Localized Surface Plasmon Resonance Spectroscopy of Triangular

Aug 15, 2008 - Aluminum for Nonlinear Plasmonics: Resonance-Driven Polarized Luminescence of Al, Ag, and Au Nanoantennas. Marta Castro-Lopez , Daan Br...
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J. Phys. Chem. C 2008, 112, 13958–13963

Localized Surface Plasmon Resonance Spectroscopy of Triangular Aluminum Nanoparticles George H. Chan, † Jing Zhao, † George C. Schatz,* and Richard P. Van Duyne* Department of Chemistry, Northwestern UniVersity, 2145 Sheridan Road, EVanston, Illinois 60208-3113 ReceiVed: May 8, 2008; ReVised Manuscript ReceiVed: June 24, 2008

The localized surface plasmon resonance (LSPR) of Al nanoparticles fabricated by nanosphere lithography (NSL) was examined by UV-vis extinction spectroscopy and electrodynamics theory. Al triangular nanoparticle arrays can support LSP resonances that are tunable throughout the visible and into the UV portion of the spectrum. Scanning electron microscope and atomic force microscope studies point to the presence of a thin native Al2O3 layer on the surface of the Al triangular nanoparticles. The presence of the oxide layer, especially on the tips of the nanotriangles, results in a significant red shift in the LSPR λmax. The refractive index (RI) sensitivity of the Al triangular nanoparticle arrays in bulk solvents was determined to be 0.405 eV/RIU. Theoretical results show that the oxide layer leads to a significant decrease in this RI sensitivity compared to unoxidized triangular nanoparticles of similar size and geometry. A comparison of Al, Ag, Cu, and Au triangular nanoparticles for a similar shape and geometry show that the LSPR λmax has the ordering Au > Cu > Ag > Al, while the full width at half-maximum satisfies Al > Au > Ag > Cu. Introduction Materials that exhibit a large negative real and small positive imaginary dielectric function are capable of supporting a collective excitation of the conduction electrons known as plasmon excitation.1 In metal nanoparticles this leads to a localized surface plasmon resonance (LSPR), which is an effect that produces strong peaks in extinction spectra, as well as strong enhancements of the local electromagnetic fields surrounding the nanoparticles.2-4 Previous work has demonstrated that the position of the LSPR extinction maximum, λmax, is sensitive to the size, shape, interparticle spacing, dielectric environment, and dielectric properties of the nanoparticle.5-8 As a result, metallic nanoparticles that support LSP resonances are promising platforms as highly sensitive optical nanosensors, as photonic components, and in surface-enhanced spectroscopies.9-20 It is well-established that Ag and Au nanoparticles support surface plasmon resonances that can be tuned throughout the UVvis-near-IR spectrum.21,22 Interestingly, a number of other metals (i.e., Li, Na, Al, In, Cu, and Ga) also meet this criterion and may possibly support surface plasmon resonances for at least part of the UV-vis-near-IR region,23-27 but there has been far less experimental work with these metals. There has been continued interest in the plasmonic properties and sensing capabilities of Al over the past 20 years.28-38 Aluminum is capable of supporting surface plasmons in the visible and UV, and it has been reported to be a substrate for surface-enhanced fluorescence37,39-41 and surface-enhanced Raman spectroscopy.42 Moreover, many proteins, fluorophores, and biological molecules of interest absorb in the UV region of the electromagnetic spectrum. It is worthwhile to explore substrates that support UV surface plasmons, which could allow for spectroscopic measurements that combine both molecular and plasmon resonance effects. * To whom corrspondence should be addressed. Telephone: (847) 4913516(R.P.V.D.); (847) 491-5657(G.C.S.). Fax: (847) 491-7713 (R.P.V.D.); (847) 491-7713 (G.G.S.). E-mail: [email protected] (R.P.V.D.); [email protected] (G.C.S.). † These authors contributed equally to this work.

