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Correlated Absorption and Scattering Spectroscopy of Individual Pt-Decorated Au Nanorods Reveals Strong Excitation Enhancement in the Non-Plasmonic Metal Anneli Joplin, Seyyed Ali Hosseini Jebeli, Eric Sung, Nathan Diemler, Patrick J. Straney, Mustafa Yorulmaz, Wei-Shun Chang, Jill E. Millstone, and Stephan Link ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b06239 • Publication Date (Web): 20 Nov 2017 Downloaded from http://pubs.acs.org on November 20, 2017

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Correlated Absorption and Scattering Spectroscopy of Individual Pt-Decorated Au Nanorods Reveals Strong Excitation Enhancement in the Non-Plasmonic Metal Anneli Joplin1,2‡, Seyyed Ali Hosseini Jebeli1,3‡, Eric Sung1,2, Nathan Diemler4, Patrick J. Straney4, Mustafa Yorulmaz1,2, Wei-Shun Chang1,2, Jill E. Millstone4, Stephan Link1,2,3* 1

Rice University, Laboratory for Nanophotonics, 2Department of Chemistry, 3Department of Electrical

and Computer Engineering, Houston, TX 77005, USA 4

University of Pittsburgh, Department of Chemistry, Pittsburgh, PA 15260, USA

‡

These authors contributed equally to this work.

KEYWORDS: bimetallic nanoparticles, surface plasmon, non-radiative relaxation, photoluminescence, photocatalysis

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ABSTRACT Bimetallic nanocatalysts have the potential to surmount current limitations in industrial catalysis if their electronic and optical properties can be effectively controlled. However, improving the performance of bimetallic photocatalysts requires a functional understanding of how the intricacies of their morphology and composition dictate every element of their optical response. In this work, we examine Au and Ptdecorated Au nanorods on a single particle level to ascertain how Pt influences the plasmon resonance of the bimetallic nanostructure. We correlated scattering, photoluminescence, and pure absorption of individual nanostructures separately to expose the impact of Pt on each component. We found that the scattering and absorption spectra of uncoated Au nanorods followed expected trends in peak intensity and shape and were accurately reproduced by Finite Difference Time Domain simulations. In contrast, the scattering and absorption spectra of single Pt-decorated Au nanorods exhibited redshifted, broad features and large deviations in line shape from particle to particle. Simulations using an idealized geometry confirmed that Pt damps the plasmon resonance of individual Au nanorods and that spectral changes after Pt deposition were a consequence of coupling between Au and Pt in the hybrid nanostructure. Simulations also revealed that the Au nanorod acts as an antenna and enhances absorption in the Pt islands. Furthermore, comparing photoluminescence spectra from Au and Pt-decorated Au nanorods illustrated that emission was significantly reduced in the presence of Pt. The reduction in photoluminescence intensity indicates that Pt lowers the number of hot carriers in the Au nanorod available for radiative recombination through either direct production of hot carriers in Pt following enhanced absorption or charge transfer from Au to Pt. Overall, these results confirm that the Pt island morphology and distribution on the nanorod surface contributes to the optical response of individual hybrid nanostructures and that the damping observed in ensemble measurements originates not only from structural heterogeneity but also because of significant damping in single nanostructures.


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Engineered nanocatalysts present a potentially transformative opportunity to overcome current limitations in industrial catalysis if we can achieve precise control over their electronic and optical properties.1-11 In particular, bimetallic nanostructures, which are composed of two metals, make excellent nanocatalysts because their electronic and optical properties are almost infinitely tunable via changes to material composition and geometry.11-16 Bimetallic nanoparticles specifically offer a convenient route to strategically combine metals with different properties to improve the catalytic performance of the hybrid nanostructure. For example, bimetallic nanostructures composed of a catalytically active metal (e.g. Pd or Pt) and a plasmonically active metal (e.g. Al, Ag, or Au) have already been shown to exhibit exceptional catalytic properties.17-24 In bimetallic nanocatalysts with a plasmonic component, enhanced catalytic performance is enabled by their distinctive optical response.7-10,24,25 Metal nanoparticles interact strongly with the oscillating electric field of incident light, causing conduction band electrons in the nanoparticle to oscillate in phase.26,27 This electron oscillation is known as the localized surface plasmon resonance and gives rise to strong absorption and scattering. Absorption by the localized surface plasmon resonance then produces energetic electrons and holes that can transfer to the catalytically active metal.7-10,28-35 Because this light interaction is highly wavelength tunable through the nanoparticle geometry, hot charge carriers with specific energies equal to the plasmon resonance can be generated for directing the selectivity of catalytic reaction. Catalytic properties can also benefit from nanostructure architectures that position the catalytically active metal in areas of high local field enhancement produced by the plasmon resonance.36 In these nanostructures, which have been termed antenna–reactor complexes, the plasmonic metal enhances direct absorption in the nanoparticle reactor and increases the availability of hot carriers.20,22-24 Improving the selectivity and efficiency of bimetallic photocatalysts requires an understanding of how their structural and electronic characteristics influence their optical properties. Therefore, meaningful optimization requires the ability to predict the influence of complex structural elements on nanoparticle optical properties including both radiative and non-radiative processes.9,14,24,31 Single particle studies are an ideal approach to reveal these relationships, and are an essential tool in decoupling effects of specific


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nanoparticle morphologies from the inherent heterogeneity in bulk nanoparticle samples. Radiative properties such as scattering and photoluminescence are straightforward to measure in single nanostructures and have previously been studied in a number of photocatalysts.18,19,22,24 However, understanding non-radiative processes (e.g. absorption) may prove even more important for engineering catalytic activity, because absorption is directly responsible for the generation of hot carriers.7-10,29-31,35 Despite the potential significance of non-radiative processes, characterization of pure absorption in individual bimetallic nanostructures has been limited to date. Here, we focus on bimetallic nanostructures composed of Au and Pt – a combination popularized by its intriguing catalytic properties in thin films and on the nanoscale. In 1980 Somorjai and colleagues found that introducing a monolayer of Pt to a non-reactive Au surface induced catalytic reactivity higher than that of Pt alone in the dehydrogenation of cyclohexene to benzene.37 Intriguingly, a similar synergistic interaction between Au and Pt emerged when these metals were combined in nanocatalysts.1719,21

For example, in 2014 Majima and colleagues reported that decorating the surface of Au nanorods

with Pt nanoparticles incited catalytic activity,18 and they have since demonstrated this phenomenon in Ptmodified Au nanoprisms as well.19 Although the favorable outcomes of combining Au and Pt have been explored in nanocatalysts, questions remain about the relationship between their optical response and catalytic properties. Previous characterization of radiative plasmon relaxation has shown that Pt damps the Au nanoparticle’s scattering and photoluminescence.18,19 However, this picture is not complete without an understanding of how the Pt also affects the non-radiative properties of Au and vice versa how the plasmonic metal changes absorption and hot carrier generation in the catalytic metal of these hybrid nanostructures. In this work, we correlated the scattering, absorption, and photoluminescence of single Ptdecorated Au nanorods. Specifically, we used photothermal absorption spectroscopy38,39 to quantify the absorption in individual Au and Pt-decorated Au nanorods directly. For the same single particles, the absorption spectrum was correlated to plasmon radiative relaxation pathways through scattering and photoluminescence spectroscopy. This combination of techniques revealed that the Pt islands significantly


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broadened the plasmon resonance of individual nanorods in both scattering and absorption. In particular, Finite Difference Time Domain (FDTD) simulations demonstrated that the majority of absorption in the hybrid nanostructure was localized in the Pt islands and that this absorption enhancement originated from the local electric field surrounding the Au nanorod core. The Pt islands were also found to reduce photoluminescence emission in the hybrid nanostructures either because absorption enhancement increases the direct production of hot carriers in Pt or charge transfer from Au to Pt occurs following electronic transitions in the Au nanorod.

