Chemistry Reference
In-Depth Information
photochemical annealing. Interestingly, energy-dispersive X-ray analysis
spectroscopy (EDS) indicated the growth of a CdS bu
er layer between the
CdSe and ZnS shell.
The Alivisatos group prepared graded structures of CdSe/CdS/ZnS
'
core/
bu
rods using metal alkyls and standard precursors.
36
This work
was initiated a
er/shell
'
er the observation that attempts to grow a ZnS layer on CdSe
d
n
1
y
4
n
g
|
3
rods o
en resulted in uneven structures with very thin (on average less than
one monolayer) shells. This was overcome by adding a small amount (8 : 1
Zn : Cd ratio) of cadmium precursor to the shell stock solution, resulting in
the formation of the bu
er layer. What is important is that the layers were
not
prepared sequentially; the bu
er layer formed when the ZnS precursors were
added to the preformed CdSe. The segregation of growth could be due to
numerous factors, including the lower solubility of CdS
versus
ZnS in TOPO,
or the tendency for zinc atoms to form more stable complexes with TOPO
than cadmium atoms do.
CdS has a lower lattice mismatch than ZnS, and this is a key factor when
growing shells on CdSe nanorods, which have a relatively
surface
than spherical dots and hence strain becomes an issue. The introduction of
abu
'
atter
'
er layer resulted in even, epitaxial growth of the shell, with no evidence
of separate CdS or ZnS particles observed. Shell ups to 6.5 monolayers thick
(on rods of
ca.
30 nm
7 nm) could be grown, with Zn : Cd ratio in the shell
being 2 : 1 in a 2 monolayer shell, 4 : 1 in a 4.5 monolayer shell, and 4.5 : 1 in
a 6.5 monolayer shell. The shell thickness was controlled by the amount of
precursor added, until a certain shell thickness was reached and the rods
developed
.
. The addition of a ZnS shell improved the surface regularity
of the structures, overcoming the stacking faults in long rods described in
Chapter 1. Interestingly, as the shell thickness increased, the re
'
tails
'
ections from
the X-ray di
value
not consistent with ZnS, CdS or CdSe phases, which was attributed to lattice
compression of the core particle.
Upon the deposition of the shell, the absorption and emission spectra red-
shi
raction (XRD) patterns shi
ed position towards a larger 2
q
ed slightly as the exciton tunnelled through the relatively low energy
barriers into the shell as described above, con
rming the CdS layer. The
quantum yields of the core particles alone was less than 1%, while particles
with a thin shell (2 monolayers) displayed quantum yields of up to 4%, and
particles with a medium shell (4.5 monolayers) displayed quantum yields of
1%, consistent with strain-induced defects of thicker layers. Under constant
laser illumination for 20 hours, the bare rods either exhibited no change in
quantum yield or a decrease, whereas the particles with shells photo-
annealed to exhibit quantum yields of up to 16% (4.5 monolayers). This
implied that a photochemical process induced an internal structural reor-
ganisation, as thermal annealing did not result in the same increase in
emission. The importance of the interface and reducing lattice strain in
preparing highly luminescent structures has been explored, and the highly
graded CdS/ZnS shell/shell particles have been suggested to be the optimum
material for such structures by supplying uniform coverage.
37
Search WWH ::
Custom Search