The Effect of Local Field Dispersion on the Spectral Characteristics of Nanosized Particles and their Composites

Abstract

Infrared (IR) spectroscopy of microand nanosized particles and their composites is currently one of the most important enabling technologies in the development of microand nanostructures and their application to various areas of science and technology. Decreasing the characteristic size of metallic, dielectric and semiconductor materials results in a dramatic alteration to their optical, electrical and mechanical properties, allowing the fabrication of new materials with unique physical properties (Lamberti, 2008; Cao, 2004). These alterations in the optical properties are related to a quantum confinement effect, as well as to a dielectric, or electrostatic, confinement effect (Cahay et al., 2001; Chemla & Miller, 1986). The effect of quantum confinement is most pronounced in semiconductor materials, where the transition from the bulk state to the microcrystalline state causes a substantial change in the band structure and an enhancement of the non-linear electrooptical properties. Dielectric, or polarisation, confinement has a wider impact, since it influences the frequencies and intensities of absorption bands in the spectra of any condensed matter, including crystalline and amorphous solids, as well as liquids. This is because considerable changes in the polarisation of micro/nanoparticles occur, depending on their form and orientation with respect to the external electromagnetic field and the details of the spatial restriction. So, the dielectric confinement effect is due to abrupt changes in the intensity of the internal (Ein(ν)), local electric field Eloc(ν), causing significant changes in the spectroscopic characteristics, depending on the direction of the external field Е(ν), and the size and shape of the submicron sized particles, or micro-objects. Dielectric confinement occurs when the absorbing material consists of micro-particles with characteristic sizes significantly smaller than the wavelength of the probe beam. These particles are generally embedded in a transparent dielectric matrix, or deposited on a transparent substrate as an ultra-thin film (Fig. 1). A good analogy to these systems is that of an aerosol suspended in air or stained glass, that is, glass doped with small metal particles (Gehr & Boyd, 1996). In the long wavelength limit, d << λ, for the determination of the spectroscopic characteristics of micro-