Environmental Symmetry Breaking Promotes Plasmon Mode Splitting

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Environmental Symmetry Breaking Promotes Plasmon Mode Splitting in Gold Nanotriangles Kyle Warren Smith, Jian Yang, Taylor Michele Hernandez, Dayne Francis Swearer, Leonardo Scarabelli, Hui Zhang, Hangqi Zhao, Nicholas Anthony Moringo, WeiShun Chang, Luis M. Liz-Marzán, Emilie Ringe, Peter Nordlander, and Stephan Link J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08428 • Publication Date (Web): 03 Oct 2017 Downloaded from http://pubs.acs.org on October 4, 2017

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The Journal of Physical Chemistry

Environmental Symmetry Breaking Promotes Plasmon Mode Splitting in Gold Nanotriangles Kyle W. Smith1, Jian Yang2, Taylor Hernandez1, Dayne F. Swearer1, Leonardo Scarabelli3,a, Hui Zhang4, Hangqi Zhao4, Nicholas A. Moringo1, Wei-Shun Chang1, Luis M. Liz-Marzán3,5,6, Emilie Ringe1,7*, Peter Nordlander2,4*, Stephan Link1,4* 1

Department of Chemistry, Rice University, 6100 Main Street, MS 60, Houston, Texas 77005, United States 2

Department of Physics and Astronomy, Rice University, 6100 Main Street, MS-550, Houston, Texas 77005, United States 3

Bionanoplasmonics Laboratory, CIC biomaGUNE, Paseo de Miramón 182, 20014 Donostia, San Sebastián, Spain 4

Department of Electrical and Computer Engineering, Rice University, 6100 Main Street, MS 366, Houston, Texas 77005, United States

5

Ikerbasque, Basque Foundation for Science, 48013 Bilbao, Spain

6

CIBER de Bioingeniería, Biomateriales y Nanomedicina, CIBER-BBN, 20014 Donostia, San Sebastián, Spain 7

Department of Materials Science and NanoEngineering, Rice University, 6100 Main Street, MS-325, Houston, Texas 77005, United States Corresponding Authors *(E. Ringe)

E-mail: [email protected]

Phone: (713) 348-2582

*(P. Nordlander) E-mail: [email protected] *(S. Link)

E-mail: [email protected]

Phone: (713) 348-5171

Phone: (713) 348-4561

Present Address a

L.S.: California NanoSystems Institute, University of California, Los Angeles, Los Angeles, California 90095, United States Department of Chemistry and Biochemistry, University of California, Los Angeles, Los Angeles, California 90095, United States

1 ACS Paragon Plus Environment


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ABSTRACT We report a single particle investigation of the polarized scattering spectra of individual Au nanotriangles (NTs) of the truncated bifrustrum type. We unexpectedly observed a wide diversity in the scattering spectra from a population of NTs with low shape polydispersity. Correlation of the optical measurements with electron microscopy revealed that the different optical responses were not due to distinct NT shapes. Rather, finite element simulations revealed that distinct polarized spectra originated from minute changes in the inclination of the NTs on the substrate. NT inclination resulted in asymmetric image charge formation in the substrate, thus breaking the degeneracy of the modes supported by the NTs. The degeneracy of the NT modes was extremely sensitive to such symmetry breaking, with inclination angles as small as 2o, producing clearly resolved, nondegenerate, orthogonally polarized plasmon modes.


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INTRODUCTION Metal nanostructures support collective oscillations of conduction band electrons called surface plasmons1-2. Plasmons have many fascinating properties, including locally enhanced electromagnetic fields3 and energy dissipation through the production of high energy or “hot” electrons4. These properties have driven extensive work toward understanding plasmons and using them for various applications including sensing5-7, imaging8-9 and catalysis10-11. Indeed, understanding how the optical properties of metal nanostructures are influenced by size, shape, and environment is a fundamental question in the field of plasmonics and is critical to the implementation of metal nanoparticles for any practical application12-14. For instance, the spatial distribution of the electric field for a given plasmon mode dictates the optimal sensing volume for an analyte in surface enhanced Raman scattering (SERS) measurements15-17. With considerable advancements in the controllable synthesis of metal nanoparticles, interesting geometries have been synthesized with low size-dispersity and high shape-yield to tailor the plasmonic properties for desired applications.18-19 Anisotropic nanoparticles such as nanostars20 and bipyramids21-22 are advantageous for SERS applications because they have tips, which are well established to increase the enhancement due to the concentrated electric fields at sharpened features. However, to optimally excite specific plasmon modes of anisotropic nanoparticles, the dependency of the plasmon response on incident light polarization must be well understood.



