3.1. Hot-Injection Technique. This technique involves the
rapid injection of a room temperature solution of precursors
into an extremely hot reaction medium in the presence of
carefully chosen surfactant molecules. The rapid injection of
the precursor solution induces the sudden supersaturation
of the solution resulting in a short burst of nucleation.
The injected solution reduces the reaction temperature and
dilutes the concentration of reactants. With the reaction
temperature dropping in response to addition of the cold
precursor solution, coupled with the low concentration of
the remaining precursor molecules, further nucleation is
prevented. The eventual growth of the nanocrystals follows
at a lower temperature than that of the nucleation process.
This sequential separation of the nucleation and growth
processes generally leads to a precise control of the size
and shape of the semiconductor nanocrystals. This latter
process forms the strength of this technique. A pioneering
example of this technique was reported by Murray et al.
[33]. They injected cadmium and selenium precursors into a
hot (300◦C) solution of tri-n-octylphosphine oxide (TOPO).
The growth temperature used was 230–260◦C. Here the
surfactant acts in fourfold. Firstly, it is as a coordinating
solvent that controls the growth process while at the same
time stabilizing the nanocrystal core. Secondly, the surfactant
binds to the surface of the nanocrystals providing a barrier
to the addition of more material to the surface of the
nanocrystals slowing down the growth kinetics [24]. Thirdly,
it serves to prevent the aggregation of particles and finally
passivates the surface of the nanocrystals.