How To Light Special Hot Spots in MultiparticleâFilm Configurations

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How to Light Special Hot Spots in Multiparticle-Film Configurations Shu Chen, Ling-Yan Meng, Hang-Yong Shan, Jian-Feng Li, Lihua Qian, Christopher T Williams, Zhi-Lin Yang, and Zhong-Qun Tian ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.5b05605 • Publication Date (Web): 18 Nov 2015 Downloaded from http://pubs.acs.org on November 21, 2015

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How to Light Special Hot Spots in MultiparticleFilm Configurations Shu Chen,a Ling-Yan Meng,a Hang-Yong Shan,a Jian-Feng Li,b Lihua Qian,c Christopher T. Williams,d Zhi-Lin Yang,* , a and Zhong-Qun Tian b

a

Department of Physics, Xiamen University, Xiamen 361005, China

b

State Key Laboratory of Physical Chemistry of Solid Surfaces and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China

c

School of Physics, Huazhong University of Science and Technology, Wuhan, 430074, China

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Department of Chemical Engineering, University of South Carolina, Columbia, South Carolina 29208, United States

* Address correspondence to (Z. L. Yang) [email protected]

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ABSTRACT: The precise control over the locations of hot spots in a nanostructured ensemble is of great importance in plasmon-enhanced spectroscopy, chemical sensing and super resolution optical imaging. However, for multiparticle configurations over metal films that involve localized and propagating surface plasmon modes, the locations of hot spots are difficult to predict due to complex plasmon competition and synergistic effects. In this work, theoretical simulations based on multiparticle-film configurations predict that the locations of hot spots can be efficiently controlled in the particle-particle gaps, the particle-film junctions, or in both, by suppressing or promoting specific plasmonic coupling effects in specific wavelength ranges. These findings offer an avenue to obtain strong Raman signals from molecules situated on single crystal surfaces, and simultaneously avoid signal interference from particle-particle gaps.

KEYWORDS: hot spots, plasmon competition and synergistic effects, shell-isolated nanoparticle -enhanced Raman spectroscopy (SHINERS), nanoparticle aggregates


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Hot spots with intense electromagnetic (EM) field enhancement play a dominant role in plasmon-enhanced spectroscopy,1-7 ultrasensitive sensors,2,

4, 8

and super resolution optical

imaging.9 For example, the probes situated within hot spots with just about 6% surface area can account for more than 85% signal in surface enhanced Raman scattering (SERS).10 Therefore, the huge enhancement factors (EFs) within hot spots make it possible to detect very few (or even single) molecules.3, 11, 12 However, due to the small spatial volumes of hot spots with high field enhancements, the precise prediction on their locations is still a major challenge in plasmonenhanced spectroscopy.7, 9 Over the past decades, metal nanoparticle aggregations placed over a dielectric film (i.e. glass or silicon) have usually been utilized to create hot spots.2, 4, 13 In such a system, the silicon or glass film just serves as a supporting substrate, with hot spots only appearing in the particleparticle gaps induced by localized surface plasmon resonance (LSPR) coupling effect.2, 4 To extend the potential application of SERS for probing chemical reactions and adsorbate’s orientation on single crystal surfaces of noble metals, particle-metal film systems have been proposed in recent years.14-20 As a result of plasmon coupling effect of particle-metal film, hot spots do exist in the particle-film junction for isolated particle-metal film systems, and this phenomenon has been confirmed by both the theoretical and experimental investigations.15-21 To overcome the weak SERS signal of the probed molecules provided by the extremely limited number of hot spots, particle aggregates over a metal film have been proposed.14, 22-25 Shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS),

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developed by our

group in 2010, has been experimentally confirmed as a particularly powerful technique to identify molecules adsorbed on a single crystal surface.26 A large number of SERS-active nanoparticles, that are coated with ultrathin shells, are randomly distributed as the aggregates or


