Chemistry Reference
In-Depth Information
To explore such systems, particles of CdS/CdSe/CdS core/shell/shell
materials were grown
123
which emitted between 520 and 650 nm with
emission quantum yields of over 40%. CdS core particle templates 2.5
5nm
in diameter were prepared using CdO and sulfur in ODE as described in
Chapter 1. Up to 7 monolayers of CdSe were grown on the template epitaxially
using the SILAR technique described earlier, with CdO/oleic acid in ODE and
selenium in ODE being added sequentially at 180
C to grow the shell to the
required thickness. The
-
d
n
1
y
4
n
g
|
3
nal CdS shell was added using the SILAR technique,
having replaced selenium with sulfur and fatty acids. Up to 5 monolayers of
the
240
C and puri
cation resulted
in particles with the highest quantum yields, of about 40%. The emission
and absorption spectra resembled those of CdSe QDs, although there was
less detail in the high-energy region of the absorption spectra. Interestingly,
the extinction coe
nal shell, followed by annealing at 230
-
rst excitonic peak showed a linear rela-
tionship with shell thickness revealing a simple way to probe the exact
dimensions of the particle. The emission position red-shi
cient at the
ed with increasing
CdSe monolayers, as expected from size quantisation e
ects. An investiga-
tion into emission lifetimes suggested the existence of free two-dimensional
excitons which decayed radiatively when the CdSe layer was 1
3 monolayers
thick; 4 monolayers resulted in an increase in non-radiative recombination
due to defect formation in the thicker layer.
124
ZnS/CdSe/ZnS structures are again typical of such materials, and have been
prepared by the growth of ZnS core particles using (Et
3
NH)
4
[Zn
10
S
4
(SPh)
16
]
(discussed in Chapter 7) in HDA at 300
C, followed by particle isolation.
125
Addition of the ZnS cores to a fresh batch of HDA at 100
C, followed by
addition of (Et
3
NH)
4
[Cd
10
Se
4
(SPh)
16
], slowly resulted in the deposition of
a CdSe shell. During shell addition, monitoring of the emission highlighted
a gradual red shi
-
.
, and growth was stopped when the desired wavelength was
reached. The
ed
ZnS/CdSe particles in HDA by adding Et
2
Zn and S(SiMe
3
)
2
/TOP dropwise at
180
C until the maximum emission was observed
in situ
, usually at 1
monolayer of ZnS. The emission from the resulting particles could be tuned
between 450 nm and 650 nm, with quantum yields of up to 60%. XPS
highlighted that the separate components of the core/shell/shell systems
were not distinct phases with sharp interfaces, but the core region existed
as a ZnS/CdSe graded alloy, although the
nal shell was then deposited on freshly isolated and puri
nal shell layer had a sharp
interface.
A multiple-shell system has also been developed containing two quantum-
con
ned materials in one single particle;
the core (zero-dimensional
con
nement), followed by a wide-bandgap bu
er layer, and a
nal shell of
narrow-bandgap material exhibiting two-dimensional
nement.
Double quantum well systems, such as CdS/HgS/CdS/HgS/CdS have previ-
ously been investigated using wet colloidal chemistry;
126,127
however,
emission from the two separate wells was hard to distinguish, possibly due to
coupling. A similar system, CdSe/ZnS/CdSe, has been prepared by an
organometallic-based route that showed two emissive regions.
128,129
CdSe
(2D) con
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