Biomedical Engineering Reference
In-Depth Information
Perfect QD
ZnS
Non-perfect QD + Dye
Non-perfect QD
Cd Se
Dye
ZnS
ZnS
Cd Se
3
Cd Se
3
1
1
2
2
CB
CB
CB
TS 2
TS 2
TS -Dye
TS 3
TS 3
TS 1
TS 1
VB
VB
VB
Scheme 4.3
Schematic presentation of a perfect TOPO-capped CdSe/ZnS QD, a non-perfect QD
and a “QD-Dye” nanoassembly as well as arbitrary excitonic and trap states (TS) and main
relaxation pathways. For perfect CdSe/ZnS QDs, calculated probability density functions
2
(
r
)
r
2
for the electron e
−
and hole h
+
are also presented. For the nanoassembly an induced ligand
detachment is shown by
broken arrows
.
1
is a hole trap,
2
and
3
are electron traps
Ψ
An alternative but qualitatively similar approach of dye-induced non-FRET can
be related to a model recently proposed by Salman et al. [
178
]. In this paper the
authors suggest that for almost all QDs observed non-exponential PL decay might
be related to the presence of a permanent electric dipole moment in the electronic
ground state. Such a dipole moment will mix the upper two allowed states within
the electronic fine structure of the lowest excited QD state. They argue that such a
mixing will result in an increase of non-radiative transitions. It is very evident that
the surface attachment of a dye molecule will increase or at least change the dipole
moment and thus the mixing of excitonic states followed by an increase of non-
radiative transitions and will thus give rise to PL quenching. Clearly, the magnitude
of such an electric dipole moment will strongly depend on dielectric properties of
the solvent as we have observed experimentally. Some of our experimental findings
