J. Phys. Chem. C 2009, 113, 2309–2315
2309
Optical Spectroscopy of Eu3+-Doped BaFCl Nanocrystals Qiang Ju, Yongsheng Liu, Renfu Li, Liqin Liu, Wenqin Luo, and Xueyuan Chen* Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Science, Fuzhou, Fujian 350002, China ReceiVed: October 18, 2008; ReVised Manuscript ReceiVed: December 12, 2008
Tetragonal Eu3+:BaFCl nanocrystals were synthesized via mild-temperature solvothermal treatment. The size of the as-grown nanoflakes was estimated to be 100-200 nm in diameter and 20-40 nm in thickness by XRD, SEM, and TEM. By means of site-selective spectroscopy at 10 K, two kinds of luminescence sites of Eu3+ were unambiguously identified. One site showing moderately resolved fluorescence lines was associated with the distorted lattice site that might be close to the surface. Another one exhibiting very sharp emission and excitation peaks was ascribed to the inner lattice site with an ordered crystalline environment. Intense and sharp emission lines from the excited states of 5D1 and 5D2 in Eu3+:BaFCl nanocrystals were observed for the first time. The lifetime of 5D0 was significantly longer than that of the microcrystal counterpart and decreased rapidly with the increase of temperature. Crystal-field levels and site symmetries of Eu3+ at the two sites have also been determined. 1. Introduction Since the rapid advances in nanosciences and nanotechnologies, particularly the development of new methods for materials synthesis, there has been growing interest in studying the optical behavior of rare-earth (RE) ions in nanomaterials.1-3 The REdoped inorganic nanophosphor is one of the most promising materials for a variety of applications in solid-state lasers, lighting and displays, and biolabels.4 The BaFCl is an alkalineearth dihalide compound that belongs to the MFX family (M ) Ca, Sr, Ba; X ) Cl, Br, I). It crystallizes in a tetragonal matlockite structure and possesses the characteristics of the layered compound, which is interesting for both fundamental reasons and applications.5,6 Besides, the BaFCl is also a suitable host into which RE ions can be embedded, and RE ions-doped BaFCl materials have also several technological applications. For instance, the luminescence of Sm2+-doped BaFCl was observed to be influenced by the pressure, which could be utilized as a pressure calibrant in diamond anvil cells.7 The Eu2+doped BaFCl as an X-ray storage phosphor, which exhibits photostimulated luminescence and offers an alternative to conventional X-ray imaging technology, is widely used in digital radiography and appears to be the most efficient material.8-10 In Sm3+-doped BaFCl nanocrystals, the efficient X-ray generation of relatively stable Sm2+ was observed, which may function as a directly photoexcitable (photoluminescence) storage phosphor.11 The Eu3+:BaFCl crystal has great potential in optical data storage, signal processing, and flat panel display devices.12 In addition, Eu3+ ions are extensively employed as an optical probe to investigate the coordination environment around the cations substituted in the crystalline lattice.13,14 When Eu3+ ions are introduced into BaFCl crystals, they may substitute Ba2+ ions and enter into the crystal lattice. Since the valence state and the ionic radius of Eu3+ differ from that of Ba2+, multiple sites of Eu3+ in BaFCl host are expected. Li et al. revealed more than 15 sites for Eu3+ in bulk BaFCl crystal and the laserexcitation-induced site-to-site conversion for persistent spectral * Corresponding author. Phone and fax: +86-591-8764-2575. E-mail:
[email protected].
