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J. Phys. Chem. C 2008, 112, 7750–7756
Cross-Relaxation and Upconversion Processes in Pr3+ Singly Doped and Pr3+/Yb3+ Codoped Nanocrystalline Gd3Ga5O12: The Sensitizer/Activator Relationship R. Naccache,† F. Vetrone,‡ A. Speghini,§ M. Bettinelli,§ and J. A. Capobianco*,† Department of Chemistry and Biochemistry, Concordia UniVersity, 7141 Sherbrooke Street West, Montréal, QC, H4B 1R6, Canada, Institut National de la Recherche Scientifique-Énergie, Matériaux et Télécommunications, UniVersité du Québec, Varennes, QC J3X 1S2, Canada, and Dipartimento Scientifico e Tecnologico, UniVersità di Verona and INSTM, UdR Verona, Ca’ Vignal, Strada Le Grazie 15, I-37134 Verona, Italy ReceiVed: December 5, 2007; ReVised Manuscript ReceiVed: March 2, 2008
The room temperature luminescence properties of Pr3+-doped gadolinium gallium garnet (GGG:Pr3+, Gd3Ga5O12:Pr3+) nanocrystals (0.1, 1, 5 and 10 mol %) were evaluated. Increasing the Pr3+ concentration in the nanocrystals resulted in a decrease of the 3P0 emission to lower lying states via the [3P0,3H4] f [3H6,1D2] cross-relaxation (CR) process. Similarly, a decrease in the 1D2 emission was observed and was attributed to the [1D2,3H4] f [1G4,3F3,4] cross-relaxation mechanism. The increase in cross-relaxation efficiency on increasing the Pr3+ concentration was attributed to the smaller average interionic distances between the dopant ions. Dominant blue/green emission due to the 3P0 f 3H4 and 3P0 f 3H6 transitions was observed after laser excitation at 457.9 nm for Pr3+/Yb3+ codoped nanocrystalline GGG samples. The blue/green emission decreased as the sensitizer (Yb3+) concentration increased in the GGG samples. The observed near-infrared (NIR) 2F5/2 f 2F 3+ ion, upon 457.9 nm excitation, suggests the presence of an energy transfer 7/2 emission from the Yb 3+ process from the Pr ions to neighboring Yb3+ ions and results in a decrease of room temperature visible emission with increasing ytterbium concentration. Upconversion emission was observed for 0.1 and 1 mol % single doped samples. The nanocrystalline GGG:Pr3+,Yb3+ codoped samples showed a notable increase in upconversion emission intensity relative to the singly doped samples excited upon 980 nm excitation radiation. Upconversion was observed to occur via an ET process for the single and codoped samples. 1. Introduction The field of nanotechnology has experienced significant advancements especially in recent years. Most notably, much of the research is being directed toward applied research domains including the development of efficient phosphor devices, as well as potential usage in biomedical applications.1–6 The constant demand for improvements on existing technology has prompted scientists to investigate the nanometer scale in search for more efficient and performing devices. As a result, nanoscience has been propelled to the forefront of research. Nanocrystals are attractive for a wealth of applications, namely, imaging and display devices, where it has been previously shown that resolution is inversely related to the particle size.7,8 Many display devices rely mainly on micrometer-sized phosphors; consequentially, the potential for higher resolution is greatly limited relative to their nanoparticle counterparts. Furthermore the implementation of nanosized phosphors will surely have significant impact on the display-device size.9–11 The introduction of plasma displays has already ushered the movement toward more efficient and compact devices. The interest in the praseodymium tripositive ion (Pr3+) stems from the multitude of transitions, which allow for detailed studies of nonradiative mechanisms such as multiphonon relaxation as well as radiative processes between the lanthanide * Corresponding author. Tel: +1-514-848-2424, extension 3350. Fax: +1-514-848-2868. E-mail:
[email protected]. † Concordia University. ‡ Université du Québec. § Università di Verona and INSTM, UdR Verona.
