The cadmium yellow paints used in impressionist and modernist paintings in early 1900s are undergoing several deterioration processes, including whitening and discoloration. The origins of such chemical and physical alterations are still under debate. The presence of structural defects may play a role in the degradation process. In general, their presence in the pigment alters the electronic structure by creating shallow and/or deep levels in the gap. When, particularly, the surface contains such defects we notice an enhancement of the reactivity in terms of adsorption of external agents (oxygen, water …).
We present a combination of theoretical ab initio simulations and photoluminescence spectroscopy on commercial CdS samples to interpret the presence of observed deep levels in the gap and a preliminary study of the influence of impurities in the complex mechanism of cadmium yellow deterioration.
PL spectroscopy has been performed on hexagonal CdS-based pigment by employing a compact spectrometer and UV-laser excitation focussed in a circular spot of 1 mm in diameter on the sample surface. All the calculations are performed within the framework of the Density Functional Theory (DFT) in the Generalized Gradient Approximation (GGA-PBE) with the use of ultrasoft pseudopotentials.