J. Phys. Chem. B 2001, 105, 12709-12713
12709
Colloidal YVO4:Eu and YP0.95V0.05O4:Eu Nanoparticles: Luminescence and Energy Transfer Processes K. Riwotzki and M. Haase* Institut fu¨ r Physikalische Chemie, UniVersita¨ t Hamburg, Bundesstrasse 45, D-20146 Hamburg, Germany ReceiVed: April 12, 2001; In Final Form: October 3, 2001
The luminescence of pure and europium-doped nanocrystalline YVO4 and YP0.95V0.05O4 and the energy transfer processes in such nanoparticles have been studied by temperature dependent luminescence spectroscopy and luminescence lifetime measurements. The results indicate thermally activated energy transfer between adjacent vanadate groups in YVO4 at temperatures above 100 K, but energy transfer to europium seems to take place from direct vanadate neighbors only. In contrast to the luminescence decay of europium, the kinetic of the vanadate luminescence strongly depends on the choice of surface capping and solvent, indicating partial quenching of the vanadate emission at surface sites. The strong competition with energy transfer to surface sites seems to be reason for the absence of energy transfer to europium from distant vanadate groups. The latter explains the low room-temperature quantum yield of 15% of YVO4:Eu colloids.
Introduction
Experimental Section
YVO4:Eu is a strongly luminescing material1 which had been used as the red phosphor in cathode ray tubes for more than 20 years. Moreover, the material is well excited by UV light, and the photoluminescence quantum yield of the europium emission is as high as 70%.2a Bulk YVO4:Eu is normally prepared by solid-state reaction at temperatures above 1200 K.2 One alternative method is growth in aqueous solutions under hydrothermal conditions, i.e., in an autoclave under elevated temperatures and pressure.3 Recently, we have used the latter method to prepare colloidal solutions of YVO4:Eu nanoparticles in the 20-30 nm size regime.4 Analysis of the line splitting and the intensity pattern of the luminescence line spectrum of europium proves that the dopant ions in the nanoparticles occupy the same crystal sites as in the bulk material, despite the low synthesis temperature.4 The quantum yield of colloids of these nanoparticles, however, is only 15% at room temperature. A problem often encountered in hydrothermal synthesis methods is the incorporation of OH species into the crystal lattice. In the case of europium-doped samples, the latter would present a serious problem because water molecules or OH groups coordinated to europium are known to quench its luminescence.5 By synthesizing YVO4:Eu nanoparticles in D2O rather than H2O, Huignard et al.6 have recently shown, however, that incorporation of OH is not the reason for the lower quantum yield. On the other hand, dispersing the nanoparticles in D2O instead of H2O increased their photoluminescence quantum yield, indicating transfer of excitation energy to the particle surface.6 In this paper, we investigate the energy transfer processes and the luminescence of pure and doped YVO4 nanoparticles by temperature-dependent luminescence and luminescence lifetime measurements. To evaluate the influence of energy transfer between adjacent vanadate groups, we also investigated the “diluted” YP0.95V0.05O4 system where the mean distance between two vanadate groups is relatively large.
YVO4:Eu nanoparticles were prepared hydrothermally as described previously.4 YP0.95V0.05O4:Eu nanoparticles were synthesized by replacing 95 mol % of Na3VO4‚10H2O by Na2HPO4‚2H2O in the recipe given for YVO4:Eu nanoparticles. As-prepared nanoparticles can be easily redispersed in water at room temperature or in TOPO (trioctylphosphine oxide) at 240 °C. Nanoparticles soluble in apolar solvents are obtained by diluting the TOPO solution with toluene and precipitating the nanoparticles with methanol. X-ray diffraction patterns of powder samples were recorded with a Philips XÅpert system. Rietveld analysis7 of the XRD data was performed using PC-Rietplus V1.1 B software (Philips).8 UV-vis absorption and photoluminescence spectra of the colloidal solutions were measured with a Cary 500 Scan spectrometer (Varian) and a Spex Fluoromax 2 spectrometer, respectively. The luminescence quantum yield of colloidal YVO4:Eu nanoparticles was determined by comparing the integral luminescence intensity of the scatter-free colloid with that of rhodamine 6G dissolved in spectroscopic grade ethanol. Luminescence decay curves were measured by using a Nd: YAG laser (Spectron Laser Systems, model SL804F10) in combination with a double monochromator (Schoeffel, model EM 200), a Hamamatsu R928 photomultiplier, and a digital oscilloscope (Hewlett-Packard, model 54522 A). The colloids were excited at 266 nm with laser pulses of low intensity (fourth harmonics of a Nd:YAG laser, 15 ns pulse duration), and the time-dependent luminescence intensity was recorded at a preselected wavelength. Cutoff filters were used when appropriate, e.g., to remove laser straylight. A closed cycle cooler cryostat (Leybold) was used for measurements below room temperature.
