Luminescence Properties of α-NaYF4: Nd3+ Nanocrystals Dispersed

Article pubs.acs.org/JPCC

Luminescence Properties of α‑NaYF4:Nd3+ Nanocrystals Dispersed in Liquid: Local Field Effect Investigation Xiaojie Xue, Zhongchao Duan, Takenobu Suzuki, Rajanish N. Tiwari, Masamichi Yoshimura, and Yasutake Ohishi* Graduate School of Engineering, Toyota Technological Institute, 2-12-1 Hisakata Tempaku, Nagoya 468-8511, Japan S Supporting Information *

ABSTRACT: Pure α-NaYF4:Nd3+ nanocrystals of various sizes from 6 to 18 nm were successfully synthesized by a solvothermal method. By controlling the doping concentration of Nd3+ ions, α-NaYF4 nanocrystals with different quantum efficiencies were prepared. The monodispersed oleate-capped nanocrystals were dispersed in several nonpolar solvents to obtain stable colloids. Under the excitation by an 803 nm laser, luminescent decay curves of the colloids were measured. The relationship between the emission lifetimes of samples and the effective refractive indices of solvents was studied. The local field correction factor for the samples of 18 nm with different quantum efficiencies is expressed by the real cavity model which indicates the local field effect is independent of the emission quantum efficiency. When the particle size decreases to 6 nm, the preferred model alters from the real cavity model to the virtual cavity model which might be derived from changing of polarizability density.

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the local field effect. By dispersing YAG:Nd3+ NCs in different solvents with various refractive indices, Boyd, et al. reported that for NCs with higher quantum efficiency the real cavity model was represented, whereas in the case of lower quantum efficiency, both the real cavity model and non-local-field-effects model were applicable.10 Zheng et al. reported that the real cavity model could explain the local field effect by using Tm3+ doped LaF3 nanoparticles.11 To the best of our knowledge, until now only these papers have investigated the local field effect by directly dispersing lanthanide-doped NCs in liquid media. In these works, however, bare NCs without any surface modification were used with the help of some surfactants which may induce negative impacts like effective refractive index shifting and some quenching effects. Moreover, NCs with cubic structures should be utilized. The simplest formulation expression of the local field correction factor could be used, due to the high symmetry.12 Therefore, colloidal samples consisting of cubic structured NCs dispersed in solvents with different refractive indexes are a much better choice for the reliable investigation of the local field effect. In this paper, we report the synthesis of oleate-capped Nd3+doped cubic phase NaYF4 NCs with various sizes by a solvothermal method. These Nd3+-doped NCs showed strong near-infrared emissions under the excitation at 803 nm. All of the NCs samples could be dispersed in some nonpolar solvents such as hexane, cyclohexane, toluene, and so on to form stable colloids. We measured the emission lifetimes of the transparent colloids of α-NaYF4 NCs with distinct Nd3+ concentrations and

INTRODUCTION In the last decades, lanthanide-doped nanocrystals (NCs) have been studied for applications in many optical fields, such as display,1,2 biolabeling,3,4 therapy, etc.5,6 When applied in such application fields, the luminescent NCs must be dispersed in some certain host media. The final material produced by the combination of a host medium and NCs would become a sort of heterogeneous composite. To control the optical properties of such heterogeneous composites, the interaction between NCs and the host should be considered. It is the local field effect that always plays a crucial role in understanding the relation between macroscopic and microscopic fields. After embedding NCs in a medium, the electric field strength acting on the NCs will not simply equal the external field due to the local field effect.7,8 Neglecting the local field effect, the lifetime of an electric dipole transition in a medium of refractive index n should be τ0/n, where τ0 is the lifetime of the electric dipole transition in free space. If the local field effect can influence the radiative dipoles, a local field correction factor l(n) should be considered.9 Many theoretical and experimental works have been performed to investigate the local field effect. Two models, virtual cavity model and real cavity model, have been mainly considered to explain the local field effect: for the virtual cavity model, the emitter dipole is placed in an imaginary cavity which has the same refractive index as the surrounding medium, whereas in the case of the real cavity model, the emitter dipole expels the medium creating a real cavity.7−10 It has been shown that the virtual cavity model (Lorentz model) can be used to explain the local field effect for lanthanide ions doped in glass hosts which are homogeneous systems, whereas in the case of NCs doped media, which are heterogeneous systems, it is still unclear which model is adequate to explain © 2012 American Chemical Society

