Probing Plasmonic Gap Resonances between Gold Nanorods and a

Jul 28, 2015 - ... of Physics, University of Otago, P.O. Box 56, Dunedin 9016, New Zealand ... Comparing the experimental data with numerical simulati...
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Probing Plasmonic Gap Resonances between Gold Nanorods and a Metallic Surface Xingxing Chen,† Yuanqing Yang,† Yu-Hui Chen,‡ Min Qiu,*,†,§ Richard J. Blaikie,*,‡ and Boyang Ding*,‡ †

State Key Laboratory of Modern Optical Instrumentation, Department of Optical Engineering, Zhejiang University, Hangzhou 310027, Zhejiang, China ‡ MacDiarmid Institute for Advanced Materials and Nanotechnology, Department of Physics, University of Otago, P.O. Box 56, Dunedin 9016, New Zealand § School of Information and Communication Technology, Royal Institute of Technology, Electrum 229, Kista 16440, Sweden ABSTRACT: The plasmonic resonances in individual gold nanorods nanoscopically coupled to a gold film with different gap spacing have been experimentally and theoretically investigated. The spectral widths, wavelengths, and optical polarizabilities of the maxima in measured single-nanoparticle scattering spectra are significantly modified as the gap distance changes in the sub-20 nm domain. Comparing the experimental data with numerical simulations reveals that these modifications arise from the complex hybridization of several dipolar and multipolar plasmon modes that are strongly localized at the gap. These plasmon gap modes have distinct resonant and spatial characteristics as a result of near-field interaction between the elongated nanorods and the gold film. Additionally, the excitation of these gap modes is highly dependent on the gap spacing. Finally, we also discuss influences of these plasmonic modes on absorption properties of the system and propose a potential application of the studied structures in facilitating photothermal conversion.

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modes13 and giving rise to large plasmon shifts and appearance of new plasmon resonances.14−17 Moreover, in this NP-film system, large field enhancement can be achieved since the incident light is extremely confined within the gap between the NP and metallic film.18−23 The nearby presence of a thin film with a high dielectric constant has also been shown to greatly alter the optical behaviors of AuNRs. For instance, AuNRs placed above metallic substrates12,24 acquire doughnut-shaped patterns in their far-field scattering image as a result of vertically excited dipole moments with respect to the substrate. Chen et al. observed Fano-like resonant features in scattering spectra of large-size AuNRs on a silicon substrate due to the interference between multipolar plasmons.25 Aubry et al. theoretically demonstrated that the field confined within the narrow gap between a metallic nanowire and a metal substrate can be drastically enhanced (>102 times).26 Recently, enhanced and polarized gap resonances have been used to trap and manipulate a metallic nanowire on a metal surface, 27 demonstrating the utility of these tunable resonances. The plasmonic gap modes between the simplest form of metallic NPs (sphere-like) and a metal film have been thoroughly investigated, demonstrating that such near-field

ollective oscillation of conduction electrons in noble metal nanoparticles (NPs), known as localized surface plasmons (LSPs), can be excited upon light irradiation, which gives rise to an increase of both optical absorption and scattering at the wavelength of plasmon resonances.1 The excitation of LSP resonances is significantly dependent on the shape of NPs. Specifically, NPs having anisotropic geometries acquire shape-induced absorption and scattering properties. For example, light scattered from gold nanorods (AuNRs) exhibits strong polarization anisotropy at spectral positions corresponding to the excitation of longitudinal plasmon modes (electron oscillation parallel with the rod length direction) and transverse plasmon modes (electron oscillation perpendicular to the rod length direction).1−3 Such anisotropic polarizability enables AuNRs to be widely used in many fundamental studies and practical applications, such as optical recording-readout,4 directional fluorescence enhancement,5 super-resolution imaging,6 photothermal conversion,7,8 and orientation sensing.8−12 On the other hand, the excitation of LSPs is also highly sensitive to the surrounding dielectric environment for NPs of fixed size and shape. For example, metal NPs placed at subnanometer separations from a metal film show distinct plasmonic behaviors compared with equivalent free-standing NPs due to the symmetry breaking of the dielectric environment induced by the metal film. Specifically, free electrons in metal NPs interact with their image charges in the metal film, leading to the hybridization of various plasmon © XXXX American Chemical Society

