Aluminum and Indium Plasmonic Nanoantennas in the Ultraviolet

Article pubs.acs.org/JPCC

Aluminum and Indium Plasmonic Nanoantennas in the Ultraviolet Michael B. Ross and George C. Schatz* Department of Chemistry and International Institute for Nanotechnology, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States S Supporting Information *

ABSTRACT: We use classical electrodynamics calculations to investigate the versatility and capability of aluminum and indium dimers with small gaps as ultraviolet plasmonic nanoantennas, focusing on the particle size and wavelength range that gives optimum near-field enhancement. We find that Al and In are highly capable plasmonic materials in the ultraviolet, even with the incorporation of Al2O3 shells on the Al spheres; however, Al is strongly influenced by quadrupole modes while In is not. Al is the optimal material in the deep-UV, while In is ideal in the near-UV and near-visible spectral regions. Unlike Au and Ag, Al and In are most effective with the lowest refractive index background media possible, with vacuum being ideal. Ag outperforms both Al and In red of ∼320 nm, but optimal surface-enhanced Raman spectroscopy enhancement factors are still substantial for Al and In, with peak |E|4 values (for dimers in vacuum with a 1 nm gap) determined to be: Al, 2.0 × 109 (at 204 nm); In, 1.2 × 109 (at 359 nm); Al/Al2O3, 1.2 × 107 (at 218 nm). For comparison, the optimal |E|4 for Au dimers is 2.8 × 1011 (at 723 nm) and for Ag is 1.3 × 1012 (at 794 nm), with background indices of 1.50 and 2.25, respectively. These data suggest that the continued exploration of Al and In as plasmonic materials could provide powerful opportunities in ultraviolet spectroscopic enhancement, fluorescence quenching, and cellular imaging.

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In33,34 and Al35−39 exist, though they do not approach the quality and diversity of those developed for noble metal nanoparticles. Al in particular has proven challenging to make, but methods using vapor deposition through a mask,26 femtosecond laser pulses,37 electrochemical methods,38 and chemical aerosols39 have been developed. Past UV-plasmonic applications with these metals include enhancement of fluorescence in relevant biomolecules above background cellular autofluorescence,40 increased solar photovoltaic efficiencies,8,41,42 nonlinear polarized ultraviolet luminescence,43 and amplified SERS due to the excitation of electronic resonances in adsorbed molecules and the stronger scattering of light (scales as ω4) at UV frequencies (3.1−12 eV, 100−400 nm).23,44 This paper explores the opportunities available for using dimers of Al and In with small gaps as substrates for SERS and other spectroscopic studies where electromagnetic field enhancements are of primary interest.13,45 For Ag and Au, it is well-known that dimers and other small clusters of nanoparticles with small gaps provide ideal substrates for SERS, as they have a combination of electromagnetic hot spots and dark plasmon modes that are ideal for enhancing electromagnetic fields for Raman scattering while minimizing interferences from Rayleigh scattering. In addition, Ag and Au are often coated with a dielectric material for chemical stability, and the higher the index of refraction of the coating, the redder

INTRODUCTION Plasmonic materials have the remarkable ability to localize light at length scales below the diffraction limit.1−4 This behavior arises from collective oscillation of the conduction electrons; in nanoparticles this is termed the localized surface plasmon resonance (LSPR). Physically, the LSPR manifests as strong absorption, scattering, and greatly amplified local electric fields near the nanostructure. By changing the size, shape, composition, or local environment of the nanoparticle, one can modify the strength and spectral location of the LSPR.3 This remarkable tailorability has significantly impacted sensing technology,5−7 solar absorption enhancement,8,9 metamaterials,10,11 and the enhancement of Raman scattering by molecules (SERS).6,12,13 Silver and gold are the most often employed plasmonic materials due to their strong LSPRs in the visible, ease of synthesis, and chemical versatility.3,14 Unfortunately, Ag and Au are scarce, expensive as commodities, and only support LSPRs in the visible-infrared region of the electromagnetic spectrum. Because of these limitations, there is considerable interest in the plasmonic properties of other metals,15,16 alloys,14 nonmetals such as graphene17,18 and indium−tin oxide (ITO),19 and a wide range of other materials.20−22 Herein, we focus on aluminum (Al) and indium (In), the most promising “poor metal” plasmonic candidates, both of which both support LSPRs in the ultraviolet (UV) region.23,24 Both Al and In have been experimentally demonstrated as plasmonic materials; however, the number of applications involving these metals (mostly concerned with extinction or SERS)25−32 is a tiny fraction of those developed with Ag and Au. Syntheses for both © 2014 American Chemical Society

