Helical-Structure-Dependent Surface-Enhanced Raman Spectroscopy

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Helical-Structure-Dependent Surface-Enhanced Raman Spectroscopy Enhancement in Gold Nanohelices Jehyeok Ryu, Seung-Hoon Lee, Yuan-Han Lee, Yu-Hsu Chang, and Jae-Won Jang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00187 • Publication Date (Web): 11 Feb 2019 Downloaded from http://pubs.acs.org on February 17, 2019

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The Journal of Physical Chemistry

Journal of Physical Chemistry C

Helical-Structure-Dependent Surface-Enhanced Raman Spectroscopy Enhancement in Gold Nanohelices Jehyeok Ryu†, Seung-Hoon Lee†, Yuan-Han Lee‡, Yu-Hsu Chang,*, ‡ and Jae-Won Jang*,† †Department

of Physics, Pukyong National University, 45 Yongso-ro, Nam-gu, Busan 48513, Republic of KOREA ‡Department of Materials and Mineral Resources Engineering, Institute of Mineral Resources Engineering, National Taipei University of Technology, 1, Sec. 3, Zhongxiao E. Rd., Taipei 10608, Taiwan R.O.C.

*E-mail: (J.-W.J) [email protected]; [email protected] (Y.-H.C) [email protected] *J. Ryu and S.-H. Lee contributed equally to this work. 1 ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Journal of Physical Chemistry C Abstract Electrical field (e-field) enhancement in gold nanohelices (AuNHs) with approximately 130 nm helical diameter (80 nm wire diameter) and various helical pitches (80  170 nm) is investigated with finite-difference time-domain (FDTD) simulation. The dimensions of the AuNHs are empirically determined from electron-microscopic images of AuNHs that were synthesized via surfactant-assisted seed-mediated growth. In contrast to Au nano-cylinders with 80 nm diameter, the e-field is effectively enhanced in the AuNHs by a longitudinally incident electromagnetic wave ( of approximately 600 nm) to the AuNHs’ axis. In particular, the dipole distribution of the AuNHs is distinguished by transverse and longitudinal incidence: higher volume and more uniform hot spots exist in AuNHs with longitudinal incidence. A maximum surface-enhanced Raman scattering (SERS) enhancement of 106 is obtained in AuNHs with a 100-nm pitch. SERS enhancement values that are obtained from the simulations are compared with experimentally measured SERS enhancement of 4-mercaptobenzoic acid that is loaded on the AuNHs. In addition, the power-law dependence of the helical gap on SERS enhancement is relatively stronger compared to the nano-gap in gold-dimer nanostructures. Hence, AuNHs are satisfactory SERS templates with resonance tuning ability due to their unique structural characteristic (helical gap).

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1. Introduction Plasmonic metal structures have attracted the attention of many researchers due to their unusual electromagnetic enhancement.1 Surface-enhanced Raman spectroscopy (SERS)2-4 is one of the applications that take advantage of the electromagnetic enhancement of the plasmonic metal structures. The sensitivity and selectivity of Raman spectroscopy can be remarkably improved via electromagnetic enhancement5-7 and chemical enhancement8-9. The enhancement factor in SERS reaches 1011 for detecting single molecules.10 The electrical field (e-field) near the surface of the metal nanostructures (NSs) is strongly related to the size and shape of the metal NSs, which is known as localized surface plasmon resonance (LSPR)11. In particular, LSPR in anisotropic metal NSs is distinguished in longitudinal and transverse modes, which demonstrates that electromagnetic enhancement in the anisotropic metal NSs is more controllable than in isotropic metal NSs. Hence, various metal NSs have been utilized in applications of SERS with electromagnetic