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Figure 1. a) Schematic illustration showing the two degenerate dipolar plasmon modes (E1 and E2) supported by NTs. Arrows indicate the direction of the induced dipole moment. b) Schematic illustration of the top and side views of the measured NTs c) Schematic illustration of the geometry of the measured NTs, with dashed lines indicating truncation planes and capital letters indicating relevant dimensions.

The plasmon modes supported by metal nanotriangles (NTs) have been studied by several groups previously23-35, with the three-dimensional geometry often being a right triangular prism. NTs have been prepared through top-down lithography for larger structures26, 33, or bottom-up chemical synthesis36-41 to produce Ag and Au triangular nanoprisms. The structural symmetry of a NT on a substrate is the ‫ܦ‬ଷ point group. The in-plane dipolar modes belong to the irreducible ‫ܧ‬ 4 ACS Paragon Plus Environment

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representation which is spanned by two degenerate orthogonal modes with polarizations perpendicular (E1) and parallel (E2) to one of edges of the NT (Figure 1a)24, 31, 42-44. For 0º polarization, only E1 is excited and for 90º polarization only E2 is excited. For any other polarizations, the response will be a superposition of these two fundamental modes. The resonance energy of these modes is determined by the edge length of the NT32 and is further influenced by rounding of the tips30. A recent report demonstrated the synthesis of Au NTs in a wide variety of sizes with 95% yield and less than 7% polydispersity45. The geometry of these NTs is a trigonal bifrustrum (Figure 1b-c). This geometry shares the three-fold symmetry of the previously considered triangular prisms, but with sharp edges and the possibility of additional asymmetry across the twin plane. This geometric model of NTs and its impact on the supported plasmon modes has not been previously considered in detail. We performed a detailed analysis of the polarized scattering of individual Au NTs of the truncated bifrustrum type with correlated electron microscopy and finite element method (FEM) simulations. We observed three distinct classes of polarized scattering spectra for the NTs, with a behavior that significantly deviated from previous reports on lithographically prepared Au NTs or chemically synthesized triangular nanoprisms. Correlated scanning electron microscopy (SEM) imaging was unable to resolve structural differences in NTs that had dramatically different polarized scattering spectra. To investigate whether unexpected geometrical features were the cause of the unique polarization responses, polarized scattering spectra were correlated with three-dimensional structural characterization using high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM). We found that the distinct polarized scattering spectra were still observed in NTs with nearly identical geometries. FEM simulations



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revealed that the degenerate dipolar plasmon modes supported by the NTs are sensitive to symmetry breaking in the environment. Small inclinations, or “tilting”, of the NT on the substrate resulted in asymmetric interaction of modes E1 and E2 with the substrate, breaking their degeneracy. A similar effect was reported by Mulvaney and coworkers46 where the inclination of decahedra nanoparticles influenced the ratio of dipolar and quadrupole mode scattering intensities, though no significant changes in resonance energy were observed as a result of differing orientation. In this report we show inclination angles as small as 2o caused clear splitting of the single peak into two orthogonally polarized resonances. Based on atomic force microscopy (AFM) measurements of the substrates, the cause of NT inclination was identified as roughness of the indium tin oxide (ITO) substrate or small excess of crystalized ligands trapped between the substrate and the NT.

