Chemistry Reference
In-Depth Information
with the hexagonal phase of ZnS upon addition of thicker shells, con
rming
the epitaxial growth of an ordered crystalline phase of ZnS.
A similar report on the preparation of CdSe/ZnS core/shell particles
described the e
ect on the incorporation of long-chain amines into the TOPO
surfactant mixture.
32
A maximum quantum yield of 60% was reported with
a shell 1.6 monolayers thick. Interestingly, the growth dynamics of CdSe
capped with hexadecylamine (HDA) and TOPO were found to be signi
d
n
1
y
4
n
g
|
3
cantly
di
erent from CdSe particles grown in TOPO. With the amine present, fast
focusing of the size distribution was observed (as opposed to defocusing,
Ostwald ripening) upon prolonged heating, therefore the narrowest size
distribution coincided with the largest particle size at a speci
ed growth
temperature. It was also observed that large amounts of amine in the
surfactant mix resulted in precipitation of nanoparticles above 200
C.
Core/shell particles of CdSe/ZnS are now standard materials when robust,
luminescent and stable particles are required. Numerous studies have been
carried out on various aspects of photophysics utilising these materials.
Their inherent stability makes them ideal candidates for various applica-
tions, and although elemental sulfur or S(SiMe
3
)
2
are the usual precursors
for the shell chalcogen deposition, other precursors such as P
2
S
5
33
and
C
6
H
11
NCS
34
have also been used.
Core/shell particles of rods have also been grown.
35
Using the chemistry
described in Chapter 1, the core rods were prepared using hexylphosphonic
acid (HPA) and multiple injections of precursors. A large aliquot of rod
sample was isolated directly without separation. This was followed by the
addition of HDA (critical for high quantum yields) and subsequent heating at
120
C for 20 minutes allowed the amine to exchange with TOPO. The
temperature was then stabilised at 190
C and the standard ZnS precursors
added dropwise. If the rods were isolated by non-solvent interactions prior to
shell growth, as is usual, the morphology of the rods changed when redis-
solved in surfactant and the shell precursors added. By avoiding isolation,
the precursors for rod growth were present during shell growth, maintaining
the anisotropic structure. The optimum ratio of Zn : S precursor was found to
be 1 : 2, with deviation resulting in lower quantum yields.
In a standard experiment, a bare rod (22
.
4 nm) exhibited a quantum
yield of 3%. Addition of 1.3 monolayers, the optimum shell thickness for
spherical particles, resulted in an increase in quantum yields to 18%.
However, further deposition increased the quantum yield up to 28% (1.7
monolayers). Increasing the shell thickness even further decreased the
quantum yield, again attributed to the lattice mismatch, which was accom-
modated in thin shells but resulted in defect-prone thicker shells. The
maximum quantum yield in this system (40%) was observed in rods 11
3
nm, with a shell 2.3 monolayers thick. Photostability experiments showed
that rods without a shell displayed a rapid reduction in quantum yield and
precipitated irreversibly from solution upon intense illumination, while
core/shell rods remained stable. Core/shell rods with a low quantum yield
exhibited an increase in quantum yield a
er irradiation, attributed to
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