Received: June 23, 2015 Revised: July 25, 2015

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DOI: 10.1021/acs.jpcc.5b06006 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C interactions exhibit very complex resonant characteristics. For example, using photoemission electron microscopy, Schertz et al. uncovered dark modes in the gap, which do not couple to the scattered fields;28 while in another study, by altering the excitation polarizations, Lei et al. revealed three types of gap resonances that possess completely different azimuthal features.29 In addition, the strength of the gap resonances is highly dependent on the spacing distance between NPs and the film. Given that (i) the complexity of gap modes between an isotropic NP (sphere-like) and a film and (ii) the morphology of narrow gaps can dramatically modify the plamonic responses,30 it is reasonable to believe that the fields confined between anisotropic metal NRs and a film should show more complex resonant structures. A very recent report provides a direct observation of the near-field optical response of AuNRs on a silicon substrate.31 Similar structures were also studied using cathodoluminescence spectroscopy,32 showing that the symmetry breaking of the dielectric environment induced by the silicon substrate contributes to the splitting of in-plane and out-of-plane resonances. However, there is very little knowledge about how the plasmonic excitation takes place within the gap between a AuNR and a metallic film, how these plamonic resonances develop as a function of gap spacing distance, and how the symmetry breaking of the dielectric environment induces the transitions of AuNRs’ optical polarizability. In this Article, we present for the first time a comprehensive study of the tightly confined plasmonic gap resonances between individual AuNRs and a metallic substrate, revealing the transition of gap modes with gap spacing and their influence on far-field scattering properties. Specifically, we have measured the scattering spectra of individual AuNRs placed at various distances above Au films using controllable dielectric spacers (schematic in Figure 1) and analyzed the polarization anisotropy of scattered light from these systems. We find several resonant maxima in their scattering spectra, and the width, spectral positions, and optical polarizability of the maxima all have significant variations as the spacing distance changes. Numerical simulations reveal that these variations arise from the coexistence of several types of gap modes that have distinct resonant and spatial characteristics, and the excitation of these modes highly relates to the gap spacing distance. From this, we also discuss the absorption properties of the studied structures and their potential application in photothermal conversion.

Figure 1. Schematic (left-top): (polarized) dark-field scattering measurement of a single gold nanorod (AuNR) deposited on a PVA film with a thickness d coated on a Au substrate. (a) Unpolarized darkfield scattering spectra of AuNRs on a glass (red curve) substrate, a Au substrate: d = 1.5 nm (green), and PVA coated Au substrates: d = 5 nm (blue) and d = 20 nm (black). The far-field scattering images for these AuNRs on each substrate are shown above. (b-g) Unpolarized scattering spectra of individual AuNRs on different substrates as a function of measurement count: (b) Au (d = 1.5 nm), (c) d = 5 nm, (d) d = 10 nm, (e) d = 20 nm, (f) d = 30 nm, (g) glass. Solid, dashed, and dotted lines indicate the average spectral positions of each resonance (see the main text for details).

and a length of ∼85 nm as measured using a scanning electron microscope (SEM). We note that the AuNRs are capped with a shell of cetyltrimethylammonium bromide (CTAB) with a thickness ∼1.5 nm in the course of the chemical synthesis process. This prevents full contact between AuNRs and substrates24 and is of particular relevance for AuNRs directly placed on a Au substrate. In this case, we use d = 1.5 nm to denote the small separation between AuNRs and the Au surface. In addition, AuNRs are also deposited on a bare glass substrate as a reference. Turning to the details of experiments, Figure 1a shows the scattering spectra measured from AuNRs on different substrates without using the polarizer (unpolarized spectra). The scattering spectrum of a AuNR on a glass substrate (red curve) exhibits a maximum at λ = 640 nm and its full width at half-maximum (fwhm) is Δλfwhm ≈ 80 nm, while light scattered from a AuNR on a Au substrate (green curve) acquires much broader spectral width (Δλfwhm ≈ 190 nm) though the wavelength of the scattering peak (λ = 628 nm) only slightly blue-shifts as compared to one on the glass substrate. If a AuNR