RESULTS AND DISCUSSION Ensemble characterization The Au and Pt-decorated Au nanorods investigated here were chemically synthesized and first characterized in ensemble using ultraviolet-visible-near infrared (UV-vis-NIR) extinction spectroscopy and transmission electron microscopy (TEM) as illustrated in Figure 1A – C. Tetrahexahedral Au nanorods were synthesized using a modified seed-mediated growth method40 and small Pt islands were grown through reduction of H2PtCl6 to form Pt-decorated Au nanorods.41 The Au nanorod extinction spectrum (Figure 1A, red diamonds) exhibits two peaks of similar height with one appearing at ~550 nm and the other at ~700 nm. The peak at 700 nm results from the oscillation of electrons along the length of the nanorod, which is known as the longitudinal surface plasmon resonance (LSPR). The peak at 550 nm arises from multiple sources including Au interband transitions, the transverse surface plasmon resonance (TSPR), and any nanosphere impurities in the nanoparticle solution. Figure 1B shows a TEM micrograph of representative tetrahexahedral Au nanorods that have an average width of 60 ± 5 nm and length of 120 ± 10 nm prior to Pt deposition. The inset of Figure 1B further illustrates the faceted tetrahexahedral nanorod geometry. TEM characterization after Pt deposition yielded a composite nanorod size of 69 ± 3 nm by 129 ± 11 nm (Figure S1). TEM also revealed that the Pt islands exhibited a variety of shapes ranging from ellipsoidal to mushroom-esque similar to those characterized on Pt-decorated Au nanoprisms.42 These irregular Pt islands had an average dimension of 6 ± 2 nm and an average aspect


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ratio of 1.7. Figure 1C displays a TEM micrograph of a single Pt-decorated Au nanorod and highlights the structure of the Pt nanoparticle coating, which is a series of elongated island architectures of relatively uniform length pendant to the Au nanorod surface. The geometry of the Pt-decorated Au nanorod sample including contact between the Au nanorod core and Pt islands was further analyzed using high resolution transmission electron microscopy (HRTEM) and scanning transmission electron microscopy-energy dispersive spectroscopy (STEM-EDS). Figure 1D – E present HRTEM characterization of a Pt-decorated Au nanorod from the same sample solution. The red inset in Figure 1E shows a close up of the lattices of the Au nanorod and Pt islands. Fast Fourier transform analysis of the Au (Figure 1E green) and Pt (Figure 1E blue) areas match to the [011] zone axes of each metal indicating epitaxial contact. The measured lattice constants for both areas are also consistent with those expected for Au and Pt: the blue area yields a lattice constant of 4.03 (Pt = 3.92)43 and the green area yields a lattice constant of 4.16 (Au = 4.08)43 which are within the error of the resolution capabilities of this particular TEM. Figure 1F displays a high angle annular dark-field (HAADF) STEM image of the Pt-decorated Au nanorods. STEM-EDS maps for Au (Au Lα line intensity) and Pt (Pt Lα line intensity) are presented in Figure 1G – H, respectively. These maps are overlaid on the HAADF image in Figure 1I revealing that Pt is observed preferentially at the edges of the composite nanostructures. This characterization confirms that under the synthetic conditions used to produce these hybrid nanostructures, the Pt islands are in contact with the Au core and form a dense layer across the Au nanorod surface.


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Figure 1. Characterization of the ensemble optical properties and morphology of Au and Pt-decorated Au nanorods. A) Comparison of the ensemble extinction spectra for the same concentration of Au nanorods (red diamonds) and Pt-decorated Au nanorods (blue circles) reveals that Pt deposition broadens the Au nanorod plasmon resonance and reduces its overall intensity by approximately 50%. B) TEM micrograph of representative Au nanorods from the same sample used for single particle measurements. The inset of Figure 1B provides a visual depiction of the tetrahexahedral Au nanorod geometry. C) A close up TEM micrograph of a representative Pt-decorated Au nanorod showing the dense coverage of small Pt islands on the entire Au nanorod surface. D – E) HRTEM micrographs of Pt-decorated Au nanorods. The red inset shows a close up of both the lattices of the Pt islands (shown in blue) and the Au nanorods (shown in green). Fast Fourier transform analysis of the corresponding areas match to the [011] zone axes of both metals indicating epitaxial contact between the Pt islands on the Au substrate. F – I) STEM-


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EDS analysis of Pt-decorated Au nanorods. F) HAADF STEM image of the Pt-decorated Au nanorods. G) Au STEM-EDS map (Au Lα line intensity). H) Pt STEM-EDS map (Pt Lα line intensity). I) Overlay of Au and Pt maps on the HAADF image. Pt is observed primarily at the edges of the nanorods. N.B. Although the main lines of Au and Pt overlap, we have used the Pt and Au Lα lines in order to avoid this issue.

The ensemble extinction spectrum shows that Pt damps the Au nanorod plasmon resonance. The extinction is reduced by a factor of 2 and the maxima of the TSPR and LSPR modes have merged into a broad featureless band (Figure 1A, blue circles). However, because extinction encompasses both absorption and scattering the origin of this damping remains ambiguous. Scattering is responsible for the majority of extinction for Au nanorods in this size range where the scattering cross section is approximately 4 times larger than the absorption cross section (Figure S11). However, Pt deposition may alter this ratio of scattering to absorption. Additionally, because ensemble measurements sample many millions of nanorods simultaneously, broadening could also originate from native polydispersity in the Ptdecorated Au nanorod morphology as it has been known that even small structural changes can affect the LSPR peak position.16,44-47 We therefore studied the optical properties of Pt-decorated Au nanorods at the single particle level in order to elucidate the impact of Pt islands on the plasmon absorption and scattering of the Au nanorod core, as well as to distinguish the effects from the role of sample polydispersity. We first present a detailed characterization of the scattering and absorption spectra of bare Au nanorods which served as a control for elucidating the interactions between Pt and Au in these bimetallic nanostructures.

