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Lithography-free approach to highly efficient, scalable SERS substrates based on disordered clusters of disc-on-pillar structures
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IOP PUBLISHING
NANOTECHNOLOGY
Nanotechnology 24 (2013) 505302 (9pp)
doi:10.1088/0957-4484/24/50/505302
Lithography-free approach to highly efficient, scalable SERS substrates based on disordered clusters of disc-on-pillar structures Rebecca L Agapov1 , Bernadeta Srijanto1,2 , Chris Fowler1 , Dayrl Briggs1 , Nickolay V Lavrik1 and Michael J Sepaniak3 1
Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996, USA 3 Department of Chemistry, University of Tennessee, Knoxville, TN 37996, USA 2
E-mail: [email protected]
Received 29 July 2013, in final form 21 October 2013 Published 27 November 2013 Online at stacks.iop.org/Nano/24/505302 Abstract We present a lithography-free technological strategy that enables fabrication of large area substrates for surface-enhanced Raman spectroscopy (SERS) with excellent performance in the red to NIR spectral range. Our approach takes advantage of metal dewetting as a facile means to create stochastic arrays of circular patterns suitable for subsequent fabrication of plasmonic disc-on-pillar (DOP) structures using a combination of anisotropic reactive ion etching (RIE) and thin film deposition. Consistent with our previous studies of individual DOP structures, pillar height which, in turn, is defined by the RIE processing time, has a dramatic effect on the SERS performance of stochastic arrays of DOP structures. Our computational analysis of model DOP systems confirms the strong effect of the pillar height and also explains the broadband sensitivity of the implemented SERS substrates. Our Raman mapping data combined with SEM structural analysis of the substrates exposed to benzenethiol solutions indicates that clustering of shorter DOP structures and bundling of taller ones is a likely mechanism contributing to higher SERS activity. Nonetheless, bundled DOP structures appeared to be consistently less SERS-active than vertically aligned clusters of DOPs with optimized parameters. The latter are characterized by average SERS enhancement factors above 107 . S Online supplementary data available from stacks.iop.org/Nano/24/505302/mmedia (Some figures may appear in colour only in the online journal)
1. Introduction
noble metal particle [9] and standing waves at the reflecting substrate [10]. Hence, optimization of DOP structures involves tuning their geometrical parameters so that the LSPR in the disc-shaped plasmonic particle and a maximum of the standing wave in its vicinity occur at a wavelength of interest [1, 6]. Additional field enhancement mechanisms can take place in pairs or larger ensembles of DOP structures separated by distances close to molecular sizes. Indeed, the concept of plasmonic dimers with nanogaps has already been very fruitful in demonstrating record values of near
Nanosized pillars capped with noble metals are hybrid plasmonic systems that combine multiple mechanisms of optical field enhancement [1, 2] and, therefore, are very promising as substrates for surface-enhanced Raman spectroscopy (SERS) [3–7]. Using such disc-on-pillar (DOP) structures with appropriate diameters and heights, it is possible to achieve synergistic coupling of the localized surface plasmon resonance (LSPR) [8] in the disc-shaped 0957-4484/13/505302+09$33.00
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c 2013 IOP Publishing Ltd Printed in the UK & the USA
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Figure 1. (a) Schematic diagram of the fabrication sequence used in the present study to create stochastic DOP arrays. (a1) A 5 nm layer of Pt is deposited onto a Si wafer with 100 nm of thermally grown SiO2 . (a2) Annealing in H2 :Ar at ∼900 ◦ C led to the formation of circular Pt islands due to metal dewetting. (a3) Pt islands serve as an etch mask during anisotropic RIE of SiO2 and Si. (a4) After removing thermal SiO2 and Pt, pillars are coated with 20 nm of conformal ALD SiO2 and metallized with Ag or Au. (b) SEM images of circular Pt islands formed as a result of dewetting a 5 nm thick Pt film. (c) SEM image of 320 nm tall nanopillar array after Pt dewetting taken at a 30◦ angle. (d) Cross-sectional SEM of 320 nm tall DOPs after completion of the fabrication sequence. (e) An optical image of a 100 mm Si wafer with such structures.
field enhancement [11–13] and enabling SERS experiments at the single-molecule level [14, 15]. Although DOP arrays with sub-2 nm inter-particle separations have been recently implemented using a combination of lithographic patterning and atomic layer deposition [16], more facile technological pathways that could enable similar SERS substrates are of great importance. In the present work we sought to establish an alternative, more scalable technological strategy that would enable highly efficient SERS substrates based on DOP structures. The majority of previous efforts in creating DOP arrays and analogous structures relied primarily on electron beam lithography (EBL) as a nanopatterning tool with the appropriate resolution [3, 6, 17–20]. When used as SERS substrates, these deterministically created structures exhibited remarkably high SERS enhancement factors (EFs) [3, 6, 17, 18, 21]. While a fully deterministic technological approach based on EBL provides excellent control over the location and sizes of plasmonic particles, aperiodic or disordered DOP arrays may have distinct advantages as SERS substrates. Indeed, previous studies of aperiodic arrays [22–24] and fractal ensembles of plasmonic particles have shown that additional enhancements of the near field intensity can be achieved due to the excitation of collective plasmonic resonances and cascade enhancement mechanisms [25–27]. It is also recognized that the magnitude of averaged SERS signals generated from microscale areas populated by heterogeneous plasmonic particles can be less sensitive to changes in incident wavelengths and angles due to a multitude of LSPRs distributed over the spectral range of interest [28]. Finally, SERS efficiency of periodic plasmonic arrays depends strongly on the incident angle and wavelength
of the probing light due to grating effects [29]. These effects are expected to be far less pronounced in randomized arrays of DOP structures. Here, we take advantage of solid state dewetting [30] in thin platinum (Pt) films as a facile means of creating stochastic arrays of circular masking patterns [21, 31, 32] with sizes similar to those in highly SERS-active DOP structures created previously by EBL patterning [6]. The thermally processed Pt layer with nanoscale circular patterns is used as a selective mask for anisotropic reactive ion etching (RIE) of the substrate material. While dewetting of gold (Au) films can lead to the formation of plasmonic structures [33] which can be subsequently used as a selective mask during RIE, our approach relies on a separate Pt masking pattern formed prior to deposition of the plasmonic metal. This allows us to achieve more precise control over the nanopillar diameters independent of the plasmonic metal thickness and avoid possible carbon contamination of the plasmonic metal during high temperature processing steps [34–37].
2. Experimental details 2.1. Fabrication The fabrication sequence is summarized in figure 1(a). In brief, single side polished single-crystal silicon (Si) wafers (100) with 100 nm of thermally grown silicon oxide (SiO2 ) were used as a starting material. A 5 nm thick layer of Pt was deposited onto a Si wafer using physical vapor deposition (PVD) in a vacuum evaporator equipped with an electron gun source (Thermonics Laboratory, VE-240). Wafers with a 2
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Pt layer were then thermally processed for 8 s at ∼900 ◦ C in a mixture of argon and hydrogen (10:1) at a pressure of 735 Torr in a cold wall furnace (Easy Tube 3000, First Nano, Ronkonkoma, NY) equipped with a radiative heat source set to its maximum power (22 kW). The resulting dewetted Pt layer then served as a mask during anisotropic RIE of the SiO2 and Si [21, 31, 32]. The RIE was carried out in an Oxford PlasmaLab system (Oxford Instruments, UK) using a combination of inductively coupled plasma (ICP) and capacitively coupled plasma (CCP). The 100 nm of SiO2 was etched in a mixture of C4 F8 and O2 at flow rates of 45 sccm and 2 sccm, respectively, at 15 ◦ C, 7 mTorr for 55 s. The anisotropic etching of Si was carried out at 10 mTorr in a SF6 :C4 F8 :Ar mixture defined by respective flow rates of 56, 25 and 5 sccm. Different nanopillar heights were obtained by varying the Si RIE time from 20 to 540 s. The heights of the pillars were extrapolated using the RIE etch time. The DOP diameters were determined using SEM (Carl Zeiss Merlin) analysis. Unless noted otherwise, RIE was followed by wet etch in a buffered oxide etchant (BOE) (6:1 NH4 F in HF). Atomic layer deposition (ALD) of SiO2 was carried out using an Oxford FlexAl tool to coat the resulting Si nanopillars with a 20 nm thick conformal layer. To create DOP structures, 25 nm of either Ag or Au was deposited on chips with nanopillars using PVD (Thermonics Laboratory, VE-240 evaporator).
