Chemistry Reference
In-Depth Information
The band edge was tuneable from
ca.
440 nm to
ca.
650 nm, with near
band edge emission ranging across the same region, with quantum yields of
40
60%. The precursors CdO and ZnO were dissolved in a mixture of oleic
acid and 2-ethylhexanoic acid prior to injection into a hot mixture of Se
-
-
S
dissolved in para
n liquid. The dissolution of the chalcogens in para
n
liquid reportedly reduced the di
erences in reactivity, an essential feature of
d
n
1
y
4
n
g
|
1
the alloyed particle formation.
Alloys are an excellent example of the tuning of the optical properties of
nanoparticles. Depending upon starting parameters, nanoparticle can be
prepared in which the band edge emission either red-shi
s
from the initial expected position for the parent binary materials.
Usually, in simple binary materials, gradual particle growth over time
results in a shrinking of the bandgap, resulting in a red shi
s or blue-shi
in the optics,
and is o
en the only option available when growing particles. CdZnSe
provides an alternative, where blue-shi
ed emission is observed in particle
growth when starting from CdSe seeds. In the case of CdSeTe, non-linear
e
d
n
4
.
ects resulted in emission that could not be attributed to either parent
material. The preparation of alloys and the associated blue shi
s and non-
linear e
ects presents further options when bandgap-engineering small
particles.
1.8 Micro
uidic Synthesis
The synthesis of nanoparticles by the green chemistry described earlier in
this chapter is a major advance. However, in practice, scale-up procedures
result in problems with precursor delivery, homogenous mixing in large
volumes and greater variations in temperature. One potential way to avoid
these problems is the use of microreactors, o
'
systems.
238
The small scales of these reactors allow excellent control of
temperature and are usually designed as
en termed
'
lab-on-a-chip
ow processes that allow accurate
control of reaction time. The use of reduced spatial dimensions to carry out
chemical reactions is attractive because of the potential to screen the process
while using the smallest possible amount of precursor. Calculations have
shown that 70 reactors in parallel with a
ow rate of 0.25 mL min
1
can
produce 10 L of product in 10 hours.
239
Initial work on the micro
uidic preparation of nanomaterials centred on
the preparation of semiconductors QDs by aqueous routes
240
and the prep-
aration of TiO
2
.
241
The micro
uidic preparation of QDs using the green
chemistry described earlier was achieved by heating cadmium acetate with
stearic acid at 130
C to give the cadmium stearate precursor.
242,243
This was
followed by the addition of TOPO under a nitrogen atmosphere, which was
then allowed to cool below 100
C. Addition of TOPSe provided the reaction
mixture, which was loaded into a syringe. The reagents were then injected
into a capillary of a known length and diameter, of which a predetermined
length was immersed in an oil bath stabilised at temperatures of 230
300
C.
The length of the capillary in the oil bath and the injection rate controlled the
-
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