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Scanning near field optical microscopy of gold nano-disc arrays fabricated by electron beam lithography and their application as surface enhanced Raman scattering substrates
Chemical Physics Letters 588 (2013) 160–166
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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Scanning near field optical microscopy of gold nano-disc arrays fabricated by electron beam lithography and their application as surface enhanced Raman scattering substrates Md. Ahamad Mohiddon a, L.D. Varma Sangani a,b, M. Ghanashyam Krishna a,b,⇑ a b
Center for Nanotechnology, University of Hyderabad, Hyderabad 500046, India School of Physics, University of Hyderabad, Hyderabad 500046, India
a r t i c l e
i n f o
Article history: Received 9 July 2013 In final form 4 October 2013 Available online 10 October 2013
a b s t r a c t Gold nano-disc arrays were fabricated on glass substrates using electron beam lithography. Scanning near field optical microscopy imaging of the patterns in the near-field illumination and far-field collection (transmission and reflection) and far-field illumination with near-field collection of signal modes is reported. An analyzer was introduced before the detector to study the effect of polarization. High resolution and good contrast are obtained in reflection detection modes compared to that of transmission mode. Polarization of the beam in the transmission mode further improves the resolution and contrast of the images. Application of these arrays as surface enhanced Raman scattering substrates is demonstrated. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Plasmonics, in very few years, has emerged as a field of science and technology that encompasses physics, chemistry, life science and engineering. Majority of plasmonic devices are based on noble metal surfaces and patterned nanostructures which are then applied in diverse areas like sensors, imaging and guiding beyond the diffraction limit, near field nanolithography, plasmonic emitters and photovoltaic cells [1–5]. An interesting phenomenon that is observed when the separation between nanoparticles of these metals is decreased is the appearance of localized surface plasmons that interact with each other and produce additional coupled oscillation modes in the region between the nanoparticles, which are called ‘hot spots’. The field intensity of hot spots is several orders higher than the incident optical field and can therefore be used for the enhancement of weak Raman and photoluminescence signals [6,7]. An important constraint to the application of plasmonics is the ability to produce periodic nanostructures for a variety of applications. Since surface plasmons are essentially a size dependent phenomenon, it is even more important that the thin films used to produce the nanostructures are of high quality. Electron beam lithography (EBL) is a powerful method for the production of highly accurate nanostructures of almost arbitrary shape down to a lateral feature size of about 30 nm, together with a precise
⇑ Corresponding author at: Center for Nanotechnology, University of Hyderabad, Hyderabad 500046, India. Fax: +91 40 23010227. E-mail address: [email protected] (M.G. Krishna). 0009-2614/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2013.10.008
control of the arrangement of the structures on a surface. EBL has been used to produce nano-disc and dot arrays [3,8]. One of the main challenges in this field is the understanding of the field distribution across patterned metal nanostructures. Scanning near field optical microscopy (SNOM) is the only possible way of direct visualizing the field distribution over these metal nanostructures [9,10]. The diffraction limit in optical imaging by illuminating the sample through a sub-wavelength aperture located at a distance much smaller than the wavelength can be overcome using a SNOM. There are different modes in which SNOMs are operated and each of these can lead to differing information about the same sample [11]. Bainier et al. compared the reflection and transmission modes of SNOM along with the apertureless SNOM imaging on a Cr test sample [12]. They demonstrated that polarized light through the uncoated fiber tip can produce better SNOM imaging. Cline and Isaacson [13] studied different modes of reflection SNOM imaging of Al stripes on a silicon substrate and concluded that near field illumination by a fiber tip and far field collection in reflection leads to better resolution and fewer artifacts during SNOM imaging. In the current work, SNOM imaging of patterned Au nanostructures has been carried out in three different modes and the optimal configuration for imaging is demonstrated. The objectives of the present work are, therefore, (1) to deposit Au thin films by magnetron sputter deposition, (2) employing these films for the realization of patterned array of Au nano-discs by a metal lift-off method and (3) imaging them using SNOM. An additional objective is to demonstrate the use of these nano-disc arrays as substrates for surface enhanced Raman scattering (SERS). SNOM imaging of the patterned Au nano-discs has been carried out both in near field
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and far field illumination configurations. In the near field illumination, images were recorded in transmission and reflection configurations. Furthermore, images were also recorded by keeping an analyzer before the detector to investigate the effects of polarization. SERS spectra of Rhodamine 6G molecule over the Au nano disc arrays are also presented.
2. Experimental Au thin films were deposited by RF magnetron sputtering in pure argon, using a customized reactor equipped with a 3 inch planar magnetron cathode and commercial high pure 2 inch Au target. Thin film depositions were performed with a RF power density of 1.5 kW/m2. Deposition pressure was maintained at 1.2 Pa, corresponding to a fixed argon gas flux in the 20 sccm range (sccm is for standard cubic centimeter per minute at STP). The base pressure was 6.7 105 Pa. 30 nm Au thin layers were deposited on a thin glass substrate (0.15 mm thickness) located downstream at 10 cm from the target. The Au nano-disc patterns with two different inter particle separation was fabricated by electron-beam lithography (EBL). The geometrical parameters of the structure were determined from the scanning electron microscopy (SEM) image. The fabrication steps for the realization of Au nanodisc pattern were (1) 150 nm PMMA 495A4 was applied on glass substrate by spin-coating, (2) EBL was performed at 30 keV using a model 9000C EBL system of CRESTEC, Japan and (3) The pattern was developed by immersion in a 3:1 IPA:MIBK solution, followed by 30 nm Au film by RF sputtering followed by lift-off in acetone. Two different