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Figure 5.3
Molecular models (a and b) and
'
particle in a box
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models (c and d) of
core and core/shell electronic structures, showing the narrowing of
the bandgap upon addition of a shell. Reprinted with permission
from X. Peng, M. C. Schlamp, A. V. Kadanavich and A. P. Alivisatos,
J. Am. Chem. Soc.
, 1997, 119, 7019. Copyright 1997 American Chemical
Society.
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mixed to form a new HOMO
LUMO gap. A more detailed explanation was
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provided by the
model (Figure 5.3c and d) using the
potentials (solid lines) and the actual hole and electron energy levels (dashed
lines). In the example shown, the potentials were estimated from bulk elec-
tron a
particle in a box
nities and ionisation potentials, showing a type I heterostructure.
However, calculations where the conduction band potential o
set between
CdSe and CdS was altered to give a gap of
0.3 eV (rather than +0.3 eV), giving
a type II, structure demonstrated the hole and electron wave functions were
insensitive to small changes in energetic barriers at the interface, suggesting
that the di
erence between type I and type II heterojunctions in this case may
be negligible.
The slight decrease in bandgap is accompanied by an increase in quantum
yield, as high as 84% for 3 nm diameter core CdSe particles with 1.8
monolayers of CdS. There are reports of core/shell particles with quantum
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