Biomedical Engineering Reference
In-Depth Information
a
b
2,5
Absorption
1
2,35
2,0
1
2
2,30
1,5
2
Photoluminescence
λ
exc
= 450 nm
2,25
1,0
80
120
160
200
240
280
320
80
120
160
200
240
280
T, K
T, K
Fig. 4.32
Temperature dependence of absorption and PL properties for alone QDs and “QD-
Dye” nanocomposites in methylcyclohexane/toluene (6:1) mixture. (
a
) Temperature dependence
of the first absorption peak (
1
, mean weighted data for two Gaussians lines) and PL maximum
(
2
,
2). The
dashed
ellipse
indicates the temperature range at which optical properties change abruptly. (
b
) Temperature
dependence of the normalized PL maximum intensity (
λ
=
450 nm) for TOPO capped CdSe/ZnS QDs (
d
CdSe
=
3.0 nm,
n
ZnS
=
exc
λ
=
450 nm) for CdSe/ZnS-AM QDs
exc
(
d
CdSe
3.0 nm, 2 ZnS monolayers) capped by long-chain amines (
1
) and for nanoassemblies
based on CdSe/ZnS-AM QDs and DPP molecules at molar ratio
x
=
1(
2
).
Thick
dashed arrows
on the curves 1 and 2 indicate temperatures at which optical properties change
abruptly. Adapted from [
75
]
=
[
C
DPP
]/[
C
QD
]
=
takes place presumably from only lower lying states belonging to surface traps
presumably.
Fitting the lowest absorption and PL band positions by Gaussian components
shows that besides the above-mentioned blue shift upon temperature lowering a
non-monotonous temperature dependence of the respective peak energies becomes
evident (Fig.
4.32
).
It is seen in Fig.
4.32
a that a non-monotonous behavior (which we name for
simplicity “kink”) is observed for TOPO-capped CdSe/ZnS QDs between 200 and
240 K, far from the glass transition temperatures for methylcyclohexane (146.7 K)
and toluene (180 K), respectively, as well as for a methylcyclohexane/toluene (6:1)
mixture (151.6 K) [
166
]. Upon lowering T the “kink” occurs at slightly different
temperatures for the first excitonic absorption band (
∼
218 K) and the PL band
(
223 K), respectively. Interestingly, that a similar “kink” is observed in the tem-
perature dependence of the PL intensity for AM-capped CdSe/ZnS QDs (Fig.
4.32
b,
curve 1). In the latter case, the “kink” in PL intensity is more pronounced and
is shifted to
∼
223 K for TOPO-capped CdSe/ZnS
QDs. Notably, this non-monotonous behavior becomes stronger for amine-capped
CdSe/ZnS QDs with attached DPP molecules at molar ratio
x
∼
237 K as compared to
∼
1,
while no “kink” is observed when following the luminescence of perylene diimide
or porphyrin molecules under the same conditions (will be presented and discussed
below). From these observations, we therefore exclude that the morphology of the
solvent matrix is changing.
=
[
C
DPP
]/[
C
QD
]
=