hole burning.15,16 Chen et al. recently observed two Eu3+ sites in Eu3+:BaFCl microcrystals, having standard and anomalous crystal-field (CF) spectra, respectively, in which the anomalous site showed unusually strong emission of 5D0 f 7F0 and the luminescence lifetime of 5D0 at both sites decreased rapidly with the temperature.17,18 To the best of our knowledge, little attention has been paid to the optical spectroscopy of Eu3+-doped BaFCl nanocrystals such as site symmetry, luminescence dynamics, and energy levels. The optical properties of Eu3+:BaFCl with nanometer dimensions may differ significantly from those of bulk materials. In this work, a facile solvothermal method without any additive is proposed to synthesize Eu3+-doped BaFCl flake nanocrystals. Two lattice sites of Eu3+ in BaFCl nanocrystals, which were not observed in the bulk counterpart, are identified by means of site-selective spectroscopy at 10 K. Intense emissions from 5D1 and 5D2 of the inner Eu3+ site are observed. The site symmetries, CF levels, and photoluminescence (PL) dynamics of Eu3+ ions at different sites are also investigated in detail. 2. Experimental Section 2.1. Sample Preparation. The BaFCl has a small solubility product constant in aqueous solution, that is, it will immediately be precipitated upon direct mixing of Ba2+, Cl-, and F-. Thus it is difficult to control the morphology and the size of the final product.11 The key factor to prepare monodisperse or uniform nanocrystals is to adjust the chemical kinetics of the precipitation.19 Due to the smaller solubility of NH4F and BaCl2 in ethanol solution, the particle growth of the precipitated BaFCl crystal is inhibited. As a result, more uniform BaFCl nanocrystals are anticipated.20 By utilizing the mixture of ethanol and water as the solvent, the relatively uniform Eu3+:BaFCl nanocrystals were synthesized through a simple solvothermal method. Briefly, 1 mmol of barium chloride (BaCl2 · 2H2O) and 0.025 mmol of europium chloride (EuCl3 · 6H2O) were dissolved in the ethanol-water solution (v/v 5:1). Meanwhile, 1 mmol of ammonium fluoride (NH4F) was also dissolved in ethanol-water solution (v/v 8:1) and the mixture was stirred at room temper-
10.1021/jp809233p CCC: $40.75 2009 American Chemical Society Published on Web 01/20/2009
2310 J. Phys. Chem. C, Vol. 113, No. 6, 2009 ature (RT) for several minutes to afford a transparent solution. The transparent NH4F solution was then dropwise added to the BaCl2 solution, and a cloudy precursor solution was formed after 10 min of stirring. The precursor solution was transferred into a 50 mL Teflon-lined autoclave, sealed, and solvothermally treated at 110 °C for 24 h. The autoclave was cooled to RT naturally and the products were deposited at the bottom of the vessel. The resulting solutions were centrifuged at 10 000 r/min for 3 min to separate the solid powder products. The powders were washed several times with distilled water and absolute ethanol in turn, and dried in air at 50 °C for 12 h to yield the final white products. To obtain the optimal doping concentration of Eu3+ ions, BaFCl nanocrystals doped with the concentrations of 0.5, 1, 2, 3, 5.5 atom % were synthesized, respectively. To investigate the influence of the atmosphere on the luminescence, the asobtained products were further heated at 300 °C for 1 h in air and argon, respectively. For comparison, the microcrystals of tetragonal Eu3+:BaFCl (0.1 atom %) were also synthesized by using the solid-state reaction method, and the detailed description of the preparation and characterization has been reported elsewhere.17 2.2. Characterization. The powder X-ray diffraction (XRD) pattern of the sample was measured with a PANalytical X’Pert PRO powder diffractometer with Cu KR1 radiation (λ ) 0.154 nm). The morphology and chemical compositions of the sample were investigated by a JEOL-2010 transmission electron microscope (TEM), equipped with the energy dispersive X-ray spectrum (EDS), and a JSM 6700F scanning electron microscope (SEM). The precise concentration of Eu3+ in the nanocrystals was determined by the Ultima2 ICP optical emission spectrometer. Emission and excitation spectra and transient decays were recorded on an Edinburgh Instruments FLS920 spectrofluorimeter equipped with both continuous (450 W) and pulsed xenon lamps. For the low-temperature measurements, samples were mounted on a closed-cycle cryostat (10-350 K, DE202, Advanced Research Systems). For site-selective spectroscopy, the excitation (or emission) monochromator’s slits were set as small as possible to maximize the instrumental resolution. The best wavelength resolution is 0.05 nm. The line intensities and positions of the measured spectra were calibrated according to the FLS920 correction curve and standard mercury lamp. 3. Results and Discussion 3.1. Structural and Morphology Characterization. The precise content of Eu3+ was determined to be 2.4 atom % by the ICP optical emission spectrometer for the sample doped with a nominal concentration of 2.5 atom %. The XRD pattern of the as-obtained product and the JCPDS card (No. 76-1368) as a reference were compared in Figure 1. As shown in Figure 1a, the diffraction peaks can be well indexed to the pure tetragonal BaFCl (JCPDS card 76-1368, Figure 1b), and no trace of characteristic peaks was observed for other impurity phases such as BaF2 or EuF3, showing that the simple solvothermal method is a feasible route to prepare pure phase Eu3+:BaFCl nanocrystals. The morphology of Eu3+:BaFCl nanocrystals was characterized by the SEM and TEM. From the SEM (Figure 2a) and TEM (Figure 2b) images, it can be seen that the samples were roughly hexagonal flake, with the diameter ranging from 100 to 200 nm and the thickness of 20 to 40 nm. The selected area electron diffraction (SAED) of a Eu3+:BaFCl nanocrystal (Figure 2c) exhibited the regular diffraction spots and could be well indexed as tetragonal BaFCl with single crystalline nature, in
Ju et al.