enhancement,

including

nanorods12-17,

nanoplates18-20,

nanocubes21-23,

nanocages24, branched nanoparticles25-27, and more-complex NSs28-30. Among the various metal NSs, it has been reported that metal nanohelices (NHs) can be used as SERS templates.31-32 Because dipole distribution is possible in metal NHs (theoretically, two consecutive dipoles per turn exist), relatively strong and more numerous hot spots belong to an individual metal helix in the transverse mode compared to other metal structures such as nanoparticles (spheres) and nanorods.33-34 Another hot spot, namely, the nanogap between turns of the metal, can be triggered in the longitudinal mode in metal NHs.34 In addition to SERS, due to their structural uniqueness, the NHs have been utilized in the other notable applications, including sensors35, actuators36-37, and optical polarizers38-39. Metal NHs, especially gold NHs (AuNHs), can be obtained via several synthetic approaches, such as electrochemical deposition and selective etching40, a solution 3 ACS Paragon Plus Environment


Journal of Physical Chemistry C method that uses a ligand-Au complex41, peptide assembly42, and a galvanic displacement reaction43. According to our previous study44, controlled growth of AuNHs is attempted via surfactant-assisted seed-mediated growth. Herein, we investigate the SERS enhancement in AuNHs with the empirical dimension that is observed in surfactant-assisted seed-mediated growth (a wire diameter of 80 nm and a helical pitch of 80  170 nm) via finite-difference time domain (FDTD) simulations. The dependence on the incident polarization demonstrates the unique electrical field (e-field) enhancement (larger enhancement with the longitudinal mode) in AuNHs. For AuNHs with 100 nm pitch, SERS enhancement (the e-field enhancement) can reach 3.2  106 at 610 nm longitudinal incident wavelength, which results from the narrowest gap distance (20 nm) between turns. For empirical SERS measurement using dye molecules (4-mercaptobenzoic acids (4MBA)) on the AuNHs, the SERS enhancement factor by up to 2.8  107 is obtained. In addition, a relatively larger power law dependence of the helical gap on SERS enhancement is observed compared to the nano-gap in gold-dimer nanostructures.45 It has been proved that AuNHs would be suitable testbeds for plasmonic applications due to their unique helical structure.

2. Experimental Methods 2.1 Preparation of AuNHs The AuNHs were fabricated via surfactant-assisted seed-mediated growth, following the previous report.44 In mixed aqueous CTAB (CH3(CH2)15N(CH3)3Br, ≧98%, Sigma –Aldrich) and hydrogen tetrachloroaurate(III) trihydrate (HAuCl4·3H2O, 99.9%, Sigma-Aldrich) solutions (100 mM and 50 mM, respectively), 0.02 M 0.45 mL sodium borohydride (NaBH4, 99.9%, SigmaAldrich) was added and the solution was stirred for five minutes to form a gold nanoseed 4 ACS Paragon Plus Environment

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Journal of Physical Chemistry C solution.46 The AuNH synthesis method involved the addition of 0.1 μL gold nanoseed solution into 20 mL of deionized water, followed by the addition of 0.0194 mL PEG (C13H27(OCH2CH2)nOH, n~12, Sigma-Aldrich), 0.736 g (0.667 mM) PVP ((C6H9NO)n, MW~55,000, Sigma-Aldrich), and 0.0242 g (3.333 mM) CTAB, after which the solution was stirred. Then, 0.045 mM 0.0182 mL HAuCl4(aq) and 0.1 M 0.056 mL ascorbic acid (C6H8O6, 99.7%, Sigma-Aldrich) were added into the solution. After the solution was evenly mixed, it was left for 20 hours at 15℃. Then, the sample was isolated via centrifugation. Repeated cleaning with deionized water and ethanol was used to remove excessive surfactants and gold nanoseeds. The morphology of the samples was characterized via FE-SEM (Hitachi S-4700).

