Chemistry Reference
In-Depth Information
with particles with a 3 nm diameter. This explained the minimum small axis
of the rods at 3 nm. The cadmium : selenium ratio was found to be a critical
variable for the preparation of rods; the higher the Cd : Se ratio, the larger the
aspect ratio of rods produced. The rate of the nucleation with di
ering ratios
of monomer could be monitored by the evolution of the 349 nm feature, with
the higher ratios of precursors taking longer to form the stable cluster,
indicating that the Se
d
n
1
y
4
n
g
|
1
phosphine complex is more reactive that the cadmium
monomer. These clusters have also been shown to be important in the
synthesis of anisotropic CdSe particles.
175
A further report con
-
rmed the growth of magic clusters with similar optical
properties,
176
and larger blue-emitting clusters of other materials have also
been reported.
177,178
CdSe magic clusters have also been prepared with stable
absorption pro
les at 395, 463 and 513 nm, all of which exhibited band edge
emission.
179,180
The organometallic-type growth of nanoparticles through
various stable con
, has been
observed for a range of materials, including CdS, CdHgS, ZnSe and ZnO.
181
The growth of teardrop-terminated rods has also been observed, similar to
those described by Manna.
153
This structure is unusual as nanoparticles of
this shape were suggested to be grown by the sudden increase in monomer
concentration in a system already forming a spherical dot, as opposed to the
consistently high monomer concentrations reported. The growth of
gurations, termed
'
discontinuous growth
'
d
n
4
.
'
close to
perfect
rods was achieved by the use of high temperatures and the octade-
cylphosphonic acid complex of cadmium, due to the fast di
'
usion and low
reactivity of long-chain monomers. It is harder for long rods to undergo
di
usion to the growing facets, hence the requirement for the higher growth
temperature. The growth of rice-shaped rods was also observed,
via
the 3D
growth phase at temperatures between 300 and 350
C using the TDPA
complex of cadmium. Multiple precursor injections resulted in a more
de
2D ripening phase the rice-shaped
particles grew into spherical dots. Multiple injections have also been used to
produce nanorods using cadmium acetate as a precursor.
182,183
During the formation of rods, branched (multipod) structures were always
formed. The population of branched particles could be increased by
controlling the monomer concentration and by reducing the growth
temperature. The prevalent factor was found to be the monomer concen-
tration; by increasing the precursor concentration, the particles grew
through short rods, to larger rods, to branched particles, feeding the growth
of the structure through the di
ned shape, although in the 1D
-
ux.
Notably, rods were also grown in a non-coordinating growth solution
(ODE) with TDPA alone, demonstrating that
usion
two ligand systems with
di
ering binding strength on various crystal facets is not necessarily required
for anisotropic growth. Also, nuclear magnetic resonance (NMR) experiments
revealed that all rods were capped with predominantly phosphonic acids,
irrespective of aspect ratio or growth conditions.
The new di
usion model for rod growth suggested that a high monomer
concentration is the prevalent factor when attempting to grow anisotropic
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