Influence of Gold Metallodielectric Partial-Shell Geometrical

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Influence of Gold Metallodielectric Partial-Shell Geometrical Irregularities on Dark Plasmon Resonances Janina Wirth, Hans D. Hallen, and Shuang Fang Lim J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b05060 • Publication Date (Web): 05 Jul 2017 Downloaded from http://pubs.acs.org on July 9, 2017

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The Journal of Physical Chemistry

Influence of Gold Metallodielectric Partial-Shell Geometrical Irregularities on Dark Plasmon Resonances Janina Wirth, Hans Hallen and Shuang Fang Lim⇤ Department of Physics, North Carolina State University, Raleigh, NC 27695, USA (Dated: July 5, 2017) ABSTRACT: The geometric asymmetry of real, fabricated gold partial-shells leads to orientation and gold partial coverage dependent local fields and scattering. We illustrate this with single particle measurements and finite element calculations. In particular, we show that the position and number of well-defined protrusions on the edge of a partial-shell qualitatively changes the spectra. The metallic protrusions result in geometrical asymmetry, which leads to excitation of the optically dark quadrupole mode as a function of incident light excitation and polarization. The far field scattering peaks result from the bright dipole resonance with contribution from the dark resonance in the presence of the partial-shell surface protrusions. With more dark modes, the overall scattered intensity decreases, reflecting the energy trapped in the local electric fields of the dark modes, until it ultimately dissipates in the metal. Introduction

Plasmonic nanoparticles absorb and scatter light at resonant frequencies, the intensity of which is dependent on the size, shape, symmetry and orientation of the nanoparticles 1–4 . The excitation of a localized surface plasmon resonance (LSPR) on these nanoparticles results in enhancement of the local electric field. Due to energy conservation, scattering must be reduced as energy flows into the LSPR fields, where eventually, it will be locally absorbed by metal or nearby materials. Hence, such nanoplasmonic structures enable light to be concentrated into nanoscale volumes and actively manipulated in new ways. Current intense interest and research may bring about applications in optical and temperature sensing 5–7 , photothermal treatment 8–12 , and in metamaterials 13 . Metallic partial-shells present a three-dimensional reduced-symmetry structure that interacts with the electric and magnetic fields of the incoming excitation. These structures di↵er from closed nanoshells, whose plasmonic resonances are less complex, and are not dependent on their orientation 14–16 . Hence, partial-shell structures, which include nanocaps, nanobowls, half-shells and nanocups 3,17–27 , demonstrate a 3D orientation dependence, which produces new scattering features such as magnetic dipole resonances that can be exploited for metamaterial design. It has been shown that symmetry breaking of a plasmonic system via metallic nanofeatures such as in gold nanorings 28 , single nanorods 29 , nanorod trimmers 30 , nanodisks 31 , interelectrode nanoscale junctions 32 , nanocrosses 33 , pillar array [34 , nanocube array 35 and nanoparticle chains 36 , lead to coupling of dipole modes with dark plasmon modes. Dark plasmon modes can be quadrupolar, multipolar, or propagating 37–39 . These modes are considered as dark because they only weakly couple with incident light and hence propagating fields do not radiate 30,36,40 . Therefore, these dark modes

⇤

[email protected]