2.2 FDTD simulations The extinction efficiency, e-field monitors, and e-field vector plots of the AuNHs were measured via three-dimensional FDTD simulation (FDTD Solutions, Lumerical Inc.). The dimensions of the AuNHs were determined by observation of the FE-SEM images of the AuNHs (Figure 1a-c). A periodic boundary (z-axis) and perfectly matched layers (PML) in the x-axis, yaxis, e-field profile, total field, and scattered field monitors were set-up and a total-field scattered field (TFSF) source was used for the simulations. A schematic diagram of the set-up is displayed in Figure S1. A mesh size of 2 nm3 and a refractive index and extinction coefficient of the Au materials that were based on the data of Johnson and Christy were used for all FDTD simulations. The extinction efficiencies of the Au cylinder and AuNHs are obtained from an extinction crosssection that is normalized by the cross-sectional area of the samples. Emax has been determined by the point with the maximum e-field in the e-field monitors.

2.3 SERS measurement 5 ACS Paragon Plus Environment


Journal of Physical Chemistry C The Au NHs (0.05 g) were dispersed in 4 mL of deionized water and then Raman dye molecules (4-MBA, 99%, Sigma-Aldrich) were loaded on the AuNHs by adding 1 mL of 4-MBA solutions of various concentrations. To fully load the 4-MBA on the AuNHs, the mixture solutions with the AuNHs and the 4-MBA were laid on the table without disturbing for overnight. SERS spectra of the AuNHs were obtained with a Raman spectrometer (NTEGRA, Nt-MDT) after putting the 4-MBA-loaded AuNHs on a piece of Si wafer and drying them. The condition for SERS measurements was 632.8 nm excitation (He–Ne laser) with a power of 3.35 mW.

3. Results and discussion Figure 1a-c shows field emission scanning electron microscopy (FE-SEM) images of AuNHs that were fabricated via surfactant-assisted seed-mediated growth. The AuNHs that are shown in Figure 1a are longer than 1 μm. As denoted by the arrows in Figure 1a, a typical helical diameter of the AuNHs is approximately 130 nm. High-resolution FE-SEM images show that a typical diameter (wire diameter) of the AuNHs is approximately 80 nm (Figure 1b) and a typical pitch of the AuNHs in the range of 80  170 nm (Figure 1b, c). According to the electron microscopic characterization of the AuNHs that were fabricated via surfactant-assistedseedmediated growth, the dimension of the AuNHs for FDTD simulations is determined via the scheme that is illustrated in Figure 1d. In detail, the length of the AuNH is sufficiently longer than the region of simulation, while the helical diameter of the AuNH is fixed as 130 nm. The wire diameter () and pitch (P) of the AuNH are set as 80 nm and 80  170 nm, respectively.


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Figure 1. (a) An FE-SEM image and (b, c) magnified FE-SEM images of AuNHs with the dimensions labelled. (d) The scheme of AuNH for FDTD simulations.




Figure 2. (a) The wavelength-dependent extinction efficiencies of AuNHs and a Au cylinder in the (top) transverse mode, (middle) longitudinal mode, and (bottom) the average of both the transverse and longitudinal modes. The insets represent the FDTD simulation scheme of each mode. (b) Schemes of the AuNH with 80 nm, 100 nm, and 150 nm pitches that were used in the FDTD simulations. (c) Graph of the gap depth (dgap) per pitch (P) of the AuNHs for various values of P. The inset defines dgap and P in the cross-sectional scheme of AuNH. In Figure 2a, pitch-distance-dependent extinction spectra of the AuNHs that are calculated via FDTD simulations are shown. As a control, extinction spectra of a Au cylinder of 80 nm diameter (the cylinder’s length is sufficiently longer than the region of simulation) are also calculated. The extinction efficiency, which is normalized by the cross-sectional area, is considered and displayed with transverse (top) and longitudinal (middle) modes and the average of the transverse and longitudinal modes (bottom) to facilitate comparison, as shown in Figure 2a. In the transverse mode of the incident light (the top of Figure 2a), both the cylinder and the 8 ACS Paragon Plus Environment