ions such as energy transfer or cross-relaxation. The latter are quite prominent in Pr3+ due to resonant energy gaps separating the various levels. Cross-relaxation processes are of interest as they are responsible for the emission quenching phenomena especially as a function of increasing dopant concentration.12–15 Previously published manuscripts have shown that emission quenching and cross-relaxation in praseodymium doped hosts can occur with dopant concentrations as low as 0.01 mol %.12–16 Consequently, concentration studies are essential in trying to elucidate the various radiationless pathways, which may be probable within a given host. The multitude of probable pathways by which cross-relaxation can occur renders praseodymium a challenging ion to investigate. The low phonon energy associated with the gadolinium gallium garnet (GGG) host (maximum phonon energy of the optical phonons is ∼600 cm-1)17 could have a significant role on the optical properties of the dopant ion. The probability of nonradiative processes tends to significantly decrease as the phonon energy decreases since more phonons would be required to bridge the energy gaps separating the various levels. Some previous investigations have been published on the structural, morphological and spectroscopic properties of Pr3+ doped nanocrystalline GGG.18–21 In these investigations, it has been shown that the lanthanide ions enter substitutionally into the GGG structure and that the average crystallite dimensions are about 50 nm, confirmed also by TEM images.19–21 Furthermore, EXAFS (extended X-ray absorption fine structure) measurements on nanocrystalline GGG:Pr3+ have shown the presence of a significant static disorder around the lanthanide ion.18,22 From the behavior of the emission line width and shift
10.1021/jp711494d CCC: $40.75 2008 American Chemical Society Published on Web 04/29/2008
Pr3+ and Pr3+/Yb3+ Doped Nanocrystalline Gd3Ga5O12 of the Pr3+ emission band as a function of the temperature, the electron–phonon coupling (EC) strength of the Pr3+ ion was observed to be notably stronger for the nanocrystalline GGG sample than for the bulk one. This behavior was attributed to the fact that the local surroundings of the Pr3+ ion are softer for the nanocrystalline sample than for the bulk one. The Yb3+ ion has been extensively used as a codopant with other lanthanide ions in many glasses and crystals (including nanocrystals)23–30 due to the fact that it possesses only one excited level (2F5/2) which can be easily excited using a radiation wavelength of 980 nm, for which many commercial diodes are readily available. This allows the possibility of populating excited states of other Ln3+ ions through Yb3+ f Ln3+ ET processes. In a previous paper,31 we reported on the luminescence and cross-relaxation in 1 mol % Pr3+ doped gadolinium gallium garnet bulk and nanocrystals through the [1D2,3H4] f [1G4,3F3,4] mechanism. In order to gain further insight on cross-relaxation mechanisms of Pr3+, a concentration study was carried out extending the investigation to the 0.5–10 mol % range. Detailed room temperature emission and upconversion studies are also presented, and resultant effects brought about by increased crossrelaxation efficiency are discussed. 2. Experimental Cubic Gd3Ga5O12 nanocrystals doped with 0.1, 1, 5 and 10 mol % Pr3+ were prepared using a solution combustion (propellant) synthesis procedure, as previously described.19,22,31 Briefly, stoichiometric quantities of the metal nitrates and a suitable amount of carbohydrazide were dissolved in water. The carbohydrazide was added to the metal nitrate in a molar ratio of 2.5:1. The solution was heated with a Bunsen flame, and after evaporation of the solvent the combustion process took place. The obtained fluffy powders were fired for 1 h at 500 °C to remove any residual nitrate ions and carbohydrazide. All nanocrystalline samples were kept in air without any further precaution. X-ray diffraction patterns in combination with TEM micrographs showed that this synthesis yields cubic GGG nanocrystals.19 In particular, transmission electron microscopy (TEM) images show that the GGG nanoparticles have an irregular shape and size distribution ranging