Structure versus Composition: A Single-Particle Investigation of

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Structure versus Composition: A Single-Particle Investigation of Plasmonic Bimetallic Nanocrystals Alison F. Smith,†,‡ Rebecca G. Weiner,† Matthew M. Bower,† Bogdan Dragnea,† and Sara E. Skrabalak*,† †

Department of Chemistry, Indiana University, Bloomington, Indiana 47405, United States NAVSEA Crane, Crane, Indiana 47533, United States

‡

S Supporting Information *

ABSTRACT: Stellated bimetallic nanostructures are a new class of plasmonic colloids in which the interplay between composition and overall architecture can provide tunable optical properties and new functionality. However, decoupling the complex compositional and structural contributions to the localized surface plasmon resonance (LSPR) remains a challenge, especially when the monometallic counterparts are not synthetically accessible for comparison. Here, stellated Au−Pd nanocrystals (NCs) with Oh symmetry are used as a model system to decouple structural and complex compositional effects on LSPR. Singleparticle correlation of the LSPR with the structure of octopodal Au−Pd NCs was achieved using optical dark-field spectroscopy with scanning electron microscopy. These measurements were compared to calculations of the optical properties of structurally similar Au-only octopods by the finite difference time domain method. This comparison enabled the role of complex composition, which was determined by scanning transmission electron microscopy−energy-dispersive spectrometry measurements, on the LSPR to be elucidated from the structural contributions. This methodology provides a powerful framework to guide the design of new plasmonic colloids through both structure and composition.

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INTRODUCTION The ability to tailor properties of metallic nanoparticles through shape1 and composition enables applications in photonics, electronics, catalysis, sensing, and medical diagnostics.2−4 Bimetallic NCs offer multifunctional opportunities within a single platform.5 Yet plasmonic colloids with bimetallic compositions are less studied than monometallic structures owing to difficulties in synthesis, characterization, and modeling. This lack of emphasis on bimetallic nanomaterials is especially true for low-symmetry particles, and there are virtually no studies on concave polyhedral particles. Concave polyhedra, however, such as stellated Au and Ag particles hold technological promise because they exhibit enhanced electric fields localized on their sharp features, offering enhanced sensing opportunities.6−9 Moreover, their packings allow for broader control of the filling factor, which can be useful in the management of losses. However, these opportunities are often limited to monometallic nanostructures. Coupling this architecture with appropriate bimetallic compositions could provide platforms for monitoring catalytic processes, hydrogen sensing, and more.5 The rational design of stellated bimetallic platforms hinges upon criteria that define the effects of both structure and composition on the localized surface plasmon resonance (LSPR). However, decoupling these contributions remains a challenge because (1) slight inhomogeneities in both structural features and composition exist from particle to particle and even within one particle; e.g., gradient alloy compositions and (2) the relationship between the optical response, embodied by © 2015 American Chemical Society

a macroscopic dielectric constant, and the local composition of bimetallics remains unclear. Methods for handling this level of complexity, a bimetallic with gradient alloy composition and therefore gradient dielectric, to predict optical properties currently do not exist.10 That is, neither the analytical tools to accurately characterize the three-dimensional (3-D) distribution of metals in bimetallic NCs nor the theoretical methods to elucidate appropriate models for such gradient dielectric structures with high throughput exist. Here, a singleparticle investigation of Au−Pd octopodal NCs as a model system is undertaken to elucidate the contributions of structure and composition to the LSPR.

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EXPERIMENTAL SECTION Single-Particle Sample Preparation. The synthesis of Au−Pd octopodal NCs was achieved using the previously reported seed mediated coreduction method.11 Two Au−Pd NC samples were used in this study. The face diagonal (FD) and tip width (TW) average and standard deviation for the smaller particle size sample were FD = 154 ± 9 nm and TW = 15 ± 4 nm, and those of the larger particle size sample were FD = 252 ± 19 nm and TW = 24 ± 4 nm. Single-Particle Correlation Methodology. Fabrication of registration markings was achieved using physical vapor deposition (PVD). An alphanumeric transmission electron Received: July 12, 2015 Revised: September 2, 2015 Published: September 9, 2015 22114

