Optimization of Nanopost Plasmonic Crystals for Surface Enhanced

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Optimization of Nanopost Plasmonic Crystals for Surface Enhanced Raman Scattering Alfred J. Baca,*,† Jason M. Montgomery,‡ Lee R. Cambrea,† Mark Moran,† Linda Johnson,† Jeanine Yacoub,‡ and Tu T. Truong§ †

US NAVY NAVAIR-NAWCWD, Research and Intelligence Department, Chemistry Branch, China Lake, California 93555, United States ‡ Florida Southern College, Chemistry Department, Lakeland, Florida 33801, United States § Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439, United States ABSTRACT: We present experimental and theoretical studies of a type of Surface Enhanced Raman Scattering (SERS) substrate composed of a metal coated square array of nanopost structures formed via soft nanoimprinting. These SERS substrates exhibit higher SERS intensities in comparison to those obtained with the corresponding square array of nanowell structures with similar spatial layouts and demonstrate multiple analyte detection using SERS. Three-dimensional finite-difference timedomain (3D FDTD) simulations qualitatively capture the key features of these systems and suggest a route to the fabrication of optimized, highly efficient SERS substrates in silico. Collectively, the ease of fabrication, high sensitivities, and predictable responses suggest an attractive route to SERS substrates for portable chemical warfare agent detection, environmental monitors, and other applications.

1. INTRODUCTION Surface Enhanced Raman Scattering (SERS) has attracted much interest in the past decade due to recent advances in nanoscale fabrication processes and theoretical modeling.1,2 Advances in these areas have enabled the design of various types of nanostructured form factors, which have been implemented in new device layouts for applications in SERS,3 nanophotonics,4 and metamaterials.5 In general, SERS substrates are composed of rough thin metal films6 or nanostructured form factors.7-15 Recently, the SERS literature has shown that large SERS enhancements can be achieved at regions or gaps between adjacent metal nanostructures due to the presence of high electric fields at these sites. Exploiting the fabrication of metalmetal interstices has been a topic of great interest and has yielded highly efficient substrates for SERS.7,8,16,17 However, the fabrication processes used to produce these materials tend to consist of serial processing techniques8,10,16,18-20 which tend to be slow, possibly not scalable to a large area format, and not to be costeffective. These disadvantages may preclude their use for most envisioned SERS applications. Therefore, despite previous important work on nanostructured SERS substrates, there remains an interest in developing cost-effective SERS substrates that can be produced over large areas and with spatially uniform amplification that can be engineered in silico for operation at different wavelengths. To this end, a potential approach for SERS substrate fabrication utilizes various forms of soft lithographic pattern techniques that are cost-effective and scalable to large areas. Our own work3 and others21,22 have shown that soft imprinting processes can be used for the fabrication of SERS substrates with good enhancement factors. In particular, our recent work on an array of nanowells,3 r 2011 American Chemical Society

an embossed thin layer of relief in a polymer material with a thin layer of metal evaporated on the surface, demonstrated its promise as a suitable substrate for SERS applications. Substrates fabricated in this way showed good Raman enhancements (∼10 5) and sufficient high levels of structure uniformity for precise twodimensional Raman mapping of surface bound monolayers. These substrates have similar optical properties of those systems that show extraordinary light transmission (EOT) effects.23 Various groups have exploited this phenomenon (i.e., EOT) for SERS and SPR sensing. For example, Rogers and Nuzzo have demonstrated, via soft lithography, that limitations of most SPR sensors (e.g., Kretschmann configuration) can be circumvented by using periodic quasi and full 3D plasmonic crystals formed via soft nanoimprinting.24-30 Research studies with plasmonic crystal substrates fabricated in this way have focused on SPR sensing and imaging, and therefore the application of these crystals in SERS represents a new research direction. The present work extends this initial work in several important ways. First, our manuscript provides a new type of SERS substrate consisting of nanopost arrays using replica molding and the detailed SERS analysis and optimization of the newly fabricated nanopost structures. Second, our approach to fabricating nanopost stamps introduces a modification step that eases the fabrication of the replicated PDMS stamps. Previous work in this area consisted of using Perfluoropolyether composite stamps which require special fabrication materials and procedures (i.e., UV source illumination under nitrogen atmosphere, use of PFPE materials) which Received: September 22, 2010 Revised: February 14, 2011 Published: March 16, 2011 7171

