Chemistry Reference
In-Depth Information
this material may limit its use.
64
Alloyed particles of PbS
x
Se
1
x
have been
prepared, and used in photovoltaic devices.
65
In this synthesis, PbOwas heated
and degassed in oleic acid and ODE at 150
C for a 1 hour, followed by a single
injection of an ODE solution of TOPSe, S(SiMe
3
)
2
and diphenylphosphine for
90 seconds, followed by cooling and dilution with hexane, and precipitation.
The resulting particles were rich in sulfur due to the increased reactivity of the
sulfur precursor and displayed emission between the typical values for PbS
(which emits at shorter wavelengths than PbSe QDs) and PbSe QDs.
Lead telluride (PbTe) is an interesting material because of its large exci-
tonic diameter, and has been prepared by similar routes to those described
above. The
d
n
1
y
4
n
g
|
7
rst report on the synthesis of PbTe QDs was described by Lu
et al.
where particles were prepared by injecting a cold solution of Pb(OAc)
3
and
TOPTe into diphenylether with oleic acid at 200
C.
66
Upon growth for 5
minutes, spherical particles could be isolated, while extended growth for 25
minutes resulted in cubic particles, although size-selective precipitation was
required for both spherical and cubic particles in order to obtain a mono-
dispersed sample (Figure 3.3). Further studies
67
in which the ratio of
precursors was widely varied with the use of a long-chain amine as a capping
agent resulted in the formation of di
ering morphologies, with ratios of
Pb : Te of 5 : 1 giving particles with a predominantly octahedral shape, while
a ratio of 1 : 5 gave cubic particles. The use of phosphonic acids as capping
agents was also found to signi
cantly slow nucleation.
A similar method was reported by Urban
et al.
who described the synthesis
of PbTe QDs.
4
In this method, Pb(OAc)
3
and oleic acid were dissolved in
squalene, and heated under vacuum at 70
C for 3 hours to remove water and
acetic acid, a key factor in preparing shape-controlled particles as described
earlier for PbSe. TOPTe was then injected at 180
C, followed by growth at
between 155 and 160
C for 2 minutes before cooling to room temperature
and isolation using solvent/non-solvent interactions. By altering the ratio of
Pb(OAc)
3
and oleic acid, the particle size could be tuned, with lower
concentration of oleic acid resulting in smaller particles due to the less stable
lead acetate reacting quickly, yielding more nuclei. Allowing a slightly longer
growth time also resulted in slightly larger particles. Using this method,
cuboctahedral particles between 4 and 10 nm could be routinely produced,
with a size distribution of about 5%. Increasing the temperature above 200
C
produced polydispersed cubic crystals and increasing the reaction time also
resulted in an increase in polydispersity. The optical properties of spherical
particles between 4 and 8 nm were also explored, with excitonic features
observed between 1800 and 2400 nm. Simultaneously, Murphy
et al.
pub-
lished a very similar synthesis
3
in which PbO and oleic acid (or a mixture of
oleic and erucic acid) were mixed with ODE and heated under argon for 30
minutes, a
.
170
C,
er which TOPTe was injected into the solution at 140
-
130
C over 6 minutes. The lower injection
and growth temperatures resulted in smaller particles than those reported by
Urban
et al.
, of 2.6
with growth temperature of 80
-
18 nm. As a result, the exciton was observed in the optical
spectra between
ca.
1000 nm and 2000 nm. Altering the precursor ratio also
-
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