J. Phys. Chem. C 2007, 111, 4529-4534
4529
Conformational Characterization of Eu3+-Doped LaF3 Core-Shell Nanoparticles through Luminescence Anisotropy Studies Enrico Bovero and Frank C. J. M. van Veggel* Department of Chemistry, UniVersity of Victoria, P.O. Box 3065, Victoria, British Columbia, Canada, V8W 3V6 ReceiVed: NoVember 22, 2006; In Final Form: January 19, 2007
Polarization techniques have been implemented to determine the size and shape of LaF3 core-shell nanoparticles in aqueous colloidal solutions, by studying the optical properties of the Eu3+ active ion inserted as a dopant in the core of the nanoparticles. A luminescence anisotropy as high as 0.4 has been measured for the 5D0 f 7 F0 transition with the sample frozen in a glass water-glycerol matrix. Time-resolved luminescence anisotropy at different temperatures and viscosity of the medium indicated that the particles present a mainly spherical shape and a size distribution very close to 10 nm. The absence of depolarization by energy transfer for a Eu3+ concentration of 5 atom % confirms that the LaF3 matrix has a high crystallinity and that each particle is a single crystal.
Introduction Nowadays, lanthanide-doped nanoparticles find many different applications such as amplifiers for fiber optics communication,1 light sources for zero-threshold lasers,2,3 displays,4 and Light-Emitting Diodes (LEDs).5 Therefore, the importance of studying them has greatly increased in the past decade. The optical features of such materials are due to transitions of electrons in the 4f shell, which are partially forbidden, since the electric dipole operator has odd parity, while the transition matrix element must have even parity. Anyhow, these transitions can take place because the 4fN states are mixed with 4fN-1nl (e.g., nl ) 5d) states of opposite parity. Different mechanisms can lead to this mixing: uneven terms in the crystal field, partial mixing with uneven vibrations in the lattice, allowed magnetic dipole transitions, and allowed electric quadrupole transitions. For this reason they generally present long luminescence lifetimes (of the order of micro- to milliseconds), which can be useful when population inversion is required, as for example in lasers or optical amplifiers. Among all the lanthanides, the emission of Eu3+ shows a lifetime of the order of milliseconds, a high quantum yield, and a high luminescence anisotropy arising from the nondegenerate 5D0 level. One of the main causes of depolarization is rotational diffusion. From the depolarization mechanisms, it is thus possible to obtain information about the characteristics of the sample, such as size and shape. Previously, studies have been carried out on Eu3+ complexes,6 showing high steady-state anisotropy. Time-resolved decay of anisotropy has generally been reported for organic or dye-labeled biomolecules,7 in order to understand their dynamic motion, but to the best of our knowledge it has never been reported in cases of Eu3+ resonant emission. In this work we studied LaF3-based core-shell nanoparticles doped in the core with 5 atom % of Eu3+ in water-glycerol mixtures. A schematic sketch of a coreshell nanoparticle is presented in Figure 1. First, the anisotropy spectra have been measured, to verify the polarization properties of the material in such environment. The anisotropy gave us important information about the crystallinity of the core-shell * Address correspondence to this authhor. E-mail:
[email protected].
Figure 1. General scheme of a LaF3:Eu3+ nanoparticle. The Eu3+ is inserted as a dopant only in the core, which is represented as a darker area in the figure.
