Spectral Control of Plasmonic Emission Enhancement from Quantum

May 26, 2010 - Japanese Journal of Applied Physics 2016 55 (1S), 01AE11 ... Martin Jahn , Sophie Patze , Izabella J. Hidi , Richard Knipper , Andreea ...
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Spectral Control of Plasmonic Emission Enhancement from Quantum Dots near Single Silver Nanoprisms Keiko Munechika, Yeechi Chen, Andreas F. Tillack, Abhishek P. Kulkarni, Ilan Jen-La Plante, Andrea M. Munro, and David S. Ginger* Department of Chemistry, University of Washington, Seattle, Washington 98195-1700 ABSTRACT The near-field effects of plasmonic optical antennas are being explored in applications ranging from biosensors to solar cells. We demonstrate that photoluminescence emission enhancement from CdSe quantum dots (QDs) can be obtained in the absence of any excitation enhancement near single silver nanoprisms. The spectral dependence of the radiative and nonradiative decay rate of the QDs closely follows the silver nanoparticle plasmon scattering spectrum. Using both experiment and theory we show that, in the absence of excitation enhancement, the ratio of radiative to nonradiative decay rate enhancement is proportional to the silver nanoparticle scattering efficiency. These results provide guidelines both for separating excitation and emission enhancement effects in sensing and device applications and for tailoring emission enhancement effects using plasmonic nanostructures. KEYWORDS Localized surface plasmon resonance, metal nanoparticles, plasmon-enhanced emission, far-field scattering, fluorescence enhancement, photoluminescence lifetime

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metal nanostructures is thus crucial to the success of plasmonic applications. Here, we focus on the spectral dependence of emission enhancement factors near single metal nanoparticles. Emission enhancement effects have been studied on a variety of nanostructures including e-beam fabricated Au or Ag nanoarrays,18–22 random colloidal aggregates,23 nanoshells,24 spherical silver nanoparticles,25,26 and silver island films.27 In particular, the importance of spectral overlap between the absorption and emission spectra of the chromophores and the plasmonic antenna has been emphasized by both our group28 and others.24,29 However, separating excitation and emission factors and systematic examination of the spectral dependence of the emission enhancement has remained a challenge. Relative contributions are often estimated from simulations,30,31 but experiments that completely isolate one factor or the other have been difficult to achieve. Even when we have been able to experimentally measure excitation enhancement factors near single metal nanoparticles, they have been coupled to some degree of emission enhancement.17 Here, we demonstrate a clean separation of these two factors by achieving emission enhancement in the absence of any excitation enhancement. Separation of the two factors allows us to independently examine the spectral dependence of emission enhancement. We take advantage of the broad photoluminescence excitation spectra of QDs and the narrow plasmonic scattering spectra from individual silver nanoprisms. We use CdSe/CdS/ZnS core/shell/shell and CdSe/ ZnS core/shell colloidal QDs as chromophores. QDs are extremely photostable and, importantly, we can excite them far above their band gap and still obtain photoluminescence

he potential to improve the performance of existing chromophores has led to the incorporation of metal nanoparticles in applications ranging from plasmonenhanced bioassays1–5 to optoelectronic devices.6–8 Recent advancements in colloidal syntheses9–13 and the availability of tools/techniques to fabricate plasmonic nanostructures14–16 have also contributed to the rapid growth in plasmonenhanced fluorescence research. Metal nanoparticles can modify the apparent brightness of nearby chromophores in two ways. First, they can increase the excitation rates of chromophores by creating localized “hotspots”. Chromophores in these areas of concentrated electric field intensity effectively absorb more light. Second, metal nanoparticles can alter the radiative and nonradiative decay rates of nearby chromophores, affecting both their quantum yield and lifetime. Although apparent brightness (increased excitation and emission) might be the key metric for an improved fluorescence assay, a plasmon-enhanced photovoltaic or photochemical reaction benefits from maximized absorption enhancement while minimizing radiative decay or nonradiative loss. Conversely, a plasmon-enhanced light-emitting diode requires an enhanced radiative rate without enhancing absorption. A better understanding and control over the relative contributions of the changes in light absorption (excitation enhancement)17 and changes in decay rates (emission enhancement and quenching) occurring near

