Upconversion Luminescence of Er and Yb Codoped NaYF4

Article pubs.acs.org/JPCC

Upconversion Luminescence of Er and Yb Codoped NaYF4 Nanoparticles with Metal Shells Minoru Fujii,* Taishi Nakano, Kenji Imakita, and Shinji Hayashi Department of Electrical and Electronic Engineering, Graduate School of Engineering, Kobe University, Rokkodai, Nada, Kobe 657-8501, Japan S Supporting Information *

ABSTRACT: Upconversion photoluminescence (PL) of a composite nanoparticle consisting of an Er and Yb codoped NaYF4 core and a Au shell is studied theoretically and experimentally. We first investigate the effects of a Au shell on the radiative and nonradiative emission rates of a dipole placed in a core, the absorption and scattering cross sections of a composite nanoparticle, and the electric field within a core at the excitation wavelength. We then synthesize the composite nanoparticle and study the PL properties. From the analyses of the PL data in combination with the data obtained by theoretical calculations, the mechanism of the enhancement and quenching of upconversion PL by the formation of a Au shell is studied.

1. INTRODUCTION Rare-earth doped upconversion materials, which convert long wavelength photons into shorter wavelength photons, have been attracting great attention in many fields, including biology,1−3 solar cells,4 three-dimensional displays,5 etc. However, due to the small excitation cross sections and small radiative decay rates of trivalent rare-earth ions caused by the parity forbidden intra-4f shell transitions, the luminescence intensity is usually low, which prevents practical applications of upconversion materials. In order to overcome the limitation, a new approach utilizing the coupling of upconversion luminescence with localized surface plasmons (SPs) supported by metal nanostrucutres has been investigated. Enhancements of upconversion photoluminescence (PL) by doping Au and Ag nanoparticles into rare-earth doped heavy metal oxide glasses were reported,6−8 and the mechanism was explained to be due to the enhanced electric fields accompanied by the excitation of SPs. Furthermore, very strong upconversion PL enhancements have been obtained by placing metal islands, metal nanowires and artificially processed metal nanostructures in the vicinity of upconversion films and particles.9−12 The enhancement factor of photoluminescence (PL) caused by the coupling of an emitter with SPs can in general be expressed as Y = |L(ωexc)|2n Z(ωflu)

PL n is usually larger than 2. Therefore, upconversion PL benefits from the enhanced incident electric field of SPs more than downconversion PL.10 In order to fully utilize the inherently suitable combination of upconversion PL and SPs, the structure of an upconversion material and a metal nanostructure should be carefully optimized. One of the interesting structures is a composite system consisting of upconversion nanoparticles and metal nanostructures. The composite structure can be building blocks for the fabrication of a variety of optoelectronic devices and also can be used in bioimaging. Different composite structures, such as a metal core with a dielectric shell,15 a dielectric core with a metal shell,16,17 a dielectric core decorated with metal nanoparticles,17−20 etc., have been considered. Among them, a dielectric core with a metal shell (Dc-Ms) structure is very attractive because the SP resonance energy of a metal shell can easily be controlled in a wide range by the core to shell ratio,21 and thus the energy can be tuned to the specific transition energies of rare-earth ions. In fact, surface enhanced Raman scattering in the near-infrared range has been observed for a composite nanoparticle consisting of a dielectric core coated by a thin metal shell.22 Furthermore, the enhancement of fluorescence emission rate of molecules placed in a metallic nanocavity has been theoretically predicted.23 Despite very interesting and promising systems, the application of the Dc-Ms structure to the upconversion PL enhancement has not been very successful. It has been reported that upconversion PL is suppressed when a continuous metal

(1)

where L(ωexc) is the enhancement factor of the incident electric field due to the concentration of the field by the excitation of SPs, Z(ωflu) is that of the quantum efficiency due to the enhancement of the radiative rate, and n is the number of photons involved in the excitation of the luminescence.13,14 In the case of downconversion PL, n = 1, while in upconversion © 2012 American Chemical Society

