Chemistry Reference
In-Depth Information
emission due to the recombination of charge carriers at the interfacial
alloyed layer.
98
CdTe/CdS and CdTe/CdSe have also been prepared in the non-
coordinating solvent ODE, using either sulfur or selenium in ODE as chal-
cogen precursors in shell deposition.
99
An interesting method of preparing
CdTe/CdSe has been reported where CdTe particles were initially made,
followed by the addition of TOPSe and either Me
2
Zn or Me
3
Al which acted as
reducing agents for the selenium.
100
The metal alkyls formed adduct
compounds, shielding the selenium and making it more reactive with the
cadmium monomer retained in solution. The resulting rectangular particles
of CdTe/CdSe had quantum yields of up to 25% and appeared more photo-
stable than the core CdTe.
It is worth noting that when examining the band structure o
d
n
1
y
4
n
g
|
3
set for CdTe
and ZnSe in bulk materials, one would expect type I behaviour from core/
shell structures prepared from these materials; however, type II optical
characteristics are routinely observed. An investigation into the deposition of
a range of compressive II
-
VI shells on a small, so
CdTe core highlighted
signi
cant lattice strain that increased the CdTe bandgap due to the applied
compressive force, which simultaneously depressed the conduction energy
band of the shell material (typically ZnSe) due to tensile strain, yielding
new bandgap o
sets consistent with type II materials (termed strain-induced
type II behaviour).
101
By choosing the correct shell materials, and by tuning
the number of shell monolayers and hence the degree of strain, the emission
could be tuned between 500 and 1050 nm with a maximum quantum yield of
60%. It is quite possible that the majority of type II structures mentioned
above based on CdTe cores do in fact exhibit strain-induced optical
behaviour.
Other type II structures include ZnSe/CdS, which again were produced
using simple precursors, such as zinc carboxylates, TOPSe, cadmium oleate
and sulfur in a non-coordinating solvent.
102
In this case, the ZnSe core
particles were prepared as outlined in Chapter 1, isolated and puri
.
ed up to
four times using solvent/non-solvent cycles, an essential step if high
quantum yields were to be realised. The shell was then deposited using the
SILAR technique. These particles exhibited emission at 480 nm, which was
shi
c core size) when more than three
monolayers of shell were deposited, consistent with the spatial separation of
charge carriers. The resulting large Stokes shi
ed to 610 nm (using one speci
was attributed to the large
o
set of the band edges. The emission had a maximum quantum yield of
18%, which could be slightly increased upon particle puri
cation. An inter-
esting related structure in which ZnSe QDs were embedded in a CdSe rod has
been reported.
103
In this example, preformed ZnSe particles were mixed with
CdS precursors and injected into a mixture of TOPO and phosphonic acids,
resulting in rods
ca.
47 nm long with the ZnSe particles (of
ca.
3.7 nm
diameter) incorporated into part of the structure. The emission of the ZnSe
particles at
ca.
400 nm was signi
er 8
minutes rod growth, consistent with type II behaviour, with a remarkably
high maximum quantum yield of 45%.
cantly red-shi
ed to
ca.
600 nm a
Search WWH ::
Custom Search