 2004, 4, 2191. (b) Meiser, F.; Cortez, C.; Caruso, F. Angew. Chem., Int. Ed. 2004, 43, 5954. (7) (a) Weber, M. J. Phys. ReV. 1967, 157, 262. (b) Sivakumar, S. F.; van Veggel, F. C. J. M.; Raudsepp, M. J. Am. Chem. Soc. 2005, 127, 12464. (c) Bovero, E.; van Veggel, F. C. J. M. J. Phys. Chem. C 2007, 111, 4529. (8) (a) Wang, Y.; Ohwaki, J. Appl. Phys. Lett. 1993, 63, 3268. (b) Takashima, M. J. Fluorine Chem. 2000, 105, 249. (c) Rambabu, U.; Amalnerkar, D. P.; Kale, B. B.; Buddhudu, S. Spectrosc. Lett. 2000, 33, 423. (9) Au, C. T.; Zhang, Y. Q.; He, H.; Lai, S. Y.; Ng, C. F. J. Catal. 1997, 167, 354. (10) Balaji, T.; Buddhudu, S. Spectrosc. Lett. 1993, 26, 113. (11) (a) Fujihara, S.; Kato, T.; Kimura, T. J. Mater. Sci. Lett. 2001, 20, 687. (b) Fujihara, S.; Kato, T.; Kimura, T. J. Sol.-Gel Sci. Technol. 2003, 26, 953. (12) (a) Barreca, D.; Gasparotto, A.; Maragno, C.; Tondello, E.; Bontempi, E.; Depero, L. E.; Sada, C. Chem. Vap. Deposition 2005, 11, 426. (b) Malandrino, G.; Perdicaro, L. M. S.; Fragala`, I. L. Chem. Vap. Deposition 2006, 12, 736. (13) Lee, J.; Zhang, Q. W.; Saito, F. J. Alloys Compd. 2003, 348, 214. (14) Sun, X.; Zhang, Y. W.; Du, Y. P.; Yan, Z. G.; Si, R.; You, L. P.; Yan, C. H. Chem. Eur. J. 2007, 13, 2320.

J. Phys. Chem. C, Vol. 112, No. 2, 2008 415 (15) Roberts, J. E. J. Am. Chem. Soc. 1961, 83, 1087. (16) (a) Langford, J. I. J. Appl. Crystallogr. 1971, 4, 259. (b) Langford, J. I. J. Appl. Crystallogr. 1973, 6, 190. (17) Mu¨ller, J. H.; Petzel, T. J. Alloys Compd. 1995, 224, 18. (18) (a) Shen, W.; Wang, X. D.; Cattrall, R. W.; Nyberg, G. L.; Liesegang, J. Electroanalysis 1997, 9, 917. (b) Chadwick, D.; McAleese, J.; Senkiw, K.; Steele, B. C. H. Appl. Surf. Sci. 1996, 99, 417. (19) (a) Pemba-Mabiala, J. M.; Lenzi, M.; Lenzi, J.; Lebugle, A. Surf. Interface Anal. 1990, 15, 663. (b) Sarma, D. D.; Rao, C. N. R. J. Electron Spectrosc. Relat. Phenom. 1980, 20, 2545. (20) (a) Yu, M.; Lin, J.; Fu, J.; Zhang, H. J.; Yan. C. H. J. Mater. Chem. 2003, 13, 1413. (b) Cooke, D. W.; Bennett, B. L.; Muenchausen, R. E.; Mcclellan, K. J.; Roper, J. M.; Whittaker. M. T. J. Appl. Phys. 1999, 86, 5308. (c) Saha, S.; Chowdhury, P. S.; Patra, A. J. Phys. Chem. B 2005, 109, 2699. (21) Aitasalo, T.; Ho¨lsa¨, J.; Lastusaari. M.; Legendziewicz, J.; Niittykoski, J.; Pell, F. Opt. Mater. 2004, 26, 107. (22) Buissette, V.; Moreau, M.; Gacoin, T.; Boilot, J. P.; Chane-Ching, J. Y.; Mercier, T. L. Chem. Mater. 2004, 16, 3767. (23) Aitasalo, T.; Ho¨lsa¨, J.; Lastusaari, M.; Niittykoski, J.; Pell, F. Phys. Status Solidi C 2 2005, 1, 272. (24) Wang, Z. L.; Quan, Z. W.; Jia, P. Y.; Lin, C. K.; Luo, Y.; Chen, Y.; Fang, J.; Zhou, W.; O’Connor, C. J.; Lin, J. Chem. Mater. 2006, 18, 2030. (25) (a) Riwotzki, K.; Meyssamy, H.; Schnablegger, H.; Kornowski, A.; Haase, M. Angew. Chem., Int. Ed. 2001, 40, 573. (b) Bourcet, J. C.; Fong, F. K. J. Chem. Phys. 1974, 60, 34. (26) Ko¨mpe, K.; Borchert, H.; Storz, J.; Lobo, A.; Adam, S.; Moeller, T.; Haase, M. Angew. Chem., Int. Ed. 2003, 42, 5513. (27) Stouwdam, J. W.; Hebbink, G. A.; Huskens, J.; van Veggel, F. C. J. M. Chem. Mater. 2003, 15, 4604. (28) (a) Lin, J. H.; You, L. P.; Lu, G. X.; Yang, L. Q; Su, M. Z. J. Mater. Chem. 1998, 8, 1051. (b) Wei, Z. G.; Sun, L. D.; Liao, C. S.; Yan, C. H. Appl. Phys. Lett. 2002, 80, 1447. (29) Lezhnina, M. M.; Ju¨stel, T.; Ka¨tker, H.; Wiechert, D. U.; Kynast, U. H. AdV. Funct. Mater. 2006, 16, 935. (30) Ho¨lsa¨, J.; Kestila¨, E. J. Chem. Soc., Faraday Trans. 1995, 91, 1503. (31) Crosswhite, H. M.; Moose, H. W. Optical Properties of Ions in Crystals; Wiley-Interscience: New York, 1967; p 467. (32) Sudarsan, V.; van Veggel, F. C. J. M.; Herring, R. A.; Raudsepp, M. J. Mater. Chem. 2005, 15, 1332. (33) Mai, H. X.; Zhang, Y. W.; Si, R.; Yan, Z. G.; Sun, L. D.; You, L. P.; Yan, C. H. J. Am. Chem. Soc. 2006, 128, 6426. (34) Lehmann, O.; Ko¨mpe, K.; Haase, M. J. Am. Chem. Soc. 2004, 126, 14935. (35) Schafer, H.; Ptacek, P.; Kompe, K.; Haase, M. Chem. Mater. 2007, 19, 1396. (36) Scheps, R. Prog. Quantum Electron. 1996, 20, 271.

Luminescent Monodisperse Nanocrystals of Lanthanide Oxyfluorides

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