Seminario J.M. Molecular and Nano Electronics. Analysis, Design and Simulation
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44 Luis A. Agapito and Jorge M. Seminario
Table 10 Comparison of the molecular orbitals for the coplanar and the perpendicular conformations of the Au–nitroOPE–Au junction. An isosurface
of value 0.02 is used for all the plots
LUMO+3 LUMO+4LUMO+2LUMO+1LUMOHOMO–4 HOMO–3 HOMO–2 HOMO–1 HOMO
neutral coplanar neutral perpendicular
Metal–molecule–semiconductor junctions 45
Table 11 Molecular orbitals for the coplanar and the perpendicular conformations of the CNT–nitroOPE–CNT junction. An isosurface of
value 0.001 is used for all the plots
LUMO+3 LUMO+4LUMO+2LUMO+1LUMOHOMO–4 HOMO–3 HOMO–2 HOMO–1 HOMO
neutral
coplanar
neutral
perpendicular
46 Luis A. Agapito and Jorge M. Seminario
Table 12 Molecular orbitals for the coplanar and the perpendicular conformations of the Au
6
–nitroOPE–Si junction. An isosurface of
value 0.001 is used for all the plots
LUMO+3 LUMO+4LUMO+2LUMO+1LUMOHOMO–4 HOMO–3 HOMO–2 HOMO–1 HOMO
neutral
coplanar
neutral
perpendicular
Metal–molecule–semiconductor junctions 47
Table 13 Comparison of the molecular orbitals for the coplanar and the perpendicular conformations of the CNT–nitroOPE–Si junction.
An isosurface of value 0.0001 is used for all the plots
LUMO+3 LUMO+4LUMO+2LUMO+1LUMOHOMO–4 HOMO–3 HOMO–2 HOMO–1 HOMO
neutral
coplanar
neutral
perpendicular
48 Luis A. Agapito and Jorge M. Seminario
We determined that the low- and high-conductance states, observed for metal–
nitroOPE–metal junctions, could still be obtained from a metal–nitroOPE–semicon-
ductor junction. The switch between the two states of conductance is because of the
conformational and charge-state changes of the molecular junctions. However, the oper-
ating point of the device should be set at higher bias voltages since a flat region of zero
current is predicted to appear at low bias voltages.
The change of conductance of the metal–nitroOPE–Si junctions due to charging effects
can be accounted by a simple electrostatic explanation. For the negatively charged junc-
tions, the ESP distribution along the junction reveals a region of negative electrostatic
potential, which repels the electrons, thus, it obstructs the flow of carriers.
Changes in conductance between the coplanar and the perpendicular conformation of
all the analyzed junctions can be explained by looking at the shape and spatial distri-
bution of their molecular orbitals. The coplanar conformations have molecular orbitals
that are more delocalized than the ones belonging to the perpendicular conformations.
This indicates that the molecular orbitals for coplanar conformations are more conduc-
tive than those from the perpendicular conformation, therefore, explaining the higher
conductance of the coplanar junctions.
Molecules can be used as electronic circuits to store binary information encoded in
two molecular bistable states: for instance, a state of high conductance and a state of
low conductance. Molecules can be arranged in crossbar architectures to create logic or
memory electronic devices. In both cases, the desirable property of bistability depends
not only on the molecule but also on the type and geometry of the contacts. Therefore,
our calculations consider the molecule and few atoms of the contacts as an entwined
unit, a molecular junction.
Advances in DFT and the increase in computational power of modern computers
have allowed us to perform full-quantum-mechanical calculations of molecular junctions
composed of up to a few hundred atoms. The continuum of electronic states, derived
from the semi-infinite nature of the contacts, has been added to the molecular junction
by using a mathematical formalism based on the Green function.
We have estimated the conductance of molecular junctions composed of the nitroOPE
molecule and two contacts of materials such as Au, Si, and carbon nanotubes. These
results are summarized in Figure 22. Two very well defined states of conductance,
requisite for digital electronics, are obtained from the nitroOPE by changing the confor-
mation and charge states of the molecule. The nitroOPE molecule is found to conduct
higher current (logic “1”) when all its phenyl rings are coplanar rather than when the
middle phenyl ring is perpendicular to the other two (logic “0”). In addition, bistable
states can be obtained by charging the molecule. When the nitroOPE is in its neutral
state, it conducts higher current (logic “1”) than when the molecule is negatively charged
(logic “0”).
We determined that when two gold contacts are used to address the molecule, the
bistable states of conductance are readily available at any bias voltage. However, when
a combination of one semiconducting and one metallic contact is used, the bistable
states are lost at low bias voltages. We found a nearly flat region in the current–voltage
characteristic of metal–nitroOPE–semiconductor junctions in the low bias voltage region.
Instead of presenting two distinguishable states, all the conformation and charge states of
the metal–nitroOPE–semiconductor junctions (coplanar, perpendicular, anion, dianion,
and trianion) showed almost indistinguishable zero conductance in the flat region. These
Metal–molecule–semiconductor junctions 49
–5 0 5
–0.3
–0.2
–0.1
0
0.1
0.2
0.3
–5 0 5
–0.3
–0.2
–0.1
0
0.1
0.2
0.3
–5 0 5
–0.3
–0.2
–0.1
0
0.1
0.2
0.3
–5 0 5
–0.3
–0.2
–0.1
0
0.1
0.2
0.3
–5 0 5
–0.3
–0.2
–0.1
0
0.1
0.2
0.3
Voltage (V)
Current (μA)
(A) (B) (C) (D) (E)
Figure 22 Summary of current–voltage characteristics for the following molecular junctions:
(A) Au
l
–S–nitroOPE–S–Au
l
, (B) Au
6
–nitroOPE–S–Au
l
, (C) CNT–nitroOPE–CNT, (D) Au
6
–
nitroOPE–Si, and (E) CNT–nitroOPE–Si
indicate that when a semiconducting contact is used, we must move the operating point
of the device to higher bias voltages.
Our calculations demonstrated that the recently synthesized CNT–nitroOPE–Si junc-
tion [29] presents the same potential as a bistable device that the Au–nitroOPE–Si
junction does. However, for the former case, charging of the junction does not lead
to two very distinguishable states; therefore, the two bistable states to consider should
come from the neutral coplanar (logic “1”) and the neutral perpendicular (“0”).
Quantum-mechanical effects cannot be disregarded whenever dealing with elec-
tronic devices of atomistic dimensions. For instance, we corroborated that a “molecular
Schottky diode” does not present the rectifying behavior associated with a macro-
scopic Schottky diode. It is most likely that future electronic devices will be purposely
engineered to take advantage of the most exotic quantum-mechanical properties for
applications such as quantum computation, quantum teleportation, and quantum cryp-
tography. Consequently, computational tools derived from the fundamental equation of
quantum mechanics, the Schrödinger equation, will be mandatory.
Acknowledgements
We thank the invaluable help of Mery Diaz Compos for her careful assistance in
several aspects of this manuscript and we highly appreciate the support from the US
Army Research Office (ARO), Defense Threat Reduction Agency (DTRA), and Defense
50 Luis A. Agapito and Jorge M. Seminario
Advanced Research Projects Agency (DARPA) as well as the research funds from the
TexasA&MUniversity.
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