Chemistry Reference
In-Depth Information
residence time. Particles of 2
4.5 nm diameter could be prepared by the
reaction in a capillary, with reaction times 7
-
-
reaction time curve was similar to that of particles grown in the batch
processes. The reaction was extremely reproducible, giving materials with
quantum yields of
ca.
1.5% and an emission range of 450
-
150 seconds. The particle size
600 nm. The size
distribution, as evidenced by the width of the exciton peak in the absorption
spectrum, was narrowed using a capillary with a smaller diameter; a capillary
with a diameter of 500
-
d
n
1
y
4
n
g
|
1
m required 4 seconds to reach reaction temperature,
whereas a capillary with a diameter of 200
m
m
m required only 0.4 seconds.
Introduction of a 0.5
L nitrogen bubble into the capillary every 3 seconds,
usually used to avoid a velocity (and hence residence time) distribution, was
also found to narrow the particle size distribution by inhibiting mixing
between segments of the precursors.
A more sophisticated version was reported using a chip-based micro-
m
uidic reactor, where channels were etched in a 100 mm diameter glass
wafer sandwich.
244
In this case, Me
2
Cd and TBPSe were mixed with DDA
and TOPO in ODE at 60
d
n
4
.
C under an inert atmosphere. A
er being
degassed, the reagents were loaded into the sample loop of a HPLC
injection valve. The precursors could (if required) be diluted before injec-
tion (100
L plug of precursors) into the reaction channel, which was
heated from below to 175
m
185
C. A
er a reaction time of 300 seconds, the
product was diluted with ODE and pumped into a capillary
-
ow cell where,
110 seconds a
er exiting the reaction channels, the emission spectra were
recorded using a
bre-optic charge-coupled device (CCD) camera. The
nanoparticles produced were approximately 2.5 nm in diameter, although
increasing the reaction temperature modestly increased particle size (from
2.44 to 2.69 nm). Decreasing the
ow (increasing the residence time in the
reactor) also increased the particle size by a similar amount, as did
increasing the concentration of precursor. Interestingly, the particle sizes
were estimated from the emission wavelengths as opposed to the absorp-
tion band edge.
There are however, problems associated with using the organometallic
approach to micro
uidic-based synthesis; the capping agent, TOPO, is a solid
at room temperature and starts to decompose upon prolonged heating,
potentially blocking the reaction channels. The cadmium precursor, Me
2
Cd,
is not ideal because of the hazards associated with its handling and the gas
evolution upon its decomposition. Work by Krishnadasan
et al.
in which
Me
2
Cd was replaced by Cd(CO
2
CH
3
)
2
resulted in particles with quantum
yields as high as 10%; however, samples prepared at temperatures of 220
C
displayed emission spectra consistent with surface defects.
245
A detailed
investigation into reaction kinetics highlighted the preferable use of high
ow rates and high temperatures to minimise the residence time distribu-
tion that ultimately led to polydispersed samples.
Another chemistry was developed to be totally compatible with the lab-
on-a-chip process; cadmium oleate and TOPSe were chosen as precursors
and dissolved in squalane, OAm and TOP, chosen because of their liquid
Search WWH ::
Custom Search