Plasmon response of nanoshell dopants in organic films: a simulation study

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Abstract

We examine the effect of an embedding medium refractive index on the plasmon resonant properties of silica core-gold shell nanoshells. The plasmon response is shifted to longer wavelengths with increasing refractive index of the dielectric host matrix, increasing in overall amplitude for nanoparticles in the dipole limit. For nanoshells of constant core-shell ratio, this plasmon shift increases with absolute particle size. We also observe that the plasmon shift is the same for nanoparticles of the same size, independent of core-shell ratio. For larger nanoshells we observe an increase in amplitude of the quadrupole plasmon resonance relative to the dipole plasmon.

Introduction

The incorporation of metallic nanoparticles into organic host materials, particularly within the context of the development of new materials combining inorganic and organic constituents, is a topic of much current interest. In addition to the chemical and materials challenges of growing or assembling inorganic–organic hybrid materials, there is a more fundamental question regarding the predictive, rational design of new and novel inorganic–organic materials based on properties or function. The recent development of nanoshells [1], metallic nanoparticles with a tunable plasmon-derived optical response based on nanoparticle structure, provides a new, systematically controllable design element that can be manipulated to control the functional properties of inorganic–organic hybrid materials in new and novel ways.

Nanoshells are dielectric core-metal shell nanoparticles whose vivid optical response is derived from their plasmon resonance. In the core-shell topology, the plasmon resonance frequency can be systematically tuned by modifying the radius r1 of the dielectric core and the total particle radius r2 [2], [3]. Relative to the plasmon resonance of a solid spherical nanoparticle of the same metal, the core-shell geometry allows the plasmon resonances to be shifted to lower energies. In our previous work with silica core-gold shell nanoshells we have tuned the plasmon response from the nominal 520 nm resonance wavelength of solid gold nanospheres across the visible region of the spectrum to nearly 3 μm in the infrared [4]. For silica core-gold shell and silica core-silver shell nanoshells, the plasmon resonant response of the fabricated nanostructures agrees quantitatively with classical electromagnetic (Mie scattering) theory [1], [5], [6]. In the small size (dipole) limit, nanoshells behave as tunable resonant optical absorbers. By scaling up the overall size of the nanoshell, a family of spectrally distinct multiple resonances are obtained, each with its own distinct relative contributions of absorption and scattering to the overall optical extinction [7].

Unlike photonic band gap based materials, whose optical properties are dependent on long range periodicity [8], nanoshells can impart new and novel functionalities into composite materials without the need for long range order. Since the extinction cross-section of nanoshells is nominally five times their physical cross-section, they can affect significant modifications in the properties of dielectric materials when present as dopants in a host matrix, at volume concentrations as low as a few percent. The simplest example of this is the incorporation of nanoshell dopants into optically transparent organic polymer films, such as polyvinyl alcohol or polydimethylsiloxane, where the plasmon resonant properties of nanoshell dopants are incorporated into the film homogeneously [9]. Nanoshells can incorporate other functionalities into dielectric host matrices besides straightforward pigmentation. One recent example is the incorporation of nanoshells into the luminescent semiconducting polymers MEV-PPV and P3OT films for photo-oxidation inhibition [10]. In these materials, photo-oxidation proceeds when the long-lived triplet exciton de-excites through excitation of singlet oxygen, which adds across the polymer chains resulting in chain scission and the formation of trap states. This photochemistry degrades the polymer’s luminescent response and, in optoelectronic applications, its device lifetime. Nanoshells with plasmon resonances tuned to the triplet exciton of the host polymer act as solid-state triplet quenchers, effectively reducing the rate of photo-oxidative degradation by more than an order of magnitude. Another recently demonstrated functionality is wavelength specific photothermal response [11]. When metallic nanoparticles are incorporated into temperature sensitive polymer host matrices, such as n-isopropyl acrylamide co-acrylamide (NIPAAm-co-AAm), the quasi-first-order phase transition of the pristine polymer can be driven optically by illuminating the composite material at a wavelength corresponding to the plasmon resonance of the nanoparticle dopant. Under resonant illumination, the nonradiative nanoparticles serve as localized heat sources within the dielectric host, inducing a highly controlled collapse of the hydrogel matrix. Such nanoparticle–hydrogel composites have been demonstrated as optically controllable drug delivery systems [12], and optomechanical materials whose wavelength response can be made color-specific by modification of the plasmon resonance of the nanoparticle dopant [13]. This wavelength-selective optomechanical collapse has immediate applications in the development of multifunctional remotely controlled mechanical devices and components.

Of primary importance in the rational design of all nanoparticle-doped organic composites is how the dielectric host medium modifies the plasmon response of the nanoparticle dopant. Increase in the dielectric constant of the embedding host medium leads to increased screening and will lower the plasmon resonance frequency of an embedded nanoparticle [14], [15], [16], [17]. A more detailed understanding of the effect of embedding medium on plasmon response shows that this plasmon resonance shift also depends on size, geometry, and orientation of the specific nanoparticle under study. For the case of a lossless and nonscattering dielectric the plasmon response of metallic nanoparticles is modeled quite accurately using Mie scattering theory. This provides an essentially predictive “design tool” for tailoring the properties of a nanoparticle–dielectric composite film or material in the low nanoparticle density limit. In this study we examine the modifications in the plasmon response of silica core-gold shell nanoshells induced by changes in the refractive index of a lossless and nonscattering embedding medium. The effect of nanoparticle size and core/shell ratio on the plasmon resonances shifts are examined, as is the effect of embedding medium refractive index on the dipole and multiple contributions to the total extinction cross-section of the nanoshell.

Section snippets

Results and discussion

Fig. 1 shows a series of Mie scattering calculations for silica core-gold shell nanoshells of core radius r1 and total radius r2 in a lossless dielectric embedding medium of increasing refractive index. Fig. 1(a) shows a series of curves depicting the wavelength shifts of the plasmon resonance maximum for nanoshells of constant core-shell ratio (r2/r1=1.1875) and increasing particle size, from 20 to 120 nm total nanoparticle radius r2. This plot clearly shows a monotonic increase in plasmon

Conclusions

We have documented the calculated variations in plasmon response to be anticipated by nanoshell dopants in lossless, nonscattering embedding media. These simulation studies provide a general set of “design rules” for the modifications of an organic medium or film’s properties based on the addition of plasmon resonant nanoshells in concentration ranges where interparticle effects can be neglected. We anticipate that this study should be of broad applicability in the design and fabrication of

Acknowledgements

The authors wish to acknowledge the Army Research Office, the Air Force Office of Scientific Research, the Robert A. Welch Foundation, and the Texas Advanced Technology Program, for financial support.

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