Molecular Sensing and Color Manipulation Based on Dimension

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C: Plasmonics, Optical Materials, and Hard Matter

Molecular Sensing and Color Manipulation Based on Dimensioncontrolled Plasmon-enhanced Silicon Nanotube SERS Substrates Yi-Chen Lai, Li-Wei Nien, Hsin-Chia Ho, Jia-Han Li, and Chun-Hway Hsueh J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00376 • Publication Date (Web): 04 Apr 2018 Downloaded from http://pubs.acs.org on April 4, 2018

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The Journal of Physical Chemistry

Figure 1. Schematic drawing showing the fabrication process of the Au-SiNT periodic nanostructure.

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Figure 2. Simulated reflectance spectra with different D/d ratios showing three pronounced resonance modes, M1, M2 and M3, indicated by dashed lines. 73x55mm (300 x 300 DPI)


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Figure 3. The electric field distribution on the surface of the suspended gold nanoring for the peak resonance of (a) M1 mode at 810 nm for D/d = 2.0, (b) M2 mode at 668 nm for D/d = 2.5 and (c) M3 mode at 566 nm for D/d = 2.5. The corresponding surface charge distributions of (a), (b) and (c) are shown in (d), (e) and (f). The intensity of electric field is given by logarithmic scale bar. 105x72mm (300 x 300 DPI)



Figure 4. SEM images of Au-SiNTs with D/d ratio of 1.99 under (a) 36000x and (b) 18000x. The scale bars are 1 µm in (a) and 2 µm in (b). 80x139mm (300 x 300 DPI)


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Figure 5. The (a) dark-field image and (b) the corresponding color gamut for Au-SiNTs with different D/d ratios. (c) Measured dark-field scattering spectra of fabricated Au-SiNTs with different D/d ratios. The scattering intensity is normalized to (0, 1). 160x124mm (300 x 300 DPI)



Figure 6. The Raman spectra of p-ATP self-assembly molecule with the excitation wavelengths of (a) 533 nm and (b) 633 nm for Au-SiNTs with different D/d ratios, and comparison of the intensity of 1580 cm–1 Stokes-shift peak between measured Raman signals and the calculated mean EF for the excitation wavelengths of (c) 533 nm and (d) 633 nm. 160x118mm (300 x 300 DPI)


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TOC Graphic 80x39mm (300 x 300 DPI)



Molecular Sensing and Color Manipulation Based on Dimension-controlled Plasmon-enhanced Silicon Nanotube SERS Substrates Yi-Chen Lai†, Li-Wei Nien†, Hsin-Chia Ho†, Jia-Han Li‡, and Chun-Hway Hsueh†,* Department of Materials Science and Engineering, ‡Department of Engineering Science and

†

Ocean Engineering, National Taiwan University, Taipei 10617, Taiwan

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ABSTRACT The system of suspended gold nanoring on silicon nanotube substrate with enhanced light harvesting and electromagnetic field enhancements was proposed in the present study. The effects of outer/inner diameter (D/d) ratio of the ring and tube on the plasmonic behavior were studied by systemic simulations and experiments. In simulations, the high order quadrupole-dipole mode was also excited in addition to the typical dipole-dipole mode, and the resonant configurations were characterized by both electric field profile and resonant surface charge distribution. Experimentally, both dark-field and Raman microscopies were conducted to examine the plasmonic behavior. The plasmon-enhanced scattering could be controlled by tailoring the D/d ratio, and the dark-field image colors could be manipulated covering the visible range. Raman spectra using two excitation wavelengths were also recorded and showed good agreement with calculated enhancement factor which, in turn, provided the evidence of the evolution of resonance mode and denoted our designed structure as a potential candidate for surface-enhanced Raman scattering applications.

