The Preparation of II–VI Semiconductor Nanomaterials - Semiconductor Quantum Dots: Organometallic and Inorganic Synthesis - page 26

position (di used) on the particle from the long axis to the short. Normal

Ostwald ripening was also observed at monomer concentrations of 0.1 - 0.2%.

Peng has also reported an in-depth analysis of the kinetic (di usion) versus

the thermodynamic models in CdSe rod growth and proposed a model that
explained the growth of faceted crystals in terms of potential. 152,169
In the CdSe model, at high monomer concentrations, the crystal grows in
the wurtzite structure on the (001

d n 1 y 4 n g | 1

) facet due to the presence of negatively
charged Se ions, uncoated by ligands because of the Lewis base character of
the surfactants. When cadmium atoms are added to this layer, they posses
three dangling bonds. This, combined with the dipole moment along the
c -axis, 170

) facet a large chemical
potential, making them the most active. The facets perpendicular to the
c -axis (100) have only one dangling bond. 171 During the 1D growth stage,
a limited amount of monomers di

gives the (001), and especially the (001

use into the di usion sphere that

) and (001) facets is
rapid due to the increased chemical potential, making the c -axis the long
axis. At this point, the di

surrounds the particle, and growth along both the (001

d n 4 .

ux is only enough for growth along the long
axis of the particle. As monomer concentration drops, the overall chemical
potential of the monomers drops, which a

usion

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) and
(001) facets. As a result, growth is consistent in all dimensions (3D growth
stage). In the third 1D/2D ripening stage, the chemical potential in solution
equals that of the surface atoms, which leads to an equilibrium. With the low
monomer concentration and hence no more reagents for growth, the
monomers start to di

ects the growth of the (001

) and (001) facets to the others, to
minimise the overall surface energy of the particle.
The role of the

use from the (001

; either HPA or n -tetradecylphosphonic acid
(TDPA) was found to control the growth rate, not the particle shape directly,
with growth in TOPO/HPA being the fastest. The presence of the strong
ligand was essential, however, to keep the monomer concentration high
enough to invoke rod growth.
Peng further investigated the growth of anisotropic CdSe by using
cadmium oxide as a precursor instead of dimethylcadmium. 172 It was noted
that structures with a slightly higher than normal aspect ratio ( ca. 6.5
compared to 3) could be obtained if an aged solution of cadmium oxide

' strong ligand '

-

phosphonic acid complex was used. It was found that the cadmium -

tetradecylphosphonic acid complex was the most stable (Cd TDPA) and

slowed the nucleation rate down relative to similar experiments with HPA
complex of cadmium. During synthesis using the stable Cd

-

TDPA complex as
a monomer, two sharp features in the absorption spectrum were observed.
The

-

rst, at ca. 285 nm, was assigned as a molecular species. The second
feature, at 349 nm, was assigned as a CdSe cluster, a thermodynamically
stable, closed shell molecular clusters up to 2 nm in diameter 173,174 consisting
of 17 Cd atoms, undetectable by TEM. These features in the absorption
spectrum were used as a measure of monomer concentration. By monitoring
the absorption spectra, the coexistence of clusters and small dots was
observed, with an excitonic peak becoming evident at ca. 540 nm, consistent

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