Biomedical Engineering Reference
In-Depth Information
Fig. 5.5
Plots of the highest five valence band state,
H
1
,
H
2
,
H
3
,
H
4
,and
H
5
, for the QDM-4.
Only the side views of the plots are shown. The
horizontal dotted lines
are marked to guide the
eye and indicate the base of the QD layers in the molecule. The intensity of the color in the plots
indicates the magnitude of the wave function: the
red color
represents the highest magnitude and
the
light blue color
represents the lowest magnitude. The energies of the valence band states are
also mentioned
particular quantum dot layer inside the molecule. This is different from the previous
study of the bilayer QD stack [
5
], where the weakly coupled quantum dot layers
at 10 nm separation resulted in atomic-like electron wave functions. In that study,
the spatial location of both the electron and the hole wave functions determined the
optical activity of the QD layers.
Previous studies on the identical bilayer QDM [
35
] and on the QDM with seven
identical QD layers [
41
] showed that the strain tends to push the electron states
towards the lower QDs in these systems. The electron wave functions for the QDM-
2 and the QDM-3 systems show similar trend. However we find that this trend is no
longer true for the molecule with the nine QD layers, QDM-4, where the electron
wave function
E
1
vanishes around the edges of the stack due to the larger strain
magnitude there (see Fig.
5.4
first row for the QDM-4).
5.3.4
Hole Wave Functions exhibit Atomic Character
Although the electrons (lighter-mass particles) are strongly influenced by the inter-
dot electronic and strain couplings of the QDs and exhibit tunnelling across the
quantum dots forming molecular-like hybridized states, the holes due to their
heavier mass remain well confined inside the individual dots and do not show any
hybridization. For example, Fig.
5.5
shows the side views of the lowest five valence
band wave functions in the QDM-4 system. The horizontal dotted lines are plotted to
mark the positions of the base of the QD layers and helps to determine the location
of a particular hole wave function inside the molecule. In this system,
H
1
and
H
3
are
inside L
2
,
H
2
and
H
5
are in L
3
,and
H
4
is in L
8
. The location of a hole state inside a
