3D Thomography and Plasmon Mapping

Using Highly Accurate 3D Nanometrology toModel the Optical Properties of HighlyIrregular Nanoparticles: A Powerful Tool forRational Design of Plasmonic DevicesEduardo M. Perassi,† Juan. C. Hernandez-Garrido,‡,| M. Sergio Moreno,§ Ezequiel R. Encina,†Eduardo A. Coronado,*,† and Paul A. Midgley‡

† INFIQC, Centro Laser de Ciencias Moleculares, Departmento de Fısico Quımica, Facultad de Ciencias Quımicas,Universidad Nacional de Cordoba, Cordoba, 5000, Argentina, ‡Department of Materials Science and Metallurgy,University of Cambridge, Pembroke Street, Cambridge, CB23QZ, United Kingdom, and §Centro Atomico Bariloche,San Carlos de Bariloche, 8400, Argentina

ABSTRACT The realization of materials at the nanometer scale creates new challenges for quantitative characterization and modelingas many physical and chemical properties at the nanoscale are highly size and shape-dependent. In particular, the accuratenanometrological characterization of noble metal nanoparticles (NPs) is crucial for understanding their optical response that isdetermined by the collective excitation of conduction electrons, known as localized surface plasmons. Its manipulation gives place toa variety of applications in ultrasensitive spectroscopies, photonics, improved photovoltaics, imaging, and cancer therapy. Here weshow that by combining electron tomography with electrodynamic simulations an accurate optical model of a highly irregular goldNP synthesized by chemical methods could be achieved. This constitutes a novel and rigorous tool for understanding the plasmonicproperties of real three-dimensional nano-objects.

KEYWORDS Gold, metallic nanoparticles, 3D, electron tomography, discrete dipole approximation.

Nanosized metal nanoparticles (NPs) and nanostruc-tures are the building blocks of the emerging andhighly promising field of plasmonics which aims to

study the properties of the collective electronic excitationsin metal nanostructures, colloquially termed surface plas-mons with the purpose of controlling, manipulating, andamplifying light on the nanometer scale.1-3 This controlcould be achieved in noble metal nanostructures able tosustain surface plasmon resonances (SPR) by an appropriateselection of the incident wavelength (that should be close tothe SPR) or by changing the size, shape, composition, anddielectric environment of the metal NP.4-7

In general, this control could be feasible if the far-fieldoptical properties (FFOP) (such as extinction, absorption,and Rayleigh scattering) and the near-field optical properties(NFOP) (the nonpropagating evanescent field generatedupon SPR excitation) and its relationship to nanoparticlemorphology is understood.

In particular, molecular plasmonics8 makes use of thedramatic change of the optical properties of molecules whenthey are coupled to the SPR, that is, enhancements of several

orders of magnitude are possible in Raman spectroscopy(surface-enhanced Raman spectroscopy (SERS),9-12 tip-enhanced Raman microscopy (TERS)13-17) and smaller butequally important enhancements in fluorescence micros-copy (metal-enhanced fluorescence (MEF)18-20). This en-hancement can be so large (of the order of 1014) that theRaman spectra of single molecules can be acquired.21-23

These enhanced optical fields of metal NPs can be useful ina variety of applications including plasmon (bio)sensors,24-26

sources for nanolithography, probes in scanning near-fieldoptical microscopy (SNOM), and optical imaging with sub-wavelength resolution in nano-optics.27-30

Recent progress in solution phase synthesis methodshave given rise to a plethora of NP morphologies such asspheroids, cubes, triangular prisms, plates, nanostars, nan-orice, branched nanocrystals, bipyramids and polyhedral(decahedral, octahedral) shapes. Anisotropic metal NPs havebeen demonstrated to have SERS activity or sensing capa-bilities superior compared to spherical shapes, and theirresonance frequencies can be tuned from the visible to thenear-infrared simply by subtle changes of their shape and/or morphology.31-33

In general, a precise three-dimensional (3D) nanometro-logical characterization is extremely important for accuratemodeling and making rigorous comparison with optical

