Colloidal Superparticles From Artificial Atoms - Molecular Self-Assembly: Advances and Applications - page 413

(amorphous) structure or a long-range ordered (supercrystalline)
structure. In this chapter we review the major efforts by the field to
construct superparticles and characterize their properties. We first
discuss the formation of superparticles from a theoretical point of
view. We then present the successful examples for the synthesis and
characterization of colloidal superparticles.

13.1 Theoretical Considerations on the

Formation of Colloidal Superparticles

Colloidal superparticles are synthesized via a bottom-up approach
using

−

The formation of
superparticles, resembling that of colloidal nanocrystals, is a phase-
transition process that includes the nucleation and growth of a new
phase. Superparticles (or nanocrystals) are the new phase created
from the old phase of their precursors.

nanocrystals as precursors.

1,22 25

26

Owing to the intrinsic
characteristics of superparticles and their precursors, the nucleation
and growth stages in the formation of superparticles exhibit four major
differences from those in the formation of colloidal nanocrystals.

1
First, the nucleation of superparticles does not have a free energy
barrier like that of nanocrystals. The molecular or ionic precursors
for nanocrystal nucleation are in the form of true solutions, and

these precursor solutes exhibit dramatically different surface
properties from those of nuclei (the new solid phase) created during
nucleation. Therefore, nucleation is accompanied by an increase in
the volume and surface of the nuclei. The change of the total free
energy during nucleation is a balance between the energy gain of
creating the new volume and the energy loss due to creating the new
surface.

4

26

The Gibbs free energy needed to form a spherical nucleus
is as follows:

4
3

3 2

D G = p

R G

+

4

p s

R (13.1)

v

where R is the radius of the nucleus, G is the energy gain per unit v

volume, and is the surface tension of the nucleus. If the nucleus is

too small, the energy gained by forming its volume is not enough to
create its surface, and nucleation is not energetically favored. It costs
energy to increase nucleus size until its radius reaches a critical
number (called critical radius, R

s

*

∆

*

). The free energy cost ( G ) to form

this critical radius is as follows: