Strong-Field-Enhanced Spectroscopy in Silicon ... - ACS Publications

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Strong-Field-Enhanced Spectroscopy in Silicon Nanoparticle Electric and Magnetic Dipole Resonance near a Metal Surface Zengli Huang,† Jianfeng Wang,*,† Zhenghui Liu,† Gengzhao Xu,† Yingmin Fan,† Haijian Zhong,† Bing Cao,‡,§ Chinhua Wang,‡,§ and Ke Xu*,† †

Suzhou Institute of Nano-tech and Nano-bionics, CAS, Suzhou 215123, People’s Republic of China College of Physics, Optoelectronics and Energy and Collaborative Innovation Center of Suzhou Nano Science and Technology, and § Key Lab of Advanced Optical Manufacturing Technologies of Jiangsu Province and Key Lab of Modern Optical Technologies of Education Ministry of China, Soochow University, Suzhou 215006, People’s Republic of China

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S Supporting Information *

ABSTRACT: Strong-field-enhanced spectroscopy in a hybrid dipole resonance system composed of a low-loss semiconductor nanoparticle and metal film is proposed and demonstrated. This hybrid Si nanoparticle on silver system is featuring extraordinary near-field enhancement and large field confinement. Extensive numerical calculations are carried out to investigate the influence of the gap size, particle diameter, and metal substrate on the near-field enhancement response in the Si particle−metal gap in order to properly model their hybridization. Our analysis reveals that this near-field enhancement originates from the strong gap magnetic resonance response by the Si nanoparticle dipole interaction with metal mirror image and metal film surface plasmon effects. We further demonstrate the strong enhanced Raman spectroscopy of a single silicon nanoparticle over Ag film with a precisely sized molecular spacer layer between them. These results illustrate the capacity and tunability of the low-loss silicon particle on the metal system on surface-enhanced spectroscopic techniques as well as possible applications in optical circuits or building new metamaterials.

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resonators,25,27 the magnetic dipole response of dielectric sphere nanoparticles can serve as efficient magnetic scatters with possible applications as metamaterials, nanoantennas, and metasurfaces.3,5,9,10,12,18 In contrast to metallic nanostructures employed in plasmonics, the near-fields in dielectric nanostructures immediately adjacent to the particles are less intense than that provided by metallic nanoparticles but can have stronger far-field scattering cross section.23,28 Importantly, the nanostructures based on low-loss dielectric materials, such as GaP and Si, can overcome heat generation and provide a lower local heating of field hotspots than plasmonic structures that can be important for gaining reliable response of the target molecules in surface-enhanced spectroscopies.23,29

INTRODUCTION Nanostructures made of high refractive dielectric or semiconductor materials have recently received considerable attention in the nanophotonics for control and manipulation of light in the near-field.1−26 They provide a novel method to directly engineer a magnetic field response at optical frequencies in addition to the electric field response in metal plasmonic structures. As a fundamental building block, the dielectric spherical nanoparticle presents both strong magnetic dipole (MD) and electric dipolar (ED) responses corresponding to the basic Mie resonances.2−6 The magnetic resonances, driven by the electric field of light that couples to circular displacement current, inducing a MD moment perpendicular to the incident electric field, have recently been demonstrated in various semiconductor nanoparticles such as silicon (Si),2,4,6,24 gallium arsenide (GaAs),12 germanium (Ge)3 and gallium phosphide (GaP).23 Similar to plasmonic nanoparticles with specially designed metallic nanostructures, such as split-ring © XXXX American Chemical Society