*Towhomcorrespondenceshouldbeaddressed.E-mail:[email protected].| Current address: Departamento de Ciencia de los Materiales, Ingenierıa Meta-lurgica y Quımica Inorganica, Facultad de Ciencias, Universidad de Cadiz, Rıo SanPedro s/n, Puerto Real, 11510, Spain. E-mail: [email protected] for review: 02/14/2010Published on Web: 05/03/2010

pubs.acs.org/NanoLett

© 2010 American Chemical Society 2097 DOI: 10.1021/nl1005492 | Nano Lett. 2010, 10, 2097–2104

measurements that now can be performed at the singlenanoparticle level using dark-field or near-field optical meth-ods.34

To date, comparisons and analysis of optical measure-ments with electrodynamic theory of single NPs have beenperformed by approximating their shape to a relativelysimple analytical geometry or idealized shape, for example,by making a parametrization of the nanoparticle surfaceusing rounded spherical tips, or by regular snipping of thenanoparticle corners.35 No theoretical description of a sys-tem with an arbitrary shape has been achieved presumablybecause a description of real objects involves surfaces thatare in general nonanalytical.

In general, the morphological features of NPs are inferredfrom transmission electron microscopy (TEM). Characteriza-tion of nanostructured objects by TEM techniques hasbecome a conventional method because it provides therequired high-spatial resolution. Although a powerful set oftechniques are routinely available in a transmission electronmicroscope, the vast majority provide only a 2D “projection”of the morphology in the form of images or thickness maps.Scanning transmission electron microscopy (STEM)-basedelectron tomography is a technique of growing importancefor 3D crystallographic and metrological studies of differentkinds of nanostructures. The use of a high-angle annulardark-field (HAADF) detector enables electrons to be detectedthat undergo high-angle scattering, and the signal is ap-proximately proportional to Z2, where Z is the atomicnumber, providing contrast sensitive to compositionalchanges. HAADF-STEM tomography allows a very precise3D description of the particle size and morphology as wellas metrological aspects at the nanoscale.36,37

Here we introduce a method that combines electrontomography, an electron microscopy methodology thatprovide precise and direct information on the nanometrol-ogy of the nanostructure, with an appropriate calculationscheme of the optical properties that can use this kind ofinformation. For that purpose, we have synthesized gold NPsusing a seed-mediated growth method, which after morpho-logical characterization using conventional 2D TEM showedthe presence of particles of irregular shape. Then, by usingSTEM-based electron tomography plus reconstruction the 3Dmetrology of a single Au NP was revealed to be highlyirregular, in contrast with the morphology that could beinferred from approximations made from conventional 2DTEM images.

Experimental and Calculations Section. Gold nanopar-ticles were synthesized in a multistep process. First a solu-tion of small gold NP seeds (solution-A) was obtained by theaddition of 0.1 mL of a 0.01 M NaBH4 solution to 5 mL of a0.1 M CTAB and 1.2510-4 M HAuCl4 solution. Then 0.5 mLof solution-A was added to 5 mL of a growth solution-B (0.1M CTAB, 2.5 × 10-4 M ascorbic acid and 1.25 × 10-4 MHAuCl4). The seeds of solution-A are allowed to react forseveral minutes in solution-B, producing a colloidal solu-

tion-C of larger anisotropic Au NPs. Then 0.5 mL of solu-tion-C are added to 5 mL of the growth solution-B and leftto react for 30 min. The final solution contains, as revealedby conventional TEM,anisotropic Au nanoparticles of evenlarger size. The global morphological and size change of Au NPswas also followed in each step by UV Visible spectroscopy. Ingeneral, a red shift of the plasmon absorption band togetherwith the appearance of a long tail is observed in each step.

Electron tomography was performed on an FEI TecnaiF20 electron microscope (FEI, Eindhoven, Netherlands) withan accelerating voltage of 200 kV. The tilt-series was ac-quired in HAADF-STEM mode (inner angle of detector ) 35mrad) using a Fischione 2020 ultrahigh-tilt tomographyholder in the tilt range of -76 to+66° with images recordedevery 2°. Many TEM parameters were controlled during theacquisition of the tilt series of projections, defocus, imageshift, specimen tilt, and the condenser lens astigmatism.Once the acquisition of the tilt series was completed, imageswere spatially aligned by cross-correlation algorithm usingInspect3D software (FEI, The Netherlands), and 3D recon-structions were achieved using a simultaneous iterativereconstruction algorithm (SIRT) of consecutive 2D slices.Visualization was performed using AMIRA 3.1 (MercuryComputer Systems).

