NANO LETTERS
Enhanced Nanoplasmonic Optical Sensors with Reduced Substrate Effect
2008 Vol. 8, No. 11 3893-3898
Alexandre Dmitriev,†,* Carl Hägglund,† Si Chen,† Hans Fredriksson,† Tavakol Pakizeh,† Mikael Käll,† and Duncan S. Sutherland‡ Applied Physics, Chalmers UniVersity of Technology, 41296 Go¨teborg Sweden, and Interdisciplinary Nanoscience Center (iNANO), UniVersity of Aarhus, 8000 Århus C Denmark Received July 30, 2008; Revised Manuscript Received September 17, 2008
ABSTRACT We present a straightforward method to double the refractive index sensitivity of surface-supported nanoplasmonic optical sensors by lifting the metal nanoparticles above the substrate by a dielectric nanopillar. The role of the pillar is to substantially decrease the spatial overlap between the substrate and the enhanced fields generated at plasmon resonance. Data presented for nanodisks and nanoellipsoids supported by pillars of varying heights are found to be in excellent agreement with electrodynamics simulations. The described concepts apply to multitude of plasmonic nanostructures, fabricated by top-down or bottom-up techniques, and are likely to further facilitate the development of novel nanooptical sensors for biomedicine and diagnostics.
Metal nanostructures support optical charge carrier resonances, the so-called localized surface plasmon resonances (LSPRs), which receive increasing attention in diverse fields of science including photonic circuitry, metamaterials, and biomedicine.1-3 One of the most interesting application areas is label-free molecular biochemosensing based on the dependence of the scattering/absorption properties of the metal nanostructures on their local dielectric environment.1,4 This dependence is typically revealed through a shift of the LSPR wavelength induced by a change in interfacial refractive index (RI), which increases if target molecules attach to the plasmonic structures. Surfacesupported arrays of noble metal nanoparticles constitute one of the most versatile platforms for this kind of RI sensing, as the shapes, sizes, compositions, and orientations of the nanoparticles can be precisely controlled and tuned by both top-down techniques, such as electron-beam lithography (EBL), and bottom-up fabrication methods, for example, colloidal lithographies.5 Several strategies have utilized this versatility in order to enhance the sensitivity of the nanoplasmonic structures, including narrowing of the spectral linewidths in ordered particle arrays, single particle measurements with reduced inhomogeneous broadening,6-8 and creating sharper nanoscopic features associated with higher field enhancements.9,10 However, surface-supported particles suffer an intrinsic drawback compared to particles in solution, that is, the fixed RI of the supporting substrate will reduce * To whom correspondence should be addressed. E-mail: alexd@ chalmers.se. Tel. + 46 31 772 51 77. † Chalmers University of Technology. ‡ University of Aarhus. 10.1021/nl8023142 CCC: $40.75 Published on Web 10/10/2008
2008 American Chemical Society
the overall RI sensitivity of the particles.11 The effect of the underlying substrate on the sensing capabilities has therefore been a recurrent subject in recent nanoplasmonic sensing studies.12-15 For example, a direct comparison of the sensing characteristics of nanodisk and nanoring arrays with similar LSP resonance wavelengths showed a markedly higher sensitivity for the latter. By employing a simple analytical approach based on the effective refractive index concept, this difference could be quantitatively understood from the much smaller contact area of the nanorings (∼17% of the particle surface in contact with the substrate) compared to the nanodisks (∼50% in contact).13 In this letter, we demonstrate a simple strategy for reducing the substrate effect by “lifting” the enhanced electromagnetic fields of the nanoparticles from the substrate by a dielectric pillar, thereby substantially enhancing the bulk refractive index sensitivity of the particle LSPR. The approach is applicable to virtually any surface-supported nanoplasmonic sensing platform, fabricated by top-down or bottom-up approaches. We also show how to actively shift the distribution of the EM field enhancement off the surface by engineering of the nanostructures shape and surface roughness, thereby increasing the availability of the high field sites for sensing the surrounding medium refractive index. Both concepts are realized without substantial formal reduction of the substrate contact area, but bring the nanoplasmonic RI sensitivity closer to the limit of solution-dispersed nanostructures while retaining the above-mentioned advantages of surface-supported sensing systems.