* To whom correspondence should be addressed.
Results and Discussion Nanocrystals without Europium Doping. The powder XRD data of YVO4 and YP0.95V0.05O4 nanoparticles are shown in Figure 1 (crosses). Comparison with the literature data for YVO4
10.1021/jp0113735 CCC: $20.00 © 2001 American Chemical Society Published on Web 12/01/2001
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Figure 3. Luminescence decay of the vanadate emission of YVO4 at 18 K. Figure 1. XRD data (crosses) of nanocrystalline YVO4:Eu and YP0.95V0.05O4 powders and Rietveld simulations (solid lines) of the patterns. Bottom: literature data of the corresponding bulk materials. The crystal structure is zircon type for both YVO4 (black, PDF No. 17-341) and YPO4 (gray, PDF No. 11-254).
Figure 4. Luminescence, luminescence excitation (broken line on the left), and UV-vis absorption spectrum of a dilute colloidal solution of nanocrystalline YVO4:Eu. The dashed line between 350 and 650 nm indicates the spectral position of the vanadate emission of pure YVO4, which is strongly quenched in the presence of europium.
Figure 2. Luminescence and UV-vis absorption spectra of dilute colloidal solutions of nanocrystalline YVO4 and YP0.95V0.05O4. Broken lines: best fits to Gaussian line shapes.
and YPO4 (vertical lines) shows that both nanocrystalline systems are obtained in the tetragonal xenotime phase known from the corresponding bulk materials. Because YVO4 and YPO4 form a homologous series of mixed crystals, the diffraction peaks of YP0.95V0.05O4 are slightly shifted away from the positions of pure YPO4 toward those of YVO4. All peaks of the nanocrystalline samples are broadened because of the small size of the crystallites. A Rietveld simulation (lines through data points) of both spectra indicates domain sizes of 21 and 17 nm for YVO4 and YP0.95V0.05O4 nanoparticles, respectively. Transmission electron micrographs (TEMs) of YP0.95V0.05O4 nanoparticles are very similar to the images of YVO4 nanocrystals given in our previous paper.4 The crystallite sizes observed in the TEMs are well in accord with the domain sizes determined by the Rietveld analyses. Figure 2 shows the absorption and luminescence spectra of colloidal solutions of pure YVO4 and YP0.95V0.05O4 nanoparticles. Both materials show a broad absorption peak, at 272 and
266 nm respectively, because of the presence of VO43- groups in the nanocrystalline lattice. The high absorption coefficient (R[272nm] ∼ 200 000 cm-1 in YVO44) indicates an optically allowed transition within the vanadate group. Both luminescence spectra consist of a broad band very similar to that observed for the corresponding bulk materials. The large Stokes shift, the similar width of the absorption and luminescence bands, and their Gaussian shapes indicate strong electron-phonon coupling. The high dilution of the solid crystals in colloidal solution allows for unambiguous determination of the apparent Stokes shift, which is about 15 400 cm-1 for both materials. The real Stokes shift is somewhat lower, however, because ESR measurements of bulk YVO4 have shown that the emitting state is a spin triplet.9 This is supported by the kinetic of the vanadate emission given in Figure 3, showing a luminescence lifetime of τ ) 1 ms at 18 K which is in strong contrast to the high absorption coefficient of the vanadate band. Directly after the laser pulse, the kinetic of the vanadate emission shows a fast component which is displayed as a sharp peak in Figure 3. To correctly display the relative intensity of the peak, we composed the figure from two measurements, one taken on a much shorter time scale. The fast component is also present in the vanadate kinetic of europium doped samples as will be shown in Figure 7 for a shorter time scale. At low temperature, the fast process represents less than 10% of the total luminescence intensity. Its origin is not yet clear. Possible explanations are the singulet emission of vanadate, a multiphotonic process under laser excitation or rapidly quenched vanadate groups at the particle surface. In fact, the luminescence
Luminescence and Energy Transfer Processes
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Figure 5. Part of the energy level scheme of YVO4:Eu. Excitation, emission, and energy transfer processes as indicated.
