Article pubs.acs.org/JPCC
Upconversion Luminescence of Rare-Earth-Doped Y2O3 Nanoparticle with Metal Nano-Cap Kaoru Yamamoto, Minoru Fujii,* Shunji Sowa, Kenji Imakita, and Kanna Aoki Department of Electrical and Electronic Engineering, Graduate School of Engineering, Kobe University, Rokkodai, Nada, Kobe 657-8501, Japan S Supporting Information *
ABSTRACT: The upconversion property of an individual composite nanoparticle consisting of a metal (Ag) nanocap and a rare-earth doped upconversion nanoparticle (Er- and Yb-doped Y2O3 nanoparticle) was studied. The structural parameters of the composite nanoparticle were chosen so that the resonant wavelengths of the electric dipole and magnetic dipole surface plasmon modes of a nanocap coincide with the upconversion luminescence peaks of Er3+. Strong modification of the upconversion spectrum was observed by the formation of a Ag nanocap. Upon excitation at 980 nm, the green (∼550 nm) and red (∼670 nm) peaks were on average 23 and 48-fold, respectively, enhanced. The strong modification of the spectral shape, i.e., the intensity ratio of the green to red luminescence, suggests that the enhancement of radiative decay rates by the two surface plasmon modes is mainly responsible for the upconversion enhancement.
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INTRODUCTION Upconverison is a nonlinear process which re-emits a photon at a shorter wavelength by absorbing more than two photons successively at longer wavelengths via long- lived intermediate energy states. Recently, rare-earth doped upconversion nanomaterials, e.g., nanorod1−3 and nanoparticles,3−5 have been extensively studied because of their potential applications as autofluorescence-free fluorescent-labeling agents in bioimaging6−9 and wavelength conversion layers in solar cells.10,11 However, because of the small absorption cross sections and emission rates of rare-earth ions due to the parity forbidden intra4f shell transitions, the upconversion efficiency is usually not high enough for practical applications. A promising approach to overcome the problem is utilizing localized surface plasmon resonances of metal nanostructures. Very large enhancement of upconversion luminescence has been reported for rare-earth doped upconversion films on which several kinds of metal nanostructures are formed.12−17 Aisaka et al.12 reported 220-fold increase in the upconversion of Er-doped Al2O3 thin films by Ag islands. An even larger enhancement factor of 450-fold was achieved by Verhagen et al.13 for 1480 to 980 nm upconversion in Er-implanted sapphire substrates by the formation of Au hole arrays. Enhancement of upconversion luminescence as large as 310-fold was also observed for a disk-coupled dots-on-pillar antenna array on which Er- and Yb-doped NaYF4 nanocrystals are placed.14 A new class of upconversion materials recently attracting much attention is composite nanoparticles consisting of upconversion nanoparticles and metal nanostructures.18−25 Composite nanoparticles with different geometries i.e., upconversion nanoparticles with Ag nanoparticle cores,22 those decorated by Au nanoparticles,20,21 Ag nanoparticles23,24 and Au nanorods,25 and those with continuous Au shells,18−21 have been reported. However, the upconversion enhancement © XXXX American Chemical Society
factors achieved by these composite structures are relatively small and in some cases, quenching of the upconversion luminescence is reported.18,20 To our knowledge, the largest enhancement factor reported is 27-fold in the case of Au nanorod decoration.25 In this work, we study upconversion composite nanoparticles consisting of upconversion nanoparticles and metal nanocaps. A metal nanocap, sometimes called semishell or half-shell, has very specific surface plasmon resonance properties.26−33 It has a magnetic dipole plasmon mode due to oscillating current loop in addition to an electric dipole mode.26,29,30 Both modes have strongly enhanced local electric fields near a nanocap rim.31,32 The resonance wavelengths of the modes can be controlled in wide ranges by the thickness and the coverage.31 The high tunability of the resonance wavelengths allows us to maximize spectral overlap between the surface plasmon resonance bands and absorption and emission peaks of rare-earth ions, and thus strong enhancement of upconversion is expected. Despite the very promising structure for the strong upconversion enhancement, upconversion nanoparticles with metal nanocaps have not been investigated in detail. In this paper, we produce Er- and Yb-codoped Y2O3 nanoparticles (Y2O3:Yb,Er nanoparticles) with Ag nanocaps and study the upconversion properties. We measure light scattering and upconversion luminescence spectra of individual nanoparticles and discuss the role of a metal nanocap for the upconversion enhancement. We demonstrate that upconversion around 550 and 670 nm are enhanced on average 23- and 48-fold, respectively, by the formation of Ag nanocaps. Received: August 20, 2014 Revised: November 10, 2014