EXPERIMENTAL METHODS NT synthesis Au NTs were synthesized according to a previously published method45. Briefly, Au seed@CTAC were prepared through standard NaBH4 mediated reduction: 300 µL of a 10 mM NaBH4 solution was added under vigorous stirring to 4.7 mL of a 100 mM cetyltrimethylammonium chloride (CTAC) solution containing 25 µL of a 50 mM HAuCl4 solution. The prepared seeds were aged for 2 hours at room temperature and then diluted ten times in 100 mM CTAC before proceeding. Subsequently, two solutions were prepared as follows: (1) 8 mL of Milli-Q water, 1.6 mL of a 100 mM CTAC solution, and 40 µL of a 50 mM HAuCl4 solution; (2) 10 mL of 50 mM CTAC solution, 125 µL of a 50 mM HAuCl4 solution, and 50 µL of a 10 mM NaI solution. Finally, 40 and 100 µL of a 100 mM ascorbic acid solution


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were added to solutions (1) and (2), respectively, and manually mixed until both solutions appeared transparent, indicating complete reduction of the Au(III) precursor to Au(I). Immediately after reaching transparency, 100 µL of the diluted seed@CTAC solution was added to solution (1). After manually stirring the solution for 1 - 2 seconds, 200 µL of solution (1) was injected into solution (2) and manually stirred for a few seconds. The growing nanoparticles were then left set undisturbed for at least 1 hour before characterization. The yield of Au NTs was about 60%. In order to separate byproducts, 210 µL of a concentrated CTAC solution (25% in weight) was added to the Au NT solution. The entire mixture was then transferred into a cylinder and left undisturbed overnight. Interparticle depletion forces47-48 drove the flocculation and precipitation of the Au NTs. The supernatant (pink-purple) was discarded, and the precipitate (forming a black patina at the bottom of the cylinder) was redispersed with 4 mL of 5 mM CTAC, immediately giving rise to a deep blue coloration. The produced Au NTs had a plasmon resonance centered at 687 nm and an edge length of 109 ± 7 nm (Figure S1). Correlated Scattering Measurements For scanning electron microscopy (SEM) correlated measurements, 10 µL of the stock Au NT solution was diluted to 100 µL with Milli-Q water. 5 µL was then spin coated onto the substrate at 500 rpm for 10 seconds followed by 3000 rpm for 30 seconds. The substrates were indium tin oxide (ITO) coated glass which were patterned through Au evaporation with an indexed transmission electron microscopy (TEM) grid mask for correlation of Au NTs between optical and electron microscopy49. The substrates were plasma cleaned in oxygen for two minutes prior to spin coating. Polarized scattering spectra were collected using a home-built dark field microscope based on an inverted microscope body (Ziess AxioObserver m1) with a hyperspectral detection system previously described50. Unpolarized light was focused onto the



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sample using a dark field oil-immersion condenser (Zeiss, numerical aperture (NA) = 1.4) with an annular aperture (Figure S2). Scattered light from the Au NTs was collected with a 50X objective (Zeiss, NA = 0.8), passed through a linear polarizer (Thorlabs, LPVIS100), and was detected with an imaging spectrograph (Princeton Instrument, Acton SpectraPro 2150i with Pixis 400 thermoelectrically cooled back-illuminated CCD camera) mounted on a linear translation stage. SEM images were collected on a FEI Quanta 400 ESEM FEG using an electron beam energy of 30 keV. For TEM correlated measurements, 100 µL of the stock Au NT solution was centrifuged at 7500 rpm for 10 minutes. The supernatant was discarded and the Au NTs were re-suspended in 1 mL Milli-Q water. This Au NT solution was then diluted 100 times, heated for 2 minutes at 40°C, and sonicated for 1 minute to minimize the population of aggregated NTs. Au NTs were then drop cast onto a 20 nm thick, 500 x 500 µm silicon nitride (Si3N4) TEM window (SiMPore, Inc.). The grid was placed on a glass coverslip and mounted onto the same microscope described above. Unpolarized white light was focused onto the sample using an air-spaced dark field condenser. Scattered light was collected using an oil-immersion objective (Zeiss, Plan-Achromat 63x, NA = 0.7) and was detected as described above. HAADF-STEM Reconstructions Correlated high angle annular dark field (HAADF) scanning transmission electron micrographs (HAADF-STEM) were obtained on Au NTs previously investigated using dark field spectroscopy. All particles were dispersed onto silicon nitride grids (SiMPore, Inc.) and analyzed on an FEI Titan Themis3 STEM operated at 300kV. Images were taken with tilt angles ranging between -65° and +65° recorded at 5° intervals. Tilt series for each individual Au NT were aligned using the TomoJ51 plug-in for FIJI52.