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RESULTS AND DISCUSSION The schematic in Figure 1 demonstrates the measurement of scattering spectra for a single AuNR on top of a poly(vinyl alcohol) (PVA) film coated on a Au substrate using a reflected dark-field microscope (see the Methods section for details in optical characterization and sample preparation). Upon unpolarized white-light illumination at an incident angle of θ = 53° with respect to the substrate normal from all azimuthal directions, the scattered light from single AuNRs is collected by an objective and then measured by a spectrometer. A rotatable linear polarizer is optionally inserted between the objective and spectrometer to analyze the polarization of scattered light. PVA films with various thicknesses (d = 5 ± 3 nm, d = 10 ± 4 nm, d = 20 ± 2 nm, and d = 30 ± 2 nm) are spin-coated as spacers to separate the AuNRs and the Au films. Note that average spacer thicknesses will be presented from now on, and the reader is referred to this section for uncertainty bounds. In our experiments, the synthesized AuNRs have a width of ∼35 nm B

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Figure 2. Un/polarized scattering spectra of the same AuNRs shown in Figure 1a on (a)/(b) a glass substrate (reference sample) or (c)/(d) 1.5 nm, (e)/(f) 5 nm, and (g)/(h) 20 nm above a Au substrate. The polarization angle φ refers to the angle between the optical axis of the polarizer and dashed lines in the scanning electron microscope (SEM) images-inset of (a), (c), (e), and (g). White scale bars in SEM images indicate 100 nm length. The blue and red dashed curves in (e) and (g) are Lorentz fits to highlight the short- and long-wavelength maxima in scattering spectra, respectively.

is slightly lifted up from the Au surface to d = 5 nm, drastic modification of scattering spectrum (blue curve) is observed, showing two resonant bands at λ = 590 nm and λ = 780 nm, respectively. Further increasing the spacing distance to d = 20 nm (black curve) leads to blue-shifts of both resonances, while the magnitude of the resonance at short wavelength (λ = 568 nm) is significantly suppressed. Substrate induced effects could also be observed from the modification of the far-field scattering patterns (images above Figure 1a) that contain not only the spectral but also the directional information on the scattered light. Specifically, different from the red dot pattern for the AuNR on glass, the scattering image of the AuNR on a Au substrate exhibits a dot-shape pattern with yellowish green color, which then turns to green and greenish yellow when d increases. It is worth noting that we do not see the doughnutshape scattering patterns observed from AuNRs on a silicon substrate,24 which represents the excitation of dipole modes vertically oriented with respect to the substrate. We note that there are fluctuations in both the geometry of studied AuNRs and the thickness of PVA spacers, which may lead to variations in the scattering spectra. In order to substantiate the reproducibility of the observed optical behaviors, we have measured over 120 single NP scattering spectra on substrates (10−30 spectra for each substrate) with different spacer thickness as shown in the intensity plots of Figure 1(b)-(g). Several findings can be summarized from these statistical measurements: (i) it is very clear to see the doublebroadened maxima width in scattering spectra of AuNRs on a Au substrate (Figure 1b) as compared to those on a glass substrate (Figure 1g), which contradicts one of the general views about AuNRs that they show relatively narrow spectral widths of LSP resonances due to damping reduction;33,34 (ii) Figure 1(c)-(f) shows that the long wavelength maxima (λ ≈ 780 nm) in scattering spectra of AuNRs at d = 5 nm above the Au substrate blue-shift as the spacing increases (indicated by solid lines in each panel); (iii) the short wavelength maxima red-shift when the gap spacing increases from d = 5 nm (dashed line in Figure 1c) to d = 10 nm (dotted line in Figure 1d), then blue-shift (dotted line in Figure 1e), and become barely observable as the spacing increases up to d = 30 nm (Figure 1f). In a NP-film system, the blue-shifting trend of LSP resonances with increasing gap spacing caused by damping of plasmon hybridization has been well studied,14−18,35 but the large

