Metamaterials with Reversible Optoelectronic and Physicochemical Properties

Introduction

The development of nanostructured materials with reversible properties [1-3] is one of the key research areas that yield various innovative functions. In particular, metamaterials with unique optical properties have attracted great interest over the last decades because of the wide-ranging possibilities for practical applications in laser optics, optoelectronics, chemical and biosensing, etc. [4- 9]. Important contributions to the creation of such metamaterials have been obtained by using plasmonic nanostructures, such as gold and silver nanoparticles, nanorods, nanowires, nanodiscs, and nanoholes [10-15]. Most works describe metamaterials developed having static physical characteristics, and only a few articles have been devoted to preparations of metamaterials that permit tuning of physical properties or, in other words, exhibit the so-called "dynamic plasmonic" behavior [16-21]. This discrepancy is due to difficulties related to physical and technological limitations especially when different types of thin composite films are considered. It is

Molecular Plasmonics: Theory and Applications Volodymyr I. Chegel and Andrii M. Lopatynskyi Copyright © 2021 Jenny Stanford Publishing Pte. Ltd.

ISBN 978-981-4800-65-5 (Hardcover), 978-0-429-29511-9 (eBook) www.jennystanford.com obvious that tuning of the physical characteristics of plasmonic colloidal solutions can be achieved more easily than for plasmonic nanostructures embedded in polymer matrices or, moreover, within solid materials due to the limited possible variation in the distance between nanoparticles. Unfortunately, only nanostructured thin-film matrices or ordered nanostructure arrays can be used for the preparation of chip-based formats that exhibit reversible properties and are suitable for practical applications. The physical aspect of this problem might be overcome if ordered arrays of shape-anisotropic nanoparticles that are sensitive to polarization of light are used. However, to fabricate such nanochips, only time- consuming and expensive nanolithographic methods are generally used, e.g., nanoimprint lithography based on specially constructed two-dimensional nanostructure array (nanoblock) molds derived from one-dimensional gratings [22].

This chapter presents the results of two specific studies related to the development of plasmonic nanocomposite matrix films exhibiting features of the "dynamic interface,” which can be prepared using quick and simple methods. The first study shows that it is possible to vary the distance between Ag nanoparticles embedded in poly(aciylic acid) (PAA) matrices in three dimensions, while the PAA network fixes their relative positions within the matrix. The Ag nanocomposite matrix developed showed reversible spectral properties both in surface plasmon resonance (SPR) and localized surface plasmon resonance (LSPR) measurements after immersion in liquids of different pH. pH-induced swelling and shrinking activity of the composite metamaterial was proposed and theoretically simulated to explain this phenomenon. The second study highlights the promising potential of electrochemical SPR method by the examples of redox switching of optical and electrical properties of nanocomposite polymeric films. Specifically, it deals with the SPR transduction of the redox transformations in electropolymerized thin Cu²⁺/polyacrylic acid films, which exhibit properties of a metamaterial with redox switching of electrorefractive, electrochromic, and conductivity functions.

Nanocomposite Polymer Matrix Containing Ag Nanoparticles with Dynamic Plasmonic Properties

Preparation of Nanocomposite Matrix

To prepare the nanocomposite matrix, known properties of some polymers to reduce the ions of plasmonic metals (Ag, Au) were exploited. For example, Gradess et al. [23] used polyvinyl alcohol to reduce Ag⁺ ions at high temperature. In the considered study [24], poly(acrylic acid) was used since polyacrylate films exhibit stronger pH-dependent swelling and shrinking properties [25]. For the reduction of metal ions, UV irradiation was used instead of heating as a more effective and rapid process [26].

Figure 9.1 Electronic absorption spectra of a PAA film during treatment with UV light that leads to formation of embedded Ag nanoparticles. Inset: Image of PAA/AgNPs matrix (in swelled state at the center of the spot and in shrunk state around). Reproduced from Ref. [24] with permission from CSIRO Publishing.

The synthesis of Ag nanoparticles (AgNPs) in PAA-based hydrogel was carried out by spincoating the water-ethanol solution of PAA containing 0.2 M AgN0₃ onto clean microscope glass substrates (for spectrophotometric measurements) and SPR chips based on thin Au film (for SPR measurements) with subsequent drying and

UV irradiation using a mercury-xenon lamp. The colorless polymer films changed color to yellow (which was evident on the transparent glass substrates) after exposure to UV light due to the formation of light-absorbing Ag nanoparticles by photoreduction.

Typical absorbance spectra of composite PAA film with embedded Ag nanoparticles, deposited on a glass substrate, acquired during the UV treatment are shown in Fig. 9.1. The gradual increase in absorbance and redshift of absorbance peak (from 420 to 440 nm) originated from the intensifying LSPR response of nanoparticles due to the increase in size and concentration of Ag nanoparticles over the course of the UV photoreduction process. These results (Fig. 9.1) demonstrate that it is possible to control the process of Ag nanoparticle formation by adjustment of UV exposure time.

