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PERSPECTIVE Darya Radziuk and Helmuth Möhwald Ultrasonically treated liquid interfaces for progress in cleaning and separation processes

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ARTICLE Long-range surface plasmon resonance and surface-enhanced Raman scattering on X-shaped gold plasmonic nanohole arrays Received 00th January 20xx, Accepted 00th January 20xx DOI: 10.1039/x0xx00000x www.rsc.org/

abc

Chao Hou,

a

c

Daniel David Galvan, Guowen Meng and Qiuming Yu*

a

A multilayered architecture including a thin Au film supporting X-shaped nanohole array and a thick continuous Au film separated by a Cytop dielectric layer is reported in this work. Long-range surface plasmon resonance (LR-SPR) was generated at the top Au/water interface, which also resulted in a long-range surface-enhanced Raman scattering (LR-SERS) effect. LR-SPR originates from the coupling of surface plasmons (SPs) propagating along the opposite sides of the thin Au film embedded in the symmetrical refractive index environment with Cytop (n = 1.34) and water (n = 1.33). The finitedifference time-domain (FDTD) simulation method was used to investigate the optimal dimensions of the substrate by studying the reflectance spectra and electric field profiles. The calculated optimal structure was then fabricated via electron beam lithography, and its LR-SERS performance was demonstrated by detecting rhodamine 6G and 4mercaptobenzoic acid in the refractive index-matched environment. We believe this structure as a LR-SPR or LR-SERS substrate can have broad applications in biosensing. 11

Introduction Surface plasmons (SPs) are surface-bound electromagnetic waves formed by collective oscillation of conduction electrons in noble metals, which propagate along the metal/dielectric interface and decay exponentially in both the metallic film and dielectric medium.1 Localized surface plasmons (LSPs) are the result of the confinement of SPs in metallic nanoparticles or nanostructures of size comparable to or smaller than the wavelength of light used to excite the plasmons. When an appropriate wavelength of light with the frequency of incident photons matches the natural frequency of LSPs, resonance occurs, called localized surface plasmon resonance (LSPR) excitation.2 This process greatly enhances the electromagnetic (EM) field close to the nanoscale metallic surface, which is generally believed to be a major factor leading to surfaceenhanced Raman scattering (SERS).3 In past decades, SERS has become a powerful technique for use in various sensing applications including environmental monitoring,4 detection of chemical and biological molecules,5,6 chemical imaging,7,8 food safety,9 and biomedical diagnostics.10 SERS offers ultrasensitive detection with the capability of directly showing molecular specificity associated with the vibrational modes of analytes. However, SERS is a near-field effect and has a low penetration

depth, which limits SERS detection of molecules in complex media or large biomolecules such as cells and bacteria. Raman signals are amplified only when the target analytes are in the proximity to the surface of the plasmonic nanostructure, as the enhanced local EM fields decay significantly beyond 2−3 12 nm from its surface. One method to overcome this limitation is to excite longrange surface plasmon resonance (LR-SPR) that could be used to enhance Raman scattering to the extended distance away 13 from the metal surface. Investigations have shown that when a thin metal film is embedded between two dielectric media with similar refractive indices, two transverse magnetic (TM) modes corresponding to two SP waves will propagate along the opposite sides of the metal film and can couple into two mixed modes: symmetric and asymmetric. The symmetric mode is referred as to the long-range surface plasmon (LR-SP) because of its lower attenuation and longer penetration depth, while the asymmetric mode is termed as short-range surface plasmon (SR-SP). It was found that when the metal film is made ultrathin (~20 nm) and sandwiched between water and a medium close to the refractive index of water, the LR-SP mode shows a maximum and SR-SP mode exhibits a minimum, therefore, around 20 nm may become the optimal thickness of 14 the thin metal film for LR-SPR sensor. The LR-SP with a narrower resonance provides larger enhancement of the electromagnetic field and has an order of magnitude lower damping compared with conventional SP waves. From the first 15,16 prediction and demonstration of LR-SP by Sarid in 1980s, LR-SPR-based biosensors have been used in various 17 applications such as LR-SP-enhanced fluorescence, LR-SPR 18 19 imaging for bioaffinity, and the detection of cells, 20-22 23 bacteria, and DNA. In such biosensing applications,