laser power to facilitate comparison across all the samples. A 100 s acquisition time at 150 µW laser power was used to collect reference Raman spectra of PAN on a plain Si surface. Raman maps were collected with 0.3 µm lateral resolution (stage movement steps), with 2 s acquisition time per step at 30 µW laser power. In order to correlate SEM images and Raman maps, the perimeter of a region of interest was marked with a focused laser beam (633 nm, 1.1 mW) using the Raman microscope before Raman signal acquisition. The resulting marks created boundaries for square areas clearly discernible with optical microscopy. The corners of the Raman maps were used to pinpoint the locations of specific pixels. SERS spectra were analyzed using the WiRE 3.4 software in the Raman system. Average SERS EFs were calculated according to the widely used procedures [38, 39] as a raw signal increase (RSI) normalized by the ratio of analyte concentration from neat BT (Raman standard) and a self-assembled monolayer of BT on the metal discs (N): Peak Area with Pillars Peak Area without Pillars − 1 RSI EF = = (1) x((π r2 )+(2πrh))t1 N πW 2 Dt2
where x is the average number of nanopillars in the beam spot, r is the average radius of a plasmonic disc, h is the average thickness of the plasmonic disc, t1 is the surface density of BT in a monolayer, W is the focused laser spot width (diameter), D is the probing depth of the beam and t2 is the volumetric density of BT as a neat liquid. Calculations of EF according to equation (1) assume that BT self-assembled on the exposed surface of the plasmonic disc contributes evenly to the measured SERS signals. This is a conservative estimate of the average SERS EF as it is likely that the majority of the measured SERS signal is generated by a small fraction of the plasmonic particle surface [17, 38].
2.2. Substrate preparation After fabrication, some Ag and Au metallized chips were exposed to benzenethiol (BT) (Sigma-Aldrich) as a test analyte commonly used to estimate the SERS EFs. The chips with metallized pillars were soaked in a solution of BT (1 mg ml−1 ) in isopropanol for 10 min. The pillars were then removed from solution and rinsed with isopropanol to remove all of the unbound BT. A few chips with Ag metallized DOPs were coated with polyacrylonitrile (PAN) as a polymeric sample that does not exhibit resonance Raman. This PAN film was deposited onto the chips with DOP arrays using plasma polymerization in a low frequency glow discharge plasma (6 kHz, 1 mA cm−2 ). The chamber was pumped to a vacuum of 100 mTorr and then refilled with a 1:1 mixture of argon and acrylonitrile to a pressure of approximately 800 mTorr. A deposition time of 60 s resulted in a PAN film of 120 nm as measured with ellipsometry on a satellite Si chip.
3. Results and discussion The thermal processing recipe and Pt layer thickness used in this study were selected based on the targeted diameters of the DOP structures. Dewetting of a 5 nm Pt layer deposited on a Si wafer with 100 nm of thermally grown SiO2 resulted in reproducible formation of circular patterns which are stochastic in nature (figure 1(b)). However, such patterns had a fairly narrow distribution of characteristic diameters (55 ± 15 nm). This distribution was within the targeted range since it corresponded to DOP diameters of 97 ± 20 nm (figures 1(c) and (d)) which are expected to exhibit a LSPR in the spectral range of interest (633–785 nm) [1, 6]. The increase in the DOP diameter compared to the diameter of the masking pattern is largely due to the 20 nm ALD SiO2 spacer layer that was deposited to separate the plasmonic particle from the Si pillar and avoid quenching of the LSPR [6]. Dewetting of Pt films slightly thicker than 5 nm resulted in patterns with substantially larger diameters. It should be noted that the high temperature annealing of Pt layers deposited directly onto Si or onto Si with much less than 100 nm SiO2 did not result in the formation of the desired
2.3. Raman spectroscopy and EF calculations All Raman spectra were collected in backscattering geometry using a Renishaw inVia Raman Microscope with a 100× objective (Leica, 100× NA 0.85 80) using either 633 nm (HeNe laser) or 785 nm (Innovative Photonic Solutions R-Type Laser Module) incident illumination. A 10 s acquisition time at 30 µW laser power was used to acquire SERS spectra in all cases except when characterizing BT on Ag metallized DOP arrays at 633 nm. The data for that series were collected at 3 µW laser power to avoid signal saturation. The reported SERS signals are normalized by the 3
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nanoscale patterns, most likely due to interdiffusion and chemical interaction between Pt and Si. The nanoscale pattern of circular Pt islands formed on Si wafers with a 100 nm SiO2 buffer layer served as a robust hard mask during the RIE of both SiO2 and Si with Si RIE times up to 360 s. All the RIE times used in the present study (except 540 s) resulted in pillars with the Pt mask remaining intact (figure 1(c)). Subsequent exposure of the chips with such pillars to BOE was used to remove both the Pt and SiO2 layers, resulting in silicon pillars with flat tops. Although Pt was not chemically attacked by BOE it was removed due to removal of the underlying SiO2 . RIE time of 540 s resulted in a lower density of pillars with sharp tips (figure 2(c)). This change in the morphology can be explained by the undercutting of Si below the Pt mask and complete removal of narrower pillars during prolonged RIE of Si. Wetting of the latter type of chips in water or isopropanol solutions followed by their drying in air resulted in irreversible bundling of adjacent pillars as shown in figure 2(d). In the final step of our fabrication sequence, deposition of either Ag or Au resulted in the formation of the plasmonic disc-shaped particles. Notably, a metal mirror layer is simultaneously formed on the Si substrate in the areas not occupied by DOP structures. Despite the directionality of the metal deposition process, some smaller nanoparticles form on the sidewalls of the pillars (figure 1(d)). Such metal nanodroplets on the pillar sidewalls are known to lead to the formation of additional hot spots [4, 16, 21]. The particular thickness of the deposited metal (25 nm) was chosen in the present study largely based on the following two considerations: (i) it was found to be optimal in previous studies of individual DOP structures [6] and (ii) films thicker than 25 nm are more likely to bridge closely spaced DOP structures [4]. Analysis of the SEM images taken after metallization revealed, however, that under our current fabrication conditions some bridging of adjacent structures does occur leading to the formation of vertically aligned clusters.
Figure 2. SEM images taken with a 30◦ tilt of metallized DOP arrays that are (a) 210 nm, (b) 500 nm and (c) 1200 nm tall. (d) 1200 nm tall DOP array where adjacent pillars have bundled after thiolization. All scale bars are 500 nm.