Figure 1. Enhancement of the RI sensitivity of surface-supported nanostructures by introducing a SiO2 supporting pillar. Typical SEM micrographs of arrays of Au nanodisks directly on glass (a, c top) or supported on SiO2 pillars (b, c bottom), scale bars 1 µm (a,b) and 100 nm (c). (d) Dependence of the RI sensitivity on spectral peak position in air for Au nanodisks directly on glass (red circles), supported on 20 nm SiO2 pillars (blue squares), supported on 80 nm SiO2 pillars (green diamonds) and Au nanoellipses (longitudinal resonance) directly on glass (red crosses) and supported on 40 nm SiO2 pillars (ruby-red cross). Cartoon models of nanodisks are shown in the inset. Thirdorder polynomial fits to data are provided as a guide to the eye. Error bars are typically smaller than the data points symbols. (e) Typical SEM micrograph of Au nanoellipses on glass. Scale bar, 1 µm.
We employ hole-mask colloidal lithography (HCL)16 to fabricate surface-supported arrays of Au nanodisks on glass. The RI sensitivity is probed by immersing the samples in the media with different refractive indices while monitoring the change in the LSP resonance by optical extinction spectroscopy (Varian Cary 500). HCL is a bottom-up technique that relies on self-assembly of negatively charged polystyrene colloidal particles (Interfacial Dynamics Corp.) onto the positively charged surface of a layer of polymer resist (polymethylmethacrylate, PMMA). Deposition of a thin metallic (Au) film over the self-assembled particles, which are subsequently removed by tape-stripping, results in an evaporation mask in the form of a short-range ordered array of nanoscopic holes with size determined by the size of the colloidal particles. By employing an oxygen plasma treatment, the polymer film is then etched through, which means that materials can be evaporated through the mask holes onto the surface. In addition, the metal mask can also be removed by the wet etch prior to materials evaporation, so that the holes in the PMMA themselves serve as an evaporation mask (see below). In all presented RI sensitivity experiments, the nanodisks thickness is kept constant at 20 nm. Tuning of the nanodisks resonances is achieved by changing their diameter, which is directly related to the size of the initial polystyrene spheres used in the initial mask. The versatility of the method can be explored in various contexts, including fabrication of layered and specially shaped nanostructures, as detailed below.16,17 3894
A typical Au nanodisks array, prepared by HCL on a glass substrate, is shown in Figure 1a (nanodisks diameters, 186 nm; height, 20 nm). The array exhibits short-range periodicity with a typical center-to-center spacing between disks of the order 2-3 disks diameters.18 The array extends over a macroscopic (cm2) surface area. It is worth noting that the nanodisks in the HCL-fabricated array are only weakly interacting, as their lateral separation exceeds the range of plasmon induced near-field coupling. Due to the lack of longrange periodicities, there is also only weak diffractive coupling between particles. The spectral signature of the arrays is therefore a good representation of the spectral response of a single nanodisk but with additional inhomogeneous broadening due to the finite size distribution in the array.18 Importantly, subsequent deposition of SiO2 and Au results in metal nanodisks supported on a silica “pillar”, which height can be controlled by the evaporation time (Figures 1b,c). Such supported nanodisks preserve their shape and topology, as can be seen from the scanning electron microscopy (SEM) side views presented in Figure 1c. However, a slight reduction of the nanodisks diameters upon introduction of the supporting pillar can be seen in Figures 1b (nanodisks diameter, 133 nm; height, 20 nm; pillar height, 40 nm). Due to the decrease in aspect ratio, this effect shifts the LSP resonance to shorter wavelengths compared to nanodisks fabricated with identical hole masks directly on the substrate. Figure 1d compares the RI sensitivity of nanodisk arrays fabricated directly on the supporting SiO2 substrates with Nano Lett., Vol. 8, No. 11, 2008
Figure 2. The RI sensitivity of surface-supported Au nanodisks, obtained through finite-element electrodynamics simulations. (a) RI sensitivity vs peak wavelength for a nanodisk directly on glass (red circles) or supported on an 80 nm SiO2 pillar. Third-order polynomial fits to data are provided as a guide to the eye. The simulated E-field distributions (total electric field amplitude) at the marked points (750 nm) are shown for a pillar-supported nanodisk (b) and a nanodisk placed directly on the substrate (c). The E-field is plotted on a logarithmic scale for improved visibility. The excitation light propagation and polarization directions are marked.