nanoparticle. Second, the time-resolved decay has been studied modulating the viscosity through simple temperature control. From the rotational diffusion of the particles it was possible to assess that the particles are mainly spherical and with a size distribution very close to 10 nm. Theory The method used to calculate anisotropy is the standard one described by Lakowicz:8
r)
I| - GI⊥ I| + 2GI⊥
(1)
where I| is the intensity of the emission when both polarizers are in the same orientation and I⊥ is the intensity when the orientation of the emission polarizer is perpendicular with respect to the excitation polarizer. The G factor is the ratio of the sensitivities of the detection system for vertically and horizontally polarized light. If the emission dipole is parallel to the excitation dipole, the photoselection of a random distribution of immobilized particles gives a maximum steadystate value of the anisotropy of 0.4. Between all the rare earth ions, Eu3+ presents a peculiar transition, in which the ground-
10.1021/jp0677849 CCC: $37.00 © 2007 American Chemical Society Published on Web 03/08/2007
4530 J. Phys. Chem. C, Vol. 111, No. 12, 2007
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state (7F0) and excited-state level (5D0) are both nondegenerate and the direction of the dipole moment is along the quantization (z) axis. The dipole moments of the transitions from the 5D0 to the other 7FJ (J ) 1-6) levels are not parallel to the z-axis, and the component for each stark state along this axis can only be observed if the spectra are well resolved. The principal causes of depolarization can be radiationless energy transfer among the ions and rotational diffusion of the particles. For a spherical particle the anisotropy decay due to rotational diffusion is described by a single exponential:
r(t) ) r0e-t/φ
(2)
where r0 is the anisotropy observed in the absence of other depolarizing processes, t is the time, and φ is the rotational correlation time. If the rotating object is not symmetric and if there are many different kinds of emitters,9 the anisotropy decay is described by the general expression n
r(t) )
{
m
Rie ∑βije ∑ i)1 j)1 -t/τi
-t/φj
n
Rie-t/τ ∑ i)1
}
+ r∞
(3)
i
where the index i runs over the n rotamers in solution, and j takes into account the m depolarizing motions of the emitter. The pre-exponential factors Ri are weighting terms and βij represents the degree of emission depolarization due to each rotational component; τi represents the lifetime of the individual emitting species. When the motion of the emitting species is hindered, this orientation does not recover to a random distribution, and the anisotropy value remains higher than zero, that is r∞. The proportionality of the correlation time on the viscosity η of the medium is expressed by the equation
φ)
ηV RT
(4)
in which V is the volume of the rotating unit and T is the absolute temperature. The viscosity for fluids with a glass transition temperature (Tg) is well described by the WilliamLandel-Ferry model,10 expressed by eq 5:
η(T) ) exp
(
)
-C1(T - Tr) C 2 + T - Tr
(5)
where C1, C2, and Tr are empiric parameters. Typically they are chosen like this: Tr ) Tg + 43 K, C1) 8.86, and C2 ) 101.6 K. By decreasing the temperature, a fluid becomes more viscous and, because the viscosity is proportional to the correlation time, the latter becomes longer and longer until the sample is frozen. At this point the luminescence emission cannot be depolarized by rotational diffusion. In a dilute vitrified solution the emitters are not only immobilized, but also far enough apart not to interact with each other, so that the depolarization due to energy transfer is unlikely to happen. Therefore, assuming that no trivial causes of depolarization are present, in such a sample it is possible to measure r0 even in a time-resolved experiment, since the value remains constant all over the duration of the luminescence emission. Experimental Section The details of the synthesis of these Eu3+-doped LnF3 coreshell nanoparticles have been reported elsewere.11 The Eu3+ was
Figure 2. Overlap of the 5D0 f 7F0 emission profile with the respective IRF.
introduced only in the core and with a concentration of 5 atom % with respect to the total Ln3+ amount in the core and the shell surface was clothed with citrate ligands to stabilize the particles from aggregation in aqueous solution and to give them a high dispersibility. The sample was dispersed in a waterglycerol mixture, in which the percentage of glycerol was 60% by volume. The glycerol was added to increase the viscosity of the medium: such a mixture shows a glass transition temperature at around 205 K.6 The anisotropy measurements on the 5D0 f 7F resonant emission decay of Eu3+ below such a temperature 0 clearly showed that the particles were immobilized. The sample was cooled with an ITC601 temperature-controlled cryostat cooled by liquid nitrogen. Photoluminescence measurements were recorded with an Edinburgh Instruments’ FLS 920 fluorimeter. The emission spectra from the 5D0 to the 7FJ levels have been measured exciting the Eu3+ at 397 nm in the 5L6 level with a 450 W Xe arc lamp. The lifetime of the same level has been measured exciting at 464 nm in the 5D2 level with 5 ns pulses at a frequency of 10 Hz from a Vibrant tuneable laser system (Model 355 IIb) with a Quantel Nd:YAG nanosecond pump laser. The percentage contribution of each lifetime component to the total decay has been calculated with the F900 Edinburgh instruments software. All the other spectra have been measured in the different polarizations with a resolution of 1 nm exciting with the laser at 577.0 nm and, for the decay curves, collecting the emission decay at 577.8 nm. The polarization of the excitation beam was oriented by a Babinet Soleil Compensator (Model BCS100) and optimized by a polarizing film (polarizing efficiency: 99.98). The polarization of the emission was selected by a Glan Thompson polarizing prism. Polarization spectra have been measured at different temperatures as TimeResolved Emission Spectra (TRES), considering the emission intensity after 2 ms, to be sure that no more contribution from the polarization of the scattered light was present. The emission from the 5D0 level to the ground-state level is weak and very narrow and the challenge in the time-resolved anisotropy measurements is to be able to collect the emitted light almost at the same wavelength of the excitation. For this purpose, a mechanical chopper was placed before the emission polarizer and was synchronized so that the emission was blocked during the flash of the laser. In this way, the shape of the emission decay was affected by two factors. The first one was that the scattered light was not completely blocked by the chopper, but a small amount was still able to reach the detector and affected its response for the first few microseconds, leading to the appearance of a fast decay. The second one was that the passage of the chopper’s blade onto the emitted beam produced an
Eu3+-Doped LaF3 Core-Shell Nanoparticles
J. Phys. Chem. C, Vol. 111, No. 12, 2007 4531 the temperature to 170 K no decay is detectable and the anisotropy value remains around 0.4 as shown in Figure 11. Discussion
Figure 3. 5D0 f 7F0 smoothed emission profiles in the VV and VH polarizations at 220 K; the solid line is the anisotropy calculated from these two curves. The anisotropy scale is on the left side and the intensity scale of the emission is on the right side.