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INTRODUCTION Plasmonics has attracted much attention in optoelectronic materials for the electromagnetic field enhancements and light harvesting in a nanoscaled volume.1 The plasmonic behavior is ascribed to the interaction between incident electromagnetic wave and subwavelength metallic nanostructure and the phenomenon is the so-called localized surface plasmon resonance (LSPR),2 which is highly dependent 3–8

on the morphology

and the surrounding environment9,10 of the metallic nanostructures. One of the

most important applications in plasmonic fields is surface-enhanced Raman scattering (SERS), which offers an effective technique to improve the efficiency of Raman detection in identifying the molecules, oxides and chemical reactions.6,11–14 Generally, SERS activity is dominated by electromagnetic field enhancements;15,16 i.e., the plasmonic behavior of the metallic nanostructure. Among the metallic nanostructures, nanoring is particularly attractive due to the exceptional electromagnetic field enhancements resulting from the plasmon coupling effect17–19 and the higher tunability in LSPR behavior by changing the outer/inner diameter (D/d) ratio.18,20 The LSPR behavior of nanoring was elucidated by the hybridization model where the LSPR was considered as the coupling interaction between nanodisk and nanocavity structures19,21,22 and it could be controlled by the D/d ratio of the nanoring.18,21,23 In addition to the metallic nanostructures, textured silicon substrates24–27 also show distinguished characteristics in light harvesting25,28,29 and provide larger surface area to allow greater numbers of molecules to be adsorbed.30 Recently, several studies have combined the metallic nanostructures with

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the textured silicon substrate for SERS applications, and it presented extraordinary Raman signal enhancements in terms of high sensitivity and good reproducibility.26,31–38 In the present work, we proposed a system of suspended gold nanoring on silicon nanotube substrate (Au-SiNT) to combine the advantages of nanoring and textured silicon substrate. Both the experimental and theoretical investigations of Au-SiNT were presented. In simulations, the D/d ratio of Au-SiNT was systematically varied to clarify the dependence of enhancement mechanism on the D/d ratio. Experimentally, Au-SiNT was fabricated by electron beam lithography and reactive ion etching. Depending on the D/d ratio of Au-SiNT, vivid color could be manipulated and observed clearly under dark-field microscopy. Finally, the Raman signals were recorded to examine the evolution of the LSPR with the D/d ratio.

METHODS Finite-Difference Time-Domain (FDTD) Simulations The far-field reflectance and near-field electric field enhancements were simulated using Lumerical FDTD Solutions.39 The schematic representation of Au-SiNT is shown in Figure S1. Each designed Au-SiNT consisted of a silicon nanotube with 380 nm height (labeled as h in Figure S1) and 30 nm-thick Au films on both the tube and the silicon substrate. The interspacing distance G was fixed at 500 nm between adjacent tubes. The inner diameter d was fixed at 280 nm, and the outer diameter D was the only variant in this work, such that the D/d ratios ranging from 1.5 to 5.0 in the interval of 0.05 were set to simulate the effects of D/d on the resonance behaviors. A plane wave of 400−850 nm

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wavelength polarized along the x−direction was illuminated from above in the negative z−direction as depicted in Figure S1. To ensure the convergence of the simulation results, various simulation z−domains were tested, and the adopted simulation region was 4000 nm with 1590 nm above the Au-SiNTs surface and 2000 nm into the silicon substrate. The perfectly matched layers were used at both the top and the bottom of the z−domain to absorb waves leaving the simulation region. Dielectric constants of Au and Si used in simulations were taken from Johnson and Christy’s paper40 and Palik’s handbook,41 respectively. The mesh size of 4 nm x 4 nm x 2 nm was adopted in the whole simulation region. The method of SERS enhancement factor calculation could be found elsewhere.42,43 The electric field intensity enhancement of each mesh cell (EFIEk,l) was calculated as EFIEk,l = (|Ek,l|/|E0|)2, where E0 is electric field of the illumination wave and Ek,l is the electric field on the top surface of gold nanoring corresponding to the mesh cell with index (k,l). To compare with experimental results of Raman spectra, the enhancement factor (EF) should be calculated, and it was calculated by multiplying the EFIEk,l of the incident laser frequency and the Stokes-shift Raman scattering frequency15 and averaging over the whole area, such that EF = ∑k∑l [EFIEk,l (ω)][EFIEk,l (ω−ωv)]/kl, where ω is the frequency of incident laser and (ω − ωv) represents the frequency of Stokes-shift scattering wave. Excitation lasers with wavelengths of 533 nm and 633 nm were used in the present study to record the Raman signals, and the Raman scattering wavelengths with Stokes shift of 1580 cm−1 corresponded to electric field enhancements at 581 nm and 703 nm, respectively.

