Probing the StructureâProperty Interplay of Plasmonic Nanoparticle

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Perspective

Probing the Structure-Property Interplay of Plasmonic Nanoparticle Transducers Using Femtosecond Laser Spectroscopy. Kenneth L. Knappenberger, Anne-Marie Dowgiallo, Manabendra Chandra, and Jeremy W. Jarrett J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/jz4001906 • Publication Date (Web): 14 Mar 2013 Downloaded from http://pubs.acs.org on March 20, 2013

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The Journal of Physical Chemistry Letters

Probing the Structure-Property Interplay of Plasmonic Nanoparticle Transducers Using Femtosecond Laser Spectroscopy. Kenneth L. Knappenberger, Jr.,* Anne-Marie Dowgiallo, Manabendra Chandra, and Jeremy W. Jarrett. Department of Chemistry and Biochemistry, Florida State University, Tallahassee, FL 32306-4390. AUTHOR EMAIL ADDRESS: [email protected] RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required according to the journal that you are submitting your paper to) CORRESPONDING AUTHOR FOOTNOTE. [email protected].

ABSTRACT. The characteristic feature of noble metal nanoparticles is the localized surface plasmon resonance (LSPR). Plasmon-supporting nanoparticles can function as a transducer because of the LSPR’s ability to amplify electromagnetic fields, and its sensitivity to changes in the surrounding dielectric. The performance of these materials in transducer applications is inherently related to nanoparticle structure. This Perspective describes the use of femtosecond laser-based spectroscopies to elucidate the nanoscale structure-property interplay. First, femtosecond time-resolved transient extinction measurements that probe the LSPR following nanoparticle photo-excitation are described. These measurements illustrate how nanostructure dimensions influence sensitivity to changes in the interfacial dielectric. The combination of single-particle nonlinear optical (NLO) measurements and ACS Paragon Plus Environment

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electron microscopy are also used to describe the symmetry of plasmon surface fields in nanoparticle assemblies. In particular, the use of continuous polarization variation-detected second harmonic generation to describe electric and magnetic dipolar contributions to NLO properties is discussed.

SYNOPSIS TOC.

KEYWORDS: plasmonics, transducers, ultrafast spectroscopy, single-particle spectroscopy, nonlinear optical spectroscopy, assembled nanoparticles, confined fluids.

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Metallic nanostructures offer unique opportunities for the utilization of electromagnetic energy. These opportunities arise because the optical and electronic properties of nanomaterials differ significantly from those of their bulk counterparts. In the case of colloidal gold nanoparticles, these properties even vary widely over the nanometer length scale, evolving from the discrete orbitals of quantum-confined nanoclusters (< 2 nm) to the collective properties of plasmonic nanoparticle (> 2nm) networks.1,2 Excitation of localized surface plasmon resonances (LSPR) in gold nanoparticles produces large local surface fields at structure-specific frequencies, allowing plasmonic metal nanostructures to function as electromagnetic antennas. In this way, light amplification by a nanoparticle transducer can be used to enhance molecular spectroscopy methods,3,4 mediate nonlinear optical processes5-10 and facilitate solar-to-electric energy conversion.11,12 The surface electromagnetic field associated with LSPRs decays rapidly into the surroundings, providing another type of transducer effect in which changes in the local refractive index are converted into a frequency shift of the plasmon mode.13 This effect has recently been exploited for trace-level detection, and LSPR peak shifts of plasmon-supporting nanoparticles have been used to detect optically transparent molecules.14,15 A significant advantage of colloidal nanoparticles is the ability to tailor their optical, electronic, and mechanical properties by controlling the nanostructure shape, size, and composition through facile laboratory synthesis.2,16,17 A plasmonic nanostructure of increasing interest is the hollow gold nanosphere (HGN),5,18-24 which consists of a thin gold shell and a fluid-filled dielectric cavity with a diameter that ranges from a few to tens of nanometers.25 Early interests in this structure stemmed from the inherent continuous tunability of the LSPR from visible to NIR frequencies.18 However, recent research has revealed several new and interesting phenomena that are unique to this nanoparticle. In comparison to similarly sized solid gold nanoparticles (SGNs), HGNs have significantly larger first hyperpolarizabilities, which can be optimized by synthetic modification of HGN total surface area and LSPR resonance.5 This attribute has potential impacts for nonlinear optical applications. HGNs also exhibit structure-dependent acoustic vibrations,23 an important design parameter for applications based ACS Paragon Plus Environment