apparent ingrowth of the emission, characterized by the increase of the emitted intensity with time, correlated to the displacing of the blade from the beam. The remaining contribution of the scattered light was eliminated by subtraction of an Instrument Response Function, which was scaled with the emission decay by setting the first points of the two curves at the same level. The IRF was measured by exciting the sample in the same conditions but at a slightly different wavelength, i.e. 572.0 nm, where the Eu3+ does not absorb, and collecting at 572.8 nm. Decay and IRF are depicted in Figure 2. As for the blade’s passage, it just affects the shape of the emission, but it does not affect at all the radiation’s polarization, which is kept even if the intensity is very low, as is shown in Figure 3. Therefore, with this experimental setup it is possible to retrieve the anisotropy right after the pulse of the laser. Results The presence of the Eu3+ in the sample was verified by measuring the emission spectrum and the luminescence decay curve reported in Figure 4. All the principal transitions from the 5D0 level have been observed, the FWHM of a single peak is around 30 cm-1. In the inset, the luminescence decay to the 7F level can be well described with a double exponential, in 1 which the longest component dominates 80% of the curve (the exponential lifetimes are indicated in Figure 4). This is a phenomenological approach for the treatment of the decay curve. The 5D0 f 7F0 emission band in Figure 5 presents a profile that is well described by a single Gaussian, with a correlation value R2 of 0.995. At room temperature the anisotropy of the emission band is zero as is shown in Figure 6, both in aqueous and in water-glycerol dispersion. The sample in the waterglycerol mixture has been cooled to 170 K, below the glass transition temperature, and this time the polarized TRES shows anisotropy very close to the 0.4 value of photoselection, as reported in Figure 7. The polarization spectra of the bands corresponding to the 5D0 f 7FJ (J ) 1-6) were quite broad and they did not show any anisotropy effect; for the sake of brevity they are not reported. The dispersion of LaF3:Eu3+ nanoparticles in the water-glycerol mixture does not show any anisotropy decay at room temperature: the anisotropy is zero from the beginning (Figure 8), while decreasing the temperature to 220 and 208 K a decay was visible. The profile of the decay in Figure 9 is a single exponential and the best fit has been obtained with eq 2, indicating a correlation time of 5.24 ms. The same situation is repeated in Figure 10, the only difference is that the correlation time is longer, i.e., 39.6 ms. Lowering
Steady-State Measurements. The presence of the Eu3+ is confirmed by the emission spectrum reported in Figure 4. The relative intensity of the 5D0 f 7FJ (J ) 1, 2, 3, 4) transitions is consistent with previous observations and has been discussed in the past.12,13 The double exponential fit is a phenomenological approach for the treatment of the decay curve of the 5D0 level. A model that takes into account the radial distribution of the Eu3+ in the core of the particles has been discussed in the past,12,13 and leads to a quite single-exponential decay for core shell particles clothed with ligands having long alkyl chains and that were dispersed in nonprotic organic solvents, where the crystal field environment of the active ions is constant all over the core. In the current case, the reason for the shortening of the lifetime in the first part of the curve should probably be ascribed to the luminescence quenching by the abundantly present OH groups from the water.14 The reason that the quenching by OH groups is still felt is due to the fact that the shell is only about 1 nm thick. In other words, the shell reduces the effect of the quenching with respect to core nanoparticles, because the distance between emitters and quenchers is increased. In fact, the average lifetime calculated from the double exponential fit (Figure 4) for core-shell nanoparticles is 5.5 ms and is indeed much longer than the 3.6 ms obtained for the core nanoparticles under otherwise identical conditions. The single Gaussian fit in Figure 5 of the 5D0 f 7F0 transition is a further confirmation that the crystal field and hence the structural environment around the Eu3+ ions is relatively homogeneous. This is fully consistent with a core-shell structure in which the Eu3+ are in the core and the shell has grown epitaxially on the core. This is quite typical for core-shell nanoparticles, because all the rare earth ions are more or less in the same kind of environment in the core. Previous XRD measurements proved indeed that the structure is highly crystalline and HR-TEM revealed that each nanoparticle is a single crystal.13 The broadness of the bands is typical for crystals, anyway, even if the structure