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Fabrication of Au-SiNT The Au-SiNT structure was fabricated on as-cut (111) p-type silicon wafer. Prior to processing, silicon substrate was immersed in acetone and ethanol sequentially and cleaned by ultrasonic oscillator for 5 min each, and then rinsed with deionized water for 5 min to remove the contamination and dust from the substrate. The fabrication process is shown schematically in Figure 1. For high-quality fabrication, electron beam lithography was used to ensure the precise control of dimension. After wafer cleaning, a layer of 300 nm ZEP520A positive electroresist produced by ZEON Corporation was spin-coated on the silicon substrate and soft-baked at 180 °C for 2 min to stabilize the adhesion between ZEP520A and the silicon substrate. Different D/d ratios of Au-SiNTs were patterned by an electron beam writer (ELS7000 ELIONIX) and subsequently treated in N50 developer for 2 min and isopropyl alcohol remover for 90 s. Then, 30 nm-thick chromium was deposited as the hard-mask using the electron beam evaporator and the sample was immersed in acetone to lift-off excess metal to complete the pattern transfer. Etching process was performed using inductively coupled plasma etching system (EIS700 ELIONIX). For the vertical nanotube structure, a pseudo Bosch process was adopted in this work. First, the etching process was performed using 9 sccm C4F8 and 1 sccm SF6 mixed gas under 500 W RF power and 200 W DC power for 15 s, which would form a passivation layer to prevent side-wall etching. Second, 5 sccm C4F8 and 5 sccm SF6 mixed gas under the same power supply was introduced to launch the etching process for 13 s. This alternate passivation-etching two-step process would result in a vertical and flat silicon nanotube substrate, and the etching depth as

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a function of the number of etching loops is shown in Figure S2. After the etching process, the sample was immersed in chromium etchant (mixture solution of HClO4 and Ce(NH4)2(NO3)6) to remove residual chromium hard-mask. Finally, a 30 nm-thick Au was deposited to form gold nanorings on the silicon nanotubes and the flat gold film on the flat portion of Si substrate. Dark-field Optical Property Measurements The dark-field scattering spectra of Au-SiNTs were collected using Axio Imager microscope (A2m Carl Zeiss) integrated with a VIS-NIR spectrometer (USB2000 Ocean Optics) under dark-field mode equipped with 5x, 0.13 NA microscope objective lens and a halogen lamp. The images were obtained using the same equipment. Raman Spectroscopy Measurements A self-assembly molecule p-aminothiophenol (p-ATP) was chosen as the analyte in the present work. The fabricated Au-SiNT was immersed in 10–3 M p-ATP solution for 24 h and subsequently rinsed in a 9:1 mixture solution of deionized water and ethanol to remove unbound p-ATP molecules to ensure monolayer adhesion. The Raman spectra were acquired by imaging spectrometer (iHR550 HORIBA) equipped with thermoelectric cooling charge-coupled device of 1024 x 256 pixels. Two incident lasers with excitation wavelengths of 533 nm and 633 nm were used, respectively, to excite Raman signal and the spot size of incident beam was around 1.5 µm under 50x microscope objective lens. The operating current was 0.9 A for 532 nm laser and the output power was 17 mW/cm2 for 633 nm laser. Measurements were performed 10 times at different positions for each Au-SiNT to examine

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the deviation and ensure the reproducibility.

Figure 1. Schematic drawing showing the fabrication process of the Au-SiNT periodic nanostructure.

