Substrate Induced Symmetry Breaking in Penta-twinned Gold

Article pubs.acs.org/JPCC

Substrate Induced Symmetry Breaking in Penta-twinned Gold Nanorod Probed by Free Electron Impact Pabitra Das* and Tapas Kumar Chini* Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 700 064, India S Supporting Information *

ABSTRACT: Cathodoluminescence (CL) spectroscopy and imaging in a high resolution scanning electron microscope (SEM) is used to probe and directly map the localized surface plasmon resonance (LSPR) modes of a penta-twinned gold nanorod deposited on a silicon substrate. Finite-difference time-domain simulation of CL enables us to gain insight into the origin of the plasmon modes. Our experimental results and simulations demonstrate that the substrate plays a very crucial role in the observed plasmonic property of gold nanorod. We have shown that, in the visible domain of the spectrum, the plasmon mode gets split into two distinct peaks due to substrate induced hybridization of in-plane and out-of-plane modes. With increasing the refractive index of the substrate, the intensity of these hybridized modes increases. We provide a detailed analysis on the origin and coupling of various plasmon modes mediated by the substrate.

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INTRODUCTION The research in noble metal nanoparticles (MNPs) in the last two decades has been mainly driven by their ability to absorb and scatter light strongly in the visible range of the spectrum.1−3 This strong absorption and scattering is caused by localized surface plasmon resonance (LSPR). LSPR exhibits localization and enhancement of the local electric field.4 The LSPR frequency depends strongly on the shape, size, and local dielectric environment of the NP. Among nanostructures of different shapes, metallic nanorods are of special importance because of their use as generic plasmonic antennas operating at optical and near-infrared frequencies. This allows directional control of light that can be exploited in versatile applications ranging from bioimaging to novel multipolar sources of photons owing to the higher order LSPR supported by these antennas.5−17 The longitudinal plasmon mode of gold nanorods is highly dependent on their length-to-diameter ratio (aspect ratio) in a linear relationship so that a slight deviation from the spherical geometry can induce distinct color changes.18−21 Consequently, nanorods are quite often discussed as an excellent tool for the manipulation of a variety of nanoscale light matter interactions. Local dielectric environment plays a very important role in the plasmonic property of MNPs, and the plasmonic response can be manipulated by changing the local dielectric environment of the MNP. When the MNP is immersed in a homogeneous dielectric medium, the local dielectric environment is symmetric, but when the MNP is put on a substrate, the homogeneity is destroyed and the symmetry is broken. Putting the MNP on a substrate is the easiest way to introduce a symmetry breaking in the system, which has enormous consequences in the plasmonic response of the MNP. Moreover, when the nanoparticle is large in size, due to the © 2014 American Chemical Society

retardation effect, multipolar plasmon modes are excited. Introduction of a substrate means the electron oscillation associated with the plasmon modes induces image charges within the substrate. The higher the refractive index of the substrate, the higher is the amount of image charge. These image charges interact with the excited plasmon modes of the nanoparticle and it causes hybridization. In most of the far field spectroscopic techniques, the metallic nanoparticle under consideration has to be deposited on a substrate. From an application point of view, such as for example, while designing a plasmonic device, the metal NP has to be put on a dielectric substrate in ultrathin single/multilayered/particle shaped nanostructured form, where the knowledge of the role of substrate in modifying the LSPR property is of utmost importance. In general, for the sake of simplicity in modeling the structure, the substrate combined with the surrounding is approximated as an effective homogeneous medium in which the MNP is embedded and as a result very rich plasmonic features arising due to the effect of substrate remain unknown. Hence accessing the precise effect of substrate on the plasmonic property of the gold NP is of utmost importance. Recently this aspect has drawn huge attention of researchers working in the field of plasmonics. Knight et al.22 demonstrated the effect of substrate induced symmetry breaking in a spherical nanoparticle on a substrate using polarization dependent dark field microscopy (DFM) measurements. Li et al.23 studied the plasmon propagation of Ag nanowires situated at different separation distances from the substrate with different dielectric constants. Wu et al.24 applied the finite-difference time-domain method to Received: September 9, 2014 Revised: October 14, 2014 Published: October 16, 2014 26284

dx.doi.org/10.1021/jp509109a | J. Phys. Chem. C 2014, 118, 26284−26291

The Journal of Physical Chemistry C

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a substrate that can result in splitting of the spectral peak. We show here that the effect of substrate can give rise to very significant changes in the LSPR mode, especially in the high energy domain of the observed far-field spectrum. As far as the local electron beam excitation and CL are concerned, the substrate effect presented here has not been demonstrated experimentally so far.

