Nanosphere lithography: tunable localized surface plasmon resonance spectra of silver nanoparticles

Nanosphere Lithography:Tunable Localized Surface Plasmon Resonance Spectra of Silver Nanoparticles

Traci R.Jensen,†Michelle Duval Malinsky,Christy L.Haynes,and Richard P.Van Duyne*

Department of Chemistry,Northwestern Uni V ersity,E V anston,Illinois60208-3113

Recei V ed:July6,2000;In Final Form:September8,2000

The wavelength corresponding to the extinction maximum,λmax,of the localized surface plasmon resonance

(LSPR)of silver nanoparticle arrays fabricated by nanosphere lithography(NSL)can be systematically tuned

from∼400nm to6000nm.Such spectral manipulation was achieved by using(1)precise lithographic control

of nanoparticle size,height,and shape,and(2)dielectric encapsulation of the nanoparticles in SiO x.These

results demonstrate an unprecedented level of wavelength agility in nanoparticle optical response throughout

the visible,near-infrared,and mid-infrared regions of the electromagnetic spectrum.It will also be shown

that this level of wavelength tunability is accompanied with the preservation of narrow LSPR bandwidths

(fwhm),Γ.Additionally,two other surprising LSPR optical properties were discovered:(1)the extinction

maximum shifts by2-6nm per1nm variation in nanoparticle width or height,and(2)the LSPR oscillator

strength is equivalent to that of atomic silver in gas or liquid phases.Furthermore,it will be shown that

encapsulation of the nanoparticles in thin films of SiO x causes the LSPRλmax to red shift by4nm per nm of

SiO x film thickness.The size,shape,and dielectric-dependent nanoparticle optical properties reported here

are likely to have significant impact in several applications including but not limited to the following:surface-

enhanced spectroscopy,single-molecule spectroscopy,near-field optical microscopy,nanoscopic object

manipulation,chemical/biological sensing,information processing,data storage,and energy transport in

integrated optical devices.

I.Introduction

The signature optical property of noble metal nanoparticles is the localized surface plasmon resonance,hereafter LSPR. When metal nanoparticles are excited by electromagnetic radiation,they exhibit collective oscillations of their conduction electrons known as localized surface plasmons.The wavelength corresponding to the extinction maximum,λmax,of the LSPR is highly dependent on the size,shape,and dielectric properties of the metal nanoparticles.1The primary consequences of LSPR excitation are selective photon absorption,scattering,and local electromagnetic field enhancement.The ability to manipulate and predict the LSPR of metal nanoparticle systems is desirable in several technological applications.For example,it has recently been demonstrated both theoretically2and experimentally3that the coupling of surface plasmons in linear chains of metal nanoparticles results in the transport of light along the direction of the chain.Also,the excitation of surface plasmons in illuminated metallic near-field scanning optical microscopy (NSOM)probes is responsible for the large enhancements in the electric fields originating from the sharply pointed tip.4 Similarly,the operation of scanning tunneling5and scanning force6,7microscopes used in FOLANT configuration also uses surface plasmon excitation to focus laser radiation in the near field of the tip where it is used to nanostructure materials by local heating.Ebbesen et al.8have also reported that the coupling of incident light with surface plasmons in a200nm thick Ag film with150nm cylindrical holes resulted in a100-fold increase in the intensity of transmitted light.The findings listed above are just a few examples of how control of the LSPR in nanoengineered materials can be used to transport optical energy or to focus large concentrations of light into small volumes.9 The ability to manipulate electromagnetic radiation in such a manner will be useful in various types of NSOMs10-13and integrated optical circuits.Other examples of applications that will benefit from the understanding and control of the LSPR include but are not limited to the following:nanoparticle manipulation by optical trapping14,15and optical tweezers,4 ultrafast optical switching,16-18optical bistability,19,20chemical and biological sensing,21,22optical filters,23,24surface-enhanced Raman spectroscopy(SERS),25-36and other surface-enhanced spectroscopies.37-41

Within this broad spectrum of applications,the LSPR of metal nanoparticle systems,particularly those of Ag and Au,is probably best known as the source of the local electromagnetic field enhancement thought to be the dominant contribution to the large intensities observed in SERS.Although thousands of studies have appeared in the literature since the discovery of SERS,42a comprehensive understanding of the enhancement mechanism(s)responsible for the106enhancement factor still remains elusive.This unsatisfactory situation has recently been greatly exacerbated by the extraordinary finding that SERS can be observed with an enhancement factor of1014-1015.26,34 Enhancement factors at this level permit the observation of signals from single molecules,both resonant and nonresonant with the laser excitation wavelength,adsorbed on colloidal Ag nanoparticles.26-34,43Emory and Nie specifically demonstrated that Ag colloidal nanoparticles of distinct sizes and shapes display enormous enhancement factors on the order of1014-1015.26,27These“hot particles”have a very narrow size range

*Author to whom correspondence should be addressed.E-mail: vanduyne@chem.nwu.edu.

†Present address:Omega Optical,Inc.,Brattleboro,VT.10549

J.Phys.Chem.B2000,104,10549-10556

10.1021/jp002435e CCC:$19.00©2000American Chemical Society

Published on Web10/21/2000

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