Chemistry Reference
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of the formed structures. The core of the tetrapod was found to be zinc
blende and the arms wurtzite, consistent with other II
VI tetrapod nano-
particles. Clearly, the surfactant plays a major role in the formation of
anisotropic materials as the low-temperature synthesis of CdS in TOPO using
the same precursor resulted in small, spherical particles. Cadmium and lead
diethyldithiocarbamates were also used as precursors in the SLS growth of
CdS
41
and PbS
35
nanorods, using bismuth nanoparticles as catalysts for
anisotropic growth. The seed growth of anisotropic particles is described in
Chapter 1. In this case, the introduction of bismuth nanoparticles catalysed
anisotropic growth, with nanowire dimension controlled by the reaction
temperature, surfactant chemistry and the diameter of the catalyst particles.
Interestingly, the wire diameters were larger than the catalyst particle
diameters, with the thickest wires obtained from the smallest particles.
Bismuth and antimony dithiophosphates, M[S
2
P(OC
8
H
17
)
2
]
3
,M
-
d
n
1
y
4
n
g
|
4
Bi, Sb,
have been used as precursors for rod formation at relatively low temperatures
(160
C) in OAm. Rods of Bi
2
S
3
, several hundred nanometres in length with
diameters of
ca.
12 nm, were simply prepared and found to clump into
bundles similar to other reports of Bi
2
S
3
reported in Chapter 4. Investigations
into the growth mechanism uncovered the initial growth (1 minute growth
time) of 7 nm diameter particles with aspect ratios of less than 3, which then
grew along the (001) axis to give rods with aspect ratios of
ca.
5a
ΒΌ
er
8 minutes further growth, which then proceeded to rod lengths of hundreds
of nanometres over 1
ξ
1.5 hours. The reaction was also found to proceed
faster at higher temperatures, producing rods of slightly altered dimensions.
Altering the concentration of precursors still resulted in rod formation, but
surprisingly decreased the length of the rods, attributed to increased seed
growth rather than increasing the length of the rod at the reaction
-
.
s earlier
stages. Slightly larger rods of Sb
2
S
3
,1
m
m in length and 45 nm in diameter,
were grown in similar conditions.
42
Similarly, zinc selenide nanorods,
ca.
24
nm in length and 6 nm in diameter displaying emission at
ca.
400 nm, have
been prepared from the thermolysis of [Zn(
i
Pr
2
PSe
2
)
2
] in TOP and HDA.
43
A similar route was developed to the preparation of MnS particles using
Mn(S
2
CNEt
2
)
2
in HDA.
44
The structures prepared and conditions required are
shown in Figure 7.2. In this case, time of synthesis was also a key variable,
with prolonged heating resulting in the growth of larger cubes or the
conversion of rods into spheres. The particles exhibited either a wurtzite
(
g
-MnS, hexagonal) or zinc blende (
b
-MnS, cubic) structure at synthesis
temperatures below 200
C, which allowed growth of branched structures, or
a rock salt (
a
-MnS) structure at temperatures above 200
C. Investigations
into the optics of the rods and spheres showed evidence of quantum
con
'
nement, with band edges between 3.3 eV (375 nm) and 3.5 eV (354 nm),
slightly wider than the bulk value of 3.2 eV (
ca.
390 nm). Notably, the Stokes
shi
ξ
for the wires was approximately twice as large as that observed for the
spheres. Emission was band edge and showed no evidence of trapping states.
By mixing cadmium and manganese diethyldithiocarbamates prior to ther-
molysis, dilute magnetic semiconducting nanorods of Cd
1
x
Mn
x
S could be
ξ
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