Biomedical Engineering Reference
In-Depth Information
Typically, the PL blue shift and intensity rise upon temperature lowering is a
commonly observed phenomenon for QDs in colloidal suspensions or in solvent-
free systems (polymeric matrices or QD solids). Generally it is explained by
reduction of thermally activated carrier trapping [
167
,
168
] and/or a decrease
of electron-phonon coupling [
10
,
80
,
165
,
168
,
169
]. Unusual “luminescence
anti-quenching” at a well-defined temperature of
T
250 K (manifested in our
experiments as “kink”) has been observed for CdTe or CdSe/ZnS QDs in toluene,
and has been related to a phase transition in the surfactant capping layer [
80
]. This
effect was connected with surface relaxation and/or surface reconstruction which
was strongly dependent on the type of capping ligands (TOPO or aminoethanethiol)
and, in turn, may cause a spatially-energetic reordering of trap states. One should
note that the existence of the “kink” has up to now only been reported with respect
to the PL properties of QDs while we detected it also in absorption spectra at
temperatures close to
∼
218 K [
75
].
When discussing the appearance of the “kink,” one has to take into account
that for TOPO- or amine-capped CdSe/ZnS QDs, a phase transition of the capping
ligand layer at low
T
may cause some strain-induced deformation of the ZnS shell.
Correspondingly, due to different thermal expansion coefficients for the semi-rigid
capping layer and the ZnS shell as well as possible strain effects, slight changes of
electron delocalization [
148
,
171
] may take place in the range of the phase transition.
In this respect, we believe that slightly different “kink” temperatures for the first
excitonic absorption band (
∼
223 K) for TOPO-capped
CdSe/ZnS QDs may be explained by the different nature of the corresponding
transitions in absorption (allowed excitonic ones [
10
-
12
]) and in emission (trap
dominated ones at low
T
).
Thus, if trap state emission dominates at low temperatures, these trap states
or their distribution might be influenced by dye molecules attached to the QD
surface due to replacing some of the surfactant (TOPO or AM) molecules.
Figure
4.33
shows the results of the corresponding temperature experiments
for “QD-porphyrin” nanoassemblies based on TOPO-capped CdSe/ZnS QD
with fixed size and various porphyrin molecules at a relative molar ratio of
x
∼
218 K) and the PL band (
∼
1.
It is seen from Fig.
4.33
a that in the case of (m-Pyr)
4
-CuP being attached to QD,
upon temperature lowering a long wavelength emission band at
=
[
C
porphyrin
]/[
C
QD
]
=
770-780 nm
appears. This emission belongs to the phosphorescence of (m-Pyr)
4
-CuP [
172
and
references herein]. The intensity of this phosphorescence is continuously increasing
upon temperature lowering (Fig.
4.33
a, inset) which is typical for Cu-containing
porphyrins [
172
,
173
]. Thus, (m-Pyr)
4
-CuP molecules may be considered as an
inner standard showing that the surrounding solvent matrix does not change
upon Temperature dependent PL intensities temperature lowering. Under the same
conditions, the PL intensity of QDs in “QD-CuP” nanoassemblies shows again
a “kink” (Fig.
4.33
b, curve 3), which, however, is now much more pronounced
(
λ
max
=
23%) than that for alone QDs (Fig.
4.33
b, curve 1). From Fig.
4.33
b it is evident
that dye attachment enhances the PL decrease at the phase transition temperature,
and the amplification is strongest for (m-Pyr)
4
-CuP molecules. We suggest that
−