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FEM Simulations The finite element method (FEM, COMSOL Multiphysics 5.1) was used to study the light scattering properties of the Au NTs. All simulation parameters were chosen to mimic the experiment. Specifically, the Au NTs had rounded corners (5 nm radius of curvature), and either laid flat or stood on a side relative to the silicon nitride substrate that was modeled with a refractive index of 2.01. Au permittivity was taken from tabulated data53. Total scattering was calculated by integrating over a collection cone equivalent to a NA = 0.7 on the substrate side, and summed over four different simulations where excitation light had an incident angle of 40o and four different azimuthal angles of 0o, 90o, 180o and 270o. The four azimuth angles were averaged to simulate the annular dark field excitation geometry used in the experiment. Polarization dependent scattering spectra were obtained by changing the polarization of the incident light. This approach was simpler than simulating a polarizer in the collection path, but consistent with the experiment based on reciprocity of received and emitted radiation from antennas54.

RESULTS AND DISCUSSION



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Figure 2. Representative examples of single NTs belonging to the three categories of polarized scattering behavior: I) NTs lying flat with a single unpolarized scattering peak. II) NTs lying flat with two orthogonal scattering peaks. III) NTs lying on an edge with an unpolarized higher energy and a linearly polarized lower energy mode. The angles labeled in the legend correspond to the orientation of the linear polarizer in the detection path. Correlated SEM images of the NTs are shown in the insets. Scale bars are 100 nm. For 46 SEM correlated NTs on an ITO substrate the population broke down into 15% class I, 35% class II, and 50% class III.


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Single particle polarized scattering measurements of NTs correlated with SEM imaging revealed three distinct classes of behavior: I) NTs lying on a triangular face that had no significant polarization response in their scattering spectra, II) NTs lying on a triangular face that had a significant polarization response, and III) NTs lying on an edge face that had a significant polarization response of the lower energy mode (Figure 2). The variation in NT response was unexpected based on the low polydispersity of the sample (Figure S1). Additional data to support the representative examples shown are available in Figures S3-S5.

The scattering behavior of class I is consistent with that previously reported for triangular prism nanostructures24, 27, 32. The scattering spectra feature a single unpolarized peak, which arises from contributions from the E1 and E2 modes (Figure 2 I). The isotropic scattering response is a direct consequence of the ‫ܦ‬ଷ symmetry and its degenerate irreducible ‫ܧ‬ representation56. The result is a highly isotropic scattering response from the NT, whose triangular plane lies perpendicular to the optical path i.e. when the linear polarizer is projected onto the triangular face. In our microscope geometry this orientation can be described as the NT lying flat on the substrate (Figure S2). SEM images confirm that NTs of class I had this orientation (Figure 2 I, inset).

The scattering behavior of class II cannot be intuitively explained using the previously described model. The orientation and geometry of the NTs in class II were indistinguishable from those in class I based on SEM images, but their scattering spectra reveal two nondegenerate modes that are orthogonally polarized and of nearly equivalent intensity (Figure 2 II).



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The splitting of the main scattering peak into two orthogonally polarized peaks indicates a structural asymmetry that breaks the degeneracy of the E1 and E2 modes.

The class III behavior is partially explained using the same interpretation as for the NTs in class I. The change in scattering spectra appears to be related to the NT’s orientation, with the edge face on the substrate rather than the triangular face (Figure 2 III). As the NT is inclined with its triangular face nearly aligned with the optical axis, the collected scattered light becomes completely linearly polarized at 675 nm. This peak corresponds to the E2 mode along the edge of the triangle in contact with the substrate. The higher energy unpolarized peak at 625 nm results from the “out-of-plane” E1 mode and will be discussed in detail below.