Figure 1. (a) X-ray diffraction pattern of the as-obtained Eu3+:BaFCl nanocrystals; (b) standard data for tetragonal BaFCl crystal (JCPDS Card No. 76-1368).
good agreement with the above XRD results. The elemental components of the as-grown Eu3+:BaFCl (2.4 atom %) nanocrystals were investigated by EDS as shown in Figure 2d, which confirmed the existence of Ba, F, Cl, and Eu in BaFCl nanocrystals. 3.2. Excitation and Emission Spectra. Because of charge imbalance and lattice distortion, multiple sites are expected for trivalent Eu ions incorporated in BaFCl nanocrystals. Due to the difficulty in preparing Eu3+:BaFCl nanocrystals by a wet chemical method, optical properties of Eu3+ ions in BaFCl nanocrystals have not been reported so far. In this work, two new sites in Eu3+:BaFCl nanocrystals were revealed by siteselective spectroscopy at 10 K. Figure 3 shows two totally different luminescence patterns of Eu3+ ions under the excitations at 392.5 and 464.8 nm, which correspond to intra f-f transitions of 7F0 f 5L6 and 7F0 f 5D2, respectively. Under the excitation at 392.5 nm, only five relatively broad emission peaks due to f-f transitions of Eu3+ were observed in the visible, which were assigned to 5D0 f 7FJ (J ) 0, 1, 2, 3, 4) (Figure 3a). These peaks with moderately resolved CF splitting suggest that Eu3+ ions are very likely located at a distorted lattice site close to the surface (hereafter referred to as site A), similar to the surface sites previously observed for Eu3+-doped ZnO and TiO2 nanocrystals.14,21 Unlike site A, when site-selectively excited at 464.8 nm, much sharper f-f emission lines peaked at 578.7, 591.9, 617.4, 660.1, and 692.2 nm were observed, which correspond to transitions of 5D0 f 7FJ (J ) 0, 1, 2, 3, 4), respectively (Figure 3b). Their line widths characterized by the value of full width at halfmaximum height (fwhm) were much smaller than that of site A, namely, decreasing from ∼4 nm (the 612-nm peaks of site A) to ∼1.1 nm (the 617-nm peak). These results indicate that Eu3+ ions occupy an inner lattice site with an ordered crystalline environment (hereafter referred to as site B). Neither site A nor site B was observed in bulk or microcrystal counterparts. It should be noted that the transition of 5D0 f 7F2 (centered at 617 nm) is the strongest among the emission peaks of site B instead of the transition of 5D0 f 7F4 (centered at 699.3 nm) for site A. Moreover, the intensity ratio of 5D0 f 7F2 to 5D0 f 7 F1 for site B is approximately two times that of site A, and the position of the most intense line of 5D0 f 7F4 for site B is noticeably blue-shifted. The different fwhm, intensity ratios, and line positions reveal a very different CF environment for Eu3+ ions at site B, which should reside in a much better crystalline lattice than site A. In addition to the characteristic transition lines from 5D0, intense emission lines from higher excited states of 5D2 and 5D1 to 7FJ were also observed in the visible. Such
Eu3+-Doped BaFCl Nanocrystals
J. Phys. Chem. C, Vol. 113, No. 6, 2009 2311
Figure 2. (a) SEM and (b) TEM images of Eu3+:BaFCl nanocrystals; (c) SAED and (d) EDS patterns of nanocrystals.