Scattering and absorption of single Au nanorods The scattering spectra of single Au nanorods follow expected trends in peak intensity and shape (Figure 2A – C) considering the distribution of nanorod sizes. Single particle scattering spectra were measured in glycerol using a homebuilt hyperspectral dark-field scattering microscope.48 The glycerol medium was chosen to keep the conditions the same across all optical measurements. FDTD simulations confirmed


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that the spectral properties of Au and Pt-decorated Au nanorods were similar in glycerol and water, which represents a common environment in catalytic experiments (Figure S13).18,19 Panels A – C in Figure 2 show the scattering spectra of three representative Au nanorods (red, orange, and blue diamonds) in order of decreasing size from top to bottom. Although arbitrary in value, the scattering intensities of the three nanorods can be directly compared to one another as well as to the scattering spectra shown below. In the long wavelength region of these spectra, the LSPR dominates with a Lorentzian peak at 768 nm, 742 nm, and 730 nm, respectively. The difference in LSPR peak position between single particle scattering spectra and the bulk extinction spectrum (Figure 1A) arises because of the glycerol medium, which increased the refractive index around the Au nanorods. Additionally, the shorter wavelength peak in the bulk extinction is largely absent from the single nanorod scattering spectra (see Figure S6 for the 500 – 550 nm region). This difference can be attributed to the missing contributions from interband absorption (Figure S11) though a small peak from the TSPR is still detectable because the Au nanorod widths were > 50 nm. The relatively large Au nanoparticle size was chosen for this work to ensure that the optical response of the Au nanorods remained detectable despite damping by Pt deposition. The overall scattering intensity varied across Au nanorods because the scattering cross section is related to the square of the nanorod volume (additional examples are given in Figure S5).47,49 Panels A – C show that the scattering intensity decreases with decreasing Au nanorod volume (A > B > C) as expected. Accounting for the Au nanorod sizes and glycerol medium, FDTD simulations reproduced all features of the experimental scattering spectra (Figure 2A – C). FDTD simulations were performed using Lumerical (see methods section and supporting information (SI) for details). SEM micrographs provided an initial size estimate and the size of each Au nanorod was then further refined to achieve agreement in the LSPR position with the experimental scattering spectrum. In each case the final size was within ± 5 nm of the one determined from the SEM images, well within the error associated based on the resolution of the SEM operated in low vacuum mode (5 - 10 nm). The intensities of the FDTD simulated scattering spectra (gray lines) in Figure 2A – C are normalized to the experimental LSPR peak height in each panel in order to enable the most direct line shape comparison. However, the relative intensities of the FDTD


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simulated scattering spectra show the same order as the experimental intensities (Figure S9) confirming that differences are primarily a consequence of Au nanorod size and shape. However, we also note that slight differences in the orientation of each nanorod on the substrate and the resulting charge interactions may contribute to intensity variations in the experimental spectra not accounted for in the simulations.49-51 The line shapes of correlated Au nanorod absorption spectra (Figure 2D – F) follow the trends seen for the Au nanorod scattering spectra. We measured the absorption of the same nanorods covering a range of 550 nm – 1000 nm using our recently described technique, photothermal absorption spectroscopy (see SI for additional details).38,39 This technique is an expansion of photothermal heterodyne imaging, which probes the heat released by a nanoparticle through a change in refractive index of the surrounding medium.44,52-59 In metal nanostructures the dissipated energy is equivalent to absorption because the energy of the absorbed photons is quickly and efficiently converted into heat.46,53,56,60 Incoherent radiative emission has a quantum yield on the order of 10-6 and while it does not contribute to heating, it can be safely neglected here.

61,62

Unlike photothermal imaging, photothermal

absorption spectroscopy incorporates a tunable white light laser to probe absorption as a function of excitation wavelength, generating an absorption spectrum.39 Figure 2D – F show single particle absorption spectra (filled red, orange, and blue diamonds) for the same Au nanorods as pictured in Figure 2A – C. All of the absorption spectra are plotted using the same scale, so that their relative intensities are comparable. However, the experimental scattering and absorption intensities are not directly comparable even for the same nanorod because they were acquired using independent techniques. The position of the LSPR in absorption and scattering is correlated for each of the Au nanorods. The absorption spectra exhibit higher relative intensities than the scattering spectra in the region of 550 – 600 nm because of contributions from the TSPR as well as Au interband transitions.


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Figure 2. Scattering and absorption spectra of 3 individual Au nanorods. Panels A, B, and C compare the experimentally measured dark-field scattering spectra (red, orange, and blue diamonds) to the FDTD simulated scattering spectra (gray lines). All experimental scattering spectra are plotted using the same scale so that their relative intensities can be directly compared. The FDTD simulated spectra are normalized to the experimental LSPR peak height to aid in line shape comparison. Correlated SEM micrographs are presented at the bottom of the figure and are circled in the same color as the corresponding Au nanorod spectra. Panels D, E, and F compare the experimental absorption spectra measured via photothermal absorption spectroscopy (red, orange, and blue diamonds) to absorption spectra simulated using FDTD (gray lines). Similar to scattering, all absorption spectra are plotted using the same intensity scale so that their relative magnitudes can be compared directly. The Au nanorod spectra show excellent agreement between experimental results and FDTD simulations for both scattering and absorption.

FDTD absorption simulations were also performed using the same experimental conditions and nanorod sizes as used to simulate scattering without any further adjustments once satisfactory agreement for the experimental and simulated scattering spectra was achieved. This procedure produced excellent agreement with the measured photothermal absorption spectra (Figure 2D – F). FDTD absorption spectra presented in Figure 2 are normalized to the experimental LSPR peak height in order to highlight spectral


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shape agreement. However, FDTD absorption simulations also reproduced differences in LSPR peak intensity between the three Au nanorods (Figure S9) showing the same trend in relative intensity as observed in experiment (F > E > D). The FDTD simulated absorption spectra (gray lines) agree with experimental results in terms of peak position as well as in the relative intensity of the short wavelength and LSPR regions. These results verify that photothermal absorption spectra represent an accurate characterization of optical absorption in metal nanostructures despite the reliance on heat as an intermediate.

Scattering and absorption of single Pt-decorated Au nanorods Unlike the spectra of the uncoated Au nanorods, the scattering spectra of single Pt-decorated Au nanorods reveal strong Pt-induced plasmon damping and an associated large variation from particle to particle. Figure 3A – F present the scattering spectra of 6 different Pt-decorated Au nanorods from the same sample solution (periwinkle, blue, navy, yellow, orange and red circles). Correlated SEM characterization of each Pt-decorated Au nanorod (bottom of Figure 3) confirmed that these spectra originated from single rod-like nanostructures. Ensemble TEM also verified that the majority of Pt-decorated Au nanorods in the sample were similarly coated with a dense layer of Pt islands (Figure 1C – I, Figure S1). Therefore, the increased heterogeneity in these scattering spectra compared to the uncoated nanorods likely resulted from differences in the Pt coatings from nanorod to nanorod; either in the distribution of Pt islands across their surfaces and/or in the morphology of the Pt islands themselves, which together result in a variety of local Au–Pt interactions within each single nanostructure (additional spectra provided in Figure S7). Scattering spectra in Figures 2 and 3 are presented using the same intensity scale which reveals that all of the Pt-decorated Au nanorods showed lower scattering intensities than those recorded for bare Au nanorods under the same conditions (glycerol medium, 3 s integration time, 3200 K white light excitation). The scattering spectra of one sub-population, represented by the spectra in Figure 3A – C, retained the signature spectral shape of an uncoated Au nanorod, i.e. a distinct LSPR mode. In these 3 example spectra, the influence of the Pt islands manifests itself as a redshift, broadening, and reduction in