Received: October 14, 2015 Revised: November 18, 2015

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DOI: 10.1021/acs.jpcc.5b10045 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. (a) Schematic of a hybrid dielectric nanoparticle on metal system. A Si sphere of permittivity εc and radius r is separated from a metal Ag half-space of permittivity εm by a nanoscale SiO2 dielectric gap of permittivity εd and height g. The inset shows the schematic of a Si sphere in air. (b) Extinction cross sections of a Si sphere in air and on Ag surface. The particle is illuminated by a plane wave from top under normal incidence, as shown in the inset. The modes ED, MD, Fst and Snd are indicated. Near-field enhancement spectra of the electric (c) and magnetic (d) field for a Si sphere in air (calculated from position A and B, shown in (e)) and a Si sphere on Ag surface (calculated from position C and D, shown in (f)). Nearfield amplitude enhancement distributions of the ED and MD resonances for a Si sphere in air (e) and of the Fst and Snd resonance modes of a Si sphere on Ag substrate (f). The size of each map is 200 nm × 200 nm. White dashed lines indicate the substrate and particle positions. For these graphics, the radius of the Si sphere is 65 nm and the SiO2 gap size g is 3 nm.

Compared to the single nanoparticle, a dielectric nanoparticle on metal (NPOM) structure should not only offer further tunability and engineering capabilities of the enhanced electric and magnetic fields but also provide sufficient control over the gap dimensions known to be very important for reliably supporting the largest field enhancement as well as reproducibly gaining a well-defined enhancement region.7,11,15,30 Up to now, a few studies have been performed on the hybrid NPOM systems consisting of high-index dielectric nanoparticle on metal films, where the dielectric nanoparticles have the electric and magnetic Mie resonance modes and their optical response should be strongly affected by the metal mirror effects and the excited propagating surface plasmon polaritons (SPPs) on the metal films, in analogy with the case of plasmonic nanoparticles.31−33 Recently, in a system of a dielectric nanoparticle dimers placed on a silver substrate, a dipolar resonance mode has been observed in the wavelength range near the metal SPPs.34 More interestingly, the metal mirror image effect could transform the oscillating electrical dipoles of the dielectric nanoparticle into the magnetic polarization currents, leading to a strong enhancement of the far-field scattering cross section.15 In addition to the far-field enhancement, simulations and experiments have demonstrated that the dielectric nanoparticle, as a scattering antennae, could convert the incoming light to evanescent waves that excites the SPPs on the metal surface and results in large electric field enhancement in the gap region.7,11

In contrast, this article reports the strong enhancements of the near-field and Raman scattering spectroscopy induced by the general electric and magnetic dipole resonance modes of a high-index dielectric nanoparticle interaction with the metal films. We systemically describe the photonic interactions between a Si nanoparticle with the strong dipole resonances and a metal film leading to large field enhancements and strong field confinement. By analyzing the finite-difference timedomain (FDTD) calculation results, the electric field enhancement in the Si particle−metal gap shows strong dependence on the gap sizes, particle diameters and metal substrates. Furthermore, surface-enhanced Raman scattering (SERS) from a monolayer molecular in the gap was used to demonstrate the performance of the configuration. Specifically, we show with experiments and numerical analysis that the particle diameter has an important influence on the enhanced Raman signals as the peak wavelength of the enhanced field is tunable in related to the coupled nanoparticle dipole resonance. Compared with the metal counterparts, the silicon NPOM system, having moderate near field enhancement, stronger far field scattering together with a lower absorption losses, provides an alternative to perform surface-enhanced spectroscopies.

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RESULTS AND DISCUSSION The hybrid silicon NPOM resonance system shown in Figure 1a consists of a silicon (Si) nanoparticle and a silver (Ag) surface, with a low refractive index silica (SiO2) gap in between. B


