New Insights into Size Effects in Luminescent Oxide Nanocrystals

Oct 7, 2009 - Mouna Abdesselem , Markus Schoeffel , Isabelle Maurin , Rivo ... Sébastien Maron , Valérie Buissette , Thierry Le Mercier , Thierry Ga...
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J. Phys. Chem. C 2009, 113, 18699–18706

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New Insights into Size Effects in Luminescent Oxide Nanocrystals G. Mialon,† S. Tu¨rkcan,‡ A. Alexandrou,‡ T. Gacoin,*,† and J.-P. Boilot† Groupe de Chimie du Solide, Laboratoire de Physique de la Matie`re Condense´e, Ecole Polytechnique, CNRS, 91128 Palaiseau, France, and Laboratoire d’Optique et Biosciences, Ecole Polytechnique, CNRS, INSERM U696, 91128 Palaiseau, France ReceiVed: July 28, 2009; ReVised Manuscript ReceiVed: September 2, 2009

We here investigate the emission properties of rare-earth-doped oxide nanoparticles with the aim to understand the commonly observed altered properties of nanoparticles as compared to the bulk counterparts. This is usually attributed to the detrimental effect of surface states that quench the excited states involved in the emission process. We study the influence of crystalline defects that are present due to the low temperature of the synthesis of 30 nm sized YVO4/Eu nanoparticles. Annealing treatments up to 1000 °C in a porous silica matrix allow the recovery of perfectly crystalline particles as colloidal suspensions and compare their properties to those of the pristine particles obtained by conventional colloid chemistry. Emission properties of pristine and annealed particles are compared with those of the bulk material. A simple model of the emission process allows an accurate fit of the luminescence decay and of the dependence of the quantum yield on europium content. Our results show that pristine particles exhibit altered emission properties mainly due to quenching from defects, among which are surface OH groups, and altered energy transfers within the particle. Annealed particles exhibit properties that are almost the same as those of the bulk material, except that the emission yield for the optimum Eu content is limited to 40 instead of 70% for the bulk material. We show that the difference may be simply explained by the difference of the radiative lifetime that results from the lower effective refractive index in the case of the particles. This effect then seems to be the ultimate limitation for the emission properties of perfectly well-crystallized nanoparticles as compared to those of the bulk material. This work provides an example of a general strategy toward the investigation of the physical properties of nanocrystals which may be altered by crystalline defects. Introduction Starting from the pioneering work on cadmium chalcogenides, luminescent nanocrystals have attracted a great deal of interest for their potential wide range of applications. Investigations have now been extended to other semiconductors such as InP1 or Si2 or other luminescent systems such as transition-metal-doped chalcogenides,3 N/V-doped diamond,4 and rare-earth-doped oxides5-9 or halides.10,11 All of these studies aim first to understand the influence of size and surface effects on the emission properties in order to further optimize emission properties. Optimized systems are further studied for applications such as labels for dynamic tracking of biological species12 or transparent or nanostructured light-emitting devices.9,13 We focus here our attention to the case of rare-earth-doped oxide materials that are well-known phosphors commonly used in light-emitting devices (lightning, displays, amplification media, lasers, etc.).14 Bulk materials are ceramics that are usually obtained through thermal treatments at temperatures that are commonly higher than 1000 °C in order to obtain the appropriate crystalline phase. Concerning nanoparticles, many of the reported elaborations do still involve high-temperature treatments of precusors, leading to aggregated powders in which the nanometer grain size is more or less controlled by lowering temperatures of calcination and/or using porogen agents (surfactant, polymers) to limit sintering and crystal growth.15 Much * To whom correspondence should be addressed. E-mail: thierry.gacoin@ polytechnique.fr. Telephone: 00.33.1.69.33.46.56. Fax: 00.33.1.69.33.47.99. † Laboratoire de Physique de la Matie`re Condense´e. ‡ Laboratoire d’Optique et Biosciences.

less work has been devoted to the direct synthesis of welldispersed crystalline particles through colloid chemistry routes. This indeed requires the possible precipitation of the desired compound in its crystalline phase through reactions between precursors in solution at moderate temperature (i.e., less than typically 300 °C) as determined by the solvent properties. This explains why only a limited number of compounds have been studied up to now, such as YVO4,5,6,16 LaPO4,7,17 Y2O3,12 and YAG/Ce.9 The case of YVO4 nanoparticles has been investigated in our group for several years.5,7,16,18 This specific compound was chosen because of (i) the excellent emission properties of the bulk material, (ii) the ability to obtain the crystallized compound through precipitation reactions in water, and (iii) the possible excitation of the emission not too far in the UV (around 280 nm) with a high absorption cross section. Two aqueous syntheses of such particles were developed, leading to particles with sizes between 7 and 50 nm, depending on the use of surface-complexing agents during the synthesis.5,16 These particles have already been shown to be interesting for original applications in the field of transparent and luminescent thin films13 and biological probes.19-22 A large part of our work was devoted to understanding the influence of the nanometer size of the particles on their emission properties.18 In accordance with most results reported by other authors on similar compounds, it was found that nanoparticles with a size higher than a few nanometers usually exhibit emission properties that are spectroscopically not very different from those of the bulk materials, except for ions located on the surface.23,24 Thus, contrary to quantum dots, these systems do

