Chemistry Reference
In-Depth Information
150 and 180
C. Prolonged growth (up to 12 hours) at 200
C resulted in larger
particles up to 5 nm in diameter (7 nm with further addition of precursor).
Particles prepared by this route exhibited band edge emission but with
substantially lower quantum yields of up to 30%.
The higher quantum yield of the
rst route described by Talapin can be
attributed to the prolonged heating, improving the crystallinity of the
samples and the constant delivery of dilute tellurium. All samples displayed
a cubic crystalline core as opposed to the hexagonal CdTe samples reported
originally by Murray, and showed evidence, although only slight, of trap
emission manifesting as a slight red-shi
d
n
1
y
4
n
g
|
1
ed shoulder in the luminescence
spectra. The use of DDA as a capping agent was found to be a key parameter
in the preparation of CdTe particles. Other long-chain amines were found to
be inferior surfactants; octylamine (OA) was found to have too low a boiling
point and dioctylamine (DOA) resulted in immediate precipitation.
Hexadecylamine (HDA) allowed particle growth but the resulting QDs had
a large size distribution relative to DDA-capped particles. A similar meth-
odology was applied to the preparation of CdSe and InP particles.
38
An in-
depth study of the reaction conditions and growth of the DDA-capped CdTe
particles was reported,
39,40
highlighting that the small initial particles
formed, emitted from a defect state, leading to exciton band edge emission
a
d
n
4
.
er several minutes growth. It was also suggested that the precursors
formed small, stable clusters at room temperatures, and that upon heating,
particle growth was found to be fast over the initial 30 minutes, with the
nal size being reached in
ca.
3 hours. In related work, stable, small
(magic) clusters of CdTe (1.9 nm in diameter) have been prepared using
n
-hexylphosphonic acid (HPA) as a capping agent. The stable particles could
be isolated due to the surface passivation and were found to exhibit broad
emission with a quantum yield of 4% in methanol, although they aggregated
due to the permanent dipole in the zinc blende cluster.
41
It is worth noting at this stage that a room-temperature aqueous route can
also be used to prepare CdTe materials of a similar optical quality to those
prepared by organometallic precursors.
42
In a typical example, a cadmium
salt and H
2
Te gas are used as precursors, and thiols are utilised as capping
agents. The routes are scalable, relatively cheap and easily reproduced, but
the resulting materials lack the degree of crystallinity obtained using
organometallic precursors. The key advantage of using aqueous methods to
make CdTe is the increased stability in ambient conditions when compared
to the extremely air-sensitive particles prepared by the organometallic route.
Thiol-capped CdTe exhibited band edge emission in a similar spectral range
as the organometallic analogues, with quantum yields up to 85% with
stability in air for months.
43
The anomalously high quantum yield for this
aqueous system is attributable in part to the chemistry and physics of the
capping agent. One explanation involves the reaction of the capping agent
with the particle; upon prolonged illumination, the thiol-capping reportedly
decomposed giving S
2
ions in solution, which reacted with the cadmium
surface of
the nanoparticle.
44
This,
in e
ect, resulted in a CdTe/CdS
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