Chemistry Reference
In-Depth Information
The slow rate of addition, low concentration of precursor solution and
speci
c addition temperature were found to be critical, ensuring the ZnS
grew heterogeneously on to the CdSe seeds instead of nucleating and
forming separate ZnS particles. Although this could not be completely
avoided, size-selective precipitation on the
nal product ensured that all ZnS
particles were removed.
Absorption spectroscopy of the CdSe core and the CdSe/ZnS core/shell
particles displayed only a slight red shi
d
n
1
y
4
n
g
|
3
in the band edge a
er addition of
ned as a shell 3.1 A thick, the
distance between planes in the (002) direction in bulk, hexagonal ZnS
29
),
attributed to a leakage of the exciton into the ZnS shell. Emission spec-
troscopy showed essentially the same position of emission a
1
-
2 monolayers of ZnS (a monolayers was de
er the addition
of the shell with an increased intensity, relating to an increase in quantum
yields from a maximum of 15% for the bare dots to 50% for the core/shell
particles. The emission wavelengths accessible were between 470 nm and
620 nm. It was noted that for the smallest bare dot size, emission was
normally dominated by trapping states giving broad white emission. Upon
deposition of the ZnS shell, intense blue emission was observed. (This
synthesis of blue-emitting CdSe/ZnS materials was later extended, using so-
called green precursors to prepare core CdSe particles 1.6
2.2 nm in size,
with wavelengths as short at 475 nm reported with quantum yields of up
to 50%.
30
)
An investigation into the optimum shell thickness probed a range of
particles with shells varying from 0.65 to 5.3 monolayers thick. Emission
increased upon shell addition up to 1.3 monolayers, then declined as a result
of the incoherent growth of the thicker shell due to lattice strain (CdSe/ZnS
particles have a 12% lattice mismatch) leading to grain boundaries and
dislocations resulting in non-radiative exciton recombination in the shell.
Similar e
-
.
ects have also been reported for CdSe with a CdS shell.
31
Modelling of the electronic structure of CdSe/ZnS gave radial probability
functions (
rst excitonic transition) for holes and electrons in bare, CdS- and
ZnS-capped CdSe particles, as shown in Figure 5.2. In bare dots, the wave
function of the electron was suggested to spread over the entire particle and
tunnel very slightly into the capping agent. In ZnS-capped particles, the
electron wave function was suggested to tunnel into the shell and the
probability of shell penetration for the hole wave function was low. Notably,
the increased delocalisation of the electron narrowed the energy gap in the
particle, although the maximum narrowing observed was only 36 meV,
depending upon particle size and shell thickness.
In CdS-capped particles, the barriers height for the electron was found to
be only 0.2 eV, less than the energy of the electron; therefore the wave
function extended deeply into the shell. The hole had a barrier of 0.55 eV, so
the wave function had a low probability of shell penetration, which was still
smaller than the hole barrier for the ZnS shell and therefore extended
further. The red shi
in the absorption feature was therefore larger, with
a maximum of
ca.
390 meV.
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