broadening and red-shift of scattering maxima have never been reported. Together with the absence of the doughnut-shape farfield scattering patterns, these unexpected scattering properties suggest that our NR-film systems acquire far more complex resonant behaviors than just simple dipole models as previously reported.12,24 The polarization anisotropy of the resonances in the scattering spectra for AuNRs on different substrates (the same samples as shown in Figure 1a) were analyzed with the help of a linear polarizer. As shown in Figure 2, the scattering spectra are measured as a function of polarization angle φ ranging from 0°−180°, which refers to the angle between the polarization axis of the polarizer and a datum line, being the dashed line in the scanning electron microscope (SEM) images, insets in Figure 2(a), 2(c), 2(e), and 2(g). For the AuNR on glass (Figure 2b), it is clear to see that the scattering intensity achieve its maximum at φ = 75°. At this angle, the polarization axis of the polarizer is parallel with the longitudinal axis of the AuNR (SEM image in the inset of Figure 2a). This result is not unexpected, since the longitudinal plasmon modes of AuNRs have much stronger oscillator strength than the transverse modes,2,3 allowing the highest scattering efficiency for light with electric field orientation parallel with the rod length. In contrast, for the AuNR on a Au substrate, the broadened maximum in the scattering spectra can be equally seen from all polarization angles (Figure 2d), suggesting possible excitations of out-of-plane resonances.12,24,25,32 However, the absence of a doughnut-shape scattering pattern in our measurement (images above Figure 1a) provides no direct evidence of dipole moments that are vertically excited with respect to the substrate. We also note that the polarized scattering spectra in Figure 2(d) exhibit a slight increasing trend toward a long wavelength region (λ ≥ 800 nm) at φ = 120°, suggesting an effective excitation of LSPs in the near-infrared (not detectable in our experiment) along the polarization that is paralleled with the longitudinal axis of the AuNR (see the SEM image in Figure 2c). Figure 2(f) shows the polarized scattering spectra for the AuNR d = 5 nm above a Au substrate. It is noted that the short wavelength maximum at λ = 590 nm shows no significant dependence on the observing polarization angles, whereas the long wavelength maximum at λ = 780 nm exhibits strong polarization anisotropy similar to the case of glass substrate. This result relates the short wavelength peak to the C

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Figure 3. Simulations for AuNRs at (a-c) d = 1.5 nm, (d-f) d = 5 nm, and (g-i) d = 20 nm above a Au film. The first (second) line of spectra demonstrates the calculated scattering cross-section of the NR-film system upon VT(VL) excitations, i.e. p-polarized white-light illumination along the y−z (x−z) plane with an incident angle θ = 53°, as shown in the schematic in each panel. The incident angle (θ = 53°) was set to mimic the real illumination conditions in experiment. Absorption cross-sections for AuNR d = 5 nm above the Au surface are shown in panels (d) and (e) as red curves. The third line of field plots exhibits the magnitude of total electric fields |E| (upper panels, viewed from the x−z plane mid cross-section of the AuNR) and its z-component Ez (lower panels, viewed from the x−y plane mid cross-section of the gap) for different modes labeled in the spectra. Bright (dark) areas in the upper panels correspond to maximum (minimum) magnitude, while red (blue) areas in the lower panels correspond to positive (negative) maximum.

broadened maxima in AuNRs on a Au film and manifests a strong connection between the long wavelength maximum and longitudinal plasmon modes. Further increasing the gap spacing up to d = 20 nm (Figure 2h), both of the short- and longwavelength maxima in the scattering spectra show strong polarization anisotropy. Specifically, the long wavelength maxima reach their maximum at φ = 110°, while the short wavelength maxima peak at φ = 20°, clearly manifesting the orthogonality between the polarizablities of these two resonances. Comparing the orientation of the AuNR (inset in Figure 2g) with these maxima and minima in the polarized spectra (Figure 2h) indicates that the long- and shortwavelength maxima directly relate to the longitudinal and transverse plasmon modes in the AuNRs, respectively.

Above all, the width, spectral positions, and polarizability of scattering maxima in the nanorod-film system show a high relevance to the gap spacing d. In order to better understand these interesting scattering behaviors, we have carried out numerical simulations to calculate the scattering cross-section of AuNRs on different substrates illuminated by a p-polarized (electric field oriented parallel with the plane of incidence as shown in schematics of Figure 3) plane-wave along the y−z plane (first line of spectra in Figure 3) or the x−z plane (second line of spectra in Figure 3) at an incident angle θ = 53°. The p-polarized wave contains both out-of-plane (perpendicular to the substrate) and in-plane (parallel with the substrate) field components, thus allowing for simultaneous excitations of both vertically and longitudinally (transversely) D