dx.doi.org/10.1021/jp109066c | J. Phys. Chem. C 2011, 115, 7171–7178

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can frustrate the processing steps of the nanopost replica stamp. In contrast to our approach, which uses a simple UVO treatment step followed by casting and curing PDMS, which is readily available and easier to process in comparison to PFPE. Third, the use of replica molding specific to these nanopost plasmonic crystals for SERS has not been previously reported. Fourth, our manuscript shows the detection of an emerging contaminant (ammonium perchlorate) using nanopost plasmonic crystal arrays and also shows the different responses of each analyte with respect to the illumination source. Fifth, we also demonstrate theoretical results on an optimized replica molded SERS substrate that not only exploits LSPR contributions but also LSPR and SPP contributions for SERS enhancements. Collectively, we believe these key points represent very important advances and also important and new extensions of previous work. The wellcontrolled, low cost, and easy fabrication of these substrates, coupled with the ability to engineer their SERS response in silico, suggests a strong potential for various modes of use in sensing and other applications.

2. EXPERIMENTAL DETAILS AND COMPUTATIONAL STUDIES 2.1. Materials. Reagents and materials were used as received without further purification. Poly(dimethylsiloxane) s-PDMS, Sylgard 184, (Dow Corning) was purchased from Ellsworth Adhesives. Materials used for the fabrication of hard PDMS (h-PDMS), such as (25-30% methylhydrosiloxane)-(dimethylsiloxane) copolymer (HMS-301), (7-8% vinylmethylsiloxane)-(dimethylsiloxane) copolymer (VDT-731), platinum-divinyltetramethyldisiloxane complex in xylene (SIP6831.2) and (1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane) (SIT7900.0), were all purchased from Gelest. NOA 61 was purchased from Norland products. Ag, SiO2, and Au pellets were purchased from Kurt Lesker. Benzenethiol (BT) was purchased from Aldrich and diluted in ethanol (100%). 2.2. Fabrication of Nanopost Plasmonic Crystals. Figure 1 shows the fabrication steps for the nanopost (NP) geometry plasmonic crystals. The process begins with a pattern of photoresist on a SiO2/Si substrate (i.e., “master”), in the geometry of a square array of cylindrical nanowells with diameters (D) from ∼0.17 to 1.12 μm, with corresponding periodicities (P) from ∼0.50 to 1.75 μm with a relief depth (RD) of ∼0.36 μm. The master used for this study supports 16 individual 4 4 mm2 patterned regions. Figure 1(A) depicts the first step in this process, which consists of mixing and casting a mixture of h-PDMS and s-PDMS composite mold that produces a replica of the “master” consisting of a nanopost array structure. The PDMS nanopost mold is then exposed to a UVO light source for 3 min. To enhance the removal of the soft stamp in the second molding step [Figure 1(B)], the UVO treated nanowell stamp was subsequently placed in a fluorination chamber for 2 h. The second molding step (Figure 1B) utilizes a soft and hard PDMS composite with the same processing conditions and PDMS thicknesses. The third molding step consists of a soft molding process, which starts with casting a few drops of NOA 61 on a glass slide and subsequently placing the nanopost stamp in conformal contact with the NOA-coated glass slide for ∼3 min and UV curing of the assembly for ∼1 h (10 mW/cm 2). Removal of the nanopost mold [Figure 1(C)] produces a square array of nanoposts on the upper-most surface of the NOA with a thickness of ∼0.3 μm (for the patterned relief area) for most

Figure 1. Schematic illustration of the processing steps for the fabrication of nanopost plasmonic crystals. (A) Molding of h-PDMS/S-PDMS against a photoresist patterned silicon master. (B) Second casting step of h-PDMS/S-PDMS against the UVO treated stamp from (A) for the production of the nanopost soft stamp. This step inverts the tone of the nanowell PDMS replica. (C) Soft nanoimprinting of the NOA 61 adhesive and (D) deposition of the SiO2 adhesion layer and metal layer onto the polyurethane molded sample.