Received: April 3, 2014 Revised: May 19, 2014 Published: May 30, 2014 12506

dx.doi.org/10.1021/jp503323u | J. Phys. Chem. C 2014, 118, 12506−12514

The Journal of Physical Chemistry C

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sphere. For multiple spheres, we adopt the generalized multiparticle Mie (GMM) method of Xu50 with extensions by Ringler.51 This method expands the local fields of interacting spheres as vector spherical harmonics; we include at least 30 spherical harmonics in all simulations for convergence. The dielectric functions used for Ag and Au are from Johnson and Christy,52 In is from Koyama,53 Al is from Lynch and Hunter, cited by Palik,54 and that for Al2O3 is a combination of Hagemann55 with Palik.54 We do not adjust the dielectric functions to account for surface scattering, which can alter the optical response of small particles, primarily through broadening of the resonance. For optimal field enhancement, the particles we identify are large enough (>10 nm) that surface scattering is not expected to play a significant role.

the plasmon resonances and the larger the SERS enhancement.46,47 However, the choice of optimum particle size, gap size, and local index for SERS and other applications for Al and In dimer structures is not known, and the quantitative comparison of these metals with Ag and Au has not been determined, so in this paper we use theoretical methods to explore these questions. We begin with a general study of the spectral versatility of Al and In compared with Ag and Au as plasmonic materials. The studies are based on generalized multiparticle Mie theory, the analytic solution to Maxwell’s equations for spheres,48 for determining optical properties. Next, the near-field profiles of dimer geometries with 1 nm gaps are investigated as a function of wavelength and nanosphere radius. It is shown that both Al and In dimers exhibit strong electromagnetic fields from dipolar plasmon resonances, while above a size threshold Al dimers show maximum enhancement due to quadrupolar plasmon resonances. We also consider the effect of changing the index of the surrounding medium on the dimer field enhancements, and we find that Al and In prefer low index surroundings while Ag and Au generally favor a higher index environment. Finally, we explore the effect of an aluminum oxide (Al2O3) shell on the optical properties of aluminum, showing that though the oxide layer diminishes the electric field strength for Al dimers, the electric fields are still well above the values needed for significant far-field scattering and near-field enhancement. These results show that dimers of Al and In are promising plasmonic materials in the UV and near-visible spectral regions.

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RESULTS General Considerations of the Plasmonic Response. Using the empirical dielectric function, we can hypothesize how a given metal will perform in different regions of the electromagnetic spectrum. Physically, the dielectric function describes the ability of a material to concentrate electric flux. The dielectric function is complex, so it has two parts, Re{ε} and Im{ε}, where Re{ε} describes the charge stored, e.g. the ease with which electrons can be polarized, while Im{ε} describes loss and absorption by the metal. It has been shown that for optimal plasmonic performance a large real contribution with a small imaginary portion is ideal.3,14 Figure 1a depicts Re{ε} for Ag, Au, Al, and In; by examining where and to what magnitude Re{ε} < 0, we can approximate the wavelength range where plasmon excitation occurs. We can see that Re{ε}for Ag is negative at wavelengths above ∼320 nm, Au is negative at wavelengths above ∼480 nm, and Al and In are negative into the deep-UV (∼100 nm). Figure 1b shows Im{ε}, which is positive by definition (for Kramers−Kronig causality), with larger values signifying increased loss in the metal and poorer quality LSPRs.15,56 It is observed that Ag has small values of Im{ε} red of ∼320 nm; thus, it is an effective plasmonic material throughout the visible. Conversely, Au has a broad transition from large to small Im{ε} from 450 to 650 nm, and it begins increasing in magnitude again at ∼750 nm. Physically, this increased Im{ε} contribution manifests as significantly broader Au resonances and lower extinction efficiencies when compared to Ag.14 Al and In, by comparison to the noble metals, show a monotonic increase in Im{ε} with increasing wavelength, with Al increasing more rapidly than In. From these data, we hypothesize that Al will be very polarizable in the deep-UV, where Re{ε} < 0 and Im{ε} is small. Also, one anticipates that In will remain an effective plasmonic material further into the visible region than Al due to the lower values of Im{ε}.23 In Figure 1c, the optimal (optimized with respect to sphere diameter) absorption efficiency Qabs is shown for each metal in vacuum (for m = 1 refractive index).49 The optimal Qabs, rather than Qext, is shown because Qabs solely describes the ability of the metal to localize light in the form of an oscillating LSPR. Qext includes the radiative scattering of photons, which for larger particles sizes contributes significantly. Because we focus on optimal near electric field strengths herein, the nonradiative Qabs is the more relevant metric for comparison between metals. Further discussion regarding the relationship between sphere size, far-field scattering, and near-fields, as well as the optimized Qext spheres, can be found in the Supporting Information. In this paper, absorption is used only when discussing isolated spheres; for all dimers extinction is used as