Received: June 15, 2012 Revised: September 17, 2012 Published: October 9, 2012 22545

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to around 803 nm. An 850 nm long-pass filter was also used to remove the scattered light from the pump LD.

with different sizes. Based on the relation between the emission lifetimes of Nd3+-doped NCs and the effective refractive indices of colloids, the concentration- and size-dependent local field effect were investigated in details.

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RESULTS AND DISCUSSION The XRD patterns of NaYF4 NCs synthesized under various molar ratios of F−/Y3+ are presented in Figure 1. All samples

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EXPERIMENTAL SECTION Materials. Ln(NO3)3·6H2O (Ln = Y and Nd) were supplied from Sigma-Aldrich. Oleic acid (OA), NH4F, and NaOH were supplied by Kishida Reagents Chemicals. All chemical reagents were used without further purification. Synthesis of Oleate-Capped α-NaYF4:Nd3+ Nanoparticles. The solvothermal method reported in other papers for the preparation of oleate-capped NaYF4 was used.13−15 Typically, 6 mL of OA, 0.2 g of NaOH, 6 mL of ethanol, and 3 mL of distilled water were mixed together under vigorous stirring. After forming a homogeneous transparent solution, 0.5 mmol of Ln(NO3)3 (99%Y3+ and 1%Nd3+) stock solution was added, and the mixture was stirred for 1 h. Then, 0.75 mL of 2 M NH4F solution was added. After stirring for 30 min, the mixture was transferred into a 25 mL Teflon-lined autoclave that was sealed and subsequently heated at 130 °C for 12 h. After cooling to room temperature, the products were collected. The as-prepared sample was denoted as N1. The purified sample was redispersed in several nonpolar solvents, such as hexane, cyclohexane, and toluene, to yield uniform transparent colloids with a concentration of 0.5 mg/mL. The experimental conditions of other samples prepared under 130 °C for 12 h by the similar procedure are listed in Table 1. Table 1. Summary of Reaction Conditions, the Corresponding Phase, and Average Size of the As-Prepared Samples sample

F−/Ln3+ ratio

Nd3+ concentration

NaOH (g)

N1 N2 N3 N4 N5 N6 N7 N8

3:1 4:1 5:1 6:1 3:1 4:1 5:1 6:1

1% 1% 1% 1% 1% 1% 1% 1%

0.2 0.2 0.2 0.2 0.4 0.4 0.4 0.4

N9

7:1

1%

0.4

N10

5:1

5%

0.2

phase

size (nm)

α-NaYF4 α-NaYF4 α-NaYF4 α+β NaYF4 α-NaYF4 α-NaYF4 α-NaYF4 α-NaYF4 + NaF α-NaYF4 + NaF α-NaYF4

6.0 ± 1.3 10.6 ± 1.6 18.3 ± 3.0

Figure 1. XRD patterns of the α-NaYF4:1%Nd3+ NCs synthesized with (a) 0.2 g and (b) 0.4 g of NaOH and different F−/Y3+ molar ratios at 130 °C for 12 h.