3.1. Optimization of DOP height height showed the best SERS performance for the probing wavelength of 633 nm. Figure 3(a) also shows SERS EFs calculated as the fourth power of the normalized maximum near field, |Emax |4 /|E0 |4 , derived from our finite difference time domain (FDTD) simulations. These simulations (see supporting information and figure S1, available at stacks.iop. org/Nano/24/505302/mmedia, for details) were performed for simplified individual DOP structures with a 100 nm diameter Au disc and pillar heights in the range of 50–275 nm. As can be seen in figure 3(a), there is a clear correlation between the theoretically predicted and experimentally observed dependences of EF on the pillar height. Since the clusters of DOP structures in our experimental system are characterized by a stochastic distribution of sub-50 nm gaps between adjacent plasmonic particles (figure 1(c)), it is important to understand how coupling in DOP clusters may affect their SERS performance. Taking into account previous studies of planar plasmonic dimers [40, 41], we
In order to optimize the height of the DOP structures, a series of DOP arrays were created while incrementally increasing the RIE time (figures 2(a)–(c)). The main idea of optimizing the DOP height is to take advantage of standing waves formed near a reflecting surface [10]. It was anticipated that the interference-enhanced SERS in our DOP arrays can be achieved when the top surface of the nanopillar coincides with the maximum of the standing wave. To evaluate SERS activity of chips with different pillar heights, we acquired SERS spectra and analyzed areas of the characteristic BT peak at 1072 cm−1 as a function of pillar height. As can be seen in figures 3(a) and (b), regardless of the probing wavelength, the strongest SERS raw signals were observed for the DOPs etched for 60 s, which corresponds to a pillar height of approximately 160 nm (table 1). This pillar height correlates well with the findings of our previous study [6] in which individual EBL patterned DOP structures with 175 nm 4
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Figure 3. The average peak area (n = 3) for the 1072 cm−1 ring breathing mode band of BT presented as a function of DOP height for DOPs metallized with (a) 25 nm of Au and (b) 25 nm of Ag. The solid lines are a guide to the eye. FDTD predictions of EF as a function of DOP height are shown in (a) with dashed lines. Table 1. Normalized peak area (NPA) for Ag and Au metallized DOP arrays using the 1072 cm−1 ring breathing mode of BT as a reference. All of the peak areas are normalized by the peak area of the DOP array etched for 60 s and metallized with 25 nm of Au. All of the DOPs fabricated were 97 (±20) nm in diameter. The heights of the nanopillars were extrapolated from the RIE etch time. The peak areas are included in the supporting information as table S1 (available at stacks.iop.org/Nano/24/505302/mmedia). Etch time (s)
Pillar height (nm)
NPA for 25Au at 633 nm
NPA for 25Au at 785 nm
NPA for 25Ag at 633 nm
NPA for 25Ag at 785 nm
20 40 60 80 100 120 360 540
50 105 160 210 265 320 500 1200
0.72 0.84 1 0.41 0.8 0.68 0.23 0.07
0.88 1.49 1.99 0.68 1.06 1.47 0.68 —
3.07 10.9 15 10.2 10.9 10.2 — —
0.26 1.02 1.45 1.01 0.41 1.05 — —
The Ag metallized DOP arrays illuminated with 633 nm incident light performed the best in this series of DOP arrays. Although the Au coated DOP arrays exhibit nearly one order of magnitude lower raw SERS signals compared to the Ag metallized arrays, they demonstrate more consistent performance at different wavelengths and are not prone to chemical degradation. Consistent with the Au dielectric function, the Au coated DOP arrays performed slightly better at the longer probing wavelength (785 nm). To elucidate whether there are particular morphologies or arrangements of individual DOP structures associated with the particularly strong SERS enhancement, high resolution SERS mapping was performed on BT treated samples fabricated close to theoretically predicted optimal parameters (nanopillars 100 nm in diameter and 160 nm in height [1, 6]). A representative SERS map from the Au metallized DOP array etched for 60 s is shown in figure 4(c). The pixels exhibiting the largest SERS signal, shown in red, corresponded to a factor of 2–3 larger peak area than the surrounding regions. SEM imaging of the regions responsible for the large SERS signals, shown in figures 4(a) and (b), revealed the morphology of the structures responsible for the high enhancements of the Raman signal. In the case of the 160 nm long DOPs, the structure consisted of a few vertically aligned DOPs that had formed a cluster during fabrication. The high SERS signals generated by optimized
anticipated that dimerization of DOP structures would lead to a red-shifted LSPR and additional near field enhancement. Our FDTD analysis of dimeric DOP structures indicates that, indeed, the spectral location of the LSPR in the DOP dimer with a 10 nm gap is red-shifted by nearly 100 nm while |Emax |4 /|E0 |4 values increased more than 100-fold. Nonetheless, the dependence of |Emax |4 /|E0 |4 on pillar height is only slightly affected by the dimerization of DOP structures. Additional details of our FDTD analysis are provided in the supporting information as figures S2 and S3 (available at stacks.iop.org/Nano/24/505302/mmedia). In addition to the 60 s etch that produced the best performing DOP structures, there is a second maximum in the SERS performance corresponding to taller pillars (figures 3(a) and (b)). This second maximum corresponds to 100 s and 120 s etch time for 633 nm and 785 nm probing wavelengths, respectively. The periodic nature of the plots shown in figures 3(a) and (b) is consistent with the spatial periodicity of the standing waves formed near the reflecting metallized surface (figure S2 in supporting information available at stacks.iop.org/Nano/24/505302/mmedia). We believe the heterogeneity and complexity of the DOP arrays used in the actual experiments are responsible for the slight difference observed between the simulated trend and the experimentally measured results shown in figure 3(a). 5
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without bundled pillar arrangements can surpass the SERS performance of similar bundled structures. In order to investigate the effects of pillar bundling on the SERS performance of DOP arrays, a second batch of DOPs were fabricated using the same fabrication process as described above, however with significantly longer Si RIE times. We anticipated that, above a certain pillar height, DOP structures would be sufficiently flexible to enable self-organization into converging bundles upon action of capillary forces as observed previously [5, 44, 45]. Indeed, by extending the RIE time to 540 s we created DOP arrays that were approximately 1200 nm tall and readily formed converging bundles after exposure to liquids (figure 2(d)). In particular, we observed bundling of these 1200 nm tall DOP arrays upon their thiolization in isopropanol solutions. DOP arrays with somewhat shorter pillars (500 nm) exhibited bundling behavior, but to a much lesser extent. Even though there are slight differences between these two groups of DOPs, meaningful information can still be gleaned from a quantitative comparison between the average performance of clusters of DOPs and bundles of DOPs. Both groups of DOPs are comprised of stochastic arrays of pillars that contain differences in their diameters and heights, even for a given sample. This heterogeneity and complexity of the arrays increases the range of usable probing wavelengths for a given sample. Also, the ability for nanopillars to self-organize into bundles depends on the mechanical properties of the pillars. The bundled structures cannot be formed from short nanopillars that are expected to be optimal for optical enhancement. In this range of diameters, the only way to increase flexibility is to increase the length of the nanopillars. Testing of various DOP arrays with the 633 nm probing wavelength shows that the bundled pillars with Au coating produced average SERS signal intensities 14 times lower in comparison to the best performing DOP arrays (see table 1). The longer RIE time required to produce nanopillars that are flexible enough to self-organize into bundles resulted in undercutting and removing some of the DOPs with smaller diameters. As a result, DOP arrays with bundled pillars had much lower average density (11 pillars per µm2 compared to 44 pillars per µm2 in the arrays with shorter DOPs). Nonetheless, the bundled DOPs performed a factor of 4 poorer than the shorter clustered DOPs, even when taking the DOP density into account. To elucidate whether there are particular morphologies of DOP bundles or arrangements of individual DOP structures associated with the SERS enhancement, we performed high resolution SERS mapping of the bundled DOP arrays and correlated these maps with the SEM images. A representative SEM image of the Au metallized 1200 nm tall DOP array and a respective Raman map acquired in the same area are shown in figures 5(a) and (b), respectively. We found that converging bundles of several DOP structures were responsible for pixels with anomalously high SERS intensities (exceeding average SERS signals 2–3 times). This is qualitatively consistent with several recent studies [5, 42, 43] which indicated that bundled structures are responsible for SERS hot spots. Nonetheless, our current results indicate that consistently
Figure 4. (a) SEM image of 160 nm tall Au metallized DOP array. (b) The area marked in red points to a specific morphology responsible for SERS intensities 2–3 times larger than the surrounding regions. (c) Map of the 1072 cm−2 peak intensity acquired from a 6 µm × 6 µm area of the sample shown in panel (a). Each pixel in the map is 0.3 µm × 0.3 µm.
DOP arrays do not necessarily require the presence of nanogaps, as previous work with individual DOP structures led to enhancement factors of the order of 109 and excellent SERS performance when their structural parameters were optimized [6]. Nonetheless, SEM images of the DOP arrays fabricated here point to the possibility that nanogap regions, such as the small gap between adjacent Au discs seen in figures 4(a) and (b), may play a significant role in the largest SERS signals observed. Unlike nanogaps in glass nanopillar arrays explored previously [4], hot spots in the DOP arrays described here are most likely located at or near the very top of the DOPs due to the slightly negative (re-entrant) sidewall profiles characteristic of the Si RIE recipes used in the present work. The bases of the DOPs are also sufficiently far apart that nanogaps are unlikely to form between the walls of adjacent nanopillars (figure 1(d)). Such hot spots located on the tops of the DOP arrays rather than within hidden gap regions would be highly advantageous for SERS analysis of hard polymeric samples that may not readily penetrate into the subsurface layers of the SERS substrate. 3.2. Analysis of comparative performance between vertically aligned clusters and converging bundles of DOPs Several reports have indicated that bundled pillar structures are associated with highly SERS-active hot spots due to nanogaps in the pinching regions [5, 42, 43]. The question, however, remains whether optimized DOP arrays 6
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Figure 5. (a) SEM image of Au metallized DOP arrays approximately 1200 nm long that formed converging bundles after thiolization. (b) Map of the 1072 cm−2 peak intensity acquired from a 6 µm × 6 µm area of the sample shown in panel (a). Each pixel in the map is 0.3 µm × 0.3 µm. The rectangle marked in panel (a) points to a typical morphology responsible for SERS intensities 2–3 times larger than the surrounding regions in the map.
