Chemistry Reference
In-Depth Information
considered.
25
Alivisatos highlighted that the kinetics of crystal growth were
also dependent on the variation of surface energy with size, and showed
prolonged growth led to growth focusing, refocusing and defocusing (Ost-
wald ripening), with an intimate link to monomer concentration. During the
focusing stage, the high concentration of the monomers exceeds the solu-
bility of the nanoparticles: all particles grow, but the smaller particles grow
faster than the larger ones and the size distribution can be focused to an
almost monodispersed sample. Below a speci
d
n
1
y
4
n
g
|
1
c monomer concentration, the
larger particles grow at the expense of the smaller ones, defocusing the size
distribution. This highlighted that the size distribution could be controlled
experimentally by maintaining a high monomer concentration. Further
nucleation studies suggest the growth of CdSe nanoparticles is reaction
limited,
26
which is supported by the earlier detailed study by Dushkin
et al.
27
To complement experimental work, detailed theoretical studies of particle
growth based on Monte Carlo simulations have also been carried out which
make suggestions as to the favoured growth regime.
28
Various models have
been utilised in examining the kinetic and thermodynamics of particle
growth; the di
d
n
4
.
erent stages of nucleation and growth have been discussed in
the context of classical nucleation theory,
29
and a barrier di
usion model has
also been developed that describes the growth kinetics in di
erent solvents,
highlighting Arrhenius behaviour dictated by solvent activation energies,
which increase with molecular weight.
30
The kinetics of nanocrystal growth is
complicated and covers many variables, yet if controlled it can be tuned to
allow the growth of di
erent-sized particles.
31,32
For example, controlling the
growth of CdSe particles is possible by choosing ligands which form
complexes of di
ering solubility, that act either as nucleating agents
(resulting in higher particle yields and smaller particles) or as growth agents
(resulting in early time ripening), allowing extremely small particles to be
prepared and stabilised.
33
Other work has suggested that the presence of
water and oxygen in the reaction mixture can lead to the etching of the small
particles that aggregate into larger particles during the Ostwald ripening
process.
34
Emission from nanometre-sized CdSe can be tuned to
ca.
600 nm while
retaining a relatively high quantum yield, and emission beyond this region
requires a di
erent material such as cadmium telluride, CdTe, a material
with a room-temperature bulk bandgap of 1.44 eV and an excitonic radius of
38 A.
35
Although the preparation of CdTe was described in the seminal organo-
metallic synthesis paper, Mikulec reported the
rst serious study of CdTe
nanoparticles prepared by a synthetic procedure based on the TOPO route.
36
The CdTe nanoparticles initially prepared by Murray
et al.
were not investi-
gated in depth and had an estimated quantum yield of below 1%, attribut-
able to the instability of the triphosphine telluride precursor. In the case of
the original TOPO route, lower growth temperatures were used to maintain
controlled growth of CdTe, which in turn led to materials with a lower
quantum yield.
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