Sarah L. Westcott

Nanoengineering of optical resonances

Abstract Metal nanoshells, consisting of a dielectric core with a metallic shell of nanometer thickness, are a new, composite nanoparticle whose optical resonance can be “designed in” in a controlled manner. By varying the relative dimensions of the core and shell, the optical resonance of these nanoparticles can be varied over hundreds of nanometers in wavelength, across the visible and into the infrared region of the spectrum. We report a general approach to the making of metal nanoshell composite nanoparticles based on molecular self-assembly and colloid reduction chemistry.

INFRARED EXTINCTION PROPERTIES OF GOLD NANOSHELLS

Gold nanoshells, nanoparticles consisting of a silica core coated with a thin gold shell, exhibit a strong optical resonance that depends sensitively on their core radius and shell thickness. Gold nanoshells have been fabricated with a peak optical extinction that can be varied across the near-infrared region of the spectrum (800 nm–2.2 μm). Multipolar plasmon resonances are clearly resolvable in the extinction spectra and agree well with electromagnetic theory. Additional resonances due to particle aggregation are also observed. The frequency agile infrared properties of these nanoparticles make them particularly attractive for a range of technologically important applications.

CONTROLLING THE SURFACE ENHANCED RAMAN EFFECT VIA THE NANOSHELL GEOMETRY

Systematic variation of the internal geometry of a dielectric core-metal shell nanoparticle allows the local electromagnetic field at the nanoparticle surface to be precisely controlled. The strength of the field as a function of core and shell dimension is measured by monitoring the surface enhanced Raman scattering (SERS) response of nonresonant molecular adsorbates (para-mercaptoaniline) bound to the nanoparticle surface. The SERS enhancement appears to be directly and exclusively due to nanoparticle geometry. Effective SERS enhancements of 106 are observable in aqueous solution, which correspond to absolute enhancements of 1012 when reabsorption of Raman emission by nearby nanoparticles is taken into account.Systematic variation of the internal geometry of a dielectric core-metal shell nanoparticle allows the local electromagnetic field at the nanoparticle surface to be precisely controlled. The strength of the field as a function of core and shell dimension is measured by monitoring the surface enhanced Raman scattering (SERS) response of nonresonant molecular adsorbates (para-mercaptoaniline) bound to the nanoparticle surface. The SERS enhancement appears to be directly and exclusively due to nanoparticle geometry. Effective SERS enhancements of 106 are observable in aqueous solution, which correspond to absolute enhancements of 1012 when reabsorption of Raman emission by nearby nanoparticles is taken into account.

Surface enhanced Raman scattering in the near infrared using metal nanoshell substrates

A metal nanoshell is a composite nanoparticle consisting of a dielectric core coated by a thin metal shell; its peak plasmon resonance wavelength is determined by the ratio of the core diameter to the shell thickness. When p-mercaptoaniline (p-MA) is in solution with gold nanoshells that have their plasmon resonance near a 1.06 μm excitation source, significant surface enhanced Raman scattering (SERS) is observed. The strongest Raman enhancements are obtained when enough gold is deposited on the silica cores to form a nearly complete metal shell. Correlations between transmission electron microscopy (TEM)-defined structure, ultraviolet (UV)-visible spectra, SERS signal strength, and electromagnetic theory show that the SERS signal is due to both the local enhancement of the dielectric field via the plasmon resonance of the nanostructure and to the localized regions of high field intensity provided by the nearly completed gold shell. Comparison with SERS enhancements on completed nanoshell structures indic...

Independent optically addressable nanoparticle-polymer optomechanical composites

We report the fabrication and characterization of optomechanically active composite materials consisting of a thermally responsive poly(NIPAAm-co-AAm) hydrogel matrix incorporating a dilute concentration of Au or silica-Au core-shell nanoparticles. Under optical illumination at the resonance absorption wavelength of the nanoparticle dopant, a dramatic volume collapse of the composite occurs due to local photothermal heating of the NIPAAm matrix. Nanoparticle dopants were chosen so that each composite was specifically optically addressable, exhibiting optomechanical behavior at independent wavelengths. Such materials can be useful as independently addressable remotely triggerable switches and gates in a wide variety of micromechanical applications.

Nanoshell-polymer composites for photothermally modulated drug delivery

Summary form only given. Optically active gold nanoshells; have been incorporated into thermally, responsive copolymers of N-Isopropylacrylamide (NIPAAm) and acryl-amide (AAm) for the purpose of photothermally modulated drug delivery. The copolymer exhibits a lower critical solution temperature (LCST) that is slightly above body, temperature. When the temperature of the hydrogel exceeds its LCST, a rapid collapse occurs, expelling any material contained within the hydrogel. The gold nanoshells initiate a temperature increase, via targeted absorption of near IR light.

Effect of adsorbed molecules on hot electron relaxation in gold nanoshells

Summary form only given. The optical response of metal nanoparticles is dominated by the electrons and thus is extremely rapid. For bulk metals, the relaxation of hot electrons is understood as an electron-phonon interaction! The relaxation of hot electrons in nanoparticles is more complex and affected by size and embedding medium. Nanoshells are nanoparticles with a thin metal shell coating a dielectric core. For nanoshells, the excitation wavelength of the collective electron oscillation (plasmon resonance) depends on the core size and shell thickness. Nanoshells with gold sulfide core and a gold shell have a plasmon resonance tunable from 600-950 nm. When p-aminobenzoic acid aniline, n-propylamine, or p-mercaptobenzoic acid were added to an aqueous nanoshell solution, the molecules were bound to the gold surface by the amine or thiol groups. Enhanced Raman signals were detected from the bound molecules due to the large local electric fields at the metal nanoshell surface, confirming the binding of the molecules. The transient bleaching of the nanoshell solutions was measured in a degenerate pump-probe experiment. The induced change in transmission occurred because of the change in the dielectric function of the gold shell for a hot electron distribution. The mechanism by which the adsorbed molecules affect the electron relaxation could be energy transfer from the hot electrons to adsorbed molecules or perturbation of the electronic potential of the metal by the bound molecules. This perturbation could induce a decrease in Coulomb screening in the nanoshells.

Plasmon-plasmon interaction between gold nanoshells and gold surfaces

Summary form only given. Gold nanoshells are colloidal particles with a dielectric core covered by a gold shell. The plasmon resonance of the nanoshells can be tuned by varying the ratio of the core/shell radii. An enhancement in the electromagnetic energy can be found, at resonance, in the region close to the nanoshell known as the near field. It has been shown theoretically and experimentally that if the evanescent near fields of a surface plasmon polariton and a particle plasmon overlap, an efficient exchange of energy from the freely propagating electromagnetic waves into surface plasmons can be achieved. Previous experiments used particles that lacked the extraordinary tunability of nanoshells. We give our sample geometry. After evaporating a layer of gold onto a glass slide, we deposit self-assembled monolayers (SAMs) of a polymer (PDDA) and Hectorite (a synthetic clay), to control the spacing between the gold surface and the nanoshells.

Optical properties and ultrafast electron dynamics in gold nanoshells

Summary form only given. We present studies of the ultrafast electron dynamics in gold nanoshells. By tuning the plasmon peak position of the nanoparticle relative to the tuning range of a cavity dumped Ti:sapphire laser, both transient bleaching and transient absorption are observed.