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hot-spots, suggesting that these resonances sustain multipolar modes of plasmon oscillations, similar to the excitation of octupolar and quadrupolar plasmon modes that have been observed in large-size AuNRs.25,32 The coexistence of these multipolar gap modes accounts for the absence of doughnutshape scattering patterns that signify the excitation of vertically oriented dipole moments. For resonance (3)/(4) in the case of d = 5 nm (Figure 3f), the electric fields are still confined within the gap, showing four/three hot-spots respectively but with suppressed amplitude at the ends of the rods. Figure 3(i) shows that for AuNRs placed d = 20 nm above a Au surface, the field distribution |E| of resonance (5) is mostly confined under the center of the nanorod, spreading through the gap volume with barely observable extension at the NR ends. Inspecting resonance (5) from the x−y plane, Ez shows extended lobes across the rod width, evidencing that the short-wavelength scattering maxima in Figures 1(e), 2(g), and 2(h) are associated with the transverse-like modes. In contrast, resonance (6) shows strong field localization at the two ends of the NR with fields diffusing into the gap volume, indicating strong dipole resonances oriented along the NR length. Together with the scattering spectra, the near-field distribution provides us a clear picture about the evolution of plasmonic gap modes with the changing gap spacing. Multipolar modes (1) and (3) sharing similar resonant characteristics are excited at small gap spacing (d ≤ 5 nm). They are responsible for the short-wavelength scattering maxima as indicated by dashed lines in Figures 1(b) and 1(c). Resonances (2), (4), and (5) represent another class of multipolar modes, gradually evolving as spacing d increases and finally forming dipole-like modes oriented parallel to the NR width. These multipolar resonances are hardly noticeable at small spacing d ≤ 5 nm due to hybridization with other modes but lead to an increase of scattering for structures with larger gap spacing (d > 5 nm), e.g. the scattering maxima indicated by dotted lines in Figures 1(d) and 1(e). Resonance (6) signifies the type of gap modes that have strong dipole moments horizontally oriented along the NR length, giving rise to the long-wavelength scattering maxima indicated by solid lines in Figures 1(c)-(f). We have discussed the plasmon modes localized at the gap between AuNRs and a Au film and their influences on scattering properties. The excitation of LSPs actually also contributes to the increase of light absorption, which leads to application in photothermal conversion. The absorption crosssection (red curves) of AuNRs d = 5 nm above a Au surface under VT and VL illuminations has been modeled and is depicted by the red lines in Figures 3(d) and 3(e). We note that under the same excitations the absorption spectra acquire similar resonant characteristics to the scattering spectra but with higher values. Our study36 on plasmonic absorbers shows that such a metal−insulator−metal (MIM) configuration allows a broadband (900−1600 nm) and nearly perfect (∼100%) light absorption for multilayer AuNRs. Most of the absorbed light is dissipated into resistive thermal energy, giving rise to a significant temperature increase in such plasmonic structures. As a result, the MIM-based absorbers provide a highly efficient platform for heating or annealing metallic NPs. In heating experiments using MIM-based absorbers,37,38 laser beams with frequencies close to the horizontal gap modes are usually chosen to illuminate samples, because MIM structures gain the highest absorption efficiency at these resonances under normal incidence. However, most of the heating experiments4,7,37−39

