Subscriber access provided by Northwestern Univ. Library

The Journal of Physical Chemistry C is published by the American ChemicalSociety. 1155 Sixteenth Street N.W., Washington, DC 20036

Article

Localized Surface Plasmon ResonanceSpectroscopy of Triangular Aluminum NanoparticlesGeorge H. Chan, Jing Zhao, George C. Schatz, and Richard P. Van Duyne

J. Phys. Chem. C, 2008, 112 (36), 13958-13963• DOI: 10.1021/jp804088z • Publication Date (Web): 15 August 2008

Downloaded from http://pubs.acs.org on March 12, 2009

More About This Article

Additional resources and features associated with this article are available within the HTML version:

• Supporting Information• Access to high resolution figures• Links to articles and content related to this article• Copyright permission to reproduce figures and/or text from this article

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 nanoparticlearrays can support LSP resonances that are tunable throughout the visible and into the UV portion of thespectrum. Scanning electron microscope and atomic force microscope studies point to the presence of a thinnative Al2O3 layer on the surface of the Al triangular nanoparticles. The presence of the oxide layer, especiallyon 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 tounoxidized triangular nanoparticles of similar size and geometry. A comparison of Al, Ag, Cu, and Au triangularnanoparticles 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 positiveimaginary dielectric function are capable of supporting acollective excitation of the conduction electrons known asplasmon excitation.1 In metal nanoparticles this leads to alocalized surface plasmon resonance (LSPR), which is an effectthat produces strong peaks in extinction spectra, as well as strongenhancements of the local electromagnetic fields surroundingthe nanoparticles.2-4 Previous work has demonstrated that theposition of the LSPR extinction maximum, λmax, is sensitive tothe size, shape, interparticle spacing, dielectric environment, anddielectric properties of the nanoparticle.5-8 As a result, metallicnanoparticles that support LSP resonances are promisingplatforms as highly sensitive optical nanosensors, as photoniccomponents, and in surface-enhanced spectroscopies.9-20 It iswell-established that Ag and Au nanoparticles support surfaceplasmon resonances that can be tuned throughout the UV-vis-near-IR spectrum.21,22 Interestingly, a number of othermetals (i.e., Li, Na, Al, In, Cu, and Ga) also meet this criterionand may possibly support surface plasmon resonances for atleast part of the UV-vis-near-IR region,23-27 but there hasbeen far less experimental work with these metals.

There has been continued interest in the plasmonic propertiesand sensing capabilities of Al over the past 20 years.28-38

Aluminum is capable of supporting surface plasmons in thevisible and UV, and it has been reported to be a substrate forsurface-enhanced fluorescence37,39-41 and surface-enhanced Ra-man spectroscopy.42 Moreover, many proteins, fluorophores, andbiological molecules of interest absorb in the UV region of theelectromagnetic spectrum. It is worthwhile to explore substratesthat support UV surface plasmons, which could allow forspectroscopic measurements that combine both molecular andplasmon resonance effects.

The optical properties of metal nanoparticles are closelyrelated to the real and imaginary parts of their wavelength-dependent dielectric constants. A convenient expression forthinking about this is provided by the quasistatic model for lightscattering from a spheroid-shaped particle. For light whosepolarization is parallel to the long axis of the spheroid, theextinction cross-section Cext is given by43

Cext ∝1λ

Im(R) ∝ 1λ

Im εi - εo

εi + εo (1)

Here λ is the wavelength, R is the polarizability, ε0 is thedielectric constant of the medium outside the particle (ε0 ) 1for vacuum), εi is the dielectric function of the metal, and isa shape-dependent parameter which varies from 2 for a sphereto infinity for a highly prolate or oblate particle. This expressionshows that the plasmon resonance occurs when the real part ofthe denominator vanishes, which means that Re(εi) ) - ε0.This indicates the real part of the dielectric constant needs tobe negative, and the narrowest resonances are associated withthe Im(εi), which is as small as possible.