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Figure 2. (a) Extinction cross sections of a Si sphere in air (blue dashed line) and on the Ag surface (solid line) as a function of gap size g from 5 to 65 nm in step of 10 nm. The inset shows the schematic of the metal mirror image effect for a silicon particle ED (upper left) and MD (upper right) mode. The modes position of ED, MD and Fst are indicated. The Fst mode gradually evolves into weak coupled ED (indicated by blue arrows) and strong coupled MD resonance mode (indicated by red arrows) as the Si nanoparticle is moved away from the metal film. The radius of the Si sphere is 65 nm. (b) The maximum extinction wavelength of the Fst resonance mode (indicated by red arrows in Figure 2a) versus nanoparticle−metal gap size: r = 45 nm (black square) and r = 85 nm (red square). The inset shows the magnetic field distributions for the cases of where the particles sizes are 45 and 85 nm and gap sizes are 1, 10 and 50 nm. The red (r = 45 nm) and black (r = 85 nm) horizontal dashed lines indicate the wavelength positions of the resonance modes for a free Si nanoparticle. (c) Maximum electric field enhancement |E|/|E0| in the gap (corresponding to the Fst mode resonance, not at a fixed wavelength) for different sphere radius r and gap size g.

interaction of the ED and MD resonance mode with the metal substrate. For the coupled ED mode (the Fst resonance mode), the pronounced electric and magnetic near-field enhancements are observed. For the case of MD interaction, enhancements are observed for the electric near-field as well, but the magnetic near-field is deeply suppressed. The simulated electric and magnetic near-field profiles at the Fst and Snd resonant wavelengths are also shown in Figures 1e and 1f. It is clearly observed that much more electric fields of the Fst and Snd mode are localized into the gap region between the particle and substrate. The magnetic hotspots from the Fst mode are confined both inside Si particle and the gap region. However, the distribution of magnetic field from the Snd mode is nearly unchanged and the total magnetic field intensity becomes much weaker compared to that of a free Si particle. Note that these local field enhancements are similar to the hotspots of the operation of a dielectric coupled nanoantenna.20,28 However, this is not a usual situation since both the accessible electric and magnetic hotspots in the gap arise from the horizontal polarization of the incident field (inset of Figure 1b). The field distributions of the coupled modes suggest that a Si sphere electric and magnetic dipole resonance can effectively couple with the metal mirror image in different ways when the sphere particle is close to the metallic surface. Thus, it is instructive to explore how the coupled modes of the hybrid NPOM system evolve as the gap size is increased gradually (Figure 2a and Figure S2). From this progression of spectra, one can see that as the nanoparticle is moved away from the metal film, the Fst mode gradually evolves into two separated resonance modesone is the suppressed ED resonance mode (indicated by blue arrows in Figure 2a), the other is the enhanced MD resonance mode (indicated by red arrows in Figure 2a). Specifically, the nanoparticle coupled ED mode is strongly enhanced as the particle is in close proximity to the metal film (g = 1 nm) but begins to slowly damp out with increased gap distance (g = 65 nm). On the contrary, the coupled MD mode is strongly enhanced at the large gap size (g = 65 nm) and first gradually weakened, then sharply enhanced and overlapped with the coupled ED mode (Fst mode, g = 1 nm). These enhancement and suppression of the extinction

It is well-known that the high-index dielectric nanoparticles can support optical resonances of both ED and MD Mie resonance.16,19,35 The MD response originates from the circular displacement current excited inside the particle by incident light (Supporting Information, Figures S1). Figure 1b shows the calculated extinction (σext) cross-section spectra for the case of a Si sphere of radius 65 nm in air (blue) and on top of a Ag substrate (black) with a 3 nm SiO2 gap. In this regime, particles made of Si material can be treated as non-Rayleigh dipole particles, as its index of refraction is real and large enough to exhibit electric and magnetic resonances.5 Consequently, for the silicon sphere in air, two clear peaks are observed which correspond to the ED mode resonating at λ = 454 nm and MD mode resonating at λ = 546 nm. And the MD resonance mode (σext ∼ 0.123 μm2) scatters much more light than that of the ED mode and is about 10 times larger than the Si particle geometrical cross section (∼0.013 μm2). This makes Si particle a very good candidate in terms of far-field enhancing performance. However, big differences of extinction spectra are observed when a Si sphere is placed on an Ag substrate with a SiO2 gap in between. The extinction cross section (∼0.35 μm2) of the Fst resonance mode resonance at λ = 486 nm becomes 5 times larger than that of a free Si particle ED mode (∼0.077 μm2). This far-field scattering enhancement can mainly attribute to the coupling of antisymmetric electric dipole image of the metal mirror, which results in an effective magnetic dipole mode and induces a high scattering cross section.15 More importantly, we observe that concomitant with the farfield extinction enhancement of the hybrid Si particle−metal system is a strong enhanced and localized electric and magnetic near-fields in the gap formed between the nanoparticle and the metal surface. As shown in Figures 1c and 1d, two clear peaks of field enhancement related to the Fst and Snd resonances are produced with the Si particle on the silver substrate. Generally, this field enhancement is obviously different from the case of SPPs mode formed by a smaller dielectric nanoparticle near a metallic surface to generate hotspots.11 For the hybrid Si particle−metal dipole system with a small gap between, the field enhancements should be mainly attributed to the C


