Efficient Energy Transfer in Monodisperse Eu-Doped ZnO

Under a typical band-edge excitation of ZnO (380 nm), a direct energy transfer from ZnO host to Eu3+ ions, together with a distinct quenching of broad...
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J. Phys. Chem. C 2008, 112, 12234–12241

Efficient Energy Transfer in Monodisperse Eu-Doped ZnO Nanocrystals Synthesized from Metal Acetylacetonates 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 Laboratory of Rare Earth Materials Chemistry and Applications, and PKU-HKU Joint Laboratory in Rare Earth Materials and Bioinorganic Chemistry, Peking UniVersity, Beijing 100871, China ReceiVed: April 5, 2008; ReVised Manuscript ReceiVed: May 30, 2008

Monodisperse ZnO:Eu nanocrystals were prepared by the codecomposition of metal acetylacetonate precursors in a mixture of oleylamine and 1-octadecene. They have been systematically characterized by means of X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, inductively coupled plasma atomic emission spectrometry, Raman spectroscopy, and photoluminescence spectroscopy. The as-obtained ZnO:Eu nanocrystals are mainly composed of nanopyramids and dot-shaped nanocrystals and have an average size in the range of 16-32 nm. The valence states of europium ions in the nanocrystals are determined as predominately +3. With increasing dopant concentration of Eu3+ ions, Eu2O3 species tend to segregate on the surfaces of nanocrystals. Under a typical band-edge excitation of ZnO (380 nm), a direct energy transfer from ZnO host to Eu3+ ions, together with a distinct quenching of broad defect emission of ZnO, was observed for the as-obtained doped nanocrystals. Introduction Since semiconductor nanocrystals feature intermediate characteristics between discrete molecules and extended crystalline lattices, they usually show extraordinary size and shapedependent material properties. The last two decades have been witnessing an unprecedented progress in the research field of semiconductor nanocrystals, especially in the investigations of controllable synthesis and related unique luminescence behaviors of pure and/or doped II-VI semiconductor nanocrystals.1 Recent achievements in transition metal doped semiconductor nanocrystals, such as diluted magnetic semiconductors (DMS), have demonstrated that introduction of hetero-ions into a semiconductor host offers a robust way to modulate material properties of the nanocrystals.2 As is well known, II-VI semiconductor nanocrystals are important host materials for doping optically active ions, and the doped semiconductor nanocrystals often exhibit high luminescence efficiency even at a room temperature.2a Lanthanide ions possessing special 4f intrashells are recognized as excellent candidates for luminescence centers of the doped II-VI nanocrystals due to their many optical advantages, such as sharp fluorescent emissions via intra- 4f or 4f-5d transition, large stokes shift, no photobleaching and long luminescent lifetime, etc. Further research predicts that an energy transfer (ET) process from excited semiconductor host to doped lanthanide ions helps the doped nanocrystals to circumvent low absorption of optically center ions with showing remarkably improved luminescent properties.3b,d As one of the most important II-VI semiconductors, ZnO with a wide band gap of 3.37 eV is an environmentally friendly and chemically stable material and is a suitable host material for the doping of luminescence centers. Lanthanide-doped ZnO nanocrystals may represent a new class of luminescent materials for advanced display and lighting applications, such as full color * Corresponding authors. Fax: +86-10-6275-4179. E-mail: (C.-H.Yan) [email protected]; (Y.-W.Zhang) [email protected].

flat panel display,4a optical communications,5a field-emission displays (FEDs)5b and infrared-to-visible-light fluorescence imaging.6 In recent years, a variety of synthesis strategies, such as hydrothermal treatment,3a,4 forced hydrolysis,6,10a sol-gel,7 coprecipitation,8 and microemulsion templating,9 have been employed to dope trivalent lanthanide ions into wide band gap host materials, and sequentially to obtain optical activity. However, owing to the intrinsic process of “self-annealing” of doped lanthanide ions,4b,c,10 the doping process is hindered by the apparent size differences between the host cations and lanthanide ions as well as big mismatches in terms of charge density and coordination environment between the two kinds of cations. Fortunately, recent research has revealed that the effective doping is directly correlated with the ability of heterogeneous ions adsorbed on a certain surface of the host nanocrystals rather than merely “self-annealing” process.10 As a consequence, successful doping of lanthanide ions into semiconductor nanocrystals may be realized by delicately manipulating surface state and morphology of the nanocrystals, and choosing suitable surfactants in nanocrystal growth solution via various chemical strategies.10b In this paper, we report a facile and effective nonhydrolytic cothermolysis method for obtaining monodisperse Eu-doped ZnO nanocrystals in high boiling solvents, and an in-depth study of the microstructure and photoluminescent properties of the doped nanocrystals. More importantly, an efficient energy transfer from ZnO host to Eu3+ ions at room temperature along with a distinct quenching of the broad defect emission of the ZnO host is observed for our doped nanocrystals. Experimental Section The synthesis was carried out using standard oxygen-free procedures and commercially available reagents. Europium (III) oxide (Eu2O3; >99.99%), zinc acetylacetonate (Zn(acac)2 · H2O; >98%, Alfa Aesar), oleylamine (OM; >80%, Acros), 1-octadecene (ODE; >90%, Acros), acetylacetone (Hacac, A. R.