display an increased lifetime, which enhances near field interactions 36 that lead to potential applications including waveguides in nanophotonic devices 41 , nanolenses 42 and far-field color imaging 43 . A loss of symmetry of the nanostructures, whether through intentional design, change in orientation, or simply unintentional defects, modify the oscillator strengths of both the bright and dark modes, which enables coupling between both modes. In this work, we will show, with both measurements and calculations, how geometric irregularities introduced by standard nanofabrication processes lead to symmetry breaking, and consequently to the appearance of dark modes and qualitative changes to the spectra. Typical fabrication of semi-shells includes deposition of a hemi-spherical shell around a dielectric core, such as silica, immobilized on a substrate via thermal evaporation 22,26 or electroless plating 3,24 . Our approach uses single particle scattering measurement, which can reveal the exact influence of the parameters such as particle geometry, orientation, and surface irregularities on the correlated measured spectra. This method is favored over conventional bulk measurements where spectra are broadened due to size distribution, non-uniform geometries or interacting/aggregated particles. We explore the influence of the semi-shell orientation, gold fractional coverage, and the position and number of gold protrusions on the scattered light. Our individual particle imaging and optical measurements allow us to present clearly the e↵ect of each parameter on the scattering spectra and compare to models. Other researchers have shown the influence of fixed orientation partial shells immobilized on a substrate 3,17–27 . In this work, we report, for the first time, experimental evidence of the influence on the spectra of defects on typical realistic, fabricated particles, of the partial shell gold fractional coverage and number and position of gold protrusions on the edge of the partial shell. Theoretical predictions of the gold fractional height and gold surface roughness have been carried out by Cortie et al 17 . However, their roughness surface models did not introduce enough asymmetry to observe the dramatic spectral changes seen in our results. Our roughening mimics that

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FIG. 1: Schematic of gold partial shell on silica nanoparticle fabrication, showing a) immobilization of silica nanoparticles on a silicon substrate, b) thermal evaporation of gold partial shell, and c) transferred gold partial shell coated nanoparticles onto a gold coated silicon substrate. The panels above show SEM images for the corresponding preparation step, and the corresponding particle size histogram obtained by AFM measurements and SEM images of bare particles.

observed in real particles and helps us to understand how the number and exact position of each gold protrusion, with or without the presence of a substrate surface, gives rise to new spectral features. We demonstrate that the gold protrusions result in geometrical asymmetry, which leads to excitation of the optically dark mode as a function of incident light excitation and polarization. The spectral position, number and intensity of the dark plasmon modes, are shown to be strongly influenced by the number and position of the protrusions, as shown both in measured and calculated far field scattering. We perform finite element modeling in order to explain the e↵ects of each gold nanofeature on the scattering. Methods

Gold semi-shells were prepared by thermal evaporation of gold onto silica nanoparticles (Fig.1). As a first step, silica nanoparticles were synthesized after the method from Stoeber et al 44 with a diameter of about 89±6.4nm (Fig.1a), and immobilized on poly-4-vinylpyridine (PVP, molecular weight, 160k) functionalized gold coated silicon substrates. Afterwards, these silica nanoparticles were used as a template to evaporate gold partial shells. Following that, the nanoparticles are transferred onto a gold coated silicon substrate via a polydimethylsiloxane (PDMS) transfer stamp 45 . This process left randomly oriented gold partial shells on a gold surface layer. For optical correlation, a wide field fluorescence image of the semi-shell nanoparticles was taken, with their positions recorded on indexed grids, using an Andor NEO sCMOS camera. Afterwards, the recorded positions were characterized with a MFP-3D-BIO Atomic Force Microscope (AFM, Asylum Research) in tapping mode to find single