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analytes are often dispersed in aqueous solutions. Therefore, the refractive index of the dielectric layer underneath the thin metal film should be matched to water (n = 1.33) in order to provide a symmetric dielectric environment. So far, a wide variety of multilayered structures have been created by using 24 18,25 different dielectric layers, including teflon, Cytop, 17,26 27 MgF2, and SiO2 . The application of LR-SPR to LR-SERS was demonstrated by probing SERS signal of 4-mercaptopyridine at a distance of ~500 nm away from a thin film of silver in a fourphase Kretschmann LR-SPR configuration composed of K9 28 prism/MgF2 (1750 nm)/silver (16 nm)/water. Therefore, it was proved that the LR-SPR configuration possessed at least 500 nm of electric field penetration depth, which is much longer than that of the conventional SPR configuration without the refractive index-matched layer MgF2 film on the other side of the thin silver film. Later work by the same group increased the SERS sensitivity of this architecture by adding Ag nanoparticles into the detection solution.26 Previously, a refractive index-matched LR-SERS substrate reported by our group demonstrated amplified SERS signals compared to conventional substrates in asymmetrical dielectric environments.29 The functional layer in the LR-SERS substrate was a ~20 nm Au thin film supporting circular nanohole array. Nanostructure arrays with other shapes having sharp vertexes could confine LSPR in the vertexes, resulting in even stronger local electric fields. In this work, we developed a plasmonic nanostructure that can generate LR-SPR when immersed in water. As illustrated in Fig. 1, the structure consists of an ultrathin (20 nm) Au film with a 2D array of X-shaped nanoholes on the top, under which a refractive index-matched Cytop layer was used to generate LR-SPs at the Au/water interface, and then another thick Au film (100 nm) is placed between the Cytop layer and the bottom glass substrate as a resonant mirror. Finitedifference time-domain (FDTD) simulations were used to investigate LR-SPR and the local electric field distribution. The optimal structure obtained from the FDTD calculations was then fabricated via electron beam lithography (EBL) due to its precise control on the dimensions. Finally, the LR-SERS effect was verified through two approaches: one is by demonstrating there are still detectable SERS signals of rhodamine 6G (R6G) at a distance of ~10 nm from the Au surface in aqueous solutions; the other is by demonstrating the SERS intensity of 4-mercaptobenzoic acid (4-MBA) self-assembled monolayers (SAMs) formed on the surface of the structure is stronger in better refractive index-matched salt solution than in the pure DI water. Therefore, this LR-SPR plasmonic nanostructure was demonstrated a promising potential as effective LR-SERS substrates for rapid detection of biomacromolecules with the capability of acquiring more biochemical characteristics.

Experimental Chemicals Methyl isobutyl ketone (MIBK), isopropanol (IPA), and potassium hydroxide (KOH) were purchased from Fisher

Fig. 1 (a) A 3D schematic and (b) a 2D top illustration of the multilayered plasmonic nanostructrure for LR-SERS. Parameters varied in FDTD simulations are Cytop thickness (T), the length (L) and width (W) of the arm within X-shaped nanohole, and the pitch (P) of the square grating array. The curvature at the tips of the cross-sectors (R) was fixed at 40 nm. The size of the Au island (I), the spacing of the opening (O) between two cross-sectors, and the gap (G) between the neighboring X-shaped nanoholes are varied along with the variation of L, W and P. The incident light is illuminated normally from the top surface with the polarization along the x-direction.

Scientific (Waltham, MA). Poly(methylmethacrylate) (PMMA, 950PMMA A6) was purchased from MircoChem (Westborough, MA). Cytop CTL-809M and additional solvent (CT-SOLV180) were purchased from Asahi Glass (Tokyo, Japan). Potassium thiosulfate anhydrous (K2S2O3) was obtained from Pfaltz & Bauer (Waterbury, CT ). Potassium hexa-cyanoferrate (III) (K3Fe(CN)6), potassium hexa-cyanoferrate (II) trihydrate (K4Fe(CN)6), rhodamine 6G (R6G) and 4-mercaptobenzoic acid (4-MBA) were purchased from Sigma-Aldrich (St. Louis, MO). All reagents were used as received without further treatment. Finite-difference time-domain (FDTD) simulations The FDTD method (Lumerical, FDTD Solutions) was used to investigate the plasmonic properties of the LR-SERS multilayered nanostructures illustrated in Fig. 1. A single Xshaped nanostructure as a unit cell was adapted in the simulations with periodic Bloch boundary conditions applied in the x and y directions, and perfectly matched layer (PML) boundary conditions applied in z-direction. The range of the xy plane was defined by the pitch (P) of the array and the range of the z-direction was defined by extending the unit cell 500 nm above the top Au/water interface and 500 nm below the Au/glass interface. A plane wave light source in the range of 600-1100 nm was placed 400 nm above the top Au/water interface and is illuminated normally from the top surface with the electric component polarized along the x-direction. A frequency monitor was placed 50 nm above the light source to collect the reflectance spectrum from the multilayered substrate. In order to collect the electric field profiles, the power monitors were placed at the top Au/water interface and every 5 nm distance above the Au/water interface (until 50 nm distance) as well as the cross-sectional x-z plane at the center of the X-shaped nanohole. The refractive indices of Cytop and glass were adapted as 1.34 and 1.507, respectively. The wavelength-dependent refractive index of gold in the simulation wavelength range was adapted from that provided in the software database. The unit cell was exposed to water (n = 1.33). A mesh size of 2 nm was added around the Xshaped nanohole in all the three dimensions. Fabrication of substrates