* To whom correspondence should be addressed, [email protected]. Received for review: 04/12/2010 Published on Web: 05/26/2010 © 2010 American Chemical Society

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FIGURE 1. Sample scheme and TEM image of the substrate. (a) A schematic illustration of the substrate structure. A monolayer of quantum dots is made by Langmuir-Blodgett deposition. Ag nanoprisms are placed on top by drop casting. (b) TEM image of two isolated nanoprisms on top of a close-packed film of quantum dots.

from their lowest band-edge excitonic state.32 These characteristics allow us to excite the QDs at a wavelength far away from the plasmon resonance and thus to minimize variations in plasmonic excitation enhancement as we systematically vary the spectral overlap of the QD photoluminescence with the plasmon resonances of individual silver nanoprisms. This also means that the excitation of the nanoprism plasmon resonance should occur only through the photoluminescence from the QDs. The use of singleparticle spectroscopy allows us to reduce inhomogeneous broadening typical in ensemble experiments and thus achieve better spectral isolation of excitation and emission. We synthesized colloidal silver nanoprisms with a distribution of scattering resonances using established protocols to obtain predominantly triangular nanoprisms of ∼10-15 nm thickness with edge lengths in the range of 50-100 nm.33 The optical properties of these particles are dominated by a strong in-plane dipole plasmon resonance. The homogeneous line widths of single Ag nanoprisms are on the order of ∼150-250 meV, arising from roughly equal contributions of radiation damping and nonradiative decay.34,35 We used quantum dots synthesized in-house36 (CdSe/ CdS/ZnS; 598 and 625 nm emission) as well as purchased (CdSe/ZnS; 550 nm emission) from Ocean NanoTech (Springdale, AR). Both the commercially available and in-house synthesized quantum dots are expected to exhibit similar enhancement behavior in the vicinity of Ag nanoprisms. Uniform QD monolayers were prepared by LangmuirBlodgett deposition37 of QD monolayers on glass substrates. A schematic illustration of the resulting sample with adsorbed Ag nanoprisms is shown in Figure 1a. High-resolution transmission electron microscopy (TEM) images of a similar sample prepared on a TEM grid (Figure 1b) show densely packed QDs (smaller round nanoparticles) underneath Ag nanoprisms. We examined the wavelength-dependent QD emission enhancement factors by correlating the darkfield scattering spectra of single Ag nanoprisms with the photoluminescence intensities and lifetimes of QDs near individual nanoprisms. The excitation wavelength dependence of the photoluminescence intensity has been studied previously.17 In this © 2010 American Chemical Society

FIGURE 2. Correlating Ag nanoprism scattering spectra, quantum dot photoluminescence intensity, and photoluminescence decays. (a) Darkfield image of an area with two silver nanoprisms on top of a homogeneous quantum dots film captured with a CCD camera with the exposure time of 348 ms. (b) Single particle scattering spectra of labeled silver nanoprisms shown in (a). The shaded graph is the emission spectrum of the quantum dots film. (c) A confocal scanning PL intensity image of the same area shown in (a) excited with a 405 nm laser with integration time of 100 ms per pixel (pixel )100 nm ×100 nm). (d) Photoluminescence decays of the quantum dots near the labeled silver nanoprisms. Background quantum dot photoluminescence decay is also plotted for comparison (gray dotted line).

paper we keep the excitation wavelength fixed: all experiments are performed with 405 nm laser excitation. We obtained darkfield images and single particle scattering spectra in transmitted light geometry and photoluminescence images using a home-built scanning confocal fluorescence lifetime imaging system using a PicoHarp 300 in timetagged time-resolved (TTTR) mode with a 405 nm picosecond diode excitation source (see Supporting Information, SI 3). Figure 2a shows a section of a typical darkfield scattering image of two isolated Ag nanoprisms on top of a LangmuirBlodgett (LB) film of 598 nm emitting QDs. The scattering spectrum for each nanoprism is shown in Figure 2b. For comparison, the QD photoluminescence spectrum is also plotted as the shaded spectrum in Figure 2b. Figure 2c shows the corresponding unmodified 2 × 2 µm2 confocal photoluminescence intensity image of the QDs in the same area. The photoluminescence intensity remains fairly uniform across the scanned area with the average “background” photoluminescence intensity of ∼16200 counts due to the QD monolayer, except for the area near nanoprism #2 which shows more photoluminescence. Here, the QDs within a diffraction-limited laser spot show a ∼20% increase in photoluminescence compared to the emission-unmodified QDs in the background, far from either nanoprism. In contrast, we observe no change in the photoluminescence intensity of the QDs near nanoprism #1. We also compare the photoluminescence lifetime of QDs at different points in the sample. Figure 2d shows 2599