The optical properties of metal nanoparticles are closely related to the real and imaginary parts of their wavelengthdependent dielectric constants. A convenient expression for thinking about this is provided by the quasistatic model for light scattering from a spheroid-shaped particle. For light whose polarization is parallel to the long axis of the spheroid, the extinction cross-section Cext is given by43

{

εi - εo 1 1 Cext ∝ Im(R) ∝ Im λ λ εi + χεo

}

(1)

Here λ is the wavelength, R is the polarizability, ε0 is the dielectric constant of the medium outside the particle (ε0 ) 1 for vacuum), εi is the dielectric function of the metal, and χ is a shape-dependent parameter which varies from 2 for a sphere to infinity for a highly prolate or oblate particle. This expression shows that the plasmon resonance occurs when the real part of the denominator vanishes, which means that Re(εi) ) - χε0. This indicates the real part of the dielectric constant needs to be negative, and the narrowest resonances are associated with the Im(εi), which is as small as possible. To see how this expression applies to Al, in Figure 1, we plot the real and imaginary parts of the dielectric functions of Al (blue curve with circles) and Ag (red curve with triangles) as obtained from Palik.44 Figure 1A shows that Ag can only show plasmon excitations at wavelengths longer than 350 nm, because the real part of its dielectric function is positive below 350 nm, while Al should be plasmonically active from 200 nm to just below 800 nm. Figure 1B shows that Al has interband transitions near 800 nm with a corresponding steep rise in the imaginary part of the dielectric constant at that wavelength. This rise has been extensively examined in the condensed matter physics literature,45,46 and it involves a transition between two different bands associated with the conduction electrons (i.e., an interband transition) which happen to be parallel for certain directions of the wavevector in the Brillouin zone very close to the Fermi energy. This behavior is quite different from the situation for silver, which has a small imaginary component of the dielectric constant in the visible, and the rise in Im(εi) below 350 nm is associated with interband transitions involving the

10.1021/jp804088z CCC: $40.75  2008 American Chemical Society Published on Web 08/15/2008

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Figure 1. Comparison of the dielectric function of Al (blue curve with circles) and Ag (red curve with triangles) from Palik between 200 and 1000 nm (A) negative real part and (B) positive imaginary part. Note that, below 300 nm, Ag exhibits interband transitions, whereas Al exhibits interband transitions at 800 nm.

localized 4d orbitals. Both parallel conduction bands and localized orbitals allow for electron scattering events which dephase the conduction electrons and thus broaden the extinction line shape of nanoparticles. This makes Al less attractive at long wavelengths where Ag is already in common use. Nevertheless, it is clear that Al has promise for significant sensing capabilities over short-wavelength portions of the electromagnetic spectrum that are otherwise not generally considered. A problem with Al, however, is that it rapidly oxidizes when exposed to the atmosphere, forming a thin Al2O3 layer that prevents further attack by oxygen.47 High-resolution transmission electron microscopy studies of aluminum nanoparticles indicate that the Al2O3 layer is approximately 2.5 nm thick and porous and is part amorphous and part crystalline.48 The presence of this oxide layer is expected to affect its plasmonic properties. Consequently, the plasmonic properties of aluminum and, in particular, the LSPR spectroscopy of aluminum nanoparticles has just recently received attention.49 In this work, we examine the optical properties of Al triangular nanoparticles fabricated using nanosphere lithography (NSL).5 The experimental results obtained from UV-vis extinction spectroscopy are compared with electrodynamics calculations based on the discrete dipole approximation (DDA) method.50 NSL involves vapor deposition through a colloidal crystal mask made using polymer nanospheres, so by selection of the nanosphere diameter (D) and the deposited metal thickness (dm), the in-plane width and out-of-plane height Al triangular nanoparticles can be controlled. This allows for systematic tuning of the LSPR throughout the UV and visible spectrum. In addition, theoretical investigations of the effects of a thin alumina layer on the surface of the triangular aluminum nanoparticles will be presented. This allows us to characterize the red shift of the LSPR λmax due to this layer which enables us to compare both experimental and theoretical LSPR properties of Al and the noble metals of similar size and geometry. Finally, the refractive index sensitivity of the NSL oxidized Al triangular nanoparticle arrays to bulk solvents will be used to provide insight on the sensitivity differences between Al and noble metal counterparts and the possibility of using Al triangular nanoparticles in sensing applications. Experimental Methods Materials. Fisher brand No. 2, 18 mm diameter glass coverslips were obtained from Fisher Scientific. S1-UV fused silica coverslips (18 mm diameter and 0.15 mm thick) with an optimum transmission range of 180 nm to 2.0 µm were