10.1021/jp907176x CCC: $40.75  2009 American Chemical Society Published on Web 10/07/2009

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not exhibit specific size effects, and this is well understood considering that the excited states are mainly localized on the sensitizer and activator species. A major difference with bulk materials concerns the lower emission yield and the higher optimum concentration of sensitizer and activator ions. For instance, YVO4/Eu nanoparticles exhibit a maximum emission yield of 15% for an optimum dopant concentration of 30%, while the bulk material has a 70% yield for a 5% Eu doping. It is usually accepted that the altered emission properties of the nanoparticles arise from surface quenching. Hydroxyl groups and other defects are indeed well-known to quench excited states of sensitizers or emit rare earth ions, thus limiting the energytransfer processes. This explains both the lower emission yield and the higher dopant optimum concentration.18 As for quantum dots, the successful application of the core-shell strategy (i.e., passivation of the particles through encapsulation with another neutral compound) supports this explanation.25 Considering that the surface state of the particles is the main limitation of their emission properties, it may be inferred in a rough approximation that the influence of the surface should vanish when increasing the size of the particles so that particles with a sufficient size should have emission properties not far from the bulk material. An important issue is thus to have an estimation of this size, that is, to know for what size nanoparticles are expected to have bulk-like properties. Concerning YVO4 nanoparticles, a quantum yield of 15% is measured for particle sizes of 7 nm.18 Surprisingly, we found almost the same emission properties in the case of particles of 30-50 nm obtained through colloidal precipitation.5 This contrasts with the expected increase of emission yield for increasing particle sizes due to a lower impact of the surface. A first explanation could be that surface defects may act as quenchers even for excited states created in the volume of the particles due to the migration of the excitation within the nanocrystal. However, another explanation could simply be that quenching occurs via defects located in the volume of the particles as a result of the low temperature of processing for the particle synthesis.15 Further investigation of size and surface effects thus requires performing studies on particles with a high crystallinity. In a recent paper,26 we developed an original process that allows the thermal annealing of particles at temperatures up to 1000 °C while preserving their size and dispersion state. This process involves the dispersion of the particles in a porous silica matrix, annealing of the composite material, and further dissolution of the silica to recover the annealed particles as a colloidal dispersion. The final particles are perfectly monocrystalline, and one can expect that defects have been totally removed through the annealing treatment. This paper presents the results concerning our investigation of the emission properties of these particles as a means to provide accurate information on “intrinsic” size effects in rare-earthdoped oxide nanoparticles, that is, eliminating the influence of defects resulting from the low temperature of synthesis used for the elaboration of dispersed particles. Experimental Section Colloidal Synthesis of Y1-xEuxVO4 Nanoparticles. An aqueous solution of yttrium and lanthanide (Ln ) Eu, Dy, or Yb/Er) nitrate with the desired molar ratio of lanthanide, ([Ln] ) 0.1 M) is added dropwise into a freshly prepared orthovanadate aqueous solution ([V] ) 0.1 M, pH ) 12.5-13) of the same volume under vigorous stirring. During the nitrate addition, a milky precipitate appears, corresponding to the formation of the solid-phase YVO4/Ln. A few drops of hydroxide sodium (1