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intensity of the LSPR, typical indicators of plasmon damping (see Figure S20 for a direct comparison).46,63 A second sub-population of Pt-decorated Au nanorods (Figure 3D – F) illustrates the extreme case of Pt-induced plasmon damping. These scattering spectra are dim and almost featureless. Although not directly observable from the SEM images, it is likely that more Pt was deposited on these nanorods. Interestingly, despite the significant variation in scattering line shapes, nanorod polydispersity is not solely responsible for the plasmon damping observed in the ensemble extinction spectrum (Figure 1A) because the broad line shape is also present in the individual spectra. Correlated absorption spectra of the same Pt-decorated Au nanorods also exhibit diverse, broad features. Absorption spectra acquired via photothermal spectroscopy are displayed for the same 6 Ptdecorated Au nanorods in panels G – L of Figure 3 (solid periwinkle, blue, navy, yellow, orange, and red circles). These spectra share most of the characteristics discussed for scattering (spectral broadening and redshift), and also further demonstrate the wide variety of optical properties observed for single Ptdecorated Au nanorods, likely because of heterogeneity in Pt island morphology (additional spectra are included in Figure S7). Similar to scattering, the absorption spectra in Figures 2 and 3 are presented using the same intensity scale to allow for direct comparison between absorption of the uncoated and Ptdecorated Au nanorods. In contrast to the bare Au nanorods, the contribution of the LSPR was minimized in roughly half of the Pt-decorated Au nanorod absorption spectra (e.g. Figure 3H, K, and L). Other Ptdecorated Au nanorod absorption spectra exhibited higher absorption intensity in the LSPR region than at shorter wavelengths (e.g. Figure 3G, I, and J), but these peaks were also much broader than those observed in the uncoated Au nanorod sample (see Figure S20 for a direct comparison). In general the absorption intensities of the bare and Pt-decorated Au nanorods were much closer in value than the scattering intensities, which were dramatically reduced in the presence of Pt. This trend is consistent with the one previously observed in the optical response of Ag-Pt core-shell nanocubes.64 These Ag-Pt coreshell nanocubes exhibited less dramatic Pt-induced spectral changes likely because of the very thin Pt shell (1 nm). However, similar to the results presented here, Pt deposition produced a clear decrease in scattering intensity and an increase in absorption.64


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Figure 3. Correlated absorption and scattering spectra from 6 different individual Pt-decorated Au nanorods. A-F) Experimental dark-field scattering spectra (periwinkle, blue, navy, yellow, orange, and red circles) of single Pt-decorated Au nanorods. These scattering spectra are presented on the same scale as the uncoated Au nanorods in Figure 2, so that their intensities can be directly compared. G-L) Experimentally measured photothermal absorption spectra (periwinkle, blue, navy, yellow, orange, and red


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circles) of the same Pt-decorated Au nanorods. Similar to the scattering spectra, the absorption spectra are displayed on the same scale as the uncoated Au nanorods in Figure 2. Correlated SEM micrographs on the bottom of the figure are circled in the same color as the corresponding absorption and scattering spectra.

Simulations confirmed that spectral changes after Pt deposition are a consequence of coupling between Au and Pt in the hybrid nanostructure. Figure 4A presents the FDTD calculated absorption of an isolated 61 × 115 nm Au nanorod (red line) and an isolated 6 nm spherical Pt island with its intensity multiplied by 1200 (blue line). In the unlikely case that the Au nanorod and Pt islands do not interact, the overall absorption of the hybrid nanostructure should be equivalent to the sum of the components evaluated separately. Figure 4B compares the sum of the two spectra shown in Figure 4A to the absorption spectrum of the Au nanorod alone, and illustrates that the sum of the spectra does not deviate significantly from the bare nanorod spectrum because the Pt islands show no obvious absorption features in this spectral window. This result is far from representative of the experimental Pt-decorated Au nanorod absorption spectra and therefore strongly suggests that Au–Pt interactions do in fact strongly alter the absorption. The same is also true for scattering, but is not presented here because the scattering cross section of a single Pt island is 4 times smaller than its absorption cross section (Figure S10). We further explored the influence of the Pt islands on the optical response through FDTD simulations using idealized geometries. The Pt islands were highly irregular in shape making it difficult to model the exact morphology of the Pt-decorated Au nanorods. From TEM micrographs and prior work,41,42 we estimated an average Pt island size of 6 ± 2 nm. However, without detailed topographic information about each individual Pt-decorated Au nanorod measured here, we are unable to account for factors like Pt island placement, surface roughness, merging of nearby Pt islands,42 or Au–Pt alloy formation, all of which could impact the optical response. Deeper insight could be obtained in the future via correlated 3D TEM tomography; however this technique is not yet readily compatible with the medium (glycerol in our case) required for photothermal spectroscopy. Despite these shortcomings, we can still apply insights from FDTD simulations to qualitatively explain trends in our experimental spectra.


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Therefore, we chose to approximate these hybrid nanostructures in simulations as hemispherical capped cylindrical Au nanorods covered in randomly distributed Pt nanospheres (Figure 4 center model). Control FDTD simulations verified that the nanorod geometry (Figure S12) as well as the shape and ordering of the Pt islands (Figures S14 and S15) did not significantly alter any of the trends observed for the overall line shapes of the scattering and absorption spectra (Figures 4 and 5). Memory limitations restricted the maximum number of Pt nanospheres in our simulations to 1200, around one quarter of the coverage anticipated from TEM characterization. To account for this discrepancy, we also simulated the same Au nanorod with a 6 nm thick continuous Pt shell, which represents a rough approximation of the hybrid nanostructure for maximum Pt coverage. The FDTD simulations predict that the addition of Pt islands to the Au nanorod damps the scattering response while simultaneously enhancing absorption. Panels C and D of Figure 4 illustrate the effects of adding Pt islands to the surface of a 61 × 115 nm Au nanorod. We simulated the scattering and absorption spectra of the Au nanorod when decorated with 200, 400, 800, and 1200 Pt nanospheres of 6 nm diameter and when coated with a continuous 6 nm thick Pt shell (approximate model geometries are depicted in the middle part of Figure 4). Simulated scattering spectra (Figure 4C) show that as the Pt nanoparticle coverage increases, the scattering maximum shifts to longer wavelengths, broadens, and loses intensity. In fact, the intensity of the Au nanorod LSPR peak decreased by 50% with the addition of only 200 Pt nanoparticles. In contrast, FDTD simulations of absorption (Figure 4D) predict an increase in intensity after Pt nanoparticle deposition in addition to redshifts and broadening. Enhanced absorption in the bimetallic nanostructure is due in part to the large imaginary component of the Pt dielectric function, which is almost four times greater than that of Au.65,66


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Figure 4. A) FDTD calculated absorption spectra of a single 61 × 115 nm Au nanorod (red line) and an isolated 6 nm Pt nanosphere with its intensity multiplied by 1200 (blue line). B) Sum of the separated absorption components plotted in panel A (dotted navy line) compared to the absorption spectrum of the bare Au nanorod alone (red line). The scattering cross section of 1200 single Pt islands is reduced compared to absorption by a factor of 4 (Figure S10) so that the sum of Pt island and Au nanorod scattering is altered even less than absorption. C) Evolution of FDTD calculated scattering spectra (colored lines) as a function of an increasing number of Pt islands on the Au nanorod surface (100, 400, and 1200 Pt islands). The Pt islands were randomly placed on the Au nanorod using a uniform random distribution of different angles for the central cylinder and end hemispheres. We have also included the spectrum of a Au nanorod with a 6 nm thick Pt shell (dotted gray line) to represent maximum Pt island coverage. D) Corresponding calculated absorption spectra for the same Pt-decorated Au nanorods. Generic models of the geometries used for FDTD simulations are depicted above panels C and D.


