Optical, infrared and electron-microscopy studies of (Cdi)n(Cdi)n metallic clusters in layered CdI2CdI2 crystals

An influence of (Cdi)n(Cdi)n metallic clusters on the optical absorption and phonon spectra of CdI2CdI2 crystals was studied. Metallic clusters of spherical shape were formed during the growth of non-stoichiometric crystals. Radii of clusters are in range from 10 to 500 nm according to scanning electron microscopy (SEM) data. The density of clusters was estimated from fractal dimension calculations. The fractal dimension of (Cdi)n(Cdi)n clusters has been found to be varied from 1.488 to 1.793. In the framework of Mie theory the spectral and size dependencies of extinction coefficients were calculated. From the experimentally obtained spectra it is evident that the metallic clusters are responsible for the bands in the transparency region of CdI2CdI2 crystals (360–430 nm) and for the peaks in the mid-infrared absorption spectra which are not present in the spectra of pure cadmium iodide. The nature of this additional optical and infrared absorption is concerned with the bulk and surface plasmons and the surface phonon modes of metallic clusters, respectively.