10.1021/jp802958x CCC: $40.75  2008 American Chemical Society Published on Web 07/19/2008

Energy Transfer in Eu-Doped ZnO Nanocrystals

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TABLE 1: ICP Analysis Results of as-obtained Eu-doped ZnO Nanocrystals samples

Eu (µg/ml)

Zn (µg/ml)

exp (molar ratio)

calc (molar ratio)

ZnO:2%Eu ZnO:5%Eu ZnO:8%Eu

11.1 32.2 8.70

190 228 47.8

0.025 0.060 0.078

0.020 0.053 0.087

grade), nitric acid (HNO3, A. R. grade), ammonia (NH3 · H2O, A. R. grade), sodium hydroxide (NaOH, >96%), absolute ethanol (C2H6O, >99.7%), and cyclohexane (C6H12, >99.5%) were used as received. 1. Synthesis of Eu(acac)3 Complex. Eu2O3 (10 mmol) was dissolved in 40 mL of deionized water by adding HNO3 to obtain Eu(NO3)3. Hacac (80 mmol) was dissolved in 40 mL of deionized water by adding NH3 · H2O (80 mmol). The above two solutions were mixed together while stirring and dilute aqueous NH3 · H2O solution was then added into the mixed solution dropwise (final pH: 6-7). The stock solution was further aged at room temperature for 12 h and then filtered. The precipitate was collected, washed by deionized water several times, and then dried in a vacuum at 60 °C overnight. The yield of the Eu(acac)3 complex was around 40-60%. 2. Synthesis of ZnO:Eu Nanocrystals. In a typical procedure for preparing ZnO:Eu nanocrystals, 1 mmol of Zn(acac)2 · H2O, a certain given amount of Eu(acac)3, 20 mmol of oleylamine, and 20 mmol of 1-octadecene were loaded into a three-necked flask at room temperature to form a slurry solution. The flask was purged at 100 °C for about 20-40 min in a temperature-controlled electromantle with vigorous magnetic stirring, so as to remove water and oxygen inside the reactor. Then, the solution was heated to 330 °C at a heating rate of around 20 °C min-1 and kept at this temperature for 60 min under an argon atmosphere. When the reaction was completed, excess ethanol was poured into the solution at a room temperature to form some white precipitates. The as-precipitated nanocrystals were collected without any size-selection by centrifugation, followed by washing several times with a mixture of cyclohexane and ethanol, drying at 60 °C overnight, and then had a yield of 50∼60%. The as-formed nanocrystals could be easily redispersed in various apolar organic solvents (e.g., cyclohexane). 3. Instrumentation. The crystal structures of as-obtained samples were identified by powder X-ray diffraction (PXRD) analysis with a Rigaku D/max-2000 diffractometer (Japan) with a slit of 1/2° at a scanning rate of 4° min-1, using Cu KR radiation (λ ) 1.5418 Å) in the 2θ range of 20-80°. The morphology and size of the nanocrystals were determined on a JEOL 200CX (Japan) low-resolution transmission electron microscopy (TEM) under a work voltage of 160 kV. Highresolution TEM (HRTEM) characterizations were performed with a Philips Tecnai F30 FEG-TEM (Holland) operated at 300 kV. The TEM samples were prepared by slowly vaporizing a dispersion of nanocrystals on carbon-coated copper grids. The X-ray photoelectron spectroscopy (XPS) measurements were carried out in an ion-pumped chamber (evacuated to 2 × 10-9 Torr) of an Escalad5 (U.K.) spectrometer, employing Al-KR radiation (BE ) 1486.6 eV) as the X-ray excitation source. The binding energy (BE) for the samples was calibrated by setting the measured BE of C 1s to 284.6 eV. The metal contents in the nanocrystals were determined by a Leeman Labs Profile spec (U.S.A.) inductively coupled plasma atomic emission spectrometer (ICP-AES), consistent with the expected values (Table 1). Room-temperature Raman spectra of the samples were