inside the nanoparticle is ordered and the Eu3+ dipoles present a main orientation along one axis, the nanoparticles in the dispersion are randomly oriented, and for example in absorption, would not present any polarization effect. Time-Resolved Emission Spectroscopy (TRES). At room temperature and in water the emission band shows zero anisotropy (Figure 6b), because the rotational diffusion randomizes the sample in a much shorter time than the 2 ms considered by the TRES spectrum. Even increasing the viscosity, dispersing the sample in water-glycerol mixture, again the rotational diffusion is not slow enough and no anisotropy is visible in the spectra in Figure 6a. Lowering the temperature below the glass transition of the water-glycerol mixture, the nanoparticles are immobilized and the anisotropy cannot be lost by rotational depolarization. The polarization spectrum shows anisotropy very close to the maximum 0.4 value for photoselection observed in Figure 7. Since the presence of energy migration is quite likely for a dopant’s concentration as high as 5 atom %, the only reason for this high anisotropy value can be that the energy transfer takes places without loss of polarization between emission dipoles with the same orientation, i.e., in a single-crystal environment. On the other hand, the 5D0 f 7FJ (J ) 1-6) emission bands hardly show any anisotropy, because even if the structure inside the particles is ordered, the broadness of the peak is enough to mix different Stark states in the manifold
4532 J. Phys. Chem. C, Vol. 111, No. 12, 2007
Bovero and van Veggel
Figure 4. Emission spectrum from the 5D0 level of LaF3:Eu core-shell nanoparticles in water. The inset shows the decay curve from the same level; τ1 and τ2 are the values of the lifetimes arising from the two-exponential fit.
5
7
Figure 5. D0 f F0 emission band of LaF3:Eu nanoparticles in water dispersion.
and what we observe is just an average value close to zero of the orientations of the emission dipoles. Time-Resolved Anisotropy Measurements. At room temperature the rotational diffusion is so fast that even the timeresolved decay measurements do not show any polarization effect (Figure 8). In particular, considering a spherical particle with a diameter of 8 nm (as was observer in previous TEM measurements),13 from eq 4 one obtains a correlation time of 58 ns, which is in fact too short to be observed on this time scale. Decreasing the temperature, the viscosity of the medium increases and the motion of the particles is slowed down. The curves in Figures 9 and 10 are well fit by a single-exponential decay as in eq 2, therefore, we can conclude that the size of the particles is very homogeneous. Moreover, it is reasonable to assume that the shape of the particles is close to spherical, otherwise we would have observed multiexponential decay, and hence a different correlation time for each axis of rotation as described in eq 3. The radius of these spheres can be calculated from eq 4, which gives the volume as a function of the viscosity. From the correlation times 5.24 ms at 220 K and 39.6 ms at 208 K, the size of the particles was determined to be 11.0 and
Figure 6. TRES spectrum at 2 ms and anisotropy values of the 5D0 f 7 F0 transition at room temperature. Part a is in a water-glycerol mixture and part b in water. The scale of the emission intensity is reported on the left side, while that for the anisotropy is reported on the right side.
9.9 nm, respectively. The small discrepancy between these two values and the high reproducibility over different measurements suggests the presence of an error of 10% on the estimation of the diameter of the particles. Both sphericity and size of the particles are in accordance with TEM and XRD measurements,13 in which the diameters were found to be in the range of 5-10 nm. The diameters calculated from the correlation times are a little bit bigger than the average size indicated by TEM and XRD, and this is perfectly reasonable, because the anisotropy decay is related to the hydrodynamic radius of the particles. The hydrodynamic diameter consists of the diameter of the whole core-shell particle, which is a rigid rotor, and the thickness of the coating of citrate ligands. Lowering the temperature below the glass transition of the medium, no anisotropy decay was observed. The value remains very close to 0.4 (Figure 10), indicating that no anisotropy was lost by rotational diffusion and hence confirming the observation from Figure 7 that the sample was immobilized in the glass and that the energy transfer does not lead to a loss of polarization.
Eu3+-Doped LaF3 Core-Shell Nanoparticles
Figure 7. TRES spectrum at 2 ms and anisotropy values of the 5D0 f 7 F0 transition at 170 K in a water/glycerol mixture. The scale of the emission intensity is reported on the left side, while that for the anisotropy is reported on the right side.
