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Tuning Equilibrium Compositions in Colloidal Cd1–xMnxSe Nanocrystals Using Diffusion Doping and Cation Exchange

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journal contribution
posted on 26.01.2016 by Charles J. Barrows, Pradip Chakraborty, Lindsey M. Kornowske, Daniel R. Gamelin
The physical properties of semiconductor nanocrystals can be tuned dramatically via composition control. Here, we report a detailed investigation of the synthesis of high-quality colloidal Cd1–xMnxSe nanocrystals by diffusion doping of preformed CdSe nanocrystals. Until recently, Cd1–xMnxSe nanocrystals proved elusive because of kinetic incompatibilities between Mn2+ and Cd2+ chemistries. Diffusion doping allows Cd1–xMnxSe nanocrystals to be prepared under thermodynamic rather than kinetic control, allowing access to broader composition ranges. We now investigate this chemistry as a model system for understanding the characteristics of nanocrystal diffusion doping more deeply. From the present work, a Se2–-limited reaction regime is identified, in which Mn2+ diffusion into CdSe nanocrystals is gated by added Se2–, and equilibrium compositions are proportional to the amount of added Se2–. At large added Se2– concentrations, a solubility-limited regime is also identified, in which x = xmax = ∼0.31, independent of the amount of added Se2–. We further demonstrate that Mn2+ in-diffusion can be reversed by cation exchange with Cd2+ under exactly the same reaction conditions, purifying Cd1–xMnxSe nanocrystals back to CdSe nanocrystals with fine tunability. These chemistries offer exceptional composition control in Cd1–xMnxSe NCs, providing opportunities for fundamental studies of impurity diffusion in nanocrystals and for development of compositionally tuned nanocrystals with diverse applications ranging from solar energy conversion to spin-based photonics.