Figure 7. Luminescence rise of the europium 5D1 level at 14 K, monitored on a faster time scale (top). The observed kinetic trace is a superposition of the vanadate emission (middle) and the europium 5D1 emission, rising with a half-life of about 0.25 µs (bottom).
Figure 6. Kinetic curves of the europium emission of nanocrystalline YVO4:Eu powder at 295 K. Top: luminescence decay of the 5D1 level. Middle: luminescence rise of the 5D0 level. Bottom: luminescence decay of the 5D0 level. The data at 14 K and of colloidal solutions at 295 K are given, too.
lifetime of the long-lived component decreases strongly with increasing temperature and, furthermore, depends on the medium the particles are dispersed in. This will be discussed in more detail below. Europium Doped Nanocrystals. Upon UV excitation, europium doped YVO4 nanoparticles exhibit strong red luminescence (Figure 4) caused by transitions within the f-electron shell of the europium ions.10 The main emission lines in the luminescence spectrum correspond to transitions from the europium 5D0 and 5D1 levels as known from the bulk material.1 Earlier measurements4 have shown that YVO4:Eu nanoparticles prepared by this method display the highest luminescence quantum yield at a europium concentration of 5 at. %, i.e., at the same dopant concentration as the bulk material. Therefore, a europium concentration of 5 at. % was used for all doped nanoparticles discussed in this paper. The luminescence of the vanadate group is much weaker as compared to the pure material, and the luminescence excitation spectrum reveals energy transfer from vanadate to europium (Figure 4). All energy transfer processes are visualized and summarized in Figure 5. The right-hand side of this figure indicates concurrent feeding of the 5D0 level by the 5D1 level, which is in fact observed in the corresponding luminescence decay curves. The 5D1 level (Figure 6, upper part) shows a lifetime of 6.6 µs, a value also found for the luminescence rise time of the 5D0 level (Figure 6, middle part). The 5D0 level subsequently decays with a luminescence lifetime of about 740 µs. This value is virtually independent of temperature and surrounding medium (Figure 6, bottom) and is roughly in accord with literature values given for the bulk material (52511 and 475 µs12). Thus, the europium luminescences of bulk and nanocrystalline YVO4:Eu are very similar. This indicates that the luminescence of europium at surface sites is either strongly quenched by all combinations of solvent and
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Figure 8. Temperature-dependent luminescence of YP0.95V0.05O4:Eu nanoparticles. Top: emission spectra of europium. Middle: emission spectra of vanadate. Bottom: luminescence decay of the vanadate emission.
Figure 9. Temperature-dependent luminescence of YVO4:Eu nanoparticles. Top: emission spectra of europium. Middle: emission spectra of vanadate. Bottom: luminescence decay of the vanadate emission.