All the optical properties presented here were calculatedusing the DDA.11 The DDA is a powerful numerical methodin which the object of interest is represented as a cubic latticeof N polarizable points. There are no restrictions as to whichof the cubic lattice sites are occupied, which means that theDDA can represent an object or multiple objects of arbitraryshape and composition. We take the ith element to have adipole polarizability Ri, with its center at a position denotedri. The details of this method have been published in severalplaces, so we will only give a brief description here. Thepolarization induced in each element as a result of interac-tion with a local electric field Eloc is

For isolated particles, Eloc is the contribution of theincident field plus the contributions from all other dipolesin particle

where E0 and k are the amplitude and the wave vector of theincident wave, respectively. A is an interaction matrix that takeinto account the interaction between each dipole with the restof the dipoles in the particle. Substituting eq 2 in eq 1 andrearranging terms in the equation, the following equation isobtained

Pi ) αi ·Eloc(ri) (1)

Eloc(ri) ) E0 exp(ik · ri) - ∑j*i

AijPj (2)

© 2010 American Chemical Society 2098 DOI: 10.1021/nl1005492 | Nano Lett. 2010, 10, 2097-–2104

where A′ is a matrix which is built out of the matrix A from eq 2where E is the incident field. By solving these complex linearequations system the polarization vector P is obtained, and withthis, theextinctioncrosssectionsandotheropticalpropertiesmaybe calculated.

Adaptive DDA Method for NPs with NonanalyticalSurface Shape. The morphological reconstruction of the NPprovides the Cartesian coordinates x, y, and z of the pointscorresponding to its surface boundary. From these points,a cubic arrange of dipole with lattice parameter d should beconstructed in order to emulate the optical response of theNP. For a given lattice parameter d, it should be fulfilled thata considerable number of points (from the surface coordi-nates given by the 3D reconstruction) are within a surfaceof area d2. If this is not the case, the resolution of the surfacereconstruction has to be improved.

For the cubic lattice construction, the minimum (xmin,ymin, zmin) and maximum (xmax, ymax, zmax) values of thecoordinates of the surface points are determined. Thecoordinates xmin, ymin, zmin are now chosen to be the centerof a new coordinate system and all the points belonging tothe NP surface are translated to this new origin. On thisreference frame, a cubic lattice of rectangular type is builtwith a number of cubes (Nx, Ny, Nz) of size d along each axesdetermined as follows: Nx ) int(xmax - xmin)/d, Ny ) int(ymax

- ymin)/d, and Nz ) int(zmax - zmin)/d, where the operator “int”returns the closest integer value of the rational number ofthis ratio. Each cube on this system will be identified with atriadic (i, j, k) and the cubes belonging to the surface of theNP are “labeled”. Then the whole rectangular cubic latticeis scanned along each direction to identify and discard theempty dipoles (i.e., those that do not belong to the NP). Bymoving along the positive z direction (k ) 0, 1, 2...) fromthe cube (i, j, 0), the cube is identified as empty until thefirst cube is labeled, or the motion is continued until Nz (ifany “labeled” cube was not found). This scanning procedureis repeated along the -z direction (k ) Nz, Nz-1, Nz-2, Nz-3...)from (i, j, Nz), the +x direction from (0, j, k), the -xdirection from (Nx, j, k), the +y axis from (i, 0, k), and the-y axis from (i, Ny, k). In this way all the nonempty cubeswill be the coordinates of the dipoles of the irregular NP.Depending on the choice, that is, if the cubes labeled on thesurface of the NP are considered to belong or not to the NP,then the volume of the reconstructed NP could be slighterlarger or smaller respectively than the real NP.

Results and Discussion. Figure 1 shows HAADF-STEMimages recorded at different tilt angles. The evident changesin the projection indicate a highly anisotropic particle shape.By inspection, a truncated prismatic geometry could beinferred from the images acquired at low tilt (Figure 1f), butvariations from this morphology are appreciated only at hightilt. This image series makes clear the need for a detailed

3D characterization of nano-objects in order to estimate theirproperties and to discern morphology effects.

Figure 2a-c shows three projections of the 3D surfacerendering of the SIRT reconstructions of this tilt series. Thenanostructures were achieved using the SIRT algorithm fora tilt series acquired using STEM HAADF images acquiredfrom-76 to+66°. The obtained results show the true shapeof the Au NP with detailed features about its facets and edgeswhich configure its anisotropic morphology.