Received: September 25, 2012 Revised: December 13, 2012 Published: December 15, 2012 1113

dx.doi.org/10.1021/jp309510s | J. Phys. Chem. C 2013, 117, 1113−1120

The Journal of Physical Chemistry C

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shell is formed on an upconversion nanoparticle16 and a nanorod,17 although formation of discontinuous metal nanoparticles on the surface of a core enhances the upconversion PL. These results suggest that the PL properties of a dielectric core with a continuous metal shell structure are more sensitive to the structural parameters than that of a dielectric core with discontinuous metal nanoparticles. However, in the core−shell structure, the guideline to design optimum structures for the upconversion PL enhancement does not exist. Especially, the relation between the structural parameters and the enhancement factors of the incident electric field and the radiative rate has not been elucidated. The purpose of this work is to study the optical response of a Dc-Ms structure in detail theoretically and experimentally, and demonstrate that the structure is useful for the upconversion PL enhancement. We first analyze the radiative and nonradiative decay rates of a dipole placed in a dielectric core surrounded by a Au shell. We demonstrate that a larger enhancement of the radiative rate than that of the nonradiative rate is possible, when the structural parameters are properly chosen. We also calculate absorption and scattering cross sections of the core−shell structure and the electric fields within a core to obtain the optimum structural parameter to enhance the incident electric field. Following the calculations, we synthesize Er and Yb codoped NaYF4 nanparticles coated with Au nanoshells and study the upconversion PL properties. We show from the analysis of experimental and calculated data that, in the core−shell nanoparticles prepared in this work, PL quantum efficiency is enhanced, while the incident electric field is attenuated, resulting in a moderate enhancement of the upconversion PL intensity.

Figure 1. The model structure used for calculations. r1 and t1 are fixed to 70 and 15 nm, respectively. t2 is changed from 0 to 55 nm. r is changed from 0 to 70 nm. The surrounding medium is water (n = 1.33).

shell. The refractive index of Au is taken from ref 26. The surrounding medium is water (n = 1.33). Figure 2 shows contour plots of calculated radiative decay rates of a dipole placed in a core. The Au shell thickness is

2. THEORETICAL CALCULATIONS 2.1. Radiative and Nonradiative Decay Rates. The normalized radiative and nonradiative decay rates of a dipole placed in a stratified sphere can be calculated by a procedure discussed in ref 24. First, the electric and magnetic fields are calculated by a recursive transfer method. The time-averaged total radiated power (radiative loss) is calculated by integrating the time-averaged Poynting vector over the surface of a sphere with an infinite radius. The ratio of the calculated radiative loss to that of a dipole in a free-space homogeneous medium results in the normalized radiative decay rates. Nonradiative loss is calculated by integrating the energy inflow per unit time and unit volume over all the absorbing regions. The normalized nonradiative decay rate is defined to the ratio of the nonradiative loss to the radiative loss of a dipole in a freespace homogeneous medium. Figure 1 shows a model structure used for the calculation. r1 is the radius of a NaYF4 core, and t1 and t2 are the thicknesses of a SiO2 shell and a Au shell, respectively. A dipole is placed at a distance r from the center of the sphere. In the calculation, r1 and t1 are fixed to 70 and 15 nm, respectively, and t2 is changed from 0 to 55 nm. The enhancement factors of the radiative and nonradiative decay rates of a dipole at position r (0−70 nm) with respect to the radiative decay rates in vacuum are calculated. In the calculation, an average was performed over different dipole orientations to obtain the rates for randomly oriented dipole sources. The refractive index (n) of a NaYF4 core used for the calculation is 1.45.25 The same value of n is used for a SiO2 shell. Therefore, the actual structure used for calculations consists of a core with the radius of 85 nm (n = 1.45) and a Au

Figure 2. Contour plots of normalized radiative decay rates. The ordinate is the position of a dipole, and the abscissa is wavelength. The thickness of a Au shell is (a) 0 nm, (b) 3 nm, (c) 5 nm, (d) 10 nm, (e) 15 nm, (f) 20 nm, (g) 25 nm, (h) 35 nm, (i) 55 nm. The decay rate is a linear scale.

changed from 0 to 55 nm in panels a to i. The ordinate is the position of a dipole, and the abscissa is the emission wavelength. Note that the scale of the decay rate is a linear scale and is the same for all the graphs. We can see that the radiative rate depends strongly on the position of a dipole and a wavelength. It also depends strongly on the Au shell thickness. When the Au shell thickness is 5 nm (Figure 2c), two modes appear. The mode around 1,000 nm extends uniformly within a core. This mode can be assigned to the dipole mode. The other one around 850 nm, which is localized near the surface of the core, can be assigned to the quadrupole mode. The dipole and quadrupole modes shift to shorter wavelength and become weak as the Au shell thickness increases. When the thickness 1114