DOI: 10.1021/acs.jpcc.5b06691 J. Phys. Chem. C 2015, 119, 22114−22121

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The Journal of Physical Chemistry C microscopy (TEM) grid was used as a mask through which Al metal was deposited as a registration marker. This method produced an alphanumeric grid pattern on an ITO/glass surface, and the resulting registration patterns are depicted in Figure S1. Au−Pd NCs were dropcast onto ITO (Delta Technologies, Inc.) substrates. A particle of interest was selected using optical imaging. That particle was then centered in the spectral window, and the spectrum of that particle was collected five times using darkfield spectroscopy. Scanning electron microscopy (SEM) images of these particles were then collected, and only symmetrically branched single-particle Au−Pd octopods were included in the data set. SEM images were obtained via a FEI Quanta 600F SEM instrument operated at 30 kV and a spot size of 3. FD and TW values were then measured, from the SEM images, using ImageJ software. Optical Imaging and Spectral Collection. The system consists of a Nikon T-DH inverted microscope with a 12 V halogen lamp light source, Nikon dark-field condenser lens with a 1.43−1.20 numerical aperture (NA), Nikon PLAN Fluor ELWD 40× lens with a 0.6 NA objective lens with a fine adjustment for working distance, Hamamatsu charge coupled device (CCD) digital camera C4742-95, Acton Research Spectra Pro 300 I spectrometer/spectrograph with a 0.3 m triple grating, Andor iDus model DU420A-BV spectral CCD camera optimized for wavelength detection in the visible range (approximately 400−800 nm), and Andor Spectra Pro software for spectral monitoring. The five spectra per particle were averaged then normalized with the source (halogen lamp) spectra using IgorPro software. 3-D contour plots were made in Origin 8.1 software using a thin-plate spline (TPS) algorithm, in which the two independent parameters (FD and TW) are plotted onto a two-dimensional grid; the grid size was set to 5 nm for FD and 1 nm for TW, and a smoothing parameter of 1 was used. The 3D wireframe plot with experimental data points and contour plots with equivalent scale values are shown in Figure S2. STEM-EDS Characterization. Scanning transmission electron microscopy (STEM) and high-resolution TEM (HRTEM) images were taken on a JEOL JEM 3200FS TEM at 300 kV and a spot size of 1 with a Gatan 4k × 4k Ultrascan 4000 camera. Energy-dispersive spectrometry (EDS) spectra were obtained with an Oxford INCA dispersive X-ray system interfaced to the JEM 3200FS TEM instrument, operating at 300 kV. Samples for TEM analysis were prepared by dropcasting a dispersed particle solution onto a carbon-coated Cu TEM grid. Grids were then rinsed with methanol. Line scan EDS measurements were obtained at the base and near a tip of individual Au−Pd NCs from both the large (FD = 252 ± 19 nm) and small (FD = 154 ± 9 nm) sample sizes as well as a (FD = 102 ± 13 nm) sample. Figure 1 and Figure S3 depict the Au regions (yellow) and the Pd regions (red), which demonstrate that Pd is located on the exterior of the Au−Pd NCs with Pd enriched at the tips. In all cases, the tip Pd concentration is greater than the base Pd concentration. Furthermore, tip Pd concentration increases with increased tip width (Figure 1 and Figure S3III,IV) within a margin of 5 atomic percent. Finite Difference Time Domain Calculations. To investigate the role of FD and TW on the LSPR, finitedifference time-domain (FDTD) numerical simulations were performed using Lumerical software. Sixteen single Au octopods with FDs of 100, 150, 200, and 250 nm, each with

Figure 1. Structure and gradient composition of a Au−Pd octopod. (a) STEM image of an individual octopod with the total atomic percentage of Au, 94%, noted in panel b. (b−d) STEM-EDS elemental mapping of corresponding octopod where Au is represented by yellow and Pd is red; (d) overlay of Au and Pd signals which are shown separately in panels b and c. STEM-EDS line scans through (e) the base of the branch and (f) near the tip of the branch illustrating Pdenrichment at the tip with the atomic percentage of Au reducing from 93% at the base to 88% near the tip.