RESULTS AND DISSCUSION Effects of D/d Ratio on Plasmonic Behaviors Simulations of optical properties were conducted to clarify the plasmonic behaviors of Au-SiNTs with D/d ratios from 1.5 to 5.0. Because excitation of LSPR would induce strong extinction (scattering and absorption), it provides a method for far-field microscopy to image the resonance frequency through the position of reflectance minimum.7,44 Thus, the reflectance spectra were simulated and are shown in Figure 2. The reflectance minima at the wavelengths less than ~500 nm result from the interband transition of Au.45 Several resonance modes were revealed in our simulations; however, only

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three pronounced resonance modes were recorded in our measurements (Figure S3), which are indicated by dashed lines in Figure 2; i.e., a weak resonance mode M1 at D/d ratios below 2.0 with wavelengths of 600−850 nm, and two strong resonance modes, M2 and M3, at D/d ratio below 3.0 with the wavelengths of 400−750 nm. It could be observed that resonance wavelengths of all the three modes red-shifted continuously with the increasing D/d ratio and it could be related to the typical evolution of LSPR.40

Figure 2. Simulated reflectance spectra with different D/d ratios showing three pronounced resonance modes, M1, M2 and M3, indicated by dashed lines.

To verify the coupling characteristics of plasmonic behaviors of these three modes, the electric field profile on the surface of suspended gold nanoring was monitored. Figure 3a shows the electric field profile of M1 resonance peak with D/d ratio of 2.0 at 810 nm, and Figure 3b and c display the electric field profiles of M2 and M3 resonance peaks with D/d ratio of 2.5 at 668 and 566 nm, respectively. Obviously, the electric field maxima in both Figure 3a (M1 mode) and b (M2 mode) are

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located on both the exterior and the interior surfaces of Au-SiNTs along the polarization direction of the incident light, and they are related to the dipole-like plasmon resonance mode. In contrast, in addition to the electric field maxima inside the interior surface, Figure 3c (M3 mode) displays four maxima on the exterior surface, which indicates the existence of quadrupole-like plasmon resonance mode. The resonant surface charge distributions on suspended gold nanorings were simulated and are shown in Figure 3d, e and f corresponding, respectively, to the cases in Figure 3a, b and c. Dipole-like resonance modes M1 and M2 exhibit similar but inverse symmetry of surface charge distribution, and correspond to parallel and antiparallel dipole-dipole moments, respectively. Meanwhile, quadrupole-like resonance mode M3 shows a single dipole moment on the interior surface, a quadrupole moment on the exterior surface, and the antiparallel direction between these two moments. On the basis of hybridization theory proposed by Pordan et al.,19 the resonant surface charges of nanoring result from the coupling between nanodisk and nanocavity. The hybridization of the nanodisk and nanocavity results in the energy splitting and generates the “bonding” and “anti-bonding” modes, where the bonding mode is characterized as the lower energy mode and the antibonding mode is referred to the higher energy mode in contrast. In the present case, the energy separation is determined by the D/d ratio. Thus, Figure 3d and e provide the evidence that resonance modes M1 and M2 could be referred, respectively, to the bonding type and the antibonding type of dipole-dipole mode. The resonant surface charge distribution in Figure 3f indicates that the resonance mode M3 is related to

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antibonding type of quadrupole-dipole mode, which could only be excited in the systems with larger sizes.

Figure 3. The electric field distribution on the surface of the suspended gold nanoring for the peak resonance of (a) M1 mode at 810 nm for D/d = 2.0, (b) M2 mode at 668 nm for D/d = 2.5 and (c) M3 mode at 566 nm for D/d = 2.5. The corresponding surface charge distributions of (a), (b) and (c) are shown in (d), (e) and (f). The intensity of electric field is given by logarithmic scale bar.

The role of silicon nanotube is not only to elevate the gold nanoring but also to enhance the light harvesting. Previous studies demonstrated that the suspended structure could increase the

