Fano Resonant Aluminum Nanoclusters for Plasmonic Colorimetric

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Fano Resonant Aluminum Nanoclusters for Plasmonic Colorimetric Sensing Nicholas S. King, Lifei Liu, Xiao Yang, Benjamin Cerjan, Henry O. Everitt, Peter Nordlander, and Naomi J. Halas ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.5b04864 • Publication Date (Web): 01 Oct 2015 Downloaded from http://pubs.acs.org on October 2, 2015

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Fano Resonant Aluminum Nanoclusters for Plasmonic Colorimetric Sensing Nicholas S. King1,4, Lifei Liu1,4, Xiao Yang1,4, Benjamin Cerjan1,4, Henry O. Everitt2,5, Peter Nordlander1,2,4, and Naomi J. Halas1,2,3,4,* 1

Department of Physics and Astronomy, 2Department of Electrical and Computer Engineering, 3Department of Chemistry, 4Laboratory for Nanophotonics

Rice University, 6100 Main Street, Houston, TX 77005 USA; 5 Army Aviation and Missile RD&E Center at Redstone Arsenal, AL 35898 USA. Abstract Aluminum is an abundant and high-quality material for plasmonics with potential for large-area, low-cost photonic technologies. Here we examine Aluminum nanoclusters, with plasmonic Fano resonances that can be tuned from the near-UV into the visible region of the spectrum. These nanoclusters can be designed with specific chromaticities in the blue-green region of the spectrum and exhibit a remarkable spectral sensitivity to changes in the local dielectric environment.

We show that such structures can be used quite generally for colorimetric

Localized Surface Plasmon Resonance (LSPR) sensing, where the presence of analytes is detected by directly observable color changes rather than through photodetectors and spectral analyzers. To quantify our results and provide a metric for optimization of such structures for colorimetric LSPR sensing, we introduce a Figure of Merit based on the color perception ability of the human eye.

*Address correspondence to: (N.J.H.) [email protected]. Keywords: Plasmon, Aluminum, Ultraviolet, Fano Resonance, Chromaticity, Figure of merit

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Aluminum is an excellent material for plasmonics, with an oxidation stability superior to silver and a functional range of plasmon resonant tunability across the ultraviolet and visible regime.1– 19

Furthermore, it is an abundant, sustainable material with the potential to transition laboratory-

based research into practical applications and commercial products. Much of the current and growing interest in Aluminum plasmonics is due to the wavelength range of the plasmon resonances obtainable with simple geometries. Its use as a potentially superior chromatic material relative to organic media for color displays has captured great interest.6,7,10,14,17,20–22A range of nanostructures, such as discs,7 holes,22 rods,6 and crosses,21 have been fabricated and studied, showing localized plasmon tuning for optical filters and other colorimetric applications. An important application in plasmonics is Localized Surface Plasmon Resonance (LSPR) sensing.23,24 This process involves monitoring the shift of the plasmon resonance of a finite nanostructure as its dielectric environment is changed. The screening charges induced in the surrounding dielectric redshift the plasmon resonance by an amount that depends on the permittivity of the dielectric: a larger permittivity results in a larger redshift. Although in principle the redshift of the LSPR provides a direct report of the dielectric permittivity surrounding the nanostructure, the permittivities of typical analytes are too similar to induce distinguishable redshifts, so this approach does not provide analyte selectivity. To overcome this problem in realistic applications, the plasmonic nanostructure is typically functionalized with molecules that can only bind specific analytes: an LSPR shift will only be observed if these analytes are present and bind to the receptors. For gold nanostructures, the highest LSPR sensitivity is for plasmon resonances in the near infrared, requiring substantial instrumentation for monitoring of the plasmon resonance, such as spectrophotometers. This relatively bulky