These projections make evident the deviation from anyregular or ideal geometry. In particular the impossibility ofdescribing the particle morphology using analytical functionsis even more evident in z-x projection (Figure 2c). Anyinformation along this projection is just the informationmissed in any 2D analysis. Note that this projection for theideal model of Figure 1f is a rectangle. From our 3D analysisit is clear that the actual shape of the NP under considerationis highly irregular with almost no symmetry axis and ir-

FIGURE 1. HAADF-STEM images of an irregular Au nanoparticle atdifferent tilting. (a-e) Series of images showing the evolution of theimage with tilting angle. (f) The three-dimensional geometry thatcould be assumed from images obtained at low tilt is sketched.

FIGURE 2. Nanometrology performed over the 3D surface renderedof an Au nanoparticle. (a-c) Different projections of the 3D surfacerendering of a tomographic reconstruction. (d-f) Ideal models ofthe nanoparticle according with the tomographic reconstruction (seeSupporting Information).

A′ ·P ) E (3)


regular surfaces that can not be emulated by any analyticalfunction. The precise knowledge of these fine details hasimportant consequences in the optical properties in this kindof nanostructures because symmetry breaking enables cou-pling between plasmon modes of different multipole orderas it is shown below. Because of the nonanalytical shape ofthe NP incorporating the richness of such fine structural

details into modeling the optical properties requires the useof a flexible method.

In recent years, there have been great advances in thedevelopment of numerical electrodynamics techniques tosimulate the optical properties of particles of arbitrary shape;the discrete dipole approximation (DDA), finite differencetime domains (FDTD), and boundary element methods(BEM) are of widespread use.35,38-45 For irregular NPs, forexample, these methods have been applied using a param-etrization of the NP surface boundaries, for example, byincluding rounding radius at the tips and corners for highlyfaceted metal NPs,35,38-45 or in some cases where the NPsurface boundary is clearly very irregular by introducingarbitrary surface roughness for the surface inside the NP.46

This kind of approximation, although useful for NPs havingsome symmetry, could be very cumbersome to apply in thepresent case, where the surface boundaries are highlyirregular. This poses the challenge of transferring all therichness of the information obtained by a precise electrontomography characterization, into a suitable computationalalgorithm able to reproduce accurately all the fine details ofthe NP surface, without making use of any parametrizationof the surface boundary.

We have chosen, among the electrodynamics methodsmentioned above, the DDA approach, which relies on adiscretization of the volume inside the NP, using a cubiclattice of dipoles as described in the methodology section,and could accommodate NPs of arbitrary shape by matchingthis cubic lattice of dipoles to the NP morphology. Electrontomography provides the coordinates, corresponding toeach point on the NP surface. From these points we wereable to make a precise 3D reconstruction of the irregular NPby implementing adaptive DDA approach, capable of repro-ducing the very irregular NP surface boundaries without anyparametrization and with a high degree of precision.

To show the important consequences of not consideringthe precise morphology of the NP on its optical response,we will compare the optical behavior of a NP whose na-nometrology information is incomplete, denoted as particle“I”, with a real NP whose actual shape and size has beendetermined accurately, which will be labeled as particle “P”.NP I consists of a truncated triangular prism as shown inFigure 1f with basal dimensions equal to the NP image seenat zero degrees in the tilt series as seen in Figure 1c. Theparticle has a uniform cross section perpendicular to thez-axis. This idealization is the usual approximation made todescribe the 3D morphology when only 2D information isconsidered. To yield meaningful comparisons, this idealizedNP has the same height parallel to the z-axis as the realnanoparticle.

The optical response should also be dependent on thedielectric environment. At this stage, we have performed thecalculations for NPs in a homogeneous dielectric environ-ment, in vacuum. The role of the substrate for irregularnanoparticles should certainly be dependent on how the NP

FIGURE 3. Comparison between the extinction, absorption, andscattering spectra of the real particle P (black lines) and the idealizedparticle I (red lines) for different polarization (given by the directionof the incident electric field E) and illumination (given by thedirection of the wavevector k). (a) Propagation and polarization alongx- and y-axes, respectively. (b) Propagation and polarization along z-and x-axes, respectively. (c) Propagation and polarization along y- andz-axes, respectively. The orientation of NPs I and P with respect to thex- and y-axes is sketched in Figure 4a and Figure 5a. In all of thespectra, the incident field is parallel to any of the three Cartesianaxes. The number of dipoles used in the calculations was around600 000 and 1 000 0000 for NPs P and I, respectively. The timerequired for computing the extinction for each wavelength wasaround 10 h using an Intel Pentium D processor 3.4 GHz.


is supported on the substrate, but this issue should be thesubject of future work.