To see how this expression applies to Al, in Figure 1, weplot the real and imaginary parts of the dielectric functions ofAl (blue curve with circles) and Ag (red curve with triangles)as obtained from Palik.44 Figure 1A shows that Ag can onlyshow plasmon excitations at wavelengths longer than 350 nm,because the real part of its dielectric function is positive below350 nm, while Al should be plasmonically active from 200 nmto just below 800 nm. Figure 1B shows that Al has interbandtransitions near 800 nm with a corresponding steep rise in theimaginary part of the dielectric constant at that wavelength. Thisrise has been extensively examined in the condensed matterphysics literature,45,46 and it involves a transition between twodifferent bands associated with the conduction electrons (i.e.,an interband transition) which happen to be parallel for certaindirections of the wavevector in the Brillouin zone very close tothe Fermi energy. This behavior is quite different from thesituation for silver, which has a small imaginary component ofthe dielectric constant in the visible, and the rise in Im(εi) below350 nm is associated with interband transitions involving the

* To whom corrspondence should be addressed. Telephone: (847) 491-3516(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: vanduyne@northwestern.edu (R.P.V.D.);schatz@chem.northwestern.edu (G.C.S.).

† These authors contributed equally to this work.

J. Phys. Chem. C 2008, 112, 13958–1396313958

10.1021/jp804088z CCC: $40.75 2008 American Chemical SocietyPublished on Web 08/15/2008

localized 4d orbitals. Both parallel conduction bands andlocalized orbitals allow for electron scattering events whichdephase the conduction electrons and thus broaden the extinctionline shape of nanoparticles. This makes Al less attractive at longwavelengths where Ag is already in common use. Nevertheless,it is clear that Al has promise for significant sensing capabilitiesover short-wavelength portions of the electromagnetic spectrumthat are otherwise not generally considered.

A problem with Al, however, is that it rapidly oxidizes whenexposed to the atmosphere, forming a thin Al2O3 layer thatprevents further attack by oxygen.47 High-resolution transmis-sion electron microscopy studies of aluminum nanoparticlesindicate that the Al2O3 layer is approximately 2.5 nm thick andporous and is part amorphous and part crystalline.48 Thepresence of this oxide layer is expected to affect its plasmonicproperties. Consequently, the plasmonic properties of aluminumand, in particular, the LSPR spectroscopy of aluminum nano-particles has just recently received attention.49

In this work, we examine the optical properties of Altriangular nanoparticles fabricated using nanosphere lithography(NSL).5 The experimental results obtained from UV-visextinction spectroscopy are compared with electrodynamicscalculations based on the discrete dipole approximation (DDA)method.50 NSL involves vapor deposition through a colloidalcrystal mask made using polymer nanospheres, so by selectionof the nanosphere diameter (D) and the deposited metalthickness (dm), the in-plane width and out-of-plane height Altriangular nanoparticles can be controlled. This allows forsystematic tuning of the LSPR throughout the UV and visiblespectrum. In addition, theoretical investigations of the effectsof a thin alumina layer on the surface of the triangular aluminumnanoparticles will be presented. This allows us to characterizethe red shift of the LSPR λmax due to this layer which enablesus to compare both experimental and theoretical LSPR propertiesof Al and the noble metals of similar size and geometry. Finally,the refractive index sensitivity of the NSL oxidized Al triangularnanoparticle arrays to bulk solvents will be used to provideinsight on the sensitivity differences between Al and noble metalcounterparts and the possibility of using Al triangular nanopar-ticles in sensing applications.

Experimental Methods

Materials. Fisher brand No. 2, 18 mm diameter glasscoverslips were obtained from Fisher Scientific. S1-UV fusedsilica coverslips (18 mm diameter and 0.15 mm thick) with anoptimum transmission range of 180 nm to 2.0 µm were

purchased from ESCO Products. Glass, Si, and S1-UV fusedsilica 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 shouldbe handled with great care!) Samples were allowed to cool andthen 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 rinsedwith copious amounts of ultrapure water.

Fabrication of Triangular Nanoparticle Arrays. NSL wasused 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-coatedonto glass, S1-UV fused silica (ESCO Products, Oak Ridge,NJ), or Si substrates and allowed to dry, forming a monolayerin a close-packed hexagonal formation, which served as adeposition mask.