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experimental setup requires stable ambient conditions, electrical power, and is not practical for applications in the field. Nanostructures that exhibit plasmonic Fano resonances are of particular interest in LSPR sensing applications. The Fano resonance is a general physical phenomenon found in systems with a broad continuum of energy levels and a narrow band of states at the same energy.25–27 In purely plasmonic systems a bright, superradiant, plasmon mode that spectrally overlaps a dark, subradiant, plasmon mode gives rise to a Fano resonance: incident light couples to the bright mode, which is coupled to the dark mode through its near field.26 The scattered light obtains a phase shift with respect to the incident light wave, and their interference produces a narrow dip in the scattering spectrum. Plasmonic Fano resonances are of particular interest in LSPR sensing because as an interference phenomenon, its resonance is highly sensitive to the dielectric environment of the nanostructure.8,28–30 While plasmonic Fano resonances appear in a variety of metallic nanostructures, plasmonic nanoclusters, also known as plasmonic oligomers, provide a remarkably robust and tunable Fano resonant structure.29,31 Here the energy, shape, and width of the Fano resonance can be tuned by changing any of a variety of its structural parameters such as the number of particles, the interparticle separation, the particle sizes, or the relative positions of the individual particles. Plasmonic Fano resonances in the near infrared (NIR) region of the spectrum have previously been demonstrated with Au nanoclusters of various geometries.

These include linear

nanoparticle chains, 2D planar nanoclusters, 3D assemblies of nanoparticles,32–37 and multilayered Au-SiO2 core-shell nanoparticles.26,27,38 Here we examine 2D Fano resonant plasmonic nanoclusters, consisting of coupled planar Al nanodisks. Aluminum nanoclusters possess Fano resonances at significantly higher energies 3


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than previously studied Au systems, with strong Fano resonances in the blue-green region of the visible spectrum extending into the ultraviolet range. (Interband transitions at energies corresponding to the red region of the spectrum strongly modify the plasmon resonant behavior of these nanostructures.) Accessing the blue-green region of the visible spectrum, where the eye is most sensitive to color variation, potentially provides a new chemical sensing platform, where chemical detection corresponding to changes in sensor coloration can be directly detectable by visualization. Other geometries are feasible for this application as well, provided that they have sufficiently sensitive and narrow resonances in the visible to allow for visual identification of the local dielectric environment. The visible color gamut perceived by the human eye spans wavelengths from nominally 400 – 700 nm. Monochromatic light is perceived as a pure saturated color, while a broader spectrum of wavelengths give rise to weaker, pastel colors. Since dipolar plasmons typically scatter a range of wavelengths, they give rise to a relatively weak color response. Recent efforts have shown that far-field diffraction effects, when combined with plasmon resonant nanostructures in an array geometry, can greatly improve the chromaticity of plasmonic systems, resulting in bright, vivid colors suitable for flat-panel displays.6 Here we demonstrate that in aluminum-based nanoclusters with Fano resonances, the color response of the plasmonic system can be improved at the individual nanostructure level. This result relies upon the use of a Fano resonance to narrow the scattering response of the plasmonic system leading to more vibrant colors. The Fano dip can be tuned from the UVA region (300 – 400 nm) through the visible regime by increasing the geometric scaling of the nanocluster. The spectral lineshape and color of the scattered light can be further controlled by modifying the nanocluster geometr Tuning both the Fano resonance and the chromaticity using these two principles enables colorimetric 4


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applications with both wavelength specific tuning and vivid color scattering from the reduced widths of the Fano lineshape. The use of a relatively complex geometry allows us to achieve these two independent tuning mechanisms. Our approach paves the way for the development of low-cost colorimetric LSPR sensors where the LSPR shifts can be monitored visually without the use of costly and bulky photodetectors and spectral analyzers.

Results and Discussion The nanocluster geometry explored in this article is a plasmonic oligomer consisting of a core disc surrounded by N satellite discs (Figure 1a). This cluster supports the subradiant and superradiant modes needed to generate a Fano resonance. The surface charge plot of the subradiant mode in Figure 1b has anti-aligned satellite and core dipole moments and possesses a weak dipole moment for the overall structure due to phase retardation effects. The superradiant mode has a strong overall dipole moment from the co-aligned satellite and core dipole moments, shown in Figure 1c. Exciting these modes and generating a Fano resonance relies upon small interparticle spacing to strongly couple the individual discs, causing a hybridization of the plasmon modes.39 Alone, the core disc generates a broad superradiant scattering peak (Figure 1d, red) with no Fano resonance. The introduction of satellite particles and the onset of a Fano resonance produce two scattering peaks which are narrower and stronger than the original (Figure 1d, black).