TWs of 5, 10, 20, and 30 nm were modeled (Figure S4). The geometry of the Au octopod was constructed from truncated hexagonal pyramids (THPs) with a base apothem of 25 nm for FD values less than 150 and 50 nm for all others in accordance with structure sizes obtained from varying the cubic seed size. The apex apothem and height correspond to TW and FD, respectively. Au−Pd octopods with Pd localized at the tips were constructed in a similar manner with Pd tips modeled as 5 nm height THPs with a base apothem of 15 nm that were positioned onto the apex of the Au THPs. The total FD values of these Pd-tipped Au THPs was kept equivalent with those of Au-only. All NCs were additionally placed onto a 100 nm thick ITO surface with four pods on the surface to mimic experimental conditions. The scattering spectra (scattering cross-section as a function of wavelength) of the NCs was calculated using FDTD. The dielectric functions for Au were fitted to optical data from Johnson and Christy,12 and Pd dielectric functions were fitted to optical data from Palik.13 Refractive index values for ITO were also taken from Palik.13 The excitation source was a plane wave with a wavelength range of 400−1300 nm, which propagated through a surrounding medium with a refractive index of 1.0. The mesh values were set to (2 nm)3 for scattering and (1 nm)3 for electric field intensity maps. The propagation direction was along the C4 symmetry axis, and the direction of electric field oscillation with respect to the NC was modeled as depicted in Figure S4. Electric field intensity maps were calculated with FDTD at the maximum wavelength positions, which were obtained from the scattering spectra. These intensity maps for an Au-only octopod with FD = 200 nm and TW = 20 nm are shown in Figure S4. A single Au−Pd particle with a FD value of 112 nm and a TW value of 16 nm was modeled as THP branches extended from a Au cubic core with side length 30 nm. The branches were modeled as Au−Pd alloyed THPs with a base apothem corresponding the 50 nm base length and a TW of 16 nm. The Au−Pd alloy concentrations were varied from 100% Au to 70% Au (atomic %) in the range of STEM-EDS values (Figure S5a). Note that these alloy atomic percentages are for the branch compositions only. Considering the Au cubic core, the 70% and 85% Au branched octopods actually represent a 77% and 90% Au octopod. This total atomic percentage of Au is calculated 22115

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The Journal of Physical Chemistry C Scheme 1. Representation of Strategy Used to Decouple Composition and Structurea

a

Trends in LSPR shifts of the Au−Pd NCs that cannot be accounted for by structure (Au-only octopodal models) are attributed to complex composition. The structural parameters of the Au−Pd octopodal NCs are inset in the upper right corner.

correlated to the optical response of the Au−Pd octopods and also used as input for the simulations of Au-only octopods. The local environment was kept constant throughout this study, with the NCs supported on an ITO surface, resulting in a C4 symmetric system. Polarization orientation effects were determined to be minimal (Figure S6) in accordance with the absence of polarization anisotropy of compositionally homogeneous metallic nanocubes.17,18 STEM-EDS provides general information about the composition of the Au−Pd octopodal NCs. Dark-field microscopy and spectroscopy of single NCs followed by imaging with SEM was used to obtain singleparticle correlation19 of 79 Au−Pd octopods from the two size distributions. Optical−SEM correlation was achieved by a combined registration and constellation method20 in which registration markings identified the general area of particles of interest. The correlation methodology is depicted in Figure S1, along with a description of the setup for dark-field microspectroscopy and sample preparation in Experimental Section. The structural parameters and position of the LSPR maximum (λmax) for each NC analyzed are summarized in Table S1. On occasion, spectra from clusters of NCs or octopods oriented in configurations other than four pods on the surface were obtained, as revealed by SEM. They have been removed from this data set as our objective was to evaluate the contributions of composition and structure to the LSPR of individual bimetallic NCs, and other configurations would disrupt the resulting C4-symmetric system. STEM-EDS mapping of a representative octopodal Au−Pd NC is shown in Figure 1a−d, and line scans at the base and near a tip of the Au−Pd octopod are depicted in Figure 1e,f. The Pd, shown in red, is located on the entire exterior of the NC, with the Pd concentration being greater at the tip of the branch than at the base of the branch. The atomic percent Au decreases from 93% at the base to 88% near the tip (Figure 1e,f). This gradient concentration has previously been characterized also by high-resolution STEM, which revealed Pd tip enrichments up to 92 atomic % Pd by lattice constant measurements,21 and larger TW values correlating with a greater Pd-tip concentration.11,16,21 STEM-EDS measurements are shown for several Au−Pd octopods with different tip thicknesses in Figure S3. The dark-field spectra and corresponding SEM images of a small (FD = 112 nm) and of a large (FD = 262 nm) Au−Pd NC are shown in Figure 2a; the maximum scattering intensity is normalized to 1. To validate the Au-only structural model used in subsequent FDTD simulations, we attempted to simulate the experimental spectrum for the small Au−Pd NC on the ITO substrate in air, considering both structure and composition.

based on volume of the cubic Au core and volume of the total octopod, which can be calculated by previously reported methods14 or Google Sketchup. As the Pd concentration of the alloyed braches increased, the LSPR blue-shifted. The 70% Au branches were then modeled with 5 nm height tips, keeping the TW constant at 16 nm and varying the Pd-tip concentrations: 70% Pd and 100% Pd (Figure S5b). As the Pd concentration of the tips increased, the LSPR red-shifted. Finally, the contact area between the 85% Au−Pd octopod and the ITO substrate was varied from a distance of 0 nm with only a vertex of the branch in contact with the ITO to a distance of 0.5 and 1 nm into the ITO surface, effectively increasing the contact area (Figure S5c).

