Chemistry Reference
In-Depth Information
of the shell. Phase transfer resulted in a slight drop to 0.10% and a slight
emission shi
of
ca.
15 nm. Other similar structures with a type II/type I
alignment have been prepared such as CdSe/CdTe/ZnSe.
120
The core CdSe/
CdTe was prepared as described earlier using the SILAR-thermal cycling
technique
88
and simply expanded to include the deposition of a
nal ZnSe
shell using similar precursors. Using this method, spherical and peanut-
shaped CdTe/CdSe particles were capped with the extra shell. The particles
were found to emit in the near-infra red as described earlier, and the addition
of the ZnSe shell slightly red-shi
d
n
1
y
4
n
g
|
3
ed and broadened the emission further,
attributed to a compressed CdSe lattice. As expected, the quantum yields
improved from
ca.
20% to between 50% and 60% upon ZnSe shell growth and
e
cient phase transfer to water was possible.
Likewise, CdS/ZnSe/ZnS core/shell/shell particles have also been prepared
using standard green precursor chemistry.
121
Core CdS particles and ZnSe
shell have staggered bandgaps as described earlier, and band edge emission
(at
ca.
400
ed by over 100 nm by
the deposition of a ZnSe shell. The addition of a ZnSe shell increased the
emission quantum yield from (a maximum of) 33% to a maximum of 56%,
and the addition of a
-
475 nm) for CdS was signi
cantly red-shi
nal ZnS shell increased the quantum yield slightly to
up to 60% while red-shi
ing the emission even further.
An interesting related advance in the formation of type II structures is the
development of a cascade core/shell/shell system, where all three energy bands
are o
set.
122
In the case of CdSe/CdTe/ZnTe, excitation resulted in the electron
residing in the CdSe core, the hole was con
ned in the ZnTe shell, and
recombination occurred across the CdTe layer. Core particles of CdSe, 3.6
.
5.7
nm in diameter, were prepared using CdO as a precursor, with CdTe shells
(
ca.
1.7 nm thick) and a
-
nal shell of ZnTe (
ca.
1.3 nm thick) deposited using
CdCl
2
, Zn(CO
2
(CH
2
)
16
CH
3
)
2
, TBPSe and TBPTe in TOPO andHDA at
ca.
200
C.
The absorption spectra remained
ca.
at 600 nm but lost the excitonic peak
upon shell formation. The core particles exhibited narrow band edge emission
at
ca.
550 nm, which shi
ed to a broad emission at
ca.
1000 nm upon CdTe
shell deposition, and
ca.
1500 nm upon deposition of the ZnTe shell. The
emission quantum yield dropped from 28%, to 0.12% upon addition of the
CdTe shell, and to 1.1
10
5
% when the
nal ZnTe shell was added.
Multiple-shell systems present a unique opportunity to build particles with
a more speci
nement; core/shell particles can be tuned to have charge
carriers in the core, the shell or across the whole particle. Multiple-shell
particles can be prepared where the charge carriers are restricted to the
middle layer,
i.e.
the
c con
rst shell. By using a wide-bandgap core particle
(template), carefully depositing a narrow-bandgap shell, followed by a
nal
wide-bandgap layer, a solution analogue of a quantum well can be produced.
In materials prepared in this manner, quantum con
ects should
only manifest themselves along the radial dimensions. Although early
studies investigated such systems, such as CdS/HgS/CdS,
13
-
17
the quality of
the nanoparticles produced was lower than that of particles produced by
organometallic-based precursors.
nement e
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