Plasmon-Enhanced Emission in Gold Nanoparticle Aggregates - The

Feb 2, 2008 - Ying Fang , Wei-Shun Chang , Britain Willingham , Pattanawit Swanglap , Sergio Dominguez-Medina , and Stephan Link. ACS Nano 2012 6 (8) ...
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J. Phys. Chem. C 2008, 112, 3103-3108

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Plasmon-Enhanced Emission in Gold Nanoparticle Aggregates Mathias Steiner,†,‡ Christina Debus,‡ Antonio Virgilio Failla,†,‡ and Alfred Johann Meixner*,†,‡ Institut fu¨r Physikalische und Theoretische Chemie, UniVersita¨t Tu¨bingen, 72076 Tu¨bingen, Germany, and Physikalische Chemie, UniVersita¨t Siegen, 57068 Siegen, Germany ReceiVed: October 2, 2007; In Final Form: NoVember 17, 2007

We have investigated the influence of the plasmon resonances of individual, spatially isolated gold nanoparticle aggregates on their emission properties using combined optical confocal and dark field scattering microscopy and spectroscopy. The emission intensity of the same aggregate is enhanced by up to 1 order of magnitude if the emission wavelengths overlap with the plasmon resonance in the corresponding white light scattering spectrum. Regardless of the specific geometry of an individual aggregate, the in situ measurement of the plasmon characteristics delivers unique information about its potential as a substrate for surface-enhanced spectroscopy and allows its characterization as a nanoscatterer, nanoemitter, or local heater.

1. Introduction Plasmon-enhanced absorption and emission of nanostructured metallic materials have been the focus of scientific and technological studies over the last few years, and numerous promising optical applications of metallic nanoparticles, tips, and aggregates have already been reported, see for example refs 1-3 and references therein. Plasmonic resonances (PRs) are determined by the boundary conditions for coherent electron oscillations in the conduction band and can be tailored by tuning the size and shape of a metallic nanostructure, for example, refs 4-6. They are responsible for a strongly localized electromagnetic field enhancement and the lightning rod (or antenna) effect, respectively, occurring in nanostructured metallic probes and substrates.7 These spatially confined and PR-enhanced electromagnetic fields have been utilized successfully for surface- and tip-enhanced microscopy and spectroscopy on the nanoscale level.8 Furthermore, the metal luminescence that originates from inter- and intraband recombination of optically excited electron hole pairs9 was found to be strongly enhanced and spectrally modified in rough surfaces and nanoparticles due to the presence of PRs.10-12 Recently, new microscopy methods have been developed that allow for the detection of individual, spatially isolated metallic nanoparticles that have dimensions of only a few nanometers, for example refs 13 and 14. Consequently, it was possible to study both experimentally and theoretically to what extent the PRs of a nanoparticle are determined by its size, shape, and material as well as by the surrounding environment.15-17 Ultimately, the luminescence spectra of gold nanoparticles turned out to be determined by their PR characteristics,18,19 which makes them promising candidates for nanophotonic applications. In many experiments, however, the PR-enhanced probe or substrate consists of randomly aggregated nanoparticles having various sizes and shapes, see for example refs 20 and 21 and references therein. The PR characteristics of such an aggregate can differ significantly from those of a single nanoparticle. If * Corresponding author. E-mail: [email protected]. † Universita ¨ t Tu¨bingen. ‡ Universita ¨ t Siegen.

the specific geometry of an individual aggregate is not formerly known, then a prediction of its PR characteristics and, hence, its utility as a substrate for PR-enhanced spectroscopy is precluded. This problem also affects novel applications of metallic nanoparticles as nonbleaching labels in bioscience22 or local heat sources in cancer therapy:23 The investigated nanoparticle aggregates exhibit optical properties that are typical for the constituents, but they also reveal significant deviations from single-particle behavior. As a result, further in situ characterization is required if single metallic nanoparticles, intentionally or unintentionally, form aggregates in a specific application.24-32 We report a straightforward optical experiment for in situ correlating the PR characteristics of an individual metallic nanoparticle aggregate with its emission properties, that is, the metal luminescence as well as the Raman-scattered light of adsorbed organic compounds, by comparing its elastic and inelastic light scattering spectra. By varying the laser excitation wavelength, we demonstrate first the spectral selectivity of the PR enhancement for the emission of individual gold nanoparticle aggregates. 2. Experimental Section 2.1. Optical Experiments. Optical studies were performed with a home-built scanning confocal microscope as sketched in Figure 1a. As excitation lasers, we used a single-line argonion laser (60X-200, American Laser Corporation) operated at 458 nm, a frequency-doubled Nd:YVO4 laser (GCL-025L, Crystal Laser) at 532 nm, and a helium neon gas laser (1137P, JDS Uniphase) at 633 nm. The laser light was focused tightly on spatially isolated nanoparticle aggregates on the sample surface (see Figure 1b) using a microscope objective (Epiplan 100x/0.75, Zeiss), and the light scattered by individual aggregates was collected through the same microscope objective. The inelastically scattered light (emission) was separated from the excitation laser light using suitable holographic notch filters (Kaiser) and guided onto the detectors. Elastic white light scattering of the same aggregate was measured using a dark field illumination technique: The light of a halogen light source (KL 2500 LCD, Schott) was coupled into an optical fiber bundle that was connected to the side of the object slide bearing the

