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quench QD emission.
81
In this example, water-soluble CdSe/ZnS QDs were
coated with a layer of poly-
L
-histidine, which is capable of immobilising Au
3+
ions at high densities. The poly(ethylene glycol) (PEG) chains on the lipid, on
to which the poly-
L
-histidine was deposited, dictated the degree of separa-
tion. Addition of a reducing agent, NH
2
OH, induced the formation of the 2
3
nm thick gold shell on the histidine layer, and addition of a thiolated poly-
ethylene glycol (PEG) was added to ensure overall colloidal stability. The
absorption pro
-
d
n
1
y
4
n
g
|
3
le of the QD was hidden by the plasmon resonance of the
gold layer at
ca.
580 nm, while the emission from the QDs was found to be
reduced from
ca.
75% to
ca.
18%, attributed to the addition of the Au
3+
ions.
The emission was also found to reduce with increasing shell thickness, being
signi
-
gold
separation could be increased by introducing polyelectrolyte monolayers
between the two inorganic layers, which hindered the di
cantly quenched when the shell thickness reached 5 nm. The QD
usion of Au
3+
to the
core that was responsible for the emission quenching. By using this spacing
technique, the emission quantum yield was increased to up to 39% by the
addition of two bilayers.
5.4 Type II Materials
Type II core/shell structures are of interest due to the unusual mechanism of
charge recombination; the band energy levels are o
set such that one charge
carrier is predominantly con
ned to
the shell. By correctly choosing materials with a notable band energy
mismatch, an e
ned to the core while the other is con
.
ectively smaller recombination gap is produced, which
results in emission wavelengths that cannot easily be obtained by the single-
core II
VI materials. Infrared-emitting type II core/shell dots have been
successfully used in oncology studies, as the wavelength, once shi
-
ed to the
near infrared region, is compatible with tissue imaging.
82
The resulting
optical properties are usually characterised by a signi
cant red shi
in the
emission pro
le with a prolonged recombination lifetime, while retaining
roughly the same absorption position (although a loss of excitonic features is
common). One must be careful not to interpret all red shi
s in core/shell QD
emission as type II, as type I structures may exhibit red-shi
ed emission
(although usually much smaller) due to leakage of charge carriers into shell
materials that possess a low potential barrier. The optical properties of these
materials can also be manipulated by the injection of electrons, resulting in
signi
cant shi
s in the emission pro
les due to spectral switching.
83
rst type II core/shell materials to be prepared by solution-based
organometallic-type routes (CdTe/CdSe and CdSe/ZnTe) were synthesised
using the metal alkyl-based methods described in Chapter 1; the band energy
diagrams are shown in Figure 5.5.
84
As a result of recombination from an
e
The
ectively indirect structure with a smaller bandgap, the emission was
shi
ed into the near infrared region. The emission could be tuned further by
controlling the size of the core particle and the shell thickness. Increasing
the particle size and the shell thickness pushed the emission further towards
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