Davydov splitting in cadmium vacancy emission, ferromagnetism and photosensitivity in manganese incorporated CdS nanocrystals

Abstract

Nanocrystals of CdS/Mn²⁺ synthesized using sodium lauryl sulphate (SLS) surfactant and two ligands, i.e. butylenediamine (BD) and propylenediamine (PD), of differing lengths show a dominant presence of the hexagonal phase. The increasing [Mn²⁺] in the CdS/Mn²⁺ crystals from BD medium leads to a red shift in the band gap absorption with increasing intensity while the optical properties are reversed in CdS/PD crystals. Such changes can be explained by the switching of the randomly blinking nanocrystals into coherently emitting nanocrystals. The photoluminescence of CdS/Mn²⁺ materials with similarities and differences between SLS/BD and SLS/PD media exhibits two emissions: at 2.25 eV due to cadmium ion vacancies exhibiting Davydov-type splitting from neighbouring Cd²⁺–Cd²⁺ vacancies, and at 2.18 eV from manganese d–d emission. Thus, addition of Mn²⁺ leads to a decrease in the intensity of vacancy emission and increase in d–d emission due to occupation of cadmium vacant sites by manganese. Interestingly, SLS/PD medium creates more defects in CdS nanocrystals than SLS/BD medium. Note that the CdS/Mn²⁺ nanocrystals made from SLS/BD medium exhibit weak ferromagnetism in pre-annealed samples, becoming stronger after annealing, while the preannealed samples from SLS/PD origin have the simultaneous presence of both superparamagnetism and ferromagnetism, which turns to complete ferromagnetism on annealing. An attempt has also been made to compare the various properties of nanomaterials made as a function of two surfactants and three diamines of differing lengths. These nanoparticles are in the size range of 10 to 15 nm. A simple photo-resistance experiment on CdS and carbon nanoparticles-based heterostructure reveals its photosensitivity.