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TOPO at 250
C, giving wires with both wurtzite and zinc blende structures,
although other bismuth compounds were also found to catalyse the wire
formation.
215
It was suggested that the bismuth compounds formed
in situ
bismuth nanoparticles, and bismuth particles were occasionally imaged on
the tips of the resulting rods.
Seeds of CdSe with di
ering crystal structures have also been used to
prepare heterostructures of di
d
n
1
y
4
n
g
|
1
erence in
crystal facets.
216
Wurtzite-structured CdSe, when added to a CdS reaction
mixture, resulted in the formation of CdSe/CdS nanorods (with the CdSe seed
shi
ering types, using the inherent di
ed to one end), whereas the use of zinc blende CdSe seeds resulted in
tetrapods, as might be expected. Unusually, the tetrapods exhibited
remarkably high-emission quantum yields of up to 60%, while the rods
exhibited quantum yields of up to 75%. These tetrapods were also shown to
be susceptible to applied pressure, with notable shi
s in the emission
wavelength making them potentially suitable optical strain gauges.
217
Similar work was reported by Carbone
et al.
, where CdSe particles were
used as seeds for the formation of extremely monodispersed and ordered
CdSe/CdS rods which self-assembled on substrates.
218
Interestingly, the rods
were found not to alloy, even at the high temperatures (up to 380
C) used
during synthesis. Again, the rods were found to have unusually high
quantum yields of up to 75% for the shorter rods, although longer rods
exhibited a much lower quantum yield. The seeds of CdSe were found to be
located at between 1/4 and 1/3 of the rod
d
n
4
.
'
s length.
1.7 Alloys
The preparation of simple compound semiconductors described above can
be extended to the synthesis of related alloy materials, which have novel
electronic pro
les dictated by the particle composition rather than just the
quantum size e
ects.
219
In some cases, alloys have optical properties unre-
lated to the parent material: for example, when preparing an alloy of CdSe
and CdTe, the e
ective mass of the excitons of both parent materials need to
be considered. In the case of CdTe, the e
ective exciton mass is substantially
lower than that of CdSe. Therefore, the exciton mass of an alloy should be
tuneable with alloy composition. In practise, a non-linear e
ect was observed
where the e
cantly lower than that
of either parent material. This manifested itself as a depression in the
absorption and emission energies, hence giving materials with unexpected
novel properties.
In the case of CdSeTe, a simple combination of precursors such as TOPSe,
TOPTe and CdO thermolysed in TOPO and HDA resulted in the alloy mate-
rial. Under the growth conditions described, tellurium was found to be more
reactive towards cadmium than selenium, allowing the preparation of two
di
ective exciton mass of the alloy was signi
erence in
kinetics.
220
The amount of tellurium or selenium could be accurately
controlled by the amount of either precursor used. By varying the amount of
ering compositional types of alloy by taking advantage of the di
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