Mechanically-Adjustable and Electrically-Gated Molecular Transistors
Mechanically-adjustable breakjunctions  and electromigration-induced breakjunctions with an electrostatic gate electrode  have both been used previously to make electrical measurements on single molecules. We have fabricated molecular devices that combine both mechanical adjustability and electrostatic gating. Our goal is to probe electron transport in single molecules as a function of controlled variations in both the molecular conformation and the energies of the electronic states in the molecule. So far we have studied C60 molecules in which we have observed a mechanically controlled charging effect. These transistors are therefore controllable with both gate voltage and mechanical motion.
Alexandre Champagne, Abhay Pasupathy, Joshua Parks, Dan Ralph.
A primary challenge in the field of single-molecule electronics is
to develop tools that can enable well-controlled, systematic
experiments. With simple techniques that
measure only a current-voltage
(I-V) curve, it can be difficult to
determine even whether a junction contains a molecule, because
nonlinear transport across
tunnel junctions or metallic shorts
can easily be mistaken for molecular signals. Efforts to
overcome this difficulty have employed
two separate strategies for systematically adjusting a molecular
device in situ, to make changes that can be compared with
theory. Mechanical adjustability, with scanning-probes or
mechanically-controlled break junctions, enables repeated
formation of molecular junctions and the assembly
of histograms to identify the resistances of single molecules.
Electrostatic gating permits control of electron transport through a molecule by
shifting its energy levels. We achieved the combined implementation of both mechanical adjustability and
electrostatic gating within a single-molecule device.
This tool can enable greatly improved understanding of molecular
transport, by allowing simultaneous control over both the device
geometry and the energies of the molecular states. We have studied devices
containing C60 molecules, and are now beginning to study carbon nanotubes, and organic molecules
showing interesting transport properties.
We have also pursued another approach to implementing both mechanical and electrostatic degrees of freedom in molecular devices. link
Mechanically-adjustable breakjunctions  are devices in which a narrow bridge of metal is suspended above a flexible substrate. By bending the substrate, the metal bridge can be broken, and the distance between the ends can be controllably adjusted, with increments of much less than a picometer. In an electromigration breakjunction, a current is passed across a narrow metal wire until it fails and forms two closely spaced contacts . By fabricating a gate electrode underneath the electromigration region, it is possible to make 3-terminal transistor devices, and by depositing molecules of interest on top of the wire before electromigration it is possible to make contact to single molecules . Our work combines the virtues of both techniques. First we make metal bridges suspended only 40 nm above a conducting substrate that will serve as the gate electrode. By using electromigration to break the bridges, we can form molecular junctions without initially having to bend the substrate. Subsequently, small deformations of the substrate adjust the size of the gap between electrodes, while the junction remains close enough to the gate electrode to allow molecular energy levels to be shifted by an applied gate voltage. Electromigration is a key factor in allowing us to use a silicon substrate since silicon is brittle and would fail before the metallic constriction did if we tried to break the junction mechanically. The use of silicon as the substrate makes it possible for us to make the breakjunctions in close enough proximity to the substrate to allow gating.
Our samples are fabricated on thin silicon wafers to allow some substrate bending. Silicon oxide is used as a spacing layer between the metal junction and the silicon substrate. Using a series of photolithography and e-beam lithography steps, and dry and wet etching, we define narrow gold bridges (30-50 nm wide) suspended 40 nm above the silicon substrate. We also make electrical contact to the wafer so that it can act as the gate electrode. Measurements are currently being made at 4.2 Kelvin, and the mechanical breakjunctions are driven with a fine threaded screw actuated by a stepper motor via a reduction gear-box.
Measurements on individual C60 molecules show current-voltage curves with Coulomb-blockage features that vary systematically as the breakjunction contacts are moved apart. By acquiring gate-voltage scans of the devices at different source-drain displacements, we have observed how mechanical motion changes the conductances and capacitances of the tunnel junctions, and the molecule’s offset charge. Both of the tunneling junctions (source-molecule and molecule-drain) are modified by mechanical motion. A contact potential arising from a work function difference between the electrodes and the molecule leads to an offset charge which changes as the electrodes are moved. We soon plan to study many other types of molecules exhibiting different transport mechanisms.
Figure 1: SEM image (78 degrees tilt) of a Au bridge suspended 40nm above a Si substrate, which is used, after electromigration, as a mechanical breakjunction.
Figure 2: I-V curves for a C60 molecule in a gold breakjunction. The spacing between the two gold electrodes is changed by 2 pm between each curve.
Figure 3: dI/dV versus bias voltage and gate voltage for a C60 device at two different electrode spacings.
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Last updated: 2005