Journal of the American Chemical Society
Article
Although the CdTe−CdS lattice mismatch is large (≈11%), the
success achieved likely results from the monolayer thinness of
the shells, and the low temperature at which the oxidative-
substitution process forming the shells was conducted.
Hollingsworth and co-workers have previously shown that
shell formation by surface-cation exchange produces large
larger surface areas than do quantum dots and rods. Thus,
achieving effective surface passivation for wires should be more
challenging, and has proven to be so. We have now shown that
the growth of a Type-I interface by a substitutive monolayer
shell formation achieves such effective passivation. New
challenges are to extend this approach to other QW systems,
and to determine if the core−shell interfaces, and thus the
optimized optical properties, remain stable over time.
36
enhancements of PL QE in PbSe quantum dots.
To our knowledge, PL QEs in the range of 15−25% are the
highest that have been achieved on an ensemble of colloidal
QWs under low excitation fluence. We previously reported
whole-wire blinking in a small fraction of single CdSe QWs in
ASSOCIATED CONTENT
Supporting Information
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sample having low ensemble PL QEs of ≈0.2%. The PL QEs
Methods for optical spectroscopy, isolation and purification of
QWs, TEM and HRTEM analyses, and mass spectrometry, and
methods to establish the step-2 photochemical process and to
in these single CdSe QWs in their bright, on states were as high
as 20%. However, these bright states were achieved only under
2
high excitation fluences in the range of 7−100 W/cm . We
argued that after the surface traps on the CdSe QWs were filled
with photogenerated carriers or excitons, radiative recombina-
tion of subsequently generated excitons was enabled,
accounting for the high single-wire PL QEs under high
AUTHOR INFORMATION
37
excitation power densities. Under low excitation power
densities, the surface traps remained largely empty, and
interaction of excitons with these traps made nonradiative
recombination the dominant exciton-relaxation pathway. Kuno
and co-workers have also reported that the PL QEs in single
colloidal QWs increase under increasing excitation intensities of
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
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up to 3000 W/cm , reaching values as high as 5−20%.
This work was supported by NSF (CHE-1012898 for W.E.B.
and DMR-0906966 for R.A.L.). Mass spectrometry was
conducted by Ms. Jing Li and Dr. Weidong Cui of the
Washington University Mass Spectrometry Resource, an NIH
Research Resource (Grant No. P41RR0954).
In contrast, the excitation fluences in the ensemble
measurements reported here were only 0.3 mW/cm , which
2
are 4−5 orders of magnitude lower than those employed in the
single-QW studies discussed above. Consequently, the high PL
QEs reported here are not achieved by the temporal
photofilling of surface traps. Instead, they are achieved from
the elimination of surface traps by the type-I core−shell
interface.
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