Conductance model of gold-molecule-silicon and carbon nanotube-molecule-silicon junctions
Luis A. Agapito, Eddy J. Bautista, and Jorge M. Seminario
Department of Chemical Engineering and Department of Electrical and Computer Engineering, Texas A&M University,
College Station, Texas 77843, USA
Received 30 July 2006; revised manuscript received 28 June 2007; published 14 September 2007
We estimate the conductance of molecular junctions composed of an oligophenylene ethynyleneOPE
molecule sandwiched between a metallic gold or carbon nanotube on one end and a semiconducting silicon
contact on the other end. Two very well defined, low and high, states of conductance logic “0” and “1” are
obtained through changes in conformation or charge states of the OPE when two metallic contacts address the
molecule. However, when a combination of a semiconducting and metallic contacts are used, the bistable states
are lost at low bias voltages where a flat region of nearly zero current in the current-voltage characteristic is
predicted regardless of the conformation or charge state.
DOI: 10.1103/PhysRevB.76.115316 PACS numbers: 31.15.Ew, 31.15.Ar, 31.70.Ks
I. INTRODUCTION
The semiconductor industry entered the nanometer regime
100 nm in the 21st century and continues today in the
race for miniaturization. The first commercial single-
molecule-based device is most likely to be built around Si.
At sizes approaching the quantum-confinement regime,
the electrical properties of silicon, and any other material,
diverge from the bulk properties. For example, studies have
shown the increase of the band gap with the decrease of the
size of the semiconducting nanostructure.
1–3
For silicon
nanowires, theoretical calculations have shown the quantum
effects are substantial at diameters below than 3 nm.
4–9
Quantum-mechanical calculations of the type presented in
this work are necessary for devices containing Si nanostruc-
tures in the quantum-confinement regime.
Advances in synthetic chemistry have allowed the direct
attachment of organic molecules on Si substrates,
10,11
open-
ing the door for hybrid organic-semiconducting devices.
Molecules, such as oligophenylene ethynyleneOPE, have
been synthesized to be used as electronic switches to store
binary information encoded in two molecular bistable states;
for instance, a state of high conductance and a state of low
conductance.
12
We consider the effect of Si contacts on the
bistable properties of a specific OPE molecule that contains a
nitro group in the middle phenyl ring, the nitroOPE molecule
1 in Fig. 1.
A Schottky diode formed when a metal and a semicon-
ductor are in intimate contact acts as a current rectifier.
Therefore, in a macroscopic metal-device-semiconductor
junction, the simultaneous use of a semiconducting and a
metallic contact implies a tremendous change in the proper-
ties of the device. In other words, the electrical behavior of
the device may collapse because of the rectifying behavior of
the contacts. The challenge is to use Si as one of the contacts
in metal-nitroOPE-Si molecular junctions without destroying
the bistable characteristic attributed to the nitroOPE mol-
ecule. The rectifying behavior has been experimentally ob-
served to vanish as the size of the metal-semiconductor junc-
tion approaches the nanometer regime, i.e., ultrasmall
Schottky diodes.
13–15
This gives hope for using Si as a con-
tact material in single-molecule-based electronic devices.
In this work, we perform quantum-mechanical calcula-
tions to assess the ability of metal-nitroOPE-Si junctions to
keep the high- and low-impedance states found in metal-
nitroOPE-metal junctions. Our study considers the different
charge states neutral, anion, dianion, and trianion as well as
the coplanar and perpendicular conformations of the ni-
troOPE molecule. Both gold and the metallic 4,4 carbon
nanotube CNT are tested as metallic contacts.
Recently, several procedures have been reported for at-
taching covalently aromatic hydrocarbons arenes to
CNTs.
16–18
Manipulation of CNTs has been limited since
they are synthesized as bundles or ropes. Because of the
tendency to agglomerate, CNTs present low solubility and
dispersion when placed in polymer matrices.
19
The ability to
attach arene “handles” to CNTs allows direct manipulation of
this amazing form of carbon, opening new possibilities for
using individual CNTs as molecular devices.
Moreover, several functionalization techniques have been
reported to react faster in metallic CNTs rather than in semi-
conducting ones,
20–22
which has allowed the separation of
CNTs based on their electronic properties, i.e., metallic from
semiconducting.
20
These advances have opened the possibil-
ity of using metallic CNTs as tips for contacting organic
molecules.
On the other hand, the synthesis of nitroOPE molecules
perpendicularly assembled on a hydride-passivated Si 111
substrate, with the top end covalently attached to a metallic
CNT, i.e., the metallic CNT-nitroOPE-Si junction shown in
Fig. 1, has been reported.
23
Computationally, the use of at-
oms with smaller atomic number, such as carbon instead of
gold, has the advantage of allowing a full-electron study of
the system, which leads to a more precise calculation.
II. METHODOLOGY: QUANTUM-MECHANICAL
CALCULATIONS
An isolated molecule has discrete electronic states, which
are precisely calculated from the Schrödinger equation.
When the molecule is attached to metallic contacts, the con-
tinuous electronic states of the contacts modify the electronic
properties of the molecule. A technique that combines the
density functional theory and the Green function
PHYSICAL REVIEW B 76, 115316 2007
1098-0121/2007/7611/11531612 ©2007 The American Physical Society 115316-1