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on nano-mechanical resonators. Finally, networks of HGNs support complex interparticle plasmon resonances that can be controlled by HGN shell thickness and particle-to-particle spatial separation.22 Increasingly, experimental data implicate the HGN cavity as the determining factor for these properties. In light of this, HGNs with fluid-filled cavities provide a convenient platform for studying the properties of fluids that are confined to nanoscale dimensions.25 All of these effects can increase the versatility of nanoparticle-based transducer devices. Predictive understanding of such devices requires that the interplay between nanoparticle morphology and the fundamental properties of these species be more thoroughly described. Laser-based spectroscopy techniques are especially well suited for studying structure-dependent properties of metal nanoparticles.5-10, 13, 21-28 Recent insights into electronic energy relaxation dynamics of hollow nanostructures and the characterization of plasmon surface fields of both isolated nanoparticles and networks of coupled nanoparticles are the subject of this Perspective. These advancements include the use of femtosecond time-resolved transient extinction measurements to determine the influences of increased surface-to-volume ratios and confined fluids on the energy dissipation and mechanical properties of HGNs as well as the utilization of single-particle second harmonic generation (SHG) measurements to quantify electric and magnetic dipolar contributions to plasmon-mediated nonlinear optical (NLO) processes in nanoparticle networks. These findings are discussed in the broader context of structure-dependent nanoparticle-based transducers. Relaxation Dynamics of Electronically Excited Hollow Nanoparticles. Metal nanoparticles with diameters exceeding a few nanometers exhibit discrete visible extinction spectra that are characterized by localized surface plasmon resonance excitations, which occur at material-specific frequencies and are sensitive to the surrounding dielectric.2 As a result, shifts in plasmon resonance peak frequencies are often used to detect surface adsorption/desorption of molecules and other changes to the interfacial dielectric. The LSPR line width is another diagnostic that informs on the properties of the nanoparticle-surroundings interface.13 For optical sensing applications, solid nanoparticles with diameters greater than several tens of nanometers are ideal systems because ACS Paragon Plus Environment

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they exhibit large scattering cross sections at optical frequencies. However, for these larger solid nanoparticles, the LSPR line width includes contributions from intrinsic (bulk) damping, interfacial scattering, and, for very large particles, radiative damping.13, 26, 27 Bulk damping results from electron scattering by phonons and defects in the lattice; the room-temperature electron mean free path in gold is approximately twenty nanometers.13 Contributions from these scattering processes often obscure the contribution of interfacial phenomena to the LSPR line width. Intrinsic damping can be reduced by using hollow nanoparticles that, unlike solid particles, have both large surface-to-volume ratios and shell thicknesses that are less than the electron mean free path. Hollow gold nanospheres are synthesized by a galvanic replacement deposition method, in which the atoms of a sacrificial cobalt nanosphere are replaced by gold atoms, forming a porous and polycrystalline gold shell that encapsulates a fluid cavity (Figure 1A). Chemical analysis performed via energy dispersive spectroscopy (EDS) confirms that the nanoshell is comprised solely of gold (Figure 1B); the cobalt is consumed during the electrodeposition process and diffuses outward from the nanostructure cavity.21, 25 The extinction spectra obtained for a series of HGNs are given in Figure 1C. As the HGN outer-diameter-to-shell-thickness aspect ratio increases, the LSPR shifts to longer wavelengths. HGN samples synthesized in our laboratory have outer diameters spanning approximately 15 nm to 77 nm and cavity diameters of 5 nm to 55 nm. The maximum shell thickness for these particles is 11 nm, well below the 20-nm electron mean free path. In order to compare the sensitivities of HGN and SGN LSPRs to interfacial electron scattering, a series of femtosecond pump-surface plasmon resonance probe measurements were performed.23 These experiments employ a broad bandwidth white-light continuum probe to monitor the LSPR at a controlled time delay following excitation by a femtosecond pump pulse. This technique is useful for studying relaxation dynamics of electronically excited metal nanostructures because it generates a nonequilibrium electron gas. As a result of this excitation, a transient bleach at the LSPR frequency features prominently in the differential pump-probe transient extinction spectra of metal nanoparticles. The hot electron gas relaxes by three successive steps: i) ultrafast electron-electron (e-e) scattering, ii) subACS Paragon Plus Environment