model the substrate effect on a nanoshell geometry and also studied the substrate mediated hybridization effect. Zhang et al.25 theoretically demonstrated the substrate induced symmetry breaking and Fano resonance in a Ag nanocube. The very local nature of the substrate effect on a Ag nanocube was experimentally and numerically studied using electron energy loss spectroscopy (EELS) and the discrete dipole approximation (DDA) by Mazzucco et al.26 The Fano resonance in a Ag nanocube on a substrate was investigated using DFM and EELS by Iberi et al.27 Studies also are reported to give an analytical formalism to access the effect of substrate and its precise effect on the LSPR modes.28 Very recently, Bosman et al.29 demonstrated the effect of substrate on chemically synthesized and lithographed nanostructures of similar shape and dimension and also demonstrated that the contact area between the particle and the substrate plays a very crucial role in determining the LSPR mode. The substrate effect on more complex structures such as decahedra30 and nanostar31 has also been demonstrated both theoretically and experimentally. In this present study, we have tackled a complex form of gold nanorodpenta-twinned nanorodand discussed its plasmonic behavior in the presence of a substrate. The pentatwinned rod is unique in the sense that it has six sharp apex on each side, which provide larger near-field enhancements at resonance wavelengths compared to its cylindrical counterpart. In a pentagonal cross-sectional geometry, the varying distances of the apex from the substrate give rise to rich features in the transverse direction, as we will demonstrate in this paper. In this context, the standard spectroscopic methods based on optical excitation of ensemble of particles are not enough to investigate the exact nature of the LSPR behavior of single particles. Although optical techniques such as DFM6,32 and near-field scanning optical microscopy (NSOM)33−35 can probe plasmons from single MNPs, those techniques have limitations in terms of diffraction limit. NSOM is constrained by the requirement of fabricating very sharp tips, and furthermore, interaction of the tip with the light in the structure often perturbs the signal of interest, making it challenging to probe the detailed spatial profile of the LSP resonance. True nanoscale resolution can be obtained only by electron microscopy based techniques.36−38 In this regard, EELS in a TEM7,8,30,39−46 and CL9,10,31,47−50 in a SEM/TEM have been shown to be excellent probes of plasmons with a very high spatial resolution, and due to the noncontact nature, they do not have undesirable effects on the measured quantities. In CL and EELS one can correlate the exact morphology with the LSPR energy, which is crucial in understanding the optical response of metal nanostructures. As no stringent condition of sample thinning is involved in SEM-CL, it can be employed as an ideal single particle spectroscopy tool to access the effect of substrate. This paper addresses several issues that are very often neglected even in a very well-studied structure such as a gold nanorod. Using CL spectroscopy and imaging, we have experimentally demonstrated, in the wavelength range 500− 700 nm, the plasmon modes of a penta-twinned gold nanorod, a far more complex structure compared to the commonly used simple spherical or cylindrical geometry, and provided twodimensional spatial maps of those modes. The experimental data was analyzed by detail FDTD simulations to establish substrate induced coupling of modes and hybridization. Our analysis shows that the higher order longitudinal plasmon mode of a rod can hybridize with a transverse mode in the presence of