Aluminum, copper, silver, or gold metal was deposited byelectron beam (e-beam) deposition (Kurt J. Lesker Axxisdeposition system, Pittsburgh, PA) with a base pressure between10-6 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 platedQCM (Fort Collins, CO). After the metal deposition, thenanosphere masks were removed by sonication in absoluteethanol (Pharmco, Brookfield, CT) for 2-3 min. The samplewas 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 geometrymode were performed using an Ocean Optics model SD2000(Dunedin, FL) or an Ocean Optics model HR4000 withunpolarized white light provided by a tungsten-halogen or adeuterium light source, respectively. The light spot diameterwas approximately 1-2 mm for experiments conducted withOcean Optics model SD2000 spectrometer and approximately1 cm for measurements conducted with Ocean Optics modelHR4000 spectrometer. The extinction maximum was locatedby calculating the zero-crossing point of the first derivative.

Scanning Electron Microscopy and Atomic Force Micros-copy. The height and the structure of the Al triangularnanoparticles were investigated with an atomic force microscope(AFM) and a scanning electron microscope (SEM). Tapping-mode AFM images were collected using a Digital Instruments

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

LSPR of Triangular Aluminum Nanoparticles J. Phys. Chem. C, Vol. 112, No. 36, 2008 13959

Nanoscope IV microscope and a Nanoscope IIIA controller(Digital Instruments, Santa Barbara, CA) on samples preparedon glass substrates. SEM images were collected using a Hitachi-4800 SEM at an accelerating voltage 10 kV and an averageworking distance of 7.1 mm on samples prepared on Sisubstrates.

Electrodynamics Calculations. The optical properties of theAl triangular nanoparticles were examined with classical elec-trodynamics calculations based on the DDA method.50,51 In allcalculations, the shape of the nanoparticles is assumed to be atruncated tetrahedron, with the Al dielectric constants taken fromthe compilation in Palik44 and the refractive index of Al2O3 takenfrom recent studies of atomic layer deposition.52 The effect ofthe glass substrate on the LSPR wavelength was treated usingeffective medium theory53 in which the particles are assumedto be embedded in a homogeneous medium and the dielectricconstant is a weighted average of that for glass and N2. Theweighting is determined by the relative fractions of the particlesthat are exposed to each medium.

Results and Discussion

Effect of Aluminum Oxide Formation. The LSPR λmax ofmetallic nanoparticles is sensitive not only to the size, shape,interparticle spacing, and the dielectric properties of the metalbut also is quite sensitive to the dielectric environment sur-rounding the nanoparticles. In particular, the effect of the nativeoxide shell is expected to result in a spectral red shift in theLSPR λmax, whereas the concomitant shrinkage of the metalliccore is expected to lead to a spectral blue shift in the LSPRλmax. To see which effect dominates, we have performed DDAcalculations for nanoparticles whose total height is fixed at 50nm and total width fixed at 90 nm, where we have replaced Alby Al2O3 in an outer layer on the nanoparticle. Figure 2A showsthat the addition of a 2 nm layer of Al2O3 on a bare Alnanoparticle (width ) 90 nm; dm ) 48 nm of Al + 2 nm ofAl2O3) leads to a red shift in the LSPR λmax of ∼13 nm. As thethickness (2-10 nm) of Al2O3 increases, the magnitude ofthe red shift in LSPR λmax increases (Supporting Information,Figure S1). This demonstrates that the contribution to the totalLSPR signal from the dielectric red shift is larger than fromthe shrinkage of the metallic core, leading to a ∼4.2 nm shiftin LSPR λmax per 1 nm increase in the oxide thickness.Moreover, no significant peak broadening or decrease in theextinction efficiency as a result of the presence of an aluminalayer is predicted. This is not surprising since the native oxide

layer is transparent and should have minimal scattering in theUV and visible regions.

