Biology Reference
In-Depth Information
26
the minimum surface energy.
Moreover, the addition of PVP has
no substantial effect on the size of the superparticles (Fig. 13.9d,e),
which further confirms that solvophobic interactions are the major
driving force behind the formation of superparticles.
Further mechanistic studies show that PVP is important in the
crystallization stage (Fig. 13.9a,g). The primary role of PVP should
be as a capping reagent to stabilize the superparticles through
repulsive steric interactions.
27
Other capping reagents such as
gelatin can also play a similar role. Without these capping reagents,
the superparticles were not stable, and their structures easily
collapsed during annealing at 80
C. In addition, TEM shows that
the annealing treatment is important in the formation of single-
supercrystalline superparticles. Before annealing, the spherical
colloidal superparticles do not exhibit superlattice (SL) fringes,
indicating that the nanocrystals are not perfectly ordered in these
superparticles (Fig. 13.9f ). This observation is consistent with the
fact that superparticle formation is a very rapid process in which
nanocrystal building blocks have not yet located their equilibrium
positions. After annealing, the spherical superparticles show very
clear superlattice fringes (Fig. 13.9g). These results suggest that the
annealing treatment is accompanied by a crystallization process
to rearrange Fe
°
nanocrystals into a single-supercrystalline
structure inside these superparticles. In the crystallization process,
the narrow size distribution of Fe
O
3
4
nanocrystal building blocks
along with sufficient surface passivation of superparticles were
found to be essential to the formation of a single-supercrystalline
structure.
O
3
4
20
These mechanistic studies reveal that the solvophobic interaction
between the nanocrystal and ethylene glycol solution is the major
driving force for the formation of spherical superparticles. Further
studiesshowthattheabilitytocontroltheinductionofthissolvophobic
interaction allows one to control the kinetics in superparticle
formation and the final size of superparticles (Fig. 13.10). According
to Eq. 13.10, the nucleation number is proportional to the active
species formation rate and inversely proportional to particle
growth rate. In a CIS synthesis, active species are the decomposed
nanocrystal micelles owing to the loss of DTAB molecules, and their
kinetic formation process is generally much faster and less tunable as
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