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The multilayered substrates were fabricated with the entire fabrication process illustrated in Fig. S1 (ESI†). Glass substrates 2 (about 1 cm ) were cleaned by ultra-sonication in soapy DI o water, DI water, acetone and IPA for 20 min each at 40 C, and were placed in a UV-O3 cleaner for 20 min. A ~100 nm thick Cytop film was spin coated on the cleaned glass substrate, o annealed at 200 C for 1 h and then exposed to an oxygen plasma cleaner (Diener, 50 W) at 5 sccm of air for 30 s to serve as an adhesion layer prior to coating Au film. A thick Au film of ~100 nm was deposited onto the substrate using a thermoevaporator (Edwards, Auto 306). The spacing 200 nm thick Cytop layer was spun cast on the thick Au film and annealed at o 100 C (lower temperature was used to avoid cracking of the Au film) for 2 h. Then a 20 nm Au-thin-film was thermally evaporated onto the Cytop layer at a deposition rate of 0.02 -6 nm/s under a background pressure of 2 × 10 mbar. A layer of PMMA electron-sensitive resist was spin coated (Microchem, 950 K) onto the 20 nm thick Au thin film and was baked at 180 o C for 90 s. The PMMA coated substrate was exposed to a fine 2 electron beam (100 kV, 3.5 nA) with a dose of 550 µC/cm in order to create a 100 μm × 100 μm array of X-shapes with the optimal parameters calculated by FDTD simulations. X-shaped nanoholes in PMMA layer were generated after development in 1:3 methyl isobutyl ketone / isopropanol (MIBK/IPA) PMMA developer for 70 s followed by an IPA rinse and a N2 blow-dry. Next, the exposed Au thin film was etched through these Xshaped nanoholes by using a standard gold etching solution containing KOH (1.0 M), K2S2O3 (0.1 M), K3Fe(CN)6 (0.01 M), 30 and K4Fe(CN)6 (0.001 M) at room temperature for 5 min and 30 s, followed by being rinsed with acetone to remove the PMMA layer, and then was rinsed by DI water. The substrate was finally placed in UV-O3 cleaner for 20 s in order to remove PMMA residuals. Characterization and SERS measurements Field-emission scanning electron microscopy (FE-SEM, Sirion XL30, FEI) and tapping mode atomic force microscope (AFM, DI MultiMode with a Nanoscope IVa controller) were used to characterize the dimensions of the nanostructures. In order to verify the LR-SERS effect of this multilayered substrate, taking SERS spectra at a long distance from the top Au/water interface is necessary. So firstly a ~10 nm thick Cytop layer was spun cast onto the Au-thin-film at 3000 RPM for 30 s using a diluted Cytop solution which was made by mixing CT-SOLV180 and stock Cytop in a 10:1 (v:v) ratio, and then annealed on a o hot place at 180 C for 90 s. The bare multilayered substrate and the Cytop layer capped multilayered substrate were immersed in 1 mM R6G aqueous solutions for 30 min to enrich R6G molecules on Au or Cytop surfaces, followed by being immersed in DI water in a home-built Teflon container covered with a cover glass to take SERS spectra. In another set of experiment, 4-MBA self-assembled monolayer (SAM) was formed on the Au surface of the multilayered substrate by soaking a UV-O3 cleaned (for 20 min) substrate in a 1 mM aqueous solution of 4-MBA for 4 h, followed by rinsing with DI water and blowing dry in a stream of air. Then the SERS spectra were taken by immersing the substrate in DI water and

NaCl solutions with different concentrations loaded in the Teflon container with a cover glass. All SERS spectra were taken on a Renishaw inVia Raman spectroscope connected to a Leica upright DMLM microscope. A 785 nm laser was focused with a 50× (N.A.= 0.8, W.D.= 2.4 mm) objective to form a laser spot size of ~2 µm × 20 µm. The laser power was measured at the focal plane to be 2.4 mW with a handheld power meter (Edmund Optics). The acquisition condition was set to 10 s and 5 times accumulations for R6G while a single accumulation for 4-MBA.

Results and discussion All the parameters that influence the plasmonic properties of this structure are labelled in Fig. 1, including the length (L), width (W), and curvature (R) of the x-shaped nanohole, periodicity (P) of the square grating array, and the thickness (T) of the Cytop dielectric layer. In addition, the size of the Au island (I), the spacing of the opening (O) between the crosssectors in the X-shape, and the gap (G) between the neighboring X-shaped nanoholes as indicated in Fig. 1b, can be calculated by using the following formulas: I = P − √2 (R is ignored), O = √2, G = P − (L + W)/√2 (only when G > 0 will independent X-shaped nanoholes exist). Among these, the key parameters varied in our FDTD simulations are L, W, P, and T (each array was identified as L-W-P-T), while R was fixed at 40 nm in accord with practical EBL fabrication. The incident light was illuminated normally from the Au thin film side with the polarization along the x-direction. The wavelength of SPR (λSPR) can be controlled by tuning these parameters, and in particular, it is highly sensitive to P. Theoretical analysis reveals that the maximum SERS signals are expected when λSPR = (λL + λR)/2, where λL is the laser excitation wavelength and λR is the 31 Stokes Raman scattering wavelength. With a 785 nm excitation laser, λR ranges from 804 to 914 nm corresponding -1 to the Stokes Raman shift of 300–1800 cm . Then, λSPR in the range of 795–850 nm is desired to achieve the maximum SERS effect. The optimal P for a 2D grating coupler can be 14 theoretically calculated using the following equation:  