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Figure 1. Schematic diagram of (a) the optical setup and (b) the sample. Images of gold nanoparticle aggregates measured with (c) a scanning electron microscope and (d) an atomic force microscope.

gold nanoparticle aggregates. As sketched in Figure 1b, the white light was guided through the object slide by total internal reflection, and, as a result, the spatially isolated nanoparticle aggregates were exposed to the evanescent optical field at the glass-air interface. Images were acquired by raster scanning the object slide with respect to the fixed microscope objective using a feedbackcontrolled scanning stage (P-517.K008, Physik Instrumente) and focusing the collected light onto a single photon counting module (SPCM-AQR-14, Perkin-Elmer). Spectra were measured by positioning spatially isolated aggregates in the focal spot of the microscope objective and guiding the collected light into a spectrograph (SpectraPro 300i, Acton Research) equipped with a grating having a groove density of 300 mm-1 and a liquidnitrogen-cooled CCD camera (LNCCD-1340/100-EB/1, Princeton Instruments). The instrument response function of the grating spectrometer was determined to have a full width at half-maximum value of ∆λ ) 0.8 nm, which corresponds to ∆ν ∼ 20 cm-1 in the actual spectral range. For spectra analysis, we subtracted from each measured spectrum a background spectrum that was recorded for the same excitation power and acquisition time. In addition, the white light scattering spectra were divided by the measured and normalized spectrum of the white light source and, finally, normalized to their respective intensity maximum. 2.2. Preparation of Gold Nanoparticle Aggregates. Gold sols were prepared according to the method reported by Lee and Meisel:33 Tetrachlorogoldacid (120 mg) was dissolved in 250 mL of threefold distilled water and heated to the boiling point. Twenty-five milliliters of a 1% solution of sodium citrate was added under vigorous stirring, and the solution was kept boiling for 1 h with constant stirring. The resulting gold sol was characterized using absorption spectroscopy (λmax ) 525 nm) and scanning electron microscopy, and we found a narrow size distribution of gold nanoparticles having an average diameter of 20 nm. The samples for the optical experiments

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Figure 2. Optical microscopy false color images of spatially isolated gold nanoparticle aggregates recorded on the same sample section. (a) White light (elastic) scattering image. (b) Confocal reflection (elastic backscattering) image. (c and d) Confocal emission (inelastic scattering) images measured for two different excitation wavelengths. The rings mark the positions where the spectra shown in Figure 3d and e were acquired.

were prepared by dispersing an aliquot of 0.5 µL of the fivefold diluted colloidal solution onto a cleaned glass object slide. During evaporation of the solvent, the gold nanoparticles formed aggregates on the glass surface with a predominant lateral extension between 200 and 500 nm, as can be seen in the scanning electron microscopy images shown in Figure 1c. Atomic force microscopy measurements on individual aggregates (see Figure 1d) revealed an average height of 40 nm while the average spatial separation was found to be on the order of one micrometer. 3. Results and Discussion In Figure 2, representative optical microscopy images of gold nanoparticle aggregates are shown that were measured from the same sample section with different optical contrast. The images consist of bright circular spots that are spatially separated in the order of one micrometer in agreement with atomic force microscopy measurements (see Figure 1d). Each spot originates from an individual gold nanoparticle aggregate with a lateral size smaller than or equal to 500 nm. By comparing cross sections through the bright spots of the laser-excited confocal images (Figure 2b-d) and the white light scattering image (Figure 2a), we found that the optical resolution of the confocal images with a full width at half-maximum value ∆x ) (0.5 ( 0.1) µm for the measured spot size is twice as good as that of the dark field scattering image acquired in non-confocal mode revealing ∆x ) (1.2 ( 0.1) µm. We proofed single nanoparticle sensitivity for both the dark field imaging technique and the confocal (back)scattering microscopy method.13,34 In the present case, however, we did not obtain significant optical response from single nanoparticles because of the low power of the white light coupled to the object slide. As a result, not all objects