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Journal of Physical Chemistry C AuNHs with the narrowest pitch (P: 80 nm) show a peak near 510 nm and the peak is red-shifted to 540 nm with the increase of the pitch. The extinction efficiency of the cylinder is smaller than that of the AuNHs over the entire wavelength region and the extinction efficiency of the AuNHs near the peak is enlarged as the pitch increases. In the transverse mode, the e-field is enhanced at the girth of the AuNHs; in the AuNHs’ cross-sectionnormal plane of incidence that includes the helical axis, parts of the girth appear similar to crests and troughs in wave geometry and behave as hot spots (Figure 3c and Figure 5a). In that case, dipoles form at the crests and troughs as in the Au wire (Figure 4). The red-shift of the peak with increasing pitch, which can be considered as the distance between the poles in the dipoles along turns of the Au wire, becomes longer as the pitch of the AuNHs increases (Figure 4). As shown in Figure 2b, a AuNH that has a narrower pitch has more hot spots (more parts that are similar to crests and troughs) in the equivalent regions of the simulation. Hence, the enlarged extinction efficiency near the peak with increasing pitch of the AuNHs demonstrates that the contribution of the hot spots in the AuNHs becomes more efficient as the pitch widens, even if the number of hot spots decreases; the hot-spot region is broader in AuNHs that have relatively wide pitch (Figure 5a). In case of extinction efficiency in the longitudinal mode, while the cylinder has no remarkable peak in the entire wavelength region, the peak of the AuNHs grows and is red-shifted from 510 nm to 590 nm as the pitch increases from 80 nm to 100 nm. Then, peak of the AuNHs decreases and is blue-shifted as the pitch increases past 100 nm. In the AuNHs with pitches of 150 nm and 170 nm, the peak disappears, similar to the cylinder; however, the extinction efficiency is lower than that of the cylinder in the wavelength region above 500 nm (the middle of Figure 2a). The maximum extinction efficiency peak for the AuNHs with 100 nm pitch can be explained by that the gap that is formed by in-between turns of the AuNHs becoming sharpest at 100 nm pitch (Figure 2c). The spectrum that is shown in the bottom of Figure 2a is average of the 9 ACS Paragon Plus Environment


Journal of Physical Chemistry C extinction efficiency spectrum in both the transverse and longitudinal modes; the AuNHs have an extinction efficiency peak in the wavelength region of 500  630 nm, in contrast to the Au cylinder. Figure 2b shows schemes of AuNHs that are used in FDTD simulations. The AuNHs with pitches of 80 nm, 100 nm, and 150 nm are typically represented to show the decreasing number of turns with increasing pitch of AuNH in the region of the FDTD simulations. In addition, the gap between the turns of AuNH with 100 nm pitch looks sharper (narrower) than that of AuNH with 80 nm and 150 nm pitch. From the geometric point of view, the AuNH with 100 nm pitch has the sharpest gap; this is confirmed by the AuNH with 100 nm pitch having the largest ratio of gap depth (dgap) to pitch (P), as shown in Figure 2c. In general, e-field enhancement strengthens as the gap sharpens between metal structures.47

Figure 3. (a) An e-field profile of a Au cylinder in the transverse mode. (b) The FDTD simulation scheme of AuNH (120 nm pitch) in the transverse mode with the monitors labelled for (c, d). (c, d) E-field profiles of AuNH (120 nm pitch) in the transverse mode that were obtained by the e-field monitors that were placed (c) perpendicular and (d) parallel to the incident light. (e) The e-field profile of a Au cylinder in the longitudinal mode. (f) The FDTD simulation scheme of AuNH (100 nm pitch) in the longitudinal mode with the monitors labelled for (g, h). (g, h) E-field profiles of AuNH (100 nm pitch) in the longitudinal mode that were obtained by the e-field monitors that were placed (g) perpendicular and (h) parallel to the incident light. The incident wavelength of 630 nm is used for (a-h). The e-field intensities in (a-h) are displayed using the same scale. 10 ACS Paragon Plus Environment