 ()

  +   =   () 



! + Δ#

%$(1)

where P is the periodicity of the grating array; (i, j) are integer scattering or diffraction orders; Re refers to the real part of a complex number; εm(λ) is the wavelength-dependent relative permittivity of metal, and εd is the relative permittivity of the adjacent dielectric media which can be expressed in terms of 2 refractive index (nd = εd) for non-absorbing materials (nd = 1.33 for an aqueous environment); Δnef denotes the difference of the propagation constant for the SPs propagating along a planar film relative to the grating array. If λSPR is set to 800 nm, P is obtained to be 604 nm from Eqn (1) to produce the optimal SERS effect for a 20 nm thick Au thin film with a 2D periodic nanostructure. This structure supports two types of SPRs: propagating SPRs and localized SPRs (LSPRs). SPRs can be excited on thin metal films using grating or prism couplers, and propagate tens to hundreds of micrometers along the metal

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surface with an electric field decaying exponentially from the metal/dielectric interface. Evanescent LSPRs can be resonantly excited around nanoholes in thin metal films, and it is intimately coupled to nonevanescent propagating light. SPRs and LSPRs can overlap and couple with one another, leading to complex reflectance spectra and electric field profiles that are sensitive to the structure and properties of the surrounding 32 media. The FDTD simulated reflectance spectrum (Fig. 2a) from a 480-80-604-200 (i.e., L-W-P-T) array shows three obvious dips at 697, 783, 960 nm, one small dip at 827 nm, and one merged dip at 760 nm. These five dips refer to five plasmonic modes, indicated as Mode I, Mode II, Mode III, Mode IV, and Mode V, respectively, which are generated upon the incident of a continuous wavelength light ranging from 600 to 1100 nm. Fig. 2b shows the plot of square of the maximum local electric field at the top Au/water interface (Emax) with respect to the electric field of incident light (E0), i.e. (Emax/E0)2 versus wavelength, revealing that the peaks in the electric field spectrum correspond to the dips in the reflectance spectrum. In order to understand the physical origins of five different modes, the profiles of electric field distribution and electric charge overlaid with the Poynting vectors are displayed in Fig. 2c for both at the top Au/water interface and at the cross-sectional x-z plane corresponding to each of the five modes. Since the Au thin film is truncated by square-arranged, X-shaped nanoholes, both SPRs and LSPRs can be generated on the top Au surface. From rows A and C (showing the electric field distribution at the top Au/water interface), Mode I generates weak electric field mainly at the four arm ends and side edges between the two-pairs of the arms. The electric charge map overlaid with Poynting vectors at the top Au surface (row B) for this mode reveals energy flows away from the arms and the cross-sectors. The distribution figures at the crosssectional x-z plane (rows D and E) show weak electric intensity at the cross-sectors in this mode and the energy flows up from the bottom surface of the Au thin film, which causes that the SPR wave at the top Au/water interface is stronger than that at the Au/Cytop interface. Row F shows the x-z plane of |Ez|, the normal z-component of the electric field, demonstrating that SPRs are visible on both sides of the Au thin film with the Xshaped hole array. Opposite charges result from the counterpropagating SPRs. The weak propagating SPR waves along the Au thin film at Mode I are displayed through the |Ez| image. In the same way, Mode II generates relatively weak electric field at the four arm ends and the cross-sectors. Energy circulations are developed in each arm and finally flow into the crosssector areas confining the energy here. From the distributions at the x-z plane (rows D, E and F), the coupling between the neighboring Au islands happens at Mode II, and the energy flows down from the top Au/water interface. The |Ez| figure shows stronger electric field at the Au/Cytop interface than that at the Au/water interface. Mode III produces the strongest electric field enhancements at the cross-sectors along the x-direction and at all the edges and ends of four arms. Strong LSPRs coupling is shown between the two edges of each arm within one X-shaped nanohole unit, which is

Fig. 2 (a) FDTD simulated reflectance spectrum and (b) the corresponding electric field spectrum as a function of wavelength for a nanostructure with the parameter combination 480-80-604-200 nm (L-W-P-T). (c) Electric field distribution profiles for all the five modes at the top Au/water interface (row A) and at the cross-sectional x–z plane (row D); the related electric charge distributions overlaid with Poynting vectors are shown in rows B and E, respectively; row C is the 3D display of the electric field intensity at the top Au/water interface; row F shows |Ez/E0| at the cross-sectional x–z plane. The scale bar represents (Emax/E0)2 on a log scale in rows A and D, and represents |Ez/E0| on the normal scale in row F.

