Broadband Dielectric-Metal Hybrid Nanoantenna -Silicon

We develop a broadband dielectric-metal hybrid nanogap resonator ... acts as a precisely-length-controlled nanogap as well as a light emitter to monit...
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Broadband Dielectric-Metal Hybrid Nanoantenna -Silicon Nanoparticle on Mirror Hiroshi Sugimoto, and Minoru Fujii ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b01461 • Publication Date (Web): 23 Mar 2018 Downloaded from http://pubs.acs.org on March 23, 2018

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Broadband Dielectric-Metal Hybrid Nanoantenna Silicon Nanoparticle on Mirror Hiroshi Sugimoto*, Minoru Fujii Department of Electrical and Electronic Engineering, Graduate School of Engineering, Kobe University, Rokkodai, Nada, Kobe 657-8501, Japan KEYWORDS Silicon nanoparticles, Mie resonance, Nanoresonator, Nanoantenna, Purcell effect

ABSTRACT

We develop a broadband dielectric-metal hybrid nanogap resonator composed of a silicon nanoparticle (Si NP) and gold (Au) flat surface. We fabricate the nanogap resonator by depositing a monolayer of colloidal quantum dots (QDs) (~2.8 nm in diameter) on Au surface followed by dropping a diluted colloidal solution of Si NPs (~150 nm in diameter). The QDs monolayer acts as a precisely-length-controlled nanogap as well as a light emitter to monitor the radiative properties of the nanogap resonator. We investigate the light scattering properties of single nanogap resonators experimentally and theoretically, and find that the coupling of the Mie resonance of Si NPs with Au surface effectively confines electromagnetic field into the nanogap in a wider wavelength range than an all-metal nanogap resonator with a comparable size. Furthermore, we show that resonance wavelength of the hybrid nanogap resonator is less

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sensitive to the gap length than that of all-metal one. We demonstrate that the broadband hybrid nanogap resonator enhances photoluminescence of a QDs monolayer integrated in the nanogap by a factor of 786.

Localized surface plasmon resonances (LSPR) in a noble metal nanostructure have been recognized as a powerful tool to confine and control electromagnetic waves in subwavelength space.1 A variety of plasmonic nanoresonators have been developed to control light matter interactions such as Raman scattering,2,3 fluorescence4,5 and nonlinear optical responses.6,7 Among them, a nanogap resonator, which consists of a couple of noble metal nanostructures separated by a sub-10 nm spacer and supports the gap plasmon modes, has superior properties as a nanoantenna.8 The gap plasmon has an extremely small mode volume and a large field enhancement, and a resultant very large Purcell factor. The large Purcell factor allows us to realize ultrafast single photon emission in a nanogap10 and strong-coupling between an emitter and the resonant mode at room temperature.11 Among many types of nanogap structures, the metal nanoparticle on mirror (MNPoM) structure composed of a metal nanoparticle and a metal film beneath it has been widely studied.12–14 The structure can be fabricated entirely by a bottomup process and is advantageous for fabricating a sub-10 nm gap with high accuracy and high reproducibility.15,16 Formation of a sub-10 nm gap with the accuracy of ~1 nm is still challenging in a typical nanofabrication method using electron beam lithography. In any types of plasmonic nanoresonators, an inherent drawback is the loss in noble metal. Due to the absorption losses, the Purcell enhancement of fluorescence by a plasmonic nanoresonator does not fully contribute to the intensity enhancement. Furthermore, local heating by the absorption17 often limits the application in bio-sensing and imaging. As an alternative of a

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plasmonic nanoresonator, a high-refractive-index and low-extinction-coefficient dielectric nanoparticle (NP), which supports the Mie resonance in the visible and near infrared (NIR) ranges, has recently been emerging as a new type of nanoantenna.18 The Mie resonator has several characteristic features such as the enhancement of a magnetic field and the directional scattering by the excitation of complex multipole modes.19–21 On the other hand, the field confinement is limited and the expected Purcell enhancement is much smaller than that of a plasmonic counterpart with a comparable size. A strategy to incorporate the benefits of the plasmonic and dielectric nanostructures is the formation of a dielectric-metal hybrid nanoresonator.22–29 As a hybrid nanoresonator, silicon (Si) NP on a metal thin film structure (SiNPoM) has been proposed.22,24,26–28 Crystalline Si has a large real part and a small imaginary part of the permittivity in a wide wavelength range covering the whole visible to NIR ranges due to the indirect nature of the energy band structure, and thus is a very suitable material for a dielectric Mie resonator. Xifré-Pérez et al.22 demonstrated for relatively large Si NP (~500 nm in diameter) that the presence of a metal mirror near a Si NP turns the electric dipole (ED) Mie resonance into a magnetic-like mode and strongly enhances the scattering cross-section. Miroshnichenko et al.24 developed an analytical method to study the optical response of a SiNPoM and demonstrated the existence of several novel substrate-induced magnetoelectic coupling between the ED and the magnetic dipole (MD) Mie resonances. Sinev et al.26 and Kuo et al.27 studied the polarization-resolved light scattering properties of Si NPs directly placed on a metal film and demonstrated the formation of electric and magnetic hot spots at the contact point. Very recently, Yang et al.28 theoretically predicted that a SiNPoM nanoantenna with a very thin dielectric spacer has a higher antenna efficiency and a Purcell factor than an all-metal nanoantenna.

