Chemistry Reference
In-Depth Information
Alivisatos reported that pure TOPO was unsuitable as a surfactant for the
reproducible synthesis of anisotropic semiconductor nanoparticles, with
growth proceeding too quickly at high monomer concentrations yielding
insoluble materials. Technical-grade TOPO was found to be more suitable,
because of the large amount of impurities that strongly coordinate to the
monomer ions and retard the growth. Impurities present in technical-grade
TOPO include HPA, which was therefore used as an additive to pure TOPO to
controllably regulate the growth of nanorods. Rapid injection of high
concentrations of precursor (Me
2
Cd and TBPSe) at 360
C into small amounts
of TOPO with a small proportion (up to 3 mol%) of HPA resulted in spherical
particles. Increasing the HPA content to between 5% and 20% reproducibly
resulted in the preparation of rods; particles with the highest aspect ratio
were obtained from a TOPO solution with 20% HPA. By controlling the
HPA : TOPO ratio, the growth rates of di
d
n
1
y
4
n
g
|
1
ering crystal faces were controlled.
Electron microscopy investigations revealed that the particles grew quickly
along the long axis a
d
n
4
.
er precursor injection, consistent with the high
monomer concentration. As the monomer concentration decreased, the
short axis grew. However, by reinjecting further precursor, the monomer
concentration could be kept high enough, resulting in larger quantum rods.
The rods exhibited emission quantum yields of
ca.
1%, which was increased
up to 5% by the addition of a wide-bandgap shell, to be discussed later.
Various shapes were obtained by controlling the of HPA : TOPO ratio,
injection rates and number of injections, yielding rods with aspects ratio of
up to 30, and particles exhibiting various morphologies such as arrows,
teardrops and tetrapods.
153,154
By using a single injection with
xed condi-
tions (injection volumes 1
2 mL, injection time <0.2 seconds) the
HPA : TOPO ratio was varied to control rod morphology and aspect ratios.
Rods of between 5 and 21 nm in length could be obtained, with aspect ratios
of up to 5 when using the optimum amount of HPA as describe above. When
using 60% HPA, particles with an arrowhead morphology were obtained.
By using the optimum growth solution, injection time of 0.2 seconds and
an injection volume of 2 mL followed by slow addition of precursor to
maintain growth, rods with an almost perfect wurtzite structure of more than
100 nm in length have been produced, with aspect ratios of up to 30. The
growth of the crystals was monitored by TEM and XRD, where the crystal face
along the long axis was distinguished by a sharp re
-
raction
pattern. The small amount of zinc blende structure could be observed in XRD
analysis and, notably, by high-resolution transmission electron microscopy
(HRTEM) as
ection in the di
'
kinks
'
in the long rods where the zinc blende structure
dominates.
From these results, three basic e
ects were noted: slow growth rates favour
spherical particles; rapid growth rates resulted in anisotropic materials with
growth along a speci
c face; and HPA emphasised growth of particle crystal
facets. In CdSe, HPA was observed to increase the growth rate of the (001
) face
relative to the others. The range of shapes observed using high concentra-
tions of HPA was attributed to the isolation of particles in di
erent stages of
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