Aluminum Nanorods - Nano Letters (ACS Publications)

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Aluminum Nanorods Benjamin Clark, Christian R. Jacobson, Minhan Lou, Jian Yang, Linan Zhou, Samuel Gottheim, Peter Nordlander, Naomi J. Halas, and Christopher J. DeSantis Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b04820 • Publication Date (Web): 22 Dec 2017 Downloaded from http://pubs.acs.org on December 22, 2017

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Aluminum Nanorods Benjamin D. Clark,1,4,‡ Christian R. Jacobson, 1,4,‡ Minhan Lou,2,4 Jian Yang,3,4 Linan Zhou,1,4 Sam Gottheim,1,4 Christopher J. DeSantis,2,4 Peter Nordlander,2,3,4 and Naomi J. Halas1,2,3,4* 1

Department of Chemistry, 2Department of Electrical & Computer Engineering, 3Department of

Physics & Astronomy, 4Laboratory for Nanophotonics, Rice University, 6100 Main Street, Houston TX 77005, USA.

ABSTRACT:

Al nanocrystals can be synthesized by high-temperature decomposition of triisobutyl aluminum, creating a mixture of nanoparticle geometries with a significant fraction (~15%) being single crystalline Al nanorods. The Al nanorods are elongated along their direction, and generally exhibit hexagonal cross sections consisting of two adjacent {111} facets separated by {100} facets on opposite sides. Dark-field scattering spectroscopy of individual Al nanorods reveals that rods of varying aspect ratio all possess transverse quadrupolar and octupolar modes in the visible (2-3 eV) and ultraviolet (3-5 eV) regimes. Theoretical modeling indicates that the longitudinal resonances of these nanorods span the near and mid-infrared regions of the spectrum. This work introduces a new class of anisotropic metal nanocrystals composed of single-crystalline Al, opening the door to highly-modifiable plasmonic nanorods from earth-abundant metals.

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Keywords: anisotropic, nanocrystals, synthesis, dark-field spectroscopy, plasmonics

Monodisperse anisotropic nanocrystals have wide usage in applications spanning photocatalysis, optoelectronics, sensing, imaging, and biomedicine.1–4 Resonant illumination of metallic nanoparticles induces oscillations of the conduction band electrons, known as localized surface plasmons.5 Elongating a metallic nanosphere into a nanorod lifts the degeneracy in the localized surface plasmon resonance frequency by breaking symmetry in one dimension, resulting in additional polarization-dependent plasmon modes at lower energies. Accordingly, nanorods typically exhibit transverse and longitudinal plasmon modes associated with their short and long axes.6,7 With increasing aspect ratio, the longitudinal plasmon resonance can be shifted to longer resonant wavelengths, enabling a tuning of the optical resonant properties with increasing nanorod length.8–10 This optical anisotropy allows one to selectively excite, or switch between, specific plasmon modes, depending upon nanorod orientation and the polarization direction of incident light, leading to applications in active chromatic displays and nonlinear optics.11–13 Although the chemical synthesis of noble metal nanorods is now quite well established, developing anisotropic nanocrystals using new materials will likely lead to new and interesting properties and applications.14–17 Aluminum is the most abundant metal in Earth’s crust, making it a potentially low-cost and sustainable candidate metal for many plasmonic applications.18 Al has a higher bulk plasmon frequency than the coinage metals Au and Ag, enabling a plasmon response that spans the electromagnetic spectrum from the ultraviolet (UV) into the infrared (IR).19,20 Lithographically prepared Al nanostructures have been shown to support localized surface plasmon resonances from the UV, through the visible region of the spectrum,21 useful for full-color displays and