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All these recent studies indicate that a SiNPoM is a very promising nanoantenna for the enhancement of light-matter interactions. However, experimental verification of the predicted effects has not been fully successful. For example, high Purcell factor of a SiNPoM nanoantenna has not been experimentally demonstrated. The difficulty arises mainly from that in producing high-quality Si NPs.30 For the formation of an efficient SiNPoM nanoantenna working in the visible range, an isolated Si NP 100-200 nm in diameter with very high crystallinity is indispensable. Unfortunately, in contrast to Au and Ag NPs,12–14 a colloidal solution of high quality size-controlled Si NPs is not commercially available. Usually, Si NPs are prepared in house by femtosecond laser-ablation and transferred to a metal-coated substrate by nanomanipulations in scanning electron microscope.26 The complexity of the process is an obstacle for the development of a SiNPoM nanoantenna. In this work, we report the formation of a SiNPoM by a solution-based process using a colloidal dispersion of almost perfectly-spherical and size-controlled crystalline Si NPs developed recently in our group.31 The formation of an almost ideal SiNPoM enables us to systematically study the light scattering properties of a single SiNPoM by precisely controlling the NP size and the spacer thickness. Furthermore, development of the solution process for the formation of a SiNPoM allows us to compare the optical response with that of an all-metal nanogap structure, i.e., Au nanoparticle on metal film structure (AuNPoM), on an equal basis. From the comparison, we experimentally demonstrate the advantages of a SiNPoM such as the broadband resonance and the insensitiveness of the resonance wavelength to the gap length. Finally, we integrate a monolayer of QDs in the gap of a SiNPoM and demonstrate the Purcell enhanced fluorescence in a broad spectral range.

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Results and Discussion Figure 1(a) shows a schematic illustration of a SiNPoM studied in this work. We calculate the scattering spectra and the electric field distribution of the structure using the MNPBEM code,32,33 which is based on the rigorous boundary element method. In the simulation, a spherical Si NP with the diameter of 140 nm is placed on a Au substrate with an infinite thickness. The thickness and the refractive index (n) of a dielectric spacer is 4 nm and 1.6, respectively. The structure is illuminated by the p-polarized light with the incident angle of 65˚. Figure 1(b) shows the scattering spectra of a Si NP on silica (n = 1.45) and a SiNPoM. The peaks at 475 and 570 nm in the spectrum of a Si NP on silica are the ED and MD Mie resonances, respectively. Although the p-polarized oblique incidence excites ED modes parallel (ED//) and perpendicular (ED⊥) to the substrate, they are not distinguished on a low-refractive-index silica substrate. On the other hand, the MD mode is purely parallel to the substrate (MD//) and that perpendicular to the substrate (MD⊥) is not excited in the p-polarization (see schematic illustration in Figure S1 in Supporting Information).26,27 In Figure 1(b), the spectral shape is significantly modified in a SiNPoM. Figure 1(c)-(e) shows the electric field (|E|/|E0|) distribution in a logarithmic scale for the SiNPoM at 475, 570 and 650 nm (indicated by allows in Figure 1(b)), respectively. The field distribution in Figure 1(c) (475 nm) is aligned to the direction of the incident electric field, which is a clear indication of the excitation of the ED mode.26,27 At 570 nm (Figure 1(d)), the field is localized at the inner surface of the sphere. This is due to the circular current along the surface of the dielectric sphere and is the signature of the excitation of the MD mode. The field distribution at 650 nm is more complicated, suggesting the overlap of multiple modes. We study the incident angle dependence

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of the scattering spectrum and found that the mode becomes strong with increasing the angle (see Figure S2 in Supporting Information). This suggests that the main component of the long wavelength mode is the ED⊥ mode coupled to the Au substrate. Among the three modes, the field enhancement in the gap is the largest in the long wavelength mode at 650 nm. Figure 1(f) shows the field distribution around the gap at 650 nm. The maximum field enhancement exceeds 50. The large field enhancement is due to in-phase coupling of the ED⊥ mode with the image dipole. Figure 1(g) plots the field enhancement averaged over the gap region (z = 0 to -4 nm) as a function of the position x. The maximum zaveraged field enhancement reaches more than 30. This value is comparable to that of a Si NP dimer (diameter: 150 nm, gap: 4 nm),34 and in between that of a AuNPoM with a 100 nm Au NP (~40) (see Figure S3 in Supporting Information) and that with a 80 nm Au NP (~20).14

Figure 1. (a) Schematic illustration of SiNPoM used for BEM simulation. (b) Calculated scattering cross-sections of Si NP on silica (black line) and SiNPoM (red line). (c-e) Electric field (|E|/|E0|) distributions (logarithmic scale) at different excitation wavelengths. (f) Enlarged

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image of (e) (linear scale). (g) Field enhancement factor averaged over the gap region (z = 0 to 4) as a function of position x.