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a monolayer over a metal film, resulting in numerous hot spots at particle-film junctions. However, different from the individual nanoparticle-film system, the hot spots might not only occur in the particle-film junctions but also in the particle-particle gaps.22, 23 Weakly adsorbed molecules on single crystal surfaces may spread into the particle-particle gaps under the optical force provided by the strong field enhancement of hot spots.27 Thus, the SERS signal from the molecules adsorbed on the single crystal surfaces will be convoluted with signal arising from adsorbed species in the particle-particle gaps, leading to the inaccurate spectral analysis. Thus, artificially controlling the locations of hot spots, which depends on the detailed geometry of the multiparticle-film system, is crucial for avoiding such interference. Although light polarization is an effective way to control the detailed locations of hot spots for multiparticle-dielectric film system,2, 3, 6 it is not easy to ensure that the hot spots are located at particle-particle gaps or particle-film junctions as we expect only through changing the polarization for particle aggregate-metallic film system. The reason for this difference lies in reflections of metal film and multiple light scattering between particles and film, weakening the polarization dependence greatly. Therefore, a comprehensive theoretical framework to precisely control the locations of hot spots in particle aggregates over a metal film is of much importance. In this work, shell-isolated particle aggregates over a gold film are selected as plasmonic systems, aiming to precisely control the locations of hot spots. With the finite-difference timedomain (FDTD) method, we systematically analyze the plasmonic modes and their contributions to the formations of hot spots by considering far field, near field and their relevance. The transfer of hot spots between the SHIN-SHIN gaps and SHIN-film junctions is demonstrated to depend on the excitation wavelength. The intrinsic physical mechanism of the hot spots transfer is analyzed and the general principle for precisely controlling the locations of hot spots in


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multiparticle-metal film configurations is proposed for the first time. We hope our findings will not only drive the development of SHINERS, but also be applicable more generally to other approaches such as infrared spectroscopy, sum frequency generation and second harmonic generation.

RESULTS AND DISCUSSION Models and Principle: The simulated models include Au/SiO2 SHIN aggregates with dimer, trimer, heptamer and nonamer structures situated onto a gold film, as illustrated in Figure 1(a), which present the most typical multiparticle systems and can be fabricated by DNA assemble method, 2, 8, 28 nanoimprint lithography, 29 or convective assembly at the confined volume.30 The diameter of the Au core, thickness of the SiO2 shell and inter-particle distance are set to 80, 1 and 1 nm, respectively. Taking the actual experimental conditions into account, the normal incident plane wave with polarization along the horizontal axis is adopted as the light source in the calculation model, as shown in Figure 1(b). The incident electric field amplitude is set as 1 V/m.

Figure 1. (a) Schematic illustrations of typical Au/SiO2 SHIN aggregates over a gold film consisting of dimer, trimer, heptamer and nonamer. (b) The spatial transfer of hot spots with only in the SHIN-SHIN gap, in both junctions, or only in the SHIN-film junctions under the different excitation wavelengths. Point A corresponds to the maximum electric field enhancement point in SHIN-SHIN gap (A region), and point B corresponds to the maximum electric field enhancement point on the gold film surface in SHIN-film junction (B region).


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In principle, when light illuminates the multiparticle-film structures (see Figure 1(a)), hot spots are usually located in the SHIN-SHIN gaps or SHIN-film junctions, as depicted in A and B regions of Figure 1(b). In general, the electric field enhancements from A and B regions maybe quite different because of the competition and synergistic interactions of the LSPR of SHINs,20, 22

near field coupling effect of SHIN-SHIN and SHINs-film,

22-25

the image effect,18, 31and the

propagating surface plasmons (PSPs) on the gold film.16, 17, 31, 32 For example, field enhancement in the A region will be significantly larger than that in the B region if the coupling effect of SHIN-film is effectively suppressed. Conversely, field enhancement in the A region will be much smaller than that in the B region if the plasmon coupling effect between SHIN and SHIN is highly suppressed. According to SERS theory, 33 the SERS intensity is proportional to |Eloc/Ein|4, where Eloc and Ein are the amplitudes of the localized electric field within the junctions and incident fields, respectively. Thus, the relative SERS intensity from the A and B regions will be about 100 if their electric field enhancement (defined as |Eloc/Ein|) ratio is over 3. Herein, we define the region with higher EM enhancement as hot spots if the relative SERS EFs between the two regions are larger than 100, and simultaneously neglect the EM enhancement of the other region. For a fixed particles-film system, it is possible to artificially adjust the precise locations of hot spots only in the SHIN-SHIN gap, only in the SHIN-film junctions, and in both of them, as shown in Figure 1(b), through the use of different excitation wavelengths. For the Au/SiO2 SHIN dimer superimposed onto a gold film, both the LSPR of an individual SHIN and the PSPs of the gold film can be excited. Under the charge-conjugate image and near field coupling effect, they can be hybridized into several plasmonic modes, 17, 31 which can be clearly identified by four distinctive modes at 525, 580, 645 and 850 nm in the scattering spectrum (see Figure 2(a)). In comparison with a bare SHIN dimer, the involvement of gold film


