Page 74 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

60 Principles and Methods

TOP/TOPO molecules, are nonaggregated and easily dispersible in organic
solvents forming optically clear dispersions.
When similar syntheses are performed in the presence of surfactant,
strongly anisotropic nanoparticles are obtained. From Cd(CH 3 ) 2 and
TOPSe in hot mixture of TOPO and hexylphosphonic acid (HPA) rod-
shaped CdSe nanoparticles can be obtained [96]. The role of adsorption
of HPA is clearly demonstrated by the change in shape of nanoparticles
with the increase in HPA concentration. An increase in the concentra-
tion of Cd precursor into the reactor also induces a higher aspect ratio
of nanoparticles exhibiting quasi-perfect wurtzite crystalline structure
(Figure 3.22). A further slow addition of monomer sustains growth of
(00-1) faces without additional nucleation giving rods exceeding 100 nm
in length with an aspect ratio over 30:1.
The role of HPA seems to be to increase the growth rate of the (00-1)
faces relative to all other faces. Further support for this argument comes
from the formation of multipods, especially tetrapods. These are remark-
able single-crystal particles consisting of a tetrahedral zinc blende core

(a) (b)

100 nm
(c) (d)
Figure 3.22 TEMs of CdSe nanoparticles obtained with surfactant to Cd
ratio of 20% HPA in TOPO. The single-injection volumes increase from
(a) to (c). Greater injection volume favors rod growth. (d) TEM of a typical
multiple injection extended rod synthesis. The average length is 34.5 ±
4.4 nm with an aspect ratio of 10:1 [96].