collected on a Jobin Yvon Raman Laboratory RAM HR800 spectrometer (France) excited with Ar+ 488 nm and He-Gd 325 nm laser beam equipped with a grating of 2400 grooves/ mm, an Olympus BX41 microscope (50 objective lens) and a charged-coupled device detector. The photoluminescence (PL) spectra of the nanocrystalline powders were recorded with an Edinburgh Instruments FLS920 steady-state fluorescence spectrometer (U. K.) at room temperature. Results and Discussion 1. Crystal Structure, Chemical Composition, Shape and Size, and Defect State of ZnO:Eu Nanocrystals. XRD. Figure 1a shows the XRD patterns of as-prepared ZnO:Eu nanocrystals. The main diffraction peaks are readily indexed to wu¨rtzite type ZnO (hexagonal structure; JCPDS: 36-1451) for both pure and Eu-doped ZnO nanocrystals. No diffraction peaks from any other chemical species such as europium oxides are detectable when the doping molar percentage of Eu (x%) is lower than 5%. By carefully analyzing the XRD patterns, we find that, when x% is higher than 2%, the peak at 29.5° attributable to cubic phase of Eu2O3 appears, and the peak intensity of Eu2O3 increases with x% from 5 to 10%. As x% higher than 8%, most reflections of cubic Eu2O3 (JCPDS: 34-0392) are distinguishable for assynthesized ZnO:10%Eu nanocrystals. In order to confirm the possibility of the substitution of Eu ions for Zn ions in as-prepared ZnO:Eu nanocrystals, the angle shift of 2θ (δ(2θ)) for the strongest peak of ZnO (101) reflection as a function of the doping molar percentage of Eu (x%) is calculated and illustrated in Figure 1b (insert shows the magnified region of (101) peak). δ(2θ) increases little as x% increases from 0 to 2%, but monotonically and significantly increases up to 0.18° as x% rises from 2 to 10%, demonstrating the presence of an effective substitution of Eu3+ for Zn2+ ions in the nanocrystals. Yang et al. reported that the solution limit in the Eu-doped system is below 0.2 wt % Eu, which corresponds to a δ(2θ) value of 0.15° for (101) peak in the XRD patterns.11c Therefore, the actual doping content of Eu2O3 in this work should be a little bit higher than 0.2 wt % Eu as x% ) 8%. Using the leastsquares refinement of cell dimensions from XRD data, the calculated Eu-doped ZnO lattice parameters are as follows: a ) b ) 3.2501 Å, c ) 5.2156 Å, V ) 47.7124 Å3 for ZnO: 2%Eu nanocrystals; a ) b ) 3.2508 Å, c ) 5.2193 Å, V ) 47.7665 Å3 for ZnO:5%Eu nanocrystals; and a ) b ) 3.2581 Å, c ) 5.2184 Å, V ) 47.9730 Å3 for ZnO:8%Eu nanocrystals. It can be observed that, compared to the pure ZnO nanocrystals (JCPDS: 36-1451) with a ) b ) 3.2498 Å, c ) 5.2066 Å, and V ) 47.6210 Å3, the lattice is expanded when introducing Eu3+ into ZnO. The lattice expansion is little when 0 < x% < 2%; whereas, the lattice expansion is significant when 2% < x% < 10%. All the above results also suggest that the solubility of Eu3+ ions in the ZnO lattice is limited, and more Eu2O3 species tend to segregate on the surfaces of the nanocrystals with increasing the doping amount of Eu.9,11 XPS. XPS analysis was employed to determine surface chemical composition of as-obtained ZnO:Eu nanocrystals. From Figure 2a, peaks assignable to core levels of Zn 2p, Zn 3p, O 1s, Eu 3d, and Eu 4d are identified. The observable peaks ascribed to the core levels of N 1s and C 1s reveal the presence of oleylamine ligands on the surfaces of the doped nanocrystals. Panels b-d of Figure 2 show XPS spectra taken from the Zn 2p, O 1s, and Eu 4d regions of the ZnO:5%Eu nanocrystals. As seen from Figure 2b, double peaks at 1044.7 and 1021.6 eV are ascribed to the core levels of Zn 2p1/2 and Zn 2p3/2 of ZnO,4a,7b respectively. The fitting of O 1s region with two-peak contribution indicates that at least two kinds of oxygen species