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Overall, these FDTD simulations correctly predict Pt-induced changes to the plasmon resonance scattering and absorption, but fail to account for the variety of spectral details showcased in experiment. In the case of scattering, the loss of intensity, broadening, and spectral redshift seen in experiment are all well accounted for in the FDTD simulations. Absorption measurements and FDTD calculations also agree in terms of broadening and redshift at this Pt coverage. However, FDTD simulations predict that the addition of Pt islands will increase the absorption intensity in the LSPR region, a trend that does not always hold true experimentally. It is observed that, in contrast to the scattering spectra that show clearly reduced intensities for all Pt-decorated Au nanorods, the intensities of the bare and Pt-decorated samples were more similar in absorption. However, in contrast FDTD simulations predict enhanced absorption especially in the LSPR region for the Pt-decorated Au nanorods. Additionally, FDTD modeling of specific Pt-decorated Au nanorods using sizes extracted from SEM did not produce line shapes consistent with experiment for scattering or absorption (Figure S19). Considering the excellent agreement between experiment and theory for the uncoated nanorods (Figure S2) and the fact that neither scattering nor absorption can be accurately modeled in case of the Pt-decorated Au nanorods (Figure S19), we conclude that the inconsistencies with FDTD simulated spectra must originate from limitations of our simplified model while the experiments are clearly necessary in order to understand the true properties of these hybrid nanostructures. Note that we carefully ensured that laser excitation did not alter the nanoparticle morphology by ensuring that the measured absorption intensity was stable over three photothermal runs (Figure S2). Given the simplifications made in the FDTD model required by the limitations in computational time and based on the complex nanoparticle morphology, it is not a surprise though that no quantitative agreement was achieved for the Pt-decorated Au nanorod spectra (Figure S19). Variations in overall Pt coverage (Figure 4, Figure S15), Pt island placement,11 and distributions of Pt island shapes (Figure S14) and sizes (Figure S16) are the likely main factors that give rise to the large spectral variations from particle to particle. In particular, an inhomogeneous Pt island size distribution that includes the presence of larger Pt islands could contribute to the lack of defined measured spectral


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features as FDTD simulations show that increasing the Pt nanoparticle size significantly distorts the spectral line shape (Figure S16).

Figure 5. A) Schematic illustration of the geometries used to calculate Au and Pt absorption separately. B-C) The absorption of a core-shell Au-Pt nanorod (total size of 61 × 115 nm) with different shell thicknesses was separated into contributions from Au (B) and Pt (C). The core-shell nanorod structure was utilized for these particular simulations to allow for straightforward integration over the Au and Pt areas. These spectra highlight how the optical properties of Au and Pt are independently affected by their interaction within the hybrid nanostructure: absorption in Pt increases while absorption in Au decreases as the shell thickens. Note that the sum of the 6 nm Pt shell spectra presented in Figure 5B – C is equal to the 6 nm Au-Pt core-shell absorption spectrum in Figure 4D. D-E) Local field enhancement (E/E0)2 maps for a bare 61 × 115 nm Au nanorod (D) and a core-shell Au-Pt nanorod with a Pt shell thickness of 3 nm (E).


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Dividing absorption into separate contributions from Au and Pt reveals that the Au nanorod acts as an antenna and enhances Pt absorption. Figure 5 shows absorption of the Au (Panel B) and Pt (Panel C) components extracted from a simulated core-shell Au-Pt nanorod as a function of Pt shell thickness. Notably, as the amount of Pt increases, absorption from the Au nanorod LSPR decreases, similar to previous studies of absorption in coupled Au-Pd nanodisks.67 In direct contrast, Pt absorption grows in intensity with significant enhancement centered at the former position of the Au nanorod LSPR. Though more Pt atoms contribute to the absorption cross section as the shell thickness grows, the increase in intensity exceeds that expected based on added volume alone. This result is readily explained by the local field enhancement.22 The total size of the core-shell structure (Au + Pt) was kept constant in these simulations, guaranteeing that the increase in absorption did not originate from a change in volume. Simulations with a constant Au core size show a similar damping of the Au nanorod absorption (Figure S18). To illustrate the electronic interaction between the Au nanorod and Pt islands, we calculated the local field enhancement, which is the square of the electric field E normalized by the excitation field E0, surrounding a Au nanorod (Figure 5D) and a Au-Pt core-shell nanorod (Figure 5E). The near field enhancement (E/E0)2 is concentrated at the tips of the Au nanorod where it remains despite plasmon damping in the hybrid nanostructure. Therefore in our experimental nanostructures, the Au nanorod acts an as antenna, amplifying absorption in the Pt islands in our experimental nanostructures, the Au nanorod acts an as antenna, amplifying absorption in the Pt islands in a mechanism similar to that seen in Pd decorated Al nanospheres (Figure S17).20,22,23 Additionally, because the imaginary part of the dielectric function of Pt exceeds that of Au by a factor of four,65,66 even limited field enhancement in the Pt islands contributes significantly to the total absorption. This explanation is consistent with the picture that adding Pt to the Au nanorod surface introduces an additional plasmon decay channel through Pt absorption especially as Pt supports direct vertical electronic excitations in the visible region.64 The impact of the local field enhancement also helps to explain heterogeneity in the measured absorption spectra. For example, the subset of Pt-decorated Au nanorods that maintain a distinct LSPR


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peak (Figure 3A-C) might have fewer Pt islands located at the nanorod tips and edges compared to their highly damped counterparts. Furthermore, the lack of significant absorption enhancement seen in experiment could originate from additional factors that interfere with the local field enhancement of the Au nanorod such as Au–Pt alloy formation. Even though these calculations were performed for the simpler core-shell geometry, the same trends are expected for Pt island-decorated Au nanorods especially as Figures 4C and 4D demonstrate that this limiting case is consistent with the spectral evolutions seen for increasing Pt island coverage.

Photoluminescence of Au and Pt-decorated Au nanorods To further understand the Au–Pt interaction, radiative relaxation following absorption in Au nanorods was

characterized

experimentally

through

correlated

photoluminescence

spectroscopy.

