Biomedical Engineering Reference
In-Depth Information
interface defects. In case that these states are subject to non-radiative deactivation
channels or decreased radiative rates, the PL will be reduced. The magnitude of
the quenching will depend on (1) the (strongly distance-dependent) amplitude of
the excitonic wave function at the trap site, (2) the number of trap sites, (3) the
microscopic nature of the site, which might be additionally influenced by the
environment (solvent), and (4) the crystal facets related to the crystal structure.
Any change of the conditions (1)-(4) will result in a change of PL properties. In
this approach non-FRET quenching of QD PL induced by attaching dye molecules
may be caused by (1) a reduced number (dependent on the facette properties) of
ligands and thus an increase of intra-band states (traps), since the dye is generally
larger than the ligand molecules and (2) a change in the electronic properties at
the attachment site since the “anchor” of the dye is different from the ones of the
(replaced) ligands. It follows from (2) that the electronic influence of the dye itself
is less important as compared to the one of the anchor groups. The general features
are shown in Scheme
4.3
with respect to the electronic properties of the QD.
Summariing, non-FRET quenching of QD PL induced by dye attachment (or
incorporation in the ligand shell) can be explained by the presence of a limited
number of empty or easily accessible sites on the QD surface followed by ligand
competitive exchange (TOPO, amines, and dyes) on various time scales. Due to
the larger volume of dyes as compared to ligands this results in the creation
of more and/or different intra-band states than in the presence of ligands only.
We would like to emphasize that also at very long time scales a still increasing
attachment of, e.g. (m-Pyr)
4
-H
2
P molecules as identified by further increasing
FRET is observed. Our present findings of inhomogeneous surface dynamics (of
ligands) for semiconductor QDs are also a prerequisite for the establishment of
the non-FRET mechanisms caused by dye molecules. We argue that non-FRET
quenching is related to depletion of capping ligands by the respective dye molecules
followed up by creating more and even new quenching states according to the
amplitude of the tunneling (“leaking”) excitonic wave function.
Temperature variation and related changes in QD absorption and emission reveal
drastic changes of the ligand shell structure in a narrow temperature range for
organic (TOPO and amine) ligands (phase transition). The effects on QD PL at
this transition become considerably pronounced upon attachment of only a few
porphyrin (or perylene diimide) molecules to the QD surface. We conclude from
temperature experiments that the ligand phase transition has impact even on the
QD core structure and exciton-phonon coupling. This investigation elaborates the
importance of (switchable) surface states for the characterization of the PL of QDs.
In this respect it should be mentioned that the environment may even induce a phase
transition of the CdSe core. Recently, in situ microdiffraction data have shown that
the CdSe QDs capped with trioctylphosphine oxide (TOPO) or hexadecylamine
(HDA) in toluene show predominantly wurtzite crystal structure, which undergoes
a phase transition to a zinc blende crystal structure upon drop-casting onto Si.
Furthermore, decreasing the size of the CdSe quantum dots enhances this phase
transition [
84
]. These effects may also lead to a redistribution of trap states which
may change the QD PL efficiency.
