Field-induced conformational changes in bimetallic oligoaniline junctions
Juan C. Sotelo,
1
Liuming Yan, Michael Wang,
1
and Jorge M. Seminario
1,2
1
Department of Chemical Engineering, Texas A&M University, College Station, Texas 77843, USA
2
Department of Electrical and Computer Engineering, Texas A&M University, College Station, Texas 77843, USA
Received 26 August 2006; revised manuscript received 23 October 2006; published 21 February 2007
We report three types of nonplanar conformations, , , and , for a neutral isolated oligoaniline molecule
as well as for an oligoaniline with Au and Pd atoms attached at its ends. Each type of conformation has several
conformers of nearly equal energies. An applied external voltage can be used to switch between conformations,
producing in the process a sharp decrease of their energies. These bias voltage-induced conformational changes
are a potential switching mechanism for two terminal molecular devices at the nanoscale domain. They cause
the conductivity of the molecule to alternate between high and low states, compensating for the behavior of
typical three-terminal devices, needed for the development of a gate-less electronics.
DOI: 10.1103/PhysRevA.75.022511 PACS numbers: 31.15.Ew, 33.15.Bh, 31.70.Ks, 31.15.Ar
I. INTRODUCTION
Molecular switches are essential components in molecular
electronics devices as they constitute the building blocks in
logic architectures. Lately, much effort has been devoted to
the identification of these switches, and to the understanding
of the intrinsic mechanisms governing their switching prop-
erties. Many types of switching mechanisms have been iden-
tified, among them, photon-induced 1, change of redox
state 2–4, and electrically induced switching 5–8 to name
a few. A typical electrically driven molecular switch consists
of a single molecule, or many of them, e.g., self-assembled
monolayer SAM, adsorbed on metal electrodes, which can
be electrically toggled between a high conductance ON
state and a low conductance OFF state, with the ON and
OFF states being stable enough for a large number of switch-
ing cycles. Recently, it has been shown experimentally that
for a wide range of temperatures oligoaniline molecular
junctions can exhibit electrical bistability and switch be-
tween a high and low conducting states in response to an
applied external bias voltage 7. It was suggested this be-
havior might be due to a redistribution of charges in the
molecule coupled with a shift in molecular conformations
7.
In this paper we show that there are three main types of
conformations for the standalone and for the extended with
electrode atoms of Au or Pd oligoaniline molecule, called ,
, and , which undergo conformational changes under the
influence of an external longitudinal electric field. In an Au-
oligoaniline-Pd junction, when equilibrium is reached at
room temperature, has the highest probability to be present
in the junction, has the smallest. Furthermore, when the
junction is subjected to an external bias voltage there is an
interconversion between the and conformations leading
to a switching behavior between high and low current
states in the junction, as observed in experiments 7.
We deal with the conformational changes by considering
first an extended oligoaniline molecule as a protojunction
where a sought after property of a molecular switch, such as
conformational changes, can be probed more easily 9,10.
Although by considering just the extended molecule, the in-
teraction with the leads of both the extending metal atoms
and the molecule is missing, and the dielectric effect of the
leads on the field is not considered, nevertheless, it can still
help elucidate the field-induced changes in molecular confor-
mation and electronic structure for the following reasons.
First, on a first approximation the interaction metal-molecule
may be considered local, with metal atoms in direct contact
with the molecule or in the near neighborhood contributing
the most. This is, in fact, just a restatement of the rationale
behind treating a molecular junction as a metal-extended-
molecule-metal system 11–14. Second, the molecular and
electronic structures of the extended molecule are those ob-
tained directly from ab initio calculations, which should al-
low for a detailed analysis at the atomic level of structural
e.g., bond length, torsional angles and electronic properties
e.g., polarization, charge distribution changes taken place
within a molecule exposed to an external field. Such a fine
level of resolution is lost when bulk electrodes are brought
into consideration because of the coarse approximations that
have to be made to treat them in present electron transport
models. Ultimately, to take full advantage of molecular de-
vices one would like to interconnect them, at most, through
small metal clusters rather than through bulk metal solids
8. Once the key properties of the extended molecule have
been established, they can be included within an appropriate
framework, such as the Green function method introduced
below, to study the current through the junction.
This paper is organized as follows. Section II introduces
the computational methods for dealing with the electronic
properties of finite clusters and bulk electrodes, and the
current-voltage I-V characterization of the junctions. Sec-
tion III presents the DOS and transmission spectra for all
conformations, as well as their I-V characteristics at fixed
geometries. The section ends with a discussion of conforma-
tional changes under applied electric fields. Finally, Sec. IV
summarizes the results of this work.
II. METHODOLOGY AND PROCEDURE
Density functional theory DFT and Green’s function
GF theory are used to study the electrical properties of
conformations of the standalone oligoaniline and oligoa-
niline junctions. Basically, three different types of calcula-
PHYSICAL REVIEW A 75, 022511 2007
1050-2947/2007/752/02251113 ©2007 The American Physical Society 022511-1