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electromagnetic field enhancements due to the reduced substrate effects,6,46,47 where the leaky field could be reduced and the hot-spot region could be increased. Also, the vertically oriented silicon nanotube would couple to the incident wave and induce Mie scattering, which generates strong electromagnetic field enhancements inside and/or outside the silicon nanotube to enhance light harvesting.29,48,49 Similar to previous studies,6 the light harvesting in silicon nanotube was highly enhanced compared to the flat silicon substrate (Figure S4) and the strong electric field enhancements were induced on the sidewall of silicon nanotube (Figure S5). Surface Morphology Seven arrays of Au-SiNTs with D/d ratios of 1.99, 2.51, 2.76, 3.59, 3.97, 4.28 and 4.92 were fabricated. The height, h, of silicon nanotube in each array was ~380 nm, the thickness of suspended gold nanoring and base film was 30 nm, and the interspacing distance G between neighboring tubes was ~500 nm. The dimensions of inner and outer diameters of each Au-SiNT were measured by the top-view SEM images (Figure S6) and are listed in Table S1. Figure 4a and b show, respectively, the enlarged and reduced SEM images of Au-SiNT with D/d ratio of 1.99. Au-SiNTs with other D/d ratios are shown in Figure S7. However, while the Au-SiNT with a uniform height could be fabricated successfully, some gold beads were deposited onto the side-wall during the evaporation process.

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Figure 4. SEM images of Au-SiNTs with D/d ratio of 1.99 under (a) 36000x and (b) 18000x. The scale bars are 1 µm in (a) and 2 µm in (b).

Dark-field Spectroscopic Analysis Dark-field spectroscopy was used to examine the dependence of plasmonic behaviors on the D/d ratio. Under the dark-field mode optical microscopy, only the scattered light can be collected and the color is generated by the scattered light. Since the LSPR would enhance the scattering intensity, the scattering peak in the dark-field spectrum could be used to verify the LSPR behavior. Figure 5a and b

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show, respectively, the dark-field image and the corresponding color gamut of Au-SiNTs with different D/d ratios. Interestingly, by systematically manipulating the D/d ratio, Au-SiNTs could produce distinct color covering the visible range as shown in Figure 5a due to the different plasmonic behaviors. The color gamut shown in Figure 5b was obtained by calculating the CIE 1931 chromatic coordinates of the scattering spectra as shown in Figure 5c. At D/d ratio of 1.99, M1 and M2 modes exist in the red and purple region, respectively, and M2 mode shows the stronger intensity than M1 modes and thus produces a near-purple color. As the D/d ratio increases, the pronounced resonance modes M1, M2 and M3 red-shift; i.e., the scattering peaks red-shift, and the color of dark-field image turns from purple to red. However, as the D/d ratio reaches above 3.59, the pronounced resonance modes (i.e. scattering peak) would red-shift beyond the visible range and become invisible. Thus, the scattering induced by the structure below the wavelength of 500 nm dominates and the color turns back to purple. Our experimental results demonstrated the relationship between plasmonic behaviors and optical properties, and that the dark-field color of Au-SiNT could be controlled by tailoring the D/d ratio.

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Figure 5. The (a) dark-field image and (b) the corresponding color gamut for Au-SiNTs with different D/d ratios. (c) Measured dark-field scattering spectra of fabricated Au-SiNTs with different D/d ratios. The scattering intensity is normalized to (0, 1).

Effects of D/d Ratio on Raman Spectroscopy Finally, SERS measurements were performed to examine the effects of D/d ratio on electric field enhancements. Based on our optical measurements and simulation results, the pronounced resonance modes would red-shift from 500 nm to 700 nm with the increasing D/d ratio. Thus, the Raman spectra were obtained using two lasers with excitation wavelengths of 533 nm and 633 nm, respectively, to

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cover the wavelength range of the resonance modes. The representative Raman spectra recorded for 10–3 M p-ATP self-assembled molecules with 533 nm and 633 nm excitation wavelengths are shown in Figure 6a and b, respectively, at different D/d ratios. The dominant Raman vibrational mode of p-ATP molecule at the characteristic Stokes shift of 1580 cm–1 was recorded as a function of D/d and is shown in Figure 6c and d, respectively, for 533 and 633 nm excitation wavelengths. It has been known that Raman signal intensity is proportional to enhancement factor (EF). Thus, the mean EF was calculated (defined in METHODS) and is plotted as a function of D/d ratio in Figure 6c and d to compare with the measured Raman signals. It could be observed that the trend of the simulation results was in good agreement with experimental measurements, and the Raman signals could reflect the near-field electric field enhancements.

