Using
CdSe/ZnSe core–shell quantum dots (QDs) as a model,
we systematically investigate the photochemical properties of QDs
with the ZnSe shells under an ambient environment, which show almost
opposite responses to either oxygen or water in comparison with CdSe/CdS
core/shell QDs. While the ZnSe shells provide an efficient potential
barrier for photoinduced electron transfer from the core to the surface-adsorbed
oxygen, they also act as a stepping stone for hot-electron transfer
directly from the ZnSe shells to oxygen. The latter process is so
effective and competes favorably with ultrafast relaxation of hot
electrons from the ZnSe shells to the core QDs, which can completely
quench the photoluminescence (PL) with saturated adsorption of oxygen
(1 bar) and initiate oxidation of the surface anion sites. Water can
slowly eliminate the excess hole to neutralize the positively charged
QDs, partially canceling the photochemical effects of oxygen. Alkylphosphinesthrough
two distinctive reaction pathways with oxygenstop the photochemical
effects of oxygen and completely recover PL. With limited thickness
(around two monolayers), the ZnS outer shells substantially slow down
photochemical effects on CdSe/ZnSe/ZnS core/shell/shell QDs but cannot
fully stop PL quenching by oxygen.
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