Mialon et al. M) are possibly added to maintain the pH above 9. The solution is then left under stirring for 30 min and purified by dialysis against pure water until its conductivity lies below 100 µS · cm-1. The YVO4 particles are then stabilized by a polymer, poly(acrylic acid) PAA,26 in the molar ratio of VO43-/PAA 1:0.05 and sonified for 5 min by a Branson sonificator at 450W. Synthesis of the Reference Bulk Materials. To synthesize the bulk material, the previous solution of particles without PAA was dried in an oven and annealed at 1000 °C for 10 h. Micronic grain size was check by electron microscopy imaging, and coherence lengths of >100-500 nm were checked using X-ray diffraction and comparing the bandwidth at around 2θ ∼ 25° with the one of a micrometer-sized commercial silicon powder. Dispersion of the Particles within a Porous Silica Matrix and Annealing Treatments. A polymeric silica sol was prepared under acidic conditions by mixing TEOS (Si(OC2H5)4), water (pH 1.25), and ethanol in the 1:5:3.8 molar ratio, and aging the mixture 1 h at 60 °C. The Pluronic PE6800 copolymer (EO73PO28EO73, Mw ∼ 8080 g · mol-1, kindly provided by BASF Europe), used as a structure-directing agent, was dissolved in ethanol at 40.4 g · L-1. The final solution was obtained by mixing the colloidal solution, the silica sol, and the PE6800 solution according to the molar ratios V/Si/PE6800 ) 1:5:0.05, and it was then dried in an oven at 90 °C for over 6 h. The resulting powder was annealed under air atmosphere at 1000 °C for 10 min after a calcination step at 500 °C for 1 h. Recovery of the Annealed Particles As an Aqueous Dispersion in Water. The silica powder containing the oxide particles was dissolved by hydrofluoric acid 2% in excess for 3 h in the molar ratio Si/HF ) 1:9. Completion of the silica removal was checked using Si elemental analysis. The hydrofluoric acid and the dissolved silica were then removed by two centrifugations at 14000 g. The precipitate was diluted in pure water, and some drops of hydroxide sodium were added to fix the pH at 9-10. The final dispersion was further stabilized by the addition of PAA (V/PAA ) 1:0.05) and by sonification in a cold bath for 5 min. Final particles were well-dispersed and with nearly the same average dimension as before annealing.26 Light Emission Characterizations. Emission spectra were recorded at room temperature with a Jobin-Yvon Triax 320 monochromator coupled with a CCD camera after excitation of the samples with a 450 W Xe lamp coupled with a JobinYvon HD10 monochromator. The signal was collected near the sample with an optical fiber. Luminescence yields were determined by comparing their integrated emission intensity (Hitachi F-4500 spectrofluometer, 150 W Xe lamp, resolution 1 nm) with the emission from a Rhodamine 6G solution having the same optical density (Cary 50 Scan spectrometer (Varian), OD < 0.3) and excited at the same wavelength (280 nm), for which diffusion effects are negligible. Fluorescence decays were recorded at room temperature by exciting the 7F0,1-5D2 transition at 465.8 nm with an argon laser line (Coherent Innova 306). The emission of the colloidal solution was detected with a photomultiplier tube (Hamamatsu R636-10) at an angle of 90° after filtering with a band-pass filter (Chroma D617/8M). The excitation beam was modulated with a mechanical chopper (Princeton Applied Research Model 197), and the data were collected using a numerical oscilloscope (Tektronix TDS 3032). Results and Discussion Structural Characterization. Details on the structural evolution of the particles as a function of the annealing temperature

Size Effects in Luminescent Oxide Nanocrystals

Figure 1. TEM image of a Y0.9Eu0.1VO4 nanoparticle before (left) and after (right) annealing at 1000 °C.

and further characterization of the final dispersion may be found in our previous publication.26 We focus here only on the difference between the pristine particles and those annealed at 1000 °C, mainly in terms of structural disorder, defects, and morphology. Figure 1 displays the transmission electron microscopy (TEM) image of representative particles before (a) and after (b) thermal annealing and after dispersion of the particles through silica dissolution. The initial particles exhibit an ellipsoidal shape, and the diffraction contrast within the particle appears granular and inhomogeneous. Annealed particles are clearly faceted, and the diffraction contrast is completely homogeneous all over the particle. The inhomogeneous contrast in the pristine particles clearly suggests the presence of grain boundaries within the volume of the particles, leading to diffraction contrast in the TEM image. N2 adsorption isotherms and BET analysis27 were performed on powders of nanoparticles to estimate the degree of sintering of the primary particles. The specific surface area was found to be about 206 and 11 m2/g for the pristine and annealed powders of particles, respectively. A clear effect of the annealing process is thus the reduction of the specific surface area by a factor of nearly 20. Interestingly, 206 m2/g corresponds to the expected surface area of spherical particles of about 7 nm in diameter. The morphology of the particles may then be explained by a mechanism of particle formation during the precipitation process that involves a first step of nucleation leading to the formation of small germs which further agglomerate to form the particle.28 Then, a partial sintering of the particle may occur as a result of the dissolution/precipitation processes that occur during aging of the particles just after their formation. This mechanism explains the observed contrast in the TEM image (Figure 1, left). It should also be noted that despite the poor crystallinity of the particles, lattice fringes are visible all along the pristine particle long axis in high-resolution TEM images of the initial particles. This suggests that the aggregation of the primary particles occurred via oriented aggregation, similarly to what was observed in other systems.29 This explains why, although the particles appear as strongly heterogeneous, the coherence length as measured from diffraction lines for pristine particles (17 nm) is smaller than their size as determined by TEM (33 nm), but not as much as could be expected. Concerning the annealed particles, lattice planes are clearly visible all over the particle volume. Annealed particles thus appear as perfect monocrystals, as confirmed by the coherence length (33 nm) that is almost equal to the TEM-measured particle size (35 nm). Furthermore, the initial ellipsoidal shape of the particles is transformed into faceted structures. This confirms the completeness of the annealing process and the total reorganization of the nanocrystal structure and morphology toward its most stable form.