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photodetectors,11,22,23 and into the mid-IR, useful for surface-enhanced infrared-absorption spectroscopy.24 Recently, Cheng et al. found that the imaginary component of the dielectric constant of single-crystalline Al is up to two times lower than that of polycrystalline Al in the UV and visible regions of the spectrum.25 The absence of grain boundaries in single-crystalline plasmonic materials reduces inhomogeneity in the free electron gas, leading to less plasmon damping in single-crystalline structures relative to polycrystalline samples.26 This substantially reduced plasmon damping should enable sharper resonances and longer plasmon lifetimes in single-crystalline Al compared to polycrystalline nanostructures of the same size and shape, such as those fabricated using lithographic methods.18 Additionally, single-crystalline Al nanoparticles are less sensitive to oxidation during formation compared to lithographically prepared Al nanostructures.21,27 Recently, a chemical synthesis of monodisperse Al nanocrystals with controlled size was reported,28 giving rise to new applications such as heterogeneous photocatalysis and new strategies for precise lineshape engineering.29,30 Decorating the surface of Al nanocrystals with catalytically active semiconductors or metals enables the design of “antennareactor” modular photocatalysts. These complexes, exploiting the plasmonic properties of the Al “antenna” and the catalytic “reactor” species, can expand the scope of plasmonic photocatalysis to industrially and environmentally important reactions, such as the selective reduction of acetylene to ethylene and the conversion of CO2 to CO.31–33 However, this synthetic strategy for Al nanocrystals has so far been limited to isotropic nanostructures. The synthesis of high-purity Al nanocrystals requires the titanium-catalyzed thermal decomposition of highly reactive aluminum hydride (alane), necessitating an inert atmosphere and aprotic anhydrous ethereal solvents.28 In this synthesis, the reduction of alane to metallic Al is accomplished by the hydride ligands, which produces thermodynamically governed isotropic


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nanocrystal shapes owing to the strength of the reducing agent. Common surfactants used to induce anisotropic growth in noble metal nanocrystals, such as cetyltrimethylammonium bromide, polyvinylpyrrolidone and sodium citrate, are incompatible with the synthesis of Al nanocrystals, since they react with the alane precursor.16,34,35 Seed-mediated approaches to anisotropic nanocrystals also require ligands that are incompatible with alane, making the synthesis of anisotropic Al nanocrystals a substantial challenge.36 High-temperature decomposition of molecular precursors in the solution phase, with the assistance of surfactants, has enabled the synthesis of monodisperse semiconductor nanorods.37 Similar thermal decomposition-based approaches with appropriate ligands have allowed for the controlled synthesis of anisotropic nanocrystals composed of face-centered cubic (FCC) metals and their alloys.38–41 Such an approach may be applicable to the synthesis of anisotropic Al nanocrystals if a suitable molecular precursor can be identified. Although alkyl aluminum compounds are more pyrophoric than alane, they are significantly more stable under inert conditions. Intriguingly, thermal decomposition of triisobutyl aluminum (TIBA) in the gas phase produced faceted but rather isotropic Al nanocrystals.42 Experimental and theoretical investigations concerning the mechanism of the gasphase pyrolysis of TIBA onto Al thin films with {111} and {100} surfaces indicate that this reaction proceeds through ß-hydride eliminations, which are faster on {111} relative to {100} Al surfaces.43,44 Since these decomposition reactions occur near 250 ºC, and since TIBA may have different decomposition rates on the various facets of a growing Al nanocrystal, TIBA appears to be a highly promising precursor candidate for solution-phase synthesis of anisotropic Al nanocrystals through high-temperature decomposition in aprotic coordinating solvents.45 In this work, we chemically synthesize anisotropic Al nanocrystals with ~15% yield by pyrolysis of TIBA in trioctylamine at 250 ºC in ~15 minutes, as shown in the reaction scheme in


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Figure 1A (also see representative images of the nanoparticle product mixture and its components, Figure S1, Supporting Information). Through structural analysis by electron microscopy, we identify the single-crystalline structure of the nanorods as the FCC lattice of Al. Using correlated dark-field spectroscopy of individual Al nanorods, their plasmonic response can be accurately modeled using the finite difference time domain (FDTD) calculations (Lumerical FDTD Solutions v8.15). This simple heat-up synthesis produces highly-faceted Al nanorods elongated along the with typical diameters of 275 ± 71 nm, lengths of 992 ± 287 nm and aspect ratios of 3.8 ± 1.5, as determined by the dimensions of 61 nanorods measured by scanning electron microscopy (SEM) from several identical reactions (twelve representative SEM images of Al nanorods are shown in Figure S2, Supporting Information). The other Al nanocrystals produced consist of comparatively isotropic shapes, including both irregular and well-formed single-crystalline octahedra and cuboctohedra, and ~10% twinned truncated trigonal bipyramidal nanocrystals, consistent with other shapes produced as side products in the synthesis of noble metal nanorods.46 As with previously synthesized Al nanocrystals, these anisotropic Al nanocrystals are passivated by the formation of a ~3 nm thick native oxide layer upon exposure to air. Powder X-ray diffraction of the anisotropic Al nanocrystals (Figure S3 of the Supporting Information) confirms the FCC lattice of the metallic Al nanocrystals. Thermogravimetric analysis of the Al nanocrystals reacting with oxygen (Figure S3 of the Supporting Information) indicates the samples are greater than 95% metallic Al while less than 4% of the total Al is incorporated in the oxide shell and less than 1% is attributed to carbonaceous ligands. To the best of our knowledge, this work demonstrates the first heat-up synthesis of colloidal Al nanocrystals and Al nanorods.45–47 The detailed protocol for the synthesis of anisotropic Al nanocrystals is provided in the Supporting Information.