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Figure 1. Fano-resonant aluminum nanocluster. a) An SEM micrograph of an N = 8 nanocluster structure (dsat = 50 nm, dcore = 90 nm) with identical spacing (xgap = 15 nm) between each element. Scale bar is 100 nm. b) Charge plot (negative – red, positive – blue) corresponding to the subradiant mode of the Fano resonance, calculated at 357 nm. It is characterized by the antialignment of satellite dipoles with the core dipole moment. c) Charge plot corresponding to the superradiant mode of the Fano resonance, calculated at 450 nm. All the dipole moments are aligned for the superradiant mode. The destructive interference of these two modes at the Fano frequency generates a dip in the scattering spectrum. d) Experimental (black dotted) and theoretical (black solid) scattering spectra for the N = 8 cluster demonstrate a Fano resonance in the UV regime. Removing the satellite particles leaves only the superradiant dipole mode of the core disc (red curves). Chromaticity Calculation. The chromaticity of a nanocluster is calculated by interpreting the experimental scattering spectrum with respect to the color matching functions of the human eye as defined by the International Commission on Illumination (Commission 6


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Internationale de l’Éclairage, CIE).40,41 First, the spectrum is multiplied by each of the three color matching functions 𝑥̅ (𝜆), 𝑦�(𝜆), and 𝑧̅(𝜆).40 These are the spectral functions defining

monochromatic red, green and blue according to human-derived data. By respectively convolving the intensity of light scattered by the nanocluster with each color matching function, integrated across the visual spectrum (from 380 to 780 nm wavelength), one obtains values referred to as the tristimulus values 𝑋, 𝑌, and 𝑍.: 780

𝑋 = � 𝐼 (𝜆)𝑥̅ (𝜆)𝑑𝑑 380 780

𝑌 = � 𝐼(𝜆)𝑦�(𝜆)𝑑𝑑 380 780

𝑍 = � 𝐼 (𝜆)𝑧̅(𝜆)𝑑𝑑 380

By normalizing each of the tristimulus values by their sum: 𝑋 𝑋+𝑌+𝑍 𝑌 𝑦= 𝑋+𝑌+𝑍 𝑍 𝑧= = 1−𝑥−𝑦 𝑋+𝑌+𝑍 𝑥=

one obtains the CIE chromaticity coordinates 𝑥, 𝑦, and 𝑧, corresponding to a point on the 1931

CIE chromaticity diagram (Fig. 2). The 𝑥 and 𝑦 coordinates of each nanocluster in Figure 2a are plotted on the 1931 CIE chromaticity diagram in Figure 2b. These coordinates can be

transformed into other useful metrics, such as the sRGB color gamut6 or the LChab color space,42 by further numerical transformations.

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Tunability of the nanoclusters. The nanocluster possesses two primary methods of tuning: scaling its size and varying the number of satellite particles. In size scaling, the number of satellite particles is kept fixed. The interparticle spacing must be kept small in order to maintain strong coupling as the two types of nanodiscs (core and satellite) are increased in size. For this tuning method, the N (number of satellite particles) and g (inter-particle spacing) parameters are fixed. The core diameter, dcore, is described as a function of the satellite diameter, dsat. 𝑑𝑐𝑐𝑐𝑐 = 2 × �

𝑑𝑠𝑠𝑠 +𝑔

360°

2×𝑠𝑠𝑠� 𝑁 �

−

𝑑𝑠𝑠𝑠 2

− 𝑔�

For Aluminum, a nanocluster with a small satellite diameter (dsat = 40 nm) generates a Fano resonance deep in the UVA region around 325 nm (Figure 2a, bottom). Most of the scattered light is ultraviolet and invisible to the human eye. As the cluster size is increased by increasing the diameter of the core and satellite discs in tandem, the spectrum is tuned to redder wavelengths. At ~2 eV, due to the influence of the Al interband transition near these energies, the long-wavelength portion of the cluster spectrum undergoes very strong damping. This damping has a profound effect on the Fano resonant lineshape, making it highly asymmetric and strongly attenuating the longer-wavelength of the two spectral peaks. This causes an unusual progression for the color response as the cluster size is increased, shifting the chromaticity abruptly, and counterintuitively, in a cyclical manner around the D65 white point of the 1931 CIE chromaticity diagram (Fig. 2b). This cycle in chromaticity from blue to green to violet and back to blue demonstrates that the Fano lineshape can reproduce any hue in the visible spectrum.

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Figure 2. Tunable Fano resonance from the UV through the visible region of the spectrum. a) Experimental (left) and theoretical (right) scattering spectra of the nanocluster Fano resonance tuning from 325 nm to 650 nm. Center, color of the scattered light and an SEM micrograph for each iteration of the N = 8 nanocluster geometry. Scale bar is 200 nm. b) The chromaticity of an aluminum nanocluster is highly sensitive to the diameter of the satellite particle (dsat, inset numbers in nm), resulting in a progression of colors that spans the sensitivity of the human eye. c) The position of the Fano resonance is tuned by increasing the diameters of the satellite and core nanodiscs in a fixed relation while maintaining a small, identical inter-particle spacing between all elements of the nanocluster to promote strong coupling. The superradiant background of the nanocluster spectrum can be tuned independently from the position of the Fano dip by varying the number of satellite particles. Obeying the constraints of this geometry, the core diameter must be increased with an increasing number of satellite particles in order to maintain a constant interparticle spacing. This broadens the 9


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scattering envelope of the superradiant mode to include the longer wavelengths of the visible regime, as shown in Figure 3, while leaving the Fano dip largely unaffected. The increased contribution from these wavelengths produces more orange and red hues, which were weak in the initial N = 8 nanocluster geometry. By maintaining a small satellite diameter, dsat = 40 nm, the Fano dip is immobilized in the UV region while the changes in N affect the scattering chromaticity. Increasing the number of satellites controls the relative weighting and spectral location of the low and high energy peaks of the Fano lineshape. These two peaks can even produce the range of “purples” by scattering both blue and red light from a single nanocluster, rather than placing isolated red and blue scattering elements within a diffraction limited space, as can be seen by eye in Supplemental Figure 3c. In theory, tuning the coupling between particles by changing the inter-particle spacing can also be used to adjust the strength of the Fano dip and thus the chromaticity. However, reproducible gaps with spacing less than 15 nm are not possible to produce with single-step lithography and so the inter-particle spacing was kept fixed at the smallest consistently achievable distance.

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Figure 3. Stationary UV Fano resonance with tunable visible scattering response. a) Experimental and b) theoretical scattering spectra of single nanoclusters with an increasing number of satellite particles, N = 8 – 12. The increasing core diameter tunes the red peak of the Fano resonance lineshape in visible regime (color inset in particle geometry). Solid squares: images of individual nanoclusters, obtained as averages of color stacked images. Maintaining a constant satellite diameter (dsat = 40 nm) and interparticle spacing (g = 15 nm) during these changes immobilizes the Fano resonance in the UV regime.

Figure of Merit for colorimetric sensors. Plasmonic sensors frequently function on the principle of a localized surface plasmon resonance (LSPR) shift induced by changes in the local dielectric environment, such as the binding of a chemical/biological agent, or a change in analyte 11


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concentration in a solution. A local dielectric change screens the charge distribution of the surface plasmon and lowers the energy of the plasmon resonance. The shift of a spectral feature, such as the peak scattering wavelength of a broad dipolar plasmon mode, is then reported as a function of some property of the dielectric medium that induced the change. We have previously shown that Fano-resonance based LSPR shifts are substantially more sensitive than simpler geometries where spectral shifts of a dipolar plasmon mode are monitored.28 Unlike Au-based LSPR sensors, where wavelength shifts are typically monitored spectroscopically at red wavelengths, an Al-based Fano resonance sensor would give rise to large spectral shifts in the blue-green region of the spectrum, where the eye is most sensitive to changes in color. This would result in direct colorimetric sensing that could be performed by simple visualization of color changes, rather than the relative complexity of spectroscopic measurements. However, the human eye’s ability to distinguish color differences varies significantly across the spectrum, so an effective colorimetric sensor must operate in a manner not constrained by this limitation.

