First principles study of the optical emission of cadmium yellow: Role of cadmium vacancies

We study the role of structural defects in the CdS-based cadmium yellow paint to explain the origin of its deep trap states optical emission. To this end, we combine a first principles study of Cd- and S- vacancies in the wurtzite (1010) CdS surface with experimental photoluminescence spectroscopy of the commercial hexagonal CdS pigment. Computational results clearly state that the presence of cadmium vacancies in the pigment surface alters the electronic structure of cadmium sulfide by forming acceptor levels in the gap of the semiconductor. Such levels are consistent with the optical emission from trap state levels detected in the CdS pigment. This finding provides a first step towards the understanding of the photo-physical mechanisms behind the degradation of this modern pigment, widely used in impressionist and modernist paintings.