J. Phys. Chem. C, Vol. 113, No. 43, 2009 18701 Emission Properties: Spectrocopy. The absorption and excitation spectra of the particles in the UV region are not significantly affected by the thermal treatment (see Supporting Information). The emission properties show several differences. Figure 2 displays the emission spectra of Y0.98Eu0.02VO4 nanoparticle powders before and after annealing, together with the spectrum of the bulk material as a reference for 280 nm UV excitation. The spectra consist of sharp lines ranging from 550 to 720 nm, which are associated with the transitions from the excited 5 D0 level to 7FJ()1-4) levels of Eu3+ activators, with a major emission corresponding to the 5D0-7F2 transition at about 621 nm. Although transitions within f electrons of rare earth ions are not very sensitive to the crystal field, their intensity depends on the symmetry of the local environment of the Eu3+ ions as described in terms of the Judd-Ofelt theory.30 In single YVO4 crystals, the Eu3+ site has D2d symmetry, and thus, the electric dipole transitions (|∆L| e 6 and |∆J| ) 2, 4, or 6) are allowed with a higher intensity as compared to that of magnetic dipole transitions (|∆J| ) 0 or 1 but not J ) 0 f J′ ) 0), which are insensitive to the surrounding symmetry. Variations of the symmetry around Eu3+ may originate from lattice distortions caused by defects or a close proximity with the surface. They can be studied by measuring the relative integrated intensity ratio of 5D0-7F2 to 5D0-7F1 transitions, known as the asymmetric ratio.31,32 After fitting the emission spectra by a sum of pseudo-Voigt functions, this ratio is found to be 21.8 ( 1.2 and 13.4 ( 3.1 for the pristine and annealed particles, respectively. Those values have to be compared to the asymmetric ratio for the bulk material, 14.9 ( 1.1. This provides a clear indication that lattice distortions exist in the pristine particles and have been almost completely removed through the annealing treatment. Further evidence for distortions can also be found by considering the inhomogeneous broadening of the 5D0-7F2 emission lines.33,34 Figure 2c displays the bandwidths of the 5D0 (A1)-7F2 (E) emission peaking at 620.6 nm (σ621) and the 5D0 (A1)-7F2 (B2) emission peaking at 616.6 nm (σ617) in the case of pristine and annealed particles with different Eu content.35 After annealing, these two bandwidths are about the same (50 cm-1), close to the bulk value (44 cm-1), and display no change with the Eu content. In the case of crude particles, both bandwidths are much higher (>60 cm-1) and increase with the europium content up to 20%. From this evolution, it must be concluded that the lattice distortions in pristine particles are partly related to the europium ions. Partial clustering of the ions in the crude particles can be discarded. Indeed, the distribution of the europium ions within the vanadate lattice is homogeneous as checked previously by Veggard’s law.5 We then conclude that the incorporation of the europium ions during the coprecipitation process induces some defects in the volume of the host YVO4 matrix which may result from a difference of hydroxylation state between the Y and Eu ions before the formation of the vanadate lattice. Finally, further information can be obtained by considering the 5D0-7F0 transition at 580.9 nm. This contribution is observed in pristine particles but almost completely vanishes after annealing (Figure 2b). Since this transition is strongly forbidden in the D2d symmetry of the Eu host lattice site in the bulk material, this represents further evidence that part of the Eu ions in the pristine particles do occupy a site, different from the bulk, for which the transition is partially allowed. In accordance with results published by other authors, especially the work of Haase at al,23 the 5D0-7F0 transition observed in the pristine particles

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Figure 2. (a) Luminescence spectra of Y0.98Eu0.02VO4 nanoparticles at λex ) 280 nm for pristine particles, annealed particles, and the bulk reference. (b) Focus in the vicinity of the 5D0-7F0 transition with a spectral resolution of 0.5 nm. (c) Evolution of the bandwidths (σ) of the two main emission peaks at 620.6 and 616.6 nm as a function of the europium concentration.