SEM and AFM images for NSL Al nanoparticles (D ) 390nm; dm ) 50 and 40 nm, respectively) on a Si and glasssubstrate, respectively, are shown in Figures 2B and 3A. Thenanoparticles are nearly triangular as expected. Upon closerexamination of the SEM image, sharp contrast between the tipsand the core of the nanoparticle is observed, which suggeststhe presence of oxides on the tips of the nanoparticles. Fromthe AFM line-scan measurements, the heights (Figure 3B) of thenanoparticles are consistent with measurements from the quartzcrystal microbalance. Notice that the nanoparticle width is 157nm from the AFM line scan. Assuming that the AFM tipbroadening effect is ∼20 nm,54 the width of the nanoparticle is∼137 nm, which is ∼1.5 times larger than the width found forAg nanoparticles fabricated with a similar nanosphere diam-eter.54 The results for Ag are very similar to what would beexpected from geometric considerations for the vapor depositionconditions, so the larger nanoparticle diameter Al particle islikely due to the wetting properties of the Al metal on the glasssurface. Therefore, to study the extinction spectra of Alnanoparticles using DDA, the nanoparticle width (still assumedto be a truncated tetrahedron) is taken to be 1.5-1.8 times thatcalculated in previous work for Ag particles on the basis ofgeometry considerations.5

Tuning the LSPR of Al Nanoparticles by Varying the In-Plane Width. Figure 4A illustrates the experimental extinctionspectra of Al nanoparticle arrays with varying in-plane widthand fixed height in a N2 environment on a glass or S1-UVsubstrate. As the nanoparticle in-plane width increases, the LSPRλmax shifts to the red. The fwhm (full width at half-maximum)of the Al LSPR (D ) 390 nm) is ∼0.65 eV, which is muchbroader than for Cu, Ag, and Au nanoparticles fabricated byNSL. This broadening in the LSPR is mainly attributed to thelarge positive imaginary contribution to the dielectric functioncompared to the other metals (see Figure 1). To confirm this,the effect of the Al nanoparticle width on the LSPR wasinvestigated using the DDA method. The calculations wereperformed on the core-shell truncated tetrahedral nanoparticleswith an Al core and 2 nm Al2O3 shell. The total width of thenanoparticle was taken to be 95, 137, 174, 206, and 230 nm,and the total height is fixed at 50 nm. The calculated extinctionspectra (Figure 4B) have LSPR line shapes similar to theexperimental ones, and we also find that an increase in the widthof the nanoparticle leads to a red shift in the LSPR λmax and abroadening in the LSPR spectra. Both the data from experimentsand calculations reveal that when the LSPR of the nanoparticlesis close to the Al interband transition (∼800 nm), the LSPR issignificantly broadened. Indeed, the spectra in red in Figure

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

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

13960 J. Phys. Chem. C, Vol. 112, No. 36, 2008 Chan et al.

4A,B are significantly broader than the others. In addition, thecalculated curve in Figure 4B has λmax at ∼760 nm and ashoulder at ∼900 nm. This shows that the Al interband transitioncan greatly affect the LSPR band of the Al triangular nanopar-ticles. Similar results were also observed in Al nanodisks.49

LSPR of Al, Cu, Ag, and Au Nanopaticles with SimilarGeometry. Figure 5A presents normalized experimental extinc-tion spectra for Al, Cu, Ag, and Au nanoparticle arraysfabricated by NSL (D ) 390 nm, dm ) 50 nm, glass substrate,in a N2 environment) with a similar shape and geometry. Theextinction maxima of Al, Cu, Ag, and Au are 508, 698, 639,and 787 nm, respectively. The approximate values of the fwhmfor the LSPR of Al, Cu, Ag, and Au are ∼0.65, ∼0.29, ∼0.36,and ∼0.40 eV, respectively. Figure 5B shows the calculatedextinction spectra for Al, Cu, Ag, and Au with the sametruncated tetrahedral geometry where the nanoparticle width is90 nm and height is 50 nm. As shown in both Figure 5A,B,interband transitions in Cu and Au do not significantly affectthe optical properties when the LSPR λmax > ∼650 nm. Incontrast, the interband transitions of Al lead to significant peakbroadening of the LSPR and a concomitant decrease in peakintensity (∼0.06 extinction units) when the LSPR λmax ap-proaches 800 nm. From the comparison of the LSPR behaviorfor Al, Cu, Ag, and Au, we conclude that the Al nanoparticlesdisplay a bluer, broader, and less intense LSPR compared tothe noble metals in the visible region. The experimental resultsobtained for Cu and Ag agree with the predicted LSPR λmax

from theoretical calculations. On the other hand, the experi-mental LSPR λmax of Al and Au are significantly red-shiftedcompared to that predicted from theory. For the case of Au,