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picosecond electron-phonon (e-ph) coupling, and iii) energy transfer from the hot nanoparticles to the surroundings on time scales spanning tens to several hundreds of picoseconds.13 This rapid dissipation of electronic energy as heat to the surroundings modifies the dielectric constant of the embedding media, which in turn influences the interfacial scattering rate. Changes in local dielectric constants can be determined when data from femtosecond pump-surface plasmon resonance probe experiments are analyzed in both the time and energy domains. The large difference in the time scales of the local heating dynamics (10s of picoseconds) and plasmon dephasing ( 0) was observed for many, but not all, structures.6,7 Most striking is the range of the experimentally determined SHG-CDR values that resulted from the SGN dimers examined. The structural specificity of the CD is summarized in Figure 9B, which portrays the SHG-CDR ratios resulting from twenty-seven different SGN dimers. Some insight into the origin of this specificity is gained by correlating singleparticle SHG data to SEM images. These SEM images revealed that the particles are not symmetric, and, most importantly, not identical. This property gives rise to unique interfacial electromagnetic field distributions and orientations, thus resulting in different SHG-CDR responses. An interparticle gap separating the SGNs that formed the dimer was distinguishable in the SEM data for all samples used to generate the data in Figure 9B. This was expected because dithiols were used as the aggregating agent.6,7,22 Taken together, the data summarized in Figures 8 and 9 confirm that second harmonic generation techniques can be used to study electromagnetic surface plasmon fields of metal nanoparticle networks. These data provide unambiguous evidence of chiral fields. However, in the case of colloidal networks, structure-specific descriptions can only be obtained at the single-particle level; these subtle structural differences (e.g. SHG-CDR) would be lost by ensemble averaging. Implicit in the unambiguous SHG-CD of SGN dimers is the contribution of magnetic dipoles to the second-order polarization. As a result, it is conceivable that colloidal metal nanoparticle networks could be used as electric and magnetic field transducers and amplify magnetic fields upon optical excitation. In order to quantify both the magnetic and electric dipolar contributions to the SH response, continuous polarization variation-detected SHG (CPV-SHG) experiments were performed.6,7 In this


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approach, second harmonic generation results from an induced nonlinear polarization, P, of the material at the incident fundamental frequency and a magnetization, M, at the harmonic frequency.46 P and M can be expressed as:

eee Pi (2ω ) = ∑ χijk (2ω ,ω ,ω )E j (ω )E k (ω )+ χijkeem (2ω ,ω ,ω )E j (ω )B k (ω )

(1)

j,k mee M i (2ω ) = ∑ χijk (2ω ,ω ,ω )E j (ω )E k (ω )

(2)

j,k

where E is the electric field component of the nonlinear scatterer, B is the magnetic induction field, χ is the susceptibility tensor, ω is the carrier frequency at the fundamental wavelength, and i, j, and k represent Cartesian coordinates. These coordinates are typically defined in the laboratory S and P frames, but, for nanoparticle dimers, they can be expressed with respect to the dimer inter-particle axis.6,7 The inclusion of P and M as nonlinear sources results in a general expression for the intensity of the experimentally measured SH intensity:6,7,46

I (2ω ) = I (ϕ ) = FP 2 (ϕ)+GS 2 (ϕ )+ HP(ϕ )S (ϕ )

2

(3)