7.0 ± 1.6 11.7 ± 1.9 14.0 ± 1.7

(N1−N4) were heated at 130 °C for 12 h. When the F−/Y3+ ratio increased from 3:1 to 5:1 (N1−N3), all diffraction peaks could be indexed to the standard data of cubic (α-) NaYF4 (JCPDS 77-2042). The decreasing half width of the diffraction peaks for those samples reveals that the particle size became larger with increasing F−/Y3+ ratio. It implies that the F−/Y3+ ratio promotes the growth of α-NaYF4 NCs. When the F−/Y3+ ratio reached 6:1, the half width of the diffraction peaks according to the α-NaYF4 remained the same, whereas some new diffraction peaks of hexagonal phase (β-) NaYF4 (JCPDS 16−0334) with weak intensity were observed. This indicates that α-NaYF4 is the major phase and β-NaYF4 is the minor phase in the sample N4. The high concentration of F− ions promoted the cubic to hexagonal phase transition process in NaYF4 NCs. According to the half width of the main diffraction peak at 27.9° which corresponds to the (111) plane of αNaYF4, the crystallite sizes calculated by the Scherrer equation were 5.8, 10.6, and 18.3 nm for samples N1−N3, respectively. Based on a similar procedure using different amounts of NaOH, samples N5−N9 with F−/Y3+ ratios from 3:1 to 7:1, respectively, were synthesized. The XRD patterns of N5−N9 are shown in Figure 1b. When the F−/Y3+ ratio was under 5:1, only pure α-NaYF4 (JCPDS 77-2042) NCs were obtained. The

17.1 ± 3.9

Characterization. The compositions and crystal structures of the samples were analyzed by X-ray powder diffraction (XRD) on a LabX XRD-6100 X-ray diffractometer from the Shimadzu company with a Cu Kα radiation source (λ = 1.5405 Å). Transmission electron microscopy (TEM) measurements were carried out on a JEOL 2100 transmission electron microscope with an acceleration voltage of 120 kV. Fourier transform infrared (FR-IR) spectra were obtained on PerkinElmer Spectrum 100 FT-IR spectrometer. The emission spectra were measured by a monochromator equipped with a Hamamatsu H10330A-75 photomultiplier tube (PMT). The output signal was amplified by the LI5640 digital lock-in amplifier. The luminescent decay curves were obtained by a 200 MHz digital oscilloscope (Yokogawa). The pumping source was an 808 nm laser diode (LD) with the wavelength adjusted 22546

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Figure 2. TEM images of α-NaYF4:1%Nd3+ NCs prepared with 0.2 g (a−c) and 0.4 g (d−f) of NaOH: F−/Y3+ molar ratios of 3:1 (a and c), 4:1 (b and d), and 5:1 (c and f). The inset in panel a is the HR-TEM of one single particle and the electron diffraction pattern, respectively.

then a drop of this colloid was dropped on a copper grid with a carbon film coating. It was clear that the as-prepared samples could be well dispersed in nonpolar solvents, like cyclohexane, due to the surface modification of OA molecules. The OA ligand capped on the surface of α-NaYF4:1%Nd3+ NCs was identified by the FT-IR spectrum shown in Figure 3. The

narrowed half width of the diffraction peaks implies that the average particle size became larger with increasing F−/Y3+ ratio. When the F−/Y3+ ratio increased to 6:1, a peak assigned to the NaF impurity appeared. When the F−/Y3+ ratio reached 7:1, the intensity of diffraction peaks of the NaF impurity enhanced significantly. This indicates that further increasing F− ions would not enhance the particle size but increase the amount of NaF. After NaF appeared, the width of the peak of α-NaYF4 stopped decreasing. This implies that the average size of αNaYF4 NCs reached a maximum under such a reaction condition. Figure 2a shows TEM images of α-NaYF4:1%Nd3+ NCs (sample N1). The as-prepared sample is monodisperse NCs of about 6 nm with irregular shapes, which is consistent with that calculated from XRD patterns by the Scherrer equation. The inset of the high-resolution TEM (HR-TEM) image shows that the as-prepared particles are high quality single crystals. The lattice fringes can be clearly distinguished. The observed interplanar distance between the lattice fringes was about 0.32 nm, corresponding to the (111) plane of α-NaYF4. The inset of the selected area electron diffraction (SAED) pattern confirmed the cubic crystal lattice of the as-prepared sample. The spotty polycrystalline diffraction rings can be assigned to the (111), (200), (220), and (311) planes of standard α-NaYF4 (JCPDS 77-2042). Figure 2b−f shows the TEM images of α-NaYF4:1% Nd3+ NCs (sample N2−N3 and N5−N7) prepared under different experimental conditions. All of the samples are well dispersed NCs. The observed average sizes are 6.0, 10.6, and 18.3 nm, as shown in Figure 2a−c, and 7.0, 11.7, and 14 nm, respectively, as shown in Figure 2d−f. It is obviously seen that the average sizes of particles observed from TEM images are in good agreement with those calculated from the Scherrer equation. To ensure the distinguishable average size for investigating the size-dependent local field effect, NCs with diameters of 6, 10, 14, and 18 nm (N1, N2, N7, and N3) were selected. The histograms of size distribution are shown in Figure S1 (Supppoting Information). For the TEM observation, the samples were dispersed in cyclohexane to form a colloid with a low concentration and