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Figure 1. (a) XRD patterns of as-prepared ZnO:Eu nanocrystals. (b) Peak shift for ZnO (101) as a function of the doping molar percentage of Eu (x%).

are present in the near surface domain of the ZnO:5%Eu nanocrystals (Figure 2c). The peak at about 530.0 eV is due to crystal lattice oxygen of dope ZnO nanocrystals, while the peak at about 531.7 eV is due to chemisorbed oxygen on the nanocrystal surfaces.12 The fitted double peaks at 141.2 and 136.1 eV are assigned to the Eu 4d3/2 and Eu 4d5/2 core levels of Eu3+ ions,13 respectively (Figure 2d). In comparison with the standard binding energy of Eu 4d5/2 (129.2 eV),14c a blueshift of 7.7 eV in binding energy is observed for the ZnO:5%Eu nanocrystals, meaning that the Eu-O distance in doped ZnO nanocrystals is quite different from that in pure Eu2O3 oxide.7a Figure 2e depicts the XPS spectra taken from the Eu 3d regions for as-obtained ZnO:Eu nanocrystals. Two intense peaks at 1164.5 and 1134.7 eV attributable to the core levels of Eu 3d1/2 and Eu 3d3/2, respectively, indicates that the oxidation states of europium ions are mainly trivalent for the doped ZnO:Eu nanocrystals.14 The atomic ratio of Eu to Zn was quantitatively determined as 0.04 for ZnO:2%Eu, 0.10 for ZnO:5%Eu and 0.16 for ZnO:8%Eu nanocrystals. For each sample, the Eu:Zn ratio determined by XPS analysis is nearly double of that obtained with ICP method (Figure 2f). This result strongly suggests that europium species predominated by Eu2O3 were remarkably enriched and segregated on the surfaces of the doped ZnO nanocrystals. TEM. TEM measurements reveal that the as-obtained ZnO: Eu nanocrystals are quite monodisperse, predominately composed of nanopyramids and dot-shaped nanocrystals (see panels a-d of Figure 3). The crystallite size is determined as (31.4 ( 3.9) nm for pure ZnO, (28.5 ( 3.6) nm for ZnO:2%Eu, (21.5 ( 4.3) nm for ZnO:5%Eu, and (16.7 ( 2.1) nm for ZnO:8%Eu nanocrystals. The sequential decrease of the crystallite size with increasing the Eu content strongly suggests that the growth rate of ZnO:Eu nanocrystals were more significantly restrained by the adsorption of more Eu species at a higher Eu(acac)3 concentration during the synthesis process. The inset in panel c of Figure 3 shows an HRTEM image of as-obtained ZnO: 5%Eu nanocrystals. The clear-cut lattice fringes shown in the HRTEM image confirm that the as-obtained ZnO:Eu nanocrystals are of high crystallinity. The interplanar spacings of 0.26 and 0.16 nm are ascribed to the (002) and (110) facet of wu¨rtzite type ZnO, respectively. Further TEM and HRTEM characterizations indicate that isolated Eu2O3 particles (encircled in the selected areas of the white marks in panel e) coexisted with Eu-doped particles as x% ) 10%, consistent with the XRD results (see Figure 1a). The interplanar spacing of 0.31 nm shown in the inset of Figure 3e is attributable to the (222) facet of cubic Eu2O3. Scanning