of the patterned arrays studied here. The total NOA thickness of the final device on the glass slide including the patterned relief area is ∼50 μm. Blanket deposition of 10 nm thick SiO2 (underlayer) and ∼40 nm thick Au (or Ag) by electron beam evaporation (Angstrom engineering Nexdep system with a sample cooling system) at a deposition pressure of 1.5 10-5 Torr (temperature of 20 C) on the molded layers of NOA completes the fabrication of the nanopost SERS substrates [Figure 1(D)]. 2.3. Transmission, Optical and Raman Characterization. Transmission measurements for nanopost plasmonic crystals were measured with a Varian 5G UV-vis-NIR. The samples were adhered onto a sample holder with a ∼0.75-mm aperture. The samples were prepared for SERS measurements by soaking the metal coated plasmonic arrays in a 15 mM ethanolic solution 7172

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of Benzenethiol (BT) for ∼1.5 h immediately after depositing the thin metal film, rinsing with copious amounts of ethanol and dried with nitrogen. The SERS spectra were collected with a Nicolet Almega XR Dispersive Raman microscope (Thermo Electron Corp. Madison, WI) using a 785 nm excitation laser and an Olympus 10X MPlan objective. A 50-μm pinhole was used as the spectrograph aperture. The resolution is ∼9-17 cm-1 with a spot size of ∼3 μm, and each spectrum is an average of 3 scans with a 10 s integration time. Scanning electron microscopy (SEM) measurements were performed on a FE-SEM (Jeol JSM-7500F) scanning electron microscope, and Atomic Force Microscopy (AFM) imaging and analysis was performed with a Digital Instruments (DI) 3100 and DI software, respectively. 2.4. FDTD Calculations. In order to understand the relative SERS response of the nanopost and nanowell geometries, we used full three-dimensional finite-difference time-domain (3DFDTD) simulations.31 Electromagnetic fields, E and H, were propagated numerically in the two substrates using similar spatial layouts as determined by our experimental results. The source, a Gaussian pulse with spectral content covering the 300-1000 nm wavelength range and polarized in the x-direction, was introduced using the total-field scattered field31 approach and propagated in the z-direction. We used nair = 1.0 and nNOA = 1.56 for the refractive indices of air and NOA, respectively. A Drude plus two-Lorentzian fit to Lynch-Hunter empirical data32 over the 300-1000 nm wavelength range was used to model the frequency dependent relative dielectric constant, εAu, for gold: εM ðωÞ ¼ ε¥ -

ω2D ω2 þ iγD ω

2

gL ωL Δε ∑ 2 2 m ¼ 1 ω - ω þ i2γ m

Lm

m

Lm ω

ð1Þ

where the parameters used are those given in ref 33. An auxiliary differential equation approach31 consistent with eq 1 was used to incorporate the dispersive εAu into the FDTD update equations. Periodic boundary conditions were implemented to mimic the periodicity of the array in the x- and y- directions, and uniaxial perfectly matched layers31 were used to absorb field components approaching the outer grid edges in the z-direction. Transmission (T) and reflection (R) spectra were calculated by taking the ratio of the transmitted and reflected power to the incident power integrated over planes above and below the structure, corresponding to the zero-order transmission and reflection, respectively. Absorption at each wavelength was then calculated via A = 1 - T - R. The time-dependent electric field was Fourier transformed at λ = 821 nm in order to calculate the time-averaged electric field intensity enhancement (g2 = |E|2/ |E0|2) at each grid point, where λ = 821 nm is the average of the excitation wavelength at 785 nm and the Raman response wavelength at 840 nm.3,34 The simulation time was 120 fs with a grid spacing in each dimension of 2 nm.

