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PbSe nanocrystals con
rmed the dipole-driven interactions through high-
energy surfaces that lead to oriented attachment, although these studies
suggested that the interaction range is theoretically shorter than experi-
mental observations.
43
Assembly through the (100) axis has also been ach-
ieved post-synthesis by the treatment of small oleic acid-capped PbSe with
pyridine.
44
The growth of these anisotropic materials is unusual as it appears that the
mechanisms are solely oriented attachment of small particles. However,
a range of straight and branched PbSe nanowires could also be prepared
using Au/Bi core/shell particles as seeds, by injecting TOPSe and the seeds
into a weakly coordinating capping agent (
e.g.
TOP or phenyl ether) and
a lead carboxylate at temperatures of 150
d
n
1
y
4
n
g
|
7
200
C.
45
The growth conditions
and resulting structures were analogous to the growth of CdSe rods described
in Chapter 1 (the
-
mechanism), and were clearly not formed by
oriented attachment of smaller particles. Although the growth temperature
did not appear to favour either straight or branched structures, the Pb : Se
ratio clearly a
'
geminate
'
ected product morphology, with Pb : Se ratios of 2 : 1
favouring straight wires whereas a 4 : 1 ratio favoured branched structures.
The diameters of the seed particles were found to dictate the width of the
wire. As mentioned, PbSe branched wires with similar structure to branched
CdSe could be grown, exhibiting Y-shaped morphologies, although in the
case of cubic PbSe T-shapes could also be observed, and also right angles
which were actually T-shaped particles which had not fully grown. Shape
control of PbSe particles was also achieved using noble metal nanoparticles
as seeds. A variety of shapes, from rods and diamonds to stars, were all
prepared rapidly at low temperatures using typical precursors by varying the
metal and reagent concentrations.
46
The role of the lead precursor counterion in particle morphology has been
explored and found to be signi
.
cant. Spherical (or quasi-spherical) particles
were obtained when the precursor (commonly lead acetate and a carboxylic
acid) was dried vigorously, removing acetate and acetic acid. If these side
products were allowed to remain, then octahedral and star-shaped particles
were predominant, due to acetic acid coordinating to the surface; the small
size of this facilitated oriented attachment of the particles.
47
Similarly, the
use of Pb(NO
3
)
2
as a precursor resulted in hollow PbSe nanoboxes, due to the
decomposition of the nitrate group, which resulted in gas bubbles that
assisted the Ostwald ripening process.
48
A more in-depth investigation into
the general reaction pathway has been undertaken by Steckel
et al.
, who
suggested two mechanisms occur during PbSe formation.
49
rst
mechanism, TOPSe acts as a Se
2
source, which reacts with the lead species
yielding TOPO, oleate anhydride and PbSe monomers. In the second, TOPSe
acts as a Se
0
source, reacting with Pb
0
which is provided by the reduction of
the lead oleate by an
in situ
generated dialkylphosphine (which might also be
present as an impurity), yielding PbSe QDs. Based on these results, the
introduction of diphenylphosphine into PbSe reactions has been carried out
in order to increase the yield of
In the
the reaction,
50,51
which is generally
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