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METHODS In 1908, Gustav Mie presented an analytic solution to Maxwell’s equations for a single sphere interacting with a plane wave.48 The full derivation can be found in several textbooks and reviews and thus will not be discussed here.49 We present only that which is necessary to obtain the extinction (Qext), absorption (Qabs), and scattering (Qsca) cross sections of a spherical particle. In its simplest form, the generalization of Mie’s solution describes both extinction and scattering efficiencies in terms of a size parameter x = 2πa/λ and electric an and magnetic bn partial wave coefficients of order n as follows: Q ext = Q sca =

∞

2 x2

∑ (2n + 1)Re[an + bn]

2 x2

∑ (2n + 1)(|an|2 + |bn|2 )

n=1

(1)

∞

n=1

(2)

Here the expanded coefficients are an =

bn =

mψn(mx)ψn′(x) − ψn(x)ψn′(mx) mψn(mx)ξn′(x) − ξn(x)ψn′(mx)

(3)

ψn(mx)ψn′(x) − mψn(x)ψn′(mx) ψn(mx)ξn′(x) − mξn(x)ψn′(mx)

(4)

The absorption efficiency is easily obtained from Qext and Qsca Q abs = Q ext − Q sca

(5)

where ψn and ξn are spherical Riccati−Bessel functions and ψ′n and ξ′n are their derivatives. The solution to eqs 3 and 4 provides the multipolar contributions to scattering, which are then summed to determine the total optical response of the 12507



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Figure 2. Maximum electric field in plasmonic nanosphere dimers with varying radii and background indices for Al and In. (a) Scheme depicting a nanoparticle dimer with a gap of 1 nm. (b) Electric field maximum in Al nanosphere dimers with increasing radii in various refractive indices. (c) Electric field maximum in In nanosphere dimers with increasing radii in various refractive indices.

Figure 1. Dielectric function and optimized absorption geometries (m = 1) for Ag, Au, Al, and In. (a) Real dielectric functions. (b) Imaginary dielectric functions. (c) Optimal plasmonic absorption by single nanospheres in a refractive index of m = 1.

with respect to wavelength. For Al dimers, we find Emax at 204 nm for 11 nm radius spheres, and for In dimers we find Emax at 358 nm for a 28 nm radius sphereboth with an index of m = 1. Interestingly, in Au and Ag, the largest Emax reported occurs with elevated background indices (i.e., m > 1). To study this, we note that by increasing either the radius or background index one can red-shift the LSPR, providing a means for identifying the global Emax structure. To investigate why Al and In do not behave in the same manner as Ag and Au, it is helpful to investigate the resonance condition of a nanosphere more thoroughly. This is most easily understood through the quasistatic (nonretarded fields, only dipolar) polarizability of a spherical particle: ε − εm α = 4πa3 ε + 2εm (6)