transmission electron microscopic-energy dispersive spectrometry (STEM-XEDS) analyses show that no isolated Eu2O3 particles present along with ZnO:Eu nanocrystals as x% is lower than 10%, suggesting that the segregated Eu2O3 species detected by the XRD and XPS methods is dispersed on surfaces of the Eu-doped nanocrystals (see Figure 3f). From the summed XEDS spectrum (see Figure 3g) extracted from the area indicated by the red square in the high-angle annular dark-field (HAADF) image of Figure 3f, the presence of Zn, Eu, and O elements is demonstrated for ZnO:8% nanocrystals (Cu peaks are due to the Cu TEM grid). The Eu:Zn ratio determined from XEDS analysis is 0.06, in agreement with the ICP result. Raman Spectroscopy. In order to obtain more information about the microstructure of as-obtained ZnO:Eu nanocrystals, their room temperature Raman spectra were measured. Group theory predicts the typical wu¨rtzite ZnO zone centered optical phonons,15 Γ ) A1 + 2B1 + E1 + 2E2, where B1 mode keeps silence and A1, E1, and E2 modes are Raman active. In addition, the A1 and E1 modes are polar and infrared active, and thus split into transverse-optical (TO) and longitudinal-optical (LO) phonons. The E2 mode consists of two modes of low- and highfrequency phonons. Figure 4a shows the Raman spectrum of as-obtained undoped and Eu-doped ZnO nanocrystals excited with 488 nm beam of Ar+ laser. The Raman peak at 433 cm-1 is attributed to the ZnO nonpolar optical phonons of high-E2 mode, which is one of the characteristic peaks of wu¨rtzite ZnO.15 The band at 327 cm-1 is assigned to the overtone of A1, the bands at 584 cm-1 corresponds to E1 symmetry with LO modes. It is generally accepted that the E1(LO) peak is caused by the defect due to O-vacancy, Zn-interstitial defect states or these complexes and free carriers. Therefore, the observation of E1(LO) peak indicates that oxygen vacancies or Zn interstitials present in the as-synthesized ZnO:Eu nanocrystals.16 Moreover, the main two Raman peaks of E1(LO) and E2(high) evidently shift toward low wavenumber as x% increases from 0 to 8%, confirming an effective substitution of Zn2+ ions with Eu3+ ions in the as-prepared nanocrystals,17 as is also revealed by the XRD method. When excited with He-Cd 325 nm laser beam, the Raman spectrum consists of four bands with frequency shifts at multiples of the 1LO zone-center with frequency of 580 cm-1 (Figure 4b). The other three resonance bands are observed at 1140, 1710, and 2296 cm-1 in turn, which are attributed to typical multiphonon processes,18 However, the 325 nm excited Raman peaks seem nearly the same for both undoped and doped ZnO nanocrystals. In addition, it should be pointed out that there is no detectable signal corresponding to any other europium species in the Raman spectra, demonstrating the doped nanoc-

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Figure 2. (a) XPS survey spectra of as-obtained ZnO:Eu nanocrystals. (b) Zn 2p, (c) O 1s, and (d) Eu 4d signals recorded for ZnO:5%Eu nanocrystals. (e) Eu 3d signals recorded for as-obtained ZnO:Eu nanocrystals. (f) Comparison of Eu:Zn ratios determined with ICP and XPS methods.

rystals keep the same structure as the ZnO when there are no apparently detectable impurities. 2. Photoluminescent (PL) Properties of ZnO:Eu Nanocrystals. ZnO Nanocrystals. Figure 5a shows the room temperature PL spectrum of as-obtained pure ZnO nanocrystals with using 325 nm excitation line of a 450 W xenon lamp. There are two emission bands on the spectrum. One is an intense and sharp blue emission band centering at around 380 nm (showing a very narrow full width at half-maximum in about 15 nm), which is attributed to the recombination of free excitons.4c,7b,19 Another is a weak and broad green emission band centering at around 536 nm, which is resulted from the presence of singly ionized oxygen vacancies or interstitial zinc ions (or other point defects).19b Further, the observation of such a strong emission band from excitons rather than from defects might imply that

the as-obtained ZnO nanocrystals hold high crystallinity or low lattice disorder.20 ZnO:Eu Nanocrystals. Figure 6a depicts the excitation spectrum of ZnO:5%Eu nanocrystals monitored at 613 nm emission of Eu3+ ions. The broad band at 380 nm and the peak at 354 nm are both attributed to the transition from valence band to conduction band (VBfCB) of ZnO semiconductor, and the two peaks at 395 and 465 nm are assigned to 7F0 f 5L6 and 7F f 5D transitions of Eu3+ ions.11a,20 The PL emission spectra 0 2 of doped nanocrystals were obtained with the excitation monitored on VBfCB transition of ZnO and on both 7F0 f 5L and 7F f 5D transitions of Eu3+ ions. 6 0 2 Figure 6b shows the emission spectra of as-obtained ZnO: Eu nanocrystals under the excitation wavelength of 380 nm, which is the typical pure ZnO exciton luminescence.19a The

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Figure 3. TEM images of as-obtained (a) ZnO, (b) ZnO:2%Eu, (c) ZnO:5%Eu, (d) ZnO:8%Eu, and (e) ZnO:10%Eu nanocrystals (insets in panels c and e show the corresponding HRTEM images). (f) STEM-HAADF image of ZnO:8%Eu nanocrystals, and (g) XEDS spectrum taken from the marked area in panel e.

