Biomedical Engineering Reference
In-Depth Information
Fig. 4.35
(
a
) PL of AM-capped CdSe/ZnS QDs in toluene at ambient temperature. The PL band
has been fitted by two Gaussian components SW and LW shown by
broken lines
.(
b
) Variation of
PL emission wavelength for SW and LW as function of observation time. Adapted from [
75
]
to QDs with TOPO ligands the PL spectrum has been fitted with two Gaussian
components, namely “short” (SW) and “long” (LW) wavelength components,
respectively (Fig.
4.35
a). After sample preparation both the PL intensity and band
shape/width change as a function of waiting time. The intensities decrease by about
a factor 4 within 10 min and stay nearly constant thereafter. During the total time
of several hours the integrated intensity ratio remains constant within experimental
error. However, the effective PL wavelengths of the two Gaussian bands change
differently as can be seen in Fig.
4.35
b. While the LW band shifts systematically
to the blue on a time scale of several hours, the SW band remains nearly constant.
We tentatively assign the SW band to the near-band edge PL (excitonic by nature),
while we assign the LW band to a broad distribution of trap states. Obviously the
center position of the trap state distribution is shifted to the blue during waiting time.
Specifically, this implies that traps with the lowest PL energy are quenched or the
traps are eliminated more effectively than the excitonic-type states.
While for alone QDs the spectral shifts for SW and LW components are
extremely small (1-2 nm) during observation time, they are enlarged following
titration by PDI-type molecules PP (see the dye structure in Fig.
4.3
). It is seen from
Fig.
4.36
that for
λ
LW
.In
both cases the shift becomes larger with increasing PP concentration. At molar ratio
x
λ
SW
the shift is at most 4 nm but increases up to 12 nm for
5 the experimental error becomes large since QD PL is already considerably
quenched. The time scale of the respective shift becomes systematically shorter
with increasing
x
. These effects reflect the local influence of attached dye molecules
on excitonic and trap states and can be understood in view of recent findings of
assembly formation as a function of concentration [
64
,
74
]. These observations
indicate that assembly formation goes in line with increased bleaching (oxidation) of
the long-wavelength component LW. Basically, also titration experiments show that
trap states emitting in the red are effectively bleached during observation time [
74
].
As was described in Sect.
4.2.2.2
the detection of the spectral properties of single
QDs and “QD-Dye” nanoassemblies shows also blue shifts of the PL bands during
observation time (see time traces of the PL intensity in the bottom of Fig.
4.13
a,
=