F, G, and H represent linear combinations of the complex-valued susceptibility tensors, which describe electric-dipolar and magnetic-dipolar nonlinear optical contributions at the fundamental and harmonic frequencies. The formalism given in Equation 3 expresses the experimentally measured SH intensity with respect to the polarization state of the incident laser carrier wave. In this way, polarization-resolved second harmonic data obtained in the laboratory can be used to quantify the contributions to the nonlinear optical susceptibilities of electromagnetically coupled plasmonic nanoparticles. The electric field vector of the fundamental laser is systematically controlled by changing the quarter-wave plate rotation angle (ϕ ). SHG obtained as a function of ϕ can be fit using Equation 3 to quantify the coefficients F, G, and H. G is the only coefficient that depends exclusively on magnetic-dipolar contributions, and, as a result, any non-zero value of G is a signature of magnetic-dipolar contributions to the NLO response of a system.6,7,46 ACS Paragon Plus Environment

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The SHG data obtained via CPV measurements for two different SGN dimers are shown in Figure 10A and 10B, along with the fit to Equation 3. The dimer structures for these particles were determined from SEM images (Figures 10C and 10D). The lack of parity of the SH line shapes with respect to ϕ is a clear indication of magnetic-dipolar contributions to the NLO properties of these nanostructures. SHG originating from purely electric-dipolar contributions would have yielded symmetric line shapes. In addition, fitting the data shown in Figure 10A and 10B to Equation 3 yielded non-zero values of G. In fact, all twenty-seven chiral SGN dimers used for data in Figure 9 return nonzero values for the value of G.6 As is the case for the SHG-CDR data, the magnitude of the magneticdipolar contributions is structure specific; the values of the parameter G varies significantly from one SGN dimer to the next. Continuous polarization variation second harmonic generation at the single-particle level allowed for the detection and quantification of structure-specific contributions to the optical properties of networked plasmonic nanoparticles that would not have been observable using linearly polarized methods.6,7 This is because SH methods that use linearly polarized light exclusively sample only two components of the nonlinear susceptibility, which is a rank three (twenty-seven element) tensor. As a result, the CPV approach is inherently more sensitive to structural changes in the electromagnetic surface fields localized in the interparticle gaps of nanoassemblies.6,7 Opportunities for future research include phase- and polarization-resolved measurements. The observed magnetic dipolar contributions may have originated from the interference of out-of-phase dipolar modes. Quantification of the phase relationship between plasmon modes that couple to form interparticle resonances will provide significant insight into the optical properties of networked nanoparticles. These data will be especially informative for describing closely spaced nanoparticles (i.e. within conductive overlap), for which coupling mechanisms are still poorly understood. One of the most recent additions to the arsenal of tools for describing structure-specific plasmonics is three-dimensional transmission electron tomography (3-D TET). 3-D TET methods can be used to generate volume reconstructions by collecting data over a series of electron beam/sample ACS Paragon Plus Environment

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incidence angles.20 This is in contrast to the averaged images obtained by data acquisition at fixed normal incidence. Figure 11 portrays a comparison of images obtained for a network of HGNs using 2-D (0 tilt) TEM (A) and from a slice taken through tomographic reconstruction (B). Several structural features are revealed upon inspection of the tomogram that are not evident in the 2-D image. Specifically, the 3-D tomographic reconstruction yields interfacial characterization that is superior to the 0-degree 2-D imaging data for the regions labeled 1-3 in Figure 11A.20 The data shown in the 2-D image (Figure 11A) indicate that interface 1 results from two HGNs spatially separated by approximately 1 nm. By comparison, the image in Figure 11B reveals a surface-surface point contact connecting the HGNs. In 2-D data, region 2 appears to be formed by surface-necked HGNs, but Figure 11B shows that this HGN interface includes a pore. Likewise, the data in Figure 11A is indicative of surface necking in region 3, but Figure 11B reveals that the HGNs have combined to form a hollow, peanut-shaped tubular structure. The interfacial structure of nanoparticle networks is a critical design parameter for optical nanoscale transducers. It is clear that, along with single-particle optical measurements, 3-D tomographic techniques will become an indispensable tool for accurately predicting and modeling structuredependent nanoscale optical properties. These data, in combination with single-particle optical measurements, will be especially useful when the spatial separation between particles is small and quantum effects may become significant. Light-driven applications featuring metal nanoparticles as functional transducers span areas such as applied spectroscopy, trace-level sensing, solar light harvesting and negative index materials. However, their successful implementation requires knowledge of structure-dependent electromagnetic coupling mechanism and LSPR field symmetries as well as a description of nanoparticle energy relaxation mechanisms. Laser-based spectroscopic methods can provide insights into each of these areas. For example, femtosecond time-resolved surface plasmon resonance spectroscopy is used to understand the effects of increased surface areas and confined fluids on nanoparticle energy dissipation. Deeper insight into the symmetry properties of electromagnetic surface fields can be gained from continuous polarization variation nonlinear optical measurements. In the future, structural control over ACS Paragon Plus Environment

