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reported however, that TOPO does not remain at cadmium sites and can in
fact shi
Se complexes
that are intimately linked to the photobrightening process where the emis-
sion quantum yield of
to selenium sites upon illumination, forming TOPO
-
er
synthesis.
22
The interaction of surfactants and the nanoparticle surface is
a key parameter, as both optical and structural properties are dominated by
the interaction between the interfaces.
Ab initio
calculations have predicted
that phosphine oxide species bind preferably to cadmium sites rather than
selenium sites, and always through the oxygen molecule (Figure 6.1). TOPO is
suggested to bind preferentially to the (011
the nanoparticles temporarily increases a
d
n
1
y
4
n
g
|
6
0) and (112
0) facets on the short
'
'
axis (
) of the particle, with binding energies of 1.23 and 1.37 eV
respectively. TOPO binds to the cadmium-terminated (0001
side
) face (
'
'
top
) with
a binding energy of 0.85 eV, but signi
cantly less tightly to the selenium-
terminated (0001) face (
) as demonstrated by the smaller binding
energy of 0.63 eV (assuming a relaxed cluster of wurtzite Cd
33
Se
33
as
a model). As this indicates, both these facets are the axis of growth for
anisotropic particles. Phosphonic acid species, present in technical-grade
TOPO and o
'
bottom
'
en included in reactions to induce anisotropic growth, are also
found to bind preferentially through the oxygen atom onto the (011
0) and
0) facets, but through the hydrogen atom on the OH group to the less
favoured selenium-terminated (0001) face. Phosphonic acids were also found
to bind stronger than TOPO to the nanoparticle surface in all cases, except on
the (112
(112
0) facet. Later results suggested phosphine oxides had a binding
energy of 3.2 eV (313.6 kJ mol
1
) with di
erences being attributed to the
partial charge on the P and O atoms being overestimated.
23
As might be
.
Figure 6.1
Modes of bonding for phosphine oxide and phosphonic acid species
onto various crystal facets of CdSe.
25
Reprinted with permission from
A. Punzder, A. J. Williams, N. Zaitseva, G. Galli, L. Manna and A. P.
Alivisatos,
Nano Lett.
, 2004,
4
, 2361. Copyright 2004 American
Chemical Society.
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