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makes the plasmonic modes more complicated (see the Figure S1).2 To further clarify the intrinsic mechanism of these modes, surface charge distributions are visualized in Figure 2(b). The mode IV at 525 nm results from the dipole resonance of an isolated Au/SiO2 SHIN, which is an uncoupled mode and also excited in the scattering spectrum of the bare SHIN dimer (see the

Figure 2. (a) Scattering spectra of SHIN dimer sited onto gold film; the scattering efficiency is normalized in the range 0~1. (b) Surface charge distributions of Au/SiO2 SHIN dimer over a gold film with the different excitation wavelengths corresponding to four scattering peaks in Figure 2(a).

Figure S1).2, 22 The dipolar distribution feature can be obtained on the single particle with a little charges gathering in the SHIN-film junctions. The other modes are assigned as the hybridized resonant modes.22-24 For the mode I as shown in Figure 2(b), the charge distributions on the two particle surfaces present a vertical pattern with anti-parallel dipole moment; together with


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massive charges accumulation on the SHIN surface nearby the junction region. Severe distortion of charge pattern on particles might be induced by the image field effect of the gold film.18, 31 Therefore, the mode I can be concluded as the dipole coupling mode between the gold film and SHIN dimer; the SHIN dimer can be regarded as a special entirety with strong coupling between the two monomers. For mode II, the surface charges of the two SHINs show the coherent oscillation with a typical dipole feature on an individual particle. Simultaneously, abundant charges also gather in the SHIN-film junctions. Thus, the mode II can be roughly considered as the dipole coupling mode of the single SHIN and gold film. The parallel dipole moment of each particle leads to a larger scattering intensity than both mode I and mode III. For mode III, the surface charges on each particle present a quadrupolar character, 22 and strong coupling also exists between particle and film, implying significant coupling between the quadruple mode of the single particle and gold film. Compared with the effective EM coupling regions in SHIN-film junctions, a smaller EM coupling region occurs in the SHIN-SHIN gap. Obviously, hot spots usually result from the plasmon coupling effect within tiny crevices including the SHIN-SHIN or the SHIN-film junctions, as represented in Figure 2(b). To quantitatively estimate the field enhancement, the detailed near field enhancement distributions for the four resonant modes are replaced by the common logarithm of SERS EM enhancement (~log10|Eloc/Ein|4), from which one can determine the maximum SERS EFs directly (see Figure S2). Besides the maximum EFs of the mode IV being in the SHIN-SHIN gap, the maximum EFs of the other modes are all located at the particle-film junction. All these modes provide the high EFs over 10 orders of magnitude. In fact, the EFs in both junctions behave differently in each plasmon mode. Similar EFs in both junctions are presented for mode I, while the enhancement in the SHIN-film junctions is slight stronger than that in the SHIN-SHIN gap under the mode II and III.


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The SERS performance is well related to the near field EM enhancement, so that identifying the relationship between the far and near fields will benefit to optimize the appropriate excitation wavelength during Raman characterization.34, 35 To unveil the relationship between the far and near field enhancement, the scattering efficiency and the electric field enhancement at two typical points (A and B) were simulated with various excitation wavelengths, as shown in Figure 3(a). Field enhancement profiles of point A and B present three resonant peaks, while the scattering spectrum shows four resonant peaks. The electric field enhancement at 525 nm is much weaker than that of the other three plasmonic modes due to the significant interband absorption.36 The residual plasmon bands show the coherent location between near and far field. Interestingly, the scattering efficiency of mode II is 4.17 and 4.03 times than those of modes I

