Biomedical Engineering Reference
In-Depth Information
K
c
as a function of QD surface area (see Fig.
4.24
b), we
obtain a nearly constant ratio between
K
c
and
K
c
(slope of 1.67
When plotting
K
c
+
K
c
/
0.09). It follows
from this observation that the number of sites accessible, e.g. via a reconstruction of
the ligand shell [
48
] is also related to the surface area. Hence there are reasonable
arguments that surface attachment takes place in two steps: (1) direct attachment and
(2) attachment following surface or surface-ligand reorganization. The crossover
between these two routes (around
x
c
) depends on the total QD surface area and
hence also on the QD size.
Finally, the strong difference in PL quenching observed for uncapped CdSe and
core/shell CdSe/ZnS QDs may be explained by a few reasons: (1) the ZnS shell is
not closed and effectively only one monolayer thick or less. This will enhance the
probability of the electron wave function
±
2
of the exciton at the attachment site
and thus PL quenching in relation to the uncapped QD by more than one order
of magnitude [
63
]. (2) The microscopic conditions are different for capped and
uncapped QD. One reason can be the formation of different (induced) surface states
[
118
]. Note that in the range of
x
ψ
1-1000 pyridine molecules do not quench the
PL in case of CdSe/ZnS QDs (see Fig.
4.16
) but very effectively in case of uncapped
CdSe QDs. This fact might indicate that even the wave function of the hole has be
considered in case of uncapped QD since pyridine is a strong hole acceptor [
39
,
44
,
105
,
106
,
156
]. (3) The binding strength of pyridine with Zn
2
+
or Cd
2
+
and thus the
attachment probability for H
2
P might be different for capped and uncapped QDs.
Concluding the results presented here show that it is possible to probe the
interface dynamics of QDs with dye molecules in solution even from ensemble
experiments, observing that a simplified picture of a spherical QD does not fully
explain the details. This is intriguing in spite of the clear correlation of the initial
quenching for small
x
with the amplitude of the exciton wave function at the QD
surface.
=
4.4.1.4
Ligand Exchange: Rates, Dynamics, Geometrical Considerations
As has been shown previously in several papers by our group [
74
] and others
[
30
-
34
,
106
,
157
-
160
] ligand dynamics play an essential role with respect to optical
properties of QDs and have considerable impact on the QD quantum yield. In
case of uncapped CdSe QDs (
d
CdSe
=
0.01 s
−
1
3.5 nm) detachment rates
k
det
=
and
0.5 M
−
1
s
−
1
=
attachment rates
k
att
have been determined for octylamine ligands
=
(OA) at concentrations [OA]
0.01-0.1 M. The corresponding equilibrium constant
K
L
k
det
for the ligands is about 50 M
−
1
. Compared to that, not only slower
[
159
,
160
] but also faster time scales have been reported [
31
]. According to these
results, ligands experience a fast exchange between surface attached ones and those
in the solvent. In case of octylamine on CdSe QDs, half of the ligand shell is
exchanged within 70 s [
30
].
In our experiments at extremely low concentration of CdSe/ZnS QDs capped
with amine ligands, a decrease of the PL is observed with a decay time of about
=
/
k
att