Figure 4. Excitation spectra of sites A and B in Eu3+:BaFCl nanocrystals at 10 K, by monitoring the emissions at (a) 588.2 and (b) 582.2 nm. The insets enlarge the differences between sites A (red) and B (black) in the region of 5L6 (c) and 5D2 (d). Figure 3. Emission spectra of sites A and B in Eu3+:BaFCl nanocrystals at 10 K, with (a) λexc ) 392.5 nm for site A and (b) λexc ) 464.8 nm for site B.
strong emissions from 5D2 and 5D1 were not observed previously in Eu3+:BaFCl materials. Their luminescence dynamics will be discussed in Section 3.5. To further reveal the multisite structure of Eu3+ in BaFCl nanocrystals, site-selective excitation spectra by monitoring the “fingerprint” emissions at 588.2 and 582.2 nm, corresponding to 5D0 f 7F1 of site A and 5D1 f 7F3 of site B, respectively, were measured at 10 K (Figure 4). The f-f transition lines of Eu3+ peaked at 297.6, 317.5, 361.3, 375.3, 393.0, 464.6, and 525.3 nm, which were assigned to the direct excitation of Eu3+ from the ground state (7F0) to different excited multiplets (5F5, 5 H3-7, 5D4, 5G2-6, 5L6, 5D2, and 5D1), were detected.22 The broader lines without apparent CF splitting of site A (Figure 4a), consistent with its emission spectrum, confirm that Eu3+
ions are located at much distorted nanolattices. In addition, a small hump centered at 320 nm was observed in Figure 4a, which was attributed to the oxygen defects (OF2-, or O2substituting one of the nearest F- around Eu3+) as previously observed in BaFCl crystals.17,23 Because of smaller particle size and higher surface-to-volume ratio, oxygen may be easily adsorbed and thus oxygen defects were formed around site A during the synthesis procedure in air. To verify that the hump was associated with the oxygen defects, the excitation spectra of the sample heated at 300 °C for 1 h under different atmospheres (air or argon) were compared. For the sample heated in air, the intensity of the hump centered at 320 nm was about three times as intense as that of the as-grown sample, whereas for the sample heated in argon, it was only one-third the intensity of the as-grown counterpart. Besides, spectral lines of the sample heated in air become more broadened, and worse resolved. It turned out that the CF environment of Eu3+ was significantly influenced by the amounts of oxygen defects. The
2312 J. Phys. Chem. C, Vol. 113, No. 6, 2009
Ju et al. TABLE 1: Partial Energy Levels of Eu3+ at Sites A and B in BaFCl Nanocrystals at 10 K and Room Temperature multiplet 7
F0 F1
7
7
F2
above facts further verify that site A is close to the surface and surrounded by defects. In contrast, much sharper and better resolved excitation lines at 297.9, 316.9, 361.3, 375.3, 394.4, 464.8, and 524.8 nm, which correspond to the transitions from 7F0 to 5F5, 5H3-7, 5D4, 5G2-6, 5 L6, 5D2, and 5D1, were observed by monitoring the transition of 5D1 f 7F3 at 582.2 nm of site B (Figure 4b). Insets c and d in Figure 4 compare the excitation spectra of sites A and B in the regions of 5L6 and 5D2, respectively. For site B, a remarkable red shift of ∼1.4 nm in the region of 5L6 was observed, and the hump centered at 320 nm typical of site A was not shown. The sharp lines and well-resolved CF splitting in both emission and excitation spectra disclose a totally different crystalline environment of site B. For comparison, the emission and the excitation spectra of the microcrystal counterpart were also measured at RT (Figure 5). Under the excitation at 266 nm, only three emission lines due to f-f transitions of Eu3+ (5D0 f 7F1,2,4) and a broadband centered at 520 nm were observed with the dominant 5D0 f7F2 transition in the emission spectrum. The emission patterns were the same as that of the normal Eu site (site I) identified in a preceding work, and the broadband was ascribed to oxygen defects (OF2-).17 When monitoring the