Au nano-disc patterns of 30 nm height with discs of (1) 100 nm diameter and inter disc separation of 50 nm (this is referred to as pattern I in the rest of the text) and (2) 120 nm diameter and inter disc separation 40 nm patterns (this is referred to as pattern II in the rest of the text) were fabricated. The fabricated Au nanoparticles are characterized by the Field emission scanning electron microscopy (FESEM) Model Ultra55 of Carl Zeiss, to establish the dimensions. SNOM imaging of Au nano-discs was carried out using a commercial near-field microscope equipped with a cantilever style SNOM probe (Model alpha 300, WITec GmbH, Germany). The SNOM tip (WITec GmbH, Germany) is a hollow-pyramid probe supported on a flexible cantilever that acts both as an optical screen and as a force sensor. The distance between SNOM tip and the sample is controlled in constant-force contact mode. The SNOM tip pyramid is coated with aluminum, thick enough to prevent any stray light from the pyramid sides, the bottom of tip has aperture with a diameter ranging between 60 and 90 nm and side wall of 100 nm. Imaging is carried out in near field and far field illumination configurations using a 532 nm, second harmonic line of Nd-YAG laser, 40 mW. The schematic representation of SNOM setup in three different configurations is presented in Figure 1. In the near field illumination configuration, laser light is focused into the SNOM tip aperture by a 20 objective. The evanescent light from the SNOM tip interacts with the patterned Au nanostructure and excites plasmons which are emitted in all directions. The transmitted part of light is collected by a 60 high numerical aperture (NA = 0.8) objective positioned below the patterned sample and it focuses tightly on the position where the SNOM probe is located (shown in Figure 1a). The collected light is first collimated, then focused to an optical fiber (100/140 lm) in front of a photomultiplier tube to satisfy a confocal detection scheme. The polarization characteristic of the transmitted light is studied by a standard rotatable polaroid filter placed in front of the detector. The reflected part of the light is collected from the side focused 3 objective (NA = 0.26), which is approximately 8.25 cm away from the tip and is positioned in a direction that is almost 87.5°
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to the incident laser direction (Figure 1b). The collected reflected signal is then sent through the optical fiber to the photomultiplier tube. In the case of far field illumination, laser light is focused on the Au nano-disc pattern using a bottom fixed 60 objective, focus spot of approximately 1 lm. The far field light interacts with the Au disc pattern and produces surface plasmons. The light resulting from the field distribution over the Au pattern is collected in the near field using the SNOM tip and is then focused into the optical fiber (100/140 lm) by the 20 objective (Figure 1c). The experimental results reported in the present study are performed using the same SNOM tip in all different configurations to avoid artifacts due to different geometries and dimensions of the SNOM tip. Furthermore, to establish that the results are tip independent, the experiments were also repeated with different SNOM tips supplied by the same manufacturer. The SERS spectra of Rhodamine 6G were recorded in air using an Nd-YAG 532 nm laser in the back scattering geometry with the same setup. A 100 objective (700 nm spot size) with a CCD detector (model alpha 300 of WiTec Germany) was used. The Raman spectra, investigated in the spectral region of 375– 2000 cm1, were recorded at five different positions and the average spectrum is considered for analysis. For the SERS study a 3.1 lM concentration Rhodamine 6G solution in methanol was drop casted on the Au disc pattern. 10 lL of R6G solution is loaded on the Au disc pattern by micropipette and allowed to evaporate completely before taking the Raman measurements.
3. Results and discussion The top view scanning electron micrograph (SEM) image of the Au nanopattern is presented in Figure 2. The SEM image shows that the circular Au nano-discs are arranged in a two dimensional close packed array configuration. Figure 2a shows that the Au nano-discs are circular in shape with a diameter of about 100 nm with centreto-centre spacing, between adjacent nano-discs, of about 150 nm yielding a gap of 50 nm. Figure 2b shows similar Au disc pattern with diameter of about 120 nm and a gap of about 40 nm. The scattering spectra of the Au disc patterns, when it is illuminated by 532 nm Nd-YAG laser is shown in the inset of the Figure 2b. Each spectrum has a broad prominent peak centered at 630 nm with a full width at half maximum of 130 nm. The large peak width indicates that the spectrum has contributions from both luminescence as well as plasmon resonance of gold. The asymmetric shape of the spectrum towards lower wavelength side is due to the high-pass filter used to block the strong Rayleigh line of the spectrum at 532 nm. The spectral width is governed by intrinsic dielectric loss of Au, extra energy relaxations induced by the plasmonic coupling and non-uniform distribution of particle size and inter particle spacing [14]. The optical field intensity distribution of Au nano-disc patterns illuminated in the near field and transmitted signal collected in the far field is presented in Figure 3a and b. The near field image consists of periodic bright spots arranged in a regular 2D packing. The photon intensities are high at the points where the Au discs are located while it is low in the inter-disc space. The high intensity observed on Au discs is due to the coupling of the evanescent field emanating from the SNOM probe with the surface plasmons on the Au nano-discs which is re-emitted as a propagating wave in the far-field. This results in enhancement of the intensity of light passing through the SNOM probe and hence the spots appear bright in the transmission SNOM image [15]. Thus, the bright spots presented in the Figure 3 correspond to the location of Au nanodiscs in the pattern. The transmission SNOM image of pattern I is displayed in Figure 3a. The resolution of the Au pattern is better along the y-direction than the x-direction probably due to small
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Figure 1. Schematic representation of SNOM in three different configurations.
Figure 2. Scanning electron microscopy image of Au disc pattern (a) diameter 100 nm, height 30 nm, inter disc separation 50 nm (b) diameter 120 nm, height 30 nm, inter disc separation 40 nm, along with the scattering spectra of the two samples with 532 nm Nd-YAG laser in inset.
Figure 3. Transmission near field images along with cross section intensity profile in the insets of the Au disc pattern (a) 100 nm/50 nm (b) 120 nm/40 nm.
