Transition Metal Doping in CdS Quantum Dots: Diffusion, Magnetism, and Ultrafast Charge Carrier Dynamics

Transition metal (TM) doping in pristine II–VI semiconductor quantum dots (QDs) is known to add several otherwise unavailable properties by introducing midgap states in the host material. Albeit being extensively investigated, the periodicity of the observed properties with respect to the electronic structure has not been attempted so far. In this work, we investigate CdS QDs doped with several different TM ions (Mn, Fe, Co, Ni, and Cu) using extended X-ray absorption fine structure spectroscopy to study dopant-induced structural perturbations and femtosecond transient absorption (TA) spectroscopy to study the ultrafast charge carrier dynamics. This provides solid evidence for the origin of magnetization in doped QDs that has been lacking despite extensive studies. Further, we demonstrate that the ionic radius and the dopant oxidation state play crucial roles in determining the dopant–anion bond lengths. Based on the investigation of the relaxation pathways of excited charge carriers using ultrafast TA spectroscopy, we hypothesize that there exists photoinduced switching between multiple oxidation states in some dopants.