from 30 to 50 nm. Visible and NIR room temperature emission spectra (457.9 nm) were obtained using a Coherent Sabre Innova, 20 W CW argon ion laser. The laser line was tuned prior to spectra collection to ensure proper functionality and maximum power output. Visible emission spectra were recorded using a JarrellAsh 1 m Czerny-Turner double monochromator. A Hamamatsu R943-02 photomultiplier with a flat spectral response between 200–850 nm was used to monitor the spectral signal. The photomultiplier signal was processed through a Stanford Research Systems SR440 preamplifier before being sent to the Stanford Research Systems SR400 gated photon counter equipped with a computer running the Stanford SR465 data collection software. The NIR emission spectra were recorded using a Jarrell-Ash ¾ meter monochromator in second order. The fluorescence signal was monitored using a North Coast EO-817P liquid nitrogen cooled germanium detector. A Stanford Research Systems SR510 lock-in amplifier equipped with a computer running the lock-in data acquisition software was used to analyze the detector signal. A Coherent Sabre Innova argon ion laser (20 W) was used to pump a Spectra Physics 375 tunable dye laser. The Rhodamine 6G dye was used to obtain the fluorescence spectra
J. Phys. Chem. C, Vol. 112, No. 20, 2008 7751
Figure 1. Room temperature visible emission spectra (λexc ) 457.9 nm) of GGG:Pr3+ nanocrystals at (a) 0.1, (b) 1, (c) 5 and (d) 10 mol % Pr3+. Note: Only 3P0 f 3H4 and 1D2 f 3H4 transitions are labeled for clarity. Inset view: Decrease in 3P0 f 3H4 emission peak intensity as a function of dopant concentration.
at the desired wavelength of 606.9 nm. The argon ion laser was also used to pump a Ti:Al2O3 laser (titanium sapphire laser, 980 nm). The dye laser and the Ti-sapphire laser were both used in the study of upconversion luminescence. Emission decay curves were measured upon excitation at 580 nm or at 290 nm with the first or the second harmonic radiation of a Quanta System D-100 dye laser (using Rodamine 6G as the dye) pumped with the second harmonic (532 nm) of a pulsed Quanta System SYL201 Nd:YAG laser. The emission radiation was collected with an optical fiber and dispersed with a Jobin Yvon HR450 half-meter monochromator equipped with a 150 lines/mm grating. A water cooled Hamamatsu GaAs photomultiplier and a Le Croy Waverunner LT342 digital oscilloscope were used to measure the emission decay curves. Decay curves of the upconversion emission were measured by modulating the continuous wave radiation at a wavelength of 606.9 and 980 nm using a Stanford Research Systems SR540 optical chopper and obtained using the aforementioned data acquisition system. 3. Results and Discussion Effect of Dopant Concentration on the Visible Room Temperature Emission of Singly Doped Nanocrystals. The optical properties of the nanocrystals were investigated as a function of the dopant concentration in the system. Following direct excitation using a wavelength of 457.9 nm (into the 1I6 energy level), blue/green, red and NIR emission was observed for the Pr3+-doped GGG nanocrystals (Figure 1). All the samples largely exhibit dominant blue/green emission from the 3P0 and 3P levels that are in thermal equilibrium so both give rise to 1 emission at RT. It is worth mentioning that the emission spectra were carefully measured under the same experimental conditions and therefore the Pr3+ ion emission intensities can be compared. From the intensities of the emission bands it can be noted that the 3P0 f 3H4 emission intensity decreases with increasing praseodymium concentration (see Figure 1, inset). It is worth remarking that the multiphonon relaxation processes (MPR) for the 3P0 and 1D2 levels are not very efficient. In fact, the energy gap between the 3P0 level and the nearest low lying one (1D2) is about 3500 cm-1, and therefore, six GGG phonons (the GGG phonon cutoff is about 600 cm-1) would be required to bridge
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Figure 2. (i) The [3P0, 3H4] f [3H6, 1D2] cross-relaxation mechanism populating the 1D2 energy level; (ii) concentration quenching of the 1 D2 f 3H4 emission via the [1D2, 3H4] f [1G4, 3F3,4] cross-relaxation mechanism.