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DOI: 10.1021/jp508443g J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
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EXPERIMENTAL METHOD Y2O3:Yb,Er nanoparticles were synthesized by a homogeneous precipitation method.34 Twelve g of (NH3)2CO, 187 mg of Y(NO3)3·6H2O, 2.3 mg of Er(NO3)3·6H2O, 2.3 mg of Yb(NO3)3·6H2O (Wako Chemicals) were dissolved in 50 mL of deionized water. After stirring, the solution is heated to 100 °C and kept for 1 h and naturally cooled down to the room temperature. Y2O3:Yb,Er nanoparticles were obtained by filtering the solution using a membrane filter. They were then annealed at 850 °C for 2 h. Finally, to suppress the quenching of upconversion due to electron−hole excitation of Ag nanocaps, a thin SiO2 shell was formed on the surface of nanoparticles by the Stöber method.35 Figure 1a shows transmission electron microscope (TEM) (H7000, Hitachi) images of Y2O3:Yb,Er nanoparticles before
The procedure for the formation of Ag nanocaps on Y2O3:Yb,Er@SiO2 nanoparticles is schematically shown in Figure 1c.29 A fused quartz plate was immersed in a 1 wt % ethanolic solution of poly-4-vinylpyridine (PVP, Mw = 160,000, Aldrich) for 2 h to functionalize the surface.29 It was then rinsed with ethanol to remove excess PVP and heated at 120 °C for 2 h. Y2O3:Yb,Er@SiO2 nanoparticles were placed on the substrate by spin coating the aqueous solution (0.2 g/L). A Ag layer of 10 to 50 nm in thickness was deposited on the particles by vacuum deposition. A vacuum chamber was first evacuated to 5.0 × 10−5 Torr and Ag was evaporated by resistive heating. The deposition rate was about 3 nm/min. To isolate Y2O3:Yb,Er@SiO2 nanoparticles with Ag nanocaps from the substrate, a cured polydimethylsiloxane resin (PDMS, SYLGARD-184, Dow Corning) (1 × 1 cm2) film was stamped onto the substrate surface and quickly peeled off from the substrate. Since van der Waals force between PDMS and Y2O3:Yb,Er@ SiO2 nanoparticles is stronger than the initial binding between the Y2O3:Yb,Er@SiO2 nanoparticles and the substrate, the PDMS films containing embedded Y2O3:Yb,Er@SiO2 nanoparticles with Ag nanocaps were obtained. Figure 1d shows a TEM image of Y2O3:Yb,Er@SiO2 with a Ag nanocap when the 20 nm thick Ag is deposited. We can clearly see the formation of a Ag nanocap. Figure 1e shows an experimental setup for the measurement of scattering and upconverson images and spectra of individual nanoparticles. For the measurements of scattering images and spectra, white light from a halogen lamp was focused onto the sample using an objective (NA = 0.9, 100×) and the scattered light was collected by the same objective. Scattered light in the region of 2 μm in diameter was selected by a spatial filter and focused onto the entrance slit of a monochromator (SpectraPro-300i, Acton Research Corp.) and detected by a liquid-N2 cooled charge coupled device (CCD) (Princeton Instruments). Upconversion images and spectra were obtained by the same setup. The excitation source was a 980 nm semiconductor laser with the power of 80 mW (9.6 kW/cm2) at the sample.
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RESULTS AND DISCUSSION Figure 2a shows a typical scattering spectrum of a single Y2O3:Yb,Er@SiO2 nanoparticle. The scattering intensity increases monotonously to the shorter wavelength. This is due to Mie scattering by the Y2O3:Yb,Er@SiO2 nanoparticle. On the other hand, two broad peaks appear in the spectrum of a Y2O3:Yb,Er@SiO2 nanoparticle with a Ag nanocap (Figure 2b). This is a typical scattering spectrum of a Ag nanocap.30 The peak around 583 nm is assigned to the electric dipole surface plasmon mode and that around 690 nm to the magnetic dipole surface plasmon mode.29 We measured the spectra of more than 30 single nanoparticles and majority of them exhibited the characteristic spectra with two peaks similar to Figure 2b, although the intensity ratio was different between particles due to slight variation of the shape of nanocaps.30 Others had distorted spectra such as triple peaks due probably to improper formation of nanocaps. The peak wavelengths of the electric dipole and magnetic dipole modes of the nanoparticles with dual scattering peaks are shown in Figure 2c. Although the peak wavelengths vary from nanoparticle to nanoparticle, the electric dipole mode has the resonance around 500−650 nm and the magnetic dipole mode around 630−800 nm. These wavelengths coincide with the green and red
Figure 1. (a) TEM image and (b) typical upconversion PL spectrum of Y2O3:Yb,Er nanoparticles excited at 980 nm. (c) Fabrication procedure of Y2O3:Yb,Er nanoparticles with Ag nanocaps. (d) Typical TEM image of Y2O3:Yb,Er nanoparticle with SiO2 spacer layer and Ag nanocap (e) Experimental setup for the measurements of scattering and upconversion spectra of individual nanoparticles.
