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The analysis, design, and simulation of molecular electronic devices using ab initio based
methods: The negative differential resistance
Jorge M. Seminario, Roy Araujo, Liming Yan, and Yuefei Ma
ABSTRACT
There are tough difficulties interpreting experiments performed to demonstrate the existence of
important characteristics needed for the implementation of molecular electronics [I]. On the other
hand, it is believed that atomistic systems can be used as electronic devices if they present highly
nonlinear current-voltage (I-V) characteristics like negative differential resistance (NDR) [2].
This is because the interconnection of even two NDR devices in series yields multivalued I-V
characteristics for the composed system, which can eventually be pro&!ammed to perform a
controllable logic function. At this point the observation of NDR in atomistic systems can not be
only associated to the molecules employed for that purpose but ir can also be associated to the
contacts used to attach the molecules. In most
cases it was assumed that the NDR is due to
metal-molecule interface. Some experiments
have confirmed that when molecules were
6'ox104
4th, -10-10
eliminated from their settings that showed
4.0~10.
NDR,
still NDK \vas found suggesting that
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Q
0.0
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NDR was most likely due IO electromigration
-2.ox10~
effects. This implies lhat earlier assignments
3rd, 0-5V
-4.OXlO~
of NDR to molecules might have been wrong
4.0x104
and that NDK 3hould be assigned to the
!I
-8.OXlO~
creation and destruction of metallic filaments.
-10
-5
0
5
10
The
on the left shows the I-V curve
v (V)
measured in our lahs on a nanocell device
provided by Tour ai Rice University. This
nanocell is a square chip with a substrate of Si02 where a discontinuouz gold film was deposited
in Franzon labs at NCSU. Nitro molecules were self-assemble on gold nanorods which were
deposited onto the box containing the gold islands. The nature of the ohservcd NDR is difficult
to determine experimentally. It may be due to the molecules or the metals alone. Similar
experiments have been carried out at Rice University without the niolecules and also NDR was
observed. Regardless of whether NDR is due to a molecule, metal. insulator. semiconductor, or
their interfaces, the mechanism of NDR may involve several procozes that allow switching due
to changes of electronic, charge, conformational, or vibrational states. Several of these processes
may involve chemical reactions yielding the creation and destruction of honds.
We show theoretically that NDR is an atomistic characteristic thnr can take place whenever a
small number of atoms is formed by any means (electrically, chemically. or mechanically). This
small number of atoms could be a molecule or could be a cluster that may include metallic atoms.
. Therefore, we demonstratc that SDK is a typical property
of small atomistic systems: w e lind that the smaller the
TB?ZERb
atomistic systems the better the nossibilities to findNDR.
The
on the left is from molecular dynamics
?@
U
simulations showing a cyclic behavior that may produce
, I .
switching in filamentary gold clusters when an excessive
@%
current is applied and is ahlc to increase the temperature
of wire. The switching is due to changes in the tunneling
probability due to the movement of the gold atoms. This
=
1
~~~
-%
.
-I,?.,
1
~~~~~~~~~~
415
sequence may even become cyclic since once the wire is broken the temperature decreases
allowing the wire to rebuild again.
However, a pure molecular switching was also reported and thoroughly studied in the past
where it was shown that the effect of NDR was
100:
due to a combination of charge and torsional
effects in the molecule [3]. Latter on, another
study was performed to demonstrate that a
dmitro molecule shown in the inset of the
on the left present NDR behavior due to
electronic and charge effects 141. It was shown
that the molecule changes radically its
molecular impedance when it goes from a
0
0.5
1
1.5
2
2.5 neutral charge state (0 charge) to the anion state
Voltage
(-1 charge) and also when it goes from the
anion to the dianion state (-2 charge). These calculations showed unambiguously that NDR is a
molecular property for the setting used in the calculation and that it was due mainly to charging
(changes in the charge state of the molecule) and electronic (changes in the molecular orbitals of
the molecule) effects.
We report high level calculations on small wires of gold atoms that show the existence of
NDR; for instance, in the case of a negatively charged chain of four gold atoms as shown in the
below (pink curve). There is a pegk centered roughly at 3.5 volts due to changes in the
electronic structure of the cluster produced by the extemal bias voltage. This behavior can be
predicted because the existence of sophisticated ab initio methods that allows to consider the
chemistry of the system. As an example, a
calculation on the same system of four gold
atoms when the effect of the extemal field due
to the bias voltage is not considered (red) yields
different results to the case when the field is
explicitly considered (blue).
In summary, NDR is a characteristic typical
of atomistic systems most likely observed in
small organic molecules having electronegative
groups, which allow trapping electrons and thus
Voltage (v)
changing the electrical characteristics of the
molecule. NDR is also due to changes in
energies and shapes of the molecular orbitals. Other types of changes due to translational,
rotational, conformational, vibrational, spin, and reactive states may also influence in the
existence of NDR; however, they are not particular of single molecules; they are also properties
of small clusters of atoms.
This work is suuuorted bv mants from the ARO and DARPA.
1
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REFERENCES
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[Z]
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_
I
J. M. Tour, L. Cheng, D.P. Nackashi, Y. Yao, A. K. Flatt, S. K. S. Angelo, T. E.
Mallouk, and P. D. Franzon, "NanoCell Electronic Memories," J. Am. Chem. Soc.,In
press, 2003.
J. M. Seminario, L. E. Cordova, and P. A. Derosa, "An ab initio approach to the
calculation of current-voltaee characteristics of ~Uromammable molecular devices," Proc.
I€€€,91, in press, 2003.
J. M. Seminario, A. G . Zacarias, and J. M. Tour, "Theoretical Study of a Molecular
Resonant T ~ e l i n Diode,"J.
g
Am. Chem. Soc.,122, 3015-3020, 2000.
J. M. Seminario, ATG. Zacarias, and P. A. Derosa, "Analysis of a Dinitro-Based
Molecular Device,"J. Chem. Phyx.116, 1671-1683,2002,
I
[3]
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