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RESULTS AND DISCUSSION This single-particle investigation used correlative optical microspectroscopy and scanning electron microscopy to obtain the frequency-dependent optical properties of single Au−Pd NCs which were compared with simulations of the optical properties of structurally analogous Au-only NCs. This way, complex compositional contributions to LSPR have been decoupled from structure. The central premise is represented in Scheme 1: trends in the LSPR shif ts of Au−Pd octopods which cannot be attributed to Au-only octopods will be attributed to the gradient Au−Pd composition. Relevant structural parameters of the model system are also defined in Scheme 1. Specifically, octopodal Au−Pd NCs, eight branched NCs with Oh symmetry and a gradient Au−Pd exterior alloy and Au interior bimetallic distribution, were synthesized by seed-mediated coreduction and selected for analysis according to structural parameters.11 This structure was selected as the model system on account of its stellation and complex bimetallic composition, which affords high sensitivity to changes in refractive index.15 The complex composition is a gradient Au−Pd alloy in which there is Pdenrichment at the tips of the octopodal structures. These octopods are synthetically accessible across a range of FD lengths and TWs, and the LSPR wavelength changes greatly with size. For example, LSPR values of between 565 and 746 nm are exhibited for octopods with FD values ranging from 61 to 143 nm when dispersed in water.16 An octopod with FD length of 112 nm was selected to validate mode assignments as the LSPR is sufficiently blue-shifted to be captured by the Si avalanche detector. However, octopods of those dimensions are difficult to characterize optically and present challenges in discerning tip features by SEM. Therefore, larger samples with two different average sizes were selected for the full study: those with 154 ± 9 nm and 252 ± 19 nm FD with a TW range of 9−30 nm (see Scheme 1). These structural parameters were 22116

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which is within the range of total %Au values determined experimentally by STEM-EDS (Figure S5a). The good match between our experimental and simulated spectra, i.e., both a distal and a proximal mode, indicates that relevant structural features are captured by our model and that the approach outlined in Scheme 1 should reveal insight into the Au−Pd distributions of these NCs. Note that the 18 nm red shift in the distal mode of the simulated spectra compared to the experimental spectra likely arises from the FDTD model not fully encompassing the complex gradient distribution of Pd. Recall that STEM-EDS elemental mapping indicates Au-rich regions at the base of octopodal branches, while high-resolution STEM reveals Pd enrichments up to 92% Pd by lattice spacing measurements at the branch tip ends.21 Of course, this compositional detail at the tip cannot be revealed by the line scan analysis in Figure 1 and Figure S3. Therefore, the effect of Pd% tip enrichment, across a range, is presented by simulation in Figure S5. These simulations also found that the relative intensity of the bands, but not positions, was sensitive to the height of the octopod from the ITO substrate, with the intensity of the LSPR band centered at λmax= 681 nm decreasing as the particle approaches the ITO substrate (Figure 2 versus Figure S5). The 112 nm FD octopod is characterized by two LSPR bands (λmax = 530 nm; λmax = 688 nm); however, the 262 nm FD Au−Pd octopod is characterized by a single LSPR band (λmax = 702 nm), within the visible spectral range. From both theoretical and experimental studies of extrinsic size effects, it is well-known that an increase in NC size will generally result in a red-shift.25,26 Thus, the observation of only one LSPR band for the larger NC suggests that a red-shift of the LSPR has moved the second band outside of the detection window, as the avalanche silicon photodiode detector is optimized for the 400−800 nm range. This shift with increasing size is consistent with ensemble measurements of different sized Au−Pd octopods in water.16 The LSPR modes corresponding to these experimental spectral bands were initially assigned using FDTD near-field calculations for the FD = 112 nm Au−Pd NC (Figure 2b,c). The EF intensity map from excitation at 547 nm indicates that the higher-energy mode is localized on the distal pods, i.e., the pods away from the ITO surface (Figure 2b). In contrast, the EF intensity map from excitation at 681 nm corresponds to the proximal pods, i.e., the pods on the ITO surface (Figure 2c). This result is expected because the distal pods are in a medium of air (RI = 1.0) and the proximal pods are on an ITO substrate (RI = 1.88 @ λ = 600 nm). Thus, the lower-energy (redshifted) mode corresponds to the proximal mode for all FD and TW values examined. Figure S4 depicts the Au-only octopodal structures constructed for the FDTD simulations. These Au-only structures have a FD ranging from 100 to 250 nm a TW ranging from 5 to 30 nm. Table S2 lists the FD, TW, and λmax values from the FDTD simulations. Figure 3a depicts the scattering spectrum of several Au-only octopods in which the maximum scattering cross sections were normalized to 1. Shown in Figure 3a is the scattering spectrum corresponding to a Au-only octopod on an ITO surface with FD = 200 nm and TW = 20 nm (green trace). The surrounding dielectric is air; full details can be found in Experimental Section. Just like the experimental and calculated spectra of the small Au−Pd octopods, two resonances are observed for the Au-only structures with the higher-energy band corresponding to the

