Biomedical Engineering Reference
In-Depth Information
To consider the differences in the temperature dependence between QDs and
QDs in nanoassemblies, one has to take into account that nanoassemblies at an
initial molar ratio x
1 were prepared in one step at ambient
temperature. At this temperature, the formation of such self-aggregated assemblies
is thermodynamically characterized by the corresponding complexation constant
[ 101 , 150 ]. Estimations according to a Poisson distribution show that at x
=
[ C CuP ]/[ C QD ]
=
=
1,
the related probabilities P i of the number of dye molecules per QD are P 0 =
0.3,
P 1 =
0.3, respectively. This implies that some QDs are “porphyrin-free”
without PL quenching at ambient temperature. Decrease of temperature might result
in a complete complexation of QDs because of the rise of the complexation constant
at low temperatures [ 177 ]. Correspondingly, this should show up in more effective
QD PL quenching. However, this is experimentally not reflected in an increase of
PL intensity up to the phase transition. We exclude that at the phase transition more
nanoassemblies are formed, since dye molecules become “frozen” in the now rigid
surfactant layer.
Summing up this part we conclude that CdSe/ZnS QDs undergo a phase
transition in the sense that the “freezing” ligand shell exerts strain on the ZnS shell,
thus creating trap states with reduced PL quantum yields. Moreover, this phase
transition changes also the CdSe/ZnS absorption, which might be explained by a
modification of the CdSe core or ZnS layer structure. Notably, it has been shown
recently [ 84 ] that the solvent environment and ligands help not only in limiting
the size of the QDs in solution but also to stabilize the crystal structure of QDs
by minimizing the surface free energy. In solution, the solvent molecules adsorbed
onto the QD surface can reduce the total surface free energy, and the surface can
undergo reconstruction when the solvent molecules leave the QD surface. In turn,
dye attachment creates new and/or more trap states, which obviously quench the PL
very effectively. Phase transition temperature and the influence of dye-induced trap
states depend critically on the type of ligand and are more pronounced for TOPO as
compared to amine ligands. This might be explained by the pyramidal structure of
TOPO, which prohibits for steric reasons a complete passivation of the QD surface
[ 64 , 178 ]. Thus, it follows from the above results and considerations that already
a few attached dye molecules noticeably influence the optical propertiesof QDs at
ligand controlled phase transitions.
0.4, P 2 =
4.5.2
Photostability of QD-Dye Nanoassemblies
As we have shown in Sects. 4.2.2.1 , 4.2.2.2 ,and 4.3.1 QD PL quenching caused
by both non-FRET and FRET is clearly visible in single “QD-Dye” nanoassembly
experiments. Here, we highlight that single functionalized dye molecules can be
considered not only as extremely sensitive probes for the complex interface time-
dependent physics and dynamics of colloidal semiconductor QDs but also partly
control reactions at surfaces.
Before discussing the influence of PDI molecules on the PL for AM-capped
CdSe/ZnS QDs in nanoassemblies, we first describe the bulk PL behavior for
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