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thickness). Figures 4a and 4b show the averaged radiative and nonradiative rates, respectively, for composite nanoparticles

reaches 55 nm (Figure 2i), the radiative rate is almost zero in the whole wavelength range. This is simply due to the fact that photons cannot escape from the sphere because of the thick Au shell. Figure 3 shows nonradiative decay rates of a dipole placed in a core. The scale of the decay rate in Figure 3a is the same as

Figure 3. Contour plots of normalized nonradiative decay rates. The ordinate is the position of a dipole, and the abscissa is wavelength. The thickness of a Au shell is (a) 0 nm, (b) 3 nm, (c) 5 nm, (d) 10 nm, (e) 15 nm, (f) 20 nm, (g) 25 nm, (h) 35 nm, (i) 55 nm. The decay rates are linear scale in panel a and logarithmic scale in panels b−i.

that in Figure 2. The scale of other graphs (Figure 3b−i) is changed because of the large variation of the nonradiative decay rate. Furthermore, logarithmic scale is used in Figure 3b−i to see the dependence more clearly. In Figure 3a, the nonradiative rate is zero in the whole range because of the absence of absorbing media. When a Au shell is formed, the nonradiative decay rate appears. Similar to the radiative rate in Figure 2, the nonradiative rate depends strongly on the position of a dipole, the wavelength, and the shell thickness. From the comparison between the radiative (Figure 2) and nonradiative (Figure 3) rates of the same structures, the longest wavelength mode in Figure 3b−f, which is localized near the surface of the core, is assigned to the quadrupole mode and those at the shorter wavelength to higher-order modes. The dipole mode is much weaker than the quadrupole mode and is not clearly seen in Figure 3. The large nonradiative rate below 500 nm, being prominent for the structure with a thick Au shell, is due to the interband transition of a Au shell. In core−shell nanoparticles prepared in this work, Er and Yb are doped uniformly in a NaYF4 core. Therefore, we average the data in Figures 2 and 3 within a NaYF4 core to obtain the wavelength dependence of radiative and nonradiative decay rates of core−shell nanoparticles. For the calculation, dipoles are assumed to be distributed within a NaYF4 core with the diameter of 70 nm, and not exist in a SiO2 shell (15 nm in

Figure 4. (a) Normalized radiative and (b) nonradiative decay rates as a function of wavelength obtained by averaging the data in Figures 2 and 3, respectively. (c) Enhancement factors of radiative (γr) and nonradiative (γnr) decay rates for a NaYF4 nanoparticle with a 20 nm thick Au shell obtained by dividing the data in panels a and b by those of a NaYF4 nanoparticle with a SiO2 shell and without a Au shell.

with different Au shell thicknesses. We can see peaks due to SP modes. The peaks shift to shorter wavelength with increasing the shell thickness. Although not shown here, without a Au shell, the spectra are very flat without any distinctive structures. Figure 4c compares the radiative and nonradiative rates when the Au shell thickness is 20 nm. To see the effects of a Au shell on the rates, they are divided by the radiative rate of a nanoparticle without a Au shell. The ordinate is thus the “enhancement factor”. In the case of Figure 4c, the enhancement factor of the radiative rate is larger than that of the nonradiative rate above 600 nm. This suggests that the enhancement of PL quantum efficiency is possible by the formation of a Au shell, if the structural parameters are properly chosen. 2.2. Enhancement of Excitation Field. To estimate the enhancement factor of the incident electric field by the formation of a Au shell, the absorption (Cabs) and scattering 1115



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for the calculation of the electric field within a core. Figure 5c shows the calculated enhancement factor of the square of the incident field at 975 nm within a core as a function of a Au shell thickness. With increasing the Au thickness, the incident field increases and reaches around 100 when the Au shell thickness is 7.5 nm. Further increase of the thickness results in the decrease of the incident electric field.

