Optical properties of metallic nanoparticles and metallic nanocomposite materials

The optical behaviour of both gold and silver nanoparticles has been studied in both experimental and theoretical aspects. In the theoretical part of our thesis, to contribute to a better understanding of particles of various shapes and configurations, the versatile Discrete Dipole Approximation (DDA) has been employed to simulate in depth the absorption spectra of single isolated oriented nanoparticles of different symmetry (nanocube, nanobar and nanoellipsoid). The effect of the plasmonic coupling and the size of spherical particles assemblies on their optical response have also been addressed. It was shown that the plasmonic coupling in the interacting particles in close proximity configuration disturbs the homogenous distribution of surface charges and results in splitting the plasmonic band into two bands. The excitations of two different bands (longitudinal and transverse bands) have been also observed in the absorption spectra of many fold symmetry particles. The diversity of the polarization factors along different symmetry axis was established as the main key for observing several bands. Thus the importance of particle shape and the different interesting possibilities offered by this single factor has been well demonstrated in the DDA calculations performed while our treatment of ensembles of nanospheres showed in detail the effect of interacting particles on the overall optical properties of actual samples. In the experimental part of this thesis, a first part is devoted to the study of the influence of dielectric host material on the optical properties of gold nanoparticles. For this purpose, gold, -poly(methyl methacrylate) (PMMA) and -gelatin nanocomposite materials have been prepared by an in-situ method. Two reduction methods (photochemical and chemical) were used to reduce the gold salt in the presence of the polymer matrix. Firstly, annealed and non annealed samples were prepared by different photochemical methods (UV-, thermal-, and MW-irradiation). Gold-poly(methyl methacrylate) nanocomposites were prepared by irradiating spin-coated films containing the polymer and the gold precursor dissolved in acetone. The reduction of gold ions resulted in the formation of gold that nucleated and grew within the polymer films. It was shown that, depending on the energy source, gold nanoparticles with different shapes could be formed. The nanocomposites prepared through the photochemical methods, showed a low sensitivity toward the environment. However, by annealing the samples at temperatures well above the glass transition temperature of the polymer, the response to dielectric environment appeared to be enhanced significantly. The increased sensitivity of the annealed sample (increase the surface particle density) was accounted for by the increased mobility of both polymer chains and gold nanoparticles in the rubbery state of the material and the presence of the monomer. The results showed that, by using adequate post-synthesis heat treatments, gold-polymer nanocomposites could be used as plasmonic sensing platforms. Secondly, gold–gelatin bionanocomposite films were prepared by the reduction of gold ions by sodium borohydride in an aqueous solution. It was shown that both the solution and the films on glass substrates contained entrapped hydrogen micro- and nanobubbles with diameters in the range of 200 nm–3 μm. The composite films having micro- and nanobubble inclusions have been found to be very stable. The optical properties of gold nanoparticles in the presence of gelatin and hydrogen nanobubbles were measured and simulated by using the discrete dipole approximation (DDA) method. The calculated localized surface plasmon resonance band was found in agreement with the experimental band position only when the presence of hydrogen bubbles around the gold nanoparticles was taken into account. The different morphological features engendered by the presence of the bubbles in the film (gelatin receptacles for the nanoparticles, gelatin hemispheres raised by the bubbles under the surface, cavities on the surface of the film, etc) are potential candidates for many applications

Simulated Optical Properties of Gold Nanocubes and Nanobars by Discrete Dipole Approximation

The absorption spectra for a gold nanocube and for a gold nanobar are calculated by using the Discrete Dipole Approximation (DDA). The results show the excitation of a single albeit broad surface plasmon (SP) band of the gold nanocube. The extinction cross section of the gold nanocube is dominated by the absorption cross section that gains importance as the width increases. Further increasing the nanocube size beyond 80 nm will result in an optical response mainly characterized by scattering properties. The absorption spectrum of the nanobar shows the excitation of both the longitudinal mode (LM) and the transverse mode (TM). The nanobar is also compared to a cylinder, a spherically capped cylinder, and a spheroid of the same aspect ratio. The band position of the TM of the nanobar is red-shifted as compared to the ones calculated for other morphologies, while the LM is either blue-shifted or red-shifted depending on the morphologies considered

Subzero Temperature Dip-Coating of Sol-Gel Vanadium Pentoxide: Effect of the Deposition Temperature on the Film Structure, Morphology, and Electrochromic Properties

Vanadium pentoxide sol-gel prepared thin films were deposited on indium-tin-oxide (ITO) substrates by dip-coating at a subzero temperature (−10°C). The structure, morphology, and optical and electrochromic properties of dense and porous vanadium oxide films coated at low temperature were determined and compared to those of the corresponding films deposited under room-temperature conditions. The results indicated that, in the films coated at −10°C, a residual compressive stress exists that would originate from the formation of microvoids during the deposition. These microvoids are preserved during the heat treatment of the films. The microvoid morphology would favor the formation of nanostructures that would be responsible for the improved electrochromic properties of the subzero dip-coated films. Low-temperature coated films, heated at 450°C for several hours, undergo the transformation from a layered to a highly uniform nanorod structure that would be an important feature for different applications

The effect of hydrogen nanobubbles on the morphology of gold–gelatin bionanocomposite films and their optical properties