Photoluminescence measurements were performed on a homebuilt confocal microscope with 488 nm laser excitation. Figure 6 panels A – C display the photoluminescence spectra of the three Au nanorods examined in Figure 2. The photoluminescence of Au nanorods tracks the plasmon resonance as expected based on previous studies of emission from metallic nanostructures.61,68 488 nm excitation provides enough energy to directly excite interband transitions in the Au nanorod and these hot carriers emit upon recombination and after internal relaxation through electron-electron scattering, resulting in photoluminescence that is amplified around the plasmon resonance.68,69 Inelastic Raman scattering may also contribute to the emission.70,71 When immersed in the glycerol medium that was used for photothermal spectroscopy, the photoluminescence intensity at the LSPR for the Au nanorods was decreased compared to the same nanorod measured in an air environment (Figure S21). Potential charge transfer from the Au nanorod to the solvent has been suggested to reduce the yield of radiative recombination and could explain this decrease.72,73 Similar to absorption in Figure 2 (see SI for direct comparison), Au nanorod photoluminescence spectra exhibit a rising intensity at shorter wavelengths. This line shape results from the blue excitation wavelength, which effectively excites interband transitions


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of higher energies. Also note that, compared to smaller Au nanorods (25 × 86 nm, Figure S21), this short wavelength emission is enhanced relative to the LSPR mode. Reduced photoluminescence in the presence of Pt islands indicates that Pt lowers the number of hot carriers in the Au nanorod available for radiative recombination. Figure 6D–I present correlated photoluminescence spectra of the same Pt-decorated Au nanorods shown in Figure 3. For these Ptdecorated Au nanorods, under the same excitation conditions the photoluminescence intensity was dramatically reduced across the entire spectrum, i.e. both at short wavelengths and at the LSPR mode. The photoluminescence was similarly affected when the Pt-decorated nanorods were measured in air confirming that the reduction in intensity was not a consequence of the glycerol environment (Figure S22). These results corroborate other investigations of bimetallic Au–Pt nanostructure photoluminescence in the literature that observed complete photoluminescence quenching for smaller Au nanostructures at high density Pt island coverage.18,19 Although a direct comparison before and after Pt coating of the same nanorod is not possible for any of our single particle measurements, we estimate that the reduction in emission varied between the different Pt-decorated Au nanorods from 25% to 80%. This variation is consistent with the polydispersity seen in the absorption spectra and assigned to the inhomogeneous Pt island decoration. In fact, the spectral shape of the photoluminescence for the Pt-decorated Au nanorods notably tracked their absorption spectra (Figure S23): when a LSPR mode is seen in absorption it is also observed in photoluminescence and vice versa.


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Figure 6. Correlated photoluminescence spectra for the same Au and Pt-decorated Au nanorods displayed in Figures 2 and 3. Photoluminescence spectra for both sample types were acquired using a laser power of 500 µW before the objective and an integration time of 20 s. A-C) Single Au nanorod photoluminescence spectra. The photoluminescence spectra track the plasmon resonance as seen in absorption and scattering, but show a smaller LSPR intensity relative to the short wavelength mode around 540 nm. The reduced intensity in the LSPR region is likely a consequence of interactions with the glycerol medium (Figure S21). D-I) Pt-decorated Au nanorod photoluminescence spectra. In all cases the photoluminescence intensity is significantly smaller with the Pt islands present at the nanorod surface. The mode corresponding to the LSPR mode is visible in the photoluminescence spectra from Pt-decorated Au nanorods in panels D and E. However, in all other cases the photoluminescence is almost completely reduced to a broad featureless emission band.

The reduction in photoluminescence intensity could be exacerbated by several factors: 1) a decrease in the absolute absorption cross section at the excitation wavelength of 488 nm, 2) a decrease in emission enhancement at the LSPR mode due to plasmon damping,69 3) a consequence of Pt localized absorption, and 4) charge transfer of hot carriers from the plasmonic to the catalytic metal as suggested by


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previous studies of Au-Pt nanostructure photoluminescence quenching, which was assigned to hot electron transfer from Au to Pt.18,19 From the FDTD calculations, we find that the absorption cross section of the hybrid nanostructure at 488 nm is increased compared to the bare Au nanorod (Figure 4). Therefore, a change in absorption cross section is not a valid explanation for the reduction of photoluminescence. We can also exclude that emission enhancement is the dominating factor because emission is quenched across the entire wavelength range while maximum emission enhancement only occurs at the LSPR mode and to a much smaller degree at shorter wavelengths. With our steady-state measurements we cannot, however, distinguish between mechanisms 3) and 4), i.e. direct production of hot carriers in Pt based on the calculated absorption enhancement in Pt (Figure 5, Figure S17) or charge transfer from Au to Pt following electronic transitions in the core metal. Ultrafast transient absorption measurements could potentially be used to resolve the mechanism in these bimetallic nanostructures.74,75 Comparing several nanoparticle architectures, e.g. those that include an insulating spacer layer of varying thickness between Au and Pt, could also help to further clarify the mechanism. Regardless of the mechanism, the reduction in photoluminescence intensity confirms that Au and Pt are strongly coupled, and more importantly for the design of nanocatalysts, that hot carriers are generated directly or by charge transfer in the catalytically active metal following photoexcitation. This result is consistent with studies of the photocatalytic activity of other bimetallic nanocatalysts that harness the plasmon resonance for hot carrier generation.22,23

64


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CONCLUSIONS In summary, our results demonstrate that structural, particle-to-particle heterogeneity is not the sole cause of damping in the bulk extinction response of Pt-decorated Au nanorods. Bare Au nanorods exhibited typical plasmonic responses for their size in both scattering and absorption with excellent agreement to FDTD simulations. In comparison, the scattering spectra of Pt-decorated Au nanorods showed features associated with plasmon damping including a redshift, broadening, and loss of intensity. Pt-decorated Au nanorods also displayed broadened and redshifted absorption spectra. Previous work on sparsely coated Au-Pt nanorods concluded that Pt’s largest impact was changing the overall nanostructure geometry.63 However, that conclusion does not adequately describe properties of the bimetallic nanostructures studied here. FDTD simulations using a simplified Pt-decorated Au nanorod model produced similar trends to experiment confirming that the Pt islands damp the plasmon resonance of individual nanostructures. However, the experimental responses were much more heterogeneous in line shape and intensity than predicted by theory. Notably, FDTD simulations suggest that the majority of absorption is localized in the Pt part of the bimetallic nanostructure and that Pt absorption is amplified by the local field enhancement surrounding the Au nanorod that remains despite plasmon damping. Therefore, we conclude that disagreement between FDTD simulations and experiment arises in part from the polydispersity of the Pt island coating. Photoluminescence studies also revealed that Pt influences the radiative relaxation of hot carriers following absorption as photoluminescence emission was considerably reduced in Pt-decorated Au nanorods. Taken together these results indicate that the plasmon resonance acting as an antenna aids in the generation of hot carriers in the catalytically active metal of bimetallic nanocatalysts either through absorption enhancement or charge transfer.