3. RESULTS AND DISCUSSION Advantages of using soft imprint lithography for SERS substrate fabrication involve the ability to (i) reuse the stamp numerous times with minimal losses in the mold quality, (ii) generate large area samples with well controlled hot spots (unlike certain self-assembly methods), and (iii) manufacture low cost molds of nanostructured designs and geometries for rapid high throughput analyses. Figure 2 depicts a schematic illustration and optical characterization of the nanopost structures formed via soft nanoimprint lithography. Samples fabricated in this way

Figure 2. (A) Schematic cross-sectional view and (B) SEM image of a representative plasmonic nanopost array. Inset in (B) shows a high resolution (tilted 30) SEM image of a row of nanopost plasmonic crystals. (C) Optical image of a completed SERS substrate.

consist of films of Au (or Ag) with square lattices of post-like nanostructures and a recessed film of Au (or Ag) with residual Au (or Ag) on the sidewalls of the relief of the nanopost features. Figure 2(A) illustrates a schematic cross-sectional view of the nanopost arrays. It should be noted that residual Au clusters are present and are distributed randomly along the sidewall area; however, the majority of the metal is present on the upper most surface of the nanopost structure. The nanometal clusters on the sidewalls have a large impact on the optical and SERS responses. The thickness of the deposited metal for both Au and Ag is ∼40 nm. Figure 2(B) shows a field emission scanning electron micrograph of a large area view of the nanopost samples and a high resolution inset depicting a 30 tilted view of the nanopost structure. Figure 2(C) shows an optical image of the completed nanopost sensor array, which supports 16 square nanopost arrays with varying dimensions. Color variations in Figure 2(C) are due to ambient lighting and diffractive effects of the different periodicities of each array. To evaluate the optical properties of the as-fabricated nanopost plasmonic crystals, we measured normal incidence transmission spectrum in air for the wavelength range of 300 2200 nm. Figure 3 shows the collected spectra from four different Au arrays with diameters of ∼0.18 μm (red), of ∼0.68 μm (black), of ∼1.06 μm (blue), and of ∼0.45 μm (green) between 400 and 1000 nm. The broad spectral features between 400 and 600 nm are related to the transmission properties of our thin films of Au/SiO2. The broad features between 600 and 1000 nm can be assigned to LSPR associated with the spatial layout of the nanopost structures, as shown by FDTD simulations (see details below). Clear assignment of each spectral peak can be difficult due to periodic nature of these subwavelength arrays and the presence of metal clusters.28,29 Our experimental setup may contribute to some of the observed losses shown in the measured transmission spectra. We show below using FDTD simulations that these plasmonic effects can be used to enhance the surface enhanced Raman scattering properties of these arrays. Optimization of these substrates was required for SERS applications. Interestingly, the results using metal coated NOA 7173



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Figure 3. Transmission spectra of Au coated nanopost plasmonic crystals with diameters of D = 0.18 μm; P = 0.58 μm (red), D = 0.68 μm; P = 1.10 μm (black), D = 1.06 μm; P = 1.64 μm (blue), and D = 0.45 μm; P = 0.73 μm (green). Dotted line corresponds to the excitation wavelength.