oriented resonances, i.e. VL (VT) excitation, under x-z (y-z) illumination. The geometric parameters of AuNRs in the simulations were set to be 88 nm in length and 34 nm in width to optimize the comparison with measured data. For the calculation of AuNRs directly deposited on a Au substrate, the AuNRs were set being suspended 1.5 nm above the Au surface to mimic the CTAB shell of the nanorod. In particular, for AuNRs d = 1.5 nm above a Au substrate, the scattering spectrum under VT excitation (Figure 3a) shows two resonances at λ = 590 nm and λ = 650 nm, while the scattering spectrum under the VL excitation (Figure 3b) exhibits similar spectral appearances and magnitudes with those under VT excitation, except the increasing tail extending to long-wavelength. The coexistence of the two resonances apparently explains the broadened maximum at λ = 628 nm in measured scattering spectra of AuNRs on a Au substrate shown in Figures 1(a), 1(b), and 2(c). The spectral and magnitude similarity between Figures 3(a) and 3(b) manifests in the observation that the two coexistent resonances are both excited by the normal field component of the p-polarized illumination, corresponding to the polarization-insensitive scattering maximum in our experiment (Figure 2d). In addition, the increasing tail in the simulated spectrum under VL excitation (Figure 3b) agrees with the experimental observation (Figure 2d) though with higher magnitude, confirming the existence of longitudinal LSP excitation in the near-infrared. In the case of d = 5 nm (Figure 3d), the VT excitation induces two resonances at λ = 520 nm and λ = 580 nm. The resonance at λ = 580 nm is barely seen as compared to the resonance at λ = 520 nm. While these two modes are still preserved in the scattering spectrum under VL excitation (Figure 3f), it is noted that an additional resonance emerges at λ = 750 nm, corresponding to the long-wavelength resonance in Figures 1(a), 1(c), 2(e), and 2(f). This calculation result has wellreproduced the measured scattering polarizability (Figure 2f), i.e. the short-wavelength resonance (λ = 590 nm) remains almost unchanged at all polarization angles whereas the longwavelength resonance (λ = 780 nm) shows strong polarization anisotropy associated with the longitudinal-like resonances. For AuNRs d = 20 nm above a Au film, only one resonance can be seen at λ = 535 nm under VT excitation (Figure 3g); while VL excitation also results in only one resonance at λ = 660 nm (Figure 3h). This result agrees very well with the measured data (Figure 2h) that the short- and long-wavelength maxima show strong and orthogonal polarization anisotropy. In order to further reveal the physical nature of the plasmonic modes associated with the scattering maxima in the measured and simulated spectra, the electric field distributions for the resonances in the NR-film system with different gap spacing are calculated and demonstrated in Figures 3(c), 3(f), and 3(i). These plasmonic modes, labeled in accordance with the notation in Figures 3(a), 3(d), 3(g), and 3(h), are inspected from the electric field magnitude |E| (upper panels) and its z-component Ez (lower panels). |E| is observed from the x−z plane mid cross-section of the AuNR, while Ez is viewed from the x−y plane mid cross-section of the gap between AuNRs and Au surface. In the case of d = 1.5 nm (Figure 3c), the |E| plots demonstrate that the electric fields are strongly localized within the gap between the AuNR and Au surface for both resonances (1) and (2), with the notation defining these resonances given in Figure 3c. More importantly, both |E| and Ez plots clearly indicate that resonance (1) exhibits four hot-spots in the gap, while resonance (2) presents three E

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Figure 4. Simulated temperature increase as a function of azimuthal angle φ for single AuNRs placed at (a) d = 5 nm and (b) d = 1.5 nm above a Au surface irradiated by p-polarized illumination with different wavelengths and azimuthal angles. The illumination power remains at 1 mW/μm2 for all wavelengths. As shown in the schematic, the AuNR is oriented horizontally along the x−z plane. φ = 0° indicates the y−z plane illumination, while φ = 90° refers to the x−z plane illumination. The incident angle is set to be θ = 53° for all illuminations. Temperature increase for AuNRs on a glass substrate is displayed as a reference in panel (b).

special applications, e.g. printing of individual anisotropic nanoparticles with fixed orientations.4,39

have been carried out using a focused laser beam through a microscope objective. As a result, the focused beam may contain significant oblique components, such as in the darkfield illumination used in our experiments. In this context, we propose that vertically excited gap resonances in MIM structures may provide another option for NP heating, especially for illuminations focused by high numerical aperture (NA) objectives, because (i) these modes can be efficiently excited under oblique incidence without biased selectivity to azimuthal direction and (ii) the vertically excited gap resonances acquire comparable magnitudes with horizontal gap modes in the case of small gap spacing, as can be seen from the scattering spectra for d = 5 nm in Figures 1(a), 1(c), 2(e), and 2(f). In order to illustrate this, we have simulated the temperature increase (ΔT) in AuNRs above a Au surface with gap spacing d = 1.5 and 5 nm under p-polarized oblique illuminations with different azimuthal angles (φ) (schematic in Figure 4). The wavelengths of illumination are chosen to match the vertically or horizontally excited resonances in each structure; while the incident angle is set to be θ = 53° mimicking the focused illumination with NA = 0.8. A typical illumination power (1 mW/μm2) for NPs absorption studies40 is used here for all incident wavelengths. Figure 4a demonstrates the temperature increase in AuNRs 5 nm above the Au surface. Specifically, under illumination at λ = 750 nm, i.e. the wavelength of horizontally excited gap modes, ΔT (red-cross in Figure 4a) experiences a sharp increase (0−156 K) with illumination direction varying from perpendicular (φ = 0°) to parallel (φ = 90°) to the NR length. In contrast, if illuminated at λ = 520 nm, i.e. the wavelength of vertically excited multipolar gap modes, ΔT (blue dotted circles in Figure 4a) remains at ∼70 K, showing a very slow variation with respect to the illumination directions. Figure 4(b) shows ΔT as a function of φ in AuNRs 1.5 nm above the Au surface under illumination at λ = 590 nm. As revealed in our previous study,40 decreasing the gap distance will greatly enhance photothermal conversion. In this case, the temperature increase shows a considerable enhancement up to ∼370 K. In addition, the enhanced absorption efficiency of the MIM configuration can be seen from the comparison with ΔT in AuNRs on a glass substrate (black-cross in Figure 4b). More importantly, ΔT does not vary significantly with the azimuthal angle, because the incoming light is effectively dissipated in the system through coupling with vertically excited multipolar gap resonances irrespective of illumination directions. Utilizing vertically excited resonances in NP heating may facilitate some