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plots of the photoluminescence decay curves for QDs near each nanoprism (solid lines) with QDs in the background (dotted gray line). While the decay curve for the QDs near nanoprism #1 is indistinguishable from that of the background QDs, the photoluminescence decay for QDs near nanoprism #2 shows both a shorter lifetime and a higher initial intensity. We propose that the differences in photoluminescence intensity and lifetime between the QDs near nanoprisms #1 and #2 in Figure 2 can be explained solely by the spectral dependence of the emission enhancement factor near the two nanoprisms. The scattering spectrum of nanoprism #2 has a significantly greater overlap with the photoluminescence peak of the QDs (centered at 598 nm) than that of nanoprism #1. To study this more quantitatively, we define the photoluminescence intensity enhancement (emission enhancement) as the ratio of the total photon counts recorded near a single nanoprism to that from the QD background (see Supporting Information, SI 6)

emission enhancement factor ) PL intensity(near nanoprism) PL intensity(background)

where krad is the average radiative decay rate of the QDs near 0 is the average radiative decay rate for the nanoprism, krad background QDs, IPL is the initial photon counts of the QDs near single nanoprisms at t ) 0 and I0PL is the initial photon counts at t ) 0 for the background QDs. The photoluminescence intensity values are extracted from the fitted parameters to compensate for the effects of the finite IRF (fwhm ∼188 ps). We obtain radiative decay rate enhancement factors of 1.02 and 3.80 for the QDs near nanoprisms #1 and #2 from Figure 2, respectively. From the measured quantum yield of the QDs (2.8% in thin film form on glass, measured using a calibrated integrating sphere, Supporting Information, SI 1) and the measured total decay rate of the background QDs (inverse of the decay time; 〈τ〉 ) 2.01 ns), k0total ) 0.498 ns-1, we are able to extract both the radiative decay rate, and also the non0 ) radiative decay rate, for the background QD sample (krad -1 0 -1 0.014 ns , k nonrad ) 0.483 ns for the QDs used in Figure 2). Using the radiative rate enhancement factors of 1.02 and 3.80 determined above, we can compute the average radiative decay rates of the QDs near nanoprism #1 and nanoprism #2 to be 0.014 and 0.053 ns-1. We also calculate the nonradiative decay rates by subtracting the radiative decay rates from the total decay rates to obtain knonrad of 0.483 and 1.51 ns-1, respectively. Given the individual decay rates, the plasmon-modified quantum yields of the QDs near nanoprisms #1 and #2 are 2.86% and 3.40%. These values, obtained from the photoluminescence decay curves, correspond to quantum yield enhancement factors of 1.02 and 1.21. These values are remarkably self-consistent with the observed increase in the total integrated brightness (emission enhancement factor, 1.02 and 1.19). We have obtained similar agreement between the photoluminescence intensity changes and the photoluminescence lifetime changes for hundreds of nanoparticles. This excellent agreement supports the contention that we have achieved complete isolation of the emission enhancement factor from excitation effects and demonstrates that we can accurately extract the average radiative and nonradiative rates for QDs near single silver nanoprisms using our experimental protocol. In order to study the spectral dependence of the emission enhancement factors more systematically, we correlated a total of 255 single nanoprism LSPR scattering spectra with the corresponding photoluminescence intensity and lifetime decay traces and repeated the above calculations using three sizes of QDs with emission wavelengths of 550, 598, and 625 nm. Figure 3 summarizes the measured emission enhancement factors (a, d, g), average lifetimes (b, e, h), and both radiative (black circles) and nonradiative (blue squares) decay rate enhancement factors (c, f, i) of these three different QD batches near single nanoprisms plotted as a function of plasmon resonance scattering peak wavelength of silver nanoprisms. The dotted traces are the Gaussian fits to the mean values in each plot. The shaded traces are the photoluminescence spectra of the corresponding QD films