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Figure 1. Synthesis and characterization of anisotropic aluminum nanocrystals. A. Reaction scheme for the production of anisotropic Al nanocrystals. B. Dark-field TEM micrograph of a representative Al nanorod. Scale bar is 250 nm. C. Corresponding electron diffractogram D. Tilt series of the same Al nanorod, beginning with the orientation of the micrograph in B and tilted by ±~44º All scale bars are 250 nm. E. Model indicating the corresponding orientation of the Al nanorod.

Structural characterization by electron microscopy and diffraction of a typical individual Al nanorod is presented in Figure 1. Following the reaction scheme (Figure 1A), both a dark-field transmission electron microscope image (Figure 1B) and its selected-area electron diffraction (Figure 1C) strongly suggest a single-crystalline structure. The electron diffraction pattern in Figure 1C displays the characteristic [100] pattern, indicating the nanocrystal is oriented with the zone axis of the electron beam normal to the (100) facet on which the nanorod is resting. SEM images of this nanorod (Figure 1D) in the same orientation (Figure 1D, middle), and tilted by ±~44º (Figure 1D, top and bottom), reveal the nanorod is highly faceted. When the nanocrystal oriented along the [100] zone axis (Figure 1D, middle) is tilted by -44º along the longitudinal axis


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of the rod, a (111) face is exposed (Figure 1D, top). Performing this tilt 44º in the opposite direction exposes an additional (111) facet that appears narrower due to an additional higher-order facet along the length of the rod. Using the nanocrystal orientation from electron diffraction (Figure 1C) and the dimensions from the SEM images at each angle (Figure 1D), we construct a model of the nanorod, shown in each perspective in Figure 1E. In the model of the nanorod in Figure 1, we include a higher-order {110} facet truncating two adjacent {111} facets (giving this particular nanorod seven sides) to accurately theoretically capture the experimentally measured scattering response of this nanorod (Figure S4 of the Supporting Information). From this data, we conclude that the single-crystalline Al nanorods synthesized by this approach generally exhibit hexagonal cross-sections, bound by two adjacent {111} facets separated by two {100} facets on opposite sites. Additional characterization of other Al nanorods by electron diffraction corroborate this structure, since nanorods grown along the direction do not display [111] diffraction patterns under typical orientations (Supporting Information Figure S5). This morphology is consistent with anisotropic nanocrystal growth along the direction, forming a shape similar to a truncated octahedron extended along the direction and producing nanorods that possess hexagonal cross-sections unless higher-order truncating facets exist between the {111} and {100} facets. This shape is consistent with the Wulff construction for FCC metal particles were the surface energy of the {111} and {100} facets are equal, except we also observe elongation in the direction of the particle.48 Growth along was also observed by high-resolution TEM and powder X-ray diffraction in Al nanowires electrochemically synthesized in a porous template.49 However, the faceting of those nanowires was not investigated. Growth along the direction is uncommon in single-crystalline noble metal nanorods, which typically grow along and exhibit


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octagonal cross-sections enclosed by four {100} and four {110} facets or perhaps higher order facets.46,50,51 Since the anisotropy of Al nanorods is crystallographically distinct from noble metal FCC nanorods, mechanistic investigations into the chemical synthesis of Al nanorods by this approach is warranted. As shown in Figure S6 of the Supporting Information, pyrolysis of TIBA in the absence of a coordinating solvent, such as hexadecane, produces irregular Al microcrystals. This structure is likely due to the absence of stabilizing ligands for the alkyl aluminum species during the initial decomposition, preventing homogenous nucleation and leading to rapid and uncontrolled growth. Remarkably, introducing a stoichiometric amount of trioctylamine, to serve as a stabilizing agent for nucleation and growth, reduces the average particle size by a factor of two and mediates the growth of well-shaped Al nanocrystals. Additionally, in pure hexadecane, TIBA begins to decompose at 225 ºC, while a stoichiometric solution of TIBA and trioctylamine in hexadecane and TIBA in pure trioctylamine both begin to decompose at 235 ºC. The 10 ºC rise in decomposition temperature suggests the formation of a dative bond where the non-bonding electrons of trioctylamine occupy the unhybridized pz orbital of the alkyl aluminum precursor, consistent with binding between hard Lewis acids and bases.52 It is possible that the additional stability imparted by complexation between TIBA and trioctylamine postpones the onset of nanocrystal nucleation, facilitating greater supersaturation of the alkyl aluminum precursors, which leads to a more homogenous nucleation event and the formation of a more uniform population of seeds. Reactions performed in trioctylamine using diisobutyl aluminum hydride (DIBAH) formed Al nanocrystals with the same morphology, suggesting that alkyl aluminum hydride intermediates may be forming during the reaction. If DIBAH is an intermediate created before the rate-limiting step, isobutylene should be produced during thermal decomposition of TIBA. To investigate this theory, we identified the gaseous byproducts from pyrolysis of TIBA in


