Spectroscopic study of Semiconductor Quantum Dots in Glass Matrix: CdSxSe1-x glasses

Besides their use as optical cut-off filters and some recent applications as non-linear optical devices and photonic dots (micrometer-sized glass spheres), the CdSxSe1-x quantum dots started to be widely used in the production of stained glass windows from about the 1920 thanks to their bright colors. When semiconductor particles are reduced in scale to nanometer dimension, their optical and electro-optical properties strongly differ from those of bulk crystals of the same composition. Since sampling is often not allowed concerning Cultural Heritage artefacts, the potentialities of two noninvasive techniques, such as Raman and Fiber Optic Reflectance Spectroscopy (FORS), have been investigated and the results of the analysis on some original glasses of different colours (from yellow to orange and deep red) and periods (from the second decade of the 20th century to present days) are reported in the present study. Raman spectroscopy allows a fast and non-destructive measure of the quantum dots composition and size, thanks to the evaluation of the frequencies and the broadening/asymmetry of the LO phonons bands respectively [1, 2], though the important role of the compressive strain arising from the glass matrix and the possible diffusion of zinc from the matrix to the nanocrystals should be taken into account when considering the optical-phonons frequency values. The incorporation of Zn has been assumed by an upward shifting of the LO band related to the most abundant anion (S or Se), while the role of the surface phonons as well as the confinement-induced scattering by phonons with a non-zero wavevectors on the Raman peaks broadening [3, 4] has been verified. The optical band gap varies from 2.42 eV (pure CdS) to 1.70 eV (CdSe). For the compositional range between 0.5<=x<=0.2, the presence of two absorption edges has been related to the contribution of both pure CdS and the CdSxSe1-x solid solution; this particular feature is probably due to the presence of unaltered cubic zinc blende structures of CdS that is not taking part to the formation of the solid solution occurring only between hexagonal CdS and CdSe. Moreover, as reported in previous works [4], the band edge tailing originating from the disorder due to the formation of weak bonds (corresponding to a deviation from the ideal bond length and angle) showed an exponentiallike dependence on the phonon energy. This subgap absorption is characterized by the Urbach edge energy and, together with the FWHM of the Raman signal, has been assumed as a good parameter to evaluate the degree of topological disorder.