(1)

In Figure 2, the emission enhancement factors for the QDs are 1.02 near nanoprism #1 and 1.19 near nanoprism #2. We also measured the corresponding changes in the photoluminescence lifetimes of the QDs near each nanoprism. We reconstructed the lifetime histograms from the TTTR data and fitted them to a stretched exponential function convoluted with the instrument response functions (IRF) using a commercial software (Fluofit Ver. 4.2, PicoQuant). Figure 2d shows that the average lifetime38 (Supporting Information, SI 5) for the QDs near nanoprism #1, nanoprism #2, and in the background are 2.01, 0.64, and 2.01 ns, respectively. We observe a 68% reduction in average photoluminescence lifetime for QDs near nanoprism #2, while the lifetime of QDs near nanoprism #1 remains similar to that of the background QDs. With the acquired data, we also calculate both radiative and nonradiative decay rate enhancement factors of the QDs near the nanoprisms. Since the initial photoluminescence intensity (t ) 0) is proportional to the radiative decay rate; I(t ) 0) ∝ krad,39,40 we can directly obtain the radiative decay rate enhancement factor, γrad, by ratioing the radiative decay rates of QDs near Ag nanoprisms to that of the background QDs. We do this by taking the ratio of the photoluminescence intensities obtained at t ) 0

γrad )

krad 0 krad

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IPL(t ) 0) 0 IPL (t ) 0)

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parts b, e, and h of Figure 3, show a similarly strong spectral dependence. For all three QD batches, the average lifetime deviates the most from the background (dotted horizontal lines) when the nanoparticle LSPR peak overlaps the emission spectra of each batch of QD films. Similarly, parts c, f, and i of Figure 3 show that both the radiative and nonradiative enhancement factors systematically increase as the nanoparticle LSPR overlaps more with the QD photoluminescence, and all reach their maxima near the QD photoluminescence peak. As one moves off resonance from the photoluminescence spectra on either side of the peak, the decay rate enhancement factors return to unity. There is little or no increase in the average radiative or nonradiative decay of QD films that emit more than ∼0.23 eV off resonance from the plasmon peak. On the basis of these data, we attribute the variations in photoluminescence enhancement and average lifetime almost exclusively to the LSPR-dependent changes to the emission enhancement factor (quantum yield). The average radiative rate data shown in parts c, f, and i in Figures 3 show that the maximum lifetime reduction occurs at the point of maximum emission enhancement and, as noted above, the observed changes in the radiative decay rates (new quantum yield) are self-consistent with the emission enhancement factors obtained by measuring the increase in the total brightness of nearby QDs (Supporting Information, SI 7). The overall width of the spectral window wherein the metal particle affects the photoluminescence lifetime/radiative rate of the QDs is an important quantity to assess, as it determines how readily emission enhancement can be tailored using spectral effects for both narrow band and broad band applications. From the data in Figure 3, this width is ∼0.23-0.24 eV, which is consistent with a simple convolution of the width of the QD photoluminescence spectrum and the average nanoprism LSPR line width. Finally, although the effects of spectral overlap dominate the data in Figures 2 and 3, we also observed variations in the brightness and emission enhancement factors near nanoparticles with nearly identical plasmon resonance peak positions. To examine the source of this variation, we compare changes in the QD decay rate and brightness as a function of relative scattering efficiency (intensity) of each nanoprism. Figure 4a shows the decay rate enhancement factors plotted against the relative scattering intensity for the same data sets taken from QDs which emit at 598 nm. A clear correlation is observed between the relative decay rate change and the nanoprisms’ relative scattering intensities. More strongly scattering nanoprisms cause both larger radiative (filled circle) and larger nonradiative (filled square) QD decay rates, but as the nanoprism scattering intensity increases, the radiative rate enhancement factor increases faster than the increase in nonradiative rate. As a result, we observe larger emission enhancements and changes in the decay rates near more strongly scattering prisms. The role of the nanoparticle scattering cross section has been em-