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Figure 4. Raman spectra of as-obtained Eu-doped ZnO nanocrystals excited with (a) Ar+ 488 nm and (b) He-Cd 325 nm laser beam.

Figure 5. Room temperature emission spectra of as-obtained ZnO nanocrystals with using 325 nm excitation line of a 450 W xenon lamp.

peaks of red emissions at 580, 591, 613, 651, and 695 nm are ascribed to the 5D0 f 7FJ line transitions (J ) 0, 1, 2, 3, 4) of Eu3+ ions, respectively. More interestingly, under this UV excitation at 380 nm, only red emissions of Eu3+ ions without the distinct background of defects emission band of ZnO are observed for the ZnO:5%Eu nanocrystals, demonstrating that the high efficient energy transfer from ZnO host to Eu3+ ions was achieved for this sample. Such an efficient ET process from ZnO host to Eu3+ ions has seldom been observed before.3a–c,4,11 The fluorescence photograph of the ZnO:5%Eu nanocrystalline powder under irradiation of a 325 nm laser diode is shown in Figure S1 (Supporting Information). However, this highly efficient ET process could not be observed for the other samples obtained at either higher (e.g., ZnO:8%Eu) or lower (e.g., ZnO: 2%Eu) doping amount of Eu3+ ions, when directly excited with ZnO exciton emission. The luminescence of ZnO:8%Eu nanocrystals is considerably weaker than those of the ZnO:2%Eu and ZnO:5%Eu nanocrystals, probably due to the presence of more nonradiative luminescent centers as a result of the segregation of more Eu2O3 on the nanocrystal surfaces.3d,7a,11 The ET process of the ZnO:2%Eu nanocrystals was less efficient than that of ZnO:5%Eu nanocrystals, possibly due to the fact that the former nanocrystals with lower doping Eu3+ concentration have fewer energy storage centers than the later nanocrystals.3d,4 In addition, one green emission band centering at around 475 nm is observed for as-obtained ZnO:2%Eu and ZnO:8%Eu nanocrystals, possibly resulted from the radiative defects at the ZnO:Eu/Eu2O3 interface.21 When the doped Eu3+ ions are directly pumped at their intrinsic excited state 7F0 f 5D0 at 465 nm, no emissions from ZnO host but only intraconfigurational transitions of Eu3+ ions