the far-field spectroscopic signature. Al exhibits the highest overall Qabs > 12 at ∼140 nm for a sphere of radius 6 nm. The optimal Qabs geometry for In also occurs in the UV, at ∼180 nm for a sphere of radius 10 nm. Finally, both Ag and Au have optimal Qabs in the visible with spheres of radii 23 and 49 nm, respectively; these are discussed in detail elsewhere.15 Optical Properties of Dimers. To investigate the potential for using spherical dimer geometries with small gaps in optics applications involving Al and In, we consider the dimer structure depicted in Figure 2a. Dimers are of interest both for strong far-field scattering and as nanoantennas with greatly enhanced local electric field profiles in the gap region.45,46,57,58 The dimers investigated here have a 1 nm gap between spheres, a value that can be considered to be a lower bound to where classical electromagnetics with bulk dielectric functions is appropriate. For smaller gaps nonlocality reductions and charge-transfer plasmons in the near-field are expected to reduce local field values compared to the classical results and change the resonance wavelengths so we do not consider this possibility.59,60 Gaps larger than 1 nm are not considered in this paper, as we are primarily interested in the largest field enhancements that can be obtained. Figures 2b (Al) and 2c (In) depict the effect of increasing sphere radius and changing background refractive index on the electric field maximum (Emax) for the dimer geometry. Here Emax is the field at the smallest gap between the particles (which is always the location of the maximum field) and is optimized

The polarizability is loosely defined as the propensity of the electrons in a sphere to be deformed, and it is easily related to the extinction and scattering of a spherical particle, which is derived in detail elsewhere.49 Here, we focus solely on the different resonance conditions possible in eq 6. By selecting only the imaginary part of eq 6 (recall that ε is complex), which is related to extinction, we arrive at the resonance condition for absorption of a spherical particle: α = 4πa3i

3 Im{ε}εm 2

Im{ε} + (Re{ε} + 2εm)2

(7)

For the LSPR of a sphere, the resonance condition is met when Re{ε} = −2εm. By substituting Re{ε} = −2εm into eq 7, we 12508



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generate the following equation for determining optimal resonant response

α = 4πa3i

3εm Im{ε}

(8)

Thus, on resonance the limiting factors of the resonance strength are the magnitudes of Im{ε} and εm, as seen in eq 8. An alternative resonance condition can be obtained from eq 7 using εm = −Re{ε}/2. Upon substitution, we obtain α = 4πa3i

−3 Re{ε} 2 Im{ε}

(9)

Here εm can be considered to float freely in order to maximize the ratio of the real and imaginary parts of the dielectric constant; through this we are able to determine the best possible resonance for the sphere.14 Of course, as discussed in the methods, higher-order vector spherical harmonic multipoles and retardation effects significantly alter the resonance strength and location for larger particles.. These retardation effects include radiative effects and damping. Radiative scattering becomes important when the particle size is large enough that the incident light is predominantly rescattered into the far-field. Below some critical size, which differs for each metal, radiative scattering results in an overall increased electric near-field.3 However, above this critical radius, dephasing of the electric field within the particle results in a net decrease in electric field due to reduced absorption and significant LSPR broadening.57 These effects are not included in the quasistatic presentation above, and thus we use generalized multiparticle Mie theory as this includes retardation effects and higher-order multipoles. Importantly, one cannot identify the optimal plasmonic geometry solely by red-shifting the LSPR or by increasing the radius.15,57 Thus, the radius and background index must be allowed to vary freely in order to maximize the LSPR and maximum near-field strength, as seen in Figure 2. For both Al and In, it is seen that increasing the background index decreases Emax. This behavior is different from that of Au and Ag, which have global Emax with background indices of m = 1.50 and 2.25, respectively (based on m iterated in increments of 0.25 and r in increments of 1). Again, this behavior can be understood via the dielectric function and eqs 7 and 8. For both Al and In, red-shifting the LSPR via the background index moves λmax red toward the interband transitions of the metal, inherent quasiparticle transitions, which damp the resonance. This is because Im{ε} is a monotonically increasing function toward the red. For Au and Ag, however, Im{ε} varies nonmonotonically through the visible. Thus, by tuning the background index to a value above m = 1, one is able to position the plasmon between local Im{ε} maxima, optimizing the electric field strength. An analytical spectral representation method that separates the effects of background index and interband transitions agrees with these results, which were presented within a geometry-independent framework.61 From this analysis we conclude that it is important to utilize Al and In structures with a background index as close to vacuum as possible to give the global optimum LSPR. In Figures 3 and 4, we investigate the electric field profile of the optimal dimer structures for Al and In, respectively. Rather than consider Emax solely a function of radius, it is helpful to consider how the spectral location of λmax changes. Figure 3a plots the same data as Figure 2b, except as a function of λmax. Surprisingly, the trend is significantly more complex than as a