Figure 2. Mode assignments for individual Au−Pd octopods. (a) Dark-field scattering spectra of a 112 nm FD and a 262 nm FD Au−Pd octopod (solid red and black curves, respectively) and FDTDcalculated scattering spectra of the 112 nm FD octopod (dashed curve). The maximum scattering intensity was normalized to 1. The smaller particle exhibits two LSPR modes and corresponds to the FDTD-calculated scattering spectra (dashed red curve) of an Au−Pd octopod with a cubic Au core, 85:15 Au:Pd atomic percent branches, and 30:70 Au:Pd atomic percent tips with the particle imbedded 1 nm into the ITO. Increasing the particle size red-shifts the LSPR to the extent that the second mode is no longer within the detection range of 400−800 nm. (b, c) FDTD-calculated EF enhancement (|E|2/|E0|2) for 112 nm FD Au−Pd octopod with a 16 nm TW at the two-peak LSPR maximum values, λmax = 547 nm and λmax = 681 nm. The proximal mode (near the ITO surface) is located at a lower energy (higher λmax). Direction of incident-light propagation, k, and incident electric field, Einc, direction is noted and is along the C4 symmetric axis.

The FDTD method has been used extensively to calculate the scattering spectra of NCs with varying shape, with good match with experimental results when the composition of the material is easily characterized.3,22−24 Here, the gradient alloy composition introduces uncertainty with regards to the Au− Pd distribution as there is no analytical tool currently available to characterize the composition in 3-D which would also enable optical characterization. Thus, a series of octopods (FD = 112 nm; TW = 16 nm) with different Au−Pd alloyed branches and Pd-tip compositions built from a cubic Au core were used as input structures, where the overall Au−Pd composition of the models was constrained by EDS analysis. Full details can be found in Experimental Section, and a selection of these FDTDcalculated scattering spectra are shown in Figure S5. As expected, changes in the bimetallic distribution result in shifts in the positions of the bands. For example, increasing the percentage of Pd at the tip from 70 to 100% shifts the distal wavelength band position from 564 to 583 nm. This finding highlights the challenges associated with accurately modeling nanostructures with complex compositions in high throughput. Still, the number and position of the bands are nearly replicated by a model consisting of a cubic Au core with alloyed (85% Au) branches and Pd enriched (70% Pd) tips (Figure 2a). Taking the cubic Au core into consideration, the 85% Au branched octopod represents an octopod composed of 90% Au in total, 22117

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compared to FDTD simulation results of Au-only octopods. Again, the NCs were selected such that two have constant FD values (red versus green traces) and two have constant TW values (red versus blue traces) as depicted in Figure 3a. As the FD increases from 200 to 250 nm, a red-shift of 94 nm occurs. As the TW decreases from 30 to 20 nm, a red-shift of 5 nm occurs. Both of these simulated results, increased FD and decreased TW, yield a red-shift of LSPR and are in agreement with the trends in the literature reports and our experimental data. For these Au-only structures, both can be explained by increased radiative damping as a consequence of secondary radiation effects.27 Although the analysis of the three particles in Figure 3 is consistent with trends in the effects of size and vertex sharpness on LSPR shifts,22,25,26 both FD and TW influence the LSPR concomitantly. Thus, an analysis that encompasses that joint influence is required. A contour plot of λmax as a function of both FD and TW is depicted in Figure 4a for the experimental

Figure 3. (a) FDTD-calculated scattering spectra of Au-only octopods. A LSPR red-shift occurs when the FD increases (red to blue plots) and when the TW decreases (red to green plots). The maximum scattering cross section was normalized to 1. (b) Dark-field spectra of Au−Pd octopods and their corresponding SEM images. The maximum scattering intensity was normalized to 1.