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Journal of Physical Chemistry C The e-field profiles of the Au cylinder and the AuNHs (120 nm and 100 nm pitch) in the transverse (Figure 3a-d) and longitudinal (Figure 3e-h) modes of the simulation schemes at an incident wavelength of 630 nm are displayed in Figure 3. The incident wavelength is fixed as 630 nm, which is determined by considering a typical laser source ( = 633 nm) in empirical Raman measurements. For the Au cylinder, the e-field is enhanced only at the surface of the cylinder in the transverse mode (Figure 3a). In contrast, remarkable e-field enhancement is not observed from the Au cylinder in the longitudinal mode, as shown in Figure 3e. In our study, SERS enhancement by the e-field is estimated as a biquadrate of the ratio of the maximum e-field in the samples (|Emax|) to the incident e-field (e-field in air without the samples, |E0|), which is expressed as |Emax/E0|4. Approximately, the obtained |Emax/E0|4 of the cylinder in the transverse mode is ten times larger compared to the longitudinal mode. In contrast, relatively larger e-field enhancement in the longitudinal mode than in the transverse mode is observed in the AuNHs (Figure 3c-d and 3g-h). Figure 3b and 3f illustrate the FDTD simulation schemes of the AuNHs in the transverse (120 nm pitch) and longitudinal (100 nm pitch) modes, respectively. AuNHs with various pitches are selected because each AuNH typically shows the maximum e-field enhancement in the transverse and longitudinal modes (Figure 5). The e-field monitors for the AuNHs are placed perpendicular (c, g) and parallel (d, h) to the incident light, as illustrated in Figure 3b and 3f. For the transverse mode, e-field enhancement occurs at the crest- and trough-like parts of the AuNH in the perpendicular monitor (Figure 3c); however, e-field enhancement in the parallel monitor rarely occurs (except slight enhancement at the gaps), as shown in Figure 3d. In contrast, e-field enhancement of the AuNH in the longitudinal mode appears similar in both the perpendicular and parallel monitors, which occurs at gaps of the AuNH, as shown in Figure 3g and 3h. |Emax/E0|4 in the longitudinal mode of the AuNH is three and five orders of magnitude larger than that in the transverse mode, as shown in Figure 3g-h and 3c-d, respectively. 11 ACS Paragon Plus Environment


Journal of Physical Chemistry C In Figure 4, the aforementioned distinguishable e-field enhancement characteristic of the AuNHs from the Au cylinder can be explained by dipole distributions, which is confirmed with e-field vector plots (with an incident wavelength of 630 nm). In the vector plots, the color of each arrow represents the magnitude of the e-field and the length of each arrow is determined by the normalized magnitude of the e-field by the maximum e-field. Moreover, the direction of each arrow indicates the direction of the e-field. For the transverse mode of AuNH (120 nm pitch) (Figure 4a), relatively strong e-field spreads and retracts near the parts that are similar to the crests and troughs of the AuNH; hence, dipoles may distribute along the wire of AuNH, as shown in Figure 4b. An individual dipole distribution of the transverse mode of the AuNH is similar to that of the longitudinal mode of a Au rod (it seems that the Au rod is placed in a perpendicular plane to the incident light along the turn of the AuNH). The longitudinal mode of AuNH (100 nm pitch), which represents a relatively strong e-field, appears across gaps of the AuNH, as shown in Figure 4c, where it can be assumed that dipoles distribute across gaps of the AuNH (Figure 4d). Because gaps of the AuNH are constructed along with the helical structure of the AuNH, the efield enhancement that is generated by the dipole distribution in the longitudinal mode is symmetrical with respect to rotation; this can explain the similar e-field profiles, which are shown in Figure 3g and 3h. The volume of hot spots (the rotationally symmetric helical gaps) in the longitudinal mode of the AuNH is much larger than that in the transverse mode. The relatively stronger and larger hot spots in the longitudinal mode of the AuNH are well matched with the much smaller enhancements in the e-field profiles, as shown in Figure 3c and 3d.