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Figure 3. (a) Scattering cross sections of a Si sphere on Ag (black), Au (red), Al (green) and PEC (blue) surface with Si nanoparticle size of r = 95 nm and gap size of g = 1 nm. (b) Maximum electric field enhancement |E|/|E0| in the gap (corresponding to the Fst mode resonance, not at a fixed wavelength) for different metal substrates and Si nanoparticle sizes. The gap size g is 1 nm.

and g. The electric field enhancement larger than 400-fold at r = 45 nm and g = 1 nm can be realized, which suggests that the surface-enhanced Raman scattering enhancements of 1010 or larger are possible. As expected, field enhancement in the gap dramatically increases as the gap decreases due to the stronger metal coupling effect. For the smaller gap thickness (g = 1 nm), the maximum electric field enhancement is observed from the enhanced ED resonance mode. In this case, due to the strong coupling between the ED mode and their metal mirror image, a localized magnetic field concentrated within the gap (r = 45 nm, inside of Figure 2b) is generated and further enhances the interaction of the particle and the illumination field; then a larger electric field enhancement is expected in the gap region between the Si nanoparticle and metal substrate. Moreover, the smaller particle diameter will lead to the larger ED coupling strength and thus the induced magnetic moment and electric field enhancement. While for a large gap thickness (g = 50 nm), the maximum electric field enhancement is observed from the enhanced MD resonance mode, where the contribution from the coupled ED mode is low. In this case, the electric field enhancement in the gap slightly increases with increasing the sphere diameter, which can be explained by the superposed ED coupling effects indicating a weak magnetic field in the gap for the bigger Si particle (r = 85 nm, inside of Figure 2b). At moderate gap thickness (d = 10 nm), the magnetic field distribution of the Fst resonance exhibits a hybrid ED resonance mode feature (r = 45 and 85 nm, inside of Figure 2b). Namely, its magnetic energy is mainly concentrated inside the Si sphere and partially in the adjacent metal−dielectric interface. As a result, the electric field enhancement shows little dependence on the sphere diameter (Figure 2c). Although the strong enhanced scattering of the hybrid Si NPOM system mainly attributes to the mirror image photonic interactions of the metal substrate,15 an alternative effect could be considered for the enhanced near field by a Si nanoparticle exciting surface plasmons on a flat metal surfaces.11 In fact, the ED and MD resonances of the high-index Si nanoparticle possessing various photon wave vector components can provide the appropriate momentum to couple light into the gap above the metal surface and excite metal SPPs. This effect of excitation of SPPs of the metal surface on the near field enhancement in the gap could be preliminarily verified by the response of the Si particle interacting with various metallic substrates. In Figure 3a, we compare the far field scattering cross sections obtained from a single Si nanoparticle on four different metal substrates with r = 95 nm and g = 1 nm. Those four configurations show similar far field scattering results only

