Biomedical Engineering Reference
In-Depth Information
mechanisms of PL quenching dynamics for “QD-Dye” nanoassemblies. Knowledge
of the ligand dynamics and surface functionalization can play an important role
in various technological fields, e.g. for the fabrication of nanostructured inks for
solution-processed photovoltaics [
76
] or printed semiconductor layers in flexible
electronics [
77
].
As far as the quantum efficiency and PL dynamics for QDs depend drastically on
the environment it is a demanding task to control the environment in a systematic
and well-defined way, e.g. by changing the temperature in a controlled manner and
observing the corresponding influence on PL for, at least, individual QDs, single
“QD-Dye” nanoassemblies and bulk solutions. From this point of view, experiments
in a wide temperature range (300-2 K) are of urgent interest because of two reasons:
(1) temperature- and size-dependence of the exciton decay of individual QDs
provides a direct way to characterize the QD energy band gap [
33
,
68
,
78
-
80
], as
well as the elucidation of the specificity of the interactions between QDs and organic
ligands and (2) the phase transition (or reconstruction) of the capping ligand shell
[
81
,
82
] or rearrangement of organic ligands upon temperature changes may affect
the PL behavior of QDs. With respect to the band gap even for pure QDs reported
results and interpretations are, however, not fully consistent with each other, while
for “QD-Dye” nanoassemblies the corresponding data are even missing. The crucial
role of the solvent and ligands on PL properties upon temperature variation has also
been demonstrated for individual QDs [
80
,
81
]. Nevertheless, there are no studies
on temperature effects in “QD-Dye” nanoassemblies. In the last case, the controlled
use of dyes, which quench the PL of the QD, is a useful tool to follow indirectly the
thermodynamics of ligand-QD interactions at various temperatures. In addition, due
to the simultaneous observation and analysis of PL quenching effects, which serve
as an indicator for the formation of an “QD-Dye” nanoassembly, there is a direct
access to surface-related processes as well as to the elucidation of PL quenching
mechanisms [
83
]. Interestingly, recent in situ diffraction data show that the TOPO-
or hexadecylamine-capped CdSe QDs in toluene exhibit predominantly wurtzite
crystal structure, which undergoes a phase transformation to zinc blende crystal
structure following drop-casting on Si, thus showing environment-induced phase
transformation of even the CdSe core [
84
].
In most cases the formation of “QD-Dye” nanoassemblies is followed by QD PL
quenching, which is studied both in bulk solutions and on a single particle detection
level. Commonly, this PL quenching is interpreted as being due to photoinduced
charge transfer (CT) [
45
,
85
-
88
] and/or energy transfer processes QD
Dye [
21
,
36
,
38
,
46
,
60
,
62
,
74
,
89
-
95
]. To date, though for lot of systems ample qualitative
evidence for the presence of such quenching processes is given, only a limited
number of papers unravel quantitatively whether the PL quenching (full or in some
cases partly, at least) can uniquely be assigned to CT [
59
,
96
] or Foerster resonance
energy transfer (FRET) for bulk solutions [
60
,
62
,
90
,
91
,
97
-
99
] and for single
“QD-Dye” nanoassemblies [
74
,
94
,
100
]. On the other hand, PL quenching may
be induced by other non-FRET processes [
36
,
96
,
101
-
103
] and be related to the
involvement of QD surface states [
23
,
56
,
58
,
83
] or photoinduced self-trapping of
charges in the dielectric medium of the environment of QDs [
67
,
104
,
105
,
124
].
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