FIGURE 3. Summary of PL enhancement, average lifetime, and radiative rate enhancement vs LSPR peak positions. The average emission enhancement factors (a, d, g), the average lifetime (b, e, h), and the average calculated decay rate enhancement factors (c, f, i, black circles are for radiative decay rates, blue squares are for nonradiative decay rates) are plotted against scattering peak positions of Ag nanoprisms on top of three different quantum dots with emission peaks centered at 550, 598, and 625 nm. LSPR peak positions are binned together by 25 nm increments along the x-axis. The average values in each bin are then plotted against LSPR peak positions in all the plots. y-error bars show the standard deviation of the mean values in all the plots. Emission spectra for all the three quantum dots (shaded spectra) are also plotted for reference. Horizontal dotted lines in the average lifetime plots (b, e, h) are the average lifetimes of the background QDs for each QD film. Red and blue dotted lines are the Gaussian fits to the mean values in each 25 nm LSPR bin.

shown for reference. The horizontal dotted lines in the lifetime plots (Figure. 3b,e,h) represent the average lifetime of the background QDs in each film (5.64, 2.01, and 3.63 ns for QDs emitting at 550, 598, and 625 nm, respectively). Together with the measured quantum yields of 10%, 2.8%, and 3.0%, we obtain the effective radiative decay rates for close-packed CdSe/ZnS (550 nm emission) and CdSe/CdS/ ZnS (598 and 625 nm emission) films of 0.02, 0.014, and 0.008 ns-1, respectively, which are consistent with the values reported for isolated CdSe and CdSe/ZnS quantum dots.41–43 The data in Figure 3 reveal the systematic dependence of the decay rates and overall emission enhancement factors as a function of spectral overlap between the QD photoluminescence and the nanoprism LSPR. The emission enhancement plots for the three QD films (Figure 3a,d,g) all show strong wavelength dependence: the maximum enhancements occur when the LSPR peaks closely overlap with the photoluminescence spectra of the QDs. We observed ∼25% emission enhancement for the 550 nm emitting QDs and ∼40-50% emission enhancements for the QDs emitting at 598 and 625 nm. The average lifetimes, shown in © 2010 American Chemical Society

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enhancement of quantum yield for more strongly scattering nanoprisms. Figure 4d shows the calculated quantum yield enhancement factors (η/η0) of chromophores with different initial quantum yield values (indicated by different symbols) calculated using the three model nanoprisms shown in Figure 4b. While large emission enhancement factors are achievable, the magnitude of the enhancement depends on both the initial quantum yield of the chromophore and the scattering intensity of the nanoprism: for high quantum yield chromophores near poorly scattering nanoprisms, quantum yield enhancement values less than unity indicate quenching rather than enhancement. While the calculated trends in parts c and d of Figure 4 are qualitatively consistent with our experimental results in Figure 4a, notably greater krad/ k0rad values with more efficient scattering nanoprism, the values are of much larger magnitudes, since the data in Figure 4d were calculated for a dipole source located exclusively in the hot spot of the nanoprism. By averaging over the entire diffraction-limited laser volume (Supporting Information, SI 11), we obtain radiative rate enhancement values which are consistent with those measured experimentally (colored empty circles, Figure. 4a). Conclusion. We have isolated the contribution of emission enhancement to plasmon-enhanced fluorescence and measured the pure emission enhancement factors for QDs near metal nanoparticles. The emission enhancement factor closely follows the spectral overlap between the QD photoluminescence spectrum and the metal nanoparticle LSPR scattering spectra. When the LSPR scattering peak matches the QD photoluminescence wavelength, we observe a strong (∼4-fold) reduction in the average photoluminescence lifetime from neighboring QDs and a concomitant increase in the photoluminescence intensity. We extracted the corresponding change in the radiative and nonradiative rate from both the lifetime data and initial intensity data and found these values in good agreement. When the photoluminescence spectrum overlaps the LSPR peak, an increase in the average radiative decay rate of up to ∼5-10 in a diffractionlimited volume can be achieved in practical thin film geometries. The spectral window over which the radiative rate of the chromophores is sensitive to the nearby nanoprism LSPR is consistent with the width of ∼0.24 eV, as determined by the photoluminescence and scattering line widths. We find that for a fixed plasmon scattering peak position, the ratio of radiative to nonradiative decay rate enhancements is directly proportional to the relative scattering cross section of the metal particle. Controlling the relative contributions of the changes in light absorption and changes in decay occurring near metal nanostructures is critical to many applications. We expect these results will provide practical guidelines for isolating emission enhancement factors independent of plasmonic excitation enhancement effects and should facilitate efforts to use spectrally tailorable plasmon resonant nanoparticles