should be attributed to surface Eu ions. This is consistent with the higher surface area of these particles as compared with the annealed ones. We nevertheless noticed that after transfer of the particles into D2O, which has been shown to limit the quenching of the Eu excited state by surface hydroxyl groups,5 we do not observe any increase of the 5D0-7F0 contribution as could be expected considering that this transition arises from Eu ions at the surface. We must then conclude that this contribution may also originate from Eu ions that are not located at the surface but are inside of the particles close to crystalline defects. Emission Properties: Efficiency. The (internal) luminescence yield F is defined as the ratio of the emitted photon number to the absorbed photon number. Following our previous work,5,18 we investigated the evolution of the quantum yield and lifetime of the 5D0-7F2 emission as a function of the europium content. Results are shown in Figure 3 in the case of pristine particles in water, pristine particles in deuterated water, annealed particles, and annealed particles in deuterated water for excitation at 280 nm. The luminescence yield of pristine particles presents a weakly marked maximum of 16% for 20% europium in water and is greatly increased (up to 40%) in deuterated water. On the contrary, and similarly to the bulk material, the luminescence yield of annealed particles shows a maximum at a doping level of 5% europium, quickly decreases at higher europium concentrations, and is not modified when the annealed colloids are transferred into deuterated water. As reported by many groups, the emission mechanism of YVO4/Eu after excitation in the UV involves an energy transfer between vanadate groups and europium ions.36 This mechanism can be decomposed in four steps, absorption of excitation energy by vanadate groups (host sensitizer) with creation of a Frenkel exciton, migration of this exciton within the VO43- sublattice, energy transfer to the europium ions (activator), and emission

Figure 3. Quantum yield versus europium content for pristine particles in H2O (filled circles), pristine particles in D2O (filled squares), annealed particles in H2O (filled triangles), annealed particles in D2O (empty triangles), and the bulk reference (empty squares). Symbols are the experimental data, and solid lines represent fits according to eq 5 (see text).

from the 5D0 level. Figure 6 (inspired by Powell37,38 and Riwotzki39) shows the proposed model for explaining the hostsensitized energy transfer in this system. From the model of Figure 5, F ) ka,r · Na,1/ωNs,0. It can easily be shown that under steady-state conditions37

F)

ka,r

ka,r β · Na,0 + ka,nr ks,nr + β · Na,0

(1)

The nonradiative recombination rate of Eu3+ can be decomposed into two terms corresponding to quenching by defects

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(kD) and to cross relaxation through multipolar interactions between Eu3+ ions (kCR), ka,nr ) kD + kCRxEus/3,40,41 where xEu is the europium content. As discussed by Poluektov and Gava,41 quadrupole-quadrupole interactions dominate in the case of Eu3+ ions, so that the multiplicity s is taken to be equal to 10. Since Na,0 is proportional to the number of europium ions, β · Na,0 can be written as β* · xEu, so that eq 1 is obtained as a function of the europium content (xEu)

F)

ka,r ka,r + kD + kCRxEu

β*xEu s,nr + β*xEu

10/3 k

(2)

Concerning the lifetime as measured by excitation at 466 nm (direct europium excitation), cross relaxation processes do not allow the simple application of a monoexponential decay fit. Inokuti and Hirayama42 provided a simple model in which the decay curves may be described by the following expression

I(t) ) I0 exp[-k0 · t - Ct3/s]

(3)

Here, k0 is the decay rate in the absence of cross relaxation processes (so that k0 ) ka,r + kD) and s is the multiplicity (equal to 10 in the case of Eu3+). C is a function of xEu and kCR and can be expressed as C ) (u · kCR)3/sxEu, where u is a constant (see Supporting Information). The decay curves may thus be described by

I(t) ) I0 exp[-(ka,r + kD) · t - xEe(u · kCR · t)3/10]

(4)

A preliminary attempt to fit our experimental data of quantum yield versus europium content and decay curves for different europium contents clearly showed that the kD parameter corresponding to quenching from defects exhibits a linear variation with the europium content. This may be expected considering that for a given amount of defects, the quenching rate is expected to depend directly on the Eu content. We thus found that a more adequate fit was obtained by decomposing kD ) kD + kD* · xEu. In this case, quantum yield and radiative decay curves are expected to follow the following equations