Plasmon-Enhanced Emission observed in the confocal images shown in Figure 2b-d appear in the corresponding white light scattering image Figure 2a. By comparing the spots marked by rings in the confocal emission images in Figure 2c and d that were measured with comparable excitation power but for two different laser excitation wavelengths, we found that the emission intensities differ significantly. To investigate how different emission intensities of individual gold nanoparticle aggregates are correlated with their PR characteristics, we analyzed the scattered white light as well as the emission using optical spectroscopy. In Figure 3a, three representative emission spectra of individual gold nanoparticle aggregates measured for different excitation wavelengths are shown. Each spectrum consists of a smooth, spectrally broad, and unstructured photoluminescence tail that we observed for all aggregates without significant PRs in the spectral range where the emission occurs. We will consider this set of spectra as a reference in the following. As can be seen in the upper spectrum shown in Figure 3a that was excited at 457 nm, the maximum luminescence intensity is spectrally located around 550 nm and the luminescence tail has a width of approximately 200 nm (710 meV). In comparison with the luminescence spectrum of bulk gold excited at 488 nm,9 the spectral distribution is broadened by around 100 nm (300 meV) and the intensity maximum is red-shifted by 30 nm (130 meV). Increasing the excitation wavelength to 532 and 633 nm, respectively, we found that the luminescence tail has a spectral width of 110 nm (380 meV) and 65 nm (170 meV), respectively, while the intensity maximum is further red-shifted by 50 nm (190 meV) and 125 nm (420 meV), respectively. An estimate of the luminescence quantum yield of the gold nanoparticle aggregates that is based on the number of acquired photon counts and the detection efficiency of the experimental setup delivered values of up to 10-8 (see ref 35), which is up to 2 orders of magnitude larger than the luminescence quantum yield of bulk gold9 and between 2 and 4 orders of magnitude smaller than the luminescence quantum yield reported for single gold nanoparticles.11,12 We speculate that the poor luminescence quantum yield observed here could result from an enhanced nonradiative decay of the optical excitation energy that is promoted by additional efficient dissipation channels occurring in complex nanoparticle aggregates. In other words, the photoinduced generation of heat in nanoparticle aggregates could be even more efficient than that in single nanoparticles. Our experimental results indicate that the luminescence properties of gold nanoparticle aggregates differ significantly from the luminescence properties of both bulk gold and individual gold nanoparticles. For modeling and quantifying the luminescence properties of randomly assembled gold nanoparticle aggregates, a novel theoretical approach supported by further experimental investigation is required. In the following, we investigate how PRs affect the aggregate’s emission spectra by comparing the spectrally resolved emission intensities of two individual gold nanoparticle aggregates labeled 1 and 2 with their corresponding white light scattering spectra. In Figure 3b, two emission spectra of the same aggregate but taken for different excitation wavelengths (λexc ) 532 nm (green) and λexc ) 633 nm (red)) and normalized with respect to excitation power and acquisition time are shown. Obviously, the emission characteristics for the two excitation wavelengths differ significantly. By comparing the emission intensities for excitation at 532 nm with the elastic scattering intensities of the same aggregate 1 shown in Figure 3c, we find that both the position of the intensity maximum and the spectral shape of the luminescence are strongly modified by the presence

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Figure 3. (a) Emission (EM) spectra of individual gold nanoparticle aggregates showing no significant plasmon resonance (PR) excited at λexc ) 457 nm (blue), λexc ) 532 nm (green), and λexc ) 633 nm (red) and shifted vertically to improve clarity. The spectral full width at halfmaximum of the luminescence tails are indicated by arrows, and the luminescence intensity maxima for excitation at 532 and 633 nm are marked by colored dashed lines. (b and d) Emission spectra of the same gold nanoparticle aggregate 1 and 2, respectively, for two excitation wavelengths λexc ) 532 nm (green) and λexc ) 633 nm (red). The gray arrows indicate the spectral shifts of the intensity maxima of the PRenhanced emission with respect to the non-enhanced spectra shown in a. The dashed ring in d highlights Raman signals from adsorbed organic compounds. (c and e) Corresponding white light scattering spectra (gray) indicating the PRs of aggregate 1 and 2. In e, the measured spectrum was fitted with a Lorentzian line shape function (black).