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Figure 6. The Raman spectra of p-ATP self-assembly molecule with the excitation wavelengths of (a) 533 nm and (b) 633 nm for Au-SiNTs with different D/d ratios, and comparison of the intensity of 1580 cm–1 Stokes-shift peak between measured Raman signals and the calculated mean EF for the excitation wavelengths of (c) 533 nm and (d) 633 nm.

Based on the measured Raman spectra and simulated mean EF for the 533 nm excitation wavelength, strong intensity was shown for D/d < 2.5 and it decreased drastically with the increasing D/d ratio, which is due to the red-shift of resonance modes. The pronounced resonance modes M1, M2 and M3 exist around 500−600 nm with the D/d ratio below 2.5 as shown in Figure 2, which would

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effectively enhance the measured Raman signal. As the D/d ratio increases over 2.5, the pronounced resonance modes red-shift to lower frequencies and result in weak Raman signals. In contrast, the strong Raman signal and enhancements could be observed in a wider range of D/d ratios for 633 nm excitation wavelength. In this case, the pronounced resonance modes M1, M2 and M3 exist in the range of 600−700 nm with the D/d ratio below 3.0 as shown in Figure 2. For D/d ratios around 2.0, the M1 mode red-shifts to lower frequencies with increasing D/d, and no pronounced resonance modes exist, which could not effectively enhance the Raman signal and hence result in lower intensity. On the contrary, the strong enhancement around D/d ratio of 2.5 is attributed to the M2 and M3 modes. As the D/d ratio increases to 2.76, this condition is exactly located in the spectral gap between M2 and M3 modes, which would lead to lower enhancements and lower intensity of Raman signal. For higher D/d ratios, the pronounced resonance modes red-shift to lower frequencies and result in lower Raman signals. The enhanced Raman signal at D/d ratio of 3.97 may result from the high-order resonance mode, but it is relatively weak due to the weak electric field enhancements compared to the pronounced resonance modes. Therefore, the plasmonic behaviors of Au-SiNTs with different D/d ratios examined by the Raman spectra of 533 nm and 633 nm excitation wavelengths verified the existence and red-shift of pronounced resonance modes M1, M2 and M3. The results showed the Raman signal enhancements and the controllability of our system.

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CONCLUSIONS In conclusion, through systematic simulations of Au-SiNTs with different D/d ratios, the optical properties and electromagnetic field enhancements were studied. Three pronounced resonance modes, bonding type dipole-dipole mode M1, antibonding type dipole-dipole mode M2 and antibonding type quadrupole-dipole mode M3, were excited. These three modes were different in the resonant surface charge distribution on the suspended nanoring surface due to the hybridization effects between nanodisk and nanocavity. In addition to the surface plasmon-induced enhancements, silicon nanotube also played an important role to induce Mie scattering and resulted in the enhanced light harvesting. The experimental results demonstrated that the color of dark-field image could be manipulated by varying the D/d ratio through changing the plasmonic behavior of the Au-SiNT. Furthermore, the evolution of the plasmonic behavior of Au-SiNTs could be traced by Raman spectroscopy, and the spectra showed good consistency with calculated results. This work offers an effective way to control the plasmonic behavior and demonstrates a promising nanostructure for SERS applications.

ASSOCIATED CONTENT Supporting Information. Schematic illustration of the Au-SiNT periodic nanostructure, the etching height as a function of the number of etching loops, measured reflectance of fabricated Au-SiNTs with different D/d ratios, simulated reflectance of planar Si substrate and simulated/measured reflectances of SiNT, electric field distribution on the longitudinal cross-section of Au-SiNTs, table containing the list

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of D and d of seven sizes of fabricated Au-SiNTs, and top/tilted view SEM images of Au-SiNTs with different D/d ratios.

AUTOHR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The work was supported by the Ministry of Science and Technology, Taiwan under Contract no. MOST 103-2221-E-002-076-MY3. We are also grateful to Nano-Electro-Mechanical-Systems Research Center at National Taiwan University for providing essential experimental instruments.

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Molecular Sensing and Color Manipulation Based on Dimension

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