Chemistry Reference
In-Depth Information
5.4.1 Inverted Core/Shell Structures
Formation of a core/shell material where the shell is a narrower bandgap
material than the core should, in theory, result in photo-generated charge
carriers residing in the shell, as this would present the lowest energy state.
A report by the Klimov group, who investigated ZnSe/CdSe particles, their
recombination lifetimes and the theoretical spatial distribution of charge
carriers in particles of varying shell thickness using an e
d
n
1
y
4
n
g
|
3
ective mass
approximation, found that the particle could be tuned between type I and
type II structures by varying the shell dimensions.
104
Taking a
xed core (ZnTe) with a radius of 1.5 nm, the calculated spatial
distribution of the density function for the charge carriers within a range of
shell thickness (CdSe) is shown in Figure 5.6b, where three distinct regions
could be seen. For shells less than 1.1 nm thick, the electron and holes were
delocalised over the entire structure while the density maximum was found
to be in the ZnTe core, despite the fact that bulk energy diagrams would
suggest carrier localisation in the shell. This behaviour is type I, where the
charge carriers were con
ned to the core. With a shell thickness of between
1.1 and 1.6 nm, the carriers were separated between the core and the shell,
which equated to type II behaviour. With shells thicker than 1.6 nm, the
.
Figure 5.6
(a) Diagram of the conduction band (right-hand side) and valence band
(le
-hand side), showing radial distribution for a 1.5 nm core ZnTe
particle with a 1.4 nm thick CdSe shell (type II system, with carriers
delocalised over the entire particle). (b) Plot of radial distribution for
electron and hole for a 1.5 nm ZnTe particle with a CdSe shell of
varying thickness, showing the three distinct regimes. Reprinted with
permission from L. P. Balet, S. A. Ivanov, A. Piryatinski, M. Achermann
and V. I. Klimov,
Nano Lett.
, 2004, 4, 1485. Copyright 2004 American
Chemical Society.
Search WWH ::
Custom Search