We will consider first the far-field optical properties, inparticular the extinction efficiency, which is the sum of thescattering and absorption efficiencies. The relative magni-tudes of the two contributions are highly dependent onnanoparticle size, with the contribution from scattering moreimportant than that of absorption as the size of the nano-particle increases.

Figure 3a-c compares the extinction, scattering, andabsorption cross section of NP P with NP I. The orientationof both nanoparticles with respect to the incident field isindicated in Figure 4a. The first, evident feature of thiscomparison is that there are, whatever the illuminationdirection, significant differences in the peak positions as wellas the relative intensities in the scattering as well as theextinction efficiencies between both NPs. These differencesare more important for polarization along the x- or z-axis.The absorption and scattering efficiencies peak respectivelyat λ ) 550 nm and at λ ) 650 nm for NP I while for NP Pthese peaks are at λ ) 508 nm and λ ) 613 nm forpolarization along the x-axis as depicted in Figure 3b. Forpolarization along the z-axis (Figure 3c) the absorption andscattering efficiencies peak respectively at λ ) 561 nm and

λ ) 634 nm for NP I and at λ ) 509 nm and λ ) 554 nm forNP P. For the remaining axis of polarization (y), althoughsome differences are evident in the extinction spectra, themaximum absorption occurs at almost the same value, λ )520 nm for both NPs while the scattering peaks at λ ) 708nm (for NP I) and at λ ) 665 nm for NP P (Figure 3a).

A large difference between the NPs is observed whencomparing the overall shape of the extinction spectra forpolarization along the x- and z-axis; for NP I there is ashoulder that is absent from the optical response of NP P.This feature is a consequence of the difference in the relativeintensities and peak positions of the absorption and scat-tering contributions for both NPs, as detailed before. It is alsoworth remarking that the scattering efficiency is greater forNP I than for NP P for polarization along the z-axis, while itis almost the same for polarization along the x- and y-axis.

Having established the important consequences that anincomplete nanometrology characterization has in the FFOP,we will now evaluate the differences on their NFOP, aquestion of paramount importance in the field of plasmon-enhanced spectroscopies, as it has been already mentionedabove.

A relevant aspect that could be answered by analyzingthe near distribution is the “nature” of the SPR mode being

FIGURE 4. Comparison between the electromagnetic-field enhancement between the real gold NP P and the idealized NP I at the samewavelength, λ ) 633 nm with wave vector and polarization as indicated in (a). (a) Three-dimensional plots showing the regions of each NPswith enhancements greater than 100. The NP orientation and wave vector and polarization direction is also indicated. (b) Field-enhancementcontours for the half plane along the z-axis, marked with a bold line in panel a. (c) Projections of the electric-field vector along this planeshowing that this mode corresponds to a dipole mode.


excited at a particular wavelength. For a dipole SPR modethe conduction electrons move back and forward in betweenthe alternating positive and negative charges generated intwo points around the NP surface. A quadrupole modecorresponds to a more complex motion of electrons be-tween two positive and two negative charges, generated ata given instant of time at the NP surface. Simple analysesbased on electrostatics will allow us to identify the SPRexcited at each resonant wavelength. Because of the prop-erty that a dipole orientates along the direction of the appliedfield, a dipole probe located in the vicinity of a chargedistribution outside the NP should orientate along the direc-tion of the field generated by these “instantaneous” chargedistributions with a direction that will depend on the chargedeveloped near the NP surface. Therefore by locating a setof probe dipoles outside the NP (with negligible polarizability,avoiding any perturbation on the field generated by thecollective response of the dipoles located within the NPboundaries), we were able to determine the charge distributionat each resonant wavelength. In the following, we will examinethese dipole vector fields that we will be shown as “vector plots”to provide qualitative insight concerning the nature of theplasmon mode being excited at each wavelength.41,42

We have chosen two wavelengths, λ ) 520 nm and λ )665 nm for the comparison that corresponds to the maxi-mum of absorption and scattering respectively of NPs P andI for polarization along the y-direction. This direction corre-sponds to where there are the less significant differencesbetween spectra (actually for this polarization direction theabsorption for particles P and I peaks at almost the samewavelength).