Figure 6. (a) Representative SERS spectrum of BT obtained from the SERS map of the 160 nm DOP array (red) and the 1200 nm DOP array (black). SEM images in the inset are representative of the 160 nm vertically aligned clusters and the 1200 nm converging bundles. (b) Peak areas plotted as a function of the number of DOPs per pixel for the 160 nm DOPs (red diamonds) and the 1200 nm DOPs (black dots).
determined by factors other than the stochastically fluctuating density of DOPs in these arrays. As our comparative analysis of SEM images and SERS maps indicates, such factors likely include subtle morphological features in DOP clusters and their bundles. While raw SERS signals are useful for comparative analysis of different SERS substrates, under identical experimental conditions, average EF is commonly used as an instrument-independent figure of merit. Our calculations of average EFs were based on approximations of uniform arrays and a diffraction-limited probing area of approximately 0.3 µm × 0.3 µm. Under these assumptions, the enhancement factors calculated for the DOP arrays fabricated here range from 9.7 × 106 to 6.6 × 107 . This figure of merit places the optimized DOP arrays described and studied here in a highly competitive category.
higher SERS signals are produced by clusters of vertically aligned DOP structures with optimized height (figure 6). The DOP arrays that were long enough to exhibit some degree of bundling, but that were not etched long enough to alter the original density (i.e. 360 s RIE etch time, 500 nm long DOPs), exhibited a wider range of SERS intensities. Some of the observed hot spots could be correlated with the presence of bundled structures, while the location of some others coincided with single DOPs (see figure S4 in supporting information available at stacks.iop.org/Nano/24/ 505302/mmedia). The intensities of the hot spots for these arrays were approximately a factor of 3 higher than those of the 1200 nm DOP arrays. However, the average SERS signal was still lower than that of the best performing 160 nm DOP array (table 1). In order to better understand how the stochastically varying density of DOP structures in the arrays studied here affects the non-uniformity of their SERS activity, sections of the SERS maps were analyzed pixel by pixel while the number of DOPs in each pixel was determined using SEM images. As can be seen in figure 6(b), each pixel in the acquired Raman maps is characterized by a SERS intensity that does not have any clear correlation with the number of DOP structures occupying that pixel. This somewhat unexpected result means that the observed variability in SERS intensity is largely
3.3. Demonstration of SERS using a polymeric analyte without resonance Raman excitation High SERS activity demonstrated using a model analyte does not always translate into more broadly applicable SERS performance, especially when generic analytes without resonance Raman excitation and those that do not chemically interact with noble metals are used. As an additional criterion 7
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By tuning the processing parameters, facile control over the geometries of the resulting DOP structures was achieved. Using the same straightforward processing sequence, vertically aligned clusters of DOP structures as well as DOP structures aggregated into converging bundles were fabricated. Although some of the bundled DOP structures exhibited very strong SERS signals, they were consistently less SERS-active than vertically aligned DOPs with optimized parameters. The latter exhibited average EFs exceeding 107 . Consistent with our previous observations [6], pillar height was found to have a strong effect on the SERS activity of the prepared samples. These experimental findings are in excellent agreement with our computational analysis applied to a simplified model of DOP structures with variable height. Raman mapping correlated to SEM images indicates that subtle changes in the DOP morphology can lead to significant changes in the SERS activity.
Figure 7. Raman spectra from a 120 nm thin film of PAN on a DOP array (160 nm, metallized with 25 nm of Ag) (red) and on a flat Si wafer reference (black). The characteristic peak from stretching vibration due to the C≡N side groups of PAN is easily observed in the SERS spectrum at 2225 cm−1 .
Acknowledgments of SERS performance, we evaluated the ability of the disordered DOP arrays to detect the signal from a polymer film that does not exhibit a resonant Raman spectrum. For this purpose a 120 nm film of polyacrylonitrile (PAN) was deposited onto the chips with DOP arrays via plasma polymerization. The characteristic peak from the stretching vibration due to the C ≡ N side groups of PAN was readily observed in the SERS spectrum at 2225 cm−1 as shown in figure 7. In the bulk Raman spectrum, this characteristic peak is barely observable above the noise in the background, even when 10× longer acquisition time with 5× higher laser power was used. The shift in the characteristic position of the C≡N peak from 2244 cm−1 in the Raman spectrum to 2225 cm−1 in the SERS spectrum is consistent with the PAN being attached to the silver metallized DOPs through the side group nitrogens [46]. This peak shift supports that the majority of the observed SERS signal originates from the first few nanometers of the polymer coating the DOP array rather than the entire thin film [47]. The ability of the aperiodic DOP arrays to pass this test for analytical performance proves their versatility in the detection of a wide range of chemical compounds.
This research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Division of Scientific User Facilities, US Department of Energy.
References [1] Polemi A, Wells S M, Lavrik N V, Sepaniak M J and Shuford K L 2010 J. Phys. Chem. C 114 18096 [2] Polemi A, Wells S M, Lavrik N V, Sepaniak M J and Shuford K L 2011 J. Phys. Chem. C 115 13624 [3] Cadwell J D, Glembocki O, Bezares F J, Bassim N D, Rendell R W, Feygelson M, Ukaegbu M, Kasica R, Shirey L and Hosten C 2011 ACS Nano 5 4046 [4] Oh Y-J and Jeong K-H 2012 Adv. Mater. 24 2234 [5] Schmidt M S, H¨ubner J and Boisen A 2012 Adv. Mater. 24 OP11 [6] Wells S M, Polemi A, Lavrik N V, Shuford K L and Sepaniak M 2011 Chem. Commun. 47 3814 [7] Wu H-Y and Cunningham B T 2011 Appl. Phys. Lett. 98 153103 [8] Murray W A and Barnes W L 2007 Adv. Mater. 19 3771 [9] Kuttge M, de Abajo F J G and Polman A 2010 Nano Lett. 10 1537 [10] Shoute L C T, Bergren A J, Mahmoud A M, Harris K D and McCreery R L 2009 Appl. Spectrosc. 63 133 [11] Im H, Bantz K C, Lindquist N C, Haynes C L and Oh S H 2010 Nano Lett. 10 2231 [12] Liang H Y, Li Z P, Wang Z X, Wang W Z, Rosei F, Ma D L and Xu H X 2012 Small 8 3400 [13] Camden J P, Dieringer J A, Wang Y M, Masiello D J, Marks L D, Schatz G C and Van Duyne R P 2008 J. Am. Chem. Soc. 130 12616 [14] Le Ru E C, Meyer M and Etchegoin P G 2006 J. Phys. Chem. B 110 1944 [15] Lim D-K, Jeon K-S, Kim H M, Nam J-M and Suh Y D 2010 Nature Mater. 9 60 [16] Caldwell J D et al 2011 Opt. Express 19 26056 [17] Hatab N A, Hsueh C-H, Gaddis A L, Retterer S T, Li J-H, Eres G, Zhang Z and Gu B 2010 Nano Lett. 10 4952 [18] Gopinath A, Boriskina S V, Premasiri W R, Ziegler L, Reinhard B M and Dal Negro L 2009 Nano Lett. 9 3922 [19] Kumar K, Duan H, Hegde R S, Koh S C W, Wei J N and Yang J K W 2012 Nature Nanotechnol. 7 557
4. Conclusion A new technological approach to large area stochastic arrays of plasmonic discs supported on silicon nanopillars was developed using a metal dewetting-assisted fabrication method. The implemented lithography-free wafer level processing sequence allows for the straightforward, rapid and reproducible formation of highly efficient large area SERS substrates. Importantly, the final step of the developed fabrication sequence is deposition of Au or Ag, which requires only a standard vacuum evaporation or sputtering tool and can be performed by the end user at the most appropriate time. For instance, in the case of Ag, which is prone to environmental degradation, this step can be postponed until shortly before SERS measurements. 8
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[34] Takagi D, Kobayashi Y, Hibino H, Suzuki S and Homma Y 2008 Nano Lett. 8 832 [35] Bhaviripudi S, Mile E, Steiner S A III, Zare A T, Dresselhaus M S, Belcher A M and Kong J 2007 J. Am. Chem. Soc. 129 1516 [36] Takagi D, Homma Y, Hibino H, Suzuki S and Kobayashi Y 2006 Nano Lett. 6 2642 [37] Lazar M D, Biris A R, Borodi G, Voica C, Watanabe F, Dervishi E and Biris A S 2013 J. Mater. Sci. 48 7409 [38] Le Ru E C, Blackie E, Meyer M and Etchegoin P G 2007 J. Phys. Chem. C 111 13794 [39] Haynes C L and Van Duyne R P 2003 J. Phys. Chem. B 107 7426 [40] Bouhelier A, Bachelot R, Im J S, Wiederrecht G P, Lerondel G, Kostcheev S and Royer P 2005 J. Phys. Chem. B 109 3195 [41] Hao E and Schatz G C 2004 J. Chem. Phys. 120 357 [42] Kim A, Ou F S, Ohlberg D A A, Hu M, Williams S and Li Z 2011 J. Am. Chem. Soc. 133 8234 [43] Barcelo S J, Kim A, Wu W and Li Z 2012 ACS Nano 6 6446 [44] Neukirch S, Roman B, de Gaudemaris B and Bico J 2007 J. Mech. Phys. Solids 55 1212 [45] Duan H and Berggren K K 2010 Nano Lett. 10 3710 [46] Xue G, Dong J, Zhang J and Sun Y 1994 Polymer 35 723 [47] Le Ru E C and Etchegoin P G 2009 Principles of Surface-Enhanced Raman Spectroscopy and Related Plasmonic Effects (New York: Elsevier)