purchased from ESCO Products. Glass, Si, and S1-UV fused silica substrates were cleaned in a piranha solution (1:3 30% H2O2:H2SO4) at 80 °C for 30 min prior to use. (CAUTION: Piranha reacts Violently with organic compounds and should be handled with great care!) Samples were allowed to cool and then rinsed repeatedly with ultrapure water (18.2 MΩ · cm; Marlborough, MA). The samples were then sonicated in a (5: 1:1 H2O:NH4OH:30% H2O2) solution for 1 h and then rinsed with copious amounts of ultrapure water. Fabrication of Triangular Nanoparticle Arrays. NSL was used to create monodisperse, surface-confined nanotriangles. Polystyrene nanospheres (∼2.2 µL) with diameters of 280, 390, 410, 500, and 590 nm were received as a suspension in water (Interfacial Dynamics Corp., Portland, OR, or Duke Scientific, Palo Alto, CA). The polystyrene nanospheres were drop-coated onto glass, S1-UV fused silica (ESCO Products, Oak Ridge, NJ), or Si substrates and allowed to dry, forming a monolayer in a close-packed hexagonal formation, which served as a deposition mask. Aluminum, copper, silver, or gold metal was deposited by electron beam (e-beam) deposition (Kurt J. Lesker Axxis deposition system, Pittsburgh, PA) with a base pressure between 10-6 and 10-7 Torr. The mass thickness and the deposition rate (0.5 Å s-1 for the noble metals and 1.0 Å s-1 for aluminum) were monitored using a Sigma Instrument 6 MHz gold plated QCM (Fort Collins, CO). After the metal deposition, the nanosphere masks were removed by sonication in absolute ethanol (Pharmco, Brookfield, CT) for 2-3 min. The sample was then placed in a home-built flow cell and introduced to N2 environment to dry the sample. UV-vis Extinction Spectroscopy. Macroscale UV-vis extinction measurements in a standard transmission geometry mode were performed using an Ocean Optics model SD2000 (Dunedin, FL) or an Ocean Optics model HR4000 with unpolarized white light provided by a tungsten-halogen or a deuterium light source, respectively. The light spot diameter was approximately 1-2 mm for experiments conducted with Ocean Optics model SD2000 spectrometer and approximately 1 cm for measurements conducted with Ocean Optics model HR4000 spectrometer. The extinction maximum was located by calculating the zero-crossing point of the first derivative. Scanning Electron Microscopy and Atomic Force Microscopy. The height and the structure of the Al triangular nanoparticles were investigated with an atomic force microscope (AFM) and a scanning electron microscope (SEM). Tappingmode AFM images were collected using a Digital Instruments

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Figure 2. (A) DDA simulation of the effect a 2 nm layer of Al2O3 on the LSPR of a NSL Al nanoparticle. The inset shows a side view of the core-shell nanoparticle. The total height and width of the nanoparticle was fixed at 50 and 90 nm, respectively. (B) SEM image of NSL Al nanoparticle arrays (D ) 390 nm; dm ) 50 nm; substrate ) Si) indicating the presence of oxide on the surface of the nanoparticles.

