Strong Coupling between Mn2+ Dopants and CdSe Nanoplatelets Enables Charge-Transfer Transition and Dual Emission

Doping transitional metals into colloidal nanocrystals can significantly modify their excited-state dynamics and enrich their optical and magneto-optical functionalities. Here we synthesize Mn-doped CdSe nanoplatelets and investigate their excited-state dynamics and light-emission mechanisms. Extensive characterizations suggest that Mn²⁺ ions are situated near the surface-region of the nanoplatelets. The atomic thinness of nanoplatelets allows for a strong host-dopant coupling, manifested as broadband charge-transfer absorption and emission (near 575 nm) between the host valence band and the dopant d-orbitals. Photoexcitation of the host leads to rapid (a few ps) electron transfer from the conduction band to the d-orbitals, and the resultant charge-transfer state decays within a few ns not only through charge-transfer emission but also generating an excited-state species (likely Mn-Mn dimer) with a characteristic near-infrared emission. These novel photophysics and photochemistry uncovered for quasi-two-dimensional Mn-doped nanocrystals form the basis for optical, magneto-optical, and energy conversion applications using such materials.