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3.4
Interaction Among Molecules in a Wire
Following the methodology described in Sect.
2.2
and concerning the post-
processing stage of the analysis flow, the interaction among molecules in a
QCA wire was evaluated, considering two inter-molecule distances: the ideal
one (1.0 nm) equal to the width of the bis-ferrocene, so that a squared QCA cell
is formed, and a lower distance equal to 0.8 nm. In Fig.
31
(A) the charges of the
two main dots (Dot1 and Dot2) are reported in case of standard inter-molecule
distance: for sake of brevity, only the first part of the wire was reported, consid-
ering only five molecules. In this case, the logic state of the molecules alternates
along the wire, but the charge displacement between the two dots is smaller and
smaller while increasing the number of molecules. In particular the fifth molecule
is in an undefined state since the dot charges are almost the same. For this rea-
son, the logic signal could be considered valid only for the first three molecules,
while from the fourth molecule on the state is not defined.
(A) Inter-molecule distance = 1.0 nm
(B) Inter-molecule distance = 0.8 nm
1
1
Dot1
Dot2
Dot1
Dot2
0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
0
0
-0.2
-0.2
M1
M2
M3
M4
M5
M1
M2
M3
M4
M5
Molecule in wire
Molecule in wire
Fig. 31.
Molecular QCA wire: dot charges of the molecules along the wire with a
molecule-molecule distance equal to 1.0 nm.
On the other hand, when the molecules are placed at 0.8 nm far from each
other, the logic state of the molecules along the wire alternates as well, as shown
in Fig.
31
(B). Moreover, the difference of charge between the two dots is still
huge enough to consider all the molecules in a defined logic state.
As additional figure of merit for this analysis, the electric field generated by
the molecules along the wire was computed in both the cases of distance. In par-
ticular, each molecule along the wire, depending on its logic state, generates an
electric field whose absolute value is maximum near the occupied dot. Figure
32
shows a top view of the electric field generated by the first five molecules of the
wire, computed at the position of an ideal receiver. The picture shows how the
picks of the electric field move following the position of the free charge inside
the molecule, but the intensity of the electric field decreases with increasing
number of molecules.
On the contrary, in case of distance equal to 0.8 nm, the positive and nega-
tive peaks of the electric fields alternate as well (Fig.
33
), but the values of the
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