Figure 3. (Top) FT-IR spectrum of NCs of 18 nm. (Bottom) digital suspensions of the dispersion of the NCs in (a) hexane, (b) octane, (c) toluene.

oleate-capped α-NaYF4:1%Nd3+ photograph of stable colloidal oleate-capped α-NaYF4:1%Nd3+ cyclohexane, (d) CCl4, and (e)

vibration bands at 1566 and 1452 cm−1 were assigned to the asymmetric (vas) and symmetric (vs) stretching vibrations of the carboxylic group of the OA molecule. The bands centered at 2925 and 2856 cm−1 are associated to the stretching vibration of methylene (−CH2) in the long alkyl chain. The bands at 1640 and 1374 cm−1 can be assigned to the stretching vibration 22547

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of CC and bending of −CH2. The broad band around 3420 cm−1 is ascribed to the −OH stretching vibration of water and OA. Since the crystal surface was capped by the hydrophobic oleate anion, the as-synthesized NaYF4:1%Nd3+ NCs were well dispersed in nonpolar organic solvents just by being sonicated for a while, and they finally became stable and clear colloids as shown in Figure 3. For the investigation of the local field effect, several nonpolar solvents of different refractive indices were selected as tabulated in Table 2. The colloids were prepared by using α-NaYF4 NCs Table 2. List of Solvents Used As Host Media and Their Refractive Indices Correspondingly solvent

refractive index

pentane hexane heptane octane cyclohexane CCl4 toluene

1.358 1.375 1.387 1.398 1.426 1.466 1.496

with the volume fraction of about 0.12%. Since the volume fraction is low, the colloids could be treated as Maxwell− Garnett composites.16 The effective refractive index neff can be given by neff 2 − nliq 2 neff 2 + 2nliq 2

=f

nnano 2 − nliq 2 nnano 2 + 2nliq 2

(1)

where nliq is the refractive index of the host liquid medium, nnano is the refractive index of α-NaYF4, and f is the volume fraction. The calculated effective refractive index neff will be used in following discussions. For the investigation of concentration-dependent local field effect, the α-NaYF4 NCs of 18 nm diameter doped with 1% and 5%Nd3+ ions denoted as N3 and N10 were synthesized. Figure 4a shows the emission spectra of these two samples under the 150 mW excitation at 803 nm. The emission bands centered at around 1054 and 1325 nm are attributed to the 4F3/2 → 4I11/2 and 4F3/2 → 4I13/2 transitions of Nd3+ ions, respectively. Obviously, the emission intensity of the α-NaYF4:5%Nd3+ NPs was much lower than that of α-NaYF4:1%Nd3+ due to the concentration quenching between Nd3+ ions pairs. The decay curves of the 1054 nm emissions of sample N3 and N10 are shown in Figure 4b. Although a double exponential function has been used to fit the emission decay curves of NCs,17 a triple exponential function defined as I = A1e−t / τ1 + A 2 e−t / τ2 + A3e−t / τ3

Figure 4. (a) Emission spectra of 1% and 5% Nd3+ doped 18 nm αNaYF4 NCs (samples N3 and N10, respectively) under 803 nm excitation. The inset shows the representative energy levels of Nd3+ ions. (b) The decay curves of samples N3 and N10 at 1054 nm emission.