Figure 3. Maximum near-fields in Al dimers for dipolar and quadrupolar resonances. (a) Field maxima for a given background index as a function of resonance wavelength. (b) Electric field magnitude for various Al dimer radii. (c) Optimal dipolar near-field map for 11 nm Al nanospheres at 204 nm. (d) Optimal quadrupolar near-field map at the maximum for 14 nm Al nanospheres at 162 nm.

function of radius. We would expect, as has been seen for Au, that λmax will increase with increasing radius and thus Emax would red-shift in a similar manner. Instead, however, we see a bimodal λmax, where the overall Emax for a given index occurs red of a comparable, but smaller, local maximum in Emax. This bimodal trend arises from a change in the nature of the plasmon mode when Emax occurs. For smaller particles Emax corresponds with the dipolar mode, however as the sphere increases in radius the quadrupolar mode exhibits significant enhancement, and at some point it becomes larger than that of the dipole resonance. Because quadrupole resonances are blue of dipole resonances, the location of Emax for a given dimer 12509



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field node is visible between the dimers in addition to the field vectors having four clear points of entry and exit to the nanoparticles, indicative of a quadrupolar plasmon mode.3 The 162 nm resonance is in the vacuum-ultraviolet (VUV) portion of the spectrum where measurements are more difficult; however, we note that an inert atmosphere, typically N2, is used by the semiconductor industry down to ∼150 nm, an added benefit here is that this would minimize oxidation of the Al. A similar analysis is performed for In dimer geometries, first investigating the relationship between Emax and λmax, in Figure 4a. Here, the relationship between Emax and λmax resembles that seen in Figure 2c as well as that for Au dimers;57 there is a single maximum in the electric field strength. Additionally, the optimal global Emax occurs at a background refractive index m = 1, like in Al. Again, this can be rationalized via the dielectric function for In. With the monotonic increase in Im{ε} toward the red, the “best” LSPR, that with the lowest losses and sharpest peaks, will tend toward the blue for the optimal sphere. Figure 4b depicts spectra for In sphere dimers of increasing radius. In these spectra, we see that the maximally enhancing peak is dipolar (up to spheres of 40 nm radius); the quadrupole is significantly lower in intensity. Finally, in Figure 4c, an electric field map is shown for the optimal In dimer geometry, 28 nm spheres with a background index m = 1. This optimum corresponds to a wavelength of 358 nm. The electric field vectors clearly depict dipolar character, in agreement with the spectra in Figure 4b. Modeling SERS Enhancements for Optimum Dimer Structures. Surface-enhanced Raman spectroscopy (SERS) is an important application of nanosphere dimer nanoantennas the strong electric field values in the gap region provide significant enhancement of molecular Raman dipoles in the region.46,47 The enhancement factor (EF) due to plasmonic near-fields scales as |E|4, allowing for the amplification molecular Raman scattering at very low concentrations or of molecules with weak Raman cross sections.13,24 Thus, in Figure 5 we investigate the viability of optimal In (28 nm radius) and Al (11 nm radius) nanospheres as SERS nanoantennas. First, Figure 5a compares the extinction efficiency (scaled for

Figure 4. Maximum near-field for indium dimers. (a) Field maxima for a given background index as a function of resonance wavelength. (b) Electric field magnitude for various In dimer radii. (c) Optimal dipolar near-field map at the maximum for 28 nm In nanospheres at 358 nm.

switches toward the blue, after which it begins red-shifting again as the sphere increases in size. This trend is seen for all indices, though it becomes less pronounced as the index increases. Individual electric field strength spectra for Al dimers are shown in Figure 3b. These electric field spectra all exhibit more than one peak, which is due to the contribution of higher order vector spherical harmonics.3,48,62 The peaks can be identified as dipolar, quadrupolar, and higher multipolar in nature where the dipolar LSPR peak is always furthest to the red and occurs in the smallest sphere, the next bluest peak is quadrupolar, and so on. For the two smallest spheres (6 and 12 nm radii) the dipole peak is the most intense. However, for the 18 nm sphere, the quadrupole (n = 2) peak is larger in magnitude than the dipolar (n = 1) peak. Finally, in the 24 nm sphere a third peak emerges toward the blue as an n = 3 multipole contribution. Thus, above a certain radius sphere (14 nm, as identified through optimization) the quadrupolar peak will provide Emax, manifesting in a λmax blue of where it would appear if it were solely due to the dipolar LSPR. Figure 3c is an electric field intensity map for the optimal dipolar Emax (11 nm Al spheres at 204 nm). The two characteristic nodes of a dipole resonance are evident around the spheres. Conversely, Figure 3d depicts the electric field intensity map surrounding the optimal quadrupolar Emax, 14 nm Al spheres at 162 nm. A third electric

