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SUMMARY:CdSe Colloidal Quantum Dots Strained by ZnxCd1-xS or ZnyCd1-ySe Al
 loyed Shells - Claudine Allen (Université Laval\, Quebec\, Canada)
DTSTART:20100423T100000Z
DTEND:20100423T110000Z
UID:TALK23730@talks.cam.ac.uk
CONTACT:6270
DESCRIPTION:Two series of colloidal quantum dot (cQD) samples were prepare
 d 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 a
 lloyed shells keep a type I band alignment with the core (though only marg
 inally for ZnyCd1 ySe shells) while their bandgap increases by ~1 eV for Z
 nyCd1 ySe and ~2 eV for ZnxCd1 xS as the Zn proportion becomes greater in 
 the shells. The resulting increase in confinement potential keeps the char
 ge carriers more confined in the CdSe core\, hence the photoluminescence (
 PL) from the ground state exciton is less redshifted by the 7 monolayers o
 f shell grown when the Zn content is higher. The whole series of samples t
 hus 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 shell
 s 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. Th
 is strain perturbs the electronic configuration and a progressive split of
  the exciton ground state with increasing x values is seen in the excitati
 on spectra. Larger strain may also relax into structural defects providing
  more non radiative recombination pathways or more charge trapping sites w
 hen located at an interface. This changes the PL blinking dynamics: the us
 ual 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 exponent
 ial cutoff appears in the tens of milliseconds range as x increases. ZnyCd
 1 ySe shells become an interesting compromise to increase the confinement 
 potential while limiting the strain to a maximum of 7% lattice mismatch wh
 en y = 1.
LOCATION:Rutherford building\, Seminar Room B
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