intermediate lifetime (τ2), and the third layer is the surface layer suffering surface defects and quenching strongly and generates the shortest lifetime (τ3). By the analysis of the triple exponential function, we obtained the lifetimes τ1 = 1.20 ms, τ2 = 497 μs, and τ3 = 150 μs for N3 and τ1 = 535 μs, τ2 = 198 μs, and τ3 = 58.8 μs for N10. Although samples N3 and N10 had the same structure and similar sizes, concentration quenching caused by high Nd3+ concentration shortened the lifetime in sample N10, compared to that of sample N3. When a dielectric sphere is embedded into a medium, the electric field strength inside the sphere does not equal the external applied electric field strength. Actually, the electric field strength inside, Elocal, equals that of applied external field, E, minus that of a field generated by induced polarized charges on the surface of the dielectric sphere, Ep.18 Then the expression of the inside electric field should be

(2)

was used in this work. This is because the calculated curves derived from the triple exponential function reproduced the measured decay curves much better than the double exponential function. Usually, two emission lifetimes have been used to explain the luminescent dynamics in lanthanidedoped NCs: the longer lifetime is derived from the emission generated in the internal part of NPs and the shorter lifetime is from the surface area.17 We propose a three layer model to interpret the luminescent dynamics. The first layer is the internal part of the NPs generating the longest lifetime (τ1), the second layer is the intermediate part, which would partially contain surface defects and quenching and generates the

E local = E − Ep

(3)

E local = l(n) (4) E where n is the refractive index of medium and l(n) is the local field correction factor which connects the macroscopic and microscopic electric field. Since the probability of spontaneous emission depends on l(n),19 the local field correction factor can 22548

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be investigated by the emission lifetime measurement. Generally, the measured emission lifetimes in experiments should be expressed as the combination of radiative and nonradiative transition lifetime by 1 τmea

=

1 1 + τrad τnonrad

(5)

where τmea, τrad, and τnonrad are the measured lifetime and the radiative and nonradiative transition lifetimes.10 The quantum efficiency η is given by the rate of radiative and nonradiative transition as

η=

A rad

A rad + A nonrad

(6)

When embedded into a medium, the radiative transition lifetime is given by8,9 τ τrad = vac l(n) (7) n where n is the refractive index of the medium, l(n) is the local field correction factor, and τrad and τvac stand for the radiative lifetimes in medium and in free space, respectively. When we consider the quantum efficiency, eqs 5−7 should be combined as ητ τmea = vac l(n) (8) n By analyzing the measured lifetimes and the effective refractive indices of colloids, the local field correction factor could be investigated. Two distinct models for the local field correction factor have been proposed: the virtual cavity model (VCM) which is given by ⎛n 2 + l(n) = ⎜ eff 3 ⎝

2⎞ ⎟ ⎠

Figure 5. Relation between the lifetimes from the internal layer (τ1) and effective refractive indices of colloids containing (a) α-NaYF4:1% Nd3+ NCs and (b) α-NaYF4:5%Nd3+ NCs.

2

solvents shown in Table 2, stable colloids were obtained. The lifetimes of Nd3+ doped α-NaYF4 NCs in solvents were smaller than those of dry powders as shown in Figure 4b. It would be due to not only the influence of the local field effect but also the quenching effect from the solvents.21 The adhered oleate ligands on the particle surface could protect the NCs from host solvents quenching. As depicted in Figure 5, it is clear that both N3 and N10 samples obeyed the RCM. Due to the distinct doping concentrations, the quantum efficiency of N3 and N10 should be different. According to the results by Boyd et al., the local field effect model depends on the quantum efficiency that under the assumption of η = 0.48, the RCM gave the best fit. If η = 1, the non-local-field-effects model provided a slightly better fit.10 However, by directly measuring the NCs with different quantum efficiencies, we found that the RCM reasonably expressed the local field effect of dispersing NCs in liquid media. The different quantum efficiencies did not have significant influences on the model changing of the local field effect. The calculation of the local field effect is based on the assumption that the radius of the spherical cavity where the optically active ions exist should be larger than that of the ions but smaller than the excitation wavelength. The size of the cavity might be a crucial factor to influence the model of local field effect. Figure 6 shows the emission decay curves of the N1, N2, and N7 dry powders under the 803 nm excitation. The lifetimes τ1, τ2, and τ3 obtained by using the triple exponential function were listed and inset in Figure 7. It was observed that the lifetimes decreased as the sample size decreased from 14 to 6 nm. It is supposed that the sample size reduction relatively