Let us first examine the vector plot at λ ) 665 nmobtained on the xy plane midway along the NP. As can beappreciated from Figure 4c, there is one region, especiallyon the NP tip at the bottom, where positive charge isdeveloped (the blue arrows, representing the direction of theprobe dipoles, points outward from the NP) while on theother two tips a negative charge is produced (the blue arrowspoints in toward the NP surface). Therefore at this wave-length a dipole resonance mode is excited in both NPs.

For excitation at λ ) 520 nm, a more complicated vectorplot is observed, taken on the same plane as before as shownin Figure 5c. In this case, there are two regions of positivecharges and two regions of negative charges, indicating thatthis SPR mode can be associated with a quadrupole modefor both NPs.

FIGURE 5. Comparison between the electromagnetic-field enhancement between the real gold NP P and the idealized NP I at the samewavelength, λ ) 520 nm. (a) Three-dimensional plots showing the regions of each NPs with enhancements greater than 5. The NP orientationand wave vector and polarization direction are also indicated. (b) Field-enhancement contours for the half plane along the z-axis markedwith a bold line in (a). (c) Projection of the electric-field vector along this plane showing that this mode corresponds to a quadrupole mode


From this analysis we conclude that the same mode isexcited at the same wavelength for both NPs. It is nowinteresting to compare the magnitude and localization of theevanescent near-field generated around the surface of eachNP for a fixed polarization along the y axis.

The magnitude of the electromagnetic-field enhancementΓ is usually defined as the square of the ratio of the modulusof the local evanescent field at a given point around the NP(|E|2) and the incident electromagnetic field (|E0|2), that is,Γ ) |E|2/|E0|2. In the so call E4 approximation, the magnitudeof the SERS intensity is approximately proportional to Γ2.

Quite clearly for both NPs, Γ is greater for the excitationof the dipole mode (520 nm) (Figure 4b) than for thequadrupole (665 nm) (Figure 5b). However both NP exhibita quite different distribution of the enhancements for bothmodes. Thus for the dipole mode of NP I, enhancementsgreater than 100 are seen just on regions localized aroundthe vertices (Figure 4a), while NP P due to its irregular shapealong the z-axis has an enhancement spread more along thisaxis (Figure 4b). The magnitude of Γ can reach values greaterthan 150 for NP P around the “bump” on the xy plane alongthe z-axis, while the enhancement is almost negligible onthe same plane for NP I, as illustrated in Figure 4a.

The 3D image of the regions with enhancement valuesgreater than 5 (Figure 5a) for the quadrupole mode indicatesthat this mode is predominantly delocalized along the sidesof NP I along the upper and lower faces, while on NP P it islocalized around the corners.

These results clearly demonstrate the importance ofconsidering the real nanoparticle morphology, since it de-picts a quite different localization and enhancement of theelectromagnetic field compared with the idealized NP forboth dipole and quadrupole excitation. The precise deter-mination of the electromagnetic-field enhancement and itslocalization in three dimensions is crucial for all plasmon-enhanced spectroscopies (SERS, TERS, MEF) and this goalcan only be achieved with the accurate nanometrologycharacterization together with a realistic 3D electrodynamicmodeling, as demonstrated in this work.

In summary, in this work we have performed a realisticnanometrology as well as a 3D modeling of the opticalproperties of a highly irregular plasmonic NP by applyingstate of the art microscopy tools and imaging algorithms inconjunction with a novel adaptive DDA approach able toreproduce subtle irregular and nonanalytical features of thenanoparticle surface. The dramatic differences on the far andnear-field optical properties that could result from an incom-plete microscopy characterization has been illustrated byrigorous 3D electrodynamics simulations.

Acknowledgment. J.C.H.G and P.A.M. acknowledge fi-nancial support from the European Union under the Frame-work 6 program for an Integrated Infrastructure Initiative,ref.: 026019ESTEEM. M.S.M. acknowledges financial sup-port of CONICET and ANPCyT. E.M.P., E.R.E., and E.A.C.

acknowledge financial support of CONICET, FONCyT. andSECYT UNC.

Supporting Information Available. A video showing a 3Dview of the nanoparticle morphology. This material is avail-able free of charge via the Internet at http://pubs.acs.org.

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3D Thomography and Plasmon Mapping