disks that have been raised above the substrate by SiO2 pillars of various heights (solid symbols). The RI sensitivity is defined as S ) (λm - λair)/(nm - nair), that is, as the LSPR peak shift per refractive index unit (RIU), and is plotted against the resonance position λair in air. The measurements were performed by immersing the nanodisks arrays in a sequence of liquids with increasing refractive index nm (water and various concentrations of ethylene glycol in water solution, spanning a refractive index from 1.33 to 1.42). It is clear that the RI sensitivity markedly increases as the height of the supporting pillars increases. For samples with λair at around 850 nm, the sensitivity more than doubles! Importantly, all sensitivity curves closely follow the expected linear dependence of RI sensitivity versus resonance position below 800 nm, while some curvature is seen in the nearinfrared range. This behavior is in line with earlier data on Au disks and nanorings and is a consequence of the wavelength dependent dielectric function of gold.13 An alternative way of expressing the RI sensitivity is to use the so-called figure of merit (FOM), defined as a ratio between the RI sensitivity and the full-width at halfmaximum (fwhm) expressed in energy units.19 Despite a slight broadening of the LSPR peak observed when the nanodisks are supported on the dielectric pillars (data not shown), the overall FOM increases from 1.7 to 2.23 for samples with resonance position at around 800 nm. Though moderate in absolute numbers, mainly due to inhomogeneous broadening contributions to the fwhm, the nanodisk FOM is thus clearly improved when the disks are raised on pillars. This is a good indication that the described concept is of general benefit to RI sensing and that the FOM’s of other surface-supported nanoplasmonic systems will be enhanced in a similar way. To verify this, we investigated arrays of oriented nanoellipsoids made by HCL (Figure 1e, ellipsoid short axis, 190 nm; long axis, 380 nm; height, 30 nm). The crosses in Figure 1d show the RI sensitivity values for the Nano Lett., Vol. 8, No. 11, 2008
longitudinal LSP resonances of two differently sized ellipsoids fabricated directly on the substrate and one array for which the ellipses have been raised on pillars. The effect is obviously analogous to the disk case. Interestingly, the RI sensitivity of the ellipsoids follow the same trend versus resonance wavelength as the nanodisks, despite the difference in electromagnetic field distribution that can be expected for the different shapes.20,21 In addition, we also investigated the sensitivity of the transverse ellipsoid resonance, which turned out to be slightly below that of nanodisks (data not shown). In order to understand how the substrate affects the RI sensitivity of the studied systems, we first make a simple analysis based on the quasi-static spheroid theory along the lines of ref 13. The disks are approximated as oblate spheroids with a dielectric response from experimental data.22 The substrate effect is included in the model through an effective refractive index, that is, the substrate (n ) 1.52) contributes a fraction S while the rest of the surrounding medium contributes a fraction 1 - S. If the embedding medium is air, we thus have neff ) 1.52 + 1.00(1 - S). In agreement with earlier work,13 we find that the RI sensitivity of nanodisks fabricated directly on the substrate are well described by a ≈50% (S ≈ 0.5) substrate contribution. In contrast, the sensitivity of nanodisks supported on 80 nm SiO2 pillars can only be described by a much lower substrate influence, of the order 20% (S ≈ 0.2), despite the fact that the fraction of the metal surface that is in physical contact with SiO2 is the same for the two types of structure. The origin of this discrepancy becomes apparent when one takes the actual local fields induced by the plasmon oscillations, as obtained from electrodynamics simulations, into account. Figure 2a shows the calculated RI sensitivity dependence for Au nanodisks situated directly on the substrate (red, nanodisks lower diameters 67, 100, 133, 170, 190, and 250; thickness, 20 nm; sidewall angle, 16°) and supported on 80 nm silica pillars (green, nanodisks lower diameters 120, 150, 3895
Figure 3. Silica pillar height-dependent experimental RI sensitivity saturation and calculated E-field decay. (a) Calculated dipolar E-field enhancement (field amplitude) of a 150 nm nanodisk on 80 nm SiO2 pillar resonating at 599 nm (plotted on a log scale). The E-field decay in panel b is plotted along the indicated arrow and normalized to the starting point (0 nm) at the bottom of the nanodisk. (b) At around 600 nm peak wavelength, the experimental RI sensitivity (red circles) saturates for a pillar height of 60 nm. The normalized E-field values (green) along the arrow indicated in panel a is also shown. (c) Same as in panel b but at a peak wavelength around 650 nm with RI sensitivity saturating at 80 nm pillar height. The E-field decay is plotted at the resonance (646.5 nm for a 150 nm nanodisk on an 80 nm pillar). Third-order polynomial fits to RI sensitivity data points are provided as a guide to the eye.