As class II behavior was most unexpected considering the absence of any obvious symmetry breaking elements in the SEM images, we will now discuss it in greater detail. Two sources of asymmetry were considered to be the cause of the distinct scattering behavior: structural asymmetry of the NTs and asymmetry in the environment. Structural asymmetries from non-uniform tip rounding or asymmetric growth away from the twin plane were considered. It has been established that tip rounding influences the resonance wavelength of plasmons in NTs30. Examples of NTs with both sharp and rounded tips were found to exhibit scattering behavior of classes I and II (Figures S3-S4). FEM simulations furthermore showed that symmetric tip rounding does not lift the degeneracy of the two plasmon modes (Figure S6a). When the tips were rounded to different degrees in the simulations, symmetry was broken and the E1 and E2 modes showed different resonance energies (Figure S6b). However, no evidence of asymmetric tip rounding was observed in the correlated SEM images of the NTs. In addition,


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FEM simulations revealed that any isosceles character in the NTs also destroys the degeneracy of E1 and E2 (Figure S6c), but again no evidence was found in any electron micrograph images. Such a geometry would furthermore be confounding from a crystallographic perspective.

Because SEM imaging is limited in both spatial and angular resolution to determine precise three-dimensional geometric parameters, scattering measurements were correlated with STEM tilt series imaging, in order to evaluate asymmetry across the twin plane. Using this method, the polarized scattering spectra were directly correlated with a three-dimensional model for the corresponding NT. This procedure revealed that some NTs had large asymmetries in growth across the twin defect (Figure S7a-b). However, NTs possessing this asymmetry were again observed to belong to both classes I and II. In fact, NTs with nearly identical structural parameters had significantly different polarized scattering responses (Figure S7c-d). This evidence indicates that structural asymmetry was not the origin of the two orthogonally polarized scattering modes.



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Figure 3. a) Schematic diagram showing a NT inclined on the substrate. The angle is exaggerated for clarity. b) Simulated scattering spectra with orthogonal linearly polarized excitation for a NT at various inclination angles, α, relative to the substrate. For α=0o, the spectra completely overlap. NT dimensions are: A = B = 18 nm, C = 95 nm.

With asymmetry in the structure of the NT ruled unlikely as the origin of the class II behavior, environmental asymmetry was considered. It is well established that dielectric substrates can produce image charges that significantly shift the resonance energy of plasmons in metal nanoparticles.57-59 If the NTs were inclined at some very small angle relative to the substrate then the effect of image charge formation would be non-uniform. For instance, if one tip of a NT was lifted away from the substrate (Figure 3a), the plasmon-induced charge 14 ACS Paragon Plus Environment

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accumulation on that tip for the E1 mode has a weaker interaction with the substrate, resulting in a blue-shifted resonance relative to the same mode for a NT lying flat on the substrate. The orthogonal E2 mode would be dominated by charge accumulation in regions of full contact with the substrate and thus remain unaffected. Therefore, the NT could appear to be flat in the SEM and TEM images, but in fact have an asymmetric environment due to a slight inclination, which results in different interaction strengths with the induced image charges in the substrate. This effect would break the ‫ܦ‬ଷ symmetry of the system and lift the degeneracy of the E1 and E2 modes.

FEM simulations revealed that inclination angles as small as 2º resulted in the main scattering peak being clearly split into two orthogonal modes (Figure 3b). A 2o inclination corresponds to only a 2.8 nm gap between the NT and the substrate for a NT with an edge length of 80 nm. Simulations were performed on a NT at different inclination angles relative to a dielectric substrate composed of Si3N4 (n = 2.01), which was the substrate in the STEM correlated measurements. Figure 3b shows that the magnitude of the splitting increases as the inclination angle increases. As a tip of the NT is lifted further away from the substrate, the interaction is further reduced and the resonance energy of E1 continues to increase. It is important to note though that resonance energies of the experimental spectra are hard to compare between the different classes because they are determined by not only the NT edge length, but also NT height, tip curvature, and inclination angle, which all vary from particle to particle and cannot easily be obtained from the correlated SEM images.