(Csca) cross sections of core−shell nanoparticles are calculated by a recurrence algorithm developed in ref 27. The results are shown in Figures 5a and 5b for Cabs and Csca, respectively. To

3. PREPARATION AND CHARACTERIZATION OF NAYF4 CORE GOLD SHELL NANOPARTICLES NaYF4 nanoparticles doped with Er (1 mol %) and Yb (20 mol %) (NaYF4:Er,Yb) were synthesized according to a hydrothermal process.31 6.4 mmol of sodium fluoride (NaF) and ethylenediaminetetraacetic acid (EDTA) (1.6 mmol) were dissolved in 10 mL of deionized water. Ten milliliters of deionized water containing Er(NO3)3·6H2O (0.016 mmol), Yb(NO3)3·nH2O (0.32 mmol), and Y(NO3)3·nH2O (1.264 mmol) was mixed into the previous EDTA solution. The mixture was then transferred into a Teflon vessel (50 mL), and deionized water was added until the total amount of the solution became 33 mL. After the pH value of the solution was adjusted to 3 by use of HNO3 solution, the vessel was tightly sealed and treated at 200 °C for 20 h and naturally cooled down to the room temperature. The NaYF4 nanoparticles were obtained by filtering the solutions using a membrane. The nanoparticles were heated at 300 °C for 2 h in a N2 gas atmosphere. A transmission electron microscope (TEM) image of a NaYF4:Er,Yb nanoparticle is shown in Figure 6a.1. The average diameter of the nanoparticles was about 140 nm. The surface of NaYF4:Er,Yb nanoparticles is coated by a silica shell to avoid direct contact with a metal shell. A silica shell was formed by a Stöber method following the procedure described in ref 32. 60 mg of NaYF4:Er,Yb nanoparticles was dispersed in a mixture of 3 mL of water and 40 mL of ethanol by ultrasonication. After the addition of 0.4 mL of ammonia (28 wt %) and 1.2 mL of tetraethyl orthosilicate (TEOS), the mixture was stirred for 1 h. Figure 6a.2 shows a TEM image of a NaYF4:Er,Yb nanoparticle with a silica shell. The average thickness of the shells was about 15 nm. A Au shell was formed by a seed-mediated electroless coating method.22,29,33 NaYF4:Er,Yb nanoparticles with a silica shell were functionalized with 3-aminopropyltrimethoxysilane (APTMS) overnight. Amine-terminated nanoparticles were decorated with small gold colloids (2−3 nm) prepared by the method described in ref 34. Figure 6a.3 shows a TEM image of a nanoparticle with Au seeds. Small Au nanoparticles are densely attached on the surface. Continuous gold shells are grown by reducing Au from a 1% solution of HAuCl4 onto the attached small Au particles in the presence of formaldehyde. Figures 6a.4 and 6a.5 show TEM images of the composite nanoparticles after the process. The amount of the HAuCl4 solution is different between Figures 6a.4 and 6a.5. In Figure 6a.4, Au nanocrystals are grown, but still a continuous shell is not formed. On the other hand, in Figure 6a.5, a continuous Au shell is formed. The Au shell thickness is about 20 nm. Figure 6b shows an electron diffraction pattern of NaYF4:Er,Yb nanoparticles with Au shells. All diffraction rings are assigned to NaYF4 (JCPDS: 06-0342) and Au (JCPDS: 04-0784). Figures 6c and 6d show photos and optical extinction spectra of aqueous solutions in which NaYF4 nanoparticles are dispersed. Panels 1−5 correspond to those in Figure 6a. The extinction in the ultraviolet (UV) to blue range is mainly due to Rayleigh scattering by NaYF4:Er,Yb nanoparticles. When Au

Figure 5. Calculated (a) absorption (Cabs) and (b) scattering (Csca) cross sections of a NaYF4 nanoparticle with a different thickness Au shell. Cabs is normalized by the volume of a composite nanoparticle (V) and Cabs by V2. (c) Square of the electric field inside a NaYF4 nanoparticle core as a function of a Au shell thickness obtained by a quasi-static approximation.

compare Cabs and Csca of particles with different sizes, Cabs and Csca are normalized by the volume (V) and the square of the volume, respectively.28 We can see several peaks due to SP resonances. For example, in the case of Au thickness of 10 nm, the broad band around 900 nm is assigned to the dipole SP mode, while the sharper one around 700 nm is the quadrupole SP mode.22,29,30 These modes shift to shorter wavelength and become weak with increasing the Au shell thickness. In Figure 5a, the absorption below 500 nm is due to interband transitions of the Au shell. In the present experimental work, we use 975 nm light for the excitation of the upconversion PL. Figures 5a and 5b indicate that, at the wavelength, the lowest order dipole SP mode dominates the optical response. Therefore, quasi-static approximation, which ignores retardation effects, can be applied 1116



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Figure 7. (a) Energy state structures of 4f shells of Yb3+ and Er3+. (b) Typical upconversion PL spectrum of NaYF4 nanoparticles with SiO2 shells in an aqueous solution excited at 975 nm. Three PL bands can be assigned to the transitions from the 2H11/2, 4S3/2, and 4F9/2 states to the 4I15/2 state of Er3+.

