Chemistry Reference
In-Depth Information
photobrightening and photodarkening, and even total decomposition under
prolonged UV exposure, as outlined in Chapter 3. This extreme air sensitivity
makes them ideal candidates for the deposition of a further inorganic shell.
The use of PbS as a shell on PbSe QDs is an excellent choice. The lattice
constants are similar (PbSe
6.12 A, PbS
5.93 A) while the bandgap has
¼
¼
been predicted to be slightly o
set
164
in what normally might be assumed to
be a type II alignment, although theoretical investigations into PbSe/PbS QD
heterostructures have predicted the behaviour to be consistent with a type I
o
d
n
1
y
4
n
g
|
3
set at the usual particle sizes because the lowest occupied molecular
orbital remains con
ned.
165
One of the earliest reports on the synthesis of PbSe described the reaction
between Pb(O
2
CCH(C
2
H
5
)C
4
H
9
)
2
and TBPSe at room temperature, giving
QDs between 2 nm and 5 nm diameter. The addition of a solution of TBPS
resulted in the formation of a PbS shell through anion exchange, which could
be controlled at 1
-
4 monolayers in thickness.
166
This resulted in the red-
shi
ing of the absorption edge assigned to electronic mixing of the PbSe and
PbS conduction states, and increasing the band edge emission of PbSe while
reducing deep trap emission, due to increased surface passivation. The
addition of more than 2 monolayers of a shell narrowed the emission pro
le,
although no quantum yield measurements were given.
The same group extended the above study by developing a traditional two-
stage synthesis of PbSe/PbS core/shell heterostructures rather than using
anion exchange.
167
PbSe core particles, 3
9 nm in diameter with an 8%
standard deviation, were synthesised as described by Murray
et al.
, by the
injection of lead oleate and TOPSe into a hot diphenyl ether. The particles
were isolated, puri
-
.
ed, then redissolved in TOP. The particles were then
mixed with TOPS, then lead oleate, and injected into hot diphenylether at 180
C, then grown at 120
C for 15 min, depositing 1
3 monolayers of PbS. The
emission was band edge, with a maximum quantum yield of 40%. Interest-
ingly, core/shell particles of PbSe/PbSe
x
S
1
x
were also reported,
167
this time
by a single injection route, in an amendment to the synthetic procedure
towards PbSe. In this synthesis, a mixture of selenium and sulfur was dis-
solved in TOP, added to the lead precursor and injected into the hot solvent.
In this manner, the PbSe nucleated primarily followed by growth of the
PbSe
x
S
1
x
shell, the stoichiometry of which could be controlled by the ratio of
precursor. The dots formed were monodispersed in nature, exhibiting
emission quantum yields of up to 55%. A further study explored the optical
properties of these materials in more depth.
168
-
in the
absorption spectra associated with oxidation of PbSe was found to be
signi
The blue shi
cantly smaller in PbSe/PbS particles, and almost entirely absent in
structures with more than 3 monolayers of PbS. Notably, the absorption edge
and emission pro
le both red shi
ed signi
cantly a
er the addition of
a single monolayer and the emission pro
increased upon further shell addition. Unusually, once 3 monolayers of PbS
had been added, the emission pro
le again narrowed. The red shi
le exhibited an anti-Stokes shi
relative to
the excitonic peak. Similarly, an anti-Stokes shi
in emission was observed in
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