Chemistry Reference
In-Depth Information
absence of a phosphine capping agent in a similar reaction reportedly yiel-
ded CdSe with a cubic crystalline phase.
72
Likewise, the use of cadmium
carboxylates and long-chain amines in ODE also resulted in small zinc
blende CdSe (although TOPSe was used as a precursor).
73
Other reports
describe the synthesis of high-quality tetrahedral zinc blende-structured
CdSe
74
onto which have been grown zinc blende-structured CdS shells.
75,76
The growth of thick (5 nm) shells on a zinc blende CdSe core has also
reportedly resulted in wurtzite structures, with a remarkable suppression of
d
n
1
y
4
n
g
|
1
uorescence intermittency,
77
a phenomena also observed in other giant
shelled QDs.
78
The optics of zinc blende-structured CdSe has been investi-
gated: although the majority of parameters were comparable to wurtzite-
structured CdSe, at larger particle sizes (>
ca.
4 nm), a weaker con
nement
e
ect was observed for the zinc blende materials, resulting in a slightly
di
erent sizing curve.
79
Interestingly, shape control of zinc blende CdSe has
been achieved using ODE and oleic acid as a solvent system, with a variation
in reaction temperature being responsible for the impressive variety of
shapes obtained, such as monodispersed cubes and tetrahedrons.
80
Peng also reported an in-depth study into the preparation of CdTe particles
in ODE, using fatty acids and phosphines as capping agents, with cadmium
oxide and tellurium powder as precursors.
81
Initial studies showed that by
altering the capping ligand concentration and chain length, the monomer
concentration could be
d
n
4
.
ne-tuned which allowed for controlled nucleation
and growth of particles. In the study on CdTe particles, ligand e
ects were
found to have a signi
cant role in the development of particle size, size
distribution, morphology and crystal phase (which was later utilised in the
synthesis of anisotropic structures
82
). The term
'
monomer concentration
'
was replaced with
, which more accurately
described monomer activity with respect to ligand stability and ultimately
how this a
'
e
ective monomer concentration
'
ected the formation of the particles. The term
'
activity coe
cient
'
was used to describe both the stability and steric e
ects on the monomers.
The preparation of samples with various shapes and crystallinities could
therefore be achieved by selecting speci
c ligand systems and concentrations
that drove the reaction to the desired product. The competition between
ligand
-
monomer stability in fatty acids and chain length in phosphines was
discussed, and a complicated relationship was uncovered.
The CdTe particles prepared in the non-coordinating solvent had quantum
yields as high as 70%, and emission line widths signi
cantly narrower than
those previously reported. The particles were again air sensitive, although the
oxidation was partially suppressed by using unsaturated fatty acids as
capping agents. Saturated fatty acids were found to produce less stable
nanoparticles due to their superior crystal packing, which provided gaps that
acted as oxygen channels.
The ODE route to CdTe has been extended, using
in situ
precipitated Cd
0
particles (100
150 nm in diameter) as precursors, generated by simply pro-
longing the heating step of CdO until a black precipitate was observed.
83,84
Injection of trioctylphosphine telluride (TOPTe) in the solution shortly a
-
er
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