Figure 6. (a) TEM images of (1) a NaYF4:Er,Yb nanoparticle, and that (2) with a SiO2 shell, (3) with a SiO2 shell and Au seeds, (4) with a SiO2 shell and Au nanoparticles, and (5) with a SiO2 and a Au shell. Inset in each panel is a schematic illustration of the structure. (b) Electron diffraction pattern of NaYF4:Er,Yb nanoparticles with a SiO2 shell and a Au shell. (c) Photographs and (d) optical extinction spectra of aqueous solutions containing NaYF4:Er,Yb nanoparticles with different shell structures.

around 520, 540, and 670 nm are assigned to the transitions from the 2H11/2, 4S3/2, and 4F9/2 states to the ground 4I15/2 state of Er3+, respectively. In the present experimental procedure, the amount of NaYF4 nanoparticles in solution is not precisely controlled, and thus the comparison of the PL intensity between samples is difficult. Therefore, in this work, we measure PL of individual nanoparticles placed on a silica substrate by spin-coating of diluted solutions, and compare the average PL intensities between samples with different shell structures. The nanoparticles on silica substrates were excited by 975 nm light, and upconversion PL images were recorded by using a fluorescence microscope with a cooled CCD camera. To obtain PL intensities of the green (2H11/2 to 4I15/2 and 4S3/2 to 4I15/2 transitions) and red (4F9/2 to 4I15/2 transition) bands separately, band-pass filters were used. Although the green PL consists of two bands, main contribution comes from the 4S3/2 to 4I15/2 transition at 540 nm. To confirm that the measured PL arises from isolated individual nanoparticles, the PL images were compared with scanning electron microscope images. Figure 8 shows PL intensities of NaYF4 nanoparticles with different kinds of shell structures. The numbers, 1−5, correspond to those in Figure 6a. The symbols are the average intensities of 5 to 7 individual nanoparticles, and the error bars are the maximum and minimum intensities. The filled squares correspond to the green PL, while the circles correspond to the red PL. The intensities are normalized to those of the samples with SiO2 shells. We can see that the upconversion PL intensities depend strongly on the shell structures, although the data are scattered in wide ranges. First, by the formation of a

seeds are attached on the surface of NaYF4:Er,Yb nanoparticles (Figures 6c.3 and 6d.3), the color becomes yellowish and a small bump due to light absorption by localized SPs of Au seeds appears in the extinction spectrum. When Au seeds grow to Au nanoparticles (Figures 6c.4 and 6d.4), the color becomes bluish and the SP resonance shifts to longer wavelength and becomes broad. The formation of a continuous Au shell results in further shift of the SP resonance wavelength and the broadening of the spectrum (Figures 6c.5 and 6d.5).

4. UPCONVERSION PL PROPERTIES Figure 7a shows the energy state diagrams of 4f shells of Yb3+ and Er3+. Figure 7b shows an upconversion PL spectrum of NaYF4 nanoparticles with SiO2 shells in an aqueous solution. The excitation source is a 975 nm laser diode, and the upconversion PL is detected by a liquid-N2 cooled charge coupled device (CCD). The excitation wavelength corresponds to the transitions from the 2F7/2 to 2F5/2 states of Yb3+ and from the 4I15/2 to 4I11/2 states of Er3+. Because of the larger absorption cross-section of Yb3+ than that of Er3+ and larger amount of Yb doped, the excitation light is mainly absorbed by Yb3+ and the excitation energy is transferred to Er3+. Three upconversion PL bands can be seen in Figure 7b. The bands 1117



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Figure 8. Enhancement factors of the PL intensity for NaYF4:Er,Yb nanoparticles with different shell structures. The symbols correspond to average intensity obtained from 5 to 7 individual isolated nanoparticles and the error bars to the maximum and the minimum of the intensities.