and 200 nm; thickness, 20 nm). The data were obtained using a commercial software package (Comsol Multiphysics), which solves the Helmholz vector wave equation for the scattered electric field in a 600 by 600 nm plane normal to the substrate and through the center of the disks/pillars. The exciting field is incident normal to the substrate in all cases. The mirror symmetry was exploited to reduce the computational load, and scattering boundary conditions appropriate for simulating a single particle were used on all external boundaries; see the supporting information in ref 23 for further details. The exciting field is incident normal to the substrate in all cases. The resonance wavelength of the localized plasmons was obtained from calculated absorption spectra. By comparing with Figure 1d, one finds that the simulations give an excellent quantitative description of the “pillar effect”. Figure 2b,c shows the corresponding field distributions for a pillar-supported and a regular nanodisk, respectively, with the same resonance wavelengths. We see that the pillar has the crucial effect of substantially reducing the overlap between the induced local field and the substrate, which means that a higher proportion of the field have access to the varying refractive index of the surrounding medium. This explains the higher RI sensitivity of the raised disks compared to the ones laying flat on the substrate. In order to further illustrate the role of the spatial distribution of the enhanced electromagnetic fields, we show in Figure 3 a comparison between the experimentally measured RI sensitivity for disks supported by pillars of different heights and calculated field decays. Figure 3b,c (red data points) gives the RI sensitivity versus pillar height for disks with resonances at around 600 nm and around 650 nm, respectively. We see that both curves exhibit a saturation behavior, that is, the sensitivity improves only up to a certain pillar height. In the same figures, we have included normalized field decays along lines normal to the substrate through the center of two raised disks with similar resonance positions; see Figure 3a. The experimental and calculated data show striking similarities. According to Figure 3b, the RI sensitivity enhancement saturates at a supporting pillar 3896
height of ∼60 nm (red data points), which correlates well to the calculated E-field values (plotted in green, calculated field decay at 599 nm, nanodisk lower diameter 120 nm, thickness 20 nm), which approach zero around 60 nm away from the nanodisk. The latter distance corresponds to a situation for which the enhanced dipolar E-field is almost completely lifted above the substrate (c.f. Figure 3a). The same trend is reproduced for the LSP resonance around 650 nm, Figure 3c, where both the RI sensitivity enhancement and the field decay flattens out at around 80 nm (calculated field decay at 646.5 nm; nanodisk lower diameter, 150 nm; thickness, 20 nm). Importantly, the field decay curves at a given resonance have the same character for all pillar heights and can be regarded as defining the maximum sensing volume around the nanodisk (see Figure S1 in Supporting Information). These saturation/decay values are further in good agreement with a recent study based on atomic layer deposition of dielectric layers on Ag nanoparticles with similar resonance positions, which showed saturation 60-80 nm away from the nanostructures depending on aspect ratio.24 An alternative approach to the concept described above is to modify the distribution of the field around a plasmonic nanostructure by engineering of the nanostructures shape to reduce the overlap between the EM field and the substrate. The HCL method provides the means of “maskless” fabrication when the thin-film evaporation mask with the pattern of nanoscopic holes, supported on the PMMA film, is removed after the holes in PMMA are etched through it. This allows it to shape the size and, most importantly, the side walls of the HCL-fabricated nanostructures.25 Figure 4 shows the result of such shape engineering, where nanodisks with straight sidewalls and supposedly increased nanoscopic roughness (Figure 4a) are compared with the regular HCL nanodisks, fabricated with the evaporation-through-the-mask procedure (Figure 4b). The difference in shape likely relates to the interaction between the evaporated material and the mask material which leads to a more rapid closing of the mask hole for the nanodisks in Figure 4b resulting in a change of disk diameter with height. Most importantly, such Nano Lett., Vol. 8, No. 11, 2008
Figure 4. Shape engineering of RI sensitivity. SEM sideview of an array of straight walled Au nanodisks (a) and regular Au nanodisks (b) on glass. (c) Experimental (solid symbols) and calculated (hollow symbols) resonance-dependent RI sensitivity of regular Au nanodisks (red circles, as in Figures 1d and 2a for experimental and calculated values, respectively) and straight-walled Au nanodisks (blue triangles). Third-order polynomial fits to data points are provided as a guide to the eye. (d) Finite-element method calculated enhanced E-field distribution around straight-walled and slightly roughened nanodisk on glass (total E-field amplitude, log scale, excitation at 720 nm). The excitation light propagation and polarization directions are marked.