[20] Ou F S, Hu M, Naumov I, Kim A, Wu W, Bratkovsky A M, Li X, Williams R S and Li Z 2011 Nano Lett. 11 2538 [21] Li W D, Ding F, Hu J and Chou S Y 2011 Opt. Express 19 3925 [22] Lawrence N, Trevino J and Dal Negro L 2012 J. Appl. Phys. 111 113101 [23] Gopinath A, Boriskina S V, Reinhard B M and Dal Negro L 2009 Opt. Express 17 3741 [24] Forestiere C, Pasquale A J, Capretti A, Miano G, Tamburrino A, Lee S Y, Reinhard B M and Dal Negro L 2012 Nano Lett. 12 2037 [25] Li K, Stockman M I and Bergman D J 2003 Phys. Rev. Lett. 91 227402 ˇ [26] Dai J, Cajko F, Tsukerman I and Stockman M I 2008 Phys. Rev. B 77 115419 [27] Dal Negro L, Feng N-N and Gopinath A 2008 J. Opt. A: Pure Appl. Opt. 10 064013 [28] Moskovits M 2013 Phys. Chem. Chem. Phys. 15 5301 [29] Baltog I, Primeau N, Reinisch R and Coutaz J L 1995 Appl. Phys. Lett. 66 1187 [30] Thompson C V 2012 Annu. Rev. Mater. Res. 42 399 [31] Lee J-M and Kim B-I 2007 Mater. Sci. Eng. A 449–451 769 [32] Strobel S, Kirkendall C, Chang J-B and Berggren K K 2010 Nanotechnology 21 505301 [33] He J-Y, Lu J-X, Dai N and Zhu D-M 2012 J. Mater. Sci. 47 668
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Lithography-free approach to highly efficient, scalable SERS substrates based on disordered clusters of disc-on-pillar structures
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IOP PUBLISHING
NANOTECHNOLOGY
Nanotechnology 24 (2013) 505302 (9pp)
doi:10.1088/0957-4484/24/50/505302
Lithography-free approach to highly efficient, scalable SERS substrates based on disordered clusters of disc-on-pillar structures Rebecca L Agapov1 , Bernadeta Srijanto1,2 , Chris Fowler1 , Dayrl Briggs1 , Nickolay V Lavrik1 and Michael J Sepaniak3 1
Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996, USA 3 Department of Chemistry, University of Tennessee, Knoxville, TN 37996, USA 2
E-mail: [email protected]
Received 29 July 2013, in final form 21 October 2013 Published 27 November 2013 Online at stacks.iop.org/Nano/24/505302 Abstract We present a lithography-free technological strategy that enables fabrication of large area substrates for surface-enhanced Raman spectroscopy (SERS) with excellent performance in the red to NIR spectral range. Our approach takes advantage of metal dewetting as a facile means to create stochastic arrays of circular patterns suitable for subsequent fabrication of plasmonic disc-on-pillar (DOP) structures using a combination of anisotropic reactive ion etching (RIE) and thin film deposition. Consistent with our previous studies of individual DOP structures, pillar height which, in turn, is defined by the RIE processing time, has a dramatic effect on the SERS performance of stochastic arrays of DOP structures. Our computational analysis of model DOP systems confirms the strong effect of the pillar height and also explains the broadband sensitivity of the implemented SERS substrates. Our Raman mapping data combined with SEM structural analysis of the substrates exposed to benzenethiol solutions indicates that clustering of shorter DOP structures and bundling of taller ones is a likely mechanism contributing to higher SERS activity. Nonetheless, bundled DOP structures appeared to be consistently less SERS-active than vertically aligned clusters of DOPs with optimized parameters. The latter are characterized by average SERS enhancement factors above 107 . S Online supplementary data available from stacks.iop.org/Nano/24/505302/mmedia (Some figures may appear in colour only in the online journal)
1. Introduction
noble metal particle [9] and standing waves at the reflecting substrate [10]. Hence, optimization of DOP structures involves tuning their geometrical parameters so that the LSPR in the disc-shaped plasmonic particle and a maximum of the standing wave in its vicinity occur at a wavelength of interest [1, 6]. Additional field enhancement mechanisms can take place in pairs or larger ensembles of DOP structures separated by distances close to molecular sizes. Indeed, the concept of plasmonic dimers with nanogaps has already been very fruitful in demonstrating record values of near
Nanosized pillars capped with noble metals are hybrid plasmonic systems that combine multiple mechanisms of optical field enhancement [1, 2] and, therefore, are very promising as substrates for surface-enhanced Raman spectroscopy (SERS) [3–7]. Using such disc-on-pillar (DOP) structures with appropriate diameters and heights, it is possible to achieve synergistic coupling of the localized surface plasmon resonance (LSPR) [8] in the disc-shaped 0957-4484/13/505302+09$33.00
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c 2013 IOP Publishing Ltd Printed in the UK & the USA
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Figure 1. (a) Schematic diagram of the fabrication sequence used in the present study to create stochastic DOP arrays. (a1) A 5 nm layer of Pt is deposited onto a Si wafer with 100 nm of thermally grown SiO2 . (a2) Annealing in H2 :Ar at ∼900 ◦ C led to the formation of circular Pt islands due to metal dewetting. (a3) Pt islands serve as an etch mask during anisotropic RIE of SiO2 and Si. (a4) After removing thermal SiO2 and Pt, pillars are coated with 20 nm of conformal ALD SiO2 and metallized with Ag or Au. (b) SEM images of circular Pt islands formed as a result of dewetting a 5 nm thick Pt film. (c) SEM image of 320 nm tall nanopillar array after Pt dewetting taken at a 30◦ angle. (d) Cross-sectional SEM of 320 nm tall DOPs after completion of the fabrication sequence. (e) An optical image of a 100 mm Si wafer with such structures.
field enhancement [11–13] and enabling SERS experiments at the single-molecule level [14, 15]. Although DOP arrays with sub-2 nm inter-particle separations have been recently implemented using a combination of lithographic patterning and atomic layer deposition [16], more facile technological pathways that could enable similar SERS substrates are of great importance. In the present work we sought to establish an alternative, more scalable technological strategy that would enable highly efficient SERS substrates based on DOP structures. The majority of previous efforts in creating DOP arrays and analogous structures relied primarily on electron beam lithography (EBL) as a nanopatterning tool with the appropriate resolution [3, 6, 17–20]. When used as SERS substrates, these deterministically created structures exhibited remarkably high SERS enhancement factors (EFs) [3, 6, 17, 18, 21]. While a fully deterministic technological approach based on EBL provides excellent control over the location and sizes of plasmonic particles, aperiodic or disordered DOP arrays may have distinct advantages as SERS substrates. Indeed, previous studies of aperiodic arrays [22–24] and fractal ensembles of plasmonic particles have shown that additional enhancements of the near field intensity can be achieved due to the excitation of collective plasmonic resonances and cascade enhancement mechanisms [25–27]. It is also recognized that the magnitude of averaged SERS signals generated from microscale areas populated by heterogeneous plasmonic particles can be less sensitive to changes in incident wavelengths and angles due to a multitude of LSPRs distributed over the spectral range of interest [28]. Finally, SERS efficiency of periodic plasmonic arrays depends strongly on the incident angle and wavelength
of the probing light due to grating effects [29]. These effects are expected to be far less pronounced in randomized arrays of DOP structures. Here, we take advantage of solid state dewetting [30] in thin platinum (Pt) films as a facile means of creating stochastic arrays of circular masking patterns [21, 31, 32] with sizes similar to those in highly SERS-active DOP structures created previously by EBL patterning [6]. The thermally processed Pt layer with nanoscale circular patterns is used as a selective mask for anisotropic reactive ion etching (RIE) of the substrate material. While dewetting of gold (Au) films can lead to the formation of plasmonic structures [33] which can be subsequently used as a selective mask during RIE, our approach relies on a separate Pt masking pattern formed prior to deposition of the plasmonic metal. This allows us to achieve more precise control over the nanopillar diameters independent of the plasmonic metal thickness and avoid possible carbon contamination of the plasmonic metal during high temperature processing steps [34–37].