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Figure 4. (a) A vector plot of the e-field of AuNH (120 nm pitch) in the transverse mode that was measured by the cross-sectional monitor. (b) A schematic diagram of the dipole distribution in AuNH (120 nm pitch) in the transverse mode. (c) A vector plot of the e-field of AuNH (100 nm pitch) in the longitudinal mode that was measured by the cross-sectional monitor. (d) A schematic diagram of the dipole distribution in AuNH (100 nm pitch) in the longitudinal mode. The distinguishable e-field enhancement in the transverse and longitudinal modes of the AuNHs with pitch and wavelength dependence is characterized as shown in Figure 5. In the transverse mode of the AuNHs (Figure 5a), the e-field enhancement seems restricted near the crest- and trough-like parts in the AuNHs, regardless of the pitch. In addition, |Emax| (marked as “X” in Figure 5a) is located near the crest- and trough-like parts of AuNHs, except for the AuNH with 100 nm pitch. |Emax| is observed at the gap of the AuNH with 100 nm pitch, where the sharpest gap of the AuNH influences it. |Emax/E0|4 in the transverse mode of the AuNHs is below 1.2  103, whereas the e-field enhancement in the longitudinal mode of the AuNH of 100 nm 13 ACS Paragon Plus Environment


Journal of Physical Chemistry C pitch has larger |Emax| than the others, even though |Emax| (marked as “X” in Figure 5b) is located at the gap of the AuNHs, regardless of the pitch (Figure 5b). In contrast to the transverse mode, the values of |Emax/E0|4 in the longitudinal mode of the AuNHs with 80 nm, 90 nm, 100 nm, and 110 nm pitches exceed 1.2  103; in particular, |Emax/E0|4 reaches 1.9  106 in the AuNH with 100 nm pitch. In Figure 5c, the values of the |Emax/E0|4 that were obtained from Figure 5a and 5b and those of the Au cylinders are displayed. Remarkable SERS enhancement is anticipated at the longitudinal mode of the AuNH with 100 nm pitch due to the sharpest gap formation (Figure 2c). However, the values of |Emax/E0|4 of the AuNHs with pitches that are smaller and larger than 100 nm are not symmetrically decreasing, which is distinguishable from the tendency of sharpness of the gap that is shown in Figure 2c (dgap/P increases relatively symmetrically increases and decreases before and after the 100 nm pitch). The non-symmetrical decrease of |Emax/E0|4 can be influenced by gap formations of the AuNH for smaller and larger pitches than 100 nm. In detail, for the AuNH with 100 nm pitch, gaps between turns of the Au wires are established as the Au wires are closer to each other. Gaps are generated by overlapped Au wires for pitches that are smaller than 100 nm, while gaps are formed by the clearly separated Au wires for pitches that exceed 100 nm. In addition, the relation between |Emax/E0|4 and the gap distance of the AuNHs is characterized, as shown in the inset of Figure 5c, where the gap distance is determined as the vertical gap distance between the turns at the middle of dgap. The power-law dependence of |Emax/E0|4 on the gap distance is observed with a power (m) of -6.5 (the magenta line in the inset of Figure 5c). For gold-dimer nanostructures (e.g., a gold bowtie nanoantenna)45, the SERS enhancement factor shows a relatively weak power law dependence on the gap distance (m ≈ 2.3) because the resonant nanocavity effects of the gold nanostructure attenuate the e-field within the gap region. According to Coulomb’s law, the e-field decays in proportion to the inverse square of the distance (E  d -2, where E denotes the e-field and d is the distance from the e-field 14 ACS Paragon Plus Environment

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Journal of Physical Chemistry C source). Hence, power-law dependence with m of -8 is anticipated in the gaps of metal nanostructures. The relatively negatively larger power law dependence (m ≈ -6.5) of the AuNHs compared to the gold-dimer nanostructures (m ≈ -2.3) originates from the distinguished gap structure of the AuNHs. A helical gap of AuNHs is formed between continuously connected Au wires; therefore, resonant nanocavity effects by metal nanostructures are hampered, compared to the gap in gold-dimer nanostructures. Moreover, the incident-wavelength dependence |Emax/E0|4 in the longitudinal mode of the AuNHs with 100 nm pitch is measured as shown in Figure 5d. |Emax/E0|4 gradually increases to 610 nm and subsequently decreases as the incident wavelength increases. The maximum value of |Emax/E0|4 is 3.2  106. Moreover, to estimate the empirical SERS enhancement in the AuNHs, the SERS enhancement factor (EFTheory) is calculated via Eq. (1)48:

EFTheory

=

|𝑬𝒍𝒐𝒄(𝛌𝒆𝒙)|𝟐 ∙ |𝑬𝒍𝒐𝒄(𝛌𝒔)|𝟐 , |𝑬𝟎|𝟐 ∙ |𝑬𝟎|𝟐

(1)

where Eloc(λex) is the local e-field at the excitation wavelength (λex) and Eloc(λs) is the local e-field at the Raman scattering wavelength (λs). In our previous report44, a 633 nm laser was used as the excitation source during the Raman scattering measurement and the main peaks of the Raman dye (4-MBA) were observed at 1088 cm-1 and 1597 cm-1. Hence, λex is set as 633 nm (the gray dashed arrow in Figure 5d) and the values of λs are determined as 680 nm and 704 nm (the dark green and red dashed arrows in Figure 5d) using the Raman shift (1088 cm-1 and 1597 cm-1). λs can be calculated from relation to the Raman shift: Raman shift = (1/λex  1/λs).48-50 From the values of |Emax/E0|2 in Figure 5d and Equation 1, EFTheory at the Raman spectrum peak positions of 4-MBA is calculated as 5.9  105 (at 1088 cm-1) and 3.9  105 (at 1597 cm-1), respectively.



Journal of Physical Chemistry C Considering the maxima of the EFTheory of the Au cylinder (~20, see Supporting Information), the EFTheory of the AuNH is four orders of magnitude larger than that of the Au cylinder.

Figure 5. (a, b) E-field profiles of AuNH in the (a) transverse and (b) longitudinal modes at 630 nm incident wavelength with various pitches. “X” denotes the position of |Emax|. (c) A graph of |Emax/E0|4 of the AuNHs (80  170 nm pitches) and Au cylinder (cyl) with 630 nm incident wavelength. E|| and E ⊥ denote the values in the longitudinal and transverse modes, respectively. The blue dashed line is a guide to eyes. The inset is a graph of the gap-distance dependence of |Emax/E0|4 of the AuNHs. The magenta solid line is a linear fitting line of the power-law dependence (m = -6.5). (d) The incident-wavelength dependence of |Emax/E0|4 of AuNH (100 nm pitch) in the longitudinal mode. The inset shows the FDTD simulation scheme of AuNH. The gray, dark green, and red dashed arrows represent the excitation wavelength (λex), and the Raman scattering wavelengths of 1088 cm-1 (λs(1088 cm-1)) and 1597 cm-1 (λs(1597 cm-1)), respectively. 16 ACS Paragon Plus Environment

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Journal of Physical Chemistry C To characterize experimentally the SERS enhancement factor (EFExp) of the AuNHs, Raman spectra of the variously concentrated 4-MBA molecules loaded AuNHs are considered. According to our previous report44, peaks that originate from 4-MBA are observed at 1088 cm-1 and 1597 cm-1, which would be assigned as ring breathing modes of vibration of the 4-MBA molecule51 and their intensity is enhanced as the concentration of 4-MBA increases. The concentration of 4-MBA dependent SERS intensity of the AuNHs adapted from the previous report44 is displayed in Figure 6a. In addition, EFExp is calculated from the values that are shown in Figure 6a via Equation 2:

