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than Dot1 (red curve), while the carbazole charge (Dot3, blue curve) does not
significantly vary. Changing the sign of the switching field the situation is dual.
For both positive and negative applied fields, the displacement of charge between
Dot1 and Dot2 is small and this represents a problem in the encoding and
identification of the two logic states. Thus, for QCA application it could be
necessary to oxidize or reduce the molecule.
Bis-ferrocene charges: Dot1, Dot2, Dot3
Dot1
Dot2
Dot3
0.2
0.1
0
-0.1
-0.2
-4
-2
0
2
4
Electric Field [V/nm]
Fig. 23. Bis-ferrocene molecule: dot charges as function of the switching field (Color
figure online).
Considering the oxidized version of the bis-ferrocene, at the equilibrium
the free positive charge is mainly delocalized between the working dots, while
the third dot is almost neutral. The application of the switching field moves the
positive charge entirely on one of the two dots, depending on the sign of the
electric field, and the third central dot remains almost neutral. The trend of
the dot charges of the oxidized bis-ferrocene as a function of the switching field
is reported in Fig. 24 : the oxidized bis-ferrocene has a non linear behavior, since
for a given value of the switching field the charge distribution “saturates”, that
means that the whole positive charge is confined in one of the two active dots
and even increasing the magnitude of the electric field the charge distribution
does not significantly vary. The results shown in Fig. 24 are important because
they highlight the QCA properties of the bis-ferrocene molecule, as well as the
suitability of a write-in system based on electric field.
Given the charge distributions of the oxidized bis-ferrocene in different bias
conditions, the generated electric field was computed as second part of the sec-
ond stage of the analysis flow (post-processing stage described in Sect. 2.2 ). In
particular, the X component (parallel to the dot axis) of the electric field ( E x )
generated by the molecule at the equilibrium was computed considering an ideal
receiver placed at the distance of 1.0 nm from the molecule along the Y axis,
so that a squared QCA cell could be formed. The values of the electric field
magnitude are reported in Fig. 25 for all the points of the XZ plain.
 
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