nanoparticles. Dark Field optical microscopy was carried out using an Axio Imager Z1.m (Carl Zeiss MicroImaging GmbH, Jena, Germany). For comparison to the calculations, the fractional coverage and angle of the particles is needed. This is retrieved from two SEM images at two tilt angles for each particle, as described in the supplementary data. We use a commercially available software package, Comsol, based on the Finite Element method, to quantify plasmonic response and the enhancement of our nanostructures during excitation. The gold semi-shells were modeled on silica nanoparticles, on gold substrates, for four di↵erent relative configurations between the incident light direction, excitation polarization and half-shell orientation. The modeled silica nanoparticle diameter and gold shell thickness was 90 nm and 15 nm,respectively. The fractional height of the gold semishell was varied from 0.2-0.5. Surface geometrical irregularities that resemble some of those on measured particles, were modeled by placing small gold hemispheres near the rim of the semi-shell edges. We modeled a perfect continuous, but thinner, gold shell of about 15 nm, in place of a real granular gold shell. The gold shell in the present modeling is assumed as a perfect continuous shell with bulk dielectric function adopted from previous experiment results 46 . We used Bruggemans e↵ective medium theory 47 to calculate the e↵ective index of the real film, nr,ef f +ni,ef f , that would accurately represent the imperfect experimental gold shell thickness. We calculate the expected measured thickness texp by requiring the same propagation loss for light passing through the perfect model film tAu as through the imperfect film by setting the Beer’s Law exponents to be the same, ni,Au .tmodel = ni,ef f .texp , with gold dielectric constant at 960 nm 46 , (nr,Au + ni,Au )2 = -46.797 + 3.0 i. We use the gold dielectric constant, the silica index of refraction of 1.52, and the gold volume fraction estimated from the electron micrographs to be 0.66. The calculated result for tmodel corresponds to the expected measured thickness texp = 21.0 nm, which is in reasonable agreement with the measured 21 nm given the uncertainty in the volume fraction measurement. In our experiment, the inclusion of the gold-coated substrate resulted in reflection from the substrate surface that adds a broad background to the measured far field scattered spectra. We subtract this gold-film background scattering from the measured spectra in order to clearly show the plasmonic modes. Correspondingly, the gold substrate scattering was also subtracted from our calculated spectra by subtracting the far field spectra of the gold layer with no particles present. Both far and near field spectra were calculated with a spectral range from 400 to 1000 nm.

Results and Discussions

Fig. 2 shows the measured far field spectra and the corresponding SEM images of a partial shell a) without any protrusions, b) with a top protrusion, c) with a side


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The Journal of Physical Chemistry 3 120 Electric Field Strength (V/m)

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Control

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FIG. 3: Calculated near field spectra comparing a) no protrusion, b) top protrusion, c) side protrusion, and d) crown of protrusions configurations. FIG. 2: Measured far field spectra and corresponding SEM images, comparing a) control: no protrusion, b) top protrusion, c) side protrusion, and d) crown of protrusions configurations. The silica cores shown in the SEM images are of 90 nm in diameter.

protrusion, and d) with a crown of protrusions. The partial shell without any protrusions, displays a peak at 680 nm attributed primarily to the dipole mode of the partial shell. The single protrusion configurations b) and c) both show an additional prominent peak from 560 to 590 nm, where this peak dominates the multi-peaked scattered spectrum of the partial shell with a crown of protrusions in (d). Hence, with the exception of a) the partial shell without a protrusion, all other configurations show at least an additional prominent peak between 560 to 590 nm. This led us to speculate that, 1) this additional peak may arise from the presence of the protrusions, and 2) that there is dependence on the intensities of the 560 to 590 nm, with respect to the 680 nm peaks, on the number of protrusions. Moreover, we observe that the total intensities of the measured peaks are highly dependent on the partial shell orientation 22 , as shown in Fig.S8. The peak positions are only weakly dependent upon orientation, and are the primary emphasis of this work. In order to understand the origin of the 560 to 590 nm peak, and its relation to the protrusion position and number, finite element modeling was performed to obtain both near field and far field scattering spectra (See Fig. S1). Fig. 3 shows the calculated near field spectra of a partial shell of fractional gold coverage of 0.2, and with its axis of symmetry oriented 6 degrees from the horizontal. In the near field spectra in Fig. 3, the partial shell only configuration, without a protrusion, shows a single peak only at 700 nm, where the low peak intensity is attributed to scattering of most of the energy to the far field. When a single protrusion is present, as seen for both configurations with top and side protrusions, peaks are obtained at 720 nm, and at 580 nm. The partial shell with side pro-