the formation of SiO2 shells. The average diameter obtained from the TEM images is 104 nm with the standard deviation of 12.2 nm. The size distribution and the electron diffraction pattern are shown in the Supporting Information (Figure S1, parts a and b). A TEM image of a nanoparticle with a SiO2 shell (Y2O3:Yb,Er@SiO2 nanoparticles) and the size distribution after the SiO2 shell formation are also shown in the Supporting Information (Figure S1, parts c and d). The shell thickness is about 8 nm. Figure 1b shows a typical upconversion spectrum of Y2O3:Yb,Er@SiO2 nanoparticles placed on a fused quartz plate. A green emission peak around 550 nm and a weaker red emission peak around 670 nm correspond to transitions from the 4S3/2 and 4F9/2 states, respectively, to the 4I15/2 ground state of Er3+. B
DOI: 10.1021/jp508443g J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 3. (a) Scattering and upconversion images of a single Y2O3:Yb,Er@SiO2 nanoparticle without Ag nanocap. The corresponding scattering and upconversion spectra are also shown. (b) Scattering and upconversion images of a single Y2O3:Yb,Er@SiO2 nanoparticle with a Ag nanocap (30 nm). The corresponding scattering and upconversion spectra are also shown. Scale bars are 1 μm.
Figure 2. Scattering spectra of single Y2O3:Yb,Er@SiO2 nanoparticles (a) without and (b) with a Ag nanocap. Two surface plasmon resonance peaks at 583 and 690 nm are assigned to electric and magnetic dipole modes, respectively. (c) Peak wavelengths of electric (■) and magnetic (●) dipole modes of single nanoparticles.
emission peaks, respectively, of Y2O3:Yb,Er nanoparticles (Figure 1b). The top images in Figure 3a show typical scattering and upconversion images of an identical single Y2O3:Yb,Er@SiO2 nanoparticle. Although a green scattering image can clearly be seen, the upconversion image is scarcely observed. The top images in Figure 3b show typical scattering and upconversion images of a single Y2O3:Yb,Er@SiO2 nanoparticle with a Ag nanocap. The scattering image changes from greenish to reddish by the formation of a Ag nanocap and the upconversion image becomes much brighter and can easily be seen in the same setup as that in Figure 3a. In the present experimental procedure, it is not possible to compare images of same nanoparticles with and without Ag nanocaps. However, we could clearly recognize the effects of Ag nanocaps in upconversion images due to the significantly different brightness. Scattering and upconversion spectra corresponding to the images are shown in Figure 3, parts a and b. We can see that Ag nanocaps not only enhance the upconversion intensity, but also change the spectral shape. Without a Ag nanocap, green luminescence around 550 nm is dominant and the ratio of the red (∼670 nm) to the green luminescence is about 0.29. With a Ag nanocap, the ratio increases to 0.61. Peak intensities of the green and red luminescence obtained for more than 30 single nanoparticles without and with Ag nanocaps are plotted in Figure 4, parts a and b, respectively. The ordinates are logarithmic scales. Although the intensities are largely scattered, we can clearly see the strong enhancement by the formation of Ag nanocaps. The average enhancement factors are 23-fold for
Figure 4. (a, b) Green and Y2O3:Yb,Er@SiO2 nanoparticles nanocaps. (c, d) Intensity ratio Y2O3:Yb,Er@SiO2 nanoparticles nanocaps. The data of more than
red upconversion intensities of (a) without and (b) with Ag of red to green upconversion of (c) without and (d) with Ag 30 single nanoparticles are shown.
the green luminescence and 48-fold for the red one. The difference in the enhancement factor between the green and red emission appears in the intensity ratio. Figure 4c and d show the red to green intensity ratio of Y2O3:Yb,Er@SiO2 nanoparticles without and with Ag nanocaps, respectively. C
DOI: 10.1021/jp508443g J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C Without a Ag nanocap, the ratio is around 0.34 and the scattering of the data is relatively small. This suggests that the local environment of Er ions in Y2O3:Yb,Er@SiO2 nanoparticles is uniform. On the other hand, the ratio increases and scatters in a wide range when Ag nanocaps are formed. This suggests that slight difference of the shape of Ag nanocaps strongly affect the red to green intensity ratio. The average ratio increases to about 0.70 and in some particles it exceeds 1.0. The enhancement factor of upconversion can be expressed as Y = |L(ωexc)|2nZ(ωflu), 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 radiative rate, and n is the number of photons involved in the excitation of the luminescence. In the present samples, n estimated by excitation power dependence of the luminescence intensities is 1.9−2.0 in green and 1.5−1.7 in red luminescence (Figure S2 in the Supporting Information).36,37 Therefore, if the upconversion enhancement is mainly due to that of the incident electric field, stronger enhancement is expected in the green luminescence than the red one. This is in contradiction with the experimental results. The larger enhancement factor in the red luminescence and relatively small light scattering intensity at the excitation wavelength (980 nm) suggest that the enhancement of the radiative decay rates is a major origin of the observed upconversion enhancement. One of the experimental evidence of the radiative rate enhancement is the observation of the shortening of the luminescence lifetime. We observed slight shortening of the lifetime by nanocap formation (see Supporting Information, Figure S3). However, the change of the lifetime is very small compared to the strong enhancement of the upconversion intensity. This may be explained by small quantum efficiency of the upconversion, typically
