Chemistry Reference
In-Depth Information
regarding this hypothesis
8
). Here, we will discuss the key surfactants as
determined by their linking functional moiety. It is worth noting that the
actual backbone of the ligand o
ers another feature that can be manipu-
lated, for example in the preparation of superhydrophobic materials by
surface exchange with
—
a wealth of possible ligands exists (including ligands not normally associated
with nanoparticle passivation,
e.g.
silanes
10
and azoles,
11
that we do not cover
here). The chemistry of the surface is not limited to the addition or removal
of ligands; chemical treatment, such as the addition of NaBH
4
to a solution of
QDs, has been reported to signi
uorinated ligands.
9
This list is not exhaustive
d
n
1
y
4
n
g
|
6
cantly improve the emission quantum yield,
by removing the surface ligand and inducing the formation of a stable
surface oxide layer.
12
6.2 Phosphine Oxide-Based Ligands
6.2.1 Tri-n-Octylphosphine Oxide (TOPO)
In Chapter 1, the evolution of surfactants for the passivation of nanoparticles
was described, through phosphate-based polymers, to monomers and
eventually the phosphine oxide-based molecules such as tri-
n
-octylphosphine
oxide (TOPO), which became a standard passivating ligand. The advantages
of using TOPO as a surfactant, solvent and capping agent include the high
boiling point, allowing reactions to proceed routinely at temperatures up to
350
C, facilitating high-temperature annealing unavailable to aqueous-
based routes, and the compatibility with organic solvents allowing
a completely inert reaction environment and hence the use of air-sensitive
precursors. Once coordinated to the particle, the long alkyl chains impart
solubility
13
to a normally insoluble solid-state material and allow it to be
manipulated like a common organic reagent (although the solvents used
must possess a signi
.
cantly high dielectric constant to overcome the van der
Waals attraction between the colloidal particles). The ligand shell is generally
robust in the case of most semiconductors, able to stand several rounds of
dispersion/precipitation before losing any degree of solubility as the
surfactant is gradually removed (although it is has been found that
trialkylphosphine oxides do not bind as strongly to metal particles,
14
highlighting the need to choose speci
c surfactants for di
ering nano-
particles). The steric properties of the alkyl groups also further a
ect particle
growth, controlling shape and morphology.
15,16
In semiconducting systems,
the ligand also blocks the electronic surface trap sites that are normally
responsible for broad emission and non-radiative charge carrier recombi-
nation, allowing clean, near band edge luminescence.
17
Surfactants also play
a key role in the carrier relaxation between excited intraband states, where
so
ligands are responsible for slow relaxation rates.
18
In the case of TOPO-capped CdSe, the TOPO binds to the surface cadmium
sites through the lone pairs of electrons on the phosphine oxide moiety,
forming dative bonds,
19,20
sometimes referred to as an L-type ligand.
21
It is
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