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CONCLUSION In conclusion, we have investigated scattering properties of individual AuNRs nanoscopically coupled to a Au film with different gap spacing using polarized single-nanoparticle darkfield microscopy. The width, spectral positions, and optical polarizability of the maxima in scattering spectra vary significantly with the distance between AuNRs and the Au substrate. Simulations show a good agreement with experimental results and reveal that the elongated geometry makes nanorods exhibit very complex plasmon hybridization behaviors when interacting with a metal film. Several types of plasmonic modes that have completely different resonant characteristics are localized at the gap. The excitations of these gap modes are highly dependent on the change of gap spacing, responsible for the interesting scattering properties observed in our experiments. Finally, we also demonstrate the influence of these plasmonic gap modes on the absorption efficiency of the NRfilm system and discuss their potential applications in facilitating photothermal conversion. In addition to the discussed application in heating process, our study can also contribute to other applications. For example, plasmonic gas sensors can be developed on the basis of the NR-film system, given that (i) the spectral properties of plasmon modes are highly dependent on the gap spacing and ambient dielectric environment and (ii) the careful selection of spacer materials can allow the spacer’s thickness and refractive index to be very sensitive to the presence of certain gas molecules.41 Furthermore, the coexistence of several different types of gap modes in the studied structures provides a high degree of flexibility for optical trapping.27 Moreover, large field enhancement within the gap and distinct radiative properties of the plasmonic gap modes make the AuNR-film system a good candidate for directional sorting of fluorescence emission.5,19,42−44

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METHODS Sample Preparation. The AuNRs with an average geometry ∼85 nm in length and ∼35 nm in width were prepared using a seed mediated method41 and confirmed by SEM imaging. The surfaces of AuNRs have been modified with poly(sodium 4-styrenesulfonate) (PSS, Mw ∼ 700,000), so they can be dispersed in ethanol without aggregation. The Au F

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The Journal of Physical Chemistry C substrates were prepared by coating 120 nm thick Au films on smooth silicon wafers using electron beam evaporation. PVA films with different thickness were then spin-coated on the Au substrates as spacers. The thickness of PVA spacers can be controlled by either adjusting the concentration of PVA in aqueous solution or changing the spinning speed and are confirmed by a surface profilometer (Dektak-XT). AuNRs dispersed in ethanol suspension are drop-casted on different substrates. Optical Characterization. A Reflection Dark-field microscope (Nikon, ECLIPSE 80) was used to measure the scattering spectra of individual AuNRs. White-light illumination from a halogen lamp is focused on the samples by a dark-field condenser, and the scattered light was collected by an objective (Nikon, CFI LU Plan Epi ELWD, 100X, NA = 0.8). The scattering spectra were recorded using a fiber-based spectrometer (Ocean Optics, QE65 pro), while the scattering image was taken by a CCD camera (Nikon, DIGITAL CAMERA HEAD DS-Fi1). Simulations. Scattering and absorption spectra were calculated using COMSOL Multiphysics, i.e. commercial software performing the finite element method in the frequency domain. We simulated the field distribution in the AuNR−film system using the Finite-Difference-Time-Domain method implemented by Lumerical Solutions. The refractive indices of PVA film and Au were taken from refs 45 and 46, respectively.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M.Q.). *E-mail: [email protected] (R.J.B.). *E-mail: [email protected] (B.D.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by New Zealand’s Marsden Fund through contract UOO-1214, the National Natural Science Foundation of China (grants 61275030, 61205030, and 61235007), the Opened Fund of State Key Laboratory of Advanced Optical Communication Systems and Networks, and the Swedish Foundation for Strategic Research (SSF) and the Swedish Research Council (VR).

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