heterogeneities in the pattern dimensions in this direction. In addition, the SNOM tip dimension is of the order of the inter-disc separation (50 nm). Therefore, any small change in dimensions of the pattern will strongly influence the contrast of the SNOM image. The dimensions of the Au nano-disc pattern can considerably modify the local optical properties. To analyze the SNOM image quantitatively, the first parameter that needs to be studied is the intensity scale along the features of the pattern. The cross section of the intensity profile along the line drawn in Figure 3a is
presented in the inset of the same figure. The intensity profile indicates good spatial resolution with approximately 100 nm disc width and 50 nm inter disc separation. The peak intensity on the Au disc is 15% higher than the background intensity, suggesting that the local electric field is enhanced on the Au nano-discs. This indicates that the excitation of surface plasmon modes plays a role in the observed enhancement of light transmission. The transmission SNOM image of pattern II in Figure 3b has lower resolution compared to that of pattern I shown in Figure 3a. The poor
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resolution is attributed to the fact that the inter-disc distance is less than the dimension of the SNOM probe. The cross section of the intensity profile along the line drawn in Figure 3b and presented in the inset of the same figure indicates that the intensity distribution is asymmetric along the selected line on the SNOM image and peak intensity of the Au disc is 10% higher than the background intensity. The contrast of the SNOM image is given by the relation; Contrast = (IAu IG)/(IAu + IG); where IAu and IG are the signal intensity on the Au disc and substrate respectively. The calculated contrast for pattern I and II are 6.2% and 4.1% respectively. Reflection SNOM (R-SNOM) imaging was carried out to improve the resolution of the images. The R-SNOM images of (a) pattern I and (b) pattern II with the corresponding cross sectional light intensity profile (in inset) along the line drawn are shown in the Figure 4. By comparing the R-SNOM images of Figure 4a and b with that of SNOM far field transmission images presented in Figure 3a and b, one can infer that both the resolution and contrast are improved in the R-SNOM images with respect to the SNOM transmission configuration. Enhanced contrast of 10% and 13% and peak intensity increase of 23% and 30% over the background intensity on patterns I and II, respectively are observed in Figure 4a and b. The unresolved SNOM image of pattern II shown in the Figure 3b is clearly separated and well resolved in the Figure 4b. However, the line profile shows that the width of the peak is slightly larger than the original Au disc pattern dimensions. This is expected in case of R-SNOM imaging, because the image contains information about the sample topography that may not be present in the transmission case. This sensitivity provides more information than the transmission mode but also complicates image interpretation. In R-SNOM imaging, the back reflected light is partially prevented from reaching the detector due to shadowing by both nano-discs and the tip when the tip is positioned between the Au nano discs. This is not the case when the tip is above the Au disc, where shadowing is purely due to the tip. Therefore, the topography of the sample will strongly influence the quantity of light reaching the detector and hence the resolution will be enhanced through shadowing effects [13]. From this study we conclude that high resolution and good contrast can be achieved in the R-SNOM configuration compared to the SNOM transmission configuration. The resolution and contrast of the image shown in Figures 3 and 4 can be further improved by recording the polarization dependent SNOM image because of the depolarization effect due to the nearfield tip and sample interactions. The linearly polarized light from the laser source is coupled into the SNOM tip aperture and the
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transmitted signal is collected by the microscope objective (as described in the experimental section). The polarization analysis of light emitted by the aperture was first measured with the bare substrate only. This was done to obtain the depolarization effects of the aperture alone, without the interaction between the aperture and the Au disc pattern. A measure of the depolarization effect is the extinction ratio which is the ratio of the maximum to the minimum intensity signal with respect to the orientation of the analyzer. Based on this, the calculated extinction ratio of polarized light keeping the SNOM tip on the bare glass substrate is 10:1. A circular analyzer placed in front of the photomultiplier tube, permits the observation of linear polarization states of the plane polarized light emitted from the SNOM tip aperture that interacts with the Au nanopattern. A series of SNOM images taken at 0°, 30°, 60° and 90° orientation of the analyzer to the direction of incident polarization, but at the same location on pattern II is shown in Figure 5a–d. The polarizer and analyzer directions are indicated by white and black arrows respectively in Figure 5a–d. In general, the cross polarization configuration, i.e. polarizer perpendicular to the analyzer will lead to high optical resolution and good image contrast compared to co-polarization configuration [16]. However, in the present case of pattern II, along with the depolarization effects due to scattering of the Au nano-disc pattern the polarization effects of excited surface plasmons will also be present. The circular analyzer collects the near field light that is emitted from the aperture after it interacts with Au disc pattern. The calculated extinction ratio from the light intensity pattern on the Au nanodisc as a function of analyzer angle is found to be 17:1. The enhancement of extinction ratio compared with the bare substrate (10:1) is due to the excitation of surface plasmons on the Au nanodisc pattern. The image presented in Figure 5a is from the same area as that shown in Figure 3b. However, the resolution and contrast are improved in Figure 5a, due to the collection of light signal through the analyzer. The cross sectional light intensity line profile drawn in Figure 5a–d is compared in inset of Figure 5d. The calculated contrasts are nearly 5.6%, 6.7%, 7.7% and 7% for the SNOM images obtained with analyzer angles of 0° (Figure 5a), 30° (Figure 5b), 60° (Figure 5c) and 90° (Figure 5d) respectively. Hence, the contrast of the image is improved by moving from collinear polarization configuration to cross-linear polarization configuration. However, the SNOM image with cross linear polarization configuration shown in Figure 5d has lost exact resemblance with the true image of pattern II. From Figure 5d it can be inferred that the light is coupled more along the y-direction of Au pattern than in the x-direction.
Figure 4. Reflection near field images along with cross section intensity profile in the insets of the Au disc pattern (a) 100 nm/50 nm (b) 120 nm/40 nm.
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Figure 5. Transmission near field images of 100 nm/40 nm Au disc pattern with analyzer angle (a) 00°, (b) 30°, (c) 60° and (d) 90°; cross section intensity profile along the lines drawn presented in the inset of Figure 5.
Aigouy et al. have carried out constant height and constant distance mode SNOM imaging and FDTD simulations of polarization of incident light on the SNOM image of Au disc arrays [8]. They also reported that the field is oriented along the polarization direction on the closely packed nano patterned Au discs. In the present case, the polarization of incident light is fixed along x-direction as shown by white arrow in Figure 5a–d. Hence, both the field confinement and the surface plasmon coupling on the nearest Au discs are along the x-direction. As the analyzer angle is rotated away from the collinear configuration, the contribution from surface plasmon coupling along the x-direction diminishes and the light scattering effect on the SNOM images starts dominating. It is important to note that the polarization properties of the incident light are preserved in the light emitted from Au surface plasmons [8]. In the third configuration of SNOM imaging, the light path is exactly inverted compared to the SNOM transmission mode. The sample is illuminated by a far field light source and a sub-wavelength SNOM probe aperture is used to sense the light passing through the sample (Figure 1c) in a configuration called ‘collection mode’. A general problem of collection mode SNOM is that scattering and interference effects contribute to the measured intensity often obscuring the signal. In the present study, the sample was illuminated by a 60 bottom objective which has an approximate spot size of 1 lm. Therefore, at any instant, surface plasmons of Au will be excited from this 1 lm2 region of the sample and the evanescent light is collected by the 50 nm SNOM aperture. The
SNOM images of pattern I and pattern II together with the cross section light intensity profile along the line drawn are presented in Figure 6a and b. The SNOM images (Figure 6a and b) have poor resolution and low contrast compared with the other two SNOM configurations. The intensity line profile presented in the inset of the corresponding figures, clearly shows that the individual Au nano discs are not well resolved indicating the overlap of the surface plasmons. The peak intensity on the Au disc is 6% and 10% higher than the background intensity and enhancement in contrast is 3% and 5% for the patterns I and II, respectively. These poor image features are due to the surface plasmon coupling across the Au nano disc pattern. Hence, from this study it is confirmed that the surface plasmons are excited both in pattern I and II which couple together to produce surface plasmon polaritons. These polaritons have a large wave vector that enables them to guide through nanostructures beyond the conventional diffraction limit. In summary, all three SNOM configurations point to the occurrence of localized surface plasmons in the Au nano-disc arrays. The line scan intensity profile in three of the SNOM imaging configurations has a non-periodic sinusoidal nature. This deviation from the periodic intensity profile is expected to be due to the rough surface of the Au disc, which is clearly visible in the SEM images (Figure 1). The rough surface on the Au discs produces additional surface effects like hotspots and artifacts in the measurements. These effects are expected to be responsible for deviation of the intensity line profile from periodicity. It has also been shown by Rodriguez-Fernandez
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Figure 6. Collection mode configuration near field images along with cross section intensity profile in the insets of the Au disc pattern (a) 100 nm/50 nm (b) 120 nm/40 nm.
