Biomedical Engineering Reference
In-Depth Information
a
b
2,5
30
35
4
QD PL
25
250K
20
30
15
130K
CuP Phosphorescence
100K
2,0
10
1
210K
5
25
T, K
0
278K
2
100
150
200
250
300
20
5
CuP phosphorescence
1,5
15
130K
250K
278K
100 K
10
210K
1,0
3
5
151.6 K
0
500
550
600
650
700
750
800
850
80
120
160
200
240
280
λ
, nm
T,K
Fig. 4.33
Temperature dependence of emission spectra (
a
) and PL intensities (
b
) for nanocom-
posites based on TOPO-capped CdSe/ZnS QDs (
d
CdSe
=
3.0 nm,
n
ZnS
=
2) and various porphyrin
molecules at the molar ratio
x
=
[
C
porphyrin
]/[
C
QD
]
=
1 in a methylcyclohexane/toluene (6:1)
mixture (
450 nm). (
a
) The short wavelength band belongs to QD PL and the with
temperature strongly increasing long wavelength band belongs to (m-Pyr)
4
-CuP phosphorescence.
The
inset
shows the temperature dependence for CuP phosphorescence bands between 77 and
285 K. (
b
) are shown for the same type of CdSe/ZnS QD (
d
CdSe
λ
=
exc
=
3.0 nm, 2 ZnS monolayers)
but various attached porphyrins: (
1
) alone QDs; (
2
)QD
+
(m-Pyr)
4
-H
2
P; (
3
)QD
+
(m-Pyr)
4
-CuP;
(
4
)QD
tetraphenylporphyrin; (
5
) QD in dried PMMA film on a quartz plate. The structure of
(m-Pyr)
4
-H
2
P molecule is shown in Fig.
4.2
. In a (m-Pyr)
4
-CuP molecule two central protons are
replaced by the Cu ion, in tetraphenylporphyrin molecule four
meso
-pyridyl rings are replaced by
four
meso
-phenyl rings. The phase transition temperature of the solvent mixture is indicated by an
arrow
. All curves are normalzed to the integrated band intensity of QD PL at 285 K. Adapted from
[
75
]
+
CuP exhibits the largest effect, since either internal molecular charge transfer states
influence the energy distribution of CdSe/ZnS QD trap states considerably or TOPO
molecules may be strongly coordinated at low temperatures with the central Cu
ion [
174
,
175
], which might increase the disorder of the surfactant capping shell.
In contrast, tetraphenylporphyrin without
meso
-pyridyl anchoring groups does not
enlarge the QD PL decrease at the “kink” (Fig.
4.33
b, curve 4) because of the
absence of self-assembly in this case. The “kink” is absent for QDs in a dried rigid
PMMA film on a quartz plate (Fig.
4.33
b, curve 5). In the latter case, the film rigidity
suppresses the reorganization of the TOPO capping layer [
80
], thus weakening a
spatially-energeticreordering of QD trap states.
Figure
4.34
shows band-edge PL decays at various detection wavelengths within
the QD PL band for TOPO-capped CdSe/ZnS QDs (
d
CdSe
=
=
3.0 nm,
n
ZnS
2) in
+
=
comparison with those for “QD
1) at 293
and 77 K. At 293 K PL decay for alone QDs is multi-exponential at various
detection wavelengths, and measured
(m-Pyr)
4
-CuP” nanoassemblies (
x
τ
values do not change much upon variation
of detection wavelength (Fig.
4.34
a, curve 1). At 77 K the QD PL decay is non-
exponential in the short wavelength region, but at the PL maximum and in the
long wavelength region the PL decay is mono-exponential exclusively. In the latter
case, the mean value
τ
is monotonically rising upon increasing the wavelength of
detection. It is evidently seen also that in the long wavelength region the
τ
values at