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trioctylamine with gas chromatography-mass spectrometry (GC-MS). The elution of isobutylene in GC-MS (Figure S7 of the Supporting Information), indicates the presence of the intermediate DIBAH, consistent with ß-hydride elimination of TIBA to DIBAH above 120 ºC.53 The GC-MS elution also reveals the ratio of isobutylene to isobutane was 9:1, indicating the majority of Al-C bond cleavage occurred through ß-hydride eliminations. In turn, ß-hydride elimination creates alkyl aluminum hydride intermediates, which thermally decompose and nucleate to form Al nanocrystals with the assistance of coordinating solvents such as trioctylamine. Greater supersaturation of the precursor and temporal control of the rapid nucleation process are expected to afford greater synthetic control over the final size, shape and abundance of anisotropic Al nanocrystals produced by pyrolysis of TIBA in trioctylamine.54

Figure 2. Orientation and polarization dependence of nanorod scattering. A. Schematic of the excitation geometry for dark-field measurements. B. SEM image of the nanorod measured with the light vectors illustrated. C. Experimental and D. theoretical scattering spectra for S and Ppolarized light, with the rod lying parallel and perpendicular to the k-vector.


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We also investigated the scattering spectrum of individual Al nanorods, using a custombuilt, dark-field microscope equipped with reflective optics for aberration-free optical measurements extending into the UV region (Figure 2).21 Complete details for the data acquisition and particle correlation are provided in the Supporting Information. All experimental spectra are unsmoothed and normalized. Consistent with other anisotropic nanostructures,55 Al nanorods exhibit a strong dependence on both orientation of the rod with respect to the incident light and on optical polarization. Different orientations of nanorods and incident polarizations alter which plasmon modes can be excited. Random particle orientations with respect to the incident light result in mixed contributions from multiple plasmon modes, so this work focuses on rods lying parallel and perpendicular to the direction of incident light, as shown in Figure 2A-B. As can be seen in Figure 2C, different combinations of orientation and light polarization excite different modes for the same nanorod. The polarization-dependent scattering spectra of the Al nanorod was calculated using the FDTD method, with the particles modeled using hexagonal cross-sections and dimensions measured from SEM images. Since the thickness and underside of the particles could not be measured, these features were varied to achieve the best fit between theoretical and experimental spectra. In the theoretical simulations, the nanorods are laying on an infinite quartz substrate and are surrounded by air with a refractive index of 1. Owing to the large size of the rods, no oxide layer was included in the models to reduce the computational cost of the simulations. In Figure S8 of the Supporting Information, we show that inclusion of the oxide layer has little effect on the plasmonic response of an Al nanorod of such large size. These models most accurately reproduce the experimental measurements when using the recently reported dielectric constant for single-crystalline Al,25 though historical data also gives the correct lineshape, but introduces


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greater broadening in the UV regime (Supporting Information Figure S8).56 Using the model of the nanorod (Figure 2A) presented in Figure 1B, the spectra shown in Figure 2D successfully reproduce the experimentally measured polarization and orientation dependent plasmonic response (Figure 2C). The perpendicularly oriented P-polarized (solid black line) and the parallel S-polarized (dashed blue line) spectra appear to have similar spectral features, but further plasmon mode analysis indicates that this arises from degenerate resonances. The resonances at 2 and 3.5 eV in the perpendicular P-polarized spectrum correspond to the transverse quadrupole and octupole resonances, respectively. The 3.5 eV resonance in the parallel S-polarized spectrum corresponds to a new mode formed by a combination of the transverse quadrupole and the longitudinal 5th-order mode. This resonance is excited by S-polarized light in both the perpendicular and parallel geometries. The 4.5 eV resonance corresponds to the transverse 4thorder mode; this mode is not seen in the other nanorod measurements we performed, and is likely due to the sensitivity of these higher order modes to the details of the nanorod geometry.