substrates for Au and Ag showed little or no observable SERS response in contrast to molded SU-8 nanostructures. In order to better understand the effects of the NOA on the deposited metal film, we performed detailed AFM imaging to elucidate the morphology of the as-deposited metal thin films. Figure 4A-D shows detailed AFM images of a 0.5 0.5 μm area of (A) Au/ NOA, (B) Au/SiO2/NOA, (C) Ag/NOA, and (E) Ag/SiO2/ NOA. Variation in the morphology of the deposited metal layers were determined by the AFM software, which revealed significant changes in the metal morphology, in the grain height (g) and the Rms surface roughness. The average grain height and roughness for Au/NOA was g = 4.56 nm, and Rms = 1.177 nm, in contrast to the Au/SiO2/NOA substrate which revealed values of g = 5.565 nm and Rms = 1.658. As shown in the AFM images, the surface morphology of the Au changes from a relatively smooth surface to a surface with a higher degree of morphology (roughness). A similar result was produce by Ag-coated films as shown in Figure 4(C),(D) but to a larger extent. The grain height of Ag/ NOA was g = 2.732 nm and Rms = 0.776 nm contrast to the Ag/ SiO2/NOA showed a g = 5.684 nm and Rms = 2.012 nm. A direct comparison to the SERS response and that of the different metal/ dielectric films is shown in Figure 4E from samples with nanopost arrays with the following spatial dimensions: D = 0.61 μm and P = 1.0 μm. The Ag (red) and Au (black) SERS spectra on SiO2/NOA are shown in the figure as solid line spectra while the dotted line spectra shows the SERS response of Ag (red) and Au (black) on NOA without a SiO2 underlayer. These results show that the quality of the metal films and the deposition conditions have a large effect on the SERS response, especially for the Au coated nanopost arrays. These results also indicate the possibility of further tuning the SERS response by using different underlayers that may provide a rougher surface or a different refractive index. These results partially illuminate the origin of the SERS response observed from these types of substrates and provide routes for optimization for future SERS substrates based on replica molded layers. In order to compare Au verses Ag coatings, we measured the SERS response of surface-adsorbed BT for each array of both Auand Ag-coated nanopost substrates. Raman spectra were collected between 3500 and 400 cm-1 with a 785 nm excitation source. Raman spectra showed no distinguishable Raman peaks at 2500 cm-1 (thiol; -SH), indicating a single monolayer of BT was formed on the surfaces of each of the nanopost plasmonic crystals. Figure 5(A) contains a plot of the SERS spectra for Au coated nanopost arrays with diameters of ∼0.18 μm (red), of

Figure 4. AFM images of nanopost arrays with different dielectric layers. (A) Au/NOA, (B) Au/SiO2/NOA, (C) Ag/NOA, and (D) Ag/ SiO2/NOA. (E) SERS spectra (from top to bottom) of nanopost arrays (D = 0.61 μm; P = 1.00 μm) with different metal/dielectric films Ag/ SiO2/NOA (red solid line) and Au/SiO2/NOA (black solid line), and Ag/NOA (red dotted line) and Au/NOA (black dotted line).

∼0.25 μm (black), of ∼0.75 μm (blue), and of ∼1.06 μm (green). The roughness (Rms) of the Au film on SiO2 /NOA is =1.658 nm and that of Ag on SiO2/NOA is =2.012 nm over a 2.5 2.5 μm area as determined by the AFM. SERS measurements over the nonpatterned substrate areas produce no measurable SERS response for either Au or Ag coated nanopost plasmonic crystal substrates, indicating that a significant aspect of the measured SERS response results from the nanopost array geometry and not due to specific thin film properties of the deposited metal. The large SERS response is due to a combination of two well-known effects: a surface-moderated chemical effect35 and, more importantly, (ii) an electromagnetic effect36 from the large electric field localization surrounding the edges and surfaces of the circular metallic discs on the top surface the nanopost plasmonic crystals and the bottom Au film [see Figure 7(C)]. Figure 5(B) depicts the Raman spectra of the analogous arrays (∼ 0.18 μm (red), of ∼0.25 μm (black), of ∼0.75 μm (blue), and of ∼1.06 μm (green))of the Ag coated nanopost arrays. The Ag arrays showed higher Raman responses in comparison to Au coated nanopost arrays, depending on the diameter of the specific array with values ranging from ∼1 up to as high as 3 increases in SERS intensities. We suspect that the larger SERS enhancements of Ag coated nanopost arrays can be partially attributed to the higher degree of surface roughness for Ag coated substrates vs Au coated nanopost substrates. The samples show good amplification uniformity over large areas and good sample to sample reproducibility. These studies indicate that SERS substrates fabricated via soft 7174



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Figure 6. SERS spectra of Au coated nanopost arrays with D = 0.17 μm; P = 0.49 μm exposed to 1 mM ammonium perchlorate and 15 mM BT collected with different excitation lasers 532 nm (red spectra) and 785 nm (black spectra).

