Chemistry Reference
In-Depth Information
Sashchiuk
et al.
also reported the synthesis of PbSe using
Pb(OOCCH(C
2
H
5
)C
4
H
9
)
2
and TBPSe, with no further capping agent added.
39
Again, this reaction was carried out in toluene at room temperature, and
apart from the synthesis of mercury selenide and telluride, are the only
published examples of the rapid synthesis of QDs by organometallic-related
methods at room temperature. The particle size (1.2
5 nm diameter) could
be controlled by altering the ratio of reagents, and placing the resulting
nanoparticle solution in the refrigerator inhibited reactions for months. The
particles displayed absorption spectra consistent with PbSe with several
excitonic features, although luminescence was dominated by trap emission.
This could easily be improved by addition of a shell, which was easily ach-
ieved by the addition of a tributyl phosphine (TBP) solution of sulfur (TBPS),
giving PbSe/PbS core/shell materials.
The same group reported the synthesis of PbSe-based assemblies based on
individual QDs by increasing reagent concentrations, reaction temperature
and time.
40
In these reactions, the structures were prepared by the rapid
injection of TBP solutions of Pb(CO
2
(CH
2
)
3
C
6
H
11
)
2
and TBPSe in TOPO at
elevated temperatures of 150
C, followed by relatively long growth periods of
15 minutes, or by gradual heating to 150
C for a prolonged period of up to
2.5 hours. The assemblies grew from individual particles into large spheres
of up to 450 nm diameter, but prolonged growth resulted in wire-like
assemblies with a helical structure, with widths increasing from 20 nm to
150 nm over time. The resulting wires were composed of individual nano-
particles, stabilised by dipole
-
d
n
1
y
4
n
g
|
7
dipole interactions. Similar results were re-
ported by Cho
et al.
who reported the use of the standard method of making
PbSe particles using diphenylether as a solvent, but using a much higher
synthesis temperature than usual (250
C), with a range of di
-
.
ering capping
agents.
41
The use of tetradecylphosphonic acid (TDPA) and oleic acid resulted
in PbSe wires up to 30
m in length, whereas the use of hexadecylamine
(HDA) resulted in PbSe nanorings, all of which consisted of individual
particles ordered by oriented attachment and fusion. Wires can also be
grown in the presence of primary amines, which exhibited a zigzag
morphology, possibly consistent with previous observations of helical
structures. This is impressive when one consider that to form a 10
m
m
m wire,
10
3
particles need to assemble, all through the (100) axis
via
dipole interac-
tions. Interestingly, the spherical particles (termed quasi-spherical because
of the faceted nature of the particles) have the highest dipole, whereas cubic
particles
have an
almost non-existent dipole moment. This is reinforced by the observation
that cubes do not form wires, but individual spheres can be fused into wires
upon mild heating. Nanowires of PbSe were also prepared using SeP(NEt
2
)
3
as a chalcogen source rather than TOPSe, allowing synthesis at a lower
temperature and for a long period of time. The rods synthesised had well-
de
—
the preferred morphology for the larger materials
—
ned optical properties and exhibited near band edge emission (quantum
yields of up to 15%), although the rods exhibited a larger Stokes shi
than
isotropic particles.
42
Theoretical investigations into the surface energies of
Search WWH ::
Custom Search