Reversible pH-Induced Changes in Optical Properties of Nanocomposite Matrix: LSPR Study

After exposure to UV, the sample was immersed in deionized water and a blue shift of the absorbance peak to ~425 nm was observed. This change in optical properties of the composite film was explicitly related to the alteration of the Ag nanoparticle's environment. It is known that the LSPR of noble metal nanoparticles depends on both dielectric properties of the surrounding medium [27] and intensity of the electromagnetic interaction between adjacent nanoparticles [28]. According to these specific plasmonic properties, a blue shift in absorbance for an ensemble of Ag nanoparticles can typically be induced by a decrease in the refractive index of environment and a decrease in electromagnetic interaction between nanoparticles (e.g., due to increase in interparticle distance). Because of the strong adsorption of water to PAA leading to hydrogel formation [25], optical evidence for the interaction of the PAA film with embedded Ag nanoparticles with deionized water can be treated as a simultaneous influence of the aforementioned mechanisms. Namely, adsorption of water by the PAA polymer network leads to reduction in its average refractive index due to the difference in their refractive indices—1.527 for PAA [29] and 1.333 for water. Additionally, water adsorption by PAA induces swelling of the polymer film due to hydrogel formation and subsequently results in an increase in the average distance between Ag nanoparticles contained in the film.

To study swelling and shrinking behavior of the composite films, they were successively immersed in deionized water, and then in aqueous 0.1 M sulfuric acid for 1 min, which led to a cyclic reversible shift in the absorbance peak (Fig. 9.2). Namely, the absorbance maximum switched between ~425 and ~411 nm for deionized water and 0.1 M aqueous sulfuric acid, respectively. It should be noted that the refractive index of aqueous 0.1 M sulfuric acid is almost identical to that of pure water [30], and the absorption spectrum should not undergo a blue shift due to change in the ambient refractive index.

This implies that a blue shift in absorbance under the influence of sulfuric acid solution was induced by a further swelling of the polymer due to the change in ambient medium pH. However, it is known that swelling of pure PAA usually occurs at high pH [31], so it was supposed that the presence of AgNPs in the PAA matrix drastically changes the conformation of this polymer most probably either during reduction of Ag⁺ ions or by chelation of Ag atom/ions by carboxylic acid in PAA.

Figure 9.2 (a) Electronic absorption spectra of PAA hydrogel film containing Ag nanoparticles after alternating immersion in deionized water and 0.1 M sulfuric acid aqueous solution, (b) Respective absorbance spectrum peak positions. Reproduced from Ref. [24] with permission from CSIRO Publishing.

Reversible pH-Induced Changes in Optical Properties of Nanocomposite Matrix: SPR Study

To verify the results of spectrophotometric measurements, SPR experiments on the same composite Ag nanoparticle-PAA film deposited on an SPR chip were carried out. To study swelling and shrinking behavior of the composite films, they were successively immersed in deionized water and then in aqueous 0.1 M sulfuric acid until SPR response stabilization. As the main mode of SPR measurements, the time dependence of the reflected light intensity from SPR chip at a fixed angle of incidence was recorded. The resulting observation of cyclic shifts in the SPR reflectance upon successive immersion of the sample into deionized water and aqueous 0.1 M sulfuric acid confirmed the swell/shrink process (Fig. 9.3).

Figure 9.3 SPR sensogram corresponding to the shrinking and swelling of the PAA hydrogel film containing embedded Ag nanoparticles, during successive immersions in deionized water and aqueous 0.1 M sulfuric acid, respectively. Reproduced from Ref. [24] with permission from CSIRO Publishing.

Theoretical Study of Nanocomposite Polymer Matrix with Dynamic Plasmonic Properties by Means of FDTD Simulations

Simulation of nanocomposite matrix optical properties was performed using the finite-difference time-domain (FDTD) method in the FDTD Solutions package (Lumerical Solutions, Inc.) by means of a model consisting of nine spherical Ag nanoparticles located in a 3x3 square grid embedded into a polymer medium resting on a glass substrate (Fig. 9.4). The diameter of nanoparticles was fixed at 12 nm taking into consideration the experimentally measured electronic absorption spectrum of a PAA/AgNPs matrix in air

(Fig. 9.1), which peaks at 440 nm. Shrunken and swollen composite films were designed by adjusting the polymer medium refractive index and interparticle distance. It should be noted that the chosen parameters differ significantly for shrunken and swollen films, and their values as well as a value of nanoparticle diameter most probably do not reflect the exact experimental parameters. These values were introduced intentionally in order to obtain more pronounced differences in the simulated optical properties and to determine the direction of absorption peak shift. However, the estimated interparticle distance in a shrunken state based on the composite film preparation procedure was equal to ~22 nm, which is near the value of 15 nm used in modelling. After obtaining the absorption spectra, two-dimensional electric field intensity distributions at light wavelengths corresponding to the absorbance peak maxima were simulated.

The swell/shrink process was simulated numerically as the change in ambient refractive index and interparticle distance in a layer of Ag nanoparticles (Fig. 9.4). The absorbance spectra (Fig. 9.5) andelectricfield intensity distributions at light wavelengths corresponding to absorbance peak maxima (Fig. 9.6) were calculated. According to modelling results, the absorbance peak undergoes a blue shift due to a decrease in the ambient medium refractive index and an increase in the interparticle distance (i.e., decrease in interparticle interaction, which is evident by a decrease in electric field intensity) during the swelling process.

Figure 9.4 Models of the (a) shrunken and (b) swollen PAA hydrogel containing Ag nanoparticles. Reproduced from Ref. [24] with permission from CSIRO Publishing.

Figure 9.5 Simulated absorbance spectra for shrunken and swollen PAA hydrogel film containing Ag nanoparticles. Reproduced from Ref. [24] with permission from CSIRO Publishing.

Figure 9.6 Simulated electric field intensity distributions for (a) shrunken and (b) swollen PAA hydrogel film containing Ag nanoparticles at light wavelengths corresponding to absorbance peak maxima in Fig. 9.S. Reproduced from Ref. [24] with permission from CSIRO Publishing.