attributed to the longer length (480 nm) and narrower width (80 nm) compared with the X-shaped nanoholes in the quasi33 3D structure (L = 420 nm, W = 140 nm) . Energy flows from the edges and ends of the arms and is then confined at the cross-sectors. It can be seen from row D that the strongest local electric field located at the cross-sectors exists in Mode III, due to the LSPR waves coupling between the neighboring Au islands. The Poynting vectors (row E) reveal that energy flows down from the top surface. The |Ez| figure demonstrates symmetric SPRs at both sides of the Au thin film at Mode III. Mode IV produces weak electric field at the similar locations as those in Mode I, while from row B, it can be seen that the charge distributions are inverse between these two modes. The Poynting vectors indicate that energy flows away from the cross-sectors and forms an energy cycle within each arm in Mode IV. The electric field at cross-sectors in the x-z plane begins getting weak again. Moreover, the same as Mode I, energy flows up from the bottom surface of the Au thin film. The |Ez| component over the top Au/water interface is

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stronger than that at the Au/Cytop interface. Mode V generates weak electric field mainly at the edges between the two-pairs of the arms resulting from the coupling of SPR waves between the nearby arm edges along the x-direction. It also confines the energy at the cross-sector areas. Obviously, Mode V has a strong coupling of SPRs generated at the Au-thinfilm/Cytop interface and the Au-thick-film/Cytop interface as shown in those distributions in the x-z plane (rows D, E and F), leading to energy flowing down from the top Au thin film surface and forming a relatively strong electric filed in the Cytop layer close to the Au thick film. In order to verify the contribution of the Au thick film to the electric field enhancement, FDTD simulations were performed on the structure of free-standing Au thin film with X-shaped nanohole arrays (Fig. S2a, ESI†) by taking away the 100 nm Au thick film and the glass substrate but keeping all the dimensions the same as in Fig. 2. The reflectance spectrum (Fig. S2b, ESI†) shows an imperfect reflection (oblique baseline) compared to that of the whole structure, demonstrating the Au thick film acts as a resonant mirror preventing the leakage of incident light. In addition, the reflectance spectrum shows only three modes for the freestanding structure. While the location of Mode III (783 nm) stays the same as that in the whole structure, the intensity of (Emax/E0)2 drops significantly from ~4000 to ~1200 (Fig. S2c, ESI†), which indicates that Mode III is a local mode originated from the Au thin film itself and its intensity is enhanced by the reflecting light from the Au thick film. The other two modes are located at 725 and 925 nm, respectively, which could be the shifts of Mode I and Mode V in the whole structure by comparing the distributions of electric field and electric charge overlaid with Poynting vectors (Fig. S2d, ESI†). Their electric field intensities are also lower compared to those in the whole structure. Therefore, it is clear that the Au thick film at the bottom not only prevents the leakage of the incident light to enhance the electric field associated with the plasmon modes, the SPRs generated at the Au/Cytop interface of the Au thick film can also interact with either the propagating or localized SPR modes at the top Au thin film with X-shaped nanoholes to create new modes. It is well known that electromagnetic (EM) and chemical enhancements are the two main enhancement mechanisms of SERS. The EM enhancement is the dominant factor, and is attributed to the strongly enhanced local electric field in the vicinity of plasmonic nanostructures. The EM enhancement contributes to the enhancement factor (EF) on the order of 106−108, while only about 102−103 for the chemical enhancement.34,35 FDTD simulations were performed to calculate the EM contribution to the SERS enhancement. The EM enhancement is proportional to the fourth power of the ratio of the maximum local electric field to the incident electric field ((Emax/E0)4), which is specifically written as: (Emax/E0)4 = |Emax(λL)/E0(λL)|2|Emax(λR)/E0(λR)|2, where Emax(λL) and Emax(λR) represent the maximum electric field intensity at the laser excitation wavelength (λL, that is 785 nm) and the Raman scattering wavelength (λR) from FDTD simulations; E0(λL) and E0(λR) represent the electric fields of the incident light at λL and

Fig. 3 (a) The FDTD simulated reflectance spectra and (b) the electric field enhancement (Emax/E0)2 versus wavelength by tuning T from 100 to 600 nm, while other parameters are kept the same with those in Fig. 2 (L = 480 nm, W = 80 nm, and P = 604 nm). (c) Calculated quality factor (QF) as a function of the thickness of Cytop (T) at the top Au/water interface (z = 0 nm), and at the planes of 10 nm (z = 10 nm) and 20 nm (z = 20 nm) distance from the top Au/water interface.