Figure 3. (a) Wavelength dependence of the electric field intensity enhancement at point A (red) and B (green); the scattering spectrum is the black curve and has been normalized in range 0~1. The wireframe with light cyan represents the R3 region in (b). (b) The electric enhancement ratio between point B and point A (defined as E (B)/E (A)) depending on the excitation wavelength. The inset presents the relative SERS enhancement (defined as E4 (B)/E4 (A)) in the wavelength range of 740~790 nm (R3). SERS (A) and SERS (B) are defined as E4 (A) and E4 (B), respectively.

and III in the far field spectra, respectively. By comparison, the field enhancement ratios between the two modes are nearly equal (in range of 1.2~1.8) in the near field region. The deviation between the far field and near field may be attributed to two factors. First, the


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excitation of PSPs on the gold surface have nonradiative characteristics, making the significant contribution of near field enhancement rather than the scattering towards the far field.37 Second, the scattering is proportional to the quantity of dipole moments, and higher order multipoles usually make few contributions to far field scattering. In the case of near field enhancement, both the dipoles and higher order multipoles critically influence its EM intensity.38 Therefore, a more reliable estimation on SERS enhancement should be carried out from the near field distribution. The optimal enhancement of point A and B can be obtained under the three resonant mode regions. The maximum SERS EFs of point A and B under the resonant mode II are ~8.64×109 and ~1.14×1010, respectively. The weaker SERS EFs are about 3.88×109 and 3.60×109 respectively for mode I. In this case, the electric field enhancement of points A and B are nearly equals. According to their relative enhancement values, the continuum wavelength range from 500 nm to 1000 nm can be divided into three regions, namely, R1, R2 and R3, as shown in Figure 3(b). In the R1 region, nearby the mode IV, the SERS enhancement of point A is over 100 times that of point B. In the R2 region, their enhanced ratios are in range from 0.01 to 100. In the R3 region, where there is a non-resonant region between mode I and II, the SERS enhancement of point B is over 100 times as that of point A. Especially, the maximum relative SERS enhancement of the two points is 1900 times under the excitation at 765 nm. This phenomenon offers a possibility to obtain strong SERS signals of molecules completely from the single crystal surface by choosing the appropriate exciting wavelength. The above results indicate that the exact locations of hot spots depend on the excitation wavelength.23 Here, six wavelengths of 514, 551, 633, 703, 785 and 837 nm were selected to identify the locations of hot spots from the side-view and top-view as shown in Figure 4. Hot spots with over 6 orders of magnitude EFs only appear in the SHIN-SHIN gap if the excitation


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wavelength is 514 nm. When excited by 551 nm laser, hot spots simultaneously appear in both junctions, but the EFs and enhanced regions of hot spots in the SHIN-film junctions are much smaller than that in the SHIN-SHIN gaps. If one increases the wavelength to the resonant regions of mode III and II (e.g. 633 nm), hot spots simultaneously present over 10 orders of magnitude EFs in the both junctions. By further shifting the excitation wavelength to 703 nm, field enhancement in the SHIN-SHIN gap gradually fades away, accompanied by 6 order of magnitude EFs. For 785 nm, hot spots can only be located in the SHIN-film junction, resulting in EFs of 9 orders of magnitude. Further increasing the wavelength to nearby resonant mode I results in a hot spot again appearing in the SHIN-SHIN gap. The changes of EFs on the gold film

Figure 4. Calculated SERS enhancement distributions at xz-plane (a) and xy-plane (b) under various wavelengths, and the corresponding excitation wavelength are marked on the left corner of the distribution. It has been normalized by the maximum enhancement in the same row in (a); the intensity has been normalized for comparison by the maximum enhanced value on gold film surface at 633 nm in (b). The EFs is shown as the scale ruler.

surface under the six excitation wavelengths can be clearly observed from the top-view in Figure 4(b). Actually, hot spots simultaneously occur in both junctions when the exciting wavelength is in the range of 560-680 nm and 830-1000 nm. Hot spots are only located in the SHIN-film


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junctions when the excited wavelength is in the range of 680-830 nm. In the remaining wavelength range, hot spots are only in the SHIN-SHIN gap, as presented in Figure S3. The deviation in wavelength ranges of R3 between Figure 3(b) and Figure S3 stems from the fact that the EM enhancement at point B is not the largest enhancement point in the SHIN-film gap (B region). As the largest enhancement point usually is located at the silica shell rather than on the surface, the relative EM enhancement ratio of SHIN-film junction to SHIN-SHIN gap (A region) is larger than that of point B to A. The phenomenon of hot spots transfer is the result of the competition and synergistic effects of the plasmon coupling between the SHIN-SHIN and SHINfilm junctions.17,