the discrepancy of the LSPR λmax between experiment andtheory was previously attributed to the difference in the wettingproperties of the noble metals on glass substrates and fromdifferences in their surface melting temperatures.8 In particular,Au triangular nanoparticles can wet the surface to produce atiny “apron” of metal around the particle, resulting in a red-shifting of the plasmon resonance relative to what is modeledby the DDA calculation. The discrepancy of the Al LSPR λmax

between experiment and theory has a similar origin. The pres-ence of the oxide layer on the tips of the nanoparticles leads toa significant red shift of the LSPR λmax.

Refractive Index Sensitivity of Al Nanoparticles. Therefractive index (RI) sensitivity of the Al triangular nanoparticlearrays was investigated to explore its use as a plasmonicrefractive index sensor. To do this, we have examined the shiftof the LSPR λmax caused by bulk solvents using extinctionmeasurements and DDA calculations. Previous work demon-strated that the LSPR λmax for noble metal nanoparticles isextremely sensitive to the external dielectric environment.10,55,56

In addition, it was found that the noble metal nanoparticle arraysfabricated via NSL experienced slight geometrical changes(rounding of the tips) during the solvent study experiments.52,55

To prevent such modifications, the noble metal triangularnanoparticle arrays were solvent-annealed to stabilize thenanoparticles prior to any spectroscopic measurements; this wasdone by monitoring the LSPR λmax until it stabilized. In thepresent application, the presence of an alumina layer was foundto act as an effective protective barrier preventing unwantedsolvent annealing.

Figure 4. (A) Extinction spectra of the Al nanoparticle arrays with varying widths (D ) 280-590 nm; dm ) 50 nm; glass substrate (visible) andUV 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 andheight ) 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.

LSPR of Triangular Aluminum Nanoparticles J. Phys. Chem. C, Vol. 112, No. 36, 2008 13961

Figure 6A shows the extinction spectra of Al nanoparticlesin different dielectric environments. The environments chosenare N2 (RI ) 1.0), H2O (RI ) 1.33), ethanol (RI ) 1.36),chloroform (RI ) 1.45), and benzene (RI ) 1.50). As shownin Figure 6A, the LSPR λmax red shifts when the RI of themedium increases as expected. The LSPR λmax versus RI ofthe medium is plotted in Figure 6B. The slope of this plot yieldsa RI sensitivity of 0.405 eV/RIU. Figure 6C illustrates theextinction spectrum of an Al nanoparticle with a 2 nm oxidelayer (total width ) 137 nm, height ) 50 nm) on a glasssubstrate on the basis of DDA calculations for various dielectricenvironments. Figure 6D shows the refractive index sensitivitypredicted from these calculations, for a 2 nm oxide layer (blackline with triangles) and bare (unoxidized) Al nanoparticle withthe same geometry (red line with circles). The RI sensitivity ofthe Al nanoparticle with 2 nm oxide layer is 1.08 eV/RIU andthat of a bare Al nanoparticle is 1.17 eV/RIU. According totheory, the presence of a thin oxide layer does not lead to asignificant decrease in the RI sensitivity of the Al nanoparticlearrays. However in the experiment, there is a significant decreasein RI sensitivity as a result of the oxide layer. This is because,in the calculations, it was assumed that the oxide layer isdistributed uniformly over the Al nanoparticle surface while theSEM measurement in Figure 2B indicates that the oxide layeris most likely relatively thicker at the nanoparticle tips. Previousstudies showed that the sharp tips are responsible for themajority of the RI sensitivity of the noble metal NSL nanopar-ticles, so a thick oxide layer at the tips will lead to a decreasein the electromagnetic field decay length ld, resulting in asignificant decrease in the RI sensitivity as previously demon-strated.52