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EXPERIMENTAL SECTION Synthesis Procedure. Penta-twinned gold nanorods in the present work are synthesized by a seed mediated growth method which is a small variant of the protocol used by Wu et al.51 and described in our recent work.48 Before doing any microscopy or CL study, the colloidal solution is diluted sufficiently with deionized water. The diluted solution is then carefully dispersed on a Si(100) substrate by dropcasting and desiccation and dried under ambient condition for 2 days and after that the nanoparticle containing specimen is inserted into the SEM chamber. This method produces a mixture of nanospheres, triangular prisms, decahedra, and faceted rods of different size. Among the randomly distributed nanoparticles, isolated single penta-twinned nanorod is selected for CL study. Cathodoluminescence Measurements. CL spectroscopy and imaging on an isolated Au nanorod on a Si substrate are performed in a ZEISS SUPRA40 SEM equipped with the Gatan MonoCL3 CL optical detection system.52 The ZEISS SUPRA40 SEM has a hot Schottky field emission gun (FEG), and the attached MonoCL3 system uses a retractable paraboloidal light collection mirror. The parabolic mirror collects light that is emitted from the sample covering 1.42π sr of the full 2π of the upper half sphere and collimates it through a hollow aluminum tube to a 300 mm Czerny−Turner type optical monochromator, and finally the signal is fed to a highsensitivity photomultiplier tube (HSPMT). Data is obtained with an electron acceleration voltage of 30 kV and beam current of ∼15 nA with a beam diameter of ∼5 nm. The electron beam is directed onto the sample surface through a 1 mm diameter hole in the mirror. To ensure maximum efficiency of light collection, the top surface of the sample is kept at the focal plane of the mirror, which lies approximately 1 mm below the bottom plane of the mirror. Before every set of experiments, this optical focal plane is adjusted with utmost care using the stepper motor controlled sample stage. The CL-SEM system can be operated in two modes, namely, monochromatic and panchromatic. In monochromatic mode, the focused e-beam is either scanned over the sample or positioned on a desired spot. The emitted light from the sample passing through the monochromator allows the emission spectra to be recorded serially in the wavelength range 500− 700 nm with a step size of 4 nm. The dwell time is selected as 0.25 s. Spectra have been averaged for each e-beam position and corrected from the substrate background. The monochromatic photon map is then built up at a selected peak wavelength of the emission spectrum by scanning the e-beam over the sample. For each e-beam position, the luminescence is collected over the entire sample. The bright pixels then correspond to the areas where the strongly excited plasmon mode emits the photons. When adding all the position dependent partial maps, obtained for each e-beam position, we obtain a full CL map of the plasmon mode associated with a particular wavelength. In the panchromatic mode, the emitted light skips the monochromator and goes directly to the HSPMT. 26285



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FDTD Simulations. To have a detailed understanding of the surface plasmon assisted photon emission from the Au nanorod and to access the effect of substrate precisely, we have performed 3D-FDTD numerical simulations (Lumerical Solutions). Maxwell’s equations are solved in discretized space and discretized time to follow the response of a material to any applied electromagnetic (EM) field (i.e., the evanescent field associated with e-beam in the case of CL). The current density associated with the e-beam is given by J(t , r ⃗) = −evuẑ δ(z − vt )δ(x − x0)δ(y − y0 )

(1)

where e is the electronic charge, and v is the velocity of electron, (x0, y0) represents the position of the electron beam, z is the direction of electron velocity, and ûz is the unit vector along the z direction. This current density can be modeled as a series of electric dipoles with temporal phase delay (z/v) (here v = 0.32c corresponds to the 30 keV electron energy used in the present experiment). We used the experimental dielectric permittivity tabulated in the CRC database for gold,53 and refractive index of 4 for silicon, wherever a substrate is used. The substrate thickness is taken to be 1 μm. A detailed description of the FDTD approach to simulate the 2D-CL image is described in our recent works.48,49

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RESULTS AND DISCUSSION CL Spectroscopy and Imaging. The SEM image of the penta-twinned gold nanorod is depicted in Figure 1. The aspect ratio of the rod (i.e., the ratio of the maximum length BE to the maximum width GI) is approximately 3.9.