Nanoscope IV microscope and a Nanoscope IIIA controller (Digital Instruments, Santa Barbara, CA) on samples prepared on glass substrates. SEM images were collected using a Hitachi4800 SEM at an accelerating voltage 10 kV and an average working distance of 7.1 mm on samples prepared on Si substrates. Electrodynamics Calculations. The optical properties of the Al triangular nanoparticles were examined with classical electrodynamics calculations based on the DDA method.50,51 In all calculations, the shape of the nanoparticles is assumed to be a truncated tetrahedron, with the Al dielectric constants taken from the compilation in Palik44 and the refractive index of Al2O3 taken from recent studies of atomic layer deposition.52 The effect of the glass substrate on the LSPR wavelength was treated using effective medium theory53 in which the particles are assumed to be embedded in a homogeneous medium and the dielectric constant is a weighted average of that for glass and N2. The weighting is determined by the relative fractions of the particles that are exposed to each medium. Results and Discussion Effect of Aluminum Oxide Formation. The LSPR λmax of metallic nanoparticles is sensitive not only to the size, shape, interparticle spacing, and the dielectric properties of the metal but also is quite sensitive to the dielectric environment surrounding the nanoparticles. In particular, the effect of the native oxide shell is expected to result in a spectral red shift in the LSPR λmax, whereas the concomitant shrinkage of the metallic core is expected to lead to a spectral blue shift in the LSPR λmax. To see which effect dominates, we have performed DDA calculations for nanoparticles whose total height is fixed at 50 nm and total width fixed at 90 nm, where we have replaced Al by Al2O3 in an outer layer on the nanoparticle. Figure 2A shows that the addition of a 2 nm layer of Al2O3 on a bare Al nanoparticle (width ) 90 nm; dm ) 48 nm of Al + 2 nm of Al2O3) leads to a red shift in the LSPR λmax of ∼13 nm. As the thickness (2-10 nm) of Al2O3 increases, the magnitude of the red shift in LSPR λmax increases (Supporting Information, Figure S1). This demonstrates that the contribution to the total LSPR signal from the dielectric red shift is larger than from the shrinkage of the metallic core, leading to a ∼4.2 nm shift in LSPR λmax per 1 nm increase in the oxide thickness. Moreover, no significant peak broadening or decrease in the extinction efficiency as a result of the presence of an alumina layer is predicted. This is not surprising since the native oxide

Chan et al.

Figure 3. (A) Tapping mode AFM image and (B) line scan of NSL Al nanoparticle arrays on a glass substrate (D ) 390 nm; dm ) 40 nm). All reported line scan values have not been deconvoluted for tip broadening effects.

layer is transparent and should have minimal scattering in the UV and visible regions. SEM and AFM images for NSL Al nanoparticles (D ) 390 nm; dm ) 50 and 40 nm, respectively) on a Si and glass substrate, respectively, are shown in Figures 2B and 3A. The nanoparticles are nearly triangular as expected. Upon closer examination of the SEM image, sharp contrast between the tips and the core of the nanoparticle is observed, which suggests the presence of oxides on the tips of the nanoparticles. From the AFM line-scan measurements, the heights (Figure 3B) of the nanoparticles are consistent with measurements from the quartz crystal microbalance. Notice that the nanoparticle width is 157 nm from the AFM line scan. Assuming that the AFM tip broadening effect is ∼20 nm,54 the width of the nanoparticle is ∼137 nm, which is ∼1.5 times larger than the width found for Ag nanoparticles fabricated with a similar nanosphere diameter.54 The results for Ag are very similar to what would be expected from geometric considerations for the vapor deposition conditions, so the larger nanoparticle diameter Al particle is likely due to the wetting properties of the Al metal on the glass surface. Therefore, to study the extinction spectra of Al nanoparticles using DDA, the nanoparticle width (still assumed to be a truncated tetrahedron) is taken to be 1.5-1.8 times that calculated in previous work for Ag particles on the basis of geometry considerations.5 Tuning the LSPR of Al Nanoparticles by Varying the InPlane Width. Figure 4A illustrates the experimental extinction spectra of Al nanoparticle arrays with varying in-plane width and fixed height in a N2 environment on a glass or S1-UV substrate. As the nanoparticle in-plane width increases, the LSPR λmax shifts to the red. The fwhm (full width at half-maximum) of the Al LSPR (D ) 390 nm) is ∼0.65 eV, which is much broader than for Cu, Ag, and Au nanoparticles fabricated by NSL. This broadening in the LSPR is mainly attributed to the large positive imaginary contribution to the dielectric function compared to the other metals (see Figure 1). To confirm this, the effect of the Al nanoparticle width on the LSPR was investigated using the DDA method. The calculations were performed on the core-shell truncated tetrahedral nanoparticles with an Al core and 2 nm Al2O3 shell. The total width of the nanoparticle was taken to be 95, 137, 174, 206, and 230 nm, and the total height is fixed at 50 nm. The calculated extinction spectra (Figure 4B) have LSPR line shapes similar to the experimental ones, and we also find that an increase in the width of the nanoparticle leads to a red shift in the LSPR λmax and a broadening in the LSPR spectra. Both the data from experiments and calculations reveal that when the LSPR of the nanoparticles is close to the Al interband transition (∼800 nm), the LSPR is significantly broadened. Indeed, the spectra in red in Figure