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Figure 4. MacAdam ellipses and perceptual uniformity across color coordinates. MacAdam ellipses encompass all indistinguishable colors around a given reference point. a) The variation of the MacAdam ellipses across the 1931 CIE xyY color space (enlarged by factor of 10) illustrate the non-uniformity of this traditional coordinate system.43 b) The Cartesian 1976 CIE L*a*b* color space (two dimensional cross-section shown) displays a more uniform distribution of size and shape in the MacAdam ellipses.44 c) The uniformity is most apparent by remapping into the cylindrical coordinates: lightness, chroma, hue (LChab, full three-dimension color space shown). d) Hue, the angular coordinate, relates closely to the peak wavelength of a Lorentzian lineshape in the visible regime. Panels a) and b) adapted with permissions from the Journal of the Optical Society of America and Color Research & Application. To describe the LSPR color changes visualized by eye, and to define a Figure of Merit for plasmonic colorimetric sensing, the chromaticity coordinates in the 1931 CIE color space are not optimal: in this system, the smallest visualizable color change (all colors contained within a structure known as a MacAdam ellipse)43,45 is non-uniform, varying greatly in size and skew across this space (Fig. 4a). Instead, here we invoke a more recent color standard, the 1976 CIE 13


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L*a*b* color space, which is organized in a three dimensional Cartesian space where the human eye response to small changes around a reference color are essentially uniform for any point in the space (Fig. 4b).42,44 The LChab color space is a simple remapping of the L*a*b* coordinates into cylindrical coordinates with more intuitive meanings: L=lightness, related to spectral peak amplitude; C=chroma, related to spectral linewidth; and h=hue, related to peak wavelength (Fig. 4c,d).

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Figure 5. Color detection of refractive index using localized surface plasmon resonance shift. a) Experimental and b) theoretical spectra of N = 10 nanocluster in an air (n=1.0), PDMS (n = 1.4), PMMA (n=1.5), and diphenyl ether (n = 1.6) environments. The colorimetric response of the system is depicted to the right of each plot. c) In the perceptually uniform LChab color space, the hue of the scattering spectra correlates linearly with the refractive index of the analyte. Inset, the progression of hue in a cylindrical cross-section of the LChab color space. d) Schematic of the simple fluid cell detector geometry used for refractive index sensing, indicating the angle of incidence and polarization of excitation light. e) Images of an individual device; diphenyl ether, PMMA, PDMS, and Air (from left to right). Demonstration of a colorimetric LSPR sensor. The human eye can detect a local change in the dielectric environment with high resolution using the color change of the LSPR shift in the visible regime. We demonstrate this by fabricating an N=10, dsat = 50 nm, g = 15 nm nanocluster, and exposing it to variations in its dielectric environment (Fig. 5). For example, for an N=10 nanocluster, a change in dielectric environment from air (n=1.0) to polymethyl methacrylate (PMMA, n = 1.5) dramatically changes the scattered light from a blue-green color to an orange color (Fig. 5a,b). Dissolving the PMMA and replacing it with polydimethylsiloxane (PDMS, n = 1.4), representing a small relative change to the dielectric environment, changes the scattering to a yellow color easily distinguished by the human eye distinct from both the bluegreen and orange cases, as seen in the images shown Figure 5e (raw data shown in Supplemental Figure 4c). Hue, the polar coordinate of the LChab space measured in degrees (modulo 360), correlates almost linearly with the refractive index of the local environment as seen in Figure 5c and Supplemental Figure 1. The magnitude of the slope, |∆hue/∆n| = 213o, can serve as a simple and practical Figure of Merit (FoM) for this colorimetric plasmonic sensing; a larger magnitude slope produces larger color changes and a more sensitive colorimetric indicator. Using a previously defined, more conventional, FoM based on peak shift per unit index change, the shifts shown in Figure 5a correspond to a FoM ≈ 1.2.46 We stress that while the figure of merit is 15