2. Experimental details 2.1. Fabrication The fabrication sequence is summarized in figure 1(a). In brief, single side polished single-crystal silicon (Si) wafers (100) with 100 nm of thermally grown silicon oxide (SiO2 ) were used as a starting material. A 5 nm thick layer of Pt was deposited onto a Si wafer using physical vapor deposition (PVD) in a vacuum evaporator equipped with an electron gun source (Thermonics Laboratory, VE-240). Wafers with a 2
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Pt layer were then thermally processed for 8 s at ∼900 ◦ C in a mixture of argon and hydrogen (10:1) at a pressure of 735 Torr in a cold wall furnace (Easy Tube 3000, First Nano, Ronkonkoma, NY) equipped with a radiative heat source set to its maximum power (22 kW). The resulting dewetted Pt layer then served as a mask during anisotropic RIE of the SiO2 and Si [21, 31, 32]. The RIE was carried out in an Oxford PlasmaLab system (Oxford Instruments, UK) using a combination of inductively coupled plasma (ICP) and capacitively coupled plasma (CCP). The 100 nm of SiO2 was etched in a mixture of C4 F8 and O2 at flow rates of 45 sccm and 2 sccm, respectively, at 15 ◦ C, 7 mTorr for 55 s. The anisotropic etching of Si was carried out at 10 mTorr in a SF6 :C4 F8 :Ar mixture defined by respective flow rates of 56, 25 and 5 sccm. Different nanopillar heights were obtained by varying the Si RIE time from 20 to 540 s. The heights of the pillars were extrapolated using the RIE etch time. The DOP diameters were determined using SEM (Carl Zeiss Merlin) analysis. Unless noted otherwise, RIE was followed by wet etch in a buffered oxide etchant (BOE) (6:1 NH4 F in HF). Atomic layer deposition (ALD) of SiO2 was carried out using an Oxford FlexAl tool to coat the resulting Si nanopillars with a 20 nm thick conformal layer. To create DOP structures, 25 nm of either Ag or Au was deposited on chips with nanopillars using PVD (Thermonics Laboratory, VE-240 evaporator).
laser power to facilitate comparison across all the samples. A 100 s acquisition time at 150 µW laser power was used to collect reference Raman spectra of PAN on a plain Si surface. Raman maps were collected with 0.3 µm lateral resolution (stage movement steps), with 2 s acquisition time per step at 30 µW laser power. In order to correlate SEM images and Raman maps, the perimeter of a region of interest was marked with a focused laser beam (633 nm, 1.1 mW) using the Raman microscope before Raman signal acquisition. The resulting marks created boundaries for square areas clearly discernible with optical microscopy. The corners of the Raman maps were used to pinpoint the locations of specific pixels. SERS spectra were analyzed using the WiRE 3.4 software in the Raman system. Average SERS EFs were calculated according to the widely used procedures [38, 39] as a raw signal increase (RSI) normalized by the ratio of analyte concentration from neat BT (Raman standard) and a self-assembled monolayer of BT on the metal discs (N): Peak Area with Pillars Peak Area without Pillars − 1 RSI EF = = (1) x((π r2 )+(2πrh))t1 N πW 2 Dt2
where x is the average number of nanopillars in the beam spot, r is the average radius of a plasmonic disc, h is the average thickness of the plasmonic disc, t1 is the surface density of BT in a monolayer, W is the focused laser spot width (diameter), D is the probing depth of the beam and t2 is the volumetric density of BT as a neat liquid. Calculations of EF according to equation (1) assume that BT self-assembled on the exposed surface of the plasmonic disc contributes evenly to the measured SERS signals. This is a conservative estimate of the average SERS EF as it is likely that the majority of the measured SERS signal is generated by a small fraction of the plasmonic particle surface [17, 38].
2.2. Substrate preparation After fabrication, some Ag and Au metallized chips were exposed to benzenethiol (BT) (Sigma-Aldrich) as a test analyte commonly used to estimate the SERS EFs. The chips with metallized pillars were soaked in a solution of BT (1 mg ml−1 ) in isopropanol for 10 min. The pillars were then removed from solution and rinsed with isopropanol to remove all of the unbound BT. A few chips with Ag metallized DOPs were coated with polyacrylonitrile (PAN) as a polymeric sample that does not exhibit resonance Raman. This PAN film was deposited onto the chips with DOP arrays using plasma polymerization in a low frequency glow discharge plasma (6 kHz, 1 mA cm−2 ). The chamber was pumped to a vacuum of 100 mTorr and then refilled with a 1:1 mixture of argon and acrylonitrile to a pressure of approximately 800 mTorr. A deposition time of 60 s resulted in a PAN film of 120 nm as measured with ellipsometry on a satellite Si chip.
3. Results and discussion The thermal processing recipe and Pt layer thickness used in this study were selected based on the targeted diameters of the DOP structures. Dewetting of a 5 nm Pt layer deposited on a Si wafer with 100 nm of thermally grown SiO2 resulted in reproducible formation of circular patterns which are stochastic in nature (figure 1(b)). However, such patterns had a fairly narrow distribution of characteristic diameters (55 ± 15 nm). This distribution was within the targeted range since it corresponded to DOP diameters of 97 ± 20 nm (figures 1(c) and (d)) which are expected to exhibit a LSPR in the spectral range of interest (633–785 nm) [1, 6]. The increase in the DOP diameter compared to the diameter of the masking pattern is largely due to the 20 nm ALD SiO2 spacer layer that was deposited to separate the plasmonic particle from the Si pillar and avoid quenching of the LSPR [6]. Dewetting of Pt films slightly thicker than 5 nm resulted in patterns with substantially larger diameters. It should be noted that the high temperature annealing of Pt layers deposited directly onto Si or onto Si with much less than 100 nm SiO2 did not result in the formation of the desired
2.3. Raman spectroscopy and EF calculations All Raman spectra were collected in backscattering geometry using a Renishaw inVia Raman Microscope with a 100× objective (Leica, 100× NA 0.85 80) using either 633 nm (HeNe laser) or 785 nm (Innovative Photonic Solutions R-Type Laser Module) incident illumination. A 10 s acquisition time at 30 µW laser power was used to acquire SERS spectra in all cases except when characterizing BT on Ag metallized DOP arrays at 633 nm. The data for that series were collected at 3 µW laser power to avoid signal saturation. The reported SERS signals are normalized by the 3
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nanoscale patterns, most likely due to interdiffusion and chemical interaction between Pt and Si. The nanoscale pattern of circular Pt islands formed on Si wafers with a 100 nm SiO2 buffer layer served as a robust hard mask during the RIE of both SiO2 and Si with Si RIE times up to 360 s. All the RIE times used in the present study (except 540 s) resulted in pillars with the Pt mask remaining intact (figure 1(c)). Subsequent exposure of the chips with such pillars to BOE was used to remove both the Pt and SiO2 layers, resulting in silicon pillars with flat tops. Although Pt was not chemically attacked by BOE it was removed due to removal of the underlying SiO2 . RIE time of 540 s resulted in a lower density of pillars with sharp tips (figure 2(c)). This change in the morphology can be explained by the undercutting of Si below the Pt mask and complete removal of narrower pillars during prolonged RIE of Si. Wetting of the latter type of chips in water or isopropanol solutions followed by their drying in air resulted in irreversible bundling of adjacent pillars as shown in figure 2(d). In the final step of our fabrication sequence, deposition of either Ag or Au resulted in the formation of the plasmonic disc-shaped particles. Notably, a metal mirror layer is simultaneously formed on the Si substrate in the areas not occupied by DOP structures. Despite the directionality of the metal deposition process, some smaller nanoparticles form on the sidewalls of the pillars (figure 1(d)). Such metal nanodroplets on the pillar sidewalls are known to lead to the formation of additional hot spots [4, 16, 21]. The particular thickness of the deposited metal (25 nm) was chosen in the present study largely based on the following two considerations: (i) it was found to be optimal in previous studies of individual DOP structures [6] and (ii) films thicker than 25 nm are more likely to bridge closely spaced DOP structures [4]. Analysis of the SEM images taken after metallization revealed, however, that under our current fabrication conditions some bridging of adjacent structures does occur leading to the formation of vertically aligned clusters.
