Mohammed Alsawafta

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.

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 nmwill 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.

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.

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.

Optical properties of two-dimensional and three-dimensional arrays of noble metal nanoparticles by the discrete dipole approximation method

The optical aspects of plasmon coupling occurring through the near-field interactions among metallic spherical nanoparticles assembled in close proximity to each other in two-dimensional and three-dimensional arrays have been examined using the discrete dipole approximation (DDA). Calculations were performed for nano-sized close-packed spheres of silver, gold or copper, hexagonally arranged in a planar monolayer target and extended gradually to three-dimensional multilayer targets with a fastened interparticle spacing. Those targets were simulated under the incident ppolarized light with an energy range of 1.5 - 4.5 eV by executing an open-source code of the DDA. The optical response of three-dimensional targets was revealed in the absorption spectra calculated at various angles of the polarized incident light, showing a blue shift of the plasmon resonance (PR) peak for both gold and copper targets. The splitting of the surface plasmon resonance (SPR) observed in the response of the two-dimensional silver system eventually disappeared into one well-defined resonance peak as the system grew in the third dimension. Moreover, to shed light on the nature of the plasmon coupling among close-packed nanospheres of different metals, we simulated a target composed of mono-sized nanospheres of the three metals placed spatially in three consecutive layers. A combined optical behaviour was thus observed through the absorption spectrum, where the plasmon peaks attributed to the silver, gold and copper interacting nanospheres emerged at the original energy values as if it was applied in isolated planar hexagonal arrays.

Electrochromic and electrical properties of layered and tubular vanadium pentoxide thin films

Due to its lithium ion intercalation capability, vanadium pentoxide is extensively studied for applications such as electrode material for pseudocapacitors, electrochromic devices, and cathodes in high capacity lithium ion batteries. Vanadium oxide has a layered structure and, in order to enhance lithium ion intercalation, templating methods are proposed in this work to create porosity in the film. The aim of this work is to evaluate how the morphology of vanadium pentoxide thin films is instrumental in obtaining a material with a high lithium ion intercalation capacity. With an appropriate morphology, the performance of vanadium oxide as electrochromic material and as cathode in lithium ion batteries can be improved significantly. For this purpose, both layered (dense and porous) and nanorod films were prepared and characterized. Scanning electron microscopy, cyclic voltammetry and electrical impedance spectroscopy measurements were used for the characterization of the different V2O5 films. The results showed the very good electrochromic properties of the porous film built up with polystyrene microspheres and triblock copolymer as templating materials as well as of the nanorod films fabricated by thermal annealing.

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 LMis 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.

Effect of symmetry breaking and substrate on the optical properties of silver sphere-like nanoparticles in different surrounding media

The effect of the substrate to the absorption spectra of silver nanoparticles of sphere-like shapes is investigated. Silver nanoparticles of broken spherical symmetry are placed on substrates of different dielectric functions at various contact angles (α). The absorption efficiency of the supported nanoparticles is calculated by using the Discrete Dipole Approximation (DDA) method. Increasing the value of α and, hence the contact area of the supported nanoparticles, results in an inhomogeneous distribution of the polarization charges over the nanoparticle-substrate system. This leads to the excitation of plasmonic bands of different characters (dipolar and quadrupole modes). The admixture of both dipolar and quadrupole modes is found to be more pronounced when a nanoparticle with highest contact area (α = 90o, hemispherical shape) is considered. The band position of the Longitudinal Mode (LM) is red-shifted with α, while the resonance wavelength of the Transverse Mode (TM) is blue-shifted.

The effect of the target size on the optical response of ultrafine metallic spherical particles arranged in a two-dimensional array

Discrete Dipole Approximation (DDA) is a computational technique to simulate the optical properties of nanostructures of different shapes, sizes, and compositions. The influence of the target size on the optical response of 5 nm-diameter nanoparticles arranged in a monolayer hexagonal array is investigated by using DDA at various incident angles of the incident light on the target considered for silver (Ag), gold (Au) and copper (Cu) nanoparticles. In our study, the target size is controlled by the number of the spherical nanoparticles used to generate the two dimensional arrays. The interparticle distance is kept constant in all the simulations. The anisotropic response of noble-metal nanoparticles is generally characterized by the excitation of the high-energy (transverse) surface plasmon (SP) mode and the low-energy (longitudinal) SP mode. Results of the simulations for the three chosen metals show an exponential dependency of the absorption efficiency of the SP modes with respect to the target size. As the target size is increased, the energy of Ag-longitudinal SP mode is red-shifted and it displays an exponential decay while the band position of the transverse mode is blue-shifted. They however overlap when the smallest target size is considered. Although, the optical response of Au and Cu nanoparticle arrays shows the same dependency on the target size as observed in the case of Ag, the positions of their respective longitudinal and transverse modes are very close, making these almost indistinguishable. The dependency of the absorption efficiency of SP modes on the incident angle is fitted linearly for Cu, Au and longitudinal- Ag modes to the target size, while the transverse-Ag mode shows an exponential fitting. No change in the Ag-SP band position is observed when the incident angle is changed, but the SP bands for both Au and Cu exhibit exponential variation behavior.