emission at 610.2 nm, the excitation spectrum in the left side of Figure 5 showed only two weak characteristic f-f transitions of Eu3+ and a dominant broadband. The strongest broadband centered at 266 nm in the ultraviolet region was attributed to an O2--Eu3+ charge transfer (CT) transition, which was usually observed in the oxides doped with trivalent europium,24-26 showing that oxygen impurities entered into the microcrystals. On the shoulder of the CT band, a hump centered at ∼320 nm, which has been observed in nanocrystals and ascribed to oxygen defects, also appeared in the excitation spectrum of microcrystals. The hump’s intensity was much stronger than that of f-f transitions of Eu3+. In contrast with the microcrystal counterpart, the absence of the O2--Eu3+ CT band, weaker PL of oxygen defects, and stronger emission and excitation lines of f-f transitions for both sites A and B in nanocrystals indicate that the solvothermal method at mild condition is an efficient route to prevent the oxygen from entering into the host, even if no reducing gas or reagent was used. From the high-resolution emission and excitation spectra at 10 and 295 K, we are able to locate and assign the CF levels of Eu3+ at the two sites. A total number of 22 for site A and 41 for site B were summarized in Table 1. The CF splitting of 7F1 of site A (169 cm-1) is about twice that of site B (88 cm-1),
site B (cm-1)
0 276 377 445 945 1080
0 338 387 426 997 1017 1085 1094 1127 1846 1875 1910 1972 2067 2681 2866 2906 3842 3910 4023
7
1897
7
2746 2832 2975
F3
Figure 5. The RT excitation (λem ) 610.2 nm) (left) and emission (λex ) 266 nm) (right) spectra of Eu3+:BaFCl microcrystals (0.1 atom %) synthesized by solid-state reaction.
site A (cm-1)
F4
7
F5
7
F6 D0 5 D1
5
5
D2
5
D3 L6
5
17274 18670 19054 21528 21561 24044 25119 25426
5
26652
5 5
27663 31496
5
33624
G2,3,4,5,6 D4 H3,4,5,6,7
F5
17280 18671 19051 21515 21547 24033 25082 25119 25138 25355 26434 26645 26731 27678 31319 31348 31505 31545 31656 33113 33658
indicating a much larger scalar rank-two CF strength of site A. Compared with the energy levels of the microcrystal counterpart previously reported by Chen et al., site A (or B) differs from either the normal site (site I) or the anomalous one (sites II) in microcrystals.17 The energy levels of the two emerging sites in nanocrystals are so close that we cannot distinguish them completely under the current experimental conditions. As a result, very weak emission lines originating from 5D2 and 5D1 of site B also appeared in the emission spectrum of site A (Figure 3a) due to the overlap between their excitation spectra. 3.3. PL from the Excited States of 5D1,2. As mentioned in Section 3.2, strong emissions from the excited states of 5D1 and 5 D2 are first observed in Eu3+:BaFCl nanocrystals. For Eu3+ ions occupying a well-crystallized site, whether the emission can occur from the excited states of 5D2 and 5D1 or not depends critically on the doping concentration of Eu3+ and the dominant vibration frequencies available in the host lattice.27 If the doping concentration of Eu3+ is high (thus short Eu3+-Eu3+ distance), PL emissions from 5D1,2 might be quenched by cross relaxation occurring between two neighboring Eu3+ ions. To investigate the influence of doping concentration on the luminescence of 5 D1, the PL intensity of 5D1 f 7F3 and the intensity ratio of 5D1 f 7F3/5D0 f 7F2 as a function of the doping concentration were
Eu3+-Doped BaFCl Nanocrystals
J. Phys. Chem. C, Vol. 113, No. 6, 2009 2313 TABLE 2: Theoretically Allowed Transition Lines of 5 D0-7FJ of Eu3+ Ions at 32 Crystallographic Point Groups transition lines of 5D0-7FJ local symmetry triclinic monoclinic orthorhomic tetragonal
Figure 6. The intensity of 5D1 f 7F3 and the intensity ratio of 5D1 f 7 F3/ 5D0 f 7F2 as a function of the doping concentration of Eu3+.