the EM properties of the thin film. Molded plasmonic crystals fabricated in this way are a current topic of study. The enhancement factors were determined using previously established methods in the literature.3 The analytical SERS enhancement factor for Au and Ag coated plasmonic crystal was found to be ∼105 and ∼106 for our best devices respectively. The EF in this case, is the ratio of the nanopost SERS signal to the Raman signal measured from the nanostructured (i.e., flat metal area). The enhancement factors for these types of SERS substrates can be difficult to qualitatively capture because the electric field is localized in a very small region at the edges of the nanopost structures and therefore, the surfaceenhanced Raman scattering originates only from a small number of molecules located in these regions. To establish the dependency of the SERS response on spatial layout, we performed detailed experimental studies of SERS response for Au and Ag coated nanopost arrays as a function of nanopost diameter. Figure 5(C) depicts the Raman intensity (integrated peak area for the Raman band at 1073 cm-1) as a function of diameter for each of the 16 individual nanopost arrays. For the Au nanopost arrays, it is clear that the SERS response is quite sensitive to the diameter of the nanopost, exhibiting a narrow maximum for D = 0.25 μm. It is interesting to note that a similar trend was seen for the Au nanowell arrays of our previous work,3 where a narrow maximum was seen at D = 0.51 μm. In the case of Ag nanopost arrays, the maximum observed SERS response is somewhat higher than that of Au, and a more broad range is seen for large SERS intensities for the Ag nanoposts, spanning D = 0.18 - 0.51 μm. We used 3D FDTD to model the experimental trend observed for Au coated nanopost arrays. Figure 5d contains a plot of the integrated electric field intensity enhancement,G2, as a function of D for Au coated nanopost arrays, such that, Figure 5. (A) SERS spectra of 15 mM BT adsorbed onto a molded nanopost SERS substrate composed of square lattices of Au and (B) Ag nanopost arrays with different diameters(D = 0.18 μm; P = 0.58 μm (red), D = 0.25 μm; P = 0.56 μm (black), D = 0.75 μm; P = 1.1 μm (blue), and D = 1.06 μm; P = 1.64 μm (green). (C) SERS response as a function of diameter for both Au (left y-axis; blue) and Ag (right y-axis; orange) nanopost arrays. (D) FDTD results for the integrated electric field enhancement (G2) as a function of D for Au coated nanopost arrays.

imprint lithography, for optimal SERS responses, should be composed of Ag coated arrays with a thin over layer of metal (e.g., Au) to control the oxidation of the Ag and possibly enhance

G2 ¼

∑i ∑j g 2

ð2Þ

where i and j index grid points in the x and z direction, respectively, for a cross-section of the structure in the xz plane and involves only those points in air for which g2 g 100. The results are plotted in Figure 5(D) and are consistent with those shown in Figure 5(C), thereby suggesting the possibility to fabricate SERS substrates with predictable responses. Figure 6 shows the dependency of Raman excitation source and the corresponding SERS response for Au coated nanopost plasmonic crystals. In this case, nanopost arrays exposed to a 7175



Figure 7. (A) Experimental SERS spectra of nanowell (red) and nanopost (black) geometries for 15 mM BT adsorbed onto plasmonic crystals. (B) Plot of absorption for the nanopost (black) and nanowell (red) geometries using 3D-FDTD simulations. (C) Plot of enhancement factor, g2 = |E|2/|E0|2, for the nanowell and nanopost superimposed on a plot of the relative dielectric constants comprising each structure. Only values of g2 between 20 and 220 are displayed, where g2 = 250 was the maximum enhancement observed for the nanowell. A maximum enhancement of g2 = 600 was observed for the nanopost.

15 mM BT solution (1.5 h) were subsequently treated with a drop (∼ 5 μL) of 1 mM ammonium perchlorate. The drop was allowed to dry and the Raman spectrum of the sample was measured with two different excitation sources. Interestingly, when the sample array was exposed to a 785 nm laser, the BT Raman bands are present with small but detectable traces of vibrational bands which correspond to perchlorate. However, upon exposure to a 532 nm laser, the BT bands are suppressed and the perchlorate bands are now detectable (higher signal intensity). This process (i.e., changing laser sources) does not change the SERS peak positions only the relative intensity of the bands related to the analytes: presumably due to a shift in the LSPR. The dependence of the polarizability tensor and Raman cross-section on the excitation frequency is a known relationship that can be exploited to yield strong Raman enhancements when the excitation frequency approaches an electronic transition of an analyte.37 Although this relationship is already mathematically