Figure 9. Decay curves of upconversion PL at (a) 540 nm (4S3/2 to 4 I15/2 transition) and (b) 670 nm (4F9/2 to 4I15/2 transition) for NaYF4:Er,Yb nanoparticles with different shell structures.

SiO2 shell, the intensity increases 2−3 times for both the green and red PL. This is due to the activation of Er3+ near the surface of NaYF4 particles by surface termination. The PL intensities do not change so much when Au seeds are formed on the surface of NaYF4 nanoparticles. The growth of Au nanoparticles from the seeds results in significant decrease in the intensities. This is considered to be due to reabsorption of emitted light by Au nanoparticles on the surface of NaYF4 nanoparticles. Interestingly, the intensities, especially that of the red PL, recover when continuous shells are formed. Compared to the PL intensities of the sample with a SiO2 shell (Figure 6a.2), the intensity of the red PL is 1.36 times enhanced, while that of the green PL is decreased to 0.74. Although the intensity values are scattered and the enhancement is small, the observation of the upconversion PL enhancement by the formation of a Au shell is an important step to achieve efficient upconversion PL from a Dc-Ms structure. Figures 9a and 9b show PL decay curves detected at the wavelengths of the 4F9/2 to 4I11/2 transition (670 nm) and the 4 S3/2 to 4I11/2 transition (540 nm), respectively. Since the PL signals of individual nanoparticles are not strong enough to measure the decay curves, the decay curves are obtained for aqueous solutions containing NaYF4:Er,Yb nanoparticles. The excitation source was modulated 975 nm light from a laser diode. The time-resolved PL signals were obtained using a single monochromator equipped with an image-intensified CCD with the minimum gate width of 2 ns. In Figure 9, decay curves of the samples with different shell structures (panels 2−5 in Figure 6a) are shown. The lifetimes are the longest for the sample with a SiO2 shell. They become short when Au seeds, Au nanoparticles, and Au shells are formed on a SiO2 shell. The shortening of the lifetime is considered to be mainly due to excitation of SPs. The lifetimes estimated by fitting the decay curves by a stretched exponential function are shown in the figure. The enhancement factor of the decay rates (γexp) by the formation of a Au shell with respect to those of the sample with a SiO2 shell is summarized in Table 1 for the green and red PL.

5. DISCUSSION From the shortening of the PL lifetime and calculated enhancement factors of the radiative and nonradiative rates, we discuss the mechanism of PL enhancement and quenching by the formation of a Au shell. As a reference to discuss the enhancement factor, we chose NaYF4:Er,Yb nanoparticles with SiO2 shells (Figure 6a.2), because the chemical environment of Er3+ and Yb3+ in a NaYF4:Er,Yb nanoparticle with a SiO2 shell is considered to be the same as those in a NaYF4:Er,Yb nanoparticle with a SiO2 and a Au shell. For the analysis, the diameter of a NaYF4 core, the thickness of a SiO2 shell, and the thickness of a Au shell are assumed to be 70 nm, 15 nm, and 20 nm, respectively. The PL quantum efficiency of NaYF4 nanoparticles with SiO2 shells is expressed as Q0 = wr/(wr + wnr), where wr and wnr are the radiative and nonradiative decay rates, respectively. It is reasonable to assume that the nonradiative rate (wnr), which is determined by the crystallinity, surface termination, etc. of NaYF4:Er,Yb nanoparticles, is not affected by the change of the electromagnetic environment by the formation of a Au shell and only the radiative rate is affected. Therefore, PL quantum efficiency is modified to be Q = (wr × γr)/[wr × (γr + γnr) + wnr], where γr and γnr are the enhancement factors of the radiative and nonradiative decay rates, respectively, by the formation of a Au shell obtained from Figure 4c. The values of γr and γnr when the Au shell thickness is 20 nm are shown in Table 1. By combining these equations, the enhancement factor of the luminescence quantum efficiency is Z(ωflu) =

(wr + wnr)γr Q = Q0 wr(γr + γnr) + wnr

(2)

and that of the luminescence decay rate is wr(γr + γnr) + wnr (wr + wnr) 1118

(3)



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Table 1. Parameters Obtained by Calculations and Experimentsa 4

S3/2 F9/2

4

a

λ (nm)

γexpb

γr

γnr

Q0 (%)

Q (%)

Z(ωflu)