Figure 2. (a) Experimental CL spectra taken from different symmetry points of penta-twinned gold nanorod on Si(100) substrate. The morphology of the rod is shown in the inset. (b) 3D-FDTD simulated CL spectra of the same morphology as shown in the inset of (a). In the inset, a panchromatic CL image has been shown. (c), (d), (e) Experimental monochromatic CL images at experimentally observed resonant wavelengths of 530, 550, and 644 nm, respectively. (f), (g), (h) FDTD simulated 2D-CL images acquired at simulated resonant wavelengths of 540, 568, and 670 nm, respectively. The length scale bar value in all the images is 200 nm.

excitation at B, is approximately 1.7. In the simulated CL spectrum, for e-beam excitation at A or C, we observe a faint dip in the spectrum at wavelength 630 nm, which is not detected in the experimental spectra. For e-beam excitation at the middle edge (point G and I), we get a blue-shifted peak compared to the other excitation points, both in experiment (530 nm) and in simulation (540 nm). For both the cases the intensity is quite small. The ratio of maximum intensity for ebeam excitation at A or C to that from G or I is 2.3 (for experimental spectra) and 1.8 (for simulated spectra). In Figure 2(c−e), we present the experimentally observed monochromatic CL photon maps at wavelengths 530, 550, and 644 nm, respectively. The simulated 2D-CL images are shown in Figure 2(f−h) at wavelengths 540, 568, and 670 nm, respectively. Figure 2(e) and (h) seem to be quite similar, showing a quadrupolar mode of plasmon oscillation pattern in both the experimental and simulated images. Overall the measurements confirm good correspondence between the experimental and the numerical findings. Figure 2(g) (the simulated CL map at 568 nm) seems to represent an octupolar mode; however, in our experiments we could not resolve it due to its low intensity and strong overlap with the intense corner modes of A and C. In simulations, this overlap is comparatively small, and we get an octupolar spatial distribution. The peak at 540 nm in Figure 2(f), although it seems to be another octupolar mode but with

Figure 1. SEM morphology of the penta-twinned gold nanorod on Si(100) substrate. BE = 597.5 nm, CF = 499 nm, GI = 153 nm. The length scale bar = 200 nm.

In this study, we focus on the site specific CL spectroscopy from different symmetry points and imaging the spatial profile of the resonant plasmon modes. In Figure 2(a), we show the CL spectra recorded from different locations of the rod indicated in the SE image (inset) in the wavelength range 500− 700 nm. The CL peak at around 550 nm, with the highest intensity, has been observed for e-beam excitation at point A and C. For excitation at point B, the intensity is small, but there are two distinct peaks separated by a dip in the spectral profile. In our experiments, the maximum intensity obtained from ebeam excitation at point B is almost 2.5 times smaller than the maximum intensity peak from point A or C. In Figure 2(b), we present the FDTD simulated CL spectra from a similar penta-twinned rod. Here also, we observed two resonance peaks separated by a sharp dip for e-beam excitation at point B. The ratio of maximum intensity obtained from ebeam excitation at point A or C, to that obtained from e-beam 26286



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the octupolar nature in the simulation arising primarily due to its overlap with the intense spectral peak at 550−560 nm, both in experiments and in simulations. We will discuss the exact origin of all these peaks later with the help of FDTD simulations. Simulated CL Spectroscopy without Substrate. Any SEM based experiment has to be performed by placing the particle on a substrate, whereas, in simulations, there are no such restrictions and we can place the particle suspended in air. From a theoretical point of view, a free-standing configuration is important, which helps us to understand the effect of the observed CL spectrum in the presence of the substrate. In Figure 3, the simulated CL spectrum of the same nanorod as

Figure 4. (a, top) Simulated CL spectra in the wavelength range 500− 3000 nm for varying refractive indexes of the substrate. (b, bottom) Zoomed view of the spectra in the wavelength range 500−1000 nm showing spectral splitting in the presence of the substrate of different indexes.