distal pods (Figure S4). The corresponding electric field (EF) intensity maps obtained for the λmax values in the scattering spectrum (Figure 3a) are depicted in Figure S4. Based on the distal and proximal LSPR mode assignments from FDTD calculations (Figure 2b,c), the two modes present in the experimental spectrum of the 112 nm FD Au−Pd NC are assigned as distal (λmax = 530 nm) and proximal (λmax = 688 nm) as shown in Figure 2a. The single LSPR mode of the 262 nm FD Au−Pd NC is assigned as distal (λmax = 702 nm), with the proximal mode being red-shifted beyond the detection range. Within the size range of the particles used in these experimental studies, only the distal LSPR mode is detected, and FD and TW effects on LSPR will refer specifically to the distal LSPR mode throughout the rest of this paper. The simulated spectra of the Au-only octopods as a function of different structural parameters is central to understanding the compositional contribution to the LSPR of these complex bimetallic NCs using the approach illustrated in Scheme 1. Use of the Au-only model octopods is particularly important because Au-only octopodal structures, to date, have not been achieved synthetically for comparison, and as previously discussed, the complex composition and heterogeneity of the Au−Pd octopods restrict the ability to model individual Au−Pd NCs with high throughput. Spectra of three Au−Pd NCs and three Au-only NCs representative of this data set are depicted in Figure 3a,b. The NCs were selected such that two have constant FD values (red versus green traces) and two have constant TW values (red versus blue traces). As the FD increases from 262 to 270 nm, a red-shift in LSPR of 6 nm occurs (Figure 3b). This result is in agreement with an increase in size resulting in a red-shift of LSPR.3,27 As the TW decreases from 25 to 23 nm, a red-shift in LSPR of 17 nm occurs. This result is also in agreement with previously reported findings that sharper tips result in a redshift in LSPR.28 These experimental results were then

Figure 4. Contour plots of LSPR λmax as a function of TW and FD for (a) experimental Au−Pd octopods and (b) FDTD-calculated Au-only octopods. Contour plots of fwhm as a function of TW and FD for (c) experimental Au−Pd octopods and (d) FDTD-calculated Au-only octopods.

Au−Pd octopods and is compared to a contour plot for the simulated Au-only octopods in Figure 4b. See Figure S2 for an additional 3-D plot with data points superimposed and contour plots with the same scale resolution. As expected, a λmax dependence on both FD and TW is observed. However, the experimental optical response depicted in Figure 4a clearly differs from the simulated optical response in Figure 4b, and this difference presumably emerges because of the complex composition of the Au−Pd octopods. For the calculated optical response, a size-dependent LSPR in which the LSPR red-shifts as the FD increases is evident in the contour plot (Figure 4b). This result is in accordance with LSPR size dependencies; an increase in size results in a red-shifted LSPR.25,26 A tipsharpness dependence on LSPR in which the LSPR red-shifts as the TW decreases is also revealed. This result is in accordance with the radiative damping that accompanies the charge confinement in small volumes.3,27 Considering the experimental optical response, a size dependence on the LSPR that aligns with the trends expressed in the Au-only octopods is observed. 22118

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in electron density of the Au core and therefore a red-shift in the LSPR. This rationale is supported by Pd−Ag bimetallic nanobar studies presented by Zhu et al. and Zeng et al. in which electron transfer between the Pd and Ag regions occurred.36,37 In addition, the presence of the low-intensity shoulder peak at ∼640 nm (Figure 3b) is indicative of multipolar modes as well as electron transfer at a bimetallic interface.36−38 X-ray photoelectron spectroscopy (XPS) analysis of Au−Pd octopods for the Au 4f5/2, Au 4f7/2, and Pd 3p1/2 regions indicates shifts to lower binding energies, which may be evidence for electron transfer between the core and tip.14 However, we note that this shift may arise from the alloy composition itself as well. The concentration of Pd localized at the tips is necessarily intertwined with TW; larger TW yields larger tip-Pd concentrations. Yet, both of these parameters affect the LSPR in opposite directions. As evident with Figure S5 where the optical response of Au− Pd NCs with different bimetallic distributions were simulated, increased Pd concentration in the Au−Pd alloy results in a different response than Pd localized on the tips of the Au−Pd NCs. This is because Pd localized on the exterior of the particle alters the local dielectric environment and creates Au−Pd interfaces. To provide a theoretical comparison to these experimental results regarding Pd-tip concentration, a new FDTD model octopod was constructed in which Pd-tips were placed onto the octopodal Au structures. FDTD simulations for this model were achieved for a FD value of 250 nm with PdTW values of 5, 10, 20, and 30 nm (Figure 5). See