Figure 2. SEM images taken with a 30◦ tilt of metallized DOP arrays that are (a) 210 nm, (b) 500 nm and (c) 1200 nm tall. (d) 1200 nm tall DOP array where adjacent pillars have bundled after thiolization. All scale bars are 500 nm.
3.1. Optimization of DOP height height showed the best SERS performance for the probing wavelength of 633 nm. Figure 3(a) also shows SERS EFs calculated as the fourth power of the normalized maximum near field, |Emax |4 /|E0 |4 , derived from our finite difference time domain (FDTD) simulations. These simulations (see supporting information and figure S1, available at stacks.iop. org/Nano/24/505302/mmedia, for details) were performed for simplified individual DOP structures with a 100 nm diameter Au disc and pillar heights in the range of 50–275 nm. As can be seen in figure 3(a), there is a clear correlation between the theoretically predicted and experimentally observed dependences of EF on the pillar height. Since the clusters of DOP structures in our experimental system are characterized by a stochastic distribution of sub-50 nm gaps between adjacent plasmonic particles (figure 1(c)), it is important to understand how coupling in DOP clusters may affect their SERS performance. Taking into account previous studies of planar plasmonic dimers [40, 41], we
In order to optimize the height of the DOP structures, a series of DOP arrays were created while incrementally increasing the RIE time (figures 2(a)–(c)). The main idea of optimizing the DOP height is to take advantage of standing waves formed near a reflecting surface [10]. It was anticipated that the interference-enhanced SERS in our DOP arrays can be achieved when the top surface of the nanopillar coincides with the maximum of the standing wave. To evaluate SERS activity of chips with different pillar heights, we acquired SERS spectra and analyzed areas of the characteristic BT peak at 1072 cm−1 as a function of pillar height. As can be seen in figures 3(a) and (b), regardless of the probing wavelength, the strongest SERS raw signals were observed for the DOPs etched for 60 s, which corresponds to a pillar height of approximately 160 nm (table 1). This pillar height correlates well with the findings of our previous study [6] in which individual EBL patterned DOP structures with 175 nm 4
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Figure 3. The average peak area (n = 3) for the 1072 cm−1 ring breathing mode band of BT presented as a function of DOP height for DOPs metallized with (a) 25 nm of Au and (b) 25 nm of Ag. The solid lines are a guide to the eye. FDTD predictions of EF as a function of DOP height are shown in (a) with dashed lines. Table 1. Normalized peak area (NPA) for Ag and Au metallized DOP arrays using the 1072 cm−1 ring breathing mode of BT as a reference. All of the peak areas are normalized by the peak area of the DOP array etched for 60 s and metallized with 25 nm of Au. All of the DOPs fabricated were 97 (±20) nm in diameter. The heights of the nanopillars were extrapolated from the RIE etch time. The peak areas are included in the supporting information as table S1 (available at stacks.iop.org/Nano/24/505302/mmedia). Etch time (s)
Pillar height (nm)
NPA for 25Au at 633 nm
NPA for 25Au at 785 nm
NPA for 25Ag at 633 nm
NPA for 25Ag at 785 nm
20 40 60 80 100 120 360 540
50 105 160 210 265 320 500 1200
0.72 0.84 1 0.41 0.8 0.68 0.23 0.07
0.88 1.49 1.99 0.68 1.06 1.47 0.68 —
3.07 10.9 15 10.2 10.9 10.2 — —
0.26 1.02 1.45 1.01 0.41 1.05 — —
The Ag metallized DOP arrays illuminated with 633 nm incident light performed the best in this series of DOP arrays. Although the Au coated DOP arrays exhibit nearly one order of magnitude lower raw SERS signals compared to the Ag metallized arrays, they demonstrate more consistent performance at different wavelengths and are not prone to chemical degradation. Consistent with the Au dielectric function, the Au coated DOP arrays performed slightly better at the longer probing wavelength (785 nm). To elucidate whether there are particular morphologies or arrangements of individual DOP structures associated with the particularly strong SERS enhancement, high resolution SERS mapping was performed on BT treated samples fabricated close to theoretically predicted optimal parameters (nanopillars 100 nm in diameter and 160 nm in height [1, 6]). A representative SERS map from the Au metallized DOP array etched for 60 s is shown in figure 4(c). The pixels exhibiting the largest SERS signal, shown in red, corresponded to a factor of 2–3 larger peak area than the surrounding regions. SEM imaging of the regions responsible for the large SERS signals, shown in figures 4(a) and (b), revealed the morphology of the structures responsible for the high enhancements of the Raman signal. In the case of the 160 nm long DOPs, the structure consisted of a few vertically aligned DOPs that had formed a cluster during fabrication. The high SERS signals generated by optimized
anticipated that dimerization of DOP structures would lead to a red-shifted LSPR and additional near field enhancement. Our FDTD analysis of dimeric DOP structures indicates that, indeed, the spectral location of the LSPR in the DOP dimer with a 10 nm gap is red-shifted by nearly 100 nm while |Emax |4 /|E0 |4 values increased more than 100-fold. Nonetheless, the dependence of |Emax |4 /|E0 |4 on pillar height is only slightly affected by the dimerization of DOP structures. Additional details of our FDTD analysis are provided in the supporting information as figures S2 and S3 (available at stacks.iop.org/Nano/24/505302/mmedia). In addition to the 60 s etch that produced the best performing DOP structures, there is a second maximum in the SERS performance corresponding to taller pillars (figures 3(a) and (b)). This second maximum corresponds to 100 s and 120 s etch time for 633 nm and 785 nm probing wavelengths, respectively. The periodic nature of the plots shown in figures 3(a) and (b) is consistent with the spatial periodicity of the standing waves formed near the reflecting metallized surface (figure S2 in supporting information available at stacks.iop.org/Nano/24/505302/mmedia). We believe the heterogeneity and complexity of the DOP arrays used in the actual experiments are responsible for the slight difference observed between the simulated trend and the experimentally measured results shown in figure 3(a). 5
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without bundled pillar arrangements can surpass the SERS performance of similar bundled structures. In order to investigate the effects of pillar bundling on the SERS performance of DOP arrays, a second batch of DOPs were fabricated using the same fabrication process as described above, however with significantly longer Si RIE times. We anticipated that, above a certain pillar height, DOP structures would be sufficiently flexible to enable self-organization into converging bundles upon action of capillary forces as observed previously [5, 44, 45]. Indeed, by extending the RIE time to 540 s we created DOP arrays that were approximately 1200 nm tall and readily formed converging bundles after exposure to liquids (figure 2(d)). In particular, we observed bundling of these 1200 nm tall DOP arrays upon their thiolization in isopropanol solutions. DOP arrays with somewhat shorter pillars (500 nm) exhibited bundling behavior, but to a much lesser extent. Even though there are slight differences between these two groups of DOPs, meaningful information can still be gleaned from a quantitative comparison between the average performance of clusters of DOPs and bundles of DOPs. Both groups of DOPs are comprised of stochastic arrays of pillars that contain differences in their diameters and heights, even for a given sample. This heterogeneity and complexity of the arrays increases the range of usable probing wavelengths for a given sample. Also, the ability for nanopillars to self-organize into bundles depends on the mechanical properties of the pillars. The bundled structures cannot be formed from short nanopillars that are expected to be optimal for optical enhancement. In this range of diameters, the only way to increase flexibility is to increase the length of the nanopillars. Testing of various DOP arrays with the 633 nm probing wavelength shows that the bundled pillars with Au coating produced average SERS signal intensities 14 times lower in comparison to the best performing DOP arrays (see table 1). The longer RIE time required to produce nanopillars that are flexible enough to self-organize into bundles resulted in undercutting and removing some of the DOPs with smaller diameters. As a result, DOP arrays with bundled pillars had much lower average density (11 pillars per µm2 compared to 44 pillars per µm2 in the arrays with shorter DOPs). Nonetheless, the bundled DOPs performed a factor of 4 poorer than the shorter clustered DOPs, even when taking the DOP density into account. To elucidate whether there are particular morphologies of DOP bundles or arrangements of individual DOP structures associated with the SERS enhancement, we performed high resolution SERS mapping of the bundled DOP arrays and correlated these maps with the SEM images. A representative SEM image of the Au metallized 1200 nm tall DOP array and a respective Raman map acquired in the same area are shown in figures 5(a) and (b), respectively. We found that converging bundles of several DOP structures were responsible for pixels with anomalously high SERS intensities (exceeding average SERS signals 2–3 times). This is qualitatively consistent with several recent studies [5, 42, 43] which indicated that bundled structures are responsible for SERS hot spots. Nonetheless, our current results indicate that consistently
Figure 4. (a) SEM image of 160 nm tall Au metallized DOP array. (b) The area marked in red points to a specific morphology responsible for SERS intensities 2–3 times larger than the surrounding regions. (c) Map of the 1072 cm−2 peak intensity acquired from a 6 µm × 6 µm area of the sample shown in panel (a). Each pixel in the map is 0.3 µm × 0.3 µm.