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Figure 4. (A) Extinction spectra of the Al nanoparticle arrays with varying widths (D ) 280-590 nm; dm ) 50 nm; glass substrate (visible) and UV substrate (UV), N2 environment). (B) Calculated extinction spectra of Al nanoparticle with 2 nm Al2O3 with varying widths of 95 (purple), 137 (blue), 174 (green), 206 (yellow), and 230 nm (red).The inset in B shows a side view of the core-shell nanoparticle; a 2 nm shell of Al2O3 surrounded a 48 nm core of Al.

Figure 5. Comparison of the LSPR of Al, Cu, Ag, and Au for (A) experiment and (B) theoretical calculations for a similar size and shape (D ) 390 nm; dm ) 50 nm; glass substrate; N2 environment). For the theoretical calculations, truncated tetrahedral nanoparticle with width ) 90 nm and height ) 50 nm is used. Note, theory indicates for a bare Al nanoparticle a LSPR λmax ) 377 nm and for an oxidized Al nanoparticle a LSPR λmax ) 390 nm.

4A,B are significantly broader than the others. In addition, the calculated curve in Figure 4B has λmax at ∼760 nm and a shoulder at ∼900 nm. This shows that the Al interband transition can greatly affect the LSPR band of the Al triangular nanoparticles. Similar results were also observed in Al nanodisks.49 LSPR of Al, Cu, Ag, and Au Nanopaticles with Similar Geometry. Figure 5A presents normalized experimental extinction spectra for Al, Cu, Ag, and Au nanoparticle arrays fabricated by NSL (D ) 390 nm, dm ) 50 nm, glass substrate, in a N2 environment) with a similar shape and geometry. The extinction maxima of Al, Cu, Ag, and Au are 508, 698, 639, and 787 nm, respectively. The approximate values of the fwhm for the LSPR of Al, Cu, Ag, and Au are ∼0.65, ∼0.29, ∼0.36, and ∼0.40 eV, respectively. Figure 5B shows the calculated extinction spectra for Al, Cu, Ag, and Au with the same truncated tetrahedral geometry where the nanoparticle width is 90 nm and height is 50 nm. As shown in both Figure 5A,B, interband transitions in Cu and Au do not significantly affect the optical properties when the LSPR λmax > ∼650 nm. In contrast, the interband transitions of Al lead to significant peak broadening of the LSPR and a concomitant decrease in peak intensity (∼0.06 extinction units) when the LSPR λmax approaches 800 nm. From the comparison of the LSPR behavior for Al, Cu, Ag, and Au, we conclude that the Al nanoparticles display a bluer, broader, and less intense LSPR compared to the noble metals in the visible region. The experimental results obtained for Cu and Ag agree with the predicted LSPR λmax from theoretical calculations. On the other hand, the experimental LSPR λmax of Al and Au are significantly red-shifted compared to that predicted from theory. For the case of Au,

the discrepancy of the LSPR λmax between experiment and theory was previously attributed to the difference in the wetting properties of the noble metals on glass substrates and from differences in their surface melting temperatures.8 In particular, Au triangular nanoparticles can wet the surface to produce a tiny “apron” of metal around the particle, resulting in a redshifting of the plasmon resonance relative to what is modeled by the DDA calculation. The discrepancy of the Al LSPR λmax between experiment and theory has a similar origin. The presence of the oxide layer on the tips of the nanoparticles leads to a significant red shift of the LSPR λmax. Refractive Index Sensitivity of Al Nanoparticles. The refractive index (RI) sensitivity of the Al triangular nanoparticle arrays was investigated to explore its use as a plasmonic refractive index sensor. To do this, we have examined the shift of the LSPR λmax caused by bulk solvents using extinction measurements and DDA calculations. Previous work demonstrated that the LSPR λmax for noble metal nanoparticles is extremely sensitive to the external dielectric environment.10,55,56 In addition, it was found that the noble metal nanoparticle arrays fabricated via NSL experienced slight geometrical changes (rounding of the tips) during the solvent study experiments.52,55 To prevent such modifications, the noble metal triangular nanoparticle arrays were solvent-annealed to stabilize the nanoparticles prior to any spectroscopic measurements; this was done by monitoring the LSPR λmax until it stabilized. In the present application, the presence of an alumina layer was found to act as an effective protective barrier preventing unwanted solvent annealing.