of the broad PR that clearly exceeds the spectral range of our spectrometer. As compared to the reference luminescence spectrum of aggregates without noticeable PR in the investigated spectral range but measured under the same experimental conditions (green curve in Figure 3a), the intensity maximum was found to be blueshifted by 30 nm (110 meV) and the luminescence distribution was spectrally narrowed by 25 nm (70 meV) following the spectral distribution of the corresponding white light scattering spectrum. In contrast, by exciting

3106 J. Phys. Chem. C, Vol. 112, No. 8, 2008 aggregate 1 at 633 nm (red), we observed that the measured emission spectrum samples the intensity maximum, spectral shape, and spectral width of the non PR-enhanced reference spectrum that is shown as a red curve in Figure 3a. In the respective spectral range, we do not observe a PR but only weak elastic light scattering from aggregate 1 (see Figure 3c). The ratio of the spectrally integrated intensities taken from the two emission spectra shown in Figure 3b delivers I(532)/I(633) ) 11. This result demonstrates the PR enhancement in gold nanoparticle aggregates. Depending on size and shape of individual gold nanoparticle aggregates, we found that both the spectral position and the spectral shape of PRs can differ significantly. As a result, the PR enhancement of the emission occurs in different spectral domains. In Figure 3d, we compare two emission spectra of the same aggregate 2 measured for different excitation wavelengths (λexc ) 532 nm (green) and λexc ) 633 nm (red)) but normalized with respect to excitation power and acquisition time. In contrast to aggregate 1, the emission measured for excitation at 532 nm samples the intensity maximum, spectral shape, and spectral width of the corresponding reference emission spectrum shown as the green curve in Figure 3a. But the emission spectrum of the aggregate using excitation at 633 nm is redshifted by around 40 nm (100 meV) with respect to the corresponding reference spectrum (red curve in Figure 3a) and samples the shape of the PR spectrally located around 700 nm (see Figure 3e). Comparing the integrated intensities in the emission spectra measured for aggregate 2 delivers a PR enhancement of I(633)/I(532) ) 4. As a result, we have demonstrated that the spectral position of PRs differs from aggregate to aggregate and, by varying the laser excitation wavelength for the same aggregate, we have verified the spectral selectivity of the PR enhancement. In case the laser excitation also spectrally overlaps with the PR, the measured plasmon enhancement can be understood as a result of the simultaneous enhancement of both emission and absorption in the investigated aggregate. The PR of aggregate 2 is well-described by a single Lorentzian line shape function having a line width of approximately 100 nm (240 meV) corresponding to a plasmon dephasing time of 5 fs. Remarkably, this result coincides with the result experimentally obtained for individual species of the same kind as the aggregate’s constituents, that is, gold nanoparticles having a diameter of 20 nm.36,37 In contrast, the spectral position of the PR of aggregate 2 is red-shifted by around 150 nm (480 meV) with respect to that one of an isolated gold nanosphere having a diameter of 20 nm.38 The observation of a single PR indicates that the geometry of aggregate 2 could be described by a solid gold disc in a first approximation.4,15,38 This interpretation is supported by results from model calculations performed for actual sample parameters in the quasistatic approximation that account for dipole contributions only.35 As a result, assuming a diameter of 400 nm and a height of 50 nm for aggregate 2, the energetically degenerated longitudinal plasmon modes in the disc plane are predicted to result in a strong PR spectrally located around 700 nm. The perpendicular oriented transversal plasmon mode of the aggregate is orders of magnitude weaker and, hence, cannot be observed in our experiments. We emphasize here that model calculations as discussed above might be useful for analyzing basic spectral features of the measured elastic scattering spectra that are correlated with the overal dimension of the investigated aggregates. They are not well-suited to model the complex optical response originating from interactions occurring on the length

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Figure 4. (a and b) White light scattering spectra (gray) indicating the plasmon resonances (PRs) of gold nanoparticle aggregate 3 and 4. The fit (black) of the measured spectrum in a is the sum of two Lorentzians (dashed lines); the spectrum in b was fitted with a single Lorentzian (black). (c and d) Emission spectra of the same aggregate 3 and 4, respectively, for two excitation wavelengths λexc ) 532 nm (green) and λexc ) 633 nm (red). Raman signals from adsorbed organic compounds are highlighted by dashed ellipses. The arrow in the upper spectrum of c and d, respectively, indicates the center wavelength of the corresponding PR at 700 nm (545 nm). The lower spectra in c and d do not spectrally overlap with a PR.