Yexpb

nb

|L(ωexc)|2

540 660

1.26 1.33

2.35 4.71

5.20 3.43

3.82 4.41

7.18 15.8

1.88 3.58

0.74 1.36

2.1 2.0

0.63 0.61

The symbols are defined in the main text. bExperimentally determined values.

placed in a core reveals that larger enhancement of the radiative rate than the nonradiative rate is possible when the structural parameters are properly chosen. Furthermore, the analyses of the absorption and scattering cross sections indicate that strong enhancement of an incident electric field within a core is possible when the shell thickness is in a proper range. The upconversion PL properties of core−shell nanoparticles consisting of an Er and Yb codoped NaYF4 core and a Au shell were prepared and the PL properties were studied. We found that the upconversion PL in the red region is slightly enhanced by the formation of Au shell and that in the green region is slightly quenched. From detailed analyses based on PL decay rates and calculated radiative and nonradiative rates, the enhancement of the quantum efficiency was demonstrated. On the other hand, the analyses revealed that the incident electric fields are attenuated by the formation of a Au shell in the present structural parameters. The theoretical and experimental studies performed in this paper indicate that the Dc-Ms structure is a promising system to obtain strong upconversion PL enhancements. However, the PL property of the system is very sensitive to the structural parameters and precise design and fabrication of the structure is indispensable.

Equation 3 corresponds to the experimentally obtained enhancement factors of the decay rates (γexp) in Table 1. From the γexp and calculated γr and γnr, Q0 and Q can be calculated. The calculated values of Q0 and Q and that of Z(ωflu) are summarized in Table 1. In Table 1, we can see that the quantum efficiencies are enhanced in both PL bands. The enhancement factor of the quantum efficiency of the red PL is about twice larger than that of the green PL. Although the PL quantum efficiency is enhanced a few times by the formation of a Au shell, the experimentally obtained enhancement factor of the PL intensity (Figure 8 and Yexp in Table 1) is not as large as that of the quantum efficiency. Furthermore, in the case of the green PL, the intensity is decreased to 0.74, although the quantum efficiency is enhanced 1.88 times. It should be noted here that the definition of the quantum efficiency in this work is the ratio of absorbed and emitted photons by a NaYF4:Er,Yb core. The smaller enhancement factors of the PL intensities than those of the PL quantum efficiencies suggest that the excitation efficiency is decreased by the formation of a Au shell. From eq 1, upconversion PL intensity is determined by the enhancement factors of the incident field and the quantum efficiency and the number of photons necessary for the excitation (n). The value of n is obtained from the slope of the PL intensity vs excitation power plot (Figure S1 in the Supporting Information). The experimentally obtained values of n are shown in Table 1. By modifying eq 1 to |L(ωexc)|2 = [Yexp/Z(ωflu)]1/n, the enhancement factor of the square of the incident field (|L(ωexc)|2) is calculated. The results are summarized in Table 1. In both PL bands, it is around 0.6. The similar values of |L(ωexc)|2 between the two bands support the validity of the analysis. The fact that |L(ωexc)|2 is smaller than 1 implies that the excitation efficiency is decreased by the formation of a Au shell. In Figure 5c, the electric field intensity within a core estimated by a quasi-static approximation is shown. In the figure, the enhancement factor of around 0.6 corresponds to the Au shell thickness of about 17 nm. This value is very close to that estimated from TEM observations (about 20 nm). This coincidence also supports the validity of the present analysis. Within the present limited structural parameters, i.e., the NaYF4 core diameter and the SiO2 shell thickness are fixed and only the Au shell thickness is changed, the condition to enhance both the quantum efficiency and the incident electric field may be very narrow. To achieve the enhancements of both and to obtain strong enhancement of the PL intensity, the core diameter, the spacer thickness, and the shell thickness should be optimized. Increasing spacer thickness may be helpful because the nonradiative rate, which is the largest on the surface of a core, can be suppressed.

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ASSOCIATED CONTENT

S Supporting Information *

Figure S1 depicting upconversion PL intensity as a function of excitation power for NaYF4:Er,Yb nanoparticles with SiO2 shells. 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 is partly supported by research grant from Izumi Science and Technology Foundation. REFERENCES

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6. SUMMARY Upconversion PL properties of composite nanoparticles consisting of a rare-earth-doped dielectric core and a metal shell were studied theoretically and experimentally. The calculation of radiative and nonradiative decay rates of a dipole 1119



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Upconversion Luminescence of Er and Yb Codoped NaYF4

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