In this paper we critically examine
the literature and provide
new data on fundamental optical properties of II–VI quantum
dots (QDs). Specifically, we examine the integrated extinction coefficients
and radiative lifetimes of different sizes of CdSe and CdTe QDs and
different shell thicknesses in CdTe/CdSe core/shell QDs. We have synthesized
particles having very high quantum yields and find that the measured
radiative lifetimes are considerably longer and have a very different
size dependence than what has been previously reported. In a simple
two-level system the integrated extinction coefficients (or oscillator
strengths) are related to the radiative lifetimes through the Einstein
relations. The situation is more complicated in the case of II–VI
QDs because of the thermal accessibility of dark states resulting
from the valence band fine structure. There are significant but not
equal populations in both bright and dark sublevels of the 1Se-1S3/2 exciton and in the dark 1Se-1P3/2 exciton. These Boltzmann populations depend on the QD size
and shape. We find that in all three cases, quantitative or semiquantitative
agreement between the measured radiative lifetimes and values calculated
from the integrated extinction coefficients is obtained only if Boltzmann
populations in all of the thermally accessible bright and dark states
are considered. We also find that the shell thickness dependence of
the radiative lifetimes of the CdTe/CdSe core/shell particles can
be quantitatively understood in terms of overlap of calculated electron
and hole wave functions. The results and analyses presented here clarify
several discrepancies in the literature.
One-photon excitation
of CdSe/CdS quantum dots (QDs) in room-temperature chloroform results
in a delayed and reversible photodarkening of the sample. There is
little or no prompt loss of photoluminescence intensity and the subsequent
photodarkening takes place on the tens of minutes time scale. The
photodarkening kinetics have been studied as a function of the solvent,
particle ligation, concentration, and shell thickness. The results
indicate that the photochemistry is the result of a “hot”
hole being transferred to a surface ligand, followed by dissociation
to a QD–/L+ ion pair. The hot hole is
generated by a trion Auger process through a surface charging mechanism.
In this mechanism, the presence of unpassivated chalcogenide surface
atoms in II–VI semiconductor QDs results in electrons that
are in an equilibrium between the valence band and chalcogenide surface
orbitals, that is, a surface cadmium vacancy has resulted in a p-type
QD. The latter state corresponds to a surface-charged QD. Photoexcitation
of a surface-charged QD produces a trion, which can undergo Auger
recombination to produce an unrelaxed hole. Following QD to ligand
hole transfer and ligand dissociation, the positively charged ligand
can react with another QD in solution causing photodarkening. Charge
recombination occurs on a slower time scale and reverses the loss
of photoluminescence intensity.
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