trusion shows a slightly lower total intensity compared to the partial shell with a top protrusion. This is because we have modeled an incident excitation angle of about 60 degrees from the vertical to match the oblique illumination of the dark field objective. Hence, any incoming excitation more efficiently scatters from a protrusion located closer to the direction of incidence, which is the top of the partial shell, as compared to on the right side of the shell. In comparison with the partial shell only configuration, the e↵ect of the added single protrusion is 2-fold. First, the protrusions increase the gold surface coverage, which shifts the initial peak at 700 nm to 720 nm, and second, it introduces an additional peak at 580 nm. Furthermore, as shown in Fig. 3, multiple peaks can be seen as more protrusions are added to the partial shell. As shown in Fig. 3 (and Figs. S3 to S7), as expected, the crown configurations increased gold surface coverage shifts the 700 nm peak further to 730 nm, and introduces two additional strong peaks at 580 nm and 670 nm, and other weaker peaks and shoulders. The additional peak at 670 nm can be attributed to the positions of the additional protrusion positions in the crown configuration (See Fig. S1(d) and S3 to S7). Qualitatively, each protrusion creates an additional possible magnetic dipole (in this wavelength range), and these dipoles mix to form the same number of normal modes. Similar effects have been observed in nanoclusters 50,51 , nanoshells 52 and in spiky nanoshells 53 . The corresponding far field calculations for scattered light collected within a 60 degree cone, were carried out for the same configurations, and are shown in Fig. 4. The 560 to 590 nm peak is notably absent. The absence of the 560 to 590 nm plasmon mode implies that it is non-radiative, which is indicative of a dark plasmon mode. The reason we observe it in our scattering measurements is likely due to the nearfield energy being scattered by grains and tiny inhomogeneities on the real particles and the thermally evaporated gold film, which exhibits small scattering inhomogeneities due to its granular or columnar nanostructures



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FIG. 4: Calculated far field spectra comparing a) no protrusion, b) top protrusion, c) side protrusion, and d) crown of protrusions configurations.

48,49

(b)!

Control

. Additionally the main dipole mode at 720 nm is the strongest intensity for the clean partial shell only. The intensity of this 720 nm peak is slightly reduced in the presence of protrusions, when located either on the side or on top. Since dark modes are dissipative, the overall scattered intensity is expected to decrease when these dark modes are excited. Further evidence for attribution of the dark plasmon mode from 560 to 590 nm, can be seen in the crown configuration. As shown in Fig.4, the overall scattered intensity is dramatically decreased, as the number of protrusions increase. The asymmetric appearance of the peaks, seen in the crown configuration, can be explained by the interference between bright and dark modes, giving rise to Fano resonances. Therefore, from both near and far field calculations, the presence of the gold protrusions disrupts the nanostructure symmetry, which then enables coupling between the bright and dark modes. Comparisons between experimental and calculated spectra show that the experimental spectra are a combination of both the near and far field calculated spectra. In order to understand the nature and origin of this 560 to 590 nm peak, 3D electric field and current density plots are calculated for all the di↵erent configurations. As seen in Fig. 5, the localization of the electric field enhancement (Fig. 5a and 5b) and current densities (Fig. 5c and 5d) at 590 nm are largely at the protrusions, while that at 720 nm are mostly localized on the partial shell itself (See Fig. S2). This is evident in both the single and multiple protrusions configurations (Fig. S3 to S6). In addition, the position of the protrusion strongly influences the electric field and current density intensities, where the top protrusion results in a higher current density than the side protrusion. This observation is also correlated with the near field spectra shown in Fig. 3, where peak intensities at 590 nm and 720 nm are lower for the side position, in comparison to the top position. We also conclude from these 3D plots that the 590 nm mode is quadrupolar in nature. The top and crown scattered spectra in Fig. 4 clearly show a splitting of the 720 nm peak. The top protrusions peak splits into

! Fig.%5!Electric!field!magnitude!E2!plots!of!(a)!top!protrusion,!and!(b)!side!protrusion,! and!current!density!plots!of!(c)!top!protrusion,!and!(d)!side!protrusion,!on!the!edge! FIG. 5: Electric field intensity E 2 plots, at 590 nm, of!partial!shells!at!590!nm.!