In order to produce the structure shown in Figure 1(a), we use a colloidal solution of Si NPs. Figure 2(a) shows the transmission electron microscope (TEM) image of Si NPs used in this work. We can see spherical Si NPs.31 The size of Si NPs can be controlled in a very wide range (2-250 nm in diameter).31 The lattice fringe in the high resolution TEM image in Figure 2(b) corresponds to {111} planes of crystalline Si. Figure 2(c) shows a TEM image of Au NPs used for the formation of AuNPoMs. The Au NPs were synthesized by seeded growth strategy.35 The size distributions of Si and Au NPs estimated from TEM images are shown in Figure 2(d). The average diameters are 130.5 and 101.5 nm for Si and Au NPs, respectively. The extinction spectra of the colloidal solutions of Si and Au NPs are shown in Figure 2(e). They exhibit the resonance almost at the same wavelength, although the origin is different. The peak of Au NPs arises from a dipolar LSPR, while that of Si NPs is due to the ED and MD Mie resonances. The broadening of the extinction peak of Si NPs is partly due to the larger size distribution, and partly due to the presence of the ED and MD modes in the spectral range. The two modes can clearly be distinguished in the scattering spectrum of a single Si NP on a silica substrate (Figure S4(a) in Supporting Information). The spectrum of a single Si NP is in good agreement with that of the simulation. In Supporting Information (Figure S4(b)), we summarize calculated extinction spectra for Si NPs 100 to 150 nm in diameters. All these spectra are within the broad extinction band in Figure 2(e). In the formation of a NP on a metal thin film structure (NPoM), precise control of the gap is crucial. In this work, we use a monolayer of colloidal Si QDs (2.8 nm in diameter) as a spacer

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and as a phosphor to monitor the performance of the nanogap resonator as schematically shown in Figure 2(f).14 The structural and optical property of the colloidal solution of the Si QDs is summarized in Supporting Information (Figure S5). The advantages of Si QDs as a spacer/phosphor in a NPoM is the high chemical stablity36 and the very broad and stable luminescence.37,38 In order to produce a NPoM, we first form a 6-amino-1-hexanethiol hydrochloride (6-AHT) self-assembled monolayer (SAM) on a vacuum evaporated Au film (200 nm in thickness). A monolayer of Si QDs is then deposited by drop-casting the colloidal solution.14,38 The formation of a monolayer is confirmed by spectroscopic ellipsometry (Figure S6 in Supporting Information). The total thickness of the dielectric spacer (SAM and Si QDs monolayer) is about 4 nm. Finally, a diluted solution (5 µL, 1 × 1010 NPs/mL) were dropped on the Si QD monolayer and dried. Optical characterization of single Si spheres A custom-built inverted optical microscope was used for dark-field backward scattering and PL measurements of single NPoMs. For the scattering measurement, a sample was placed facedown onto the stage and illuminated by a halogen lamp through a dark-field objective lens (100×, NA = 0.9). The scattered light was collected by the same objective lens. To measure the spectra, the scattered light was transferred to the entrance slit of a monochromator (SpectraPro-300i, Acton Research Corp.) and detected by a liquid-N2 cooled CCD (Princeton Instruments). For the PL measurement, a NPoM was excited by 405 nm light fro m a semiconductor laser via the objective lens. The power density at the sample was reduce to ~2 W/cm2 to avoid local heating and degradation of Si QDs. For polarization resolved scattering measurements, the sample was illuminated by collimated white light through the polarizer with an oblique incidence (65˚). Scattering light from individual NPoMs was collected by a reflective objective lens (40×, NA = 0.5) and analyzed by a monochromator (iHR550, Horiba) equipped with a liquid-N2 cooled CCD (Spectrum OneCCD3000).

ASSOCIATED CONTENT Supporting Information. The following file is available free of charge. Schematics, detailed description of estimation of spacer thickness and quantum efficiency, Supplementary Figures of additional simulation and experimental data (PDF)

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AUTHOR INFORMATION Corresponding Author *[email protected] ACKNOWLEDGMENT This work was partly supported by the 2015 JST Visegrad Group (V4)−Japan Joint Research Project on Advanced Materials and JSPS KAKENHI Grant Number 16H03828.

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