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Figure 6. Extinction spectra of NSL Al nanoparticles in the presence of a layer of oxide in various solvents (width ) 137nm; dm ) 50 nm; on glass substrate) (A) experiment and (C) theory, and the refractive index sensitivity m for (B) experiment and (D) theory for Al nanoparticle with 2 nm Al2O3 (black line with triangles) and bare Al nanoparticle (red line with circles).

Figure 6A shows the extinction spectra of Al nanoparticles in different dielectric environments. The environments chosen are N2 (RI ) 1.0), H2O (RI ) 1.33), ethanol (RI ) 1.36), chloroform (RI ) 1.45), and benzene (RI ) 1.50). As shown in Figure 6A, the LSPR λmax red shifts when the RI of the medium increases as expected. The LSPR λmax versus RI of the medium is plotted in Figure 6B. The slope of this plot yields a RI sensitivity of 0.405 eV/RIU. Figure 6C illustrates the extinction spectrum of an Al nanoparticle with a 2 nm oxide layer (total width ) 137 nm, height ) 50 nm) on a glass substrate on the basis of DDA calculations for various dielectric environments. Figure 6D shows the refractive index sensitivity predicted from these calculations, for a 2 nm oxide layer (black line with triangles) and bare (unoxidized) Al nanoparticle with the same geometry (red line with circles). The RI sensitivity of the Al nanoparticle with 2 nm oxide layer is 1.08 eV/RIU and that of a bare Al nanoparticle is 1.17 eV/RIU. According to theory, the presence of a thin oxide layer does not lead to a significant decrease in the RI sensitivity of the Al nanoparticle arrays. However in the experiment, there is a significant decrease in RI sensitivity as a result of the oxide layer. This is because, in the calculations, it was assumed that the oxide layer is distributed uniformly over the Al nanoparticle surface while the SEM measurement in Figure 2B indicates that the oxide layer is most likely relatively thicker at the nanoparticle tips. Previous studies showed that the sharp tips are responsible for the majority of the RI sensitivity of the noble metal NSL nanoparticles, so a thick oxide layer at the tips will lead to a decrease in the electromagnetic field decay length ld, resulting in a significant decrease in the RI sensitivity as previously demonstrated.52 Conclusions In conclusion, our experiments show that Al triangular nanoparticle arrays fabricated by NSL are capable of supporting

surface plasmons in the near-UV and visible regions of the spectrum. We demonstrate that the presence of a thin native aluminum oxide layer leads to a red shift in the LSPR λmax. In addition, when the nanoparticle height is fixed and the nanoparticle width is increased, a blue shift in the LSPR λmax is observed. These trends all agree with the predictions from theory. In addition, both experiment and theory demonstrate that when the LSPR λmax is close to the Al interband transition at 1.5 eV or 800 nm, there is a significant broadening in the LSPR spectra. This effect, as well as the effect of Al oxides, is in agreement with the recent study of Al nanodisks by Langhammer et al.49 However, our work has also included the refractive index sensitivity of the NSL fabricated Al nanoparticles, where we find the observed result is smaller than is predicted and smaller than for Ag nanoparticle arrays of the same size and shape in the visible region. This reduction in RI sensitivity arises because oxidation occurs preferentially at the tips of the nanoparticles, and it is the tips where electromagnetic hot spots produce the strongest contribution to the presence of adsorbed molecules. This conclusion likely applies to any anisotropic Al nanoparticle shape in which sharp features are present, and it points to the use of shell-shaped structures, rather than structures with sharp points, as a possible direction for producing Al particles where RI sensitivity is less sensitive to oxidation. This is the first paper which has provided data such as that in Figure 5 in which the extinction spectrum of aluminum nanoparticles has been compared with that of Ag, Cu, and Au for particles with the same structure. This possibility arises through the use of nanosphere lithography to make the particles. Other methods for making Al nanoparticles (such as wet chemistry methods) lead to structures which are different from those which can be made for the other metals, and therefore give spectra which are hard to compare. Comparison of the LSPR properties for Cu, Ag, Au, and Al particles with similar shape and geometry show that LSPR λmax is ordered Au > Cu