with 1 V/m far field excitation at 60 degrees towards right from ’z’ (vertical) and polarized along the x direction (out of the page), to simulate a dark field illumination from above, of partial shells, with (a) top protrusion, with color range 50 V 2 /m2 , and (b) side protrusion, with color range 80 V 2 /m2 , and current density plots with color range 106 A/m2 of (c) top protrusion model, and (d) side protrusion model. !the

two, which can be understood heuristically by the consideration that this magnetic dipole peak corresponds to current oscillation around the bowl of the partial shell, and the protrusion versus clear areas of the rim represent di↵erent physical distances around the bowl, hence di↵erent resonant wavelengths. The longer distance including the protrusion corresponds to a longer wavelength: the new peak. The quantitative understanding must also include the slight shift to shorter wavelengths of the 720 nm peak. This is due to a mixing of the wave resonances that causes the energy levels to repel, as is often observed with electron waves in clusters using perturbation theory 53,54 . Essentially, the normal modes of the system include a superposition with energy shifts. The superposition is also evident in where the current flows. The peaks in the crown configuration, as shown in the near field spectra in Fig. 3, at 590 nm, 670 nm, 740 nm (shoulder at 790 nm), and 880 nm (shoulder at 900 nm), are attributed to the interaction of both the partial shell and protrusions (See also Figs. S3 to S6), and include many more possible modes, with mixing driving the 720 nm peak and a derivative to even shorter wavelengths. The ring of protrusions strongly disrupts the partial shell symmetry, leading to multiple peaks as shown in Fig. 3 (See Fig. S7), and the resultant drop in scattering intensity in the far field. We also observe in Fig. S7 that the position of the pair protrusions contributes to splitting of the primary dipole mode into spectral pairs. A further analysis of the e↵ect of the gold fractional coverage was carried out, in Fig.6, showing the calculated scattering spectra of partial-shells, with fractional gold coverage, from 0.2 0.5, with no protrusions. In our calculations, an additional quadrupole peak at 590 nm is observed as gold fractional coverage is increased to 0.4 and 0.5, even with no protrusion. The metallic structure has become large


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The Journal of Physical Chemistry 5

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the case and hints that the capacitive viewpoint rather than a wave viewpoint is more predictive for a qualitative understanding of the local fields. Conclusions

FIG. 6: Dependence of calculated far field scattering on the partial shell fraction.

We show that realistic processing-induced particle surface inhomogeneities give rise to nanoparticle asymmetry and the subsequent generation of dark plasmons with qualitative changes to the spectra. This e↵ect is not typically observed in ensemble measurements. Through careful measurements at the single particle level, we correlate the metallic protrusion number and position to the appearance of the dark plasmon modes. Associated Content

enough to support a quadrupole for wavelengths allowed by the gold dielectric constant. The position of the 720 nm peak is also seen to shift to longer wavelengths with increasing gold fractional coverage. This is expected as the size of the metallic fraction or the particle size is increased. Note that the 590 nm feature also shifts to the right of the gold dielectric constant cuto↵ with increasing gold coverage. When we compare the influence of gold fractional coverage with that of the protrusions, we see similar trends in the emergence of the 590 nm peak and also the shifts in the peak positions. The e↵ect of the protrusions is to 1) increase the gold fractional coverage, and 2) disrupt the geometry of the nanostructure, where both e↵ects lead to increased coupling between bright and dark modes. One might expect that the crown of protrusions should di↵er from the clean, protrusion free partial shell only by having a larger fractional coverage, since both the size of and spacing between the protrusions is small compared to the wavelength over the entire partial shell perimeter. Our results show that this is not

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Supporting Information. Additional current density plots for all sample configurations (S1-S9). This material is available free of charge via the Internet at http://pubs.acs.org. Author Information

Corresponding [email protected].

Author

*(S.F.

Lim)

Email:

Acknowledgements

This work was financially supported by the National Science Foundation CBET 1067508 grant and the NC State Undergraduate Research Grant. References

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! Fig.%5!Electric!field!magnitude!E2!plots!of!(a)!top!protrusion,!and!(b)!side!protrusion,! and!current!density!plots!of!(c)!top!protrusion,!and!(d)!side!protrusion,!on!the!edge! of!partial!shells!at!590!nm.! !



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Influence of Gold Metallodielectric Partial-Shell Geometrical

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