λR, respectively. In this work, a quality factor (QF) was adopted to evaluate EF of this multilayered plasmonic nanostructure, which gauges the average SERS intensity over the entire range 36 of Stokes Raman shifted frequencies: &' =

( -. ,-. (/ )  ,-. ( )  + + + 123 + * ) 45 ,0 (/ ) ,0 ( )

(2)

where λR ranges from 804 nm to 914 nm as mentioned previously, and β is a normalization constant defined as β = λmax – λmin. λmax is 914 nm, and λmin is chosen as 785 nm, which means that the integral from the laser excitation wavelength to the maximum Raman scattering wavelength under investigation was adopted in this work to quantify the cumulative contributions from inelastically scattered surface plasmons. The value of QF then became a judging criterion during the control of the key parameters in our FDTD simulations. To obtain the optimal dimensions of the structure, the thickness (T) of the Cytop layer was tuned first. The reflectance and the electric field intensity as a function of wavelength for the structures with T varying from 100 to 600 nm are shown in Figs. 3a and b, respectively. While four of the five modes shift with the variation of T, only Mode III remains the same wavelength around 785 nm. This is because Mode III is mainly due to the coupling of the SPRs that originate from the opposite sides of the Au thin film, while the thickness of the Cytop layer modulates the intensity of this mode through the resonant mirror effect due to the existence of the Au thick film. Fig. 3c shows the quality factors (QFs) versus the Cytop thickness at the planes of z = 0, 10 and 20 nm, i.e., at the plane

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of the Au/water interface and the planes that are 10 and 20 nm above the Au/water interface, respectively. The QFs show the same trend as a function of the Cytop layer thickness T, exhibiting two maxima at T = 200 and 500 nm, respectively. The QFs decrease by around two orders of magnitude at z = 10 nm compared to those at z = 0 nm, while a slower decrease of about one order of magnitude when extending the distance from 10 to 20 nm. Keeping the thickness of Cytop fixed at 200 nm, L, W, and P were next tuned sequentially. Fig. 4a shows the electric field intensity as a function of wavelength for the nanostructures with the length (L) varied from 480 to 580 nm in an increment of 20 nm. With the increase of L, Mode II and Mode IV become more remarkable and remain almost stationary, while Mode III gets sharper and exhibits a slight red shift. Mode V shows a significant red shift. The variation of intensities at Mode I, Mode II, and Mode V can be neglected during the calculation of QF as they are located out of the integral range (785−914 nm). The intensity of Mode IV increases, while the intensity of Mode III first increases and then decreases, therefore, the strongest value at 785 nm occurs when L = 540 nm. The corresponding electric field distributions of the five modes at the top Au/water interface and the cross-sectional x-z plane for three selected lengths (L = 500, 540 and 580 nm) are displayed in Fig. S3 (ESI†). The variations of the five modes are all due to the fact that longer L causes the coupling between the neighboring arms to strengthen, so the related Mode IV and Mode V get stronger, at the same time weakens Mode I and Mode II, while the localized Mode III experiences the process of energy transferring from the former modes to the latter modes. Keeping the length fixed at 540 nm, the width (W) of the X-shaped nanohole was then varied from 60 to 100 nm with an increment of 10 nm. Fig. 4b shows that by widening W, Mode I and Mode II red shift slightly, while Mode III, Mode IV, and Mode V blue shift. In consideration of QF, the intensities of Mode IV decrease, while the intensity of Mode III first increases and then decreases with the strongest electric field appearing at W = 80 nm. Since the width (W) of the Xshaped nanohole directly influences the opening (O) between the two tips of the X-shaped nanohole, this parameter plays a key role to affect the coupling between the LSPRs generated from the neighboring Au islands. Taking the structures with W = 60, 80, and 100 nm for example (Fig. S4, ESI†), the LSPRs coupling of the neighboring Au islands moves from Mode IV (W = 60 nm), to Mode III (W = 80 nm), and then Mode II (W = 100 nm). For the 785 nm laser, the intensity of Mode III is a dominant value to obtain high QF. Therefore, W = 80 nm is the optimal dimension because of the strongest LSPRs coupling of the neighboring Au islands at Mode III. The most obvious longrange effect is also exhibited for the structure with W = 80 nm. Besides, with the increase of W (i.e., the extension of the opening), Mode I and Mode II become stronger, while Mode IV and Mode V become weaker, which is reverse with the situation of tuning L. Finally, the pitch (P) was varied from 580 to 640 nm in order to obtain the optimal structure. Fig. 4c shows the electric field intensity as a function of wavelength for the structures with fixed L, W and T but varied P. Mode I

and Mode II show almost no change while Mode III, Mode IV,

Fig. 4 (a-c) The simulated electric field enhancement (Emax/E0)2 versus wavelength with varying L, W and P, respectively. (d) Calculated QF as a function of P at the top Au/water interface (z = 0 nm), and at the planes of 10 nm (z = 10 nm) and 20 nm (z = 20 nm) distance from the Au/water interface.