22, 23

As a result, hot spots should appear in both junctions.22 However, the

relative EM enhancement between SHIN-SHIN gap and SHIN-film junctions varies with the specific plasmon modes. For R2 regions, which mainly include the resonant regions of the three hybridization modes, the EM coupling between SHIN-SHIN and SHIN-film is nearly identical, thus the hot spots appear simultaneously in both junctions. However, for the R3 region, which is the non-resonant regions between the mode I and mode II, the coupling effect in SHIN-film junction will be stronger than that in the SHIN-SHIN gap because the gold film can be regarded as a free electron reservoir in comparison with the particle.19 Thus, the hot spots in the SHINfilm junctions provide the majority of the EM enhancement. For the residual regions (R1), hot spots appear in the SHIN-SHIN gap as a result of the weak LSPR coupling of the dipole mode on the SHIN surface. Thus, the hot spots can switch between the SHIN-SHIN gap and SHIN-film junctions by choosing the approximate wavelength range. It should be noted that the plasmon coupling effect between particle-particle in the SHIN dimer-film system will decrease significantly when the light polarization on the xy-plane gradually deviates from the dimer axis. Accordingly, hot spots in the SHIN-film junctions are more easily obtained.20


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Besides the SHIN dimer, other multiparticle aggregates that contain more junctions can also serve as the high performance SERS substrates.2,

8, 39

Herein, the Au/SiO2 SHIN aggregates

situated onto the gold film were configured as shown in Figure 1(a). For the SHIN aggregatesfilm systems, their optical properties mainly depend on the near field coupling effect of the particles-gold film and particle-particle along the electric field polarized direction. For a SHIN trimer or tetramer with a fixed light polarization (as presented in Figure 1), scattering spectra show the similar profiles to the dimer-film system, except for the slight red-shift of the four plasmon coupling modes. Meanwhile, the location transition of hot spots can also be observed as illustrated in Figure S4. For the SHIN trimer and tetramer, hot spots situate in the interparticle gaps, the SHIN-film junctions, and simultaneously both junctions under 514 nm, 785 nm and 633 nm, respectively. Figure 5(a) shows the scattering spectrum of the Au/SiO2 SHIN heptamer-gold film system. The quadruple coupling mode of single SHIN-film and the dipole coupling mode of single SHIN-film are respectively located at 588 and 690 nm, exhibiting the slight red-shift in comparison with dimer-film system. The dipole coupling mode between SHIN heptamer and gold film at 975 nm is significantly red-shifted with the rising number of nanoparticles in the aggregates.39 It should be noted that a new mode marked by the red arrow in Figure 5(a) appears at 658 nm, which stems from the coupling between particle and film evidenced by the charge redistribution on particles and film surface in Figure 5(b) and Figure S5(b). A majority of surface charges accumulate on the SHIN-film junctions, especially on the left and right SHIN-film junctions, whilst weak accumulation is observed at the interparticle junctions. The distribution patterns of the EM and electric vector show that this mode is mainly induced by the coupling between SHIN and film. Meanwhile, this mode is only excited in the electric field enhancement


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spectrum of point B rather than point A, as shown in Figure S5(a). In this case, the strong EM coupling effect between SHIN-film results in huge field enhancement as high as 11 orders of magnitude, so that it can be considered as a "gap mode". The significant distinctions between SHIN-film and SHIN-SHIN indicates that the high-quality SERS signal can be created onto the single crystal surface with negligible signal disturbance from SHIN-SHIN gaps under the gap

Figure 5. (a) The normalized scattering spectrum of heptamer aggregates on gold film. (b) SERS EM enhancement distribution of gap mode. (c). The relative electric enhancement ratio between point B and point A with excitation wavelength. The inset presents the relative SERS enhancement between point B and point A nearby the gap mode. (d) SERS enhancement distributions under excitation wavelength of 530 nm and 633 nm, and the SERS enhancement distributions were normalized by their maximum enhancement under 633 nm in heptamer and nonamer aggregates, respectively.