are observed (Figure 6c); whereas, when exciting the sample at 395 nm on 7F0 f 5L6 transitions of Eu3+ ions, red emissions of Eu3+ ions together with a rather weak broad green emission (500-550 nm) arising from the defects of ZnO nanocrystals is observed. In addition, the 4f-4f intrashell transitions of 5D0 f 7F at 613 nm is the strongest. Therefore, the Eu3+ ions 2 emissions in as-obtained ZnO:5%Eu nanocrystals are clearly sensitized under the direct Eu3+ ions excitation.3,4 As is well demonstrated, Eu3+ ions are good signal probe for the local crystal lattice environment of lanthanide ions in the host materials. The relative intensity of typical electric-dipole 5D f 7F transition depends on the local symmetry of Eu3+ 0 2 ions, and it is very sensitive to the crystal lattice environment of the Eu3+ ions;22 whereas, the typical magnetic dipole transition 5D0 f 7F1 is relatively insensitive to the local symmetry of Eu3+ ions. Therefore, the host crystal field symmetry, in which the Eu3+ ions are situated, can be partially reflected by the ratio of 5D0 f 7F1 and 5D0 f 7F2 transitional probabilities. When fixed the excitation wavelength to 380 nm, the intensity ratio of 5D0 f 7F2 to 5D0 f 7F1 (I613 /I591) is 8.2, 7.8, and 2.4 for ZnO:2%Eu, ZnO:5%Eu, and ZnO:8%Eu nanocrystals, respectively (Figure 6b). The remarkably decrease in the I613/I591 ratio with increasing Eu3+ ions concentration suggests that the symmetry around the Eu3+ ions in ZnO nanocrystal host should increase with Eu3+ ions concentration for our ZnO:Eu nanocrystals. Possibly due to the fact that more Eu2O3 species are segregated on the surfaces of the ZnO:Eu nanocrystals at a high doping content of Eu3+ (as revealed by XRD and XPS analyses) with forming a core/shell like nanostructure, higher symmetry are likely favored for the Eu3+ ions doped in the ZnO host for the as-obtained nanocrystals. Under this condition, with increasing the doping concentration of Eu3+, the emission from the 5D0 f 7F2 transition of the Eu3+ ions decreases for as-obtained ZnO:Eu nanocrystals. Energy Transfer Process. It is known that the much faster (>102 times) radiative and nonradiative decay of excitons in ZnO than ET rate of RE3+ makes the direct ZnOfRE3+ energy transfer physically impossible.23 As demonstrated before in several systems such as ZnO:(F,Eu),24a,c,f ZnO:(N,Eu),24b,f SiC:(N,Er),24d,e and nanosheet-based ZnO:Eu microspheres,4b,c one effective way to facilitate the ET process is to introduce a proper trap center (intrinsic or extrinsic defects) in the ZnO host. In this work, to account for the origin of the observed efficient ET process of characteristic emissions of Eu3+ ions, we put forward a simple scheme to illustrate the mechanism of excitation and emission processes for as-obtained ZnO:5%Eu nanocrystals, as shown in Figure 7.

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Figure 6. (a) Room temperature excitation spectrum of as-obtained ZnO:5%Eu nanocrystals (λem ) 613 nm). (b) Room temperature emission spectra of as-obtained ZnO:Eu nanocrystals (λex ) 380 nm). Room temperature emission spectra of as-obtained ZnO:5%Eu nanocrystals under (c) λex ) 465 nm and (d) λex ) 395 nm.

Figure 7. Plausible energy transfer process for as-obtained ZnO:5%Eu nanocrystals under λex ) 380 nm (solid and dashed lines depict radiative and nonradiative transitions, respectively). Here, VB, CB, and DS are the abbreviations of valence band, conduction band, and defect states, respectively.

At first, a large amount of carriers are excited from the VB to CB of ZnO nanocrystals (Process 1) under 380 nm wavelength. Then, they are trapped at energy storage centers of defect complexes (intrinsic defects), as resulted from the substitution of Zn2+ by Eu3+ in the nanocrystals (Process 2). Finally, the Eu3+ ions at the excited-state of the defects of centers give radiative decays of characteristic Eu3+ ions emissions at 591 nm (5D0 f 7F1, Process 3), 580 nm (5D0 f 7F0, Process 4), and 613 nm (5D0 f 7F2, Process 5), respectively. Conclusions In this paper, we have demonstrated a facile and one-step synthesis of monodisperse Eu-doped ZnO nanocrystals in hot-

boiling organic solvents of oleyamine/1-octadecene by using metal acetylacetonate as precursors. The valence states of europium ions in doped nanocrystals are determined as predominately +3. The as-obtained ZnO:Eu nanocrystals are predominately composed of nanopyramids and dot-shaped nanocrystals, and have an average size in the range of 16-32 nm. With increasing dopant concentration of Eu3+ ions, more Eu2O3 species tend to be enriched and segregated on the surfaces of the nanocrystals. Under the typical UV excitation of ZnO host (380 nm), the direct energy transfer from ZnO host to Eu3+ ions, along with a distinct quenching of broad defect emission of ZnO, was observed for the as-obtained doped nanocrystals, possibly due to the formation of intrinsic trapping centers aroused from the substitution of Zn2+ by Eu3+ in the nanocrystals. Therefore, the developed synthetic approach has provided a robust way for the fabrication of ZnO nanomaterials doped with optically active impurities. To get a comprehensive understanding of the corresponding luminescent mechanisms and the key structural feature of the doped nanocrystals (such as the exact location and concentration of the Eu3+ ions), further work will be done with site-selective excitation spectroscopy and high-end X-ray absorption techniques. Acknowledgment. We gratefully acknowledge the financial aids 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). Supporting Information Available: Fluorescence photograph of as-obtained ZnO:5%Eu nanocrystalline powder under

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