J. Phys. Chem. C, Vol. 111, No. 12, 2007 4533
Figure 10. The anisotropy decay at 208 K in a water/glycerol mixture.
Figure 11. Time dependence of anisotropy at 170 K.
Figure 8. Time-resolved anisotropy in a water-glycerol mixture at room temperature.
particles in solution, which make the particle seem bigger or smaller, so it can be used only if all the interactions in the solution or dispersion are absent or known. In fact, we have not been able to obtain reliable information regarding the size of these nanoparticles from DLS. Furthermore, DLS is not able to give any information about the inner structure of the particle in which the active species is located. Conclusions
Figure 9. The anisotropy decay at 220 K in a water/glycerol mixture.
Advantages of the Method. The method applied in this work is a straightforward way to obtain information about the size, shape, and structure of the particle in which the emitter is lodged. It confirms information from other techniques, but more importantly, it gives information otherwise not easily obtainable. TEM, for example, retrieves the same information from a bidimensional image and does not give any idea about the axis perpendicular to the surface of the sample. In principle also DLS could provide information about the size and shape, but the size of the particle is very close to the edge of sensitivity, which generally goes from a few nanometres to 6 µm,15 depending on the refractive index of the medium. Moreover, the results are affected by the presence of unexpected interactions between
Core-shell LaF3 nanoparticles show a high anisotropy for the 5D0 f 7F0 emission band of Eu3+, inserted as a dopant in the core with a concentration of 5 atom %. The anisotropy value of 0.4 in a water-glycerol mixture cooled below its glass transition temperature is the same as that predicted from photoselection and let us evince that the core of the particle is a single crystal as the reason why the intraparticle energy transfer does not work as a significant pathway for depolarization of the emission. The anisotropy decay at a temperature close to the glass transition indicated that the size of the particles is very homogeneous, with an average value of 10 nm for the diameter. Acknowledgment. The Natural Science and Engineering Research Council (NSERC) of Canada, the Canada Foundation for Innovation (CFI), and the British Columbia Knowledge Development Fund (BCKDF) of Canada are gratefully acknowledged for financial support. References and Notes (1) Barber, D. B.; Pollock, C. R.; Beecroft, L. L.; Ober, C. K. Opt. Lett. 1997, 22, 1247. (2) Hebbink, G. A.; Stouwdam, J. W.; Reinhoudt, D. N.; van Veggel, F. C. J. M. AdV. Mater. 2002, 14, 1147.
4534 J. Phys. Chem. C, Vol. 111, No. 12, 2007 (3) 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) Justel, T.; Nikol, H.; Ronda, C. Angew. Chem., Int. Ed. 1998, 37, 3085. (5) Dabbousi, B. O.; Bawendi, M. G.; Onitsuka, O.; Rubner, M. F. Appl. Phys. Lett. 1995, 66, 1316. (6) Reifenberger, J. G.; Snyder, G. E.; Baym, G.; Selvin, P. R. J. Phys. Chem. B 2003, 107, 12862. (7) Sharma, J.; Tleugabulova, D.; Czardybon, W.; Brennan, J. D. J. Am. Chem. Soc. 2006, 128, 5496. (8) Lakowicz, J. R. Principles of FLuorescence, 2nd ed.; Kluwer Acedemic: New York, 1999. (9) Bialik, C. N.; Wolf, B.; Rachofsky, E. L.; Ross, J. B. A.; Laws, W. R. Biophys. J. 1998, 75, 2564.
Bovero and van Veggel (10) Williams, M. L.; Landel, R. F.; Ferry, J. D. J. Am. Chem. Soc. 1955, 77, 3701. (11) Sudarsan, V.; Sivakumar, S.; van Veggel, F. C. J. M.; Raudsepp, M. Chem. Mater. 2005, 17, 4736. (12) Stouwdam, J. W.; Hebbink, G. A.; Huskens, J.; van Veggel, F. C. J. M. Chem. Mater. 2003, 15, 4604. (13) Sudarsan, V.; van Veggel, F. C. J. M.; Herring, R. A.; Raudsepp, M. J. Mater. Chem. 2005, 15, 1332. (14) Ning, L.; Lodi, L.; Trioni, M. I.; Tubino, R.; Edvardsson, S.; Brivio, G. P. J. Phys.: Condens. Matter 2007, 19, 1. (15) Berne, B. J.; Pecora, R. Dynamic Light Scattering: with applications to Chemistry, Biology, and Physics; Dover Publications: Mineola, NY, 1976.