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The origin of the NT inclination was likely due to a combination of surface roughness and non-uniform ligand distribution underneath the NT as they dried on the substrate. SEM correlated single particle measurements of NTs were performed on an ITO substrate had significant surface roughness. Tapping mode AFM measurements of the ITO substrate revealed a root mean squared surface roughness of 4.5±0.8 nm (Figure S8a). STEM correlated single particle measurements were performed on Si3N4 substrates, which were, however, nearly atomically thin. On this smoother substrate, the incidence of mode splitting in flat lying particles decreased from 70% in the measurements performed on ITO to 30%. Due to the fragile nature of these substrates their surface roughness cannot be measured with AFM directly. Instead, a solution containing cetyltrimethylammonium chloride (CTAC) of the same concentration as in the NT sample was drop cast on a glass substrate. AFM measurements of this sample showed a surface roughness of 3.3±0.1 nm that was clearly due to formation of small CTAC crystals (Figure S8b-c). The measured magnitudes of surface roughness for either substrate were sufficient to produce the inclination angles required for the observed symmetry breaking. We note that CTAC has a refractive index of n=1.38 (Sigma-Aldrich, #292737), which is greater than that of air, but significantly lower than silicon nitride (n=2.02) or ITO (n=1.83) substrates. A CTAC crystal was not included in the simulations due to unknown crystal size or location. More detailed modeling including a CTAC crystal would not materially change the circumstance of a lower refractive index environment surrounding a raised NT tip, relative to two NT tips near the surface of the substrate, though the magnitude of the resulting mode splitting may be impacted by the ratio of refractive indices. A CTAC crystal only 2.8 nm high between the substrate and the NT could result in a 2o inclination angle for a triangle face with an 80 nm edge length.


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Figure 4. Specific examples of class II and III behavior with TEM correlation. a, d) HAADFSTEM tilt series images of the single NTs. Scale bars are 100 nm. b, e) Measured and calculated polarized scattering spectra. The angles labeled in the legend correspond to the orientation of the linear polarizer in the detection path. c, f) Calculated charge plots of the NTs at peak wavelengths under different excitation polarization angles. The charge plot calculated at 605 nm in panel f has the charge intensity doubled for clarity in the visualization. The NT in panels a-c had dimensions of A = 8 nm, B = 45 nm, and C = 105 nm and was simulated with an inclination angle of 3º. The angle depicted on the charge plot is exaggerated for clarity. The NT in panels d-f had dimensions of A = 14 nm, B = 36 nm, and C = 110 nm and was simulated with an inclination angle of 54.75º. This tilt angle is determined by the crystallographic structure of the NTs.



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A combination of polarized scattering spectra correlated with HAADF-STEM images at various tilts and FEM simulations enabled us to clearly identify the modes contributing to the scattering spectra. A HAADF-STEM tilt series was collected on NTs belonging to class I, II, and III (Figures 4a, 4d, and S9). Accurate NT dimensions could be extracted this way, including any asymmetry in growth away from the twin defect, i.e. differences in lengths A and B (Figure 1). These dimensions were used in FEM simulations to calculate the polarized scattering spectra of the NTs. The angle of inclination of the NT could not be extracted from the HAADF-STEM images due to the necessarily low contrast of the substrate. Therefore, the inclination of the NTs was approximated based on agreement between simulated spectra compared to the experimental results.

The class II NT in Figure 4b had the characteristic two orthogonally polarized scattering peaks which were well reproduced by simulation. An inclination of only 3º was required to achieve good agreement with experimental results. It is worth noting that this small change in geometry results in an energy difference of 100 meV between the two modes. Charge plots of the two peaks reveal clearly that the high energy mode is the E1 mode and the low energy mode is the orthogonal E2 mode (Figure 4c). This assignment is in full agreement with the argument that the E1 mode is blue-shifted as the tip is raised away from the substrate and the effect of image charges is therefore reduced. A weak high energy mode at 550 nm appears in the experimental spectra and was also resolved in the simulated data, though its intensity was smaller and could be assigned to a higher order mode based on the calculated charge distribution (Figure S10). FEM simulations furthermore show that the mode splitting persists under normal incident excitation, and thus was not a result of the high angle, dark field excitation (Figure S11).


