Structure–Property Relationship between Copper Ions and Hot Electron Relaxation in Colloidal Quantum Dots

The structure and physical properties of materials, such as elementary exciton processes, are closely interconnected. Doping heteroatoms into a host crystal enables modulation of the physical properties without altering the intrinsic electronic structure. Doping copper ions in colloidal semiconductor quantum dots (QDs) has been shown to suppress hot electron relaxation. However, the relationship between the local environment of the heteroatoms in the host crystal and the physical properties has been insufficiently explored. In this study, CdSe and InP QDs were synthesized in the presence of copper ions (Cu:CdSe and Cu:InP QDs, respectively). Femtosecond transient absorption spectroscopy was performed to investigate the hot electron relaxation dynamics, and X-ray absorption fine structure (XAFS) measurements were performed to elucidate the local environment of the copper ions. Hot electron relaxation was slower in the Cu:CdSe QDs than in the CdSe QDs, whereas no such trend was observed for the Cu:InP QDs. XAFS analyses indicated that the copper ions substituted for cadmium ions in the CdSe QDs, whereas the copper ions segregated on the surface of the InP QDs. This behavior can be explained by the solid solubility of the respective materials: CdSe and Cu2Se are miscible, facilitating substitution, whereas InP and Cu3P are immiscible, resulting in the formation of a heterostructure. The combination of time-resolved spectroscopy and XAFS measurements is an effective method for elucidating structure–property relationships, and it will aid in comprehensively understanding the effect of heteroatoms in the host crystal on the physical properties.