the gap. Moreover, the gap between the 1D2 level and the 1G4 level is larger, about 7000 cm-1, and thus more than 10 GGG phonons are needed to bridge the energy gap. Therefore, both the multiphonon relaxation rates from the 3P0 and 1D2 levels are very low. The 3P0 emission quenching is most probably due to a cross-relaxation (CR) mechanism between Pr3+ ion pairs involving the 1D2 or the 1G4 levels. A possible CR mechanism could be [3P0, 3H4] f [3H6, 1D2], sketched in Figure 2. This mechanism was already proposed for similar systems such as Pr3+ doped YAG.16 This mechanism implies that an increase in the 1D2 emission on increasing the Pr3+ concentration would be expected; however, as this level is populated by the abovementioned mechanism, it could be easily depopulated via the [1D2, 3H4] f [1G4, 3F3,4] pathway. What is in fact observed is a decrease in intensity of the 1D2 emission with respect to the 3P one (see Figure 1). This decrease is attributed to efficient 0 CR mechanisms depopulating the 1D2 level which are obviously enhanced by increasing the dopant ion concentration. Work by Chen et al.2 showed an increase in emission intensity for the 3P and 1D states with increasing Pr3+ content but only up to 0 2 0.3 mol %, after which a decrease in the emission intensity was observed for 1D2 state and 3 mol % for the 3P0 state. Chen et al. also reported that almost total fluorescence quenching was observed for the 1D2 state at 10 mol %. The remarkable decrease in emission quenching noted for the 1D2 state suggests that concentration quenching is much more significant for this level relative to the 3P0 one and likely depends on the distance separating neighboring Pr3+ ions which surpasses a critical separation distance (∼10 Å).14 The average ion distance d in crystalline materials can be estimated using the equation32
d)
3 ( 4πN )
1⁄3
(1)
where N is the ion density. From eq 1 and the crystallographic data reported by Krsmanovic et al.20 for Pr3+ doped GGG samples prepared using the same propellant synthesis, the average Pr-Pr distance results to be 27, 12, 7.2 and 5.7 Å for the 0.1, 1, 5 and 10 mol % Pr3+ doped GGG nanocrystalline samples, respectively. From a comparison of these distances with the above-mentioned critical separation distance (∼10 Å) it appears therefore plausible that the CR processes are not very relevant for the 0.1 mol % doped sample, while they are operative for the more concentrated ones.
Figure 3. Emission decay curve of the 1D2 f 3H4 transition for the 5 mol % doped GGG:Pr3+ sample (λexc ) 580 nm, λem ) 606.9 nm).
TABLE 1: Effective Decay Times (τeff in µs) for the 3P0 and 1D Pr3+ Energy Levels for the Pr3+-Doped GGG 2 Nanocrystalline Samples sample level 3
P0 D2 3 P0 1
λexc (nm)
0.1 mol %
1 mol %
5 mol %
10 mol %
457.9 457.9 606.9
32 280 108
18 110 50
7 11 30
2 3 27
Efficient concentration quenching of the 1D2 f 3H4 emission may therefore occur via the [1D2, 3H4] f [1G4, 3F3,4] mechanism31 (see Figure 2). This cross-relaxation mechanism is plausible due to the match in energy gap separating the involved states.28 Any energy mismatch can be likely bridged using one intrinsic phonon energy of the system. Consequently, this process becomes highly probable in the most concentrated nanocrystals. Emission Decay Curves for Singly Doped Nanocrystals. The room temperature emission decay curves for the Pr3+-doped GGG nanocrystalline samples obtained upon excitation at 290 or 580 nm show a clear nonexponential behavior, even for the most diluted sample (0.1 mol %) . As an example, the emission decay curves for the 1 mol % Pr3+-doped nanocrystals are shown in Figure 3. From the emission decay curves, we calculated the effective emission decay times, τeff, using the eq 2:33
τeff )
∫ tI(t) dt ∫ I(t) dt
(2)
where I(t) represents the luminescence intensity at time t corrected for the background and the integrals are evaluated in a range 0 < t < tmax where tmax . τeff. The τeff values are reported in Table 1 and Figure 4. The nonexponential behavior of the decay curves suggests the presence of a significant CR process between the Pr3+ ions and/or the presence of more than one site in which the dopant ions could be accommodated in the garnet crystal lattice.18,34,35 For instance, Lupei et al.34 found that for Tm3+ doped GGG single crystals the rare earth ions are accommodated in normal dodecahedral c-sites and also in perturbed c-sites by a close nonstoichiometric defect. Nonetheless, Courrol et al.35 showed the presence of three nonequivalent crystalline sites for the lanthanide ions in a Eu3+ doped GGG single crystal, with site symmetries which are not typical of the regular garnet sites.