from the ED and MD periodically modulation with different gap size reflect the particle dipole interaction with the metal mirror image in different resonance phases. Compared with a Si sphere in an air, the illuminating field from a Si sphere near a metal interface includes two parts. One is the source wave directly illuminating at the sphere. The other is the reflected field at the particle dipole resonance position from the metal interface. Consequently, when the dipole resonance is in phase (out of phase) with the field propagating away from the metal interface or the mirror image, the reflected field and consequently the particle scattering is enhanced (inhibited). One can find different statements concerning this subject.36,37 Note that for the intimate distance between the particle and metal film, the ED mode of the Si particle would produce an opposite electric dipole in the metal substrate with comparable amplitudes and opposite phase (antisymmetric ED image). This phenomenon leads to the excitation of a large magnetic moment in the gap and leads to large field enhancement in the gap region similar to the case of substrate induced bianisotropy effect.38 Figure 2b further shows the effects of particle and gap size dependence on maximum extinction wavelength of the Fst resonance mode (indicated by red arrows in Figure 2a) and the corresponding magnetic field feature. The maximum extinction wavelength of the Fst resonance mode gradually shifts from the ED mode to the MD mode, which reflects the resonance phase difference between the particle dipole and their mirror image at various gap size.36 The tunability of the ED and MD extinction maximum is critical for the broad spectral tuning of a single Si nanoparticle. Besides, it is important to see that for the case of the 45 nm particle radius and 1 nm gap the strongest magnetic field hotspot from the coupled ED mode is generated in the gap as well as partially localized inside the nanoparticle and partially in the metal substrate. This magnetic field hotspot in the gap would be expected from the enhanced circular displacement currents induced by the coupling between the ED mode and their metal mirror image, which makes the Fst mode gain the feature of gap magnetic dipole resonance and enable the incident field penetrate to the gap region. The gap magnetic feature evolves and decreases in intensity as the gap is increasing. At the 85 nm particle radius and 50 nm gap size, the map shows an enhanced MD mode feature with an elongated magnetic tail in the gap region. To get further insight into near field enhancement from the metal mirror effects, Figure 2c shows the dependence of the maximum field enhancement (|E|/|E0|) in the gap (corresponding to the Fst mode resonance, not at a fixed wavelength), on r D


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Figure 4. Electric field enhancement along dashed line in the inset of (a) shows the field confinement in the gap region with different nanoparticle diameters for g = 1 nm (a), g = 10 nm (b) and g = 50 nm (c). (d) The normalized fwhm/d (indicated in the inset of (b)) of the electric field in the gap at the maximum f value for different gap sizes and nanoparticle diameters.

Figure 5. Absorption and scattering cross sections (a), and electric field enhancement |E|/|E0| in the gap (b), for the cases of a Si sphere of radius 95 nm, an Au sphere of radius 85 nm and an Ag sphere of radius 85 nm on different metal substrates with the gap size g = 1 nm.

particle size decreases, the peak wavelength of the Fst resonance mode gradually blue shifts (Figure S3a) and approaches the respective SPPs wavelength of the these metals. As expected, the closer the peak wavelength of the coupling resonance mode for different metals to their respective SPPs wavelength, the more enhanced near field in the gap region would be. Consequently, the Si nanoparticle on the Au substrate has the strongest near field enhancement and this near field enhancement exhibits the stronger dependence on Si particle size than other Ag and Al substrates. Thus, in addition to the interaction of metal image, the strong coupling between the metal SPs and the dipole mode plays a key role in the nearfield enhancement in the hybrid NPOM system. In plasmonic systems, field localization is generally correlated with local field enhancement, f = |E|/|E0|, where E is the local electric field and E0 is the electric field of the incident wave. In the present context, the gap between the Si particle and the metal substrate provides an approach to store electromagnetic energy from the strong interaction between the particle and