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established, these results still provide an interesting data set demonstrating that the SERS enhancement is different to these two analytes at different excitation wavelengths. Instead of looking at the Raman enhancement as a function of excitation frequency we demonstrate the ability to “tune” the enhancement of one analyte relative to another, on the same surface, using this relationship. This result indicates the possibility of using the nanopost arrays for the detection of multi species, all on the same substrate, with different excitation sources in a way that can enhance or suppress undesired spectral response of common interferents and enhance those of the species of interest. As an initial comparison between the nanopost and nanowell geometries, we fabricated a separate set of nanowell and nanopost samples and measured the SERS response of an array of Au coated nanowells and nanopost structures with similar spatial dimensions. Due to shrinkage rates and other processing variables, the exact dimensions of the two different geometries vary to some extent even with implementing the same master for stamp fabrication. The nanowell geometry had the following spatial dimensions D = ∼188 nm, P = ∼519 nm, RD = ∼200 nm and the nanopost geometry D =∼220 nm, P =∼523 nm, RD =∼200 nm with a metal thickness of ∼24 nm. Figure 7(A) shows the Raman spectra of 15 mM BT adsorbed onto nanowell and nanopost plasmonic crystals that were fabricated under similar conditions. The results show that the nanopost array, for this particular geometry, shows much higher Raman response ∼8 in comparison to the SERS response from the nanowell plasmonic crystal array. To understand the SERS response and illuminate the key physics of the above-mentioned nanopost and nanowell arrays, we performed detailed 3D FDTD simulations using similar spatial layouts as determined by our experimental results. In particular, P = 520 nm, D = 200 nm, RD = 200 nm, and tm = 24 nm, where tm is the gold metal thickness. Figure 7(B) shows the calculated zero-order absorption spectra for the nanopost and nanowell arrays. A broad LSPR absorption (A) peak is seen for the nanopost at 888 nm, with an absorption value of A = 0.54 at 821 nm, while the nanowell shows a sharp LSPR absorption peak at 790 nm, with A = 0.29 at 821 nm. Figure 7(C) shows the corresponding plot of the time-averaged electric field enhancements, g2 = |E|2/|E0|2, at λ = 821 nm superimposed on a plot of the relative dielectric constants comprising each structure. Maximum enhancements of g2 = 120 and g2 = 600 were observed for the nanowell and nanopost, respectively. This factor of 5 increase in g2 for the nanopost suggests a 25 times greater Raman response (g4) in the nanopost when excited with 785 nm light due to a nearby shape resonance, consistent with the results presented in Figure 7(A). However, we note that variation in the spatial dimensions of these crystals can cause the difference observed in the measured SERS results. Similar calculations of g2 computed at 790 nm, at which a sharper shape resonance exists for the nanowell and not for the nanopost, indeed yields maximum enhancements of g2 = 470 and g2 = 400 for the nanowell and nanopost, respectively, suggesting that the nanowell would produce a higher Raman response. Experimental results of 790 nm LSPR tuned Au coated nanowells vs nanopost arrays show a qualitative agreement and consistent with our theoretical predictions (data not shown). While FDTD calculations of the transmission spectra for the nanopost substrates qualitatively agree with experimental results, we have not been able to get quantitative agreement due to the sensitivity of the transmission spectrum to defects caused by isolated grains of 7176



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periodicity, P, is given by the following:33 1=2 P εðλSPP Þ λSPP ¼ Re εðλSPP Þ þ 1:0 ðm21 þ m22 Þ1=2

ð3Þ

where ε is the complex dielectric of the metal, and m1 and m2 = 0, 1, 2, .... Using P = 806 nm and eq 1 for ε, we find that the (m1,m2) = (1,0) BW-SPP occurs at λSPP = 820 nm.34 Experimental verification of these results are currently underway, and our future work is geared toward developing new masters (fabricated via electron beam lithography methods) for the fabrication of novel form factors for SERS.