capping molecules employed or that our acidic treatment with a complexing agent removes most of the europium ions from the surface.4 Shortly after the laser pulse, the kinetic curves of both europium levels are disturbed by the short-lived component of the vanadate luminescence, the latter of which extents into the wavelength region of the europium lines (compare with Figure 4). Figure 7 shows that the shape of the 5D1 kinetic (at 14 K) can be decomposed into the decay of the pure vanadate luminescence, measured at 480 nm, and a second component with a rise time of about 0.25 µs. Because all europium ions are excited via energy transfer from vanadate groups, the latter indicates an upper limit of about 250 ns for the energy transfer rate to europium. The value presents an upper limit only, because the 5D1 level of europium is not populated directly but via a relaxation process involving several higher states of europium (see Figure 5). Relaxation via the 5D2 level, for instance, is evident from the weak 5D2 luminescence easily identified in the luminescence spectrum at higher magnification.4 Taking into account the time necessary for relaxation to the 5D1 level, we conclude that energy transfer to europium is even faster than 250 ns and, hence, occurs predominantly from vanadate groups not far away from the europium site. In contrast, luminescence rise times between 6 and 14 µs have been reported12 for the europium 5D1 level of bulk YVO4:Eu and can be explained by
transport of excitation energy from distant VO43- groups to europium. This energy transfer process via adjacent vanadate groups is believed to be one reason for the high photoluminescence quantum yield of bulk YVO4:Eu.13 Energy transfer between vanadate groups is known to be a thermally activated process and, with a rate sufficiently high, occurs between nearest neighbors only.14 To investigate the luminescence of our nanoparticles in the absence of this process, we additionally prepared YP0.95V0.05O4:Eu nanoparticles, where most vanadate groups are well separated from each other due to the “dilution” with phosphate.13 Figure 8 shows temperature dependent luminescence spectra of the europium ions and the vanadate groups in this “dilute” system. The vanadate luminescence at low temperatures is predominantly long-lived with a lifetime similar to that of undoped material, indicating that the luminescence originates from isolated vanadate groups having no europium neighbors. With increasing temperature, radiationless processes become dominant, and the luminescence intensity and luminescence lifetime of these VO43- groups decreases. Because strong Eu3+ luminescence is observed upon excitation of the vanadate band, additional vanadate groups must exist which transfer their energy to europium. The luminescence intensity of europium is scarcely affected by temperature, although all europium ions are excited via
Luminescence and Energy Transfer Processes
Figure 10. Luminescence decay of the vanadate emission of nanocrystalline YVO4:Eu powder (line) and colloidal solutions in water (dashed line) and in hexane (TOPO-capped particles, dotted line).
vanadate groups and despite the strong temperature dependence of the vanadate luminescence. We, therefore, conclude that energy transfer from a vanadate group to europium is much faster than the radiationless processes at this vanadate group. The latter indicates that energy transfer to europium takes place predominately from nearest vanadate neighbors. This is not unexpected, because the phosphate content of the YP0.95V0.05O4 lattice strongly hampers a multistep energy transport to europium via several vanadate groups. Next, we discuss the “concentrated” system YVO4:Eu, where energy transfer between adjacent vanadate groups takes place, if the temperature is not too low. In principle, the europium luminescence quantum yield could benefit from this energy transfer, because the dopant may be excited even by distant vanadate groups. However, the upper part of Figure 9 shows that the intensity of the europium luminescence does not increase at higher temperatures, i.e., in the presence of energy transfer between vanadate groups. In accord with the luminescence lifetime measurements discussed above, we therefore conclude that energy transfer from distant vanadate groups to europium is negligible not only in the dilute YP0.95V0.05O4:Eu