with small variations in the scattering resonance intensity and wavelength position (Figure S3). In fact, the reflectivity properties of these three metal surfaces (Ag, Au, and Al) are very similar to those of PEC in the visible range, except as λ < 560 nm, the reflectivity of metal Au is sharply decreased (Figure S4). Therefore the observed far field scattering enhancement could be attributed to the metal mirror image effect similar to ref 15. However, refer to the maximum electric near field enhancement of the Fst mode; these four configurations show significantly different dependence on the Si particle diameter. As shown in Figure 3b, for the PEC substrate, the electric near field enhancement of the Fst mode has the lowest value and shows slightly smaller dependence on the particle diameter. This weaker electric near field enhancement by the PEC substrate is linked to only the metal image charge contributions because the PEC substrate does not have any plasmonic effect. In comparison, the metal Al possess only weak plasmonic in the visible wavelength range and metal Au film gets the strong plasmonic resonance toward red wavelength. When the Si E


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Figure 6. (a) Schematic of enhanced Raman spectroscopy from a Si nanoparticle on a functionalized Ag surface. (b) Raman spectra of a functionalized Ag surface and a Si nanoparticle deposited on a functionalized Ag substrate. The inset shows the SEM image of a Si nanoparticle (scale bar = 200 nm).

alternative candidate to perform surface-enhanced spectroscopies. In order to demonstrate that the field enhancement in the gap just by placing Si nanoparticles on flat metal surfaces can greatly enhance the Raman scattering of target molecular, we prepared a NPOM SERS substrates consisting of well-separated Si nanoparticles residing on a monolayer p-aminothiophenol (PATP) spacer layer covering a silver mirror. Briefly, the fabrication process consists of preparing an Ag film on mica substrate by e-beam evaporating 100 nm of silver at a deposition rate of 0.5 Å s−1. The Ag substrates were incubated in PATP solution to coat a monolayer PATP on flat silver substrate and the respective Si nanoparticles were then adsorbed onto the monolayer by dip-coating the substrate in an aqueous Si NPs (single-crystalline undoped silicon nanoparticles, Alfa Aesar) solution. To study the Raman scattering from the Si NPOM structure, the concentration of suspensions and the absorption time were controlled such that the Si NPs were well separated from each other. These low Si NP coverages were purposely selected to minimize NP−NP coupling so as to restrict the single-particle Raman properties to that originating from the hotspots located in the gap between the Si NP and its underlying silver film with negligible contribution from hotspots located between neighboring SiNPs. Raman spectra of Si NPOM structure were recorded using a LabRam HR confocal microprobe Raman system (Horiba Jobin Yvon, France) equipped with 2400 grooves/mm holographic gratings and a thermoelectrically cooled CCD detector. The samples were excited by a 633 nm He−Ne laser through a 100× objective lens with a numerical aperture (NA) of 0.7 to focus the laser resulting in a laser spot size of around 2 μm in diameter. The Raman signal was collected in confocal mode using a 50 μm pinhole. The signal accumulation time for the CCD detector was 1 s. The Si particles enhanced Raman scattering effect is studied at the single-particle level with a 633 nm laser excitation source as shown in Figure 6a. An intense PATP Raman scattering signal is observed from single Si particle (silicon diameter, d = 190 nm) on a PATP-functionalized silver substrate (Figure 6b). The SEM image of the same particle is recorded and shown in the inset of Figure 6b, from which we could get the Si particle diameter. From the enhanced Raman scattering spectrum, distinct resonances corresponding to a collection of vibrational modes specific to PATP molecule can be readily identified.39 Without attachment of Si NPs, rather weaker Raman signal