shape engineering results in a remarkable RI sensitivity enhancement of the surface-supported nanodisks so that the RI sensitivity of the straight-walled nanodisks essentially exceeds that obtained by supporting the nanodisks on dielectric pillars (c.f. Figures 4c and 1d). Following the data points in Figure 4c (blue for straight-walled nanodisks, initial PS spheres sizes 80, 140, and 190 nm were used), the RI sensitivity at 879 nm reaches a 473 nm peak shift/RIU as compared to an RI sensitivity of 80 nm pillar-supported disks at 856 nm with RI sensitivity of 422 nm peak shift/RIU, taken from Figure 1d. A comparison of regular and straightwalled disks yields a FOM increase from 1.081 (at 605 nm) to 1.714 (at 626 nm). The origins of this RI sensitivity enhancement can also be traced by electrodynamics simulations. In particular, the nanodisk with straight side-walls but introduced nanoscopic roughness at the sides (simulated by a large number of small rectilinear blocks), exhibits that a sizable E-field redistribution occurs (c.f., Figure 4d, as compared to Figure 2c) in the sense that a larger portion of the enhanced field becomes available to the surrounding media through lifting it off the substrate. We note that the simulated RI sensitivity for this configuration (hollow blue triangles in Figure 4c) is in better agreement with the experimental sensitivity data than several other tested configurations (e.g., nanodisks with simple straight walls, “cogwheel”-shaped nanodisks, nanodisks with a “roughened” top rim, and so forth). It is likely that the remaining deviation compared to experiment probably relates to the irregular nature of the actual nanoscopic corrugation present at the sidewalls of experimentally obtained nanodisks, which is not straightforwardly accounted for in the calculations (c.f. Figure 4d). In summary, we have demonstrated that supporting nanoplasmonic structures on dielectric pillars is a route to enhanced bulk refractive index sensitivity. Obviously, the effect of the substrate cannot be completely eliminated using surface-supported nanoplasmonic sensors. However, the described simple approach substantially reduces the substrate influence by letting a larger portion of the enhanced field to be exposed for sensing. Overall, we envision that the described strategies for RI sensitivity enhancement of Nano Lett., Vol. 8, No. 11, 2008
surface-supported nanoplasmonic sensors have general applicability for a large variety of top-down and bottom-up nanofabricated optical label-free sensing platforms. It is particularly appealing that all major advantages of the nanofabricated surface-supported geometries for sensing applications, that is, high spectral tunability, array-type integration, reproducibility, and high-degree of topological control/orientation, are retained. In addition, as the method makes a larger portion of the enhanced EM fields sensitive to changes in the surrounding medium, it can be effectively applied to enhance optical label-free biochemosensing of large biomolecules with extended surface-linking chemistry. The presented RI sensitivity enhancement strategy can therefore significantly benefit further development of nanoplasmonic-based optical sensors applicable in real-world biomedical and clinical biochemosensing and diagnostics. Acknowledgment. D.S. acknowledges funding from FNU in Denmark. This work was supported by the Swedish Research Council (VR) and the Swedish Foundation for Strategic Research (SSF). Supporting Information Available: Figure S1 shows field decay from the point below the nanodisk to the point at the substrate (A to B on the inset) when the SiO2 pillar height h is changed from 20 to 80 nm (respective colored curves). Inset: E-field amplitude distribution for 120 nm nanodisk on 80 nm pillar. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Anker, J. N.; Hall, W. P.; Lyandres, O.; Shah, N. C.; Zhao, J.; Van Duyne, R. P. Nat. Mater. 2008, 7, 442. (2) Lal, S.; Link, S.; Halas, N. J. Nature Photonics 2008, 1, 641. (3) Shalaev, V. M. Nature Photonics 2007, 1, 41. (4) Stewart, M. E.; Anderton, C. R.; Thompson, L. B.; Maria, J.; Gray, S. K.; Rogers, J. A.; Nuzzo, R. G. Chem. ReV. 2008, 108, 494. (5) Yang, S.-M.; Jang, S. G.; Choi, D.-G.; Kim, S.; Yu, H. K. Small 2006, 2, 458. (6) Hicks, E. M.; Zou, S.; Schatz, G. C.; Spears, K. G.; Van Duyne, R. P.; Gunnarsson, L.; Rindzevicius, T.; Kasemo, B.; Ka¨ll, M. Nano Lett. 2005, 5, 1065. (7) Raschke, G.; Kowarik, S.; Franzl, T.; So¨nnichsen, C.; Klar, T. A.; Feldmann, J.; Nichtl, A.; Kuerzinger, K. Nano Lett. 2003, 3, 935. 3897
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Nano Lett., Vol. 8, No. 11, 2008
