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CdSe Colloidal Quantum Dots Strained by ZnxCd1-xS or ZnyCd1-ySe Alloyed Shells

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Two series of colloidal quantum dot (cQD) samples were prepared with CdSe cores and ZnxCd1 xS or ZnyCd1 ySe shells varying the relative alloy concentration of Zn : Cd between x(or y) = 0 and x(or y) = 1. Both alloyed shells keep a type I band alignment with the core (though only marginally for ZnyCd1 ySe shells) while their bandgap increases by 1 eV for ZnyCd1 ySe and 2 eV for ZnxCd1 xS as the Zn proportion becomes greater in the shells. The resulting increase in confinement potential keeps the charge carriers more confined in the CdSe core, hence the photoluminescence (PL) from the ground state exciton is less redshifted by the 7 monolayers of shell grown when the Zn content is higher. The whole series of samples thus shows continuous wavelength tuning of visible emitted light over 50 nm with shell composition instead of the usual tuning with cQD size. Further manipulation of the exciton ground state is possible with ZnxCd1 xS shells since the core-shell lattice mismatch in this case increases from 4% to 11% with the Zn : Cd ratio, creating significant strain in the cQDs. This strain perturbs the electronic configuration and a progressive split of the exciton ground state with increasing x values is seen in the excitation spectra. Larger strain may also relax into structural defects providing more non radiative recombination pathways or more charge trapping sites when located at an interface. This changes the PL blinking dynamics: the usual power-law distribution of on and off times is followed when x is small (at least up to the hundreds of milliseconds timescale), but an exponential cutoff appears in the tens of milliseconds range as x increases. ZnyCd1 ySe shells become an interesting compromise to increase the confinement potential while limiting the strain to a maximum of 7% lattice mismatch when y = 1.

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