Chemistry Reference
In-Depth Information
The phase-transferred materials exhibited a reduced emission quantum
yield, and some materials (HgTe) displayed a shi
in emission pro
le.
t of using thiol compounds is in the monitoring of shell
growth on emitting cores. As thiols almost immediately quenched the
emission of a
Another bene
(unshelled) CdSe QD and only reduced the emission on
shelled particles, addition of a thiolated compound such as thiophenol is
a simple and e
'
naked
'
d
n
1
y
4
n
g
|
6
ciency of a shelling method (although it
is dependent on the solution used).
178
Phenothiazine, another such
compound used for optimising the shell thickness of a core/shell system,
also worked by a similar method, although this ligand was suggested to
coordinate through an amine group.
179
ective test for the e
6.6.2 Phosphine-Based Surfactant Exchange
Although thiolated ligands are extremely versatile for facilitating surfactant
exchange, their usually detrimental e
ects on particle luminescence have
driven investigations towards less disruptive capping agents. Naturally, as
the majority of synthetic routes use surfactants related to phosphine and
phosphine oxide, more complicated, engineered phosphine-based ligands
have been developed that can be used in exchange reactions giving the
desired functionality while retaining the e
cient linkage.
As mentioned previously, a common problem in the incorporation of
TOPO-capped particles into electronic devices is the monolayer of surfactant
on the surface, which signi
cantly reduces electron transfer.
180
To circum-
vent this problem, Milliron
et al.
have designed an electroactive oligothio-
phene ligand with a phosphinic acid functionality to allow strong
coordination to a nanoparticle surface,
181
a similar concept to the thiolated
aniline capping agent mentioned above. Simple coordination to the CdSe
particle surface, in this case, was not the only factor; correct energy level
alignment between the particle and the organic group and solubility in
a solvent convenient for further processing are all requirements for ligands
designed for such applications. The energy level alignment was addressed by
the modular synthesis of the ligand that enabled speci
.
cally engineered
conjugation lengths, and the usually poor solubility of the oligothiophene
was recti
ed by the inclusion of alkyl chains on the thiophene units inducing
the required solubility. The free oligothiophene ligands
uoresced with
quantum yields in the region of 15%, but quenched when linked to CdSe
nanoparticles. The emission of the nanoparticle/ligand complex varied with
the length of the oligo backbone, as the electronic structure was staggered
and hence the nature of the charge/energy transfer varied with electronic
alignment, either slightly enhancing or reducing emission. Likewise,
branched (dendron) oligothiophene/phosphinic acid ligands have been used
to cap CdSe nanoparticles, using the branched structure to
ll the ligand
sphere around the particle. When using branched oligothiophene, the QD
emission was completely quenched; this was attributed to the successful
electron transfer between the ligand and nanoparticle.
182
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