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METHODS General materials and methods. Hexadecyltrimethylammonium bromide (CTAB, 99%), chloroplatinic acid (H2PtCl6, 8 wt. % in H2O), hydrochloric acid (HCl, >99.999% trace metal basis), hydrogen tetrachloroaurate trihydrate (HAuCl4•3H2O, 99.999%), L-ascorbic acid (99%), silver nitrate (AgNO3, 99.9999%), sodium borohydride (NaBH4, 99.99%), and glycerol (C3H8O3, 99.5%) were obtained from Sigma Aldrich (St. Louis, MO) and used as received. 11-amino-1-undecanethiol hydrochloride (AUT, 99.2%) was purchased from Dojindo (Rockville, MD) and used as received. NANOpure™ water (Thermo Scientific, > 18.2 MΩ•cm) was used for all washing, synthesis, and purification protocols as well as in the preparation of all solutions. All stock solutions were aqueous and prepared fresh before each reaction, unless otherwise noted. All glassware and Teflon-coated stir bars were washed with aqua regia (3:1 ratio of concentrated HCl and HNO3 by volume) and rinsed thoroughly with water. Caution: Aqua regia is highly toxic and corrosive and requires the use of personal protective equipment. Aqua regia should be handled in a fume hood only.

Synthesis of Au nanorods. Au nanorods were synthesized by modifying previous literature protocols.40 Au seeds were prepared by adding 0.25 mL of a 0.01 M solution of HAuCl4 to 9.75 mL of 0.1 M CTAB in a 20 mL scintillation vial equipped with a stir bar. Meanwhile, a 0.01 M solution of fresh NaBH4 was prepared. Then, 0.6 mL of 0.01 M solution of NaBH4 was rapidly injected into the Au precursor-CTAB solution while vigorously stirring. The resulting seed solution was stirred for one minute, and then was covered loosely with parafilm and allowed to rest for three hours. A growth solution was prepared by sequentially adding the following to a 100 mL Erlenmeyer flask and stirring after each addition: 40 mL of 0.1 M CTAB, 2 mL of 0.01 M HAuCl4, 0.4 mL of 0.01 M AgNO3, 0.8 mL of 1.0 M HCl, and 0.32 mL of 0.1 M ascorbic acid. Upon addition of the ascorbic acid, partial reduction of the Au precursor causes the color of the solution to change from orange to clear. After preparation of the growth solution, the aged seeds were gently swirled in a water bath (37 °C) to dissolve any CTAB that may have recrystallized during the aging period. Next, the seed solution was diluted 50× by adding 200 µL of the seeds to 9.8 mL


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of 0.1 M CTAB. 200 µL of the diluted seeds were then rapidly injected into the growth solution while swirling. The growth solution was then mixed for three seconds and was allowed to rest undisturbed overnight. Initial growth of the Au nanorods is indicated by the gradual appearance of a faint orange-red color over the course of 30 minutes.

Purification of Au nanorods. To purify the resulting Au nanorods from unreacted reagents and/or excess surfactant, the solution was centrifuged at 2200 rcf for 5 minutes (Spectrum mini-centrifuge SC1006-R). After centrifugation, the supernatant was removed, and the pellet was resuspended in 1 mL of H2O. This washing process was repeated once more in order to yield a purified Au nanorod pellet. Au nanorods were then resuspended in water to a desired concentration as determined by extinction spectroscopy, where concentration was measured as the optical density (O.D) at λmax (~700 nm, see below for details pertaining to UV-vis-NIR measurements) of the LSPR. The solution of purified Au nanorods was then diluted with water to an O.D. of 1.0 for further use.

Deposition of Pt on Au nanorods. Deposition of Pt on Au nanorods was performed using similar methods to previous protocols.41 Briefly, 1 mL of the Au nanorod stock solution (O.D. of 1.0 at λmax) was added to a 1.5 mL centrifuge tube. To this solution, 20 mM ascorbic acid was added followed by the same volume of 2 mM H2PtCl6 (to maintain a constant ratio of 1:10 H2PtCl6:ascorbic acid) with mixing after each addition. The amount of Pt deposition could be altered by increasing the total amount of Pt precursor added to 1 mL of 1 O.D. of Au nanorods. For lower extents of Pt deposition, 10 µL of 20 mM ascorbic acid/2 mM H2PtCl6 was added, whereas for higher degrees of Pt deposition 75 µL of 20 mM ascorbic acid/2 mM H2PtCl6 was added, while all ratios of Pt precursor to reducing agent were held constant. After allowing one hour for nanoparticle growth, the reaction mixture was purified from excess reagent by centrifugation (5 minutes at 2200 rcf) to a pellet and removal of the supernatant. This purification process was repeated once more to yield pure Pt-decorated Au nanorods. After removal of the supernatant, the particles were resuspended in 1.0 mL of H2O by brief sonication (~10 s).


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AUT functionalization of Pt-decorated Au nanorods. To functionalize the Pt-decorated Au nanorods with AUT, 1.0 µL of 1 mM AUT was added to 1.0 mL of the product solution, and mixed at 800 rpm (Eppendorf, Thermomixer® R mixer-incubator with 1.5 mL block) for 12 hours at room temperature. After functionalization with AUT, the Pt-decorated Au nanorods were purified three times by centrifugation (5 minutes at 2200 rcf) and resuspended in 1.0 mL of H2O.

UV-Vis-NIR extinction spectroscopy. Colloids were measured by UV-Vis-NIR extinction spectroscopy using a Cary 5000 spectrophotometer (Agilent, Inc.). Spectra were baseline corrected to the spectrum of water for optical density measurements.

Transmission electron microscopy (TEM). After Pt deposition, the resulting nanoparticle products were purified by centrifugation (vide supra). The purified Pt-decorated Au nanorod pellet was then resuspended in 100 µL of H2O by briefly vortexing the solution (~ 5 s) followed by brief sonication (~ 5 s). A 10 µL aliquot of the concentrated, purified particles was drop cast onto a Formvar-backed (Ted Pella, Formvar on 400 mesh Cu) or ultra-thin carbon (Ted Pella, Carbon Type A on 300 mesh Cu) TEM grid. A Hitachi H-9500 ETEM (Nanoscale Fabrication and Characterization Facility (NFCF), Peterson Institute of Nanoscience and Engineering (PINSE), University of Pittsburgh) operating at 200-300 kV was used for all imaging. Additional scanning electron microscopy (SEM) characterization is included in the SI. SEM characterization was performed on silicon wafer substrates (University Wafer, p-doped) with a 200 nm thermal oxide (SiO2) layer. The substrates were first cleaned by sonication in ethanol for 5 minutes and then rinsed with ethanol and dried under air. A 10 µL aliquot of the concentrated, purified particle solution (as described above) was then drop cast onto the substrate and allowed to dry. Samples were imaged using a Zeiss Sigma 500VP at 20 kV (Nanoscale Fabrication and Characterization Facility, Peterson Institute of Nanoscience and Engineering, University of Pittsburgh).


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Scanning transmission electron microscopy energy dispersive X-ray spectroscopy (STEM-EDS). Elemental maps were collected using a JEOL JEM 2100F electron microscope (NFCF, PINSE, University of Pittsburgh). Oxford Aztec software was used for data processing and generation of elemental maps. EDS spectra were collected using a beryllium double tilt holder (JEOL #31640) with a STEM probe diameter of 1.0 nm. The EDS was acquired using 1024 channels from 0 to 10 keV. Elemental maps were collected for 15-20 minutes and the site autolock feature (Oxford Aztec software) was used to correct for sample drift during acquisition, with a pixel dwell time of 100 µs and a pixel resolution of 1024 × 1024. EDS maps were populated using signal from the Au and Pt Lα line intensities measured from 9.61 to 9.81 and 9.34 to 9.54 keV, respectively.