et al. that increased roughness can cause red shift in the plasmon resonance peak of Au nanoparticles [17]. One of the main applications of periodic Au nanostructures in which surface plasmons are excited is for use as surface enhanced Raman scattering (SERS) substrates for molecular detection. SERS is a high sensitive analytical tool for molecular detection, down to the single molecule detection limit. SERS enhances the Raman signal of Rhodamine 6G (R6G) by a factor 1010–1011 [18,19]. The two major contributions for such huge enhancement of Raman signal are electromagnetic (EM) enhancement and chemical enhancement. In chemical enhancement a charge transfer takes place between the adsorbed molecules (R6G) and the metallic surface (Au nanodisc) [20–22]. The EM enhancement is due to the surface plasmon field of the metal nanostructures. A high EM field exists between the two adjacent metal nanostructures, called ‘hot spots’. In the present case of Au nanopattern, along with the surface plasmon effect, the periodic arrangement of Au nano-discs leads to the generation of a large number of hot spots across the surface leading to large SERS effect on R6G. The SERS spectra of R6G adsorbed molecule on patterns I and II along with the bare substrate spectrum are presented in the Figure 7. It is seen that the bare substrate does not give any Raman signal. However, when the Au nano-disc arrays are introduced there is a very large enhancement in the Raman signal. The Raman shift peaks exactly matches with that of the Raman
spectra of R6G [23]. The absorption peak for R6G at 528 nm is in the vicinity of excitation wavelength of 532 nm, which results in strong fluorescence. Therefore, apart from the R6G Raman signal, a huge back ground signal is also enhanced on the Au nanostructures. This can be attributed to the enhancement of R6G fluorescence and Au luminescence. The surface plasmons of Au nanodisc pattern enhanced the Raman and fluorescence signal of R6G. The enhancement of these signal are expected to be due to the increase in the apparent cross-section of the molecules, which is ascertained quantitatively in terms of the SERS enhancement factor [24,25].
SERS enhancement factor ¼ ðISERS NRS Þ=ðNSERS IRS Þ where ISERS and IRS are the intensities of the same band for the SERS and normal Raman spectra, NRS is the average number of molecules in the scattering volume for the Raman (non-SERS) measurement and NSERS is the average number of adsorbed molecules in the scattering volume for the SERS experiments. Under identical experimental conditions, i.e. for constant laser wavelength, laser power, microscope objective, spectrometer, etc., and for same analyte on a SERS substrate, with possibly different concentrations for Raman scattering (cRS) and SERS (cSERS) the above SERS enhancement factor relation is modified as [24,25]
SERS enhancement factor ¼ ðISERS C RS Þ=ðIRS C SERS Þ
Figure 7. Raman signals recorded from Rhodium 6G molecules (3 lM) in methanol measured over Au disc pattern (a) 100 nm/50 nm (b) 120 nm/40 nm along with the bare glass cover slip.
We have used 0.25 M R6G in methanol solution to measure the normal Raman spectra on the bare glass substrate. The strong Raman peak centered around 1331 cm1 has been used for the calculation of the SERS enhancement factor. The fluorescence background is subtracted by a polynomial background removal and the area under the peak has been used to determine the SERS enhancement factor. The area under the peak (ISERS) for pattern I and pattern II are 1197 and 3492 respectively, whereas IRS = 2458. The calculated SERS enhancement factors are found to be 4 104 and 2 105 for pattern I and pattern II respectively. The extent of enhancement is found to be higher on pattern II compared to that on pattern I due to larger EM field enhancement on the former than later. While the enhancement factors observed in the present study are lower than the best values reported in literature they demonstrate the efficacy of these Au nano-disc arrays for application as SERS substrates. Further studies are ongoing to improve the SERS signals.
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4. Conclusions In summary, near field optical microscopy of Au nano-disc arrays patterned using electron beam lithography is demonstrated. Far field and near field illumination effects on the field distribution and their consequence for the quality of near field imaging is revealed. Transmission SNOM study confirms that the dimensions of SNOM probe aperture should be less than the inter particle features of the pattern to obtain high quality near field images. Reflection SNOM imaging is superior to Transmission SNOM but the best resolution and contrast are achieved using polarization SNOM. It is demonstrated that resolution and contrast can be improved by polarizing the beam transmitted through the pattern with the collinear polarizer configuration being superior to the cross linear polarizer configuration. All three SNOM configurations point to the occurrence of localized surface plasmons in the Au nano-disc arrays. Proof-of-concept use of these nano-disc arrays as SERS substrates for the detection of Rhodamine 6G is also demonstrated.
Acknowledgements Authors acknowledge the facilities provided by Centre for Nanotechnology funded by the DST and the School of Physics (University of Hyderabad, India) under the UGC-CAS, UPE programs. Authors acknowledge Mr. G. Lakshmi Narayana Rao for his support in sample fabrication. One of the authors, Dr. Md. Ahamad Mohiddon acknowledges Prof. N. Siva Kumar, School of Life Science,
University of Hyderabad and UPE (No. A-R7) for the financial assistantship during this work. References [1] S.A. Maier, H.A. Atwater, J. Appl. Phys. 98 (2005) 011101. [2] S. Lal, S. Link, N. Halas, Nat. Photonics 1 (2007) 641. [3] J. Aizpurua, P. Hanarp, D.S. Sutherland, M. Kall, G.W. Bryant, F.J.G. de Abajo, Phys. Rev. Lett. 90 (2003) 057401. [4] F. Hao, C.L. Nehl, J.H. Hafner, P. Nordlander, Nano Lett. 7 (2007) 729. [5] V.M. Shalaev, Nat. Photonics 1 (2007) 41. [6] H. Xu, J. Aizpurua, M. Kall, P. Apell, Phys. Rev. E 62 (2000) 4318. [7] D.M. Kuncicky, S.D. Christesen, O.D. Velev, Appl. Spectrosc. 59 (2005) 401. [8] L. Aigouy et al., J. Appl. Phys. 110 (2011) 044308. [9] D.W. Pohl, W. Denk, M. Lanz, Appl. Phys. Lett. 44 (1984) 651. [10] B. Hecht, H. Heinzelmann, D.W. Pohl, Ultramicroscopy 57 (1995) 228. [11] N.F. Van Hulst, M.H.P. Moers, B. Bolger, J. Microsc. 171 (1993) 95. [12] C. Bainier et al., Appl. Opt. 42 (2003) 691. [13] J.A. Cline, M. Isaacson, Ultramicroscopy 57 (1995) 147. [14] S.H. Chang, B.Y. Lin, T.Y. Cheng, J.K. Wang, Phys. Status Solidi RRL 4 (2010) 259. [15] T.Y. Cheng, H.H. Wang, S.H. Chang, J.Y. Chu, J.H. Lee, Y.L. Wang, J.K. Wang, Phys. Chem. Chem. Phys. 15 (2013) 4275. [16] T.M. Yatsui, M. Kourogi, M. Otsu, Appl. Phys. Lett. 71 (1997) 1756. [17] J. Rodriguez-Fernandez, A.M. Funston, J. Perez-Juste, R.A. Alvarez-Puebla, L.M. Liz-Marzan, P. Mulvaney, Phys. Chem. Chem. Phys. 11 (2009) 5909. [18] S.I. Bozhevolnyi, M. Xiao, J.M. Hvam, J. Opt. Soc. Am. A 16 (1999) 2649. [19] B. Pettinger, K. Krischer, G. Ertl, Chem. Phys. Lett. 151 (1988) 151. [20] A.M. Michaels, J. Jiang, L. Brus, J. Phys. Chem. B 104 (2000) 11965. [21] K. Ray, R. Badugu, J.R. Lakowicz, Langmuir 22 (2006) 8374. [22] N. Akbay, J.R. Lakowicz, K. Ray, J. Phys. Chem. C 116 (2012) 10766. [23] G. Krishna Podagatlapalli, S. Hamad, S.P. Tewari, S. Sreedhar, M.D. Prasad, S. Venugopal Rao, J. Appl. Phys. 113 (2013) 073106. [24] E.C. Le Ru, E. Blackie, M. Meyer, P.G. Etchegoin, J. Phys. Chem. C111 (2007) 13794. [25] F.J. Bezares et al., Plasmonics 7 (2012) 143.