Figure 5. Far-field LSPR scattering and SERS enhancement factors for Al and In nanoantennas. (a) Electric field strength (solid) and scaled extinction (dashed, 20× for In and 10× for Al) for the optimal dimer geometries. SERS enhancement factor maps (|E|4) for Al (b, 11 nm radius at 204 nm) and for In (c, 28 nm radius at 358 nm). 12510



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Figure 6. Effect of an Al2O3 shell on the plasmonic response of Al spheres. (a) Optimal single sphere absorption with an increasingly thick Al2O3 shell (0.5 nm increments). (b) Near-field strength (solid) and scaled extinction (dashed line, 5×) for the field intensity maximum dimer geometry of Al/Al2O3 (13 nm Al, 2.5 nm Al2O3 shell). (c) Near-field and (d) enhancement factor maps at 218 nm.

comparison) and the electric field strength of the dimers. Recently, “dark” plasmon modes, which do not scatter far-field light efficiently, have been shown to have comparable or even larger local electric field values than the “bright” modes normally associated with an LSPR.46 In our comparison, these “dark modes” would manifest where the extinction efficiency has no peak but the electric field value does. However, for both Al and In, the electric field maxima are within ∼20 nm of the LSPR, so the only modes relevant in this case can be considered to be bright. This is likely due to the fact that the optimized nanoparticles we are considering are significantly smaller than was the case for the (∼50 nm radius) Au nanoparticles where the dark modes were produced by quadrupolar effects. In experimental studies, excitation of an Al or In dimer with a laser near the LSPR should provide ample near-field strength for Raman scattering enhancement. In Figures 5b and 5c, EF maps are plotted for the optimal dimer structures. The largest EF is observed in the gap: |E|4 = 2.0 × 109 for Al (at 204 nm) and 1.2 × 109 for In (at 359 nm). For the quadrupole resonance (14 nm Al spheres at 162 nm) the largest EF seen is |E|4 = 1.8 × 109; the electric field map for this mode can be found in the Supporting Information. For comparison, the optimal Au sphere dimer (radius 25 nm, m = 1.50) with a gap of 1 nm has |E|4 = 2.8 × 1011 at 723 nm.57 The optimal Ag sphere dimer (radius 10 nm, m = 2.25) with a gap of 1 nm has an |E|4 = 1.3 × 1012 at 794 nm; this, to the best of our knowledge, is the first report of the optimal Ag dimer properties. Thus, though Al and In do not provide as large EF as Ag and Au, they can be expected to provide Raman signal 109 times above normal, which is well within detection limits. Additionally, because Rayleigh scattering is much more intense at shorter wavelengths (scales as 1/λ4), more overall signal is expected at the UV wavelengths that are plasmonically accessible with Al and In nanoparticles. A major challenge in the use of Al as a plasmonic material is that it forms a 2−3 nm Al2O3 layer immediately upon

introduction of oxygen.26,27 Thus, unless the structures are stored under ultrahigh-vacuum conditions indefinitely, the oxide layer is an experimental reality. With generalized multiparticle Mie theory, a core−shell structure can easily be simulated; though it should be noted that this assumes that the interface between Al and Al2O3 does not introduce additional complexities that cannot be modeled with a dielectric function.49 In Figure 6, we investigate the role of an Al2O3 shell on an Al nanosphere, in both the single nanoparticle and dimer case. Figure 6a depicts the optimized single sphere absorption efficiency for Al nanospheres with increasing oxide layer thickness, ranging from no oxide layer (6 nm Al, dark blue) to a 3 nm thick oxide shell (10 nm Al, gray) in 0.5 nm increments. As the thickness of the oxide layer increases, the Qabs maximum red-shifts in location and decreases in efficiency. We hypothesize that the thicker Al2O3 shells red-shift the Qabs maximum further to the red, toward the interband transitions (higher Im{ε}) in Al, resulting in a poorer LSPR. This behavior is similar to that seen in Figure 2 when the index of the surrounding medium is increased. The results in Figure 6a indicate that it is important to investigate the Emax values for Al/Al2O3 to see what role the oxide layer has on the local electric field values. High-resolution transmission electric microscopy studies suggest that Al nanostructures adopt a 2.5 nm Al2O3 shell so we will use that thickness in the rest of our analysis.63 Figure 6b plots the electric field strength and scaled far-field extinction of the optimal Al/Al2O3 dimer (13 nm Al, 2.5 nm Al2O3). First, it is clear that the near-field maximum and the far-field LSPR overlap. Additionally, the quadrupole mode is much less pronounced than it was in the uncoated Al dimers (Al result in Figure 5a). Indeed, for all Al/Al2O3 dimer structures (2.5 nm shell) investigated, the quadrupole was lower in intensity than the dipolar resonance. This change compared to Al reflects the short-ranged nature of fields associated with the quadrupole mode. Figures 6c and 6d depict the electric field map and EF map, respectively, for the optimal dimer geometry. Only two 12511