Single particle sample preparation. Coverslips were first treated in a dilute solution of ammonium hydroxide and hydrogen peroxide (1:4:20 NH4OH:H2O2:H2O). The solution was heated to ~ 60 ˚C and the coverslips were immersed and sonicated for 15 min. The coverslips were then rinsed twice via sonication in Millipore water for 15 minutes. Finally, the coverslips were dried with N2 and treated with UV-light in ambient conditions for 2 minutes to improve immobilization of the nanorods on the substrate. The Au and Pt-decorated Au nanorod solutions were sonicated for at least 5 minutes prior to deposition and then dropcast on a freshly UV-treated coverslip. After 1 minute, the solution was wicked away from the coverslip and the sample was dried again with N2. The samples were also patterned with an indexed grid prior to optical measurements to enable correlation of the same single nanorods across different optical techniques and SEM. The patterning process involved taping an uncovered Cu TEM grid (Ted Pella, Index 1 100 mesh Cu) to the sample and then evaporating a 2 nm Ti film followed by a 15 nm Au film onto the coverslip. Removal of the grid after evaporation left an indexed Au film on the coverslip, which we utilized for correlation and as the standard in photothermal absorption spectroscopy. Before optical measurements, the sample was embedded in glycerol by creating a liquid cell from a 150 µm thick silicone spacer and an additional coverslip.


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Single particle dark-field scattering measurements. Dark-field scattering measurements were performed on a custom hyperspectral set-up based on an optical microscope. The sample was excited through a high numerical aperture (NA) oil immersion dark-field condenser (NA = 1.4) with a broadband halogen lamp. The scattering response was collected through an oil immersion 63x objective with an NA of 0.7. This set-up is described in detail in Byers et al.48 Briefly, after scattered light exits the microscope it enters a rectangular slit of the spectrograph and is reflected by a grating to generate a two dimensional image of vertical position versus wavelength on a charge-coupled device (CCD) camera. During acquisition the spectrograph is translated on a LabView controlled stage so that the slit samples the entire region of interest creating a hyperspectral image. The exposure time for dark-field scattering measurements was fixed at 3 s per pixel. Nanoparticles are identified as bright spots in the image. Dark-field scattering spectra were extracted by integrating the intensity of the bright pixels and then correcting with measured background signal and a white light standard to account for the spectral lineshape of the excitation.

Single particle photoluminescence measurements. The photoluminescence spectra of single Au and Ptdecorated Au nanorods were measured in a confocal geometry.62 A 488 nm laser was focused on the sample through an oil immersion objective lens (63×, NA = 0.7). Emission was collected using the same objective and back-scattered 488 nm light was removed using a 488 nm notch filter. The photoluminescence signal was directed using a flip mirror to either an avalanche photodiode (APD) to obtain a raster scanned image of the sample or a CCD camera attached to a spectrograph to acquire single particle photoluminescence spectra. The power of the 488 nm laser was limited to 0.50 mW before the objective lens in order to avoid damaging the nanostructure. Three raw photoluminescence spectra were measured with an exposure time of 20 s for each nanorod and background spectra were also obtained under the same conditions. Photoluminescence spectra were analyzed by subtracting the average background signal from the average raw signal and then multiplying by a correction factor to account for any wavelength dependence of the collection optics as described previously.62


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Single particle photothermal absorption spectroscopy. Photothermal measurements were conducted using a homebuilt set-up described in previous publications38,39 (instrument diagram provided in Figure S3). For these experiments we used a 532 nm probe beam and a white light laser with a built in acousto-optical tunable filter (AOTF) as the heating beam. The probe and heating beam powers were limited to approximately 75 µW and 100 µW (recorded at 700 nm) at the sample, respectively. The heating beam was modulated at 30 kHz by applying a square wave function from a function generator to the AOTF. The intensity of the probe beam after the sample was monitored with a Si photodetector and lock-in amplifier. Transmitted light from the heating beam was removed with a 532 nm notch filter placed before the photodetector. The AOTF was operated using a custom LabView program to obtain absorption spectra (details explained in SI). The LabView program automatically recorded spectra of each nanoparticle found in a 30 × 30 µm image. Before removing the sample, a standard Au film spectrum was also measured under the same conditions. The data for each nanoparticle was analyzed by multiplying the average intensity for each wavelength by a correction factor derived from the measured Au film spectrum and a simulated Au film spectrum as described previously.38,39 For each nanoparticle, an absorption spectrum was measured using orthogonal excitation polarizations and both the visible (VIS) and nearinfrared (NIR) AOTF channels. The intensity of the NIR component of each spectrum was adjusted to match the VIS channel intensity in the region of 700 – 750 nm.38 Then the spectra obtained in each orientation were averaged to determine the unpolarized absorption response.

Finite Difference Time Domain (FDTD) simulations. In order to interpret the absorption and scattering spectra, FDTD simulations were performed using the Lumerical software. In our simulations, the scattering and absorption cross sections were calculated for plane wave excitation at normal incidence to the sample. The incident light was circularly polarized to mimic the unpolarized light used in dark-field scattering measurements and the unpolarized photothermal absorption spectra. The refractive index of glycerol and glass were assumed to be 1.47 and 1.52, respectively. To calculate absorption using the


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integration of losses inside of the metal nanoparticles (Figure 5), electric field data was first extracted from every mesh grid point.76 The field was then integrated over the entire volume of the Au nanorod or the volume of the Pt shell and multiplied by the imaginary part of the permittivity for the corresponding metal. 76 More details concerning FDTD simulations are presented in the SI.

SEM correlation following optical characterization. After optical measurements, the coverslip was prepared for SEM imaging by wicking away the glycerol, rinsing with ethanol for ~ 15 seconds, and drying with N2. SEM micrographs of single Au nanorods and Pt-decorated Au nanorods after optical characterization were obtained using a FEI quanta 450 SEM operated in low vacuum mode.


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications Website. Additional electron microscopy characterization, photothermal instrument diagram, additional single nanorod absorption and scattering spectra, control FDTD simulations, and photoluminescence measurements in different media.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] Present Addresses M.Y.: ASELSAN Research Center, Ankara 06370, Turkey Author Contributions P.J.S. and N.D. synthesized and characterized Au and Pt-decorated Au nanorods. S.A.H.J. performed dark-field scattering measurements and FDTD simulations. Photothermal absorption spectroscopy measurements were performed by A.J. with assistance from E.S., M.Y., and W.-S.C. SEM correlation after optical characterization was completed by A.J. E.S. performed the photoluminescence measurements. The project was supervised by S.L. and J.M. and the manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. A.J. and S.A.H.J. contributed equally to this work.

ACKNOWLEDGEMENTS We acknowledge support from the Robert A. Welch Foundation (C-1664) and the Air Force Office of Scientific Research (MURI FA9550-15-1-0022). M.Y. acknowledges financial support from the SmalleyCurl Institute at Rice University through a Carl & Lillian Illig Postdoctoral Fellowship. A.J.


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acknowledges support by the National Science Foundation Graduate Research Fellowship Program under Grant No. 1450681. We also thank Hangqi Zhou and Peter Nordlander for stimulating discussions.


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