(9)

where neff is the effective refractive index of the colloidal in our case, and the real cavity model (RCM) which is given by8,9,20 ⎛ 3n 2 ⎞2 eff ⎟ l(n) = ⎜ 2 ⎝ 2neff + 1 ⎠

(10)

By fitting the emission decay curves of the various colloidal samples to the decay function given by eq 2, we obtained the emission lifetimes of samples N3 and N10 in liquid media with different refractive indices. The relationships between the effective refractive index and emission lifetimes are shown in Figure 5. Since the local field effect influences the electric field on the whole nanoparticle, each layer, such as the internal, intermediate and surface layer, would have the similar lifetime change tendency, when the refractive index of surrounding medium changes. Due to the strong quenching from the surfaces which would influence the emission lifetime, however, it was suggested that it was difficult to obtain reproducible results on local field effect in surface layer.10 It was the case for our results. The intermediate layer would have the similar effect, as it suffers the influence of the surface layer. We plotted the relation between the lifetime generated in the internal layer (τ1) and effective refractive indices. Equations 9 and 10 have been used in the analysis of the results shown in Figure 5. The least-squares fitting method was used to analyze the measured data. In the fitting process, τvac was treated as adjustable values. By dispersing Nd3+-doped α-NaYF4 NCs (N3 and N10) in 22549

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Figure 6. Decay curves of α-NaYF4:1%Nd3+ powders with sizes of 6, 10, and 14 nm under an 803 nm excitation.

enhances the influence of surface quenching that magnifies the nonradiative transition probability and then reduces the measured radiative lifetime, according to eq 2. The details about surface quenching and size effect are not concerned in this paper. We will discuss them in another paper. By using the colloids containing α-NaYF4:1%Nd3+ with sizes of 6, 10, and 14 nm (N1, N2, and N7), the emission lifetimes were measured. The relationship between the emission lifetime of the internal part (τ1) and the effective refractive indices of colloids is shown in Figure 7. This figure depicts that, in case of 14 nm α-NaYF4 NCs (N7), the relation between the measured lifetimes and effective refractive indices tends to agree with the RCM. When the particle size decreased to 10 nm (N2), although the RCM was still fitted, some data points in a small range seemed to match the VCM better, as shown in Figure 7b. When the particle size decreased to around 6 nm (N1), the lifetime fitted the VCM well. It implies that the RCM is an adequate model to express the local field effect for comparatively larger particles. The theoretical work by Lagendijk et al. showed that, based on the doping type, interstitial and substitutional, the most suitable model will be changed between the VCM and the RCM: In the case of interstitial doping, the VCM is relevant, whereas in the case of substitutional doping, the RCM is relevant.22 In our case, although the doped Nd3+ ions in α-NaYF4 NCs substitutively occupied the site of Y3+,23 the VCM was the preferred model to explain the local field effect in NCs with 6 nm average size. It is interesting to observe the dependence of the local field effect model on the particle size. This model change due to the reduction of particle size might be attributed to the following reasons: (1) It is pointed out that, because embedding NPs in media expels the media and creates real cavities, the RCM should be reasonable.22 When the particle sizes decrease, the volume that NPs expels will reduce. If the size is much smaller, the expelled volume could be ignored. Thus the composite materials containing NPs could be considered as a uniform medium where the VCM is reasonable. (2) Considering the more general case of a cavity of radius a embedded into a medium with effective refractive index neff, and the polarizability α of the emitter inside this cavity, the lifetime is given by24 ητvac τmea = 2 3neff 2 neff 2 3 2

(

(2neff + 1) − (2α / a )(neff − 1)

)

Figure 7. Relationship between the lifetimes from the internal layer (τ1) and effective refractive indices of colloids containing α-NaYF4:1% Nd3+ NCs with sizes of (a) 14, (b) 10, and (c) 6 nm.