Pr3+ and Pr3+/Yb3+ Doped Nanocrystalline Gd3Ga5O12
Figure 4. Change of the 3P0 and 1D2 effective decay times as a function of Pr3+ and Yb3+ concentration in singly and codoped GGG nanocrystals, respectively (λexc ) 457.9 nm).
On the other hand, Daldosso et al.18 showed that for Eu3+ doped nanocrystalline GGG samples prepared by propellant synthesis the rare earth ions are accommodated in several nonregular garnet sites, giving rise to distinct bands in the visible emission spectrum. For nanocrystalline GGG powders prepared by combustion synthesis, it has been shown that two cubic garnet phases with slightly different lattice parameters are present. These results indicate that the lanthanide ions in the present materials can experience very different local environments and therefore they can give rise to different emission features.19 All these results highlight that the lanthanide ions in the present materials can experience very different local environments and therefore can give rise to different emission features. However, in particular for the 0.1 mol % doped sample for which the CR processes are most probably not important, the nonexponential behavior of the decay curve could also be due to the presence of a notable amount of disorder affecting the sites in which the lanthanide ions are accommodated. In fact, a notable amount of disorder affecting the Pr3+ environment was also evidenced from an EXAFS analysis of the nanocrystalline materials under investigation.22 Due to the large surface to volume ratio typical of nanostructured systems, a significant percentage of lanthanide ions lie on the surface of the nanoparticle in distorted sites. The effective lifetimes of the 3P0 and 1D2 level of Pr3+ are shown as a function of the lanthanide concentration in Figure 4. The decay times of the 3P0 and 1D2 levels of Pr3+ ion in a 1 mol % Pr3+ doped GGG single crystal resulted to be 25 and 220 µs, respectively.36 The decay times of the 3P0 and 1D2 levels are significantly longer than those found for the 1 mol % Pr3+ doped nanocrystalline sample under investigation (18 and 110 µs for the 3P0 and 1D2 levels, respectively, see Figure 4). The shortening of the effective decay times for the nanocrystalline samples is attributed to the higher degree of site distortion of the Pr3+ ions lying in surface sites, as mentioned above. The effective lifetimes of the 3P0 level were found to vary from 32 to 2 µs for the 0.1 mol % to 10 mol % dopant concentration. This is in agreement with the proposed cross-relaxation mechanism. In contrast, the 1D2f 3H4 transition lifetime varied from 280 µs (0.1 mol % Pr3+) to 3.0 µs (10 mol %). The strong decrease of the decay time on increasing the Pr3+ concentration points to a strong quenching of the 1D2 emission, as also evidenced from the emission spectra (see Figure 1). Emission from the 1D2 level is nonetheless detectable, albeit weak, due to a population of this state through the [3P0, 3H4] f
J. Phys. Chem. C, Vol. 112, No. 20, 2008 7753 [3H6, 1D2] mechanism as the quenching is not complete (see Figure 2). It is noteworthy to mention that, even at