F)

ka,r ka,r + kD + kD* · xEu +

xEu 10/3 (k kCRxEu s,nr /β*) +

xEu

(5) I(t)/I0 ) exp[-(ka,r + kD + kD* · xEu) · t - xEu(u · kCR · t)3/10] (6) Here, ka,r represents the radiative emission rate of the Eu ion. This value may be inferred from the Judd-Ofelt Theory through the relation ka,r(5D0) ) A0-1 · F, where F can be calculated from the emission spectra of the samples (see Supporting Information). F values were found to be 14.8 ( 1.8, 12.02 ( 1.6, and 19 ( 1.3 for pristine, annealed, and bulk material, respectively. These values were found not to depend strongly on the Eu content. Considering that 5D0-7F1 is a magnetic dipole transition, A0-1 is expected to be almost independent of distortions and thus to depend only on the cubic power of the refractive index.43 The ka,r value for the bulk material is deduced from the reported lifetime of the 5D0 emission and the quantum yield, leading to ka,r ) 1138 s-1 and A0-1 ) 59.945 s-1 for a refractive index of 2 (bulk YVO444). Assuming a refractive index of 1.33 for the particles dispersed in water (as expected considering a volumic fraction less of than 0.1%), this leads to ka,r values of 260 and 213 s-1 for crude and annealed particles, respectively. We may conclude from these results that distortions in the crude particles induce an increase of the radiative lifetime of the 5D0 level, but this variation is of less importance compared to the effect of the effective refractive index. From the previous analytical expressions F ) f(xEu) and I(t), the values for the kD, kD*, kCR, and ks,nr/β* parameters were determined in order to obtain the best fit with the experimental data. Results are presented in Table 1, and the corresponding fitted curves are presented together with the experimental data in Figures 3 and 4, respectively. All experimental curves may be fitted quite well using eqs 5 and 6 derived from the simple model of Figure 5. It is also remarkable that we found similar values for the parameters of the model from the fit of the quantum yield versus europium content data and from the fits of the luminescence decays for samples with very different Eu contents (2 and 40%). On the basis of the evolution of these parameters for our different samples, we may then discuss the relationship between the structure of the particles and their emission properties. We first consider the changes induced by the transfer of the pristine particles into D2O. In this case, the main parameter

TABLE 1: Parameters Obtained by Fitting the Experimental Data of Luminescence Decay (2 and 40% Europium Content) and Quantum Yield versus Europium Content with Equations 5 and 6, Respectivelya Lifetime -1

ka,r (ms ) kD (ms-1) kD* (ms-1) ukCR (ms-1)

ka,r (ms-1) kD (ms-1) kD* (ms-1) kCR (ms-1) ks,nr/β* a

2% 40% 2% 40% 2% 40%

pristine H2O

pristine D2O

annealed H2O

annealed D2O

bulk

0.260 0.65 0.65 2.7 2.7 0.0005 0.0005

0.260 0.45 0.75 0 0 0.0005 0.0005

0.213 0.8 0.8 3 3 450 450

0.213 0.8 0.8 0 0 450 450

1.138 0.75 5 0 0 2000 2000

0.26 0.22 2 0.3 0.18

F ) f(%Eu) 0.26 0.2 0 2 0.08

0.213 0.22 0 450 0.01

0.213 0.22 0 450 0.01

1.138 0.22 0 1500 0.008

The indicated values of ka,r were calculated as explained in the text.

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Figure 4. Normalized emission decays from the 5D0 level excited at 466 nm for nanoparticles containing 2 (left) and 40% (right) europium; pristine particles in H2O (a), pristine particles in D2O (b), annealed particles in H2O (c), annealed particles in D2O (d), and the bulk reference (e). Solid lines represent the experimental data, and symbols correspond to the fits according to eq 6 (see text).

Figure 5. Model for the emission process in Y1-xEuxVO4. Ns,i and Na,i correspond to the number of sensitizers (s ) VO43-) and activators (a ) Eu3+) at the level i, respectively. The rate of exciton creation by excitation of VO43- ions is designed as ω. The ks,1 and ks,nr are the nonradiative recombination rates from the 2 and 1 excited states of the sensitizer ions, respectively. The energy-transfer rate from sensitizer to activator, or the coupling constant, is characterized by β. The ka,1 and ka,nr are the nonradiative recombination rates from the 2 and 1 excited states of the europium ion, respectively; ka,r is the spontaneous emission rate of the Eu3+.

