Chemistry Reference
In-Depth Information
where the thiol group hydrolyses, reportedly inducing a thin CdS bu
er layer
on the CdTe particle and e
ectively resulting in a wide-bandgap protective
shell.
89
This is a common explanation given for the strong emission from
thiol-capped CdTe, but it fails to explain why CdSe particles capped with
thiols usually display low-emission quantum yields
90
despite the fact we have
already described CdSe/CdS core/shell materials as highly luminescent.
Clearly, the electronic structure of the thiol, the position of the energy levels
and their interaction with charge carriers plays a signi
d
n
1
y
4
n
g
|
6
cant role.
91
Almost all
reports of thiols as capping agents utilise ligands with the thiol groups at the
end of the chain, although the use of isomers where the thiol group is
elsewhere in the molecule has been shown to have advantages. For example,
the use of 2-mercaptopropionic acid in the synthesis of CdS QDs has been
shown to increase colloidal stability and increase photoluminescence relative
to similar materials formed using 3-mercaptopropionic acid as a stabiliser,
due to secondary carboxylate stabilisation and the steric crowding of the
methyl group.
92
The exposure of preformed CdSe particles prepared by organometallic
routes to thiols resulted in a complicated and usually detrimental interac-
tion, notably quenching the luminescence and reducing stability in solu-
tion.
93
A notable study on CdSe QDs with octadecanethiol has demonstrated
that a single thiol molecule quenched the emission by 50%.
94
A red shi
in
the emission peak has also been observed upon thiol addition, attributed to
an electronic contribution from the adsorbed ligands.
58
By way of compar-
ison, CdSe particles prepared by aqueous routes using thiols alone as sta-
bilisers have extremely low quantum yields, highlighting the unsuitability of
thiols as surfactants for CdSe.
95
Aldana has described in depth the instability
of aqueously dispersed thiol-capped CdSe in the presence of oxygen and UV
light,
96
reporting several distinct stages of photoinstability. The
.
rst phase
was the photo-oxidation of the ligand, catalysed by the nanocrystals, yielding
disul
des were water-soluble, the nanoparticles
precipitated out of solution as they lost the capping agent. In the presence of
excess free thiols in solution, the disul
des. Where the disul
des were expelled from the ligand
shell by free ligands in solution and resulted in the nanoparticles remaining
in solution a while longer. Where the disul
des were insoluble in water
a micelle formed within which the nanoparticle oxidised, became smaller
and precipitated out when the micelle became too unstable to support the
particle.
The introduction of small thiol molecules to polymer-coated CdSe/ZnS
core/shell particles also displayed a remarkable suppression of
uorescence
intermittency (blinking).
97,98
This was attributed to the electron-rich species
donating electrons to surface trapping states, reducing the traps ability to
accept electrons from the nanoparticle and reducing the rate of blinking. This
inspired a further study to determine the nature of QD/thiol interactions,
where polymer-coated CdSe/ZnS particles were exposed to
b
-mercaptoethanol
(BME).
99
er exposure of QDs to low concentrations
of the thiol, the emission quantum yield of the particles almost doubled.
It was observed that a
Search WWH ::
Custom Search