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the materials. The wavelength of emission could be tuned by varying the
selenium and tellurium composition, the particles with the higher selenium
content emitting further towards the red end of the spectrum, and, to a lesser
degree by varying the particle size which was dictated by the reaction time.
The particles could not be capped by ZnS (10% success rate in capping),
attributed to the lattice mismatch but could successfully be capped by CdS
which surprisingly reduced the emission quantum yield. The reduced
emission did however recover in most cases, attributed to photoannealing of
the particles giving quantum yields above 30%.
The use of alloyed materials was extended to the preparation of the related
CdTeSe/CdZnS QDs, utilising graded CdTeSe alloyed core particles. In this
case, attempts to grow either a CdS or a ZnS shell failed, and a method of
depositing a CdZnS shell using metal carboxylates as precursors with tri-
octylamine (TOA) was reported. The TOA was found to be essential, resulting
in the formation of a high-quality zinc blende shell on the zinc blende core
particles. The resulting particles emitted at 780
d
n
1
y
4
n
g
|
3
800 nm, with emission
quantum yields of 50%, which dropped to 30% upon phase transfer to water
using phospholipids.
70
Alloy-based core/shell materials have also been prepared that emit in the
blue region of the visible spectrum, in a simple one-pot reaction. In
a typical reaction CdO and an excess of Zn(CO
2
CH
3
)
2
and oleic acid were
dissolved and dried in ODE, followed by the injection of a solution of
sulfur in octadecene (ODE/S) at 300
C.
71
This was followed shortly a
-
er by
the injection of tributylphosphine (TBPS), overcapping the particles with
ZnS, yielding Cd
1
x
Zn
x
S/ZnS, QDs,
ca.
9 nm in diameter, with a wurtzite
crystalline structure. The particles exhibited an absorption band edge at
ca.
450 nm, with band edge emission also observed with quantum yields of
42
.
-
81%. The emission of the core/shells was reported to slightly blue-shi
a
usion of the zinc atoms
from the shell into the core. Notably, a related material, CdZnSe/ZnSe,
exhibited continuous non-blinking photoluminescence, a signi
er shell deposition, attributed to the intradi
cant
advance in the preparation of materials with controllable, stable optical
properties.
72
Alternative Core/Shell Structures
An interesting alternative shelling material, AsS, has been reported and used
to passivate CdSe particles, giving CdSe/AsS QDs.
73
In this example, the
C
4
H
9
NH
2
-
S cluster was coordinated to the CdSe surface by gentle heating
in isopropanol, ultimately resulting in the formation of the AsS shell,
followed by addition of long-chain amines or TOP to help passivate the core/
shell particle. The shelling technique could be applied to a wide range of
CdSe sizes, increasing the quantum yield from
ca.
5% to up to 50%, resulting
in strong emission across the visible spectrum. The particle exhibited up-
conversion and were used in imaging HeLa cells, despite having arsenic in
the shell material.
As
-
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