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Scanning near field optical microscopy of gold nano-disc arrays fabricated by electron beam lithography and their application as surface enhanced Raman scattering substrates Md. Ahamad Mohiddon a, L.D. Varma Sangani a,b, M. Ghanashyam Krishna a,b,⇑ a b
Center for Nanotechnology, University of Hyderabad, Hyderabad 500046, India School of Physics, University of Hyderabad, Hyderabad 500046, India
a r t i c l e
i n f o
Article history: Received 9 July 2013 In final form 4 October 2013 Available online 10 October 2013
a b s t r a c t Gold nano-disc arrays were fabricated on glass substrates using electron beam lithography. Scanning near field optical microscopy imaging of the patterns in the near-field illumination and far-field collection (transmission and reflection) and far-field illumination with near-field collection of signal modes is reported. An analyzer was introduced before the detector to study the effect of polarization. High resolution and good contrast are obtained in reflection detection modes compared to that of transmission mode. Polarization of the beam in the transmission mode further improves the resolution and contrast of the images. Application of these arrays as surface enhanced Raman scattering substrates is demonstrated. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Plasmonics, in very few years, has emerged as a field of science and technology that encompasses physics, chemistry, life science and engineering. Majority of plasmonic devices are based on noble metal surfaces and patterned nanostructures which are then applied in diverse areas like sensors, imaging and guiding beyond the diffraction limit, near field nanolithography, plasmonic emitters and photovoltaic cells [1–5]. An interesting phenomenon that is observed when the separation between nanoparticles of these metals is decreased is the appearance of localized surface plasmons that interact with each other and produce additional coupled oscillation modes in the region between the nanoparticles, which are called ‘hot spots’. The field intensity of hot spots is several orders higher than the incident optical field and can therefore be used for the enhancement of weak Raman and photoluminescence signals [6,7]. An important constraint to the application of plasmonics is the ability to produce periodic nanostructures for a variety of applications. Since surface plasmons are essentially a size dependent phenomenon, it is even more important that the thin films used to produce the nanostructures are of high quality. Electron beam lithography (EBL) is a powerful method for the production of highly accurate nanostructures of almost arbitrary shape down to a lateral feature size of about 30 nm, together with a precise
⇑ Corresponding author at: Center for Nanotechnology, University of Hyderabad, Hyderabad 500046, India. Fax: +91 40 23010227. E-mail address: [email protected] (M.G. Krishna). 0009-2614/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2013.10.008
control of the arrangement of the structures on a surface. EBL has been used to produce nano-disc and dot arrays [3,8]. One of the main challenges in this field is the understanding of the field distribution across patterned metal nanostructures. Scanning near field optical microscopy (SNOM) is the only possible way of direct visualizing the field distribution over these metal nanostructures [9,10]. The diffraction limit in optical imaging by illuminating the sample through a sub-wavelength aperture located at a distance much smaller than the wavelength can be overcome using a SNOM. There are different modes in which SNOMs are operated and each of these can lead to differing information about the same sample [11]. Bainier et al. compared the reflection and transmission modes of SNOM along with the apertureless SNOM imaging on a Cr test sample [12]. They demonstrated that polarized light through the uncoated fiber tip can produce better SNOM imaging. Cline and Isaacson [13] studied different modes of reflection SNOM imaging of Al stripes on a silicon substrate and concluded that near field illumination by a fiber tip and far field collection in reflection leads to better resolution and fewer artifacts during SNOM imaging. In the current work, SNOM imaging of patterned Au nanostructures has been carried out in three different modes and the optimal configuration for imaging is demonstrated. The objectives of the present work are, therefore, (1) to deposit Au thin films by magnetron sputter deposition, (2) employing these films for the realization of patterned array of Au nano-discs by a metal lift-off method and (3) imaging them using SNOM. An additional objective is to demonstrate the use of these nano-disc arrays as substrates for surface enhanced Raman scattering (SERS). SNOM imaging of the patterned Au nano-discs has been carried out both in near field
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and far field illumination configurations. In the near field illumination, images were recorded in transmission and reflection configurations. Furthermore, images were also recorded by keeping an analyzer before the detector to investigate the effects of polarization. SERS spectra of Rhodamine 6G molecule over the Au nano disc arrays are also presented.