and Mode V all red shift. Fig. S5 (ESI†) shows the variation of electric field distributions for three pitches (P = 580, 604 and 620 nm). The pitch (P) affects the coupling between the Xshaped units, so the variations of the electric field intensities at all the five modes are similar with tuning W (i.e., the opening). The QF as a function of pitch was plotted for the QF at z = 0, 10 and 20 nm (Fig. 4d). The pitch of 585 nm shows the highest QF. Thus, the structure was optimized to be L-W-P-T = 6 540-80-585-200 nm with the highest QF of 4.52 × 10 at the Au/water interface. The reflectance spectrum and the electric field intensity of the optimal structure are displayed in Figs. 5a and b, respectively. The five obvious dips in the reflectance spectrum and five peaks in the electric filed intensity spectrum are in accord with the five modes. The electric field distribution images and the electric charge profiles overlaid with the Poynting vector maps both at the top Au/water interface and at the cross-sectional x-z plane are shown in Fig. S6 (ESI†). The distributions of these five modes for the optimal structure (LW-P-T = 540-80-585-200 nm) are similar to those for the structure (L-W-P-T = 480-80-604-200 nm) displayed in Fig. 2c, while the much stronger electric field of the optimal structure is attributed to the extended length of the X-shaped nanohole and abbreviated pitch resulting in stronger LSPRs coupling between the neighboring edges of each arm and between the

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ARTICLE thin film supporting X-shaped nanohole arrays, SERS spectra

Fig. 6 (a) SEM images at low magnification and (b) high magnification of the LR-SERS substrate with the optimal parameters (L-W-P-T = 540-80-585-200 nm) fabricated via EBL. (c) A 4 μm × 2 μm AFM image of this structure and (d) a line profile showing ~25 nm thick Au thin film with the X-shaped nanohole array.

Fig. 5 (a) The FDTD simulated reflectance spectrum and (b) the plot of electric field 2 intensity (Emax/E0) versus wavelength of the optimal structure (L-W-P-T = 540-80-585200 nm). (c) The decay of calculated QF as a function of distance from the top Au/water interface for the optimal long-range structure and the conventional quasi3D structure reported in ref. 33.

function of the distance from the Au/water interface (Fig. 5c), the electric fields of the long-range plasmonic multilayered nanostructure developed in this work indeed decay more slowly with distance from the Au surface compared to the previously reported X-shaped quasi-3D conventional 33 structure. Finally, a video of the FDTD simulations for the optimal structure is shown in Video S1 (ESI†), which clearly shows the broadband time domain variation of the electric field at the cross-sectional x-z plane, the top Au/water interface, and the Au/Cytop interface of the thick Au film. The multilayered nanostructure substrates with the optimal parameters determined by FDTD simulations were fabricated via EBL followed by chemical etching and lift-off. The SEM image (Fig. 6a) shows the large-area uniform morphology of the top Au thin film with the 2D X-shaped nanohole array. A magnified SEM image (Fig. 6b) further shows the well-defined X-shaped nanoholes. A 4 μm × 2 μm AFM topographic image (Fig. 6c) also displays the highly uniform Xshaped nanoholes. The thickness of the top Au thin film is determined to be ~25 nm from a line profile of the AFM image (Fig. 6d). In order to verify the long-range SERS effect of the multilayered nanostructure substrates, R6G was used as a Raman reporter molecule to measure the SERS signals at the top Au/water interface and at a distance of ~10 nm from the top Au/water interface. For the purpose of maintaining the refractive index matching condition on both sides of the Au

Fig. 7 (a) SERS spectra of 1 mM R6G adsorbed on the Au surface of the optimal LRSERS substrate (the blue curve) and on the surface of the optimal LR-SERS substrate capped with ~10 nm Cytop layer (the red curve). (b) SERS spectra of 4-MBA SAM on the Au surface of the optimal LR-SERS substrate immersed in DI water and different concentrations of salt solution. (c) The plot of the relative intensity at 1078 cm-1 Raman shift in (b) versus the concentration of salt solution.

were taken by immersing the substrates in DI water in a homebuilt Teflon container covered with a cover glass after immersing in 1 mM R6G aqueous solutions for 30 min. Cytop was spun cast on the top of the Au thin film to create a ~10 nm distance from the Au surface while also to maintain the symmetrical dielectric environment. Fig. 7a exhibits the SERS spectra of R6G obtained on a multilayered substrate with the Au thin film with 2D X-shaped nanohole array (the blue curve) and on a multilayered substrate capped with ~10 nm Cytop on the Au thin film with 2D X-shaped nanohole array (the red

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neighboring Au islands. On the basis of the plot of QF as a