Conclusions

In conclusion, our experiments show that Al triangularnanoparticle arrays fabricated by NSL are capable of supporting

surface plasmons in the near-UV and visible regions of thespectrum. We demonstrate that the presence of a thin nativealuminum oxide layer leads to a red shift in the LSPR λmax. Inaddition, when the nanoparticle height is fixed and the nano-particle width is increased, a blue shift in the LSPR λmax isobserved. These trends all agree with the predictions fromtheory. In addition, both experiment and theory demonstrate thatwhen the LSPR λmax is close to the Al interband transition at1.5 eV or 800 nm, there is a significant broadening in the LSPRspectra. This effect, as well as the effect of Al oxides, is inagreement with the recent study of Al nanodisks by Langham-mer et al.49 However, our work has also included the refractiveindex sensitivity of the NSL fabricated Al nanoparticles, wherewe find the observed result is smaller than is predicted andsmaller than for Ag nanoparticle arrays of the same size andshape in the visible region. This reduction in RI sensitivity arisesbecause oxidation occurs preferentially at the tips of thenanoparticles, and it is the tips where electromagnetic hot spotsproduce the strongest contribution to the presence of adsorbedmolecules. This conclusion likely applies to any anisotropic Alnanoparticle shape in which sharp features are present, and itpoints to the use of shell-shaped structures, rather than structureswith sharp points, as a possible direction for producing Alparticles where RI sensitivity is less sensitive to oxidation.

This is the first paper which has provided data such as thatin Figure 5 in which the extinction spectrum of aluminumnanoparticles has been compared with that of Ag, Cu, and Aufor particles with the same structure. This possibility arisesthrough the use of nanosphere lithography to make the particles.Other methods for making Al nanoparticles (such as wetchemistry methods) lead to structures which are different fromthose which can be made for the other metals, and thereforegive spectra which are hard to compare. Comparison of theLSPR properties for Cu, Ag, Au, and Al particles with similarshape and geometry show that LSPR λmax is ordered Au > Cu

Figure 6. Extinction spectra of NSL Al nanoparticles in the presence of a layer of oxide in various solvents (width ) 137nm; dm ) 50 nm; onglass substrate) (A) experiment and (C) theory, and the refractive index sensitivity m for (B) experiment and (D) theory for Al nanoparticle with2 nm Al2O3 (black line with triangles) and bare Al nanoparticle (red line with circles).

13962 J. Phys. Chem. C, Vol. 112, No. 36, 2008 Chan et al.

> Ag > Al, while the fwhm satisfies Al > Au > Ag > Cu.Further work is being carried out to investigate the utility ofNSL Al triangular arrays as a substrate for ultraviolet surfaceenhanced Raman spectroscopy.

Acknowledgment. This work was supported by the NationalScience Foundation (Grants EEC-0118025, CHE-0414554, andBES-0507036), the Air Force Office of Scientific ResearchMURI program (Grant F49620-02-1-0381), DTRA JSTO Pro-gram (Grant FA9550-06-1-0558), AFOSR/DARPA ProjectBAA07-61 (Grant FA9550-08-1-0221), and the MRSEC pro-gram of the National Science Foundation (Grant DMR-0520513)at the Materials Research Center of Northwestern University.

Supporting Information Available: Effect of Al2O3 thick-ness on the LSPR of Al nanoparticles. This material is availablefree 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.; VanDuyne, 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. B2003, 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.

(22) Nehl, C. L.; Liao, H. W.; Hafner, J. H. Nano Lett. 2006, 6, 683.(23) Bloemer, M. J.; Buncick, M. C.; Warmack, R. J.; Ferrell, T. L. J.

Opt. Soc. Am. B 1988, 5, 2552.(24) Creighton, J. A.; Eadon, D. G. J. Chem. Soc., Faraday Trans. 1991,

87, 3881.(25) Zeman, E. J.; Schatz, G. C. J. Phys. Chem. 1987, 91, 634.(26) Chan, G. H.; Zhao, J.; Hicks, E. M.; Schatz, G. C.; Van Duyne,

R. P. Nano Lett. 2007, 7, 1947.(27) Langhammer, C.; Yuan, Z.; Zoric, I.; Kasemo, B. Nano Lett. 2006,

6, 833.(28) Boyd, G. T.; Rasing, T.; Leite, J. R. R.; Shen, Y. R. Phys. ReV. B:

Condens. Matter Mater. Phys. 1984, 30, 519.(29) Gao, Y.; Lopezrios, T. Surf. Sci. 1988, 198, 509.(30) Liao, P. F.; Stern, M. B. Opt. Lett. 1982, 7, 483.(31) Lopez-Rios, T.; Gao, Y.; Vuye, G. Chem. Phys. Lett. 1984, 111,

249.(32) Lopez-Rios, T.; Pettenkofer, C.; Pockrand, I.; Otto, A. Surf. Sci.