nanomaterial but also in YVO4:Eu nanoparticles. The latter, however, does not indicate that no energy transfer at all takes place between vanadate groups. This is evident from the temperature dependence of the vanadate luminescence. At temperatures below 50 K, the vanadate luminescence of YVO4: Eu is most intense and shows a long luminescence lifetime with components of up to several 100 µs (Figure 9, bottom). With increasing temperature, the luminescence lifetime decreases much faster than in the case of YP0.95V0.05O4 nanoparticles, indicating an additional channel in YVO4:Eu for radiationless recombination. It is very likely that this channel is energy transport via vanadate to the particle surface, because, in contrast to the lifetime of the europium luminescence, the lifetime of the vanadate luminescence depends on the medium the particles are dispersed in. The latter is displayed in Figure 10, showing
J. Phys. Chem. B, Vol. 105, No. 51, 2001 12713 that the lifetime of the vanadate emission is shorter for TOPO capped particles dispersed in hexane than for particles in aqueous solution and the dry state. Thus, at sufficiently high temperatures, energy transfer via vanadate groups takes place in nanocrystalline YVO4:Eu as well as in bulk YVO4:Eu, but in the former case, the final step is deactivation of the excited vanadate state at a surface site rather than excitation of europium. The low quantum yield of 15% at room temperature may therefore be strongly improved by terminating the surface of YVO4:Eu nanoparticles with a material growing epitactically on YVO4, provided that transport of excitation energy through this material is negligible. YPO4 used in this paper to dilute the YVO4 lattice may be a suitable coating material, because YVO4 and YPO4 are isomorphous and have similar lattice constants and the excitation energy cannot be transferred via phosphate groups. Therefore, YVO4:Eu/YPO4 core/shell particles may be a promising nanocrystalline system, probably combining high luminescence quantum yield with high chemical stability. Acknowledgment. We thank J. Kolny for measuring the XRD spectra. We greatly acknowledge funding of this project by the German Science Foundation (DFG). References and Notes (1) Brecher, C.; Samelson, H.; Lempicki, A.; Riley, R.; Peters, T. Phys. ReV. 1967, 155, 178. (2) (a) Wanmaker, W. L.; Bril, A.; ter Vrugt, J. W.; Broos, J. Philips Res. Rep. 1966, 21, 270. (b) Schwarz, H. Z. Anorg. Allg. Chem. 1963, 323, 44. (c) Palilla, F. C.; Levine, A. K.; Rinkevics, M. J. Electrochem. Soc. 1965, 112 (8), 776. (3) (a) General Electric; U.S. Patent 3,479,296, 1969. (b) Phillips, M. L. F. Proc. SPIE 1995, 2408, 200. (4) Riwotzki, K.; Haase, M. J. Phys. Chem. B 1998, 102, 10129. (5) (a) Harrocks, W. DeW., Jr.; Schmidt, G. R.; Sudnick, D. R.; Kittrel, C.; Bernheim, R. A. J. Am. Chem. Soc. 1977, 99, 2378. (b) Harrocks, W. DeW., Jr.; Sudnick, D. R. Science 1979, 206, 1194. (c) Harrocks, W. DeW., Jr.; Sudnick, D. R. Acc. Chem. Res. 1981, 14, 384. (d) Hole, R. C.; Harrocks, W. DeW. Jr. Inorg. Chim. Acta 1990, 171, 193. (e) Frey, S. T.; Chang, C. A.; Pounds, K. L.; Harrocks, W. DeW., Jr. Inorg. Chem. 1994, 33, 2882. (6) Huignard, A.; Gacoin, T.; Boilot, J.-P. Chem. Mater. 2000, 12, 1090. (7) Young, R. A. The RietVeld Method; Oxford University Press: Oxford, 1993. (8) Fischer, R. X.; Lengauer, C.; Tillmanns, E.; Ensink, R. J.; Reiss, C. A.; Fantner, E. J. Mater. Sci. Forum 1993, 287, 133. (9) (a) Barendswaart, W.; van Tol, J.; van der Waals, J. H. Chem. Phys. Lett. 1985, 121 (4-5), 361. (b) Barendswaart, W.; Weber, R. T.; van der Waals, J. H. J. Chem. Phys. 1987, 87 (7), 3731. (10) (a) Judd, B. R. Phys. ReV. 1962, 127, 750. (b) Ofelt, G. S. J. Chem. Phys. 1962, 37, 511. (c) Peacock, R. D. Struct. Bonding (Berlin) 1975, 22, 83. (11) Levine, A. K.; Palilla, F. C. Appl. Phys. Lett. 1964, 5 (6), 118. (12) Venikouas, G. E.; Powell, R. C. J. Lumin. 1978, 16, 29. (13) (a) Blasse, G.; Bril, A. J. Electrochem. Soc. 1968, 115 (10), 1067. (b) Blasse, G. Philips Res. Repts. 1968, 23, 344. (c) Blasse, G. Philips Res. Repts. 1969, 24, 131. (d) Blasse, G.; Bril, A. Philips Techn. ReV. 1970, 31, 304. (e) Powell, R. C.; Blasse, G. Struct. Bonding (Berlin) 1980, 42 (II), 70. (14) (a) Hsu, Ch.; Powell, C. J. Lumin. 1975, 10, 273. (b) Blasse, G. Energy Transfer Processes in Condensed Matter; Plenum Press: New York, 1984.