depicted in Figure 6. With the increasing concentration of Eu3+, the PL intensity of 5D1 f 7F3 increased gradually, reached the maximum at 2.5 atom %, and then decreased due to the concentration quenching at higher doping concentrations. In contrast, the intensity ratio of 5D1 f 7F3/5D0 f 7F2 gradually decreased with the concentration, suggesting that the population ratio of 5D1 to 5D0 decreases at higher doping concentration. Moreover, the populations of 5D1,2 can also be diminished via nonradiative multiphonon relaxation if the maximum phonon frequency of the host lattice is large enough. It has been proposed by Blasse that the radiative rate is approximately equal to the nonradiative transition if the energy gap amounts to four to five times the maximum phonon frequency of the host lattices.28 The maximum phonon frequency of the BaFCl crystal was observed to be as low as 294 cm-1,17 and the energy gaps of 5D1-5D0 and 5D2-5D1 were estimated to be 1771 and 2464 cm-1, respectively. Correspondingly, it needs at least 6 and 8 phonons to bridge the gaps of 5D1-5D0 and 5D2-5D1, giving rise to much smaller nonradiative relaxation rates from 5D1,2 and thus much longer lifetimes of 5D1,2 (as will be shown in Section 3.5). However, despite the very low doping concentration of Eu3+ in BaFCl microcrystals (0.1 atom %), no PL emissions from 5D1,2 were observed. Because of abundant oxygen defects formed in the microcrystals, the Eu3+-O2cluster can be easily formed and the localized vibrational frequency of the axial Eu3+-O2- cluster was estimate to be 600 cm-1,17 much larger than the maximum phonon energy of BaFCl. Due to the much enhanced de-excitation to 5D0 via such nonradiative relaxation paths, no radiations from 5D1,2 were observed in the microcrystal counterpart. 3.4. Local Structures of Eu3+. It is well-known that the 5D0 f 7F2 lines of Eu3+ ions are of electric dipole (ED) nature and very sensitive to site symmetry, whereas the 5D0 f 7F1 lines of Eu3+ ions are of magnetic dipole (MD) nature and insensitive to site symmetry. According to the J-O theory,29,30 the former ED is permitted on condition that Eu3+ ions occupy a site without an inversion center. The intensity ratio of 5D0 f 7F2 to 5 D0 f 7F1 may provide structural hints such as the distortion of CF environment and the site symmetry. Radiative transitions from 5D0 to levels with J ) 0 or odd J (J ) 3, 5) are both ED and MD forbidden, and only weak transitions from 5D0 to these levels are observed due to CF induced J-mixing effect.17 Moreover, the 5D0 f 7F0 transition is only allowed in the following 10 site symmetries: Cs, C1, C2, C3, C4, C6, C2V, C3V, C4V, and C6V, according to the ED selection rule.18 On the basis of the above analyses associated with the J-O theory and the selection rules for ED and MD transitions, the allowed transition lines of 5D0 f 7FJ (J ) 0, 1, 2, 3, 4, 5, and 6) for Eu3+ ions at 32 crystallographic point groups are summarized in Table 2.31-33
trigonal
hexagonal
cubic
point group
0
1
2
3
4
5
6
C1 Ci Cs C2 C2h C2V D2 D2h C4 C4V S4 D2d D4 C4h D4h C3 C3V D3 D3d S6 C6 C6V D6 C3h D3h C6h D6h T Td Th O Oh
1 0 1 1 0 1 0 0 1 1 0 0 0 0 0 1 1 0 0 0 1 1 0 0 0 0 0 0 0 0 0 0
3 3 3 3 3 3 3 3 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 1 1 1 1 1
5 0 5 5 0 4 3 0 2 2 3 2 1 0 0 3 3 2 0 0 2 2 1 1 1 0 0 1 1 0 0 0
7 0 7 7 0 5 6 0 3 2 4 3 3 0 0 5 3 4 0 0 2 1 2 3 2 0 0 2 1 0 1 0
9 0 9 9 0 7 6 0 5 4 4 3 3 0 0 6 5 4 0 0 2 2 1 4 3 0 0 2 1 0 1 0
11 0 11 11 0 8 9 0 6 4 5 4 5 0 0 7 5 6 0 0 3 2 3 4 3 0 0 3 1 0 2 0
13 0 13 13 0 10 9 0 6 5 7 5 4 0 0 9 7 6 0 0 5 4 3 4 3 0 0 3 2 0 1 0