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The behavior of NTs in class III can be understood as a further extension of the substrate effects discussed when explaining class II, but with much larger inclinations. The trend of energy splitting and intensity reduction of the higher energy mode observed in Figure 3 can be extrapolated to the spectra observed for NTs resting along an edge face. Both the SEM and STEM correlated images clearly reveal this NT orientation (Figures 2 III and 4d). The scattering spectra show an intense, linearly polarized, low energy mode and a weak, unpolarized, high energy mode (Figure 4e). The low energy mode remains dominated by the E2 resonance along the edge of the NT in contact with the substrate, while the high energy mode is due to the E1 resonance oriented away from the plane of the substrate (Figure 4f). The reduction in scattering intensity and the polarization response of the high energy mode is due to the orientation of the E1 dipole. When the NT is highly inclined on its edge face the E1 dipole is almost completely aligned with the optical collection axis, reducing the scattering intensity as anticipated based on the radiation pattern of a classical dipole antenna. The unpolarized character of the high energy peak is also a result of the orientation of the dipole perpendicular to the plane of the substrate and its resulting projection on the polarizer in the detection path. Light collected by the objective from a dipole aligned parallel with the optical path will result in a completely isotropic polarization response.60 An additional weak, high energy peak identified as a quadrupolar mode at 605 nm was observed in the FEM simulations, though this was not observed in our experimental data due to its low intensity and the broad E1 peak centered at 625 nm.

CONCLUSIONS In summary, we conclude that the degenerate plasmon modes supported by trigonal bifrustrum type Au NTs are sensitive to symmetry breaking induced by the environment. Specifically,



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asymmetric image charge formation in a dielectric substrate due to inclined NTs resulted in dramatic degeneracy breaking between modes. FEM simulations showed that inclination angles as small as 2o result in clearly resolved, nondegenerate, orthogonally polarized plasmon modes. This phenomenon resulted in structurally similar NTs exhibiting very distinct polarized scattering spectra in single particle dark field measurements despite a low degree of shape polydispersity. This work is critical to the interpretation and understanding of the spatial localization of electromagnetic fields produced by Au NTs for sensing applications.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: BLANK. Characterization of the NTs including UV-vis ensemble measurements and TEM imaging, additional scattering measurements, additional FEM simulations, additional correlated STEM images, and substrate characterization by AFM.

AUTHOR INFORMATION Author Contributions K.W.S., L.S., W.-S.C., and S.L. designed research. L.S. synthesized and performed initial characterization of the NTs and was supervised by L.L.-M. K.W.S. performed SEM correlated scattering measurements and was supervised by S.L. T.H. performed the STEM correlated scattering measurements and was supervised by S.L. E.R. performed the STEM tilt series


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measurements. D.F.S. analyzed the STEM tilt series data and was supervised by E.R. J.Y., Ha.Z. and Hu.Z. performed the FEM simulations and were supervised by P.N. N.A.M. performed the AFM measurements. K.W.S. and S.L. wrote the paper in collaboration with all authors. Notes The authors declare no competing financial interests.

ACKNOWLEDGEMENTS This work was funded by the Robert A. Welch Foundation (C-1664 to S.L., C-1222 to P.N), the Army Research Office (MURI W911NF-12-1-0407 to S.L. and P.N.), the National Science Foundation (CHE1507745 to S.L.), and the European Research Council (ERC Advanced Grant 267867 Plasmaquo to L.L.-M.). K.W.S., T.H., and D.F.S. acknowledge that this material is based upon work supported by the National Science Foundation Graduate Research Fellowship Program (0940902).



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TOC GRAPHIC


Environmental Symmetry Breaking Promotes Plasmon Mode Splitting

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