Gold–gelatin bionanocomposite films are prepared by the reduction of gold ions by sodium borohydride in an aqueous solution. It is shown that both the solution and the films on glass substrates contain entrapped hydrogen micro- and nanobubbles with diameters in the range of 200 nm–3 μm. The optical properties of gold nanoparticles in the presence of gelatin and hydrogen nanobubbles are measured and simulated by using the discrete dipole approximation method. The composite films having micro- and nanobubble inclusions have been found to be very stable. The calculated localized surface plasmon resonance band is found in agreement with the experimental band position only when the presence of hydrogen bubbles around the gold nanoparticles is taken into account. The different morphological features engendered by the presence of the bubbles in the film (gelatin receptacles for the nanoparticles, gelatin hemispheres raised by the bubbles under the surface, cavities on the surface of the film, etc) are described in detail and considered for potential applications. This work is highly relevant to the new and exciting topic of nanobubbles on surfaces and interfaces

Fermented juices as reducing and capping agents for the biosynthesis of size-defined spherical gold nanoparticles

Gold nanoparticles (AuNPs) are of scientific and industrial significance; however, the traditional synthesis methods employ toxic compounds. Hence, non-toxic and environmentally friendly AuNPs synthesis methods are of special interest. Here, AuNPs were produced using four solutions of fermented grape juices. UV/Vis absorption spectrophotometry and transmission electron microscopy indicated that AuNPs synthesized with a solution based on semi-sweet red grapes were mostly spherical with narrow size distribution (average diameter of 82.1 ± 36.2 nm). AuNPs of similar spherical morphology but smaller size were obtained using a solution based on semi-dry red grapes (57.1 ± 16.4 nm). A large variety of AuNPs shapes and broader size distribution were produced when solutions based on semi-sweet or dry white grapes were applied. In this case, the average sizes of the AuNPs were 271.6 ± 130.2 nm and 76.0 ± 47.2 nm, respectively. Using energy dispersive X-ray spectroscopy, Au, C, and O were detected, confirming formation of biogenic AuNPs in all cases. Mie theory calculations for AuNPs synthesized with the aid of solutions based on red grapes suggest that their optical properties are different and best suited for distinct downstream applications. Attenuated total reflectance Fourier transform infrared spectroscopy, the Folin-Ciocalteu assay, and the Bertrand's method were used to examine bioactive compounds present in the solutions applied for synthesis. Phenolics, and to a lesser extent reducing sugars, were identified as likely playing a significant role in reduction and stabilization of the AuNPs. These results display the great potential of these solutions for green synthesis of size defined AuNPs, and illustrate that different grape varieties may be used to obtain AuNPs with unique properties. Keywords: Nanostructures, Bioreduction process, Phenolics, Reducing sugars, Mie scatterin

Plasmonic Modes and Optical Properties of Gold and Silver Ellipsoidal Nanoparticles by the Discrete Dipole Approximation

The discrete dipole approximation (DDA) is used to model the absorption efficiency of isolated gold (Au) and silver (Ag) ellipsoidal nanoparticles. The characteristics of the plasmonic bands of those nanostructures depend strongly on the size and orientation of the particles in both the lab and target frames. At specific rotation and incident angles, the desired plasmonic mode can be excited. The result of the simulation shows the possibility of excitation of three plasmonic modes—one longitudinal mode (LM) and two transverse modes (TM)—corresponding to the redistribution of the polarization charges along each principal axis. At oblique incidence of the incoming light, both the Au LM and a hybrid Au TM are observed whereas three more distinct plasmonic modes can be found in the case of the Ag particle. The effect of length distribution on the characteristics of the plasmonic bands is also examined for the three principal axes. The band position of the plasmonic bands associated with the electronic oscillation along each principal axis is found to vary linearly with the axis length. The linear variation of the band position of the LM is steeper as compared with the one found for the other modes

Investigation of the Validity of the Universal Scaling Law on Linear Chains of Silver Nanoparticles

Due to the wide range of variation in the plasmonic characteristics of the metallic nanoparticles arranged in linear arrays, the optical spectra of these arrays provide a powerful platform for spectroscopic studies and biosensing applications. Due to the coupling effect between the interacting nanoparticles, the excited resonance mode is shifted with the interparticle separation. The change in the resonance energy of the coupled mode is expressed by the fractional plasmon shift which would normally follow a universal scaling behavior. Such a universal law has been successfully applied on a system of dimers under parallel polarization. It has been found that the plasmon shift decays exponentially over interparticle spacing. The decay length is independent of both the nanoparticle and dielectric properties of the surrounding medium. In this paper, the discrete dipole approximation (DDA) is used to examine the validity of extending the universal scaling law to linear chains of several interacting nanoparticles embedded in various host media for both parallel and perpendicular polarizations. Our calculations reveal that the decay length of both the coupled longitudinal mode (LM) and transverse modes (TM) is strongly dependent on the refractive index of the surrounding medium nm. The decay constant of the LM is linearly proportional to nm while the corresponding constant of the TM decays exponentially with nm. Upon changing the nanoparticle size, the change in the peak position of the LM decreases exponentially with the interparticle separation and hence, it obeys the universal law. The sensitivity of coupled LM to the nanoparticle size is more pronounced at both smaller nanoparticle sizes and separations. The sensitivity of the coupled TM to the nanoparticle size on the other hand changes linearly with the separation and therefore, the universal law does not apply in the case of the excited TM