This table may be useful in the assignment of the possible Eu3+ site symmetry from the observed spectra. For Eu3+ ions at site B, the ED transition of Eu3+ was much stronger than the MD transition, suggesting that Eu3+ ions at site B possess a low-symmetry site without an inversion center. The 5D0 f 7F0 transition, which is only allowed in the 10 point group symmetries aforementioned, was observed in the emission spectrum. In tetragonal BaFCl structure, Ba2+ ions occupy a site of C4V symmetry. The substitution of Ba2+ with Eu3+ may result in significant lattice distortion and thus descend the original C4V site symmetry to C4, C2V, C2, Cs, and C1 according to the branching rules of the 32 point groups.34 As shown in Table 2, if Eu3+ ions at site B situate at C4V or C4 site, only two lines for J ) 0 f J ) 1 transition and two lines for J ) 0 f J ) 2 transition are allowed. If Eu3+ ions are situated at C2V, there should be three lines for J ) 0 f J ) 1 transition and four lines for J ) 0 f J ) 2 transition. As a matter of fact, three lines for 5D0 f 7F1 transition and five lines for 5D0 f 7F2 transition of Eu3+ at site B were resolved. It infers that the actual site symmetry of Eu3+ at site B is very likely reduced to a lower symmetry than C2V (that is, C2, Cs, or C1), due to the difference of ionic radius and charge imbalance between Eu3+ (r ) 0.095 nm) and Ba2+ (r ) 0.135 nm).18,35 For Eu3+ ions at site A, the transition of 5D0 f 7F0 was also present in the emission spectrum (Figure 3a), which to some extent indicates a low-symmetry site without an inversion center for site A. Besides, three lines of the 5D0 f 7F1 transition of site A can be well-resolved in the emission spectrum, evidencing that it may occupy C2V, C2, Cs, or C1, according to the selection rules in Table 2. Generally, the smaller the intensity ratio of 5D0 f 7F2/5D0 f 7F1 is, the higher symmetry the local structure.36 As shown in the emission
2314 J. Phys. Chem. C, Vol. 113, No. 6, 2009
Figure 7. The 10 K luminescence decays from 5D0 of Eu3+ at sites A and B under the site-selective excitation at 392.5 and 464.8 nm, respectively.
spectrum of site A, the intensity of 5D0 f 7F1 (MD) is obviously stronger than that of 5D0 f 7F2 (ED), implying that it possibly occupies the highest site symmetry of C2V. To determine the actual symmetry of site A, nevertheless, additional evidence such as EXAFS may be needed to characterize the local structure of Eu3+. 3.5. Luminescence Lifetime. To probe the CF environment experienced by Eu3+ ions at multiple sites, luminescence decays of 5D0 for sites A and B were measured at 10 K (Figure 7) by monitoring the site-selective emissions of 5D0 f 7F1 (591.8 nm) and 5D0 f 7F2 (617.2 nm), respectively. Similar to the closeto-surface sites for Eu3+ doped in ZnO or TiO2 nanocrystals,14,21,37 the decay curve for Eu3+ at site A fitted well to a single exponential function, which thus excluded the possibility of a superposition of different decays from various sites and indicated a homogeneous CF environment around Eu3+ at site A. The luminescence lifetime of 5D0 for Eu3+ at site A was determined to be 3.9 ms at 10 K. Unlike site A, the decay curve of site B exhibited a fast rise at the very initial stage followed by a single exponential decay when excited to the 5D2 state (464.8 nm). The luminescence lifetime of 5D0 for Eu3+ at site B was determined to be 3.6 ms at 10 K, which was comparable to that of site A. Such behavior is similar to that of Eu3+-doped TiO2 nanocrystals,21 but very different from that of Eu3+-doped ZnO nanocrystals where the PL lifetime of the close-to-surface site was significantly shorter than that of the inner site.14 Interestingly, the PL lifetimes of 5D0 for Eu3+ at sites A and B were both found to be significantly dependent on the temperature despite their large energy gap between 5D0 and its next low-lying 7F6 (∼12 000 cm-1). The 5D0 lifetimes of sites A and B at RT were determined to be 1.85 and 2.80 