metal substrate, leading to subwavelength optical confinement. This is confirmed by resolving the electromagnetic field enhancement in x directions (Figure 4a−c). In order to determine the field confinement in different gap sizes and particle diameters, Figure 4d shows the normalized fwhm/d (d = 2r) of the electric field in the gap at the maximum f value. The field distribution shows two lobes, and the fwhm is calculated from the edge of one lobe to the edge of the particle at half-maximum, as shown in the Figure 4b. As expected, the fields become more confined as the thickness of the gap layer is decreased and nearly approach as value of ∼0.13 (fwhm/d) for r = 45 nm and g = 1 nm, meaning that in theory around λ/40 mode confinement in one dimension can be achieved. Thus, this particle on metal system can be considered as electromagnetic energy concentrators at the nanoscale dimensions, much smaller than the light wavelength. We note that when the thickness of the gap layer is increased, the dependence of the electric field confinement on the particle diameter varies from positive to negative, which reflects that the main contributions of electric field enhancement in the gap are changed from the enhanced coupling ED to MD mode with g increasing. Furthermore, it is very instructive to compare the far-field and near-field enhancement obtained for the Si nanoparticle on metal with the one calculated for the metal counterparts. Figure 5a shows the absorption and scattering cross sections in the visible range for the case of a Si particle of radius 95 nm on Ag, an Au particle of radius 85 nm on Au and an Ag particle of 85 nm radius on Ag. It can be seen that the scattering cross section of the hybrid Si NPOM structure is much larger than that of Au and Ag counterparts. This makes Si NPOM structure a very good competitor versus metal counterparts in terms of far-field enhancing performance. More importantly, the Si absorption cross section is as much as 3 times smaller than that of Au. For the case of Ag, the absorption cross section is reasonably larger than that of Si in most of the visible range. This is because that the imaginary part of the permittivity of the Si material is much smaller than that of the other two metals as λ > 560 nm (Figure S5). Figure 5b shows the electric field enhancement in the gap for the NPOM structure with the resonance wavelength of Si, Au and Ag nanoparticle in the visible range. Unlike in the case of the far field scattering shown in Figure 5a, the metal NPOM structure offers a (as much as thrice) better near electric field enhancement as compared with Si. However, considering the stronger far-field enhancement together with the lower absorption losses, the Si NPOM structure is a promising F


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Figure 7. (a) Raman spectra of a functionalized Ag surface and a Si nanoparticle deposited on a functionalized Ag substrate for different Si nanoparticle sizes. (b) The electric field enhancement |E|/|E0| spectra of a Si nanoparticle on Ag system as a function of Si nanoparticle diameters from 100 to 220 nm in steps of 10 nm. The gap size g is 1 nm. The evolution of field enhancement peaks from the coupled ED and MD mode with different particle sizes is indicated by two blue dashed lines. The black dashed line indicates the 633 nm laser position. (c) SERS intensities for the 1148 cm−1 of PATP molecule with excitation wavelength of 633 nm compared with calculated near-field enhanced factors (EF) for different Si nanoparticle diameter. For the calculated size-dependent EF, the SiO2 gap size is 1 nm.

from the PATP monolayer on Ag film is detected. The Si NPs results in up to 2 orders of magnitude enhancement in SERS response compared to that of Ag film only substrate. To study the effect of size-tunable metal coupling silicon dipole resonance on the Raman scattering enhancing, we examined the Raman intensity of PATP molecule with various Si nanoparticles sizes (d = 40−250 nm) coupled with Ag substrate. We found that the intensity of the enhanced Raman signal (1148 cm−1, peak counts) of the PATP molecule reached a maximum at a Si nanoparticle size of 200 nm (Figure 7a). For a Si NP of 190 nm, the electric field enhancement in the gap is the highest at the wavelength of 633 nm due to the ED resonance mode coupling to metal substrate, which is clearly seen from the simulation results. In Figure 7b, we show the effect of Si NP size on the field enhancement in the gap over the visible range in the Si NPOM system. Clearly, the field enhancement peaks by the coupled ED (Fst) and MD (Snd) resonance mode shift to red with increased NP diameter. For the 633 nm laser excitation wavelength, the maximum electric field enhancement coincidentally corresponds to the ED coupled resonance mode of a silicon nanoparticle with size of 190 nm (Figure 7b). The simulated size-dependent Raman enhancement factor (EF ∼ |E/E0|4) in the particle−Ag gap correctly reproduces the maximum SERS activity for Si NP on metal configuration. As shown in Figure 7c, a good match between the experimental and simulated Raman enhancement is obtained within the excitation source wavelength of 633 nm. Generally, the SERS EF can be calculated by ratio of the SERS intensity per adsorbed molecule and the normal Raman intensity per bulk molecule. However, it is complicated to determinate the number of molecules that yield the Raman signal and their contribution to the SERS. To approximate the EF on performing a SERS experiment, various parameters need to be considered, such as the enhanced field spatial distribution, the orientation of the molecules, the experimental limitations in resolution, or the oxidation layer of the metal nanoparticles. In the Si NPOM configuration, the calculated optical near-fields show that the highest field intensity is in the nanoparticle− metal surface gap region. Since the EF in SERS scales approximately with |E|4, the calculations indicate that the maximum EF could be as high as 107 at Si particle size d = 190 nm for the excitation laser wavelength of 633 nm (Figure 7c). Nevertheless, the EF would be limited to a highly localized area