change concerns the kD*, which corresponds to the quenching of the Eu emission through energy transfer to a nearby defect. Considering that the only change in the sample is the replacement of OH by OD, it is clear that acceptor defects in the pristine particles are surface OH groups, that is, OH groups accessible to exchange by the solvent.45 Since the kD* parameter goes to 0 after transfer into D2O, we may conclude that OH groups are almost the only species involved in the kD* parameter. Now considering the evolution related to the annealing treatment, the major evolution concerns the values of kCR and ks,nr/β*. The drastic increase of kCR corresponds to a strong improvement of the cross relaxation efficiency. Concerning the decrease of ks,nr/β*, it can be explained either by a suppression of some quenching of the VO43- leading to a decrease of ks,nr or an improvement of the VO43--VO43- and VO43s-Eu3+ transfer rate (β*). The latter effect could be explained by the fact that the annealing treatment reduces distortions and thus re-establishes the almost optimal angle of 170° for the V-O-Y (and thus V-O-Eu) bonds as in a perfect YVO4 crystal,46 leading to a better overlap of electron wave functions. Concerning the value of kD, which corresponds to the quenching rate independent of the europium content, we note that its value remains almost the same in all samples, either annealed or not, nearly identical to the value obtained for the bulk materials. We may thus infer that this quenching process

is intrinsic to the YVO4 compound, independent from defects or distortions. The kD may thus correspond to multiphonon relaxation, which is known to occur in bulk YVO4,47 and may explain its limited emission yield of 70% for the optimal Eu content.48 Dielectric Confinement Effect. From the above discussion, one can conclude that the main effect of the annealing process is to improve the efficiency of energy transfers, as can be seen from the evolution of the cross relaxation rate (kCR) and the vanadate to europium energy-transfer rate (ks,nr/β*). As discussed above, the spectroscopic properties of the annealed particles are similar to those of the bulk, and the quantum yield versus europium content curve in the case of the annealed particles has almost the same shape as that for the bulk material. However, the emission yield of the particles (40%) is still significantly lower than that of the bulk material (70%). An important difference between the bulk material and the nanoparticles concerns the radiative rate of emission that is strongly decreased in nanoparticles (0.213 s-1) as compared with that of the bulk (1.138 s-1). This difference can be explained by dielectric effects since the radiative rate depends strongly on the effective index of refraction of the medium surrounding the emitters.49,50 In the quantum yield versus europium content curves as described by eq 5, the radiative rate of emission appears directly as ka,r but also indirectly in the expression kCR as derived from the model of Inokuti and Hirayma (see Supporting Information). In this model, kCR is proportional to k0, which is the recombination rate of the Eu excited state in the absence of Eu-Eu interactions. The kCR is thus proportional to k0 ) ka,r + kD + kD* · xEu. Using the values determined for the annealed particles and considering the bulk value of the radiative rate of recombination (ka,r ) 1.138 ms-1), we obtain a value for kCR of 1411 ms-1. We note that the obtained value for kCR is not far from the value determined for the bulk material (1500 ms-1). The dashed line in Figure 6 shows the curve obtained from eq 5 using the same parameters as the ones determined for the annealed particles but using the bulk value of ka,r and the corresponding new value of kCR. This curve then corresponds to the expected properties of the annealed particles as if they were in the same dielectric environment as in the bulk material. The resulting curve is now very close to the bulk curve (filled squares), with almost the same value of optimum quantum yield (about 70%) for a similar Eu content of about 5%. Thus, the difference of radiative lifetime between nanoparticles and the

Size Effects in Luminescent Oxide Nanocrystals

Figure 6. Quantum yield versus europium content curves as derived from eq 5. Fit of the experimental data for annealed particles (filled circles) and bulk materials (filled squares). Calculated curve using the parameters of the annealed particles, except for ka,r, for which the bulk value was used (dotted line). Calculated curves derived from parameters determined for annealed particles (empty circles) and bulk materials (empty squares) assuming kD ) 0.

bulk, which results from a dielectric effect, is sufficient to explain the altered emission yield of the particles as compared to the bulk material. We may thus conclude that annealed particles exhibit almost the best emission properties that can be expected for particles dispersed in a medium with a lower refractive index (n ) 1.33) than that of the bulk (n ) 2). Another interesting result from the model is that the impact of dielectric effects on the difference of emission yield between the annealed particles and the bulk is all the more important if nonradiative recombination pathways are active in the bulk, that is, if the value of kD is important. In the case of YVO4, the determined value of kD is about 0.22 ms-1, and the optimum yield of the annealed particles is measured to be about 40%, while it is 70% in the bulk. Now, if we consider the case of a material with no “intrinsic” nonradiative recombination pathways (such as multiphonon relaxation), then the value of kD is expected to be almost 0. In that case, using kD ) 0 in eq 5 and all other parameters determined for annealed and bulk materials, respectively, the optimum quantum yield is found to be 76 and 81% for annealed particles (in water) and bulk material, respectively (Figure 6). The difference in this case is much lower; dielectric effects are less pronounced. Thus, dielectric effects modulate the relative influence of radiative to nonradiative recombination pathways by modifying the radiative emission rate, whereas nonradiative processes are less sensitive to dielectric effects. We note that a similar explanation is given for the plasmonic enhancement of the emission properties of luminescent dyes close to a metal nanostructure.51 Conclusion The purpose of this work was to investigate the emission properties of 30 nm YVO4/Eu particles, considering that surface effects in this size regime would be limited. The idea was that emission efficiency was, in this case, limited by defects in the volume of the particles, as observed by a strong contrast in the TEM observation of the pristine particles. An original process was used for the annealing of the particles at 1000 °C, leading

