Chemistry Reference
In-Depth Information
state at room temperature.
246
The reactor was a continuous-
ow channel
with an internal diameter of 250
m and a reaction length of 14.6 cm
maintained at a temperature of 180
m
320
C. The two precursors were
delivered in separate channels to a mixer prior to injection into the reac-
tion channel, avoiding the room-temperature formation of clusters that
a
-
nal nanoparticle diameter. Again, particle size
could be tuned to between
ca.
2.2 nm and
ca.
2.8 nm by careful alteration
of
ect the reproducibility in
d
n
1
y
4
n
g
|
1
ow rate, temperature and precursor composition. By maintaining
a constant precursor concentration, it was observed that higher tempera-
tures and residence times increased the particle diameter, although size
distribution was compromised at high
ow rates or low temperatures. By
keeping the concentration of cadmium ions constant and by varying the
amount of TOPSe, e
ective tuning of the particle size was achieved while
maintaining an acceptable size distribution. The quantum yields of parti-
cles prepared by this procedure (28
cantly higher than the
other particles prepared on a chip; this was attributed to the presence of
the amine capping group. Over an 8 hour run, the absorption spectra of all
samples were identical, highlighting the stability of the system. The same
group also reported the use of supercritical hexane in a continuous-
-
51%) were signi
d
n
4
.
ow
process, which overcame problems with solvents such as viscosity, solu-
bility and di
usivity.
247
Reactions were carried out in silicon/Pyrex micro-
reactors, which allowed high-pressure reactions. The reactors consisted of
a nitrogen-purged channel 400
m, with a 0.1 m long mixing
zone at room temperature and a 1 m long reaction zone at 350
C. The
CdSe particles were prepared using a similar solvent chemistry as
described immediately above, under a pressure of 5 MPa. The use of the
supercritical solvent resulted in more nuclei forming due to a higher
supersaturation of precursors, which resulted in the narrowing of the size
distribution.
The preparation of CdSe/ZnS core/shell particles by a multi-step process in
a microcapillary has been reported,
248
by the addition of Et
2
Zn and S(SiMe
3
)
2
in TOP
via
a ceramic micromixer to preformed CdSe particles prepared as
described above. In this case, two oil baths and two di
m
m
250
m
erent temperatures
were used to prepare the particles, with the emission intensity being
increased
vefold upon capping the particles with the inorganic shell. Slower
ow rates resulted in a decrease in the photoluminescence, due to a thicker
shell than necessary being deposited. The same group and others have also
reported the use of a single-source precursor to deposit the ZnS shell on to
preformed CdSe.
249,250
It is also possible to carry out ligand-exchange reac-
tions on chip immediately a
er shell deposition, to provide water-soluble
particles that are potentially useful in biology.
251
An important advance in the chip-based synthesis was described by
Krishnadasan
et al.
, who developed their earlier work by including an in-line
spectrometer to feedback the optical properties into an algorithm, which
then altered reaction conditions to obtain the required emission wave-
length.
252
This intelligent optimisation of the synthetic parameter has
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