Chemistry Reference
In-Depth Information
The origins of this work can be found in the early vapour
-
liquid
-
solid
(VLS)
205
and solution
solid (SLS)
206
work where anisotropic growth of
a macroscopic wire can be catalysed by a molten metal particle. In CdSe-
seeded growth, the actual concentration of precursor does not need to be
high: the more dilute the reaction mixture, the straighter the rods, attributed
to a decrease of selenium in the vicinity of the Au/Bi spheres. A dilute
solution also increased the nanowires
-
liquid
-
d
n
1
y
4
n
g
|
1
erence
between the catalysed longitudinal growth and uncatalysed transverse
growth. A growth temperature of 330
'
diameter, highlighting the di
350
C favoured straight rods, whereas
growth temperatures starting at 280
C favoured bend rod growth. By
controlling the cadmium : selenium ratio, the diameter of the wires could
be controlled; increasing the ratio increased the diameter of the wires
(optimum ratio of 7 : 1), and decreasing the ratio by increasing the
concentration of TOPSe resulted in branching of the structures. Branching
could be avoided by using larger Au/Bi seed particles, although Bi
2
Se
3
was
observed in systems that utilised thicker bismuth shells. The addition of
a small amount of TOP to a system usually expected to produce straight wires
produced branched structures, although the mechanism of branched
growth from such
-
d
n
4
.
is unknown. The rods also exhibited random
regions of both zinc blende and wurtzite structures, as determined by TEM
investigations. (Essentially the same reaction has been reported, using
Me
2
Cd as a precursor, which resulted in much thinner wires when compared
to the same reactions using CdO.
207
) Branched structures such as tripods,
V-shaped and Y-shaped crystals were formed with a zinc blende crystal core.
As well as Y-shaped structures, highly ordered structures were also observed.
Notably, the diameter of the branches did not vary, making the structures
ideal candidates for molecular wires. In the case of multipods grown by the
seeded method, most appear to have three arms, as opposed to the four arms
grown by the high-concentration routes, attributable to surface energy
consideration of the seed where three arms may be thermodynamically
stable.
Grebinski
204
suggested four mechanisms relevant to the growth of seeded
particles: uncatalysed growth (di
'
doping
'
ssion (which depended
on the seed particle to split into either two or three particles and rod growth
to occur in di
usion); catalytic
—
however, this is not supported by the variety of experimental observations);
collision (which has already been described as an unsuitable mechanism for
describing the anisotropic particles); and germinate nanowire nucleation,
which has been assigned as the growth mechanism for the seeded particles
described.
The germinate nucleation mechanism suggested multiple wires were
grown from the surface of a single particle where su
erent directions to promote V-shaped or Y-shaped rods
cient precursor is
present not only to supersaturate the system, but also to initiate the growth of
at least one arm. It was suggested that following nucleation, two nanowires
merge to form a branched structure. As the wires grow and become
comparable to the size of the dot, the wires join and form a grain boundary,
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