EFExp

𝑰𝑺𝑬𝑹𝑺

𝑵𝑩𝒖𝒍𝒌

= 𝑰𝑩𝒖𝒍𝒌  𝑵𝑺𝑬𝑹𝑺

,

(2)

where ISERS and IBulk are the peak intensities of the SERS and bulk Raman spectra. The numbers of molecules in the SERS and bulk Raman spectra are denoted as NSERS and NBulk, respectively. Raman spectra of 4-MBA powders are used to determine IBulk/NBulk (see Supporting Information). The 4-MBA-concentration dependence of EFExp for the AuNHs is shown in Figure 6b. EFExp of the AuNHs increases as the concentration decreases, where inverse dependence on the concentration of the 4-MBA molecules ([4-MBA]) (EFExp ∝ [4-MBA]-1) is observed. EFExp on the order of 107 is obtained from the 0.02 ppm 4-MBA loaded AuNHs, which exceeds the value of EFTheory (~105) that was obtained in Figure 5d. In case of the EFTheory, only e-field enhancement, except chemical enhancement, is considered; this would result in the larger EFExp of the AuNHs.




Figure 6. (a) The SERS peak intensity dependence on the concentration of the 4-MBA adapted from the previous report44. (b) The SERS enhancement factors (EFEXP) of AuNHs with various concentrations of 4-MBA are calculated via Eq. (2).

4. Conclusions The wavelength and polarization of incident-light-dependent e-field enhancement in AuNHs of empirically observed dimensions is characterized by FDTD simulation measurements. In addition, SERS enhancement of the AuNHs is examined by both theoretical calculation based upon the FDTD simulation results and experimentally measured dye-molecule-concentrationdependent SERS spectra. In the longitudinal mode of the AuNH with 100 nm pitch, e-field enhancement is maximal when the wavelength of the incident light is approximately 600 nm. In the result, several findings on the AuNHs’ noticeable characteristics are obtained: (1) The e-field enhancement in the longitudinal mode of the AuNHs is stronger than that in the transverse mode, which is the opposite tendency compared to the Au cylinder. (2) While dipoles distribute along the wire of the AuNHs in the transverse mode, dipole distribution across the helical gap of the AuNHs is observed in the longitudinal mode. Due to the rotationally symmetrical dipole distribution in the longitudinal mode, the volume of hot spots in the longitudinal mode is much 18 ACS Paragon Plus Environment

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Journal of Physical Chemistry C larger than that in the transverse mode of the AuNHs. (3) Relatively negatively larger power-law dependence (m ≈ -6.5) of EFTheory on the gap distance of the AuNHs compared to gold-dimer nanostructures (m ≈ -2.3) is observed due to the hampering of resonant nanocavity effects in the AuNHs. (4) The maximum value of EFExp (~ 107) is obtained from 0.02 ppm dye molecules that are loaded onto AuNHs, which agrees well with EFTheory being on the order of 105 when only efield enhancement is considered. In the end, it is concluded that AuNHs are useful materials as SERS templates owing to their unique structural characteristic of hot spots (helical gaps), by means of the complementary characterization via FDTD simulation and experimental SERS measurements.

ASSOCIATED CONTENTS Supporting Information. Detailed set-up for the FDTD simulations, e-field profiles in the xy-plane monitor with transverse incidence, e-field profiles in the xy-plane monitor with longitudinal incidence, movies of the efield profiles of the AuNHs, control spectra for the SERS measurements, extinction efficiency of short AuNHs, SERS enhancement of the Au cylinders. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEGMENTS This research was supported by Basic Science Research Program through the National Research Foundation

of

Korea

(NRF)

funded

by

the

Ministry

of

Education

(NRF-

2015R1A1A1A05027681, NRF-2018R1D1A1B07045244). Prof. Chang thank the Ministry of Science and Technology of Taiwan (MOST 106-2113-M-027-006) for financial support.




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Table of Contents


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(a)

(b) 108

400

at 1088 cm-1 at 1597 cm-1

300

106

200

105

100

104

0.01

0.1

1

at 1088 cm-1 at 1597 cm-1

107

EFExp

SERS intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41


10

100

1000

0.01

0.1

1

ppm

ppm


10

100

1000

Helical-Structure-Dependent Surface-Enhanced Raman Spectroscopy

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