2. Experimental Au thin films were deposited by RF magnetron sputtering in pure argon, using a customized reactor equipped with a 3 inch planar magnetron cathode and commercial high pure 2 inch Au target. Thin film depositions were performed with a RF power density of 1.5 kW/m2. Deposition pressure was maintained at 1.2 Pa, corresponding to a fixed argon gas flux in the 20 sccm range (sccm is for standard cubic centimeter per minute at STP). The base pressure was 6.7 105 Pa. 30 nm Au thin layers were deposited on a thin glass substrate (0.15 mm thickness) located downstream at 10 cm from the target. The Au nano-disc patterns with two different inter particle separation was fabricated by electron-beam lithography (EBL). The geometrical parameters of the structure were determined from the scanning electron microscopy (SEM) image. The fabrication steps for the realization of Au nanodisc pattern were (1) 150 nm PMMA 495A4 was applied on glass substrate by spin-coating, (2) EBL was performed at 30 keV using a model 9000C EBL system of CRESTEC, Japan and (3) The pattern was developed by immersion in a 3:1 IPA:MIBK solution, followed by 30 nm Au film by RF sputtering followed by lift-off in acetone. Two different Au nano-disc patterns of 30 nm height with discs of (1) 100 nm diameter and inter disc separation of 50 nm (this is referred to as pattern I in the rest of the text) and (2) 120 nm diameter and inter disc separation 40 nm patterns (this is referred to as pattern II in the rest of the text) were fabricated. The fabricated Au nanoparticles are characterized by the Field emission scanning electron microscopy (FESEM) Model Ultra55 of Carl Zeiss, to establish the dimensions. SNOM imaging of Au nano-discs was carried out using a commercial near-field microscope equipped with a cantilever style SNOM probe (Model alpha 300, WITec GmbH, Germany). The SNOM tip (WITec GmbH, Germany) is a hollow-pyramid probe supported on a flexible cantilever that acts both as an optical screen and as a force sensor. The distance between SNOM tip and the sample is controlled in constant-force contact mode. The SNOM tip pyramid is coated with aluminum, thick enough to prevent any stray light from the pyramid sides, the bottom of tip has aperture with a diameter ranging between 60 and 90 nm and side wall of 100 nm. Imaging is carried out in near field and far field illumination configurations using a 532 nm, second harmonic line of Nd-YAG laser, 40 mW. The schematic representation of SNOM setup in three different configurations is presented in Figure 1. In the near field illumination configuration, laser light is focused into the SNOM tip aperture by a 20 objective. The evanescent light from the SNOM tip interacts with the patterned Au nanostructure and excites plasmons which are emitted in all directions. The transmitted part of light is collected by a 60 high numerical aperture (NA = 0.8) objective positioned below the patterned sample and it focuses tightly on the position where the SNOM probe is located (shown in Figure 1a). The collected light is first collimated, then focused to an optical fiber (100/140 lm) in front of a photomultiplier tube to satisfy a confocal detection scheme. The polarization characteristic of the transmitted light is studied by a standard rotatable polaroid filter placed in front of the detector. The reflected part of the light is collected from the side focused 3 objective (NA = 0.26), which is approximately 8.25 cm away from the tip and is positioned in a direction that is almost 87.5°
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to the incident laser direction (Figure 1b). The collected reflected signal is then sent through the optical fiber to the photomultiplier tube. In the case of far field illumination, laser light is focused on the Au nano-disc pattern using a bottom fixed 60 objective, focus spot of approximately 1 lm. The far field light interacts with the Au disc pattern and produces surface plasmons. The light resulting from the field distribution over the Au pattern is collected in the near field using the SNOM tip and is then focused into the optical fiber (100/140 lm) by the 20 objective (Figure 1c). The experimental results reported in the present study are performed using the same SNOM tip in all different configurations to avoid artifacts due to different geometries and dimensions of the SNOM tip. Furthermore, to establish that the results are tip independent, the experiments were also repeated with different SNOM tips supplied by the same manufacturer. The SERS spectra of Rhodamine 6G were recorded in air using an Nd-YAG 532 nm laser in the back scattering geometry with the same setup. A 100 objective (700 nm spot size) with a CCD detector (model alpha 300 of WiTec Germany) was used. The Raman spectra, investigated in the spectral region of 375– 2000 cm1, were recorded at five different positions and the average spectrum is considered for analysis. For the SERS study a 3.1 lM concentration Rhodamine 6G solution in methanol was drop casted on the Au disc pattern. 10 lL of R6G solution is loaded on the Au disc pattern by micropipette and allowed to evaporate completely before taking the Raman measurements.
3. Results and discussion The top view scanning electron micrograph (SEM) image of the Au nanopattern is presented in Figure 2. The SEM image shows that the circular Au nano-discs are arranged in a two dimensional close packed array configuration. Figure 2a shows that the Au nano-discs are circular in shape with a diameter of about 100 nm with centreto-centre spacing, between adjacent nano-discs, of about 150 nm yielding a gap of 50 nm. Figure 2b shows similar Au disc pattern with diameter of about 120 nm and a gap of about 40 nm. The scattering spectra of the Au disc patterns, when it is illuminated by 532 nm Nd-YAG laser is shown in the inset of the Figure 2b. Each spectrum has a broad prominent peak centered at 630 nm with a full width at half maximum of 130 nm. The large peak width indicates that the spectrum has contributions from both luminescence as well as plasmon resonance of gold. The asymmetric shape of the spectrum towards lower wavelength side is due to the high-pass filter used to block the strong Rayleigh line of the spectrum at 532 nm. The spectral width is governed by intrinsic dielectric loss of Au, extra energy relaxations induced by the plasmonic coupling and non-uniform distribution of particle size and inter particle spacing [14]. The optical field intensity distribution of Au nano-disc patterns illuminated in the near field and transmitted signal collected in the far field is presented in Figure 3a and b. The near field image consists of periodic bright spots arranged in a regular 2D packing. The photon intensities are high at the points where the Au discs are located while it is low in the inter-disc space. The high intensity observed on Au discs is due to the coupling of the evanescent field emanating from the SNOM probe with the surface plasmons on the Au nano-discs which is re-emitted as a propagating wave in the far-field. This results in enhancement of the intensity of light passing through the SNOM probe and hence the spots appear bright in the transmission SNOM image [15]. Thus, the bright spots presented in the Figure 3 correspond to the location of Au nanodiscs in the pattern. The transmission SNOM image of pattern I is displayed in Figure 3a. The resolution of the Au pattern is better along the y-direction than the x-direction probably due to small
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Figure 1. Schematic representation of SNOM in three different configurations.
Figure 2. Scanning electron microscopy image of Au disc pattern (a) diameter 100 nm, height 30 nm, inter disc separation 50 nm (b) diameter 120 nm, height 30 nm, inter disc separation 40 nm, along with the scattering spectra of the two samples with 532 nm Nd-YAG laser in inset.
