Chemistry Reference
In-Depth Information
Similarly, a second-generation polybenzyl ether dendron with a phospho-
nate group has been used to cap CdSe nanoparticles. In this case, the den-
dron was added during the synthesis rather than used a
erwards as a surface
exchange reagent. The resulting material exhibited enhanced emission.
Electropolymerization of the pendant carbazole groups was readily achieved,
but red-shi
ed the QD absorption and quenched the emission, attributed to
the polymerisation step, li
d
n
1
y
4
n
g
|
6
ing the ground state of the carbazole groups
above the ground state of the CdSe.
246
6.7 Inorganic Ligands
The majority of exchange reactions utilise organic-based ligands as a vast
number of compounds are available, providing coordinating groups and
pendant functionalities. A drawback with the use of organic ligands is the
insulating properties this induces in the
nal nanomaterials, which is
problematic if the
nal application requires the unhindered passage of
a charge carrier. A simple, yet elegant way to circumvent this problem is to
use inorganic complexes. Kovalenko
et al.
have reported the use of metal
chalcogenide complexes (MCCs) in e
ective surfactant exchange reactions,
in which compounds such as (N
2
H
5
)
4
Sn
2
S
6
completely replaced the organic
surface cap on a variety of nanoparticles with ions such as Sn
2
S
6
4
.
247,248
The
exchange reaction proceeded smoothly in solution, leaving the particles
essentially physically and optically unchanged, and soluble in a wide range of
solvents yielding all-inorganic colloids. When deposited as
.
lms, QDs and
gold nanoparticles both showed impressive conductivity values, consistent
with the inorganic ligands facilitating e
cient charge transfer, which led to
the fabrication of QD based
ect transistors. Although MCCs are
themselves precursors for chalcogenide materials, their primary use in these
cases was as molecular ligands, although gentle heating resulted in the
preparation of the relevant semiconducting phase, producing composite
materials. A wide range of QDs and metals of various types and structures
were reported with numerous MMCs, giving a potentially enormous of vari-
ation of conducting nanoparticle solids. A range of nanomaterials (PbS,
PbTe, Bi
2
S
3
) have been functionalised with the Zintl ion Sb
2
Te
3
MCCs, and
used to prepare thin
eld-e
lms of nanostructured thermoelectrics with relatively
large values of the
gure of merit,
ZT
.
249
6.8 Encapsulation Driven by Hydrophobic
Interactions
Although surfactant exchange is a versatile proven chemistry, this process is
not always ideal. As the capping ligands are an inherent part of a nano-
particle
s structure, changing the surface capping chemistry alters the elec-
tronic properties, and applications such as solar energy conversion rely on
the intimate electronic link between solid-state material and organic ligand.
'
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