Chemistry Reference
In-Depth Information
For distinctly low concentrations, the e
ect was slow to manifest, but
remained stable for over 12 days once observed. Large concentrations of thiol
resulted in quenching of the photoluminescence in less than 1 day. The initial
increase of the photoluminescence was attributed to the thiol reducing the
number of electron traps, while the higher concentration resulted in the
formation of hole traps. To further investigate the identi
cation of the active
species, pH dependency studies of a set concentration of BME were under-
taken. At high pH, the thiol primarily dissociated into the thiolate, which was
found to decrease photoluminescence quantum yields, again attributed to
the presence of hole traps, whereas at low pH, exposure of the protonated
species to the nanoparticle was found to increase emission quantum yield.
Control experiments, exposing the nanoparticles and thiols to environments
where thiolates could not form, resulted in no change in optical properties.
These results suggest that thiolates and not thiols are the active species
responsible for both positive and negative e
d
n
1
y
4
n
g
|
6
ects of thiol-related species.
Therefore it is more accurate to say that at low concentrations, thiolates
provide electrons for the trapping sites, while at higher concentrations, thi-
olates act as hole traps. Interestingly, the transfer of a hole to a thiolate ion
results in the formation of a thyil radical, RS
_
, which could react with another
thyil radical to form RSSR, the disul
de observed in thiol-capped CdSe. The
thiolate-trapped hole can also recombine with an electron from the nano-
crystal, giving deep trap emission, or decay non-radiatively, either way
reducing band edge emission. The charge transfer and blocking of surface
sites was con
rmed by theoretical work
88
and further experimentation with
mercaptopropanoic acid
100
which concurred that thiols can be used without
reducing the emission quantum yield if the thiol concentration is optimised.
In related work, the quenching of the band edge emission of phosphine/
amine-capped CdSe by 3-mercaptopropionic acid has also been accompanied
by an enhancement of trap emission of up to 5%, but substitution of the
amine with the thiol created selenium vacancies.
101
Interestingly, this
phenomenon was not observed in QDs capped solely with phosphine/
phosphine oxide.
Aldana has also shown that thiolate-capped nanoparticles of CdE (E
.
¼
S,
Se, Te) undergo ligand dissociation from the particle surface when exposed
to relatively low pH systems, precipitating (undamaged) particles. The
particles were redispersed in solutions with a signi
cantly higher pH, dis-
playing a distinct hysteresis curve, attributed to the di
ering processes
governing precipitation and redispersion, namely ligand protonation and
deprotonation.
102
This process was found to be size (and hence bandgap)
dependent, showing the Gibbs reaction energy (
D
r
G
o
) for the formation of
surface-ligand bonds for materials of di
ering diameters. This size/binding
energy relationship has, however, been suggested to be coincidental, and the
changing binding energies attributed to the di
ering crystal facets on the
nanoparticle surface.
103
In related work, a thiolated spin trap has been
coordinated to TOPO-capped CdSe QDs, and the nature of disul
de bond
cleavage investigated.
104
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