CdSe-Dot/CdS-Rod/PbS-Dot Nanocrystals by Partial Cation Exchange Reaction

Dual-emissive nanorods with fluorescence in both the visible and infrared range are prepared by a combination of a CdSe-nanocrystal-seeded growth of CdS nanorods and a successive partial Cd-to-Pb cation exchange. We show that the exchange reaction, which involves Pb halides in oleylamine, starts at the tip of the rods, leading to the formation of CdSe-dot/CdS-rod/PbS-dot nanocrystals (DRDs). Besides these DRDs, the reaction product also contains shorter nanorods and spherical quantum dots. Their fraction strongly depends on the amount of lead halide precursor and the reaction time. The reaction mechanism is investigated in detail, such that by carefully adjusting the reaction conditions, it is possible to synthesize DRDs of distinct PbS-dot sizes with yields of over 95%. The resulting DRDs are crystalline and show a CdSe-fluorescence band in the visible range at 600 nm and also a fluorescence band in the near-infrared at 1440 nm, resulting from the PbS part of the rods. The dual emission is confirmed by single-DRD spectroscopy. For that, scanning fluorescence and transmission electron microscopy imaging are correlated, and time-resolved single-photon counting experiments simultaneously in the near-infrared and visible range are performed.