ms, respectively. Similar decay behaviors of 5D0 have also been observed in Eu3+-doped BaFCl microcrystals, where the energy transfer processes were the dominant nonradiative relaxation mechanism and thus led to the temperature-dependent lifetime of 5D0.17 To investigate the influence of oxygen defects on the 5 D0 lifetime, PL decays of site A were measured and compared for those samples heated at 300 °C in different atmospheres. For the sample heated in argon, the 5D0 lifetime was estimated to be 2.20 ms at RT, while it was only 0.8 ms for the sample heated in air. The variation of the 5D0 lifetime of site A, being associated with oxygen defects in these samples, further confirms that Eu3+ ions at site A possess a CF environment surrounded by defects. It should be noted that the observed lifetimes of 5 D0 for Eu3+ at sites A and B in nanocrystals are remarkably longer than that of the microcrystal counterpart (0.779 and 0.305 ms for sites I and II at 3 K, respectively).17 The much longer
Ju et al.
Figure 8. The 10 K luminescence decays from 5D1 and 5D2 of Eu3+ at site B when directly pumping to 5D2 at 464.8 nm.
PL lifetimes in BaFCl nanocrystals are very likely induced by (1) very different CF surroundings experienced by Eu3+ at sites A and B in nanocrystals that might lead to smaller spontaneous radiative transition rates and (2) a smaller filling factor of nanocrystals that leads to smaller effective index of refraction and longer radiative lifetime, as often observed in Eu3+ doped nanophosphors.38 PL decays from 5D1,2 of Eu3+ at site B were also investigated in detail. Because of the strong emissions from 5D1,2 we observed, the lifetimes of 5D1 and 5D2 can be determined by direct decay measurement instead of the indirect method.37,39 As shown in Figure 8, the decay curve of 5D1 fitted well to a single exponential function, whereas the decay of 5D2 deviated slightly from a single exponential. By utilizing the single exponential fit, the lifetimes of 5D1 and 5D2 were determined to be 2.87 and 1.36 ms at 10 K, respectively. So far, PL emissions from 5D1 and 5D2 were not observed in bulk or microcrystal counterparts. Such long luminescence lifetimes of 5D1 and 5D2 were not reported previously for Eu3+:BaFCl crystals. The longlived 5D0, 5D1, and 5D2 of Eu3+ at site B show unambiguously that the nonradiative relaxation rates from these excited states are significantly diminished in nanocrystals. 4. Conclusions A simple solvothermal method has been developed to synthesize Eu3+:BaFCl nanocrystals, in which the water and ethanol solutions play a key role in controlling the nucleation kinetics. Site-selective optical spectra have provided clear evidence for the existence of two nonequivalent sites of Eu3+ in BaFCl nanocrystals, which were not observed in bulk counterparts. Due to the lattice distortion, the site symmetry of the local structure surrounding Eu3+ was descended from C4V to C2V or lower for one site close to the surface, and to C2, Cs, or C1 for another inner lattice site. Compared to the microcrystal counterpart, the 5D0 lifetimes at both sites, with a magnitude of several milliseconds, are significantly prolonged. Intense emission lines from the unusual long-lived 5D1 and 5D2 of site B have been observed in BaFCl nanocrystals, and these emissions were influenced by the doping concentration and the maximum phonon frequency available in the host lattice. Acknowledgment. This work is supported by the Knowledge Innovation and One Hundred Talents Programs of the Chinese Academy of Sciences, the National Natural Science Foundation of China (Grant Nos. 10504032 and 10774143), the 973 program (No. 2007CB936703), and the Science and Technology Foundation of Fujian Province (No. 2007I0024).
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