Based on the polarizability α and radius a, a suitable model could be decided.20,22 Generally, if 2α/a3 → 0, which implies the polarizability is very small (an empty cavity) or radius a is very large, the model becomes the RCM; if the polarizability density inside the cavity equals that in the medium, the VCM is introduced based on the Clausius−Mossotti relation. The cubic structure usually has a lower polarizability which is almost 0, due to the high symmetry. The surrounding liquid media also have low polarizability. When the sample size is larger, like 14 nm, the low polarizability and the difference of polarizability density between inside the cavity and outside the cavity will lead to the RCM. When the sample size is 6 nm, however, the size reduction enhances the value of polarizability density inside the cavity. If the polarizability density inside the cavity becomes comparable with that of surrounding liquid media, the VCM

(11)

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(14) Zhang, F.; Wan, Y.; Yu, T.; Zhang, F. Q.; Shi, Y. F.; Xie, S. H.; Li, Y. G.; Xu, L.; Tu, B.; Zhao, D. Y. Angew. Chem., Int. Ed. 2007, 46, 7976−7979. (15) Xue, X. J.; Qin, W. P.; Zhang, D. S.; Zhao, D.; Wei, G. D.; Zheng, K. Z.; Wang, L. L.; Wang, G. F. J. Nanosci. Nanotechnol. 2010, 10, 2028−2031. (16) Liang, X.; Wang, X.; Zhuang, J.; Peng, Q.; Li, Y. Adv. Funct. Mater. 2007, 17, 2757−2765. (17) Dolgaleva, K.; Boyd, R. W. J. Opt. Soc. Am. B 2007, 24, A19− A25. (18) Kittel, C. Introduction to Solid State Physics; Wiley: New York, 1975. (19) Pukhov, K. K.; Basiev, T. T.; Orlovskii, Yu. V. Opt. Spectrosc. 2011, 111, 386−392. (20) Rikken, G. L. J. A.; Kessener, Y. A. R. R. Phys. Rev. Lett. 1995, 74, 880−883. (21) Wang, F.; Wang, J.; Liu, X. Angew. Chem., Int. Ed. 2010, 49, 7456−7460. (22) Lagendijk, Ad.; Vries, P. Phys. Rev. Lett. 1998, 81, 1381−1384. (23) Joubert, M. F.; Linares, C.; Jacquier, B.; Cassanho, A.; Jenssen, H. P. J. Lumin. 1992, 51, 175−187. (24) Böttcher, C. J. F. Theory of Electric Polarization; Elsevier: Amsterdam, 1973.

would become reasonable. Therefore, when embedding NCs in nonpolar liquid with low polarizability, the local field effect model would obey the RCM if the particle size is comparably larger. However, if the particle size is small, like 6 nm, the local field effect will follow the VCM. As discussed above, these would be the possible mechanisms which cause the local field effect model changes for NCs as we observed.

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CONCLUSION In summary, pure α-NaYF4 NCs have been successfully synthesized by a facile solvothermal method. By changing the experimental conditions, α-NaYF4 NCs with different sizes and Nd3+ concentrations could be synthesized. The as-prepared samples can be directly dispersed in some nonpolar solvents forming stable colloids. By using these colloids, concentrationand size-dependent local field effects were investigated. In the case of larger particle size, the real cavity model was the most fitted model which is independent of the quantum efficiency. When the particle size reduced to 6 nm, the virtual cavity model expressed the experimental results well.

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ASSOCIATED CONTENT

S Supporting Information *

Additional information about the histograms of sample size distribution (Figures S1 and S2), emission lifetime values and corresponding weight factor (Table S1), and the FTIR spectra of α-NaYF4:1% Nd3+ NCs kept in toluene for different times (Figure S3). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by MEXT, the Support Program for Forming Strategic Research Infrastructure (2011-2015). REFERENCES

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