Internal energy transfer from nanocrystals to Co2+ ions at Bi2S3 tetrahedral sites embedded in host glass

Abstract

We experimentally evidenced the internal energy transfer (IET) in Co-doped Bi2S3 Nanocrystals (NCs) embedded in borosilicate host glass produced by the fusion method. Transmission electron microscopy images showed quantum dots in glass with an average size of 5.0 nm and interplanar distances of around d212 = 0.280 nm, which corresponds to the Bi2S3 semiconductor. Energy dispersive X-ray spectroscopy analysis confirmed the chemical elements Bi, S and Co in the NCs. X-ray diffraction showed the peaks associated with the crystalline planes that are characteristic of the Bi2S3 orthorhombic crystalline structure. The shift to greater angles of the (212) plane with increasing Co-content confirms doping in the orthorhombic unit cell. The changes in the optical absorption (OA) and photoluminescence (PL) spectra are strongly related to the tetrahedral (Th) symmetry of the Co2+ ions within the Bi2S3 NCs. The Racah and crystal field energy parameters confirm the symmetrical environment of the dopant ion. We clearly observe in the PL and OA spectra radiative and non-radiative IET from Bi2S3 NCs to Co2+ Th ion energy states. The valleys and peaks of the emission and absorption bands correspond exactly to the coordinate state of the [CoS4]6- ion. The PL and OA spectra reinforce the internal energy transfer process when analyzing the increased overlapping of the Bi2S3 NCs’ exciton bands and the decrease in stimulated emissions from Co transitions at Th sites.