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Figure 3. Polarization dependence of plasmonic modes in single-crystalline Al nanorod. A. SEM image of the measured rod, with light vectors illustrated. B. Experimental and theoretical darkfield scattering spectra. C. Calculated full scattering spectra D. Charge distribution maps for the resonance modes present in (C).

Because perpendicular orientation of the direction of incident light provides the most straightforward separation of the transverse and longitudinal modes in this nanorod, we use this orientation to examine Al nanorod plasmon modes in greater detail (Figure 3). With this orientation of incident light, Al nanorods exhibit clear resonances across the visible and into the UV wavelength range, most notably when excited with P-polarized light (Figures 3A, B). Due to the


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limited volume and direction of the collection cone, changes in relative mode intensity and shifts in resonance energy (for example, a 0.5 eV shift for the transverse quadrupole mode) can occur when comparing the collected spectrum to that produced by the full scattering sphere of the nanoparticle.57 The collection cone of the instrument preferentially collects different modes based on the shapes of their scattering lobes, so the collected intensities can differ from the total amount of light scattered for a specific plasmon mode. The full scattering spectrum shown in Figure 3C shows that modes are closely overlapping, while the collected scattering is affected by interference between adjacent modes, resulting in sharper peaks and shifts in the resonance energy. Despite these shifts, the resonances observed in the total scattering sphere correlate directly with those observed in experiment, with the same modes are present in both spectra. This correlation allows the use of the full scattering spectrum, which often has clearer resonance behavior, for plasmon mode analysis. Comparison of the experimental spectra in Figure 3B and the charge-density maps (Figure 3D) corresponding to the peaks in the calculated spectra in Figure 3C allows for identification of the plasmon modes that produce the experimentally observed scattering features. This analysis shows that the observed transverse resonances are primarily higher-order quadrupole and octupole plasmon modes. While these modes are typically considered dark, with stronger absorption than scattering, the large diameter of the nanorods allows these dark modes to have significant radiative coupling to the far field, mediated through optical phase retardation and coupling with the broadened dipole mode. As a result, the typically absorptive modes are clearly observed in the scattering spectrum. Additionally, the measured longitudinal scattering (Figure 3B) is shown to consist of very high order modes, 4th order and higher, which overlap such that no clear peaks can be observed, resulting in the featureless, broadband scattering observed in the measurement.


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After achieving good agreement between experiment and theory in the measured spectral region, using a simulated structure with a hexagonal cross section, the theoretical scattering spectra for the measured nanorods were extended into the infrared spectral region (Figure 3C) to further investigate their plasmonic behavior. FDTD calculations predict that these nanorods have a strong resonance in the mid infrared region of the spectrum, corresponding to the longitudinal dipole mode. This resonance is predicted to be relatively sharp, not broadened due to the size of the particle, as is the case for the transverse dipole mode. This clear, sharp resonance is likely due to the long length and large aspect ratio of the nanorod, causing it to act as an antenna for mid-infrared light. Metal nanorods have been shown to support a strong longitudinal dipole resonance when their length is approximately λ/2 due to electromagnetic field confinement in the transverse direction.58


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Figure 4. Comparison of selected Al nanorods. A. SEM images of the Al rods, with light vectors illustrated. B. Experimental scattering spectra collected with P-polarized light. C. Corresponding theoretical FDTD spectra.

Comparison of several representative nanorods in Figure 4 shows that despite nanoparticle-tonanoparticle differences in longitudinal and transverse length (Figure 4A), the transverse modes of the nanorods are similar in energy, with visible quadrupole and UV octupole modes (Figure