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the symmetry of nanoparticle surface fields could lead to enhancement mechanisms for other higherorder nonlinear spectroscopy techniques. Using nanoparticles as an electromagnetic antenna, it may be possible to localize sculpted fields to small volumes. This emerging area will provide many exciting opportunities for researchers in the area of Physical Chemistry. ACKNOWLEDGMENT. Our work on nanoparticle assemblies has benefited tremendously from collaborations with S. Stagg. We acknowledge S. Link and G. Hartland for helpful discussions on nanoparticle optical properties. Various aspects of this research were supported by the Air Force Office of Scientific Research (FA9550-10-1-0300), the National Science Foundation (CHE-1150249) and the American Chemical Society Petroleum Research Foundation (51233-DNI6).

Biographies. Kenneth L. Knappenberger, Jr. is an assistant professor in the Department of Chemistry and Biochemistry at Florida State University. He received his Ph.D. from the Pennsylvania State University in 2005 under the mentorship of A.W. Castleman, Jr. From 2005-2008, he was a postdoctoral researcher at University of California, Berkeley in the group of Stephen R. Leone. His main research interests include understanding the optical properties of nanoparticles and their assemblies as well as their development for nanoparticle-based transducers. (http://www.chem.fsu.edu/~klk/KLK_group/) Anne-Marie Dowgiallo is a Ph.D candidate in Chemistry at Florida State University, working under the guidance of Kenneth L. Knappenberger, Jr. She received her B.S. in Chemistry from Towson University in Maryland in 2008. Her primary research interest is ultrafast energy relaxation dynamics in nanoparticles. Manabendra Chandra is a postdoctoral researcher at Florida State University with Kenneth L. Knappenberger, Jr. He received his Ph.D from the Indian Institute of Science with P. K. Das in 2009. His current research is nanoparticle plasmonics studied at the single-particle level.


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Jeremy W. Jarrett is a graduate student in Chemistry at Florida State University, working with Kenneth L. Knappenberger, Jr. He received a B. S. in chemistry from Florida State University in 2011. His research interests include nonlinear optics and single-particle spectroscopy.

Quotes: Femtosecond laser-based spectrospcopies are especially well suited for studying structure-dependent properties of metal nanoparticles. – page 3. Because the plasmons dephase so rapidly, the local environment is essentially static during the plasmon coherence time, and spectroscopic measurements of the LSPR with femtosecond time resolution provide a snapshot of the local dielectric environment. – page 6. The combination of the plasmonic hollow nanosphere morphology, which confines fluids to nanoscale volumes, and the sensitivity of femtosecond pump – LSPR probe measurements to the local environment provides a unique experimental platform for understanding thermal and dielectric properties of confined fluids. – page 8. In the case of colloidal networks, structure-specific descriptions can only be obtained at the singleparticle level. – page 12 It is conceivable that colloidal metal nanoparticle networks could be used as electric and magnetic field transducers and amplify magnetic fields upon optical excitation. – page 12. Continuous polarization variation second harmonic generation at the single-particle level allowed for the detection and quantification of structure-specific contributions to the optical properties of networked plasmonic nanoparticles that would not have been observable using linearly polarized methods…., the CPV approach is inherently more sensitive to structural changes in the electromagnetic surface fields localized in the interparticle gaps of nanoassemblies. - page 14


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Figure 1. (A) TEM image of a representative hollow gold nanosphere. (B) Energy dispersive spectrum demonstrating that the hollow nanostructures consist of gold; Cu peaks arise from the sample grid and are not indicative of contamination. Taken together, the TEM image and the EDS data indicate that the HGNs are composed of a gold shell and a hollow, fluidfilled cavity. (C) Normalized extinction spectra for select

HGN

samples.