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mode. Similar to the SERS enhancement ratio between point B and point A of the dimer on the gold film surface, the entire wavelength range can also be divided into three regions (R1, R2 and R3), as presented in Figure 5(c). The hot spots only locate in the SHIN-SHIN gaps, in the SHINfilm junctions or in both junctions under R1, R3 and R2, respectively. Differently, besides the non-resonant region of 730-820 nm, the range of R3 is broadened to the resonant regions of the gap mode. Moreover, the SERS enhancement ratio between point B and point A is about 10000 under the gap mode (Figure 5(c), insert). Previous investigation40 also has shown that the red shifting magnitude of the plasmon resonance depends on the SHIN number during self-assembly, so that more shifts are observed in the nonamer SHINs over gold film system. It is easy to realize the transfer of hot spots in heptamer-film or nonamer-film if appropriate excitation wavelengths are selected as shown in Figure 5(d) and Figure S5(c). Different from the SHIN dimer-film system, the far and near field properties for heptamer-film just show slight changes under different light polarizations in xy-plane because of the rotation symmetry. More details are presented in Figure S6.

CONCLUSIONS In conclusion, we have revealed hot spot transfer in SHIN aggregates over the metal film, resulting from the plasmon competition and synergistic effects between the interparticle and particle-film junctions. Therefore, the locations of hot spots can be efficiently controlled in the SHIN-SHIN gaps, the SHIN-film junctions or both junctions, by choosing the approximate excitation wavelength ranges based on the plasmon modes. As the number of particles in the aggregates rises, a new gap mode between particle and film can be also excited, besides the dipolar coupling mode of SHIN dimer-film, the dipolar coupling mode of single SHIN-film, and


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the quadrupolar coupling mode of single SHIN-film. The relative SERS EFs between B and A regions can be further enlarged if the wavelength of the excitation laser matches the gap mode in heptamer or nonamer aggregates. Thus, the wavelength range corresponding to the locations of hot spots only in the SHIN-film junctions can be also broadened to the resonant regions of the gap mode. Current investigation not only provides a strong theoretical framework prediction that will underpin potential applications of SHINERS, but also shows the potentially promising applications in other optical spectroscopic approaches.

SIMULATION METHOD The fundamental principle of FDTD is well documented in the literature.41 The FDTD method is applied to simulate the scattering, EM field distributions and other optical properties.2 Perfectly matched layer boundary conditions are used in the simulations to avoid unphysical reflections around the structure. The simulation time in the all simulations is set as 1000 fs, which is long enough to ensure calculation convergence. Non-uniform FDTD mesh method was adopted to save the computation resources and calculation time. In the simulation, the Yee cell size in the SHIN-SHIN and SHIN-film gap regions was set to be 0.25 nm×0.25 nm×0.25 nm while 0.5 nm×0.5 nm×0.5 nm Yee cell size was used in other regions. The frequency dependent optical constant of Au was obtained from the literature, 42 and the refractive index of SiO2 is 1.46.42 In the simulation, the xy-plane dimension of the gold film with 100 nm thickness is infinite. It should be noted that, for more accurate results, the model introduced in reference 43 should be taken to perfectly match the actual experimental light source.43 Conflict of Interest: The authors declare no competing financial interest.


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Acknowledgements. Financial supports from National Natural Science Foundation of China (21173171, 11474239, 51371084) and Fundamental Research Funds for the Central Universities (2012121013) are highly acknowledged. We thank Professor Tien-Mo Shih for helpful discussions. Supporting Information Available: the scattering spectrum of bare Au/SiO2 SHIN dimer without the gold film, the SERS enhancement distributions under the four plasmon modes, the transfer of hot spots by continuously changing the excitation wavelength range, hot spot transfer in Au/SiO2 SHIN trimer and tetramer-gold film systems, near field properties of heptamer and nonamer-gold film systems and the plasmonic properties of the SHIN heptamer-gold film system under different light polarizations. This material and the available free of charge via the internet at http://pubs.acs.org.

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The locations of hot spots in multiparticle-film configurations are precisely controlled in the particle-particle gaps, the particle-film junctions, or in both by suppressing or promoting specific plasmonic coupling effects.


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How To Light Special Hot Spots in MultiparticleâFilm Configurations

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