Biomedical Engineering Reference
In-Depth Information
4.5
Influence of Single Dye Molecules on Temperature
and Time-Dependent Optical Properties of QDs
in Nanoassemblies
In previous sections we presented a microscopic description of QD PL quenching
upon formation of nanoassemblies at ambient temperature which takes place in
competition with capping ligand (surfactant) dynamics and is accompanied by the
reorganization of the capping ligand shell. As we mentioned above PL quantum
yield and PL dynamics of QDs and “QD-Dye” nanoassemblies depend drastically
on the dielectric properties of the embedding environment [
62
,
64
,
67
,
106
,
140
]. In
addition, the properties of these systems are sensitive to changes in temperature [
2
,
31
,
78
-
80
,
82
,
83
]. As a part of the detailed analysis of self-assembly processes in
“QD-Dye” nanoassemblies, this section is devoted to the analysis of the influence of
variations of the surface structure such as a ligand-induced phase transitions at low
temperatures as well as photodegradiation of QD optical properties and how they
are related to nanoassembly formation. Our results are related to temperature (77-
300 K)-dependent optical properties of “QD-porphyrin” nanoassemblies as well as
to long-term temporal evolution of PL spectra (photodegradation) of QDs assembled
with PDI molecules. In case of “QD-porphyrin” nanoassemblies we will report on
the temperature-dependent formation of CdSe/ZnS trap states in competition to QD
near band edge states, while the temporal variation of trap and near band edge states
will be presented for “QD-PDI” nanoassemblies.
4.5.1
Ligand-Related Phase Transitions in QD-Dye
Nanoassemblies
Experiments on alone of CdSe/ZnS QDs (
d
CdSe
=
2) and “QD-
porphyrin” nanoassemblies with participation of (m-Pyr)
4
-H
2
P molecules have been
performed in a methylcyclohexane/toluene (6:1) mixture in a temperature range
from 77 to 300 K. These experiments have shown [
65
,
75
] that upon temperature
lowering, the band-edge absorption (maximum of the first excitonic transition) for
alone QDs is blue shifting which is typical for these QDs [
165
]. The temperature
dependence of the PL peak energy behaves similar as the band-edge absorption,
that is a blue shift upon temperature lowering is observed. The analysis of QD
PL spectra dependence on the excitation wavelength at 295 K reveals that in
addition to size-dependent site selection (observed for QD PL [
118
]) the presence
of at least two luminescent states is characteristic to the QDs under study. Upon
temperature lowering PL spectra are not only shifted to the blue but also the line
width (FWHM) becomes essentially narrower, while the PL intensity becomes
higher. Based on considerations presented recently [
75
] it may be concluded the
QD PL quantum efficiency increases upon temperature lowering and PL emission
3.0 nm,
n
ZnS
=