shift is also accompanied by an almost 81% decrease in CL intensity. More interesting changes in the spectrum take place in the wavelength range 500−1000 nm. The distinct Q, O, and T modes get split, and we see new peaks at different wavelengths. Figure 4(b) depicts the zoomed view of the spectra of Figure 4(a) in the wavelength range 500−1000 nm. We also note that the overall intensity of the peaks below 800 nm wavelength increases with the increasing refractive index of the substrate. This is exactly the opposite behavior of the dipolar modes located in the far IR region. In the final part of this section we will discuss all these issues with the help of an image charge model. To obtain a more detailed nature of the peaks, we select the spectrum for substrate index 2. For this value of substrate index, the major dipolar mode and the other modes below 800 nm are distinct. The simulated CL spectrum of Figure 5(a) depicts six peaks at wavelengths 2336, 1190, 867, 715, 653, and 557 nm. We plotted the near-field intensity (|E|2) maps along different planes shown in the inset of Figure 5(a) at wavelengths 2336, 653, and 567 nm. We present the field intensity distribution and vector maps for other peaks (867, 715 nm) in the Supporting Information. As we can clearly see from Figure 5(b−d), the broad peak at around 2336 nm is dipolar in nature, whereas the resonances at 653 and 563 nm sustain quadrupolar and octupolar modes of plasmon oscillations along the length of the rod. We recall here that for a free-standing particle the quadrupolar (Q) and octupolar (O) modes were observed at wavelengths 816 and 588 nm, respectively (Figure 3). Our general understanding about the presence of a substrate beneath a nanorod is that all the modes red shift as we increase the refractive index of the substrate. The change is most dramatic in the dipolar mode, and it is relatively small for other higher order modes.50 So the presence of a longitudinal quadrupolar and octupolar mode at

Figure 3. FDTD simulated CL spectrum in the wavelength range 500−3000 nm, for a free-standing (no substrate) nanorod of the same dimension as used in Figure 2(b). The schematic of the simulated structure is shown in the inset. Four distinct resonant modes at wavelengths 1745, 816, 588, and 533 nm have been identified. From the near-field intensity (|E|2) maps (shown in right inset) taken at planes marked by red (XY) and green (XZ) rectangles of the left inset schematic, one can easily identify the resonances are due to dipolar (D), quadrupolar (Q), octupolar (O), and transverse (T) modes of plasmon oscillations, respectively.

before, but without the substrate, has been shown. Four resonant modes at wavelengths 1745, 816, 588 and 533 nm have been observed. From the inspection of the spatial distribution of electric field intensity maps taken from the XY plane (red rectangle), one can easily identify the resonant modes at 1745, 816, and 588 nm to be dipolar (D), quadrupolar (Q), and octupolar (O) modes of plasmon oscillation. The resonant mode at 533 nm appears to be a transverse dipolar mode (T) extending from the end of the rod (B) to the corners, such as A or C. Simulated CL Spectroscopy in the Presence of Substrate. The situation is quite complicated and interesting in the presence of a substrate beneath the nanoparticle. The total surface area in contact with the substrate is approximately (500 nm × 92 nm), which means that a substantial effect of the substrate will be observed in the CL spectrum. In Figure 4(a), the CL spectra of the pentagonal nanorod sitting on the substrate with various indexes have been shown. The major dipolar mode red shifts from 1745 to 2789 nm as we vary the refractive index of the substrate from 1 to 2.5. This dramatic red 26287