J. Phys. Chem. C, Vol. 113, No. 43, 2009 18705 to perfectly crystalline particles whose properties could be compared to those of the pristine particles and the bulk reference materials. Both the experimental emission decays and the dependence of the quantum yield on europium content could be fitted using two equations obtained from a simple model for the emission process. The main parameters are the quenching rate of the europium excited state, the energy-transfer rate from vanadate to europium, and the rate of cross relaxation between europium ions. From the evolution of these parameters for our different samples, we conclude that the altered emission from the pristine particles is explained by surface quenching (considering some porosity within the particles) and strongly altered energy-transfer processes. Annealing of the particles leads to a drastic improvement of their emission properties that become similar to those of the bulk, except for their emission yield (40 instead of 70% for the bulk). It appears from our model that this difference can be well understood simply by considering that the radiative emission rate for the bulk is much higher than that for the nanoparticles due to a higher effective refractive index. We thus conclude that our annealed particles present the best emission properties that can be expected for YVO4/Eu nanoparticles dispersed in a given medium. We also note that the dielectric effect, by diminishing the radiative emission rate, enhances the importance of nonradiative effects that are present in the bulk. We thus expect that, provided a perfect crystallinity, lower differences due to the dielectric effect and higher emission yields could then be obtained for nanoparticles made of compounds that exhibit higher emission yields in the bulk state. The experimental validation of these predictions is now under consideration, beginning with the determination of the emission yield of YVO4/Eu nanoparticles dispersed in a medium of high refractive index such as TiO2. This work provides an example of a general strategy toward the investigation of the physical properties of nanocrystals whose physical properties may be altered by crystalline defects. Acknowledgment. The authors thank D. Casanova for setting up the luminescence decay measurements. Supporting Information Available: Additional absorption and excitation results and calculation details. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Xu, S.; Kumar, S.; Nann, T. J. Am. Chem. Soc. 2006, 4, 1054– 1055. (2) Wilson, W. L.; Szajowski, P. F.; Brus, L. E. Science 1993, 262, 1242–1244. (3) Counio, G.; Esnouf, S.; Gacoin, T.; Boilot, J. P. J. Phys. Chem. 1996, 100, 20021–20026. (4) Treussart, F.; Jacques, V.; Wu, E.; Gacoin, T.; Grangier, P.; Roch, J. F. Physica B 2006, 376, 926–929. (5) Huignard, A.; Gacoin, T.; Boilot, J. P. Chem. Mater. 2000, 12, 1090–1094. (6) Haase, M.; Riwotzki, K.; Meyssamy, H.; Kornowski, A. J. Alloys Compd. 2000, 303-304, 191–19. (7) Buissette, V.; Giaume, D.; Gacoin, T.; Boilot, J.-P. J. Mater. Chem. 2006, 16, 529–539. (8) Louis, C.; Bazzi, R.; Marquette, C. A.; Bridot, J.-L.; Roux, S.; Ledoux, G.; Mercier, B.; Blum, L.; Perriat, P.; Tillement, O. Chem. Mater. 2005, 17, 1673–1682. (9) Kasuya, R.; Kawano, A.; Isobe, T.; Kuma, H.; Katano, J. Appl. Phys. Lett. 2007, 91, 111916. (10) Heer, S.; Ko¨mpe, K.; Gu¨del, H. U.; Haase, M. AdV. Mater. 2004, 16, 2102–2105. (11) Stouwdam, J. W.; Hebbink, G. A.; Huskens, J.; van Veggel, F. C. J. M. Chem. Mater. 2003, 15, 4604–4616. (12) Michalet, X.; Pinaud, F. F.; Bentolila, L. A.; Tsay, J. M.; Doose, S.; Li, J. J.; Sundaresan, G.; Wu, A. M.; Gambhir, S. S.; Weiss, S. Science 2005, 307, 538–544.

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