Biomedical Engineering Reference
In-Depth Information
Fig. 11.2
PL as a function of applied electric field for a single QD (
left
) and QDM (
right
).
Identification of the QDM states is aided by the similarities between the positive, negative, neutral,
and multi-exciton states [
2
].
Panel B
shows the region of neutral exciton anticrossing indicating
the formation of molecular bonding and antibonding orbitals for the hole states. Detail from a
positively charged exciton in the QDM is displayed in
panel A
, where the additional structure is
due to the fact that anticrossings occur in both the initial and final states. The nomenclature is as
described in the text
QD [
20
-
23
]. To facilitate the discussion of the various exciton states we will
introduce the following nomenclature,
e
B
e
T
h
B
h
T
X
q
,wheree
B(T)
represents the number of
electrons in the bottom (top) dot and similarly
h
B(T)
represents the number of holes
in the bottom (top) and where
q
is the charge state of the exciton. The position of
the electron and hole participating in the recombination are underlined, for example
a positively charged exciton, where the two holes reside in separate QDs, may
recombine via the hole in either the bottom
1
0
1
0
1
1
X
+
)
1
1
X
+
)
(
(
dot.
Perhaps the most obvious similarity is the onset of the emission from negatively
charged exciton states. This is indicated by the decrease of the intensity of the
neutral exciton emission (
1
X
0
and
1
0
or top
10
X
0
)at
∼
36 kV/cm and the subsequent increase
2
0
of the negative trion emission (
1
X
−
and
1
0
X
−
) approximately 6 meV lower
in energy. This well-understood shift in energy due to Coulomb interactions is
observed in both spectra and provides a good reference point from which to identify
the additional lines. For example, the lines which extend to high electric fields along
with, but a few meV higher in energy than, the neutral exciton are identified as
arising from positively charged exciton configurations (
2
X
+
and
1
0
20
X
+
). It is in these
states that the most obvious differences between the single QD and the QDM can be
observed. In the QDM a signature x-pattern is seen which arises from the symmetric
and antisymmetric molecular hole wavefunctions and the anticrossing of these levels
with applied electric field. These anticrossings occur in both the ground and excited
states providing the rich features observed.