in the particle−metal gap (Figure 1f), and the average EF is ∼105 on the bottom surface of the nanoparticle. In comparison, we observe that the Raman intensity of Si nanoparticle on the silver substrate with the particle diameter of 190 nm is about 100 times higher over that on the Ag film only substrate (Figure 6b). Since most of signal enhancement comes from the bottom surface of the nanoparticle, which accounts for 1/10 of the area for the Si nanoparticles, and Ag film substrate also has a near field intensity enhancement (∼10), we estimate the average experimental EF to be ∼104, a value which is an order of magnitude lower than the calculated one. The difference of maximum EF and correspond nanoparticle size between experiment and theory may result from differences between the nanoparticle parameters used for the simulations and the deposited Si nanoparticles. The oxide layer around the silicon nanoparticle may lead to a decrease in the EF of the nanoparticle. In addition, both the deposited nanoparticle and silver surface have roughness and this effect of roughness is difficult to take into account using the FDTD method.

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CONCLUSIONS In this work, we have proposed and demonstrated a strong enhancement of the near-field and Raman scattering spectroscopy by the general electric and magnetic dipole resonance modes of a high-index Si nanoparticle interaction with the metal films. This hybrid Si nanoparticle on silver system is featuring extraordinary field enhancement (about 400-fold) and large field confinement (λ/40 in one dimension). Using numerical analysis of this geometry, we found that the field enhancement in the Si particle−metal gap, resulting from metal image charge interactions and metal film surface plasmons coupling, is strongly dependent on gap size, particle diameter and metal substrate. We further demonstrate the strong enhanced Raman spectroscopy of a single silicon nanoparticle over Ag film with a precisely sized molecular spacer layer between them. Specifically, we show with experiments and numerical analysis that the particle diameter has an importance influence on the enhanced Raman signals. Compared to the metal counterparts, the near-field enhancement of the low loss silicon NPOM system offers new approaches for other surfaceenhanced spectroscopic techniques such as surface-enhanced fluorescence, infrared spectroscopies and even Si tip enhanced Raman spectroscopy. G


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b10045.

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Numerical calculations; Figures S1−S5 (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (J.W.). *E-mail [email protected] (K.X.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported from the Instrument Developing Project of the Chinese Academy of Sciences (Nos. YZ200939 and YZ201341), the Natural Science Foundation of China (Nos. 61325022, 11435010, 11327804, 11204347, 61404159 and 61475184), the Natural Science Foundation of Jiang Province (No. BK20140394), the Ministry of Science and Technology of China (No. 2010DFA22770), National Hightech Research and Development Program of China (863 Program) (No. 2013AA031601, 2014AA032605 and 2015AA034601) and National Program on Key Basic Research Program of China (973 Program) (No. 2012CB619305).

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