scale of the randomly assembled constituents and, in particular, spatially localized effects resulting in surface-enhanced Raman scattering (SERS) of adsorbed molecules as will be discussed in the following. On top of the PR-enhanced emission spectrum of aggregate 2, we observe narrow spectral features marked in Figure 3d by a dashed circle. These features result from SERS of organic compounds that are adsorbed on the surface of the aggregates. Most of the Raman signals could be associated with citrat ions and their reaction products acting as surfactants of the gold nanoparticles in solution.39,40 Even though the poor spectral resolution in our experiment does not allow for isolating all individual Raman lines, we correlate in the following the overall visibility of Raman signals with the PR characteristics and the luminescence of the investigated aggregates.41,42 In Figure 4a and b, white light scattering spectra of two aggregates labeled 3 and 4 are shown having strong PRs spectrally located around 700 and 545 nm, respectively. In contrast to the PRs measured for aggregates 2 and 4, the PR measured for aggregate 3 cannot be fitted by a single Lorentzian line shape function. The splitting of the PR into two Lorentzian contributions as observed here could be explained by assuming geometrical excentricity of aggregate 3; that is, its overall geometry is approximated by an ellipsoid rather than a disc. In this case, the longitudinal plasmon modes associated with the ellipsoid’s short and long axes are not longer energetically degenerated and, hence, have different resonance wavelengths.35 Emission spectra of aggregate 3 were recorded for two different laser excitation wavelengths (λexc ) 532 nm (green) and λexc ) 633 nm (red)) and are shown in Figure 4c as a function of the Raman shift. As expected, the aggregate’s

Plasmon-Enhanced Emission luminescence intensity as well as the Raman signals of surfactant molecules in the fingerprint spectral region (highlighted by the dashed ellipse) are maximized for laser excitation at 633 nm because the emission wavelengths overlap with the corresponding PR shown in Figure 4a. To improve clarity, the center wavelength of the PR around 700 nm is indicated by an arrow in the Raman spectrum shown Figure 4c. In contrast, the emission spectrum of the same aggregate excited at 532 nm shows significantly less luminescence and no SERS signals at all because of the absence of plasmon-enhancement. For a few aggregates, however, the observed visibility maximum of the SERS signals was not correlated to the spectral position the measured PR. In Figure 4d, the emission spectra of aggregate 4 acquired for two different excitation wavelengths (λexc ) 532 nm (green) and λexc ) 633 nm (red)) are compared. Like for aggregate 3, we would expect to obtain maximum visibility of SERS signals for emission wavelengths that overlap spectrally with the PR of the aggregate (see Figure 4b). For the upper spectrum in Figure 4d that was excited at 532 nm, both excitation and emission wavelengths overlap well with the broad PR having a center wavelength of 545 nm (indicated by an arrow). Even though the luminescence intensity excited at 532 nm is strongly PR-enhanced with respect to the one excited at 633 nm, the visibility of SERS signals (highlighted by the dashed ellipse) is larger on top of the non-PR-enhanced emission spectrum. This result indicates that the visibility of SERS signals as well as the SERS enhancement cannot be exclusively assigned to the influence of PRs occurring in random nanoparticle aggregates. The Raman signals obtained from aggregate 4 imply that the resonance Raman condition as an intrinsic property of the adsorbed compound rather than the enhancement due to the presence of the aggregate’s PR is responsible for the high visibility of the SERS signals observed here. However, the aggregate’s geometry in the vicinity of SERS active molecules leading to localized “hot emission spots” as well as the position and orientation of the adsorbed molecules with respect to the substrate have also been recognized to strongly influence the observed SERS enhancement, see for example refs 21 and 4345 and references therein. To decompose these contributions experimentally, the tunability of the laser excitation wavelength and polarization as well as application of electron or atomic force microscopy and near-field optical techniques on the same aggregate is needed. 4. Conclusions We have compared white light scattering spectra and emission spectra measured on the same individual gold nanoparticle aggregates with lateral dimensions up to 500 nm. The spectral properties of the plasmon resonances were found to be determined by the individual geometry of each aggregate. We observed an emission enhancement of up to 1 order of magnitude for individual aggregates depending on the spectral overlap between the emission spectrum and the plasmon resonance. Furthermore, we found that the presence of Raman lines on top of the gold luminescence is not strictly correlated with the spectral position of the plasmon resonance. The results demonstrate that we can in situ correlate the plasmon resonance characteristics of individual, randomly assembled, and geometrically complex nanoparticle aggregates with their emission properties. This is particularly useful for chemical, biological, and medical applications where the aggregation of individual metallic nanoparticles cannot be avoided.

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