Chemistry Reference
In-Depth Information
The emission from TOPO-capped particles was initially band edge without
the need for a further inorganic shell, highlighting the high optical quality of
materials prepared under an inert atmosphere.
In this case, the steric properties of TOPO/TOP play a key role; reducing the
chain length to butyl groups or smaller resulted in uncontrolled growth. The
actual coordination of the capping agent to the crystallite slowed growth
kinetics, allowing for the controlled growth at elevated temperatures. The
a
d
n
1
y
4
n
g
|
1
nity of the phosphine oxide group for the surface metal (controlled by
tuning the chain length of the alkyls chains)
21
also e
ected the growth rate;
the increased Lewis base character,
i.e.
, the more electron donating, the
stronger the binding and the slower the growth. The presence of the capping
agent was also essential for the band edge emission and relatively high initial
quantum yields of
ca.
10% (for CdSe) were observed, which dropped by
orders of magnitude upon removal of the surface ligand. Notably, it has since
been discovered that the emission quantum yield of QDs can be increased up
to a maximum of 75%
via
the surface treatment with sodium borohydride
(NaBH
4
). The addition of small amounts of the reagent reduced the ligand,
removing the surfactant while allowing the surface cadmium to oxidise,
yielding a cadmium oxide layer that enhanced the emission. Prolonged
exposure or an excess of reducing agents resulted in the precipitation of the
nanoparticles, although addition of the optimum amount of NaBH
4
was
achievable and the treated nanoparticles were found to be stable for up to
a year in ambient conditions.
22
The surface ligand could be removed by re
d
n
4
.
uxing the nanoparticles in
pyridine, which resulted in ligand exchange, removing the TOPO and leaving
the more labile pyridine coordinated to the particle surface. This was initially
used as in intermediate for other capping agents and will be discussed later.
Murray also reported that although Lewis base ligands are generally utilised
for capping agents, Lewis acids, such as alkylboranes and alkylaluminium
species could also be used to passivate the surface.
21
The particles were crystalline as determined both by powder X-ray
di
raction (XRD) and by transmission electron microscopy (TEM). The XRD
patterns displayed re
ections consistent with wurtzite (hexagonal) struc-
tured nanoparticles, with the distinction between hexagonal and cubic
structure being lost in particles below
ca.
2 nm in diameter (Figure 1.2).
Modelling studies of the di
raction patterns suggested each nanoparticle
contained one stacking fault.
This route quickly became the standard method to prepare high-quality
II
VI nanoparticles, with CdSe becoming the most studied material. A
notable amendment to the preparation of CdSe was reported by Bowen
Katari,
23
where the selenium was dissolved in TBP rather than the longer TOP
in an attempt to achieve a higher surface coverage, although comparisons
between QDs prepared by both methods revealed little di
-
erence. In an
attempt to prepare larger samples with absorption features beyond
ca.
580
nm, the synthesis temperature was increased to 350
C, which was followed
by several minutes
continuous heating at
ca.
320
Ca
'
er precursor
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