TW effects, however, are not as straightforward and will be discussed in two parts. To understand the dependence on TW most easily, the experimentally obtained contour plot can be divided into two sections, as denoted by the dashed line in Figure 4a. These sections correspond to octopods with TWs below 16 nm and TWs above 16 nm. The LSPR primarily red-shifts as TW decreases from 16 to 9 nm, with some variation of the extent of red-shift in this region as evident by the nonlinearity of the curves in Figure 4a. This red-shift in LSPR is expected, from a structural analysis, as TW decreases (increase in FD/TW aspect ratio)29 and (sharper tips) due to volume confinement,3,27,30 which has been shown for structures that maintain the same aspect ratio yet have varying sharp features, such as nanorice versus nanobars.22 For this portion of the data (TW range of 16−9 nm), the variation is consistent with structural effects. Composition may play a role in LSPR position here; however, the trend, LSPR red-shifts with decreasing TW, can be attributed to structure. Surprisingly, the LSPR also primarily red-shifts as TW increases from 16 to 30 nm, again with some variation of the extent of red-shift in this region. This result cannot be structurally explained by the effect of tip width on the LSPR response. Wider tips allow the charge distribution to be more dispersed; therefore, a wider tip or vertex structure typically exhibits a blue-shifted LSPR compared to its sharp tip/vertex counterpart.3,27 Furthermore, this result is in accordance with previously reported ensemble measurements of Au−Pd octopods in which an increase in TW (decrease in the FD/ TW aspect ratio) results in LSPR red-shift16 and opposes that of Au-only branched structures.29 This finding highlights the complex composition of the Au−Pd octopods and variations in this composition as a function of octopodal structure, as discussed in more detail shortly. The full width at half-maximum (fwhm) of an LSPR band increases with increasing size (volume) in the size regime of our particles because of increased radiation damping;25,31 it also shows a dependence on composition, with an increase with increasing Pd concentration in Au−Pd NCs.32−34 This result is also in accordance with previously reported ensemble measurements.16 A contour plot of the fwhm for the LSPR bands as a function of FD and TW is depicted in Figure 4c for the experimental Au−Pd octopods and compared to the simulated Au-only octopods in Figure 4d. A dependence on both TW and FD is evident for the Au−Pd NCs. The trend observed in the region corresponding to NCs with TWs less than 16 nm and FDs less than 190 nm is similar to the calculated fwhm trend, i.e., that of increasing fwhm with increasing FD and TW for the Au-only octopods (Figure 4d). This region corresponds to Au− Pd NCs with a low concentration of Pd at their tips, i.e., those NCs most similar in composition to the Au-only octopods. Otherwise, differences between the experimental and simulated results are observed, again highlighting the contribution of Pd (Figure 1) to the LSPR response. One explanation for the observed red-shift in LSPR for increasing TWs of the Au−Pd octopods is departures in the local dielectric environment due to Pd on the exterior, particularly the tips of the branches, which has a larger refractive index than Au and the medium. Another possibility for the origin of the red shift is charge transfer. Specifically, Pd has a higher work function, 5.55 eV, compared to that of Au, 5.1 eV.35 This difference would result in electron transfer from the Au core to the Pd-rich tips, which would result in a decrease

Figure 5. FDTD-simulated scattering spectra of 250 nm FD Auoctopods with Pd-tips; model images (inset). As the TW increases from 10 to 20 nm, the LSPR red-shifts (blue to green plot). The LSPR continues to red-shift as the TW is increased from 20 to 30 nm (green to red plot). There is no to little shift from 5 to 10 nm (black to blue plot). The scattering cross section was normalized to 1. The distal modes for the Au-octopods with different Pd tips are enlarged in the inset.

Experimental Section for modeling details. These results indicate a red-shift in LSPR with increasing tip width, with the red-shift being greater as the tip width increased, in accordance with the experimental findings. Because numerical simulations have the bulk dielectric constants of Au and Pd as input parameters, the result is independent of the chargetransfer hypothesis. In other words, departures in the local dielectric resulting from increased Pd-tip concentration may account for the red-shift observed. This finding does not exclude, however, the charge-transfer hypothesis. Moreover, 22119

