Exciton Energy Shifts and Tunable Dopant Emission in Manganese-Doped Two-Dimensional CdS/ZnS Core/Shell Nanoplatelets

The ability to dope transition-metal ions into semiconductor nanocrystals (NCs) allows for the introduction and exploitation of new extrinsic properties in the original intrinsic material. Although the synthesis of doped zero-dimensional quantum dots and one-dimensional nanorods/nanowires has been widely reported, transition-metal ion-doped two-dimensional (2D) NCs have been less explored. In this study, we developed a one-pot synthesis of Mn²⁺-doped 2D CdS (i.e., Mn:CdS) nanoplatelets (NPLs). Successful Mn doping inside the CdS NPL lattice was confirmed by electron paramagnetic resonance and X-ray diffraction measurements. Surprisingly, only CdS photoluminescence (PL), without contribution from Mn PL, was observed in the Mn:CdS NPLs, regardless of Mn doping concentration. To address the issue of poor thermal stability and improve the optical properties of the 2D Mn:CdS NPLs, we synthesized ZnS shell-passivated Mn:CdS/ZnS core/shell NPLs using a single-source shelling precursor method, which allows for ZnS surface passivation of NPLs at relatively low temperatures, while being thermally adaptable to ensure minimal NPL degradation. An extremely large exciton red shift (∼420 meV), upon ZnS shell passivation, was observed because of the increased effective thickness of the CdS core NPLs. Steady-state and time-resolved emission measurements indicate that the host-dopant energy-transfer efficiency and Mn-Mn interactions within the 2D Mn:CdS/ZnS core/shell NPLs can be fine-tuned via the dopant concentration, resulting in an intense Mn PL as well as tunable dual-band emission from the host NPLs and Mn dopants. Magnetic measurements indicate intrinsic spin states in the 2D NPLs and complex magnetic interactions at high doping concentrations, including antiferromagnetic exchange between dopants and possible dopant-surface state interaction.