Chemistry Reference
In-Depth Information
cadmium, either a homogenous (prepared under cadmium-limited condi-
tions) or a graded structure (prepared under cadmium-rich conditions) could
be synthesised. In cadmium-limited conditions, the composition of the
particle was determined by the relative growth rates of CdSe and CdTe and
not by the cadmium concentration, as cadmium was common to both
species and cancelled out from the kinetic equations. Therefore the
composition remained constant. In the case of the cadmium-rich reaction,
a tellurium-rich core was produced during nucleation due to the faster
reaction rate of tellurium as compared to selenium. As the tellurium was
removed from the reagent mixture, selenium became the predominant
species in the subsequent growth step. Once all reagents were consumed,
growth stopped and a graded structure was produced. This might be thought
of as a CdTe/CdSe core/shell material (to be discussed in Chapter 5) without
a discrete boundary. Ostwald ripening, where smaller particles grow on the
surface of larger ones, was not considered problematic as the particles
formed before this growth mechanism became important.
Both types of alloyed particles consisted of a wurtzite crystal core 2
d
n
1
y
4
n
g
|
1
d
n
4
.
-
8nmin
diameter and were extremely
uorescent, with quantum yields of 30
-
60%.
Due to the non-linear optics of alloyed particles, emission was signi
cantly
shi
ed towards the red end of the visible spectrum. This is an excellent
example of the tuneability of the optical properties of nanoparticles. It
should be stressed that the non-linear e
ect is not necessarily related to the
size quantisation e
ects that shi
the band edge in nanoparticles, and that
'
has been previously observed in bulk alloys of CdSeTe.
221
In
fact, optical bowing has also been observed in CdS
x
Te
1
x
alloyed particle
prepared by similar methods, where the emission was shi
optical bowing
'
ed to beyond 700
nm, despite CdS emitting at
ca.
425 nm and CdTe emitting at
ca.
575 nm.
222
In a similar manner, Zn
x
Cd
1
x
S (with
x
between 0.10 and 0.53) alloy
nanoparticles have been prepared using cadmium and zinc oleates with
sulfur in ODE, with oleic acid as a capping agent.
223
The particle diameter was
controlled between 2.4 nm (
x
0.10), while the emission
was band edge, extremely narrow (FWHM of up to 18 nm) with reported
quantum yields of up to 50%. The emission could be tuned towards the blue
end of the spectrum by increasing the zinc content, giving a range from 391
nm (Zn
0.53
Cd
0.47
S) to 474 nm (Zn
0.1
Cd
0.9
S). The FWHM of 14 nm (Zn
0.1
Cd
0.9
S)
is one of the narrowest emission pro
¼
0.53) and 4 nm (
x
¼
les for QDs reported at room temper-
ature. All particles exhibited a wurtzite structure. The di
raction pattern
displayed a shi
to larger angles as zinc content increased due to a decrease
in lattice parameter
c
, consistent with Vergard
rming the alloy
composition. The particles were homogenous and did not possess a graded
structure despite the faster growth rate of CdS relative to ZnS. The particle
homogeneity was attributed to high-temperature-induced internal atom
di
'
s law,
224
con
le.
Similar results were observed with the zinc blende analogue of the same
system, obtained by using cadmium acetate or stearate as precursors and
amines as capping agents.
225
Chloride or sulfate salts of cadmium could also
usion, which was also used to explain the narrow emission pro
Search WWH ::
Custom Search