Figure 3. Transmission near field images along with cross section intensity profile in the insets of the Au disc pattern (a) 100 nm/50 nm (b) 120 nm/40 nm.
heterogeneities in the pattern dimensions in this direction. In addition, the SNOM tip dimension is of the order of the inter-disc separation (50 nm). Therefore, any small change in dimensions of the pattern will strongly influence the contrast of the SNOM image. The dimensions of the Au nano-disc pattern can considerably modify the local optical properties. To analyze the SNOM image quantitatively, the first parameter that needs to be studied is the intensity scale along the features of the pattern. The cross section of the intensity profile along the line drawn in Figure 3a is
presented in the inset of the same figure. The intensity profile indicates good spatial resolution with approximately 100 nm disc width and 50 nm inter disc separation. The peak intensity on the Au disc is 15% higher than the background intensity, suggesting that the local electric field is enhanced on the Au nano-discs. This indicates that the excitation of surface plasmon modes plays a role in the observed enhancement of light transmission. The transmission SNOM image of pattern II in Figure 3b has lower resolution compared to that of pattern I shown in Figure 3a. The poor
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resolution is attributed to the fact that the inter-disc distance is less than the dimension of the SNOM probe. The cross section of the intensity profile along the line drawn in Figure 3b and presented in the inset of the same figure indicates that the intensity distribution is asymmetric along the selected line on the SNOM image and peak intensity of the Au disc is 10% higher than the background intensity. The contrast of the SNOM image is given by the relation; Contrast = (IAu IG)/(IAu + IG); where IAu and IG are the signal intensity on the Au disc and substrate respectively. The calculated contrast for pattern I and II are 6.2% and 4.1% respectively. Reflection SNOM (R-SNOM) imaging was carried out to improve the resolution of the images. The R-SNOM images of (a) pattern I and (b) pattern II with the corresponding cross sectional light intensity profile (in inset) along the line drawn are shown in the Figure 4. By comparing the R-SNOM images of Figure 4a and b with that of SNOM far field transmission images presented in Figure 3a and b, one can infer that both the resolution and contrast are improved in the R-SNOM images with respect to the SNOM transmission configuration. Enhanced contrast of 10% and 13% and peak intensity increase of 23% and 30% over the background intensity on patterns I and II, respectively are observed in Figure 4a and b. The unresolved SNOM image of pattern II shown in the Figure 3b is clearly separated and well resolved in the Figure 4b. However, the line profile shows that the width of the peak is slightly larger than the original Au disc pattern dimensions. This is expected in case of R-SNOM imaging, because the image contains information about the sample topography that may not be present in the transmission case. This sensitivity provides more information than the transmission mode but also complicates image interpretation. In R-SNOM imaging, the back reflected light is partially prevented from reaching the detector due to shadowing by both nano-discs and the tip when the tip is positioned between the Au nano discs. This is not the case when the tip is above the Au disc, where shadowing is purely due to the tip. Therefore, the topography of the sample will strongly influence the quantity of light reaching the detector and hence the resolution will be enhanced through shadowing effects [13]. From this study we conclude that high resolution and good contrast can be achieved in the R-SNOM configuration compared to the SNOM transmission configuration. The resolution and contrast of the image shown in Figures 3 and 4 can be further improved by recording the polarization dependent SNOM image because of the depolarization effect due to the nearfield tip and sample interactions. The linearly polarized light from the laser source is coupled into the SNOM tip aperture and the
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transmitted signal is collected by the microscope objective (as described in the experimental section). The polarization analysis of light emitted by the aperture was first measured with the bare substrate only. This was done to obtain the depolarization effects of the aperture alone, without the interaction between the aperture and the Au disc pattern. A measure of the depolarization effect is the extinction ratio which is the ratio of the maximum to the minimum intensity signal with respect to the orientation of the analyzer. Based on this, the calculated extinction ratio of polarized light keeping the SNOM tip on the bare glass substrate is 10:1. A circular analyzer placed in front of the photomultiplier tube, permits the observation of linear polarization states of the plane polarized light emitted from the SNOM tip aperture that interacts with the Au nanopattern. A series of SNOM images taken at 0°, 30°, 60° and 90° orientation of the analyzer to the direction of incident polarization, but at the same location on pattern II is shown in Figure 5a–d. The polarizer and analyzer directions are indicated by white and black arrows respectively in Figure 5a–d. In general, the cross polarization configuration, i.e. polarizer perpendicular to the analyzer will lead to high optical resolution and good image contrast compared to co-polarization configuration [16]. However, in the present case of pattern II, along with the depolarization effects due to scattering of the Au nano-disc pattern the polarization effects of excited surface plasmons will also be present. The circular analyzer collects the near field light that is emitted from the aperture after it interacts with Au disc pattern. The calculated extinction ratio from the light intensity pattern on the Au nanodisc as a function of analyzer angle is found to be 17:1. The enhancement of extinction ratio compared with the bare substrate (10:1) is due to the excitation of surface plasmons on the Au nanodisc pattern. The image presented in Figure 5a is from the same area as that shown in Figure 3b. However, the resolution and contrast are improved in Figure 5a, due to the collection of light signal through the analyzer. The cross sectional light intensity line profile drawn in Figure 5a–d is compared in inset of Figure 5d. The calculated contrasts are nearly 5.6%, 6.7%, 7.7% and 7% for the SNOM images obtained with analyzer angles of 0° (Figure 5a), 30° (Figure 5b), 60° (Figure 5c) and 90° (Figure 5d) respectively. Hence, the contrast of the image is improved by moving from collinear polarization configuration to cross-linear polarization configuration. However, the SNOM image with cross linear polarization configuration shown in Figure 5d has lost exact resemblance with the true image of pattern II. From Figure 5d it can be inferred that the light is coupled more along the y-direction of Au pattern than in the x-direction.
Figure 4. Reflection near field images along with cross section intensity profile in the insets of the Au disc pattern (a) 100 nm/50 nm (b) 120 nm/40 nm.
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Figure 5. Transmission near field images of 100 nm/40 nm Au disc pattern with analyzer angle (a) 00°, (b) 30°, (c) 60° and (d) 90°; cross section intensity profile along the lines drawn presented in the inset of Figure 5.