1982, 121, L541.(33) Tillin, M. D.; Sambles, J. R. Thin Solid Films 1988, 167, 73.(34) Srituravanich, W.; Fang, N.; Sun, C.; Luo, Q.; Zhang, X. Nano

Lett. 2004, 4, 1085.(35) Ctistis, G.; Patoka, P.; Wang, X.; Kempa, K.; Giersig, M. Nano

Lett. 2007, 7, 2926.(36) Buerker, U.; Steinmann, W. Phys. ReV. Lett. 1968, 21, 143.(37) Malicka, J.; Gryczynski, I.; Gryczynski, Z.; Lakowicz, J. R. J. Phys.

Chem. B 2004, 108, 19114.(38) Janz, S.; Bottomley, D. J.; Vandriel, H. M.; Timsit, R. S. Phys.

ReV. Lett. 1991, 66, 1201.(39) Bharadwaj, P.; Anger, P.; Novotny, L. Nanotechnology 2007, 18,

044017.(40) Kostov, Y.; Smith, D. S.; Tolosa, L.; Rao, G.; Gryczynski, I.;

Gryczynski, Z.; Malicka, J.; Lakowicz, J. R. Biotechnol. Prog. 2005, 21,1731.

(41) Ray, K.; Chowdhury, M. H.; Lakowicz, J. R. Anal. Chem. 2007,79, 6480.

(42) Dorfer, T.; Schmitt, M.; Popp, J. J. Raman Spectrosc. 2007, 38,1379.

(43) Kelly, K. L.; Coronado, E.; Zhao, L.; Schatz, G. C. J. Phys. Chem.B 2003, 107, 668.

(44) Lynch, D. W.; Hunter, W. R. In Handbook of Optical Constantsof Solids; Palik, E. D., Ed.; Academic Press: New York, 1985; p 350.

(45) Brust, D. Phys. ReV. B: Solid State 1970, 2, 818.(46) Szmulowicz, F.; Segall, B. Phys. ReV. B: Condens. Matter Mater.

Phys. 1981, 24, 892.(47) Trunov, M. A.; Umbrajkar, S. M.; Schoenitz, M.; Mang, J. T.;

Dreizin, E. L. J. Phys. Chem. B 2006, 110, 13094.(48) Ramaswamy, A. L.; Kaste, P. J. Energ. Mater. 2005, 23, 1.(49) Langhammer, C.; Schwind, M.; Kasemo, B.; Zoric, I. Nano Lett.

2008, 8, 1461.(50) Draine, B. T.; Flatau, P. J. J. Opt. Soc. Am. A 1994, 11, 1491.(51) Jensen, T.; Kelly, L.; Lazarides, A.; Schatz, G. C. J. Cluster Sci.

1999, 10, 295.(52) Whitney, A. V.; Elam, J. W.; Zou, S. L.; Zinovev, A. V.; Stair,

P. C.; Schatz, G. C.; Van Duyne, R. P. J. Phys. Chem. B 2005, 109, 20522.(53) Malinsky, M. D.; Kelly, K. L.; Schatz, G. C.; Van Duyne, R. P. J.

Phys. Chem. B 2001, 105, 2343.(54) Jensen, T. R.; Malinsky, M. D.; Haynes, C. L.; Van Duyne, R. P.

J. Phys. Chem. B 2000, 104, 10549.(55) Jensen, T. R.; Duval, M. L.; Kelly, K. L.; Lazarides, A. A.; Schatz,

G. C.; Van Duyne, R. P. J. Phys. Chem. B 1999, 103, 9846.(56) Malinsky, M. D.; Kelly, K. L.; Schatz, G. C.; Van Duyne, R. P.

J. Am. Chem. Soc. 2001, 123, 1471.

JP804088Z

LSPR of Triangular Aluminum Nanoparticles J. Phys. Chem. C, Vol. 112, No. 36, 2008 13963