LSPR of Triangular Aluminum Nanoparticles > Ag > Al, while the fwhm satisfies Al > Au > Ag > Cu. Further work is being carried out to investigate the utility of NSL Al triangular arrays as a substrate for ultraviolet surface enhanced Raman spectroscopy. Acknowledgment. This work was supported by the National Science Foundation (Grants EEC-0118025, CHE-0414554, and BES-0507036), the Air Force Office of Scientific Research MURI program (Grant F49620-02-1-0381), DTRA JSTO Program (Grant FA9550-06-1-0558), AFOSR/DARPA Project BAA07-61 (Grant FA9550-08-1-0221), and the MRSEC program of the National Science Foundation (Grant DMR-0520513) at the Materials Research Center of Northwestern University. Supporting Information Available: Effect of Al2O3 thickness on the LSPR of Al nanoparticles. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Willets, K. A.; Van Duyne, R. P. Annu. ReV. Phys. Chem. 2007, 58, 267. (2) Mie, G. Ann. Phys. 1908, 25, 377. (3) Bohren, C. F.; Huffman, D. R. Absorption and Scattering of Light by Small Particles; John Wiley & Sons: New York, 1983. (4) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer-Verlag: Heidelberg, Germany, 1995; Vol. 25. (5) Haynes, C. L.; Van Duyne, R. P. J. Phys. Chem. B 2001, 105, 5599. (6) Sherry, L. J.; Jin, R.; Mirkin, C. A.; Schatz, G. C.; Van Duyne, R. P. Nano Lett. 2006, 6, 2060. (7) Sherry, L. J.; Chang, S. H.; Schatz, G. C.; Van Duyne, R. P.; Wiley, B. J.; Xia, Y. Nano Lett. 2005, 5, 2034. (8) Huang, W. Y.; Qian, W.; El-Sayed, M. A. Nano Lett. 2004, 4, 1741. (9) Nie, S. M.; Emory, S. R. Science 1997, 275, 1102. (10) Dieringer, J. A.; McFarland, A. D.; Shah, N. C.; Stuart, D. A.; Whitney, A. V.; Yonzon, C. R.; Young, M. A.; Zhang, X. Y.; Van Duyne, R. P. Faraday Discuss. 2006, 132, 9. (11) Chen, K.; Durak, C.; Heflin, J. R.; Robinson, H. D. Nano Lett. 2007, 7, 254. (12) Zhao, J.; Das, A.; Zhang, X. Y.; Schatz, G. C.; Sligar, S. G.; Van Duyne, R. P. J. Am. Chem. Soc. 2006, 128, 11004. (13) Yonzon, C. R.; Jeoung, E.; Zou, S.; Schatz, G. C.; Mrksich, M.; VanDuyne, R. P. J. Am. Chem. Soc. 2004, 126, 12669. (14) Moran, A. M.; Sung, J.; Hicks, E. M.; Van Duyne, R. P.; Spears, K. G. J. Phys. Chem. B 2005, 109, 4501. (15) Haes, A. J.; Chang, L.; Klein, W. L.; Van Duyne, R. P. J. Am. Chem. Soc. 2005, 127, 2264. (16) Haes, A. J.; Hall, W. P.; Chang, L.; Klein, W. L.; Van Duyne, R. P. Nano Lett. 2004, 4, 1029. (17) Jiang, J.; Bosnick, K.; Maillard, M.; Brus, L. J. Phys. Chem. B 2003, 107, 9964. (18) Li, Y.; Lee, H. J.; Corn, R. M. Anal. Chem. 2007, 79, 1082. (19) Sung, J.; Hicks, E. M.; Van Duyne, R. P.; Spears, K. G. J. Phys. Chem. C 2007, 111, 10368. (20) Sung, J.; Hicks, E. M.; Van Duyne, R. P.; Spears, K. G. J. Phys. Chem. C 2008, 112, 4091. (21) Murray, W. A.; Suckling, J. R.; Barnes, W. L. Nano Lett. 2006, 6, 1772.

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