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relatively low, the perceived color difference to the human eye is dramatically different depending on the dielectric environment of the nanocluster. The refractive index of a static or dynamic unknown environment can be rapidly identified using this colorimetric sensing technique. The schematic in Figure 5d illustrates a sensor with aluminum nanoclusters fabricated directly onto a fused silica observation window. The oxide shell of the nanostructure protects the underlying aluminum from reacting with organic solvents and polymers (see Supplemental Figure 2). The beam path of the probing optics is isolated from these environmental changes, eliminating any need to realign the system with each measurement. Note that due to the high rotational symmetry of the structure both s- and ppolarized light behave nearly identically for this excitation angle. Real-time identification of the analyte refractive index can be achieved by comparing the measured hue of the scattering spectrum in CIE LChab space with the linear trend predicted from numerical models (see Supplemental Figure 1).

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Figure 6. Custom color indicators for refractive index sensing. Three different nanocluster geometries, N = 8-10, dsat = 40 nm, g = 15 nm demonstrate unique reference colors in an air environment. When immersed in a refractive index (PMMA, n = 1.5) the color change can be immediately detected by the human eye or an RGB detector. Based on the tuning parameters for Al Fano clusters described earlier, a plasmonic colorimetric sensor can be designed to a specific color regime. Figure 6 demonstrates how the reference color in an air environment changes by increasing the number of satellite particles in the nanocluster, while maintaining a fixed gap size (15 nm) and satellite diameter (40 nm). When coated with a PMMA polymer film, the color reported by each nanocluster is different. The optimal nanocluster geometry for a given task will maximize the |∆hue/∆n| figure of merit and report a distinct color difference between the reference and test cases. The M and L cones of the 17


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human eye and the RGB channels of modern sensors (in an effort to emulate their biological counterpart) are highly sensitive to the range of hues between green and orange. Based on this sensitivity, the N = 10, dsat = 40 nm nanocluster geometry is properly tuned to generate an extremely strong colorimetric sensitivity for reporting the change from an air to a PMMA environment.

Conclusions We have demonstrated ultraviolet and visible Fano resonances using plasmonic aluminum nanoclusters. The nanocluster geometry possesses the flexibility to tune the position of the Fano resonance and simultaneously engineer the far-field scattering response. Previously demonstrated in the near-infrared region of the spectrum with Au structures, this advanced spectral lineshape and its associated characteristics (narrow line width features, high field enhancements, etc.) can be extended to the visible and ultraviolet range by fabricating the nanoclusters in Aluminum. This enables the development of plasmonic colorimetric sensors, where changes in the dielectric environment of the nanostructure can be easily visualized. Because of the promise of this potential application, we propose a Figure of Merit based on the device-independent, human eye response to color change, for evaluating plasmonic colorimetric LSPR sensors. The properties we report here are likely to lead to the development of Aluminum sensing platforms for visual readout for a large variety of applications. Experimental Methods Nanocluster Fabrication. Silica substrates were cleaned by sonication in acetone for 5 minutes, rinsing in isopropyl alcohol (IPA), and drying under flowing nitrogen. A 70 nm thick resist layer (PMMA 950, Microchem) and a conductive ESPACER layer (Showa Denko) were 18