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4B). Although the rods appear variable in shape and the spectral line-shape is sensitive to particle geometry, these particles can be modelled as single-crystalline structures using hexagonal cross sections for predicting their resonance energies. Modelling the particles using alternate crosssections fails to capture the correct plasmonic behavior of the Al nanorods. Variations of the edgelengths of the four {111} and two {100} facets along the length of the rod (Figure S9 of the Supporting Information) enables the calculation of theoretical scattering spectra (Figure 4C) in good agreement with experimental spectra (Figure 4B) for various rods. The facets at the ends of the all models were carefully constructed because variations in the geometry at the ends of the rod can introduce shifts in the UV scattering of the nanorods, though the visible and infrared scattering is unaffected by these changes (Figure S8 of the Supporting Information). This comparison shows the consistency with which different nanorods still exhibit very similar quadrupole modes in the 2-3 eV region and octupole modes in the 3-5 eV region for P-polarized incident light. The similarity between the experimental and theoretical spectra provides further evidence that the Al nanorods grow along and generally have hexagonal cross sections despite variations in morphology. The energy of the transverse plasmon mode becomes less dependent on nanorod length than for smaller nanorods, and agrees well with infinite circular cylinder scattering theory.38 Small variations in the surface structure impacts their scattering properties much less than their overall size. Due to the variability in length and aspect ratio between Al nanorods, the S-polarized response varies from particle to particle. The longitudinal dipole modes of the different rods are calculated to span from the NIR to the mid-IR regions of the spectrum with increasing nanorod length. The theoretical scattering spectra of the longitudinal dipole mode of the nanorods in Figure 4 are provided in Figure S10 of the Supporting Information. As the length of the nanorods increases, the


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intensity of longitudinal resonance increases and redshifts. The dipole mode also becomes sharper as the length of the rods increases. Our calculations, and those of others, indicate that the longitudinal resonances of the Al nanorods can be tuned across the visible region when the rods are scaled down in overall size by a factor of five to ten.59 Producing smaller Al nanorods remains a synthetic challenge that, once resolved, will have profound implications on the development of anisotropic plasmonic nanocrystals for sustainable plasmonics. In summary, we have demonstrated the synthesis of single-crystalline Al nanorods with polarization-dependent plasmon resonances spanning the visible and UV regions of the spectrum. Structural analysis confirms that the nanorods typically share a hexagonal cross-section and growth along the direction. Experimental scattering spectra of individual Al nanorods supported by theoretical calculations showed that their optical properties in the visible and UV regions are dominated by higher order transverse quadrupole and octupole modes.

FDTD

calculations also indicate that these nanostructures support strong mid-IR longitudinal dipole modes, acting as mid-IR antennas. Strongly resonant modes spanning from the mid-IR into the UV, can enable a variety of potential applications across this wide energy range. Because of their polarization-dependent plasmonic response and highly faceted nature, these single-crystalline Al nanorods may interact strongly with Al thin films, which would dramatically alter and narrow the spectral features of their plasmonic response.30 These nanorods may also be useful for nonlinear optics, for example, for harmonic generation, providing resonant enhancement at very disparate energies.60 Developing further control over the dimensions of Al nanorods is a major goal moving forward, since shrinking the nanorods from their current dimensions would further enhance features such as field enhancement and optical tunability, making them useful building blocks for sustainable plasmonics.


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ASSOCIATED CONTENT Supporting Information. Representative TEM and SEM of anisotropic Al nanocrystals and histogram of shape distributions. Powder X-ray diffraction, thermogravimetric analysis and differential scanning calorimetry of anisotropic Al nanocrystals. Experimental and theoretical scattering spectra of the rod in Figure 1. Electron diffraction of Al nanorod near [111] zone axis. SEM of reactions with stoichiometric amounts of trioctylamine and in pure hexadecane. GC-MS elution profiles of the gaseous byproducts and associated mechanism for the pyrolysis of TIBA. Control simulations showing the effects of variations in end-cap geometry, dielectric constant for Al, and the inclusion of an oxide shell. Hexagonal cross-sections of the models used for the rods in Figures 3 and 4. FDTD calculations of the longitudinal mode for the rods in Figure 4. The following files are available free of charge at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected]. Author Contributions ‡These authors contributed equally to this manuscript. B.D.C. and C.R.J. contributed equally to this work. B.D.C. developed the nanoparticle synthesis; B.D.C., C.J.D., and C.R.J. performed the physical characterization; C.R.J. and S.G. performed


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the optical measurements; M.L. and J.Y. performed the theoretical simulations; B.D.C. and L.Z. performed GC-MS characterization. All authors analyzed the results and contributed to the preparation of manuscript and discussions. P.N. and N.H. supervised the research. ACKNOWLEDGMENTS This research was financially supported by the Army Research Office (MURI W911NF-12-10407), the Air Force Office of Scientific Research Multidisciplinary Research Program of the University Research Initiative (MURI FA9550-15-1-0022), the Defense Threat Reduction Agency (HDTRA1-16-1-0042) and the Welch Foundation under grant C-1220 (N.H.) and C1222 (P. N.). C. J. acknowledges graduate fellowship support by the Department of Defense (DoD) through the National Defense Science and Engineering Graduate Fellowship (NDSEG).

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