Chemistry Reference
In-Depth Information
referred to as
and extended to
pseudo-core/shell material.
62
In a seminal example of growth doping, ZnSe
core particles were prepared as described in Chapter 1 and the growth
arrested by reducing the growth temperature. The dopant ions (Cu) were
then added to produce the small doped structure, with an emergence of the
doped emission being observed at
ca.
525 nm. Further growth of ZnSe
resulted in the structural embedding of the dopant ions well within the
particle structure, avoiding the usual problem with doping; ensuring the
ions remain in the interior of the QD. As recombination occurred between
a ZnSe generated electron and a hole from the d-orbital of the Cu
2+
ions, the
resulting emission could be red-shi
'
nucleation doping
'
or
'
growth doping
'
d
n
1
y
4
n
g
|
3
ed with changing particle size due to
the overall change in ZnSe bandgap. In this example, no core/shell structure
was prepared. However, in nucleation doping, the dopant precursor
(
e.g.
manganese carboxylates) and host precursor (ZnSe) were added
together in the initial injection, resulting in the immediate nucleation of
both species. By then tailoring the growth conditions, only the host material
(ZnSe) grew, resulting in essentially a core/shell species with a MnSe/ZnSe
structure. The optical properties were consistent with manganese-doped
ZnSe, with emission at
ca.
580 nm from the
4
T
1
/
6
A
1
transition inMn
2+
and
no emission attributable to ZnSe; these materials were therefore referred to
as doped nanostructures despite having, essentially, a core/shell structure.
In the case of MnSe/ZnSe, pure MnSe cores of up to 4 nm were observed,
with an overall zinc blende crystalline structure detected. The emission
quantum yields of MnSe/ZnSe were up to 30%, with the emission tuneable
between 575 nm and 595 nm with increasing shell thickness. In this case,
the shi
.
eld splitting of the dopant ion
becoming smaller. Further work on this system highlighted the importance
of the di
was attributable to the crystal
used interface between the core MnSe particles and the ZnSe shell,
and the use of small MnSe clusters.
63
Related to this is the preparation of
ZnSe/ZnMnS/ZnS core/shell/shell materials, using standard techniques.
64
The particles consisted of 2.5 nm core ZnSe particles, with a further
1.5
2 nm shell. This material was again developed to overcome issues with
doping simple QDs, and emission from the Mn
2+
state was found to be poor
until the
-
nal shell layer was deposited. Design of the structure took into
account several factors; the preference for Mn
2+
to interact with a ZnS matrix
rather than a ZnSe one, and the increased e
ciency of impurity emitters
when they are located within a shell or on a nanoparticle edge. The
absorption edge red-shi
ed with the deposition of an increasing number of
shells, while the emission from the Mn
2+
state reached an average quantum
yield of 25%. Quantum yields of up to 65% have also been reported when
a thick ZnS shell was deposited, in conjunction with speci
cligandchem-
istry and a speci
c ratio of precursors.
65
Similar results have been achieved
by doping copper on to the surface of small (
ca.
2.9 nm diameter) ZnSe
particles, followed by further shell growth, yielding particles with emission
quantum yields of up to 20%.
66
Larger particles (>3.6 nm) were found to be
too unreactive to adsorb the dopant ions onto the surface.
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