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the shell (not from the formation of a type II structure) with quantum yields
increasing to up to 6%. Addition of a further shell to aid phase transfer into
water was essential, although the deposition of a ZnS shell failed, possibly
due to the lattice mismatch. A shell of ZnSe was successfully grown using
Me
2
Zn and TOPSe, which resulted in a further red shi
of up 15 nm, with
a
nal quantum yield of 3.5% and particles 5.78 nm in diameter. An inter-
esting observation has been reported by Mokari
et al.
, where addition of
a gold precursor to InAs QDs resulted in di
d
n
1
y
4
n
g
|
3
usion of gold into the QD,
resulting in a crystalline gold core with an amorphous InAs or In
2
O
3
shell.
143
Impressively, the absorption spectrum of InAs was maintained although the
excitonic
ne features were lost.
Although InAs QDs were the
rst core materials to be used in the synthesis
of heterostructures based on III
V materials, their uses are primarily limited
to infrared applications, such as tissue imaging where they have been
extremely successful. The more common III
-
V material is InP which, in
theory, has a wider spectral range, notably across the visible region. The
problem with InP QDs is the inherently low-emission quantum yield, which
rarely reaches 5% without surface etching procedures with HF, as described
in Chapter 2. The material is therefore an ideal candidate for shell deposi-
tion, as the poor emissive properties demonstrably improved upon surface
treatment.
Micic
et al.
were the
-
rst to explore the possibility of improving the
optical properties of InP QDs by depositing a shell layer,
144
shortly a
er the
rst reports of InAs-based core/shell particles were described by Cao
et al.
With reference to bulk systems where Zn
0.475
Cd
0.525
Se
2
is an ideal lattice
match for InP, a shell of ZnCdSe
2
was deposited on core particles of InP.
The QDs of InP with average sizes between 2.5 and 4.5 nm in diameter were
synthesised as described in Chapter 2, then precipitated and dispersed in
pyridine (in a similar style to the
.
rst report of CdSe particles being
prepared for a CdS shell).
40
The shells were deposited in pyridine at 100
C
by the dropwise addition of Me
2
Zn, Me
2
Cd and TBPSe at a ratio of 1 : 1 : 4
to ensure complete formation of the shell, followed by addition of OAm to
the solution. The formation of small ZnCdSe
2
particles also occurred,
although these were easily removed by size-selective precipitation due to
their poor solubility in pyridine. Shells of up to 5 nm thick were added to
InP cores, with particles up to 20 nm in total diameter being reported
which still exhibited excellent colloidal stability due to OAm on the particle
surface.
A red shi
er
shell deposition, consistent with the exciton leakage, which became more
pronounced with the addition of further monolayers. Band edge emission
with quantum yields of 5
was observed in the absorption spectra of InP/ZnCdSe
2
a
10% were observed upon shell growth on 2.2 nm
and 4.2 nm cores, which exhibited little or no emission prior to deposition.
The emission could be tuned from
ca.
600 nm for a 3 nm InP QD with a 1.5
nm shell, to
ca.
700 nm for the same core with a 5 nm shell. Interestingly, InP
QDs etched with HF to initially improve the emission could not be e
-
ectively
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