Colloidal Superparticles From Artificial Atoms - Molecular Self-Assembly: Advances and Applications - page 437

surface energy is a determining factor in the morphology of surfaces
and interfaces. Short-range-ordered superparticles often exhibit
isotropic particle-packing symmetry, and thus the minimization of
their surface energy can only lead to a spherical equilibrium shape.

39

Supercrystalline superparticles possess an anisotropic structure,
and in principle, they can adopt shapes other than spheres. However,
because the internal energy of these superparticles is often smaller
than their surface energy, the minimization of their surface energy also
results in a spherical shape. Indeed, all the colloidal superparticles
discussed ex supra exhibit a spherical shape.

− − −

1,20,22 25,28,36 38,40 42

To
induce the formation of nonspherical superparticles, one approach is
to decrease the surface energy of superparticles and make it smaller
than their internal energy, but decreasing surface energy could cause
the formation of a short-range-ordered structure, which again results
in spherical superparticles under equilibrium conditions. Another
approach is to increase the strength of the interparticle interactions
in superparticles, making their internal energy comparable to or
larger than their surface energy. On the basis of this approach, we
have synthesized cylindrical supercrystalline superparticles from
long CdSe/CdS semiconductor nanorods.

21
These cylindrical superparticles were made via a CIS synthesis
using CdSe/CdS nanorods (55.4

± ±

0.2
nm in diameter). This CIS synthesis includes two major steps: (1)
preparation of water-soluble nanocrystal micelles using DTAB; and
(2) growth of superparticles by adding a nanocrystal micelle aqueous
solution into ethylene glycol, in which an annealing treatment (at 80
°

2.6 nm in length and 4.4

C) is important to improve the supercrystallinity of the resulting
superparticles. In this annealing treatment, suitable capping

ligands are critical for stabilizing the superparticles. PVP and
gelatin were found to be suitable ligands for making stable spherical
superparticles. However, these ligands cannot effectively stabilize
the surface of CdSe/CdS nanorod superparticles, often resulting in
superparticle products with aggregation and poor solubility (Fig.
13.13a,b). These results are due in part to the fact that these ligands
have a weak affinity to the (100) facets of CdSe/CdS nanorods. To
overcome this difficulty, we used dual-interaction ligands (e.g., dithiol
functionalized Tween-20, Tween-SH)

21

to passivate these anisotropic
superparticles in the growth step. These dual-interaction ligands
have a strong affinity for CdSe/CdS nanorods because the ligands
bind onto the surface of the nanorods through both coordinate