Atomic scale visualization of the single molecule junctions formed with two equivalent pathways (left) and one pathway (right), including the bonding to the tips of two gold electrodes and a schematic of the external electrical circuit.

In a paper published in Nature Nanontechnology
on September 2, 2012, scientists from the U.S. Department of Energy’s
(DOE) Brookhaven National Laboratory and Columbia University’s
departments of Chemistry and of Applied Physics explore the laws that
govern electronic conductance in molecular scale circuits.

“Everyone
who has worked with basic electronic circuits knows that there are some
simple rules of the road, like Ohm’s Law,” explains collaborator Mark
Hybertsen, a physicist at Brookhaven’s Center for Functional
Nanomaterials (CFN). Hybertsen provided the theory to model the observed
circuit behavior with the CFN’s computational tools. “For several years
we have been asking fundamental questions to probe how those rules
might be different if the electronic circuit is shrunk down to the scale
of a single molecule.”

Conductance
measures the degree to which a circuit conducts electricity. In a
simple circuit, if you hook the resistors up in parallel, the electrons
can flow through two different paths. In this case, the conductance of
the full circuit will simply be the sum of the conductance of each
resistor.

However,
in a molecular circuit, the rules that govern current flow now involve
fundamental quantum mechanics. In most single-molecule circuits, the
molecules do not behave like conventional resistors; instead, the
electrons tunnel through the molecule. When the molecule offers two
pathways in parallel, the wave-like movement of an electron can
dramatically change the way conductance adds up. For several years,
experts in nanotechnology have suspected—but not proven—that quantum
interference effects make the conductance of a circuit with two paths up
to four times higher than the conductance of a circuit with a single
path.

In
order to investigate these quantum mechanical effects further, the
scientists needed to construct their own controllable nano-size
circuits. Working with Ronald Breslow’s group at Columbia, they designed
and synthesized a series of molecules to use in the experiment.

“Reliably
making a circuit from a single molecule is really challenging,” says
Latha Venkataraman, a Columbia Engineering Applied Physics professor
whose group perfected the method used to make the molecular circuits.
“Imagine trying to touch the two ends of a molecule that is only ten
atoms long.”

To
make the circuits, Venkataraman’s group adapted a scanning tunneling
microscope (STM) apparatus to repeatedly press a sharp gold tip into
another gold electrode and then pull it away. When this junction breaks,
there is a moment when the gap between the two pieces of gold is a
perfect fit for the molecule. Once the circuit system is set up, the
conductance measurement is fast and can be repeated thousands of times
to get statistically reliable data.

Using
this approach, the scientists discovered that the molecules with two
built-in pathways like the one visualized in the figure at right had a
conductance that was greater than the sum of each arm’s conductance,
although the increase was not as large as they had anticipated. In order
to understand this effect better, Columbia’s Hector Vasquez worked with
Hybertsen to computationally simulate the quantum mechanical
transmission of an electron through each circuit.

“Both
the measurements and the simulations show that the molecules with two
parallel paths can have a conductance that is bigger than two times that
of molecule with a single path,” said Hybertsen. “This is the signature
that the quantum interference effect is playing a role.”

The
group suspects that other factors, such as the nature of the molecule’s
bond to the electrodes, need to be considered when calculating the
conductance of a molecular circuit. They are currently looking into
other central questions about molecular electronics, including how the
device changes when different metals are used.

This
research was funded primarily by the National Science Foundation and
the New York State Office of Science, Technology, and Academic Research.
Columbia’s Rachid Skouta and Severin Schneebeli synthesized the
experiment molecules with Ronald Breslow and Masha Kamanetska carried
out the conductance measurements. The CFN at Brookhaven Lab is supported
by the DOE’s Office of Science.

Probing the conductance superposition law in single-molecule circuits with parallel paths

Source: Brookhaven National Laboratory