The

LSPR

maximum

wavelength increases from 550 to 710 nm as the outer-diameter-to-shell-thickness

aspect

ratio

increases from 3 to 12.


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Figure 2. (A) Two-dimensional transient extinction image obtained for a hollow gold nanosphere following 400-nm excitation. (B) Transient extinction data obtained from the data in panel A by recording the differential amplitude at 585 nm (blue) and 660 nm (red). (C) Localized surface plasmon resonance central wavelength recorded as a function of pump-probe delay time. Taken together, these time-dependent data reflect changes in the nanoparticle volume and surrounding dielectric. (D) Comparison of the change in the LSPR line width for hollow (open circles) and solid (filled circles) nanospheres following excitation by a femtosecond laser pulse. The data in panel D demonstrate the enhanced sensitivity of hollow nanostructures to changes in interfacial dielectric induced by laser heating. Reprinted from reference 23.


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Figure 3. Fourier transformation of transient absorption time-domain data for a series of hollow gold nanospheres. The outer radii were (A) 10, (B) 15, (C) 25, (D) 28, and (E) 40 nm. The inner-to-outer radius aspect ratios were (A) 0.38, (B) 0.46, (C) 0.60, (D) 0.67, and (E) 0.75. Reprinted from reference 23.


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Figure 4. Room-temperature zero-point electron-phonon coupling times for HGNs (open circles) and SGNs (closed circles) as a function of aspect ratio and inverse radius, respectively. Reprinted from reference 24.


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Figure 5. (A) Nanoparticle-to-surroundings energy transfer half times (τET) of HGNs plotted as a function of their total surface area. These HGNs have cavity radii ranging from 3.3 to 27.5 nm. The data reflect a clear discontinuity in linear surface area dependence for cavity radii less than 15 nm. (B) HGN (open circles) and SGN (filled circles) energy transfer half times (τET) plotted as a function of HGN shell thickness, or SGN radius. The experimental data are also plotted with calculated size-dependent interfacial thermal conductivities (colored lines). Reprinted from reference 25.


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Figure 6. (A) Femtosecond transient extinction spectra of isolated (red) and aggregated (blue) hollow gold nanospheres. The samples were excited by 405-nm light (500 nJ/pulse) and probed at 500-fs pumpprobe time delay. (B) Transient bleach recoveries compared under the same excitation conditions. The time-domain data were recorded by monitoring the differential signal at the maximum wavelength for both samples. Reprinted from reference 21.


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Figure 7. Experimental extinction spectra comparing isolated HGN (black) and HGN aggregates induced by ethane dithiol (blue) and dimerized cysteine (red). Truncated spectra (normalized at respective LSPR maxima) are shown in the inset to show more clearly the structure-dependent peak shifts. Reprinted from reference 22.


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Figure 8. Experimental second harmonic generation depolarization ratios [I2ω(x)]/[I2ω(y)] plotted as a function of inter-particle-gap-to-diameter ratio (D/2r). Data were obtained from approximately fifty different solid gold nanosphere dimers for each D/2r value. Reprinted from reference 7.


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Figure 9. (A) Second harmonic intensity traces from a single solid gold nanosphere dimer. The data are obtained using either left (LCP) or right (RCP) circularly polarized excitation. (B) Summary of second harmonic generation-circular difference ratios (SHG-CDR) from several dimeric assemblies of solid gold nanospheres. Reprinted from reference 6.


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Figure 10. (A and B) Second harmonic generation line shapes obtained from continuous polarization variation measurements on two different SGN dimers. Filled circles represent the experimental data and the solid red line is a fit to the data using Equation 3. The line shapes given in panels A and B were determined for the specific structures portrayed in the images and panels C and D, respectively. Reprinted from reference 7.


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Figure 11. Electron microscopy of HGN aggregates. (A) 0 tilt electron micrograph of an HGN aggregate. (B) Slice through the center of the tomographic reconstruction of the particles shown in (A). Three interfacial regions of interest are highlighted in panel A and described in the main text. Regions that portray pinholes not observable in the 0 tilt image (panel A) are designated by arrows in panel B. Scale bar: 200 Å. Reprinted from reference 20.


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