Chemistry Reference
In-Depth Information
CHAPTER 6
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6
Ligand Chemistry
6.1 The Functions of Ligands
The capping agent is a key parameter when synthesising nanomaterials
using organometallic and inorganic precursors. Depending on your point of
view, the capping agent can be thought of primarily as a stabilising agent,
providing colloidal stability, hindering uncontrolled growth and agglomer-
ation. This simplistic view can be expanded; the passivating ligands are, as
described in Chapter 1, also intimately linked to the nucleation process,
tuning the availability of monomers, and are important when considering
the growth process. The ligands control the rate of growth, particle
morphology, reaction pathways
1
and the particle size distribution.
At a deeper level, the electronic structure of the passivating ligands
contribute to the overall electronic and optical pro
.
les of the nanoparticles,
blocking surface states and hence a
ecting emission yields and spectral
position, while ligand exchange has also been shown to shi
energy levels in
quantum dots (QDs).
2
Surfactants may also block catalytic sites, reducing the
catalytic ability of some nanomaterials,
3
and are even intimately linked to the
particle
s magnetic properties.
4,5
Some capping agents are chemically labile,
and react with the nanoparticles producing new materials.
6
When considering biological applications, the solubility of the particles is
directly linked to the surface functionalities. Such applications must also
take into account the overall particle size and the hydrodynamic diameter of
the particle, which is again dictated by the surface decorations and func-
tional groups. How surfactants interact with particles is a key concept and is
surprisingly complex. For example, complicated stripe-like patterns on
particle surfaces have been both theoretically predicted and allegedly
observed in some nanoparticle systems
7
(although some controversy remains
'
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