DOP arrays do not necessarily require the presence of nanogaps, as previous work with individual DOP structures led to enhancement factors of the order of 109 and excellent SERS performance when their structural parameters were optimized [6]. Nonetheless, SEM images of the DOP arrays fabricated here point to the possibility that nanogap regions, such as the small gap between adjacent Au discs seen in figures 4(a) and (b), may play a significant role in the largest SERS signals observed. Unlike nanogaps in glass nanopillar arrays explored previously [4], hot spots in the DOP arrays described here are most likely located at or near the very top of the DOPs due to the slightly negative (re-entrant) sidewall profiles characteristic of the Si RIE recipes used in the present work. The bases of the DOPs are also sufficiently far apart that nanogaps are unlikely to form between the walls of adjacent nanopillars (figure 1(d)). Such hot spots located on the tops of the DOP arrays rather than within hidden gap regions would be highly advantageous for SERS analysis of hard polymeric samples that may not readily penetrate into the subsurface layers of the SERS substrate. 3.2. Analysis of comparative performance between vertically aligned clusters and converging bundles of DOPs Several reports have indicated that bundled pillar structures are associated with highly SERS-active hot spots due to nanogaps in the pinching regions [5, 42, 43]. The question, however, remains whether optimized DOP arrays 6
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Figure 5. (a) SEM image of Au metallized DOP arrays approximately 1200 nm long that formed converging bundles after thiolization. (b) Map of the 1072 cm−2 peak intensity acquired from a 6 µm × 6 µm area of the sample shown in panel (a). Each pixel in the map is 0.3 µm × 0.3 µm. The rectangle marked in panel (a) points to a typical morphology responsible for SERS intensities 2–3 times larger than the surrounding regions in the map.
Figure 6. (a) Representative SERS spectrum of BT obtained from the SERS map of the 160 nm DOP array (red) and the 1200 nm DOP array (black). SEM images in the inset are representative of the 160 nm vertically aligned clusters and the 1200 nm converging bundles. (b) Peak areas plotted as a function of the number of DOPs per pixel for the 160 nm DOPs (red diamonds) and the 1200 nm DOPs (black dots).
determined by factors other than the stochastically fluctuating density of DOPs in these arrays. As our comparative analysis of SEM images and SERS maps indicates, such factors likely include subtle morphological features in DOP clusters and their bundles. While raw SERS signals are useful for comparative analysis of different SERS substrates, under identical experimental conditions, average EF is commonly used as an instrument-independent figure of merit. Our calculations of average EFs were based on approximations of uniform arrays and a diffraction-limited probing area of approximately 0.3 µm × 0.3 µm. Under these assumptions, the enhancement factors calculated for the DOP arrays fabricated here range from 9.7 × 106 to 6.6 × 107 . This figure of merit places the optimized DOP arrays described and studied here in a highly competitive category.
higher SERS signals are produced by clusters of vertically aligned DOP structures with optimized height (figure 6). The DOP arrays that were long enough to exhibit some degree of bundling, but that were not etched long enough to alter the original density (i.e. 360 s RIE etch time, 500 nm long DOPs), exhibited a wider range of SERS intensities. Some of the observed hot spots could be correlated with the presence of bundled structures, while the location of some others coincided with single DOPs (see figure S4 in supporting information available at stacks.iop.org/Nano/24/ 505302/mmedia). The intensities of the hot spots for these arrays were approximately a factor of 3 higher than those of the 1200 nm DOP arrays. However, the average SERS signal was still lower than that of the best performing 160 nm DOP array (table 1). In order to better understand how the stochastically varying density of DOP structures in the arrays studied here affects the non-uniformity of their SERS activity, sections of the SERS maps were analyzed pixel by pixel while the number of DOPs in each pixel was determined using SEM images. As can be seen in figure 6(b), each pixel in the acquired Raman maps is characterized by a SERS intensity that does not have any clear correlation with the number of DOP structures occupying that pixel. This somewhat unexpected result means that the observed variability in SERS intensity is largely
3.3. Demonstration of SERS using a polymeric analyte without resonance Raman excitation High SERS activity demonstrated using a model analyte does not always translate into more broadly applicable SERS performance, especially when generic analytes without resonance Raman excitation and those that do not chemically interact with noble metals are used. As an additional criterion 7
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By tuning the processing parameters, facile control over the geometries of the resulting DOP structures was achieved. Using the same straightforward processing sequence, vertically aligned clusters of DOP structures as well as DOP structures aggregated into converging bundles were fabricated. Although some of the bundled DOP structures exhibited very strong SERS signals, they were consistently less SERS-active than vertically aligned DOPs with optimized parameters. The latter exhibited average EFs exceeding 107 . Consistent with our previous observations [6], pillar height was found to have a strong effect on the SERS activity of the prepared samples. These experimental findings are in excellent agreement with our computational analysis applied to a simplified model of DOP structures with variable height. Raman mapping correlated to SEM images indicates that subtle changes in the DOP morphology can lead to significant changes in the SERS activity.
Figure 7. Raman spectra from a 120 nm thin film of PAN on a DOP array (160 nm, metallized with 25 nm of Ag) (red) and on a flat Si wafer reference (black). The characteristic peak from stretching vibration due to the C≡N side groups of PAN is easily observed in the SERS spectrum at 2225 cm−1 .
Acknowledgments of SERS performance, we evaluated the ability of the disordered DOP arrays to detect the signal from a polymer film that does not exhibit a resonant Raman spectrum. For this purpose a 120 nm film of polyacrylonitrile (PAN) was deposited onto the chips with DOP arrays via plasma polymerization. The characteristic peak from the stretching vibration due to the C ≡ N side groups of PAN was readily observed in the SERS spectrum at 2225 cm−1 as shown in figure 7. In the bulk Raman spectrum, this characteristic peak is barely observable above the noise in the background, even when 10× longer acquisition time with 5× higher laser power was used. The shift in the characteristic position of the C≡N peak from 2244 cm−1 in the Raman spectrum to 2225 cm−1 in the SERS spectrum is consistent with the PAN being attached to the silver metallized DOPs through the side group nitrogens [46]. This peak shift supports that the majority of the observed SERS signal originates from the first few nanometers of the polymer coating the DOP array rather than the entire thin film [47]. The ability of the aperiodic DOP arrays to pass this test for analytical performance proves their versatility in the detection of a wide range of chemical compounds.
This research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Division of Scientific User Facilities, US Department of Energy.
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4. Conclusion A new technological approach to large area stochastic arrays of plasmonic discs supported on silicon nanopillars was developed using a metal dewetting-assisted fabrication method. The implemented lithography-free wafer level processing sequence allows for the straightforward, rapid and reproducible formation of highly efficient large area SERS substrates. Importantly, the final step of the developed fabrication sequence is deposition of Au or Ag, which requires only a standard vacuum evaporation or sputtering tool and can be performed by the end user at the most appropriate time. For instance, in the case of Ag, which is prone to environmental degradation, this step can be postponed until shortly before SERS measurements. 8
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