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quantum yields of up to 20% and a full width half maximum (FWHM) of
approximately 100 meV. The particles could be isolated just seconds a
er
precursor injection, and were found to be irregular in size and shape. Pro-
longed mild heating resulted in more monodispersed spherical particles,
which could be isolated by the usual solvent/non-solvent interactions. The
excitonic peak of the isolated particles suggested a relatively wide size
distribution; however, but unusually, when isolated in a solvent, the exci-
tonic peak and emission peak blue-shi
d
n
1
y
4
n
g
|
7
ed slightly and sharpened, indicating
continued growth (digestive ripening) yielding spherical monodispersed
particles. The oleic acid was found to coordinate to the surface lead sites,
whereas TOP was found to passivate the surface sulfur sites.
56
This improved
passivation was the origin of the increased emission quantum yields (up to
80%), along with the improved optical pro
les and stability in TOP-coated
PbS.
57
QDs of PbS prepared by this method have successfully been incorpo-
rated into light-emitting devices.
58
QDs of PbS have also been found to be air
sensitive, in a similar manner to PbSe.
59
Smaller PbS particles, with less
sulfur on the surface and denser capping agent concentrations exhibited trap
defects introduced by the formation of PbSO
3
, which merely extended carrier
lifetime. The larger faceted particles, with sulfur-rich crystal planes such as
(111) which were prone to oxidation, exhibited trapping species from PbSO
4
formation which acted as recombination centres resulting in device deteri-
oration. In contrast, a size-dependent oxidation was also found by Peterson
and Krauss
60
who found that smaller particles were more prone to oxidation,
and suggested that charge trapping to an optically dark state along with
photo-oxidation resulted in the decrease in emission. Nitrogen purging was
also reported to result in brief photobrightening of the particles.
Related to the initial synthetic route is a method brie
.
y described by
Joo
et al.
, where monodispersed cubic particles of PbS 6
13 nm in diameter
were prepared by the injection of sulfur in OAm into a solution of PbCl
2
complexed to OAm at 90
C, followed by growth for 1 hour at 220
C.
61
A
further similar report described the synthesis of crystalline, monodispersed
PbS using PbCl
2
suspended in OAm in more depth,
62
with the addition of
sulfur in OAm at 120
C. The particles were again isolated by non-solvent
interactions, and the reaction could be used to produce up to 1.5 g of puri
-
ed
product. Interestingly, the material was found to be lead-rich, and X-ray
photoelectron spectroscopy (XPS) con
Cl mono-
layer shell on the surface. The size of the particles was controlled by altering
the viscosity of the solution by amending the Pb : amine ratio. The increase
in viscosity resulted in a decrease in the mass transfer coe
rmed the presence of a Pb
-
cient, resulting in
the particles growing in a di
usion-controlled reaction, essential for focusing
the growth of the nanoparticles. The materials again emitted at
ca.
1200
-
1600 nm, with a FWHM of approximately 62 meV and quantum yields of up
to 40%; further studies revealed the extinction coe
cient.
63
The use of
elemental sulfur rather than the toxic and noxious-smelling S(SiMe
3
)
2
is
a clear bene
ering chalcogen
precursors, such as thioacetamide, CH
3
NH
2
S, although safety issues with
t. Reports exist regarding the use of di
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