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spin coated onto the substrates prior to electron beam patterning (FEI Quanta 650). The resulting patterns were developed in a 3:1 solution of IPA:MIBK (methyl isobutyl ketone) to create a mask of the nanocluster geometries. 99.999% pure aluminum (Kamis) was deposited onto the mask to a thickness of 35 nm by electron beam evaporation at a base pressure of 2x10-7 torr. The samples were then submerged in a room temperature acetone bath overnight. Liftoff of the surrounding film was performed by briefly sonicating the substrate in the acetone bath. The substrates were extracted from the bath under flowing acetone, then IPA, and finally dried under flowing nitrogen. The use of resist only twice the thickness of the final metal was necessary for the successful formation of 15 nm gaps between particles. Optical Measurements. A nitrogen-purged, incoherent, continuum source (Energetiq LDLS) was used to produce light in the ultraviolet and visible regime (200 – 700 nm). The light was filtered by a monochromator and broadband polarizer before being focused onto a sample with an incidence angle of 65°. Scattered monochromatic light was collected by a UV-grade reflective objective (NA = 0.50, Edmund Optics) in a dark field configuration. A UV-sensitized CCD array (Princeton Instruments) was used to image a 500 x 500 micron region of interest for each wavelength scanned by the monochromator (5 nm intervals). Integrating the spatially correlated pixels in each image provides a spectrum for every nanocluster geometry in the field of view. Raw spectral data were corrected for dark current contributions and the spectral dependence of the source using a white calibration standard (Labsphere). Alternatively, the calibrated monochromatic images can be stacked to form a color image of the sample region. Stacking several images corresponding the red, green, and blue CIE color matching functions recreates the same effect as the color filter array on commercial digital cameras. Averaging the color data of non-saturated pixels illuminated by a single nanocluster provides the true color of 19


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the scattered light. The entire apparatus was enclosed in a nitrogen purge box to mitigate oxygen and ozone absorption in the ultraviolet regime. Finite Difference Time Domain Simulations. The Al nanoclusters were simulated on top of a semi-infinite SiO2 substrate. All Al discs are cylinders sized to nominal experimental diameters and heights. Each disc includes a 3 nm Al surface oxide layer and a 5 nm radius of curvature on all the exposed edges. The dielectric responses of the surface oxide and the silica substrate were simulated using tabulated data.47 The dielectric function of bulk Al was calculated by mixing Al (91%) and Al2O3 (9%) values based on the Bruggeman effective medium approximation of the experimental material composition determined by x-ray photoelectron spectroscopy (XPS).4,48 The Al nanoclusters were excited by P- or S- polarized light at a 65o oblique incidence angle from the air side of the sample. Scattering spectra were calculated using the finite difference time domain (FDTD) method by integrating the far-field intensity in the solid angle corresponding to the numerical aperture of the objective.

Associated Content Supplemental Information Supplemental figures describe: (1) the theoretical scattering spectra and color of a N=8 nanocluster in various refractive index environments; (2) the stability of the aluminum nanocluster after exposure to polymer coatings and thorough rinsing in an organic solvent; (3) experimental, theoretical, and 20


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visual optical specta of single nanoclusters of different N; and (4) experimental, theoretical, and visual optical spectra of N = 10 nanocluster in different dielectric environments. This material is available free of charge via the Internet at http://pubs.acs.org. Author Information Corresponding Authors: *Email: (N.J.H.) [email protected], *Email: (P.N.) [email protected] Author contributions: N.S.K. performed particle nanostructure fabrication and experimental work. L.L., X.Y., carried out theoretical study and computational modeling, P.N., H.O.E. and N.J.H. conceived the project and supervised the research. All authors contributed to the development of the manuscript. Notes: The authors declare no competing financial interests. Acknowledgements We would like to thank Bob Zheng, Nathaniel Hogan, Mark Knight, Andrea Schlather, Ali Sobani, Stephan Link, Surbhi Lal, Tad Finnegan, and Roswell Easton King III for technical and editorial support. This material is based upon work supported by an in house Army Research Office grant, W911NF-12-1-0251, the Office of Naval Research Grant N00014-10-1-0989, the Defense Threat Reduction Agency Grant HDTRA1-11-1-0040, and the Robert A. Welch Foundation Grants C-1220 and C-1222. References (1)

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Fano Resonant Aluminum Nanoclusters for Plasmonic Colorimetric

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