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nodes are visible in the electromagnetic field map in Figure 6c, providing further proof that the dipole mode is the main contributor to Emax at this wavelength (218 nm). Figure 6d depicts the EF for the same dimer; the maximum value is 1.2 × 107 |E|4, a 2 order of magnitude decrease from the optimal Al dimer. However, 107 is still a substantial increase in Raman signal and is well within detection limits for such a short wavelength.6 Thus, though the Al2O3 layer diminishes the plasmonic properties of Al, our data suggest that Al/Al2O3 nanoantennas are a powerful means of electromagnetic field enhancement and absorption.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by AFOSR MURI grant FA955011-1-0275 and by the Northwestern Materials Research Center under NSF grant DMR-1121262. M.B.R. gratefully acknowledges support through the NDSEG graduate fellowship program. Computational time was provided by the Quest High-Performance Computing facility at Northwestern University, which is jointly supported by the Office of the Provost, the Office for Research, and Northwestern University Information Technology.

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DISCUSSION We have used generalized Mie theory to explore plasmon resonances and local electric field profiles for Al and In nanosphere dimers with a small gap. Al dimer nanoantennas were demonstrated to produce strong near-field profiles at both dipolar and quadrupolar resonance wavelengths. An optimal enhancement factor of 2.0 × 109 (at 204 nm) is observed at the dipole driven resonance. For In dimers, the plasmonic response is primarily dipole driven and an optimal SERS enhancement factor of 1.2 × 109 (at 359 nm) occurs. Finally, it is shown that the presence of a 2.5 nm Al2O3 layer in an Al nanosphere dimer does not preclude it from being a relevant plasmonic material, demonstrating tunable plasmon resonances in the ultraviolet and an optimal enhancement factor 1.2 × 107 (at 218 nm). This result is consistent with our analysis of the effect of changing the refractive index of the surrounding medium, where the red-shifts associated with increasing index produced smaller enhancements for Al and In, in contrast to the generally larger enhancements seen for Au and Ag under the same circumstances. This arises because the imaginary part of the dielectric function is a monotonically increasing function of wavelength for Al and In, while it has a more complicated wavelength dependence for Au and Ag and is a decreasing function for Au above 450 nm and for Ag above 300 nm. Thus, we conclude that Al should be the primary plasmonic material of use in the deep-UV (315−100 nm) and that In is the optimal material in the near-UV (400−315 nm), though both exhibit high quality LSPRs throughout the ultraviolet region. Though our work mostly focuses on local electric field enhancement, this is merely an introduction to the potential for aluminum in plasmonics. For example, the highly dispersive dielectric function of Al in the UV will likely result in high quality and long-lived plasmon lifetimes, which are crucial to plasmonmediated chemistries and studying photophysics on femtosecond time scales.

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ASSOCIATED CONTENT

S Supporting Information *

Optimal extinction efficiencies and a discussion of its relationship to the optimal absorption efficiencies for Ag, Au, Al, and In spheres is included; additionally, a comparison between extinction, absorption, and E2 is made for the largest sphere studied, a 73 nm Au radius, and smallest sphere studied, a 7 nm Al radius; the optimal SERS enhancement factor field map for the quadrupolar resonance in 14 nm Al spheres. This material is available free of charge via the Internet at http:// pubs.acs.org.

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Aluminum and Indium Plasmonic Nanoantennas in the Ultraviolet

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