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transverse dipolar mode along the base corners to the tip of the nanorod (Figure 5(e)), whereas, at wavelength 567 nm, one can see a transverse dipolar mode along the base corners to the adjacent side corners (Figure 5(f)). The spectra of hybridized plasmon modes are characterized by more than one peak, which is observed for all the index of the substrate greater than 1. Although both the in plane (longitudinal) and out-of-plane modes are already present and excited by the electron beam in a free-standing nanorod, the substrate mediating the hybridization breaks the symmetry and allows these modes to be optically active/visible. While the 550 nm peak shows a longitudinal octupolar mode (not resolved in CL experiments), the 664 nm indicates a longitudinal quadrupole mode. In both of these wavelengths, there is also a transverse mode of plasmon oscillations extending from the base corners to the adjacent side corners or the middle apex. At the moment, no particular plasmon modes of oscillation are attributed at wavelengths of 715, 867, and 1190 nm. We present the nearfield intensity map and the vector maps taken from the XY plane in the Supporting Information. In a few recent pieces of literature it has been shown that substrate induced symmetry breaking may lead to Fano resonance.20,25 The fundamental condition to be fulfilled for the occurrence of Fano resonance is a weak coupling and interference between a dark and bright plasmon mode, where the coupling is controlled by symmetry breaking. The symmetry breaking can be induced easily by depositing the particle in a substrate, which is done here. We have also found a dip structure in the observed spectrum (Figure 2(a,b)). However, we have seen here hybridization between a longitudinal octupolar and a transverse dipolar mode, both of which are bright modes, where the condition of Fano resonance is not fulfilled. A detailed study to explore these aspects is underway. Image Charge Model. The rich spectral features in the presence of the substrate can be understood with the help of an image charge model.22,55 For example, the simplest form of the nanoparticle that we can think about is a sphere. When a sphere is in air or embedded in a homogeneous medium, it has three degenerate dipolar LSP modes along three directions (Figure 6(a)). Hence the dipolar plasmon modes of a sphere are 3-fold degenerate. The introduction of a dielectric substrate beneath the sphere reduces the symmetry and lifts the degeneracy, giving rise to a splitting of the dipolar LSPR. In such a case, the interaction between the nanoparticle and the dielectric substrate can be viewed as the metal nanoparticle interacting with its own image inside the substrate. The image charge magnitude is reduced by a factor (ε − 1)/(ε + 1), where ε is the dielectric permittivity of the substrate.22 Two situations may arise in the presence of a substrate. For horizontal dipolar modes of the sphere along the X and Y directions (Figure 6(b)), the image dipole (p′) is directed opposite to the particle dipole (p), causing a reduced coupling with the external electromagnetic wave. But, as we see in Figure 6(c), for the transverse dipolar plasmon mode of the sphere (along the Z direction), the particle dipole is in the same direction with the image dipole, causing a strong coupling with the external electromagnetic excitation. We can extend this simple picture to our pentagonal nanorod. We have observed a decrease in intensity of the major dipolar mode with increasing index of the substrate, as depicted in Figure 4(a). As we see in Figure 6(d), the image charge for the longitudinal dipolar mode produces another dipole antiparallel to the original dipole, causing a

Figure 5. Detailed analysis of splitting of plasmon modes in the presence of substrate (refractive index 2). (a) Simulated CL spectrum in the wavelength range 500−3000 nm. (b−d) Near-field intensity map in the XY plane (marked as a red rectangle in the inset of (a)) and vector plot of the E field overlaid at different resonant wavelengths. (f and g) Near-field intensity (|E|2) map in the XZ plane (marked as a green rectangle in the inset of (a)) and vector plot of the E field overlaid at different resonant wavelengths.

wavelengths 653 and 588 nm is completely the opposite of what is expected for the nanorod sitting on a substrate. A possible explanation for this effect is given below: In the presence of the substrate, hybridization effects come into play, and this hybridization is mediated by the image charges induced in the presence of the substrate.22,25,54 The free space longitudinal octupolar mode (O) and the transverse dipole mode (T) hybridize in the presence of the substrate, and what we get as a result are quadrupolar and octupolar modes along the longitudinal direction mixed with dipolar modes along the transverse directions. The vector map of the electric field along the XZ direction at wavelength 653 nm shows a 26288



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beam excitation. The experimental and simulated CL maps in the visible range, at wavelengths 550 and 644 nm (Figure 2), look like dipolar and quadrupolar modes of plasmon oscillation of the rod at a first glance. However, by simulation, we demonstrated that these modes are indeed mixtures of the transverse dipolar mode and the horizontal quadrupolar mode. With increasing refractive index of the substrate, the intensity of the LSPR modes in the visible region increases for a nanorod. As we have seen, this high energy region of the spectrum is dominated by mostly higher order longitudinal LSPR modes and the transverse dipole mode which are coupled with each other. By a simple image charge model, we confirmed that this intensity increase is due to the presence of a transverse mode. Moreover, these mixed modes have a sharper FWHM compared to the dipolar one, which might have applications in sensing and in plasmonic solar cells. So a high index substrate can be deliberately used to enhance the LSPR response in the visible range of the spectrum.