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The Journal of Physical Chemistry C

rich tips or from local Au dielectric environment changes in this compositionally gradient structure. The complex relationship between TW and Pd concentration at the tips gives rise to a tug-of-war between the structural (TW) and compositional (tip-Pd concentration) contributions to the LSPR, which are intertwined. The effect of Pd concentration dominates at wide TWs corresponding to high Pd-tip concentration. Manipulation of both of these parameters allows for LSPR tuning. We envision that these guidelines can be applied to other bimetallic systems. Moreover, this approach to decoupling the structural and complex compositional contributions to the LSPR by single-particle correlation may serve as a framework for studying other bimetallic platforms.

certain experimental features are not reproduced by simulations using bulk dielectric constants, as discussed in the following. The lack of shift between a Pd-TW of 5 and 10 nm (Figure 5) suggests that there is a threshold Pd concentration for manipulating the LSPR position. This idea is also supported by previously reported ensemble measurements for Au−Pd octopods with an average FD value of 105 nm and TW value of 14 nm, which exhibited a blue-shifted LSPR compared to that of Au−Pd octopods with an average FD value of 95 nm and TW value of 15 nm.16 Recall that there was a 17 nm LSPR red-shift when decreasing the TW from 25 to 23 nm (Figure 3b). The LSPR red-shift depicted in Figure 3b may be solely based on structural contributions, yet the red-shift at a smaller TW value contradicts the guidelines presented here. The Au− Pd composition varies among Au−Pd octopods, and these compositional complexities and variations were considered from previous studies as well as presented here (Figure 1).16 The LSPR red-shift is now attributed primarily to increased Pd concentration resulting in either local departures from the Au dielectric constant due to Pd or increased electron transfer from the Au core to the Pd-rich tip. This result further supports that Pd-tip concentration inhomogeneities, which would give rise to the slope discontinuities in the contour plots, may exist from particle to particle.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b06691. Polarization anisotropy, additional correlation methodology, and FDTD details (PDF)

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AUTHOR INFORMATION

Corresponding Author

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*E-mail: [email protected].

CONCLUSIONS In summary, the compositional contribution (specifically localized Pd on the tips of the Au−Pd octopods) to the optical response was revealed as structural effects (FD and TW) were decoupled from the overall optical response of Au− Pd octopods. In fact, the intertwined effects of TW and Pdconcentrations were unraveled. The plasmonic response of Au−Pd NCs with varying FD and TW values were compared to that of FDTD-simulated Au-only NCs. The differences in optical response between the Au−Pd and Au-only octopods were attributed to the complex composition of the Au−Pd octopods, which have Pd-rich tips. Thus, the following guidelines for tuning the LSPR through structure and complex composition have been determined: (1) There is a FD (size) dependency on LSPR for stellated bimetallic NCs. Both the simulated and experimentally determined plasmonic response support this design principle in which an increase in FD results in a red-shifted LSPR. (2) There is a TW dependency on LSPR for stellated bimetallic NCs, and structural ef fects dominate the LSPR response of Au−Pd octopodal NCs at tip widths less than 16 nm. Both the simulated and experimentally determined plasmonic response support this design principle in which a decrease in TW results in a redshifted LSPR. (3) Tip width and complex compositional effects are intertwined. If one component of the bimetallic is localized on the exterior, there will be departures in the local dielectric environment and/or an interface in which charge transfer may occur. In the case of the Au−Pd system presented here, the structural and complex compositional effects oppose one another. Compositional effects dominate the LSPR response at tip widths greater than 16 nm, where the Pd localized at the tips is in greater concentration. As TW is increased, there is a red-shift in the LSPR. This finding is interesting; the localization of Pd on the tips plays an important role in the plasmonic response of the Au−Pd NC, which is not explained by the simulated Au-only structures. The plasmonic response of the Au−Pd NCs likely arises from electron transfer from the Au core toward the Pd-

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The optical characterization of NCs and simulations were supported by Indiana University (IU) start-up funds and IU’s Office of the Vice President for Research and the Office of the Vice Provost for Research through the Faculty Research Support Program. Synthesis of NCs was supported by the U.S. National Science Foundation under grant CHE-1306853. SES is a Cottrell Scholar (Research Corporation), Alfred P. Sloan Fellow, and Camille Dreyfus Teacher-Scholar. The authors gratefully acknowledge fruitful discussions with Emilie Ringe of Rice University and Maryam Zahedian, Eun Soh Kohl, and Lu Dai of Indiana University. AFS acknowledges the support from a NSWC Crane PhD Fellowship. BD acknowledges support from U.S. Army Research Office under award W911NF-13-10490.

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DOI: 10.1021/acs.jpcc.5b06691 J. Phys. Chem. C 2015, 119, 22114−22121