Aigouy et al. have carried out constant height and constant distance mode SNOM imaging and FDTD simulations of polarization of incident light on the SNOM image of Au disc arrays [8]. They also reported that the field is oriented along the polarization direction on the closely packed nano patterned Au discs. In the present case, the polarization of incident light is fixed along x-direction as shown by white arrow in Figure 5a–d. Hence, both the field confinement and the surface plasmon coupling on the nearest Au discs are along the x-direction. As the analyzer angle is rotated away from the collinear configuration, the contribution from surface plasmon coupling along the x-direction diminishes and the light scattering effect on the SNOM images starts dominating. It is important to note that the polarization properties of the incident light are preserved in the light emitted from Au surface plasmons [8]. In the third configuration of SNOM imaging, the light path is exactly inverted compared to the SNOM transmission mode. The sample is illuminated by a far field light source and a sub-wavelength SNOM probe aperture is used to sense the light passing through the sample (Figure 1c) in a configuration called ‘collection mode’. A general problem of collection mode SNOM is that scattering and interference effects contribute to the measured intensity often obscuring the signal. In the present study, the sample was illuminated by a 60 bottom objective which has an approximate spot size of 1 lm. Therefore, at any instant, surface plasmons of Au will be excited from this 1 lm2 region of the sample and the evanescent light is collected by the 50 nm SNOM aperture. The
SNOM images of pattern I and pattern II together with the cross section light intensity profile along the line drawn are presented in Figure 6a and b. The SNOM images (Figure 6a and b) have poor resolution and low contrast compared with the other two SNOM configurations. The intensity line profile presented in the inset of the corresponding figures, clearly shows that the individual Au nano discs are not well resolved indicating the overlap of the surface plasmons. The peak intensity on the Au disc is 6% and 10% higher than the background intensity and enhancement in contrast is 3% and 5% for the patterns I and II, respectively. These poor image features are due to the surface plasmon coupling across the Au nano disc pattern. Hence, from this study it is confirmed that the surface plasmons are excited both in pattern I and II which couple together to produce surface plasmon polaritons. These polaritons have a large wave vector that enables them to guide through nanostructures beyond the conventional diffraction limit. In summary, all three SNOM configurations point to the occurrence of localized surface plasmons in the Au nano-disc arrays. The line scan intensity profile in three of the SNOM imaging configurations has a non-periodic sinusoidal nature. This deviation from the periodic intensity profile is expected to be due to the rough surface of the Au disc, which is clearly visible in the SEM images (Figure 1). The rough surface on the Au discs produces additional surface effects like hotspots and artifacts in the measurements. These effects are expected to be responsible for deviation of the intensity line profile from periodicity. It has also been shown by Rodriguez-Fernandez
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Figure 6. Collection mode configuration near field images along with cross section intensity profile in the insets of the Au disc pattern (a) 100 nm/50 nm (b) 120 nm/40 nm.
et al. that increased roughness can cause red shift in the plasmon resonance peak of Au nanoparticles [17]. One of the main applications of periodic Au nanostructures in which surface plasmons are excited is for use as surface enhanced Raman scattering (SERS) substrates for molecular detection. SERS is a high sensitive analytical tool for molecular detection, down to the single molecule detection limit. SERS enhances the Raman signal of Rhodamine 6G (R6G) by a factor 1010–1011 [18,19]. The two major contributions for such huge enhancement of Raman signal are electromagnetic (EM) enhancement and chemical enhancement. In chemical enhancement a charge transfer takes place between the adsorbed molecules (R6G) and the metallic surface (Au nanodisc) [20–22]. The EM enhancement is due to the surface plasmon field of the metal nanostructures. A high EM field exists between the two adjacent metal nanostructures, called ‘hot spots’. In the present case of Au nanopattern, along with the surface plasmon effect, the periodic arrangement of Au nano-discs leads to the generation of a large number of hot spots across the surface leading to large SERS effect on R6G. The SERS spectra of R6G adsorbed molecule on patterns I and II along with the bare substrate spectrum are presented in the Figure 7. It is seen that the bare substrate does not give any Raman signal. However, when the Au nano-disc arrays are introduced there is a very large enhancement in the Raman signal. The Raman shift peaks exactly matches with that of the Raman
spectra of R6G [23]. The absorption peak for R6G at 528 nm is in the vicinity of excitation wavelength of 532 nm, which results in strong fluorescence. Therefore, apart from the R6G Raman signal, a huge back ground signal is also enhanced on the Au nanostructures. This can be attributed to the enhancement of R6G fluorescence and Au luminescence. The surface plasmons of Au nanodisc pattern enhanced the Raman and fluorescence signal of R6G. The enhancement of these signal are expected to be due to the increase in the apparent cross-section of the molecules, which is ascertained quantitatively in terms of the SERS enhancement factor [24,25].
SERS enhancement factor ¼ ðISERS NRS Þ=ðNSERS IRS Þ where ISERS and IRS are the intensities of the same band for the SERS and normal Raman spectra, NRS is the average number of molecules in the scattering volume for the Raman (non-SERS) measurement and NSERS is the average number of adsorbed molecules in the scattering volume for the SERS experiments. Under identical experimental conditions, i.e. for constant laser wavelength, laser power, microscope objective, spectrometer, etc., and for same analyte on a SERS substrate, with possibly different concentrations for Raman scattering (cRS) and SERS (cSERS) the above SERS enhancement factor relation is modified as [24,25]
SERS enhancement factor ¼ ðISERS C RS Þ=ðIRS C SERS Þ
Figure 7. Raman signals recorded from Rhodium 6G molecules (3 lM) in methanol measured over Au disc pattern (a) 100 nm/50 nm (b) 120 nm/40 nm along with the bare glass cover slip.
We have used 0.25 M R6G in methanol solution to measure the normal Raman spectra on the bare glass substrate. The strong Raman peak centered around 1331 cm1 has been used for the calculation of the SERS enhancement factor. The fluorescence background is subtracted by a polynomial background removal and the area under the peak has been used to determine the SERS enhancement factor. The area under the peak (ISERS) for pattern I and pattern II are 1197 and 3492 respectively, whereas IRS = 2458. The calculated SERS enhancement factors are found to be 4 104 and 2 105 for pattern I and pattern II respectively. The extent of enhancement is found to be higher on pattern II compared to that on pattern I due to larger EM field enhancement on the former than later. While the enhancement factors observed in the present study are lower than the best values reported in literature they demonstrate the efficacy of these Au nano-disc arrays for application as SERS substrates. Further studies are ongoing to improve the SERS signals.
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4. Conclusions In summary, near field optical microscopy of Au nano-disc arrays patterned using electron beam lithography is demonstrated. Far field and near field illumination effects on the field distribution and their consequence for the quality of near field imaging is revealed. Transmission SNOM study confirms that the dimensions of SNOM probe aperture should be less than the inter particle features of the pattern to obtain high quality near field images. Reflection SNOM imaging is superior to Transmission SNOM but the best resolution and contrast are achieved using polarization SNOM. It is demonstrated that resolution and contrast can be improved by polarizing the beam transmitted through the pattern with the collinear polarizer configuration being superior to the cross linear polarizer configuration. All three SNOM configurations point to the occurrence of localized surface plasmons in the Au nano-disc arrays. Proof-of-concept use of these nano-disc arrays as SERS substrates for the detection of Rhodamine 6G is also demonstrated.
Acknowledgements Authors acknowledge the facilities provided by Centre for Nanotechnology funded by the DST and the School of Physics (University of Hyderabad, India) under the UGC-CAS, UPE programs. Authors acknowledge Mr. G. Lakshmi Narayana Rao for his support in sample fabrication. One of the authors, Dr. Md. Ahamad Mohiddon acknowledges Prof. N. Siva Kumar, School of Life Science,
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