FIGURE 4. Decay rates vs relative scattering intensity of Ag nanoprisms. (a) Changes in both radiative (filled black circles) and nonradiative decay rates (filled blue squares) plotted as a function of relative scattering intensities of nearby Ag nanoprisms. Both values are the average changes in the decay rates from the experimental data sets with the QD film emitting at 598 nm. Relative scattering intensity values are binned together by increments of 0.045 along the x-axis, and then the average value in each bin is plotted. y-error bars are the standard deviations of the mean values in each bin. Colored empty circles are the FDTD calculated radiative rate enhancement factors which was averaged over the entire diffraction-limited laser volume. (b) LSPR scattering spectra of three different Ag nanoprisms which show the same LSPR peak positions and different relative scattering intensities. SEM images of corresponding nanoprisms are also shown as the inset. (c) FDTD calculated decay rates of a single chromophore plotted as a function of calculated scattering intensities of Ag nanoprisms simulated using the labeled nanoprisms’ geometry shown in (b). Filled black circles 0 0 indicate the krad/krad and filled blue squares are the knonrad/krad . (d) Calculated quantum yield enhancement factors of chromophores with varying intrinsic quantum yields (indicated by different symbols) plotted as a function of calculated scattering intensities. Each color of symbol corresponds to the labeled nanoprisms in (b). All the dashed/dotted lines in each plot are linear fits to the data points.

phasized previously,24 but such a direct correlation has not been observed due to competing contributions from excitation enhancement. To understand this observed trend, we performed finite difference time domain calculations to examine the changes in the decay rates and quantum yield using a point source (dipole) located near the nanoprisms for three different nanoprism shapes which experimentally showed nearly identical LSPR peak wavelengths but varying degrees of relative scattering intensities (Supporting Information, SI 8 and SI 9). Figure 4b shows the LSPR scattering spectra of the nanoprisms along with their scanning electron microscopy (SEM) images (inset) from which we extracted the model geometries. Figure 4c shows calculated radiative decay rates (krad/k0rad, filled circles) and nonradiative decay rates (knonrad/k0rad, filled squares), plotted versus calculated scattering intensity for the three model nanoprisms shown in Figure 4b. As observed experimentally, the slope for krad/ k0rad is higher than that of knonrad/k0rad, leading to an overall © 2010 American Chemical Society

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in applications ranging from biosensors to thin-film optoelectronics.44

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Acknowledgment. This paper is based on work supported by the Air Force Office of Scientific Research and the National Science Foundation (DMR 0520567), with additional contributions from the Office of Naval Research. D.S.G. thanks the Research Corporation Cottrell Scholars Program, Alfred P. Sloan Foundation Fellowship Program, and Camille Dreyfus Teacher-Scholar Program for additional support. We would like to also thank Professor Sarah L. Keller for her generosity for the use of the LangmuirBlodgett deposition system. Supporting Information Available. Experimental details of silver nanoprism synthesis, quantum dot synthesis, sample fabrication, experimental details of time-resolved photoluminescence, image and fitting analysis protocol, FDTD calculations; extinction spectra of nanoprism; absorption and emission spectrum of all the three quantum dots; LSPR scattering spectra before and after multiple laser scans; FDTD calculation of decay rates and quantum yield; comparison of radiative decay rate enhancement factors (dipole source vs averaged field intensity); FDTD calculated field intensity around three different nanoprisms in Figure 4. This material is available free of charge via the Internet at http:// pubs.acs.org. REFERENCES AND NOTES (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16)

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