Colloidal Superparticles From Artificial Atoms - Molecular Self-Assembly: Advances and Applications - page 414

3

ps

16
3

D G

*

=

(13.2)

2

() G v

∆

*

is the nucleation energy barrier—it governs the initiation and
domination of nanocrystal nucleation.

G

26

In addition, the existence
of a nucleation energy barrier is important to the separation of the
nucleation and growth stages in nanocrystal synthesis, which is
critical to the formation of monodisperse nanocrystals.

14
In contrast to nanocrystal synthesis, superparticle precursors
(i.e., nanocrystal building blocks) are in the form of a colloidal
solution, which exhibits a bi-phase nature.

27

Each nanocrystal retains
its own surface boundary and thus has a separate existence from
the dispersion solution. The chemical and physical properties of
these precursors are nearly identical to those of the resulting nuclei
or superparticles.

23,28

Therefore, superparticle nucleation does not
require additional free energy to create the surface of superparticles,
but it results in a decrease in the total particle surface area owing
to the binding of nanocrystal building blocks. As a consequence,
superparticle nucleation does not exhibit the free energy barrier
observed in nanocrystal nucleation. The lack of a thermodynamic
nucleation energy barrier makes it difficult to achieve the separation
of nucleation and growth in superparticle synthesis. Concurrent
nucleation and growth often result in particles with a poor size
distribution.

14

−

Thomson equation does not apply to
superparticle systems. Superparticles do not exhibit size-dependent
“solubility” because their surface properties are very close to those
of their nanocrystal building blocks. However, colloidal nanocrystals
exhibit size-dependent solubility owing to a surface effect governed
by the Gibbs

Second, the Gibbs

−

Thomson equation: The solubility of nanocrystals
increases as their size decreases.

29

Importantly, this size-dependent
property enables a kinetic-controlled nanocrystal-size focusing
mechanism. Under this mechanism, smaller particles grow faster

than the larger particles, and thus the size distribution of the
nanocrystals decreases with growth.

30

30
Third, nanocrystal building blocks (i.e., atoms) exhibit limited
valence, while superparticle building blocks (i.e., colloidal
nanocrystals) have a multivalent property that is dependent on
their surface functionality and surface area.

23

On the one hand, this
multivalent property enables the rational control of nanocrystal