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growth. The particle morphology was tracked from rods, to pencils, to
'
pine
trees
. The teardrop-shaped particle was noted as a particularly good example
of shape control; the shape developed as a particle grew at low monomer
concentrations and slow injection volumes, followed by rapid injection of
monomer; the particles favoured a spherical morphology, developing a tail
when the monomer concentration was increased.
The growth of anisotropic particles has also been reported by
gradual addition of the chalcogen precursor, not by increasing the
concentration of precursors or by altering the reaction chemistry.
155
Another notable low-temperature route to anisotropic CdSe structures
involved the thermolysis of a CdCl
2
-
'
d
n
1
y
4
n
g
|
1
octylamine complex and
in situ
gener-
ated [CH
3
(CH
2
)
7
NH
3
]
+
[CH
3
(CH
2
)
7
NHC(
O)Se]
at 70
C.
156
The resulting
wurtzite-structured CdSe nanoribbons were isolated by precipitation using
ethanol (containing TOP), and were found to be 1.4 nm thick, 10
]
20 nm wide
and micrometres in length. The nanoribbons had extremely clear excitonic
transitions and band edge emission and a surprisingly high quantum yield
of 1
-
d
n
4
.
2%.
Other studies have provided low-temperature (160
C) routes to CdSe
nanorods using cadmium naphthenate, TOPSe and HDA.
157
Di
-
ering struc-
tures have also been observed; intermittent addition of high concentrations
of precursor (CdCl
2
and TBPSe) to a hot TOPO solution resulted in the
formation of pyramid-type structures estimated at
ca.
60 nm high, each side
approximately 70 nm long.
158
It is also worth noting that almost all aniso-
tropic CdSe structures have emission quantum yields signi
cantly lower than
spherical CdSe particles. This is attributed to the increased surface area
resulting in more surface states which act as charge trapping sites, and the
increased delocalisation (and hence reduced overlap) of charge carrier wave
functions, resulting in a lower probability of recombination.
Other anisotropic cadmium chalcogenide particles have also been
prepared. For example, maze-like structures have been synthesised using
similar precursors, taking advantage of the inherent polytypism and long-
range order in CdSe.
159
Hyperbranched particles of CdSe and CdTe have also
been prepared by varying precursor concentration and type.
160
Other chem-
ical routes have also been explored; cadmium diethyldithiocarbamate was
used as a single-source precursor to CdS rods and bi-, tri- and tetrapods,
when thermolysed in HDA, giving particles with zinc blende cores and
wurtzite arms.
161
Examination of the facets suggested a similar growth
mechanism to that of CdSe rods and tetrapods. Similarly, cadmium thio-
semicarbazide gave nanorods when thermolysed in TOPO
162
(single-source
precursors are discussed in a later chapter).
CdS rods, bipods and tripods were prepared by increasing the amount of
sulfur precursor in the green chemical routes to sul
des described previ-
ously, utilising CdCl
2
and sulfur as precursors in OAm. Similarly, sulfur and
Cd(CO
2
CH
3
)
2
, when dissolved in OA and added sequentially to HDA at
140
C, followed by further additions over approximately 3 hours, resulted in
particles of varying morphologies.
163,164
Notably, the morphology and crystal
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