Figure 8. Plot of electric field intensity enhancements (g2) for optimized Au nanopost (A) and nanowell (B) arrays. Values of g2 > 100 are displayed in white. The nanopost geometry consisted of D = 200 nm, P = 700 nm, and RD = 250 nm, leading to a factor of 9 increase in the predicted SERS response. The nanowell geometry consisted of D = 400 nm, P = 806 nm, and RD = 120 nm, leading to factor of 100 in predicted SERS response due to interactions of BW-SPPs and LSPR resonances.

gold on the sidewalls of the nanopost. The effect of these defects on the calculated transmission spectrum has been previously reported.28 As a result, soft imprint lithography allows for the fabrication of substrates engineered such that they are composed of both nanowell and nanopost geometries within the same array and suggest a powerful and versatile platform for sensing different molecules with possibly improved sensitivities toward a particular analyte. Finally, we looked at the potential for using 3D-FDTD simulations as an in silico route to optimization of our SERS substrates. For the Au-coated nanopost geometry, we chose as a control the array described above that led to the highest SERS response with dimensions of RD = 200 nm, D = 224 nm, and P = 584 nm with max g2 = 600. As a first approximation, we considered RD, D, and P to be uncoupled. That is, we first maximized g2 with respect to RD, then with respect to D, and then with respect to P. With RD = 250 nm, D = 200 nm, and P = 700 nm, we found a max g2 = 1800, a factor of 3 higher electric field intensity enhancement and 9 times higher SERS response. Figure 8a shows the resulting electric field intensity distribution. Values of g2 > 100 are shown in white. For the Au-coated nanowell geometry, we chose as a control the array with geometry given by RD = 200 nm, D = 200 nm, P = 520 nm and g2 = 120. With RD = 120 nm, D = 400 nm, and P = 806 nm, we found g2 = 1110, almost an order of magnitude greater electric field intensity enhancement, giving rise to 100 greater SERS response! Figure 8b shows the resulting electric field intensity distribution. The reason for the large enhancement is the coupling of BW-SPPs with the LSPR of the hole. The wavelength needed to excite a BW-SPP at an air/metal interface with a

4. CONCLUSIONS We have presented theoretical and experimental results of nanopost plasmonic crystal arrays formed by soft imprint lithography. This simple and easy to use method affords the fabrication of SERS substrates with good spatial uniformity and reproducible SERS response in contrast to SERS substrates fabricated via certain colloid assembly methods. The as-produced substrates are highly versatile, and the process lends itself to large area fabrication of unusual format optical nanostructures for applications in sensing and other emerging areas in nanophotonics. Our analysis shows that for a particular spatial dimension and LSPR wavelength, nanopost arrays show a higher SERS response in comparison to the corresponding nanowell structure. Further analysis shows the ability to engineer the SERS response for these nanopost structures in good agreement with theoretical predictions, suggesting a route to high performance SERS substrates in silico that can exploit not only enhancements of LSPRs but also of coupled SPP-LSPRs as well, leading to a predicted 100 enhancement over previous substrates considered. We show that careful selection of metal dielectric layers is critical to the production of SERS responses. Fully optimized nanopost arrays (e.g., metal layer thicknesses, Au-Ag nanopost arrays, variation in dielectric constant of the support substrate and others) may show better SERS response and, therefore, suggest possible use for applications in environmental monitoring and others. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the NAVAIR Independent Applied Research (IAR) program managed by Scott Munro. The computational portion of this research used resources of the National Energy Research Scientific Computing Center, which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. A.J.B. would like to thank Prof. John A. Rogers for generously donating masters used in this work. J.M.M. and J.Y. would like to thank Jeffrey M. McMahon for his parallel 3D FDTD code. A.J.B. would like to thank Dan Connor for SEM measurements and Drs. M.J. Roberts and G. Ostrom and Mr. Tyler A. Cain for helpful discussions. Use of the Center for Nanoscale Materials was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. 7177



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’ NOTE ADDED AFTER ASAP PUBLICATION This article was published ASAP on March 16, 2011. Text changes have been made in section 2.2, section 3, and the captions to Figures 3, 5, 6, and 7. Figure 5 has also been modified. The correct version was published on March 25, 2011.

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