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Figure 6. (a) A free-standing sphere with three degenerate modes. (b and c) Schematic of the image charge distributions induced by the substrate for a sphere. For the horizontal mode in (b), the dipoles p and p′ oppose each other. For the transverse mode in (c), p and p′ add up. (d) Longitudinal dipolar mode for a penta-twinned nanorod in the presence of the substrate. (e-f) Two transverse dipole modes and their image configuration in the presence of the substrate.

CONCLUSIONS Localized cathodoluminescence spectroscopy is used to demonstrate the substrate effect on LSPRs of a penta-twinned gold nanorod in the wavelength range 500−700 nm. The LSPR modes hybridize in the presence of the substrate. The far-field information on hybridized LSPR modes is experimentally given at the nanoscale. Substrate-induced LSPR splitting was discussed under two types of image charge configurations. The horizontal modes have lower frequencies and reduced CL intensities compared to those in a free-standing environment because of image charge cancellations. On the contrary, the transverse modes have higher energies and stronger CL intensities because the image charges add up. Hence the presence of a dielectric substrate does not necessarily damp all LSPR modes. Local plasmonic effects arising due to the presence of substrate have important consequences for a range of applications, such as, surface-enhanced spectroscopy, sensing, optical device designing, nonlinear photonics, and plasmon enhanced solar cells. The methodology presented in this paper will open a new way toward the analysis of plasmon mode coupling in the presence of a substrate.

reduced coupling with the electron beam. As the substrate dielectric constant increases, the ratio of the magnitude of the image charge to the original charge approaches 1, reducing the coupling between the electron beam and the plasmon mode, which explains why we see reduction of the CL intensity of the dipolar plasmon mode (Figure 4(a)). We have already demonstrated with the help of FDTD simulations that the high energy transverse dipole modes of plasmon oscillations in the presence of a substrate are mixed with the longitudinal higher order (high energy) modes. With increasing substrate index, the highest energy peak in Figure 4(b) increases more than the next higher energy peaks. For refractive index of the substrate n = 2, the highest and the next highest energy peaks occur at 567 nm (2.19 eV) and 653 nm (1.90 eV), respectively. The vectorial distribution of the electric field in the XZ plane as shown in Figure 5(e and f) demonstrates that the origins of the transverse modes are different for wavelengths 567 and 653 nm. This is schematically depicted in Figure 6(e and f). For the longitudinal component, the image dipole is oriented opposite to the particle dipole, causing a reduced coupling with the external electromagnetic excitation. But for the transverse component, the particle dipole and the image dipole add up. Consequently, the higher the refractive index of the substrate, the stronger is the coupling with the external electromagnetic radiation. The strongest coupling will occur when the particle dipole as well as the image dipole will be exactly perpendicular to the interface. Figure 5(e and f) and Figure 6(e and f) demonstrate that the two transverse dipoles in our case are not exactly perpendicular to the interface. The 567 nm mode makes an angle of 72° whereas the 653 nm mode makes an angle of 54° w.r.t. the interface plane. For each mode the horizontal (longitudinal) component cancels and the vertical component adds up, causing a stronger mode for the 72° situation (567 nm) compared to that of the 54° case (653 nm). This is observed in the experimental CL spectrum of Figure 2(a), where we find a higher intensity for the 550 nm CL peak. We have addressed several new aspects of the plasmonic response of a penta-twinned gold nanorod to local electron

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ASSOCIATED CONTENT

S Supporting Information *

Field intensity distribution and vector maps. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: (P.D.) [email protected]. Fax: +91 33 2337 4637. *E-mail: (T.K.C.) [email protected]. Fax: +91 33 2337 4637. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Mathieu Kociak of Université Paris Sud, France, for fruitful discussions. REFERENCES

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