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curve). For the bare multilayered substrate, the clearly observed characteristic peaks of R6G are assigned to C–C–C -1 ring in-plane bending (613 cm ), C–H out-of-plane bending -1 -1 (775 cm ), C–H in-plane bending (1016 and 1196 cm ), C–O–C -1 stretching (1317 cm ), and aromatic C–C stretching (1363, -1 37,38 1509, 1602 and 1648 cm ). On the substrate capped with a -1 ~10 nm Cytop layer, peaks at 775, 1196, 1363, and 1509 cm can still be recognized even though they are much weaker. These results confirm that the fabricated substrates with the optimal structure predicted by FDTD simulations exhibit the long-range SERS effect. However, several factors may cause variance between the experimental conditions and the FDTD simulation conditions. The thickness of Au thin film is ~25 nm, not 20 nm used in FDTD simulations. The thicker Au film could reduce the coupling of SPRs in the opposite interfaces of the Au thin film. In addition, the refractive index of the spacing Cytop layer (~200 nm) might be away from that of the bulk Cytop. The spacing Cytop layer was exposed to high energy electrons during EBL, which could change the refractive index of the Cytop layer due to the chemical variation of Cytop. The refractive index of an extremely thin capping Cytop layer (~10 nm) may also be away from the value of the bulk Cytop. Therefore, in order to better match the refractive indices on both sides of the Au thin film, we conducted another set of experiment by immersing a multilayered substrate with preformed 4-MBA SAMs on the top Au surface in pure DI water and NaCl solutions with different concentrations loaded in the Teflon holder to measure SERS signals. It is reported that the refractive indices of NaCl solutions increase linearly with the increase of concentrations. The concentrations of NaCl solutions were varied from 0 to 20% in order to vary the 39 refractive indices from 1.33 to 1.37. The SERS spectra are shown in Fig. 7b, with the characteristic peaks at 1078 and -1 1588 cm corresponding to ν12 and ν8a aromatic ring 40,41 vibrations, respectively. With the increase of NaCl concentrations, SERS intensity is also increased, indicating that the refractive index of aqueous solution indeed affects the SERS performance of the substrate and the better refractive index-matched aqueous solution with the underneath Cytop layer, the stronger the SERS intensity will be. Fig. 7c shows the -1 plot of relative intensity of the SERS signal at 1078 cm obtained from different NaCl solutions to that in pure DI water. It can be clearly noticed that the SERS intensity enhances with the increase of the NaCl concentrations, indicating that the spacing Cytop layer might have a larger refractive index.

Conclusions In summary, we have presented a multilayered nanostructure that can generate long-range SERS effect in aqueous solutions by staking a Au thin film (20 nm) supporting X-shaped nanohole array on top of a water refractive index-matched Cytop layer and a continuous Au thick film (100 nm) coated on a glass substrate. FDTD simulations were performed to investigate the plasmonic properties, to understand the physical origins of the modes displayed by this multilayered

structure, and to obtain the optimal structure that exhibits strong electric fields by the plasmonic resonances within the of interest Stokes Raman scattering range. The optimal structure was determined to have the length, width and pitch of the Xshaped nanohole of 540, 80 and 585 nm, respectively, and the Cytop layer thickness of 200 nm, which exhibited the highest 6 quality factor (QF) of 4.52 × 10 at the top Au/water interface. The symmetrical dielectric environment on both sides of the Au thin film indeed strengthened the local electric field intensity by giving high QFs, and also slowed the decay of QF from the Au/water interface compared to the conventional Xshaped quasi-3D structure. By tuning the length, width and pitch of X-shaped nanoholes and the thickness of Cytop layer, it was figured out that the increase of local electric fields was attributed to the coupling of SPRs on both sides of the Au thin film with the X-shaped nanohole array while the continuous Au thick film functioned as a resonant mirror to prevent the leakage of incident light. Particularly, Mode III at ~785 nm was strongly associated with the Au thin film supporting the Xshaped nanohole array. Its location remained the same with or without the presence of the Au thick film, while its electric filed intensity was significantly enhanced with the presence of the Au thick film. Although the long-range SERS effect was verified by demonstrating the detectable SERS signals when immersing a fabricated substrate capped with a ~10 nm Cytop layer in a R6G aqueous solution, the variation of refractive indices of Cytop layers due to exposure to strong electron beam and extremely thin layer could change the refractive index matched condition that is critical to the LR-SPRs. The stronger SERS intensity was obtained when detecting preformed 4-MBA SAMs on the top Au surface in the better refractive index-matched environment created by salt solutions. This novel multilayered nanostructure, showing strong long-range SERS effect, has promising potential in the applications of SERS-based biosensors for the rapid detection of large biological entities in aqueous solutions.

Conflict of interest There are no conflicts to declare.

Acknowledgements The authors gratefully acknowledge the financial support provided by the National Science Foundation (NSF, CBET 1159609) and the University of Washington (UW) Royalty Research Fund. CH acknowledges a fellowship from the China Scholarship Council. Raman and SEM experiments were performed at the Molecular Analysis Facility (MAF) and EBL was carried out at the Washington Nanofabrication Facility (WNF). The MAF and WNF are part of the National Nanotechnology Coordinated Infrastructure (NNCI) at UW supported by the NSF (ECCS-1542101).

References

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Text: X-shaped gold plasmonic nanohole array embedded in refractive index-matched dielectric media are designed and optimized as a long-range SERS substrate.

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